TRANSACTIONS OF THE BOSE RESEARCH INSTITUTE, CALCUTTA, VOL. II, 1919 LIFE MOVEMENTS IN PLANTS BY SIR JAGADIS CHUNDER BOSE. Kt.. m.a.. d.Sc.. cs.i., c.i.e., PROFESSOR EMERITUS. PRESIDENCY COLLEGE. DIRECTOR. BOSE RESEARCH INSTITUTE. WITH 128 ILLUSTRATIONS CALCUTTA BENGAL GOVERNMENT PRESS. 1919 PUBUSHED BY THE BOSE RESEARCH INSTITUTE. CALCUTTA. w TRANSACTIONS OF THE BOSE RESEARCH INSTITUTE, CALCUTTA. VOL. II, 1919 LIFE MOVEMENTS IN PLANTS BY SIR JAGADIS CHUNDER BOSE, Kt.. m.a.. d.Sc. c.s.i.. c.i.e. PROFESSOR EMERITUS. PRESIDENCY COLLEGE. DIRECTOR, BOSE RESEARCH INSTITUTE. WITH 128 ILLUSTRATIONS / ^^ (( 1 ^ ■ CALCUTTA BENGAL GOVERNMENT PRESS. 1919 PUBLISHED BY THE BOSE KESfcAKCH INSTITUTE. CALCUTTA. WORKS BY THE SAME AUTHOR. RESPONSE IN THE LIVING AND NON- LIVING. . With 117 Illustrations, 8vo. 10s. millionth of an inch per second, the sensi- tiveness of this apparatus thus rivals that of the spectro- scope. The increasing refinement in our experimental methods cannot but lead to important advances towards a deeper understanding of underlying reactions in the living organism. I shall here draw attentioi to only a few of the important results given in the present volume. The tropic effect of light has been shown to have a definite rela- tion to the quantity of incident light. A complete tropic curve has been obtained from sub-minimal to maximal stimulation which shows the inadequacy of Weber's law, for the sub-minimal stimulus induces a qualitative differ- ence in physiological reaction. It has further been shown that the prevalent idea that perception and helioiropic excitation are two distinct phenomena is without any foundation. With reference to the effect of ether waves on plants, 1 have given an account of my discovery of the response of all plants to wireless stimulation, the results being similar to that induced by visible light. The perceptive range of the plant is thus infinitely greater than ours ; for it not only perceives, but also responds to different rays of the vast ethereal spectrum. The results obtained by the method of geo-electric response show that the responsive reaction of the root IV is in no way different from that of the shoot, the opposite movements being due to the fact that in the shoot the stimulation is direct, and in the root it is indirect. Full description is given of the new method of physiological exploration by means of the electric probe, by which the particular layer which perceives the stimulus of gravity is definitely localised. The method of electric probe is also found to be of extended applica- tion in the detection of physiological changes in the interior of an organ. An important factor of nyctitropic movements, hither- to unsuspected, is the effect of variation of temperature on geotropic curvature. This and other co-operative factors have been fully analysed, and a satisfactory explanation has been offered of various types of diurnal movement. A generalisation has been obtained which explains all the diverse movements of plants, under all modes of stimulation : it has been shown that dh^ect stimulation induces contraction and reta^^dation of growth, atid that indirect stimulation induces an expansion and acceleration of growth. Another generalisation of still greater importance is the establishment of identical nature of physiological reaction in the plant and the animal, leading to advances in general physiology. Thus the discovery of a method for immediate enhancement or inhibition of nervous impulse in the plant led to my success in the control of nervous impulse in the animal. Another important discovery was the dual nervous iinpulaes in plants, and I have very recently been able to establish, chat the nervous impulse generated in the animal nerve by stimulus is not single, but double. The study of the responsive phenomena in plants must thus form an integral part of physiological investigation into various problems relating to the irritability of all living tissues, and without such study the investigation must in future remain incomplete. October 1919, J. C. BOSE. CONTENTS. PART III. TROPISM IN PLANTS. XXII.— THE BALANCED CRESCOGRAPH. PA«E. Principle of the Metliod of Balance — Compensating move- ment— Grrowtli-scale — Sensitiveness of the Crescographic Balance— Effect of CO.,— Effect of anaesthetic,-, . . 255 XXIIL— ON TROPIC MOVEMENTS. Complexity of the problem — Contradictory nature of responses — Two classes of tropic responses — Longitudinal transmission of effect of stimulus — Transverse trans- mission of effect of stimulus — Modificat'on of tropic curvature by conducting power of tissues and diCEerential excitability of the organ ...... 2Gfl XXIV.— TROPIC CURVATURE WITH LONGITUDINAL TRANS- MISSION OF EFFECT OF STIMULUS. Dual impulses, positive and negative, caused by stimulus — Direct and Indirect stimulus — Tropic effect of Indirect stimulation . • . . . . . . . . 271 ii LIFE MOVEMENTS IN PLANTS XXV.— TROPIC CURVATURE WITH TRANSVERSE TRANSMISSION OF EFFECT OF STIMULUS. PAGE, Turgor variation under transverse transmission oE stimulus- effect — Tropic responses of pulvinated and growing- origans to unilateral tcimulatinn — Direct unilateral stimu- lation— Indirect unilateral stimulation — Dillerence of effects induced by Direct and Indirect stimulation — Laws of tropic curvature ....... 279 XXVL— MECHANOTROPISM : TWINING OF TENDRILS. Anomalies of mechanotropism — Effects of indirect and direct electric stimulation on growtli of tendril — Effect of direct and indirect meclianical stimulus — Immediate and after-effect of stimulus — Inhibitory action of stimulus — Response of less excitable side of the tendril — Relative intensities of responses of upper and under sides of tendril of Passiflora — Negative curvature of tendril 288 XXVIL— ON GALVANOTROPISM. Polar eff(3cts of electric current on growth — Effect of anode and cathode on growth ..... 301 XXVIII.— ON THERMONASTIC PHENOMENA. Effect of temperature — Different thermonastic organs — Two types of response : Positive and Negative — Effect of rise of temperature and of stimulus on thermonastic organs — Law of thermonastic reaction .... 305 CONTENTS iii XXIX.— ON PHOTOTROPISM. PAGE. Complexity of piohleni of phototropic reaction — Action of liglit — Positive phototropic curvature of pulvinated organs — Positive phototropic curvature of growing organs — Plienoineuon of recovery — Immediate and after- ortect of light on growtli — Latent period of phototropic reaction — Growtli variation induced by flash of light from a single spark — Maximum positive curvature under continued action of light ...... 31.3 XXX.— DlA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM. Differential excitability of two i;alves of pulvinus of Mimosa — Transformation of positive to negative curvature — Tropic effect under sunlight — Negative phototropism of slioot and root ........ 328 XXXI.— REL'ATION BETWEEN THE QUANTITY OF LIGHT AND THE INDUCED PHOTOTROPIC CURVATURE. Effect of increasing intensity of light on pulvinated and growing organs — Effect of increasing angle — Effect of duration of exposure ....... 338 XXXII.— THE PHOTOTROPIC CURVE AND ITS CHARAC- TERISTICS. Summation of stimulus — General consideration — The general characteristic curve — Characteristics of simple phototropic curve — Variation of susceptibility for excitation in different parth of the curve — Effect of sub-minimal stimulus — The complete phototropic curves of pulvinated and QTOwing organs — Limitation of Weljor's law . 346 f] iv LIFE MOVEMENTS IN PLANTS XXXIII.— TRANSMITTED EFFECT OF PHOTIC STIMULATION. Effect of light applied on tip of Sctaria — Response of growing region to unilateral stimulus — Effect of simul- taneous stimulation of the tip and the hj-pocotyl — Algebraical summation of effects of direct and indirect stimuli .......... 302 XXXIV.— ON PHOTONASTIC CURVATURES. Phototropic response of anisotropic organs — Positive pnra-heliotropism — Negative para-heliotropisni — Responses of pulvi nated and growing organs to light . . 378 XXXV.— EFFECT OF TEMPERATURE ON PHOTOTROPIC CURVATURE. Effect of temperature on excitability — Effect of tempera- ture on conduction — Phototropic response of tendrils — Seasonal variation of phototropic curvature — Antagonistic effects of light and of rise of temperature . . 388 XXXVL— ON PHOTOTROPIC TORSION. Torsional response to light — Effect of different modes of lateral stimulation — Effect of differential excitability on the direction of torsion — Laws of torsional response — Complex torsion under light — Advantages of the Method of Torsional Response — The Torsional Halance — Determination of the direction of stimulus . 397 XXXVII.— RADIO-THERMOTROPISM. Effect of infra-red radiation — Positive radio-thermotropism — Dia-radio-thermotropism — Negative radio-thermotropism 410 CONTENTS XXXVJll.— RESPONSE OF PLANTS TO WIRELESS STIMULATION. PAGE. Effects of different rays of spectrum on t;ro\vtli — The wireless system — Meciianical and electrical responses of Mimosa to Hertzian waves — Effect of wireless stimula- tion 01) growth of plants , . . . . .416 XXXIX.— GEOTROPISM. Direction of the stimulus of gravity — The Geotropic Recorder — Determination of the character of geotropic reaction — Theory of statoliths — Determination of the latent period — The complete geotropic curve — Determin- ation of effective direction of stimulus of gravity — Algebraical snnimation of effects of geotropic and photic stimulus — Analogy between the effects of stimulus of light and of gravity — Relation between the directive angle and geotropic reaction — Differential geotropic excitability ......... 425 XL.— GEO-ELECTRIC RESPONSE OF SHOOT. Electric response to direct and indirect stimulation — Experimental arrangement for obtaining geo-electric response — Geo-electric response of the upper and lower sides of the organ — Method of Axial Rotation — Charac- teristics of geo-electric response — Physiological character of geo-electric response — Effect of differential excitabi- lity of the organ — Law determining the relation between angle of inclination and geotropic effect — Method of Vertical Rotation — Electric response through an entire cycle — Relation between angle of vertical rotation and intensity of geo-tropic reaction ..... 442 yi LIFE MOVEMENTS IN PLANTS XLI.— MECHANICAL AND ELECTRICAL RESPONSE OF ROOT TO VARIOUS STIMULI. PAGE. Mechanical and electrical response to Direct stimulation — Mechanical and electrical response to Indirect stimula- tion— Effect of unilateral stimulation applied at tlie root- tip 4fil XLII.— GEO-ELECTRIC RESPONSE OF ROOT. Geo-electric response of the root-tip — Electric response in the growing region of root — Differential effect between tiie tip and the growing region — Geo-perception at the root-tip .......... 467 XLIIL— LOCALISATION OF GEO-PERCEPTIVE LAYER BY .MEANS OF THE ELECTRIC PROBE. Principle of the method of electric exploration — The Electric Probe — Electric exploration of the geo-percep- tive layer — Geo-electric reaction at different depths of tissues — Microscopical examination of the maximally excited layer — Influence of season on geo-electric response — Tests of insensitive specimens — Reaction at lower side of the organ — The Method of Transverse Perforation 478 XLIV.— ON GEOTROPIC TORSION. Arrangement for torsional response — Algebraical summation of geotropic, and phototropic effects — Balance of geo- tropic by phototropic action — Comparative balancing- effects of white and red lights — Effect of coal gas on photo-geotropic balance ...... 503 CONTENTS vii XL v.— ON THERMO-GEOTROPISM. PAGE. Necessary conditions for geotropic curvature — Modify iuj;; influence of temperature on geotropic curvature — Magnetic analogue — Tropic equilibrium under varying intensities of stimulus — EfEect of variation of tempera- ture on geotropic torsion — Variation of apo-geotropic curvature under thermal change — EfEect of variation of temperature on dia-geotropic equilibrium . . 509 PART IV. NIGHT AND DAY MOVEMENTS IN PLANTS. XLVL— DIURNAL MOVEMENTS IN PLANTS. Complexity of tiie problem — The difiEerent factors in- volved— Autonomous movements — Epinasty and hypo- nasty — L'csitive and negative thermouasty — Thermo- geotropism — Classification of diurnal movements — Discriminating tests for classification — Diurnal variation of light and of temperature ..... 523 XLVIL— DIURNAL MOVEMENT DUE TO ALTERNATION OF LIGHT AND DARKNESS. Experimental arrangements — The Quadruples Nyctitropic Recorder — Diurnal movement of the leaflet of Cassia alata — Effect of variation of temperature — Effect of variation of light — Diurnal movement of the terminal leaflet of Desmodium gyrans — The 'midday sleep' . 535 XL VIII.— DIURNAL MOVEMENT DUE TO VARIATION OF TEMPERATURE AFFECTING GROWTH. Tropic and nastic movements — Distinction between thermo- nastic and therrao-geotropic action — Diurnal movement of Nymphcea — Action of light — Efl:ect of variation of temperature 546 Viii LIFE MOVEMENTS IN PLANTS XLIX.— DAILY MOVEMENT IN PLANTS DUE TO THEKMO- GEOTROPISM. I'AGE. Cliaracteristics of thenno-geotrcpic movements — Diurnal iiiov'imerit of Palm trees — Diurnal icoveinent of procum- bent stems and of leaves — Continuous diurnal record for successive thermal uoon — Modification of tlie diurnal curve — Effect of fluctuation of temperature — Effect of restricted pliability of the organ — Effect of age — Effect of season — Reversal of the normal rhythm — Effect of constant temperature — Diurnal movement in inverted position ........ 554 L.— THE AFTER-EFFECT OF LIGHT. Electric after-effect of light — After-effect at pre-maximum, maximum, and post-maximum — Tropic lesponse under light, and its after-effects at pre-maximum, maximum, and post-maximum ....... SGi) LI.— THE DIURNAL MOVEMENT OF THE LEAF OF MIMOSA. Four different phases in the diurnal record of Mimosa — Different factors determining its diurnal move- ment— Diurnal variation of geotropic torsion — Auto- nomous pulsation of tiie leaf of Mimosa — The Photometric Recorder — Effect of direct light — The evening spasmodic fall of the leaf — Diurnal movement of the amputated petiole — Diurnal curve of the petiole of Cassia alata — Hesponse of Mimosa to darkness at different parts of the day — After-effect of light at pre-uiaximutn, maximum, and post-maximum . . 076 ILLUSTRATIONS. FIGLKE. 93. 94. 95. 9(r. 97. 98. 99. 100. 101. 102. 103. 104. 105. 10«. 107. 108. 109. 110. 111. Ii2. 114. Arrangement for compensation of growth-movement by equal subsidence of plant-holder Photograph of the Balanced Crescograph Balanced Crescographic record . . . . . Record showing the effect of CO.^ .... Ilffect of ether and of chloroform . . ' . Diagrammatic representation of effects of Indirect and Direct stimulation ....... Tropic curvature of Grinum ...... Turgor variation caused by Indirect stimulation Response of himona leaf under transverse transmis- sion of effect of electric stimulus . . . . Diagrammatic representation of Indirect and Direct stimulation of tendril ..... Record by Method of Balance .... Variation of growth under direct stimulation Positive curvature of tendril of Cueurbita Diagrammatic representation of effects of Indirec and Direct unilateral stimulation of tendril Retardation of rate of growth under cathode Acceleration of rate of growth under anode Thermonastic and radionaslic response^ of petal of Zephyranthes ....... The Thermonastic Kecorder ..... Negative thermonastic respcmse of Nymphcea Successive positive responses of the terminal leaflet of bean plant ....... Positive response and recovery under moderate photo tropic stimulation ...... Persistent positive curvature under stronger stimulation 21 257 258 260 265 266 275 276 281 282 290 291 292 295 296 303 303 308 309 310 317 318 318 LIFE MOVEMENTS IN PLANTS FIGURE. 115. Immediate and afier-effect of stimulus of light on growth ........ 116. Latent period for photic stimulation 117. Effect of a single electric spark on growth . 118. Responses of Mimosa leaf to light from above . 119. Responses of Mimo»a leaf to light from below 120. Record of effect of contiimous application of light on upper half of pulvinus of Mimosa . 121. Positive and negative phototropic response Oryza ........ 122. Leaf of Desinodium gyratis ..... 123. The Oscillating Recorder 124. Tropic effect of increasing intensity of light ou the leaflet of Desmodium gyrans .... 126. Tropic effect of increasing intensity of light on growing organ (Crinum) ..... 126. The Collimator 127. Effect of angle of inclination of light on tropi curvature of pulvinated organ 128. Effect of angle of inclination on growth-curvature 129. Effect of increasing duration of exposure to light 130. Effect of continuous electric and photic stimulation on rate of growth ..... 131. Characteristic curve of iron .... 132. Simple characteristic curve of phototropic reaction 133. Complete phototropic curve of pulvinated organ 134. Complete phototropic curve of growing organ 135. Arrangement for local application of light 136. Response of seedling of Setaria to light . 137. Effect of application of light to the growing hypo cotyl of Setaria ...... 138. Response to direct and indirect photic stimulus 139. Diagrammatic representation of the effects of direct and indirect stimulation of Setaria 140. Photonastic curvature of creeping stem of Mimosa pudica ........ 141. Positive phototropic response of Erythrina indica PAGE. 320 324 325 330 330 331 335 339 340 341 341 342 343 343 344 348 351 351 358 359 367 368 370 373 376 380 382 ILLUSTRATIONS XI FIGURE. 142. 143. 144. 145. 146. Response of leaflet of Mimosa to light Response of leaflet of Averrhoa to light . Diagramnidtio representation of different types of photo tropic response ...... Phototropic curvature of tendril of Passiflora Effect of rise of temperature on phototropic curva ture ...... ... 147. .\fter-effict of rise of temperature .... 148. Arrangement for record of torsional response . 149. Record of torsional response of pulvinus of Mimosa pudica ........ l.iO. Leaflets of Cassia alata ..... 151. Positive response to thermal radiation 152. Record of positive, neutral, and reversed negative curvature under thermal radiation . 153. Diagrammatic representation of the wireless system 154. Mechanical response of Mimosa leaf to electric waves 155. Electric response of Mimosa to Hertzian wave . 156. Record of responses of growing organs to wireless stimulation ....... 157. The Quadruplex Geotropic Recorder 158. Effect of alternate application of cold on upper and lower sides of the organ .... 159. Geotropic response of flower stalk of Tuho-rose 160. Geotropic response of Tropceoluvi 161. The Complete Geotropic Curve .... 162. Diagrammatic representation of photic and geotropic stimulation ....... 163. The effect of super-imposition of photic stimulus 164. Diagrammatic representation of the mechanical and electrical response ...... 165. Diagrammatic representation of geo-electric response 166. Diagrammatic representation of Methods of Axial and Vertical Rotation ...... 167. Diagrammatic representation of the geo-electric res ponse of the shoot ...... 168. Geo-electric response of the petiole of Trojpceolum 21 A PAGE. 383 383 384 392 394 395 399 400 404 413 414 419 420 420 422 428 430 433 433 435 436 436 443 447 449 450 452 di LIFE MOVEMENTS IN PLANTS FKiUnE. PAGE. 169. Geo-electric response of the scape of Uriclis . . 453 170. Mechanical and electric response to indirect stimula- tion 1 • • ■ 463 171. Diagrammatic representation of mechanical and elec- tric response of root . . . . . . 464 172. Diagrammatic representation of geo-electric response of root-tip ......... 469 173. Diagrammatic representation of geo-eleotric response of growing region of root . . . . . 471 174. Diagrammatic representation of the geo-perceptive layer ......... 480 175. The Electric Probe 483 176. Transverse section showing continuous geo-perceptive layer {Bri/ophyllum) ...... 48H 177. Curve of geo-electric excitation in different layei-s of Nymphcea ... ... . .497 178. Curve of geo-electric excitation in Bryophylluiii . 497 179. Diagram of arrangement of geotropic torsional response ......... 503 180. Additive effect of stimulus of gravity and light . 505 181. Algebraical summation of geotropic aud phototropic actions ......... 505 182. Comparative balancing effects of white and red liglits 506 183. Effect of coal gas on photo-geotropic balance . 507 184. Diagram of magnetic balance . . . . . 511 185. Effect of variation of light on phototropic equilibrium 512 186. Effect of variation of tempeiature on geotropic torsion . . . . . . . • • 514 187. Simultaneous records of variation of temperature, on up and down movement, and of torsion of the leaf of Mimosa ' ' ' ' . , . . ol8 188. Arrest of pulsatory movement of leaflet of Desino- dium gyrans by light ...... 528 189. Effect of unilateral light oxi hyponastic movement . 529 190. The Nyctitropic Recorder 537 191. Effect of sudden darkness on leaflet of Cada alata . 539 ILLUSTRATIONS xiU FIGURE. PAGE. 192, Diurnal movement of the leaflet of Cassia alata . 540 19.3. The day and night position of the petiole nud terminal leaflet of Desmodium gyrans . . . '141 194. Diurnal record of the terndnal leaflet of Desmodium gyrans . . . . . . . 542 195. Photograph of closed flower of Nymphcra during day OfiO 196. Photograph of open flower of Nymphcea at night . 550 197. Action of light on the petal of Nymphcea . . . 551 198. Diurnal movement of tlie petal of Nymph(va . 552 199. Diurnal record of the Sijbaria Palm . . . 556 200. Diurnal record of inclined Palm, geotropicaliy curved procumbent stem of Trvpo'ohim, and dia- geotropic leaf of Palm ...... 557 201. Diurnal record of leaves of Dahlia, Papya, and Croton 558 202. Duirnal record of procumbent stem of Tropreolnin, and leaf of Dahlia for two successive days . 560 203. Abolition of the diurnal movement under constant temperature {Tropcvolnm) ..... 565 204. Efl^ect of inversion of plant on diurnal movement of Tropceolum ........ 567 205. Electric response of the leaf stalk of Bryophyllum under light . . . . . . . . 57.1 206. Diagrammatic representation of electric after-effect of photic stimulation ...... 571 207. After-effect of pre-maximum photic stimuiaiiim . 574 208. After-effect of maximum photic stimulation . . 574 209. After-effect of post-maxinnim photic stimulation . 574 210. Diurnal record of Mimosa in summer and winter . 577 211. Record of diurnal variatioii of torsion in Mimosa leaf 582 212. Continuous record of automatic pulsation of Mimosa leaf 585 213. Photometric record showing variation of intensity of light from morning to evening .... 586 214. Record of loaf of Mimosa after amputation of sub- petioles 589 XIV LIFE MOVEMENTS IN PLANTS FIGURE. PAGE. 215. Diurnal record of Cassia leaf ..... 591 216. Post-maximum after-effect of light on response of leaflet of Cassia 592 217. Effect of periodic alternation of light and darkness on response of Mimosa leaf ..... 594 218. Pre-raaximum after-effect of light in Mimosa . . 595 219. After-effect at maximum ...... 595 220. Post-maximum after-effect exhibiting over-shooting below position of equilibrium ..... 595 PART III. TROPISM IN PLANTS. I XXII.— THE BALANCf:D CRESCOGRAPH By Sir J. C. BosE. We shall in the succeeding series of papers denl with the subject of tropism in general. Different plant organs undergo curvature or bending, sometimes towards and at other times away from the stimulus which induces it. The problem is very intricate ; the possibility of its solu- tion will depend greatly on the accurate determination of the immediate and after-effects of various stimuli on the responding organ. The curvature induced in the growing organ is brought about by variation, often extremely slight, of the rate of growth ; the result, moreover, is liable to be modified by the duration and point of application of stimulus. The difficulties connected with the problem can only be removed by the detection and measurement of the minutest variation in growth, and by securing a continuous and automatic record of the entire history of the change. In the chapter on High Magnification Crescograph an account is given of the apparatus which I have devised by which the rate of growth may be magnified from ten thousand to ten millions times. It is thus possible to measure the imperceptible growth of plants for a period shorter than a single second. The variation of normal rate of growth is also found by measuring successive growth records on a stationary plate at regular intervals, say of ten seconds, or from the flexure in the growth-curve taken on a moving plate (p. 163). I was next desirous of exalting the sensitiveness to a still higher degree by an independent method, which would 256 LIFE MOVEMENTS IN PLANTS not only reveal very slight variation induced in the rate of growth, but also the latent period and time-relations of the change. For this purpose I at first devised the Optical Method of Balance* which was considered at the time to be extremely sensitive. The spot of light from the Optical Lever (which magnified the rate of growth) was made to fall upon a mirror to which a compensating movement was imparted so that the light-spot after double reflection remained stationary. Any change of rate of growth — acceleration or retardation — was at once detected by the movement of the hitherto stationary spot of light in one direction or the other. A very careful manipulation was required for the adjustment of the Optical Balance ; the record moreover was not automatic. For these reasons I have been engaged for several years past in perfecting a new appa- ratus by which, (1) the balance could be directly obtained with the utmost exactitude, (2) where an attached scale would indicate the exact rate of growth, and (3) in which the upsetting of the balance by external stimulus would be automatically recorded, the curve giving the lime rela- tions of the change. PRINCIPLE OF THE METHOD OF BALANCE. I shall take a concrete example in explanation of the method of balance. Taking the rate of growth per second of a plant to be ^—^ inch or 0*5 fx per second (equal to the wave length of sodium light), the tip of the plant will be maintained at the same point in space if we suc- ceeded in making the plant-holder subside exactly at the same rate. The growth-elongation of the plant will then be exactly balanced by a compensating movement down- wards. The state of exact balance is indicated when the * "Plant Response"— p. 413. THE BALANCED CRESCOGRaPH 257 recording lever of the Crescograph traces a horizontal line on the moving plate. Overbalance or underbalance will deflect the record below or above the horizontal line. COMPENSATING MOVEMENT, For securing exact balance the holder of the plant P, in the given example, will have to subside at a rate of ^~^ SG Fig. 93. — Arrangement for compensation of growth-movement by equal subsi- dence of plant-holder; S, adjusting screw for regulation of speed of rotation; G, governor; W, heavy weight; P, plant-holder. inch per second. This is accomplished by a system of reducing worm and pinion, also of clock wheels (Fig, 93), The clock at first used for this purpose was worked by the usual balance wheel. Though this secured an average balance yet as each tick of the clock consisted of sudden movement and stoppage, it caused minute variation in the 258 LIFE MOVEMENTS IN PLANTS rate of subsidence ; this became magnified by the Cres- cograph and appeared as a series of oscillations about a mean position of equilibrium. This particular defect was obviated by the substitntion of a fan governor for the balance wheel. But the speed of rotation slows down with the unwinding of the main spring, and the balance obtained Fig. 94. — Photographic repvoduction of the Balanced Crescograpb. L, L^, magni- fying compound lever. R, recording plate. P. plant. C, clockwork for oscillation of the plate and lateral movement. G. governor. M, circular growth-scale. V, plant- chamber. at the beginning was found to be insufRcient later on. The difficulty was finally overcome by the use of a heavy THE BALANCED CRESCOGRAPH 2oy weight W, in the place of coiled spring. The complete apparatus is seen in figure 94. For purpose of simplicity of explanation, I assumed the growth rate to have a definite value of r^^ inch per second. But the rate varies widely in different plants and even in the same plant at different days and seasons. In practice the rate of growth for which compensation has to be made varies from j^y^o ^^ 25~ooo~ iii^h, or from 0"17/x to I'O/x per second. We have thus to secure some means of continuous adjustment for growth, the rate of which could be continuously varied from one to six times. This range of adjustment I have been able to secure by the com- pound method of frictional resistance and of centrifugal governor. As regards frictional resistance the two pointed ends of a hinged fork rub against a horizontal circular plate not shown in the figure. By means of the screw head S, the free ends of the fork spread out and the circumfer- ence of the frictional circle continuously increased. The centrifugal governor is also spread out by the action of the adjusting screw. By the joint actions of the frictional control and the centrifugal governor, the speed of rotation can be continuously adjusted from 1 to 6 times. When the adjusting screw is set in a particular position, the speed of rotation, and therefore the rate of subsidence of plant-holder, remains absolutely constant for several hours. The attain- ment of this constancy is a matter of fundamental import- ance, and it was only by the employment of the compound system of regulation that I was able to secure it. The method of obtaining balance now becomes extremely simple. Before starting the balancing movement by clock regulation, the plant is made to record its magnified growth by the Crescograph. The compensation is effected as follows : the speed of the clockwork is at the beginning adjusted at its lowest value, and the pressure of a button 260 LIFE MOVEMENTS IN PLANTS starta the balancing movement of the plant downwards. On a^ccount of partial balance the record will be found to be less steep than before ; the spead of the clock is gradually Fig. 95. — Balanced Creacographic record: (a) sh nving effect oi underbalaiic and (6) overbalance of about 3 per cent. (Magnification 2,000 times.) increased till the record becomes perfectly horizontal under exact balance. Overbalance makes the record slope down- wards. In figure 95 is seen records of underbalance (a) and overbalance {b), to the extent of about 3 per cent. It will thus be seen that the effect of an external agent may be detected by the upsetting of the balance ; an up- movement indicates (unless stated to the contrary) an enhancement of the rate of growth above the normal ; and a down-movement, on the other hand, a depression of the normal rate. Calibration. — The cahbration* of the instrument is obtained in two different ways. The rate of subsidence of the plant-holder, by which the balance is obtained, is strictly proportional to the rate of rotation of the vertical spindle and the attached train of clock-wheels. A striker is attached to one of the wheels, and a bell is struck at each complete revolution. The clockwork is adjusted THE BALANCED CRESCOGRAPH 261 at a medium speed, the bell striking 35 times in a minute. A microscope micrometer is focussed on a mark made on the plant-holder, and the amount of subsidence of the mark determined after one minute ; this was found to be 0'0525 mm. As this fall occurred after 35 strokes of the bell the subsidence per stroke was 0*0015 mm. Determination of the absolute rate of growth. — If growth be found balanced at N strokes of bell per minute, the rate of subsidence per second = N x -^— mm. per second = N X '000025 mm. per second = N X "025 fi per second = M X 10~'' inch per second. Example. — The growth of a specimen of Zea Mays was found balanced when the number of strokes of the bell was 20 times in a minute. Absolute rate of growth = 20 x '025 fi = 0*5 /x per second or = 20 X 10-3 inch or = ' ^ 50,000 " '» If we take the wave length of sodium light X as our standard, the growth in length per second is equal to X. This will give us some idea of the sensitiveness of the Crescograph employed in recording the movement of growth. GROWTH-SCALE. The Balanced Crescograph enables us not merely to determine the absolute rate of growth, but the slightest fluctuation in that rate. Indicator Scale. — All necessity of calculation is obviated by the scale provided with the apparatus. The speed of clockwork which brings about the balance of growth is determined by the position of the adjusting screw S, the 262 LIFE MOVEMENTS IN PLANTS gradual lowering of which produces a continuous diminu- tion of ppeed. A particular position of the screw thei-efore indicates a definite rate of subsidence for balancing growth. By a simple mechanism the up or down movement of the screw causes rotation of an index pivoted at the centre of a circular scale. Each division of the scale is calibrated by counting the cori-psponding number of strokes of the bell per minute at different positions of the adjusting screw. The scale is calibrated in this manner to indicate different rates of growth from 0"2 fjL to 1*2 fx per second. The determination of the rate of growth now becomes extremely simple. Few turns of the screw bring about the balance of growth and the resulting position of the index against the circular scale automatically indicates the absolute rate. The procedure is even simpler and more expeditious than the determination of the weight of a substance by means of a balance. SENSITIVENESS OF THE CRESCOGRAPHIC BALANCE. Perhaps the most delicate method of measuring lengths is that afforded indirectly by the spectrum of a light. A good spectroscope resolves differences of wave lengths of Dj (=0-5896 /x) and D.. (=0-5890) i.e. of 1 part in a thousand. The average rate of growth of Zea Mays is of this order ; being about 0-5 yu. per second. Let us consider the question of the possibility of detecting a fractional variation of the ultra-microscopic length by means of the Balanced Cresco- graph. In reality the problem before us is more intricate than simple measurement of change ui length ; for we have to determine the rate of variation of length. The sensitiveness of the balance will, it is obvious, depend on the magnifying power of the Crescograph. By the Method of Magnetic Amplification referred to in page 170, I have succeeded in obtaining a magnification of ten THK BALANCED CRBSCOGRAPH 263 million times. In this method a very delicate astatic system of magnets undergoes deflection by the movement of a magnetised lever in its neighbourhood. A spot of light reflected from a small mirror attached to the astatic system, thus gives the highly magnified movement of the rate of growth, which may easily be raised to ten million times. I shall in the following describe the results obtained with this easily managed magnification of ten million times. Determination of sensitiveness .