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PLANT RESPONSE 



I 

WORKS BY THE SAME AUTHOR. 



RESPONSE IN THE LIVING AND NON- 
LIVING, With 117 Illustrations. 8vo. 105. 6d. 

1902. 

ELECTRO-PHYSIOLOGY OF PLANTS. 

(To be published shortly.) 



LONGMANS, GREEN, & CO., 39 Paternoster Row, London, 
New York and Bombay. 



PLANT RESPONSE 



AS A MEANS OF 



PHYSIOLOGICAL INVESTIGATION 



BY 



JAGADIS CHUNDER BOSE, -M.A., D.Sc. 

PROFESSOR, PRESIDENCY COLLEGE, CALCUTTA 



WITH ILLUSTRATIONS 



LONGMANS, GREEN, AND CO, 

39 PATERNOSTER ROW, LQ-NDON 
NEW YORK AND BOMBAY' 

1906 
AH rights reserved 



PREFACE 



THE investigations described in the present volume have 
been an outcome of my work on the Similarity of Responsive 
Phenomena in Inorganic and Living Matter, first com- 
municated as a Memoir to the Science Congress at Paris, in 
August ipoo, 1 and subsequently expanded into greater detail 
in my book on ' Response in the Living and Non-Living/ 
The electrical responses described in the Memoir referred to, 
had been obtained by the method of conductivity variation. 
The same problem was next attacked by a different mode of 
investigation, response being now obtained by electromotive 
variation. 52 Believing in the continuity of responsive pheno- 
mena in the inorganic and organic, I undertook on that 
occasion to demonstrate by the same method the electrical 
response of ordinary plants, and to show that every plant, 
and every organ of every plant, was excitable. It was then 
generally believed that so-called ' sensitive ' plants alone ex- 
hibited excitation by electrical response, and the proposition 
that ordinary plants also showed excitatory electrical response 
to mechanical stimulation, and that such response was appro- 
priately modified under physiological changes, was much 
controverted. I have to thank Professor Sidney H. Vines, 

1 * De la Generality des Phenom&nes Moleculaires produits par PElectricit 
sur la Mature Inorganique et sur la Matiere Vivante.' (Travaux du Congrts 
International de Physique. Paris, 1900.) 

2 Paper read before Royal Society, June 6, 1901. Also Friday Evening 
Discourse, Royal Institution, May 10, 1901. 



vlii PLANT RESPONSE 

at that time President, for the facilities which he then afforded 
me for the full publication of my results in the ' Journal ' of 
the Linnean Society, and for the warm interest which he has 

* 

manifested in my work, both then and later. 

I next undertook to demonstrate that ail the important 
characteristics of the responses exhibited by even the most 
highly differentiated animal tissues, were also to be found in 
those of the plant 1 

In my previous investigations I had shown that the tissues 
even of ordinary plants gave electrical signs of excitatory 
response. I now undertook an inquiry as to why they should 
not also exhibit response by mechanical indications ; and I 
was surprised to discover, that ordinary plants, usually re- 
garded as insensitive, gave motile responses, which had 
hitherto passed unnoticed. 

From the point of view of its movements a plant may be 
regarded in either of two ways : in the first place as a 
mysterious entity, with regard to whose working no law can 
be definitely predicated, or in the second place, simply as a 
machine, transforming the energy supplied to it, in ways 
more or less capable of mechanical explanation. Its move- 
ments are apparently so diverse, that the former of these 
hypotheses might well seem to be the only alternative. 
Light, for example, induces sometimes positive curvature, 
sometimes negative. Gravitation, again, induces one move- 
ment in the root, and the opposite in the shoot. From these 
and other reactions it would appear as if the organism had 
been endowed with various specific sensibilities for its own 
advantage, and that a consistent mechanical explanation 
of its movements was therefore out of the question. In spite 
of this, however, I have attempted to show that the planl 
may nevertheless be regarded as a machine, and that its 
movements in response to external stimuli, though apparently 

1 Paper read before Royal Society, February 4, 1904. 



PREFACE ix 

so "various, are ultimately reducible to a fundamental unity 
of reaction. 

This demonstration has been the object of the present 
work, and not that treatment of known aspects of plant- 
movements which is to be found detailed together with the 
history of the subject, in standard books of reference on 
plant physiology, such as those of Sachs, Pfeffer, Strasburger, 
Darwin, Francis Darwin, Vines, and Detmer. 

In analysing plant-movements the greatest complexity 
arises from the confusion of effects due to internal energy 
and external stimulus respectively. I have, however, been 
able to discriminate the characteristic expressions of these 
two factors, and thus to disentangle the complex phenomena 
which result from their combined action. Another very 
obscure problem is found in the nature of so-called ' spon- 
taneous or autonomous ' movements. By the discovery, 
however, of multiple response, and by the continuity which I 
have been able to establish, as existing between multiple and 
autonomous responses, it has been found possible to demon- 
strate that there are, strictly speaking, no c spontaneous ' 
movements, those known by this name being really due to 
external stimulus previously absorbed by the organism. 
Thus all the experiments have tended to show that the 
phenomenon of life does not, as such, connote any intrusion 
into the realm of the organic of a force which would interfere 
with that law of the Conservation of Energy which is known 
to hold good in the inorganic world. 

The elucidation of the fact that such varied and obscure 
phenomena in the life-processes of the plant, as, for instance, 
growth and the ascent of sap, are fundamentally due to the 
same excitatory reactions as are seen otherwise exemplified 
in the simple mechanical response now familiar to us, con- 
stituted a further result which, at the outset of the investiga- 
tion, was little to be foreseen. 



X PLANT RESPONSE 

It has been shown finally that there is no physiological 
response given by the most highly organised animal tissue 
that is not also to be met with in the plant. This was proved 
in detail in the case of the identical polar effects induced in 
both by electrical currents ; in the conduction of the excitatory 
impulse to a distance ; in the possibility of detecting the 
excitatory wave in transit and measuring its rate ; and in the 
appropriate modification of its velocity by different agencies, 
even in the case of ordinary plants ; in the passing of multiple 
into autonomous response in vegetable tissues ; in the light 
thrown by this phenomenon on the causes of rhythmicity in 
animal tissues ; in the similar effects of drugs on animal and 
vegetable tissues, and in the modifications introduced into 
these effects by the factor of individual ' constitution.' This 
identity of effects, indeed, as between the responses of plant 
and animal, is so deep and so extended, that it is to be 
anticipated that as several of the obscure problems of animal 
physiology have already been found elucidated by means of 
these researches carried out on plants, so others will be 
found capable of explanation by similar means in the near 
future. 

In conclusion, I wish to say that from my assistant, 
Mr. J. Roy, and my pupils, Messrs, A. C. Basu, S. C. Acharya, 
S. Chakravarty, N. Roy, and S. Goswami, I have received 
able assistance at various periods during the course of these 
long and extended investigations. 

J. C. BOSE. 

PRESIDENCY COLLEGE, CALCUTTA: 
July 1905. 



CONTENTS 

PART L SIMPLE RESPONSE 
CHAPTER I 

THE PLANT AS A MACHINE 

I'AGE 

Responsive movements in plants Work done by plant Plant as a fnachine 
Indicator-diagrams Physiological response-curves Pulse-records 
Cardiagrams Modification of pulse by poison* and other agencies 
Automatic response in plants Optical Lever Recorder Effect of 
external agencies on automatic pulse-beat in plants i 

CHAPTER II 

MECHANICAL RESPONSE TO STIMULUS 

Molecular derangement caused by stimulus Expression in change of form, 
contraction Mechanical model Myograph Response by differential 
contraction in pulvinated plant-organs Longitudinal response in plants 
Response of plant to all forms of stimulus Plant chamber Practic- 
able forms of graduated stimulus Electro-thermic stimulator Stimula- 
tion by condenser discharge Response-recorder Advantage of counter- 
poise Response of Biophytum to thermal stimulation Response to 
* condenser discharge Absolute measurements of motile effect and of 
work performed Effect of load Definite determination of threshold of 
response Determination of variation of excitability by measurement 
of minimally effective stimulus ........ 10 

CHAPTER III 

ON THE UNIVERSALITY OF SENSITIVENESS IN PLANTS 
AS DEMONSTRATED I*Y MEANS OF ELECTRICAL RESPONSE 

Arbitrary classification of plants into sensitive and ordinary Method of 
electro-motive variation for detecting state of excitation Hydraulic 
model Excitation of vegetable tissue, like that of animal tissue, induces 
galvanometric negativity Methods of direct and transmitted excitation 
Electrical and mechanical response alike record molecular derange- 
ment and recovery Similarities in simultaneous record of mechanical 



XII PLANT RESPONSE 

I'AGB 

and electrical response True excitation has a concomitant negative 
turgidity- variation, negative mechanical response or fall, and galvano- 
metric negativity These are true physiological responses, and are 
abolished at death Abnormal positive mechanical and electrical 
responses brought about by positive turgidity-variation Direct and 
indirect effects of stimulation Discrimination of differences of excit- 
ability by electric test Excitability of plant-tissues in general Re- 
sponsive power characteristic of matter 29 

CHAPTER IV 

ON CONDITIONS FAVOURABLE TO THE CONSPICUOUS 
EXHIBITION OF MECHANICAL RESPONSE 

Differences of degree of motile sensibility in sensitive plants so called 
Response of anisotropic organ brought about by differential contraction 
Production of response by artificial variation of turgidity Variation 
and counter- variation of turgesccnce, causing two opposite responsive 
movements Differences between hydrostatic and true excitatory effects 
Distinction of plants as ordinary and sensitive, arbitrary Sensitive 
plants may be excited, yet give no mechanical response Certain con- 
ditions necessary to exhibition of differential response Balanced action 
as result of diffuse stimulus on radial organ Slight differential contraction 
of pulvinus magnified by long petiolar index ..... 43 

CHAPTER V 

MECHANICAL RESPONSE IN ORDINARY LEAVES 

Pulvinoid and pulvinus Demonstration of mechanical response in ordinary 
leaves Response of Artocarpits similar to that of Biophytum Response 
to stimulus, even in old tissues, by expulsion of water Localisation of 
motile organ in ordinary leaves Conducting properties of various tissues 
Lamina is not the perceptive organ Response in ordinary leaves, 
though sluggish, yet comparable in extent to that of Mimosa Peculiar 
phenomenon of fatigue-reversal seen in Mimosa observed also in ordinary 
plants - Periodic reversals ......... 53 

CHAPTER VI 

LONGITUDINAL RESPONSE OF RADIAL ORGANS 

Absence of lateral response movements in radial organs due to mutually 
antagonistic effects of equal contractions of diametrically opposite sides 
Lateral response in radial stem of Walnut under unilateral stimulation 
Also in pistil of Mu$a~~+ Diffuse stimulation of radial organ causes 
longitudinal contractionThe * Kunchangraph 'Longitudinal contrac- 
tion of stamens of Cynerea not unique Similar longitudinal responses 



CONTENTS Xiii 

PAGE 

obtained with stems, roots, tendrils, petioles, stamens, and styles of 
ordinary plants- Also in fungi Responsive contraction in Passi/lora* 
comparable in extent with that in Cynerea Longitudinal response in 
plants modified by the physiological variations due to age, season, and 
chemical agencies 66 

CHAPTER VII 

RESPONSIVE CURVATURE OF MOLECULARLY ANISOTROPIC 

ORGAN 

Molecular anisotropy artificially induced by one-sided cooling Cooled side 
less responsive Diffuse stimulation causes concavity of the uncooled, 
that being relatively the more excitable Local fatigue diminishes excit- 
ability Diffuse stimulation now causes concavity of the unstrained side 
Similar anisotropy induced in plagiotropic organs, by unilateral action 
of light The lower or shaded side of such organs relatively more excit- 
able Diffuse stimulation causes current of response from Jower to upper, 
and also concavity of lower half Responses of plagiotropic Cucurbita 
and Convolvuhts Differences in excitabilities of outer and inner surfaces 
of tubular organ Complex response due to successive excitations of two 
antagonistic halves of an anisotropic organ Response of spiral tendrils 
by uncurling Response in certain cases by contraction of the spiral or 
curling Writhing movement in spiral tendril under strong stimulation . 82 

CHAPTER VIII 

RELATION BETWEEN STIMULUS AND RESPONSE 

Ineffective stimulus becomes effective by repetition Two types of response 
in contractile animal tissues, cardiac and skeletal Response of cardiac 
muscle on ' all or none ' principle ; parallel case in Biophytwn In 
skeletal muscle, increasing stimulus causes increasing response, which . 
tends to reach a limit Parallel results in longitudinal and electrical 
response of plants Effect of superposition of stimuli Tetanus . . 94 



PART II. MODIFICATION OF RESPONSE 
UNDER VARIOUS CONDITIONS 

CHAPTER IX 

ON THE UNIFORM, FATIGUE, AND STAIRCASE EFFECTS 

IN RESPONSE 

Uniform response in plants Staircase effect Fatigue due to molecular 
strain Fatigue in plant-responses Periodic fatigue Fatigue under 
continuous stimulation Explanation of anomalous erection of leaf of 



XIV PLANT RESPONSE 

PAGE 

Mintosa under continuous stimulation Conductivity and excitability of 
tissue diminished through incomplete protoplasmic recovery Relatively 
greater fatigue in a motile than conducting organ Disappearance of the 
motile excitability earlier than conductivity Refractory period Absence 
of responsive effect when stimulus falls within refractory period . . 103 



CHAPTER X 

THEORIES CONCERNING DIFFERENT TYPES OF RESPONSE 

The chemical theory of response Insufficiency of the theory of assimilation 
and dissimilation to explain fatigue and staircase effects Similar 
responsive effects seen in inorganic substances Molecular theory 
When molecular recovery is complete, responses uniform : when incom- 
plete, fatigue brought about by residual strain Fatigue under con- 
tinuous stimulation, in inorganic substance, in plant, and in muscle 
Staircase effect brought about by increased molecular mobility : examples 
seen in inorganic substance, and in living tissues No sharp line of 
demarcation in the borderland between physical and chemical pheno- 
mena Molecular changes attended by changes of chemical activity 
Unequal molecular strain gives rise to a secondary series of chemical 
actions Volta-chemical effect and by-products Supposition that re- 
sponse always disproportionately larger than stimulus, not justified 
Existence of three types : (i) response proportionate to stimulus; (2) 
response disproportionately greater than stimulus ; (3) response dis- 
proportionately less than stimulus Instances of stimulus partially held 
latent : staircase and additive effects ; multiple response ; renewed 
growth 116 



CHAPTER XI 

EFFECT OF ANAESTHETICS, POISONS AND OTHER CHEMICAL 
REAGENTS ON LONGITUDINAL RESPONSE 

Response modified by physiological change Carbonic acid causes depression, 
and transitory exaltation as after-effect Gradual abolition of response in 
hydrogen and restoration by access of air Chemical agents cause con- 
traction or relaxation of plant-tissue Effect of alcohol causing temporary 
exaltation of response followed by depression and protracted period of 
recovery Ether causes relaxation and temporary depression of response 
Explanation of anomalous action of ether on stimulated Mimosa leaf 
Abolition of response by hydrochloric acid Response restored by 
timely application of ammonia Abolition of response by poisonous 
reagent Similarity of effect of chemical agents on the response of animal 
and vegetable tissues 129 



CONTENTS XV 

CHAPTER XII 

EFFECT OF TEMPERATURE 

I'AGF 

Temperatures optimum, maximum, and minimum Diminution of electrical 
response by cooling Temporary or permanent abolition of response due 
to cold Characteristic differences exhibited by different species 
Mechanical response of Biophytum and autonomous response of 
Desmodium arrested by cold Prolongation of latent period Diminution 
of longitudinal mechanical response by cold Diminution of electrical 
response of plants by rise of temperature Similar diminution seen in 
longitudinal mechanical response Increase of excitability due to cyclic 
variation of temperature . . . . . . . , 139 

CHAPTER XIII 

ON THE DEATH-SPASM IN PLANTS 

Difficulty of determining exact moment of death Various post-mortem 
symptoms afford no immediate indication Ideal methods for determina- 
tion of death-point Realised in four different ways : (a) Determination 
by electrical method (b) Determination by spasmodic lateral movement 
at moment of death Experiments with Mimosa Death-contraction a 
true physiological response Continuity of fatigue and death Death- 
point earlier in young tissues Composite spasmodic movement (//) 
Determination of death-point in tendril of Passijlora, by sudden move- 
ment of uncurling (c) Determination of death-point by method of volu- 
metric contraction of hollow organ, causing expulsion of contained water 148 

CHAPTER XIV 

THE DETERMINATION OF THE CRITICAL POINT OF DEATH 
BY INVERSION OF THE THERMO-MECHANICAL CURVE 

Death-spasm in anisotropic organ due to differential longitudinal contraction 
In radial organ the death-contraction is purely longitudinal Death- 
point determined from point of inversion of a thermo-mechanical curve 
The complete record thus constitutes a curve of life-and-death, the two 
being separated by the death-point Characteristic thermo-mechanical 
curve as resultant of variation of temperature and variation of length 
The necessity of specifying the rate of rise of temperature The thermo- 
mechanical curve characterised by sharp and definite inversion at point 
of death No inversion of thermo-mechanical curve after death of plant 
Death-contraction under heat-rigor in plant analogous to similar 
phenomenon in animal The Morograph^ a perfected form of apparatus 
for determining critical point of death Remarkable identity of thermo- 
mechanical curves obtained with two similar specimens Death -point 
almost as definite as a physical constant Vanishing of point of inversion 
with age Determination of death-point under cold-rigor Constancy of 
death- point 159 



PLANT RESPONSE 



CHAPTER XV 

EFFECT OF VARIOUS AGENCIES ON DEATH-RESPONSE : 
THERMOGRAPHS OF REGIONAL DEATH 

PAGE 

Lowering of death-point by fatigue Modification of characteristic thermo- 
mechanical curve by the action of chemical agents Comparison- 
Morograph Duplication of rigor-point Death-response a physiological 
response and not due to coagulation Death-movements of flowers 
Approximate constancy of death-point of florets in a capitulum Definite 
interval between death-point and discoloration-point Translocation of 
discoloration-point by various agencies Thermographs of regional 
death Thermograph of local fatigue Thermographic investigation of 
electrotonic excitation . . . 176 



PART III. EXCITABILITY AND 
CONDUCTIVITY 

CHAPTER XVI 

ON EXCITATORY POLAR EFFECTS OF CURRENTS 

Hydro-mechanical theory of excitation in plants Theory of protoplasmic 
change Crucial tests applied by means of polar excitation Mono-polar 
and Bi- polar methods of excitation Advantages of study of polar 
excitation in plant -tissues as compared with animal Effects of feeble 
E.M.F. Effect of moderately high E.M.F. Experiments with highly 
excitable tissues 189 

CHAPTER XVII 

ON CONDITIONS OF REVERSAL OF NORMAL POLAR EFFECTS 

IN LIVING TISSUES 

Effect of high E.M.F, Effects at two stages, A and B Experimental 
verification of A stage effect Similar effects seen in protozoa Experi- 
mental verification of complete reversal at B stage Law of polar effects 
under high E.M.F. Investigation on polar effects by death-response 
Reversal of polar effects as due to fatigue, or tissue-modification In- 
vestigation of polar effects by glow-response of fireflies . . . . 200 

CHAPTER XVIII 

ON CONDUCTIVITY AND EXCITABILITY 

Receptive excitability, conductivity, and motile excitability Molecular 
Model Modification of motile excitability : (a) by anaesthetics () by 



CONTENTS XVli 

PAGE 

cold (c) by fatigue Variation of conductivity : (a) by cold (6) by 
rise of temperature (c) by fatigue (d) by anaesthetics Variation of 
receptive excitability by ether Conductivity versus excitability 
Abolition of motile excitability without abolition of conductivity 
Hydro-mechanical theory of transmission of stimulus untenable . .216 



CHAPTER XIX 

ON ELECTROTONUS 

The anode acts as a block to the transmission of stimulus Opposite effect 
of kathode Experiments on Biophytum y showing variations of con- 
ductivity by anode and kathode respectively Experiments on Mimosa^ 
showing increase of motile excitability at or near the kathode, and 
diminution of motile excitability at or near the anode Curious c develop- 
ment ' of response, near the kathode . . . . . . .231 

CHAPTER XX 

ON THE VELOCITY OF TRANSMISSION OF EXCITATORY WAVES 

IN PLANTS 

Difficulties in accurate determination of velocity of transmission, due to 
unknown variations of excitability arising from injury, and variations of 
conductivity through fatigue A perfect method of obtaining accurate 
and consistent results Relative advantages of studying conduction in 
plants as compared with animals Determinations of velocity of trans- 
mission in centripetal and centrifugal directions Preferential conductivity 
in centrifugal direction Diminution of conductivity and excitability by 
fatigue Within a certain critical interval, organ ' refractory ' to further 
stimulus Increased velocity of transmission with increasing stimulus 
Measurement of diminution of conductivity by cold Fibro-vascular 
elements the best conducting channels Conductivity lengthwise greater 
than crosswise Electric mode of determination of velocity of trans- 
missionIndifferent parenchymatous tissues practically not transmitters 
of excitation Comparative tables showing velocity of transmission in 
various plant and animal tissues ........ 238 

CHAPTER XXI 

ON DETECTION OF EXCITATORY PULSE DURING TRANSIT BY 
ELECTROTACTILE AND ELECTROMOTIVE METHODS 

Pfeffer's experiment on expulsion of water from excited cells Author 's 
experiment on a delicate method of detecting excitatory expulsion of cell- 
sap Chemical method of determining velocity of transmission of excita- 
tion Electrotactile detectorDemonstration of passage of excitatory 

a 



XVtii PLANT RESPONSE 

I'AGE 

contractile wave by means of electrotactile method Determination of 
velocity of transmission of excitation in ordinary plants by electromotive 
method Excitatory versus hydro-mechanical movement of water . , 254 

.CHAPTER XXII 

THE LATENT PERIOD AND REFRACTORY PERIOD 

The determination of the latent period in Mimosa Experimental arrange- 
ments for obtaining automatic record Prolongation of latent period by 
cold Spark-record for determination of latent period Prolongation of 
latent period by fatigue Sluggishness of the response of Philanthus 
urinaria, also long latent period and very protracted period of recovery 
Latent period reduced under strong stimulation Response in 
Biophytum on the ' all or none ' principle Definite value of effective 
stimulus Phenomenon of refractory period in Biophytum Parallelism 
of responses in Biophytum and in cardiac muscle Additive effects 
Inappropriateness of term 'refractory period 5 Energy in excess of 
effective stimulus held latent for subsequent manifestation . . . 264 



PART IV. MULTIPLE AND AUTONOMOUS 

RESPONSE 

CHAPTER XXIII 

ON MULTIPLE RESPONSE 

Multiple electromotive responses due to a single strong stimulus Multiple 
electrotactile responses Multiple mechanical responses in Biophytum 
Cyclic variations in multiple responses Multiple retinal excitations 
Intermittent pulse in man and plant Semi-automatism Continuity of 
multiple and automatic response Conversion of Biophytum into auto- 
matically responding plant ; conversion of Desmodium into ordinarily 
responding plant Similar polar effects of current in Biophytum and in 
Desmodium leaflet at standstill Moderate stimulus in Biophytum and 
in Desmodium at standstill produces single response ; and strong stimulus, 
multiple response 279 

CHAPTER XXIV 

AN INQUIRY INTO THE CAUSES OF AUTONOMOUS MOVEMENTS 

Production of pulsatory movements as after-effect of energy absorbed 
Physical analogue Localisation of seat of automatic excitation in D 
medium Demonstration of multiple response to a constant stimulus : 
(i) Chemical (2) Electrical (3) Stimulus of light Multiple response 



CONTENTS xix 

PAGE 

to constant stimulus of light, in : (a ) retina~() Biopkytum -~(c) Dt$- 
modium~(4\ Thermal- Induction of automatism in Biophytum at 
favourable temperature (5) Of internal hydrostatic pressure Absorp- 
tion of external energy and its absorption by the plant in latent form 
True meaning of c tonic ' condition Cause of rhythmicity After-effect, 
and its relative persistence . . 295 



CHAPTER XXV 

INFLUENCE OF VARIOUS CHEMICAL REAGENTS ON THE 
AUTONOMOUS RESPONSE OF DESMODIUM GYRANS 

The recorder and experimental chamber Absolute measurement of period 
and amplitude of Desm0dtum-osci\la,tion Responsive significance of up 
and down movements deduced from (a) analogy with response of 
Mimosa ; (b) test of increased internal hydrostatic pressure ' Systolic ' 
contraction and * diastolic J expansion of Desmodium pulvinus Mode of 
application of chemical reagents Action of chemical reagents modified 
by : tonic condition of plants ; strength of solution ; and duration of 
application Effect of anaesthetics Effect of alcohol Effect of carbonic 
acid Effects of ammonia and of carbon disulphide Effect of copper 
sulphate solution, either when applied externally, direct on the pulvipus, 
or internally Spark -record of Zter/wdYwjw- pulsation . . . . 315 



CHAPTER XXVI 

EFFECTS OF TEMPERATURE ON AUTONOMOUS RESPONSES 

Increase of frequency and diminution of amplitude of pulsation with rising 
temperature Converse effect of fall of temperature Similar effect in 
cardiac pulsation Effect of the reduction of temperature to the thermo- 
tonic minimum Explanation of diminution of amplitude of pulsation 
with rise of temperature Anomalous use of the word ' relaxation ' 
Simple versus additive character of individual pulsation . . . 329 

CHAPTER XXVII 

SIMILARITIES OF RHYTHMIC RESPONSE IN VEGETABLE AND 

ANIMAL TISSUES 

The similarities) in their fundamental characteristics, of rhythmic tissues, 
animal and vegetable: (i) In responses (2) In possession of long re- 
fractory periods (3) In incapability of tetanus Theories regarding the 
causation of heart-beat The similarities of rhythmic tissues, animal and 
vegetable, as seen in : (I) The effects of internal hydrostatic pressure 
(2) The effects of variation of temperature (3) The periodic groupings 

a 2 



XX PLANT RESPONSE 

PAGE 

of response (4) The effect of barium salt (5) The antagonistic actions 
of acid and alkali Identity of rhythmic phenomena in animal and 
vegetable tissues 344 



PART V. ASCENT OF SAP 
CHAPTER XXVIII 

. SUCTIONAL RESPONSE AND ASCENT OF SAP 

Inadequacy of existing theories of ascent of sap General considerations 
regarding cellular activity and resultant propulsion of water The 
Shoshungraph Balanced Shoshungraph for determining variations of 
suction Hydrostatic and Hydraulic Methods of Balance . . . 359 

CHAPTER XXIX 

MODIFICATION OF SUCTIONAL RESPONSE 

Effect of temperature on suction by three methods of inquiry: (i) Un- 
balanced method of Shoshungraph : (a) Action of cold (6) Action of 
moderate rise of temperature (2) Method of Hydrostatic Balance : (a) 
Action of cold Reversal of normal direction of flow (b) Action of 
warm water (3) Method of Hydraulic Balance (a) Action of cold 
(b) Effect of warm water Explanation of suction when the root is 
killed by boiling water Stimulation renews suctional activity in plant 
whose suction has come to a standstill Osmotic versus excitatory action 
Abolition of suction by poison Suctional activity continued until 
whole plant is killed by poison ........ 372 

CHAPTER XXX 

THE PHENOMENON OF PROPULSION OF SAP 
AND ITS VARIOUS EFFECTS 

The mechanics of the ascent of sap : (a) Uni-directioned flow (b) Initiation 
of multiple rhythmic excitations Connection between conduction of 
excitation and conduction of sap Rapidity of ascent of sap accounted 
for by stimulatory action Positive and negative pressures due to one 
cause (i) Positive pressure (2) Negative pressure (3) Irregular 
variations of pressure Direct conduction and conduction by relays 
Excretion of water Excretion of nectar Translocation of organic food- 
substances Mechanical response to suctional activity Effect of warmth 
Effect of cold Explanation of the drooping of leaves during frost 
Explanation of response and recovery Antagonistic actions of internal 
energy and external stimulus 389 



CONTENTS XXI 



PART VI. GROWTH 
CHAPTER XXXI 

THE RECORD OF GROWTH-RESPONSE 

PAGE 

The simple Crescograph The Balanced Crescograph Rhythmic growth- 
response Growth-response and excitatory response Law of direct and 
indirect effects of excitation Positive turgidity- variation as indirect effect 
of excitation Mechanical test Significance of c inner stimuli ' . . 409 

CHAPTER XXXII 

THE EFFECTS ON GROWTH OF INTERNAL ENERGY 
AND EXTERNAL STIMULUS 

Characteristics common to growth and to other forms of rhy thtnic response : 
(i) Periodic groupings (2) Effect of external stimulus in renewal of 
growth when at temporary standstill (3) Renewal of growth-pulsation 
by positive turgidity-variation (4) Effect of increased internal hydro- 
static pressure (5) Effect of ascent of sap on growth Effect of tempera- 
ture on growth Comparison of various types of multiple response 
Effect of external tension on growth Effect of direct application of 
stimulus on the growing region Similarities between motile and growth 
responses Direct and indirect effects of stimulus, and laws of growth . 424 

CHAPTER XXXIII 

ON THE RELATION BETWEEN TEMPERATURE AND GROWTH, 
AND THE ACCURATE DETERMINATION OF OPTIMUM AND 
MAXIMUM POINTS 

General consideration of difficulties of accurate determination of effects of 
temperature on growth Four accurate methods : (i) Method of discon- 
tinuous observations Accurate regulation of temperature by electrolytic 
rheostat (2) Method of continuous observations Thermo-crescent 
curve Determination of the optimum point (3) Method of balance 
(4) Method of excitatory response Translocation of the optimum point 441 

CHAPTER XXXIV 

ON AN ATTEMPT TO DETECT AND MEASURE LATENT STIMULUS, 
AND ON THE STUDY OF PERIODIC AFTER-EFFECTS 

Positive and negative after-effects Extreme delicacy of the Method of 
Balance Detection of absorbed stimulus by negative after-effect 



XXII PLANT RESPONSE 

PAGE 

Constancy of sum of direct and indirect after-effects Latent component 
almost vanishing above the optimum Variation of receptivity Direct . 
and indirect response of plant in sub-tonic condition Table showing 
direct and indirect effects at different temperatures Is the change 
induced by stimulus always of an explosive chemical character ? Rela- 
^ tion between stimulus and response in different tonic conditions After- 
* effect Factors which determine periodic after-effects :, (i) "Stimulus of 
light (2) Temperature (3) Chemical stimulus (4) Turgidity Con- 
tinuous photographic record of the pulsations of Desmodium Record of 
petiodic variation of rate of growth Continuous photographic record 
of periodic variations of transpiration Continuous photographic record 
of the variation of the rate of growth Annual rings and seasonal 
periodicity 456 



CHAPTER XXXV 

AN INVESTIGATION INTO THE DIFFERENT EFFECTS OF DRUGS 
ON PLANTS OF DIFFERENT * CONSTITUTIONS ' 

General consideration of the problem f Constitution f and the elements 
which determine it Methods of investigation Action of carbonic acid 
gas Action of ether Effect of solution of sodium carbonate Effect of 
solution of sugar Effect of alcohol Effect of acids Effect of alkali 
Antagonistic action of alkalis and acids Action of strong solution of 
sodium chloride Effect of poisonous solution of copper sulphate 
Opposite effects of the same dose on different constitutions Opposite 
effects of large and small doses 477 



PART VII. GEOTROPISM, CHEMOTROPISM, 
AND GALVANOTROPISM 

CHAPTER XXXVI 

THE RESPONSIVE CURVATURES CAUSED BY GRAVITY. 
NEGATIVE GEOTROPISM 

Statement of the problem of apogeotropic response Mode in which stimu- 
lation is brought about : radial-pressure theory, and theory of statoliths 
Mechanics of responsive movement Experiment demonstrating re- 
sponsive curvature as brought about by unilateral pressure of particles 
Record of curvature induced by gravitation Record of different rates 
of curvature when specimen is held at angles of 45 and 135 to the 
vertical Determination of the true character of apogeotropic response 
-Responsive curvature of acellular organs Curvature of grass haulm 
under gravity Growth of grass haulm on a klinostat . 493 



CONTENTS xxiii 

CHAPTER XXXVII 

THE RESPONSIVE PECULIARITIES OF THE TIPS OF 
GROWING ORGANS 

PAGE 

Difference between shoot and root in their response to stimulus of gravity 
Difference in character of response between tip and growing region of 
root Scope of the investigation Electrical investigation Responsive 
results of: i. Longitudinal transmission of effect of stimulus from tip ; 
(a) Moderate unilateral stimulation ; (6) effect of stronger unilateral 
stimulation 2, Direct unilateral stimulation of growing region- 
Moderately strong stimulus 3, Transverse transmission of effect of 
stimulus ; (a) moderate stimulation ; (b) stronger stimulation 
Mechanical response inferred from observed electrical response 
Tabular statement ' . . . .512 

CHAPTER XXXVIII 

INQUIRY INTO THE LAWS OF RESPONSIVE GROWTH- 
CURVATURES 

Scope of the investigation : i. Mechanical response to unilateral stimula- 
tion of the tips of shoot and root : (a) Moderate stimulus (b) Stronger 
stimulation 2. Effect of unilateral stimulus, applied at the responding 
growing region: (a) Moderate stimulus (A) Strong or long-continued 
stimulus Experiments on the direct and indirect effects of stimulus on 
Mimosa : (a) Direct stimulation (b) Indirect stimulation, longitudinal 
transmission (c) Indirect stimulation, transverse transmission The 
curious response of Arisama Table showing responsive effects common 
to pulvini, pulvinoids, and growing organs Laws of responsive growth- 
curvature ............ 524 

CHAPTER XXXIX 

INQUIRY INTO POSITIVE GEOTROPISM 

No specific difference as regards their responses, between shoot and root 
Darwinian curvature Localisation of geotropic sensibility at the root- 
tip Experiments as to whether amputation of root-tip abolishes 
excitability The tip of the root the organ of gravi-perception The 
perceptive versus the responding organ True perceptive region . . 536 

CHAPTER XL 

ON CHEMO-TROPISM AttD GALVANO-TROPISM 

General difficulties of the investigation How to overcome these difficulties 
Three distinct methods of testing results: (i) by variation of longitudinal 



XXIV PLANT RESPONSE 

PAGE 

growth ; (2) by responsive movement of pulvinus; and (3) by growth* 
curvature Method of application of chemical reagent Effect of alkali 
Effect of acid Effect of copper sulphate Action of sugar solution 
Chemo-tactic movements Explanation of anomalous osmotic or plas- 
molytic action Excitatory versus plasmolytic reaction in pulvinus of 
Mimosa : (i) Favourable tonic condition (2) Ordinary tonic condition 
Polar effects of currents inducing growth-curvatures Localised polar 
effects on pulvinus Anodic and kathodic effects on longitudinal growth 
Generalised law of polar excitation in plants Galvano-tropic response 
The indirect effect of polar excitation The effect on growth of 
* electrification ' of soil 546 



PART VIIL HELIOTROPISM 
CHAPTER XLI 

FUNDAMENTAL RESPONSIVE ACTION OF PLANT-TISSUES 
TO STIMULUS OF LIGHT 

Diversity of movements induced by light Differentiation of responsive 
movements Action of light on tissues in sub-tonic condition Effect of 
light on pulvinated organs Effect of diffuse stimulation of light on 
non-growing radial organs Retai ding effect of light on longitudinal 
growth Phenomenon of oscillation under long-continued stimulation- 
Similarity of responsive reaction under light and under other forms of 
stimulation 565 

CHAPTER XLII 

POSITIVE HELIOTROPISM 

Introduction Theory of de Candolle Inadequacy of de Candolle's theory 
Definition of terms positive and negative Darwin's theory of 
modified circumnutation Response of terminal leaflet of Desmodium 
Extreme sensitiveness of some plant-organs to light Merging of 
multiple in continuous response Orientation induced by light The 
perceptive region in the terminal leaflet of Desmodium Heliotropic 
response in radial organ Magnetically controlled recorder Heliotropic 
response of hypocotyl of Sinapis Recovery and theory of recti-petality. 579 

CHAPTER XLIII 

NEGATIVE HELIOTROPISM 

Incomplete parallelism between actions of light and of gravitation 
Theoretical considerations Recording microscope Negative hello- 



CONTENTS XXV 

I>AGB 

tropic curvature induced by stimulation of the tips of root and shoot 
Intermediate phases between positive and negative heliotropic response : 
(a) neutralisation by transverse transmission ; (d) neutralisation by 
transverse transmission, with multiple response Localised sensitiveness 
to light and transmission of excitatory effect Negative heliotropism of 
a radial organ Gradual transition from positive to negative, through 
intermediate phase of neutrality Apparent heliotropic insensitiveness of 
certain tendrils Negative heliotropism of tendril of Vitis . . , 597 

CHAPTER XLIV 

EFFECT OF INVISIBLE RADIATION AND EMANATIONS 

t 

Effect of temperature and its variations Demonstration of fundamental 
effect of thermal radiation^on growth Response to successive uniform 
stimuli of thermal radiation Effect of continuous unilateral stimulation 
Effect of electrical waves on growth Response of Mimosa to electric 
radiation Action of high frequency Tesla current * 614 

CHAPTER XLV 

ON PHOTONASTIC PHENOMENA AND ON DIURNAL SLEEP 

Photonasty and para-heliotropism Response of Tropaolum ma/us Re- 
sponses of plagiotropic stems : (a) Mimosa (b) Ipom&a (c) Cucurbita 
Daily periodic movements of plagiotropic stems Responsive movements 
of pulvinated organs Pulvinated organs showing positive heliotropic 
movement : (a) Response of terminal leaflet of Desmodium (t>) Response 
of leaflet of Robinia (c) Responsive movements of leaflets of Erythrina 
indica and Clitoria ternatea The negative heliotropic type of response : 
(a) Response of pulvinus of Mimosa (b) Diurnal sleep of Oxalis (c) 
Diurnal sleep of Biophytum Directive versus non -directive action of 
light General view of responsive curvatures induced in different organs 
by unilateral application of light 621 

CHAPTER XLVI 

ON DIA-HELIOTROPISM AND DIA-GEOTROP1SM 

Difficulty of distinguishing between effect of light and other reactions- 
Theories of Frank and De Vries Subsidiary factors: (i) Epinasty and 
hyponasty ; (2) Effect of gravity ; (3) Effect of suctional activity and 
of turgescence; (4) Modification of effect by characteristic limits of 
flexibility Discrimination of the part played by heliotropism in the 
movement of the leaf Proof of absence of any specific dia-heliotropic 
tendency in leaves The lamina not the perceptive organ Principal 
types of the response of leaves to stimulus of light Positive type of 
response : mango leaf Negative type of response : leaf of Artocarpus . 640 



XXVi PLANT RESPONSE 



CHAPTER XLVII 

TORSIONAL RESPONSE TO HELIOTROPIC AND GEOTROPIC STI- 
MULUS : AUTONOMOUS TORSION AND ITS VARIATIONS 

PACK 

Torsional effect Method of recording torsional response Torsional re- 
sponse under the lateral action of light Torsional response to other 
forms of lateral stimulation Torsional response of compound strip of 
ebonite and stretched india-rubber Modification of torsional response 
by artificial variation of the relative excitabilities of the two halves 
Laws of torsional response Demonstration of differential geotropic ex- 
citation in a dorsi-ventral organ Torsional response to lateral geotropic 
stimulation Modification of torsional geotropic response by artificial 
variation of differential excitability Autonomous torsion : Effect of 
temperature Effect of light Effect of electrical current Effect of 
gravity The twining of stems 656 



CHAPTER XLVIII 

NYCTITROPIC MOVEMENTS 

Comparison between nyctitropic and autonomous pulsations Diurnal move- 
ment of plagiotropic stem Supposed distinction between nyctitropic 
and other movements of response to stimulus of light Diurnal response 
of leaf of Biophytum Diurnal response of primary petiole of Mimosa 
Periodic itrtpulses acting on the leaf Periodic impulses contributed by 
the plant as a whole Other modes of exhibition of diurnal periodicity 
of hydrostatic tension Forced vibration and its periodic after-effects 
Physical analogue Impressed periodic vibrations in organ originally 
radial 676 



CHAPTER XLIX 

ON PULSATORY RESPONSE AND SWIMMING MOVEMENTS AS 
INITIATED AND MODIFIED BY LIGHT AND OTHER FORMS 
OF STIMULUS 

Investigations on the influence of light on the lateral leaflet of Desmodium 
gyrans : (a) in sub-tonic condition ; (/>) in normal tonic condition 
Changes induced in existing anisotropy of Desmodium leaflets Reversal 
under intense stimulation seen in all forms of response The swimming 
movements of ciliated organisms Fundamental resemblance between 
the swimming responses and the ordinary heliotropic responses in radial 
organs Phototactic movements : (a) Two natural types of responsive 
movements; () Responsive movement positive, negative, or inter* 
mediate, according to intensity of stimulation Directive action of light 
Thermotaxis Galvanotaxis Chcmotaxis . . . . 689 



CONTENTS xxvii 



PART IX.-GENERAL SURVEY AND 
CONCLUSION 

CHAPTER L 

REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 

PAGE 

Responsive contraction Kunchangraphic records Direct and indirect 
effects of stimulation Various forms of responsive expression ; (a) 
Lateral motile response by differential contraction ; (b) Suctional re- 
sponse ; (c) Growth-response ; (d) Torsional response ; (e) Death 
response ; (/) Thermographs of regional death ; (g) Electrical response 
Different types of response : (a) Uniform response ; (d) Fatigue ; 
(c) Staircase response ExcitabilityConductivity Polar effects of 
currents Multiple response Continuity of multiple and autonomous 
responses The ascent of sap ........ 703 

CHAPTER LI 

RESPONSIVE GROWTH-CURVATURES IN PLANTS 

Longitudinal growth and its variations Effect of temperature on growth 
Responsive growth-curvature under unilateral stimulation: I. Direct 
unilateral stimulus on the responding organ : (a) Positive response 
under moderate stimulation ; () Intermediate or neutral response ; (c) 
Negative response ; (d) Dorsi-ventral positive response ; (e) Dorsi-ventral 
response which may become negative 2, Indirect effect of uni- 
lateral stimulation : (a) Negative response ; (b) Positive response 
Responsive action under stimulus of gravity Heliotropic action in radial 
organs Heliotropic action in plagiotropic and dorsi-ventral organs 
Phototactic movements Nyctitropic movements . . . . 7^5 

CHAPTER LII 

ON PHYSIOLOGICAL RESPONSE, 
AND ITS CONTINUITY IN PLANT AND ANIMAL 

Vitalism - Fundamental unity of physiological response in plant artd animal 

Variation as induced by external forces . . 740 



CLASSIFIED LIST OF EXPERIMENTS ..... 755 
INDEX ............. 765 



ERRATUM 
Page 409, line I of analytical contents, for Growth-Recorder read Crescograph. 



ILLUSTRATIONS 



FIG. 1'AGE 

I* Record of Healthy Adult and Senile Human Pulse (Broadbent) . . 3 

2. Effect of Muscarin in arresting Pulsation of Frog's Ventricle (Gushing) 4 

3. Record of Human Pulse (Broadbent) ...... 4 

4. Demonstration Optical Pulse- Recorder . . . +. . . . 7 

5. Death of Plant, and Arrest of Pulsation, by Poison .... 8 

6. Death, and Arrest of Pulsation, in Leaflet of Desmodium by Strong 

Electric Shock 8 

7. Response of India-rubber 12 

8. Mechanical Lever Recorder 12 

9. Series of Contractile Responses in Muscle . . , . 13 
10, Photographic Record of Longitudinal Contractile Response in 

ordinary Stamens (Brownea ariza) 13 

n. Differential Lateral Response of Compound Strip . . . 13 

12. Plant Chamber and Recorder 16 

13. The Electro-thermic Stimulator . . 17 

14. Diagram of Connections for Stimulation by Condenser Discharge . 18 

15. Photographic Record of Response and Recovery of Mimosa, taken on 

a slowly moving drum 21 

1 6. Photographic Record of Response in different specimen, taken on a 

faster-moving drum . . 21 

17. Response of Biophytunt to Thermal Stimulation .... 24 

1 8. Response of Biophytttm to Electric Stimulation 24 

19. Effect of Load ... 25 

20. The Spiral Spring-Recorder 26 

21. Isometric Response of Mimosa 26 

22. Hydraulic Model for Explanation of Electric Response , . . 31 

23. Electrical Response in Plant by Method of Block . . . -32 

24. Electric Response Recorder 33 

25. Method of Transmitted Stimulation 33 

26. Simultaneous Mechanical and Electrical Responses in Biophytum . . 34 

27. The Abnormal Positive preceding the Normal Negative in Mechanical 

and Electrical Responses in Biophytum 37 

28. Response of Selenium to the Stimulus of Light 39 



XXX PLANT RESPONSE 

*KJ> PAGE 

29. Response of Metal abolished by the action of ' Poison ' (Oxalic Acid) . 40 

30. Responses of quickly reacting Biophytum, and sluggish Philattfhtts 

urinaria, under moderate and under stronger Stimulation . . 44 

31. Artificial Hydraulic Response of Mimosa 46 

32. Response of Ordinary Leaf (Artocarpus) 56 

33- Response of Leaves of Ordinary Plants to Electric Stimulation . . 64 

34. Alternate opposite-directioned Responses obtained by the successive 

Unilateral Stimulations of opposite sides of Pistil of Musa . . 68 

35. The Kunchangraph 73 

36. Diagrammatic Representation of Apparatus for Periodic Stimulation 

of Plant . 75 

37. Response of Stem of Cuscuta to Electric Stimulation . . . . 77 

38. Photographic Record of Responses of Style of Datura alba to Thermal 

Stimulation 78 

39. Responses of Plagiotropic Stem of Cucurbita 86 

40. Responses of Plagiotropic Stem of Convolvuhis . . .86 

41. Response of Bifurcated Alliitm Tube by sudden Collapse . . . 87 

42. Responses of Hooked Tendril of Passiflora 90 

43. Response by Coiling of spirally-cut Alliiim Peduncle . . . . 91 

44. Ineffective Stimulus made Effective by Repetition 95 

45. Additive Effect in Electrical Response 95 

46. Mechanical Responses to Stimuli increasing in Arithmetical Progression 96 

47. Curve showing Relation between Stimulus and Response . , . 96 

48. Increased Electrical Response with Increasing Vibrational Stimuli < 

(Cauliflower-stalk) 97 

49. Genesis of Tetanus in Muscle 98 

50. Photographic Record of Genesis of Tetanus in Mechanical Response of 

Plants (Style of Datura alba} 99 

51. Uniform Electrical Responses (Radish) 104 

52. Staircase Effect in Longitudinal Mechanical Response of Plant (Style 

of Eucharis} 104 

53- Fatigue in Longitudinal Mechanical Response of Plant (Style of 

Datura) 105 

54. Fatigue shown in Electrical Response, when sufficient Time is not 

allowed for Full Recovery 105 

55. Alternate Fatigue in Electrical Responses of Petiole of Cauliflower ; 

in Multiple Electric Responses of Peduncle of Biophytum ; in 
Multiple Mechanical Responses of Leaflet of Biophytum ; and in 
Autonomous Responses of Desmodium 106 

56. Rapid Fatigue under Continuous Stimulation in Muscle, and in 

Leafstalk of Celery 107 

57. Fatigue under long-continued Stimulation in the Contractile Response * ; 

of Plants * . . 108 

58. Photographic Record of Periodic Fatigue under Continuous Stimula- 

tion in Contractile Response (Filament of Uridis Lily) . . * 108 

59. Photographic Records of Normal Response of Mimosa to Single 

Stimulus (upper figure), and to Continuous Stimulation (lower 
figure) . . 109* 



ILLUSTRATIONS XXXI* 

FIG* .. I'AGE 

60. Ineffectiveness of Stimuli, owing to Increasing Fatigue, in Miniosa . 113 

6 1. Fatigue-Reversal in Arsenic, under Continuous Stimulation of Hertzian 

Radiation . . . . ^%^ 119 

62. Automatic Record of Fatigue in the Contrqtijfile Response of India- 

rubber under Rapidly Succeeding Thermal Shocks . . .120 

63. Preliminary Staircase Increase, followed by Fatigue, in the Response 

of Galena to Hertzian Radiation 121 

64. Preliminary Staircase Increase, followed by Fatigue, in the Response 

of Style of Eucharis 321 

65. Preliminary Staircase, followed by Fatigue, in the Responses of 

Muscle (Brodie) 122 

66. Staircase Increase in Electrical Response of Petiole of Bryophyllum, 

rendered sluggish by cooling . . . 4 , . .122 

67. Mechanical Response in Tendril of Passiflora in which Growth 

was originally at Standstill ........ 127 

68. Effect of Carbonic Acid Gas on Longitudinal Contractile Response . 131 

69. Effect of Hydrogen Gas . . . . , 132 

70. Photographic Record showing Effect of Carbon Bisulphide in Abolish- 

ing Response . .132 

71. Effect of Vapour of Alcohol 134 

72. Effect of Ether 135 

73. Effect of HC1 Vapour 136 

74. Action of Chlorine 137 

75. Diminution of Response in Eucharis Lily by Lowering of Temperature 140 

76. After-effect of Cold on Ivy, Holly, and Eucharis .... 141 

77. Effect of Cold on Longitudinal Response , . . . 143 

78. Effect of Rise of Temperature on Electrical Response . . . 144 

79. Effect of Rise of Temperature on Longitudinal Contractile Response 

of Plant ............ 144 

80. Effect of Rising and Falling Temperature on the Electrical Response 

of Scotch Kale (stimulus constant) . . . . . .145 

81. Effect of Cyclic Rise and Fall of Temperature on Longitudinal 

Mechanical Response in Plant 146 

82. Determination of Death-point in Allium Tube by Observation of 

Volumetric Contraction, causing sudden Expulsion of Water . . 157 

83. The Thermometric Spiral and Optic Lever of the Morograph . . 165, 

84. The Morograph . . . . . . . .167 

85. Thermo-mechanicai Curve obtained Photographically (Coronal Fila- 

ment of Passiflora} 1 68 

86. Thermo-mechanicai Curve of Two Different Specimens of Style of 

Datura alba, obtained from Flowers of the same Plant , . .169 

87. Thermo-mechanicai Records of Young Specimen of Spirogyra ; Older 

Specimen of same ; and Style of Datura alba 170 

88. Thermo-mechanicai Record of Leaf of Mimosa . . . .172 

89. Diagrammatic Representation of Mono-polar Excitation . . . 193 

90. Bi- polar Excitation of Mimosa 194 

91. Bi-pojar Excitation of Biophytum 194 

Make-katbode and Bjreak-^node, Effects in Biophytum . . . 195 



KXXfi PLANT RESPONSE 

no, 

93. Record of Responses of Leaflet of Biophytum^ showing Responses 

occurring at Kathode at Make and not at Break ; and at Anode at 
Break and not at Make . . . 196 

94. Effects of Ascending and Descending Currents, on Highly Excitable 

Specimen of Mimosa 197 

95. Molecular Model Exhibiting Excitability at the Receptive Area ; 

Conductivity of Intervening Region; and Mechanical Response 

of Terminal Responder ."..218 

96. Effect of Ether in the Abolition of Motile Excitability . . .220 

97. Experimental Demonstration of Effects of Cold and Anaesthetics in 

Abolishing Conductivity 222 

98. Diagrammatic Representation of Experiment on Biophytum . . 224 

99. Effect of Anode as Block 233 

100. Experiment showing the Transmission of Excitatory Wav6 through 

Kathodic Area, and its Stoppage by the Anode . 233 

101. Demonstration of Simultaneous Opposite Effects of Anode and 

Kathode on Transmission of Excitation* 234 

1 02. Diagrammatic Representation of Electrical Connections in Mimosa to 

Exhibit Variation of Motile Excitability, induced by Anode and 
Kathode 235 

103. c Developing' Action of Kaf hod e 236 

104. Diagrammatic Representation of Electrical Connections for Determi- 

nation of Velocities of Centrifugal and Centripetal Transmissions . 242 

105. Curve showing Decline in Heights of Responses, with Diminishing 

Periods of Rest 246 

106. Eiectrotactile Method for Detection of Excitatory Wave during 

Transit 259 

107. Eiectrotactile Response in Stem of Mimosa . . . . . 260 
j 08. Experimental Arrangements for Simultaneous Recording of Mechani- 
cal and Electrical Responses . . , , , .261 

109. Apparatus for Automatic Record ....... 266 

1 10, Photographic Record of Response of Mimosa^ Exhibiting the Latent 

Period in its Variation 268 

III* Electric Spark Record, showing Increase of Latent Period by Fatigue, 

in Successive Responses of a Leaf of Mimosa 269 

112. Response of Biophytum ; Electrical Stimulus having been Applied 

at the Pulvinus of the Motile Leaflet 270 

113. Additive Effects seen in Responses of Biophytum to Stimuli which 

Fall outside the Refractory Period 274 

114. Multiple Electrical Responses due to Single Strong Stimulus . . 280 
115* Multiple Eiectrotactile Response in Stem of Ajfynosa, due to Single 

Strong Thermal Stimulus 280 

11 6. Multiple Mechanical Response of Biophytum, due to a Single 

Thermal Stimulus ......... 283 

117. Multiple Response in Biophytum . . . * . 285 

1 1 8. Multiple Response in Biophytum 285 

Mitftiple Response in Biophytum^ showing Cyclic Groupings of * 

Amplitude and Period *< 286 



ILLUSTRATIONS xxxiii 



no. 

1 20. Multiple Response In Biophytum^ showing Cyclic Groupings . . ,286 

121. Intermittent Human Pulse (Broadbent)* ...... 288 

122. Internrittence in Pulsation of Biophytum . . ..... 289 

123. Multiple Response of Biophytum under the Continuous Action of 

Light .., 303 

124. .Induction ,of Autonomous Response .in Biophytum^ at Moderately 

High Temperature of 35 C ........ 305 

125. Experimental Apparatus for Making Records of Pulsation of Desmo- 

diuni *..,...... 316 

126. Photographic Record of Pulsation of Desmodium . . . 3*7 

127. Photographic Record of Uniform Pulsations in Desmodium . 318 

128. Displacement of Mean Position of Vibration of Desmodium Leaflet 

by Increased Internal Hydrostatic Pressure ..... 320 

129. Method of Application of Chemical Agent to Cut End of Petiole . 321 

130. Photographic Record of Effect of Ether Vapour, Large Dose , . 324 

131. Photographic Record of Effect of Alcohol ...... 325 

132. Photographic Record of Effect of Carbonic Acid Gas , . . . 325 

133. Photographic Record of Effect of Copper Sulphate Solution Applied 

on the Pulvinus .......... 326 

134* Spark-record of Single Pulsation in Leaflet of Desmodium . . 327 

135. Photographic Records of Autonomous Pulsations in Desmodium^ 

showing Increase of" Amplitude and Decrease of Frequency, with 
Lowering of Temperature ........ 332 

136. Effect of Lowering of Temperature in Producing Increase of Ampli- 

tude and Decrease of Frequency in Pulsation of Frog's Heart . 333 

137. Photographic Record of Pulsations of Desmodium during Continuous 

Rise of Temperature ....... , . 333 

138. Record of Pulsations of Desmodium at Different Temperatures . 334 

139. Record of Pulsations of Frog's Heart at Different Temperatures 

(Pembrey and Phillips) ......... 334 

140* Effect of Cooling to Thermo- tonic Minimum on Pulsation of Desmo- 

dium ............ 335 

141. Effect of Rapid Cooling by Ice-cold Water ..... 336 

142. .Photographic Record of Pulsation of Desmodium^ showing Sub-pulses 

during slower Up Movement, as Nodules ..... 340 

143. Photographic Record of Cyclic Groupings in Autonomous Pulsations 

of Desmodium) showing Sub-pulses ....... 341 

144. Photographic Record of Autonomous Pulsation of Desmodium^ 

showing Hourly Period ........ 341 

145. Record showing that Rhythmic Tissue of Desmodium is Incapable of 

being Tetanised ..,,,,,,,. 346 

146. Curve showing Relation between Temperature and Period of 

Pulsation in Desmodium ..... . . . 349 

147* Curve showing Relation between Temperature and Period of Pulsa- 

tion in the Heart of a Frog ........ 349 

14$. -Simple Alternation of Pulsation in Desmodium . 350 

149. Periodic Groupings of Pulsation in Desmodium ..... 350 

5P. .Simple Alternation of Pulsation in Frog's Heart (Pembrey and Phillips) 350 



PLANT RESPONSE 

FIG* PAGI 

151* Effect of Barium Chloride Solution on Desntodium . . . * 351 

152. Arrest of Beat of Ventricle of Frog at Diastole by Application of . / 

Acid . . . . . t " , . * 351 

153. Systolic Arrest of Heart-beat by Dilute NaHO Solution (Gaskell) * 352 

154. Arrest of De&modium Pulsation at * Diastole ' by. Application of 



155. Arrest of Pulsation of Desmodium at ' Systole ' by Application of 

Dilute Alkali . * . . . . * * ^ 4 353 

156. Diagrammatic Representation of the Shoshungraph , 365 

157. Photograph of Shoshungraph ........ 370 

158. Effect of Cold on Suction ....... w 374 

159. Curve showing Normal Suction at 23 C M Increased Suction. at 

35 C, and the After-effect persisting on Return to Normal Tem- 
perature. ........... 375 

160. Method of Hydrostatic Balance . . . . . . , 375 

161* Record obtained by Method of Hydrostatic Balance of Successive 

Applications of Cold and Warm Water ,...., 378 

162. Record, obtained by Method of Hydraulic Balance, of Successive 

Effects of Cold and Warm Water ...... 380 

163. .Effect of Strong KNO, Solution ....... ,384 

164. Effect of Strong NaCi Solution ....... 384 

165. Effect of Copper Sulphate Solution ....... 386 

166. Record showing Recovery to be Hastened by the Increase of Internal 

Activity which is caused by Application of Warm Water to the 
Roots , 401 

167. Diagrammatic Representation of Balanced Crescograph . * , 414 

168. Complete Apparatus for Crescographic Record under Ordinary and 

Balanced Conditions . . . . . , . .416 

169. Multiple Growth-responses (Peduncle of Crocus} . . . 417 
170* Responses of Leaf of Artocarpus to Thermal Stimulation . . 420 
171. Renewal of Growth-pulsation by .Thermal Stimulus in Tamarindus . \ 

indica originally at Standstill ........ 425 

1 72* Initiation of Erectile Response in Leaf by Supply of Water to partially . . .. 

Drought-rigored Mitnosa . . . . . . 426 

173. Initiation of Growth-pulsation by Small Supply -of Water to 

Drought- rigor ed Seedling of Cucurbita ...... 426 

174. Curve showing Relation between Internal Hydrostatic Pressure and 

Rate of Growth (Crinwn Lily) ,..._*. 429 
[75. Photographic Record showing the Slow Pulsations of Large Amplitude 
of Desmodium Leaflet at 30 C to become very much Quickened 
. and Reduced in Amplitude at 42, C ..... . , * , 431 

176* Growth-pulsation seen in Seedling of Balsam , . . * 432 

177. .Photographic Record of Responses of Mature Style of Datura alba 

to External Thermal Stimulus . . ... . , *>..,; 434 

178. * Photographic Record of. Responses, of Style of Datura alba in which 

, Growth had come to a Temporary Stop . ., ' ." t ^ . * 431 
.Photographic Record of Response of Growing Style of Datura alba /;; : 
, io External Stimulus , T f . f ..,.* ..:-.. ^ ; . m .i 



ILLUSTRATIONS 



1 80. Balanced Record of Response in Growing Peduncle of Euckaris 

, Lily to Electrical Stimulation .....*. * . . * 436 
i8z. -Semicircular Electrolytic Rheostat interposed in Heating Cir- 

cuit t <> , . 9 > 443 

182. Record of Growth [ in Crinum at Temperature of 34* C. and 

35 ^* * * * * * ^ * * * * * * 445 

183. .Thermo-crescent Curve of Growth in Crinum Lily under Continu- 

ously Increasing Temperature. ....... 447 

184. Curve showing Relation between Temperature and Rate of Growth, 

as deduced from the Thermo-crescent Curve . . . . 449 

185. Series of Responses of Growing Organ of Crinum Lily,, taken under 

Balanced Conditions at Three Different Temperatures . . . 460 
1 86. Curve showing the Relation between Stimulus and Response in the 

same Organ, under the two different Tonic Conditions of 30 C. 

(upper curve) and 37 C. (lower curve) . . . . . 467 

187. Continuous Photographic Record of Autonomous Pulsation of Des- 

medium gyrans from 6 P.M. to 6 A. M ....... 470 

1 88. Hydrometric Apparatus for Recording Continuous Variation of Rate 

of Growth ..,.... 471 

189. Photographic Record showing Variation of Rate of Transpiration in 

Cucurbita % from 3 P.M. to 12 P. M ........ 472 

190. Continuous Photographic Record of Variation of Rate of Growth in 

Four Days' Old Seedling of Oryza sativa> from 3 P.M. till 9 A.M., 
that is during Eighteen Hours . ...... 473 

191. Continuous Photographic Record of Variation of Rate of Growth in 

Seedling of Tamarindus indica> a Fortnight Old, from 3 P.M. to 

3 A.M., ...* 474 

192. Effect of Carbonic Acid Gas on Growth . . . . . . 480 

193. Balanced Records of Effect of Ether on Growth ..... 481 

194. Excitatory Effect of Dilute Splutiqn of Sodium Carbonate on 

Growth .......... 481 

195. Acceleration 6f Growth by Application of Solution of Sugar . . 482 

196. Spasmodic Alternations of Growth under Alcohol .... 483 

197. Unbalanced Record showing Action of Acid in Causing Relaxation 

and Ultimate Arrest of Growth . , . . ... 483 

198. Unbalanced Record showing the Action of Alkali, and the Antago- 

nistic Action Of Subsequent Application of Acid .... 484 

199. Effect o( Strong Solution of NaCl on Rate of Growth, as Modified by 

Different Constitutions of Specimens ...... 484 

200. The Effect of Different Constitutions in Determining the Resistance 

Offered to Poisons. The Action of 5 per Cent. Solution of Copper 
Sulphate ...... 4$7 

201. The Effect of Favourable Induced Constitution in Enabling Plant to 

, Shake off, Resujt of Toxic .Dose of Cppper Sulphate . . . . 487 

202. Opposite Effects of Large and Small Doses of Poison , . .488 

203. Diagrammatic Representation showing Differential Effect of Weight 

/ oq Lateral Walk of Cells ,,*,, 494 
304. Diagrammatic Representation of a Multicellolar Organ . , . 495 



PLANT RESPONSE 

PAGE 

205. Diagrammatic Representation of Experiment showing Curvature 

, . Induced by Unilateral Pressure Exerted by Particles . * . 497 

206. Record of Responsive Curvature induced in Crinum under Expert- 

, mental Conditions shown in fig. 205 ...... 498 

207. Record of Apogeotropic Response in Scape of Uriclis Lily . . . 500 

208. .Response Records showing Differences in Rate of Curvature 

according as Specimen is held at Angles of 45 and 135 . . 501 

209. Diagrammatic Representation of Different Positions of a Single 

Cell) according as the Specimen is held at an Angle of 45 
or 135, showing Consequent Redistribution of Statoliths (after 
F. Darwin) 502 

210. Effect on Apogeotropic Movement of Application of Ice-cold Water 

to Upper and Lower Surfaces alternately of a Horizontally laid 
Crinum Lily *......,. 504 

211. Effect on Apogeotropic Movement of Temporary Applications of 

Cold alternately to Upper and Lower Surfaces of Horizontally 
laid Scape of Uridis Lily 505 

212. Experimental Connections for obtaining Electrical Response due to 

Direct and Indirect Effects of Stimulation 515 

213. Record of Positive Electrical Variation, indicating Positive Turgidity- 

Variation (represented by Down Curve), induced in Growing 
Region by Moderate Stimulation on same side of Tip . . . 517 

214. Record showing Galvanometric Positivity subsequently Neutralised 

under Transmission of True Excitatory Effect, due to Continuance 

of Moderate Stimulation of the Tip 517 

215. Record showing Neutralisation and Reversal of Electrical Response 

at Responding Region, under Strong Stimulation of Tip . . . 518 

216. Record showing Negative Electrical Response represented by Up 

Curve, indicating Negative Turgidity-Variation due to Direct 
Stimulation . .. . * 518 

217% Record showing Positive Electrical Variation indicating Positive 
Turgidity-Variation of Distal Point, under Moderate Stimulation of 
Proximal > 5*9 

218. The Relative Electrical and Turgidity Variations of two Diametri- 
ally Opposite Points when Strong and Long-continued Stimulus 
is applied * S 21 

19. Mechanical Responses of Shoot and of Root to Unilateral Stimulus 
applied at the Tip . . * 

220. Mechanical Response of Root of Bindweed to very strong Unilateral 

Stimulation applied at the Tip 527 

22 1 . Mechanical Responses of Peduncle of Crocus and Root of Bind- 

weed to Unilateral Thermal Stimulation at the Growing Region . 527 

222. Diagram showing the various Responsive Effects induced at the 

Growing Region . *...* > 5 2 9 

Experimental Arrangement for obtaining' Records on Smoked 
.Drum of Responses given to Direct and Indirect Stimulation by 

Leaf of Mintosa 53 

Mechanical Responses of Leaf of Mimos* .... 531 



ILLUSTRATIONS - 

FIG. *A0B 

225. Erectile Response of Leaf of Mimosa due to Transmission of Indirect 

Effect to Distal Side, when Proximal is Stinttilated , . > . 532 

226. Curious Response of Arisama '. '. , . . . . 533 

227. Curves showing Effect of Amputation on Rate of Growth and 

Response in Root of Bindweed 541 

228. Response of Leaf of Mimosa in Favourable Tonic Condition to 

Chemical Stimulus of 3 per Cent Salt Solution - . . -55* 

229. Response of Leaf of Mimosa in Ordinary Tonic Condition to the 

Chemical Stimulus of 10 per Cent. Solution of Salt . . . . 552 

230. Polar Effects of Currents due to Localised Application on Upper 

Half of Pulvinus of Erythrina indica ...... 555 

231. Effects of Anode and Kathode on Variation of Rate of Growth in 

Root of Bindweed exhibited by Balanced Growth-record . . * 557 

232. Responsive Curvature in Scape of Crinum Lily by Unilateral 

Application of Anode and Kathode 558 

233. Effect in Acceleration of Rate of Growth of Seedling of Oryza sativa 

of Current through the Soil 560 

234. Longitudinal Contraction and Retardation of Growth under Light in 

Hypocotyl of Sinapis nigra 572 

235. Balanced Record of Variation of Growth in Flower-bud of Crinum 

Lily under Diffuse Stimulation of Light 574 

236. Oscillatory Response of Arsenic acted on Continuously by Hertzian * 

Radiation . . 575 

237. Multiple Response to Light of Terminal Leaflet of Desmodium . . 584 

238. Response of Terminal Leaflet of Desmodium to Strong Light from 

Above \ 586 

239. Response of Terminal Leaflet of Desmodium to Sunlight acting from 

Below 587 

240. Diagrammatic Representation 01 the Magnetically Controlled 

Recorder 589 

241. Heliotropic Chamber and Magnetically Controlled Recorder . . 591 

242. Heliotropic Response of Sinapis 593 

243. Heliotropic Response of Sinapis to Sunlight 593 

244. Microscope Recorder 599 

245. Record of Response of Root of Sinapis nigra 60 1 

246. Positive Heliotropic Movement of Terminal Leaflet of Desmodium 

Converted by Strong and too Long-continued Stimulus of Light 
into Oscillatory Movement 604 

247. Response of Hypocotyl of Sinapis nigra ....%. 609 

248. Responses to Successive Uniform Stimuli of Thermal Radiation in 

Pistil of Musa 616 

249. Response of Hypocotyl of Tamarindus indica to Continuous Stimu- 

lation of Thermal Radiation 617 

250. Effect of Electric Waves on Growth . . . . . .618 

251. Response to Diurnal Light and Darkness of Plagiotropic Stem of 

Ipomcea held vertical * * 625 

252. Response to Daily Periodic Light and Darkness of Plagiotropic Stem 

of Cuturbita maxima .* 626 



kxxvil! PLANT RESPONSE 

, , , - PGE 



253. Positive Heliotropic Response of Leaflet of Robinia to Sunlight *' 

Acting from' Above ........ . . 629 

254. Positive Heliotfopic Response of Leaflets of Erythrina indica . . 629 

255. Responses of Mimosa to Sunlight of not too long Duration . . . 631 

256. 'Response of PulviriUs otAfimosa to Action of Continuous Light from 

Above ' * . . . , . . . . f . . 632 

257. 'Responses of Ofalis to Sunlight ..... , . 633 

258. Negative Multiple Response in Biophytum when Acted on from - 

Above by Strong Sunlight . ... . . ... . 634 

259. Different Limits of Flexibility ..... , . . 647 

260. Movement of Terminal Leaflet of Desmodium placing itself Parallel 

to Incident Horizontal Light ...... . 649 

261. Curve showing Time-relations of the Responsive Angular Movement 

of Terminal Leaflet of Desmodium under Light, as represented in 

the last figure ........... 649 

262. Record showing that Lamina is not the Perceptive Organ , .651 

263. " Positive Heli6tropic Movement of Leaf of Mangifera indica under 

Sunlight acting from Above ....... . 653 

264. * Negative Heliofropiq Response of Leaves of Artocarpus under Suq- 

light acting from Above . . ..... 654 

265. Tprsional Response of Terminal Leaflet of Desmodium under the 

* Action of Lateral Sunlight . ~ . ~ ". . . . . . 657 

266. Torsion -recorder , ..... . . . , . 658 

267. Torsional Response of Leaf of 'Mimosa when Laterally Stimulated 

by Sunlight ........... 659 

268. Torsional Response of Leaf of Mimosa to Lateral Chemical 

Stimulus * ...... ' . . 660 

269. Tprsional Response of Complex Strip of Ebonite and India-rubber 

Under Lateral Action of Strong Radiation ..... 660 

270. Records showing Modification of Torsional Response under Induced 

Variations of Differential Excitabilities in Pulvinus of Mimosa . 662 

271. Records showing Torsional Response to Geotropic Stimulus and 

Induced Modifications in Terminal Leaflet of Erythrina indica . 666 

272. Retardation and Reversal of Normal Torsional Movement by the 

unilateral Action of Light in Porana paniculata . . . . 670 

273. Photographic Record of Diurnal Movement of Petiole of Biophytum 

from 5 P.M. till 9 A.M .......... 680 

274. Continuous Records of the Diurnal Movement during Thirty-six 

* Hours in Two Specimens of Mimosa ...... 68 1 

275. Initiation of Multiple Response in Lateral Leaflet of Desmodium 

originally at Standstill ......... 690 

276. Photographic Record of Autonomous Pulsations in Lateral Leaflet 

of Desmodium gyrans under Action of Sunlight, showing Periodic 
Reversals .......... 692 

277. Model of an Artificial Plant ..... . % 79 

278. Diagrammatit Representation of a Windmill with Attached Dynamo 

and Accumulator . . . . . * 742 



PART I 
SIMPLE RESPONSE 



CHAPTER I 

THE PLANT AS A MACHINE 

Responsive movements in plants --Work done by plant Plant as a machine 
Indicator-diagrams -Physiological response-curves Pulse-records Cardia- 
grams Modification of pulse by poison and other agencies Automatic 
response in plants .-Optical Lever Recorder Effect of external agencies on 
automatic pulse-beat in plants. 

FROM the moment when the germinating seedling bursts 
its seed-coat, a complex series of movements is initiated. 
The radicle turns downwards, the plumule up. Underground, 
the root gropes its way towards moist places, and contrives 
to avoid hard stones and obstacles. Above ground, the stem 
is seen to bend, as if in search of light. Tendrils twine about 
a support. These, amongst many visible movements, are 
striking enough, but within the unruffled exterior of the 
plant-body there are others, energetic and incessant, which 
escape our scrutiny. 

Now all these activities are but so many expressions of 
the response of the plant to the various stimuli by which 
the living organism is constantly being excited. The plant- 
organ sometimes responds locally to the direct impact of 
stimulus, and at other times the effect is conveyed away by 
conducting elements analogous to nerves, and the responsive 
changes are manifested at a distance. Thus the most varied 
and important functions of plant-life are brought about by 
the power of the tissue to respond to stimulus, that is to say, 
by the irritability of the plant-cells. 

In such fashion, work is performed continuously by the 
organism, as if by a machine, and the magnitude of the work 
performed is often very considerable, as is seen, for instance, 
when sprouting seedlings break through a pavement 

B 



2 PLANT RESPONSE 

The few instances enumerated by no means exhaust the 
activities that go on in the living machine of the plant ; they 
only suffice to give us a glimpse into the complexity of its 
functions. We can arrive at a comprehensive idea of such 
multifarious and obscure phenomena only by coming to 
understand the machine itself, and trying to disentangle the 
processes by which the various stimuli supplied by the 
environment bring about the appropriate responsive move- 
ments in the organism. This is difficult to do, inasmuch as 
the intricate internal machinery is hidden from our view. 

Indicator-diagrams. Though the interior be thus con- 
cealed, however, it is still not impossible, by careful observation 
of external actions, to gain some conception of the hidden 
mechanism. Let us take, for example, the analogous case 
of a steam-engine. That we may be able to infer at any 
moment the efficiency of various hidden parts of the machine, 
we attach, to the moving piston, a recording apparatus, and 
from the diagram thus obtained we are able to judge of the 
working efficiency of the engine. The upstroke is followed 
by a .downstroke, and a recording pen traces for us, on 
moving paper, the responsive movements. But an irregu- 
larity may suddenly take place in the curve. This is due 
to some internal obstruction. On removal of the cause, the 
amplitude of the record is restored, and the pulsating strokes 
resume their normal frequency. 

In dealing with living machines also, we may use 
similar contrivances, in order to gain some indication of 
their efficiency ; and by means of indicator-diagrams, or 
' response-curves/ thus obtained, we are able to gather much 
information as to the physiological perfection or imperfection 
of the living machine. 

Pulse-records as indicators of physiological efficiency. 
We shall first take as an example that responsive pulsation 
with which we are most .familiar, our own heart-beat. As 
in the steam-engine the energy of heat brings about the 
responsive movement of the piston, so in the heart, some 
internal stimulus brings about responsive pulsations. A 



THE PLANT AS A MACHINE 3 

sudden contractile movement is followed by relaxation. By 
a series of these, the blood is forced in a pulsating manner 
through the arteries, and we perceive the pulsatory movement 
at the wrist. Physicians by feeling such pulse-throbbings 
are able to pronounce on the condition of a patient. Or 
the movements may be recorded by means of a lever- 
arrangement, in which the short arm of the lever rests on 
the throbbing pulse. Its longer arm, provided with a tracing 
point, records these pulsatory movements on a travelling 
band of paper which is moved by clock-work at a uniform 
rate. It is on this principle that the instruments known as 
sphygmographs are constructed, and the response-records, or 
sphygmograms, reveal the physiological condition of the 
individual at the time (figs, i and 3). 




FIG. I. Record of (a) Healthy Adult and (f>) Senile Human 1'iilse 

(Broadbent) 

The heart-record, however, has been still more directly 
obtained, in the case of the lower animals, by attaching one 
end of the lever to the apex of the heart itself. Each con- 
traction and subsequent recovery is now recorded, in the 
manner which has been indicated. If we know the rate at 
which the recording surface is travelling, or if we make time- 
marks at regular intervals, we are able to determine the 
frequency of pulsation. The record also gives us the amplitude 

of each pulse. 

If now these records are to furnish reliable indications 
of the internal condition of the living machine, then, any 
circumstance which affects this internal condition must reveal 
itself in the external record. And this is found to be 
the case. For example, the effects of age are seen in 
the accompanying record (fig. i); and that of poison by 



B 2 



4 PLANT RESPONSE 

the gradual waning of pulsation, culminating in arrest at 
the moment of death (fig. 2). Similarly, changes of heat 

and cold, and the influence of 
various drugs (fig. 3), are all 
discernible from the modifica- 
tions which they induce in 
the pulse-record. 

For the purpose of studying 
the actions by which the plant 
MC;. 2. Kflccl of Muscarin in arrest- res P onds to the various Stimuli 

Pulsation of Frog's Ventricle of its environment, I have been 




(dishing) 11,1- ,1 

,. . .. , . able to devise apparatus, by 

The arrow indicates the moment ol l L 

application of reagent in this and means of which records of its 

following. responsive pulsations may be 

made. In the matter of automatic pulsations, we have in 
plants many instances which have not hitherto been recog- 
nised ; but in one case which is well known, that of Desmodium 
gyrans Hedysarum gyrans^ the telegraph-plant we observe 
pulsatory movements of its lateral leaflets, which, as I shall 




Fu;. 3. Record of Human Pulse 
(a) before and (b) after Inhalation of Nitrite of Aniyl. (Broadbent.) 

elsewhere show, exhibit a resemblance to those of the animal 
heart, a resemblance which is not merely superficial, but is 
the result of causes fundamentally the same. 

This telegraph-plant grows wild on the Gangetic plain, 
where its Indian name is Bon Charal or 'outcast of the 
forests,' and where the peasant belief is that it dances to the 
clapping of the hand. It is a papilionaceous plant with 
trifoliate leaves, of which the terminal leaflet is large, and the 
two lateral very small. Each of the latter is inserted on the 
petiole by means of a motile organ known as a pulvinus. 



THE PLANT AS A MACHINE 5 

These lateral leaflets, when in normal condition, go on con- 
tinuously, and apparently spontaneously, executing approxi- 
mately up and down movements, each of which takes from 
two to four minutes to complete. 

The great difficulty in recording the pulsatory movements 
of Desmodium arises from the extreme slenderness of these 
lateral leaflets. This is such that in attaching to them a 
recording lever, however light, its weight, and the friction of 
the writing-point, are sufficient to bring their movements to 
a stop. I have, however, succeeded in overcoming this 
difficulty by devising a recording Optical Lever. 

This lever consists of a very light aluminium wire, or, 
which is still better, the stripped quill of a peacock's tail 
feather, this being extremely light, and sufficiently rigid for 
the purpose. The two arms of the lever" are unequal. 
The fulcrum rod rests on frictionless supports of glass or 
agate. The same rod carries a light mirror. A thread of 
cocoon silk is stuck to the motile leaflet by a minute drop 
of shellac varnish. The far end of the thread is looped, 
and fixed at any suitable notch on the arm B of the lever. 
The other arm of the lever has a light sliding counterpoise. 
It will thus be seen that by gradually shifting the silk loop 
nearer the fulcrum, the magnification may be increased. 
When the automatically moving leaflet executes a downward 
movement, the arm B is pulled down, and there is a rotation 
of the fulcrum rod with its attached mirror. A spot of light 
reflected from the mirror is thus suddenly moved downwards 
from its original position. It will be observed that by 
moving the recording surface further away, the magnification 
may be still more enhanced. A wide latitude of magnifica- 
tion may thus be obtained, by changes in the effective length of 
arm of the lever, and by variation of the distance of the record- 
ing surface. Thus, for example, for purposes of demonstration, 
with a screen at a distance of five metres, it is easy to exhibit 
a pulsatory movement magnified to as much as one metre in 
amplitude. But in the case of the illustrations in the present 
book, it has not been found necessary to have any magnifica- 



6 PLANT RESPONSE 

tion whatever, since the movement of the leaflet is itself 
considerable. 

A very light counterpoise is used, as will be seen later, to 
exert a slight pull on the leaflet in an upward direction when 
necessary, the sliding arrangement enabling us to vary the 
amount of this tension. It will thus be seen that the leaflet is 
practically free from constraint, and any movement, however 
slight, is easily detected. When the leaflet falls, the spot of 
light moves, say downwards, and vice versa. The record of 
the entire response down movement followed by up may 
thus be made on a vertical revolving drum, whose speed is 
regulated by clock-work. .The magnification of the record 
having been determined previously, and the speed of the 
drum being known, the response-curve gives the absolute 
movement and the time-relations of such movement 

Instead of using a vertical drum, it is more convenient to 
record on the revolving surface of a horizontal drum. The 
up and down movement of the spot of light may now be 
converted into lateral, or left and right movement, by means 
of a second mirror suitably inclined. The finer adjustment 
of the reflected spot of light may be brought about by means 
of a milled head with which the second mirror is provided. 
A photographic record may be obtained by wrapping over 
the drum surface a sensitised roll-film. But since these 
movements are comparatively slow, it is easy to obtain the 
record more simply by following the spot of light with a 
recording pen, which slides on a horizontal guide-bar, parallel 
with its movements. 

These response-records can be traced on a large scale in 
the presence of an audience, by the use of the Demonstration 
Recorder, which consists of a twin-drum, over which is wrapped 
an endless band of paper to serve as the recording surface 
(fig. 4). Elastic bands pass over the two drums, one of which 
is kept revolving by clock-work. The excursion of the spot of 
light is now followed by means of a sliding ink-well, from 
which projects the ink-sponge. By this means, the tracing 
of the response-curve, and its various modifications under the 



THE PLANT AS A MACHINE 



action of different influences, can be made visible to the 
whole audience. It is thus possible to obtain records of these 
pulsatory movements, by attaching the Optical Lever to the 
leaflet of an intact plant. Or we may detach a petiole, 





FIG. 4. Demonstration Optical Pulse- Recorder 

H, Arm of Optical Lever, attached to moving leaflet ; L, Ray of light, which 
after two reflections from the two mirrors falls on the recorder ; c, Clock, 
which keeps twin-drum- on which is wrapped the recording paper 
revolving ; H, Horizontal guide-bar ; K, Ink-well, with projecting 
sponge. 

carrying the leaf, and place it in water, in which case it 
will remain alive as long as a couple of days, executing its 
accustomed pulsatory movements during a considerable time. 
The effect of any given agency, say poison, on the living 



8 



PLANT RESPONSE 



machinery may now be observed, as graphically indicated in 
the waning and final arrest of the pulse-record (fig. 5). Or 
the plant may be killed by passing through it excessively 
strong electric shocks, after which the occurrence of death 
will be indicated by the arrest of pulsation (fig. 6). 

Thus we see not only the similarity between the pulsations 
of Desmodium and those of cardiac muscle, but also how 
similarly both are affected by external agencies, such as poison. 
Later, we shall study the effects of other physiological in- 





FIG. 5. Death of Plant, and Arrest of 
Pulsation, by Poison 



FIG. 6. Death, and Arrest ot 
Pulsation, in Leaflet olDesmo- 
dium by Strong Electric Shock 



fiucnces on both. In the present chapter, however, it has been 
my aim to show that these pulse-records give us a reliable 
indication of the very obscure modifications of the life-processes 
initiated in the living tissues by various external factors. 
Speaking generally, we may say that an exciting reagent 
exalts the pulse, a depressing reagent reduces the amplitude 
of pulsation, and a poison arrests it permanently, this arrest 
being death. 

In the cases which we have chosen as examples, there is 
the advantage of a store of latent energy, which maintains the 
pulsation by providing an internal source of stimulus. This 
internal stimulation, as will be shown later, is really derived 
from external sources, the absorbed energy having been 
held latent in the plant. We shall in the next chapter take 
up a very much simpler case, in which the plant has no such 
reserve, but responds immediately to external stimulus. 



THE PLANT AS A MACHINE 



SUMMARY 

A plant, like a machine, responds either to the impact of 
external forces, or to energy that is latent within. 

As the working efficiency of an engine is exhibited by 
indicator-diagrams, so the physiological efficiency of a living 
machine may be inferred from the character of its pulse- 
records. 

Agencies which depress the physiological condition of a 
tissue, also depress its responsive pulsation. At the death 
of a tissue there is a permanent arrest of pulsation. 



CHAPTER II 

MECHANICAL RESPONSE TO STIMULUS 

Molecular derangement caused by stimulus Expression in change of form, 
contraction Mechanical model Myograph Response by differential con- 
traction in pulvinated plant-organs Longitudinal response in plants 
Response of plant to all forms of stimulus Plant chamber Practicable forms 
of graduated stimulus Electro-thermic stimulator -Stimulation by condenser 
discharge Response-recorder Advantage of counterpoise Response of 
Bfophytutn to thermal stimulation Response to condenser discharge Absolute 
measurements of motile effect and of work performed Effect of load Definite 
determination of threshold of response Determination of variation of excita- 
bility by measurement of minimally effective stimulus. 



of the phenomena of plant-life are so striking as the 
conspicuous mechanical movements of certain plants, like 
Mimosa, commonly known as sensitive ' in contradistinction 
to i ordinary ' plants. These movements, take place in 
response to various forms of stimulation, such as^is^causqd by 
mechanical touch or application of heat. It will be shown, 
however, in the course of the present book that this division 
of plants into sensitive and ordinary is arbitrary, since all 
plants are sensitive that is to say, react to stimulus. The 
plant, throughout its life, is constantly responding to stimuli, 
external and internaJL Some of its responses are manifested 
in mechanical movements which are too striking to be over- 
looked. Others, not so obvious, have passed hitherto un- 
noticed. But in both these cases changes of form occur in 
the tissue, in consequence of stimulation. In some instances, 
owing to conditions which will be explained later, these 
changes produce little visible effect. In others, the responsive 
change of form is displayed in a striking manner, owing to 
certain advantageous circumstances of structure, and to the 
possession of a magnifying arrangement 



MECHANICAL RESPONSE TO STIMULUS 1 

The shock of stimulus causes molecular derangement in 
the tissue of the plant, and it is this fundamental mole- 
cular change that finds expression in mechanical movement. 
It finds independent expression also in electrical move- 
ment. For the* conspicuous display of mechanical response 
certain peculiar structural arrangements are, as has been 
said, advantageous ; but for the exhibition of electrical 
response, the molecular change itself, which is concomitant 
to excitation, is the only condition. This subject of the 
electrical response of plants, however, I treat in detail else- 
where. 1 For the present we are concerned only with the 
question of mechanical response to stimulus. We have not 
only to determine the existence of such response, but also to 
ascertain under what conditions it occurs, and. by what means 
it is brought about. 

The whole sequence of molecular events initiated by 
stimulus and expressed as mechanical response, may be very 
simply illustrated by means of an india-rubber model. We 
take a piece of stretched india-rubber, attached to a recording 
lever. The rubber is enclosed in a tube in which there is also 
enclosed a spiral of thin German-silver wire, by which the india- 
rubber may be subjected to the momentary action of heat 
The quantity of heat generated is regulated by the strength 
and duration of an electrical current flowing through the 
heating wire. This application may be uniform for successive 
experiments, or increased at will. 

Longitudinal response. The thermal stimulus causes a 
molecular rearrangement in the substance of the india-rubber, 
in consequence of which the piece becomes shorter and 
broader. This sudden longitudinal shortening is recorded by 
the lever as the first half of the responsive movement. As 
the substance gradually recovers from the effect of the 
momentary stimulation, the molecules return to their normal 
position, with a concomitant *"C*i ration of the india-rubber to 
its original form. During this second half of the J Jlv ""~s we 

1 Bose, Response in the Living and Non- Living (Messrs. Longmans, Green 
& Co.). Bose, Electro-Physiology of Plants. 



12 



PLANT RESPONSE 




obtain the curve of recovery. 1 If we apply similar stimuli 
successively, we obtain successive responses which are alike 
(fig. 7). But if stronger stimulus be applied, by means of 
stronger heating current, the amplitude of response will be 
correspondingly increased. 

In the simple instance which we have considered, the 
response-record was obtained by taking advantage of the 
sudden contraction of the india- 
rubber. In the response of con- 
tractile animal muscle, we obtain 
response-records in exactly the 
same manner (fig. 8), and such 
records are known as myographs 

(ng- 9). 

Similar contraction in length, or 
LONGITUDINAL RESPONSE under 
the action of stimulus, has been 
$hown byl Pfeffer to occur in the 

FIG, 8. Mechanical Lever 
Recorder 

The muscle M with the attached 
bone is securely held at one 
end, the other end being con- 
nected with the writing lever. 
Under the action of stimulus 
the contracting muscle pulls 
the lever, and moves the 
tracing point to the right 
over the travelling recording 
surface P, When the muscle 
recovers from contraction the 
tracing point returns to its 
original position. See on P 
the record of muscle-curve. 





Fir.. 7. Response of India-rubber 

Thermal stimulus for I second at 
intervals of two minutes. 



filament of the sensitive stamens of Cynerece. I shall, how- 
ever, show in Chapter IV. that such longitudinal contraction 
under stimulus is not unique, but a phenomenon very exten- 
sively exhibited by plant-tissues, as seen in the series o r 
uniform r-- -v -^^7-, o btam^ from *c stamen 
rafl g orcl?nary plant, which is here given (fig. 10). 

Such models made of catgut and stretched caoutchouc have been used by 
Engelmann for explaining muscle response. 



MECHANICAL RESPONSE TO STIMULUS 



Differential response. But the responsive movement 
in plants is more generally produced by differential contractile 
movement, and a mechanical model again will clearly show 
how such movements are brought about. We take two 
equal strips of unequally contracting 
substances, which are glued together 
throughout their length. The two 
strips consist of ebonite and the 
relatively more contractile caoutchouc. 





FIG. 9. Series of Contractile 
Responses in Muscle 



IMC.. IO. Photographic 
Record of Longitudinal 
Contractile Response 
in ordinary Stamens 
(firownia ariza) 



If such a compound strip be held horizontally, with the 
more contractile element below, and if we subject it to 
thermal stimulation in the manner described above, the 
result will be a responsive curvature downwards, the more 
contractile caoutchouc forming the concave surface. Thermal 




FIG. 11. Differential Lateral Response of Compound Strip 
^Thermal stimuli applied at intervals of three minutes. 



stimulus may be applied, as in the last case, by sending a 
momentarv heating current through an enclosing spiral of 
German-silver wire, the responses being recorded in the usual 

manner (fig. 1 1). 

In such cases, where the upper and lower elements are 



14 PLANT RESPONSE 

unequally contractile, we obtain a DIFFERENTIAL RESPONSE, 
the more contractile becoming concave ; and it is evident 
that such movements must take place in a direction perpen- 
dicular to the plane of separation. 

Typical cases of mechanical response in plants are 
obtained from pulvinated organs. A good example of this 
is found at the insertion of the petiole in Mimosa pudica. 
When such an organ is stimulated, it is the lower half that 
undergoes the greater contraction, and the leaf is depressed 
by the concavity thus produced. It is generally assumed 
that the upper half of the pulvinus is not excitable, but this, 
as I shall show later, is an error. The responsive movement, 
however, is due to the differential contraction of the two 
halves, and, as already explained, takes place in a direction 
perpendicular to the plane which separates them. Such 
differential response will be found characteristic of all organs 
possessing dorsi-ventral differentiation. 

Whenever the plant is subjected to any sudden dis- 
turbance, the sensitive leaf reacts by a fall, which is brought 
about by the hinge-like mechanism at the pulvinus. The 
sudden disturbance which induces the fall constitutes the 
stimulus. The leaf responds when it is shaken, or cut, or 
when a prick is applied to it, or when a sudden variation of 
temperature is produced, as by touching it with a hot wire, 
or with ice, or when an electrical shock is passed through it, 
or if it be acted on by certain chemical reagents, or a beam 
of strong light be thrown on it. All these constitute the 
various forms of stimuli mechanical, thermal, electrical, 
chemical, and photic. 

We have next to study the relation between the intensity 
of the stimulus and the extent of response under varying 
conditions ; that is to say, we have to determine the 
* threshold of response,' in other words, the minimum intensity 
of stimulus that will be just sufficient to initiate reaction. We 
have then to observe the repeated response of the plant to 
repeated stimulation, whether uniform or gradually increasing. 
We have to detect the signs of fatigue if there be any, and 



MECHANICAL RESPONSE TO STIMULUS 15 

discover after what period of rest this disappears. We have 
also to record the exact time-relations of these phenomena. 
And further we have to study the effects of various external 
agencies in modifying the response. 

In order to carry out these investigations, it will be 
necessary first to arrange for placing the plant under suitable 
conditions for experiment. The next point is the devising of 
facilities for applying a stimulus of known intensity, which 
can be repeated, or increased by definite amounts, at will. 
And, lastly, there must be some means of obtaining an exact 
record of the response, from which the absolute movement 
of the responding organ and its time-relations may be 
deduced. 

Experimental plant chamber. As regards the first of 
these, it is advisable to ^ have a special plant chamber within 
which the specimen can be subjected to the necessary con- 
ditions. This chamber may consist of a base-board and a 
movable cover. The framework of the latter is of wood, 
with glass panes. In order to give easy access to the plant 
during experiment, one side of the cover has a hinged 
window. The recording Optical Lever is placed inside the 
chamber, and the glass cover protects the recorder from any 
accidental disturbance caused by air-currents. 

In connection with this, it is also important to provide 
arrangements for producing changes of temperature, and 
maintaining the changed condition uniform, for the required 
length of time. This is most satisfactorily accomplished by 
means of a heating coil placed inside the chamber, the 
temperature being regulated by suitable, adjustment of the 
electrical current, sent into the coil through proper electrodes. 
Other necessary accessories consist of appliances for the 
purpose of stimulation, and facilities by which a constant 
current can be made to flow through the tissue, in experiments 
on the effect of electric currents on the excitability of plants. 
Details regarding these will be given later. The plant may 
be maintained in favourable humid conditions by placing 
wet blotting-paper inside the chamber (fig. 1 2). 



i6 



PLANT RESPONSE 



The most important question with regard to the application 
of suitable stimulus is, as has been said, that it should be 
capable of exact measurement, of uniform repetition, and of 
definite increase or decrease at will. Another point which 
must be borne in mind is that the application of stimulus 
should not, by causing injury, change the excitability of the 
organ. As, moreover, a magnified record of the responsive 
movement is to be made immediately after the application, 
any stimulus which causes the slightest jar must necessarily 




FIG. 12. Plant Chamber and Recorder 
The glass cover is not shown. 

be avoided. And , for these reasons the mechanical form 
of stimulation is inappropriate to the investigation. The 
three most perfect modes of stimulation which I have 
been able to render practicable are, then, the thermal, the 
electrical, and the stimulus of light. The action of the last 
will be described in detail in another chapter, and we shall 
for the present confine our attention to the first two. 

Electro-thermic stimulator. Thermal stimulus may be 
applied very easily by touching the plant with a hot wire, but 



MECHANICAL RESPONSE TO STIMULUS 17 

it is difficult by this means to ensure the uniformity of 
successive stimuli, inasmuch as the wire cannot be heated 
repeatedly to the same temperature, or made to touch the 
same point, many times in succession, with an equally effec- 
tive contact. This difficulty is removed by means of what 
I have named the ELECTRO-THERMIC STIMULATOR. This 
consists of a thin M-shaped wire of platinum, with thick 
copper leads. It is slipped over the petiole which carries the 
sensitive leaflets. By now sending through it a current of 
definite intensity and duration, we can raise its temperature 
to any point we wish, and thus secure the application of a 
known intensity of stimulus at will. The elasticity due to 
the peculiar form of the thermal stimulator gives a definite 
and constant pressure 
of contact (fig. 13). 

The observer ap- 
plies the stimulus with 
his left hand, by press- 
ing a tapping -key 
which is interposed in 

an electric circuit, for 

, c ., t , , . FIG. 13. The Electro-thermic Stimulator 

a definite short time- J 

With his right-hand t he records on the revolving drum the 
exact moment of this application. This mode of thermic 
stimulation is, as will be shown presently, very efficient. 

Electric stimulation. I have been able, however, to 
employ a mode of stimulus still more perfect, that, namely, of 
the electrical discharge from a condenser. Other forms of elec- 
trical stimulation may be used, such as those given by means 
of constant or induction currents. But these are liable, not 
only to cause more or less permanent internal changes by 
polarisation, but also to induce fatigue of the tissue. It will 
be shown in a later chapter that, on making the circuit, excita- 
tion takes place at the point where the current leaves the 
tissue that is to say, at the kathode and not at the anode, 
or point of entrance. By appropriate connections shown in 
the diagram (fig. 14), the point to be excited can be made 

C 




1 8 PLANT RESPONSE 

kathode during ' charge,' when the key is pressed. When the 
key is released, the circuit is * discharged,' and the current 
flows in the opposite direction. 

The given point B is, as has been said, excited by being 
made kathode at the moment of charge. The immediately 
succeeding discharge produces no exciting effect, but it wipes 
off any residual polarisation effect caused by charge. The 
plant-tissue is thus maintained in as completely normal a 




Kir.. 14. Diagram of Connections for Stimulation by Condenser Discharge 

Pressure of key K charges the condenser c through the plant. Release of key 
brings it in contact with M, discharging the condenser through the plant. 
I., responding leaflet attached to recording lever by thread s. 

condition as possible. The excitation produced in the plant 
by current to or from the condenser, I shall, for simplicity, 
designate as ' stimulation by condenser discharge.' 

In the course of the present chapter we shall study the 
response of the leaf of Mimosa, shown by its fall, and also 
that of other sensitive plants, exhibited by the closure of 
the leaflets, as in the case of Biophytum sensitivum. One 
difficulty encountered in obtaining successive responses, in 
these latter cases, was due to the fact that the responding 



MECHANICAL RESPONSE TO STIMULUS 

leaflets, after each downward response, would sometimes 
remain persistently closed, for an indefinite period, thus pre- 
venting the continuation of the experiment In cases where 
the leaflets are completely closed, one naturally regards the 
position as one of fatigue, or complete insensitiveness, 
because no further mechanical response is then obtainable. 
This depressed position, however, may not be indicative of 
total want of sensibility, for the apparent absence of response 
may really be due to the fact that further closure of the 
leaflets is a mechanical impossibility. We may consider an 
analogous instance in the case of animal tissues, muscle 
floating in mercury for example. The tissue remains per- 
sistently contracted after a single stimulus, and further 
response is impossible. But if, again, the muscle be stimu- 
lated while under tension, it responds to each stimulation, 
the process of recovery being aided by the external tension. 

Practical importance of counterpoise. Acting on 
this idea, it appeared to me that if we applied an external 
tension, the restoration of the leaflet to the natural outspread 
position might be helped, and the difficulty solved of ob- 
taining the uniform repetition of effects of successive stimuli 
at brief and regular intervals of time. I therefore placed 
a small sliding counterpoise on that arm of the lever which 
was not attached to the leaflet. This was found to fulfil its 
purpose. For in observing the effects of successive stimuli 
on different leaflets, I found that while neighbouring leaflets, 
not under tension, closed up after a few stimulations, and 
gave no further response, the leaflet which was attached to 
the lever, and which was under some slight tension, recovered 
its normal outspread position in the course of three to five 
minutes, and continued to respond in a normal manner to a 
long series of successive stimuli. 

We shall now proceed to observe' the actual responses 
obtained. The object here is not to investigate the peculiar 
or specific reaction of any one sensitive plant in particular, 
but the effects found universally among motile plant-organs. 
The occurrence of such effects in plants exhibiting all 

C 2 



20 PLANT RESPONSE 

degrees of mechanical sensibility from tho.se in which it 
is shown in an extreme degree, to others again in which it is 
apparently almost non-existent will be demonstrated in this 
and succeeding chapters. 

In order to study the responsive movements of plants, 
we may take either the leaflets or the main petiole of 
Mimosa pudica. The leaflets, however, in this case are so 
excessively sensitive that even the contact for experimental 
adjustment is sufficient to produce a closure from which they 
do not recover for a considerable time. The pulvinus of the 
main petiole, on the other hand, is considerably less sensitive. 
Of intermediate sensibility are the leaflets of Biophytum 
sensitivum, which on the whole furnish the most suitable 
specimens for the general purposes of these experiments. 
This plant, which is known to be sensitive, grows in a wild 
state near Calcutta, and is so common as to be considered 
a weed. It is a low-growing herb, with simply pinnate 
leaves, each bearing from ten to sixteen pairs of leaflets. 
A better specimen could hardly perhaps be found for the 
exhibition of some of the most important characteristics of 
mechanical response. It is not, under ordinary conditions, 
excessively sensitive. A gentle touch does not, as a rule, 
produce the closing effect, but under specially favourable 
circumstances its sensitiveness may equal, if not surpass, that 
of the Mimosa leaflets. The closing of the leaflets takes place 
not upwards as in Mimosa, but in the downward direction. 
I shall presently give details of the response obtained with 
Biophytum. But as this plant is not universally obtainable, 
and as it flourishes only for a short season, during and after 
our tropical rains, it may be best first to give an account of 
experiments made on the more generally accessible leaf of 
Mimosa. 

Response of Mimosa. As the responsive movement of 
the leaf of Mimosa is of considerable extent, no magnifica- 
tion is necessary for the record. Indeed, on the contrary, 
for the illustrations in the present work, the records had fre- 
quently to be taken on a reduced scale. This was accomplished 



MECHANICAL RESPONSE TO STIMULUS 



21 



by attaching the leaf to the long arm of the recording Optic 
Lever, and shortening the distance of the recording surface. 
The records given in figs. 15 and 16 were automatically 
obtained by the impression of 
the moving spot of light on a 
sensitive film wrapped about 
the recording drum. The leaf 
was excited by a single strong 
induction-shock. In order to 
obtain the complete curve of 
response and recovery the 
double process being accom- 
plished in the course of about 
seven minutes the first record 
was taken on a slowly moving 
drum. For the detailed study of the characteristic time- 
relations of the first part of the curve, again, two more 
records were obtained, one with a moderate (fig. 15) and the 




Fi<;. 15. Photographic Record of 
Response and Recovery of Mimosa , 
taken on a slowly moving drum. 
Record shows - actual movement 
reduced to one- third. 




FIG. 16. Photographic Record of Response in different specimen, taken 
on a faster-moving drum showing only first part of the curve. Each 
division of time-scale = *5 second. 

other with a rapid speed of drum (fig. no). The last of 
these enables us to obtain time-measurements which are 
accurate to less than -fa of a second. The method by 



22 PLANT RESPONSE 

which these rapid records are obtained will be described in 
Chapter XX 1 1. 

From records obtained on a fast-moving drum, with a 
fairly average specimen of Mimosa, it is found that the 
leaf does not respond to stimulus immediately, there 
being a latent period of -$fa of a second before it begins 
to move. The maximum fall is attained in the course of 
2 seconds after the shock. After attaining the position 
of maximum depression, the leaf remains in its contracted 
position for a further period of about thirty seconds. It then 
begins slowly to recover, and perfect recovery takes place in 
the further course of six minutes. The record given was 
obtained from the leaf of a plant which was one year old, and 
in the summer season. It will be remembered that these 
responsive curves are modified by the physiological condition 
of the plant ; thus, for example, the time taken by the leaf of 
a vigorous young plant for recovery may be as short as four 
minutes, whereas an older specimen in winter may require as 
long a period as eighteen minutes. We may thus obtain 
from the record an idea of the physiological condition of the 
specimen. 

By means of photography the taking of the record is made 
extremely simple, but there arc certain disadvantages insepar- 
able from this method, which render the devising of other 
means essential. For example, the motile sensibility of plants 
like Mimosa and Biophylum is profoundly modified in dark- 
ness. In the case of the records given, the plants have been 
kept outside in the light, and brought in immediately before 
experiment. But even then, after remaining in the dark for 
half an hour or so, the leaves 6f Mimosa become abnormally 
erected, till it can hardly be believed that the plant is sensi- 
tive, for it often becomes irresponsive to the hardest blow. 
Biophytum leaflets, again, in the same circumstances undergo 
closure. For these reasons, long-continued experiments in 
a dark room are an impossibility. Various sensitive plants, 
again, flourish only for a short-lived season, and during that 
period some hundreds of experiments have to be carried out. 



MECHANICAL RESPONSE TO STIMULUS 23 

This necessitates some method of record more expeditious 
than that of photography. 

Fortunately, the responsive movements of these sensitive 
organs are relatively slow, usually requiring several minutes 
for completion. And it is quite easy to follow the excursion 
of the responding spot of light, with the recording pen, on a 
horizontal drum. There are some few special investigations, 
such as those on exceedingly short latent periods, in which 
automatic records by photography are a necessity, but for the 
majority of the records the second method is all that is 
required. By the latter means, moreover, we overcome the 
serious difficulty occasioned by the variation of sensibility 
which the plant undergoes when kept long in a dark chamber. 
When the second method is employed, the specimen may be 
placed in a well-lighted and well-ventilated room, and under 
these conditions it is found to maintain its sensitiveness 
unchanged for a considerable length of time. The fact that 
the spot of light reflected on the drum becomes inconspicuous 
in the surrounding daylight is overcome by placing in front 
of the recording drum a special hood with a long horizontal 
slit. The back of this hood curves over the head of the 
observer, and the spot of light then appears very bright. 

Response of Biophytum. I shall next deal with the 
responses obtained from Biophytum. In fig. 17 are given two 
successive responses to two successive thermal stimuli. It 
will be noticed how uniform these responses are. The up- 
curve represents the fall of the leaflets, and the subsequent 
down-curve of the response exhibits its gradual return to 
the normal outspread horizontal position. An abnormal 
erectile twitch will be noticed at the beginning of each of 
these responses. This effect is usually present when a 
stimulus of whatever nature is applied at a distance from 
the responding leaflet. Its cause will be explained later. 
It should be stated here that stimulus was in this case 
applied at a distance of 35 mm. from the responsive leaflet, 
and that the true excitatory reaction, by the depression of 
the leaflet, took place fifteen seconds after the application. In 



24 PLANT RESPONSE 

other words, the excitation travelled the intervening distance 
with a speed of 2*3 mm. per second. The abnormal erectile 




FIG. 17. Response of Riophytum to Thermal Stimulation 

Stimulus was applied at some distance from the responding leaflet* 
Thick dot represents moment of application ot stimulus. 




FIG. 1 8. Response of Biophytum to Electric Stimulation 
Stimulus was applied directly on the pulvinus. Ordinate represents absolute 



movement in mm. 



effect, however due, as will be explained later, to hydrostatic 
disturbance took place almost instantaneously. 



MECHANICAL RESPONSE TO STIMULUS 2$ 

The next figure (fig. 18) gives successive responses of 
Biophytum to condenser discharge, the pulvinus of the leaflet 
being directly excited. It will be noticed that in this case 
of direct stimulation, no abnormal erectile twitch is present. 
From the magnification of the record the absolute value of 
the movement is known, and in the present case it was r88 
mm. The force exerted by the leaflet during its responsive 
movement was found equivalent to that exerted by the 
weight of 17 milligrammes. The total work performed by 
the leaflet during each responsive movement is therefore 
nearly equal to 1,600 millimetre-milligrams. 

Effect of load. In order to observe the effect of load 
on the response-curve, I added a slight additional counter- 
poise to the other arm of the 
lever. The record (fig. 19, a) 
shows the response-curve when 
the acting load is the weight of 
the lever ; (d) shows the effect 
of the additional load. It will 
be seen that while the height of 
the responses was diminished, yet 
the period of recovery was very 
much reduced, from five minutes to Fl - T 9-. u Effect , of 

' . (a) without, and (i>) with, 

less than three, under the increased additional load 

load. 

Isometric record. The method of observing response 
employed in the foregoing results was that of recording the 
movement of the leaf. Similar methods are known in Animal 
Physiology as isotonic. There is, however, an interesting 
method corresponding to that known in Animal Physiology 
as the isometric where, in obtaining records of responses, 
actual movement is almost abolished. The contraction of 
the more excitable half of the pulvinus exerts a certain 
tension, or pull. The object, under the isometric method of 
experiment, is to obtain records of varying responsive ten- 
sions of excited tissues, the physical movement being at the 
same time restrained. This I have been able to accomplish, 




26 



PLANT RESPONSE 







in the case of plant response, by the construction of a spiral 
spring-recorder, the movement of whose index is approxi- 
mately proportional to the tension. 
The recorder is constructed of a fine 
flattened spiral spring. Springs of 
this description have the peculiarity 
that, when they are stretched by 
tension, the free end of the spiral 
rotates round the axis of the 
spring. A slight rotation may be 
magnified by means of a mirror 
and reflected spot of light (fig. 20). 
This arrangement is specially 
appropriate to the leaf-stalk of 
Mimosa, where the pull exerted 
by the excited leaf is consider- 
able. Fig. 21 gives the isometric 
response of Mimosa. 

Minimally effective stimulus 
in Biophytum. It is well to 
mention here that at least in the case of Biophytiim 
the minimal intensity of stimulus necessary to cause response 

is very well defined. With 
a certain specimen for 
example, when the plant 
was excited by the dis- 
charge from a *oi micro- 
farad condenser, charged 
to seven volts, there was 
no response. But when 
the condenser was charged 
to nine volts the discharge 
, I ^^"~ 1 "" _ . ^ -^^^^ always produced a larere 

PIG. 21. Isometric Response of Mimosa J 

and definite response. 

Charging of the condenser to nine and seven volts alternately 
would in the one case produce response, and in the other 
none. If now, by the action of an external agency, the 



FIG. 20. The Spiral Spring- 
Recorder 

The leaf is practically prevented 
from moving. The tension 
exerted by excited leaf causes 
rotation of index or mirror. 




MECHANICAL RESPONSE TO STIMULUS 2J 

excitability of the tissue be increased, the seven-volt con- 
denser charge, before inadequate, will become adequate. 
Conversely, if by the action of an external agent the excita- 
bility of the tissue be depressed, the nine-volt charge, which 
was formerly effective, will become now ineffective. I find, 
for instance, that lowering of the temperature will, by in- 
creasing molecular sluggishness, reduce excitability. Hence 
a minimally effective stimulus becomes ineffective when 
the tissue is cooled. Conversely, a rise of temperature 
produces the reverse effect, namely, increase of excitability. 
This was seen in a particular experiment with Biophytum, 
where the minimally effective stimulus necessary at 30 C. 
was found to be reduced to two-thirds when the temperature 
was raised to 35 G. 

Having thus obtained a reliable stimulus, whose value may 
be measured with precision, and which is capable of being 
repeated, and having also discovered an arrangement by which 
the effect of a given stimulus is invariably exhibited by a 
uniform response-record, we are now in a position to attack 
various physiological problems, as regards the influence of 
given external agencies on the conductivity and excitability 
of the plant-tissue. 

SUMMARY 

Longitudinal responses are given by radial organs. 

A differential response, causing lateral movement, is 
given by an organ in which the excitability of one half is 
different from that of the other ; and the movement takes 
place in a direction perpendicular to the plane of separation 
of the two halves. Such responses are characteristic of dorsi- 
ventral plant-organs. 

In the responses of the sensitive organs of plants we 
notice : a short latent period ; a period during which the 
excitatory movement attains its maximum ; and a period of 
slow recovery. 

When the stimulus is applied at a distance, a preliminary 
abnormal erectile twitch is occasionally observed, which is 



28 PLANT RESPONSE 

due to hydrostatic disturbance. The true excitatory response 
takes place later. 

Besides the isotonic response, obtained by recording the 
actual movements of the excited leaf, it is also possible 
to record the isometric response where the movement is 
restrained, and the variation of tension caused by the con- 
traction of tissue is alone recorded. 

The intensity of a minimally effective stimulus in the 
case of Biophytum is definite. This value undergoes appro- 
priate variation with the variation of excitability of the 
organ. 



CHAPTER III 

ON THE UNIVERSALITY OF SENSITIVENESS IN PLANTS 
AS DEMONSTRATED BY MEANS OF ELECTRICAL RESPONSE 

Arbitrary classification of plants into sensitive and ordinary Method of electro- 
motive variation for detecting state of excitation Hydraulic model Excita- 
tion of vegetable tissue, like that of animal tissue, induces galvanometric 
negativity Methods of direct and transmitted excitation Electrical and 
mechanical response alike record molecular derangement and recovery 
Similarities in simultaneous record of mechanical and electrical response 
True excitation has a concomitant negative turgidity-variation, negative 
mechanical response or fall, and galvanometric negativity^ These are true 
physiological responses, and are abolished at death Abnormal positive 
mechanical and electrical responses brought about by positive turgidity- 
variation -Direct and indirect effects of stimulation Discrimination of differ- 
ences of excitability by electric test Excitability of plant-tissues in general 
Responsive power characteristic of matter. 

WE have seen that when stimulus is applied to a sensitive 
organ like the pulvinus of Mimosa there is a fall of the leaf, 
which fall is due to the excitatory contraction of the more 
excitable lower half of the pulvinus. 

Ordinary plants are said to give no motile indications, 
hence they are usually regarded as insensitive. 1 It is difficult, 
however, to conceive that while the protoplasm of certain 
plants reacts to stimulus, that of others should not do so. On 
the other hand, it may be that the absence of mechanical 
response in these ordinary plants is not due to any want of 
excitability, but rather to the fact that conditions favourable 
to the conspicuous exhibition of motile effects do not in 

1 Vines has already drawn attention to the possibility of error here : We 
must be careful not to assume that irritability is restricted to growing and to 
motile organs. For all we know to the contrary, it is possessed by the proto- 
plasm of all plant organs, and if in any case the action of a stimulus is not 
followed by a responsive movement, we must, before we assume the absence of 
irritability, assure ourselves that the structure of the organ is such that a move- 
ment is a mechanical possibility. 'Vines, Physiology of Plants, 1 886, p. 372. 



3O PLANT RESPONSE 

such cases exist. What these conditions are will be detailed 
in the next chapter, where it will also be shown that excita- 
tion of an organ may take place, even where there is little 
mechanical indication of the fact, owing to antagonistic and 
balanced contractions. 

Electrical response. It is my intention, in the course of 
the present work, to offer a complete demonstration of all the 
phenomena of excitation in plants, by means of mechanical 
response alone. But the conclusions to which we shall be led 
by the study of this response will receive irrefragable support, 
if they can also be established independently by some mode 
of investigation altogether different. Such a mode of in- 
quiry, namely the electrical, and the conclusions to which it 
leads, will be fully described in the companion volume to this 
work, on the Electro-Physiology of Plants. Meanwhile it is 
convenient in this place to enter upon a short elucidation of 
the principle of that method, in order that we may be able, 
while considering the results of mechanical response, to make 
casual references to confirmatory results of independent ob- 
servations obtained by means of the electrical method. 

It has been said that under the action of stimulus excited 
cells undergo contraction, and that owing to the consequent 
expulsion of water, the turgidity of the tissue is diminished. 
Thus one expression of the molecular change induced by 
stimulus is a negative variation of turgidity. But this 
molecular change may also be detected by means of other 
concomitant physical changes. For instance, the electrical 
level or potential of a given point may, owing to the ex- 
citatory molecular change, undergo variation, relatively to 
another point which is unexcited. A hydraulic model will 
serve to make this point clear (fig. 22). 

Let us imagine a flexible pipe of india-rubber, with bent 
ends of glass-tube, filled with water, and held in the middle 
by a clamp c. It is also supported in the stable horizontal 
position by spiral springs. If a single blow, say upwards, 
be now given to the end A, the level of the pipe at that end 
will be raised, and there will be a resultant flow of water from 



UNIVERSALITY OF SENSITIVENESS IN PLANTS 31 

, 

A to B, or away from the struck end. The intensity of the 
current is determined by the height to which the struck end A 
has been raised, and this again depends on the intensity of 
the blow. Hence the intensity of the current is a measure 
of the intensity of the stimulus or disturbance. The flow 
subsides with the return of the pipe to its equilibrium posi- 
tion. If the pipe had been disturbed throughout, the level 
would have been raised equally at both ends, and there would 
have been no flow. The object of the clamp is, therefore, to 
confine the disturbance to one side. If the blow had been 




FIG. 22. Hydraulic Model for Explanation of Electric Response 

When an upstroke is given to A, a responsive current flows from A to 

and vice versa. 



given on the B side, the direction of the responsive flow would 
have been reversed. 

The principle of electromotive response in plants is 
exactly similar to this. The plant tissue is clamped at 
C (fig. 23), and a stimulus is given at one end, say A. The 
electrical level of that side is now found to be raised, it 
becomes electro-positive, or like the copper in a voltaic com- 
bination. The responsive current thus flows in the tissue 
from A to B, or away from the excited point. In the 
external circuit containing the galvanometer, it flows, of 



PLANT RESPONSE 



(a) 




course, in the opposite direction, that is from B to A. 1 The 
excited point A is thus electro-positive, but in physiological 
text-books it has been ambiguously termed negative. In 
order therefore to keep touch with the older terminology and 
yet avoid the implied error, I shall refer to the excited point 
as ' galvanometrically negative.' 

If the B end of the specimen be now excited, the direction 
of the responsive current will be reversed. With greater 
intensity of stimulus the electrical response will be found 
correspondingly increased. The record of such responses is 

obtained on a revolving drum, 
by following the deflection of 
a spot of light reflected from 
the galvanometer mirror in a 
manner precisely similar to 
that employed with the Optic 
Lever (fig. 24). 

Current of response when Method Of transmitted 

A is stimulated-* \ 

Current of response when \J Stimulation. In the Simple 

/> is stimulated < r 

FIG. 23. Electrical Response in riant case just described, the tissue 
by Method of Block was st i mu l a ted directly and 

(a) The plant is clamped at c, between , ,, , . , 

A and H. locally at the end A, say by 

(/>) Responses obtained by alternately tors j ona l vibration. There is, 
stimulating the two ends, btimula- J 

lion of A produces upward response, however, another method of 

and that of B downward. transmitted stimulation, by 

which a stimulus, say by application of cut, or hot wire, 
or electrical shock, is given at a point X some distance 
away, say to the right. The excitation now travels with a 
velocity characteristic of the specimen, and when it reaches 
the proximal electrode produces galvanometric negativity 
of that point. The interval of time which elapses between 
the application of stimulus and response will therefore depend 
on the velocity of transmission and the distance of the point 

1 The failure to understand this point clewrly has been the source of many 
grave errors in some physiological text books. From the fact that the current in 
the external circuit is seen to flow in the direction of A, it has been erroneously 
supposed that that point is negative, or zinc-like. See Bose, Response in the 
Living and Non- Living. 




UNIVERSALITY of SENSITIVENESS IN PLANTS 



33 



of application. It will also be remembered that the state of 
excitation is attended by an expulsion of water, or negative 
turgidity- variation. After causing galvanometric negativity 
of the proximal contact, the excitation may reach the distal, 
and bring about reversal of response, thus constituting a 





FIG. 24. Electric Response Recorder 

diphasic variation. If, however, the distal point be very far, 
the excitation may by transmission through the long tract 
become so enfeebled as to produce practically no effect at 




FIG. 25. Method of Transmitted Stimulation 

Stimulus applied to the right at x . Excitation reaches right contact fust, 
causing galvanometric negativity of the point. 

that point, in which case we obtain only the monophasic 
response of the proximal point. 

Simultaneous mechanical and electrical events, 
ensuing on excitation. We may prove that these electrical 
responses are undoubtedly signs of excitation, by choosing 

D 



34 PLANT RESPONSE 

for the electrical experiment a plant in which the state of 
excitation is independently manifested by mechanical response. 
If now these electrical and mechanical responses be indeed 
only two different expressions of the same thing that is 
to say, of a molecular disturbance and recovery which is 
concomitant to excitation and recovery from excitation 
then we should expect that, on taking a simultaneous record, 
the two responses would be shown to be initiated at the same 
moment, and to bear some general resemblance to each other. 
In the following record it will be seen that this is found to be 
the case (fig. 26). 

To recapitulate : let us take the concrete example of the 
Mimosa leaf. When the pulvinus is excited, owing to the 



Fir.. 26. Simultaneous Mechanical (M) and Electrical (E) Responses in Biophytum 
These responses are seen to take place at the same moment. 

molecular change induced by stimulus, there is an expul- 
sion of water, or negative turgidity-variation, and in the 
absence of restraint this is attended by the normal negative 
mechanical response, or fall of the leaf. If, now, electrical 
connections have been made, one with the pulvinus, and the 
other with a distant point on the stem, it will be found that 
the excitatory change is attended by a strictly concomitant 
electrical change, the current of response flowing away from 
the excited point, which in other words becomes galvano- 
metrically negative. 

All these events will perhaps be more easily realised if 
we remember that excitation, in the typical case of Mimosa 



UNIVERSALITY OF SENSITIVENESS IN PLANTS 35 

fives rise simultaneously to (a) contraction of the cells, with 
oncomitant negative turgidity-variation ; (b) negative me- 
:hanical response, or fall of the leaf ; and (c) galvanometric 
legative variation. Had the leaf been physically restrained 
>y any means, the mechanical response would have been 
prevented, although the negative turgidity-variation con- 
:omitant to excitation would have taken place just the same. 
But this internal change would have been imperceptible, and 
n that case we could still have detected the effect of excita- 
:ion by means of the electromotive response. As a matter 
:>f fact it is found that the electrical response always takes 
place in answer to effective stimulation, even in cases where 
the mechanical response is rendered impossible. We thus see 
>hat galvanometric negativity is a certain indication of the 
excitatory contraction of a cell, whether or not the effect of 
juch contraction be outwardly manifested by mechanical 
Tiovement. The detection of the state of excitation by the 
electric test is thus unfailing, and of universal application. 
By the employment of this electric mode of investigation, I 
have shown that not sensitive plants alone, but every plant, 
ind also every organ of every plant, is excitable. 

True excitatory negative versus hydrostatic positive 
variation. It has been supposed that the galvanometric 
negativity consequent on stimulation maybe due to mechani- 
cal movement of water in the tissue. But I have shown that 
this cannot be the case. For while it is true that the pro- 
duction of water-movement by sudden forcing of water into 
a tissue does cause electrical variation, yet it must be noted 
that the sign of this electrical change is always one of 
galvanometric positivity, which is opposite to that of the 
true excitatory response. The intensity of the true negative 
electrical response, moreover, varies with the physiological 
activity of the tissue, and is abolished with its death. The 
electrical variation due to mere water-movement, however, 
may take place even in a dead tissue, and is, as has been 
said, of positive sign. 

If a piece of living tissue be subjected to direct stimula- 

D 2 



36 PLANT RESPONSE 

tion that is to say, be locally disturbed, say by torsional 
vibration two effects will be produced: first, the negative 
turgidity-variation, which is the true excitatory effect, with 
its attendant negative electrical variation ; and, second, the 
electrical effect due to hydrostatic disturbance or water- 
movement, which is positive. Of these two opposed electrical 
effects, the first, or true excitatory variation, is generally 
speaking much the stronger. It therefore completely masks 
the second, or effect of water-movement, and the resultant 
response is the normal negative variation. The water-move- 
ment effect may, however, be unmasked by killing the tissue, 
and then applying the same torsional vibration as before. 
The result is now a positive electrical response. 

Or the positive effect may be made to exhibit itself 
separately, under favourable conditions in a living tissue, by 
the method of indirect stimulation, that is to say, by the 
application of stimulus at a distance. When such a distant 
point is stimulated, there is a sudden expulsion of water 
from that point, due to stimulation. This gives rise to a 
wave of increased hydrostatic pressure (with its attendant 
positive turgidity-variation), which travels with a relatively 
great velocity. The true excitatory variation, travelling at 
its slower rate, reaches any given distant point much later. 
The two effects ought thus to be divided from each other by 
some interval of time. 

We should therefore expect on stimulation of a sensitive 
plant to find the hydrostatic disturbance, with its attendant 
positive turgidity-variation reaching the distant motile 
organ the earlier of the two. And since the negative 
turgidity-variation due to excitation causes a fall of the leaf, 
the positive turgidity-variation due to hydrostatic disturbance 
should be expected to produce an abnormal positive or 
erectile movement, and the same positive turgidity-varia- 
tion should also find a simultaneous electrical expression 
in the abnormal positive response. The true excitatory 
response with its attendant negative turgidity-variation 
should cause, later the normal negative mechanical response, 



UNIVERSALITY OF SENSITIVENESS IN PLANTS 37 



and also the normal negative electrical response. We have 
already seen, in fig. 17, an instance of this abnormal positive 
followed by the normal negative mechanical response, in 
experiments with Biophytuui. This abnormal positive re- 
sponse, being a matter of the intensity of the blow delivered 
by the water-movement, can only exhibit itself under favour- 
able conditions. It is thus possible in mechanical response to 
have cither the normal response preceded by the abnormal, or 
the normal response alone. But whenever we have an abnormal 
mechanical response, due 
to positive turgidity- 
variation, we have also 
simultaneously an ab- 
normal positive electrical 
response. In fig. 27 is 
shown a simultaneous re- 
cord of the two, in which 
there is a preliminary ab- 
normal positive mechani- 
cal response, and a syn- 
chronous positive elec- 
trical response, followed 
in both cases by the 
normal responses. 1 It 
should be stated here that 
this positive turgidity- 
variation, which is re- 
ferred to as abnormal, is 

of very great importance, and will be seen in Chapter XXX. 
to be directly responsible for growth. It will be found 

1 It must be understood that this positive electrical response, being dependent 
on the excitatory expulsion of water at a distant point, is, in a certain sense, a 
physiological response. For it is the excitability of that distant point which 
determines the positive turgidity and concomitant positive electrical variation of 
the point under examination. The contraction of the excited point gives rise to 
a hydrostatic disturbance, by which a movement of water is brought about. 
Such a disturbance, then, will be indifferently designated as the hydrostatic, or 
hydraulic, wave. 




FIG. 27. The Abnormal Positive preceding 
the Normal Negative in Mechanical and 
Electrical Responses in Biophytum 

< represents the moment of application of 
stimulus. The upper is the mechanical 
and the lower the electrical record. The 
records downward indicate erection of the 
leaf or galvanometric positivity. 



38 PLANT RESPONSE 

helpful in the future if we uniformly distinguish (r) the true 
excitatory or normal as the direct, and (2) this positive or 
abnormal as the indirect effects of stimulation. 

Discrimination of differences of excitability by electric 
test. Not only does the electrical response enable us to detect 
the state of excitation, but I have been able further to devise 
an electrical test by which differences in the natural excita- 
bility of two points might be distinguished. The demonstra- 
tion of the existence of two electrical responses, one of which 
alone, namely the negative, constitutes the true excitatory 
effect, is of much theoretical interest. For the hydraulic, or 
positive electrical effect, has been mistaken for the true 
excitatory response in plants. As the excitatory effect in 
animal tissues, moreover, was known to be negative, this fact 
was supposed to indicate a difference between the proto- 
plasmic reactions of animal and vegetable. But the experi- 
ments which I have just described conclusively prove that 
such a difference does not exist, the sign of response in 
animal and vegetable being the same. They offer us an 
explanation, further, of the source of error. A more detailed 
account of this subject will be found in my forthcoming work 
on the Electro-Physiology of Plants. 

Many of the motile phenomena of mechanical response 
which we shall have to study in the course of the present 
work are modified by differences of excitability at different 
parts of a tissue. In the case of the primary pulvinus of 
Mimosa for example, we can see how the responsive fall is 
brought about by the evidently greater excitability of the 
lower half of the pulvinus. But in organs which apparently 
exhibit little motility, it is impossible from inspection to 
know whether all parts of the tissue are equally excitable, 
and, if not, which parts exhibit the greater excitability. 
Such variation of excitability is often due to invisible molecular 
differentiation, and eludes visual scrutiny. Fortunately, as 
already said, I have been successful in devising a mode 
of electrical investigation by which this differentiation is 
detected with the greatest certainty. This method will be 



UNIVERSALITY OF SENSITIVENESS IN PLANTS 39 

found fully described elsewhere, but the results may be 
summarised as follows : On simultaneous excitation of two 
points, the current of response flows in the tissue from the 
more excitable point A to the less excitable point B ; con- 
versely, if the direction of the responsive current is from A to 
B, the point A may be taken as the more excitable. By 
means of the unfailing discrimination of the differences of 
excitability in a tissue which this method renders possible, it 
will be shown in the course of the present work that many ot 
the anomalies of growth-curvature receive a most complete 
and satisfactory explanation. 




FIG. 28. Response of Selenium to the Stimulus of Light 
(Conductivity variation method.) 

The electrical response given by plant-tissues in general, 
as described in this chapter, is obtained by means of the 
difference of electrical potential or electromotive variation, 
induced as between the excited and unexcited portions of the 
tissue. 

There is another method of detection by means of 
those changes of electrical conductivity which are concomi- 
tant to excitation in the substance under experiment 1 It 
should be borne in mind that the various responses, obtained 
by the mechanical, by the electromotive, or by the conduc- 
tivity-variation method, are merely different expressions of 

1 Bose, Molecular Changes produced in Matter by Electric Waves, Roy. 
Soc. Proc. i oo. 



1 LA NT KESPONSK 



that fundamental molecular change which underlies excita- 
tion, and which disappears on the restoration of molecular 
equilibrium. 

Universality of responsiveness in matter. If we take 
a plant-tissue and subject it to a sufficient degree of cold, its 
responsive power will be found to disappear. It reappears, 
however, on the return of the tissue to the normal tempera- 
ture. The power of response is thus seen to depend on the 
molecular condition of the substance. 

Now this irritability, or power of responding to stimulus, 
may be vaguely regarded as a characteristic property of 

living substances ; and we 
may evade the difficulty of 
any attempt at a real ex- 
planation by describing it 
as a ' vital ' phenomenon. 
But if we regard all such 
phenomena as due ulti- 
mately to physico-chemical 
actions, we cannot rest 
satisfied with what is, after 
all, a mere descriptive 
phrase. Progress can only 
be made in scientific in- 
quiry by attempting gradu- 
ally to discard all such 
assumptions of the working 
of mystical forces in favour 
of simpler and more rational 




FIG. 29. Response of Metal abolished 
by the action of 4 Poison ' (Oxalic 
Arid) 



The record to the left is the normal 
response, and the line to the right 
shows abolition of response on 
application. 

explanations. 

By following the electrical method of inquiry, which has 
just been described, I have been able to prove that the power 
of responding to stimulus, and, under certain conditions, the 
arrest of this power, is the characteristic not of organic matter 
only, but of all matter, both organic and inorganic ; ] and that 
in general the various agencies which bring on the modifica- 

1 Hose, /vVsyV//\<' /// the Lit'ing and Non-Tit ///;'. 



UNIVERSALITY OF SENSITIVENESS IN PLANTS 41 

tion of response in one case such as fatigue, temperature 
changes, stimulating or depressing chemical reagents act 
in the same way in the other. 

The capability of responding, so long regarded as the 
peculiar characteristic of the organic, is also found in the 
inorganic, and seems to depend in all cases, both qualitatively 
and quantitatively, on the condition of molecular mobility. 
In the course of the present work, then, the term ' physio- 
logical ' is to be understood as a convenient expression 
for the phenomena of plant or animal tissues under in- 
vestigation, and not as in any sense opposed to the word 
* physical.' 

SUMMARY 

Stimulus causes molecular derangement in matter. The 
conditions of molecular upset and return to the state of equi- 
librium correspond to the state of excitation and recovery 
from that state. 

The molecular disturbance is attended by various physico- 
chemical changes in the properties of the substance, among 
the most important of which in a living tissue may be men- 
tioned (i) contraction of the excited cell, and expulsion 
of water ; (2) electromotive variation ; (3) conductivity- 
variation. 

The true excitatory change causes negative variation of 
turgidity, with depression or negative mechanical response 
of the leaf, and galvanometric negativity. The intensity of 
these effects varies with the physiological condition, being 
totally abolished by those molecular changes which are con- 
comitant with the death of the tissue. 

By means of electrical response it is found that, not 
sensitive plants alone, but every plant and every organ of the 
plant is excitable. 

Positive turgidity-variation, by whatever means produced, 
causes positive mechanical response or, in the case of 
Mimosa, erection of the leaf and galvanometric positivity. 



42 PLANT RESPONSE 

A pulse of positive turgidity-variation is often propagated in 
consequence of the sudden expulsion of water from a stimu- 
lated point at a distance. 

The laws of true electric response to excitation may be 
summarised as follows : 

A. The indirect effect of stimulation is a positive electrical 
variation, indicating a positive turgidity- variation. 

B. The direct effect of stimulation causes negative turgidity- 
variation, concomitant with a negative electrical variation, 

(1) The direction of this responsive current in plant- 

tissues, as in animal, is from the more to the less 
excited portions of the tissue ; or, the excited point 
is galvanometrically negative. 

(2) On simultaneous excitation of two points, the current 

of response in the tissue is from the more to the less 
excitable. 

(3) Conversely, if the responsive current be from A to B, 

the point A may be taken as the more excitable. 



CHAPTER IV 

ON CONDITIONS FAVOURABLE TO THE CONSPICUOUS 
EXHIBITION OF MECHANICAL RESPONSE 

Differences of degree of motile sensibility in sensitive plants so called Response 
of anisotropic organ brought about by differential contraction Production ot 
response by artificial variation of turgidity Variation and counter-variation ot 
turgescence, causing two opposite responsive movements Differences between 
hydrostatic and true excitatory effects- Distinction of plants as ordinaiy and 
sensitive, arbitrary - Sensitive plants may be excited, yet give no mechanical 
response Certain conditions necessary to exhibition of differential response 
Balanced action as result of diffuse stimulus on radial organ Slight 
differential contraction of pulvinus magnified by long petiolar index. 

BY means of the electric mode of investigation detailed 
in the preceding chapter, I have shown that all vegetable 
organs, whether of ordinary or of sensitive plants, are excit- 
able. Hence the supposed absence of motile indications, in 
ordinary plants, is not to be considered as due to want of 
sensitiveness, but to the lack of proper conditions for mechanical 
movement. What these conditions are will be described 
presently. But before entering upon such considerations, I 
first wish to demonstrate that even in the matter of the 
pulvinar motility itself there is no abrupt division, but a very 
gradual transition, from those plants in which it is scarcely 
perceptible, to others which exhibit it in a marked degree. 

Range of sensitiveness in sensitive plants. Three 
typical instances of plants possessing extremely sensitive, 
moderately sensitive, and almost insensitive leaflets, are : 
(i) Mimosa pudica, (2) Biophytum sensitivum,*x\& (3) Phil- 
anthus urinaria. In Philanthus the small leaves borne on 
pulvini are arranged on two sides of the long twig or petiole, 
in a manner somewhat resembling the arrangement of leaflets 
in Biophylum. The plant appears on casual inspection to be 



44 I'LANT KKSrONSE 

wholly insensitive, ordinary mechanical stimulation having no 
effect on its leaves. Hut if we apply strong thermal or 
electrical stimulus to the end of the twig bearing the leaves, 
then they begin to close very slowly, in serial succession. 
But whereas with Biopliytum the response begins almost 
instantaneously, the maximum being reached in less than a 
second, and complete recovery attained in about four minutes, 
in the leaf of Philanthus nrinaria the latent period, for 
moderate stimulus, is as long as three minutes, the maximum 
reached in not less than forty-five minutes, and complete 




FIG. 30. Responses of (a] quickly reacting Biophytttm, and (b) sluggish 
Philanthtm nrinaria, under moderate and (<) under stronger Stimulation 

recovery may require from two to three hours (fig. 30). It is 
seen, then, that even with regard to the so-called sensitive 
plants, there is a wide range of sensitiveness. In some, a 
slight shock produces quick reaction, in others a very intense 
stimulation is necessary to initiate response, and the reaction 
itself is very sluggish. 

Anisotropy necessary to lateral response. We shall 
now try to understand a little more of the mechanics which 
cause this responsive curvature. In these pulvinated organs, 
one thing that is noticeable is that the organ is not isotropic, 
that is to say, its properties are not the same in all directions. 



CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE 45 

The isotropic condition is seen in the case of radial organs, 
like cylindrical stems or peduncles. One way of showing 
this is to try to bend such a stem in all directions, when it 
will be found that equal forces produce equal bending in any 
direction. This, however, is not the case with dorsi-ventral 
organs, such as the pulvinus. This is more pliable in a 
vertical than in a lateral plane. 

The mass of cells which constitute the lower half of the 
pulvinus, in Mimosa for example, is larger than that of the 
upper. The lower half is also the more excitable. The leaf 
remains in a balanced horizontal position, under the action of 
two opposing forces, the tensions of the opposite halves of the 
pulvinus, these tensions being modified by the turgidity of 
the cells. PfefTer and Sachs have shown that under stimula- 
tion there is an expulsion of water from these excitable cells. 
This may be seen if we watch the cut end of a pulvinus very 
attentively after the disappearance of the excitation due to 
cut, and during the application of a new stimulus. The 
application of this stimulus is followed by a visible escape of 
water from the cut end. A more striking demonstration of 
this fact will be given in Chapter XXI. 

Stimulus, then, induces diminution of the turgidity of the 
organ, by expulsion of water from the excited tissue, and I 
shall show by experiment how this negative variation of 
turgidity, owing to the dorsi-ventral inequality of the organ, 
causes the depression or fall of the leaf. 

Response by artificial turgidity- variation. We may 
fix air-tight the cut end of a branch of Mimosa bearing leaves 
in a U-tube filled with water. When this is done, a quantity 
of water is sucked up, and owing to this increase of turgidity 
the leaves will be forced to assume a highly erect or almost 
vertical position. After several hours this excessive turgidity 
will disappear, and the leaf will then assume a more or less 
horizontal position. The other end of the tube may now be 
connected alternately with a vacuum and with a force-pump, 
by means of which a diminution or increase of internal pres- 
sure may be induced at will. When connected with the 



46 PLANT RESPONSE 

former, or vacuum-pump, water is sucked away, or expelled 
from the plant and its organs ; when, on the contrary, the 
pressure is increased by connection with the force-pump, 
water is forced in. From the curve given below (fig. 31), 
it will be seen that the expulsion of water from the organ 
actually causes the fall of the leaf, and that the forcing of 
it back brings about erection. From the processes involved 
in this artificial response and recovery, we can see clearly 
how in the true response to stimulation we have a two-fold 
process of (i)^the expulsion j of water caused by stimulus, 
bringing about the depression of the leaf, and (2) the return 




Fie. jr. Artificial Hydraulic Response of Alimosa 

The plant was subjected to diminished pressure up to #, and to normal 
pressure to , after which the pressure was increased. The effect 
of diminished pressure, in the depression of the leaf, continues for a 
while. The ordmate represents movement of tip of leaf in cm., 
abscissa represents time. 

of water into the organ, bringing about the restoration of the 
leaf to its original position, or recovery. And since the 
lower half of the organ is the more contractile, it is evident that 
in this lower half there must be relatively greater expulsion 
and absorption during response and recovery. 1 

Two possible types of response. We must bear in 
mind that the entire response consists of these two alternating 
processes, and that the recovery, or restoration of turgidity, 

1 As a normal type I have taken the pulvinus of Mimosa^ the excitability of 
the lower half of \\luch is gieater, and where the responsive movement is 
down. But there may be other cases. If the excitability of the upper half be 
relatively the greater, excitation will in such cases cause upward responsive 
movement. In what follows, unless the contrary be stated, I shall speak of the 
normal type. 



CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE 47 

is not a passive but an active process ; for when the tissue is 
killed it remains flaccid. Throughout the phenomenon of 
response, the essential factor is the variation of turgidity 
from, and its return to, the normal. This variation, as 
generally seen, consists of a fall below, and recovery to, the 
original level of turgescence. And this, as we have seen, is 
accompanied by the sequence of the fall and rise of the leaf. 
But theoretically it should be quite possible to bring about a 
responsive movement by means of the counter-variation, 
namely, an increase, followed by diminution of turgescence. 
In such a case the concomitant movement would be a rise 
and fall, instead of the opposite. Something of this kind will 
be observed in studying the daily periodic movements of the 
Mimosa leaf, where the rise and fall of the leaf will be found 
to synchronise with alternating increase and diminution of 
hydrostatic pressure. 

Abnormal hydrostatic and true excitatory effects. 
Certain effects due to the variation of turgor above and back 
to the normal, will be seen in growth-responses, to be treated 
later. For the present we may find an instance in the 
abnormal erectile twitch, which has already been noted, in 
certain responses of Biophytum (fig. 17). In that case, as 
was explained, the pulse of increased pressure, due to the 
expulsion of water from the distant stimulated point, was 
the first to reach the motile organ, causing erection. The 
true excitatory effect reached the same organ later, with the 
normal effect of depression. The abnormal erectile effect 
may be produced artificially, say by sudden forcing in of 
water. But, in the case mentioned, the pulse of increased 
pressure which brought about this effect was due to stimula- 
tion of a distant point. I shall henceforth, as stated before, 
distinguish the two effects as (a) the direct and (b] the 
indirect effects of stimulation. When a tissue is directly 
stimulated, there is produced a negative turgidity-variation ; 
normal negative mechanical response, or fall of the motile 
leaf; and normal electric response of galvanometric nega- 
tivity. The velocity of transmission of this excitation is 



48 PLANT RESPONSE 

relatively slow, and, as has been said, definite, and charac- 
teristic of the plant under normal conditions. This velocity 
varies, as will be seen later, in the case of different plants, from 
a rate of about '5 to about 15 mm. per second. And this 
true excitatory response, mechanical or electrical, undergoes 
appropriate modifications, according to the physiological 
changes of the tissues, and is abolished at death. 

The hydrostatic effect, on the other hand, may be seen, 
in the form of a preliminary twitch, when the specimen is 
indirectly stimulated that is to say, when the stimulus is 
applied at a distance from the point where the responsive 
effect is observed. The hydrostatic effect gives rise to posi- 
tive turgidity-variation ; abnormal positive or ' up ' mechanical 
response ; and abnonna 1 gal vanometric positivity. The velocity 
of transmission of this hydrostatic disturbance is relatively 
very great, being about several hundreds of mm. per second. 

The conditions of exhibition of excitation by lateral 
response. We shall next consider the question of that 
division of plants into sensitive and ordinary which has 
led to the impression that only the former are excitable. 
And, first, we shall study the conditions which are favourable 
to the exhibition of the motile effect, according to which it 
is customary to estimate the sensitiveness of the plant. We 
have seen that in plants like Mimosa it is the difference in 
excitability between the two halves of the motile organ 
which makes it possible for it to exhibit the state of excita- 
tion by means of lateral movement. If, then, through any 
circumstance, this difference of excitability as between the 
two halves of the organ be diminished or abolished, a plant 
which is undoubtedly sensitive will appear insensitive, as 
judged by the mechanical test. The reductio ad absurdum 
is reached when the same plant is sensitive and insensitive at 
the same time. 

As an instance of this, we may take the plant Biophytum. 
In consequence of age, the differential excitability of the 
pulvini of the leaflets disappears. Hence, when an old leaf 
is excited, its leaflets give no motile indication, and we are 



CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE 49 

apt to consider it as insensitive. But on applying the test 
of electrical response, we discover that, though there is 
no mechanical indication, excitation is nevertheless present. 
Again, if the stimulus be sufficiently strong, the wave of 
excitation will pass through the old leaf, without producing 
any visible effect, and on reaching the younger will be 
manifested by the conspicuous motile response of their 
leaflets. 

Again, we have seen that one of the conditions for the 
production of the responsive movement was the expulsion 
of water from the excited tissue. Hence, if this expulsion 
of water be in any way impeded, mechanical response may 
not take place. This may be seen in the following experi- 
ment : The cut end of a Mimosa stem is placed in water. 
A large amount of water is now found to be absorbed, and 
an abnormal turgidity is produced in the tissue, in conse- 
quence of which the leaves are erected almost vertically. 
If stimulus be now applied, there is no responsive movement 
owing to the difficulty of the expulsion of water from the 
gorged tissue. But the specimen is found to exhibit its 
state of excitation by electrical response, thus proving that 
not its sensitiveness, but its power of manifesting it mechani- 
cally, has been arrested. 

We must bear in mind that, in these cases of differential 
response, the efficiency of the motile apparatus depends 
upon a delicacy of poise as between the two halves of the 
organ, which is capable of being easily upset, under the 
action of stimulus. This poise is determined by the antago- 
nistic tissue-tensions of the two halves, and this again must 
be modified by the distribution of water, or the relative 
turgor- variations, in the two halves. Any deviation from 
the normal distribution of turgidity might, therefore, be ex- 
pected to affect the exhibition of the motile effect. Thus, 
early in the morning, owing to excess of turgor-tension, 
the leaflets of Biophytum show hardly any response, and 
their motility disappears altogether, when the turgor is 
raised still higher, on wet days. But later in the day, when 



50 PLANT RESPONSE 

the periodic turgor-tens<on characteristic of the morning has 
passed off, the plant exhibits normal mechanical responses. 
We have again seen that when the plant Mimosa is placed 
for a time in a dark room, and an abnormal condition of 
turgor as seen in the erection of its leaves is induced, 
the strongest blow will often produce no mechanical 
response. 

It is thus clear that the fulfilment of certain conditions 
is necessary, in order that a plant may exhibit its state 
of excitation, by mechanical response. The absence of this 
response is therefore no proof of the insensitiveness of the 
plant. Having thus shown that sensitive plants so called 
may under certain conditions fail to give motile indications, 
we shall in the next chapter see whether, on the other hand, 
ordinary plants- -commonly assumed to be insensitive may 
not be found to exhibit mechanical response to excitation ; 
such mechanical response having been hitherto overlooked, 
either in consequence of our own imperfect observation, or 
from the fact that in radial organs excitatory reactions would 
be likely to have a multi- radial character, which would cause 
them to balance each other. 

Explanation of absence of lateral response in radial 
organs. Taking first, then, the case of a radial stem, that is 
to say, one whose properties are the same in all directions, 
we shall find that a single stimulus applied simultaneously 
on all sides in other words, a diffuse stimulus would, even 
if it produced responsive contraction, result in no visible 
lateral movement like that seen in Mimosa leaf. This would 
be due to the fact that the contractions at various diametri- 
cally opposite points would be antagonistic, and balance each 
other. Though lateral movement would thus not take place 
in a radial organ, yet it is possible under favourable circum- 
stances, as will be shown in the next chapter, to obtain longi- 
tudinal contraction as seen in muscle. Lateral movement in 
response to a diffuse stimulus can therefore take place only 
when there is some difference of excitability as between two 
opposite halves of an organ. The movement will then be 



CONDITIONS FAVOURABLE TO MECHANICAL RESPONSE $L 

brought about by the greater contraction and resultant con- 
cavity of the more excitable. 

Differential contraction effect in Mimosa magnified 
by the petiolar index. We are impressed with the magni- 
tude of the responsive movement of the Mimosa leaf. It is 
worth while to remember, however, that the fundamental 
differential contraction of the organ by which this is brought 
about is very inconspicuous. If the petiole be amputated, 
we shall hardly notice the responsive action of the pulvinus. 
But the petiole, with its attached secondary petioles, acts as 
a long index, by which the curvature produced at the 
pulvinus on excitation is highly magnified. Many plant- 
movements, which now pass unnoticed, would have arrested 
our attention had there been in their case any such magnify- 
ing index. 

SUMMARY 

An anisotropic organ is unequally excitable on its two 
differentiated sides. 

In a dorsi-ventral organ, like the pulvinus of Mimosa, 
lateral response is brought about by the differential con- 
traction of the two halves. 

In such dorsi-ventral organs, owing to the differentiation 
of the two halves, an increase of turgidity causes erection, 
and a diminution the depression, of the leaf. 

Hence two opposite kinds of responses are possible, 
(a) that which is usually seen, due to the negative variation 
of turgidity, followed by recovery to the normal ; and (b} the 
counter-variation, that is to say, a variation of turgidity 
above the normal causing abnormal erectile response and 
recovery. 

The ordinary mechanical response of dorsi-ventral organs 
being dependent on (a) the difference in excitability of the 
two halves, and () on the expulsion of water from the ex- 
cited organ, it follows that any condition which dimimsnes 
or abolishes this difference, or prevents the expulsion of 
water, will render the mechanical response impossible. A 



E 2 



52 PLANT RESPONSE 

plant may thus be sensitive, and yet fail to exhibit mechani- 
cal response. Hence the absence of mechanical response is 
no indication of the plant's insensibility. 

In a radial organ under diffuse stimulation there can be 
no lateral response, owing to the balanced and mutually 
antagonistic character of the contractions produced. 

The excitatory differential contraction in Mimosa would 
be inconspicuous but for the magnification produced by the 
long pctiolar index. 



CHAPTER V 

MECHANICAL RESPONSE IN ORDINARY LEAVES 

Pulvinoid and pulvinus -Demonstration of mechanical response- in ordinary 
leaves Response of Arlocarpus similar to that of Biophytutn Response 
to stimulus, even in old tissues, by expulsion of water Localisation of 
motile organ in ordinary leaves Conducting properties of various tissues 
Lamina is not the perceptive organ Response in ordinary leaves, though 
sluggish, yet comparable in extent to that of Mimosa Peculiar phenomenon 
of fatigue-reversal seen in Mimosa observed also in ordinary plants Pciiodic 
reversals. 

WE have seen, in the course of the last chapter, that even a 
sensitive plant will fail, under certain conditions, to give any 
mechanical indication of its state of excitation. We shall 
now proceed to determine whether, on the other hand, 
motile response may not be detected in the case of ordinary 
plants. 

It has already been said that the popular division of 
plants into sensitive and ordinary is purely arbitrary. It 
is the erroneous impression consequent on the use of these 
terms which is responsible for the fact that inquirers have 
accepted without hesitation the assumption on which the 
classification rests. Had such not been the case, it must 
long ago have been discovered that the leaves of even 
ordinary plants respond to stimulus by mechanical move- 
ments, in precisely the same manner as do those of sensitive 
plants. 

We have seen that the condition necessary for the pro- 
duction of lateral responsive movement is, that the organ be 
anisotropic ; that is to say, there must be a differentiation 
as between the upper and lower halves. The petioles of 



54 PLANT RESPONSE 

ordinary leaves are obviously anisotropic, their upper and 
lower halves being quite unlike. We might, therefore, expect 
to find in them the exhibition of differential response. There 
is, however, some difficulty in detecting these responsive 
movements of ordinary leaves in an unmistakable manner, 
inasmuch as flexibility is not so great in ordinary petioles 
as in those which are provided with a pulvinus. Moderate 
stimulus therefore causes in these relatively smaller move- 
ments, and unless some form of stimulation can be used 
which brings about no mechanical disturbance of the plant 
itself, it is impossible to discriminate the true responsive 
movement. I have, however, described modes of stimula- 
tion, by the electro-thermic stimulator and by electric 
shocks, by means of which this difficulty is overcome. By 
the use of the Optic Lever, further, a magnified record 
of the responsive movement and its time-relations may be 
obtained. 

Pulvini proper and pulvinoids. We have seen that the 
responsive effect is caused by the turgidity-variation due to 
stimulus. Such motile indications can occur with facility 
only in tissues which are not yet hardened. We may there- 
fore expect to find motile effects throughout the anisotropic 
petiole and its prolongations, especially in young leaves. 
After a certain time, in many instances, it is only portions of 
the petiole, such as those at the junctions of the petiole with 
the stem and lamina, that remain flexible. These flexible 
points are sometimes rather swollen or cushion-like, and may 
be seen though in a much less developed condition than in 
Mimosa in the leaves of many ordinary plants. But such 
areas, in ordinary leaves, are usually regarded as non-motile, 
and therefore functionally distinct from the true pulvini in 
sensitive plants. I shall therefore, for the sake of convenience, 
distinguish between pulvini proper and these pulvinoids. But 
I shall show presently that, contrary to the usual belief, these 
pulvinoids also are fully sensitive. Functionally, then, we 
have in young leaves a diffuse pulvinoid, which is capable of 
mechanical motility, throughout the length of the petiole. 



