-/-/

3 0

= O

■

■ a

; ru

i _j

I

t-=\

^=^^

\ 1=3

^ -

a

=>— ^

r-q

□

m

a

GROWTH AND

TROPIC MOVEMENTS

OF PLANTS

By the Same Author

THE PHYSIOLOGY OF THE ASCENT OF SAP
With 95 Diagrams. 8vo, i6s. net.

RESEARCHED ON THE IRRITABILITY OF
PLANTS

With 190 Illustrations. 8vo, 7s. 6d. net.

RESPONSE IN THE LIVING AND NON-
LIVING

With 117 Illustrations. 8vo, ids. 6d. net.

PLANT RESPONSE AS A MEANS OF
PHYSIOLOGICAL INVESTIGATION
With 278 Illustrations. 8vo, 21s. net.

COMPARATIVE ELECTRO-PHYSIOLOGY
A Physico-Physiological Study. With 406 Illustra-
tions and Classified List of 321 new Experiments.
8vo, 1 6s. net.

PLANT AUTOGRAPHS AND THEIR
REVELATIONS
8vo, 7s. 6d, net.

THE NERVOUS MECHANISM OF PLANTS
With 82 Illustrations. 8vo, i6s. net.

THE MOTOR MECHANISM OF PLANTS
With 242 Illustrations. 8vo, 21s. net.

THE PHYSIOLOGY OF PHOTOSYNTHESIS
With 60 Illustrations. 8vo, 16s. net.

LIFE MOVEMENTS IN PLANTS.

Transactions of the Bose Research Institute, Calcutta. 8vo.
Vol. I. With 92 Illustrations. 15s. net.
Vol. II. With 128 Illustrations. 20s. net.
Vol. III. and IV. In one volume. With 107 Illustra-
tions. 20s. net.
Vol. V.

COLLECTED PHYSICAL PAPERS

With a Foreword by Sir J. J. Thomson, O.M., F.R.S.
With 123 Illustrations. 8vo, los. net.

THE LIFE AND WORK OF SIR JAGADIS C.

BOSE, M.A., D.Sc, LL.D., F.R.S., C.S.I., CLE.
By Patrick Geddes, late Professor of Botany, Uni-
versity College, Dundee, St. Andrews University, and
Professor of Sociology and Civics, University of
Bombay. With Portraits and Illustrations. 8vo,
1 6s. net.

13 ^^'

GROWTH AND
TROPIC MOVEMENTS

OF PLANTS

BY

SIR JAGADIS CHUNDER BOSE

M.A., D.SC, LL.D., F.R.S., C.S.I. , CLE.
CORRESPONDING MEMBER, ACADEMY OF
SCIENCES, VIENNA ; MEMBER OF INTER-
NATIONAL COMMITTEE OF INTELLECTUAL
CO-OPERATION, LEAGUE OF NATIONS ;
FOUNDER AND DIRECTOR, BOSE
RESEARCH INSTITUTE
CALCl'TTA

WITH 22g ILLUSTRATIONS

LONGMANS, GREEN AND CO.
LONDON ♦ NEW YORK ♦ TORONTO

1929

LONGMANS, GREEN AND CO. LTD.

39 PATERNOSTER ROW, LONDON, E.G. 4

6 OLD COURT HOUSE STREET, CALCUTTA

53 NICOL ROAD, BOMBAY

MOUNT ROAD, MADRAS

LONGMANS, GREEN AND CO.

55 FIFTH AVENUE, NEW YORK
221 EAST 20TH STREET, CHICAGO
TREMONT TEMPLE, BOSTON
128-132 UNIVERSITY AVENUE, TORONTO

Made in Great Britain

To His Highness

BHUPINDAR SINGH MAHINDAR BAHADUR

MAHARAJADHIRAJ OF PATIALA

G.C.S.I., G.C.I.E., G.C.V.O., G.B.E.

MAJOR-GENERAL, A.D.C.

FOR HIS ENLIGHTENED AND GENEROUS INTEREST

IN THE BosE Research Institute

.THIS BOOK is inscribed
BY THE AUTHOR

PREFACE

Plant-physiology has been enriched by contributions to
knowledge made by a succession of eminent and devoted
workers. In regard to my own investigations on the subject,
it may not be out of place to state that, being originally
intended for the profession of medicine, I had the great
■advantage of pursuing at the University of Cambridge
the study of the life-reactions of both plants and animals.
Since then two subjects have claimed my closest attention —
the physics of inorganic and the physiology of living matter.

Being at one time called upon to teach physiology of
plants to a student preparing for a University degree in
Science, the various questions raised by the unsophisticated
mind of my pupil led me to realise that some of the theories
which I had passively imbibed in my student days were
by no means infallible. I also came to recognise that the
main difficulty which stood in the way of deeper knowledge
was the absence of sufficiently sensitive means for detecting
the internal activities of plant-life. I therefore devoted many
years to the invention and construction of various Auto-
matic Recorders of extreme delicacy and precision, which
enable the plant itself to write down the inner workings
of its life.

Unfounded speculation has often obstructed the advance
of knowledge ; facts must supersede speculation, for it is
not the preconceived bias of the observer, but unimpeach-
able facts that alone can lead to the establishment of sound
theory. This accounts for the attitude of detachment
that I maintained for many years. In the preface to my

Vlll PREFACE

' Plant Response ' (1906) I stated that the aim of my work was
the demonstration of the unity of physiological mechanism
of the plant with that of the animal, as evidenced by the
script of the plant, and not the treatment

of known aspects of plant-movements which is to be found detailed,
together with the history of the subject, in standard books of
reference on plant-physiology, such as those of Sachs, Pfeffer,
Strasburger, Darwin, Francis Darwin, Vines and Detmer.

My investigations were commenced with the study of
the mechanical response given by plants to stimulation
(' Plant Response,' 1906). Since then they have been greatly
extended by instrumental appliances of extraordinarily
high magnification, the employment of which at one time
created some misgiving that the records obtained might be
due to physical disturbance and not to physiological reac-
tion. It would be an unpardonable omission of any investi-
gator to fail to take complete precautions against all possible
purely physical effects. The obvious method is to perform
parallel experiments with two plants, one of which is alive
and the other dead. The vigorous response of the living
plant, contrasted with complete absence of all response in the
dead, gives conclusive evidence in regard to the physiological
character of the reaction. This test has been carried out
by competent authorities, who reported [Nature, May 6,
1920) that my Magnetic Crescograph, with its magnification
of one to ten million times, gives correct record of what is
undoubtedly the physiological response of the plant.

The results obtained by the method of mechanical
response were confirmed in every detail by the method of
electric response, the essential feature of which is the
negative electromotive variation which is caused by stimu-
lation. In my ' Comparative Electro-Physiology ' (1907)
special precautions were enjoined for avoiding the errors
into which investigators are likely to fall in the employment
of these powerful aids to investigation.

I have in this, and in my previous works, employed
several independent methods of experimentation, whose

PREFACE IX

concordant testimony could leave no doubt as to the
authenticity of the newly discovered facts. Advanced in-
vestigation necessarily depends on the use of specially devised
instrumental appliances and on the proper understanding
of the technique of new methods, which can only be acquired
after sufficient training. The perfect reliability of my highly
sensitive instruments has been repeatedly verified at various
scientific centres both in the West and in the East. The
technical knowledge of the new methods can be gained
after a certain amount of practice. Professor Hans Molisch,
lately Director of the Plant-Physiological Institute, Vienna
University, during his recent visit to my Institute, was
able to repeat, with invariable success, many of the experi-
ments described in this volume. His account of some of
these will be found in Nature of August 4, 1928, and of
April 13, 1929.

It must be confessed that I was completely baffled, at
the beginning of my researches, by many results which were
as astonishing as they were unexpected. It was long per-
sistence and careful comparison of numerous records that
ultimately led to the elucidation of all anomalies. Attention
had hitherto been centred on the end-effects, which, taken
alone, are misleading. Examination of continuous records
under prolonged stimulation showed clearly the gradual
transformation of what may be regarded as the normal
reaction to its very opposite. With such basic facts to
work upon, it was afterwards possible to discover the causes
of transformation.

I may here briefly refer to a few of the more important
results. For instance, the excitation of plants by radiation
had come to be regarded as more or less confined within
the narrow range of the visible spectrum. Experiments on
the effects of wireless waves and of a high-frequency alter-
nating field of electric force prove, on the contrary, that
the sensitivity of plants to the ethereal spectrum extends
far beyond the infra-red region.

An unexpected factor in the modification of normal

X PREFACE

tropic curvature has been discovered in the transverse
conduction of excitation across the organ, by which the
response becomes gradually transformed from the positive
to the negative. The irritabiUty of the root has been found
to be in no way different from that of the shoot. A new
method for the physiological discrimination of gradation of
excitability in different layers of a tissue has led to the dis-
covery that there are certain cells in the lower half of the
pulvinus of Mimosa pudica which are far more sensitive and
active than the rest.

In the investigation of geotropism, the exact direction of
the incident stimulus has been determined, as also the funda-
mental reaction under this mode of stimulation. The device
of the Photo-Geotropic Balance has made possible the com-
parison of the effects of two modes of stimulation on an
identical organ. Investigation of the various characteristics
of the geotropic reaction has been greatly facilitated by the
discovery of geo-electric response, first described in my
' Comparative Electro-Physiology ' (1907). The geo-per-
ceptive layer has been locahsed by the Electric Probe, and
a probable explanation had been suggested of the opposite
geotropic curvature of the root and the shoot in the ex-
perimentally established fact that the stimulation of the
responding region of the root is indirect, whereas it is
direct in the shoot.

Another important result is the demonstration of the
torsional response of dorsiventral organs under different
modes of lateral stimulation, and the establishment of the
Law of Torsional Response. The extension of this particular
method of inquiry has led to the solution of various problems
connected with the torsion of twining stems.

Finally, from the fully demonstrated facts that direct
stimulation induces contraction while indirect stimulation
causes expansion, a wide generalisation has been established,
which includes within its scope the diverse tropic movements
of plant-organs.

I have endeavoured to link all the observed facts together

PREFACE XI

by a consistent and logical interpretation. This interpre-
tation may possibly be subject to modification, but it will
serve until something better has been suggested. The facts
on which the theoretical interpretation is based are, how-
ever, irrefutable.

I conclude with an extract from the preface of my
* Comparative Electro-Physiology ' (1907) that by the new
methods employed not only

a deeper perception of unity (of physiological mechanism of plant
and animal) has been made available, but also many regions of
inquiry have been opened out which had at one time been regarded
as beyond the scope of experimental exploration.

I take this opportunity to express once more to my as-
sistants and scholars my thanks for the most efficient help
rendered in these difficult and prolonged investigations.

J. C. BosE.

BosE Research Institute, Calcutta,
May 1929.

CONTENTS

CHAPTER I

PAGE

INTRODUCTORY i

CHAPTER II

THE HIGH MAGNIFICATION CRESCOGRAPH

Necessity for shortening the period of experiment — The Optical
Lever — Automatic High ]\Iagnitication Crescograph — Automatic
record of rate of growth — Oscillation Method for elimination of
friction in record — Time-relation of response obtained from dotted
record — List of specimens with average rate of growth — The
Stationary and the Moving Plate Methods — Determination of
absolute rate of growth — Effect of variation of temperature —
Precaution against physical disturbance — Pulsation in growth —
Magnification of fifty million times by Magnetic Crescograph . 5

CHAPTER III

THE BALANCED CRESCOGRAPH

Principle of the Method of Balance — Method of Compensation by
Falling Weight — Method of Compensation by Inclined Plane —
Scale for measurement of absolute rate of growth — Formula for
determination of absolute rate of growth by Method of Inclined
Plane — Experimental verification of formula — The Time-Factor
— Effect on growth of duration of the application of carbonic acid
gas . . . . . . . . . . .22

CHAPTER IV

EFFECT OF VARIATION OF TEMPERATURE ON GROWTH

Error arising from sudden variation of temperature — Thermal
regulator for gradual change of temperature — Determination
of cardinal points of temperature — The temperature optimum —
The temperature minimum — Contractile death-spasm at 60° C.

^dOlG

XIV CONTENTS

PAGE

— Method of continuous observation — The Thermocrescent
Curve — Determination of absolute rate of growth at different
temperatures ......... 34

CHAPTER V

EFFECT OF CHEMICAL AGENTS ON GROWTH

Effect of stimulants — Effect of anaesthetics — Effect of carbonic acid
gas — Effect of vapour of ether — Revival of growth by ether —
Effect of chloroform — Effects of sulphuretted hydrogen and of
ammonium sulphide — Effect of copper sulphate solution —
Stimulating action of poison in minute doses .... 43

CHAPTER VI

RELATION OF TURGOR AND TENSION TO GROWTH

Arrest of growth under drought — Revival of growth after irrigation
— Effect on growth of change in rate of ascent of sap — Effect of
irrigation with cold and with warm water — Effect of positive and
negative variation of turgor — Effect of gain or loss of water
on motile and growing organs — Change of turgor under directive
movement of sap — Effect of artificial increase of internal hydro-
static pressure — Effect of external tension . . . .51

CHAPTER VII

EFFECT OF ELECTRIC STIMULATION ON GROW^TH

Similar excitatory reaction under all modes of stimulation — Struc-
tural modifications for perception of external stimulation — The
Electric Stimulator — Determination of the latent period — Eft'ect
of increasing intensity and duration of stimulation — Continuity
between incipient and actual contraction — Similarity of excitatory
response in pulvinated and in growing organs .... 62

CHAPTER VIII

EFFECT OF LIGHT ON GROWTH

Normal retardation of growth under light — The latent period —
Effect of increasing intensity of light — Similarity of effect under
continued electric and photic stimulation — After-effect of light in
enhancement of growth — Effects of different rays of spectrum —
Effect of thermal rays ........ 71

CONTEXTS XV

CHAPTER IX

ACCELERATION OF GROWTH IN SUBTONIC PLANTS

ON STIMULATION

PAGE

Acceleration of growth under light— Modifying effect of tonic condi-
tion on response to stimulation — Continuity between abnormal
and normal response — Enhanced rate of growth under subminimal
stimulation — Revival of arrested growth under photic stimula-
tion— Acceleration of growth in subtonic organ under electric
stimulation ...••••••• °°

CHAPTER X

EFFECT OF MECHANICAL STIMULATION ON GROWTH

■ Effect of feeble and of moderate stimulation — Effect of wound —
Twining of tendrils — Tactile pits in certain tendrils — Effect of
indirect stimulation on growth of tendril — -Effect of direct electric
^ stimulation — Immediate and after-effect of mechanical stimulation

on growth of tendril — Positive tropic curvature under unilateral
mechanical stimulation — Negative tropic curvature under
indirect unilateral mechanical stimulation — Electric response of
proximal and distal sides of the tendril— Contractile response of
the less excitable side of tendril — Inhibitory action of stimulus . 87

CHAPTER XI

THE PHOTOTROPIC EFFECT OF LIGHT

The hvpothesis of specific positive and negative irritability of
different organs— The Phototropic Recorder— Positive photo-
tropic response of pulvinated organs — Positive curvature of
growing organs — Determination of the latent period — Induced
variation of growth bv Hght from a single spark — :\Iaximum
positive curvature under continued Hght — Relation between
quantity of light and induced curvature — Effect of increasing
intensity of hght — Effect of increasing duration of exposure —
Effect of the angle of the incident light loi

CHAPTER XII

THE MECHANISM OF PHOTOTROPIC CURVATURE

The effects fof direct and indirect stimulation — Enhancement of
turgor in leaf of Mimosa under indirect stimulation — Dual impulses
generated on stimulation, a quick positive and a slow negative
— Longitudinal transmission of impulse — Effect of transverse
transmission of impulse across the stem under photic stimula-
tion— Transverse transmission of impulse und-er electric and

h

PAGE

Il6

xvi CONTENTS

thermal stimulation — Effect of direct and indirect stimulation
on growing organ — Effect of indirect and direct radio -thermal
and electric stimulation — Electric response to direct and indirect
stimulation — Laws of Effect of Direct and of Indirect Stimula-
tion ..•••••••••

CHAPTER XIII

DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM

Neutrahsation caused by conduction of excitation across the organ-
Complete neutrahsation in thin organs — Fatigue-relaxation of
directly stimulated proximal side — Negative phototropic curva-
ture under prolonged unilateral action of strong light— Supposed
phototropic ineffectiveness of sunhght — So-called negative
phototropism of roots — The Root-Recorder — Effect of unilateral
stimulation of root-tip — Effect of indirect unilateral stimulation
of shoot — Effect of simultaneous stimulation of the tip and of the
growing region of root — Phototropic curvature of root of Sinapis
— Preliminary positive curvature transformed into negative by
transverse conduction of excitation . . . . -133

CHAPTER XIV

THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS

Susceptibility for excitation not constant but variable at different
parts of phototropic curve — The simple phototropic curve-
Effect of stimulation on subtonic organ — Complete phototropic
curve of a pulvinated organ — Four distinct stages in the complete
curve of a growing organ , . . . . . . .148

CHAPTER XV

THE PHOTONASTIC PHENOMENA

Phototropic and photonastic effects — Fall of leaflet of Averrhoa
when acted on by light from above or below — Exhibition of
opposite effect by leaflet of Mimosa — Photic Stimulator— Effect
of moderate photic stimulation of upper surface of pulvinus of
Mimosa — Effect of stronger light — Velocity of transmission
through upper half of organ — Effect of light on lower half of
pulvinus — Characteristic effects of appHcation of strong light
on upper and lower halves of the organ— Physiological detection
of gradation of excitability in lower half of pulvinus — Erectile
twitch of hydraulic impulse — Effect of radio-thermal stimulation
of upper half of pulvinus — Effect of strong photic stimulation of
lower half of pulvinus — Response of leaflet of Averrhoa to
strong hght from above — Effect of strong light applied below
— Effect of strong light on leaflet of Mimosa applied above and
be'ow^Universal laws of responsive movements of plant-organs
— Mechanical and electric responses of pulvinated and growing

CONTENTS XVll

PAGE

organs — Classification of principal types of response — Modifica-
tion of response by tonic condition by point of application,
by intensity of stimulus, and by differential excitability
of the organ — Continuity between phototropic and photonastic
movements . . . • • • • • • '157

CHAPTER XVI

RADIO-THERMOTROPISM

Antagonism between thermal and radio-thermal effects — Radio-
thermal Stimulator — Effect of indirect stimulation by thermal
radiation — Positive radio-thermotropism — Dia-thermotropism —
Negative thermotropism — Response of root to thermal radia-
tion— Continuity between positive, dia-, and negative radio-
thermotropism . . . . • • • • • ^74

CHAPTER XVn

RESPONSE OF PLANTS TO WIRELESS STIMULATION

Effect of high-frequency alternating field of electric force on plants — •
Response of IMimosa — Effect of rapidly alternating field on
gro\vi:h — Acceleration of grou-th under moderate stimulation —
Revival of gro\\'th under high-tension alternating current —
Retardation of growth under stronger stimulation — Response to
wdreless stimulation — The wireless system — Effect of feeble
stimulation — Effect of strong stimulation — Acceleration of
gro\\i;h in subtonic specimens — Effect of moderate stimulation
on active growth . . . . • • • • • 183

CHAPTER XVni

DIURNAL MOVEMENTS OF PLANTS

Movements complicated by the effects of numerous factors — The
Self- Recording Radiograph — The Selenium cell — The Wheatstone
Bridge — The Galvanograph — Radiogram of variation of light at
different hours of day — Irregular variation in stormy weather —
Simultaneous record of variation of temperature and of light for
24 hours — Effects of variation of light and of temperature . . I95

CHAPTER XIX

EFFECT OF RECURRENT LIGHT AND DARKNESS
ON MOVEMENTS OF PLANTS

Method of continuous automatic record of movements for 24 hours
— Constant irrigation— Vertical illumination— The Recorder

4 1 i

XVlll CONTENTS

pag£

• — The Thermograph — Response of leaflet of Cassia alata —
Effect of variation of temperature — Effect of variation of hght — ■
Diurnal movement of leaflet of Cassia — Diurnal movement of
terminal leaflet of Desmo£^a<m ^7m;?s ..... 207

CHAPTER XX

THERMONASTIC MOVEMENT OF NYMPHAEA

Nastic and tropic movements — Positive thermonastic response of
Zephyranthes — Effect of variation of temperature — Responsive
movement under radiation — Diurnal movement of Nymphaea

Effect of recurrent light and darkness — Ineffectiveness of

geotropic stimulus — Thermonastic Recorder — The diurnal record
— Effect of rise and fall of temperature on thermonastic move-
ment—Two types of thermonastic movement, positive and
negative . . . . • • • • • .216

CHAPTER XXI

THE DIURNAL MOVEMENT OF THE ' PRAYING ' PALM

The diurnal movement of the Faridpore Palm — ^Physiological cause
of the movement — The Automatic Recorder for the diurnal
movements of trees — -The diurnal record — The effect of light —
Effect of variation of temperature — Periodic Movement of the
Sijbaria Palm — Diurnal movement of Kentia Palm — The Diurnal
Indicator 224

CHAPTER XXn

DIURNAL MOVEMENTS OF PLANTS RENDERED
ANISOTROPIC BY^ GRAVITY^

Diurnal movement of procumbent stem of Mimosa — Records of
stem and leaf — Diurnal record of Tropaeolum under constant
temperature — Diurnal variation of tension and bulk in various
organs — Diurnal variation of tissue-tension in Mimosa — Effect
of reversal of induced anisotropy on diurnal movement — Analogy
with thermonastic phenomena — Effect of transpiration on
periodic movement — Persistence of diurnal movement of stem
and leaf after abolition of transpiration . . . . .236

CHAPTER XXni

PHOTOTROPIC TORSION

Response by torsional movement — The Torsional Recorder — Effect
of lateral stimulation by light — Directive action of stimulus-
Effect of different modes of lateral stimulation — Effect of lateral

CONTENTS XIX

PAGE

stimulation by thermal radiation — EfEect of differential excita-
bility on the direction of torsion — Laws of Torsional Response-
Advantages of Method of Torsional Response — The Torsional
Balance .....••••• 250

CHAPTER XXIV

THE AFTER-EFFECT OF LIGHT

Immediate and aftereffect of light— Determination of aftereffect
by electric response — Characteristic after-effects at pre-maximum,
at maximum, and at post maximum — Modification of after-effect
by duration of incident stimulation — Iropic response to hght and
its aftereffect — Aftereffect at pre maximum — Aftereffect at
maximum — Post maximum after effect in Cassia . . .259

CHAPTER XXV

THE DIURNAL MOVEMENT OF THE LEAF OF MIMOSA

Diurnal record of :Mimosa exhibiting four phases of reaction — Com-
plexity attributable to algebraical summation of several effects :
(i) of thermo geotropic reaction, (2) of immediate effect of photic
stimulus, and (3) of after-effect of hght — Autonomous pulsation of
the leaf— Spasmodic fall of the leaf in the evening— Diurnal move-
ment of petiole after removal of sub-petioles — Diurnal variation
of geotropic torsion of the pulvinus — Characteristic after-effects
of light on Mimosa — Abrupt fall of leaf in the evening is a post-
maximum after-effect of photic stimulation — Diurnal curve of
petiole of Cassia ....••••• ~^1

CHAPTER XXVI

GEOTROPISM

Apparent difference in sign of response of shoot and root under
geotropic stimulation— Uncertainty of fundamental reaction,
whether contraction or expansion— Direction of geotropic
stimulus — The Geotropic Recorder — Determination of the true
character of geotropic reaction by electrical method— Deter-
mination of the latent period— The complete geotropic curve-
Determination of direction of geotropic stimulus— Algebraical
summation of effects of light and gravity— Analogy between
phototropic and geotropic reactions— Relation between directive
angle and geotropic reaction— Differential geotropic excitation . 281

XX CONTENTS

CHAPTER XXVII

EFFECT OF ANAESTHETICS ON GEOTROPIC RESPONSE

PAGE

Effect of anaesthetics on growth — Effect of season on geotropic
response — Effect of chloroform on geotropic response of growing
and of pulvinated organs — Effect of ether on geotropic curvature
— Effect of carbonic acid gas on geotropic response of growing
organs — Effect on winter- and on spring-specimens — Effect of
carbonic acid gas on geotropic response of pulvinated organ-
Reversal of geotropic response under continued action of
carbonic acid gas . . . • • • • • -293

CHAPTER XXVIII

GEOTROPIC TORSION

Torsional response of dorsiventral organs to lateral geotropic
stimulation — Determination of the direction of stimulus of
gravity from torsional response — Balance of geotropic by photo-
tropic reaction — Comparative balancing effect of white and red
light — Effect of coal-gas on photo-geotropic balance . . . 306

CHAPTER XXIX

THE GEO-ELECTRIC RESPONSE OF THE SHOOT

Experimental arrangements for observing geo-electric response—
Non-polarisable electrodes — Securing iso-electric condition of
electrodes — ^Three different methods of experimentation — The
Method of Contact with sensitive and indifferent points — The
Method of Axial Rotation— The Method of Vertical Rotation-
Characteristics of geo-electric response — The latent period of
electric response — Physiological character of geo-electric response

Relation between angle of inclination and geotropic excitation

— ^The Law of Sines . . . • • • • • 3 ^ 3

CHAPTER XXX

LOCALISATION OF GEO-PERCEPTIVE LAYER
BY ELECTRIC PROBE

Electric excitation at different layers under geotropic stimulation —
The Electric Probe — Method of experiment — Prick-reaction —
Geo-electric response at different depths in the petiole of
Tropaeolum— Maximum excitability at geo-perceptive layer —
Decline of geotropic excitability at distance from perceptive
layer — Opposite reactions in the upper and lower halves — Method
of Transverse Perforation — LocaUsation of geo-perceptive layer —

CONTENTS XXI

PAGE

The * starch- sheath,' the perceptive layer — Evidence of in-
sensitive specimens — The electric, geotropic and microscopic
tests — ^Duplication of geo-perceptive layer in shoot of Calendula. 326

CHAPTER XXXI

RELATION BETWEEN ANGLE OF INCLINATION AND
GEOTROPIC EXCITATION

Excitatory geotropic reaction at 45° and 90° — Geotropic excita-
tion at various angles — Excitatory reaction at 45° and 135° —
Greater geotropic excitability at apical than at basal end of
sensitive statocysts . . . . . . . .348

CHAPTER XXXn

THE CRITICAL ANGLE FOR GEOTROPIC EXCITATION

Excitation disproportionately lower at smaller than at larger angles
of inclination — Determination of the critical angle — Method of
alternate inclination to right and to left — The effect of repetition
in lowering the critical angle . . . . . -357

CHAPTER XXXni

THE RESPONSE OF THE ROOT TO DIFFERENT STIMULI

Irritable root-tip separate from responding growing region — ^Nlechani-
cal response to unilateral stimulation of root-tip — Response to
direct unilateral stimulation of growing region — Electric re-
sponse to indirect stimulation — Geo-electric response of root-tip
— Electric response in the growing region — Additive action-cur-
rent at the tip and at the growing region — Geo-perception at the
root-tip — Difference in the geotropic response of shoot and root —
Difference between effects of geotropic and photic stimulation . 365

CHAPTER XXXIV

THE MECHANISM OF THE TWINING STEM

Characteristic response of anisotropic organs — Preliminary adjust-
ments— Autonomous torsion and circumnutation — Rates of
circumnutation at different hours of day — Torsional rotation
round a vertical axis — Torsional activity of different internodes
— Contact-stimulus and twining — Method of optical magnifica-
tion for investigation of torsional response — Automatic Recorder

XXll CONTENTS

PAGE

— EfEect of variation in the rate of ascent of sap — Effect of
irrigation with warm water — EfEect of plasmolytic withdrawal
of water — EfEect of lowering of temperature — Effect of rise of
temperature — The critical fatal temperature — Effect of poison —
EfEect of chloroform — EfEect of ether . . . . -375

CHAPTER XXXV

EFFECT OF DIFFUSE STIMULATION ON
AUTONOMOUS TORSION

Response to electric stimulation — EfEect of strong stimulation :
retardation or actual reversal of torsion — EfEect of minimal
stimulation — Modifying effect of subtonicity — EfEect of indirect
stimulation — EfEect of thermal shock — EfEect of mechanical
stimulation — EfEect of direct stimulation by light — Effect of
indirect stimulation — EfEect of red and of blue light — EfEect of
geotropic stimulation on the erect and the inverted plant . 395

CHAPTER XXXVI

EFFECT OF UNILATERAL STIMULATION ON
AUTONOMOUS TORSION

The numerous factors complicating torsional movement — Diurnal
variation of torsion — EfEect of unilateral stimulation by light
■ — Unequal effect of light on the difEerent sides — No effect on
neutral sides (N, N'), but maximum effect, positive or negative,
on flanks (A, R) — ^Universal Laws of Torsional Response —
Torsional response of lhe pulvinated organ — -Torsional response of
the twisting stem— Effect of unilateral stimulation by gravity —
Unequal effects at difEerent sides — Combined effects of geotropic
and photic stimulation in concurrence or in opposition . . 406

CHAPTER XXXVn

GENERAL REVIEW 420

INDEX 437

ILLUSTRATIOiNS

HG. PACE

1. Compound lever of High Magnification Crescograph . . 8

2. Mechanism for oscillation of plate . . . , .10

3. Complete recording apparatus . . . . . .11

4. Crescographic records of the absolute rate of growth, on a

stationary plate . . . . . . . .14

5. Effect of temperature, on a moving plate . . . .15

6. Effect of variation of temperature on a dead branch, taken on

moving plate . . . . . . . .17

7. Effect of temperature recorded on a stationary plate . . 18

8. Record of a single growth pulse of Zephyranthes . . .18

9. Diagram of Method of Balance by Falling Weight . . 24

10. Record showing condition of overbalance and underbalance

of growth , . . . . . . . .25

11. Diagram of Method of Inclined Plane . . . . .27

12. The complete apparatus for Balance by Inclined Plane . 29

13. Record showing the effect of Carbonic Acid Gas on growth . 31

14. Arrangement for variation of temperature of plant-chamber . 35

15. Records of growth, on stationary plate, at different tempera-

tures .......... 36

16. The Ihermocrescent Curve ...... 39

17. Curve showing relation between temperature and rate of

growth ......... 41

18. Effect of Carbonic Acid Gas on growth .... 44

19. Effect of vapour of Ether on growth ..... 45

20. Effect of vapour of Chloroform ...... 47

21. Effect of Sulphuretted Hydrogen (Wheat seedhng) . . 49

22. Effect of Copper Sulphate solution . . . . .49

23. Growth at standstill revived after irrigation with water . . 52

24. Effect of irrigation on growth . . . . . -53

25. Ihe effects of alternate application and withdrawal of water

on growth ,...,.... 55

26. Response of the pulvinus of Mimosa to irrigation and to

withdrawal of water ....... 56

27. Induction Coil ........ 63

28. Time-relations of the response of a growing organ to electric

stimulus of increasing intensity ..... 64

29. Records of contractile response under increasing intensities of

electric stimulation of 0-25, i, and 3 units ... 66

30. Effect of continuous electric stimulation .... 67

31. Contractile response of leaf of INIimosa .... 68

Xxiv LIST OF ILLUSTRATIONS

FIG. PAGE

32. Contractile response of a growing organ under electric shock

(Crinum) ......... 69

33. Effect of light in retardation of growth .... 72

34. Determination of the latent period ..... 73

35. Effect of light of increasing intensity in retardation of growth 74

36. Effects of continuous electric and photic stimulation recorded

on a moving plate (Crinum) ...... 75

37. Direct and after-effect of light recorded by Method of Balance

(Allium) ......... 76

38. Response of subtonic specimen to stimulus of light . . 77

39. Accleration of growth under subminimal stimulus of light

(Kysoor) ......... 83

40. Stimulus reviving growth at standstill (Allium) ... 84

41. Electric stimulation enhancing rate of growth of subtonic

plant (Wheat) ........ 85

42. Effect of mechanical friction on growth . . . ,88

43. Effect of pin-prick on growth ...... 88

44. Diagrammatic representation of Method for the indirect and

direct electric stimulation of tendril . . . .91

45. Record by jMethod of Balance, showing acceleration of growth

of tendril induced by electric stimulation (Cucurbita) . 91

46. Variation of growth of tendril induced by direct electric

stimulation ......... 92

47. Positive curvature of tendril of Cucurbita under unilateral

contact-stimulus ........ 94

48. Diagrammatic representation of effect of indirect and direct

unilateral stimulation of a tendril (Passiflora) ... 95

49. Electric response at the distal and proximal sides of a tendril

under unilateral mechanical stimulation (Vitis) . . 96

50. The Phototropic Recorder for pulvinated organ and for grow-

ing organ ......... 103

51. Positive tropic effect of light on pulvinated organ . . .104

52. Positive response of leaflets of Erythrina indica. (Para-helio-

tropic response) . . . . . . . .104

53. Positive phototropic curvature of a growing stem . . .105

54. Latent period of response to light in a pulvinated organ . 106

55. Effect of a single electric spark in causing variation of growth 108

56. Tropic effect of increasing intensity of light, 1:2, on the

positive up-response of terminal leaflet of Desmodium . 109

57. Curve showing the relation between intensity of light and

retardation of rate of growth (Crinum) . . . .110

58. Tropic effect of increasing intensity of light, 1:2:3, on

growing organ (Crinum) . . , .. . . .111

59. Effect of increasing duration of exposure of i : 2 : 3 minutes

on phototropic curvature of growing organ (Crinum) . .112

60. The Collimator for application of light at various angles . 112

61. Effect of angle of incidence of light on tropic curvature of

pulvinus of Desmodium . . . . . .113

62. Records of tropic curvature of growing bud of Crinum on

alternate stimulation by light at 45° and at 90° . .113

63. Longitudinal transmission of impulse in stem of Mirnosa . 119

123

123

I30

LIST OF ILLUSTRATIONS XXV

FIG. PAGE

64. Records of effect of longitudinal transmission of impulse . 119

65. Effect of transverse transmission of impulse under photic

stimulation ......... 122

66. Effect of transverse transmission of impulse under electric

stimulation .........