- Experiment 99. — A seed- ling of Zea Mays was placed on the Crescographic Balance ; and the magnetic amplification, as stated above, was ten million times. With 18 strokes of the bell per minute the spot of light had a drift of + 266 cm. per minute to the right ; this is because the growth was underbalanced. With faster rate of clock movement, i.e., 21 strokes in 68 seconds or 18*53 strokes per minute, the drift of the spot of light, owing to overbalance, was to the left at the rate of - 530 cm. per minute. Thus (1) 18 strokes per minute caused a drift of + 266 cm. per minute. (2) 18"53 strokes per minute caused a drift of - 530 cm. per minute. Hence by interpolation the exact balance is found to correspond to. 18*177 strokes per minute. Therefore the absolute rate of growth = 18-177 X 0-025 fi per second. = 0-45/i, per second. = 0-000018 inch per second. We learn further from (1) and (2) that a variation of ii-— — ^^ produces a change of drift of the spot of light 18-177 ^ f & from + 266 to - 530 cm., i.e., of 796 cm. per minute. 22 2G4 LIFE MOVEMENTS IN PLANTS As it is easy to detect a drift of 1 cm, per miuiite a 0"53 variation of , ^ , __ ' _„ . , or 1 part in 27,000 may thus be lo"177 X 7^0 detecred by the Method of Balance. The spectroscopic method enabled us, as we saw, to detect change of wave length 1 part in a thousand. The sensibility of the Balanced Crescograph is thus seen to rival, if not surpass that of the spectroscope. For obtaining a general idea of the sensitiveness, the absolute of growth in the instance given above was 0'0001'r (movement away from stimulus). Though these curvatures result from protoplasmic reactions, yet the jjositive ciirvi- ture is not necessarily associated with positive protopla^^mic reaction. It will be shown that the curvature of an organ is determined by the algebraical summation of effects induced at the proximal and distal sides of the responding orjjfan. LONGiTUbiif aL transmission of effect of stimulus 273 Physiologists have not been aware of the dual character of the impulse generated by stimulns, and the term " trans- mission of stimulus" is thus misleading since its effect may be an expansion, or its very opposite, contraction. It is therefore necessary to discriminate the effect of one from the other : the impulse which induces an increase of turgor, expansion, and galvanometric positivity will be distinguished as positive, in the sense that it causes an enhancement of turgor. The other, which induces diminution of turgor and contraction, wilt be termed as the excitatory impulse. Transmission of the latter is dependent on conducting power of the tissue ; the positive impulse is practically independent of the conducting power. In animal physiology again, there is no essential difference between the effect of the direct and indirect stimulation. In a nerve-and-muscle preparation, for ex- ample, indirect stimulation at the nerve induces the same contraction as the direct stimulation of the muscle. The only difference lies in the latent period, which is found to be longer under indirect stimulation by the time interval necessary for the excitation to travel along the conducting nerve. It is probable that stimulus gives rise to dual impulses in the animal tissue as in the plant. But the detection of the positive impulse in the animal nerve is rendered exceedingly difficult on account of the high velocity of conduction of excitation. I have explained that the separate effects of the two impulses can only be detected if there is a sufficient lag of the excitatory negative behind the positive, so that the relatively sluggish responding organ may exhibit the two impulses one after the other. In a highly conducting tissue the lag is very slight, and the negative will therefore mask the positive by its predominant effect. In spite of the difficulty involved in the problem, I have recently been successful in demonstrating the dual impulses in the animal nerve. ^74 LIFE MOVEMENTS IN PLANTS In any case it is important to remember the following characteristic effects of indirect stimulation. TAULE XXn. — SHOniNG THE EFKECT OF INDIKECT STIMULATION. Intensity of stimulus. Character of intervening tissue Responsive effect. Moderate Feeble Highly conducting Non-conducting Sami-conducting 11 11 ••■ Contraction. Expansion. Expansion followed by contraction. Expansion. These effects of indirect stimulation have been fully demonstrated in the case of pulvinated organs (p. 136) and growing organs (p. 215). Having demonstrated the fundamental reactions of direct and indirect stimulation, we shall next study the tropic effects induced in growing organs by the effect of unilateral application of indirect stimulus. Experiment 108. — I have already explained, how thermal radiation is almost as effective in inducing contraction and retardation of growth as th3 more refrangible rays of the spectrum. The thermal radiation was produced by the heating of a platinum spiral, short of incandescence, by the passage of an electric current. The intensity of radiation is easily varied by adjustment of the current by means of a rheostat. The experimental specimen was a flower bud of Crinum. It was held by a clamp, a little below the region of growth. Stimulus was applied below the clamp so that the transmitted effect had to pass LONGITUDINAL TRANSMISSION OF EB'FECT OF STIMULUS 275 through S, the securely held tissue (Fig. 98). A feeble stimulus was applied on one side, at the invlifferent Fig. 98. — Diagrammal ic representation of effects of indirect and direct sti- mulation. Continuous arrow represents the indirect stimulation, and the curved continuous arro w above, the induced negative curvature : dotted arrow indicates the application of direct stimulus, and the dotted curve above, the induced positive curvature. point about ',\ cm, below the region of growth. The positive effect of indirect stimulus reached the region of growth on the same side, bringing about an accelera- tion of growth with expansion and convexity, the resulting movement being negative or away from the stimulus. The latent period was ten seconds, and maximum negative movement was completed in the further course of ten seconds, after which there was a recovery in the course of 75 seconds. A stronger stimulus S' gave a larger response ; but when the intensity was raised still higher to S", the excitatory negative impulse overtook the positive within 15 seconds of its commencement ; the convex was 276 LIFE MOVEMENTS IN PLANTS thus succeeded Ly the concave curvature (Fig. 99). Direct application of stimulus at the growing region gave rise to a positive curvature. Fig. 99.— Tropic curvature of Crinum to unilateral indirect stimulation of increasing intensities : S, S' of moderate intensity induced negative tropic effect (movement away from the stimulated side) ; stronger stimulus S" gave rise to negative followed by positive. Successive dots at intervals of 5 seconds Magnifi- cation 100 times. The effect of feeble stimulus transmitted longitudinally is thus found always to induce convexity, a negative curvature and movement away from stimulus. I have obtained similar responsive movement of negative sign with various plant organs, and under various formg of stimuli. Thus in the stem of Dregea volubilis the longitudinally transmitted effect of light of moderate LONGITUDINAL TRANSMISSION OP EFFECT OF STIMULUS 277 iutensily was u negative curvature ; direct application of light on the growing region gave, on the other hand, a positive curvature an6, 216). The resulting curvature is thus brought about by the joint effects of direct sti- mulation of the proximal, and indirect stimulation of the distal side. We may now recapitulate some of the important facts relating to tropic curvatures : Indirect stimulation gives rise lo dual impulses, positive and negative ; of these the positive impulse is practically independent of the conducting power of the tissue ; but the transmission of the excitatory negative impulse is tlependent on the conducting power. No tissue is a perfect conductor, nor is any a perfect non-conductor of excitation, the difference is a question of degree. In a petiole or a stem the conducting power along the direction of length is considerable, but very feeble in a transverse direction. In a semi-conducting tissue, a feeble stimulus will transmit 1284 LIE*E MOV^mSnTS In PLANTS* only the positive impulse ; strong or long continued sti- mulation will transmit both positive and negative impulees, the positive preceding the negative. The transmitted posi- tive gives rise to increase of turgor, expansion, and accelera- tion of rate of growth ; the negative induces the opposite reaction of diminution of turgor, of contraction, and of retardation of rate of growth. Transverse transmission is only a particular instance of transmission in general ; the only difference is that the conducting power for excitation is very much less in the transverse than in the longitudinal direction. Owing to feeble transverse conductivity, the transmitted impulse to the distal side often remains positive ; it is only under strong or conti- nued stimulation that the excitatory negative reaches the distal side and neutralises or reverses the previous positive reaction. If the distal is the more excitable side, the reversed response will appear as pronounced negative. I give a table which will clearly exhibit the effects of stimulus on the proximal and distal sides of the responding organ. TABLK XXIV. — SHOWING RESPONSIVE EFFECTS COMMON TO PULVINI AND GROWING ORGANS UNDER UNILATERAL STIMULATION. Effect of direct stimulation on Effect of ii.direct stinuilatioii on proximal side. i distal side. Diminution of tiu-gor Galvanometric negativity Contraction and concavity Increase of turgor. Galvanometric positivity. Expansion and convexity, When stimulus is stroug or long continued, the true excitatory effect is conducted to the distal side, neutralising or reversing the first response. TRA!!JSVERSE TRANSMISSION OF EFFECT OF STIMULUS 28:') The diagram whicli I have already given (Fig. W) clearly explains the different tropic effects induced by changing the point of application of stimulus. We may thiis have stimulus applied at the responding region itself (Direct Stimulation) or at some distance from it (Indirect Stimu- lation). The final effect will be modified by the con- ducting power of the tissue. DIRECT UNILATERAL STIMULATION. Type I. — The tissue has little or no power of trans- verse conduction : stimulus remains localised, the proximal side undergoes contraction, and the distal side expansion. The result is a positive curvature. Type 11. — The tissue is transversely conducting. Under strong and long continued stimulation the excita- tory impulse reaches the distal side, neutralising or reversing the first effect. INDIRECT UNILATERAL STIMULATION. Tifpe I. — The intervening tissue is an indifferent con- ductor : transmitted positive impulse induces ex- pansion and convexity on the same side, thus giving rise to negative curvature {i.e., away from stimulus). Type II. — Intervening tissue is a fairly good conductor : tlie effect of positive impulse is over-powered by the predominant excitatory negative impulse, the final result is a concavity and positive curva- ture, with movement towards the stimulus. m LIS^E MOVEMENTS IN PLANTjS. The following is a tabular statement of the different effects induced by Direct and Indirect stimulation. TABLE XXV. — SHOWIN'G UIFKERKNCE OF EFFECTS INDUCED BY DIRECT AND INDIRECl' STIMTLATION. Stimulation. Nature of the tissue. Final effect. Direct (Feeble) Indirect ,, Direct (Strong) Indirect ,, Semi-conducting tissue. Better conducting tissue. V 51 11 Positive curvature. Negative curvature. Neutral or negative curvature. Negative followed by positive curvature. The results of investigations already described, enable us to formulate the general laws of tropic curvature applicable to all forms of stimuli, and to all types of responding organs, pulvinated or growing. LAWS OF TROPIC CURVATURE. I, {a) DIRECT APPLICATION OP UNILATERAL STIMULUS OF MODERATE INTENSITY, INDUCES A POSITIVE OR CONCAVE CURVATURE, BY THE CONTRACTION OF THE PROXIMAL AND EXPANSION OF THE DISTAL SIDE. (h) UNDER STRONG OR LONG-CONTINUED STIMULATION, THE POSITIVE CURVATURE IS NEUTRALISED OR REVERSED, BY TRANSVERSE CONDUCTION OF EX- CITATION ; THIS EFFECT IS ACCENTUATED BY THE DIFFERENTIAL EXCITABILITY OF THE TWO SIDES OF THE ORGAN. Transverse transmission of effect of stimulus 287 2. {a) indirect application of unilateral stimu- lus of feeble intensity induces a negative curvature. (h) in a conducting tissue the excitatory effect being TRANSMITTED UNDER STRONG AND LONG CONTINUED STIMULATION, INDUCES A POSITIVE CURVATURE, IL will thus be seen that the tropic effect is modified by : (1) the point of application of stimulus, (2) the intensity and duration of stimulus, (;>) the conducting power of tissue in the transverse direction, (4) the relative excitabilities of the proximal and distal sides of the organ. In the following series of Papers the tropic effects of various forms of stimuli will be studied in detail. SUMMARY, In a semi-conducting !issue Direct stimulation induces a diminution ot turgor and contraction, Indirect stimula- tion inducing the opposite effect of increase of turgor and expansion. Unilateral stimulation thus induces a positive curvature by the joint effects of contraction at the proximal, and expansion at the distal side. Under strong and long continued unilateral stimula- tion, the excitation at the proximal side is transmitted to the distal side. Transverse conduction thus neutralises or reverses the normal positive curvature. XXVI.— MECHANOTROPISM: TWINING OF TENDRILS By Sir J. C. BoSE, Assisted hi/ GURUPRASANNA DAS. In response to the stimulus of contact a tendril twines round its support. Certain tendrils are uniformly sensitive on all sides ; but in other cases, as in the tendril of Passiflora, the sensitiveness is greater on the under side. A curvature is induced when this side is rubbed with a splinter of wood, the stimulated under side becoming concave. This movement may be distinguished as a move- ment of curling. There is, as I shall presently show, a response where the under side becomes convex, and the curvature becomes reversed. As regards perception of mechanical stimulus, Pfeffer discovered tactile pits in the tendrils Cucurhitacece. These pits no doubt facilitate sudden deformation of the sensi- tive protoplasm by frictional contact. No satisfactory ex- planation has however been offered as regards the physio- logical machinery of responsive movement. The difficulty Twining of TfiNr>liiLS 289 of explanation of twining movements is accentuated by a peculiarity in the response of tendrils which is extremely puzzling. This anomaly was observed by Fitting in tendrils which are sensitive on the under side : "If a small part of the upper side and at the same time the whole of the under side be stimulated, curvature takes place only at the places on the under side which lie opposite to the unstimulated regions of the upper side. The sensitiveness to contact is thus as well developed on the upper side as on the under side, and the difference between the two sides lies in the fact that while stimulation of the under side induces curvature, stimulation of the upper side induces no visible result, or simply inhibits curvature on the under side, according to circumstances."* Here then we have the inexplicable phenomenon of a particular tissue, itself incapable of response, yet arresting the movement in a neighbouring tissue. The problem before us may be thus stated : Is the move- ment of the tendril due to certain specific sensibility of the organ, on account of which its reactions are characteristically different from other tropic movements ? Or, does the twining of tendril come under the law of tropic curvature that has been established, namely that it is brought about by the contraction of the directly stimulated proximal side, and the expansion of the indirectly stimulated distal side ? I shall now describe ray investigations on the effects of direct and indirect stimulus on the growth of tendril ; I * Jost— /6t(/— p. 490. 290 LIFE MOVEMENTS IN PLANTS have in this investigation studied the effect not merely of mechanical, but also of other forms of stimuli. I shall also describe the diverse effects induced by mechanical st mulus under different conditions. From the results of these experi- ments I shall be able to show that the twining of the tendril comes under the general law of tropic curvature ; that the curvature results from the contraction of the proximal and expansion of the distal side. Finally I shall be able to offer a satisfactory explanation of the inhibition of response of the tendril by the stimulation of the opposite side of the organ. OENERAL EFFECTS OF INDIRECT AND DIRECT ELECTRIC STIMULATION ON THE GROWTH OF TENDRIL. For this experiment I took a growing tendril of Cucurbita in which the sensitiveness is more or less uniform on all sides. The tendril was suitably mounted on the Balanced Cresnograph, which records the variation of the rate of growth induced by immediate and after-effect of stimulus. The specimen is held in a clamp as in the diagram (Fig. 102), the tip being suitably attached to the recording lever. For indirect stimulation feeble shock from an induction coil is applied at the two electric connections Fi«. io-2.-i)iagramn,atic below the damp. Direct stimulus is repiedentation of indirect applied by means of electric connections and direct stimulation of ^^^ above and the Other below the clamp. tendril TWINING OF TENDRILS 291 Effect of Indirect Stimulus : Experiment 106. — The growth of the tendril was exactly balanced, and the record became horizontal. Indirect stimulus was next applied below the clamp ; this is seen to upset the balance, with the resulting up-curve which indicates a sudden acceleration of growth above the normal. This acceleration took place within ten seconds of the application of Fig. 103.— Record by Method Stimulus, and persisted for three of Balance, showing acceiera- minutes ; after this the normal rate tion of growth of tendril (up- ^^^ ^^^^,^^ became restored, as seen curve ■) induced by indirect sti- muiation. {Cucurbita.) by the balanced record once more becoming horizontal (Fig. 103). Effect of Direct Stimulus : Experiment 107. — The in- cipient contraction induced by direct stimulation is so great that the record obtained by the delicate method of balance cannot be kept within the plate. I, therefore, took the ordinary growth-curve on a moving plate. The first part of the curve represents normal growth ; stimulus of feeble electric shock was applied at the highest point of the curve. This is seen (Fig. 104) to induce an immediate contraction and reversal of the curve which persisted for two and half minutes, after which growth was slowly renewed. Tht^ most interesting fact regarding the after- effect of stimulus is that the rate of growth became actually enhanced to three times the normal. This is clearly seen in the record (upper half of the figure) taken 20 minutes 292 LIF'E MOVEMENTS IN PLANTS . ' after stimulation, where the curve is far more erect than that of th«^ normal rate of growth before stimulation. Fui. 104. — Variation of growth induced by direct stimuiation. Firat part of the curve showB normal rate of growth. Direct stimulation induces contraction (reversal of curve). After-effect of stimulus seen in highly erect curve in upper part of record, taken 20 minutes after. The effects of Indirect and Direct stimulation of the tendril are summarised below : (1) Indirect stimulation induces a sudden enhancement of rate of growth, followed by a recovery of the normal rate. TWINING OF TENDRILS 293 (2) Direct stimulatiou induces a retardation of the rate of growth which may culminate into an actual contraction. The after-effect of direct stimulus of moderate intensity is an enhancement of the rate ofijrowth. The experiments describ.Ml above demonstrate the effects ot tlireet and indirect electrical stimulus. I shall now proceed to show that mechanical stimulus induces effects which are similar to those of electric stimulus. EFFECTS OF DIRECT AND INDIRECT MECHANICAL STIMULUS. Kff'ect of Direct meclianical. stimulus : Experiment 108. — In this case I took a tendril of Gucurhita, and attached it to the ordinary High Magnification Crescograph, the record of which gives the absolute rate of its normal growth, and the induced variation of that rate. The tendril was stimulated mechanically by simultaneous friction of its different sides. The immediate effect was a retardation of growth, the reduced rate being less than half the normal. There was a recovery on the cessation of the stimulus ; the rate of growth was even slightly enhanced after an interval TABLE XXVI. — SHOWING THE IMMEDIATE AND AFTER-EFFECT OK MECHA- NICAL STIMULATION ON TENDRIL {CuCurbitu), Normal rate of growth ... ... ... 0'44 H- per sec. Retarded rate iniinediutely after stimulation ... 0'20 M- „ „ Hecoverv and enhancement after 15 minutes ... 0'50 /a ,, ,, of 15 minutes. Table XXYI shows the immediate and after-effects of mechanical stimulation on growth. 294 LIFE MOVEMENTS IN PLANTS The immeJiate and after-effects of mschauical stimulus on the tendril are therefore the same as that of electric stimulus. The incipient contraction under direct mechani- cal stimulus, moreover, is not the special characteristic of ten) the distal or co.wex si 1 ■ unln-go.^s an immeliata enhancement of growth. * PfeHet— Ibid— \ol 111, p. 57. TWINING OF TENDRILS 295 I give below a record given by h tendril of Cucurhltd in response to unilateral contact of short duration (Fig. 10')). Successive ^h the application, of ^riiuulus on the upper side 24 296 LIFE MOVEMENTS IN PLANTR of the tendril of Passiflora did not induce any response, yet it inhibited the normal response of the under side. The results of experiments which I have described will, however, afford a satisfactory ex- planation of this curious inhibition. It has been explained, that the curvature of the tendril is due to the joint effects of diminished turgor and contraction at the directly stimulated side, and an enhancement of turgor and ex- pansion on the opposite side. In the diagram seen in figare 106, the left is the more excitable side, and contraction will induce con- cavity of the stimulated side. But if the opposite or less excitable side of the tendril be stimulated at the same time, then the transmitted effect of indirect stimulus will induce enhancement of turgor and expansion on the left side, and thus neutralise the previous effect of direct stimulus. An inhibition of response will thus result from the stimulation of the . opposite side. A difficulty arises here from the fact that the upper side of the tendril (the right side in Fig. 106) is supposed to be inexcitable and non-contractile. Hence there may be a misgiving that the stimulation of the non-motile side may not induce the effect of indirect stimulus (an increase of turgor and expansion) at the opposite side, which is to inhibit the response. But I have shown that even a non-contractile organ under stimulus generates both Fig. lot). — Diagrammatic re- presentation of effects of In- dii-ect and Direct unilateral stimulation of the tendril. Indirect stimulation, I, induces movement away from stimulated side (negative curvature) repre- sented by continuous arrow. Direct stimulation, D, induces movement towards stimulus (positive curvature) indicated by dotted arrow. TWINING OP TENDRILS 297 the impulses, positive and negative. This is seen illustrated in figure 100, where the rigid stem of Mimosa was subjected to unilateral stimulation ; the effect of indirect stimulus was found to induce an enhancement of turgor at the diametri- cally opposite side, and thus caused an erectile movement of the motile leaf. Electric investigations which I have carried out also corroborate the results given above. Here also stimulation of a non-motile organ at any point, induces at a diametrically opposite point, a positive electric varia- tion indicative of enhanced turgor. It will thus be seen that inhil)ition is possible even in the absence of contraction of the upoer side of the tendril ; hence the contraction of the directly stimulated side is neutralised by the effect of indirect stimulation of the distal side. RESPONSE OF LESS EXCITABLE SIDE OP THE TENDRIL. It is generally supposed that the upper side of the tendril of Passijlora is devoid of contractility. This is how- ever not the case, for my experiments show that stimula- tion of the upper side also induces contraction and con- cavity of that side, though the actual movement is rela- tively feeble. Experiment J09. — In order to subject the question to quantitative test I applied feeble stimulus of the same intensity on upper and lower side alternately. Successive stimuli were kept more or less uniform by employing the foUovsring device. I took a flat strip of wood 1 cm. in breadth, and coated 2 cm. of its length with shellac varnish mixed with fine emery powder. On drying the surface became rough, the flat surface was gently pressed against the area of the tendril to be stimulated, and quickly drawn so that the rough surface 2 cm. x 1 cm. was rubbed against the tendril in each experiment. Stimulation, thus produced, induced a responsive movement of each side of 24 a 298 LIFE MOVEMENTS IN PLANTS the organ. The extent of the maximam movement was measured by the microscope micrometer. The following results were obtained with four different specimens. TABLE XXVII. — SHOWING THE RELATIVE INTENSITIES OF RESPONSES OF THE UI'l'Elt AND UNDER SIDE OK TENDRIL {Passijlora). Movement induced by stimulation of under side, A. Movement induced by stimulation of upper side, B. Ratio --. A (1) 85 divisions ... (2) 106 (3) 60 (4) 80 „ 14 divisions 15 8 „ 10 1/6 1/7 1/7 1/8 It will thus be seen that the upper side of the tendril is not totally inexcital)le, its power of contraction being about one-seventh that of the under side. NEGATIVE CURVATURE OP THE TENDRIL. I shall now describe certain remarkable results which show that under certain definite conditions the tendril moves away from the stimulated side. I have explained, how in growing organs the effect of unilateral stimulus longitudinally transmitted, induces an expansion higher up on the same side to which the stimulus is applied, re- sulting in convexity and movement away from the stimulus, (cf. Laws of Tropic Curvatures, p. 286). As the reaction of tendril is in no way different from that of growing organs in general, it occurred to me that it would be possible to indue* in it a negative curvature by application of indirect unilateral stimulu'^. twIniNg Of tendrils '29^ Experiment 110. — A tendril of Passijiora was held in a clamp, as in the diat^ram (Fig. 1U6) in which the left is the more excitable side of the or^ran. The responsive movement of the tendril is o])served by focussing a read- ing microscope on a mark on the upper part of the tendril. Direct mechanical stimulation at the dotted arrow makes the tendril move in the same direction, the response being positive. But if stimulus be applied on the same side below the clamp the tendril is found to move away from stimulus, the response being now iiefjative. This reversal of response, as previously stated, is due to the fact that the transmitted effect of indirect stimulus induces an acceleration of growth higher up on the same side, which now becomes convex. The result though unexpected, is in every way parallel to the response of the flower bud of Crinam, in which the normal positive res- ponse was converted into negative by changing the point of application of stimulus, so that it became indirect (p. 216). SUMMARY, The response of tendril is in no way different from that of growing organs in general. Direct stimulus, electrical or mechanical, induces an inci- pient contraction ; the after-effect of a feeble stimulus is an acceleration of growth above the normal. Indirect stimulus induces an enhancement of the rate of growth. Under unilateral mechanical stimulus of short duration the directly excited proximal side undergoes contraction, the indirectly stimulated distal side exhibits the opposite effect of expansion. The induced curvature is thus due to the joint effects of the contraction of one side, and the expansion of the opposite side. 300 LIFE MOVEMENTS IN PLANTS As the after-effect of direct stimulus is an acceleration of fifrowth above the normal, the stimulated side undergoes an expansion by which the recovery is hastened. Unilateral application of direct stimulus induces a posi- tive curvature, but the same stimulus applied at a distance from the responding region induces a negative curvature. The tendril of Pdssijlora is excitable both on the upper and under sides ; the excitability of the under side is about seven times greater than that of the upper side. Stimulation of one side of the tendril induces an ex- pansion of the opposite side, even in cases where the con- tractility of the stimulated side is feeble. The response to stimulation of the more excitable side of the tendril is thus inhibited by the stimulation of the opposite side. This is because of the neutralisation of the effect of direct by that of indirect stimulation. XXVII.