MECHANICAL RESPONSE IN ORDINARY LEAVES 55 

Later, we find the pulvinoid localised at one or two points 
only, such as the stem and laminal junctions, the motile 
function persisting here for some considerable period. And 
finally, in the markedly motile pulvini of the so-called 
' sensitive ' plants, we have the property of motility manifested 
throughout a still greater length of time. It will thus be 
seen that we can scarcely draw any sharp line of demarcation 
between pulvini proper and the pulvinoids of ordinary 
plants. And when the leaves are old, both alike cease to be 
motile. 

Mechanical response of Artocarpus. I shall now 
describe the method by which I have obtained records of 
these mechanical responses in ordinary leaves. For this 
purpose I took a pot-grown specimen of Artocarpus integri- 
folia, or Jack-fruit plant, the leaves of which are stiff and, as 
far as the eye can judge, singularly inappropriate for the 
exhibition of motile effects, and selected for my investigation 
the third leaf from the top of a stem, this being neither too 
young nor too old. I have said that in an anisotropic petiole 
the response ought to take place by the induced concavity 
of the more excitable half, whether upper or lower. In this 
case it was impossible to know from inspection which was 
the more excitable. But I have described, in the last chapter, 
an electrical method by which differences of excitability, arising 
from molecular or anatomical differentiation, can be distin- 
guished. It was there explained that the electrical current 
of response flows from the relatively more to the relatively 
less excitable. And on repeating the electric experiment, 
in the present case, I found the responsive current to flow 
from below upwards. This proved that in Artocarpus the 
lower surface of the petiole was, as in the case of Mimosa, 
the more excitable of the two. Hence, if we should obtain 
the mechanical response from this apparently non-sensitive 
leaf, we might expect that it would be downwards. In order 
to produce stimulation, I used the electro- thermic stimulator, 
in the manner already described. The stimulus was first 
applied at a point on the petiole 3 mm. from the laminal 



I'LANT RESPONSE 



junction. From the record given below (fig. 32), it will be 
seen that the responses arc similar, even in minute details, 
to those' obtained with sensitive Biophytuin leaflets. 

It will be seen that we have first the abnormal erectile 
twitch, due to transmitted hydrostatic disturbance, which 
takes place almost instantaneously. The true responsive 
effect is found to follow, after an interval of four seconds. 
From this we obtain the velocity of transmission as 75 mm. 

per second, that is to 
say, about one-third the 
rate found in Biophytum. 
A somewhat incomplete 
recovery took place in 
the course of ten mi- 
nutes. The successive 
responses are found to 
be nearly uniform ; but 
if only shorter interven- 
ing periods of rest be 
allowed, they exhibit 
marked fatigue. That 
the motile region is 




FlG. 32. Kesponv uf Oidiuai) Leaf (.///t>- 
r </;/// s) 

Note preliminary erectilr t\\krh. 



somewhere near the junction of the lamina with the petiole, 
was proved by the fact that on bringing the stimulator 
nearer to, or further away from, this point, the responses 
underwent corresponding increase or diminution. 

I obtained similar responses by subjecting the lower 
pulvinoid, that, namely, at the junction of the petiole with 
the stem, to similar stimulus, which was now applied on the 
adjacent stem. In the preceding experiment the Optic 
Lever was attached to the lamina, but in this case, in order 
to avoid possible complications from the responses of two 
pulvinoids in succession, I tied a thin stiff wire to the inter- 
vening portion of the petiole, and the Optic Lever was 
attached to this wire, instead of to the lamina. By this 
means the response of the lower pulvinoid alone was 
recorded. 



MECHANICAL RESPONSE IN ORDINARY LEAVES $? 

Excitatory reaction in older tissues. One very impor- 
tant and hitherto undecided question, which is answered 
by this experiment, is as to whether all tissues, even the 
old, give contractile response by expulsion of water, or 
by negative turgidity- variation. In the pulvinus such an 
effect is made evident by the differential contractile move- 
ment produced. In young tissues it will be shown that 
we have effects exactly similar. But in older tissues no 
such movement is observable. Of this fact, two alternative 
explanations are possible: (i) there may be no excitatory 
expulsion of water in old tissue ; (2) such expulsion may 
occur, while movement is at the same time rendered 
impossible, by the inflexible condition of the tissue. We 
have already seen how, whenever there is negative turgidity- 
variation due to excitation, there is also a simultaneous 
galvanometric negativity. Conversely, induced galvanometric 
negativity may be taken as an indication of the excitatory 
expulsion of water. And from this indication the con- 
clusion is arrived at that, even in relatively old tissues, this 
excitatory expulsion occurs, since these tissues on excitation 
exhibit galvanometric negativity. 

I was desirous, however, to obtain additional proof ot 
this excitatory expulsion of water in the older tissues, and 
from the preliminary positive twitch seen in the mechanical 
response (fig. 32) the existence of such action is clearly 
proved. For the preliminary erectile twitch observed in the 
leaf, when the rigid part of the petiole or of the stem at 
a distance beyond the petiolar junction is excited, can 
only be explained by a pulse of increased pressure towards 
the motile area, initiated by water expelled from the 
stimulated point. Independent experiments, carried out by 
other methods, also tend to show that, though the power of 
excitatory contraction undergoes diminution with age, it 
does not, generally speaking, disappear entirely. 

We saw in Biophytum that when stimulus was applied 
at a distance, the hydrostatic disturbance produced the 
preliminary erectile twitch. But no such effect was observed 



$8 PLANT RESPONSE 

when the pulvinus was directly stimulated, the result now 
being the true excitatory fall alone. In the case of Arto- 
carpus I obtained the same result Direct stimulation was 
here applied, at the motile region, by electric induction 
shocks, the electrical connections being made on two sides 
of the pulvinoid, by means of thin flexible spirals of tinsel 
which did not offer any obstruction to the free move- 
ment of the responding leaf. The response thus obtained 
was normal, and unattended by any preliminary positive 
twitch. 

The motile responsiveness of Artocarpus^ then, is seen 
to follow that of Biophytum or Mimosa, even in details. 
An instance of this is found in the fact that in all three 
cases abnormal increase of turgidity is unfavourable to the 
manifestation of mechanical response. Thus, just after the 
rainy season, the Artocarpus is highly turgid, and response 
not easily obtainable. But in November, when the turgor 
has become moderate, its motile indications are once more 
made conspicuous. 

If the motile region be extended, or diffuse, the responsive 
action when the stimulus is applied at any single point becomes 
very complicated. For we have, first, the preliminary hydro- 
static pulse, travelling along the whole length of the motile 
organ, and producing a somewhat long-continued erectile 
effect, which is, therefore, unlike the quickly exhausted 
erectile twitch that took place during the short passage of 
the hydrostatic pulse through the restricted pulvinoid. And, 
secondly, follows, in the wake of this, a wave of negative 
turgidity-variation concomitant to the passage of true excita- 
tion. Such complicated effects may be avoided, however, by 
applying electrical stimulus simultaneously throughout the 
whole area. The electrical form of stimulation has therefore 
certain advantages, but it is apt to bring on quick fatigue of 
the tissue. 

Localisation of motile areas. 1 shall now deal with some 
further difficult points, in connection with the motile response 
of the leaf. And first comes the question of localising 



MECHANICAL RESPONSE IN ORDINARY LEAVES 59 

the motile area itself. Next comes that of the possibility 
of excitation reaching the motile organ from a djstance, 
through certain specific tissues, and the determination of the 
nature of such tissues. By using the experimental methods 
which I have described, it is possible to determine these 
important points. But, before doing this, it will be well to 
enter upon the theoretical considerations in connection with 
the subject. 

As regards the first point, it has been shown that any 
flexible anisotropic organ is capable of lateral response owing 
to differential action. In the leaf, when young, the power of 
movement extends throughout the length of the petiole and 
its prolongation, the midrib. But when older, as has been 
said, the motile power becomes localised at the pulvinoids or 
pulvini. This conclusion is verified by experiment. The 
young petiole is found motile throughout its length, but when 
older the existence of certain specialised areas is made evident 
by the fact of the strong response obtained, when stimulus is 
applied on such areas, and its rapid diminution on application 
at gradually increasing distances. For example, in the case 
of Artocarpus leaf, when this distance from the pulvinoid is 
increased to I cm. the normal response to moderate stimulus 
disappears altogether. In this connection it is well to bear 
in mind the fact, which will be fully demonstrated later, that 
the distance to which the effect of stimulus is transmitted 
depends not only on the conductivity of the tissue, but also 
on the intensity of the stimulus. A moderate intensity 
of stimulus is only effective in producing motile indications 
when the application is on, or very near, the pulvinoid. We 
thus see, as regards motile organs, that there is a strict 
continuity between those cases in which the property of 
motility is diffused through a large area, and others in which 
it is contracted to one or more definite points, this last form 
culminating in the maximum flexibility of pulvini proper. 

The petiole of the leaf of Biophytum itself gives an excellent 
example of a diffuse pulvinoid. It is not provided with any 
specialised pulvinus, such as that of Mimosa, or of its own 



60 PLANT RESPONSE 

leaflets. But if this petiole be subjected to any form ot 
stimulus, it responds by a depression of the leaf as a whole. 
On the cessation of stimulus there is the usual recovery. In 
a subsequent chapter I shall describe in detail the responses 
of the petiole of Biophytum to different forms of stimulation. 

Conducting* properties of various tissues. With 
regard to the next point, this is to say, the transmission 
of excitation to a distance, it will be shown in Chapter XX. 
that the transmission of the excitatory state is brought about 
by the propagation of protoplasmic changes from point to 
point We may therefore expect that tissues in which there 
is more or less uninterrupted protoplasmic continuity will be 
those which will, other things being equal, show the greatest 
power of transmitting stimulus. Even in such cases it will 
be understood that the transmission becomes enfeebled with 
distance. Now the tissues in which this continuity is most 
uninterrupted are the fibre-vascular elements. Hence, stimu- 
lus applied on the petiole, or its prolongation, will reach the 
motile organ, with the greater intensity, the nearer the point 
of application is to the motile region. We should expect, 
on the other hand, that indifferent tissues, like leaf parenchyma 
proper, would prove to be, owing to the more or less complete 
cellular partitions, incapable of transmitting stimulus to a 
distance. These considerations I have been able to verify 
experimentally by means of electric response. I shall here, 
however, describe experiments in which the same conclusions 
are established by the method of mechanical response. 

Moderately strong electrical stimulus from an induction 
coil was applied on the petiole of Artocarpus at 'points 
increasingly far from the laminal pulvinoid ; these produced 
motile responses which diminished rapidly with distance, as 
was described in the last experiment. But when such 
stimulus was applied on the lamina, at a point relatively near 
the pulvinoid, there was no response. This shows that the 
lamina is not to any extent the perceptive organ, and that 
stimulus received on such an area does not cause movement 
of the leaf as a whole. 



MECHANICAL RESPONSE IN ORDINARY LEAVES 6l 

It will thus be seen that the petiole when young is both 
the motile and the transmitting organ. With increasing 
age, certain areas become inflexible, and the power of 
motility is narrowed to the still remaining points of flexi- 
bility the pulvinoids. But the inflexible petiolar portion 
still retains the power of transmitting stimulus. Using the 
language of Animal Physiology, we might say, therefore, 
that a young petiole is, functionally, nerve and muscle 
combined. Later there is a differentiation into motile pulvi- 
noids, corresponding to muscle, and the rest corresponding 
to conducting nerve. The pulvinoid, however, has still the 
power of transmitting stimulus. 

To serve the purpose of a concrete example, I have 
taken as a typical case the Artocarpus leaf. I shall now, 
however, show that responsive motile effects are obtainable 
from the leaves of plants in general. In the record given 
(fig. 32) I applied only moderate stimulus, in order that the 
magnified response might be brought within the limited 
space of record. From this, it might be supposed that the 
extent of responsive movement in ordinary leaves is rela- 
tively much smaller than what is obtained with the sensitive 
Mimosa. But this is by no means the case. By using 
stronger stimulus a response is obtained, in the case of many 
ordinary leaves, which in its extent is strictly comparable 
with that of Mimosa. Of this, I shall here proceed to give 
examples. But, before doing so, we must remind ourselves 
that the promptitude of mechanical response is a matter of 
the delicacy of poise of the motile apparatus. In the case of 
Philanthus, which is provided with distinct pulvini, we found 
that response took place very sluggishly, and was evoked 
only by relatively strong stimulus. The difference between 
the response in this case and in that of Mimosa is, however, 
not of kind, but of degree. 

Response of ordinary leaves comparable with that of 
Mimosa. When we come to the response of ordinary leaves, 
we find, under strong stimulation, considerable extents of 
movement, even rivalling those of Mimosa, but taking place 



62 PLANT RESPONSE 

with a comparative sluggishness which, generally speaking, 
is not so great as that of Philanthus. In the following 
experiments various leaves were excited by strong electric 
shocks, sent through the entire length of the petiole, and 
responsive movements were, as a rule, observed, the leaves in 
these instances falling downwards from their normal more or 
less horizontal position. Of these I shall give only three 
instances. A particular leaf of Bryophyllum calcynum, which 
is unprovided with a pulvinoid, was 5 cm. in length. After 
tetanic shocks of three minutes, the leaf fell, the distance 
traversed by the. tip being 2*1 cm. A leaf of Canna indica, 
again, unprovided with any pulvinoid and not too young, 
was 20 cm. in length. On the application of tetanic shocks 
for one minute, the leaf fell, the distance traversed by the 
tip being as much as 14 cm. The third instance is a leaf 
of Ficus religiosa, 14 cm. in length. After three minutes of 
continuous stimulation, the leaf fell, the tip passing through a 
distance of 12 cm. It will thus be seen that the movement 
caused by stimulation in ordinary leaves is quite considerable. 

It may, however, be objected that the fall of the leaf is 
really due simply to its weight, acting on the petiole when 
rendered flaccid. Even in this case the flaccidity would be 
directly due to stimulation, and the movement of the Mimosa 
leaf itself must be regarded as being of a similar nature. 
But in order to prove that the movement of the leaf is 
mainly due to the active differential contraction of the upper 
and lower halves, and in order to eliminate completely the 
effect of weight, I arranged the experiments as follows : 
The plant was held with the leaf directed vertically down- 
wards. The movement due to the greater contraction of the 
lower half of the organ, in consequence of the true excitatory 
effect, ought now to raise the leaf against the force of gravity. 
And this, as will be seen from the records (fig. 33), was found 
to be the case. 

In order to compare the angular movements of some 
ordinary leaves, and their time-relations, with those of 
Mimosa^ when subjected to strong electric shocks, J took 



MECHANICAL RESPONSE IN ORDINARY LEAVER 63 

records of such movements in these different cases on the 
same, revolving drum. And with the purpose of making the 
angular movements, in the various instances of long and 
short leaves, comparable, I rendered the virtual length the 
same in all cases, by attaching longer or shorter indicators, 
as was necessary, and the movement of the tip of the indi- 
cator or its projection was then recorded on the drum. The 
longest leaf experimented on in this series was Mimosa, 
having a length of 5 cm. In this case no additional indicator 
was attached ; and in other cases the indicator-lengths which 
were added made each leaf have a virtual length of 5 cm. 

Fatigue-reversals in ordinary leaves, as in Mimosa. 
In comparing the response- records of various specimens, it 
is necessary to understand one peculiarity which is observed 
in certain responses under long-continued stimulation. In 
order to do this, I must anticipate a certain characteristic 
of Mimosa response, which will be more fully described in 
Chapter IX, It is found that the leaf of Mimosa gives imme- 
diate response to stimulus, by a fall or contraction of the 
lower half. But when the stimulus is long continued, the 
leaf rises gradually to its original position, or even, some- 
times, beyond this. This position is, however, only apparently 
normal, being really a posture of fatigue. For whereas, 
while the leaf is fresh, it responds to stimulus by depression, 
it is now, though it occupies the same position, entirely in- 
susceptible of excitatory depression. In exhibiting the effect 
of long-continued stimulation on ordinary leaves, I shall be 
able to show that they resemble Mimosa, not only in the 
extent of their responses, but also, in certain cases, even with 
regard to this specific characteristic. 

In the records given (fig. 33) the first curve is of Mimosa 
leaf. It should be remembered that the experiments in all 
these cases were carried out with the leaves held perpendicu- 
larly downwards, the excitatory movement being produced 
by strong and continuous tetanic shocks. The excitatory 
effect recorded, therefore, is due to the differential contraction 
of the two halves of the motile organ, lifting the leaf against 



64 PLANT KKSPONSK 

gravity. In the specimen of Mimosa (a) the maximum 
responsive deviation of 25 was reached in a little more than 
one second, after which occurred the peculiar reversal already 
referred to. This, which was due to fatigue, was completed 
in seven and a half seconds. The next curve (ft] was ob- 
tained from the young leaf of Citrus decumana. It will 
be observed that here we have a practical duplication of the 
curve of Mimosa, the only difference lying in the longer 
period necessary for the completion of its different parts. 
The responsive deviation in this case is 30, that is to 




1' K; - 33- Response of Li-uves of Ordinary Plant* to Klcctiic Stimulation 
The orclinate represents the angular movement in degrees. 

say, even greater than that of Mimosa. This was attained 
in eleven seconds, and reversal completed in twenty five 
seconds. The extent of these responsive movements, being 
due to differences of excitability as between the two halves, 
will, it is easy to understand, be modified by the age of the 
leaf. This fact is illustrated by the next curve (r), obtained 
with an older leaf of the same plant. Here the responsive 
movement takes place through only 10, and there is only a 
partial reversal. 

The instances given may be considered as typical, the 
responses obtained with specimens of other plants being 
simply variations of these. For example, while some leaves 



MECHANICAL RESPONSE IN ORDINARY LEAVES 65 

under continuous stimulation exhibit complete, and some 
partial, reversals, there are again other species of plants whose 
leaves do not exhibit any reversal at all. Still others, again, 
exhibit periodic reversals. The effects produced are thus 
seen to depend on the species of the plant, on the age of the 
leaf, and on the intensity and duration of the stimulus. 

It will thus be seen that an anisotropic organ like the 
petiole responds to stimulus by differential contraction of its 
upper and lower halves, and that the mechanics of such 
movements in the case of ordinary leaves are precisely the 
same as in those of the leaves of sensitive plants such as 
Mimosa. In the next chapter I hope to show that even 
radial organs are not insensitive, but exhibit responsive con- 
tractile movements. 

SUMMARY 

The motile organs of ordinary leaves, owing to dorsi- 
ventral differentiation, give mechanical response by differential 
contraction, in a manner precisely the same as does the 
motile organ of Mimosa. 

In a young leaf, the petiole and its prolongation act as a 
diffuse pulvinoid ; later, the motile area becomes restricted 
to certain points, generally speaking, the junctions of petiole 
and stem, and petiole and lamina. There is no sharp line of 
demarcation between pulvinoids and pulvini proper. 

Even in relatively old and rigid tissues, response to stimu- 
lus is by excitatory expulsion of water. 

Responsive movements take place in ordinary leaves, 
when the stimulus is applied at or near the motile organ. 

The lamina is not, generally speaking, the perceptive 
organ. The effect of stimulus remains in this case localised, 
and is not transmitted to a distance. 

Organs like the petiole, which contain fibre-vascular 
elements, are capable of conducting stimulus to a distance. 

Under continuous stimulation there may be reversal due 
to fatigue in ordinary leaves as in Mimosa. In some cases, 
periodic reversals are also observed, 

K 



CHAPTER VI 

LONGITUDINAL RESPONSE OF RADIAL ORGANS 

Absence of lateral response movements in radial organs due to mutually antagonistic 
effects of equal contractions of diametrically opposite sides Lateral response 
in radial stem of Walnut under unilateral stimulation Also in pistil of Musa 
Diffuse stimulation of radial organ causes longitudinal contraction --The 
' Kunchangraph ' Longitudinal contraction of stamens of Cynerete not unique 
--Similar longitudinal responses obtained with stems, roots, tendrils, petioles, 
stamens, and styles of ordinary plants Also in fungi Responsive contrac- 
tion in Passiflora, comparable in extent with that in Cynerece Longitudinal 
response in plants modified by the physiological variations due to age, season, 
and chemical agencies. 

WE have now considered the phenomenon of responsive 
lateral curvature in plant-organs such as leaves, and have 
seen that it is brought about by the differential contraction 
of two unequally excited halves. But in the case of radial 
organs, under diffuse stimulus, such responsive movements 
arc not usually noticeable, hence these organs have generally 
been regarded as insensitive. 

It is, however, conceivable, as already said, that this 
absence of responsive movement might be due to the occur- 
rence of simultaneous contractions in diametrically opposite 
sides, which would balance each other, and thus permit of no 
lateral movement. The correctness of this supposition might 
be tested in either of two ways. We might, for instance, 
apply a unilateral instead of a diffuse stimulus, and so 
bring about a lateral contraction. Or we might use a diffuse 
stimulus, and look for the contraction in length of the speci- 
men as a whole. 

Experiments showing equal and opposite reactions of 
radial organs, If we apply local and unilateral stimulus, 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 67 

we may expect contraction of the acted side alone. The 
stimulated side should thus become concaye. I have been 
able to verify this by experimenting with, amongst others, the 
radial stem of a young Walnut plant ; stimulus was applied 
in a region about 4 cm. from the tip, this being a plastic 
area, just below the zone of growth. There are practical 
difficulties in the application of unilateral stimulus, which 
might easily be overcome by the use of photic stimulus. In 
the present chapter, however, I intend to deal with forms of 
stimulation other than that of light. In the experiment on 
Walnut, therefore, I made use of the stimulus of electric 
shocks from an induction coil. The electrical connections 
were made with one side only of the radial stem. Small 
quantities of kaolin paste, moistened with normal saline 
solution, were placed on the stem at two points, at a distance 
of 2 cm. one above the other, and these were connected with 
the terminals of the induction coil by means of flexible 
spirals of tinsel. Electrical excitation was now produced by 
tetanising shocks for the requisite length of time. With 
regard to this, it must be pointed out that excitation given by 
such means cannot be strictly confined to one side of the 
organ alone, since the current will pass through the further 
side also ; but as most of it will take the shortest path, 
excitation will be relatively greater on the side of the electric 
connections. The responses were recorded as usual, by 
attaching the stem to the Optic Lever. 

Strong electrical shocks being now applied during a 
period of five seconds on one side of the stem, a responsive 
concavity of that side was produced, the amplitude of the 
response being eight divisions. The effect attained its 
maximum in forty seconds, after which the recovery was 
partially completed, in six minutes. On now exciting the 
opposite side of the same stem, a response which was 
practically the same i.e. 7-5 divisions was obtained, the 
curvature being now in the opposite direction. Hence it is 
clear from these two experiments, that when a radial organ 
is simultaneously excited on all sides, there can be no lateral 



V 2 



68 



I'LANT RKSI'ONSE 



curvature, owing to the fact that opposite reactions neutralise 
each other. 

I succeeded in obtaining a singularly perfect demonstration 
of the response of a radial organ to local and unilateral stimula- 
tion, by paying special attention to the selection of the specimen 
and mode of stimulation. 1 took a pistil of Musaparadisiaca, in 
which I was able to localise with great distinctness the zone of 
growth, in this particular case of not greater extent than 2 mm. 
The stimulation, which was thermal, was effected by placing 
on one side of the selected area, but not in actual contact with 

it, the outer point of 
a small V-shaped 
piece of platinum 
wire which could be 
put in circuit with a 
battery. An exactly 
similar arrangement 
was made on the 
other side of the 
organ, at the dia- 
metrically opposite 
point. By sending a 
heating current from 
the battery through 
either of the plati- 
num pieces for a de- 
finite length of time, 
the tissue in that region is stimulated locally by thermal 
radiation. In this manner two opposite points of the tissue 
may be alternately subjected to equal stimulation. The 
accompanying figure (fig. 34) gives the record of such 
an experiment. The up-responses here represent the con- 
cavity produced by stimulating on one side, say the right, 
and the down-responses the same on the left. It will thus 
be .seen that equal and opposite responses were obtained, 
equal alternate stimulation of the two sides. The recovery, 
it will be noticed, is not complete, the curvature caused by 




I'M.. 34. Alternate opposite-directioned Re- 
sponses obtained by the successive Unilateral 
Stimulations of opposite sides of Pistil of 
Mus a 



K, 



oiixe-i of light side, upwards; i, re- 
sponses of left side, downwards. 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 69 



stimulus being partially fixed by growth. The responses, 
moreover, afford some slight indication of fatigue. 

But though there is thus no lateral movement, owing 
to the antagonistic character of the simultaneous longi- 
tudinal contractions on all sides, we might nevertheless 
hope to detect some responsive contraction in the length of 
the organ as a whole. And perhaps it will be well to con- 
sider at this point what would be the most favourable con- 
dition for the exhibition of such contraction. Taking the 
parallel instance of contractile animal tissue, namely muscle, 
we can easily see that if this were attached throughout its length 
to a rigid structure such as bone, contractile movement would 
have been an impossibility. Similarly, contractile vegetable 
tissues when attached to hard elements, such as wood, must 
be prevented from exhibiting contractile movements. The 
vegetable tissues, therefore, which ought to be best fitted to 
exhibit this effect, will be comparatively deficient in hardened 
fibro-vascular elements, and will consist largely of prosenchy- 
matous cells, relatively longer in one direction, and very 
elastic and highly extended by turgidity. The diminution 
of this turgidity by stimulus might then be expected to 
produce a relatively large degree of longitudinal contraction. 1 

Excitatory contractions in Cynereae. This shortening 
in length is most strikingly exhibited by the filaments of the 
stamens of Cynerece. This subject has been specially studied 
by Pfeffer, and the account which I give here is epitomised 
from Sachs* ' Physiology of Plants.' 2 The filaments of the 
stamens of Centaurea jacea are free from each other, but the 
anthers cohere, forming a tube. The stamens are, in the 
unexcited state, strongly curved convexly outwards. If now 
the filaments are all excited simultaneously, say by mechanical 
touch, there is produced a downward withdrawal of the 

1 I have shown that galvanometric negativity is an unmistakable indication of 
the excitatory contraction of a vegetable tissue. Employing this test, I find that 
all living vegetable tissues are excitable. But the mechanical contraction pro- 
duced will, other things being equal, be very marked only in long prosenchymatous 
tissues, and in a mass of parenchyma relatively less marked. 

1 Sachs' Physiology of Plants, Engl. ed. 1887, pp. 651-2. 



7O PLANT RESPONSE 

anther-tube, in consequence of the general shortening. After 
a few minutes the filaments again elongate, returning to 
their original arched position. They are now once more 
irritable. Again, if one of the filaments be freed, and 
stimulation be applied to its outer surface, this convex surface 
becomes concave, returning to the original convex form during 
recovery. But if the inside of the filament be touched the 
reverse action takes place, that is to say, the inner side now 
becomes strongly concave. On excitation, the expelled 
water passes from the excited cells into the intercellular 
spaces. This excitatorily expelled water was observed by 
Pfeffer to well forth from the cut end of the filament. He 
also studied the contractile shortening, using a magnification 
of one to two hundred diameters, and found it to amount in 
various cases to from 8 to 22 per cent, of the original length. 

It will be seen that in this case of Cyncrece we find those 
conditions of extensibility and elasticity which are so charac- 
teristic of muscle, to be present, though not in an extreme 
degree. In muscles, contraction is brought about by re- 
distribution of fluid, as between the constituent isotropic and 
anisotropic elements. In Cynerece, also, the phenomenon is 
not altogether different, for here it is known to be brought 
about by a redistribution of water as between the cells and 
intercellular spaces. It will be well to remember here that 
the effect of stimulus acting on an organ from without is, in 
general, to force the expelled water inwards, whence it may 
be driven, through intercellular spaces and fibre-vascular 
elements, out of the excited region, into the interior, or the 
rest, of the plant. On the cessation of stimulus, the water, 
thus expelled under tension, returns into the contracted cells, 
and this fact aids the process of recovery. 

I have already demonstrated in the beginning of this 
chapter the contraction and concavity of the excited side of 
an organ, in response to unilateral stimulus. The responsive 
concavity of either side of the filament of Cynerea when 
excited, as observed by various investigators, is simply ar 
instance of this. 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 7 1 

The filaments of Cynerea cannot be regarded as, strictly 
speaking, radial organs, for their tangential diameters are 
nearly twice as much as the radial, hence their response 
cannot be considered as entirely unaffected by differential 
action on the two sides. 

I shall now proceed to demonstrate that the radial organs 
of plants exhibit response by longitudinal contraction, just as 
muscle is seen to contract under stimulus, and that such 
contraction, so far from being distinctive of any specific plant, 
or of any organ of such plant, is characteristic of all radial 
organs of plants in general. It will also be Shown that the 
lateral response of anisotropic organs is to be regarded as an 
instance of differential longitudinal contraction. 

The Kunchangraph. But it is here necessary to give 
a full account of the experimental arrangements by means 
of which this demonstration has been rendered possible, and 
to show that the results thereby obtained are ^reliable, 
consistent, and capable of the highest quantitative exactitude. 
In this ' Kunchangraph,' l which records the contractile response 
of the plant, as the Myograph that of the animal, we have, 
first, the plant chamber proper, which is made small, in order 
that the conditions, and variations of conditions, whose effects 
are to be studied, may be easily and rapidly changed. The 
lower end of the organ, securely held by means of a cork, is 
immersed in a small test-tube containing water, and fixed in 
the middle of the base-board. The upper end of the organ 
is connected with one arm of the Optic Lever by means of 
a thread. The fulcrum-rod, carrying the reflecting mirror, 
projects outside the chamber. A thin glass cylinder, not 
shown in the figure, may be used to cover the projected 
portion of the fulcrum-rod, thus protecting the mirror from 
the disturbance caused by air-currents. 

One important condition which I find essential to the 
maintenance of uniformity of sensitiveness is that the plant 
shall be surrounded by a moist atmosphere, whose humidity 
is constant. An air-bag is kept under suitable pressure, and 

1 From the Sanskrit knnchan = contraction. The is here pronounced as in /////. 



72 PLANT RESPONSE 

air, bubbling through water, is made to enter the chamber 
through an entrance-pipe. The exit-pipe is connected with 
an aspirator. By proper manipulation of the stop-cocks of 
the air-bag and the aspirator, a gentle stream of humid air is 
kept in constant circulation through the chamber. A modi- 
fication of this arrangement enables us to study the effect of 
various gases and vapours on the excitability of the organ. 
A series of responses is first taken, under normal conditions, 
that is to say, when the plant is surrounded simply by a 
moist atmosphere. By now turning a three-way tap in 
a given direction, the water-vapour can be made to pass 
through a vessel filled with a given gas, before reaching the 
plant chamber. Or the pure gas can be introduced alone. 
The series of responses now obtained shows both the pre- 
liminary and the permanent effects of the gas. For it is now 
easy, by means of the three-way cock, to shut off the gas, 
and to, re-establish the first or normal condition. The re- 
sponses next given afford an indication of the after-effect of 
the gaseous reagent. It will be seen that by this means the 
effects of various gases and vapours can be studied with the 
greatest ease (fig. 35). 

The next point to be dealt with is that of the mode of 
application of a uniform or graduated stimulus. For electrical 
stimulation we may use induction currents. For this purpose, 
a sliding induction coil of Du Bois-Reymond's pattern is 
used. The intensity of the shock is here graduated, by 
bringing the primary coil nearer and nearer to the secondary. 
Non-polarisable electrodes make suitable connections with 
the upper and lower ends of the organ. Electrical stimu- 
lation is often preferable on the whole, but unless vigorous 
specimens are used, it is apt when strong to induce fatigue. 
This may be avoided by the use of moderate stimulus and 
high magnification. Or we may, if desired, use thermal 
stimulus, which is effected by means of heat generated 
in a continuous length of thin German-silver wire which 
surrounds the whole length of the specimen, in the form of 
a cylindrical cage. This wire cylinder is fixed appropriately 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 



73 



".( ' ,j ,'.,,1W t, .. ,UVS 

,f; ':>r'f l i';'i;',r;:^ t ^ l ^!, 1 J l ;*ii! 

1 1' ','1! , n ''5i r V "l"i'P ht ' ' trl 1 ' V ^'! d'' 1 -"^, 




FIG. 35. The Kunchangraph 

The plant enclosed in semi-cylindrical wire heating cage, H, seen open. The 
plant is attached to the Optic Lever. Light proceeding from focussing tube, L, 
after reflection from optical mirror, M, falls on the recording drum, l>. Stimu- 
lation is periodically effected on closure of electrical circuit, containing storage- 
battery, s, by the rotating rod, R. Air from bag not shown is passed through 
water-vessel to right, and circulated through plant chamber. The vessel to left 
is the aspirator. The middle vessel contains ether or other chemical sub- 
stance, which is made to displace air in plant chamber, by manipulation of 
stop-cock. G, the clock-governor. 



74 PLANT 

in wooden or ebonite forms, which are made in halves and 
hinged, so that one half of the cylinder may be swung back 
in order to afford easy access to the specimen, for the purpose 
of adjustment. When closed, the wire is in complete circuit 
with the electrodes outside. By sending through this wire 
a strong current of short duration, the sudden rise of tem- 
perature generated in the chamber causes the stimulation of 
the tissue. This heat is quickly dissipated, again, by the 
stream of air charged with vapour, which is in constant 
circulation. The effectiveness of stimulation will depend on 
the range, and also on the suddenness, of the temperature- 
variation. The required thermal stimulus is thus most easily 
effected by electrical means, the degree of rise of temperature 
being determined by the strength of the current acting 
on the heating circuit, and the requisite suddenness of 
variation being the result of temporary completions of the 
circuit of definite and short duration. This mode of stimu- 
lation, I shall, for convenience, designate as stimulation by 
thermal shocks. 

It is very difficult, using only the hand, to attain the 
necessary precision in making these brief and equal com- 
pletions of the circuit several times in succession. Hence, 
the stimulus not being strictly uniform, the responses are 
apt to become unequal. I have overcome this difficulty, 
however, by the construction of a closing key regulated by 
clock-work, which enables successive stimuli, of equal in- 
tensity and duration, to be applied automatically, at pre- 
determined intervals. This is accomplished by means of a 
clockwork arrangement, which enables uniform and successive 
electrical or thermal shocks to be applied, while at the 
same time the intervening periods between successive shocks 
may be so adjusted as to allow for complete recovery. 
A radial arm, carried on the axle-rod of the clock, at each 
complete revolution strikes against a balanced key, which 
completes the electric circuit. The intervals of successive 
stimulation may be determined by regulating the speed 
of the clock. This may be done by suitably inclining the 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 75 

blades of the air-vane governor (fig. 36). The diagram 
shows the mode of making successive closures of the electric 
circuit for giving . thermal shocks. And the same arrange- 
ment serves to close the primary of the induction coil. 
The duration of stimulus, depending upon that of the closure 
of circuit, may be adjusted by varying the length of the 
radial arm. The effective intensity of stimulation may be 
increased by applying three or four shocks in rapid 




B 



FIG. 36. Diagrammatic Representation of Apparatus for Periodic 

Stimulation of Plant 

H, the wire cylinder made in hinged halves, periodically heated when 
the electric circuit is closed by tilting over of balanced key, K, when 
pressed by rod, R. v, air-vane of clock, by which period of rotation 
is adjusted. 

succession, instead of one. For this purpose the radial arm 
carries at its end a small plate, of which the margin is 
divided into three or four teeth as the case may be. It is 
thus possible to produce a stimulation which consists of the 
requisite number of summated shocks. Or, instead of the 
clock, we might, for the purpose of producing brief and 
definite closures, have a metronome. But this is a less 
perfect arrangement than that of the clock. 



76 PLANT RESPONSE 

By wrapping a sensitive film round the recording drum, 
all these response-records may be obtained photographically. 
Thus the. whole process of stimulation and its record may be 
rendered automatic. The records given in this and succeeding 
chapters have been obtained sometimes by photography, and 
sometimes by employing the simpler process of following the 
spot of light with a recording pen. 

From a knowledge of the magnification produced by the 
Optic Lever, and the height of the responses, it is easy to 
calculate the actual contraction produced by the stimulus. 
From the length of the -specimen experimented on we can 
also determine the coefficient of responsive contraction that 
is to say, the absolute contraction for unit length. When the 
magnification of the lever, the length of various specimens, 
and the strength of the stimulus are all kept constant, then 
the heights of the responses in different cases will be found 
to afford us a measure of the mechanical excitabilities of 
the different specimens. 

Demonstration of universality of excitatory longi- 
tudinal contraction in radial organs. I shall now describe 
my experiments on the longitudinal contraction of various 
radial organs. The first of these was performed on a straight 
radial internode of Cuscuta^ which was attached to the 
recording Optical Lever in the usual manner. Tetanis- 
ing shocks were given for twenty-five seconds at a time, 
from an induction coil, and the successive responses were 
obtained, at intervals of two minutes (fig. 37). The contractile 
effect persisted, even after the cessation of stimulation, for 
a period of five seconds, after which there was recovery, 
which was completed in a period of ninety seconds. It will 
be seen from the record that the successive responses were 
uniform. 

I obtained similar responses from the root of a water- 
growing plant of the Bindweed family. Successive responses 
were obtained at intervals of two minutes, the stimulus in 
each case consisting of tetanising shocks of forty-five seconds. 
The maximum contraction was attained fifteen seconds after 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 77 

the cessation of stimulus ; and recovery was completed after 
a further period of one minute. In this case the responses 
exhibited fatigue. It may be stated here that, speaking 
generally, the period required for recovery is dependent on 
the strength of stimulus. With moderate intensity of 
stimulation, recovery is complete within a comparatively 
short period. But it is protracted, or indefinitely delayed, 
when the stimulus is strong. Again, if successive stimuli be 
applied, before recovery is complete, the responses will be 
found to be additive. 

In order to convey some idea of the amount of contraction 
produced by stimulus, I shall here give a detailed account of 
an experiment on longitudinal contraction, the specimen 




FIG. 37. Response of Stem of Ciucnta to Electric Stimulation 

used being a young stem of the species of Bindweed 
(Convolvulus] already referred to. The length experimented 
on was 5 cm. On passing through this tetanising shocks 
of five seconds 5 duration, a maximum contraction was 
found to occur in the course of two minutes. The magnifi- 
cation used for record was fifty times, and the extent of 
contractile response recorded was 7-5 cm. Hence, the 
actual contraction was 1*5 mm., in a stem whose length 
was 50 mm. The contraction produced is thus 3 per cent, 
of the original length. 

Similar contractile response may be obtained with other 
forms of stimulation, and I shall now describe that induced 
by thermal stimulus, the specimen used being the radial 




78 PLANT RESPONSE 

style of Datura alba. Experimenting with the style has 
the special theoretical advantage that, owing to the soft 
nature of the tissue, the effect recorded is purely of 
longitudinal contraction. In specimens which have not 
this characteristic to the same extent, and which may be 
anisotropic, like the filaments of Cynerece^ the contraction is 
not always purely longitudinal. There is a tendency, owing 
to differential contraction, to the production of curvature. 
""" For these reasons, a limp and thread-like 

style fulfils the ideal requirements of an 
experiment for obtaining true longitudinal 
contraction. With this specimen of Datura 
I applied thermal stimulus at intervals of 
FIG. 38. Photo- two minutes, in the manner already de- 
graphic Record of scribed. The records show how extremely 

Responses of Style ... . //% n _. 

of Datura alba to uniform the responses are (fig. 38). The 
Thermal Stimuia- same longitudinal contraction may also be 
obtained from plants other than phanero- 
gams. On applying electrical stimulus to the stalk of the 
fungus Agaricus I obtained a contraction of 2 per cent, of 
the original length. 

Remarkable extent of contraction in coronal filaments 
of Passiflora. There are some plants, again, in which the 
extent of the excitatory contraction is very great. For 
instance, the filamentous corona of Passiflora quadrangularis 
often gives a contraction of as much as 20 per cent. 

It will thus be seen that not only is the phenomenon of 
longitudinal excitatory contraction present in all plants, but 
that such excitatory movements in some which are supposed 
to be insensitive, rival in extent those of the typically sensi- 
tive filaments of Cynerece> which are said to exhibit a con- 
traction of from 8 to 22 per cent. 

In order to obtain a suitable record a magnification of 
only twenty to thirty times is necessary in the case of the 
highly excitable tissue of the corona of Passiflora. In less 
excitable specimens, a magnification of 100 would be enough. 
The advantage of relatively high magnifications in general 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 79 



lies in the fact that they necessitate only moderate intensities 
of stimulation, which have the advantage of not fatiguing the 
tissue. 

Modification of excitatory contraction by physiological 
conditions. That these contractions are the expressions of 
true excitatory response is proved by the fact that they are 
modified by whatever affects the physiological condition of 
the tissue. Thus, for example, they undergo a temporary 
abolition under the action of anaesthetics, and a permanent 
abolition under the action of poisons. This will be demon- 
strated in more detail in a later chapter. They also exhibit 
very interesting modifications, according to the age of- the 
specimen and the season of the year, as might theoretically 
have been expected. In experiments on this subject, under- 
taken with the filamentous corona of Passiflora^ stimulation 
was produced by tetanising electric shocks, and the maximum 
contraction was measured by means of a micrometer. The 
following results show, in condensed form, the effect of age 
on excitatory contraction. 

TABLE SHOWING EFFECT OF AGE ON EXCITATORY CONTRACTION 
(Coronal filaments of Fassiflora </.) 

[Length of specimens, 21 mm. Time, February, i.e. end of winter.] 



Age of Specimen. 



Absolute 
contraction. 



Mean 
contraction. 



Percentage of 
contraction. 



Flower already 


opened ((/>) i 

((<) : i 


[6 mm. 
i *5 mm. 
[45 mm. 


1 [ 1*51 mm. 7-2 per cent. 


One day before 


( () 1 
opening-^ (t>) ; 

1(0 ! 

1 


8 mm. 
7 mm. 
65 mm. 


r 171 mm. 8'i per cent. 


Bud . 


(() I 
](*) , 

1(0 i 


65 mm. 
*6 mtn. 
5 mm. 


! i *6 mm. 7-6 per cent. 

! ' 



It will thus be seen that when the physiological activity 
of the specimen is at its greatest, that is to say, just before 



80 PLANT RESPONSE 

the opening of the flower, the excitatory contraction is also 
at its maximum. 

In order next to determine the effect of season on the 
excitatory contraction, we shall compare the mean percent- 
age of contraction in winter with that obtained in spring, in 
the month of April. The average contraction in winter may 
be taken as 7*6 per cent. But in spring I obtained (i) with 
a specimen 12 mm. long, a contraction of 17 mm., i.e. 14 per 
cent. ; and (2) with a second specimen 10 mm. long, a con- 
traction of 2 mm., Le. of 20 per cent The mean of these, 
17 per cent, representing the contraction in spring, is thus 
seen to be about two and a half times that obtained in 
winter. 

We have now ascertained the universal occurrence of 
longitudinal contraction in the organs of plants, and we have 
seen that lateral response cannot take place under diffuse 
stimulation in a strictly radial organ, owing to the antago- 
nistic character of the equal and simultaneous responsive 
contractions on diametrically opposite sides. 

Lateral response, however, as we have seen, will take place 
in a radial organ when stimulus is not diffuse, but unilateral. 
Such lateral response is only possible under diffuse stimulus, 
when the excitability of the two opposite halves is different, 
that is to say, when the organ is anisotropic. In such cases, 
as in the petioles of leaves, for example, the very striking 
lateral movement is simply the result of differential longi- 
tudinal contraction. The differentiation of the upper and 
lower halves is anatomically evident in the case of dorsi- 
ventral organs. But the anisotropy may often be undistin- 
guishable to the eye. For an organ, originally radial, may 
become molecularly bilateral, owing to the unequal action of 
external forces on diametrically opposite sides. It will be 
shown in the next chapter that such molecular differentiation 
gives rise to physiological differentiation, and there I shall 
be able to trace a continuity between the longitudinal 
response of radial organs and the lateral response of dorsi- 
ventral organs through intermediate types. 



LONGITUDINAL RESPONSE OF RADIAL ORGANS 8 1 

SUMMARY 

In a radial organ, unilateral stimulation causes contrac- 
tion, and consequent concavity of the acted side. 

Under diffuse stimulation there is no resultant lateral 
response, owing to the balanced and mutually antagonistic 
character of the contractions. 

A radial organ under diffuse stimulation exhibits longi- 
tudinal contraction. 

Longitudinal contractions are observed in the radial 
organs of all plants. 

Such responsive contractions as seen in some * ordinary ' 
plants are strictly comparable in extent to those which are 
known to occur in the sensitive filaments of Cynerea. 

These excitatory responses are modified by*all those 
agencies which affect the physiological condition of the plant. 



G 



CHAPTER VII 

RESPONSIVE CURVATURE OF MOLECULARLY 
ANISOTROPIC ORGAN 

Molecular anisotropy artificially induced by one-sided cooling Cooled side less 
responsive Diffuse stimulation causes concavity of the uncooled, that being 
relatively the more excitable Local fatigue diminishes excitability Diffuse 
stimulation now causes concavity of the unstrained side Similar anisotropy 
induced in plagiotropic organs, by unilateral action of light The lower or 
shaded side of such organs relatively more excitable Diffuse stimulation 
causes current of response from lower to upper, and also concavity of lower 
half Responses of plagiotropic Cuiurbita and Convolvulus Differences in 
excitabilities of outer and inner surfaces of tubular organ Complex response 
due to successive excitations of two antagonistic halves of an anisotropic 
organ Response of spiral tendrils by uncurling Response in certain cases by 
contraction of the spiral or curling Writhing movement in spiral tendril 
under strong stimulation. 

WE have seen that in a radial organ, owing to balanced 
actions, there is no lateral response to diffuse stimulus ; and 
that in dorsi-ventral organs, where there is pronounced 
anisotropy as seen in anatomical differentiation lateral 
movement is produced by means of differential action. I 
am now about to demonstrate the fact that these phenomena 
are not sharply divided, but merge gradually one into the 
other, through intermediate types. 

Molecular anisotropy induced by unilateral application 
of cold or of excessive stimulation. If we take a hollow 
radial petiole of Gourd (Cucurbita maxima) growing erect, we 
shall find, on application of diffuse stimulus, that it gives no 
responsive curvature, but exhibits the simple longitudinal 
contraction of a radial organ. We may now take this petiole/ 
and split it into two equal halves, throughout almost its 
entire length. We have now a single specimen bifurcated, 
the forked divisions being equal in every respect One tbrk, 



RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 83 

say A, is now immersed in ice-water, the other being dipped 
in water at the ordinary temperature. The two halves are 
next taken out of the water and bound together, from their 
free ends upwards, so as to make once more a single tube. 
The petiole is now held by the uncut part vertically, with 
a long index projecting downwards from its lower end. 
The petiole was, as will be remembered, originally radial; 
but now, by the local application of cold to one half, a 
certain molecular differentiation has been induced, and the 
particles in the cooled, or A half, are now therefore " more 

# 

sluggish and irresponsive than those of the B side. This 
induced molecular differentiation, moreover, is invisible to 
the eye, and had we not observed the process by which it 
was brought about, it would have been impossible, from mere 
visual inspection, to know which side had been subjected to 
cold. The application of diffuse stimulus, however, reveals it 
at once ; for on passing electrical shocks along the length 
of the specimen, the relatively more excitable, or uncooled B 
half, becomes concave. It is clear, then, that at this moment 
the B half is the more excitable, and stimulus acts on 
it preferentially. But after long-continued stimulation, the 
B half becomes overstrained, and its excitability undergoes 
diminution or fatigue. At the same time, by means of 
repeated shocks, the sluggishness of the A half has been 
gradually made to disappear ; that side now regains its 
excitability ; and, the excitation of A becoming thus rela- 
tively greater, the curvature of the specimen is reversed. 

I have said that excitability is diminished under the 
molecular strain caused by over-stimulation. I shall now 
demonstrate this by another and independent experiment. A 
new specimen, slit like the last, is taken, and one half, say A, 
is alone subjected to strong excitation, by sending electric 
shocks along its length. The two halves are again brought 
together, and on now subjecting the whole petiole to electric 
stimulation, it is found that the fresh, or previously unexcited, 
half is that which becomes concave, thus proving that 
the fresh half is the more excitable, and that strong or 

G 2 



84 PLANT RESPONSE 

long-continued stimulation diminishes the excitability of a 
tissue. 

From these two experiments it will be seen that loss 
of excitability may be produced in, amongst others, two 
different ways. First, there is the molecular sluggishness, 
induced, as we have seen, by cooling, which is, however, 
only temporary, the original sensitiveness being restored on 
warming. Secondly, we may have loss of sensibility due 
to overstrain, owing to strong or long-continued stimulation. 
This strain-effect, with its attendant loss of excitability, may, 
if excessive, prove more or less permanent. It will thus be 
seen that in consequence of unilateral stimulation, a mole- 
cular differentiation is produced, in consequence of which an 
organ originally radial becomes physiologically anisotropic. 
The unstimulated portion of the organ is now the relatively 
more excitable, and on diffuse stimulation becomes concave. 
Molecular anisotropy induced under natural condi- 
tions. We shall next observe the induction of such mole- 
cular anisotropy under natural conditions. The stem of a 
young Gourd is at first erect and strictly radial ; but later it 
bends over, and then assumes a creeping habit. Under these 
conditions, its upper surface is subjected to the unilateral 
action of vertical light, the lower half being shaded and 
protected. The organ is no longer radial, then, but plagio- 
tropic, and from what has been said already, we shall expect 
its upper or exposed surface, in consequence of the prolonged 
action of stimulus of sunlight, to be less excitable than the 
lower or shaded surface. This inference I have been able to 
verify by means of the electric mode of investigation. I find 
that on simultaneously exciting both sides of such a stem, 
a current of response flows from the lower to the upper 
surface ; hence it will be seen, according to the third law of 
electrical response, as enunciated at the end of Chapter III., 
that this lower side is the more excitable, and ought to 
become concave under diffuse stimulation. 

Such induced concavity in response to diffuse stimulation 
I have found in the case of various plagiotropic stems, for 



RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 85 

example, in those of Cucurbita and Convolvulus. In order 
to demonstrate the greater contraction of the shaded side 
which is seen as responsive curvature while eliminating the 
effect of gravity, I have employed two different modes of 
experiment. In the first, stems are held with their tips 
vertically downwards, and electric shocks are passed through 
them ; a curvature is then produced by the greater contrac- 
tion of the shaded side, in consequence of which the free end 
of the stem is lifted up against the force of gravity, The 
second method consists in supporting the stem horizontally 
in such a way that the plane which divides the previously 
shaded and unshaded sides is vertical : on strong stimulation, 
the stem now moves in the horizontal plane, in a definite 
direction which is determined by the induced concavity of 
the shaded and more excitable side. Here we see the plagio- 
tropic stem behaving like the pulvinus of Mimosa, the more 
excitable side becoming concave under diffuse stimulation in 
both. 

Response of plagiotropic stems. In order to obtain a 
series of responses, made from plagiotropic stems, demon- 
strating their similarity to those obtained from the pulvini of 
sensitive plants like Mimosa, we may use specimens of 
Cucurbita or Convolvulus, selecting the last internode of 
the stem as the most sensitive. The cut ends of the 
specimens are placed in water, and the abnormal turgidity 
thus produced may sometimes at first cause erratic responses ; 
but after a while these become very regular. Stimulation 
is produced by thermal shocks, the specimen being held 
erect, within an inclosing spiral of heating wire, in the 
manner already described. The responses are now given 
in the form of lateral movements, which are recorded by 
the use of a magnetically controlled horizontal recorder, fully 
described in a subsequent chapter. The responses might 
easily have been recorded also by the use of the Optic Lever, 
which was employed in the case of Mimosa. But my object 
in the present case was to eliminate as far as possible 
the effect of gravity, and this could not have been done by 



86 



PLANT RESPONSE 



holding the specimen horizontally, as would be necessary 
in using the Optic Lever. 

In the records obtained with Cucurbita (fig. 39), it will 
be noticed that the recovery was not very complete, even 
though an interval of five minutes elapsed between successive 




FIG. 39. Responses of Piagiotropic Stem of Cnnirbita 

stimuli. In the case of Convolvulus (fig. 40), however, the 
recovery was almost complete in two minutes. But of the 
two, Cucurbita was the more sensitive, Convolvulus requiring 
a stimulus about four times as great, in order to produce the 




FIG. 40. Responses of Piagiotropic Stem of Convolvulus 

same amplitude of response. Both records show evidence 
of fatigue. 

Response by collapse of divergent halves of Allium 
peduncle. The effect of the anisotropy thus induced by the 
unilateral action of light may also be exhibited by taking 



KKSPOXSIYK CURVATURE OF ANISOTUOPIC OKC.AN 87 



the hollow petiole of Cucurhita or the peduncle of A/liuin. 
Of these two specimens, the latter is the more sensitive, and 
reacts far more quickly. It will be observed that the out- 
side of such tubular organs growing erect has been long 
exposed to light, whereas the inside has been protected from 
it. If we now split the specimen for a few centimetres of its 
length, then the freed halves, owing to the differences of 
tension as between the 
outside and inside, will be 
found to curve outwards, 
that is to say, the inner 
side becomes convex. In 
the previous experiments, 
where the two halves were 
rejoined, anisotropy having 
been induced, we observed 
the effect of the relatively 
greater contraction of one 
of the two halves. But we 
are now about to study 
natural differences of ex- 
citability as between the 
inner and outer surfaces of 
each half. As the outer 
surface has already been 
exposed to the continuous 
action of stimulus of light, 
we should expect the inner or protected side to be relatively 
the more excitable. The effect of diffuse stimulus should 
then be to straighten the curled halves, by producing a 
greater contraction of the more excitable inner side, which 
is at present convex. The experiment is carried out by 
dipping the freed ends in a beaker of water, the undivided 
portion being held in a clamp (fig. 41). Electrical con- 
nections are made with the upper part, and with the water 
in the beaker in which the free ends of the specimen are 
dipped. After passing a few shocks from an induction coil 




KK;. 41. Response of Bifurcated Allinm 

Tube by sudden Collapse 
E and E f are connected with induction 

coil, by means of which the plant is 

stimulated electrically. 



88 PLANT RESPONSE 

through the specimen, the divergent curled portions are seen 
to fall together, the observed effect being very like the 
sudden collapse of the divergent leaves of a gold-leaf electro- 
scope. The experiment described becomes very striking 
when magnified by optical projection. 

In the response-phenomena of anisotropic organs we meet 
with instances in which the continuous action of stimulus 

* 

gives rise to alternate movements up and down. One factor 
of this obscure phenomenon may be determined by a modi- 
fication of the experiment just described. Since we have 
seen that in a typically anisotropic or dorsi-ventral organ 
the excitabilitics of the two halves are different, there must 
also exist a difference of time-relations as between their 
responses ; that is to say, the beginning of response, the 
attainment of the maximum, and so on, will take place earlier 
in the one half than in the other. The response of the organ 
as a whole will thus be the resultant of the curves of response 
of its constituent halves ; and since these latter differ, in 
amplitude and phase, we are in a position to understand how 
we may have great variations in the resultant effect. In 
order to show the difference of phase I shall take a simple 
case of induced anisotropy. One half of the bifurcated 
peduncle is cooled by immersion for a time in ice-water. 
The two halves are now dipped in a vessel of water, as in the 
last experiment, and electric shocks are passed through the 
peduncle as a whole. It will now be found that we obtain 
successive, instead of simultaneous, excitations of the two 
halves. For the uncooled half responds at once, whereas the 
cooled half only begins to respond after ten or more seconds. 
It is thus clear that had the two halves been joined to form 
a single organ, the observed response would have been a 
compound of these two constituent responses. The first part 
of this response would be due to the active contraction of the 
uncooled half, but, later, the contraction of the cooled half 
would reverse this first movement. 

Response by uncurling*. Having now studied the 
anisotropy, and consequent differences of excitability, caused 



RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 89 

by the unilateral stimulus of light, we shall next proceed to 
consider the similar effects induced by unilateral mechanical 
stimulation. We shall find, if we touch a tendril unilaterally, 
that it responds to this one-sided stimulus by the concavity 
of. the excited side, and we obtain a curvature. After a 
more or less prolonged contact, this curvature becomes fairly 
permanent. Thus, by means of unilateral excitation, The 
originally radial tendril like the unilaterally excited plagio- 
tropic organ has been made to become bilateral and 
anisotropic, and the excited concave surface should now be 
relatively less excitable than the convex. 

A tendril of Passiflora was taken, in which a curvature 
of half a spiral had been induced by stimulus of unilateral 
contact. The straight lower end of the tendril was. now fixed 
in a clamp, the hooked end being attached to the Optic 
Lever. A spiral of tinsel made one of the electrical contacts 
at the hooked end, the other being made at the clamp. An 
electrical shock of moderate intensity was now passed through 
the length of the tendril. From what has been said already, 
we should expect that diffuse excitation would now produce 
concavity of the more excitable, or convex, side of the hooked 
tendril. We should expect, in other words, that the electric 
stimulation would have the effect of undoing the existing 
curvature, or straightening out the curved tendril. Such a 
responsive uncurling would, if it occurred, relax the tension 
on the Lever, and cause a concomitant movement upwards 
of the spot of light. On the cessation of stimulus, again 
provided this have been not too strong, there should be a 
restoration of the tendril to its original curvature. Greater 
intensity of stimulus, or stimulus of longer duration, should, 
other things being equal, produce greater responsive move- 
ment. And the recovery from such stronger stimulation 
would require a relatively longer time. 

All these theoretical considerations are found fully verified 
in the record given below (fig. 42). It will there be seen that 
a stimulus of moderate electric shock, lasting fifteen seconds, 
produced a correspondingly moderate response of fourteen 




90 PLANT RESPONSE 

divisions, and the recovery was completed in eight minutes. 
Stimulus of longer duration, that is to say, of twenty seconds, 
was next applied, and the response was correspondingly 
greater, that is to say, twenty-one divisions, recovery taking 
place in the longer period of eleven minutes ; and finally, 
stimulus was applied for a still longer period, that is to say, 
thirty seconds, the response, of thirty-five divisions, being now 

correspondingly great, and 
recovery requiring six- 
teen minutes. 

Tt will be noticed that 
in this case the amplitude 
of response and the period 
of recovery varied almost 
in direct proportion with 

FIG. 42. Responses of Hooked Tendril of ,1 i . i , 

Passiflora the duration, determining 

(</) Response to electric shock of fifteen the effective intensity of 

seconds duration ;(/,) of twenty seconds ; th stimulus. Thus the 
(c) of thirty seconds. Note increase in 

height of response and lengthening of restoration to the Original 
period of recovery with increasing , r -ru ~> 

stimulus. position of equilibrium 

takes place quickly when 

the stimulus is feeble ; but the period is prolonged, when 
stimulus is strong ; or recovery may even be postponed in- 
definitely, after very strong stimulation. Recovery, when it 
does occur in such a case, may only be partial, a permanent 
after-effect being left. 

The effects here described are obtained most easily by 
direct stimulation of the organ. But similar results may 

nevertheless be exhibited bv means of transmitted stimulation. 

j 

To show this, we may take a spiral of Passiflora tendril, and 
apply strong electric stimulation through two points at its 
lower end. The transmitted stimulus, reaching the free spiral 
end, causes response by uncurling, and variation of the twist. 
Response by curling 1 . -In the case of the experiments 
just described, stimulus has been found to produce uncoiling 
of the spiral. From this, however, it must not be too hastily 
concluded that similar effects will ensue in every case. The 



RESPONSIVE CURVATURE OK ANISOTROPIC ORGAN 91 




fundamental phenomenon to be kept in mind is the greater 
contraction of the more excitable side. This might give rise 
to curling or uncurling, according to individual circum- 
stances. I shall now describe 
an experiment which illustrates 
the opposite, or curling, action 
of this particular form of re- 
sponse. 

If we cut a petiole of 
Cucurbita or a peduncle of 
Alliuni corkscrew- wise, so as to 
form a spiral strip, and pass 
electric shocks through this 
prepared specimen, the index 
at its lower end shows very 
energetic movement, but of 
coiling. Here we must bear in 
mind that the inside of the 
spiral of Passiflora has been 
formed by the stimulus of con- 
tact, and is thus the less excit- 
able. Diffuse stimulation in 

such a case, therefore, will cause the contraction of the con- 
vex surface, with the result of uncoiling. But in this spiral 
preparation of Alliuni or Cucurbita, it is the outside which has 
been long acted on by light, and it is the inner or concave 
side, therefore, which remains the more excitable. Hence, 
under stimulus, it is this more excitable inner side which 
becomes still more concave with the result of coiling (fig. 43). 

Writhing movements of excited spiral tendril. The 
most striking of this series of results were obtained, however, 
with long spiral tendrils of Passiflora, which were not too 
old. Very strong stimuli of electric shocks were sent through 
the entire length of these spirals, with results so striking and 
unmistakable as to furnish a final refutation of the popular 
assumption which distinguishes between animal and vege- 
table tissues as relatively motile and non-motile. These 



FIG. 43. Response by Coiling of 
spirally-cut Allittm Peduncle 

Through E and E' are passed elec- 
trical shocks from an induction 
coil. 



92 PLANT RESPONSE 

spiral tendrils, on receiving electric stimulation, began 
instantly to uncurl their free ends, as they did so, sweeping 
through large arcs and then straightened themselves out. 
Startling as this was, however, it was not all. I have already 
alluded to the phenomena of successive excitations and 
alternating fatigue, of the different sides of an anisotropic 
organ, under continuous stimulation. Often, owing to this 
peculiarity, the tendrils after their first uncoiling action* 
showed, though with less vigour, the movement of recurling. 
These violent contortions were strongly suggestive of the 
writhing of a worm under torture. 

Though the response to the stimulus of strong electrical 
shock is the most vigorous, yet this responsive movement of 
uncoiling can also be obtained by other forms of stimulation, 
such as the thermal and chemical. For this purpose we may 
dip the spiral tendril into hot water, or into dilute sulphuric 
acid. The differential contractile response may then be 
observed. 

We have thus traced out in unbroken continuity the 
various types of mechanical response as seen in plants. To 
begin with, we have observed the responsive longitudinal 
contraction, pure and simple, of a strictly radial organ. Next, 
in the case of plagiotropic stems, the same longitudinal 
contraction, but acting differentially, produced lateral move- 
ment, the differential action being the result of induced 
molecular anisotropy and consequent difference in the 
excitabilities of the two halves, as a result of which, a 
plagiotropic stem functions as a diffuse pulvinoid. From 
this we pass to the anatomical anisotropy, which may be 
observed in the dorsi-ventral petioles of ordinary leaves. 
Here we find a tendency in the diffuse pulvinoid to become 
contracted to certain definite areas ; and the responsive 
movement, in such cases, also, is brought about by differential 
longitudinal contraction of the upper and lower halves. And, 
finally, we discover the culminating type of such differentia- 
tion for the purpose of motile efficiency, in the pulvinus of the 
so-called ' sensitive ' plants, where also responsive movement 
is brought about by differential longitudinal contraction. 



RESPONSIVE CURVATURE OF ANISOTROPIC ORGAN 93 

It has thus been clearly established that there is no 
specific sensibility of the dorsi- ventral organ which is in 
any way distinct from that of radial organs the responsive 
lateral movements of leaves being merely a special or dif- 
ferential form of the longitudinal contraction which has thus 
been found to be widely prevalent. 

SUMMARY 

When the two sides of an organ become unequally 
excitable by reason of molecular differentiation, a resultant 
lateral response, due to differential longitudinal contraction, 
is obtained, the more excitable side becoming concave. This 
molecular differentiation may be induced artificially by uni- 
lateral application of cold, or of strong stimulation. 

This molecular differentiation occurs under natural con- 
ditions in plagiotropic stems, the upper surface being acted 
on by stimulus of vertical sun-light 

In a hollow tubular organ, such as the petiole of Citcurbita, 
or the peduncle of Alliuw, the outer surface, which is 
constantly acted on by light, is found to be less excitable 
than the inner surface. 

The spiral formed, by unilateral stimulus of contact, in 
such tendrils as that of Passiflora is less excitable on the 
already stimulated, or concave, than on the outer, or convex, 
side ; diffuse stimulation, causing greater contraction of the 
more excitable convex side, gives rise here by differential 
contraction to the responsive movement of uncurling. 

Molecular anisotropy culminates in dorsi-ventral in- 
equality, as seen in the petioles or in the pulvini of leaves. 
Here, too, diffuse stimulation causes lateral response, by 
inducing concavity of the more excitable half. 

Dorsi-ventral organs do not possess any specific sensi- 
bility different from that of radial organs, the lateral 
responsive movement being the result of the differential 
longitudinal contraction of two unequally excitable halves. 

The universal law of responsive movement is : Mechanical 
response takes place by the concavity of the more excited side. 



CHAPTER VIII 

RELATION BETWEEN STIMULUS AND RESPONSE 

Ineffective stimulus becomes effective by repetition Two types of response in 
contractile animal tissues, cardiac and skeletal Response of cardiac muscle 
on ' all or none ' principle ; parallel case in Biophytum In skeletal muscle, 
increasing stimulus causes increasing response, which tends to reach a limit 
Parallel \ results in longitudinal and electrical response of plants Effect of 
superposition of stimuli Tetanus. 

THE application of stimulus to a tissue initiates a series of 
events which culminates in the contraction of the excited 
cells. It is easily seen that a certain minimum intensity 
of stimulus is necessary in order to bring the excitatory 
condition of the tissue to the threshold of response. In the 
case of the electrical stimulation of Biophytum^ we found this 
minimum stimulus-intensity to be of a very definite order. 
In the production of longitudinal response also a certain 
minimum amount of stimulus, either electrical or thermal, is 
necessary in order to evoke response. 

Additive effect. A thermal shock which is singly 
ineffective, may become effective by repetition. Below is 
given a record which shows this. A single shock produced 
by the closure for one second of an electrical circuit, acted 
on by six volts, was found to be ineffective in inducing 
mechanical response. But when the same stimulus was 
repeated six times in succession, it gave rise to a moderately 
large response (fig. 44). This additive effect I also find in the 
electric response of plants (fig. 45). And it is well known in 
the case of animal tissues. 

In the case of contractile animal tissues, again, we have 
two distinct types of response. The first is that of cardiac 
muscle, which is said to be on the ' all or none ' principle. 
That is to say, on applying a gradually increasing stimulus, 



RELATION BETWEEN STIMULUS AND RESPONSE 95 



we presently arrive at the threshold of response, where the 
response becomes at once the maximal possible. In this 
case, then, the minimal response is also maximal. We shall 





FlG. 44. Ineffective Stimulus made 
Effective by Repetition 

The line to the left shows that single 
stimulus produced no mechanical 
response. The curve to the right 
shows the effect produced when 
stimulus had been repeated six times. 



FIG. 45. Additive litfect in Electrical 
Response 

(a) A single stimulus of 3 vibration pro- 
duced little or no effect, but the same 
stimulus when rapidly superposed 
thirty times produced the large 
effect (/>). Leafstalk of turnip. 



find, in Chapter XXII., that an exactly similar type of 
response is afforded by Biophytum, where the minimally 
effective stimulus suddenly produces maximal effect. 

Relation between stimulus and response in animal 
and vegetable. The second type of contractile response is 
shown by skeletal muscle. Here, after reaching the threshold 
of response, increasing stimulus causes increasing response, 
which, however, tends to reach a limit. Exactly parallel 
effects are seen in the case of plants, in the longitudinal 
responses exhibited by different radial organs. 

The experimental method by which this is demonstrated 
is as follows: The intensity of thermal stimulus may be 
appropriately increased, as has been explained, by increasing 
the value of the heating current, the circuit being always 
closed for a certain definite time, say one second. The 
resistance of the heating coil being kept constant, the 
thermal effect is proportional to the square of the current. 
If, then, we use currents which increase as the square root of 
the natural numbers, the successive thermal effects will be 
increased in arithmetical progression. The currents to be 
used are previously adjusted, by means of an ammeter, and 



PLAN 1 RESPONSE 




KlG. 46. Mechanical Kc.spon-.c-, to Stimuli in- 
creasing in Arithmetical Progression 



suitable external resistances. In this way I have obtained 
successive responses to thermal stimuli, which increased in 

arithmetical progres- 
sion. And we see 
from the records in 
what manner the cor- 
responding responses 
undergo an increase 
(fig. 46). 

It will be seen 
from this figure that 
with a stimulus re- 
presented by unity, 
the response was 1*5 
division. With a 
stimulus twice as great, the corresponding response was 
slightly greater than twice as much, that is to say, it was 3*5. 
The intensity of response went on increasing, but with 

special acceleration when the 
stimuli were four and five. 
After the stimulus of six 
there was a tendency for the 
response to approach a limit. 
The subjoined curve (fig. 47) 
shows the relation between 
the increasing stimuli and the 
corresponding responses. 

A nother interesting fea- 
ture of these response-curves 
is one which has already been 
referred to, the prolongation 
of the period of recovery with 
increasing stimulus. In the 
present case, with the stimulus of unit-intensity, the recovery 
was completed in forty seconds. With an intensity twice as 
great, it required fifty-six seconds for the restoration of 
equilibrium. And this increase in the period of recovery 




Ku;. 47. Curve showing Relation be- 
tween Stimulus and Response 

The abscissa represents the stimulus, 
and ordinate the height of response. 



RKLATION HETWKEN STIMULUS AND KKSl'ONSH 97 

continued progressively, until, with the stimulus-intensity of 
eight, the time required for restoration to the original con- 
dition was as much as 4 minutes 40 seconds, or exactly seven 
times that necessitated by unit-stimulus. 

All these peculiarities are observable in the electrical 
responses of plants, as will be seen from the record in fig. 48. 
The stimulus applied was vibrational, and was increased in 
amplitude in successive experiments, from 2*5 to 7*5 to 
10 to 1 2 -5. It will be seen that here also as in the case 




,24 

FIG. 48. Increased Electrical Response with Increasing Vibrational Stimuli 

(Cauliflower-stalk) 

Vertical line to right = *i volt. Stimuli applied at intervals of three minutes. 

of the response of skeletal muscle, and the longitudinal 
response of plants, the amplitude of response increasing with 
increasing stimulus tends to approach a limit. These curves 
also show the increase in the period required for complete 
recovery, but in a manner different from that of fig. 46. In 
fig. 46 the time-intervals were suitably increased, to allow of 
complete recovery. In the case of the electrical responses, 
however, stimuli were applied at equal intervals of time 
throughout. This was enough to bring about complete 

II 



PLANT RESPONSE 

recovery in the case of the first two responses. But after- 
wards it was not sufficient, so that recovery in the last three 
instances was more and more incomplete, as seen by the 
tilting upwards of the base-line of the responses (fig. 47). 

TAHI.E SHOWING THK INCREASED E.M. VARIATION 
CRD i:v INCREASING STIMULUS' 



Angle of \ihi.uion. 


Electromotive response 







2-5 


044 volt. 


5 


075 >. 


7-5 -090 


IO' -IOO ,, 


12-5 -106 ,, 



Tetanus. Having now observed the effect produced by 
single stimuli, we shall proceed to study the effects of similar 




Fio. 49. (lenesis of Tetanus in Muscle 

Record to left shows incomplete tetanus, with moderate frequency of 
stimulation. Record to right shows tetanus more complete, with 
greater frequency of stimulation (Brodie). 



stimuli when superposed. In muscle, we find that when 
stimuli succeed each other with great rapidity, the effect of 
the second stimulus becomes superposed on that of the first, 
which has not had tiine^to disappear. The result is a fusion 
of effects, more or less complete. With moderate frequency 
of stimulation we thus obtain incomplete tetanus, which, with 

1 Rose, Response in the Living and Non-Living, p. 53. 




RELATION BETWEEN STIMULUS AND RESPONSE 99 

increasing frequency of stimulation, becomes more and more 
complete (fig. 49). 

As regards the mechanical response of plants, I obtained 
similar tetanic effects with the longitudinal contractions of 
the pistil of Datura alba (fig. 50). Stimuli were here applied 
at intervals of ten seconds, which was too short an interval, 
when compared with the natural period of recovery, lasting 
about two minutes. Hence we obtained incomplete tetanus. 
This incomplete tetanus became more complete when the 
stimulation- frequency was increased, 
successive stimuli being now applied at 
intervals of five seconds. It may be 
noted here that, in the tetanus both of 
muscle and of plant, the effects of 
individual stimuli, when rapidly suc- 
ceeding, become so merged as to appear Kl(; . 50 . Photographic 
continuous. It is only after the maxi- Hecord of Genesis of 

Tetanus in Mechanical 

mum effect has been attained, that the Response of Plants 
individual effects of stimuli sometimes (Style of Z>/I/AI) 
become distinguishable by slight oscillatory movements of 
the curve. In the case of rhythmic cardiac muscle, how-- 
ever, there is no tetanus ; and similarly, as described in 
Chapter XXVII., we find no tetanus in the rhythmic 
vegetable tissue of Desmodium. 

SUMMARY 

There is a minimal intensity of stimulus necessary to 
initiate response. 

A stimulus, singly ineffective, becomes effective on repe- 
tition. 

Increasing intensity of stimulus produces increasing 
response, which, however, tends to approach a limit. 

The effects of rapidly succeeding stimuli in plant-tissues, 
as in animal, become fused, individual effects being then in- 
distinguishable. A maximum contractile effect is then pro- 
duced depending on the intensity of stimulus. 

II 2 



IOC PLANT RESPONSE 

In all the above respects, we find that the responses of 
plants in general exhibit the closest parallel to the responses 
of skeletal muscle in animals. 

But in the animal a different type of response is exhibited 
by certain rhythmic tissues like cardiac muscle. The re- 
sponse here is on the all or none ' principle, and such a tissue 
cannot be tetanised. In the parallel instances of rhythmic 
vegetable tissues, the same characteristics are present ; that 
is to say, the responses are on the * all or none ' principle, and 
there is no tetanus. 



PART II 

MODIFICATION OF RESPONSE UNDER 
VARIOUS CONDITIONS 



CHAPTER IX 

ON THE UNIFORM, FATIGUE, AND STAIRCASE EFFECTS 

IN RESPONSE 

Uniform response in plants Staircase effect Fatigue due to molecular strain 
Fjatigue in plant-responses Periodic fatigue Fatigue under continuous 
stimulation Explanation of anomalous erection of leaf of Mimosa under con- 
tinuous stimulation Conductivity and excitability of tissue diminished through 
incomplete protoplasmic recovery Relatively greater fatigue in a motile than 
conducting organ Disappearance of the motile excitability earlier*than con- 
ductivity Refractory period Absence of responsive effect when stimulus 
falls within refractory period. 

THE mechanical response of plants is fundamentally due, as 
we have seen, to those molecular changes which are the 
result of stimulus. These changes bring about contractions 
of the excited cells, in consequence of which water is ex- 
pelled, and we obtain longitudinal response in radial organs, 
or lateral movement in dorsi-ventral organs, the latter being 
simply a special case of differential longitudinal contraction. 
On the cessation of stimulus the expelled water is reabsorbed, 
and the organ resumes its original position. In the case, for 
example, of the leaves of Mimosa, this position of equilibrium 
is, approximately speaking, at an angle of 45 above the 
horizon, and this, for convenience, may be called the erect 
position. After a period of rest, then, molecular equilibrium 
being re-established, the protoplasm recovers its original 
properties, of which excitability is one, and response takes 
place on stimulation as before. This resumption by the leaf 
of its original position may thus be taken as a rough indica- 
tion of the restoration of its original protoplasmic properties. 
But this is only true in a general way, for there may be 
cases, as we shall see, in which the apparent return of the leaf 



1O4 



I 'LA NT KKSI'ONSK 



to its original position is deceptive, and docs not really indi- 
cate a complete protoplasmic recovery. 

Uniform responses. If the motile organ, however, be 
restored, by an appropriate period of rest, to exactly its 
original molecular condition, and therefore to its original 
condition of excitability, it is clear that we ought to be 




Fir.. 51. Uniform Electrical Responses (Radish) 

able to obtain uniform responses to uniform stimuli. That 
this is true has been shown to be the case, with regard to the 
responses of the leaflet of Biophytum, and the longitudinal 
contractions of various radial organs (figs. 18 and 38). 
By taking electromotive instead of mechanical responses, 
I obtained a similar result, of uniform responses to uniform 

stimuli, from various species of plants 
(fig. 51). In the case of muscle, also, the 
responses are found to be uniform, if 
intervening periods of rest be allowed* 
sufficient for full recovery (fig. 9). 

'Staircase' effect. It is sometimes 
FIG 52. Staircase Ef- f und th t tissue falls into a sluggish 

feet m Longitudinal ^ > f** 5 

Mechanical Response condition, and successive stimuli, by 
/w/,r; a /!j increasing molecular mobility, have the 

effect of gradually enhancing the re- 
sponses, which are seen to increase in a ' staircase ' manner. 
I give here an instance of this effect (fig. 52) in the case of 
longitudinal response, obtained with a style of Eucharis Lily. 




UNIFORM, FATK1UE, AND STAIKCASK EFFECTS IO5 




FIG. 53. 



Fatigue in Longitudinal 

< 



was allowed. When the period 
allowed for recovery was reduced 
to half a minute, there was rapid 
fatigue ; the second pair *f re- 
sponses shows tbis immediate 
effect ; the third pair of responses 
are to the tenth and eleventh 
stimuli. 



Fatigue. It has been said that, when sufficient time is 
allowed for protoplasmic recovery, the responses are uniform, 
but that, if sufficient time be not 
allowed, molecular recovery will 
be incomplete, and the tissue 
will remain in a strained con- 
dition. Under these circum- 
stances, it is obvious that there 

will not be a complete restora- i n the case of the first pair of re- 

tion of the original protoplasmic sponses, a sufficient interval for 

L l recovery, namely one minute, 

excitability, hence successive re- 
sponses will exhibit a diminution 
or fatigue. The following record 
(fig. 53) shows this in the case 
of longitudinal response. Uni- 
form stimuli were first applied 
at intervals of one minute, by which time the recovery was 
complete ; and these responses of twenty divisions are seen 
to be large and uniform. 
The stimuli were next 
applied at intervals of half 
a minute, and the response 
at once fell to eleven divi- 
sions. Now, owing to the 
effect of cumulative strain, 
the succeeding responses 
at this interval underwent 

COntillUOUSdiminution, and FK;. 54. Fatigue shown in Electrical Re- 
i i r 11 i . sponse, when sufficient Time is not allowed 

had fallen as low as to f Fu] 'j Recovcry 

five divisions at the tenth j n () stimuli were applied at intervals of one 

stimulus. In the next 
figure (fig. 54), fatigue is 
shown in the electrical 




niinute ; in (/;) the intervals were reduced 
^ half a minute ; this caused a diminution 
of response. In (r) the original rhythm is 
restored, and the response is found to been - 

hanced t ' o nearly its iginal value (Radish) . 



responses of plants under 
the same conditions, sufficient time for recovery, that is to 
say, not being allowed. Similar instances of fatigue are well 
known in the case of muscle. 



I06 PLANT RESPONSE 

Fatigue being principally clue to residual strain, it is to 
be expected that, other things being equal, strain will be 
more persistent with stronger stimulus, as has been shown in 
the last chapter. It was there shown also that the period 
required for recovery from a strong was more protracted 
than from a moderate stimulus. From this it follows that 
the stimulation-frequency which will exactly allow for com- 
plete recovery, and so give rise to uniform responses, in the 
case of a moderate stimulus, will not be sufficient for stronger 
stimulus. Hence, keeping the intervals constant, we may 




IMC. 55. Alternate Fatigue (a) in Electrical Responses of Petiole of 
Cauliflower ; (f>) in Multiple Electric Responses of Peduncle of Bio- 
fihvtunt ; (<) in Multiple Mechanical Responses of Leaflet of Bio- 
ptivtitM ; and (</) in Autonomous Responses of Dcsmodium 

obtain uniform responses to moderate, and diminished or 
fatigue-responses to strong stimulus. 

There is another curious phenomenon, of alternate or 
periodic fatigue, which I have often observed in the response 
of plants. The simplest type of such periodic fatigue is that 
in which responses wax and wane, in regular alternation. 
Such alternate fatigue is sometimes seen in the electrical 
response of plants, the multiple response of Biophytum, and 
also in the autonomous response of Desmodium (fig. 55). 
Curiously enough, I have sometimes obtained similar alter- 
nate responses with the compound strip of ebonite and india- 
rubber, previously described. There are other cases of 
response in plants, where the variations are cyclic in type, that 



UNIFORM, FATIGUE, AND STAIRCASE EFFECTS IO7 



ta,) t 




is to say, they consist of groups of responses, which wax and 
wane alternately. 

Fatigue under continuous stimulation. In connection 
with the induction of fatigue under, long-continued stimu- 
lation, I shall now anticipate certain results regarding the 
character of the responses given by matter universally 
results to be described in detail in the next chapter. It will 
there be shown that if we represent a given molecular change, 
caused by stimulus, as positive, the continuation of stimulus 
will at first increase that change, until it has attained a 
maximum, after which, under 
the still continued action of the 
same stimulus, there will be a 
reversal or change to the nega- 
tive. In a living tissue, then, 
where the incidence of stimulus 
causes contraction, we may be 
prepared to find that the same 
stimulus, long continued, will 
bring about a reversal of this 
effect, or, that is to say, a re- 
laxation. 

In any case, such a reversal .,, 

J ' r IG. 

is illustrated in the correspond- 
ing phenomenon in contractile 
muscle. It is there found 
that, whereas the first effect of stimulus is contraction, the 
same stimulus, when too long continued, brings about relaxa- 
tion to the original form. In the electrical response of 
plants under continuous stimulation, I also find this peculiar 
fatigue-reversal (fig. 56). 

I have also obtained similar fatigue- reversals in the longi- 
tudinal response of radial organs. In fig. 56, (a), a series of 
tetanic electric shocks was continuously applied during a 
period of four minutes. The maximum contraction was 
attained in the course of one minute, after which there was 
a reversal, or relaxation. Under this condition of fatigue- 




Rapid Fatigue under Con- 
tinuous Stimulation in (a) Muscle ; 
(I)) Leaf-stalk of Celery (Electrical 
Response) 



io8 



PLANT RESPONSE 



reversal, the tissue is incapable of the normal excitatory con- 
traction, but after a period of rest of about seven minutes it 




FH;. 57. Fatigue under long-continued Stimulation in the Contractile 

Response of Plants 

(<t) Stimulation by tetanising electric shocks ; (ft) stimulation by rapidly 
succeeding thermal shocks. Continuous lines represent action during 
stimulation ; dotted lines represent after-effect (coronal filament of 
ra^ magnification forty times). 



again gives response, which is at first normal, and then, after 
reaching a maximum, becomes reversed. The second re- 
sponse is, however, seen to be smaller than the first. 

I obtained parallel results 
under the action of rapidly 
succeeding thermal shocks 

(fig- 57, J). 

I have already described 
curious instances of alter- 
nating fatigue exhibited in 
successive single responses 
to single stimuli (fig. 54). 
A very curious and interest- 
ing effect of this nature 
occurring under continu- 
ous electric stimulation, is 

shown in tho accompanying photographic record (fig. 58) 
of responses given by the filament of Uriclis Lily. It will 




FIG. 58. Photographic Record of 
Periodic Fatigue under Continuous 
Stimulation in Contractile Response 
(Filament of Uriclh TJly) 



UNIFORM, FATIGUK, AND STAIRCASF EFFFCTS 



ICQ 



there be noticed that the maximum contraction is attained in 
the course of three minutes, after which there is a fatigue- 
relaxation, which continues up to the eleventh minute. 
There then occurs a second, though .much feebler, response, 
after which comes a slow and continuous reversal action. 

So-called ' anomalous ' response in Mimosa. In con- 
nection with the subject of fatigue, I shall here enter upon 
the explanation of certain well-known responsive effects in 
the case of Mimosa which 
have hitherto been regarded 
as anomalous. It is generally 
found that an erect leaf of 
this plant is sensitive, that is 
to say, when stimulated it 
becomes depressed. In this 
depressed position it is ap- 
parently insensitive, hence 
we are apt to assume that the 
erect posture is one of sensi- 
tiveness, depression indicating 
the reverse. It will be found, 
however, that if a Mimosa leaf 
be continuously stimulated 
by successive blows or taps, 




Fir.. 59. 1 'holographic Records of 
Normal Response of Mimosa to 
Single Stimulus (upper figure), and 
to Continuous Stimulation (lower 
(figure) 

In the latter case the leaf is erected in 
spite of continuous stimulation. 



in the manner of Pfefifer's 

experiment, the leaf will at 

first fall. But, though the 

blows be continued, the 

petiole will, after a time, 

return to its normal erect position. In this erect posture, 

however, further blows prove to have no effect upon it, the 

leaf being now insensitive. 

I give above a photographic record of this effect in 
Mimosa (lower record, fig. 59), continuous stimulation in this 
case having been produced by tetanic electric shocks. It 
will be noticed that after its responsive fall the leaf returns 
to the erect position, in spite of the fact that stimulus is 



1 10 PLANT RESPONSE 

being continued. It is important to note that the two records 
were taken with the same specimen, and in immediate suc- 
cession to each other. The first, or upper, records the 
response of the leaf to a single stimulus and its recovery ; 
the lower gives the response to continuous stimulation. In 
appearance the two records are singularly alike. But though 
the leaf at the end of each response occupies the same 
position, the molecular conditions in the two cases are, as 
will be shown presently, entirely different, inasmuch as in 
the first, renewed response was immediately obtained, thus 
showing its sensitive condition ; while in the second, the 
organ was insensitive, and could give no response until after 
a period of rest of about ten minutes. 

The explanation of this apparent anomaly is quite clear 
from the experiments which have already been described, 
showing that under continuous stimulation the normal longi- 
tudinal contraction undergoes reversal and passes into relaxa- 
tion, as is also the case with continuously excited muscle, 
the motile response of Mimosa being only an instance of 
differential longitudinal contraction. We obtain here also the 
usual sequence of first, normal contraction, and second, the 
fatigue-relaxation, or posture of erection. 

To sum up, then, it is clear that our association of the 
erect position with sensitiveness is not always correct, for the 
leaf may assume this posture as the result of fatigue. That 
its position in this case, however, though outwardly imitating 
that of sensitiveness, is profoundly different, is at once 
revealed on application of stimulus. The leaf is now irre- 
sponsive. But if we allow it a period of rest of some eight 
or nine minutes in summer, or double that time in winter 
the internal molecular equilibrium is re-established. But of 
this internal readjustment the leaf gives no visible indication. 
It remains in the same unchangingly erect position. During 
the course of the cycle, then, it has passed from the normal 
erect to the normal depressed, and thence to the abnormal 
erect position. It next passes, without any outward change, 
from this abnormal erect to the normal erect position, after 



UNIFORM, FATIGUE, AND STAIRCASE EFFECTS III 

a period of repose. And this return of the leaf to its normal 
condition is testified by its once more responding to stimulus. 
Fatigue of conductivity and excitability. It has already 
been pointed out that the protoplasmic properties of a tissue 
cannot be restored to their original condition after stimulation, 
without the intervention of a suitable period for the re- 
establishment of molecular equilibrium. This fact I shall 
now demonstrate by additional experiments. 

The restoration of the normal protoplasmic condition in 
a tissue may be tested by observing the recovery of some of 
those properties which are capable of measurement. One of 
these is its conductivity, measured by determining the speed 
with which excitation travels through the tissue, the method 
of which determination will be fully described in Chapter XX. 
Under normal conditions this velocity is constant If we 
excite the tissue, and measure the rate at which the excitation 
travels, and if we then allow a sufficient interval of rest for 
complete protoplasmic recovery and again determine this 
velocity, we shall find that the two are the same. But, if the 
necessary resting interval be not allowed, recovery being 
incomplete, there will remain a residual strain. The velocity 
of transmission of excitation will, under these circumstances, 
be found to be reduced. 

Another protoplasmic property which is capable of 
measurement is the excitatory contraction seen in motile 
organs, measurable by the amplitude of response. We 
have seen that this amplitude is constant under normal 
conditions, and when sufficient intervening periods of rest 
are allowed. But diminution of the intervening resting period 
produces diminution of the amplitude of response. 

From what has been said it follows that, if the intervening 
resting periods of a tissue be continuously diminished, there 
will be a continuously increasing residual strain, and this 
might be detected by the consequent continuous decrease 
of conductivity and motile excitability. I have been able 
to verify this deduction by an experiment on a leaf of 
Biophytum, the details of which will be found in Chapter XX. 



112 PLANT RESPONSE 

It will there be seen that, as the resting period was gradually 
shortened to half a minute, the conductivity underwent di- 
minution from the normal r88 mm. to 1-54 mm. per second, 
that is to say, by 18 per cent. The reduction of motile 
excitability was found, however, to be still more marked, 
the height of response being reduced from the normal thirty- 
four to one division, that is to say, by as much as 97 per cent. 
On still further reducing the period of rest, from half a minute 
to ten seconds, it was found that there was no motile response 
whatever. The tissue is thus seen to be altogether incapable 
of response during a certain refractory period. The time- 
value of this refractory period not only differs in different 
plants, but also varies with the physiological condition. In 
Biophytum> normally speaking, it is ten seconds, and in 
Mimosa about one minute. 

Earlier abolition of motile excitability than of con- 
ductivity. We have seen in the last experiment, that while 
the conductivity of the petiole was reduced by 1 8 per cent, 
the motile excitability of the attached leaflet underwent a 
diminution of 97 per cent. This shows that motile excitability 
disappears earlier than conductivity. The reason will be 
apparent if we consider the difference between the two 
expressions of protoplasmic excitation. We have, in the 
conduction of the state of excitation from point to point, 
a direct expression of the transmission of the molecular 
change initiated by stimulus. The motile response, however, 
is a somewhat remote consequence of the series of events 
which follows on the fundamental molecular change. Inter- 
mediate occurrences are the permeability variation and the 
contraction, in this case differential, which it produces. The 
movement of the leaf is a result of all of these, and depends 
for its complete fulfilment on certain favourable circumstances. 
In any case, there are mechanical obstacles which have to 
be overcome in forcing the expelled water through channels 
of escape. That this must involve some degree of waste of 
force is partly seen in the fact that all excitations do not 
produce response, it being necessary that the stimulus should 



UNIFORM, FATIGUE, AM> STAIRCASE EFFECTS 113 

exceed a certain minimum value in order to produce any 
movement at all. If, again, the escape of water should be 
resisted, owing to the peculiar condition of the tissue, which 
has already been described (p. 49), then even a strong 
stimulus would be unable to bring about movement. 

Analysis of the different phases in the response of 
Mimosa under continuous stimulation. Having now con- 
sidered in detail some of those changes of protoplasmic pro- 
perties which are brought about by the action of stimulus, we 
are enabled to study the effect of long-continued stimulation 




FlG. 60. Ineffectiveness of Stimuli, owing to Increasing Fatigue, 

in Mimosa 

In the left-hand figure stimuli were applied at intervals of 3-5 minutes. 
These evoked feeble responses. In the right-hand figure stimuli were 
applied at intervals of two minutes. Response now became incon- 
spicuous. Where stimuli were applied at intervals of one minute no 
effect was produced. The leaf was refractory. 



from a somewhat different point of view. We saw that with 
incomplete recovery, protoplasmic excitability was pro- 
gressively diminished. In order to demonstrate this, in the 
case of Mimosa, I obtained responses at intervals of 3-5 
minutes, the uniform stimulus of condenser discharge being 
employed. The responses, which had been uniform, when 
stimulus was applied after complete recovery, at intervals of 
about eight minutes, were now found to be very much reduced 
(fig. 60). In a second series of experiments on the same 
specimen, the intervening periods of rest were still further 

I 



114 PLANT RESPONSE 

reduced, to two minutes ; and it will be noticed that, owing 
to increasing incompleteness of recovery, the responses 
were here reduced to the merest indications of twitches. 
When the intervening periods were still further shortened 
to less than one minute, the stimuli fell within the refractory 
period of the tissue, and produced no indication whatsoever 
of their effect. As an extreme instance, we have the effect 
of continuous stimulation, already described on p. 109. To 
be precise, we must remember that in this gradual abolition 
of response, under quickening stimulation, we not only see 
the action of diminished excitability, but also of diminished 
conductivity. 

The erection of the leaf of Mimosa^ by the relaxing action 
of fatigue, may also be assisted by the later contraction of 
the upper half of the pulvinus. For we have seen that in an 
anisotropic organ, the less excitable half responds subsequently 
to the more excitable. The contraction of the upper half 
of the pulvinus in Mimosa would produce erection of the 
leaf, and that this might sometimes happen appears probable 
from the fact that in the erection of the leaf under continuous 
stimulation it is occasionally found to be lifted above its 
normal position. 

SUMMARY 

Stimulus, by causing molecular derangement, brings about 
mechanical response ; and by molecular transmission of 
disturbance from point to point, the excitation is conducted 
to a distance. Excitatory mechanical response and conduction 
of excitation are different expressions of the effect of stimulus 

After a period of rest from the action of stimulus, there 
is a restoration of molecular equilibrium. The protoplasmic 
properties of excitability and conductivity are then completely 
restored. Under such normal conditions responses are uniform. 

A tissue in a sluggish condition has its molecular mobility 
increased by the action of successive stimuli. This produces 
the * staircase ' effect, of gradually enhanced responses. 

When sufficient time is not allowed, there is a residual 



UNIFORM, FATIGUE, AND STAIRCASE EFFECTS 115 

molecular strain. The conductivity and excitability of an 
organ are thus diminished, and the responses undergo 
diminution, in consequence of cumulative residual strain. 

Fatigue is greater in a motile than in a conducting organ. 
Motile excitability disappears earlier than conductivity. 

The anomalous erection of the Mimosa leaf is brought 
about by the combined effects of diminution of conductivity, 
abolition of excitability, and possibly, in some instances, by the 
subsequent excitation of the upper half of the pulvinus also. 



CHAPTER X 

THEORIES CONCERNING DIFFERENT TYPES OF RESPONSE 

The chemical theory of response Insufficiency of the theory of assimilation and 
dissimilation to explain fatigue and staircase effects Similar responsive effects 
seen in inorganic substances Molecular theory When molecular recovery is 
complete, responses uniform : when incomplete, fatigue brought about by 
residual strain Fatigue under continuous stimulation, in inorganic substance, 
in plant, and in muscle Staircase effect brought about by increased molecular 
mobility : examples seen in inorganic substance, and in living tissues No sharp 
line of demarcation in the borderland between physical and chemical phenomena 
Molecular changes attended by changes of chemical activity Unequal 
molecular strain gives rise to a secondary series of chemical actions Volta- 
chemical effect and by-products Supposition that response always dispropor- 
tionately larger than stimulus, not justifiedExistence of three types: (i) 
response proportionate to stimulus ; (2) response disproportionately greater than 
stimulus ; (3) response disproportionately less than stimulus Instances of 
stimulus partially held latent : staircase and additive effects ; multiple response ; 
renewed growth. 

IT has already been shown, in previous chapters, that the 
various types of response met with in animal tissues are 
exactly paralleled, even in detail, in the response of plants ; 
and numerous further instances of this fact will be met with 
in the course of the present work. It would thus appear 
that the theoretical explanation of either class of responses 
must be applicable to the other also. Existing theories 
regarding animal response, however, have not yet been found 
sufficient to meet all the difficulties of the case, and it is 
probable that the larger data now made available by the 
inclusion of response in plants, may go far to throw light 
on certain obscurities which are at present regarded as 
perplexing. 

Chemical theory of assimilation and dissimilation. 
The theory which is generally accepted at present may be 
referred to briefly as chemical According to it, living matter 



DIFFERENT TYPES OF RESPONSE 117 

is maintained in a state of equilibrium by the two opposed 
chemical processes of building up or assimilation, and break- 
down or dissimilation. Stimulus causes a down or dissimi- 
latory change, which is again compensated, during recovery, 
by the building up, or assimilative change. In the case of 
uniform responses, the two processes exactly balance each 
other. But on occasions when the down change is the greater 
of the two, the potential energy of the system falls below 
par, for the building-up process cannot then sufficiently 
repair the chemical depreciation caused by the downward 
change. Hence occurs diminution of response, or fatigue, 
which is supposed to be further accentuated by the produc- 
tion and accumulation of deleterious * fatigue-stuffs.' The 
disappearance of fatigue, after a period of rest, is explained 
by the renovating action of the blood supply, which Is also 
regarded as the means of carrying away the fatigue-stuffs. 

A serious objection to these explanations, however, lies in 
the fact that even excised and bloodless muscles exhibit 
recovery from fatigue, after a period of rest. In isolated 
vegetable tissues, again, where there is no active circulation 
of renovating material, the same effect, and its removal after 
a period of rest, are observed. 

Thus the difficulties met with in explaining fatigue accord- 
ing to a purely chemical theory are great enough. But still 
greater are those which we encounter when we come to deal 
with the staircase effect typically shown in cardiac muscle 
in which successive responses to uniform stimuli exhibit a 
gradual enhancement of amplitude. Here the result obtained 
is in direct opposition to the theory described ; for in this 
particular case, we have to assume that the same stimulus 
which is usually supposed to cause a chemical break-down 
becomes efficient to produce an effect exactly the reverse. 
It is true that the heart, usually speaking, is charged with 
blood ; but this particular staircase increment of response, 
under uniform stimulation, is observed even in the initial 
twitches of bloodless muscle (fig. 64), and here there can be 
no question of a supply of renovating blood. 



Il8 PLANT RESPONSE 

Parallel types of response in living organic, and in 
inorganic matter. Such being the difficulties involved in 
the explanation of a single class of phenomena, on the 
chemical hypothesis of assimilation and dissimilation, it may 
be well next to turn our attention to the conclusions suggested 
by the observation of response in matter generally. And for 
this purpose it is best to take the responses obtained from 
inorganic matter in particular, the hypothetical assimilation 
and dissimilation being in that case out of the question. 
With regard to the mode of observation, I have already 
explained how the molecular derangement consequent on 
stimulus may be studied, either (i) by recording the change 
of form ; or (2) by recording the variation of conductivity ; 
or (3) by recording the electromotive variation. 

As an example in the first place of responsive contraction 
in inorganic matter, we may select for our investigation the 
response of india-rubber, under thermal stimulation. In this 
case, chemical changes, either up or down, are impossible. 
The second, or conductivity variation method, may be used 
in the case of metallic powders, the stimulus being that of 
Hertzian radiation. In this case also chemical action 
may be excluded, the experimental material being usually 
placed in naphtha. In the third case, again, where response is 
obtained by means of the electromotive variation, under 
mechanical stimulus, the substance used is platinum, the 
most chemically inactive of metals, electrolytic contacts being 
made by water. 1 The possibility of chemical action is thus 
reduced to a minimum, and the assimilatory change entirely 
excluded. 

It will be found, however, that in all these cases of in- 
organic response, in which substances, physically and chemi- 
cally widely unlike, are subjected to diverse forms of stimula- 
tion, and have their responses tested and recorded by absolutely 
different methods, the results obtained are exactly parallel. 
All alike, when sufficient intervening periods of rest are 

1 For details of these investigations and results, see Bose, Response in the 
Living and Non-Living. 



DIFFERENT TYPES OF RESPONSE 119 

allowed, give uniform responses to uniform stimuli. And 
when the period of rest is shortened, all alike exhibit 
fatigue. 

From the conditions of experiment it is clear that 
these effects are physical or molecular. The molecular 
derangement caused by stimulus is thus gauged by the 
amplitude of response. Recovery is brought about by the 
restoration of molecular equilibrium, and for this purpose it 
has now become evident that the process of assimilation is 
not essential. When sufficient time, however, is not allowed 
for recovery, we have a residual molecular strain, and a 
substance in this strained condition is less responsive, as 
seen in the diminished height of its response. Fatigue is 
thus due to molecular strain, and its 
cumulative effects. But when the 
fatigued substance is allowed sufficient 
time for the strain to disappear, its 

i . u-i-'i. I.L FIG. 61. Fatigue- Reversal 

subsequent responses exhibit the nor- in Arsenic> * nder Con . 

mal amplitude. tinuous Stimulation of 

r , ...... . Hertzian Radiation. 

It was explained in the last chapter ..... 

. . The horizontal line repre- 

that in the case of Mimosa, as in that sen ts the duration ot 




of muscle, a complete reversal of 

response is brought about by extreme variation method. 
fatigue, under continuous stimulation. 

The following record shows a similar reversal in Arsenic, 
under the continuous stimulation of electric radiation (fig. 61). 
It was only after a sufficient interval of rest that this sub- 
stance gave renewed normal response. It may be added that 
these fatigue-reversals, as in the longitudinal response of the 
Uriclis Lily, are sometimes found to be recurrent. 

This curve of fatigue-reversal in Arsenic under continuous 
stimulation was obtained by recording the changes of electric 
conductivity in the substance. A still more striking analogy 
with the mechanical records of fatigue in plants and animals 
is afforded, however, by the automatic record given in fig. 62 
of contractile responses in india-rubber. When this substance 
is excited by rapidly succeeding thermal shocks, we obtain 



I2O 



I'L \NT KKSl'OXSi; 



first, thc normal contractile effect, and secondly, the relaxation 
due to fatigue, in a manner exactly similar to that which 
characterises thc fatigue-reversals of Mimosa and of skeletal 
muscles. In the present case, the india-rubber attained its 
maximum contraction in the course of two minutes, after 
which there was a continuous relaxation. 

In this response of india rubber, and in its fatigue-reversal, 
we have an analogy with thc response of living animal tissues, 

such as muscle, so close as to 
compel us to the conclusion 
that both alike are phenomena 
of molecular response. A 
mere contraction of the india- 
rubber might have been sup- 
posed to be due to the specific 
action of heat on that sub- 
stance. And had this been 
all, successive thermal shocks 
would have had the effect of 
continuously increasing the 
,, , . ... . ,,, . contraction, till a limit was 

r ic. 62. Automatic Record of ratigm- 

in the Contractile Response of India- reached. But if, Oil the Other 

xtwrnid Shil:kl <apidly SucccedinK Hand, molecular excitability 

The first effect of stimulus is contrac- bc a fact r in thc process 
tion, hut this passes into relaxation of response, WC might then 
under continued stimulation. The , \ 

time-maiks represent intci\ais of expect, under certain condi- 
half a minute. tions of fatigue, a loss of 

molecular excitability. That 

this is actually the case is shown by the fatigue-reversal, 
attended with relaxation, which is seen in the figure. 

A muscle, again, in this condition of fatigue-relaxation, 
after a period of rest -during which thc molecules have 
time for recovery from their state of strain becomes 
once more excitable without any visible change. We found 
a like phenomenon occurring in thc case of fatigued and 
relaxed Mimosa^ under continuous stimulation. In that case 
it was, as we saw, a period of rest of about eight or ten 




DIFFERENT TYPES OK RESPONSE 



121 



minutes, which made the fatigued Mimosa once more con- 
tractile. Returning to india-rubber, we find that here also 
a period of rest of ten minutes enables it to recover 
its excitability, and once more exhibit its responsive con- 
traction. 

From these facts it would appear that in order to explain 
the phenomenon of response, and its various modifications by 
fatigue and other factors, we have no option but to regard it 
as an expression of the molecular responsiveness of matter 
in general. 

We next turn to the converse phenomenon of staircase 
response. Since response is an expression of molecular 





K,. 63. Preliminary Staircase In" 
crease, followed by Fatigue, in the 
Response of Catena to Hertzian 
Radiation 

(Conductivity variation method) 



Flo. 64. Preliminary 
Staircase Increase, fol- 
lowed by Fatigue-, in 
the Response of Style 
of Enchari\ 



derangement, it follows that its extent will, other things being 
equal, depend on the degree of molecular mobility. This 
being so, it is conceivable that a substance, at first in a 
sluggish condition, may, by the impact of successive stimuli, 
have its molecular mobility gradually increased, with a corre- 
sponding enhancement of its response. In fig. 63 we have an 
example of this staircase effect, in the responses of Galena. 
It is to be noticed that this effect occurs at the beginning of 
the series of responses, as we should expect. After the 
attainment of maximum mobility, the phenomenon of over- 
strain is seen, with its accompanying diminution of response, 
or fatigue. I have obtained similar results in the longitudinal 



122 



I'I,\NT RESPONSE 



response of plants (fig. 64). It need only be mentioned, 
further, that exactly the same preliminary staircase effect, 
reaching a maximum, and followed by fatigue, is found in 
the responses of muscle (fig. 65). 

The inference that the occurrence of the staircase effect 
is due to the gradual removal of molecular sluggishness, 
receives further support from the following experiment on 
a plant where sluggishness is brought on artificially, and 

made to disappear gradually. 
Sluggishness may be induced 
in a tissue by cooling, this con- 
dition being made to disappear, 
by the gradual return of the 
substance to the ordinary tem- 
perature of the room. On carry- 
ing out such an experiment, 





t 65. Preliminary Staircase, 
followed by Fatigue, in the 
Responses of Muscle (Brodie) 



FIG. 66. Staircase Increase 
in Electrical Response of 
Petiole of Bryophyllum^ 
rendered sluggish by 
cooling 



and recording the successive electrical responses to successive 
uniform stimuli, I obtained, as I had expected, a marked 
staircase effect (fig. 66). 

Merging of physical into chemical phenomena. 
I have explained that responsive phenomena arc primarily 
due to the molecular distortion caused by stimulus, that is to 
say, they are of a physical character. Fatigue has also been 
shown to be primarily due to residual strain. But it must be 
borne in mind that in the borderland between physics and 
chemistry there is no sharp line of demarcation. For ex- 
ample, yellow, phosphorus under the stimulus of light is 



DIFFERENT TYPES OF RESPONSE 123 

converted into the molecular red, or allotropic variety. This 
molecular change, however, is also attended by a concomi- 
tant change in the chemical activity, phosphorus in the red 
condition being less active chemically than in the yellow. 

Under certain circumstances, further, it is possible to 
have a secondary series of chemical events following upon a 
condition of unequal molecular strain. We have seen that a 
homogeneous living tissue, when unstimulated, is iso-electric. 
When stimulated, however, an electromotive difference is 
induced, as between the more and the less acted parts of 
the tissue. The result is an electrical current attended by 
chemical changes. As a consequence of such volta-chemical 
action, when prolonged, by-products (fatigue-stuffs ?) may be 
accumulated, and these may have a depressing effect on the 
activity of the tissue. Hence, just as after very prolonged 
activity of a voltaic element it is necessary to renew the 
active element and change the electrolyte surcharged with 
by-products, so, after sustained activity of a living tissue, 
the process of renewal, or renovation, will be necessary. 
Thus we see how, upon the fundamental molecular derange- 
ment, a chain of very various chemical events may follow as 
its after-effect. And it is only by going in this way to the 
very root of the phenomenon, that we can avoid the many 
contradictions with which we are confronted by the chemical 
theory. 

I have therefore aimed at demonstrating the universal 
existence in matter of the property of responsiveness, and 
by taking the simplest cases and excluding as many com- 
plicating factors as possible, have attempted to show further 
how this power of response is modified by those conditions 
which occasion fatigue, or its converse, the staircase effect. 
Having thus cleared the ground, it is possible to take up 
cases of greater complexity. 

Different modes of transformation of stimulus. It has 
been assumed that response is brought about by a sudden 
explosive chemical change, the stimulus acting as if on a 
trigger, for the release and run-down of potential chemical 



124 PLANT RESPONSE 

energy. This implies that response is disproportionately 
greater than stimulus, and that the responsive change is 
attended by an evolution of heat and chemical by-products. 
It would follow, however, from such rapid depreciation, that 
fatigue must be the invariable consequence of any series of 
responses. But it is notorious that in the responses of nerve, 
not only is there no fatigue, but neither is there any evolution 
of heat, nor occurrence of chemical change, that can be 
detected. 

As the simplest case, we have hitherto considered the 
substance acted on to be neutral in its character, that is to 
say, not active in the sense of being able to absorb stimulus 
and hold it latent. But if we regard the living organism as 
a machine, three different cases are conceivable. These arc : 
first, that in which the responding substance simply con- 
verts the energy received as stimulus into response ; secondly, 
that in which the responding substance possesses a large 
amount of energy, some of which is set free by the action of 
the stimulus ; and lastly, that in which the responding sub- 
stance is capable of absorbing and holding latent, to a greater 
or less extent, the stimulus which it receives. 

(1) Response proportionate to stimulus. The first of these 
types may be illustrated by a responding circuit which con- 
tains a magnetic motor say a galvanometer translating cur- 
rent into motion. The source of stimulus may be an external 
battery periodically closed by a tapping key. The waste of 
energy by the production of heat may be supposed to be 
brought down to a minimum in this circuit by using a feeble 
current and reducing the resistance. Uniform stimuli will 
now cause uniform responses of the responder. Response 
will be proportionate to stimulus, and there will be no chemi- 
cal, and no appreciable thermal, change in the responder. 
This is a state of things which may be said to approximate 
closely to the responsive peculiarities of the nerve. 

(2) Response disproportionately greater than stimulus: re- 
sponding substance reduced below par. As an example of the 
second type, we have to imagine a responding system which 



DIFFERENT TYPES OF RESPONSE 12$ 

contains a large amount of energy. We may suppose it to 
consist of a storage battery and a galvanometer, the circuit 
being normally incomplete. External stimulus may, by some 
easily arranged mechanism, close the responding circuit 
periodically. In this case, the responses might be made dis- 
proportionately larger than the stimulus. There will then be 
a progressive run-down of the latent energy of the system, 
and the responses will show diminution, or true fatigue. The 
system, at the end of the experiment, will be found to be 
below par. This case may be paralleled by that of highly 
excitable tissue which is wasted under excessive and long- 
continued stimulation. 

(3) Stiimilus wholly or partially absorbed. The third type 
of substance we have supposed to be one which is capable of 
absorbing and holding latent, to a greater or less extent, the 
stimulus which it receives, and under this we may have 
several important sub-cases. 

. (a) Staircase and additive effects. The absorbed stimulus 
may gradually enhance the molecular mobility, with gradual 
enhancement of response. This is seen exemplified in the 
staircase effect. Another example of the same thing is 
probably to be found in the singly ineffective stimulus which 
becomes effective on repetition. Evidently in this case, the 
energy of the first few stimuli is held latent in the tissue and 
added up until it reaches the threshold of response. 

(b) Multiple response. We can next sec the possibility 
of a very interesting case of stimulus becoming latent. A 
spring which is immersed in a viscid fluid may, on receiving 
a feeble blow, give a single vibrational response. But if the 
blow be powerful, this single strong stimulus will give rise to 
a multiple series of vibrational responses. Again, a phos- 
phorescent substance acted on by light absorbs it, and, on 
the cessation of incident stimulus, continues to give up the 
excess of latent energy thus acquired in the form of luminous 
vibration. A selenium cell again, when acted on by a single 
strong flash of light, I have found to give what may be 
regarded as two responses, one strong and the other feeble. 



126 PLANT RESPONSE 

From certain metallic particles, again, when exposed to a 
single strong flash of electric radiation, I have obtained 
pulsatory responses. From the consideration of all these 
cases, I was led to investigate the question whether a plant- 
tissue, when acted on by a single strong stimulus, could be 
found to give similar repeated responses. From this I was 
led to the discovery of Multiple Response in plants, a pheno- 
menon which I find to be very prevalent, and which I shall 
describe fully in Chapter XX. 

(f) Response disproportionately less than stimulus : respond- 
ing substance raised above par. It is the last of these sub- 
cases, however, which we could least easily have foreseen. It 
is as if here the active expression of the incident stimulus were 
bifurcated. Returning once more to the mechanical model, 
we may imagine the responding portion of the system to 
contain, besides the mechanically responding galvanometer, 
two plates of lead, immersed in dilute sulphuric acid, by 
which the energy of the stimulating current is partially stored 
up. The external stimulating current has now to do two 
things, first, to cause mechanical response in the galvano- 
meter, and secondly, to store up an increasing amount of 
latent energy in the second part of the responding system. 
It is evident here that, owing to increasing storage, the 
mechanical response of the galvanometer may become pro- 
gressively less. We are, perhaps, too apt to ascribe to 
1 fatigue ' all cases of diminution of responses. For in this 
case we shall have an appearance of fatigue which is not due 
to the run-down of energy, but to its actual increase, in the 
responding system. 

I have been able to discover a parallel case to this in the 
response of plants, a case, that is to say, in which the response 
is disproportionately smaller than stimulus, successive stimuli 
nevertheless causing an increase of the energy of the system. 
I took for my experiment a straight tendril of Passiflora in 
which growth had undergone arrest It may be pointed out 
here that a certain tonic condition is necessary to the con- 
tinuation of growth. This tonic condition is determined, as 



DIFFERENT TYPES OF RESPONSE 



127 



will be shown later, by the sum total of energy latent in 
the tissue. From the curve of response given in fig. 67, it 
will be seen that at the beginning of the experiment, the 
length of the tendril being constant, the first part of the 
record was horizontal, instead of descending, as would have 
been the case had the tendril been growing. The con- 
tractile shortening brought about by stimulus is here 
represented upwards. The stimulus of induction shock 
was now applied at intervals of one minute, and the 
record shows that the recovery, instead of stopping short at 
the level of the horizontal line, has proceeded beneath, 
thus indicating that stimulus has been effective not only 
in producing contractile response, but also in afterwards 
initiating growth! (Cf. fig. 178.) 
And further, as the stimulus 
goes on causing not only 
mechanical response, but also 
accelerated growth, we see that 
the successive mechanical re- 
sponses are undergoing dimi- 
nution. The application of 
stimulus ceased at the end of 
the fourth response, and we 
observe active growth proceed- 
ing from this point as a result 
of the energy absorbed from 
the previous stimuli. We have 
thus demonstrated the very 
curious case in which the build- 
ing up process is attended by an actual diminution of re- 
sponse, and where, after stimulation, the energy of the 
responding organ, instead of being reduced, is raised above 
par. This experiment explains why an athlete in constant 
practice does not waste away, but actually increases, muscle. 




Pa^sijiora in which Growth was 
originally at Standstill 

Stimulation, besides producing 
mechanical response, initiated 
the opposite movement of 
growth, as shown by the slope 
of the base-line downwards. 



128 PLANT RESPONSE 

SUMMARY 

^rfhe principal types of response seen in animal tissues are 
lound also in the responses of plants and of inorganic 
substances. 

Three types of responses are possible : (i) that in which 
response is proportionate to stimulus ; (2) that in which 
response is disproportionately greater than stimulus ; and (3) 
that in which all or part of the stimulus is, for a longer or 
shorter time, absorbed by the tissue and held latent. 

The subsequent effect of stimulus which is held latent 
may s-ometimes be seen in singly ineffective stimulus which 
becomes effective on repetition ; or in the staircase response, 
consequent on the enhancing of molecular mobility by thd 
partial absorption of previous stimulus. 

The fact that stimulus may be held latent for a time, and 
subsequently find expression, is strikingly shown in the 
occurrence of multiple response, in answer to a single strong 
stimulus. 

It is also possible for the incident stimulus to become 
divided in its expression, part of it finding an outlet directly 
in mechanical response, and part becoming latent, and causing 
accelerated growth. The gradual augmentation of the latter 
of these may cause a corresponding diminution of the former. 
An appearance of fatigue is thus brought about, which is 
misleading, as the decrease of mechanical response is in this 
case due. not to a diminution, but to an increase of latent 
energy in the responding substance, as shown by its capacity 
for renewed growth. 



CHAPTER XI 

EFFECT OF ANESTHETICS, POISONS AND OTHER CHEMICAL 
REAGENTS ON LONGITUDINAL RESPONSE 

Response modified by physiological change Carbonic acid causes depression, 
and transitory exaltation as after-effect Gradual abolition of response in 
hydrogen and restoration by access of air Chemical agents cause contraction 
or relaxation of plant-tissue Effect of alcohol causing temporary exaltation of 
response followed by depression and protracted period of recovery Ether 
causes < relaxation and temporary depression of response Explanation of 
anomalous action of ether on stimulated Mimosa leaf Abolition of response 
by hydrochloric acid Response restored by timely application of ammonia 
Abolition of response by poisonous reagent Similarity of effect of chemical 
agents on the response of animal and vegetable tissues. 

THE subject of our inquiry, in this and succeeding chap- 
ters, will be the determination of those influences which 
bring about the variation of conductivity and excitability in 
plant-tissues. The variation of excitability induced in a 
tissue by chemical reagents, may be studied through the 
consequent modifications of the responses. Various forms 
of response which . may be used for this investigation are 
(i) electrical, (2) autonomous, (3) growth, and (4) the simple 
mechanical response about to be described. A description 
of the modifications induced in electrical response under the 
action of chemical reagents will be found on pp. 73-80 of 
my book, ' Response in the Living and Non-Living.' The 
effects of chemical reagents on autonomous pulsation and 
on growth are given in detail in Chapters XXV, XXVII, 
and XXXV of the present work. But it may be said here, 
that the results obtained by all these different methods are 
concordant, and that the simp'e method of mechanical 
response now to be detailed is to be regarded as an additional 
corroboration. It should also be borne in mind that the 

K 



130 PLANT RESPONSE 

effects of chemical reagents are subject to modification, as is 
fully explained later (pp. 322 and 477), by the tonic con- 
dition of the specimen. 

The method of procedure consists in first obtaining a 
series of responses to uniform stimuli under normal con- 
ditions, and then a similar series after the application of the 
reagent. The uniform individual stimuli in both series arc 
applied at intervals which allow of complete recovery. The 
intensity of stimulus, and the time-intervals, are kept constant 
throughout the experiment. The chemical agent may take 
the form of gas or vapour, or it may consist of a liquid. In 
the former case, by the turning of the three-way stop-cock 
described on p. 72, water-vapour is passed through the 
plant chamber, and the responses thus obtained are taken as 
the normal. By a quick manipulation of the stop-cock, the 
gaseous or vaporous reagent is next introduced, and the 
responses now obtained exhibit the physiological modifica- 
tion induced by it. The effect produced by some agents is 
permanent, by others transitory. This difference may be 
demonstrated by turning the three-way cock once more, so as 
to allow water-vapour to replace the specific gaseous medium. 
The responses now obtained exhibit the after-effect 

The difficulty in this investigation lies in the selection of 
specimens which exhibit complete recovery, together with 
uniformity of successive responses. For this purpose we 
may take any radial organ, one of the most excitable, and 
therefore suitable, of these being the filament of the corona 
of Passiflora. In this case high magnification is not necessary, 
and very moderate stimulus is sufficient. Failing Passiflora, 
I have frequently used with success the staminal filaments of 
the Uriclis Lily, and Brownca arisa> and the styles of Datura 
and Euchans Lily. It should be borne in mind that the 
excitability of the tissue is to a certain extent influenced by 
seasonal conditions, being different under different circum- 
stances of time and weather. 

It is best to choose specimens from flowers which are 
already open, and in which growth has just ceased. Although, 



CHEMICAL REAGENTS ON LONGITUDINAL RESPONSE 131 



as already stated, this is not the most excitable period for the 
tissue, yet it affords us the advantage of simplified conditions, 
inasmuch as, owing to cessation of growth, the line of record 
before stimulation, or the base-line, now remains horizontal. 
Stimulation, producing contraction, is represented by up 
movements of response, and the recovery brings the curve 
back to the base-line. 

Effect of carbonic acid gas. I shall now proceed to 
describe a few typical experiments, on the modification of 
response by chemical reagents, out of a large number, which 
were performed on the radial organs of plants of various 
kinds, in the course of the season. And first we shall take 
the effect of carbonic acid. 
The effect of this gas, after 
the lapse of about half an 
hour, is one of considerable 
depression. By this time 
the responses are reduced 
to about half their normal 
value, and this depression, 
though slow, is continuous. FIG 6g Efl - ect of Carbonic Add Gag 

This may be looked for as on Longitudinal Contractile Response 

the general effect, after a 
certain length of time, of 
the action of carbonic acid 

gas. The immediate result of the sudden introduction of an 
abnormal factor may, however, be slightly different in different 
cases, according to the tonic condition of the tissue. This 
immediate effect is sometimes one of brief depression followed 
by equally brief exaltation, to be succeeded by the true 
depression. Or there may be a short exaltation, followed by 
the true depression. The restoration of the normal con- 
dition, however, is generally followed in the case of carbonic 
acid gas by a gradually increasing exaltation of the response, 
which may culminate in double its ordinary height, and after 
this it again attains the normal (fig. 68). 

Effect of hydrogen gas. We next investigate the 




(a) Normal response ; (b) after exposure 
to carbonic acid ; (c) transient exalta- 
tion after readmission of air. 



K 2 



132 



IT. ANT RESPONSE 



effect of hydrogen gas. The characteristic effect of depres- 
sion occurs in this case after a much longer interval than 
is required by carbonic acid. The immediate result of 
application is very erratic and various. There may some- 
times be an exaltation, or even the contrary, a reversal, of 



AAAA 



FIG. 69. Effect of Hydrogen Gas 

((?) Normal response ; (6) after twelve hours' exposure to H. ; 
(f) slow revival of response after readmission of air. 

the normal response. But after a twelve hours' exposure to 
the action of this gas the responses of the tissue are so much 
diminished as to approach very near abolition. On now, 
however, allowing air charged with water-vapour to displace 
the hydrogen gas, the responses undergo a steady revival 

(fig. 69). There is here no sudden 
exaltation, such as is the general after- 
effect of carbonic acid gas. 

Effect of carbon disulphide. 
We have seen that the depressing 
effect of hydrogen takes place very 
slowly. This is owing to the fact 
that this gas acts here rather as an 
agent for cutting off that supply of 
FIG. 70. Photographic Re- oxygen that is necessary to the 
St.? S,pWe C ' ,n maintenance of the normal life of the 
Abolishing Response plant, than as a direct poison. But 
Normal response, seen to we have other gases which are actively 

left, abolished by intro- . . . t f .. . 

auction of vapour of CS , toxic, and in such cases the diminution 

or abolition of response takes place 

with greater rapidity. Such an agent may be found in the 
vapour of carbon disulphide (fig. 70). 




CHEMICAL REAGENTS ON LONGITUDINAL RESPONSE 133 

I may here draw attention to the great advantage offered 
by the study of the variation of longitudinal response, in 
determining the nature of the action of various chemical 
agents. The modifications which these agents produce in 
the lateral response of the pulvini of sensitive plants are 
not so simple, inasmuch as we have to deal in these cases 
with differential action. In radial organs, on the other 
hand, the response-record gives us indications of the specific 
action of each modifying agent. In the response itself there 
are, it must be remembered, two factors which have to be 
distinguished, namely, contraction in response to stimulus, 
and the power of recovery from contraction, or relaxation. 
The diminution and final abolition of response may be 
brought about, then, in two different ways. The effect of a 
given agent may be to diminish the normal relaxation which 
brings on recovery. Successive stimuli will in that case 
produce a cumulative residual contraction, which places the 
tissue in a state of strain, in consequence of which subsequent 
responses become enfeebled. We may, on the other hand, 
have an agent whose effect is to produce abnormal relaxation. 
The contractile impulse due to stimulus is in this case 
opposed by the abnormal relaxation induced by the agent, 
and we have, in this case also, an enfeeblement and abolition 
of response. 

The comparison of the time-relations of the normal and 
modified curves, together with the trend of the base-line up 
or down, will show the nature of the reaction, in an unmis- 
takable manner. All these facts are clearly demonstrated 
in the experiments and curves given below. 

Effect of alcohol. I shall next describe the action of 
the vapour of alcohol. Generally speaking, the immediate 
effect in this case is one of exaltation, though individual 
idiosyncrasies may sometimes be present, which cause 
depression from the very beginning. The general effect of 
this reagent, however, appears to be a prolongation of the 
period of recovery. So what is gained by brief exaltation 
is lost again by induced sluggishness. Thus, from the result 



134 PLANT RESPONSE 

recorded on a fast-moving drum, using the style of Datura 
alba^ I find that the height of the normal response was eleven 
divisions, and complete recovery took place in one minute 
and a quarter. During the first period of exaltation, after 
the application of alcohol, the height of the response was 
increased to sixteen divisions, that is, practically half as 
much again. But the period of recovery was protracted to 
four and a half minutes, or nearly three times the period of 
normal recovery. These considerations will fully explain 
the series of responses under the continued action of alcohol- 
vapour, given in fig. 71, where (a) shows normal response, 
(7?) the immediate and transitory exaltation, and (c) which 




FIG. 71. Eflect of Vapour of Alcohol 

(ti) Normal response ; (/>) immediate temporary exaltation on introduction 
of alcohol ; (r) subsequent depression. 

was taken after fifteen minutes' further application the 
diminished responses in which the contraction remainders 
are a marked feature. On blowing off the alcohol-vapour, 
however, and substituting fresh air, the tissue is found to 
recover slowly its normal excitability. If, instead of alcohol- 
vapour, dilute alcoholic solution be applied, the depressing 
effect is immediate and very great, 

Effect of ether. We now pass on to the question of the 
action of the anaesthetic agent, ether. This produces a relaxa- 
tion so great as to be incapable of proper representations 
within the limits of the diagram (fig. 72), where it is merely 
indicated by the dotted line. It is to be remembered that 
contraction is shown by lines upward, and recovery, or 



CHEMICAL REAGENTS ON LONGITUDINAL RESPONSE 135 



relaxation, by lines downward. Owing to the predominance 
of this relaxing tendency, it will be seen that true contrac- 
tile movement is here very much diminished. Even after 
the relaxation has attained its 



maximum, the responses remain 
insignificant. When the speci- 
men is not too long etherised, 
the blowing in of fresh air 
brings on gradual restoration. 
It may be mentioned here that 
ether also produces relaxation 
of animal tissue. 

Explanation of anomalous 
effect of ether on Mimosa. 
This experiment on the effect 




FIG. 72. Effect of Ether 



of ethor aflnrHs i verv satis Arrow marks moment of application. 

oi ecncr anoras a very sans- This producc(1 rc i axat i on a nd 



depression of response. Air 
substituted at x , and there is 
subsequent recovery of response. 



factory explanation of a phe- 
nomenon in the recovery of 
Mimosa, which has hitherto 

been regarded as anomalous. The outspread position of 
the leaflets is generally regarded as one of sensitiveness ; but, 
when they have just closed, in consequence of stimulation, 
if they be subjected to ether-vapour, they open out. Though 
now, however, mimicking the appearance of sensitiveness, 
they are in fact over-relaxed, a condition which is one of 
relative insensitiveness. The explanation may be gathered 
from the record in fig. 72. It is necessary here to give 
specific meanings to certain terms which have been used 
somewhat indefinitely. We know that stimulation causes 
the fall of the leaf, by differential contraction, and that the 
organ recovers, or 'relaxes,' into its original form after a period 
of rest. This term ' relaxation,' then, may be properly used 
as the converse of ' contraction.' But, in consequence of the 
expulsion of water from the organ after stimulation, it becomes 
flaccid, and this condition also is sometimes vaguely described 
as ' relaxed.' In my own use of the word, however, I shall 
confine myself to denoting by it that process which is the 



130 



PLANT RESPONSE 



opposite of contraction, and which therefore brings about a 
position contrary to that effected by stimulus. 

We have seen that ether produces a relaxing effect which 
is more rapid than the process of relaxation that brings about 
recovery. And we have seen how, in consequence of this 
excessive relaxation, excitatory contraction is diminished or 
abolished. Hence, we see how a stimulated Mimosa leaflet 
under ether relaxes into an outspread position, which never- 
theless is indicative of no re- 
newed sensitiveness such as ac- 
companies true recovery. 

Effect of vapour of hydro- 
chloric acid. I shall now deal 
with the case of strongly 
poisonous agents, of which 
hydrochloric acid may be taken 
as typical. On passing the 
vapour of hydrochloric acid into 
the plant chamber there was 
produced a great relaxation, 
and the responses underwent a 
rapid diminution which ended 
in abolition. The effect of this 
poison is so persistent that 
the blowing-in of fresh air did 

nothing to revive the response. But the timely application 
of vapour of ammonia is found to act as an antidote, restoring 
the response Cfig. 73). 

Effect of chlorine gas. This gas also produces a marked 
depression of excitability, which, under long-continued action, 
brings about the permanent abolition of response. The 
accompanying photographic record (fig. 74) shows the effect 
very clearly. The normal responses to the left arc seen to 
be very rapidly diminished after the application of this gas, 
the response being reduced to one- eighth of its original value 
in the course of nine minutes. There are other important 
considerations in connection with this question, of relaxation 




Fie. 73. Effect of I1C1 Vapour 

Arrow indicates moment of ap- 
plication. Depressing effect 
neutralised by antagonistic 
action of NH, at x . 




CHEMICAL REAGENTS ON LONGITUDINAL RESPONSE 137 

or contraction as the direct effects of chemical agents, which 
it would be out of place to treat in detail here. It need only 
be stated that these effects, which can be very accurately and 
continuously recorded by the arrangement ot the Optic 
Lever, are very suggestive. They are found to be modified 
by the tonic condition of the tissue, the strength of the agent, 
and the duration of application. Thus, an effect of relaxation 
may, after a time, pass into the opposite, of contraction. And 
since these relaxations or contractions of the tissue have a 
modifying influence on the 
response, much light on the 
obscure subject of the effect 
of drugs becomes possible 
through this study. I have 
been able already to obtain 

several curious and interest- 

. /- i - i T 1' IG - 74- Action of Chlorine 

ing results, of which I may .,, t . . , , . , 

' I holographic record, showing normal 

here refer to one, in which effect to the left, depressed and 

tWO drugs, either of which ah^tdbolished after introduction 

when applied singly would 

abolish response, and produce death, are made, when 
applied in succession, to act as antidotes to each other. It 
is my intention to show in the course of this book that all 
the physiological phenomena of the animal have the closest 
correspondence with similar phenomena in the plant, and 
this being so, an investigation carried out on the lines 
indicated, with plants, is likely to be of very great import- 
ance, practically as well as theoretically. 

SUMMARY 

The responsive contractions of an organ afford a reliable 
indication of the excitability of the tissue. 

The physiological changes induced in plant-tissues by the 
action of chemical reagents are outwardly manifested by 
modification of response. 

Certain agents, producing great relaxation, reduce the 



138 PLANT RESPONSE 

power of responsive contraction. Others produce the 
opposite effect, thus protracting the natural period of 
recovery. 

Hydrogen gas produces a gradual diminution of response, 
which is restored to its original value on the readmission 
of air. 

Carbonic acid causes depression, but the restoration to 
normal conditions is generally followed by temporary exalta- 
tion above the normal. 

Vapour of alcohol causes gradual, and solution of alcohol 
rapid, depression. This action may be preceded by tem- 
porary exaltation. The recovery-period is very much pro- 
tracted. 

As in the animal, so also in the plant-tissue, ether causes 
marked relaxation. The depression of response increases 
progressively with the exposure ; on blowing off the vapour, 
response is not only restored but may even show an exalta- 
tion. The opening of the Mimosa leaflets under ether is not 
indicative of true recovery but of over-relaxation. 

A poisonous reagent, like hydrochloric acid or chlorine, 
permanently abolishes the response. 

Reagents which individually abolish response may, by 
their antagonistic character, act as antidotes to one another. 



CHAPTER XII 

EFFECT OF TEMPERATURE 

Temperatures optimum, maximum, and minimum Diminution .of electrical 
response by cooling Temporary or permanent abolition of response due to 
cold Characteristic differences exhibited by different species Mechanical 
response of Biophytum and autonomous response of Desmodium arrested by 
cold Prolongation of latent period Diminution of longitudinal mechanical 
response by cold Diminution of electrical response of plants by rise ot 
temperature Similar diminution seen in longitudinal mechanical response 
Increase of excitability due to cyclic variation of temperature. 

ONE of the factors which modify response in plants is 
temperature. It is known in a general way that certain 
temperatures are favourable, and others unfavourable, to 
physiological activity. It is generally understood further 
that there is a certain optimum, in the case of each species, 
above or below which the excitability of the plant undergoes 
diminution. After this, on reaching a certain maximum or 
minimum temperature, as the case may be, excitability is 
abolished, and if these unfavourable conditions be long main- 
tained the plant is apt to be killed. But the problem of the 
precise determination of such points has hitherto offered 
insuperable difficulties. 

Effects of cold : (a) Diminution or abolition of electrical 
response. Already, however, by adopting the electrical mode 
of investigation, I had been able to overcome these difficulties ; 
for I had found that the amplitude of the electrical responses, 
under different temperature-conditions, afforded a means of 
measuring the excitability of a tissue at the respective points. 
And now, by the use of mechanical response, I am enabled 
again to investigate the same problem by new and indepen- 
dent means. From the results obtained it will be seen that 



140 



PLANT RESPONSE 



(b) 



(c) 



each method furnishes a remarkable corroboration of the 
other. I shall now proceed to describe these various results. 
As an effect of low temperature I have found, by the use 
of the electrical method, that response undergoes a very 
great diminution. For example, the subjecting of a petiole 
of Eucharis Lily to a temperature of 2 C. almost abolished 

its excitability (fig. 75). 
When the specimen, 
however, was restored to 
the normal temperature 
the original response 
reappeared, and some- 
times with even greater 
amplitude than at first. 

When the plant is 
maintained at a very low 
temperature for a con- 
siderable length of time, 
the normal electrical 
response disappears al- 
together, and the spe- 
cimen undergoes per- 
manent death. In this 
respect, different species 
of plants have charac- 
teristic powers of resist- 

FlG. 75. Diminution of Response in Eucharis ance. For example, the 

Lily by Lowering of Temperature tro pj ca l plant, Euckaris 

(a) Normal response at 17 C. ., 

(*) The response almost disappears when plant Lllv> after an exposure 

is subjected to -2-C. for fifteen minutes. Q f twenty-four hours to 

(c) Revival of response on warming to 20 C. J 

a temperature of o C., 

on being subsequently restored to its normal temperature, 
gives no sign of revival by response ; whereas the hardier 
Holly and Ivy, when subjected to the same treatment; do 
exhibit signs of renewed life (fig. 76). 

(b) Prolongation of latent period^ or abolition of lateral 
and autonomous responses. Turning now to mechanical 



EFFECT OF TEMPERATURE 14! 

response, I find in the case of the plant Biophytum that the 
effect of too great cold, or of long-continued exposure to low 
temperature, is the abolition of its lateral response. But 
when the temperature is, relatively speaking, only slightly 
lowered, induced sluggishness is shown in a very interesting 
manner. Whereas, at the normal temperature, say 23 C, 
the response of the leaflet of Biophytum takes place imme- 
diately, after moderate cooling, on the contrary,, the latent 





/ 

Ivy Holly Eucharis 

FIG. 76. After-effect of Cold on Ivy, Holly, and Eucharis 

Normal response ; (/>) response after subjection to freezing temperature 

for twenty-four hours. 



period is increased, and response does not begin to take place 
until from one to two seconds after the application of stimulus. 
Lowering of temperature also abolishes the autonomous 
response of Desmodium. 

(c] Diminution of longitudinal response. With regard to 
longitudinal response, I have been able to demonstrate the 
effect of cold, by taking a specimen of the coronal filament 
of Passiflora. The specimen was kept for fifteen minutes 
in ice, after which electrical stimulus was applied with no im- 
mediate contractile effect. Control specimens, on the other 
hand, exhibited considerable contraction. These contractions 
were measured in both cases by means of a micrometer. The 
specimens taken for experiment were all 21 mm. in length. 
The average contraction of the control specimens was 
i '5 mm. The cooled specimen, as said before, exhibited 
no immediate response. But on continuing stimulation for 
two minutes, contraction began to take place slowly, reaching 
a maximum of only -5 mm. It must be borne in mind 



142 PLANT RESPONSE 

that the specimen was all this time undergoing gradual 
warming by the temperature of the room. 

Another way of demonstrating the effect of cold on the 
longitudinal response of this corona, is to take three filaments ; 
of these (a) has been subjected to cold, (b) is normal, and (c} 
has been killed by immersion in hot water at 60 C. The right- 
hand ends of the filaments are so arranged as to lie perfectly 
even, and a moist cotton thread touches these ends. A 
brass spring presses against the left-hand ends. The thread 
and the brass spring serve the purpose of two electrodes, by 
which shocks can be sent through the three specimens at the 
same time. 

The right-hand end of the object, so arranged, is placed 
within the field of a microscope of low magnifying power 
which has a micrometer eye-piece. At the beginning the 
ends of the specimens lie in a straight line, but after the 
passage of a shock for a period of a few seconds it is found 
that, while the normal () shows the maximum contraction, 
the cooled (a) exhibits very little, and the killed (c) none 
at all. 

The difficulty in conducting experiments on cooling, lies 
in the fact that the inertness due to cold is liable to disappear 
with more or less rapidity when the specimen, for experi- 
mental purposes, is exposed to the temperature of the room, 
which is about 23 C. It cannot be kept immersed in ice- 
cold water, as the exciting current will then to a great 
extent pass through the conducting water itself. The only 
practicable way, therefore, is to subject the specimen to 
prolonged cold, and make a rapid observation afterwards. 

I next attempted to obtain a series of responses, when 
the temperature of the space in which the specimen was 
placed was very gradually lowered. As recovery from the 
stimulus of electrical shock constitutes a very prolonged 
process, I had to use thermal stimulation. But this intro- 
duced the difficulty of itself raising to a certain extent the 
temperature of the space. I had been fortunate enough, 
however, to secure a few specimens of the style of Datura 



EFFECT OF TEMPERATURE 143 

which were extremely sensitive to cold, and thus, notwith- 
standing the disadvantages incidental to the experiment, 
I was able to obtain the very interesting records shown 
below. The experiment was carried out as follows : 

The specimen was mounted in the special plant chamber. 
By pioper manipulation of the stop-cocks it was possible 
to send at will "through the chamber, first, air at the 
ordinary temperature under which conditions the normal re- 
sponses were taken and, secondly, air which had been cooled 
by ice, reducing the temperature of the chamber by several 
degrees. The responses then 
obtained showed the effect of cool- 
ing. And, lastly, ordinary air was 
re-introduced, and the responses 
at this temperature -again re- 

corded. Fig. 77 shows that while FlG - 77- . Affect of Cold on 
Longitudinal Response 

the amplitude of the normal . . . . , ,. , 

r (a) Normal response ; (f>) after 

responses Was eight divisions, cooling; (<) during gradual 




that of the responses at reduced ' " rmal 



temperature was only three, and 

that, on the restoration of normal conditions, the responses 
increased in a staircase manner tending to return to their 
original value. 

Effect of rise of temperature : (a) on electrical re- 
sponse. So much for the results of cooling ; we have now 
to study the effect of rise of temperature. And, first, I 
shall refer to observations made by means of electric 
responses. It may be said that the optimum degree of 
temperature for the excitability of the plant must be under- 
stood to vary with different species. In several cases, how- 
ever, I have found it to lie at about 22 C. But it must 
be premised that this optimum of response refers to passive 
tissues only, that is to say, to those in which there is no 
growth. The optimum temperature for growth may be 
different. In the ordinary response of passive tissues, heat 
has only to bring about a condition favourable to mobility. 
In the case of growing tissues, however, something addi- 



'44 



PLANT RESPONSE 



tional has to be effected, namely, the acceleration of growth. 
We may therefore expect that the optimum temperature 
for growth will be relatively higher than that of simple 
response. And this I generally find to be the case. When 
the temperature is much raised, the electrical response of a 
plant is seen to undergo diminution (fig. 78). It is to be noted 
that the temperatures referred to are dry, or" atmospheric only. 
This explains why, even when the temperature was raised to. 
65 C, which is above the fatal point, response was still 



20"C 




40C SOT 



55C(w) 



1 min 

FIG. 78. Effect of Rise of Temperature on Electrical Response 



observed. Owing to the relative non- conductivity of the 
plant, and the evaporation from its surface, the tissue does 
not actually attain the temperature of the surrounding air. 
When, however, it was subjected for some time to a water- 
temperature of 55 C. response disappeared, by the death of 
the specimen. 

(&) On longitudinal mechanical response. I give next 
(fig. 79) the effect of rise of temperature on longitudinal mecha- 
nical response. The specimen was a 
filament of the corona of Passiflora, 
the stimulus used being thermal. In 
order to subject the plant to definite 
temperature conditions, the adjust- 
ments were made by means of a 
subsidiary heating coil, placed inside 
the chamber. Any given temperature above the normal 
could thus be maintained constant, for the required length of 
time, by simple adjustment of the strength of the heating 
current. The results thus obtained in mechanical response 




FIG. 79. Effect of Rise of 
Temperature on Longi- 
tudinal Contractile Re- 
sponse of Plant 



EFFECT OF TEMPERATURE 



145 



will be seen to be exactly parallel to those already given in 
electrical response. 

The responses are seen to be very much diminished at 35C, 
and almost to disappear at 55 C, which, as will be shown later, 
is near the death-point. Heat-rigor began to manifest itself 
a few degrees above this point, in the contracting of the speci- 
men as a whole. This last phenomenon will be treated at 
greater length in a subsequent chapter. I have already alluded 
to the experiment in which, when the plant is killed by ex- 
cessively high temperature, the response disappears altogether. 

Effects of cyclic variation of temperature : (a) on 
electrical response. I detected another very curious result 



19* C 



30 C C 



25C 




50 C 





30C 




50C 



Temperature 
fattiny 



70 C 




Temperature rising >- 

FIG. 80. Effect of Rising and Falling Temperature on the Electrical 
Response of Scotch Kale (stimulus constant) 



in the course of my investigations on the effect of temperature 
by means of electrical response. This was, a marked increase 



W\ MV AM 



146 PLANT RESPONSE 

of sensitiveness, which often appears as the after-effect of a 
preceding cyclic variation of temperature. That is to say, if 
we take a series of responses while the temperature is rising, 
and afterwards a similar series while the temperature is falling, 
it is found that during the process of cooling, the responses 
are markedly enhanced in amplitude, as compared with those 
given at the corresponding temperatures during heating. 
This is seen in a very clear manner in fig. 80. 

(b} On longitudinal mechanical response. I have ob- 
served a very similar phenomenon in the longitudinal 
mechanical response of, for example, the coronal filaments of 
Passiflora. An ascending scries of responses was taken at 

temperatures of 25, 35, and 45 C. 
They exhibited a regular decrease, 
as has already been explained. On 
now, however, taking records of re- 
sponses during cooling, it was found 

that the response at 35 C. was 50 per 

KM:. Si. Mk-ct of Cyclic . t r iU . , ~ , , 3 \ 

Rise and Fall of Tempe- cent - greater than it had been when the 

rature on Longitudinal temperature was ascending (fig. 81). 
Mechanical Response m & v " ' 

riant I have already explained that 

response is found to be abolished 

when the plant is killed, by raising the temperature above 
a certain maximum point. The exact determination of 
this point has hitherto been a matter of great uncertainty. 
I shall in the next two chapters, however, explain several 
methods by which this investigation may be carried out with 
great precision. 

SUMMARY 

The electrical and mechanical responses of plants undergo 
diminution under the influence of cold. 

The latent period of response is prolonged by lowering of 
temperature. 

When the temperature is raised above an optimum, the 



EFFECT OF TEMPERATURE 147 

responses, both electrical and mechanical, undergo a diminu- 
tion. 

Prolonged exposure to excessive cold or heat brings on 
abolition of response. Owing to cold or heat-rigor this 
abolition may become permanent, indicating the death of the 
plant. 



CHAPTER XIII 

ON THE DEATH-SPASM IN PLANTS 

Difficulty of determining exact moment of death Various post-mortem symptoms 
afford no immediate indication Ideal methods for determination of death- 
point Realised in four different ways : (a) Determination by electrical method 
(d) Determination by spasmodic lateral movement at moment of death 
Experiments with Mimosa Death-contraction a true physiological response 
Continuity of fatigue and death Death-point earlier in young tissues Com- 
posite spasmodic movement (b ! ) Determination of death-point in tendril of 
rassiflora, by sudden movement of uncurling (<) Determination of death- 
point by method of volumetric contraction of hollow organ, causing expulsion 
of contained water. 

IT is known that when the temperature to which it is 
subjected is raised above a certain maximum, a plant is 
killed. But it is very difficult to determine at what exact 
temperature this takes place. One reason of the difficulty 
lies in the fact that hitherto a sure criterion of death, which 
would give an immediate and reliable indication of its occur- 
rence, has not been generally available. Its various symptoms, 
such as drooping, withering, discoloration, and the escape of 
coloured cell-sap, do not manifest themselves at the moment 
of death, but at some time indeterminately later. Even 
when a plant has been subjected to a temperature in excess 
of the fatal degree, it continues to appear fresh and living, 
and it is not till after some greater or less interval that the 
death-symptoms are seen. Various investigators have taken 
up different indications as the criteria of death, and this fact 
accounts for discordance in the results, which would already 
have been sufficiently uncertain even had a common standard 
been decided upon. 

Exact methods of determination of death-point. - A 
good method for the determination of the death-point would 



THE DEATH-SPASM IN PLANTS 149 

consist in watching the waning of a given effect, characteristic 
of the living condition. Still better would be the discovery 
of some effect suddenly and strikingly manifested at death. 
But the ideal method would be found, if some effect could be 
detected which at the moment of death would undergo 
sudden reversal to its opposite. In this last case, there 
would not be even that minor degree of uncertainty which is 
incidental to the determination of the exact vanishing-point 
of a waning effect. I have been successful in devising four 
distinct means, by which the death-point might be detected 
with precision, and "it will be shown that all these different 
modes of investigation enable results to be obtained which' 
corroborate each other in a remarkable manner. The four 
means are : (a) the method of electrical response"; (b) that 
method by which the point of death is determined from the 
occurrence of a spasmodic movement, in a dorsi-ventral or 
anisotropic organ ; (c} that method which depends on the 
sudden expulsion of water at the moment of death from a 
hollow organ, previously filled with liquid ; and (d) the 
method in which the death-point is determined from the 
sudden reversal of a thermo-mechanical response-curve. I 
shall, in the course of the present chapter, describe the first 
three of these, leaving the fourth method to be treated in the 
next chapter. 

(cT) Determination of the death-point by electrical response. 
As regards the electrical method, I have shown elsewhere l 
that the response of normal galvanometric negativity is cha- 
racteristic of the living condition of a plant-tissue. When the 
plant is killed, by any means whatsoever, this normal response 
disappears. At the moment of death from rise of temperature, 
therefore, we shall have the abolition of the normal negative 
excitatory response. But at or beyond this point, on the other 
hand, we may have the positive response of hydrostatic dis r 
turbance replacing the true excitatory effect. By this electrical 
mode of investigation, I have been able to determine the 
death-points of different plants. In the following table, for 

, Response in the Living and Non-Living, p. 62. 



ISO 



PLANT RESPONSE 



example, the specimens tested were radishes, and the experi- 
ment was conducted during the winter season in England. 
It would appear from the results given, that in these six cases 
response begins to be abolished at temperatures varying from 
35 to 55 C. It will be shown later that the death-point 
depends on the season, being a few degrees lower in winter 
than in summer. 



TABLE SHOWING EFFECT OF HIGH TEMPERATURE IN ABOLITION OF 
RESPONSE AND DEATH OF PLANT 



Galvanometric re- I 

Temperature spouse of specimen ! 

at given temp. 

17 C 70 dns. 



(I) 



(2) 



(3) 



C7 C 
\ J J *~" 

17 C. 
53 C. 



,50 c. 



,...4 
,160 ., 



,1 



17 C 100 







Galvanometric re- 
Temperature sponse of specimen 

at given temp. 

i I7 3 C 80 dns. 



60 C o 




By observation of the spasmodic movement of lateral 
response, I now turn to the second of the four methods 
which I have named, that in which a spasmodic lateral 
movement is looked for, in a dorsi-ventral or anisotropic 
organ, at the moment of death. It has been shown that 
when an electric shock of moderate intensity is applied 
to an anisotropic organ, say the leaf of Mimosa, response 
occurs, in consequence of molecular derangement, and recovery 
takes place on restoration of molecular equilibrium. If the 
shock, however, be excessive, response occurs, it is true, 
but there is no subsequent recovery, owing to the fact 
that the molecular derangement has passed beyond the 
point where restoration was possible. There is thus a per- 
manent, irreversible 'set,' and the organ is now said to be 
killed. 

At death, then, a sudden irreversible molecular derange- 



THE DEATH-SPASM IN PLANTS 151 

ment is produced. It follows that if we could bring on very 
gradually those conditions which cause death, then, on arrival 
at the critical point, we might expect the irreversible mole- 
cular derangement to occur abruptly. If, further, throughout 
this process, the organ could be protected from stimulation, 
we might expect that this sudden molecular derangement 
would also be attended by a correspondingly sudden evidence 
of excitation, which would in this case, however, be at once 
the first indication of excitation and the last sign of life. 
This spasmodic movement we shall designate as the death- 
response. 

As regards the protection of the experimental organ from 
accidental stimulation, it is to be remembered that excitation 
under ordinary circumstances depends upon some sudden 
variation of environmental conditions. A sudden change of 
temperature may thus act as a stimulating agent and produce 
depression of the leaf. But a gradual change will not act as 
a stimulus. The effect of such a gradual variation, on the 
contrary, as will be shown presently, is to produce no excita- 
tory contraction whatsoever. 

If now we take a specimen of Mimosa and place it 
suddenly in warm water, say at 35 C, a responsive collapse 
of the leaves will at once occur. But if the plant be placed 
in water at the ordinary temperature of the room, and the tem- 
perature gradually raised say at a rate of i per one minute 
and a half, or thereabouts there will be no responsive 
downward movement whatsoever. On the contrary, owing 
to absorption of water by the organ, and also to the re- 
laxing physiological action of heat, a delicate method of 
record will show a slight and continuous movement upwards. 
This proceeds till we reach a degree of temperature which 
proves to be the death-point. For example, in the case of a 
particular experiment in summer, with a young leaf of 
Mimosa^ when the temperature of 59 C. was reached, there 
was a sudden spasmodic movement of the leaf downwards. 
This was, in fact, the death-throe of the plant. In winter, 
after a spell of cold weather, when the physiological condition 



152 PLANT RESPONSE. 

of the tissue was somewhat depressed, this spasmodic move- 
ment was found to take place at 53 C, which exactly agrees 
with that of radish, under similar conditions. That this was 
the true death-point of the Mimosa specimen was proved 
when, on trying the electrical test, it was found that the 
normal electrical response had disappeared. If, again, one 
branch of Mimosa on the intact plant be bent over, and 
subjected in the manner described to the death- temperature, 
we find, on examination after a considerable lapse of time, 
that whereas the leaves of the rest of the plant are still fresh 
and healthy, reacting to stimulus, those of this branch may 
be seen, from their dried and shrivelled condition, to be 
quite dead. 

Death-response a true physiological response. It will 
be shown that this death-response is a true physiological 
response. Under normal conditions, it will be found to be 
extremely definite, even in different plants. But, under 
physiological modification, it varies appropriately with the 
season, age, condition of freshness or fatigue, and the action 
of chemical reagents. I shall first, then, demonstrate the 
effect of age on the death-point. Thus, on immersing a 
branch of Mimosa in water whose temperature is raised con- 
tinuously, we find that the spasmodic movement of death 
occurs earlier in the young leaves than in the old. Young 
seedlings, again, have a lower death-point than mature plants. 
The following table gives results bearing on this fact. The 
death-points of different plants in the same season and of the 
same age are, however, so definite as to be almost like a 
physical constant. This will appear from the following 
tabular results, and also from results given in the next 
chapter. A fact which will be explained later must be 
stated here. In these experiments with continuously rising 
temperature, it is found that the first spasmodic movement 
downwards is succeeded by a later, upwards, by which time 
the temperature has risen a few degrees. The temperatures 
of both movements are given in the accompanying table, 
corresponding to their occurrence in two leaves, one old and 



THE DEATH-SPASM IN PLANTS 



153 



one young, of each plant tested. These experiments were 
carried out at the beginning of spring. 