67. Effect of transverse transmission of impulse under radio-

thermal stimulation .......

68. Effect of indirect unilateral stimulation (Crinum) . .126

69. Effect of indirect and direct electric stimulation on growth of

Crinum ......... 127

70. Effect of continuous indirect electric stimulation (Cosmos) . 128

71. Diagrammatic representation of electromotive response to

indirect stimulation of effectively increasing intensity . 129

72. Positive, diphasic, and negative electric responses (Bryophyllum

calycinum) ......... 129

73. Electric response at distal and proximal sides under unilateral

stimulation .........

74. Effect of electric shock of short duration in inducing contraction

followed by normal recovery (Vigna Catjang) . . .136

75. Effect of continuous electric stimulation showing fatigue-

relaxation (Vigna Catjang) . . . . . .137

76. Phototropic reversal under continued action of strong light

(Vicia Faba) 138

77. Positive, dia- and negative phototropic response of Oryza

under continued unilateral stimulus of intense light . .138

78. The Root-Recorder . . . . . . . .140

79. Negative response of the root to indirect stimulation (Ipomoea) 141

80. Negative response of shoot under indirect unilateral photic

stimulation (Vicia Faba) . . . . . .142

81. Effect of simultaneous unilateral stimulation of the tip and

of the gro^Wng region of root (Ipomoea) . . . .143

82. Effect on shoot and root of unilateral exposure to light

(Sanchezia) . . . . . . . . .144

83. Response of root of Sinapis under continued unilateral action

of hght 146

84. Simple characteristic curv^e of phototropic reaction . .150

85. Complete phototropic curve of a pulvinated organ . . 154

86. Complete phototropic curve of a growing organ (Zea Mays) . 155

87. The incandescent electric lamp-holder .... 159

88. Arc-lamp \\-ith totally reflecting prism for application of strong

light vertically downwards or upwards . . . .160

89. Records of effect of feeble light and of strong hght on upper

half of pulvinus (Mimosa) ...... 160

90. Effect of feeble and of strong light on lower half of pulvinus

(Mimosa) 162

91. Diagrammatic representation of the different layers in a

longitudinal section of pulvinus . . . . .165

92. Triphasic response on application of hght (Mimosa) . .166

93. Parallel effect of radio-thermal stimulation . . . .166

94. Effect of application of strong light on lower half of pulvinus. 166

95. Response of leaflet of Averrhoa to strong light from above . 168

XXvi LIST OF ILLUSTRATIONS

PAGE

96. Effect of strong light applied below (Averrhoa) . . .168

97. Effect of strong light on upper side of Mimosa leaflet . .169

98. Effect of strong light on lower side of leaflet . . .169

99. Diagrammatic representation of different types of response

under unilateral stimulation . . . . . -172

100. Radio thermal Stimulator . . . . • . .176

loi. Effect of indirect radio thermal stimulation . . . -177

102. Diphasic response under more effective stimulation . -177

103. Positive response to stimulation by short exposures to thermal

radiation . . . • . • • • .178

104. Response of pulvinus of Mimosa to thermal radiation . .179

105. Record of positive, neutral, and negative curvature under

continued unilateral action of thermal radiation (Dregea) . 180

106. Response of root to unilateral and continuous thermal radia-

tion (Ipomoea) . . . • . • • .181

107. Effect of high-frequency alternating electric field on response

of pulvinus of Mimosa . . . . . • .185

108. Effect of high-tension alternating field on growth . .186
io8a. Revival of growth in Wheat-seedhng, normal record without

balance . . . . • • • • .187

109. Effect of strong intensity of field in retarding growth (Wheat) . 188
no. Diagrammatic representation of method employed for obtain-
ing response to Wireless Stimulation . . . .189

111. Record of responses to electric wave with the Balanced

Crescograph . . . • • • • .190

112. Acceleration of growth by wireless stimulation in a subtonic

specimen (Wheat) . . . . • • .191

113. Retardation of rate of growth under strong wireless stimulation

(Crinum) . . . • • • • • .192

114. Effect of wireless stimulation on unbalanced growth . .193

115. The Self-Recording Radiograph . . . . . .199

116. Radiogram of variation of intensity of Hght from the sky

during 12 hours in winter ...... 202

117. Record of diurnal variation of hght and of temperature in

summer .......•• 203

118. Leaflets of Cassia alata : open in daytime, and closed in

evening ....•.••• 207

119. The Automatic Recorder of diurnal movements of plants . 208

120. Effect of sudden darkening . . . . • .211

121. Diurnal movement of the leaflet of Cassia alata . . .212

122. The day and night positions of the petiole and terminal

leaflet of Desmodium gyrans . . . . • .213

123. Record of diurnal movement of the terminal leaflet of

Desmodium gyrans . . . • • • .214

^^"^'l Thermonastic responses of petal of Zephyranthes . . . 218

125- )

126. Nymphaea closed in daytime . . . • • .219

127. Nymphaea open at night .....♦• 219

128. The Thermonastic Recorder ...... 220

129. Negative thermonastic response of Nymphaea . . . 221

130. Diurnal record of Nymphaea . . . . • .222

List OF Illustrations

XXVll

FIG.

131-
132.

133-
134-

^55-
136.

137-

13S.

139.

140.
141,
142.

143-

144.

145-
146. 1

147.
14S.
149.
150.

I5I.

152.

153-

154-

155-
156.

157-

158.

159-
160.
161.
162.
163.
164.
165.
166.

The ' Praying ' Palm : the morning position
The ' Praying ' Palm : the afternoon position
Automatic Recorder of movements of trees and plants .
Record of diurnal movement of the ' Praying ' Palm (Phoenix
sylvestris) .........

Record of the Sijbaria Palm from noon for 24 hours

The Diurnal Indicator .......

Diurnal curve of movement of procumbent young stem of
^Mimosa pudica ........

Diurnal cur\e of the procumbent stem of Tiopaeolum majus,

and of the leaf of Dahlia, for two successive days
Abolition of diurnal movement in Tropaeolum under constant
temperature, and its restoration under normal daily
variation .........

Diurnal record of adult leaves of Dahlia, Papaya, and Croton .
Effect of inversion of plant on diurnal movement
Record of Kentia Palm before and after abolition of tran-
spiration .........

Parallel record in a dia-geotropic leaf (Er^-thrina indie a)
Diagrammatic representation of the Torsional Recorder
Clockwise and anti-clockwise torsional responses of pulvinus

of Mimosa to stimulation of its right and left flanks
Clockwise torsion under stimulation of right flank by thermal

radiation .........

Anti-clockA\-ise torsion under stimulation by thermal radiation

applied to left flank of pulvinus (Mimosa)
Electric response of the petiole of Br\-ophyllum under con-
tinuous photic stimulation ......

Semi-diagrammatic representation of electric after-effects due

to photic stimulation .......

After-effect of pre-maximum stimulation of terminal leaffet of

Desmodium g\Tans .......

After-effect of stimulation at maximum of terminal leaflet of

Desmodium gyrans .......

After-effect of post-maximum stimulation of terminal leaflet of

Desmodium g}-rans

light

on response of leaf of

Post-maximum after effect of

Diurnal record of ^Mimosa leaf in summer and in winter
Record of automatic pulsation of ^limosa leaf in the forenoon
Photometric record sho\\-ing the variation of intensity of

hght from morning to evening
Record of leaf of Mimosa after removal of sub-petioles
Record of diurnal variation of torsion in Mimosa leaf
Effect of periodic alternation of darkness and light
Pre-maximum after-effect of light on ^limosa
^laximum after-effect of light on Mimosa
Post-maximum after-effect of light on ^limosa
Diurnal record of Cassia leaf
The Quadruplex Geotropic Recorder .
Geotropic response of peduncle of Tuberose

PAGE

224

225
227

228
232

233
236

238

240
241

244

247
248

252

253
254

254

261

261
264
264
264

265
26S
270

271

272

274
276

277

277

277

279
283

2S5

XXViii LIST OF ILLUSTRATIONS

FIG.

PAGE

167. Geotropic response of petiole of Tropaeolum . . . 285

168. The complete geotropic curve (Crinum) .... 287

169. Diagram of response to light or gravity (Crinum) . . 288

170. Additive effect of photic stimulus ..... 288

171. Geotropic curve of a spring- and a winter specimen of

Tropaeolum ......... 295

172. Effect of Chloroform on geotropic response .... 296

173. Effect of Chloroform on geotropic response of the terminal

leaflet of Desmodium ....... 297

174. Effect of Ether in enhancing geotropic response (petiole of

Tropaeolum) ........ 298

175. Effect of Ether vapour on geotropism .... 299
176.) Effect of CO2 on geotropic response of a winter-specimen
177.) and of a spring-specimen of Tropaeolum .... 300

178. Effect of CO2 on geotropic response of Tropaeolum . . 301

179. Method of record of geotropic response of j\Iimosa . . 303

180. Effect of CO2 on geotropic response of Mimosa . . . 304

181. Effect of CO2 on geotropic response of Erythrina indica . 304

182. Diagram of arrangement for recording torsional response to

geotropic stimulation (Mimosa) ..... 306

183. Clockwise torsional response under geotropic stimulation . 307

184. Anti-clockwise torsional response under geotropic stimulation 307

185. Additive effect of stimulation by gravity and by light . 309

186. Algebraical summation of geotropic and phototropic action . 309

187. Comparative balancing effects of white and red light . .310

188. Effect of coal gas on photo-geotropic balance . . • 311

189. Diagrammatic representation of a multicellular organ laid

horizontally and exposed to geotropic stimulation . - 3^4

190. Experimental arrangement for subjecting shoot to geotropic

stimulation . . . . . . . • -315

191. Geo-electric response of the physically restrained scape of

Eucharis Lily . . . . . . • .316

192. Successive responses (Polianthes tuberosa) .... 3^6

193. Diagrammatic representation of geo-electric response . .319

194. Diagrammatic representation of the Method of Axial Rotation

and of Vertical Rotation ...... 320

195. Geo-electric response of Anthurium ..... 322

196. Diagrammatic representation of the geo-perceptive layer in

unexcited vertical and in excited horizontal position . 327

197. The Electric Probe . . . . . . • .328

198. Intensity of geo-electric response at different depths in the

petiole of Tropaeolum ....... 332

199. Curve showing distribution of geotropic excitability . . 335

200. Photo-micrograph of transverse section of upper half of hori-

zontally laid stem of Impatiens . . . • .337

201. Longitudinal section of upper and lower halves of geo-

tropically curved stem of Eclipta . . . . • 337

202. Distribution of geotropic excitability in the peduncle of

Nymphaea ......••• 34°

203. Distribution of geotropic excitability in the shoot of Bryo-

phyllum ......••• 34°

LIST OF ILLUSTRATIONS XXIX

FIG. PAGE

204. Transverse section of the stem of Bryophyllum . . . 341

205. Diagrammatic representation of inducing the geotropic

reaction of the shoot by the Method of Reversal . . 349

2g6. Alternate geo-electric response at + 45° and — 45°, also

+ 90° and — 90° (jNIethod of Vertical Rotation) . , 350

207. Records of geo-electric response at various angles (Tropaeolum) 353

208. Geo-electricresponseat 45°, 135°, and 45° in sequence . . 355

209. Curves showing values of sines and of reactions . . . 359

210. Abrupt geo-electric response at an inclination of 31°

(Tropaeolum) ........ 361

211. Diagrammatic representation of mechanical and electric

response of root to indirect stimulation .... 366

212. Diagrammatic representation of geo-electric response of root-

tip . . . . . . . . . . 368

213. Diagrammatic representation of geo-electric response in

growing region of root . . . . . . -370

214. Method of measurement of Circumnutation of a twining shoot 379

215. Measurement of relative activities of successive internodes . 379

216. Torsion of the ridges of stem. Stem twining anti-clockwise

round support (Thunbergia coccinea) . . . .381

217. Method of Optical Magnification ..... 383
2t8. Automatic Recorder of Torsion ...... 3S5

219. Death-spasm at fatal temperature of 60° .... 390

220. Effect of Formaldehyde on torsion ..... 39I

221. Effects of Chloroform and Ether on torsion . . . 392

222. Direction of normal torsion reversed under electric stimu-

lation (Thunbergia) ....... 396

223. Automatic record of diurnal variation of torsion . . . 408
[Diagrammatic representation of torsional response of aniso-'

"^' - tropic organs to unilateral stimulation by light ; pulvinus ■ 412
' of INIimosa and twisting stem

226. Optical ]\Iethod of observing rate of torsional movement under

unilateral stimulation by gravity . . . . .414

227. Torsional response of the stem to stimulation of its

different sides by gravity . . . . . .415

228. Combined effects of simultaneous geotropic and photic stimu-

lation on autonomous torsion . . . . . .418

GROWTH AND TROPIC
MOVEMENTS OF PLANTS

CHAPTER I

INTRODUCTORY

_ Growth and Tropic Movements of Plants

ERRATA.

Page 30, line 17, foy ' seconds ' read ' minutes.'

,, 97, fourth line from bottom, for ' fiat ' read ' rough.'

,, 118, last line but one, for ' 30 ' read '20.'

,, 348, ninth line from bottom, the word ' the ' should be inserted before

' mechanical.'
., 352, second line from bottom, for ' reaction ' read ' reactions.'
,, 392, seventh line from bottom, for 'has ' read 'have.'
,, 410, Table LIV, for 'rotation ' read ' torsion.'

Bose, Growth and Tropic Movements.

ana laii 01 lemperature. iney may act on organs wnicn
exhibit all degrees of physiological differentiation, from
the radial to the dorsiventral. An identical stimulus some-
times induces one effect, and at other times precisely the
opposite. Thus, under unilateral stimulation by light of
increasing intensity, a radial organ exhibits first a positive,
then a dia-phototropic, and finally a negative movement ;

GROWTH AND TROPIC
MOVEMENTS OF PLANTS

CHAPTER I

introductory

Growth and Tropic Movements of Plants

In my ' Motor Mechanism of Plants/ published a year ago,
detailed accounts were given of investigations on the motor
mechanism of adult members, such as the leaves of sensitive
and other plants. A very large and extensive class of
phenomena still remained to be dealt with, namely, the
movements of growing organs induced under external
stimulation. The consideration of these responsive move-
ments of growing organs forms the main subject of the
present volume.

The movements induced in a growing organ by external
stimulation are extremely diverse. The effective agents
are manifold — the stimulus of contact, of electric current,
of gravity, of radiation visible and invisible, and the rise
and fall of temperature. They may act on organs which
exhibit all degrees of physiological differentiation, from
the radial to the dorsiventral. An identical stimulus some-
times induces one effect, and at other times precisely the
opposite. Thus, under unilateral stimulation by light of
increasing intensity, a radial organ exhibits first a positive,
then a dia-phototropic, and finally a negative movement ;

2 CHAP. I. INTRODUCTORY

the positive movement is towards, and the negative away
from, the stimulus. Strong sunHght brings about a para-
hehotropic movement, often described as the ' Midday sleep,'
in which the apices of leaves or leaflets turn either towards
or away from the source of illumination. In the leaflet of
Averrhoa Caramhola the movement is downward, which-
ever side is illuminated with strong light ; in the leaflet
of Mimosa the movement under similar circumstances is
in the opposite direction. Such photonastic movements,
apparently independent of the directive action of light,
have been regarded as phenomena unrelated to photo-
tropic reaction, due to a different kind of irritability with
a different mode of response. Pfeffer,^ after enumerating
the apparently anomalous effects induced by light, which is
only one out of numerous factors in operation, dwells on the
inadequacy of the various explanations that have been
advanced, and concludes by saying 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 con-
veniently abbreviated form, without explaining why such
curvature is possible or how it is produced.'

Greater difficulties are encountered in explaining the
effects of other modes of stimulation. An attempt is made
in this volume to discover and explain the common and
fundamental reaction which underlies the various responses
given by growing organs under all modes of unilateral
stimulation which induce tropic curvature. The tropic
movements are essentially due to unequal growth induced
in the two opposed sides of the organ. What now are
the characteristic modifications of growth at the proximal
and distal sides of the organ under various modes of
stimulation ? Do they induce reactions essentially similar
or specifically different ?

1 Pfeffer, Physiology of Plants, trans, by A. J. Ewart, vol. ii., p. 74
(Clarendon Press).

SUBJECTS TO BE DISCUSSED 3

In spite of much theorising, no clear idea has yet been
reached concerning the ultimate mechanism of growth in
the individual cell. It is not possible to go beyond the
statement that the growth of the cell depends upon its
turgidity, the degree of which, and also the rate of growth,
are regulated by the cell-protoplasm . Fundamentally, there-
fore, the mechanism of growth is the same as that of other
plant-movements.

For the purpose of observing the rate of growth in length,
auxanometers magnifying about 20 times have been in
general use. Even with this magnification it takes several
hours to measure the rate of growth and the change induced
in it by any particular stimulus. The external conditions
cannot, however, be maintained constant for such a pro-
tracted period, and the observed result is vitiated by the
effect of change in these factors.

The possibility of accurate investigation, therefore, lies
in reducing the period of the experiment to a few minutes
during which the external conditions can be maintained
constant, thus making possible the accurate determination
of the variation of rate of growth induced by any one mode
of stimulation. This necessitates the devising of a method
of high magnification for the record of growth.

Among the subjects discussed in the present volume are :

1. Absolute determination of the rate of growth by
several sensitive and exact methods.

2. Quantitative determination of changes induced in
growth under variation of external conditions.

3. The modifying influence of tonic condition on response
to external stimulation.

4. The effect of radiant energy through a wide range
of the ethereal spectrum.

5. The effects of direct and indirect stimulation on
growth.

6. Tropic curvature under unilateral stimulation.

7. The modification of tropic curvature by transverse
conduction of excitation.

4 CHAP. I. INTRODUCTORY

8. The relative effect of unilateral stimulation of shoot

and root.

g. Thermonastic and thermo-geotropic phenomena.

10. Mechanical and electric response under geotropic
stimulation.

11. Torsional response under lateral stimulation.

12. Phenomenon of autonomous torsion.

CHAPTER II

THE HIGH MAGNIFICATION CRESCOGRAPH

The essential difficulty in investigations on growth arises
from its extraordinary slowness. The average rate of longi-
tudinal growth in a plant is about Toihuu inch per second,
a length which is half that of a single wave of sodium light.
Even with the magnifying growth-recorders hitherto
employed, it takes several hours to detect and measure its
rate. For the accurate investigation of the effect of any
given agent on growth, it is necessary to keep all other
conditions, such as light and heat, strictly constant through-
out the experiment ; even if this were possible, there would
probably be some autonomous variation of the rate of
growth during such lengthy periods. Considerable un-
certainty is inevitable in results obtained by a method
which involves a long time for observation. The element
of uncertainty can only be eliminated by reducing the
period of the experiment to a few minutes, but that
necessitates devising a method of very high magnification
and an automatic record of the magnified rate of growth.

The Optical Lever

The problem of high magnification w^as first solved by
the employment of my Optical Lever, where an axis carrying
a mirror undergoes rotation proportional to the growth
elongation. The reflected spot of light magnified the
movement of growth from looo to 10,000 times. The
vertical movement of the spot of light was converted into

6 CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

a horizontal movement by means of a mirror suitabl}^
inclined. The excursion of the spot of light was followed
by means of a pen on a drum revolving at a known rate ;
or the record was obtained automatically by photography.
Hence a curve was obtained whose ordinate gave growth-
movement, and the abscissa time.^

Records thus obtained opened out a very extensive
field of investigation on growth and its variation under
the manifold influences of the environment. The photo-
graphic method is automatic, but it necessitates the
discomfort and inconvenience of a dark room ; the results,
moreover, cannot be followed visually. The other method
of obtaining the curve of growth by following the excursion
of the spot of light with a pen is far more convenient,
but the results in this case are likely to be affected by
personal error. In order to obviate all these difficulties
I devised a direct method, in which the plant by its own
autograph exhibits the absolute rate of growth and any
induced variation in an extremely short period of time.

The Automatic High Magnification Crescograph

I secured high magnification by means of a compound
system of two or more levers.^ The plant is attached to the
short arm of a lever, the long arm of which is attached to
the short arm of the second lever. If the magnification
by the first lever be m, and that by the second 11, then the
total magnification will be mn.

The numerous practical difficulties encountered were
principally due (i) to the great tension exerted by the
two levers on the plant ; (2) to the possible stretching of
the connectors by which the plant is attached to the first
lever, and the first lever to the second ; and (3) to the
friction at the fulcrums. I will describe the most recent
method employed to overcome these difficulties.

1 Plant Response (1906), p. 412.

2 Proc. Roy. Soc, B, vol. 90 (1919). P- 3^5-

AUTOMATIC CRESCOGRAPH 7

Weight of the levers. — The tension exerted by the weight
of even low magnification auxanometers is found to modify
the normal rate of growth. In the compound system of
levers employed in my apparatus, the first lever has to be
made rigid in order to exert a pull on the second without
undergoing any bending. In order to secure the rigidity
of the first lever, it must have a certain minimum cross-
section. This entails not only an increase of weight and
of tension on the plant, but also increased friction at the
fulcrum. The conditions necessary to overcome these
difficulties are : (i) construction of a very light lever
possessing sufficient rigidity, and (2) arranging the two
levers in such a way that the tension on the plant may be
reduced to any extent desired, so as practically to eliminate
it. It may be stated here that the second lever serving
as the writer can be made of fine glass fibre of extreme
lightness.

For the construction of the first lever I use ' navaldum,'
the well-known alloy of aluminium which possesses con-
siderable rigidity: A thin strip of this material about
30 cm. in length is taken, rigidity being imparted to it by
giving it a T-shape. A specially prepared thin strip of
bamboo has been found to be a satisfactory substitute for
the metal.

Each of the two levers is nearly balanced by a counter-
poise W (fig. i) . The horizontal fulcrum-axis of each lever
is supported in a fork provided with appropriate jewel-
bearings.

Attacher and Connector. — The first of these terms
designates the contrivance for connecting the plant with
the first lever ; and the second, the flexible connection
between the first and the second lever. For the Attacher
I use a thin glass hook, which does not stretch, nor does
it undergo any appreciable change of length on variation
of temperature. The upper end of the hook can be placed
on notches in various positions on the first lever, greater
magnification being produced when the hook is brought

8

CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

nearer the fulcrum. The Connector, which joins the first
to the second lever, has to be flexible. For this I employed
different devices, of which the following is simple and
effective. The connection is made by a thin waxed string
which has been previously kept stretched for several days
by a weight. A moderate tension does not stretch the
prepared string any further, nor is the string affected by
any hygroscopic change.

[Z>=

w

Fig. I . Compound lever of High Magnification Crescograph.

, plant attached to arm of lever, l ; l', second lever with bent
tip for tracing record ; w, w', balancing counterpoise ; each
lever is carried by a fork ; the transverse fulcrum-bar of the
lever ends in pin-points which rest in agate cups, one on each
side of the fork.

The attachment of the plant to the first lever may be
to the short arm of the lever, in which case the growth-
elongation is recorded as a down-curve. It is perhaps
more natural to associate upward growth with an up-curve ;
this may be secured by attaching the plant to the long
arm of the lever (fig. i). The curvature in the record is
slight, and is practically neghgible in the middle portion
of it.

automatic record of growth 9

Automatic Record of the Rate of Growth

Another obstacle to obtaining an accurate record of the
curve of growth arises from the friction of contact of the
bent tip of the writing lever with the recording smoked
surface of glass. This difficulty was removed by an oscillat-
ing device (fig. 2) ; the smoked glass plate G is made to
oscillate, to and fro, at right angles to the tip of the writing
lever ; the oscillations are at regular intervals, say once
in a second. The bent tip of the writer thus comes
periodically into contact with the recording plate during its
extreme forward movement. The record of growth there-
fore consists of a series of dots, the distance between them
representing magnified growth during a second.

The approach of the recording surface to the writing
lever has to be rendered very gradual, otherwise the sudden
stroke of the plate causes an after-oscillation of the WTiting
lever, resulting in multiple dots which spoil the record.
In order to obviate this, a special contrivance had to be
devised by which the speed of approach of the plate was
gradually reduced to zero on contact with the writer ;
the rate of recession should, on the other hand, continuously
increase from zero to maximum. The writer will in this
manner be gently pressed against the glass plate, marking
the dot, and then set free. It is only by strict observance
of these conditions that the disturbing effect of after-
vibration of the lever can be obviated.

The oscillating device consists of an eccentric E, a crank
K actuated by clockwork, and a slide S, supported on ball-
bearings, which carries the smoked glass plate. The
eccentric consists of a cylindrical rod of glass mounted
eccentrically. A semi-rotation of the eccentric in one
direction pushes the recording plate gradually forwards,
while that in the opposite direction causes it to recede
(fig. 2). The eccentric is actuated by the crank K, which
in its turn is acted on bv clockwork which releases a re-
volving wheel at intervals of i, 2, 10 or 15 seconds, according

10

CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

to the requirement of the particular experiment. The
complete apparatus is seen in fig. 3, in which the revolving
wheel for actuating the oscillating mechanism is clearly
shown.

I used at first a pair of parallel eccentrics, but in the
newest type of apparatus with improved to-and-fro sliding
arrangement, one eccentric is found to be quite sufficient.
A very important condition for success is the securing of

Fig. 2. Mechanism for oscillation of plate.

E, eccentric ; k, crank ; s, slide ; p, holder for glass plate g ;
L, recording lever. Clock releases string, c, for lateral move-
ment of the plate.

perfect smoothness of movement during the oscillation of
the plate. A horizontal slide, moving on ball-bearings,
carries the vertical plate-holder. The slide is so perfect
in action that a puff of air is by itself sufficient to move
the free plate-carrier either backward or forward. The
plate may thus be maintained in its to-and-fro oscillation
with very little expenditure of force, and the power required
from the clockwork is therefore very small. In my later
instruments an electric oscillating device has been success-
fully employed, which simplifies the matter still further.
An electric current flows intermittently through a coil of

AUTOMATIC RECORD OF GROWTH

II

wire which sucks in a rod of soft iron attached to the plate-
carrier. The force required for bringing about the oscillatory
movement thus acts directly, without any intervention of
the eccentric.

The amplitude of oscillation of the plate is about 3 mm.
It is important that the vertical recording plate should be

Fig. 3. Complete apparatus.

p, plant ; s, s', micrometer screws for raising or lowering the
plant ; c, clockwork for periodic oscillation of plate ;
w, rotating wheel.

so adjusted that its distance from the tip of the writer will
remain the same during the excursion of the index, as well
as during the lateral displacement of the plate moved by
the clockwork. Failure to secure this makes the dot-marks
unequally distinct ; in the worst cases some of the dots

12 CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

may even be missing. The difficulty is obviated by accurate
adjustment of the plate in a vertical plane by means of
regulating screws.

Experimental Accessories

The soil in a flower-pot is liable to be disturbed by
irrigation and the record be consequently vitiated. This is
obviated by wrapping the root, imbedded in a small quantity
of soil, in a piece of cloth, the lower end of the plant being
held securely by the clamp of the plant-holder. In order
to subject the plant to the action of gases and vapours,
or to variation of temperature, it is enclosed in a cylindrical
chamber constructed of a sheet of mica, with an inlet and
an outlet pipe. The chamber is maintained in a humid
condition by means of a sponge soaked in water. Vapours
and gases can be easily introduced or removed from the
plant-chamber ; variation of temperature is simply effected
by blowing in heated or cooled water-vapour.

