Life Movements in Plants, Volume II, 1919

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Title: Life Movements in Plants, Volume II, 1919
Author: Bose, Sir Jagadis Chunder
Language: English
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*** Start of this LibraryBlog Digital Book "Life Movements in Plants, Volume II, 1919" ***

TRANSACTIONS
OF THE
BOSE RESEARCH INSTITUTE, CALCUTTA,

VOL. II, 1919

LIFE MOVEMENTS IN PLANTS

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

CALCUTTA
BENGAL GOVERNMENT PRESS
1919

PUBLISHED BY
THE BOSE RESEARCH INSTITUTE, CALCUTTA.

WORKS BY THE SAME AUTHOR.

RESPONSE IN THE LIVING AND NON-LIVING.
With 117 Illustrations, 8vo. 10_s._ 6_d._ 1902

PLANT RESPONSE: AS A MEANS OF PHYSIOLOGICAL INVESTIGATION.
With 278 Illustrations, 8vo. 21_s._ 1906

COMPARATIVE ELECTRO-PHYSIOLOGY. A PHYSICO-PHYSIOLOGICAL STUDY.
With 406 Illustrations, 8vo. 15_s._ 1907

RESEARCHES ON IRRITABILITY OF PLANTS.
With 190 Illustrations, 8vo. 10_s._ 6_d._ _net_ 1913

LIFE MOVEMENTS IN PLANTS, VOL. I.
With 92 Illustrations, 8vo. 10_s._ 6_d._ 1918

LONGMANS, GREEN & CO.
London, New York, Bombay and Calcutta.

PREFACE TO VOLUME II.

I have in the present volume dealt with the intricate phenomena of
different tropisms. The movements in plants under the stimuli of the
environment--the twining of tendrils, the effect of temperature, the
action of light inducing movements sometimes towards and at other times
away from the stimulus, the diametrically opposite responses of the
shoot and the root to the same stimulus of gravity, the day and night
positions of organs of plants--these, and many others present such
diversities that it must have appeared a hopeless endeavour to discover
any fundamental reaction applicable in all cases. It has therefore been
customary to assume different sensibilities especially evolved for the
advantage of the plant. But teleological argument and the use of
descriptive phrases, like positive and negative tropism, offer no real
explanation of the phenomena. Thus to quote Pfeffer "When we say
that an organ curves towards a source of illumination, because of its
heliotropic irritability we are simply expressing an ascertained fact in
a conveniently abbreviated form, without explaining why such curvature
is possible or how it is produced.... Many observers have unfortunately
devoted their attention to artificially classifying the phenomenon
observed, and have entirely neglected the explanation of causes
underlying them." He also adds that in regard to the phenomenon of
growth and its variations, an empirical treatment is all that is
possible in the present state of our knowledge; but deduction from
results of experimental investigation "still remains the ideal of
physiology, and only when this ideal has been attained, shall we be able
to obtain a comprehensive view of the interacting factors at work in the
living organism."

In my previous work on "Plant Response" (1906) I described detailed
investigations on irritability of plants which I carried out with highly
sensitive recorders. The plant was thus made to tell its own story by
means of its self-made records. The results showed that there is no
specific difference in physiological reaction of different organs to
justify the assumption of positive and negative irritabilities. A
generalisation was obtained which gave a complete explanation of diverse
movements in plants. The results were fully confirmed by an independent
method of inquiry, namely that of electric response, which I have been
able to elaborate so as to become a very important means of research.

The investigations described in the present volume not only support the
conclusions reached in my earlier works, but have led to important
additions. It is evident that the range of our investigation is limited
only by our power of recording the rate of plant-movement, that is to
say, in the measurement of length and time. In these respects the
instruments that I have been able to devise have surpassed my sanguine
expectations. The Resonant Recorder traces time-intervals as short as a
thousandth part of a second, while my Balanced Crescograph enables us to
measure variation of rate of growth as minute as 1/1000 millionth of an
inch per second, the sensitiveness of this apparatus thus rivals that of
the spectroscope. 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 attention 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 relation 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_ difference in physiological reaction. It has
further been shown that the prevalent idea that perception and
heliotropic excitation are two distinct phenomena is without any
foundation.

With reference to the effect of ether waves on plants, I 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 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 application in the
detection of physiological changes in the interior of an organ.

An important factor of nyctitropic movements, hitherto 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 direct stimulation induces contraction and retardation of growth,
and 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 impulses in
plants, and I have very recently been able to establish, that 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.
PAGE.
Principle of the Method of Balance--Compensating movement--
Growth-scale--Sensitiveness of the Crescographic Balance--
Effect of CO_{2}--Effect of anæsthetics 255

XXIII.--ON TROPIC MOVEMENTS.

Complexity of the problem--Contradictory nature of
responses--Two classes of tropic responses--Longitudinal
transmission of effect of stimulus--Transverse transmission
of effect of stimulus--Modification of tropic curvature by
conducting power of tissues and differential excitability
of the organ 268

XXIV.--TROPIC CURVATURE WITH LONGITUDINAL TRANSMISSION OF
EFFECT OF STIMULUS.

Dual impulses, positive and negative, caused by stimulus--
Direct and Indirect stimulus--Tropic effect of Indirect
stimulation 271

XXV.--TROPIC CURVATURE WITH TRANSVERSE TRANSMISSION OF EFFECT
OF STIMULUS.

Turgor variation under transverse transmission of
stimulus-effect--Tropic responses of pulvinated and growing
organs to unilateral stimulation--Direct unilateral
stimulation--Indirect unilateral stimulation--Difference of
effects induced by Direct and Indirect stimulation--Laws of
tropic curvature 279

XXVI.--MECHANOTROPISM: TWINING OF TENDRILS.

Anomalies of mechanotropism--Effects of indirect and direct
electric stimulation on growth of tendril--Effect of direct
and indirect mechanical 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

XXVII.--ON GALVANOTROPISM.

Polar effects 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

XXIX.--ON PHOTOTROPISM.

Complexity of problem of phototropic reaction--Action of
light--Positive phototropic curvature of pulvinated
organs--Positive phototropic curvature of growing
organs--Phenomenon of recovery--Immediate and after-effect
of light on growth--Latent period of phototropic reaction--
Growth variation induced by flash of light from a single
spark--Maximum positive curvature under continued action of
light 313

XXX.--DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM.

Differential excitability of two halves of pulvinus of
_Mimosa_--Transformation of positive to negative
curvature--Tropic effect under sunlight--Negative
phototropism of shoot and root 328

XXXI.--RELATION 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 CHARACTERISTICS.

Summation of stimulus--General consideration--The general
characteristic curve--Characteristics of simple phototropic
curve--Variation of susceptibility for excitation in
different parts of the curve--Effect of sub-minimal
stimulus--The complete phototropic curves of pulvinated
and growing organs--Limitation of Weber's law 346

XXXIII.--TRANSMITTED EFFECT OF PHOTIC STIMULATION.

Effect of light applied on tip of Setaria--Response of
growing region to unilateral stimulus--Effect of
simultaneous stimulation of the tip and the hypocotyl--
Algebraical summation of effects of direct and indirect
stimuli 362

XXXIV.--ON PHOTONASTIC CURVATURES.

Phototropic response of anisotropic organs--Positive
para-heliotropism--Negative para-heliotropism--Responses
of pulvinated and growing organs to light 378

XXXV.--EFFECT OF TEMPERATURE ON PHOTOTROPIC CURVATURE.

Effect of temperature on excitability--Effect of temperature
on conduction--Phototropic response of tendrils--Seasonal
variation of phototropic curvature--Antagonistic effects of
light and of rise of temperature 388

XXXVI.--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 Balance--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

XXXVIII.--RESPONSE OF PLANTS TO WIRELESS STIMULATION.

Effects of different rays of spectrum on growth--The wireless
system--Mechanical and electrical responses of _Mimosa_ to
Hertzian waves--Effect of wireless stimulation on 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--Determination of
effective direction of stimulus of gravity--Algebraical
summation 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--Characteristics
of geo-electric response--Physiological character of
geo-electric response--Effect of differential excitability
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

XLI.--MECHANICAL AND ELECTRICAL RESPONSE OF ROOT TO VARIOUS
STIMULI.

Mechanical and electrical response to Direct stimulation--
Mechanical and electrical response to Indirect stimulation--
Effect of unilateral stimulation applied at the root-tip 461

XLII.--GEO-ELECTRIC RESPONSE OF ROOT.

Geo-electric response of the root-tip--Electric response in
the growing region of root--Differential effect between the
tip and the growing region--Geo-perception at the root-tip 467

XLIII.--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-perceptive
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 geotropic by
phototropic action--Comparative balancing effects of white
and red lights--Effect of coal gas on photo-geotropic balance 503

XLV.--ON THERMO-GEOTROPISM.

Necessary conditions for geotropic curvature--Modifying
influence of temperature on geotropic curvature--Magnetic
analogue--Tropic equilibrium under varying intensities of
stimulus--Effect of variation of temperature on geotropic
torsion--Variation of apo-geotropic curvature under thermal
change--Effect of variation of temperature on dia-geotropic
equilibrium 509

PART IV.

NIGHT AND DAY MOVEMENTS IN PLANTS.

XLVI.--DIURNAL MOVEMENTS IN PLANTS.

Complexity of the problem--The different factors involved--
Autonomous movements--Epinasty and hyponasty--Positive and
negative thermonasty--Thermo-geotropism--Classification of
diurnal movements--Discriminating tests for classification--
Diurnal variation of light and of temperature 523

XLVII.--DIURNAL MOVEMENT DUE TO ALTERNATION OF LIGHT AND DARKNESS.

Experimental arrangements--The Quadruplex 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

XLVIII.--DIURNAL MOVEMENT DUE TO VARIATION OF TEMPERATURE
AFFECTING GROWTH.

Tropic and nastic movements--Distinction between
thermonastic and thermo-geotropic action--Diurnal
movement of _Nymphæa_--Action of light--Effect of
variation of temperature 546

XLIX.--DAILY MOVEMENT IN PLANTS DUE TO THERMO-GEOTROPISM.

Characteristics of thermo-geotropic movements--Diurnal
movement of Palm trees--Diurnal movement of procumbent stems
and of leaves--Continuous diurnal record for successive
thermal noon--Modification of the 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 response under light, and
its after-effects at pre-maximum, maximum, and post-maximum 569

LI.--THE DIURNAL MOVEMENT OF THE LEAF OF _MIMOSA_.

Four different phases in the diurnal record of _Mimosa_--
Different factors determining its diurnal movement--Diurnal
variation of geotropic torsion--Autonomous pulsation of the
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_--Response of _Mimosa_ to darkness
at different parts of the day--After-effect of light at
pre-maximum, maximum, and post-maximum 576

ILLUSTRATIONS.

FIGURE. PAGE.

93. Arrangement for compensation of growth-movement
by equal subsidence of plant-holder 257

94. Photograph of the Balanced Crescograph 258

95. Balanced Crescographic record 260

96. Record showing the effect of CO_{2} 265

97. Effect of ether and of chloroform 266

98. Diagrammatic representation of effects of Indirect and
Direct stimulation 275

99. Tropic curvature of _Crinum_ 276

100. Turgor variation caused by Indirect stimulation 281

101. Response of _Mimosa_ leaf under transverse transmission
of effect of electric stimulus 282

102. Diagrammatic representation of Indirect and Direct
stimulation of tendril 290

103. Record by Method of Balance 291

104. Variation of growth under direct stimulation 292

105. Positive curvature of tendril of _Cucurbita_ 295

106. Diagrammatic representation of effects of Indirect
and Direct unilateral stimulation of tendril 296

107. Retardation of rate of growth under cathode 303

108. Acceleration of rate of growth under anode 303

109. Thermonastic and radionastic responses of petal of
_Zephyranthes_ 308

110. The Thermonastic Recorder 309

111. Negative thermonastic response of _Nymphæa_ 310

112. Successive positive responses of the terminal leaflet
of bean plant 317

113. Positive response and recovery under moderate phototropic
stimulation 318

114. Persistent positive curvature under stronger stimulation 318

115. Immediate and after-effect of stimulus of light on growth 320

116. Latent period for photic stimulation 324

117. Effect of a single electric spark on growth 325

118. Responses of _Mimosa_ leaf to light from above 330

119. Responses of _Mimosa_ leaf to light from below 330

120. Record of effect of continuous application of light
on upper half of pulvinus of _Mimosa_ 331

121. Positive and negative phototropic response of _Oryza_ 335

122. Leaf of _Desmodium gyrans_ 339

123. The Oscillating Recorder 340

124. Tropic effect of increasing intensity of light on the
leaflet of _Desmodium gyrans_ 341

125. Tropic effect of increasing intensity of light on
growing organ (_Crinum_) 341

126. The Collimator 342

127. Effect of angle of inclination of light on tropic
curvature of pulvinated organ 343

128. Effect of angle of inclination on growth-curvature 343

129. Effect of increasing duration of exposure to light 344

130. Effect of continuous electric and photic stimulation
on rate of growth 348

131. Characteristic curve of iron 351

132. Simple characteristic curve of phototropic reaction 351

133. Complete phototropic curve of pulvinated organ 358

134. Complete phototropic curve of growing organ 359

135. Arrangement for local application of light 367

136. Response of seedling of _Setaria_ to light 368

137. Effect of application of light to the growing hypocotyl
of _Setaria_ 370

138. Response to direct and indirect photic stimulus 373

139. Diagrammatic representation of the effects of direct
and indirect stimulation of _Setaria_ 375

140. Photonastic curvature of creeping stem of _Mimosa pudica_ 380

141. Positive phototropic response of _Erythrina indica_ 382

142. Response of leaflet of _Mimosa_ to light 383

143. Response of leaflet of _Averrhoa_ to light 383

144. Diagrammatic representation of different types of
phototropic response 384

145. Phototropic curvature of tendril of _Passiflora_ 392

146. Effect of rise of temperature on phototropic curvature 394

147. After-effect of rise of temperature 395

148. Arrangement for record of torsional response 399

149. Record of torsional response of pulvinus of _Mimosa pudica_ 400

150. Leaflets of _Cassia alata_ 404

151. Positive response to thermal radiation 413

152. Record of positive, neutral, and reversed negative
curvature under thermal radiation 414

153. Diagrammatic representation of the wireless system 419

154. Mechanical response of _Mimosa_ leaf to electric waves 420

155. Electric response of _Mimosa_ to Hertzian wave 420

156. Record of responses of growing organs to wireless
stimulation 422

157. The Quadruplex Geotropic Recorder 428

158. Effect of alternate application of cold on upper and
lower sides of the organ 430

159. Geotropic response of flower stalk of Tube-rose 433

160. Geotropic response of _Tropæolum_ 433

161. The Complete Geotropic Curve 435

162. Diagrammatic representation of photic and geotropic
stimulation 436

163. The effect of super-imposition of photic stimulus 436

164. Diagrammatic representation of the mechanical and
electrical response 443

165. Diagrammatic representation of geo-electric response 447

166. Diagrammatic representation of Methods of Axial and
Vertical Rotation 449

167. Diagrammatic representation of the geo-electric response
of the shoot 450

168. Geo-electric response of the petiole of _Tropæolum_ 452

169. Geo-electric response of the scape of _Uriclis_ 453

170. Mechanical and electric response to indirect stimulation 463

171. Diagrammatic representation of mechanical and electric
response of root 464

172. Diagrammatic representation of geo-electric response of
root-tip 469

173. Diagrammatic representation of geo-electric 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 (_Bryophyllum_) 488

177. Curve of geo-electric excitation in different layers of
_Nymphæa_ 497

178. Curve of geo-electric excitation in _Bryophyllum_ 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 and phototropic
actions 505

182. Comparative balancing effects of white and red lights 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 temperature on geotropic torsion 514

187. Simultaneous records of variation of temperature, on up and
down movement, and of torsion of the leaf of _Mimosa_ 518

188. Arrest of pulsatory movement of leaflet of _Desmodium
gyrans_ by light 528

189. Effect of unilateral light on hyponastic movement 529

190. The Nyctitropic Recorder 537

191. Effect of sudden darkness on leaflet of _Casia alata_ 539

192. Diurnal movement of the leaflet of _Cassia alata_ 540

193. The day and night position of the petiole and terminal
leaflet of _Desmodium gyrans_ 541

194. Diurnal record of the terminal leaflet of _Desmodium
gyrans_ 542

195. Photograph of closed flower of _Nymphæa_ during day 550

196. Photograph of open flower of _Nymphæa_ at night 550

197. Action of light on the petal of _Nymphæa_ 551

198. Diurnal movement of the petal of _Nymphæa_ 552

199. Diurnal record of the Sijbaria Palm 556

200. Diurnal record of inclined Palm, geotropically curved
procumbent stem of _Tropæolum_, and dia-geotropic
leaf of Palm 557

201. Diurnal record of leaves of _Dahlia_, _Papya_, and _Croton_ 558

202. Diurnal record of procumbent stem of _Tropæolum_, and
leaf of _Dahlia_ for two successive days 560

203. Abolition of the diurnal movement under constant
temperature (_Tropæolum_) 565

204. Effect of inversion of plant on diurnal movement
of _Tropæolum_ 567

205. Electric response of the leaf stalk of _Bryophyllum_
under light 571

206. Diagrammatic representation of electric after-effect
of photic stimulation 571

207. After-effect of pre-maximum photic stimulation 574

208. After-effect of maximum photic stimulation 574

209. After-effect of post-maximum photic stimulation 574

210. Diurnal record of Mimosa in summer and winter 577

211. Record of diurnal variation 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 leaf of _Mimosa_ after amputation of sub-petioles 589

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-maximum 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.

XXII.--THE BALANCED CRESCOGRAPH

_By_

SIR J. C. BOSE.

We shall in the succeeding series of papers deal 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
solution 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 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[1] 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.

[1] "Plant Response"--p. 413.

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
apparatus 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
time relations 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 1/50,000 inch or
0·5 µ, 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 succeeded
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 downwards. The state of exact balance is indicated
when the 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.

[Illustration: FIG. 93.--Arrangement for compensation of growth-movement
by equal subsidence of plant-holder; S, adjusting screw for regulation
of speed of rotation; G, governor; W, heavy weight; P, plant-holder.]

COMPENSATING MOVEMENT.

For securing exact balance the holder of the plant P, in the given
example, will have to subside at a rate of 1/50,000 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 rate of subsidence; this
became magnified by the Crescograph and appeared as a series of
oscillations about a mean position of equilibrium. This particular
defect was obviated by the substitution 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 at the beginning was found
to be insufficient later on. The difficulty was finally overcome by the
use of a heavy weight W, in the place of coiled spring. The complete
apparatus is seen in figure 94.

[Illustration: FIG. 94.--Photographic reproduction of the Balanced
Crescograph. L, L', magnifying compound lever. R, recording plate.
P, plant. C, clock work for oscillation of the plate and lateral
movement. G, governor. M, circular growth-scale. V, plant-chamber.]

For purpose of simplicity of explanation, I assumed the growth rate to
have a definite value of 1/50,000 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 1/150,000 to 1/25,000 inch, or from 0·17 µ to
1·0 µ 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 compound 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 circumference 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 attainment of this constancy is a matter of fundamental
importance, and it was only by the employment or 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 starts the
balancing movement of the plant downwards. On account of partial balance
the record will be found to be less steep than before; the speed of the
clock is gradually increased till the record becomes perfectly
horizontal under exact balance. Overbalance makes the record slope
downwards. In figure 95 is seen records of underbalance (_a_) and
overbalance (_b_), to the extent of about 3 per cent.

[Illustration: FIG. 95.--Balanced Crescographic record: (_a_) showing
effect of underbalance and (_b_) overbalance of about 3 per cent.
(Magnification 2,000 times.)]

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 calibration 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 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 × ·0015/60 mm. per second
= N × ·000025 mm. per second
= N × ·025 µ per second
= N × 10^{-5} 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 × ·025 µ = 0·5 µ per second
or = 20 × 10^{-5} inch "
or = 1/50,000 " "

If we take the wave length of sodium light [Greek: lambda] as our
standard, the growth in length per second is equal to [Greek: lambda].
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 gradual lowering of which produces a continuous
diminution of speed. A particular position of the screw therefore
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 corresponding 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 µ to 1·2 µ 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 D_{1} ( = 0·5896 µ) and D_{2} ( = 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 µ 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 Crescograph. In
reality the problem before us is more intricate than simple measurement
of change of 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 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 seedling 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 × 0·025 µ per second.
= 0·45 µ per second.
= 0·000018 inch per second.

We learn further from (1) and (2) that a variation of
(18·53 - 18)/18·177 produces a change of drift of the spot of
light from +266 to -530 cm., _i.e._, of 796 cm. per minute.
As it is easy to detect a drift of 1 cm. per minute a variation of
0·53/(18·177 × 796), or 1 part in 27,000 may thus be detected 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 0·00018 inch per second, and the
Balanced Crescograph was shown capable of discriminating a variation
of 1 part in 27,000; hence it is possible to detect by this means a
variation of 1/1,500 millionth of an inch per second.

This method of unprecedented delicacy opens out a new field of
investigation on the effect of changes of environment in modification of
growth; instances of this will be found in subsequent chapters. I give
below accounts of certain demonstrations which will no doubt appear as
very striking.

After obtaining the exact balance a match was struck in the
neighbourhood of the plant. This produced a marked movement of the
hitherto quiescent spot of light, thus indicating the perception of such
an extremely feeble stimulus by the plant.

Breathing on the plant causes an enhancement of growth due to the joint
effects of warmth and carbonic acid gas. A more striking experiment is
to fill a small jar with carbonic acid and empty it over the plant. A
violent movement of the spot of light to the right demonstrates the
stimulating effect of this gas on growth.

The method described above is excessively sensitive; for general
purposes and for the method of direct record, a less sensitive
arrangement is sufficient. I give below accounts of several typical
experiments in which the recording form of Crescograph was employed, the
magnification being only 2,000 times.

[Illustration: FIG. 96.--Record showing the effect of CO_{2}. Horizontal
line at beginning indicates balanced growth. Application of CO_{2} at
arrow induces enhancement of growth shown by the up-curve followed by
depression, shown by the down-curve. Successive dots at intervals of 10
seconds. (Seedling of wheat.)]

_Effect of carbonic acid on Balanced growth: Experiment 100._--I have
already shown that carbonic acid diluted with air induces an enhancement
of the rate of growth, but its long continued action induces a
depression (p. 185). I shall now employ the Method of Balance in
studying the effect of CO_{2} on growth. It should be remembered in this
connection that the horizontal record indicates the balance of normal
rate of growth. An up-curve exhibits the induced enhancement and a
down-curve, a depression of growth. In the present experiment after
obtaining the exact balance, pure carbonic acid gas was made to fill up
the plant-chamber at the point marked with an arrow (Fig. 96). It will
be seen that this induced an almost immediate acceleration of the rate,
the latent period being less than five seconds. The acceleration
continued for two and half minutes; the accelerated rate then slowed
down, became enfeebled, and the growth returned for a short time to the
normal as indicated by the horizontal portion at the top of the record;
this proved to be the turning point of inversion from acceleration into
retardation of growth. The stronger is the concentration of the gas the
earlier is the point of inversion. With diluted carbonic acid the
acceleration may persist for an hour or more.

EFFECT OF ANÆSTHETICS.

_Effect of Ether: Experiment 101._--Dilute vapour of ether is found to
induce an acceleration of rate of growth which persist for a
considerable length of time. This is seen in the upsetting of the
balance upwards on the introduction of the vapour (Fig. 97a.).

[Illustration: FIG. 97.--(_a_) Effect of ether, acceleration of growth,
(_b_) effect of chloroform preliminary acceleration followed by
depression.]

_Effect of Chloroform: Experiment 102._--The effect of chloroform vapour
is relatively more depressing than ether. Application of chloroform is
seen to induce at first an acceleration which persisted for 50 seconds,
but after this depression set in (Fig. 97b). Prolonged application of
the anæsthetic is followed by the death of the plant.

SUMMARY.

In the Method of Balance the movement of growth upwards is compensated
by an equal movement of the plant downwards, with the result that the
record remains horizontal.

The effect of an external agent is immediately detected by the upsetting
of the balance, up-record representing acceleration above normal, a
down-record the opposite effect of depression below the normal rate.

The latent period and the after-effect of stimulus may thus be obtained
with the highest accuracy.

The sensitiveness of the Method of Balance may be raised so as to
indicate a variation of rate of growth smaller than 1/1000 millionth of
an inch per second.

XXIII.--ON TROPIC MOVEMENTS

_By_

SIR J. C. BOSE.

The diverse movements induced by external stimuli in different organs of
plants are extremely varied and complicated. The forces in operation are
manifold--the influence of changing temperature, the stimulus of
contact, of electric current, of gravity, and of light visible and
invisible. They act on organs which exhibit all degrees of physiological
differentiation, from the radial to the dorsiventral. An identical
stimulus may sometimes induce one effect, and at other times, the
precisely opposite. Thus under unilateral stimulation of light of
increasing intensity, a radial organ exhibits a positive, a
dia-phototropic, and finally a negative response. Strong sunlight brings
about para-heliotropic or 'midday sleep' movement, by which the apices
of leaves or leaflets turn towards or away from the source of
illumination. The teleological argument advanced, that in this position
the plant is protected from excessive transpiration, does not hold good
universally; for under the same reaction, the leaflets of _Cassia
montana_ assume positions by which the plant risks fatal loss of water.
In _Averrhoa carambola_ the movement is downwards, whichever side is
illuminated with strong light; in _Mimosa_ leaflet the movement, under
similar circumstances is precisely in the opposite direction. The
photonastic movement, apparently independent of the directive action of
light, has come to be regarded as a phenomenon unrelated to phototropic
reaction, and due to a different kind of irritability, and a different
mode of response. So very anomalous are these various effects that
Pfeffer, after showing the inadequacy of different theories that have
been advanced, came to the conclusion that "the precise character of the
stimulatory action of light has yet to be determined. When we say that
an organ curves towards a source of illumination because of its
heliotropic irritability, we are simply expressing an ascertained fact
in a conveniently abbreviated form, without explaining why such
curvature is possible or how it is produced."[2]

[2] Pfeffer--_Ibid_--Vol. II, p. 74.

The contradictory nature of the various responses is however not real;
the apparent anomaly had lain in the fact that two definite fundamental
reactions of opposite signs induced by stimulus had not hitherto been
recognised and distinguished from each other. The innumerable variations
in the resultant response are due to the summation of the effects of two
fluctuating factors, with further complications arising from: (1)
difference in the point of application of stimulus, (2) the differential
excitability of the different sides of the responding organ, and (3) the
effect of temperature in modifying tropic curvature. It is therefore
most important to have the means for automatic record of _continuous_
change in the response brought about by various factors, which act
sometimes in accord, and at other times in conflict. The autograph of
the plant itself, giving a history of the change in response and its
time-relations, is alone decisive in explanation of various difficulties
in connection with plant movements, as against the various tentative
theories that have been put forward. The analysis of the resulting
effect, thus rendered possible, casts new light on the phenomena of
response, proving that the anomalies which had so long perplexed us, are
more apparent than real.

One of the causes of uncertainty lay with the question, whether response
changed with the mode of stimulation. I have, however, been able to show
that _all forms of stimuli_ induce a definite excitatory reaction of
contraction (p. 218).

Tropic movements induced by unilateral action of stimulus may, broadly
speaking, be divided into two classes depending on the point of
application of stimulus:

In the first, the point of application of unilateral stimulus is not on
the responding organ itself, but at some distance from it. The question
therefore relates to LONGITUDINAL TRANSMISSION of effect of stimulus.

In the second, unilateral stimulus acts directly on the responding
organ. For the determination of the resultant movement, it is necessary
to take account of effects induced on the two sides of the organ. The
side adjacent to the stimulus I shall designate as the _proximal_, and
the diametrically opposite as the _distal_ side. The question to be
investigated in this case relates to TRANSVERSE TRANSMISSION of effect
of stimulus. It will be shown that the resulting movement depends on:--

(_a_) whether the tissue is a conductor or a non-conductor of
excitation in a transverse direction, and

(_b_) whether it is the proximal, or the distal side of the
organ that is the more excitable.

In connection with the response to environmental changes, a source of
uncertainty is traceable to the absence of sufficient knowledge of the
physiological effect of heat, which has been regarded as a form of
stimulus: it will be shown that heat induces two distinct effects
dependent on conduction and radiation. We shall in the succeeding
chapters, take up the study of the physiological effects induced by
changes in the environment.

XXIV.--TROPIC CURVATURE WITH LONGITUDINAL TRANSMISSION OF EFFECT OF
STIMULUS

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

I have in previous chapters explained that the direct application of
stimulus gives rise in different organs to contraction, diminution of
turgor, fall of motile leaf, electro-motive change of galvanometric
negativity, and retardation of the rate of growth. I have also shown
that indirect stimulation (_i.e._ application of stimulus at some
distance from the responding organ) gives rise to a positive or erectile
response of the responding leaf or leaflet (indicative of an increase of
turgor), often followed by normal negative response. The positive
impulse travels quickly. The interval of time that elapses, between the
application of stimulus and the erectile response of the responding
leaf, depends on the distance of the point of application, and the
character of the transmitting tissue: it varies in different cases from
0·6 second to about 40 seconds. The positive is followed by a slower
wave of protoplasmic excitation, which causes the excitatory fall. The
velocity of this excitatory impulse is about 30 mm. per second in the
petiole of _Mimosa_, and about 3 mm. per second in _Biophytum_. The
positive followed by the negative thus gives rise to a diphasic
response. The excitatory impulse is much enfeebled during transit: the
negative impulse may thus fail to reach the responding organ, if the
stimulus be feeble or if the intervening distance be long or
semi-conducting. Hence moderate stimulus applied at a distance gives
rise only to positive response; direct application of strong stimulus
gives rise, on the other hand, to the normal negative. By employing the
electric method of investigation, I have obtained with ordinary tissues
the positive, the diphasic, and the negative electric response, in
correspondence with the responses given by a motile organ (p. 214). The
mechanics of propagation of the positive and the negative impulse are
different. It is therefore necessary to distinguish the quick
_transmission_ of the positive impulse from the slow _conduction_ of the
negative impulse due to the propagation of excitatory protoplasmic
change.

It should be borne in mind in this connection that all responsive
movements are ultimately due to protoplasmic changes which are beyond
our scrutiny. We can infer the nature of the change by the concomitant
outward manifestations, which are of two kinds: the _positive_,
associated with increase of turgor, expansion, and galvanometric
positivity, and the _negative_ with concomitant decrease of turgor,
contraction, and galvanometric negativity. Thus positive and negative
reactions indicate the fundamental protoplasmic changes of opposite
characters.

The movement and curvature induced by stimulus have, for convenience,
been distinguished as _positive curvature_, (movement towards stimulus),
and _negative curvature_ (movement away from stimulus). Though these
curvatures result from protoplasmic reactions, yet the _positive
curvature_ is not necessarily associated with _positive protoplasmic
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 organ.

Physiologists have not been aware of the dual character of the impulse
generated by stimulus, and the term "transmission 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, will 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 example, 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.

In any case it is important to remember the following characteristic
effects of indirect stimulation.

TABLE XXII.--SHOWING THE EFFECT OF INDIRECT STIMULATION.

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 103._--I have already explained, how thermal _radiation_ is
almost as effective in inducing contraction and retardation of growth as
the 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 through S, the
securely held tissue (Fig. 98). A feeble stimulus was applied on one
side, at the indifferent point about 3 cm. below the region of growth.
The positive effect of indirect stimulus reached the region of growth on
the same side, bringing about an acceleration 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 thus succeeded by the
concave curvature (Fig. 99). Direct application of stimulus at the
growing region gave rise to a positive curvature.

[Illustration: FIG. 98.--Diagrammatic representation of effects of
indirect and direct stimulation. Continuous arrow represents the
indirect stimulation, and the curved continuous arrow above, the induced
negative curvature: dotted arrow indicates the application of direct
stimulus, and the dotted curve above, the induced positive curvature.]

[Illustration: 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 Magnification 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 forms of stimuli. Thus
in the stem of _Dregea volubilis_ the longitudinally transmitted effect
of light of moderate intensity was a negative curvature; direct
application of light on the growing region gave, on the other hand, a
positive curvature and movement towards light.

Thus while the effect of direct unilateral stimulation is a positive
curvature, the effect of indirect stimulation is a negative curvature.
The following table gives a summary of results of tropic effects under
unilateral application of indirect stimulus.

TABLE XXIII.--SHOWING TROPIC EFFECT OF UNILATERAL APPLICATION OF
INDIRECT STIMULUS.

SUMMARY.

In sensitive plants stimulus applied at a distance induces in the
responding region an expansion indicative of increase of turgor.

The effect of indirect stimulation is also exhibited by an electric
change of galvanometric positivity, indicative of enhancement of turgor
and expansion.

Indirect stimulus induces in growing organs an enhancement of rate of
growth.

Unilateral application of stimulus causes an expansion higher up on the
same side to which the stimulus is applied; the result is an induced
convexity, a movement away from the stimulus, _i.e._, a negative
curvature. Direct stimulus applied unilaterally at the responding region
induces, on the other hand, a positive curvature.

XXV.--TROPIC CURVATURE WITH TRANSVERSE TRANSMISSION OF EFFECT OF
STIMULUS

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

We have next to consider a very large class of phenomena arising out of
the direct stimulation of one side and its transversely transmitted
effect on the opposite side. The unilateral stimuli to which the plant
is naturally exposed are those of contact, of light, of thermal
radiation, and of gravity. There is besides the stimulation by electric
current. I shall presently show that these tropic curvatures are
determined by the definite effects of direct and indirect stimulations.

Under unilateral stimulus, the proximal side is found to become concave
and the distal side convex; the organ thus moves towards stimulus,
exhibiting a positive curvature. This movement may be due: (1) to the
diminution of turgor, contraction or retardation of rate of growth of
the proximal side, (2) to the increase of turgor, expansion or
acceleration of rate of growth on the distal side, or (3) to the joint
effects of contraction of the proximal and expansion of the distal side.

As regards the reaction of the proximal side, it has been shown that
direct stimulation induces local contraction in a pulvinated organ, and
retardation of growth in a growing organ. The effect induced on the
distal side had hitherto remained a matter of uncertainty. In regard to
this we must bear in mind that it is the effect of indirect stimulus
that reaches the distal sided, inducing an enhancement of turgor and
expansion of that side.

For obtaining a complete explanation of tropic curvatures in general, it
is important that the induction of enhanced turgor at the distal side
(by the action of stimulus at the proximal side) should be corroborated
by independent methods of enquiry. One of the methods I employed for
this purpose was electrical. Two electric connections were made, one
with the distal point (diametrically opposite to the stimulated area),
and the other, with an indifferent point at a distance. On application
of stimulus of various kinds, the distal point was found to exhibit
galvanometric positivity, indicative of enhancement of turgor.[3]

[3] "Plant Response"--p. 519.

I have since been able to devise a new experiment by which the
enhancement of turgor on the distal side is demonstrated in a very
striking manner.

I have shown (p. 39) that the movement of the motile leaf of _Mimosa_ is
a reliable indicator of the state of turgor, increase of turgor inducing
erection, and diminution of turgor bringing about the fall of the leaf.
I shall employ the mechanical response of the leaf to demonstrate the
enhancement of turgor induced by transverse transmission of effect of
stimulus.

[Illustration: FIG. 100.--Increased turgor due to indirect stimulation,
inducing erection of Mimosa leaf: (_a_) diagram of the experiment, point
of application of stimulus indicated by arrow. (_b_) erectile response
(shown by down-curve) followed by rapid fall (up-curve) due to
transverse conduction of true excitation. (Successive dots at intervals
of 10 seconds.)]

TURGOR-VARIATION UNDER TRANSVERSE TRANSMISSION OF STIMULUS-EFFECT.

_Unilateral photic stimulation: Experiment 104._--A _Mimosa_ plant was
taken, and its stem was held vertical by means of a clamp. We apply a
stimulus at a point on one side of the stem, and observe the effect of
this on the state of turgor at the diametrically opposite side. In my
first experiment on the subject of detection of induced change of
turgor I employed the stimulus of light. A narrow beam from a small arc
lamp was made to fall on the stem, at a point diametrically opposite to
the motile leaf, which was to serve as a indicator for induced variation
of turgor at the distal side. The leaf was attached to the recording
lever, the successive dots in the record being at intervals of ten
seconds. Stimulation by light caused a positive or erectile movement
within 20 seconds of application. The positive response afforded a
conclusive proof of the induction of an increase of turgor at the distal
point. When the stimulus is moderate or of short duration, the response
remains positive. But with strong or prolonged stimulation, the slower
excitatory negative impulse is conducted to the distal point and brings
about the sudden fall of the leaf (Fig. 100). In the present case the
excitatory impulse reached the motile organ 200 seconds after the
initiation of the positive response. The stem was thin, only 2 mm. in
diameter. The velocity of excitatory impulse in a transverse direction
is thus 0·01 mm. per second; transverse transmission is, for obvious
reasons, a much slower process than longitudinal transmission of
excitation; in the _Mimosa_ stem this is about 4 mm. per second.

[Illustration: FIG. 101.--Response of leaf of _Mimosa_ under transverse
transmission of electric stimulus. (Compare this with fig. 100.)]

_Unilateral electric stimulation: Experiment 105._--In order to show
that the effects described above are not due to any particular mode of
stimulation but to stimuli in general, I carried out an additional
experiment, the stimulus employed being electrical. Two fine
pin-electrodes were pricked into the stem, opposite to the responding
leaf of _Mimosa_; these electrodes were placed vertically one above the
other, 5 mm. apart. After a suitable period, allowed for recovery from
mechanical irritation, feeble tetanising electric shock was passed
through the electrodes. The responsive effects at the distal side of the
stem is precisely similar to those induced under unilateral photic
stimulation; that is to say, the first effect was an erectile movement
of the leaf, indicative of an induced enhancement of turgor; the
excitatory negative impulse then reached the distal point and caused a
sudden fall of the leaf (Fig. 101).

The experiments that have just been described are of much significance.
An organ like the stem of _Mimosa_, since it exhibits no contraction,
may appear insensitive to stimulation; but its perception of stimulus is
shown by its power of transmitting two characteristic impulses, one of
which is the positive, giving rise to an enhancement of turgor, and the
other, the true excitatory negative, inducing the opposite reaction or
diminution of turgor. Unilateral stimulation gives rise to both these
effects in all organs: pulvinated, growing, and non-growing. It was the
fortunate circumstance of the insertion of the motile leaf on one side
of the _Mimosa_ stem that enabled us to demonstrate the important facts
given above.

