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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