Any quickly growing organ of a plant will be found
suitable for the following experiments ; complications
arising from circumnutation may be avoided by employing
either radial organs, such as flower peduncles, buds, or the
pistils of certain fiowTrs, or the hmp leaves of various
species of grass. It is advisable to select specimens in
which the growth is fairly uniform. I append a repre-
sentative list of different specimens in which, under favour-
able conditions of season and temperature, the rates of
growth may be as high as those given below :

Table I. — Average Rate of Growth ix Different

Specimens.

Peduncle of Zephyranthes
Leaf of grass
Pistil of Hibiscus flower
Seedling of Wheat
Flower-bud of Crinum
Seedling of Scirpus Kysoov

o- 7 mm. per hour
I • lO
I -20

1 -Go

2 -20

3-00

The specimen employed for experiment may be an
entire plant ; it is, however, more convenient to employ

AVERAGE RATE OF GROWTH 1 3

a detached organ, the exposed cut end being wrapped
in a moist cloth. The shock-effect of section disappears
in the course of a few hours, after which the isolated organ
renews its normal rate of growth. Of the different
specimens, the plant Scirpus Kysoor offers special advan-
tages ; its leaves are much stronger than those of Wheat
and other grasses, and can bear a certain amount of pull
without harm. Its rate of growth under favourable seasonal
conditions is considerable : some specimens were found
to have grown more than 8 cm. in the course of 24 hours,
or more than 3 mm. per hour. This was during the rainy
season in the month of August : but a month later the rate
of growth had fallen to less than i mm. per hour.

I will now describe certain typical experiments which
will show (i) the extreme sensibihty of the Crescograph,
and (2) its wide applicabihty in different investigations.
The capabihty of the apparatus in the accurate determina-
tion of the time-relations of responsive changes in the rate
of growth will be described in a later chapter.

In order to ensure regularity in the rate of growth, the
plant should be kept in uniform darkness or in uniformly
diffused light. So sensitive is the recorder that it shows
a change of growth-rate due to the slight increase of illumi-
nation caused by the opening of an additional window.
One-sided light should be avoided, as it gives rise to dis-
turbing phototropic curvature. Temperature and hygro-
metric condition must be kept constant. After observing
these precautions, the growth-rate of vigorous specimens is
found to be very uniform.

The records are taken either on a stationary plate or
on a plate which moves past the writer at a uniform rate.

Stationary Plate Method. — A record, which is vertical,
is first taken, to ascertain the normal rate of growth.
Then, in order to study the effect of some changed condition,
the recording plate is moved i cm. to the left ; the tip of
the writer is brought once more to the bottom by means of
the fine screw adjustment S (see fig. 3) and another record

14 CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

taken. The increase or diminution of the space between
successive dots in the second record, as compared with the
first, at once demonstrates the stimulating or depressing
nature of the changed condition.

Moving Plate Method. — The smoked glass plate in this
case, as already stated, is carried laterally past the
writer. The record is now a curve, the ordinate represent-
ing growth-elongation and the abscissa the
time. The increment of length divided by
the increment of time gives the absolute rate
of growth at any part of the curve. As long
as the growth is uniform, so long the slope of
the curve remains constant. When an agent
enhances the rate of growth, there is an up-
ward flexure in the curve ; a depressing agent,
on the other hand, lessens its slope.

Determination of Absolute Rate of

Fig. 4^. Cresco- GrO\VTH

graphic records.

I7y^piatto*f?he Experiment i.— The record of growth
absolute rate of obtained with a vigorous specimen of 5.

srrowth. Second _^ , , . . , . .

record taken Kysoof ou a stationary plate is given m
after 15 min- f^^ . -pj^g oscillation frequency of the

utes. Magnifi- o t -^ j

cation 10,000 plate w^as once m a second, the magnm-
times.(Kysoor.) (.^^jqj^ employed being 10,000 times. The

magnified growth-movement was so rapid,
that the record consists of a series of short dashes instead
of dots.

After the completion of the first vertical record, a second
record was taken after an interval of 15 minutes. The
magnified record for 4 seconds is 38 mm. in the first record :
it is precisely the same in the record taken 15 minutes
after. The growth-elongation during successive intervals of
I second is practically the same throughout, being 9-5 mm.
This uniformity in the spacings demonstrates not onl}^ the
regularity of growth under constant conditions, but also

ABSOLUTE KATE OF GROWTH

15

the reliability and perfection of the apparatus. It also
shows that by keeping the external conditions constant, the
normal growth-rate can be maintained uniform for at least
15 minutes. As the growth when magnified 10,000 times
is nearly i cm. per second, and as it is quite easy to measure
0-5 mm., the Crescograph permits the record of a growth-
elongation of 0-00005 mm., that is to say, the sixteenth part
of a wave of red light. The absolute rate of growth, more-
over, can be determined for a period as short as 0-05 second.

Fig. 5. Effect of temperature.

Record on moving plate, where diminished slope of curve in the
second part denotes retarded rate under cold (Kysoor).

These facts will give some idea of the great possibilities
of the Crescograph for investigations on growth.

As the period of the experiment is very greatly shortened
by the method of high magnification, I have, in the deter-
mination of the absolute rate of growth, adopted a second as
the unit of time and [i or micron as the unit of length — the
micron being o-oooooi metre, or o-ooi mm.

If in be the magnifying power of the compound lever,
and / the average distance between successive dots in milli-
metres at intervals of / seconds, then :

I

Rate of growth = — X 10^ [i per sec

7nt

l6 CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

In the record given / = 9*5 mm., m = 10,000, t = 1 second.
Hence rate of growth

= ^ ^ X 10^ LL per second = o -95 ll per sec.

lOOOO

I will now give a few typical examples of the employ-
ment of the Crescograph for the investigation of growth ;
the first example demonstrates the influence of temperature.

Experiment 2. Effect of lowering the temperature. — The
specimen employed was Kysoor, and the record was taken
on a moving plate. The first part of the curve represents
the normal rate of growth ; moderate coohng produced a
diminished slope of the curve (fig. 5), demonstrating the
retardation of growth induced by cold.

Precaution against Physical Disturbance

Experiment 3. — There may be some misgiving about
the employment of such high magnification ; it may be
thought that the accuracy of the record might be vitiated
by physical disturbance, such as vibration. In physical
experimentation far greater difficulties have been over-
come, and the problem of securing freedom from vibration
is not at all formidable. The whole apparatus needs only to
be placed on a heavy wooden bracket screwed on to the wall
to ensure against mechanical disturbance. As an additional
precaution, a thick sheet of felt is interposed between the
base of the Crescograph and the surface of the bracket to
function as a shock-absorber. To what extent this freedom
from mechanical disturbance has been realised will be found
from the inspection of the first part of the record in fig. 6,
taken on a moving plate. A thin dead twig was substituted
for the growing plant, and a perfectly horizontal record not
only demonstrated the absence of growth-movement but
also of all disturbance. There is also another element of
physical change, against which precaution has to be taken
in experiments on variation of the rate of growth with

PHYSICAL DISTURBANXE I7

rising temperature. In order to determine its character and
extent, a record was taken, with a dead twig, of the effect
of raising the temperature of the plant-chamber through 10°.
The record, with a magnification of 2000, shows that there was
an expansion during the rise of the temperature, after which
there was a cessation of physical movement, the record
becoming once more horizontal. The obvious precaution to
be taken in such a case is to wait for several minutes for the
attainment of steady temperature. The movement caused

Fig. 6. Effect of variation of temperature on a dead branch,

taken on moving plate.

Horizontal record shows absence of growth and freedom from
physical disturbance. Physical expansion on application of
heat at arrow is followed by horizontal record on attainment
of a steady temperature. (^Magnification 2000 times.)

by physical change abates in a short time, whereas the
change in the rate of grow^th brought about by physiologi-
cal reaction is persistent.

Having demonstrated the extreme sensitiveness and
reliability of the apparatus for quantitative determination,
I now demonstrate its wide applicability for various
researches relating to the influence of external agencies in
modification of growth.

Experiment 4. Effect of variation of temperature on
growth. — The record was taken on a stationary plate with
a different specimen of Kysoor. The magnification was
reduced to 2000 ; but the interval between successive dots

i8

CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

was increased to 5 seconds. The middle record N (fig. 7)
was taken at the temperature of the room. The temperature
was next lowered by a few degrees and the record C shows
the result. Finally, the record H was obtained when the
temperature was raised a few degrees above that of the
room. It will be seen that under the action of cold
the space between successive dots became shortened, ex-
hibiting a diminution in the rate of growth. Heat, on the

Fig. 7. Fig. 8.

Fig. 7. Effect of temperature recorded with stationary plate.

N, normal rate ; c, retarded rate under cold ; h, enhanced rate

under heat. (Kysoor.)

Fig. 8. Record of a single growth-pulse of Zephyranthes.
(Magnification 10,000 times.)

other hand, caused a widening of the interval between
successive dots, thus demonstrating enhancement of the
rate.

Calculating from the data supplied by the record :

The absolute value of normal rate is 0*457 y. per sec.
Diminished rate under cold . . 0'ioi(i, ,, ,,

Enhanced rate under heat . . 0-737 [i ,,

Experiment 5. Pulsation in growth. — As a further
example of the capability of the Crescograph, I give the

MAGNETIC CRESCOGRAPH I9

record of a single pulse of growth obtained with the peduncle
of Zephyranthes sidphiirea (fig. 8). The magnification
employed was 10,000 times, the successive dots being at
intervals of i second. It will be seen that the growth-pulse
commenced with a sudden elongation, the maximum rate
being 0 -4 [jl per second. The pulse of elongation exhausted
itself in the course of 15 seconds, after which there was a
partial recovery lasting for 13 seconds, the period of the
complete pulsation being 28 seconds. The resultant growth
is therefore the difference between the elongation and the
recovery. Had not a very highly magnifying arrangement
been used, the growth w^ould have appeared to be continu-
ous; this is especially the case when the rate is very high.
Less favourable conditions appear to promote exhibition
of the pulsation, instances of which will be described later.

The Magnetic Crescograph

The highest magnification obtained with two levers is,
as already stated, 10,000 times. It might be thought that
further magnification would be possible by a compound
system of three levers. There is, however, a practical
limit to the number of levers that can be employed. The
slight overweight of the last lever becomes greatly multi-
plied, exerting considerable tension on the plant and thus
interfering with the normal rate of growth. The friction
at the bearings also becomes increased, obstructing the free
movement of the writing lever. For securing further
magnification, the idea of more material contacts had
to be abandoned. A method of magnification without
additional contact had therefore to be devised. A
magnetised rod of steel was made to function as the first
lever, the magnified movement of which causes a very large
deflection of a delicately suspended astatic system of
magnets. The indicator is a spot of light reflected from
a mirror carried by the deflected astatic system. The
magnetic lever itself gives a magnification of 50 times, and

20 CHAP. II. HIGH MAGNIFICATION CRESCOGRAPH

the deflected system a further magnification of 20,000 times,
the total magnification being thus a millionfold. This
was verified by moving the short arm of the lever through
0-005 mm., by means of a micrometer screw. The resulting
deflection of the spot of light at a distance of 4 metres was
then found to be 5000 mm. or a total magnification of
a million times. With a more sensitive apparatus the
magnification was increased even to 50 million times. ^

A concrete idea of this order of magnification is formed
when it is realised that this is 30,000 times greater than
that produced by the highest powers of the microscope.
With such a magnification a single wave of sodium light
would appear lengthened to about 3000 cm.

Demonstration by Magnetic Crescograph

With this apparatus I have been able to give striking
demonstrations of various phenomena of growth before
large audiences at different scientific centres. Employing
even the slowly growing flower-bud of the Crinum Lily (the
average rate of growth of which is only 0-0006 mm. per
sec), a magnification of a million times was found to be
m.ore than ample. The excursion of the indicating spot
of light exhibiting the movement of growth was found to
be 300 cm. in 5 seconds, the scale being placed at a distance
of 4 metres. The temperature of the room was 30° C

Experiment 6. — The plant-chamber was then cooled
to 26° C. by the blowing in of cool water-vapour. The time
taken by the spot of light to traverse the 300 cm. length of
the scale was now 20 seconds, the growth-rate being thus
depressed to one-fourth. Under continuous lowering of
temperature, the rate was slowed down till there was arrest
at 21° C. Warm vapour was next introduced which
gradually raised the temperature of the chamber to 35° C.
The spot of light now rushed across the scale in a second
and a half, that is to say, growth was enhanced to more

^ Plant Autographs (1927), p. 95.

SUMMARY 21

than three times the normal rate. The entire series of the
above experiments on the effect of temperature on growth
was thus completed in less than 15 minutes.

Summary

The High IMagnification Crescograph gives automatic
record of growth at a magnification of 10,000 times.
Growth movement as minute as the sixteenth part of a
wave-length of red light can thus be detected.

Growth appears to be a pulsatory phenomenon. The
resultant growth in each pulsation is the difference between
elongation and recovery.

The influence of external conditions on variation of
rate of growth is recorded by two methods.

In the Stationary Method the increase or diminution
of the distance between successive dots of the vertical
record demonstrates the stimulating or depressing nature
of the changed condition.

In the Moving Plate Method the record is a curve,
the ordinate representing growth-elongation, and the
abscissa, time. A stimulating agent causes an upward
flexure of the normal curve ; a depressing agent, on the
other hand, lessens the slope of the curve.

The Magnetic Crescograph makes possible the demonstra-
tion of the principal phenomena of growth and its variations
before a large audience, the magnification produced being
from one to fifty million times.

CHAPTER III

THE BALANCED CRESCOGRAPH

The Crescograph described in the previous chapter gives
records of the enhancement or depression of the rate of
growth induced by external change. The enhancement of
growth is shown by the increase of spacing between the suc-
cessive dots of the vertical record taken on a stationary
plate, or by the upward flexure of the curve inscribed on a
moving plate. Depression of the rate is, on the other hand,
indicated by the shortening of the distance between the
successive dots or by the downward flexure of the curve.

Though these methods are highly sensitive, yet it
requires very careful inspection of the records to detect a
change induced in the rate of growth, when such a variation
is very slight. It was therefore necessary to devise a new
method which would instantly show, by the up or down
movement of an indicator, the accelerating or retarding
effect of an external agent. For this purpose I first em-
ployed the Optical Method of Balance,^ which was con-
sidered at that time to be very sensitive. The spot of
light reflected from the Optical Lever (which magnified the
rate of growth) was made to fall upon a second mirror to
which a compensating movement was imparted, so that
the light-spot, after double reflection, remained stationary.
Any change in the rate of growth — an acceleration or a
retardation — was detected by the movement of the hitherto
stationary spot of light, in one direction or the other.

A still more perfect apparatus was next devised which
possesses several important advantages ; the exact balance

^ Plant Response (1906), p. 413.

METHOD OF BALANCE 23

is here secured with ease and certainty ; the apparatus
has an attached scale which indicates the actual rate of
growth, and the upsetting of the balance by a stimulant
or a depressor is automatically recorded.

Principle of the Method of Balance

The plant is made to descend at the exact rate at which
its growing tip is rising : the latter is attached in the usual
manner to the High Magnification Crescograph. When
growth is exactly balanced, the record is a horizontal line
instead of an ascending curve as in the ordinary method.
The apparatus so adjusted is found to be extremely sensitive.
The minutest change induced in the rate of growth by the
environment is at once indicated by the upset of the balance
and recorded as an up- or a down-curve.

The compensating movement of descent. — A regulating
contrivance had to be devised for the subsidence of the
plant at the same rate as its growth-elongation. The re-
quired method is somewhat analogous to the compensating
device for an astronomical telescope, which neutralises the
effect of the earth's movement round her axis once in
24 hours. The problem is, however, far more difficult ;
for instead of compensating for a definite rate, adjustments
had to be made for balancing widely varying rates of
growth of different plants, and even of the same plant under
different conditions. The problem w^as solved by the
employment of two different methods of compensation :
A, the Method of Falling Weight, and B, the Method
OF Inclined Plane.

A. Method of Compensation by Falling Weight

By means of a train of revolving clock- wheels actuated by
the fall of a weight, the plant is made to subside at the same
rate as that at which it is growing.^ It will be presently
explained how the gradual turning of the adjusting screw S in
a right-handed or a left-handed direction causes a continuous

^ Life JMovements in Plants (1919), p. 257.

24

CHAP. III. THE BALANCED CRESCOGRAPH

decrease or increase in the rate of the compensating fall
(fig. 9). Growth thus becomes accurately balanced, so that
the tip of the plant remains exactly at the same level.

I take a concrete example in explanation of the adjust-
ment for compensation. Different plants exhibit consider-
able difference in the rate of their growth ; in a large number

£ e

Fig. 9. Diagram of Method of Falling Weight.

Compensation of growth-movement effected by equal sub-
sidence of the holder supporting the plant (p). Adjusting
screw (s) regulates the speed of the governor (g), and the
rate of subsidence of the plant, w, heavy weight actuating
clockwork.

of cases the rate varies from 0 • 17 [i to i -o jx per second, the
range being about six times. The construction of this par-
ticular apparatus enables the balance to be secured within
these limits. The adjusting mechanism consists of a centri-
fugal governor and a frictional resistance. The two arms of
the governor can be increasingly outspread by a right-handed
turn of the screw S {see fig. 9). This increases not only the

METHOD OF FALLING WEIGHT 2$

inertia of the revolving governor, but also the friction of the
lower points of the arms of the governor which rub against
a horizontal circular plate, not shown in the figure. In-
creasing inertia and friction both tend to slow down the
speed of the rotation of the vertical spindle on which the
rate of subsidence of the plant depends. A left-handed
turn of the adjusting screw S causes, on the other hand, a
continuous increase in the rate of subsidence.

When the adjusting screw has been set at a particular
position, the balancing rate of subsidence of the plant
remains constant for many hours. The attainment of

Fig. io. Record showing {a) condition of overbalance, and
(b) underbalance of growth. (Magnification 2000 times.)

this constancy is a matter of fundamental importance in
accurate investigation by this method.

The ease and exactitude of securing the balance are shown
in the records (fig. lo, a and h). In each of these the exact
balance is seen in the first part of the record at a definite
position of the adjusting screw. A slight turn of the screw
to the right reduced the rate of subsidence, resulting in the
upset of growth-balance upwards (fig. lo, a). A turn of
the screw in the opposite direction caused overbalance of
the growth-rate, the resulting curve being downwards
(fig. 10, b).

Calibration. — This is effected as follows : the rate of

26 CHAP. III. THE BALANCED CRESCOGRAPH

subsidence of the plant-holder which balances the rate of
growth is, as already stated, proportional to the rate of
rotation of the vertical spindle with the dependent train
of revolving clock-wheels. A striker is attached to one of
the wheels and a bell is struck at each complete revolution.
The clockwork is adjusted by the governor to revolve at
a medium speed, the bell striking 35 times in a minute.
A microscope-micrometer is focused on a mark made on
the plant-holder, and the amount of subsidence of the mark
is determined during one minute ; this was found to be
0-0525 mm. As this fall occurred when the bell was
striking 35 times in a minute, the subsidence per stroke
was 0-0015 mm. From this it is possible to determine the
absolute rate of growth.

Determination of the absolute rate of growth. — Supposing
that balance occurred at N strokes of bell per minute, the
rate of balancing subsidence = N X 0 • 0015 mm. per minute ;
= N X 0-000025 mm. per second; = N x 0-025 [i per
second.

Experiment 7. — The growth of a specimen of Zea Mays
was found to be balanced when the number of strokes of
the bell was 20 per minute (absolute rate of growth
= 20 X 0-025 [-^ = 0'5 y- per second) ; the length thus
measured is equal to the wave-length of sodium light. This
affords some idea of the sensitiveness of the Crescographic
Method of Balance.

Growth-scale. — All necessity for calculation is obviated
by the scale attached to the apparatus. The speed of the
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 the
growth. 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

METHOD OF INCLINED PLANE 27

at different positions of the screw. Rates of growth from,
say, 0-2 (X to I -2 (JL per second can thus be found directly
from the readings of the scale.

The determination of the rate of growth now becomes
extremely simple. A few turns of the screw cause an exact
balance of growth, the index indicating 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.

B. Method of Compensation by an Inclined Plane

With the apparatus already described, it is possible to
balance growth within the range of 0-17 [i to I'g [l per

Fig, II. Diagram of Method of Inclined Plane.

I, inclined plane pushed forward by revolving screw s, at rate
adjusted by rotation of vertical spindle v. The lower end
of rod R of the plant-holder rests on the inclined plane. {See
text.)

second ; other instruments have to be employed when the
rate of growth is quicker or slower. In order to meet all
requirements in a single instrument, it became necessary

28 CHAP. III. THE BALANCED CRESCOGKAPH

to devise a universal apparatus based on a different method,
that of the incUned plane.

Imagine the plant-holder so held that its lower pointed
end rests on an inchned plane which slopes down to the left ;
suppose further that the inclined plane is being pushed
forward to the right at a uniform rate. This will result in
the gradual subsidence of the plant-holder, the rate depend-
ing on the angle of inchnation (fig. ii). When the angle is
reduced to zero, the plane becomes horizontal, and there is
no subsidence, the balancing arrangement being thus put
out of operation. The rate of subsidence is, on the other
hand, gradually enhanced by a continuous increase in the
angle of inclination.

The rate of forward movement of the inclined plane has
been assumed to be constant. It is obvious that greater
speed of movement of the plane will correspondingly increase
the rate of subsidence. The problem of balancing widely
different rates of growth w^as solved by appropriate modifica-
tions of the angle of inchnation of the plane and the rate
of its forward movement. In practice, instead of a solid
inclined plane, a board is employed the inchnation of which
can be varied.

The rate of subsidence for balancing growth will thus
depend on the following

1. On the constant of the particular apparatus ;

2. On the angle of inclination ; and

3. On the rate of forward movement of the plane as
determined by the period of rotation of the vertical
spindle.

Taking these factors into consideration the formula for

absolute measurement of

J K tan i
the rate of growth [i per second = 7

K is the constant of the apparatus ;
i is the angle of inclination ; and

t is the period of a complete revolution of the vertical
spindle in seconds.

METHOD OF INCLINED PLANE

29

The angle of inclination of the plane can be read on the
circular scale. The interval between two successive strokes

Fig. 12. The complete apparatus for Balance by Inclined

Plane.

A, screw for adjustment of angle of inclination of plane i, read
in circular scale ; p, rod supporting plant-holder enclosed in
outer tube t.

of a bell gives the period of a complete revolution of the
vertical spindle.

A photographic reproduction of the complete balancing
apparatus, reduced one-third, is given in fig. 12.

30 chap. iii. the balanced crescograph

Experimental Verification of the Formula

Experiment 8. — The reliability of the apparatus and the
accuracy of the working formula were tested by independent
measurements obtained (i) by the method of balance, and
(2) by direct micrometric measurement of growth of a
seedling of Wheat.

Measurement bv the Balance Method :

The constant of the apparatus = 10-45.

The balancing angle of inclination = 23-5°.

The time of a single revolution of spindle = 12 seconds.

Hence absolute rate of growth in [ji per second

^ 10-45 X tan 23-5°
12

= 0-381 . . . . . (i)

Direct measurement by Microscope-Micrometer . — The
micrometer was focused on a mark on the tip of the plant,
and the growth-elongation after an interval of 2 hours and
35 minutes (155 seconds) was found to be 3-5 mm.

The rate of growth \l per second

^3-5 X 1000
155 X 60
= 0-376 . . . . (2)

The two results are practically the same, the difference
being in the third place of decimals.

The Time-Factor

The factor of time is a very important element in the
responsive growth-variation. Growth is not immediately
affected by an external change ; there is a delay in the
reaction, designated as the latent period ; this has to be
accurately determined, as also its modification under different

THE TIME-FACTOR 3 1

conditions. The effect induced by an external agent is,
moreover, modified by the duration of its influence. The
intensity of the reaction may thus increase at the first
stage, reach a climax and then undergo an actual decline.
No method had hitherto been available for immediate
detection and record of these changes, moment after

Fig. 13. Record showing the effect of Carbonic Acid Gas on

growth.

Horizontal hne at the beginning indicates balanced growth.
Application of carbonic acid gas induces preliminary en-
hancement of growth, shown here by up-curve. The
growth-rate becomes normal for a while with subsequent
depression, shown by down curve. Successive dots at
intervals of 10 seconds. (Wheat.)

moment, as they occur. The Method of Balance offers, in
this respect, unique advantages.

The following are the characteristics of the curves
obtained by the balancing method. The initial horizontal
record indicates balance of the normal rate of growth ;
the delay in the responsive variation is the latent period ;
an up-curve represents enhancement of the rate of growth,
a down-curve indicating depression. If during the course

32 CHAP. III. THE BALANCED CRESCOGRAPH

of the experiment the curve becomes once more horizontal,
it indicates the resumption of the normal rate. The follow-
ing results give a striking demionstration of the influence of
the time-factor.

Experiment 9. Effect of Carbonic Acid Gas on growth. —
After obtaining the balance of growth with a seedling
of Wheat, the plant-chamber was filled with CO2 diluted
with air, the moment of application being marked with an
arrow in the record. There was an almost immediate
acceleration of the rate of growth, the latent period being
less than 5 seconds. This enhanced rate continued for
120 seconds, slowing down gradually to the normal and
remaining so for 20 seconds as indicated by the horizontal
record at the top of the curve. Continued action of CO2
induced, however, an inversion of the curve, the down-
record indicating a rate below the normal (fig. 13). Strength
of dose exerts its characteristic influence, very dilute CO2
inducing acceleration, while stronger concentration or longer
application of the gas brings about a quick reversal
from acceleration to retardation, culminating in arrest of
growth.

Two different methods are thus available for investi-
gations on growth ; the ordinary Method of Crescographic
Magnification, and the excessively sensitive Method of
Balance. The results obtained by the second method will
be found not merely to confirm those obtained by the first,
but also to afford important information regarding the
influence of the time-factor in its relation to growth.

Summary

In the Method of Balance the upward movement of
growth is compensated by a corresponding subsidence of
the plant. Two different methods of balance have been
successfully employed : (i) that of the Falling Weight,
and (2) that of the Inclined Plane.

In the condition of balance, the record remains horizontal.

SUMMARY oo

The effect of an external agent is quickly detected by
the upsetting of the balance, upwards or downwards. An
up-curve represents acceleration above, and a down-curve
depression below, the normal rate.

The influence of the time-factor is shown in the changing
flexures of the record.

D

CHAPTER IV

EFFECT OF VARIATION OF TEMPERATURE ON GROWTH

Accurate determination of the effect of temperature on
growth presents many difficulties due to the presence of the
numerous comphcating factors. In order to reduce these
to a minimum, it is advisable to carry out the experiment
with an identical plant, observing its rate of growth at
different temperatures. The period of the experiment should,
moreover, be as short as possible, so as to eliminate the
complication which arises from, the diurnal variation of
the rate of growth. A quick change of the temperature
of the plant-chamber is therefore necessary, which un-
fortunately introduces unexpected difficulties ; for a rapid
change of temperature acts as an excitatory shock, inducing
contraction of the growing organ. This drawback can be
obviated to a great extent by securing a gradual instead
of an abrupt variation of temperature. The temperature
of the plant-chamber has, therefore, to be gradually raised
or lowered. This has been secured in the following manner :
a cylinder of thin metal with its base closed serves as the
plant-chamber P, kept in a humid condition by a piece of
moist sponge (fig. 14). The cylinder P is enclosed in
a larger vessel T.R., fiUed with water and serving as the
thermal regulator. There are two reservoirs, H and C,
containing hot and ice-cold water respectively. Opening
of the stopcocks Si and S3 allows hot water to stream
through the thermal regulator, the excess escaping through
the outflow pipe O. The opening of So and S3 allows, on
the other hand, a stream of cold water to flow through the

VARIATION OF TEMPERATURE

35

thermal regulator. The rate of heating or cooling of the
regulator depends on the rate of inflow of hot or cold water.
To effect a gradual rise of temperature in the plant-chamber,
the stopcock Si is first opened. The experimenter, keeping
his hand on the stopcock S3, then carefully adjusts the
rate of inflow ; he has before him a clock-hand which goes
round once in a minute, and with previous practice he is
able to adjust the inflow, so that the rate of rise of tempera-
ture of the plant-chamber, indicated by the thermometer T,

Fig. 14. Arrangement for variation of temperature of plant-
chamber.

p, plant-chamber enclosed in thermal regulator t.r. ; h and c,
reservoirs containing hot and cold water. Variation of
temperature of thermal regulator adjusted by proper
manipulation of stopcocks s^, Sg, and s^, Sg. o, outflow pipe.
Plant attached to recording lever w. "t, thermometer.

is half a degree every 30 seconds or one degree every
minute. The mass of water in the thermal regulator
acts as a stabihser, preventing any sudden fluctuation of
temperature. A similar method is also employed for the
gradual lowering of temperature. The adoption of this
device for ensuring gradual variation of temperature was
found to eliminate the erratic changes in the rate of growth
which had previously proved to be so extremely baffling.

36 chap. iv. variation of temperature

Method of Discontinuous Observation

Determination of cardinal points of temperature. — The
cardinal points are not the same for different plants ; even
in the same species they are modified by the climate to
which the plants have been habituated. The results
obtained in the tropics must necessarily be different from
those in colder climates. The following experiments were

Fig. 15. Records of growth, on stationary plate,
at different temperatures.

carried out in Calcutta in the spring season, when the
average temperature is about 30° C.

Experiment 10. The optimum temperature. — High mag-
nification records of the growth of Kysoor were taken on a
stationary plate for selected temperatures of 30°, 35°, 40°,
and 45° C, each of these being kept constant during the
period of observation. The records (fig. 15) show that the
rate of growth increases with the rise of temperature to an
optimum, above which the rate undergoes depression. In
the different records given in the figure, the time-interval

METHOD OF CONTINUOUS OBSERVATION 37

between the successive dots is the same ; but for completing
the same length of growth-elongation the number of dot-
intervals is different. At 30° C. it is eight, at 35° it is six,
at 40° it is ten, and at 45° it is twenty-two. The rate of
growth at 35° is thus nearly one and a half times that at
30° ; at 45°, on the other hand, the rate has fallen to nearly
a third. In some cases there was no growth even at 40°,
when pulsations were exhibited in each of which the up-
and down-curves are nearly equal.

For Kysoor the optimum temperature is about 35°, but
there are plants in which the optimum is as low as 28°,
and continues for about the next eight degrees.

Experiment 11. The temperature minimum. — The ex-
periment was carried out with Kysoor subjected to a
continuous lowering of temperature by adjusting a flow of
ice-cold water into the thermal regulator. It was found
that the rate of growth underwent continuous depression,
till arrest took place at 22° C. The temperature was
then gradually raised, growth being feebly revived at 23°.
The minimum temperature may therefore be regarded as
22-5° C.

Experiment 12. Effect of higher temperatures. — Growth
was found to be greatly retarded at 55° ; at 60° there
occurred a sudden spasmodic contraction, which I have
shown elsewhere to be the spasm of death. ^ This spasmodic
death-contraction is the immediate effect of the fatal
temperature. Prolonged exposure to a temperature a few
degrees below 60° would no doubt be followed by the death
of the organ.