TABLE SHOWING DEATH-POINTS IN OLD AND YOUNG LEAVES OF 
DIFFERENT SPECIMENS OF MIMOSA 



Specimen 


Young I^eaf-organ 


Old Leaf-organ 




Temperature 
corresponding to 
fall 


Temperature 
corresponding to 
later erection 


Temperature 
corresponding to 
fall 


Temperature 
corresponding to 
later erection 


i. 
ii. 
in. 


C. 

59*5 

59 
60 


C. 

61-5 
61-5 
62 


C. 
60-8' 
6l 

61-2 


C. 
62 
63 
63-6 


Mean . 


59'5 


617 


6l 


. 62-9 



'From these results it will be seen that there is a mean 
difference of r5 C. between the death-responses of old and 
young leaves. It would thus appear that the age of a cell 
must be the occasion of a certain amount of protoplasmic 
change, as manifested in the retardation of death-response. 
We may also infer that sudden change to unfavourable 
physiological circumstances before the plant has accommo- 
dated itself to the changed condition will tend to lower the 
death-point. This fact I found illustrated during the preva- 
lence of an extraordinary wave of cold, which supervened 
recently, during the progress of these experiments. I then 
found that the mean death-point, in the case of various plants, 
was reduced by several degrees. In Mimosa it fell from the 
average of 59 C. to 55 C., t.e. as much as 4 C. We have 
thus seen that the physiological differentiation concomitant 
on protoplasmic change is attended by variation of death- 
point. 

Explanation of the subsequent erection. In animal 
tissues, the contraction produced by rigor mortis is succeeded 
by a relaxation. The contractile death-spasm in a plant is, 
similarly, followed by the relaxation seen in the subsequent 



154 PLANT RESPONSK 

erection of the leaf. There is another and very interesting 
point of view, from which we may see in this phenomenon the 
continuity of fatigue with death. In the curve of reversal, 
due to fatigue, in Mimosa^ we saw that the first contraction, 
induced by strong and long-continued stimulation, passed into 
subsequent relaxation. In this latter state, the molecular 
condition was such that responsiveness was abolished. It was 
as if, in other words, the tissue had passed into a temporary 
state of death. It is true that, if the stimulation had not 
been excessive, the organ would recover its sensitiveness, 
after a period of rest. But this transient would pass imper- 
ceptibly into the permanent condition of death if, on the 
other hand, stimulation had been excessive. In that case, 
after the fatigue-reversal, the tissue would remain permanently 
irresponsive. 

I have already said that the death-spasm is an instance 
of excitatory response to intense stimulation, and we should 
therefore expect the same kind of effect to be produced as is 
caused by excessive stimulation, that is to say, a preliminary 
contraction, followed by relaxation, after which there is no 
recovery. Again, we saw that in the fatigue-reversal of 
Mimosa, the subsequent erection of the leaf, mainly due as it 
was to relaxation, was possibly also aided by the later con- 
traction of the less excitable upper half of the pulvinus. 
Similarly, it might be expected that the death-contraction of 
the less excitable upper, would take place slightly later than 
that of the lower, half of the pulvinus. We have also seen 
that the excitability of a tissue declines with age, and this 
decline would naturally be greater in the more excitable half. 
Thus the difference of excitability as between the two halves 
would at the same time tend to disappear. As, then, this 
spasmodic movement in dorsi-ventral organs is a true instance 
of differential excitatory response, it would appear that the 
younger the organ, the greater is the excitatory spasm caused 
by death, and in experimenting with Mimosa I have found 
that at the death-point hardly any spasmodic movement is 
shown by old leaves. These considerations will also explain 



THE DEATH- SPASM IN PLANTS 155 

why older leaves give less motile indication than the young, 
in response to stimulus in general. 

(b'} By observation of spasmodic movement of uncurling. 
I shall now proceed to demonstrate that the death- move- 
ment which we have seen in Mimosa is in its essentials 
characteristic of anisotropic response in general ; and this 
may be shown by taking a spiral tendril of Passiflora which 
has become anisotropic by curling. In order to detect and 
measure with ease the responsive movements of uncurling 
and curling the experiment is arranged as follows : a light 
index is attached to the tip of the spiral, and the whole is 
immersed co-axially in a glass cylinder filled with water. A 
strip of paper, marked with degrees, is wrapped round the 
outside of the cylinder on the plane of the index. The 
temperature of the water is now raised very gradually, and 
the responsive excursion of the index is read on the graduated 
circle formed by the paper. 

It is to be borne in mind that the true excitatory response 
of the tendril is given by uncurling, which here corresponds 
to the fall of the leaf of Mimosa ; thus the movement of 
erection would be represented by that of curling. In the 
case of Mimosa we saw that the first effect during rise of 
temperature (due to absorption of water and relaxation) was 
slow and gradual erection. On the arrival of the death- 
point of the organ, however, this preliminary rise was 
succeeded by a sudden responsive fall. The subsequent re- 
laxation of death then produced an opposite movement, of 
erection. Similarly, in the death-response of the spiral 
tendril of Passiflora, we observe parallel phases. There was 
first a slow and continuous movement of curling, during the 
preliminary stages of warming. But this movement ceased 
when a temperature of 57 C. was reached, and the tendril 
remained stationary for a time. At 59 C., however, there 
was produced a sudden excitatory response of death by an 
uncurling, executed with great rapidity, an angular movement 
of 360 degrees being described by the index during the course 
of the next few degrees of rise in temperature. With regard 



I 56 PLANT RESPONSE 

to the short stationary period, it is to be borne in mind that 
the death-point depends on the age of the tissue, and in the 
tendril we have different parts in different stages of growth. 
Hence while the uncurling movement of death was being 
initiated in younger portions, older parts of the tendril were 
still moving in an opposite direction. The outcome of these 
antagonistic movements was a resultant pause, which only 
lasted for a little while, and was followed by the vigorous 
movement of uncurling, caused by the death-contraction of 
the whole tissue. After the completion of the uncurling 
movement, there followed the opposite, namely, the move- 
ment of post mortem relaxation. In a second experiment 
with a younger specimen of tendril, I obtained results almost 
identical. Here the uncurling response of death began at 
one degree of temperature earlier, namely at 58 C, and the 
index moved through 1 50 degrees of the circular scale. 

On the subject of the death- contraction of the radial 
organs of ordinary plants, I shall speak in some detail in 
the next chapter, and shall there describe the perfected 
apparatus by which the thermo-mechanical response can be 
continuously recorded, the curve exhibiting the death -point 
with great precision. 

(c] By observation of volumetric contraction^ causing sudden 
expulsion of water. or the present I shall describe only 
the third of those methods which I have enumerated for the 
determination of the death- point, that namely which depends 
on the sudden expulsion of water, at the moment of death, 
from the hollow organ of an ordinary plant, previously filled 
with liquid. The specimen used for the present demonstration 
will be the peduncle of Allium > although there are .many 
tubular organs of various species of plants which are more 
or less suitable for these experiments. 

We cut a length of about 10 cm. from the middle of a 
peduncle of Allium, rejecting the too young and too old 
portions at top and bottom. As the presence of air-bubbles 
is likely to be disturbing to the experiment, the water used 
must have been previously boiled. The specimen is placed in 



THE DEATH SPASM IN PLANTS 



157 




a vessel of this water, and as a further precaution against air- 
bubbles clinging to the interior of the tube, the whole may 
be put inside the receiver of an air-pump and subjected to 
a repeated partial vacuum. The removal of air-bubbles may 
also be effected by rinsing the tube of Alliuin in water 
containing a small quantity of ether, and immediately after- 
wards washing with ordinary water. This must be done, 
however, with caution, as the presence of an appreciable 
quantity of ether would be likely to affect the excitability of 
the tissue. Before commencing the experiment, it is advis- 
able to allow the specimen to 
remain immersed in water for 
about half an hour, by which 
time it becomes fully turgid. 

The lower end of the Alliuin 
tube filled with water is closed 
by a piece of solid glass rod, 
and the upper end is also closed 
with a piece of glass tube, 
having a capillary bore. A 
graduated scale is placed behind 
this latter, so as to measure the 
movement of the water- index, or 
this movement may be con- 
tinuously recorded on a revolving 
drum (fig. 156). The Allium preparation is now placed in a 
vessel of water, and subjected to a gradual rise of temperature 
in the manner already described. 

If, at the temperature corresponding to death, there 
should now be a sudden excitatory contraction of the Allium 
tissue, the volumetric change thus produced will force out 
the contained water, and we shall observe a relatively rapid 
expulsive movement of the water-index. From the curves 
given above in fig. 82, it will be seen that this occurs at 
a temperature of 59 C. in a younger, and at 63 C. in an 
older specimen. Previous to this, there was an inward 
movement of the water-column corresponding to the gradual 



FIG. 82. Determination of Death- 
point in Allium Tube by Ob- 
servation of Volumetric Con- 
traction, causing sudden Expul- 
sion of Water 

A, record given by older specimen ; 
death-point 63 C. B, record 
of younger specimen ; death- 
point 59 C. 



158 PLANT RESPONSE 

preliminary rise of the Mimosa leaf or curling of the Passiflora 
tendril. But at the death-point this movement was at first 
arrested, and then reversed with accelerated speed. The 
condition of this particular specimen was afterwards tested 
by the electrical method, when it was found that, while a 
portion of the peduncle previously cut off gave the normal 
response of living tissue, the specimen which had been 
subjected to the death- temperature gave no response. 

SUMMARY 

The death-point of a plant, under heat-rigor, is concomi- 
tant to the disappearance of the true excitatory response of 
galvanometric negativity. After this point is passed, hydro- 
static disturbance generally gives rise to the reverse positive 
response. 

When an anisotropic organ like the pulvinus of Mimosa is 
gradually raised in temperature, then, at a certain critical 
temperature, a spasmodic movement is produced which 
proves to be the death -response of the organ. This is a true 
excitatory response. The critical death-points of similar 
specimens are very definite and practically identical. 

The death-point is modified by the physiological condition 
of the tissue. Other things being equal, death occurs earlier 
with a young than with a mature tissue. 

The death-response of an anisotropic organ is composite. 
In Mimosa it consists of a down, followed by an up, move- 
ment. This is due to the death-contraction being followed 
later by the post-mortem relaxation of the organ. 

In the spiral tendril of Passiflora^ the death-response is 
given at the critical point by a sudden uncurling. 

In the case of the hollow peduncle of Allium> the death- 
response is exhibited by volumetric contraction, producing 
sudden expulsion of contained water. 



CHAPTER XIV 

THE DETERMINATION OF THE CRITICAL POINT OF DEATH 
BY INVERSION OF THE THERMO-MECHANICAL CURVE 

Death-spasm in anisotropic organ due to differential longitudinal contraction In 
radial organ the death-contraction is purely longitudinal Death-point deter- 
mined from point of inversion of a thermo-mechanical curve The complete 
record thus constitutes a curve of life-and-death, the two being separated by 
the death-point Characteristic thermo-mechanical curve a.s resultant of varia- 
tion of temperature and variation of length The necessity of specifying the 
rate of rise of temperature The thermo-mechanical curve characterised by 
sharp and definite inversion at point of death No inversion of thermo- 
mechanical curve after death of plant Death-contraction under heat-rigor in 
plant analogous to similar phenomenon in animal The Morograph, a perfected 
form of apparatus for determining critical point of death Remarkable identity 
of thermo-mechanical curves obtained with two similar specimens Death-point 
almost as definite as a physical constant Vanishing of point of inversion with 
age Determination of death-point under cold-rigor Constancy of death-point. 

WE have seen how the death-point can be determined in 
anisotropic organs, by the occurrence of a spasmodic lateral 
movement. We have also seen that this death -movement is 
an excitatory response, at once initiated and terminated at 
the point of death. It has been shown further that the 
anisotropic is simply an instance of differential longitudinal 
response. It follows that if the death-spasm in the aniso- 
tropic organ be indeed caused by true excitation, then from 
a radial organ, at the moment of death, we should obtain a 
purely longitudinal contraction. We may look upon this phe- 
nomenon, again, from a purely molecular standpoint. We can 
then see that if death be brought about by a sudden mole- 
cular change, such an event might be expected to exhibit 
itself in a correspondingly sudden change of form. Let us 



l6o PLANT RESPONSE 

then take a radial tissue, and subject it to a gradual rise of 
temperature, taking a continuous record of its variation in 
length. From what has already been said it will be under- 
stood that, the variation being gradual, no responsive con- 
tractile effect will be induced during the process, but, on the 
contrary, some relaxation. At the death-point, however, a 
sudden inversion of the curve, due to death-contraction, may 
be expected to appear, and thus the whole record will consti- 
tute a curve of life-and-death, this point of inversion separating 
the two. Should the inversion prove to be very abrupt, the 
turning point will afford us a means of determining the 
temperature at which death occurs, with very great accuracy. 

In order to prove, further, that this specific response is 
definitive, we may, after passing the death-point, bring the 
tissue back to its original temperature once more, and repeat 
the process. The record ought now to show no inversion 
characteristic of a transition from life to death. 

Means of obtaining thermo-mechanical record. I shall 
now proceed to describe the manner in which I have obtained 
this thermo-mechanical record. I took a specimen of a radial 
organ, in this case the long style of Datura, and fixed it to a 
small glass rod which in its turn was fastened to a weight, 
the whole being placed in a vessel of water. The free end of 
the style was attached to one arm of the Optic Lever. The 
bath was now warmed gradually, a thermometer indicating 
the rise of temperature. Variations of length corresponding 
to the rise of temperature were progressively recorded, from 
the movement of the spot of light. For this purpose the 
mode of procedure was as follows : The vertical movement of 
the spot of light occasioned by the variation of length of 
the specimen was converted into lateral, by reflection from 
a second mirror. The paper wrapped about the recording 
drum was divided into millimetres. It was required that the 
abscissa of the thermo-mechanical curve should represent 
temperature, and the ordinate the corresponding length. 
The position of the spot of light, at any given temperature, 
was marked on the drum. At each rise of temperature of 



DETERMINATION OF CRITICAL POINT OF DEATH l6l 

1 C. the drum was rotated, say, through a distance of 2 mm., 
and the position of the spot recorded. In this way, by con- 
necting the recorded points, a curve was obtained, in which 
the length corresponding to each temperature was known. 
In this curve an abrupt inversion, due to sudden death- 
contraction, was found to occur at about 59 C. The curve 
thus obtained, however, though the successive points recorded 
were very near each other, is the result of intermittent 
observations. Again, two observers were required, one to 
read the temperature, and the other to take the record. It 
was therefore subject to error of thermometric reading. 

Means of obtaining automatic record. For this reason 
I was desirous of obtaining a curve which should be con- 
tinuous and practically automatic, the plant itself being made 
to record its own variations of length, and its own death-point. 
The problem resolves itself into that of making the reflected 
spot of light partake of two motions simultaneously, namely, a 
horizontal movement proportional to the change of tempera- 
ture, and a vertical movement proportional to change of length. 
The horizontal, or thermometric, component of the movement 
I secured as follows : I constructed a thermo-electric element 
of iron and nickel, one junction of which was placed in melting 
ice, and the other junction in the vessel of water containing 
the specimen whose temperature was being subjected to change. 
This element was placed in circuit with a resistance box and 
a sensitive reflecting galvanometer. The amount of the 
movement of the galvanometer spot of light could now, by 
interposing suitable resistance, be brought to any appropriate 
value. In my experiments, with a particular galvanometer, 
the movement of this spot of light, for each degree of rise of 
temperature, was 2-5 mm. 1>. one-tenth of an inch when 
the recording surface was at a distance of 125 cm. from the 
galvanometer. This extent of movement was quite sufficient 
for the purposes of the experiment, as it enabled estimates to 
be made with ease, correct to one- fifth of a degree. By inter- 
posing smaller resistances, however, one-twentieth of a degree 
could easily be discriminated. The excursion of the spot of 

M 



1 62 PLANT RESPONSE 

light was now found to be strictly proportional to the rise of 
temperature. 

In order to combine this horizontal thermometric move-^ 
ment with that vertical movement occasioned by the variation 
in length of the specimen, the vertically moving spot of light 
from the Optic Lever was thrown on the galvanometer- 
mirror. The apparatus, it should be mentioned, was so 
arranged that the two mirrors were as close together as 
possible. The spot of light now, having been reflected 
from two mirrors, directly described a curve in which the 
abscissa gave temperature- variation, and the ordinate, varia- 
tion of length. When the source of light is a point, that is 
to say, a pinhole with electric light behind the excursion of 
the reflected ray upon a photographic plate will produce an 
automatic record. Or the movement of the light may be 
followed continuously with a pen. 

Conditions for securing accurate death-point. Here 
I must point out certain conditions which must be kept in 
view if we are to obtain a very definite .death-point. We 
know that if a plant be placed in an unfavourable environ- 
ment, or in a temperature much above the optimum, for a 
prolonged period, death will ultimately ensue. But inas- 
much as these temperatures would only cause the death of 
.the plant by indirect and cumulative action through pro- 
gressive derangement of the several functions, they cannot 
in themselves be said to constitute death-points. To be 
scientifically precise in such a determination it is necessary 
that we should discover a temperature which is of itself 
efficient to initiate sudden death. On the other hand, again, 
as the contraction of death is a phenomenon of response, 
we see that it must have a certain latent period. Some 
interval elapses, moreover, during which the tissue is attaining 
the temperature of the bath in which it is placed. Now if 
the rate of rise of temperature be too rapid, then, owing 
to the lag caused by these last two factors, by the time the 
death-response commences, the recorded temperature may 
have gone beyond the actual death-point. 



DETERMINATION OF CRITICAL POINT OF DEATH 163 

We thus arrive at two conditions which must be regarded 
as mutually somewhat antagonistic. In the first place, in 
order to obtain the immediate point of death, it is essential 
that the plant undergo an exposure which is not too pro- 
longed. Jn order, on the other hand, to make due allowance 
for the latent period and for attainment of the surrounding 
temperature, the rate of rise must be gradual and definite. 
In the case of tissues which are not too thick, the latter of 
these conditions is amply fulfilled by a rate of i C. per 
minute and a half. We see, therefore, that in precise deter- 
minations of the death- point, the rate of rise must always be 
specified. 

With thick stems, however, owing to relative want of 
thermal conductivity, the attainment of surrounding tem- 
perature and occurrence of death throughout the whole of 
the organ is a very protracted process. The experiments 
which I shall describe were made with specimens which 
were not too thick, death at the fatal point being ensured, 
when the rate of rise of temperature was that prescribed, 
namely, i C. per every minute and a half. 

This definite rate of rise may be secured by using an 
electric heating apparatus, such as is commonly employed 
for boiling a tea-kettle. The current from the street-mains, 
which is 220 volts, heats water too rapidly. But the desired 
rate may be obtained by interposing an electrolytic rheostat 
of copper sulphate, the current being brought to a suitable 
value, by separation of the two electrodes through which the 
current enters and leaves the electrolyte. 

In this case, when placing the specimen in the experi- 
mental bath, it is advisable to secure it to a bent glass rod, 
which rests outside. For if it is placed in the metallic vessel 
itself, the record will be subject to a certain disturbance, 
owing to the expansion of the supporting metal while heating. 
The expansion of the glass rod is so small as to be negligible. 
In this way, using for experiment a filament of the corona 
of Passiflora, I obtained a record, showing a very abrupt 
inversion, corresponding to the death-point, which was 

M 2 



PLANT RESPONSE 

between 59 and 60 C. I shall presently have occasion 
to describe in detail the various characteristics of this 
curve. 

Having described the apparatus with which these curves 
were recorded, it is necessary to point out the difficulties 
which were encountered in working with it. It must be 
remembered that the excursion of the spot of light, in this 
case, represented a high magnification of the actual move- 
ments involved. The spot of light, moreover, was re- 
flected from two separate instruments, and was liable to be 
disturbed by the slightest jar or tremor in either of them. 
Though the instruments were placed on a steady stone 
pedestal, even this precaution could not be made wholly 
effective, in the heavy traffic of a town. It was only, there- 
fore, in intervals of quiet that approximately perfect results 
could be obtained. This difficulty led me to the devising of a 
much simpler and more perfect instrument, which I shall 
designate as the Morograph. 1 This is a small and portable 
apparatus, self-contained, in which the necessity of a galvano- 
meter is obviated. By its means, moreover, the record is 
unaffected by any earth-vibration. 

The Morograph. The thermometric record is produced 
by means of the curling and uncurling of a spiral compound 
strip, of two metals, having different coefficients of expansion. 
In order to give strength and steadiness, this helix, which is 
about 2-5 cm. in diameter, is made of somewhat thick strips 
of brass and tinned iron, soldered together. By increasing 
the number of turns in this spiral, the extent of movement 
per degree in the thermometric record may be increased 
at will. In my own Morograph^ a helix of three circles 
was found to answer all requirements. The last half-circle 
of the lower end of the spiral is fixed to a heavy circular 
stand of brass, 3 cm. in diameter. The topmost half-circle, 
on the other hand, has had the tinned-iron strip cut off, and 
therefore consists of brass alone. It will thus be Understood 
that a line drawn diametrically across this last half-circle 

1 This word is derived from the Sanskrit root mrf, Latin /tiers, death. 



DETERMINATION OF CRITICAL POINT OF DEATH 



would rotate round a vertical axis passing through the centre 
of the spiral, under the influence of the differential expansion 
or contraction produced in the compound strip of metal by 
rise or fall of the temperature, When the outside of the 
circumference of the spiral consists of the more expansible 
metal, brass, then a rise of temperature will produce the 
movement of curling. The difficulty in the construction of 
this part of the apparatus lies in securing equal angular 
rotation of the diameter about the axis, with every equal rise 
of temperature. These indications 
were at first extremely irregular. 
I was able, however, to remove all 
traces of irregularity by careful 
and repeated annealing. In any 
case the thermometric indications 
of the compound helix may be 
previously calibrated. 

The axis of the Optic Lever 
one arm of which is attached to 
the plant-specimen, and which is 
to give the record of its variation 
in length with rise of temperature 
is now supported on the diameter 
of this last half-circle of the helix 
and is thus rotated bodily, with rise 

of temperature (fig. 83). And it *'i. 83- The Thermometric 
... .. , ,t .1 ._ Spiral and Optic Lever of 

will thus be seen that the motion t he Morograph 

of the spot of light, reflected from 

the single mirror of the Optic Lever, is a resultant of two 
movements, which take place at right angles to each other 
namely, the horizontal movement, due to thermometric 
variation and the corresponding vertical movement, due to 
the changes of length of the experimental plant-tissue. 
Owing to the fact that the spot of light in this apparatus is 
reflected only once, it is extremely bright. 

I shall now proceed to describe the manner in which the 
plant is mounted, and other accessories of the apparatus. 




I?56 PLANT RESPONSE 

The circular brass stand on which the helix is mounted 
has at the centre a small tube, in which the lower part of 
the specimen is clamped. The plant-organ thus occupies the 
vertical axis of the spiral, its upper end being connected by 
a thread with the short arm of the Optic Lever. It may be 
pointed out here, as is better explained in the diagram, that 
in order to give room to the specimen, the axis of the Lever 
is made to rest upon T-pieces, which are erected at the two 
ends of the diameter of the helix. The plant-organ being 
thus placed at the centre, the inclosing spiral thermometer 
gives an accurate indication of the temperature to which it is 
exposed. 

The circular stand, supporting both the specimen and the 
recording apparatus, is placed in what I shall describe as the 
inner thermal cylinder, within the circumference of which 
the base fits exactly, while the helix is free, to the extent of 
25 cm. all round. This internal cylinder is made of copper, 
coated with silver. It is filled with water and placed inside 
an outer, or heating, cylinder of brass, which is also filled 
with water. Heat is applied, by means of a spirit-lamp, to 
the bottom of the outer cylinder ; thus the water in the inner 
vessel is subjected to equal heat, on all sides at the same 
time. Had the heat been applied directly to the inner 
cylinder, convection-currents would have caused great dis- 
turbance of the recording spot. With these precautions, 
however, there is no trace of such disturbance. 

The whole apparatus is supported on a steady stand. 
Below it is the spirit-lamp, which may be raised or lowered 
till a distance is found which gives us the standard rate of 
rise of temperature, that is to say, i C. per minute and a 
half. Above the apparatus and on a sliding holder is the 
electric lamp, with focussing lens ; the light from this falls 
veitically on the mirror of the Optk Lever, which is inclined 
at an angle of 45 to the horizon. The horizontally reflected 
light is then thrown on a screen, which carries either semi- 
transparent recording paper or a photographic plate. In 
the former case the observer, standing behind the screen, 



DETERMINATION OF CRITICAL POINT OF DEATH 167 

traces the movement of the spot of light with a pencil. The 
whole recording apparatus and the source of light being thus 
placed on the same stand, any ordinary disturbance will 
affect all equally, and cause no irregularity therefore in the 
movement of the recording spot (fig. 84). 




FIG. 84. The Morograph 

Record may be taken by following excursion of spot of light on screen to the left. 
For photographic record, plate-holder is substituted for screen. 



I have given a great deal of space to the description of 
these details, because on them depends the accuracy and 
perfection of the results. The record may now be made 
on any scale of magnification, without misgiving. In fact 



168 I'l.ANT RESPONSE 

I have obtained very perfect records even when the passing 
traffic was at its thickest How true this is may be seen 
from the photographic record of a thermo-mechanical curve, 
given in fig. 85, 

It will be noticed from the curve that, as the temperature 
rose, there was a continuous preliminary elongation, which 

was suddenly reversed by the exci- 
tatory contraction at the death-point, 
found in this case to be 59'6 C. 
If desired, the photographic curve 
itself may be made to indicate the 
different temperatures at different 
parts of the curve. This is secured 
by interrupting the light for a time 
at, say, every half degree of rise of 
temperature. As in the anisotropic 
death-responses, described in the last 
chapter, we have in this case also, 
though not shown in the present 
KM-.. 85. Thermo-mechanicai record, the post-mortem relaxation 

Curve obtained Photo- 1-1 

graphically (Coronal Fila succeeding the contraction of rigor 

ment of l\issijtora} mortis 

The first or down part of the T t , , , , , ,, , 

curve shows relaxation, In order to show that the mole- 

imt on reaching death- cular change which occurred at the 

point, at 59-6 C., there is . . . . 

a sudden inversion of the point of inversion was indeed the 
curve, due to spasmodic j rrcve rsiblc death-change, I took the 

death-contraction. & 

curve once more, after allowing the 

specimen to return to its original temperature. The curve 
now obtained showed no reversal-point. 

Remarkable agreement between thermo-mechanical 
curves of similar specimens. It was pointed out in the last 
chapter that the death-point is almost as definite as a phy- 
sical constant. And not only is this true of the death-point, 
which I find in different phanerogamous specimens, under 
normal conditions, to be almost invariably close upon 60 C., 
but it is also more or less true of the whole curve, those given 
by similar specimens being almost identical In this way 




DETERMINATION OF CRITICAL POINT OF DEATH 169 




the thermo-mechanical curve is, in a sense, characteristic of 

the plant in a given condition. This is well seen in the two 

records which I have obtained from the styles of two flowers 

both on the point of opening of a single plant of Datura alba 

(fig. 86). These two curves are so extraordinarily similar in 

all their parts, that I was obliged, 

in printing them, to raise the 

origin of one slightly above that 

of the other. If the point of 

origin had been allowed to 

remain the same in both, one 

would have been superposed 

upon the other, so as to prove 

almost indistinguishable. On 

minute examination, however, 

I find that the death-point of 

one differs from that of the 

other by about $ of a degree. 

The possibility of securing 
such uniformity of results, en- 
ables us to attempt an investigation on the influence of 
various agencies. For any deviation from the standard 
characteristic curve will then form an indication of the action 
of such agents. 

Standardisation of curves. Different plants, again, will 
exhibit differences in their characteristic curves, and in order 
to render these strictly comparable with one another, we 
must know the absolute value of relaxation or contraction in 
each part of the curve. By absolute value, is here meant the 
amount of relaxation or contraction per unit-length of the 
specimen. This is rendered simpler if we adopt a uniform 
standard for all specimens ; that is to say, the horizontal dis- 
tances representing temperature may in the standard curves 
be -i inch (2*5 mm.) per degree. Vertical distances, again, 
of -j inch maybe made to represent a relaxation or contraction 
of one part in a thousand. The standardisation is carried out 
in the following way : first, the recording surface is moved, 



FIG. 86. Thermo- mechanical Curve 
of Two Different Specimens of 
Style of Datura a/ha, obtained 
from Flowers of the same Plant 



I/O 



PLANT RESPONSE 



till one degree of rise of temperature produces a horizontal 
movement equal to *i inch. After this, keeping the distance 
of the recording surface constant, the length of the short arm 
of the Lever, to which the plant is attached, is so adjusted 
that the vertical magnification is two hundred times. The 
length of specimen used, unless the contrary is stated, is 

always 5 cm. A movement of the 
light-spot through a vertical distance 
of one division (T inch) will then 
represent an expansion' or contraction 
of one part in two hundred of a 
specimen whose length is 5 cm., that 
is to say, one part in a thousand of 
a specimen whose length is one 
centimetre. In fig. 85, the original 
record has been reduced to one- 
fourtb. The distance between two 
horizontal lines represents a contrac- 
tion or relaxation of I per cent. 

In order to exhibit the differences 
in the characteristic curves of different 
specimens, or of the same specimen 
at different ages, I append three re- 
cords taken under the same standard 

I- a; 87. Thermo-mechanieal conditions: (l>) that of the Style of 
Records of s, Young Speci- J . 

of s/>iro$yni ; s' Older Datura alba ; (s) of a young specimen 




The distance between two specimen of the same (fig. 87). In 
horizontal lines represents tnese t h ree experiments, the rate of 

a contraction or relaxation * 

..i i |vi ( ( nt. rise of temperature and other circum- 

stances having been the same, it is 

instructive to compare the different parts of the different 
curves. 

Trfking first the curve of Datura^ we find its death-point to 
occur at 60 C. The relaxation undergone by the specimen 
during the rise of temperature from 35 C. to the death- point, 
was at the mean rate of 2' I parts per thousand per degree for the 



DETERMINATION OF CRITICAL POINT OF DEATH I/r 

unit-length. This, for convenience, we "shall call the coefficient 
of relaxation. But after the death-point, the sign of response 
undergoes an abrupt change to the negative, that is, contrac- 
tion, the coefficient of contraction being fifty per thousand, or 
nearly twenty-four times the coefficient of relaxation. 

The next specimen whose curve (s) is given was young 
Spirogyra of light-greenish .colour. From the slight differen- 
tiation of these simple algal forms, and from their lack at this 
young stage of any efficient protecting envelope, we should 
expect them, to offer but feeble resistance to the effect of heat, 
and we find the death-point lowered to 47, that is to say, 
13 below that of the* phanerogam Datura. Along with this, 
we find also a difference in the coefficients of relaxation and 
contraction. The mean coefficient of relaxation was in this 
case 'ooi, and that of contraction '007. 

Vanishing: of point of inversion with age. The older 
specimen of Spirogyra (s'), taken from the same place, had 
its point of inversion raised by 4, the death-point being 
therefore at 51 C. There is a further and interesting 
difference ,as between curves for young and old specimens- 
In the younger specimen there was produced a very consider- 
able contraction due to rigor, and this was followed after a 
time by the usual post-mortem relaxation. But in the older 
specimen the rigor was relatively slight and the subsequent 
relaxatibn took place much earlier. We thus see that there 
is a great loss of contractile power in old tissues. In still 
older specimens the contraction tends to vanish altogether, 
and we have no line of demarcation to mark the moment of 
transition from life into death. In connection with this, it is 
interesting to note that, whereas the death-spasm in young 
leaves of Mimosa is very vigorous, old leaves exhibit little or 
no spasmodic lateral movement at death. , 

Cold-rigor. ---Turning from the effe'ct caused by con- 
tinuous rise of temperature, I shall now proceed to the con- 
sideration of the effect produced by the reverse process of 
continuous fall to the.tainimum temperature. Here also, 
as in the case of the curve for rising temperature, there is a 



172 PLANT KKSrONSE 

sudden inversion at a definite minimum point of temperature. 
That is to say, just as we observe a sudden contraction 
when the point of heat-rigor is reached, so also we obtain 
a similar sudden contraction at a point corresponding to the 
cold- rigor. For example, with the style of Eucharis Lily, 
which is very susceptible of depression by cold, I found the 
death-point to be at about i C. The experimental diffi- 
culties for the determination of cold-rigor are, however, very 
great, owing to the fact that facilities do not exist for con- 
tinuous lowering of temperature to zero or below. 

Thermo-mechanical record of Mimosa. At the begin- 
ning of this chapter it was stated that the death-spasm in an 

anisotropic organ, such as that of the 
pulvinus of Mimosa, was an instance 
of differential longitudinal excita- 
tory contraction. The accompanying 
curve (fig. 88) was obtained by means 
of the Morograph. We must remem- 
ber that in this case we are dealing 
with a differential action. In the 
first part of the curve, therefore, we 
do not obtain such marked relaxation 

Thermo-mechanical as in radial rgans, where WC obtain 

Record <>f Leaf of Minima noil antagonised and direct change 

The spasmodic contraction of form. But when we reach a tern- 
shown here as downward . 1-1 i , 

movement took place at perature which corresponds to the 
54 c, the normal death- death-point, that is to say, 54 C., 

point l>emg lowered by 5 r > j JT > 

owing to prevailing cold, there is a sudden downward move- 
Subsequent erectile move- t j t m t u romemhorod that 
ment took place at 60 c. meni - u musl De rcmemDerea mat 

this particular experiment was carried 

out just after the spell of cold weather, when the death-points 
of plant-organs were found to be lowered by several degrees. 
After the downward movement, which commences at 54 C., 
we sec that there is an equally abrupt upward movement, 
beginning at $9 C., due to post-mortem relaxation aided by 
the later contraction of the upper half of the organ. 




DETERMINATION OF CRITICAL POINT OF DEATH 173 

A few words may be said here with regard to these 
successive movements. As in animals the rigor mortis is 
succeeded by relaxation, so also in radial organs, as has been 
said, we see relaxation succeeding the death-contraction. It 
may then be asked whether the second half of the present 
curve, in fig. 87, giving the rise of the leaf, does not simply 
represent a similar relaxation, in the case of the pulvinus of 
Mimosa. But we have to notice that, in taking records with 
the Lever, the weight of the Lever ensures the indication of 
any passive relaxation of the specimen. If we inspect a 
Mimosa leaf, however, during the death- spasm, the leaf 
being free, i.e. unconnected with the Lever, we find that it, 
after its first fall, becomes again almost vertically erected, 
evidently in consequence, at least to some extent, of some 
process of active contraction, which must be that of the upper 
half of the organ. Had there merely been a general relaxa- 
tion of the whole pulvinus, caused by death, then the weight 
of the leaf might have caused it to fall. 

The slope of the curve of relaxation, again, is, generally 
speaking, relatively gentle. Its comparative steepness, in the 
case of Mimosa, after the passing of the death- point, seems to 
indicate that the movement of relaxation was partially aided 
by later contraction of the upper half of the pulvinus. 

Constancy of death-point. Before concluding the present 
chapter, I must refer to the remarkable fixity of the death- 
point in all the phanerogamous plants which have come 
under my observation in normal conditions. Thus, on re- 
peating my experiments at the end of spring, by the perfected 
method of morographic record, I invariably found that the 
point of inversion was at, or within T V of a degree of, 60 C. 
Other and less perfect modes of investigation, such as the 
spasmodic lateral movement of a dorsi-ventral organ, the 
movement of uncurling, the sudden expulsion of water, and 
those opening and closing movements of flowers which are to 
be described in the next chapter, enabled us to obtain death- 
points which were not very different from this. I give below 



174 



PLANT RESPONSE 



tabular statement which makes it possible to see at a 
glance how concordant these results are. 

TABLE SHOWING DEATH-POINTS OBTAINED BY DIFFERENT METHODS 



Specimens 



r Coronal filament of Passiflora . 
I [Six specimens used. Each gave] 
I Style of Hibiscus 
([Four specimens. Each gave] 

J Sty" 6 ^ *D atura 
* 1 [Four specimens. Each gave] 

15. Pulvinus of Mimosa. Young 

16. Spiral tendril of Passilora 

17. Peduncle of Alliitm. Young 

1 8. Flower of French Marigold 

19. Flower of fpom&a . 



Method 



Morograph. 



J* 



Spasmodic lateral movement. 
Movement of uncurling. 
Expulsion of contained water. 
Opening or closing of flower. 



Death- 
point 



C. 

60 

60 
60 



59 
59 

59 
62 



It will thus be seen that, using very diverse methods and 
specimens, we nevertheless always obtain a death-point which 
is very near 60 C. 

SUMMARY 

In radial organs, the death-contraction due to heat- 
rigor is abrupt, and takes place at a definite temperature, 
which is to be regarded as the death-point. 

In the thermo-mcchanical curve given by the Morograph, 
the point of inversion is the death-point 

When death has taken place, a repetition of the experi- 
ment shows no inversion. 

The death-point, due to heat-rigor, in phanerogamous 
plants, under normal conditions, is found, though obtained 
by various methods, to be very close on 60 C. 

Under the action of continuous lowering of temperature 
^here is produced, at a definite minimum degree, a spasmodic 
:ontraction, due to cold-rigor. 

^ The death -contraction in plants is in every respect 
iimilar to the same phenomenon in the animal, and is an 
nstance of true excitatory effect As in the animal, so also 



DETERMINATION OF CRITICAL POINT OF DEATH 

in the plant, this rigor of death is succeeded, after a time, 
by passive relaxation of the tissue. 

The thermo- mechanical curves of two similar specimens 
that is to say, two specimens of the same plant, having the 
same previous history are found to be practically identical. 

Different plants have different characteristic curves. The 
curve of the same plant also exhibits a variation of its 
characteristics under changed conditions of age, experiment, 
and previous history. 



CHAPTER XV 

EFFECT OF VARIOUS AGENCIES ON DEATH-RESPONSE: 
THERMOGRAPHS OF REGIONAL DEATH 

Lowering of death-point by fatigue Modification of characteristic thermo- 
mechanical curve by the action of chemical z.igi.vte Comparison-Morograph 
Duplication of rigor-point Death- response a physiological response and not 
due to coagulation Death-movement of flowers Approximate constancy of 
death-point of florets in a capitulum Definite interval between death-point 
and discoloration- point -- Translocat ion of discoloration-point by various 
agencies Thermographs of regional death Thermograph of local fatigue 
Thermographic investigation of electrotonic excitation. 

I HAVE shown that the death-contraction is a phenomenon 
of excitatory response. We might expect from this that 
various conditions which affect the excitability of a plant 
would also have a modifying influence upon the characteristic 
thermo-mechanical curve of death- response. One such 
modification would lie in the translocation of the point of 
inversion, or, in other words, in the displacement of the 
death-point. In order to test this inference we might subject 
the plant to the influence of various agents which modify the 
physiological condition, and observe the consequent modi- 
fication of the death-response. We have already seen how 
the physiological modification induced by age causes dis- 
placement of the death-point. We have seen, further, how 
unfavourable seasonal conditions, such as sudden prevalence 
of cold, will lower the death-point by several degrees. We 
shall now study the effect of other agencies, such as fatigue, 
and the action of chemical reagents, in producing displace- 
ment of the death- point 

Effect of fatigue. In the course of these experiments, 
fatigue was produced by means of tetanising electric shocks, 



VARIOUS AGENCIES ON DEATH-RESPONSE 177 

care being taken that these should not be strong enough 
to kill the plant. I have carried out experiments on this 
subject with two different classes of specimens : first, with 
anisotropic pulvinated organs, like that of Mimosa, where the 
death-spasm is shown by lateral movement ; and secondly, 
with radial organs, like the style of Datura, where the death- 
point is determined by thermo-mechanical inversion. In 
experimenting on Mimosa, I took a batch of young leaves of 
the same age, whose death-point was found to be at, or close 
upon, 59 C. After fatigue caused by moderate stimulation, 
however, the death-point was found to be at 56 C., that is to 
say, it had been lowered by 3. 

Working with Datura pistil, the death-point of which was 
never normally lower than 60 C., it was found when fatigued 
to be at 41 C. The lowering in this case was therefore 
about 19. It will thus be seen that fatigue does lower the 
death-point of a plant, the degree to which it does so 
depending on the extent of the fatigue. In the course of the 
present chapter, I shall be able to demonstrate once more the 
lowering of the death-point through fatigue, by means of an 
altogether different mode of investigation. 

Effect of chemical reagents. Similarly I find that 
death-response is modified by the action of various chemical 
reagents. We have seen how characteristic is the thermo- 
mechanical curve of each plant, under definite conditions 
We found, for example, that two styles of two different flowers 
on the same plant, having had the same previous history, gave- 
curves which were practically identical. Specimens thus re- 
sembling each other are not difficult to obtain in spring, when 
there is no sudden variation of weather conditions, or from 
plants grown under glass, in an unchanging environment. The 
characteristics of the thermo-mechanical curve being so con- 
stant, the effect of a given agent will then be indicated by 
certain variations from the normal. Thus, on taking a 
thermo-mechanical curve the specimen used being the style 
of Datura, subjected to the action of a 2*5 solution of copper 
sulphate I found that the form of the curve was much 

N 



178 PLANT RESPONSE 

modified in contrast with the normal curve. The most 
striking difference lay in the lowering of the death point by 
3C. 

Comparison-Morograph. In order to facilitate the 
investigation into the modification of the curve, by various 
agents, I have devised a Comparison- Morograph, by means of 
which the thermo-mechanical curves of two similar speci- 
mens, one under normal and the other under modified con- 
ditions, can be taken simultaneously. We use here two 
recording Optic Levers, supported on a single thermometric 
helix. The normal specimen is placed in the internal 
cylinder in the ordinary way and attached to one of the 
Optic Levers. The second specimen, contained in a small 
cylindrical tube, filled with the given chemical reagent, is also 
placed inside the helix and the plant is attached to the 
second Lever. The spots of light are so adjusted that one 
lies immediately above the other. The two specimens are 
thus subjected to the same temperature-variations, and the 
variation of the second curve from the standard exhibits the 
effect of the reagent. 

I can here barely indicate the very extended line of 
inquiry thus opened out. With regard to the general effect 
of drugs on death-response, it may be said that the displace- 
ment of the rigor- point varies with the tonic condition of the 
tissue, the nature of the drug, and the strength of the solution. 
Out of several possible cases, I shall here give only a few 
simple instances. 

Duplication of rigor-points. One very curious effect 
of certain chemical reagents, such as ether, lies in the ex- 
hibition of two distinct points of rigor, instead, as normally, 
of one. This effect is very easily seen, in the spasmodic 
death response of Mimosa. We take a specimen, and subject 
it to continuous rise of temperature, in water, which contains 
a small quantity of ether. It will be remembered that, under 
normal conditions, the first down movement of the single 
composite spasm of death-response took place in young 
leaves at an average temperature of 59*5 C, the second 



VARIOUS AGENCIES ON DEATH-RESPONSE 



179 



upward movement being at 617 C. Under ether, however, 
we have the peculiar phenomenon of two composite spasms 
separated by an average interval of about 27 C. This 
effect will be understood from the following table, which 
gives the results obtained with two different specimens, B 
and C, the specimen A being heated in water without ether, 
and thus constituted a standard. 

TABLE SHOWING DUPLICATION OF RIGOR BY ETHER 



Specimen 


Preliminary Spasm 


Final Spasm 


Fall 


Erection 


Fall 


Erection 


Under normal conditions 
(Leaf (i) young 
A ( Leaf (2) older 


Absent 
Absent 


Absent 
Absent 


59 C. 
60 C. 


6i-5C. 
62 C. 


After application of ether 
(Leaf (i) young 
B lLeaf(2)ofder 


31 C. 

32 c. 


37 C. 
38 C. 


59-5 C. 
60 C. 


6i-5C. 
62-5 C. 


After application of ether 
rLeaf (i) young 
^ 1 Leaf (2) older 


32 c. 

33 C. 


39 C. 
40 C. 


58-5 c. 

59 C. 


62-5 C. 
64 C. 



Another table is here given, showing the results obtained 
when the water contained a small quantity of hydrochloric 

acid. 

EFFECT OF HYDROCHLORIC ACID 



Specimen 


Preliminary Spasm 


Final Spasm 




Fall 


Erection 


Fall 


Erection 


Leaf (I) 


29-5 c. 


31 c. 


63 C. 


66-2 C. 


Leaf (2) 


29-8 C. 


3i '5 C. 


63 C. 


66-8 C. 



From these results several interesting observations arise, 
the most striking of which is the occurrence of the pre- 
liminary spasm itself, separated by so large an interval from 
the final death- response. This duplication of the rigor-point 



N 2 



ISO PLANT RESPONSE 

is not a distinctive effect of the action of anaesthetics as such, 
since hydrochloric acid and various other chemical reagents 
give a similar result. It would be premature to pronounce 
on the significance of this very suggestive phenomenon. 

A plausible suggestion, which offers itself, is that the 
approach of molecular rigidity concomitant with death, 
which here appears imminent by the action of the reagent, 
as seen in the preliminary spasm, is tided over, or counter- 
acted, by the molecular mobility conferred on the tissue, 
through the rising temperature of the bath. Should this 
inference prove to be correct, these experiments might 
throw an interesting light on an ancient practice, still current 
in India amongst an old class of quack-doctors, by which 
cases of snake-bite are said to be cured, under a treatment 
whose essential feature is the application of hot water and 
steam, with accessory incantations! The % same principle 
may also be the basis of the alleged hot- water and steam 
cures of more modem practitioners. 

The duplication of the rigor-point by the action of ether 
is also noticeable in the thermo-mechanical curve given 
by a radial organ. Thus, in a curve given by the style of 
Datura, the preliminary rigor-point was found to be at 
36-5 C., the second being at 53 C. 

As has been said, the effect produced by various poison- 
ous reagents depends on the tonic condition of the tissue, as 
well as on the nature of the drug. In those cases in which 
the rigor is not duplicated, there is a translocation of the 
death-point, which, as far as I have yet seen, is invariably 
lowered. Thus, in an experiment already described with 
the style of Datura, I found this translocation, under the 
action of dilute copper sulphate solution, to be from the 
normal 60 to 57 C. 

Death-response not due to coagulation. From the 
experiments which have been described, it is evident that the 
death-response, like other modes of excitatory response, is 
appropriately modified by all those influences which affect 



VARIOUS AGENCIES ON DEATH-RESPONSE l8l 

the physiological condition of the tissue. The rigor of 
spasmodic contraction at death is, therefore, not to be 
regarded as due to any coagulative action. And, indeed, the 
theory of a connection between rigor and coagulation is now 
generally discredited. 1 

By the methods described, then, it is possible to study the 
effect of various agencies in the modification of death-response : 
in the case of anisotropic organs, by observation of their 
lateral responsive movements, and in that of radial organs, 
by the translocation of the point of inversion. I was next 
desirous of discovering some still simpler means of determin- 
ing the effects of various conditions in a qualitative manner. 
This might be accomplished if we had a number of organs 
exactly similar to one another, which would give some 
unmistakable sign of death-response, at the point of occur- 
rence, either immediately, or at some definite interval after- 
wards. A certain number of these organs might then be 
taken as standard, and the others subjected to the action 
of various modifying influences. Any differences between 
the temperatures concomitant with post-mortem symptoms 
would now indicate the modifications produced by these 
agents. 

In a certain sense, such an experiment may be carried 
out with a number of leaves on the same plant of Mimosa. 
But in such a case the organs to be compared are not very 
numerous, and different leaves of exactly the same age 
cannot be secured. 

Death-response in flowers. This led me to investigate 
whether, amongst flowers, specimens could be obtained which 
would exhibit a death-movement at the critical temperature. 

1 ' The causes which determine the varying resistance of different plants to 
heat are quite unknown. The fact that a temperature of from 20 C. to 40 C. 
kills certain plants, shows that in their case death is not the result of coagulation 
of the plant-albumin. Further, some plants grow at 75 C., i.e. at>ove the 
temperature at which egg-albumin coagulates. Coagulation need not always 
occur, for we must remember that the acid and alkali albumins are not coagulated 
by heat.' Pfeffer, Physiology of Plants, English edition, 1903, vol. ii. p. 230. 



1 82 PLANT RESPONSE 

And I found that many flowers did so in a marked degree. 
Thus, for example, in two different specimens of Convolvulus^ 
both full-blown, the flower being subjected to rising tempera- 
ture, the corolla-bell folded up at exactly 62*5 C. 

We thus see the possibility of obtaining flowers which, 
having had the same previous history, are likely to exhibit the 
death- movement more or less at the same point. I thought 
that such a collection of similar specimens might probably be 
obtained in a small space, from the capitulum of a composi- 
taceous flower, and as a matter of fact I succeeded in finding 
several. The nature of the movement, whether up or down, 
and its more or less pronounced character, appeared to 
depend in these cases on the age of the flower. 

In connection with this question, we must remember that 
in flowers, as in leaves, we may have in a single specimen 
alternating hyponastic and epinastic growths. It is therefore 
conceivable that the death- movements of old and young 
flowers may take place in different directions, and that at 
some stages there may be little or no motion of any kind. 
However this may be, I have found that in the double Indian 
marigold at the temperature of 62-5 C. the florets arranged 
themselves in two groups, the outer and lower whorls turn- 
ing down, and the younger or central whorls rising up, 
at the critical temperature. In the case of some of the 
large garden daisies, yellow and white, I found the critical 
temperatures to lie between 61*5 C. and 63 C., the death- 
movement consisting of a folding up in some cases, and a 
curving down in others. If the flower have been subjected 
to uniform illumination on all sides, then the movement of 
all the florets will take place within a degree or so. In the 
French marigold, grown in India, the florets of the ray fold 
up, at from 59 C. to 60 C. From these experiments we 
see that, the death-point for all the flowers on the same 
capitulum being about the same, it might be possible to 
treat one-half of the florets of a single flower-head as normal 
or standard, while using the rest for comparative study on 
the influence of various agencies. 



THERMOGRAPHS OF REGIONAL DEATH 183 

Method of thermographs of regional death. But I 

have found out another and distinct method for detect- 
ing the effects of various agencies. And this method is 
not only very interesting in itself, but it enables other 
obscure problems to be attacked in a satisfactory manner. 
It depends on the taking of Thermographs of Regional 
Death. 

It is known that amongst the symptoms which occur at 
some indefinite interval after death is that of discoloration. 
Although this phenomenon is not concomitant with death, 
yet the temperature-interval between the two can in many 
cases be rendered definite. Thus for example, when the blue 
Convolvulus is subjected to rising temperature at the normal 
rate, it shows death-movement at 62*5 C. But there is as 



yet no sign of discoloration. When the temperature, however, 
rises to 70 C. the heating water begins to undergo discolora- 
tion from the escaping cell sap. It would appear probable, 
from various experiments which I have carried out, that 
discoloration does not begin at the point of death-contrac- 
tion, but occurs at or about the point of the subsequent 
relaxation. But in the case of Convolvulus there is no strik- 
ing change seen in the flower itself, for the loss of colouring 
matter is gradual. In the style of Datura alba, however, we 
have a more definite change of colour. This organ, from 
being milk-white, becomes brown at a temperature of 64 C., 
that is to say, 4 above the death-point, when the temperature 
of the bath is rising at the ordinary rate. In the petals of 
Scsbania coccineum> again, under similar conditions, the change 
of colour is very striking. Rich crimson here turns into pale 
blue, at a fairly definite temperature of 67 C. The most marked 
and easily observed of all these changes is seen, however, in 
the mauve petals of Passiflora quadrangularis, which normally 
becomes colourless at a temperature of 70 C. The filamentous 
corona of the same flower again, in which the filaments are 
barred by purple rings at intervals, loses its colour normally 
at 68 C. The death-point of these filaments is, it should be 
remembered, 60 C. We thus find on raising the temperature 



1 84 PLANT RESPONSE 

in each of these cases, at the standard rate of i per 1-5 
minute, that not only is there a definite death-point, 
evidenced by sudden contraction, but that the discoloration- 
point is separated from this, by a definite temperature-interval. 
And since we have found the death-point to be translocated 
by the influence of various agencies, we may expect the 
discoloration-point also to be displaced, in a similar manner, 
under the same conditions. 

Development of thermographs. This being so, it ought 
to be possible to ' develop ' images of local death. We take 
a coloured petal, say of Passifl.ora, and placing two circular 
electrodes diametrically opposite to each other, with the petal 
between, pass tetanising shocks, which are of sufficient 
intensity to fatigue, but not to kill, the tissue. When the 
electrodes are removed, there is nothing by which the eye 
may distinguish the zone of fatigue. In order now to 
develop this invisible picture, we have simply to subject the 
specimen to the ordinary bath, with rising temperature. For 
we have seen, from experiments already described, that the 
power of the tissue to resist death is lowered by fatigue. In 
the case of the present specimen, therefore, the fatigued area 
will die, and undergo subsequent discoloration, earlier than 
the rest of the petal. In carrying out this experiment, the 
area of fatigue was found developed as a white image on a 
purple background, at a temperature of 45 C. It should be 
remembered that, as said before, the lowering of the death- 
point varies with the amount of the fatigue ; hence the point 
of discoloration may be found as low as 40 C. or as high as 
50 C. and upwards. If the petal be removed from the bath 
as soon as development begins, the image will remain. But 
if it be maintained under the rising temperature, the thermo- 
graph will vanish, with the death and discoloration of the 
background. 

Again, I took two similar styles of Datura. One of these 
I kept as standard, and passed tetanising shocks through the 
other. On subjecting both to rising temperature in a single 



THERMOGRAPHS OF REGIONAL DEATH 185 

bath, the fatigued style underwent the change from white to 
brown at 56 C., whereas the test specimen was not discoloured 
until 64 C. 

Determination by thermographic method of relative 
excitatory effects of anode and kathode. This thermo- 
graphic method also enables us to attempt the solution of 
other recondite problems, such as that of the relative excita- 
tory effects of the anode and kathode. It will be shown in 
the next chapter that when the electromotive force is not too 
excessive, it is the kathode which causes excitation when the 
circuit is made or completed. This fact will be demonstrated 
there by experiments undertaken with sensitive plants, in 
which the excitatory effect is indicated by the mechanical 
response of the motile leaflets. No such means is available, 
however, in the case of ordinary, or so-called non-sensi- 
tive, tissues. In such cases, therefore, I shall undertake 
to demonstrate the same fact, but by means of death- 
response. 

We have seen that a tissue which has already been 
excited, is more fatigued than one which has not, and a 
fatigued tissue is, as we have seen, subject to death, and 
subsequent discoloration, at lower temperatures than the 
unfatigued. Hence, if excitation be caused in the kathodic 
region at make, death-discoloration ought to occur there 
earlier than in the anodic. This I have been able to 
demonstrate in the following manner : I took two similar 
petals, or two halves of the same petal, of Passiflora. The 
two were held side by side in a glass vessel full of water, 
at a distance of 3 cm. from each other, the temperature 
being gradually raised. When the temperature is about 
50 C. a current is sent from a battery so that it enters 
by one petal, and leaves by the other. It is now found 
that the discoloration of death takes place earlier at the 
kathode than at the anode. The value of the difference is 
about 4 C. I carried out this experiment on the petals of 
the crimson Sesbania coccineum also. 



1 86 



PLANT RESPONSE 



TABLE SHOWING EFFECT OF ELECTROTONUS ON DEATH-POINT 

Specimen 



1 . Petal of Passiflora 



2. Filament of Passiflora . 



3. Petal of Sesbania . 





Temperature of Discoloration 


Anode 


Kathode 


. 


63 C. 


59 C. 


a . 


60 C. 


56 C. 





64 C. 


6iC. 



The effect here described takes place, as has been said, 
where the electromotive force is not excessive. Under these 
conditions, it is the kathode which is the more excitable. I 
have, however, discovered a very curious case of inversion of 
excitation which occurs when the E.M.F. exceeds a certain 
value. With high electromotive force, then, it is the anode 
w^hich excites at make of the circuit. The demonstration of 
this fact by means of mechanical response, and subsidiary 
proof by means of death-response, will be given in detail in a 
subsequent chapter. 

SUMMARY 

The death-point is lowered by fatigue, the amount of 
lowering depending on the intensity of fatigue. 

The characteristic thermo-mechanical curve is modified 
and the point of inversion translocated by the action of 
chemical reagents. 

Certain reagents produce a duplication of the rigor-point 

The death-point is translocated to a temperature lower 
than normal by the action of poisonous reagents. 

Under standard conditions, there is a definite interval be- 
tween the death point and discoloration-point of vegetable 
tissue. 

Hence it is possible to obtain thermographs of localised 
effects of various agents. 

The excitatory effect of kathode is demonstrated by the 
earlier discoloration produced there. 



PART III 

EXCITABILITY AND CONDUCTIVITY 



CHAPTER XVI 
ON EXCITATORY POLAR EFFECTS OF CURRENTS 

Hydro-mechanical theory of excitation in plantsTheory of protoplasmic change 
Crucial tests applied by means of polar excitation Mono-polar and lii-polar 
methods of excitation Advantages of study of polar excitation in plant-tissues 
as compared with animal Effects of feeble E. M.F. Effect of moderately high 
E. M.F. Experiments with highly excitable tissues. 

HAVING observed, by means of mechanical responses, the 
various excitatory effects which are caused in plants by 
stimulation, and the influence of different agencies in modify- 
ing these excitatory effects, it is now desirable to make an 
inquiry into the manner in which excitation takes place, 
and into the method by which it is transmitted to a distance. 
There has been a great deal of uncertainty regarding this 
subject, and the prevailing view is that which holds the trans- 
mission of excitation to be due to the propagation of hydro- 
static disturbance. 

Mechanical theory. According to this theory, it is 
supposed that stimulus causes a mechanical disturbance, 
bringing about an alteration of the hydrostatic equilibrium. 
The propagation of excitation in plants is thus regarded as 
nothing more than the transmission of this hydro-mechanical 
disturbance. 

We know, however, that the transmission of hydrostatic 
disturbance takes place with relatively great rapidity, while 
these excitatory effects in the case of plants travel sometimes 
as slowly as I mm. or less per second. I have shown, more- 
over, that its responses, both mechanical and electrical, 
are profoundly modified by the physiological condition of the 
plant. There is, for example, an optimum temperature at 
which response is at a maximum, any change, whether above 



PLANT RESPONSE 

or below, inducing depression. Anaesthetics, moreover, tem- 
porarily, and poisons permanently, abolish response. It will 
be shown further, in Chapter XVIII, that the transmission 
of excitation may be very much diminished, or even arrested, 
by the application of cold or ether. 

Theory of protoplasmic change. It is thus seen that the 
hydro mechanical theory is incapable of explaining the facts 
of the case. I shall now, therefore, proceed to demonstrate 
that the excitatory change in plants is brought about in the 
same manner as in animals, and that the transmission of 
excitation depends upon the propagation of protoplasmic 
changes, in the one case as in the other. This may be 
determined by a crucial experiment as to whether vegetable 
tissue exhibits those peculiar polar effects of the electric 
current on excitability, which are seen in the protoplasm of 
animal tissues. In the animal tissue, for example, it is the 
kathode that, under normal conditions, produces excitation, 
the effect of the anode being the reverse. In the case of animal 
tissues, again, the anode will even act as a block to the trans- 
mission of stimulus. 

Crucial tests applied by means of polar excitation. 
Such effects arc incapable of explanation by the hydro- 
mechanical theory, and if we succeed in discovering similar 
phenomena in the case of vegetable tissues, we shall establish 
the existence of a fundamental property of protoplasm 
common to the animal and vegetable alike. With this end 
in view I have carried out numerous experiments on plants, 
both sensitive and ordinary. As specimens of the former 
class, I used Biophytum, Mimosa, and Averrhoa. The in- 
vestigation resolves itself into the determination of the differ- 
ences of excitatory effects, at the anode and kathode, both at 
make when the circuit is completed, and at break when 
it is interrupted. The presence of the excitatory effect is 
indicated in the case of ' sensitive ' plants by the mechanical 
responses of the motile organ. In order to separate the 
effects of the anode and kathode, we may use the Mono-polar 
method, i.e. have one electrode near a motile organ, and the 



EXCITATORY POLAR EFFECTS OF CURRENTS 

other very distant from it (fig. 89). If the plant is not very 
excitable the effect produced at the distant point will not 
reach the motile organ, and we shall obtain the isolated effect 
of a particular electrode. Again, if we wish to observe the 
effects at both the electrodes simultaneously, we may employ 
the Bi-polar method, in which both electrodes will be placed 
at or near the motile organs. The most suitable means for 
the application of electrical stimulus will be either a constant 
electrical current from a voltaic battery, or the discharge from 
a charged condenser. 

We have again to study the respective effects of feeble, 
moderate, and excessively strong electromotive forces. 

In experimenting on polar excitation in animal tissues, 
a nerve-and-muscle preparation is generally used, the ex- 
citation of the nerve being studied by means of the indication 
given by the terminal motile organ, the muscle. On the 
other hand, experimenting on Biophytum for instance, the 
petiole acts as the conductor of stimulus, and is provided 
with-- not a single terminal motile organ, but a number of 
lateral motile organs, viz. the ' sensitive ' lateral leaflets. 
The analogous case in animal tissue would be a hypothetical 
nerve, provided with a hypothetical series of contractile 
muscles attached to it laterally. The relative advantage 
possessed by such a vegetable organ is, that the changes in 
the excitabilities, throughout every portion of the excitable 
conducting tissue, are visibly manifested. 

I experimented altogether on some hundreds of specimens. 
Some of these were very sensitive ; others only moderately 
so. The results under normal conditions were perfectly 
consistent. As it would entail much mere repetition to relate 
every one of these experiments, I shall here give only typical 
instances in detail. While I was studying the effect of the 
establishment or cessation of a constant 'current I made a 
practice whenever the leaves or leaflets recovered within a 
moderate time from the effects of the stimulus of a current 
flowing in one direction of trying a second experiment on 
the same plant, by reversing the direction of the current, so 



192 PLANT RESPONSE 

that in the reversal experiment, what was formerly anode 
became kathode, and vice versa. 

In this way corroborative reversal effects were obtained. 
In experiments with condensers it was not necessary, in order 
to reverse the electrodes, to reverse the battery connection, 
for owing to the special arrangements of the electric circuit 
(fig. 14) the anode at * charge ' became kathode at ' discharge.' 

In studying the effects of increasing intensity, in the case 
of constant current, I simply add to the number of storage 
cells, and in this way obtain increasing voltage. The strength 
of the condenser discharge is increased by increasing the 
voltage of the charging circuit. With the same tissue, where 
the resistance is constant, the current increases with the 
acting E.M.F. Hence, increasing E.M.F. here connotes also 
increasing current. But we may have a very high E.M.F. 
and, owing to high resistance of the tissue, only a feeble 
current. From the trend of the various experiments that I 
have carried out, it would appear that the characteristic polar 
effects are determined more by the intensity of the E.M.F. 
than by that of the current. In the case of the present 
investigation, as we must also bear in mind, the experiments 
were performed with many different specimens, the excitability 
of some being greater than that of others. 

Effect of feeble E.M.F. The first experiments of this 
series were carried out by the method of mono-polar ex- 
citation. The first specimen employed was Mimosa^ one 
electrode, the kathode, being connected with the pulvinus, 
and the anode, at some distance, with the main stem. The 
electromotive force used was ten volts. The leaf-stalk fell 
at make of the circuit. The leaf was found to recover, 
after a due interval, during the continuation of the current. 
The current was now broken, but this produced no responsive 
effect whatsoever. The current was next reversed, the pulvinus 
being made anode. But this anode-make did not produce 
any excitatory effect, neither did the succeeding anode-break. 
From these experiments we see (a) that a feeble E.M.F. 
excites at the kathode at make ; (b) that the excitation takes 



EXCITATORY POLAR EFFECTS OF CURRENTS 



193 




place during the variation of current, but not when the cur- 
rent has attained a constant value ; and (c) that there is no 
excitation at either kathode- break or anode-make or break. 

I next used a specimen vtBiophytum, the E.M.F. employed 
being eight volts. The kathode was at first at the lower end of 
the leaflets, the anode being on the main stem (fig. 89). At 
make there was an excitatory wave at the kathode. This 
travelled outwards and produced depression of four pairs of 
leaflets. On reversing the current, the new anode did not 
produce any effect at make, nor did 
its break produce any excitation. It 
will be shown presently that it -is 
necessary to have a certain moderate 
intensity of current in order that the 
anode-break may cause excitation. 

The Affect on Biophytum of a 
continuous current at the kathode is 
to bring about a more or less pro- 
longed 'contraction,' the period of 
recovery being thereby much pro- 
tracted. With the leaf-stalks of 
Mimosa, however, the effect is not 
so marked. With this plant, never- 
theless, I have been able to observe certain antagonistic effects 
of anodic and kathodic actions ; that is to say, while there is 
slow recovery from kathodic contraction, on reversing the 
current, there is often an impulse of relaxation, the recovery 
being thereby suddenly hastened. But it must be under- 
stood that these particular effects are liable to modification, 
being dependent on the physiological condition of the tissue. 

I next repeated these experiments on the effect of feeble 
E.M.F. producing excitation by means of condenser dis- 
charge. The results obtained were precisely the same as 
with constant current. That is to say, with relatively feeble 
charge, the excitation took place at the kathode at make, 
and not at the anode. The great advantage of excitation 
by the method of condenser discharge is, that the total 

O 



FIG. 89. Diagrammatic re- 
presentation of Mono- polar 
Excitation 

B was kathode, which at make 
gave rise to excitatory 
wave, causing depression 
of four pairs of leaflets. 
The electric connections 
are always made with non- 
polarisable electrodes. 



194 



PLANT RESPONSE 




FIG. 90. Bi-polar 
Excitation of 



quantity of electricity passing through the tissue is very 
small, and the changes produced in the substance of the 
specimen are therefore slight. 

The next group of experiments was carried out by the bi- 
polar method of excitation, which enables us to make simul- 
taneous observations of the effects at anode and kathode. As 

in the previous cases, the specimen used 
for the first experiment was Mimosa^ the 
E.M.F. employed being twelve volts. 
,tr - Connections were made with the pulvini 

of two neighbouring leaves (fig. 90). On 
make, the kathodic leaf-stalk fell ; there 
was no action at the anode. At break, 
there was no action. On now reversing 
the electrodes and making the anode 
kathode, the leaf-stalk which had not 
previously responded fell under kathodic excitation. There 
was no effect on the anodic leaf-stalk, nor was there any 
effect on either at break. 

For the succeeding experiment 1 used a leaf of Bio- 
phylum and an E.M.F. of eight volts, the electrical con- 
nections being as shown in the diagram (fig. 91). On 
completing the circuit, the 

excitation was discharged at 
the kathode, and the wave 
proceeded in both directions 
from the kathodic point, three 
pairs of leaflets being de- 
pressed towards the stem and 
two in the interpolar region. 
There was no effect at break 
at either electrode. On reversal, the new kathode, formerly 
the anode, became the point of excitation, as evidenced by 
the depression of contiguous leaflets. 

Similar results were obtained when excitation was 
produced by condenser discharge. Thus with a condenser 
having a capacity of *oi microfarad, charged to eight volts, 



K 




91. 



Bi-polar Excitation of 
Biophytaw 

At make an excitatory wave proceeded 
in two directions from kathode, 
but none from anode. 



EXCITATORY POLAR EFFECTS OF CURRENTS 



195 



response was observed at the kathode at charge. The 
excitatory wave travelled in both directions, and five pairs 
of leaflets were depressed. There was no effect at the anode. 
At discharge, the former anode became kathode, and there 
was a responsive movement of the leaflets near that point. 

In the following table, the characteristic polar effects of 
feeble E.M.F. may be seen at a glance. 

TABLE SHOWING THE POLAR EFFECTS OF FEEHLE E.M.F. 



Kathode 



Make 



Response 



Break 



No response 



Anode 



Make 



No response 



Break 



No response 



Effects of moderate E.M.F. From this point onwards 
it will be found sufficient to describe the results obtained 
by means of the bi-polar method of excitation, this mode 
of investigation being 
more complete than 
the "mono-polar. 

With regard to 
the first of these, a 
number of experi- 
ments were performed 
on a single specimen 
of Biophytum, using 
an E.M.F. of 24 volts. 
The excitatory wave 
at make was found 
to be initiated at ka- 
thode, and to travel 
in both directions, 
causing the depres- 
sion of nine pairs of 

leaflets. The forward half of this wave of excitation only 
stopped at one pair of leaflets before the anode (fig. 92). This, 




FIG. 92. Make-kathode and Break-anode 
Effects in Biophytum 

(a) Shows effect at make, excitation being 
produced at kathode ; (A) shows effect at 
break, excitation being now produced at 
anode. 



o 2 



J'LANT RESPONSE 

as will be seen later, is due to the depressing action of a strong 
anode. There was no action at the anode itself at make ; at 
break, there was no action near the kathode, but there was 
excitation at the anode, as was shown by the fall of three 
contiguous pairs of leaflets. The direction of the current was 
now changed, the poles being thus reversed, and eight pairs of 
leaflets fell at the new kathode, in and out. There was, how- 
ever, no effect at the new anode at make ; but at break, the 
reverse was the case, leaflets falling near the anode, and no 
response occurring at the kathode. This experiment was 
repeated three times on the same specimen, and the results 
were in every case similar. I give (fig. 93) a pair of records 




FIG. 93. Records of Responses of Leaflet of Biophytiun, showing 
Responses occurring at Kathode at Make and not at' Break ; and at 
Anode at Break and not at Make 



in ttiophytum leaflet, showing the opposite character of the 
effects of make and break at the anode and kathode respec- 
tively. The E.M.F. used in this particular experiment was 
sixteen volts. 

In the former series of experiments, it was seen that 
there was no break-anode effect when the K.M.F. was 
feeble. The present experiments show us that the break- 
anode is effective when the E.M.F. is moderately strong. 

1 obtained similar effects when stimulation was produced 
by means of condenser discharge, the experiments being 
carried out on Biophytum. 

From the investigation just described, it will be seen 
that with moderate FM.F. we obtain response from the 



EXCITATORY POLAR EFFECTS OF CURRENTS 



197 



kathode at make, and from anode at break. The following 
tabular statement exhibits these various effects in a concise 

form. 

TABLE SHOWING EFFECTS OF MODERATE E.M.F. 



Kathode 



Make 



Response 



Break 



No response 



Anode 



Make 



No response 



Break 



Response 



Experiments with highly excitable tissues. In ex- 

perimenting on the polar excitation of animal tissues, using 

a nerve-and-muscle preparation, it is found that when the 

proximal end of the nerve is made kathode, that is to say, 

when the current is ascend- 

ing, the indicating muscle "' 

shows response. This is 

due to the make action of 

the kathode. At break 

also response occurs ; but 

this is due to the trans- 

mitted action of the break 

., ,. r ,1 i :. 4. 

excitation of the distant 

anode. When the current 





Effects of Ascending and De- 
urrents, on Highly Excitable 

Specimen of Mimosa 

In tho % ure to the left, the anode is below 
and kathode above, and current ascend- 
ing. In the figure to the right, the 
kathode below, anode above, and cur- 

ren t descending. 



is reversed that 1S to say 

' \ * 

made to descend, there is 
i ~ ~~* ^^^ 4-^ 4-U^v 

also response, due to the 

make excitation of the 

distant kathode. When the current is broken, response 

takes place again, in consequence of the break of the 

proximal anode. All these cases are rendered possible by 

the high conducting power of the intervening tissue, the 

nerve. 

I have been able to obtain precisely similar results, by 
selecting very highly excitable specimens of Mimosa. One 
electrode was placed at the junction of stem and petiole, the 



198 PLANT RESPONSE 

second being at a distance of about 3 cm. lower on the stem. 
In this case the stem, or certain of its elements, acted as the 
conducting nerve, the leaf serving as the terminal indicator 
(fig. 94). With such an arrangement, using a plant of 
exceptionally high excitability, and E.M.F. of moderate 
intensity, I have obtained the following results : 

1. Current ascending (a). At make, the leaf-stalk fell. 
This was due to the direct make action at the kathode (b], 
At break there was also a response. This was due to the 
transmitted break-anode excitation reaching the leaf-stalk. 

2. Current descending (a). At make, the excitation of 
the distant kathode reached the leaf-stalk, the current at 
the anode not being sufficiently strong to act as an effective 
block, (b) At the stoppage of the current, there was another 
response of the leaf-stalk. This was due to the break effect 
of the anode in the immediate vicinity. 

The following tabular statement shows at a glance the 
effects which are apparent at the terminal organ : 

TABLE SHOWING THE EFFECT OF MODERATE E.M.F. ON HIGHLY 

EXCITABLE MIMOSA 



Asrending 



Make 



Descending 



Break , Make Break 



Response Response Response Response | 

The experiments described above show that the excitation 
produced in plant-tissues by an electrical current is not 
indiscriminate, but selective, or polar, in its action. The 
effects seen here are of precisely the same nature as those 
observed in animal tissues. The exhibition of such polar 
effects completely disproves the hydro-mechanical theory of 
excitation in plants. They point unmistakably, on the 
other hand, to the existence of some fundamental property 
of protoplasm, common to animal and vegetable alike, which 
under normal conditions finds an identical expression in 
the two, of kathodic excitation at make, and anodic at break. 



EXCITATORY POLAR EFFECTS OF CURRENTS 



SUMMARY 

'Polar effects are observed in plants in every way similar 
Fo those obtained from animal tissues. The laws of polar 
excitation in plants are as follows : 

A. With feeble E.M.F. the kathode excites at make, and not 
at break. The anode excites at neither make nor break. 

B. With moderately strong E.M.F. the kathode excites at 
make, and not at break. The anode excites at break, and not at 
make. 



CHAPTER XVII 

ON CONDITIONS OF REVERSAL OF NORMAL POLAR 
EFFECTS IN LIVING TISSUES 

Effect of high E.M.F. - Effects at two stages, A and B Experimental verifica- 
tion of A stage effect Similar effects seen in protozoa Experimental verifi- 
cation of complete reversal at B stage Law of polar effects under high E.M.F. 
Investigation on polar effects by death-response Reversal of polar effects as 
due to fatigue, or tissue-modification Investigation of polar effects by glow- 
response of fireflies. 

THE phenomena ot polar excitation which have been 
observed in animal tissues are summarised in the formula 
which is known as Pfliiger's Law, viz. that excitation takes 
place at the kathode at make, and the anode at break. It 
has been found, however, by Kiihnc, Verworn, and others, 
that in the case of the protozoa the polar effects are exactly 
the opposite; that is to say, in these instances, it is the. 
anode which excites at make. The inference has hence been 
drawn, that Pfliiger's law was inapplicable in the case of un- 
fibrillated protoplasm. 

That this assumption, however, is incorrect, I have 
already shown, by the fact that the undifferentiated proto- 
plasm of the plant-body gives rise to polar effects which are 
in every way identical with the normal polar effects seen in 
animal tissues. 

It occurred to me that the study of polar effects in plants 
might throw some light on this anomaly, and that I might 
thus be able to trace out the stages by which the one effect 
was gradually transformed into the other, determining further 
the conditions which were effective in predisposing a tissue 
towards this reversal. 

Effect Of high E.M.F. In carrying out this investigation, 



REVERSED POLAR EFFECTS IN LIVING TISSUES 2OI 

I soon discovered that the value of the acting electromotive 
force had an important influence on polar excitation. I 
found that under an increasing E.M.K the excitation pro- 
duced at the kathode underwent, first an increase, and then, 
on reaching a maximum, a decrease, which might even 
become negative. The changes produced at the same time 
at the anode were exactly the opposite. There was thus a 
progressive variation, resulting in an exchange of the excita- 
tory properties of the anode and kathode. 

My first observation with regard to this question was 
made in the course of my investigations on the determination 
of the velocity of transmission of excitation (Chapter XX.), 
and on the effect of increasing intensity of stimulation 
on this velocity. I found that, using for instance thermal 
stimulation, when this was strong, it was transmitted with a 
greater velocity than when it was feeble. Hence the speed 
with which the effect of stimulus travels in a given tissue 
may be held to afford a measure of the effective intensity of 
the stimulus. In order, however, to apply a stimulus which 
might be increased by known amounts, I next tried the electric 
mode of stimulation, expecting to produce an increasingly 
effective intensity of stimulus by increasing the E.M.F., excita- 
tion being produced at the kathode at make. In the course 
of a particular experiment, on a leaf of Biophytum, I found 
that as the E.M.F. was augmented from eight to thirty-two 
volts, the excitatory value of kathode at make was also 
increased, as shown by the fact that the velocity of the 
transmission of excitation was raised from 3*27 mm. per 
second, in the former case, to 3*83 mm. in the latter, an. 
increase, that is to say, of 17 per cent. 

But on raising the E.M.F. still higher, I found that, the 
velocity of transmission from this point underwent a progres- 
sive decrease. From this it would appear that the excitatory 
effect of the kathode at make had an optimum value, in this 
case of thirty-two volts, beyond which there was a decline. 
This optimum value would naturally undergo a certain varia- 
tion with the nature and condition of the tissue. Now, as the 



202 PLANT RESPONSE 

excitatory power of the kathode at make is seen to undergo 
a gradual diminution, beyond this optimum, it follows that at 
some certain high E.M.F. the excitation produced by it would 
be zero. In other words, the kathode would cease to excite. 

Possibility of two distinct stages of reversal, A and B. 
Further, since the effects at anode and kathode are, generally 
speaking, contrary in character, we might expect a corre- 
sponding change, but of opposite nature, to make its appear- 
ance progressively at the anode. In other words, it might 
happen that at a certain stage in the raising of the E.M.F. 
the exciting value of the kathode would be considerably 
diminished, and that the anode would begin to show 
excitatory effect. This might be designated as the A stage. 

On still further raising the E.M.F. the same contrary- 
directioned change might be expected to continue progres- 
sively at the anode and kathode, and to reach a stage at 
and beyond which it would be the anode which excited at 
make, while the kathode produced either no excitation or 
actual depression. This might be designated as the B stage. 
We should then have a complete reversal of the normal 
polar effects. 

If we exhibit these inferences, as to the relative excitatory 
powers of anode and kathode with increasing E.M.F., by 
means of curves, whose abscissae represent the E.M.F. while 
their ordinates give the corresponding excitatory values, 
the kathode curve would first rise to a maximum, and 
then fall continuously, till, reaching the zero line, it might 
even proceed still further fn the negative direction, thus 
representing depression. The anodic curve, on the contrary, 
would at first descend in the negative direction, thus indicat- 
ing increasing depression of excitability^ until it reached a 
negative maximum, after which there would be a reversal, 
and it would begin to ascend and -reach the zero-line. Here 
the anode would cease to depress. After this it would 
proceed upwards in the positive direction, indicating a con- 
tinuously increasing power of excitation. The anodic and 
kathodic curves in the course of this ascent would cross at a 



REVERSED POLAR EFFECTS IN LIVING TISSUES 203 

certain point, indicating that both of them now excited in 
about equal degrees. This would constitute the A stage. 
Beyond this would be the B stage, where the anode alone 
would excite. 

I now proceeded to subject these theoretical inferences to 
the experimental test The special difficulty of this investiga- 
tion lies in the fact that it is necessary to discriminate the 
direct from the transmitted effect at the two electrodes, with 
absolute certainty. For if the two points be not at a sufficient 
distance, and if the conductivity of the intervening tissue be 
great, the true effect of one electrode may be rapidly trans- 
mitted, and appear at the other. For these reasons, the 
plant Riophytum is not in this case a very suitable subject for 
experiment, its conductivity being great, and its opposite 
leaves not at a sufficient distance from each other. Mimosa, 

^ 

however, may be made to serve the purpose, for, though its 
conductivity is great, it is possible to select two leaves on 
different branches of the same plant which are very far 
apart. The plant Averrhoa is also appropriate, its conduct- 
ing power being relatively slight. Electrical connections 
may, in this case, also be made with opposite leaves widely 
apart. In the case of Mimosa the excitatory effect is made 
visible by the fall of the leaf, in the case of Averrhoa by the 
depression of the leaflets. It is thus possible to render the 
effect of the two electrodes mutually distinct. It is also 
possible to distinguish the transmitted excitation, if any, by the 
serial depression of the intervening leaves or leaflets during 
the passage of the wave of excitation. 

As these excitatory effects are dependent on the physio- 
logical condition of the tissue, we should expect that the 
E.M.F. which produces reversal would vary with different 
plants and their physiological conditions. The highest constant 
E.M.F. available for my own investigations was 220 volts, 
that being the pressure in the street-mains. I therefore 
hoped that I might be fortunate enough to find plants in a 
state to exhibit the expected reversal within this value. I shall 
now proceed to describe actual experiments with various 



2O4 PLANT RESPONSE 

plants, and first I shall take those in which the A stage was 
exhibited, that is to say, those in which the anode as well as 
the kathode showed excitation at make. 

Experimental verification of A stage effects. With 
the plant Biophytum^ I have always found, without exception, 
that up to thirty-two volts, or thereabouts, the polar effect was 
normal ; that is to say, excitation was produced at the kathode 
at make and not at the anode. On using an E.M.F. of forty- 
eight volts, however, with a certain specimen, I obtained 
excitatory response at make, at both anode and kathode. 
That this anodic effect was not due to transmission of ex- 
citation from the kathode, was seen in the fact that some 
of the interpolar leaflets were not affected, as all would have 
been had the wave of excitation passed from kathode to 
anode. 

I shall next describe experiments made on Mimosa, in 
which, as has been said, the two electrodes can be separated 
by a longer tract of tissue. In the case of this plant, the 
value of the E.M.F. which is required to bring on the A stage 
effect, is much higher than in Biophytum. I have occasionally 
obtained it with no, but more usually with 220 volts. In 
order to show how at this stage the anodic and kathodic 
effects tend to become interchangeable, 1 shall describe three 
experiments. 

In the first of these, an E.M.F. of 1 10 volts was used. At 
make, the kathodic leaflets fell energetically, while the anodic 
fell but slightly, and after a little delay. Here we see that 
though reversal is setting in, yet the normal kathodic effect 
is relatively predominant. 

In the second of these experiments, I used 220 volts. 
The anodic fall now took place slightly earlier than the 
kathodic. The current was maintained till the leaflets 
recovered. On now breaking the circuit, there was a slight 
anode-break excitation, but none at the kathode. In this 
case, though from the slight priority of the anodic excitation 
we infer some predominance of the anode, yet the fact that 
the effect at break is normal shows that we are still in the 



REVERSED POLAR EFFECTS IN LIVING TISSUES 2O5 

transition stage. In the next experiment, the tendency to 
reversal will be shown to have become predominant. 

In this third experiment with another specimen, an 
E.M.F. of 220 volts was again used. At make, there was 
an immediate energetic fall of the anodic leaflet, while that 
at the kathode was slight, and delayed for some time. At 
break, moreover, there was no effect on the anode, and a 
slight and delayed excitatory effect was distinctly perceptible 
at the kathode. From this we see that the anode is now 
appropriating the normal action of the kathode, and vice versa, 
reversal having set in unmistakably. 

Reversed action in protozoa. These experiments will 
probably be found to explain the assumed anomaly in the 
case of protozoa. In experimenting on Actinosphcerium, for 
example, Verworn found that ' at closure of the current in 
the first place, the pseudopodia, both on the anodic and 
kathodic side of the globular body, become varicose and 
begin to contract. If the circuit be opened the pseudopods 
on the kathodic side become varicose in about the same 
degree as had taken place immediately after the closure of 
the circuit.' 1 In this experiment, where both anode and 
kathode exhibit excitation at make and only the kathode at 
break, we have a case exactly parallel to that of the third 
experiment with Mimosa, which has just been described. 
That, as in the case of plant-tissues, a fairly high E.M.F. was 
instrumental in producing reversal appears probable, from 
the fact that it is specially mentioned in the account of the 
experiment, that * in consequence of the high resistance in the 
circuit, a comparatively high E.M.F. had to be used.' 

Experimental verification of B stage effect. I could 
not obtain with Mimosa at 220 volts complete cessation of 
excitation at the kathode at make, but I succeeded in doing 
so with Averrhoa^ in the autumn and winter seasons. With 
this plant, I observed all these A stage effects, which have 
already been described in the case of Mimosa ; and that of 
completed reversal, or the B stage effect, was obtained in 

1 Biedermann, Electro- Physiology, English edition, 1896, vol. i. p. 302. 



206 



PLANT RESPONSE 



more than a dozen instances, out of which I shall give an 
account of two. 

In the first of these, the two electric contacts were made 
on the same leaf, at a distance of 5 cm. from each other. At 
make, using an E.M.F. of 220 volts, excitation was produced 
at the anode only, and the depression of successive leaflets 
proceeded towards the kathode, but was arrested at one pair 
in advance of that point, the kathode apparently acting here 
as a depressor. 

In the second experiment, the electric contacts were made 
at a great distance from each other, with middle points of two 
opposite leaves. At make, excitation was produced at the 
anode only. At break, however, it took place at the kathode 
and not at the anode. We have here a complete reversal 
of the normal polar effects under the action of very high 
E.M.F. 

The following tabular statements show at a glance the 
polar effects at both A and B stages, under a high E.M.F. : 

TABLE SHOWING EFFECT OF HIGH E. M.F. A Stage. 



Kathode 



Anode 



Make 



Break 



Make 



Break 



Moderate response 



i 

Occasional response Moderate response Occasional response 



TABLE SHOWING EFFECT OF EXCESSIVELY HIGH E.M.F. B Stage. 



Kathode 



Anode 



Make 
No response 



Break 
Response 



Make 
Strong response 



Break 
No response 



Law of polar effects under high E.M.F. We have now 
traced out that process of continuous change by which under 
a gradually increasing E.M.F. there is produced a reversal of 
normal polar effects, and we thus arrive at the following law 



REVERSED POLAR EFFECTS IN LIVING TISSUES 2O/ 

Under high E.M.F. at the A stage both anode and kathode 
excite at make; at break there is occasional excitation at 
either anode or kathode. Under excessively high E.M.F. at the 
/> stage the anode excites at make and the kathode at break. 

Without this addition the law of polar excitation is in- 
complete ; and I shall have occasion, in my work on the 
Electro-Physiology of Plants, to show its application in 
explaining certain excitatory electromotive phenomena which 
would otherwise have remained obscure. 

Investigation on polar effects by death-response. 
I have already explained in Chapter XV. that the death- 
point of an excited tissue is lowered below the normal. This 
made it possible to devise a test by whose means it might be 
determined which of the two electrodes produced excitation. 
Thus, on taking two similar petals of Passiflora, and making 
one anode and the other kathode, it was found with a 
moderate E.M.F. that death-discoloration took place at the 
kathode, at a temperature of 4 C. lower than at the anode 
(p. 185), thus proving that under these conditions it was 
the kathode which produced excitation, I was now desirous 
of rinding out whether the same test could not be applied to 
the demonstration of the reversed effect due to high E.M.F., 
and in this connection I shall give an account of an experi- 
ment on the coloured petals of Sesbania coccineum, the death- 
discoloration of which occurs normally at 65*5 C. Taking 
two similar petals, and using the high E.M.F. of 220 volts, 
I found on sending a current that death-discoloration took 
place at the anode, at 60 C., that is at 6 below the normal. 
The discoloration point of the kathode was also lowered, 
but only slightly, being 2^ below the normal. We thus see 
that with a high E.M.F. it is the anode "which is more ex- 
citable at make. It is clear from this that the reversal of the 
normal polar effect has set in, the anodic excitation being 
considerably predominant. 

Reversal of polar effects as due to fatigue or tissue- 
modification. It has now been demonstrated that an exces- 
sively strong E.M.F. is one of the conditions by which the 



208 PLANT RESPONSE 

reversal of normal polar effects may be brought about. We 
shall next study other circumstances which may also be 
efficient to induce this reversal. This subject assumes the 
greater importance from the difference of opinion which exists 
among investigators in animal physiology as to the possi- 
bility of such reversal. The question has not yet, as. far as 
I am aware, been definitely settled. Thus, ' Aeby thought 
he had proved that under certain conditions, more par- 
ticularly with progressive fatigue of the preparation, the 
normal reaction in which the excitatory action of the 
kathode far exceeds that of the anode was exactly reversed. 
Aeby's experiments, however, are by no means unimpeach- 
able, as both Engelmann and Hering pointed out later. 
Engelmann, also, came to the conclusion later, that such a 
complete reversal of phenomena (i.e. of the. law of polar ex- 
citation) might take place. But until it has been determined 
by unexceptional experiments, there must be great scepticism 
in regard to such statements.' l 

We now turn our attention to that of the changed 
condition of the tissue by which the normal polar response 
may become reversed, and in this regard the experiments 
which I shall describe are very instructive, as these changes 
are there seen to gccur progressively. I took a specimen of 
Mimosa and carried out on it five consecutive experiments. 
The two electrodes were attached to the pulvini of different 
leaves on the same stem, and the E.M.F. used was fifty volts ; 
an interval of about seven minutes was allowed in each case 
for recovery. For easy inspection, the results are given in 
somewhat tabular form. 

(1) At make Leaves fell both at kathode and anode. 
The kathodic fall was earlier and more energetic. 

At break No decisive effect observed at either 
electrode. 

(2) At make The kathodic leaf fell, and the anodic fall 
was slight. 

At break No action at kathode, but energetic fall 

1 Biedermann, Efatro-PhysiolQgy, English translation, 1898, vol. i. p. 271. 



REVERSED POLAR EFFECTS IN LIVING TISSUES 209 

at anode. The action of anode-break was here much 
stronger than that of kathode-make. 

(3) At makeFatt of kathodic leaf; no action at anode. 
At break No action at kathode ; response at anode. 

(4) At make Kathodic action became feeble, and anode- 
fall, though at make, the more pronounced of the two. 

At break No action at either electrode. 

(5) At make. No action at kathode; feeble action at 
anode. 

At break No action at cither electrode. 

In tracing out the changes which are here taking place at 
each electrode, we are struck by their progressive character. 
If we fix our attention first on the kathode, we find that the 
normal effect in the first of the series is gradually, diminished, 
till it disappears in the last. Again taking the anode, we 
find a still more remarkable change, of a periodic character. 
In the first experiment, we observe the most pronounced 
abnormality, or reversal of the series, inasmuch as there was 
response at make and none at break. In the second, the 
response is tending towards normal, the anode-make effect 
being feeble, and the break strong. In the third, the anodic 
response has become normal, for there is no action at make, 
but excitation at break. In the fourth, we again see a ten- 
dency towards reversal, inasmuch as again there is response 
at make and none at break. The same state of things, 
though in a less degree, occurs in the fifth experiment. 

We have thus observed two different conditions, each of 
which may contribute to produce this reversal of polar effects. 
These are, firstly, the influence of a high E.M.F., which, at or 
beyond a certain critical value, will produce reversal ; and, 
secondly, certain tissue-modifications similar to those which 
we have observed during the progress of fatigue. It is clear 
that with light tissue-modification the critical value of the 
E.M.F. at which, under normal conditions, reversal of polar 
effects would take place, will be lowered. This, the experi- 
ments on Mimosa just described clearly show ; for in them 
we see that reversal has set in at the relatively low E.M.F. of 



2IO PLANT RESPONSE 

fifty volts, whereas normally in Mimosa the critical value is con- 
siderably above a hundred volts. These tissue-modifications 
sometimes proceed so far that I have occasionally observed 
reversal in the case of this plant even with a moderate 
E.M.F. 

I was next desirous of determining whether these different 
types of polar effects normal, transitional, and reversed 
could not be demonstrated in some novel and striking manner, 
in the case of animal tissues. It occurred to me that the 
intermittent flashes of light emitted by the firefly might 
be simple expressions of rhythmic excitation, a subject 
which will be dealt with in detail in Chapter XXIII. The 
emission of light, or an increased intensity of emission on 
the part of the insect, would in that case be indicative of 
the state of excitation, and this mode of excitatory expression 
I shall designate as glow-response. 

Investigation of polar excitation by glow- response. 
I may here state in anticipation that I have succeeded in 
demonstrating, by means of this glow-response, all the prin- 
cipal characteristic effects of (a) normal response, due to 
moderate electromotive force ; (/>) the reversed effect due to 
high electromotive force ; and (c) the reversed effect due to a 
modified condition of the tissue. It may be pointed out 
further, that some specimens gave the normal, and others, 
owing to a modified condition of the tissue, the reversed 
effect ; but that the results obtained from any given in- 
dividual were always consistent and characteristic. 

I shall first describe certain results which were frequently 
observed, and which are entirely analogous to those described 
in a previous chapter as given by a nerve-and-muscle pre- 
paration, and highly excitable tissue of Mimosa (p. 197). We 
there saw that while the current was ascending, the excira- 
tion exhibited by the terminal organ at make w,as due to 
direct action of the proximal kathode. Excitation was also 
produced at break, and this was due to the transmission of 
the distal anode-break effect. Again, when the current was 
reversed, excitation was exhibited in a corresponding manner, 



REVERSED POLAR EFFECTS IN LIVING TISSUES 211 

through the action of the distal kathode make and the 
proximal anode-bfleak. 

The firefly under natural conditions emits flashes of light 
at intervals of about three seconds, from two discs, situated 
on the ventral surface of its tail, 

We select a specimen and make suitable electrical con- 
nections, one with the bead, and the other with the luminous 
disc. The natural luminescence of the insect is moderate 
and intermittent ; but on now passing through it a descending 
current from a battery having an E.M.F. of twelve volts, the 
light at once becomes persistent and very brilliant. We 
must bear in mind that the luminous discs stand here in the 
place of the terminal motile indicator, of the ncrve-and- 
muscle or Mimosa preparation, and that the state of excitation 
is indicated in them by the increase of luminescence instead 
of by an excitatory movement. This glow-response, then, is 
due to the action of the proximal kathode-make. The induced 
brilliance slowly dies down, and in the course of a minute and 
a half becomes very feeble. If the circuit be now broken, 
a single intense flash is produced, due to the excitation of 
the distal anode-break. The insect now recovers from the 
state of induced excitation, and begins once more to exhibit 
its natural intermittent flashes. We next pass the current 
in the reverse, that is to say ascending, direction. The light 
again becomes persistent and brilliant, owing to the excitatory 
action of the distal kathode-make. During the continuation 
of the current, the light wanes and becomes feeble. But when 
the circuit is broken, there is once more seen a single flash of 
intense light, due to the action of the proximal anode-break. 

In order to ensure a simpler condition for experiment by 
eliminating the nervous conduction of excitation, I next 
isolated the double disc, and found that the detached ot^an 
maintained its excitability for a couple of hours or more. 
The discs now emitted a light which was somewhat feeble 
but not intermittent. Electrical connections were then made 
with the two discs, by means of fine cotton threads, moistened 
with saline solution, and an E.M.F. of sixteen volts was used. 

P 2 



212 PLANT RESPONSE 

At make the kathodic disc was found to become very 
brilliant, and there was no effect on the anodic. In some 
instances, indeed, the anode became dimmer than usual, thus 
showing the depressing influence of the anode. At other 
times, again, the luminous excitation of the kathode irradiated 
and encroached upon the anodic region. At break it was 
the anode which flashed out, showing excitation. These 
results, as will be seen, are entirely normal. I shall next 
describe experiments which illustrate the reversed effect 
sometimes observed with excessively high E.M.F., and at 
other times due to a modified condition, of the tissue. 

With regard to the production of the reversed effect under 
a high E.M.F., some difficulty is encountered owing to the 
proximity of the two discs of the luminous organ. The effect 
of one electrode is thus liable to encroach on the region of the 
other. But specimens are occasionally obtained in which, the 
conducting power of the tissue being feeble, each effect is 
practically confined to its own area, though the excitatory 
E.M.F. may be high. 

In the following investigation it is to be noted that 
successive experiments were carried out on the same speci- 
men, without disturbing the electrodes. By proper manipula- 
tion of the key, the current was made to flow now in one 
direction, then in another, or the acting E.M.F. was changed 
from low to high at will. The differences of the results 
observed must therefore have been due, cither in the first 
case to the reversal of anode and kathode, or in the second 
case to the difference in intensity of the E.M.F. 

A specimen was taken of the detached luminous organ, 
and electrical connections were made with the two discs, by 
means of moistened threads. An E.M.F. of ten volts was 
first used, and the effect at make was a brilliant illumination 
of the kathode-disc. During the continuation of the current 
this gradually waned, but at the break of the circuit a 
brilliant flash appeared at the anode. Thus we have, in the 
present case, the normal effect with moderate E.M.F. I next 
used with the same specimen the high E.M.F. of fifty volts. 



REVERSED POLAR EFFECTS IN LIVING TISSUES 21$ 

The luminescence at make now took place at the anode, 
and at break at the kathode. On reversing the current, the 
new anode, formerly kathode, gave responsive illumination, 
and at break the new kathode responded. In these results, 
therefore, it will be seen that we have an instance of reversal 
of polar effects, under excessively high E.M.F. 

These reversed effects are usually observed with a high 
E.M.F. ; but sometimes, as has been said, owing to a modified 
condition of the tissue, they may be obtained, under the 
action of even a moderate E.M.F. I shall now give a very 
interesting example in which we can trace the process of 
reversal owing to the modification induced by fatigue, in a 
manner somewhat similar to the last experiment described in 
the case of Mimosa (p. 209). 

I took a fresh specimen of the detached organ, and carried 
out four successive experiments on it, observing the effects 
at both make and break, the E.M.F. used being twenty volts. 
In order to present these results at a glance, I shall again put 
them in a somewhat tabular form. 

(1) At make Luminous response at kathode, which 
irradiates slightly towards anode. 

At break Little effect at anode, but natural luminosity 
of the kathode falls below par. This shows the depressing 
action of kathode-break. 

(2) At make Luminous response at both anode and 
kathode. 

At break Luminous response at kathode only. These 
effects, especially that of break -excitation at kathode, show 
that the condition of reversal has set in. This will become 
still more pronounced in the succeeding experiments. 

(3) At make Luminous response appears at anode and 
irradiates slowly towards kathode. 

(It will be seen that we have here a complete reversal of 
the effects observed in (i) at make.) 

At break No immediate effect is at first observed ; 
later, a flash passes from anode to kathode. 

(4) At make Luminous response at anode. 



214 PLANT RESPONSE 

At break No effect at anode, but feeble augmentation 
of luminosity at kathode. 

These results afford us some insight into that obscure 
phenomenon of the modified condition of tissue by which 
reversal of response is brought about. 

We have seen that in the case of Biophytum, the polar 
effects are always found to be normal, within rather a wide 
range of E.M.F., that is to say, up to about thirty volts. The 
kathode here excites at make, and the anode at break. I 
have carried out several hundreds of experiments with this 
plant, but have not once come across any deviation from this 
normal action. 

As the E.M.F. was progressively increased, however, we 
found in this and other plants a tendency towards the 
reversal of these normal polar effects. During the first, or 
A, stage of this reversal, the excitatory value of the kathode 
was seen to undergo a diminution, and the anode, which 
normally had a depressing influence, was observed to have 
its property reversed, and to produce excitation. The result 
during this stage, therefore, was the exhibition of excitation 
at both kathode and anode at make. 

With still higher E.M.F. the B stage was reached, and 
here there was a complete reversal of the normal polar effects. 
It was then found that the anode produced excitation at 
make, and the kathode at break. This reversal of polar 
effects under a high E.M.F. was further demonstrated by 
means of Death-response in plants, and Glow-response in 
animals. 

We have also seen that in consequence of progressive 
molecular change induced by fatigue, the normal polar effect 
tended to be reversed, and we have been able to trace the 
successive stages of such a reversal, in experiments on the 
plant Mimosa and on the firefly. 

And, finally, specimens are occasionally found which, 
owing to molecular modifications of their tissues modi- 
fications that a knowledge of their previous history could 



REVERSED POLAR EFFECTS IN LIVING TISSUES 21$ 

alone enable us to explain tend to exhibit abnormal polar 
effects. 

SUMMARY 

Under high E.M.F. the normal polar excitation tends to 
be reversed. In the A stage, both the anode and kathode 
excite at make, and either kathode or anode at break ; in the 
B btage that is, with excessively high E.M.F. it is the 
anode which excites at make, and the kathode at break. 

The firefly under excitation exhibits glow-response. 
Under moderate E.M.F. it shows normal polar effects. 
Under a high E.M.F. it, like the plant, exhibits a reversal 
of these polar effects. Under fatigue, or other tissue- 
modification, normal polar effects tend to undergp reversal. 



CHAPTER XVIII 

ON CONDUCTIVITY AND EXCITABILITY 

Receptive excitability, conductivity, and motile excitability - Molecular model 
Modification of motile excitability : (a) by anesthetics (b) by cold (c) by 
fatigue Variation of conductivity : (a) by cold (k] by rise of temperature 
(r) by fatigue -(</) by anaesthetics Variation of receptive excitability by ether 
--Conductivity versus excitability Abolition of motile excitability without 
alxdition of conductivity Hydro-mechanical theory of transmission of stimu'us 
untenable. 

HITHERTO we have been concerned mainly with the pecu- 
liarities of the responding organ, by which the state of 
excitation is outwardly manifested. Very often, the respond- 
ing organ is not directly stimulated, but a distant point is 
acted on by stimulus, and the state of excitation is trans- 
mitted through the intervening distance, by the conducting 
power of the tissue. In the actual life of a plant it is 
frequently the case that the stimulus impinging on a recep- 
tive area is transmitted along conducting channels, and is 
manifested, on reaching some responsive organ. The whole 
cycle of events is something like a telegraphic circuit, in 
which the message taken at a transmitting station is sent to a 
distance along conducting wires, and produces a signal at the 
distant or responding station. 

In our experiments, for example, on Biophytum^ as given 
in the previous chapter, the stimulus was applied at the pe- 
tiole, and was conducted along certain channels. On reaching 
the specialised motile organ the pulvinus this transmitted 
stimulus caused a responsive depression of the leaflet. The 
petiole, during conduction of stimulus, was excited, but there 
was no conspicuous external evidence of this, because, first, 
the contractility of the tissue was relatively feeble, and 



CONDUCTIVITY AND EXCITABILITY 2 17 

second, the differential excitability, on which responsive 
curvature depends, was also slight. It is only at the pul- 
vinated organ that the state of excitation is conspicuously 
exhibited by motile response. 

In the case of the plant, therefore, we have to study the 
excitable property of the receptive area, the conducting 
property of the transmitting tissue, and that property of the 
responding organ by which the excitatory effect is outwardly 
manifested. It will be convenient to distinguish the excit- 
ability at the point of application of stimulus from that of 
the motor region, by using a specific term for the former. I 
shall therefore designate it as receptive excitability, or merely 
as receptivity, whereas the excitability of the motor region 
will be described simply as excitability. At the point of 
application, the stimulus comes from outside, and produces 
internal changes. In the motile region, the internal excita- 
tory disturbances are manifested outwards. In the physio- 
logical study of excitation, some confusion is apt to arise 
from the failure to discriminate between these three factors. 
And this confusion becomes greater in those cases in which 
the area of motile excitability coincides with that of 
receptivity. 

Molecular model. The state of excitation being ulti- 
mately due to molecular upset from the position of equi- 
librium, we can understand that such a disturbance is 
propagated from molecule to molecule, till it reaches the 
responding organ. We may, perhaps, be enabled to 
visualise this better by means of a mechanical model. The 
individual molecules in our model should hold a position of 
stable equilibrium. When disturbed from this stable poise, 
they should return automatically to the equilibrium position ; 
and further, the derangement of one molecule should cause a 
subsequent disturbance of the next, and this disturbance 
should be transmitted from point to point 

These conditions are realised in the case of the following 
model, which consists of a row of small suspended spheres of 
cork, within each of which is placed a magnetic needle. Each 



2l8 



PLANT RESPONSE 



sphere is now in stable equilibrium, under the directive action 
of the earth, and the mutual action of the needles ; hence the 
north pole of each needle, represented by the arrow-head, 
points to the north, which is, say, to the left. The disturb- 
ance of any individual sphere, say E, brings about the 
disturbance of its neighbour, and, owing to the mutual 
magnetic action between contiguous north and south poles, a 
derangement initiated in this way is transmitted onwards. 
Such a disturbance may be initiated by means, for instance, of 




Vic,. 95. Molecular Model Exhibiting (<r) Excitability at the Receptive 
Area ; (/>) Conductivity of Intervening Region ; and (< ) Mechanical 
Response of Terminal Responder 

Disturbance is initiated at the sphere connected with E, by the magnetic 
action of the electro-magnet seen to the right. This disturbance is con- 
ducted by the intervening spheres and reaches the terminal responder, R. 
Molecular viscosity is increased by immersion of attached dampers in 
viscous fluid. 



a small electro-magnet, placed at right angles to the molecular 
magnet in E. This electro-magnet is magnetised for a short 
time by the tapping of a key, which closes an electric current, 
causing a rotation of the sphere E. The intensity of this 
disturbing force, the stimulus, may be increased at will, by 
appropriate exaltation of the strength of the magnetising 
current (fig. 95). 

In such a row of molecules, then, that to the extreme 
right, E, is the point at which we shall initiate molecular 
disturbance. That is to say, it corresponds to the receptive 



CONDUCTIVITY AND EXCITABILITY 219 

point The intermediate row, C, is the conductor of disturb- 
ance ; and the last molecule, R, which may be provided with 
an index, or a reflecting mirror, by means of which the dis- 
turbance can be made conspicuous, represents the motile 
responder. 

We shall next observe how the extent of the distortion of 
each of the molecules from the position of equilibrium by a 
given force that is to say, the amplitude of its response 
is modified by the factor of molecular mobility. Under the 
action of certain agencies the freedom of molecular movement 
may be retarded, by variation'] of elasticity or of viscosity. 
We may, with our model, imitate the resultant molecular 
sluggishness, by means of dampers, which are seen in the 
diagram, attached to each sphere. The extent of damping is 
capable of increase by immersion of the damper in a viscous 
fluid. The response-curve of this particular sphere may now 
be taken by the usual method of a reflected spot of light. 
The curves thus obtained will show, firstly, that, the disturb- 
ing force remaining the same, diminished molecular mobility 
is attended by diminution of amplitude of response ; secondly, 
that this diminution may become so marked that visible 
response may disappear ; thirdly, that though, with a given 
moderate disturbance, response may thus be in abeyance, yet 
it may be restored if the disturbing force be made sufficiently 
strong ; and fourthly, that the sluggishness thus induced may 
also be exhibited by delay in the initiation of response, that 
is to say, by the prolongation of the latent period. 

From such considerations, it is clear that if an agency 
which reduces molecular mobility be applied on the receptive 
area, then, inasmuch as the initiation of excitation is prevented, 
there will be no response exhibited by the motile organ, 
although the conducting power of the intervening tissue, 
and Ae motility of the responding organ, remain unchanged. 
Again, if the intervening conducting tissue be subjected to 
loss of molecular mobility by any means, the power of 
conduction will be either very much retarded, or abolished, 
the receptivity and excitability of the terminal points 




22O PLANT RESPONSE 

remaining unaffected. And, finally, the excitability of the 
motor region may be depressed by certain agencies, and 
the stimulation, initiated at the receptive point, and trans- 
mitted through the intervening conducting channels, will 
nevertheless fail to find expression. We shall next proceed 
to demonstrate experimentally the influence of various 
agencies on the receptivity, on the conductivity, and on the 
excitability of the tissue. 

Variation of motile excitability : (a) Under anesthetics. 
First we shall take the variation of excitability in the motor 
region. Let us then select a leaf of Biophytum and apply 
ether to the two terminal pairs of leaflets beyond D. 
Thermal stimulus is then applied at x , by touching with a 

hot wire (fig. 96). As the 
receptivity of the point of 
application, and the con- 
ductivity of the intervening 
"'" 9< " K c?oaL aS&ft Ab0 ' Ui0n tteue, remain unimpaired, 
Ether is applied to the two pairs of leaflets the excitatory disturbance 
to the right of i> ; stimulus is applied proceeds in the normal 

at x . Excitation travels up to D, and , . 

cannot pass beyond. manner to the point D, a 

fact seen by the successive 

depressions of the leaflets. Owing, however, to the abolition 
of their excitability, the last two pairs remain unaffected. 

A similar loss of excitability, due to the action of ether, 
may be demonstrated in Mimosa. On taking a stem pro- 
vided with three motile leaves, A, B, and C, the pulvinus of B 
is touched with ether, and thermal stimulus is applied 
between A and B. The excitation is transmitted in both 
directions, up and down, as seen by the fall of the leaves A 
and c. But the intermediate leaf B fails to respond, showing 
that its excitability has been abolished by the ether. 

(b] By effect of cold. The prolonged applicatidh of 
cold, also, will produce, as would be expected, molecular 
sluggishness, with consequent loss of motor excitability. 
This may be shown by touching the small pulvinus of a 
leaflet of Biophytum with ice- water. If stimulus now be 



CONDUCTIVITY AND EXCITABILITY 221 

applied on the petiole, it will be found that this particular 
leaflet will not respond. This loss of excitability will, how- 
ever, be temporary, disappearing as the leaflet returns to its 
ordinary temperature, when it will be found to respond as 
usual. 

A moderate application of cold does not altogether 
abolish the response, but the molecular sluggishness induced 
is shown in the prolongation of the latent period of response. 
It was found, for example, in an experiment on Biophytum 
that the latent period was sometimes prolonged by several 
seconds (p. 268). 

(c) By effect of fatigue. We have already seen (p. 113) 
how the motile excitability of the plant-tissue is diminished 
by fatigue, as shown in the diminution of successive responses, 
when the intervening periods of rest are not sufficient for 
complete recovery. We have seen, too, that under strong 
and long-continued excitation the motile excitability is 
abolished ; and that it can be restored after the lapse of a 
sufficiently long res-ting period. 

Variation of conductivity We shall next examine how 
the transmission of stimulus from point to point is affected 
by various external agencies. And, first, we shall refer back 
to the mechanical model (fig. 95). We there saw how the 
sluggishness, induced in the intermediate molecules by 
plunging the dampers to a greater or less depth in a viscous 
liquid, retarded the transmission of disturbance through them. 
When this induced sluggishness is slight, the propagation will 
merely be slowed below the normal ; but when the sluggish- 
ness induced is great, the disturbance will not reach the 
responder R. 

(a) By effect of cold. We shall now proceed to investi- 
gate the effect of induced molecular sluggishness on the 
conductivity of a plant-tissue ; and for this purpose we 
shall first observe the influence of cold. In an experiment 
on Biophytum, I found that the normal velocity of trans- 
mission, depending on the conductivity, was 37 mm. per 
second ; but on subjecting the tissue to moderate cold, the 



222 



PLANT RESPONSE 




Flo. 97. Experimental Demonstration 
of Effects of Cold and AnrestliHics 
in Abolishing Conductivity 

Cold or ethi'i applied at K; stimulus 
at x cannot be transmitted across R, 
and there is no effect on the 
motile leaflets. 



velocity of transmission was reduced to 1*3 mm. per 
second, or nearly to one-third of its original value (p. 249) ; 
a still greater application of cold produces a temporary 
abolition of conductivity. This may be shown by touching 

a given portion, E, of the pe- 
tiole with ice, when moderate 
stimulus applied below such 
a point will not be transmitted 
across the lethargic area, and 
the motile leaflets beyond will 
remain unaffected. The nor- 
mal conductivity will, how- 
ever, be restored when the 
tissue regains the temperature 
of the surrounding atmo- 
sphere, and a second similar 

application of stimulus will then be found to be conducted 
to the motile leaflets, producing successive depressions 

(fig- 97). 

(/;) By rise of temperature. We have seen how, in 

consequence of the molecular sluggishness induced by cold, 
the conductivity of the tissue is lowered. A rise of tempera- 
ture might therefore be expected, by increasing molecular 
mobility, to enhance the conducting power. That this is 
the case is shown in detail in Chapter XX. In a leaf of 
Biophytuni) for instance, it was found that a velocity of 37 
mm. per second at 30 C. was increased at 35 C. to 7*4 mm,, 
and at 37 C. to 9*1 mm. per second. Thus, by a rise of 
temperature of from 30 C. to 37 C. the conductivity of the 
tissue was increased to nearly three times its initial value. 

(c] By effect of fatigue. We have already seen (p. 111) 
that motile response, and the transmission of excitation, are 
both alike expressions of the protoplasmic changes induced 
by stimulus. We there saw also that just as fatigue of 
motile excitability was exhibited by diminished motile 
response, so too a diminished speed of transmission exhibits 
fatigue of conductivity. An experiment will be described 



CONDUCTIVITY AND EXCITABILITY 223 

later (p. 245), which shows that in that case, under moderate 
fatigue, conductivity was diminished by 18 per cent, of its 
normal value. 

The following experiments give us a further and striking 
demonstration of the diminution or abolition of conductivity 
under fatigue. If we take a leaf of Mimosa, and excite it, by 
snipping off a terminal leaflet, borne on one of the four sub- 
petioles, the stimulus, transmitted along the narrow con- 
ducting channel of that sub-petiole, and passing through the 
large channel of the petiole, will, on reaching the pulvinus, 
cause the fall of the leaf. After a suitable period of rest, 
the leaf will re-erect itself. If now the operation be several 
times repeated, by stimulating the same sub-petiole, it will 
be found eventually that the leaf no longer responds. That 
this is due to the fatigue in conductivity of the sub-petiole 
may be proved, by snipping a leaflet off a second sub- 
petiole, which will be found to conduct the stimulus, and 
produce depression of the leaf, as did the first sub-petiole 
when fresh. It will be noticed here that the excitation which 
abolished the conductivity of the first sub-petiole, did not 
abolish that of the main petiole. This is due to the fact that 
the somewhat enfeebled stimulus on reaching the petiole is 
spread over a larger channel, and therefore the strain-effect 
which it produces there is relatively much less. 

(d) By effect of anesthetics. We shall now study the effect 
of anaesthetics on conductivity. This may be shown by the 
local application of ether to the petiole, in the intermediate 
portion of a Biophytum leaf, beyond, say, the first three pairs 
of leaflets. Stimulus applied below this area will be con- 
ducted to it, as seen by the fall of intervening leaflets, but its 
further passage will be blocked, and neither the leaflets of 
the etherised area, nor those beyond, will show response. 
That this abolition of conductivity, however, is only tempo- 
rary, is seen when the stimulus is repeated after blowing off 
the ether vapour. All the leaflets, from first to last, will now 
be found to respond. If etherisation, however, be carried 
too far, the abolition of conductivity persists for a long time, 



224 



PLANT RESPONSE 



and its restoration may not take place for one or more 
hours. 

An interesting experiment, on the abolition of conduc- 
tivity under ether, was performed with a specimen of Bio- 
pltytiun having eight leaves of fairly equal sensitiveness. Of 
these, four, taken alternately, had ether applied on those 
portions of their petioles which were next to the stem. On 
now applying strong thermal stimulus on the stem, the state 




Kic. 98. l)iagi.inun,itic RcjutMiitation of Experiment on Biophytum 

Kther was applied on the alternate petioles marked I, 2, 3, 4. Stimulus 
at x is prevented from acting on the leaflets of these leaves. 

The same diagram also represents the subsequent experiment on variation 
of receptive excitability. Ether is applied at K instead of on the 
petioles. Stimulus applied at E now produces no excitation. 

of excitation radiated to all the leaves. But the passage of 
stimulus through the four etherised petioles was blocked, 
and no effect was produced on their leaflets. The leaflets of 
the non-etherised leaves, however, promptly responded, falling 
one after another from the centre outwards (fig. 98). 

Variation of receptivity by anaesthetics. Lastly, we 
shall inquire into the variation of excitability at the point of 
application of stimulus, that is to say, into the modification 
of the plant's receptivity, under the action of an external 
agent. It is to be borne in mind that stimulus coming from 



CONDUCTIVITY AND EXCITABILITY 22$ 

without directly affects the outer layer of the tissue, and the 
excitation may then proceed inwards and in lateral directions, 
by conduction. 

The effect of ether in diminishing receptive excitability 
may be demonstrated by taking, as in the last case, a specimen 
of Biophytum. We first test the specimen by applying a 
moderate stimulus on the stem at E. The excitation thus 
initiated at the receptive area is transmitted to the leaves, 
and causes depression of their leaflets. When these have 
recovered, ether is applied locally on the area* E. On now 
repeating the stimulation, we find that none of the leaflets 
respond. Since the conductivity of the intervening tissue 
and the excitability of the motile organs have remained 
unaffected, it is clear that the failure to respond is in this 
case due to the depression of receptive excitability by ether. 

A tissue, however, whose superficial excitability is depressed 
in this way, may still retain the power of conduction. This is 
shown by applying stimulus on the stem, as in the last ex- 
periment, but at x , below the etherised ring E. The stimulus 
is now shown to be transmitted, by the fall of the motile 
leaflets. The explanation of this difference probably lies in 
the fact that the molecular torpidity induced by the etherisa- 
tion does not extend very deep, unless it has been excessive 
and long-continued. In that case, the internal layer of the 
tissue, remaining unaffected, would serve as the channel of 
conduction. This view is supported by the fact which I 
have noticed, that it is much easier to produce a complete 
block to the passage of stimulation, when a relatively thin 
tissue, such as the petiole of a leaf, is etherised. It is much 
more difficult, on the other hand, to do this with a thick 
stem. 

We saw from the molecular model (fig. 94) that though 
when the molecules were sluggish no response could be 
obtained to moderate stimulus, yet when the stimulus was 
very strong response could be brought about. Similarly, in 
experimenting on plants, I have found it possible, by careful 
graduation of etherisation, to arrange matters in such a way 



226 PLANT RESPONSE 

that while moderate intensity of stimulus, applied on the 
etherised area, failed to evoke a responsive movement of the 
distant leaflet, a powerful stimulus was able to do so. 

Receptivity versus motile excitability. At the begin- 
ning of the present chapter, I drew attention to the necessity 
of discriminating between the functions of receptivity and 
motile excitability. It is only by carefully distinguishing these 
that we can possibly come to an understanding of certain 
apparent contradictions. Let us suppose that stimulus is 
applied on a motile organ, say the pulvinus of Mimosa. In 
this particular case, the areas of receptivity and motile excit- 
ability are coincident. By the reception of stimulus the motile 
machinery is eventually set in motion. The mobility of the 
superficial particles will thus determine the receptivity and 
the inner mechanism of the organ, the motile excitability. 
The motile excitability is measured by the amplitude of 
response. Receptivity, on the other hand, may be partially 
discriminated by (i) the length of the latent period, and (2) 
the value of the minimally effective stimulus. 

When a tissue is cooled, say to 7 C. or lower, its recep- 
tivity and motile excitability both undergo diminution. Hence 
the latent period is prolonged (p. 268), and the stimulus 
which was formerly effective becomes ineffective. In such a 
case, where the two factors conspire, it is difficult to distin- 
guish between the relative effects of receptivity and motile 
excitability. But when, on the other hand, the temperature 
is raised, say to 35 C., the amplitude of contractile response, 
by which we are in the habit of gauging the motile excitability, 
is generally speaking diminished (fig. 79). Hence we are 
apt to infer that excitability in general is decreased at 35 C. 
But if we test this question by means of the minimally 
effective stimulus, we arrive at a very different conclusion. 
For example, taking a specimen of Biophytum at 30 C., 
I found that the minimally effective stimulus was given by 
a condenser charged to twenty- two volts, whereas when the 
temperature was raised to 35 C. the minimally effective 
timulus was a charge of fourteen volts. It is clear from 



CONDUCTIVITY AND EXCITABILITY 22; 

this that the excitability at 35 C. is higher than at 30 C. 1 
Hence we arrive at two conclusions directly opposed to each 
other. 

This apparent anomaly completely disappears, however, 
in the light of the distinction between the receptive and 
motile excitabilities ; for it was said that it was the 
mobility of the superficial particles which determined the 
receptivity, and this is evidently enhanced by rise of tempera- 
ture. The amplitude of mechanical response, however, by which 
we measure the motile excitability, is not solely, dependent on 
molecular mobility. This mechanical response is, as we have 
seen, brought about by diminution of turgor, and any agent 
which produced increase of turgor would act antagonistically, 
and thus diminish the motile expression of excitation. For 
example, we have seen that a pulvinus of Mimosa, when 
highly turgid, failed to show any motile response, though 
excited (p. 49). Now, it will be shown (p. 400), that rise 
of temperature has the effect of increasing turgor. Hence 
the diminution of mechanical response with increasing 
temperature does not indicate diminution of excitability in 
general, but rather the setting in of an antagonistic force, 
whose influence will be to increase the force of recovery from 
molecular distortion. It should be mentioned, however, 
that there is a limit to the enhancement of excitability by 
rise of temperature ; for the molecular disturbance caused 
by heat will when excessive be detrimental to response. 

Excitability versus conductivity. The same considera- 
tions which have thus enabled us to distinguish between 
receptivity and motile excitability, will also enable us to see 
the difference between motile excitability and conductivity. 
We have seen, for example, that at 35 C. the conductivity in 
a given specimen of Biophytum was almost three times as 
great as at 30 C. in spite of the fact that, as just explained, 
contractile response is considerably diminished at high 

1 It will be found in Chapter XXX. that growth, which is a phenomenon of 
excitatory response, is, in the case of many plants, at its maximum at or near 
35 C. 

Q 2 



228 PLANT RESPONSE 

temperatures. This distinction between the effects of conduc- 
tivity and of excitability is especially important, since by its 
means we are enabled to explain certain facts apparently 
anomalous, which seem at first sight to lend support to the 
hydro-mechanical theory of excitation. I have shown that, 
under normal conditions, the intensity of excitation must 
exceed a certain value before it can be manifested as 
mechanical response. I have also shown that under un- 
favourable circumstances, motile excitability is abolished 
earlier than conductivity. An excited tissue may thus 
conduct stimulus, without itself exhibiting any motile indica- 
tion. Numerous examples of such a state of things may be 
cited. It must be borne in mind that the mechanical 
indication of the state of excitation can be afforded by a 
pulvinated organ, only when there is some difference of 
excitability as between its upper and lower halves. If this 
difference of excitability be in any manner reduced or 
diminished, there will be a failure of the mechanical response. 
In old leaves of Biophyttim^ for example, not only is the 
general excitability diminished, but the differential excitability 
also has disappeared. Hence, excitation of such leaves gives 
rise to no local excitatory response of the leaflets. But that 
the leaf is still nevertheless excitable, and can transmit that 
state of excitation, is shown by the fact that on stimulating 
it strongly, the leaflets of younger leaves at a distance are, 
after a time, seen to be depressed in serial succession. This 
proves that, though unable itself to give the motile indication, 
the leaf was capable of receiving and transmitting the state 
of excitation. Similarly, it may be shown that a tissue whose 
motile excitability is temporarily abolished, by, say, the 
application of ether, may, nevertheless, be the conductor of 
stimulation. 

Jn order to demonstrate this, let us take a plant of 
Biophytum> and expose some of the leaflets of a particular 
leaf to ether-vapour. Strong stimulation of that portion of 
the petiole which bears them, will now fail to induce move- 
ment of the leaflet in the etherised region ; but the excitation 



CONDUCTIVITY AND EXCITABILITY 229 

is found to be conducted through the anaesthetised area, and 
to produce responsive depression, not only of the leaflets 
beyond, but also of those of other leaves. 

This experiment is important in its relation to the theory 
of the mode of transmission of excitation. I have already 
adduced conclusive proofs that the conduction of stimulus is 
dependent, not on the mere mechanical transmission of 
hydrostatic disturbance, but on the propagation of proto- 
plasmic changes. Strong support has been lent to the 
hydro-mechanical theory by a classical experiment in which 
the pulvinus of a leaf of Mimosa was chloroformed. On 
then strongly exciting the leaflets of this leaf, the ex- 
citation was found to be conducted across the anaesthetised 
pulvinus and to produce depression of leaves beyond. At first 
sight it was natural to suppose that, as the motile* excitability 
of the pulvinus was abolished by chloroform, the conductivity 
must also have been abolished. It was therefore inferred 
that, unlike the conduction of stimulus in animal tissues, 
where such transmission takes place by the propagation of 
protoplasmic changes, the conduction of excitation in the 
plant was purely mechanical. It will be seen, however, that 
the assumption on which this conclusion is based that con- 
duction must necessarily be abolished, with the abolition of 
motor excitability has been invalidated by the experiments 
which I have just described. 

In the present chapter, then, it has been shown that those 
agencies which, like cold, anaesthetics, and fatigue, diminish 
molecular mobility, also diminish the excitability and conduc- 
tivity of the plant- tissue. I shall in the next chapter describe 
a series of experiments on the profound excitatory changes, 
of opposite character, which are induced in the experimental 
tissue, by the passage of an electrical current, the nature of 
such changes being dependent on the question whether the 
current enters or leaves the tissue at a given point. It must 
be added that this series of observations will be found to 
offer a further disproof of the hydro-mechanical theory of 
conduction of stimulus. 



230 PLANT RESPONSE 

SUMMARY 

Motile excitability is temporarily abolished by anaes- 
thetics. 

Strong application of cold produces a temporary abolition 
of motile excitability. Moderate application of cold prolongs 
the latent period. 

Similarly, fatigue produces a diminution or abolition of 
motile excitability ; and this is restored, after a sufficient 
period of rest. 

Conductivity, similarly, undergoes diminution as the 
effect of cold, anaesthetics, and fatigue. 

Receptive excitability, again, undergoes diminution or 
abolition by the action of similar agencies. 

Conductivity may persist even after the abolition of 
motile excitability. Hence a strong stimulus may be con- 
ducted through a region which exhibits no motile excit- 
ability. 



CHAPTER XIX 

ON ELECTROTONUS 

The anode acts as a block to the transmission of stimulus Opposite effect of 
kathode Experiments on Biophytum, showing variations- of conductivity by 
anode and kathode respectively Experiments on Mimosa, showing increase of 
motile excitability at or near the kathode, and diminution of motile excitability 
at or near the anode Curious ' development ' of response, near the kathode. 

WE have seen in the last chapter that on account of the 
diminished molecular mobility caused by physical and 
chemical agents, the response underwent a diminution. It 
was also seen that this reduction of molecular mobility found 
expression in the diminution of conductivity and excitability. 
External agents, like cold and ether, produce a temporary 
reduction of mobility, after which there is a revival to the 
original condition on the removal of the depressing agents. 
But certain other agents, such as poisons, produce permanent 
immobility, from which there is no recovery of response. The 
tissue is then said to be ' killed.' 

Returning now to the molecular model, described in the 
last chapter, we see that while stimulus causes molecular 
upset, yet, at the same time, the force which restores the 
molecule to its equilibrium position, or, in other words, that 
which determines its stability, resists such an upset. Let us 
then first imagine the molecular model to be under the 
moderate directive action of the earth's magnetism. The 
stability of the individual molecule will thus be neither too 
great nor too small, and we shall call this, for convenience, 
the normal stability. This stability may further be increased 
by increasing the external directive force with the help of an 
auxiliary magnet, arranged in a suitable manner. Or it may 



232 PLANT RESPONSE 

be decreased, below the normal, by the action of an external 
magnet which reduces the earth's directive force. 

On now obtaining responses to a uniform disturbing 
force, under these three conditions of normal, increased, and 
diminished stability, we shall find that while in the first case 
we get moderate response, in the second the response is 
very much diminished (and may even disappear entirely, when 
the stability is very great), and in the third it becomes exalted. 

An-electrotonus and kat-electrotonus. I shall now 
proceed to show the opposite effects of the anode and 
kathode on molecular responsiveness, during the passage of 
an electrical current through a plant-tissue. This change, 
induced by an electrical current, is known as electrotonus, 
and the effect due to the kathode is distinguished as kat- 
clectrotonus, while that due to the anode is known as an- 
electrotonus. It is probable that here, also, the variation of 
sensibility is brought about by the variation of molecular 
mobility, and that this is induced by an increase or diminu- 
tion in the conditions of stability, as in the model. These 
opposite variations of the susceptibility to excitation, due to 
the anode and kathode respectively, will be demonstrated by 
the changes which they induce . in the conductivity and 
excitability of the tissue. 

In the chapter on the Excitatory Polar Effects of Currents, 
the intensity of the E.M.F. used was such that the excitation 
caused by the kathode was visibly manifested in the motile 
effects to which it gave rise. In the present chapter, how- 
ever, we shall have to deal with latent excitatory effects, the 
E.M.F. used not being sufficient to give rise to any imme- 
diate external reaction. In the cases referred to, again, the 
distinctive action of the anode could not be demonstrated 
inasmuch as under ordinary conditions it could not give 
rise to any motile indication. It will now, however, be 
shown that the effect of the anode is one of depression, or 
the opposite of that of the kathode. In studying variations 
of conductivity we have to remember that when the conduc- 
tivity of a tissue is great, the state of excitation is transmitted 



ELECTROTONUS 



233 



K 




99- 

A, 



Effect of Anode as Block. 
Anode ; K, Kathode 

The progressive wave of excitation, 
initiated at x , stopped by anode, 
one pair of leaflets to its left. 



either with greater velocity or to a greater distance ; but if 
conductivity be in any way diminished, the distance to which 
the excitatory disturbance will be transmitted, will be corre- 
spondingly reduced. 

The anodic block. In order to demonstrate the de- 
pressing action of the anode, I took a leaf of Biophytum, and 
sent a current through portions of it, entering at A, the anode, 
and leaving at K, or kathode 
(fig. 99). The E.M.F. used was 
two volts, and was thus insuffi- 
cient to cause responsive action. 
In this and the following expe- 
riments, it will be understood, 
unless the contrary is stated, 
that the intensity of the elec- 
trotonic currents was not such 
as to create any direct action at the kathode. Thermal 
stimulus was now applied at x, and the excitatory wave was 
found to be stopped at a distance of one pair of leaflets to 
the left of A. This shows that the depressing effect of the 
anode acts as a block to the passage of stimulus, and that 
such depressing action extends to some distance beyond the 
anode itself. 

Experiments showing differences of anode and 
kathode. In order to show that the kathode acts differently 
from the anode, not offering 
a block, but rather facili- 
tating the passage of stimu- 
lation, I performed another 
experiment on a leaf similar 
to the last In that case, 
the anode was near the 
point of application of 

Stimulus. I now made the Stimulus was applied at x . 

nearer electrode kathode. 

On next applying the usual stimulus, the excitatory wave 
passed on through the kathodic area, producing successive 




FIG. 100. Experiment showing the 
Transmission of Excitatory Wave 
through Kathodic Area, and its Stop- 
page by the Anode 



234 



PLANT RESPONSE 




FIG. 101. Demonstration of Simultaneous 
Opposite Effects of Anode and Kathode 
on Transmission of Excitation 

Stimulus applied in interpolar region at x . 
Excitation is transmitted through great 
distances in the kathodic region, but 
limited in the anodic. 



'all of leaflets, and was only stopped by the depressing action 
jf the anode, which this time extended to a distance of two 
pairs of leaflets to the left of A (fig. 100). 

The next experiment was devised to show the opposite 
effects of anode and kathode simultaneously. For this, the 

stimulus was applied in the 
interpolar region, half-way 
between the two. Two 
wave-systems were found 
to start from the excited 
point in opposite directions. 
That towards K not only 
reached K, but passed 
beyond it, causing the 
depression of all the leaf- 
lets, six pairs in number, 
on that side. But the excitatory wave that travelled towards 
A passed through only two pairs of leaflets, and was stopped 
at a point one pair to the left of the anode (fig. 101). 

Electrotonic variation of motile excitability. We have 
seen that protoplasmic excitability finds expression in, 
among other things, the conductivity and motile response 
of the tissue. We have seen also how the former, that is to 
say, the conductivity, is modified in opposite ways by the 
influence of the anode and kathode. I shall now proceed to 
describe experiments in which the opposite character of the 
effects at anode and kathode is still more strikingly demon- 
strated by the exaltation at kathode, and depression at anode, 
of the motile excitability. 

I used two pairs of electrodes, the first pair, KA, for the 
purpose of producing stimulation ; and the second pair, K'A', 
in order to produce variation of excitability, through electro- 
tonus (fig. 102) ; or vice versa. The first pair was applied on 
the stem, the kathode K being in contact with the pulvinus of 
the lateral leaf. K'A' were applied on the petiole of that leaf. 
The plant was very sensitive, and in order that there 
should be no responsive fall, by the direct and unaided 



ELECTROTONUS 



235 




excitatory action of K, the current due to an E.M.F. of two 
volts was reduced, by separating A from K, and thus inter- 
posing a greater resistance. A 
distance was thus found />. 
10 cm. such that, on complet- 
ing the AK circuit, the excitation 
was not sufficient to produce 
response of the leaf. 

The AK circuit was now 
opened, and adjustments made 
with the second circuit A'K', 
with an E.M.F. of two volts, in 
such a way that, on complet- 
ing that circuit alone, there 
was no response of the leaf. 
Owing to the shorter length of 
petiole available i.e. 3 cm. 
the current could not be reduced 
to the requisite amount by 
simply increasing the interpolar 
distance. An external resist- 
ance had therefore to be added, 
in order to attain the desired 
condition. Thus either circuit, 
acting alone, was ineffective. 

Kat-electrotonic increase of excitability. Now y in order 
to show the increase of excitability in the pulvinus at K, as 
induced by the neighbourhood of kathode K', we first com- 
plete the A'K' circuit. This, as has been said, is ineffective. 
But now, on making the AK circuit, its previously ineffective 
stimulus becomes effective, and the leaf responds. From 
this it will be seen that during the passage of a current 
through the A'K' circuit, a point in the neighbourhood of the 
kathode K', that is to say, the pulvinus, is rendered more 
excitable. 

This experiment may be varied by first making the AK, 
which is now the electrotonic, circuit, and then completing A'K' 



FIG. 102. Diagrammatic Repre- 
sentation of Electrical Connec- 
tions in Mimosa to Exhibit 
Variation of Motile Excitability, 
induced by Anode and Kathode 

In the first of these experiments, 
the A'K' circuit is electrotonic 
and the AK circuit excitatory. 
In the second and third experi- 
ments the AK circuit is made 
electrotonic, and A'K' excitatory. 
In the third experiment, A and K 
are reversed. 



236 PLANT RESPONSE 

for the purpose of stimulation. It is then found that the 

hitherto ineffective stimulus of A'K' is thus rendered effective. 

An-electrotonic depression of excitability. The de- 

pressing action of the anode has been already demonstrated 
in the case of Biophytum (p. 234). The following experiment 
exhibits the same effect in a different manner in the case of 
Mimosa. In this instance, I used an E.M.F. of four volts in 
each of the two circuits AK and A'K'. When each circuit 
was made separately, the leaf responded by depression. 
At make, then, of one of the circuits the leaf responds, but as 
the stimulus is only effective at make, the leaf recovers 
during the continuation of the current. After this, on the 
second circuit being completed, the excitement at make 
again caused response. 

The experiment was now modified in the following way. 
The AK circuit was reversed, the pulvinus becoming anode. 
The excitation of the distant kathode, however, was still 
strong enough to cause response of the leaf. The current 
was kept on till the leaf recovered. On now making the 
A'K' circuit, the leaf did not respond. Thus the stimulus of 
A'K' at make, which was formerly effective, now became in- 
effective, by the depressing action of A. 

Developing action of kathode. Another experiment, 
showing the latent excitatory action of the kathode, is very 

striking. This experiment, 
however, is somewhat diffi- 
cult, as it requires a very 
delicate adjustment of the 
stimulus. The specimen 
Km. io.j. 4 Developing ' Action of used was a leaf of Biophytum. 

Kathode \ . . /y , , 

,..,., ,. , A current insufficient to pro- 

A subnummal stimulus applied at x , *. 

ineffective to produce excitation of dllCC any direct excitation 




though 

the circuit AK(fig. 101). The 
point of special difficulty was to apply a stimulus of exactly 
subminimal intensity at x , so as not to excite the adjacent 
leaflet I have sometimes succeeded in obtaining this condition. 



ELECTROTONUS 237 

The effect of this imperceptible stimulus, then, which passed 
through the nearer pair of leaflets, without giving any sign of 
its presence, became suddenly ' developed ' on reaching the 
further pair of leaflets, R (fig. 103), which were rendered more 
excitable by the neighbourhood of the kathode. 

These peculiar variations of excitability, induced by the 
action of the anode and kathode, as well as those caused by 
other physical and chemical agencies, are exactly similar to 
what are observed in animal tissues under the same influences. 
They bring out, further, the essential unity of physiological 
response, as seen in the highly differentiated protoplasm of 
the animal and the undifferentiated protoplasm of plant 
tissue. 

SUMMARY 

The anode acts as a block to the transmission of stimulus. 

The effect of the kathode is opposite to that of the 
anode. 

Motile excitability is diminished at or near the anode, so 
that previously effective stimulus becomes ineffective. 

Motile excitability is exalted at or near the kathode. 
Stimulus previously ineffective here becomes effective. 



CHAPTER XX 

ON THE VELO'CITY OF TRANSMISSION OF EXCITATORY 

WAVES IN PLANTS 

Difficulties in accurate determination of velocity of transmission, due to unknown 
variations of excitability arising from injury, and variations of conductivity 
through fatigue A perfect method of obtaining accurate and consistent results 
Relative advantages of studying conduction in plants as compared with animals 
Determinations of velocity of transmission in centripetal and centrifugal direc- 
tions Preferential conductivity in centrifugal direction ~ Diminution of con* 
ductivity and excitability by fatigue Within a certain critical interval, organ 
* refractory ' to further stimulus Increased velocity of transmission with in- 
creasing stimulus Measurement of diminution of conductivity by cold Fibro- 
vascular elements the best conducting channels Conductivity lengthwise greater 
than crosswise - Electric mode of determination of velocity of transmission 
Indifferent parenchymatous tisstrs do not transmit stimulation Comparative 
tables showing velocity of transmission in various plant and animal tissues. 

IN the last two chapters the effects of various agencies on the 
power of conduction were demonstrated qualitatively. It is 
important, however, to obtain, if possible, the quantitative 
values of this conductivity and its variations. The absolute 
value of conduction can be obtained from the determination 
of the velocity of transmission of excitation through the 
tissue. This determination of velocity may be made roughly, 
by observing the time taken for the application of a stimu- 
lus, say by cut or hot wire contact at a given point, to produce 
motile effects on a leaflet at a known distance. 

A result thus obtained, however, would, for reasons to be 
given presently, prove very indefinite, and no two such results 
in succession could be trusted to agree. In order to ascer- 
tain the exact quantitative effects of various agencies on 
conductivity, we must first be completely assured that our 
determinations of velocity under normal conditions are trust- 
worthy. 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 239 

Difficulties in exact determination of velocity of trans- 
mission of excitation. In the course of t,he investigation 
carried out on this subject, I have found that the discrepancies 
between the velocities, determined in the way described, are 
largely to be accounted for, first, by indefinite changes of 
excitability at the point of application, due to the injury 
caused by excessive stimulation ; and, second, to the changes 
of conductivity, caused by fatigue, in the rest of the tissue. 
I have also found that the velocity of transmission is only 
a determinate quantity when the intensity of stimulus is 
constant. It undergoes variation, with changes in the stimu- 
lation-intensity. 

These difficulties are met by using a stimulus which does 
not cause injury, and which can be repeated at uniform 
intensity. Such a stimulus is given by means of the con- 
denser discharge. As regards the changes of conductivity 
due to fatigue, I have found that fatigue is removed, and 
conductivity fully restored, after a definite period of rest, 
which, in the case of Biophytum, is about four to five minutes. 

The next difficulty to be overcome is concerned with the 
question of recording the exact moment of application of 
stimulus, and that of the initiation of response at a distant 
leaflet. A further source of uncertainty in the last respect, 
lies in the existence of an unknown latent period of the 
leaflet, which may delay the visible response, even after the 
effect of stimulus has reached the point at the base of the 
leaflet. 

It is evident that the times of application of such rude 
modes of stimulation as cut, or contact of hot wire, cannot be 
accurately determined, and the exact moment of the beginning 
of the responsive movement of the motile leaflet is equally 
difficult to ascertain by the unaided eye. These difficulties 
are, however, removed, if we use the discharge from a con- 
denser as our mode of stimulation, and the magnified move- 
ment of the spot of light from the Optic Lever, as the 
indicator of the commencement of response. The observer, 
following the spot of light from the Optic Lever, makes two 



240 PLANT RESPONSE 

marks on the revolving drum one when the discharge-key 
is pressed, at the moment of application of stimulus, and 
another when the spot begins to move, that is to say, at the 
commencement of response. It is then easy, knowing the 
rate of movement of the drum and the distance between the 
two marks, to determine the exact time-interval between 
the two. 

There then remains only the question of allowing for the 
loss of time due to the latent period of the responding organ. 
This is accomplished by means of a separate experiment, in 
which the stimulus is directly applied at the base of the 
motile organ. The latent period thus ascertained is sub- 
tracted from the time-interval already determined, and we 
have thus the true time of transmission of excitation through 
the given distance ; from this the velocity, or rate of trans- 
mission per second, may be deduced. 

Exact determination of velocity. I now give an 
account of an actual experiment for the determination of 
the velocity of transmission of excitation in the petiole of 
Biophytum. The two points A and B are connected in the 
circuit of a condenser through the usual non-polarisable 
electrodes (fig. 14). An indicating leaflet, L, is attached to 
the Optic Lever, by which the exact moment of its response 
may be recorded on the revolving drum. If now we make B 
kathode, during the charge of condenser, an excitatory wave 
will start from B and travel inwards towards L, in this par- 
ticular case in a centripetal direction, i.e. towards the main 
stem. A mark is made, as already explained, on the re- 
volving drum, at the exact moment when the tapping-key 
excites the plant. In this case, the capacity of the condenser 
was 'Oi microfarad, and E.M.F. twelve volts. The time of 
the stimulus reaching L, as indicated by the movement of the 
spot of light, is also marked, as explained, on the revolving 
drum. As has been said before, the time- interval is accu- 
rately determined from the speed of the revolving drum. As 
an additional precaution, the same time-interval is taken by 
means of a stop-watch. 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 241 

From a separate experiment, by direct stimulation of the 
base of the petiolule, it was found that the latent period of 
the leaflet was so small a fraction of a second, as, for our 
present purpose, to be negligible. In this way, in my first 
experiment, I found the time taken by the excitation to travel 
the distance of 27 mm. between B and L to be 14-3 seconds. 
I allowed the plant a period of rest of three minutes, and 
again performed the experiment under similar conditions. 
The time taken was found to be 14-5 seconds, which is 
practically the same, within experimental error, as the result 
first obtained. The slight difference was due to the residual 
effect of fatigue. In any case, the extreme difference between 
the two results amounts to less than 1-4 per cent. or 7 per 
cent, from the mean value of 14'4. From this we find that in 
the particular plant under experiment the velocity ^of trans- 
mission in a centripetal direction was r88 mm. per second. 
In order to show how consistent successive results are, I give 
successive time-intervals taken by stimulus in two different 
cases, to travel the intervening distances. 

Case i. Time-interval in first experiment I2'6 seconds. 

second 12-9 

Case 2. first 14-8 

second 15 

In all these cases, the second experiment was undertaken 
after an interval of rest of three minutes. The slight re- 
tardation uniformly observed is due, as already explained, to 
residual fatigue. It is, however, so small as to be negligible. 
At any rate, making allowance for all possible sources of 
uncertainty, the variation of these determinations will be less 
than 2 per cent. 

We have to remember that, owing to the slow velocity of 
transmission of impulses in plants, and also to the com- 
paratively great length of tissue, that can, when necessary, 
be brought under examination, the total interval of time that 
has to be observed may be made as large as twenty to forty 
seconds. In such periods, a mean error of even -2 second 

R 



242 



PLANT RESPONSE 



would hardly produce an inaccuracy of i per cent in the result 
This would compare favourably with the determinations that 
have been made of the velocity of transmission of nervous 
impulses in animals. In a frog's nerve, for example, owing 
to the high velocity and comparatively short length of nerve 
available for experiment, the total interval of time which has 
to be observed is of the order of some thousandths of a 
second. To obtain an accuracy within i per cent here, 
would mean the recording and measuring of an interval of 
something like the ^0^-$$ part of a second. 

The velocity of transmission in a given plant is found, 
under normal conditions, to be constant It varies in different 
species, and even in the same species the value changes with 
the season of the year and the physiological condition of the 
specimen. A velocity determined in winter under less 
favourable physiological conditions, is very much lower than 
the velocity of transmission in the same plant in summer. 

The exact determination of the velocity of nervous im- 
pulses in animals has, therefore, been a matter of some 

uncertainty. For example, 
llclmholtz found this ve- 
locity in man to be about 
thirty-three metres per 
second. Some recent de- 
terminations, again, give 
a value twice as great 
Owing, moreover, to the 
difficulty in exactly dis- 
criminating the rising part 
of the curve, the same re- 
cord may be interpreted to 
give results which differ 
from each other by as much 
as 20 per cent 1 
I shall next pass to the 




FIG. 104. Diagrammatic Representation 
of Electrical Connections for Deter- 
mination of Velocities of Centrifugal 
and Centripetal Transmissions 

A and H are the electrodes, and i, the 
indicating leaflet. 



Preferential conductivity.- 

consideration of the very curious and interesting pheno- 

1 Nature, 1903, pp. 105, 151. 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 243 

menon of preferential conductivity, by which it is seen that 
the state of excitation travels through a tissue with greater 
facility in one direction than in the opposite. For the 
purpose of this demonstration, I took a fresh leaf of Bio- 
phytum. The two condenser connections (capacity - oi micro- 
farad charged to twelve volts) were made at A and B 
(fig. 104). The indicating leaflet L was situated somewhere 
between. On charge, B became the kathode, and an ex- 
citatory wave was started in a centripetal direction. On 
the abatement of this wave, the condenser was discharged ; 
A now became the kathode, and an excitatory wave was 
transmitted in a centrifugal direction. From these two 
successive experiments, we are now able to determine the two 
velocities in opposite directions in the same leaf. 

The following table exhibits the results obtained in this 
manner with two different specimens : 



SPECIMEN I 



Direction 

Centripetal (B 
Centrifugal (AL). 



Distance 


! Time 


22 '5 mm. 
45 mm. 


1 1 *2 seconds. 
! i5' 2 



Velocity 



2 mm, per second. 
2 '9 mm. ,, 



SPECIMEN II 



Direction 

Centripetal (BL). 
Centrifugal (AL). 


Distance 

28 mm. 
39-5 mm. 


Time 

" 
15*2 seconds. 

17-5 . 


Velocity 

I -84 mm. per second. 
2*2 mm. ,, 

. .* 



Excitatory discharge preferentially directed. These 
two observations show that the velocity is greater in the 
centrifugal direction. In some instances I have found the 
centrifugal velocity to be nearly twice as great as the centri- 
petal. These experiments seem to indicate that under 
certain conditions, excitatory discharges will take place 
preferentially in one direction only. We may imagine any 
intermediate point in the midrib of the leaf, to be acted on 
locally by a gradually increasing or accumulating stimulus from 



R 2 



244 PLANT RESPONSE 

external sources. Immediately on this reaching the threshold 
of response, it will give rise to an excitatory discharge. And 
it is clear that this excitation will be transmitted preferentially 
along the line of least resistance, that is to say, in the 
direction of the greatest conductivity, or outwards. I have 
been able to obtain further experimental verification of this 
conclusion, by applying a gradually increasing stimulus of 
condenser discharge to an intermediate point on a petiole of 
Biophytum. When this stimulus had reached a certain value, 
it was found that while excitation, as indicated by the fall of 
the leaflets, travelled through a great distance forwards, or 
outwards, its transmission backwards was extremely limited. 
Could we have adjusted the stimulus, so as to have been 
slightly above the threshold of response, there would have been 
no transmission backwards. When stimulus, on the other 
hand, is excessive, the entire excitatory effect cannot be carried 
forward ; there is an overflow backwards ; and under these 
conditions excitatory movements take place in both directions. 

Effect of fatigue on velocity of transmission. We shall 
next deal with the modification of the velocity of transmission 
by fatigue. Specimens of Biophytiun were used for the 
purposes of this investigation, and experiments were per- 
formed by ascertaining the times taken for the transmission 
of a repeated uniform stimulus, through the same distance, 
under shortening periods of rest. 

A stimulus was given to a leaf of Biophytum, and the 
record of time taken the transmission being in a centripetal 
direction. The plant was now given an interval of rest of 
three minutes. Stimulus was again applied, and the time- 
records obtained in the usual manner. The next stimulus was 
applied after a resting-interval of* two minutes, the following 
after one, and the last after half a minute, the time of trans- 
mission and the records of response being taken throughout. 
From the results given below, it will be seen how regular is the 
decrease in velocity with the increase of fatigue. The 
distance to be traversed, 27 mm., was kept the same in all 
cases. The time taken at the beginning, when the plant was 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 245 

fresh, to traverse this distance was 14*3 seconds. In the next 
experiment, when the stimulus was applied after three minutes, 
there was a slight residual fatigue, and this prolonged the 
time to 14*5 seconds. On the third occasion, a still shorter 
time, namely two minutes, was allowed for rest, and the rate 
of transmission became slower, the time being now 157 
seconds. The next interval of rest was still further shortened, 
to one minute, and the time of transmission was correspond- 
ingly increased to 16*4 seconds. And lastly, when the 
stimulus was given after an interval of only half, a minute, the 
velocity was still further retarded, the time now taken being 
I7'5 seconds. 

The following table gives the different velocities under 
increasing fatigue, and the heights of the corresponding 
responses. It has already been said that the distance through 
which the stimulus was transmitted was in all cases the same, 
namely 27 mm. 

TABLE SHOWING VARIATIONS OK VELOCITY or TRANSMISSION AND OK 
AMPLITUDE OF RESPONSE IN BIOPHYTUM, WITH INCREASING FATIGUK 



Time 


Height 
of response 




Velocity 




14*3 seconds 


34 dns. i -88 


mm. per 


second 


14-5 ,, 20 1-86 


f J 


9 1 


157 ,. 


I4-5 172 


M 





16-4 ,, 


2-5 1-64 


M 


) > 


I7'5 j I>0 >. 


i'S4 


M 


n 



Intervals of rest j 

I 

The plant fresh i 
3 minutes ! 

2 minutes j 

i minute | 

i minute ! 



It will be seen from this table that, while the variation of 
velocity due to the difference of conductivity between three 
minutes' rest and an indefinitely longer period is slight, there 
is a considerable diminution of this velocity when the resting- 
periods are still further shortened. It will be noted, more- 
over, that increasing fatigue is shown not only by a regular 
decrement in the speed of transmission, but also in an inde- 
pendent and still more striking manner by a steady diminution 
in the heights of the responses themselves. 

The following curve (fig. 105) shows the variation of 



246 



PLANT RESPONSE 



motile excitability under shortened periods of rest. This 
curve when produced will cut the abscissa. Such a point 
would mark the time-interval between two successive stimuli, 

at which the response would 
be zero, that is to say, the 
motile excitability would be 
abolished. In other words, 
the leaf would, when the 
resting- interval was short- 
ened to this period, prove 
refractory to stimulus. In 
Chapter XX 1 1 the existence 
of this theoretical refractory 
period will be demonstrated 

,, , . T . ,. . by experiment, 

r KI. 105. Curve showing Dec-line in J r 

Heights of Responses, with Diminish- It is thus seen that Owing 

ing Periods of Rest 




Abscissa represents resting-periods, and 
ordinate heights of iesponse 



to imperfect protoplasmic 
recovery, or, in other words, 
to residual molecular strain, 
not only is the conductivity of the tissue gradually diminished, 
but the excitability also. Thus we obtain some idea of the 
processes by which fatigue is brought about. 