The underlying reactions, which give rise to tropic curvature, could
have been foretold from the Laws of effects of Direct and Indirect
stimulation, established in previous chapters (pp. 136, 216). The
resulting curvature is thus brought about by the joint effects of direct
stimulation 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 to 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 dependent 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 only the positive impulse; strong or long
continued stimulation will transmit both positive and negative impulses,
the positive preceding the negative. The transmitted positive gives rise
to increase of turgor, expansion, and acceleration 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 continued
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.

TABLE XXIV.--SHOWING RESPONSIVE EFFECTS COMMON TO PULVINI AND GROWING
ORGANS UNDER UNILATERAL STIMULATION.

The diagram which I have already given (Fig. 98) clearly explains the
different tropic effects induced by changing the point of application of
stimulus. We may thus have stimulus applied at the responding region
itself (Direct Stimulation) or at some distance from it (Indirect
Stimulation). The final effect will be modified by the conducting power
of the tissue.

DIRECT UNILATERAL STIMULATION.

_Type I._--The tissue has little or no power of transverse
conduction: stimulus remains localised, the proximal side
undergoes contraction, and the distal side expansion. The
result is a positive curvature.

_Type II._--The tissue is transversely conducting. Under
strong and long continued stimulation the excitatory impulse
reaches the distal side, neutralising or reversing the first
effect.

INDIRECT UNILATERAL STIMULATION.

_Type I._--The intervening tissue is an indifferent
conductor: transmitted positive impulse induces expansion 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:
the effect of positive impulse is over-powered by the
predominant excitatory negative impulse, the final result is
a concavity and positive curvature, with movement towards the
stimulus.

The following is a tabular statement of the different effects induced by
Direct and Indirect stimulation.

TABLE XXV.--SHOWING DIFFERENCE OF EFFECTS INDUCED BY DIRECT AND INDIRECT
STIMULATION.

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.

1. (_a_) DIRECT APPLICATION OF UNILATERAL STIMULUS OF
MODERATE INTENSITY, INDUCES A POSITIVE OR CONCAVE CURVATURE,
BY THE CONTRACTION OF THE PROXIMAL AND EXPANSION OF THE
DISTAL SIDE.

(_b_) UNDER STRONG OR LONG-CONTINUED STIMULATION, THE
POSITIVE CURVATURE IS NEUTRALISED OR REVERSED, BY TRANSVERSE
CONDUCTION OF EXCITATION; THIS EFFECT IS ACCENTUATED BY THE
DIFFERENTIAL EXCITABILITY OF THE TWO SIDES OF THE ORGAN.

2. (_a_) INDIRECT APPLICATION OF UNILATERAL STIMULUS OF
FEEBLE INTENSITY INDUCES A NEGATIVE CURVATURE.

(_b_) IN A CONDUCTING TISSUE THE EXCITATORY EFFECT BEING
TRANSMITTED UNDER STRONG AND LONG CONTINUED STIMULATION,
INDUCES A POSITIVE CURVATURE.

It 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,

(3) 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 tissue Direct stimulation induces a diminution of
turgor and contraction, Indirect stimulation 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 stimulation, 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 by_

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 movement 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 _Cucurbitaceæ_. These pits no doubt facilitate
sudden deformation of the sensitive protoplasm by frictional contact. No
satisfactory explanation has however been offered as regards the
physiological machinery of responsive movement. The difficulty 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."[4]

[4] Jost--_Ibid_--p. 490.

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 movement 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 my investigations on the effects of direct and
indirect stimulus on the growth of tendril; I 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 stimulus under different conditions. From the
results of these experiments 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.

GENERAL 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 Crescograph, 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 below the clamp. Direct stimulus is applied by
means of electric connections one above and the other below the clamp.

[Illustration: FIG. 102.--Diagrammatic representation of indirect and
direct stimulation of tendril.]

_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 stimulus, and persisted for
three minutes; after this the normal rate of growth became restored, as
seen by the balanced record once more becoming horizontal (Fig. 103).

[Illustration: FIG. 103.--Record by Method of Balance, showing
acceleration of growth of tendril (up-curve) induced by indirect
stimulation. (_Cucurbita._)]

_Effect of Direct Stimulus: Experiment 107._--The incipient 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. The 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 after stimulation, where the curve is far more
erect than that of the normal rate of growth before stimulation.

[Illustration: FIG. 104.--Variation of growth induced by direct
stimulation. First part of the curve shows 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.

(2) Direct stimulation 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 of growth._

The experiments described above demonstrate the effects of direct 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.

_Effect of Direct mechanical stimulus: Experiment 108._--In this case I
took a tendril of _Cucurbita_, 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 of 15 minutes. Table XXVI shows the immediate
and after-effects of mechanical stimulation on growth.

TABLE XXVI.--SHOWING THE IMMEDIATE AND AFTER-EFFECT OF MECHANICAL
STIMULATION ON TENDRIL (_Cucurbita_).

+--------------------------------------------------------------+
|Normal rate of growth 0·44 µ per sec. |
|Retarded rate immediately after stimulation 0·20 µ " " |
|Recovery and enhancement after 15 minutes 0·50 µ " " |
+--------------------------------------------------------------+

The immediate and after-effects of mechanical stimulus on the tendril
are therefore the same as that of electric stimulus. The incipient
contraction under direct mechanical stimulus, moreover, is not the
special characteristic of tendrils, but of growing plants in general.
For I have shown (page 203) that the growth of flower stalk of
_Zephyranthes_ is also retarded after mechanical friction, from the
normal rate 0·48 µ to 0·11 µ after stimulation. We shall find later that
different plant organs, after moderate stimulation, exhibit acceleration
of growth as an after-effect. The phenomenon of responsive reaction of
tendril is therefore not unique, but similar to that of other organs
under all forms of stimulation. The only speciality in tendril is that
owing to anatomical peculiarities, the perceptive power of the organ for
mechanical stimulus is highly developed.

We are now in a position to offer an explanation of the induced
concavity of the stimulated side of the tendril, and its recovery after
brief contact. The experiments that have been described show that:

(1) the proximal side contracts because it is directly stimulated,
and the distal side, being indirectly stimulated, expands; the
curvature is thus due to the joint effects of contraction of one
side, and expansion of the opposite side, and

(2) the recovery of the tendril after brief contact is hastened by
the after-effect of stimulus, which is expansion and acceleration
of growth.

The results given above will also be found to explain Fitting's
important observations[5] that (_a_) the stimulated side of the tendril
undergoes transient contraction with subsequent acceleration of growth,
and that (_b_) the distal or convex side undergoes an immediate
enhancement of growth.

[5] Pfeffer--_Ibid_--Vol. III, p. 57.

I give below a record given by a tendril of _Cucurbita_ in response to
unilateral contact of short duration (Fig. 105). Successive dots in the
record are at intervals of three seconds. The latent period was ten
seconds, and the maximum curvature was attained in the course of two and
a half minutes. The curvature persisted for a further period of two
minutes after which recovery was completed in the course of 12 minutes.
Feeble stimulation is attended by a recovery within a short period, but
under strong stimulus the induced curvature becomes more persistent.

[Illustration: FIG. 105.--Positive curvature of tendril of Cucurbita
under unilateral stimulus of contact at x.]

INHIBITORY ACTION OF STIMULUS.

I have referred to the remarkable observation of Fitting that though
the application of stimulus on the upper side 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 explanation 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 expansion on the opposite side. In the
diagram seen in figure 106, the left is the more excitable side, and
contraction will induce concavity 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.

[Illustration: FIG. 106.--Diagrammatic representation of effects of
Indirect and Direct unilateral stimulation of the tendril. Indirect
stimulation, I, induces movement away from stimulated side (negative
curvature) represented by continuous arrow. Direct stimulation, D,
induces movement towards stimulus (positive curvature) indicated by
dotted arrow.]

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 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 diametrically 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 variation
indicative of enhanced turgor. It will thus be seen that inhibition is
possible even in the absence of contraction of the upper 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 OF THE TENDRIL.

It is generally supposed that the upper side of the tendril of
_Passiflora_ is devoid of contractility. This is however not the case,
for my experiments show that stimulation of the upper side also induces
contraction and concavity of that side, though the actual movement is
relatively feeble.

_Experiment 109._--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 following 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.×1 cm. was rubbed against
the tendril in each experiment. Stimulation, thus produced, induced a
responsive movement of each side of the organ. The extent of the
maximum 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 UPPER
AND UNDER SIDE OF TENDRIL (_Passiflora_).

+------------------------------------------------------------+
|Movement induced by | Movement induced by | B |
| stimulation of under | stimulation of upper | Ratio ---. |
| side, A. | side, B. | A |
+----------------------+-------------------------------------+
|(1) 85 divisions | 14 divisions | 1/6 |
|(2) 106 " | 15 " | 1/7 |
|(3) 60 " | 8 " | 1/7 |
|(4) 80 " | 10 " | 1/8 |
+------------------------------------------------------------+

It will thus be seen that the upper side of the tendril is not totally
inexcitable, its power of contraction being about one-seventh that of
the under side.

NEGATIVE CURVATURE OF 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, resulting 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 induce in it a negative curvature by application of indirect
unilateral stimulus.

_Experiment 110._--A tendril of _Passiflora_ was held in a clamp, as in
the diagram (Fig. 106) in which the left is the more excitable side of
the organ. The responsive movement of the tendril is observed by
focussing a reading 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 _negative_.
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 _Crinum_, in which the normal positive response 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 incipient
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.

As the after-effect of direct stimulus is an acceleration of growth
above the normal, the stimulated side undergoes an expansion by which
the recovery is hastened.

Unilateral application of direct stimulus induces a _positive_
curvature, but the same stimulus applied at a distance from the
responding region induces a _negative_ curvature.

The tendril of _Passiflora_ 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 expansion of the
opposite side, even in cases where the contractility 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[6] 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.

[6] "Irritability of Plants," p. 212.

I have also carried out further investigations on the polar effect of
current on the autonomous activity of the leaflet of _Desmodium gyrans_.
These rhythmic pulsations can be recorded by my Oscillating Recorder.
Each pulsation 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 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 _expansion_ 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 opposite effects on growth.
For this experiment I took a specimen of _Kysoor_ and determined the
region where growth was maximum. A piece of moist cloth was wrapped
round this region to serve as one of the two electrodes. The second
electrode was placed in the neighbouring indifferent region where there
had been a cessation of growth.

_Effect of Cathode: Experiment 111._--The particular specimen of
_Kysoor_ had a normal rate of growth of 0·48 µ per second. On
application of the cathode the rate was reduced to 0·14 µ per second, or
to less than a third. This will be seen in record (Fig. 107), where N is
the normal rate of growth and K, retarded rate under the action of the
cathode.

[Illustration: FIG. 107.--Retardation of rate of growth under the action
of cathode (_Kysoor_).]

[Illustration: FIG. 108.--Acceleration of rate of growth under anode
(_Kysoor_).]

_Effect of anode: Experiment 112._--If the cathode induced a
retardation, the anode might be expected to induce an acceleration of
growth. But in my first experiment on the action of anode, I could
detect no perceptible variation of rate of growth. In trying to account
for this failure, I found that the specimen employed for the experiment
had normally a very rapid rate of growth. It appeared that an induced
acceleration would be brought out more conspicuously by choosing a
specimen in which the growth-rate was low, rather than in one in which
it was near its maximum. Acting on this idea, I took another specimen of
_Kysoor_ in which the normal rate was as slow as 0·10 µ per second. On
applying the anode to the growing region, there was an enhancement to
one and half times the normal rate (Fig. 108).

TABLE XXVIII.--EFFECT OF ANODE AND CATHODE ON GROWTH (_Kysoor_).

Specimen A Normal rate 0·10 µ per sec.
Acceleration under anode 0·155 µ per sec.

Specimen B Normal rate 0·48 µ per sec.
Retardation under cathode 0·14 µ per sec.

The effects given above take place under the action of a feeble current.
Strong current on the other hand induces a retardation or an arrest of
growth.

I have in the above experiments demonstrated the normal effect of anode
in inducing expansion and acceleration of rate of growth; the cathode
was shown to induce contraction and retardation of growth. Unilateral
application of anode and cathode thus induces appropriate curvatures in
pulvinated and in growing organs.

SUMMARY.

The effects of an electric current on growth is modified by the
direction of current. A feeble anodic current enhances the rate of
growth; a cathodic current on the other hand induces a retardation of
the rate. Strong current, both anodic and cathodic, induces a
retardation.

XXVIII.--ON THERMONASTIC PHENOMENA

_By_

SIR J. C. BOSE,

_Assisted by_

SURENDRA CHANDRA DAS.

In describing thermonastic curvatures Pfeffer says that "a special power
of thermonastic response has been developed by various flowers, in which
low temperatures produce closing movements, and high temperatures,
opening ones. The flowers of _Crocus vernus_ and _Crocus luteus_ are
specially responsive, as also those of _Tulipa Gesneriana_ for these
flowers perceptibly respond to a change of temperature of half a degree
centigrade."[7]

[7] Pfeffer--_Ibid_, Vol. III, p. 112.

We have hitherto studied the response of various organs to _stimulus_;
we have now to deal with the effect of thermal variation. Does rise of
temperature act like other forms of stimuli or is its action different?
We have therefore to find:

(1) The physiological effect of variation of temperature.

(2) Whether thermonastic irritability is confined only to
certain classes of organs, or is it a phenomenon of very wide
occurrence?

(3) Whether variation of temperature induces in anisotropic
organs only one type of response, or two types, positive and
negative.

(4) The law which determines the direction of responsive
movement.

EFFECT OF RISE OF TEMPERATURE.

As regards the effect of rise of temperature we have seen that, within
normal limits, it induces expansion and acceleration of the rate of
growth. Stimulus, on the other hand, induces precisely the opposite
effect. Hence the physiological reaction of steady rise of temperature
is, generally speaking, antagonistic to that of stimulus. This
conclusion is supported by numerous experiments which I have carried out
with various plant organs. Example of this will be found in the present
and subsequent chapters.

DIFFERENT THERMONASTIC ORGANS.

The only condition requisite for the exhibition of response is the
differential excitability of an anisotropic organ. It is therefore
likely to be exhibited by a large variety of plant organs, such as
pulvini, petioles, leaves, and flowers, and my results show that this is
actually the case. This particular sensibility, moreover, is not
confined to delicate structures, but is extended to rigid trees and
their branches.

Before proceeding further, it is necessary to draw attention to the
confusion which arises from the use of the common prefix '_thermo_' in
thermonasty and thermo-tropism. With regard to this Pfeffer says "It is
not known whether radiated and conducted heat exercise a similar
thermotropic reaction."[8] I shall show that the reactions to radiant
heat, and to conducted heat (rise of temperature) are of opposite
character, radiation inducing contraction, and rise of temperature,
expansion. It is therefore advisable to distinguish the thermal, or
temperature effect, from the radio-thermal effect of infra-red
radiation.

[8] Pfeffer--_Ibid_, Vol. III, p. 177.

TWO TYPES OF RESPONSE.

As regards the effect of variation of temperature I shall proceed to
show that there are two distinct types, which I shall, for convenience,
distinguish as the _Positive_ and _Negative_.

Positive thermonastic reaction is exhibited by organs in which the upper
half is the more excitable. Response to rise of temperature is by
_downward_ or _outward_ movement. In floral organs this finds expression
by a movement of opening. In illustration of this may be cited the
examples of the well known Crocus and also of _Zephyranthes_.

Negative thermonastic movement is shown by organs in which the lower
half is the more excitable. Here the response to rise of temperature is
by an _upward_ or _inward_ movement. I shall show that an example of
this is furnished by the flower of _Nymphæa_ which closes under rise,
and opens during fall of temperature.

POSITIVE THERMONASTIC RESPONSE.

_Response of Zephyranthes: Experiment 113._--Viewed from the top, the
inner side of the petal of a flower is the upper side. The _Crocus_
flower under rise of temperature opens outwards by expansion of the
inner side, which must be the more excitable. As _Crocus_ was not
available in Calcutta, I found the flower of _Zephyranthes_ (sometimes
called the Indian _Crocus_) reacting to variation of temperature in a
manner similar to that of _Crocus_, that is to say, the flower opens
under rise and closes with a fall of temperature. For obtaining record
all the perianth segments but one was removed. This segment was attached
to the recording lever. On lowering of temperature through 5°C. there
was an up-movement, or a movement of closure. Rise of temperature
induced, on the other hand, a movement of opening.

[Illustration: FIG. 109.--Thermonastic and radionastic responses of
petal of _Zephyranthes_ C, closing movement due to cooling, and H,
opening movement due to warming; R, closing movement due to
heat-radiation. Note opposite responses to rise of temperature and to
thermal radiation.]

_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 inner half of the perianth; hence the inner
half of the organ is relatively the more excitable.

[Illustration: FIG. 110.--The Thermonastic Recorder. T, metallic
thermometer attached to the short arm of the upper lever; the specimen
of _Nymphæa_, N, has one of its perianth leaves attached to the short
arm of the second lever by a thread. C, clockwork for oscillation of the
plate.]

NEGATIVE THERMONASTIC RESPONSE.

_Response of_ Nymphæa: _Experiment 115._--Many of the Indian
_Nymphæaceæ_ have their sepals and petals closed during the day, and
open at night. I find that the perianth leaves of this flower are
markedly sensitive to variation of temperature. The Thermonastic
Recorder employed in this investigation is shown in figure 110. The
record given in figure 111 shows that the perianth segment, subjected to
a few degrees' rise of temperature, responded by an up-movement of
closure, due to greater expansion of the outer half. The latent period
was 6 seconds, and the maximum effect was attained in the further course
of 21 seconds. This experiment shows that the thermonastic response of
this flower is of the negative type.

[Illustration: FIG. 111.--Negative thermonastic response of _Nymphæa_.
Application of warmth at the vertical mark induced up-movement of
closure, but stimulus of electric shock at arrow induced rapid
excitatory down movement of opening. Successive dots at intervals of a
second.]

_Effect of stimulus: Experiment 116._--In the positive type of
thermonastic organs, where rise of temperature induced a movement of
opening, stimulus induced the opposite movement of closure (Expt. 114).
We shall now study the effect of stimulus on the movement of _Nymphæa_,
which undergoes closure during rise of temperature, as seen in the first
part of the record in figure 111. Stimulus of electric shock was applied
at the point marked with an arrow; the response is seen to be by a
movement of opening. Here also we find the effects of rise of
temperature and of stimulus to be antagonistic to each other. This will
be clearly seen in the following tabular statement.

TABLE XXIX.--SHOWING THE EFFECT OF RISE OF TEMPERATURE AND OF STIMULUS
ON THERMONASTIC ORGANS.

In _Nymphæa_ it is the outer side of the perianth that is relatively the
more excitable since diffuse electric stimulus induces a movement of
opening due to the greater contraction of the outer side. It is by the
greater expansion of this more excitable side that the movement of
closure is effected during rise of temperature.

From the results of experiments given above we arrive at the
following:--

LAW OF THERMONASTIC REACTION

RISE OF TEMPERATURE INDUCES A GREATER EXPANSION OF THE MORE EXCITABLE
HALF OF AN ANISOTROPIC ORGAN.

SUMMARY.

Thermonastic movements are induced by the differential physiological
effect of variation of temperature on the two halves of an anisotropic
organ.

Rise of temperature induces greater expansion, and enhancement of rate
of growth of the more excitable half of the organ; lowering of
temperature induces the opposite effect.

Two types of thermonastic movements are met with, the _positive_
exhibiting a movement of opening during rise of temperature; in these
the inner half of the organ is relatively the more excitable. Example of
this is seen in the _Crocus_ and in _Zephyranthes_.

In the _negative_ type, rise of temperature induces a movement of
closure. Here the outer half of the organ is the more excitable. The
flower of _Nymphæa_ belongs to this type.

The effect of stimulus is antagonistic to that of rise of temperature.
In positive thermonastic organs stimulus induces a movement of closure;
in the negative type it induces a movement of opening.

XXIX.--ON PHOTOTROPISM

_By_

SIR J. C. BOSE.

In different organs of plants the stimulus of light induces movements of
an extremely varied character. Radial organs exhibit tropic movements in
which the position of equilibrium is definitely related to the direction
of incident stimulus. Nastic movements under the action of light are, on
the other hand, regarded as curvatures of the organ which show "no
relation to the stimulus but is determined by the activity of the plant
itself".[9] There are thus two classes of response to light which seem
to be unrelated to each other. Returning to the directive action of
light, radial stems often bend towards the light, while certain roots
bend away from it. It may be thought that this difference is due to
specific difference of irritability between shoot and root, the
irritability of the former being of a positive, and of the latter, of a
negative character. But there are numerous exceptions to this
generalisation. Certain roots bend towards the light, while a stem,
under different circumstances, moves towards light or away from it.
Again an identical organ may exhibit a positive or a negative curvature.
Thus the leaflets of _Mimosa pudica_ acted on by light from above fold
upwards, the phototropic effect being _positive_. But the same leaflets
acted on by light from below exhibit a folding upwards, the phototropic
effect being now _negative_. Effects precisely the opposite are found
with the leaflets of _Biophytum_ and _Averrhoa_. They fold downwards
whether light acts from above or below. Finally, a radial organ in
found to exhibit under light of increasing intensity or duration, a
positive, a dia-phototropic, or a negative phototropic curvature.

[9] Jost--_Ibid_, p. 428.

In these circumstances the theory of specific positive and negative
irritabilities is untenable; in any case, it throws no light on the
phenomenon of movement. The difficulties of the problem are thus clearly
stated by Pfeffer: "When we say that an organ curves towards a source of
illumination, because of its heliotropic irritability and we are simply
expressing an ascertained fact in a conveniently abbreviated form,
without explaining why such curvature is possible or how it is
produced.... Many observers have unfortunately devoted their attention
to artificially classifying the phenomenon observed, and have entirely
neglected the explanation of causes underlying them."[10]

[10] Pfeffer--_Ibid_, Vol. II, p. 74.

COMPLEXITY OF PROBLEM OF PHOTOTROPIC REACTION.

The complexity of phototropic reaction arises from the summated effects
of numerous factors; for explanation of the resultant response it is
therefore necessary to take full account of the individual effect of
each of them.

Among these operative factors in phototropic reaction may be
mentioned:--

(1) The difference of effects induced by light at the proximal
and distal sides of the organ.

(2) The modification of the latent period with the intensity of
stimulus.

(3) The after-effect of stimulus.

(4) The modifying influence of tonic condition on response.

(5) The effect of direction of light.

(6) The effect of intensity of light.

(7) The effect of duration of stimulation.

(8) The transmitted effect of light.

(9) The effect of unequal excitability in different zones of
the organ.

(10) The effect of transverse conduction in modification of
the sign of response.

(11) The effect of temperature on phototropic action.

(12) The modification of response due to differential
excitability of the organ.

(13) Nastic and tropic reactions.

(14) The torsional effect of light.

The sketch given above will give us some idea of the complexity of the
problem. In this and in the following papers I shall describe the
investigations I have carried out on the subjects detailed above.

ACTION OF LIGHT.

I have shown that there is no essential difference between the responses
of pulvinated and growing organs, that diminution of turgor induced by
stimulus brings about contraction in the one, and retardation of the
rate of growth in the other. Indirect stimulation, on the other hand,
induces an expansion and acceleration of the rate of growth. The
experimental investigation on the tropic effect of light may therefore
be carried out both with pulvinated and growing organs.

As regards the effect of direct stimulus of light on growing organs we
found (p. 208) that it induces an incipient contraction, seen in
diminution of the rate of growth; this incipient contraction culminates
in an actual contraction under increasing intensity of light. The
contraction under direct stimulation is also observed in pulvinated
organs. When light acts from above the upper half of the pulvinus
undergoes contraction, resulting in erection of the motile leaf or
leaflets. As regards the effect of indirect unilateral stimulus of light
on the distal side of the organ, we found that its effect is an
enhancement of turgor (p. 281). Hence the positive tropic curvature
under light is brought about, as in the case of other forms of stimuli,
by the contraction of the proximal, and expansion of the distal sides of
the organ.

Various analogies have been noticed between phototropic and geotropic
reactions, and it has been supposed that the two phenomena are closely
related to each other. This has even led to assumption that there are
phototropic particles which function like statoliths in geotropic
organs. There is, however, certain outstanding difference between the
two classes of phenomena. In the case of light, the incident energy is
entirely derived from the outside. But in geotropism, the force of
gravity by itself is ineffective without the intervention of the weight
of cell-contents to exert pressure on the sensitive ectoplasm, and thus
induce stimulation. This aspect of the subject will be treated in
greater detail in a subsequent chapter.

POSITIVE PHOTOTROPIC CURVATURE.

I shall now describe the phototropic effect of unilateral light in
pulvinated, and in growing organs. From the explanation that has already
been given, it will be understood that the side of the organ directly
acted on by light undergoes contraction and concavity.

_Tropic curvature of pulvinated organs: Experiment 117._--For this
experiment I employed the terminal leaflet of the bean plant. The
source of illumination was 32 c.p. electric lamp, enclosed in a metallic
tube with circular aperture for passage of light. The leaflet was
attached to an Oscillating Recorder. Light was applied on the upper half
of the pulvinus for 20 seconds; this induced an up-movement of the
leaflet, due to the contraction of the upper half of the organ. Recovery
took place in course of 8 minutes (Fig. 112).

[Illustration: FIG. 112.--Successive positive responses of the terminal
leaflet of bean. Light applied from above for 20 seconds; complete
recovery in 8 minutes.]

POSITIVE PHOTOTROPIC CURVATURE OF GROWING ORGANS.

_Effect of moderate stimulation: Experiment 118._--I shall presently
show that the intensity of phototropic reaction depends on the
intensity and duration of the incident light. A moderate and effective
stimulation may thus be produced by short exposure to strong light. For
my present experiment I took a stem of _Dregea volubilis_, and applied
light from a small arc lamp to one side of the organ for 1 minute; this
induced a positive curvature followed by complete recovery on the
cessation of light (Fig. 113).

[Illustration: FIG. 113.--Positive curvature under moderate phototropic
stimulation. Note complete recovery (_Dregea_).]

[Illustration: FIG. 114.--Persistent positive curvature under stronger
stimulation (_Dregea_).]

_Effect of strong stimulation: Experiment 119._--After recovery of the
stem of the last experiment, the same light was applied for 5 minutes.
It is seen that the curvature is greatly increased (Fig. 114). Thus the
phototropic curvature increases, within limits, with the duration of
stimulation. The curvature induced under stronger stimulation remained
more or less persistent. In certain instances there was a partial
recovery after a considerable length of time; in others curvature was
fixed by growth.

PHENOMENON OF RECOVERY.

On the cessation of stimulus of moderate intensity the heliotropically
curved organ straightens itself; similar effects are also found in other
tropic curvatures. Thus a tendril straightens itself after curvature
induced by contact of short duration. The theory of rectipitality has
been proposed to account for the recovery, which assumes the action of
an unknown regulating power by which the organ is brought back to a
straight line; but beyond the assumption of an unknown specific power,
the theory affords no explanation of the mechanism by which this is
brought about.

The problem before us is to find out the means by which the organ
straightens itself after brief stimulation. It will also be necessary to
find out why there is no recovery after prolonged stimulation. We have
thus to investigate the after-effect of stimulus of various intensities
on growth, and the Balanced Method of recording Growth offers us an
unique opportunity of studying the characteristic after-effects.

IMMEDIATE AND AFTER-EFFECT OF LIGHT ON GROWTH.

As regards the effect of light I have already shown:

(1) that a sub-minimal stimulus induces an acceleration of
growth, but under long continued action the acceleration is
converted into normal retardation (p. 225),

(2) that a stimulus of moderate intensity induces the normal
retardation of the rate of growth.

It is evident that there is a _critical intensity_ of stimulus, above
which there is a retardation, and below which there is the opposite
reaction of acceleration. This critical intensity, I have found to be
low in vigorous specimens, 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 _above par_, and an
acceleration in others, in which it is _below par_. The following
experiments will demonstrate the immediate and after-effect of light of
increasing intensity and duration.

[Illustration: FIG. 115.--Immediate and after-effect of stimulus of
light on growth. (_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. (_b_) Immediate and after-effect of
stronger light: immediate effect, a retardation; after-effect, recovery
to normal rate without acceleration.]

_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 or further away from the plant. Two inclined mirrors were
placed behind the plant so that the specimen was acted on by light from
all sides. A seedling of wheat was mounted on the Balanced Crescograph,
and record was first taken under exact balance; this gives a horizontal
record. The up-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 phototropic effect is proportional 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 opposite direction, the resulting down-curve
showing an acceleration of the rate of growth above the normal. This
acceleration persisted for a time, after which the normal rate of growth
was restored, as seen in the curve becoming once more horizontal. _The
after-effect of light of moderate intensity is thus a temporary
acceleration of rate of growth above the normal._

_Effect of strong light: Experiment 121._--The same specimen was used as
in the last experiment. By bringing the source of light to a distance of
25 cm. the intensity 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 acceleration. 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 stimulation. On the cessation of
light, the after-effect showed no enhancement of rate of growth, the
recovery from retardation to the normal rate being gradual. In the next
experiment (the result of which is not given in the record) the
intensity of light was increased still further; the retardation now
became very marked, and it persisted for a long time even on the
cessation of light.

We thus find that:

(1) The immediate effect of light of moderate intensity is a
preliminary acceleration, followed by normal retardation. The
acceleration is the effect of sub-minimal stimulation. The
immediate after-effect is an acceleration above the normal.

(2) The immediate effect 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 complete recovery
after moderate stimulation, and also the absence of recovery after
strong stimulation. The immediate after-effect of moderate stimulation
is shown to be an acceleration of rate above the normal. Returning to
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). One is the 'up' or A-change, associated with
increase of potential energy of the system, and the other is associated
with 'down' or D-change, by which there is a run-down or depletion of
energy. With moderate stimulation 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-change, finds expression
in enhancement of the rate of growth. The depletion of energy under
strong stimulation is, however, too great to be compensated by the
A-change.

LATENT PERIOD OF PHOTOTROPIC REACTION.

With reference to the latent period Jost thus summarizes the known
results:[11] "The latent period of the heliotropic 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 hypocotyl of
_Helianthus_, and 50 minutes in the epicotyl of _Phaseolus_. If one of
these organs be unilaterally illuminated 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."

[11] Jost--_Ibid_, p. 473.

With regard to the question of relation of the latent period to the
intensity of stimulus I have shown (p. 166) 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 reduced
under 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.

[Illustration: FIG. 116.--Latent period for photic stimulation at
vertical line. Successive dots at intervals of 2 seconds. (_Erythrina
indica_).]

_Determination of the latent period: 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 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 experiment described above, the latent period is to be
understood in this sense. But in the extract given above, Jost uses the
term 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 retardation of growth? For this
investigation I employed the very sensitive method of the Balanced
Crescograph.

[Illustration: FIG. 117.--Effect of a single electric 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.]

GROWTH-VARIATION BY FLASH OF LIGHT FROM A SINGLE SPARK.

_Experiment 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 1/100,000th 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 in the first part of the record. The spark gap was placed at a
distance of 10 cm. from the plant; there was the usual arrangement of
inclined mirrors for illumination 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-effect 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 maximum, (see below Fig. 132) beyond
which there is no further change. This is due to absence of transverse
conductivity in the organ. The modifying effect of transverse
conductivity 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.

XXX.--DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA 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 negative heliotropism"[12]. No explanation has
however been offered as to why the same organ should exhibit at
different times, a positive, a neutral, and a negative irritability.
These changing effects exhibited by an identical organ is thus
incompatible with the theory of specific sensibility, assumed in
explanation of characteristic differences in phototropic response.

[12] Jost--_Ibid_--p. 462.

In regard to this I would draw attention to an important factor which
modifies the tropic response, namely, the effect of transverse
conduction of excitation. I shall presently describe in detail a typical
experiment of the effect of unilateral stimulus of light on the
responsive movement of main pulvinus of _Mimosa pudica_. 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 _Mimosa_.

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 pulvinus 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.

_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 pulvinus is
shown by the fact that the response of the lower half of the pulvinus to
ten seconds' exposure is even larger than that given by the upper half
under the prolonged exposure of 240 seconds.

[Illustration: FIG. 118.--Series of up-responses of _Mimosa_ leaf to
light applied on upper half of pulvinus.]

[Illustration: FIG. 119.--Down-responses given by the same plant on
application of light from below.]

TRANSFORMATION OF POSITIVE TO NEGATIVE PHOTOTROPIC CURVATURE.

_Experiment 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
expansion 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_ curvature. The
arrival of transmitted excitation 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 pulvinus was 1·5 mm., 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 of excitation under strong light was
found to vary in different cases from 50 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
0·010 mm. per second in the stem (p. 282).

[Illustration: 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 transverse conduction of excitation. Up-movement
shown by down curve, and _vice versâ_.]

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.

(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
curvature also depends on the pliability of the organ.

(3) In anisotropic organs where the distal side is
physiologically the more excitable than the proximal, 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 curvature 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. But 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 sides is
reduced in amount or vanishes altogether. Owing to the weak transverse
conductivity of the tissue, while the effect of a feeble stimulus
remains localised, that of a stronger stimulus is conducted across it.

Oltmanns found that the seedling of _Lepidium sativum_ assumed a
transverse or dia-phototropic position under intense and long continued
action of light of 600,000 Hefner lamps. He regards this as the
indifferent position. But the neutralisation of curvature is not,
as explained before, due to a condition of indifference, but to the
antagonistic 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 volubilis_ under the prolonged unilateral action of arc-light.
The first effect was positive; this was gradually and continuously
neutralised under exposure for two hours; even then the neutralisation
was not complete. I shall presently adduce instances where the
neutralisation was not merely complete, but the final effect was an
actual reversal 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 forthcoming, that "direct sunlight is too
bright to bring about heliotropic curvature, only diffuse, not direct
sunlight has the power of inducing heliotropic movements."[13] But we
cannot conceive of light suddenly losing its phototropic effect 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 rise 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
the other hand, that the first effect of strong light is a positive
curvature, and that under its continuous action the positive effect
becomes neutralised (cf. Fig. 121). In the study of phototropic action,
the employment of strong light has many advantages, since the period of
experiment is, by this means, materially shortened. The continuous
record then gives an epitome of the various phases of reaction.

[13] Jost--_Ibid_--p. 464.

NEGATIVE PHOTOTROPISM.

I shall next show the continuity of responsive phototropic effects, from
the positive curvature to the negative, through the intermediate phase
of neutralisation. I have in the preceding paragraph described an
experiment where under a given intensity and duration of exposure the
excitations of the proximal and distal sides bring about neutralisation,
the organ assuming a dia-phototropic position. If the intensity or
duration of the stimulating light be further increased, it is easy to
see that while excitation transmitted to the distal side is being
increased, the excitatory contraction on the proximal side may, at the
same time, be decreased owing to fatigue brought on by over-stimulation.

In connection with this it should be borne in mind that the pulvinus of
_Mimosa_ exhibits under continuous stimulation, a fatigue relaxation
instead of normal contraction. Similar effects are known to take place
in animal muscles. The effect of relatively greater excitation will thus
give rise to negative phototropic curvature. The transverse conductivity
of organs of diverse plants will necessarily be different. The
neutralisation and reversal into negative will thus depend on three
factors: the transverse conductivity of the organ, the intensity, and
duration of stimulus.

_Neutralisation and reversal under increased intensity of light:
Experiment 127._--It is advisable to employ thin specimens (in which the
transverse distance is small) for the exhibition of reversal effect. I
took a hypocotyl of _Sinapis nigra_ and subjected it to unilateral
action of light from a 16 candle-power incandescent electric lamp placed
at a distance of 10 cm. A maximum positive curvature was induced in the
course of 50 minutes. The intensity of light was afterwards increased by
bringing the lamp nearer to a distance of 6 cm. This resulted in a
process of neutralisation of the preceding response; after an exposure
of 70 minutes the specimen assumed a dia-phototropic position in which
it remained in equilibrium. Sunlight was next applied, and in the
further course of 30 minutes there was a pronounced reversal into
negative phototropic curvature.

[Illustration: FIG. 121.--Positive and negative phototropic responses of
_Oryza_ under continued unilateral stimulus of intense light from arc
lamp.]

_Neutralisation and reversal under continuous stimulation: Experiment
128._--In the last experiment the different changes in the response were
brought about by successive increase in the intensity of light. In the
present experiment, very strong light was applied from the beginning,
and continuous record was taken of the change in the response. In order
to reduce the period of experiment I employed a mercury vapour lamp
which emits the most effective violet and ultra-violet rays. The
specimen used was a seedling of the rice plant (_Oryza sativa_). The
first effect of light was a positive curvature which attained its
maximum; after this there was a neutralisation in less than six minutes
after the application of light. The further continuation of light
induced a pronounced negative curvature (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 organ, under different conditions of intensity
and duration of stimulus, exhibits _positive_ phototropic,
_dia_-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 excitatory reaction, remaining either localised or
increasingly transmitted to the distal side.

NEGATIVE PHOTOTROPISM OF 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 negative
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 the tip, the stimulus of light is not
necessarily restricted in the area of its action. That there is no true
analogy between 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 _Sinapis_, it
will be shown (p. 376) to be brought about by algebraical summation of
the effects of direct and indirect photic stimulus.

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 incident 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 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 neutralised 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 _positive_-phototropic, the _dia_-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,

_Assisted by_

SURENDRA CHANDRA DAS, M.A.

I shall in this chapter describe experiments in support of the important
proposition that _the intensity 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 hold good for the
median range of stimulation; the deviation from this proportionality at
the two ends of the range of stimulation--the sub-minimal and
supramaximal--is, as we shall find, capable of explanation, and will be
fully dealt with in the next chapter.

The quantity of light incident on the responding organ depends: (1) on
the intensity of light, (2) on the angle of inclination or _the
directive angle_,[14] and (3) on the duration of exposure. I shall give
a detailed account of the investigation relating to the individual
effects of each of these factors on the tropic reactions not merely in
pulvinated but also in growing organs.

[14] The directive angle [Greek: th] is the angle of inclination of
the rays of light to the responding surface. The angle
[Greek: th] is complementary to the angle of incidence _i_ in
optics. Sin [Greek: th] = Cos i.

EFFECT OF INCREASING INTENSITY OF LIGHT ON TROPIC CURVATURE.

