Plant Response as a Means of Physiological Investigation

LONGMANS, GREEN, & CO., 39 Paternoster Row, London, New York and Bombay. 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 communicated as a Memoir to the Science Congress at Paris, in August 1900,1 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.2 Believing in the continuity of responsive phenomena 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 exhibited excitation by electrical response, and the proposition that ordinary plants also showed excitatory electrical response to mechanical stimulation, and that such response was appropriately modified under physiological changes, was much controverted. I have to thank Professor Sidney H. Vines,

1 ' De la Generality des Phenomenes Moleculaires produits par l'Electricite sur la Matiere Inorganique et sur la Matiere Vivante.' {Travaux du Congris International de Physique. Paris, 1900.) 2 Paper read before Royal Society, June 6, 1901. Also Friday Evening Discourse, Royal Institution, May 10, 1901. 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 all 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 regarded 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 movements 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 movement 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 plant may nevertheless be regarded as a machine, and that its movements in response to external stimuli, though apparently

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 plantmovements 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 ' spontaneous 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 demonstrate that there are, strictly speaking, no ' 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, constituted a further result which, at the outset of the investigation, was little to be foreseen. 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. Responsive movements in plants— Work done by plant — Plant as a machine — 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

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 — Practicable forms of graduated stimulus — Electro-thermic stimulator — Stimulation by condenser discharge — Response-recorder — Advantage of counterpoise— Response of BiophyUtm 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

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 — 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 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 differences of excitability by electric test — Excitability of plant-tissues in general— Re- sponsive power characteristic of matter 29

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 turgescence, 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 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 43

Pulvinoid and pulvinus — Demonstration of mechanical response in ordinary leaves — Response of Artocarpus 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

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 Cytierea not unique— Similar longitudinal responses obtained with stems, roots, tendrils, petioles, stamens, and styles of ordinary plants — Also in fungi— Responsive contraction in Passi flora, comparable in extent with that in Cynerece — Longitudinal response in plants modified by the physiological variations due to age, season, and chemical agencies . . . . . . ; ... , 66

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 Cucurbita 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 . 82

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 BiophyHcm — 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 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

Mimosa 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 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 crTemical actions— Volta-chemical effect and by-products — Supposition that response always disproportionately larger than stimulus, not justified — Existence of three types : (1) 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 . . . . . . . . . . . .116

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 . . . . . . . . . .129

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

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— (b) Determination by spasmodic lateral movement at moment of death — Experiments with Mimosa — Death-contraction a true physiological response — Continuity of fatigue and death — Deathpoint earlier in young tissues — Composite spasmodic movement — {b') Determination of death-point in tendril of Passiflora, by sudden movement of uncurling — (c ) Determination of death-point by method of volumetric contraction of hollow organ, causing expulsion of contained water 148

Death-spasm in anisotropic organ due to differential longitudinal contraction — In radial organ the death-contraction is purely longitudinal — Deathpoint 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 thermomechanical 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 thermomechanical 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 coldrigor— Constancy of death-point ........... 159

Lowering of death-point by fatigue— Modification of characteristic thermomechanical 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 acapitulum — 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

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 Effect of high E.M.F, — Effects at two stages, A and B — Experimental verification of A stage effect — Similar effects seen in protozoa— Experimental 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

Receptive excitability, conductivity, and motile excitability— Molecular Model — Modification of motile excitability : (a) by anaesthetics — (/>) by cold — {c) by fatigue — Variation of conductivity : (a) by cold — (3) 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

The anode acts as a block to the transmission of stimulus — Opposite effect of kathode — Experiments on Biofihytum, 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 . . . . . . .231 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 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 transmission— Indifferent parenchymatous tissues practically not transmitters of excitation — Comparative tables showing velocity of transmission in various plant and animal tissues ........ 238

Pfeffer's experiment on expulsion of water from excited cells— Author's experiment on a delicate method of detecting excitatory expulsion of cellsap— Chemical method of determining velocity of transmission of excitation— Electrotactile detector — Demonstration of passage of excitatory contractile wave by means of electrotactile method — Determination of velocity of transmission of excitation in ordinary plants by electromotive method — Excitatory versus hydro-mechanical movement of water . . 254

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

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 automatically responding plant ; conversion of Desmodium into ordinarily responding plant — Similar polar effects of current in Biophytum and in Desmodium leaflet at standstill — Moderate stimulus in Biophytum and in Desmodium at standstill produces single response ; and strong stimulus, multiple response . ., . . . . . . -279

Production of pulsatory movements as after-effect of energy absorbed — Physical analogue -Localisation of seat of automatic excitation in Desmodium— Demonstration of multiple response to a constant stimulus : (1) Chemical — (2) Electrical— (3) Stimulus of light- Multiple response to constant stimulus of light, in : (a) retina — (b) Biophytum —(c) Desmodiuni — (4) Thermal— Induction of automatism in Biophytum at favourable temperature — (5) Of internal hydrostatic pressure — Absorption of external energy and its absorption by the plant in latent form — True meaning of ' tonic ' condition — Cause of rhythmicity — After-effect, and its relative persistence 295

The recorder and experimental chamber — Absolute measurement of period and amplitude of Z>^w^'«/^-oscillation — Responsive significance of up and down movements deduced from (a) analogy with response of Mimosa ; (b) test of increased internal hydrostatic pressure — ' Systolic ' contraction and ' diastolic ' expansion of 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 pulvinus, or internally — Spark -record of Desmodiumpulsation . . . 315

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 thermotonic 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 The similarities, in their fundamental characteristics, of rhythmic tissues, animal and vegetable: (1) In responses — (2) In possession of long refractory periods — (3) In incapability of tetanus — Theories regarding the causation of heart-beat— The. similarities of rhythmic tissues, animal and vegetable, as seen in : (1) The effects of internal hydrostatic pressure — (2) The effects of variation of temperature— (3) The periodic groupings "■*-■— __ a 2