Effect of intensity of stimulus on velocity. We have 
next to study the variation in the velocity of transmission of 
the excitatory condition, with increasing strength of stimulus. 
In the case of animal nerve, it has been ascertained by 
different observers (Helmholtz, Vintschgau, and Pick) that 
the velocity of transmission of the nervous impulse is not 
independent of the strength of stimulus, but increases with 
increasing intensity. The experimental verification of this 
with conducting animal tissue is, however, extremely difficult, 
owing principally to the shortness of time involved. 

1 have carried out an investigation on this subject with 
vegetable tissues, which shows in an unmistakable manner 
that velocity does undergo an increase, with increasing 
stimulus. These experiments were carried out with leaves of 
Biophytuw. I first demonstrated this in a qualitative manner, 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 247 

by using thermal stimulation. It was thus found that the 
excitation caused by a strong, would travel with a greater 
velocity than that due to feeble stimulus, the one being 
sometimes double the other. I next tried to obtain quan- 
titative results, by using a form of stimulus which was 
measurable, and could be increased in a graduated manner. 
For this I employed the method of stimulation by condenser 
discharge, the stimulus being increased by increasing the 
E.M.F. that charged the condenser (-01 microfarad). 

In one series, with a stimulus of eight-volt charge, the 



velocity found was r8 mm. per second. When the stimulus was 
increased by charging the condenser to twelve volts, there was 
an increase of velocity to 1*9 mm. per second. And finally, 
with a sixteen-volt charge, the velocity was found to be 2*1 mrn. 
per second. These velocities referred to centripetal trans- 
mission. In the next series, the experiments were carried 
out on a much more excitable leaf, and the velocity was 
determined in a centrifugal direction ; with a charge of eight 
volts, the velocity was 3*27 mm. per second ; with sixteen volts, 
it rose to 376 mm. ; and with thirty-two volts, it became 
3*83 mm. per second. The two following tables exhibit these 
results of increasing velocity with increasing stimulus : 

TAHLKS SHOWING INCREASE OF VELOCITY WITH INCREASING STIMULUS 

Specimen I. Centripetal transmission. 
The distance traversed by stimulus was 27 nun. 

i 

Velocity 

i -8 mm. per second 
1-9 mm. ,, 
2-1 mm. ,, 

Specimen //. Centrifugal transmission. 
Distance traversed by stimulus was 38 mm. 



Stimulus 




Time 


01 microfarad 

55 

55 


charged 

5 5 


to 8 volts 

12 

16 


14-9 seconds 

I2'8 " 



Stimulus Time 



01 microfarad charged to 8 volts 1 1 '6 seconds 



Velocity 



3*27 mm. per second 



16 ,, 10-2 i 372mm. 

,, 24 ,, io-i ,, j 376 mm. 

32 9*9 i 3' 8 3 mm. 



f 



248 / PLANT RESPONSE 

The stimulus, it is to be remembered, is increased by 
increasing the voltage. But, on an undue increase of this 
charging voltage, to about forty volts or upwards, I have 
often found that the velocity undergoes an actual diminution. 
This is to be ascribed to the fact, which was demonstrated in 
the chapter on Polar Effects of Currents, that the excitatory 
value of the kathode reaches a limit, with a certain E.M.F., 
and that if the E.M.F. be carried far beyond this, the excita- 
tory effect is reversed, that is to say, it is now the anode 
that excites (p. 206). We have then the curious case of a 
negative direction, as it were, of transmission. For whereas, 
with moderate voltage, the excitatory disturbance travels in the 
interpolar region from kathode to anode, it now, with exces- 
sive voltage, travels in the opposite direction, from the anode 
towards the kathode. 

We can now see with what great accuracy it is possible to 
measure these changes of velocity, from which we can deduce 
the variations of conductivity, not merely qualitatively, but 
also quantitatively. This opens out to us the further possi- 
bility of studying the quantitative effects of various external 
agencies in modifying conductivity. I shall here relate a 
simple experiment which affords an example of the method 
to be followed in such an investigation. 

Effect of lowering of temperature on velocity of trans- 
mission. In order to study the effect of lowered tempera- 
ture on conductivity, I applied ice-cold water over an area of 
10 mm. of the conducting petiole in Biophytuin. The length 
of the conducting tissue experimented upon was 38 mm., and 
the time taken for the stimulus of a condenser-discharge 
(ten volts and 'Oi microfarad) under normal conditions, 
i.e. before the application of ice-cold water, to traverse this 
length, was IOT seconds, giving a velocity of 376 mm. per 
second. But after the application of ice-cold water, the con- 
ductivity was so diminished that the transmitted excitation 
did not produce any response of the motile leaflet ; on 
allowing the temperature of the cold water applied, however, 
to rise a few degrees, the stimulus was found to be effective ; 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 249 

but the velocity of transmission was now found to be much 
reduced. Instead of 10*1 seconds being necessary for trans- 
mission through the entire length, it was now found to take 
I4'8 seconds. The difference of 47 seconds here, represents 
the additional time taken for transmission through the lomm. 
length of cooled tissue. In other words, whereas transmis- 
sion through 10 mm. of normal tissue had taken about 
2*6 seconds, it now took about 2*6 + 47, or 7*3 seconds ; that 
is to say, the conductivity was reduced by cooling to nearly 
one-third. 

Effect of rise of temperature on velocity. It has been 
proved that conductivity is reduced by lowering of tempera- 
ture. We should therefore expect that a rise of temperature 
would produce the opposite, namely, an increase of conduc- 
tivity. That this is the case was shown by an experiment 
on a leaf of BiopJiytuin. ('are was taken that the leaf should 
not be too young, since, as will be shown later, the effect of a 
rise of temperature on a young leaf is to initiate automatic 
response. I found that in this specimen excitation travelled 
a distance of 41 mm. in a centrifugal direction in 1 1 seconds, 
the temperature being 30 C. The velocity at this tempera- 
ture was therefore 37 mm. per second. On now raising the 
temperature to 35 C. the time taken for transmission was 
reduced to half, i.e. 5*5 seconds. The temperature was next 
raised to 37 C., and the time was now found to be further 
reduced to 4*5 seconds, the velocity being thus 9*1 mm. per 
second, or nearly three times as great as at 30 C. 



TABLE SHOWING EFFECT OF RISE OF TEMPERATURE ON VELOCITY 
OF TRANSMISSION IN BIOPHYTUM 



I Temperature 



C. 

30 

35 
37 



Distance 

41 mm. 
41 mm. 
41 mm. 



Time 



Velocity 



II seconds 37 mm, per sec. 

__ ^ ___ 

5*5 seconds ! 7*4 mm. per sec. 



4 '5 seconds 9*1 mm. per sec. 



250 PLANT RESPONSE 

Channels for conduction of effect of stimulus. Before 
concluding this chapter, it is important to consider the channels 
through which stimulus is conducted with the greatest facility. 
Since the conduction of stimulus is due to the transmission 
of protoplasmic change, it is clear, as already said in a 
previous chapter (p. 60), that such changes will . be con- 
ducted most easily along those paths in which there is least 
interruption of protoplasmic continuity. It is evident, there- 
fore, that certain elements in the fibro- vascular bundles will 
furnish the best conducting medium for the transmission of 
stimulus. It also follows that in the fibro- vascular tissue 
itself, the conduction along the length would be more rapid 
and complete than across. 

On the other hand, the cells of indifferent tissue, such as 
the parenchyma of the leaf, are divided from each other by 
more or less complete septa, the fine filaments by which 
neighbouring cells may be protoplasmically connected being 
so minute that the conduction of stimulus through such im- 
perfect channels must be exceedingly feeble. 

These theoretical conclusions I have been able to verify 
by direct measurement of conductivity in different kinds of 
tissue. In this investigation, as motile tests of the state of 
excitability were not available, I devised an electrical method 
to be referred to briefly in the next chapter, and described 
more fully elsewhere by which to attack the problem. 

Using this method of investigation, I found that plant- 
organs which contained fibro- vascular elements, such as the 
stem, peduncle, and petiole, were the best conductors of the 
state of excitation, and that conduction in such organs is 
much greater along the length than across it ; in the 
peduncle of Musa, for example, the conductivity lengthwise 
is three times as great as that crosswise ; and finally I found 
that though indifferent tissues like the parenchyma are 
directly excitable, yet there is practically no transmission of 
that state of excitation through such tissue. 

From anatomical and other considerations, Dutrochet and 
Haberlandt came to the conclusion, that it was certain ele- 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 251 

ments in the fibro-vascular bundle which were concerned in 
transmitting the disturbance in Mimosa. This transmission 
was, however, regarded rather as a hydro-mechanical than 
as a true excitatory propagation. Such a conclusion, 
as we have already seen, appeared at one time to be 
probable in the light of the experiment on the trans- 
mission of excitation through a narcotised area. I have, 
however, already shown on p. 229, that abolition of motile 
excitability need not always imply the abolition of conduc- 
tivity. Haberlandt describes an experiment according to 
which the excitation in Mimosa is said to have been propa- 
gated over dead tracts of the petiole, these* portions having 
been destroyed by scalding. But it is extremely difficult to 
ensure the death of interior tissue by such means as super- 
ficial scalding. I have found that a portion of a plant-tissue 
when subjected locally to the action of boiling water, after- 
wards exhibited signs of true excitatory electric response. 
It is only by prolonged immersion in boiling water that one 
can be quite sure that the interior tissue is really killed by 
scalding, and unless this is done thoroughly it is easy to see 
that the inner cells may conduct the stimulus. 

There is, moreover, another possibility, that of pseudo- 
conduction, by which the effect of stimulus might appear to be 
transmitted across dead areas. My meaning will perhaps be 
clearer if we imagine two isolated muscle-preparations, one 
of which is attached to a striking lever, under which is the 
second. Supposing stimulus to be applied to the first of 
these, we can see that it would cause the lever to strike the 
second muscle, thus causing excitation. In this way, the 
effect of a stimulus applied to the first muscle would appear 
to have been transmitted to the second, completely isolated 
from it. In reality, however, this was not a case of true, but 
of pseudo-conduction, the excitation of the second muscle 
being started de novo by the blow of the lever, itself only a 
secondary effect of the excitation of the first. 

Similarly, we may have, in Haberlandt's experiment, two 
living tissues isolated from each other by an intervening area 



252 PLANT RESPONSE 

of dead tissue. A strong stimulus applied to the first of these 
will now cause an excitatory expulsion of water, which will 
be transmitted across the dead area, and impart a mechanical 
blow to the second living tissue, thus setting up excitation 
de. novo in that portion of the petiole. 

Velocities of transmission in various plant and animal 
tissues. I give below a number of determinations of velo- 
cities of transmission made with different plants, both 
ordinary and sensitive, the electrical method of determination 
having been used in the case of the former ; and with this, 
for purposes of comparison, a series of values that have been 
determined in the case of animal tissues. The respective 
values given in the table refer to the maximum velocities 
obtained. In this connection, it should be remembered 
that the velocity of transmission depends on the intensity of 
stimulus. The intensity of stimulus, again, is diminished in 
the course of transmission through a* long tract. Hence the 
velocity near the point of application of stimulus is relatively 
great, and becomes less the further the stimulus travels. In 
order, therefore, to make the different results comparable, my 
experiments have been made on equal lengths of tissue, 
namely, 7*5 cm. in each case, the stimulus applied being also 
the same. 

TAHLES GIVING VELOCITIES OF TRANSMISSION OF EXCITATORY WAVE 

(a) Animal 



Subject 



Nerve of AnoJon .... 
Nerve of Eledone (observed by Uexkiill) 



Velocity 

10 mm. per second 
5 to i mm. per second 



(b) Sensitive Plants 



Subject 



Mimosa pudica : Petiole . 
Neptunia oleracea : Petiole 



Velocity 



14 mm. per second 



i ! mm. 



Biaphytum sensitivum : 

Petiole of direction centripetal . . 2'i mm. 

Petiole of direction centrifugal . . 3*8 mm. 



Peduncle of . 



37 mm. 



TRANSMISSION OF EXCITATORY WAVES IN PLANTS 253 

* 

(c) Ordinary Plants 



Subject I Velocity 



Ficus religiosa : Stem . . , . | 9-4 mm. per second 
Cncitrbita\ Tendril . . . . 1 5- mm. 

Jute : Stem 

Artocarpus : Petiole .... 



3*5 
54 mm. 



J5 



J ) 



5> 



It will thus be seen that the velocity of transmission in 
conducting plant-tissue is not very different from that in the 
conducting tissue of certain animals. 

SUMMARY 

Successive determinations of velocity of transmission are 
consistent when the stimuli are uniform, and when inter- 
vening periods of rest, sufficient for complete protoplasmic 
recovery, are allowed. 

Velocities of transmission are not the same in centripetal 
and centrifugal directions. In Biophytum, for example, the 
centrifugal velocity is greater than the centripetal. 

When a given point in a plant-tissue is gradually raised 
in excitability, the consequent excitatory discharge takes 
place preferentially in one direction. 

Conductivity and excitability are both diminished by the 
increasing fatigue consequent on shortened intervals of rest. 
When the resting-period is shortened below a certain critical 
interval, the motile organ proves ' refractory ' to further 
stimulus. 

The velocity of transmission is not the same for all 
intensities, but increases with increasing stimulation. 

The velocity of transmission is diminished by lowering, 
and increased by raising, of temperature. 

The fibro- vascular elements are the best channels for con- 
^uction of stimulus : in them, the transmission lengthwise is 
greater than crosswise. Indifferent parenchymatous tissue 
has little or no power of conducting stimulus. 

The velocity of transmission in plants is not of an 
altogether different order of magnitude from that in certain 
animal tissues. 



CHAPTER XXI 

ON DETECTION OB' EXCITATORY TULSE DURING TRANSIT 
BY ELECTROTACTILE AND ELECTROMOTIVE METHODS 

Pfeffer's experiment on expulsion of water from excited cells Author's experiment 
on a delicate method of detecting excitatory expulsion of cell-sap Chemical 
method of determining velocity of transmission of excitation Electrotactile 
detector Demonstration of passage of excitatory contractile wave by means 
of electrotactile method Determination of velocity of transmission of exci- 
tation in ordinary plants by electromotive method Excitatory versus hydro- 
mechanical movement of water. 



THE fact that the excitatory wave is propagated with a 
constant and measurable velocity was demonstrated, with 
regard to sensitive plants, in the last chapter, the arrival of 
the wave from a distance at the motile organ being detected 
by means of mechanical response. As ordinary plants, on 
the other hand, do not possess such efficient motile indicators, 
some other method, more universally applicable, is necessary 
in order to show that in these also the state of excitation is 
transmitted from point to point. 

Before describing the two methods which I have devised 
for this purpose, I shall give an account of a very interesting 
experiment, depending on chemical reaction, by which I have 
been able not only to demonstrate in a striking manner the 
expulsion of water from excited cells, but also to make a 
rough determination of the velocity of transmission. It was 

^ 

shown by Pfeffer that on exciting the lower side of a pulvinus, 
water oozes out from the cut end. On taking a detached 
pulvinus, and stimulating the lower surface with a blunt 
needle, he found that the organ curved downwards, and a 
drop of water was seen to escape from the cut end. 



ELECTROTACTILE DETECTION OF EXCITATORY PULSE 255 

* 

Chemical method of detecting excitatory expulsion of 
cell-sap. My own experiment differs from this in several 
particulars. Since there is a general impression that certain 
specialised tissues in the pulvinus are alone excitable, it was 
my object to show that cells which do not exhibit any motility 
will also give rise to the expulsion of water by excitatory 
contraction, and I desired further to utilise this effect for the 
determination of the velocity of transmission of excitation in 
that tissue. The essentials to this purpose were : some means 
of detection of the excitatory expulsion at the exact moment 
of its occurrence ; some means of marking accurately the 
moment of application of stimulus ; and, lastly, the use of a 
fairly long tract of tissue, in order that the interval between 
application of stimulus at one end, and the manifestation of 
its reaction at the other, might be of a duration capable of 
exact measurement. For this purpose I took petioles of 
Mimosa and the non-motile stems of the same plant, and 
placed their cut ends in very dilute solution of sodium chloride. 
So dilute a solution of salt does not, as I find, appreciably 
affect the excitability of the tissue. Selecting one of the 
specimens, say a petiole, I adjusted the electrothermic 
stimulator at a distance of, say, 4 cm. from its lower or cut 
end, the specimen being held vertical by means of a clamp. 
The vessel of salt solution in which it had hitherto been 
placed was now removed. The end of the petiole was 
carefully rinsed, to remove all traces of salt from the outside, 
and a small beaker of very dilute silver nitrate solution was 
substituted. At this point it became necessary to finish the 
experiment rapidly, as silver nitrate solution is likely after a 
time to affect the excitability of the tissue. Momentary 
thermal stimulus was given' by brief closure of the electric 
circuit The excitation then travelled through the intervening 
4 cm. of tissue, with a velocity characteristic of the plant. 
When it reached the cut end, the excitatory contraction 
produced an expulsion of cell-sap containing the salt 
solution previously absorbed. This expulsion was instantly 
made visible by the formation of a dense white precipitate 



256 PLANT RESPONSE 

of silver chloride. This was sometimes seen to be projected 
as a white vortex ring. The interval between the application 
of stimulus and this projection was found to be three seconds. 

It is thus clear that the excitatory wave is not one of 
hydrostatic disturbance, for such a disturbance would be 
transmitted with very much greater velocity. The velocity 
of transmission in this petiole is seen to have been 1 3 mm. 
per second, or practically the same as that obtained by a 
separate experiment on the fall of the leaf with similar 
specimens. 

In a similar manner I determined the velocity of trans- 
mission through the stem. The stimulus was applied at a 
distance of 5 cm. from the cut end, and the chemical pre- 
cipitation was observed, after an interval of five seconds. The 
velocity in the stem is thus seen to be 10 mm. per second. 
A repetition of this experiment with another piece of stem 
from the same plant gave similar results. 

It is thus seen that cells through which excitation is 
proceeding undergo excitatory contraction, in consequence of 
which there is an expulsion of water forwards, in the direction 
of propagation. And this effect is produced, not in pulvinated 
organs alone, but in others which are not usually regarded as 
motile. This result is also arrived at independently by the 
method of electrical response. 

The electrotactile method. I shall next give an account 
of the much more delicate methods which I have succeeded 
in devising for the detection of the excitatory impulse during 
transit, the first of which is the electrotactile method. It is 
easy to understand that while the wave of excitation is passing 
through any given section of tissue, it must produce there 
certain form-changes, infinitesimal though they may be. Had 
our sense of touch been more delicate, we might perhaps have 
been able to perceive this pulse. It occurred to me, however, 
that it might be possible, if it existed, to obtain its indication 
by means of an electrical method of detection, the sensitiveness 
of which could be exalted at will. 

As regards the pulse of form-changes we can see that two 



ELECTROTACTILE DETECTION OF EXCITATORY PULSE 257 

* 

different effects are possible. For example, in the case of 
muscle with parallel fibres, the wave of excitation will travel 
onwards from the excited point, the contraction produced 
giving rise to expansion of the muscle in a direction at right 
angles to that of propagation. In the intestinal' muscle, 
however, owing to a different distribution of the fibres, the 
propagated wave is one of constriction. Now, it is clear that 
these two kinds of muscle, placed within enclosing contacts, 
will give rise, during the passage of excitatory waves, in the 
one case to an increase of pressure, and in the other to its 
diminution. 

We are thus prepared to see that if similar contractile 
waves pass through a vegetable tissue, they may be detected 
by means of a concomitant variation of pressure a variation 
which may prove to be either an increase, or a diminution, 
according to the particular disposition of the contractile 
elements in the tissue. If the tissue, again, should happen to 
be anisotropic, the same wave of contraction may appear to 
give rise to a diminution of pressure in one direction, and an 
increase in tKat at right angles. In any case the excitatory 
wave, in the course of its transmission through any given area, 
might be expected to produce variation of pressure, as between 
two diametrically opposite leading-points. 

I am now about to describe the electrical device which 
I have used for the detection of such transient pressure- 
variations, concomitant to the passage of excitation through 
vegetable tissues. It is known that the electrical resistance 
of contact varies with pressure, and on this principle depends 
the construction of the microphone. But a loose contact, 
such as would be favourable for microphonic use, is unsuitable 
for our present purpose, by reason of the disturbance to which 
it is subject from atmospheric vibration. The necessity to 
be met, therefore, is that of the adjustment of an electric 
contact, which shall not be subject to resistance-variation 
from atmospheric disturbance, and which shall, at the same 
time, be sensitive to the pressure-variation effected by the 

excited tissue. 

s 



2$8 PLANT RESPONSE 

I overcame the difficulty regarding extraneous dis- 
turbance by using, instead of a loose, a steady pressure- 
contact, capable of the finest adjustment, by means of a 
micrometer screw. The next problem lay in choosing 
contact material of great sensitiveness. For this purpose 
I used different materials, the sensitiveness of some being 
very great, while that of others was moderate. Carbon 
contacts belonged to the latter class, but had the advantage 
of being easily adjustable. Various metallic powders, how- 
ever, such as that of bronze, were considerably more sen- 
sitive, but at - the same time required greater care for 
adjustment. 

The most important factor in the arrangements, by which 
the sensitiveness of the contact may be exalted to a very 
high degree, lies in the proper adjustment of the electro- 
motive force acting on the contact circuit. The sensitiveness 
to pressure-variation increases with increasing electromotive 
force, being greatest when this is just short of a certain 
critical value, after which the electric contact is observed to 
become unstable, and give rise to spontaneous oscillatory 
variations. For the purpose of easy adjustment of the 
electromotive force I use a sliding potentiometer. The 
tissue, say a stem of Mimosa, is placed between points, 
A and H, and by means of the micrometer screw, s, it is 
pressed against the spring B. The electric contact, say oi 
carbon points, is between B and C, which are in circuit with 
a galvanometer, and the potentiometer. The pressure i 
so adjusted that a feeble current flows through the gal- 
vanometer. In order to increase the sensitiveness of the 
detector, the electromotive force may be gradually increased 
by proper adjustment of the potentiometer, short of the 
critical point (fig. 106). The deflected spot of light from 
the galvanometer will remain steady, provided the adjust- 
ments have been properly made. 

The tissue is now stimulated at a distant point, care 
being taken that it is not in any way jarred mechanically. 
Stimulation without jar is effected, however, without difficulty 



ELEd'KOTACTILK DETECTION OI< EX( ITATORY PULSE 259 

by the use of the electrothermic stimulator. In any case, it 
is easy to discriminate between the effect of any such ex- 
traneous disturbance and the true excitatory effect in transit 
for which we are looking ; for the effect of the former, if it 




FIG. 106. Electrotaclile Method for Detection of Excitatory Wave 

during Transit 

Stem placed under light pressure between B and sliding rod c. A, B, elec- 
tric contact points, the pressure of which is delicately adjusted by 
micrometer screw, M. A moderate current flows round the circuit, 
including the contact points, A K, and the galvanometer, o, the 
E.M.F. being suitably adjusted by the potentiometer slide, P Stimu- 
lation of stem effected by momentary closure of key, K, in circuit 
with electrothermic stimulator. Excitatory svave reaching the zone H, c, 
causes pressure- variation, with concomitant galvanometric response. 

occurs, is immediate, whereas that due to excitation takes 
place after a definite interval from the application of stimulus. 
In the experiment of which the record is given in fig. 107, 
stimulus was applied at a distance of 4 cm. The excitatory 
wave reached the experimental zone after an interval of five 

S 2 



26o 



PLANT RESPONSE 



seconds, and indicated its presence by a diminution of 
pressure on the contact (fig. 107), resulting in sudden 
diminution of current After the passage of the excitatory 
wave the tissue was restored to its original condition, as 
shown by its once more exerting its normal pressure. This 
process of response and recovery by variation of pressure 
is exhibited by the galvanometer record. The recovery is 
usually found to be attained in the course of a half to one 
minute, according to the intensity of stimulus. As the result 
of this experiment, we find the velocity in this particular 

stem to be 8 mm. per second, which 
is very near the determination already 
made by other methods, with different 
specimens of stem of Mimosa. 

Frequently as 1 have obtained this 
response during transit by diminution 
of pressure, its opposite, that is to say, 
response by increase of pressure, is by 
no means uncommon. I have already 
explained how it is possible for excita- 
tory contraction to give rise to two 
such opposed effects, in consequence 
either of different dispositions of the 
contractile elements in the tissue, or 
by the presence of anisotropy in the 
organ. By means of the electrotactile 
method, then, we are able to demon- 
strate the passage of the excitatory wave, and also to measure 
its velocity, in tissues which are not motile. 

The electromotive method. I shall now describe the 
second, or electromotive, method which I have used for the 
detection of the excitatory wave during its passage through 
a vegetable tissue. I have already explained that when the 
plant-tissue is directly excited, the state of excitation is 
invariably accompanied by an electromotive variation, the 
excited point becoming galvanometrically negative (p. 32). 
Hence, when an excitatory wave is transmitted through the 




Fie. 107. Electrotactile 
Response in Stem of 
l\liinosa 

Stimulus applied at the mo- 
ment x , at a distance 
of 4 cm. below the 
detector. 



ELECTROMOTIVE DETECTION OF EXCITATORY PULSE 26 1 



tissue in any direction, from the stimulated point, there must 
always be an electromotive wave as its strict concomitant. 
The moment, therefore, when the excitation reaches a given 
point may be determined by observing the arrival of this 
excitatory electrical disturbance of galvanomctric negativity, 
and for the detection of such an excitatory wave the gal- 
vanometer takes the place of the motile leaflet. 

In order to prove that the excitatory mechanical and 
electrical effects are strictly concomitant, it is only necessary 




FIG. 108. Experimental Arrangements for Simultaneous Recording of 
Mechanical and Electrical Responses 

Stimulus applied by thermo-electric stimulator, s, at A. Excitation reach- 
ing B causes mechanical response of leaflet, which is recorded by 
optical lever on drum at M. Simultaneous galvanometric negativity 
recorded at E. 

to perform an experiment on a plant, such as Biophytmn, 
which is provided with motile leaflets. We attach one of the 
indicating leaflets to the Optic Lever, and connect its base B 
with one of the electrodes of the galvanometer, the second 
electrode of which is connected with a distant point of the 
leaf (fig. 1 08). The two spots of light, one from the Optic 
Lever indicating mechanical, and the other from the gal- 
vanometer indicating electrical, response are adjusted to lie 



262 PLANT RESPONSE 

one above the other on the same revolving drum. On 
applying a stimulus, say thermal, at A, it will be found, 
after the lapse of a definite interval, that both spots of light 
are deflected simultaneously, proving the concomitance of the 
mechanical and electrical effects. Such a record has been 
given already, in fig. 26. If we make a mark on the 
revolving drum at the moment of the application of stimulus, 
and a second mark when the electrical (and, in this particular 
case, also the mechanical) response is initiated, we can, with 
the previous knowledge of the speed of the drum, determine 
the time taken by the excitation to travel from A to B, and 
thus find the velocity of transmission for that specimen by 
electrical means. 

A further refinement of this method lies in the use of two 
galvanometers instead of one, the slight lag of response, 
caused by galvanometric inertia, being in this way eliminated. 
Particulars regarding this will be found elsewhere. 

Jt will be seen, then, that in the electromotive method 
we have a second means by which to determine the velocity 
of transmission of excitation in what are known as ordinary 
plants. I shall now describe experiments performed by this 
method. The peduncle of Biophytum is leafless. That it 
does transmit stimulus is seen nevertheless when we excite 
it at any point. Excitation will then be found transmitted 
through it to the main stem, from which it travels outwards 
to different leaves, a fact evidenced by the serial fall of 
leaflets in a centrifugal order. In such a peduncle 1 have 
determined the velocity of transmission by the electromotive 
method. The distance between the points A and B was 
4*6 cm.; the time taken for transmission was 127 seconds. 
The velocity is thus found to be 37 mm. per second. 

Similarly, in the stem of Ficus religiosa, I found velocity of 
transmission to be 9*4 mm. per second, which is almost the same 
as that in the stem of the so-called ' sensitive ' plant Mimosa. 
From the experiments carried out on the electrotactile 
method it will be seen that excitation is conducted along 
a plant-tissue from cell to cell, as a contractile wave. We 



ELECTROMOTIVE DETECTION OF EXCITATORY PULSE 263 

% 

have also seen that water is expelled from an excited and 
therefore contracted cell. It is further clear that when an 
excitatory wave, is proceeding in any direction, this cell-to- 
cell passage of excitation will give rise to a cell-to-cell 
contraction, the result of which will be a forward movement 
of water, which will have the velocity, not of hydrostatic 
transmission, but of the excitatory wave. The hydrostatic 
disturbance is quite distinct, being transmitted with great 
rapidity, and its presence has been shown in the preliminary 
abnormal response of erection in leaflets of Biophytum 
(p. 24). But that propulsion of water which is con- 
comitant to the passage of true excitation is very much 
slower, having the same speed as that of excitation itself. 

SUMMARY 

The direct effect of stimulus is not transmitted by means 
of hydrostatic disturbance, but by a cell-to-cell propagation 
of excitation. 

This transmission of excitation from cell to cell is attended 
by a cell-to-cell contraction. 

The passage of such a contractile wave may be detected 
by the electrotactile method, which thus enables us to 
determine the velocity of transmission of excitation, even 
in tissues which are not motile. 

In consequence of the concomitance of the excitatory 
wave with cellular contraction, water is moved forward pro- 
gressively, with a velocity and in a direction the same as that 
of excitation. 

This movement of water is not brought about by any 
hydro-mechanical action, but is the direct effect of the con- 
tractile wave due to excitation. The hydrostatic disturbance, 
when present, is transmitted with very great velocity, and its 
effect is seen in the abnormal preliminary response of erection, 
exhibited, for example, by the leaflets of Biophytum. 

The velocity of transmission of excitation in ordinary 
plants may also be found by determining the velocity of the 
concomitant electromotive wave. 



CHAPTER XXII 
THE LATENT PERIOD AND REFRACTORY PERIOD 

The determination of the latent period in Mimosa Experimental arrangements 
for obtaining automatic record Prolongation of latent period by cold Spark- 
record for determination of latent period Prolongation of latent period by 
fatigue - Sluggishness of the response of Philanthus urinaria, also long latent 
period and very protracted period of recovery - Latent period reduced under 
strong stimulation Response in Biophytum on the ' all or none ' principle 
Definite value of effective stimulus Phenomenon of refractory period in 
Biophytum Parallelism of responses in Biophytum and in cardiac muscle 
Additive effects Inappropriateness of term 'refractory period ' Energy in 
excess of effective stimulus held latent for subsequent manifestation. 

HAVING explained the means by which it is possible to 
apply a quantitative stimulus of uniform or increasing 
intensity, and also how the responsive effect and its time- 
relations are accurately recorded, we shall next turn to the 
study of the various characteristics of the response itself, as 
given for example by the plant Mimosa or Biophytum. In 
order that we may inspect different parts of the response- 
curve in greater detail, it will be necessary to take the record 
on a fast-moving drum, so that the curve may be drawn 
out, and its several features more easily distinguished. As 
we wish, moreover, to study the excitatory effect on the 
motile organ, stimulus will be applied directly on the 
pulvinus. 

Automatic method of record. As it will be necessary 
in the course of the following investigation to measure the 
times of reaction accurately, to small fractions of a second, 
the record must be obtained automatically. It is to be 
remembered that, as said before, response in vegetable tissues 
is relatively more sluggish than in animal, and it is super- 
fluous to arrange for measurements of time up to more than 



LATENT PERIOD AND REFRACTORY PERIOD 265 



hundredths of a second. The experimental method that I 
am about to describe would, however, enable us, if necessary, 
to make determination of time-intervals of one- tenth this 
magnitude, the question being only one of using a recording 
drum with the requisite increased speed. The drum used for 
these experiments was one constructed by Verdin, and 
provided with a very perfect governor. Some little time 
elapses after starting the drum before it acquires uniform 
speed, which it afterwards maintains, however at least 
during the short time required for the experiment with 
great perfection. The record is not taken until this uniform 
condition is attained. The mirror of the Optic Lever throws 
a spot of light upon a sensitive photographic film, wrapped 
round the revolving drum. In order to produce records of 
the required rapidity, I employ sunlight, proceeding from a 
pinhole, which after reflection from the mirror of the Optic 
Lever falls on the drum, appropriately focussed by means of 
a condensing lens placed at the end of a focussing tube, as 
seen in the figure. It is understood that, the record to be 
obtained being photographic, this experiment is carried out 
in a dark room, the sunlight required for the record being 
directed upon the pinhole by a heliostat outside. 

The stimulus consists of a single strong break-shock, 
from a RuhmkorrT's coil, one electrode of which, by means of 
non-polarisable connections, is attached to the pulvinus of a 
leaf, and the other to the main stem lower down, of a speci- 
men of Mimosa. The shock is applied by the recording 
drum itself, at a particular moment in the course of its 
revolution ; and at the same instant the curve of response 
begins to record itself automatically. These two acts of 
imparting stimulus, and of opening a shutter by which the 
recording ray of light is allowed to fall upon the moving film 
are performed simultaneously ; and both alike are initiated 
by the stroke of a rod which is fixed to the axis of the drum 
underneath. 

This rod, which I shall designate as the striker, at a certain 
period in the revolution of the drum, impinges upon a balanced 



266 



PLANT RESPONSE 



electric key, K,, thus closing an electro-magnetic circuit, and 
so releasing the shutter s, which is immediately in front of 
the pinhole, by which sunlight is admitted. This drop of the 
shutter, simultaneously producing a break of the Ruhmkorffs 
coil circuit, gives an excitatory electrical shock to the plant. 




FIG. 109. Apparatus for Automatic Record 

When the speed of the drum has become uniform, K., is closed, and the 
striker, in connection with the drum, closes the electro-magnetic 
shutter circuit. The dropping of shutter, s, interrupts by K., the primary 
circuit of the induction coil, the secondary of which gives a shock to 
the plant. The fall of the leaf pulls down the Optical Lever, o, pro- 
ducing record on drum. K.,, short-circuiting key of secondary. 

The electro- magnetic circuit of the shutter is interrupted 
by a key, K a , and this is kept open till the speed of 



the drum has become uniform. On closing K 9 , the circuit 



is still incomplete ; -but the striker, impinging against the 



LATENT PERIOD AND REFRACTORY PERIOD 267 



balanced key, K J? completes the circuit, and actuates the 
shutter, 

A thread connects the shutter with one arm of a second 
balanced key, K 3 . This arm is so overweighted that when 
freed, it causes two prongs, at the opposite extremity of the 
lever, which complete the primary circuit of the Ruhmkorff's 
coil, to be lifted out of their cups of mercury, and thus the 
circuit, is interrupted. But the thread is of such a length 
that when the shutter is set, so as to close the pinhole, 
the prongs, dipping into the cups of mercury, complete 
the primary circuit. The overweighted arm of K 3 falls, 
with the drop of the shutter lifting up the prongs, and 
thus suddenly interrupts the primary current, giving rise 
to a break-shock in the secondary, which passes through 
the plant. During the course of the preliminary adjustment, 
when primary circuit of the coil is made, a make-shock. is 
produced, but this is prevented from affecting the plant by 
a key, K 4 , which short-circuits the secondary. When the 
adjustment has been made, this short-circuiting key is opened. 

Briefly to recapitulate the procedure : The drum, carrying 
the sensitive film, is released, and begins to revolve. The 
key K 2 of the shutter-circuit is kept open, until a uniform 
speed is attained. It is then closed. The striker connected 
with the drum now closes the balanced key, K, ; the shutter 
drops, and simultaneously interrupts at K 3 the primary current 
of the induction coil, thus causing an excitatory shock to be 
given to the plant. 

The determination of the latent period. It will be seen 
from the upper of the two photographs, given in fig. 1 10, 
that for a period of T %- of a second the record remains hori- 
zontal. This represents the latent period, after which the 
tissue begins to respond. For a further period of half a 
second the leaf is seen to fall with a considerable and approxi- 
mately uniform speed. The rate of movement of the tip of 
the leaf is now 7 1 mm. per second. After this the leaf con- 
tinues to fall, but with a diminishing speed, till the maxi- 
mum contraction fall is reached. From records obtained on 



268 



PLANT RESPONSE 



slower-moving drums, I find that this is attained in different 
specimens, in a period of 1-5 to 2*5 seconds after the shock. 
This maximum contraction persists for a further period of 
about thirty seconds. The leaf now begins to erect itself, 
and full recovery is attained in the course of a further period 
of about six minutes. These statements refer to reaction in 
vigorous Mimosa, at a favourable season of the year, like 
summer. In an unfavourable season, like winter, however, 
the reaction becomes very sluggish, and recovery is not then 

complete in less than 
eighteen minutes, or 
three times the normal 
period. 

Effect of cold on 
latent period. I shall 
next refer to the slug- 
gishness induced by 
cold, prolonging the 
latent period. The ex- 
treme instance of this is 
seen when iced water is 
applied to the pulvinus, 
and too great cooling 
being thereby effected, 
the response is abolished. 
With moderate cooling 
the latent period is found 
to be prolonged to 
several seconds. This 




FIG. no. Photographic Record of Response 
of Mimosa^ Exhibiting the Latent Period 
in its Variation 

The upper curve was taken under normal 
conditions, and the lower when the pul- 
vinus was slightly cooled. Time-marks 
= tenths of a second. Original record 
reduced to half. 

[The duplication, which \\ill he observed in 
each record, is due to the fact that the 
heliostatic mirror was silvered behind, 
thus producing two reflections, one from 
the surface of the glass and the other from 
that of the silver.] 



effect cannot conveni- 
ently be shown, however, within the limits of a fast record. 
In order, therefore, to show the comparative effect of cold on 
the latent period, in the case of the same specimen whose 
record is seen in the upper of the two photographs in fig. 1 10, 
I was careful to cool the pulvinus very slightly. In the lower 
of these photographs it will be seen that the latent period has 
become prolonged, from the normal jVn to nnr f a second. 



LATENT PERIOD AND REFRACTORY PERIOD 269 

The rate of responsive movement is also seen to have under- 
gone considerable diminution. In the first, or normal, case, 
during half a second after the commencement of response, the 
rate of movement was, as said before, 71 mm. per second. 
In the second case, however, after slight cooling, it is seen to 
have been reduced to 26 mm. per second, or almost one-third 
of the original rate. 

Record by means of electric sparks : prolongation of 
latent period by fatigue. In order to overcome the difficulty 
of the insensitiveness caused by keeping the plant in a photo- 
graphic dark room, I have recently devised a method of 




FIG. ill. Electric Spark Record, Showing Increase of Latent Period by 
Fatigue, in Successive Responses of a Leaf of Mimosa 

Latent period in normal topmost record seen to be ^ second ; this increased 
in next taken I minute before full recovery to T : ]-J| y second; latent 
period further increased in last case taken 3 minutes before full 
recovery to {^ second. Note also progressive change in slope of 
curve. 

record by means of a series of punctures produced by electric 
sparks on a recording paper surface. The sparks occur at 
the short gap between the end of the long arm of the record- 
ing aluminium lever, and the drum, these being connected 
respectively with the two electrodes of a Ruhmkorfif's coil. 
The electrical disturbance does not affect the plant, as the 
leaf is attached to the lever by a long silk thread. One 
great advantage of this method lies in the fact that the time- 
intervals, which may be as short as desired, are indicated by 
the distance between successive punctures, which are deter- 
mined by the frequency of the vibrating interrupter of the 
coil. In the case of which the record is given (fig. 1 1 1) the 



2/O PLANT RESPONSE 

interval between successive sparks was T ' r , of a second. By 
this means, it was found that increasing fatigue induced a 
corresponding increase in the latent period of a leaf of 
Mimosa, from the normal ffi {J to -ffa of a second. 

Response of Biophytum. I shall now proceed to de- 
scribe effects essentially similar to the last, seen in Biophytum. 
In order to be able to observe in detail the various responsive 
peculiarities of the curve, subsequent to stimulation, records 
were taken in this case on a much slower-moving drum. The 
record given in fig. 112 shows the mechanical response to 
stimulation produced by discharge of condenser (capacity *oi 




KK;. 112. Response of Biophytum ; Electrical Stimulus having been 
Applied at the- Pulvinus of tlie Motile Leaflet 

The ihu'k dot represents the moment of stimulation. 

microfarad, charged to nine volts). The exact moment of 
stimulation is marked on the record. 

It will he seen that the leaflet begins to respond almost 
instantaneously. The maximum contraction in this case, 
being considerably more rapid than in that of Mimosa^ is 
almost attained in the course of half a second. An interest- 
ing point to be noticed in the record is the flattening of the 
top of the curve (fig 112). That is to say, the maximum 
contraction persists for a considerable time before recovery 
begins. In the present case this lasted for ten seconds. This 
period varies in different specimens, from a maximum of ten to 
about two seconds. But in this particular specimen the period 



LATENT PERIOD AND REFRACTORY PERIOD 271 

* 

in question remained at least approximately constant, in suc- 
cessive experiments. After this there was commencement of 
recovery, which was completed as seen by the return of the 
spot of light to its exact original position in the course of 
five minutes. In order to form an idea of the consistency of 
results which may be expected from a good specimen, I re- 
peated this experiment six times in succession, commencing 
the record each time at the same point on the recording 
surface as before. The degree to which all these curves 
coincided with each other in detail is almost incredible. 

* 

Their rising portions, their flat tops, and their gradual descent 
during recovery, were all so coincident that the six successive 
curves appeared as but one. < 

In specimens of plants which were not in good condition, 
fatigue was shown by the gradual diminution in height of 
successive responses. For accurate standard experiments it 
is therefore necessary to have specimens which are vigorous. 

We have already seen the prolongation of the latent 
period which is induced by cold, ir|i the case of Mimosa. 
I have obtained similar results also in working with Bio- 
phytum. For example, in a certain experiment, moderate 
cooling induced a prolongation of two seconds in the latent 
period. When the plant was allowed to return to the sur- 
rounding temperature of the room, however, the increase of 
latent period disappeared. 

Latent period diminished by increased intensity of 
stimulus. It has been said before that there are innumer- 
able gradations between the extreme cases of motile sensi- 
bility in plants. As regards motility, an extremely sensitive 
leaflet was that of Mimosa pudica. Somewhat less quickly 
reacting were those of Biophytum sensitivum, and we had in 
the leaflets of Philanthus urinaria an instance of extreme 
sluggishness. 

The latent period of the leaflet of Philanthus, under 
moderate stimulus, is as long as three minutes, and the maxi- 
mum contraction is not attained under forty minutes ; but 
with a stronger stimulus the latent period is reduced to less 



2/2 PLANT RESPONSE 

than one minute, and the maximum contraction takes place 
in a relatively short period of about fifteen minutes (fig. 30, 
b and c, p. 44). 

Response of Biophytum on the ' all or none ' principle. 
It is well known that in the case of a contractile skeletal 
muscle, there is a minimal intensity of stimulus which is 
necessary in order to produce contraction. From this point 
onwards, as the stimulus is gradually increased, the response 
increases, till a maximum contraction is arrived at, beyond 
which still further increase of stimulus produces no increase 
in effect. In cardiac muscle, however, the range of stimulus 
between minimal and maximal is practically narrowed to a 
point, so that the minimally effective stimulation is also at 
once maximal. It is to be remembered, at the same time, 
that the differences between cardiac and skeletal response are 
a question of degree, rather than of kind. 

Curiously, the response of Biophytum is, in this respect, 
somewhat similar to cardiac response. In an experiment 
with a particular specimen of Biophytum^ the intensity of 
stimulus was increased by successive increments of the E.M.F. 
used for charging the condenser. With an E.M.F. of seven 
volts there was no response. With a charge of nine volts 
there was always a response, and this was maximal. A 
charge of eight volts was almost on the threshold of response. 
That is to say, when I started experimenting, the leaflet was 
in a somewhat sluggish condition, and an eight-volt charge 
was ineffective. But after obtaining response to a nine-volt 
charge, I could obtain, response also at eight volts. This was 
due to the fact that molecular inertness had been removed by 
the preceding effective shock. Thus we have two determi- 
nate values of stimulation, giving respectively maximum 
response and absence of response, the charges, namely, of 
nine and seven volts. The effective stimulus is, of course, 
constant for a given individual, but differs with the excita- 
bility of different specimens. Here, then, we have an instance 
of the ' all or none ' effect. The leaflet either responds to 
the utmost, or not at all. 



LATENT PERIOD AND REFRACTORY PERIOD 273 

Refractory period. We shall next consider the pecu- 
liarities of the refractory period which I .have discovered in 
the case of plant-tissues ; and for the material of this investi- 
gation we shall use the plant Biophytum^ taking, in fact, the 
very specimen whose successive responses have already dis- 
played such remarkable consistency (fig. 1 1 2). Such uni- 
formity in successive responses is only possible when we 
allow sufficient time of rest for complete protoplasmic re- 
covery, by which the excitability is fully restored. But it has 
been shown in Chapter XX. that if sufficient, time of rest be 
not allowed, the protoplasmic recovery is incomplete, and the 
excitability is diminished. Hence the extent of response, 
which is an outward indication of excitability, is diminished, 
and this effect is known as fatigue. 

We also arrived, in the same chapter, at the theoretical 
conclusion that there is a minimum resting interval, the dimi- 
nution of which results in such a loss of excitability as to 
abolish response, and this period we know as the Refractory 
Period, because the leaf then apparently takes no account of 
stimulus, or is 'refractory' to it (fig. 105). We shall now 
enter into greater detail regarding the peculiarities of this 
refractory period. After taking the six curves in response to 
separate single stimuli which were so extraordinarily similar, 
I proceeded to take a curve of response to two equal stimuli 
of the same intensity as before namely, nine volts, charging 
oi microfarad the two stimuli following each other at an 
interval of one second. The application of the second stimu- 
lus appeared to produce no effect, the extent and general 
character of response being the same as in the case of single 
stimulus, with only the difference of a slight elongation of the 
flattened top of the curve. I next tried the effect of two 
stimuli at an interval of five seconds. The leaflet was still 
refractory to the second stimulation, but when I applied it at 
an interval of ten seconds, the second stimulus became effec- 
tive. It will thus be seen that Biophytum has rather a long 
refractory period, during which, as far as can be seen, it takes 
no account of the impact of a nf w stimulus. This refractory 

T 



274 



I'LANT RESPONSE 



period is a matter of several seconds, but varies somewhat 
with different specimens. I have, again, in some cases 
observed a very curious phenomenon of two refractory 
periods. 

We thus find several very interesting parallelisms between 
the response of BiopJiytum in plants and that of cardiac 
muscle in animals. We find in both that the minimal 
response is also the maximal, increasing stimulation pro- 
ducing no increase of response. In the response-curve itself 

the flattened top is common 
to both ; both have a pro- 
longed refractory period ; 
and we shall see later that 
in both there is a tendency 
to the production of mul- 
tiple rhythmic responses. 

In all these respects the 
responses of Biophytum 
resemble the cardiac re- 
sponses, rather than those 
of skeletal muscle. But 
they have one peculiarity 
in which they share the 
characteristics of the re- 
sponses of skeletal muscle. 
In the responses of cardiac 
muscle, successive effects 
are not additive, perhaps because that muscle undergoes 
the maximum contraction possible. In Biophytum^ however, 
while any effective stimulus whether minimal or largely 
super-maximal will produce response of the same extent, 
yet this response, though the greatest possible for a single 
stimulus, is not the utmost of which the leaflet is capable. 
Hence, if we superpose successive stimuli, taking care that 
they do not fall within refractory periods, we shall obtain 
an extremely interesting response, showing the separate 
additive effects. I give here two curves, exhibiting these 




FIG. 113. Additive Effects seen in Re- 
sponses of Biophytum to Stimuli which 
Fall outside the Refractory Period 

The record to the left shows the effect of 
stimuli, applied at intervals of one, 
and that to the right of half a minute. 



LATENT PERIOD AND REFRACTORY PERIOD 275 

effects of superposition of stimuli. In the right-hand record 
in fig. 113 the stimuli were applied at intervals of thirty 
seconds. The successive responses, except the last, show 
a regular decrease. In the left-hand record, successive 
stimuli were applied at intervals of a minute, and appear 
much more equal. 

But even in these additive effects we find one peculiarity 
which is also characteristic of cardiac response. In the latter 
case, though on repetition of stimulus there is no summation 
of height of response, yet the apex-time .of the second 
response is shorter than that of the first. We see this in 
the case of Biophytum> in the right-hand curve. The first 
response has a slightly rounded top, but. this is reduced to an 
acute angle in the second. The record, having t*een reduced 
to one-eighth for reproduction, does not show this so plainly 
as does the original. 

Though each single response of Biophytum is maximal, 
yet from fig. 1 1 3 we have seen that this maximal response 
does not represent the utmost movement of which the leaflet 
is capable. In the particular plant here used for experiment, 
the maximal response to individual stimulus was always 
about thirty-eight divisions, but four superpositions produced 
a total movement of ninety-four divisions. It must be 
remembered that such effects can only be possible when 
the second stimulus is timed to fall at the expiration of 
the preceding refractory period. 

If the absolute value of each individual minimally effective 
stimulus be represented by s, and if the whole be added 
together, or, in other words, if a stimulus of 48 be given 
at once, we may regard such a stimulus as made up of one 
minimally effective, plus three others which fall within the 
refractory period, and are thereby rendered totally ineffective. 
In other words, we may regard a very strong stimulus as 
made up of so many minimally effective stimuli. It is as 
if the first effective fraction alone acted, the succeeding 
portions, which arrive within the refractory period, being- 
inoperative. 



T 2 



2/6 PLANT RESPONSE 

In view of certain other phenomena not altogether dis- 
connected, it seems unfortunate that the term * refractory 
period 1 should be used with its present significance. For 
this term might be held to imply that the tissue refuses to 
take any account of the superfluous energy that is impressed 
upon it It is more likely, however, that by some peculiar 
mechanism, the superfluous stimulus z>. what is over and 
above the amouot s necessary for producing maximal re- 
sponse is prevented from overflowing. This excess of 
energy may, then, at least in some cases, remain latent, 
to be manifested at a later period in the form of excitatory 
impulses. Such impulses, again, attuned by some regulating 
process, may give rise to periodic or rhythmic overflow. 
That this is actually the case will be demonstrated in the 
next chapter, 

SUMMARY 

The latent period of response is protracted by cold. 
It is also protracted by fatigue. It is shorter under strong, 
than under moderate stimulation. 

In vigorous specimens of Btophytum leaflet, the mini- 
mally effective stimulus is also maximal ; under normal 
conditions, this minimally effective stimulus has a definite 
value. 

There is also a definite refractory period in the response 
of Btophytum. If a second stimulus fall within this refractory 
period, it appears to produce no effect. 

The response of the leaflet of Biophytum resembles in 
many respects that of cardiac muscle. In both, response is 
on the 'all or none' principle, and both exhibit a relatively 
long refractory period. 



PART IV 

MULTIPLE AND AUTONOMOUS 

RESPONSE 



CHAPTER XXIII 

ON MULTIPLE RESPONSE 

"Multiple electromotive responses due to a single strong* stimulus Multiple 
electrotactile responses Multiple mechanical responses in Biophytitm 
Cyclic variations in multiple responses Multiple retinal excitations Inter- 
mittent pulse in man and plant Semi-automatism Continuity of multiple 
and automatic response Conversion of Biophytitm into automatically 
responding plant ; conversion of Desmodium into ordinarily responding 
plant Similar polar effects of current in Biophytum and in Desmodium 
leaflet, at standstill Moderate stimulus in Biophytum and in Desmodium at 
standstill produces single response ; and strong stimulus, multiple response. 

I HAVE already explained in Chapter III. that the excitatory 
wave initiated by a stimulus has a concomitant electro- 
motive wave. If the plant experimented on is provided with 
motile leaves or leaflets, the excitation is evidenced by the 
simultaneous mechanical response of the motile organ, and 
the electrical response of galvanometric negativity. 

Multiple electromotive responses due to single strong 
stimulus. In my investigations on electrical response in 
plants, I was surprised to find that whereas a single mode- 
rate stimulus gave rise to a single electrical response, a very 
strong stimulus very often initiated a multiple series of 
responses. I have obtained such multiple responses to a 
single stimulus with all kinds of plants, ordinary and ' sensi- 
tive, 1 and under the action of various forms of stimulus 
chemical, thermal, and mechanical. 

To obtain these multiple electrical responses in an unmis- 
takable manner it is necessary that the excitation should 
reach one electrical contact and not the other, for in the latter 
case there would be complications arising from diphasic 
variation. The necessary condition may be fulfilled by 



280 



PLANT RESPONSE 



applying stimulus near the proximal contact, the other 
contact being at a relatively great distance. The periodicity 
of these multiple electric responses I find to vary in different 




KIG. 114. Multiple Electrical Responses due to Single Strong Stimulus 

(a) In Mimosa due to thermal, and (If) to chemical stimulation ; (r) in"] 
peduncle of Biophytum^ due to thermal stimulus. [N.B. This series 
persisted for two hours.] (c/), in hypocotyl of Tamarindus indica> due 
to stimulus of cut* 

cases from about a half to five minutes. These multiple 
electrical responses, resulting from a single strong stimulus, 

sometimes persist for as long as 
two hours (fig. 1 14). 

Multiple electrotactile re- 
sponses. In the course of my 
experiments on the electrotactile 
method of detecting the excitatory 
wave, I discovered multiple waves, 
initiated by a single strong stimulus, 
passing through the tissue. In 
fig. 115 I give a record of such 
multiple responses in the stem of 
Mimosa, in which it will be seen 
that there are four such responses, 
with an average period of one 
minute each. 

Now, as these pulsations are 
signs of the presence of excitation, 
it follows from the occurrence of such multiple responses 
that a stroncr .stimulus mnv cn've rise to a multinlo series 




FIG. 115. Multiple Electro- 
tactile Response in Stem of 
Mimosa^ due to Single Strong 
Thermal Stimulus 

(Original record reduced to ^.) 



MULTIPLE RESPONSE 28 1 

of excitations. I was therefore led to expect that this 
fact might be demonstrated in another and still more con- 
vincing manner, if I should succeed in finding a sensitive 
plant which exhibited these multiple excitatory waves by 
repeated movements of the indicating motile leaflets. 

Certain peculiarities of Biophytum which I had previously 
discovered led me to think that I might find in this plant the 
opportunity I sought ; for we have seen in the last chapter 
that in Biophytum a certain minimal intensity of stimulus 
induced the maximal mechanical response. With such 
a stimulus we obtain only one response, and if we apply 
a stimulus of very much more than minimal intensity it 
produces no greater mechanical effect. What, then, happens 
to the excess of stimulus ? This excess may be wasted as 
heat, or it may continue to exist in some latent form, 
and this latent stimulus may subsequently be given out 
rhythmically. 

Multiple mechanical responses. In view of these facts 
I expected a strong stimulus to give rise to those periodic 
waves whose existence I had been led to suspect from the 
observation of the recurrent electromotive and electrotactile 
waves in various plants. As a matter of fact it has been 
noticed that .Biophytum^ when strongly excited, exhibits two 
successive movements of mechanical response, the leaflets not 
completing their closure at once, but in the course of two 
twitches succeeding each other. But I expected to detect 
a larger number of pulsatory movements in response to a 
single strong stimulus. 

In order to do this, however, it was necessary to prevent 
the complete closure of the leaflets, by which the further 
exhibition of mechanical response was made impossible. 
For this purpose I used the Optic Lever, with the light 
counterpoise (p. 19). In this way I succeeded in demon- 
strating, through mechanical response, what had already 
been demonstrated electrically, the fact that a single strong 
stimulus, of whatever form thermal, mechanical, electrical, 
or photic will induce, not one but a multiple series of 



282 PLANT RESPONSE 

rhythmic responses in a plant. Thus on applying a thermal 
stimulus to the petiole of Biophytum near its insertion on the 
stem, an excitatory wave was found, as usual, to produce 
successive depressions of leaflets in a centrifugal order. But 
after a while . the existence of a second excitatory wave 
became evident, by a second series of closures of leaflets. 
That the wave was due not to excitation reflected in some 
way from -the tip of the leaf, but to a second wave starting 
from the original point of stimulation, was made evident by 
the fact that the successive fall of leaflets was again centrifugal, 
from near the stem to the tip of the leaf. From the records 
now obtained by means of the Optic Lever, attached to one 
of the leaflets, it was interesting to observe the very numerous 
rhythmic pulsations, often as many as twenty, now made 
-visible. 

As an instance of the simplest form of such multiple 
response, I give the following record, which was obtained 
under the thermal stimulation of the electrothermic stimu- 
lator. The point of application was close to the responding 
leaf. The average period of these pulsations was about 
thirty seconds (fig. 116), It may be stated here that the 
period of multiple responses of Biophytum is found to vary 
from about fifteen seconds to three minutes or so, depending 
on the condition of the plant and the intensity of the 
stimulus. 

The question suggests itself, since multiple electromotive 
and electrotactile excitations are observed in Mimosa^ why 
should not this plant also exhibit them mechanically ? The 
answer to this is probably found in the fact that the Biophy- 
tum leaflet is light, and easily exhibits fluctuating impulses, 
whereas the impulsive fall of the heavier Mimosa leaf persists, 
owing to greater momentum, till it is more or less complete. 
Again, unless an organ has at least partially recovered from 
fatigue, it is not susceptible of fresh excitation. The period 
of full recovery in the pulvinus of Mimosa is very long, 
being about seven minutes. We saw, in studying fatigue in 



MULTIPLE RESPONSE 283 

Mimosa, that when a second stimulus succeeds a first, after 
an interval of less than one minute, it produces no responsive 
effect. If, then, a second wave of excitation arrives within 
the refractory period of the first, we must expect that it will 
remain mechanically ineffective. In the multiple electro- 
tactile responses seen in fig. 1 14, these successive excitations 
are seen to have occurred at intervals of one minute, com- 
plete recovery being accomplished in that time. In the 




FIG. 116. Multiple Mechanical Response of J-iiof/iy/inn, due to a Single 

Thermal Stimulus 



stem, then, the period of recovery is very much shorter than 
in the pulvinus, and the same waves of excitation which 
at intervals of one minute produce response in the one 
case, prove ineffective in the other. Had these periodic 
waves of multiple excitation been three or four times as 
slow as they are, we might have been able to observe 
multiple mechanical responses of the leaf of Afimosa. In 
connection with this, I may state that I once observed 
a second mechanical response to a single strong stimulus 



284 PLANT RESPONSE 

in the leaf of Mimosa. In this case the second response 
occurred four minutes after the first. 

We have seen that multiple electrical response is obtained 
when any form of strong stimulation is employed. I now 
tried to find out whether multiple mechanical response could 
be produced by chemical stimulation. One of the middle 
leaflets of a Biophytum leaf was attached to the recording 
Optic Lever, and I applied a drop of sulphuric acid I cm. 
away from the recording leaflet in the direction of the tip 
of the leaf. This was found to give rise to five vigorous 
recurrent pulsations. 

Thus, though a single moderate stimulus evokes but 
a single response, yet under strong stimulation we obtain 
not only the immediate consequent response, but also a 
surplus of energy which remains over and is held latent in 
the tissue, to be given out later, after a shorter or longer 
interval, in the form of recurrent responses. From these 
experiments it is clear that a rhythmic series of effects 
need not have a periodic antecedent cause. As regards 
these pulsatory movements, it was shown on page 47 that 
the fall of the motile leaflet was due to a pulse of diminution, 
and its erection to a pulse of restoration, or increase, of 
turgidity. In these multiple responses, then, we have the 
expression of rhythmic variations of turgidity initiated by 
stimulus. 

Among ordinary responses that is to say, single response 
to single stimulus we have observed three types, depending 
on the excitability of the tissue. When the excitability 
remains uniform, the responses are uniform. When the 
excitability undergoes a gradual diminution, there is a cor- 
responding depression of response that is to say, fatigue 
supervenes. And when the excitability increa~ses by degrees, 
there is a correspondent enhancement of response known 
as the 'staircase* effect. In addition to these, I have, as 
explained before, also noticed some curious instances in which 
the excitability of the tissue appeared to undergo periodic 



MULTIPLE RKSl'ONSK 



28 5 



uniform stimuli exhibited periodic fluctuations (p. 106). We 
may thus have an alternate, periodic, or cyclic fluctuation of 
excitability exhibited in the responses. In the first of these, 
the responses are alternately large and small. In the last, 
we may have either an ascending or descending series, which 
is periodically repeated. 

Cyclic variation in multiple response. In multiple 
responses also we find all these types. And if the seat of 
origin of the multiple excita- 
tion be at a distance from the 
responding leaflet, we have 
in this fact an added element 
of variation. For we have 
now to consider, not simply 
the periodic variations of ex- 
citability of the motile organ, 
but also those of the excita- 
bility of the intervening tissue, 
causing periodic changes of 
conductivity. Hence, in re- 
cords obtained of multiple 
responses, where the seat of excitation is at a distance, we 
have a complex combination of variations of amplitude arid 
of period in the pulsations. 

Confining our attention to the period alone, we find that 
multiple responses may be classified under three headings : 




FIG. 117. Multiple Response in 
Biophytum 

The rhythm, at first slow, becomes 
quicker. 




Fiu. 118. Multiple Response in /->ia/>Jn'/u>n 
Quick rhythm becoming slo\\. 

those in which the successive periods are fairly uniform 
(fig. 1 1 6) ; those in which the pulsations are at first slow and 
later become quicker (fig. 117) ; and those which begin with 



286 



I'LANT KKSI'ONSK 



rapid pulsatiun.s, and afteruards slow down (fig. 1 1 8). And 
finally we may have any combination of these (figs. 119 and 
120). 




l't<'. MM Multiple Kcspon.se in Biophytiini, showing Cyclic Groupings 

of Amplitude and Period 

These cyclic changes of amplitude and period are seen in 
all types of rhythmic multiple responses, of which I shall 
show that the autonomous responses of Desmodinm and the 
growth-responses of all plants are only special cases. These 

characteristics are seen not only in 
the rhythmic responses of vegetable, 
but also in those of animal, tissues. 
The cause of the latter is still regarded 
as very obscure. Hence the study 
of similar phenomena in plants, under 
simpler conditions, may be expected 
to throw much light on the subject. 

How striking, even in their more 
intricate details, are the similarities 
between multiple responses in plant 
and animal, will be shown in the next 
three chapters. For the present I 
shall confine myself to the considera- 
tion of two of the most obscure 
instances in animal tissues, and 
to be paralleled in the case of the 




!' K;. 120. Multiple Re- 
sponse in BiopJiytinn, 
showing Cyclic Group- 
ings 



shall show them 
vegetable. 

Recurrent visual impulses. It is known that when we 
look for some time at a strongly illuminated object and after- 
wards close the eyes, we see the same image repeating itself 
many times in succession ; and of this phenomenon, so far 



MULTIPLE RESPONSE 287 

% 
% 

as I am aware, no satisfactory explanation has hitherto been 
offered. The phenomena which have been described in the 
case of vegetable tissues show, however, that this is nothing 
but an instance of multiple excitations in the retina caused 
by strong stimulation. In the case of the plant these recurrent 
excitations express themselves mechanically as twitches ; in 
the case of the firefly as flashes of light ; and in the retina 
as recurrent visual images. In these recurrent visual impulses 
are found all the peculiarities which I have observed in 
recurring excitatory impulses in Biophytum. . This is shown, 
not only in simple cases, but also in the most complicated. 
I shall presently give, in illustration of this, two parallel 
instances of multiple excitation in Biophytum and multiple 
excitation in retina (see table, p. 288), in which the successive 
intervals undergo similar cyclic changes. 

In order to measure accurately the intervals between 
successive visual images, I use a special stereoscope, based on 
my discovery of the phenomenon of binocular alternation of 
vision, and by this means I have found that the after-effects 
of light in the two eyes are not simultaneous but alternate 
that is to say. when the after-image in one eye is most vivid, 
that in the other eye has just vanished, and vice versa. For 
accurate measurement of this periodicity, it is necessary to 
have the effect on one eye distinguished from that on the 
other. For this purpose I have been accustomed to use a 
stereoscope containing, instead of photographs, incised plates 
with two inclined cuts, the right eye seeing the slit inclined 
to the right, and the left eye that inclined to the left. When 
the observer looks through this stereoscope turned towards 
a bright sky, his two eyes are acted on by strong stimulus 
of light. On closing the eyes, periodic visual impulses are 
sent from the strongly stimulated retina just as the stimu- 
lated area of Biophytum was found to send out periodic 
impulses. 1 

1 Bose, ' Binocular Alternation of Vision,' Response in the Living and Non- 
Living, p. 175. 



288 



PLANT RESPONSE 



TAHLK SHOWING CYI LI< \AKIAIION OK MULTIPLE RESPONSES 
IN BlOPHYTUM AND IN RETINA 



Cyclic variation in the periodicity of the 
multiple response of Biophytutn 



Cyclic variation in the recurrent visual 
impulses 



Interval between 1st & 2nd responses 52'' Interval between 1st & 2nd responses 7" 
2nd,, 3rd ,, 37" ,, ,, 2nd 3rd 5-25" 

M 59 3 rc * )> 4^h ,, 25 9 , ,? 3 rt * 4^h ,, 0*5 



5 J 
M 



59 
J5 



4th 5th 
5th 6th 
6th ,, 71 h 



,, 
,, 



25 
22" 

30" 
52" 



5 i 

JJ 
11 



> ? 

> > 
) ) 

n 



4th 
4th sth 



5th 
6th 



,, 



6lh 
;th 



9 ? 
? > 
5 J 
11 



7' 

9" 

ii" 



In both these cases, therefore, we begin with a compara- 
tively long interval, which grows shorter, and then, again, is 
progressively lengthened. 

Intermittent pulsation. We have now dealt with one 
of the two instances of correspondence between obscure pulsa- 
tory phenomena in animal and vegetable. The second, which 
remains to be considered, is that known to physicians as ' the 




FIG. 121. Intermittent Human Pulse (Broad bent) 

intermittent pulse' (fig. 121). * The term " intermittent " is 
employed to designate the pulse when a beat is occasionally 
missing from time to time, while the pulse in the intervals is 
perfectly regular. It is a remarkable variety of pulse, and is 
perhaps the least capable of explanation of any. The inter- 
mission may happen at regular and definite periods, every 
four, six, or up to twenty beats ; or the number of interven- 
ing pulsations may vary. The intermittent pulse may be 
habitual and constant, and in this case is more likely to be 
at definite intervals, or it may be occasional only, under the 
influence of some disturbing cause. . . . Occasionally nervous- 
ness and fatigue will render the intermissions more frequent.' ' 

1 Rroadbent, The Pulse, p. 125. 



MULTIPLE RESPONSE 289 

In connection with this, I have observed, among the 
types of cyclic variation in multiple response in plants, an 
instance in which every third response was missing, the period 
of each second beat being thus approximately twice as long 
as that of the first. It was easy to see that this was an 
instance of alternating fatigue, causing the particular record- 




FIG. 122. Interiniitence in Pulsation of Biophylum 

ing leaflet to just miss the response every third time (p. 122). 
That the excitatory wave arrived in regular sequence, was 
seen by the fact that the neighbouring leaflets pulsated at 
regular intervals. 

Semi-automatism. The plant Biophytum growing in 
the open, under favourable conditions of heat and light, some- 
times becomes so excessively sensitive that motile impulses 
are generated, the stimulus causing which it is often difficult 
to localise. A particular leaflet may have been moved by a 
puff of wind ; or the alighting of a small insect, or the accidental 
grazing of an adjacent blade of grass, may have been the 
original source of the impulse. But this is enough to set all 
the leaflets of the plant quivering in an extraordinarily lively 
manner. For from the excited leaflet, the impulse travels 
inwards, the leaflets falling in centripetal succession. The 
excitatory wave then reaches the stem and overflows to the 
other leaves. But this time the progressive closure of the 
leaflets proceeds in a centrifugal or outward direction. 
Before the first discharge, however, going through the 
numerous avenues, can exhaust itself, the second impulse of 
the multiple response may begin ; and in this way the leaf- 
lets exhibit most lively movements without any immediate 

u 



290 PLANT RESPONSE 

antecedent cause. In some of these cases the origin of the 
impulse can be traced, but there are others in which no 
external source of stimulus can be assigned. And here we 
find ourselves passing imperceptibly into the obscure region 
of automatism. 

There is much resemblance between such semi-automatic 
phenomena in Biophytum and the well-known instance of 
spontaneous movements exhibited by Desmodium gyrans, the 
successive mechanical responses of which exhibit all the 
peculiarities of multiple response as seen in Biophytum. In 
Desmodium as in Biophytum we have similar cyclic changes 
of period and of amplitude. Just as in the case of Biophytum, 
when we cannot determine the source of stimulus, we are 
tempted to regard the phenomenon as automatic, so in the 
case of Desmodium gyrans it is our own inability to trace out 
the origin of stimulus that leads us to regard the periodic 
movements of the plant as automatic. 

Biophytum^ then, under ordinary circumstances, exhibits 
a single response to a single stimulus ; but when the stimulus 
is strong, a single stimulation will produce multiple responses, 
and these will persist long after the cessation of the primary 
stimulus. Under exceptionally favourable circumstances 
periodic movements occur, apparently without any exciting 
cause. I have again found, as will be described more fully in 
Chapter XXIV., that Biophytum itself under favourable cir- 
cumstances of light and warmth exhibits persistent and long- 
continued autonomous pulsations, which are in no way 
distinguishable from those of Desmodium. Even the periods 
of vibration are, generally speaking, similar, inasmuch as in 
both these plants, under different circumstances, I have 
recorded pulsations, the periodicities of which vary from 
one minute or less to four or five minutes. Thus Biophytum 
forms a connecting link between those plants which exhibit 
only ordinary response (single stimulus, single response) and 
those in which mechanical movements appear to be auto- 
matic. Biophytum is also particularly interesting, be- 
cause in its case we find the same plant, under different 



MULTIPLE RESPONSE 29 1 

* 

circumstances, behaving in either of these ways ; that is 
to say, giving ordinary response, or exhibiting automatic 
movements. 

Continuity of multiple and automatic response. If there 
be thus no real breach of continuity between multiply and 
automatically responding plants, it should be possible, under 
suitable circumstances, to obtain from such a characteristi- 
cally automatic plant as Desmodium all those peculiarities of 
responsive indication which were found in Biophytum. The 
latter, it will be remembered, under the condition of over- 
excitability, consequent on the absorption of abundance of 
energy of heat and light, became converted from an ordinarily 
responding into an automatically responding plant. Con- 
versely we might expect Desmodium^ under the opposite 
circumstances, to pass from the condition of giving automatic 
to that of giving ordinary response. 

This inference I have been able to verify, for I succeeded 
in obtaining the requisite conditions for converting the auto- 
matically responding Desmodium into an ordinarily responding 
plant, in three different ways : firstly, by artificially reducing 
the absorbed energy of the plant, under cautious cooling, care 
being taken that this did not produce permanent cold-rigor ; 
secondly, by keeping a plant for some days in a dark room, 
at uniform temperature ; and thirdly, by selecting a plant for 
experiment in the most unfavourable season of the year that 
is to say, in the winter, when it was already exhausted by 
flowering. It will be seen that the principle adopted in each 
of these cases consisted in depriving the plant of its excess of 
energy. The third method was the most satisfactory of all, 
since the leaflets were in a state of natural standstill. In 
every instance, the automatic movements of the leaflets came 
to a stop, and the motile organ was reduced, as will be shown, 
to the ordinarily responding condition. In choosing the 
leaflets for experiment, it must be remembered that their 
sensitiveness is liable to disappear with age. In Biophytum y 
for example, very old leaflets show no response. In experi- 
menting, therefore, on leaflets of Desmodium which have been 

U 2 



292 PLANT RESPONSE 

brought to a state of standstill, care should be taken to select 
those which have not permanently lost their motility, but in 
which automatic movements have simply come to a stop for 
want of a sufficient reserve of latent energy. 

Polar excitation of Desmodium leaflet in a state of 
standstill. We have seen how the leaflets of Biophytum 
showed kathodic excitation at make. I shall now show that 
a Desmodium leaflet, which has been brought to a state of 
standstill, either naturally or artificially, will exhibit similar 
effects of polar excitation ; and first, I shall take a case in 
which arrest was produced by cautious application of cold. 
I placed one electrode on the petiolule of the leaflet, and the 
other on the main petiole of the leaf, and applied the stimu- 
lus of condenser discharge, the petiole being made kathode. 
In this way I obtained from the Desmodium leaflet a series 
of single responses to single moderate stimuli. 

I next selected specimens which, in the winter season, had 
come to a state of natural standstill, and tried on them the 
polar effects of a constant current. Electrical connections 
were now made at the bases of the petiolules of lateral 
leaflets in two neighbouring leaves. In completing the elec- 
trical circuit, one of these leaflets would be under anodic, and 
the other under kathodic, action. I found that a consider- 
able electromotive force was necessary to initiate the excita- 
tory reaction. Thus, using thirty volts, I found that the 
kathodic leaflet responded at make, and the anodic at break. 
This shows that, as regards polar effects, Desmodium in a 
state of standstill gives exactly the same responses as does 
Biophytum. 

In the case of Biophytum^ moreover, we found that with 
high E.M.F., we arrived at a phase of response, the A stage, 
in which both the anode and the kathode caused excitation 
at make ; and in Desmodium in a state of standstill 
I obtained an exactly similar result, for, on now using a 
higher E.M.F. of forty-eight volts, with the same specimens 
as in the last experiment, I obtained excitation at make, at 
both anode and kathode. 



MULTIPLE RESPONSE 293 

Multiple response caused by strong stimulation in 
Desmodium. I next tried to find out whether Desmodium 
in a state of standstill would give multiple responses to a 
strong chemical stimulation, as I had found Biophytum to do. 
Remembering how successive twitches are produced in a 
frog's muscle, in a nerve muscle preparation, when the nerve 
is touched with salt, I applied a strong solution of the same 
reagent to the petiolule of the arrested Desmodium leaflet. 
This gave rise to a series of four vigorous mechanical 
pulsations. 

In order again to show that multiple response could be 
initiated by strong thermal stimulus, in Desmodium as in 
Biophytum, I selected a plant whose leaflets were in a state, 
of natural standstill. A fairly strong stimulus was applied 
by means of the thermal stimulator, at a point on the petiole 
half a centimetre above the insertion of the motile leaflet, in 
precisely the same manner as was ordinarily done with 
Biophytum. The Desmodium leaflet now gave a multiple 
series of responses, exactly similar to those obtained from 
Biophytum. The first occurred three and a half minutes after 
the application of stimulus. The successive rise and fall were 
then uninterrupted. The average period of each response in 
the series was approximately 4-5 minutes. The multiple 
responses gradually declined in amplitude, and came to a 
stop after the thirteenth oscillation, It is thus seen that 
a strong stimulus will give rise to a multiple series of re- 
sponses in the case of Desmodium, precisely as in that of 
Biophytum. 

It is now clear that there is no rigid line of demarcation 
between multiple and automatic responses. An ordinarily 
responding plant like Biophytum, which gives a single response 
to single moderate stimulus, and multiple response to strong 
stimulus, will, under very favourable circumstances, that is to 
say, when it has absorbed an excess of energy from without, 
become automatically responding ; and, conversely, the pro- 
nouncedly automatic Desmodium will, under unfavourable 
circumstances, that is to say, when the sum total of its latent 




PLANT RESPONSE 

energy has fallen below par, be reduced to the condition of 
an ordinarily responding plant, giving single response to 
single moderate stimulus, and multiple response to strong 
stimulus. 

SUMMARY 

On application of a strong stimulus, of whatever nature, 
to a vegetable tissue, a multiple series of electromotive re- 
sponses is produced. These multiple responses may also be 
observed by the electrotactile method. 

These multiple excitations, in consequence of a single 
strong stimulus, may be observed in Biophytum as multiple 
mechanical responses. 

These multiple responses may be uniform in character, or 
may exhibit cyclic variations, similar to those observed in the 
rhythmic pulsations of animal tissues. 

As, in the case of a plant-tissue, a strong stimulus causes 
multiple excitations, so, in the retina, strong stimulus of 
light causes multiple visual excitations, seen in recurrent after- 
images, which have the same characteristics as the multiple 
after-effects of stimulation in Biophytum. 

There is no strict line of demarcation between the 
phenomena of multiple and of automatic response. Under 
very favourable circumstances that is to say, when it has 
absorbed an excess of energy from without an ordinarily 
responding plant like Biophytum will become converted into 
an apparently automatically responding plant, like Desmodium. 

Conversely, under unfavourable circumstances that is 
to say, when the sum total of its energy is below par an 
automatically responding plant like Desmodium^ will become 
converted into an ordinarily responding plant like Biophytum. 
Its leaflets then come to a state of standstill. 

Desmodium leaflets in a state of standstill respond to 
stimulus in exactly the same way as do those of Biophytum. 
To moderate stimulus, both give single response ; the polar 
effects of currents in both are the same ; and strong stimula- 
tion causes a multiple series of responses in both. 



CHAPTER XXIV 

AN INQUIRY INTO THE CAUSES OF AUTONOMOUS 

MOVEMENTS 

Production of pulsatory movements as after-effect of energy absorbed Physical 
analogue Localisation of seat of automatic excitation in Desmodium 
Demonstration of multiple response to a constant stimulus: (i) Chemical 
(2) Electrical (3) Stimulus of light Multiple response to constant stimulus 
of light, in : (a) retina (6) Biophytunt (c) Desmodiuiii (4) Thermal 
Induction of automatism in Biophytum at favourable temperature (5) Of 
internal hydrostatic pressure Absorption of external energy and its absorption 
by the plant in latent form True meaning of ' tonic" condition Cause of 
rhythmicity After-effect, and its relative persistence. 

IN living tissues, both animal and vegetable, we find 
numerous cases of spontaneous periodic movements, to 
which no direct exciting cause is apparently assignable. 
We confess our inability to trace out the exciting cause 
by classing such phenomena as automatic. Among well- 
known examples of automatic movements in animal tissues 
may be cited the pulsatory action of the heart. In the 
vegetable kingdom, also, such movements are very numerous, 
and are of various degrees of rapidity, from quick pulsations 
of some few seconds in duration, to others which occupy 
periods of several hours. It is to be remembered that these 
spontaneous movements take place in plants under favourable 
circumstances, i.e. under that totality of the optimum degrees 
of light, temperature, turgidity, and so on, which is vaguely 
referred to in vegetable physiology as the tonic condition. I 
intend to set forth presently experimental considerations 
which will, I hope, serve to make clear the precise significance 
of this term. 

As already said, to call such movements automatic is 



296 PLANT RESPONSE 

only one way of evading the difficulty of finding out their 
actual cause. When we see these responsive indications 
given by a moving leaf or leaflet, we can but feel it 
necessary to trace them to the impulses, internal or external, 
by which they must have been occasioned. How are such 
periodic impulses caused, and where is their seat? The 
investigations which follow are intended to throw light on 
this, one of the most obscure problems in Physiology. In 
previous chapters I have demonstrated the fact that the 
antecedent cause of a periodic effect need not itself be 
periodic. A stimulus may remain long latent in a tissue, 
and this latent stimulus may subsequently give rise to 
periodic excitations. 

Such periodic expression of the absorbed energy is not 
without analogy in the world of physics. For example, we 
take a glass tube of moderate diameter and push into it, to a 
certain distance from the free end, a piece of wire gauze, 
which is then heated over a Bunsen burner. On removal of 
the heating flame, we observe the after-effect of this absorbed 
thermal energy, in pulsating movements of the air-column, 
which give rise to a musical note, whose pitch is determined 
by their periodicity. We have here a physical analogue to 
a case previously described, in which the thermal energy 
absorbed by a tissue of Biophytum was afterwards manifested 
in long-continued periodic movements of the leaflets. In the 
experiment with the glass tube, the periodicity of aerial 
pulsation is determined by the size, shape, and temperature of 
the pulsating column. Similarly, the periodicity of multiple 
response in the Biophytum leaflet is determined by the various 
constants of the cell-complex which is the seat of the movement 

We saw in Biophytum that the region to which strong 
external stimulus was applied, became the place in which it 
was accumulated, and the source of subsequent excitation. 
We further saw that there is no rigid line of demarcation 
between plants which exhibit multiple responses and those 
which show autonomous movements, the same plant passing 
from one category to the other, according to circumstances. 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 297 

Thus Biophytum, which normally speaking gives ordinary 
single or multiple response, may under favourable circum- 
stances exhibit automatic response. On the other hand, 
Desmodium, which normally speaking exhibits automatic 
response, will under unfavourable circumstances become 
converted into an ordinarily responding plant. 

As already stated, in the case of Biophytum, the point of 
application of stimulus becomes also the source of subse- 
quent multiple excitations. An observer, unacquainted with 
the position of this point, might succeed in determining it, by 
watching the order of pulsation of the leaflets. For if we 
suppose the stimulus to have been applied at some point in 
the middle of the leaf, the observer will in that case notice 
periodic waves of excitation proceeding in opposite directions, 
and giving rise to the closure of successive leaflets from a 
common point outwards, thus clearly indicating the position 
of the point from which successive excitations are initiated. 
Had the stem, on the other hand, been stimulated, and thus 
become the source of excitation, these successive impulses 
would have begun by arriving at the first pair of leaflets, 
and thence would have passed through all the leaflets to 
the tip of the leaf, in a centrifugal order. Again, had the 
stimulus remained latent at the tip of a particular leaf, the 
successive excitatory waves would then have proceeded 
through the leaf in a centripetal order, and on reaching the 
stem would have radiated outwards, or in a centrifugal 
sequence once more, throughout the other leaves. There 
are other ways also, presently to be described, by whose 
means we can obtain an idea of the direction in which the 
excitatory wave is travelling. 

Localisation of seat of origin of autonomous excitation 
in Desmodium. Our next inquiry is into the very obscure 
question of the point of origin of the so-called autonomous 
excitation of Desmodium. I have already shown that, if the 
plant absorbs a certain amount of energy in excess of that 
required for immediate response, the surplus is stored up, to 
be given out subsequently in the form of pulsating waves of 



298 PLANT RESPONSE 

excitatory disturbance. But the difficulty is to determine 
the point at which the latent energy is stored up, and from 
which the excitatory disturbances subsequently proceed. As 
we are accustomed to think that stimulus must be due to 
some sudden variation, one is tempted to suppose, as the 
simplest explanation, that in the case of autononwus move- 
ments the stimulation is caused by variations in the rate of 
absorption of food material, which is probably being carried 
on, in an intermittent manner only, by the roots and leaves. 
In this case, if the stimulus proceed from the root, we may 
expect the excitation to travel to the motile leaflets through 
the stem, outwards, or in a centrifugal direction. If the 
assimilating leaves, however, be the source of excitation, we 
shall look to see the wave of excitation proceeding from 
without inwards, or centripetally. 

In order, therefore, to localise the source of stimulation 
in Desmodium gyrans, for example, I first tried to find 
out in which direction the excitatory impulse proceeded. 
Unfortunately in this case, the leaf being provided with only 
a single pair of motile leaflets, it is impossible to obtain the 
indication of direction which is given by Biophytum, by the 
successive closure of leaflets in a definite order. I was 
therefore obliged to have recourse to other expedients. 

I have already shown how the transmission of stimulus 
can be arrested by local application of ether, or by the anodic 
block. If the pulsating stimulus, therefore, should be central, 
that is to say, proceeding from the stem, we should then 
expect the application of ether, or the anodic block, at the 
junction of the leaf with the stem, to arrest the movement of 
the leaflets. If, on the other hand, the source of the stimula- 
tion should be peripheral, a block produced in the manner 
described, at a point between the terminal expanded leaflet 
and the small motile leaflets, would prevent the response of 
the latter. In carrying out -these experiments, however, I 
found that no such arrest took place in either case. 

The fact that the source of stimulation is not central, is 
also made evident by the following consideration. Had it 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 299 

v 

been so, the responding movements would have been syn- 
chronous with that of the hypothetical source, and all the 
leaflets of the plant would in that case have had the same 
period of vibration. As a matter of fact, however, different 
leaflets have different periodicities. 

That the source of stimulation, again, is neither central 
nor peripheral, may be shown by applying two tight liga- 
tures, one behind and the other in front of the motile leaflets. 
The passage of the excitatory impulse, should this be 
transmitted from either direction, will by this means be 
completely arrested. On carrying out this experiment, how- 
ever, the periodic pulsation was not affected. 

Again, the leaf may be completely detached from the 
plant, and the terminal leaflet amputated. But the periodic 
pulsation still proceeds as before, just like the persistent beat 
of the isolated heart of a frog. 

Thus we find that the isolated motile leaflet continues to 
manifest an evolution of energy in the form of pulsating 
movements, which cannot be derived from either of the 
hypothetical sources, whether central or peripheral. It 
maintains this activity for a long time, when kept under 
favourable conditions. It thus becomes clear that in the case 
of Desmodium the power of maintaining rhythmic pulsation 
is local, and resides in the tissue of the motile leaflets. 

This does not exclude the possibility of other periodically 
moving organs obtaining the pulsating excitatory impulses 
from a distant point. Such a case may well happen when, 
the conductivity of the intervening tissue being very great, 
and the excitability of the motile organ high, stimulation, 
though enfeebled by transmission through a long tract, yet 
remains above that critical intensity which would cause 
effective response. 

In the previous chapter, I showed that if Desmodium be 
kept in the dark for a sufficiently long period but not so 
long as to produce permanent rigor or if a specimen be 
taken in the unfavourable season of the year, its autonomic 
movements would be found to have come to a stop for the 



300 PLANT RESPONSE 

time. This was because the energy stored up in the tissue 
had become exhausted. If, then, a single stimulus were 
given, say, by condenser discharge, a single response would 
be found to ensue. This showed that in such a case the 
arrest of the automatic movements was not due to the 
abolition of motility in the leaflet, but only to exhaustion of 
the energy stored up, which would have given rise to oscilla- 
tion. It was also shown that if Desmodium in a state of 
standstill were subjected to a single strong thermal stimulus, 
it exhibited a multiple series of rhythmic responses. 

Multiple response to constant stimulus. Having 
observed the production of multiple responses as an after- 
effect of a single strong stimulus, I shall now proceed to 
demonstrate the production of similar multiple responses by 
the action of a constant excitatory condition. 

1. Chemical. I have shown that in Biophytum, and in 
Desmodium at standstill, rhythmic pulsations are produced 
by the constant action of a chemical stimulant. Analogous 
phenomena are also known in animal tissues ; in the isolated 
heart in a state of standstill, rhythmic pulsation can be renewed 
by chemical stimulation. 

2. Electrical. I have often observed that when a strong 
current is sent through Averrhoa carambola, a number of 
rhythmic pulsations are found to take place. In animal 
tissues, again, similar rhythmic excitations have been ob- 
served, not only in cardiac muscle, whose rhythmicity is 
so marked a characteristic, but also in skeletal and other 
muscles. 

3. Stimulus of light : (a) On retina. And now before I 
describe the experimental demonstration of periodic excita- 
tion in plants, as caused by constant stimulus of light, I shall 
refer in detail to certain remarkable periodic effects which I 
have observed in the retina, under the action of the same 
stimulus. I showed in the last chapter that strong stimulus 
of light gives rise in the retina to multiple responses, in the 
form of recurrent after-images. I shall now prove that during 
the continuance of constant light, pulsatory visual effects are 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 301 

produced. These pulsations are not usually noticed in visual 
sensation, owing firstly to the absence of a standard of com- 
parison, and secondly to the fact that though the impressions 
on the individual retinae undergo variation, the sum total of the 
two remains constant I have been able to provide the neces- 
sary comparison -standards by having two distinguishable 
images produced in the two eyes, the fluctuation in the visual 
excitation in one eye being thus capable of detection by com- 
parison with that in the other. It would have been impossible 
to detect this fluctuation had the excitatory variation taken 
place in the two eyes simultaneously, i.e. if the maximum 
excitation in the one had occurred at the same moment as the 
maximum excitation in the other. But I have found that, as 
regards excitation, there is a relative difference of phase, of 
half a period, between the two eyes, so that the maximum effect 
at the given moment in one eye corresponds to the minimum 
in the other. It is owing to this fact that the periodic excita- 
tions in each retina are brought out in an unmistakable 
manner by the following experiment, which consists in look- 
ing at two slits through the modified stereoscope described in 
the previous chapter. One of the two slits inclines to the 
right and the other to the left, and on looking through these 
at a bright sky, the right eye perceives a bar of light turned, 
say to the right, and the left eye a bar turned to the left, 
the resultant impression being that of an inclined cross. 

When the stereoscope is turned to a bright sky, and the 
cross looked at steadily for some time, it will be found, owing 
to pulsatory excitation in each eye, that when one arm of 
the cross begins to be dim, the other becomes bright, and 
vice versa. These periodic fluctuations are perceived con- 
tinuously under the constant action of light 1 

I shall now proceed to the demonstration of the very 
interesting rhythmic movements caused in plants under 
constant light- stimulus. As we wish to prove that the 
cause of automatic movements lies in the action of some 

1 The experiment will be found described in detail in my book, Response in 
the Living and Non-Living , p. 175. 



302 PLANT RESPONSE 

continuous stimulation, it is necessary to satisfy ourselves 
that this continuous source is the sole cause of the rhythmic 
movements. This will appear conclusive, if it is made clear 
that the plant does not possess any intrinsic energy of self- 
movement, but only the power to regulate rhythmically the 
overflow of energy supplied. 

The demonstration will then be made rigorous, if we take 
a plant which is not manifesting automatic movement, and 
cause it to exhibit such action by the application of constant 
external stimulus. For the purpose of this experiment, then, 
we may take the leaflet of Desmodium in a state of standstill, 
or that of Biophytum under nor mal rrmHifWic wVnVVi ac 
have seen, may be regarded as practically equivalent to 
Desmodium in a standstill condition. If therefore we can 
succeed in making Biophytum exhibit rhythmic movements 
under the continuous action of a given external energy, we 
shall not need to look for the explanation of the so-called 
automatic movements of Desmodium to some periodically 
varying stimulus, any constant stimulus being then proved 
fully competent to produce the rhythmic pulsations. 

(b) On Biophytum. I first subjected the motile leaflet of 
Biophytum to light of comparatively short duration, i.e. two 
minutes, by throwing sunlight upon it from a reflecting 
mirror. There was no immediate response, but after a latent 
period of one minute the leaflet gave a single response. I 
next subjected the plant to the continuous action of light 
during ten minutes. I obtained rhythmic pulsations, there 
being two responses in the given time. I then cut off the 
light, and the rhythmic action was stopped. After an 
interval of several minutes I again applied light for ten 
minutes, and this gave rise to similar rhythmic responses. 
The procedure was repeated once more with the same results. 
With another specimen, I applied light for a still longer 
period, i.e. twelve minutes, and I obtained three pulsations. 
It .will thus be seen that we have here a regulated outflow of 
energy ; the plant absorbs energy continuously, but gives it 
out in a pulsating manner (fig. 123). In this way, a number 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 303 

of autonomic pulsations may be obtained in Biophytinn under 
the continuous stimulation of light. The recovery not being 
complete after each pulse, the result is a progressive folding 
downwards of the leaflets, and this makes it impossible after 
a time to observe further responses. I shall presently describe, 
however, another method of supplying the plant with energy, 
by thermal means, in which case 
the autonomous pulsation in Bio- 
phytum is prolonged indefinitely. 

(c) On Desmodium leaflet at 
standstill. Turning now to the 
leaflet of Desmodium in a state of 
standstill, I find that application 
of sunlight initiates rhythmic move- 
ments. In a particular experiment, 
light was continuously applied for 
half an hour, and there were pro- 
duced in that time seven vibrations. 
The responses were found to under- 
go 'staircase' increase by increas- 
ing absorption of energy, and the 
resultant increase of molecular 
mobility produced in the tissue. On the stoppage of light 
the rhythmic pulsations persisted for some time, the ampli- 
tude, however, undergoing diminution owing to the run-down 
of absorbed energy. 

4. Thermal : (a) Induction of autonomous response 
in Biophytum. I shall now describe an experiment of very 
great theoretical importance, by which I have able to convert 
a specimen of Biophytum from an ordinarily into an auto- 
matically responding plant. Guided by the theoretical in- 
ference which I have already stated, that it is an excess of 
energy that brings about the condition of automatism, I 
subjected a specimen of Biophytum to the constant stimulus 
of a moderately high temperature. I first placed the speci- 
men in a chamber whose temperature was 37 C, and it was 
astonishing to see the younger (and therefore more excitable) 




FIG. 123. Multiple Response 
of Biophytttm under the 
Continuous Action of Light 



304 



PLANT RESPONSE 



leaflets, at first quiescent, break into a sustained 'series of 
uninterrupted pulsatory movements, which they kept up 
throughout the maintenance of the condition of high tempera- 
ture. It was interesting also to observe the quickening of 
vibration by absorption of energy. The pulses were at first 
slow, each having a period of four minutes, but they became 
steadily more rapid, till they had reached a frequency of two 
vibrations in a minute. A continuous record of these periods 
was made during one hour, and the following tabular state- 
ment exhibits the results : 

TABLE SHOWING PERIODS OF SUCCESSIVE PULSATIONS IN BIOPHYTUM 
WHEN TEMPERATURE is RAISED TO 37 C. 



No. of 
pulse 



I 


4 u 


2 


4 


3 


4 


4 


4 


5 


4 


6 


3 


7 


3 


8 


3 


9 


3 


10 


2*5 



Period 



minutes 










1 


No. of 
pulse 


Period 


No. of 
pulse 


Period 


II 


2 -5 minutes 


21 '5 minute 


12 


2*5 M 


22 -5 ,, 


13 


2-5 n 


23 '5 *> 


14 


2*5 ,, 


24 '5 


i '5 


2 


25 '5 


16 


I *5 minute 


26 , -5 


17 


i '5 99 


27 '5 


18 


* M 


28 -5 ,. 


19 


I 


29 


5 


20 


5 : 30 


5 



Having thus found that a high temperature was favour- 
able to the initiation of automatic movements in Biophytum> 
I was next desirous of determining the minimum temperature 
at which such responses could be induced. For this purpose 
I took a fresh specimen of Biophytum, and cautiously raised 
the temperature of the chamber from 23 C. upwards. When 
29 C. had been reached, I obtained the first pulse of auto- 
nomous response, the period being rather slow that is to say, 
8'5 minutes and by the time the temperature had gone up to 
35 C. this period had become shortened to two minutes. We 
find here a phenomenon identical with that of Desmodium, 
where the frequency of vibration is found to be increased with 
rising temperature. 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 305 

In Desmodium, the autonomous movement is initiated at 
a certain more or less definite temperature, which is about 
17 C This we may call the critical thermo-tonic condition. 
Below this critical degree, Desmodium ceases to be autonomic, 
and becomes an ordinarily responding plant. In Biophytum, 
similarly, the critical thermo-tonic point is about 29 C. 
Above this, the young leaflets are autonomic, and below it % 
ordinarily responding. The difference between Desmodium 
and Biophytum in this respect lies, therefore, in the fact that 
their critical thermo-tonic points are about twelve degrees 
apart. 

In the case of Biophytum, when the temperature is main- 
tained at a uniform favourable degree, the periods of the 




FlG. 124. Induction of Autonomous Response in Biophytum^ 
at Moderately High Temperature of 35 C. 

Note the diminution of amplitude of response with the gradual loss of 
latent energy, consequent on falling temperature. The pulsations 
came to a stop below 29 C. 



autonomous pulsations become very regular. It has been 
said that these pulsatory movements are maintained by 
means of energy absorbed, and with Biophytum I found an 
added opportunity of demonstrating this fact. A particular 
plant had been kept at a uniform temperature of 35 C., 
under which the young leaflets gave autonomous responses. 
The heating current, by which this temperature was main- 
tained uniform, was now stopped, and the chamber gradually 

x 



306 PLANT RESPONSE 

cooled down. One of the young leaflets had been attached 
to the Optical Lever, and records were now taken con- 
tinuously. It was very interesting to observe how, with the 
gradual loss of absorbed energy, the pulsating movements of 
the plant became diminished in amplitude (fig. 124) till they 
came to a stop. We shall find exactly the same thing in the 
case of Desmodium. 

I referred in the last chapter to certain observations of 
my own, in which the leaflets of Biophytum were found, under 
favourable circumstances of light and temperature, to give 
rise to apparently spontaneous movements, which could not 
be traced to any definite varying external cause. From the 
experiment just described, however, we see that it was the 
continuous stimulus of favourable temperature and light com- 
bined that caused this rhythmic movement, which appeared 
as automatic. 

(//) Resumption of automatic movements in Desmo- 
dium. Similarly in Desmodium, if the isolated leaflet in which 
all movement has been brought to a stop, but not to a condi- 
tion of permanent rigor, be raised in temperature, and if the 
tissue be maintained uniformly at this higher point, the 
pulsatory movement will be found to commence and persist 
for a long time. The initiation and maintenance of the 
responses here, again, is undoubtedly due to the renewed 
supply of energy. 

A new difficulty now arises, however, from our habit 
of regarding stimulation as dependent upon some sudden 
variation of external conditions. For we are here confronted 
with a case in which uniform and continuous application of 
heat produces stimulation. It may be urged that there has 
been some variation of environmental conditions, in the fact 
of the rise of temperature before the constant point was 
reached. But it has to be remembered, that this rise was 
purposely made very gradual, and even if a single stimulation 
had been caused by this preliminary thermal variation, it 
would have evoked only a single response, or at most a few 
multiple responses But how are we to account for these 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 307 

k 

long-continued periodic pulsations, which are kept up during 
the whole time in which the leaflets are maintained at an 
unvarying given temperature ? 

We are thus led to see that stimulation is not in all cases 
dependent upon the occurrence of a sudden change in the 
exciting cause. On the contrary, excitation may be pro- 
duced, as we have seen, under a constant and uniform 
exciting condition. If we analyse the multiple responses 
produced in Biophytum as the after-effect of the application 
of a strong thermal stimulus, we find, as said before, that 
some part of this thermal energy remains latent, and after- 
wards gives rise to recurring pulsations. 

5. Of internal hydrostatic pressure. The isolated animal 
heart, when in a state of standstill, is found to renew its 
excitatory pulsation under an increase of internal hydrostatic 
pressure. I shall show later (p. 349) that a Desmodium 
leaflet, similarly, when in a state of standstill, can be made 
to resume its excitatory rhythmic activity, by increasing the 
internal hydrostatic pressure. 

The true meaning of * tonic ' condition. We have thus 
seen that under the continuous action of a constant source of 
external stimulus, multiple responses are produced. We have 
also seen that the excess of energy absorbed remains latent 
in the tissue, in consequence of which, even on the cessation 
of external stimulation, the pulsatory movements are main- 
tained for a longer or shorter time. It is thus the excess of 
stimulus absorbed which renders the tissue excitable, or 
'tonic.' Hence we may have tonicity imparted by light, 
photo-tonus ; by favourable temperature, thermo-tonus ; by 
electric current, electro-tonus ; by internal hydrostatic pres- 
sure, hydro- tonus ; or by the presence of favourable chemical 
substances, chemo-tonus. We have seen that each one of 
these, *by itself, was competent to give rise to multiple 
responses. It has been shown further that there is no hard- 
and-fast line between such multiple and automatic responses, 
the one passing imperceptibly into the other. 

We have now seen that the tonic condition of the plant is 

X 2 



308 PLANT RESPONSE 

determined by the sum total of the latent energy derived 
from all the above-named factors. This internal factor, 
of latent energy, will be shown to play a very important 
part in all response-phenomena, and as it thus becomes 
necessary to have some convenient means of referring to it, 
we shall henceforth designate it as the Internal Energy of 
the plant. 

Automatic movements in plants are thus only exhibited 
under favourable tonic conditions. It has been shown that 
the plant displays rhythmic activity when subjected to different 
forms of constant stimulus, and we have now investigated 
separately the effects of such constant stimuli chemical, 
electrical, thermal, photic, and hydrostatic. As has been 
explained before, if a given stimulus be not of sufficient 
intensity to evoke visible response, yet the absorbed energy 
may render the tissue capable of responding to another 
subsequent stimulus, which by itself would have been in- 
effectual ; in other words, the tissue is made excitable by the 
presence of a stimulus which has not of itself been adequate 
to cause response. When this latent excitability exceeds a 
certain amount, any further increase may be expressed in a 
visible manner by mechanical pulsation. Now, taking the 
various forms of stimulus to which the plant is constantly 
exposed namely, warmth, light, moisture, and the action of 
the various chemical reagents, organic and inorganic, present 
in it or absorbed by it it is clear that each of these exerts its 
stimulating effect independently ; any one by itself may then 
make the tissue excitable to the verge of response ; in this 
condition, though there is no outward sign of the fact, there 
is a considerable amount of latent energy ; and the incidence 
of any second and additional form of stimulation is now 
sufficient to precipitate the excitation in visible form. These 
considerations will show how, by the cumulative and additive 
effects of all the forms of constant stimulation mentioned 



above, the plant may become so highly excitable as to 
manifest the fact by giving rise to responses which appear as 
automatic. The tonic condition is thus the latent excitatory 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 309 

condition, which is determined by the sum total of all these 
exciting factors. 

In connection with these facts, it is well to bear in mind 
that the excitability imparted by a stimulus does not always 
increase continuously with the intensity. On the contrary, 
there may be an optimum intensity beyond which excitability 
may be diminished. 

We have now seen that the energy which expresses itself 
in pulsatory movements may be derived by the plant, either 
directly from immediate external sources ; or from the excess 
.of such energy, already accumulated and held latent in the 
tissue, aided by the incidence of external stimulation ; or from 
an excessive accumulation of such latent energy alone. In 
the last case, however, if the plant were kept isolated from all 
external supply of energy, it is clear that its reserve would 
become exhausted, and its automatic movements would cease. 
I have already described an experiment in which this arrest 
of pulsation took place in the case of a specimen of Desmodium 
kept in a dark room. We then saw that revival of response 
was only brought about when fresh stimulus was applied. 
And we have also seen the converse, namely, the ordinarily 
responding Biophytum, when supplied with excess of energy, 
become automatically responding. 

Cause of rhythmicity. Having, then, seen that it is a 
constant source of energy, external or internal the latter 
being really derived from a previous absorption of external 
stimulus which maintains the so-called automatic movements 
in plants, we have still to determine how it is that a latent 
or constantly acting external stimulus can find only periodic 
expression ? In connection with this we have seen how, 
when the minimal factor of stimulating intensity is exceeded, 
there is a manifestation of the fact by visible response. I 
have also shown (p. 245) that after each excitatory discharge 
there is a marked diminution of both conductivity and 
excitability ; and a new stimulus, or existing excess of 
stimulus, owing to the loss of these properties, is retarded 
for a time from producing any effect on the motile tissue. It 



3IO PLANT RESPONSE 

is only after the lapse of an interval that the protoplasm 
regains its original properties. There is thus an oscillatory 
variation of conductivity and excitability. It will therefore 
be seen how, under the circumstances, a constant stimulus, 
or a stimulus which is latent in the tissue, can find an excitatory 
expression only in a pulsating manner. Perhaps a physical 
model will enable us to visualise this process. Imagine a 
reservoir into which flows a constant supply of water. An 
elastic conducting pipe is led from the reservoir, and this pipe 
is constricted by a compressing spring. On the far open end 
of this pipe abuts the flattened end of an indicating lever. 
When water has been supplied for some time, its level is 
gradually raised, producing an increasing pressure. At a 
certain point, when the pressure becomes sufficiently great, 
the spring which keeps the elastic tube constricted gives way, 
and there is an impulsive discharge of water, which, impinging 
against the lever, gives rise to a visible response. But the 
yielding spring again closes, the tube is once more constricted, 
and thus by the oscillation of the spring which regulates the 
conduction of water, pulsating hydraulic impulses are kept 
up. On account of the oscillating mechanism, the outflow, 
and consequent mechanical response, are periodic, though the 
supply is constant. In this model the first period, before the 
pressure of water becomes sufficient to force open the spring, 
corresponds to the latent period in plant response ; the 
oscillation of the flow of water corresponds to the oscillation 
of conductivity ; and the responding lever corresponds to the 
motile leaflet. Similarly, an ill-fitting spring tap is thus often 
thrown into a pulsating movement, by the constant pressure 
of water from the main, and there is then seen a rhythmic 
play of the water- jet. 

* After-effect ' and its relative persistence. In the ex- 
periment on Biophytum under the continued action of light, we 
saw that for a period of one minute, during which the plant was 
absorbing light, there was no response. Then after this latent 
period of one minute, the energy absorbed reached the verge 
of response. The excitatory discharge was followed by a 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 311 

retardation of conductivity and excitability, which was, how- 
ever, gradually recovered, and there were produced a second 
and then successive periodic responsive movements. 

In the hydraulic model, if the capacity of the reservoir be 
small, the cessation of the water-supply will cause an immediate 
cessation of the rhythmic movement of the indicating lever. 
In Biophytum, similarly, we found that the periodic response 
continued as long as energy was supplied, and that the 
movement soon stopped on the stoppage of the supply of 
external energy. But if the capacity of the reservoir be great, 
the accumulation may be sufficient to maintain the oscillation 
for a considerable length of time, even after the main supply 
is cut off. And we see that in Desmodium the responsive 
movements continue in a persistent manner, though the 
immediate source of stimulation be interrupted. A similar 
difference in the persistence of after-effects, depending on 
relative capacities for storage of energy, is seen in the two 
classes of inorganic substances, which are distinguished as 
fluorescent and phosphorescent. In the first, the responsive 
emission of light caused by a preceding excitation is 
extremely short-lived ; but in the latter it continues long 
after the light stimulus has ceased to act. Thus Desmodium 
may continue to exhibit rhythmic movements, though not at 
the moment exposed to the marked action of any special 
source of stimulus. But for the display of long-continued 
rhythmic movements it should previously have absorbed a 
considerable amount of energy from an external source in 
other words, it must have been exposed to those circum- 
stances which produce a favourable tonic condition. 

We have thus obtained some insight into that very ob- 
scure phenomenon which is known as the after-effect. By 
the inertia of the organism there is a certain loss of time 
before response begins to take place, and this determines the 
latent period. But when the stimulus has already initiated 
movement, the responding organ will, through the same inertia, 
continue to show this movement even when the stimulus has 
ceased to act. There is another factor, however, which 



312 PLANT RESPONSE 

determines the persistence of this after-effect, namely, the 
larger or smaller capacity of the tissue itself to hold energy 
latent within it. 

In the attempt to investigate the cause of automatic 
movements in Desmodium, there were three points of inquiry 
which had to be determined. First, there was the question 
of the seat of excitation ; second, that of the nature of the 
stimulus which maintained the rhythmic pulsation ; and third, 
the determination of the process by which constant stimulus 
found periodic expression. 

I have shown that the seat of excitation in Desmodium is 
neither central nor peripheral, but is in, or in immediate con- 
tiguity with, the motile tissue itself, which resembles in this 
respect the animal heart, the seat of excitation there also 
being in the cardiac tissue itself (Chapter XXVI.). 

I have also shown that as the cause of excitation it is 
not necessary to have any sudden variation. A constant 
stimulus, of whatever nature, is found efficient to produce 
excitation. 

I have also demonstrated that periodic pulsation is pro- 
duced in Desmodium at standstill, by a constant thermal stimu- 
lus ; also that periodic pulsations are produced in Biophytum 
and in Desmodium by the constant action of a chemical 
stimulant. I have shown further that rhythmic excitation 
was produced in Averrhoa carambola by the passage of a 
constant electric current. And, finally, I have shown that, 
just as in the retina, under constant stimulus of light we have 
periodic visual excitations, so also in Biophytum^ under con- 
stant stimulus of light we obtain periodic excitations which 
give rise to rhythmic movements. In the animal tissue, 
similar multiple rhythmic responses are met with under 
constant stimulus. 

Since we have found it to be a fundamental characteristic 
of the tissue of a rhythmically-responding plant like Bio- 
phytum or Desmodium to give a response which cannot be 
increased by any excess in the stimulus-intensity (maximal 
response or none), we should expect that the excess over the 



INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 313 

minimally effective stimulus must remain latent in the tissue. 
This is evidenced by the fact that on applying a very strong 
stimulus we obtain, not a single but multiple responses. 
Thus all the various forms of constant stimulus to which it is 
exposed warmth, light, moisture, and the different chemical 
reagents, organic and inorganic, present in or absorbed by 
it become latent, and the sum total of all these stimulating 
factors determines its ' tonic ' condition. 

When this accumulated latent energy exceeds a certain 
value, it is visibly manifested in the form of the so-called 
* automatic ' movements. 

The periodicity of the excitatory discharges which give 
rise to rhythmic movements in a plant that is under constant 
latent stimulus, is brought about by the peculiarity which 
has been demonstrated, that after each discharge the con- 
ductivity and excitability of the tissue are diminished, and 
are only gradually regained. This oscillation in the conduc- 
tivity regulates the outflow of energy, and causes the 
rhythmicity of the responsive movements. 

SUMMARY 

In Desmodiuni) the seat of excitation of the lateral leaflets 
lies in the motile organ itself. 

ultiple response is produced in the plant, as in the ani- 

l, under constant thermal, chemical, or electrieal stimulus. 

The retina, under constant stimulus of light, exhibits 
periodic visual pulsations. 

Similarly, the leaflet of Biophytum t and that of Desmodium 
at standstill, under continuous stimulation of light, exhibit 
rhythmic mechanical pulsations. 

Biophytum> when raised to a temperature of about 29 C, 
becomes automatically responding. Under these circum- 
stances, the only difference between the so-called automatism 
of Biophytum and Desmodium is that, in the latter case, the 
critical thermo-tonic condition is arrived at about 12 C. 
earlier. 



314 PLANT RESPONSE 

The energy which expresses itself in pulsatory movements 
may be derived by the plant, either directly from immediate 
external sources, or from the excess of such energy, already 
accumulated and held latent in the tissue, aided by the 
incidence of external stimulation, or from an excessive 
accumulation of such latent energy alone. 

By * tonic ' condition is meant the latent excitatory con- 
dition of the plant, as determined by the sum total of the 
stimulating factors which are, or have been, derived from its 
environment. In other words, the tonic condition depends 
on the internal energy of the plant. 

In rhythmic tissues, a constant stimulus, external or 
internal, finds pulsatory expression in consequence of the 
oscillatory variation of conductivity and excitability. 

The duration of rhythmic movements, in the absence of 
any external exciting cause, depends on the amount of 
energy previously absorbed and held latent in the plant. 
The persistence of this after-effect, therefore, depends also on 
the greater or less capacity of the tissue for storage of energy. 
These rhythmic movements thus appear to be automatic, but 
when the reserve is exhausted, the so-called automatic move- 
ments come to a stop. Renewal of pulsatory movements can 
then take place only on the supply of fresh energy from 
without. 



CHAPTER XXV 

INFLUENCE OF VARIOUS CHEMICAL REAGENTS ON THE 
AUTONOMOUS RESPONSE OF DESMODIUM GYRANS 

The recorder and experimental chamber Absolute measurement of period and 
amplitude of Desmodt'um-oscillsition Responsive significance of up and down 
movements deduced from (a) analogy with response of Mimosa ; (b) test of 
increased internal hydrostatic pressure * Systolic ' contraction and ' diastolic ' 
expansion of Desmodi^^m pulvinus Mode of application of chemical reagents 
Action of chemical reagents modified by : tonic condition of plants ; strength 
of solution; and duration of application Effect of anaesthetics Effect of 
alcohol Effect of carbonic acid Effects of ammonia and of carbon disul- 
phide Effect of copper sulphate solution, either when applied externally, 
direct on the pulvinus, or internally Spark -record of Z>;//<w///-pulsation. 

HAVING thus, in the last chapter, traced the causes of 
autonomous movements in plants, I shall now, taking Desmo- 
dium gyrans as the type, describe the effect of various 
agencies on the so-called ' spontaneous ' responses of its 
lateral leaflets. 

With regard to the experimental arrangements for 
making the record, I have already in Chapter I. described 
how records may be obtained, using the intact plant for 
experiment. The automatic movements of the leaflets, how- 
ever, persist, even after the petiole bearing them is detached, 
the cut end being kept in water. Under proper conditions, 
the rhythmic pulsations of the detached specimen will continue 
for a couple of days. In order, therefore, to subject the motile 
organ to various modifying conditions, it is much more con- 
venient to use such a specimen than the whole plant. 

The recorder and experimental chamber. As the 
extent of the movement of the tip of the leaflet is consider- 
able, no magnification is necessary for the record. A single 
cocoon thread is attached to the middle of the leaflet by a 



316 TLA.NT RESPONSE 

drop of shellac varnish, the other end of the thread being tied 
to the longer arm of the Optic Lever, which is 30 cm. long. 
The distance of the recording drum from the mirror is also 
30 cm. The length of the lever arm being thus equal to the 
distance of the drum, and the extent of the angular move- 
ment being by reflection doubled, it will be seen that the 
movement is magnified twice. The thread is attached, 
however, to the middle of the leaflet, and the record there- 




I'U.. 125. ICxperimental Apparatus for Making Records of Pulsation 

of f>,'smodiutn 

Leaflet, P, mounted in U-tube in paint chamber, and attached to long arm 
of Optical Lever, L; M, mirror attached to fulcrum-rod; n, recording 
drum ; i, o, inlet and outlet pipes for gases and vapours introduced 
into plant chamber ; c, electric heating coil. 

fore gives the movement of the tip unmagnified. The records 
given in some of the figures are thus without magnification. 
In others, again, the records are on a reduced scale. 

It will be understood here that the extent of movement 
will vary with the length of the leaflet, some of these being 
very small, and others relatively large. 

In the apparatus shown in fig. 125, the smaller chamber 
contains the motile leaflets mounted in a tube filled with 
water. By means of an inlet-pipe, different gases may be 



CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 317 



introduced into the chamber for any desired length of time, 
after which fresh air may be re-introduced. In order to 
produce variations of temperature there is a coil of wire for 
electric heating. 

The automatic movements of the leaflet, both up and 
down, take place in some cases as a number of jerks, which 
rnay pass gradually into continuous movement. In others 
they are continuous from the beginning. From the normal 
or highest position, the leaflet, generally speaking, sinks some- 
what rapidly. Having reached its maximum depressed position, 
there is a pause, after which there is rathc'r a slow return to 
its original position. The up and down motion is in some 
cases approximately straight. In other cases, the sub-petiole 
of the leaflet is slightly twisted after its descent, and the 
curve described becomes more circular. For "the purpose of 
the present investigation, specimens were selectee! in which 
the movement of the leaflet took place gradually, without 
jerks, the up and down move- 
ments being approximately in a 
straight line. 

Absolute measurement of 
period and amplitude of Des- 
modium - oscillation. The 
period of a single oscillation 
varies with the temperature and 
the tonic condition of the plant. 
In winter it may be as long as 
five, in summer as short as two 
minutes, and when the tempera- 
ture is artificially raised, the 
period may be even further re- 
duced to one minute. As a con- 
crete example, affording a clear 
idea of the general characteristics 
of the pulsatory movement of Desmodium, I shall give 
the following results, obtained from a photographic record 
which gives the extent of the absolute movement (fig. 126). 




FIG. 126. Photographic Record of 
Pulsation of Desmodium 

The up movement of the record 
Jmeans down movement of the 

i leaflet in this and all subsequent 
records. 



PLANT RESPONSE 



The period of the complete vibration was in this case 
3*5 minutes, of which the down movement was accom- 
plished in the course of 1*5 minute. The up movement 
was relatively slower, and was accomplished in two minutes. 
The mean amplitude of pulsation that is to say, the 
vertical distance travelled by the tip of the leaf was 
25 mm. The fact that the down movement is, generally 
speaking, relatively the quicker is seen visually demonstrated 
in the photographic record of uniform pulsations obtained 

with another specimen 
(fig. 127). In connection 
with this, certain pecu- 
liarities of photographic 
action should be borne 
in mind. It is found 
that a short exposure 
gives an image which 
in the case of a line is 
very thin and sometimes 
consists of only the 
faintest impression; but 
when the exposure is 
prolonged the line is 
much thickened. Hence, 
in the records, the faint or more sharply outlined portions 
indicate responsive down movements which were relatively 
rapid. In fig. 127, therefore, these differences of line afford 
us a graphic representation of the various rates of movement, 
and durations of pause, in the different parts of the curve. In 
all the photographic records given here and elsewhere, we 
arc thus able to distinguish the down movements by the 
relative thinness of the recording line. In fig. 134, at the 
end of this chapter, will be found a spark-record of the 
pulsation of Dcsmodium in which the different rates of move- 
ment at different stages of the response can be distinguished 
;at a glance. 

In the multiple responses of Biophytum^ though, generally 






;, 127. Photographic Record of Uniform 
Pulsations in Desmodinm 

Period of each pulsation = 27 minutes. 



CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 319 

speaking, the down movement is more rapid, yet at times the 
up and down movements approximate to each other in this 
respect. I have occasionally found similar instances in the 
pulsatory movement of Desmodium, when there would be 
only a slight difference between the rates of the up and down 
movements. The normal rates again may even become 
reversed under the influence of external agencies. 

Responsive significance of up and down movements. 

v, 

A question of some difficulty and importance arises here as 
to the significance, whether of contraction or relaxation, of 
these up and down movements. We saw in the cases of 
Mimosa and Biophytum that the down movement was due 
to the relatively greater contraction of the lower half of the 
pulvinus, and the up movement to recovery or relaxation. 
It must be borne in mind that the upper half of the pulvinus 
of Mimosa, like the lower, is also excitable, though in a minor 
degree, as will be shown in a subsequent chapter. We have 
further seen that the more excitable half responds earlier to 
stimulus. The depression of the leaf, then, is due to the 
earlier and greater contraction of the lower half of the organ ; 
and its subsequent erection, to a natural expansive recovery, 
possibly aided by the later and feebler contraction of the 
upper half of the pulvinus. Arguing from analogy, we may 
regard the movement of the Desmodium leaflet as essentially 
similar to this. For here too we find, generally speaking, 
that the down movement is the more energetic, and the up 
movement relatively the slower, of the two. Hence we 
may infer that in Desmodium the down movement is due 
to contraction and the up movement to relaxation. 

Test by increased internal hydrostatic pressure. It 
will be shown on page 349 that when Desmodium in a sub- 
tonic condition undergoes arrest of pulsation, an increase of 
internal hydrostatic pressure is found to renew the rhythmic 
activity. With fairly high internal pressure, the frequency of 
the pulsation is increased, though the amplitude is decreased. 
In order now to test the responsive significance of the up and 
down movements respectively, I tried the effect of an increase 



320 



PLANT RESPONSE 




of internal hydrostatic pressure in shifting the vibration-limits 
of the Desmoditim leaflet. 

It will be remembered that in the case of Mimosa the 
effect of this increased internal hydrostatic pressure was to 
cause the erection of the leaf above its normal position. If, 
then, the same result should follow in the case of Desmodium, 
we should be justified in inferring that the mechanics of the 

motile organ were similar in the 
two cases. The increase of in- 
ternal hydrostatic pressure was 
in this case effected by the same 
method as was employed in that 
of Mimosa that is to say, the 
cut end of the petiole of Des- 
modium was placed in one limb 
of a U-tube, filled with water, and 
the pressure was increased by 
adding water, so as to raise the 
FIG. 128. Displacement of Mean . level at the free end of the tube. 

Position of Vibration of Des- A ii i a 4. r r> j- i- 

medium Leaflet by Increased As the leaflet of Desmodium, how- 

Internnl Hydrostatic Pressure ever) J s J n constant oscillation, 

we must regard the mean of its 
vibration-limits as the normal 
mean, and the extent of the 
leaflet's erection under increase 
f hydrostatic pressure must be 
measured by the shifting upwards 

Oi this mean. 

J n carry i n g O Ut this experi- 

' L 

meat, I found, on applying the 
increase of internal pressure exerted by a water-column of 
10 cm., that, as will be seen in fig. 128, the lower limit 
of oscillation was displaced upwards in the record by 
11*5 mm. and the upper limit by 3*5 mm. The mean 
position was thus raised by 7*5 mm. As the magnification 
of the record was in this particular case five times, it will 
be seen that the pressure exerted by a height of 10 cm. of 



upper limit ; l,, lower limit ; 
M, mean position under normal 
conditions ; u', I/, M', corre- 
sponding positions under in- 
creased internal hydrostatic 
pressure ; MM' is the extent 



rally speaking, produces a 

movement towards expansion, 
and tends to diminish the 
amplitude of the pulse, while 
mcreasing the frequency. 



CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 32! 



water had thus produced an absolute displacement of the 
mean position of the leaflet, of 1-5 mm. Considering these 
facts, it becomes reasonable to regard the motile indications 
of Desmodium as similar to those of Mimosa ; hence the 
down position of the Desmodium leaflet may be regarded 
as one of contraction, the up position being one of relaxation. 
Thus in Desmodium the down position of the leaflet corre- 
sponds to the systolic contraction, and the up position to the 
diastolic expansion of the heart. 

Mode of application of chemical reagents. Having 
now to some extent determined the character of the move- 
ments of the Desmodium 
leaflet, we shall proceed 
to observe in detail the 
modification of their 
movements by the action 
of various chemical re- 
agents. Three different 
methods of application 
may be employed. In 
the first place, the chemi- 
cal reagent may be dis- 
solved in the water in 
which the specimen is 
placed. The solution will 
thus reach the motile 

organ by the same process as that which brings about the 
ascent of sap. The characteristic action of the chemical 
reagent will be demonstrated in the subsequent modification 
of the responses. 

This method of applying a reagent at one point in this 
instance the cut end of the petiole and observing the sub- 
sequent physiological effect on the distant motile organ, is of 
special interest and importance in the case of poisons. For 
it serves to elucidate the obscure question of the ascent of 
sap through tissues that have been killed by poison (p. 385). 

In order to observe and compare the responses of the 

y 




FIG. 129. Method of Application of 
Chemical Agent to Cut End of Petiole 



$22 PLANT RESPONSE 

giv'en plant, before and after the application of the solution, 
it is necessary that the continuity of the record should be in 
no way disturbed. For this purpose I insert the specimen 
in the arrangement shown in the diagram (fig. 129). One 
end of the tube is connected with a funnel, F, by means of an 
india-rubber tubing ; the other end is provided with a stop- 
cock. The tube is first filled with water, and the stop-cock, 
S, closed. The normal responses of the leaflet are now taken, 
with the petiole in water. Next, by proper manipulation of the 
stop-cock, the water is allowed to run out, and its place is taken 
by the chemical solution which is passed in at the funnel. The 
record which is now taken exhibits the effect of the drug. 

The second of our three methods of experiment is that of 
direct application that is to say, touching the motile organ 
itself with a drop of the solution. In this case, the modifi- 
cation of response takes place rapidly. And, lastly, gaseous 
substances may be introduced or withdrawn from the plant 
chamber, by means of suitable inlet and outlet pipes. 

Action of chemical reagents modified by . tonic con- 
dition of tissue ; strength of solution ; duration of appli- 
cation. There are certain general considerations of a very 
significant kind which it will be well to specify at this point. 
Though the fundamental effect of any given reagent is 
definite, yet certain minor variations of this effect, due to 
the constitution of the individual plant, are liable to occur, 
and these are often extremely suggestive. Two successive 
experiments, for example, were performed to determine the 
action of the poison mercuric chloride on different specimens ; 
one of these was extremely vigorous, and the other the 
reverse. The effect of the poison on the robust specimen 
consisted in depressing the pulsation, and reached its maximum 
some fifteen minutes after application. But subsequently the 
plant appeared to shake off the influence 'of the poison, and 
the pulses slowly recovered, till in the course of an hour they 
had once more become normal. The effect on the weakly 
specimen, however, was somewhat different. Instead of 
depression, the immediate result of application was a transi- 



CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 323 

tory exaltation of the amplitude of oscillation, which was 
doubled, though at the same time the rhythm became slowed. 
After ten minutes, however, these pulsatory movements grew 
irregular and depressed, and the plant succumbed to the 
action of the toxic agent, its pulsation undergoing complete 
arrest forty-five minutes after application. In the case of a 
weakly specimen, again, on filling the chamber with an 
atmosphere of carbonic acid gas, the pulsations soon come to 
a stop, and unless fresh air be quickly introduced, this arrest 
becomes permanent But with a vigorous specimen, the depres- 



sion produced by this gas is very slow, and the permanent 
arrest does not take place till after a considerable lapse of time. 

The effect of an agent, again, depends on the strength 
of the solution, and the duration of application. A solution 
which, in larger quantities, would produce depression, will 
often, if given in very diluted form, cause the exaltation of 
response. The sudden introduction of an agent which may 
ultimately produce arrest, may act as a transitory stimulus, 
bringing about a preliminary augmentation of response, to be 
followed later by depression and arrest. The application of a 
deleterious substance, again, for a short time, will cause a 
temporary depression, from which there is revival ; but too 
long-continued action of the same reagent will cause 
permanent arrest. Besides all these, there is the interesting 
phenomenon of accommodation, by which the plant becomes 
gradually accustomed to the action of any adverse circum- 
stance, and is thus rendered immune. 

Effect of anaesthetics. Taking a specimen of Desmo- 
dium, I passed ether-vapour into the plant chamber. The 
pulsatory movement which had hitherto been fairly uniform 
now showed a transitory exaltation, and then fell with 
remarkable regularity of decrement. The response next 
showed an equally regular tendency towards the gradual 
recovery of its previous amplitude, but with longer period, 
this being protracted, from the normal two to four minutes. 
Finally, the pulsation of this specimen was abolished, the 
leaflet remaining in a position of relative relaxation, thirty 

Y 2 



324 



PLANT RESPONSE 




minutes after the first introduction of ether into the 
plant chamber. On blowing off the ether-vapour, there 

was in this case no revival 
of response. In those cases, 
however, in which the ether- 
vapour is more diluted with 
air, or applied for a shorter 
period, the depression is 
temporary only, and is fol- 
lowed by revival. But if 
a larger quantity of ether- 
vapour be at once introduced, 
a permanent arrest, in the 
relaxed position, quickly en- 
sues. Before this happens, 
there may be one or two 
spasmodic flutterings, but 
these quickly subside, as will 
be seen in the photographic 
record given (fig. 130). 

Effect of alcohol. The effect of this reagent is much 
modified by the tonic condition of the specimen. For 
example, in the case of a weaklier plant, the application, 
even of dilute solutions, induces rapid diminution and arrest 
of pulsation. More vigorous specimens can, however, with- 
stand the deleterious effect of this drug, and bear stronger 
doses. In the photographic record here given (fig. 131) a 
5 per cent, solution is seen to induce a greater regularity 
and amplitude of pulsation. The subsequent application of a 
10 per cent, solution causes a moderate, and 1 5 per cent, a still 
greater, depression. The application of a 5 per cent, solution 
to a weakly specimen, however, is found, as said before, to 
induce a depression so great as to cause a speedy arrest. 

Effect of carbonic acid. The first effect of the intro- 
duction of this gas is sometimes one of exaltation. This is 
however, brief, and is followed by depression and sudden 
permanent arrest of pulsation in weaklier specimens. In 



FIG. 130. Photographic Record of 
Effect of Ether Vapour, Large Dose 

Arrow marks moment of application. 
Pulsation arrested in up, or relaxed, 
position. It has to be remembered 
that the up movement of the record 
corresponds to a do\\n movement 
of the leaf. 



CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 



other cases, again, an earlier re-introduction of fresh air into 
the chamber is sufficient to restore the specimen to its 




FIG. 131. Photographic Record of Effect of Alcohol 

Pulsations to left show normal response, f Marks application of 5 per 
cent, solution ; f ' application of 10 per cent. ; f ' application of 15 per 
cent. 

natural pulsatory activity. I give here (fig. 132) a photo- 
graphic record of the effect of this gas on a vigorous speci- 
men, in which its action 
is seen to be somewhat 
gradual. One curious phe- 
nomenon which I have no- 
ticed in connection with 
the effect of this gas, is 
that when it remains stag- 
nant in the chamber its 
depressing effect is much 
more rapid than when a 
current is allowed to stream 
through. 

In the manner which I FIG. 132. Photographic Record of Effect 

of Carbonic Acid Gas 

have just described, I have Arrow marks moment of application, 
carried out further a number 

of experiments on the action of various gases and vapours, 
such as ammonia, carbon disulphide, and others, all of which 




326 



PLANT RESPONSE 



arc found to cause an arrest of the rhythmic movements of 
Desmodium. It is not necessary to go into these in detail, 
the experiments already given affording sufficient informa- 
tion for their successful repetition. 

I will only mention here the very interesting and impor- 
tant fact, that I find acids and alkalies, generally speaking, 
to produce effects which are in a certain physiological sense 
antagonistic. These effects, together with the influence of 
temperature on rhythm, and the action of tetanising electric 
shocks on the autonomous movements of Desmodium^ will be 
found fully described in Chapters XXVI. and XXVII., where 
the remarkable parallelism of their influence on rhythmic 
animal and vegetable tissues will be demonstrated. I shall con- 
clude the present chapter by describing the action of a strongly 
poisonous reagent on the pulsatory movements of Desmodium. 
Effect of copper sulphate solution. I carried out two 
experiments on similar specimens to test the effect of 

this reagent. In the 
first, the solution was 
applied directly on the 
pulvinus, and produced 
a very quick arrest of 
its rhythmic activity 
(fig. 133). In the 
second case, the ap- 
plication was made at 
the cut end of the 
petiole, as already de- 
scribed, which was at a 
distance of 2 cm. from 
the pulvinus. In this 
case the arrest took 
place much later, that 
is to say, thirty minutes after the application. This delay was 
due to the fact that the poisonous solution had to ascend 
the intervening distance before it could affect the rhythmic 
activity of the tissue at the pulvinus. This experiment will 




FIG. 133. Photographic Record of Effect of 
Copper Sulphate Solution Applied on the 
Pulvinus 

Arrow marks moment of application. 



CITEMirAL REAGENTS ON AUTONOMOUS PULSATION 327 



be found important, as touching a later investigation on 
the ascent of sap. It is to be noticed that the petiole 
allows the poisonous solution which kills it to pass upwards 
through it. 

Spark-record of pulsation of Desmodium. Before 
ending this chapter, I shall give a spark-record of a single 
pulsation in a leaflet of Desmodium. The successive sparks 
were produced at intervals of 
5 seconds, and a glance at 
the record affords a visual 
demonstration of the peculiar 
characteristics of the move- 
ment of the leaflet. The 
up line as usual indicates 
down movement. It is thus 
seen that, after a pause in 
the highest position, a sudden 
excitatory impulse is de- 
veloped, which is gradually 
exhausted, as the lowest 
position is reached. The up movement takes place more 
gradually, and at a much slower rate. The results are shown 
in the following table. 

TAKLE SHOWING RATES OF MOVEMENT AT DIFFERENT STAGES OF 

PULSATION IN DESMODIUM. 




FIG. 134. Spark-record of Single Pul- 
sation in Leaflet of Desmodium 

Interval between successive sparks 
= 5 seconds. 



Down movement 



Up movement 



Total period 
Average rate 
Maximum rale . 
Duration of pause 



45 seconds 
6 1 mm. per sec. 

'7 55 M 

40 seconds 



Total period 
Average rate 
Maximum rate . 
Duration of pause 



7 seconds 
4 mm. per sec. 

"5 ' > " 
35 seconds 



SUMMARY 

The effect of a chemical reagent on a plant is modified tc 
some extent by the tonic condition of the tissue. A vigorous 
plant will, generally speaking, withstand for a considerable 
time the action of deleterious agents ; a weakly specimen 
succumbs more quickly. 



328 PLANT RESPONSE 

The effect of a reagent depends also on the strength of 
the solution. A reagent which, in strong solution, induces 
depression, may, if given in small quantities, cause exalta- 
tion. 

The effect of a reagent depends also on the duration of 
application. The temporary depression produced by a short 
application is overcome by the self-accommodation of the 
plant But it will succumb to too long or too strong an 
application of the same reagent. 

The effects of the various reagents on autonomous response 
are, generally speaking, similar to their effects on simple 
response. 

The depressed position of the leaflet of Desmodium 
represents a ' systolic ' contraction, and the up position a 
c diastolic ' relaxation, of the motile organ. Increased internal 
hydrostatic pressure increases the extent of the relaxed or 
1 diastolic ' limit. 

The effect of too strong an application of ether is to 
abolish response, the arrest of pulsation usually taking place 
in a relaxed position. The immediate effect of application is 
generally a transient exaltation of response. 

The effect of vapour of alcohol is usually a transient 
exaltation. If the application be prolonged, the result is a 
permanent arrest of pulsation. 

Carbonic acid sometimes produces a transient exaltation 
of response, and always a subsequent depression, which, 
under the long-continued action of this gas, may pass into 
permanent arrest. 

Copper sulphate solution, when applied directly on the 
pulvinus, quickl^ causes arrest of pulsation ; but if the cut 
petiole be allowed to absorb the solution, the final arrest 
does not take place till after the lapse of a certain period, 
required for the solution to ascend to the motile organ. 



CHAPTER XXVI 

EFFECTS OF TEMPERATURE ON AUTONOMOUS RESPONSES 

Increase of frequency and diminution of amplitude of pulsation with rising 
temperature Converse effect of fall of temperature ^Similar effect in cardiac 
pulsation Effect of the reduction of temperature to the thermo-tonic minimum 
Explanation of diminution of amplitude of pulsation with rise of temperature 
Anomalous use of the word * relaxation ' Simple versus additive character 
of individual pulsation. 

> 

IT is my intention in the course of the next chapter to 
make a comprehensive review of the similarities in all their 
characteristics of rhythmically responding tissues, both vege- 
table and animal. In the present chapter, then, we shall 
confine our attention to a study in detail of the influence 
of temperature in modifying the amplitude and period of 
rhythmic autonomous responses, exemplified in the case of 
plant-tissues by Desmodium and in that of the animal by 
cardiac muscle. 

As then we are about to study the effect of temperature 
on the period and amplitude of vibration of the Desmodium 
leaflet, it is clear that our first difficulty must be the securing 
of a specimen in which both are, to begin with, more or 
less uniform ; for the pulsation of Desmodium^ like that of 
the isolated frog's heart used for experiments, is often 
irregular. It is thus only by careful selection that one can 
obtain suitable experimental subjects. A moderate increase 
of the internal hydrostatic pressure, however, will often have 
the effect of rendering the responses sufficiently uniform. 

Regulation of temperature. The second difficulty in 
this investigation lies in subjecting the plant to the required 
rise or fall of temperature. A rise of temperature may be 
secured by any one of three different methods, (i) A spirit 



330 PLANT RESPONSE 

flame may be applied underneath a bath of water in which 
the leaflets are placed. The temperature is thus gradually 
and continuously raised, and the successive pulsations, 
corresponding to different temperatures, are recorded in the 
usual manner by means of the Optic Lever. (2) Water 
at the required temperature may be syphoned into the bath, 
and the responses taken in the ordinary manner. (3) The 
air chamber in which the specimen is placed may be subjected 
to electric heating. The use of temperature may now be 
very accurately regulated by adjustment of the current, and 
records of pulsations may be taken at different and deter- 
minate temperatures. 

This last is the most perfect method, the two former, 
dependent as they are on the immersion of the specimen in 
a bath of water, having as compared with it many disadvan- 
tages. For the natural freedom of movement of the leaflet 
is hampered by the water, and more troublesome still is the 
difficulty which at times arises from capillary action in the 
partially immersed cocoon-thread, by which the leaflet is 
attached to the Optic Lever outside the bath. 

But we have not the same perfect facilities for lowering 
temperature in a gradual and continuous manner, as for 
raising it. This may be accomplished, however, sufficiently 
well for our purposes: (i) by placing fragments of ice in 
the air chamber ; or (2) the pulvinus of the leaflet may 
be touched with cold water which has been reduced to 
the required temperature by means of ice. I find, however, 
(3) that a much better method is that of placing in the air 
chamber a coil of thin-walled metallic tubing, preferably 
of highly conducting copper. When cooled brine is made 
to circulate through this coil, the temperature of the chamber 
is lowered, and by regulation of the flow, by means of stop- 
cocks, it is possible to produce an adjustment of cooling. 

Effect of temperatures maximum and minimum. 
Autonomous vibrations come to a stop when the temperature 
is sufficiently lowered. The temperature minimum at which 
this occurs depends, as we should expect, on the nature of 



TEMPERATURE AND AUTONOMOUS RESPONSES 331 

the specimen whether Desmodium^ Biophyttim> or cardiac 
muscle and also, with similar specimens, on the tonic condi- 
tion. With regard to the first of these points, we have seen 
that the autonomous vibration of Biophytum comes to a 
stop below 29 C. The pulsation of Desmodium is said to 
be arrested at 22 C., but I find that this is a matter which 
is much modified by the tonic condition of the particular 
plant. With vigorous specimens I have seen that the vibra- 
tion may persist even at so low a temperature as 17 C. 
The thermo-tonic minimum in Biophytum and Desmodium 
thus shows a difference, as already said, of about 12 C. ;, 
and in the case of the frog's heart this is still lower, and is 
said to be about o C. 

When, again, the specimens are raised to a maximum 
temperature, the pulsations come to a stop. The tempera- 
ture at which this takes place depends in part on the 
condition of the specimen. For example, with the frog's 
heart, this maximum is sometimes at a temperature so low 
as 38 C. In other cases, pulsation may be detected even at 
so high a temperature as 44 C. Similarly, in Desmodium, 
I have found that the maximum temperature at which arrest 
took place was sometimes as low as 35 C. ; but in certain 
specimens it did not occur till 45 C. A plant may, 
again, be accustomed gradually to high temperatures, and 
under these circumstances the maximum may be raised as 
much as 3 C. or 4 C. higher. 

Effect of temperature on period and amplitude of 
response. The most marked phenomenon of effect of 
temperature on automatic pulsations, whether animal or 
vegetable, lies, however, in the fact that the period and 
amplitude are both affected. When the temperature is 
lowered, the amplitude of pulsation is enhanced, while the 
frequency is diminished. Conversely, with rise of tempera- 
ture, the amplitude is diminished, and frequency augmented. 
This is true not only of the pulsations of the rhythmic 
tissue of Desmodium^ but also of those of the animal heart. 
This is seen in the two following records, where the first 



332 PLANT RESPONSE 

set of responses in each series gives pulsations at the 
temperature of the room, while the second set in each gives 
pulsations of greater amplitude and smaller frequency, 
due to the lowering of temperature by several degrees 
(figs. 135, 136). Conversely, as already said, when the 
temperature is raised, the frequency is increased and the 
amplitude decreased. This is seen in a general way in the 
following photographic record (fig. 137), which I took with 




Vl^" MS- Photographic Records of Autonomous Pulsations in Des- 
viodinm, showing Increase of Amplitude and Decrease of Frequency, 
with Lowering of Temperature 

The pulsation.s to the left were taken at the ordinary temperature of the 
room, 29 C. Those to the right were taken when the temperature 
had been lowered to 25 C. 

Desmodium y the temperature being continuously raised, from 
30 C. to 39 C. It will be seen how progressive in character 
is the diminution of amplitude and increase of frequency in 
these responses. 

In another set of experiments in which I took records 
of the responses of Desmodinm at various definite ascending 
temperatures, I found that at 19 C. the period of a single 
oscillation was 4*3 minutes. At 22 C. this was reduced to 



TEMPERATURE AND AUTONOMOUS RESPONSES 333 



3*2 minutes, or nearly to two-thirds of the period at 19 C. 
At 28 C. it was found to be again reduced to 2'i minutes, 
or half. But at 40 C. it 
was only 1*4 minute, or 
one-third. Thus, while 
in 4*3 minutes, at 19 C. 
there is only a single 
beat, there are two beats 
at 28 C. and three beats 
at 40 C. in the same 
time. I give below (fig. 
138) a record of these 
responses at various 
temperatures. The re- 
cord given afterwards 

(fig- r 39) shows how 

similar is the effect of 

temperature on the 

amplitude and rhythm 

of the pulsation of the 

animal heart. When 

the temperature of Dcsmodium is raised above 40 C. there 

appears to be an arrest of pulsation. But this need not be 




FIG. 1 36. Effect of Lowering of Tempera- 
ture in Producing Increase of Amplitude 
and Decrease of Frequency in Pulsation 
of Frog's Heart 

The pulsations to the left represent normal 
pulsations at the temperature of the 
room. Those to the right were taken 
at a temperature several degrees lower 
(after Brodie). 




FIG. 137. Photographic Record of Pulsations of Desmodium during 
Continuous Rise of Temperature 



regarded as due to heat-rigor. For in magnified records 
I have often noticed that here we may have very much 



334 



PLANT RESPONSE 



quicker pulsations, but of an amplitude so small as usually 
to pass undetected. It is only under prolonged exposure 
to this relatively high temperature, or after exposure to a 

temperature above 50 C., that true 
heat-rigor sets in. This maximum 
temperature varies in individual cases 
with the tonic condition of the plant. 
Effect of the reduction of 
temperature to the thermo-tonic 
minimum. It has been said that, 
generally speaking, the amplitude of 
response increases with the lowering 
of temperature. It is evident, how- 
ever, that this process must have a 
limit. For we know that pulsation 
vanishes at the thermo-tonic mini- 
mum ; before this reduction of ampli- 
tude to zero it is clear that there must 
be some point where the increase of 
amplitude due to continuous cooling 
must undergo reversal, or diminution. 
And this is what we should theoreti- 
cally expect, for since it is the absorbed 
thermal energy that maintains the pulsation, it follows that, 
when this is diminished below par, the vibrational energy 




FIG. 138. Record of Pulsa- 
tions of Desmodium at 
Different Temperatures 




FIG. 139. Record of Pulsations of Frog's Heart at Different 
Temperatures (Pembrey and Phillips) 



should also undergo diminution. We had an illustration ot 
this (p, 305), when the automatically vibrating Biophytum 



TEMPERATURE AND AUTONOMOUS RESPONSES 335 

at 35 C. was allowed to descend to the thermo-tonic 
minimum. It was in that case found that there was a regular 
diminution of amplitude, and the pulsation afterwards dis- 
appeared below 29 C. (fig. 124). 

I was next desirous of determining whether this theoretical 
inference could be verified in the case of Desmodium. For 
this purpose I rapidly cooled the plant, by means of the 
cooling coil, through which ice-cold brine was passed, a 




FK;. 140. Effect of Cooling to Thermo-tonic Minimum on Pulsation 

of Desmodiuin 

The first two pulses to the left were taken at the normal temperature of 
29 C. ; those to the right during continuous cooling. The first 
response of the latter series occurred at 22 C. ; the second at 20 C. ; 
and the third at 17 C. Note that while in normal responses there 
are no sub-pulses, these are seen with increasing distinctness as the 
pulsation becomes continuously slower. They first make their appear- 
ance during the up movement of the first pulsation under cooling. In 
the two successive pulsations they are seen more and more clearly in 
the down as well as the up movements. 

photographic record being taken of the pulsations all the 
time. It will be noticed that the first effect of cooling was 
the normal increase of amplitude and prolongation of period. 
This latter which at the temperature of the room had had 
a value of three minutes was now prolonged to nearly six 
minutes. But the most interesting fact was that, as the 
thermo-tonic minimum was approached, the amplitude was 
reduced (fig. 140). 



PLANT RESPONSE 

In the next experiment cooling was produced more 
suddenly, by application of cold water at about 4 C. to 
the pulvinus. We observe in this case how the quick 
reduction to the thermo-tonic minimum reduced the ampli- 
tude till the pulsation had come to a stop. I then allowed 
the leaf to return gradually to the temperature of the room, 
and it is very interesting to note the effect of increasing 




FIG. 141. Effect of Rapid Cooling by Ice-cold Water 

Normal pulsations recorded to the left. Effect or application of ice-cold 
water is seen in the production of diminished amplitude and abolition 
of pulsation. Gradual return to the temperature of the room revives 
the pulsation in a staircase manner, the period remaining approximately 
constant. Note that cooling, in this and previous figure, displaced the 
pulsation in a downward or contracted direction. Gradual warming, 
conversely, is seen in this figure to produce the opposite displacement 
towards relaxation. 



absorption of thermal energy from its surroundings. The 
increased energy thus absorbed is seen to give rise to 
increased amplitude of oscillation, in a staircase manner, 
\yhich gradually approaches the original pulsation (fig. 141). 
In both these figures it will be noticed that cooling dis- 
places the pulsation in a downward or contracted direction. 
And in the last series of fig. 141 we see that the raising of the 



TEMPERATURE AND AUTONOMOUS RESPONSEsS 337 

temperature displaces it upwards or towards relaxation. 
These facts are of great importance, and should be borne in 
mind in reference to the explanation of the cause of variation 
of amplitude and period, which I shall bring forward. 

Explanation of diminution of amplitude of pulsation 
with rise of temperature. We have thus seen, as we should 
theoretically have expected, that with the increased absorp- 
tion of energy at a higher temperature, the amplitude of 
vibration is also increased. How is it, then, that with the 
still further increase of absorption of energy, at still higher 
temperatures, the amplitude should undergo a diminution ? 
When approaching the maximum point, where heat-rigor 
takes place, we can understand that the excitability of the 
tissue would be very much decreased, with a consequent 
reduction of amplitude of pulsation. But at temperatures of 
25 C. to 30 C. excitability of the tissue could not be 
diminished. Indeed, I shall in Chapter XXXIII. adduce 
considerations to show that it must, at that temperature, be 
highly excitable, and we should have expected that this 
would have produced, in addition to the increased energy, an 
augmentation of vibrational amplitude. But, instead of this, 
we obtain the curious result which has been described, of 
a diminution, in the cases both of cardiac muscle and of 
Desmodium. 

It might be suggested that if increased activity, due to 
rise of temperature, increased the frequency of vibration, 
then this fact would be sufficient in itself to account for a 
diminution of amplitude ; for in this case, less time being 
allowed for each single vibration, its extent must be cur- 
tailed. But this consideration alone would not explain all the 
facts of the case ; for we have seen that on approaching 
the thermo-tonic minimum, though the frequency of vibra- 
tion is reduced, and the period very much extended, yet 
the amplitude is also at the same time decreased (fig. 140). 
We thus see that the question of internal energy is important 
in this connection. An increase of internal energy may be 
expressed, as I shall show, in two different ways ; either, that 

z 



338 PLANT RESPONSE 

is to say, by increase of amplitude or by increase of frequency 
of pulsation. 

Increased internal energy, shown by: (a) Increase of 
amplitude, period remaining constant. And we shall, as the 
simpler of the two, consider that case in which the latent energy, 
or tonic condition of the plant, is below par, that is to say, the 
case in which it is near the thermo-tonic minimum. The effect 
of increased absorption of energy with rising temperature 
would here be indicated by increasing amplitude of pulsation, 
the period remaining constant. Conversely the reduction of 
latent energy with falling temperature would be indicated, 
when the period is constant, by the fall of amplitude. This 
we find fully illustrated in records obtained with Biophytum 
and Desmodium. In the former, when nearing the thermo-tonic 
minimum, it is found that while the period remains approxi- 
mately constant, that is to say, two and a half minutes, the 
amplitude of pulsation falls from 8 divisions at 35 C. to 5*5 
divisions at 32 C. (fig. 124). In Desmodium, again, we find 
a converse case. Here, while the plant is rising from the 
thermo-tonic minimum to the normal condition, the amplitude 
of pulsation is seen to increase progressively, while the period 
of 2'4 minutes remains constant (fig. 141). 

(d) Increase of frequency. Taking next the case of a plant 
in the ordinary tonic condition, we find that the increase of 
internal activity, due to the greater absorption of energy 
during a rise of temperature, is exhibited by a higher frequency 
of vibration. The reason why, with this increase of frequency, 
there is a diminution of amplitude of pulsation, is now to be 
explained. 

We have seen that an increase of internal energy, as 
caused by rise of temperature, brings about an increase of 
turgor, and that this increased turgor hastens the process 
of recovery, and by acting antagonistically to the con- 
tractile phase of responses diminishes its amplitude. In- 
creased internal pressure also, generally speaking, increases 
the frequency of vibration. If, then, the rise of temperature 
increases the turgor of Desmodmm, as we have found it to do 



TEMPERATURE AND AUTONOMOUS RESPONSES 339 

in plants in general, then the diminution of the amplitude of 
its vibration with higher temperature is explained ; and I 
shall be able to adduce independent proof that this is actually 
the case in Desmodium> for we have seen that the external 
indication of internal increase of turgor is the expansion of 
the organ, which produces a movement upwards, the same as 
that of relaxation. We found in the last series of responses 
in fig. 141, moreover, that when the temperature was raised 
gradually from the thermo-tonic minimum, the leaflet was 
more and more erected, or 'relaxed.' Cooling, conversely, 
produced diminution of turgor, and an opposite movement in 
the direction of fall or contraction (figs. 140 and 141). 

The anomalous use of the word * relaxation.' We have 
seen that the motile organ of Desmodium^ when anaesthetised, 
is brought to a state of standstill in a position of relaxation. 
Its tonic condition, by virtue of which it exhibits contractile 
response, has thus been abolished. We may then regard 
ether as having brought about a loss of tone, or as having 
reduced the tissue to the a-tonic condition. 

An apparently similar position of relaxation may, however, 
be attained by the active process of expansion, which is the 
result of an increase of internal turgor. It would thus appear 
that we are liable to form many wrong inferences, as to the 
tonic changes undergone by the organ, if we too hastily 
conclude that expansion is always caused by loss of tone. 

Simple versus additive character of individual pulsa- 
tions. One question, regarding which opposite views have 
been put forward hitherto, is that of the simple or composite 
nature of the individual pulsations of cardiac muscle. The 
movement of systole may, for example, be regarded as con- 
sisting either of a single or of the additive effect of several 
constituent contractions. In the production of tetanus in the 
case of muscle and also in that of contractile vegetable tissue 
we have seen several individual contractions, when following 
each other with sufficient rapidity, become merged in one appa- 
rently continuous contraction (figs. 49, 50). When less rapid, 
however, they are individually distinguishable. Similarly, 

7. 2 



340 PLANT RESPONSK 

in the case of cardiac pulsation, it is possible that one appa- 
rently simple contraction may in reality consist of several, 
which are rendered indistinguishable by the great rapidity 
of their succession. 

Now, since the pulsatory response of Desmodium is in 
every way so similar to that of cardiac muscle, its analysis 
might be expected to throw much light on this question ; 
and this more especially since it possesses the added advantage 
that its pulsation is executed in a period about one hundred 
times as long as that of cardiac muscle. The constituent 

elements of each pulsation, if such 
exist, ought thus, owing to the 
relative slowness of the movement, 
to be much more easy of detection. 
Now, it is often seen in watching 
the pulsatory movements of Des- 
modium that they proceed some- 
what discontinuously, or by jerks. 
Under favourable circumstances, 
nevertheless, the movements of 
the leaflet up and down become 
apparently continuous. When 
Ku; 142. Photographic Record suc h pulsations arc recorded on a 

of Pulsation of Desiodtnm y ~ 

showing Sub-pulses during rather rapidly moving drum, the 

slower Up Movement, as ,. , , f , t , 

Nodules discrete nature of the movement 

can be brought out more easily in 

the case of the up movement, which is relatively slow. This 
is shown in a very interesting manner in the accompanying 
photographic record (fig. 142), where the subsidiary pulsations 
make themselves visible as nodules ; places where the move- 
ment is slow appear thicker, on account of the photographic 
irradiation-effect. During the course of the single up move- 
ment recorded in this photograph, we may count as many as 
twenty-five of these sub-pulses. It has been said that these 
subsidiary movements are more easily detected when the 
general movement is relatively slow ; and in connection with 
this, it is extremely interesting to note the record of pulsations 




TEMPERATURE AND AUTONOMOUS RESPONSES 34! 