-ON GALVANOTROPISM By Sir J. C. BosE, Assisted by GURUPRASANNA DaS. Before describing the effect of unilateral application of an electrical current in inducing tropic curvature, I shall give an account of the polar effect of anode and cathode on the pulvinated and growing organs. In my previous work * on the action of electrical current on sensitive pulvini I have shown that : — (1) at the 'make' of a current of moderate intensity a contraction takes place at the cathode ; the anode induces no such contractile effect ; (2) at the ' make ' of a stronger current both the anode and cathode induce contraction. I have also carried out further investigations on the polar effect of current on the autonomous activity of the leaflet of Desmodimn gyrans. These rhythmic pulsations can be recorded by my Oscillating Recorder. Each pulsa- tion consists of a sudden contractile movement downwards, corresponding to the systole of a beating heart, and a slower up movement of diastolic expansion. Application of ""Irritability of Plants." p. 21'.>. 302 Lit'E MOVEMENTS IN PLANTS cathode at the pulvinule was found to exert a contractile, reaction, exhibited either by the reduction of normal limit of diastolic expansion, or by an arrest of movement at systole. The effect of anode was precisely the opposite ; the induced expansinti was exhibited either by reduction of normal limit of systolic contraction, or by arrest of pulsation at diastole. From the above results it is seen that with a feeble current : (1) contraction is induced at the cathode, and (2) expansion is brought about at the anode. These effects take place under the action of a feeble current. Under strong currents, contraction takes place both at the anode and the cathode. POLAR EFFECT OF ELECTRICAL CURRENT ON GROWTH. The object of this investigation was to determine whether anode and cathode exerted similar discriminative and opposita effects on growth. For this experiment I took a specimen of Kysoor and °C. there was an up-movement, or a movement of closure* Fig. 109.- -Thermonastic and r£ldionastic responses of petal of ZephyrantUes C closing movement due to cooling, and H, opening movement due to warming . K closing movement due to heat-radiation. Note opposite responses to rise of temperature and to thermal radiation. Rise of temperature induced, on the other hand, a movemeni of opening. Effect of thermal radiation : Experiment 114. — I stated that the effect of thermal radiation acts as a stimulus, inducing a reaction which is antagonistic to that of rise of temperature. In verification of this, I subjected the specimen to the action of infra-red radiation acting from all sides. The result is seen in the responsive movement of closure (Fig. 109 R). These experiments demonstrate clearly that the responses to rise of temperature and thermal radiation are of opposite signs. As a movement of closure was induced by the diffuse stimulus of thermal radiation, it is evident that this must have been brought about by the greater contraction of the THERMONASTIC PHENOMENA 309 inner half of the perianth ; hence the inner half of the organ is i*elatively the more excitable. NEGATIVE THERMONASTIC RESPONSE. Response of Nymphaea : Experiment 115. — Many of the Indian Nyynphoeaceoe have their sepals and petals closed Fig. 110. — The Thermonastic Kecorder. T, metallic thermorueter attached to the short arm of the upper lever ; the specimen of Nymphcea, N, has one of its perianth leaves attached to the short arm of the second lever by a thread. C' clock work for oscillation of the plate. (luring the day, and open at night. I find that the perianth leaves of this flower are markedly sensitive to :U0 LIFE MOVEMENTS IN PLANTS Vtiriatiou of temperature. The Therinonastic Recorder em- ployed in this investigation is showti in figure 110. The record given in figure 111 shows that the perianih seg- ment, subjected to a few degrees' rise of temperature, responded by an up-movement of closure, due to greater expansion of the outer lialf. The latent period was 6 seconds, and the maximum etlect was attained in the further course of 21 seconds, Tliis experiment shows that the thermonastic response ot this flower is of the negative type. Effect of stifHiilus : Experiment 116. — In the positive type of thermonastic organs, where rise of temperature induced a movement of opening, stimulus induced the oppo- site movement of closure (Expt. 114). We shall now study the effect of stimulus on the movement of Nymphcea, which undergoes closure during rise of temperature, as seen Fig. 111. — ]?), (2) that a stimulus of moderate intensity induces the normal retardation of the rate of growth. 320 LIFE MOVEMENTS IN PLANTS It is evident that there is a critical intensity of stimu- lus, above which there is a retardation, and below which there is the opposite reaction of acceleration. This criti- cal intensity, I have found to be low in vigorous speci- mens, and high in sub-tonic specimens. Thus the same intensity of stimulus may induce a retardation of growth in specimens the tonic condition of which is ahovi par, and an acceleration in others, in which it is heloiv par. The following experiments will demonstrate the immediate and after-efifect of light of increasing intensity and duration. Effect of light of moderate intensity : Experiment 120. — The source of light was a small arc lamp placed at a distance of 50 cm., the intensity of incident light was increased or decreased by bringing the source of light nearer Fig'. 115. — Immediate and after-effect of stimulus of light on ejrowth. (a) shows immediate effect of moderate light to be a transitory acceleration (down-curve) followed by retardation (up-curve). The after-effect on cessation of light is an acceleration (down-curve) followed by restoration to normal. (6) Immediate and after-effect of stronger light : immediate effect, a retardation ; after-effect, recovery to normal rate without acceleration. or further away from the plant. Two inclined mirrors were placed behind the plant so that the specimen was PHOTOTROPISM 321 acted on by light from all sides. A seedling of wheaf was mounted on the Balanced Crescograph, and record was first taken under exact balance ; this , gives a hori- zontal record. The np-curve represents retardation, and down-curve acceleration of rate of growth. The source of light was at first placed at a distance of 50 cm. from the plant, and exposure was given for 4 minutes at the point marked with an arrow (Fig. 115a). We shall find in the next chapter that the intensity of photntropic effect is jjrojjortioaal to the quantity of incident light. This quantity at the beginning proved to be sub- minimal, and hence there was an acceleration at the beginning. Continued action induced the normal effect of retardation, as seen in the subsequent resulting up-curve. On the cessation of light, the balance was upset in an oppo- site direction, the resulting down-curve showing an accelera- tion of the rate of growth above the normal. This acce- leration persisted for a time, after which the normal rate of growth was restored, as seen in the curve becoming once more horizontal. IVie after-effect of light of moderate intensity is thus a temporary acceleratio)i of rate of groivth ahove the normal. Effect of strong light : Experiment 121. — The same speci- men was used as in the last experiment. By bringing the source of light to a distance of 25 cm. the inten- sity of light was increased fourfold ; the duration of exposure was kept the same as before. The record (Fig. 115b) shows that a retardation of rate of growth occurred from the very beginning without the preliminary accelera- tion. This is for two reasons : (1) the increased intensity was now above the critical minimum, and (2) the tone of the organ had become improved by previous stimu- lation. On the cessation of light, the after-effect showed no enhancement of rate of growth, the recovery from re- tardation to the normal rate being gradual. In the next 322 LIFE MOVEMENTB IN PLANTS experiment (the result of which is not given in tlie record) the intensity of light was increased still further ; the retardation now Itecame very marked, and it persisted for a long time even on the cessation of light. We thus find that : (1) The immediate efl'ect of light of moderate intensity is a preliminary acceleration, followed by normal retardation. The acceleration is the effect of sub-minimal stimulation. The imme- diate after-etl'ect is an acceleration above the normal. (2) The immesliate eflCect of strong light is a retardation from the beginning ; the immediate after-effect shows no acceleration, the growth rate being gradually restored to the normal. (3) Under very strong light the induced retardation is very great, and this persists for a long time even on the removal of light. The experiments described explains the reasons of com- plete recovery after moderate stimulation, and also the absence of recovery after strong stimulation. The imme- diate after-effect of moderate stimulation is shown to be an acceleration of rate above the normal. Returning tt) tropic curvature, the contraction at the proximal side induced by unilateral light is thus compensated by the accelerated rate of growth on the cessation of light. There is no such compensation in the case of strong and long continued action of light ; for the after-effect of strong light shows no such acceleration as the immediate after- effect. We may perhaps go a step further in explaining this difference. Stimulus was found to induce at the same time two physico-chemical reactions of opposite signs (p. 144). PHOTOTROPISM 323 One is the 'up' or A-change, associated with increase of potential energy of the system, and the other is asso- ciated with ' (h)wn ' or D-change, by which there is a run-down or depletion of energy. With moderate stimula- tion the A-and-D effects are more or less comparable to each other. But under strong stimulation the down-change is relatively greater. Hence on cessation of moderate stimulation the increase of potential energy, associated with A-chaiige, finds expression in enhancement of the rate of growth. The depletion of energy under strong stimula- tion is, however, too great to be compensated by the A-change. LATENT PERIOD OF PHOTOTROPIC REACTION. With reference to the latent period Jost thus summa- rizes the known results :* " The latent period of the helio- tropic stimulus has already been determined. According to Czapek it amounts to 7 minutes in the cotyledons of Avena and in Phycomyces ; 10 minutes in hypocotyls of Sinapis alba and Beta vulgaris, 20 minutes in the hypo- cotyl of Helianthus, and 50 minutes in the epicotyl of Fhaseolus. If one of these organs be unilaterally illumi- nated for the specified time, heliotropic curvature ensues afterwards in the dark, that is to say, we meet with an after-effect in this case as in geotropism. We are quite ignorant, however, as to whether and how the latent period is dependent on the intensity of light." With regard to the (juestion of relation of the latent period to the intensity of stimulus I have shown (p. 16(5) that the latent period is shortened under increasing intensity of stimulus. In the case of tropic curvature induced by light, I find that the latent period is reduceu under * Jost — Ibid, p. 473. 324 LIFE MOVEMENTS IN PLANTS increasing intensity of light. The shortest latent period found by Czapek, as stated before, was 7 minutes. But by employing high magnification for record, I find that the latent period of phototropic action under strong light to be a question of seconds. Determination of the latent 2^^1'iod : Experiment 122. — I give a record of response (Fig. 116) of the terminal leaflet of Erythrina inidca to light acting from above. The recording plate was made to move at a fast rate, the Fig. 116. — Latent, period for photic stimulation at vertical line. Successive dots at intervals of 2 seconds. {Erythrina indica). successive dots being at intervals of 2 seconds. The latent period in this case is seen to be 35 seconds. By the employment of stronger light I have obtained latent period which is very much shorter. The term latent period is used in two different sense. It may mean the interval between the application of stimulus and the initiation of response. In the experi- ment described above, the latent period is to be understood in this sense. But in the extract given above, Jost uses the terra latent period as the shortest period of exposure necessary to induce phototropic reaction as an after-effect. What then is the shortest exposure that will induce a PHOTOTROPISM 325 retardation of growth ? For this investigation I employed the very sensitive method of the Balanced Crescograph. GROWTH-VARIATION BY FLASH OF LIGHT FROM A SINGLE SPARK. Experime)it 123. — I stated that the more intense is the light, the shorter is the latent period. The duration of a single spark discharge from a Leyden jar is almost instantaneous, the duration of discharge being of the order of ^p^^QQO th of a second. The single discharge was made to take place between two small steel spheres, the light given out by the spark being rich in effective ultra-violet rays. The plant used for the experiment was a seedling of wheat. It was mounted on the Balanced Crescograph, and its normal growth was exactly compensated as seen. Fig. 117.— Effect of a single elect ric spark on variation of growth. Record taken by Balanced Crescograph. Up-curve shows induced retardation of growth; the after- effect is an acceleration (down-curve) followed by restoration to normal. in the first part of the record. The spark gap was placed at a distance of 10 cm. from the plant ; there 320 LIFE MOVEMENTS IN PLANTS was the usual arrangement of inclined mirrors for illu- mination of the plant. The flash of light from a single spark is seen to induce a sudden retardation of rate of growth which lasted for one and half minutes. The record (Fig. 117) shows another interesting peculiarity of acceleration as an after-efiect of moderate stimulation. After the retardation which lasted for 90 seconds, there is an acceleration of growth above the normal, which persisted for 6 minutes, after which the rate of growth returned to the normal. In order to show that the induced variation is due to the action of light and not to any other disturbance, I interposed a sheet of ebonite between the spark-gap and the plant. The production of spark produced no effect, but the removal of the ebonite screen was at once followed by the characteristic response. MAXIMUM POSITIVE CURVATURE UNDER CONTINUED ACTION OF LIGHT. The positive curvature is, as we have seen, due to the contraction of the proximal side and expansion of the distal side. The curvature will increase with growing contraction of the proximal side ; a maximum curvature is however reached since : (1) the contraction of the cells must have a limit, (2) the bending organ offers increasing resistance to curvature, and (3) the induced curvature tends to place the organ parallel to the direction of light when the tropic effect is reduced to a minimum The pulvinus of Erythrina exemplifies the type of reaction in which the positive curvature reaches a maxi- mum, (see below Fig. 132) beyond which there is no further PHOTOTROPISM 327 change. This is due to absence of transverse conductivity in the organ. The modifying effect of transverse conducti- vity on response will be dealt with in the next chapter. SUMMARY. The positive phototropic curvature is brought about by the joint effects of the directly stimulated proximal, and indirectly stimulated distal side. The phototropically curved organ undergoes recovery after brief stimulation. The recovery after moderate stimulation is hastened by the previously stimulated side exhibiting an acceleration of the rate of growth above the normal. The after-effects of photic and mechanical stimulation are similar. The latent period of photic reaction is shortened with the increasing intensity of light. The seedling of wheat responds to a flash of light from an electric spark, the duration of which is about a hundred thousandth part of a second. Tissues in which the power of transverse conduction is negligible, the positive phototropic curvature under continued action of light attains a maximum without subsequent neutralisation or reversal. 26 XXX. -DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM By Sir J. C. BosE, Assisted by (tURUprasanna Das. I have explained how under the action of unilateral light the positive curvature attains a maximum. There are, however, cases where under the continued action of strong light the tropic movement undergoes a reversal. Thus to quote Jost : " Each organism may be found in one of the three different conditions determined by the light intensity, viz. (1) a condition of positive heliotropism, (2) a condition of indifference, (3) a condition of nega- tive heliotropism"*. No explanation has however been offered as to why the same organ should exhibit at different times, a positive, a neutral, and a negative irrita- bility. These changing effects exhibited by au identical organ is thus incompatible with the theory of specific sensibility, assumed in explanation of characteristic differ- ences in phototropic response. In regard to this I would draw attention to an important factor which modifies the tropic response, namely, the eff'ect of transverse conduction of excitation. I shall presently describe in detail a typical experiment of the fjQet— /fc»rf-p.462, DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 329 effect of unilateral stimulus of light on the responsive movement of main pulvinus of Alifnosa piidica. The results will be found of much theoretical interest, since a single experiment will give an insight to all possible types of phototropic response. Before describing the experiment I shall demonstrate the tropic reactions of the two halves of the pulvinus of Jlitnosa. UNEQUAL EXCITABILITY OF UPPER AND LOWER HALVES OF PULVINUS TO PHOTIC STIMULATION. I have by method of selective amputation shown that as regards electric stimulation the excitability of the upper half of the pulvinus is very much less than that of the lower half (p. 85). I have obtained similar results with photic stimulation. Tropic effect of light acting from above : Experiment 124. — Light of moderate intensity from an incandescent electric lamp was applied on the upper half of the pul- vinus of Mimosa for 4 minutes ; this induced a contraction of the stimulated upper half and gave rise to an up or erectile response. On the stoppage of light recovery took place in the course of ten minutes. The phototropic curvature is thus seen to be positive. A series of such positive responses of the upper half of the pulvinus is given in figure 118. 9 Effect of light acting from below : Experiment 125. — Light was now applied from below ; this also induced a contraction of the lower half of the pulvinus, causing a down-movement (Fig. 119). As the responsive movement is towards light, the phototropic effect must be regarded as positive. The greater excitability of the lower half of the 2(3 k 330 LIFE MOVEMENTS IN PLANTS pulvinus is shown by the fact that the response of the lower half of the pulvinus to ten seconds' exposure is Fig. 118. Fig. 119. Fig. 118. — Series of up-responses of Aiinwsa leaf to light applied on upper half of pulvinus. Fig. 119. — Down-responses given by the same plant on application of light from below. even larger than that given by the upper half under the prolonged exposure of 240 seconds. TRANSFORMATION OF POSITIVE TO NEGATIVE PHOTO- TROPIC CURVATURE. J^xperiment 126. — A beam of light from a small arc lamp was thrown on the upper half of the pulvinus. After a latent period of 5 seconds, a positive curvature was' initiated, by the contraction of the upper and expan- sion of the lower side of the organ. But under continued action of light, the excitatory impulse reached the lower half of the organ, causing a rapid fall of the leaf, and a negative curvatare. The arrival of transmitted excita- tion at the more excitable distal half of the organ is clearly demonstrated by the very rapid down-movement, seen as the up-curve in the record (Fig. 120). In sensitive specimens this movement is so abrupt and rapid, that the writing lever is jerked off above the recording plate before making a dot on it. The thickness of the DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 3:U pulvinus was 1-5 ijira., the distance which the excitatory impulse has to traverse to reach the lower half would thus be about 0-75 mm. The period for transverse transmission Fig. 120. — Record of effect of continuous application of light on upper half of pulvinus of Mimosa leaf. Note erectile response (positive curvature) followed by neutralisation and pronounced reversal into negative due to trans- verse conduction of excitation. Up-movement shown bj' down curve, and vice versa. of excitation under strong light was found to vary in different cases from oO to 80 seconds. The velocity of transmission of excitation in a transverse direction through the pulvinus is about 0*011 mm. per second, which is not very different from O'OIO mm. per second in the stem (p. 282). Returning to the main experiment we find that : (1) As a result of unilateral action of light, there was positive phototropic curvature which lasted for 50 seconds. 332 LIFE MOVEMENTS IN PLANTS (2) Owing to the internal conduction of excitation the positive effect underwent neutralisation by the excitatory contraction of the distal side. This neutralisation depends on four factors : (a) on the intensity of the stimulus, (b) on the conductivity of the organ in a transverse direction, (c) on the thickness of the intervening tissue, and (d) on the relative excitability of the distal as compared to the proximal side. The extent of positive curva- ture also depends on the pliability of the organ. (3) In anisotropic organs where the distal side is physiologically the more excitable than the pro- ximal, the internally diffused excitation brings about a greater contraction of the distal, and the positive phototropic curvature becomes reversed to a very pronounced negative. The effect of the internally diffused stimulus is thus the same as that of external diffuse stimulus. (4) When the stimulus is applied on the more excitable half of the organ, the result is a predominant contraction of that half, which cannot be neutralised by the excitation conducted to the less excitable half of the organ. As the curva- ture is towards the stimulus, the phototropic curvature thus remains positive, even under continued stimulation. The positive curvature is due to the differential action of unilateral stimulus on the proximal and distal sides. Fut when a strong light is made to act continuously on one side of an organ, the excitation becomes internally diffused, and the differential effect on the two f^ides *s reduced in amount or vanishes altogether. Owing to the weak transverse conductivity of the tissue, while the effect of a feeble stimulus remains localii^ed, that of a stronger stimulus is conducted across it. t)lA-PHOTOTROPlSM AND NEGATIVE PHOTOTROPISM 333 Oltmanns lound that the seedlirig of Lepidium sativtim assumed a. transverse or dia-phototropic position under intense and long continued action of light of ()00,()00 Hef- ner lamps. He regards this as the inditierent position. But tlie neutralisation of curvature is not, as explained before, due to a condition of indifference, but to the anta- gonistic effects of the two opposite sides of the organ, the proximal being stimulated by the direct, and the distal by the transversely conducted excitation. I obtained such neutralisation with Dregea volahilis under the prolonged unilateral action of arc-light. The first effect was positive ; this w^as gradually and continuously neutralised under ex- posure for two hours ; even then the neutralisation was not complete. I shall presently adduce instances where ihe neutralisation was not merely complete, but the final effect was an actual reveisal into negative response. SUPPOSED PHOTOTROPIC INEFFECTIVENESS OF SUNLIGHT. I may here consider the remarkable fact that has been observed, but for which no explanation has been forth- coming, that "direct sunlight is too bright to bring about heliotropic curvaiure, only diffuse, not direct sunlight has the power of inducing heliotropic movements."* But we cannot conceive of light sudcenly losing its phototropic eff'ect by an increase of intensity. The experiment just described will offer full explanation for this apparent anomaly. Feeble or moderate stimulus remains, as we have seen, localised, hence the contraction of the proximal side gives ri^e to positive curvature. But the intense excitation caused by sunlight would be transmitted to the distal side and thus bring about neutralisation. It is the observation of the final result that has misled observers as to the inefficiency of direct sunlight. A continuous record of the response of the organ shows, on * Jost— /6i "»o a jjuDi light from arc lamp. tive curvaiure which attained its maximum ; after this there was a neutralisation in less than six minutes after the application of light. The further 336 LIFE MOVEMENTS tH PLANTS continuation of light induced a pronounced negative cur- vature (Fig. 121). I shall in the next chapter give other instances which will show that all organs (pulvinated and growing) possessed of power of transverse conduction, exhibit a transformation of response from positive to negative under continued action of strong light. Thus an identical oi-gan, under different conditions of intensity and duration of stimulus, exhibits positive photo- tropic, rff'«-phototropic, and negative phototropic curvatures, proving conclusively that the three effects are not due to three distinct irritabilities. The responsive movements are, on the other hand, traced to a fundamental excita- tory reaction, remaining either localised or iiicreasingl)'^ transmitted to the distal side. NEGATIVE PHOTOTROPISM OP ROOTS. From the analogy of opposite responses of shoot and root to stimulus of gravity', it was surmised that the root would respond to light by a negative curvature. This was apparently confirmed by the negat-ve phototropic curvature of the root of Sinapis. The supposed analogy is however false ; for while the stimulus of gravity acts, in the case of root, only on a restricted area of t'le tip, the stimulus of light is not necessarily restricted in the area of its action. That there is no true analogy be- tween the action of light and gravitation is seen from the fact that while gravitation induces in the root a movement opposite to that in the stem, in the case of light, this is not always so ; for though a few roots turn away from light, others move towards the light. As regards negative phototropic response of the root of iSinapisy it will be shown (p. 376) to be brought about by DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 337 algebraical sunmiation of the effects of direct and indirect photic Btimulus. SUMMARY. The normal positive phototropic curvature is modified by transverse conduction of true excitation to the distal side of the organ. The extent of neutralisation or reversal due to internal conduction of excitation from the proximal to the distal side of the organ depends : (a) on the intensity of the inci- dent stimulus, (b) on the conductivity of the organ in a transverse direction, (c) on the thickness of the intervening tissae, and (d) on the relative excitability of the distal as compared to the proximal side. The dia-phototropic position is not one of indifference, but of balanced antagonistic reactions of two opposite sides of the organ. The supposition that direct sunlight is phototropically ineffective is unfounded. The response is fully vigorous, but the first positive curvature may in certain cases be neutra- lised by the transmission of excitation to the distal side. Under light of strong intensity and long duration, the transmitted excitation to the distal side neutralises, and finally reverses the positive into negative curvature. The positi ue-phototroTpic, the d/a-phototropic, and the negative phototropic curvatures are not due to three distinct irritabilities but are brought about by a fundamental excitatory reaction remaining localised or increasingly transmitted to the distal side. XXXI.