The intensity of light was increased in successive experiments, in
arithmetical progression 1:2:3 by suitably diminishing the distance
between the plant and the source of light, and the resulting tropic
curvatures recorded.

[Illustration: FIG. 122.--Leaf of _Desmodium gyrans_, with the terminal
large, and two lateral small leaflets. These latter exhibit automatic
pulsations.]

_Effect of increasing intensity of light on the pulvinus of_ Desmodium
gyrans: _Experiment 129._--The source of light was a 50 candle-power
incandescent lamp, and the duration of exposure was 1 minute. The
specimen employed was a terminal leaflet of _Desmodium gyrans_ (Fig.
122) the pulvinus of which is very sensitive to light. It is more
convenient to manipulate a cut specimen of the leaf, instead of the
whole plant. The petiole is placed in water contained in a U-tube; the
depressing effect of wound passes off in the course of an hour or so.
Light of increasing intensity is applied from above; this induces a
contraction of the upper half of the pulvinus, and the resulting
response is recorded by means of the Oscillating Recorder (Fig. 123).

[Illustration: FIG. 123.--The Oscillating Recorder (From a photograph).]

The first record was obtained under a given intensity, and the second,
under an intensity twice as great. The tropic effects are seen to
increase with the intensity (Fig. 124). If the tropic curvature
increased proportionately to the intensity, the two responses should
have been in the ratio of 1:2; the actual ratio was however slightly
greater, _viz._ 1:2·6. In this connection it will be shown in the next
chapter, that strict proportionality holds good only in the median
range, and that the susceptibility for excitation undergoes an increase
at the beginning of the phototropic curve.

[Illustration: FIG. 124.--Tropic effect of increasing intensity of
light 1:2; on the response of terminal leaflet of _Desmodium gyrans_.]

[Illustration: FIG. 125.--Tropic effect of increasing intensity of
light 1:2:3 on growing organ (_Crinum_).]

_Effect of increasing intensity of light on the tropic curvature of
growing organs._--As the tropic curvature is primarily due to the
retardation of growth induced by light at the proximal side of the
organ, it will be of interest to recapitulate the results I obtained (p.
208) on the effects of increasing intensity of light on growth itself.
The normal rate of growth of the specimen in the dark was 0·47 µ per
second; this was reduced to 0·29 µ under an intensity of one unit, to
0·17 µ under two, and to 0·10 µ under three units. Growth became
arrested when the intensity was raised to four units. Thus increasing
intensity of light induces an increasing retardation of growth at the
proximal side of the organ. This aided by the effect of indirect
stimulus at the distal side brings about an increasing positive
curvature.

_Experiment 130._--The flower bud of _Crinum_ was used for the
experiment, the source of light being a small arc lamp. The duration of
exposure was one minute. Increasing intensity of light gave rise to
increasing positive curvatures (Fig. 125) in the ratio of 1:2·5:5 under
increasing intensities which varied as 1:2:3.

THE EFFECT OF INCREASING ANGLE.

The quantity of light which falls on an unit area of the responding
organ varies as sin [Greek: th] where [Greek: th] is the directive angle
_i.e._ the angle made by the rays with the surface. Some allowance has
to be made for the amount of light reflected from the surface, this
being greater at 45° than at 90°.

[Illustration: FIG. 126.--The Collimator.]

_Tropic response of pulvinus of_ Desmodium gyrans: _Experiment
131._--For application of light at various angles an incandescent
electric lamp was mounted at one end of a brass tube, a collimating
lens being placed at the other (Fig. 126). The parallel beam of light
from the collimator could be sent at various angles by rotating the
collimator tube round an axis at right angles to the tube. The specimen
employed was the terminal leaflet of _Desmodium gyrans_; light 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
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
sin 90°/sin 45° = 1·4.

[Illustration: FIG. 127.--Effect 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°. (_Desmodium gyrans_).]

[Illustration: FIG. 128.--Series of tropic curvatures of growing bud of
_Crinum_ to alternate stimulation by light at 45° and 90°.]

_Tropic response of growing organs: Experiment 132._--Similar experiment
was carried out with the flower bud of _Crinum_, 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 experiment. 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° enable us to find out, whether any variation of excitability
occurred during the course of the experiment and make allowance for it.
The records show that stimulation 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 second 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 133._--The specimen employed for the experiment was a flower
bud of _Crinum_ in a slightly sub-tonic condition. Successive responses
exhibited on this account, a preliminary negative[15] before the normal
positive curvature. The successive durations of exposure were for 1, 2,
and 3 minutes. The amplitudes of responses (Fig. 129) are in the ratio
of 1:2·5:5.

[15] An explanation of this preliminary effect will be found in the
next chapter.

[Illustration: FIG. 129.--Effect of increasing duration of exposure
1:2:3 minutes, on phototropic curvature. Note preliminary negative
response. (_Crinum_).]

We may now recapitulate the tropic effects of light of increasing
intensity, directive angle, and duration of exposure. It has been shown
that the tropic effect is enhanced under increasing intensity of light;
it is also increased with the angle increasing from grazing to
perpendicular 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 conclusion that _phototropic effect
increases with the quantity of incident light_. It will be shown in the
next chapter that strict proportionality of cause and effect holds good
in the median range of stimulation, and the slight deviation 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.

Tropic curvature increases with the directive angle, the effect being
approximately proportional to sin [Greek: th], where [Greek: th] is the
angle made by the rays with the responding surface.

Tropic curvature also increases with the duration of exposure.

The intensity of induced tropic effect is determined by the quantity of
incident light.

XXXII.--THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS

_By_

SIR J. C. BOSE.

When a plant organ is subjected to the continued action of unilateral
stimulus of light, it exhibits increasing tropic curvature, which in
certain cases reaches a limit; in other instances 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 terminology 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 distinguish between cause and effect, between external
stimulus and the excitation induced by it. As regards stimulus itself I
have shown elsewhere[16] that its effective intensity becomes summated
by repetition. This was demonstrated by the two following typical
experiments carried out with the pulvinus of _Mimosa_.

[16] "Irritability of Plants"--p. 54.

(1) The intensity of a single electric shock of intensity of 0·5 unit
was found to be ineffective in inducing 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 repetition of 20 times for the
stimulus to become effective.

The total stimulus in the first case was 0·5 × 4 = 2, and this was found
to be the same as 0·1 × 20 = 2 in the second case. Thus the intensity of
stimulus is increased by repetition; in the limiting case where the
interval between successive stimulus is zero, the stimulus becomes
continuous. Bearing in mind the additive effects of stimulus we see that
its effective intensity increases with the _duration_ of application.
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 characteristics of the
phototropic curve, which gives the relation between increasing stimulus
and the resulting excitation. As regards stimulus we found that its
effectiveness 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
obtained 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 of the 'excitation curve' (Fig. 130) exhibits
increasing incipient contraction (retardation of growth) culminating in
an arrest of growth; the abscissa represents increasing stimulus
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 excitatory 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 curves 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.

[Illustration: FIG. 130.--Effects of continuous (_a_) electric, and
(_b_) photic stimulation on rate of growth. Abscissa represents duration
of application of stimulus. Note induced retardation, and arrest of
growth.]

For facility of explanation of what follows, I shall have to use a new
and necessary term, _susceptibility_, to indicate the relation of cause
and effect, of stimulus and resulting excitation. _Susceptibility_ is
thus = Excitation/Stimulus. Different organs of plants exhibit unequal
susceptibilities; 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[17]; 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 distortion in inorganic matter under
increasing intensities of an external force.

[17] I shall use the term _stimulus_ in preference to _stimulation_,
for the latter is often taken in the sense of the resulting
excitation.

THE CHARACTERISTIC CURVE.

Thus the molecular upset and rearrangement, in a magnetic substance
under increasing magnetising force are inferred from the curve obtained
by means of appropriate magnetometric or galvanometric 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 _susceptibility_ 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 determined 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
characteristic excitation curve, by making the plant organ record its
tropic movement under continuous action of light applied on one side of
the organ.

[Illustration: FIG. 131.--Characteristic curve of iron under increasing
magnetising force. (After Ewing).]

[Illustration: FIG. 132.--Simple characteristic curve of phototropic
reaction. Excitation increases slowly in the first part and rapidly in
the second; it is uniform in the third, and undergoes decline in the
fourth part (_Erythrina indica_).]

_Experiment 134._--I give below the characteristic curve of excitation
(Fig. 132) of the pulvinus of _Erythrina indica_, traced by the plant
itself, and exactly reproduced by photomechanical process. A parallel
beam of light from a Nernst 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 successive dots represent the corresponding
increments of excitation. The gradient at any point of the
curve--increment of excitation divided by increment of stimulus--gives
the susceptibility for excitation at that point. The following 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 TROPIC CURVE.

+--------------------------------------+
| Successive points | Susceptibility |
| in the curve. | for excitation. |
+-------------------+------------------+
| 1 ... ... | 0 |
| 2 ... ... | 0·187 |
| 3 ... ... | 0·44 |
| 4 ... ... | 0·625 |
| 5 ... ... | 0·875 |
| 6 ... ... | 1·25 |
| 7 ... ... | 1·87 |
| 8 ... ... | 3·12 |
| 9 ... ... | 5·0 |
| 10 ... ... | 6·25 |
| 11 ... ... | 8·75 |
| 12 ... ... | 8·87 |
| 13 ... ... | 8·12 |
| 14 ... ... | 6·6 |
| 15 ... ... | 4·4 |
| 16 ... ... | 2·5 |
| 17 ... ... | 1·87 |
| 18 ... ... | 1·5 |
| 19 ... ... | 1·12 |
| 20 ... ... | 0·937 |
| 21 ... ... | 0·75 |
| 22 ... ... | 0·562 |
| 23 ... ... | 0·375 |
| 24 ... ... | 0·25 |
| 25 ... ... | 0·187 |
| 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 vary 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 susceptibility for excitation then falls
rapidly, and increase of stimulus induces no further increase of tropic
curvature. The maximum 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 curve for electric
stimulus (Fig. 130a), and will also be met with in the curve for
geotropic stimulus (Fig. 161).

I may here refer incidentally to the three types of responses exhibited
by an organ to successive stimuli of uniform intensity; these appear to
correspond to the three different 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 last 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 conduction, 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 description of
effective stimulus should be so very indefinite. A stimulus which is
just sufficient to evoke excitatory _contraction_ is termed the minimal,
stimulus below the threshold being tacitly regarded as ineffective. The
employment of sensitive recorders has, however, enabled me to 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, but 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 responsive contraction.

The _critical_ stimulus varies in different 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 hand, 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 distinguished as the "A" and "D"
change (pp. 143, 223). The assimilatory 'building up', A change, is
associated with an increase of potential energy of the system; the
dissimilatory 'break down', D change, on the other hand, is attended 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 "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 detection 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 retardation, 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 contraction 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 two opposite reactions succeed each other too quickly 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 conflicting
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.

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, exhibit under continued stimulation a periodic up and
down-movement, which persists even on the cessation of the stimulus. The
persistence of after-oscillation depends, moreover, on the intensity and
duration of previous stimulation.[18]

[18] "Plant Response"--p. 293, etc.

The facts given above cannot be explained by the prevalent theory that
stimulus acts merely as a releasing 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 responding 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 phototropic curve in
which the tropic curvature, on account of the favourable tonic condition
and strong intensity of 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 incident 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
negative tropic curvature; continued action of stimulus, however,
converts the preliminary negative into the usual positive. The
preliminary negative curvature may be detected by the use of a
moderately sensitive recorder with a magnification 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 stimulation, 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 difficult 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 pulvinated organ: Experiment 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 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.

[Illustration: 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 response due to
sub-minimal stimulus. The positive increases, attains a maximum, and
undergoes a reversal. Successive dots at intervals of 30 seconds.
Abscissa represents duration of exposure and quantity of incident light.
(Terminal leaflet of _Desmodium gyrans_.)]

_Complete phototropic curve of growing organs: Experiment 136._--I
obtained very similar effects by subjecting the seedling of _Zea Mays_
to unilateral light from an arc lamp for two hours. The characteristic
of this curve is similar to that given by the terminal leaflet of
_Desmodium gyrans_. At the first stage, the sub-minimal stimulation is
seen to induce a negative curvature, transformed into positive after an
interval of 10 minutes. The maximum positive curvature is reached after
50 minutes, and neutralisation completed in a further period of 43
minutes (Fig. 134). After this the response became transformed into
negative.

[Illustration: FIG. 134.--Complete phototropic curve of a growing organ
(_Zea Mays_).]

In a complete phototropic curve we may thus distinguish 4 distinct
stages:--

(1) The stage of sub-minimal stimulation.

(2) The stage of increasing positive curvature culminating in
a maximum.

(3) The stage of neutralisation.

(4) The stage of complete reversal into negative.

The curve thus crosses the zero line of the abscissa twice; the first
crossing takes places _upwards_ at the critical point of stimulation
which demarcates the sub-minimal from the minimal. The second crossing
downwards occurs beyond the point of complete neutralisation.

In a tissue in which transverse conductivity is absent, and the stimulus
applied from the beginning is above the minimal, the simple tropic
curve is confined to the second stage (see Fig. 132).

WEBER'S LAW.

If we neglect the preliminary negative portion under sub-minimal
stimulus, the curve of excitation under increasing photic stimulation
obeys what is known as Weber's law. This is equally true of modes of
stimulation other than that of light as is seen in figure 130 of the
contractile effect of continued electric stimulus on growth; the
excitatory effect is also seen to reach a limit.

Weber's law is applicable for a limited range of stimulation. For the
quantitative relation fails in the region of sub-minimal stimulus, where
the physiological reaction is _qualitatively_ different, namely
expansion instead of contraction. This holds good even in the case of
animal tissues, for here also my recent experiments show that two
opposite reactions--expansion and contraction--take place under
stimulus, and that a very feeble stimulus tends to induce expansion
instead of contraction. The responsive reaction of a kitten under gentle
caressing strokes must be _qualitatively_ different from that of a blow.
The psychological effects under the two treatments evidently differ
qualitatively rather than quantitatively.[19]

[19] "It has been argued by James that the feeling does not cause,
but is caused by the bodily expression.... Münsterberg
concludes that the feeling of agreeableness is the mental
accompaniment and outcome of reflexly produced movements of
extension, and disagreeableness of the movement of flexion."
Schäfer--Text Book of Physiology, Vol. II, p. 975 (1900).

SUMMARY.

The excitation curve exhibits a slow ascent in the first part; in the
second part the gradient is steep, indicating rapid rise in excitation;
in the third part it is uniform; and in the last part the curve rounds
off and the rate of ascent becomes very small.

The susceptibility for excitation is feeble at the beginning; it
increases very rapidly with increasing stimulus; finally it undergoes a
fall, increase of stimulus inducing no further enhancement of
excitation.

In a complete phototropic curve the first part is negative; this is due
to the physiological expansion induced by sub-minimal stimulus. The
curve then crosses the abscissa upwards, and the positive curvature
reaches a maximum. This is followed by neutralisation and reversal into
negative; the curve crosses the zero line and proceeds in the negative
direction.

Weber's law is not applicable for the entire range of stimulation. The
quantitative relation fails in the region of sub-minimal stimulus, where
the physiological reaction is _qualitatively_ different.

XXXIII.--THE TRANSMITTED EFFECT OF PHOTIC STIMULATION

_By_

SIR J. C. BOSE,

_Assisted by_

JYOTIPRAKASH SIRCAR, M.B.

Plant organs exhibit, as we have already seen, a heliotropic curvature
under direct stimulation. Still more interesting is the transmitted
effect of light giving rise to a curvature. Thus if the tip of the
seedling of wheat be exposed to light, the excitation is transmitted
lower down into the region which acts as the responding organ. Growth is
very active in this particular zone, and the change of growth, induced
by the transmitted effect of stimulus, brings about a curvature by which
the tip of the seedling bends towards light. The seedling thus appears
to be differentiated into three physiological zones subserving three
different functions. The tip is the perceptive zone, the intervening
distance between the tip and the growing region is the zone of
conduction, and the growing region is the responsive zone. These
differentiations are shown in a striking manner by certain Paniceae,
_Setaria_ for example. In this seedling the tapering sheathing leaf or
cotyledon is about 5 mm. in length, and it is the upper part of the
cotyledon that is most sensitive to light. Below the sheathing leaf is a
narrow length which will be distinguished as the hypocotyl, and where
growth is very active. The apex of the leaf perceives the stimulus, and
the effect is transmitted to the hypocotyl, which responds by becoming
curved so that the seedling bends towards light.

It is necessary here to make special reference to the confusion that
arises from want of precision in the use of the term stimulus, used
indifferently to denote both the cause and the resulting effect. An
external agent, say light, causes certain excitatory change in the
tissue, and we refer to the agent which induces it, as the _stimulus_.
Thus in the instance cited above, light is the _stimulus_, and it is the
stimulus-effect that is transmitted to a distance. But in physiological
literature no distinction is made between the stimulus and its effect,
hence arises frequent use of the phrase 'transmission of stimulus'. It
is obvious that it is not light but its effect that is transmitted.

Such want of precision in the use of the term stimulus would not have
seriously affected the truth about the description of facts, had the
transmitted effect been only of one kind. In a nerve-and-muscle
preparation, the velocity of transmission of excitation is so great,
that it completely masks the positive impulse (assuming the existence of
such an impulse). The effect of indirect stimulation is, therefore, the
same as that of direct stimulation. Any indefiniteness in the use of the
term stimulus for its transmitted effect does not, in animal physiology,
seriously militate against the observed facts. But lack of precision in
the employment of the term in plant physiology leads to hopeless
confusion. For owing to the semi-conducting nature of vegetable tissue,
the transmitted effect is not of a definite sign, but may be positive or
negative; in the first case, the response is by expansion, in the
latter, by contraction. Thus the transmitted effect will be very
different in the two cases, according as the intervening tissue is a
good or a bad conductor. These facts accentuate the urgent necessity of
revision of our existing terminology.

I have shown that the effects of other forms of stimuli are also
transmitted from the perceptive to the responding region along the
intervening path of conduction. Thus the petiole of _Mimosa_ perceive
any form of stimulus applied to it, and the induced excitation is
conducted to the distant pulvinus to evoke the familiar responsive fall
of the leaf. The pulvinus, moreover, perceives and responds to direct
stimulation. In a nerve-and-muscle preparation the responding muscle is
alike perceptive and responsive.

But in _Setaria_ we meet with certain characteristics of reaction which
are quite inexplicable. Thus if

"the seedling be illuminated on one side, a sharp heliotropic curving
takes place at the apex of hypocotyl. The curvature makes itself
apparent only if the cotyledon be illuminated from one side whether the
hypocotyl be exposed to light or not. If the cotyledon be shaded and the
light be permitted to fall on one side of the hypocotyl, no heliotropic
curving takes place. Hence we may conclude that it is only the cotyledon
that is sensitive to the light stimulus, and it is only the hypocotyl
which can carry out the movement. The excitation which the light effects
in the cotyledon must be transmitted to the hypocotyl and curvature
takes place only from such a transmitted excitation. We have thus in
this case a definite organ for the perception of the stimulus of light,
viz., the cotyledon, and as Rothert has shown, it is more specially the
apex of that organ that is the sensitive part: on the other hand, the
motile organ, the hypocotyl, is some distance away from the sensitive
organ, and in it the power of perception is entirely absent. From the
behaviour of these organs we may draw the further conclusion that
perception and heliotropic excitation are two distinct phenomena, which
depend on different properties of the protoplasm and which are
independent of each other.... We may, therefore, conclude from this
experiment that these two types of excitation are fundamentally distinct
processes, for it is only after indirect or transmitted and not after
direct excitation that a reaction occurs in the case of the seedlings of
the Paniceae".[20]

[20] Jost--_Ibid_--p. 468.

The noteworthy deductions on the above facts are:--

(1) That the motile organ in _Setaria_ is totally devoid of
perception, since direct action of light induces no effect.

(2) That perception and heliotropic excitation are two
distinct phenomena, which depend on different properties of
the protoplasm, and which are independent of each other.

Though the conclusions thus arrived at appear to follow from the facts
that have been observed, yet it is difficult to accept the inference,
that a responding organ should be totally devoid of the power of
perception, and that excitation and perception are to be regarded as
dependent on different properties of protoplasm. It therefore appeared
necessary to re-investigate the subject of the perceptive power of the
cotyledon, and the responding characteristics of the hypocotyl.

The criterion employed for test of perception is the movement induced in
response to stimulus. The responsive _mechanical_ movement is rendered
possible only by the contractility of the organ, and mechanical and
anatomical facilities offered by it for unhampered movement. The
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 transmitted 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
investigations have shown that every living tissue not only perceives
but also responds to stimulation.[21] Hence considerable doubt may be
entertained as regards the supposed absence of perception in the
hypocotyl of _Setaria_.

[21] "Response in the Living and Non-Living"--p. 17.

I 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 stimulation.

(3) Summated effects of direct and indirect stimulation.

EXPERIMENTAL ARRANGEMENTS.

_The Recorder._--The pull exerted by the tropic curvature of the
seedling is very feeble; it was therefore necessary to construct a very
light and nearly balanced 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 recording
plate oscillates to and fro once in a minute; the successive dots give
therefore the time relations of the responsive movement. The positive
curvature towards light is recorded as an up-curve, the negative
curvature being represented by a down-curve.

[Illustration: FIG. 135.--Arrangement for local application of light to
the tip and the growing region. O, O', apertures 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
pivot and worked by string, T.]

_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 135 is free from this objection and offers
many advantages. A metallic screen has two holes O and O'; these
apertures are illuminated by a parallel beam of light from an arc lamp.
A lens focusses the light from O, on the hypocotyl, and that from O',
on the tip of the cotyledon. A rectangular pivoted shutter S, lies
between the apertures O and O'. 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 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.

[Illustration: FIG. 136.--Response of seedling of _Setaria_ to
unilateral stimulation of the tip applied at dotted arrow.

Note preliminary negative curvature reversed later into positive.]

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 about 30 seconds and
the negative movement continued to increase for 25 minutes (Fig. 136).
This result, hitherto 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 transmitted along
the parenchyma which is moderately conducting; 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 reach the growing region,
and by its predominant reaction neutralise and 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 excitatory 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 excitation is not neutralised by
transverse conduction (see below).

[Illustration: FIG. 137.--Effect of application of light to the growing
hypocotyl at arrow induced positive phototropic 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_).]

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 experimental 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 obtained 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 which I
shall presently describe will, however, offer an explanation of the
prevailing error. On continuing the action of unilateral light, the
positive curvature after attaining a maximum in the course of 15
minutes, underwent 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. 136) which
continues for several hours. The explanation of this neutralisation
under direct stimulation of the growing region is found in the fact that
transverse conduction of excitation induces contraction at the distal
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 neutralisation, 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 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 _Passiflora_ (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 curvature, 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. I 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 longitudinally transmitted effect of
indirect stimulus in _Setaria_ may, likewise, be summated with the
effect of direct stimulus. 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.

_Experiment 139._--I have explained, (Expt. 126) that unilateral
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 transmitted
excitation only suffices to induce neutralisation 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).

[Illustration: FIG. 138.--(_a_) Diagrammatic representation of direct
application of light (v) on the pulvinus and the indirect application on
the stem (-->) (_b_) Record of effect of direct stimulus, positive
curvature followed by neutralisation. Superposition of the positive
reaction of indirect stimulus induces erectile up-response followed by
down movement due to transmitted excitatory impulse (_Mimosa_).]

Two different impulses are thus initiated from the effect of indirect
stimulus. In the present case the positive 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 understanding of the
characteristic response of Paniceae to transmitted phototropic
excitation.

(1) Local stimulation of the tip gives rise to two impulses, 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 antagonistic reactions of two opposite sides of the organ.

(3) The effects of direct and indirect stimulations are independent of
each other; hence, on simultaneous stimulations 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 convexity and movement away from
stimulus (negative curvature). This is diagrammatically shown in Fig.
139. 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).

[Illustration: FIG. 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 following
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.

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 phototropism of the root of
_Sinapis_.

_Negative phototropism of root of_ Sinapis: _Experiment 140._--For
investigation of the negative phototropism of the root of _Sinapis
nigra_ I took record of its movement under unilateral action of light by
means of a Recording Microscope, devised for the purpose.[22] 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 growing region at some distance from the tip.
The intervening distance between the tip and the growing region is
practically 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 resultant responsive movement was induced which was away from
light.

[22] "Plant Response"--p. 604.

Thus in the root of _Sinapis_, the expansive effect of indirect
stimulation of the tip is superposed on that of direct stimulation of
the growing region (neutral or slightly 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 preliminary
negative curvature, which is reversed later by the excitatory negative
impulse into positive curvature.

Contrary to generally accepted view the hypocotyl not only perceives but
responds to light. The positive curvature induced by direct stimulation
is, however, neutralised by transverse conduction of excitation.

The effects of direct and indirect stimulus are independent of each
other; the final effect is determined by their algebraical summation.

XXXIV.--ON PHOTONASTIC CURVATURES

_By_

SIR J. C. BOSE,

_Assisted by_

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 anatomico-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 movement will further
be shown to be distorted by the lateral action of light.

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 therefore 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. Subjected thus to diffuse 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 respectively (Fig. 140). The fundamental action by which
all these were determined was the induced concavity of the under or
normally shaded side, which was the more excitable. I obtained similar
results with various other creeping stems.

[Illustration: FIG. 140.--Photonastic curvature of creeping stem of
_Mimosa pudica_: in the central figure the stem is seen to be vertical:
action of diffuse light induced appropriate curvatures by greater
contraction and concavity of the more excitable lower or shaded side, as
seen in figures to the right (_b_) 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 from above, exhibit concavity of the shaded and more excitable
side.

PARA-HELIOTROPISM.

Under strong sunlight, the leaflets of various plants move sometimes
upwards, at other times downwards, so as to place the blades of leaflets
parallel to incident light. This 'midday sleep' has been termed
_para-heliotropism_ by Darwin. It has been 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 _positively_ para-heliotropic, and
those which fold downwards as _negatively_ para-heliotropic. This is
merely for convenience of description. There is no specific irritability
which distinguishes one from the other.

POSITIVE PARA-HELIOTROPISM.

_Para-heliotropic response of_ Erythrina indica _and of_ Clitoria
ternatea: _Experiment 142._--For the purpose of simplicity 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 curvature 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 was no neutralisation or reversal,
demonstrating the absence of transverse conduction (_cf._ Fig. 132).

[Illustration: FIG. 141.--Positive para-heliotropic response of leaflets
of _Erythrina indica_.]

_Para-heliotropic movement of leaflets of_ Mimosa pudica: _Experiment
143._--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 immediate 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 negative 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 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 side of the organ.

[Illustration: FIG. 142.--Response of leaflet of _Mimosa_ to light
applied below: transient positive followed by pronounced negative
curvature.]

[Illustration: 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 diffuse 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 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 excitable lower side. Illumination from below
gave rise only to strong positive response. Thus in _Averrhoa_ 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 fact, that in _Mimosa_ leaflet it is the upper
half of the pulvinule that is more excitable; while in _Averrhoa_ and in
_Biophytum_ the lower is the more excitable half of the organ.

[Illustration: FIG. 144.--Diagrammatic representation of different types
of phototropic response. (See text.)]

As a summary of the tropic action of light I shall give diagrammatic
representations of various types of phototropic response, including the
photonastic (Fig. 144). The direction of the arrow indicates the
direction of incident light. Dotted specimens are those which possess
transverse conductivity. Thick lines represent the more excitable side
of an anisotropic or dorsiventral organ. The size of the circles, with
positive and negative signs, represents the amplitude and sign of
curvature.

_a._ Radial thick organ, in which transverse conduction is
absent. Curvature is _positive_, _i.e._, movement towards
light. The result will be similar when light strikes in an
opposite direction, _i.e._, from right to left.

_b._ 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 excitable 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
conductivity. Sequence of curvature: transient positive,
quickly masked by predominant negative. Light striking on the
more excitable side will give rise only to _positive_. The
response in relation 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) 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
stimulus 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.

SUMMARY.

There is no line of demarcation between tropic and nastic movements.

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 _Erythrina
indica_ and of _Clitoria ternatea_ thus fold upwards, the apices of the
leaflets pointing towards the sun.

Internally diffused 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 PHOTOTROPIC 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 temperature and by seasonal
change; certain organs again are apparently erratic in their phototropic
response.

SEASONAL CHANGE OF PHOTOTROPIC ACTION.

Sachs observed a positive phototropic curvature in the stems of
_Tropæolum 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 _Passiflora_ even
under the action of strong light. The tendrils of _Vitis_ and
_Ampelopsis_ 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 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 different tissues. The following table
shows the enhancement of excitability of _Mimosa_ at different
temperatures, the testing stimulus being the same.

TABLE XXXII--SHOWING VARIATION OF EXCITABILITY OF PULVINUS OF
_Mimosa_ AT DIFFERENT TEMPERATURES.

+-----------------------------------+
|Temperature.|Amplitude of response.|
+------------+----------------------+
| 22°C. | 2 divisions. |
| 27°C. | 16 " |
| 32°C. | 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 effect of rise of temperature is complex; it
enhances the excitability 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 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 temperature 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
temperature; 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 to light is due 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 temperature it is
evident that at a certain temperature the tropic effect will be exactly
neutralised by transverse conduction. Lowering of temperature, by
reducing the transmission 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 antagonistic 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 _Passiflora_,
supposed to be insensitive to the action of light.

_Phototropic response of the tendril of_ Passiflora: _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°C. 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 takes place at an intermediate temperature, and the
organ thus appears insensitive to light.

SEASONAL VARIATION OF PHOTOTROPIC CURVATURE.

[Illustration: FIG. 145.--(_a_) Positive curvature of tendril of
_Passiflora_ at 15°C.; (_b_) negative phototropic curvature at 30°C.]

Reference has been made of the phototropic curvature of _Tropæolum_ and
of Ivy undergoing a change from positive in autumn to negative in
summer. The experiment described above shows that rise of temperature,
by enhancing transverse conductivity, transforms the positive into
negative heliotropic curvature. The reversal of the phototropic
curvature of _Tropæolum_ and Ivy, from positive in autumn to negative in
summer, finds a probable explanation 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
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 winter.

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
temperature, as previously stated, introduces another element of
variation by its effect on conductivity. Transverse conduction 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. 146a). 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
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).

[Illustration: FIG. 146.--Effect of rise of temperature on phototropic
curvature. (_a_) normal positive curvature followed by recovery, (_b_)
reversal of positive into negative curvature by rise of temperature at
(H). (Pea seedling).]

_After-effect of rise of temperature: Experiment 147._--The after-effect
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 restoration 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 cessation of light (Fig. 147b).
This reversal of response as an after-effect of rise of temperature was
in this case found to persist for several hours. I experimented with the
same specimen next day when the response was found restored to the
normal positive.

[Illustration: FIG. 147.--After-effect of rise of temperature,
persistent negative curvature: (_a_) response one hour after rise
of temperature; (_b_) series of negative responses after 4 hours
(successive stimuli applied at vertical lines).]

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 expansion 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 phototropic curvature.

Tendrils of _Passiflora_, supposed to be phototropically insensitive,
exhibit positive curvature at low, and negative curvature at a
moderately high temperature.

The change of phototropic curvature exhibited by _Tropæolum majus_ and
Ivy, from positive in autumn to negative in summer, is probably due to
the effect of temperature. Higher temperature with enhanced transverse
conductivity in summer, may thus convert positive into negative
curvature.

The physiological effects of rise 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.

In 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
the torsions cannot generally be regarded as due to the combination of
two curvatures, we are completely in the dark as to the mechanics of
their production."[23]

[23] Jost--_Ibid_--p. 465.

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.

The experiments 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 light
but by all forms of stimulation,

(4) that the direction of the torsional response depends on
the direction of the incident stimulus and the differential
excitability of the organ, and

(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 pulvinus of
_Mimosa_, light of moderate intensity and of short duration applied on
the upper half induces a slow up-movement, 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 _dorsiventral_ 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 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 petiole (Fig. 148). The end
of the aluminium wire is attached to the short arm of a recording lever;
there is thus a compound magnification of the torsional movement. The
Oscillating 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.

[Illustration: FIG. 148.--Diagrammatic representation for record of
torsional response. H, thin glass hook: A, aluminium wire attached to
petiole for magnification of torsional movement. T, silk thread for
attachment to recording lever.]

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 lower halves of the organ.
The observer standing in front of the leaf is supposed to look at the
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, 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 stimulated 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 underwent differential excitation. When light
struck on the left flank, the responsive torsion was anti-clockwise; the
responsive reaction thus made _the upper and the less excitable half of
the pulvinus face the stimulus_. Figure 149 gives a 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.

[Illustration: FIG. 149.--Record of torsional response of pulvinus of
_Mimosa pudica_.]

DIRECTIVE ACTION OF STIMULUS.

_Experiment 149._--If now the direction of stimulus be changed so that
light strikes on the right flank instead 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 chemical stimulation: Experiment 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 responsive torsion against the hands of a clock. Chemical
stimulation of the right flank induced, on the other hand, 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 radiation: 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.

_Geotropic stimulus._--The stimulus of gravity induces, as I shall show
in a subsequent chapter, a similar responsive torsion, the direction of
which is determined by the direction of the incident stimulus.

EFFECT OF 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 torsional response
is, moreover, shown to be determined by the direction of incident
stimulus, and the differential excitability of the organ. This latter
may be reversed by the local application of various depressing agents on
the normally more excitable lower half of the pulvinus. Under this
treatment, the lower half of the pulvinus may be rendered relatively the
less excitable. Lateral application 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 contractile stretched India-rubber; if such a strip be held
securely at one end in a clamp, and if the lateral flank, consisting
half of ebonite and half of India-rubber, be subjected to 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 borne in mind in
this connection that the torsional response of pulvinus is brought about
by differential _physiological_ contraction of the organ, the movement
being abolished at death.

From the results given above, we arrive at the following:--

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 DIMINUTION 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 describe 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 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 twistings 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
movements 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.

_Torsional movement of leaflet of_ Cassia alata: _Experiment
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
morning 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 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.

[Illustration: FIG. 150.--Leaflets of _Cassia alata_: open in daytime,
and closed in evening.]

For obtaining an explanation of these complex movements under different
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 excitabilities 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
independently 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 under the diffuse light. A very light index
was attached to the leaflet for magnifying the subsequent torsional
movement. A strong beam of parallel light from an arc lamp was thrown
down on the pulvinule from above; this fell at the junction of the more
excitable outer with the less excitable inner half of the organ, the
plane of separation of the two unequally excitable halves being, as
previously explained, vertical. I have shown that under lateral
stimulation, a differentially excitable organ undergoes torsion by which
the less excitable half is made to face the stimulus. Since it is the
inner half of the organ that is the less excitable, the attached leaflet
becomes twisted so as to expose its (former infolded) surface upwards,
at right angles to the incident light.

As a confirmatory test, strong light was made to strike the pulvinule
from _below_ with the result that the leaflets exhibited an opposite
torsion by which their surfaces faced downwards, so as to be at right
angles to light that struck them from below.

Under normal conditions sunlight comes from above; stimulation thus
takes place at the junction of the two differentially excitable halves
of the organ, the plane of separation of which is vertical. The torsion
induced makes the less excitable inner half turn in such a way that the
inner surfaces of the leaflets are placed perpendicular to the incident
light.

ADVANTAGES OF THE METHOD OF TORSIONAL RESPONSE.

The torsional response not only affords a new method of enquiry on the
reaction of various stimuli, but it also possesses certain advantages.
For instance in studying the response of the leaf of _Mimosa_ under
light, the records were taken of the movement of the leaf in a vertical
plane. But the responsive up-movement, induced by light acting from
above, is opposed by the weight of the leaf. But in the torsional
response, the leaf rests on the hooked glass support and the movement is
thus free from the complicating factor of the weight of the leaf. Again
the pulvinus of _Mimosa_, for example, is sometimes subject to
spontaneous variation of turgor, on account of which it exhibits an
autonomous up or down movement. In the ordinary method of record the
true response to external stimulus may thus be modified by natural
movement of the leaf. But in the torsional method, the autonomous up or
down movement is restrained by the hooked support, and the response to
lateral stimulus is unaffected by the spontaneous movement of the leaf.
The torsional method, moreover, opens out possibilities of inquiry in
new directions, such as the comparison of the excitatory effects of
different stimuli by the Method of Balance, and the determination of the
effective direction of geotropic stimulus.

THE TORSIONAL BALANCE.

A beam of light falling on the left flank of the pulvinus of _Mimosa_
induces a torsion against the hands of the clock. A second beam falling
on the right flank opposes the first movement; the resultant effect is
therefore determined by the effective stimulation of the two flanks. The
pulvinus thus becomes a delicate index by which two stimuli may be
compared with each other. The following experiment is cited as an
example of the application of the method of phototropic balance.

_Experiment 155._--Parallel beam of light from a small arc lamp passing
through blue glass falls on the left flank of the pulvinus; a beam of
blue light also strikes the pulvinus from the right side, and the
intensity of the latter is so adjusted that the resultant torsion is
zero. Blue glass is now removed from the left side, the unobstructed
white light being allowed to fall on the left flank of the pulvinus.
This was found to upset the balance, the resultant torsion being
anti-clockwise. This showed that white light induced greater excitation
than blue light. We next interpose a red glass on the left side, with
the result that the balance is upset in the opposite direction. This is
because the phototropic effect of red light is comparatively feeble.
We may thus compare the tropic effect of one form of stimulus against
a totally different form, phototropic against geotropic action
for example. It is enough here to draw attention to the various
investigations rendered possible by the method of balance. Concrete
examples of some of these will be given in a subsequent chapter.

DETERMINATION OF THE DIRECTION OF STIMULUS.

I have shown that the torsion, clockwise or anti-clockwise, is
determined by the direction of incident stimulus. Hence it would be
possible to determine the direction of incident stimulus from the
observed torsional movement. In the case of light, the direction of
incident stimulus is quite apparent. But it is difficult to determine
the direction of stimulus which is itself invisible. In such cases, the
torsional movement gives us infallible indication of the effective
direction of stimulus. The application of this principle will be found
in a later chapter.

SUMMARY.

Lateral stimulus induces a torsional response in a dorsiventral organ.
This is true of all modes of stimulation.

The responsive torsion is determined by the direction of incident
stimulus, and the differential excitability of two halves of the organ,
the torsion being such that the less excitable half of the organ is made
to face the stimulus.

The twist exhibited by various leaves and leaflets under light finds its
explanation from the demonstrated laws of torsional response.