Method of Continuous Observation

The above method of observation, though not ideally
perfect, was fairly satisfactory ; a minor defect was, how-
ever, discovered as the result of further observation. The
plant, according to the method described, was surrounded by

^ Plant Response (1906), p. 1O8 ; Comparative Electro-physiology (1907),
pp. 202, 546.

38 CHAP. IV. VARIATION OF TEMPERATURE

air, which is a bad conductor of heat. The plant could not,
therefore, quickly assume the temperature of the outside
regulator. In the method of continuous observation which
was contemplated, it was essential that the temperature
of the plant itself should undergo continuous and rapid
change.

An additional difficulty arose from the radiation of rays
of heat from the sides of the thermal regulator, which
induces retardation of growth. The enhancement of the
rate of growth by rise of temperature is thus antagonised
by the effect of radiation. The difficulties arising from the
non-conductivity of the air and from thermal radiation
were removed by filling the inner plant-chamber with water,
which is a better medium for transport of heat. Further
experiments showed that the plant quickly assumed the
temperature of the water in which it w^as immersed, provided
that the thermal rise was at a uniform rate of, say, i° C.
per minute. Heat being conveyed by the water, the dis-
turbing effect of radiation was eliminated.

The elongation recorded by the Crescograph during rise
of temperature may be due (i) to physical expansion, (2) to
expansion resulting from absorption of water by the plant,
and (3) to an enhanced rate of growth. The relative values
of the first two factors in comparison with growth-elongation
were found by carrying out parallel experiments with two
different specimens, in one of which growth had already
been completed whilst in the other it was still in full activity.
Records of the two specimens were taken under the same
magnification for a rise of 20° C. In order to keep the
record of the actively growing organ within the plate, the
magnification had to be reduced to 250 times. The joint
effects of thermal expansion and absorption of water in
the non-growing plant gave an elongation of only i mm. ;
whereas, under similar circumstances, the elongation of the
actively growing plant was more than 100 mm. Compared
with the physiological reaction, the physical effects were
quite negligible.

THERMOCRESCENT CURVE

39

The Thermocrescent Curve

I was desirous of devising a method by which an auto-
matic and continuous curve could be obtained recording
the rate of growth between 22° and 40° C. The entire
record had to be completed within the short period of
18 minutes, at a rate of rise of temperature of 1° C. per
minute. Previous experiments, as already stated, showed

•

1

•

1

1

•

•

1

1

1
•

• •

, •

t

•

I

1

'

— l—

_J_

1

— L-

1 1.,

22=

30'

40*-

Q^nvfve/t^itiJinjc.

Fig. 16. The Thermocrescent Curve.

Ordinate represents increment of growth ; abscissa, increment

of temperature.

that at this rate of continuous rise of temperature there is
practically no lag in the plant assuming the temperature
of the bath, especially when it is a thin specimen.

Experiment 13. — The specimen employed was Kysoor,
in which arrest of growth occurred at the minimum tempera-
ture of 22° C. The temperature was now raised at the
uniform rate of 1° C. per minute. The curve of growth was
taken on a moving plate which travelled at the rate of

40

CHAP. IV. VARIATION OF TEMPERATURE

5 mm. per minute. The recording lever inscribed successive
dots at intervals of a minute, during which the temperature
rose 1°. A Thermocrescent Curve was thus obtained, the
ordinate representing increment of growth, and the abscissa
the time as well as the rise of temperature. The curve
(fig. i6) shows growth at standstill at 22° C. The growth
was at first slowly increased with the rise of temperature,
and then more quickly. At 33° C. the rate of increase of
growth was very rapid, attaining its maximum at or about
34° C. At higher temperatures the rate underwent a rapid
decline, being reduced at 39° C. to about a fifth of the

optimum rate.

Determination of absolute rate of growth at different tem-
peratures.— The Thermocrescent Curve [see fig. 16) gives
sufficient data for the calculation of absolute rates of growth
at different temperatures. The vertical distance between
successive dots is the increment of growth in i minute for
a rise of 1° of temperature between T and T'. If / repre-
sents the magnified growth-elongation in millimetres for a
period of 60 seconds, and m the magnifying power of the
recorder, then the absolute rate of growth for the mean

temperature

is found from the formula

Absolute rate of growth at

T + T' / X io3

[jL per second.

2 ~ m X 60
The results thus calculated are given below in a tabulated
form.

Table II. Rate of Growth for Different Temperatures (Kysoor).

Temperature

Growth

Temperature

Growth

22° C.

o-oo /z per sec.

31° c.

0-45 ,u per

sec.

23° c.

0 -02 fX

32° c.

o-6o u.

24° c.

0 -04 /i ,,

33° C.

o-So ft

25° c.

o-o6 [i

34° C.

0 • 92 ,u

26° c.

o-o8 fi

35° C.

0 • 84 «

27° c.

0-I2 fl

36° C.

o-6^ fl

28° c.

o-i6 fi

37° C.

O- 48 fl

29° c.

0'22 fi

38° C.

0-30/1

30° c.

0-32 fl

39° C.

o- 16 /Z

SUMMARY

41

I give below a curve (fig. 17) constructed from the above
data, showing the relation between temperature and rate
of growth.

The methods described possess important advantages.
In place of measuring the average change in the rate of
growth in a number of plants, the variation is recorded in
an identical specimen. The employment of magnification
reduces, moreover, the period of the whole experiment to

"^

""

A

V,

\

i

\

I

\

i

\

\

I

\

/

\

y

/

\

Lr

/

1

iW

25'

3

0'

3d°

4

0°

ofe/rnfimxttUne,

Fig. 17.

Curve showing relation between temperature and
rate of growth.

about 20 minutes, and thus eliminates the complication
arising from the diurnal variation in the rate of growth.

Summary

Methods of experimentation have been described for
rapid determination of the effect of variation of temperature
on the rate of growth.

Radiant heat induces a retardation of growth which
antagonises the acceleration due to rise of temperature;
the gradual rise of temperature of the plant has therefore

42 CHAP. IV. VARIATION OF TEMPERATURE

to be SO effected as to eliminate this source of error. A
continuous record of growth under uniform rise of tem-
perature gives a Thermocrescent Curve, from which
the absohite rate of growth at different temperatures can
be found.

Different plants exhibit characteristic differences in
their cardinal points of temperature. In the tropical plant
Kysoor the minimum temperature for arrest of growth is
22-5° C, the optimum is about 34° C. The growth-rate
declines as the temperature rises further, and becomes
arrested at or about 55° C. A sudden spasmodic death-
contraction occurs at 60° C.

CHAPTER V

THE EFFECT OF CHEMICAL AGENTS ON GROWTH

The record of the effect of chemical agents on growth
may be taken on either a stationary or a moving plate.
The method of application is as follows : for the study of
the action of gases and vapours, the plant is enclosed in a
cylindrical chamber constructed of a sheet of mica provided
with inlet and outlet pipes for circulation of different gases
and vapours, the chamber being maintained in a humid
condition by pieces of sponge soaked in water. In the case
of liquid chemical agents, the specimen is suitably mounted
in a glass cylinder filled with water, and the normal rate of
growth recorded. This remains constant for about an hour,
in the course of which the experiment should be concluded,
as prolonged deprivation of oxygen affects the growth of
the plant. After taking the normal record, the chemicals
are added to the water, and the strength of solution gradually
increased. The chemical agents may be broadly divided
into three classes : (i) stimulants ; (2) anaesthetics ; and
(3) poisons.

Effect of Stimulants

Experiment 14. Hydrogen Peroxide. — A i per cent,
solution of this agent was often foimd to enhance the
rate of growth ; in some cases the enhancement was as
much as two and a half times the normal rate.

Effect of Anaesthetics

The results given below indicate that Carbonic Acid Gas,
Ether, and Chloroform act as mild or strong anaesthetics
in the above order.

44

CHAP. V. EFFECT OF CHEMICAL AGENTS

Effect of Carbonic Acid Gas

Experiment 15. — The immediate effect is a very marked
acceleration of growth. With a seedhng of Onion (Alhum)
the increase was found to be two and a half times. With

the flower-bud of Crinum the rate
was enhanced threefold, from the
normal o -25 [x to 0-75 Jjl per second.
After the preliminary enhancement,
there was a depression of growth
within 15 minutes of the application,
the rate being now reduced to 0 • 15 a
per second (fig. 18). These effects
taking place equally in light and
darkness prove that the phenomenon
is independent of photosynthesis.
The immediate and subsequent effects
of CO2 on growth have already been
demonstrated by the highly sensitive
method of balance {cf. fig. 13).

Fig. 18. Effect of Car-
bonic Acid Gas on
growth.

a, normal rate ; b, en-
hanced rate as the
immediate effect ; c,
retarded rate after pro-
longed application
(Crinum).

Effect of Vapour of Ether

The following may serve as typical
examples of results obtained with
various plants. Among these may
be mentioned the seedlings of Wheat ;
stems of Helianthus and of Dahlia ;
petiole of Tropaeolum ; tendril of Cucurbita ; peduncles of
Hibiscus, Centaurea, Daffodil, and of Allium ; the flower-
bud of Crinum Lily and the pistil of Datura. The effects
observed were essentially similar in all cases, of which three
representative examples will be described in detail.

Experiment 16. Seedling of Wheat. — The specimen was
an intact seedling with roots ; it exhibited a fairly rapid
rate of growth, as shown in the first part of the record. On

EFFECT OF ETHER 45

application of ether the growth-rate became very greatly
enhanced in less than 15 seconds and this persisted for
a considerable length of time, as shown by the erection
of the curve and wider spacings between the successive
dots (fig. 19, a). Prolonged application of ether, however,

Fig. 19. Effect of vapour of Ether on growth.

a, enhancement of the rate (\A'heat seedhng) ; b, enhancement
followed by depression (Crinum) ; c, renewal of growth of
stem in a state of standstill (Helianthus).

subsequently caused a depression of the rate, not shown in
the record.

Experiment 17. Crinum Lily. — The result was similar
to that obtained with the seedling of Wheat. The accelera-
tion occurred within 30 seconds of the application of ether
vapour ; the enhanced rate persisted for a period of more
than 2 minutes, after which the depressing effect of pro-
longed application is shown, the response curve (fig. 19, h)
tending to become horizontal.

Experiment 18. Stem of Helianthus. — As the cut

46 CHAP. V. EFFECT OF CHEMICAL AGENTS

specimen was in a state of arrested growth, the effect of
apphcation of ether is of much interest. The record
(fig. 19, c) shows that ether brought about a renewal of growth
previously in a state of standstill. This renewal occurred
after an apphcation of several minutes, the growth persisting
for a long time.

Effect of Chloroform

Experiment 19. Petiole of Tropaeolum.- — The typical
effects of chloroform on growth in different organs are
given below. The preliminary effect of chloroform vapour,
like that of other anaesthetics, is an acceleration of the rate
of growth. With the petiole of Tropaeolum this occurred in
less than 2 minutes and persisted for a period of 90 seconds.
After a further period of 45 seconds grow^th became arrested ;
subsequently there was an abrupt contraction due to death-
spasm (fig. 20, a).

Experiment 20. Peduncle of Centaurea. — The specimen
was in a state of arrested growth ; application of chloroform
vapour induced a vigorous renew^al of growth in the course
of 30 seconds. The renewal of growth occurred under a
small dose of the anaesthetic which persisted for 3 minutes,
after which there was an arrest followed by the spasmodic
death-contraction (fig. 20, b).

Experiment 21. Crinum Lily.— The specimen in this
case was also in a state of arrested growth. Application
of chloroform renewed the growth in less than 30 seconds
(fig. 20, c). Under the continued action of chloroform the
revived growth, which had persisted for 2 minutes and
15 seconds, was arrested ; this was followed by a violent
spasmodic death-contraction.

Experiment 22. Pistil of Datura. — The effect of chloro-
form on normal growth was a great enhancement which
occurred in the course of 30 seconds (fig. 20, d). This
persisted for nearly 2 minutes, after which there was an
arrest and subsequent spasmodic death-contraction.

EFFECT OF CHLOROFORM

47

Under chloroform a growing organ thus exhibits a
prehminary acceleration of growth followed by contraction,
which may be either feeble or very intense. The con-
traction by itself should not be regarded as the sign of
death, for there are agents which induce a temporary
contraction from which a revival is possible. The test of
the death-spasm is that it is an irreversible change, from
which the plant cannot be revived by substituting fresh air

Fig. 20. Effect of vapour of Chloroform.

a, preliminary acceleration followed by arrest and death-spasm
(Tropaeolum) ; h, revival of growth (Centaurea) ; c, revival
of growth, arrest, and death-spasm (Crinum) ; d, enhance-
ment, arrest, and death-spasm (Datura).

for the anaesthetic. The contraction under the prolonged
action of chloroform (by which even the interior of the organ
becomes affected) may be taken as the death-spasm, since
fresh air fails to revive the plant. Another interesting
phenomenon observed after chloroforming the plant is
the profuse deposit of minute drops of liquid on the
surface. This is due to the forcing out of the sap
during death-contraction, the escape being facilitated by
the increased permeability of the cell-protoplasm. Dark
spots of discoloration soon begin to appear and spread

48 CHAP. V. EFFECT OF CHEMICAL AGENTS

rapidly over the surface, and the organ exhibits rapid
wilting.

It is instructive to compare the death-spasm under an
anaesthetic with the parallel effect of stimulation by a
strong electric shock. The plant recovers from moderate
stimulation and responds again to fresh stimulation. But
under excessive stimulation, such as that induced by
strong electric shocks, the excitatory contraction passes
from a reversible to an irreversible condition associated with
death. Both pulvinated and growing organs exhibit a
violent contraction from which there is no recovery. In
the case of anaesthetics likewise, a mild dose induces a
contraction, recovery taking place after substitution of
fresh air. But under a stronger dose the violent con-
tractile spasm proves to be the spasm of death.

Effect of Poisonous Agents

This refers to gases or liquids which cause depression
and subsequent death of the plant.

Sulphuretted Hydrogen

Experiment 23. — This gas not only exerts a great
depressing effect, but is also toxic in its action, which
accounts for the impossibility of growing many plants in
a town, the air of which contains traces of this gas. For
example, Biophytum sensitivum, which flourishes and main-
tains its high sensitivity seven miles out of Calcutta in the
suburban area, soon succumbs when brought within the
town. The record (fig. 21) shows the marked retardation
of growth when the plant is exposed to HoS even for
15 minutes. In the present case the growth-rate was
reduced to half.

Experiment 24. Ammonium Sulphide. — This reagent
in dilute solution retards growth, and in stronger and long-
continued dose acts as a poison. The following results

COPPER SULPHATE

49

with a Wheat-seedling were immediately obtained under
different strengths of solution :

Normal rate .

0'5 per cent, solution

0-30 [JL per second
0-15 [^ ,,

2 -0

o-o8

!^

Copper Sulphate Solution

Experiment 25. — A minute dose of copper sulphate
solution of about o-i per cent, was found to induce

Fig. 21

Fig. 22,

Fig. 21. Effect of Sulphuretted Hydrogen (Wheat-seedling).
Fig. 22. Effect of Copper Sulphate solution.

immediate enhancement of the rate of growth, whereas
a stronger dose of 5 per cent, caused depression of the rate
(fig. 22), a remarkable instance of a minute dose of poison
acting as a stimulant. The following data relate to the
effect of I per cent. CUSO4 solution on the rate of growth.
The normal rate of growth of a Wheat-seedhng was 0-45 [i
per second ; application of i per cent, solution reduced
the rate to 0-13 [jl per second. Long-continued action of
the solution caused the death of the plant.

E

50 chap. v. effect of chemical agents

Summary

The effect of a chemical agent on growth is determined
by the strength of the dose, the duration of appHcation,
and the tonic condition of the tissue. A poisonous substance
in minute doses often has the effect of enhancing the rate
of growth.

The effect of any chemical agent on various growing
organs, such as the stem, the petiole, the flower-bud, and
the pistil, is essentially the same.

The various anaesthetics induce more or less similar
reactions on growth ; of these carbonic acid gas may be
regarded as a mild and chloroform as a strong anaesthetic.

Carbon dioxide induces a preliminary enhancement of
the rate of growth ; its continued action is followed by
decline and arrest of growth. The influence of CO2 is the
same in darkness as in light, the effect being thus inde-
pendent of photosynthesis.

A small dose of ether induces a great enhancement in
the rate of growth ; a large dose paralyses growth, but
timely substitution of fresh air is followed by revival.

The effect of continued action of chloroform vapour on
growth is as follows : at the first stage there is acceleration ;
at the second stage growth becomes arrested ; at the third
stage there is a violent contraction which is the spasm of
death.

A small dose of ether or chloroform renews growth which
had previously been in a state of standstill. This no doubt
explains the action of these anaesthetics in forcing growth
of dormant buds in winter.

Sulphuretted hydrogen retards growth and acts as a
poison ; the effect of ammonium sulphide is similar.

Copper sulphate, in minute doses, often has the effect
of enhancing the rate of growth. A stronger dose, or
a prolonged action, however, induces a depression which
culminates in the death of the plant.

CHAPTER VI

RELATION OF TURGOR AND OF TENSION

TO GROWTH

An important condition for growth is the supply of
water to the growing region, so that the cells may be in a
turgid state. The favourable condition of turgor can only
be assured by the pumping up of water from the soil,
growth being thus dependent upon the ascent of sap.
I will first describe the effects induced in growth by
changes in the rate of ascent of sap.

Growth Revived after Irrigation

Experiment 26. Growth-arrest and revival.- — The ex-
periment was carried out with a seedling of Cucurbit a 12 cm.
in height, growing in a small pot. Under excessive drought
the growth of the plant had been brought to a state of
standstih. The stem was held at its lower end in a clamp
and 2 c.c. of water was supphed to the root, growth becoming
revived after a latent period of 11 seconds. This delay
was due to the time taken for the water to reach the growing
region, and to impart sufficient turgidity to the cells for
the initiation of growth. It was interesting to find the
growth-record exhibiting pulsations {see fig. 23). The
small quantity of water supplied was sufficient to maintain
growth for only 3 minutes, after which it came to a stand-
still. Another 2 c.c. of water was next applied, resulting in
a renewal of growth for about 4 minutes or so ; the latent
period was, however, considerably reduced. The shortening

52

CHAP. VI. RELATION OF TURGOR AND TENSION

of the latent period was due to the fact that on the second
occasion there was less loss of time in getting the cells
sufficiently turgid for the renewal of growth. The response
of growth thus followed for a time after each doling out of
water.

Experiment 27. — A similar result was obtained with a
seedling of Vicia Faha, the growth of which, on account of

drought, was in a state
of standstill. Applica-
tion of a small quantity
of water produced a
short-lived revival of
growth, which occurred
in a pulsatory manner
(fig. 23). A second
application produced a
similar revival, the
amplitude and fre-
quency of pulsation
being quicker than in
the last case. Applica-

FiG. 23. Growth at standstill revived . .

after irrigation with water. tionof a larger quantity

A small quantity of water revived growth of W^ater produced a
for a short time. Note pulsation of • , , • i r

growth. (Vicia Faba.) persistent revival of

growth, the constituent
pulsations of which followed each other with such rapidity
that it appeared to be continuous.

Effect on Growth of Variation
OF Ascent of Sap

I have shown elsewhere that the rate of ascent of sap
is increased when warm water is used for irrigation, and
decreased when cold water is used for the same purpose.
Since activity of growth is normally dependent on the
ascent of sap, the rate of growth may be expected to be
appropriately modified by irrigation with either warm or
cold water.

EFFECT OF ASCENT OF SAP

53

Experiment 28. Effect of irrigation with cold and with
warm water. — A specimen of Kysoor with a quantity of soil
surrounding the root (enclosed in a small bag) was employed
for this experiment. The lower part of the plant was
securely fixed on a stand, the tip being attached to the
Crescograph. The specimen was then subjected to partial
drought, water being withheld for a day. This caused a
depressedrate of growth, but
not complete arrest. The
record D (fig. 24) exhibits
the depressed rate of growth
under partial drought.
Cold water was then applied
to the root and the effect is
shown in record C. Finally,
the record H was obtained
after irrigation with warm
water. It will be seen that
the spacings between suc-
cessive dots at intervals of
10 seconds in the three re-
cords are very different.
While a given growth-elon-
gation under drought took
place in 19 x 10 seconds, a
similar lengthening occurred
after irrigation with cold
water in 13 x 10 seconds, and
after irrigation with warm
water in 3 X 10 seconds.
Enhancement of the rate of ascent of sap by irrigation with
warm water is thus seen to have increased the rate of
growth more than six times (fig. 24).

The interval between irrigation and responsive variation
of growth will obviously depend (i) on the intervening
distance between the root and the region of growth, and
(2) on the vital activity underlying the ascent of sap. This
activity is increased by the action of warm water. In the

Fig. 24. Effect of irrigation on
growth.

D, record of grou-th under partial
drought ; c, acceleration after
irrigation with cold water ;
H, enhanced acceleration of
gro^vth after irrigation with
warm water (S. Kysoor).

54 CHAP. Vr. RELATION OF TURGOR AND TENSION

case described, the increased rate of growth on irrigation
with cold water took place after 70 seconds ; but the
responsive growth-elongation after application of warm
water occurred in less than 20 seconds.

In regard to the effect of irrigation with warm water,
certain precautions have to be taken, for sudden application
of hot water is liable to induce excitatory contraction : it
is therefore advisable to commence irrigation with tepid
and end with warm water. The transport of warm water
to the growing region may, however, introduce a complica-
tion of enhancement of growth by the rise of temperature.
This uncertainty may be obviated by waiting for the tissue
to return to the temperature of the room. The persistent
rate of growth may then be regarded as solely due to the
enhanced activity of the ascent of sap.

Experiment 29. Temporary and persistent enhance-
ment of growth. — The results obtained with the peduncle of
Zephyranthes will be found to be of interest in this con-
nection. Its rate of growth under partial drought was
found to be 0 • 04 [X per second ; application of warm water
increased the rate to 0-20 [x per second. After 15 minutes
the rate fell to 0-13 [i, and after an hour to the permanent
rate of 0 • 08 [x per second. It will be noted that even then
it was twice the initial rate before irrigation.

I give below a table which shows the immediate effect
on growth of irrigation with cold and with warm water,
the persistent effect being given in Table IV :

Table III. — Effect of Irrigation with Cold and
WITH Warm Water.

Specimen

Condition of experiment

Rate of growth

Kysoor

Peduncle of Zephy- 1
ranthes '

Dry soil

Irrigation with cold water

Irrigation with warm water

Dry soil

Irrigation with warm water

0-2I /i per sec.
0-30 (1

1-33/^

0-04 //

0-20 /(

1

variation of turgor

Effect of Positive and Negative Variation

OF Turgor

55

The acceleration of growth by the enhancement of
turgor caused by increased rate of ascent of water having
been demonstrated by the last experiment,
I then proceeded to observe the effect of
diminution of turgor in the very same organ.

Experiment 30. Effect of alternate
supply and withdrawal of water. — The rate
of growth of the peduncle of Zephyranthes
in the condition of partial drought was,
as already stated, 0-04 ]x per second,
increased to 0-20 [l after irrigation with
warm water. The permanent rate of growth
after irrigation was found to be o-o8 \l
per second. A strong solution of KNO3
was then applied to the root in order to ^^*^; ^5- The
withdraw water, with the result that the nate application

and withdrawal
of water on
growth,

N, normal rate
under drought ;
H, enhanced rate
after irrigation
with warm
water ; n', sub-
sequent per-
manent rate ; p,
diminished rate
of growth after
plasmolytic
withdrawal of
water (Zephy-
ranthes).

rate of growth quickly declined to 0-03 (x
per second, being nearly one-third the
previous rate (fig. 25). The induced de-
pression was thus greater than that under
condition of drought. The table shown on
page 56 is a statement of the results.

The results given show that the rate
of growth is enhanced, within limits, by an
increase of turgor due to more rapid supply
of water ; withdrawal of water, on the
other hand, brings about a retardation or
negative variation in the rate of growth.
Protoplasmic activity underlies both the movement of
growth and the ascent of sap.

Effect of Gain or Loss of Water on Motile

AND Growing Organs
In my previous work, ' The Motor Mechanism of Plants,'
it has been shown that the response of a motile organ is

56 CHAP. VI. RELATION OF TURGOR AND TENSION

Table IV. — Effect of Alternate Variations of Turgor
ON Growth (Zephyranthes).

Condition of experiment

Rate of growth .

Dry soil ......

Application of warm water
Steady groA\'th after one hour
Application of KNO3 solution

0-04 tx per second
o-2o,a „
o-o8 n ,,

0-03 IX ,,

essentially similar to that of a growing organ. Illustrative
examples of this will be given in a later chapter. In order

Fig. 26. Response of the pulvinus of Mimosa to irrigation
and to withdrawal of water.

Increased turgor by application of water at point marked with
vertical arrow induced erectile movement. Diminution of
turgor by application of KNO3 solution at the point marked
with the horizontal arrow brought about the fall of the
leaf. Successive dots at intervals of 10 seconds. (The
down-curve represents up-movement and vice versa.)

to observe the effect on a motile organ of the supply or
withdrawal of water, Mimosa in a condition of drought
may be taken, when its leaves are in a somewhat drooping

DIRECTIVE MOVEMENT OF SAP 57

condition. The result of watering the roots is a renewal
of suctional activity by which water is supplied to the
pulvinus, causing its expansion and the erectile movement
of the leaf. The record (fig. 26) exhibits this in a clear
manner. Water was supplied to the root at the point
marked by the vertical arrow, and the erectile movement
occurred after 10 seconds, the delay being due to the time
taken for the ascending water to reach the pulvinus. In
order to ascertain the effect of withdrawal of water, a
5 per cent. KNO3 solution was rapidly applied to the root
at the horizontal arrow. The effect of the consequent
withdrawal of water was the fall of the leaf, which occurred
in the course of 40 seconds.

The increase of turgor by more rapid supply of water
finds thus two parallel expressions, namely, an erectile
movement of the leaf of Mimosa and an enhanced rate
of growth in a growing organ. Withdrawal of water,
resulting in a diminution of turgor, causes, conversely, a
fall of the leaf and a retardation of the rate of growth.

Change of Turgor under Directive Movement

OF Sap

The movement of growth, as well as the erectile
movement of a pulvinated organ, is generally ascribed to
increase of turgor. But this cannot take place without
an adequate supply of sap. The law which governs the
directive movement of sap is that it follows the stimulation
gradient from the stimulated to the unstimulated region.'^
The turgor is diminished at the point from which the
sap is expelled, and becomes increased where the sap is
accumulated.

The effects of quicker or slower rate of ascent of sap
on movement of pulvinated and of growing organs are
summarised under A and B.

^ The Motor Mechanism of Plants (192S), p. 357.

58 CHAP. VI. RELATION OF TURGOR AND TENSION

A.

Enhanced rate of ascent of sap

I

Gain of water by the organ
Increase of Turgor

Erectile movement of Enhanced rate of growth

Mimosa leaf. in growing organ.

B.

Diminished rate of ascent of sap

I

Loss of water

Decrease of Turgor

Movement of fall of Retardation of rate of

Mimosa leaf. growth in growing organ.

The correspondence between the responsive movement
of the leaf of Mimosa and that of variation in the rate
of growth as explained above may be summarised as
follows :

. I. An increase or positive variation of turgor due to
enhanced rate of ascent of sap induces an erection
or positive response of the leaf of Mimosa, and a
positive variation or enhancement of the rate of
growth.
2. A diminution or negative variation of turgor due to
withdrawal of water induces a fall or negative
response of the leaf, and a negative variation or
retardation of the rate of growth.

INTERNAL HYDROSTATIC PRESSURE

59

Effect of Artificial Increase of Internal
Hydrostatic Pressure

Experiment 31. — The plant was mounted watertight
in the short hmb of a U-tube, and subjected to increased
hydrostatic pressure by increasing the height of water in
the longer limb. The records were taken only when the
rate of growth under the changed condition had become
uniform, which occurs, generally speaking, in the course
of about 5 minutes. Increasing internal hydrostatic pres-
sure was found to increase the rate of growth up to a
critical point, after which there was a decline. It is inter-
esting to note that the motility of the pulvinus of Mimosa
also undergoes a marked diminution under the condition
of excessive turgor.

The critical pressure beyond which growth exhibits
retardation is different in diverse species of plants. In
Kysoor it is about 4 cm. pressure of water ; in Crinum Lily
it is as high as 30 cm. The curve which gives the relation
between internal pressure and rate of growth is S-shaped.
It rises slowly in the first part, and more quickly in the
second ; it then becomes horizontal and finally reversed,
indicative of retardation.

The following tabular statement gives the effect of
increased internal hydrostatic pressure on the rate of growth
in two different specimens of Kysoor.

Table V. — Effect of Increased Hydrostatic Pressure

ON Growth.

Specimen

Hydrostatic pressure

Rate of growth

No. I.

Normal

2 cm. pressure

4 cm.

o-i8 ^ per second

0-20 /i ,,

0 • 1 1 /i , ,

No. II.

Normal

I cm. pressure

3 cm.

4 cm.

0-13 [J. ,,
0-20 /I ,,
o-i8 fi ,.
0-15 ft ,,

go chap. vi. relation of turgor and tension

Effect of External Tension

Excessive tension is usually found to retard the rate
of growth. This is one of the defects of the ordinary auxano-
meter, in which the recording lever exerts a considerable
pull on the plant.

Experiment 32. — The effect of gradual increase of tension
on growth is observed as follows. The recording levers of the
Crescograph are at first so balanced that very little tension
is exerted on the plant, the record now giving the normal
rate. The tension is then graduahy increased from i grm.
to 10 grm. When the growing organ is subjected to an
increase of tension, the immediate effect is an excitatory
contraction. But this transient effect disappears in a short
time, after which it is easy to observe the permanent effect
of tension on the rate of growth.

Table VI. — Effect of Tension on Growth (Crinum).

Tension

Rate of growth

0 (Normal) .....
4 grams .....

6 „

8 ,.

10 ,,

0-41 a per second
0-44 /f ,,
0-48 ^« ,,
0-52 li ,,
0-40 /{ ,,

The results given in the table above were obtained
with Crinum ; they show that the growth-rate increases
with tension till a limit is reached, after which there is
retardation.

Summary

Increase of turgor due to rise of sap after irrigation
enhances the rate of growth. Enhanced rate of the ascent
caused by irrigation with warm water induces a further
augmentation of the rate of growth.

The length of the latent period preceding the enhance-
nient of growth depends on the distance of the growing

SUMMARY 6l

region from the irrigated root. Irrigation with warm water
reduces the latent period.

Diminution of turgor caused by withdrawal of water
depresses the rate of growth.

Artificial increase of internal hydrostatic pressure up
to a critical degree enhances the rate of growth.