— THE RELATION BETWEEN THE QUANTITY OF LIGHT AND THE INDUCED PHOTOTROPIC CURVATURE By Sir J. C. BosE, Asftisted by SURENDRA Chandra Das, m.a. I SHALL in this chapter describe experiments in support of tlie important proposition that the intf.nsity of phototropic action is dependent on the quantity of incident light. The proportionality of the tropic effect to the quantity of light will be found to hokl good for the median range of stimu- lation ; the deviation from this proportionality at the two ends of the range of stimulation — the sub-minimal and supra- maximal— is, as we shall finht was applied for a minute in the two successive experiments for the two angles of 45*^ and 90°. The record (Fig. 127) shows that the phototropic Fig. 127. Fig. 128. Fig. 127. — EflEect. of angle of inclination of light on the tropic curvature of pulvinus. The first response is to light at 45^ and the second, to 90^. (Betmodium gyrans). Fig. 128. — Series of tropic curvatures of growing bud of Crinnm to alternate stimulation by light at 4.5^ and 90^. effect increases with the directive angle. In the present case the ratio of the two effects is 1*6 : 1, which is not very different from the ratio 7^7^=1'4 sin 45° I'vopic response of growing organs : Experiment 132. — Similar experiment was carried out with the flower bud of Crimini, held vertical. Light was applied alternately at 45° and 90°, in two successive series. The object of this was to make due allowance of possible variation of excitability of the organ during the course of the experi- ment. I have explained (p. 147), how the excitability of a tissue in a condition slightly below par, is increased by the action of previous stimulation. Series of responses obtained under alternate stimulations at 45° and 90^ 27 344 LIFE MOVEMENTS IN PLANTS enable us to find out, whether any variation of excit- ability occurred during the course of the experiment and make allowance for it. The records show that stimula- tion did enhance the excitability of the organ to a small extent. Thus the first stimulation at 45° induced an amplitude of response of 5 mm. ; the sacond stimulation at 45° i. e. the third response of the series, induced a slightly larger response 7 mm. in amplitude. Similarly the two responses at 90° gave an amplitude of 9 mm. and 11 mm. respectively (Fig. 128). Taking the mean value of each pair, the ratio of tropic effects for 90° and 45 ' is = 10/6 = 1*7 nearly. EFFECT OF DURATION OF EXPOSURE. Experiment IBS. — The specimen employed for the experi- ment was a flower bud of Crinum in a slightly sub-tonic condition. Successive responses exhibited on this account, a preliminary negative* before the normal positive curvature. The successive durations of exposure were for 1, 2, and Fig. 129.— Effect of increasing duration of exposure 1:2: 3 minutes, on photo- tropic curvature. Note preliminary negative response. {Crinum). 3 minutes. The amplitudes of responses (Fig. 129) are in the ratio of 1 : 2*5 : 5. ♦ An explanatiop of this preliminary effect wjll be found in the nest chapter. QUANTITY OP LIGHT AND PHOTOTROPIC CURVATURE 345 We may now recapitulate the tropic effects of light of increaaing intensity, directive angle, and duratioa of ex- posure. It has been shown that the tropic effect is en- hanced under increasing intensity of light ; it is also in- creased with the angle increasing from grazing to perpendi- cular incidence. And finally, the tropic effect is enhanced with the duration of exposure. Taking into consideration the effects of these different factors we arrive at the con- clusion that pJiolotropic effect increases with the quantity of incident light. It will be shown in the next chapter tliat strict proportioaality of cause and effect holds good in the mecjian range of stimulation, and the slight de- viation from this, above and below the median range, is due to the fact that susceptibility for excitation is low at these two regions. SUMMARY. Increasing intensity of light induces increasing tropic curvature. 0 Tropic curvature increases with the directive angle, the etfect being approximately proportional to sin 0, where 6 is the angle made by the rays with the responding sur- face. Tropic curvature also increases with the duration of exposure. The intensity of induced tropic effect is determined by the quantity of incident light. 27 a xxxii.-thp: phototropio curve and its characteristics By Sir J. C. BosE. When h plant organ is subjected to the continued action of unilaterial stimulus of light, it exhibits in- creasing tropic curvature, which in certain cases reaches a limit ; in other insiances a reversal takes place, seen in neutralisation, or in the conversion of the positive into negative curvature. I shall in this chapter enter into a detailed study of the phototropic curve, and determine its characteristics. As the vague terminalogy at present in use has been the source of much confusion, it is necessary here to define clearly the various terms which will be employed in this investigation. It is first of all necessary to distin- guish between cause and effect, between external stimulus and the excitation induced by it. As regards stimulus itself I have shown elsewhere* that its effective intensity becomes summated by repetition. This was demonstrated by the two following typical experiments carried out with the pulvinus of Mimosa. (1) The intensity of a single electric shock of intensity of 0'5 unit was found to be ineffective in inducing ♦ " Irritability of Plants " — p. 54, THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 347 excitation ; but it became effective on being repeated four times in rapid succession. (2) The same specimen was next subjected to a feebler stimulus of intensity of 0*1 unit, and it required a re- petition of 20 times for the stimulus to become effective. The total stimulus in the first case was 0'5x4=2, and this was found to be the same as 0'lx20=2 in the second case. Thus the intensity of stimulus is increased by repetition ; in the limiting case where the interval between successive stinmlus is zero, the stimulus becomes continuous. Bearing in mind the additive effects of sti- mulus we see that its effective intensity increases with the duration of applica*^ion. This important conclusion found independent support from the results of Experiment 133 given in the last chapter. We shall now take up the general question of the characteristic^ of the phototropic curve, which gives the relation between increasing stimulus and the resulting exci- tation. As regards stimulus we found that its effective- ness increases with the duration of application. The induced excitation in growing organs may be measured by concomitant retardation of growth caused by stimulus. In the excitation curves which will be presently given, the abscissae represent increasing stimulus and ordinates the resulting excitation. This excitation curve may be ob- tained by making the plant record on a moving plate its retardation of growth by means of the High Magnification Crescograph. I reproduce below two records of the effects of continuous photic and electric stimulation. The ordinate 3*^ LIFE MOVEMENTS IN PLANTS of the 'excitation curve' (Fig. 180) exhil:)ils increasing in- cipient contraction (retardation of growth) ciilminatin<» in an Fig. 130. — Effects of continuous (a) electric, and (6) photic stimulation on rate of growth. Abscissa represents duration of application of stimulus. Note induced retardation, and arrest of growth. arrest of growth ; the abscissa represents increasing stimu- lus consequent on increased duration of application. The record shows that the incipient contraction is slight at the first stage ; it increases rapidly in the second stage ; finally, it declines and reaches a limit. The excita- tory reaction is thus not constant throughout the entire curve of excitation, but undergoes very definite and characteristic changes. We shall find similar characteristics in the phototropic carves under unilateral stimulus which will be given presently. The explanation of the similarity is found in the fact that the tropic curvature is also due to incipient contraction or retardation of the rate of growth, which remains confined to the directly stimulated proximal side of the organ. For facility of explanation of what follows, I shall have to use a new and necessary term, siiscejjtibility, to indicate the relation of cause and efi"ect, of stimulus and u- •. 4.- cr 4 1 Tt ■ ^u Excitation resultmg excitation. Susceptibthty is thus = -rrr. v ° x- c/ btimulus THE PHOTOTROPIC OURVE AND TTft CHARACTERISTICS 349 Different organs of plants exhibit unequal suscepti- bilities ; some undergo excitation under feeble stimulus, while others require more intense stimulus to induce excitation. But even in an identical organ the susceptibility undergoes, as we have seen, a characteristic variation, being feeble at the beginning of the excitation curve, considerable in the middle, and becoming feeble once more towards the end of the curve. The most difficult problem that faces us is an explanation of this characteristic difference in different parts of the tropic curve. GENERAL CONSIDERATIONS. Before entering into the fuller consideration of the subject, it will be helpful to form some mental picture of the phenomena of excitation, however inadequate it may be. The excitation is admitted to be due to the molecular upset induced by the shock of stimulus* ; the increased excitation results from increasing molecular upset brought on by enhanced stimulus. The condition of molecular upset or excitation may be detected from the record of any one of the several concomitant changes, such as the change of form, (contraction or expansion) or change of electric condition (galvanometric negativity or positivity). These means of investigation are not in principle different from those we employ in the detection of molecular dis- tortion in inorganic matter under increasing intensities of an external force. THE CHARACTERISTIC CURVE. Thus the molecular upset and rearrangement, in a magnetic substance under increasing magnetising force are * I shall use the term stimulus in preference to stitnulation, for the latter 19 often taken in the senae of the resulting excitation. 350 LIFE MOVEMENTS IN PLANTS inferred from the curve obtained by means of appropriate magnetometric or galvanometrie methods. I reproduce the characteristic curve of iron (Fig. 131) in which the abscissa represents increasing magnetising force, and the ordinate, the induced magnetisation. This characteristic curve, giving the relation of cause and effect, will be found to be highly suggestive as regards the similar characteristic curve which gives the relation between increasing stimulus and the resulting enhanced tropic effect in vegetable tissues. The parallelism will be found to be very striking. Inspection of figure 131 shows that, broadly speaking, the curve of magnetisation may be divided into four parts. In the first part, under feeble magnetic force, the slope of the curve is very small ; later, in the second part, as the force increases, the curve becomes very steep ; in the third part the slope of the curve remains fairly constant ; and finally in the fourth part, the curve rounds off and the rate of ascent again becomes very small. The suscep- tihility for induced magnetisation is thus very feeble at the beginning ; under increasing force, in the second stage, the susceptibility becomes greatly enhanced; in the third stage, the susceptibility remains approximately constant ; and in the fourth stage it becomes diminished. We shall presently find that the susceptibility for excitation also undergoes a similar variation at the four different stages of stimulation. CHARACTERISTICS OF SIMPLE PHOTOTROPIC CURVE. I have shown (Fig. 130) the relation between the stimulus and the resulting excitation, the latter being deter- mined from the diminution of the rate of growth. Under unilateral action of light, the excitatory contraction gives rise to tropic curvature. We may thus obtain the charac- teristic excitation curve, by making the plant organ record THE PHOTOTROPIC CURVB AND ITS CHARACTERISTICS 351 its tropic movement under continuous action o£ light applied on one side of tlie organ. Experiment 134. — I give below the characteristic curve of excitation (Fig. Ili2) of the pulvinus of Erytht'ina indica, traced bj- the plant itself, and exactly reproduced by photo- mechanical process. A parallel beam of light from a Nernst . (1 5 / ^ / - / u / 2 , / o / < / Q / Ul / U / 3 / 2 / Z g o X . 5e ;; •' '' >•• MAGNETIC rORCE STIMULUS Fig. 131. Fig. 132. Fk4. 131. — Characleristic curve of iron under increasing magnetising force. (.A.fter Ewing). Fig. 132. — Simple characteristic cnrve of photolropic reaction. Excitation increases slowly in the first part and lapidiy in the .second : it is uniform in the third, and undergoes decline in the fourth part {Erythrina indica). lamp was thrown on the upper leaf- of the pulvinus, and the increasing positive curvature was recorded on a smoked glass plate which was moved at an uniform rate. The successive dots are at intervals of 20 seconds ; the horizontal distances between successive dots are equal, and represent equal increments of stimulus ; the vertical distances between 352 LIFE MOVEMENT'S IN PLAJffS successive dott« represent the corresponding increments of excitation. The gradient at any point of the curve — incre- ment of excitation divided by increment of stimulus — gives the susceptibility for excitation at that point. The follow- ing table will show how the susceptibility for excitation undergoes variation through the entire range of stimulus. The average susceptibility for each point has been calculated from the data furnished by the curve. TABLE XXX. — SHOWING THE VARIATION OF SUSCEPTIBILITY FOR EXCITATION AT DIFFERENT POINTS OF THE TBOPIC CURVE. Successive points Susceptibility Successive points Susceptibility in the curve. for excitation. in the curve. for excitation. 1 ... 0 14 ... 6-6 2 ... DM87 15 ... 4-4 3 ... 0-44 16 ... 2-5 4 ... 0-625 17 ... 1-87 5 ... U-875 18 ... 1-5 6 ... 1-25 19 ... 1-12 7 ... 1-87 20 ... 0-937 8 ... 3-12 21 ... 0-75 9 ... 5-0 22 ... 0-562 10 ... 6-2 f) 23 ... 0-375 11 ... 8-75 24 ... 0-25 12 ... 8-87 25 ... 0-187 13 ... 8-12 26 ... 0-062 The induced excitation is seen to be increased very gradually from the zero point of susceptibility, known as the latent period at which no excitation takes place. In the second part of the excitation curve, the rate of increase is very rapid; the maximum rate is nearly reached at point 11 of the curve and remains fairly constant for a time. This is the median range where equal increment of stimulus induces equal increment of excitation. The sus- ceptibility for excitation then falls rapidly, and increase of stimulus induces no further increase of tropic curvature. THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 35;i The maxim -iin tropic curvature was, in the present case, reached in the course of nine minutes. The attainment of this maximum depends on the excitability of the tissue, and the intensity of incident stimulus. The characteristics that have been described are not confined to the phototropic curve but exhibited by tropic curves in general. Similar characteristics have been found in the c irve for electric stimulus (Fig. liK'a), and will also be m^t with in the curve for geotropic stimulus (Fig. 161). I may here reftsr incidentally to the three types of responses exhibited by an organ to successive stimuli of uniform intensity ; these appear to correspond to the three dilferent regions of tropic curve ; in the first stage, the plant exhibits a tendency to exhibit a 'staircase' increase of response ; in the intermediate stage, the response is uniform ; and in the lust stage, the responses show a 'fatigue' decline. For purpose of simplicity, I first selected the simple type of phototropic curve, where ,the specimen employed was in a favourable tonic condition, and the stimulus was, from the beginning, above the minimal. Transverse con- duction, which induces neutralisation or reversal into negative, was moreover absent in the specimen. I shall now take up the more complex cases : (1) where the condition of the specimen is slightly sub-tonic, (2) where the stimulus is gradually increased from the sub-minimal, and (3) where the specimen possesses the power of transverse conduction. EFFECT OF SUB-MINIMAL STIMULUS. It is unfortunate that the terms in general use for des- cription of effective stimulus should be so very indefinite. A stimulus which is just sufficient to evoke excitatory con- traction is termed the minimal, stimulus below the thres- hold being tacitly regarded as ineffective. The employ- ment of sensitive recorders has, however, enabled me to 354 LIFE MOVEMENTS IN PLANTS discover the important fact that stimulus below the minimal, though ineffective in inducing excitatory contraction, is not below the threshold of perception. The plant not merely perceives such stimulus, bur also responds to it in a definite way, by expansion instead of contraction. I shall designate the stimulus below the minimal, as the sub-minimal. There is a critical point, which demarcates the sub-minimal stimulus with its expansive reaction from the minimal with its re- sponsive contraction. The critical stimulus varies in ditterent species of plants . Thus the same intensity of light which induces a retardation of growth in one species of plants will enhance the rate of growth in another. Again, the critical point will vary with the tonic level of the same organ ; in an optimum condition of the tissue, a relatively feeble stimulus will be sufficient to evoke excitatory contraction ; the critical point is therefore low for tissues in tonic condition which may be described as above par. In a sub-tonic condition, on the other nand, strong and long continued stimulation will be necessary to induce the excitatory reaction. The critical point is therefore high, for tissues in a condition below par. Stimulus below the critical point will here induce the opposite physiological reaction, i.e., expansion. The physico-chemical reactions underlying these opposite physiological responses have, for convenience, been distin- guished as the " A " and " D " change (pp. 143, 22'6). The assimilatory 'building up', A change, is associated with an increase of potential energy of the system ; the dissimi- latory ' break down ', D change, on the other hand, is attenil- ed by a run-down of energy. Stimulus was shown (p. 225) to give rise to both these reactions, though the A effect is, generally speaking, masked by the predominant D effect. The " A " change is quicker in initiation, while the '"D" effect developes later; again THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 355 the "A" effect under moderate stimulation may persist longer. Thus owing to the difference in their time-relations the A effect is capable of being unmasked at the onset of stimulus or on its sudden cessation. For the detec- tion of the relatively feeble expansive A effect, a special recorder is required which combines lightness with high power of magnification. The earlier expansive reaction and acceleration of rate of growth, followed by normal retarda- tion, are often found in the response of growing organs. The corresponding effect of unilateral stimulation, even when direct, is a transient expansion at the proximal side, inducing a convexity of that side and movement away from stimulus (negative curvature) ; this is followed by contrac- tion and concavity with normal positive curvature. The interval between the A and D effects is increased with increasing sub-tonicity of the specimen. But it nearly vanishes when the excitability of the specimen is high and the iwo opposite reactions succeed each other too quick- ly for the preliminary A reaction to become evident. It is probable that in such a case the conflict between the two opposite reactions prolongs the latent period. But in other instances a preliminary expansive response is found to herald the more pronounced contractile response. Example of this is seen in figure 129 given in page 344. The A effect was detected in the records referred to above by its earlier appearance. Its longer persistence after moderate stimulation, is also to be found on the cessation of moderate stimulation. This was seen in the acceleration of growth which was the after-effect of stimulation (Figs. 104, 115). The presence of two conflict- ing physiological reactions is also made evident on sudden cessation of long continued stimulation. This particular phenomenon of " overshooting " will be more fully dealt with in a subsequent chapter. 356 LIFE MOVEMENTS IN PLANTS Owing to the difference in the time relations of the two opposing activities, A and D, a phase difference often arises in their respective maxima. It is probably on this account that rhythmic tissues originally at standstill, exhi- bit under continued stimulation a periodic up and down- movement, which persists even on the cessation of the stimulus. The persistence of after-oscillation depends, more- over, on the intensity and duration of previous stimu- lation.* The facts given above cannot bs explained by the prevalent theory that stimulus acts merely as a releas- ing agent, to set free energy which had been previously stored up by the organism, like the pull of a trigger causing explosion of a charged cartridge. It is true that in a highly excitable tissue, the external work performed and the run down of energy are disproportionately greater than the energy of stimulus that induces it. But in a sub-tonic tissue,- stimulus induces an effect which is precisely the opposite ; instead of a depletion, there is an enhancement of potential energy of the system. Thus the resjionding leaf instead of undergoing a fall becomes erected ; growing organs similarly exhibit a ' building up ' and an acceleration of rate of growth, in contrast with the usual ' break down ' and depression of the rate. It is obvious that these new facts relating to the action of stimulus necessitate a theory more comprehensive and satisfactory than the one which has been in vogue. THE COMPLETE PHOTOTROPIC CURVE. I have explained the characteristics of the simple photo- tropic curve in which the tropic curvature, on account of the favourable tonic condition and strong intensity of * " Plant Response "—p. 293, etc. THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 357 incident • light, was positive from the beginning, and in which the curvature reached a maximum beyond which there was no subsequent reversal. If the intensity of the stimulus be feeble or moderate, the quantity of light inci- dent on the responding organ at the beginning may fall below the critical value, and thus act as a sub-minimal stimulus. This induces as we have seen (p. 344) a nega- tive tropic curvature ; continued action of stimulus, how- ever, converts the preliminary negative into the usual posi- tive. The prelimijiary negative curvature may be detected by the use of a moderately sensitive recorder with a magni- fication of about 30 times. It is comparatively easy to obtain the preliminary negative response in specimens which are in a slightly sub-tonic condition. Semi-conducting tissues exhibit under continued stimu- lation, a neutralisation and reversal into negative (p. 331). Since this reversal into negative usually takes place under prolonged exposure to exceedingly strong light, it is diffi- cult to obtain in a single curve all the different phases of transformation. I have, however, been fortunate in obtaining a complete phototropic curve which exhibits in a single specimen all the characteristic changes from a preliminary negative to positive and subsequent reversal to negative. I shall describe two such typical curves obtained with the terminal leaflet of Desmodium gyrans and the growing seedling of Zea Mays. Complete phototropic curve of a jjulvinated organ : Evperiment 135. — A continuous record was taken of the action of light of a 50 c. p. incandescent lamp, applied on the upper half of the pulvinus of the terminal leaflet of Desmodium gyrans. This gave rise : (1) to a negative curvature (due to sub-minimal stimulus) which lasted for 3 minutes. The curve then proceeded upwards, at first slowly, then rapidly ; it then rounded off, and reached a 358 LIFE MOVEMENTS IN PLANTS maximum positive value in the course of 18 minutes ; after this the curve underwent a reversal, and complete neutralisation occurred after a further period of 24 minutes (Fig. 133). Beyond this the induced curvature is negative. Fig. 133. — Complete phototropic curve given by pulvinated Eq. organ. Positive curvature above, and negative curvature below the horizontal zero line. Preliminary negative phase of responste due to .sub-minimal stimulus. The posi- tive increases, attains a maximum, and undergoes a reversal. Successive dots at intervals of 30 seconds. Abscissa represents duration of exposure and quantity of incideut light. (Terminal leaflet of Desmodium /jyram.) Complete phototfipic curve of . 468. 366 ' LIFE MOVEMENTS IN PLANTS petiole of Mimosa when locally stimulated does not itself exhibit any movement. The fortunate circumstance of the presence of a motile pulvinus in the neighbourhood enables us to recognise the perceptive power of the petiole, since it transmits an impulse which causes the fall of the leaf. There is no motile pulvinus in ordinary leaves, and stimulation of the petiole gives rise to no direct or trans- mitted motile reaction ; from this we are apt to draw the inference that the petiole of ordinary leaves are devoid of perception. This conclusion is, however, erroneous, since under stimulus the petiole exhibits the electric response characteristic of excitation. Moreover my electric investi- gations have shown that every living tissue not only perceives but also responds to stimulation.* Hence con- siderable doubt may be entertained as regards the supposed absence of perception in the hypocotyl of Setaria. T shall in the present paper describe my investigations on the mechanical response of Setaria under direct and indirect stimulation which will be given in the following order : — (1) The response to unilateral stimulation of the tip of the seedling. (2) The response of growing hypocotyl to direct sti- mulation. (3) Summated effects of direct and indirect stimula- tion. EXPERIMENTAL ARRANGEMENTS. The Recorder. — The pull exerted by the tropic curva- ture of the seedling is very feeble ; it was therefore necessary to construct a very light and nearly balanced * " Response in the Living and Non-Living " — p. 17. TRANSMITTED EFFECT OF LIGHT 367 recording lever. A long glass fibre is supported by lateral pivots on jewel bearings. The seedling is attached to the short arm of the lever by means of a cocoon thread. The reeordiag plate oscillates to and fro once in a minnte ; the successive dots give therefore the time relations of the responsive movement. The positive cur- vature towards light is recorded as an up-curve, the negative curvature being represented by a down-curve. Arrangement for local stimulation by light. — The device of placing tin foil caps on the tip employed by some observers labours under the disadvantage, that it causes mechanical irritation of the sensitive tip. The appliance seen in figure 13") is free from this objection and offers S h 0 1 Fig. 1.35. — Arrangement for local application of light to the tip and the growing region. 0, O', aperture.^ on a metallic screen. Light is focussed by a lens on the tip, and on the growing region at o, o.* Figure to the right shows front view of the shutter resting on a pivol and worked by string, T. many advantages. A metallic screen has two holes 0 and 0^; these apertures are illuminated by a parallel beam of light from an arc lamp. A lens focusses the light from 0, on the hypocotyl, and that from 0\ on the tip of the cotyledon. A rectangular pivoted shutter S, lies between the apertures 0 and 0'. In the intermediate position of the shutter, light acts on both the tip and the growing region. The shutter is tilted up by a pull on the thread T, thus cutting off light from the growing region ; release of the thread cuts off light from the tip. Thus by proper manipulation of the shutter, the tip or the growing hypocotyl, or both of them, may be subjected to 368 LIFE MOVEMENTS IN PLANTS the stimulus of light. The experiment was carried out in a dark room, special precaution being taken that light was screened off from the plant except at points of localised stimulation. EFFECT OF LIGHT AT THE TIP OF THE ORGAN. Experiment 137. — If the tip of the seedling of Setaria be illuminated on one side, it is found that a positive curvature {i.e., towards light) is induced in the course of an hour or more. But in obtaining record of the seedling by unilateral stimulation of the tip, I found that the immediate response was not towards, but away from light (negative curvature). The latent period was Fig. 136.