The direction of incident stimulus may be determined from the responsive
torsion of a dorsiventral organ.

The Method of Torsional Balance enables us to compare the excitatory
efficiencies of two different stimuli which act simultaneously on the
two flanks of the organ.

XXXVII.--RADIO-THERMOTROPISM

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

We have studied the tropic curvature induced by different rays of light.
We saw that while the more refrangible rays of the spectrum were most
effective, the less refrangible rays were ineffective. Below the red,
there are the thermal rays about whose tropic effect very little is
definitely known.

The intricacies of the problem are very great owing to the difficulty of
discriminating the effect of temperature from that of radiation; to this
must be ascribed the contradictory results that have been obtained by
different observers, of which Pfeffer gives the following summary:[24]

[24] Pfeffer--_Ibid_--Vol. III, p. 776.

"In addition to the action of ultra-red rays which are associated with
the visible part of the spectrum, dark heat-rays of still greater wave
length, as well as differences of temperature may produce a thermotropic
curvature in certain cases. Wortmann observed that seedlings of
_Lepidium sativum_ and _Zea Mays_, as well as sporangiphores of
_Phycomyces_ curved towards a hot iron plate emitting dark heat-rays.
Steyer has, however, shown that the sporangiphore of _Phycomyces_ has no
power of thermotropic reaction.... Wortmann observed that the seedling
shoot of _Zea Mays_ was positively, but that of _Lepidium_ negatively,
thermotropic.... Steyer, however, found that both plants were positively
thermotropic. Wortmann has also investigated the radicles of seedlings
by growing them in boxes of saw-dust, one side being kept hot, the other
cold."

It will be noted that in the investigations described above,
thermotropic reaction has been assumed to be the same under variation of
temperature (as in experiments with unequally heated saw-dust), and
under radiation from heated plate of metal. With reference to this Jost
maintains that "so far as we know, thermotropism due to _radiant_ heat
cannot be distinguished from thermotropism due to _conduction_."[25]

[25] Jost--_Ibid_--p. 480.

The effect of temperature, within optimum limits, is a physiological
expansion and enhancement of the rate of growth. The effect of visible
radiation is, on the other hand, a contraction and retardation of
growth. Should radiant heat act like light, the various tropic effects
in the two cases would be similar; the temperature effect would in that
case be opposite to the radiation effect. In order to find whether the
thermal radiation produces tropic curvature similar to that of light, we
have to devise a crucial experiment in which the complicating factor of
rise of temperature on the responding organ is eliminated.

_Experiment 156._--I have described the effect of light applied
unilaterally to the stem of _Mimosa_, at a point diametrically opposite
to the indicating leaf (_Expt._ 104). It was shown that the effect of
indirect stimulus induced at first an erectile movement of the leaf, and
that this was followed by a fall of the leaf on account of transverse
transmission of excitation. In the present experiment I applied thermal
radiation instead of light. The source of radiation was a spiral of
platinum wire heated short of incandescence by means of electric
current. The intensity of incident radiation could thus be maintained
constant, or increased or decreased by approach or recession of the
radiating spiral. The effect of unilateral stimulus of heat-rays was
found exactly similar to that of light; _i.e._, there was at first an
erectile movement due to indirect stimulation, followed by the fall of
the leaf due to transmitted excitation. It will be noticed that under
the particular condition of the experiment, the responding pulvinus was
completely shielded from temperature-variation. The reaction to thermal
radiation is thus similar to that of light.

As regards the effects of rise of temperature and radiation I have shown
that they are antagonistic to each other (pp. 211, 308). Thus in
positive types of thermonastic organs like the flower of _Zephyranthes_,
while rise of temperature induces a movement of opening, radiation
causes the opposite movement of closure. Again, in the negative type
exemplified by _Nymphæa_, rise of temperature induces a movement of
closure; radiation on the other hand, brings about the opposite movement
of opening. The tropic effect of thermal radiation thus takes place in
opposition to that of rise of temperature, and the resultant effect is
therefore liable to undergo some modification, depending on the relative
sensibility of the organ to radiation and to variation of temperature.

The facts that have been given above prove that infra-red radiation is
as effective a mode of stimulation as the more refrangible rays of the
spectrum. Phototropic and radio-thermotropic reactions would therefore
prove to be essentially similar. The following experiments fully confirm
the similarity of the two reactions.

POSITIVE RADIO-THERMOTROPISM.

_Experiment 157._--I shall now describe the normal reaction of a growing
organ to the unilateral stimulus of thermal radiation. Figure 151 gives
a record of response of the stem of _Dregea_ to stimulus of short
duration; the induced curvature is positive or towards the source of
heat. On the cessation of stimulus, there is a recovery which is
practically complete, and which takes place at a slower rate than the
excitatory positive curvature. Repetition of stimulus gives rise to
responses similar to the first. _Successive stimuli of moderate
intensity thus give rise to repeated responses of growth curvature._ An
arbitrary distinction has been made between the responses of pulvinated
and of growing organs. The former is distinguished as a movement of
variation, with its supposed characteristic of repeated response. But
the experiment described shows that this is also met with in the
response by growth curvature. It is only under long continued
stimulation that the curvature is fixed by growth.

[Illustration: FIG. 151.--Positive response to short exposure to
thermal radiation. Successive dots at intervals of 5 seconds.
(_Dregea volubilis._)]

DIA-RADIO-THERMOTROPISM.

The positive curvature is induced by retardation of growth at the
proximal side, and enhancement of growth at the distal side. This
latter effect is, as we have seen, brought about by the effect of
indirect stimulation.

But under long continued action of stimulus, the negative or excitatory
impulse reaches the distal side, inducing diminution of turgor and
retardation of the rate of growth. This leads to neutralisation, the
organ placing itself at right angles to the orienting stimulus.

[Illustration: FIG. 152.--Record of positive, neutral and reversed
negative curvature under continued action of thermal radiation. The
negative response went off the plate. Successive dots at intervals
of 5 seconds. (_Dregea volubilis_).]

_Experiment 158._--This neutralisation is seen in the record given in
figure 152, where under continuous unilateral stimulation, the growing
organ exhibited its maximum positive curvature, after which the movement
became arrested by the arrival of the excitatory impulse at the distal
side, on account of which the first positive curvature became
neutralised. Further continuation of stimulus caused a reversal into
negative in the course of 7 minutes. It will thus be seen that in
inducing phototropic curvature, the heat rays in sunlight play as
important a part as the more refrangible rays of the spectrum.

SUMMARY.

The effects of rise of temperature and of radiation are antagonistic to
each other.

Under unilateral action of thermal radiation a positive curvature is
induced by the retardation of growth at the proximal, and acceleration
of growth at the distal side of the organ.

There is a complete recovery on the cessation of stimulus of moderate
intensity and short duration. Repeated responses may thus be obtained
similar to repeated responses in pulvinated organs. In certain tissues
the power of conduction in a transverse direction is wanting; excitation
remains localised at the proximal side, and the responsive curvature
remains positive.

In other cases, there is a slow conduction of excitation to the distal
side. The result of this under different circumstances is
dia-radio-thermotropic neutralization, or a reversed negative curvature.

In inducing phototropic curvature, the heat rays in sunlight play as
important a part as the more refrangible rays of the spectrum.

XXXVIII.--RESPONSE OF PLANTS TO WIRELESS STIMULATION

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

A growing plant bends towards light, and this is true not only of the
main stem but also of its branches and attached leaves and leaflets.
Light affects growth, the effect being modified by the intensity of
radiation. Strong stimulus of light causes a diminution of the rate of
growth, but very feeble stimulus induces an acceleration. The tropic
effect is very strong in the ultra-violet region of the spectrum with
its extremely short wave length, but the effect declines practically to
zero as we move towards the less refrangible rays--the yellow and the
red with their comparatively long wave length. As we proceed beyond the
infra-red region, we come across the vast range of electric radiation,
the wave lengths of which vary from 0·6 cm., the shortest wave I have
been able to produce, to others which may be miles in length. There thus
arises the very interesting question, whether plants perceive and
respond to the long ether waves including those employed in signalling
through space.

At first sight this would appear to be very unlikely; for the most
effective rays are in the ultra-violet region with wave length as short
as 20 × 10^{-6} cm.; but with electric waves used in wireless signalling
we have to deal with waves 50 million times as long. The perceptive
power of our retina is confined within the very narrow range of a
single octave, the wave lengths of which lie between 70 × 10^{-6} cm.
and 35 × 10^{-6} cm. It is difficult to imagine that plants could
perceive radiations so widely separated from each other as the visible
light and the invisible electric radiation.

But the subject assumes a different aspect, when we take into
consideration the _total_ effect of radiation on the plant. Light
induces two different effects which may broadly be distinguished as
external and internal. The former gives rise to movement; the latter
finds no outward manifestation, but consists of an 'up' or assimilatory
chemical change, with concomitant increase of potential energy. Of the
two reactions then, one is dynamic attended by dissimilatory 'down'
change; the other is potential, associated with the opposite 'up'
change. In reality the two effects take place simultaneously; but one of
these becomes predominant under definite conditions.

The modifying condition is the _quality_ of light; with reference to
this I quote the following from Pfeffer: "So far as is at present known,
the action of different rays of the spectrum gives similar curves in
regard to heliotropic and phototactic movements, to protoplasmic
streaming and movements of the chloroplastids as well as the photonastic
movements produced by growth or by changes of turgor. On the other hand,
it is the less refrangible rays which are most active in
photo-synthesis."[26] The dynamic and potential manifestations are thus
seen to be complementary to each other, the rays which induce
photo-synthesis being relatively ineffective for tropic reaction and
_vice versâ_.

[26] Pfeffer--Vol. II, p. 104.

Returning to the action of electric waves, since they exert no
photo-synthetic action they might conceivably induce the complementary
tropic effect. These considerations led me to the investigation of the
subject fourteen years ago, and my results showed that very short
electric waves induce a retardation of rate of growth; they also produce
responsive movements of the leaf of _Mimosa_, when the plant was in a
highly sensitive condition.[27] The energy of the short electric waves
is very feeble, and undergoes great diminution at a distance; hence the
necessity of employment of a specimen of plant in a highly sensitive
condition.

[27] "Plant Response"--p. 618 (1905).

I resumed my investigations on the subject at the beginning of this
year. I wished to find out whether plants in general perceived and
responded to the long ether waves which reached it from a distance. The
perception of the wireless stimulation was to be tested not merely by
the responsive movement of sensitive plants, but by diverse modes of
response given by all kinds of plants.

Stimulus induces, as we have seen, three different types of response in
plants. It causes excitation in sensitive plants like _Mimosa_, in
consequence of which the leaf undergoes a fall; this is the mechanical
response to stimulus. Stimulus also induces electric response in plants,
both sensitive and ordinary, the excited tissue undergoing an electric
change of galvanometric negativity. Finally, the effect of stimulus on
growing plants is a variation in the rate of growth, an acceleration
under feeble, and a retardation under strong stimulus. I undertook to
investigate the effect of electric waves on plants by the methods of
mechanical and of electrical responses, and also by that of induced
variation of growth.

[Illustration: FIG. 153.--Diagrammatic representation of method employed
for obtaining response to wireless stimulation. Transmitting apparatus
seen to the right. Receiving aerial connected to upper part of plant,
the lower part of the plant or the flower-pot being connected with the
earth.]

THE WIRELESS SYSTEM.

For sending wireless signals, I had to improvise the following
arrangement, more powerful means not being available. The secondary
terminals of a moderate sized Ruhmkorff's coil were connected with two
cylinders of brass, each 20 cm. in length; the sparking took place
between two small spheres of steel attached to the cylinders. One of the
two cylinders was earthed, and the other connected with the aerial 10
meters in height. The receiving aerial was also 10 meters in height and
its lower terminal led to the laboratory, and connected by means of a
thin wire to the experimental plant growing in a pot; this latter was
put in electric connection with the earth (Fig. 153). The distance
between the transmitting and receiving aerial was about 200 meters, the
maximum length permitted by the grounds of the Institute.

MECHANICAL RESPONSE OF _Mimosa_.

_Experiment 159._--One of the leaves of _Mimosa_ was connected with the
aerial by means of a thin tinsel of loose wire, which did not interfere
with the free movement of the leaf. This latter was attached to the
recording lever. Wireless signals induced a responsive fall of the leaf
(Fig. 154) which was gradual as under action of light, and not so abrupt
as under a mechanical blow.

[Illustration: FIG. 154.--Mechanical response of leaf of _Mimosa_ to
electric wave.]

[Illustration: FIG. 155.--Electric response of _Mimosa pudica_ to
wireless stimulation.]

THE ELECTRIC RESPONSE.

_Experiment 160._--The leaf of _Mimosa_ was in this experiment held
securely, and two electrical connections made, one with the less
excitable upper and the other with the more excitable lower half of the
pulvinus. The incident ether-wave induced an electric response in the
pulvinus, the more excitable lower half exhibiting galvanometric
negativity. On the cessation of stimulus there was a recovery (Fig.
155).

It is not at all necessary to employ the sensitive _Mimosa_ for
exhibition of electric response; for this is universally exhibited by
all plants. The only condition for electric response is that the points
of electric contacts should be made with two unequally excitable areas
in the plant. This may be secured by artificial means as by causing
'injury' to one point of contact.[28] It is however much better to take
advantage of the natural difference of excitability of two different
areas in the organ as in the pulvinus of _Mimosa_. This difference of
excitability is also found between the inner and outer sides of a hollow
tubular organ as in the peduncles of various lilies. I was thus able to
secure specimens which were far more sensitive to the action of electric
waves than the pulvinus of _Mimosa_.

[28] "Comparative Electro-Physiology"--p. 149.

EFFECT OF WIRELESS STIMULATION ON GROWTH.

There now remains the very interesting question as to whether the effect
of long ether waves induce any variation of growth. The results given
below show that growing plants not only perceive but respond to the
stimulus of electric waves. The effects to be presently described are
exhibited by all plants.

I shall, however, content myself in describing a typical experiment
carried with the seedling of wheat. The specimen was mounted on the
Balanced Crescograph, and the growth exactly balanced. This gives a
horizontal record; an acceleration of growth above the normal is, in
the following records, represented by a down curve, and a retardation
by an up-curve.

_Effect of feeble stimulus: Experiment 161._--I first studied the effect
of feeble stimulus. This was secured by decreasing the energy of sparks
of the radiator. The response was an acceleration of rate of growth as
seen in figure 156a. The analogy of this with the accelerating effect of
sub-minimal intensity of light (p. 224) is very remarkable.

[Illustration: FIG. 156.--Record of responses to electric wave by the
Balanced Crescograph (_a_) response to feeble stimulus by acceleration
of growth, (_b_) response to strong stimulus by retardation, (_c_)
responses to medium stimulation--retardation followed by recovery.
Down-curve represents acceleration, and up-curve retardation of growth:
(Seedling of wheat.)]

_Effect of strong stimulus: Experiment 162._--The maximum energy
radiated by my transmitter, as stated before, was only moderate. In
spite of this its effect on plants was exhibited in a very striking
manner. The balance was immediately upset, indicating a retardation of
the rate of growth. The latent period, _i.e._, the interval between the
incident wave and the response, was only a few seconds (Fig. 156b). The
record given in the figure was obtained with the moderate magnification
of 2,000 times only. But with my Magnetic Crescograph, the magnification
can easily be raised ten million times; and the response of plant to the
space signalling can be exalted in the same proportion.

Under an intensity of stimulus slightly above the sub-minimal, the
responses exhibit retardation of growth followed by quick recovery, as
seen in the series of records given in Fig. 156c.

A remarkable peculiarity in the response was noticed during the course
of the experiments. Strong stimulation by ether waves gives rise, as we
have seen, to a very marked retardation of the rate of growth. Repeated
stimulation induces fatigue, and temporary insensitiveness of the organ.
Under moderate fatigue the effect is a prolongation of the latent
period. Thus in a particular experiment the plant failed to give any
response to a short signal. But after an interval of five minutes a
marked response occurred to the wireless stimulus that had been received
previously. The plant had perceived the stimulus but on account of
fatigue the latent period was prolonged, from the normal 5 seconds to as
many minutes.

SUMMARY.

Plants not only perceive, but also respond to long ether waves employed
in signalling through space.

Mechanical response to wireless stimulation is exhibited by the leaf of
_Mimosa pudica_.

All plants give electric response to the stimulus of long ether waves.

Growing plants exhibit response to electric waves by modification of
rate of growth. Feeble stimulus induces an acceleration, while strong
stimulus causes a retardation of the rate of growth.

The perceptive range of the plant is far greater than ours; it not only
perceives but responds to the different rays of the vast ethereal
spectrum.

XXXIX.--GEOTROPISM

_By_

SIR J. C. BOSE.

No phenomenon of tropic movement appears so inexplicable as that of
geotropism. There are two diametrically opposite effects induced by the
same stimulus of gravity, in the root a movement downwards, and in the
shoot a movement upwards. The seeming impossibility of explaining
effects so divergent by the fundamental reaction of stimulus, has led to
the assumption that the irritability of stem and root are of opposite
character. I shall, however, be able to show that this assumption is
unnecessary.

The difficulty of relating geotropic curvature to a definite reaction to
stimulus is accentuated by the fact that the direction of the incident
stimulus, and the side which responds effectively to it are not clearly
understood; nor is it known, whether the reaction to this stimulus is a
contraction, or its very opposite, an expansion.

Taking the simple case of a horizontally laid shoot, the geotropic
up-curvature is evidently due to differential effect of the stimulus on
upper and lower sides of the organ. The up-curvature may be explained by
one or the other of two suppositions: (1) that the stimulus of gravity
induces contraction of the upper side; or (2) that the fundamental
reaction is not a contraction but an expansion and this of the lower
side. The second of these two assumptions has found a more general
acceptance.

Tropic curvatures in general are brought about by the differential
effect of stimulus on two sides of the organ. Thus light falling on one
side of a shoot induces local contraction, the rays being cut off from
acting on the further side by the opacity of the intervening tissue. But
there is no opaque screen to cut off the vertical lines of gravity,[29]
which enter the upper side of a horizontally laid shoot and leave it by
the lower side. Though lines of force of gravity are transmitted without
hindrance, yet a differential action is found to take place, for the
upper side, where the lines of force enter, becomes concave, while the
lower side where they emerge becomes convex. Why should there be this
difference?

[29] I shall in what follows take the _direction_ of vertical lines
of gravity as that of movement of falling bodies, from above
towards the centre of the earth.

For the removal of various obscurities connected with geotropism it is
therefore necessary to elucidate the following:

1. The sign of excitation is, as we found, a contraction and concomitant
galvanometric negativity. Does gravitational stimulus, like stimulus in
general, induce this excitatory reaction?

2. What is the effective direction of geotropic stimulus? In the case of
light, we are able to trace the rays of light which is incident on the
proximal side and measure the angle of inclination. In the case of
gravity, the invisible lines of force enter by one side of the organ and
leave by the other side. Assuming that the direction of stimulus is
coincident with the vertical lines of gravity, is it the upper or the
lower side of the organ that undergoes effective stimulation?

3. What is the law relating to the 'directive angle' and the resulting
geotropic curvature? By the directive angle (sometimes referred to as
the angle of inclination) is meant, as previously explained, the angle
which direction of stimulus makes with the responding surface.

4. We have finally to investigate, whether the assumption of opposite
irritabilities of the root and the shoot is at all justifiable. If not,
we have to find the true explanation of the opposite curvatures
exhibited by the two types of organs.

Of these the first three are inter-related. They will, however, be
investigated separately; and each by more than one method of inquiry.
The results will be found to be in complete harmony with each other.

I propose in this and in the following chapters to carry out the
investigations sketched above, employing two independent methods of
enquiry, namely, of mechanical and of electrical response. I shall first
describe the automatic method I have been able to devise, for an
accurate and magnified record of geotropic movement and its time
relations.

THE GEOTROPIC RECORDER.

The recorder shown in figure 157 is very convenient for study of
geotropic movement. The apparatus is four-sided and it is thus possible
to obtain four simultaneous records with different specimens under
identical conditions. The recording levers are free from contact with
the recording surface. By an appropriate clock-work mechanism, the
levers are pressed for a fraction of a second against the recording
surfaces. The successive dots in the record may, according to different
requirements, be at intervals varying from 5 to 20 seconds. The records
therefore not only give the characteristic curves of geotropic movements
of different plants, but also their time durations. For high
magnification, I employ an Oscillating Recorder, the short arm of the
lever being 2·5 mm., and the long arm 250 mm., the magnification being a
hundredfold; half that magnification is, however, sufficient for general
purposes.

[Illustration: FIG. 157.--The Quadruplex Geotropic Recorder.]

DETERMINATION OF THE CHARACTER OF GEOTROPIC REACTION.

The observed geotropic concavity of the upper side of a horizontally
laid shoot may be due to excitatory _contraction_ of that side, or it
may result from passive yielding to the active responsive _expansion_ of
the lower side. The crucial test of excitatory reaction under geotropic
stimulus is furnished by investigations on geo-electric response. When a
shoot is displaced from vertical to horizontal position, _the upper side
of the organ is found to undergo an excitatory electric change of
galvanometric negativity_ indicative of diminution of turgor and
_contraction_. The electric change induced on the lower side is one of
galvanometric positivity, which indicates an increase of turgor and
expansion. The tropic effect of geotropic stimulus is thus similar to
that of any other mode of stimulation, _i.e._, a contraction of the
upper (which in the present case is the proximal) and expansion of the
lower or the distal side. The vertical lines of gravity impinge on the
upper side of the organ which undergoes effective stimulation.

In order to show that the concavity of the upper side is not due to the
passive yielding to the expansion of the lower half, I restrained the
organ from any movement. I have explained that excitatory electric
response is manifested even in the absence of mechanical expression of
excitation; and under geotropic stimulus, the securely held shoot gave
the response of galvanometric negativity of the upper side. Hence the
fundamental reaction under geotropic stimulus is excitatory
_contraction_ as under other modes of stimulation.

Finally, I employed the additional test of induced paralysis by
application of intense cold. Excitatory physiological reaction is, as we
know, abolished temporarily by the action of excessive cold.

_Experiment 163._--I obtained records of mechanical response to
determine the side which undergoes excitation under geotropic stimulus,
the method of discrimination being local paralysis induced by cold. I
took the flower-scapes of _Amarayllis_ and of _Uriclis_, and holding
them vertical applied fragments of ice on one of the two sides. I then
laid the scape horizontal, first with cooled side below, the record
showed that this did not affect the geotropic movement. But on cooling
the upper side, the geotropic movement became arrested, and it was not
till the plant had assumed the temperature of the surroundings that the
geotropic movement became renewed. Figure 158 shows the effect of
alternate application of cold, on the upper and lower sides of the
organ.[30] In the shoot, therefore, it is the upper side of the organ
that becomes effectively stimulated. Before proceeding further I shall
make brief reference to the highly suggestive statolithic theory of
gravi-perception.

[30] "Plant Response"--p. 505.

[Illustration: FIG. 158.--Effect of alternate application of cold on the
upper and lower sides of the organ. Application of cold on upper side
(down-pointing arrow) induces arrest of geotropic movement. Application
below (up-pointing arrow) causes no arrest.]

THEORY OF STATOLITHS.

With regard to the perception of geotropic stimulus there can be no
doubt that this must be due to the effect of weight of cell contents,
whether of the sap itself, or of the heavy particles contained in the
cells, exerting pressure on the sensitive plasma. The theory of
statoliths advocated by Noll, Haberlandt and Nemec (in spite of certain
difficulties which further work may remove) is the only rational
explanation hitherto offered for gravi-perception. The sensitive plasma
is the ectoplasm of the entire cell, and statoliths are relatively heavy
bodies, such as crystals and starch grains. Haberlandt has found
statoliths in the apo-geotropic organs like stems.[31] When the cell is
laid horizontal, it is the lower tangential wall which has to support
the greater weight, and thus undergo excitation. In the case of
multicellular plants laid horizontally, the excitation on the upper side
is, as we have seen, the more effective than on the lower side. This
inequality, it has been suggested, is probably due to this difference
that the statoliths on the upper side press on the inner tangential
walls of the cells while those on the lower side rest on the outer
tangential walls.

[31] Haberlandt--"Physiological Plant Anatomy"--p. 603.

When the organ is held erect, the action of statoliths would be
symmetrical on the two sides. But when it is laid horizontal a complete
rearrangement of the statoliths will take place, and the differential
effects on the upper and lower sides will thus induce geotropic
reaction. This _period of migration_ must necessarily be very short; but
the reaction time, or the latent period, is found to be of considerable
duration. "Even in rapidly reacting organs there is always an interval
of about one to one and a half hours, before the horizontally placed
organ shows a noticeable curvature, and this latent period may in other
cases be extended to several hours."[32] This great difference between
the _period of migration_ and the _latent period_ offers a serious
difficulty in the acceptance of the theory of statoliths. But it may be
urged that the latent period has hitherto been obtained by relatively
crude methods, and I therefore undertook a fresh determination of its
value by a sensitive and accurate means of record.

[32] Jost--_Ibid_, p. 437.

DETERMINATION OF THE LATENT PERIOD.

As regards the interpretation of the record of geotropic movement, it
should be borne in mind that after the perception of stimulus a certain
time must elapse before the induced growth-variation will result in
curvature. There is again another factor which causes delay in the
exhibition of true geotropic movement; for the up-movement of stems, in
response to the stimulus of gravity, has to overcome the opposite down
movement, caused by weight, before it becomes at all perceptible. On
account of the bending due to weight there is a greater tension on the
upper side, which as we have seen (p. 193), enhances the rate of growth,
and thus tends to make that side convex. The exhibition of geotropic
response by induced contraction of the excited upper side thus becomes
greatly delayed. In these circumstances I tried to discover specimens in
which the geotropic action would be quick, and in which the retarding
effect of weight could be considerably reduced.

_Geotropic response of flower stalk of Tuberose: Experiment 164._--For
this I took a short length of flower stalk of tuberose in a state of
active growth; the flower head itself was cut off in order to remove
unnecessary weight. After a suitable period of rest for recovery from
the shock of operation, the specimen was placed in a horizontal
position, and its record taken. The successive dots in the curve are at
intervals of 20 seconds, and the geotropic up-movement is seen to be
initiated (Fig. 159) after the tenth dot, the latent period being thus 3
minutes and 20 seconds, the greater part of which was spent in
overcoming the down-movement caused by the weight of the organ.

[Illustration: FIG. 159.--Geotropic response of flower stalk of tube
rose: preliminary down-movement is due to weight.]

[Illustration: FIG. 160.--Geotropic response of petiole of _Tropæolum_:
latent period shorter than 20 seconds.]

_Geotropic response of petiole of_ Tropæolum: _Experiment 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 _Tropæolum_, 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 down-movement, and the
geotropic up-movement was commenced within a few seconds after 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 GEOTROPIC 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 geotropic equilibrium (cf. page 353).
The period of completion of the curve varies in different specimens from
a few to many hours.

_Experiment 166._--The following record was obtained with a bud of
_Crinum_, the successive dots being at intervals of 10 minutes. After
overcoming the effect of weight (which took an hour), the curve rose at
first slowly, then rapidly. The period of uniformity of movement is seen
to be attained after three hours and continued for nearly 90 minutes.
The final equilibrium was reached after a period of 8 hours (Fig. 161).

[Illustration: FIG. 161.--The Complete Geotropic curve (_Crinum_).]

For studying the effect of an external agent on geotropic 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 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 summation of
effect with that of a different stimulus whose direction is known, (2)
the relation between the directive angle and geotropic reaction, and (3)
the torsional response under geotropic stimulus.

EFFECT OF ALGEBRAICAL SUMMATION.

_Experiment 167._--A flower bud of _Crinum_ is laid horizontally, and
record taken of its geotropic movement. On application of light on the
upper side at L, the responsive movement is enhanced, proving that
gravity and light are inducing similar effects. On the cessation of
light, the original rate of geotropic movement is restored (Fig. 163).
Application of light of increasing intensity from below induces, on the
other hand, a diminution, 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 geotropic 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.

[Illustration: FIG. 162.--Stimulus of light or gravity, represented by
arrow, induces up curvature as seen in dotted figure.]

[Illustration: 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.]

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 gravity.
The analogy between the effects of light and of gravity is very
close[33]; 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 described 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 _Crinum_. In both, the upper side undergoes contraction and the
movement of response carries the 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_
phototropism, 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 _Zenithotropism_ and of the
root as _Nadirotropism_.

[33] Exception to this will be found in page 336, where explanation
is offered for the difference.

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. The 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 geotropic action at
various angles will be fully described in the next chapter. In the
ordinary method of experimentation 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 application of cold,
anæsthetics like chloroform, and of poisonous potassium cyanide
solution.

The different sides of a dorsiventral organ are unequally excitable to
different 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 of "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. 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 geotropic"; but an identical organ cannot be supposed to
possess two different specific sensibilities. The normal horizontal
position assumed by the leaf is, 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. Experiments 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 the
organ.

SUMMARY.

The stimulus of gravity is shown to induce an excitatory reaction which
is similar to that induced by other forms of stimulation. The direct
effect of geotropic stimulus 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.

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. According 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 _Tropæolum_ 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
statoliths.

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 excitabilities of a dorsiventral organ modifies its
position of geotropic equilibrium.

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 preceding 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
contraction 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 excitation can be detected by the induced electromotive change
of galvanometric negativity. This electrical indication of excitation
may be observed even in plants physically restrained from exhibiting
response by mechanical movement.[34]

[34] "Comparative Electro-Physiology," p. 20.

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 curvatures are brought
about by the joint effects, of 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 effects in response to
unilateral stimulus. We have to determine the induced electrical
variation at the proximal side A, and at the distal side B.

[Illustration: 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.

(_b_) 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 negativity of the directly stimulated proximal
A, and galvanometric positivity of the indirectly stimulated
distal point 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 one terminal of the galvanometer, the
other terminal being led to an indifferent or neutral point N on the
leaf. 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 galvanometrically negative._ The "action" current lasts during
the application of stimulus and disappears on its cessation.

_Electric response to indirect stimulation: Experiment 169._--We have
also seen that application of stimulus at A causes indirect stimulation
of the distal point B resulting in an increase of turgor and expansion.
The corresponding 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 stimulated point thus becomes
galvanometrically positive._

Having thus obtained the separate effects at A and B, we next modify the
experiment for obtaining the joint effects. For this purpose the neutral
point N is discarded and A and B connected directly with the indicating
galvanometer. 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:--

TABLE XXXIII.--ELECTRIC RESPONSE TO DIRECT UNILATERAL STIMULUS.

Galvanometric negativity is thus seen to indicate the effect of direct
stimulus, and galvanometric positivity that of indirect stimulus. We
thus see the possibility of electric 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 galvanometer 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
millionth 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-polarisable electrodes._--The electrical connections with the plant
are usually made by means of non-polarisable electrodes (amalgamated
zinc rod in zinc-sulphate solution 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 misnomër; 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 get 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 potential between two
pieces of the same metal, say of platinum, 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, alternating
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.

The direct method of contact described above is extremely convenient 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.

[Illustration: 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 galvanometric negativity of A. In the figure to the left,
rotation is through -90° A being below; the electric response is by
induced galvanometric positivity of A. For simplification of diagram,
vertical position of sepal is not always shown in the figure.]

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 _Nymphæa_,
the peduncle of which is sensitive to geotropic action. One electrical
contact is made with a 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 the neutral point, N, and A. This natural current
may be allowed to remain, the action current due to geotropism being
_superposed on 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.

_Induced 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 stalk 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).[35]

[35] For detailed account cf. Chapter XLIII.

Having thus found that the upper side of the organ under geotropic
stimulus becomes galvanometrically negative, 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 stimulation: that (1) of Axial
Rotation, and (2) of Vertical Rotation.

[Illustration: FIG. 166.--Diagrammatic representation of the Method of
Axial Rotation H, and of Vertical rotation V (see text).]

METHOD OF AXIAL ROTATION.

In the method of Axial Rotation, the organ is held with its long axis
horizontal (Fig. 166 H). We have seen that the geotropic action
increases with the angle which the responding surface of the organ makes
with the 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.

_Experiment 172._--I shall, as a typical example, give a detailed
account of experiments with the petiole of _Tropæolum_ 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, mechanical expression by contraction and
concavity, with positive or up-curvature.

[Illustration: FIG. 167.--Diagrammatic representation of the
geo-electric response of the shoot (see text).]

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 of considerable intensity often
exceeding 15 millivolts. The characteristic electric change is shown
diagrammatically in figure 167 in which the middle figure shows the
symmetrical or zero position. On rotation through +90° (figure to the
right) A occupies the upper and B the lower position. A is seen to
exhibit induced change of galvanometric 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 electric response under
geotropic stimulus which appear to be highly significant. According to
statolithic theory

"Geotropic response begins as soon as an organ is deflected 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 horizontally, 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 different
organs."[36]

[36] Haberlandt--_Ibid_--p. 598.

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 0·1 mm.; and the stimulus will
be perceived after the very short interval taken by the statoliths to
fall through a space shorter than 0·1 mm. This may be somewhat delayed
by the viscous nature of the plasma, but in any case the period for
perceptible displacement of the statoliths should be very short, about a
second or so, and the latent period of perception of stimulus should be
of this order.

The mechanical indication of response to stimulus is delayed by a period
which is somewhat indefinite; for the initiation of responsive growth
variation will necessarily lag behind the perception of stimulus.

[Illustration: FIG. 168.--Geo-electric response of the petiole of
_Tropæolum_.]

_Experiment 173._--The mechanical response with its drawbacks is thus
incapable of giving an accurate value of the latent period. The
electrical method of investigation labours under no such disadvantage,
since the excitation is here detected even in the absence of movement.
The perception of stimulus will thus be followed by response without
undue delay. I shall in this connection give a record of electric
response of the quickly reacting petiole of _Tropæolum_, when the angle
of inclination is increased from zero 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 deflection practically
disappeared in the further course of a minute and a half (Fig. 168).
There was a small "excitation remainder". But with vigorous specimens
the recovery is complete.

[Illustration: FIG. 169.--Geo-electric response of the scape of
_Uriclis_.]

The latent period of quickly reacting petiole of _Tropæolum_ is thus
about 5 seconds, a value which is more consonant with the idea of
particles inducing excitation by their fall through an exceedingly short
distance. In very sluggish organs latent period may be as long as a
minute (Fig. 169), which is considerably shorter than an hour, the
generally accepted value. Further even in the electric response, the
latent period will be delayed beyond the period of perception. For this
perception takes place in some unknown sensitive layer in the interior
of the tissue, while electric contact is made with the epidermis
outside. It is obvious that certain time must elapse before the
excitation, initiated at the sensitive layer, should reach the
epidermis. Under ideal conditions of experiment which will be described
in a subsequent chapter, the latent period for geotropic excitation, I
find, to be sometimes as short as a second.

PHYSIOLOGICAL CHARACTER OF GEO-ELECTRIC RESPONSE.

The intensity of the electro-motive variation is found to depend on the
physiological vigour of the specimen. The _Tropæolum_ plant, used for
most of the above experiments, are at the best condition of growth in
Calcutta in February; after this the plants begin to decline in March
and die off by the end of April.

_Experiment 174._--In February the intensity of electric response was
nearly double of that in March; it was only in March that I made
quantitative determination of the induced electro-motive force between
the upper and lower contacts on rotation of the specimen from zero to
90°. The E. M. F. was determined by the potentiometer method. I give
below the following typical values obtained with two different
specimens:--

Specimen Induced E. M. F.
(1) 12 millivolts.
(2) 15 "

In the most favourable season the induced electro-motive force is likely
to exceed the above value very considerably.

_Effect of Age._--While a young petiole gave the above value, an old
specimen from the same plant exhibited no response. The plants were in a
dying condition in April and all indications of electrical reaction were
found abolished. The physiological character of the response was also
demonstrated by first obtaining the normal electric response in a
vigorous specimen; after death, by immersion in boiling water, the
plant gave no electric response to geotropic stimulus.

EFFECT OF DIFFERENTIAL EXCITABILITY OF THE ORGAN.

I have hitherto described the geo-electric effect of radial and
isotropic organs. The induced E. M. F. at 90° was found practically the
same whether A was above and B below, and _vice versâ_. In the
mechanical response of the pulvinus of _Mimosa_, the geotropic
excitability was, however, found to be greater in the lower half than in
the upper (p. 440). I wished to investigate the question of differential
geotropic excitability anew, by means of electric response.

_Experiment 175._--Electric connections with the galvanometer were made
with the upper and lower halves of the pulvinus, the organ being placed
in the vertical or neutral position. The angle of inclination was then
increased to 90° in the positive and negative directions alternately.

TABLE XXXIV.--DIFFERENCE OF GEO-ELECTRIC RESPONSE OF UPPER AND LOWER
HALVES OF THE PULVINUS OF _Mimosa_.

+---------------------------------------------------------+
| Specimen. | Position of particular | Induced E. M. F. |
| | half of pulvinus. | |
+-----------+--------------------------+------------------+
| (1) | { Upper half above | 23 millivolts. |
| | { Lower half above | 30 " |
| | | |
| (2) | { Upper half above | 16 " |
| | { Lower half above | 29 " |
+---------------------------------------------------------+

In the former case the upper half of the pulvinus occupied the
up-position; in the second case the up-position was occupied by the
lower half of the pulvinus. In both cases strong electric responses were
obtained, the upper point of contact being always galvanometrically
negative. There was, however, a difference between the two responses,
the excitatory electro-motive variation was invariably greater when the
lower half of the organ occupied the favourable up-position. This will
be seen from the results of two typical experiments in table given
above.

The electrical mode of investigation thus leads to confirm the result
obtained with mechanical method that the lower half of the pulvinus of
_Mimosa_ is geotropically more excitable than the upper half.

RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.

In the Method of Axial Rotation, the condition of the experiment is
ideally perfect; in the neutral position the sides A and B are both
parallel to the vertical lines of gravity, and are little affected by
geotropic reaction. As the specimen is rotated on its long axis the
vertical component of the force of gravity increases with the angle of
inclination. The hypothetical statolithic particles will become
displaced all along the cell, and the vertical pressure exerted by them
will also increase with the angle.

The geo-electric response will then afford us a measure of the intensity
of excitation induced at various angles of inclination. The mechanical
response on account of its inherent defects does not afford us the true
relation between the angle of inclination and intensity of geotropic
reaction. But the electric method of inquiry is free from the defects of
the mechanical method.