FIG. 143. Photographic 
Record of Cyclic Group- 
ings in Autonomous Pul- 
sations of Dtsinodimn, 
showing Sub-pulses 



seen in fig. 140, where successive pulsations of the same 
leaflet, at first apparently continuous, are made to exhibit the 
sub-pulses with growing distinctness, as the period becomes 
progressively slowed down by cooling. 
First we are enabled to observe the 
sub-pulses during the up movement, 
and afterwards during the down move- 
ment also. I give another photo- 
graphic record also (fig. 143), in which 
these subsidiary pulses are seen at both 
the 'systole' and ' diastole 1 (see also 
fig- 1 7$)- Arguing from analogy, 
therefore, it becomes highly probable 
that any single pulsation of the heart 
also may be made up of similar dis- 
crete and constituent elements. 

While dealing with this question, it must be borne in 
mind that in each case the contractile organ as a whole is 
made up of a mass of in- 
dividually contractile ele- 
ments, the sum of whose 
separate and additive ac- 
tions it is which is seen 
as a single contraction and 
expansion of the whole 
organ. Thus, in Desmo- 
dium for example, a single 
pulsation of the motile 
organ is made up of sub- 
sidiary pulselets. The 
unit-pulses, again, may 
themselves be grouped in 
larger systems, either as 
the alternate waxings and 
vvanings seen in periodic 
groupings (figs. 143 and 149), or in those periodicities of 
the order of an hour or so, which the plant sometimes 




FIG. 144. Photographic Record of Auto- 
nomous Pulsation of Desmodium, 
showing Hourly Period 
[Speed of drum = 5 cm. in one hour.] 



34 2 PLANT RESPONSE 

exhibits, under the influence of periodic variations of tem- 
perature and other factors. An extremely interesting example 
of such an hourly periodicity is given above (fig. 144), where 
the mean plane of vibration is itself seen to exhibit a periodic 
up-and-down oscillation. There is, again, the still larger 
periodicity of diurnal variation of day and night. We thus 
see how complex may be these wave-systems, in which, 
superposed over large waves, are smaller waves, and on the 
latter still smaller wavelets. 

SUMMARY 

In Desmodium> as in cardiac muscle, rise of temperature 
produces increased frequency with diminished amplitude of 
pulsation. This is true within a certain normal range of 
temperature. When the temperature of Desmodium is reduced 
to a thermo-tonic minimum which is about 17 C. but subject 
to certain individual variations the amplitude of pulsation, 
owing to the loss of internal energy, is decreased till there is 
an arrest. If now the temperature be gradually raised, the 
pulsations, owing to the absorption of energy, become again 
increased in a staircase manner, the period remaining approxi- 
mately constant. 

Under normal tonic conditions, the decrease of amplitude 
of pulsation with rising temperature when this is not 
excessive is not indicative of loss of excitability. It is due 
to the increase of internal energy, which hastens recovery and 
acts antagonistically to the responsive movement of contrac- 
tion. The same explanation is probably applicable to the 
diminished amplitude of pulsations with rise of temperature, 
observable in cardiac response. 

Rise of temperature, by increasing the internal energy 
and consequent turgor of the plant, causes expansion of the 
organ, thus bringing about a shifting of the pulsation towards 
' diastole/ 

* 

A converse effect, or shifting towards ' systole,' is seen in 
De$modium > as the result of cooling. 



TEMPERATURE AND AUTONOMOUS RESPONSES 343 

The process of relaxation in the motile organ may be the 
result of entirely different causes. It may be a consequence 
of a loss of tone, such as results from narcotisation ; or it 
may be brought about by excessive turgor, caused by increased 
internal energy. 

The apparently simple rhythmic pulsations of Desmodium 
can be analysed and shown to consist of subsidiary minor 
pulsations. The ordinary pulsations, again, may show hourly 
and daily periodicities. 



CHAPTER XXVII 

SIMILARITIES OF RHYTHMIC RESPONSE IN VEGETABLE 

AND ANIMAL TISSUES 

The similarities, in their fundamental characteristics, of rhythmic tissues, animal 
and vegetable : (l) In responses (2) In possession of long refractory periods 
(3) In incapability of tetanus Theories regarding the causation of heart-beat 
The similarities of rhythmic tissues, animal and vegetable, as seen in : (i) 
The effects of internal hydrostatic pressure- (2) The effects of variation of tem- 
perature (3) The periodic groupings of response (4) The effect of barium 
salt (5) The antagonistic actions of acid and alkali Identity of rhythmic 
phenomena in animal and vegetable tissues. 

I HAVE, in the course of the previous chapters, shown the 
remarkable general similarities, extending through numerous 
details, between the responses in animal and vegetable tissues. 
These similarities, however, become still more striking when 
we compare the special characteristics of those plant and 
animal tissues which exhibit the property of rhythmicity that 
is to say, those tissues which, under the action of a single 
strong stimulus, give rise to a multiple and rhythmic series of 
responses, such responses, under favourable circumstances, 
passing into the so-called automatic movements. 

In animal tissues, such rhythmic movements may be 
observed in their perfection in the case of cardiac muscle. 
The isolated heart, when brought to a state of temporary 
standstill, will give, in answer to a single stimulus, a single 
response, or to a sufficiently strong stimulus a multiple series of 
rhythmic responses ; and under favourable circumstances it 
will give automatic responses for a considerable length of time. 

Similarly, the plant Biophytum, under exceptionally 
favourable circumstances, exhibits what are apparently auto- 
matic responses ; and again, under ordinary conditions it 



RHYTHMIC RESPONSE IN PLANT AND ANIMAL 345 



gives a single response to a single stimulus, or if the stimulus 
be sufficiently strong, a multiple series of rhythmic responses. 
In the plant Desmodium^ under favourable tonic conditions 
we observe automatic movements ; but when under less 
favourable circumstances, as for instance owing to the 
unfavourable season, it is brought to a state of standstill, it 
gives a single response to a single stimulus. When the 
stimulus, however, is strong, we have seen that it gives rise to 
a multiple series of responses, in a manner precisely like that 
of Biophytum under similar circumstances. It will thus be 



seen that Biophytum in its ordinary condition may be 
regarded as equivalent to Desmodium in a state of standstill. 
Similarities, in their fundamental characteristics, of 
rhythmic tissues, animal and vegetable : (i) In responses. 
In the matter of response, we have found that the rhythmic 
automatic movements of cardiac muscle are repeated in 
Desmodium under ordinary tonic conditions, and in Biophytum 
under exceptionally favourable circumstances. In a state of 
standstill, all three give a single response to a single moderate 
stimulus, and a multiple series of rhythmic responses to a 
sufficiently strong stimulus. The following tabular statement 
exhibits this parallelism in a concise form : 

TABULAR STATEMENT SHOWING SIMILARITIES IN THE RESPONSES OF 
RHYTHMIC ANIMAL AND VEGETABLE TISSUES 



At standstill 



Moderate stimulus 



Cardiac muscle 



Desmodium 



Biophytum 



Single stimulus, 
single response 



Do. 



Do. 



Sufficiently strong 
stimulus 



Single stimulus, 

multiple series of 

rhythmic 

responses 



Do. 



_ ! 



Do. 



Under favourable 
conditions 



Automatic 

rhythmic 

movements 



Do. 
Do. 



346 PLANT RESPONSE 

(2) In possession of long refractory period. In order to study 
in detail the characteristics of response in Desmodium, I took 
a plant in which the leaflets had come to a state of natural 
standstill. To such a specimen I applied the stimulus of a 
condenser discharge ; it was found, as stated already, that 
a rather high electromotive charge (twenty-four volts) was 
required to produce response. The first few responses were 
somewhat feeble, owing to the sluggish condition of the tissue ; 
they then increased in a ' staircase ' manner till they became 
uniform, the period of a complete response being now about 
six minutes. From this point on, the responses were the 
maximal possible, and a higher E.M.F. produced no notice- 
able increase. The most characteristic feature of these 
responses was the possession of a long refractory period, 
which we have also found to be characteristic of the response 
of Biophytum. With this specimen of Desmodium I found 
that when a second stimulus was given after three minutes, 
there was no further response. But a stimulus given after 
three and a half minutes was effective. It may be mentioned 
here that the length of the refractory period varies somewhat 
with the condition of the tissue, being relatively longer when 
that is sluggish. 

(3) In incapability of tetanus. The rhythmic tissue of 
Desmodium thus resembles cardiac tissue, in the possession 




KM;. 145. Record showing that Rhythmic Tissue of Desmodium is 
Incapable of being Tetanised 

After the first two pulsations, strong tetanising electric shocks were applied 
continuously. No tetanic effect was produced, but the pulsation 
became somewhat irregular. 

of a marked refractory period. There is again another 
interesting similarity. A rhythmically beating cardiac tissue 
cannot be thrown into tetanus by quickly recurring electric 



RHYTHMIC RESPONSE IN PLANT AND ANIMAL 347 

shocks. An automatically moving Desmodium leaflet is also 
incapable of being thrown into a state of tetanus (fig. 145). 
Rapidly succeeding shocks do not produce tetanic contraction, 
though some irregularity may occur in the pulsation ; exces- 
sively strong shocks kill the plant, and the pulsation is then 
permanently arrested. 

Theories regarding the causation of heart-beat 
Having thus seen how similar are the phenomena of rhyth- 
micity in cardiac muscle and in plants, we may proceed to 
inquire into the theories which have been proposed to 
account for the automatic pulsation of the heart It has been 
suggested : 

(1) That discrete impulses are sent out from certain 
motor nerve-centres in the heart to the muscular tissue, thus 
causing the periodic heart-beat. Assuming the correctness 
of this theory, however, the difficulty is merely transferred, 
for we have still to account for the rhythmic excitation of the 
nerve. But that the rhythmic heart-beat is not fundament- 
ally due to rhythmic impulses from nerve-centres, has been 
proved from facts discovered by various observers : (a) that 
the isolated ganglion-free apex of the frog's heart may be 
thrown into rhythmic activity by stimulus ; it has also been 
shown by Gaskell (b} that the apex of the tortoise-heart, 
which is free from nerve-cells, is capable of rhythmic move- 
ments ; and (c) it is found that even in the embryo, before 
any connection with the central nervous system has been 
established, there is a rhythmic heart- pulsation. 

(2) That cardiac muscle may have the inherent property 
of rhythmicity. This explanation, however, by itself, is incom- 
plete, for it takes no account of the stimulus which must 
exist, in order to give rise to rhythmic expression. 

(3) That the pulsation of the heart is maintained by some 
' inner stimuli,' its rhythmicity being brought about by the 
long refractory period peculiar to cardiac muscle. 

Independent light, however, may be expected to be 
^hrown on the question of the causation of spontaneous 



348 PLANT RESPONSE 

rhythmic action in cardiac muscle, by the consideration of . 
similar phenomena in plants, especially since it can be shown 
that their similarity is manifested under numerous varying 
conditions, and extends to fundamental characteristics. 

We have already seen that there is a similarity of funda- 
mental characteristics between the response of cardiac muscle 
and that of rhythmic vegetable tissue. 

Similarities of rhythmic tissues, animal and vege- 
table. We shall now, therefore, observe in detail those other 
and more special similarities which are exhibited in a com- 
mon modification of response under varying external condi- 
tions, by Desmodium and cardiac muscle alike, and I shall first 
describe the remarkable effect produced on both by internal 
pressure. 

( i ) The effects of internal hydrostatic pressure. It is found 
that a heart which has come to a condition of standstill may 
be set into rhythmic activity by filling the cavity of the heart 
with liquid. Endo-cardiac pressure is thus found to act as a 
stimulus. 

In Desmodium , when the season is favourable, the tissue 
is in a turgid condition, and there is a considerable internal 
hydrostatic pressure, which we have seen to be advantageous 
to the maintenance of rhythm. This turgid condition 
depends on the ascent of sap, which, as I shall show in 
Chapter XXVIII., depends again on the rhythmic activity of 
certain tissues. Thus, in the summer season, we have the 
conditions most favourable for the maintenance of rhythmic 
activity in Desmodium. But with the approach of winter 
the vigour of the plant and its turgid condition undergo a 
marked decline, and the autonomous movement of the leaflet 
then comes to a stop. 

It appeared to me that this cessation of movement might 
to a great extent be due to the diminution of internal 
hydrostatic pressure, and I undertook experiments to see 
whether the pulsatory movement could be renewed by an 
increase of this pressure, just as increased endo-cardiac pres- 
sure was found to renew the beating of the heart. The 



RHYTHMIC RESPONSE IN PLANT AND ANIMAL 349 



experiment was carried out by mounting a detached petiole 
containing the motile leaflets at one end of a limb of a 
U-tube. Any desired 
pressure could be ex- 
erted by varying the 
height of the second 
limb, which was con- 
nected with the first by 
india-rubber tubing. By 
exerting internal pres- 
sure in this manner, 
I was able to pro- 
duce vigorous rhythmic 
movements of leaflets 
which were, before this, 




Vic,. 146. Curve showing Relation between 
Temperature and Period of Pulsation in 
Desmodium 



in an absolutely qui- Abscissa represents temperature, and ordinate 

J \ time, in tenths of a minute. 

escent condition. The 

beneficial effect of this constant internal pressure was further 
seen demonstrated by the extreme regularity and persistency 
of the rhythmic beats. 
Even in the best season 
of the year, the pulsations 
arc irregular, and come to 
an occasional stop ; but 
under the action of internal 
pressure, I have found the 
detached leaflet to main- 
tain its rhythmic activity 
unimpaired for nearly one 
hundred hours. In con- 
nection with this question 

of thf> inn-oner* nf in tor 
Ot the increase Ol intCl- 

nal pressure, it should be 

mentioned here that, after Ordinate represents time, in tenths of a 

the normal condition of 

turgidity has been established, a further increase of internal 

pressure is found to increase the frequency of pulsation. 




FIG. 147. Curve showing Relation 
between Temperature and Period of 
Pulsation in the Heart of a Frog 

jsents tir 
second. 



350 



PLANT RESPONSE 



Excessive pressure, however, brings on irregularity, or even 
stoppage, of autonomous movement. 

(2) The effects of variation of temperature. I have shown 
in the last chapter how perfectly similar are the effects of tem- 
perature in causing variations of the ampli- 
tude and frequency of pulsation in rhythmic 
tissues, both vegetable and animal. As 
regards the effect of temperature on the 
period, the similarity is strikingly exhibited 
in the two curves given above, showing 
the relation between temperature and 
period in Desmodium and in cardiac muscle. 
It will be seen that in both (figs. 146 and 
147) the fall of period with increase of 
temperature is at first rapid and then slow. 
(3) Periodic groupings of response. 
Another very remarkable similarity be- 
tween the pulsations of Desmodium and of 
cardiac muscle lies in their exhibition of 
periodic groupings, very simple types of which, given by 
Desmodium, are seen in the photographic records (figs. 148, 
149). In fig. 146 we have an alternate waxing and waning 
of pulsation, and a remarkably similar record, given by frog's 




IMC.. 148. Simple 
Alternation of 
Pulsation in /)es- 
inodiutn 





FIG. 149. Periodic Groupings 
of Pulsation in Desmodiinn 



FIG. 150. Simple Alternation 
of Pulsation in Frog's 
Heart (Pemhrey and 
Phillips) % 



heart, is seen in fig. ] 50. These groupings are ot various 
degrees of complexity, one type of such heart-beats being 
that known as Luciani's groups, where the successive groups 
are separated by a long pause. Now, similar groups with 



RHYTHMIC RESPONSE IN PLANT AND ANIMAL 351 



intermediate pause are also seen in the pulsation of Des- 
wodium. I have again shown that periodic rhythms of various 
degrees of complexity are seen, not only in the pulsations of 
Desmodium, but also in the multiple responses of Biophytum, 
and even in the electrical responses in plants. 

(4) Effects of barinm salt. There are agencies, such as 
internal pressure and certain chemical reagents, which induce 
regularity of pulsation in 
irregularly pulsating rhyth- 
mic tissues. Conversely, 
there are others, which pro- 
duce the opposite effect, that 
is to say, a regular pulsation, 
after such an application be- 
comes irregular. 

When the heart pulsations 
are regular, it is found that 
addition of Veratrin disturbs 
the uniformity. Remember- 
ing the general similarity of the action of Veratrin and barium 
salts, I applied a 5 per cent, solution of this substance to 
the pulvinus of Desmodium leaflet, which was executing very 
regular vibrations. The application of this reagent disturbed 
the regularity of the pulsation, and somewhat irregular 




Fi. 151. Effect of Barium Chloride 
Solution on Desmodium 

The regular pulsation, seen to the left, 
becomes irregular after application 
of this reagent. 




FIG. 152. Arrest of Beat of Ventricle of Frog at Diastole by Application 

of Acid at Arrow 

Record to be read from right to left (Gaskell). 

groupings were at once established (fig. 151). In another ex- 
periment with the same reagent, as the application of heat is 
known to neutralise the action induced by Veratrin and barium 
salts, I raised the temperature of the plant chamber, with the 
result that the beats became regular once more. 



352 



PLANT RESPONSE 



(5) Antagonistic actions of acid and alkali. But the most 
remarkable of all the similarities seen in the pulsations of 
rhythmic tissues of animal and vegetable, is that of the 
antagonistic actions of acid and alkali. Acid induces in 
the case of the heart a relaxed or diastolic standstill, whereas 




Ki<. 153. Systolic Arrest of Heart-beat by Dilute NallO Solution (Gaskell) 

alkali induces an effect exactly the opposite, the standstill 
being brought about at systolic contraction (figs. 152 and 153). 
It is also known that the standstill caused by one of these 
reagents can be counteracted by the antagonistic action of 
the other. 

It is astonishing to find that exactly the same effects 
are produced by these reagents on the tissue of Desmodium. 

I first tried the effect of dilute hydro- 
chloric acid, which, as will be seen, 
produced an arrest of pulsation in 
the c diastolic' or relaxed position 
(fig. 154). I next tried the effect 
of alkali dilute solution of sodium 
hydrate and it will be seen that this 
produced an arrest of pulsation in 
the 'systolic' or contracted position 
(fig. 155). In records of the effect of 
this reagent on other specimens, in 
which its action had not proceeded 
so far, there was a continuous dimi- 
nution of pulsation with a shifting 

towards the systole ; and when an acid was now applied, 
the antagonistic character of its action to alkali was clearly 
shown, by a gradual revival of response, with a shifting 
towards the diastole. In the present record, the systolic 
standstill caused by alkali was allowed to proceed far, and 




FK;. 154. Arrest t>[ /v*. 
modium Pulsation at 
4 Diastole ' by Applica- 
tion of Acid 



RHYTHMIC KKSl'ONSK IN I'LXVP \\1) .\\IM\I, 353 

yet on application of the acid there is seen to be a slight 
revival of pulsation and a final arrest towards diastole. 

Identical nature of rhythmic phenomena in animal 
and vegetable tissues. We have thus found the responsive 
phenomena of cardiac muscle to be in every respect similar 
to those observed in the rhythmic tissues of Desmodinm and 




FIG. 155. Arrest of Pulsation of Desmodinm at ' Systole ' by Application 

of Dilute Alkali at T 

Acid was next applied at !, and the record shows its antagonistic action. 

Biophytnm, the response of the latter being regarded as 
practically that of Desmodinm in a state of standstill. We 
have seen that in these rhythmic animal and vegetable 
tissues the fundamental characteristics are identical : 

(1) In all of them stimulus gives either maximum response 
or none. 

(2) They all exhibit a long refractory period, during 
which additional stimulus produces apparently no effect. 

(3) They are all incapable of being tetanised. 

And further, as regards the influence of various external 
agencies on the two classes of rhythmic tissues, animal and 
vegetable, the effects are also remarkably similar. Internal 

A A 



354 PLANT RESPONSE 

hydrostatic pressure renews in them rhythmic activity. All 
of them exhibit under certain circumstances similar cyclic 
groupings. The effects of chemical reagents arc similar on 
both classes. Rise of temperature quickens the rhythm and 
reduces the amplitude of pulsation in a manner exactly 
similar in both ; and, finally, the effects of chemical reagents, 
even in the matter of antagonistic actions, are alike in the two 
classes. From a consideration of all these, it would appear 
that in studying response in rhythmic animal and vegetable 
tissues, we are dealing, not with two distinct but with a 
single class of phenomena. We are thus justified in 
ascribing the rhythmic action of the heart to those same 
causes which we have found to originate and maintain the 
rhythm of Desmodium or Biophytum. We have seen that 
these plants absorb energy continuously from the various 
forms of stimulus mechanical, thermal, chemical, and other 
to which they are subjected. This absorbed energy 
remains latent in the tissue, and determines its tonic con- 
dition, which is simply the sum total of these latent stimu- 
lating factors. When the sum of these factors exceeds a 
certain value, it will find expression outwardly in the form of 
excitatory discharges. This discharge, however, is not single 
and continuous, but intermittent. After each partial excita- 
tory discharge, there is a diminution of conductivity and 
excitability which are only restored gradually. The long 
refractory period is merely an expression of this peculiar 
property. Owing to this periodic oscillation of conductivity 
and excitability, the constant latent stimulus finds expression 
in a rhythmic manner. Under favourable circumstances, 
there is a large surplus of accumulated energy, and long- 
continued responses, apparently automatic, are thus produced. 
Under less favourable conditions, when the stored-up energy 
is not great, a single stimulus gives rise to a single response, 
or, when the stimulus is stronger, to a multiple series of 
jhythmic responses ; and this statement is true of tissues 
exhibiting ' spontaneous movements/ not only in the case of 
the plant, but also in that of cardiac muscle. 



RHYTHMIC RESPONSE IN PLANT AND ANIMAL 355 

* 

It is impossible to conceive that there could be movement 
without an exciting cause. Automatism is said to be one of 
the properties of protoplasm. It will be seen, however, from 
the evidence which I have adduced in cases where experi- 
mental investigation is possible, that, strictly speaking, there 
is no such thing as automatism. Only under the action of a 
stimulus can a living tissue give responsive indications. The 
impact of an external stimulus may give rise to an imme- 
diate expression, or it may partly or wholly be reserved in 
latent form for subsequent manifestation. ' Inner stimuli ' are 
simply external stimuli absorbed previously and held latent. 
An animal or a plant is thus an accumulator which is constantly 
storing up energy from external sources, and numerous mani- 
festations of life often periodic in their character are but 
responsive expressions of energy which has~ been derived 
from external sources and held latent in the tissue. 

SUMMARY 

The rhythmic tissue of Biophytum may be regarded as 
equivalent to that of Desmodium in a state of standstill. 

Both alike, when at standstill, give a single response to a 
single moderate stimulus, and multiple response to a strong 
stimulus. 

Both, when the sum total of latent energy is above par, 
give apparently ' automatic ' responses. 

The rhythmic tissues of both plants exhibit a long re- 
fractory period. 

The automatically responding leaflet of Desmodium is 
incapable of being tetanised. 

An artificial increase of internal hydrostatic pressure 
renews pulsation in a Desmodium which was previously in a 
state of standstill. 

The effect of rise of temperature on Desmodium is to 
produce a shortening of period and decrease of amplitude of 
oscillation. 

The automatic responses of Desmodium often exhibit 
periodic groupings, of various degrees of complexity. 

A A 2 



356 PLANT RESPONSE 

Certain reagents tend to make irregular pulsations in 
Desmodium fegular. Conversely, other reagents, like barium 
salts, induce irregularity in regular pulsations. The irre- 
gularity induced by the latter reagent may, however, be 
counteracted by a rise of temperature. 

The effects of acids and alkalis on the pulsatory move- 
ments of Desmodium are antagonistic, acids inducing arrest 
of pulsation in a relaxed position, while alkalis induce 
its arrest in a contracted position. The standstill induced 
by one reagent may therefore be counteracted by the use of 
the other. 

By all these, the rhythmic phenomena of the plant are 
seen to be identical with those of the animal. 

The pulsation of the animal heart is thug to be ascribed 
to the same causes as bring about and maintain the rhythmic 
pulsations of Desmodium. 



PART V 

ASCENT OF SAP 



CHAPTER XXVIII 
SUCTIONAL RESPONSE AND ASCENT OF SAP 

Inadequacy of existing theories of ascent of sap General considerations regarding 
cellular activity and resultant propulsion of water The Shoshungraph 
Balanced Shoshungraph for determining variations of suction Hydrostatic 
and Hydraulic Methods of Balance. 

THERE are few phenomena in plant- life which have attracted 
keener interest and inquiry than that process of transport by 
which water is carried, from below the surface of the earth 
to the tops of the tallest trees. The obscurity of the subject 
is so great, and the secondary co-operating agencies so 
numerous, that the inquirer is apt to be led into the error 
of confining his attention to some one of them alone, imagin- 
ing it to be the principal element in the problem. In 
studying this subject, then, our first effort must be to distin- 
guish between the essential factor and others which are 
merely subsidiary. 

For a statement of the inadequacy of these subsidiary 
factors to the solution of the problem, it is only necessary to 
refer to the summary of Strasburger and Pfeffer regarding 
existing theories of the ascent of sap : l 

The theory of atmospheric pressure is discredited, inas- 
much as water is known to be lifted, in certain cases, to many 
times the height of the water-barometer. 

The theory of capillarity is inadequate, inasmuch as 
continuous capillaries are absent, and the height to which 
liquids could be raised by such means would not, moreover, 
approach that of an ordinary tree. 

.' Pfeffer, Physiology of Plants, English translation, 1903, vol. i. p. 222, 
et seq. ; Strasburger, Text-book of Botany, English translation, 1903, p. 188. 



360 PLANT RESPONSE 

The theory of osmotic action cannot be considered satis- 
factory, since such action is too slow ; besides which, there is 
no fixed distribution of osmotic substances, such as would 
account for the necessary transportation-current 

The theory of root-pressure, again, is open to the objection 
that it cannot possibly account for the maintenance of a 
sufficient force during the process of active transpiration, when 
root-pressure is found to be negative. Moreover, this root- 
pressure itself requires an explanation. 

There is, however, another theory, due to Dixon, Joly, 
and Askenasy, which has apparently more to support it than 
any of those yet mentioned. According to this, the ascent is 
brought about by transpiration from the leaves. The fluid 
in the mesophyl cells of the leaves becomes concentrated by 
evaporation ; thus osmotic attraction is set up by the leaves, 
and the suction thereby exerted is supposed to be transmitted 
backwards, as far as the roots, through cohering columns of 
water. The difficulties in the way of this theory lie (i) in 
explaining how a slow- osmotic action could produce so rapid 
a water-current ; (2) in the absence of any conclusive proof 
that, under actual conditions within the plant, the water- 
column could have sufficient tensile strength ; and, lastly, 
(3) in the fact which I shall demonstrate, that, when evapo- 
ration is not taking place in the leaves, the transport of water 
is still very considerable, and that, besides, other related 
phenomena, like exudation pressure, continue to take place even 
in the complete absence of evaporative activity in the leaves. 
I shall, moreover, be able to show that the movement of water 
often takes place in the plant in a direction opposite to that 
which would be the case if osmotic action were alone involved. 

'Thus,' to quote PfefTer, 'a satisfactory explanation of 
the means by which the transpiration-current is maintained 
has not yet been brought forward. If no vital actions take 
partln it, then it is obvious that we have only an incomplete 
knowledge of the causes at work, and of the relationship of 
the different factors concerned.' 1 

1 Pfeffer, Physiology of Plants, English translation, 1903, p. 224. 



SUCTION AL RESPONSE AND ASCENT OF SAP 361 

There then remains the question as to whether living 
cells by some unknown physiological activity might not be 
instrumental in effecting this transport of water. But the 
experiments of Hartig, Bohm, and Strasburger have been 
held to contradict such a possibility. Thus, Strasburger 
set the cut ends of trees in tubs of poison, such as copper 
sulphate solution. The poison ascended to the leaves, a 
distance, in the tallest trees, of twenty-one metres. Now, if 
such violent protoplasmic poisons ascend the trunk, it is clear 
that they must kill all the cells lying in their path. That 
the living cells of the stems could not be necessary to the 
rise of sap was taken to be a necessary inference from 
this experiment. Strasburger also killed portions of the 
stems of living trees by heat, and yet the upper living and 
leafy portion was found to remain turgid for a few days. 
Another well-known experiment which was held to negative 
the theory of protoplasmic activity, was that in which 
boiling water was poured on the roots, when the plant con- 
tinued to transpire, in spite of the roots having been killed. 

From such considerations, Strasburger was led to con- 
clude that ' the supposition that the living elements in any 
way co-operate in the ascent of the transpiration-current is 
absolutely precluded.' 1 

I shall nevertheless show that the ascent of sap is funda- 
mentally due to the physiological activity of living cells, 
and that the experiments described above in no way negative 
this, being, on the contrary, capable of a different, and very 
satisfactory, explanation. Many difficulties connected with 
the problem of the ascent of sap will be found to disappear, 
when the physiological activity of living tissues is once 
clearly established as the essential factor. But a vague 
assumption of protoplasmic activity will not be sufficient for 
the elucidation of the phenomenon. It will be necessary 
to show further how this excitatory activity is initiated, 
and by what means a definite-directioned flow is imparted to 
the sap. 

1 Strasburger, Text-book of Botany ', English translation, 1903, p. 188. 



362 PLANT RESPONSE 

We have seen in Chapter XXI. that when a strong 
stimulus is applied to the base of any organ, say a stem, 
a multiple series of excitatory waves is propagated onwards, 
such multiple responses being detected by electrotactile or 
electromotive pulsations. It was also shown in Chapter 
XXVI. that such multiple, passing into automatic, response, 
may be induced by the action of a constant stimulus. It 
was further demonstrated in Chapter XXI. that these excita- 
tory waves, and the concomitant cell-to-cell contraction, would 
produce a movement of water forwards, along the direction 
of propagation. This series of excitatory waves, proceeding 
from the base of the organ, and propelling water forwards, 
must then cause a deficit of water behind. If, however, the 
base of the organ be kept supplied with water, this deficit will 
be made up by suction. 

It is thus seen that by such rhythmic activity a one- 
directioned movement of water may be produced. Just as 
the various effects produced by multiple or autonomous 
response in, for example, the electromotive and electro- 
tactile responses, and in the multiple mechanical responses of 
Desmodium give us an indication of the degree of rhythmic 
activity exhibited by the tissue, so, in the rate of this water- 
movement also, we have an additional means of measure- 
ment. This would be analogous to the measurement of 
the rhythmic activity of the heart by a determination of the 
rate of flow of the circulating blood. In the case of the 
plant this rate of movement may be measured, either by 
means of the propulsion of water forwards or by the suction 
exerted behind. 

The ascent of sap in the plant, then, may be brought 
about by the rhythmic activity of the tissue. How this 
activity is initiated will be discussed later. Meanwhile it 
is clear that if the movement of sap be really an expression 
of protoplasmic activity, then any physiological modifica- 
tion which tends to increase that activity will also tend to 
increase the rate of movement ; and part passu any physio- 
logical condition which tends to depress the activity, will 



SUCTIONAL RESPONSE AND ASCENT OF SAP 363 

* 

correspondingly express itself in a diminished rate of move- 
ment. The movement of sap is thus taken to be another 
expression of that autonomous activity (multiple response) of 
the plant, which we have already seen exhibited locally by 
the motile tissue of Desmodium. The conclusive test of this 
would lie in proving that all those agencies which acted 
in a given way on the autonomous response of Desmodium, 
act also in the same way in modifying the rate of movement 
of sap.. In other words, just as an exciting reagent will in 
the one case induce a greater amplitude, frequency, or 
both, of oscillation, above the normal, so in the other the 
excitatory nature of a given reagent may be expected to 
exhibit itself by an increase above the normal, in the rate of 
flow. A depressing reagent, on the contrary, should produce 
the opposite effect in both. That is to say, just as we may 
study the multiple or rhythmic excitability of a tissue 
through mechanical, electromotive, or electrotactile response, 
so here, in hydraulic response, or the determination of changes 
in the rate of flow of sap, we have an independent mode of 
investigating the same phenomenon. 

The great difficulty of this investigation lies in the 
absence of a method by which these changes can be imme- 
diately recorded. In other words, we require some simple 
means of making a direct record, which will show, in a 
continuous manner, the changes produced by the various 
agencies, enabling us to distinguish their immediate effects, 
after effects, time-relations, and so on. 

We have seen that the propulsion of water forward by the 
tissue is attended by a suction behind. 1 The quantity of 
water sucked up in a definite period will therefore give us an 
indication of the rate of movement of sap in the plant. Thus 
from the readings afforded by the water-index of a potometer, 
and the times at which such readings are taken, we may derive 

1 The forward movement of water, and the suction exerted, in the same tissue, 
are not necessarily equal in all cases. Part of the water sucked up may be 
deviated to increase the turgidity of the cells themselves. Suction may neverthe- 
less be taken as a measure, other things being equal, of rhythmic activity. 



364 PLANT RESPONSE 

the rate of movement. But these readings are necessarily 
discontinuous, and important phases of change are thus apt 
to be overlooked. They are, again, subject to error ;" nor do 
they give us, at sight, the rate of flow, nor the changes in that 
rate, at any given moment. In order to determine such varia- 
tions, a laborious process of construction of curyes, from the 
experimental data, must, generally speaking, be undertaken. 

It was therefore necessary to devise an apparatus by 
means of which curves might be obtained direct, so, that a 
mere inspection would be sufficient to inform us as to the 
normal rate of suction, and the influence of external factors 
on that rate. This suction will be shown to be a physio- 
logical phenomenon ; its variation, therefore, will enable us 
to measure the physiological influence of various external 
agencies. It must be borne in mind that under ordinary 
conditions there is a normal rate of suction. The incidence 
of an external stimulus will change this rate, and the variation 
of rate which results is thus a measure of the effect produced 
by the stimulus. Similarly, we measure a force by noting 
the variation which it produces in the rate of movement of a 
uniformly moving body. The variation of the rate of suction 
may thus be taken as constituting a form of response to 
stimulus, which for the sake of simplicity we shall designate 
as Suctional Response. 

The Shoshungraph. For the purpose of subjecting the 
plant to various conditions, and also in order to obtain the 
record of the resultant suctional response, I have constructed 
an apparatus to which I have given the name of the 
Shoshungraph. 1 It consists of (i) an arrangement by which 
the specimen may be rapidly subjected to the action of 
different excitatory or depressing agents ; (2) a potometric 
tube, by which the constant changes of suctional activity are 
measured ; (3) a contrivance by means of which the move- 
ments of the water-index, with their time-relations, are 
recorded. The principal parts of this instrument are shown 
diagrammatically in fig. 156. v is the plant-vessel, in which 

1 From Sanskrit, Shoshun suction. 



SUCTIONAL RESPONSE AND ASCENT OF SAP 



365 



the specimen is mounted by means of a water-tight india- 
rubber cork. The vessel is closed at the bottom also by 
means of a large cork through which enter four tubes. One 
of these, controlled by the stop-cock A, is connected with 
the capillary potometer-tube. The second, controlled by A', 
leads to the compensating vessel c, filled with water. The 
interior ends of these two tubes reach almost to the top 
of the plant-vessel. The third, with stop-cock B', leads to 




FIG. 1 56. Diagrammatic Representation of the Shoshungraph 

v, plant-vessel ; c, compensator ; R, reservoir. The potometer-tube is 
controlled by stop-cock, A ; compensator by A' ; and reservoir by B'. 
B is the stop-cock of the outflow pipe ; D, recording drum driven by 
clockwork ; P, collar carrying recording pen, seen magnified above. 
The water-index is followed by appropriate manipulation of the 
wheel, w. 



the reservoir R, from which water at various temperatures 
or different chemical reagents may be introduced. The 
fourth is an outlet-tube controlled by the stop-cock B. 

Adjustment for Unbalanced Record. The great 
difficulty in connection with delicate experiments arises from 
the presence of air-bubbles in the plant-vessel, which are not 
easy to expel. This is done, however, by means of an escape- 
tube, with a stop-cock, which runs through the upper cork, 



366 PLANT RESPONSE 

and is not shown in the figure. The stop-cock A' is opened, A, 
B, and B' being closed. Water from the compensator c thus 
passes into the plant-vessel, and the air-bubbles which have 
been accumulating at the top of the vessel escape, with the 
water which is driven out at the escape-tube. When all are 
expelled then the stop-cock of the escape-tube, and A', are 
closed. The potometer stop-cock A is now opened, and the 
water-index is adjusted at any point desired, by manipulation 
of the stop-cocks B and A'. A temporary opening of the 
stop-cock A' of the compensator C causes the water-index 
to move to the left ; whereas, when the stop-cock B, of the 
outflow pipe, is opened, it moves to the right. After this 
preliminary adjustment the stop- cocks A', B, and B' are closed, 
the potometer tap A being kept open ; the movement of the 
water-index per unit-time now gives us the normal rate of 
suction of the specimen. 

We come next to the question of making a direct record 
of the rate of movement. For this purpose, a writing pen is 
fitted on the potometric tube, by means of a brass collar. 
This brass collar has a rectangular opening, which enables 
us to watch the water-index. It has also stretched across it 
a fine wire, which is kept always coincident with the water- 
index. This wire is parallel with, and placed vertically above, 
the recording pen. The collar is attached to a thread which 
passes round small pulleys. One end of this thread carries 
a counterpoise and the other is wound round a wheel, W, 
which can be so manipulated as to make the index-wire 
follow the movements of the water- column. When the wheel 
is wound, the index moves to the right ; when it is slightly 
released, the weight of the counterpoise makes it move to 
the left. The weighted recording pen rests with its point 
on a revolving drum, D, covered with paper for the record ; 
this drum is kept revolving by clockwork at a known and 
adjustable speed. When the water-index is followed in the 
way described, there is produced a direct record of water- 
movement in the plant. A curve is thus traced, the ordinate 
of which represents the quantity of water sucked up, and the 



SUCTION AL RESPONSE AND ASCENT OF SAP 367 

abscissa the time. The slope of the curve thus gives the 
rate of movement. As long as the suction is uniform, the 
slope remains constant. If any exciting agency increases 
the rate of suction, there is an immediate flexure in the curve, 
which thus becomes steeper. A depressing agent lessens 
the slope of the curve. And when suction is abolished, the 
record becomes horizontal. 

By this arrangement, then, we are enabled, simply and 
accurately, to obtain a direct and continuous record ; and as 
the necessity for taking readings is obviated, a large number 
of experiments can be performed very quickly, with little 
trouble. The flexure in the curve affords immediate visible 
indication of the effect of any particular agency. The value 
of each division of the ordinate is found once for all by 
determining the volume of unit-length of the potometer-tube. 
A previous determination at the beginning of the experiment 
of the rate of movement of the drum, gives us the time- 
value of each division of the abscissa. Knowing these, we 
can determine the absolute rate of suction at any period of 
the curve required. Responsive variations of suction are 
more easily detected when the normal curve is almost 
equally inclined to the ordinate and abscissa that is to say, 
when it makes an angle of about 45 with either. This is 
most easily accomplished if we keep the potometer-tube 
always the same, and merely adjust the speed of the drum. 

The Balanced Shoshungraph. According to the simple 
method of making records which has just been described, we 
observe the responsive effect by means of flexures produced 
in the curve under the action of various agencies. If the 
effect of an agency be slight, the change in the slope of the 
curve will be proportionately small and liable to escape 
detection. In order to bring the sensitiveness of the instru- 
ment to its highest, I have devised the Method of Balance, 
by which the slightest responsive variation is made to exhibit 
itself in a marked manner. For the purpose of many delicate 
investigations, it is not so necessary to know the normal rate 
of suction, as the variations positive and negative in that rate. 



$68 PLANT RESPONSE 

An agency which induces the positive variation will then 
be excitatory, while that which induces the negative is 
depressing. In order to carry out my investigations along 
these lines, I have employed two different methods of balance. 
In one the Hydrostatic Method of Balance the natural 
suction of the plant is arrested by a counter-hydrostatic 
pressure suitably applied. The effect of an external agent 
is now studied by the direction, positive or negative, and 
the extent to which it disturbs the static equilibrium thus 
established. Experiments carried out on this method will 
be described in the next chapter. 

The second or Hydraulic Method of Balance depends 
upon an application of compensation, by means of which 
under normal conditions the water-index is kept stationary, 
though the suctional movement in the plant is in no way 
disturbed. According to this hydraulic method, the normal 
rate of suction, when balanced, gives rise to a neutral, or 
horizontal, line in the record ; while an exciting agent 
produces an inclination upwards ; and a depressing agent a 
declination downwards. The balance, by which under normal 
conditions the neutral line is secured, is obtained by allowing 
water to enter the plant-vessel from the compensator c at a 
rate exactly equal to that of its withdrawal by suction. 

In practice, this adjustment is roughly made by opening 
the stop-cock A' in connection with C, to a greater or less 
extent. Over-balance causes movement of the water-index 
to the left ; under-balance to the right ; and when the adjust- 
ment is perfect, the water-index becomes stationary. After 
making the preliminary adjustment, the final balance may be 
obtained, by very careful and gradual movement of the com- 
pensating reservoir up or down. When the reservoir is 
raised the flow is increased, owing to the greater difference 
of level established, as between the reservoir and the plant- 
vessel. The stand on which the vessel C is placed is pro- 
vided with a rack and pinion, by means of which the necessary 
adjustment of height is made. 

Introduction of changed conditions. We have thus 



SUCTIONAL RESPONSE AND ASCENT OF SAP 369 



seen how the normal rate of suction and its variations may 
be recorded accurately. The next difficulty to be overcome is 
that of introducing the changed conditions without creating 
any disturbance, thus practically maintaining the continuity 
of record. It will be necessary to observe, among other 
things, the immediate and after-effects of cold and heat, as 
well as those of various chemical reagents. This is accom- 
plished by the right manipulation of the four stop-cocks. 
Let it be supposed that we wish to study the immediate 
and after-effects of cold. Up to this time the stop-cocks 
A and A' have been opened B and B' being closed and the 
balanced horizontal record taken. For the sake of simplicity 
I refer to the stop-cock A' as the only one which opens and 
closes the communication with the compensator C. In reality, 
however, there is a second stop-cock in its neighbourhood, 
by which the balancing adjustment is first made, A' being 
employed for opening or closing communication under such 
an adjustment. The reservoir R is now filled with cold 
water ; A and A' are next closed thus arresting the water- 
index and B and B' opened. The water then leaves the 
plant-vessel by the overflow pipe, and its place is taken by 
cold water from R. After this, the stop-cocks B and B' are 
once more closed, and A and A' opened. The index, being 
now released, indicates by its movement the excitatory 
or depressing effect of cold. It must be remembered that 
the index was previously adjusted to balance. Should the 
effect of cold prove to be excitatory, the rate of suction 
would be increased ; under-balanced by the supply of water 
from C, the index would move to the right, thus giving 
positive response. If, on the contrary, the effect should be 
depressing, the rate of suction would be decreased, and the 
water from the compensator would produce an over-balance, 
causing a movement of the index to the left, or negative 
response. By now filling the reservoir R with water at 
the ordinary temperature, and repeating the operation, the 
original condition is re-established, and the effect of re-esta- 
blishment of old conditions observed. In a similar manner, 

B B 



370 PLANT RESPONSE 

we may study the immediate and after-effects of rise of 
temperature, and of the various chemical reagents. 




FIG. 157. 1 h< t< gmph of Shoshungraph 

v, plant- vessel ; R, reservoir ; c, compensator, whose balancing height is 
adjusted by rack and pinion, s ; K, key for manipulation of four- way 
stop-cock ; r, recording pen, with counterpoise M, manipulated by 
wheel, \\. The drum is rotated by the clock at uniform speed. 



SUCTIONAL RESPONSE AND ASCENT OF SAP 371 

In order to make the explanation easier, I have described 
all these stop-cocks and their serial opening and closing 
separately ; but this arrangement, apart from its clumsiness, 
would involve a certain loss of time. In many of these 
experiments, it must be remembered, it is necessary to know 
the immediate effect produced. I have therefore simplified 
the procedure by the use of a special key, K, by turning 
which, in one direction or another, the requisite alternate 
openings and closings are accomplished. 

Thus, on turning the key-handle to the left, A and A' are 
opened, and B and B' closed. The balanced record is now 
taken. As the handle is now being turned to the right, the 
stop-cocks A and A' are first closed, arresting the index. Con- 
tinued turning to the right opens B and B', by which the modi- 
fying reagent is introduced into the plant- vessel. A sudden 
turning of the key to the left now closes B and B', and opens 
A and A', thus releasing the index and enabling the record 
to be taken once more, under the changed conditions. The 
whole process is thus made so rapid, that modified conditions 
can be established, and the record renewed, within the short 
interval of less than one minute. The photograph of the 
completed apparatus is seen in fig. 1 57. Having now given 
in detail all the experimental arrangements, I shall in the 
next chapter describe the physiological modifications induced 
by different agencies, as exhibited by the suctional response. 
The periodic variation of ascent of sap may be recorded 
continuously and automatically by photography. Another 
method of recording transpiration will also be found described 
on page 472. 

SUMMARY 

This chapter gives a description of the Shoshungraph, by 
means of which the rate of suction and its variations may be 
indicated and recorded. The sensitiveness of the apparatus 
is very much increased by the Hydrostatic and Hydraulic 
Methods of Balance. 

B B 2 



CHAPTER XXIX 

MODIFICATION OF SUCTIONAL RESPONSE 

Effect of temperature on suction by three methods of inquiry : (i) Unbalanced 
method of Shoshungraph : (a) Action of cold (b) Action of moderate rise of 
temperature (2) Method of Hydrostatic Balance : (a) Action of cold Re- 
versal of normal direction of flow (b) Action of warm water (3) Method of 
Hydraulic Balance : (a) Action of cold (b) Effect of warm water Explana- 
tion of suction when the root is killed by boiling water - Stimulation renews 
suctional activity in plant whose suction has come to a standstill Osmotic 
versus excitatory action Abolition of suction by poison Suctional activity 
continued until whole plant is killed by poison. 

I SHALL now proceed to prove that the movement of water 
in plants is mainly due to rhythmic excitation of the tissue, 
and that evaporation from the leaves, osmotic action, and 
so on, are only co-operating factors, of subsidiary importance. 
We have seen in Chapter XXI. that any part of a stem when 
excited will -become the seat of rhythmic activity, and that 
this pulsatory excitation causes movement of water. I shall 
now demonstrate a similar phenomenon by means of suc- 
tional response, eliminating from some of the typical ex- 
periments all auxiliary factors, such as osmotic action and 
evaporation from leaves. I have shown in the last chapter 
how the effect of suctional activity may be continuously 
recorded by means of the Shoshungraph. We saw also 
that the effects of various agents, exciting or depressing, were 
to be detected, under the unbalanced method, by appropriate 
variation in the slope of the curve. By the balanced method, 
whether hydrostatic or hydraulic, the derangement of the 
balance upwards indicates an increase of activity, and its 
derangement downwards a decrease. I shall first demon- 
strate the fact that these observations, though obtained by 
such various methods, are all reliable and mutually consis- 



MODIFICATION OF SUCTIONAL RESPONSE 373 

tent, by subjecting the plant to the action of an agent whose 
general effect is well known, and recording the results by all 
three methods. For this purpose we shall take the influence 
of low and moderately high temperatures. 

Modification of suctional response by various agencies. 
We have seen that any sudden variation of temperature 
acts as a stimulus in itself. Thus, if we touch the pulvinus 
of Mimosa or Biophytum with ice, there is a responsive 
twitch. This may be taken as the preliminary effect. Pro- 
longed application of cold, however, abolishes excitability. 
If the ascent of sap be really a phenomenon of excitation, 
we may expect to find a sudden application of cold, appro- 
priately made, producing a preliminary augmentation, followed 
by the depression and arrest of suction. An application of 
hot water might be expected, on the other Hand, to bring 
about the contrary effect, that is to say, an increase in the 
rate of suction. I shall now describe the experimental results 
obtained by the three methods. 

Effect of temperature on suction: (i) Unbalanced 
Method, (a) Action of cold. As I did not know what might 
be the effect of injury on the suctional activity of the plant, 
I selected intact specimens for my first experiments. For 
this purpose I took cuttings of Croton, a plant whose stem 
when placed in water will develop roots in a few weeks' 
time. When the roots were well developed, the specimen 
was fitted in its place in the apparatus. In other cases, 
I took pot-grown specimens and placed them in water, so 
that the earth was dissolved away. Violence to the rootlets 
was thus avoided. I may here state, however, that I found, 
in the course of these experiments, chat there is no essential 
difference between the effects exhibited in such intact plants 
and those observed in the case of cut branches. All that is 
necessary in the latter case is that the specimen should be 
mounted in the apparatus and left for some time, in order 
that the effect of the disturbance caused by cut may pass. 
The record afforded by a specimen thus mounted gives the 
normal rate of suction. The attainment of constancy of 



374 



PLANT RESPONSE 



external conditions is gauged by the uniform inclination to 
the curve, and it may be well to mention here that through- 
out the investigation every experiment was begun with this 

test. 

The normal rate of suction, in the first experiment, in a 
Cretan at the temperature of the room (23 C.), was eight 
cubic mm. per minute. On now applying cold water at a 
temperature of 4 C. to the root, by appropriate manipulation 
of the stop-cock, the rate showed the preliminary excitatory 
effect, due to sudden variation of temperature, by an increased 




Fi<;. 158. Effect of Cold on Suction 

The first part of the record shows the normal rate of suction at 23 C. 
Asterisk marks moment of application of cold water at 4 C., which is 
seen to produce a preliminary exaltation, followed by arrest of suction. 
Abscissa represents time in minutes ; ordinatc, tin- quantity of water 
sucked up in milligrammes or cubic mm. 

rate during the first two minutes, of eighteen cubic mm. per 
minute, or 2*25 times the normal value. But this temporary 
exaltation gradually passed away, till there was an almost 
complete arrest, ten minutes after the first application (fig. 
158). This arrest by cold was not found to be permanent; 
for it disappeared on the return to a higher temperature, as 
will be seen in the first part of the next figure (fig. 1 59), 
which was taken after water at 23 C. had been substituted 
for the cold water in the vessel. In this second curve, the 
rate of suction is found to return almost to the normal 



MODIFICATION OF SUCTIONAL KKSJ'ONSK 



375 



degree, being now about seven instead of eight cubic mm. 
per minute. 

(ft) Action of moderate rise of temperature. I next tried 
the effect of a rise of temperature. This experiment was per- 
formed with the same specimen as the last, in which the 
return normal rate of suction at 23 C. had already been 
determined to be seven cubic mm. per minute. On now 




FIG. 159. Curve showing 
Normal Suction at 23 C. ,- 
Increased Suction at 35 
C., and the After-effect 
persisting on Return to 
Normal Temperature 

This experiment was earned 
out on the same specimen 
as the last. 




FIG. 1 60. 



Method of 
Balance 



Hydrostatic 



A short length of stem, r, has its as- 
censional water-movement balanced 
by superincumbent water-column of 
variable height. 



applying water at 35 C. it will be seen that a very steep rise 
was induced in the curve, indicating an increased suctional 
rate of fifty-eight cubic mm. per minute, more than eight 
times the rate at 23 C. On now once more substituting 
water at 23 C. the rate became lowered, though not to the 
original degree (fig. 159). It must be remembered, with 
regard to this, that the movement of sap depends on the cell- 
activity of the entire plant, and that the tissue has by this 



376 PLANT RESPONSE 

time absorbed some quantity of hot water, which cannot 
immediately be displaced by the water of ordinary tempera- 
ture which is applied at the roots. The rate of suction, 
therefore, could not at once revert to the normal, but must be 
expected for some short period to show a slight enhancement. 
The curve shows that on the return to 23 C. the rate fell 
from fifty-eight to fourteen instead of to the original eight 
cubic mm. per minute. 

(2) Method of Hydrostatic Balance : (a) Action of 
cold. For this experiment I took a Croton stem cut at both 
ends, the lower end being placed in the plant-vessel of the 
apparatus. There was now found to be a considerable 
movement of water upwards. This movement was arrested 
by suitable hydrostatic pressure, the upper end of the stem 
being connected with an india-rubber tubing filled with 
water and ending in a funnel (fig. 160). To prevent evapora- 
tion, the surface of the water in the funnel was covered 
with a thin film of oil. Rather a high hydrostatic pressure 
was required to produce a balance. When the pressure of a 
column of 75 cm. was applied, there was still a movement of 
water upwards in the tissue, at so great a rate as ten cubic 
mm. per minute. 

In this experiment, then, the root having been cut off, 
there is still a considerable propulsion of water upwards 
through the stem. It is thus clear that root-pressure is not 
the essential factor in the ascent of sap. As the leaves had 
also been cut off, and the cut end of the stem covered by 
water sealed with a film of oil, evaporation from the leaves, 
and the osmotic action thereby produced, are also seen to be 
excluded. These, like root-pressure, therefore, cannot consti- 
tute the essential factors in the process of suction. But since, 
on the contrary, any small length of the stem' is competent 
to show this water-movement, the required activity must 
reside in the tissue of the stem. 

A balance was finally obtained, by the pressure of a 
water-column of 105 cm. This equilibrium is not to be re- 
garded as merely the result of equality between the upward 



MODIFICATION OF SUCTIONAL RESPONSE 377 

pressure, exerted by suction from below, and the downward* 
pressure of the superincumbent water-column. There are 
reasons for thinking that there may also be an important 
additional factor, of opposed rhythmic activities, balancing 
each other. For when a cut branch is placed in water, the 
lower end becomes over-turgid, and this, as we know, is 
a condition for the initiation of rhythmic activity. The 
upper end of the branch not being so turgid, activity will 
be greater below than above. We therefore obtain a 
movement of water from the more to the less active. But 
if the leafy end of the branch be immersefd in water, and 
the cut end held in the air, it is known that the direction of 
the flow becomes reversed. This is evidently due to the fact 
that it is now the upper end which is over-turgid, and there- 
fore relatively the more active. Similarly, in the case of the 
balance described, the activity of the lower end, which deter- 
mines the flow upwards, was opposed and balanced by the 
increased activity of the upper half, induced by increased 
hydrostatic pressure. 1 The experiment which I am about 
to describe, besides demonstrating the effects of cold and 
warmth, also lends strong support to the view that the direc- 
tion of the resultant movement of sap is determined by the 
relative activities of the two ends of the stem. 

When the balance had been obtained, as already described, 
with a pressure of water of 105 cm., the record on the drum 
became horizontal, as has been explained. Cold water 
was now applied to the specimen at its lower end. As 
sudden cooling constitutes a stimulus, while its continued 
action produces depression, we should expect a transient 
augmentation of the activity of the lower end of the specimen, 
followed by its diminution and arrest. The record should 
therefore show a preliminary movement of water upwards, 
followed by the reversal of the current, which should now, as 
the permanent effect of cold applied below, be from the 

1 Rhythmic activity is, in general, increased by an increase in the hydrostatic 
pressure. But there is a limit to this. Excessive pressure, above a certain critical 
point, is found to depress rhythmic activity (p. 350). 



378 



FT, ANT RESPONSE 



more active upper, to the less active lower end, or downwards. 
The record will be seen to verify this anticipation in every 
particular (fig. 161). Cold water was applied below repre- 
sented by x in the record which before application was 
horizontal. After this we observe a movement of water 
upwards, at a rate of four cubic mm. per minute. The upward 
flow continues for about seven minutes, by which time the 



mgrsl 



10' \I5' 20' / 25 



FIG. 161. Record obtained by Method of Hydrostatic Balance of 
Successive Applications of Cold and Warm Water 

After obtaining static balance, with hori/ontal record not shown in the 
figure, cold water was applied below at moment x . The transitory 
excitation thus occasioned causes movement of water upwards, followed 
by depression and reversal of flow downwards. Warm water next 
applied at f causes second reversal and flow of water upwards. 



activity of the lower end of the specimen has become so 
depressed as to cause reversal of the flow, which is now from 
above downwards at an average rate of about six cubic mm. 
per minute. This record was taken continuously for some 
time, when hot water was quickly substituted for cold, in the 
plant-vessel. A cross marks this point in the record. 

Action of warm water. The result was not only that 



MODIFICATION OF SUCTIONAL RESPONSE 379 

* 

its sluggishness was now obviated, but that the lower end of 
the specimen was actually rendered the more excitable of 
the two, and we observe a second reversal of the direction 
of current, which now flows upwards. 

(3) Method of Hydraulic Balance. This Hydraulic 
Method of Balance is much easier to carry out than the 
arrest of movement by hydrostatic pressure. It does not, 
moreover, in any way interfere with the normal movement of 
water through the plant. For the balance is obtained and 
the index rendered stationary, by the simple device already 
explained, of allowing a subsidiary flow of water to enter the 
plant-vessel, at a rate exactly sufficient to compensate for the 
loss by ascent of sap. 

(a) Action of cold. This experiment was performed on a 

*r 

leafy branch of Croton. The balanced horizontal record was 
first taken at 22 C, after which ice-cold water was passed 
into the vessel. A record was made of the immediate or 
preliminary excitatory effect of this cold water on the rate of 
suction, and continued during the return of the water to the 
temperature of the room. In this record, then, we shall find, 
besides the immediate, the continued effect of cold, and 
subsequently the effect of gradual restoration to an ordinary 
temperature. In the present record (fig. 162) we see the 
transient and permanent effects of cold exhibited as before. 
According to the method of Hydraulic Balance, as has been 
explained, an ascending line in the record means a positive 
variation, or increase of the rate of suction over the normal. 
A descending line, on the contrary, denotes a negative 
variation, or diminution of the rate of suction below the 
normal. And a horizontal line shows return to the original 
rate. In the first, or upper, of the two curves in fig. 162, 
the immediate effect of cold is seen in a very marked positive 
variation. After the lapse of about five minutes, the effect 
of continued cold is seen in the depression, which shows 
itself by the reversal of the curve. This depression continues, 
till the temperature of the vessel has returned to that of the 
room, which takes place in the course of about forty minutes. 



380 PLANT RESPONSE 

The normal rate of suction is now re-established, as seen in 
the fact that the record becomes horizontal. 

(b) Effect of warm water. I next tried the effect on the 
same specimen of water at a higher temperature, falling by 
degrees to the temperature of the room. We should expect 
in such a case, to obtain the excitatory effect of rise of 
temperature, with subsequent approach to the normal, with- 
out any reversal, indicative of the transition from exaltation 
to depression, such as was observed in the case of application 
of cold. From the second and lower of the two curves 




FIG. 162. Record, obtained by Method of Hydraulic Balance, of Successive 

Effects of Cold and Warm Water 



shown in fig. 162, it will be seen that this is the case. 
The curve at first rises abruptly, and it continues to rise, 
though with decreased speed, till, when the temperature of 
the vessel is once more normal, it becomes horizontal, 
indicating the resumption of the original rate of suction. 

We have thus seen, by three different methods of inquiry 
those namely of the Unbalanced Shoshungraph, the Hydro- 
static Balance, and the Hydraulic Balance that the re- 
cords, though of different forms, exhibit effects of which the 
interpretations are identical. For delicate experiments, the 



MODIFICATION OF SUCTION AL RESPONSE 381 

Hydraulic Method of Balance will be found most sensitive': 
for ordinary purposes, however, the unbalanced Shoshun- 
graph is simple and efficient ; and in the investigations which 
follow I shall use this method only. 

Explanation of suction, when the root is killed by 
boiling water. - 1 shall now take up the apparently anoma- 
lous case in which, when the root has been killed, by pouring 
boiling water over it, the suction of the plant is nevertheless 
maintained. In such an experiment, the normal record was 
first taken, and on allowing boiling water to enter the plant- 
chamber there was a steep rise in the rec'ord, showing the 
excitatory action due to the application of hot water. The 
boiling water was now passed in continuously for several 
minutes, so as to ensure the killing of that portion of the 
plant which was immersed in the vessel. On allowing the 
water in the vessel to return to the temperature of the room, 
it was found that suction continued, at an even greater than 
the original normal rate. 

This result would at first appear to show that protoplas- 
mic activity had nothing to do with the ascent of sap. And 
the objection would have been fatal, if the rhythmic activity 
which produces suction had been confined to the roots alone. 
But such activity is present to a greater or less extent 
throughout every zone of the plant, and it is by the com- 
bined action of all these that the ascensional movement is 
maintained (p. 376). Thus, when hot water is poured on the 
root, its first effect is a sudden increase of the activity of that 
organ, by which warm water is carried to the higher zones, 
there as a stimulating agency to increase this rhythmic 
activity. It must be remembered that on reaching the 
stem above the vessel, the hot water itself is considerably 
cooled. Hence the only portion of the plant which is killed 
is that which is actually immersed in the boiling water, 
or in immediate contiguity with it. The unkilled portions 
above continue their suctional activity unabated. 

I have said that the suction, on the return of the water 
to its old temperature, continued to take place at a greater 



PLANT RESPONSE 

than the original rate. This was due to the fact that, instead 
of the extremely attenuated channels of the root-hairs, 
through which suction normally takes place, there was now 
substituted the whole mass of the root, acting virtually as a 
wet rag, tied round the base of the living stem ; and indeed 
it was found that, whereas the stem outside the vessel was 
turgid, the portion within was limp and soft. The mass of 
water which it was thus possible to suck up directly, by 
means of the broad -sectioned stem, was evidently much 
greater than could have been the case through the interven- 
tion of the resistant organically conducting channels of the 
rootlets. 

We must not forget the obvious fact that a plant is a 
colony of more or less independent living cells, each of which 
maintains its physiological activity as an individual. The 
death of one group does not necessarily, therefore, arrest 
the physiological activity of its neighbours. The plant is 
finally killed only when every one of its cellular elements has 
undergone death. 

Further proof that suction is an excitatory response. - 
We have seen in the case of rhythmic Desmodmm, when it is 
kept for a long time under unfavourable circumstances, that its 
activity comes to a stop owing to the run-down of stored-up 
energy. We also saw how the application of thermal stimulus 
would re-initiate this activity. Again, if we keep a cut branch 
of any plant in water, after a few days its suctional activity, 
as is well known, disappears. This abolition of suction is 
attributed to the blocking of the cut end by mucilage and 
bacterial growths, and the making of a fresh section is found 
to renew the activity. 

But, though the blocking of the cut end of the stem by 
outgrowths does, no doubt, obstruct the passage of water, 
yet the total abolition of suction may not be due to this 
cause alone. It may be induced, in part at least, by depres- 
sion of the rhythmic activity of the tissue, owing to the run- 
down of its latent energy. The making of a fresh section 
does not decide this, question, for, in doing this, we apply 



MODIFICATION OF SUCTIONAL RESPONSE 383 

the strong mechanical stimulus of a cut. I therefore devised 
an experiment which appears to show that this run-down 
of energy does in such a case constitute a factor in the 
abolition of suction. Without in any way disturbing the 
mucilaginous end of the stem, which had ceased to exhibit 
its suctional activity, I supplied it with water somewhat above 
the ordinary temperature. This thermal stimulation at once 
initiated renewed suctionai activity with great vigour, just as 
its rhythmic mechanical activity was renewed by Desmodium 
on the application of similar stimulus. 

Osmotic versus excitatory action. Though, under the 
co-operation of a favourable disposition of osmotic substances, 
the suctional activity of the tissue may be increased, yet I 
have shown that suction is normally maintained even with- 
out the co-operation of this factor (p. 376). " I shall now 
proceed to show that this suction may increase even in 
opposition to osmotic action. And such a demonstration will 
further prove the excitatory physiological nature of the 
processes which bring about the ascent of sap. Among 
various solutions of salt, some are physiologically neutral in 
their effects ; l of these, potassium nitrate may be taken as 
an example. Others, again, like strong solutions of sodium 
chloride, act as excitatory agents. The application of this 
last reagent is known to initiate rhythmic excitation in animal 
tissues. Similar effects have been shown to be brought about 
by this reagent, in the case of Biophytum and Desmodium. 

Thus, in a strong solution of potassium nitrate, we have 
a reagent whose physiological action is more or less neutral 
while its osmotic action is pronounced, and in a strong 
solution of common salt we have an agent which is both 
excitatory and osmotic at the same time. If, then, we 
apply KNO 3 solution to the base of a cut stem, placed 
in the Shoshungraph, water will be osrfiotically withdrawn 
from the plant, in opposition to normal suction, and the 

1 It should, however, be remembered that solutions, even of inactive salts, 
above a certain strength, will induce physiological depression, and thus bring 
about diminution of transpiration. 



PLANT RESPONSE 



normal suctional rate will be somewhat reduced. This is 
seen in the accompanying record (fig. 163), which I obtained 
with a cut branch of Croton. The normal rate of suction 
was in this case twenty-six cubic mm. per minute. After the 
application of potassium nitrate solution this was found 
to be reduced to seventeen cubic mm. per minute. But if, 
instead of this, we apply strong solution of sodium chloride, 
two antagonistic effects will be produced. One, due to 





IMC. 163. Effect of Strong 
KNO, Solution 

The first record shows the nor- 
mal and the second the de- 
pressed rate of suction caused 
by the reagent. 



FIG. 164. Effect of Strong NaCl 
Solution 

The first iccord shows the nor- 
mal and the second the ex- 
alted rale of suction caused 
by the reagent. 



osmotic action, will oppose suction ; and the other, due 
to the excitatory nature of the reagent, will accelerate 
suction. The resultant effect will, then, be modified by the 
excitability of the experimental plant itself. In some cases 
we should expect to find the excitatory reaction predomi- 
nant, and in others the osmotic. In repeating the experi- 
ment, on different specimens of Croton, I have found these 
theoretical inferences to be fully verified. As the more 
interesting of the two cases, I give a record (fig. 164) in 



MODIFICATION OF SUCTIONAL RESPONSE 385 



which the excitatory effect is shown by the very great 
increase in the rate of suction induced . after the application 
of the reagent. We have here a very great enhancement of 
the ascensional movement of water in spite of the osmotic 
attraction of the solution, which alone would have retarded 
the normal rate. 

The action of poisonous reagents. In studying the 
effect of poison on the rhythmic activity of Desmodium> we 
found that this was modified by the tonic condition of the 
plant. Thus a vigorous specimen was shown to be much 
less affected by poison than one which was weakly. Certain 
poisons, again, act more quickly than others in inducing 
the death of the plant. We have also seen, in that experi- 
ment in which the root was killed with hot water, that 
upper and unkilled portions of a specimen will continue 
to exhibit suctional activity when lower parts are killed ; and 
also that, in general, the killed area offers no barrier to the 
passage of water through its dead tissues. 

That a poison can easily pass through killed tissues, 
owing to the suctional activity of cells higher up, we have 
seen in our experiments on Desmodium, when the cut end 
of the petiole was placed in copper sulphate solution (p. 326). 
It is fortunate that in this case, during the ascent of poison, 
we have areas, the activity of which is indicated visibly 
by the rhythmic motile indications of the pulvini of the 
inserted lateral leaflets. That copper sulphate solution 
arrests rhythmic activity and induces death, is seen by 
the rapid stoppage of pulsation, when we apply it directly 
on the pulvini of the pulsating leaflets. When it is applied, 
however, at the cut end of the petiole, the arrest of 
pulsation takes place much later, this delay being due to 
the time taken for the poison t9 ascend through the 
intervening distance. This shows clearly that successive 
zones are killed one after another, and that the death of .a 
point below does not stop the suction above. From this 
experiment it is evident that the application of poison at 
the root, or the cut end of a stem, does not in general arrest 

CC 



386 PLANT RESPONSE 

suction, until the whole plant is killed. And from the 
Shoshungraphic records we find that the final arrest occurs 
after an appropriately long period. 

In this connection, I shall describe some very interesting 
results of rapid arrest of suction, which I have often obtained 
by the action of poison. I was already familiar with a fact 
which I had come across while studying the effects of various 
chemical reagents on the longitudinal response of radial 
organs namely, that death was attended in such cases either 
by an abnormal contraction or by an abnormal relaxation. 
These two effects were liable, again, to be modified by the 




FIG. 165. Effect of Copper Sulphate Solution 

The first part of the record shows the normal rate 01 suction. The asterisk 
denotes the time of application of the poisonous reagent. 

tonic condition of the tissue. In now studying the effect 
of solution of copper sulphate on the suctional activity of 
Cretan, I noticed certain peculiarities in the record, which 
appeared to be related to the results just described. These 
peculiarities, it should be mentioned, were specially notice- 
able in those specimens which were experimented on during 
the month of April, that is to say, at the end of the Indian 
spring. 

In a particular experiment the normal suctional rate 
had been fifteen cubic mm. per minute. On the application 
of copper sulphate, the suctional movement was quickly 



MODIFICATION OF SUCTION AL RESPONSE 387 

* 

arrested, and this was followed almost immediately by a 
slight movement in the negative direction, showing that, by 
some spasmodic contraction, water was being expelled from 
the tissue. This phase was succeeded by an almost com^ 
plete arrest of suction, there being now only the feeblest 
ascensional movement (fig. 165). Within a short period 
after this, on washing off the poisonous reagent, it was 
found that the arrest had been temporary only, suction being 
renewed at the rate of eleven, instead of the normal fifteen, 
cubic mm. per minute. 

I applied the poison once more, and allowed it to act for 
thirty-six hours. The arrest was then found to be permanent 
that is to say, the substitution of fresh water induced no 
revival of response, the plant being killed throughout. 

Strasburger, as we have seen, in his experiments on the 
effect of poisonous reagents on plants, found that the reagent 
is carried to the top of the tallest tree ; from this fact it was 
inferred that since all the cells in the path of the poisonous 
solution must necessarily be killed by its action, therefore 
the activity of living cells was not the essential factor in the 
ascent of sap. But I have proved that the ascent of sap 
is brought about, not by any localised group of cells in a 
particular region, but by cells which extend throughout the 
length of the plant. Even after some of these have died, 
therefore, by the access of poison, those above are still active, 
and will continue to exhibit suction till they in their turn are 
finally killed. It will thus be evident that the movement 
of ascent cannot be completely abolished till the poison 
has effectively reached the very top. As all the living cells 
are actively concerned in the work of suction, this con- 
veyance of poison to the top of the plant is what was 
to be expected. Only after such conveyance, indeed, could 
permanent arrest possibly take place, and, in fact, Strasburger 
himself mentions that the movement of water did come to a 
stop when the poison reached the top of the tree. 



era 



388 PLANT RESPONSE 

SUMMARY 

Rhythmic activity being as a rule exalted by rise of 
temperature, suctional response, as one of its effects, also 
undergoes an increase. 

Suction being an expression of excitatory response, the 
direction of the resultant movement of sap is determined by 
the relative excitabilities of the two ends of a tissue. Under 
certain circumstances, the normal direction of movement of 
sap may be reversed. 

The application of cold produces a transient excitation, 
and thus causes a preliminary enhancement of suction. Pro- 
longed application of cold, causing a depression of excit- 
ability, brings about arrest of suction. 

As suction is produced by the rhythmic activity of the 
tissue of the entire plant, local death, as by scalding or 
application of poison, does not cause its arrest, until the 
whole plant is killed. 

When the sum total of the latent energy of the tissue 
that is to say, its tonic condition is below par, its rhythmic 
suctional activity comes to a stop ; fresh application of 
stimulus, however, renews this activity. 

Osmotic substances, as regards their stimulatory action, 
may be either neutral or excitatory. If such a solution be 
applied at the root, there will in the former case be a diminu- 
tion, and in the latter, if the excitatory action be relatively 
great, an increase of suction. 

The application of poison abolishes local excitability and 
power of suction. In some cases this arrest of suction may 
,occur quickly. But the totai abolition of suction by poison 
only takes place, for reasons already explained, on the death 
of the plant as a whole. 



CHAPTER XXX 

THE PHENOMENON OF PROPULSION OF SAP 
AND ITS VARIOUS EFFECTS 

The mechanics of the ascent of sap : (a) Uni-directioned flow (b) Initiation of 
multiple rhythmic excitations Connection between conduction of excitation 
and conduction of sap Rapidity of ascent of sap accounted for by stimulatory 
action Positive and negative pressures due to one cause ( i ) Positive pressure 
(2) Negative pressure (3) Irregular variations of pressure Direct con- 
duction and conduction by relays Excretion of water Excretion of nectar 
Translocation of organic food-substances Mechanical response to suctional 
activity Effect of warmth- 1 - Effect of cold Explanation of the drooping of 
leaves during frost Explanation of response and recovery Antagonistic 
actions of internal energy and external stimulus. 

THERE are various phenomena connected with the transport 
of water in plants, which are at present considered as entirely 
distinct. Thus, for example, when a plant is cut above the 
root, the exuding water exerts a considerable positive pressure 
on a manometer, this being known as root or exudation 
pressure. But when manometers are inserted in lateral holes 
bored in the trunk of a tree, a negative pressure is observed. 
These facts have led to the inference that there exist, iri a 
transpiring plant, two independent forces, one of suction, and 
the other of pressure. The negative pressure, again, which 
is, generally speaking, maximal at the top of a tree, falls to 
a minimal value near the root. " But this fall is characterised 
by very' irregular fluctuations, the negative pressure, in a 
zone below, being sometimes greater than that at a given 
distance above. : 

I shall now proceed to show, however, that these very 
various results are not actually due, as supposed, to the 
operation of distinct forces, but are, on the contrary, so many 
different effects, under different conditions, of the rhythmic 



390 PLANT RESPONSE 

cellular activity of the plant-tissue. I shall show that this 
activity is sufficient to account not only for the phenomena 
of the excretion of solution from pores and nectaries, but also 
for other phenomena, whose connection with it has been little 
suspected. 

The mechanics of the ascent of sap. We have seen 
that throughout the plant there are active rhythmic tissues, 
which by their excitatory activity bring about the movement 
of water. We have next to inquire, therefore, as to how this 
excitatory action is initiated, and further in what manner so 
many activities in different zones of the stem are correlated, 
so as to give rise to a uni-directional flow, generally upwards ; 
for the rhythmic activity which may cause any given group 
of cells to act as a pump, would not alone be sufficient to 
account for the regulated, one-directioned flow of sap, since, 
while one such group propels water in one direction, there is 
no obvious reason why another should not propel it in the 
opposite. 

(a) Uni-directioned flow. In connection with the trans- 
mission of multiple excitatory waves, such, for example, as 
those which we have seen in Biophytum, it is important to 
note that these repeated excitations are all initiated at the 
original point of stimulation, and are propagated in proper 
sequence from point to point outwards. Thus, after each 
wave of excitation is -exhausted, it starts anew from the 
original point. This sequence of multiple excitations is 
exactly parallel to that which is observed in multi- ciliated 
tissues in which the cilia repeatedly contract in sequence. 
Thus a single cilium at one end gives, as it were, a signal 
which is followed serially by the rest. 

This being so, it is clear that if such a multi-ciliated 
tissue take the form of a hollow tube, with the ciliated surface 
inwards, and if the tube be filled with water, then, owing to 
this peculiarity of the multiply-responding cilia, the water 
will always be driven in one direction. A somewhat similar 
phenomenon occurs in the blood- circulation of animals, where 
the sinus giving the signal, the rhythmic contraction of the 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 39! 

heart proceeds towards the ventricle, and the pumping-action 
thus initiated produces a one-directioned flow of fluid. 

In the leaf of Biophytum, strongly excited at, say, the 
inner end, we have similar rhythmic excitations, passing in 
regular succession from leaflet to leaflet, the innermost leaflet, 
which is near the seat of multiple excitation, giving the 
signal to the rest. And just as rigid sequence is observable 
in the movements of motile cilia, so it is also seen in the 
plant, by the depression in orderly series of its lateral motile 
leaflets. 

(ft) Initiation of multiple rhythmic excitdtions. We have 
just seen that in multiple rhythmic response, the multiple 
waves of excitation proceed serially outwards, from the point 
of excitation. We also saw in Chapter XXI. that the wave 
of excitatory contraction is attended by the * propulsion of 
water in the direction of propagation of excitation. It 
remains to be determined, then, with regard to the ascent of 
sap in a plant, how this one-directioned propagation of 
excitation is initiated. 

In the case of an intact plant, the root is acted on con- 
stantly by various forms of stimulation, among which are 
(i) contact with the soil; (2) friction of the growing organ 
against rough surfaces ; (3) turgor of its own tissues, due to 
absorption of water ; and there may also be in addition, 
stimulation by (4), chemical substances of various kinds in 
the soil, which possibly.exert some excitatory influence. All 
these factors, separately or in combination, serve to set up 
multiple rhythmic excitation at the extremity of the plant, 
and the excitatory effect is transmitted upwards, preferably 
along certain better-conducting tissues. Similarly, in the 
case of a cut branch placed in water, rhythmic excitation is 
initiated at the lower end by excessive turgor, just as we 
found rhythmic movements to be initiated in Desmodium 
leaflets at standstill, by the artificial increase of internal hydro- 
static pressure, that is to say, by excessive turgor (p. 348). 

Connection between conduction of excitation and con- 
duction of sap. It was said in the case of the intact plant 



392 PLANT RESPONSE 

that excitation is transmitted upwards by conducting tissues 
and we have already seen, in studying transmission of excita- 
tion (p. 250), that the fibro-vascular elements are those which 
conduct best. It is therefore to be expected that the move- 
ment of water, which is itself an excitatory effect, should 
also follow by preference the length of the fibro-vascular 
elements ; and it is here worthy of note that the ascent of 
sap fs known to take place preferably along such channels. 
Again, I have shown (p. 250) that the power of conducting 
excitation is greater along the length of the plant than 
across ; and we find also that the same is true as regards the 
transport of water, which is known to be greater in the direc- 
tion of length. Conduction of excitation takes place with 
very great slowness across parenchymatous tissues, and the 
same is the case as regards the conduction of water through 
such a tissue. If, further, a plant were to be excited from 
above, instead of below, the transmission of excitation would 
be downwards, instead of upwards. We might then conceive 
of the possibility of reversing the direction of the normal 
transport of water. In such a case, rhythmic excitation would 
need to be initiated at the top, instead of at the bottom, of the 
plant. This may be seen in the experiment already referred 
to, of placing a leafy branch upside down, with its leaves in 
water. Rhythmic activity due to excessive turgor being 
now initiated at the upper extremity of the branch, the water- 
movement is reversed, and sap exudes from the cut end of 
the stem. Turning back, however, to the subject of the trans- 
mission of excitation, we have found, it will be remembered, 
in the case of Biophytum that this takes place with greater 
rapidity in a centrifugal than in a centripetal direction, that 
is to say, in the .direction of the normal transport of water. 
It is therefore interesting to note that a reversed water- 
movement is, in general, known to be somewhat less rapid 
than the normal flow. 

Rapidity of ascent of sap accounted for by stimula- 
tory action. One great difficulty with regard to the ascent 
of sap has lain in its relatively great rapidity. No theory 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 393 

hitherto suggested has been held to account for this. As, 
however, we now find that the ascent of sap is due to the 
propulsive energy of vigorous excitatory contraction pro- 
ceeding from cell to cell, the rapidity of the movement is 
easily understood. 

We thus see how by the action of this cellular machinery, 
set in motion by stimulus, an upward movement of water 
takes place. We have in fact an active chain of pumps, 
working throughout the length of the plant, partly carrying 
water themselves, and partly pumping it into the better con- 
ducting vessels of the xylem ; and there is 'no limit to the 
height to which it may, by such means, be lifted. 

Positive and negative pressures due to one cause. Let 
us suppose an india-rubber pipe, open at its upper end, and 
provided throughout its length with a series oT pumps, one 
above the other, each of these being independently engaged 
in raising water upwards. The individual activity of these 
several pumps may or may not be uniform, but, provided 
that they are sufficiently numerous, when the pipe is placed 
in connection with a supply of water, the result of their com- 
bined action will be that water will be sucked in at the lower 
end, and ejected at the upper, in a uniform stream. 

If now we confined our attention to the lowermost pump, 
it would appear to us to be forcing water up ; if, on the other 
hand, we observed the uppermost pump alone, it would 
appear to be sucking water up ; and if, finally, we selected 
some intermediate point for scrutiny, we should discover that 
the pumps above were sucking, and those below pressing 
water upwards. Thus, one single effect, namely, the rhythmic 
activity of pumps, is made to appear various, by simply 
changing the point of view. Again, certain peculiarities of 
variation of pressure may appear in the pipe as a whole or 
in particular parts of it, depending on the rates of supply and 
removal of water. 

(i) Positive pressure. We may now suppose the 
aperture of escape at the upper end to be narrowed. The 
water pumped into the flexible pipe being thus in a state of 



394 PLANT RESPONSE 

compression, will produce a bulging or over-turgidity, and a 
manometric tube inserted laterally will indicate a positive 
pressure. Or if, under these circumstances, we make a cut in 
the pipe, we shall observe exudation under pressure. Now, 
this corresponds to the exudation pressure, causing so-called 
'bleeding,' which we find on making incisions in plants in 
spring time, when the loss of water by transpiration is feeble, 
the buds being still unfolded ; or even in summer, if trans- 
piration be by any means prevented. 

In connection with this exudation of sap, we have to 
remember that the whole question is one of income and ex- 
penditure. Generally speaking, the loss by transpiration is 
less at the end of winter or the beginning of spring than in 
other seasons. Thus the wild date palm, or Phoenix sylvestris, 
yields considerable quantities of sugary sap from incisions in 
the stem, at the end of winter or the beginning of spring. In 
the case of the Palmyra palm (Borassus flabelliformis) of 
Bengal, however, the increase of cellular activity in summer 
more than compensates for the loss by transpiration, and sugary 
juice is collected in the height of summer at the top of the 
tree by incisions in the peduncle. The pressure exerted by 
the sap may be gathered from the fact that these trees are 
often more than a hundred feet high. 

(2) Negative pressure. We may next suppose that in 
the chain of pumps, those at the upper end are the most active, 
and the aperture of escape wide open, the removal of water 
being further aided by evaporation. The loss of water being 
thus greater than the supply, it is clear that there will be a 
negative pressure in the pipe, and the mercury in a testing 
lateral manometer will be sucked in. This corresponds to 
the negative pressure exhibited by an actively transpiring 
plant In such a case it will be noted that the activity of the 
rhythmic cells, which is the fundamental cause of the ascent 
of sap, is further aided by the evaporation from the leaves. 
Concentration of cell-sap also, and the osmotic action 
thereby produced, may then constitute an additional auxiliary 
factor. 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 39$ 

When the stem of an actively transpiring plant is cut across, 
the stump of the plant does not always immediately show 
bleeding, but often, on the contrary, will suck in water. This 
is generally ascribed solely to the existence of negative pres- 
sure in the stem. There is another element in the problem, 
however, which is generally overlooked. By such stimulus 
as that of amputation, excitation must be produced at the 
cut end of the stem, which is propagated downwards, and 
tends to induce a reversal of flow. But when the negative 
tension and the excitatory effect, due to stimulus of cut, have 
both subsided, exudation will begin to take place. 

(3) Irregular variations of pressure.- It is evident 
from what has been said that the hydrostatic pressure at any 
given zone of the tissue will depend on the relative activities 
of cells below and cells above. Cells immediately above, by 
their activity, produce suction or negative pressure ; and 
those immediately below, an increase or positive pressure. 
The resultant pressure at any individual zone depends, 
therefore, on the algebraical summation of these. Now, 
though all the cells throughout the length of the plant may 
be active, yet there will be some difference in their activities. 
Or the same cells again, at different periods of life and 
different conditions, may undergo variations of their activity. 
There are, in nature, many disturbing influences which 
produce local variations of excitability ; hence, by the dis- 
tribution of cells whose excitabilities are irregular, we may 
obtain a variation of internal pressure from the top to the 
bottom of the plant, which is not uniform but fluctuating. 

Direct conduction and conduction by relays. I have 
already said that the movement of water, being mainly 
brought about by excitatory reactions, will take place 
preferentially along conducting channels. We have seen that 
every tissue possesses the power of conduction to a greater 
or less degree ; in parenchymatous cells, however, owing to 
the presence of numerous more or less complete septa, trans- 
mission is enfeebled, whereas in prosenchymatous fibro- 
vascular elements it is very much facilitated. Thus, for 



396 PLANT RESPONSE 

conduction through long tracts, the fibro-vascular tissues are 
the most favourable. But even in this case the transmission 
of the excitatory effect may be much enfeebled by distance. 
Or an interposition of parenchymatous elements may offer a 
relative obstruction to the transmission. In such cases there 
might be pseudo-conduction by means of ' relays/ For as an 
example of the last case we may imagine a mass of excitable 
parenchymatous tissue, against which a conducting tissue 
abuts. This mass, being supplied with water by the conduct- 
ing elements, may become over-turgid, and thus rhythmic 
activity may be initiated in it de novo. 

That rhythmic activity may under favourable circum- 
stances be started locally in a mass of excitable tissue, we 
have seen in the case of the pulvinus of Desmodium. For the 
fact that activity in this case was not due to any transmitted 
impulse was proved in the experiment on the localisation of 
the excitable area, when it was found that an isolated leaflet, 
if sufficiently turgid, could pulsate. Under natural condi- 
tions, the necessary turgidity is maintained by the cellular 
activity of the tissue below. It is worth while to remember, 
in this regard, that the characteristic pulsation of the leaflet 
has no immediate connection with that rhythmic activity of 
the plant-tissue which brings about the ascent of sap. The 
period of pulsation of the leaflet is determined by certain 
constants of its cell-complex. We may in fact have various 
vibration-periods in different organs of the same plant, the 
different oscillations being brought about by the turgidity 
caused by the ascent of the sap, just as the same electrical 
current may give rise to various frequencies of vibration of 
different electro- magnetic vibrators included in the same 
circuit. 

Excretion of water. The rhythmic activity of a mass 
of excitable cells is seen again in the case 'of such as ac- 
tively excrete water. A very striking example is that of 
Colocasia esculentum, in which the successive expulsions of 
water-drops noticed by Musset were as many as eighty-five 
in one minute. 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 397 

, A distinction is sometimes made between this excretion " 
due to the special local activity of a certain group of cells 
and the somewhat passive excretion of "water from water- 
pores, which is said to be caused by the general pressure of 
exudation. But this difference is really one of degree and 
not of kind. All cells are excitable, and the exudation 
pressure itself is produced by cellular activity. As regards 
excitability, however, we may have a transition from 
moderately to highly excitable cells distributed in a continu- 
ous or discontinuous manner. Even in the stem we have 
seen that there are cases of irregular distribution, bringing 
about irregularities of water-pressure. Extreme instances of 
these are found in Desmodium, and in the actively excreting 
cells of Colocasta, where highly excitable cells are localised in 
special areas. A test which is sometimes insisted on as a 
means of distinguishing between the active and so-called pas- 
sive excretions is, that in the latter case the flow ceases from 
the excreting organ, as soon as the branch is cut off. But 
this , is not by any means a satisfactory proof of the absence 
of active excreting cells in the latter. We saw that the only 
distinction between the activities of the multiply-responding 
tissues of Biophytum and Desmodium, lay in the fact that the 
latter had the capacity to hold latent a large amount of 
energy by which the rhythmic activity was maintained even 
on the cessation of a directly exciting cause. In the case of 
Desmodium^ indeed, if the tonic condition be above par, and 
the leaf as a whole be cut off and isolated, without any supply 
of water, the rhythmic activity will be maintained for a con- 
siderable time, though the turgidity is undergoing constant 
diminution. But when the tonic condition of the plant is 
below par, its rhythmic activity comes to a stop, and can 
only be maintained by an artificial increase of internal 
hydrostatic pressure. Similarly in the case of water-excreting 
organs, we have some which under a favourable condition can 
maintain their activity for a considerable period, even when the 
supply of water is cut off, while in other instances activity can 
be maintained only under favourable conditions of turgidity. 



398 PLANT RESPONSE 

Excretion of nectar. It is often assumed that the 
excretion of nectar is due to plasmolytic action. The 
excreted solution of sugar dries up by evaporation, and this 
by plasmolysis draws up more from behind. We have seen 
that the ascent of sap in the plant is brought about mainly 
by rhythmic activity, and that concentrated solutions in the 
leaves may help this movement osmotically. In the case of 
nectaries, the presence of concentrated sugar solution outside 
may thus help continued excretion, but this does not explain 
the initiation of excretion, which could only have been caused 
by the rhythmic activity of cells. 

Translocation of organic food-substances. Though 
the flow of organic food-materials towards places where there 
is a deficit may be brought about by diosmosis, yet such a 
mode of diffusion must be extremely slow. A more rapid 
transport than could be produced by this means would appear 
to be a necessity. There are certain considerations moreover 
which may be brought forward, tending to show that this 
translocation of food -materials receives considerable aid from 
stimulatory actions. Czapek's observation, too, that there is a 
cessation of translocation in a chloroformed leaf-stalk, points 
to the inference that it is a physiological process. 

We know that an excitatory action proceeds from the 
more to the less stimulated. Now the accumulation of a 
large quantity of organic food-material may of itself act as a 
stimulating agent in a given case ; thus an excitatory move- 
ment would proceed from cell to cell, from places where there 
was excess to places where there was deficit. In this way 
large quantities of food-materials may be rapidly transported 
by excitatory reaction, through conducting channels. And 
such transport is also possible by stimulatory action even 
through unspecialised cells. These ordinary cells are known 
to be connected with each other, by means of pores and 
plasmic threads. Mr. Horace Brown has shown (Phil. Trans. 
vol. cxciii.) that transport of fluid may take place through 
such a * multi-perforate septum ' with almost as great rapidity 



PROPULSION OF -SAP AND ITS VARIOUS EFFECTS 399 

as if no closing membrane were present. It is clear 
that by the contracting expulsive action of excited cells, 
transport of materials might be brought about more rapidly 
than by mere osmotic action. 

Mechanical response to suctional activity. The effect 
of that internal rhythmic activity which, as we have seen, is 
caused by the absorption of energy from various forms of 
stimulus, is to induce an increase of turgidity throughout 
the plant I shall next proceed to describe a mechanical 
means of obtaining some indication of this internal 
activity. 

I have shown that owing to the dorsi-ventral differentiation 
in the pulvinus of Mimosa, when the turgidity of the organ is 
increased, the leaf is erected ; and when the turgidity is 
decreased, as by the direct action of an external stimulus, the 
leaf is depressed. Similar effects occur not in pulvinated 
organs alone, but in all dorsi-ventral organs, in which the 
lower half is more excitable than the upper. Thus, for 
example, the lower half of the petiole carrying the lateral 
leaflets of Biophytum is more excitable than the upper half. 
It is true that the leaf possesses a slightly developed pulvinus ; 
the differential excitability on which the motile response 
depends is not, however, confined to this pulvinus alone, but 
extends throughout the length of the petiole. The petiole 
thus acts like a diffuse pulvinoid. 

The action of stimulus on the diffuse pulvinoid of Biophytum 
and on the pulvinus of Mimosa is to produce the fall of the 
leaf. And just as the leaf of Mimosa was erected by increased 
turgidity, so in Biophytum also we should expect to have, with 
similar increase of turgidity, a similar responsive movement 
of erection ; and this I find to be the case. The erection of 
the leaf of Mimosa or Biophytum is therefore a mechanical 
indication of an increase of turgidity or of positive turgidity- 
variation, by whatever means this may have been induced. 
Such an increase may be brought about by an augmentation 
of suctional activity. And the up movements of dorsi-ventral 



400 PLANT RESPONSE 

petioles in which the lower surface is the more excitable may 
thus be taken as mechanical indications of such added 
activity. 

Effect of warmth. We have already seen that applica- 
tion of warm water to the root of a plant, increases its suctional 
activity. I was able to demonstrate this fact by means of the 
mechanical response of Mimosa and Biophytum. On making 
the application, the leaves of both plants responded by erec- 
tion to a position which in the case of Mimosa was 9 mm. 
and in the case of Biophytum 12 mm. above normal. A rise 
of temperature we have also seen to have the effect of enhanc- 
ing the internal energy of the plant. By doing this, it brings 
about an erection of the leaves. 

Effect of cold. We have seen, on the other hand, that 
the effect of cold is to stop rhythmic activity, and thereby to 
arrest the ascent of sap. The motile indication given by the 
leaf would in this case be the opposite to that of positive 
turgidity-variation, that is to say, would consist of a fall or 
droop. On applying ice-cold water to the root of Mimosa 
I found that the effect of cooling was to induce a lowering or 
drooping of the leaf, to a position of 3 mm. below the normal. 
In Biophytum the corresponding fall was through 5 mm. 
Lowering of temperature, by depressing the internal energy 
of the plant, has the same effect. 

Explanation of the drooping of leaves during frost 
These experiments offer an explanation of that drooping of 
leaves which is observed in frost, and of the disappearance of 
this drooping when a plant is restored to a warmer atmosphere 
indoors ; for we have seen that when the internal energy of 
the plant is normal, or above par, its suctional activity and 
consequent turgidity are high, this favourable internal condi- 
tion being outwardly exhibited by the erection of the leaves. 
But when the internal energy is below par, the reverse effect 
is seen in their droop or fall. 

Explanation of response and recovery. It has been 
stated, when describing the effect of stimulus in inducing a 
fall of the leaf, and its subsequent erection, that the latter 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 401 



movement, namely, that of recovery, was an active and not a 
passive process. Certain experiments that I am about to 
describe will enable us to analyse response and recovery 
still more closely. 

When an organ is locally excited, the contraction induced 
results in an expulsion of water, thus reducing the turgidity 
of the organ, and the consequence of this is a mechanical 
fall of the leaf. Some internal activity now forces the water 
back into the organ from 
which it was expelled, and 
we have the recovery or 
erection of the leaf. That 
it is the internal energy 
of the plant to which this 
recovery is due, will be 
seen clearly from the 
following experiment. A 
leaf of Biophytuin falls, as 
has been said before, when 
it is stimulated, whatever 
be the form of stimulus. 
I shall explain in my 

The first part of the record shows the fall 
of the leaf, due to direct stimulation by 




EIG. 166. Record showing Recovery to be 
Hastened by the Increase of Internal 
Activity which is caused by Application 
of Warm Water to the Roots 



chapter on the effect of 
stimulus of light, that the 
effect of photic stimulus 
is the same as that of 
any other form, while its 
special advantage is that 



sunlight during thirty minutes. Stimulus 
is next cut off at point marked by 
interruption of record. After-effect 
persists for five minutes, and there is a 
subsequent slow recovery. Warm water 
applied to root at moment x , with the 
result of quickening the rate of recovery 
by fifteen times. 



it may be applied without 

causing any mechanical disturbance, and that its intensity can 

be very easily regulated. 

On subjecting a petiole of Biophytum, therefore, to the 
action of sunlight, the leaf responded by depression, the 
average rate of fall of the tip of the leaf being -16 mm. per 
minute. In half an hour it had passed through almost 5 mm. 
On now shutting off the light, the after-effect persisted for 
another five minutes, when there followed recovery by slow 

D D 



4O2 PLANT RESPONSE 

erection of the leaf at a rate of *I2 mm. per minute. Warm 
water was now poured on the root, thus suddenly increasing 
the internal activity of the plant. Now, if it be true that 
recovery is brought about by this factor of internal energy, 
then the increase of internal energy ought to produce a 
sudden augmentation of the rate of the recovery. That this 
is the case will be seen from the record (fig. 166), where it will 
be noticed that the enhanced rate of recovery from this point 
is i '8 mm. per minute, that is to say, fifteen times the normal 
rate. 

Antagonistic actions of internal energy and external 
stimulus. We thus see that it is the internal energy of 
the plant vaguely known as a favourable tonic condition 
which actively determines the recovery of the organ. This 
will explain the fact which I have mentioned elsewhere, that 
in summer, when the internal energy is considerable, the leaf 
of Mimosa recovers from the effect of stimulus in about six 
minutes, whereas in winter, when the internal energy is low, 
the same process may take as long as eighteen minutes. We 
thus see that as regards mechanical response the external 
stimulus and internal energy act antagonistically. Local 
external stimulus induces a diminution of turgidity, while 
internal energy causes an increase of turgidity. Thus when 
the internal turgidity is very great it opposes the mechanical 
response to external stimulus. This we saw in the case of 
over-turgid leaves of Mimosa^ and in those of Artocarpus 
during the rainy season, which, though excited, did not exhibit 
mechanical response to stimulation (pp. 49, 58). 

This will be clearly understood also from an attentive 
consideration of the experiment, the record of which is given 
in fig. 1 66. In that case, had the warm water which in- 
creased the internal activity been applied earlier at the root, 
that is to say during the application of external stimulus, 
the induced internal turgidity would then have been so great 
as to arrest the responsive down movement of the leaf. The 
amplitude of the response to external stimulus would thus 
have undergone diminution or even abolition. 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 403 

We must remember, however, that the internal energy 
which maintains the normal turgid condition is itself the 
result of energy previously absorbed from external sources ; 
and if the plant be cut off from these sources of external 
energy, then its own tonic condition will fall below par. Thus 
the leaves of many plants are seen to droop when kept too 
long in darkness, and exposure to light makes them recover 
their natural position of normal turgidity. Hence in the case 
of a plant whose condition is sub-tonic the leaves may be 
made turgid by exposure to light ; but after the attainment of 
the normal tonic condition, exposure to strong light will 
bring about the proper contractile response to stimulus, with 
the characteristic external motile indication of diminished 
turgidity, 

We have thus seen the various effects produced by the 
internal or latent energy of the plant. We have seen it bring 
about the ascent of sap by means of the increased activity of 
the plant-cells. It was seen to produce exudation pressure, 
and excretion of nectar from intact plants. We have traced 
it out to its appropriate expression by the lateral movements 
characteristic of positive turgidity-variation, in the case of 
anisotropic or dorsi-ventral organs. We have seen, too, how 
necessary it is to the production of recovery of an organ from 
the action of an external stimulus, the effects of local ex- 
ternal stimulation, and of this internal activity, being opposed 
in character. Any increase of this internal energy is thus a 
factor tending to hasten the recovery of the organ from 
stimulation, and when it is sufficiently great, it may, by its 
antagonistic action, reduce the amplitude or even abolish 
response to external local stimulus. We have again seen 
this internal activity finding motile expression in the auto- 
nomous movements of leaflets of Desmodium ; but, for its 
mechanical exhibition, we need not confine our attention to 
the sensitive plants so called, for we shall find the same 
internal activity exhibited mechanically by all plants, in their 
rhythmic growth-responses, to be described in the following 
chapters. 



D D 2 



404 PLANT RESPONSE 



SUMMARY 

The ascent of sap is fundamentally due to excitatory 
reaction, its uni-directioned flow being brought about by the 
passage, from point to point, of the co-ordinated excitatory 
reaction, propelling water forward. 

This rhythmic excitation is initiated in the intact plant 
at its root, by stimulus of contact with soil, the friction of 
the growing organ against rough surfaces, the excessive tur- 
gidity caused by the absorption of water, and possibly by 
the chemical stimulus of substances present in the soil. In 
the case of cut branches placed in water, the excessive 
turgidity at the cut end initiates rhythmic activity. Again, 
if stimulus be applied at the top instead of at the root, the 
direction of water-conduction may be reversed, along with 
the reversal of propagation of excitation. 

The following facts show the intimate relation between 
the conduction of stimulus and conduction of water : 

(a) The movement of water takes place preferentially 
through the fibro-vascular elements, these being also the 
better conductors of excitation. 

(b) The conduction of excitation along a plant is greater 
than across. The same is true of its power of transport of 
water. 

(c) Though conduction of excitation may take place 
either upwards or downwards, yet there is a preferential 
direction for such conduction. The same is true of the trans- 
port of water. 

The same movement of water produced by the co- 
ordinated rhythmic activity of cells throughout the plant 
appears as either suctional or pressure movement, according 
to the point of view. 

When the removal of water from the plant is in any way 
arrested, a positive pressure is produced, owing to the exces- 
sive accumulation of water. When, on the other hand, the 



PROPULSION OF SAP AND ITS VARIOUS EFFECTS 405 

1k 

loss of water by transpiration is greater than the supply, a 
negative pressure will be observed. 

The ascent of sap primarily due to cellular activity, may 
be secondarily aided by evaporation from the leaves, and the 
osmotic action of the concentrated cell sap in the leaves. 

Owing to the distribution of unequally active cells, an 
irregular variation of pressure is induced in the stem. 

The excitatory movement may be transmitted to a dis- 
tance by conduction, or there may be conduction by ' relays.' 
An isolated mass of highly excitable tissue may thus be 
excited de novo. 

The excretion of water and of nectar are phenomena of 
cellular activity, analogous to that which brings about the 
ascent of sap. 

The translocation of food-material is also probably due, 
at least in part, to excitatory reaction. 

The internal activity of the plant, causing increase of 
turgidity, may be detected mechanically by that erection of 
the leaf which is characteristic of the positive turgidity- 
variation. 

Any increase of internal activity is exhibited in dorsi- 
ventral organs, such as the petioles of Mimosa, Biophytum^ 
and Artocarpus, by the erection of the leaf. Thus, when the 
internal energy of the plant is increased by a rise of tem- 
perature, the leaves become erected. Conversely, under the 
action of cold, on account of the diminution of the latent 
energy, the opposite effect, or droop, is induced. This 
explains the drooping of various leaves during frost, and 
their subsequent erection when brought into a warmer atmo- 
sphere. 

This internal energy is also an important factor in 
bringing about the recovery of an organ from the effect of 
external local stimulus. The effect of external local stimulus 
in causing the diminution of turgidity of an organ is thus 
antagonised by the internal activity, which causes an increase 
of turgidity. 



406 PLANT RESPONSE 

The internal energy, when sufficiently great, may thus 
hasten the recovery of the organ from the effect of 
stimulus. 

This increased internal energy may also reduce the am- 
plitude, culminating in the total abolition of mechanical 
response, as seen in over-turgid Mimosa, or in Attocarpus 
during the rains. 



PART VI 

GROWTH 



CHAPTER XXXI 

THE RECORD OF GROWTH-RESPONSE 

The simple Growth-Recorder The Balanced Growth-RecorderRhythmic 
growth-response Growth-response and excitatory response Law of direct 
and indirect effects of excitation Positive turgidity-variation as indirect effect 
of excitation Mechanical test Significance of ' inner stimuli. ' 

ONE of the most characteristic manifestations of life is 
growth. The question then arises, whether this particular 
manifestation is to be regarded as a distinct and specific 
phenomenon, unlike all others, or whether it may be possible 
to trace a connection between it and those responsive re- 
actions with which we are already familiar. 

The occurrence, in response to stimulus, of numerous 
growth-curvatures, sometimes positive and sometimes nega- 
tive in character, offers us again a problem of very great 
complexity. It is sometimes supposed that stimulus retards, 
and sometimes that it accelerates, growth. But it is difficult 
to understand how the same influence can produce opposite 
effects. Then, again, there is intruded upon the problem 
the unknown effect of * inner stimuli.' From all these it will 
be seen that the subject of growth and growth- movements is 
one of extreme obscurity, and that the difficulties which 
baffle us can only be met satisfactorily if we are able to 
analyse and follow out, one by one, the various elements that 
enter into the problem. 

We have seen in the Desmodium leaflet at standstill, and 
in that of Biophytum under ordinary circumstances, that 
when the latent energy is not excessive, we . obtain a single 
.movement in response to a single stimulus. When the sum 
total of the latent energy of the tissue, however, is above par, 



4 I PLANT RESPONSE 

it is manifested in a rhythmic manner, by periodic variations 
of turgidity, bringing on responsive movements. It was 
also stated that responsive movements under the action of 
stimulus took place in ordinary young tissues, these move- 
ments being lateral when the tissue was anisotropic, and 
longitudinal when it was strictly radial. It would follow, 
then, that when the sum total of the latent energy in such 
tissues was above par, they might be expected like the 
leaflet of Biophytum or Desmodium under similar conditions 
to exhibit their rhythmic excitation by repeated lateral or 
longitudinal movements. I shall now proceed to show that 
in the case of these young tissues, under favourable circum- 
stances, this multiple rhythmic excitation finds expression in 
the responsive movement known as growth. 

In the majority of instances an organ is not absolutely 
radial ; hence, during growth, we obtain the lateral respon- 
sive movements which are known as circumnutation. In 
bilateral growing organs, these movements are to and fro, 
in strict parallelism to the to and fro movements of the 
leaflet of Biophytum. In such instances the axis of bilate- 
rality is fixed, and the responsive movement takes place in 
a definite plane. In the leaflet of Desmodium also rectilinear 
movements are often observed ; but as a rule, owing to the 
revolution of the bilateral axis, the movement of the leaflet is 
circular or elliptical, and in the case of growing organs, from 
the same cause, circular or elliptical movements of nutation 
are common. The ideally simple and most interesting 
example of this multiple rhythmic activity is seen, however, 
in the growth- movements of radial organs, these being longi- 
tudinal ; for there is not in this case that complication which 
arises from the gradual shifting of the bilateral axis seen in 
the growth of anisotropic organs. 

In order to prove the identity of these rhythmic growth- 
movements with multiple response, we have to show 
( i ) that such rhythm is a characteristic of growth ; (2) that 
each pulsatory growth-movement of the series exhibits all 
the characteristics of true response ; (3) that the series itself 



THE RECORD OF GROWTH-RESPONSE 411 

* 

is characterised by the same cyclic variation which we have 
observed in the case of multiple response ; (4) that just as the 
application of appropriate stimulus renews pulsation in a 
Desmodium at standstill, so, in a plant with growth at stand- 
still, appropriate stimulation renews pulsatory growth ; and, 
lastly (5), that the modifying influence of external agents is 
similar in both cases. 

From a series of observations, taken at intervals of several 
minutes, on Spyrogyra princeps, Hofmeister found that growth 
undergoes fluctuation, the first and second .maximal points in 
his series of observations being separated by an interval of 
forty-four minutes, and the second and third by an interval 
of ninety-five minutes. Such experiments, however, have 
laboured under the great disadvantage of discontinuity, and 
in order to overcome this I undertook to devise some appa- 
ratus by whose means growth-pulsations might be recorded 
continuously, in such a way as to give not only the period, but 
also the individual peculiarities of each pulsation. And I may 
here forestall matters to say that by such means I have been 
able to detect longitudinal pulsations two hundred times as 
quick as those observed by Hofmeister. 

Conditions to be kept in view. Before attempting to 
demonstrate the pulsatory character of growth-movements, 
however, I shall point out certain facts which it is essential 
to remember. We have seen that under rapidly succeeding 
excitations, the separate responsive effects become merged, 
and a response is produced, which is apparently continuous, 
although the stimuli themselves were discontinuous. This 
is seen, for instance, in the first part of the tetanic curve 
(figs. 49, 50). But when once the maximum responsive 
effect is produced, as there can be no further additive 
effect, the subsequent responses show themselves in a series 
of fluctuations of the top of the tetanic curve. In that 
form of response, which we are now considering, as there is 
no maximum limit, the additive effect of growth continues 
indefinitely. It is thus clear that when rhythmic excitation 
is very rapid, it may produce a growth-movement which 



412 PLANT RESPONSE 

appears to be continuous. But when this rapidity is not 
excessive, it should be possible, by employing sufficient 
magnification and a suitably quick rate of movement of 
the recording surface, to display its actual pulsatory cha- 
racter. The sensitiveness of this mode of detection becomes 
again very much increased if we employ the method of 
balance, or compensation, which will be described presently. 
We thus require a high magnification, and some means of 
continuous record. 

A high magnification may be produced by microscopic 
optical projection, but this labours under the great dis- 
advantage that the specimen is subjected to the strong and 
unilateral stimulus of light, by which its normal growth- 
movements are greatly modified. The ordinary auxano- 
metric method, again, cannot be employed, (i) because the 
magnification produced is not sufficiently great ; and (2) 
because the inertia of the wheel, and the unavoidable friction 
of the apparatus, themselves combine to obliterate the quick 
pulsations of the growth -response. 

The Crescograph. All these difficulties were over- 
come by the use of my Optical Lever, for making growth- 
records. The lever is made extremely light, and the fulcrum- 
rod rests on agate planes. When the tip of the growing 
organ is attached by a thread to the short arm of the Lever, 
the length of the latter being *5 cm., and when the record- 
ing surface is at a distance of 2*5 metres, a magnification 
of 1,000 times is obtained. This is in most cases more than 
sufficient. But, when necessary, a magnification of 10,000 
times can easily be secured. In order to avoid any dis- 
turbance due to vibration in the room, the apparatus is 
supported on a steady bracket, fixed on the wall. Using 
these ordinary precautions, records are obtained with this 
instrument which are absolutely free from external disturb- 
ance. 

The Balanced Crescograph. In records of growth 
we obtain a sloping curve whose abscissa represents time ; 
and ordinate, the elongation produced in the organ during 



THE RECORD OF GROWTH-RESPONSE 413 

V 

that time. A variation of growth will produce a variation 
in the slope of the curve. When this variation of growth, 
however, is slight, the variation of slope of the curve is so 
small as not to be detected. It was in order to detect and 
measure such small variations that I devised the Method of 
Balance, in which the average rate of growth is represented 
by the horizontal line of balance, any fluctuation, even the 
slightest, appearing as a deviation from this horizontal. The 
influence of various agencies, again, may be displayed in a 
marked manner, by using this method ; for in such cases we 
do not so much require the rate of growth itself, as the 
variation i.e. acceleration or retardation-- in the normal 
rate, which is induced by one agent or another. 

The principle of the Method of Balance consists in 
making the spot of light which is moving in response to 
growth become stationary, by subjecting it to a compen- 
sating movement. An example will make this clear. We 
shall suppose the average rate of growth to be r2 mm. 
per hour. This will cause an excursion of the moving spot 
of light, from, say, left to right, through 1,200 mm. by the 
end of the hour, in that case where the magnification is 1,000. 
Had the growth been uniform, this would have meant a 
movement of 20 mm. per minute. But if not uniform, the 
rate might sometimes have risen above, and at others fallen 
below, this average. If now we subject the spot of light to a 
uniform compensating movement, such as by itself would 
have made it move from right to left of the recording sur- 
face, to the extent of 1,200 mm. by the end of the hour, we 
shall find that, being acted on by these two opposite move- 
ments, of growth and compensation, the spot will remain 
approximately on a single base line of compensation. The 
fluctuations, or variations, which have occurred in this average 
rate of growth, will, however, be recorded as deviations 
to one side or other of this mean neutral line. Thus it 
will be seen that the slightest deviation from a uniform rate 
of growth, will be found displayed by the record of the 
moving spot of light. We are further enabled, from our 



414 



PLANT RESPONSE 



knowledge of the speed of the recording-drum, and the 
balancing rate, and from an inspection of the curve itself, to 
determine not only the periodicities, but also the absolute 
value of the rate of variation of growth, at any given 
moment 

The compensating movement to which I have referred is 
effected by means of an hydraulic device. The spot of light 

from the Optical 
Lever falls upon a 
mirror attached to a 
second lever, or to a 
rotating wheel. The 
arm of the lever, or 
a thread which is 
passed round the 
wheel, is attached to 
a float on the surface 
of a cylinder of water. 
Water is escaping 
from this cylinder, by 
means of a syphon 
arrangement, at a rate 
which can be adjusted 
with the greatest 
nicety. The float can 
thus be made to dc- 




Diagrammatic Representation of 
Balanced Crescograph 



FIG. 167. 

Hfilarfvf1 f~*r<r t rorrarVi 

scend at any speed 

M, attached to fulcrum-rod, resting on knife- that is desired, this 
edges. A and A' ; L'. lever attached to float, , , , 

F F, with second mirror, M, attached to its ful- descent producing a 
crum-rod ; B, balancing wheel adjusting differ- rotation of the second 
ences of level of the two limbs of syphon, s. 

lever or of the wheel. 

We have, then, two mirrors, of which one is rotated in one 
direction by the growth-movement of the plant, and the 
second in the opposite direction by the descent of the float. 
A spot of light reflected on the two mirrors will thus remain 
stationary when the precise balance is effected, by proper 
regulation of the escape of water from the cylinder (fig. 167). 



THE RECORD OF GROWTH-RESPONSE 415 

* 

This outflow of water is roughly adjusted by opening the 
stop-cock, to a greater or less extent. The finer adjustment 
is then effected by the suitable variation of the difference of 
level in the two limbs of the syphon. One end is con- 
nected with a flexible india-rubber tube, which is attached to 
a string passing over a pulley, and fixed to the adjustable 
wheel B, attached to the observer's table. Rotation of the 
wheel in one direction will depress this end of the syphon, so 
increasing the flow, and in the other direction will raise it, so 
diminishing the escape of water. It will be noticed that the 
rate of outflow of the water is not in any way affected by the 
variation of level of the water in the cylinder. It simply 
depends on the difference of level between the two ends of 
the syphon. The rate of descent of the float is. thus regulated 
with the utmost nicety, till an absolute balance is obtained. 
The .observer recognises the condition of balance when there 
is no drifting of the spot of light on the recording drum. 
The adjusting wheel is now fixed at this position of balance. 
If any agent should induce an acceleration of growth, the 
balance is disturbed, and the spot of light moves, say, to the 
right, or in a positive direction. Any agent which induces 
retardation will, on the other hand, cause a deflection of the 
spot of light in a negative direction. Should there be any 
natural fluctuations in the growth, the oscillation of the spot 
of light will give an indication of the fact. 

The record is made in the usual manner on a revolving 
drum. Fig. 168 illustrates the complete apparatus, which 
enables us to obtain a record under balanced or by closing 
the stop-cock of the syphon, also under unbalanced con- 
ditions. 

The wheel is graduated, and the absolute value of the 
compensatory movement, at any position of the circular scale, 
can be previously calibrated, by fixing the plant-mirror, and 
observing the extent of movement of the spot of light on the 
drum, due to the subsidence of the float in a given time. 

For the exhibition of pure longitudinal growth-response, 
the most perfect specimens are the growing radial styles and 



416 



PLANT RESPONSE 



stamens of flowers. There are also other organs, which are 
more or less strictly radial, such as the peduncle and the 
hypocotyl. But in these latter cases care should be taken 
that the specimen have been so grown, that its different sides 
have been subjected to uniform conditions of light or dark- 
ness ; for one-sided illumination tends, to produce amsotropy. 




FlG. 1 68. Complete Apparatus for Crescographic Record under Ordinary 

and Balanced Conditions 



With specimens of all the types mentioned, I have obtained 
multiple growth-responses when the constituent pulsations 
were not too rapid. In some cases, indeed, the effects 
were so marked that they did not even require a balancing 
arrangement to render them conspicuous. As an instance 
of this, I shall give a record of the growth-response of a 



THE RECORD OF GROWTH-RESPONSE 



417 



vigorously growing peduncle of Crocus which brings out in 
an interesting manner the mechanics of growth (fig. 169). 

Rhythmic growth-response. We saw that when the 
sum total of energy is above par, a tissue becomes self- 
excitatory in a multiple or rhythmic manner, giving rise 
to periodic turgidity-variations. There may thus be respon- 
sive pulsations of increased turgidity, each followed by 
slow recovery from 
such excess. With 
each such pulse, a 
transient elongation 
of the growing tissue 
will be produced, and 
the succeeding slow 
recovery will be more 
or less incomplete. 
This incompleteness 
is due to the deposit 
of material which 
fixes growth. The 
irreversible or per- 
manent growth-effect 
produced by each 
pulsation, will thus 
be measured by the 
responsive elongation 
minus the recovery. _ - .. . . . , . 

* Fir.. 169. Multiple drowth-responses 

This is well illustrated (Peduncle of 

in fig. 169, where 
three separate sets of 
responses are given, 
taken from a single specimen in the course of the day. 
Growth, as will be shown, is not uniform throughout the 
day, but exhibits variation, in consequence of changing 
conditions, such as that of temperature. But for a short 
interval of time, the rate of growth under a constant en- 
vironment, it may be taken as uniform. In the present 

E E 




The orclinate represents the extent of responsive 
elongations in mm. ; the abscissa, time in 
seconds. 



41$ 



PLANT RESPONSE 



instance the maximum rate was as high as "0035 and the 
minimum as low as *ooio mm. per minute. Confining our 
attention to the uppermost of these series (c\ we find that 
the responsive elongation is very quick, and the recovery 
slow and incomplete. The average period of a single pulse 
is twenty seconds. The results of series (c) are given in the 
following table : 

TABLE SHOWING PULSATIONS OF GROWTH 



Number of 
pulse 


Responsive 
elongation 


Recovery 


Permanent 
growth 


Time 


I. 


0020 mm. 


ooio mm. 


ooio mm. 


20 seconds 


2. 


0023 


0008 


0015 


22 


3- 


0018 


0005 


0013 


18 


4- 


0021 


'OOI2 


0009 


20 


Total 






0047 


80 



It will be seen from this table that the total growth 
in eighty seconds is '0047 mm., giving an average rate of 
growth of '0035 mm. per minute. Had a magnifying 
arrangement not been used, this average rate of growth, 
shown by the dotted base line, would have appeared as con- 
tinuous growth. By the magnification of the responsive 
curve, however, we are enabled to see that such a rate is in 
reality made up of numerous fluctuating growths of which it 
is an average. 

Growth-response and excitatory response. If we 
compare these multiple growth-responses with the multiple 
mechanical responses of Biophytum (fig. 116), their similarity 
is at once evident. As in that case, so here also, recovery is 
not complete, and the series of effects produced is therefore 
additive in both cases. With Biophytum, however, when 
the leaflet is depressed to the utmost, a limit is reached ; 
but in growth there is no such limit, and the summation 
of effects may go on indefinitely, until senility and death 
supervene. 



THE RECORD OF GROWTH-RESPONSE 419 

Yet there is a certain difference between these responses. 
In the case of Biophytum, the response is due to a sudden 
diminution, but in growth, to a sudden increase, of turgidity. 
This might at first sight appear anomalous, but I shall 
presently show that both are expressions of an excitatory 
reaction ; for we have seen that when the responsive organ 
of Biophytum is directly excited there is an expulsion of 
water, and the response is brought about by negative turgidity - 
variation. This variation we shall for the sake of con- 
venience distinguish as the direct effect of stimulation. We 
saw in the last chapter, however, that an increase of the 
internal energy of a plant gives rise to an opposite re- 
sponse, that is to say, one characteristic of positive turgidity - 
variation. The two responsive turgidity-variations, then, both 
negative and positive, are alike in being expressions of the 
excitatory reaction, though the negative variation is the effect 
of external stimulus applied directly to the responding organ, 
while the positive variation is to be regarded as the effect 
of the internal energy of the plant. This internal energy 
may itself have been derived previously by the plant from 
external sources of stimulation, or the internal energy of a 
given point may result from the application of a stimulus at a 
distance. The pumping-in of water by the stimulated root is 
an example of the latter case ; the cells are thus made tense, 
and the potential energy of the tissue is raised above par. 
Again, we may conceive of another interesting instance as 
follows. When stimulus is applied at a distance the excita- 
tory expulsion of water gives rise to a wave of increased 
turgidity, which produces an abnormal positive response, and 
this positive turgidity-variation we shall designate as the 
indirect effect of stimulation. The wave of true excitation 
may in this case reach the organ after that of positive 
turgidity-variation, and give rise to the direct effect of stimu- 
lation, that is to say, depression of the leaflet We may next 
imagine that the seat of stimulus is at so great a distance 
from the responding organ, that the transmitted excitation 
becomes too much enfeebled, by the long tract which has to 



420 PLANT RESPONSE 

be traversed, to produce the excitatory negative response. 
In this case, nevertheless, the sudden expulsion of water at 
a distance will give rise to a wave of increased turgidity, 
which will reach the responding organ, and produce there 
only that response which is characteristic of positive turgidity- 
variation. 

Positive response as indirect effect of excitation. 
Numerous experiments have been described exhibiting the 
negative turgidity-variation as the direct effect of stimulation. 
I have already described the production of positive turgidity- 




1' i<;. 1 70. Responses of Leaf c f - Irtocarpus to Thermal Stimulation 

Thick dots show points o! application of stimulation. In a stimulus 
was applied near the responsive pulvinoid and gave rise to normal 
response of fall here represented as up movement preceded by pre- 
liminary erectile twitch. In b stimulus was applied at a greater 
distance. The true excitatory effect did not reach the organ, and we 
obtain positive erectile response of positive turgidity-variation, here 
represented as down. 



variation as the indirect effect of stimulation (p. 400). A 
fuller demonstration of this, by the electrical method, will be 
found in Chapter XXX VI I. I shall here give an experiment 
which establishes the fact by means of mechanical response. 
In fig. 32 (reproduced in fig. 170, #) was shown a series of 
normal mechanical responses of negative turgidity-variation 
obtained from the leaf of Artocarpns. The conductivity 
of the petiole in this case is relatively feeble, and these 
normal responses, preceded by preliminary positive twitches, 
\\crc obtained when stimulus was applied at a distance ot 



THE RECORD OF GROWTH-RESPONSE 421 

3 mm. from the responsive pulvinoid. I then repeated the 
experiment with the same leaf, but applying stimulus at the 
greater distance of 5 mm. The responses now consisted of 
a series of up movements of the leaf, indicative of positive 
turgidity- variations (fig. 170, ), the direct effect of stimulus 
not reaching the organ. 

Now, turning our attention to the growing organ, we find 
that the fibro-vascular element, which possesses the power of 
conduction to a high degree, is not yet fully established in 
the zone of growth. If, then, contiguous to the growing 
zone there be a mass of active tissue thrown into a state of 
rhythmic excitation, it is to be expected that the indirect 
effect of such stimulation will alone act, and give rise in the 
region of growth to pulsations of increased turgidity. It will 
be remembered from the last chapter that water is conducted 
by preference along the fibro-vascular elements ; and since 
these strands end below the zone of growth, it is clear that 
there must be in this region an accumulation of water, and 
consequent over-turgidity of the tissue ; a condition which is, 
as we know, sufficient to initiate rhythmic excitation. 1 This 
region, then, acts like the actively excitable tissue of Colocasia^ 
which, as we saw, gives rise to spasmodic expulsions of water. 
In the latter case there is, however, a channel by which the 
water escapes, thus relieving the pressure on the tissue ; but 
the growing organ offers only a cul de sac, and the constant 
repetition of hydrostatic blows thus effects those positive 
turgidity- variations that are to result in the responsive 
elongations and incomplete recoveries of the tissue, bringing 
about growth-movements. 

* Inner stimuli.' It is thus seen that growth represents 
the indirect effect of stimulus ; its motive power residing 
in the rhythmic activity of the internal tissues of the plant. 
This rhythmic activity has been shown to be, in its turn, 
the result of the tonic condition of the plant, that is to say, 
of the sum total of energy previously absorbed, and held 

1 Or the over-turgidity of the growing region may be sufficient of itself to 
initiate rhythmic activity. 



422 PLANT RESPONSE 

latent in the tissue (p. 314), which we have designated as 
the internal energy. We have thus succeeded in defining 
the actual nature of those * inner stimuli ' to which the 
phenomenon of growth is usually vaguely ascribed. 

From what has been said, it is clear that the responsive 
peculiarities -of the growing region are not per se in any 
way different from those of any other excitable tissue, the 
apparent contrast between negative and positive turgidity- 
variations that is to say, between responsive contractions and 
responsive expansions having been shown to depend upon 
the fact that in one case we see the direct, and in the other 
the indirect, effects of excitation. If this be so, it follows that 
the direct application of external stimulus to a growing tissue 
ought to have the normal effect of excitatory contraction. 
In other words, while the action of the so-called ' inner 
stimuli,' or internal energy, gives rise, as explained above, to 
responsive expansions, the direct effect of external local 
stimulation must be the production of responsive contractions. 
That this is the case, will be shown in the next chapter. 

SUMMARY 

Growth is a phenomenon of multiple response. 

Each of these multiple growth-responses consists of a 
sudden elongation, due to a pulse of increased turgidity, 
followed by an incomplete recovery. The difference between 
elongation and recovery is the irreversible growth-effect. 

Such responses, when very rapid, appear as continuous. 

In ordinary excitatory response there is a pulse of 
diminished, and in growth-response a pulse of increased, 
turgidity. Both are, however, effects of excitatory reaction. 

When a tissue is locally excited, it gives a response of 
negative turgidity-variation, that is to say, of contraction. 
This is the direct effect of stimulus. 

When the source of stimulation is behind, and the inter- 
vening tissue does not conduct excitation, then the excitatory 
expulsion of water finds expression in a positive turgidity- 



THE RECORD OF GROWTH-RESPONSE 423 

variation, which produces an expansion of the responding 
zone of growth. The growth-response is thus the indirect 
effect of stimulation. 

The rhythmic activity of internal tissue supplies its motive 
power to the zone of growth. This activity, depending on 
the tonic condition of the plant, constitutes the 'inner 
stimuli ' to which growth is to be ascribed. 



CHAPTER XXXil 

THE EFFECTS ON GROWTH OF INTERNAL ENERGY 
AND EXTERNAL STIMULUS 

Characteristics common to growth and to other forms of rhythmic response : 
(i) Periodic groupings (2) Effect of external stimulus in renewal of growth 
when at temporary standstill (3) Renewal of *growth-pulsation by positive 
turgidity-variation (4) Effect of increased internal hydrostatic pressure 
(5) Effect of ascent of sap on growth Effect of temperature on growth 
Comparison of various types of multiple response Effect of external tension 
on growth Effect of direct application of stimulus on the growing region 
Similarities between motile and growth responses Direct and indirect 
effects of stimulus, and laws of growth. 

HAVING shown in the last chapter that growth is a form 
of multiple or rhythmic response, I shall now proceed to 
demonstrate in detail the fact that in it also are found various 
phenomena which are characteristic of rhythmic response in 
general. 

(i) Periodic groupings.- -In the multiple response of 
Biophytum> and in the autonomous response of Desmodium> 
we have noticed the occurrence of various periodic groupings, 
the simplest of which consisted of an alternate waxing and 
waning of the pulses. In multiple growth-responses, similarly, 
we are able to detect such groupings, of which the simplest 
was shown in fig. 169 (c). When a continuous series of 
records is taken, extending over some time, these groupings 
undergo various changes, as is illustrated in that figure, the 
three series (a), (), and (c) having been taken with the same 
plant at different intervals. It will there be seen that the 
pulse-records in (c) represent an alternate waxing and waning 
of amplitude ; that in those of () the responses are small at 
the beginning and large at the end ; and, finally, that in the 



INTERNAL ENERGY AND EXTERNAL STIMULUS 425 



responses of (a) this order is reversed, the amplitude being at 
first great, and undergoing a steady decrease to the end. 

(2) Effect of external stimulus on growth when at 
standstill It will be remembered that the rhythmic excita- 
tion of Desmodium comes to a standstill under unfavourable 
circumstances that is to say, when the sum total of internal 
energy has fallen below par ; and when this has happened, 
the application of fresh external stimulus is found to renew 
the activity. Growth-response, similarly, comes to a stop 
when the plant is in an unfavourable \ 

condition with regard to light, tem- 
perature, or moisture. I shall now 
show that under such circumstances 
the application of external stimulus 
is found to be competent to renew 
growth. Taking a specimen of the 
hypocotyl of Tamarindus indicus, in 
which growth had come to standstill, 
I stimulated the plant by thermal 
means, and this was found to renew 
the multiple response of growth, as 
is clearly seen in fig. 171. The pulses 
are here characterised by interesting 
periodic variations. The period of 
each is relatively long, the average 
value being about six minutes. In 
some other cases the renewed pulsa- 
tions were so rapid as almost to 
appear continuous. When the tonic condition of the plant 
was very low, the energy supplied by brief stimulation was 
only sufficient to maintain the rhythmic growth-activity 
for a short time, and after this the plant would again return 
to the state of standstill. It will thus be seen that when the 
tonic condition of the plant is below par, the applied external 
stimulus is absorbed, and, becoming latent, serves as internal 
energy for the production of growth-response (cf. p. 462). I 
shall adduce other instances of this in the course of the present 
chapter. 




FJ<;. 171. Renewal of 
Growth-pulsation by Ther- 
mal Stimulus in Tamarin- 
di<\ indie us originally at 
Standstill 



426 



PLANT RESPONSE 




FIG. 172. Initiation of 
Erectile Response in leaf 
by Supply of Water to 
partially Drought-rigored 
Mimosa 



(3) Renewal of growth by positive turgidity-varia- 
tion. We have seen that in Desmodium in a state of standstill, 
increased internal hydrostatic pressure renewed the rhythmic 

activity. It was also stated in the last 
chapter, that growth is a responsive 
expression of the positive turgidity- 
variation. We have seen further that 
the mechanical expression of the 
positive turgidity-variation in a dorsi- 
ventral organ takes the form of 
erectile response. Thus this erectile 
response and growth-elongation are 
to be regarded as two different forms 
of expression of the same internal 
activity. 

If we take for example a plant of Mimosa which is under- 
turgid, for want of sufficient supply of water, but not to the 
extent of drought-rigor, the leaves are found to assume a 

certain horizontal position, corre- 
sponding to the degree of tur- 
gidity. If we now supply the 
plant with water, poured on at 
the roots, the consequent sudden 
increase of suctional pumping 
activity is seen in the positiveerec- 
tile response of the leaf (fig. 172). 

,, , ... .. , ,, ., Similarly, when an ordinary 

IMG. 173. Initiation of Growth- J > y 

pulsation by Small Supply of plant, under the same circum- 

Water to Drought-rigored Seed- , . ., ,, , , 

ling of Cucurbit* stances, has its growth brought 

The first thick dot represents to a standstill, the growth-elonga- 
appiication of water, which t ion is found to be renewed on 

induces growth for three minutes 

only. A second application at the application of a fresh supply 
the second dot renews it for the of water This experiment was 

second time, I 

carried out on a seedling of 

Cucurhita 12 cm. in height, growing in a small pot, which 
had come to growth-standstill for reasons described. Two cc. 
of water was supplied to the dry soil about the roots, and 




INTERNAL ENERGY AND EXTERNAL STIMULUS 427 

growth-response was initiated after a latent period of eleven 
seconds (fig. 173). 

It will be remembered that the positive turgidity-variation, 
on which growth depends, is hydrostatically transmitted at a 
much quicker rate than the state of excitation itself. In the 
present case, the responsive elongation in the growing region at 
a distance of about 12 cm. took place eleven seconds after the 
application of water at the root. There was a certain loss of 
time before the cells in the growing region became of sufficient 
turgidity to initiate growth. The small supply of water which 
had been given was enough to maintain growth for three minutes 
only, after which the plant came again to a standstill. Another 
2 cc. of water was now applied, and the latent period was 
found reduced, as we should have expected, being now three 
seconds only. This was due to the fact that the cells had not 
now to absorb water before they could be sufficiently turgid. 
This renewed growth-activity was again, however, exhausted 
after about three minutes ; and it was very interesting to 
observe how the response of growth followed, for a little while, 
after each such doling out of water. 

(4) Effect of increased internal hydrostatic pressure. 
We found, in the case of Desmodium, when the cut petiole, carry- 
ing the motile leaflets, was subjected to increased hydrostatic 
pressure, applied by means of the U-tube, that the rhythmic 
activity of the leaflet, as shown by its quickening, was thereby 
increased (p. 320). We saw also in that case that this increased 
frequency, due to increased pressure, reached an optimum, 
and that beyond this, under excessive pressure, the pulsations 
became irregular, or even came to a stop. 

In order, then, to study the effect of hydrostatic pressure 
on growth, I mounted the specimens in this case an entire 
seedling of Balsam and a cut flower of Crinum in U-tubes 
supplied with water, and proceeded to take records, first 
under normal conditions i.e. the level of water in the two 
limbs being the same and then under a gradually increasing 
hydrostatic pressure. These records are made, it should be 
said, only when the rate of growth under the changed 



428 



PLANT RESPONSE 



conditions has become uniform. This occurs, generally 
speaking, after a period of variation which does not exceed 
two or three minutes. I give here a table embodying the 
results of the experiments on Balsam, and on Crinum> which 
show how increase of internal hydrostatic pressure increases 
the rate of growth up to an optimum, after which there is a 
diminution of growth. 

TABLE SHOWING EFFECT ON GROWTH OF INCREASED INTERNAL 

HYDROSTATIC PRESSURE 



Balsam seedling 


Crinum Lily 


Pressure 


Rate of growth 


Pressure 


Rate of growth 


Normal 
5 cm. 
10 cm. 
15 cm. 


0034 mm. per minute 
0036 ,, 
0060 ,, ,, 
0095 ,, ,, 


Normal 
10 cm. 
20 cm. 
30 cm. 


0038 mm. per minute 

OO44 >f 99 
0142 ,, ,, 
0036 


20 cm. 


0130 ,, ,, ,, 







The curve seen in fig. 174 exhibits graphically the rela- 
tion between these internal pressures and corresponding 
growths in the case of Balsam seedling. It will be seen 
that after a certain moderate rate of growth has been 
attained by increase of pressure, the curve becomes a straight 
line ; that is to say, after this point, equal variation of pres- 
sure produces equal variation in the rate of growth. But in 
the first part of the curve, where the rate of growth is feeble, 
an equal increase of pressure causes a disproportionately 
large increase in the rate of growth. This is still more 
strikingly shown when the growth, to begin with, is zero 
that is to say, at standstill; in such a case, -by gradually 
increasing the internal pressure, we arrive at a point where 
growth begins abruptly, after which increasing pressure 
causes an increasing rate of growth. But if the pressure be 
now brought back to a point just below that at which growth 
was initiated, it is found not to be arrested, but to persist. 
Thus the curve does not here return upon itself. 

Since the various growth-curvatures are brought about by 
the variations of internal hydrostatic pressure caused by the 



INTERNAL ENERGY AND EXTERNAL STIMULUS 429 




action of external stimulus, this quantitative demonstration 
of the effect of internal pressure on growth, is of much 
theoretical importance. 

I have already shown the connec- 
tion between mechanical response and 
growth-response, and demonstrated 
the fact that the erectile mechanical 
response and growth-elongation are 
but different expressions of increased 
internal activity. I shall now show 
how the suctional and growth re- 
sponses are related to each other, and 
in what manner the action of the 
former affects the latter. 

(5) Effect of ascent of sap on 
growth. We have already seen that 
the positive turgidity-variation on 
which growth depends is brought 
about, under normal conditions, by 
the ascent of sap. As regards the 
latter, we have seen that when the 
root is subjected to the stimulating 
action of warm water there is a 
sudden augmentation induced in the rate of suction. The 
application of cold water, on the other hand, induces the 
converse effect. The mechanical response of the plant to 
warm or cold water, applied at the base, was shown to be 
manifested in the erection or depression of the leaves of 
Mimosa or Biophytum (p. 400). 

These agents are seen in the following experiments to 
produce parallel effects on growth. A growing Crinum Lily 
was taken, and its normal rate of growth ascertained to be 
005 mm. per minute. Ice-cold water was now applied at its 
base, and this was found to cause an almost immediate arrest 
of growth. As the temperature, however, was gradually 
restored to that of the surroundings, the rate of growth was 
also slowly recovered. Five minutes after, the rate was only 



1C. 174. Curve showing 
Relation between Internal 
Hydrostatic Pressure and 
Rate of Growth (Crinum 
Lily) 



43O PLANT RESPONSE 

ooi mm. per minute, or one-fifth of the original rate. It was 
only after about half an hour that the original rate of growth 
was once more attained. 

I next applied warm water at the base, with the result 
that the rate of growth was almost instantaneously enhanced 
to '125 mm. per minute, or twenty-five times the normal! 
That this effect was not due to the rise of temperature as 
such, is shown by the fact that it was almost instantaneous, 
and that, moreover, as will be shown in the next chapter, the 
maximum rate of growth of Crinum at the optimum tem- 
perature is only about three or four times as great as the 
normal. 

From these experiments we see that the energy applied 
at the root is transmitted hydraulically to the growing region 
by the ascent of sap, where a certain amount of work is per- 
formed in causing an increase of turgidity, and thus producing 
in the cells a state of tension. Growth is now caused not 
simply by the presence of water, but rather by the energy 
conveyed by that water. It is well to bear in mind, at this 
point, that the mobility or plasticity of the responding grow- 
ing region is also an important factor in the production of 
growth ; for if the molecular mobility of the zone of growth 
be in any way reduced, the transmitted pressure, which was 
formerly effective, will now become ineffective to bring about 
growth. 

(6) Effect of temperature on growth. We have seen, 
in studying the pulsatory movements of Desmodium, that the 
rise of temperature, within certain moderate limits, increased 
the rhythmic activity of the plant, as shown in the increased 
frequency of pulsation. At a maximum temperature, again, 
above 40 C. these movements almost disappeared, there 
being now produced very rapid oscillations, of so small an 
amplitude as to be almost incapable of detection. This will 
be seen in fig. 175, where the normal pulsations, of a period 
of 2*5' at 30 C., are seen reduced to a period of only 10" at 
42 C. With this, the amplitude also is so far reduced as to 
be visible only on very careful inspection. We meet with 



INTERNAL ENERGY AND EXTERNAL STIMULUS 431 

corresponding phenomena in growth -response. It will be 
shown in the next chapter that the rate of growth increases 
with the rise of the temperature up to a certain optimum. 
At a determinate maximum, however, which is about 44 C, 
growth is arrested, but this arrest does not, as we have just 
seen in the corresponding instance of Desmodium, imply a 
total cessation of internal activity. In making experiments 
on a seedling of Balsam, I obtained the record shown in the 
upper part of fig. 176, the temperature being 34 C., which is 
below the optimum. The average period of a single pulse 




FIG. 175. Photographic Record showing the Slow Pulsations of Large 
' Amplitude of Desmodiutn Leaflet at 30 C. to become very much 
, Quickened and Reduced in Amplitude at 42 C. 



was in this case I2'5 seconds, and owing to the considerable 
amplitude of each pulsation, combined with its incomplete 
recovery, the average of the resultant rate of growth was as 
much as '074 mm. per minute. 

On now raising the temperature to 44 C. I obtained the 
lower of the two records in fig. 176, showing no resultant 
growth. It is interesting to observe the process by which 
the cessation of growth comes about in this case. For it is 
clearly seen from the record that there is no cessation of 
activity. On the contrary, we find that the frequency of 
oscillation has become increased from four pulsations to ten, 



432 



PLANT RESPONSE 




in the course of 50". The average period has thus fallen 
from 12-5" at 34 C. to 5" at 44 C. The amplitude of pul- 
sation at the same time is found to be decreased, and this, 

with the fact that recovery 
is now complete, accounts 
for the resultant cessation 
of growth. 

Comparison of vari- 
ous types of multiple 
response. At this point, 
it is" worth while to com- 
pare two or three types 
of multiple response. In 
Biophytum we have seen 
that, by reason of incom- 
plete recoveries from nega- 
tive turgidity- variation, 
the multiply-responding 
leaflet gradually becomes 
depressed below its ori- 

Upper reeoid shows slow puls.ilions with pinal level InCOntrasttO 

incomplete recoveries at 34 ( . Lower . 

record shows quickened pulsations with this we have ill growth- 
complete recovers at 44 c. The resp onse those incomplete 

magnification employed is 500 times. * * 

recoveries from positive 

turgidity-variations which have the effect of gradual elonga- 
tions In the pulsation of Desmodium again we have an 
intermediate instance where, response and recovery being 
equal, the responding organ is ultimately neither raised nor 
depressed. It is interesting to note, therefore, that in raising 
the temperature of a growing organ to the maximum, and 
thus abolishing the resultant elongation, we bring on a con- 
dition of equality of response and recovery which in so far 
resembles the pulsation of Desmodium. 

Effect of external tension on growth. Great advan- 
tages are afforded by the method of magnified record, which 
enables us to detect instantly the immediate and after effects 
on the specimen of any changes of external conditions. It is 



IMC. 176. Growlh-pul.sation seen in 
Seedling of 



INTERNAL ENERGY AND EXTERNAL STIMULUS 433 

thus easy to obtain exact records of the effect of tension on 
growth. The normal record is first taken, with the very 
slight tension exerted by the recording lever itself. The 
short arm of the lever, *5 cm. in length, is, it should be 
remembered, attached to the growing organ. A rider, half a 
gramme in weight, can be placed on the longer arm of the 
lever, at distances of *5, i, i'5, 2, 2-5, or 3 cm. from the ful- 
crum. The effective tension may thus be gradually in- 
creased, and the corresponding effects on growth recorded. 
It may be stated here that, generally speaking, any sudden 
change of external conditions, such as sudden cooling, 
sudden warming, or sudden variation of tension, acts on the 
organ as an external stimulus ; and I shall presently show 
that an external stimulus always induces a contraction or 
retardation of growth. When the organ is 'subjected to 
sudden increase of tension, the preliminary effect of con- 
traction occurs therefore, as we should expect. But after 
this temporary disturbance has disappeared we are able to 
observe the permanent effect of increase of tension on growth. 
For these experiments I took different specimens of Crinum 
Lily, and the results obtained show that increase of tension 
enhances the rate of growth. This increase, however, appears 
to reach a limit at a certain optimum point, beyond which 
increase of tension would seem to retard growth. The follow- 
ing table exhibits the results of two experiments on different 
specimens of Crinum. 

TABLE SHOWING EFFECT OF TENSION ON RATE OF GROWTH 



Specimen A 


Specimen B 


Tension 


Rate of growth 


Tension 


Rate of growth 




i 








mm. - 





mm. 


Weight of lever alone 


0040 per min. 


Weight of lever 


0036 per min. 


I gr. -h lever 


ono 


'Sgr- + 


0070 


2 gr. + 


0140 


I gr. + 


oioo 


3g r - + 


0045 


i '5 g r - + 


0130 






2 gr. + 


0170 




|i 2'5 g r - + 


0050 



F F 



434 PLANT RESPONSE 

Effect of direct application of stimulus on the grow- 
ing region. Many of the phenomena of growth-curvature 
are brought about, as we shall see, by means of the changes 
induced by external stimulus in the rate of growth, and there 
is much misconception as to whether the effect of stimulus is 
to enhance or to retard growth. I shall be able to show that 
this misconception is the result of the complexity of the 
problem, depending (i) on the tonic condition of the tissue, 
(2) on whether the stimulus is internal or external, and (3) 
on the point of application of stimulus. The fundamental 
effect of stimulus is, however, very definite. 

But at the beginning of our investigation we are met by a 
question of great importance, namely, as to whether the effect 

of stimulus on a growing, is in any 
way different from that on a stationary, 
organ. We have seen, for example, 
that a fully grown style of Datura, in 
which growth has come to a stop, 
FIG. 177. Photographic exhibits on the application of local 
Record of Responses of stimulus the usual contractile response 

Mature Style of Datura L 

alba to External Thermal (fig. I 77). On the completion of re- 

Slimulus covery again, the organ returns to its 

original length. Hence the base-line of a series of these 
responses is horizontal. 

I shall now pass, by means of intermediate links, from 
this to the response of growing organs. And first 1 shall take 
the response of a style of Datura, in which, for want of a 
sufficient supply of internal energy, growth has come to a 
temporary stop. On applying thermal stimulus, at intervals 
of a minute, the first five responses are seen to be practically 
like those of the stationary style of Datura (fig. 178, cf. 177). 
But a portion of the stimulus applied is being absorbed and 
held latent in the organ, thus increasing the internal energy, 
or tonic condition. The result of this is seen in the renewal 
of growth at the sixth response. The stimulus now, therefore, 
finds bifurcated expression in maintaining response, and in 
renewing growth, as is seen by the trend downwards of the 




INTERNAL ENERGY AND EXTERNAL STIMULUS 435 



hitherto horizontal base-line. This bifurcation causes the 
first contractile response of the now growing organ that is 
the sixth response of the record to be smaller than usual. 
But as the tonic condition is established, and the molecular 
mobility of the responding organ is increased, the contractile 
response becomes larger, and growth goes on at a certain 
steady rate. From this intermediate link we pass on to the 
case of response to stimulus 
of a style of Datura which is 
in a state of uniform growth 
(fig. 179). Here also we find 
that stimulus produces the 
normal contractile effect. All 
these clearly demonstrate that 
the response of growing 
organs is in no way different 
from that of stationary organs. 
In a stationary organ, sti- 
mulation produces negative 
turgidity-variation, resulting 
in contraction. The same 
contraction is seen in growing 
organs, causing a temporary 
retardation of the rate of 
growth. 

The retardation of growth 
which is caused in a growing 
organ by external stimulus 
may be exhibited in a some- 
what different way. We may use the Method of Balanced 
Record, by which the normal rate of growth is made to appear as 
a neutral horizontal line. For the purpose of this experiment 
I took the growing peduncle of a Eucharis Lily. The balanced 
record (fig. 180) is here seen to be horizontal, save for 
minute autonomous oscillations about the neutral line. Sti- 
mulation, in this case, was produced by tetanic electric shocks 
from an induction coil, which were applied on the growing 




FIG. 178. Photographic Record of 
Responses of Style of Dattira alba, 
in which Growth had come to a 
Temporary Slop 

The up curve shows contraction. As 
long as the base-line is horizontal, 
growth is seen to be at standstill. 
Renewal of growth at sixth re- 
sponse, after which growth-elon- 
gation is shown by the trend of 
the base-line downwards. 



F F 2 



436 



PLANT RESPONSE 



organ, by means of non-polarisable electrodes, connected, 
one a little above, and the other a little below, the growing 
region. According to the conditions of the experiment, the 
balanced horizontal base-line will represent uniform growth 

under normal turgidity. An 
up curve will here represent a 
rate of growth below the normal, 
or a retardation ; a down curve, 
on the contrary, will represent 
a rate of growth above the 
normal, or an acceleration. 
When stimulus is applied a 
responsive negative turgidity- 
variation is induced, in conse- 
quence of which there is a 
temporary disturbance of the 
balanced growth-record. The 
curve thus produced exhibits 
the effect of stimulus, and recovery from that effect. It will 
be observed from the figure that on the application of tetanic 
electric shocks for two seconds, a responsive retardation of 




KIG. 179. Photographic Record of 
Response of Growing Style 
of Dattira alba to External 
Stimulus 




FIG. 1 80. Balanced Record of Response in Growing Peduncle of 
Eucharis Lily to Electrical Stimulation 

Up curve here represents retardation of growth. First response is to 
stimulus of two seconds', the second to stimulus of three seconds', 
duration. 



growth was induced, as seen in the up curve, and that there 
was a recovery after an interval of nine minutes from the time 
of application of stimulus. It will also be noticed that 
during the process of recovery in this particular case, the rate 



INTERNAL ENERGY AND EXTERNAL STIMULUS 437 

of growth is above the normal, which is due to the fact that 
the internal energy has been augmented by the absorption of 
the stimulus applied. 1 A stimulus of longer duration, that is 
to say of three seconds, was next applied, and the responsive 
retardation had now a greater amplitude than before, the 
period of restoration being also longer, that is to say, sixteen 
minutes. 

Similarities between motile and growth responses. 
We have, then, in growth- response an exact parallel to the 
mechanical responses given by pulvinated or anisotropic 
organs. The following tabular statement will show the 
reason of this fundamental parallelism between responses 
whose modes of indication are so widely different : 

TABULAR STATEMENT SHOWING COMPARATIVE EFFECTS OF STIMULUS IN 
PULVINATED AND GROWING ORGANS 



Mechanical response 



Effect of normal turgidity : 

Normal horizontal position of leaf. 
Local action of external stimulus : 



Growth response 



Contraction ; Contraction ; 



Effect of normal turgidity : 
Uniform rate of growth. 
Local act, on of external stimulus : 



Diminution of turgidity ; 

and concomitant depression of leaf. 

Action of internal energy exhibited by 
(a) Recovery : 

Re-establishment of turgidity and 
gradual return of leaf to normal 
horizontal position. 
(b) Increased hydrostatic pressure : 
Erection of leaf. 



Diminution of turgidity ; 

and concomitant depression of rate 

of growth. 

Action of internal energy exhibited by 
(a) Recovery : 

Re-establishment of turgidity and 
gradual return of organ to normal 
rate of growth. 

() Increased hydrostatic pressure : 
Increased rate of growth. 



In studying the mechanical response of plants we found 
that direct application of stimulus to the responding organ 
always produced a response characterised by the negative 
turgidity-variation, that is to say, a depression of the leaf. 
We also saw that internal energy, inducing positive turgidity- 
variation, caused the opposite response, that is to say, an 
erection of the leaf ; and that an increase of the internal energy 

1 Other records showing the effect of external stimulus on growth will be found 
in Chapter XXXIV. 



43$ PLANT RESPONSE 

of the organ was, in some cases, brought about by that in- 
creased suctional activity by which energy was transmitted, 
by means of water forced into the responding organ. This 
increased suctional activity was due in its turn to external 
stimulation of the roots. Or a similar transmission of energy 
to the responding organ might be the result of stimulation 
applied at a distance on the stem, in consequence of which a 
wave of positive turgidity-variation would .travel towards the 
responding organ from the excited point. For the exhibition 
of this latter effect, however, it is necessary that the direct 
excitatory effect of stimulation should not be conducted to 
the organ. This condition is met if the point of stimulation 
be at a sufficient distance, or when the intervening tissue is 
not a good conductor of excitation. Again, the plant as a 
whole, it must be remembered, has its internal energy raised, 
as the after-effect of the absorption of stimulus from its sur- 
roundings. 

Direct and indirect effects of stimulus and laws of 
growth. In the phenomenon of growth-response we have 
a case which is exactly parallel. Direct stimulation of a 
growing organ always induces a negative turgidity-variation, 
with a concomitant responsive retardation of growth. Any- 
thing, on the other hand, which increases the internal energy, 
brings about the opposite effect, of positive turgidity-variation, 
with concomitant responsive increase in the rate of growth. 
This may be induced by a favourable rise of temperature, 
or by stimulating the root, and so increasing the ascent of 
sap. It may also be brought about by stimulating a distant 
point, and so causing a wave of positive turgidity-variation 
to be transmitted to the responding organ, the stimulated 
point being at a sufficient distance to prevent the direct effect 
of stimulus from reaching it. And, finally, the energy ab- 
sorbed from external stimulus may, as an after-effect, increase 
the internal energy of the plant. The increase of internal 
energy, under all these different conditions, we shall for the 
sake of convenience designate as the INDIRECT EFFECT OF 
STIMULUS. 



INTERNAL ENERGY AND EXTERNAL STIMULUS 439 

The laws of growth are therefore : 

(1) The response of a growing organ is the same as that of a 
stationary organ. Direct application of stimulus, inducing con- 
traction, retards the rate of growth. 

(2) The effect of indirect stimulation is to increase the internal 
energy, and thus augment the rate of growth. 



SUMMARY 

The multiple response of growth is characterised by all 
the peculiarities seen, for instance, in the autonomous response 
of Desmodium. 

The rhythmic responses of growth exhibit periodic 
groupings. 

External stimulus is found to renew growth in organs in 
which, owing to the deficit of internal energy, it had come to 
a temporary standstill. 

The increase of internal hydrostatic pressure, up to an 
optimum, increases the rate of growth. 