— Response of seedling of Setaria to unilateral stimulation of the tip applied at dotted arrow. Note preliminary nesrative curvature reversed later into positive. about IJO seconds and the negative movement continued to increase for 25 minutes (Fig. 13G). This result, hitherto TRANSMITTED EFFECT OF LIGHT 369 unsuspected, is not so anomalous as would appear at first sight. Indirect stimulus, unilaterally applied, has been shown to give rise to two impulses : a quicker positive and a slower excitatory negative. The former induces a convexity on the same side, and a movement away from stimulus (negative curvature) ; the excitatory negative, on the other hand, is conducted slowly and induces contraction and concavity, and a movement towards the stimulus (positive curvature). In semi-conducting or non-conducting tissues, the excitatory negative is weakened to extinction during transit, and the positive reaction with negative curvature persists as the initial and final effect. But in Setaria the excitatory negative impulse is trans- mitted along the parenchyma which is moderately conduct- ing ; the speed of transmission of heliotropic excitation is, according to Pfeffer, one or two mm. in five minutes or about 0*4 mm. per minute. Thus under the continued action of light, the excitatory impulse will reacl) the grow- ing region, and by its predominant reaction neutralise ai\d reverse the previous negative curvature. Inspection of figure 136 shows that this is what actually took place ; the intervening distance between the tip of the cotyledon and the growing region in hypocotyl was about 20 mm,, and the beginning of reversal from negative to positive curvature occurred 29 minutes after application of light. The velocity of transmission of ex- citatory impulse under strong light is thus 0*7 mm. per minute. The positive curvature continued to increase for a very long time and became comparatively large. This is for two reasons : (1) because the sensibility of the tip of the cotyledon is very great, and (2) because the positive curvature induced by longitudinally transmitted 370 LIFE MOVEMENTS IN PLANTS excitation is not neutralised by transverse conduction (see below). RESPONSE TO UNILATERAL STIMULUS IN THE GROWING REGION. Experiment 138. — The growing region of the hypocotyl of Setaria is supposed to be totally devoid of the power of perception. In order to subject the question to experi- mental test, I applied unilateral light on the growing region of the same specimen, after it had recovered from the effect of previous stimulation. The response now ob- tained was vigorous and was ab-initio positive. Direct stimulus has thus induced the normal effect of contraction and concavity of the excited side. The belief that the hypocotyl of Setaria is incapable of perceiving stimulus is thus without any foundation. The further experiment Fig. 137. — Effect of application of light to the growing hypocotyl at arrow induced positive pbototropic curvature followed by neutralisation. Application of indirect stimulus at dotted arrow on the tip gave rise at first to negative, subsequently to positive curvature. (Seedling of Setaria). which I shall presently describe will, however, offer an explanation of the prevailing error. On continuing the TRANSMITTED EFFECT OF LIGHT 371 action of unilateral light, the positive curvature after attaining a maximum in the course of lo minutes, under- went a diminution and final neutralisation (Fig. 137). On account of this neutralisation the seedling became erect after an exposure of 30 minutes ; in contrast with this is the increasing positive curvature under unilateral illumination of the tip (Fig. 336) which continues for several hours. The explanation of this neutralisation under direct stimulation of the growing region is found iti the fact that transverse conduction of excitation induces contraction at the tlistal side of the organ and thus nullifies the positive curvature. The seeming absence of tropic effect under direct stimulation is thus not due to want of perception, but to balanced antagonistic reactions on opposite sides of the organ. EFFECT OF SIMULTANEOUS STIMULATION OF THE TIP AND THE HYPOCOTYL. Though stimulation of the hypocotyl results in neutra- lisation, yet the illumination of one side of the organ including the tip and hypocotyl is found to give rise to positive curvature. This will be understood from the following experiment. After the neutralisation in the last experiment light was also applied to the tip from the right side at the dotted arrow (Fig. 137). The record shows that this gave rise at first to a negative curvature (away from light) ; under the continued action of light, however, the negative was subsequently reversed to a positive curvature, towards light. Inspection of the curve shows another interesting fact. The positive curvature induced by direct stimulation is very much less than that brought out by indirect stimulation. This is due to two reasons: (1) the sensitiveness of the tip of the organ is, as is well known, greater than that 372 LIFE MOVEMENTS IN PLANTS of the hypocotyl, (2) the positive curvature under direct stimulation cannot proceed very far, since it is neutralised by transverse conduction of excitation. It will be seen from the above that the illumination of the tip practically inhibits the neutralisation and thus restores the normal positive curvature. The question now arises as to how this particular inhibition is brought about. ALGEBRAICAL SUMMATION OF THE EFFECTS OF DIRECT AND INDIRECT STIMULATIONS. An instance of inhibition, though of a different kind, was noticed in the response of the tendril of Passijiora (p. 296) ; the under side of the organ is highly sensitive, while the upper side is almost insensitive. Stimulation of the under side of the tendril induces a marked cur- vature, but simultaneous stimulation of the diametrically opposite side inhibits the response. This neutralisation could not be due to the antagonistic contraction of the upper side since the irritability of that side is very slight. 1 have shown that the inhibition results from the two antagonistic reactions, contraction at the proximal side due to direct stimulation and expansion caused by the positive impulse from the indirectly stimulated distal side. "We have in the above an algebraical summation of the effects of direct and indirect stimulations. The longitu- dinally transmitted effect of indirect stimulus in Setaria may, likewise, be summated with the effect of direct sti- mulus. The phenomenon of algebraical summation is demonstrated in a very striking and convincing manner in the following experiment, which I have been successful in devising. TRANSMITTED EFFECT OF LIGHT 373 Experiment 139. — I have explained, (Expt. 126) that unila- teral application of stimulus of light on the upper half of the responding pulvinus of Mimosa induces an up or positive curvature, followed by a neutralisation and even a reversal into negative, the last two effects being brought about by transverse conduction of excitation to the distal side. When the incident light is of moderate intensity, the trans- mitted excitation only sutiices to induce neutralisation Fig. 138. — (a) Diagrammatic representation of direct application of light (|) on the pulvinus and the indirect application on the stem (— ■^) (b) Record of eflfect of direct stimulus, positive curvature followed by neutralisation. Super- position of the positive reaction of indirect stimulus induces erectile up-response followed by down movement due to transmitted e.^citatory impulse (Mimosa). without further reversal into negative ; while in this state of balanced neutralisation let us apply indirect stimulus by throwing light on the stem at a point directly opposite to the leaf (Fig. 138). Two different impulses are thus initiated from the effect of indirect stimulus. In the present case the positive 374 LIFE MOVEMENTS IN PLANTS reached the responding pulvinus after 30 seconds and induced an erectile movement of the leaf ; the excitatory negative impulse reached the organ 4 minutes later and caused a rapid fall of the leaf. The record (Fig. 138) shows further that the previous action of direct stimulus which brought about neutralisation, does not interfere with the effects of indirect stimulus. The individual effects of direct and indirect stimulus are practically independent of each other; hence their joint effects exhibit algebraical summation. We are now in a position to have a complete under- standing of the characteristic response of Paniceae to transmitted phototropic excitation. (1) Local stimulation of the tip gives rise to two im- pulses, positive and negative. The former induces a transient negative movement (away from light) ; the latter causes a permanent and increasing positive curvature towards light. (2) Local stimulation of the growing hypocotyl gives rise to positive curvature, subsequently neutralised by the transverse conduction of excitation to the distal side. The absence of tropic effect in the growing region is thus due not to lack of power of perception, but to balanced anta- gonistic reactions of two opposite sides of the organ. (3) The effects of direct and indirect stimulations are independent of each other ; hence, on simultaneous stimula- tions of the tip and the growing hypocotyl, the effects of indirect stimulus are algebraically summated with the effect of direct stimulus (neutralisation). The indirect stimulation of the tip on the right side gives rise to two impulses, of which the expansive positive reached the right side of the responding region earlier, inducing TRANSMITTED EFFECT OF LIGHT 875 convexity and movement away from stimulus (negative curvature). This is diagrammatically shown in Fig. 189 Had the intervening tissue been non-conducting, the slow excitatory negative impulse would have failed to reach the responding region, and the negative curvature induced by the positive impulse would prove to be the initial as well as the final effect. In the case of Setaria^ however, the excitatory impulse reaches the right side of the organ after the positive impulse ; the final effect is therefore an induced concavity and positive curvature (movement towards stimulus). Fk4. 139. — Diagrammatic representation of the effects of direct and indirect stimulus on the response of Setaria. Direct stimulation, represented by thick arrow gives rise to antagonistic concavities of opposite sides of responding hypocotyl, resulting in neutralisation. Indirect stimulus represented by dotted arrow gives rise to two impulses the quick positive impulse represented by a circle, and the slower negative impulse represented by crescent (concave). The results given above enable us to draw the follow- ing generalisations : — 1. In an organ, the tip of which is highly excitable, the balanced state of neutralisation, induced by direct stimulation of the responding region, is upset in two different ways by two impulses generated in consequence of indirect stimulation at the tip. Hence arises two types of resultant response : — Type A. — If the intervening tissue be semi-conducting, the positive impulse alone will reach the growing region and induce convexity of the same side of the organ giving rise to a negative curvature. Type B. — If the intervening tissue be conducting the transmission of the excitatory impulse will finally give rise to a positive curvature. 29 376 LIFE MOVEMENTS IN PLANTS Type B is exemplified by the seedling of Setaria where the transmission of excitatory impulse from the tip upsets the neutral balance and induces the final positive curvature. Example of type A is found in the negative phototro- pisni of the root of Sinajjis. Negative photntropism of rant of Sinapis : Expe^nment 140.— For investigation of the negative phototropism of the root of Swapis nigra I took record of its movement under unilateral action of light by means of a Recording Microscope, devised for the purpose.* When the root-tip alone was stimulated by unilateral light, the root moved away from the source of light. This was due to the longitudinal transmission of positive impulse to the grow- ing region at some distance from the tip. The intervening distance between the tip and the growing region is practi- cally non-conducting, hence the excitatory impulse could not be conducted from the tip. After a period of rest in darkness, I next took record of its movement under direct unilateral illumination of the growing region ; the result was at first a positive movement ; but this, on account of transverse conduction of excitation under continued stimulation, underwent a neutralisation and slight reversal. In taking a third record, in which both the tip and growing region were simultaneously subjected to unilateral stimulation of light, I found that a result- ant responsive movement was induced which was away from light. Thus in the root of Sinapis., the expansive eH'ect of indirect stimulation of the tip is superposed on that of direct stimulation of the growing region (neutral or slightly * •• Plant KesiiDiise" — p. lioi. TRANSMITTED EFFECT OF LIGHT 377 negative). The final result is thus a movement away from light or a negative phototropic curvature. SUMMARY. The effect induced by stimulus of light is transmitted to a distance, in a manner precisely the same as in other modes of stimulation. In the Paniceae, the local unilateral stimulation of the tip of the cotyledon induces positive curvature in the growing hypocotyl, at some distance from the tip. This is due to transmitted excitatory effect- of indirect stimulation ; the earlier positive impulse induces a })reliminary negative curvature, which is reversed later by the excitatory negative impulse into positive curvature. Contrary lo generally accepted view the hypocotyl not only perceives but responds to light. The positive curva- ture induced by direct stimulation is, however, neutralised by transverse conduction of excitation. The effects of direct and indirect stimulus are inde- pendent of each other ; the final effect is determined by their algebraical suniuiation. 29 A XXXIV.— ON PHOTONASTIC CURVATURES By Sir J. C. BoSE, Assisted hy GURUPRASANNA DaS. PhototropiC response, positive or negative, is determined by the directive action of light. But photonastic reaction is supposed to belong to a different class of phenomenon, where the movement is independent of the directive action of light. I shall, however, be able to establish a continuity between the tropic response of a radial and the nastic movement of a dorsiventral organ. The intermediate link is supplied by organs originally radial, but subsequently rendered anisotropic by the unilateral action of stimulus of the environment. In a dorsiventral organ, owing to ana- tomico-physiological differentiation, the responsive movement is constrained to take place in a direction perpendicular to the plane of separation of the two unequally excitable halves of the organ. Even in such a case, it will be shown, that light does exert a directive action ; the direction of PHOTONASTIC CURVATURES 379 movement will further be shown to be distorted by the lateral action of li^jrht. PHOTOTROPIC RESPONSE OF ANISOTROPIC ORGANS. The different sides of a radial organ, such as the young stem of Mimosa, are equally excitable. The response to unilateral light of moderate intensity is therefore positive ; owing to equal excitabilities of the two sides the response of the opposite sides are alike. Diffuse stimulation there- fore induces no resultant curvature. If, however, the plant is allowed to form a creeping habit, the excitabilities of the dorsal and ventral sides will no longer remain the same. Thus in the creeping stem of Mimosa the lower or the shaded side is, generally speaking, found to be the more excitable. In fact such anisotropic stem of Mimosa acts somewhat like the pulvinus of the same plant. Diffuse stimulation induces, in both, a concavity of the more excitable lower half with the down movement of the leaf or the stem. Experiment 141. — I took four creeping stems of Mimosa in vigorous condition and tied them in such a manner that their free ends should be vertical. The shaded sides of the four specimens were so turned that each faced a different point of the compass — east, west, north and south. Subject- ed thus to dift'use stimulus of light from the sky, they all executed curvatures. The specimen whose under side faced the east, became bent towards the east ; the same happened to those which faced north, south, and west, that is to say they curved towards the north, south, and west respec- tively (Fig. 140). The fundamental action by which all these were determined was the induced concavity of the under or normally shaded sitle, which was the more excitable 380 LIFE MOVEMENTS TN FLANTS I obtained similar results with various other creeping stems. Fig. 140. — Photonastic curvature of creeping stem of Mimosa pudica : in the central figure the stem is seen to be vertical: action of diftiise light induced appropriate curvatures by greater contraction and concavity of the more excitable lower or shaded side, as seen ia figures to the right(6) and left(c). It has been shown that under prolonged unilateral stimulation, excitation becomes internally diffused ; this gives rise to an effect similar to that of external diffuse stimulus. Under strong light the shaded side becomes concave, and thus press against the ground or the support ; this will be the characteristic response of creeping stems in which the shaded side is the more excitable. The facts given above will probably explain the response of midribs of leaves, of the creeping stem of Lysimachia, all of which, in response to the action of strong light acting photOnastic curvatures 381 from above, exhibit concavity of the shaded and more excitabU' s^ide. PARA-HELIOTROPISM. Under strong sunlight, the leaflets oO various plants move sometimes upwards, at other times downwards, so as to pUice the blades of leaflets parallel to incident light. This 'mi(l(biy sleep' has been termed para-?ieliotropism by Darwin. It has I)een thought that para-heliotropic action has nothing to do with the directive action of light since many leaflets either fold upwards or downwards, irrespective of the direction of incident light. I shall for convenience distinguish the leaflets which fold upwards under light as iiositiveln para-heliotropic, and those which fold downwards as Jiegatively para-heliotropic. This is merely for convenience of description. There is no specific irritabality which distinguishes one from the other. POSITIVE PARA-HELIOTROPISM. Para-heliotrupic response of Erythrina indict and of Olitoria ternatea : Experiment 142. — For the purpose of sim- plicity I have described the type of movement of these leaflets as upwards; but the actual direction in which the leaflets point their apices is towards the sun. Both the plants mentioned here are so remarkably sensitive that the leaflets follow the course of the sun, in such a way that the axis of the cup, formed by the folding leaflets at the end and the sides of the petiole, is coincident with the rays of light. The pulvinus makes a sharp curva- ture which is concave to light, the blade of the leaflet being parallel to light. I have taken record of continuous action of strong light acting on the responding pulvinus of the leaflets from above. The result is an increasing positive curvature which reached a limit (Fig. 141). There 382 LIFE MOVEMENTS IN PLANTS was no neutralisation or reversal, demonstrating the absence of transverse conduction (c/. Fig. 132). Fig. 141. — Positive para-heliotropic response of leaflets of Eryihrina indica. Para-heliotro2nc movement of leaflets of Mimosa pudica : Experiment 148. — These leaflets, as previously stated, fold themselves upwards, when strongly illuminated either from above or below. Diffuse electric stimulation also induce a closing movement upwards ; hence the upper half of the pulvinule of these leaflets are the more excitable. In order to obtain a continuous record of the leaflet under the action of unilateral light, I constructed a very delicate recording lever magnifying about 150 times. Light of moderate intensity from a 100 candle-power incandescent lamp was applied on the less excitable lower side of the pulvinule. The record (Fig. 142) shows that the imme- diate response is positive, or a movement towards the light. But owing to transverse conduction, through the thin and highly conducting pulvinule, the response was quickly reversed into a very pronounced negative, or movement away from light. Had a delicate means of obtaining magnified record not been available, the slight positive twitch, and the gradual transition from positive to nega- tive phototropic curvature would have passed unnoticed. Application of light from above gave, on account of the greater excitability of the upper half of the pulvinule, a pronounced positive response or movement towards light. The anomaly of an identical organ appearing as positively heliotropic when acted by light from above, and negatively PHOTONASTIC CURVATURES 383 heliotropic when acted from below, is now fully removed. The response of the leaflets is also seen to be determined by the directive action of light, though the short-lived response of the less excitable lower side is quickly masked by the predominant reaction of the more excitable upper aide of the organ. Fig. 142. Fig. 143. Fig. 142.— Response of leaflet of Mimosa to light applied below : transient positive followed by pronounced negative curvature. Fig. 143. — Response of leaflet of Averrhoa, to light applied above: transient positive followed by pronounced negative curvature. Up-curve represents up-movement, and down-curve, down-movement. NEGATIVE PARA-HELIOTROPISM. Response of leaflet of Averrhoa carambola : Experiment 144. — The leaflets of this plant, and also those of Biophytum sensitivum fold downwards under action of strong light, applied above or below. In these leaflets difl'use electric stimulation induce a fall of the leaflets demonstrating the greater excitability of the lower half of the pulvinule. The analysis of reaction under light is rendered possible from 384 LIFE MOVEMENTS IN PLANTS the record of response of leaflet of Averrhoa, given in Fig. 143. Light of moderate intensity from an incandescent electric lamp acted from above : the result was a feeble and short-lived positive response, quickly reversed to strong negative by transmission of excitation to the more excit- able lower side. Illumination from below gave rise only to strong positive response. Thus in AverrJioa the effect of continuous light applied above or below is a downward movement ; in Mimosa the movement is upwards. The explanation of this difference lies in the facl, that in Mimosa leaflet it is the upper half of the pulvinule that is more excitable ; while in AverrJioa and in Biophytum the lower is the more excitable half of the organ. As a summary of the tropic action of light I shall give diagrammatic representations of various types of phototropic ifs^^ »-—* (a) (d) % (-¥■') (- -) 0 c«. © e o Fiu. 144. — Diagrammatic representation of different types of phototropic response. (See text.) response, including the photonastic (Fig. 144). The direc- tion of the arrow indicates the direction of incident light. Dotted specimens are those which possess transverse conductivi- ty. Thick lines represent the more excitable side of an aniso- tropic or dorsiventral organ. The size of the circles, with PHOTONASTIC CURVATURES 385 positive ami negative sigus, represents the amplitude and sign of curvature. a. Radial thick organ, in which transverse conduction is absent. Curvature is positive, i.e., movement to- wards light. The result will be similar when light strikes in an opposite direction, i.e., from right to left. h. Radial thin organ. There is here a possibility of transverse conduction. Sequence of curvature : positive, neutral, and negative. Reversal of direction of light gives rise to similar sequence of responses as before (e.g., seedling of Sinapis). c. Anisotropic thick organ ; transverse conduction possible. Thick line represents the more excitable distal side. Sequence of curvature : positive, neutral and pronounced negative. When light strikes from opposite direction on the more excitable side the curvature will remain positive, since the pronounced reaction of the more excit- able side cannot be neutralised or reversed by transmitted excitation to the less excitable distal side {e.g., leaf of Mimosa). In the absence of transverse conduction, the curvature remains positive (e.g., leaflet of Erythrina). d. Anisotropic thin organ with high transverse con- ductivity. Sequence of curvature : transient posi- tive, quickly masked by predominant negative. Light striking on the more excitable side will give rise only to positive. The response in rela- tion to the plant, will apparently be in the same direction whether light strikes the organ on one side or the opposite (e.g., leaflets of Mimosa Averrhoa and Biophytum). I have shown that tissues in sub-tonic condition exhibit an acceleration of the rate of growth under stimulus (p. 224) 386 LIFE MOVEMENTS IN PLANTS the corresponding tropic reaction would therefore be away from stimulus or negative curvature. The tonic condition is, however, raised to the normal by the action of stimu- lus itself, and the tropic curvature becomes positive. I give below a table which will show at a glance all possible variations of phototropic reaction. TABLE XXXI. — MECHANICAL RESPONSE OF PULVINATED AND GROWING ORGANS UNDER LIGHT. Description of tissue. I Tissue sub- ionic. II Normally excitable organ under uni- lateral light. A. Organ radial. B. Dorsi- ventral organ. tll Rhythmic tissue. Action. Stimulus causes increase of internal energy. A 1. Moderate light, causing excitatory contraction of proximal and positive expan- sion of distal. A 2. Strong light. Excitatory effect transmitted to distal, neutralising first. A .8. Intense and long-continu- ed light. Fatigue o\ proximal and excitatory contraction of distal. B 1. Excitatory contraction of proximal predominant, owing either to greater excitability of proximal or feeble transverse conductivity of tissue. B 2. Transmission of excitation through highly conducting tissue to more excitable lower or distal. Greater contraction of distal. Considerable absorption of energy, immediate or prior. Effect observed. Expansion or enhanced rate of growth, e.g.^ Pileus of Coprinus drooping in darkness, made re-turgid by light. Renewed growth of dark rigored plant exposed to light. 1. Curvature towards light, e.g., llower bud of Crinnm. 2. Neutralisations, e.p., seedling of Setaria. 3. Reversed or negative response, e..(/., seedling of Zea ^lays . 1. Positive response, e.g., up- ward folding of leaflets in so- called " diurnal sleep " of Ery- thrina indica and Clitoria ternatea. 2. Negative response, e.g., down- ward folding of leaflets in so- called "diurnal sleep " of Bio- phytum and Averrhoa. Initiation of multiple response in Desmodinm gyrans previous- ly at standstill ; multiple response under continuous action of light in Biophytum. SUMMARY. There is no line of demarcation between tropic and nastic movements. PHOTONASTIC CURVATURES 387 In a differentially excitable organ the effect of strong unilateral stimulus becomes internally diffused, and causes greater contraction of the more excitable side of the organ. In the absence of transverse conduction, the positive curvature reaches a maximum without neutralisation or reversal. The leaflets of Ernjtliriiia indica and of Clitoria tei'natea thus fold upwards, the apices of the leaflets pointing towards the sun. Internally dift'used excitation under strong light induces greater contraction of the more excitable half of the pulvinule, causing upward folding of Mimosa leaflet, and downward folding of the leaflets of Biophytum and Averrhoa. XXXV.— EFFECT OF TEMPERATURE ON PHOTOTROPIO CURVATURE By Sir J. C. BosE, Assisted by GURUPRASANNA DaS. I SHALL in this chapter deal with certain anomalies in phototropic curvature, brought about by variation of tem- perature and by seasonal change ; cerlain organs again are apparently erratic in their phototropic response. SEASONAL CHANGE OP PHOTOTROPIC ACTION. Sachs observed a positive phototropic curvature in the stems of l^ropceolum majus in autumn ; but this was reversed into negative in summer ; similarly in the hypocotyl of Ivy, the positive curvature in autumn is converted into negative curvature in summer. Certain organs are apparently insensitive to the action of light. Thus no phototropic response is found in the tendril of Passijiora even under the action of strong light. The tendrils of Vitia and Ampelojsis exhibit, according to Wiesner, positive phototropism under feeble, and negative phototropism under strong light. The anomalies referred to above may be explained by taking into consideration the modifying influence of TEMPERATURE ON PHOTOTROPIC CURVATURE oSO temperature on the excitability, and the conductivity of the organ. EFFECT OF TEMPERATURE ON EXCITABILITY. The excitability of an organ is abolished at a low temperature ; it is enhanced by a rise of temperature up to an optimum. The temperature minimum and optimum varies in ditferent tissues. The folio sving table shows the enhancement of excitability of Mimosa at different temperatures, the testing stimulus being the same. lABLE XXXH— SHOWINd VARIATION OK EXCITABILITY OK I'ULVINUS OF Mimosa at differext temperatures. Teraporature. Amplitude of re^sponse. 22 OC. ... 2 divisions. 270c. ... 