_Experiment 176._--The specimen was rotated so that the angle of
rotation was 45°, and the maximum electric response observed. The angle
was next increased to 90° and the reading for the enhanced response
taken. The ratio of the geo-electric response at 90° and 45°, thus
affords us a measure of the effective stimulations at the two angles.
I give below a table which gives results obtained with 24 different
specimens.

TABLE XXXV.--RELATION BETWEEN ANGLE OF INCLINATION AND GEOTROPIC EFFECT.

+-------------------------------------------------------------+
|No. of specimen.| Galvanometric deflection. |Ratio b/a.|
| +---------------------------------+ |
| | (_a_) at 45° | (_b_) at 90° | |
+----------------+---------------+-----------------+----------+
| 1 | 70 divisions | 110 divisions | 1·5 |
| 2 | 30 " | 45 " | 1·5 |
| 3 | 90 " | 126 " | 1·4 |
| 4 | 70 " | 100 " | 1·4 |
| 5 | 21 " | 33 " | 1·6 |
| 6 | 30 " | 50 " | 1·6 |
| 7 | 12 " | 20 " | 1·6 |
| 8 | 14 " | 20 " | 1·4 |
| 9 | 10 " | 16 " | 1·6 |
| 10 | 45 " | 75 " | 1·5 |
| 11 | 25 " | 40 " | 1·6 |
| 12 | 14 " | 20 " | 1·4 |
| 13 | 13 " | 20 " | 1·5 |
| 14 | 30 " | 50 " | 1·5 |
| 15 | 38 " | 54 " | 1·4 |
| 16 | 50 " | 75 " | 1·5 |
| 17 | 55 " | 90 " | 1·5 |
| 18 | 13 " | 20 " | 1·5 |
| 19 | 17 " | 25 " | 1·4 |
| 20 | 80 " | 130 " | 1·5 |
| 21 | 15 " | 22 " | 1·4 |
| 22 | 45 " | 75 " | 1·5 |
| 23 |135 " | 220 " | 1·6 |
| 24 | 55 " | 93 " | 1·5 |
+-------------------------------------------------------------+
| Mean ratio = 1·49 |
+-------------------------------------------------------------+

The mean ratio 1·49 may thus be regarded as the relative
geotropic effects at 90° and 45°; this is practically the
same as Sin 90°/Sin 45° = 1·4. Hence we arrive at the
following law:

_The intensity on geotropic action varies as the sine of the
directive angle._

METHOD OF VERTICAL ROTATION.

I have hitherto described results obtained with the Method of Axial
Rotation; I shall now take up the second method, that of Vertical
Rotation, diagrammatic representation of which is given in figure 166V.
The specimen is held vertical and two electrical contacts, A and B, made
with the two lateral sides; it is then rotated round a horizontal axis
perpendicular to the length of the specimen. Rotation may be carried in
a right-handed direction with increasing angle with the vertical. The
point A is thus subjected to enhanced geotropic stimulation and exhibits
increasing electric change of galvanometric negativity; continuous
decrease of angle of inclination to zero by rotation in the reverse
direction causes a disappearance of the induced electric change. The
rotation is next continued in the negative direction by which the point
B is increasingly subjected to geotropic action. B is now found to
exhibit excitatory reaction, the current of response having undergone a
reversal. Rotation to the right and left will be distinguished by plus
and minus signs.

ELECTRIC RESPONSE THROUGH AN ENTIRE CYCLE.

_Experiment 177._--When the specimen is vigorous, characteristic
response with its changing sign may be obtained through an entire cycle
from 0° to +45° to +90°; then back to 45° to 0° to -45° to -90°. With
less vigorous specimens the responses becomes enfeebled under fatigue. I
give below the results of a typical experiment carried out with a
vigorous specimen, the response being distinguished as - when A is
above, and + when A is below, the inversion bringing about a reversal
direction of the responsive current.

+------------------------------------------------------+
|Angle of inclination |+45°|+90°|+45°| 0°|-45°|-90°|
|-------------------------+----+----+----+---+----+----|
|Galvanometer deflection |-19 |-35 |-18 | 0 |+14 | +25|
+------------------------------------------------------+

RELATION BETWEEN ANGLE OF VERTICAL ROTATION AND INTENSITY OF GEOTROPIC
REACTION.

The relation between the angle of inclination and the resulting
geotropic action has already been determined by the Method of Axial
Rotation. The ratio between the geotropic effects at 90° and 45° was
thus found to be 1·49, which is nearly the same as Sin 90°/Sin 45°. I
was next desirous of determining the relative excitations at the two
angles by the Method of Vertical Rotation. It is necessary here to refer
to certain differences of condition in the two methods. In the Axial
Method, the hypothetical statoliths are distributed uniformly through
the length of the cell, and rotation round the long axis causes
displacement of the statoliths, the resulting pressure thus increasing
with the sine of the angle of inclination. But in the case of vertical
rotation through 45° to the right, the statoliths originally at the base
of the cell accumulate to the right hand corner of the cell; a portion
of the basal side of the cell is thus subjected to pressure. When the
angle is increased to 90° the statoliths pass along the whole length
including the basal and apical sides of the cell; but the excitability
of the apical half may prove to be greater than that of the basal half.
Hence excitatory geotropic effect is not likely to vary strictly as in
sine of angle of inclination.

Whatever the reason may be, I find as a result of experiments with 12
different specimens that the mean ratio of the effects at 90° and 45°,
obtained by the Method of Vertical Rotation, is 1·8:1 which is greater
than 1·49:1 obtained by the Method of Axial Rotation, this latter value
being practically the same as Sin 90°/Sin 45°.

SUMMARY.

It is shown that the state of excitation under direct stimulus is
exhibited by an electrical change of galvanometric negativity; the
effect of indirect stimulus induces, on the other hand, an electrical
change of galvanometric positivity. The negative electric change
corresponds to contraction and diminution of turgor; the positive
electric change indicates, on the other hand, an expansion and increase
of turgor.

The electric response to geotropic stimulus is studied by the two
methods of Axial and Vertical Rotation. The upper side of a horizontally
laid shoot is found to undergo an excitatory change of galvanometric
negativity.

In quick reacting organs the latent period of geo-electric response is
about 5 seconds, and the maximum excitation is induced in the course of
2 minutes.

The geo-electric response is due to physiological reaction. The
intensity of response declines with age and is abolished at the death of
the plant.

Under symmetrical conditions, the intensity of geotropic reaction is
found proportional to the sine of the angle of inclination.

Electric investigation shows that the lower half of the pulvinus of
_Mimosa_ is geotropically more excitable than the upper half.

XLI.--THE MECHANICAL AND ELECTRICAL RESPONSE OF ROOT TO VARIOUS STIMULI

_By_

SIR J. C. BOSE.

In the last chapter we studied the electric response of the shoot to the
stimulus of gravity, and found that the excitatory effect of that
stimulus is similar to that of other forms of stimulation. Before taking
up the subject of the geo-electric response of the root to gravitational
stimulus, I shall describe the effects of other forms of stimuli on the
mechanical and electrical response of the root.

In connection with this subject, it should be borne in mind that the
responsive curvature in the root takes place in the sub-apical growing
zone which is separated by a certain distance from the tip. The stimulus
is therefore direct when applied at the responding growing region; it is
indirect when applied at the tip of the root. The intervening distance
between the root-tip and the responsive zone of growth is
semi-conducting or non-conducting.

I shall proceed to give an account of my investigations on the response
of the root to direct and indirect unilateral stimulation. We shall
study:--

(1) The Mechanical response to Direct unilateral stimulus.

(2) The Electrical response to Direct unilateral stimulus.

(3) The Mechanical response to Indirect unilateral stimulus.

(4) The Electrical response to Indirect unilateral stimulus.

MECHANICAL RESPONSE TO DIRECT STIMULUS.

As the geotropic responses of the shoot and the root are opposed to each
other, the object of the investigation is to find out; whether the
response of the root to various stimuli is specifically different from
that of the shoot. We have seen that tissues in general respond to
direct unilateral stimulus by contraction of the proximal and expansion
of the distal side, the tropic curvature being thus _positive_. We shall
now determine whether direct unilateral stimulation of the root induces
a tropic movement which is similar or dissimilar to that exhibited by
the shoot.

_Experiment 178._--In experimenting with roots of various plants I
obtained results which are precisely similar to that of the shoot. The
movement of the root was observed by means of a reading microscope
focussed on the tip of the organ. I employed various forms of stimuli,
mechanical, thermal, and chemical. Unilateral application of these on
one side of the growing region gave rise to a _positive_ tropic
curvature, resulting in a movement towards the stimulus. These
experiments confirm Sachs' observation that unilateral application of
stimulus in the region of growth induces positive curvature of the root.

ELECTRICAL RESPONSE TO DIRECT STIMULATION.

I next undertook an investigation on the electric response of the root
to direct unilateral stimulation.

_Experiment 179._--The terminals of the galvanometer were suitably
connected with the two diametrically opposite points A and B in the
growing region of the root. Stimulus was now applied very near the point
A, the various stimuli employed in different experiments being: (1)
mechanical, (2) chemical, and (3) thermal. In every instance the
excited point A becomes galvanometrically negative. This shows that the
response of the root is in no way different from that of the shoot.

MECHANICAL RESPONSE TO INDIRECT STIMULUS.

Before describing the effect of indirect stimulus on the root, I shall
recapitulate its effects on ordinary tissues. I have shown that the
effect of indirect unilateral stimulus is to induce a movement away from
stimulus. This was shown to be the case with the bud of _Crinum_ (p.
275) and the tendril of _Passiflora_ (p. 291). The mechanical and
electric response to indirect stimulation in the shoot is shown in the
diagrammatic representation (Fig. 170). I shall now proceed to describe
the mechanical response induced by unilateral stimulation of the root
tip. As the responding region of growth is at some distance from the
tip, the stimulation is therefore indirect.

[Illustration: FIG. 170.--Mechanical and electrical response to indirect
stimulation at dotted arrow. In figure to the left, the point A, on the
same side undergoes expansion, with responsive mechanical movement away
from stimulus indicated by continuous arrow. In figure to the right,
indirect stimulus at dotted arrow induces electric response of
galvanometric positivity at A, indicative of increase of turgor and
expansion.]

_Experiment 180._--I employed at first mechanical stimulus of moderate
intensity by rubbing one side of the tip of the root of _Bindweed_; this
induced a movement away from stimulus. Unilateral application of dilute
acid gave rise to a similar response. Thermal stimulus of moderate
intensity also induced responsive movement away from the stimulus (Fig.
171).

Darwin in his _Movements of Plants_ described experiments on the
responsive behaviour of the tip of the radicle. He produced unilateral
stimulation in three different 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 application of unilateral
stimulus at the root-tip.

[Illustration: FIG. 171.--Diagrammatic representation of mechanical and
electric response of root to indirect stimulus applied at the tip _a_.
Figure to the left shows responsive movement away from stimulus. The
electric response to indirect stimulus is indicated in the figure to the
right; the point on the same side exhibiting galvanometric positivity.
The shaded part indicates the responsive region of growth at some
distance from the 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 the tip to various modes of
unilateral stimulation. Mechanical stimulation was effected by
emery-paper friction or by pin-prick; chemical stimulation was produced
by application of dilute hydrochloric acid. Thermal stimulation was
caused by the proximity of electrically heated platinum wire. In every
case the response was by _induced galvanometric positivity at A_ (Fig.
171). This electrical variation took place within about ten seconds of
the application of stimulus; the interval would obviously depend on the
length of path to be traversed by the transmitted effect of indirect
stimulation.

The galvanometric positivity at A indicated that there was induced at
that point an increase of turgor and expansion, 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 effects 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 XXXVI.--EFFECT OF INDIRECT STIMULUS UNILATERALLY APPLIED AT THE
ROOT-TIP.

+--------------------------------------------------------------+
|Effect at the proximal side A in the | Effect at the distal|
| growing region. | side B. |
+--------------------------------------+-----------------------+
|Galvanometric positivity, indicative | Negligible. |
| of increase of turgor and expansion. | |
|--------------------------------------------------------------|
|The corresponding tropic curvature is negative, _i.e._, a |
|movement away from stimulus. |
+--------------------------------------------------------------+

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 advantage 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-growing. Indirect
stimulus has been shown to give rise, in both shoot and root, to a
_negative_ tropic curvature in 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 stimulations that have
been established regarding plant response in general (p. 231).

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 movement away from stimulus.

The corresponding electric response induced is galvanometric 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.

XLII.--GEO-ELECTRIC RESPONSE OF ROOT

_By_

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 irritability in the two organs, specially
evolved for the advantage of the plant. The root is thus supposed to be
characterised by "positive" and the shoot by "negative" geotropism.

As regards response 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
exploration 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, 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 geotropic 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 be distinguished
as _a_ and _b_, the corresponding points higher up in the growing region
being A and B. When the root is vertical the electric conditions of the
two diametrically opposite points are practically the same. But when the
root is rotated in a vertical plane through +90° a geo-electric response
will be found to take place; the direction of the responsive current
disappears when the root is brought back 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.

_Experiment 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 1·5 mm. from the extreme end.
Owing to the very small size of the tip this is by no means an easy
operation. Two platinum points tipped with kaolin paste are very
carefully adjusted so as to make good electric contacts at the two
opposite sides, 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 top and also from the sides; for
the latter purpose, I use paddings of cotton wool.

[Illustration: FIG. 172.--Diagrammatic representation of geo-electric
response of root-tip. The middle figure shows root in vertical position.
Rotation through +90° places _a_ above, which becomes galvanometrically
negative. Rotation through -90°, places _b_ above and makes it
negative.]

After due observance of these precautions the electric response obtained
is found to be very definite; when the root is made horizontal, by
rotation of the root through +90°, the point _a_ is above, and the
responsive current is found to flow from _b_ to _a_, _the upper side of
the tip_ becoming galvanometrically negative; when the root is brought
back to the vertical, the responsive current disappears; rotation
through -90° makes the point _b_ occupy the upper position, and the
responsive current is from _a_ to _b_; the upper side thus exhibits in
every case, an excitatory electric change of galvanometric negativity
(Fig. 172). The root-tip thus exhibits the characteristic response to
direct stimulation. Experiments carried out with 12 different specimens
gave concordant results. The following table gives the absolute values
of electro-motive force induced at the tip under geotropic stimulus.

TABLE XXXVII.--GEO-ELECTRIC RESPONSE OF THE ROOT TIP (_Vicia Faba_).

+--------------------------+
|Specimen.|Induced E. M. F.|
+---------+----------------+
|1 | 0·0005 volt. |
|2 | 0·0011 " |
|3 | 0·0010 " |
|4 | 0·0015 " |
+--------------------------+

ELECTRIC RESPONSE IN THE GROWING REGION.

_Experiment 183._--I next undertook an investigation on the electric
variation induced in the growing region under the stimulus of gravity.
The experimental difficulties are here greatly reduced, since the
available area of contact for galvanometric connection is not so
restricted as in the case of the root-tip. The specimen is securely
mounted so that the root is vertical. It is next rotated in the vertical
plane through +90°, so that the point A in the growing region occupied
the upper position. The electric response in the growing region took
place in a short time and was very distinct. The induced electric change
at A was now galvanometric _positivity_ indicative of increase of
_turgor and expansion_.

The series of experiments were carried out in the following order. The
specimen was first rotated through +90° so that A was above. The
responsive electric variation rendered it galvanometrically positive.
The root was rotated back to neutral position when the current
disappeared. The root was next rotated through -90° and the responsive
current became reversed, the upper B becoming electro-positive (Fig.
173). The alternative rotations through +90° and -90° were carried out
six times in succession with consistent results. The interval allowed
between one stimulation and the next was determined by the period of
complete recovery. Growing fatigue was found to increase this period; at
first it was seven minutes, at the second repetition it was ten minutes,
and at the third time it was prolonged to fifteen minutes.

[Illustration: FIG. 173.--Diagrammatic representation of geo-electric
response of growing region of root. (_a_) Rotation through -90° makes B,
galvanometrically positive. (_b_) Vertical and neutral position. (_c_)
Rotation through +90° places A above and renders it galvanometrically
positive. (_d_) Additive effect on current of response, root-tip a
negative, and growing region A positive.]

I give below the series of electric responses induced by alternate
rotations through +90° and -90°. The upper position was occupied by A in
the odd series, and by B in the even series. In every case the upper
side became galvanometrically positive.

TABLE XXXVIII.--GEO-ELECTRIC RESPONSE OF ROOT IN THE REGION OF GROWTH.

+---------------------------------------------------------------+
| Odd |Galvanometer deflection| Even |Galvanometer deflection|
|series.| A, positive. |series.| B, positive. |
+-------+-----------------------+-------+-----------------------+
| 1 | 20 divisions. | 2 | 18 divisions. |
| 3 | 16 " | 4 | 18 " |
| 5 | 10 " | 6 | 12 " |
+---------------------------------------------------------------+

ADDITIVE ACTION-CURRENT AT THE TIP AND THE GROWING REGION.

It has been shown that under geotropic stimulus the upper side of the
tip, _a_, becomes galvanometrically negative, 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 response becomes
enhanced.

_Experiment 184._--The root was at first held vertical, 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 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.

TABLE XXXIX.--INDUCED E. M. F. VARIATION BETWEEN THE TIP AND THE GROWING
REGION (_a_ NEGATIVE AND A POSITIVE).

+-------------------------------------------------------+
|Geotropic stimulation. | Resulting electric response.|
+------------------------+------------------------------+
|First stimulation | 140 divisions. |
|Second " | 130 " |
|Third " | 130 " |
|Fourth " | 123 " |
|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 growing region
shows that it is the effect of indirect stimulus that reaches it.

From these facts it will be seen that the tip perceives the stimulus and
thus undergoes excitation, and that owing to the intervening tissue
being a semi-conductor of excitation, it is the positive impulse that
reaches the growing region and induces there an expansion and a 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 stimulus of gravity[37]; he found
that removal of the tip abolished the geotropic response of the root.
Objection has been raised about the shock-effect of operation itself
being the cause of abolition of response. But subsequent observations
have shown that Darwin's conclusions are in the main correct.

[37] "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 perceiving 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 Faba_, _Phaseolus multeflorus_ and _Lupinus
albus_, 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. Charles Darwin's views were therefore in
the main correct."--Haberlandt--_Ibid_, p. 748.

The experiments which I have described on the 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 experiments
exhibit in an identical _uninjured_ organ: the excitatory 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 +90°, 0° and -90°. The effect at the growing zone is
precisely the opposite to that at the tip, _i.e._, an expansive
reaction which results from the effect of indirect stimulus, in contrast
to the contractile reaction due to direct stimulation.

We may now proceed a step further and try to obtain some idea of 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 geotropic 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 must be borne in
mind _that this holds good only in the case of gravitational 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 stimulations, 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 _Sinapis_ is an exceptional
phenomenon for which full explanation has been given in page 376.

We shall next consider whether the particular distribution of the
falling starch-grains (which offers a rational explanation 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 perceptive 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
stimulus 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 irritability of the root has been shown to
be in no way different from that of other organs; an uniformity is thus
found to exist in the reaction of all vegetable tissues.

SUMMARY.

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.

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 irritabilities for
the shoot and the root, since tissues in general exhibit positive or
negative curvatures according as the stimulus is direct or indirect.

XLIII.--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 _external_ 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 _perceiving_ geotropic stimulus by
means of certain localised sense organs, which send out impulses in
response to which neighbouring cells carry out the movement of
orientation in a definite direction. Are the sensitive 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 accessible, the problem would appear to be beyond the reach of
experimental investigation.

It is true that post-mortem examination of sectioned tissues under the
microscope enables us to form a probable hypothesis as regards the
contents of certain cells causing geotropic irritation; we have thus the
very illuminating theory of statoliths propounded by Noll, Haberlandt
and Nemec. But for the clear understanding of the _physiological
reaction_ which induces the orientating movement, it is necessary 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-perceptive cell in the
interior of the organ for carrying out various physiological tests would
appear to be very extravagant; 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.

[Illustration: FIG. 174.--Diagrammatic representation of the
geo-perceptive layer in unexcited vertical, and in excited horizontal
position. (See text.).]

METHOD OF EXPLORATION BY THE ELECTRIC PROBE.

The principle of the new method will be better understood if I first
explained the steps of reasoning by which I was led to discover it. The
experiments described in Chapter XL showed that the upper surface of a
horizontally 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 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-perceptive layer G, which occupies the upper position. This
sensitive layer perceives the stimulus and is therefore the focus of
irritation; the state of excitation is, as we have seen, detected by
induced galvanometric 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 diminishing 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 different 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, however, 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 indicate 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 the electric indication of excitation 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
geotropic 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 their 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 presupposes no hypothesis of
geotropic action. I start with the observed fact that an organ under the
stimulus of 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 contractile
reaction is detected by electro-motive change of galvanometric
negativity, and the opposite expansive reaction by a change of
galvanometric positivity. 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 the probe, and the after-effect of
wound on variation of excitability.

THE ELECTRIC PROBE.

[Illustration: FIG. 175.--The Electric Probe. Figure to the left
represents one electric contact made with sepal of _Nymphæa_, 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.]

The wound-irritation is, however, reduced to a minimum by making the
probe exceedingly thin. A fine platinum 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 insulated 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
transversely from one end to the other; the average diameter of the
capillary is about 0·15 mm. The other end of the platinum wire comes out
of the side of the tubing and is led to one terminal of the
galvanometer, the other being connected with an indifferent point in the
organ. The probe can be gradually pushed into the plant-organ by
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 acts 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 with 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 a
steady value in the course of about 5 minutes.

_Effect of wound on excitability._--I have shewn (p. 81) that severe
wound caused by transverse section induced a temporary 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 _Nymphæa_ 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 was the shoot of
_Bryophyllum_, one contact being made with the side of the stem, and the
other with an indifferent point on the leaf which was always held
vertical. In a particular experiment, the probe was introduced into the
stem through 0·4 mm. and a feeble galvanometric negativity was induced
as the wound-effect. After an interval of 5 minutes, this attained a
steady value of -15 divisions. On the rotation of stem through +90°, the
point A was above and a very much larger deflection of -82 divisions was
obtained, being the result of summation of wound and geo-electric
effects. On restoration of the plant to vertical position the
geo-electric reaction disappeared, leaving the persistent 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 excitatory
reactions at different layers of the organ. The following table gives
true values of geo-electric reaction at different layers of the stem as
the probe entered it by steps of 0·4 mm.

TABLE XL.--SHOWING THE GEO-ELECTRIC REACTION AT DIFFERENT DEPTHS OF
THE ORGAN (_Bryophyllum_).

+-------------------------------------------+
|Position of the| Geo-electric excitation |
|probe. |(galvanometric negativity).|
+---------------+---------------------------+
| Surface | 5 divisions. |
| 0·4 mm. | -20 " |
| 0·8 " | -24 " |
| 1·2 " | -22 " |
| 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 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 be experimentally localised by
measuring the depth of intrusion of the probe for maximum deflection of
galvanometric negativity._

_Localisation of geo-perceptive layer in_ Nymphæa: _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 _Nymphæa_. The
electric reaction in _Nymphæa_, even under the prevailing unfavourable
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 XLI.--SHOWING THE DISTRIBUTION OF INDUCED GEO-ELECTRIC EXCITATION
IN DIFFERENT LAYERS (_Nymphæa_).

+--------------------------------------------+
|Position of probe.|Galvanometric deflection.|
+------------------+-------------------------+
| Surface | 0 divisions. |
| 0·4 mm. | -16 " |
| 0·8 " | -42 " |
| 1·2 " | -20 " |
| 1·6 " | -10 " |
| 2·0 " | -2 " |
| 2·4 " | 0 " |
+--------------------------------------------+

It will be seen that as in _Bryophyllum_, so in _Nymphæa_, the
geo-electric excitation increased at first with increasing depth of the
tissue till at a depth of 0·8 mm. of the particular specimen the induced
excitation attained a maximum value. The excitatory effect then
declines till it vanished 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 _Bryophyllum_ 3·6 mm. in diameter the geo-perceptive
layer was found at a depth of 0·6 mm., it occurred at the greater depth
of 0·8 mm. in a thicker specimen, 5 mm. in diameter. In _Nymphæa_ also
the perceptive layer was found at a depth of 0·8 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 microscope. I
also wished to find out from microscopic examination, the cause of
certain differences noticed in the determinations of the perceptive
layer in _Bryophyllum_ and in _Nymphæa_. 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 that the
sensitive layer in _Bryophyllum_ was continuous round the axis. In
_Nymphæa_, 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 passage of the probe. Thus in a
particular experiment with _Bryophyllum_ the point of maximum geotropic
excitation was found to be at a distance of 0·8 mm. from the surface. By
means of the micrometer slide in the stage and the micrometer eye-piece,
the internal layer 0·8 mm. from the surface was examined; the particular
sensitive layer S was recognised as the _continuous_ 'starch sheath' or
endodermis containing unusually large sized starch grains (Fig. 176).
These often occurred in loosely cohering groups of 8 to 10 particles,
and their appearance is very different from the small sized irregularly
distributed grains in other cells.

Examination of the microscopic section of the flower stalk of _Nymphæa_
showed 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 gaps, are arranged in a circle.

[Illustration: FIG. 176.--Transverse section showing continuous
geo-perceptive layer S; enlarged view S' of cell of endodermis
containing group of large starch grains. (_Bryophyllum_).]

I give below a number of experimental determinations of the
geo-perceptive layer in different specimens together with the
micrometric measurement of the distance of the 'starch sheath' from the
surface, the transverse section being made at the place where the probe
entered the shoot. Eight different determinations are given, three for
_Bryophyllum_ and five for _Nymphæa_.

TABLE XLII.--SHOWING THE POSITION OF THE GEO-PERCEPTIVE LAYER AND
OF 'STARCH SHEATH' IN DIFFERENT SPECIMENS.

+-----------------------------------------------------------+
| Specimen. | Distance of | Distance of the |
| | geo-perceptive layer | starch sheath |
| | from surface. | from surface. |
| | (Method of | (Microscopic |
| | electric probe.) | measurement.) |
+---------------+----------------------+--------------------+
|_Bryophyllum_: | | |
| | (1) 0·6 mm. | 0·6 mm. |
| | (2) 0·8 " | 0·8 " |
| | (3) 0·8 " | 0·8 " |
| _Nymphæa_: | | |
| | (1) 0·6 " | 0·6 " |
| | (2) 0·8 " | 0·8 " |
| | (3) 0·8 " | 0·8 " |
| | (4) 1·0 " | 1·0 " |
| | (5) 1·4 " | 1·4 " |
+-----------------------------------------------------------+

Thus in all specimens 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 investigation. The statolithic theory has been adversely
criticised 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 special geotropic sense-organs--a striking instance of
anatomico-physiological division of labour."[38]

[38] Haberlandt--_Ibid_, p. 597.

In the instances of _Bryophyllum_ and _Nymphæa_ given above, the
geo-perceptive layer localised by means of the electric probe is
definitely found to be the endodermis containing large sized starch
grains.

INFLUENCE OF SEASON ON GEO-ELECTRIC RESPONSE.

I shall now describe certain modifications in response, 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 Kysoor_ 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 advance of season in seedlings of _Zea Mays_, where a
very rapid fall in growth takes place in the course of a fortnight. The
intensity of geotropic responses, both mechanical and electrical, of
_Tropæolum_ declines rapidly in the course of a month from February to
March (p. 454). The flowers of _Nymphæa_ 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 abolished.

Much time had to be spent in perfecting the apparatus, and it was not
till the beginning of August that the investigations could be properly
started; the responsive 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 the advance of the season. This may partly
be due to unfavourable season, and partly to high temperature. In the
middle of the season the responses were extremely feeble on warm days,
but on cool mornings they became suddenly enhanced, to decline once more
by the middle of the day. I could sometimes succeed in enhancing the
sensitiveness by placing the specimen in a cold chamber. It thus
appeared that certain internal change unfavourable for geo-perception
takes place at high temperatures, and that the sensitive condition could
sometimes be restored by artificial cooling. But later in the season,
the internal change, whatever it may be, had proceeded too far, and
artificial cooling did not restore the sensitiveness of the specimen.
What are the physico-chemical concomitants which distinguish insensitive
specimens, in which the electric indications had declined almost to the
vanishing point?

TEST OF INSENSITIVE SPECIMENS.

I shall now describe the various physico-chemical concomitants 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, _below 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 physiological condition of the tissue
falls _below par_, the sign of response undergoes a reversal into one of
_galvanometric positivity_. The same reversal under condition of
sub-tonicity 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
insensitive 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 geotropic reaction: Experiment 188._--I took four different
specimens of _Bryophyllum_ and _Nymphæa_, and held them horizontal.
These plant organs had, earlier in the season, exhibited very strong
geotropic effect, the shoot curving up through 90° in the course of ten
hours or less. But these specimens obtained later in the season
exhibited 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
_Bryophyllum_ and _Nymphæa_ 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 be associated with the sensitiveness of the perceptive layer.

REACTION AT LOWER SIDE OF THE ORGAN.

There remains now the important question of 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 expansion.
It may be urged that the effect of one of the two sides might result
from the passive yielding to the definite reaction induced on the
opposite side. Investigation by the electric method enables us, however,
to discriminate 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 therefore 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 successive
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 introduced from
above, and successive readings for the response taken for different
positions of the probe as it enters the organ from the upper side and
comes out ultimately at the lower side. This I shall call the METHOD OF
TRANSVERSE PERFORATION. The intrusion of the probe on the upper side
gives, as we have seen, increasing negative deflection of the
galvanometer which reaches a maximum at the perceptive layer. Passage of
the probe to still greater depths give deflections which decline to
zero. But when the probe comes within the influence of the perceptive
layer of the under side, the electric indication, as we shall presently
find, undergoes a reversal.

ELECTRIC EXPLORATION OF THE LOWER SIDE OF THE ORGAN.

I shall first describe the results obtained from the first method, the
probe entering the organ from the lower side.

_Experiment 189._--The investigation was carried out with the stem of
_Bryophyllum_, and the flower stalk of _Nymphæa_. The probe was made to
enter the organ through 0·4 mm. and the geo-electric effect found, on
rotation of the flower stalk of _Nymphæa_ from the vertical to the
horizontal, was a deflection of +6 divisions of the galvanometer. _The
change induced at the lower side by geotropic stimulus is thus
galvanometric positivity, indicative of enhancement of turgor, and, of
expansion._ Intrusion of the probe through 0·6 mm. gave rise to an
increased positive geo-electric response. That the sign of electric
response depended on the relation of the side of the organ to the
vertical lines of gravity was demonstrated by alternate rotation of the
plant through +90° and -90°, the probe remaining at a definite position.
Rotation through +90° brought A above, and rotation through -90° brought
A below. When the probe was in the _up_ position the geo-electric
response was negative, but when rotation through -90° brought it
_below_, the response became positive. Thus with an identical contact in
the plant, the electric response underwent reversal from negative to
positive. This will be understood from the following table.

It will thus be seen that physiological change induced at any point is
modified by its relation to vertical lines of gravity. When the point is
above, the induced change is _negative_, when below, the induced change
is _positive_.

I shall next describe the variation of effect at different layers of the
under side of the organ.

_Experiment 190._--A complete set of readings of the geo-electric
reaction at different layers of the organ was taken, as the probe
entered the lower side by successive steps of 0·2 mm. The following
table gives the results obtained with a specimen of _Nymphæa_.

TABLE XLIII.--ELECTRIC EXPLORATION OF DIFFERENT LAYERS ON THE LOWER SIDE
OF THE ORGAN (_Nymphæa_).

+-----------------------------+
|Position of the|Galvanometer |
| probe. | deflection. |
+---------------+-------------+
| Surface | 2 divisions.|
| 0·2 mm. | 4 " |
| 0·4 " | 8 " |
| 0·6 " |16 " |
| 0·8 " |20 " |
| 1·0 mm. |32 " |
| 1·2 |16 " |
| 1·4 " |12 " |
| 1·6 " | 4 " |
| 1·8 " | 0 " |
+-----------------------------+

It is thus seen that just as in the upper so also in the lower side, the
electric variation undergoes at first an increase which attains a
maximum; beyond this point the electric change undergoes a rapid
decline. The induced electric change on the upper and lower sides are,
however, different, galvanometric _negativity_ in one case and
_positivity_ in the other.

The maximum galvanometric _negativity_ of the upper side was found to
occur at the geo-perceptive layer. We may next inquire about the
anatomical characteristic of the layer in the lower side of the organ
which exhibits the maximum galvanometric _positivity_. Microscopic
section of the specimen employed in the above experiment showed the
particular layer to be the starch crescent which lies above the vascular
bundle. Thus the same geotropic layer which when placed above shows the
maximum galvanometric negativity, exhibits maximum positivity when
placed below.

METHOD OF TRANSVERSE PERFORATION.

_Experiment 191._--I next carried out a complete exploration of the
interior of the organ along the diameter. The probe started from the
upper surface, and came out at the lower by successive steps of 0·2 mm.,
the corresponding geo-electric effects being observed at each step. It
has to be borne in mind that the successive readings were obtained by
rotation from vertical to +90° (A above); the rotation was never carried
out in the negative direction through -90°. But the probe entering from
above passed the central axis, and entered a region where the
galvanometric indication was transformed from negative to positive. The
following table gives the results obtained with the flower stalk of
_Nymphæa_.

TABLE XLIV.--SHOWING THE INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE
FLOWER STALK OF _Nymphæa_ (diameter = 6·8 mm.)

+---------------------------------+
| Position of | Galvanometer |
| probe. | deflection. |
+--------------+------------------+
| Surface ... | - 10 divisions. |
| 0·2 mm. ... | - 26 " |
| 0·4 " ... | - 40 " |
| 0·6 " ... | - 50 " |
| 0·8 " ... | - 62 " |
| 1·0 " ... | - 72 " |
| 1·2 " ... | - 88 " |
| 1·4 " ... | -108 " |
| 1·6 " ... | - 72 " |
| 1·8 " ... | - 44 " |
| 2·0 " ... | - 30 " |
| 2·2 " ... | - 18 " |
| 2·4 " ... | - 10 " |
| 2·6 " ... | - 5 " |
| 2·8 " ... | - 2 " |
| 3·0 " ... | 0 " |
| 3·2 " ... | 0 " |
| 3·4 " ... | 0 " |
| 3·6 mm. ... | 0 divisions. |
| 3·8 " ... | 0 " |
| 4·0 " ... | 0 " |
| 4·2 " ... | + 2 " |
| 4·4 " ... | + 4 " |
| 4·6 " ... | + 5 " |
| 4·8 " ... | + 11 " |
| 5·0 " ... | + 22 " |
| 5·2 " ... | + 38 " |
| 5·4 " ... | + 46 " |
| 5·6 " ... | + 39 " |
| 5·8 " ... | + 32 " |
| 6·0 " ... | + 24 " |
| 6·2 " ... | + 18 " |
| 6·4 " ... | + 12 " |
| 6·6 " ... | + 6 " |
| 6·8 " ... | + 3 " |
+---------------------------------+

[Illustration: FIG. 177.--Curve of geo-electric excitation in different
layers of _Nymphæa_. Ordinate represents geo-electric excitation;
abscissa, distance from upper surface of flower stalk. The diagrammatic
section underneath shows the position of geo-perceptive layer
(starch-sheath) corresponding to maximum induced galvanometric
negativity and positivity on the two sides.]

[Illustration: FIG. 178.--The curve of geo-electric excitation in
different layers of _Bryophyllum_.]

A curve constructed from the data given above is seen in figure 177. The
diameter of the flower stalk was 6·8 mm. The negative geo-electric
reaction is seen to undergo an increase till it attains a climax at the
depth of 1·4 mm. It then undergoes a continuous diminution till it
becomes zero at the depth of 3 mm.; this neutral zone extends through 1
mm. When the probe enters a depth of 4·2 mm. measured from the upper
side, it enters a region affected by the perceptive layer situated on
the under side, the opposite physiological reaction being indicated by
induced electric change of galvanometric positivity. This positivity
reaches a climax at a depth of 5·4 mm. measured from the upper side, and
1·4 mm. when measured from the lower side. The points of maximum
positivity and negativity are situated symmetrically on the opposite
sides of the organ. The electric variation of maximum positivity on the
lower side is comparatively feeble, less than half the corresponding
maximum negativity on the upper side. Microscopic section showed that
the geo-perceptive layers were the same as the starch-crescents.

_Experiment 192._--I carried out similar experiments with the shoot of
_Bryophyllum_. The results are given in Table XLV; the curve of the
electric distribution along the diameter is seen in figure 178. The
characteristics of this curve are the same as that of _Nymphæa_. The
maximum galvanometric negativity occurred at the depth of 0·6 mm., and
of positivity at a corresponding point on the opposite side.

TABLE XLV.--SHOWING INDUCED GEO-ELECTRIC DISTRIBUTION ACROSS THE STEM
OF _Bryophyllum_ (diameter = 3·6 mm.).

+--------------------------------+
| Position of | Galvanometric |
| probe. | deflection |
+---------------+----------------+
|Surface | 0 divisions.|
|0·2 mm. | -24 " |
|0·4 " | -45 " |
|0·6 " | -63 " |
|0·8 " | -21 " |
|1·0 " | - 9 " |
|1·2 " | - 6 " |
|1·4 " | - 3 " |
|1·6 " | 0 " |
|1·8 " | 0 " |
|2·0 " | 0 divisions.|
|2·2 " | 0 " |
|2·4 " | + 3 " |
|2·6 " | + 4 " |
|2·8 " | + 9 " |
|3·0 " | +36 " |
|3·2 " | +21 " |
|3·4 " | + 9 " |
|3·6 " | 0 " |
+--------------------------------+

Microscopic examination showed that the electric maxima in _Bryophyllum_
coincided with the diametrically opposite points in the continuous
endodermic ring. In _Bryophyllum_ as in _Nymphæa_, the excitatory
galvanometric negativity of the upper geo-perceptive layer is greater
than the induced positivity of the lower layer in the ratio of about
2:1. But in a depressed condition of the tissue, the excitatory reaction
is the first to disappear and the positive reaction persists, though
with diminished intensity.

The geo-electric distribution in vigorous specimens seems to indicate
that under the stimulus of gravity a marked excitatory reaction
(contraction) takes place in the layer of cells contiguous to the upper
geo-perceptive layer, and, a less marked positive reaction (expansion)
occurs in layers contiguous to the lower perceptive layer.