The effect of increased internal hydrostatic pressure is 
exhibited in the case of the leaf of Mimosa by an erectile 
response. In a growing organ the effect of increased turgidity 
is shown by growth-elongation. A drought-rigored Mimosa 
on being supplied with water responds by erection of the 
leaf ; and similarly, a plant in which growth, owing to drought- 
rigor, has come to a standstill, responds, on being supplied 
with water, by renewed growth-eloftgation. When the root 
of Mimosa is supplied with ice-cold water, the responding 
leaf, owing to the consequent arrest of ascent of sap, becomes 
depressed. The increased suctional activity, again, which is 
caused by a supply of warm water to the root, induces an 
erectile response of the leaf. Similarly, corresponding varia- 
tions in the rate of ascent of sap, brought about in a growing 
plant by the application of cold and warm water to the roots, 
cause respective depressions and accelerations of the rate of 
growth. * 

The energy supplied at the roots is hydraulically trans- 



440 PLANT RESPONSE 

mitted to the growing region, and finds expression in the 
work of growth. 

A rise of temperature, up to the optimum, enhances the 
rate of growth. At a maximum temperature of about 44 C. 
growth is apparently arrested. This is not, however, due to 
any rigor or arrest of internal activity, but to the fact that in 
each pulsation of growth the constituent response and recovery 
are now equal. There is thus no resultant growth-elongation. 
At such a temperature the amplitude of pulsation is re- 
duced, and the frequency increased, as in Desmodium. 

Longitudinal tension has the effect, up to an optimum, of 
increasing the rate of growth. 

The effect of external stimulus on a growing, is precisely 
the same as on a stationary, organ, that is to say, a respon- 
sive contraction. On account of this contraction, and con- 
comitant negative turgidity-variation, growth is retarded, as 
the direct effect of the action of external stimulus. 

External stimulus, however, when absorbed and held 
latent by the tissue, has the effect of increasing the internal 
energy of the plant. This indirect effect of stimulus causes 
acceleration of growth. 



CHAPTER XXXIII 

ON THE RELATION BETWEEN TEMPERATURE AND GROWTH, 
AND THE ACCURATE DETERMINATION OF OPTIMUM 
AND MAXIMUM POINTS 

General consideration of difficulties of accurate determination of effects of 
temperature on growth Four accurate methods: (i) Method of discon- 
tinuous observations Accurate regulation of temperature by electrolytic 
rheostat (2) Method of continuous observations Thermo-crescent curve 
Determination of the optimum point (3) Method of balance (4) Method of 
excitatory response Translocation of the optimum point. 

DETERMINATIONS of the effect of temperature on growth are 
usually carried out either by observing a single plant for 
several hours in succession, or by obtaining the average 
growth of various groups of plants, kept under different 
temperatures. In the latter case the individual peculiarities 
of different specimens cause them to give values which are 
more or less discrepant, but this fact is to some extent 
neutralised by taking the average of a large number. These 
methods, however, are all very laborious, and the results not 
highly consistent. Whichever method be adopted, we have 
to remember that in all cases in which observations stretching 
over periods of several hours are required, the growth of the 
plant will be liable to spontaneous variations, resulting from 
the periodicities impressed upon it by the changing conditions 
of its environment. It is, perhaps, owing to this fact that the 
results obtained by various authorities have been so widely 
divergent. For example, the optimum temperature for 
growth of Zea mats was found by Sachs to be 34 C. and by 
Koppen to be 30-2 C. 

Clearly, the perfect method would be one in which we 
should be able to measure the effects of temperature alone on 



442 PLANT RESPONSE 

the growth of a particular plant, so that the varying factors 
of age and constitution, or tonic condition, should remain 
constant. And, further, it should be possible to carry out 
this determination of growth at different temperatures in a 
time so short that the spontaneous variations of the plant, 
if any, would be insignificant. Only by such a method 
could we hope to obtain results which would be reliable and 
consistent. 

Keeping these considerations in view, I have been fortunate 
enough to be able to devise four distinct methods of determin- 
ing the effect of temperature on the rate of growth, the 
perfection of which may be gauged from the fact that the 
results of all agree with and corroborate each other .within a 
fraction of a degree. With some of these, the entire experi- 
ment on the different rates of growth at various temperatures, 
ranging from ordinary through optimum to maximum, can 
be carried out within about half an hour. The determination 
of any single cardinal point, such as the optimum, can always 
again be made within five minutes. The investigation thus 
gains by simplicity and experimental accuracy, and observa- 
tions may be made on many different specimens within a 
very short period. I shall now proceed to describe these 
different methods. 

(i) The method of discontinuous observations. We 
shall first take the method by which rates of growth are 
recorded at different temperatures, say one degree apart. 
Some means of raising the temperature to exactly the 
required point, and maintaining it unchanged during the 
time of experiment, is an essential condition of all these 
investigations. This I have been able to accomplish in the 
following way. The electrical heating coil inside the plant 
chamber is put in connection with an external battery. The 
heat given out, and the consequent permanent rise of 
temperature in the chamber, depend on the intensity of the 
current that flows through the heating coil. This current 
again may be progressively regulated by the interposition of 
an electrolytic rheostat in the circuit, the resistance of which 



RELATION BETWEEN TEMPERATURE AND GROWTH 443 

can be subjected to gradual variation. The electrolytic 
rheostat consists of two semicircular troughs (fig. 181). By 
turning the handle, say to the right, the electrolytic resistance 
interposed is continuously increased, diminishing the current, 
and hence diminishing the temperature inside the chamber. 
Rotation of the handle in the opposite direction produces the 
opposite effect, that is to say, raises the temperature inside 
the chamber. Thus, by proper manipulation of the handle of 
the rheostat, the chamber can be raised to any temperature, 




FIG. 181. Semicircular Electrolytic Rheostat inteiposed in Heating Coil 

Current enters the first trough, filled with zinc sulphate solution, by the 
electrode, z', and is led to the second trough by the diagonal metallic 
connector, D lV. By turning the index-arm, I, clockwise, the interposed 
resistance is increased, and the heating current thus diminished. Rota- 
tion in the opposite direction diminishes the resistance and increases 
the heating current. 



which can then be maintained uniform for any length of time, 
by keeping the rheostatic resistance constant. From a 
previous experiment, the temperature-values of the position 
of the index in connection with the handle can be ascertained 
and marked. Thus by turning the handle to any given index 
number, say of 31 C, the temperature of the chamber will be 
found to be raised permanently to that value. A delicate 
thermometer graduated in twentieths of degrees is placed in 
the chamber, and affords an independent indication of the 



444 PLANT RESPONSE 

temperature thus attained. It is also necessary, for reasons 
to be fully explained in a subsequent chapter, that the 
specimen should not receive thermal radiation from the 
heating coil, as such radiation, I find, retards growth. A 
shield of mica, opaque to thermal radiation, is interposed 
between the heating coil and the plant, which is thus subjected 
only to the action of changes of temperature. For many of 
my experiments I selected specimens of the Crinum Lily, on 
account of its extreme regularity of growth, which is so 
uniform that on adjusting the record under balance, the 
external conditions being constant, the line of record remained 
horizontal for a period of. certainly over an hour. 

The rate of growth at the temperature of the room, say 
30 C, is first taken on the recording drum, which is covered 
with paper divided into millimetres. The horizontal distance 
or abscissa represents time, which, with the particular speed 
of drum which I used was 6 mm. per minute. The ordinate 
represents growth-elongation, and as the growth-recorder, or 
crescograph, produces a magnification of 1,000 times, I mm. 
distance of the ordinate is equal to an actual growth of 
*ooi mm. The ordinate corresponding to an abscissa of 
6 mm. would thus be equivalent to a growth in thousandths 
of a millimetre per minute. 

The rheostat handle is now turned to the index-number 
corresponding to the raising of the temperature of the chamber 
by i C. There is first a variable period of rise of temperature, 
after which a permanent degree is attained. During this 
preliminary stage, variation of temperature acts as a stimulus, 
giving rise to responsive contraction or retardation of growth. 
But after this transient disturbance, the growth attains a 
constant rate, characteristic of the given temperature. These 
peculiarities will be better understood on following the record 
given in fig. 182 reduced here to half the original size 
which was taken with a specimen of Crinum Lily, during ten 
minutes. The record during the first five minutes is for the 
temperature of 34 C. It will be seen that in two minutes 
the growth-elongation is fifteen divisions, and as each division 



RELATION BETWEEN TEMPERATURE AND GROWTH 445 



represents *ooi mm., the rate of growth is thus '0075 mm - 
per minute. The rheostatic handle was afterwards turned to 
the mark 35 C. With the particular battery power used in 
this case, the permanent rate of rise to 35 C. was attained 
after a period of three minutes. It will be seen from the record 
that the stimulus of sudden variation of temperature caused 
a contractile twitch, after which 
growth proceeded at a very 
rapid rate during the variable 
period. But as soon as the 
temperature of the chamber had 
attained a permanent condition 
- i.e. 35 C. the rate of growth 
became constant The attain- 
ment of this constant rate was 
practically simultaneous with 
the attainment of the permanent 
temperature condition. The 
lag, in any case, if it existed, 
could not be more than fifteen 
seconds. 

One curious and interesting 
fact to be fully explained later, 
which was noticed in the course 
of the experiments, was that 
the amount of contractile twitch 
went on increasing during the 
variable period, as the tempera- 
ture was raised each time i C., 
from 30 to 35 C., but after 

this point practically disappeared. The permanent rate of 
growth, then, at a temperature of 35 C., is, as will be 
seen from the figure, twenty-four divisions per two minutes, 
or "OI2 mm. per minute. In this way, by taking successive 
records at different temperatures, I obtained the following 
rates of growth, in the cases of Crinum Lily and the peduncle 
of Crocus. 




FIG. 182. Record of Grouth in 
Crinum at Temperature of 34 C. 
and 35 C. 

The dotted line represents the 
variable period of temperature 
change. Note the contractile 
twitch and transient highly ac- 
celerated growth which follows. 
The rate of growth became con- 
stant when the temperature be- 
came permanent at 35 C. 



446 



PLANT RESPONSE 



Crinum Lily 


Peduncle of Crocus 


Temperature 


Growth in mm. per minute 


Temperature 


Growth in mm. per minute 


30 c. 


0036 


30 c. 


0059 


31 c. 


0040 


33 C. 


oioo 


32 C. 


OO5O 


35 C. 


0145 


33 C. 


0055 


36 c. 


0062 


34 C. 


0075 


38 C. 


0030 


35 C. 

36 c. 


0085 

0050 


40 C. 


ooio 


37 C. 


0035 







(2) Method of continuous observations. The method 
which I have just described gives us results which, though 
obtained at closely consecutive temperatures, are nevertheless 
discontinuous. There is, besides, some loss of time involved 
during the variable period, in addition to which there is the 
factor of transient stimulation during sudden changes of 
temperature. For these reasons I was anxious to perfect 
some method by which the curve of growth should afford a 
continuous means of obtaining the rate of growth at all 
temperatures. I also wished to eliminate from this record 
the preliminary disturbance caused by sudden change of 
temperature. 

I was enabled to do this in practice by bringing about a 
gradual and continuous rise of temperature, instead of the 
former sudden variations by steps. This was effected by 
turning the handle of the electrolytic rheostat at a rate so 
graduated that the rise of temperature within the chamber 
was uniform. As it was necessary to complete the experiment 
within not too long a period, I found that a rise of i C. per 
2'5 minutes was sufficient to meet the requirements of the case. 
This means a rise of *i C. in fifteen seconds. An observer 
watches a delicate thermometer, which is placed in the plant 
chamber, with his hand on the handle of the rheostat. By 
means of this, and a chronometer beating seconds, he is able 
to regulate the uniform rise of temperature with the greatest 
nicety. Should the rate be too quick, it may be reduced by 
the slightest turn of the handle towards the increase of 



RELATION BETWEEN TEMPERATURE AND GROWTH 447 

resistance, or vice versa. After a little practice the process of 
regulation becomes almost instinctive. The record of growth is 
now taken continuously on the revolving drum, and the thermo- 
crescent curve obtained under these conditions of continuous 
variation of temperature is seen to be extremely rcgular,giving 
data by which we may determine the rate of growth at any 
point of the curve (fig. 183). The revolving drum gave, as 
already said, a movement of the recording surface of 6 mm. per 



30" 31 32" 33' 34' 35 36 37* 38* 39 40 41 42 43 44 



FIG. 183. Thermo-crescent Curve of Growth in Crinum Lily 
under Continuously Increasing Temperature 

minute. That length of the abscissa would therefore repre- 
sent one minute of time ; and since the rise of temperature 
was regulated, at i C. of rise per 2-5 minutes, intervals of 
15 mm. in the abscissa also represent i C. in temperature. 
With the magnification used, i mm. of the ordinate represents 
a growth-elongation of a thousandth part of a mm. In order, 
therefore, to obtain from this curve the rate of growth at any 
given temperature, say at 34 C., we have to find the elonga- 



448 



PLANT RESPONSE 



tion per minute at that point. The rise of temperature in 
one minute, then, under the experimental conditions described, 
is through -4 C. The growth-elongation .of the specimen, 
therefore, while the temperature is rising from 33*8 C. to 
34'2 C., gives us the rate of growth for the mean temperature 
of 34 C. This is found in the magnified record to be 
10 mm. The absolute value of the rate of growth is thus 
01 mm. per minute. In this way we can determine from the 
curve the rate of growth corresponding to any temperature. 
It will thus be seen how in the course of an experiment 
lasting for thirty-five minutes only, we are able to obtain 
data which give us the various rates of growth through a 
wide range of temperatures. 

This operation can, moreover, be made entirely automatic. 
The breadth of the circular electrolytic trough may be appro- 
priately varied at different parts of the circle, so that turning 
the handle through equal arcs raises the temperature of the 
plant chamber by equal degrees. The handle of the rheostat 
may then be rotated by the recording drum itself. Hence 
in the record, equal lengths of the abscissa will represent not 
only equal times, but also equal rises of temperature. And 
finally by taking the record photographically, the whole pro- 
cess becomes automatic. From the data furnished by fig. 1 84 
we obtain the following table : 

TABLE SHOWING RATES OF GROWTH AT DIFFERENT TEMPERATURES IN 

FLOWER OF CRINUM LILY. 







i 




Temperature 


Growth in mm. per minute 


* Temperature 


Growth in mm. per minute 


30 c. 


0040 


I 36 c. 


0070 


31 c. 


0057 


37 C. 


0045 


32 c. 


0075 


1 38 C. 


0030 


33 C, 


0087 


< 39 C. 


002O 


34 C. 


oioo 


1 40 C. 


OOI2 


35 C. 


OIIO 


I A<* C 

42 L,. 


0005 


35*5 C- 


0X13 


A<J r* 
i 43 <-" 


0002 



The curve shown in fig. 1 84 exhibits the relation between 
these various temperatures and their corresponding rates of 
growth. It is here seen that the rate gradually rises till we 



RELATION BETWEEN TEMPERATURE AND GROWTH 449 

approach the optimum point, which in the present case is 
35*5 C. After this there is a steep fall, and growth is almost 
abolished at a maximum temperature at or near 45 C. This 
arrest of growth does not mean arrest of internal activity. I 
have in the last chapter (fig. 1 76) explained why, in spite of 
the persistence of internal activity, there is in this case no 
resultant growth. It is to be borne in mind that as the curve 
of growth in fig. 183 is continuous, the rate of growth at 




FIG. 184. Curve showing Relation between Temperature and Rate of 
Growth, as deduced from the Thermo-crescent Curve in fig. 181 

points intermediate to those specified may be determined 
from it. 

From a large number of experiments which I have carried 
out on Crinum Lily, I find that the optimum temperature is 
very constant, not varying by as much as one-tenth of a 
degree from the mean value of 35'5 C. as the optimum tem- 
perature. In the first portion of the curve, as the temperature 
rises from 30 C. to 35*5 C. the rate of growth is seen to 
increase, from -004 to -01125 mm. per minute, or to nearly 
three times its first value. The fall, beyond the optimum, is 
steeper than this rise. At 37 C. the rate of growth has 

G G 



450 PLANT RESPONSE 

fallen to '0045 mm, per minute, or almost the same as at 
30 C. Thus, while 5*5 C. of rise of temperature before the 
optimum enhanced the rate of growth by three times, a 
further rise of only 1*5 C. beyond that point was sufficient to 
bring it back to almost the same value as at 30 C. The 
individual characteristics of each specimen are seen, not by 
any perceptible variation of the optimum point, but rather by 
differences in the steepness, during rise or fall, of the curve. 
With some specimens, for example, the increase of rate of 
growth during an equal rise of temperature from 30 C. to 
3 5 '5 C. is only half of that seen in the figure. The steepness 
of fall, on the other hand, beyond the optimum may be 
much greater ; that is to say, a rise of i C. or less above 
the optimum will sometimes reduce the rate of growth to its 
value at 30 C. 

(3) The Method of Balance. I shall now describe an 
extremely delicate method of determining the rate of growth 
at different temperatures, which is especially suited for the 
exact determination of the optimum point. A balanced line 
of record is first obtained by the turning of the balancing 
wheel of the Crescograph (fig. 168). This regulates the 
difference of level of the syphon tube, until the spot of light 
is stationary at the .temperature of the room. As the tem- 
perature is now raised and the rate of growth increased, the 
balancing wheel has to be rotated, say to the right, in order 
to keep the spot of light stationary. The reading of the 
circular scale at different temperatures thus gives the balanced 
readings for the corresponding rates of growth at those 
temperatures. A previous calibration of the value of the 
circular scale enables us to determine the absolute growth- 
movements at various temperatures. For the determina- 
tion of the optimum point, however, this is not necessary. 
All that has to be done in this case is to keep the spot, 
which would otherwise drift to the right under a constantly 
increasing rate of growth, on the point of balance, by the 
right-handed rotation of the balancing wheel. This must be 
done as long as the temperature and rate of growth are 



RELATION BETWEEN TEMPERATURE AND GROWTH 451 

ascending towards the optimum. On reaching and passing 
that point, however, it is found that the spot of light, which 
has hitherto tended to move to the right, now has its move- 
ment suddenly reversed to the left, thus necessitating a 
corresponding reversal of the balancing rotation. This 
turning point is extremely sharp and well defined, and enables 
us to make an accurate determination of the optimum tem- 
perature within less than a tenth of a degree. From a 
previous knowledge that the optimum point lies, say, between 
35 C. and 36 C., the rise of temperature from 35 C. to 36 C. 
within the chamber may be adjusted to take place in five 
minutes, that is to say a rise of one-twentieth of a degree per 
fifteen seconds. The second observer, watching the delicate 
thermometer in the plant chamber, calls out at every twentieth 
of a degree of rise of temperature. The first observer, at the 
recording drum, notes the temperature of the turning point. 
It has been said before that the permanent rate of growth 
for any given temperature is always established in less than 
fifteen seconds of reaching it. The possible error, owing to 
this lag, could not therefore exceed one-twentieth of a degree. 

TABLE SHOWING CIRCULAR READINGS OF BALANCING WHEEL AT 
DIFFERENT TEMPERATURES. 



Crinutn Lily 



Temperature 


Circular reading 


30 c. 







31 c. 


70 




32 c. 


171 




33 C. 


270 




34 C. 


405 




35 C. 


538 




35*4 ^* 


600 




36 C. 


494 




37 C. 


354 




38 C. 


140 




The turning point of another specimen 
found to be 35*5 C. 


was 





Hypocotyl of Balsam 


>g 


Temperature 


Circular reading 




30 C. o 




31 C. 12 




32 C. 


42 




33 C. 


86 




34 C. 


146 




34/6 C. 


170 




35 C. 


112 




36 C. 


85 




37 C. 


37 




38 C. 


5 D 


vas 


The turning point of another specimen was 
found to be 34*4 C. 



I give above two sets of readings of the balancing wheel, 
made during two experiments for the determination of the 



G G 2 



452 PLANT RESPONSE 

optimum point for Crinum Lily and the hypocotyl ot Balsam 
The balanced reading at 30 C. is taken as zero. The ad- 
justment of the stop-cocks for regulation of outflow was 
different in the two cases. 

(4) The method of excitatory response. The method 
which I am about to describe and by which the relative 
rates of growth at different temperatures are afforded in- 
directly is one of much theoretical importance, for it proves 
what I have already suggested, that growth is a phenomenon 
of excitatory response. This being so, it would follow that 
the reason why growth is at its optimum at about 35 C. in 
the case of most tropical plants, is that the excitability of 
the tissue is greatest at that temperature. The different 
excitabilities at different temperatures might further be 
expected, this being true, to offer an independent indication 
of the characteristic rate of growth of the tissue at those 
temperatures. 

The excitability of the tissue can be tested, in the case 
of radial organs, by its longtitudinal contractile response to 
external stimulus, which, as we have seen, will be represented 
in growing organs by a retardation of growth, proportionate 
to the excitability. We must bear in mind, at this point, 
certain differences between responsive effects in mature and 
in growing organs. In the former, owing to the increase of 
internal energy brought about by rise of temperature, the 
tissue becomes over-turgid and the internal hydrostatic 
pressure is greatly increased. The contractile action of 
external stimulus is thus strongly resisted by the tissue, 
which in this way antagonises the normal extent of response 
(p. 338). Similarly, a closed india-rubber ball, fully distended 
with water, will not yield to any great extent when struck. 
But if we have, instead, a tube through which water is 
running, the flexible pipe when struck will yield, and cause a 
proportionate retardation or reversal of current behind. We 
have a case somewhat analogous in a growing organ. For 
here the tissue cannot be regarded as closed, since it is 
constantly elongating. It therefore represents, not a static 



RELATION BETWEEN TEMPERATURE AND GROWTH 453 

condition of rest, but a dynamic condition of equilibrium, 
and it will offer little effective resistance to excitatory con- 
traction. We shall therefore expect that in growing organs 
similar stimuli will induce responsive effects varying in pro- 
portion to the changes of excitability in the tissue, under 
different condition^. 

We saw, in the case of Crinum Lily, that the optimum 
temperature was near 35 C, and that at this optimum the 
rate of growth was something like one and a half to three 
times as great as at 30 C. At 37 C. we saw, further, that 
the rate of growth was again reduced, and had become equal 
to, or lower than, that at 30 C. We might therefore expect 
that, on recording the retardation of growth in response to 
external stimulus at three definite temperatures, say 30 C., 
35 C., and 37 C., we should find it to be greatest at 35 C. 
being in fact at that point about one and a half to three 
times as great as at 30 C. The response at 37 C., on the 
other hand, which is beyond the optimum, would be much 
less than at 35 C., being equal to, or even less than, that at 
30 C 

TABLE SHOWING VARIATION OF EXCITATORY MECHANICAL RESPONSE 
AT DIFFERENT TEMPERATURES 



Stimulus 


Response at 
temperature of 30 C. 


Response at 
temperature of 35 C. 


Response at 
temperature of 37 C. 


Electrical I. 
II. 


10*5 divisions 

22 


37 divisions 
3i 


10-5 divisions 
I3'5 


Thermal III. 
IV. 


32 . 

18 


46 
28 


24 

12 



I have made numerous experiments completely bearing 
out these conclusions. The mode of experimental proce- 
dure is as follows : a balanced record is taken at the given 
temperature, and the growing organ is then subjected to a 
definite intensity of stimulation, which may consist of tetanic 
thermal or electrical shocks, lasting for twenty seconds. 
Records are then made of the resulting contractile retarda- 



454 PLANT RESPONSE 

tion of growth at the different required temperatures. Three 
responses were taken at each temperature, and were found to 
be practically the same. Some of these records will be 
given in the next chapter (fig. 185). I have given the 
results of four such experiments, carried out on different 
specimens. ^ 

The translocation of the optimum point. We have 
thus seen how constant is the optimum point in the same 
species, under normal conditions ; but, since we found that 
the otherwise constant death-point was liable to be shifted 
under the disturbance caused by the sudden variation of 
external conditions (p. 172), so it would appear probable that 
the optimum point also would be liable to transposition under 
the influence of similarly disturbing causes. The optimum 
point of the Crinum Lily has been seen to lie, normally 
speaking, between 35*4 C. and 35*5 C. After a night of 
heavy rain and gale, however, I found that the optimum 
point of a specimen of this Lily had fallen to 34*6 C. Under 
the action of a poison like copper sulphate, again, admi- 
nistered in such dilution as not to kill, but only to retard 
growth, I have observed the optimum point to be lowered 
to 34'5 C. In the case of dilute solution of sugar, however, 
which induces as we shall see in the next chapter an 
increase of growth-activity, I have found the optimum point 
to be raised to 36*6 C. 

Thus under normal conditions the optimum temperature 
for each species is extremely definite. But circumstances 
which increase or decrease the rate of growth abnormally, 
operate also to transpose the optimum point, in the same 
manner as the death- point was found to be translocated by 
external influences. 

SUMMARY 

The difficulties usually encountered in the accurate deter- 
mination of the effect of temperature on growth have been 
successfully overcome in the case of four distinct methods. 

In taking records of growth at different temperatures, it 



RELATION BETWEEN TEMPERATURE AND GROWTH 455 

is found that the variation from one to another acts as a 
stimulus, and induces a transient retardation of growth. 

But this cause of disturbance is eliminated when the rise 
of temperature is made gradual and continuous. In this 
way, by taking a continuous record of growth under uniform 
rise of temperature, a thermo-crescent curve is obtained, that 
gives data from which the absolute values of growth at all 
temperatures may be obtained. From this curve we are also 
able to obtain an accurate determination of the optimum and 
maximum points. 

The Method of Balance also affords us, by means of a 
sharply defined turning point, an exact indication of the 
optimum point. 

The optimum point is very definite, and under normal 
conditions is always constant for a given species ; but just 
as the death-point was found liable to be shifted under 
abnormal external conditions, so the optimum point also is 
apt to be transposed under similarly disturbing causes. 

That growth is a phenomenon of excitatory response is 
demonstrated by the fact that the growth- rate is increased or 
decreased at different temperatures, in proportion to the 
excitability of the tissue at the same points, as indicated by 
its contractile response. 



CHAPTER XXXIV 

ON AN ATTEMPT TO DETECT AND MEASURE LATENT 
STIMULUS, AND ON THE STUDY OF PERIODIC AFTER- 
EFFECTS 

Positive and negative after-effects Extreme delicacy of the Method of Balance 
Detection of absorbed stimulus by negative after-effect Constancy of sum of 
direct and indirect after-effects Latent component almost vanishing above the 
optimum Variation of receptivity Direct and indirect response of plant in 
sub-tonic condition Table showing direct and indirect effects at different 
temperatures Is the change induced by stimulus always of an explosive 
chemical character ? Relation between stimulus and response in different 
tonic conditions After-effect Factors which determine periodic after-effects : 
(l) Stimulus of light (2) Temperature (3) Chemical stimulus (4) Tur- 
gidity Continuous photographic record of the pulsations of Desmodittm 
Record of periodic variation of rate of growth Continuous photographic 
record of periodic variations of transpiration Continuous photographic record 
of the variation of the rate of growth Annual rings and seasonal periodicity. 

BY regarding the plant as a machine, as we did in the cpurse of 
the earlier chapters, we were enabled to understand the possi- 
bility of its absorbing, and holding latent, more or less of the 
incident stimulus (p. 124). The experimental demonstration 
of this would, however, be difficult, in the case of the ordinary 
response of motile organs ; for though we have seen that ex- 
ternal stimulus and the absorbed internal energy are opposite 
in their responsive effects, yet in the ordinary records of 
mechanical response it is not easy to discriminate that part 
of the effect which is due to the latter element ; for while 
it is true that the presence of internal energy would tend to 
hasten the recovery, it is still impossible to distinguish with 
certainty a recovery so hastened from one which is natural. 
The fact that excess of stimulus is transformed into latent 
energy is demonstrated, however, by the occurrence of 
multiple response. 



DETECTION OF LATENT STIMULUS 457 

Positive and negative after-effects. We have seen that 
when a moderately strong stimulus acts on a responding 
organ, a short time elapses before the initiation of the re- 
sponse, and this is known as the latent period ; but when 
response has been initiated, it persists for some time, even on 
the cessation of the stimulus, and this is known as the after- 
effect I shall, however, for important reasons, which will 
appear later, further distinguish it as the positive after-effect. 
By the term positive after-effect, then, is meant the continua- 
tion of a response evoked by external stimulus, on the cessation 
of that stimulus. We may, for example, 'imagine a heavy 
elastic spring immersed in a viscous fluid. If this be sub- 
jected to a sudden compressional blow, then, after a short 
latent period, it will begin to undergo compression, and this 
compressional movement will continue for some time, even 
on the cessation of the blow that caused it, thus exhibiting a 
positive after-effect. But a spring compressed in this manner 
contains some amount of latent or potential energy, on 
account of which it next begins to expand, exhibiting a 
movement opposite to the first. This second movement, due 
to the latent energy, we may distinguish as the negative after- 
effect. This negative after-effect, it should further be stated, 
may sometimes be separated from the direct effect by a con- 
siderable interval of time. This may be seen in a viscous 
wire subjected to a torsional impulse. After the twisting 
has ceased, some time elapses before the wire is seen to 
begin the contrary, or negative, movement of untwisting, 
which is accomplished very slowly, and may even take hours 
to complete. 

Now, it occurred to me that, in the response of growth, 
it was possible to find a means of detecting whether the 
external stimulus in the case of living tissue might, or might 
not, become partially latent, to be similarly manifested later, 
in the form of the negative after-effect ; for if we take a 
balanced horizontal record of growth, then the direct effect 
of external stimulus will be seen in that retardation which 
is shown in the shifting of the line here, for convenience of 



458 PLANT RESPONSE 

inspection, represented upwards (fig. 185). On the cessation 
of external stimulus, if the recovery of the excited region be 
merely passive, it is evident that this ascending line will 
gradually return to the horizontal, as in the third record of 
fig. 185 ; that is to say, the retarded will be exchanged by 
degrees for the normal rate of growth. But if some portion 
of the external stimulus be held latent in the tissue, this will 
go to increase the internal energy of the plant Now, we 
have already seen that the effect of augmented internal 
energy is exhibited in an increase of the rate of growth 
above the normal, shown in a balanced response-curve by an 
opposite movement to that of retardation, constituting the 
negative after-effect. Such a negative after-effect, consisting 
of an enhanced rate of growth, will persist until the energy 
thus held latent is exhausted, when the curve will again 
return to the horizontal. Thus, the up curve will represent 
the direct effect of external stimulus, and the down curve the 
acceleration of growth due to absorbed stimulus, or the nega- 
tive after-effect. 

Extreme delicacy of the Method of Balance. Such 
transient variations in the rate of growth, occurring as the 
expression of the absorbed fraction of incident stimulus, 
would have been incapable of detection by the ordinary 
auxonometric method of growth-record ; for here, owing to 
the relatively slight magnification which is possible, it takes 
nearly half an hour to obtain data from which the normal 
rate of growth may be inferred. Another half-hour's obser- 
vation would be necessary before we could infer the occur- 
rence of variation under changed conditions, and it is clear 
that, during a period relatively so long, the plant may 
undergo spontaneous changes. The after-effects, however, 
which we now wish to detect, are found to take place imme- 
diately, and to last for a few minutes only, in the case of 
moderate stimulation. Even with our crescographic arrange- 
ment, though the usual magnification is a thousand times, the 
variation constituting the after-effect is seen only in a slight 
change of the slope of the curve ; but when the Method of 



DETECTION OF LATENT STIMULUS 459 

Balance is employed, the ordinary magnification is enough 
to show, in a very marked manner, all the phases of these 
transient variations. The records here reproduced have 
been in fact reduced to one-third of the originals. 

The sensitiveness of the arrangement can be very much 
exalted by observing the balanced line of light with its devia- 
tions, through a telescope placed at a distance. In this way, 
I have been able to detect a variation from the normal rate of 
growth, of so little as a two millionth part of I mm. per second, 
within so short a period of observation as ten seconds. 

Detection of absorbed stimulus by negative after- 
effect. We must now revert to the question of the detection 
of latent stimulus by an increase in the rate of growth, 
and we shall first take that simple case in which there is no 
loss of energy from irreversible effects due to molecular 
friction. The energy of external stimulus will here find 
complete expression in doing external and internal work. 
If the external stimulus remain constant, the sum of these 
two that is to say, the direct or immediate, and the indirect 
effects will also remain constant ; but if, under the same 
circumstances, one of these factors, say the direct effect, 
should for any reason be enhanced, we might then expect 
that its complement, the indirect effect, would undergo a 
corresponding diminution ; while, if the direct effect should 
be small, the indirect effect would show augmentation. 

These theoretical considerations are found very strikingly 
verified in the experiment which I shall now describe. I 
first took a balanced record of growth in a specimen of 
Crinum Lily, at 30 C. This was then subjected to thermal 
shocks for five seconds. The direct response, as will be seen 
from the first record in fig. 185, which is reduced to one-third 
of the original, was a retardation of growth represented by 
33 divisions. On the cessation of stimulus, however, the rate 
of growth did not at once return to the normal, but exhibited 
the effect of absorbed energy by an acceleration shown in the 
down curve, and represented by 13 divisions, after which it 
became normal. The same stimulus was now repeated, and 



460 



PLANT RESPONSE 




the direct effect was a retardation of 31, which was followed 
by an augmentation of 14 divisions. Thus the sum of these 
two effects is practically constant, being in one case 46, and 
in the other 45, divisions. 

Constancy of sum of direct effect and indirect after- 
effect. This constancy, however, becomes still more remark- 
able when the same plant is raised to a temperature of 35 C. 
and subjected once more to the same stimulation. The direct 
effect is now shown b)' a retardation which may be repre- 
sented as 39, and the in- 
direct by 9, divisions. In 
the second response of this 
second series, we have a 
direct effect of 37, and an 
indirect effect of 8, divi- 
sions. Thus the sum of 
the first direct and indirect 
effects is 48, and the sum 
of the second 45, divisions, 
the mean of the two at 
35 C. being 46*5 divisions, 
while the mean at 30 C. 
was 45*5 divisions. We 
have found, then, not only 
that the sum of direct and 
indirect effects at a given 
temperature is practically 
constant when stimulus is the same, but also that this sum 
itself remains approximately constant at different tempera- 
tures within the optimum ; and, further, we see that as the 
excitability is increased in approaching the optimum, the 
direct effect also increases at the expense of the indirect. 
In other words, when the tissue is at its optimum tonic 
condition, its capacity for the absorption of stimulus being 
already fully satisfied, the external stimulus tends to be 
immediately expended, in direct response, allowing relatively 
little to become latent. 



FIG. 185. Series of Responses of Growing 
Organ of Crimtw Lily, taken under 
Balanced Conditions at Three Different 
Temperatures 

On comparing these records it will be seen 
thai the direct effect increases up to 
the optimum, and that the indirect 
effect of accelerated growth decreases. 
Beyond the optimum, at 37 C., there 
is no latent component, as shown by- 
recovery from direct effect to normal 
rate of growth. 



DETECTION OF LATENT STIMULUS 461 

Latent component almost vanishing above the opti- 
mum. As an extreme instance of this, we may take the 
response at a temperature beyond the optimum, say at 37 C. 
Here, by its environmental conditions, the plant is already 
supplied with an excess of energy. And besides this, there 
is the fact which we have already noticed, that its general 
excitability is diminished, so as to be equal to, or less than, 
that at 30 C. At these two temperatures, then of 30 C. 
and 37 C. we have two conditions of excitability more or 
less the same, but with a different history. 

Below the optimum there is an unsatisfied capacity for 
absorption of stimulus, whereas above it this capacity has 
been fully met. It would therefore appear that the power of 
the tissue to hold stimulus latent is diminished progressively 
up to the optimum, till beyond that point it practically 
disappears. I obtained a remarkable confirmation of this 
inference in the course of my experiments. In the experi- 
ment just described, for example, the direct response was 
a retardation of twenty-four divisions, and there was no 
indirect effect, showing that little or no stimulus had become 
latent (fig. 185). 

Variation of receptivity. In the experiments described, 
the sum of the direct and the indirect effects, up to the optimum, 
had been found to be approximately constant, that is to say, 
a total response of about forty -six divisions. At 37 C., how- 
ever, we see the total response reduced to twenty-four divisions 
without any latent component. And since the total response 
measures for us the amount of stimulus taken up by the 
tissue, it would appear that at 37 C. not only is the 
power to hold stimulus latent lost, but also that the general 
receptivity of the tissue is very much reduced. It is thus 
seen that the condition of a tissue modifies its receptive 
power ; hence it is possible for different parts of the same 
organ say, for instance, the tip and the growing region 
being in different conditions, to possess different receptivities. 

Direct and indirect response of plant in sub-tonic 
condition. Turning from this case of excess of energy to 



462 PLANT RESPONSE 

that in which it is below par, the plant being in a sub-tonic 
condition of arrested growth, we find that external stimulus 
gives rise to little expression in direct response. External 
stimulus is found under such circumstances mainly to increase 
the store of internal energy, in consequence of which we 
obtain the indirect response of renewed growth. In such a 
condition, the plant has a great capacity for the absorption of 
external energy. After growth has commenced, the energy 
of incident stimulus finds bifurcated expression, by inducing 
direct or immediate response, and by the indirect or negative 
after-effect, of enhanced rate of growth (p. 434). The sum 
total of these two external stimulus being the same 
remains approximately constant, till the optimum tonic con- 
dition is reached. The direct and indirect effects are thus, 
up to this point, complementary to each other. After 
passing this point, however, when the plant is possessed of 
excess of energy, its power of absorbing stimulus appears to 
undergo diminution. The direct effect of stimulus is then 
found reduced, and there is no negative after-effect, due to 
the absorbed component of the external stimulus. 

From these considerations we are enabled to understand 
the curious growth-response that was observed in varying the 
temperature of the plant from 34 C. to 35 C. (fig. 182, 

P- 445). 

The change of temperature was in that case accomplished, 

as will be remembered, by changing the intensity of the 
electrical heating current. The change from 34 C. to 35 C. 
thus produced was not, however, brought about at once, but 
took place in the course of a period of three minutes. We 
had consequently the stimulating effect of variation of tem- 
perature bringing about contraction, followed by the accele- 
rated rate of growth which constituted the negative after- 
effect of that stimulus, plus the accelerated rate of growth 
due to rising temperature. That the first of these two factors 
played a considerable part in this acceleration, is seen from the 
fact that the permanent increase in the rate of growth charac- 
teristic of the higher temperature of 35 C. is smaller than the 



DETECTION OF LATENT STIMULUS 



463 



acceleration that precedes it. Incidentally, we see here the 
difference between the temperature-effect per se, and the 
stimulating effect of sudden variations of temperature, con- 
stituting thermal shocks. 

In the simple case we have just studied, where the whole 
amount of incident stimulus was expressed in work external 
and internal, without any loss from molecular friction, the 
sum total of the two forms of response was found approxi- 
mately the same under a constant stimulus ; but in other 
cases, where a certam amount of energy is wasted in over- 
coming molecular sluggishness, the results will be slightly 
different, for at a temperature below the optimum a por- 
tion of the stimulus will be wasted in overcoming such 
sluggishness, whereas near the optimum temperature the 
loss entailed on this account will be very slight. Hence, the 
sum of direct and indirect responses, near the optimum, will 
in such cases be somewhat greater than at a temperature 
several degrees lower. I give below a table which shows at 
a glance the direct effect and the indirect after-effect, 
obtained at the three temperatures of 30 C, 35 C, and 
37 C. respectively, with three different specimens, one of 
which was a rice-seedling (Orysa sativd), and the two others 
flower-buds of Crinum Lily. Each response given is the 
mean of three. 

TABLE SHOWING DIRECT AND INDIRECT EFFECTS AT THREE DIFFERENT 

TEMPERATURES 



Specimen 


Tempera- 
ture 


Direct effect 
(retardation of 
growth) 


Indirect effect 
(acceleration of 
growth) 


I . Seedling of J 
Oryza sativa, j 


30 C. 

35 C. 
37 C. 


15 divisions 
20 
10 ,, 


10 divisions 

6'5 
o 


2. Crinum Lily - 


30 C. 
35 C. 
37 C. 


32 divisions 

38 
24 


13-5 divisions 

8-5 
o 


/ 
3. Crinum Lily \ 


30 C. 
35 C. 
37 C. 


1 8 divisions 
28 

12 


17 divisions 

13 

,, 



Total effect 



25 divisions 
26-5 ,, 
10 ,, 

45 "5 divisions 

46-5 

24 

35 divisions 



12 



99 
99 



464 PLANT RESPONSE 

It will be seen from the readings given by specimens I and 
2, that up to the optimum the total response is approxi- 
mately constant, while at 37 C. there is no expression of 
energy held latent. The condition of specimen 3 having 
been at starting somewhat sub-tonic, the total effect is 
heightened at the optimum, for reasons which have been 
explained. It has already been said that the proportion of 
stimulus held latent will be greater with the degree of sub- 
tonicity of the plant. That this was the condition of 
specimen 3, then, is demonstrated, not only by the increase 
of the total effect at the optimum temperature, but also by 
the fact that at 30 C. so large a proportion of the stimulus 
as almost exactly one-half is held latent. At 37 C. here, as 
in the other cases, there was no latent component. 

Is the change induced by stimulus always of an 
explosive character? It has generally been supposed that 
stimulus causes response by an explosive chemical change. 
According to this theory, the stimulus acts as upon a trigger, 
to release suddenly a large amount of energy previously held 
latent in the tissue. The response is thus assumed to be 
always disproportionately larger than the stimulus, and to be 
brought about by chemical degradation, or dissimilation of 
the living tissue. The tissue, thus reduced below par, is sup- 
posed to be restored by the process of assimilation. 

In Chapter X., however on Theories concerning Different 
Types of Response I adduced considerations, showing that 
there are cases of responsive phenomena in which this 
description would not hold good ; that is to say, there are 
instances in which the response cannot be due to a chemical 
down change of explosive character. Nor is it always true 
that response is disproportionally larger than stimulus. The 
series of experiments which has just been described offers 
conclusive evidence on this point, of a quantitative character. 
Further, if the theory of an explosive down change had been 
the real explanation of response in general, then it is clear 
that in the case of the response induced by stimulus in grow- 
ing organs, recovery would have taken place slowly, and 



DETECTION OF LATENT STIMULUS 465 

would have culminated in the restoration of the original rate 
of growth ; but we actually find, on the contrary, that, 
immediately following the responsive retardation, there is 
an acceleration of growth above the normal, and the true 
recovery, or restoration of the normal rate, takes place only 
after this. Thus there is here, instead of a run down, an 
actual increase, of the energy of the system. 

Again, from this constancy of the sum of the immediate 
and the after-effects of stimulus, we can see that, as in an 
inorganic system, so also in a living organism, the law of the 
Conservation of Energy holds good. For while at the 
optimum point the entire stimulus finds expression in direct 
response (stimulus being here equal to response), below the 
optimum the direct response is less than the stimulus, the 
missing fraction being left to find expression in the negative 
after-effect. 

Relation between stimulus and response in different 
tonic conditions. The experiments which I have described 
were carried out under the thermal form of stimulation, which 
is, as already explained, the most satisfactory in practice. 
We shall hereafter come across instances of the after-effect of 
accelerated growth, as caused by stimulus of light. And a 
similar after-effect has already been seen to be caused by 
electrical stimulus (p. 436). But for the clear demonstration 
of this particular effect, electrical stimulus is not very suit- 
able, inasmuch as it is apt to induce fatigue and, through 
electrical polarisation, a certain amount of tissue-change. I 
shall, however, describe an experiment, using this mode of 
stimulation, which, by its responsive indications, will demon- 
strate certain differences in the internal conditions of tissues, 
according as they are sub- or super-tonic. 

I have shown that the general excitability of the tissue at 
30 C. is superficially the same as that at 37 C. or thereabouts ; 
that is to say, the direct responses at these two temperatures 
are in some cases approximately the same ; but we have seen 
that there is a difference of molecular condition at these two 
points, for at 30 C. the tissue is capable of holding a portion 

H H 



466 PLANT RESPONSE 

of the incident stimulus latent, by which its molecular mobility 
becomes enhanced, whereas at 37 C. there is already the 
fullest molecular mobility. If now we apply at 30 C. stimuli 
which increase, say, in arithmetical progression, we see that by 
the very reception of the increasing stimuli the tissue is made 
to approach more and more closely to the optimum condition, 
at which point, as we have seen, the whole of the stimulus is 
given up immediately, in the form of direct response. Hence 
the curve showing the relation between stimulus and response 
in a tissue that is in a condition below the optimum will be 
steep, and somewhat convex to the abscissa which represents 
the stimulus ; but at 37 C., when no molecular sluggishness 
has to be overcome, we may expect the response to increase 
proportionately with the stimulus; that is- to say, the curve 
showing the relation between stimulus and response will now 
tend to be a straight line. 

In order to apply electrical stimulation whose intensity 
was increased by known amounts, I used an induction coil, 
the primary coil of which was completely within the secondary. 
The ordinary method of Du Bois-Reymond's sliding coil was 
not very suitable in this case, because the increasing intensity 
obtained by sliding the coil inwards is merely qualitative. 
The required definite increase of induction -shock I secured 
by suitable augmentations in the value of the current that 
flowed round the primary coil. In order to determine these 
values, a preliminary experiment was carried out. A storage 
battery was in circuit with the primary coil, which had 
interposed in it also an ammeter and a rheostat. By 
increasing the resistance, a moderate current, say c, read by 
the ammeter, was adjusted to flow round the circuit. The 
secondary circuit contained a ballistic galvanometer, which 
by its throw indicated the intensity of the induced current at 
make or break of the primary circuit. When the first current, 
C, had given an induced current, which caused a deflection of, 
say, 30, it was increased to C ; , when the deflection due to 
induction was found to be 50 ; and lastly a third value, or , 
was found, whose induction -effect was 70. In this manner, 



DETECTION OF LATENT STIMULUS 



467 




induction-shocks, increasing in arithmetical progression as 3 
to 5 to 7, were determined. Tetanic shocks of the described 
intensity 3, 5, 7 the total application of each group being 
a period of 5", were now given, response being taken after 
each. In this manner two series of responses were obtained at 
the temperatures of 30 C. and 37 C. At both temperatures 
with the stimulus-intensity of 3, the 
responses given by this particular 
specimen were the same. But while 
at 37 C. the lower of the two 
curves, showing the relation between 
stimulus and response, is a straight 
line, that at 30 C., the upper of the 
two, is seen to ascend more steeply 
and to exhibit convexity to the 
abscissa (fig. 186). 

After-effect. The phenomenon 
which is generally referred to as the 
after-effect is characterised by so 
many complexities as to have been 
regarded as highly perplexing. After 
the foregoing analysis, however, by 
which it has been resolved into two 
elements, much of this obscurity 
will be found to have disappeared. 
We saw that the first effect induced 
by a very strong or long-continued 
external stimulus consisted in bring- 
ing about the continuation of the 
direct effect itself, its persistence 
depending on the intensity and 
duration of that stimulation. And 

this we have distinguished as the positive after-effect. A com- 
ponent part of the external stimulus, however, as we also saw, 
becomes latent, thus increasing the internal energy ; and the 
expression due to this element the negative after-effect is 
opposite in sign to the effect of direct stimulus and its positive 



KK;. 1 86. Curve showing 
the Relation between 
Stimulus and Response 
in the same Organ, under 
the two different Tonic 
Conditions of 30 C. (upper 
curve) and 37 C. (lower 
curve) 

The abscissa represents the 
intensity of stimulus, and 
the ordinate the extent of 
response. The response 
was the same in both cases 
when the stimulus-intensity 
was 3 ; but later, the 
curve for 30 C. is seen 
to rise more steeply than 
that at 37 C., exhibiting 
at the same time convexity 
to the abscissa. 



H H 2 



468 PLANT RESPONSE 

after-effect. Thus a plant under natural conditions, acted 
upon by different stimuli, gives to each stimulus direct re- 
sponse, direct after- response, and indirect response. But as 
all these individual stimuli do not act, or cease to act, 
simultaneously, we can easily understand the infinite com- 
plexity of the combinations which take place between the 
direct and indirect, effects of stimuli of unlike forms, whose 
maxima, instead of being coincident, are superposed on 
each other, with various differences of phase. 

Factors which determine periodic after-effects. We 
shall now proceed to enumerate some of the most important 
stimulating factors instrumental in modifying the response of 
growth. 

(1) Stimulus of light. If we take the average rate of 
growth during the twenty-four hours as the normal, then the 
direct effect of this stimulus will appear as a retardation of that 
normal rate of growth, and, after the long-continued action 
of the whole day's illumination, this retardation may persist 
for a time as the positive after-effect. Later, however, on 
account of the stimulus which has been absorbed and held 
latent, we shall observe an acceleration of growth above the 
normal, or a negative after-effect. The persistence, again, of 
this negative after-effect will depend on the amount of the 
energy held latent by the tissue. The diurnal sequence of 
light and darkness will, after long repetition, impress itself 
upon the organism, and, other factors remaining constant, 
will find expression as periodic retardation and acceleration 
of growth during day and night ; such periodicity continuing 
to show itself, for some time, even when the plant is kept in 
continuous darkness. 

(2) Temperature. The effect of temperature up to the 
optimum point will, by increasing the internal energy, prove 
favourable to growth. Thus, the temperature during day- 
light will usually be favourable, with the exception of the 
tropical noon, when it may be excessive. A certain amount 
of heat, again, may be stored up in the plant to give the after- 
effect. At night, if the fall of temperature be very great, 
there will be, relatively to this factor, a retardation of growth. 



DETECTION OF LATENT STIMULUS 469 

(3) Chemical stimulus. This we see supplied by the salts 
taken up by the roots, and by the process of photo-synthesis 
in the leaves. The amount of the former supply is depen- 
dent not only on the richness of the soil, but also on the 
suctional activity of the plant ; and the latter on the effective 
intensity of light. 

(4) Turgidity. The turgid condition of the plant will 
depend, firstly, on the supply of water ; secondly, on the 
suctional activity ; and thirdly, on the relative absence of a 
loss of water by transpiration. We have seen how watering 
the roots of the plant will cause an immediate response of 
enhanced growth, and the seasonal periodicity induced by 
this cause may be observed in a very striking manner in 
tropical countries. 

The suctional activity, depending as ft does on the 
internal energy of the plant, will tend to be augmented by 
an advantageous temperature, and by the after-effect of the 
absorbed stimulus of light ; but the turgid condition will be 
reduced by active transpiration, which is relatively greater 
in the daytime. 

We thus see how numerous are the factors which co- 
operate to bring about periodic fluctuations in the rate of 
growth during every twenty-four hours. Of all these factors, 
the alternation of day and night is the most pronounced in 
its action. The curve of growth, then, will exhibit not only 
a large wave of alternation due to the diurnal period, but also 
a number of sub-waves. And even beyond these, superposed 
upon them, we may expect, when the magnification is suffi- 
ciently great, to observe systems of still smaller wavelets, 
caused by the rhythmicity of growth. 

Continuous photographic record of the pulsations 
of Desmodium. Before describing the periodic diurnal 
variation of that autonomous response which we know as 
growth, it occurred to me that a continuous record of another 
form of autonomous response that is to say, of the rhythmic 
pulsations of Desmodium might prove interesting. I was 
fortunate enough to succeed in obtaining a very good record 



470 



PLANT RESPONSE 



of these pulsations during a period of twelve hours, by means 
of photography. The record began at 6 P.M. and ended at 
6A.M. During this time there were no fewer than 180 con- 
stituent pulses, and it will be noticed that these again fall 
into groupings, whose average period is a little over an hour, 
there being about ten such groups in the course of twelve 
hours (fig. 187). 

Returning now to the question of periodic growth-fluctua- 
tions, Sachs and others have measured the different rates of 
growth at various hours within the twenty-four, and from the 
data thus obtained have constructed curves which showed 




'. M. 



9 P.M. 



12 




12 3 A.M. 6 A.M. 

Kir.. 187. Continuous Photographic Record of Autonomous Pulsation of 
DesModiitw g}vans from 6 P.M. to (> A.M. 

The lower record is in continuation of the upper. 

the periodicity of the rate of growth. These curves exhibit 
the daily period in a marked manner ; but the subordinate 
waves are more or less obliterated, in consequence of the 
fact that the data from which the curves were constructed 
were obtained from discontinuous observations. The curves 
thus deduced show marked differences also, according as 
their points were determined frequently or at long intervals. 
Record of periodic variation of rate of growth. For 
this reason it appeared to me important to devise means by 
which, not the growth, but the variations, of its rate might 
be automatically recorded, directly and continuously, for any 



DETECTION OF LATENT STIMULUS 



471 



length of time. The curve thus obtained ought instanta- 
neously to mark the fluctuations of rate throughout the 
period in question. This I have been able to accomplish 
by means of a modified Method of Balance. From the 
description of that method already given, it will be under- 
stood that after the establishment of the average balance, if 
the rate increases, the curve will move upwards. When, 
after this, the rate of growth returns to the normal, the 
balance will be re-established, and the curve become again 
horizontal ; but if the growth at any time should fall below 
the average, the curve will 
descend. In this way pe- 
riodic fluctuations in the 
rate of growth may be 
recorded. 

For the purposes of the 
modified method of record, 
the compensating arrange- 
ment used for balance has 
to be somewhat altered. 
In recording these long 
periodic changes, we have 
fluctuations of larger am- 
plitude than those of au- 
tonomous pulsation. The 
spot of light which is 
thrown from the mirror of 
the experimental Optic 
Lever upon the second, or a ^ ., , f ,. , . 

r ' F, float, supporting the plant, which is 

attached by thread to optic lever, L. 
\v, wheel, for adjustment of balancing 




FIG. 1 88. llydrometric Apparatus for 
Recording Continuous Variation of 
Rate of Growth 



IS 



compensating, mirror 

thus apt to fall outside overflow. 

the range of the latter. In 

order to overcome this difficulty I mount the plant on the 

float itself, and adjust the outflow of water from the 

cylinder, so that the upward growth of the plant is, at a 

given moment, exactly compensated by the descent of the 

float supporting it. Deviations above or below the balanced 

rate of growth may then be followed with a recording pen, 



472 



PLANT RESPONSE 



or will record themselves on a sensitised photographic film 
wrapped round the revolving drum. 

In carrying out such a continuous record, certain pre- 
cautions are necessary. Owing to transpiration, the float on 
which the plant is mounted will become lighter, causing an 
ascensional movement of the record. In order to obviate 
this, it is only necessary (i) to weight the float to such an 
extent that the variation caused by transpiration is negli- 
gible ; and (2), in addition to this, the diameter of the cylin- 
drical float may be so increased as to reduce still further the 
ascensional movement due to loss of weight by transpiration. 
By these means the error from this source may be reduced 

to any extent desired. In the 
figure of the apparatus which is 
here given (fig. 188), the specimen 
was a young seedling of Oryza 
sativa, in which transpiration was 
relatively little. I shall presently 
give photographic records obtained 
in the manner described. 

Continuous photographic re- 
cord of periodic variations of 
transpiration. By a somewhat 
similar method we are enabled to 
determine the periodic variation of 
the rate of transpiration. In this 
case, the plant is mounted with its 
roots in a test-tube, which acts like 
a float, and is partially filled with 
water, but not so full as to make it 
sink. The test-tube containing the 
plant is attached to one arm of the 
Optic Lever, and the outflow from 
the outer cylinder so adjusted that 

the ascensional movement of the test-tube, due to its loss of 
weight by transpiration, is exactly balanced by the subsidence 
of the water-level of the water that buoys it up. In order 




Fl< J. 1 89. Photographic Record 
showing Variation of Rate 
of Transpiration in Citcitr- 
fritd, from 3 P.M. to 12 I'.M. 

The nite is seen to undergo 
enhancement till about 
11.30 r. M., after which 
there is a rapid fall of the 
rate of transpiration. 



DETECTION OF LATENT STIMULUS 473 

to prevent the loss of water from the cylinder by evaporation, 
a film of oil covers the surface. In this way I obtained 
the accompanying record of variation of rate of transpiration 
in a young specimen of Cucurbita, from 3 P.M. to 12 P.M. 
(fig. 189). It will be noticed that in this case there is an 
enhancement of transpiration which continues with a single 
fluctuation till past 1 1 P.M., after which there is a sudden 
depression of the rate. 

Continuous photographic record of the diurnal varia- 
tion of the rate of growth. I shall first give a photographic 
record (fig. 190) taken from a seedling of Vryza sativa, only 




FIG. 190. Continuous Photographic Record of Variation of Rate of 
Growth in Four Days' Old Seedling of Oryza saliva, from 3 P.M. 
till 9 A.M., that is during Eighteen Hours 



four days old. The diurnal periodicity has already, it will be 
seen, become fairly impressed, though it is not yet sufficiently 
powerful to mask, to any great extent, the subsidiary periodi- 
cities induced by other factors. The record, it must be 
remembered, was taken in continuous darkness, being com- 
menced at 3 P.M., when it was balanced. From this time to 
9 P.M. there were three pulsations. From 6 till after 8 P.M. 
there was depression of the average rate of growth, after 
which it rose somewhat rapidly till 12.30 A.M., exhibiting 
during that period two groups of two pulsations each. There 
was now a quick fall for the next half-hour, and after this 



474 PLANT RESPONSE 

the growth-rate rose more or less continuously till 8 A.M. The 
growth-rate then began to fall during the course of the day. 

The second record was taken with a seedling of Tama- 
rindus indica fourteen days old, the diurnal periodicity being 
thus deeply impressed. It was placed in the dark room, 
mounted on the float, and the balanced record begun at 
3 P.M. It will be seen that, as the positive after-effect of 
the day's illumination, there was a depression of the rate 




FlG. 191. Continuous Photographic Record of Variation of Rate ot 
Growth in Seedling of Tamarindtts indica, a Fortnight Old, from 
3 i'.M. to 3 \.M. 

Owing to positive uker-effect of daylight, theie is a depletion of rate of 
growth, although the seedling was now placed in a dark rconi. In the 
evening, howe\er, the rate began to rise. 

of growth, though the plant was kept in the dark. This 
persisted for two hours, till 5 P.M., after which the rate showed 
increase, there being three pulsations before the end of the 
record, at 3 A.M. (fig. 191). 

Annual rings of wood and seasonal periodicity. The 
different growths of wood in spring and autumn, leading to 
the formations known as 'annual rings,' constitute a pheno- 
menon of growth not yet fully explained. I may here point 
out an important factor in connection with this subject. It 



DETECTION OF LATENT STIMULUS 475 

has already been demonstrated that growth is a phenomenon 
of excitatory reaction, being at its maximum when the 
excitability of the tissue is greatest. Thus varying expres- 
sions of growth at the two seasons, as seen in the production 
of different sized cells during spring and autumn, would 
appear natural, if they could be correlated to differences of 
excitability, characteristic of those seasons. Now all modes 
of testing degrees of excitability lead to the conclusion that 
while it is very great in spring and summer, it is very much 
enfeebled in autumn and winter. Thus, in the latter season, 
contractility under stimulation, velocity of transmission of 
excitation, and the electrical response of a tissue, are all 
found to undergo a marked diminution, as compared with 
spring and summer. 

SUMMARY 

On the cessation of strong stimulus the responsive 
movement continues for a time in the same direction. This 

* 

is the positive after-effect. 

A portion of the incident stimulus is absorbed and held 
latent, thus increasing the latent energy of the plant On 
the cessation of stimulus this latent component, either 
immediately or after a time, finds expression in an opposite 
responsive movement. This is the negative after-effect. 

In the case of growth-response, the positive after-effect 
consists in the persistence for a time of the retardation of 
growth, and the negative after-effect exhibits itself as an 
acceleration of the rate of growth above the normal. 

With moderate stimulus and under normal conditions 
the sum of the direct effect and the negative after-effect (due to 
the latent component) remains constant up to the optimum ; 
that is to say, the sum of the external work (direct effect) 
and internal work (negative after-effect) done by the stimulus 
is the same. The direct and the negative after-effect of 
stimulus are thus complementary. 

At the exact optimum almost the whole stimulus will 
find expression in direct response, there being little or no 



476" PLANT RESPONSE 

latent component. In a sub-tonic condition, on the other 
hand, a greater proportion of the stimulus is temporarily held 
latent, and expresses itself as the negative after-effect, the 
direct responses being here correspondingly diminished. 

Above the optimum there is no latent component, and the 
general receptivity of the organ shows great diminution. 

From the constancy of the sum of the direct and indirect 
effects it is demonstrated that, with regard to some forms of 
response at least, response is not disproportionately greater 
than stimulus. Thus the theory that response must always 
be due to an explosive chemical change does not hold good. 

The curve showing the relation between stimulus and 
response is appropriately modified by the tonic condition of 
the tissue. 

As each stimulus of every form thus finds expression in 
direct response, direct after-effect, and indirect after-effect, 
and as there are many forms of stimulus which under natural 
conditions act on the plant, whose maxima, instead of being 
coincident, are superposed on each other, in "varying differ- 
ences of phase, highly complex periodicities are induced, and 
find expression in the various forms of plant response. 

Among such varying factors of stimulation may be 
mentioned the diurnal alternation of light, temperature, 
chemical stimulus, and varying turgescence. 

The induced periodicities which result from the conditions 
described may be seen in the periodic groupings which 
appear in a continuous record of the autonomous pulsations of 
Desmodium for example. 

The autonomous response of growth, as the result of the 
periodically acting stimuli mentioned, exhibits not only a 
large wave of alternation, due to the diurnal period, but also 
a number of sub-waves. But the impression made on the 
organism by the diurnal period is the deepest of these, and 
tends in an old plant to subordinate all others in a marked 
degree. In the responses of a seedling a few days old, how- 
ever, the minor waves are very distinct. 



CHAPTER XXXV 

AN INVESTIGATION INTO THE DIFFERENT EFFECTS OF 
DRUGS ON PLANTS OF DIFFERENT 'CONSTITUTIONS' 

General consideration of the problem 'Constitution' and the elements which 
determine it Methods of investigation Action of carbonic acid gas Action 
of ether Effect of solution of sodium carbonate Effect of solution of sugar 
Effect of alcohol Effect of acids Effect of alkali Antagonistic action of 
alkalis and acids Action of strong solution of sodium chloride Effect of 
poisonous solution of copper sulphate Opposite effects of ^ the same dose on 
different constitutions Opposite effects of large and small doses. 

WE have already studied the effect of various chemical 
agents on the physiological condition of plants, as seen 
from the modifications induced by them in ordinary and in 
autonomous responses. We have also noticed the remark- 
able similarity between these effects in the two cases of 
plant and animal tissues, and I have drawn attention to 
the great practical utility of these investigations, inasmuch 
as the experiments carried out on plants may be made 
to throw light on many obscure phenomena regarding 
the effects of drugs on the animal system. One puzzling 
fact, however, which is encountered in medical practice, is, 
that the same drug will often produce varying effects on 
different individuals ; and this is vaguely ascribed to dis- 
similarity of ' constitution.' 

Similarly, we have sometimes seen different effects to 
occur in plants under the action of a single given drug. 
A large dose of some depressing agent, for example, though 
it will ultimately produce the depressing effect, will not 
always do so at once, for in some cases there will be a pre- 
liminary period of exaltation of response. The same toxic 
dose, again, which will in some instances kill the plant, will 



478 PLANT RESPONSE 

in others fail to do so, the plant being ultimately able to 
shake off the depressing influence after an interval of 
struggle. In studying suctional response, again, we found 
that copper sulphate applied at the roots induced, in some 
cases, an immediate arrest of suction, while in other instances 
this arrest did not take place till after a long time. The 
depression of suction which was induced by the application 
of strong sodium chloride, again, was in some cases imme- 
diate, and in others preceded by a fairly long period of an 
exalted rate of suction (pp. 384, 385). All these variations 
of results we regarded as due to individual differences of 
constitution, or of tonic condition. 

Thus we can only hope to arrive at a complete knowledge 
regarding the action of any given drug if we first obtain a 
precise understanding of what is meant by constitution, and 
of how, for experimental purposes, specimens of a definite 
characteristic constitution can be secured, while others can be 
subjected to an ascertained variation in a pre-determined 
manner. It will thus be made possible to study the effect of 
a drug, by applying it (i) in solutions of different strengths 
to a number of plants of identical constitution ; and (2) in a 
single strength of solution to specimens of definitely varying 
constitutions. 

' Constitution ' and the elements which determine it. 
The first factor in determining constitution will consist of those 
properties which have been impressed upon the plant by its 
heredity. The second will depend upon its environment. 
The sum total of the energy absorbed by the plant from its 
surroundings we have already designated as the tonic con- 
dition. It is clear that we may secure the factor of a constant 
heredity by taking either seedlings from the same batch of 
seeds, or organs from the same plant. These, again, when 
maintained under the same environmental conditions, in 
respect of temperature and other circumstances, will give us 
plants having practically the same constitution. In order, 
next, to obtain specimens of different but well-ascertained 
constitutions, it is only necessary to keep these under known 



EFFECTS OF DRUGS ON PLANTS 479 

differences of tonic condition. Thus, all other factors being 
maintained constant, we may have three clearly defined 
states, in the case, say, of Crinum Lily, according as it is 
kept at a temperature of 30 C, 34 C, or 37 C. The first 
of these we may regard as the normal ; the second as 
near the optimum ; and the third as intermediate between 
optimum and maximum. The excitability of the plant kept 
at the optimum will be the greatest ; but though the excita- 
bilities of those at 30 C. and 37 C. will be approximately 
the same, yet, in the latter case, the plant will possess an 
excess of latent energy which will be wanting in the former. 
Having thus secured these definite artificial constitutions of 
different values, some of the investigations given at the end 
of this chapter will show how free from uncertainty the 
action of drugs may be made, and how rational an expla- 
nation can be given of the observed variations of effect. 

Methods of investigation. I shall now proceed to 
describe the general methods of experiment, in studying the 
effects of drugs, from the modifications which they induce in 
growth -response. There are two ways of doing this. Accord- 
ing to the first, we take a record of the growth before and 
after the application of the reagent. From the variation 
then seen in the rate of growth the excitatory or depressing 
nature of the drug may be ascertained. The second, or 
Method of the Balanced Crescograph, is much more delicate ; 
it exhibits each transient variation of response, and its time- 
relations, with perfect clearness. The balanced horizontal 
record, which is first taken, indicates the normal rate of 
growth. A deviation upwards from this horizontal line will 
indicate accelerated growth ; a return to the horizontal will 
mean a regaining of the normal rate ; and a deviation down- 
wards will show responsive retardation. The specimens used 
for this investigation were Crinum Lilies, and, unless stated to 
the contrary, the experiments were carried out at the normal 
temperature of 30 C. 

Action of carbonic acid gas. I first give a record of 
.the effect of carbonic acid gas on growth (fig. 192), taken by 



480 



PLANT RESPONSE 



___ 

the Unbalanced Method. The normal rate of growth was 
006 mm. per minute. On passing CO 2 into the plant 
chamber the immediate effect was an acceleration, the rate 
for the next five minutes being '009 mm. or i times the 
normal. Under the continued action of carbonic acid, how- 
ever, the rate underwent a rapid diminution, and, as is seen 

by the slope of the curve 
becoming horizontal, growth 
was arrested fifteen minutes 
after the introduction of 
carbonic acid into the plant 
chamber. On the re-intro- 
duction of fresh air the growth 
was slowly renewed, and 
gradually returned to its 
original rate. The effect of 
carbonic acid on growth- 



10' 15' 20' 25' 



Fio. 192. Effect of Carbonic Acid 
Gas on Growth 



arrow indicates the introduction of 
fresh air, followed by the revival 
of growth. Record taken by Un- 
balanced Method. 



The first arrow indicates introduction response, then, is a prelimi- 
of CCX,, which induces preliminary 

enhancement of growth, followed nary exaltation, followed by 

by subsequent arrest. The second depression and arrest, which 

arrest, if the action be not too 
long continued, proves to be 
only temporary. 

Action of ether. This experiment shows the curious 
difference of results which occurs, according as an applica- 
tion is external or internal. In this and in the following 
cases, with the exception of the experiments on acids and 
alkalis, I shall use the Balanced Method, as this brings out 
even transient variations in a very striking manner. The 
balanced horizontal record is seen to the left of each figure. 
In curve a of fig. 193 is shown the effect of an external appli- 
cation of this reagent, ether vapour being introduced into 
the plant chamber. It will be seen that there was an imme- 
diate retardation of growth, which lasted for more than a 
minute. This was followed by an acceleration of growth, 
which lasted for two minutes. There was then a depression, 
which continued, and culminated in the arrest of growth. 



EFFECTS OF DRUGS ON PLANTS 



481 




In b is shown the effect of an internal application, made by 
replacing the water, by which the cut end of the stem was 
normally surrounded, with a 3 per cent, solution of ether. 
It will be seen that, as the specimen sucked up the solution, 
the immediate effect of this 
internal application was an 
enhancement of the rate of 
growth, and that this was 
followed by depression, lead- 
ing to arrest of growth. The 
preliminary depression seen 
in a is thus wanting in the 
case of the internal applica- 
tion. It might be thought 
that this first depression, as 
seen in the case of the ex- 
ternal application, was due to 
a slight cooling, caused by 
the introduction of ether. We 
know, however, that while 

lowering of the temperature, as long as that is below the 
optimum, would suffice to retard growth, a similar lowering 
of it when above the optimum would have the opposite effect 
of acceleration. Now I find that 
this preliminary retardation of growth, 
seen in fig. 193, a, takes place in 
exactly the same manner when the 
experiment is repeated with the 
specimen at 38 C. It cannot, there- 
fore, be due to the suggested cooling, 
which must in any case, under the i,-j (1 . I94 . 
experimental conditions, have been 
extremely slight There is, however, 
another explanation, which more nearly meets the require- 
ments of the case. We know that any sudden variation of 
environmental conditions is apt, generally speaking, to act as 
a stimulus, and the effect of direct stimulation is always to 



FIG. 193. Balanced Records oi 
Effect of Ether on Growth. Up 
Curves represent Acceleration 
Above, and Down Curves Re- 
tardation Below, the Normal 

a Effect of external application, and 
b of internal application of ether. 
Successive thick dots in the base- 
line indicate time in minutes in 
this and following records. Arrow 
shows moment of application. 




Excitatory Effect 
of Dilute Solution of So- 
dium Carbonate on Growth 



T T 



482 PLANT RESPONSE 

induce contraction, which would in the present case take 
the form of a transient retardation of growth. 

Effect of solution of sodium carbonate. In the course 
of the following experiments the chemical reagents arc 
administered internally, by applying the solution at the cut 
end of the specimen. A dilute solution of sodium carbonate 
is known to increase excitability in the case of animal tissues. 
T find that this holds good in the case of the growth-response 
of plants, growth being accelerated, as will be seen in 
fig. 194, where the balanced record suddenly gives place to 
an ascending curve. But in the case of chemical reagents in 
general, and of this especially, it must be remembered that 
the strength of the solution, or dose, is an important element 
in the result. A '5 per cent, solution of sodium carbonate 
was always found in these experiments to be an excitant ; 
but as the strength of the solution was increased, the excita- 
tory effect was found to be gradually diminished, until at 

2 per cent, it became neutral. If 
now the strength were still further 
increased, say to 5 per cent., the 
effect was an actual depression of 
the rate of growth. 

Effect of solution of sugar.- -I 
next give a record (fig. 195) showing 
the effect of the application of sugar 
Kir,. 195 Acceleration of j n a 2 per cent, solution. This is 

(.rovvth by Application of 

Solution of Sugar seen to induce a responsive accelera- 

tion of growth. In the case of the 

Crinum Lily, this acceleration is found to occur even under a 
5 per cent, solution of sugar ; but very much stronger solu- 
tions induce depression. 

Effect of alcohol. We have hitherto observed different 
reagents inducing a more or less uniform acceleration or 
depression. In the case of alcohol, given in 5 per cent, 
solution, however, we obtain a very curious instance of 
alternating spasmodic effects (fig. 196) ; that is to say, the 
growth at one moment exhibits a sudden acceleration, and 




EFFECTS OF DRUGS ON PLANTS 



483 




Vic.. 196. , Spasmodic Alternations 
of Growth under Alcohol 



at the next a sudden depression, such alternations being 
continued for a considerable length of time. On repeating 
the experiment at the higher temperature of 34 C. I found 
that these spasmodic alterna- 
tions became still more violent 
that is to say, of greater ampli- 
tude, though less frequent. At 
a much higher temperature, how- 
ever, the effect of alcohol was 
an immediate depression. Still 
stronger solutions caused arrest 
of growth at all temperatures. 

Effect of acids. We found, in the case of the autono- 
mous responses of Desmodiitm^ as of cardiac muscle, that 
acids induced relaxation, and that the long-continued action 
of such a reagent, or a strong solution, would bring about 
arrest in the relaxed position. Curiously enough, then, in 
the case of growth, which I have shown 
to be an instance of multiple or autono- 
mous response, I find an effect exactly 
parallel. In order to hasten the result 
I used a somewhat strong solution, 
namely 4 per cent., of hydrochloric acid. 
This, as will be seen (fig. 197), caused 
a marked relaxation, and growth came 
to a standstill some six minutes after- 
wards. In this experiment, and the 
following on the effect of alkali, the 
record was taken under unbalanced 
conditions. 

Effect of alkali. The effect of alkali in the case of Des- 
modimn, and also of cardiac muscle, is to produce arrest in 
the contracted position ; a similar effect is strikingly exhi- 
bited in the growth-record given in fig. 198. The first part 
of this record shows the normal rate of growth. A 3 per 
cent, solution of sodium hydrate was then applied at the point 
marked with the downward arrow ^. It will be noticed 




FIG. 197. Unbalanced 
Record showing Ac- 
tion of Acid in Causing 
Relaxation and Ulti- 
mate Arrest of Growth 



I I 2 



PLANT RESPONSE 




that this induces a very great contraction, and that in the 
course of seven minutes there occurs an arrest of growth, in 
a contracted position. It will also be seen that the specimen, 

after the application, actually be- 
came shorter than it had been 
before. The alkali therefore had 
the effect not merely of arresting 
growth, but also of causing an 
active contraction of the tissue. 

Antagonistic action of alkalis 
and acids. We saw that in the 
autonomous responses of Des- 
nwdium and cardiac muscle, the 
state of standstill induced by the 
action of either acid or alkali was 
neutralised and counteracted by the 
antagonistic action of the other (cf. 
fig. 155). I have detected precisely 
the same peculiarity in the case of 
growth-response also. It was seen 
in the course of the last experiment that growth was brought 
to a state of standstill, in the contracted position, by the action 



FIG. 198. Unbalanced Record 
showing the Action of Al- 
kiili, and the Antagonistic 
Action of Subsequent Appli- 
cation of Acid 

The downward arrow ( j) indi- 
cates application of alkali, 
which induces arrest of 
growth in contracted posi- 
tion. The upward arrow ( t ) 
indicates application of acid, 
which by its antagonistic 
action renews growth. 




FlG. 199. Effect of Strong Solution of NaCl on Kate of Growth, as 
Modified by Different Constitutions of Specimens 

At a temperature of 30 C. there is an immediate depression. Near the 
optimum there is a well-marked resistance, and preliminary acceleration 
before depression sets in. The same is true to a less extent at 37 C. 
These records were taken under balanced conditions. At 44 C. there 
was normally no growth, but this was temporarily initiated under the 
action of strong NaCl solution. Record taken by Method of Balance. 

of alkali (fig. 198). At this point, as marked by the upward 
arrow (t)> acid was applied. This reagent now had the effect 



EFFECTS OF DRUGS ON PLANTS 485 

as will be seen from the record, of neutralising the previous 
contractile arrest, and in the course of two minutes it had 
brought about the renewal of growth. 

Action of strong solution of sodium chloride. In the 
course of the investigation on suctional response we noticed 
that the effect of this reagent, when applied to the root, was 
not always to cause a diminution of suction, as might have 
been expected had osmotic action been the only factor. This 
reagent, on the contrary, usually operated to bring about a 
preliminary acceleration of suction. And I have already ex- 
plained that this was due to its excitatory" character, acting in 
opposition to the osmotic action set up by the strong solu- 
tion. It is usually supposed that the excitatory effect of 
strong solution of salt on animal tissues is due to the osmotic 
withdrawal of water ; but we have here seen that similar 
excitation is induced in vegetable tissues without any with- 
drawal of water. Hence the usual theory of the action of 
salt solution in causing excitation is rendered very doubt- 
ful. We found, in fact, that when the tonic condition of the 
tissue was favourable, the excitatory reaction predominated, 
and there was a consequent enhancement of suctional 
activity. In other instances, where the tonic condition was 
less favourable, osmotic action predominated, and there was 
a consequent depression of the normal rate of suction (p. 385). 
It is thus seen that the factor of variation in these two 
different cases was the constitution, or tonic condition, of the 
plant ; we are now able to study with precision the influence 
exercised by constitution, in causing the plant to struggle 
against, or succumb to, the action of adverse external cir- 
cumstances. The action, in causing variation of suction, of a 
strong solution of salt, applied internally through the cut end 
of the stem, can also be studied by means of growth-response ; 
for while increased suctional activity will give rise to a posi- 
tive turgidity- variation, with concomitant enhancement of the 
rate of growth, diminished suctional activity will have the 
Dpposite effect, of retarding the rate of growth. We shall 
next observe the effect of this reagent on different specimens 



486 PLANT RESPONSE 

of Crinum Lily, which had been taken from the same flower- 
head, and in which different constitutions were artificially 
induced by carrying out the experiments at 30 C, 34 C, 
37 C., and 44 C. The records were ta'ken under balanced 
conditions. 

On making an application of a solution so strong as 10 
per cent, to a specimen at the normal temperature of 30 C., 
the result was always a depression which set in immediately ; 
but when the tonic condition of the plant was exalted, by 
raising the temperature to a point near the optimum that is 
to say, to 34 C. the same reagent was found to induce an 
excitatory effect, its depressing action being postponed for 
a considerable length of time. The effect obtained at the 
temperature of 37 C. was very instructive. Responsive 
excitability at this temperature has been shown to be almost 
the same, if not indeed slightly lower, than that at 30 C. 
But we should remember that in these two cases we have very 
different histories ; for if in the former that is, at 30 C. 
we have a normal amount of latent energy, then it is clear 
that in the latter that is to say, at 37 C. there must be an 
excess of latent energy. And as the result of this we see 
that, while at 30 C. the growth-response showed immediate 
depression, at 37 C. it offered a considerable resistance, as 
seen in the temporary exaltation of response. Still more 
interesting, however, was the effect at 44 C. At this tem- 
perature it will be remembered that there was an apparent 
arrest of growth, often supposed to be due to the setting-in 
of heat- rigor. I have shown, however, that, so far from this 
being the case, there is still at 44 C. a good deal of rhythmic 
activity, the cessation of growth being due to the fact that in 
the multiple response of growth each constituent response and 
recovery had become equal. The presence of this activity at 
44 C. becomes quite clear, when we find that the application ol 
salt at this temperature has the effect of renewing for a time 
the resultant growth which had been in abeyance (fig. 199). 

Effect of poisonous solution of copper sulphate. The 
influence of constitution in determining resistance to adverse 



EFFECTS OF DRUGS ON PLANTS 



487 




circumstances is made still more striking when we observe 
the action on the plant of strongly poisonous reagents, such 
as 5 per cent, solution of copper sulphate. At the normal 
temperature of 30 C. its application, as will be seen in the 
record (fig. 200), induces a 
very rapid depression, which 
soon culminates in permanent 
arrest. But at 34 C. the 
resistance offered is consider- 
able, actually exhibiting it- 
self in temporary exaltation. 

A , o /-. . , FIG. 200. The Effect of Different 

At 44 C., again, we observe Constitutions in Determining the 

the poisonous reagent actually Resistance Offered to Poisons. 

. . . . The Action of 5 per Cent. Solution. 

initiating growth, as in the of Copper Sulphate 

last case. 

Opposite effects of the same dose on different consti- 
tutions. In the case last considered, we studied the pheno- 
menon of the resistance offered by the plant to a toxic dose 
largely in excess of the fatal amount. In spite of the 
excessive dose we found that, when a favourable constitution 
was artificially induced, the plant 
succumbed, it is true, but only 
after considerable struggle. I shall 
now proceed to show how, when 
the artificial constitution is suf- 
ficiently favourable, the plant, 
instead of succumbing to an 
ordinarily fatal dose, can shake off 
the effect, and may even be stimu- 
lated by it. Thus I found that a 
i per cent, solution of copper 
sulphate induces depression and 
is ultimately fatal at 30 C. ; but 

when the same dose is applied to a plant at 34 C. the effect 
is seen in a marked exaltation (fig. 201), which continues for 
a fairly long period, after which it shakes off the effect of the 
poison altogether, and resumes its normal rate of growth. 




FIG. 201. The Effect of Favour- 
able Induced Constitution in 
Enabling Plant to Shake off 
Result of Toxic Dose of 
Copper Sulphate 




488 PLANT RESPONSE 

Opposite effects of large and small doses. I have 
shown the toxic effect of a i per cent, solution of copper 
sulphate at the normal temperature of 30 C. If, however, 
the dose be reduced to '2 per cent, we shall find that its 
action becomes stimulatory (fig. 202). This is an interesting 
illustration of the general fact that a poisonous reagent, if 
given in sufficiently minute doses, will act as an excitant. 

In conclusion, a survey of the effects of drugs, both 
stimulatory and poisonous, reveals the striking fact that the 
difference between them is a question of quantity. Sugar, 

for instance, which is stimulat- 
ing when given in solutions of, 
say, i to 5 per cent., becomes 
depressing when the solution 
is very strong. Copper sul- 

FIG. 202. Opposite Effects ot phate again, which is regarded 
Large and Small Doses of i 1 

Poison as a poison, is only so at I per 

A solution of i per cent, copper cen t- and upwards, a solution of 
sulphate produces depression - 2 per cent, being actually a 

and death, but '2 per cent. r J 

exercises stimulating action. stimulant. Ine dinerence be- 

tween sugar and copper sulphate 

is here seen to lie in the fact that in the latter case the range 
of safety is very narrow. Another fact which must be borne 
in mind in this connection is that a substance like sugar is 
used by the plant for general metabolic processes, and thus 
removed from the sphere of action. Thus continuous absorp- 
tion of sugar could not for a long time bring about sufficient 
accumulation to cause depression. With copper sulphate, 
however, the case is different. Here, the constant absorption 
of the sub-toxic stimulatory dose would cause accumulation 
in the system, and thus ultimately bring about the death of 
the plant. 

SUMMARY 

Chemical reagents arc found to have the same effect on 
the response of growth as on ordinary response, or as on the 
autonomous response of Desmodium gyrans. 



EFFECTS OF DRUGS ON PLANTS 489 

Carbonic acid gas induces a preliminary acceleration, 
followed by retardation and arrest of growth. On re- 
admission of fresh air growth is revived. 

Ether, when applied internally, causes a preliminary 
acceleration, followed by retardation and arrest of growth. 
Minor differences of effect may be observed if the applica- 
tion be made externally. 

A very dilute solution of sodium carbonate is an exci- 
tant, and accelerates growth ; but stronger solutions cause 
retardation. 

Solution of sugar also stimulates growth, if the strength 
of solution be not excessive. 

Alcohol causes spasmodic alternations of growth. Too 
strong a solution arrests growth. 

As in the case of autonomous response of Desmodium, 
so in the response of growth also, acids and alkalis are 
found to have antagonistic effects. Acid causes relaxa- 
tion and ultimate arrest of growth ; alkali causes arrest 
of growth in the contracted position. The arrest brought 
about by one of these agents may be counteracted by the 
other. 

The reaction of a specimen to a chemical reagent is 
determined by the strength of the dose, and by the duration 
of application. 

A stimulating agent if given in too strong a dose causes 
depression. A poisonous reagent, again, if given in a suffi- 
ciently small dose, acts as an excitant. 

A clear insight into the nature of ' constitution,' so called, 
as a factor in determining the reaction of a specimen to a 
given drug, is afforded by the induction of definite arti- 
ficial constitutions. This may be carried out by subject- 
ing the plant to four different typical temperature con- 
ditions the ordinary, the optimum, post-optimum, and 
maximum. 

It is then found that a plant which has been made to 
acquire excess of internal energy can struggle against, or 
even overcome, the influence of such adverse circumstances as 



490 PLANT RESPONSE 

the action of poison, whereas under ordinary circumstances 
it quickly succumbs. 

The nature of the response is thus seen to be determined 
in a definite manner by the constitution, or tonic condition, 
of the plant, this factor being in its turn dependent on the 
sum total of the energy which is latent in the organism. 



PART VII 

GEOTROPISM, CHEMOTROPISM AND 
GALVANOTROPISM 



CHAPTER XXXVI 

THE RESPONSIVE CURVATURES CAUSED BY GRAVITY. 
NEGATIVE GEOTROPISM 

+ 

Statement of the problem of apogeotropic response Mode in which stimulation 
is brought about : radial -pressure theory, and theory of statoliths Mechanics 
of responsive movement Experiment demonstrating responsive curvature as 
brought about by unilateral pressure of particles Record of curvature induced 
by gravitation Record of different rates of curvature when specimen is held 
at angles of 45 and 135 co the vertical Determination of the true character 
of apogeotropic response Responsive curvature of acellular organs Curvature 
of grass haulm under gravity Growth of grass haulm on a klinostat. 

IT is well known that growing organs exhibit certain 
directive movements under the influence of gravitation. 
Horizontally laid shoots, for example, bend upwards, or 
against the direction of gravity ; while roots react in pre- 
cisely the opposite way that is to say, they bend so as to 
lie in the direction of the force of gravity. In the case of 
the various forms of stimulation hitherto studied, the action 
of the plant is well defined and intelligible, consisting of con- 
cavity of the excited side, in response to a stimulus which is 
clearly understood. But in the case of geotropism the mode 
in which stimulation is brought about is not quite evident, and 
we find, moreover, two directly opposed effects brought about 
by apparently the same force of gravity : in the root, as 
already said, a positive movement, that is to say a movement 
in the direction of gravity ; and in the shoot a negative move- 
ment, away from the direction of gravity. 

The seeming impossibility of explaining effects so divergent 
as due to a single common cause, has led to the modern 
idea that these responsive movements are ' executed at the 
suggestion of changes in the environment, not as the direct 



494 



PLANT RESPONSE 



and necessary result of such changes ; ' or in other words that 
' light and gravitation could be classed together as external 
agencies acting, not directly, but in some unknown indirect 
manner.' 1 

There are two distinct points to be borne in mind in 
connection with the effect of gravity : first, the question as 
to how gravity exercises stimulation ; and secondly, that of 
how, in answer to this stimulus, a definite responsive curvature 
is induced. 

Mode in which stimulation is brought about. Now it 
is clear that, as regards the former of these points, the only 
conceivable way in which gravity could produce stimulation 

is by the effect of weight ; 
not, that is to say, by the 
weight of the plant as a whole, 
but by the differential effect 
of weight in the cells. Two 
important theories have been 
advanced which offer very 
rational explanations of the 
means by which gravi-percep- 



B 





C 




B 




A 









FIG. 203. Diagrammatic Representa- 
tion showing Differential Effect 
of Weight on Lateral Walls of 
Cells 



In the figure to the right the cell is 
laid horizontally, and the lateral 
wall, l), has to bear greater weight 
than c (after Francis Darwin). 



tion may be induced. Accord- 
ing to these, the necessary 
differential weight-effect may 
be due to the weight of the 
cell-contents, whether of the 

sap itself, or of those heavy particles like starch-grains which 
are contained in it. When, therefore, the cell is laid horizon- 
tally, it is the lower tangential wall which has to support the 
relatively greater weight (fig. 203). This theory of hydrostatic 
or radial pressure was suggested by Pfeffer, and supported by 
Czapek. The other theory, of statoliths advocated by Noll, 
Haberlandt, and Nemec substitutes for the weight of the 

1 A luminous rtsumj of our present state of knowledge on this subject, with 
all its difficulties and obscurities, is contained in the addresses of Francis Darwin 
delivered before the British Association in 1891 and 1904. From these I have 
made the quotations which appear in the text. Figs. 203, 204, and 209 appear 
as illustrations of the latter address in Nature of September 8, 1904. 



RESPONSIVE CURVATURESNEGATIVE GEOTROPISM 495 

water-column the weight of certain relatively heavy bodies, 
such as starch-grains, differentially exercised upon the lower 
tangential walls. In the case of multicellular plants, laid hori- 
zontally (fig. 204), EE and E'E' may be regarded as regions in 
which stimulation is caused by the weight of the particles. 
The effects produced on 
the upper and lower c 
halves are evidently an- 
tagonistic, and in spite of 
this we obtain in the case 
of shoots a resultant cur- 
vature upwards. This 



.1 1 1. 



E 
M 



shows that the stimulation c , 
of one half must be greater 

than that of the Other. FlG - 2 4- Diagrammatic Representation 
_, ,. , of a Multicellular Organ 

The inequality must be 

On the upper side the statohths act on the 
due to this difference, that inner, and on the lower side on the 

the statoliths in EE rest Darn'm^^^ 1 Wa " ^ FrandS 
on the inner, and in E'E' 

on the outer, tangential wall. It would thus appear that one 
of these must be less excitable than the other. 

Mechanics of responsive movement. From these 
hydrostatic or statolithic differences of weight, bearing on the 
ectoplasm of the cell, it is understood that gravi- perception 
arises, that is to say that the plant perceives the direction of 
gravity. But there is no explanation as to how stimulation 
is produced ; nor is there any satisfactory explanation as to 
the mechanics of the responsive curvature. It is generally 
supposed, as has been said, that this curvature is not the 
direct and necessary result of some environmental change, 
but that it is an instance 'of a plant reading a signal and 
directing its growth' accordingly. It is supposed further 
that in an apogeotropic cell the curvature takes place by the 
development * of relatively accelerated growth on the side on 
which the pressure is greatest.' 

This view, that the curvature induced in some unknown 
way in a geotropic organ, by gravitation, depends upon the 



496 PLANT RESPONSE 

accelerated growin of the convex side, is apparently supported 
by Elfving's experiments on grass haulms in which growth 
had been at standstill. ' He found that the pulvini of grass 
haulms placed on the klinostat increase in length. This 
experiment shows incidentally that the klinostat does not 
remove but merely distributes equally the geotropic stimulus ; 
also that geotropic stimulus leads to increased, not to 
diminished, growth. The same thing is proved by the simple 
fact that a grass haulm shows no growth in its pulvinus while 
it is vertical, so that when curvature begins (on its being 
placed horizontally) it must be due to acceleration on the 
convex, since there is no growth on the concave side in which 
retardation could occur.' l 

In the cases of response to stimulation, hitherto studied, 
however, it will be remembered that the fundamental effect 
was always a contraction and concomitant retardation of 
growth, the expansion and acceleration of growth being 
always a secondary and indirect effect ; but in the case of 
the response to stimulus of gravity, the interpretation of 
results which has just been quoted would make it appear 
that the responsive action brought about by gravity was 
essentially distinct in its character from the responsive re- 
action with which we have hitherto been familiar. 

Experiment demonstrating responsive curvature as 
brought about by unilateral pressure of particles. My 
own object in the course of the present chapter is, however, 
to demonstrate the fact that the curvature of an apogeotropic 
plant-organ in response to gravitation is the result of the 
direct effect of stimulus, its reactive peculiarities being in no 
way different from those other instances of the response of 
the plant to stimulation, which we have already studied 
And in order to simplify my explanation, I shall here 



1 B.A. Report, 1891, p. 671. It ought to be mentioned here that in other 
plants when placed on the klinostat this increased rectilinear growth was not 
observed, leading to the supposition that in such cases a simultaneous increase 
and decrease of growth-rate on opposite sides of the rotating plant is produced. 
Jbid. ; Nature, 



RESPONSIVE CURVATURES NEGATIVE GEOTROPISM 497 



describe a typical experiment showing the excitatory effect 
of pressure in inducing responsive curvature. That unilateral 
pressure or contact does induce curvature is known in the 
case of tendrils ; but in the present experiment I shall show 
that this responsive reaction is not peculiar to tendrils alone, 
but is exhibited by all organs alike. I was also desirous of 
making the action on my experimental specimens in every 
way parallel to that of gravitationally excited tissue in which 
the statolithic particles exert 
their weight on a particular 
side of the responsive organ. 

I took a specimen of 
Crinum Lily and mounted it 
vertically. I next took a thin 
strip of india-rubber, the inner 
surface of which was studded 
with iron particles, adhering 
by means of shellac varnish. 
This was adjusted laterally, 
on one side of its zone of 
growth, so as almost, but not 
quite, to touch the specimen. 
On the opposite side was 
placed an electro-magnet, 
which when excited attracted 
the strip to which the iron 
particles adhered, and thus 

produced a unilateral pressure on the specimen, the magnetic 
particles functioning as so many statoliths (fig. 205). A 
recording microscope, which will be fully described in a later 
chapter, was now focussed on the index, I, attached to the 
specimen. 

Before the application of pressure, the quiescent condition 
of the specimen had been ascertained by noting the stationary 
position of the index in the field of view of the microscope. 
On now applying unilateral pressure by exciting the magnet, 
we might expect to obtain two different effects. The first oi 

K K 




FIG. 205. Diagrammatic Repre- 
sentation of Experiment showing 
Curvature Induced by Unilateral 
Pressure Exerted by Particles 

F, flower-bud of CHnmn ; s, india- 
rubber strip studded with iron 
particles attracted by electro- 
magnet, M, causing unilateral 
pressure on growing region ; I, 
index attached to flower. 



498 PLANT RESPONSE 

these would be the sudden magnetic pull which would cause 
a movement of the flower aivay from the pressed side. This 
effect would be instantaneous, and quickly come to a stop. 
The second would be the excitatory effect on the specimen 
of the unilateral irritation caused by the pressure of the 
iron particles. If this response, like response in general, 
were to take the form of a contraction, a curvature concave 
to the source of stimulus would be produced, by which the 
flower should be seen to bend towards the stimulated side. 
This stimulatory effect, unlike the mechanical disturbance, 
would go on increasing with time. In this way it is easy to 
demonstrate that unilateral irritation of pressure of particles 
induces contraction, which in growing organs retards the 

normal rate of growth, 
the side acted upon 
thus becoming concave 
(fig. 206). It should 
also be borne in mind 
that in consequence of 
,, , ", TT \ ~ this responsive con- 

r IG. 206. Record of Responsive Curvature __ r 

induced in Crinum under Experimental traction some of the 

Conditions shown in fig. 205. Magnifica- P5mp ii e rf wa ter finds 

lion twenty-five times CXpCllCd water nnab 

its way to the opposite 

side, thus increasing its turgidity, with consequent acceleration 
of rate of growth and convexity of that side. The experi- 
mental proof of this latter phenomenon will be given in the next 
chapter. Moreover, in consequence of the curvature induced 
by this contraction of the excited side, the further side will 
be stretched, and tension is, as we know, an agent tending to 
increase the rate of growth. Thus we see that in this case 
the contraction of the excited side is the active factor, and the 
convexity of the further side is the secondary or subsidiary 
effect in the growth-curvature. 

If then such stimulation by unilateral weight-effect, whether 
statolithic or hydrostatic, be the cause of the apogeotropic 
curvature induced by gravitation, it follows that the active 
factor in the process lies in the contraction of the upper side, 




RESPONSIVE CURVATURES NEGATIVE GEOTROPISM 499 

and that the expansion and convexity of the lower must be 
merely the subsidiary effect. But it is usually supposed, as 
we have seen, that the predominatingly active factor in these 
gravitation-curvatures is the accelerated growth of the convex 
side. Hence the crucial experiment by which the correctness 
of one hypothesis or the other may be determined will 
evidently consist in demonstrating which of the two, contrac- 
tion or expansion, is actually the essential element in the 
responsive growth-curvature. And if, further, we should 
succeed in proving that contraction was the essential element, 
we should then have established a unity, as between the 
phenomena of response to gravitation and those which are 
the results of other forms of stimulation. But before I 
describe this particular investigation I must explain the 
method of continuous record, which I employ for observing 
gravitational curvatures. 

Record of curvature induced by gravitation. We have 
in the Optical Lever a means by which the responsive effect 
of gravitation-curvature and its variations may be recorded 
quickly and continuously, and with as great a magnification 
as is desirable. The horizontally laid specimen, say the scape 
of Uriclis Lily, has its terminal upper end attached to one arm of 
the Lever, the other arm being weighted with a slight counter- 
poise. A continuous record is then taken, on a revolving 
drum, from which we obtain the responsive curvature and its 
time-relations. It will be seen from the record that the scape 
first bent down during a period of forty minutes, after which 
the effect of gravitation was seen by the reversal of the 
curve, which indicated the curving up proper to gravitation 
(fig. 207). 

It is sometimes thought that this preliminary lapse of 
time before the appearance of the gravitation-effect, some- 
times known as the presentation time, is the interval necessary 
for the statoliths to fall ; but I shall presently describe some 
experiments which will show that the time taken for variation 
in response to the varying action of gravitation is very much 
shorter, probably less than a minute. In the present case we 

K K 2 



500 PLANT RESPONSE 

must remember that the movement in response to gravitation 
has to overcome opposite mechanical movements before it 
can be made perceptible at all ; for we have, firstly, the 
weight of the organ, which tends to make it bend down ; and 
secondly, on account of this bending, we have greater tension 
on the upper surface, which, as we have seen, increases the 




Kir.. 207. Record of Apogeotropie Response in Scape of Uriclis Lily 

The up curvature due to apogeotropic action proper commenced forty 
minutes after the specimen was laid horizontally. 

rate of growth, tending to make that side convex. The 
differential effect due to gravity has to overcome all this, and 
becomes visible only when it has done so. After this stage 
has been reached, the rate of movement upwards goes on 
increasing until a fairly constant rate is attained. 

Record of different rates when specimen is held at 
angles of 45 and I35. Czapek has found that the effective 
stimulus of gravitation is greater when the organ is held at 
135 to the vertical than when held at 45. This difference 
of effect can be obtained quantitatively with great accuracy 
by taking successive records with the specimen in the two 
positions ; and in order to eliminate the effect of any chance 
disturbance, or of spontaneous variation, I took four alternate 
records, first with a specimen at 45, then at 135, then back 
once more at 45, and then again at 135, each position 
being maintained just long enough for the attainment of 
the permanent effect. The change from one position to the 
other was made quickly, in the course of less than a minute. 
The experiment was carried out with the unopened flower of 
Crinum Lily, in which we have already found that the rate of 
growth is very regular and considerable. 



RESPONSIVE CURVATURES NEGATIVE GEOTROFISM 5OI 



The first record was taken after the curvature movement, 
due to gravitation at an angle of 45, had attained a constant 
value. The tip of the flower was then found to be moving 
at a rate of four divisions per minute. On changing the 
angle to 135, the rate of movement showed an imme- 
diate increase, and attained in the course of five minutes 
a permanent rate of 15-5 divisions per minute. It has been 
said that the experimental adjustment of change of position 
took only about a minute 
to effect, yet on renewing 
the record we find that the 
effective increase thus esta- 
blished in the gravitation- 
stimulus was immediatelv 



perceived and responded 
to by the organ, in an 
accelerated rate of curva- 
ture. The permanent in- 
creased rate at 135 is thus 
found to be nearly four 
times that at 45. The 
organ was now returned 
to its position at 45, and 
the rate once more fell till 




FIG. 208. Response Records showing Dif- 
ferences in Rate of Curvature according 
as Specimen is held at Angles of 45 
and 135 



it became nearly equal to 

what it had originally been 

at 45, being only very 

slightly greater. The organ 

was once more placed at 135, and the rate again rose, till it 

reached slightly beyond its former value at that angle. The 

ratio of the second rates determined at 45 and 135 was also 

found to be almost as I is to 4 (fig. 208). 

With reference to the cause of this difference of gravita- 
tional effect, Ilaberlandt suggested that it may lie in the 
fact that the weight of the statoliths in 45 position is on the 
basal half, while at 135 it is on the apical half (fig. 209) 
The next ooint is. to account for the greater reaction of the 



502 



PLANT RESPONSE 




Different Positions of a Single Cell, accord- 
ing as the Specimen is held at an Angle of 
45 or 135, showing Consequent Redistribu- 
tion of Statoliths (after F. Darwin) 

A, apical, B, basal, ends of cell. 



apical half. Some light may, perhaps, be thrown on this 
subject from the results of my experiments on Biophytum. 
I found that on subjecting this plant to the favourable tonic 

condition of rise of tem- 
perature, the younger 
leaflets began to show 
spontaneous excitatory 
response much earlier 
than the older leaflets. 
This shows that in an 
excitable tissue the 
younger portions are, 

FIG. 209. Diagrammatic Representation of generally speaking, the 

TTN? ff A ft .. _ f . C* 1 ^ ^* _ 1 1 ^_^_ M J ^^ 

more sensitive. This 
will probably account 
for the difference of 
geotropic reaction in 
the case under consideration, for the apical halves of the cells 
are relatively younger than the basal halves. 

Determination of the true character of apogeotropic 
response. I shall now proceed with the crucial determi- 
nation of the true nature of gravitational response. In the 
diagrammatic representation of the multicellular organ, we 
have the upper and lower responding layers of cells repre- 
sented by E and E' (fig. 204). Each of these may be one or 
more layers in thickness. Since in apogeotropic organs the 
curvature is upwards, this response may be due (i) to the 
relative expansion or acceleration of growth of the lower side, 
or (2) to the relative contraction or retardation of growth of 
the upper side. As regards the first hypothesis, the curva- 
ture induced in grass haulms has been assumed, as stated 
before, to have proved that the gravitational response is one 
of accelerated growth. On the other hand, the curvature 
may be due to the active contraction of the upper side, the 
response then being of the same nature as was seen demon- 
strated by the irritating pressure of the magnetic particles. 
The crucial experiment in deciding between the two alter- 



RESPONSIVE CURVATURES NEGATIVE GEOTROPISM 503 

native hypotheses will lie in determining which factor is 
actively concerned in the production of responsive curvature. 
If the lower should prove to be the side actively concerned, 
then the response will be one of expansion ; the activity of the 
upper, on the other hand, would demonstrate the contractile 
nature of the response. 

The principle of the mode of investigation which I adopted 
in order to decide this question will be understood, if we 
remember that motile response can be abolished temporarily 
by application of cold. Thus, if we cool the pulvinus of 
Mimosa, it ceases to exhibit any responsive contraction 
under stimulation. Now, in a horizontally placed organ, if 
the continued responsive curvature be due to the excitatory 
contraction of the upper side, then local application of cold 
on that side ought to arrest it. The application of cold 
on the lower side, however, should produce little effect. But 
if, on the other hand, the lower side should be actively con- 
cerned in the production of response, then the application 
of cold on that side would have the effect of arresting 
the curvature, while its application on the upper would 
have little or no effect. In connection with this, it should 
be remembered that a twitch may sometimes be produced 
by the transient excitation due to sudden variation of tem- 
perature, but the effect will be short-lived. The permanent 
arrest of hitherto continuous responsive movement due to 
gravitational stimulus will only take place when the active 
side has its power of response abolished by cold. 

An experiment carried out in this manner would thus 
decide the question as to whether it is the upper or the lower 
side that is actively concerned, and also the question as to 
whether the gravitational response, like all other forms of 
response to direct external stimulus, is, or is not, funda- 
mentally one of contraction. 

In carrying out experiments on the principle described 
above, I first took a record of the responsive curvature of a 
Crinum Lily, which was lying horizontally. When a uniform 
rate of upward movement had been attained, the tip of the 



504 PLANT RESPONSE 

bud moving at a rate of '13 mm. per minute, ice-cold water 
was applied on the upper surface, by means of a strip of 
cloth, at the point marked in the figure by a downward 
arrow (4) (fig. 210). In consequence of this the movement 
is seen to be retarded, and in the course of five minutes it 
came almost to a stop. The cloth was now removed, at the 




FIG. 210. Effect on Apogeotropic Movement of Application of Ice-cold 
Water to Upper and Lower Surfaces alternately of a Horizontallyilaid 
Crinntn Lily 

The first part of the curve shows normal up movement, which is arrested 
in consequence of the application of cold on the upper surface, at the 
moment marked with a downward arrow ( j). On removal of the 



application at point marked {' 4< , ^c normal apogeotropic movement is 

renewed, and continues unaffected by the application of cold below at 
the moment marked with an upward arrow (t). 

moment marked in the curve, and the movement of response 
to gravitation recommenced and tended gradually to attain 
its original value, with the return of the upper surface to the 
normal temperature. Ice-cold water was next applied on 
the lower surface, at the moment marked by an upward 
arrow (t), and it will be seen that this produced no percep- 
tible effect on the rate of responsive curvature, A similar 



RESPONSIVE CURVATURES NEGATIVE GEOTKOl'ISM 505 

experiment was performed with a long flower-scape of 
Uriclis Lily laid horizontally. The attachment to the re- 
cording lever was made with the upper end of the specimen, 
the lower end being held in a clamp. The specimen was 
30 cm. long, and the responsive movement was found to be 
23 mm. per minute (fig. 211). Here, too, ice-cold water was 
applied to the upper and lower surfaces alternately, four 
times in succession, and it will be seen from the figure that 
an application of cold to the upper surface caused arrest of 




FlG. 211. Effect on Apogcotropic Movement of Temporary Applications 
of Cold alternately to Upper (|) and Lower (*) Surfaces of Horizontally 
laid Scape of Uriclis Lily 

Application above is seen to produce arrest of movement, while application 

below has no perceptible effect. 



the responsive movement, while a similar application below 
produced no effect that could be detected. 

These experiments conclusively prove that the funda- 
mental responsive effect induced by stimulus of gravitation 
is not acceleration, but contraction, or retardation of growth, 
precisely similar to the action of other forms of stimulus. 

Though, under cooling, there cannot be any exhibition of 
mechanical response to gravitation, yet it appears that the 
effect of gravitation may be held latent in the organ, as will 



506 PLANT RESPONSE 

be seen from the following experiment. I took three long 
scapes of Uriclis Lily and laid them horizontally, packed in 
ice. As long as they were in the ice there was no responsive 
curvature. After an hour they were taken out of the ice 
and held erect, and by the time that they were restored to 
the temperature of the room it was found that the top of the 
scape in each case was bent from 1*3 to 1*5 cm. in the 
direction of what had been the upper surface when they 
were horizontal. This was evidently due to the fact that the 
cold brought about an arrest of growth ; but the geotropic 
stimulus remained latent, to express itself in a responsive 
movement later, when growth was renewed, under the action 
of a favourable temperature. 

Responsive curvature of acellular organs. There is 
one point, in connection with the induction of gravitational 
curvature, which might at first sight appear anomalous. In 
the case of the negative geotropic curvatures of multicellular 
organs, the fact that it is the upper side which is relatively 
effective, and that the curvature is the result of its responsive 
contraction and concavity, is evident from the experiments 
already described on the local application of cold. When 
we take an acellular apogeotropic organ, however, such as the 
stalk of the sporangium of Mucor, we find that it is the 
irritated lower side, differentially acted on by weight, whether 
of sap or statoliths, that becomes convex. This looks at first 
sight as if the effect of irritation were, as it is generally 
supposed to be, to induce acceleration of growth, and 
attendant convexity. 

We must in this case, however, bear in mind the position 
of the surface on which the stimulus acts. In our experi- 
ment on the irritation produced by the pressure of magnetic 
particles it was the outside surface of the organ that was 
acted on by stimulus, and it was that side that became 
concave. In the case of the organ with a single row of cells 
as described, however, it is the internal surface which is so 
irritated, and if we take as our object of observation that 
internal surface, we shall find that, as in other cases, so here 



RESPONSIVE CURVATURESNEGATIVE GEOTROPISM 507 

also, the response is by contraction and concavity of the 
excited surface. The convexity of the outer is thus to be 
taken as the inevitable result of the concavity of the inner. 

The fact that in the acellular organ response actually 
takes place by the concavity to stimulus of the surface acted 
upon, is further seen in the response of such organs to stimulus 
of light. In the case of geotropic stimulus it is the internal 
surface of the lower side of the organ which is irritated by 
the differential weight of the cell-contents, and becomes con- 
cave to the stimulus thus acting upon it. In the case of light, 
on the other hand, stimulus acts from the outside, and it is 
thus the outer or external surface, say, of the same side, 
which becomes concave. Thus stimulus, acting on the same 
side in one case from within, and in the other from without, 
induces responsive curvatures in opposite directions. 1 

1 The explanation of the responsive curvatures of acellular organs has hitherto 
offered many difficulties. In a multicellular organ, acted on unilaterally by stimu- 
lus, there is a difference induced in hydrostatic pressure as between the two 
opposite sides. The diminished turgidity of the proximal, and increased turgidity 
of the distal, explains the induced curvature. In an acellular organ, however, 
there cannot be this difference of hydrostatic pressure on the two opposite sides. 
But the considerations which I shall now offer may perhaps be found to meet the 
difficulties of the case. The fundamental effect of stimulus is, as we know, to 
induce protoplasmic contraction. Hence unilateral stimulation acting on an 
acellular organ may be expected to induce contraction and concavity of the 
proximal side of the ectoplasmic layer, the result of which will be a curving over 
of the organ. As a result of this, the ectoplasmic layer of the distal side will be 
subjected to tension, which is, as we know, an influence that accelerates growth. 
Hence the retardation of growth on the proximal, due to contraction, and its 
acceleration on the distal under increased tension, will combine to produce growth- 
curvature. 

A problem of somewhat greater complexity arises in the case of stimulus of 
light traversing a transparent acellular organ. Let us suppose such a vertical 
organ to be acted upon horizontally by rays of light from the right-hand side. 
We have in this case to consider the separate effects of stimulus of light on four 
different surfaces : (i) the outer ectoplasmic layer of the proximal side P0; (2) 
the inner ectoplasmic layer of the proximal side P* ; (3) the inner ectoplasmic 
layer of the distal side D* ; and (4) the outer ectoplasmic layer of the distal side 
Do. The contractions of the outer surface of the proximal Po, and the inner 
surface of the distal Dz\ would induce a curvature to the right ; those of the inner 
surface of the proximal Pi and the outer surface of the distal Do a curvature to 
the left. But it is evident that as light passes through the organ there must be 
loss of intensity by absorption. Hence the sum of effective intensities at Po and 



508 PLANT RESPONSE 

Curvature of grass haulms under gravity. We shall 
now take up the consideration of the curvatures induced in 
grass haulms, laid horizontally, when growth had originally 
been at standstill. It will be remembered that it was the 
appearance of curvature under such circumstances in the 
pulvinus of the grass haulm that gave the strongest support 
to the theory, that negative gravitational curvature in general 
was due to the increase in the rate of growth on the convex 
side, rather than to its retardation from active contraction on 
the concave. In the present case it was argued that since, 
at the beginning of the experiment, the upper side of the 
pulvinus was not undergoing growth, it was clear that growth 
there could not be retarded. The curvature, therefore, must 
be due to the induction of growth on the convex side, under 
the stimulation of gravity. 

This misconception has arisen from the supposition that 
all curvatures must be induced by differential growth. I 
have shown, however, (i) that contraction takes place in a 
stationary organ in response to stimulus ; (2) that the 
unilateral stimulation of such an organ induces concavity ; 
and (3) that retardation of growth in a growing organ is 
itself the result of the contractile effect of stimulus. Now, in 
a horizontally laid grass haulm in which growth has ceased, 
the upper side which we have found to be relatively the 
more effective will contract under stimulus of gravity 
That this is the case is seen from the fact that this upper 
surface is found to become actually shorter than it was 
before. But, as regards the convexity of the lower surface, 
the water expelled from the actively contracting upper 
side will reach the lower, and the increased turgidity thus 
produced is sufficient, as we have already seen, to re- 
initiate growth in a dormant tissue. This explains the 
renewed growth and convexity of the lower side. Thus the 
curvature of the grass haulm cannot be held to support the 

at Di will be greater than the corresponding sum of effects at Pi and Do, As 
the result of this difference we shall have a right-handed or positive heliotropic 
curvature. 



RESPONSIVE CURVATURES NEGATIVE GEOTROPISM 509 

view that the fundamental action of gravitational stimulus is 
to increase the rate of growth ; it shows, on the contrary, that 
contraction under stimulation is the active factor. 

Growth of grass haulms on a klinostat. I shall here 
adduce certain considerations which may further serve to 
explain the difference between the stationary or feebly grow- 
ing grass haulms, and other normally growing organs, as 
regards the effects induced in them by the rotation on the 
klinostat. It is found that in grass haulms, when subjected 
to the rotation of the klinostat, growth is recommenced, or 
increased ; while in other normally growing plants there is 
no such increase of rectilinear growth. For an explanation 
of this difference we have to recall the effect on growth of 
increased internal hydrostatic pressure, with the consequent 
increase of turgidity, which has already 'been described 
(p. 428). It was there shown that, when the plant was grow- 
ing at a moderate rate, the curve of relation between increase 
of turgidity and increase of growth was practically a straight 
line ; that is to say, any increase or diminution of turgidity 
would then produce a proportionate increase or decrease of 
growth. But this relation did. not hold good when the 
natural rate of growth was feeble or absent. In the latter 
cases, increase of pressure induced an effect that was dis- 
proportionately large. And this was specially the case when 
the growth of an organ had come to a temporary condition 
of standstill. In that case, when the pressure was gradually 
increased, growth was found at a certain point to be abruptly 
renewed, and to go on increasing with the increase of pres- 
sure, at a rate disproportionately large. If, now, the pressure 
be once more brought down to what it was just before the 
point was reached at which growth was started, we find that 
growth is not arrested, but persists. Thus the net result 
of these alternations of pressure is a positive resultant 
growth. We have, thus, two distinct cases: (i) that in 
which the normal rate of growth is moderate, and in which 
alternate increase and diminution of pressure, acting for 
equal lengths of time, will induce equal increase and decrease 



510 PLANT RESPONSE 

of growth alternately, the total growth during the whole of 
the time being the same as it would have been without alter- 
nation ; and (2) that in which, the original rate of growth 
having been feeble or absent, equal alternations of pressure 
have the net effect of causing a positive increase of rectilinear 
growth. Now, in an ordinary growing plant, rotating on the 
klinostat, we have an instance of the first of these two cases ; 
for if at any given moment the side A be below, and the 
side B above, then B under stimulus of gravity will undergo 
contraction ; hence there will be a diminution of local tur- 
gidity, and the expelled water will produce an increase of 
turgidity on the opposite side A. At the end of a semi- 
revolution of the klinostat, however, this state of things will 
be exactly reversed, and the alternating effects being thus 
equal and opposite, there will be no resultant increase of 
rectilinear growth ; but in the second case that is to say, of 
stationary or feebly growing grass haulms the resultant 
effect is not nil, but a positive increase of rectilinear growth. 



SUMMARY 

It has been shown that the effect of gravitational'stimulus 
on an apogeotropic organ is fundamentally the same as 
that of any other form of stimulus, namely, a responsive 
contraction. 

A rational explanation of the mode in which geotropic 
stimulus acts, is afforded by the radial-pressure theory of 
Pfeffer and Czapek, or by the statolithic theory of Noll, 
Haberlandt, and Nemec the essential element of both lying 
in the hypothesis that stimulus is caused by means of the 
weight of the cell-contents acting differentially on the inner 
wall of the horizontally placed cell. 

That the unilateral pressure of particles is competent to 
induce responsive curvature of the organ, has been shown 
experimentally by pressure resulting from the magnetic 
attraction of iron particles. 



RESPONSIVE CURVATURES NEGATIVE GEOTROPISM $11 

<* 

It has generally been supposed that the active factor in 
apogeotropic curvature was the accelerated rate of growth on 
the convex side of the organ ; but it has here been shown 
by crucial experiments on the unilateral application of cold, 
that the active factor is really the responsive contraction 
and retardation of growth on the concave side. 



CHAPTER XXXVII 

THE RESPONSIVE PECULIARITIES OF THE TIPS OF 

GROWING ORGANS 

Difference between shoot and root in their response to stimulus of gravity 
Difference in character of response between tip and growing region of root 
Scope of the investigation Electrical investigation Responsive results of : i. 
Longitudinal transmission of effect of stimulus from tip ; (a) Moderate uni- 
lateral stimulation ; (/) effect of stronger unilateral stimulation 2. Direct 
unilateral stimulation of growing region Moderately strong stimulus 3. 
Transverse transmission of effect of stimulus ; (a) moderate stimulation ; (6) 
stronger stimulation Mechanical response inferred from observed electrical 
response Tabular statement. 

WE have seen in the last chapter that the responsive effect 
of gravitational stimulus in an apogeotropic organ is of the 
same nature as that of any other form of stimulation. In 
positively geotropic organs like roots, however, this would 
seem not to be the case, for here the responsive curvature 
is in the opposite direction. Thus, a root .placed horizontally 
bends in the direction of gravity, and not away from it. 

It may be urged that there is some polar difference 
between shoot and root, on account of which, if the response 
of the one be regarded as positive, that of the other must be 
negative ; but I have shown that, so far from this being the 
case, the response of the root to stimulus is precisely the 
same as that of all other organs, the shoot included, for all 
alike under stimulation exhibit contraction (p. 76). 

We shall first see, then, whether in the root there is any 
difference, as regards the action of gravity, from, for instance, 
the stem. We know that the growing stem with regard to 
the gravitational stimulus is both the perceptive and respond- 
ing organ ; for we may cut and isolate any portion of it, 



RESPONSE OF TIPS OF SHOOT AND ROOT 513 

and it will still show an apogeotropic curvature. But the 
case is quite different with the root. Here the perceptive 
organ and the responding organ are, as will be seen in the 
next chapter, distinct and separate from one another. 

Difference in character of response between tip and 
growing region of root. We shall next turn our attention 
to the peculiar characteristics of the tip of the root as dis- 
tinguished from the responding region of growth. Darwin, on 
applying moderate artificial stimulus unilaterally to the tip 
of the root, found that it moved away frpm the source of 
stimulus ; whereas Sachs, on applying similar unilateral stimu- 
lus to the responding growing region directly, found that it 
moved towards it. Thus we see that two different effects are 
induced, according as stimulation is applied on^ the responsive 
zone itself, or transmitted to it from the distant tip ; and in 
this fact we may perhaps find a clue to the explanation of 
the opposite effects produced by stimulus of gravity on 
positive and negative geotropic organs. 

No explanation has yet been offered of the opposite 
characters of these responsive effects induced by similar 
stimuli, according as they are applied on the responsive zone 
or on the tip of the root. It was suggested by Darwin that 
' the tip of the radicle is endowed with diverse kinds of 
sensitiveness ; and that the tip directs the adjoining growing 
parts to bend to or from the exciting cause, according to the 
needs of the plant.' l These diverse kinds of sensitiveness 
have in his view been acquired by the tip of the root, for the 
final advantage of the plant. 

The question, then, which we must investigate is, as to 
whether the peculiar sensitiveness of the tip has been specially 
evolved by the burrowing root, or is characteristic of the tips 
of growing organs in general. Should the latter prove to be 
the case, we have next to account for this characteristic itself. 
I have already suggested, as a possible explanation of the 
difference between the responsive actions in apogeotropic 
and geotropic organs, that in the one case stimulus acts 

1 Movements of Plants , pp. 552 and 573. 

LL 



5 14 PLANT RESPONSE 

directly, producing a movement towards^ while in the other 
the effect of stimulus is transmitted from a distance, produc- 
ing a responsive movement in the opposite direction. As 
against this assumption, however, we are confronted with the 
fact, which we shall find later, that in some instances of 
transmitted stimulus of light, the responding organ bends 
towards, and not away from, the source of stimulation. 

Scope of the investigation. We have therefore to 
determine (i) whether or not the tips of all growing organs 
behave alike ; (2) why the behaviour of the tip is different 
from that of the responsive zone of growth ; and (3) under 
what circumstances the transmitted effect of stimulus causes 
an organ to move towards, and under what, to move away 
from, the source of stimulation, at the same time ascertaining 
clearly the mechanics of such movements. For the purpose 
of this investigation I shall first use the electrical method of 
inquiry, which I have already fully described, since it has 
the unique advantage of offering unerring indications under 
the most difficult experimental conditions ; and shall study by 
its means the characteristic differences of response as between 
the tip and growing region of a single organ, in this case the 
shoot. 

Electrical investigation. It will be shown that growth- 
curvature is produced by unequal variations of turgidity on 
the two sides of the responding organ. This variation can 
be detected with great certainty by electrical means. I have 
already explained how a positive turgidity-variation gives a 
concomitant electrical variation of galvanometric positivity ; 
and that the true excitatory effect of negative turgidity- 
variation gives rise to a concomitant electrical change of 
galvanometric negativity. For the present experiments 
I took specimens of various growing plants such as Bryo- 
phyllum, Cucurbita, and others and the results obtained 
from all were alike. In order to obtain unmistakable indica- 
tions of the effect produced in the responding zone, one 
electrical connection was made at a point in the growing 
region, A (fig. 212, a), and the other with a point so distant 



RESPONSE OF TIPS OF SHOOT AND ROOT 515 

that the effect of stimulation could not reach it. As 
parenchymatous tissues offer great resistance to the conduc- 
tion of stimulus, it is an advantage to make this second 
contact with a leaf. In the present investigation we have to 
study the effect of unilateral stimulation of varying intensity 
and duration, when applied either at the tip T, or at C, near 
the responding point A, the latter being in the same longi- 
tudinal line as the excited points T and C. We shall also 
study the effect of stimulation of A, on the transverse point 




Experimental Connections for obtaining Electrical Response 
due to Direct and Indirect Effects of Stimulation 

(a) electrical connections for detection of electrical changes at A, caused 
by indirect effect of stimulus, from excitation of same side of distant 
tip, T, and direct effect of stimulus from excitation of adjacent point, c ; 
(A) electrical connections for detection of electrical changes at B, caused 
by excitation of diametrically opposite point, A ; (<:) electrical connec- 
tions for detection of relative electric variations of diametrically opposite 
points, A and B, clue to excitation of c, adjacent to A. 



B (fig. 212, ), and the resultant effects on A and B, when 
the point C, near A, is stimulated (fig. 212, c). 

Before proceeding further, it will be well to consider the 
theoretical conclusions to which we are led from the demon- 
strations already made of the turgidity-variations caused by 
stimulation. The tip T consists, as we know, of undifferen- 
tiated tissue, which is a relatively bad conductor of stimula- 
tion. Moderate stimulus, then, at T, might be expected to 
induce local excitatory contraction, and the water thereby 
pelled would originate a wave of positive turgidity-varia- 
n ; this would reach A, a point on its own line, with greater 
effect than the transversely placed B. The moderate stimula- 
tion of T would thus produce a positive turgidity-variation 



L L 2 



$16 PLANT RESPONSE 

at A on its own side. This transmitted effect of increased 
turgidity we have already distinguished as the indirect effect 
of stimulation. 

But all cells conduct stimulus more or less efficiently, the 
difference being one only of degree. Hence an indifferently 
conducting tissue, such as that of the tip, will conduct the 
true excitatory state only if the stimulation be sufficiently 
strong and long continued. The transmitted effect of this 
true excitation may then be expected to produce negative 
turgidity-variation, and concomitant galvanometric nega- 
tivity, at A. 

When the stimulus is applied, however, at or near the 
responding point A, we may expect to obtain the direct effect 
of stimulation that is to say, a negative turgidity-variation 
and the concomitant galvanometric negativity. 

To sum up, then, it may be expected that moderate 
unilateral stimulus applied at the tip T will give rise on the 
same side of the responding region to the indirect effect of 
stimulation, which is an increase of turgidity exhibited by a 
positive electrical variation. The direct effect of stimulus, 
whether immediate or transmitted, always produces a negative 
turgidity-variation, evidenced by galvanometric negativity. 
This effect may be obtained either by the local application 
of moderate, or by the distant application of strong, stimulus. 

In making the electrical investigation which is now to be 
described, I employed various forms of stimulation thermal, 
mechanical, chemical, and the stimulus of light. Mechanical 
stimulus may be applied by friction of emery paper, or by 
means of a pin-prick. Chemical stimulation is applied by 
touching the point with a brush which has been moistened in 
hydrochloric or sulphuric acid. Very dilute acid produces 
moderate, strong acid a more intense, stimulation. The most 
perfect mode of stimulation is by means of incident light, the 
intensity of which may be varied at will. The effect ot 
stimulus of light, however, will be fully described in the 
chapter on heliotropism. Another form of stimulation which is 
also very suitable is the thermal. A short piece of platinum 



RESPONSE OF TIPS OF SHOOT AND ROOT 



517 



wire, heated by electrical current, is placed in the neighbour- 
hood of the point to be stimulated. The intensity of stimulus 
in this case is regulated by varying either the strength of the 
heating current, or the distance of the heating wire from the 
point to be excited. All the different forms of stimulation 
will be shown to produce the same results. 

Responsive results of : i. Longitudinal transmission of 
effect of stimulus from tip : (a) Moderate unilateral stimula- 
tion. Using the thermal mode of stimulation applied at T (fig. 
212, a), I obtained positive electric-variation at A, the record 
of which is given in fig. 213. A similar result was obtained 





FIG. 213. Record of Positive 
Electrical Variation, indicating 
Positive Turgidily- Variation 
(represented by Down Curve), 
induced in Growing Region 
by Moderate Stimulation on 
same side of Tip. Time- 
marks = minutes 



FIG. 214. Record showing 
Galvanometric Positivity 
subsequently Neutralised 
under Transmission of 
True Excitatory Effect, 
due to Continuance of 
Moderate Stimulation of 
the Tip 



with the mechanical stimulation oi a pin-prick. The same 
was again observed on effecting stimulation by dilute acid. 

(b) Effect of stronger unilateral stimulation. I next pro- 
duced a somewhat stronger thermal stimulation by suitably in- 
creasing the heating current. This gave rise to the electrical 
indication of the preliminary positive turgidity- variation, as the 
immediate effect. But the long-continued action of the stimulus 
caused the transmission of the true excitatory, which, reaching 
1, caused the neutralisation of the previous effect (fig. 214). 
With another specimen, I next applied to the tip a still stronger 
unilateral stimulus. This caused a brief positive electrical 
tnrgidity variation, followed by a reversed response of 



5i8 



PLANT RESPONSE 



galvanometric negativity, thus proving that the strong 
excitatory effect transmitted to the organ had not only 
neutralised, but also reversed, the previous effect (fig. 215). 

2. Direct unilateral stimulation of growing region : 
Moderately strong stimulus. When stimulus is applied near 
the responding organ, say at C (fig. 212, a\ there is always 





Fit;. 215. Record showing Neutralisation 
and Reversal of Electrical Response 
at Responding Region, under Strong 
Stimulation of Tip 

This indicates that the first positive turgidity - 
variation, due to indirect stimulation, is 
converted into negative turgidity-varia- 
tion of transmitted true excitation. 



FIG. 216. Record showing 
Negative Electrical Re- 
sponse represented by Up 
Curve, indicating Negative' 
Turgidity Variation due 
to Direct Stimulation 



produced at A a negative electrical, indicating a negative 
turgidity, variation (fig. 216). 

Thus we have obtained, using the same moderate stimulus, 
two opposite effects, of positive and negative turgidity-varia- 
tions, according as the point of application was at the distant 
tip or in the vicinity of the responding organ. It is evident, 
moreover, that there is a .continuity between these two 
extreme effects, for, as \vc gradually shift the point of applica- 
tion from the distant point nearer to the responding organ 
we observe corresponding intermediate changes, from pure 
positive, through the neutral, due to equal positive and 
negative effects, to pure negative. 



RESPONSE OF TIPS OF SHOOT AND ROOT 



519 



3. Transverse transmission of effect of stimulus : 

(a) Moderate stimulation. We have seen that the direct appli- 
cation of stimulus at a point has the local effect of a nega- 
tive turgidity-variation. It is of great theoretical importance 
that the effect of this stimulus on the diametrically opposite 
point should be clearly demonstrated. As conductivity has 
already been shown to be very feeble across a tissue (p. 250), 
we might expect that it would be 
the indirect effect, that is to say the 
positive turgidity-variation, which on 
stimulation of A would reach the 
diametrically opposite, or distal, 
point B. The experiment is carried 
out, by making galvanometric con- 
nections with B and L, and applying 
stimulus at A (fig. 212, b\ 

On now applying moderate sti- 
mulus at A, we obtain a positive elec- 
trical response, indicating positive 
turgidity-variation, at B (fig. 217) 
a result precisely the same as was 
obtained by stimulating the distant 
tip. The effect, then, of stimulating 
any point is to induce a negative 
turgidity-variation of the point itself, 
and a positive turgidity-variation of the diametrically opposite 
point. 

(b) Stronger stimulation. When stronger stimulus is 
applied, however, at A, the true excitatory effect is gradually 
transmitted across the tissue, and we obtain at B the neu- 
tralisation of the preliminary positive effect, as in fig. 214. 
And, lastly, when the point A is very strongly stimulated, the 
responsive effect on the diametrically opposite point r. is a 
transient positive, followed by a strong negative variation, 
as in fig. 215. 

We have thus studied the separate effects produced at 
A and B by stimulation of a point near A. The differential 




FIG. 217. Record showing 
Positive Electrical Varia- 
tion indicating Positive 
Turgidity -Variation of Did' 
tal Point, under Moderate 
Stimulation of Proximal 



52O PLANT RESPONSE 

effect as between A and B can be inferred by the algebraical 
summation of these separate effects. Or it can be directly 
obtained by making electrical connections with the dia- 
metrically opposite points A and B, as in fig. 212, c, and 
applying gradually increasing stimulus at C near A. 

It will be remembered that these opposite effects of 
direct and indirect stimulation have received independent 
demonstration, in both mechanical and growth responses. 
In Biophytum and in Artocarpus the positive turgidity- 
variation was proved to be the indirect effect of stimulation, 
by the erectile responses of the motile organs (pp. 24, 420). 
In the case of growth- response, again, the indirect effect of 
stimulation, with the concomitant positive turgidity-variation, 
was shown in the increased rate of growth (p. 430). The 
effect of direct stimulation in inducing a negative turgidity- 
variation, again, was exhibited in Biophytum and Artocarpus, 
by the responsive depression of the motile organs. In the 
case of growth-response, it was exhibited by contraction and 
concomitant retardation of growth. 

Mechanical response inferred from observed electrical 
response. From the results of the electrical investigation on 
these turgidity-variations, induced by direct and indirect 
effects of stimulus, as just described, we are led to conclude 
that: 

i. (a) Moderate unilateral stimulation of the tip gives 
rise by longitudinal transmission to the indirect effect of 
stimulus, namely a" positive turgidity-variation, on the same 
side of the distant responding-organ. This will give rise to 
acceleration of growth and convexity of that side, and by 
the consequent responsive movement the tip will be carried 
away from the source of stimulation. 

(fr) When the unilateral stimulation of the tip is a little 
stronger, the indirect effect of stimulus is neutralised by the 
subsequently transmitted true excitatory effect, and there is 
no resultant action. 

Very strong unilateral stimulation of the tip gives rise by 
longitudinal transmission to the direct effect of stimulus, 
namely a negative turgidity-variation, on the same side of 



RESPONSE OF TIPS OF SHOOT AND ROOT 



521 




the distant responding organ. This will give rise to retarda- 
tion of growth and consequent concavity of that side, the tip 
being carried, by the responsive 
movement, towards the source of 
stimulation. 

2. (a) Stimulation of a point 
at or near the responding region 
of growth will induce a negative 
turgidity-variation as the direct 
effect of stimulus, and at the 
diametrically opposite point a 
positive turgidity-variation as the 
indirect effect of stimulus. This 
will give rise to concavity of the 
proximal with convexity of the 
distal sides. The mechanical 
effects of negative turgidity- 
variation on one side and posi- 
tive turgidity-variation on the 
other side are thus additive. In 
this way, the concavity of the 
proximal and convexity of the 
distal conspire to bring about the 
resultant curvature. 

(b) When the stimulus applied 
at A is stronger, the effect is a 
negative turgidity-variation at A, and later, owing to the 
transmission of stimulus across the tissue, the positive is 
succeeded by a negative turgidity-variation of B. The 
resultant effect obtained by algebraical summation thus tends 
to become zero. When the stimulus at A, however, is still 
stronger and of longer continuance, the negative response of 
A will be found to undergo a gradual diminution owing to 
fatigue, while the transmitted effect at the diametrically 
opposite point undergoes an increase. Under these circum- 
stances the responsive negative change at B will predominate 
over that at A. The final result may thus be a relative 
negativity of B, that is to say an effect opposite to that seen 



FiG. 2l8. The Relative Elec- 
trical and Turgidity Variations 
of two Diametrically Opposite 
Points, A and B, when Strong 
and Long-continued Stimulus 
is applied near A (cf. fig. 
212, c). 

We see here (l), in the up curve, 
the negative variation of the 
proximal, followed by (2) 
neutralisation, followed by (3) 
reversal, that is to say, rela- 
tive negativity of B. Note the 
multiple response which here 
makes its appearance. 



522 



PLANT RESPONSE 



in case (a). These different phases of the effect induced by 
long-continued application of strong stimulus at A namely 
the relative negativity of A, followed by neutralisation, 
followed by reversal or relative negativity of B are well 
seen in the record given in fig. 218. Translated into terms 
of the resulting mechanical response, this would mean : (i) a 
movement of the organ towards the stimulus ; (2) neutralisa- 
tion of this movement ; and (3) a pronounced movement 
away. I give here a tabular statement which shows at a 
glance the various electrical effects and the corresponding 
mechanical responses which are theoretically to be inferred 
from them, the experimental verification of these inferences 
being given in the next chapter. 

TABULAR STATEMENT OF ELECTRICAL EFFECTS AND INFERRED 
MECHANICAL RESPONSES 



Stimulation 



I 



Electrical and turgidity effects 
in the responding region 



Mechanical response 
theoretically inferred 



I . Unilateral, of tip, on 

side A : 
(a) Moderate stimulus. 



Positive electrical and i Convexity of A, and 



positive turgidity vari- 
ation of same side A. 



movement of tip away 
from stimulus. 



(b) Stronger stimulus. ! Positive and subsequent No resultant effect. 

j negative effects neu- 
tralise each other. 



(c) Very strong and long- 
continued stimulus. 



2. Direct action of 
stimulus on respond- 
ing region : 

(a) Moderate stimulus. 



(/>) Stronger stimulus. 



(e) Very strong and long- 
continued stimulus. 



Positive twitch, followed 
by strong negative elec- 
trical and turgidity 
variations, of same 
side, A, 



Negative variation of side 
acted upon, and posi- 
tive variation of dia- 
metrically opposite 
side. 



Positive and subsequent 
negative effects neu- 
tralise each other. 

Positive twitch, followed 
by strong negative 
electrical and turgidity 
variations of the op- 
posite side, B. 



Transient twitch away 
from, followed by 
strong movement to- 
wards, stimulus. 



Concavity of the excited, 

with convexity of the 

opposite side. 
Responsive movement of 

organ towards source 

of stimulus. 



No resultant effect. 



Resultant mechanical 
response opposite to 
that in 2 (a) ; i.e. 
movement away from 
the stimulus. 



RESPONSE OF TIPS OF SHOOT AND ROOT $23 

The verification of these theoretical inferences will be 
given in full in the following chapter. 

SUMMARY 

From electrical investigation we find that the responsive 
peculiarities of the tip of the root are not characteristic of 
that organ alone, but of the tip of the shoot also. 

The different effect between stimulation applied to the tip 
and to the growing region of an organ lies in the fact that 
from the former it not being a good conductor of excitation 
only the indirect effect, that is to say the positive turgidity- 
variation, is transmitted to the responding organ. In the 
latter case, however, direct excitation gives rise to negative 
turgid ity- variation. 

Moderate unilateral stimulation of the tip, therefore, gives 
rise on the same side of the responsive region to increased 
turgidity, which, translated into mechanical response, means 
a negative curvature or movement away from stimulus. 

Stronger and long-continued stimulation of the tip causes 
transmission of the direct effect. This will gradually neu- 
tralise or even reverse the first effect, the positive turgidity- 
variation giving place to negative. These events translated 
into terms of mechanical response mean a change from 
negative to positive response, or movement towards stimulus. 

Direct unilateral stimulation of the responsive region 
causes negative turgidity-variation of the proximal and 
positive turgidity-variation of the distal. The mechanical 
expression of these will be a movement towards stimulus. 

By strong and long-continued action the stimulus ceases 
to be unilateral and becomes internally diffused. The excita- 
tion thus reaches the distal side. The difference of turgidity- 
variation on the proximal and distal is thus gradually 
abolished, or even reversed. The corresponding mechanical 
response will be a neutralisation or reversal into negative, 
that is to say a movement away from the stimulus. 



CHAPTER XXXVIII 

INQUIRY INTO THE LAWS OF RESPONSIVE GROWTH- 
CURVATURES 

Scope of the investigation : I. Mechanical response to unilateral stimulation of the 
tips of shoot and root : (a) Moderate stimulus (b) Stronger stimulation 
2. Effect of unilateral stimulus, applied at the responding growing region : 
(a) Moderate stimulus (b) Strong or long-continued stimulus Experiments 
on the direct and indirect effects of stimulus on Mimosa : (a) Direct stimula- 
tion (b) Indirect stimulation, longitudinal transmission (c) Indirect stimula- 
tion, transverse transmission The curious response of an Aroid Table showing 
responsive effects common to pulvini, pulvinoids, and growing organs Laws 
of responsive growth-curvature. 


WE were able, at the end of the last chapter, to pass in 
review the theoretical conclusions to which we had been led 
by the electrical mode of investigation, as to the responsive 
movements which might be expected to follow on the 
unilateral application of stimulus to the tip and the growing 
region respectively. I shall now proceed to submit these 
theoretical conclusions to experimental verification, by taking 
records of the mechanical movements actually induced. 

It will be understood here that my object is (i) to show 
that the peculiar response given by the tip of the root is 
characteristic of the tip of the shoot also ; and (2) to demon- 
strate the effect of unilateral stimulation on the growing 
region. We have therefore to study the effects which are 
induced at the growing region by the action of stimuli of 
different intensities, according as they are applied unilaterally 
at the distant tip, or, locally, on the growing region itself. As 
a specimen of the shoot-tip, we may employ either the tip of 
a stem or that of an unopened flower-bud. This latter, 
composed mainly as it is of indifferently conducting elements^ 



LAWS OF RESPONSIVE GROWTH-CURVATURES 525 

has all the characteristics of the tip of the organ, while the 
peduncle below often represents the area of quickest growth, 
and functions as the responding region. Another advantage 
of the unopened flower-bud, again, lies in the fact that its 
upper end is not covered over with appendages like that of 
the stem. The unopened buds, with peduncles, of Crocus, 
then, will be found suitable for this investigation. As 
specimens of the roots, again, the long straight water-roots of 
Bindweed are very suitable for these experiments. 

The records of the responsive movements are taken by 
means of the recording microscope fully described in a 
subsequent chapter. The quiescence of the organ, before the 
application of stimulus, is tested by the fact that the record 
is a horizontal line. The occurrence of an up curve in the 
record represents responsive movement towards, while a 
down curve means movement away from", the stimulus. In 
this investigation, as in the preceding, various forms of 
stimulation have been employed. Mechanic^ stimulus is 
applied by friction of emery-paper. The jar produced by 
this causes a temporary disturbance of the image in the field 
of view of the microscope ; but this soon subsides, and the 
excitatory movement commences after a short latent period, 
increasing steadily until the effect of stimulus is exhausted. 
The chemical form of stimulation has the advantage of 
producing no mechanical jar. The stimulation produced by 
light, which is the most perfect, will be described in the 
subsequent chapter on heliotropism. Thermal stimulation is 
effected by holding a platinum wire, heated by the electrical 
current, in more or less proximity to the point in the tissue 
which is to be excited. 

i. Mechanical response to unilateral stimulation of the 
tips of shoot and root : (a) Moderate stimulus. Applying a 
single mechanical stimulus of emery-paper friction, of mode- 
rate intensity, unilaterally to the bud of Crocus, a movement 
was induced in the responding organ, which carried the tip 
away from the source of stimulation. This movement 
persisted for four minutes, after the application of this single 



526 PLANT RESPONSE 

stimulus. I performed a similar experiment on the tip of 
the root of Bindweed, the response being precisely the same. 
The rate of movement was in this case somewhat slower, but 
persisted for eight minutes. The application unilaterally of 
dilute sulphuric acid brought about exactly similar results in 
both cases. The unilateral application of thermal stimulus of 
moderate intensity, again, to the tip, induced movement away 

from the source of stimulation in 
both shoot and root, as will be seen 
from the record given below (fig. 219). 
Stronger stimulation. A 




somewhat stronger stimulation of the 
same character caused a movement 
away, followed by movement towards, 
Fir.. 219. Mechanical Re- the source of stimulus, the resultant 

spouses of Shoot, s, and cc , . , , ir 

of Root, u, to Unilateral effcc t being neutral. We now pass 
Stimulus applied at the on t o the effects induced by still 

stronger stimulation. I have already 

In this and follo%mg records 1-11 

the down curve indicates explained how, when very strong 
negative movement or away stimu i us j s applied unilaterally at the 

from source of stimulus. ^ * * J 

The up curve indicates tip of an organ, its first and transitory 

positive or movement to- rr , . , - , ... . . .., 

wards source of stimulus. cffect ls * to induce a positive turgidity- 

The time-marks represent variation on the same side of the 
minutes (cf. fig. 213). . 

growing region. J^rom this we in- 

ferred the occurrence of a convexity on that side, which 
would carry the tip away from the source of stimulus. The 
direct effect of stimulus next reaches the responding region, 
reversing the first effects and causing a negative turgidity- 
variation, which would, pari passu, induce a concavity, and 
carry the tip towards the source of stimulation. 

In carrying out the experimental verification of these 
mechanical movements on a bud of Crocus, I found that an 
application of strong sulphuric acid on one side of the bud 
caused it to move first away from, and then very energetically 
towards, the direction in which the application was made. I 
next tried to determine the effect of a strong unilateral 
application of thermal stimulus on the root-tip of Bindweed. 



LAWS OF RESPONSIVE GROWTH-CURVATURES 



52 



7 



Here, too, after a transient movement away, there was an 
energetic movement towards the stimulating heated wire 

(fig. 220). 

2. Effect of unilateral stimulus applied at the respond- 
ing growing region, I shall now show that when stimulus 
is applied near the growing region, it induces effects which 
are opposite to those resulting from stimulation of the tip. 





FIG. 220. Mechanical Response 
of Root of Bindweed to very 
strong Unilateral Stimulation 
applied at the Tip 

This causes a preliminary negative, 
followed by a positive, move- 
ment, that is to say towards the 
source of stimulus (cf. fig. 215). 



FIG. 221. Mechanical Re- 
sponses of Peduncle of 
Crocus, s, and Root of 
Bindweed, R, to Unilate- 
ral Thermal Stimulation 
at the Growing Region 

The responses are positive 
and towards the source of 
stimulus (cf. fig. 216). 



(a) Moderate stimulus. When moderate stimulation of 
any kind is applied unilaterally in the growing region, the 
consequent negative turgidity-variation of the side directly 
excited makes it concave ; and a positive turgidity-variation 
due to the indirect effect of stimulation occurs at the distal 
side, making that side convex. Thus the induced concavity 
of the proximal, and convexity of the distal, both conspire to 
cause a movement of the organ towards the source of stimula- 
tion. This is seen in the following records obtained with the 
peduncle of Crocus and the root of Bindweed, the stimulus 
used having been thermal (fig. 221). 



28 PLANT RESPONSE 



Strong or long- continued stimulus. I have already 
explained that with moderate stimulation the negative 
turgidity-variation and concomitant contraction of the proxi- 
mal, and the positive turgidity-variation and concomitant 
expansion of the distal, conspire to induce a positive respon- 
sive curvature ; but when the stimulus is strong or long- 
continued the excitation is conducted across the tissue to 
the distal side, which, now contracting, antagonises and 
reverses the action of the proximal. We have seen this ex- 
emplified in the electrical response given in fig. 2 1 8, where the 
first positive electrical response of the distal, indicating positive 
turgidity-variation, was afterwards neutralised and converted 
into negative by the transverse conduction of excitation. 

In the case of responsive growth-curvature we obtain 
results precisely similar. Long-continued unilateral stimula- 
tion is here often found to neutralise the first or normal 
effect. Or if, again, the unilateral stimulation^ be very strong, 
the, proximal side is liable to become fatigued, and the 
response of the distal to transmitted stimulus being thus 
predominant, a responsive movement occurs which is reversed 
or negative, that- is to say, away from the source of stimula- 
tion. Examples of this will be seen in Chapter XLII. 
I shall also presently give a demonstration of similar 
preliminary effects with subsequent transverse conduction, 
giving rise to reversed effects, in the case of the motile 
response of Mimosa. 

We have thus seen that growth-curvature is induced by 
unequal variations of turgidity, on the diametrically opposite 
sides of the growing region. We have seen that the effect 
of indirect stimulation is a positive turgidity-variation. When 
a feebly conducting tissue is unilaterally subjected to moderate 
stimulation, the direct excitatory effect cannot be transmitted 
far, and it is the indirect effect which reaches the responding 
region, R (fig. 222), and induces convexity. Hence we obtain 
the typical examples of this effect by stimulating the tip, T, 
of either root or shoot. The sensibility of these regions is 
itself in no way different from that of any other portion of 



LAWS OF RESPONSIVE GROWTH-CURVATURES 



529 



T 



R.-. 



I 

V 



the plant-tissue, for they respond to direct external stimula- 
tion by contraction. But their power of transmitting stimulus 
is relatively feeble. For this reason, under ordinary circum- 
stances, they transmit only the indirect effect of stimulus, 
and it is only when the unilateral stimulus is very strong 
that the direct excitatory effect is transmitted, inducing the 
opposite to the usual result, in the 
responsive concavity of the same side 
of the growing region. 

The fact that the sensitiveness of 
the tip is not fundamentally different 
from that of the growing region may 
be demonstrated by applying stimulus 
to a given point D in the growing 
region, and observing the responsive 
effect induced at R, diametrically 
opposite. The power of the tissue to 
conduct stimulus transversely being 
feeble, the result is in this case 
the same as in that of the ordinary 
longitudinal transmission from the 
tip ; that is to say, it is the indirect 
effect that reaches the diametrically 
opposite point, R, and induces con- 
vexity there, this effect being aided, 
as it happens, by the concavity of 
the proximal side, due to the direct 
effect of stimulation. Here again, as before, a stronger or 
long-continued stimulus may later transmit the direct effect, 
and neutralise or reverse this first responsive curvature. A 
third case arises when unilateral stimulus is applied at L, 
lower down on the stem, at some distance from the respond- 
ing region, and if this be sufficiently feeble, it will be the 
indirect effect which will reach the same side of the respond- 
ing region, and produce convexity there. Stronger or long- 
continued stimulation will in this case, as before, neutralise or 
reverse the first effect. 

M M 



L..- 



FIG. 222. Diagram showing 
the various Responsive 
Effects induced at the 
Growing Region, R 

When moderate stimulus is 
applied unilaterally at 
the tip, T, or distant 
point, i,, it is the indirect 
effect that reaches R, and 
produces convexity. The 
same convexity of R is 
induced by stimulation 
of the transverse point, D; 
but here the induced 
curvature is aided by the 
concavity of the directly 
excited D. 



530 



PLANT RESPONSE 



I have been at some pains to make these examples of 
the direct and indirect effects of stimulus clear ; for all the 
complex curvatures of growth, which appear at first sight so 
anomalous, are ultimately resolvable into these. And since 
the subject is so important, I shall add still another demon- 
stration, which is capable of easy repetition, and will be 
found to be striking and convincing. 

Experiments on the direct and indirect effects of 
stimulus on Mimosa. -We have now seen that all growth- 
curvatures may be analysed 
into, (i) that contraction, with 
consequent concavity, which is 
concomitant to the negative 
turgidity-variation that consti- 
tutes the direct, or transmitted 
direct, effect of stimulus ; and (2) 
that expansion, with consequent 
convexity, that is concomitant to 
the positive turgidity-variation, 
which constitutes the indirect 
effect of stimulus. Now, the 
negative turgidity-variation is 
exhibited in the case of the 
motile leaf of Mimosa by depres- 
sion, and the positive turgidity- 
variation by erection. 

I shall now explain the 
experimental arrangements by 
which the plant itself may be 

made to record these opposite effects. The indicating leaf is 
attached to the short arm of a long writing lever. This lever 
consists of the quill of a long tail-feather of a peacock. Its 
short arm, I cm. in length, is tied by a thread to the petiole 
of the leaf. A fine needle is passed through the quill, and 
rests on frictionless supports which may be glass tubes. 
The longer arm of the lever, 10 cm. in length, has a piece 
of bent aluminium, with a sharp point, tied to the end, 




Fu;. 223. Kxpciimciital Arrange- 
ment for obtaining Records 
on Smoked Drum of Responses 
given to Direct and In- 
direct Stimulation by Leaf of 
Alimosa 

Thermal stimulator at s produces 
direct stimulation, and conse- 
quent fall of leaf. Moderate 
stimulation, at a distant point, 
s,,, gives rise to indirect effect 
of erection. 



LAWS OF RESPONSIVE GROWTH-CURVATURES 



531 



to serve as a writer. This writing-point is, by the elasticity 
of the feather, pressed lightly against the smoked-paper 
surface of a vertical revolving drum (fig. 223). 

When the leaf is under no stimulation, either direct or 
transmitted, the record is a horizontal line. But true excita- 
tion produces a depression of the leaf, causing an up curve. 
The erectile response, on the 
other hand, which is due to 
indirect stimulation, produces 
a down curve. The records 
given in the following figures 
are accurate reproductions of 
some which were taken in this 
way (fig. 224). 

(a) Direct stimulation. 
Stimulus is applied by the close 
proximity of a V-shaped plati- 
num wire, heated electrically. 
Its intensity is varied by varying 
either the distance of the stimu- 
lating wire or the intensity of the 
heating current. When stimula- 
tion is now applied directly at S, 
that is to say, near the respond- 
ing organ, response takes place 
by a negative turgidity-varia- 
tion, producing a fall of the leaf. 
This is seen in the up curve 
(fig. 224, a). 

(b) Indirect stimulation, lon- 
gitudinal transmission. Sti- 
mulus of moderate intensity is 

now applied lower down on the same side of the stem, at 
s,,. This is observed first to induce the positive turgidity- 
variation, causing erectile response, which is due to the indirect 
effect of stimulus, and is shown in the preliminary down 
twitch of the curve (fig. 224, b). Later, the direct effect is trans- 




FIG. 224. Mechanical Responses 
of Leaf of Mimosa 

(a) record of responsive fall when 
stimulus applied near the re- 
sponding organ (cf. fig. 221); 
(b) response when stimulus is 
applied on same side, but at 
greater distance, s /r A pre- 
liminary erectile response is here 
followed by the true excitatory 
depression. This is due to the 
indirect effect first transmitted 
being succeeded by the direct. 
Had the stimulus applied been 
feebler, or more .distant, there 
would have been only the first, 
or indirect erectile effect, similar 
to fig. 225 (cf. fig. 220). 



M M 2 



532 



PLANT RESPONSE 




mitted, causing the fall of the leaf, as shown in the up curve. 
When the stimulus is feebler, or applied at a still greater 
distance, the indirect effect alone reaches the organ, and only 

the erectile response, due to posi- 
tive turgidity-variation, results, being 
similar to that shown in the next 
record (fig. 225). 