16 ., 320C. ... i 36 ,. Below 20^C. the excitability of the pulvinus of Mimosa is practically abolished. The excitability increases till an optimum temperature is reached, above which it undergoes a decline. Though rise of temperature enhances excitability up to an optimum, there is an antagonistic reaction induced by it which opposes the excitatory contraction. The physiological reaction of a rise of temperature, within normal range, is expansion and this must oppose the contraction induced by stimulus. Hence the elLect of rise of temperature is complex ; it enhances the excita- bility which favours contraction, while tending to oppose this contraction by the induced physiological expansion. As a result of these opposite reactions there will be a critical temperature, below which the contractile effect 390 LIFE MOVEMENTS IN PLANTS will relatively be greater than expansion ; above the critical point, expansion will be the predominant effect. The critical temperature will obviously be different in different organs. The positive curvature may thus- be increased by a slight rise, while it may be neutralised, or even reversed by a greater rise of temperature. The induced variation of excitability due to change of temperature is not the only factor in modifying tropic curvature, for variation of conductivity also exerts a marked effect. EFFECT OF TEMPERATURE ON CONDUCTION. The conducting power of an organ is greatly enhanced with rise of temperature. Thus in Mimosa the velocity of transmission of excitation is doubled by a rise of tempera- ture through 9^C. (p. 100). An organ which is practically non-conducting at a low temperature will become conducting at a higher temperature. Thus at a low temperature the organ may be non- conducting, and the excitatory contraction under unilateral stimulus will remain localised at the proximal side ; this will give rise to a positive curvature. But under rising temperature, the power of transverse conduction will be increased and the excitation will be conducted to the distal side. The result of this will be a neutralisation or reversal into negative curvature (p. 139). A positive curvature is thus reversed into negative by change of excitability and conductivity, induced by rise of tempera- ture ; examples of this will be given presently. PHOTOTROPIC RESPONSE OF TENDRILS. I shall here adduce considerations which will show that the apparent anomalies regarding the response of tendrils TEMPERATURE ON PHOTOTROPIC CURVATURE 301 to light is line to the variation of transverse conductivity of the organ. In a semi-conducting tissue, while the excitatory effect of feeble stimulus remains localised at the proximal side, the effect of stronger stimulus is conducted to the distal side. This explains the positive phototropic curvature of tendrils of Vitis and Ampelopsis under feeble light, and its reversal into negative curvature under intense light. As the conducting power is increased with rise of tem- perature it is evident that at a certain temperature the tropic effect will be exactly neutralised by transverse con- duction. Lowering of temperature, by reducing the trans- mission of excitation to the distal side, will restore the positive curvature. Enhancement of conduction under rise of temperature will, on the other hand, increase the anta- gonistic reaction of the distal side and give rise to a negative curvature. I shall in verification of the above, describe experiments which I have carried out on the phototropic response of the tendril of Passljiora., supposed to be insensitive to the action of light. Phototrojiic response of the tendril of Passifiora : Experiment 145. — The tendril was cooled by keeping it for a long time in a cold chamber, maintained at 15'^C. The effect of unilateral light on the cooled specimen was found to be positive ; the tendril was next allowed to assume the temperature of the room which was 30^0. The response was now found to have undergone a change into negative. The positive and negative phototropic curvatures of an identical organ at different temperatures is seen in the two records given in figure 145. Neutralisation 30 392 LIFE MOVEMENTS IN PLANTS takes place at an intermediate temperature, and the organ thus appears insensitive to light. SEASONAL VARIATION OF PHOTOTROPIC CURVATURE. Reference has been made of the phototropic curvature of Tropceolum and of Ivy undergoing a change from Fig. 145. — (a) Positive curvature of tendril of Passiflora ai 15"C. ; (6) negative phototropic curvature at 30^C. positive in autumn to negative in summer. The experi- ment described above shows that rise of temperature, by enhancing transverse conductivity, transforms the positive inio negative heliotropic curvature. The reversal of the phototropic curvature of Tropceolum and Ivy, from positive in autumn to negative in summer, finds a probable explana- tion in the higher temperature condition of the latter season. This inference finds independent support from the fact previously described (p. 100) that while the velocity of TEMPERATURE ON PHOTOTROPIC CURVATURE 393 conduction of excitation in the petiole of Mimosa is as high as 30 mm. per second in summer, it is reduced to about 4 mm. in late autumn and early \vinter. ANTAGONISTIC EFFECTS OF LIGHT AND OF RISE OF TEMPERATURE. I have explained the complex effect of rise of temperature on phototropic curvature. Rise of temperature, within limits, enhances the excitability, and therefore the positive curvature under light. Its expansive reaction, on the other hand, opposes the contraction of the proximal side, which produces the normal positive curvature. Rise of tempera- ture, as previously stated, introduces another element of variation by its effect on conductivity. Transverse conduc- tion favoured by rise of temperature promotes neutralisation and reversal ; the resultant effect will thus be very complicated. I give below account of an experiment where the induced positive curvature under light underwent a reversal during rise of temperature. Reversal of tropic curvature under rise of temperature : Experiment 146. — The specimen employed for this experiment was a seedling of pea, enclosed in a glass chamber, the temperature of which could be gradually raised by means of an electric heater. Provisions were made to maintain the chamber in a humid condition. The temperature of the chamber was originally at 29°C., and application of light on one side of the organ gave rise to positive curvature, followed by complete recovery on the cessation of light (Fig. 14t)a). The next experiment was carried out with the same specimen ; while the plant was undergoing increasing positive curvature under the continued action of light, the temperature of the 30 a 394 LIFE MOVEMENTS IN PLANTS chamber was gradually raised from 29° to 33°C. at the point marked with arrow. It will be seen that the positive curvature became arrested, neutralised, and finally reversed into negative (Fig. 146b). Fig. 146. — Effect of rise of temperature on phototropic curvature, (a) normal positive curvature followed by recovery, (6) reversal of positive into negative curvature by rise of temperature at (.H). (Pea seedling). After-effect of rise of temperature : Experiment 147. — The after-ej[fect of rise of temperature exhibited by this specimen was extremely curious. The temperature of the chamber was allowed to return to the normal, and the experiment repeated after an hour ; the response was now found to be negative (Fig. 147a). It appeared probable that the temperature in the interior of the tissue had not yet returned to the normal, and an interval of four hours was therefore allowed for the res- toration of the tissue to the normal temperature of the room. The response still persisted to be negative, as seen in the series of records obtained under successive stimulations of light of short duration ; these negative responses exhibited recovery on the TEMPERATURE ON PHOTOTROPIC CURVATURE 395 cessation of light (Fig. 1471)). This reversal of response as an after-effect of rise of temperature was in this case found to Fig. 1-47. — After-effecc of rise of temperature, persistent negative curvature : (a) response one hour after rise of temperature; (.6) series of negative responses after 4 hours (successive stimuli applied at vertical lines). persist for several hours. I experimented with the same speci- men next day when the response was found restored to the normal positive. SUMMARY. Rise of temperature, within limits, enhances the general excitability of the organ. This has the effect of increasing positive phototropic curvature. But the physiological expan- sion induced by rise of temperature exerts an antagonistic effect. The transverse conductivity is increased with the rise of temperature ; this favours neutralisation and reversal of photo- tropic curvature. Tendrils of Passifiora, supposed to be phototropically insensitive, exhibit positive curvature at low, and negative cur- vature at a moderately high temperature. 396 LIFE MOVEMENTS IN PLANTS The change of phototropic curvature exhibited by Tropceolum majus and Ivy, from positive in autumn to negative in summer, is probably due to the effect of tem- perature. Higher temperature with enhanced transverse conductivity in summer, may thus convert positive into negative curvature. The physiological effects of vise of temperature and the stimulus of light are antagonistic to each other. Rise of temperature tends to neutralise or reverse the positive phototropic curvature. The after-effect of temperature is often very persistent. XXXVI.— ON PHOTOTROPIC TORSION By Sir J. C. BosE, Assisted by SuRENDRA Chandra Das. Is addition to positive or negative curvatures light induces a responsive torsion. With regard to this Jost says : — " The mechanics of the torsions have not as yet been fully explained. It has long been believed that these torsions were occasioned only by the action of a series of external factors, such as light, gravity, weight of the organ which individually led to curvatures, but in combination induced torsions ; but later investigations have shown that torsions might appear when light only was the functional external factor. ... If (he torsions cannot generally be regarded as due to the combina- tion of tv>'0 curvatures, we are completely in the dark as to the mechanics of their production."* A leaf when struck laterally by light undergoes a twist, so that the upper surface is placed, more or less, at right angles to the incident rays ; as no explanation was available for this movement, the suggestion has been made that the particular reaction is for the advantage of the plant. I shall presently show, that it is possible to reverse this normal torsion and thus make the upper surface of the leaf move away from light. * Jost -/6k/— p. 465. ;^98 LIFE MOVEMENTS IN PLANTS The expei-imeuts which I shall presently describe will, it is hoped, throw light on the obscure phenomenon. I shall be able to show : (1) that the torsional response is not dependent on the combination of two curvatures, (2) that it is also independent of the effect of weight, (3) that it may be induced not merely by stimulus of lignt but by all forms of stimulation, (4) that the direction of the tor.sional response depends on the direction of the incident stimulus and the differential excitability of the organ, acd (5) that there is a definite law which determines the torsional movement. EXPERIMENTAL ARRANGEMENTS. I shall first describe a typical experiment on the responsive torsion under the action of light. We have seen that in the pnlvinus of Mimosa, light of moderate intensity and of short duration applied on the upper half iuduces a slow up-move- ment, while the stimulus of light applied below induces a more rapid down-movement. The difference is due to the fact that the lower half of the pulvinus is relatively the more excitable. Vertical light thus induces a movement in a vertical plane. But an interesting variation is induced in the response under the action of lateral light. A stimulus will be called lateral when it acts on either the right or left flank of a dorsiventi'al organ. We shall presently find that a dorsiventral organ responds to lateral stimulus by torsion. The present series of experiments were carried out with the leaf of Mimosa, and in order to eliminate the effect of weight and also for obtaining record of pure torsion, I employed the following device. The petiole was enclosed in a hooked support made of thin rod of glass, the petiole resting on the concavity of the smooth surface. Friction and the PHOTOTROPIC TORSION 399 effect of weight is thus practically eliminated ; the looped support prevented up or down movements, and yet allowed perfect freedom for torsional response. This latter is magnified by a piece of stout aluminium wire fixed at right angles to the Fig, 148. — Diagrammatic representation for record of tors^ional response. H, thin glass hook : A, aluminum wire attached to petiole for magnification of torsional movement. T, silk thread for attachment to recording lever. petiole (Fig. 148). The end of the aluminium wire is attached to the short arm of a recording lever ; there is thus a com- pound magnification of the torsional movement. The Oscillat- ing Recorder gave successive dots at intervals which could be varied from 20 seconds to 2 minutes. Time-relations of the response may thus be obtained from the dotted record. With the experimental device just described, we shall be in a position to study the effect of various stimuli applied at one flank of the pulvinus — at the junction of the upper and low^er halves of the organ. The observer standing in front of the leaf is supposed to look at ihe stem. Torsional response will then appear as a movement either with or against the hands of the clock. The torsional response, right-handed or left-handed, 400 LIFE MOVEMENTS IN PLANTS will presently be shown to depend on the direction of incident stimulus. In figure 149, anti-clockwise torsion is recorded as an up-curve ; clockwise rotation is recorded as a down- curve. ACTION OF STIMULUS OF LIGHT. Experiment 148. — The pulvinus of the leaf was stimu- lated by a horizontal beam of light thrown in a lateral direction ; the areas contiguous to line of junction of the upper and lower halves of the anisotropic organ thus under- went differential excitation. When light struck on the left flank, the responsive torsion was anti-clockwise ; the respon- sive reaction thus made the upper and the less excitable half of the pulvinus face the stimulus. Figure 149 gives a Fiw. 149. — Record of torsional response of pulvinus of Mimosa pudica. record of the torsional movement when light struck the left flank of the organ ; on the cessation of stimulus the response is followed by recovery. DIRECTIVE ACTION OF STIMULUS. Experiment 149. — If now the direction of stimulus be changed so that light strikes on the right flank instead PHOTOTROPIC TORSION 401 of the left, the responsive torsion is found to be reversed, the direction of movement being clockwise. Here also the responsive movement is such that it is the less excitable upper half of the organ that is made to face the stimulus. It will thus be seen that the torsion, anti-clockwise or clockwise, depends on two factors, namely the direction of stimulus, and the differential excitability of the organ. EFFECT OF DIFFERENT MODES OF LATERAL STIMULATION. I shall now proceed to show that the torsional response is induced not merely by the action of light, but by all forms of stimulation. Effect of cliemical stimulation : E tperiment 150. — Dilute hydrochloric acid was at first applied on the left flank of the pulvinus along the narrow strip of junction of the upper and lower halves. This gave rise to a respon- sive torsion against the hands of a clock. Chemical stimulation of the right flank induced, on the other band, a torsional movement with the hands of a clock. Here also the direction of stimulus is found to determine the direction of responsive torsion. Effect of thermal radiatio?i : Experiment 151. — I next employed thermal radiation as the stimulus ; the source of radiation was a length of electrically heated platinum wire. It is advisable to interpose a narrow horizontal slit, so as to localise the stimulus at the junction of the upper and lower halves of the pulvinus. Stimulus applied at the left flank induced left-handed or anti-clockwise torsion ; application at the right flank gave rise to right- handed torsion. Geotro]nc stimulus. — The stimulus of gravity induces, as I shall show in a subsequent chapter, a similar 402 LlE'E MOVEMENTS IN PLANTS responsive torsion, the direction of which is determine*! by the direction of the incident stimulus. EFFECT OP DIFFERENTIAL EXCITABILITY ON THE DIRECTION OF TORSION. Under normal conditions, the torsional response under light places the upper surface of the leaf or leaflets at right angles to light. That this movement is not due to some specific sensibility to light is shown by the fact that all modes of stimulation, chemical, thermal or gravitational, induce similar responsive torsion. The tor- sional response is, moreover, shown to be determined by the direction of incident stimulus, and the differential excita- bility of the organ. This latter may be reversed by the local application of various depressing agents on the nor- mally more excitable lower half of the pulvinus. Under this treatment, the lower half of the pulvinus may be rendered relatively the less excitable. Lateral applica- tion of light now induces a torsional movement which is the reverse of the normal, so that the upper surface of the leaf moves away from light. The advantage of the plant cannot, therefore, be the factor which determines the directive movement ; the teleological argument often advanced is, in any case, no real explanation of the phenomenon. In all the instances given above, and under every mode of stimulation, the responsive movement makes the less excitable half of the pulvinus face the stimulus. The torsional response is, in reality, the mechanical result of the differential contraction of a complex organ, which is fixed at one end and subjected to lateral stimulation. I have been able to verify this, by the construction of an artificial pulvinus consisting of a compound strip, the upper half of which is ebonite, and lower half the more PHOTOTROPIC TORSION 403 contractile stretched Imlia-rubber ; if such a strip be held securely at one end in a clamp, and if the lateral flank, consisting half of ebonite and half of India-rubber, be subjected to radiation, and record taken in the usual manner, it will be found that a torsional response takes place which is similar to that of the pulvinus of Mimosa. The above experiment was devised to offer an explanation of the mechanics of the movement. It should, however, be Itorne in niiiul 'u\ this connection that the torsional response of pulvinus is brought about by differential pliysinlogical contraction of the organ, the movement being abolished at death. From the results given above, we arrive at the follow- ing : — LAWS OF TORSIONAL RESPONSE. 1. An ANISOTROPIC ORGAN, WHEN LATERALLY EXCITED by any stimulus, undergoes torsion by which the less excitable side is made to face the stimulus. 2. The intensity of torsional response increases WITH THE differential EXCITABILITY ; WHEN THE ORIGINAL DIFFERENCE IS REDUCED, OR REVERSED, THE TORSIONAL RESPONSE UNDERGOES CONCOMITANT DIMINU- TION OR REVERSAL. Having thus established the laws that guide torsional response, I shall try to explain certain related phenomena which are regarded as highly obscure. I shall also des- cribe the application of the method of torsional response in various investigations. COMPLEX TORSION UNDER LIGHT. The leaves of the so-called "Compass plants" exhibit very complex movements, these being modified according to 404 LIFE MOVEMENTS IN PLANTS the intensity of incident light. Thus in compass plants the leaves, under moderate intensity of light in the morning or in the evening, turn themselves so as to expose their surfaces to the incident rays. But under intense sun light, the leaves perform bendings and twistiugs so that they stand at profile at midday. I have not yet been able to secure "Compass plants" at Calcutta. I shall, however, describe my investigations on the complicated torsional movemenis exhibited by certain leaflets by the action of vertical light. The results obtained from these will show that torsional movements, even the most complex, are capable of explanation from the general laws that have been established. Toi'siuiial movement of leaflet of Cassia alata : Experi- ment 152. — These leaflets are closed laterally at night but place themselves in an outspread position at day time. The character of the movement is, however, modified by the intensity of light. With moderate light in the morn- ing the leaflets open out laterally. But under more intense light, the pulvinules of the leaflets exhibit a torsion by which the formerly infolded surfaces of the leaflets are Fig. 150. — Leaflets of Cassia alata : open in daytime, and closed in evening. exposed at right angles to light from above (Fig. 150)_ Such complicated movements, in two directions of space, are also exhibited by other leaflets which are closed at night in a lateral direction. PHOTOTROPIC TORSION 405 For obtaining an explanation of these complex move- ments under cliflFerent intensities of light, we have first to discover the particular disposition of the two halves of the pulvinule which are unequally excitable ; we have next to explain the responsive movements under the directive action of moderate and of intense light. Determination of differential excitahilities of the organ : Experiment 153. — In the leaflet of Cassia the movement of opening under diffuse stimulation of light can only be brought about by the contraction of the outer half, which must therefore be the more excitable. This is indepen- dently demonstrated by the reaction to an electric-shock. On subjecting the half closed leaflets to diffuse electric stimulation, they open outwards in a lateral direction. The disposition of the unequally excitable halves of the pulvinule is thus different from that of the main pulvinus of Mimosa. In the latter, the plane that divides the two halves is horizontal, the lower half being the more excitable. Thus in the pulvinule of Cassia the plane that separates the two unequally excitable halves is vertical, the outer half being the more excitable than the inner. By inner half is here meant that half which is inside when the leaflets are closed. Effect of strong vertical light : Experiment 154. — When the plant is placed in a moderately lighted room, the leaflets open out laterally to the outmost. This is brought about by the contraction of the more excitable outer half of the organ. If strong light be thrown down from above, a new movement is superposed, namely, of torsion by which the leaflets undergo a twist and thus place their inner surface at right angles to the vertical light. In order to investigate this phenomenon in greater detail I placed the plant in a well lighted room, the leaflets being three quarters open un(i. Fig. Inlt. — Geotropic rei-ponse of flower stalk of tube ro«' : preliminary down- movement is due to weight. Fig. 160. — Geotroinc response of petiole of Tiofxpolion : latent period shorter than 20 seconds. the geotropic up-movement is seen to be initiated (Fig. ir)9) after the tenth dot, the latent period being thus li minutes and 20 seconds, the greater part of which was spent in overcoming the down-movement caused by the weight of the organ. Geotropic response of petiole of Tropaeolum : Experi- ment 165. — I expected to obtain still shorter latent period by choosing thinner specimens with less weight. I therefore took a cut specimen of the petiole of Tropceo- lum^ and held it at one end. The lamina was also cut off in order to reduce the considerable leverage exerted by it. The response did not now exhibit any preliminary 434 LIFE MOVEMENTS IN PLANTS down-movement, and the geotropic up-movement was com- menced within a few seconds Hfter placing the petiole in a horizontal position (Fig. 160). The successive dots in the record are at intervals of 20 seconds and the second dot already exhibited an up-movement ; the latent period is therefore shorter than 20 seconds. It will thus be seen that the latent period in this case is of the same order as the hypothetical period of migration of the statoliths. I may state here that I have been successful in devising an electric method for the determination of the latent period, in which the disturbing effect of the weight of the organ is completely eliminated. Applying this perfect method, I found that the latent period was in some cases as short as a second. The experiment will be found fully described in a later chapter. THE COMPLETE GEOTEOPIC CURVE. The characteristics of the geotropic curve are similar to those of other tropic curves. That is to say the susceptibility for excitation is at first feeble ; it then increases at a rapid rate ; in the third stage the rate becomes uniform ; and finally the curvature attains a maximum value and the organ attains a state of geotro- pic equilibrium (cf. page 353). The period of comple- tion of the curve varies in diff:erent specimens from a few to many hours. Experiment 166. — The following record was obtained with a bud of Cri?iu?n, the successive dots being at intervals of 10 minutes. After overcoming the efiEect of weight (which took an hour), the curve rose at first slowly, then rapidly. The period of uniformity of move- ment is seen to be attained after three hours and GEOTROPISM 435 coutiuued for nearly 'JO minutes Tiie final equilibrium was reached after a period of 8 hours (Fig. 161). Fig IGl.— The Conipleie Geotropic carve (Crininn). For studying the effect of an external agent on geotro- pic action, the period of uniform movement is the most suitable. Acceleration of the normal rate (with enhanced steepness of curve) indicates that the external agent acts with geotropism in a concordant manner ; depression of the rate with resulting flattening of the curve shows, on the other hand, the antagonistic effect of the outside agent. DETERMINATION OF EFFECTIVE DIRECTION OF STIMULUS. The experiments which have been described show that it is the upper side (on which the vertical lines of 436 LIFE MOVEMENTS IN PLANTS gravity impinge) that undergoes excitation. The vertical lines of gravity must therefore be the direction of incident stimulus. This conclusion is supported by results of three independent lines of inquiry : (1) the algebraical summa- tion of effect with that of a different, stimulus whose direction is known, (2) the relation between the directive angle and geotropic reaction, and (H) the torsiotuil response under geotropic stimulus. EFFECT OF ALGEBRAICAL SUMMATION. Experiment 167. — A flower bud of Grinmn is laid horizontally, and record taken of its geotropic move- ment. On application of light on the upper side at L, c:^ Fig. 16-J. Fig. 163. Fig. 162. — Stimulu? of light or gravity, represented bj' arrow, induces up cur- vature aa .seen in dotted figure. Fig. 163. — The effect of super-imposition of photic stimulus. The first, third, and fifth parts of the curve, give normal record under geotropic stimulus. Rate, of up-movement enhanced under light L. the responsive movement is enhanced, proving that gravity and light are inducing similar etfects. On the GEOTROPISM 4:>7 cessation of light, tlif original rate of geotropic movement is restored (Fig. 163). Application of light of increasing intensity from below induces, on tlie other hand, a diminu- tion, neutralisation, or reversal of geotropic movement. Light acting vertically from above induces a concavity of the excited upper side in consequence of which the organ moves, as it were, to meet the stimulus. The geo- tropic response is precisely similar. In figure 162 the arrow represents the direction of stimulus which may be rays of light or vertical lines of gravity. ANALOGY BETWEEN THE EFFECTS OF STIMULUS OF LIGHT AND OF GRAVITY. In geotropic curvature we may for all practical purposes regard the direction of stimulus as coinciding with the vertical lines of gravitj'. The analogy between the effects of light and of gravity is very close* ; in both the induced curvature is such that the organ moves so as to meet the stimulus. This will be made still more evident in the investigations on torsional geotropic response describetl in a subsequent chapter. The tropic curve under geotropic stimulus is similar to that under photic stimulus. The tropic reaction, both under the stimulus of light and of gravity, increases similarly with the ' directive ' angle. These real analogies are unfortunately obscured by the use of arbitrary terminology used in description of the geotropic curvature of the shoot. In figure 163 records are given of the effects of vertical light and of vertical stimulus of gravity, on the responses of the horizontally laid bud of Crimim. In both, the upper side undergoes contraction and the movement of response carries the ■ Exception to this will be found in page HHi-i. where esplanatiori is offered for the difference. 438 LIFE MOVEMENTS IN PLANTS organ upwards so as to place it parallel to the incident stimulus. Though the reactions are similar in the two cases, yet the effect of light is termed positive phototro- pism, that of gravity negative geotropism. I would draw the attention of plant-physiologists to the anomalous character of the existing nomenclature. Geotropism of the shoot should, for reasons given above, be termed positive instead of negative, and it is unfortunate that long usage has given currency to terms which are misleading, and which certainly has the effect of obscuring analogous phenomena. Until the existing terminology is revised, it would perhaps be advisable to distinguish the geotropism of the shoot as Z^'nitliotropisni and of the root as Nadiro- t?' op ism. RELATION BETWEEN THE DIRECTIVE ANGLE AND GEOTROPIC REACTION. When the main axis of the shoot is held vertical, the angle made by the surface of the organ with lines of force of gravity is zero, and there is no geotropic effect. Tie geotropic reaction increases with the directive angle ; theoretically the geotropic effect should vary as the sine of the angle. I shall in the next chapter describe the very accurate electrical method, which I have been able to devise for determination of relative intensities of geotropic action at various angles. Under perfect conditions of symmetry, the intensity of effect is found to vary as the sine of the directive angle. This quantitative relation fully demonstrates that geotropic stimulus acts in a definite direction which coincides with the vertical lines of gravity. The conditions of perfect symmetry for study of geo- tronic action at various angles will be fully described in GEOTROPTSM 439 the next chapter. In the ordinary method of experiment- ation with mechanical response the organ is rotated in a vertical plane. The geotropic movement is found increased as the directive angle is increased from zero to 90^. DIFFERENTIAL GEOTROPIC EXCITABILITY. It has been shown that geotropic stimulus acts more effectively on the upper side of the organ. The intensity of geotropic reaction is, moreover, modified by the excitability of the responding tissue. It is easy to demonstrate this by application of depressing agents on the more effective side of the organ. The rate of geotropic up-movement will be found reduced, or even abolished by the local applica- tion of cold, anaesthetics like chloroform, and of poison- ous potassium cyanide solution. The different sides of a dorsiventral organ are unequally excitable to diff'erent forms of stimuli. I have already shown (p. 85) that the lower side of the pulvinus of Mimosa, is about 80 times more excitable to electric stimulus than the upper side. Since the effect of geotropic stimulus is similar to that of other forms of stimuli, the lower side of the pulvinus should prove to be geotropically more excitable than the upper side. This I have been able to demonstrate by different methods of investigation which will be described in the following chapters. Under ordinary circumstances, the upper half of the pulvinus is, on account of its favourable position, more effectively stimulated by geotropic stimulus ; in consequence of this the leaf assume a more or less horizontal position fif " dia-geotropic " equilibrium. But when the plant is inverted the more excitable lower half of the organ now occupies the favourable position for geotropic excitation. 38 440 LIFE MOVEMENTS IN PLANTS The leaf now erects itself till it becomes* almost parallel to the stem. The response of the same pulvinus which was formerly " dia-geotropic " now becomes " negatively geo- tropic " ; but an identical organ cannot be supposed to possess two different specific sensibilities. The normal horizontal position assumed by the leaf i-<, therefore, due to differential geotropic excitabilities of the two sides of a dorsiventral organ. I have explained (p. 401) that when the pulvinus of Mimosa is subjected to lateral stimulation of any kind, it undergoes a torsion, in virtue of which the less excitable half of the organ is made to face the stimulus. Experi- ments will be described in a subsequent chapter which show that geotropic stimulus also induces similar torsional response. The results obtained from this method of enquiry give independent proof : (1) that the lower half of the pulvinus is geotropically the more excitable, and (2) that the direction of incident geotropic stimulus is the vertical line of gravity which impinges on the upper surface of of the organ. SUMMARY. The stimulus of gravity is shown to induce an excita- tory reaction which is similar to that induced by other forms of stimulation. The direct effect of geotropic stimu- lus is an incipient contraction and retardation of rate of growth. The upper side of a horizontally laid shoot is more effectively stimulated than the lower side, the excited upper side becoming concave. Electrical investigation also shows that it is the upper side that undergoes direct stimulation. GEOTROPISM 441 Tropic reactions are said to be positive, when the directly stimulated side undergoes contraction with the result that the organ moves to meet the stimulus. Accord- ing to this test, the geotropic response of the stem is positive. The geotropic response is delayed by the bending down of the horizontally laid shoot. Reduction of weight is found to shorten the latent period ; in the case of the petiole of Tropceolum this is shorter than 20 seconds. The latent period of geotropic response is found to be of the same order as the " migration period " of the hypothetical stato- liths. The complete geotropic curve shows characteristics which are similar to tropic curves in general. In a dorsiventral organ the geotropic excitabilities of the upper and lower sides are different. In the pulvinus of Mimosa the geotropic excitability of the lower half is greater than that of the upper half. The differential exci- tabilities of a dorsiventral orgau modifies its position of geotropic equilibrium. 