It is remarkable that physiological reaction of opposite kinds should
occur on the upper and lower sides of an organ under the identical
stimulus of gravity. The difference of reaction may conceivably be
connected with the fact that the vertical lines of gravity enter by the
upper, and leave by the lower side of the organ. The statolithic
particles rest on the inner tangential walls of the perceptive cells of
the upper layer, and on the outer tangential walls of the lower layer.
Similar difference of physiological reactions of a polar character are
also known in responses of plants under the action of an identical
electric current; here with different ionic distributions, contraction
takes place at the kathode, and expansion at the anode.

The geo-electric reactions that have been described were obtained under
unfavourable conditions of climate and of temperature. But under better
conditions the reaction becomes very greatly enhanced, as would appear
from the following account of results which I obtained on two separate
occasions in the beginning of August. The season had not become quite as
unfavourable as towards the end of the month, but the prevailing sultry
weather had caused great depression of the geo-electric excitability.
On the first occasion referred to, thunderstorm had broken out at night,
and it was refreshingly cool in the morning. It was with the utmost
surprise that I noted the astonishing violence of the geo-electric
response which the plants gave that morning; the maximum response
hitherto obtained was about 100 divisions of the galvanometer scale; but
on the present occasion the displacement of the plant, from vertical to
horizontal position, induced responsive deflection so great that the
galvanometer spot of light flew off the scale of 3,000 divisions. I was
at first incredulous of the results and wasted the valuable occasion in
trying to discover some hidden source of error. Subsequent tests showed
that my misgivings were groundless, and that the extraordinary large
deflection was really due to geo-electric reaction. On the second
favourable occasion, which lasted for three hours (during the cool hours
of the morning), I was able to secure a number of important
observations. Thus displacement of the flower stalk of _Nymphæa_ through
+90° was immediately followed by geo-electric response, the deflection
being about 3,000 divisions of the scale. The latent period hardly
exceeded a second; the return of the plant to the vertical position was
quickly followed by electric recovery which was complete. The above
results were obtained with the same specimen time after time without a
single failure. The successive responses showed no sign of fatigue.
Another remarkable effect was noticed during gradual increase of the
angle of inclination. Nothing happened till a critical angle was
reached, which was roughly estimated to be about 33°; when this critical
angle was exceeded by a single degree, there was a sudden precipitation
of geo-electric response. The experiments were repeated time after time
with the identical result. It appeared as if some frictional resistance
obstructed the displacement of the geotropic particles accumulated at
the basal end of the cell, and it was not till the organ had been
tilted beyond 33° that this resistance to sliding was overcome.

SUMMARY.

The electric distribution induced in an organ under the stimulus of
gravity may be mapped out by means of an exploring Electric Probe.

The induced galvanometric negativity of the upper side of an organ
(indicative of excitation) undergoes variation in different layers of
the organ. The excitatory reaction attains a maximum value at a definite
layer, beyond which there is a decline.

The geo-perceptive layer is experimentally localised by measuring the
depth of intrusion of the probe for maximum deflection of galvanometric
negativity.

The geo-perceptive layer thus determined is found to be the starch
sheath which contains a number of large-sized starch grains.

The power of geo-perception undergoes seasonal variation. It is also
lowered by high temperature.

The geo-electric response undergoes decline with growing sub-tonicity of
the specimen; such specimens exhibit abnormal positive electric response
under the stimulus of prick and feeble curvature under geotropic
stimulus. The large-sized starch-grains, normally observed in the
endodermis, are found to disappear in specimens which have become
geo-electrically insensitive.

The electric response of the lower side of the organ to gravitational
stimulus is of opposite sign to that of the upper side. The electric
distribution on the lower side exhibits variations in different layers,
the maximum positivity occurring at the perceptive layer. In vigorous
specimens the excitatory negative electric change on the upper side is
greater than the positive electric change on the lower side. Depressed
condition of the tissue is attended by a relatively greater decline of
the negative in comparison with the positive.

The induced electric variation on the upper and on the lower side
indicates that the layers of tissue contiguous to the upper perceptive
layer undergoes contraction, while those contiguous to the lower
perceptive layer undergoes expansion.

XLIV.--ON GEOTROPIC TORSION

_By_

SIR J. C. BOSE,

_Assisted by_

GURUPRASANNA DAS.

I have explained that in a dorsiventral organ, lateral application of
various stimuli induces a responsive torsion by which the less excitable
side is made to face the stimulus (p. 403). I shall in this chapter show
that the effect of stimulus of gravity is in every respect similar to
other forms of stimulation.

[Illustration: FIG. 179.--Diagram of arrangement for torsional response
under geotropic stimulus. The less excitable upper half of pulvinus is,
in the above figure, to the left and the torsional response is
clockwise.]

The direction of force of gravity is fixed, and we have to arrange
matters in such a way that the geotropic stimulus should act on the
dorsiventral organ in a lateral direction. In the following experiments
the pulvinus of _Mimosa_ is taken as the typical dorsiventral organ.
For lateral stimulation, the plant is placed on its side, so that the
vertical lines of gravity impinge on one of the two flanks of the organ.
In regard to this, I shall distinguish two different positions, _a_ and
_b_. In the _a-position_, the apex of the stem and the upper half of the
pulvinus are to the left of the observer, and in _b-position_, the apex
of the stem and the less excitable upper half of the pulvinus are to the
right. The arrangement for obtaining record of the torsional response
under _a-position_ is shown in figure 179.

_Torsional response in a- and b-positions: Experiment 193._--When the
leaf is in _a-position_, the geotropic torsion is found to be with the
movement of the hands of a clock. In the _b-position_, on the other
hand, the torsion is against the hands of a clock. In both these cases
the _geotropic torsion makes the less excitable upper half of the
pulvinus face the vertical lines of gravity_. The incident stimulus is
vertical, and it is the upper flank, consisting of the upper and lower
halves of the pulvinus (on which the vertical lines of gravity impinge)
that undergoes effective stimulation.

_Algebraical summation of geotropic and phototropic effects: Experiment
194._--We are, however, able to adduce further tests in confirmation of
the above. If the direction of the incident geotropic stimulus is
vertical, and should it act more effectively on the upper flank, it
follows that stimulus of light acting from above would enhance the
previous torsional response due to geotropism. In the above case, the
lines of gravity and the rays of light coincide. The effect of rays of
light acting from below should, on the other hand, oppose the geotropic
torsion. The additive effect of stimulus of light and gravity is seen
illustrated in figure 180. The first part of the curve is the record of
pure geotropic torsional movement. Light from above is applied at L;
the rate of movement is seen to become greatly enhanced. Light is next
cut off, and the enhanced rate induced by it is also found to disappear,
the response-curve being now due solely to geotropic action. The effect
of geotropism in opposition to phototropism will be found in the
following experiments, where the opposing action of light of different
intensities is seen to give rise to a partial, to an exact, or to an
over-balance.

[Illustration: FIG. 180.--Additive effect of stimulus of gravity G, and
of light L. Application of light at--L increases torsional response.
Removal of light restores original geotropic torsion.]

[Illustration: FIG. 181.--Algebraical summation of geotropic and
phototropic actions. Light applied below at--L, opposes geotropic
action. Cessation of light restores geotropic torsion. Cessation of
light is indicated by L within a circle.]

BALANCE OF GEOTROPIC BY PHOTOTROPIC ACTION.

_Photo-geotropic balance: Experiment 195._--I shall here describe in
detail the procedure for obtaining an exact balance. A parallel beam of
light from a small arc lamp is reflected by means of an inclined mirror,
so as to act on the pulvinus below. An iris diaphragm regulates the
intensity of incident light. The first part of the curve is the record
of geotropic torsional movement. Light of a given intensity was applied
below at a point marked -L (Fig. 181); this is seen to produce an
over-balance, the phototropic effect being slightly in excess. The
intensity of incident light was continuously diminished by regulation of
the diaphragm till an exact balance was obtained as seen in the
horizontal part of the record. It is with great surprise that one comes
to realise the fact that the effect of one form of stimulus can be so
exactly balanced by that of another, so entirely different, and that the
stimulus of gravity could be measured, as it were, in candle powers of
light! After securing the balance, light was cut off, and the geotropic
torsion became renewed on the cessation of the counteracting phototropic
action.

[Illustration: FIG. 182.--Application of white light at--L in opposition
causes reversal of torsion. Red light R, is ineffective, and geotropic
torsion is restored. Reapplication of white light causes once more the
reversal of torsion.]

_Comparative balancing effects of white and red lights: Experiment
196._--White light was at first applied at -L in opposition to geotropic
movement. The intensity of light was stronger than what was necessary
for exact balance, and its effect was at first to retard and then
reverse the torsional response due to geotropism. When thus
overbalanced, red glass was interposed on the path of light at R. As the
phototropic effect of this light is feeble or absent, the geotropic
torsion became predominant as seen in the subsequent up-curve. The red
glass was next removed substituting white light at -L to act once more
in opposition; the result is seen in the final over-balance, and
reversal of torsion (Fig. 182).

[Illustration: FIG. 183.--Effect of coal gas on photo geotropic balance.
Geotropic torsion, G, is exactly balanced by opposing action of light
-L. Application of coal gas at C, at first caused enhancement of
phototropic action with resulting reversal. Prolonged application
induced depression of phototropic reaction, geotropic action thus
becoming predominant.]

_Effect of coal gas on the balance: Experiment 197._--The method of
balance described above opens out new possibilities in regard to
investigations on the relative modifications of geotropic and
phototropic excitabilities by a given external change. Traces of coal
gas are known to enhance the phototropic excitability of an organ while
continued absence of oxygen is found to depress it. The experiment I am
going to describe shows: (1) the enhancement of phototropic excitability
on the introduction of coal gas, and (2) the depressing effect of excess
of coal gas and of the absence of oxygen. After obtaining the normal
curve of geotropic torsion, light was applied below at -L, and exact
balance was obtained in the course of two minutes as seen in the top of
the curve becoming horizontal. Coal gas was now introduced in the
plant-chamber at C. This induced an enhancement of phototropic effect
with resulting over-balance seen in the reversal of torsion. This
enhancement persisted for more than three minutes. By this time the
plant-chamber was completely filled with coal gas, and the resulting
depression of phototropic action is seen in the second upset of the
balance, this time in favour of geotropic torsion (Fig. 183). It would
seem that the cells which respond to light are situated nearer the
surface of the organ than those which react to geotropic stimulus. Hence
an agent which acts on the organ from outside, induces phototropic
change earlier than variation in geotropism.

SUMMARY.

Under lateral action of geotropic stimulus, a dorsiventral organ
undergoes torsional response by which the less excitable half of the
organ is made to face the stimulus.

The direction of incident geotropic stimulus is the same as the
direction of vertical lines of gravity. Under geotropic stimulus it is
the upper side of the organ that undergoes effective stimulation.

The effects of gravity and of light become algebraically summated under
their simultaneous action. Light may be made to act in opposition to the
stimulus of gravity. By suitable adjustment of the intensity of light,
the two torsions become exactly balanced.

This state of balance is upset by any slight variation in one of the
opposing stimuli.

The relative modification of geotropic and phototropic excitabilities by
an external agent, is determined by the resulting upset of the
photo-geotropic balance.

XLV.--ON THERMO-GEOTROPISM

_By_

SIR J. C. BOSE.

I shall in this chapter investigate the effect of variation of
temperature on geotropic response. We have to bear in mind in this
connection, that for the exhibition of geotropic curvature two
conditions are necessary: (1) the presence of a perceptive organ to
undergo excitation under the stimulus of gravity, and (2) the motility
of the organ. A motile organ, including both the pulvinated and growing,
will exhibit no geotropic effect on account of the depression of the
power of perception through seasonal or other changes, or in the entire
absence of the perceptive organ. The organ may, on the other hand,
possess the geo-perceptive apparatus, but no visible movement can take
place in the absence of motility of the tissue.

As regards the modifying influence of temperature on geotropic
curvature, the effect will depend on two factors:

(1) the influence of variation of temperature on
geo-perception by the sensitive layer, and

(2) the modifying effect of temperature variation
on the motile reaction.

I have in Chapter XLIII adduced facts which appear to show that the
power of geo-perception declines at high temperatures. As regards motile
reaction, we have seen that in _Mimosa_ it increases from a minimum to
an optimum temperature beyond which there is a depression (p. 55). As
the optimum temperature for geo-perception is not necessarily the same
as that for responsive curvature, the result is likely to be very
complex.

The case becomes simpler after the attainment of maximum curvature.
Enhanced temperature has a tendency to diminish the tropic curvature, as
we found in the arrest and reversal of phototropic curvature under the
application of warmth (p. 393); it appears as if rise of temperature
induced a relatively greater expansion of the contracted side of the
organ.

I shall now describe the effect of rising temperature on geotropic
curvature in general, including torsion. A horizontally laid shoot
curves upwards under geotropic action; a dorsiventral organ, owing to
the differential excitabilities of its upper and lower sides, places
itself in the so-called dia-geotropic position. A dorsiventral organ,
moreover, exhibits a torsional movement under lateral stimulus of
gravity.

In the geotropic movements we are able, as stated before, to distinguish
three different phases (cf. Fig. 161). In the first, the movement
initiated undergoes an increase; in the second, the rate of movement
becomes more or less uniform; and in the last phase, a balance takes
place between the tropic reaction, and the increasing resistance of the
curved or twisted organ to further distortion.

The question now arises whether this position of geotropic equilibrium
is permanent, or whether it undergoes modification in a definite way by
variation of temperature. I shall proceed to show that the position of
equilibrium undergoes a change in one direction by a rise, and in the
opposite direction by a fall of temperature. I shall use the term
_thermo-geotropism_ as a convenient phrase to indicate the effect of
temperature in modification of geotropic curvature and torsion.

I shall first deal with the effect of variation of temperature on
geotropic torsion. Under the continued action of stimulus of gravity the
torsion increases till it reaches a limit; for the twisted organ
resists further distortion and a balance is struck when the twisting and
untwisting forces are equal and opposite. In this state of equilibrium
the effect of an external agent, say of variation of temperature, will
bring about an upset of the balance. The torsion will be increased if
the external agent induces an enhancement of geotropic action; it will,
on the other hand, be decreased when it induces a diminished reaction.

[Illustration: FIG. 184.--Magnet M causes deflection of the needle _n
s_, suspended by a thin wire. Increase of magnetisation of M increases
deflection, while decrease of magnetisation diminishes the deflection.]

A physical analogy will make this point clear; imagine a small magnetic
needle suspended by a thin wire; the earth's directive force is supposed
to be annulled by the well known device of a compensating magnet. A
second and larger magnet M is now placed at right angles to the
suspended needle; N will repel _n_ and attract _s_, and a deflection
will be produced, the deflecting force of the magnet M being balanced by
the force of torsion of suspending wire (Fig. 184).

The state of equilibrium will however be disturbed by variation of the
magnetic force of M. It is known that a rise of temperature diminishes
magnetisation while lowering of temperature increases it. Hence the
deflecting force of the magnet will be diminished under rise of
temperature with concomitant diminution of deflection of the needle and
the torsion of the wire. Fall of temperature, on the other hand, will
cause an increase of deflection and of torsion. The physical
illustration given above will help us to understand how the
physiological effect of variation of temperature may bring about changes
in geotropic curvature and torsion.

TROPIC EQUILIBRIUM UNDER VARYING INTENSITIES OF STIMULUS.

The following experiment will show that the position of tropic
equilibrium is not fixed but subject to variation under changes of
effective stimulation.

[Illustration: FIG. 185.--Effect of variation of intensity of light on
phototropic equilibrium. Increase of intensity of light from L to L'
produces an increased positive curvature and a new state of balance.
Diminished intensity of light _l_ brings about a new balance at a lower
level. The cessation of light (_l_ within a circle) restores the normal
position of the organ.]

_Experiment 198._--I have explained how a maximum tropic curvature is
induced under continued action of light. Employing the pulvinus of
_Erythrina indica_ I applied light on the upper half of the pulvinus:
(1) of medium intensity L, (2) of strong intensity L', and (3) of feeble
intensity _l_. The source of light was an arc lamp; the intensity of
light was varied by means of a focussing lens, which gave a parallel, a
convergent or a divergent beam, with corresponding increase or
diminution of intensity of light. Light was in each case continued till
equilibrium was reached. Inspection of figure 185 shows that the
position of equilibrium depends on the intensity of stimulation; the
balance is 'raised' under increased and 'lowered' under decreased
intensity.

In the case of geotropism the stimulus is constant, but its tropic
effect, we shall presently see, undergoes variation with changing
temperature.

EFFECT OF VARIATION OF TEMPERATURE ON GEOTROPIC TORSION.

_Modification of geotropic torsion: Experiment 199._--The _Mimosa_ plant
was placed on its side, so that the pulvinus was subjected to lateral
geotropic action. In response to this it underwent torsion, the upper
half of the pulvinus tending to place itself so as to face the vertical
lines of gravity. This torsional response was recorded as an
up-movement; on the attainment of equilibrium the record became
horizontal. The plant was now subjected to a cyclic variation of
temperature, and the resulting variation of torsion recorded at the same
time. The temperature of the plant chamber was gradually raised from the
normal 30° to 34° C. and then allowed to return to the normal; finally
the temperature was lowered to 26°C. Rise of temperature was effected by
means of an electrical heater placed inside the chamber with a vessel of
water placed above it. Care has to be taken that the rise of temperature
is gradual, since a sudden variation often acts as a stimulus. The water
in the vessel not only keeps the chamber in a humid condition but also
prevents sudden fluctuation of temperature. After the temperature had
been raised to 34°C., the heating current was stopped and the door of
the plant chamber gradually opened, so as to allow the temperature to be
restored to the normal. Cooled air was next introduced into the chamber
till the temperature fell to 26°C. Figure 186 exhibits clearly the
effect of variation of temperature on geotropic torsion. The maximum
torsion had been attained at 30°C. and the first part of the record is
therefore horizontal. Warmth was applied at H, and after a latent period
of ten minutes, the geotropic torsion underwent a continuous diminution
till a new state of equilibrium was reached at 34°C. This took place
shortly after the stoppage of the heating current at (H). On return to
normal temperature the torsional balance was restored to its original
position of equilibrium. Application of cold at C, is seen to bring
about a new state of balance with an increase of geotropic torsion.

[Illustration: FIG. 186.--Effect of variation of temperature on
geotropic torsion. Application of warmth at H diminishes the geotropic
torsion; return to normal temperature (H) restores the original torsion;
cooling at C, increases the geotropic torsion.]

The position of geotropic equilibrium is thus seen to be modified by
variation of temperature, the tropic effect being diminished with the
rise, and enhanced with the fall of temperature.

It may be thought that the phenomenon just described may not be
different from ordinary thermonasty, exhibited by the perianth leaves of
_Crocus_ and _Tulip_ in which a rise of temperature induces a movement
of unfolding, and a fall of temperature brings about the opposite
movement of closure. In these cases the movement is determined solely
by the natural anisotropy of the organ, and not by the paratonic action
of a directive external force. Thus the inner side of the perianth
leaves undergoes an expansion with rise of temperature attended by the
opening of the flower; this movement of opening does not undergo any
change on holding the flower in an inverted position.

But the torsional movement of the leaf of _Mimosa_, and the induced
variation of torsion under change of temperature are not solely
determined by the natural anisotropy of the organ; it is, on the
contrary, regulated by the directive action of the stimulus of gravity.
The pulvinus in normal position does not exhibit any geotropic torsion
and in the absence of an antecedent torsion change of temperature cannot
induce any variation in it. It is only after the pulvinus had become
torsioned under the lateral action of geotropic stimulus that a
responsive variation is induced in it by the action of changing
temperature.

The change in torsion is, moreover, determined in reference to the
paratonic action of incident geotropic stimulus. This will be clearly
understood from the tabular statement given below.

TABLE XLVI.--SHOWING THE EFFECT OF RISE OF TEMPERATURE ON GEOTROPIC
TORSION.

By right flank in the above table is meant the side of the pulvinus to
the right of the observer facing the leaf of the plant held in the
normal position. When the plant is laid on its left side in the
_a_-position, the right flank will be above and the responsive torsion
under geotropic stimulus becomes right handed or with the hands of a
clock (Cf. Fig. 179). When the plant is laid on its right side, the left
flank will be above and the geotropic torsion becomes left handed or
against the hands of the clock.

It will be seen from the above that in whatever way the experimental
condition may be varied, the movement in response to variation of
temperature is determined in relation to the antecedent geotropic
torsion. The geotropic effect whether left-handed or right-handed
torsion is always diminished by the rise of temperature, and enhanced by
the fall of temperature.

VARIATION OF APO-GEOTROPIC CURVATURE UNDER THERMAL CHANGE.

I shall now proceed to show that variation of temperature not merely
induces variation of geotropic torsion but also of geotropic curvature.
I shall first demonstrate the effect of thermal change on geotropic
curvature of the shoot, and then demonstrate its effect on dia-geotropic
curvature of leaves.

_Experiment 200._--A specimen of _Tropæolum majus_ grown in a small
flower pot, is laid on its side. Under geotropic action the shoot
becomes curved, the upper side becoming concave and the lower side
convex. The end of the stem is attached to the recording apparatus; when
the plant is subjected to a rise of temperature, the movement induced
shows that the geotropic effect has undergone a diminution, the
curvature exhibiting a flattening; lowering of temperature, on the other
hand, increases the geotropic curvature. Other instances of this will be
found in a subsequent chapter. The diurnal movement of the 'Praying
Palm' is a striking example of the effect of variation of temperature in
modification of geotropic curvature (p. 30). Rise of temperature is
thus shown to diminish geotropic torsion of dorsiventral organs, and the
apo-geotropic curvature of radial organs. We have next to study the
effect of temperature variation on the dia-geotropic equilibrium of
leaves.

EFFECT OF VARIATION OF TEMPERATURE ON DIA-GEOTROPIC EQUILIBRIUM.

In the normal position of the plant, the leaf of _Mimosa_ assumes, under
geotropic action, an equilibrium position which is approximately
horizontal. I shall proceed to show that this position of equilibrium
also undergoes appropriate variation under changing temperature, the
leaf undergoing a fall during rise, and an erection during fall of
temperature.

I stated that the torsional response is one of the means of recording
geotropic effect and its variations. In the ordinary position of the
plant, the geotropic variation will be indicated by the responsive up or
down movement of the leaf in a vertical plane. Taking the leaf of
_Mimosa_, we have thus the means of studying the effect of variation of
temperature by two independent means of inquiry, namely, by record of
ordinary responsive movement in a vertical plane, and also by record of
torsional response. The variation of temperature which induces these
movements may be simultaneously recorded by means of a differential
metallic thermometer. The Multiplex Recorder employed for this research
consists of three recording levers. A photographic reproduction of the
apparatus will be found in a subsequent chapter (see Fig. 190). The
first lever is attached to the leaf of _Mimosa_ placed in the normal
position; the second lever records the torsional response of _Mimosa_
leaf, the plant being placed on its side; the third lever attached to
the differential metallic thermometer gives a continuous record of
variation of temperature.

[Illustration: FIG. 187.--Simultaneous record (_a_) of variation of
temperature, (_b_) of up or down movement of leaf of _Mimosa_, and (_c_)
of variation of torsion. Rise of temperature is attended by fall of leaf
and diminution of torsion, fall of temperature inducing the opposite
effect.]

_Effect of variation of temperature: Experiment 201._--Special
arrangement was made for gradual variation of temperature in the plant
chamber. Two rectangular metallic vessels each 50 × 30 × 6 cm. were
placed on opposite sides of the plant chamber, and warm water was made
to circulate through them; this device ensured a steady rise of
temperature. The flow of warm water was then stopped and the plant
chamber was allowed to cool down; the fall of temperature was at first
moderately rapid, but later on the rate of cooling became extremely
slow; on account of this the temperature of the plant chamber, towards
the end of the experiment remained higher than the normal temperature
outside. The rate of rise and fall of temperature during the entire
course is illustrated in the thermo-graphic (_a_) tracing (Fig. 187);
the record (_b_) exhibits the movement of the leaf in a vertical plane,
rise of temperature being attended by a diminution of geotropic
curvature resulting in the fall of the leaf, the fall of temperature
inducing the opposite effect. In record (_c_) is seen the responsive
variation of geotropic torsion, rise of temperature inducing a
diminution and fall of temperature causing an enhancement of torsion.
The results obtained by diverse methods thus prove that the geotropic
effect is diminished under rise, and increased under fall of
temperature.

SUMMARY.

The position of equilibrium under geotropic action is not fixed but
undergoes change with variation of temperature.

The geotropic curvature and torsion are increased by lowering of
temperature, and decreased by rise of temperature. This is equally true
of apo-geotropic and dia-geotropic curvatures.

PART IV.

NIGHT AND DAY MOVEMENTS IN PLANTS.

XLVI.--DIURNAL MOVEMENTS IN PLANTS

_By_

SIR J. C. BOSE.

The subject has long been a perplexing one, and its literature is
copious. After a good many years of experimental investigation, I have
succeeded in analysing the main factors concerned in the many phenomena
which have been described as Nyctitropism. The results of the researches
are given in a sequence of five papers, which may be read separately,
yet will be seen as so many chapters of what has been a single though
varied investigation.

The different chapters are:

1. Daily movements in relation to Light and Darkness.

2. Daily movements due to Variation of Temperature affecting
Growth.

3. Daily movements due to Variation of Temperature affecting
Geotropic Curvature.

4. The Immediate and After-effect of Light.

5. Diurnal Movement of the leaf of _Mimosa_ due to combined
effects of various factors.

Nyctitropic movements are thus described by Jost[39]:

"Many plant organs, especially foliage and floral leaves take up,
towards evening, positions other than those they occupy by day. Petals
and perianth leaves, for example, bend outwards by day so as to open the
flower, and inwards at night so as to close it.... Many foliage leaves
also may be said to exhibit opening and closing movements, not merely
when they open and close in the bud but also when arranged in pairs on
an axis, they exhibit movements towards and away from each other. In
other cases, speaking generally, we may employ the terms _night
position_ and _day position_ for the closed and open conditions
respectively. The night position may also be described as the _sleep
position_." After reviewing the various theories proposed, he proceeds
to say "that a completely satisfactory theory of nyctitropic pulvinus
movements is not yet forthcoming. Such a theory can only be established
after new and exhaustive experimental research."

[39] Jost--_Ibid_, p. 500.

The difficulties of the experimental reinvestigation here called for
towards clearing up and explanation of the subject are sufficiently
great; they are further increased by the fact that these diurnal
movements may be brought about by different agencies independent of each
other. Thus in _Crocus_ and in _Tulip_, the movement of opening during
rise of temperature has been shown by Pfeffer to be due to differential
growth in the inner and outer halves of the perianth. I shall in this
connection show that a precisely opposite movement of closing is induced
in _Nymphæa_ under similar rise of temperature. I shall for convenience
distinguish the differential growth under temperature variation as
_Thermonasty_ proper. Again certain leaflets open in light, and close in
darkness in the so-called sleep position. Intense light, however,
produces the 'midday sleep'--an effect which is apparently similar to
that of darkness. The determining factor of these movements is the
variation of light.

There are other instances of diurnal movement, far more numerous, which
cannot be explained from considerations given above. It has therefore
been suggested that the "Day and night positions may arise by the
combined action of geotropism and heliotropism. Thus Vochting (1888)
observed in the case of _Malva verticillatta_, that the leaves, when
illuminated from below, turned their laminæ downwards during the day,
but during the night became erect geotropically. The sleep movements in
leaves and flowers, referred to above, cannot however be explained by
assuming such a combination of heliotropism and geotropism."[40]

[40] For further information on the subject of Nyctitropism, _cf._--
Pfeffer--_Ibid_, Vol. II (1903), p. 112;
Jost--_Ibid_, pp. 500, 507;
Vines--Physiology of Plants (1886), pp. 406, 543.

I commenced my investigation on nyctitropism five years ago, after
having perfected an apparatus for continuous record of the movements of
plants throughout day and night. A contrivance, described further on,
has been devised for obtaining a record of diurnal variation of
temperature. I have also succeeded recently, in perfecting a device for
automatic record of variation of intensity of light. It has thus been
possible not only to obtain a continuous record of the diurnal movement
of the plant, but also obtain simultaneous record of those changes in
the environment which might have an influence on the daily movement. I
have in this way collected several hundred autographs of different
plants throughout all seasons of the year. The records thus obtained
were extremely diverse, and it was at first impossible to discover any
fundamental reaction which would explain the phenomenon. While in this
perplexity my attention was directed two years ago to the extraordinary
performances of the "Praying Palm" of Faridpore, in which the geotropic
curvature of the tree underwent an accentuation during fall of
temperature, and a diminution during rise of temperature.

The discovery of this new phenomenon led me to the inquiry whether
Thermo-geotropic reaction, as I may call it, was exerted only on Palm
trees, or whether it was a phenomenon of universal occurrence. I
therefore extended my investigation on various geotropically curved
procumbent stems of _Ipoemia_, _Basella_, and of _Tropæolum majus_.
Here also I found that diurnal variation of temperature induced a
periodic movement exactly similar to that in Palm trees.

I next wished to find whether the Thermo-geotropic reaction observed in
stems was also exhibited by lateral organs such as leaves, which being
spread out in a horizontal direction are subjected to the stimulus of
gravity. I found that in a large number of typical cases, a periodic
movement took place which was exactly similar to that given by rigid
trees and trailing stems. A standard curve was thus obtained which was
found to be characteristic not only of trees and herbs, but also of
leaves. The stem and leaves _fell_ continuously with the rise of
temperature, from the minimum at about 6 in the morning to the maximum
at about 2 p.m. They erected themselves with falling temperature from 2
p.m. to 6 a.m. next morning.

In the diurnal record of _Mimosa_ I met, however, with an unaccountable
deviation from the standard curve, for which I could not for a long time
find an adequate explanation. Subsequent investigations showed that the
deviation was due to the introduction of additional factors of
variation, namely of immediate and after-effects of light.

COMPLEXITY OF THE PROBLEM.

I have already referred to the great difficulty of explanation of
nyctitropism from the fact that the diurnal movements may be brought
about by different agencies independent of each other. It is, moreover,
not easy to discriminate the effect of one agency from that of the
other.

The combined effects of different factors will evidently be very
numerous. This will be understood from consideration of the number of
possible combinations with only two variables, geotropism and
phototropism. The effect of geotropism may be strong _G_, or feeble,
_g_. Similarly we may have strong effect of light _L_, or feeble effect
of light _l_. Light may exert positive phototropic action +_L_ or
negative action -_L_. Thus from two variables we obtain the following
eight combinations:

_G_ + _L_; _G_ - _L_; _G_ + _l_; _G_ - _l_;
_g_ + _L_; _g_ - _L_; _g_ + _l_; _g_ - _l_.

The number of possible variables are, however, far more numerous as will
be seen from the following:

_Geotropism._--The effect of geotropic stimulus on horizontally placed
organs is one of erection. But this stimulus, which is constant, cannot
by itself give rise to periodic movements. It has however been shown
that variation of temperature has a modifying influence on geotropic
curvature (p. 519).

_Phototropism._--The action of unilateral light is to induce a tropic
curvature, which in some cases is positive, in others negative (p. 386).
In addition to these effects induced during the incidence of light, we
have to take account of the after-effects on the cessation of light.

_After-effects of light._--I find two very different effects, depending
on the intensity and duration of previous illumination. Of these the
most important is the phenomenon of 'overshooting' which occurs on the
cessation of light of long duration. This particular reaction, to be
fully described, will be found to offer an explanation of certain
anomalous effects in diurnal movement.

_Periodic variation of turgor._--I have shown (p. 39) that artificial
enhancement of turgor in the plant induces an erectile movement of the
leaf of _Mimosa_, diminution of turgor inducing the opposite movement of
fall. Kraus and Millardet have shown that a diurnal variation of tension
takes place in the shoot of all plants, which is presumably indicative
of variation of turgor. This variation of turgor in the shoot must have
some effect on the lateral leaves. But the leaves are subjected to
conditions which are absent in the stem. The erect stem is, for example,
free from geotropic action, whereas the lateral leaf is subject to it.
The effect of turgor variation in the shoot on the movement of leaves
may be, and often is, overpowered by the predominant geotropic action. I
shall, later on, refer to this question in greater detail.

[Illustration: FIG. 188.--Arrest of pulsatory movement of leaflet of
_Desmodium gyrans_ by light from above and gradual restoration on
cessation of light. Up-movement represented by up-curve.]

_Autonomous movements: Experiment 202._--The lateral organ, say the leaf
or leaflet, may have an autonomous movement of its own. In some, the
autonomous movement may be relatively quick; the complete pulsation in
_Desmodium gyrans_ may be as short as a minute or so. I find that this
autonomous movement becomes modified or even arrested by the paratonic
effect of light. This is seen in figure 188, where light applied from
above is seen to arrest the pulsation; the normal activity is, however,
restored on the stoppage of light.

[Illustration: FIG. 189.--Effect of unilateral light on hyponastic
movement of the cotyledon of _Pepo_. Application of light indicated by
arrows; light acting from below retards, acting from above accelerates
the movement. The last part of the curve in each shows recovery on the
stoppage of light.]

_Epinasty and Hyponasty: Experiment 203._--There are other autonomous
movements which are relatively slow. Even in an erect stem there may be
a to and fro oscillation. In such a case the effect of an external
stimulus, say of light, is one of algebraical summation. The following
is the summary of results of unilateral action of light on the nutating
hypocotyl of a pea seedling:

Figure 189 exhibits the effect of light applied alternately above or
below the cotyledon of _Cucurbita Pepo_. On account of the more rapid
growth of the lower side, the cotyledon was exhibiting a hyponastic
up-movement. Application of light from above enhanced the existing rate
of movement, whereas light acting from below retarded the movement. Here
we have instances of photo-hyponastic modification of natural movement.
Similarly epinastic organs will, normally speaking, have their natural
down movement retarded by light from above, and accelerated by light
from below. If the periodicity of the autonomous movements coincides
with the periodicity of the external stimulus, the resulting movement
will be determined by algebraical summation; it will be very pronounced
when the two effects are concordant. If the two periodicities do not
agree, the interference effects will become extremely complicated.

_Positive thermonasty._--Rise of temperature inducing differential
growth brings about the _closure_ of the flower. Fall of temperature on
the other hand induces a movement of opening. Example of this has
already been given in the responsive movement of _Nymphæa_.

_Negative thermonasty._--The opposite type of movement is exhibited by
_Crocus_ and _Tulip_. Pfeffer has shown that a rise of temperature
induces in these flowers, a quicker rate of growth of the inner side of
the perianth. Rise of temperature thus induces a movement of opening,
and a fall of temperature brings about the opposite movement of closure.
I shall presently describe the effects of both positive and negative
thermonasty, in diurnal movements of flowers.

_Thermo-geotropism._--I have already described the accentuation of
geotropic curvature during the fall, and a flattening of curvature
during the rise of temperature (p. 519). The influence of this factor on
diurnal movement will presently be treated in fuller detail.

There are thus more than ten variables, and the resulting effect due to
their combinations will exceed a thousand. This will explain why
attempts at explanation of the phenomenon of nyctitropism had hitherto
proved so baffling. It is indeed a difficult task to disentangle the
full explanation of each given case in the vast complexity. It is,
however, possible, by a process of judicious elimination, to reduce the
difficulties which at first appear to be insurmountable.

In the periodic movement of plants there are several factors which are
predominant, others being of minor importance. The important factors are
the effects of light and darkness, of variation of temperature on
differential growth, and of thermal variation on geotropic curvature.

For facility of treatment, I shall first take the three ideal types: (1)
where the variation of light is the important factor, (2) where the
movement is due to differential growth under variation of temperature,
and (3) where thermal variation induces changes in geotropic curvature.
I shall then take up the movement of the leaf of _Mimosa_ where the
combined effects of numerous factors give rise to a highly complex
diurnal curve. There remains now the difficulty of discriminating the
three types which approximate to the ideal.

DISCRIMINATING TESTS FOR CLASSIFICATION.

_Predominant effect of light and darkness._--Turning first to the case
where light exerts a predominant influence, the obvious test of keeping
the plant in continuous darkness or continuous light is not practicable.
One would think that if the movement was due to periodic variation of
light, such movement would disappear under constant light or darkness.
But owing to the persistence of after-effect, the periodic movement
previously acquired is continued for a long time.

There is, however, another possibility of discrimination. The effect of
variation of light will be most marked at the two periods, early in the
morning when the light appears, and in the evening when it disappears.
In the tropics there is little twilight; in Calcutta, the sun rises in
summer at about 5-30 a.m., and sets at 6-30 p.m. In winter the sun rises
an hour later, and the sunset is an hour earlier. The average dawn may
therefore be taken approximately at 6 a.m., and the average sunset at 6
p.m. Unlike the diurnal variation of temperature which is gradual, the
change from light to darkness or from darkness to light is very abrupt.
If we succeed next in obtaining a continuous curve of the diurnal
movement of the plant, the phototropic action would be evidenced by some
flexures of the curve in the morning and towards evening.

The other two types of daily movement depend on the diurnal variation of
temperature, and there is some difficulty in distinguishing the effect
of variation of light from that of temperature, since both are connected
with the appearance and disappearance of the sun.

_Diurnal variation of light and of temperature._--There are certain
differences, however, which enable us to distinguish the two variations.
Light appears in the morning, say at 6 a.m., becomes most intense at
noon; after 4 p.m. the light wanes, and darkness sets in quickly after 5
p.m. and remains persistent till next morning. The course of variation
of temperature is somewhat different. The minimum temperature is
attained in my green house at about 5 a.m. in summer, and at about 7
a.m. in winter. The maximum temperature is reached at about 3 p.m. in
summer, and about 1 p.m. in winter. The range of daily variation in
summer may be taken to be from about 23° C. to 34° C.; in winter it is
from 16° C. to about 29° C. The above gives the normal variation and not
the sudden fluctuations that occur during uncertain weather conditions.

The temperature remains constant for nearly an hour during the period of
transition from falling to rising temperature, and _vice versâ_. The
average period of minimum temperature may be taken at 6 a.m., which I
shall distinguish as the _thermal-dawn_. The average period for maximum
temperature, the _thermal-noon_, is at 2 p.m. Variations from these
average periods at different seasons do not amount to more than an hour.

The light-dawn and thermal-dawn are more or less coincident, while the
thermal-noon is two hours later than the light-noon. A change in the
diurnal curve of movement due to thermal variation will thus be detected
at about 2 p.m. If the curve of daily movement of the plant-organ
closely resemble the diurnal thermographic curve, there can then be no
doubt of the causal relation of variation of temperature in the
production of the periodic movement. Two different classes of phenomena,
as already stated, arise however from the variation of temperature,
_thermonasty_ and _thermo-geotropism_. In the former, the movement is
autonomous, and determined in relation to the plant; in the latter, the
movement is related to the direction of external stimulus of gravity.
Further tests will be given later, to distinguish the phenomenon of
Thermonasty from that of Thermo-geotropism.