(c} Indirect stimulation, transverse 
transmission. We next obtain the 
very interesting case in which feeble 
stimulus is applied at the transverse 
point s,. The record (fig. 225) shows 
that we have here an erectile response 
due to the positive turgidity-variation 
of indirect stimulation. But if this 
transverse stimulus be made strong 
or be long continued, the direct effect 
is transmitted somewhat later, and, 
in that case, we obtain a fall of the 
leaf, preceded by the positive erectile 
twitch, which is similar to that shown 
in the previous record (fig. 224, <). 

The curious response of an 
Arisaema. This fact will explain a 
very remarkable phenomenon which I 
have noticed in certain species of Arisama, that grow on the 
mountains round Darjeeling, at a height of about 7,000 feet. 
This plant, before flowering, consists of a long petiole bearing 
a terminal whorl of leaflets, which are arranged like rays in a 
strictly horizontal plane. Later, however, the inflorescence, 
borne on a peduncle enclosed within its spathe, breaks out 
from one side of this petiole. Unilateral mechanical stimu- 
lation is thus undoubtedly brought about, and gives rise to 
indirect stimulation on the distal side, which, as we have just 
seen, causes an erectile mechanical response. In the case of 
this Arisicuia, it is a striking fact that immediately after flower- 
ing, the most distal leaflet of the whorl that is to say, the 



FlG. 225. Erectile Re- 
sponse of Leaf of Mimosa 
due to Transmission of 
Indirect Effect to Distal 
Side, when Proximal, s,, 
is Stimulated 

If stimulus were stronger, 
this would be followed 
by the fall of the leaf 
due to the later trans- 
mission of true excitation. 
The response would then 
become like that of fig. 
224 (b) (cf. fig. 219). 



LAWS OF RESPONSIVE GROWTH-CURVATURES 



533 



leaflet which is situated in the diametrically opposite line 
hitherto horizontal, becomes abruptly vertical (fig. 226). 
This is the only leaflet which stands out from its fellows, 
and it is invariably found to be situated on the line diametri- 
cally opposite to the flower, such differentiation being induced 
only after flowering. 

All the variations exhibited by diverse forms of response 
electrical, mechanical, responsive acceleration or retarda- 
tion of growth, and growth-curvatures are only so many 




FIG. 226. Curious Response of Arisnema 

Before flowering the leaflets lie in a horizontal plane, as seen in the figure 
to the left ; but when inflorescence breaks through unilaterally, the 
indirect stimulation of the distal side causes erection of the diametrically 
opposite leaflet, as seen in figure to the right. 

expressions of these two fundamental phenomena, the effects 
of direct and indirect stimulation. It is these two variables 
which, conjoined with stimulus unilateral or diffuse, give 
us all those manifold effects that at first sight would appear 
to belong to different classes of phenomena. That such a 
unity does actually underlie them all may be seen at a 
glance from the following concise statement of responsive 
effects, induced in pulvini, pulvinoids, and in growing regions 
which act as pulvinoids, 



534 



PLANT RESPONSE 



TABLE SHOWING RESPONSIVE EFFECTS COMMON TO PULVINI, 
PULVINOIDS, AND GROWING ORGANS 



Stimulation diffuse 



i. DIRECT EFFECT. 

Negative tu rgidit y- 
variation. 

Galvanometric nega- 
tivity. 

Depression of motile 
leaf. 

Retardation of 
growth. 



Stimulation unilateral 



Effect on proximal side 



3. DIRECT EFFECT. 

Negative turgidity- 
variation. 

Galvanometric nega- 
tivity. 

Contraction and 
concavity. ! 



Effect on distal side 



CORRESPONDING 
EFFECT. 

Positive turgidity- 
variation. 

Galvanometric posi- 
tivity. 

Expansion and con- 
vexity. 



When stimulus is strong or long continued, the 

true excitatory effect passes to the distal side, 

neutralising or reversing the first response. 



2. INDIRECT EFFECT. 

Positive turgidity- 

variation. 
Galvanometric posi- 

tivity. 
Erection of motile 

leaf. 
Acceleration of 

growth. 



5. TRANSMITTED 

DIRECT EFFECT. 
Negative turgid ity- 

variation. 

Galvanometric nega- 
tivity. 

Contraction and 
concavity, in re- 
sponsive region. 

7. INDIRECT EFFECT. 

Positive turgidity- 
variation. 

Galvanometric posi- 
tivity. 

Expansion and con- 
vexity in respon- 
sive region. 



6. 



CORRESPONDING 
EFFECT. 

Positive turgidity- 
variation. 

Galvanometric posi- 
tivity. 

Expansion and con- 
vexity in respon- 
sive region* 



8. CORRESPONDING 

EFFECT. 
Negligible. 



REMARKS. It will be remembered that the indirect effect is a secondary con- 
sequence of the direct contractile effect of stimulus on the excited point. Thus 
the motive power is the active contraction of that point. The indirect effect, 
described as No. 7, is exemplified by the moderate unilateral stimulation of the 
tip of shoot or root. When this stimulus is stronger, or sufficiently long 
continued, we have a transmission of the direct effect of stimulus, and case No. 7 
is displaced by case No. 5. 



1 The induction of concavity, of either upper or lower side of the pulvinus, 
by local stimulation will be found demonstrated in a subsequent chapter. 



Confining our attention to the effects induced at the 
growing region, we arrive at the following laws of growth- 



LAWS OF RESPONSIVE GROWTH-CURVATURES 535 

Laws of responsive growth-curvature. It must be 
remembered here that the effect of indirect stimulation is to 
cause an increase in the rate of growth, and that of direct 
stimulation a retardation of the rate. By a positive effect is 
meant a responsive mechanical movement towards, and by 
negative^ away from, the source of stimulation. 

1. Unilateral stimulus of moderate intensity, applied at the tip 
of root or shoot, gives rise to a negative effect, the tip being moved 
away. 

2. Stronger or long- continued stimulus, applied unilaterally 
at the tip of root or shoot being conducted gradually to the 
growing region results in a neutralisation of the first negative 
by a subsequent positive effect. Or there may be a resultant 
positive, due to the predominance of the transmitted effect. 

3. Direct unilateral stimulation of moderate intensity on the 
growing region causes a positive response or movement towards 
stimulus. 

4. Strong or long-continued unilateral stimulation of the 
growing region, on account of the transmission of effect to the 
distal side, may give rise either to neutralisation of the normal, 
or to a reversed or negative effect, that is to say, to movement 
away from stimulus. 

SUMMARY 

All the mechanical effects induced at the responsive 
growing region of either root or shoot, by unilateral stimula- 
tion, may be summarised as follows, it being understood that 
positive response means movement towards, and negative, 
movement away from, stimulus : 

(1) Positive response'^ induced, first, by direct unilateral 
application of stimulus on the growing region ; second, by the 
long-continued unilateral application of moderately strong 
stimulus at the tip. 

(2) Negative response is induced, first, by the unilateral 
ipplication of feeble stimulus at the tip ; and second, by long- 
continued unilateral application of strong stimulus at the 
growing region, causing fatigue of the proximal, and trans- 
mission of true excitation to the distal, side. 



CHAPTER XXXIX 

INQUIRY INTO POSITIVE GEOTROPISM 

No specific difference as regards their responses between shoot and root 
Darwinian curvature Localisation of geotropic sensibility at the root-tip 
Experiments as to whether amputation of root-tip abolishes excitability The 
tip of the root the organ of gravi- perception The perceptive versus the 
responding organ True perceptive region. 

WE have in the course of the last chapter studied in detail 
the expression in growth-curvatures of the direct and indirect 
effects of stimulus, and have found them to be, under known 
conditions, very definite. We have seen that the responsive 
effects produced at the tips of root and shoot are in no way 
different from those which have been observed in other parts 
of the tissue. The specific sensitiveness hitherto so generally 
attributed to the root-tip is thus found not to be justified. 
The differences of effect which have been noticed, according 
as tip or responding region is the point of stimulation, we have 
seen to be due merely to the transmission of the indirect instead 
of the direct effect. I shall next proceed to show that the 
opposite curvatures induced in shoot and root by stimulus of 
gravitation are really due, not to two distinct sensibilities 
positive and negative, but to the action of a single stimulus 
which acts in one case directly, and in the other indirectly. 

Darwinian curvature. It has thus been shown that the 
responsive curvature manifested by the radicle on stimulation 
of its tip is not characteristic of roots alone, but, under 
similar circumstances, of the shoot also. It has not, then, 
been specially evolved for the advantage of the plant. As 
proving this, the abrupt conclusion of certain experiments 
of my own has a peculiar interest. The tip of the root 



INQUIRY INTO POSITIVE GEOTROPISM 537 

was in these cases subjected to strong unilateral stimulation 
by the proximity of a heated platinum wire. Instead of 
responding, however, by movement away, as would have 
been the case had the advantage of the plant been the 
primary object of its action, the root-tip gave only a 
preliminary spasmodic twitch in the negative direction, and 
then reversed its movement ; the organ now turned towards 
the heating wire, fell upon it, and was burnt. 1 There is thus 
no protective adaptation here, any more than in the case of 
the moth which is impelled to throw itself upon the destroy- 
ing flame. No choice exists for either of these, for, in both 
alike, the movement is due to the working of the inexorable 
laws which govern the phenomenon of response. In the 
case of the plant, that increased turgidity which is the in- 
direct effect of a distant stimulus always induces an increased 
rate of growth ; but when the stimulus is sufficiently strong or 
long continued, its direct or true excitatory effect, being trans- 
mitted, brings about retardation of growth. And each oi 
the diverse curvatures of growth, induced by the unilateral 
application of stimulus, forms a particular case of the work- 
ing out of these two laws, of the direct and indirect effects oi 
stimulation. 

Having now arrived at certain definite conclusions as 
to the mechanism of the responsive curvature induced by 
unilateral stimulation of the tip of root or shoot, it will be 
well to pass in rapid review the exhaustive series of experi- 
ments on the responsive behaviour of the tip of the radicle, 
which were carried out by Darwin, more especially as some 
of the cases which he noted as somewhat exceptional will be 
found, in view of the investigation which I have described, 
to be susceptible of very simple explanation. He pro- 
duced unilateral stimulation in three different ways, first, by 
attaching minute fragments of cardboard to one side of the 

1 The same thing happens, in a still more striking manner, when the wire i: 
placed in front of the growing region, and suddenly heated. The excitatory eflFecl 
on the proximal side is then so great that the organ at once rushes upon the 
stimulating wire and is scorched. This occurs before the distal side could b< 
excited by the transmitted effect of stimulus. 



PLANT RESPONSE 

root-tip by means of gum or shellac varnish. The moderate 
and constant irritation which was produced in this way 
was usually found to induce a convexity on the same side of 
the growing region. His second method was chemical. 
He touched one side of the tip with silver nitrate. This 
also, generally speaking, induced a convexity similar to the 
last. The third and last method of unilateral stimulation 
employed by Darwin was a slanting cut, resulting, in the 
majority of cases, in the same responsive curvature as before. 
All these cases, it will be understood, are illustrations of the 
indirect effect of moderate stimulation. 

But we have seen in the second law of responsive 
growth-curvature that under the long-continued* action of 
a stimulus sufficiently strong, this first, or negative, effect is 
neutralised by the subsequent conduction of excitation (p. 52). 
The degree of such conduction and the power of neutralisa- 
tion will obviously depend on the conducting power of the 
particular specimen. Now, it was found by Darwin, in 
several of his experiments, that the negative effect was 
followed by neutralisation. This he regarded as somewhat 
exceptional. We have seen, however, that such a result, 
under certain circumstances, was to be expected. 

We have also seen, in the same law of responsive growth- 
curvature, that when the transmitted stimulus is strong, 
it induces a reversed or positive curvature. And in some 
of Darwin's experiments also, especially those in which he 
used the strong stimulation of caustic, this was observed. 

Finally, in some six cases, Darwin found that on 
applying unilateral stimulation to the tip there followed 
an induced concavity. With regard to this it need only 
be said that the sensibility of the tip to direct stimulation 
is not specifically different from that of any other tissue, 
All tissues contract, in response to direct stimulation. The 
difference between the contractile powers of the tip and 
the growing region is one merely of degree, that of the 
latter being relatively greater, and for certain reasons, to 
be described later, the more evident. Visible contraction 



INQUIRY INTO POSITIVE GEOTROPISM 539 

of the tip can only be seen under considerable stimulation, 
or when that part of the tissue is highly excitable. 

Localisation of geotropic sensibility at the root-tip. 

I next turn to Darwin's very important demonstration of 
geotropic perception as residing in the root-tip. He showed 
this by extensive researches on the fact observed by Ciesielski, 
that on the decapitation of the root-tip the geotropic action 
is found to disappear. Various objections have been urged 
against this view, which I shall be able to show to be 
groundless. 

Sachs, for instance, has argued that if such sensibility 
resided in the root-tip, there is no reason why the tip of 
the shoot should not exhibit the same. This argument is 
not, however, valid, inasmuch as the statolithic or other 
elements, which by their weight cause, stimulation, may 
be concentrated locally in the root-tip, and more diffusely 
distributed in the shoot. And that the general sensibility 
of the root-tip to other forms of stimulation is not different 
from that of the shoot-tip I have already fully demonstrated. 
Francis Darwin has shown, again, that localised gravi- 
perceptive areas may occur in organs other than the root. 
In the seedling of Sorghum^ for example, he finds this sensi- 
bility to reside in the cotyledons. 

The second objection that has been raised is that the 
shock of amputation might either abolish the general ex- 
citability or arrest the growth of the root, on which the 
responsive growth- curvature depends. With regard to the 
abolition of excitability, it is true that the effect of a strong 
stimulus, such as that of cut, would persist for some time, 
yet there is always recovery, after a longer or shorter 
interval. I have tested the persistence of the fatigue caused 
by amputation, by the electrical method, and I find that 
with certain plants, such as Bryophyllum, the recovery from 
the effect of amputation-shock is very rapid, taking place 
in the course of about a minute. The longest persistence 
of the after-effect which I have been able to observe was 
in the case of Colocasia^ where it lasted for a period of 



540 PLANT RESPONSE 

from forty-five to sixty minutes. It is thus unlikely that 
amputation would permanently abolish the power of renewed 
response. 

With regard, next, to the contention that amputation 
would be liable to arrest that growth of the root on which 
responsive curvature is supposed to depend, it is to be 
borne in mind that though growth-activity is a sign of 
excitability, yet we may have excitability without growth. 
Hence it is quite possible that responsive curvature might 
be initiated in a tissue which was not beforehand in a con- 
dition of growth by the stimulus of gravity, as indeed was 
found to be the case with grass haulms. 

Investigation on supposed abolition of excitability by 
amputation of root-tip. The question as to whether am- 
putation of the tip abolishes the general excitability of the 
growing region of the root, can only be decided finally by 
the direct observation of the variation, if any, in the mechani- 
cal response of the root to stimulus after amputation. Should* 
there be no such variation, then the researches of Ciesielski 
and Darwin having proved that a root deprived of its tip 
does not respond to gravity it will follow that the tip of the 
root alone is geotropically sensitive. That this is so that is 
to say, that amputation does not permanently modify the 
general sensibility of the root I shall next proceed to 
demonstrate. 

The experiments that have been carried out by different 
observers, for the purpose of determining whether amputation 
of the root-tip produces any modification of the rate of 
growth, have led to very various results. Some have found 
that it has the effect of inducing an accelerated rate of growth ; 
others, again, state that it induces no change whatever in the 
rate ; and still others find that it causes retardation of 
growth. All these discrepancies are reconciled, however, 
when we remember, as I have demonstrated, that indirect 
stimulus causes increase, and direct stimulation retardation, 
of growth. Moderate stimulation, therefore, by section of the 
extreme tip, giving rise to the transmitted effect of indirect 



INQUIRY INTO POSITIVE GEOTRO1TSM 



541 




stimulus, would accelerate growth ; while a section made in 
a better-conducting tissue, or nearer the growing region, with 
its stronger stimulation, would transmit the direct effect, with 
concomitant retardation of growth. But a stimulation of 
intermediate intensity would initiate no change. 

I shall, however, describe an investigation by which the 
question of the variation of responsive excitability in the 
growing region by amputation is put to the test of direct 
experiment. For this purpose I mounted a straight water- 
root of Bindweed in my Crescograph, and took a record of 
its normal rate of growth. It should be stated that the 
root was attached to the Optic 
Lever at a distance of 3 mm. 
below the tip, which was thus 
left free for amputation. The 
normal rate of growth was 
considerable, being '0105 mm. 
per minute. I next took a 
record of three successive con- 
tractile responses to thermal 
shocks (fig. 227). After this, 
without disturbing the ar- 
rangement, the root-tip was 
amputated, to a distance of 
2 mm., and a record of the 
growth was again taken after an interval of fifteen minutes. 
It was now found to be practically the same as before amputa- 
tion, the rate being *oioo mm. per minute. The contractile 
responses to thermal shocks of the same intensity as before 
were also, to all intents and purposes, the same. With another 
specimen the rate of growth before section was '0066 mm. per 
minute ; after amputation of 3 mm. of tip, the rate was 
0055 mm. The mechanical responses before and after section 
were almost the same. These experiments, then, show that 
amputation of the root-tip does not abolish the general ex- 
citability of the responding region. The fact that the gravita- 
tional response of the root, however, is abolished by the removal 



FIG. 227. Curves showing Effect of 
Amputation on Rate of Growth 
and Response in Root of Bindweed 

(a) continuous line indicates normal 
rate of growth. Dotted curves 
represent three responses to three 
successive uniform stimuli ; (t>) 
exhibits the rate of growth and 
responses after amputation. It 
will be seen that there is practically 
very little variation. 



542 PLANT RESPONSE 

of the tip, proves that, as pointed out by Darwin, the 
tip is, with regard to gravity, the perceptive organ of 
the root. 

The tip of the root the organ of gravi-perception. The 

inference that it is the root-tip which is the perceptive organ 
has also been arrived at by Pfeffer and Czapek, pursuing 
an independent method of investigation which discarded 
amputation. 

There is again the very suggestive fact that the starch 
grains whose weight, according to the statolithic theory, 
brings about stimulation, occur generally at the root-tips 
alone. Thus three different, lines of research lead to the 
conclusion that it is in that region that the gravi-perceptive 
power resides, the power of response by curvature being 
behind, in the region of growth. 

The perceptive versus the responding organ. In view 
of much existing vagueness on this subject, it is desirable 
here to obtain clear and well-defined conceptions as to how 
far the differences commonly insisted on between perceptive 
and responding organs are justified. To take the familiar 
instance of Mimosa, we know that when stimulus is applied, 
for example, on the stem, there is no perceptible motile effect 
in the stem itself, but excitation is markedly manifested at 
the pulvinus. From this it has been erroneously supposed 
that the stem merely perceived, and did not respond to, 
stimulus. It has, however, been shown in previous chapters 
that every part of the plant-tissue responds to stimulus by 
contraction, the visible and striking manifestation seen at the 
pulvinus being simply the result of certain accessory ana- 
tomical facilities which happen to exist at that point. 

Thus, every part of the plant-tissue is both perceptive 
and responsive to external stimulus, to a greater or less 
extent. When we come to the growing organ, responsive 
curvature is induced by the concavity of the excited side. 
And as the zone of growth is, generally speaking, 
the region of the greatest excitability, the greatest cur- 
vature usually takes place there, that region functioning 



INQUIRY INTO POSITIVE GEOTROPISM 543 

in this respect as a diffuse pulvinoid. It should be re- 
membered, however, that the excitability of a tissue does 
not disappear on the cessation of growth, and responsive 
curvatures are thus sometimes seen to extend even beyond 
that zone. 

Contraction takes place, as we have seen, in all tissues, but it 
is relatively much less in the very young and the very old. The 
responsive contraction at the tip is therefore slight and com- 
paratively inconspicuous. Indeed, it can only be made visible 
under very strong stimulation. That contraction is induced, 
however, is shown in the fact that a wave of positive turgidity- 
variation reaches the zone of growth by excitation of the tip. 
This can only be brought about by the expulsion of water, 
which is the result of excitatory contraction at that point. 
Under direct stimulation, then, every tissue, whether at the tip 
or in the growing region, undergoes a similar contraction. 
The sensitiveness, or sign of response, of all tissues is therefore 
the same ; but about these signs we are liable to be deceived 
if we pay attention to the conspicuous effects alone, for the 
local application of a unilateral stimulus at the tip gives rise 
to little perceptible concavity, while its transmitted indirect 
effect, lower down in the growing region, induces a relatively 
great convexity. Moreover, the whole length of the organ 
above the point of greatest convexity acts as a kind of lever- 
index, causing high magnification of the responsive movement. 
The tip, on the other hand, possesses no such magnifying 
index. These considerations will clearly show that there is 
no difference in kind between the sensitiveness of the tip 
and that of the growing region, both alike having the power 
of perceiving and responding, in different degrees, to external 
stimulation in general. Just as the tip itself merges physically 
into the growing region, so these respective responses pass 
imperceptibly into one another. Though moderate unilateral 
stimulus of the root-tip causes the characteristic movement 
away from stimulus, yet strong stimulation induces movement 
towards it Stimulus applied on other parts of the plant 
also will induce movements similar to that caused by mode- 



544 PLANT RESPONSE 

rate stimulation of the tip, provided the effect transmitted 
to the growing region be indirect. 

True perceptive region. There is a sense, however, in 
which it is true that the root-tip alone is the perceptive 
region, but this statement applies exclusively to the stimula- 
tion caused by gravity ; for, in order that this may take 
place, the presence of statolithic or other elements through 
which it can act is necessary. But if such elements be 
concentrated at the root-tip, it is clear that that region alone 
can become the seat of stimulation. This is not the case 
with other forms of stimulus, which act directly, the pre- 
sence or absence of statoliths being a matter of indifference. 
This distinction is important, since the outward resemblance 
of effects at the root-tip, in the two cases of stimulation by 
light and stimulation by gravity, has sometimes led to the 
assumption that statolithic bodies formed the medium of 
excitation in both. That such, however, is not the case is 
now evident. 

In the geotropic root, then, it is the indirect effect of 
moderate stimulus, unilaterally transmitted from the distant 
tip, that causes the responsive curvature. In the case of 
the apogeotropic stem, on the other hand, it is the direct 
stimulation of the statolithic elements distributed throughout 
that region that induces the responsive curvature ; and since 
I have shown that the curvature caused by direct is opposite 
to that which is the result of indirect stimulation, it will be seen 
that the opposite curvatures observed in root and shoot are 
not due to different sensibilities possessed by the two organs. 

It is to be noted, moreover, that although the statolithic 
or radial pressure theory affords a very rational explanation 
of the mode in which stimulation is brought about by 
gravity, yet the explanation which I have offered, of the 
occurrence of opposite geotropic effects in root and shoot, 
does not in any way depend upon the ultimate validity of 
this or any other particular theory. The aim of my demon- 
stration has been to show that, through whatever means the 
stimulus of gravity may act, it is inevitable from the fact 



INQUIRY INTO POSITIVE GEOTROPISM 545 

that the stimulation of the shoot is direct, and that of the 
root indirect that a single identical stimulus should in 
the two cases induce opposite responsive effects. 

Thus, though for the sake of convenience it is necessary 
to distinguish the upward curvature of the stem from the 
downward curvature of the root, by such terms as apogeo- 
tropic and geotropic, yet these words must not be allowed to 
connote two different sensibilities to the action of gravity ; 
for the sensibility of irritable tissues to stimulus is always of 
one kind, and of one kind only. 

SUMMARY* 

The responses induced by stimulation of the tips of shoot 
and root are similar under similar circumstances. 

The amputation of the tip of the root does not, normally 
speaking, abolish the excitability of the growing region of 
the root. 

The abolition of geotropic action in the root, after 
amputation of the tip, shows therefore, as pointed out by 
Ciesielski and Darwin, that with regard to gravity it is the 
tip that is the perceptive organ. This conclusion is also 
supported by the experiments of Pfeffer and Czapek, in 
which amputation was not included. The statolithic particles, 
again, through whose weight stimulation by gravity is pro- 
bably brought about, are, generally speaking, found con- 
centrated at the root-tip. 

Direct stimulation has been shown to induce a responsive 
movement in one direction, and fadirect stimulation in the 
opposite. The opposite geotropic curvatures in apogeotropic 
and geotropic organs are therefore due, not to the possession 
of different sensibilities, but to the fact that in the former 
stimulation is direct, and in the latter indirect, 



N N 



CHAPTER XL 

ON CHEMO-TROPISM AND GALVANO-TROPISM 

General difficulties of the investigation How lo overcome these difficulties 
Three distinct methods of testing results : ( I ) by variation of longitudinal 
growth ; (2) by responsive movement of pulvinus ; and (3) by growth-curva- 
ture Method of application of chemical reagent Effect of alkali Effect of 
acid Effect of copper sulphate Action of sugar solution Chemo-tactic 
movements Explanation of anomalous osmotic or plasmolytic action Excita- 
tory versus plasmolytic reaction in pulvinus of Mimosa : ( i ) Favourable tonic 
condition (2) Ordinary tonic condition Polar effects of currents inducing 
growth-curvatures Localised polar effects on pulvinus Anodic and kathodic 
effects on longitudinal growth Generalised law of polar excitation in plants 
Galvano-tropic response The indirect effect of polar excitation The effect on 
growth of ' electrification ' of soil. 

IT will be understood that growth-curvatures take place so 
slowly, that the effects of various external agents in inducing 
them can only be observed visually after the lapse of con- 
siderable intervals of time, which may be a matter of hours, 
or even of days. This fact is sufficient of itself to introduce 
elements of complication into the problem ; for (i) the 
plant may in that time undergo unknown spontaneous 
variations ; (2) the fundamental effect of any given stimulus 
is subject, when too long continued, to reversal, as we have 
seen in the case of the leaf of Mimosa, which from normal 
contraction passes into fatigue-relaxation, under the long- 
continued action of stimulus ; (3) the subjection of a plant 
during too long a period to an abnormal condition may 
sometimes cause derangement of its general functions ; and 
(4) there is the further element of variation which depends 
upon the point of application of stimulus itself, since we have 
seen that stimulation, acting directly on the responding organ, 
may produce one effect, and indirectly exactly the opposite. It 



CHEMO-TROPISM AND GALVANO-TROPISM 547 

has doubtless been due to the action of these sources of varia- 
tion that the observations made by investigators have so often 
been contradictory. 

All these uncertainties disappear, however, when we 
employ the high magnification, and the method of continuous 
record, which have been described ; for by these means we 
are enabled to follow the different phases of effect which occur 
immediately on the application of stimulus, and during its 
continuation. As these curvatures, moreover, are the results 
of the unilateral application of stimulus, expressed in the 
one-sided modification of longitudinal growth, the previous 
demonstration of the effect of the diffuse application of the 
same agent on growth itself enables us to infer the result 
to be looked for. The experiment is thus resolved into a 
verification of the inference. 

Again, I have shown that the growing region acts like a 
diffuse pulvinoid. Experiments then, by unilateral applica- 
tion of the particular stimulus to a true pulvinus, offer us 
another and independent means of testing the results arrived 
at. Thus we have no fewer than three distinct methods of 
testing the action of a given agent in the induction of growth- 
curvature, which, if they corroborate each other, may be 
regarded as affording a rigorous demonstration of the results. 
These are : first, the variation of longitudinal growth under 
the diffuse action ; second, the responsive movement of a 
pulvinus under the unilateral action ; and, third, the curvature 
which represents the modification of growth induced by the 
unilateral action of a given stimulus or agent. 

Method of application of chemical reagent. In dealing 
with problems involving the unilateral application of chemical 
reagents particularly, the experimental arrangements which I 
am about to describe will be found suitable. In order to 
obviate the complications which might result from the indirect 
effect of stimulus applied at a distance, the chemical reagent 
should be applied directly on the growing region. For this 
purpose the petals of various flowers, while in an active state 
of growth, are very appropriate, and I have used particularly 

N N 2 



548 PLANT RESPONSE 

those of the Indian Champaca (Michetia Champacd). The 
claw of a detached petal is held in a clip, and the petal 
is immersed in the inverted position in a small glass 
trough of water, of cubical shape. Inside the trough, 
and almost touching one face of the petal, say with its 
own right surface, is a partition of mica, in which is a long 
narrow slit. Thus the petal is in the left-hand compart- 
ment of the cube. A microscope of low magnification is 
focussed on the marginal point of its tip. The required 
chemical solution is now dropped by means of a pipette 
into the right-hand compartment. Thus it is diffused 
through the narrow slit, and acts directly and virtually 
unilaterally on the proximal side of the petal. If the 
particular agent should be one which accelerates growth, that 
is to say induces expansion, a growth-curvature will be 
induced, the proximal side becoming convex ; l but if its 
action is to induce depression or retardation of growth, the 
permanent effect will be a concavity of the proximal. The 
movements induced are observed by means of the microscope. 

Effect of alkali. In studying the effect of various drugs 
on growth in Chapter XXXV. we saw that the characteristic 
effect of alkali was to produce a contraction and retardation. 
In the present experiment, on the unilateral application of 
sodium hydrate in the manner described, the proximal side 
of- the petal was found to become concave, the tip being 
carried towards the agent. 

Effect of acid. The general effect of acid was found, as 
will be remembered, to be opposite to that of alkalis, namely 
relaxation. On now applying solution of HC1 unilaterally 
to a petal of Champaka, the result was the convexity of the 
proximal side. 

Effect of copper sulphate. This I have found to arrest 
growth, and its unilateral application in the present case was 
observed to induce concavity of the proximal side of the petal. 

1 The sudden introduction of a reagent may occasionally give rise to a transient 
excitatory contraction, but the effects described here are the permanent growth' 
effects. 



CHEMO-TROPISM AND GALVANO-TROPISM 549 

Action of sugar solution. A dilute solution of this agent 
has already been shown to accelerate growth. Its unilateral 
application was found, in the present case, to bring about 
convexity of the proximal side of the specimen ; but very 
strong solutions of sugar were found, as we have seen, to 
retard growth, and the unilateral application of such a solu- 
tion was now found to induce concavity of the proximal side. 

Chemo-tactic movements. We have now seen that the 
excitatory or depressing action of various agents is indicated, 
in the case of the growing organ, by a curvature one way or 
the other. Similar movements are induced also in pulvinated 
organs ; but in organs which are capable of multiple re- 
sponse we shall expect to find that the corresponding effects 
induced by such agents will consist ' in similar movements 
often repeated that is to say, in the initiation of such 
multiple response, or in its appropriate modification. It will 
be shown in Chapter XLIX. that the swimming movements 
executed by ciliated organisms form an instance of such 
multiple response. It will further be shown in that chapter 
that the organs respond to stimulus, whether chemical, 
photic, thermal, or electrical, by swimming either towards or 
away from it. 

Explanation of anomalous osmotic or plasmolytic 
action. An attempt has sometimes been made to explain 
the responsive movements of plant-organs as the result of 
supposed osmotic variations within the tissue. Thus it was 
held by De Vries that the growth -curvatures of multi- 
cellular organs were brought about by an increase of osmotic 
substances on the convex side, giving rise to augmented 
hydrostatic pressure. It was afterwards ascertained, how- 
ever, that the convex side of the curved organ did not con- 
tain any greater quantity of osmotically active substances 
than the concave. The method of plasmolysis of De Vries 
is often used for the determination of the differential osmotic 
activity and turgescence on the two sides of a curved organ. 
After plasmolysis, the curve induced in the organ is flatter 
than before, and from this it has been inferred that the cell- 



550 PLANT RESPONSE 

sap of the convex side of the organ is more powerfully 
osmotic than that on the concave. 

But various effects have been observed, of which the 
occurrence of plasmolysis alone affords no explanation. 
Thus, the diminution of curvature, which De Vries showed 
to be a consequence of plasmolysis, was demonstrated by 
Noll, in the case of recently curved organs, to be only the 
second phase of the effect ; for at the beginning of the 
operation in these cases, as he pointed out, the curvature of 
the organ was actually increased. This opposition of effects, 
the increase of curvature, followed by the flattening of the 
curve, as the result of plasmolysis, has not hitherto met with 
any satisfactory explanation. 

These obscurities in plasmolytic reactions have arisen 
from the fact that the excitatory action of some of the 
plasmolysing reagents has not hitherto been recognised. 
That such an influence may be exercised, and sometimes in 
opposition to osmosis, was, however, demonstrated in my 
experiments on suctional response, where it was seen that a 
strong solution of sodium chloride applied at the roots, 
instead of arresting suction by the withdrawal of cell-sap, 
actually enhanced the rate of suction for some time (p. 383). 
It was also shown in a later chapter, dealing with the 
effects of chemical reagents on growth, that when favourable 
tonic conditions had been induced, fairly strong salt solutions, 
instead of the usual retardation of growth, brought about a 
temporary enhancement, followed by depression (p. 486). 
We thus see that the responsive movement is modified 
(i) by the condition of the tissue, and (2) by the duration of 
the application of the solution. Now, in an already curved 
organ, we have an induced anisotropy, or difference of con- 
dition, on the two opposite sides. The effects of chemical 
reagents, then, on such an organ will be complex, for they 
will differ on the two sides, in intensity, and also in phase of 
reaction. The observed responsive movement in increasing 
or diminishing the existing curvature will thus represent the 
algebraical summation of the effects on the two sides. 



CHEMOTROPISM AND GALVANO-TROPISM 



Excitatory versus plasmolytic reaction in pulvinus of 
Mimosa: (i) Favourable tonic condition. I have already 
stated that a moderately strong solution of sodium chloride 
is a stimulating reagent. It is therefore to be expected that 
its diffuse application on the pulvinus will bring about a fall 
of the leaf ; and this, as we shall presently see, is found to be 
the case. We saw, however, in studying the effect of sodium 
chloride on growth-response, that under the favourable tonic 
condition induced by the optimum temperature of 34 to 
35 C. it gave rise to a preliminary 
relaxation, followed later by contrac- 
tion. From this it occurred to me, 
that in the parallel response of the 
pulvinus of Mimosa we might expect, 
under similar favourable conditions, 
to observe two opposite responsive 
movements : first, a preliminary ex- 
pansion, exhibited by erection of the 
leaf, and afterwards a contraction, 
exhibited by the depression of the 
leaf. 

In carrying out the experiment, 
I cautiously raised the temperature 
of the pulvinus to 34 C. and then 
applied to it a 3 per cent, solution of 
sodium chloride at the same tem- 
perature. The responses were auto- 
matically recorded by the leaf on 
smoked paper wrapped about the 

revolving drum. It will be seen (fig. 228) that the preliminary 
effect was a movement of relaxation or erection, which was 
completed in the course of one minute, and was followed by 
the opposite movement, or fall of the leaf. In these two 
opposite responsive movements, then, occurring in succession 
to each other, we have an analogous case to that of the two 
opposite and succeeding modifications of curvature, observed 
by Noll in a curved organ, immersed in salt solution. 




FIG. 228. Response of 
Leaf of Mimosa in 
Favourable Tonic Con- 
dition to Chemical Sti- 
mulus of 3 per CYnt. 
Salt Solution 

The preliminary response 
is here seen to be 
erectile. 



552 PLANT RESPONSE 

(2) Ordinary tonic condition. It will thus be seen that 
inferences drawn from experiments on plasmolysis may lead 
us to very wrong conclusions unless we take full account of 
the possible excitatory action of the particular solution. 
This is made strikingly evident by the following experiment. 
If we apply a strong solution say 10 per cent. of sodium 
chloride to the pulvinus of Mimosa, then, arguing entirely 
from the theory of plasmolysis, we must expect that the 
withdrawal of water will cause flaccidity of the tissue, and 
so bring about the fall of the leaf. This flaccidity, further 
increasing with the duration of application, would tend also to 
increase the fall of the leaf progressively. But a similar fall 




FlG. 229. Response of Leaf of Mimosa in Ordinary Tonic Condition 
to the Chemical Stimulus of 10 per Cent. Solution of Salt 

Under continuous stimulation the normal responsive fall is followed by 

fatigue-relaxation. 

may, on the other hand, be due to an excitatory reaction 
caused by strong salt, and there is nothing at first sight 
which would enable us to distinguish the one from the other. 
We know, however, as regards Mimosa, that under a con- 
tinuous application of stimulus -as, for instance, rapidly 
succeeding mechanical or electrical shocks the first fall 
of the leaf is succeeded by a return to the erect posi- 
tion. If the predominant efTect of salt, therefore, in the 
given instance, be to induce a responsive fall by excitation, 
then its continuous operation should give rise to a subse- 
quent erection. And from the automatic curve recorded on 
the smoked drum this is found to be the case (fig. 229) ; 



CHEMO-TROPISM AND GALVANO-TROPISM 553 

for it demonstrates that the fall of the leaf of Mimosa under 

* 

the action of a strong solution of salt was due, not to 
plasmolytic, but to excitatory action. 

Polar effects of currents inducing growth-curvatures. 

The galvano-tropic effects noticed by various observers in the 
case of growing plant-organs have been contradictory, because 
of the many complicating factors which might be expected to 
arise, not only from the differences of anodic and kathodic 
effects, but also from the varying points of application, and 
intensity and duration of current. All the obscurities and 
anomalies in connection with this subject will, however, be 
found to disappear, when they are related to the funda- 
mental polar effects, which have already been established as 
applying to the excitation of plant-organs (Chapter XVI.). I 
have shown that the kathode excites at make, and the anode at 
break. I shall now be able to show further that the galvano- 
tropic effects observed are all deducible from these. Inci- 
dentally, too, owing to certain special advantages afforded 
by growth-response, we shall be able to discover additional 
effects which could not have been detected by the mechanical 
response of the pulvinus alone. We have hitherto investi- 
gated the polar effects of an electrical current acting on the 
pulvinus as a whole ; but in order to bring these funda- 
mental effects into the clearest prominence, and to establish 
their universality, I shall now study (i) the reaction induced 
in a limited area of the pulvinus ; (2) the variations of longi- 
tudinal growth which result from the electrical actions of 
anode and kathode ; and (3) the growth-curvature induced 
by the unilateral action of anode or kathode on a growing 
region. 

Localised polar effects on Pulvinus. It is usually 
believed that the responsive effect seen in a pulvinus is 
entirely due to the excitability of the lower half of the organ. 
I have already shown that it is really due to the differential 
excitability of the upper and lower halves, and that the 
upper half is also contractile under stimulus, though in a less 



$54 PLANT RESPONSE 

degree than the lower. This will be seen fully demon- 
strated in heliotropic response, where the localised action of 
moderate light on the upper surface will be found to induce 
a movement upwards. In the present case, a given electrode, 
carrying current, will be applied locally on the upper half of a 
pulvinus, the second electrode being applied at a consider- 
able distance on the stem, and the specific result recorded. 

For such local application, it will be understood that a 
large pulvinus is an advantage, and therefore I used for my 
experiment the pulvinus of Erythrina indica> the sensitive- 
ness of which is not so marked as that of Mimosa. As 
preventing the diffusion of stimulus to the lower side, this 
lessened sensitiveness of the organ constitutes an added 
advantage for the purposes of my experiment. The respon- 
sive effect, it must be remembered, can be magnified to any 
extent that is desired by the Optic Lever, and in the present 
investigation a magnification of 200 times was employed. 
If the effect of the given electrode (laid on the upper sur- 
face), then, be to induce a responsive contraction, we can see 
that the normally horizontal leaf will be moved upwards, 
whereas a responsive expansion or relaxation would bring 
about a movement downwards. In order, however, to show 
the essential unity of all the different kinds of responses, 
whether by mechanical or growth movements, we shall 
regard them as cases respectively of the two fundamental 
phenomena of expansion and contraction. Expansion thus 
causes convexity or acceleration of growth, represented in 
these records by up curves, while contraction, concavity, or 
retardation of growth, is shown by down curves. 

In my experiment on the upper half of the pulvinus of 
Erythrina, I first made it anode, the E.M.F. used being 
twenty volts. In previous experiments on the anodic and 
kathodic reactions of the pulvinus, it was the differential 
effect that was observed, and the magnification employed for 
the record was only slight. Hence the pure effect of anode- 
make was found to be inconspicuous, whereas the anode- 
break induced the usual marked contractile action. In the 



CHEMO-TROPISM AND GALVANO-TROPISM 



555 




present case, however, no complicating differential action 
being involved, and the magnification being considerable, we 
are able to detect a make-effect, which is contrary to the 
break-effect at anode that is to say, it induces an expan- 
sion and consequent convexity. This anode-make expan- 
sion is quick, and soon reaches its maximum. At break the 
usual contractile effect is induced, 
but this is not here very clearly dis- 
tinguishable from the movement of 
a natural recovery (fig. 230). 

The current was now reversed, 
making the upper half of the pul vinus 
kathode. This induced a contraction 
and concavity which, unlike the 
quickly exhausted effect of the 
anode-make, went on increasing for 
some time. At kathode-break we 
see not only the cessation of the 
contractile effect, but probably also 
an expansion, aided by the natural 
process of recovery. Other experi- 
ments will be described presently, 
which will clearly show that not 
only do anode and kathode induce 
opposite effects, but that each at its 
own make and break exhibits re- 
actions, which, though not of equal 
intensity, are of contrary signs. 
These interesting opposite effects, 
at make and break respectively, 
are not easily observed in muscle under ordinary condi- 
tions, inasmuch as muscle which is in the usual state 
of expansion cannot^ be further expanded ; but Bieder- 
mann has found that smooth or cross-striated muscles, 
which are partially contracted, exhibit local expansion at the 
anode-make. Again, he found in the case of cardiac muscle 
that the kathode-break also gave rise to local expansion. 



Fid. 230. Polar Effects ot 
Currents due to Localised 
Application on Upper Half 
of Pulvinus of Erythrina 

indica 

Up curve, in this and in fig. 
232, represents expansion 
and convexity. Down 
curve represents contrac- 
tion and concavity. Con- 
tinuous curve represents 
the action at make. The 
dotted curve shows the 
effect at break. A;;/ = con- 
vexity induced ut anode- 
make. \b = responsive con- 
cavity at anode-break. 
K/// = induced concavity at 
kathode-make. K/; = ex- 
pansion induced at kathode- 
break. The time-marks in 
this and following curves 
represent minutes. 



556 PLANT RESPONSE 

These opposite effects at make and break, which I have 
already demonstrated in the case of the pulvinar response 
of vegetable tissue, will next be exhibited in a still more 
striking manner in the case of growth-response. 

Anodic and kathodic effects on longitudinal growth. 
We thus pass from the question of the motile response of 
pulvinated organs to the polar effects of currents on the rate 
of growth. From a demonstration of the fundamental 
action here, we shall be able to infer the curvature which 
will be induced by the unilateral application of polar 
currents in the growing organ. Experiments on growth, 
especially when conducted by the Method of Balance, have 
the unique advantage that the opposite responsive effects, of 
expansion and contraction, are clearly distinguishable, and 
not liable, under any circumstances, to be confused with the 
natural process of recovery ; for, as I have explained 
before, when a balanced horizontal record is taken of growth, 
an expansion or acceleration of the rate of growth will give 
rise to a deflection, represented, say, by an up curve. 
Recovery to the normal condition will now be indicated by 
a horizontal record. Contraction or retardation, similarly, 
will be represented by a deflection of the record downwards. 
Using this method, then, I have studied the effects of anode . 
and kathode on the growing regions of both root and shoot, 
the second electrode being placed at a very great distance 
from the first, so that its effect may be considered non- 
existent. It may be said here that the results obtained 
were the same for both root and shoot, and I shall now 
describe the effects observed in the case of an experiment on 
the root of Bindweed, using an E.M.F. of twenty volts. One 
of the electrodes a moist strip of cloth is wrapped about the 
growing region ; the second, as said before, being at a great 
distance! The balanced horizontal record of growth is first 
taken, and the growing region is made kathode. From the 
down curve in the record (fig. 231), it will be seen that a 
responsive retardation of growth is induced, which goes on 
increasing for a considerable time during the maintenance 



CIIKMOTROPISM AND GALVANO-TROPISM 



557 




of the current The current was now interrupted, and as an 
effect of the kathode-break we obtain an expansion and 
acceleration of growth above the normal, as seen from the 
up curve. This acceleration persisted for nearly three 
minutes, after which the growth-rate became again normal. 
The current was now sent in a reversed direction, and the 
result of this anode-make was a sudden expansion and 
acceleration of growth, which, as in 
the case of motile response, persisted 
for a relatively short time, when the 
growth-rate, as seen from the return 
to the horizontal, became once more 
normal. The current was now in- 
terrupted, and as the result of anode- 
break we have a contraction and 
retardation of growth, which per- 
sisted for nearly a minute and a 
half, after which the growth-rate 
became again normal. From this 
experiment we again see that not 
only are the anodic and kathodic 
effects opposite, but that the effects 
on each of make and break are also 
opposite. It is also seen that the 
expansional effect caused by anode- 
make is relatively smaller, and 
occurs, and is completed, more 
rapidly than the kathode-make con- 
traction, whose effect is more per- 
sistent. From these data we are 
able to arrive at a more comprehensive law of polar ex- 
citation in the case of vegetable tissues than was given 
in Chapter XVI , including not only ordinary mechanical 
responses, but also the modifications induced in growth. 
It will be understood that with regard to growth, expan- 
sion means acceleration of growth, and contraction means 
retardation. 



FIG, 231. Effects of Anode 
and Kathode on Variation 
of Rate of Growth in Root 
of Bindweed exhibited by 
Balanced Growth -record 

Down curve represents con- 
traction and retardation of 
growth, here and in fig. 233. 
Up curve represents expan- 
sion and acceleration of 
growth. K;;/ = contraction 
at kathode-make ; K/; = ex- 
pansion at kathode-break ; 
A/;/ = expansion at anode- 
make, and A = contraction 
at anode-break. It will be 
noticed in this and other 
figures that the contraction 
at kathode-make is stronger 
and more persistent than 
the expansion at anode- 
make. 



558 



PLANT RESPONSE 



The generalised law of polar excitation in plants is as 
follows : The kathode induces contraction at make, and expansion 
at break. The anode induces expansion at make, and contraction 
at break. 

Galvano-tropic response. From the demonstration just 
made of the fundamental polar effects on growth, we can 
easily infer the responsive curvature that would be induced 
in a growing organ by the unilateral action of anode or 
kathode. I shall, however, show that the effects directly 
observed are in strict conformity with these deductions. For 

these experiments I took the long scape 
of Crinum Lily, and on its growing 
region I applied one electrode, the other 
being connected with a distant point. 
On now making one side of the growing 
region anode, there was induced on that 
side a sudden and short-lived responsive 
expansion and convexity, seen in the 
up curve (fig. 232). At break, the 
opposite effect occurred, consisting 
partly of contraction and partly of 
natural recovery. The current was 
now reversed, and the effect of the 
unilateral kathode-make was the induc- 
tion of a very active and relatively long- 
continued contraction and concavity, to 
be seen from the down curve. The 
kathode-break next gave rise to the 
opposite effect, which was made up 

partly of expansion, and partly of natural recovery. Thus 
in these gal vano- tropic effects of currents on growing organs, 
we find what is simply a special case of the fundamental 
polar effects of currents on growth in general. 

The indirect effect of polar excitation. For the pur- 
pose of clearly demonstrating the fundamental action of 
currents, I have taken in the first place the simplest cases of 
the direct action of anode and kathode on the growing 




FIG. 232. Responsive 
Curvature in Scape of 
Crinum Lily by Uni- 
lateral Application of 
Anode and Kathode 

A/// expansion at anode- 
make ; Ad contrac- 
tion at anode-break ; 
KW = contraction at 
kathode-make, and 
Kb expansion at 
kathode-break. 



CHEMO-TROPISM AND GALVANO-TROPISM 559 

responding region itself ; but when similar applications are 
made, at a point distant from the responding region, we can 
foresee the fact that variations of these effects will occur. 
Thus the kathode-make, whose direct action induces contrac- 
tion and concavity, will now, acting from a distance, bring 
about, by means of expelled water, the indirect effect of 
expansion and convexity. This can be demonstrated by 
selecting a growing and undetached petal of Champaca^ in 
which the growing region is diffuse. The lower half of this 
petal is held in a clamp, the free upper half being attached 
to the Optic Lever for observation of its responsive movement. 
If now we make one side of the clamped half kathode (the 
other electrode being connected with the rest of the plant at 
a distance), then, at make of kathode, the indirect effect of 
stimulation, reaching the same side of the free end of the 
petal, gives rise to expansion and convexity that is to say, 
the opposite effect to that which would have occurred had 
that region been directly subjected to kathodic action. The 
effect of the kathode on a distant growing organ is thus an 
acceleration of growth. 

The effect on growth of ' electrification ' of soil. 
Empiric attempts have been made to discover whether the 
maintenance of an electrical current through the soil might 
or might not be made to have the effect of accelerating the 
growth of plants. This problem can only be satisfactorily 
solved, however, from accurate knowledge of the direct and 
indirect effects on growth of currents under the given condi- 
tions. Since growth is determined by the direction of current, 
it is not clear that currents flowing through the soil from right 
to left, or left to right, could give rise to a single effect. 
Looking, again, at the root, on which the current acts, we can 
see that one side will be anode and the other side kathode, 
and as the actions of these are known to be antagonistic, it is 
at first sight difficult to see how there can be any resultant 
excitatory effect at all. 

We are, however, enabled to obtain a clear understanding 
of the subject by an attentive comparison of the responsive 



560 PLANT RESPONSE 

effects of anode and kathode (figs. 230, 231). It is to be borne 
in mind that the growth of a plant, other factors remaining the 
same, is dependent on its suctional activity. And this again 
depends on the excitation of the root. Now, we saw that 
though the effects of anode and kathode at make arc opposite, 
yet they are at the same time not equal. The anodic effect 
is relatively small and short-lived, and the kathodic effect is 
stronger and more persistent. Hence, though the effects on 
the two halves of the root may be electrically opposed, yet 
there will be a resultant differential effect of excitation, due 
to the predominance of kathodic. action. Hence an electrical 
current, whatever be its direction, would excite the roots, 
causing a greater suctional activity, with consequent enhance- 
ment of growth. This 
deduction I have been able 
to verify by experimenting 
on the variation of the rate 
of growth of seedlings of 

-,, !? * A i * r Oryza sativa when the soil 
JIG. 233. Effect in Acceleration of Rate , ^ 

of Growth of Seedling of Oryza sativa in which they grow was 

of Current through the Soil subjected to a current, 

flowi "S " ' ne, and 

(I) from left to right (-->), and (2) rom again in the opposite 
right to left (<-). Dotted line shows .. . ^p, , 

effect on cessation of current. Record direction. 1 he growth- 

of growth taken under balanced con- reC ord of a single SCed- 
ditions. 

ling was first taken under 

balanced conditions (fig. 233). The electrical current was 
next sent through the soil from left to right, and during 
the continuation of this current, we see from the up curve 
the acceleration of growth above the normal that took 
place. The current was now stopped, and the former 
induced acceleration was replaced by a brief retardation, 
as seen in the down curve, after which the normal rate 
was restored. The current was next reversed, and yet 
the response was one of accelerated growth, and on the 
stoppage of the current after a brief retardation there was 
again a restoration of the normal rate. This experiment 




CHEMO-TROPISM AND GALVANO-TROPISM 561 

was carried out using an E.M.F. of ten volts, the two elec- 
trodes being applied at a distance of 10 cm. from each other. 
This meant a potential gradient of one volt per cm. This 
relatively strong voltage was applied for the sake of obtaining 
measurable response within a short time. But a much 
smaller E.M.F. is found to produce similar effects, though 
the action is slower. The application of a strong E.M.F. has 
the disadvantage of inducing fatigue, which is a drawback 
not present in the use of a feeble E.M.F. It is, however, 
shown that, during the continuation of a current through the 
soil, the rate of growth of a plant is enhanced. 

SUMMARY 

The effects -of unilateral chemical and galvanic excita- 
tion may be studied, both from pulyinar and growth 
movements. 

The unilateral application of alkali gives rise to positive 
curvature, that is to say, a movement towards the stimulat- 
ing agent 

Acids, under similar circumstances, give rise to negative 
curvature. 

The unilateral application of copper sulphate gives rise, by 
retardation of growth, to a concavity or positive responsive 
movement 

The unilateral action of dilute solution of sugar, by 
enhancing the rate of growth, induces negative response. A 
strong solution, however, gives the reversed or positive 
response. 

Effects fundamentally similar are expressed by swimming 
organisms in appropriate movements to and from sources of 
chemical stimulation. 

The assumption that the variation of curvature which 
occurs when an already curved organ is placed in strong 
solutions of salt is due to the action of plasmolysis is not 
always justified, for such solutions also exert characteristic 
excitatory effects. 

The pulvinus of Mimosa in a favourable tonic condition 



562 PLANT RESPONSE 

reacts to the action of salt, at least for a time, by an erectile 
responsive movement. Under a less favourable tonic con- 
dition, it responds by depression. That this latter is not 
entirely due to plasmolytic action is seen from the fact that 
the leaflet subsequently becomes erected, as is the case under 
continuous stimulation. 

The localised and unilateral action of anode and kathode 
on a pulvinated organ is as follows : 

The anode-make causes expansion, inducing convexity. 
This effect attains its maximum in a short time. 

The anode-break induces contraction and concavity. 

The kathode-make induces contraction and concavity. 
This effect is stronger and more persistent than the opposite 
effect of expansion at anode-make. 

The kathode-break induces expansion and convexity. 

The polar effects on longitudinal growth are as follows : 

The anode-make causes expansion and acceleration of 
growth. This effect attains its maximum in a short time. 

The anode-break causes contraction and transient retarda- 
tion of growth, 

The kathode-make causes contraction and retardation of 
growth. This effect is stronger and more persistent than 
the opposite effect of acceleration at anode-make, 

The kathode-break induces an expansion and transient 
acceleration of growth. 

The localised and unilateral action of anode and kathode . 
on a growing organ, results from the unilateral exhibition of 
those growth-variations which have just been described. 
Since a growing organ is virtually a diffuse pulvinoid, the 
effects are exactly similar to those seem in a pulvinated 
organ, already described. 

Owing to the fact that kathodic action is relatively 
stronger than anodic, a feeble or moderate current flowing 
through the soil exerts an excitatory action on roots, by 
which the suctional activity of the plant is increased. The 
result is an increased rate of growth, which is independent 
of the direction of flow of the current through the soil. 



PART VIII 
HELIOTROPISM 



CHAPTER XLI 

FUNDAMENTAL RESPONSIVE ACTION OF PLANT-TISSUES 

TO STIMULUS OF LIGHT 

* 

Diversity of movements induced by light Differentiation of responsive movements 
Action of light on tissues in sub-tonic conditions Effect of light on pulvinated 
organs Effect of diffuse stimulation of light on non-growing radial organs 
Retarding effect of light on longitudinal growth Phenomenon of oscillation 
under long-continued stimulation Similarity of responsive reaction under 
light and under other forms of stimulation. 

THERE is perhaps no other phenomenon in the plant-world 
which is at once so striking and so universal as that of the 
response which is evoked from plants by the stimulus of 
light. Under this influence the plant as a whole, and every 
part of it, is tremulous. Not only does the growing stem 
curve towards or away from the incident rays, but every 
leaf under their action is thrown into a state of periodic daily 
rhythm. By the absorption of light, again, the plant is 
energised. Thus the two factors, of external stimulus and 
internal energy, whose manifestations are so opposite in 
character, are brought into play 

Diversity of movements induced by light. Light, as a 
form of external stimulus, evokes responsive movements, which 
appear to be extremely diverse in their nature. Radial organs, 
for example, such as stems, under certain conditions, direct 
themselves towards the light, and under others, again, away 
from it. Leaves, however, behave quite differently. These 
are said to possess the peculiarity of placing themselves with 
their surfaces at right angles to the light ; but even this 
is not a property universally exhibited, for there are certain 
leaves which place themselves in a direction coinciding with 



566 PLANT RESPONSE 

that of light, either towards or away from it. Some plants, 
again, close their leaves or leaflets on the approach of night, 
in the so-called ' position of sleep ; ' while a further complica- 
tion arises from the fact that an apparently similar * sleep ' 
movement is produced, in these or others, by the action of 
the noonday sun. 

Light, again, appears in some cases to initiate movement, 
as was seen in Desmodium at standstill, and in others to arrest 
it, as is said to happen with the spontaneous movements of 
Trifolium pratense. Certain swarm-spores, moreover, appear 
to be attracted by light, swimming towards it, with energetic 
beats of their cilia ; while others, on the contrary, are affected 
in the opposite manner, and swim away. Or the same 
specimen may be found to swim, now towards, and again 
away from, light, swinging backwards and forwards like a 
pendulum. 

We thus see that not a single responsive effect of light 
has been observed in the case of plant-organs of which an 
example directly to the contrary may not be found. For 
this reason it has appeared hopeless to attempt to unify these 
phenomena, and this fact has left investigators with little 
option but to tend towards a ' belief in the individuality of 
the plant in deciding what shall be the effect pn it of external 
conditions.' * 

So far we have been considering only the diversity of the 
responsive effects which are induced by light in plants. 
When we come, however, to the further question of the 
responsive mechanics by which the stimulus of light evokes 
these movements, we are confronted at the outset of our 
inquiry by the fact that, as Pfeffer says, ' the precise character 
of the stimulatory action of light has yet to be determined.' 2 

Differentiation of responsive movements. It is cus- 
tomary, in treating of plant physiology, to. draw sharp lines of 
demarcation between the different classes of movements which 
are to be attributed to the action of light, ascribing each to 

1 Francis Darwin, B.A. Report, 

3 Pfeffer, Physiology of Plants, English translation, 1903, vol. ii. p. 101. 



RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 567 

some unknown specific sensibility. Thus, Sachs differentiates 
some of the principal effects as follows : 

' In the case of that stimulation of light which produces 
waking and sleeping, the stimulus lies in the variations of the 
intensity of light ; it is not the light as a constant force which 
effects these movements, but the varying intensity. A further 
great difference between the heliotropic curvatures and those 
which bring about the sleep movements, lies in the fact that 
the organ can make heliotropic curvatures in all directions. 
The movement of waking and sleeping, on the contrary, only 
takes place in one plane, which divides the leaf and motile 
organ symmetrically, and it is thus unimportant here in what 
direction the rays of light fall upon the motile organ, but only 
important that light is present at all, or increases or decreases 
in intensity. The above will suffice for the distinction of the 
movement of waking and sleeping from the heliotropic 
curvatures.' l 

Finally, he summarises the differences of motile effects as 
follows : 

' We may thus say shortly, the movements of waking and 
sleeping are called forth by paratonic light stimulus, whereas 
the spontaneous movements of the same leaves are in- 
dependent of any light stimuli, but probably dependent on 
phototonus. Heliotropic curvatures, on the contrary, have 
nothing to do with phototonus.' 

It will be found, however, that all these effects, sharply 
differentiated as they are, may be seen in one and the same 
organ. We may take for example the terminal leaflet of 
Desmodium gyrans. This exhibits under favourable condi- 
tions autonomous movements, whose period is short. It 
exhibits also daily periodic movements, with the very long 
period of twenty-four hours. It further, as I shall show, 
exhibits positive heliotropic curvature when exposed to one- 
sided illumination. Of these effects, it is supposed that the 
autonomous movement is independent of the paratonic action 
of light, but probably dependent on phototonus. The daily 

1 Sachs, Physiology of Plants, English translation, p. 628. 



568 PLANT RESPONSE 

periodic movement is held to be dependent on paratonic 
effects and phototonus. The heliotropic effect is ascribed to 
the continuous action of light, independent of phototonus. 

Thus in the same organ we have to postulate various 
irritabilities and mechanisms, in order to account for its 
multifarious movements. Are there, then, independent or 
different irritabilities, coexisting simultaneously in the same 
organ ? Such a state of things is so difficult to imagine, that 
it prompts us to try to look at the problem in a fresh light, 
divested of all assumptions which are incapable of experi- 
mental proof. 

Approaching the matter thus directly, then, we see that, 
instead of so many different irritabilities, there may possibly 
be, fundamentally, but a single phenomenon of irritability, 
finding expressions apparently diverse, in consequence of the 
anatomical or physiological differentiations of the responding 
organ. If this should be so, the question will resolve itself 
into three separate inquiries. First, what is that responsive 
action which constitutes the characteristic effect of stimulus 
of light ? Second, is such response to light unique in 
character, or is it a single instance of that universal 
phenomenon of contraction which we have seen to be the 
response of all excitable cells to stimulus in general ? And, 
lastly, in what manner do the various anatomical and physio- 
logical differentiations of responding organs operate to modify 
the expression of this response ? 

Action of light on tissues in sub -tonic condition. Before 
proceeding to a decisive demonstration of the nature of the 
effect of light on excitable tissues in a normal condition, 
however, I shall briefly refer to the suggestion which has 
been offered, that light in some unknown way induces a 
lessening of turgor, which brings about diminution of growth. 
This theory could not hitherto find acceptance for want of a 
precise knowledge of the exact nature of the stimulatory 
action of light, and of the relative significance of absorbed 
energy in promoting growth. Against the assumption that 
light diminished turgor, it was urged that the pileus of 



RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 569 

Coprinus drooped in darkness, and became turgid on restora- 
tion to light ; and the further supposition that light dimi- 
nished growth was held to be negatived by the instance 
of a dark-rigored plant, in which growth, so far from being 
retarded, was accelerated or renewed by simple exposure to 
light 

In both these instances we notice the abnormal condition 
induced in the plant. We must bear in mind that that 
rhythmic activity which is essential to growth depends, like 
all other rhythmic activities, not only on the turgor of the 
tissue, but also on the energy which it has absorbed. We 
have seen that when the sum total of independent stimulating 
factors present in the plant is adequate to raise its tonic condi- 
tion above par, then rhythmic activity is initiated or renewed. 
Among these factors are, as has been shown : (a) a proper 
condition of turgidity ; (fr) favourable temperature ; and (c] 
that previous absorption of energy of light which determines 
what we may distinguish as the phototonic condition. 

The fact that, besides turgor, a certain amount of energy 
is also necessary to initiate growth has been fully demon- 
strated in a previous chapter. Taking now the case of the 
dark-rigored plant, we see that the arrest of its growth is due 
to a deficit of absorbed energy in this instance, phototonus. 
Under these conditions, the renewed exposure to light would 
be sufficient, by supplying the missing factor, to re-initiate 
rhythmic activity and consequent growth. The drooping of 
the pileus of Coprinus in darkness is another expression of 
the sub-tonic condition of the plant. It must be remembered 
that the suctional activity which determines turgor is itself 
dependent on the rhythmic activity, and therefore on the 
tonic condition, of the plant. The critical element of this 
tonic condition may in certain cases be the absorption of 
light. I have noticed a similar drooping in flowering plants 
kept in the dark for a long time. Exposure to light, restoring 
the tonic condition, is in such cases, as also in Coprinus, 
sufficient to restore the normal turgidity of the plant. 

In connection with this, I may draw attention to the fact 



570 PLANT RESPONSE 

that the after-effect of absorbed stimulus in maintaining a 
favourable tonic condition is more prolonged in some cases 
than in others. In Biophytum, for example, the rhythmic 
activity by which the leaflets are thrown into pulsation is 
maintained only as long as the stimulating factors are acting. 
In Desmodium, on the other hand, there is a considerable 
capacity for storage of energy, and rhythmic activity persists 
for a long time, even on the removal of external stimulating 
agencies. 

We thus see that the question of turgidity-variation alone 
is not sufficient to explain the action of light on a plant. 
We have also to take into account the important element of 
energy. We have briefly considered the effect of light on 
tissues in an abnormal condition ; but our main inquiry 
concerns itself with the precise nature of the stimulatory 
effect of light on tissues which are in a normal tonic condition. 
I propose to demonstrate the character of this action by 
three independent lines of investigation. First, we shall 
study the action of light on pulvinated organs, in order to see 
whether or not this stimulus produces the same kind of 
differential contractile effect as other forms of stimulation. 
Secondly, discarding those complications which inevitably 
result from the differentiated structure of the pulvinated 
organ, I shall proceed to determine the precise nature of the 
fundamental form-change undergone by a tissue when 
excited by light. For this purpose I shall subject a radial 
organ to diffuse stimulation of light, observing whether or not 
under these circumstances it exhibits longitudinal contraction. 
And, lastly, I shall study the effect of light in inducing 
changes in the rate of growth, and shall also try to find out 
whether the fundamental responsive action discovered in the 
case of stationary, that is to say of non -growing, organs, 
might not be capable of explaining the observed variations of 
growth under the action of light. 

Effect of light on pulvinated organs. If a strong 
beam of light be applied to the pulvinus of the leaf of Mimosa 



RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 571 

or to the pulvini of the leaflets of Biophytum, it is known that 
a responsive movement, similar to that evoked by any other 
form of stimulus, is induced. Now, we have seen that the 
response of such pulvini is given by means of differential 
longitudinal contraction. If, then, the effect of stimulus of 
light in this case is to produce a differential longitudinal 
contraction, we ought to be able to obtain from a radial 
organ, subjected to stimulus of light from all sides, a respon- 
sive longitudinal contraction. 

Effect of diffuse stimulation of light on non-growing 
radial organs. In order to demonstrate this, I took a radial 
style of Datura, and subjected it to the stimulus of light from 
all sides, by throwing a beam of sunlight which struck it on 
one side directly, and on others by reflection from properly 
inclined mirrors. On exposing the specimen to this stimulus 
for a period of four minutes, a responsive contraction of seven 
divisions was induced. On the cessation of light, there was 
recovery in the further course of a period of nine minutes. I 
next applied stimulus of light for six minutes, and a respon- 
sive contraction of eleven divisions was then induced, with 
a subsequent recovery on the stoppage of light, which was 
completed in fifteen minutes. We thus see that the effect 
of stimulus of light in producing responsive contraction is 
precisely the same as that which is the result of any other 
form of stimulation ; that a feeble or short-lived stimulus 
induces a corresponding effect, from which recovery takes 
place, in a comparatively short time; and that a strong 
stimulus unless it induce fatigue will bring about a 
considerable contractile effect, from which the recovery is 
accomplished in a proportionately longer time. Again, I find 
that continuous stimulation of light produces a maximum 
tetanic effect, and that too long-continued action, bringing 
about fatigue, may induce fatigue-reversal contraction 
passing into relaxation as we found in the case of Mimosa, 
and of various radial organs which were subjected to too 
long continued action of stimulus. 



5/2 PLANT RESPONSE 

Retarding effect of light on longitudinal growth. We 

thus see that the fundamental contractile effect of stimulus is 
precisely the same in the case of light as in that of any other 
form of stimulation. We know also that the response of a 
growing organ is the same as that of one which is not 
growing. It has been shown further, that in a growing 
organ the contraction clue to direct action of stimulus had 
the effect of retarding growth. Now, from the fact that the 
effect of light is the same as that of other forms of stimu- 
lation, it follows that the result of its direct action on a 
growing organ should be to produce contraction, and 




FIG. 234. Longitudinal Contraction and Retardation of Growth under 
Light in Hypocotyl of Sinapis nigra 



The first part of the curve shows the normal rate of growth. Arrow 
indicates moment of application of diffuse light, which is seen not only 
to retard growth, but also to induce a marked contraction. The 
second arrow indicates moment of withdrawal of light, and dotted 
portion of the curve shows recovery. 

resultant retardation of growth. Though this conclusion, 
however, was thus clearly established in theory and by other 
experiments, using different forms of stimulus, I yet thought 
it important to test the matter with regard to the specific 
action of light, under conditions so simple as to bring out the 
fundamental phenomenon unmistakably. 

In order to do this, I took a seedling ot Sinapis nigra in 
which the hypocotyl was strictly radial, and made a record 
of its longitudinal growth (fig. 234). Its normal rate of 
growth, seen in the curve as movement upwards, was at the 
rate of '015 mm. per minute. On now applying light to 
the specimen on all sides at once, the growth is seen to 



RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 573 

undergo rapid diminution till arrested, as seen by the curve 
becoming horizontal at its highest point. The continued 
application of light now proceeds to cause a marked contrac- 
tion, the maximum rate of which is *O2 mm. per minute. It 
is therefore to be noticed that, in this particular experiment, 
light not only retarded growth, but also produced an actual 
shortening of the plant. On the cessation of light, growth was 
slowly recommenced, but the average rate of growth during 
the sixteen minutes following the stoppage of light was only 
ooi mm. instead of the normal rate of '015 mm. per minute. 
This rate was, however, gradually increased in the absence 
of light, and after a certain interval became normal again. 

It must be borne in mind, in connection with this, that 
though the immediate and after-effects of incident light are 
here seen as retardation of growth, nevertheless the rate, 
after the lapse of a certain interval from the cessation of 
stimulus, may be again increased above the normal in con- 
sequence of the enhancement of the tonic condition of the 
plant, by its absorption of energy of light. 

In the last experiment we saw that responsive contraction 
not only arrested growth, but made the tissue actually 
shorter. In other instances the contraction and resulting 
retardation are not so great. Thus in a second experiment 
with Sinapis, the normal rate of growth was *O2 mm. per 
minute, which during the continuance of light fell to one- 
tenth of this, or -002 mm. per minute. Thus the effect of light 
on the growing organ is always a contraction, which may 
in some cases induce a mere retardation, but in others 
culminates not even in cessation of growth, but in an actual 
shortening of the responding tissue. 

The effects thus described occur in plants which are in 
normal tonic condition. But we have seen that when the 
specimen is, on the other hand, in a sub-tonic condition, 
absorption of enegy in any form from outside will, at first, 
by increasing the internal energy, serve to accelerate growth ; 
and that afterwards, when the normal tonic condition has 
been attained, external stimulus will have the normal effect 



574 



PLANT RESPONSE 



10' 15' "20' 25' SO"' 35' 40' 45' 



of retarding growth. In order to verify this inference I took a 
growing flower-bud of Crinuin Lily, which had been previously 
kept in the dark. And for the further purpose of detecting 
even the transitory variations, I used the delicate method of 
balanced record. On now subjecting the specimen to the 
stimulation of sunlight, acting on it from all sides, I observed 
a preliminary acceleration of growth, which lasted one 

minute (fig. 235). By 
this time the plant had 
evidently attained its nor- 
mal tonic condition, and 
the continued action of 
light resulted in a retarda- 
tion of growth, as seen in 
the rapid descent of the 
curve. The light was 
next shut off, and the 
after-effect of absorbed 

FIG. 235. Balanced Record of Variation energy is seen in the CO11- 
of Growth in Flower-bud of Crinum sem , en( . o rrP W;,fi n n of 
Lily under Diffuse Stimulation of Light se q uen t acceleration Ol 

Continuous lines represent the effect during the rate of S rOwth abovc 

application of light, the dotted line on the normal. This ao 
withdrawal of light. The plant was , . 1 t r r 

originally in a sub-tonic condition, and CCleratlOn lasted for four 

minutes, after which the 




application of light at x , after short 
latent period, induces preliminary ac- 
celeration of growth. After this follows 
the normal retardation. On withdrawal 
of light, in the dotted portion of the curve 
is seen the negative after-effect, followed 
by return to the normal rate of growth. 
A second and long-continued application 
of light induces retardation, followed by 
oscillatory response. 



plant returned to almost 
its normal growth, as seen 
by the record approximat- 
ing to the horizontal. The 
plant may at this point 
be regarded as in ordinary 
tonic condition. Light was again applied, and retardation of 
growth is immediately shown by the descent of the curve. 
There is now no preliminary acceleration of growth, as in the 
case when the plant was sub-tonic. Under the long-continued 
action of light, there is now seen the very interesting 
phenomenon of the induction of autonomous pulsations of 
the rate of growth, whose period is about ten minutes. 



RESPONSIVE ACTION OF PLANT-TISSUKS TO LIGHT 575 

Phenomenon of oscillation under long-continued 
stimulation. We have seen, in Chapter XXIV., that 
when a tissue is subjected to continuous stimulation, so 
that it becomes possessed of excess of energy, its response 
becomes pulsatory in consequence of periodic variations of 
its excitability. The exhibition of this variation of excita- 
bility we have, for the sake of convenience, designated as 
periodic fatigue. The occurrence of autonomous response is 
intimately connected with this periodic variation. In some 
tissues it may be exhibited only once, while in others it may 
be repeated indefinitely. For example, in Mimosa, under 
continuous stimulation, we see a single complete pulsation, 
consisting of the contractile response, 
and the subsequent fatigue-relaxation, 
which looks like recovery (fig. 59). 
But, in the longitudinal response of 
the style of Uriclis Lily to continuous 
stimulus, we observed two pulsations 
(fig- 58). And in Biophytum and 
Desmodium we see these repeated 
indefinitely, constituting what we FIG. 236. Oscillatory Rc- 

. ,,. , , sponse of Arsenic acted 

know as multiple or autonomous J n Continuously by 

response. Hertzian Radiation 

It is very interesting to note, in Taken by method of con- 

, , . ductivity variation. 

connection with this, that even in- 
organic substances, under continuous stimulation, exhibit 
this pulsatory response. I give here a record (fig. 236) 
of the oscillatory response of arsenic when acted on con- 
tinuously by Hertzian radiation. 

Or we may view this induction of multiple response in a 
radial vegetable organ, again, from a different standpoint. 
By the action of stimulus, it is easy to see that antagonistic 
hydrostatic actions may be set up as between the excited 
organ and the rest of the plant. Thus, the direct effect of 
continuous stimulation is contraction, with increasing expul- 
sion of water from the excited organ into the rest of the 
plant, the hydrostatic reaction which this will induce in the 




5/6 PLANT RESPONSE 

rest of the plant will also constantly increase. The occur- 
rence of oscillatory action, under these balanced and opposed 
forces, is what might be expected. Moreover, in the re- 
sponding organ itself, we see the action of opposed forces 
to be induced ; for while the direct effect of local stimulus is 
to cause contraction, the absorbed stimulus is meanwhile 
increasing the internal energy, the result of which is the 
opposite expression in expansion. Thus, from what has 
been said, it would appear that in all these namely, the 
hydrostatic action and reaction between the excited organ 
and the rest of the plant, the opposed effects of external 
stimulus and internal energy, and the presence of an excess 
of latent energy, with periodic variations of excitability we 
have so many factors, which would all contribute to a 
common result, in the oscillatory character of the responsive 
expression. 

But when we come to the case of anisotropic or dorsi- 
ventral organs, we find an additional element making for 
alternation of effects, for when such an organ is diffusely 
stimulated, we obtain a differential response. But as the 
constitutions of the two anisotropic halves are different, the 
fatigue produced on opposite sides will not be simultaneous 
but alternate. Similar effects will also appear under uni- 
lateral stimulation of a radial organ. For here, too, the 
organ, owing to the relatively greater fatigue of one side, 
becomes molecularly anisotropic, and the stimulus by its long- 
continued action becomes internally diffused. 

Similarity of responsive reaction under light and other 
forms of stimulation. It has thus been fully demonstrated 
that the fundamental response of the plant to the stimulus of 
light takes place, like that to all other forms of stimulation, 
by contraction, leading, in the case of growing organs, to 
retardation of growth. Hence the unilateral stimulus of 
light may be expected to induce curvatures similar to those 
observed under other forms of stimulation, that is to 
say : 

i. The direct effect of moderate unilateral stimulus of 



RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 577 



light on the growing region will be a responsive con- 
cavity. 

2. The effect of such stimulus of light, acting on the tip 
of either shoot or root, will be a convexity of the same side of 
the responding region. 

3. Strong or long-continued stimulation of light acting 
on the growing region will induce a neutral or reversed 
effect 

4. The response, under certain conditions of continuous 
stimulation, may be characterised by pulsations. 

It has been shown in previous chapters that the responsive 
action of anisotropic or dorsi-ventral organs is not funda- 
mentally different from that of radial organs, the seeming 
differences being accounted for by differential action. In 
explaining the various effects of stimulus of light, then, the 
assumption of various sensibilities in the plant is unjustified. 

I shall attempt, therefore, to trace out the manner in 
which one fundamental effect of responsive contraction is 
made to find diverse expressions, owing to the anatomico- 
physiological differentiation of the responding organ. And as 
the supposed different specific sensibilities do not exist, I 
shall designate all movements and curvatures induced by 
light as heliotropic, the signs positive, negative, and dia- being 
used only for descriptive purposes. An investigation into the 
action of stimulus of light, then, must apply itself to the 
following points : 

1. The effect of unilateral stimulus, of varying intensity 
and duration, on the tips and growing regions respectively of 
radial organs. 

2. The induction of autonomous movement by the absorp- 
tion of energy of light. 

3. The action, of stimulus of light on molecularly aniso- 
tropic and on dorsi-ventral organs. 

4. The direct and after-effects of light, in inducing move- 
ments of daily periodicity. 

These are the questions which will be specially dealt with 
in the course of the following chapters. 

p P 



PLANT RESPONSE 



SUMMARY 

The action of the diffused stimulus of light on a radial 
organ is, like that of other forms of stimulation, to induce a 
longitudinal contraction. The action on mature does not 
differ from that on growing organs. In a growing organ the 
induced contraction has the effect of retarding growth. 

The response to light may thus consist not merely of a 
retardation of growth, but sometimes also of an actual 
shortening of the responding organ. 

When an organ is in a sub-tonic condition, absorption of 
energy of light may give rise to a transient acceleration of 
the rate of growth ; but when the plant has attained the 
normal tonic condition, response is by the usual retardation of 
growth. 

The after-effect of stimulus of light may consist of a 
simple continuation of the characteristic responsive contrac- 
tion or retardation of growth. This constitutes the positive 
after-effect. There may also be a negative after-effect, con- 
sisting of expansion or acceleration of the rate of growth. 

Under the long-continued action of light, fatigue is 
induced. There is sometimes an exhibition of periodic fatigue 
with oscillatory response. 

The response to light is not different from that evoked by 
any other form of stimulation. The various responsive 
movements which occur under the action of light are thus 
explicable without the assumption of the possession by 
different organs of different specific sensibilities to light. 



CHAPTER XLII 

POSITIVE HELIOTROPISM 

Introduction Theory of de Candolle Inadequacy of de Candolle's theory 
Definition of terms positive and negative Darwin's theory of modified circum- 
nutation Response of terminal leaflet of Desmodium Extreme sensitiveness 
of some plant-organs to light Merging of multiple in continuous response 
Orientation induced by light The perceptive region in the terminal leaflet of 
Desmodium Heliotropic response in radial organ Magnetically controlled 
recorder Heliotropic response of hypocotyl of Sinapis Recovery and theory 
of recti-petality. 

HAVING demonstrated the fact that the stimulus of light 
induces contraction in mature organs, and retardation of 
growth in growing organs, in precisely the same manner as 
any other form of stimulus, we shall now proceed to study 
in detail the various effects produced by the unilateral 
application of light. Such application, long continued, is 
seen to induce movement of the organ, in some cases towards 
the light, in others away from it, and in still other organs to 
induce a position at right angles to it. While it is doubtless 
convenient to distinguish such external effects as positive, 
negative, and dia-heliotropic, it is nevertheless unfortunate 
that these terms carry with them an assumption that the 
movements in question are due to as many distinct sensi- 
bilities on the part of the plant-organs. I shall, however, 
endeavour to show that there is in the plant but one funda- 
mental sensibility to light, as to other forms of stimula- 
tion, which finds expression in contraction under its direct 
action ; and that the resulting movements of the organs are 
dependent on the question as to whether stimulus acts 
directly or indirectly, unilaterally or diffusely, and also, 
on anatomical or physiological peculiarities of structure 
The complexity of the subject being very great, it has often 

P P 2 



580 PLANT RESPONSE 

been found difficult to resolve a given movement into its 
components, and this has led to the abandonment of the 
attempt to relate these various movements to any single 
basic reaction. So far was this carried that, in spite of the 
well-known observation that a radial organ, illuminated with 
different intensities on two different sides simultaneously, 
bends in the direction of the more intense illumination, 
Sachs found himself compelled to believe that it was the 
direction and not the intensity of light that determined the 
responsive movement. 

Theory of de Candolle. It is appropriate to make here 
a brief mention of the theory of de Candolle, which has 
hitherto met with unmerited neglect. 'De Candolle started 
from the known fact that light retards growth, and explained 
growth-curvature as due to the relatively greater growth of 
the shaded, inducing concavity of the lighted, side. This 
explanation of the mechanics of such movements constitutes 
an important advance, though it does not take full account 
of all the factors of the problem. This theory of de Candolle 
has, however, been discarded, in consequence of the difficulty 
which it presented of explaining the action of the negatively 
heliotropic organs, in which the lighted side is found to be 
convex. Extending this theory to cases* of negative helio- 
tropism, it was regarded as a logical inference that light 
should here accelerate growth. It is doubtful, however, 
whether such an inference is justifiable. In any case this 
was negatived by the researches of Miiller-Thurgau, F. 
Darwin, and Wiesner, who showed that light retarded general 
growth in negative as well as in positive heliotropic organs. 
It will be shown, however, in the course of succeeding 
chapters, that though the circumstances which modify the 
response of the plant, so that it is exhibited as negative 
heliotropism, are somewhat complicated, yet they in no way 
detract from the theory of de Candolle as applied to positive 
heliotropism. 

Inadequacy of de Candolle's theory. The flaw in his 
theory lies rather in the fact that he regarded the normal 



POSITIVE HELIOTROPISM 581 

rate of growth under shade as the active factor in growth- 
curvature, elongation on the lighted side being retarded, where- 
as in the case of positive light-curvatures the motive-power 
really lies in the active responsive contraction of the lighted 
side, the expelled water from which, reaching the opposite, 
may further cause an increase of growth above the normal. 
This fact, that it is the active contraction of the lighted, and 
not the passive growth of the unlighted, side that is actually 
the efficient cause of heliotropic curvature, will be made clear 
by taking an extreme case in which there is no growth. 
Here, if heliotropic curvature had been due simply to differ- 
ential growth, the occurrence of curvature would have been 
an impossibility. But heliotropic curvatures are observed in 
organs which have come to growth-standstill. Again, grass 
haulms, in which growth is arrested, exhibit curvatures due 
to the contraction of the lighted, and the renewal of growth 
(due to the increased turgescence caused by expelled water) 
on the unlighted, side, in precisely the same manner as they 
were observed to do under the action of gravitational 
stimulus. 

I shall now proceed to describe some typical experiments 
on heliotropic effects as induced by the unilateral application 
of light. And since it has been shown that the responses of 
growing organs are not essentially different from those of 
pulvinated organs, it will be helpful to begin by observing the 
effect of the unilateral application of light on the latter, 
especially as these have the advantage of showing relatively 
rapid reactions. In order to obtain a pulvinus which 
approximates in character to that of a radial organ, a speci- 
men must be selected in which the difference of excitability, 
as between the upper and lower halves, is as small as possible. 
This may be found in the pulvinus of the large terminal 
leaflet of Desmodium. 

Definition of terms * positive ' and ' negative.' It will 
be demonstrated, in this and succeeding chapters, that how- 
ever various in type may be the responsive movements 
induced by light, they are not due to the different specific 



582 PLANT RESPONSE 

sensibilities of different organs, but can all be shown to form 
only special instances of a single fundamental effect. And this 
fundamental effect is the same in growing, in stationary, in 
radial, and in anisotropic organs. In describing the respon- 
sive actions induced by light, it will, however, be necessary to 
distinguish the direction of movement, in relation to the 
stimulating light, by clearly defined terms. I shall therefore 
designate all movements, of whatever organs, towards light 
as positively heliotropic, and all movements away from light 
of whatever organs, as negatively heliotropic. 

Darwin's theory of modified circumnutation. Before 
describing the response, it is necessary to say a few words 
regarding Darwin's view of heliotropic movements as a 
modified form of circumnutation. According to this, the 
already existing movement (of circumnutation) had only to 
be increased in some one direction, and lessened or stopped 
in others, in order to become heliotropic or ap-heliotropic, as 
the case might be ; 1 but, in order to prove conclusively that 
heliotropic curvatures were caused by the modification of the 
pre-existing movements, it would be necessary to show that 
they did not take place in those organs from which circum- 
nutation was absent, and this would constitute the crucial 
test of the theory. The difficulty of obtaining such a speci- 
men is, however, so great, that Darwin, although he noted 
the point, was unable to apply the test. 

I have shown elsewhere that circumnutation is only a 
particular manifestation of that multiple or autonomous 
response of plants which is due to an excess of energy, 
previously absorbed. In order to obtain a plant-organ com- 
pletely at standstill, therefore, it would be necessary to find 
a specimen in which there was no such excess of latent 
energy. The fact that circumnutation was absent could then 
be ascertained by means of the high magnification obtainable 
from the recording apparatus which I have already employed. 
Another difficulty lay in the fact that the use of light, how- 
ever feeble, for purposes of observation, would be apt of itself 

1 Darwin, The Movements of Plants, p. 449. 



POSITIVE HELIOTROPISM 583 

to give sufficient stimulus to initiate multiple responses 
which had not been present before. This might have 
appeared incredible had I not, in the course of my experi- 
ments, had reason to know how extraordinarily sensitive 
plants may become to the influence of light, instances of 
which will be found in the investigations presently to be 
described. In view, indeed, of the vitiation of results which 
might be caused in this way, I was compelled to devise 
special means for obviating the use of any light whatsoever 
for the observation of responsive curvature. 

Response of the terminal leaflet of Desmodium. In 
order, then, to determine the important question of whether or 
not light will produce heliotropic movement in a plant devoid 
of circumnutation, I acted on the idea that the withdrawal of 
superfluous energy was the essential preliminary condition, 
and chose for my purpose, as already said, the large terminal 
leaflet of a specimen of Desmodium gyrans in which all 
movement had come to a stop the plant having been 
exhausted by flowering, and by the unfavourableness of the 
season, which was winter. And further, in order that there 
should be no storage of energy derived from light, I kept 
this plant for one day in a dark room. To eliminate the 
necessity of using light for purposes of observation, I attached 
the leaflet by cocoon fibre to the arm of the Optic Lever 
Recorder. The plant itself was enclosed in a dark box, and 
thus protected from any access of light. Through a trap- 
door in the box, light for stimulation could be thrown down 
on the leaf at the desired moment. The preliminary absence 
of any autonomous movement in the plant was seen by the 
quiescence of the spot of light reflected from the mirror 
attached to the recording lever. 

I now subjected the terminal leaflet of the selected speci- 
men to light from a candle, this being thrown down on the 
leaflet vertically by means of a mirror, the effective distance 
of the candle from the leaf being twenty centimetres. The 
leaflet, which had previously been quiescent, began to respond 
after a latent period of ten seconds, and during the course of 



5 8 4 



PLANT RESPONSE 



an exposure of twenty minutes executed five complete 
oscillations (fig. 237). The notable points in this record are : 
(i) that a perfectly quiescent organ is made to give multiple 
response by the stimulus of light ; (2) that molecular 
sluggishness appears to be gradually removed by the con- 
tinuous absorption of energy, and the successive responses 
exhibit an enhanced or ' staircase ' effect ; and (3) that from 
the tendency of the scries of curves to tilt towards the light, 
the organ is seen to exhibit a resultant positive movement. 

It will thus be seen that we have here not a modifica- 
tion of an existing movement, but a series of multiple 

responses, with a trend in 
a particular direction, that 
is to say towards the 
light, constituting a posi- 
tive heliotropic movement. 
In a growing organ also, 
which was previously de- 
void of circumnutation, we 
shall be able to observe 
the induction of a simi- 
lar movement. As in the 
case of the movements of 
growth, so also in those of 
heliotropism, we are often 
able to detect multiple 

constituent pulsations, especially at the commencement of 
response when stimulus is moderate. It will be understood 
here that, under the unilateral contraction induced in the 
excited side of the organ by the stimulus of light, a hydrostatic 
disturbance is set up, the expelled water being forced to the 
opposite side a state of things calculated to show pulsation 
in a marked degree. The final resultant curvature, then, as we 
have seen under the action of other forms of unilateral stimulus 
also (p. 521), represents the joint effects of the concavity of 
the proximal and the convexity of the distal sides. When 
light acts on the organ continuously for some time, the 




Fie,. 237. Multiple Response to Light 
of Terminal Leaflet of Desnwdinm 

The moment of application is marked 
by x . The arrow shows the direction 
of light, i.e. from above. The 
numbers in the abscissa represent 
time in minutes. 



POSITIVE HELIOTROPISM 585 

multiple responses become more frequent by the increased 
absorption of energy, and their individuality is lost. 

The extreme sensitiveness of some plant-organs to 
light. I shall now say a few words about the extraordinary 
sensitiveness of some plants to the stimulus of light. Darwin 
gives a striking example of this in a case where the cotyledons 
of Phalaris canariensis, after three hours of continuous exposure 
to a small lamp at a distance of twelve feet, became doubtfully 
curved towards the light, and after seven hours and forty 
minutes from the first exposure were plainly, though slightly, 
curved towards the lamp. The candle-power of the lamp is not 
given, but it may be taken to be about four candles. Reducing 
this to the standard distance of one metre, we find four candles 
at a distance of twelve feet (four metres approximately) to 
be equal to a quarter candle at a distance of one metre. If 
three hours' exposure induced a doubtful curvature, then the 
smallest amount of light to be effective must have been 
| or 75 candle-hour ; the candle-hour giving an indication 
of the quantity of light that had to be absorbed by the plant 
in order to induce a movement that was just perceptible. 

Now, with the terminal leaflet of Desmodium, exposure to 
the light of a candle at a distance of 20 cm. for ten seconds 
was sufficient to initiate responsive movement. This when 
reduced to standard conditions is equivalent to "07 candle- 
hour. In other words, we find, as far as these two experi- 
ments can determine the point, that the terminal leaflet of 
Desmodium in this experiment was at least ten times more 
sensitive than Darwin's cotyledons of Phalaris canariensis. 

Darwin, however, mentions another instance which is more 
like the sensitiveness of which I have just given an example. 
He has been using a small wax taper, in order to observe the 
cotyledons of Phalaris ; he used this light for one or two 
minutes at each observation, and observed the seedlings 
seventeen times in the course of the day, in consequence of 
which he found that zigzag responsive movements had been 
induced. 

I must again point out here that the specimen of 



586 



PLANT RESPONSE 



Desmodium which exhibited such remarkable sensitiveness 
had been specially chosen, as being in the least favourable 
tonic condition. And yet I could not even strike a match 
near this plant without inducing responsive movements. 
This will indicate the extreme sensitiveness of certain plants 
to light, and show that it is necessary to make our 
observations of induced movements without its aid. The 
manner in which this was done will be described presently. 

Merging of multiple in continuous response. We 
found that light acting from above on the terminal leaflet of 

Desmodium gyrans gave rise to multiple 
responses. As the light was acting con- 
stantly on the upper half, there was a 
cumulative contractile effect on that half. 
The consequence of this was a trend of 
the series of curves towards the light, or 
an incipient positive heliotropic move- 
ment. I shall now proceed to show how 
these constituent multiple movements 
may often, if not always, merge into one 
continuous movement. 

For- this experiment I used the same 
leaflet as in the last, the only difference 
being that I now applied the strong 
stimulus of sunlight from above. The 
response induced is seen in fig. 238. 
During the first impact of the stimulus a 
pulsatory movement may sometimes be 
observed. But the response soon becomes 
a continuous movement upwards. Generally speaking, the 
constituent multiple marks are to be seen under feeble 
or moderate stimulation, and a continuous movement when 
the stimulus is strong. In the present case, the average 
rate of movement of the tip 
1*5 mm. per minute (fig. 238). 
there was persistence for some 




FIG. 238. Response 
of Terminal Leaflet 
of Desmodium to 
Strong Light from 
Above 

Abscissa gives time in 
minutes, and or- 
dinate movement in 
millimetres. 



of the leaf was almost 
On the stoppage of light 
time of the after-effect of 



light. This was succeeded by recovery. The persistence 



POSITIVE HELIOTROPISM 



5*7 



of the after-effect varies widely, depending on the condition 
of the tissue as well as the intensity of stimulus. 

Orientation induced by light. When the leaflet, with 
its sensitive motile organ, is exposed to strong sunlight, the 
heliotropic movement continues till the organ becomes par- 
allel to the direction of light. The question now arises, 
in this as in other cases of heliotropic movement, why should 
the movement come to a stop when the organ reaches this 
parallel position ? 

A partial answer to this question may be found in the 
fact that such movements depend upon the effective intensity 
of light which is absorbed, and this 
effective intensity is greatest at perpen- 
dicular incidence, and becomes reduced 
to nearly zero as the rays of light are 
rendered more and more oblique by the 
responsive movement of the organ. This 
consideration alone, however, would not 
wholly explain the orientation of the 
organ, parallel to the direction of light, 
for we know that the directive impulse 
caused by light persists for some time, 
and this would cause the organ to over- 
shoot the parallel position. In order, 
therefore, to obtain a satisfactory explana- 
tion, I undertook the following experi- 
ment, which, as will be seen, completely 
meets the difficulties of the case. 

I now took the same leaflet of DCS- 
medium as was used in the previous ex- 
periments, and caused sunlight to strike 
the pulvinus vertically, from below upwards, by means of a 
suitably inclined mirror. I obtained, as will be seen (fig. 239), a 
continuous responsive movement downwards i.e. towards the 
direction of the light. The average rate of movement of the 
tip of the leaf was in this case about 2-5 mm. per minute. 
This is somewhat greater than the upward rate of movement, 




FIG. 239. Response 
of Terminal Leaflet 
of Desmodimn to 
Sunlight acting from 
Below 

The dotted portion of 
the curve represents 
the after-effect and 
recovery on the 
cessation of light. 



$88 PLANT RESPONSE 

and is probably due to the fact that the excitability of the 
lower half of the pulvinus is slightly greater than that of the 
upper. 

We are now in a position to understand the reason of 
orientation ; for if we suppose light to be incident from a 
position slightly above the pulvinus, it will curve upwards, 
owing to positive heliotropism, till it has become parallel 
with the rays of light. Should there be any over-shooting of 
this position owing to after-effect, the pulvinus will then 
begin to curve downwards, because the light will now be 
acting from below. Permanent equilibrium can thus only be 
attained when the plant-organ has become parallel to the 
direction of light 

The perceptive region in the terminal leaflet of Des- 
modium. In connection with the response of the Desmodium 
leaflet to light, it is interesting to note that the pulvinus is 
not only the responding, but also the perceptive region ; 
for, throwing the light on the leaflet alone, and protecting 
the pulvinus with an opaque shield of black paper, we 
find that no responsive movement takes place ; conversely, 
if the pulvinus alone be exposed, and the rest of the leaflet 
shaded, we observe the normal action. 

Heliotropic response in radial organs. Having observed 
the peculiarities of heliotropic movement in a pulvinated 
organ, I shall now describe the experimental arrangements 
for studying the same problem in non-pulvinated growing 
organs. It is understood that there is no essential difference 
between the two movements. They are both caused by the 
same contractile effect, due to the stimulus of light, the only 
difference being that, whereas in the pulvinated organ the 
recovery is complete, in the growing organ it remains more 
or less incomplete, the curvature being fixed by growth. 

Magnetically controlled recorder. The great difficulty 
which stands in the way of accurate investigation is the 
question of how to take a continuous time-record of the 
heliotropic curvature of the growing organ without being 
under the necessity of using light for purposes of observation, 



POSITIVE HELIOTROPISM 589 

i procedure which, as we have seen, causes disturbance. 
The free end of a normally growing organ, when acted on, 
;ay by horizontal light, bends towards it. Thus the problem 
s to obtain a continuous time-record of this movement, from 
vhich the latent period, the actual rate of movement, the 
ifter-effect, and other related effects might be ascertained. 
[ have been able to solve this difficulty by devising a mag- 




FIG. 240. Diagrammatic Representation of the Magnetically Controlled 

Recorder 

I.L', the Optic Lever, to one arm of which the plant is attached by a 
thread ; M, the mirror, with small magnet, NS, attached behind. The 
lever is rotated to dotted position by heliotropic curvature of the plant, 
diagrammatically represented disproportionately magnified. 

netically controlled recorder, the principle of which will be 
understood from the accompanying diagram (fig. 240). 

The principal part of this recording instrument consists 
of a magneto-metric arrangement. Attached to a long 
aluminium lever, LL/, is a vertical T-piece, V. This T-piece, 
V, carries a reflecting mirror, behind which is a short magnet, 
NS. The whole arrangement is freely suspended by a silk 
thread. By means of a controlling magnet not shown in the 



590 PLANT RESPONSE 

figure, the suspended lever may be adjusted in any con- 
venient azimuth. 

The free end of the growing plant is attached to one end 
of the lever, L, being on its own level, at a distance of, say, 
20 cm. from the line of suspension, the attached thread being 
at right angles to the lever. There is a slight tension of the 
thread, due to the magnetic force of the controlling magnet, 
which tends to draw the lever away from the plant. This 
tension, however, is very slight When the growing organ is 
acted on by light in a horizontal direction, parallel to the 
attached thread, and therefore at right angles to the lever, 
then if the action of light be to induce a positive heliotropic 
effect, the rotation of the suspended magnetic system, owing 
to the pull exerted by the curving organ, will be, when seen 
from the top, in the direction opposite to that of the hands of 
a watch. 

If, however, the heliotropic effect be negative, the top of 
the growing organ will move in the opposite direction. But 
it has been said that there was a slight tension of the thread, 
owing to the action of the controlling magnet. This being 
now released, the lever will move to a proportionate extent 
in the same direction as the hands of a watch. A spot of 
light reflected from the mirror magnifies these movements, 
and a record of this moving spot of light on a revolving 
drum gives the response-curve. Instead of this magnetic 
control, it would also be possible to use, as the controlling 
force, the torsion of a fine metallic wire. 

By using a shorter lever arm and increasing the distance 
of the revolving recording drum from the mirror, a wide 
range of magnification up to 500 times may easily be 
obtained. For ordinary purposes a magnification of ten 
times is all that is necessary. 

The movement of the spot of light is proportionate to 
the heliotropic movement of the plant, at least when the 
amount of that movement is not excessive. Thus, know- 
ing the magnification produced by the system, and the 
rate of the revolving drum, we can determine from the 



POSITIVE HELIOTROPISM 591 

response-curves the absolute movement, and the rate of such 
movement 

I now give a description of the complete apparatus 
actually used (fig. 241), consisting of the Recorder and the 
Heliotropic Chamber. In a dark chamber is placed the 
plant, attached to the Lever as explained before, A portion 
of the vertical piece with attached mirror, M, projects outside 
the chamber. It will be seen that by this arrangement the 




FIG. 241. Heliotropic Chamber and Magnetically Controlled Recorder 

Heliotropic chamber seen in the middle of the apparatus. Guide-bars to 
right and left carry sliding lamps, L and L'. Exposure is given by 
pressing key, K, which raises shutter, s. The corresponding shutter 
to the right is not shown in the figure. M, mirror of the Optic Lever ; 
c, controlling magnet. 

plant can be completely protected from light, while its move- 
ments are at the same time recorded by the spot ot light 
thrown from the mirror, M, upon the recording drum, without 
the possibility of its reaching the plant within the chamber. 

The chamber carries two projecting graduated arms or 
guide-bars, one to the right and the other to the left, over 
which slides the holder for a candle, or incandescent, or 
Nernst's electric lamp, LI/ ; but if a still stronger light be 



592 PLANT RESPONSE 

desired, sunlight may be reflected in the required direction 
by a mirror. From whatever source, the light can be made 
to strike the plant horizontally through the slit, which is 
usually covered with a sliding shutter, s. By manipulating 
a key, K, the shutter is raised. Thus exposure may be 
made at any moment, and continued for the length of time 
desired. 

That part of the record which is made on the revolving 
drum before exposure begins, gives an indication of the 
quiescent condition of the plant. The moment and duration 
of exposure are found from corresponding marks made on 
the recording surface, at the instants of opening and closing 
the shutter. 

It will be seen that we have means of controlling the 
intensity of illumination within wide limits by (i) the use of 
different sources of light as enumerated above, and (2) varia- 
tion of the distance of the source of light, at least when this 
is artificial. 

Thus we are able to illuminate the plant from the right 
or left flanks, or from both simultaneously, and by lights of 
equal or of different intensities, at will. Again, by covering 
the slit with a second plate, provided with suitable apertures, 
we are enabled to subject any part of the plant, whether tip 
or growing region, or both, as desired, to the stimulus of 
light, and thus to determine the characteristic response of 
each. All these considerations will show the facilities 
afforded by this apparatus for carrying out a great number 
of diverse experiments. I shall, however, content myself 
here with the description of a few necessary examples. 

Heliotropic response of hypocotyl of Sinapis. As 
examples of radial organs, exhibiting the positive helio- 
tropic effect, I took seedlings of Sinapis nigra. They were 
attached to the lever, as indicated above, and at least half an 
hour was permitted to elapse, in order to remove the lasl 
possible trace of excitation due to contact. The attainment 
of the quiescent condition was ascertained from the stationary 
position of the spot of light. 



POSITIVE HELIOTROPISM 



593 



In this experiment, on a seedling of Sinapis, light was 
allowed to strike the growing organ horizontally. The 
specimen was very sensitive, and the source of light employed 
at first was a candle placed at a distance of 20 cm. acting on 
the plant for three minutes. The responsive movement began 
within five seconds, and though the light was cut off, there 
were produced three multiple responses, after which the plant 
underwent a complete recovery and resumed its former 
position. Sunlight was next applied, and a continuous move- 
ment towards the light was induced (fig. 242). 




FIG. 242. Heliotropic Response of 
Sinapis 

Application of candle-light for three 
minutes gave three multiple re- 
sponses ; f application of sunlight 
gave rise to continuous response. 




FIG. 243. Ileliotropic Response of 
Sinapis to Sunlight 

Dotted line shows after-effect and 
recovery on cessation of light. 



Ill another case sunlight was applied for twelve minutes. 
The response commenced almost immediately on application, 
and the average rate of movement was I mm. per minute. 
In this particular case, the positive after-effect persisted for 
five minutes, even on the stoppage of light, after which 
there was a gradual recovery (fig. 243). 

Recovery and theory of recti-petality. On the cessation 
of stimulus, there is a more or less perfect recovery of the 
radial organ from its induced curvature. I have already 
demonstrated the fact that the growing organ acts as a 
diffused pulvinoid. We have just seen that the primary 
action of light is to induce similar motile effects in both 

QQ 



594 PLANT RESPONSE 

pulvinated and growing organs. We have also seen that in 
both, on the cessation of stimulus, there is a tendency towards 
recovery. In the case of the growing organ, when stimulus 
is moderate, recovery is fairly complete ; but when the 
stimulus is very strong and long-continued, some part of the 
induced curvature is rendered permanent through fixation by 
growth. 

An attempt has been made in the case of Vochting's 
Theory of Recti-petality to account for this recovery, by 
assuming the action of an unknown regulating power which 
would tend always to bring the organ back to a straight line ; 
but, beyond the assumption of an unknown specific power, 
this theory affords no explanation of the, mechanism by 
which recovery is brought about, and I am able to adduce 
considerations which obviate the necessity for thus assuming 
the existence of any such specific agency as that of recti- 
petality. 

In a growing organ which is radial, the tip of the grov/ing 
region being free, the vertical direction is that in which there 
is least obstruction to growth, and as long as all the lateral 
tensions are the same in all directions, there is no reason why 
the organ in the course of its upward growth should bend 
permanently on any one side more than on another. We 
have therefore the normal growth of radial organs in a 
straight line ; but when stimulus acts unilaterally on the 
growing region, a sequence of events ensues, which has already 
been fully explained : 

(1) Active contraction is induced during the continuance 
of stimulus, on the proximal or excited side, with concomitant 
diminished turgidity, and retardation of growth. 

(2) The water thus expelled is forced, against tension, 
into the growing cells of the distal side, raising their 
turgescence and consequent rate of growth above par. 

(3) The curvature thus induced is maintained as long as 
the difference of hydrostatic pressure on the two sides is 
continued, by the persistent contraction of the proximal, 
under the action of stimulus. 



POSITIVE HELIOTROHSM 595 

(4) When the stimulus ceases to act, the active contraction 
which forced the water against tension to the distal side comes 
to an end, and there is a rebound of the expelled water to 
the proximal side. Thus the increased growth of the distal 
falls, and the decreased growth of the proximal rises, to the 
normal rate of growth. The unequal tensions on the two 
sides, which previously maintained the curvature, being now 
equalised, the organ shows a tendency to straighten itself. 

(5) I have also shown that a tissue which has been 
subjected to stimulus, having absorbed energy and held it 
latent, exhibits it on the cessation* of external stimulus, in 
the form of a temporary negative after-effect, that is to say 
an acceleration of growth above the normal. As a result o 
this fact, the stimulated side of the organ will show an active 
tendency to neutralise the previous curvature, and return to 
the straight line. 

It is thus seen, from facts which I had already established 
regarding the nature of the after-effect of stimulus, that the 
recovery of the organ is fully explained, without postulating 
the existence of any specific power, such as that of recti- 
petality. 

SUMMARY 

The responsive movement of the plant-organ towards 
light is due to the excitatory contraction of the side acted 
upon. The curvature of a growing organ towards light is 
brought about by the joint action of the induced concavity 
of the proximal and the convexity of the distal sides. The 
former is the result of the contraction, negative turgidity- 
variation, and retardation of growth caused by the stimulus. 
The latter comes about by the positive turgidity-variation 
due to forcing-in of expelled water, expansion and accelera- 
tion of growth of the distal side. 

Such responsive movements take place in organs previously 
devoid of circumnutation. 

The sensitiveness of certain plant-organs to heliotropic 
stimulus is very great. The terminal leaflet of Desmodium 

QQ2 



596 PLANT RESPONSE 



responds under the briefest exposure to feeble candle- 
light. This necessitates the making of observations on 
heliotropic effects without the aid of light. 

The perceptive region for the stimulus of light in the 
case of the terminal leaflet of Desmodium is the pulvinus. 

There is no essential difference between the heliotropic 
response of a growing and a pulvinated organ. 

On the cessation of stimulus the recovery of a pulvinated 
organ is complete ; and this is more or less true also of the 
recovery of a growing organ from response, if the stimulus 
have not been excessive. 

For the explanation of this recovery in a growing organ, 
it is not necessary to assume the existence of any specific 
power such as recti-petality. The cessation of the difference 
of hydrostatic pressure on the two sides such difference 
being only maintained during the action of stimulus 
together with the accelerated rate of growth on the proximal 
side, which constitutes the negative after-effect, are quite 
sufficient to explain the recovery from induced curvature. 



CHAPTER XLIII 

NEGATIVE HELIOTROPISM 

Incomplete parallelism between actions of light and of gravitation Theoretical 
considerations Recording microscope T Negative heliotropic curvature 
induced by stimulation of the tips of root and shoot Intermediate phases 
between positive and negative heliotropic response : (a) neutralisation by 
transverse transmission ; (A) neutralisation by transverse transmission, with 
multiple response Localised sensitiveness to light and transmission of 
excitatory effect Negative heliotropism of a radial organ Gradual transition 
from positive to negative, through intermediate phase of neutrality Apparent 
heliotropic insensitiveness of certain tendrils Negative heliotropism of tendril 
of Vitis. 

WE have seen in the chapter on the response due to gravita- 
tion, that the responsive curvature of the root is opposite in 
character to that of the stem, this fact having led to the 
assumption of specific sensibilities as characteristic of the 
root-tip. It was there shown, however, that the responsive 
characteristics of the root were not actually different from 
those of the shoot, and that the differences in their observed 
responses were simply a consequence of the fact that in the 
one case the stimulus of gravity acted indirectly, and in the 
other directly, upon the responding growing organ. This 
assumption that the root possessed a definite sensitiveness 
characteristically different from that of the stem, was ap- 
parently supported by certain differences in heliotropic action 
also, as between shoot and root ; for example, while the 
hypocotyl of Sinapis bends towards the light, the root is 
found to bend away from it. 

Incomplete analogy between action of light and 
gravitation. But I have already explained the fact that the 
supposed analogy is false ; for while the stimulus of gravity 
acts, in the case of the root, only on a restricted area of 



PLANT RESPONSE 

the tip, the stimulus of light is not necessarily restricted 
in the area of its action. Again, whereas the stimulation 
caused by statolithic particles is moderate, that caused by 
light may be of any degree of intensity. The fact that 
there is no true extended analogy between the action of 
light and that of gravitation, is seen from the fact that, while 
gravitation in the case of the root induces a movement 
opposite to that induced in the stem, in the case of light this 
is not always so ; for though a few roots turn away from 
light, in others there is either no resultant movement, or 
movement towards the light. Again, while the shoot makes 
a definite curvature with reference to the direction of gravity, 
in the case of light we shall observe that though under 
moderate stimulation it turns towards it, yet it will sometimes 
under stronger stimulation be found to move away. The idea 
that positive and negative heliotropic curvatures are due to 
two distinct sensibilities could not be better disproved than 
by the fact, which will be demonstrated shortly, that the same 
organ can be made under different conditions of illumination 
to exhibit the two opposite effects. 

Discarding, then, the theory of any specific sensibility, we 
shall now proceed to show how the movement away from the 
stimulating light, the so-called negative heliotropic curvature, 
is brought about. 

Theoretical considerations. From the movements al- 
ready demonstrated (p. 535) as taking place in plant-organs 
in response to stimulus unilaterally applied, we can see the 
possibility of such movement becoming negative, or away 
from stimulus, under three different conditions : 

(1) Under longitudinal transmission of the indirect effect 
of stimulus, when, for example, moderate stimulus is applied 
to the tip of either shoot or root. 

(2) Under transverse conduction of the direct excitatory 
effect of stimulus to the distal side of a radial organ, the 
proximal side being fatigued by excessive stimulation ; and 

(3) Under the transverse transmission of excitation to the 
distal side of an anisotropic organ, the distal side being the 



NEGATIVE HEUOTKOPISM 599 

more excitable. In this last case, we may obtain a very 
pronounced negative response in consequence of the relatively 
greater natural excitability of the distal side. 

We shall see in the course of this and the following 
chapters how heliotropic movements other than positive are 
actually brought about under these different conditions, and 
in the present chapter we shall study cases which are illustrative 
of the first two. 




FIG. 244. Microscope Recorder 

Plant mounted in cubical glass trough with root in water. Light strikes 
root, R, unilaterally from the right side. Movement of root observed 
by microscope, M, the inclined transparent disc of glass, G, giving at 
the same time the reflected image of the recording pen, r. 

Recording microscope. Since the growing root has to 
be kept in water, for the purpose of studying the phases of its 
responsive curvature, the method hitherto employed of obtain- 
ing records by the Optic Lever is inapplicable. I therefore 
devised a different method of observation-- that of the Record- 
ing Microscope (fig. 244). The method of record will be 
understood from the figure, where in a cubical glass trough 
a piece of the stem of Bindweed, with its water root, K, is 
securely fixed on the surface of the water. Light is made to 



600 PLANT RESPONSE 

strike the root unilaterally, say from the right side. This 
pencil of light may be so thrown as to act locally on the root- 
tip, or on the growing region, or on both at the same time. 
The movement of the root towards or away from light is 
observed through the microscope focussed on the tip. The 
eye-piece end of the microscope has a disc of glass adjusted 
at an angle of 45 to the vertical. The observer sees the tip 
of the root directly through the transparent disc, and at the 
same time the reflected image of the recording point of the 
pen, lying against the revolving drum below. The two 
images are at the beginning of the experiment coincident, 
and the responsive movement of the tip of the root, which 
takes place afterwards, is easily followed by the observer with 
the recording pen. Thus we obtain the response-record on 
the moving surface. This method of the recording microscope 
can always be used when attachment to the Optic Lever is not 
possible or not desired. 

Negative heliotropic curvature induced by stimulation 
of the tips of root and shoot. I have by this method ob- 
tained various records of the responses of the root and shoot 
to the unilateral stimulus of light applied at the tip. Of these 
I shall give, as a typical example, the record of the root of a 
seedling of Sinapis nigra y suitably mounted in the cubical 
trough by means of a cork. The curve seen to the left of 
fig. 245 represents the negative movement, or movement 
away from light, of this root, when the tip alone was 
unilaterally stimulated. This movement was due therefore 
to the indirect action of the stimulus on the growing 
responding region. After a period of rest in darkness I next 
took a record of its movement resulting from the direct 
unilateral illumination of the growing region. I now ob- 
tained a positive responsive curvature, as seen to the right of 
fig. 245. It will be noticed that this particular movement 
was relatively smaller than the preceding. We must here 
remember that the receptivity of an organ is not the same in 
all its different parts, and the greater negative response 
induced in this case by the indirect action of stimulus on the 



NEGATIVE HELIOTROPISM 



601 



0' 10' 20' 30' 



0' 10' 20' 30'40' 



tip is probably due to the higher degree of receptivity 
possessed by that part of the organ. In taking a third record 
in a case in which both tip and growing region were simul- 
taneously subjected to unilateral stimulation of light, I found 
that a resultant responsive movement was induced, which was 
away from light. 

That this negative movement, induced by stimulation of 
the root-tip, is not due to any specific sensitiveness of the 
root as such, is seen from the fact that on local stimulation of 
the tip of the shoot, e.g. the flower-bud of Crocus, I obtained 
a responsive movement 
away from, whereas uni- 
lateral stimulation of the 
growing region of the pe- 
duncle induced a move- 
ment towards, light. 

Apart from this pos- 
sible factor, however, of 
the greater receptivity of 
the tip, there is another, 
which tends to make the 
positive curvature of the 
growing region of the 
root relatively ineffec- 
tive. This region, being 
acted on unilaterally by 

light, the proximal excitation often passes by conduction 
to the distal side, thus neutralising the first positive action. 
Instances of this will be given in greater detail presently. 

The negative curvature induced by the action of the tip, 
depending as it does on the indirect transmission of stimula- 
tion, requires as a condition the relative non-conductivity of 
the intervening tissue to the passage of true excitation. 
Hence, if the conductivity of such a tissue be not sufficiently 
feeble, or if the intensity of stimulus be too great, we shall 
find that the direct effect of stimulus is transmitted to the 
growing organ, and a positive curvature is induced. This 



FIG. 245. Record of Response of Root of 
Sinapis nigra 

The curve seen to the left shows the negative 
response due to stimulation of root-tip. 
The curve to the right exhibits positive 
response on stimulation of growing region. 



6O2 PLANT RESPONSE 

explains the positive heliotropic curvature exhibited by many 
roots. 

Intermediate phases between positive and negative 
heliotropic response. I shall next proceed to demonstrate 
the induction of negative heliotropic movements in radial 
shoots, a phenomenon which, for reasons already explained, 
has no parallel in the case of geotropic action (p. 544). As it 
has already been said that there is no specific sensibility which 
determines the positive or negative character of the heliotropic 
response, it would be interesting to trace out the transitions 
by which the normal positive is gradually transformed into 
the negative movement. We have seen that when stimulus 
is applied unilaterally to a growing region, the positive 
curvature at first induced is jointly due to the contraction 
caused by direct stimulation of the proximal and the expan- 
sion caused by the indirect stimulation of the distal ; but 
when the stimulus is strong or long-continued, excitation is 
transmitted from the proximal to the distal, the contraction 
of which latter now neutralises the first effect Hence the 
normal positive curvature disappears. 

(a) Neutralisation by transverse transmission. The con- 
siderations just related explain the curious anomaly that has 
been observed, by which, while feeble or moderate stimulus of 
light, or interrupted light, gives rise to well-marked positive 
heliotropic curvature, the continuous application of stronger 
light induces a relatively feeble effect. Thus under moderate 
lighting we often observe strong heliotropic curvature, which 
disappears under strong sunlight The curvature induced 
is, as we have seen, due to the differential action of unilateral 
stimulus, on the proximal and distal sides ; but when a strong 
light is used the stimulus becomes internally diffused, and the 
differential effect on the two sides is reduced in amount or 
vanishes altogether. Such internal diffusion is due to the fact 
that, owing to the weak transverse conductivity of the tissue, 
while a feeble stimulus is not conducted across it, a stronger 
stimulus is. This consideration, together with the fact that 
the conductivity of a tissue undergoes seasonal variation, 



NEGATIVE HELIOTROPISM 603 

will be found to offer a satisfactory explanation of various 
anomalies in heliotropic response. Sinapis y for example, 
exhibits a strong positive , effect in winter, while in hot 
weather its action is very feeble. It may be supposed that 
this is due, in some unknown way, to a greater rapidity of 
growth in warm than in cold seasons. That this, however, 
cannot be the reason, will be seen from the fact which I have 
demonstrated, that that contractile response of the plant to 
external stimulus on which curvature depends is greatest 
when the rate of growth is at its optimum. The real 
explanation lies in the fact that the neutralisation, or reversal 
of normal positive response, caused by transverse conduction, 
takes place more easily in warmer seasons, the general con- 
ducting power being then great. This accounts for the 
feebler positive response in summer, which culminates in 
certain instances in an actual reversal into negative (p. 623). 

(b} Neutralisation by transverse transmission, with multiple 
response. Thus if stimulus be sufficiently strong or long- 
continued, the positive curvature will become neutralised, 
and the organ will return to its original position. I have, 
however, observed an interesting modification of this neu- 
tralisation, in which it is attended by oscillatory move- 
ments to and fro about the mean position. We have seen 
that unilateral stimulus, when its action is long continued, 
becomes diffused, and thus both sides of the organ become 
excited. The tissue, moreover, is now possessed of an excess 
of energy a condition conducive to the production of 
multiple response. This fact, together with the periodic 
and alternate variation of excitability on the two sides, is 
then found to give rise to oscillatory movements of the kind 
described. I give below a record which shows the initiation 
of these oscillatory movements when the organ had been too 
long subjected to unilateral stimulus. It will be remembered 
that the pulvinus of the terminal leaflet of Desmodium 
executes a positive heliotropic movement, the record of 
which has been given in fig. 238. In winter, when the 
conductivity of the tissue is feeble, the leaflet curves towards 



604 PLANT RESPONSE 

the light to the maximum extent possible, and remains in 
that position as long as the light acts. But we have seen 
that in summer the stimulus is more likely to be internally 
transmitted to the distal side, the positive effect being thus 
gradually neutralised. Thus, in the course of an experiment 
during the summer on the pulvinus of the terminal leaflet of 
Desmodium, I found, on subjecting it to sunlight from above, 
that for the first forty minutes the leaflet rose continuously, 
its tip having moved during that time through a little more 
than 4 cm. After this there was induced, instead of the 
continuous movement upwards, a pulsatory movement up 

and down (fig. 246). After a series 
of such movements the leaflet was 
gradually depressed, the former 
positive curvature being thus 
neutralised. 

The supposed localisation of 
sensitiveness to light, and the 
transmission of excitatory effect. 
I have fully explained the mari- 
ner in which the effect of stimulus 

FIG. 246. Positive Heliotropic of H S ht a PP lied at a S iven P oint is 

Movement of Terminal Leaf- transmitted to the distant growing 

let of Desmodium Converted , . , . . . 

by Strong and too Long- organ, and the mechanics by which 
continued Stimulus of Light t h e curvature is induced. In con- 

into Oscillatory Movement . .... 

nection with this, a peculiar phe- 
nomenon has been observed, which has led to the belief 
that in seedlings, like that of Avena saliva, the zone for 
the perception of heliotropic stimulus is confined to the 
upper region, or tip of the shoot. This conclusion is based 
on Darwin's observations on the unilateral effect of light 
on these seedlings. It was found that, generally speaking, 
when the lower part of the cotyledon was alone exposed to 
the unilateral light the upper part being covered with a cap 
of tinfoil or with an opaque glass tube there was little 
curvature induced ; but when such light was allowed to act 
on the upper part of the seedlings the curvature was con- 




NEGATIVE HCLIOTROPISM 605 

siderable. From this it was concluded that sensitiveness to 
light was mainly confined to the upper part of the plant, and 
that this determined the curvature ; but to this conclusion, 
that it was the upper rather than the lower part that was 
sensitive to light, Darwin found and recorded several excep- 
tions, which he regarded as inexplicable. In the case of six 
seedlings, for instance, of which the upper parts were covered 
with opaque shields, there was as much curvature induced as 
in seedlings which were unshielded. In these, therefore, there 
must have been sensitiveness in the lower parts also, thus 
negativing the conclusion, drawn from other and more 
numerous experiments, that it was characteristic of the upper 
alone. 

In order to see if these discrepancies were not capable of 
explanation, I undertook an investigation into the heliotropic 
action of light on the seedlings of A vena sativa. The method 
of screening the upper part of the seedlings from light, which 
has usually been employed by Darwin and others, labours 
under the disadvantage that the weight and contact of the 
tinfoil or the blackened glass tube are not unlikely themselves 
to set up a certain mechanical irritation, which may have the 
effect of causing an unknown disturbance in the result. I 
was therefore desirous of keeping the delicate seedling free 
from the irritating contact of caps in the course of my own 
experiments. For this reason I arranged for the localised 
application of light on upper or lower or both parts of the 
organ at will, by the method which has already been described 
of throwing a pencil of light on the required spot in the plant 
placed in the heliotropic chamber (p. 592). The resultant 
movement of the organ was now continuously observed and 
recorded, by means of the Recording Microscope which has 
been described. 

For the sake of clearness I may here forestall matters, 
by saying that the observed results fall under two types, 
according to whether the given specimen possesses feeble or 
high conductivity that is to say, power of transmitting 
stimulus to a distance. Taking first the case in which the 



606 PLANT RESPONSE 

organ possesses feeble conductivity, I have found that when 
the extreme tip was stimulated unilaterally, by using a pencil 
of light from a sixteen candle-power incandescent electric lamp, 
the result was a negative movement that is to say, a move- 
ment away from light. This is exactly parallel to the 
response to stimulus of the unopened flower-bud of Crocus 
and the root-tip of Sinapis, in both of which the indirect 
effect at the growing region, of stimulus applied on the tip, 
was seen to be a convexity of the side acted upon, with 
consequent negative movement of the tip. 

I next applied unilateral stimulus to the same specimen 
a little lower down, and now, owing to the better conductivity 
of this part of the tissue, the excitation itself was transmitted 
to the growing region, inducing concavity and positive 
heliotropic movement. The same effect was found to be 
produced when stimulus was applied on the growing region 
itself. It must be remembered that in this case the con- 
ducting power of the tissue is not high, hence there is no 
transmission of stimulus to the distal side, by which, as 
we have seen, the positive curvature would be neutralised. 
The long-continued action of light on one side here tends 
only to increase the positive curvature to a maximum. Thus 
when one side of the entire seedling is acted upon by light, 
while the response of the extreme tip tends to induce a slight 
negative, all the other parts, from immediately below it to 
the growing region, conspire together to exhibit a much 
stronger positive heliotropic action. The result is therefore a 
movement towards the light. Thus we see that in the case 
of seedlings having feeble conductivity the curvature will 
be positive, whether it is the lower part only or the entire 
plant which is exposed to the one-sided action of light. In 
this fact we find the explanation of those exceptional cases 
observed by Darwin, in which the seedling was found to bend 
towards the light, in spite of the upper part being covered. 

We shall next take up that type of response in which the 
tissue of the specimen is rather better conducting. In this 
case, when the upper part of the organ is locally stimulated, 



NEGATIVE HELIOTROPISM 607 

the excitation is longitudinally transmitted to the growing 
region lower down, and induces a concavity there which 
increases with the duration of the stimulus ; but if the 
stimulus of light be applied directly on the growing region 
itself, instead of on the upper part of the specimen, then, by 
reason of the transverse transmission of excitation to the 
distal side, we obtain a state of things in which there is no 
resultant curvature at all. In this case, then, the direct effect 
of stimulus on the proximal side of the growing region is 
balanced or neutralised by the transmitted effect on the distal 
side ; but this condition of balance will be upset if the uni- 
lateral stimulus of light, hitherto acting on the growing 
region alone, be allowed to act simultaneously on the upper 
part of the specimen also. The longitudinally transmitted 
stimulus from the upper part being now added to the direct 
excitation of the proximal side of the growing region, causes 
an over-balance of responsive effect on that side, resulting in 
a positive heliotropic curvature. The fact that there is no 
heliotropic movement, when only the lower part of the 
seedling is unilaterally acted on by stimulus, is thus not due 
to any absence in that region of heliotropic sensibility, but to 
the neutralisation of the proximal effect by the equal excita- 
tion of the distal. Such transmission of stimulation along 
the length of the organ is observed to take place in a specially 
marked manner in the cotyledon of graminaceous plants. 
We may account for this by the fact that such organs are 
parallel-veined that is to say, the fibre-vascular elements, 
which we already know as good conductors of excitation, run 
along their length. This is no doubt the reason of their 
ready transmission of stimulus to a distance. 

Negative heliotropism of a radial organ. We have 
seen that when moderate stimulus acts unilaterally on a 
growing organ a positive curvature is induced, and that, with 
stronger or long-continued stimulation, this reaches the distal 
side, producing neutralisation. We shall now proceed tc 
trace out the continuity of responsive heliotropic effects, from 
the positive curvature to the negative, through the inter- 



608 PLANT RESPONSE 

mediate phase of neutralisation. We have seen that at a 
certain definite intensity of illumination the excitations 
of proximal and distal sides, balancing each other, cause 
neutralisation. If now the intensity of stimulating light be 
further increased, it is easy to see that while the stimulation 
transmitted to the distal side, with the concomitant contrac- 
tion, is being increased, the excitatory contraction of the 
proximal will be at the same time decreased, owing to the 
fatigue brought on by over-stimulation. The result then 
will be the greater contraction of the distal side, with a 
consequent negative heliotropic curvature of the organ. 

These considerations show the mechanical action of 
heliotropic stimulus in causing : (i) positive heliotropic curva- 
ture under moderate illumination ; (2) the neutralisation of this 
action under stronger illumination ; and (3) the conversion 
of the normal positive into negative when illumination is 
excessive. Thus the observation made by Oltmanns, on 
young seedlings of Lepidium^ subjected to varying intensities 
of light, an observation of which there has been hitherto no 
satisfactory explanation, is fully accounted for. Oltmanns 
subjected a row of seedlings of Lepidium to the action of 
sunlight, diverging from the focus of a lens. The seedlings 
nearest the focus were thus subjected to the strongest stimu- 
lation, those further from this point being under gradually 
decreasing infensities of light. It was then found that 
the seedlings nearest the focus, which were subjected to the 
strongest degree of light, exhibited negative heliotropic 
curvature, while others, further away, and therefore subjected 
to less intense illumination, did not show any effect at all 
(neutralisation), and others again, which were still further 
away, and therefore under only moderate intensity of 
illumination, exhibited positive curvature. 

Gradual transition from positive to negative, through 
intermediate phase of neutrality. I shall, however, give a 
still more conclusive verification of the theoretical inferences 
which I have just set forth regarding the gradual transition 
of positive heliotropic response into negative, through the 



NEGATIVE HELIOTROPISM 



609 



intermediate neutral, in consequence of the increasing internal 
diffusion of stimulus with increasing intensity of stimulating 
light. This I shall do by a continuous record taken from a 
single plant under changing conditions of increasing illumina- 
tion, in which record, further, we shall be able to follow all 
the phases of responsive change, from positive to negative. 
Taking a hypocotyl of Sinapis nigra, I subjected it to the 
unilateral action of light from a sixteen-candle-power incan- 
descent electric lamp, placed at a distance of 10 cm. from 
the specimen. The plant, hitherto quiescent, began to move 
towards the light, as shown 
by the up curve in the 
record (fig. 247), the maxi- 
mum being attained in 
the course of fifty minutes. 
The intensity of incident 
stimulus was now in- 
creased, by bringing the 
lamp to a distance of 6 cm. 
from the specimen, at the 
moment marked with the 
downward arrow. It will 
be seen that this resulted 
in a process of neutralisa- 
tion of the preceding re- 
sponse, and that this 
became complete in the 
course of a further exposure of seventy minutes, the hypo- 
cotyl being then erect and free from curvature, having thus 
placed itself at right angles to the incident light. Still 
stronger illumination of sunlight was now applied, at the 
point marked x . This induced, as is seen in the down curve, 
a very marked reversed or negative heliotropic response. 

Thus, in other words, in an identical organ, under different 
conditions of illumination, the plant turning towards the 
light exhibits the positive heliotropic, at right angles to the 
light the dia-heliotropiC) and away from the light the negative 

R R 




FIG. 247. 



Response of Hypocotyl of 
Sinapis nigra 



The first curve shows positive response 
induced by incandescent electric lamp 
at distance of 10 cm. Increased in- 
tensity of light applied at arrow (^) 
by bringing lamp to distance of 6 cm. 
causes neutralisation. Reversal, or nega- 
tive response, when sunlight applied 
at x . 



6iO PLANT RESPONSE 

heliotropic effect, thus conclusively proving that the induction 
of these three effects is not due to the presence of three 
distinct and characteristic sensibilities. These different 
responsive movements are thus traced to a single pheno- 
menon of contractile response. 

This may be made equally clear from another point of 
view, as illustrations namely of the general law that response 
takes place by the contraction of the more excited, or the 
relatively more excited. With moderate stimulation it is the 
proximal side of the organ that is .excited, and, becoming 
concave, gives rise to a positive heliotropic curvature. On 
the application of somewhat stronger stimulation, however, 
when excitation is transmitted to the distal side, there is a 
case in which the excitation of the proximal and distal are 
equal, and the differential excitation being thus zero, there is 
no resultant response. The organ, standing thus at right 
angles to the light, and moving neither towards nor away 
from it, will now appear to be either dia-heliotropic or else 
irresponsive to heliotropic stimulus. 

But under still stronger stimulation two effects are in- 
duced simultaneously: (i) a physiological anisotropy, by the 
fatigue and loss of excitability of the proximal, in conse- 
quence of which the distal becomes the more excitable ; and 
(2) the internal diffusion of the stimulus, which, now acting 
on the physiologically anisotropic organ, induces concavity of 
the distal, that is to say a negative heliotropic curvature. 

1 shall now proceed to give further examples of that 
transverse transmission of excitation, in consequence of which 
an organ appears either to be irresponsive to heliotropic 
stimulus or to give negative response. 

Organs apparently insensitive to light. Many vegetable 
organs exhibit no resultant movement under the action of 
light, and are therefore supposed to be insensitive to it ; but 
this inference could be accepted only on the theory of a 
specific sensibility, in which case some organs would be 
without it, while in others it would be characterised by 
certain inherent positive or negative properties. We have 



NEGATIVE HELIOTROPISM 6 1 1 

seen, however, that such a theory is untenable. The apparent 
absence in some organs of sensibility to light may perhaps, 
then, be explicable, not as want of sensitiveness, but as the 
neutralisation of effect, by the equal excitation of the two 
opposite sides. 

In the case of certain seedlings of Avena> it has already 
been shown that the so-called insensitiveness of the lower part 
of the organ was due to this cause. Many tendrils, again, 
according to Mohl and others, are heliotropically insensitive. 
Thus, for example, on subjecting the tendril of Passiflora to 
lateral sunlight, there is practically no responsive movement ; 
but as the tendril is a highly conducting organ, we might 
expect that its responsive movement would be neutralised by 
the transverse transmission of excitation. It occurred to me 
that this question, as between a characteristic insensitiveness, 
and a sensitiveness with equal excitation of two sides, might 
be tested by artificial reduction of the conducting power by 
cooling. Under such circumstances, if any sensitiveness 
existed, a one-sided excitation by light would remain 
localised, and induce concavity, or positive heliotropic move- 
ment. On carrying out this experiment, I found that the 
selected tendril of Passiflora now exhibited a marked positive 
heliotropic movement by the induced concavity of the side 
acted upon. 

Negative heliotropism of tendril of Vitis. The tendril 
of Vitis is adduced as the type of those organs which exhibit 
the negative helibtropic effect. 1 therefore undertook an 
investigation on this organ, to determine whether it would 
not be possible to explain its negative movement without 
postulating the existence, in its case, of a specific heliotropic 
sensibility of negative sign. When sunlight strikes it on one 
side it is found that it moves away from the light. If, now, 
this movement be really due to the intensity of stimulus, 
causing it to be rapidly conducted to the distal side, and at 
the same time giving rise to the fatigue of the proximal, then 
we should expect that the application of moderate unilateral 
illumination would induce the positive heliotropic movement. 

R R 2 



6l2 PLANT RESPONSE 

The crucial test would thus lie in the observation of the 
responsive movement under moderate unilateral stimulation. 

I placed a tendril in a dark room, and subjected it to light 
of moderate intensity from a sixteen-candle-power electric 
lamp, placed at a distance of 1 5 cm. This induced an active 
movement of the tendril towards the light, or positive helio- 
tropic response. I then brought the lamp nearer, to a distance 
of 5 cm., thus increasing the intensity of light. The active 
positive movement was now quickly reversed into a move- 
ment away, or negative heliotropic response. This experiment 
once more demonstrates the fact that positive and negative 
heliotropic responses are not due to two specific sensibilities of 
opposite sign. 

The negative heliotropic curvature in an organ originally 
radial, which we have just studied, was due to the physio- 
logical anisotropy induced by the stimulus itself; but there 
are organs in which anisotropy is already developed in 
various degrees of perfection, and in them we shall be able to 
observe varying intensities of this negative heliotropic effect. 
This will be discussed in detail in succeeding chapters. 

SUMMARY 

Negative curvature is induced under the action of light in 
two different ways : ( I ) by the indirect effect of the moderate 
stimulation of the tip of root or shoot ; and (2) by the 
transversely transmitted effect of strong or long-continued 
stimulus acting on the distal side of an organ, when the 
proximal has become fatigued. The parallelism between 
geotropic and heliotropic effects is thus incomplete. 

The direct effect of stimulus, unilaterally applied, may be 
longitudinally transmitted, and cause responsive movement 
in the distant growing region. 

The unilateral effect of stimulating light on the growing 
region varies with its intensity, as follows : 

i. Moderate intensity induces the normal positive re- 
sponsive movement. 



NEGATIVE HELIOTROPISM 613 

2. (a) Stronger or long-continued stimulus, on account 
of its internal diffusion by transverse transmission, causes 
neutralisation of effect. In other words, the organ, after 
first moving towards the light, returns to its original position 
at right angles to it, or assumes the dia-heliotropic attitude. 
This neutralisation, depending as it does on transverse con- 
duction of stimulus, can only take place when the stimulus is 
very strong, or when the organ is highly conducting. The 
first of these considerations explains the fact that while 
moderate stimulation causes positive curvature, stronger 
stimulation has no resultant effect. The effect of the second 
factor (the higher conductivity of the tissue), which is brought 
about by a warmer season, is seen when the transversely 
transmitted effect causes neutralisation. The apparent 
absence of heliotropic effect in various tendrils is due not 
to any want of sensibility, but to this neutralisation by 
transverse conduction of the effect of stimulus. 

(b) The transverse transmission of stimulus to the distal 
side is sometimes attended by oscillatory responsive move- 
ments, owing to periodic or alternate fatigue. 

3. With still stronger unilateral stimulation, the organ 
becomes for the time being anisotropic, owing to the fatigue 
of the proximal side. The internally diffused stimulus then 
induces negative curvature, through the relatively greater 
excitability and contraction of the distal half of the organ. 
The negative heliotropic movement of the tendril of Vitis 
is explained by these considerations. This tendril, under 
moderate unilateral stimulus, exhibits positive heliotropic 
movement ; but stronger stimulation, in consequence of 
transverse transmission and unilateral fatigue, gives rise to 
negative heliotropic movement. 

The statement that the different responsive curvatures 
brought about by light are not due to different sensibilities 
possessed by different organs, is proved by the fact that the 
same organ exhibits continuous changes, from positive to 
negative through neutral, under different intensities of 
stimulation. 



CHAPTER XLIV 
EFFECT OF INVISIBLE RADIATION AND EMANATIONS 

Effect of temperature and its variations Demonstration of fundamental effect of 
thermal radiation on growth Response to successive uniform stimuli of 
thermal radiation Effect of continuous unilateral stimulation Effect of 
electrical waves on growth Response of Mimosa to electric radia* Jrm 
Action of high frequency Tesla current. 

WE have now studied the curvature effects induced in plants 
by those ethereal vibrations that lie within narrow limits, and 
are known as visible light, There are, however, other vibra- 
tions outside this range, the ultra-violet and the infra-red. 
The excessively quick vibrations beyond violet are known to 
produce very marked heliotropic effects. But below the red, 
again, we have comparatively long waves which give rise to 
thermal, and others, still longer, to electrical radiation. 

In studying the curvature-effects on plants of invisible 
radiations of low frequency, it is necessary to distinguish 
carefully between the action of radiation as such and the 
subsidiary effect of temperature. 

Effect of temperature and its variation. In this in- 
vestigation it becomes especially important to distinguish 
the temperature from the radiation-effect, and I shall pre- 
sently describe a very decisive experiment by which the 
effects of the two may be clearly distinguished. The effect 
of temperature up to the optimum is, as we have seen, to 
increase the internal energy of the plant, in consequence of 
which there is an acceleration of the rate of growth ; but 
variation of temperature acts as an external stimulus, and 
would thus be effective in inducing a transient retardation of 
growth. A part of the stimulus, however, is held latent, as 



EFFECT OF INVISIBLE RADIATION AND EMANATIONS 615 

we have seen, in the tissue, so long as the temperature is 
below the optimum, to give rise later to an acceleration of 
the rate of growth. Hence, frequent variations of tempera- 
ture below the optimum will, by reason of these alternate 
retardations and accelerations, produce little total effect on 
the rate of growth.; but above the Optimum, the stimu- 
lating action of variation of temperature will retard growth, 
and as there is here no latent factor, this will not be made 
up by any subsequent acceleration (p. 461). Hence, the 
total effect of such variation will be a retardation. This 
consideration explains the different conclusions to which 
observers have been led as to the effect of frequent variation 
of temperature on growth. 

Demonstration of fundamental effect of thermal radia- 
tion on growth. In the usual experiments on the effect 
of thermal radiation on the induction of growth- variation, 
a difficulty arises in distinguishing between the effect of 
thermal radiation and that of temperature. In order, then, 
to determine the fundamental effect of thermal radiation, 
the experiment must be so arranged that the radiation whose 
effect is to be observed causes no change of temperature. 
I have been able to accomplish this by mounting the growing 
organ in a plant chamber surrounded by a wide heating coil 
of platinum wire. Between the coil and the specimen there 
is a cylinder of mica, which is opaque to thermal radiation. 
By means of a string attached to it from above, this cylinder 
may be alternately lifted and lowered. The plant is attached 
to the Crescograph, and a balanced record is taken of its 
growth when the shield is down, and when, by maintaining a 
current through the heating coil, the chamber has already 
been brought to a steady temperature of 34 C. As every- 
thing inside the chamber has now attained a steady tempera- 
ture, the movement of the shield up and down will produce 
no change in this condition. By now raising the mica 
shield, we can subject the specimen to the action of thermal 
radiation, which proceeds from the heated spiral wire, with- 
out producing any variation of temperature. When the 



6l6 PLANT RESPONSE 

mica shield is again dropped the action of radiation on the 
organ is cut off. 

Experimenting in this manner, and obtaining a horizontal 
record with the shield down, we find, when the shield is 
lifted, that there is at once an upsetting of the balance, which 
indicates a retardation of growth. When the shield is once 
more allowed to drop, the deflected record becomes again 
horizontal, indicating the restoration of the original rate of 
growth. This experiment conclusively shows that radiation 
by itself acts as an external stimulus, retarding the rate of 
growth, It will be remembered that this is quite distinct 
from the effect of temperature, which, up to the optimum, 
always induces the opposite result, namely, an enhancement 
of the rate of growth. The same distinction is also to be 
borne in mind in dealing with the excitatory effect of light, 
for incident light also has the twofold effect of causing 
external stimulation and at the same time raising the tem- 
perature of the tissue. 

Response to successive uniform stimuli of thermal 
radiation. --Having thus demonstrated the fundamental 
action of thermal radiation, we shall now study the effect of 

the unilateral appli- 
cation of this form 
of stimulus, which 
will be found similar 
to that caused by 
visible light. The 
responsive effect of 
successive uniform 

FIG. 248. Responses to Successive Uniform .:.^,,i^ *.:,->., ^^ u 
Stimuli of Thermal Radiation in Pistil of Afusa stimulations may be 

studied by placing a 

V-shaped platinum wire with its point opposite to the 
region of growth, and heating it periodically by definite 
currents of short duration ; the flashes of thermal radiation 
thus produced bring about the usual responsive concavity. 
These responses and recoveries are recorded in the usual 
manner. I give in fig. 248 a series of such responses 




EFFECT OF INVISIBLE RADIATION AND EMANATIONS 6l/ 

of the pistil of Musa, in some specimens of which the 
growing region is found to be narrow and sharply defined. 
In this case the V-shaped radiator was placed opposite to 
this region. In these responses we see that there is a con- 
siderable amount of recovery after each transient stimulation, 
and also a certain degree of fatigue. 

Effect of continuous unilateral stimulation. We shall 
next study the effect of continuous unilateral stimulation. 
For this experiment I took the hypocotyl of Tamarindus 
indica. As the growing region in this case is extended, 




FIG. 249. Response of Hypocotyl of Tamarindus indica to Continuous 
Stimulation of Thermal Radiation 

I used a thermal radiator, which consisted of a linear plati- 
num wire, 5 cm. in length, placed parallel to one side of 
the growing region. Under the action of continuous uni- 
lateral radiation, an increasing positive curvature that is to 
say, towards the stimulus was induced, till a maximum 
effect had been attained (fig. 249). With specimens of other 
plants I obtained either the neutralisation or the reversal of 
this curvature, as it might happen, in consequence of the 
internal diffusion of stimulus to the distal side. This neutra- 
lisation is brought about by equal excitation of the proximal 
and distal sides, in precisely the same manner as in the 



6l8 PLANT RESPONSE 

case of light. It is seen in the return of the organ to its 
original position, or by oscillations about that position as the 
mean ; but in cases where the proximal becomes fatigued, 
the responsive contraction of the distal gives rise to a nega- 
tive curvature. In all these cases we see thermal radiation 
inducing the same curvature effects positive, neutral, and 
negative as were found to be induced by visible light 

Effect of electric waves on growth. I shall next describe 
the effect of Hertzian waves in inducing responsive curvatures ; 
and first, in order to obtain the fundamental effect on growth 
itself, I took an electric radiator consisting of a rod 5 cm. 




FlG. 250. Effect of Electric Waves on Growth 

Connection with electric vibrator at downward arrow (|) is seen to arrest 
growth and to induce contraction, by which the specimen undergoes 
an actual shortening. 

& 

in length, excited by oscillatory discharge from a Ruhmkorffs 
coil. The specimen was a flower-bud of Crinum Lily, whose 
upper and lower ends were connected by means of thin wires 
with the two ends of the electric radiator. The natural 
growth-record under unbalanced conditions was first taken 
(fig. 250), and the specimen was now subjected to the action 
of electric waves at the point marked in the record with a 
downward arrow. It will be seen that this gradually 
diminished the rate of growth, till at the end of five minutes 
it was completely arrested ; and as the action of the electric 
waves continued, the contractile effect by which growth is 
arrested is seen to be carried further, actually bringing about 
a shortening in the length of the specimen. 



EFFECT OF INVISIBLE RADIATION AND EMANATIONS 619 

Response of Mimosa to electric radiation. In the last 
case, in order to subject the specimen to the intense action of 
electric waves, the radiator was electrically connected with 
the responding organ, which was diffusely stimulated by it, 
and exhibited a longitudinal response of contraction. If, 
however, we wish to observe the effect of the unilateral action, 
it will be necessary that the radiation shall act on one side 
only. A difficulty is here met with, however, owing to the 
relatively greater length of these electric waves, which, like 
those of sound, do not cast shadows, but curl round corners. 
In order to produce unilateral action, then, the length of 
these waves has to be reduced to a minimum. By using 
small spheres of platinum as the source of radiation, I 
succeeded in obtaining short electric waves of about I cm. in 
length ; but the intensity of such radiation is somewhat 
feeble, and with them I could only occasionally obtain 
responsive curvatures of growth. I was, however, more 
successful in obtaining responsive effects from the pulvinus 
of Mimosa. When the small radiator which I have described 
was placed at a distance of a few centimetres from the lower 
half of a pulvinus, and when the radiator was allowed to act, 
a depression of the leaf of this plant occurred after an 
exposure of nearly half a minute to the continuous action. 
It should be mentioned that for the demonstration of this 
effect it is the younger leaves which are suitable, the older 
not being sufficiently sensitive. With regard to the electric 
waves, it is to be borne in mind that to them water is opaque, 
while ebonite is highly transparent. Hence the interposition 
of a sheet of ebonite, between the radiator and the plant, 
does not stop the responsive action, while a parallel-sided 
trough of water in the same place would effectually prevent 
the passage of the rays, and thereby put an end to the 
action. 

Action of high frequency Tesla current If one electrode 
of a Tesla coil be put in connection with the growing organ, 
this electric variation of high frequency is found to cause 
an arrest of growth. Even the contiguity of wires carrying 



620 PLANT RESPONSE 

these high frequency currents is often found to retard growth. 
It is probable that this is due to certain material emanations 
that proceed from the wire. A coil of iron wire was made to 
surround the growing organ, and during the excitation of 
this coil the normal growth-rate was found to be retarded ; 
but there was no such retardation when a glass cylinder was 
interposed between the specimen and the coil. During the 
short period of the experiment there was but little rise of 
temperature within the coil, and the retardation of growth 
could not have been due to any thermal radiation. 

SUMMARY 

Variation of temperature acts as a stimulus. Below the 
optimum the direct effect of variation of temperature is a 
retardation of growth, and the negative after-effect, owing to 
absorption of stimulus, is an acceleration of growth. Hence 
repeated variations of temperature below the optimum have 
little resultant effect on growth. 

Above the optimum, stimulus is not held latent, and there 
is no negative after-effect of accelerated growth. Hence, 
repeated variations of temperature above the optimum will 
retard growth. 

While rise of temperature up to the optimum accelerates 
growth, thermal radiation, as such, acts as a stimulus, and 
retards growth. 

The unilateral action of thermal radiation is similar to 
that of luminous radiation that is to say, thermal radiation, 
when of moderate intensity, gives rise to positive, and when 
strong, to neutral, or negative movement. 

Electric waves induce retardation of growth. The uni- 
lateral application of electric radiation is also found to induce 
responsive movements. 

High frequency Tesla currents retard growth. 



CHAPTER XLV 
ON PHOTONASTIC PHENOMENA AND ON DIURNAL SLEEP 

Photonasty and para-heliotropism Response of Troptcoltim majus Responses 
of plagiotropic stems : (a) Mimosa (f>) Iponicua (<;) Ciicnrbita Daily periodic 
movements of plagiotropic stems Responsive movements of pulvinated organs 
Pulvinated organs showing positive heliotropic movement : (a) Response of 
terminal leaflet of Desinoditwi(b) Response of leaflet of Rolnnia (c) Re- 
sponsive movements of leaflets of Erytkrina indica and Clitoria ternatea The 
negative heliotropic type of response : (a) Response of pulvinus of Afimosa 
(f>) Diurnal sleep of Oxalis (f) Diurnal sleep of Biophytum Directive versus 
non-directive action of light General view of responsive curvatures induced 
in different organs by unilateral application of light. 

I SHALL now enter upon the investigation of a very large 
class of phenomena, brought about by the action of light, 
which have hitherto been regarded not only as obscure, but 
also as totally unrelated to each other. These phenomena 
may be described in a general way as the exhibition of the 
differential effects of light on anisotropic or dorsi-ventral 
organs. Such effects may again be divided for convenience 
into two classes, according as they are exhibited either by 
growing or by mature and pulvinated organs. The respon- 
sive curvature in the former of these cases, due to the 
differential growth induced by light, we shall designate as 
photonastic. It must be understood that there is in reality, 
as we shall see, no fundamental difference between the 
responsive curvatures induced in growing organs, whether 
radial or dorsi-ventral, and those in pulvinated organs ; but 
it is nevertheless suitable for the purposes of this investigation 
to treat them under separate headings. 

Photonasty and para-heliotropism. In experimenting 
with dorsi-ventral shoots, De Vries found that when light 
was applied to the lower or normally shaded surface of, for 



622 PLANT RESPONSE 

example, runners of Lysimachia Nummularia, the result was 
a concavity of that side ; and when strong light was applied 
on the upper surface, the result was still the concavity of the 
normally shaded, and now distal, side. He obtained similar 
results also with midribs of leaves. Again, in the case of 
Marchantia thallus, it was found by Frank and Sachs that it 
was the normally shaded side which became concave, whether 
light was applied above or below. 

De Vries explained the curvature away from light, when 
the dorsal surface was illuminated, by the assumption that 
the organ was negatively heliotropic ; but the concave 
curvature induced when the lower surface of the same organ 
was illuminated necessitates the description of that surface, 
at least, as positively heliotropic. Thus we are driven to 
assume that the two surfaces of the organ are endowed at the 
same time with two opposite heliotropic properties, the obvious 
impossibility of which has led to the idea that in this class of 
phenomena we have not to deal with the directive action of 
light at all. 

In the case of certain pulvinated organs I shall be able to 
show an exact parallel to these phenomena that is to say, an 
induction under light of concavity in the lower surface, whether 
light be applied from above or below. This being so, it is 
clear that the only theory of the phenomena which could be 
satisfactory would be one which would apply equally to both. 

Closely connected with the same inquiry is the pheno- 
menon of para-heliotropism, or the so-called diurnal sleep. 
Under the intense illumination of midday, the leaves or 
leaflets of certain plants take up positions which outwardly 
resemble those which they adopt at night. In some cases 
again, in which the normal daylight position is outspread, and 
the nocturnal one of downward folding, the effect of light at 
midday on the leaflets is to induce a folding upwards. 
These phenomena have not yet been satisfactorily explained. 
Darwin suggested that such habits had been acquired for the 
special purpose of avoiding too intense an illumination. I 
shall be able, however, to offer a simple and inclusive ex- 



PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 623 

planation, applicable to the phenomena both of photonasty 
and of para-heliotropism. It will be noticed that in the first 
case we have to deal with the effect of light on all growing 
dorsi-ventral organs, and in the second with its effect on 
mature dorsi-ventral pulvini ; and in dealing with both 
these classes I shall pass step by step from the consideration 
of simple to that of more complex types. 

The response of Tropaeolum majus. Sachs found that 
when the stem of Tropceolum majus is exposed to intense 
and long-continued unilateral illumination, a negative helio- 
tropic curvature is induced ; but when the plant is exposed 
to moderate unilateral illumination it exhibits positive helio- 
tropic movement. The explanation of this difference is 
made quite clear from the experiments which I have already 
described, with reference to seedlings "of Sinapis nigra and to 
the tendril of Vitis ; for it has been shown that in these 
cases moderate unilateral stimulus of light caused a positive 
heliotropic effect ; whereas, under the action of intense stimu- 
lation, a transient anisotropy was induced, on account of 
which the excitability of the over-stimulated proximal side 
was diminished. Hence the distal side was rendered the 
relatively more excitable, and the intense stimulus becoming 
internally diffused caused a concavity of the distal side, and 
gave rise to a negative heliotropic effect (p. 609). 

The negative effect is thus due to anisotropy, and internal 
diffusion of excitation from the proximal to the distal sides, 
this latter factor evidently tending to be facilitated by any 
agency that increases the conductivity of the tissue (p. 603). 
We have also seen elsewhere that this power of conductivity 
is very much augmented in summer (p. 475). In connection 
with this, it is interesting to note that the negative heliotropic 
curvature of Tropceolum majus, due to transmitted excita- 
tion, has only been observed by Sachs during summer, while 
in autumn the effect observed by him was always positive, 
owing to stimulus remaining localised. Another instance of 
the same kind is furnished by the hypocotyl of Ivy (Hedera 
helix\ which from the normal positive heliotropic curvature, in 



624 PLANT RESPONSE 

autumn, as observed by Darwin, passes into the exhibition of 
a strongly negative heliotropic condition in summer. 