3;i A XL.— GEO-ELECTRIC RESPONSE OF SHOOT By Sir J. C. BosE, Assisted by Satyendra Chandra Guha, m.sc. The experiments that have been described in the pre- ceding chapter show that the upper side of a horizontally laid shoot undergoes excitatory contraction, in consequence of which the organ bends upwards. The fundamental geotropic reaction is, therefore, not expansion, but contrac- tion which results from all modes of stimulation. In confirmation of the above, I wished to discover and employ new means of detecting excitatory reaction under geotropic stimulus. In regard to this, I would refer to the fact which I have fully established that the state of excit- ation can be detected by the induced electromotive change of galvanometric negativity. This electrical indication of excitation may be observed even in plants physically res- trained from exhibiting response by mechanical movement.* ELECTRIC RESPONSE TO STIMULUS. Before giving account of the results of investigations on the detection of geotropic excitation by means of electric response, I shall describe a few typical experiments which will fully explain the method of the electrical investigation, and show the correspondence of mechanical and electric responses. I have explained how tropic cur- vatures are brought about by the joint effects, of * "Comparative Electro-Physiology." ]). 20. GEO-ELECTRIC RESPONSE OF SHOOT 443 contraction of the directly excited proximal side A, and the expansion of the distal side B. In the diagram of mechanical response to stimulus (Fig. 164a) the excitatory contraction is indicated by - sign, and the expansion, by + sign. The resulting movement is, therefore, towards the stimulus as shown by the curved arrow. I shall now describe the corresponding electric eifects in response to unilateral stimulus. We have to determine the induced electrical variation at the proximal side A, and at the distal side B. Electric response to direct stimulation : Experiment 168. — For the determination of electric response at the directly excited proximal side A, we take a shoot with a lateral leaf. The point A, which is to undergo stimulation, is connected with Fig. 164. — Diagrammatic representation of the mechanical and electrical response to direct unilateral stimulation indicated by arrow : — (a) Positive mechanical response (curved arrow) due to contraction of directly stimulated A, and expansion of indirectly stimulated B. (6) Electric response of induced galvanometric negativity of A under direct stimulation (c) Electric response of induced galvanometric positivity at the distal point B. (d) Additive effects of direct and indirect stimulations ; galvanometric nega- tivity of the directly stimulated proximal A, and galvanometric positivitv of the indirectly stimulated distal point B. one terminal of the galvanometer, the other terminal being led to an indifferent or neutral point N on the leaf. 444 LIFE MOVEMENTS IN PLANTS Application of any form stimulus at A, gives rise to an electric current which flows through the galvanometer from the neutral to the excited point A (Fig. 164b). The directly stimulated point A thus becomes galvanometricalli) negative. The " action " current lasts during the application of stimulus and disappears on its cessation. Electric response to indirect stimulation : Experimerd 169. — We have also seen that application of stimulus at A causes indirect stimulation of the distal point B result- ing in an increase of turgor and expansion. The corres- ponding electric change of the indirectly stimulated point B is found in the responsive current, which flows now through the galvanometer from the indirectly stimulated B to the, neutral point N (Fig. 164c). The indirectly stimu- lated j^oint thus becomes galvanometrically positive. Having thus obtained the separate efl^ects at A and B, we next modify the experiment for obtaining the joint eff'ects. For this purpose the neutral point N is discarded and A and B connected directly with the indicating gal- vanometer. On stimulation of A that point becomes negative and B positive, and the current of response flows through the galvanometer from B to A. The deflection is increased by the joint electrical reactions at A and B (Fig. 164d). The results may thus be summarised : — TABI.K XXXUI. — ELECTRIC RESPONSE TO DIRECT UNILATERAL STIMULUS. Electrical change at the proximal side A. Galvanoinotric negativity indicative of contraction and diminution of turgor. Electrical change at the distal side B. Galvanometric positivity indicative of expansion and increase of turgor. The corresponding tropic curvature is positive movement towards stimulus. GBO-BLECTRIC RESPONSE OP SHOOT 445 Galvanometric negativity is thus seen to indicate the effect of direct stimulus, and galvanometric positivity that or indirect stimulus. We thus see the possibility of elec- tric detection of the effects of geotropic stimulation. This method would, moreover, enable' us to discriminate the side of the organ which undergoes greater excitation. EXPERIMENTAL ARRANGEMENTS FOR OBTAINING GEO- ELECTRIC RESPONSE. Returning to the investigation on electric response to geotropic stimulus, the specimen of plant is at first held erect ; two electrodes connected with a sensitive galvano- meter are applied, one to an indifferent point, and the other to one side of the shoot. The sensitiveness of the galvanometer was such that a current of one milllionth of an ampere produced a deflection of the reflected spot of light through 1,000 divisions of the scale. An action current is produced on displacement of the plant from vertical to horizontal position. Non-pularisable electrodes. — The electrical connections with the plant are usually made by means of non-polaris- able electrodes (amalgamated zinc rod in zinc-sulphate solu- tion and kaolin paste with normal saline), I at first used this method and obtained all the results which will be presently described. But the employment of the usual non-polarisable electrodes with liquid electrolyte is, for our present purpose, extremely inconvenient in practice ; for the plant-holder with the electrodes has to be rotated from vertical to horizontal through 90-'. The reliability of the non-polarisable electrode, moreover, is not above criticism. The zinc-sulphate solution percolates through the kaolin paste and ultimately comes in contact with the plant, and seriously affects its excitability. The name non-polarisable electrode is in reality a misnomer ; 446 LIFE MOVEMENTS IN PLANTS for the action current (whose polarising effect is to be guarded against) is excessively feeble, being of the order of a millionth of an ampere or even less ; the counter polarisation induced by such a feeble current is practically negligible. The idea that non-polarisable electrodes are meant to ged rid of polarisation is not thus justified by the facts of the case. The real reason for its use is very different ; the electrical connections with the plant has to be made ultimately by means of two metal contacts. If we take two pieces of metal even from the same sheet, and put them in connection with the plant, a voltaic couple is produced owing to slight physical differences between the two electrodes. Amalgamation of the two zinc rods with mercury reduces the electric difference but cannot altogether eliminate it. I have been able to wipe off the difference of poten- tial between two pieces of the same metal, say of plati- num, and by immersing them in dilute salt solution from a voltaic couple. The circuit is kept complete for 24 hours, and the potential of the two electrodes by this process is nearly equalised. A perfect equality is secured by repeated warming and cooling of the solution and by sending through the circuit, alternat- ing current which is gradually reduced to zero. I have by this means been able to obtain two electrodes which are iso-electric. The specially prepared electrodes (made of gold or platinum wire) are put in connection with the plant through kaolin paste moistened with normal, saline solution. Care should be taken to use opaque cover over the plant-holder, so as to guard against any possible photo-electric action ; moistened blotting paper maintains, the closed chamber in a uniform humid condition. GEO-ELECTRIC RESPONSE OF SHOOT 447 The direct method of contact described above is extremely convenieni in practice ; the resistance of contact is considerably reduced, and there is no possibility of its variation during the necessary process of rotation of the plant for subjecting it to geotropic action. GEO-ELECTRIC RESPONSE OF THE UPPER AND LOWER SIDES OF THE ORGAN. We have next to discover the electric change induced by geotropic stimulus on the upper and lower sides of the organ. For this purpose it is necessary to find a neutral point which is not affected by the inclination of the organ from vertical to horizontal position. For the present experiment, I employed the flower of the water lily Nymphcea, the peduncle of which is sensitive to geo- tropic action. One electrical contact is made with a Fig. 165. — Diagrammatic representation of geo-electric response. The middle figure represents vertical position. In figure to the right rotation through +90^ has placed A above with induced electric change of galavauometric negativity of A. In the figure to the left, rotation is through -90^ A being below: the electric response is by induced galvanometric poaitivity of A. For simplification of diagram, vertical position of sepal is not always shown in the figure. sepal, which is always kept vertical ; the other electric contact is made at the point A, on one side of the flower stalk (Fig. 165). On making connections with a sensitive galvanometer a very feeble current was found, which was due to slight physiological difference between 44S LIFE MOVEMENTS IN PLANTS the neutral point, N, and A. This natural current may be allowed to remain, the action current due to geotropism being superposed ott, it; or the natural current may be neutralised by means of a potentiometer and the reflected spot of light brought to zero of the scale. hiduced electric variation on upper side of the organ : Experiment 170. — While the sepal is held vertical, the stalk is displaced through +90=' so that the point A is above. Geotropic stimulation is at once followed by a responsive current which flows through the galvanometer from N to A, the upper side of the organ thus exhibiting excitatory reaction of galvanometric negativity (Right-hand figure of 166). When the stalk is brought back to vertical position geotropic stimulation disappears, and with it the responsive current. Electric response of the lower side: Experiment 171. — The stal'k is now displaced through - 90^ ; the point A, which under rotation through +90** pointed upwards, is now made to point downwards. The direction of the current of response is now found to have undergone a reversal ; it now flows from A on the lower side to the neutral point N ; thus under geotropic action the lower side of the organ exhibits galvanometric positivity indicative of increase of turgor and expansion (Left-hand figure 166).* Having thus found that the upper side of the organ under geotropic stimulus becomes galvanometrically nega- tive, and the lower side, galvanometrically positive, we make electric connections with two diametrically opposite points of the shoot A and B, and subject the organ to alternate rotation through + 90° and - 90°. The electro- motive changes induced at the two sides now became algebraically summated. I employ two methods for geotropic * For detailed account cf . Chapter XLIII. GEO-ELBCTRIC RESPONSE OF SHOOT 449 Stimulation : that (1) of Axial Rotation, and (2) of Vertical Rotation. METHOD OF AXIAL ROTATION. In the method of Axial Rotation, the organ is held with its long axis horizontal (Fig. 16(JH). We have seen that the geotropic action increases with the angle which the responding surface of the organ makes with the C Crpoi Fig. 166. — Diagrammatic representation of the Method of Axial Kotation H, and of Vertical rotation V (see text). vertical lines of gravity. When the organ is held with its length horizontal, the angle made by its two sides, A and B, with the vertical is zero and there is thus no geotropic effect. There is, moreover, no differential effect, since the two sides are symmetrically placed as regards the vertical lines of force. The plant is next rotated round its long axis, the angle of rotation being indicated in the circular scale. When the rotation is through +90 A is above and B below ; this induces a differential geotropic effect, the upper side exhibiting excitatory electric change of galvanometric negativity. 450 LIFE MOVEMENTS IN PLANTS Experiment 172. — I shall, as a typical example, give a detailed account of experiments with the petiole of Tropmolum which was found so highly excitable to geotropic stimulus (p. 434). The specimen was held horizontal with two symmetrical contacts at the two sides, the electrodes being connected in^ the usual manner with the indicating galvanometer. When the plant is rotated through +90° there is an immediate current of response, the upper side becoming galvanometrically negative. This excitatory reaction on the upper side finds, as we have seen, mecha- nical expression by contraction and concavity, with posi- tive or up-curvature. The differential stimulation of A and B disappears on rotation of the axis back to zero position, and the induced electro-motive response also disappears at the same time. If now the axis be rotated through - 90°, A will become the lower, and B the upper and the excited side. The electro-motive change is now found to have undergone a reversal, B becoming galvanometrically negative. This induced electro-motive variation under geotropic stimulus is Fig. IriT. — ^Uiagranimatic lepreseatation of the geo-electric response of the shoot (see text). of considerable intensity often exceeding 15 millivolts. The characteristic electric change is shown diagrammatically in figure 167 in which the middle figure shows the symme- geo-electric response op shoot 451 trical or zero poi-ition. On rotation through + 1)0° (figure to the right) A occupies the upper ami B the lower position. A is seen to exhibit induced change of gaiva- nometric negativity. Rotation through - 90° reverses the current of response, as B now occupies the upper and A the lower position. CHARACTERISTICS OF GEO-ELECTRIC RESPONSE. There are certain phenomena connected with the elec- tric response under geotropic stimulus which appear to be highly significant. According to statolithic theory " Geotropic response begins as soon as an organ is deflect- ed from its stable position, so that a few starch-grains press upon the ectoplasts occupying the walls which are underneath in the new position ; an actual rearrangement of the starch-grains is therefore not an essential condition of stimulation. As a matter of fact, the starch-grains do very soon migrate on to the physically lower walls, when a positively or negatively geotropic organ is placed hori- zontally, with the result that the intensity of stimulation gradually increases attaining its maximum value when all the falling starch-grains have moved on to the lower region of the ectoplast. The time required for the complete rearrangement of the statoliths may be termed the period of migration ; its average length varies from five to twenty minutes in diflFerent organs."* Stimulation, according to the statolithic theory, is induced by the displacement of the particles. The diameter of the geotropically sensitive cells is considerably less than O'l mm. ; and the stimulus will be perceived after the very short interval taken by the statoliths to fall through a space shorter than (>1 mm. This may be somewhat delayed by the viscous nature of the plasma, but in anj' case the period * Haberlandt— /6i8.— Geo-electiic response of the petiole of Tropceohim. to 90^. The responsive movement of the galvanometer spot of light was initiated in less than 5 seconds and the maximum deflection was reached in the course of 90 seconds. The angle was next reduced to zero, and the GEO-ELECTRTC RESPONSE OP SHOOT 45.^ deflection practically disappeared in the further course of a minute and a half (Fig. 16vith responsive mechanical movement away from stimulus indicated by con- tinuous arrow. In figure to the right, indirect stimulus at dotted arrow induces electric response of galvanometric posi- tivitj- at A, indicative of increase of tur- gor and expansion. 484 LIFE MOVEMENTS IN PLANTS Darwin in his Movements' of Plants described experi- ments on the responsive behaviour of the tip of the radicle. He produced unilateral stimulation in three differc^nt ways, first by attaching minute fragments of cardboard to one side of the root-tip ; this moderate and constant irritation was found to induce a convexity on the same side of the growing region, with the resulting negative movement, i.e., away from stimulus. His second method was chemical, one side of the tip being touched with silver nitrate ; the third method of stimulation was a slanting cut. All these methods induced a movement away from stimulus. ELECTRICAL RESPONSE TO INDIRECT STIMULATION. The next investigation was for the determination of the electrical change induced in the growing region by applica- tion of unilateral stimulus at the root-tip. Experiment 181. — One of the two electrical connections with the galvanometer is made at one side of the growing region A, the other connection being made with the diametrically opposite point B. Unilateral stimulus was applied at the root tip a, of the bean plant and on the same side as A. I subjected Fig. 17L-Diagrammatic repre- ^^^ jj^ ^^ various modeS of uni- sentation of mechanical and electric response of root to indirect stimulus lateral Stimulation. Mechanical applied at the tip a. Figure to stimulation was effected by emery- the left shows responsive move- p • ,. ^ • . , f ' . , ,p, paper friction or by pin-prick ; ment awa}' from stimulus. The r f j i f • electric response to indirect stimulus chemical Stimulation waS pro- is indicated in the figure to the , ,, i-x- c til . ^, ^, . , ., ., duced by application or dilute right ; the point on the same side •' ^^ exhibiting gaivanometric positivity. hydrochloric acid. Thermal sti- The shaded part indicates the j^ulation was Caiised by the responsive region of growth iit some distance from the tip. proximity of electrically heated platinum wire. In every case the response was by induced galvanometnc positivity at A (Fig. 171). This MECHANICAL AND ELECTRICAL RESPONSE OP ROOT 465 electrical variation took place within about ten seconds of the application of stimulus ; the interval would ob- viously depend on the length of path to be traversed by tho transmitted effect of indirect stimulation. The galvanometric positivity at A indicated that there was induced at that point an increase of turgor and ex- pansion, in consequence of which the organ would move away from stimulus. Thus both by the mechanical and electrical methods of investigation we arrive at an identical conclusion that the efiPects of unilateral stimulus at the tip of the root gives rise to a movement, by which the organ is moved away from the source of stimulus ; since tropic movement towards stimulus is termed positive, this opposite response must be regarded as negative. TABLE KXXVI. — EFFECT OF INDlREl.'T STIMULJS UNILATERALLV APPLIED AT THE ROOT-TIP. Effect at tlie proxitnal side A in the Effect at the distal growing i-f^gioii. side B. Galvanometric positivity, indicative of increase of turgor and expansion. Negligible. Tlie corresponding tropic ciirv;itiire is negative, /.e., a movement away from stinmhjs. The root-tip when burrowing its way underground comes in contact with hard substances and moves away from the source of irritation. The irritability of the root-tip is generally regarded as being specially evolved for the ad- vantage of the plant. But reference to experiments that, have been described shows that this reaction is not unique but exhibited by all plant organs, growing and non-grow- ing. Indirect stimulus has been shown to give rise, in both shoot and root, to a negative tropic curvature in 4GG LIFE MOVEMENTS IN PLANTS contrast to the positive curvature brought about by direct stimulation ; the response of the root is therefore in no way different from that of vegetable tissues in general. It will also be seen that an identical stimulus induces two opposite effects, according as the stimulus is applied at the tip or at the growing region itself. In the former case, the stimulus is indirect, and in the latter case it is direct. The results are in strict conformity with the laws of effects of direct and indirect stimiilations that have been established regarding plant response in general (p. 281). SUMMARY. In the root, the responsive region is in the zone of growth. The tip of the root is separated from the region of response by a semi-conducting or non-conducting tissue. Direct unilateral stimulus (applied at the region of growth) 'induces a positive curvature by the contraction of the proximal and expansion of the distal side. The electrical response to direct unilateral stimulus is galvanometric negativity of the proximal, and galvanometric positivity of the distal side. Indirect unilateral stimulus induces expansion of the proximal side resulting in negative curvature and move- ment away from stimulus. The corresponding electric response induced is galvano- metric positivity of the proximal side. The responses of the root, to both direct and indirect stimulations, are precisely similar to those in the shoot. The assumption of specific irritability of the root as differing from that of the shoot, is without any justification. XLIL— GEO-ELECTRIC RESPONSE OF ROOT Bu Sir J. C. BosE, Assisted by Satyendra Chandra Guha. The effects of various stimuli, direct and indirect, on the response of the root have been described in the last chapter. These responsive reactions have been found to be in no way different from those of the shoot. But the shoot and the root exhibit under the stimulus of gravity, responsive movements which are diametrically opposite to each other. These opposite effects of an identical stimulus have been regarded as due to specific differences of irrita- bility in the two organs, specially evolved for the advantage of the plant. The root is thus supposed to be charac- terised by " positive " and the shoot by " negative " geotro- pism. As regards r'esponse to other forms of stimuli, the root has been shown to behave like the shoot. We have now to inquire whether the reaction of the root to gravitational stimulus is specifically different to that of the shoot. The electric method of investigation described in the last chapter, holds out the possibility of discovering the character of the responsive reaction induced in the root by its displacement from vertical to horizontal position ; we shall, moreover, be able to make an electrical explora- tion of the root-tip and the zone of growth, and thus determine the qualitative changes of response, induced in two regions of the root under the action of gravitational stimulus. For the detection of geotropic action in the shoot, 4:6S LIFE MOVEMENTS IN PLANTS electric contacts were made at two points diametrically opposite to each other. Displacement of the shoot from vertical to horizontal position induced excitatory change of galvanometric negativity at the upper side of the organ, demonstrating the effect of direct stimulation of that side ; this excitatory reaction of the upper side finds independent mechanical expression in the induced contraction and concavity of that side of the organ. I employ a similar electric method for detection of geotropic excitation of the root, responses to geo tropic stimulus being taken at the root-tip and also at the zone of growth in which geotropic curvature is effected. I shall now proceed to give a detailed description of the characteristic .electric responses of the tip and of the growing region. The two diametrically opposite contacts at the tip will l)e distinguished as a and i, the corresponding points higher up in the growing region being A and B. When the root is vertical the electric conditions of the two diame- trically opposite points are practically the same. But when the root is rotated in a vertical plane through -f 90° a geo-electric response will be found to take place ; the direction of the responsive current disappears when the root is brought l)ack to the vertical. Rotation through -90° gives rise again to a responsive current, but its direction is found reversed. GEO-ELECTRIC RESPONSE OF THE ROOT-TIP. Experitnetit 182. — I took the root of the bean plant and made two electric contacts with the diametrically opposite points, a and b, of the root-tip at a distance of about I'D mm. from the extreme end. Owing to the very small GEO-BLECTRIC RESPONSE OB^ ROOT 469 size of the tip this is by no means an easy operation. Two phitinum points tipped with kaolin paste are very carefully adjusted so as to make good electric contacts at the two opposite sitles, without exerting undue pressure. For geotropic stimulation the root has to be laid horizontal, and as the root of the bean plant is somewhat long and limp, displacement from the vertical position is apt to cause a break of the electric contact. This is avoided by supporting the root from the t i 6 18 divisions. IB 12 „ 1 ADDITIVE ACTION-CURRENT AT THE TIP AND THE GROWING REGION. It has been shown tliat under geotropic stimulus the upper side of the tip, a, becomes galvaiiometrically nega- tive, while the point A, higher up in the growing region, becomes galvanometrically positive. If now we make the two galvanometric connections with a and A, the induced electric difference is increased, and the galvanometric res- ponse becomes enhanced. Experiment 184. — The root was at first held viertical, and two electric contacts made with a and A. In this neutral position there is little or no current. But as soon as the root was laid horizontal, an electro-motive response was obtained which showed that, a was galvanometrically negative, and A galvanometrically positive (Fig. 173d). The induced electric response disappeared on restoration of the root to the vertical position. I give below the results of typical experiments with a vigorous specimen which gave strong electric response. It was possible to repeat GBO-ELECTRIC RESPONSE OF ROOT 473 the geotropic stimulation six times in succession, the results being perfectly consistent. The responses taken in succession exhibited slight fatigue, the first deflection being 140 divisions, and the sixth 115 divisions of the galvanometer scale. TAUI.K XXXIX. — INDUCED E. M. V. VARIATION BETWEEN THE TIP AND THE GBOWING REGION (a NEGATIVE AND A POSITIVE). Geotropic stimulation. Resulting electric response. Firtit stimulation 1 140 divisions. 1 Second ,, 1 30 ., Third 130 „ F'ourth „ U'3 „ Fifth 127 „ Sixth ,, 115 „ The results of experiments 182 and 183 are summarised as follows : — (1) the induced galvanometric negativity at root tip indicates direct stimulation of the tip, and (2) the induced galvanometric positivity of the grow- ing region shows that it is the effect of indirect stimulus that reaches it. From these facts it will be seen that the tip per- ceives the stimulus and thus undergoes excitation, and that owing to the intervening tissue being a semi-couductor of excitation, it is the positive impulse that reaches the 35 A 474 LIFE MOVEMENTS IN PLANTS growing region and induces there an expansion and ti convex curvature. GEO-PERCEPTION AT THE ROOT TIP. The results given above fully confirm Charles Darwin's discovery that it is the root tip that perceives the stim- ulus of gravity *; he found that removal of the tip abolished the geotropic response of the root. Objection has been raised about the shock-eflLect of operation itself being the cause of abolition of response. But subsequent observations have shown that Darwin's conclusions are in the main correct. The experiments which I have described on tiie geo- electric response of the root tip and of the growing region offer convincing proof of the perception of the stimulus at the tip, and the transmission of the effect of indirect stimulus to the growing region. These experi- ments exhibit in an identical uninjured organ : the excita- tory reaction at the upper side of the tip, the cessation of excitation, and the excitation of the opposite side of the tip, following the rotation of the organ through -H 90°, 0° and -90°.