I shall in the succeeding papers describe the principal types of diurnal
movements as sketched above. The success of the investigation greatly
depends on the elaboration of automatic apparatus of precision, which
gives a continuous record of the diurnal movement of different plant
organs. The description of this Nyctitropic Recorder will be given in
the next paper.

SUMMARY.

The obscurities in the nyctitropic movement of plants arises from the
presence of numerous complicating factors.

In the diurnal movement of plants the most important factors are the
effects of light and darkness, of variation of temperature on
differential growth, and of thermal variation on geotropic curvature.

These three classes of phenomena may be discriminated from each other by
the following tests. The effects of light and darkness are most
pronounced in the morning when light appears, and in the evening when
light disappears. A pronounced flexure in the diurnal curve at these
periods indicates the dominant character of the phototropic action. The
effect of light can also be distinguished from that of temperature from
the fact that the period of maximum intensity of light, or _light-noon_,
is about two hours earlier than the _thermal-noon_, at which the
temperature is maximum.

A flexure of the diurnal curve about thermal noon, at which an inversion
takes place from rise to fall of temperature, indicates the effect of
temperature. The additional test of the effect of temperature is
furnished by the close resemblance of the diurnal curve of the plant
with the thermographic record for 24 hours.

Two different classes of phenomena arise from variation of
temperature--Thermonasty and Thermo-geotropism. In the former the
movement is autonomous and determined by the differential
growth-activity of the two sides of an anisotropic organ. In the latter
the movement is not in relation to the plant but directed by the
external stimulus of gravity.

XLVII.--DIURNAL MOVEMENT DUE TO ALTERNATION OF LIGHT AND DARKNESS

_By_

SIR J. C. BOSE,

_Assisted by_

LALIT MOHAN MUKHERJI, B.Sc. (_Nawroji Scholar_).

The nyctitropic movements of the leaflet of _Cassia alata_ and of the
terminal leaflet of _Desmodium gyrans_ furnish us with typical examples
of the recurrent effects of light and darkness. The petiole of _Cassia_
contains a number of paired leaflets each of which is about 5 cm. long
and 2·5 cm. broad. The leaflets are extremely sensitive to light; at
night each pair of leaflets fold themselves in a forward direction (see
Fig. 150). With the appearance of light they open at first in a lateral
direction; later on there is a twist of the pulvinus by which the inner
surface of the leaflets faces light coming from above (p. 405). I shall
show that the diurnal movements of the leaflets are predominantly due to
phototropic action.

Before proceeding further it will be necessary to give a general
description of the experimental method employed, and of the apparatus by
which diurnal movements are recorded.

EXPERIMENTAL ARRANGEMENTS.

The diurnal record is often taken continuously for several days, and it
is therefore necessary to take precautions against the disturbing effect
of watering the plant. The record is also liable to be affected by the
twist induced by light when it acts on one side of the organ.

_Irrigation._--There is, as is well known, a periodic variation of
turgor in the plant. This normal variation is, however, disturbed by
watering the plant at irregular intervals. Precaution against this was
taken by placing the three flower pots on a long trough filled with
water (Fig. 190). The height of water in the trough is always maintained
constant by a syphon.

_Vertical illumination._--The direction of sunlight changes from morning
to evening, and the leaves exhibit appropriate phototropic movements or
torsions under changing directions of lateral light. In order to obviate
this, a special chamber was constructed, which allowed light from the
sky to fall vertically on the plant through a sheet of ground glass
which covered the roof. The sides and the base of the chamber are
impervious to light. A narrow slit covered with red glass allows
inspection of the curve during the process of record.

_The Ventilator._--A revolving ventilator, acted on by the wind, sucks
the air away from the chamber, thus ensuring constant supply of fresh
air, without causing any disturbances of the record.

_The Recorder._--The Oscillating Recorder employed is of the quadruplex
type carrying four recording plates (Fig. 190). The first lever records
the daily variation of temperature. The other three are attached to
three different specimens of the same plant, or to three different
plants. In the former case, three records are obtained of the same
species of plant, under identical external condition. If they agree in
all essentials, the periodic curve may be taken as characteristic of the
given plant. A very great saving of time is thus ensured, and it is
thus possible to obtain characteristic curves of numbers of different
species of plants within the short period of a season. The quadruplex
recorder enables us also to obtain simultaneous records under identical
external condition of leaves of different age of the same plant, or of
leaves of three different species of plant. I have for the last five
years taken records of numerous plants at all seasons of the year. The
autograph of the plant is often so characteristic that it is possible to
name it by mere inspection of its daily record.

[Illustration: FIG. 190.--The Nyctitropic Recorder with four writing
levers. The flower pots are placed in a trough filled with water to a
constant height. The first two levers are shown in the figure to record
movements of leaves, the third to record movement of a horizontally laid
shoot; the fourth lever attached to a differential thermometer, T,
records diurnal variation of temperature.]

_Thermograph._--For obtaining a continuous record of diurnal variation
of temperature, I use a compound strip, T, made of brass and steel.
Variation of temperature induces a curvature of the compound strip which
is recorded by means of the attached lever. The oscillation of the plate
takes place once in fifteen minutes, and the successive dots thus
produced give time records of the diurnal curve. The record thus
consists of a series of dots. An additional device makes the plate
oscillate three times in rapid succession at the end of each hour; the
hourly dot is thus thicker than others. The movement of the plant,
corresponding to the particular variation of temperature at any period,
may thus be easily determined. I shall now give a typical example of
diurnal movement induced by variation of light and darkness.

DIURNAL MOVEMENT OF THE LEAFLET OF _Cassia alata_.

The leaflet of _Cassia alata_ exhibits a movement of opening in the
morning, and it remains outspread throughout the day. It then begins to
close before evening and remains closed throughout the night. The
problem before us is to find out the relative importance of variation of
temperature and of light in the diurnal movement of the leaflets.

In the daytime the light is increasing till midday; there is, on the
other hand, a rapid decline of light after 5 p.m. and uninterrupted
darkness at night. As regards temperature there is a continuous rise
from morning till the thermal noon at 2 p.m., after which the fall of
temperature is continuous till next morning. The opening of the leaflets
in the daytime may therefore be due to the summated effects of rising
temperature and increasing light, the closure, on the other hand, being
due to falling temperature, and to darkness. The individual effect of
each of these factors is not known and it is therefore necessary to
determine the effects of variation of temperature and of light.

EFFECT OF VARIATION OF TEMPERATURE.

_Experiment 204._--The plant was enclosed in a glass chamber and exposed
to diffuse light. The experiment was commenced at midday, when the
leaflets were open; the light was kept uniform while temperature was
artificially increased by means of an electric heater placed in the
chamber, and decreased by introducing cold air into the plant chamber.
One of the leaflets was attached to the recording lever and its
movement, up or down, indicated the movement of opening or closure. The
records showed that rise of temperature induces a movement of closure,
while that of fall brings about the movement of opening.

[Illustration: FIG. 191.--Effect of sudden darkening at arrow, produces
movement of closure (up-curve). Restoration of light induces opening
movement (down-curve). Successive dots at intervals of 15 minutes.
(Leaflet of _Cassia_.)]

EFFECT OF VARIATION OF LIGHT.

_Experiment 205._--This experiment was also carried out at midday, when
the leaflets were open. The horizontal record in figure 191 represents
the stationary expanded condition of the leaflet; a black cloth was put
over the glass chamber at 1 p.m., and the effect of darkness was
recorded for one hour. Darkness is seen to initiate a movement of
closure which increased at a rapid rate; the black cloth was removed
after an hour, and the movement of opening under light was completed in
the course of five quarters of an hour. It is thus seen that the
leaflets are extremely sensitive to the action of light.

The experiments that have just been described on the effects of rise of
temperature, and of light, show that they are antagonistic to each
other. In the forenoon the opening movement under light has to be
carried out against the closure movement due to rise of temperature.
Light, therefore, is the predominant factor in the diurnal movement of
the leaflets of _Cassia_. The closure effect of darkness at night, on
the other hand, overpowers the tendency of movement of opening due to
fall of temperature.

[Illustration: FIG. 192.--Diurnal movement of the leaflet of _Cassia
alata_. Closure movement commenced at 5 p.m. and completed by 9 p.m.
Leaflets began to open at 5 a.m.]

DIURNAL MOVEMENT OF THE LEAFLET OF _Cassia alata_.

_Experiment 206._--I next obtained the diurnal record of the leaflet,
from 4 p.m. till 1 p.m. next day. The leaflets remain open from 1 p.m.
to 4 p.m. and the record of this period is therefore omitted. In the
diurnal record (Fig. 192) the first thick dot was made at 4 p.m. and
successive thick dots are at intervals of an hour, the thinner dots
being at intervals of 15 minutes. It will be seen that a rapid movement
of closure was initiated at 5 p.m. when the light is undergoing a rapid
diminution. The movement of closure is completed at about 9 p.m. The
leaflets remain closed till 5 a.m. next morning, after which they begin
to open; this opening may commence even an hour earlier. It should be
borne in mind in this connection, that since light and rise of
temperature are antagonistic in their reactions, the effects of light
and fall of temperature would be concordant; and the opening in the
early hours may possibly be hastened by the low temperature in the
morning. The leaflets open to their utmost by 9 a.m., and they remain
open till the afternoon. The plant is so extremely sensitive to light
that any slight fluctuation is followed by responsive movement of the
leaflet. Thus the transitory passage of a cloud is marked in the record
by a short-lived closure movement.

DIURNAL MOVEMENT OF THE TERMINAL LEAFLET OF _Desmodium gyrans_.

Both the petiole, and the terminal leaflet of this plant exhibit very
marked nyctitropic movement. The petiole is raised and becomes almost
erect in the evening, while the pulvinus of the terminal leaflet
exhibits a sharp curvature downwards (Fig. 193).

[Illustration: FIG. 193.--The day and night positions of the petiole and
terminal leaflet of _Desmodium gyrans_.]

_Experiment 207._--The petiole was held fixed, and the terminal leaflet
attached to the recording lever. I have already explained that light
falling on the pulvinus from above, induces an up-movement of the
leaflet, which is thus erected under light of moderate intensity. If the
light be strong, the transversely conducted excitation induces a partial
neutralisation; very intense light may even cause a reversal into
down-movement. Under natural conditions, day-light acting from above
induces an up-movement; darkness, on the other hand, induces a rapid
movement of fall. The leaflets sometimes exhibit autonomous pulsations;
but the diurnal movement is very strong and the daily curve appears as a
single large pulse on which smaller autonomous pulsations may become
superposed.

[Illustration: FIG. 194.--Diurnal record of the terminal leaflet of
_Desmodium gyrans_. Up-curve represents movement of closure.]

The diurnal curve (Fig. 194) exhibits a sudden flexure at about 5 p.m.
on the rapid waning of afternoon light till, by 6-30 p.m., it becomes
closely pressed against the petiole, by the rapid fall of the leaflet.
The discriminating test, between effects of variation of temperature and
of light, lies in the fact that the flexure of the diurnal curve takes
place in the former at about 2 p.m. when temperature undergoes change
from ascent to descent; in the case of light, the change in the
intensity of light begins to be marked about three hours later. In the
diurnal curve of _Desmodium_ the record shows little change at 2 p.m.,
showing that the leaflet is not affected to any great extent by the
variation of temperature; it is, however, strongly affected by change in
light as seen in the rapid closure movement about 5 p.m. The leaflet
remains tightly closed throughout the night and begins to open and
spread out early in the morning at about 5 a.m. This up-movement is also
very rapid and the leaflet assumes the fullest outspread position by 7
a.m. It remains in this position till the afternoon, after which the
cycle becomes repeated. As the leaflet is very sensitive to light, the
position of equilibrium of the leaflet is liable to be disturbed by the
slightest variation of light and the fluctuation of light from the sky
often gives rise to a wavy outline in the record. The leaflet, moreover,
has a tendency to exhibit rhythmic pulsations.

In the leaflets of _Cassia_ and _Desmodium_, the daily movement is thus
brought about by the predominant action of recurrent light and darkness.

MIDDAY SLEEP.

I shall here briefly recapitulate the results given in greater detail in
an earlier paper (p. 352). I have shown that the midday closure of
leaflets is brought about by the excitatory action of strong sunlight.
The responsive movement of motile pulvinus under diffuse stimulus is
determined by the greater contraction of the more excitable half of the
organ. Under the action of the midday sun the leaflets of _Mimosa_
undergo a folding upwards, whereas the leaflets of _Averrhoa carambola_
a folding downwards. The explanation of the difference lies in the fact
that in the leaflets of _Mimosa_ it is the upper half, and in _Averrhoa_
it is the lower half of the pulvinule, that is the more excitable. This
difference may be demonstrated by the action of diffuse electric shock
under which the leaflets of _Mimosa_ exhibit an upward, and those of
_Averrhoa_ a downward, closure. I have also shown that conduction of
excitation takes place across the pulvinule; hence the strong excitation
caused by sunlight becomes internally diffused, and brings about the
responsive movements, the direction of which is determined by the more
excitable half of the pulvinule.

SUMMARY.

Rise of temperature induces a movement of closure of the leaflet of
_Cassia_, fall of temperature inducing the opposite movement.

Artificial darkness induces a closure of the leaflets, the closure being
completed in the course of an hour. On readmission of light, the
leaflets become fully expanded in the course of one hour and a quarter.
The leaflets are extremely sensitive to light, closure movement being
induced by the transitory passage of a cloud.

The effect of rise of temperature is antagonistic to the action of
light. The movement of opening during the course of the day is due to
the effect of light overpowering the effect of rise of temperature.

Under daily variation of light and darkness, the movement of closure is
initiated at about 5 p.m., when the light is undergoing a rapid
diminution. The movement of closure is complete by 9 p.m. The leaflets
remain closed till about 5 a.m. next morning, after which they begin to
open and become fully expanded by 9 a.m.

The terminal leaflet of _Desmodium gyrans_ exhibits a diurnal movement
which is very similar to that of _Cassia_. It begins to open early in
the morning and remains outspread during the whole day; the leaflet
exhibits a rapid down-movement after 5 p.m. and becomes closely pressed
against the petiole in the course of about two hours.

The midday sleep of leaflets of _Mimosa_ and _Averrhoa_ is due to the
excitatory action of strong sunlight on the pulvinule, the more
excitable half becoming contracted under excitation. In _Mimosa_
leaflets it is the upper, and in _Averrhoa_, it is the lower half of the
pulvinule that is the more excitable. It is in consequence of this that
the diffuse excitation of strong sunlight causes the leaflets of
_Mimosa_ to fold upwards, those of _Averrhoa_ to fold downwards.

XLVIII.--DIURNAL MOVEMENT DUE TO VARIATION OF TEMPERATURE AFFECTING
GROWTH

_By_

SIR J. C. BOSE,

_Assisted by_

LALIT MOHAN MUKERJEE.

It has been stated that there are two classes of diurnal movements
caused by variation of temperature; one of these is due to differential
growth induced on two sides of the organ, and the other is brought about
by the induced variation of geotropic curvature. The former may be
distinguished as _Thermonastic_, and the latter as _Thermo-geotropic_
movement. Before laying down the criteria to distinguish the one class
of phenomenon from the other, it would be advisable to refer to the
somewhat arbitrary distinction that has been made between nastic and
tropic reactions.

TROPIC AND NASTIC MOVEMENTS.

The explanation, which I shall offer about the night and day movements
in plants, has been reached through the study not only of pulvinated,
but also of growing and fully grown organs. A distinction is made
between the movement due to growth, and the 'variation movement' due to
change of turgor. I have shown (p. 239) that the same diminution of
turgor which induces a contraction in a pulvinus, also induces in a
growing organ an incipient contraction, and retardation of growth.
Enhancement of turgor, on the other hand, induces in both the opposite
effect of expansion. Unilateral stimulus induces curvature, and there
is no essential difference in the production of such curvatures in
pulvinated, growing, and fully grown organs. The exhibition of
nyctitropic movement by the fully grown, and rigid 'Praying Palm' is a
striking demonstration of the unity of response of all plant organs.

As regards the distinction between the tropic and nastic movements, it
will be found that there is no sharp line of demarcation between the
two. A movement is said to be _tropic_, when unilateral stimulus acts on
an organ and induces in it a directive movement. Curvature induced by
diffused stimulus on a dorsiventral or anisotropic organ (with
differential excitabilities of the two halves) is termed _nastic_.
Daylight is supposed to act diffusely (_i.e._, equally on all sides) on
leaves; this is, however, not strictly true, since the light from sky
above is stronger than from ground below. Moreover, the tropic action of
unilateral light may become nastic by internal diffusion of excitation.
This is seen in the response of the pulvinus of _Mimosa_ to light acting
from above. The leaf at first moves upwards towards the stimulus, the
response being positively phototropic. But under the continued action of
light, excitation becomes internally diffused, and the leaf undergoes a
fall by the greater contraction of the more excitable lower half of the
organ (p. 331). No sharp distinction can therefore be made between the
movements of growth and of variation, between tropic and nastic
curvatures.

The employment of the term 'nastic' is, however, convenient when used in
a well-defined and restricted sense. "We speak of tropism when the organ
takes up a resting position definitely _related to the effective
stimulus_. Nastic movements, on the other hand, are curvatures which
bring about a particular position _in relation to the plant_, and not to
the direction of the stimulus".[41] It will sometimes be necessary, in
the course of this paper, to discriminate the movements which are
autonomous from others which are paratonic, _i.e._, brought about by
external stimulus, to the former class belongs a large number of
automatic activities ranging from the quick pulsations of _Desmodium
gyrans_ to the slow movements, exhibited by epinastic and hyponastic
organs. Under the category of nastic movements may also be included
those of the flower of _Crocus_ and _Tulip_, in which variation of
temperature induces differential growth on two sides of the organ. The
direction of the movement, though initiated by change of temperature, is
determined by the difference of growth-activity on the two sides. In
these instances of nastic movement, the induced curvature is in relation
of the plant; the opening of the flower due to rise of temperature will
remain the same, whether the flower be kept in an erect or in an
inverted position. Had the movement, on the other hand, been paratonic,
that is to say, due to the external stimulus of gravity, the responsive
movement would have been determined not in relation to the plant but to
the direction of external force of gravity.

[41] Strasburger--"Text-book of Botany" (1912), p. 300.

In the description of direction of responsive movements, confusion is
likely to arise unless the point of view be carefully defined. An
up-movement of a leaf or a petal means approach towards the growing
point of the axis. This may be variously described as movement of
closure or of folding. A down-movement may, on the other hand, be
described as a movement of opening or of unfolding. If the plant be held
inverted, two different effects will be noticed depending on the
character of the movement, whether nastic or tropic. In the case of
nastic movement, the former up-movement in erect position would appear,
on inversion of the plant, to be a down-movement; but in relation to the
plant the closure movement will remain closure movement, whether the
plant be held in the normal position or upside down. If, on the other
hand, the direction of movement be determined by the paratonic effect of
external stimulus, gravity for example, an up-movement due to fall of
temperature will continue to be an up-movement, whether the plant be
held in its normal or inverted position. The responsive movement in
relation to the plant will, however, be different; the closure movement
will, on inversion, be reversed into a movement of opening. The reversal
of closure into an opening movement or _vice versâ_ will thus be a test
of the paratonic effect of external stimulus.

We may thus distinguish thermonastic from thermo-geotropic action by the
following tests:

1. Thermonastic movements are, generally speaking, due to differential
growth, and are therefore characteristically present in growing organs.
Thermo-geotropic action is independent of growth.

2. Thermonastic movements take place in relation to the plant, and is
not determined by external force of a directive nature. Opening or
closing movement will remain unchanged after inversion of the plant. But
thermo-geotropic reaction being determined by the external stimulus of
gravity, becomes reversed on inversion of the plant. Closure movement is
thus converted into opening movement, and _vice versâ_.

I shall now take up the diurnal movement due to variation of growth
induced by change of temperature. Of this the flower of _Nymphæa_
furnishes an example.

[Illustration: FIG. 195.--Nymphæa closed at daytime.]

[Illustration: FIG. 196.--Nymphæa open at night.]

[Illustration: FIG. 197.--Response to light applied successively for 1
minute. Down-curve shows movement of opening followed by recovery in
darkness. (_Nymphæa_).]

DIURNAL MOVEMENTS OF _Nymphæa_.

The flower of _Nymphæa_ remains closed during the day and opens at
night. Figures 195 and 196 are from photographs of the day and night
positions of the flower. The closure and opening movements of this
flower have been regarded as being mainly due to recurrent variations of
light and darkness.[42] If the opening be due to darkness, closure of
the flower should take place in the morning with the appearance of
light. But the flowers often remain open till ten or eleven in the
forenoon. I have sometimes succeeded in keeping the flower open for
greater part of the day by lowering the temperature of the
plant-chamber. The movement of the flower thus appeared to be associated
with variation of temperature rather than of light.

[42] Pfeffer--Ibid, Vol. III. p. 122.

_Action of light: Experiment 208._--I investigated the effect of light
on the movement of opening or of closing of the flower. One of the
petals was attached to the recording lever; light from an arc lamp was
made to act diffusely on the petal; this was done by means of two
inclined mirrors by which the divergent horizontal beam of light was
thrown on the upper and lower sides. The record in figure 197 shows that
light induced a movement of opening, followed by closure in darkness.
Since light induces a movement of opening, and darkness brings about a
closure, the opening of the flower at night could not be due to
darkness. We have therefore to look for a different cause for the
diurnal movement of the flower.

_Effect of variation of temperature._--I have already described an
experiment which proves that rise of temperature induces a movement of
closure of the floral leaves of _Nymphæa_, lowering of temperature
producing the opposite effect (p. 311).

From the study of the action of light and of variation of temperature,
it will be seen that the flower of _Nymphæa_ is acted on in the evening
by two antagonistic forces; darkness induces a movement of closure, and
fall of temperature gives rise to a movement of opening. Since the
flower opens in the evening, the predominant effect is that of falling
temperature.

The above conclusions are fully borne out by the diurnal record which I
obtained with _Nymphæa_.

[Illustration: FIG. 198.--Diurnal record of _Nymphæa_. Upper record
gives variation of temperature; the up-curve representing fall, and
down-curve rise of temperature. The lower record exhibits the movement
of the flower, up-curve representing the opening, and down-curve the
closure of the flower.]

_Experiment 209._--One of the perianth leaves was attached to one of the
recording levers, the differential thermometer being attached to the
other. It will be seen (Fig. 198) that the movement of the flower
follows very closely the curve of variation of temperature. The flower
was tightly closed in the day time; and the perianth leaves began to
open out in the evening at first slowly, then very rapidly, and the
flower becoming fully expanded by 10 p.m. at night. Though the
temperature continued to fall, there was no possibility of further
expansion beyond the maximum. The temperature began to rise after
passing through the minimum at 6 a.m., and the movement of closure set
in with rising temperature, the flower becoming completely closed by 10
a.m. That geotropism has little effect is seen from the fact that the
inversion of flower does not interfere with the normal opening or
closing of the flower.

The phenomenon of diurnal movement of _Nymphæa_ is therefore
thermonastic, the floral leaves exhibiting movement of opening at night
owing to fall of temperature. _Luffa acutangula_, which opens in the
afternoon, and closes early in the morning, gives a diurnal record
similar to that of _Nymphæa_.

SUMMARY.

The flower of _Nymphæa_ exhibits a movement of closure during rise of
temperature, and of opening during fall of temperature.

It is shown further that the effects of light and of rise of temperature
are antagonistic to each other. Light is shown to induce in _Nymphæa_
the movement of opening, and darkness to cause the movement of closure.
The diurnal movement of _Nymphæa_ is not therefore due to periodic
variation of light and darkness, but to the predominant effect of
variation of temperature.

The diurnal record shows that the perianth leaves begin to open in the
evening with falling temperature, and the flower becomes fully expanded
by 10 p.m. The movement of closure sets in with rising temperature in
the morning, and the flower becomes fully closed by 10 a.m.

XLIX.--DAILY MOVEMENT IN PLANTS DUE TO THERMO-GEOTROPISM

_By_

SIR J. C. BOSE,

_Assisted by_

LALIT MOHAN MUKHERJI.

Of the vast number of daily movements perhaps the largest proportion is
due to thermo-geotropic reaction and its modifications. Thermo-geotropic
movements have the following characteristics:

1. The organs are sensitive to the stimulus of gravity and the periodic
movements are brought about by variation of geotropic curvature under
change of temperature.

2. The movement is not confined to growing organs, but is also exhibited
by organs which are fully grown and even by rigid trees.

3. The periodic movement is closely related to the diurnal variation of
temperature. Fall of temperature from thermal-noon (about 2 p.m.) to
thermal-dawn (about 6 a.m.) is attended by a movement of erection; rise
of temperature from thermal-dawn to thermal-noon is followed, on the
other hand, by a reverse movement of fall.

That the movement is primarily due to variation of temperature will be
demonstrated in two different ways:

(_a_) by the change of normal rhythm of movement by
artificial transpositions of periods of maximum and minimum
temperature, and

(_b_) by the abolition of periodic movement through
maintenance of constant temperature.

That the phenomenon is not nastic, but paratonic will be demonstrated:--

(_a_) by the reversal of closure into opening movement and
_vice versâ_, in consequence of inversion of the plant upside
down, and

(_b_) by the diurnal variation of torsional movement, the
direction of which is dependent on the directive action of
the stimulus of gravity.

I shall now describe the diurnal movement of various geotropically
curved plant-organs; the most striking example of this is furnished by
the 'Praying' Palm of Faridpore, already described. I shall here
recapitulate some of the important features connected with the
phenomenon.

DIURNAL MOVEMENT OF PALM TREES.

Movements similar to that of the Faridpore Palm (p. 12) are found in
other Palm trees growing at an inclination from the vertical. I
reproduce once more the diurnal curve given by the Sijberia Palm
together with the curve of daily thermal variation (Fig. 199). It will
be seen that the two curves resemble each other so closely that the
curve of movement of the tree is practically a replica of the
thermographic record. There can therefore be no doubt of the movement
being brought about by variation of temperature; rise of temperature is
attended by the movement of fall of the tree and _vice versâ_. The
record was commenced at noon; the temperature rose till the maximum was
reached at about 3 p.m. and the tree also reached its lowest position at
3-45 p.m., the lag being 45 minutes. The temperature fell continuously
after the maximum at 3 p.m., to the minimum at 6 a.m. next morning. In
response to the falling temperature, the tree exhibited a movement of
erection. The temperature rose after 6 a.m. and the movement of the
tree became reversed from ascent to descent.

[Illustration: FIG. 199.--Diurnal record of the Sijberia Palm. Upper
curve gives variation of temperature, and the lower curve the movement
of the tree.]

I have already shown: (1) that the diurnal movement just described is
due to physiological reaction, and that the movement is abolished at the
death of the plant; (2) that light has little or no effect, since the
thick bark and bases of leaves screen the living tissue from the action
of light; (3) that transpiration has practically no effect on the
periodic movement, since such movement takes place in other plants
completely immersed under water; thus _Ipomoea aquatica_, a water
plant, kept under water, gave the normal diurnal curve similar to that
of the palm. The modifying effect of transpiration was in this case,
completely excluded. I obtained similar effect with geotropically curved
stem of _Basella cordifolia_ (p. 25); (4) that the weight of the
plant-organ as such, has little effect on the diurnal curve, since an
inverted plant continues for a few days to exhibit the periodic
movement, in spite of the antagonistic effect of weight. A different
experiment will be described (see p. 582) where the effect of weight was
completely neutralised and the plant-organ gave, nevertheless, the
normal diurnal curve.

[Illustration: FIG. 200.--Diurnal record of inclined palm tree, of
geotropically curved procumbent stem of _Tropæolum_ and the
dia-geotropic leaf of palm. Note general similarity between diurnal
curve of plants and the thermographic record.]

I have also shown that the diurnal movement is determined by the
modifying influence of temperature on geotropic curvature. Rise of
temperature opposes or neutralises the geotropic curvature; fall of
temperature, on the other hand, accentuates it. The particular diurnal
movement was not confined to the palm trees, but was exhibited by all
plant-organs subjected to the stimulus of gravity.

DIURNAL MOVEMENT OF PROCUMBENT STEMS AND OF LEAVES.

[Illustration: FIG. 201.---Diurnal records of leaves of _Dahlia_,
_Papaya_ and _Croton_.]

_Experiment 210._--In order to demonstrate the continuity of the
phenomenon of diurnal movement I took various stems growing in water or
land for my experiment. The plants were laid horizontally, till the
stems bent up and assumed the stable position of geotropic equilibrium.
In figure 200 is given records of the inclined palm tree, of procumbent
stem of _Tropæolum_, and the leaf of the palm tree. The very close
relation between the temperature-variation and the movement of different
plant-organs is sufficiently obvious.

I shall next give a series of diurnal records of leaves of different
plants such as those of _Dahlia_, _Papaya_ and _Croton_ (Fig. 201). In
these also fall of temperature induces an up-movement while rise of
temperature causes a fall of the leaf. I shall presently refer to the
'personal equation' by which the record of one plant is distinguished
from another.

CONTINUOUS DIURNAL RECORD FOR SUCCESSIVE THERMAL NOONS.

_Experiment 211._--The diurnal record given above, was taken from
ordinary noon at 12 o'clock to noon next day. The diurnal curve becomes
much simplified if the record be taken from _thermal-noon_ (at about 2
p.m.) to the thermal noon next day. The plant-organ becomes erected
during falling temperature from thermal-noon to thermal-dawn next
morning, and undergoes a fall during rise of temperature from
thermal-dawn to thermal-noon. The subsequent diurnal records will
therefore be given for 24 hours commencing with 2 p.m. In figure 202 is
given diurnal records of geotropically curved stem of _Tropæolum_ and
the leaf of _Dahlia_ for two days in succession.

The thermal record shows that there was a continuous fall of temperature
from thermal-noon at 2 p.m. to the thermal-dawn at 6 a.m. next morning,
that is to say, for 16 hours. Rise of temperature through the same range
occurred in 8 hours from 6 a.m. till 2 p.m. The average rate of rise of
temperature was thus twice as quick as that of fall. This is clearly
seen from the slopes of thermal curve during thermal ascent and descent.
The record of the movement of the plant shows a striking parallelism;
the different plant-organs became erected from thermal-noon to
thermal-dawn, and underwent a fall from thermal-dawn to thermal-noon.
The descent of the curve is, as in the case of thermal curve, relatively
more abrupt. The records on two successive days are very similar, the
slight difference being due to the physiological depression consequent
on prolonged maintenance of the plants in a closed chamber.

[Illustration: FIG. 202.--Diurnal curve of the procumbent stem of
_Tropæolum majus_, and the leaf of _Dahlia_ for two successive days. In
the thermographic record the up-curve represents fall, and down-curve
rise of temperature.]

MODIFICATION OF THE DIURNAL CURVE.

I shall now proceed to explain the modifications that may occur in the
standard thermo-geotropic curve.

_Turning points._--In the bulky Palm, the reversal of movement from fall
to rise or _vice versâ_ takes place about an hour after the thermal
inversion. This lag is partly due to the time taken by a mass of tissue
to assume the temperature of the surrounding air. There is, moreover,
the question of physiological inertia which delays the reaction. In
leaves this lag may be considerably less or even absent. In certain
cases the reversal of movement may take place a little earlier than the
temperature inversion. It should be remembered in this connection, that
in response to temperature change, the leaf is often displaced to a
considerable extent from its 'mean position of equilibrium'; moreover
the force of recovery is greatest at the two extreme positions. These
considerations probably explain the quick return of the leaf to
equilibrium position. The slow autonomous movement of the leaf may
sometimes prove to be a contributory factor.

_Effect of irregular fluctuation of temperature._--In settled weather
the diurnal rise and fall of temperature is very regular. But under less
settled condition, owing to the change of direction of the wind, the
temperature curve shows one or more fluctuations, specially in the
forenoon. It was a matter of surprise to me to find the plant-record
repeating the fluctuations of thermal record with astonishing fidelity.
This common twitch in the two records is seen in the record of the
Sijberia Palm (Fig. 199). Certain plants are extremely sensitive to
variation of temperature; so much so that these physiological indicators
of thermal variation are far more delicate than ordinary thermometers.

_Effect of restricted pliability of the organ._--A leaf is more pliable
in one direction than in the other. The pulvinus of _Mimosa_, for
example, allows a greater amount of bending downwards than upwards; in
consequence of this the leaf in its fall becomes almost parallel to the
internode below; the up-movement is, however, far more restricted. The
leaf in its most erect position still makes a considerable angle with
the internode of the stem above it. If the leaf-stalk of a plant be
restricted in its rise the erectile movement at night will reach a
limit, and the top of the curve will remain flat. This is seen,
illustrated in the record of the leaf of _Croton_ (Fig. 202), which
attains its maximum erection at 9 p.m. and the subsequent curve remains
flattened till 7 a.m.; after this the leaf begins to execute its
downward movement. In other cases, the range of up-movement is very
great and the plant-organ erects itself continuously till morning. In
certain cases the impulse of up-movement carries the organ beyond the
stable position of equilibrium; after this the leaf begins to retrace
its path slowly; the down-movement due to rise of temperature is,
however, far more abrupt, and easily distinguishable from the previous
slow return.

It will thus be seen that though the diurnal record consists of an
alternating up and down curve, yet these minor characteristics or
'personal equation' of the plant confers on the record a certain stamp
of individuality.

_Effect of age._--In the floral leaves of _Nymphæa_ the thermonastic
movement is of positive sign; that is to say, an erection of the petal
during rise, and a fall during the lowering of temperature. The
corresponding movement of leaves would therefore be an erection of the
leaf in day-time, and a fall of the leaf at night. The periodic curve
of such leaves would be of opposite sign to the standard
thermo-geotropic curves given above. The leaf of _Nicotina_ is adduced
as an example of a leaf which exhibits a movement of fall at night. But
the fully grown and horizontally spread leaf I find that gives the
normal record. The very young growing leaves give a different and
somewhat erratic curve. The difference between growing and fully grown
leaves is explained by the fact that the former would be affected by
thermotropism, and the latter by thermo-geotropism. Young leaves exhibit
moreover a pronounced hyponasty or epinasty, which would naturally
modify the diurnal curve.

Certain interesting variation is met with in the diurnal record of
sprouting leaves of _Mimosa_ in spring. The movements of leaves grown
later in the season, as will be explained in a later chapter, are very
definite and characteristic. But the young leaves in spring exhibit no
definite diurnal curve, but a series of automatic pulsations, the
unsuspected presence of which in all leaves of _Mimosa_ will be
demonstrated in a subsequent chapter. Later in the season, the leaf
becomes tuned, as it were, to the periodic variation of the environment;
the automatic movements become suppressed, and the diurnal periodicity
becomes deeply impressed on the organism.

_Effect of season._--The diurnal curve may also be modified by the
seasonal variation of any one of the effective factors. _Tropæolum
majus_, for example, exhibits positive phototropic action in one season
and a negative reaction in a different season. These seasonal variations
must necessarily modify the diurnal curve.

I shall now proceed to demonstrate the determining influence of thermal
variation, and of stimulus of gravity on the thermo-geotropic movements.
The striking similarity of the thermograph, and the record of movement
of plants demonstrate the causal relation between temperature variation
and diurnal movement, of which the two additional tests described below
offer further confirmation.

REVERSAL OF NORMAL RHYTHM.

The normal diurnal movement is, as we have seen, a fall during rise of
temperature from morning to afternoon, and a rise from afternoon till
next morning. I succeeded in reversing the normal rhythm of _Basella_ by
reversing the normal variation of temperature at the two turning points,
in the morning and in the afternoon. The plant was subjected to falling
temperature in the morning and to rising temperature in the afternoon.
The normal movement now became reversed, _i.e._, an erection instead of
fall in the forenoon and a fall instead of rise in the afternoon (p.
28).

EFFECT OF CONSTANT TEMPERATURE.

The second test which I shall employ is the effect of maintenance of
constant temperature, which should wipe off, as it were, traces of
periodic movement. It was necessary for this investigation to maintain
the plant chamber at constant temperature throughout day and night. The
usual thermostat is virtually a recess in a double-walled chamber filled
with water, the chamber being covered with a heat insulating material.
But this contrivance is unsuitable for the plant chamber which is to
contain good sized plants, and the recording apparatus. The problem of
maintaining a large air-chamber at constant temperature presented many
difficulties which were ultimately overcome by the device of an
extremely sensitive thermal regulator.

_The Thermal Regulator._--I shall in a future paper give a complete
account of the large thermostatic air-chamber. The important part of the
apparatus is an electro-thermic regulator which interrupts the heating
electric current as soon as the temperature of the chamber is raised a
hundredth part of a degree above the predetermined temperature. The
automatic make and break of the current takes place in rapid succession,
and the temperature of the chamber is thus maintained constant within
tenth of a degree, throughout day and night.

[Illustration: FIG. 203.--Abolition of diurnal movement in _Tropæolum_
under constant temperature, and its restoration under normal daily
fluctuation. The upper record is of temperature and the lower of plant
movement.]

_Diurnal record of_ Tropæolum _under constant temperature: Experiment
212._--The normal record of geotropically curved _Tropæolum_ is already
given in figure 202. In repeating the record I maintained the plant at
constant temperature for 24 hours; the result of this is seen in the
first part of the record (Fig. 203). The thermal record is practically
horizontal, and the diurnal record of the plant shows no periodic
movement. The thermal regulator was on the next day put out of
operation, thus restoring the normal diurnal variation of temperature.
The record of the plant is seen to exhibit once more its normal periodic
movement.

I have in the chapter on thermo-geotropism (p. 515) shown that the
diurnal movement of a geotropically curved organ is determined in
reference to the direction of force of gravity. This will be seen
demonstrated in an interesting manner in the two following experiments
on the effect of inversion of the plant on daily movement.

DIURNAL MOVEMENT IN INVERTED POSITION.