Responses of plagiotropic stems. It is found that 
under feeble diffuse illumination the Gourd-plant (Cucurbitd) 
for example, grows erect. In the open, however, if, on 
account of its own weight, or by the action of the wind, it 
is once made to bend, the stem is brought into a position 
where its upper surface is constantly acted on by strong 
light, whilst its lower is shaded. By the continuous action 
of the stimulus of light the upper surface is now rendered 
less excitable, and a permanent anisotropy (plagiotropism) 
is induced, such as we saw transiently exhibited in the 
hypocotyl of Sinapis (p. 609) under the short experimental 
exposure of a specimen to intense unilateral illumination. It 
has already been shown in Chapter VII. that such plagio- 
tropic stems are more excitable on their lower or shaded 
side, and that under diffuse stimulation response is by con- 
cavity of that side. Hence a recumbent plagiotropic stem 
of this description, acted on by strong vertical light, which 
becomes internally diffused, will always exhibit concavity of 
the lower surface, in consequence of which it will be closely 
pressed against the ground. 

Such a plagiotropic organ, then, acted upon dorsally, 
shows a concavity of the ventral surface, or a negative helio- 
tropic effect ; but if the ventral, or normally shaded, side 
be itself acted upon directly by light, the result will still be 
the concavity of the lower or more sensitive surface that is 
to say, a positive heliotropic effect. In the former case, then, 
we have an example of the differential response of an aniso- 
tropic organ to diffuse stimulation, by concavity of the more 
excitable ; and in the latter, the direct contraction and con- 
cavity of the surface acted upon, which happens in this case 
to be also the more excitable. This will explain why, in the 
midribs of leaves, in plagiotropic shoots of Lysimachia^ and 
in the thallus of Marchantia, we always observe the concavity 
of the shaded and more excitable side, in response to the 
action of strong light, whether it is applied from above or below. 



PHOTON ASTIC PHENOMENA AND DIURNAL SLEEP 625 

(a) Mimosa. This fact, that in plagiotropic stems under 
the diffuse stimulation of light it is the more excitable 
shaded side which becomes concave, I have been able to 
demonstrate by numerous other experiments ; for example, 
taking four creeping stems of Mimosa, I tied them in such a 
manner that their free ends should be vertical. The shaded 
sides of the four specimens were turned so as to face each a 
different point of the compass 

east, west, north, and south. 
Subjected thus to the diffuse 
stimulation of light from the 
sky, they all executed curva- 
tures. The specimen whose 
under side faced the east be- 
came bent towards the east. 
The same happened to those 
which faced north, south, and 
west that is to say, they 
became curved towards the 
north, south, and west re- 
spectively. The fundamental 
responsive action by which 
all these were determined 
was the induced concavity of FlG - W- Response to Diurnal Light 

, and Darkness of Plagiotropic Stem 

the under, or normally shaded, O f Ipomaa held vertical 

side, which is the more CXClt- The curvature is seen to increase with 

able. 

(b) Ipomosa. Another ex- 
ample was that of the creeping 
stem of Ipomosa. This I tied 

up vertically with its end free, at 10 A.M., the normally 
shaded side being represented in the diagram to the left 
(fig. 249). For purposes of the record I placed behind 
it a piece of paper, on which its different positions were 
traced from time to time. It will be seen that by I P.M 
ft had become considerably curved, the normally shaded 
side being, concave. This concavity had become still more 

S S 




the progress of day, the shaded side 
to the left becoming more and more 
concave. The dotted figures repre- 
sent positions of gradual recovery 

at s J 



626 



PLANT RESPONSE 



marked by 4 P.M. The next record was taken at 7 P.M., 
when the sun had gone down, and the source of stimulus 
was therefore removed. The dotted portion of the record 
shows the partial recovery which had now taken place, 
and whose extent was still further increased by 10 P.M. 

This partial undoing of the 
induced curvature is mainly 
due to the recovery which 
is always observed on ces- 
sation of stimulus. It 
might, however, be urged 
that geotropic action, ab- 
sent when the plant was 
vertical, and coming into 
increasing effectiveness 
with its growing horizon- 
tal ity, played a large part in 
bringing it about. In order, 
then, to eliminate this ele- 
ment, I next experimented 
on a stem of Cucurbita. 

(c] Cucurbita. This 
plagiotropic stem was held 
horizontally and sideways, 
in such a way that the 

ventral surface being here represented p i ane wh j ch divided its 

to the right. The action of gravitation r 

is thus practically eliminated. Cumula- dorsal and ventral halves 
tive effect of day's illumination is seen vprtiral Th<* haH<r1 

in progressive concavity of normally WES veitlCEl. 1 He Shaded 

lower or shaded surface. Dotted figures side is here represented in 

show positions of recovery after dark. t .. , . . 

the diagram to the right 

(fig. 250). Owing to this particular arrangement of the 
plant it will be seen that the responsive heliotropic move- 
ment must take place in a horizontal plane, and that on it 
geotropism will have no influence. The free end of the 
plagiotropic stem had been naturally somewhat curved up- 
wards, and this appeared, when placed sideways, as a slight 
curve to the left, in its position at 7 A.M. Under the stimulus 




FIG. 252. Response to Daily Periodic 
Light and Darkness of Plagiotropic 
Stem of Cucurbita maxima 

The specimen held horizontally, with plane 
of dorsi-ventral division vertical, the 

ice 1 
to the right. The action of gravitation 



PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 62; 

of daylight by 10 A.M. the stem is seen to have taken up a 
strongly curved position, with concavity of the shaded side. 
This process is seen from the figure to have been progressive, 
but on the approach of darkness there was recovery, the 
return being considerable at 7 P.M., and still greater at 
10 P.M. It may be stated here that such recoveries are never 
quite complete, part of the curvature being fixed by growth. 
Thus in a naturally growing plagiotropic stem, the portion 
which was one day slightly lifted above the ground is on the 
next, owing to this residual effect, made to lie closely against 
it, so that by the action of light the growing stem is pressed 
progressively closer to the earth. 

Daily periodic movement of plagiotropic stem. From 
this demonstration of light-curvature and recovery, it will be 
seen that the free organ has a daily periodicity in virtue of 
which, in the daytime, it moves gradually downwards, and 
at night, owing to recovery, upwards. In this plagiotropic 
stem, then, we see, as will be shown later, the first induction 
of that nyctitropic movement which is more strikingly dis- 
played in dorsi-ventral leaves. 

Responsive movement of pulvinated organs. The 
demonstration which I have just made of the peculiar 
responses of anisotropic organs to the stimulus of light, as 
exemplified in the case of plagiotropic stems, will be found 
still more strikingly applicable to pronouncedly dorsi-ventral 
organs like pulvini. In the last-named instance, indeed, as 
motility is great, the investigation has the advantage of con- 
cerning itself with responsive effects which take place very 
quickly. 

From what has already been said, we shall be prepared 
to meet with two different types of responses, according as 
the transverse conductivity of the organ is feeble or consider- 
able. In the former of these cases, under unilateral stimu- 
lation, there is no internal diffusion of stimulus, and the side 
acted on, whether above or below, will respond by concavity ; 
but in the latter case that is to say, where the transverse 
conductivity of the tissue is great it will be the more excit- 

S S 2 



628 PLANT RESPONSE 

able side which, under strong unilateral stimulation of either, 
will become concave. Supposing the lower side to be the 
more excitable, then, strong stimulation, whether of dorsal 
or of ventral, will bring about concavity of the lower ; but 
here we must bear in mind the possibility of an effect which 
will be the result of moderate stimulus. The differential 
effect, which brings about the concavity of the lower, even 
when it is the dorsal surface that is excited, depends on 
the internal diffusion of stimulation, and this in turn is 
dependent not only on the conductivity of the tissue, but 
also on the intensity of the stimulus. If, then, a feeble or 
moderate stimulus be applied on the dorsal surface, even of 
a highly conducting organ, the result will be a concavity of 
that surface. The long-continued action of moderate stimu- 
lus will, however, bring about a gradual percolation, and 
the first response due to localised, will give place to the 
differential effect of diffuse, stimulus. If the stimulus again 
be very much stronger, this reversal will take place much 
more quickly. We thus see that responsive movements may 
be positive, neutral, or negative, according to the strength 
and duration of the stimulus. 

Pulvinated organ showing positive heliotropic move* 
ment : (a) Terminal leaflet of Desmodium. We shall now 
take the case of a pulvinus in which the conductivity is 
feeble, and in which we should therefore expect to obtain 
positive response. I have already given response-curves in 
illustration of this positive response, whether it be the upper 
or lower surface of the pulvinus which is subjected to light 
(p. 586). It was also shown that the leaflet moved towards 
the light, and that in such cases it was the pulvinus, and not 
the lamina, which was both the perceptive and responding 
region. 

(b) Response of leaflet of Robinia. The leaflets of Robinia^ 
under the vertical light of the noonday sun, exhibit what is 
known as diurnal sleep that is to say, they fold themselves 
upwards. This is simply an instance of the positive helio- 
tropic effect common to all those pulvinated organs in which 



niOTONASTIC PHENOMENA AND DIURNAL SLEEP 629 



transverse conductivity is feeble. I give here (fig. 253) a 

response-record obtained with a leaflet of Robinia under the 

action of sunlight from above. A similar response, but in a 

downward direction, is obtained when 

light is made to act from below ; 

but as, under natural conditions, 

light always acts from above, the 

leaflets are found, when the acting 

daylight is sufficiently strong, to 

exhibit what is known as diurnal 

sleep, or the para-heliotropic effect, 

in an upward direction. I have 

obtained numerous similar records 

with other leaflets, which fold upwards 

under the action of sunlight. 

c) Responsive movements of leaflets Dotted line represents re- 




Fir.. 253. Positive IJelio- 
tropic Response of Leaf- 
let of Kobinin to Sunlight 
Acting from A hove 



r r? ./ . / 7 x- xv, covery on cessation of 

0/ Erythnna indica and of Chtona s , imu i us . Time -marks 

tematea.or the Sake of simplicity represent intervals offnc 

I described the movement of Robinia 

leaflet as upward ; but the actual direction is one which 

more or less accurately coincides with that of incident 

sunlight. As further examples of this particular type of 

diurnal sleep movement, I may mention the leaflets of 

Erythrina indica and Clitoria ternatca I 

(Indian name Aparajitd). Both of v(l\/ 

these are so remarkably sensitive that 

they follow the course of the sun, in 

such a way that the axis of the cup 

formed by the folding leaflets at the 

end of the petiole is coincident with Fio. 254. Positive lielio- 

. r i- i i. i i-* tropic Response of Leaf- 

the rays of light, and continues so letj j f of E ,ythnna indica 
from about 1 1 A.M. till about 3 P.M. The leaflets form a hollow 
The negative heliotropic type cup with direction of 

incident sunlight as axis. 

of response. We shall now pass 

on to the second, or negative, heliotropic type of response 
due to the internal diffusion of stimulus ; and in order 
to show that there is a continuity between these and the 




630 PLANT RESPONSE 

former instances, I shall here refer once more to the 
case, already mentioned, of the heliotropic response of the 
terminal leaflet of Desmodium, which in winter is always 
positive, but in a given experiment, under the conditions of 
greater transverse conductivity which are brought on in 
summer, exhibited, after two hours of continuous exposure to 
vertical sunlight, a neutralisation of the previous positive 
effect (p. 604). 

(a] Response of pulvinus of Mimosa. The second type of 
response will be the better understood if we first study in detail 
all its characteristics. As I have already said, these negative 
heliotropic responses result from the transverse diffusion of 
stimulus across the tissue, which brings about the concavity of 
the more excitable lower half of the organ. We can easily, 
in the continuous response-record to be given presently, 
detect this gradual process of the percolation of stimulus 
through the tissue, when light is applied from above. As 
there is in the case of Mimosa a considerable mass of inter- 
vening tissue between the upper and lower surfaces of the 
pulvinus, it follows that unless the stimulus applied be 
excessive, there will be a certain interval of time required 
for its passage. We should therefore expect that on the 
application of light to the dorsal surface there would be a 
local contraction and concavity of that side, raising the 
leaf up, and causing a preliminary positive response ; but 
this movement will be arrested, and gradually reversed, so 
soon as the stimulus reaches the lower side, and begins to 
induce antagonistic contraction there ; and after this, the 
greater excitability of the lower will be manifested by its 
greater contraction, as seen in the negative response, or 
depression of the leaf. All this will be clearly understood 
from the series of records given below. 

It is necessary, however, before describing the effect of 
the continuous application of light from above, to analyse 
the responsive sensibilities of the two sides of this organ ; 
and this is the more desirable since, in the case of Mimosa, 
it is commonly assumed that excitability characterises only 



PHOTONASTK' PHKNOMKNA AND DIURNAL SLKKP 63 1 



the lower half of the organ, the responsive movement of the 
pulvinus being due to that factor alone. In the course of the 
present work, however, I have frequently stated that the 
upper half also was excitable, and that the usual responsive 
movement was due to the differential excitability of the two. 
It is now easy, using localised stimulus of light, to submit 
this question to a crucial test. 

First I took a record of the responsive movement of the 
leaf of Mtwosa, when the upper half of the pulvinus alone 
was subjected to stimulus of sunlight. Fig. 255, a, shows 
the moderate positive, 
or in this case upward, 
movement, which was 
the result of this sti- 
mulation. By means 
of a properly inclined 
mirror light was now 
thrown vertically up- 
wards, so as to strike 
the lower half of the 
pulvinus. The con- 
sequent positive re- 
sponsive movement, 
in this case down- 
wards, is seen to be 
much stronger (fig. 

2 55> ^)i on account of 

the greater excitability of the lower half of the organ. The 

differential character of the responsive movement under 

externally diffused stimulation is shown in fig. 255, c, which 

is a record of the response given by the pulvinus when both 

its upper and lower sides were simultaneously acted upon by 

light. 

It will be seen that in this record, then, we have a case of 
response to externally diffused stimulus. We shall next 
observe the effect of stimulus which has become internally 
diffused, owing to conduction from the upper to the lower 




Fie.. 255. Responses of Mimosa to Sunlight of 
not too long Duration 

(a] Light acting on pulvinus from above ; (/>) light 
acting on pulvinus from below ; (r) light acting 
simultaneously from above and below. Dotted 
line represents recovery on cessation of light. 



632 



PLANT RESPONSE 




half of the pulvinus. We have just seen that in the case of 
externally diffused stimulation both sides of the organ are 
acted upon at the same moment, and the differential response 
is downward from the beginning ; but when continuous 
stimulus is applied on the dorsal surface, it is at first 
unilateral, and only afterwards becomes internally diffused. 
We therefore obtain in this case (fig. 256), as was theoretically 

inferred, all the phases of re- 
sponse, positive, passing through 
neutral, into negative. 

A very interesting feature of 
this record is the after-effect, on 
the cessation of stimulus, which 
is represented by a dotted line. 
In Chapter XXXI V., while deal- 
ing with the detection of the 
latent factor, it was explained 
that there are two distinct after- 
effects, positive and negative 
(p. 457). In the former, the 
movement is simply a continua- 
tion of the effect seen when 
stimulus was acting ; but the 
latter, or negative, after-effect, 
being due to the increase of 
latent energy by absorption of 
incident stimulus, finds expres- 
sion in an opposite movement In the case of the response 
of growth we saw that the positive after-effect consisted of a 
persistent retardation, and the negative of an acceleration, 
of growth. In the case of pul vitiated organs, however, the 
negative after effect is generally indistinguishable from the 
movement of recovery ; but in the present instance we find 
it clearly exhibited in the fact that the after-effect not only 
causes recovery to the original position, but carries the leaf 
to a distance beyond. 

The positive heliotropic response of the leaf, then, persists 



FIG. 256. Response of Pulvinus 
of Mimosa to Action of Con- 
tinuous Light from Above ap- 
plied at Moment marked with | 

Positive heliotropic movement 
caused by excitation of upper 
half neutralised by transmission 
to distal side, and ultimately 
reversed owing to greater ex- 
citability of lower half. Dotted 
line represents recovery on ces- 
sation of light. Note iinal 
erection of leaf above original 
position as negative after-effect 
by absorbed stimulus. 



PIIOTONASTIC PHENOMENA AND DIURNAL SLEEP 633 



only as long as it takes the stimulus to reach the distal side 
of the pulvinus. When strong concentrated sunlight is 
applied from above, therefore, the duration of this upward 
movement is reduced, and it appears only as a slight twitch. 
The same effect is produced under less intense light when, 
as in thinner pulvinated organs, the distance to be traversed 
by stimulus is not so great. 

(b) The diurnal sleep of Oxalis. In consequence of strong 
vertical illumination, as at noonday, negative heliotropic 
response, with downward 
folding of the leaves, takes 
place in those pulvinated 
organs in which there is 
internal diffusion of sti- 
mulus, and in which the 
lower side of the pulvinus 
is much more excitable 
than the upper. As ex- 
amples of this, we shall 
take the cases of Oxalis 
and Biophytuin. In the 
former, if light of moderate 
intensity, say from a lamp, 
be applied on the dorsal 
or upper surface, an up- 
ward, or positive, heliotro- 
pic curvature is induced. 
If the same light be 
applied below, a positive 
heliotropic movement, in this case downwards, is again 
induced ; but the fact that the lower side is much the 
more excitable is seen when the upper and lower surfaces 
are excited simultaneously, for there is now a downward 
movement (fig. 257, a). Thus external diffuse stimulation 
induces downward movement, and we shall also find that 
stimulus internally diffused has the same effect. This is seen 
by throwing a strong beam of sunlight on the upper surface, 




Fid. 257. Responses of Owl it to Sun- 
light 

(a) Shows greater excitation of lower half 
and downward response when both 
upper and lower are simultaneously 
subjected to stimulus of light ; (/>) shows 
downward or negative response owing 
to internal diffusion of stimulus when 
upper surface only is acted on by 
strong light. Arrows indicate the di- 
rections of incident light. 



G34 



PLANT RESPONSE 



when the leaflet is found to respond by depression (fig. 255, b ). 
Here, then, we have a repetition of those movements which we 
have already seen in the case of plagiotropic stems, where 
strong illumination of whichever surface always had the same 
effect, that is to say the induced concavity in the lower. 

(c] Diurnal sleep of Biophytuin, In the case of Oxalis 
and in that of Mimosa the responsive movement is more 
or less continuous ; but in such a pronouncedly multiply- 
responding organ as that of Biophytum, the effect of internally 
diffused stimulus on the more excitable lower half of the 
pulvinus is to induce depression by a series of multiple 

responses. This is well seen in 
the following record (fig. 258), 
where under the action of strong 
sunlight from above the leaflet is 
undergoing a progressive fall. A 
record of a similar effect in the 
case of Biophytuin is seen in fig. 
123, the fall being in that case 
represented by an up curve. The 
specimen there was somewhat 
sluggish, and there were three 
pulsations in the course of ten 
minutes, making an average of 3*3 
minutes to each. In the present 
instance, however, there were six 
pulsations in the course of fifteen 

minutes each, that is to say, having an average period of 2*5 
minutes. Similarly in Averrhoa the fall of the leaflet occurs 
in a pulsating manner, an instance of which is given by 
Darwin. 1 

The directive versus non-directive action of light. It 
will be well at this point to enter into the question of arbitrary 
distinction which is commonly made as between heliotropic 
action in radial organs and the same action in the anisotropic 
or dorsi-vcntral. It is clear, however, that such a distinction 

1 Darwin, Movements of Plants, p. 333. 




FIG. 258. Negative Multiple 
Response in Biophytiim 
when Acted on from Above 
by Strong Sunlight 



PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 635 

is no longer possible when the fundamental unity of effects 
in the two cases has been perceived. 

But it is customary to make a further distinction between 
these effects, on the ground that the movement of the radial 
is determined by, while that of the anisotropic is independent 
of, the directive action of light. In this matter again I shall 
be able to show further that no such line of demarcation can 
be drawn. It will then be seen that the differences between 
the two classes of phenomena are only apparent. 

In the case of radial organs, we saw that the direct action 
of moderate stimulus of light, inducing concavity of the 
proximal, gave rise to positive heliotropic movement. The 
same is true of the similar direct action of a moderate 
intensity of light on, say, the pulvinus of Mimosa, where the 
upper and lower halves of the organ both exhibit positive 
response. With stronger light, again, the radial organ 
develops an induced anisotropy, by which the proximal 
side, owing to fatigue, becomes the relatively less excitable. 
Under this condition, the proximal side of the radial organ 
corresponds to the less excitable upper half of the pulvinus 
of Mimosa, and the distal to the more excitable lower half. 
This differentiation is seen to be continuous throughout the 
three cases of the radial organ, the plagiotropic stem, and the 
pulvinus, although in the first of these it is transient, lasting 
only during the action of stimulus. In all these cases alike, 
strong unilateral stimulus, acting on the less excitable 
proximal or upper surface, becomes internally diffused, and 
causes movement away from stimulus, or negative heliotropic 
response, in a direction which is perpendicular to the plane 
of separation of the two anisotropic halves. Thus there is 
no difference between the negative heliotropic responses of 
a radial and a pulvinated organ. 

The distinction between the two cases of the permanent 
and temporary differentiation of the organ has been illustrated 
by Sachs by a reference to the induction of polarity in steel 
and in soft iron respectively. According to this analogy, 
Marchantia behaves to intense light like steel to a magnet, 



636 PLANT RESPONSE 

retaining permanently the induced polarity. Tropaolum 
ma/us, on the other hand, behaves like soft iron, assuming a 
definite but temporary polarity, which disappears when the 
influence of light, like that of the magnet, is withdrawn. This 
illustration, however, though vivid, is likely to be misleading, 
for it suggests that, under strong stimulus, the normal pro- 
perties of the organ pass into a polar or opposite condition. 
But there is no such change. Light merely induces a 
difference of excitability in the two sides. That there is no 
reversal of heliotropic sensibility is shown by the fact that 
either side, when excited by moderate intensity of stimulus, 
gives positive response. The necessary condition for the 
exhibition of negative heliotropic response is not only the 
differential excitability of the two sides of the organ, but also 
the internal diffusion of stimulus. 

Owing, however, to the permanence of this differentiation 
in a pulvinated organ, there will be certain conditions under 
which the action of light will appear to be non-directive, 
for it is the diffuse stimulus, however produced, that brings 
about the responsive concavity of the more excitable lower 
half of the pulvinus, and this internal diffusion will take 
place just the same, whatever be the flank of the organ on 
which the strong external stimulus may have been applied. 
In the case of the radial organ, on the other hand, the 
differentiation between the less excitable proximal and more 
excitable distal is not fixed, but changes according to the 
side acted upon at the time by light. 

The different responsive movements induced by light 
positive, negative, dia-heliotropic, and para-heliotropic have 
hitherto been ascribed, as I have already had occasion to 
point out, to as many specific sensibilities possessed by the 
plant. They have now, however, been demonstrated to be 
but so many examples of the general law that plant-organs 
respond to stimulus, by the induced concavity of the relatively 
more excited. 

There is thus no fundamental discontinuity between the 
responses of radial and of dorsi-ventral organs, or between 



PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 637 

positive heliotropic and negative heliotropic response. This 
will be seen still more clearly in the following statement, 
applying to the two extreme cases of radial and of dorsi- 
ventral organs, the plagiotropic constituting an intermediate 
link between the two. 

General view of responsive curvatures induced in 
different organs by unilateral application of light 

1. Radial organs : (a) Positive response-, stimulus localised, 
the proximal side more excited and concave ; e.g. Sinapis 
under moderate unilateral light (p. 609). 

(b) Intermediate or neutral response : stimulus internally 
diffused, causing equal excitation of proximal and distal sides ; 
the organ takes up a neutral (so-called dia-heliotropic) posi- 
tion, at right angles to light ; e.g. Sinapis under moderately 
strong unilateral light (p. 609) ; certain tendrils apparently 
insensitive. 

(c) Negative response : strong stimulus internally diffused, 
which also induces anisotropy, the distal being then the more 
excitable ; concavity of the more excited distal ; e.g. seedling of 
Sinapis, and tendril of Vitis under strong unilateral sunlight 
(pp. 609, 611). 

2. Dorsi-ventral organs : (a) Positive response : stimulus 
localised by relative non-conductivity of organ : the more 
excited proximal side concave ; e.g. positive movement of 
terminal leaflet of Desmodium ; the diurnal sleep movements 
of Robinia, Erythrina indica, and Clitoria ternatea (p. 629). 

() Intermediate or neutral response : stimulus internally 
diffused ; upper and lower halves equally excited ; e.g. 
terminal leaflet of Desmodium, under several hours of 
vertically acting sunlight (p. 604). 

(c) Negative response : strong stimulus acting from above 
on highly conducting organ, of which lower half is con- 
siderably more excitable ; concavity of the more excitable 
lower half ; e.g. fall of Mimosa leaf under strong illumination 
from above ; diurnal sleep movements of Oxalis, Biophytum, 
Averrhoa. - - 



638 PLANT RESPONSE 

SUMMARY 

The responsive action of growing organs being not 
essentially different from that of pulvinated organs, a 
similar explanation must be applicable to the two classes of 
phenomena. 

Owing to long-continued unilateral excitation by light, the 
upper side of a plagiotropic stem is rendered the relatively 
less excitable. Hence such organs may be regarded as 
equivalent to diffuse pulvinoids, of which the upper side is 
the less excitable. 

In both these cases, of pulvini and diffuse pulvinoids, the 
local application of moderate stimulus on either side induces 
normal positive response. 

But with long-continued application of strong stimulus 
two types of response may be obtained, according as the 
organ is characterised by a feeble or high power of trans- 
verse conductivity. In the former of these cases the stimu- 
lus will remain localised, and the response will be positive. 
In the second, the stimulus will become internally diffused, 
and if the lower side be the more excitable, stimulation of 
the upper will give rise to the concavity of the lower, con- 
stituting negative response. 

As examples of the first of these classes may be 
mentioned the diurnal sleep movements of Robinia, Erythrina 
indicci) and Clitoria ternatea> in which vertical illumination 
induces upward folding of the leaflets. 

As representing the second class, we have both plagio- 
tropic and pulvinated organs. 

The plagiotropic stems of Mimosa, Ipomoea, and Cucur- 
bita exhibit increasing concavity of the lower side with the 
duration of the day's illumination. 

A periodic downward movement is thus induced in 
them, which reaches its maximum at the end of the day. 
The reverse movement, of gradual erection, occurs during the 
night. 

In pulvinated organs, negative heliotropic response, with 



PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 639 

downward folding of the leaflets, is exhibited in the diurnal 
sleep movements of Oxalis and Biophytum, 

In Mimosa the application of moderate intensity of light 
from above induces upward or positive heliotropic move- 
ment. Strong and long-continued application of light on 
the same upper surface, however, owing to the internal 
diffusion of excitation, induces concavity of the lower and 
more excitable side, or negative heliotropic movement. Both 
these responses may be regarded as cases of the directive 
action of light ; but if the organ be excited by stimulus 
which is externally diffuse, the responsive movement is still 
downwards. The directive action of light has thus passed 
into non-directive. 



CHAPTER XLVI 

ON DIA-HKLIOTROPISM AND DIA-GEOTROPISM 

Difficulty of distinguishing between effect of light and other reactions Theories 
of Frank and De Vries Subsidiary factors : (i) Epinasty and hyponasty ; 
(2) Effect of gravity ; (3) Effect of suctional activity and of turgescence ; 
(4) Modification of effect by characteristic limits of flexibility Discrimination 
of the part played by heliotropism in the movement of the leaf Proof of 
absence of any specific dia-heliotropic tendency in leaves The lamina not 
the perceptive organ Principal types of the response of leaves to stimulus of 
light Positive type of response : mango leaf Negative type of response : 
leaf of Artocarpus. 

WE shall now take up the question of the effects of 
illumination on ordinary leaves, which are generally supposed, 
on account of some special dia-heliotropic sensibility, to have 
the habit of placing themselves at right angles to incident 
light We have seen in the last chapter that the movement 
of anisotropic organs under heliotropic stimulation is not the 
result of any specific sensitiveness, but constitutes a simple 
instance of the response of plant-organs to all forms of 
stimulus, the response being appropriately modified in this 
particular case by the anatomical and physiological pecu- 
liarities of the responding organ. We have been able to 
demonstrate the continuity of this responsive phenomenon 
by analysing it in two extreme cases of the anisotropic 
differentiation, those namely of the piagiotropic stem, in 
which we see one of its earlier phases, and of dorsi-ventral 
pulvinated organs, in which it attains its highest development. 
In the movements of ordinary leaves we have what is merely 
an intermediate stage between these, hence any explanation 
which would elucidate their action must be one which is 
applicable to both the extremes. 



DIA-HELIOTROPISM AND DIA-GEOTROPISM 64! 

Difficulty of distinguishing- between effect of light and 
other reactions. In the case of the response of pulvinated 
organs, we had the advantage, owing to their great motility, 
of being able to refer each particular responsive movement 
to the immediate stimulating action of incident light. In 
ordinary leaves, however, the movements being very sluggish, 
it takes so long a time for any given responsive action to 
attain the requisite magnitude for ordinary observation, that 
other factors of variation intervene, and it becomes difficult 
to know how much of the resultant response is due to 
heliotropic stimulus as such. It is this great difficulty of 
disentangling the response due to light, which is the proper 
subject of the inquiry, from numerous other subsidiary factors 
that has led to the existing divergence of views among 
observers. 

In the course of the present chapter, therefore, I shall 
shortly enumerate those various agencies which are subsidiarily 
instrumental in bringing about the ultimate position assumed 
by the leaf. I shall then describe a method by which helio- 
tropic action proper can be discriminated with certainty from 
other influences. The relation between the fundamental action 
in response to light, which has already been demonstrated, and 
the heliotropic response of the leaves will thus be made 
apparent. But before entering upon these questions I shall 
briefly allude to the principal existing theories on this 
subject. 

Theory of Frank. There are at present two main types 
of opinion with regard to the question of the effect of light on 
leaves. Frank and his school account for that action of leaves 
by which they place themselves with their flat surfaces perpen- 
dicular to the direction of incidence of the rays of light, or of 
the action of gravity, by assuming that dorsi-ventral organs 
possess a peculiar property of sensitiveness to the directive 
action of light and gravity. This they designate as Transverse 
or Dia-heliotropism, and Transverse or Dia-geotropism. It is 
supposed that the habit has been acquired for the advantage 
of the plant, inasmuch as the leaves, by placing themselves 

T T 



642 PLANT RESPONSE 

in this particular position, are enabled to absorb the largest 
amount of sunlight. Such arguments, however, do not throw 
any light on the mechanism by which the movement is 
brought about. It is even somewhat difficult to understand 
how this generalisation, that leaves place themselves at right 
angles to light, has come to be accepted as a universal fact ; 
for it is only necessary to make a visit to the open forest in 
order to see that, so far from this being the case, many leaves 
place themselves vertically upwards, others downwards, and 
the rest in all possible intermediate angles between the two. 
All that can be claimed on behalf of the dia-heliotropic 
position is that no deviation from it is greater than phis or 
minus 90. 

Theory of De Vries. De Vries, however, is in disagree- 
ment with this theory. He establishes the importance of the 
unequal rates of growth in the upper and lower halves of the 
leaves i.e. cpinasty or hyponasty as a factor in bringing 
about their ultimate attitude. He next assumes, in accordance 
with the generally accepted view, the existence of two opposite 
responsive reactions, positive and negative, in regard to light 
and gravity. He then proceeds to show that there can be no 
necessity for the further assumption of a third or dia-heliotropic 
tendency in the leaves, since epinasty and hyponasty, their 
different combinations with either of the opposite actions 
of positive and negative heliotropism and geotropism, and 
considerations of the weight and balance of parts, are all 
factors which take a share in determining the position ulti- 
mately to be assumed by the dorsi-ventral organ with regard 
to light. 

I shall now attempt to demonstrate the fact that the 
responses of ordinary leaves are in every way similar to 
those of the pulvinated, the mechanics of whose movements 
have already been fully described. It will further be^ shown 
how and under what circumstances the normal positive passes 
into negative response, through certain intermediate phases. 

Subsidiary factors. But before undertaking either of 
these inquiries, it will be well to enumerate briefly the 



DIA-HELIOTROPISM AND DIA-GEOTROPISM 643 

subsidiary factors which combine with the heliotropic action 
proper in determining the final position of the leaf. 

I. Epinasty and hyponasty. These unequal growths of 
one side or the other, De Vries believed to be brought about 
by some spontaneous unknown cause in the plant itself. 
Detmer, however, came to the conclusion that, as regarded 
epinasty, it was not spontaneous, but induced by the action 
of light. This conclusion was based on the observations 
(i) that cotyledons of Cucurbita remained closed up, in 
continuous darkness, but opened out when subjected to light ; 
and (2) that the primordial leaves of P/taseolus y kept in 
darkness, remained folded, and only opened out on illumina- 
tion. The investigations of Vines, however, though he 
supports the contention of Detmer with regard to Phaseolus, 
have led him to a different view in the case of Cucurbita, 
where he finds the epinastic movement to be induced even 
in the absence of light. He has therefore come to the 
conclusion that the phenomena of epinasty and hyponasty 
are spontaneous, and not directly due to the action of 
illumination. 

These differences of opinion, however, have arisen from 
the obscurity in which autonomous or spontaneous movement 
has been involved, and they may be expected to disappear 
when its true nature is clearly perceived. These alternate 
movements of growth are in fact only another example of 
multiple or autonomous response, the difference between 
them and those other forms with which we are already 
acquainted lying in the greater slowness of their period, and 
in the relative fewness of the pulsations that can be exhibited. 
In the case of autonomous pulsation or circumnutation of 
stems, since the growth on which they depend is indefinitely 
prolonged, we have an indefinite continuance of these pulsa- 
tions. In the case of leaves, however, the organ usually 
completes only half its swing, whether epinastic or hyponastic, 
and by that time further pulsation is arrested, owing to the 
cessation of growth ; yet in some cases there is more than 
a semi-pulsation, as where hyponastic movement is followed 

T T 2 



644 PLANT RESPONSE 

by epinastic. It has already been shown that autonomous 
response in general can take place only when the internal 
energy or the sum total of the latent stimulating 'factors is 
above par. This is equally true of the autonomous response 
of growth itself. In the case of autonomous epi- or hypo- 
nastic pulsations, therefore, a certain amount of internal 
energy is essential for their initiation. This is in some cases 
supplied by other forms of stimulus, but there may be others 
in which light is the critical factor. In order, then, to study 
the directive action of light on dorsi-ventral organs under 
normal tonic conditions, we must be able to determine the 
characteristic influence which will be exerted by the stimulus 
of incident light in modification of the already existing 
movement of the organ. This inquiry" is therefore exactly 
parallel to our previous study of the action of light in 
modifying the existing growth-movements of radial organs. 
In that case the variations induced in the ordinary rate of 
movement afforded us a measure of the effect of the stimulus 
of light. In the case of dorsi-ventral organs such as leaves, 
similarly, the effect of light can be correctly inferred only by 
observing the variations which it induces in their existing 
movements. The manner in which this is done will be 
described presently. 

2. Effect of gravity. In long-continued experiments on 
the curvatures induced by light, the observed movement is 
also modified in part by the influence of geotropism. This 
geotropic action in leaves is by some investigators believed 
to be of two types, negative and positive. Others, again, 
regard it as dia-geotropic. I shall, however, adduce con- 
siderations which will show that the upper and lower halves 
of dorsi-ventral organs exhibit differential excitability to geo- 
tropic as to other forms of stimulus. In order to neutralise 
the geotropic action, and thus study the heliotropic effect, 
alone, Francis Darwin mounted the plant on a rotating 
klinostat. It is true that in a strictly radial organ the 
geotropic effect is successfully neutralised by rotation on the 
klinostat, since in this case the geotropic sensitiveness of the 



DIA-HELIOTROPISM AND DIA-GEOTROPISM 645 

different flanks is the same ; but this, as will appear from 
certain experiments to be described later, cannot be said of 
dorsi-ventral organs, for in these geotropic sensitiveness is 
different in the upper and lower halves. In my own experi- 
ments on the action of light, however, I shall be able to give 
results which are but little affected by geotropism. 

3. Effect of suctional activity and of turgescence. There 
are two other factors not hitherto taken into account which 
exert considerable influence in determining the attitude 
finally assumed by the leaf. These are the general condition 
of turgescence of the plant, and the limits of flexibility 
which characterise the particular responding organ. The 
first of these is, as we have seen, dependent on the suctional 
activity. Now, the mechanical response of a plant-organ is 
the result, as we already know, of the expulsion of water 
from the excited tissue ; but if the tissue be over-turgid 
expulsion of water is opposed, and the responsive movement 
is thereby reduced or abolished, as we have seen in the case 
of Mimosa in a condition of excessive turgor. The same 
phenomenon I have again seen manifested, in a remarkable 
manner, in the difference of the movements made by the 
leaves in response to light according as they were normally- 
or super- turgid. Thus, in a small plant of Artocarpus, 
grown in a pot, and in the autumn season, when the suctional 
activity was not great, the leaves responded to light by 
making a progressive angle with the vertical, so that, under 
the long-continued action of light, they first reached the 
horizontal position and then fell many degrees below it ; but 
in the rainy season, when they held themselves abruptly 
vertical in consequence of excessive turgor, the action of 
light produced little or no responsive movement. In the 
autumn season, again, the limited system of roots and rootlets 
possessed by this plant, when grown in a pot, allows it only 
a moderate degree of turgescence, and in this condition it 
readily responds to light ; but a large tree of the same 
species, growing in the open and possessed of a highly 
ramified and extensive root-system, will during the same 



646 PLANT RESPONSE 

season, its turgescence being great, maintain its leaves in a 
vertical position, but little affected by the action of light. 
From this it will also become clear that any influence, such 
as long maintenance under darkness, which modifies the 
suctional activity, is liable to render the response of the plant 
abnormal. 

4. Modification of effect by characteristic limits of flexi- 
bility. I shall here deal with another important factor in 
the determination of the final position assumed by the 
leaf that is to say, with the anatomical peculiarities which 
determine the limits of flexibility. Let us take the petiole 
bearing the terminal leaflet of a leaf of Desmodium. We 
now suppose this leaflet to be outspread, in such a position 
that its midrib is in a continuous straight line with the 
petiole. This upper line we shall know as the dorsal line. 
It consists of two parts or components, the laminal and the 
petiolar. In this particular position they form a straight 
line ; but the movement of the leaflet takes place with the 
point of junction as the hinge. We shall then distinguish 
that particular position of the dorsal line in which the laminal 
and petiolar halves are continuous and straight as neutral, 
and angular movements above or below will be measured 
accordingly. In the case of Desmodium^ when the terminal 
leaflet has reached the neutral position, it cannot, owing to 
the anatomical peculiarity of the joint, be bent further up- 
wards ; but it can be bent in the opposite direction, that is 
downwards, until the leaflet lies along the under side of the 
petiole, the curvature being then through 180. Thus the 
limits of flexibility of this leaflet may be expressed by the 

o 
formula : that is to say, its flexibility above the 

neutral position is o, and below, 180. Now, the petiole 
itself, in the case of Desmodium^ for example, may, and does, 
occupy many different positions with regard to the stem. 
Let us suppose it at a given moment to subtend an acute 
angle, being thus more or less vertical, and suppose the 
terminal leaflet to be horizontal. If light now acts from 



DIA-HELIOTROPISM AND DIA-GEOTROPISM 



647 



above, the leaflet will move continuously upwards till the 
lamina has reached the neutral position that is to say, till the 
midrib constitutes a straight prolongation of the petiole ; but 
if light act on the leaflet from below, it will bend downwards 
(fig. 259, a). If the petiole, however, at the beginning of the 
observation be horizontal instead of vertical, and the lamina 
be in the neutral position, then vertical light cannot, owing 
to the anatomical peculiarities of the pulvinar joint, carry the 
leaflet further above the dorsal line (fig. 259, b}. Or we may 



:/ J 

M/ 

J 





f> 



FIG. 259. Different Limits of Flexibility 

Vertical light on terminal leaflet of Desmodium causes (a) up movement 
till the dorsal line is a continuous straight line ; light applied below 
causes movement downwards below this neutral line through 180. 
In b is shown neutral position, after reaching which there is no further 
movement upwards. In c is shown movement of terminal leaflet of 
Erythrina indica upwards, under vertical illumination, through 180 
above the neutral line. 

again take as an example the terminal leaflet of Erythrina 
indica (fig. 259, c). The limit of flexibility is in this case 
represented by an angle of almost 180 above the neutral, 
whereas downwards its limit is about 90. The formula is 

1 80 
thus . Now, when this leaflet is acted upon by light 

90 

from above, it may become almost doubled upon the petiole 
upwards, just as we found the terminal leaflet of Desmodium 
to be almost doubled downwards. Hence we see that 
though the heliotropic effect of light is always the same, yet 



648 PLANT RESPONSE 

the position of the leaflet in space that is to say, its relation 
to the vertical line is largely modified not only by the limits 
which the anatomical structure of the laminal pulvinus or 
pulvinoid imposes upon its flexibility, but also by the angle 
which the petiole makes with the stem ; for there is, 
generally speaking, a second pulvinus or pulvinoid at the 
junction of the petiole and the stem, and this petiolar 
pulvinus is, in its turn, more or less sensitive to stimulation 
(P- 59)- We have thus obtained some idea of the anatomical 
elements, regarding the petiole and its joints, which enter 
into the complex question of the final position assumed by 
the leaf in space. 

Discrimination of the part played by heliotropism in 
the movement of the leaf. In studying the heliotropic 
effect we are concerned only with the action of light itself, 
and not with the resultant effect, due to various co-operating 
factors. When the action of each of these is definitely 
understood, it becomes a simple problem to understand the 
effects due to their combination. The difficulty of this 
investigation has hitherto lain, as already said, in the fact 
that, owing to the generally sluggish nature of respon- 
sive movements in ordinary leaves, a long time must be 
allowed to elapse before they become measurable, and during 
this long period other factors may become operative in 
unknown ways. The effect of light, however, can easily be 
discriminated by the use of the Optic Lever for record, for this 
allows us any degree of magnification which may be desired. 
Thus the natural curve gives us the resultant effect of all the 
pre-existing factors under normal tonic conditions, and its 
subsequent variations under the incidence of light at once 
exhibits the distinctive action of that stimulus. On the 
withdrawal of light, again, the recovery from the induced 
variation affords an additional corroboration of the inference 
that the variation itself had been due to the action of light. 
In consequence, moreover, of the delicacy of this means of 
continuous record, the characteristic effect can be detected 
in the course of a few minutes, thus eliminating the unknown 



DIA-HKLIOTKUJ'ISM AND DIA-GEOTROPtSM 



649 



changes which are liable to occur during the lapse of long 
intervals of time. The action of light is, generally speaking, 
so predominant over that of the subsidiary factors, that a 
high magnification of record is not necessary. Under these 
circumstances, that is to say under low magnification, the 
record before the application of light is practically a hori- 
zontal line. Under vertical illumination of the upper side of 
the leaf, then, deviation above this horizontal represents 
a positive heliotropic movement, while deflection in the 
opposite direction signifies the 
negative. 

But before I proceed to 
give details of my experi- 
ments, I shall adduce facts 
which will show that there is 
no inherent tendency in the 
leaves to move in such a 
manner as to place themselves 
at right angles to the light. 

Proof of absence of any 
specific dia-heliotropic ten- 
dency in leaves. A Des- 
modium plant was taken, and 
the petiole fixed horizontally, 

FIG. 261. Curve showing Time- 
relations of the Responsive 
Angular Movement of Terminal 
Leaflet of Desmodium under 
Light, as represented in the 
last figure 

The original angle of 45 between 
the surface of the leaflet and the 
direction of incident light was 
reduced to 13 in the course ot 
25 minutes. 





FIG. 260. Movement of Ter- 
minal Leaflet of Desmodium 
placing itself Parallel to 
Incident Horizontal Light 



the terminal leaflet being at an angle of 45 with the horizon. 
Sunlight was then made to strike it horizontally (fig. 260). li 
now the leaflet had a dia-heliotropic tendency, it is clear that 
its movement would be such as to increase its angle with the 
horizon to 90, thus bringing the light to strike it at right 



650 PLANT RESPONSE 

angles. Movement in the other direction that is to say, 
towards the decrease of the angle would, on the other hand, 
indicate that the effect of light was to induce the normal 
positive heliotropic response. From the record (fig. 261) it 
will be seen that the latter was the case that is to say, 
the leaflet moved continuously, tending to become parallel 
to the light, having in the course of twenty-five minutes 
moved from its position at an angle of 45 to one at 13 to 
the horizon that is to say, at an average rate of angular 
movement of about 1*3 per minute. 

The lamina not the perceptive organ. If, again, the 
object of the so-called dia-heliotropic movement had been 
the absorption by the upper surface of the lamina of the 
largest possible amount of light, it would have been neces- 
sary for the lamina to be the perceptive organ, determining 
its movement according to the direction of stimulus ; but 
that this is not the case may be demonstrated by subjecting 
the lamina alone to the action of light, and covering the 
pulvinus with a small opaque shield of black paper. On 
doing so all movement is found to be arrested. If now, on 
the other hand, the leaflet be covered with opaque paper, and 
the pulvinus be left exposed, the usual heliotropic movement 
is found to take place. This conclusively proves that, as 
regards the response of the leaf to light, the lamina is not the 
perceptive organ, and therefore that a supposed advantage to 
itself cannot be the important factor in determining its move- 
ment. That the lamina is not the perceptive organ has 
indeed been shown already in the case of ordinary leaves, 
when subjected to forms of stimulus other than light (p. 60). 
We saw this, for example, in the case of a leaf of A rtocarpus 
when subjected to electrical and thermal stimulation. It was 
in that case shown that since the lamina consisted of a mass 
of non-conducting parenchymatous tissue, local excitation 
might be caused by the incidence of stimulus, but could not 
be effectively transmitted to the distant pulvinus or pulvinoid 
by which the movement of the leaf was brought about. The 
only case in which such transmission is possible to some 



DIA-HELIOTROPISM AND DIA-GEOTROPISM 651 

extent in leaves is in the parallel-veined leaves of monocoty- 
ledons. This fact, that the lamina is not in general the per- 
ceptive organ with regard to stimulus, say of light, is still 
further demonstrated in the following experiment, performed 
on the leaflet of Erythrina indica. Sunlight was first applied 
locally to the lamina of this leaflet by means of a suitably 
inclined mirror, at the point in the record which is marked x 
(fig. 262). It will be noticed that during ten minutes of such 
application not the slightest responsive movement was in- 
duced. Keeping everything else the same, the light was 



5 IQ f 15' 20' 25' 30' 35' 40' 45' SO' 55' 



FIG. 262. Record showing that Lamina is not the T'erceptive Organ 

Light applied vertically at x on lamina of Erythrina indica, and con- 
tinued for ten minutes, produces no response ; but when applied on 
the pulvinus at the arrow ( \ ) there is immediate positive response. 
Dotted line shows positive after-effect and recovery on cessation of 
light. 

now shifted by a movement of the mirror, and thrown 
directly on the pulvinus, at the point in the record which is 
marked with an arrow (t). It will be observed that the leaf 
began to respond immediately by a movement towards the 
light, and in the course of seventeen minutes' exposure its 
tip moved through a distance of 20 mm. or at an average 
rate of a little over I mm. per minute. On the withdrawal 
of light the movement persisted, owing to the positive after- 
effect, for a period of seven minutes, after which recovery 
began. The marked difference between the quiescent con- 



652 PLANT RESPONSE 

dition of the leaflet during the ten minutes' exposure of the 
lamina to the action of light, and its energetic movement 
immediately on the application of light to the pulvinus, 
shows once more, in a striking manner, that it is the latter 
organ and not the lamina whose perception of light is effec- 
tive in initiating the responsive action. 

Principal types of the response of leaves to stimulus 
of light. I shall now proceed to show that the directive 
effect of light on leaves is very definite. In studying the 
response of anisotropic organs that is to say, plagiotropic 
shoots and dorsi-ventral pulvinated leaves we have seen 
that there are two extreme types of response, of which the 
first is exhibited by organs possessing feeble transverse con- 
ductivity, and the second by those in which the transverse 
conductivity is great, and the lower side the more excit- 
able. In the first of these types, light acting on the organ 
from above gives rise to a positive heliotropic response. In 
the latter, long-continued application of strong light from 
above gives rise to internal diffusion of stimulus, causing 
concavity of the more excitable lower half, with the result of 
negative heliotropic response. Intermediate between these 
we have seen that there are cases of the equal excitation ot 
the upper and lower halves of the organ, bringing about the 
so-called dia-heliotropic position. As examples of the two 
extreme types within which lie the responses of all ordinary 
leaves I shall here give records of the movements of leaves 
of Mango (Mangifera indicd] and of A r tocarpus. 

Positive type of response : Mango. The leaves of 
this plant when young are bent abruptly downwards by the 
sharp curvature of the short petiole. In the course of a week 
or so, however, they rise, and gradually attain a position at 
or above the horizontal, a process in which the action of 
light is an important agent This will be seen from the 
following record of the movement of a Mango leaf when 
acted upon by sunlight from above (fig. 263). The record 
before the application of light was practically horizontal, 
showing that there was little natural movement ; but the 



DIA-HELIOTROPISM AND DIA-GKOTROPISM 



653 



application of light induced an energetic movement upwards 

that is to say, a positive heliotropic response, the tip of the 

leaf moving through a distance of 35 mm in the course 

of sixty-five minutes 

that is to say, at an 

average rate of '53 mm. 

per minute. On the 

cessation of light there 

was a slow recovery, 

which was found to be 

only partial. 

Negative type of 
response : Artocar- 
pus. As an example 
of negative response by 
the internal diffusion 
of stimulus, due to the 
high transverse con- 
ductivity of the tissue, 
and the greater excita- 



10' 20' 30' 40' 50' 60'70' 80' 90' 



FIG. 263. Positive Heliotropic Movement of 
Leaf of Mangifera indica under Sunlight 
acting from Above 



Dotted line shows recovery on cessation of 
light. 



bility of the lower half 
of the pulvinoid, I give 
here two records, from 
leaves of different ages, borne on the same plant, under the 
action of strong sunlight from above (fig. 264). The upper of 
these two records was taken from a young leaf, which was 
second in order from the top of the shoot, and the lower from 
the fourth. It is generally found that motile sensitiveness is at 
its greatest in leaves which are neither too young nor too old. 
In the present experiment the upper of the two leaves, 
which was very young, gave a negative heliotropic response, 
the tip moving through 1 5 mm. in the course of eighty 
minutes, or at an average rate of almost '2 mm. per minute. 
In the case of the lower leaf the rate of the responsive move- 
ment was more rapid, being on an average *5 mm. per minute. 
As regards leaves, then, their responsive movements under 
light have been shown to be very definite, and simply 



654 



TLA XT RKSl'ONSE 



determined by the fact that it is always the more excited 
side of the motile organ that undergoes contraction and 
concavity. That various types of this effect arise is due only 
to the unequal excitabilities of the two halves, and to the fact 

that the stimulus remains 
in some cases localised, 
while in others, owing to 
the better transverse con- 
ductivity of the tissue, it 
becomes internally diffused. 
The position ultimately 
taken up by the leaves is 
thus determined primarily 
by the action of light, and 
secondarily by that of 
various subsidiary factors, 
which are: (i) the natural 
movement of the organ, 
due to hyponasty or epi- 




FIG. 264. Negative Heliotropic Response 
of Leaves of Artocarpus under Sunlight 
acting from Above 



The upper record exhibits the response of 
a very young leaf, second in order from 
the top of the shoot. The lower record 
shows that of an older leaf, fourth in 
order on the same shoot. 



nasty ; (2) the differen- 
tial excitability of the 
two halves of the dorsi- 
ventral organ under the 

stimulus of gravity ; (3) the turgescent condition of the plant, 
as determined by its suctional activity ; and (4) the limits of 
flexibility of the organ, as determined by its anatomical 
peculiarities. It is out of the innumerable possible combina- 
tions of these factors that the variety of attitudes ultimately 
assumed by the leaves directly arises. 

With regard to the nature of geotropic action on leaves, 
it will be shown in the next chapter that here also, as in the 
case of dia-heliotropism, the assumption of a dia-geotropic 
property is unnecessary ; for the observed effects are ex- 
plained by the fact, which I shall demonstrate, that a dorsi- 
ventral organ possesses differential geotropic sensibility. 



DIA-HELIOTROPISM AND DIA-GEOTROPISM 655 

SUMMARY 

The position ultimately assumed by a leaf is determined 
by heliotropic action and other subsidiary factors. 

These subsidiary factors are: (i) the natural movement 
of the organ, due to epinasty or hyponasty ; (2) the differ- 
ential excitability to gravitational stimulus of the two halves 
of the organ ; (3) the effect of suctional activity and of 
turgescence ; and (4) the modification of effect by the 
characteristic limits of flexibility of the organ. 

As regards the action of light, the lamina is not the 
perceptive organ, and there is no specific dia-heliotropic 
sensitiveness possessed by the leaves. 

The sensibility of the pulvinoid of a leaf is essentially 
similar to that of a pulvinus. There are two main types of 
response given by pulvinoids : (i) that exhibited when the 
conductivity of the organ is feeble, and vertical light induces 
movement upwards, or positive response ; and (2) that exhibited 
when conductivity is great, and the lower half is the more 
excitable, inducing movement downwards, or negative response. 

The various attitudes assumed by the leaves are the joint 
effects of these responsive actions, and their modification by 
epinasty or hyponasty, the differential action of gravity, the 
turgescent condition of the plant, and the limits of flexibility 
of the pulvinus or pulvinoid. 



CHAPTER XLVII 

v 

TORSIONAL RESPONSE TO HELIOTROPIC AND GEOTROPIC 
STIMULUS: AUTONOMOUS TORSION AND ITS VARIATIONS 

Torsional effect Method of recording torsional response Torsional response 
under the lateral action of light Torsional response to other forms of lateral 
stimulation Torsional response of compound strip of ebonite and stretched 
india-rubber Modification of torsional response by artificial variation of the 
relative excitabilities of the two halves Laws of torsional response Demon- 
stration of differential geotropic excitation in a dorsi ventral organ Torsional 
response to lateral geotropic stimulation Modification of torsional geotropic 
response by artificial variation of differential excitability Autonomous torsion: 
effect of temperature Effect of light Effect of electrical current Effect of 
gravity The twining of stems. 

IT has been shown in the course of the last chapter that the 
movement of the leaf in response to light constitutes a 
simple instance of that general reaction of plant tissues to 
stimulus with which we have now become familiar, and that 
no specific sensibility requires to be postulated in order to 
account for it. 

Torsional effect. There is one effect, however, which 
has hitherto appeared to be inexplicable, except on the 
supposition that some such specific sensibility had actually 
been acquired by the leaf for the definite purpose of sub- 
serving the advantage of the plant by placing the upper 
surface of the lamina at right angles to incident light. A 
leaf when struck laterally by light undergoes a torsion, which 
carries the upper surface of the lamina into such a position 
that it faces the light No result could seem at first sight 
more conclusively to support the theory of dia-heliotropic 
sensibility. Before going further into this question, I shall 
give records of the actual effect observed. In fig. 265 is 
shown a curve which exhibits the increasing torsion induced 



HELIOTROFIC TORSION AL RESPONSE 657 

by lateral application of sunlight in the terminal leaflet of 
Desmodium. A light index was attached transversely to 
the lamina, by means of which, and with the help of a pro- 
tractor, the gradually increasing torsion was measured at 
definite intervals of time. It will be seen from the curve 




FIG. 265. Torsional Response of Terminal Leaflet of Desmodium under 

the Action of Lateral Sunlight 

Abscissa represents time in minutes, and ordinate angular movement 

in degrees. 

thus obtained, of which the abscissa represents time, and 
the ordinate the induced angular torsion, that in this 
case, within twenty-five minutes, a torsion of 14 had been 
induced. 

Method of recording 1 torsional response. For the 
purpose of certain investigations, presently to be described, 
in connection with this torsion, it became necessary to devise 
special experimental arrangements by which a continuous 
record of the rate of torsion and its variations could be 
obtained. The mode in which this was accomplished will 
be understood from the illustration in fig. 266. A mirror, 
carried on a light spring-clip made of aluminium, is slipped 
over and attached to the petiole, at a short distance from the 
pulvinus or pulvinoid, which is the seat of the torsional 
movement. As the object is to record only the torsion, the 

U U 



658 



PLANT RESPONSE 



vertical up and down movement, if there be any, is prevented 
by a lateral support, which has at its end a smooth bent rod 
of glass, in the concavity of which the free petiole rests. If 
there be any responsive torsional movement, a spot of light 
reflected from the mirror will now undergo a vertical deflec- 
tion. This is, for convenience of the record, converted into 
lateral by reflection from a second mirror suitably inclined. 
The response record is then taken in the usual manner on 




KIG. 266. Torsion- recorder 

M, mirror slipped over petiole by light aluminium clip behind ; G, bent 
glass piece supporting petiole to prevent vertical movement. 

a revolving drum. The absolute value of the angular 
movement shown in the record can be determined from a 
knowledge of the distance from the mirror of the recording 
drum. 

Torsional response under the lateral action of 
light. The pulvinus of the leaf, say of Mimosa, is now 
stimulated by throwing upon it a horizontal beam of light, 
which strikes it laterally. By the word lateral is here meant 
either the right or left flank of the pulvinus or pulvinoid, 
consisting of part of the upper and part of the lower aniso- 
tropic halves. This causes a responsive torsion, by which the 
petiole is rotated, the tendency being for the upper, or less 
excitable, half to face the stimulating agent. The up curve, 
then, in this and the following records will represent a torsional 
movement by which the less excitable upper side is made to 



HELIOTROPIC TORSION AL RESPONSE 



659 




face the source of stimulus. In the present case (fig. 267) 

we see that the responsive torsion took place during the 

application of light, and that on the cessation of stimulus 

there was a positive after-effect, followed by recovery. If 

light were applied laterally on the 

opposite flank, the torsion would be 

found to take place in the opposite 

direction, the law which determines 

such movement being, as said before, 

that the less excitable is always 

turned towards the stimulus. In 

connection with this, we have to 

notice, in the first place, that the 

responsive torsion takes place not 

when the lamina, but when the 

pulvinus, is acted on. laterally by 

light ; and, secondly, the particular 

torsion in question results from the 

differential action of stimulus, by its 

lateral application to a complex 

organ, the lower half of which is the 

more excitable. 

Torsional response to other forms of lateral stimu- 
lation. The supposition that this torsional response is due 
to a specific sensibility to light, evolved for the advantage of 
the plant, will be found entirely untenable if it can be shown 
that the same movement is manifested under the same 
conditions in response to other forms of stimulus. Thus, on 
bringing a heated wire near that side which was previously 
excited by light, I obtained exactly the same torsional 
response of the same sign. Still more interesting is the 
excitation of the same lateral side by chemical stimulus, say 
a strong solution of common salt. In the record given 
(fig. 268) we see how similar in every way are this response 
and that evoked by light. The upward arrow (t) indicates 
the application of chemical stimulus to the same flank as had 
previously been stimulated by light, of which the record is 



FIG. 267. Torsional Re- 
sponse of Leaf of Mimosa 
when Laterally Stimu- 
lated by Sunlight 

The ordinate gives the ab- 
solute angular movement 
in degrees. The dotted 
line shows positive after- 
effect and recovery on 
stoppage of light. 



U u 2 



66o 



PLANT RESPONSE 



given in fig, 267. The downward arrow (I) indicates the 
application of the chemical solution to the opposite flank. It 
will be noticed that we have in consequence, in the dotted 
portion of the record, a reversal of the first torsional response. 
Torsional response of compound strip of ebonite and 
stretched india-rubber. Enough has already been said to 
demonstrate the fact that the torsion induced in leaves by the 
lateral application of light is not due to any specific sensibility 
to light as such. I shall next therefore proceed to show that 
the same torsion is the mechanical result of the differential 





FIG. 268. Torsional Response of Leaf 
of Mimosa to Lateral Chemical 
Stimulus 



FIG. 269. Torsional Re- 
sponse of Complex Strip 
of Ebonite and India- 
rubber under Lateral 
Action of Strong Radia- 
tion 



Upward arrow (f) indicates moment 
of application of stimulus to one 
flank; (|) indicates application of 
stimulus to opposite flank, with effect 
of reversing torsional movement. 

contraction of a complex organ, which is fixed at one end, 
and subjected to lateral stimulation. I have in a former 
chapter described an artificial model of the pulvinus of 
Mimosa^ which consisted of a compound strip, the upper half 
of which was ebonite, and the lower the more contractile 
stretched india-rubber. If such a strip be held securely at 
one end in a clamp, and if the lateral flank, consisting half 
of ebonite and half of india-rubber, be subjected to the 
strong action of light, records being taken in the usual 
manner, it will be found that a torsional response takes place 
which is in every respect similar to that of the pulvinus of a 



HELIOTROPIC TORSION AL RESPONSE 66 1 

leaf, the less contractile ebonite being turned so as to face 
the light (fig. 269). 

Modification of torsional response by artificial varia- 
tion of the relative excitabilities of the two halves. 

I find it necessary to go still further into this subject of 
torsional response, as by its means I have been enabled to 
solve a question of very great importance, that, namely, of the 
different excitabilities of the two halves of the anisotropic 
organ to geotropic stimulus. The fact that it is the differential 
character of the excitabilities of these two halves that brings 
about the torsion of a dorsi-ventral organ under lateral stimu- 
lation may be still further established in a very interesting 
manner by inducing artificial variation in the existing excita- 
bilities. The torsion depends, as said before, on the difference 
of excitability as between the two. If, then, we could render 
the lower and more excitable half gradually less and less 
excitable, till its differential excitability had disappeared, the 
organ would thus have been rendered virtually radial. The 
torsional effect might then be expected to vanish, and a 
simple curvature towards stimulus to result, without torsion, 
as in the case of other radial organs. Let us next suppose 
the reduction of excitability to be carried still further, till the 
lower half of the pulvinus have been rendered less excitable 
than the upper. On the theory of torsional response which 
has just been advanced, it is the less excitable lower half 
which should now twist round to face the stimulating light. 
In other words, there would then be a reversal of the original 
torsion. Thus, as the excitability of the lower half becomes 
gradually reduced, the intensity of the normal positive 
torsional response by which the upper half was made to face 
the stimulus would be gradually first decreased to zero, and 
then reversed to negative, as the excitability of the lower 
became first equal to, and then less than, that of the upper half. 
If, on the other hand, the excitability of the upper half 
be reduced, the existing differential excitability as between 
upper and lower would then be still further increased, and 
the intensity of the positive torsional response would be 




662 PLANT RESPONSE 

enhanced. All these conclusions will be found exactly 
verified in the records given in fig. 270. In the first part of 
the left-hand figure we see the normal positive torsional 
response of Mimosa under the lateral action of light. The 
excitability of the lower half of the pulvinus was then 

reduced by the local appli- 
cation of chloroform, at the 
moment represented by the 
arrow from below (t). It 
will be seen that the response 
now undergoes reversal, 
owing to the fact that it 
is the upper side that is 
the relatively more excit- 

Fio. 270. Records showing Modifica- able. Ill the figure to the 
tion of Torsional Response under . , , , 
Induced Variations of Differential ri g ht > again, IS Shown the 
Excitabilities in Pulvinus of Mimosa effect Oil the torsional re- 
in left-hand figure the normal torsion is sponse of the leaf, of increas- 
seen to be reversed by application of 

chloroform to the lower half (f), tng the already existing 

reversing the differential excitability differenc e as between the ex- 

of the organ. In the right-hand 

record the normal responsive torsion citabilities of the two halves, 

is enhanced by application of chloro- , . . r . 

form to the upper half (|), increasing hen that of the upper IS 

the natural differential excitability of reduced by the local applica- 

the organ. . .,, 

tion of chloroform. It will 

be seen that this application as marked by the arrow from 
above (|) caused an enhancement of torsional response, as 
seen in the greater steepness of the curve. 

Laws of torsional response. From these experiments 
we arrive at the following laws : 

1. An anisotropic organ, when laterally excited, under- 
goes torsion, by which the less excitable side is made to face 
the stimulus, 

2. Stimulus remaining constant, the intensity of torsional 
response increases with the differential excitability. But when 
the original difference is in any way reduced or reversed, 
the torsional response undergoes concomitant diminution or 
reversal. 



GEOTROPIC TORSIONAL RESPONSE 663 

3. An organ which, under lateral stimulation, responds by 
torsion, is always physiologically anisotropic, and the side 
which is made to face the stimulus is the less excitable. 

Demonstration of differential geotropic excitation in 
a dorsi-ventral organ. From this experimental demon- 
stration, then, we have obtained a new means of discrimi- 
nating the relative . excitabilities of the two halves of an 
organ, since that side which is turned by the responsive 
movement to face the given stimulus is relatively the less 
excitable to it. It will be remembered that we found that 
the reason why certain dorsi-ventral organs showed a tendency 
to assume a horizontal position under the action of vertical 
light was not that they possessed dia-heliotropic sensibility, 
but was due to the differential excitability of the two 
halves under stimulus in general, including that of light. 
Again, in the case of the action of gravity, it is found that 
such organs exhibit a similar tendency to place themselves 
horizontally ; and the assumption of a specific dia-geotropic 
sensibility is not necessary, if it can be shown that the upper 
and lower sides are unequally sensitive to this stimulus 
also. We may first take an ideally simple case. We have 
seen that in a radial apogeotropic stem, when laid hori- 
zontally, it is only the upper 4ialf that is effectively 
stimulated by geotropic action ; and there was reason to 
believe that this was due to the fact that it was the inner 
tangential wall of the upper side in contrast to the less 
excitable outer wall of the lower side that was excited by 
the statolithic or other influence of weight (pp. 495, 503). If 
now, for any reason, the excitability of the upper half of the 
horizontally placed radial organ be abolished, the geotropic 
response of that effective half will disappear, and the organ 
will remain horizontal, as if unaffected by stimulus of gra- 
vity. This state of things we have already realised, when 
the excitability of the upper half was artificially diminished 
by the local application of cold, and geotropic response was 
seen to be arrested (p. 504). Now, a horizontally placed 
radial organ which has been rendered unequally excitable 



664 PLANT RESPONSE 

in the two halves, by the reduction of excitability of the 
upper, is virtually a plagiotropic or dorsi-ventral organ, and 
we can thus see why a true plagiotropic organ, laid hori- 
zontally, has a tendency to remain in that position. It 
would remain in that position absolutely if the excitability 
of the outer tangential wall of the lower side were actually 
zero, and the general excitability of the upper half of the 
dorsi-ventral organ had also completely vanished ; but if 
these two values were not both zero, various effects would 
occur, according to the relative differential excitabilities of 
the two halves. 

I shall now describe the line of investigation by which 
it is possible to demonstrate the fact that the two halves of 
an anisotropic organ possess different excitabilities with 
regard to geotropic stimulus ; but before entering upon this, 
it is necessary to obtain a clear idea of the direction of 
response, in relation to direction of the force which causes 
stimulation of gravity. In the case of an apogeotropic stem, 
laid horizontally, we have vertical lines of force striking the 
organ from above, and the curvature induced makes it turn 
upwards to meet, as it were, these lines of force. We have 
here an example which is analogous to the response of the 
organ to the rays of light, in which the responsive move- 
ment consists in a similar curvature upwards to meet them. 

The fact that the upper and lower halves of an aniso- 
tropic organ are unequally excited by light has just been 
demonstrated by means of torsional response, where it was 
shown that lateral excitation induced a twisting movement 
by which the less excitable was made to face the incident rays. 
By a similar torsional response of the anisotropic organ we 
are able not only to demonstrate the unequal geotropic 
excitability of the upper and lower halves, but also, by 
noting which of the two is made to face the lines of gravita- 
tional force, to determine which it is that is the less excitable 
to this particular form of stimulus. 

Torsional response to lateral geotropic stimulation. 
For this experiment I took a leaf of Erythrina indica, whose 



GEOTROPIC TORSIONAL RESPONSE 665 

pulvinus I find to be very sensitive to geotropic action. 
Under normal conditions the leaflets of this plant place 
themselves horizontally. I now took one of these, say the 
terminal, and adjusted the plant so that its lateral edge was 
vertical. The dorsal and ventral halves of its pulvinus were 
thus equally subjected to geotropic action. This experiment, 
it is to be noted, was carried out in a dark room, where 
the only stimulus acting was geotropic. It will be seen, from 
the first part of the left-hand record in fig. 271, that under 
geotropic stimulus a torsional response was here obtained, 
which was exactly similar to that given under the action of 
light in fig. 270. The angular movement was in this case 
at the rate of about 'H per minute, and it was the dorsal 
surface of the organ that was eventually turned upwards 
that is to say, so as to face the lines, of force of gravity. 
This experiment, then, conclusively demonstrates the fact 
that the two halves of the dorsi-ventral organ are unequally 
sensitive to geotropic stimulus, and that it is the upper which 
is the less sensitive. 

Modification of torsional geotropic response by arti- 
ficial variation of differential excitability. The fact that 
the upper half is, under normal conditions, the less excitable 
to geotropic stimulus is capable of further and striking 
demonstration, by the tests of reversal, and acceleration of 
torsional response. I have already explained, as will be 
remembered, that if the existing difference of excitability 
were reversed, the direction of torsional response would also 
be reversed ; and that if, on the contrary, the difference were 
by any means increased, the rate of torsional movement would 
be correspondingly accelerated. In the first part of fig. 271 
is seen the natural torsional response of the terminal leaflet 
of Erythrina indtca. The excitability of the lower half of 
the pulvinus was now abolished, by the local applica- 
tion of chloroform, at the point marked with an upward 
arrow, and the torsional response to geotropic stimulation 
was thus found to be reversed, from plus '11 to minus -05 
per minute. Hence it was now the lower half of the pul- 



666 



PLANT RESPONSE 



vinus artificially rendered the less excitable of the two 
that was found turning upwards, to face the rays of incident 
force of gravity. 

The converse of this experiment would consist in still 
further reducing the excitability of the upper half of the 
pulvinus, in which case the normal differential excitability 
of the two halves would be increased, and the geotropic 
response enhanced. The right-hand record in fig, 271 shows 

that this is what actually 
occurs. The first part of 
the record displays the 
normal torsional response, 
and that which follows, 
after the local application 
of chloroform on the 
upper surface indicated 
by the arrow from above 
exhibits, by its in- 
creased steepness, the en- 
hanced character of this 




FIG. 271. Records showing Torsional 
Response to Geotropic Stimulus and 
Induced Modifications in Terminal Leaf- 
let of Erythrina indica 



response. 

We have thus seen, 



In the figure to the left is seen the normal 
torsion under lateral geotropic action, 
and its reversal when the differential 
excitability of the organ is reversed by b meang of torsional re _ 
the application of chloroform to the J 
lower half (f). In the figure to the 
right is seen the enhancement of the 
torsional response when the natural 
differential excitability is increased by 
the application of chloroform to the 
upper half of the organ 



sponse, that the differ- 
ential character of the 
excitability of the two 
halves of an anisotropic 
organ under the stimulus 
of light is in every way paralleled by their differential ex- 
citability to the stimulus of gravity. The geotropic response 
of the organ, as a whole, is neither simple positive nor 
negative, but may be more fittingly described as differential. 
And as we saw with regard to photic stimulus that there was 
no need for the assumption of any specific dia-heliotropic 
sensibility in plagiotropic stems or in leaves their move- 
ments being fully accounted for by the mechanical con- 
siderations arising out of their differential excitability to 



AUTONOMOUS TORSION 667 

stimulus in general so similarly with regard to geotropic 
stimulus, there is no necessity for the assumption of any 
specific dia-geotropic sensibility. 

Autonomous torsion. We have now seen how torsional 
response is induced in an anisotropic organ. We have next 
to deal with that interesting class of phenomena which 
consists of the natural torsional movements of growing 
organs. It is to be remembered that, in referring to these 
torsional movements, we mean the torsional growth-move- 
ments of certain stems themselves about their own axes. 
The term positive torsion in such cases means a movement 
which appears, when looked at from above, to take place in 
the same direction as the movement of the hands of a watch, 
the term negative being used in the contrary sense. 

Such torsional movements are found, to occur in the stems 
of climbing plants. In some of these they are of a positive, 
and in others of a negative character, while in still others 
again they alternate, the natural torsion being, say, positive 
at one period, and negative at another. 

Now, we have seen in the case of leaves that,, owing to 
different rates of growth in the antagonistic upper and lower 
halves of the petiole, autonomous epinastic or hyponastic 
movements occur in a rectilinear direction. When the 
growth of the upper half is predominant, the movement of 
the organ is downwards, and vice versa. More complex, 
however, is that case in which the line of maximum growth 
is a spiral revolving about an axis, thus bringing about a 
growth-movement which causes a torsion of the organ round 
that axis. In the case of epinasty and hyponasty, it was 
said that the rectilinear movement induced was due, not to a 
total absence of growth on either side, but to the relatively 
greater growth of one of the antagonistic halves. Similarly, 
in these torsional movements we have to remember the 
existence of antagonistic elements in the stem, and it is the 
predominant growth of one of these, the right-handed or left- 
handed, that brings about the resultant positive or negative 
torsion. 



668 PLANT RESPONSE 

Effect of temperature on autonomous torsion. In 
order to take a record of these torsional movements, a stem 
is taken and held securely fixed at one end say the lower 
the other being left free. On this free end, the mirror, 
by the reflected light from which the record is to be made, 
is attached ; and a long thread from above is tied to the tip 
of the specimen, in order to prevent its falling to one side 
by its own weight. The limpness of this supporting thread 
allows the torsional movements of the specimen to occur 
without hindrance. 

In taking records of the natural torsional response of 
plants at various temperatures, I have obtained results which 
are in a general way similar to the records made of longi- 
tudinal growth-response (p. 446) that is to say, the response 
is enhanced up to the optimum, which is at or near 35 C. 
Above this, response is diminished. These facts will be seen 
from the following table, which gives the absolute rate of 
the angular movements of the torsional response at different 
temperatures. 

The specimens employed in these and the following 
experiments were climbing stems of Parana paniculata> a 
plant belonging to the Campanulacece, which normally ex- 
hibits a strong negative torsion. 

TABLE GIVING THE ABSOLUTE RATE OF ANGULAR MOVEMENTS 
OK TORSIONAL RESPONSE AT VARIOUS TEMPERATURES 



Temperature 



28 C. 
32 C. 
35 C. 
37 C. 



Torsional movements 



O9 per minute 

18 

282 

18 



O 



P 



An interesting effect was observed, however, in this case 
of torsional response, when the temperature was raised 
above 43 C., which was apparently different from what 
occurred in ordinary longitudinal growth-response at the 
same temperature. It was found in the latter case that at 
44 C. or thereabouts growth underwent an apparent arrest ; 



AUTONOMOUS TORSION 669 

but this was shown not to be due to any actual arrest 
brought about by heat-rigor, for the rhythmic activity 
induced at this high temperature was even greater than 
before (p. 432). Now, in the case of torsional response, I find 
that though the rate of torsion undergoes a continuous 
decrease up to 42 C. or 43 C., yet beyond this an unex- 
pected increase is induced. Thus in the experiment just 
described, the torsional movement at 45 C. was at least 
temporarily enhanced to *8 per minute. This phenomenon 
may be due to abnormal relaxation at these high temperatures. 
Or there is another possible explanation. It has been said 
that the resultant torsion is due to the differential growth- 
activities of two antagonistic elements. Now, it may be that 
at 44 C. or thereabouts the growth of the less excitable of 
these antagonistic elements may undergo the same kind of 
arrest as we have seen to occur in the longitudinal growth of 
a radial organ. The sudden increase observed at and above 
44 C. in the rate of natural torsion may then be due to the 
withdrawal of the resistance previously offered by the growth- 
activity of the antagonistic element. 

Effect of increased suctional activity. Another in- 
teresting point, in connection with the occurrence of auto- 
nomous torsion, lies in the fact that its rate is enhanced by 
any means which tends to increase internal energy. One 
example of this has just been seen in the effect induced by 
rise of temperature. Another is found in the application of 
warm water to the base of the specimen, a process which has 
already been shown, in the case of radial organs, to cause, by 
means of the consequent increase of suctional activity, a 
sudden enhancement of the rate of autonomous growth 
(p. 430). I find, similarly, that the application of warm 
water to its base enhances the torsional response of a torsion- 
ing plant. 

Effect on natural torsion of unilateral application of 
light. We have seen that when an anisotropic organ is 
laterally excited by external stimulus, a response torsion 
takes place, by which the less excitable side is made to face 



6;o 



PLANT RESPONSE 



the stimulus. Now, in a naturally torsioning organ, such an 
induced torsional movement must obviously be opposite in 
direction to the natural movement caused by internal energy. 
This will be found to be illustrated in the modification in- 
duced in autonomous torsion by the unilateral application of 
light, as shown in fig. 272, where the first part of the curve 
shows the normal negative torsion of the plant. At the 
point marked with the upward arrow, light from a thirty-two 
candle-power electric lamp was allowed to strike the stem 
from one side. It will be seen that the excitatory effect of 
this external stimulus is first exhibited in the retardation of 




FIG. 272. Retardation and Reversal of Normal Torsional Movement 
by the unilateral Action of Light in Parana fanifiila/a 

Light applied at f retards, and in the course of two minutes reverses, the 
normal movement of negative torsion, making it positive. On cessation 

of light at ( f \ there is recovery of the original negative torsion. 



the normal rate of torsion, culminating in its abolition, suc- 
ceeded by actual reversal of direction. The normal negative 
was thus converted into positive torsion. On the cessation 
of the external stimulus the normal torsion was gradually 
resumed. The effect described is best observed in vigorous 
specimens in which the natural torsional movement is fairly 
strong. 

In studying the action of light on autonomous longi- 
tudinal growth, we found that when a series of responses to 
the action of light were recorded, the first effect of incident 



AUTONOMOUS TORSION 671 

light, if the specimen were in a sub-tonic condition, would 
be, by increasing the internal energy, to give rise to an 
enhanced rate of growth ; but when the normal tonic 
condition had been attained by absorption of light, the 
subsequent responses would be by a movement opposite to 
that of growth-elongation that is to say, a contraction or 
retardation of growth. In experimenting on the effect ot 
light on the autonomous torsional movements of the stems 
of certain specimens of Ipomcea y I have met with results 
exactly parallel that is to say, the natural rate of torsional 
movement, in this case negative, was transitorily enhanced 
by the first incidence of light, but declined and underwent 
reversal after continued action of stimulus. In the second 
and subsequent responses this preliminary enhancement was 
found to have disappeared, the effect o light now being a 
retardation and subsequent reversal of the natural torsional 
movement. 

Effect of electric current. I have often observed a 
very interesting effect, as induced by a constant current 
flowing along the length of the organ. During the continu- 
ance of the current the normal torsional response is first 
decreased and then reversed. On the cessation of current 
there is recovery and restoration of the normal direction of 
torsion. If the current maintained be strong or long con- 
tinued, the induced reversal may become more or less 
persistent. Such an induced reversal of torsion, moreover, 
is independent of the direction of current. 

Effect of gravity. I shall next describe a series of 
effects which I have only been able to obtain with any 
degree of certainty under favourable conditions. Where 
natural torsion is feeble the retarding effect of gravity which 
I am about to describe is such as to arrest torsion, and it is 
difficult to decide whether such arrest is accidental or in- 
duced. When the specimen, on the other hand, is too 
vigorous, the retardation is not easily observed, probably 
because it tends to be masked by the natural movement. In 
practice, therefore, the best specimens are those characterise^ 



6/2 PLANT RESPONSE 

by moderate vigour and uniformity of natural torsion during 
a considerable length of time. 

In studying the effect of geotropic stimulus on radial 
organs, we have seen that its effective intensity was greater 
when the specimen was inclined at an angle of 135 to the 
vertical than when at 90 (p. 501). It occurred to me, 
then, that the effect of geotropic stimulus might possibly be 
detected, by means of variations induced in natural torsional 
response at different angles of inclination. Thus, we might 
first take a record of the normal rate of torsion, occurring 
when the tip of the organ was in its natural position upwards. 
We might next take a record of torsion, the stem being held 
horizontal ; and we might finally take a record with the tip 
held vertically downwards. 1 The effect of geotropic stimulus 
might then be seen in the retardation induced in the normal 
torsion. I give here a summary of results obtained from two 
different specimens of Parana paniculata, and from a specimen 
of Ipomcea. 

In the first case, the record taken in the normal upright 
position gave a rate of movement of negative torsion through 
minus twenty-three divisions of the scale per minute. The 
record of torsion in the horizontal position was found to give 
a very much diminished rate, being now only minus seven divi- 
sions per minute ; and finally, with the tip held downwards, 
the torsional movement of the specimen was found to have 
undergone an absolute reversal to positive that is to say, the 
tip now moved with the hands of a watch at a rate of plus 
five divisions of the scale per minute. 

In the second case, the normal negative torsion in the 
erect position was at the rate of minus twenty divisions per 

1 Here we must bear in mind, with regard to the effective angle of inclination, 
a certain difference as between radial and torsioning organs. In the former, the 
line of growth may be taken as vertical, and coincident with the axis, whereas in 
the latter it is spiral, or inclined at some undetermined angle to the axis of the 
organ. Hence any inclination of the growth-line of a radial organ is measured 
by the angle between the organ and the vertical. In the case of the torsioning 
organ, however, when it is held vertically downwards, its growth-line makes an 
angle not of 180 to the vertical, but of 180 less by the degree of its inclination 
o the axis. 



ANTONOMOtJS TORSION 

minute. This was reduced in the horizontal position to 
minus fourteen divisions per minute, and in the vertically 
downward position it was found to have undergone reversal 
\& plus nine divisions per minute. 

In the next series of experiments I took records from 
specimens held alternately up and down and up again. 
This was, done in order to eliminate the effect of any chance 
variation. The specimen employed was Ipomcea. The tor- 
sional response in the first up position was at the rate of 
minus sixteen divisions per minute. When held in the in- 
verted position, the rate was found -reduced to minus ten 
divisions per minute, and when once more placed in its 
normal vertical position, the Ipomcea exhibited an increased 
rate of movement that is to say, the torsion now took place 
at the rate of minus twenty divisions of the scale per minute. 

All these results tend to show that the action of stimulus 
of gravity is to retard the autonomous torsion, this effect 
being at a maximum when the specimen is in a position at 
1 80 from the vertically upright. 

The twining of stems.- The twining movements of 
certain stems are the result of various contributing factors, 
the relative values of which may differ in different cases. 
One such factor, suggested by Von Mohl and denied by 
others, may lie in the irritability of the stem to the contact 
of its support. Such response to unilateral pressure was found 
to occur in various organs (p. 497), and probably plays an 
important part, in some cases, in the phenomenon of twining. 

Some connection would also seem to exist, in many in- 
stances, between autonomous torsion and twining. In the 
first place, most of the twining plants also exhibit autonomous 
torsion. Again, just as we have various types of torsioning 
organs some characterised by positive, others negative, and 
others again by positive alternating with negative, or negative 
with positive, torsions -so in twining stems also, we see some 
which exhibit positive-directioned twining, others negative, 
and others again alternately one and the other. A plant, 
moreover, which has twined, shows, if inverted, a tendency 

x x 



6*74 

towards reversal or untwining. This is analogous to the 
effect of inversion on autonomous torsion, in which, as we 
have seen, the original torsion tends to be reversed. 

The effect of gravity is known to be a very important 
factor in the induction of twining. It is believed that we 
have here a third type of geotropic action, which is neither 
positive nor negative, but lateral in its character. Under 
lateral geotropism, a curvature is induced in the twining stem 
in a horizontal plane. Since we have found, however, that 
even such diverse effects as positive and negative geotropic 
actions are not to be regarded as due to different specific 
sensibilities, it would be interesting to inquire whether lateral 
geotropism also is not simply a case of ordinary apogeo- 
tropism in combination with some other tendency, say that 
of autonomous torsion. 

At any rate, that this erectile or apogeotropic action is 
always an element in the process, is shown by the fact that the 
coils in the older portion of the stem become drawn out, show- 
ing that the ascending movement predominates in that region. 

The whole subject is, however, extremely complicated, 
owing to the presence of many factors and their different 
relative intensities. And each of these may again be subject 
to modification, under the action of external stimulus, as was 
seen to be the case with autonomous torsion under the in- 
fluence of light, gravity, and the variation of internal energy. 

SUMMARY 

The observed effect of torsion, by which the upper 
surface of the leaf is turned to face the light, is not due to 
any specific dia-heliotropic sensibility. 

Similar torsional response is induced by the lateral 
application of any other form of stimulus, such as thermal 
or chemical. 

Such torsional response is also obtained from a com- 
pound strip made of two unequally contracting inorganic 
materials, such as ebonite and stretched india-rubber, when 
"timulus is applied to one of the lateral flanks. 



TORSIONAL RESPONSE TO STIMULUS 675 

The general law of induced torsional response is that the 
less excitable side of the organ is made to face the incident 
stimulus. If the excitability of the lower half of the pulvinus 
be artificially abolished by the local application of chloro- 
form, the torsional responsive movement is reversed. The 
leaf now executes a movement by which its lower surface is 
made to face the light. 

On artificially increasing the natural difference, as between 
the two halves, by altogether abolishing the excitability of 
the upper, the normal torsional response is accelerated. 

That the upper and lower halves of a dorsi-ventral organ 
are unequally sensitive to geotropic stimulus is shown by the 
fact that they give torsional response, under which the 
less excitable half is carried so as to face the incident lines 
of gravitational force. An artificial increase of this natural 
difference of excitability has the effect of accelerating the 
rate of torsional response. The artificial reversal, on the 
other hand, of these relative excitabilities has the effect 
of reversing the direction of the natural responsive move- 
ment. 

From these considerations it is seen that none of the 
responsive movements caused in dorsi-ventral- organs by 
light or gravitation are due to any specific dia-heliotropic or 
dia-geotropic sensibility, and that they are in reality due 
to the differential excitability of the two halves of the organ. 

The autonomous torsional movement of growth increases 
in rate up to an optimum temperature, after which it begins 
to decrease. There may be a second acceleration after the 
attainment of the first minimum. 

This autonomous torsional movement is retarded or even 
reversed in sign by unilateral stimulus of light. A constant 
current, flowing along the length of the organ, often retards 
or reverses the normal autonomous torsion. The stimulus 
of gravity also has the effect of retarding or reversing normal 
torsional movement. The effective intensity of this stimulus 
appears to be greatest when the organ acted upon is in a 

position at 180 from the vertically upright. 

x x a 



CHAPTER XLVIII 

NYCTITROPIC MOVEMENTS 

Comparison between nyctitropic and autonomous pulsations Diurnal movement 
of plagiotropic stem Supposed distinction between nyctitropic and other 
movements of response to stimulus of light Diurnal response of leaf of 
Biophytum Diurnal response of primary petiole of Mimosa Periodic 
impulses acting on the leaf Periodic impulses contributed by the plant as a 
whole Other modes of exhibition of diurnal periodicity of hydrostatic tension 
Forced vibration and its periodic after-effects Physical analogue Impressed 
periodic vibrations in organ originally radial. 

IN addition to the other effects of light which we have been 
studying, there is also a periodic movement, induced by the 
alternation of day and night, which consists of a pulsation 
executed by the responding organs in the very long period 
of twenty-four hours. In its pulsatory character this move- 
ment resembles the so-called autonomous pulsations of such 
leaflets as those of Desmodium gyrans. But besides the fact 
that it has the long and definite duration of twenty-four 
hours, whereas the autonomous pulsations of the leaflets of 
Desmodium are short and variable in period, there are other 
differences between the two. 

Comparison between nyctitropic and autonomous 
pulsations. In the first place it will be noticed with regard 
to the daily periodic movement that at any given time all the 
motile organs of the same plant are going through the same 
phase. This rhythm therefore is in a manner controlled by 
the plant as a whole. In the case of autonomous movements 
so called, on the other hand, the seat of rhythm is localised 
in the motile organ of that particular leaflet whose pulsations 
are being observed, and there is no necessary coincidence of 
period, as between any two leaflets on the same plant. The 



NYCTITROPIC MOVEMENTS 677 

period of autonomous vibration is much modified, again, by 
rise or fall of temperature, and other conditions ; whereas the 
daily period is unaffected by these circumstances. This is 
due to the fact that the autonomous pulsations are akin to 
free or natural vibrations, whereas the daily periodic move 
ments are, as we shall see, of the nature of forced vibra- 
tions. 

As the nyctitropic movement in the primary petiole of 
Mimosa is of a simple character, free from those other 
complicating considerations which have to be taken into 
account in the case of the leaflets, this organ has been chosen 
as the typical specimen by many investigators. We shall 
therefore consider in detail all its peculiarities in regard to 
this movement, but I shall at the same time endeavour by 
the use of the comparative method to trace the evolution of 
the movement, as first seen in plagiotropic stems, more clearly 
manifested in certain dorsi-ventral organs like ordinary leaves, 
and strongly exhibited in pulvinated leaves, such as those of 
Mimosa. 

Supposed distinction between nyctitropic and other 
movements of response to stimulus of light. This nycti- 
tropic movement has been sharply distinguished from ordinary 
response to the stimulation of light, by the statement that 
while the latter depends on the direction, this is determined 
only by the varying intensity of illumination. As a further 
distinction, it is also insisted that, whereas the responsive 
curvature induced by light takes place in all directions, 
nyctitropic movement always occurs in a single definite plane. 

But whenever we subject the pulvinus of Mimosa to the 
action of strong light, we obtain response by a greater or 
smaller depression or fall, which occurs in a definite vertical 
plane, and is precisely the same in character as that which is 
exhibited slowly during the course of the whole day, and 
attains its maximum in the evening. This response to strong 
light, moreover, is always the same, on whichever side of 
the pulvinus the stimulus may act that is to say, it is 
independent of the direction of light. In analysing this 



678 PLANT RESPONSE 

further (p. 635) we saw that it constituted a true instance of 
phototropic reaction, and that this particular movement of 
fall in a definite direction was due to the stimulus being 
diffused, whether internally or externally, and by differential 
action inducing concavity of the lower and more excitable 
half of the pulvinus. 

In the assumption by the leaf at evening, then, of its 
lowest position, we have nothing which is specifically different 
from this responsive movement, as caused by the action of 
light on the anisotropic organ ; but this same fall, when it 
attains its maximum in the evening, as a phase of the 
nyctitropic movement, is usually said to be due, not to the 
stimulus of light, but to the variation in that stimulus, or to 
the effect of on-coming darkness. Now, if it had been true 
that the diminution of light, or on-coming of darkness in the 
evening, had acted in some unknown manner as a stimulus 
to bring about the fall, then we should have found that such 
a fall was at that time extremely rapid, and that it persisted 
on the withdrawal of light during the night. As a matter of 
fact, however, it is found that this movement of the petiole 
downwards has been taking place progressively throughout 
the day, and that the fall of the leaf at evening is merely a 
continuation of this previous movement, and not markedly 
more rapid at that time than before, if the increased 
mechanical moment due to the closing of the secondary 
petioles and their leaflets be eliminated by their removal. 
Nor does the petiole remain in this depressed position, but 
begins after an interval to erect itself, till in the morning, or 
earlier, it has attained its highest degree of erection. So far, 
again, from darkness being efficient to cause the depression 
of the leaf, it is well known that on artificially darkening the 
Mimosa plant the leaves respond by erection. 

It is thus clear that we must seek some other explanation 
of the nyctitropic movement, and as it is known that such 
periodic movement persists for a time, even when the plant 
is kept for days in continuous darkness, the inquiry resolves 
itself into the two questions: (i) how are these diurnal 



NYCTITROPIC MOVEMENTS 679 

motile impulses generated ? and (2) how is it that the move- 
ment will still persist in the absence of a periodically exciting 
cause ? 

Diurnal movement of plagiotropic stem. In order to 
understand clearly the periodic action of light, in induc- 
ing diurnal movements, it is best to take as our starting- 
point the simplest type of anisotropic organ, in which this 
effect may be observed, and for this we may select the 
plagiotropic stem of, say, Cucurbita. This creeping stem is 
acted on, under natural conditions, by vertical light from 
above, the excitatory effect of which, by long-continued 
action, reaches the lower and more excitable side. The 
stimulus thus becomes internally diffused, and there is also a 
certain amount of externally diffused light from the environ- 
ment. Hence it happens that, by the contraction of the more 
excitable lower half, the free end of the stem becomes progres- 
sively depressed, under the continued action of light, during 
the course of the day (p. 627). We have seen that in conse- 
quence of this the greatest depression occurs about the time 
of evening, and that on the cessation of the stimulus of light 
there is a recovery, .the stem erecting itself more and more as 
the night advances (fig. 252). Thus this nyctitropic depression 
at nightfall is not due to the on-coming of darkness, but repre- 
sents the cumulative responsive effect oj the day's illumina- 
tion. In fact we may regard the responsive movement 
down, and recovery up, which are executed in the course of 
the diurnal period, as parallel to that phenomenon with which 
we are already familiar, of a single response to a single 
transient stimulus. The only difference lies in the fact that, 
while in this latter case the whole process is completed in a 
few minutes, in the former the stimulus acts continuously 
during something like twelve hours, and the recovery is 
allowed to take place during the course of the night Thus, 
in the ordinary case of records of uniform responses to 
uniform stimuli, the successive stimuli, each lasting for a few 
seconds, are given at intervals of some minutes, but in the 
case of the diurnal responses we have a series of stimuli, 



68o 



PLANT RESPONSE 



each lasting for twelve hours, and the beginning of each 
separated from the next by a period of twenty-four hours. 

Diurnal responses of leaf of Biophytum. From this 
simple instance of anisotropy we pass on to a more highly 
differentiated case of dorsi-ventrality, as seen in leaves. We 
have already seen that the petiole of Biophytum is only 
provided with a moderately developed pulvinus, and that the 
petiole also, as a whole, acts as a diffuse pulvinoid. In this 

case also, by the in- 
ternally and externally 
diffused action of light 
during the course of 
the day, the petiole is 
progressively depressed, 
from its position of 
highest erection in the 
morning to its lowest 
depression in the even- 
ing. Recovery takes 
place again in the ab- 
sence of the stimulus of 
light, and the leaf once 
more assumes its highest 




KJ. 273. Photographic Record of Diurnal 
Movement of Petiole of Biophytum from 
5 P.M. till 9 A.M. 

First part of record shows maximum depres- 
sion to be reached at 6 P.M. and maximum 
erection at 7 A.M. After the latter hour 
the leaf again begins to exhibit its usual 
daytime depression. 



position by morning. If, 
however, in the course 
of the afternoon, say, at 
five o'clock, the plant be 
taken to a dark room, 

then, owing to the positive after-effect, the leaf will still con- 
tinue to fall for about an hour, and then begin to erect itself. 
Owing to the daily periodicity impressed on the plant, of which 
we shall presently speak in greater detail, the leaves continue 
to exhibit the diurnal movement even when kept in continuous 
darkness. I give here (fig. 273) an interesting photographic 
record of the diurnal movement of the leaf of Biophytum 
from 5 P.M., when it was brought into the dark room, till 
9 AM. next morning. It will here be seen that, owing to the 



NYCTITROPIC MOVKMKNTS 68 1 

positive after-effect, the leaf continued to fall till 6 P.M., and 
that after this it erected itself by a series of five pulsations, 
until its highest position was attained at about 7 A.M. It 
next began to exhibit the impressed effect of day, and the 
leaf then fell rapidly. 

Diurnal response of primary petiole of Mimosa. In 
Mimosa we merely find the repetition of those movements 
whose evolution we have already traced through the plagio- 
tropic stem and the dorsi-ventral petiole. This is made very 




FIG. 274. Continuous Records of the Diurnal Movement during Thirty- 
six Hours in Two Specimens of JMimosa 

The continuous line represents the movement of the one-year old, and the 

dotted line that of the six-months old specimen. The maximum 

depression is seen to take place at six in the evening, and the recovery 

nearly completed by midnight;. This maximum erection is slightly 

augmented at dawn. 

apparent in the continuous records which I took of the 
diurnal movement in two specimens of Mimosa, the records 
being continued during a period of thirty-six hours (fig. 274). 
The two plants were placed in an open verandah, and 
long light indices made, as already described, of peacock's 
quills, were so attached to the petiole as to form prolonga- 
tions. Records were taken on a vertical revolving drum. 
A vertical thread was suspended in front of the drum, and 
the point at which the moving index cut this vertical line 
was marked at every fifteen minutes. Thus the record gives 



682 PLANT RESPONSE 

a measure of the angular movement at different parts of the 
day. The dotted line shows the diurnal movement of the 
younger of the two plants, which was six months old, and the 
continuous line that of the other, which was one year old. The 
two, as will be seen, are practically the same. It should be 
mentioned here that, when this record was taken, 6 A.M. and 
6 P.M. were the hours of sunrise and sunset, there being no 
twilight. The records show that the leaves exhibited the 
erectile or recovery effect with great rapidity during the first 
part of the night, this movement being almost completed by 
i A.M. After this there was but slight upward movement, 
until about 7 A.M., from which time onwards, during twelve 
hours, they fell continuously. The maximum depression was 
reached at almost exactly 6 P.M., and then the leaves again 
rose, repeating their former movement of recovery. 

Periodic impulses acting on the leaf. The periodic 
movements down during the day, and up during the rright, 
are thus seen to be due to periodic impulses of stimulus 
and recovery acting on the responding leaves. It must be 
pointed out here that the process of recovery is not alto- 
gether passive. We saw in the records of Mimosa under 
light (fig. 256) that, on the cessation of light, the responding 
leaf continued to move down for a while owing to the 
positive after-effect. Later, however, owing to the latent 
energy which it had acquired .by the absorption of light, it 
exhibited the negative after-effect in an erection which 
carried it beyond the original position. Hence we can see 
that the increased internal energy due to previous absorption 
of light plays an important part in that movement of erection 
which is initiated shortly after nightfall. We may therefore 
say that the diurnal movements of the leaves are brought 
about by two alternate periodic impulses, those, namely, 
of external stimulus and internal energy. The persist- 
ence of the effect of each of these impulses will depend on 
the intensity of the two factors respectively, and also on 
the capacity of the tissue for absorbing stimulus. For 
example, the positive after-effect by which the leaf continues 



NYCTITROPIC MOVEMENTS 683 

to be depressed, even on the cessation of the stimulating day- 
light, may, in some cases, be short-lived, and in others may 
continue for a considerable period. The return movement 
will in the latter case be somewhat delayed. Again, the 
internal energy which hastens recovery may, in certain cases, 
bring about the utmost erection of which the leaf is capable 
at some time earlier than the ensuing morning. Thus, for 
example, in the response of Biophytum, the highest position 
(fig. 273) was attained at about 7 A.M., while in the records of 
these particular Mimosa leaves the corresponding point was 
reached very much earlier, that is to say at I P.M. 

Periodic impulses contributed by the plant as a 
whole. In addition, however, to those periodic impulses 
whose seat is in the responding leaf, there is another concor- 
dant impulse contributed by the plant as a whole which 
intensifies the diurnal movement. The plant is subjected 
during the day to the stimulus of light, which causes con- 
traction of the exterior tissues, and thereby tends to drive 
the water inwards. The turgidity of the cortex is thus 
progressively diminished during daylight. But at night the 
water which has thus been driven into the central reservoir ol 
the plant will flow outwards. In this also the absorbed 
energy will play an important part. The rhythmic activity 
of the cortical tissues being thereby increased, they will suck 
water outwards from the central reservoir, just as the rootlets 
suck it from the ground. The result of this alternation ol 
external stimulus during the day, and internal stimulus during 
the night, will be a periodic inflow and outflow a diminu- 
tion and increase of tension the first half of the cycle being 
completed in the daytime, and the second half in the night. 

I have shown (p. 46) that the leaf of Mimosa is 
erected whenever the internal hydrostatic tension is arti- 
ficially increased, and depressed when it is diminished ; and 
we have seen how such variations are induced by the alter- 
nation of day and night, not only in individual petioles, but 
throughout the plant as a whole. The alternate ebb and 
flow of the water, from the central reservoir, will thus be 



684 PLANT RESPONSE 

indicated by the periodic erection and depression of all the 
leaves synchronously, which will thus act, so to speak, as 
signal-flags. 

Other modes of exhibition of diurnal periodicity of 
hydrostatic tension. There are also other modes besides 
that of mechanical response of the leaves by which this 
diurnal periodicity of tension can be detected ; and Millardet 
has shown that the maximum tension in Mimosa occurs at 
dawn, when the primary petiole is in its most erect position. 
The minimum tension, again, occurs in the evening, when the 
leaf occupies its most depressed position. Kraus, further, has 
found the organs of the plant diminish in bulk from morning 
till afternoon, and that the reverse process takes place from 
afternoon till morning. Growth itself, again, is well known 
to exhibit a diurnal periodicity. It is interesting, however, 
to realise that this is simply the mode by which a radial 
organ exhibits, in a form of longitudinal response, what was 
otherwise exhibited in Mimosa as lateral response. The 
periodic variation of tension induced by the diurnal period 
has been seen to manifest itself in Mimosa in two alternate 
movements, positive and negative. Similarly we shall find, 
in growth- response, positive and negative variations above 
and below the normal or average rate. If we take a 
balanced record of growth, representing its average rate, a 
downward line will indicate retardation or negative response, 
while an upward line will indicate an acceleration of growth, 
or positive response ; and in thus recording variations of the 
rate of growth, by the balanced method, for a period of 
twenty-four hours, we obtain, as we have seen (fig. 191), 
a curve which closely resembles that of the nyctitropic 
movement. 

Forced vibration and its periodic after-effects. We 
have thus traced out the two alternating impulses the direct 
effect of external stimulus and its direct after-effect, and the 
internal stimulus with the negative after-effect which induce 
the forced vibration of the responding organ. The periodic 
effects of protoplasmic contraction and expansion, induced 



NYCTITROPIC MOVEMENTS 68$ 

alternately in the plant under alternating light and dark- 
ness, thus leave a molecular impress, and this impression is 
deepened by repetition, finding subsequent expression even 
when the primary alternating cause is absent. The length 
of time during which such after-oscillation persists will 
depend, amongst other things, on the depth of the molecular 
impression. 

Physical analogue. An analogous physical phenome- 
non will make the point clearer. If a wire be taken and 
subjected to alternating molecular strains, say by giving it 
alternate positive and negative twists, the wire being held in 
each of these twisted positions for a time, then, even after 
stoppage of such alternate twisting movements, the released 
wire will continue to vibrate to and fro in expression of the 
release of impressed latent strains, consequent on previous 
forced vibrations. 

That such forced vibrations may persist in a plant has 
been shown by F. Darwin and D. Pertz, who subjected a 
plant to alternating geotropic stimuli, by which a rhythmic 
movement was found to persist for a time, even on the 
stoppage of stimulation. Czapek and Wiesner obtained 
similar after-vibrations with alternating phototropic stimuli. 

Impressed periodic vibrations in organ originally 
radial. I shall here give a very remarkable instance of such 
forced periodic vibrations, as induced in an originally radial 
organ. I had a row of sunflowers planted, in a line which 
ran accurately east and west, at the season when the sun 
moved daily in an almost vertical plane. The plants were 
thus stimulated in the morning from the east, and in the 
afternoon from the west. All these plants, then, by helio- 
tropic action, followed the path of the sun during the course 
of the day, and during the course of the night, again, there 
was recovery. At first the diurnal swing east and west was 
only through a small angle, but under the action of these 
repeated periodic impulses it became larger and larger, like 
that of a pendulum under regular and repeated blows. 
When the plants had grown to a height of one metre, it was 



686 PLANT RESPONSE 

a remarkable sight in the early mornings to see all the six, 
with their upper halves bent over equally to the east, and in 
the evenings equally to the west. One curious phenomenon 
connected with this consisted in the fact that not only was 
the nightly recovery completed by I A.M., but the upper part 
of the shoot was already carried over to the east, just as we 
found the Mimosa leaf to be erected to the highest position by 
midnight in consequence of the presence of internal energy. 

Analysis of constituent impulses causing nyctitropic 
movement. Nyctitropic movements are thus brought about 
by two different periodic factors, themselves induced by the 
periodic action of light and darkness. These periodic forces 
acting on the pulvinus are : 

1. The differential heliotropic effect on the pulvinus 
itself. By the stimulus of light, externally and internally 
diffused, the dorsi-ventral leaf is progressively depressed 
during the day. The reverse process takes place at night, 
by means of natural recovery, aided by internal energy, 
which gives an impulse opposite to that of external stimulus. 

2. A periodic inflow and outflow of water taking place 
in the plant as a whole, by the recurrent action of light and 
darkness. This, acting on the dorsi-ventral pulvinus, causes 
periodic movement of the petiole. 

3. Both these periodic forces are concordant in their 
action on the pulvinated organ, and give rise to periodic 
movements of large amplitude. 

SUMMARY 

The nyctitropic movement of such leaves as that of 
Mimosa is believed to be distinguished from heliotropic 
action proper by the facts that (i) it takes place in a definite 
plane, and (2) it is caused, not by light as a constant force, 
but by its variation of intensity, the fall of the petiole in the 
evening being thus ascribed to the on-coming of darkness. 

With regard to the first of these points, I have shown 
that, even under heliotropic action proper, all dorsi-ventral 
organs move in a definite plane ; and as regards the second, 



MOVEMENTS 687 

it has been shown that the fall of the leaf in the evening is 
not due to the action of on-coming darkness, but to the 
cumulative stimulation of the day's illumination that is to 
say, to the action of light as a constant force. 

It is possible to trace out the gradual evolution of this 
periodic diurnal movement, taking as the simplest type that 
of the plagiotropic stem. Under the action of stimulus, 
which is internally or externally diffused, the lower and more 
excitable side of such a stem undergoes progressive con- 
cavity, the lowest position being attained in the evening. 
At night, however, on removal of the stimulus of light, re- 
covery takes place by erection of the stem. A diurnal up- 
and-down movement is thus induced. 

A similar effect is observed in the petiole of Biophytum. 
In the leaf of Mimosa the action is precisely the same. 
Here, owing to the direct action, and the positive after- 
effect of light, the leaf is depressed progressively till evening. 
At night, however, recovery takes place by an erectile move- 
ment. This is not due to a passive recovery merely, but is 
aided by the negative after-effect, consequent on the storage 
of internal energy by the day's illumination. By this active 
internal impulse the leaf attains its highest position some 
time before dawn. The alternate impulses acting periodi- 
cally on the leaf are thus : (i) the direct effect of continuous 
stimulation of light, and (2) the opposite impulse due to 
internal energy. 

In addition to these alternate impulses the plant as a 
whole contributes periodic impulses, which are concordant 
with the periodic impulses in the leaf. The external tissues 
of the plant, acted on by light stimulus, contract and drive the 
water inwards, into the central reservoir. At night a reverse 
movement of water takes place. As a result of this alter- 
nation of external stimulus during the day, and internal 
stimulus during the night, there is a periodic inflow and out- 
flow, a diminution and increase of tension, and these varia- 
tions of tension are indicated by the periodic depression and 
erection of all motile leaves synchronously. 



88 PLANT RESPONSE 

The fact that these periodic forces in leaves and in plant 
act concordantly causes the periodic movements to be of large 
amplitude. 

The periodic diurnal variations of internal hydrostatic 
tension are also exhibited by periodic variations in the rate 
of growth. The curves of the diurnal periodicity of growth 
and of the nyctitropic movement of the leaf are therefore 
similar. 

This forced diurnal vibration, being often repeated, gives 
rise to periodic after-effects, which persist for a time even on 
the cessation of the periodically exciting cause. 



CHAPTER XLIX 

ON PULSATORY RESPONSE AND SWIMMING MOVEMENTS 
AS INITIATED AND MODIFIED BY LIGHT AND OTHER 
FORMS OF STIMULUS 

Investigations on the influence of light on the lateral leaflet of Desmodium gyrans : 
(a) in sub-tonic condition ; (b) in normal tonic condition - Changes induced in 
existing anisotropy of Desmodium leaflets Reversal under intense stimulation 
seen in all forms of response The swimming movements of ciliated organisms 
Fundamental resemblance between the swimming responses and the ordinary 
heliotropic responses in radial organs Phototactic movements : (a) Two 
natural types of responsive movements ; (l>) Responsive movement positive, 
negative, or intermediate, according to intensity of stimulation Directive 
action of light Thermotaxis Galvanotaxis Chemotaxis. 

WE have already studied in detail the autonomous responses 
of the leaflets of Desmodium ; but there are also other 
leaves which execute similar pulsatory movements, and we 
have now to investigate the effect of the stimulus of light on 
such multiply-responding organs, either in initiating these 
movements or in modifying them. We have also another 
instance of such multiple pulsations, in the swimming move- 
ments of ciliated organisms, of which the causes are at 
present regarded as very obscure. The treatment of these I 
include within the present chapter, because I hope to show 
that they really belong to a class of phenomena essentially 
similar to the autonomous movements of Desmodium> and 
that from this point of view all their seeming peculiarities 
may be very satisfactorily explained. 

In the first place, I shall take up the question of multiply- 
responding leaves or leaflets. Concerning the effect of light 
on these, there is a certain amount of discrepancy of observa- 
tion. Strasburger, for example, referring to such movements 

y y 



6pO PLANT RESPONSE 

in the lateral leaflets of Desmodium gyrans, says they are in 
no way disturbed by variation in the intensity of light. 
According to Pfeffer, similarly, the autonomous movements 
of the leaflets of Trifoliiim pratense are not affected by 
illumination. But Strasburger again states that the leaflets 
of this plant, on exposure to light, cease their oscilla- 
tions. 

Investigations on the influence of light on the lateral 
leaflet of Desmodium gyrans. (a) In sub-tonic condition. 
A clearer insight into this subject will, however, be obtained 
when I describe my experiments on Desmodium gyrans. I 
have shown that when this plant is in a sub-tonic condition it 
behaves like an ordinarily responding plant, such as Biophy- 
tum ; a single moderate stimulus then evokes a single response, 
and strong stimulus multiple responses. Thus a leaflet of Des- 
modium in a state of standstill has multiple response initiated 
by the incident stimulus of light. The following record 




Fir.. 275. Initiation of Multiple Response in Lateral Leaflet of 
Desmodium originally at Standstill 

Light applied at x and continued till the end of the sixth response, as 
shown by the thick line. The responses show a staircase increase 
with increase of absorbed energy. Pulsations persist for a short time 
even on the cessation of stimulus. 



(fig. 275) shows this clearly. The leaflet was in a quiescent 
condition, but during the application of light it exhibited 
multiple responses, which, owing to the increasing absorption 
of energy, showed a staircase enhancement of amplitude. On 
the cessation of light the energy absorbed maintained the 
pulsation for some time. 

(If) In normal tonic condition. When the specimen, how- 
c^er, is in a favourable tonic condition, its pulsations being 



PULSATORY RESPONSE 691 

apparently autonomous, the application of excessive stimulus 
of strong light often brings about fatigue, and under these cir- 
cumstances I have obtained two different types of response, 
according to the particular condition of the tissue. In the 
first of these, progressive fatigue, under the continued action 
of light, is shown by the gradual diminution of amplitude of 
pulsation. If the strong stimulus of light, which causes this 
fatigue, be applied for a short time only, the pulsation whose 
amplitude was diminished, again on the cessation of stimulus 
recovers its first amplitude in a staircase manner ; but if the 
light be long continued, fatigue becomes so great that the 
pulsatory movements are brought to a stop, at least for a 
considerable period. We have seen again that an organ 
sometimes exhibits fatigue somewhat differently from this, 
that is to say in a periodic manner. In accordance with 
this fact the leaflets of Desmodium are sometimes seen 
to display periodic fatigue that is to say, the pulses wax 
and wane in groups, an example of which is seen in 
fig. 276. 

Thus we see that in a leaflet at standstill, light, by 
supplying energy, initiates autonomous movement ; while in 
an actively pulsating leaflet, strong illumination may bring 
about such fatigue as to cause cessation of movement At 
still other times, again, owing to the favourable condition of 
the tissue, fatigue is slight, and there is no arrest of activity. 
These facts are sufficient to explain the discrepancy already 
mentioned, in the observations made by Strasburger and by 
Pfeffer, on the action of the leaflets of Trifolium pratenst 
under light. 

Changes induced in existing anisotropy of Desmodium 
leaflets. A very interesting consideration arises at this 
point as to the differential fatigue caused in the anisotropic 
organ by strong stimulus. We have seen that the lower half 
of such an organ is the more excitable, hence we can see the 
probability of fatigue being relatively greater there. Now, it 
is the naturally greater excitatory contraction of the lower 
half of the organ which causes the downstroke of the leaflst 



V V 2 



692 PLANT RKSPONSK 

to be the quicker ; but if stimulus produce greater fatigue of 
the lower half, we can see that its movement will be made 
somewhat slower, and the upstroke will then become the 
relatively quicker of the two. 

This anticipation finds remarkable verification in the 
photographic record given in fig. 276. We see there, in the 
first or normal record, from the fineness and steepness of the 
upward line, representing the downstroke, that this is the 
quicker movement of the two ; but after the application of 
strong stimulus of light, at the point marked with an upward 



Kh 1 .. 276. Photographic Record of Autonomous Pulsations in Lateral 
Leaflet of Desmodium gyrans under Action of Sunlight, showing 
Periodic Reversals 

Light applied continuously from arrow onwards. In the first two 
responses, representing the normal, the downstroke of the leaflet, 
represented by the up curve, is relatively the quicker. After the 
application of light, this relation is gradually reversed, till in the 
fourth and fifth pulses after application it is the upstroke, represented 
by the down curve, which is pronouncedly the quicker. This reversal 
is in its turn reversed at the eighth pulsation. 

arrow, we observe a gradual change, by which the existing 
normal difference between up and down strokes is first abolished 
and then reversed. In the next complete response, after the 
application, we see that the two strokes have become equally 
rapid. In the next, the downstroke has become distinctly 
the slower, and this goes on progressively till we see in the 
fifth of this series a very remarkable degree of difference 
between the two, the upstroke being now much the quicker. 
These reversals are found to be recurrent, further on in the 
record. Such alternate changes of excitability on the two 
sides we have noticed even in the case of radial organs ; for 



PULSATORY RESPONSE 693 

we have seen that such an organ, when acted on unilaterally 
by strong light, often exhibits to-and-fro oscillations, due to 
alternate fatigue of the two sides. 

We thus find, starting with a Desmodium leaflet in a 
state of standstill, that moderate intensity of light initiates 
normal pulsatory movements, in which the downstroke is 
quicker than the upstroke ; but, in a pulsating leaflet, under 
intense or long-continued light, these beats are reversed, that 
is to say the upstroke becomes the quicker. Under the 
action of continuous light, again, these reversals themselves 
may become periodic or recurrent. 

Reversals under intense stimulation seen in all forms 
of response. We have already seen that autonomous pulsa- 
tion is simply ordinary response repeated, owing to excess of 
energy ; and that the Desmodium leaflet is an ordinary aniso- 
tropic organ, in which response may be initiated by any form 
of stimulation thermal, photic, chemical, or electrical. We 
have just seen, further, in the case of photfc stimulus, that 
the character of the response may be reversed by the in- 
tensity of the stimulation. Thus the normal response, in 
which the downstroke is more energetic than the upstroke, 
may be exchanged for a type of response in which the up- 
stroke is quicker than the down. This law of reversal of 
response with varying intensity of stimulation has already 
been shown to be illustrated in different types of response. 
For example, in the case of chemical stimulation, we found 
that the leaf of Mimosa when subjected to the action of 
a dilute solution of sodium chloride gave an erectile response, 
whereas when the solution was stronger it responded by 
depression (pp. 551, 552). In the response of growth, again, 
very dilute and very strong solutions of a given reagent were 
shown to produce opposite effects. Thus, while dilute solu- 
tion of sugar accelerated, a very strong solution retarded, the 
rate of growth (p. 488). 

In the case, then, of an organ which is capable of multiple 
response, we find that any form of stimulus is competent to 
initiate such response, and that, with regard to certain charac- 



694 PLANT RESPONSE 

teristics, such as the relative quickness of the up or down 
movement, it may be exhibited in opposite ways, according 
to the intensity of stimulus. 

The swimming movements of ciliated organisms. It 
is the automatic character of ciliary movements which has 
hitherto rendered them a subject of such great perplexity, 
for these movements are independent of any nervous system 
for their initiation or control. The impulses that lead to 
ciliary motion arise in the cilia themselves. There is, again, 
some difficulty in understanding the mechanics of these 
movements. 

Their automatism may, however, be explained by those 
considerations which are now familiar to us in similar in- 
stances that is to say, initiation of movement by external 
stimulus, and its maintenance by excess of latent energy. 
There is, again, the closest resemblance, from a mechanical 
point of view, between the movements of the cilia and the 
movements of Desmodium leaflets. In both cases alike, one 
of the component strokes is quicker than the other. Again, 
though in both cases we meet with instances in which the up 
and down movements take place in the same plane, yet there 
are also others in which more complicated paths, whether 
circular or elliptical, are described. The fact that one move- 
ment is quicker than the other, and that the strokes are 
lateral, shows that we have in the cilium an anisotropic 
organ, essentially similar to the pulsating pulvini of the 
lateral leaflets of Desmodium. The only difference between 
the two cases lies in the fact that we have in the pulvinus a 
multicellular, and in the cilium a unicellular, organ. 

Let us suppose a detached petiole of Desmodium^ bearing 
the lateral leaflets, to be thrown into water contained in 
a glass trough ; let us further suppose these leaflets to be in 
a sub-tonic condition, that is to say in a state of standstill. 
If this vegetable organism be now stimulated by sunlight 
of moderate intensity, striking it horizontally from one side 
of the vessel, it will be found that rhythmic excitation is 
initiated, and that the stroke backwards is much quicker and 



PHOTOTAXIS 695 

more energetic than that forwards. The consequence of this 
will be, in the vegetable organism as in the case of a man 
swimming, its forward propulsion. This result depends upon 
the fact that the lower half of the anisotropic organ is in this 
case the more excitable. Reversal of the relative activities 
of the two halves of the dorsi-ventral organ was, however, 
seen to occur in the case of the leaflets of Desmodium 
when the intensity of stimulation was very great. Such a 
reversal, under excessive stimulation, would give rise then 
to a swimming movement in the opposite direction. We also 
saw such reversals under continuous stimulation of light 
becoming periodic (fig. 276). The corresponding swimming 
response would thus consist of a movement to and fro. 

Supposing, however, that the excitability of the upper 
half of the motile organ had been the greater, it is clear that 
the normal excitatory response would have taken the form ol 
a backward or negative swimming movement. We thus see 
the possibility of normal responsive swimming movements ol 
two different types, according to the particular half of the 
anisotropic motile organ which is the more excitable. Taking, 
again, that type of swimming in which the response is posi- 
tive or forward, a stronger intensity of stimulation may give 
rise to a reversal, or negative movement. And from what 
has already been said, it will be seen that similar responsive 
movements may also be expected under forms of stimulation 
other than light. 

Similarity between swimming responses and the 
ordinary heliotropic responses of radial organs. Though 
at first sight it would appear as if there were no con- 
nection between the simple responsive curvatures of radial 
organs and the apparently complicated responsive move- 
ments of swimming, yet on a closer analysis we shall find 
that there is little essential difference between the two ; 
for we have seen that growth itself, or growth-curvature, 
is simply a phenomenon of multiple responsive movements, 
which, owing to the rapidity of the individual responses, 
appears continuous. Hence, when, under moderate stimula- 



696 PLANT RESPONSE 

tion, the organ moves towards the light, or exhibits a positive 
response, this means that the resultant of its multiple move- 
ments is towards the stimulus, like the resultant movement 
of the ciliated organism towards light. Similarly, under 
strong photic stimulation, the negative heliotropic movement 
of the organ corresponds to the swimming of the ciliated 
organism away from light. In the intermediate case, again, 
where the stimulated organ shows no resultant curvature, but 
oscillates about a mean position, we have an instance which 
is paralleled, in the case of the ciliated organism, by alter- 
nate swimming backwards and forwards. 

Again, just as in the heliotropic response of a radial 
organ, the minor pulsations by which it is brought about are 
too rapid and minute to be easily detected, and we can per- 
ceive only the resultant movement of the organ as a whole, 
so in the case of the ciliated organism the individual beats 
cannot easily be perceived, and we infer their presence from 
the resultant motion of the organism as a whole. 

Phototactic movements. From the fundamental de- 
monstrations which I have already given of the character- 
istics of multiple response, and its modification by relative 
variations of contractility, as between the upper and lower 
halves of the responding organ, it will be found that the 
multifarious responsive movements of ciliated organisms, 
under various forms of stimulus of differing intensities, will 
have been elucidated. 

(#) Two natural types of responsive movements.- -The two 
opposite types of movement, positive and negative, which we 
have now theoretically anticipated, are found to be com- 
pletely illustrated in the case of swarm-spores under the 
action of light. Thus, for example, in the case of Botrydium 
granulatum, they respond by a positive swimming move- 
ment, or motion towards the light. Again, while certain 
varieties of Ulothrix exhibit the positive effect, by movement 
towards light, there is another variety which gives the nega- 
tive response, by swimming away from it. This opposition 
of effects is obviously due, as we have anticipated, to a 



PHOTOTAXIS (597 

natural difference of relative excitabilities, as between the 
upper and lower halves of the anisotropic swimming organ 

(P- 695). ^ 

() Responsive movements positive, negative \ or intermediate, 

according to intensity of stimulation. Turning now to the 
question of different movements in the same organism as 
modified by the varying intensity of illumination, examples 
are furnished by the observations of Stahl and Strasburger. 
These investigators find that the swarm- spores, generally 
speaking, when the intensity of light is moderate, move 
towards it, and when stronger, away. 

In the lateral leaflets of Desmodium, again, under con- 
tinuous illumination, we have observed recurrent reversals of 
the direction of the more rapid of the two strokes which con- 
stitute each individual pulsation. Even this phenomenon 
finds a curiously exact parallel in the movements of certain 
swarm-spores of Ulothrix under the continuous action of 
light, as noticed by Strasburger. These he finds first to 
retire from the light, then to remain stationary, and again to 
return towards the light, only then to begin the whole 
process over again, thus moving to and fro for some time 
like a pendulum. 

Directive action of light. The movement of the ciliated 
organism, then, whether towards or away from it, is parallel 
to the direction of incident light. But a difficult question 
arises here as to how the organ perceives this direction as it 
were, and by what mechanism it determines its own course 
accordingly. At first sight, there appears no reason why 
rhythmic beats caused by stimulus should not produce 
propulsive movement in any direction. In this connection, 
turning to the case of Ulothrix, we find the organism pro- 
vided with symmetrical pairs of cilia. We also know that 
the excitatory effect of stimulus of light depends upon its 
angle of incidence. If, then, light strike the two cilia of a 
given pair asymmetrically, they will undergo unequal excita- 
tion, causing them to execute a turning movement. Thus a 
stable condition can only be arrived at when excitation is 



698 PLANT RESPONSE 

equal in the two cilia, and this can clearly happen only when 
the organism has so orientated itself that its axis is parallel 
to the direction of the incident rays ; and it is evident that 
in this position the equal beats of both cilia must result either 
in progressive or in retrogressive motion, parallel to the direc- 
tion of the incident rays. 

Thermotaxis. Thermal stimulation also causes respon- 
sive movements towards or away from the source of stimula- 
tion ; and these reactions, again, are found to change their 
signs, with varying intensities of stimulus. Thus, Paramcecia 
swim towards the warmer side of a vessel which is unequally 
heated, provided the temperature of the warmer side does not 
exceed 24 C. The response is in this case, then, seen to be 
positive ; but when the temperature of the heated side is 
higher than 28 C. the Paramcecia are found to swim away, 
thus exhibiting a negative response. 

Galvanotaxis. Similar multiple response finds expres- 
sion in certain Infusoria, in swimming movements of posi- 
tive and negative character. Thus Verworn finds, for 
example, that under the excitation of an electrical current, 
Polytoma swims away from the kathode, and Pleuronema 
towards it. 

In these galvanotactic movements, also, we meet with 
the same recurrent reversals with which we are already 
familiar in the case of the leaflets of Desmodium. In work- 
ing with Paramcecium, Arthur W. Greely L found that after 
being subjected for about half an hour to a moderate current, 
the organisms which had previously gathered round the 
kathode reversed their action, and moved towards the anode, 
only to dart back immediately, again, towards the kathode. 

I have demonstrated in Chapter XXXVI. the opposite 
responsive changes which occur under the action of acids 
and alkalis respectively. This explains the appropriate 
modification of galvanotactic response in Paramcecium, 
according as the organism has been reared in an acid or in 

1 Science has lost a very promising worker in the early death of this investi- 
gator. His experiments on Paramcetium are very valuable and suggestive. 



CHEMOTAXIS 699 

an alkaline culture. Thus Greely has found that while 
alkali-reared Paramcecia invariably give the initial response 
by swimming towards the kathode, the reverse is generally 
the case with those which have been subjected to acid that 
is to say, the latter as a rule begin by swimming towards the 
anode. 

Chemotactic movements. Similar effects are, again, 
observed under chemical stimulation. The opposite reactions 
of acids and alkalis which have already been described are the 
occasion of opposite chemotactic responses. Thus Jennings 
found Paramcecia moving towards, or showing positive reaction 
to, acids, whereas to alkalis they exhibit negative response, 
or movement away. Chemotactic reaction, again, is modified 
by the strength of the solution, that is to say by the intensity 
of stimulation. The opposite effects of strong and feeble 
doses (p. 488) are here illustrated by the fact that in many 
organisms positive response is observed under the action of a 
weak solution, and negative under a strong. Connected with 
the subject of chemotaxis is the interesting phenomenon of 
the response of the antherozoids of ferns and of Selaginella 
to malic acid, as discovered by Pfeffer. In the response of 
growth we found that dilute solutions of sugar gave rise to 
one kind of response, that is to say to acceleration of growth, 
and that with very strong solutions, say about 10 per cent., 
the opposite occurred that is to say, retardation (p. 482). 
Now, it is found by Pfeffer that whereas the dilute solution of 
a malate exerted an attractive influence on the antherozoids, 
a 10 per cent, solution had a repellent effect. 

SUMMARY 

A rhythmic vegetable organ, in a state of standstill, has 
its autonomous movement renewed by a. supply of energy 
from incident light. 

Too strong an intensity of light may, by causing fatigue, 
arrest such movement. Or the greater fatigue of the more 
excitable half of the organ may cause a reversal of the relative 
rapidities of the up and down beats, 



700 PLANT RESPONSE 

In Desmodium, under the continuous stimulation of strong 
light, these reversals are often recurrent. The downstroke, 
which is at first quicker, becomes less quick than the upstroke, 
and this may be again and again reversed. 

In a ciliated organism the swimming movements are 
explicable by similar unequal up and down strokes of the 
anisotropic cilia. When the downstroke is quicker, the 
organism propels itself forward. When the upstroke is quicker, 
there is a movement backwards. Such swimming movements, 
due to multiple response, are initiated by stimuli of various 
forms. There are two natural types of these responses : 
positive movement, or swimming towards, and negative, or 
away from, stimulus. These are determined by the relative 
excitabilities of the upper and lower halves of the cilium. 

Unilateral light exerts a directive action on responsive 
swimming movements, because the only stable position is one 
in which pairs of cilia are equally excited. The axis of the 
organism is thus orientated till parallel with incident light. 

Strong stimulation of light, for reasons described, causes a 
reversal of the normal movement. Alternate periodic fatigue 
causes a responsive movement of the organism to and fro. 

Under moderate unilateral thermal stimulus the organism 
exlribits a positive swimming movement, and under stronger 
a negative movement. 

Similar responses to stimulus are exhibited under galvanic 
excitation. The normal response here also undergoes reversal 
under long-continued stimulation ; since the actions of acids 
and alkalis are antagonistic, organisms reared under acid 
and alkali cultures tend to exhibit opposite reactions under 
galvanotaxis. 

Under chemical stimulation, swimming organisms exhibit 
multiple responses. Acid and alkali, owing to their an- 
tagonistic actions, bring about opposite responses. The 
same reagent again occasions opposite responses, according 
to the amount of the dose. Thus antherozoids of ferns are 
attracted by the dilute solution of a malate, but stronger 
solutions exert a repellent action. 



PART IX 

GENERAL SURVEY AND CONCLUSION 



CHAPTER L 

REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 

Responsive contraction Kunchangraphic records Direct and indirect effects of 
stimulation Various forms of responsive expression ; (a) Lateral motile 
response by differential contraction ; (6) Suctional response ; (c) Growth- 
response ; (d) Torsional response ; (e) Death response ; (f) Thermographs of 
regional death ; (g) Electrical response Different types of response : (a) 
Uniform response ; (b) Fatigue ; (c) Staircase response Excitability 
Conductivity Polar effects of currents Multiple response Continuity of 
multiple and autonomous responses The ascent of sap. 

WE have now studied in detail the manner in which a plant 
reacts to the varied forces of its environment, and although 
its response to them finds modes of expression which appear 
highly diverse, yet we have found that on analysis these are 
all reducible to two very simple and well-defined factors, of 
responsive contraction and expansion. 

Responsive contraction. In vegetable cells, as in other 
protoplasmic bodies, it has been shown that the impact of all 
external stimulus evokes responsive contraction ; and this 
we found to be true, not in the so-called sensitive plants 
alone, but in all plants. Taking the simple case of a radial 
vegetable organ, such as a stem, style, stamen, or other 
filamentous structure, we find that it undergoes longitudinal 
contraction under stimulation. Here, then, we have a phe- 
nomenon which is analogous to the contraction of muscle, 
The amount of contraction of these ordinary vegetable 
organs, further, is sometimes very considerable, as we saw in 
the case of the coronal filament of Passiflora, where it was 
as great as 20 per cent, of the original length. Such 
responsive contraction takes place, moreover, under all forms 
of stimulation, mechanical, thermal, electrical, photic, and 
chemical ; and we have found that all the various move- 



704 PLANT RESPONSE 

ments of plants which are seen in nature, under the action of 
external stimulus, are but different expressions of a single 
fundamental response by contraction. 

Kunchangraphic records. By taking advantage of this 
responsive contraction, we are able to study all the physio- 
logical modifications induced in the vegetable tissue by 
various reagents, with as great ease and certainty as similar 
phenomena can be studied in animal muscle, using myographic 
records. By such study, again, we are once more led to 
see how misleading has been the superficial distinction be- 
tween sensitive and non-sensitive plants, since the latter, 
or so-called ordinary, plants also exhibit contraction under 
stimulation. 

Direct and indirect effects of stimulation. The living 
organism is thus a delicately responding machine, whose 
responsive movements are brought about by external stimulus ; 
but this complex machinery has also the power of holding 
part of the energy of the external stimulating shock latent, 
for a longer or shorter time, so that part only may find 
immediate expression, while the rest is stored up as internal 
energy to be given out after the lapse of an intervening 
period. These two factors, of external stimulus and internal 
energy, again, induce opposite effects, of contraction and 
expansion respectively. And the infinite multiplicity of 
responsive processes in the life-cycle of the plant is brought 
about by their mutual play. That the combination of these 
two elements in varying degrees of each, finding expression in 
different ways, creates a tangle which would at first sight appear 
inextricable, can be easily understood. And it was the bewilder- 
ment which this fact imposed upon the observer, that drove 
us to postulate the existence of an unknown and indefinable 
vital force, whose mysterious working was to be held to 
account for the occurrence of all those phenomena that we 
were otherwise unable to explain. 

It is possible, however, as we have found, going back step 
by step, to trace out the different expressions of these two 
distinct factors, of external stimulus and internal energy, and 



REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 705 

to show, moreover, how the latter may be derived from the 
former. This may be-clearly and easily seen by taking a simple 
case, in which we excite the stem by the application of external 
stimulus, on a zone a few centimetres below its upper end. 
As the direct effect of this stimulation, a contraction takes 
place in that zone. By this contraction an active expulsion 
of water, with local negative turgidity-variation, is brought 
about, and the expelled water is driven outwards in both 
directions from the excited zone. Following the course 
of the water which is thus forced upwards, we have found 
that it produces an increase of turgidity, or positive turgidity- 
variation, with consequent distension or expansion of all the 
cells above the stimulated area. Work is thus performed on 
these cells, in consequence of external stimulus, by which 
their latent energy is increased. 

The energy of the stimulus applied in one has thus been 
conveyed to another regfon, hydraulically, there to give rise 
to an effect of increased turgidity and expansion, which was 
designated as the indirect effect. Thus the effect of external 
stimulus is seen to be twofold namely, first by its direct 
action to induce local contraction, and secondly by its indirect 
effect, of increasing the internal energy, to bring about an 
expansion. The expressions of direct and indirect stimula- 
tion are thus seen to be opposite in character, and we have 
seen how they find opposite modes of expression in the case 
of each form of response. 

Various forms of responsive expression. The next 
question to be passed in review is that of the various modes 
in which the response of the plant was seen to find expression ; 
and here we found various responsive* phenomena which are 
characteristic of life, and apparently entirely unrelated, to be 
ultimately dependent upon this fundamental inter-action 
between, on the one hand, the contraction due to external 
stimulus, and on the other the expansion due to internal 
energy. 

(a) Lateral motile response by differential contraction. 
Taking first the responsive mechanical movement of motile 



706 PLANT RESPONSE 

organs, we find that its gradual evolution can be traced from 
the simplest case, namely, the longitudinal response of a 
radial organ. In such a radial organ, if one side be rendered 
in any way the less excitable say by the unilateral applica- 
tion of cold or anaesthetics, or by the fatigue induced by 
long-continued unilateral stimulation we have an induced 
anisotropy. On now subjecting the organ to diffuse stimula- 
tion, the relatively more excited side will undergo the 
greater contraction, and the response will thus be by that 
movement which results from the concavity of the more 
excited. We find many instances again of the various stages 
through which this anisotropy passes before it culminates in 
the dorsi-ventral pulvinus ; thus, for example, in a spirally 
formed tendril, where concavity has been induced by the 
unilateral excitation of that side, the concave surface is less 
excitable than the convex ; and such a tendril, when 
diffusely excited by strong electrical stimulus, exhibits the 
excitatory effect by extraordinary writhing movements due 
to the relatively greater excitation and concavity of the 
originally convex side (p. 92). A plagiotropic stem, again, 
whose upper side is fatigued by the action of sunlight, 
exhibits on diffuse stimulation a downward movement, due 
to the greater excitatory contraction of the more excitable 
lower half (p. 86). The phenomenon of differential response, 
then, whose various preliminary stages we have thus traced, 
comes to its greatest perfection in the dorsi-ventral pulvinus 
of such plants as Mimosa, for we have found that in the 
last-named organ the characteristic responsive movement is 
not brought about by the action of excitable cells restricted 
to the lower half. That the upper half also is excitable is 
shown by the fact that on applying to it localised stimulus, 
say of light, its cells contract and raise the leaf (p. 631). The 
fall of the leaf, on the application of diffuse stimulation, is 
thus the result of the greater excitability of the lower. This 
fall, again, is not due to mere flaccidity caused by the ex- 
pulsion of water from the excited organ, but to the actual 
differential contraction of the lower half, whose activity may 



REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 707 

be gauged by the tension it can be made to exert on a 
spiral spring (p. 26). As regards mechanical response, then, 
a single law that response is always by concavity of the 
more excited side will be found universally applicable. In 
a dorsi-ventral organ, like the pulvinus of Mimosa, we have 
seen that response to diffuse stimulation is always by the 
contraction of the more excitable lower half. The same 
law, however, is also applicable even in the case of a radial 
organ excited unilaterally, for here the side acted on is 
relatively the more excited, and response is by concavity of 
that side. 

A radial organ, acted upon from one side, undergoes 
concavity of that side, and consequent movement towards 
the stimulus. The same is true of pulvinated organs, when 
either the upper or lower half is acted upon locally by 
stimulus. As in radial organs, then, so also here, under these 
circumstances, we obtain instances of the directive action of 
stimulus ; but when stimulus is either internally or externally 
diffused, we obtain from a dorsi-ventral pulvinus the greater 
contraction of the more excitable half, and movement is 
thus made, for anatomical reasons, to occur in the direction 
at right angles to the plane which separates the two aniso- 
tropic halves. 

Even in Mimosa, the contraction of the pulvinus itself is 
not very great, but the contractile movement is highly 
magnified by the attached petiolar index. When a radial 
organ is diffusely stimulated there is no lateral movement at 
all, and from this fact it has been erroneously inferred that 
ordinary plants are not sensitive. In fact, however, every 
plant is sensitive, and exhibits contractile response, which 
is shown, in the case of radial organs, by longitudinal con- 
traction under diffuse stimulus, and by lateral response under 
unilateral stimulus. 

The direct effect of stimulus on the pulvinus of Mimosa, 
causing a negative turgidity-variation of the organ, finds 
appropriate expression in the responsive movement of fall. 
But we have seen that the expression of the indirect effect 

Z Z 2 



708 PLANT RESPONSE 

stimulus is opposite in character to that of the direct effect. 
The indirect effect of stimulus, or the increase of internal 
energy, thus results in an erectile response, as seen in the 
erection of the leaf in Mimosa, or Biophytum, when their 
internal energy is in any way enhanced, say by rise of 
temperature (p. 400). Taking all kinds of response, it will be 
found universally true that if the increase of internal energy 
give rise to one form of expression, the impact of external 
stimulus evokes the opposite. 

(#) Suctional response. It has been shown that con- 
tractile response gives rise to a forward impulsion of water ; 
hence by the excitatory contraction of the root-cells the 
movement of water upwards is initiated. When such con- 
tractile movement is not single, but repeated or multiple, 
continuous propulsion of water is maintained. The rate of 
this propulsion therefore affords us a means of measuring the 
rhythmic activity of the plant-tissue. 

(c) Growth response. This pumping-in of water causes 
the transmission of energy to the distant growing region, atd 
as by this the internal energy of the plant is increased, it 
finds expression there in a pulsatory expansion, which is the 
movement of growth. Following each pulse of expansion, 
there is a recovery which is incomplete, owing to fixation of 
growth-material. The resultant growth is thus the irreversible 
effect of the entire process. This growth-movement is another 
expression of that indirect effect of stimulation which we 
have already considered, by which a distant excited point 
gives rise to a progressive train of waves of positive turgid ity- 
variation. The action of these waves may be seen not only 
in the movement of expansion at the growing point, but also 
by the erectile response of an intervening motile organ. This 
will be understood from the following diagram of an artificial 
plant (fig. 277), which shows how contractile action at the 
base, giving rise to an hydraulic wave, causes two different 
expressions of (a) motile response of the lateral organ, or leaf, 
and (#) growth-expansion of the terminal growing point. 
The pulvinus of this artificial motile organ consists of an 



REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 709 




india-rubber tubing, the lower half of which is thinner, and 

therefore more expanding or responsive, than the upper. 

The upper end of the tube, representing the end of the plant, 

is closed over with a thin elastic strip of india- rubber. When 

now there is contractile action at the base, the pulse of 

increased hydraulic pressure thus induced is found to bring 

about a practically simultaneous erection of the artificial 

lateral leaf L, and an expansion or bulging outwards of the 

terminal yielding body G. The _.^ 

analogy in the case of the latter 

would be still more perfect if 

we imagined it covered, instead 

of with india-rubber, with some 

plastic substance which would set 

quickly on elongation or expansion,- 

thus representing the permanent 

growth-effect. From this we see 

the connection between growth and 

those other responses with which 

we are already familiar. The only 

difference between the responsive 

expansion of pulvinated organs and 

this responsive expansion of growth 

lies in the fact that in the former 

recovery is perfect, and in the latter 

imperfect. It must also be borne 

in mind that growth may be 

initiated locally at the growing region if the sum total 

of energy, directly or indirectly supplied to it, be above 

par. 

We further see in the case given that growth-response is not, 
strictly speaking, { of its own accord ' or spontaneous ; for the 
energy of a definite stimulus, causing contractile movement 
at the base, is transmitted hydraulically, and. performs the 
work of growth.. It would be as accurate to describe the 
work done by hydraulic machinery as spontaneous, ignoring 
the energy that had set the pump in action, as to call growth 



P'iG. 277. Model of an Arti- 
ficial Plant 

Sudden compression of india- 
rubber bulb, R, causes 
hydraulic wave upwards, 
downward movement being 
prevented by valve v. 
This wave of increased 
pressure causes erection of 
the leaf, L, and the expan- 
sion of the terminal septum 
into G. 



7IO PLANT RESPONSE 

spontaneous, without recognising or tracing that energy 
which in the form of stimulus must have been supplied to 
some part of the plant machinery. 

We have seen that a plant in a state of growth-standstill 
has its activity renewed when a stimulus is applied to the 
distant root, and that when the amount thus supplied is 
exhausted, the activity ceases. In growth, then, which is 
regarded as so characteristic a phenomenon of vital action, 
we see the law of the conservation of energy holding 
good, as in an ordinary inorganic system. The plant thus 
expresses the absorbed energy, by a responsive expan- 
sion, either of erection or of increased rate of growth, 
according to the particular organ of response which is 
concerned, both of these constituting cases of work done 
by internal energy, or indirect effect of stimulus ; but where 
the responding organ is directly excited by external stimulus, 
we obtain a contractile response of the organ, with expulsion 
of water, or negative turgidity-variation. The hydraulic cur- 
rent in our model is now reversed, and opposite responsive 
movements take place, by the fall of growth below the 
normal rate, and by depression of the leaf. If now we take 
a balanced record of growth, the impact of a uniform series 
of external stimuli will be found to give rise to a series of 
responses by depression of the rate, followed by recovery, 
exactly similar to those records of depressions of the leaf, 
with recoveries, which are obtained from pulvinated organs. 

We may also see the expression of external stimulus and 
cessation of stimulus, in the appropriate periodic variations 
of growth, and movements of motile organs, which occur 
under the stimulating action of daylight, and the withdrawal 
of such stimulus at night. Thus in the daytime we see 
a response consisting of depression of the rate of growth, 
which corresponds to the depression of the leaf, say of Mimosa, 
and at night a recovery, or enhancement of growth, and 
erection of the motile organ. The daily, periodic curves 
obtained of this growth-variation and responsive mechanical 
movement are very similar. 



REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 711 

(//) Torsional response. Another interesting type of 
response occurs when an anisotropic or dorsi-ventral organ 
is stimulated laterally. Under such conditions, a responsive 
torsion is induced, by which the less excitable side is made 
to face the external stimulus. The extent of the response 
depends on the intensity of stimulus, and the differential 
excitability of the anisotropic organ. In dorsi-ventral organs, 
the p!ane of anisotropy is fixed. But in certain climbing 
plants, this plane revolves in a positive or negative direction, 
and under the internal activity of growth, an autonomous 
torsional movement is thus observed. The opposition of the 
effects of internal energy and external stimulus is here seen, 
when such an organ is uni-laterally acted on, say by light. 
The autonomous torsional movement is then found to be 
retarded, or even reversed. 

(e) Death response. There is another curious phenomenon 
of response, which takes place at a certain definite point 
When an organ is gradually raised in temperature, the 
internal energy is increased, and the organ exhibits a re- 
sponsive expansion, if radial by elongation, or if pulvinated 
by erection ; but when the death-point is reached, a sudden 
and irreversible molecular change takes place, attended by 
an excitatory contraction. In the curve of thermo-mechanical 
response we here find a sharply defined point of reversal, 
which affords us an exact index of the death-point. This 
death-point is very definite in plant-organs under normal 
conditions ; in phanerogamous plants it is very near 60 C. 
Physiological modification of the tissue, moreover, may be 
gauged by the transposition of the otherwise definite death- 
point (p. 185). 

(/") Thermographs of regional death. Just as there is a 
definite point of reversal in the thermo-mechanical curve, so 
there is also a point of discoloration which is, under standard 
conditions, at a determinate interval from the death-point. 
A particular region, physiologically changed, may thus be 
thermally 'developed,' and made to exhibit as a thermo- 
graph, a picture of localised physiological variation (p v i 



712 PLANT RESPONSE 



Electrical response. We have, lastly, to consider 
briefly the electrical mode of responsive expression. The 
excitatory contraction, with negative turgidity-variation, of 
a vegetable, as of an animal tissue, is accompanied by an 
electrical variation of galvanometric negativity. The in- 
direct effect of stimulus with its positive turgidity-variation, 
moreover, has also a concomitant electrical expression of 
galvanometric positivity (p. 37). 

Different types of response : (a) Uniform response. 
These various expressions of response are brought about by 
the fundamental molecular change induced by stimulus. 
When stimulus is moderate, and sufficient time allowed for 
recovery from molecular distoition to the original condition, 
a series of responses to uniform stimuli will be uniform ; 
but if very strong stimulus be applied, recovery will only be 
completed after a long interval. 

(b) Fatigue. If successive stimuli be applied before 
complete recovery has taken place, the successive responses 
will exhibit diminution, or fatigue. Under strong and long- 
continued stimulation the plant-tissue exhibits, as in the 
case of tetanised muscle, a fatigue-reversal that is to say, 
the contracted tissue passes into what is apparently its 
original expanded condition ; but the difference between 
the normal condition and the condition of fatigue-reversal 
is seen in the fact that, while the former is sensitive to fresh 
stimulation, the latter is insensitive. The fatigued tissue, 
however, resumes its original excitability after a period of 
rest. This fatigue-reversal explains the erection of the 
Mimosa leaf under continuous stimulation (p. no). We 
observe similar fatigue-reversals, even in inorganic substances 
like india-rubber, where the normal contraction under thermal 
stimulation passes into relaxation under the long-continued 
action of such stimulation ; and the india-rubber becomes 
sensitive again only after a sufficient period of rest (p. 120). 
In connection with this, we sometimes meet with the very 
curious case of alternate or periodic fatigue, both in living 
and in inorganic substances. The simplest type of this 



REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 713 

occurs when, under uniform stimuli, responses are alternately 
large and small. These alternations sometimes show them- 
selves in groups. Under continuous stimulation, again, this 
periodic fatigue exhibits itself by response of an oscillatory 
character. 

(t) Staircase response. The tissue is sometimes in a 
relatively sluggish condition, and by the absorption of 
stimulus the molecular mobility is gradually increased. The 
effect of this is seen in the amplitude of successive responses, 
increasing in a staircase manner. 

It is usually supposed that response is brought about by 
a chemical run-down of energy, of an explosive character. 
The external stimulus is thus supposed to act as it were 
on a trigger, to release the latent energy. The response 
is hence assumed to be disproportionately larger than the 
stimulus. That this cannot, however, hold good in all 
cases is clear ; for the tissue is often found to absorb a 
certain proportion of the incident stimulus, the immediate 
expression of response being thus disproportionately smaller 
than stimulus. The energy of the system is now found, 
instead of being lowered, to be raised above par. The 
internal energy thus held latent is sometimes seen, as 
in the case of strongly stimulated Biophytum> to find 
expression later by multiple response. In the case of 
growth-response, again, it is a variable fraction of incident 
stimulus that finds immediate expression in the direct effect 
of retardation of growth, whereas the absorbed component 
gives rise to the subsequent responsive effect of an enhanced 
rate of growth. Referring to the former of these as the 
direct, and to the latter as the indirect, effect of stimulus, it 
is found that the sum of the two remains approximately 
constant. Below the optimum tonic condition it is found 
that the indirect effect is relatively the larger, but near the 
optimum this relation is reversed, and the direct effect is 
the larger. In a sub-tonic condition stimulus produces little 
or no direct effect, it being utilised to produce the indirect 
effect of enhanced growth. At the optimum, on the other 



7 14 PLANT RESPONSE 

hand, practically the whole of the incident energy is expressed 
in direct response, there' being little or no absorbed element 
(p. 460). A similar series of considerations may be applied 
to the response of mature pulvinated organs. In this case, 
however, the indirect effect of stimulus will find expression 
in an enhancement of the rate of recovery of the organ. It 
is, however, difficult always to discriminate with certainty 
between the natural and an enhanced rate of recovery ; 
but on turning to growth-response we find that, by using 
the balanced method of record, it is easy to distinguish 
between the direct and indirect effects of stimulus, since 
these are here shown by curves in opposite directions. 
This method moreover affords us some means of measuring 
the relative magnitudes of the two factors in the response. 

Excitability. It is interesting to find that an agency 
which induces a variation of excitability produces a similar 
modification of all the different forms of response. In this 
way the long-continued application of cold has the effect of 
lessening excitability, and the response of a motile organ is 
thus found to be temporarily diminished or abolished. At 
the moment of application, however, owing to the fact that 
any sudden variation of environment acts as a stimulus, its 
effect is the induction of an excitatory movement. These 
phenomena are repeated with curious exactness in the case 
of suctional response. On the application of ice-cold water 
to the root, the immediate effect is a transient exaltation of 
suction, followed later by depression and arrest (p. 375). In 
growth-response, also, growth is diminished or arrested by 
this agency. 

Another effect of the moderate application of cold is to 
induce a molecular sluggishness by which the latent period is 
increased. A moderate rise of temperature, on the other 
hand, increasing the molecular mobility, has the contrary 
effect, of reducing the latent period. 

Anaesthetics, again, induce a diminution or abolition of 
excitability, as is seen by their effect on the various forms of 
response. 



REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 715 

The excitability of a tissue which has not recovered fully 
from previous strong stimulation is found to be impaired. 
The fatigue-effect only disappears after the lapse of a period 
of rest. If, then, the resting intervals between successive 
stimuli be gradually shortened, the motile responses will be 
found to be progressively diminished, and a time arrives 
when the succeeding stimulus evokes no response, the tissue 
having become as it were refractory. The minimum interval 
during which the tissue remains thus irresponsive is known 
as the refractory period. In Biophytum, under normal condi- 
tions, this period is about ten seconds in duration (p. 273). 

Conductivity. It is usually supposed that the trans- 
mission of the excitatory effect, as seen in sensitive plants 
like Mimosa, is merely the transmission of a hydro- 
mechanical disturbance, and therefore unlike the transmis- 
sion of the excitatory effect in 'animal tissues. It has, 
however, been shown that this is not the case, for here, as in 
the animal, transmission of excitation takes place by the 
propagation of protoplasmic changes. It was shown further 
that the hydrostatic disturbance was transmitted with a 
relatively great rapidity, whereas the true excitatory effect 
has a slower and definite velocity, characteristic of the 
particular specimen. This velocity, moreover, is found to be 
modified, and that in a manner precisely similar, by all those 
agencies which modify the velocity of transmission of excita- 
tion in animal tissues. Thus, cold, anaesthetics, and fatigue 
are all influences which reduce the velocity of transmission. 
As an example of this, we saw, in a certain specimen of 
Biophytum, in which the normal velocity was 3-8 mm. per 
second, that slight cooling reduced it to 1-3 mm. per secdnd, 
or almost to one-third. Conversely, the raising of the 
temperature from 30 C. to 37 C. increased the velocity from 
37 to 9*i mm. per second. Strong stimulus is found to be 
conducted further and more rapidly than feeble or moderate. 
It is found in the case of animal tissues, again, that on 
account of its physiologically depressing effect, the anode 
acts as a block to the transmission of excitation ; and the 



7l6 PLANT RESPONSE