The law of the directive movement of sap (which induces
change of turgor) is that it follows the stimulation-gradient
from the stimulated to the unstimulated region.

There is a correspondence between the responsive
movement of the leaf of Mimosa and the movement of
growth. An increase or positive variation of turgor, due
to enhanced rate of ascent of water, induces the erection or
positive response of the leaf of Mimosa, and the positive
variation or enhancement of the rate of growth. A diminu-
tion or negative variation of turgor, due to withdrawal of
water, induces the fall or negative response of the leaf of
Mimosa, and the negative variation or retardation of the
rate of growth.

External tension, within limits, enhances the rate of
growth.

CHAPTER VII

EFFECT OF ELECTRIC STIMULATION ON GROWTH

In plant-physiology the word ' stimulus ' is often used in
a very indefinite manner. This is probably due to the
different meanings which have been attached to the term.
An agent is said to stimulate growth when it induces an
acceleration of growth. The normal effect of stimulus is,
as will be presently shown, the retardation of growth. It
is probably on account of lack of precision in the use of the
term that statements are often vaguely made of a stimulus
sometimes accelerating and at other times retarding growth.
In order to avoid any ambiguity, the terms stimulus and
stimulation are here used in a sense as definite as in animal
physiology. An induction electric shock, a sudden variation
of temperature, mechanical stimulation, for example, bring
about excitatory contraction in the muscle. These various
forms of stimulation also cause excitatory contraction of
the motile pulvinus of Mimosa pudica. The question
suggests itself whether such diverse forms of stimulation
evoke similar or different reactions in the growing organ.

An opinion prevails, however, that different modes of
stimulation induce reactions which are specifically different.
The results of investigation to be given in this and in the
following chapters will show that this is by no means the
case ; for all kinds of stimulation of effective intensity
induce excitatory response of the nature of mechanical
contraction and of electromotive variation of galvanometric
negativity. The perception of the stimulus and the con-
sequent reaction ultimately arise from the excitation

EFFECT OF ELECTRIC STIMULATION

63

of sensitive protoplasm. This is sometimes facilitated
by special anatomical structures such as tactile hairs, by
which mechanical stimulus becomes accentuated, and by
lens-shaped epidermal cells for focusing the stimulus of light
on the protoplasm.

In regard to the effect of different modes of stimulation
on growth, the subjects to be considered in this and sub-
sequent chapters are the following :

1. The Effect of Electric Stimulation.

2. The Effect of Stimulus of Light.

3. The Effect of Mechanical Stimulation.

4. The Effect of Thermal Radiation.

5. The Effect of Stimulus of Gravity.

Effect of Electric Stimulation

A form of stimulation which is extensively used in physio-
logical investigations is the electric stimulus of an induction

h

\-5 Sr\

1

^

UIIII|IIII|III|

4)0 810 2iO 1

^

t>i=4W

Fig. 27. Induction CoiL {See text.)

shock, which can be easily graduated by the use of the
well-known sliding induction coil (fig. 27), in which the

64 CHAP. VII. EFFECT OF ELECTRIC STIMULATION

approach of the secondary to the primary coil, indicated
by the higher reading of the scale, gives rise to increasing
intensity of stimulation.

Determination of Latent Period and Time-
Relations OF Response

Experiment 33. Latent period in electric stimulation. —
A feeble current was applied for i second to the growing

region of a bud of Crinum Lily
by means of two electrodes, one
above and the other below. The
record was taken on a moving
plate, the magnification employed
being 2000 times, and the suc-
cessive dots made at intervals
of 2 seconds. It was a matter
of surprise that the growth of
the plant was affected by an
intensity of stimulus below even
the limit of human perception.
As regards the relative sensitive-
ness of the plant and the animal,
experiments described elsewhere
show that the leaf of Mimosa
pudica, when in favourable con-
dition, responds to an electric
stimulus which is one-tenth the
minimum intensity that causes perception in a human
being. For convenience, I designate the intensity of in-
duction current that is barely perceptible to man as the
unit of electric stimulus.

When an intensity of 0-25 unit was applied to the
growing organ, it responded to it by a retardation of growth.
Inspection of fig. 28 shows that a flexure was induced in
the curve in response to the stimulus, denoting retarda-
tion of growth. The latent period in this case was 8 seconds.

Fig. 28. Time-relations of
the response of a growing
organ to electric stimulus
of increasing intensity.

jMoment of stimulation shown
by thick horizontal line.
Successive dots at intervals
of 2 seconds. (Crinum).

INTENSITY AND DURATION

6:^

The noi-mal rate was restored after 5 minutes. The in-
tensity of current was next raised from 0-25 unit to i unit.
The second record shows that the latent period was reduced
to 4 seconds, and a relatively greater retardation of growth
occurred under the action of the stronger stimulus. The
recovery to the normal rate was not attained until after a
period of 10 minutes. The intensity of stimulus was finally
raised to 3 units ; the latent period was now reduced to
I second ; -the retardation induced was so great as to cause
a temporary arrest of growth.

Table VII. — Time-Relatioxs of Responsive Growth-Variation
UNDER Electric Stimulation (Crinum).

Intensity of
stimulus

0-25 unit

I -o

^•o units

Latent period

8 seconds

I second

Normal rate

0-62 fx per second

0-62 ,« ,,

0-62

/^

Retarded rate

O

•49 /'. per second

0-25 ,« ,.

Temporan- arrest of
growth

Effect of Intensity and Duration of Stimulation

In the last experiment the effect of increasing intensit}^
of stimulation in shortening the latent period and in pro-
longing the period of recovery was observed. In the two
following experiments the effect of increasing intensity
and duration of stimulus was the special object of investiga-
tion. The specimens employed were the buds of Crinum
Lily, which were subjected to successive increase of intensity
of stimulation. After each record sufficient time was
allowed for recovery before application of the next
stimulation.

Experiment 34. Effect of increasing intensity of stimula-
tion.— The duration of the stimulation, which was 5 seconds,
was kept the same in successive records, the intensity being
increased from i to 2, and finally to 4 units. The normal

66 CHAP. VII. EFFECT OF ELECTRIC STIMULATION

rate of growth of the bud of Crinum Lily was 0-35 (x per
second. On the apphcation of electric stimulus of unit
intensity for 5 seconds, the rate became reduced to 0-22 [jl
per second. When the stimulus was increased to 2 units,
the rate of the retarded growth was o-oy [i per second.
When the intensity was raised to 4 units, there was complete

Fig. 29. Records of contractile response under increasing
intensities of electric stimulation of 0-25, i, and 3 units.

Records to be read from above downwards. Short vertical lines
indicate moments of application of stimulus (Crinum),

arrest of growth. In fig. 29 are given records of another
series of experiments which show the effects of increasing
intensity of stimulation in retarding growth.

Experiment 35. Effect of continuous stimulation, — The
effect of continuous electric stimulation of increasing in-
tensity will be seen in the record (fig. 30) taken on a moving
plate. On application of continuous stimulation of increasing
intensity, an increased flexure was produced in the curve,
which denoted greater retardation in the rate of growth.

INCIPIENT AND ACTUAL CONTRACTION 67

When the intensity of stimulus was raised to 3 units there
was an actual contraction.

Fig. 30. Effect of continuous electric stimulation. Intensities
of stimuli, 0-5, I, and 3 units; corresponding records from
left to right.

Note actual shortening of the organ under 3 units (Crinum).

Continuity between Incipient and Actual

Contraction

It will thus be seen that external stimulation induces
a reaction which is of opposite sign to that of normal grow^th-
elongation. This retardation may be conveniently described
as ' incipient ' contraction ; for under increasing intensity of
stimulus the contractile reaction opposing growth-elonga-
tion becomes more and more pronounced ; at an inter-
mediate stage this results in an arrest of growth ; at a
further stage it culminates in an actual shortening of the
organ. There is no break of continuity between these stages.
I shall, therefore, use the term ' contraction ' in a wider
sense, including the ' incipient ' stage which finds expression
in retardation of growth.

In Table VIII are given the results of typical experi-
ments on the effect of stimulation of increasing intensity
and duration.

68 CHAP. VII. EFFECT OF ELECTRIC STIMULATION

Table VIII. — Effect of Intensity and Duration of Electric
Stimulation on Growth (Crinum).

Duration of application

Intensity

Rate of growth

5 seconds

» »

9 >

Normal

1 unit

2 units

4 ..

0-35 ju per second
0-22 /x ,,
0-07 fj. ,,
Arrest of growth

Continuous stimulation

Normal
o '5 unit

I

3 units

0-30 fx per second

0-20 ft ,,
0-09 jX ,,
Contraction

Contractile Response of Pulvinated and of

Growing Organs

Very striking is the similarity between the contractile
response of the leaf of Mimosa and that of the growing

organ. For the purpose of
comparison I first give the
record of Mimosa under
moderate electric stimula-
tion. The up-curve exhibits
the contractile movement,
the successive dots being
at intervals of a tenth of a
second ; the recovery is slow,
and the successive dots in
the down-curve are at inter-
vals of 10 seconds (fig. 31).

Experiment 36. Contrac-
tile response of a growing
organ. — A very similar re-
sponse was obtained with
a growing bud of Crinum
under electric stimulation
of moderate intensity, the recorder employed giving
a magnification of 1000 times. In fig. 32 the normal

Fig. 31,

Contractile response of leaf
of Mimosa.

Up-curve (successive dots at intervals
of o • I second) indicates responsive
contraction. Down-curve shows
recovery (successive dots at inter-
vals of 10 seconds).

SUMMARY 69

growth-elongation is represented as a down-curve. On the
apphcation of stimulus, the normal expansion was suddenly
reversed to excitatory contraction ; the latent period was
I second, and the period of attainment of maximum con-
traction 4 minutes. The organ
recovered its original length
after a period of 7 minutes.
Repeated stimulation gave rise
to repeated responses as in the
case of Mimosa.

Arbitrary Distinction be-
tween Responses of Pulvi-
NATED and Growing Organs

The growing organ, when
subjected to successive stimu-
lations, gives a contractile
response in every way similar
to the mechanical response of
Mimosa. An arbitrary dis-
tinction has been drawn be-
tween the response of pulvinated and that of growing organs.
The movement of the former has been distinguished as
one of variation adapted for repetition an infinite number
of times, w^hereas a growing organ has been supposed to be
incapable of exhibiting repeated response. The experiment
described proves that there is no such basic distinction
between the two classes of phenomena.

Fig. 32. Contractile response of
a growing organ under electric
shock (Crinum).

Successive dots at intervals of 4
seconds. Vertical lines below
represent intervals of i minute.
(Magnification 1000 times.)

Summary

In normal conditions, electric stimulation induces incipient
contraction exhibited by retardation of the rate of growth.
Growth may be affected by an intensity of stimulation
below the range of human perception.

70 CHAP. VII. EFFECT OF ELECTRIC STIMULATION

The latent period in the responsive variation of growth is
shortened under stronger stimulus, but the period of recovery
becomes protracted.

Under increasing intensity of stimulation the con-
tractile reaction becomes more and more pronounced. At
a critical intensity of stimulus, growth becomes arrested ;
under a still stronger intensity there is an actual shortening
of the organ. Continuity thus exists between incipient and
actual contraction.

The distinction between the responses of pulvinated and
growing organs is arbitrary. Growing organs are capable
of repeated response, just as are pulvinated organs.

CHAPTER VIII

EFFECT OF LIGHT OX GROWTH

I SHALL first deal with the effect of hght on vigorously
growing organs, leaving the consideration of the abnormal
reaction to light, the underlying cause of which was at
first difficult to trace, for the next chapter. The effect of
light is studied by taking the normal record of growth in
darkness or under uniformly diffused feeble light, and
then records under increasing intensities of illumination.
The subjects discussed in the present chapter are :

1. Normal effect of hght on growth.

2. Determination of the latent period of response.

3. Effect of increasing intensity of light.

4. Effect of continuous stimulation.

5. Immediate and after-effect of light.

6. Effects of different rays of the spectrum.

Method of Experiment

The plant was placed in a glass chamber kept moist.
Strong light was obtained from a small arc-lamp with a
self-regulating device for ensuring steadiness of illumination.
An incandescent electric bulb was also employed as a less
strong source of light. Two inclined mirrors were placed
close behind the plant so that light acted on it from all
sides.

Normal Effect of Light

Experiment 37. — The record of normal growth N, of
Kysoor, was at first taken in darkness on a stationary plate.

72

CHAP. VIII. EFFECT OF LIGHT OX GROWTH

The effect of light is shown in the record S, where shortening
of the spacings between successive dots indicates retardation
of growth (fig. 33). For the same extent of growth-elonga-
tion there are 12 spacings in darkness and 24 spacings in

^^^^^HF^-^^

'^. .*• ' ^ ■

^^^^^^^^1

;^|jSH|

H^^^H

^H^mir':.'/.^

^^^^^^^^^H

■ . ■ .-. N ■»•' I •■, •■ J- li

|^¥i^:'^l

^^^^^^H

^^^^^^^^^H

^^i/ V-'. '^^IH

I^^^^^^H

^ .^^^^^H

^^^^^^^H

^^'^^^^l^■

^^^^^^^^^^^^1

^^^^^^^^^H

'.^Wi'^^H^^H

l^^^^^l

■^ v^ij^^^^^^l

^^^^^^^^^^^^1

'-' f*v ^ ?:*■ ^^^^^^^H

^^^^^^^^^^H

'V/^^^^^^^l

H^^^^^^^^^^^^^^^l

. " '" V . '^^^H^^H

-.•..-v'./^, -'^^^^H

^^^^^^^^^^^^1

^^H

''-' ''Sm

^^^^J

. - ^^^^^^B

^jM^fyu^^gp^i

A-' ^ VA_ll^^M

^B^^^S»«^u^w^ ^SB^^^I

^ . ^^^^^B^^^^^^H

^^H^Emj^'vi >^^^H

'''.''--'/-^'V'^H^^I

' ^mH^H

B^^Hn ' ' -'>' ^1

ir^^P^^^^I

nit?<- >-.„w

:^ wTTj^^B

^^^B

H^^^

^^^^^^H

''^^^^^^^^^jHH^H

HH^^^^V' ^IsH^^I

^^^^^

^^H

Fig. 33. Effect of light in retardation of growth.

N, normal ; s, retarded rate in response to light ; n', recovery-
half an hour after cessation of light (Kysoor).

light. The rate of growth under the action of light was
thus reduced to half. Record N' was taken once more
in darkness after an interval of half an hour, showing
restoration of the normal rate.

The Latent Period

A prevalent impression exists that a considerable period,
from several minutes to an hour, intervenes between the
incidence of light and the responsive variation of growth.
This over-estimate must be due to the absence of sufficiently

INTENSITY Oi- LIGHT 73

delicate means of observation ; for my High Magnification
Recorder indicated, in some cases, response within a period
as short as 5 seconds of exposure to Hght. In other cases
the latent period was found to vary from 15 seconds to
several minutes.

Experiment 38. Determination of the latent period. —
The record of the effect of an arc-light on a seedling of

Fig. 34. Determination of the latent period.

Light apphed at thick dot. Successive dots at intervals of

5 seconds (Cucurbita).

Cucurbita was taken on a moving plate (fig. 34). The first
part of the record gives the normal rate of growth ; light
was applied at the thick dot, and the flexure of the curve
after the seventh dot indicates the responsive retardation.
As the successive dots are at intervals of 5 seconds, the
latent period in this case was 35 seconds.

Effect of Increasing Intensity of Light

Experiment 39. — The specimen employed was Crinum ;
it was subjected to light emitted by a half- watt incandescent
electric lamp of 200 candle-power. The intensity of light
when the lamp was placed at a distance of 100 cm. from
the plant was taken as the unit. Much feebler light would

74

CHAP. VIII. EFFECT OF LIGHT ON GROWTH

have been sufficient, but would have required a very long
exposure. The intensity was increased by bringing the

lamp nearer the plant ; marks were made
on a horizontal scale so that the intensity
of incident light increased at the succes-
sive marks of the scale as 1:2:3, and
so on. The duration of exposure was
the same in all cases, namely, 5 minutes.
After each exposure to light, suitable
periods of rest were allowed for the
plant to recover its normal rate of
growth. The records in fig. 35 show in-
creasing retardation induced by stronger
intensities of light. Table IX gives the
result obtained with another specimen.

Fig. 35. Effect of
light of increasing
intensity in retar-
dation of growth.

First record normal ;
second and third
records under in-
tensities of 2 and
3 units (Crinum).

Effect of Continuous Light

Experiment 40. — The effect of con-
tinued light of moderate intensity in
bringing about increasing retardation of
growth will be seen in fig. 36, h, side by
side with the record of effect of con-
tinuous electric stimulation of constant intensity on growth
(fig. 36, a). In both cases the effect of continuous stimula-
tion is seen to be essentially similar, namely, iricreasing
retardation culminating in arrest of growth. This is true

Table IX. — Effect of Light of Increasing Intensity on
THE Rate of Growth (Crinum).

Intensity of light

Rate of growth

0 (Normal)

1 unit

2 units
?> ..

0-47 n per second
0-29 jX ,,
0-17 PL ,,
0-09 /i ,,
Arrest of growth

AFTER-EFFECT OF LIGHT 75

of stimulus of moderate intensity. Under a more intense
stimulation the incipient contraction does not end in the
arrest of growth, but the responding organ undergoes an
actual shortening of its length.

Fig. 36. EfEects of continuous (a) electric and (b) photic stimu-
lation recorded on a moving plate (Crinum).

Immediate and After-Effect of Light

The Balance Method of observation offers, as previously
indicated, a unique opportunity of discovering the charac-
teristics of different responsive phases, both during the
exposure and after the cessation of light. I will describe
two illustrative examples with two different species of
plants, one of which was in a vigorous condition of growth,
while the other was in a slightly sub tonic condition.

Experiment 41. Direct and after-effect of light. — The
specimen was a flower-stalk of Allium mounted on the
Balanced Crescograph. The index showed the normal
rate of growth to be 0-37 [x per second. After obtaining
the balance, the plant was subjected to light from a
small arc-lamp, being illuminated on all sides by suitably
inclined mirrors. The successive dots in the record are at
intervals of 10 seconds. The moment of incidence of light
is indicated by a vertical arrow, and cessation of light by
a horizontal arrow within a circle (see fig. 37).

76 CHAP. VIII. EFFECT OF LIGHT ON GROWTH

The record shows that the balance, after a latent period
of 40 seconds, was upset upwards, indicative of retardation
of growth. The total duration of application of light was
about 5 minutes ; the retardation persisted for a further
period of a minute and a half. After this the rate of
growth became normal for about 2 minutes, as seen in the
approximately horizontal record at the top of the curve,

l^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^l

-^ ' '- ' '^'' ' '^ «fl^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^H

. '•' ■'

'''^^^^^^^^^t^^^^^^^^^^^^H^^^^^^^^^^^^^^^^^^^^I

LfcS^^^^^^^^^^M^^^^^^^^^^^^M^^^^^^^^^^^^^^^^^^B

--«'.-.- ' jJbH^H^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^H^^^^^^^^^^^H

.JiiJHI^BI^^^^^^^^^^^^^^^^^^^H

^.',-' M"^U::

ihhI^^^^^SI^^^^^^^^^^^^^^^^I

' ::v"^ ^- '-Oi,:'."^-:'

.^^^P^Hb^^B|^^^^^^^^^^^^^^^^^^^^^^^^H

i v■■^:v^^/?:^i^.^:'■/'

■kSH^^IH^^^^^^h

•■->>■ ' -f^- ■ ■ -■--'• jO .•;. .»<

IS^IB^^I^^I^B^^^^^^^^^H

^^^^■E^/ ::"'' '^ -'.-~>S

^^^Q^^DHHBPmB^^P^^|Hi^^^^^^^^^^^^^^^^^^^^^^K^^^^^^^^^^^^^^^^^^^^^^^^^^^H

^^^^^Eji^^iii-ili^.;^

BV^w^^^^^^^H^^^^^^H

i^^v'*'*^ ^^^^^^^^^^^^^^^^^^^^^^IBJ^^^^^B^^^^^^^^^^^^^^^^^^I

^^r ' '^BHB^BI^^^^^^^^H^^^^^^^^^^^^^^^^^^I

^^^^^^P^* ^^^Hi

B 1 Ihrl^BlB^^^^^^^W

.i^^ -\V,, .;,., ,.,,..

zm-r^-^

"^t-.:-:i>--'^;. ^

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^1

■ ^'-r, *■■/.>.■;

*> 'S^^^^^^^^^^^^^^^^^^^^^^^^^^^^^H

' M ■ • ^.

• , 'V- ^^ *<-.',_

r.v'vtj.'''- 1 1 ^^^^^^^^^^^^^^^^^^^1

Fig. 37. Direct and after-effect of light recorded by Method

of Balance (Allium),

characterised by two small pulsations. The most interesting
after-effect of stimulation by light was then exhibited in the
down-curve, indicative of enhancement of the rate of growth
above the normal, which persisted for 8 minutes. After
this, the growth-rate was restored to normal as indicated
by the horizontal record (fig. 37).

Experiment 42. Effect on a slightly suhtonic specimen. —
A seedhng of Wheat was mounted on the Balanced Cresco-
graph and record was first taken under exact balance,
giving a horizontal line. Light from the arc-lamp was
now apphed for a few minutes. The record shows, by the

LIGHT AND SUBTONICITY 77

preliminary down-curve, a transitory acceleration of growth.
This was followed by the normal retardation as shown b}'
the up-curve. On the cessation of light, there was a
restoration of the normal rate as seen in the horizontal
record at the top of the curve ; but after a while the balance
was upset in the opposite direction, exhibiting a rate above
the normal. Finally, the rate of growth became restored

Fig. 38. Response of subtonic specimen to stimulus of light.

Note preliminary acceleration of growth followed by retardation
(up-curve) ; after-effect, an enhanced rate of growth followed
by recovery (seedling of Wheat).

to the norm.al, as seen in the curve becoming once more
horizontal (fig. 38).

The curves given in figs. 37 and 38 are essentially similar.
The only difference is in the preliminary acceleration of
growth which, as will be shown in the next chapter, is the
temporary effect of stimulation on a subtonic tissue.

A second record was taken with the identical specimen,
the intensity of light being increased by bringing the arc-
light nearer the plant. In this experiment retardation of
growth occurred from the very beginning. This was for
two reasons : (i) the increased intensity of light was now

78 CHAP. VIII. EFFECT OF LIGHT ON GROWTH

above the critical minimum which will be shown later to
demarcate the abnormal from the normal response ; (2) the
tonic condition of the organ was now improved by previous
stimulation.

The important result regarding the after-effect of moderate
intensity of light is that, for a brief period, the rate of growth
is enhanced above the normal. This will be shown to be the
characteristic after-effect of all modes of stimulation, being
a factor of much importance in accelerating the recovery
of the organ from the effect of brief stimulation.

Effects of Different Rays of the Spectrum

Previous observers have found that it is the more
refrangible rays which exercise the greatest influence upon
growth and tropic curvature. The relative effects of lights
of different colours are, however, more precisely indicated
by the curve of response to the action of different rays.
For this purpose I first employed monochromatic light
from different parts of the spectrum, produced by a prism
of high dispersion. In practice the usual colour-filters
were found very convenient, as they allow the application
of more intense light. Certain complications arise from
the slight rise of temperature due to the absorption of
radiant energy by the organ. Moderate rise of temperature
has been shown to enhance the rate of growth (p. 36), while
radiation, in general, causes a retardation. In spite of this,
it is easy to demonstrate the predominant effect of certain
rays in retarding growth.

Experiment 43. Effect of red and of yellow light. — The
highly sensitive Balanced Method of record fully supported
the results previously obtained, that these rays are practi-
cally ineffective in inducing any retardation in the rate of
growth.

Experiment 44. Effect of blue light. — On application of
light even for such a short period as 34 seconds, the effective-
ness of the blue rays became fully demonstrated. For the

SUMMARY 79

responsive retardation occurred in the course of 14 seconds
and the depression of the rate was two-fifths of the normal.

Experiment 45. Effect of infra-red rays. — In passing
from the highl}^ refrangible blue to the less refrangible red
rays, the responsive retardation of growth undergoes a
diminution or even abolition. Proceeding further into the
infra-red region of thermal rays, these are found to become
highly effective in inducing very marked retardation in the
rate of growth.

The curve drawn with the wave-length of light as
abscissa, and the effectiveness of the ray as ordinate, shows
a fall towards zero from the blue to the red ; the curve
however shoots up on proceeding further into the region of
the infra-red towards the invisible thermal rays.

Summary

The normal effect of light is retardation of the rate of
growth, which is, in fact, incipient contraction.

The latent period may in some cases be as short as
5 seconds, in others it varies from 15 seconds to several
minutes. The latent period is shortened under stronger
intensity of light. Increasing intensity of light induces in-
creasing retardation culminating in an arrest of growth.

The response to continuous stimulation by light is
essentially similar to that to continuous electric stimulation.

The after-effect of brief and moderate stimulation by
light is a short-lived acceleration of growth above the
normal rate.

The effectiveness of different rays of the visible spectrum
in retarding growth undergoes a decline from the blue to
the red rays. The thermal rays in the infra-red region are,
however, very effective in retarding growth.

CHAPTER IX

ACCELERATION OF GROWTH IN SUBTONIC PLANTS ON

STIMULATION

A ITER investigation of the normal effect of electric and
photic stimulation in retarding growth, described in the two
previous chapters, it was a matter of considerable surprise
to me that the responses were occasionally positive ; that
is to say, an acceleration of growth instead of the normal
negative response of retardation. After giving an account
of these positive responses, I shall attempt to trace the
cause of the abnormahty.

Acceleration of Growth under Light

Experiment 46.— The specimen employed was Kysoor,
which was exposed to the action of strong hght for 5 minutes.
Its normal rate of growth was 0-3 [j. per second ; but after
exposure to light there was an enhancement of the rate of
growth to 0-40 tjL per second.

The plant was then subjected to the continued action
of light for half an hour, which caused a transformation of
the response from the abnormal positive acceleration to the
normal negative retardation of growth. The question now
arises : What is the characteristic of the organ which pre-
disposed it to give a positive response, and how did the
positive response at the beginning become transformed into
the normal negative under the continued action of light ?

Referring to fig. 33 (p. 72), in which normal retardation
of growth occurred in Kysoor, it was found that its rate of
growth was as high as o-8 [x per second. But in Experi-
ment 46, in which there was an acceleration of growth

ABNORMAL AND NORMAL RESPONSE 8l

under stimulation, the rate of growth of that particular
specimen of Kysoor was feeble, being as low as 0-3 (j,
per second. As the activity of growth is an indication
of a healthy tone, the enfeebled rate of growth was a sign
of the subtonic condition of the plant. It thus appeared
that, other things being the same, the abnormal positive
is the characteristic response of the subtonic plant.

In the parallel phenomenon of response of a pulvinated
organ such as the leaf of Mimosa, I have shown (i) that
when the tonic level of the pulvinus is helow par, the response
is abnormal positive, exhibited by the erectile movemicnt of
the leaf ; and (2) that as a result of continuous stimulation
the tonicity is raised to a condition of par. The 'response
is now changed into normal negative as indicated by the
fall of the leaf.^ In the responsive variation of growth,
likewise, the abnormal positive response of the subtonic
specimen was transformed into the normal negative in
consequence of improvement of tonic condition attained
under continued photic stimulation. The above facts lead
to the following generalisation :

1. That while strong light induces retardation of growth

in an organ whose tonic condition is normal or
above par, it induces acceleration in an organ whose
condition is helow par ;

2. That by the action of the stimulus of light itself,

a subtonic organ is raised to a condition of par,
with concomitant transformation of its response to
that of normal retardation.

Continuity between Abnormal and Normal

Response

The tonic condition of a growing organ may vary widely ;
of this the following are the two extreme cases : (i) the
optimum, and (2) the subtonic. In the optimum condition

^ The Motor Mechanism of Plants (1928), p. 51.

82 CHAP. IX. GROWTH IN SUBTONIC PLANTS

the rate of growth is very rapid, while in the subtonic
growth is feeble or even arrested. There are all possible
gradations between the two extremes.

The responsive reaction depends then on two important
factors : the tonic condition of the organ and the effective
intensity of stimulation. The effectiveness of stimulation
is determined, not only by its intensity, but also by its
duration.

These theoretical conclusions mav be summarised thus :

1. A subminimal stimulus is gradually transformed into
a minimal and then to a maximal, under prolonged duration
of stimulation.

2. A strong stimulus may prove to be subminimal for
a short time at the moment of its application, specialty
when the organ is in a slightly subtonic condition.

The possible combinations of the effects of these varying
factors are very numerous, and it is therefore necessary to
confine attention to the typical cases which, from the
theoretical point of view, may be expected to give the
positive response of enhanced rate of growth :

First, a normal organ under short exposure to a sub-
minimal stimulus ; and

Secondly, a subtonic organ under moderate and not
excessively prolonged stimulation.

Enhanced Rate of Growth under Subminimal

Stimulation

Experiment 47. — The abnormal positive response was
obtained even with a moderately vigorous specimen, when
the intensity of the incident light was feeble, as seen in the
record of growth taken on a moving plate (fig. 39). The
slope of the first part of the curve show^s the normal rate ;
stimulus was applied for a short time at the fifth dot,
and the sudden erection of the curve demonstrates the
enhanced rate of growth. This persisted for a time, after
which the rate returned to the normal. Continued exposure

REVIVAL OF GROWTH 83

to feeble light, or to stronger light, converted the accelera-
tion into the normal retardation.

Opposite effects of feeble and strong stimulation. —Thus
while strong stimulation induces retardation of the rate of
growth, feeble stimulation causes an enchancement of the
rate. In the wide range of stimulation between minimal
and maximal there is therefore a critical intensity above
which there is a retardation and below which there is

Fig. 39. Acceleration of growth under subminimal stimulus of

light (Kysoor).

an acceleration. This critical intensity varies in different
species of plants.

As chemical substances often cause stimulation, the
opposite effects of small or large doses of the same drug
may perhaps afford parallel phenom.ena.

Revival of Growth Previously at Standstill

Experiment 48.— An organ falls to a condition of extreme
subtonicity when it is maintained for a long time under
unfavourable conditions. A peduncle of Allium was kept
in the dark for a fortnight, after which its growth was found
to have been practically arrested. The plant attached to
the Crescograph then gave an almost horizontal record on

84 CHAP. IX. GROWTH IN SUBTONIC PLANTS

a moving plate. Exposure to strong light from an arc-
lamp for 4 minutes was, however, effective in reviving the
growth, as indicated by the erection of the curve (fig. 40).
In a vigorous specimen of Allium, on the other hand, the
effect of light has been shown to be a retardation of growth
Icf. fig. Z7)- ^^^^ results offer conclusive evidence that the
sign of response, negative or positive, is dependent on the
tonic condition of the organ.