- The effect at the growing zone is precisely the opposite to that at the tip, i.e., an expansive '^ " This view has been the subject of a considerable amount of controversy. Wiesner denies the localisation of geotropic sensitiveness. Czapek, on the other hand, supports Darwin's theory. Recently Picard has attacked the problem in a new way (and) concludes that not only the root tip but also the entire growing zone is capable of pevceiving gravitational stimuli. ... As both Picard's experimental method and his interpretation are open to criticism, the author has repeated his experiments with a more satisfactory apparatus. He finds that in Vicia Faha, Phaseolus multejlorus and Lupinns alhus, both apex and growing zone are geotropically sensitive, the former being by far the more sensitive of the two, and the curvature of the growing zone being without a doubt largely induced by secondary stimuli transmitted from the apical region. ; harles Darwin's views were therefore in the main correct." — Haberlandt — Ibid, p. 748. GEO-ELECTRIC RESPONSE OF ROOT 475 reaction which results from the effect of indirect stimu- lus, in contrast to the contractile reaction due to direct stimulation. We may now proceed a step further and try to obtain some idea oi the difference in the mechanics of geotropic stimulation of the shoot and of the root, to account for the different responses in the two organs. The reason of this difference lies in the fact that in the shoot the perceptive and responding region is one and the same ; every cut-piece of stem exhibits the characteristic geo- tropic curvature. In the root the case is different ; for the removal of the sensitive root-tip reduces or abolishes the geotropic action ; the region of maximum geotropic perception is thus separated from that of response. It tnusi b3 l)orne in mind th it this holds good only in the case of ffvavitatiotial stimulus, for the decapitated root still continues to respond to other forms of stimulation such as chemical or photic. The cause of this difference in ' the reactions to geotropic and other stimuli lies in the fact that in the latter case, energy is supplied from outside. But in geotropism the force of gravity is by itself inoperative ; it is only through the weight of the cell contents that the stimulus becomes effective. Want of recognition of this fundamental difference has led many observers in their far-fetched and sweeping attempt, to establish an identity of reaction of the root to geotropic and photic stimula- tions, in spite of facts which plainly contradict it. Thus the root moves away from the incident vertical line of gravity ; but under light, the root very often moves towards the stimulus. The negative phototropic response of the root of Si}mpis is an exceptional phenomenon for which full explanation has been given in page 376. We shall next consider whether the particular distri- bution of the falling starch-grains (which offers a rational 476 LIFE MOVEMENTS IN PLANTS explanatiow of geotropic stimulation) in the shoot and in the root, is capable of furnishing an explanation of the different geotropic responses in the two organs. In this connection, the results of investigation of Haberlandt and Nemec are highly suggestive. Haberlandt finds statoliths present in the responding region of the stem ; the geotropic stimulation of the stem is therefore direct. Nemec's investigation on the distribution of statoliths in the root show, on the other hand, that it is the central portion of the root cap that contains the falling starch grains, and this would account for the indirect geotropic stimulation of the root. The theory of statoliths is, however, not essential for the explanation of the opposite geotropic effects in the shoot and in the root. The observed fact, that the percep- tive region in the root is separated from the responding region, is sufficient to explain the difference of geotropic action in the two organs. Through whatever means the stimulus of gravity may act, it is inevitable, from the fact that the stimulation of the shoot is direct and of the root indirect, that an identical stimuhis should in two cases induce responsive reactions of opposite signs. It will thus be seen that the postulation of two different irritabilities in the shoot and in the root is wholly unnecessary and unwarranted by facts. For the irrita- bility of the root has been shown to be in no way dift"erent from that of other organs; an uniformity is thus' found to exist in the reaction of all vegetable tissues. SUMMAKY. On subjection of the tip of the root to the stimulus of gravity, the upper side exhibits excitatory reaction of galvanometric negativity. This shows that the root-tip undergoes direct stimulation. GEO-ELECTRIC RESPONSE OF ROOT 477 The electric response in the growing region above the stimulated point of the root-tip is positive, indicative of increase of turgor and expansion. This is due to the effect of indirect stimulus. The stimulus of gravity is perceived at the root-tip ; it is the effect of indirect stimulus that is transmitted to the responding region of growth. In contrast with the above is the fact that the growing region of the shoot is both sensitive and responsive to geotropic stimulus. As the effects of direct and indirect stimulation on growth are antithetic, the responses of shoot and root to the direct and indirect stimulus must be of opposite signs. There is no necessity for postulating two different irri- tabilities for the shoot and the root, since tis-iues in general exhibit positive or negative curvatures according as the stimulus is direct or indirect. XLIIL— LOCALISATION OF GEO-PERCEPTIVE LAYER BY MEANS OF THE ELECTRIC PROBE By Sir J. C. BoSE, Assisted by Satyendra Chandra Guha. The obscurities which surround the phenomenon of geotropism arise : (1) from the invisibility of the stimulating agent, (2) from want of definite knowledge as to whether the fundamental reaction is contractile or expansive, and (3) from the peculiar characteristic that the stimulus is only effective when the exter?ial force of gravity reacts internally through the mass of contents of the sensitive cells. The experiments that have been detailed in the foregoing chapters will have removed most of the difficulties. But beyond these is the question of that power possessed by plants of jjerxeiving geotropic stimulus by means of certain localised sense organs, which send out impulses in response to which neighbouring cells carry out the move- ment of orientation in a definite direction. Are the sensi- tive cells diffusely distributed in the organ or do they form a definite layer ? Could we by the well established method of physiological response localise the sensitive cells in the interior of the organ ? As the internal cells are not acces- sible, the problem would appear to be beyond the reach of experimental investigation. It is true that post-mortem examination of sectioned tissues undei- the microscope enables us to form a probable hypothesis as regards the conieuts of certain cells causing geotropic irritation ; we have thus the very illuminating loCalisatiok of geo-perCeptive layer 479 theory of statoliths propouiKk'tl by Noll, Haberlandt and Nemec. But for the clear understanding of the physiological reaction which induces the orientating movement, it is neces- sary to get hold, as it were, of a single or a group of sensory cells in situ and in a condition of fullest vital activity ; to detect and follow by some subtle means the change induced in the perceptive organ and the irradiation of excitation to neighbouring cells, through the entire cycles of reaction, from the onset of geotropic stimulus to its cessation. The idea of obtaining access to the unknown geo-percep- tive cell in the interior of the organ for carrying out various physiological tests would appear to be very extra- vagant ; yet I could not altogether give up the thought that the obscure problem of geotropic action might be attacked with some chance of success, by means of an electric probe which would explore the excitatory electric distribution in the interior of the organ. But the experimental difficulties which stood in the way were so great that for a long time I gave up any serious attempt to pursue the subject. And it is only when the present volume is going through the press that the very first experiments undertaken proved so highly successful that I am able to give a short account of the more important results, which cast a flood of light on the obscurities of geotropic phenomena. The new method has opened out, moreover, a very extensive range of investigation on the activities of cells in the interior of an organ, and enabled me to localise the conducting ' nerve ' which transmits excitation in plants. These and other results will be given in the next volume. METHOD OF EXPLORATION BY THE ELECTRIC PROBE. The principle of the new method will be better under- stood if I first explained the steps of reasoning by which 480 LIFE MOVEMENTS IN PLANTS I was led to discover it. The experiments described in Chapter XL showed that the upper surface of a horizon- tally laid shoot exhibits sign of excitation by induced galvanometric negativity ; that this was due to the stimulus of gravity was made clear by restoration of the plant-organ to the vertical position, when all signs of electric excitation disappeared. Now the skin of the organ on which the electrode was applied could not be the perceptive organ, for the removal of the epidermis did not abolish the geotropic action ; the perceptive layer must therefore lie somewhere in the interior. As every side of a radial organ undergoes geotropic excitation, the geo-perceptive cells must therefore be disposed in a cylindrical layer, at some unknown depth Fig. 174. — Diagrammatic representation of the geo-perceptive layer in nnexcited vertical, and in excited horizontal poKition. (See text.). from the surface. In a longitudinal section of the shoot, they would appear as two straight lines G and G' (Fig. 174). In a vertical position the geo-perceptive layer will remain quiescent but rotation through -|- 90° would initiate the excitatory reaction. Let us first centre our attention to the geo-percc^ptive layer G, which occupies the upper position. This sensitive layer perceives the stimulus and is therefore LOCALISATION OP GEO-PERCEPTIVK LAYER 4.S1 the focus of irritutiou ; the state of excitation is, as we have seen, detected by induced galvanoinetric negativity, and the electric change would be most intense at the perceptive layer itself. As the power of transverse conduction is feeble, the excitation of the perceptive layer will irradiate into the neighbouring cells in radial directions with intensity dimini- shing with distance. Hence the intensity of responsive electric change will decline in both directions outwards and inwards. The distribution of the excitatory change, initiated at the perceptive layer and irradiated in radial directions is represented by the depth of shading, the darkest shadow being on the perceptive layer. Had excitation been attended with change of light into shade, we would have witnessed the spectacle of a deep shadow (vanishing towards the edges) spreading over the (.lifferent layers of cells during displacement of the organ from vertical to horizontal ; the shadow would have disappeared on the restoration of the organ to the vertical position. Different shades of excitation in different layers is, how- ever, capable of discrimination by means of an insulated electric probe, which is gradually pushed into the organ from outside. It will at first encounter increasing excitatory change during its approach to the perceptive layer where the irritation will be at its maximum. The indicating galvanometer in connection with the probe will thus indi- cate increasing galvanometric negativity, which will reach a maximum value at the moment of contact of the probe with the perceptive layer. It will be understood that the surface electric reaction under geotropic stimulus, which we hitherto obtained, would be relatively feeble compared to the response obtained with direct contact with the maximally excited perceptive layer. When the probe passes beyond the perceptive layer 482 LIFE MOVEMENTS IN PLANTS the electric iudicatioa of excittition will undergo decline and final abolition. The characteristic effects described above are to be found only under the .action of gravitational stimulus ; they will be absent when the organ is held in a vertical position and thus freed from geotropio excitation. I have hitherto spoken of the excitatory effect of the upper layer ; there must be some physiological reaction on the lower perceptive layer, though of a different character, represented diagrammatically by vertical shading. Had the physiological reaction on the lower side of a radial organ been the same as on the upper, geotropic curvature would have been an impossibility, for similar reactions on opposite sides would, by th«ir antagonistic effects, 'have neutralised each other. After this preliminary explanation, I shall give a detailed account of the experiments and results. It is to be borne in mind that the investigation I am going to describe pre- supposes no hypothesis of geotropic action. I start with the observed fact that an organ under the stimulus oi gravity, exhibits responsive movement. I ascertain the nature of the underlying reaction by electric tests ; I have, in my previous works, fully demonstrated that the excitatory con- tractile reaction is detected by electro-motive change of galvanometric negativity, and the opposite expansive reaction by a change of galvanometric posifcivity. With the electric probe I ascertain whether geotropic irritation is diffuse, or whether it is localised at any particular depth of the organ. I map out the contour lines of physiological reaction with its heights and depths of excitation. I shall now proceed to describe the results of electric exploration into the interior of the organ. The trouble I foresaw, related to the irritation caused by the passage of LOCALISATION OF GEO-PERCEPTIVE LAYER 483 the probe, and the after-effect of wound on variation of excitability. THE ELECTRIC PROBE. The wound-irritation is, however, reduced to a minimum by making the probe exceedingly thin. A fine platinum Fig. 175. — The Electric Probe. Figure to the left represents one electric contact made with sepal rf Nymphcea, and the other, with the flower-stalk by means of the probe ; the included galvanometer is represented by a circle. Figure to the right an enlarged view of the probe. wire 0*06 mm, in diameter passes through a glass tubing drawn out into a fine capillary, and fused round one end of the platinum wire which protrudes very slightly beyond the point of fusion ; the exploring electrode is thus in- sulated except at the protruded sharp point of the platinum wire. The length of the capillary is about 6 mm., just long enough to pass the experimental plant-organ trans- versely from one end to the other ; the average diameter of the capillary is about Olo mm. The other end of the platinum wire comes out of the side of the tubing and is led to one terminal of t^e galvanometer, the other being connected with an indifferent point in the organ. The probe can be gradually pushed into the plant-organ by 484 LIFE MOVEMENTS IN PLANTS rotation of a screw head, one complete rotation causing a forward movement through 0'2 mm. (Fig. 175). Wound-reaction. — I have shown that a prick acta as a mechanical stimulus, and in normal excitable tissues induces an excitatory change of galvanometric negativity. This wound-reaction increases with the extent of the wound, and the suddenness wilh which it is inflicted. On account of the fineness of the probe, it insinuates itself into the tissue rather than make any marked rupture ; the probe again is introduced very gradually ; with these precautions the wound-reaction is found to be greatly reduced. The immediate effect of the prick is a negative deflection of the galvanometer, which declines and attains * steady value in the course of about 5 minutes. Effect of wound on excitabilitij. — I have shewn (p. 81) that severe wound caused by transverse section induced a tempor- ary abolition of irritability in Mimosa, but that the normal excitability was restored in the course of an hour. A prick from a thick pin was shown to depress temporarily the rate of growth, the normal rate being restored after an interval of 15 minutes (p. 202). In the case of geo-electric excitability, the depressing effect of the passage of the probe, I find, to disappear in the course of about 10 minutes. For a choice of experimental material we have to find specimens which are not merely geotropically sensitive, but also exhibit large electric response under stimulus. In both these respects the shoot of Bryophyllum and the flower stalk of Nympluea give good results. ELECTRIC EXPLORATION FOR GEO-PERCEPTIVE LAYER BY MEANS OF THE PROBE. Experiment 185. — I shall now proceed to give a detailed account of the experiments. The first specimen employed LOCALISATION OF GEO-PERCEPTIVE LAYER 485 was the shoot of Bryophyllum , one contact being made with the side of the stem, and the other with an indifl"(irent point on the leaf which was always held vertical. In a particular experiment, the probe was introduced into the stem through O-l mm. and a feeble galvanometric negati- vity was induced as the wound-effect. After an interval of ") minutes, thie attained a steady value of-l/i divisions. On the rotation of stem through +90°, the point A was al)ove and a very much larger deflection of - l)tained, being the result of summation of wound and geo- electric effects. On restoration of the plant to vertical posi- tion the geo-electric reaction disappeared, leaving the persis- tent wound reaction of -15 divisions unchanged. The true geo-electric reaction at a point 0*4 mm. inside the stem was thus -67 divisions which is the difference between - 82 and -15 divisions. I obtained in this manner the excitatoiy reactions at different layers of the organ. The following table gives true values of geo-electric reaction at different layer.< of the stem as the probe entered it by steps of 0*4 mm. TABLE XL. — SHOWINO THE OEO-ELECTKIC EEAOTION AT DIFFERENT DEPTHS OF THE ORGAN {Bryophyllum). Position of the Geo-electric excitation probe. (gal van ometric negativity). Surface - 5 ilivisions. 0'4 luin. - 20 0-8 „ - 24 ,, 1-2 „ - 22 11 1-6 „ - 18 ., 2-0 „ - 14 ,, 2-4 „ - 10 2-8 ,. - 5 ,, 3-2 „ ■ - 0 " The results given above, typical of many others, show that there is a definite layer in the tissue which undergoes 486 LIFE MOVEMENTS IN PLANTS maximum excitation under the stimulus of gravity, and that this excitation irradiates with diminishing intensity in radial directions inwards and outwards. The geo-perceptive layer may thus he experimentally localised by measuring the de^jtJi of intrusion of the probe for maximum deflection of galvanometric negativity. Localisation of geo-perce2)tive layer in Nympha3a : Experiment 186. — I employed the same method for the determination of the perceptive layer of a different organ namely, that of the flower stalk of Nymphcea, The electric reaction in Nymphcea^ even under the prevailing unfavour- able condition of the season, was moderately strong, being about three times greater than in Bryophyllum. A dozen observations made with different specimens gave very consistent results of which the following may be taken as typical. The probe was in this case, as in the last, moved by steps of 0*4 mm. at a time. Other examples will be given later where readings were taken for successive steps of 0 2 mm. TABLE XI.I. — SHOWING THE DISTRIBUTION OF INDUCED GEO-ELECTRin EXCITATION IN DIFFERENT LAYERS {NymplKKo). Position of probe. Galvanometric deflection. Surface 0 (]ivi?^ions. 0'4 luni. - 16 0-8 „ - 42 1-' „ - 20 1(5 ,. - 10 2-0 ,. _ 2 2-4 .., 0 It will be seen that as in Bryophyllum, so in Nymphma^ the geo-electric excitation increased at first with increasing depth of the tissue till at a depth of 0-8 mm. of the parti- cular specimen the induced excitation attained a maximum LOCALISATION OP GEO-PERCEPTIVE LAYER 487 value. The excitatory eH'ect then declines till it vanisheii at a depth of 2*4 mm. The depth of layer at which maximum excitation takes place varies to some extent, according to the thickness of the shoot. Thus while in a thin specimen of BryophyUtim 3't'» mm. in diameter the geo-perceptive layer was found at a depth of 0*0 mm., it occurred at the greater depth of 0"'S mm. in a thicker specimen, f) mm. in diameter. In Nympfuea also the perceptive layer was found at a depth of OvS mm. in a thin and at a depth of 1 "4 mm. in a thick specimen. Having thus succeeded in localising the geo-perceptive layer by experimental means, it was now possible to examine the anatomical characteristics of the layer by examining it under the miscroscope, I also wished to find out from microscopic examination, the cause of certain differences uoticed in the determinations of the perceptive layer in BryophyUum and in Nymj^Jicea. In the former the probe always encountered the maximally excited geo-perceptive layer from whichever point of the surface it entered the organ ; this indicated thai the sensitive layer in Bryophyl- him was continuous round the axis. In Nymphcea^ however, the probe occasionally missed the sensitive layer; but a new point of entry led to successful localisation of the perceptive layer ; this was probably due to the particular layer not being continuous but interrupted by certain gaps. MICROSCOPIC EXAMINATION OF THE MAXIMALLY EXCITED LAYER. The specimens were taken out after the electric test, and the transverse sections made at the radial line of the 36 4«8 LIFE MOVEMENTS IN PLANTS passage of the pi'obe. Thus in a particular experiment with Brijophyllum the point of maximum geotropic excitation was found to be at a distance of 0*8 mm. from the surface. By means of tlie micromoter slide in the stage and the micrometer eye-piece, the internal layer OvS mm. from the surface was examined ; the particular sensitive layer S was re- cognised as the continu- ous 'r^tarch sheath' or endodermis containing unusually large sized starch grains (Fig. 17(»). These often occurred in PitJ. 17(5. — Traii8ver?e section showing con- tiiiuoii8 geo-perceptive layer S : enlarged view loOSCly Cohering groups S' of cell of endodermis containing group of of 8 tO 10 particles, and large starch grains. {Bn/ophyihrn). their appearance is very (lifferent from the snia'l si/.etl irregularly distributed grains in other cells. Examination of the microscopic section of the Hower stalk of Nymphcea shovved that the 'starch sheath' was not continuous but occurred in crescents above the vascular bundles which are separated from each other. The occasional failure of electric detection of the perceptive layer is thus due to the probe missing one of the crescents, which with intervening gap.-', are arranged in a circle. I give below a number of experimental determinations of the geo-perceptive layer in difl'erent s[)ecimens together with the micrometric measurement of the distance of the 'starch sheath' from the surface, the transverse section being mads at the place where the probe entered the shoot. LOCALISATION OF GEO-PERCEPTIVB LAYER 489 Eight dillereut determinations are given, three for Bryu- phylluni and five for Nymphcea. TAliht: XMl. — SHoWINt; 'IHE I'dSlTlON OK THK (iEO-l'KBCKPTl VE 1 AVER A.NI" OF ' STARCH SHEATH ' IN DIFFERENT SPBCIMRNS. Distance of i;eo-perceptive Distance of the starch . lavor from surface. sheatli from surface. SpiH'iinoii. ' (Method of electric probe.) (M icroscopic measurement.) ! Bryojthyllnm : (1) 0-6 mm. 0-6 mm. (2) 0-8 0-8 „ (3) 0-8 „ 0-8 „ Niimphaa : (1) U-6 „ O-C. „ (2) 0-8 „ 0-8 „ (3) 0-8 „ 0-8 „ (4) 1-0 10 „ (r)) 1-4 ., 1-4 „ •Thns in all speciraens examined, the experimentally determined geo-perceptive layer coincided with the ' starch sheath.' The theory of statoliths thus obtains strong support from an independent line of experimental inves- tigation. The statolithic theory has been adversely criti- cised because in simpler organs the geotropic action takes place in the absence of statoliths. There is no doubt that the weight of the cell contents may in certain cases be effective in geotropic stimulation ; it may nevertheless be true that " at a higher level of adaptation, the geotropically sensitive members of the plant-body are furnished with spt-cial geotropic sense-organs — a striking instance of anato- mico-physiological division of labour."* In the instances of Bryophyllum and Nymphwa given above, the geo-perceptive layer localised by means of the Haberlandt — Jb\d, p. 597. J6 A 490 LIFE MOVEMENTS IN PLANTS electric probe is definitely found to be the endodermis con- taining large sized starch grains. INFLUENCE OF SEASON ON OEO-ELECTRIC RESPONSE. I vshall now describe certain modifications in responst^, which result from the change of season and also from condition of high temperature. Physiological reactions, generally speaking, are much affected by different seasons ; thus the seedlings of Scirpus Kijsoor exhibit a very rapid rate of growth of 3 mm. per hour in August, but a month later the growth-rate declines to only 1 mm. per hour. I find similar depression of growth with the ad- vance of season in si^edlings of Zea Mays, where a very rapid fall in growth takes place in the course of a fort- night. The intensity of geotropic responses, both mechanical and electrical, of Tropceohim declines rapidly in the course of a month from February to March (p. 454). The fiowers of Nymplioea began to appear by the end of June when the flower stalks exhibited strong geo-electric response. But later in the season, by July and the beginning of August, the response underwent continuous decline, and by the end of August the response was nearly abolishetl. Much time had to be spent in perfecting the appara- tus, and it was not till the beginning of August that the investigations could be properly started ; the res- ponsive indications were, however, marked and definite, though relatively feeble compared to those obtained at the beginning of the season. The decline of the geo-electric response was to a certain extent also due to the prevailing high temperature. Effect of high temperature. — I shall in the next chapter describe experiments which show that geotropic response is diminished under rise of temperature. The specimens employed for localisation of geo-perceptive layer exhibited, as stated before, a decline of geo-electric response with LOCALISATION OF GEO-PERCEPTIVE LAYER 491 the advance of the season. This may partly be due to unfavourable season, and partly to hi^'h temperature. In the middle of the season the responses were extremely feeble on warm ilays, but on cool mornings they became suddenly enhanced, to decline once more by the middle of the day. I coukl sometimes succeed in enhancing (he sensitiveness by placing the specimen in a cold chamber. It thus appeared that certain internal change unfavourable for geo-perception takes place at high tem- peratures, and that the sensitive condition could some- times be restored by artificial cooling. Hut later in the season, ihe internal change, whatever it may be, had pro- ceeded too far, and artificial cooling did not restore the sensitiveness of the specimen. What are the physico- chemical concomitants which distinguish insensitive speci- mens, in which the electric indications had declined almost to the vanishing point ? TEST OF INSENSITIVE SPECIMENS. I shall now d 'scribe the various physico-chemical con- comitants which accompany the condition of relative insensibility. I have found three different tests ; the electric, the geotropic, and the microscopic, by which the sensitive could be distinguished from the insensitive condition. The following tests were made on insensitive specimens. Electric test : Experiment 187. — By the end of August the geo-electric indications given by the probe had, as stated before, almost disappeared. The tonic condition of the specimen, heloiv par, was independently revealed by the response to prick of the probe ; this, in vigorous specimens, is by an electric response of galvanometric negativity. But the response to prick in sub-tonic specimens is very different. I find that when the physio- logical condition of the tissue falls below pai\ the sign of response undergoes a reversal into one of galvanometric 492 LIFE MOVEMENTS^ IN PLANTS positivity. The same reversal under condition of snb-toai- city was also shown to take place in growth, where under the stimulus of light a positive acceleration took place, instead of normal retardation of growth (p. 221). In the present investigation, the insensiti7e specimens were found to give abnormal positive electric response to the stimulus of prick made by the probe. The prick-effect in fact often gave me previous indication as to the suitability of the particular specimen for exhibition of geo-electric response. Test of ijeotroijic reaction: Experiment 188. — I took four different specimens of Bryophyliiun and Nymplicea, and held them horizontal. These plant organs had, earlier in the season, exhibited very strong geotropic effect, the shoot carving up through 90^ in the course of len hours or les.-j* But these specimens obtained later in the season exhil)ited very feeble curvature, which hardly amounted to 10 ' degrees, even after prolonged exposure to geotropic action for 24 hours. Test of microscopic examination. — I next made sections of Bryophytlum and Nympluea and on examining them under the microscope discovered certain striking changes. A fortnight ago the group of large starch grains stained with iodine were the most striking feature of the starch sheath. But now these starch grains could not be found in any of the numerous specimens examined. The presence of the starch grains thus appears to ba associated with the sensitiveness of the perceptive layer. REACTION AT LOWER SIDE OF THE ORGAN. There remains now the important question or" the physiological change induced on the lower side of the horizontally laid shoot. The physiological reaction of two sides of the organ must be different, since the upper side exhibits contraction and the lower side expan- sion. It may be urged that the effect of one of the LOCALfSATlON OV (JKO-PERCEPTIVE LAYER VX\ two sides might result from the passive yielding to the definite reaction iiuiuced on tiie opposite side. Investiga- tion l)y the electric method enables us, however, to dis- criminate the two reactions from each other, since the electric response characteristic of the induced physiological change takes place in the organ, even under condition of restraint by which movement is prevented. We shall there- fore investigate the geo-electrical. reaction on the lower side of the securely held organ, and find out whether the induced electric change undergoes any variation in different layers from below upwards. There are two different ways in which the electric explorations of the lower side of the organ may be carried out. In the first method, the probe is introduced from below, and saecessive readings for geo- electric response taken as the probe enters the organ by successive steps. It is understood that the true geotropic effect is found from difference of galvanometer readings in vertical and horizontal positions. In the second method, the probe is intro „ 2-8 „ ... - 2 ,. 3-0 ., ... 0 3-2 „ ... 0 „ 3-4 „ ... 0 .., Position of Galvanoinfter probe. 1 deflection. 3'6 mm. 0 divisions. 3-8 „ ... 0 ,. j 4-0 ., ... 0 „ 4-2 „ ... + 2 ., 4-4 ,. + 4 ,. 4-6 + 5 „ 4-8 „ ... -f-11 ., 5-0 „ + 22 „ 52 , ... + 38 5-4 „ ... + 46 ,. 5-6 ., + 39 „ 1 5-8 ,. + 32 „ 1 6-0 ,, ... + 24 6-2 „ ... + 18 „ 6-4 „ ... + 12 „ 6-6 „ + 6 „ 6-8 „ ... + 3 ., LOCALISATION OK (iKO-PERCEPTlVE LAYER 497 A curve constructed from the data given above is seen in figure 177. The diameter of the flower stalk was (v