I have already referred to the distinction that is made between nastic
and paratonic movements. In the former the movement is autonomous and in
relation to the plant, and in the latter it is due to an external force
which determines the direction of movement. In nastic reaction, closure
movement would persist as a closure movement[42]; but should the
direction of movement be determined by the stimulus of gravity, closure
movement would, on inversion, be reversed into an opening movement.
Viewed from an external point of view an up-movement in the latter case
would, after readjustment on inversion, become an up-movement, though in
so doing, the expansion should be transferred from the upper to the
lower side of the organ. It is to be understood in this connection, that
some time must lapse before this readjustment is possible, and that the
former movement may continue, in certain cases, as a persistence of
after-effect.

[42] By closure is meant movement of opposite pairs of
leaf-organs towards each other.

I succeeded in demonstrating the paratonic effect of geotropic stimulus
on the periodic movement of the palm leaf, by holding the plant in an
inverted position (p. 24). On the first day of inversion, the diurnal
record was erratic, but in the course of 24 hours, the leaf readjusted
itself to its unaccustomed position, and became somewhat erected under
geotropic action. After the attainment of this new state of geotropic
equilibrium, the leaf gave the record of down-movement during rise, and
up-movement during fall of temperature, movements which in reference to
the plant are the very opposite to those in a normal position. But seen
from an external point of view, rise of temperature caused in both
normal and inverted positions, a down-movement indicative of diminished
geotropic curvature; fall of temperature, on the other hand, brought
about an erectile movement, thus exhibiting enhancement of geotropic
curvature.

[Illustration: FIG. 204.--Effect of inversion of the plant on diurnal
movement. (_a_) Normal record, (_b_) record 24 hours after inversion and
(_c_) after 48 hours (_Tropæolum_).]

_Experiment 213._--A still more striking result exhibiting the phase of
transition was given by the geotropically curved stem of _Tropæolum_.
Its diurnal curve and the subsequent changes after inversion are given
in figure 204. In (_a_) is seen the normal diurnal curve; the specimen
was inverted, and it took an entire day for the plant to readjust itself
to the new geotropic condition. The record (_b_) was recommenced on the
second day after inversion; the persistence of previous movement is seen
in the reversed curve during the first half of the second day; but in
the second half the record became true, and the third day the inverted
plant gave a record which, from an external point of view, was similar
to that given by the plant in the normal position.

SUMMARY.

A continuity is shown to exist between the thermo-geotropic response of
rigid trees, stems, and leaves of plants.

The diurnal record exhibits an erectile movement from thermal-noon to
thermal-dawn, and a movement of fall from thermal-dawn to thermal-noon.

In contrast with thermonastic movement which takes place in growing
organs, thermo-geotropic movement takes place in fully grown organs
including rigid trees. The thermonastic movement is independent of the
direction of gravity, while in thermo-geotropic reaction, the stimulus
of gravity exerts a directive action.

The effect of variation of temperature on the diurnal movement is
demonstrated by induced change of normal rhythm, by artificial
transposition of periods of thermal inversion, and by the abolition of
periodic movement under constant temperature.

The effect of stimulus of gravity on the diurnal movement is
demonstrated by the effect induced on holding the plant upside down.
The direction of the daily movement is found to be determined by the
directive action of the stimulus of gravity.

L.--THE AFTER-EFFECT OF LIGHT

_By_

SIR J. C. BOSE,

_Assisted by_

SURENDRA CHANDRA DAS.

We have considered two types of diurnal movement, one due to the
predominant action of variation of light, and the other, to that of
changing temperature. There are, however, other organs which are
sensitive to variations both of light and of temperature. The effect of
light is, generally speaking, antagonistic to that of rise of
temperature; hence the resultant of the two becomes highly complex.

Still greater complexity is introduced by the different factors of
immediate and after-effect of light. This latter phenomenon is very
obscure, and I attempted to determine its characteristics by electrical
method of investigation. A fuller account of after-effect of light on
the response of various plant-organs and of animal retinæ will be found
elsewhere.[43] I shall here refer only to one or two characteristic
results which have immediate bearing on the present subject.

[43] "Comparative Electro-Physiology"--p. 392.

Direct stimulation under light induces excitatory reaction, which is
mechanically exhibited by contraction, and electrically by induced
galvanometric negativity. Under continuous stimulation, the excitatory
effect, either of positive curvature or of induced galvanometric
negativity, is found to attain a maximum. This is often found to undergo
a decline and reversal; for under continuous stimulation there is a
fatigue-decline, as seen in the relaxation following normal contraction
in animal muscle. The positive tropic curvature, and the induced
galvanometric negativity may thus undergo a decline, and neutralisation.
This neutralisation is also favoured, in certain cases, by transverse
conduction of excitation to the distal side.

The character of the after-effect will presently be shown to be modified
by the duration of previous stimulation, the different phases of which
will for convenience, be distinguished as pre-maximum, maximum and
post-maximum. Since stimulus simultaneously induces positive "A" and the
negative "D" changes (p. 143), their intensities will undergo relative
variation during the continuance and cessation of stimulus. The
after-effect will therefore exhibit unequal persistence of the expansive
"A" and contractile "D" reaction at different phases of stimulation.

ELECTRIC AFTER-EFFECT.

Confining our attention to the electric response, it is found that under
continued action of light the excitatory galvanometric negativity
increases to a maximum, after which there is a decline, and
neutralisation. Figure 205 gives the galvanographic record of the
electric response of the leaf stalk of _Bryophyllum_ under light; the
up-curve represents increasing negativity which, after attaining a
maximum, undergoes neutralisation as seen in the down-curve. I shall,
with the help of the diagram given in the next figure, describe and
explain the various after-effects I observed on sudden stoppage of
light: before the attainment of maximum, at the maximum, and after the
maximum.

[Illustration: FIG. 205.--Electric response of the leaf-stalk of
_Bryophyllum_ under continuous photic stimulation. Increasing negativity
represented by up-curve; neutralisation by down-curve.]

[Illustration: FIG. 206.--Diagrammatic representation of electric
after-effect of stimulation. Pre-maximal stimulation produced by
stoppage of light at _a_, gives rise to continuation of previous
response followed by recovery. Stoppage of light at maximum _b_ gives
rise to recovery to equilibrium position. Stoppage of light at
post-maximum _c_, gives rise to over-shooting below zero line.]

_After-effect of pre-maximum stimulation: Experiment 214._--Light is
applied at arrow and stopped in different experiments at _a_, _b_, and
_c_ (Fig. 106). Continuous stimulation induces increasing galvanometric
negativity; when stimulus is stopped at _a_ before the maximum, the
after-effect is a persistence of excitatory galvanometric negativity,
which carries the response record higher up; after a certain interval
recovery takes place and the record returns to the zero line of normal
equilibrium. The after-effect of pre-maximum stimulation is thus a
short-lived continuance of response followed by recovery.

_After-effect at maximum: Experiment 215._--In this the photic stimulus
was continued till the attainment of maximum, when light was suddenly
removed at _b_. The after-effect was no longer a persistence of
responsive movement, but disappearance of negativity and recovery to
zero line of equilibrium.

_Post-maximum after-effect: Experiment 216._--In this light was
continued till there was a complete neutralisation, the curve of
response returning to zero line; to all outer seeming the responsive
indication of the tissue is the same as before excitation. But stoppage
of stimulus at _c_ causes an over-shooting at a rapid rate far _below_
the zero line; and it is after a considerable period that the curve
returns to the zero line of equilibrium.

The condition at post-maximum _c_ is thus one of dynamic equilibrium
where two opposite activities, "A" and "D," balance each other; for had
the condition of the 'neutralised' tissue been exactly the same when
fresh, cessation of stimulus would have kept the galvanometric spot of
light at the zero position.

The electric investigation described above shows that the after-effect
is modified by duration of stimulation, and that:

(1) the after-effect of pre-maximum stimulation is the
continuation of response in the original direction (upward,
and away from zero line), followed by recovery,

(2) the after-effect of the maximum is an electric recovery
towards zero position, and

(3) the after-effect of post-maximum stimulation is an
over-shooting _downward_ below the zero line.

TROPIC RESPONSE UNDER LIGHT AND ITS AFTER-EFFECT.

I shall now describe the after-effect of light as seen in mechanical
response, and the results will be found parallel to those given by the
electric response. The specimen employed is the terminal leaflet of
_Desmodium gyrans_, the pulvinus of which is very sensitive to light.
Pulvinated organs, generally speaking, exhibit a diurnal variation of
turgor in consequence of which the position of equilibrium of the leaf
or leaflet undergoes a periodic change. But this equilibrium position of
the organ remains fairly constant for nearly two hours about midday, the
variation of temperature at this period being slight. We may therefore
obtain the pure effect of light by carrying out the experiment at this
period, and completing it within a short time to avoid complication
arising from the autonomous variation of turgor.

The period of experiment of the plant may be shortened by a choice of
suitable intensity of light; a given tropic effect induced by prolonged
feeble light may thus be obtained by short exposure to stronger light.
The source of light for the following experiment was a 50 c.p.
incandescent lamp. The intensity was increased to a suitable value by
focussing light on the upper half of the pulvinus by means of a lens.
The intensity was so adjusted that the maximum positive curvature was
attained in the course of about 6 minutes, and complete neutralisation
after an exposure of 17 minutes.

_Pre-maximum after-effect: Experiment 217._--Light was allowed to act on
the upper half of the pulvinus for two minutes and twenty seconds; this
induced an up-movement _i.e._, a positive curvature. On the stoppage of
light the up-movement continued for one minute and twenty seconds, after
which the down-movement of recovery was completed in six minutes and
twenty seconds (Fig. 207). The immediate after-effect is thus a movement
upward, away from the zero line of equilibrium. The result is seen to be
the same as the electric after-effect of pre-maximum stimulation.

[Illustration: FIG. 207.--Light applied at arrow, and stopped at the
second arrow within a circle. After-effect of pre-maximum stimulation
is continuation of positive curvature followed by recovery.]

[Illustration: FIG. 208.--After-effect at maximum; recovery towards zero
position of equilibrium.]

[Illustration: FIG. 209.--After-effect at post-maximum is a rapid
overshooting below the position of equilibrium. Light was applied in all
cases on upper half of pulvinus of terminal leaflet of _Desmodium
gyrans_.]

_After-effect at maximum: Experiment 218._--Application of light for 5
minutes and twenty seconds induced a maximum positive curvature.
Stoppage of light was followed at once by recovery which was completed
in about 10 minutes (Fig. 208).

_After-effect at post-maximum: Experiment 219._--As the plant was
fatigued by previous experiments, a fresh specimen was taken and light
was applied continuously on the upper half of the pulvinus. This gave
rise first to a maximum positive curvature, subsequently diminished by
transverse transmission of excitation. Neutralisation took place after
application of light for 17 minutes. On the stoppage of light, there was
a sudden overshooting _below_ the zero line (Fig. 209), and the rate of
the movement on the cessation of light was nearly twice as quick as
during the process of neutralisation.

SUMMARY.

The after-effect of light is modified by the duration of exposure to
light.

Under continued action of light, the electric response of galvanometric
negativity in plants attains a maximum after which it undergoes decline,
and neutralisation.

The electrical after-effect exhibits characteristic differences
depending on the duration of previous exposure to light.

The pre-maximal after-effect is a temporary continuation of response
under light followed by recovery.

The after-effect at the maximum is a recovery to the normal equilibrium.

The after-effect at post-maximum is an 'overshooting' below the position
of equilibrium.

The immediate and after-tropic response of light are similar to the
corresponding photo-electric effects.

The pre-maximum after-effect is a continuation of positive tropic
movement followed by recovery; the after-effect at maximum is a recovery
to the normal equilibrium position of the organ. The post-maximum
after-effect is an overshooting below the position of normal
equilibrium.

LI.--THE DIURNAL MOVEMENT OF THE LEAF OF _MIMOSA_

_By_

SIR J. C. BOSE.

In the standard curve of nyctitropic movement under thermo-geotropism
described in a previous paper, the diurnal record consisted of an
up-curve from thermal-noon to thermal-dawn, and a down-curve from the
thermal-dawn to thermal-noon. The responding organ, which may be an
inclined stem or a horizontally spread petiole, underwent an erection
during the decline of temperature, and a fall with the rise of
temperature. The diurnal record of the _Mimosa_ leaf appears, however,
to be totally different.

[Illustration: FIG. 210.--Diurnal record of _Mimosa_ in summer, and in
winter. Leaf rises from 2 to 5 p.m., when there is a spasmodic fall.
Leaf re-erects itself from 9 p.m. to 6 a.m. after which there is a
gradual fall till 2 p.m. with pulsations. The upper-most record gives
temperature variation, up-curve representing fall of temperature and
_vice versâ_.]

_Experiment 220._--I obtained the diurnal record of _Mimosa_ (Fig. 210)
for twenty-four hours commencing at 2 p.m. which is the thermal-noon.
The summer and winter records are essentially the same; the only
difference is in the greater vigour of movement exhibited by summer
specimens. The diurnal movement of the leaf is very definite and
characteristic; for the curves taken five years ago do not differ in
any way from those obtained this year. The record may conveniently be
divided into four phases.

_First phase._--The leaf erects itself after the thermal-noon up to 5 or
5-30 p.m. The temperature, it should be remembered, is undergoing a fall
during this period.

_Second phase._--There is a sudden fall of the leaf in the evening which
continues till 9 p.m. or thereabout.

_Third phase._--The leaf erects itself till thermal-dawn at about 6 a.m.
next morning.

_Fourth phase._--There is a fall of the leaf during the rise of
temperature from thermal-dawn to thermal-noon. The uniformity of the
fall is, however, interrupted by one or more pulsations in the forenoon.
These pulsations are more frequent in summer than in winter.

It will thus be seen that the difference between the normal
thermo-geotropic curve, and the curve of _Mimosa_ is not so great as
appears at first sight. With the exception of the spasmodic fall in the
evening, the diurnal curve shows an erectile movement during lowering of
temperature, and a movement of fall during rise of temperature. I shall
presently explain the reason of the sudden fall in the evening, and of
the multiple pulsations in the forenoon.

I have, moreover, been able to trace a continuity in _Mimosa_ itself,
between the standard thermo-geotropic reactions and the modification of
it by the action of light. The young leaves which sprout out at the
beginning of spring take some time to become adjusted to the diurnal
variation. There are two intermediate stages through which the leaves
pass before they exhibit their characteristic diurnal curve. Slow
rhythmic pulsations are at first seen to occur during day and night. At
the next stage the leaves exhibit the diurnal movement of fall from
thermal-dawn to thermal-noon, and movement of erection from thermal-noon
to thermal-dawn next morning, the record being in every way similar to
the standard thermo-geotropic curve. It is only at the final stage that
there is a spasmodic fall in the evening which we shall find is the
characteristic after-effect of light.

Before proceeding further I shall refer briefly to the theory of
Millardet in explanation of the diurnal movement of the leaf of
_Mimosa_. He found that the tension in stems, and presumably its turgor,
is increased with rise and decreased with fall of temperature. The
movement of the lateral leaf may, therefore, be due to the induced
variation of tension in the main axis. Had this been the case the
minimum tension would have occurred at the minimum temperature in the
morning, and the leaf should have undergone a maximum fall. The maximum
temperature attained in the afternoon should have, on the other hand,
brought about the maximum erection. The observed facts are, however, the
very opposite to these. Kraus and Millardet also found that light and
darkness had great influence on the tension, which increases in darkness
and diminishes in light. The tension at dawn may therefore be a
resultant of the depressing effect of low temperature opposed by the
promoting effect of darkness, the latter being the predominant factor.
The erect position of _Mimosa_ leaf in the morning may thus be accounted
for by the resultant increase of tension of the stem. The explanation of
the movements of the leaves is thus to be attributed to the variation of
tension in the main axis to which the leaves are attached; this leads to
the conclusion that the leaf movement should be determined in relation
to the plant, and not in relation to the external stimulus. I shall,
however, describe a crucial experiment in the course of this paper,
which will show that the direction of stimulus of gravity has a
determining influence on the periodic movement. The sudden fall of the
leaf before evening is again inexplicable from the theory of periodic
variation of tension.

The complexity in the diurnal movement in _Mimosa_ arises from the fact
that there are three factors whose fluctuating effects are different at
different parts of the day. The effect at any particular hour results
from the algebraical summation of the following factors: (1) the
thermo-geotropic action, (2) the immediate effect of photic stimulus and
(3) the after-effect of light. The leaf of _Mimosa_ has, moreover, as I
shall show, an autonomous movement of its own. I shall take up the full
consideration of the subject in the following order:

1. _The thermo-geotropic reaction._--A crucial experiment will be
described which demonstrates the effect of thermo-geotropism in the
diurnal movement of the leaf of _Mimosa_.

2. _Autonomous pulsation of Mimosa._--The natural pulsation of the plant
is obscured by the paratonic effect of external stimuli. I shall explain
the method by which the natural pulsation of the leaf becomes fully
revealed.

3. _The immediate effect of light._--This is not constant, but will be
shown to undergo a definite variation with the intensity and duration of
light. A very great difficulty in the study of effect of daylight at
different parts of the day is introduced on account of the absence of
any reliable recorder for measurement of fluctuation of light. I shall
describe a device which gives a continuous record of photic variation
for the whole day.

4. _The after-effect of light._--The spasmodic fall of the leaf of
_Mimosa_ towards the evening presents the most difficult problem for
solution. I shall first describe the diurnal movement of another plant
which presents characteristics similar to those of _Mimosa_. I shall
also demonstrate the various after-effects of light at different parts
of the day. These results will offer the fullest explanation of the
sudden fall of the leaf towards evening.

As regards the sudden fall of the leaf about evening, Pfeffer regarded
it as due to increased mechanical moment of the secondary petioles
moving forward on the withdrawal of light. I shall, however, in the
course of this paper show, that the characteristic movements occur even
after complete removal of the sub-petioles. In the following experiment,
carried out with the intact plant, the effect of possible variation of
weight is completely eliminated. In spite of this, the diurnal movement
exhibited its characteristic phases including sudden movement in the
evening.

The experiment I am going to describe will exhibit the diurnal curve
obtained by an entirely different method, and will clearly exhibit the
thermo-geotropic effect, as well as the immediate and after-effect of
light.

DIURNAL VARIATION OF GEOTROPIC TORSION.

I have shown that the pulvinus of _Mimosa_, subjected laterally to the
action of stimulus of gravity, exhibits a torsional response. When the
_Mimosa_ plant is laid sideways, so that the plane of separation of the
upper and lower halves of the pulvinus is vertical, geotropic stimulus
acts laterally on the two halves of the differentially excitable
pulvinus. When the less excitable upper half is to the left of the
observer (see Fig. 179), the responsive torsion under geotropic stimulus
will be clock-wise, the less excitable upper half of the pulvinus being
thereby made to face the vertical lines of gravity. When the plant is
turned over to the other side (the less excitable upper half being now
to the right of the observer) the induced torsion will be counter
clock-wise. The response is therefore determined by the directive action
of stimulus of gravity. Light has also been shown to give rise to
torsion (p. 400). Light acting in the same direction as the stimulus of
gravity, _i.e._, from above, enhances the rate of torsion, the curve of
response being due to the joint effects of light and gravity.

[Illustration: FIG. 211.--Record of diurnal variation of torsion in
_Mimosa_ leaf. Up-curve represents increase and down-curve decrease of
geotropic torsion.]

_Experiment 221._--I obtained 24 hours' record of variation of torsional
response of _Mimosa_, commencing with thermal-noon at 2 p.m. It is to be
borne in mind that increase of torsion indicates increase of geotropic
action, just as the erectile movement of the leaf in the normal position
indicates the enhanced geotropic effect. Inspection of figure 211 shows
that the fall of temperature after thermal-noon was attended by increase
of torsion. The curve went up till about 5 p.m., as in the ordinary
record of _Mimosa_. The torsion suddenly decreased with the rapid
diminution of light after 5 p.m. The torsion then increased with falling
temperature from 9 p.m. till thermal-dawn next morning. After 6 a.m.
there is a continuous diminution of torsion till 5 p.m.

We may now summarise the diurnal variation of torsion exhibited by
_Mimosa_. The torsion undergoes a periodic increase during the fall of
temperature from afternoon till next morning, and a diminution during
rising temperature from morning till afternoon. A sudden diminution of
torsion occurs at about 5 p.m. due to the disappearance of light. The
torsional record is, to all intents and purposes, a replica of the
record of periodic up and down movements of the leaf.

This method of torsion has several advantages over the ordinary method.
First, the petiole being supported by the loop of wire, the weight of
the leaf has no effect on the curve of response. In the second place,
the periodic variation of turgor of the stem, as suggested by Millardet,
will not in any way affect the record. Variation of turgor can only
cause a swing to and fro, in a direction perpendicular to the plane
which divides the pulvinus into upper and lower halves; it can in no way
induce a torsional movement, or a variation of the rate of that
movement.

_The automatic pulsation of the leaf of Mimosa._--The occurrence of the
pulsatory response in the morning record of _Mimosa_ led me to search
for multiple activity in the response of the pulvinus. I have in my
previous investigation on the electric response of _Mimosa_ obtained
multiple series of responses to a single strong stimulus. Blackman and
Paine have recently shown that an isolated pulvinus of _Mimosa_ exhibit
multiple mechanical twitches under excitation.[44]

[44] Blackman and Paine--"Annals of Botany" January 1918.

Even under normal conditions, the sprouting young leaves in March, as
already stated, exhibit automatic pulsations throughout the day and
night; in older leaves tuned to diurnal periodic movements, these
natural pulsations are more or less suppressed. But in the forenoon,
several pulsations are exhibited even by the old leaves.

The question may now be asked: Why should the pulsations occur
preferably in the morning? In connection with this I shall refer to the
suppression of the pulsatory activity of _Desmodium gyrans_ when the
leaflet was pulled up by the action of light (cf. Fig. 188). The leaf of
_Mimosa_ executes a very rapid movement of erection at night, and the
natural pulsations are thereby rendered very inconspicuous. These
pulsations may, however, be found in the night record of young leaves.
The general occurrence of pulsations in the forenoon is probably due to
the fact that the resultant force which causes the down-movement is at
the time relatively feeble--the operative factors being: (1) the action
of the rising temperature which induces down-movement, and (2) the
action of light which in the forenoon opposes this movement. It will
thus be seen that the forces in operation in the forenoon are more or
less in a state of balance, hence conditions for exhibition of natural
pulsations are more favourable in the morning than in other parts of the
day.

_Experiment 222._--I next tried to discover conditions under which the
plant would exhibit its normal rhythmic activity during the whole course
of 24 hours. The external stimuli which may interfere with the
exhibition of its automatic pulsations are those due to gravity and
light. They act most effectively on the pulvinus, when that organ is
more or less horizontal and therefore at right angles to the direction
of the incident stimulus; they act least effectively on the pulvinus
when the organ is parallel to the direction of the external force. This
latter condition may be secured by holding the plant upside down, when
the pulvinus bends up and the leaf becomes erect and almost parallel to
the vertical lines of gravity and to vertical light from above. The
leaf, now relatively free from the effects of external stimulus, was
found to exhibit its autonomous pulsations for more than seven days. I
reproduce two sets of records (Fig. 212) for 24 hours each, obtained on
the first and the third day. The average period of a single pulsation is
slightly less than six hours; but this is likely to be modified by the
age of the specimen and the temperature of the environment.

One of the factors that determines the diurnal movement of the leaf is
the immediate and after-effect of light. The movement under the action
of light, is modified by the intensity and duration of illumination. The
experimental investigation of the subject offers many difficulties,
principally owing to the absence of any reliable indicator for the
varying intensity of light during the course of the day.

[Illustration: FIG. 212.--Continuous record of automatic pulsation of
_Mimosa_ leaf. The two series are for the first and the third day.]

THE PHOTOMETRIC RECORDER.

This difficulty I have been able to overcome by the automatic device for
continuous record of the variation of light. The electric resistance of
a selenium cell undergoes diminution with the intensity of light that
falls on it. The photo-sensitive cell was made the fourth arm of a
Wheatstone bridge, the resistance of the cell being exactly balanced
when the shutter of the sensitive cell was closed. The selenium receiver
was pointed upwards against the sky. Precaution was taken that it was
protected from the direct action of sunlight. On opening the shutter a
deflection of the index of a sensitive galvanometer was produced, and
the deflection increased with increasing intensity of diffuse skylight.
The special difficulty was in securing automatic record of the
galvanometer deflections. This was obtained by a special contrivance of
an oscillating smoked glass plate, the up and down oscillation being at
intervals of 30 minutes. A detailed account of this apparatus will, with
its possibilities for meteorology, be given in a future paper. I
reproduce the record obtained in my greenhouse on the 5th March (1919),
which gives a general idea of the variation of the light from morning to
evening (Fig. 213). The record shows that the light began to be
perceptible at 5-30 a.m., and that the intensity increased rapidly and
continuously till it reached a climax at noon, after which it began to
decline slowly. The decline of intensity of light was very abrupt after
5 p.m., the effect being reduced to zero at 6-30 p.m.

[Illustration: FIG. 213.--Photometric record showing variation of
intensity of light from morning to evening. Successive dots are at
intervals of 30 minutes.]

THE EFFECT OF DIRECT LIGHT.

Under natural conditions, the leaf of _Mimosa_ is acted on by light from
above, and it is generally supposed that the pulvinus is positively
phototropic, that is to say, it curves upwards till the leaf is placed
at right angles to the direction of light. My investigations show,
however, that the phototropic effects vary from positive to negative
through an intermediate stage of neutralisation, these depending on the
intensity and duration of exposure. When light acts continuously on the
upper half of the pulvinus, there follows the following sequences of
reaction:

(1) The leaf is at first erected by the contraction of the upper half of
the pulvinus due to direct action of light acting from above.

(2) Under continuous stimulation of the upper half of the pulvinus by
light, the excitation is slowly conducted to the lower half across the
pulvinus. In consequence of this transmitted excitation, the lower half
begins to contract and thus neutralises the first effect of erection.
The upper half of the pulvinus is less contractile than the lower half,
and the neutralisation is due to the full contraction of the upper half
antagonised by slight contraction of the lower half. The horizontal
position of the leaf under light is therefore the result of balance of
the two antagonistic reactions. If the incident light be very strong,
the more intense transmitted excitation induces greater contraction of
the lower half, and bring about a resultant down-movement (_cf._ p.
331).

Let us consider the effect of daily variation of light on _Mimosa_; we
have here to take account both of intensity and duration. The intensity
of light is seen to undergo a continuous increase which reaches a climax
at noon; it then begins to decline slowly and the diminution of
intensity of light is very abrupt after 5 p.m.

Under natural conditions the following phototropic effects are observed
during the course of the day: light acting from above induces an
up-movement of the leaf; but this is opposed by the thermo-geotropic
fall of the leaf due to rise of temperature. As the two opposing effects
are nearly balanced, any fluctuation of the relative intensity of the
two gives rise to the pulsatory movements often seen in the forenoon;
the _Mimosa_ leaf has moreover an autonomous movement of its own. Under
continued action of light neutralisation begins to take place after 1
p.m. (_cf._ _Expt._ 135). Later in the day the phototropic effect may
become negative; reversal into this negative takes place under the joint
action of intensity and duration of light; it takes place earlier under
strong, and later under feeble, light.

THE EVENING SPASMODIC FALL OF THE LEAF.

I shall now deal with the difficult problem of the sudden fall of the
leaf after 5 p.m. Pfeffer regarded this sudden fall in the evening as
due to the increased mechanical moment of the secondary petioles moving
forward on the withdrawal of light. But the following experiment shows
that the increased mechanical moment cannot be the true explanation of
the fall.

[Illustration: FIG. 214.--Record of leaf of _Mimosa_ after amputation of
sub-petioles. The leaf fell up to 2-30 p.m., and rose till 5 p.m., after
which there is a spasmodic fall. (Successive dots at intervals of 15
minutes.)]

_Diurnal movement of the amputated petiole: Experiment 223._--In my
present experiment the possibility of variation of mechanical movement
was obviated by cutting off the end of the petiole, which carried the
sub-petioles. The cut end was coated with collodion flexile to prevent
evaporation. The intense stimulus caused by amputation induced the
excitatory fall of the leaf, but it recovered its normal activity after
a period of three hours or so. The diurnal record of the leaf was
commenced shortly after 1 p.m.; it will be noticed that the leaf, though
deprived of the weight of its sub-petioles, still exhibited a sudden
fall at about 5 p.m. (Fig. 214). The fall of the leaf cannot therefore
be due to increased mechanical moment. The effect of weight was,
moreover, eliminated in torsional response (_Expt._ 221). In spite of
this the leaf exhibited a sudden movement after 5 p.m.

Pfeffer has in his 'Entstehung der Schlafbewegung' (1907) offered
another explanation of the sudden fall of the leaf of _Mimosa_. This,
according to him, is not the direct effect of diminished intensity of
light in the evening, but is due to the release of the leaf from the
phototropic action of light, which, so long as it is sufficiently
intense, holds the leaf in the normal position with its upper surface
at right angles to the incident rays. Thus, on being set free from the
strong action of light, the leaf moves in accordance with the preceding
condition of tension; and as this is low the leaf falls, soon to rise
again as the tension increases in prolonged darkness.

The above explanation presupposes: (1) that the tension was continuously
decreasing till the evening, and (2) that as soon as the phototropic
restraint which held the leaf up was removed it fell down in accordance
with the prevailing diminished tension.

Referring to the first point, an inspection of the diurnal curve of
_Mimosa_ shows that the leaf had no natural tendency to fall towards the
evening. There was on the contrary a movement of erection, on account of
fall of temperature after the thermal-noon (Fig. 210). As the natural
tendency of the leaf was to erect itself, the removal of phototropic
restraint cannot therefore induce a movement of fall.

As regards the factor of light, the effect in the afternoon is a
down-movement on account of transverse conduction of excitation; but the
leaf is prevented from exhibiting this down-movement by the
thermo-geotropic up-movement due to fall of temperature after the
thermal noon. I shall presently describe experiments on the pure effect
of light, which will show that the action of continued photic stimulus
induces a down-movement of the leaf in the afternoon.

The results of experiments that have been described show that the sudden
fall of the leaf in the evening could not be due to:

(1) increased mechanical moment,

(2) the natural tendency of the leaf to fall towards evening
against phototropic action by which the leaf is held up.

The above explanations having proved unsatisfactory we have to search
for other factors to account for the fall of the leaf on the cessation
of light. In this connection I was struck by the extraordinary
similarity of the diurnal curve of the petiole of _Cassia alata_ with
that of _Mimosa_.

[Illustration: FIG. 215.--Diurnal record of _Cassia_ leaf. Note
similarity with diurnal record of _Mimosa_.]

DIURNAL CURVE OF THE PETIOLE OF _Cassia alata_.

_Experiment 224_.--The leaf of _Cassia_ exhibits as in the leaf of
_Mimosa_ a slight erectile movement after the thermal-noon at 2 p.m.,
there is next a sudden fall after 5 p.m., which continues about 9 p.m.;
after this the leaf exhibits a continuous rise with the fall of
temperature, till the climax is reached about 6 a.m. in the morning; the
leaf then undergoes a fall with rise of temperature, there being a
number of pulsatory movements in the forenoon, evidently due to unstable
balance under the opposing effects of light and of rise of temperature
(Fig. 215).

The reason of this similarity between the records of _Cassia_ and
_Mimosa_ was found in the fact:

(1) That the main pulvinus of the leaf of _Cassia_ is, like the pulvinus
of _Mimosa_, differentially excitable, the lower half being more
excitable than the upper. This is demonstrated by sending a diffuse
electric shock through the leaf, the response being by a fall of the
leaf due to the greater contraction of the lower half of the pulvinus.
The leaf recovered after an interval of 20 minutes, the curve of
response being similar to that of _Mimosa_. The only difference between
the two organs is in the lesser excitability of the pulvinus of
_Cassia_, on account of which a greater intensity of shock is necessary
for producing the responsive fall.

(2) The responses to light are the same in both as will be seen in the
following experiment.

[Illustration: FIG. 216.--Post-maximum after-effect of light on response
of leaf of _Cassia_. There is an over-shooting on cessation of light at
arrow within a circle.]

_Experiment 225._--In _Cassia_, as in _Mimosa_, light acting from above
induces at first an erectile movement which reaches a maximum; after
this there is a neutralisation and reversal. In the record given in
figure 216, light from a small arc lamp acting on the upper half of the
pulvinus for 48 minutes gave the maximum positive curvature; this was
completely neutralised by further exposure to light for 20 minutes.
Light was cut off at neutralisation and there was a sudden fall beyond
the equilibrium position, which was more rapid than the movement under
light. The after-effect of prolonged exposure is thus an 'over-shooting'
beyond the normal position of equilibrium.

RESPONSE OF _Mimosa_ TO DARKNESS AT DIFFERENT PARTS OF THE DAY.

I now tried the effect of darkness on the movement of _Mimosa_, and was
surprised to find that while artificial darkness caused a sudden fall of
the leaf in the afternoon, it had no such effect in the forenoon.

_Experiment 226._--Successive records were taken of the effect of
artificial darkness for two hours, alternating with exposure to light
for two hours. The plant was subjected to darkness by placing a piece of
black cloth over the glass cover from 12 to 2 p.m., it was exposed to
light from 2 to 4 p.m. and darkened once more from 4 to 6 p.m.

The record given in figure 217 shows that the leaf had been moving
upwards under the action of light (positive phototropism); darkness
commenced at the point marked with a thick dot. _The after-effect on the
stoppage of light is seen to be in the same direction as under light_;
this persisted for ten minutes followed by recovery which was complete
by 2 p.m., as seen in the horizontal character of the curve. On
restoration of light (at the point marked with the second thick dot) the
leaf moved upwards till the positive phototropic movement attained a
maximum in the course of an hour and twenty minutes, after which
neutralisation set in, and by 4 p.m. the positive phototropic effect had
become partially neutralised. Artificial darkness at the third thick dot
caused a rapid down-movement which overshot the position of equilibrium.
The difference of after-effect in the forenoon and in the afternoon lies
in the fact that in the first case it was the pre-maximum after-effect;
but in the second case the after-effect was post-maximum. I have already
shown in the previous chapter that the pre-maximum after-effect of light
is a short-lived movement in the same direction as under light, while
post-maximum after-effect was a rapid over-shooting downwards beyond the
equilibrium position. These characteristics are also found in the
after-effects of light in _Mimosa_.

[Illustration: FIG. 217.--Effect of periodic alternation of light L, and
of darkness D, on the response of _Mimosa_ leaf. The first darkness
causes the pre-maximal after-effect of slight erection followed by
recovery. The subsequent application of light from 2 to 4 p.m. caused
erectile movement followed by partial neutralisation by 4 p.m. Stoppage
of light at the third thick dot caused a sudden fall of leaf _below_ the
position of equilibrium.]

The responses of _Mimosa_ on the cessation of light described above took
place in the course of experiments which lasted for more than six hours.
Objection may be raised that during this long period the temperature
variation must have produced certain effects on the response. In order
to meet this difficulty, I carried out the following experiments which
were completed in a relatively short time. I have already explained how
the period of experiment could be shortened by suitable increase of the
intensity of light. The experiment was commenced inside a room at noon
and completed by 2 p.m.; the temperature variation during this period
was less than 1°C.

[Illustration:

FIG. 218.--Pre-maximum after-effect of light in _Mimosa_.

FIG. 219.--After-effect at maximum.

FIG. 220.--Post-maximum after-effect exhibiting an 'over-shooting'
below position of equilibrium.

In the above records light was applied at arrow, and stopped at
the second arrow enclosed in a circle.]

_After-effect at pre-maximum: Experiment 227._--Light from an 100 c.p.
incandescent lamp was focussed on the upper half of the pulvinus of
_Mimosa_ for 8 minutes, after which the light was turned off. The
after-effect was a persistence of previous movement followed by recovery
(Fig. 218).

_After-effect at maximum: Experiment 228._--Continued action of light
for 18 minutes induced maximum positive curvature as seen in the upper
part of the curve becoming horizontal. On the stoppage of light, there
was a recovery to the original position of equilibrium (Fig. 219).

_After-effect at post-maximum: Experiment 229._--A fresh specimen of
plant was taken for this experiment; it exhibited maximum positive
curvature after an exposure of 20 minutes; continuation of light for a
further period of 17 minutes produced complete neutralisation. Stoppage
of light at this point, gave rise to a rapid down-movement (Fig. 220)
below the equilibrium position.

The experiments that have been described show that the rapid fall of the
leaf of _Mimosa_ in the afternoon is due to 'over-shooting' which is the
after-effect of prolonged action of light.

We are now in a position to give a full explanation of the different
phases of diurnal movement of the leaf of _Mimosa_. The fall of the leaf
commences from its highest position at thermal-dawn at 6 a.m. in the
morning and continued till the thermal-noon at 2 p.m. This is the
thermo-geotropic reaction due to rise of temperature. In the forenoon
the phototropic action is positive, and the fall of the leaf, due to
rise of temperature, is brought about in opposition to the action of
light. The temperature begins to fall after 2 p.m. and the leaf begins
to erect itself, and in the absence of any disturbing factor would have
continued its up-movement till next morning. But light undergoes a rapid
diminution after 5 p.m. and the after-effect of light is an
'over-shooting' in a downward direction. This fall continues till about
9 p.m., after which the leaf erects itself under thermo-geotropic action
of falling temperature, the maximum erection being attained at the
thermal-dawn at about 6 a.m.

SUMMARY.

The very complex type of nyctitropic movement of the primary petiole of
_Mimosa_ results from the combined effects of thermo-geotropism and
phototropism.

With the exception of a small portion of the curve in the evening, the
diurnal curve of _Mimosa_ is similar to the standard thermo-geotropic
curve, where the leaf exhibits an erectile movement from thermal-noon to
thermal-dawn, and a fall from thermal-dawn to thermal-noon.

Investigations show that the leaf of _Mimosa_ has an autonomous movement
of its own, which persists throughout twenty-four hours.

The torsional response of _Mimosa_ exhibits a diurnal variation similar
to that exhibited by the leaf in normal position.

The leaf of _Cassia alata_ exhibits a diurnal movement of the same type
as that of _Mimosa_.

The spasmodic fall of the leaf towards evening is not due to the
increased mechanical moment caused by the forward position of the
sub-petioles. The record of the leaf with amputated sub-petioles
exhibits the sudden fall in the evening as that of the intact leaf.

The evening fall of the leaf of _Mimosa_ is shown to be due to the
post-maximum after-effect of light, which causes an 'over-shooting', the
leaf undergoing a fall below the position of equilibrium.

B. S. Press--5-11-1919--19754J--750--R. D'S.

*** End of this LibraryBlog Digital Book "Life Movements in Plants, Volume II, 1919" ***

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