Fig. 40. Stimulus reviving growth at standstill (Allium).

The next problem to be considered is whether the
acceleration of growth in a subtonic organ under light is
due to its action as a stimulus, or to its possible photo-
synthetic effect. The experimental specimens usually
employed were flower-stalks or stems in which chlorophyll
was absent. The question can, however, be finally settled
by finding whether a different form of stimulus, such as
a tetanising electric current, which cannot possibly exert
any photosynthetic action, also induces acceleration of
growth in a subtonic organ.

effect of electric stimulation 85

Acceleration of Growth under Electric

Stimulation

Experiment 49. — I took a subtonic seedling of Wheat,
whose rate of growth was as low as 0-05 [i per second.
After electric stimulation, the rate was found to be
enhanced to o-i2 [i, per second or about 2 J times. I give
I wo records obtained with different specimens (fig. 41). 1 he
two vertical records to the left of the figure were taken

Fig. 41. Electric stimulation enhancing rate of growth
of subtonic plant (Wheat).

N, normal record ; s, after stimulation.

on a stationary plate. The closeness of the dots in N shows
the feeble rate of growth of the subtonic specimen. After
application of electric stimulus the record S shows, by
the wider spacings between successive dots, the induced
enhancement of growth.

In the second experiment the records (fig. 41, b) were
taken on a moving plate. The specimen was so extremely
subtonic that its normal record N appears almost hori-
zontal. The marked erection of the curve S after stimula-
tion demonstrates the induced acceleration of growth.

86

CHAP. IX. GROWTH IN SUBTONIC PLANTS

Table X. — Acceleration of Growth in Subtonic Specimens
BY Electric and Photic Stimulation.

Specimen

Stimulus

Rate of growth

Wheat seedling

Normal

After electric stimulation

0-05 n per second
0-I2 [i ,,

Kysoor .

Normal

After 5' exposure to light

0-30 ^« per second
0-40 IX „

The effect of direct stimulation upon growth Las thus
been shown to be modified by the tonic condition of the
plant, there being an enhancement of the rate when the
plant is in a state of subtonicity. Is there any other con-
dition under which stim.ulus enhances the rate of growth ?
I take up the question in a future chapter.

Summary

The sign of the response of an organ is dependent on its
tonic condition.

When the pulvinus of Mimosa is in a subtonic condition,
the response to stimulation is positive, that is expansion
and erectile movement, instead of the negative response of
contraction and resulting fall of the leaf.

As the result of continuous stimiulation the tonicity is
raised to a condition of par, the abnormal positive response
being converted into the normal negative.

The effect of stimulation on growth is modified in a
parallel manner, according to the tonic condition of the
organ.

When the organ is in a subtonic condition, it responds
to stimulation by an enhancement of its rate of growth.

In extreme cases, growth in a state of standstill becomes
revived under stimulation.

Continuous stimulation of a subtonic organ by light or
by electric current raises the tonic condition of the growing
tissue, the response of acceleration becoming transformed
into one of normal retardation.

CHAPTER X

EFFECT OF MECHANICAL STIMULATION ON GROWTH

Amongst the mechanical stimuh which induce excitatory
contraction in Mimosa may be mentioned the irritation
caused by rough contact, by a prick or by a wound. Friction
causes moderate stimulation, from which the excited
pulvinus recovers within a short time. But a prick or a
cut induces a far more intense and persistent excitation ;
the recovery becomes protracted, and the overstimulated
pulvinus remains contracted for a long period.

I now describe the effect of mechanical irritation on
growing organs, which will be found to be essentially
similar to that on the pulvinus. For moderate stimulation
I employ rough contact or friction ; for more intense
stimulation, a prick or a cut.

Effect of Mechanical Stimulation

Experiment 50. — ^The experiment was carried out with
the peduncle of Zephyranthes, which had a normal rate of
growth of o • 18 [JL per second. It was subjected to mechani-
cal stimulation, the surface being rubbed with a piece of
cardboard. This caused a retardation of growth, the
depressed rate being o-ii (jl per second, or three-fifths the
normal rate. The normal rate of growth was restored
after this moderate stimulation within a comparatively
short period of rest. After 15 minutes the rate became
0-14 (JL per second, and complete recovery was attained
after an hour when the rate became o-i8 y. per second, as

88 CHAP. X. EFFECT OF MECHANICAL STIMULATION

at the beginning (fig. 42). The growth-rate is greatly
depressed under intense stimulation, and the period of
recovery then becomes very much protracted,

I have often been puzzled by the fact that specimens
apparently vigorous exhibited little or no growth after
attachment to the Crescograph. After waiting in vain
for an hour, I had to discard them for others with equally

Fig. 42,

Fig. 43.

Fig. 42. Effect of mechanical friction on growth.
N, normal rate ; f, retarded rate immediately after friction ;
A, partial recovery after 15 minutes. Successive dots at
intervals of 5 seconds (Zephyranthes).

Fig. 43. Effect of pin-prick on growth.

N, normal rate ; w, immediate effect after wound ; a, partial

recovery after an hour (Zephyra,nthes).

unsatisfactory results. One of these specimens happened
to be left attached to the recorder overnight, and I was
greatly surprised to find that the specimen which had shown
no growth the previous evening, now exhibited vigorous
growth after being left to itself for 12 hours. I then reahsed
that the temporary arrest of growth had been due to stimu-
lation caused by the somewhat rough handling during
the process of mounting and attachment of the specimen
to the recorder.

The prolongation of the period of recovery after intense
stimulation is demonstrated by the following experiments.

WOUND

89

Effect of Wound

A prick causes an intense excitation in Mimosa ; I
tried the effect of this form of stimulation on a growing
organ.

Experiment 51. — The specimen was the same as had
been employed in the last experiment. After moderate
stimulation by friction it had, in the course of an hour,
completely recovered its normal rate of growth of o-i8 y.
per second. The stimulus of a pin-prick was now applied ;
the actual injury to the tissue was relatively slight, but the
retardation of growth induced by this relatively intense
stimulation was very great. With moderate mechanical
friction the rate had fallen from 0 • 18 [j, to 0 • 11 [j. per second,
i.e. to three-fifths the normal rate ; but the prick induced
a depression of growth from o-i8 ^ to 0-05 [i per second,
i.e. to less than a third of the normal rate. After 15 minutes
the rate recovered from 0-05 11 to o -o? u- per second. After
moderate friction the recovery was completed after an
hour ; but after the prick the recovery at the end of an
hour was onl}^ three-fourths of the normal, the rate being
now 0-12 [jl per second (fig. 43). I next applied the more
intense stimuation • induced by a longitudinal cut. This
caused a depression of the growth-rate to 0-04 a per
second. A transverse cut was found to be far more effec-
tive in retarding growth than a longitudinal slit.

Table XI. — Effect of Mechanical Stimulation and of Wound

ON Growth (Zephyranthes).

Nature of stira lus

Condition

Rate of growth

Mechanical friction

Normal rate

Immediately after stimulation

15 minutes after stimulation

60

0 • 18 fj. per second
0 • II ;£ , ,
0-14 pL ,,
o-iS fi ,,

Prick with needle

Normal rate

Immediately after stimulation
15 minutes after stimulation
60 ,, ,, ,,

0 • 1 8 // per second
0-05 pL ,,
0-07 /£ ,,
0-I2 j« ,,

90 CHAP. X. EFFECT OF MECHANICAL STIMULATION

The effect of mechanical stimulus on growth is thus
similar to those of other modes of stimulation, such as
electric and photic. Moderate frictional stimulation in-
duces incipient contraction, shown by retardation of growth,
recovery being completed within a short time ; but intense
stimulation, caused by a wound, gives rise to a greater and
more persistent retardation.

I next describe the movement of curling induced in a
tendril bj^ unilateral mechanical stimulation.

Mechanotropism : Twining of Tendrils

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 that side is rubbed with a splinter of wood,
the stimulated side becoming concave. This movement
may be distinguished as one of curling. There are, as wilJ
be presently shown, instances where the under side becomes
convex, the curvature being thereby reversed.

As regards contrivances for enhanced perception of
mechanical stimulus by the tendril, Pfeffer discovered
tactile pits on the tendrils of Cucurbitaceae. These no
doubt facilitate sudden deformation of the sensitive proto-
plasm by frictional contact. No satisfactory explanation
has, however, been offered as regards the physiological
mechanism of the responsive movement.

The twining must result from the modification of the
normal rate of growth under the stimulus of contact.
Further, this modification must be different on the two
sides of the organ : the proximal, on which the stimulus is
directly applied; and the distal, on which the stimulus can
only act indirectly. Investigations had therefore to be
undertaken to detect and record the effect of direct stimu-
lation, and also of indirect stimulation, that is, when the
stimulus acts at a distance.

GROWTH OF THE TENDRIL 9I

I have studied the effects of direct and indirect stimula-
tion on the growth of the tendril, employing not onl}^
mechanical but also other kinds of stimuli as well, the
fundamental reaction being essentially similar in all cases.
I will first describe the effect of electric stimulation, since
the impact of this stimulus does not produce any mechanical
disturbance.

Effects of Indirect and Direct Electric Stimulation
ON THE Growth of a Tendril

For this investigation I took a growing tendril of
Cucurbita, the sensitiveness of which is more or less uniform

Fig. 44. Fig. 45.

Fig. 44. Diagrammatic representation of ^Method for the Indirect

and Direct Electric Stimulation of tendril.
Fig. 45. Record by Method of Balance, showing acceleration
of growth of tendril (up-curve), induced by indirect electric
stimulation (Cucurbita).

on all sides. The specimen was held in a clamp, and the
tip suitably attached to the recorder. For indirect stimula-
tion, a feeble current from an induction coil was applied
by two electric connections below the clamp. Direct
stimulation was effected by sending the current through
the length of the organ, the two electrodes being placed
the one above and the other below the clamp (fig. 44).

Experiment 52. Effect of indirect electric stimulation. —
The tendril was mounted on a Balanced Crescograph, the

02 CHAP. X. EFFECT OF MECHANICAL STIMULATION

indications of which give the immediate and the after-effect
of stimulation. After securing exact balance, the record was
horizontal. Indirect stimulation w^as now appHed below

the clamp ; this upset the
balance, show^n by the
resulting up-curve, which
indicates a sudden acceler-
ation of growth above the
normal. This acceleration
took place within lo
seconds of the application
of stimulus and persisted
for 3 minutes. The normal
rate of growth then be-
came restored, the record
becoming once more hori-
zontal (fig. 45).

Experiment 53. Effect
of direct electric stimula-
tion.-— The contraction
induced by direct stimu-
lation is so great that the
record obtained by the sen-
sitive method of balance
cannot be kept within the
plate. I therefore took
a Crescographic record of
the growlh-curve without
balance. The first part
of the curve represents
normal grow^th; stimulus
of a feeble electric current
w^as then directly applied
near the highest point of the curve. This induced an
immediate contraction *and reversal of the curve, the
contraction persisting for 2 J minutes ; growth w^as then
slowly renewed (fig. 46). The most interesting fact re-
garding the after-effect of stimulation is that the rate of

Fig. 46. Variation of growth of tendril
induced by direct electric stimulation.

First part of the curve shows normal
rate of growth. Direct stimulation
induced contraction (reversal of
curve). After-effect of stimulation
seen in highly erect curve in upper
part of record, taken after 20 minutes
(Cucurbita).

/

STIMULATION OF THE TENDRIL 93

growth became greatly enhanced, even more than three
times the normal. This is clearly seen in the record (upper
half of the figure) taken 20 minutes after stimulation, the
curve being far more erect than that of the normal rate of
growth before stim^ulation.

The effects of indirect and direct electric stimulation
of the tendril are, therefore, as follows :

1. Indirect stimulation induces sudden enhancement

of the rate of growth, followed by recovery to the
normjal rate.

2. Direct stimulation induces a retardation of the rate

of growth. 1 he after-effect of direct stimulation of
moderate intensity is a short-lived enhancement of
the rate of growth above the normal.

I now proceed to show that mechanical stimulation
induces effects which are ver}^ similar.

Effects of Direct and Indirect Mechanical

Stimulation
Experiment 54. Effect of direct mechanical stimulation.
I took a growing tendril of Cucurbita, and after taking a
record of its normal rate of growth, subjected it to feeble
mechanical stimulation by rubbing its different sides. The
im.mediate effect was a retardation from the normal rate of
0-44 }x to 0-20 [X per second, the reduced rate being less
than half the normal. The tendril recovered its normal
rate of growth after the feeble stimulation ; in fact, the
effect after 15 minutes was even a shght acceleration above
the normal, the growth-rate being 0-59 [j, per second. The
results are given in the following table :

Table XII. — The Immediate and After-Effect of Mechanical
Stimulation on Growth of Tendril (Cucurbita).

Normal rate of growth

Retarded rate immediately after stimulation

Recovery and enhancement after 15 minutes

0-44 ;« per second

0-20 {.i ,,

The effect of unilateral stimulation will be next considered,

94 chap. x. effect of mechanical stimulation

Effect of Direct Unilateral Mechanical

Stimulation

The previous experiment gives the effect of diffuse
mechanical stimulation of the tendril. I now describe
the effect of direct unilateral stimulation applied at the
growing region.

Experiment 55. — The tendril of Cucurbita was mounted
on the Balanced Crescograph, and the record obtained with
a single recording lever. The first part of the curve shows

Fig. 47. Positive curvature of tendril of Cucurbita
under stimulus of unilateral contact at x.

the horizontal record of balanced growth. Unilateral
contact-stimulation w^as effected between two successive
dots (which were at intervals of 3 seconds) with the object
of not disturbing the record. Positive curvature was
induced in the course of about 10 seconds and attained its
climax in about 2 minutes, after which the tendril slowly
recovered, as shown in the horizontal curve in the upper
part of the record (fig. 47). Feeble stimulation is attended
by a recovery within a short time. Under strong stimula-
tion the curvature becomes more persistent.

INDIRECT UNILATERAL STIMULATION

95

Effect of Indirect Unilateral Mechanical

Stimulation

Experiment 56. — A tendril of Passiflora was held in a
clamp as in the diagram, (fig. 48) . The responsive movement
of the tendril was observed by focusing a reading-micro-
scope on a mark on the upper part
of the tendril. Direct mechanical
stimulation made the tendril move
towards the stimulated side, the
response being positive, as was also
found in the last experiment. The
stimulus was next transferred to a
point below the clamp, but on the
same side as before. This gave rise to
a negative responsive movem.ent, i.e.
in a direction away from the stimulated
side. This reversal into negative tropic
curvature is due to the fact that the
transmitted effect of indirect stimula-
tion induces an acceleration of growth
on the same side higher up, producing
a convexity in the growing region.

The different effects of direct and
indirect unilateral stimulation are
clearly in dicated in the diagram (fig. 48) .

Fig. 48. Diagrammatic
representation of effect
of indirect and direct
unilateral stimulation
of a tendril (Passiflora) .

Indirect stimulation i in-
duces movement away
from stimulated side
(negative curvature),
represented by con-
tinuous arrow above.
Direct stimulation d
induces positive curva-
ture, indicated by
dotted arrow above.

Electric Response of Proximal
AND Distal Sides

The effects of unilateral stimulation
on the two sides of the organ were
next determined by the test of elec-
tric response, in which excitatory reaction is detected by
electric change of galvanometric negativity, while expansive
reaction is indicated by galvanometric positivity.

Experiment 57.— Taking a tendril, I made suitable
electric connections for two series of experiments with an
identical specimen. In the first, an electric connection

96 CHAP. X. EFFECT OF MECHANICAL STIMULATION

was made with the proximal side to the right (the proximal
side being the side to be directly stimulated), the second
connection being made with a distant indifferent point.
Similar connections were also made for the second series,
the two electric contacts being respectively made with a

distal point diametrically opposite to
the proximal, and with an indifferent
point at a distance. A sensitive gal-
vanometer was included in the electric
circuit for record of the responses at
the proximal and distal points.

Mechanical stimulation was effected
by friction applied very near the
proximal point. The record first taken
was that of the effect on the distal side
of this indirect stimiulation. The
down-record, followed by recovery,
indicates a positive electric variation,
indicative of increase of turgor, ex-
pansion and enhancement of growth
at the distal point. Record was next
taken of the effect of the direct stimu-
lation on the proximal side. The
larger up-response indicates strong
galvanometric negativity, indicative of
diminution of turgor, contraction and
retardation of growth (fig. 49).
These phenomena of responsive reaction in the tendri]
will be shown to be by no means unique, but similar to
those of other organs under all forms of stimulation. The
only speciahty in the tendril is that, owing to its structure,
the perceptive power of the organ for mechanical stimula-
tion is highly developed.

The following is a brief summary of the facts estabhshed
in regard to the responsive characteristics of the tendril,
and the tropic curvature induced in it by unilateral
stimulation :

Fig. 49. Electric re-
sponse at the distal
and proximal sides of
a tendril under uni-
lateral mechanical
stimulation.

Galvanometric positiv-
ity is induced at distal
side (down-curve),
while galvanometric
negativity (up-curve)
is exhibited at the
proximal side (Vitis
quadrangularis).

THE LESS EXCITABLE SIDE 97

1. The proximal side contracts because it is directly

stimulated, the expansion of the distal side being
due to indirect stimulation. The curvature is
brought about by the joint action of contraction on
the one side and expansion on the opposite side.

2. The recovery of the tendril after brief stimulation is

hastened by the after-effect of stimulation, which
is an active expansion and enhancement of growth
{cf. Experiments 53, 54). .

The contraction of the directly excited side, and the
expansion of the indirectly stimulated distal side of the
organ, will explain Fitting's important observation ^ that
in a unilaterally stimulated tendril there is (i) an accelera-
tion of growth on the convex side, and (2) a contraction
on the concave side. Fitting ascertained that the tendril
became straightened by the renewal of active growth on the
excited side.

Response of the Less Excitable Side of Tendril

It is generally supposed that in the tendril of Passifiora
the upper side is devoid of moto-excitability. My experi-
ments show, however, that direct stimulation does induce
contraction and concavity of that side, though the actual
movement is relatively feeble, as shown below.

Experiment 58.— Feeble stimulus of the same intensity
was applied on the upper and under sides of the tendril
alternately. Successive stimuli were rendered more or less
uniform by the following device. A flat strip i cm. in
breadth was coated 2 cm. of its length with shellac varnish
mixed with fine emery powder. On drying, the surface
became rough ; the flat surface was then gently pressed
against the area of the tendril to be stimulated, and quickly
drawn sideways so as to rub the upper or the under side of
the tendril in each experiment. Stimulation, thus effected,
induced a responsive movement of both sides of the organ.

1 PfefEer, ibid. voL iii. p. 57.

n

98 CHAP. X. EFFECT OF MECHANICAL STIMULATION

The extent of the maximum movement was measured
by the microscope-micrometer. The following results were
obtained with four different specimens :

*

Table XIII. — Showing the Relative Intensities of Response of
THE Upper and Under Sides of the Tendril (Passiflora).

Movement induced by stimulation
of under side, A

Movement induced by stimjlation p .. ^
of upper side, B ^^^'° A

(i) 85 divisions

(2) 106

(3) 60

(4) 80

14 divisions

15

8

10

1
t>

1

T

1
T

1

8"

The upper side of the tendril is therefore not entirely devoid
of moto-excitability, its power of contraction being about
one-seventh that of the under side.

Inhibitory Action of Stimulation

The following puzzling phenomenon was observed by
Fitting in tendrils w^hich are specially 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 sensitive-
ness 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.' ^

It certainly seems to be anomalous that one side of the
organ, apparently inexcitable, should inhibit the response of
the opposite side. The results of my experiments already
described afford a satisfactory explanation of this curious
phenomenon of inhibition.

^ Jost, Lectures on Plant Physiology, trans, by R. J. Harvey Gibson,
p. 490 (Clarendon Press).

SUMMARY 99

It has been shown that direct and indirect stimulation
induce two opposite reactions : direct stimulation induces
diminution of turgor, contraction and retardation of growth
{cf. Experiments 53, 54, 55, 57) ; indirect stimulation induces,
on the other hand, increase of turgor, expansion and
enhancement of the rate of growth [cf. Experiments 52,
56, 57). The indirect stimulation is transmitted not only
longitudinally, but also across the organ.

Applying these principles to the explanation of the
anomalous case described by Fitting, the reason why only
those points on the directly stimulated under or proximal
side exhibit response which are opposite to corresponding
unstimulated areas on the upper or distal side, would appear
to be this : that the remaining unresponsive points on the
under or proximal side are opposite to corresponding
stimulated areas on the upper or distal side from which
indirect stimulation is transmitted which has an inhibitory
effect.

The effects of direct and indirect stimulation and the
general mechanism of tropic curvature are treated of in
greater detail in Chapter XII.

Summary

The response' of a tendril is in no way different from
that of growing organs in general.

Mechanical stimulus directly applied induces incipient
contraction, that is, retardation of the rate of growth, the
effect being similar to those induced by other modes of
stimulation.

Stimulus of contact or of friction induces a moderate
retardation in the rate of growth. On the cessation of
stimulation, the normal rate is restored within a short
time.

Under unilateral mechanical stimulation of short dura-
tion, the directly excited proximal side undergoes contraction,
the indirectly stimulated distal side exhibits the opposite
reaction of expansion. The induced curvature is thus

100 CHAP. X. EFFECT OF MECHANICAL STIMULATION

due to the joint effects of the contraction of the one side,
and the expansion of the opposite side.

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

Direct application of unilateral stimulus induces a
positive curvature, but the same stimulus applied indirectly
at a distance from the responding region induces a negative
curvature.

The dorsiventral tendril of Passiflora is excitable on both
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 expansion
of the opposite side, even in cases where the excitability
of the stimulated side is feeble.

The response to direct stimulation of the more excitable
side of the tendril is thus inhibited by stimulation of the
opposite side, as seen in Fitting's experiment. This is a
case of the neutralisation of the effect of direct by that of
indirect stimulation.

CHAPTER XI

THE PHOTOTROPIC EFFECT OF LIGHT

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 the incident light. Such stems often bend towards the
light, while roots, generally speaking, are supposed to bend
away from it. It might be thought that this is due to a
specific difference of irritabihty between shoot and root,
the irritabihty of the former being of a positive, and of
the latter of a negative, character. There are, however,
numerous exceptions to this hasty generahsation.

The intensity of the light has a modifying influence
on the character of the response. Thus, under unilateral
photic stimulation of increasing intensity and duration, a
radial organ may exhibit a positive, a dia-phototropic, and,
finally, a negative response. Strong sunlight brings about
a para-phototropic movement in which the apices of the
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 desiccation by
transpiration, does not hold good universally ; for under
strong light the leaflets of Cassia alata assume a position
by which the plant risks excessive loss of water.

x\n identical organ, as previously stated, may appear to
exhibit sometimes a positive and at other times a negative
response. Thus the leaflets of Mimosa pudica acted on by
light from above fold up towards the light, the phototropic

102 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT

effect being positive. But when the leaflets are acted on
by hght from below, then they also exhibit an upward
folding, the phototropic effect being now negative. Precisely
opposite effects are exhibited by the leaflets of Biophytum
and Averrhoa. They fold downwards whether light acts
from above or from below.

In these circumstances the hypothesis of the positive or
negative irritabihties of different organs is quite untenable.
Phototropic reactions are complex, for they represent the
summated effects of numerous factors. The individual
effect of these can only be ascertained by careful inspection
of the curves of response recorded continuously under the
action of light. The mechanism of tropic curvature will
be dealt with in detail in the next chapter.

Positive Phototropic Curvature

In this chapter I describe :

1. The positive curvature of pulvinated and growing
organs exposed to light.

2. Determination of the latent period.

3. The immediate and the after-effect of light.

4. The relation between the quantity of light and the
responsive curvature.

I have already shown (p. 68) that there is no essential
difference between the responses of pulvinated and those of
growing organs, direct stimulation inducing contraction in
both. The experimental investigation of the tropic effect
of hght has been carried out with both pulvinated and
growing organs.

Phototropic Recorder

A very sensitive method had to be devised for obtaining
accurate record of phototropic movements of pulvinated
and of growing organs. The writing lever is made of fine
glass fibre (fig. 50). The fulcrum-rod supported on jewel-

PHOTOTROPIC RECORDER

103

bearings has a small wheel (smaller than represented in the
figure) to which is attached the leaf (upper figure) or the
stem (lower figure). The up-movement of the leaf under

Fig. 50. The Phototropic Recorder : for pulvinated organ
(upper figure), for growing organ (lower figure).

vertical Hght, or the bending of the stem in response to
unilateral photic stimulation, is recorded as an up-curve
on an oscillating smoked-glass plate.

Positive Phototropic Response of Pulvinated

Organs

Experiment 59. Response of pulvinated organs. — For
this experiment I employed the terminal leaflet of Phaseolus.
The source of illumination was a 32-candle-power electric
lamp, enclosed in a metallic tube with a circular aperture for
passage of the light. The leaflet was attached to the Photo-
tropic Recorder. Light was applied on the upper surface of
the pulvinus for 20 seconds ; this induced an up-movement
of the leaflet, due to the contraction of the upper half of the

104

CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT

organ. Recovery took place in the course of 8 minutes
(fig. 51). Similar effects were observed with the leaflets of

Fig. 51. Positive tropic effect of light on pulvinated organ.

Responses to successive stimulations by light. Up-curve repre-
sents up-movement, and down- curve represents recovery.
(Phaseolus.)

Erythrina indica and of Clitoria Ternatea. The movement
under light from above has for simplicity been described

Fig. 52.

Positive response of leaflets of Erythrina indica.
(Para-heliotropic response.)

as upward. But the actual direction in which the leaflets
point their apices is towards the source of light. Both
these plants are so remarkably sensitive that the leaflets

CURVATURE OF GROWING ORGANS

105

follow the course of the sun in such a way that the axis of
the cup, formed by the terminal and lateral leaflets, is
coincident with the sun's rays (fig. 52).

Positive Phototropic Curvature of Growing

Organs

For this I employed with success the young stems of
numerous plants such as Dregea voUihilis, Vicia Faha, and

■^■1

^^^^^^^^^^^^^^^^Bi ^

^^^^^^^^^^^^^^^^^1

^^^^^^^^^^^^^^^bitaMrita<v

^^^^^^^^^^^^1

^^^■^^^^^■^■fif t , : '

^^^^^^^^^^H

^^^^^B^^^^^^^^^^^H^^' -'

^^^H

^^^H^^^^^^^^^^^^^HiMk ■•"'

^^^H

^B^H^^^^^HKii:'

^^^1

^^^^H

H^^^^^^^^B^' w.'

^^^^^^^^^^^^^^H

^^^^^^^^^^^^■|~ik-.'<>'' :'

^^^^^^^^1

■^^^^^^^■[>^:>w--

^^^^^^^^^^H

^^^^^^^^^^^H^^^^^^^^Ba^^bt*' r

^^^^^f

B^^^^^^Hr ^' '^

^^H

B^^^^^^^^^^^V^^^^^^^^^^^Ks • '*'^

^^1

^^^^^^H^^l

^^^^H

^^^^^^^^^^H

■^WHwn^: f : "1

^^^^^^^^^^H

^^^TT^^^^Tj^' -'y ,--:'-■■ ';■

^^^^^^^^^^^^^^pi

^^^HUML;' '-■■'''■' fv^Bitt^^''^'' '■ '-■

^^^^^H[

^^^^^^^^^H^^^Q

^^^^K^ ■^'- '1^^^^^^^ "'-' ■

^^»

H^:>miJ

■HhT, A'..'. ..-' TiBwH^^^n '^^^H

^^Ba

^^^^^HHH^^^^H

«A>.i^v, ■>>..- VI--.. ^tA«

^^K^^l

^^^^^^^^^^H

■teDliiK^'%^'<^%vv '\'';t.y: ' /" ^^|

^^^^^^^^^^H

^^^^^^^H'"' ^^^H

i^^^^^l

Fig. 53. Positive phototropic curvature of a growing stem.
Light apphed at vertical arrow and withdrawn at horizontal
arrow within circle. Successive dots at intervals of 10 seconds.
(Vicia Faba.)

others. The reaction of some of these was relatively feeble
and sluggish, whereas in others it was far more energetic.

Experiment 60. Positive phototropic curvattire of Vicia
Faba. — Light from a 25-candle-power Pointolite acted on
one side of the young stem at the most active growing
region, the record being taken on a smoked plate oscillating

I06 CHAP. XL PHOTOTROPIC EFFECT OF LIGHT

once in lo seconds. The moment of application of light
is indicated by the up-pointing arrow, and its removal by
the horizontal arrow within circle, the total duration of
exposure being 60 seconds. It will be noted that the
response occurred within a short time of the exposure, and
that the tropic movement persisted for nearly 3 minutes
after the cessation of light, when there was a recovery
which was practically complete (fig. 53).

It may be said in general that there is a quick recovery
after stimulation of moderate intensity and duration. But
the curvature induced under stronger stimulation remains
more or less persistent ; in extreme cases it becomes fixed
by permanent growth.

Determination of the Latent Period

The latent period of phototropic response is usually
regarded as of long duration, from several minutes to an
hour or so. The shortest latent period for the cotyledon

Fig. 54. Latent period of response to light in, a pulvinated
organ. Response occurred after 14 dots which were at
intervals of 2 seconds. (Erythrina.)

of Avena was found by Czapek to be 7 minutes. Little
is known about the relation of the latent period to the
intensity of light.

Experiment 61. Latent period of Erythrina. — The re-
cording plate was made to move at a fast rate, the successive

RESPONSE TO A SIX(';LE SPARK I07

dots being at intervals of 2 seconds. Record of response
was taken under moderate intensity of light acting from
above, and the latent period was found to be as short as
28 seconds (fig. 54). Under the action of stronger light
the latent period was further shortened. The latent period
for retardation of growth under diffuse stimulation by light
has also been found of the same order, namely, 35 seconds
[cf. fig. 34).

Two different meanings are attached to the expression
latent period. It may connote the interval between the
application of stimulus and the initiation of response, being
in the cases just described of the order of about 30 seconds ;
or it may be taken to mean the shortest period of exposure
to stimulus for ensuring response. Taking the case of light,
it may well be asked what is the shortest exposure to light
for inducing a retardation of growth in extremely sensitive
plants ? For this investigation I employed the highly
sensitive method of the Balanced Crescograph.

Growth-Variation Caused by Flash of Light

FROM A Single Spark

Experiment 62. — The duration of a spark-discharge
from a Leyden jar is almost instantaneous, being of the
order of TTrorni 0 of ^ second. A single discharge was made
to take place between two small steel spheres, the light
given out by the spark being most effective in retarding
growth. The plant employed was a seedling of Wheat
mounted on the Balanced Crescograph ; after exact com-
pensation of its normal grow^th, the record was horizontal.
The spark gap was placed at a distance of 10 cm. from the
plant, which was also illuminated from behind by reflected
rays from a suitably inclined mirror. The flash of light
from a single spark is seen to have induced a sudden retarda-
tion of the rate of growth, which lasted for 1 1 minutes.
The record shows another interesting pecuharity, namely,
acceleration as an after-effect of moderate stimulation.

I08 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT

After the retardation, which lasted for lOO seconds, there
was an acceleration of growth above the normal, persist-
ing for 5 minutes, after which the rate of growth returned
to normal (fig. 55).

Fig. od- Effect of a single electric spark in causing variation of

growth.

Record taken by Balanced Crescograph. Spark at arrow in-
duced retardation of rate (up-curve) ; after-effect is accelera-
tion above normal (dowoi- curve) followed by return to
normal rate. (Wheat.)

In order to show that the induced variation is due to
the action of hght and not to any electric disturbance, I inter-
posed a sheet of ebonite between the spark-gap and the
plant ; there was now no response when the spark passed.

Maximum Positive Curvature under Continued

Action of Light

One important factor in the production of positive
curvature is the contraction of the proximal side of the
organ. The curvature becomes greater and greater with
increasing contraction of that side. A limit of curvature
is, however, reached because :

QUANTITY OF LIGHT IO9

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.

Relation between Quantity of Light and
Induced Curvature

I now describe the effects of increasing intensity and
duration of exposure to hght on a pulvinated and on a
growing organ. The pulvinus of the terminal leaflet of
Desmodiiim gyrans was employed for the former, and a
growing bud of Crinum for the latter.

Effect of Increasing Intensity of Light

Experiment 63. Effect on pulvinus of Desmodium. — A
petiole bearing the terminal leaflet was suitably mounted
in a U-tube filled with water. The
wound-effect of section passed off
in the course of an hour. Light of
increasing intensity was successive-
ly applied from above, producing
increasing contraction of the upper
half of the pulvinus and increased
up-movement of the leaflet, re-
sulting in enhancement of the
amplitude of response. The first
record was obtained under light
of a given intensity, and the second
under an intensity twice as great
(fig. 56). If tropic curvature
increased in proportion to light-
intensity, then the two responses
would have been in the ratio of

1:2. The actual ratio was however slightly greater,
namely, 1:2-6. It will be shown, in a succeeding chapter,

Fig. 5G. Tropic effect of in-
creasing intensity of light,
I : 2, on the positive up-
response of terminal leaf-
let of Desmodium.

no CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT

that Strict proportionality holds good only in the median
range of stimulation, and that in a complete phototropic
curve the susceptibility to excitation undergoes an increase
at the beginning of the curve (p. 150).

The effect of light of increasing intensity on tropic
curvature of growing organs will now be considered. Since

4

15

^

>

15

:4_\ ..

•3 \

'1 _v

II I IK

0 12 S 4

Fig. 57. Curve showing the relation between intensity of
light and retardation of rate of growth (Crinum) .

phototropic curvature is mainly due to one-sided retardation
of growth, I will recapitulate the results previously obtained
on the effect of light of increasing intensity on growth
itself. The normal rate of growth of Crinum in the dark
was 0*47 [JL per second ; this was reduced to 0-29 \x under
the intensity of light of i unit, to 0-17 (j, under 2, and
to 0-09 [JL under 3 units; growth became arrested when
the intensity was raised to 4 units (p. 74). The curve of
variation of growth under increasing intensity of light is

IxNCREASING DURATION III

given in fig. 57, which shows that the retardation of growth
is at first rapid and then tends to reach a hmit. This
must also be true when hght acts only on one side of the
organ, the retardation of growth of the directly stimulated
proximal side of the organ contributing to bring about the
positive curvature.

Fig. 58. Tropic effect of increasing intensity of light, 1:2:3,

on growing organ (Crinum).

Experiment 64. Tropic curvature of a growing organ. —
The flower-bud of Crinum was used for this experiment.
The intensity of light acting on one side of the organ was
increased by bringing the source of light nearer to it,
the duration of exposure being in all cases kept the same,
namely, i minute. Increasing intensity of light in the ratio
of I : 2 : 3 gave rise to increasing positive curvature (fig. 58)
in the ratio of 1:2-5:5.

Effect of Increasing Duration of Exposure

Experiment 65. — The specimen of Crinum was in a
slightl}^ subtonic condition ; the responses therefore were
a short-lived negative preceding the normal positive. The
duration of successive exposures was for i, 2, and 3

112 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT

minutes. The amplitudes of the responses are in the ratio
of I : 1-25 : 5 (fig. 59)-

Fig. 59. Effect of increasing duration of exposure of
1:2:3 minutes on phototropic curvature of growing
organ (Crinum).

Effect of the Angle of the Incident Light

The quantity of Hght which falls on a unit area of the
responding organ varies as sin 0, where 0 is the directive
angle — i.e. the angle made by the rays with the surface.

Some allowance has to be

made for the loss of light
O \ I 1 reflected from the surface,

this being greater at 45°
than at 90°.

Experiment 66. Re-

FiG. 60. The Collimator for application , . * 7 • j j

of light at various angles. ■ SpOUSC of a pulvmated

organ. — For application
of light at various angles, an incandescent electric lamp
was mounted at one end of a brass tube, a colhmating
lens being placed at the other end (fig. 60). The parallel
beam of light from the Collimator could be thrown at
various angles by rotating the coUimator-tube round an
axis at right angles to the tube. Light was directed for
a minute, in the two successive experiments with the

RESPONSE OF GROWING ORGANS

113

pulvinus of Desmodium, at the angles of 45° and 90°. The

record (fig. 61) shows that the phototropic effect increases

with the directive angle. In the present case the ratio of

the two effects is i-6 : i, which is not very different from

^, ^. sin 90°
the ratio —. — ^-5 = i •4.
sm 45° ^

Experiment 67. Response of growing organs. — A parallel

experiment was carried out with the flower-bud of Crinum

Fig. 61,

Fig. 62.

Fig. 61. Effect of angle of incidence of light on tropic curvature

of pulvinus of Desmodium.
The first response is to light at 45°, and the second at 90°.

Fig. 62. Records of tropic curvature of growing bud of Crinum
on alternate stimulation by light at 45° and at 90°.

held vertical. Unilateral light was applied alternately at
45° and 90° in two successive series. It may be said in
general that the excitability of a tissue in a condition slightly
below par is increased by previous stimulation. From the
series of responses obtained under alternate stimulation at
45° and at 90° it is possible to ascertain whether any varia-
tion of excitability had occurred during the course of the
experiment, in order to 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°

114 CHAP. XI. PHOTOTROPIC EFFECT OF LIGHT

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 amplitudes of
9 and II mm. respectively (fig. 62). Taking the mean
value of each pair, the ratio of tropic effects for 90° and 45°
is = 10/6 = 1-7, nearly — a value which is slightly greater
than the ratio of the sines of the two angles.

The tropic effect of light as affected by increasing
intensity, duration, and change of directive angle, may now
be recapitulated : (i) the tropic effect is enhanced under
increasing intensity of light ; (2) it is increased with the
duration of exposure ; and finally (3) it is increased with
the directive angle from grazing to perpendicular incidence.
Taking into consideration the effects of these different
factors the conclusion is that the phototropic effect increases
with the quantity of incident light. It will be shown in
a subsequent chapter that strict proportionality of cause
and effect holds good only in the median range of stimula-
tion, and the slight deviation from this is due to the fact
that the susceptibility for excitation is feeble below that
range.

Summary

Phototropic response is similar in pulvinated and in
growing organs.

One important factor in positive curvature is the con-
traction of the directly stimulated proximal side of the
organ. The modifying effect of another factor will be
described in the next chapter.

The induced curvature is followed by complete recovery
after brief stimulation by light. The recovery is hastened
by an acceleration of the rate of growth above the normal
of the previously stimulated side. The recovery is pro-
longed after strong and long-continued stimulation.

The latent period for phototropic reaction is very much
shorter than has been previously supposed. In certain

SUMMARY 115

plants it is only about 30 seconds instead of minutes. The
latent period is shortened under increased intensity of
light.

The shortest period of exposure necessary to induce
responsive contraction under the action of light is extremely
short in certain cases. The seedling of Wheat responded
to a flash of light from an electric spark, the duration of
which is about a hundred thousandth part of a second.

Tropic curvature increases with the intensity and dura-
tion of exposure to light. It also increases with the directive
angle, the effect being approximately proportional to sin 0,
where 0 is the angle made by the rays with the surface of
the responding organ.

Within the median range of stimulation the intensity
of induced tropic effect is proportional to the quantity of
incident light.

CHAPTER XII

THE MECHANISM OF PHOTOTROPIC CURVATURE

Attention was mainly directed in the previous chapter to
the effect of direct stimulation on the proximal side of the
organ. It was shown that the positive phototropic curvature
is attributable to the contraction of the stimulated side.

The question now arises whether the stimulus of light,
acting on the proximal side of the organ, also induces
a reaction at the distal side ; if so, whether this helps or
opposes the positive curvature. It has been shown in a
previous chapter that unilateral mechanical stimulation
induces not only contraction and retardation of growth
of the proximal side, but also expansion and acceleration
of growth of the distal side, the resulting curvature being
due to these conjoint effects.

Does photic stimulation induce effects parallel to those
produced by mechanical stimulation ? And what are the
effects, if any, of the impulse transmitted from the stimulated
to the unstimulated side ?

The Effects of Direct and Indirect Stimulation

In order to analyse the effects of stimulation, it has been
necessary to devise a number of independent methods of
investigation, the concordant results of which lead to a
convincing conclusion. The questions to be answered are :
Is the effect of stimulation to generate a single impulse or
two impulses ? Is the impulse modified in transmission by

DIRECT AND INDIRECT STIMULATION II7

the distance traversed and by the conducting capacity of
the tissue ? To these questions an answer is obtained from
the study of the effects of Direct and Indirect Stimulation :
by Direct stimulation is imphed appHcation of stimulus at
or near to the responding organ ; by Indirect stimulation,
application of stimulus at some appreciable distance from
the responding organ.

Direct stimulation. — Taking the case of an excitable
shoot of Mimosa, direct stimulation of the pulvinus of a
leaf at once causes it to fall. The same effect is produced
when the stimulus is applied to the petiole, for the petiole
is a fairly good conductor.

Indirect stimulation. — Continuing the experiment with the
identical shoot, the same stimulus is now applied to the
stem, with the result that, instead of faUing as previously,
the nearest leaf shows an erectile movement, which may
or may not be followed by a fall.

These different effects suggest that stimulation generates,
in the stimulated tissue, two impulses. The one, which
induces the fall of the leaf, has been shown to be excitatory,
that is, of the nature of protoplasmic excitation, and since
the response which it evokes is designated negative, so the
impulse itself may conveniently be termed negative also.
The other, which induces the rise of the leaf, that is a positive
response, may likewise be termed the positive impulse.
But what is its nature ? On grounds which have been fully
discussed in previous works, more especially in the ' Ascent
of Sap ' (p. 247), I have come to the conclusion that it is
an hydraulic impulse, of the nature of a wave of increased
hydrostatic pressure which, originating in the contraction
of the cells at the point of stimulation, travels to the pulvinus,
causing it to expand. The two impulses produce opposite
effects : the excitatory nervous impulse causes contraction
of the responding cells, and is attended by galvanometric
negativity ; the hydraulic impulse causes expansion of the
cells, and is attended by galvanometric positivity. More-
over, the hydrauHc impulse resulting from stimulation

Il8 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

travels considerably faster than the nervous, and is not
dependent upon the conducting power of the tissues in
which it travels.

There are thus two distinct impulses initiated by stimu-
lation, the universaHty of which is demonstrated :

1. By the identical effects induced by modes of stimu-

lation so diverse as thermal, electric, and photic ;

2. By the longitudinal and the transverse transmission

of two impulses generated by stimulation ;

3. By the opposite reactions induced in growing organs

by the two impulses ; and

4. By the electric detection of the two impulses.

It is necessary to explain here that it is not the stimulus
but its effect that is transmitted to a distance. The
phrase transmission of stimulus has, however, come into
general use, and will be occasionally employed in the place
of the more correct phrase, transmission of impulse.

These points are well illustrated in the following experi-
ments. Compared with that of the petiole, the conducting
power of the stem of Mimosa is relatively feeble, as evidenced
by the fact that the velocity of transmission of excitation
is only 2 mm. as against 30 mm. per second in the petiole.
The semi-conductivity of the stem makes it easier to obtain
records of the effects of the two impulses.

Experiment 68. Longitudinal transmission of impulse
under radio-thermal stimulation. — The method of experi-
mentation is illustrated in fig. 63. The distance of the
point of application of stimulus on the stem from the
responding leaf was, in successive experiments with an
identical specimen, reduced from S to Si and finally to Sg.
The application of stimulus at a point opposite to the
indicating leaf is represented by S3. The radio- thermal
stimulus of moderate intensity was kept constant, the
intervening distance in successive experiments being re-
duced from 80 mm. to 30 mm., and finally to 10 mm.
The record was taken on a fast-moving plate, the successive

TRANSMISSION OF IMPULSE

119

dots being at intervals of a second. The resulting responses
given in fig. 64 should be read from below upwards.

S----

f-Sz
-•-Si

r

- -s

Fig. 63.

Fig. 64.

Fig. G3. Longitudinal transmission of impulse in stem of

Mimosa.

Stimulus successively applied at s, s^ and s,. Transverse applica-
tion of stimulus indicated by S3.

Fig. 64. Records of effect of longitudinal transmission of
impulse, to be read from below upwards.

Positive, diphasic and negative response to stimulation at s
s, and So. Erectile response in this and m the follo^g
represented by a down-curve, responsive fall bemg mdicated
by an up-curve. Successive dots at intervals of a second.

I first applied the stimulus S at a distance of 80 mm. ;
the transmitted impulse gave rise only to a positive erectile
movement of the leaf followed by partial recovery, there
being no negative response of the fall of the leaf. The

120 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

positive response occurred after an interval of 3 • 5 seconds ;
the failure of the negative impulse to reach the pulvinus
may be ascribed to the feeble conductivity of the stem.

Stimulus was now applied at Si at a distance of
20 mm. The response was positive, an erectile movement
of the leaf; this was followed by the excitatory negative,
a rapid fall of the leaf as indicated by the scratch-line
in the up-curve. The positive response was initiated
after an interval of 2 seconds, while the negative occurred
after 10 seconds. The velocity of the positive impulse under
indirect stimulation is thus greater than that of the excita-
tory negative impulse of 2 mm. per second, obtained by
dividing the distance of 20 mm. by the interval of time,
10 seconds.

Finally, the stimulus was applied at S2 at a distance of
10 mm. from the pulvinus. The response was now only
excitatory negative, which was initiated after 5 seconds,
the velocity of conduction being the same as before, namely,
2 mm. per second. The positive response is here masked
by the more intense excitatory negative reaction.

1 he results afford a basis for a rational explanation of
various tropic movements, which is only possible by taking
account of the two impulses, the existence of which is
fully established.

Experiment 69. Impulse under indirect electric stimula-
tion.— Exactly parallel effects were obtained under electric
stimulation — that is to say, the response was positive when
the stimulus was applied at a relatively long distance ; a
diphasic response, positive followed by negative, occurred
when the stimulus was applied at a short distance. Applica-
tion of stimulus at or near the responding organ gave rise
only to negative response.

I will next show that similar positive and negative re-
sponses are obtained when the stimulus is applied on one side
of the stem, the transmission being now in the transverse
instead of in the longitudinal direction.

transmission of impulse 121

Transverse Transmission of Impulse

Of greater importance is the transverse transmission
of impulse from the proximal to the distal side. It is
obvious that the conductivity across the stem must be very
much less than that along its length. Hence application of
stimulus to the stem of Mimosa diametrically opposite to the
indicating leaf, is equivalent to indirect stimulation of that
leaf. I will describe the effects of photic, electric, and
radio-thermal stimulation which result from the transmission
of impulse across the semi-conducting stem.

Experiment 70. Transverse transmission of impulse
under photic stimulation. — A narrow beam from a small arc-
lamp was made to fall on the stem, at a point diametrically
opposite to a motile leaf which was attached to the recording
lever, the successive dots in the record being at intervals
of a second. Stimulation by light caused a positive or
erectile movement of the leaf indicator within 5 seconds of
apphcation. The positive response affords conclusive proof
of the induction of increase of turgor at the distal point of
the stem to which the leaf was attached. When the stimulus
is moderate or of short duration the response remains
positive. But under strong or prolonged stimulation the
leaf falls, showing that the slower excitatory negative
impulse is conducted to the distal point (fig. 65). It will
be noted that the fall is slow at the beginning ; it then
becomes suddenly rapid, as indicated by a scratch-line
instead of a dot in the record.

The excitatory impulse reached the motile pulvinus
35 seconds after the initiation of the positive response. The
velocity of the negative impulse, it should be remembered,
depends on the intensity of the stimulus ; the stimulus
was moderately strong and the stem was thin, only 2 mm.
in diameter. The velocity of the excitatory impulse in the
transverse direction in this case was 0-05 mm. per second.

Experiment 71. Transverse transmission under electric
stimulation. —In order to show that the effects described

122 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

are not due to any particular mode of stimulation but to
stimulation in general, I carried out the following additional
experiments. Two fine pin-electrodes were inserted 5 mm.
apart into the stem of Mimosa exactly opposite to the
particular responding leaf. After a suitable period allow-
ing for recovery from mechanical irritation, a tetanising

Fig. 65. Effect of transverse transmission of impulse under
photic stimulation. Preliminary erectile response due to
positive impulse (down-curve), followed by fall of leaf (up-
curve) due to transverse conduction'of excitation. (Mimosa.)

electric current of moderate intensity was passed through
the electrodes. The responsive effects on the distal side
of the stem were precisely similar to those induced under
unilateral photic stimulation — that is to say, the first effect
was an erectile movement of the leaf brought about by
the positive hydraulic impulse ; the excitatory negative
impulse then reached the distal side after an interval of
31 seconds and caused a fall of the leaf (fig. 66). The
velocity of conduction of excitation was o-o6 mm. per
second.

TRANSMISSION OF IMPULSE

123

Experiment 72. Transmission under radio-thermal
stimulation. — Application of radio-thermal stimulus gave
rise to results essentially similar. The intensity of stimula-
tion was relatively stronger, and the negative excitatory
impulse reached the distal side (after the hydraulic positive)
in the course of 25 seconds (fig. 67), the velocity of the
impulse being o -08 mm. per second.

Fig. 66.

Fig. 67.

Fig. 66.

Effect of transverse transmission of impulse under
electric stimulation E.

Fig. 67. Effect of transverse transinission of impulse under

radio-thermal stimulation T.

Note parallel effect exhibited in figs. 65, 66, 67 ; also slow fall of
leaf in the commencement becoming suddenly rapid, as
shown by the scratch line. (Mimosa.)

The experiments that have been described are of much
significance. A rigid organ like the stem of Mimosa may
appear insensitive to stimulation, since it exhibits no
responsive movement ; but its perception of stimulus is
shown by its power of initiation and transmission of two
characteristic impulses to a distance, one of which is the
positive giving rise to an enhancement of turgor at the
distal side, and the other the true excitatory negative
inducing the opposite effect of diminution of turgor. Uni-
lateral stimulation gives rise to both these effects in all
organs — pulvinated, growing and non-growing. It is the

124 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

fortunate fact of the existence of the motile leaf of Mimosa
that makes possible the most convincing demonstration of
the reactions underlying the mechanics of tropic curvature.

To recapitulate : Stimulation gives rise to dual impulses,
positive and negative ; of these the positive impulse is
not entirely dependent on the conducting power of the
tissue, but the propagation of the excitatory negative
impulse is greatly dependent on the conducting power.
No tissue is a perfect conductor, nor is any a perfect non-
conductor of excitation, the difference being a question of
degree. A semi-conducting tissue, on feeble stimulation,
will transmit only the positive impulse ; on strong or long-
continued stimulation it will transmit both positive and
negative impulses, the positive preceding the negative.
The transmitted positive gives rise to increase of turgor
and expansion ; the negative induces the opposite reaction
of diminution of turgor and contraction.

In cases where transverse conductivity is feeble, the
impulse transmitted to the distal side (indirect effect of
stimulation) is positive, while the directly stimulated proxi-
mal side is negative. The tropic curvature is in such
cases positive, being brought about as the conjoint effect
of contraction of the proximal and expansion of the distal
side. It is only under strong and long-continued stimula-
tion that the excitatory negative impulse reaches the distal
side, and may thus neutrahse the positive curvature. Con-
sideration of this aspect of the subject is deferred to the
next chapter.

Effects of Direct and Indirect Stimulation on

A Growing Organ

The characteristic effects of direct and indirect stimu-
lation have been definitely established by experiments
carried out with Mimosa. It has been shown that direct
stimulation induces contraction and diminution of turgor,
while indirect stimulation induces the opposite reaction of
expansion and increase of turgor. What are the effects

EFFECTS OF STIMULATION 125

of these reactions on a growing organ ? Diminution of
turgor has been shown to induce a retardation of growth,
the incipient contraction culminating in a marked con-
traction (pp. 67, 68, 74). Enhancement of turgor due to
indirect stimulation, on the other hand, has been shown to
induce expansion and acceleration of growth (p. 91). The
following experiments w^ere so devised that the growing
organ should itself record the responsive variations of
growth under direct and indirect stimulation.

Experiment 73. Effect of direct and indirect radio-thermal
stimulation. — The experimental specimen was a flower-bud
of Crinum, held by a clamp a little below^ the region of growth
{cf. fig. 48). Radio-thermal stimulus was apphed below the
clamp, so that the transmitted impulse had to pass through
the securely held intervening tissue. The stimulus was
unilaterally apphed at a point about 30 mm. below the
region of active growth. The first stimulation was feeble,
and brought about an acceleration of growth on the same side
with expansion and convexity, the resulting movement being
negative, or away from the stimulus ; this effect is attri-
butable to the positive impulse transmitted to the region of
growth. The latent period was 5 seconds, and the maximum
negative movement was completed in the further course of
10 seconds, after which there w^as a recovery in the course of
a minute and a half. A stronger stimulus S' gave a larger
response; but when the intensity was raised still higher
to S", the positive was overtaken by the excitatory nega-
tive impulse within 15 seconds of the commencement of the
positive response ; the convex w^as then succeeded by a
concave curvature, the response being therefore diphasic
(fig. 68). When the stimulation was direct, that is, applied
at or near the region of growth, the response was a cur-
vature towards the stimulus.

I will briefly summarise the results obtained with growing
organs under direct and indirect stimulation. The effect
induced by feeble stimulus applied at a distance from the
growing region is a positive variation or acceleration of

126 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

growth : the effect is negative, i.e. retardation of growth,
when the stimulus is appHed at the responding region
of growth ; under intermediate conditions, the growth-
variation is diphasic, positive acceleration followed by
negative retardation.

Fig. 68. Effect of indirect unilateral stimulation.

s, s', induced negative tropic effect (movement avrayfrom
stimulated side) ; stronger stimulus s" gave rise to
negative followed by positive. (Crinum.)

Experiment 74. Effects of indirect and direct electric
stimulation.— In the place of radio-thermal, I next appHed
electric stimulation. Taking a growing bud of Crinum
I determined the region of its active growth ; lower down
a region was found where the growth had practically dis-
appeared and could therefore be regarded as an indifferent
region. In order to observe the effect of indirect stimulation
on the rate of growth, I applied two electrodes on this

EFFECTS OF STIMULATION

127

indifferent region about i cm. below the region of growth.
On application of diffuse and moderate electric stimulation
of short duration, the response was an acceleration of growth

Fig. 69. Effect of indirect and direct electric stimulation on
growth of Crinum, taken on a moving plate.

Dotted arrow shows the indirect application of stimulus, with
consequent acceleration of growth (highly erect curve).
Direct application of stimulus at the second arrow induced
retardation of rate of growth which culminated in actual
contraction (down-curve). Successive dots are at intervals
of 5 seconds. (Magnification 2000 times.)

which persisted for nearly a minute, after which there was a
resumption of the normal rate of growth. In this particular
case the interval of time between the application of stimulus

Table XIV. — Accelerating Effect of Indirect Stimulation

ON Growth (Crinum).

Specimen

Condition of experiment

Rate of growth

No. I.

No. II.

Normal

After indirect stimulation

0-2I // per second
0-26 n ,,

Normal

After indirect stimulation

0-25 fi per second
0-30 fi ,,

and the responsive acceleration of growth was 12 seconds.
The interval varies in different cases from i second to
20 seconds or more, depending on the intervening distance
between the point of application of stimulus and the re-
sponding region of growth. I give a record (fig. 69) obtained

128 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

with a different experiment which shows, in an identical
specimen, (i) an acceleration of growth under indirect and
(2) a retardation of growth under direct stimulation.

Experiment 75. Effect of long-continued indirect stimu-
Jation. — In the case of Mimosa, it was shown that indirect

Fig. 70. Effect of continuous indirect electric stimulation E.

Preliminary enhancement of growth followed by excitatory con-
traction. Interval ofsuccessive dots 10 seconds. (Cosmos.)

stimulation induced at first a positive response, w^hich was
transformed into negative under prolonged action. Parallel
results were obtained with Cosmos under continuous electric
stimulation ; there was a preliminary enhancement of growth
shov/n by the up-curve, followed by excitatory contraction
(down-curve) due to the conduction of true excitation (fig. 70).

Electric Response to Direct and Indirect

Stimulation

Employing the electric method of investigation, I have
obtained with different organs the positive, the diphasic, and
the negative electric responses corresponding to the mechani-
cal responses given by both motile and growing organs.

Experiment 76. Electric Response. — I took a leaf of Bryo-
phyllum calycinum, and made suitable electric connections.

ELECTRIC RESPONSE I29

one with the midrib and the other with a distant in-
different point on the lamina. Radio-thermal stimulation

Fig. 71. Diagrammatic representation of electromotive response
to indirect stimulation of effectively increasing intensity.

was successively applied, the point of application being
gradually brought nearer from S to S2 (fig. 71). When the

Fig. 72. Positive, diphasic, and negative electric responses
(Bryophyllum calycinum).

1^0 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

-o

intervening distance was 20 mm. the response was one of
galvanometric positivity. Reduction of the distance to
10 mm. gave rise to a diphasic response, positive followed
by negative. When the intervening distance was further
reduced to 3 mm. the response was one of galvanometric
negativity (fig. 72). This is an independent proof that
positive and negative impulses are generated under
indirect stimulation.

Fig. 73. Electric response at distal and proximal sides under

unilateral stimulation.

a, electropositive response at distal, and b, electronegative
response at proximal side (Helianthus annuus).

Experiment 77. Electric response at proximal and distal
sides under unilateral photic stimulation. — I took a young
flower-stalk of Sunflower (Helianthus) and made suitable
electric connections with diametrically opposite sides of the
organ for the record of the electric response. On applica-
tion of strong light on one side of the organ, the electric
response at the distal side was positive, indicative of
expansion and increase of turgor ; the electric response at
the proximal side was, on the other hand, negative, demon-
strating contraction and diminution of turgor (fig. 73).

The results obtained by widely different methods of
investigation are thus found to be essentially similar ; the

LAWS OF STIMULATION

131

different methods employed are the mechanical response of
motor organs, the response of growing organs by variation
in the rate of growth, and the response by electromotive
variation. It has thus been possible to formulate the Laws
OF Direct and Indirect Stimulation :

1. Direct Stimulation :

The effect of moderate intensity of stimulation acting
on organs in favourable tonic condition, is
contraction, diminution of turgor, negative
mechanical and electric response, negative
variation (retardation) of the rate of growth.

2. Indirect Stimulation :

{a) The effect of feeble stimulation is expansion, in-
crease of turgor, positive mechanical and electric
response, positive variation (acceleration) of the
rate of growth.

(b) The effect of prolonged application of stimulus of
moderate intensity is a diphasic response, posi-
tive mechanical or electric response followed by
the negative ; an acceleration followed by a
retardation of growth. If the intervening tissue
be highly conducting, the transient positive
effect becomes masked by the predominant
negative.

These fundamental effects of direct and indirect stimula-
tion are instrumental in bringing about various tropic
curvatures. The following table gives the responsive effects
induced in pulvini and in growing tissues.

Table XV. — Showing Responsive Effects Common to Pulvini
AND Growing Organs under Unilateral Stimulation.

Effect of direct stimulation on
proximal side

Effect of indirect stimulation on
distal side

Diminution of turgor and con-
traction
Galvanometric negativity
Contraction and concavity

Increase of turgor and expansion

Galvanometric positivity
Expansion and convexity

132 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE

Positive tropic curvature is brought about by the con-
joint expansion of the distal and contraction of the proximal
side.

Summary

The response of an organ is modified by the point of
application of the stimulus.

The closest parallelism has been established between the
response to stimulation given by pulvinated and by growing
organs respectively. Conditions which give rise to negative
mechanical or electric response also give rise to negative
variation or retardation of growth. This is also true of
positive mechanical and electric response and positive
variation or enhancement of growth.

Effective stimulation is shown to give rise to two distinct
impulses : one of these, the positive, is of a hydraulic nature ;
the negative, on the other hand, is of an excitatory charac-
ter. The positive is transmitted quickly ; the latter, being
a phenomenon of conduction of protoplasmic change, is
propagated slowly. The positive impulse gives rise to
expansion, the excitatory negative to contraction.

Feeble stimulus, especially when acting on a subtonic
organ, gives rise only to positive response.

The results of investigation of the effect induced by all
forms of stimulation lead to the establishment of the follow-
ing law : direct stimulation induces contraction ; indirect
stimulation gives rise to expansion.

Direct stimulation of the responding region causes a
contractile fall of the motile leaf, and a retardation of growth
in a growing organ. The transmitted or indirect effect of
stimulus applied at a distance is to induce an erection of
the leaf and an acceleration of the rate of growth.

Tropic movements illustrate the laws of direct and
indirect stimulation. The directly excited proximal side
undergoes contraction, the opposite distal side undergoes
expansion ; these two factors conspire to produce a positive
curvature.

CHAPTER XIII

DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM

I HAVE explained how under the unilateral action of 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 organ may be found in one of the
three different conditions determined by the light intensity,
viz. (i) a condition of positive heliotropism, (2) a condition
of indifference, (3) a condition of negative heliotropism.' ^
No satisfactory explanation has, however, been found as to
why the same organ should exhibit at different times a
positive, a neutral, and a negative response. The exhibition
of these different effects by an identical organ is incom-
patible with the theory of specific sensibility, often assumed
in explanation of characteristic differences of phototropic
response.

Oltmanns found that the seedling of Lepidium sativum
assumed a 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 neutralisa-
tion of curvature is, as will be presently explained, not due
to a condition of indifference, but to antagonistic effects
induced at the two opposite sides of the organ.

Phenomenon of Neutralisation

Neutralisation, partial or complete, is principally due
to the transverse conduction of excitation across the stem.

^ Jost, ibid. p. 462.

134 CHAP. XIII. DIA-PHOTOTROPISM

This was demonstrated by the experiment with the stem of
Mimosa, in which Hght was appUed on the proximal side, oppo-
site to the indicating motile leaf on the distal side. After
the preliminary erectile response, due to indirect stimulation,
the leaf exhibited a fall on account of the conduction of
the excitatory impulse across the stem {cf. Experiment 70)
which induced a diminution of turgor at the distal side.
This would antagonise the effect induced at the proximal
side by direct stimulation, and neutralise it.

The extent of the neutralisation will therefore depend
(i) on the transverse conductivity of the tissue, and (2) on
the intensity and duration of the incident stimulation.
Other things being equal, neutralisation will be incomplete
in a thick, and complete in a thin, stem. The following
experiments were undertaken in verification.

Experiment y^. Partial neutralisation. — The moderately
thick stem of Dragea volubilis was exposed to the unilateral
action of an arc-light. The first effect was a positive
phototropic curvature, which, after reaching a maximum,
was partially neutralised under continued action of the
light for 2 hours.

Experiment 79. Complete neutralisation. — The thin stem
of a young seedling of Phaseolus does not offer so great
a resistance to the transverse conduction of excitation as a
thick stem. When light from Pointolite of 30 candle-power
was unilaterally applied on the seedling, the maximum
positive curvature was induced in about 2 minutes. This
curvature was, however, completely neutralised under the
continued action of light for 5 minutes.

Strong and prolonged unilateral stimulation does not,
however, end in mere neutralisation, which places the organ
at right angles to light, regarded as the dia-phototropic
position. The transformation is carried still further ; thus,
three stages of phototropic action may be distinguished :
positive at the beginning, neutraHsation or dia-phototropic
attitude -as the intermediate, and negative phototropism as
the final effect. How^ is this final transformation effected ?

FATIGUE-RELAXATION I35

Fatigue-Relaxation

There is a difficulty in connection with the reversal of
positive curvature into negative which cannot be explained
simply by the conduction of excitation transversely from the
stimulated to the opposite side ; for, in a radial organ, the
contraction of the distal side cannot be greater than that of
the directly excited proximal side. There must therefore
be an additional factor in operation, which has to be
discovered.

The following experiments prove that it is fatigue-relaxa-
tion which occurs in a tissue under continued stimulation.

As an example, in illustration of this fact, may be cited
the erectile movement of the leaf of Mimosa after continuous
stimulation. When the leaf is subjected to brief electric
stimulation there is an immediate fall of the leaf, due to
active contraction of the more effective lower half of the
pulvinus. The leaf then exhibits normal erectile recovery.
When the leaf is subjected to continuous stimulation it
also exhibits a preliminary fall with subsequent erection
of the leaf, this erection being due to fatigue-relaxation
of the lower half of the organ. ^ The result is not unHke
the contraction of a muscle passing into relaxation under
continuous stimulation.

The preliminary phases culminating in fatigue-relaxation
in growing organs I propose to demonstrate :

1. Under different intensities of stimulation, successively

applied to the plant ; and

2. Under uniform strong intensity, but increasing

duration of application.

I will first describe the general effects of brief and of pro-
longed electric stimulation on growing organs.

Experiment 80. Normal contraction and recovery after
brief electric stimulation. — I took a specimen of Cow-pea
{Vigna Catjang) and obtained record of its normal rate of

1 The Motor Mechanism of Plants (1928), p.

136

CHAP. XIIT. DIA-PHOTOTROPISM

growth on a moving plate. The ascending part of the
curve (fig. 74) represents the normal rate ; direct applica-
tion of electric stimulus at horizontal arrow (intensity
2 units and duration 2 seconds) induced contraction indi-
cated by the down-curve. Slow recovery occurred till the
normal rate of growth was restored. After this normal
recovery, the plant exhibited cojitraction on fresh stimulation.
Experiment 81. Fatigue-relaxation of growth under

continued stimulation. — After
the normal recovery the
identical specimen was con-
tinuously subjected to an
electric stimulus of 2 units.
The response now was a pre-
liminary down-curve of con-
traction, succeeded by an up-
curve of relaxation (fig. 75).
An outward resemblance will
be noticed in the two records
(figs. 74, 75). The inner differ-
ence lies in the fact that after
apparent recovery consequent
on fatigue-relaxation , the plant
does not exhibit contraction on
fresh stimulation, as it does
after normal recovery.

In the experiments described
the stimulus employed was electric. I next investigated
the effect of intense photic stimulation on a growing
plant.

Experiment 82. Fatigue-relaxation under strong light. —
I took a seedling of Oryza, and strong light was made to act,
by means of inclined mirrors, on all sides of the plant.
The result obtained at the first stage was normal contraction
and retardation of growth, followed later by . relaxation
under the prolonged stimulation.

From the above facts it is clear that stimulation induces

B

^H

H

^H

J

Fig. 74. Effect of electric shock of
short duration at arrow induces
contraction, followed by normal
recovery (Vigna Catjang).

NEGATIVE PHOTOTROPIC CURVATURE I37

definite reactions which modify the rate of growth, i.e. a
contraction under moderate, and relaxation under intense
and prolonged, stimulation. Hence tropic curvature brought
about by one-sided action of light would likewise be subject
to modification, according
to the intensity and dura-
tion of the incident illu-
mination, by two factors :
(i) the transverse conduc-
tion of excitation to the
distal side, and (2) the
fatigue-relaxation at the
proximal side. This ex-
planation is based on the
following experiments.

Negative Phototropic
Curvature

Experiment 83. Effect ^ig. 75. Effect of continuous electric

of varying intensity of HM. stimulation at arrow onwards in-

•^ -/ o .y J o duces contraction followed by fatigue

The growing organ was relaxation (Vigna Catjang).

moderately thin ; it was

subjected to unilateral action of light from a i6-candle-power
incandescent 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 then increased by
bringing the lamp nearer, the intervening distance being
reduced to 6 cm. After an exposure of 70 minutes to the
stronger light the specimen assumed a dia-phototropic
position of equilibrium. Sunlight was next applied, and
there was a pronounced reversal into negative phototropic
curvature in the course of 30 minutes.

Experiment 84. Phototropic reversal under continued
action of strong light. — The change of response from positive
to negative, described in the previous experiment, was
brought about by successive increase in the intensity of

138 CHAP. XIII. DIA-PHOTOTROPISM

light. In the present case, very strong light from an arc-
lamp was applied from the beginning, and a continuous

Fig. 76, Phototropic reversal under continued action of
strong light L (Vicia Faba).

record taken of the change in the sign of response of the
young shoot of Vicia Faba. The first effect was a positive

curvature which attained its
maximum in the course of
5 minutes ; after this there was
neutralisation, succeeded by nega-
tive curvature (fig. 76).

Experiment 85. Transformation
of positive into pronounced negative
in Oryza. — Since transverse con-
duction is more rapid in a thin
specimen, I expected to obtain
a quicker reversal in Oryza.
Light from an arc-lamp produced
maximum positive curvature in
the course of 2 minutes, after
which there was neutralisation.
Subsequently complete and pro-
nounced negative phototropic

Fig. 77. Positive, dia- and ■, j • ^^i

negative phototropic re- reversal occurred m the course
sponse of Oryza under of a further minute and a half

continued unilateral

stimulus of intense light. (hg. 77/'

negative phototropism of roots i39

Supposed Phototropic Ineffectiveness of Sunlight

A brief reference may be made here of the apparently
anomalous phenomenon that ' direct sunlight is too bright to
bring about heliotropic curvature ; only diffuse and not direct
sunlight has the power of inducing heliotropic movements.' ^
It is inconceivable that sunlight should have lost all photo-
tropic power because it is so bright. The experiments just
described give an adequate explanation of the apparent
ineffectiveness of bright light. It has been shown that the
tropic curvature, under moderate intensity of light, does
not undergo any neutralisation, but that under very high
intensity of bright light transverse conduction occurs, which
causes the undoing of the curvature. This is demonstrated by
the record already given of the continuous action of strong
light, showing that the normal positive curvature at the
beginning became neutralised later.

Negative Phototropism of Roots

The abohtion of geotropic reaction in the root, after
amputation of the tip, has led to the conclusion that the
tip is the perceptive organ, the responding organ being at
the growing region at a short distance from the root-tip.
On the analogy of the opposite geotropic responses of shoot
and root, the hasty generalisation has been made that the
sign of response of root to light is opposite to that of the
stem, a negative instead of a positive curvature. The
conclusion was apparently supported by the negative
phototropic curvature of the root of Sinapis. The supposed
analogy is, however, false ; for while in the case of the
root the stimulus of gravity acts only on the restricted
area of the tip, the stimulus of light is not necessarily so
restricted, since it can act not only on the tip, but also on the
region of growth. That there is no universal analogy between
the action of light and gravitation is seen from the fact that
while gravitation induces in the root a movement opposite

1 Jost, ibid. p. 464.

140

CHAP. XIII. DIA-PHOTOTROPISM

to that in the shoot, hght does not always produce this
opposite reaction ; for though some roots turn away from
light, others move towards it.

The difficulty encountered in obtaining a direct record
of the phototropic curvature of the root was great. It was
overcome by the use of a sensitive recorder, and by the
employment of roots which possessed a moderate amount of
rigidity, so that the induced curvature could exert a sufficient
pull on the recording lever. One of the most suitable plants
is Ipomoea reptans, which floats on ponds, the roots normally
growing vertically downw^ards. The aerial roots of certain
plants were also found suitable for the investigation.

The Root-Recorder

The Recorder (fig. y^) consists of a very light writing lever
W, the axis carrying an aluminium wheel of a small diameter.

W

Fig. 78. The Root-Recorder. {See text.)

A string S, appropriately attached to the wheel, is tied to
the tip of the root T, the growing region G being a httle
above the tip. The stem St is fixed on a piece of cork by
the pin P, the root projecting vertically downwards. The
plant is suitably mounted in a rectangular trough of mica.

INDIRECT STIMULATION OF ROOT I4I

kept in a proper humid condition by pieces of sponge soaked
in water. There are two small openings, one to the right
for the passage of light through a narrow horizontal
diaphragm D, the other to the left for the passage of the
string attached to the recorder. The cork supporting the
root can be adjusted up or down, so that either the root-tip
or the growing region or both can be exposed to the action

of light.

The following experiments were carried out to determine
the effect of unilateral photic stimulation (i) on the root-
tip, and (2) on the growing region of the root.

Effect of Unilateral Stimulation of the

Root-Tip

Experiment 86. — The tip of the root of Ipomoea was
subjected to the unilateral action of light for 70 seconds.

Fig. 79. Negative response of the root to indirect stimulation

(Ipomoea).

Vertical line indicates moment of application, and horizontal
arrow within circle, of the withdrawal of light.

The source of light was a loo-candle-power PointoHte.
The curvature induced was found to be negative or away
from the source of light, initiated within 15 seconds of the
exposure to Hght. The negative movement continued for
I minute after the cessation of light, after which there was
complete recovery (fig. 79).

142 CHAP. XIII. DIA-PHOTOTROPISM

Since the responding growing region is at some distance
from the tip, the stimulation of the root was indirect in this
experiment. The negative curvature of the root in response
to indirect stimulation is by no means unique, for a similar
effect is induced by indirect stimulation of the shoot, as
shown in the following experiment.

Effect of Indirect Unilateral Stimulation

OF Shoot

Experiment Sy. — I took a young shoot of Vicia Faba,
and after finding its region of maximum growth, applied
unilateral light at a distance of 5 cm. below that region, the

Fig. 80. Negative response of shoot under indirect
unilateral photic stimulation (Vicia Faba).

moment of application being indicated by the vertical
arrow in the record. The stimulation was, therefore,
indirect. Light is seen to have induced negative curvature,
shown by the down-curve, which persisted for a while
after the cessation of light marked by the horizontal arrow
within the circle. There was a subsequent recovery which
was complete (fig. 80).

I have obtained a similar negative response on the

STIMULATION OF ROOT I43

unilateral stimulation of the tip of the shoot, when the tip
happened to be at a certain distance from the region of active
growth. The final result is somewhat dependent on the
conducting power of the intervening length of tissue. In
cases where the tissue is highly conducting, the excitatory
negative impulse, reaching the growing region, induces a
positive curvature.

Effect of Simultaneous Stimulation of the Tip
AND OF the Growing Region of Root

When the growing region of the root is directly stimu-
lated by unilateral light, the response is a positive curva-

FiG. 81. Effect of simultaneous unilateral stimulation
of the tip and of the growing region of root (Ipomoea).

Successive dots at intervals of 10 seconds.

ture in contrast with the negative curvature induced
by indirect stimulation {cf. also Experiment 86). I next
observed the effect of simultaneous direct and indirect
unilateral stimulation of the root.

144 CHAP. XIII. DIA-PHOTOTROPISM

Experiment 88. — In a specimen of Ipomoea both the
tip and the growing-point were simultaneously stimulated
b}^ unilateral light from a loo-candle-power Pointolite.
The effect of stimulation of the tip alone would, as already
shown, be negative, while that of the growing region would be
positive. Since the resultant effect was positive, the response
of the growing region, at least in the present case, was
relatively the more effective. The positive response occurred
20 seconds after the incidence of light and reached a maxi-

,<:^gp^

Fig. 82. Effect on shoot and root of unilateral exposure to light

(Sanchezia).

mum after 70 seconds. After this there was a reversal of
response from positive to negative, due to transverse con-
duction of excitation across the organ from the proximal
to the distal side (fig. 81). The extent of this reversal
obviously depends (i) on the transverse conductivity and
thickness of the root, and (2) on the intensity and duration
of the illumination.

Three characteristic types of response of the root to light
are met with :

1. Positive curvature ;

2. Neutralisation ; and

3. Negative curvature.

Referring to the third type, a pronounced negative
curvature occurs under prolonged unilateral stimulation of

ROOT OF SINAPIS I45

the root, especially when it is thin, a condition which
facilitates transverse conduction of excitation to the distal
side.

Should the power of transverse conduction in the shoot
be less than that in the root, then under the action of the
same unilateral stimulation b}^ light the stem would exhibit
a positive, while the root would show a negative, curvature.
This is seen in the illustration (fig. 82), reproduced from
a photograph, of the responsive curvature of stem and
root of Sanchezia nobilis. The stem, being thick, failed
to transmit excitation to the distal side ; the phototropic
curvature was therefore positive. But transverse conduc-
tion occurring in the root induced the negative phototropic
curvature.

Phototropic Curvature of Root of Sixapis

I next investigated the effect of unilateral light on the
thin root of Sinapis by taking a continuous record of its
movements. The root was too thin to give a direct record
by exerting a pull on the recording lever. The experiment
was therefore modified as follows :

Experiment 89.^ — The plant was mounted with its root
irhmersed in a cubical glass vessel. A microscope with
micrometer eye-piece was focused on the tip of the root.
Light from a lOO-candle-power Pointolite was applied
laterally on one side, say the left. The incidence of the
lateral light at first induced a movement towards the light
(positive curvature), which went on increasing for 15 minutes,
after which there was a turning away from light. There
was neutralisation in the course of about 28 minutes, after
which an increasing negative . curvature was produced.
The following table gives readings of the scale taken every
5 minutes, the positive readings indicating the movement of
the root towards, and the negative readings that away from,
the light.

146 CHAP. XIII. DIA-PHOTOTROPISM

Table XVI. Effect of Unilateral Light on the Root

OF SiNAPIS.

Responsive movement, Positive

Intervals of time

and Negative

0

5 minutes

0

+ I-o

10

+ 1-8

15

+ 2-1

20 ,,

+ 2-0

25

+ I-3

30

+ 0-5

35

— 0-3

40

- 0-9

45

~^'\ '

50

'

— 1-8

55

— 2-1

60

- 2-4

The curve obtained from the above data (fig. 83) clearly

shows how the root at first moves towards the light

anc

a +2

k

/

\

/

\

1^

/

\

5 +»

J

\

/

\

.!

/

>L

•w /

\

1°

f

\

\

R.

\

b

\

5 -I

>

\

X

&>

N

1

1-2

<

O 20' 40' 6O'

€

Time

Fig. 83. Response of root of Sinapis under continued unilateral

action of light.
The sequence of response is positive, neutralisation, and

pronounced negative.

then moves away from it, passing through the stage of
neutralisation,

SUMMARY 147

Summary

The normal effect of direct unilateral stimulation by light
is a positive curvature.

Transverse conduction of excitation to the distal side
induces neutralisation, that is, a dia-phototropic attitude
of the organ.

Stronger and long-continued action of light transforms
the positive into a negative curvature. An important con-
tributory factor in the reversal of response is the fatigue-
relaxation of the proximal side of the organ.

Investigation of the action of light on the root shows
that the irritability of the root is in no way different from
that of the shoot.

The effect of unilateral illumination of the tip of the
root is the indirect stimulation of the growing region by
transmitted impulse; this induces acceleration of the rate
of growth on the stimulated side which causes negative
curvature. The shoot gives a similar response to indirect
stimulation.

In a thick root, in which there is no transmission of
excitation to the distal side, the response is a positive
phototropic curvature.

In a thin root, such as that of Sinapis, the sequence of
response is positive, dia-phototropic, and finally negative
due to the transmission of excitation across the thin organ.

It was want of knowledge of the preliminary positive
curvature of the root that led to the erroneous inference
that the root possesses an irritability specifically different
from that of the shoot.

CHAPTER XIV

THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS

The characteristics of the phototropic curve will be studied
in greater detail in this chapter, any point in the curve
exhibiting the relation between the stimulus of light and
the effect induced by it. The effective intensity of stimula-
tion has been shown to depend on the duration of exposure
(p. 112). It has been further shown (i) that in the simpler
cases the reaction under continued unilateral stimulation by
light is one of increasing phototropic curvature ; (2) that
the tropic curvature is modified by the transverse conduction
of excitation across the organ from the proximal to the
distal side.

I will first consider the contractile reaction induced in
a growing organ under continued action of diffuse external
stimulation, either electric or photic. The reaction-curve
is obtained by making the plant record, by means of the
High Magnification Crescograph, the increasing retardation
of its growth (incipient contraction). Two records of
the effects of continuous electric and photic stimulation
have already been given (cf. fig. 36), in which the ordinate
of the curve represents the incipient contraction and the
abscissa the duration of application of the stimulus. The
contraction was seen to be slight at the first stage ; it
increased rapidly in the second, after which it declined and
reached a limit at the third stage. The excitator\^ con-
traction is thus not constant throughout the whole curve,
but undergoes very definite and characteristic variation.
To facilitate explanation of certain characteristic effects,

THE SIMPLE PHOTOTROPIC CURVE I49

I use the necessary new term susceptibility to indicate
the relation between the stimulus and the resulting
excitatory contraction ; this latter will often be designated
by the shorter term excitation :

.-.,., Excitatory contraction

Susceptibility = -^rr- \

•^ -^ Stimulus

Different organs of plants exhibit unequal suscepti-
bihties ; some undergo excitation under feeble stimulation,
while others require more intense stimulation to induce it.
Even in one identical organ the susceptibility will be found
to undergo a characteristic variation, being feeble at the
beginning, considerable in the middle, and becoming feeble
once more towards the end.

The Simple Phototropic Curve

The simple phototropic curve is obtained, as already
explained, by making the plant-organ record its movement
under continuous action of light applied on one side. Curves
were obtained in this way, of both pulvinated and growing
organs.

Characteristic Curve of Erythrina indica

Experiment 90.— A parallel beam of light from a Nernst
lamp was thrown on the upper half of the pulvinus of
a leaf of Erythrina, and the increasing positive curve was
recorded on a smoked-glass plate which was moved past the
writer by a clockwork at a uniform rate. The pulvinus, it
should be remembered, does not possess any transverse con-
ductivity. The record (fig. 84) is exactly reproduced from
the original by photomechanical process. The successive
horizontal dot-intervals of 20 seconds represent equal
increments of stimulation during successive equal lengths
of exposure. The vertical distances between the dots
represent, on the other hand, the corresponding increase

150 CHAP. XIV. THE PHOTOTROPIC CURVE

of tropic curvature. The gradient at any point of the curve,
the increment of tropic curvature divided by the increment
of stimulation, gives the susceptibihty for that point. The
following table shows how the susceptibility undergoes
variation through the whole range of the curve. The

Fig. 84. Simple characteristic curve of phototropic reaction.
Excitatory contraction 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).

average susceptibility for each point has been calculated
from the data furnished by the curve.

The induced contraction which results in tropic curva-
ture is seen to be increased very gradually from the zero
point of susceptibility, at which there is no responsive move-
ment. After this, the increase of susceptibility is very rapid,
the maximum rate of increase being reached at the point 11
of the curve. In the median range the susceptibihty is
constant, each equal increment of stimulation inducing an

STIMULATION OF SUBTONIC ORGAN

151

equal amount of tropic curvature. The maximum positive
curvature in the present case was attained in the course of
9 minutes. The period of the attainment of this maximum
depends on the moto-excitabihty of the tissue and on the
intensity of the incident stimulus. The phototropic curve
(cf. fig. 86) of growing organs exhibits characteristics similar
to those of pulvinated organs.

Table XVII. — The Variation of Susceptibility at Different
Points of the Tropic Curve.

Successive points

Susceptibility for

Successive points

Susceptibility for

in the curve

excitatory contraction

in the curve

excitatory contraction

I

1

0

14

6-6

2

0-187

15

4-4

3

0-44

16

2-5

4

0-625

17

1-87

5

0-875

18

1-5

6

1-25

19

I -12

7

1-87

20

0-937

8

3-12

21

0'75

9

5'0

22

0-562

10

6-25

23

0-375

II

8-75

24

0-25

12

8-87

25

0-187

13

8-12

26

0-062

The organ, of which the phototropic curve is given in
fig. 84, was in optimum condition ; the power of transverse
conduction was practically absent. I now take up the
more complex case of an organ which was in a shghtly
subtonic condition at the beginning, and also possessed
transverse conductivitv.

Effect of Stimulation of Subtonic Organ

It is unfortunate that the terms usually employed in
the description of stimulus are so indefinite. A stimulus
which is just sufficient to induce excitatory contraction is
termed minimal, while an intensity below this minimal
[subminimal stimulus) is tacitly assumed to be ineffective.

152 CHAP. XIV. THE PHOTOTROPIC CURVE

The employment of sensitive recorders has, however,
enabled me to discover the important fact that subminimal
stimulus is bv no means ineffective. It induces a definite
reaction of expansio7i, antecedent to the normal contrac-
tion. A critical intensity demarcates the subminimal from
the minimal stimulus, the respective responses being of
opposite signs, an expansive positive and a contractile
negative.

I have already explained that the critical intensity of
stimulus varies in different species of plants. Thus the
same intensity of light which induces a retardation of growth
(negative variation) in one species may cause an enhance-
ment of the rate (positive variation) in another. The critical
point, moreover, depends on the tonic level of the organ ;
in an optimum condition the critical point is low, inas-
much as a feeble stimulus induces the excitatory reaction.
In a subtonic tissue, on the other hand, the critical point
is high, necessitating a relatively strong and long-continued
stimulation to induce the normal reaction. Stimulation
itself is found to raise the tonic level of the tissue, so that
the response is transformed from expansion to normal
contraction.

The physico-chemical processes underlying these opposite
responses have, for convenience, been distinguished as the
* A ' and ' D ' changes. The expansive assimilatory ' build-
ing-up ' process A is associated with an increase of potential
energy of the system. The contractile dissimilatory ' break-
down ' D is, on the other hand, concomitant with an
evolution or run-down of energy.^

Returning to the consideration of the action of uni-
lateral stimulation by light in inducing phototropic curva-
ture of an organ in a slightly subtonic condition, the directly
stimulated side exhibits expansion at the first stage, causing
a negative curvature away from light. But by the con-
tinuous action of stimulus the subminimal becomes minimal
and then maximal. The negative will thus be transformed

^ The Motor Mechanism of Plants (1928), p. 56.

CURVE OF PULVINATED ORGAN 153

into normal positive curvature towards the light, which
will reach a maximum value.

The organ has been assumed to possess the capacity for
transverse conduction. After the attainment of maximum
positive curvature, the conduction of excitation across the
organ from the proximal to the distal side will bring
about a complete neutralisation, to be succeeded by an
actual reversal into negative curvature. These theoretical
considerations will now be subjected to experimental
tests.

The reversal of positive into negative phototropic
curvature under strong unilateral illumination usually
takes place under an exposure so prolonged that it is difficult
to represent the different transformations in a single curve
that can be reproduced on a page. This can only be done
wdth a thin specimen, so that the transverse conduction
of excitation which induces reversal may take place within
a reasonable time. A complete phototropic curve was thus
obtained in an identical specimen, which exhibited all the
characteristic transformations. For a pulvinated organ,
I employed the thin pulvinus of the terminal leaflet of
Desmodimn gyrans. For growing organs, young seedlings
of Zea Mays were found suitable. In both the above cases
the specimens were in a slightly subtonic condition.

Complete Phototropic Curve of a Pulvinated

Organ

Experiment 91. — A continuous record was taken (fig. 85)
of the action of light from a 50-candle-power incandescent
electric lamp incident on the upper half of the pulvinus of
the terminal leaflet of Desmodium. This gave rise first
to the abnormal negative curvature induced by subminimal
stimulation. The curve then proceeded upwards in the
direction of positivity, at first slowly, then rapidly. The
maximal positive curvature was attained in the course of

154

CHAP. XIV. THE PHOTOTROPIC CURVE

i8 minutes and remained constant for a short time, as seen
in the more or less horizontal record at the top of the curve.
After this, transverse conduction of excitation from the

Fig. 85. Complete phototropic curve of a pulvinated organ.
Positive curvature above, and negative curvature below,
the horizontal zero line. Preliminary negative phase of
response due to subminimal stimulus. The positive in-
creases, attains a maximum, and undergoes a reversal.
Successive dots at intervals of 30 seconds. Abscissa repre-
sents duration of exposure and quantity of incident light.
(Terminal leaflet of Desmodium gyrans.)

proximal to the distal side began to take place, inducing
neutralisation which was completed in the course of
24 minutes. Subsequently to this, there was a reversal of
phototropic curvature into a pronounced negative.

Complete Phototropic Curve of a Growing Organ

Experiment 92. — ^A seedling of Zea Mays was sub-
jected continuously to unilateral light from a small arc-
lamp for 2 hours. The characteristics of this curve are
very similar to those of Desmodium. The subminimal
stimulation induced, at the first stage, a negative curvature ;
this was transformed into positive after an exposure of

CURVE OF GROWING ORGAN 155

10 minutes, and neutralisation was completed after a further
exposure of 43 minutes. The subsequent response became
completely transformed into negative (fig. 86).

In the complete phototropic curve of pulvinated and of
growing organs, four distinct stages may be distinguished :

1. The stage of subminimal stimulation giving abnormal

negative curvature ;

2. The stage of increasing positive curvature which

reaches a maximum ;

3. The stage of neutraHsation ; and

4. The stage of complete reversal into pronounced

negative curvature.

Fig. 86. Complete phototropic curve of a growing organ

(Zea Mays).

The phototropic curve crosses the zero line of the abscissa
twice ; the first crossing takes place upwards at the critical
point of stimulation which demarcates the subminimal
from the minimal. The second crossing downwards occurs
beyond the point of complete neutralisation.

Neglecting the preliminary negative due to the action
of subminimal stimulus, the tropic curve under photic
stimulation obeys what is known as Weber's Law. But this
is appHcable only for a limited range of stimulation ; it fails

156 CHAP. XIV. THE PHOTOTROPIC CURVE

in the region of subminimal stimulation, where the physio-
logical reaction is qualitatively different, being expansion
instead of contraction.

Summary

A simple phototropic curve exhibits, in the first part,
a slow ascent ; in the second part the gradient is steep,
indicative of a rapid increase of positive curvature ; in
the third part the rate of increase is uniform ; and in the
last part the curve tends to become horizontal, indicative
of maximal positive curvature.

The susceptibility to stimulation is not the same, but
varies in different points of the curve. It is feeble at the
beginning, it increases rapidly at the second stage, and
reaches uniformity at the median range. Further on, the
susceptibility undergoes a rapid decline.

Complicating factors are introduced by a subtonic
condition of the tissue, and by the conduction of excitation
across the organ from the proximal to the distal side. The
complete phototropic curve exhibits, in such circumstances,
a negative curvature in the first part, due to physiological
expansion under subminimal stimulation. The curve then
crosses the abscissa upwards, after which the curvature
attains its positive maximum. Subsequent to this is the
stage of neutralisation brought about by fatigue of the
directly stimulated proximal side, and by percolation of
excitation across the organ to the distal side. Further
stimulation causes the curve to cross the zero line in the
downward direction, the phototropic curvature being reversed
into negative.

CHAPTER XV

THE PHOTONASTIC PHENOMENA

Phototropic response, positive or negative, has been shown
to be determined by the directive action of hght. There is,
however, a different class of phenomena already referred
to in previous chapters which is supposed to be independent
of the directive action of incident stimulus. This is the
so-called photonastic action of light. Strong sunlight, for
example, brings about a para-phototropic movement, by
which the apices of leaves or leaflets turn towards or away
from the strong source of light. The upward movement of
the leaflet of Erythrina has already been described. Far
more anomalous are the movements of the leaflets of
Averrhoa Cammhola and of Mimosa pudica. In Averrhoa
the effect of strong light from above is a movement down-
wards, away from light. In Mimosa the movement under
similar circumstances is precisely the opposite.

The above description relates to the action of light from
above. What happens when the hght acts from below ?
The results are apparently even more inexplicable, as would
appear from the detailed accounts given below.

Response of the leaflet of Averrhoa. —Strong light acting
in a downward direction from above induces, as already
stated, a movement downwards, away from light. Speak-
ing from the phototropic point of view, this may be
described as negative phototropism. But when strong light
is incident on the leaflet in an upward direction from below
the response is also a down-movement towards the light, and
has therefore to be described as positive phototropism. This
leads to the paradoxical conclusion that an identical organ
possesses two different irritabilities, negative and positive.

158 CHAP. XV. PHOTONASTIC PHENOMENA

Response of the leaflet of Mimosa. — Equally anomalous
appears to be the response of the Mimosa leaflet. Strong
light acting from above induces a positive phototropic
movement upwards. But when the direction of the incident
light is changed so as to act from below, the responsive
movement is still upwards, that is to say, a negative photo-
tropic movement away from light.

Such paradoxical reactions led to the employment of
the term photonasty to describe this class of phenomena,
supposed to be totally unrelated to normal phototropic
action and due to a different kind of irritability and a
different mode of response. Is there really a hiatus between
phototropic and photonastic reactions, or is there a possi-
bility of discovering continuity between them ? Investi-
gations on the effect of light on the main pulvinus of Mimosa,
acting alternately from above and below, gave a valuable
clue to the solution of the problem.

It is necessarv in this connection to bear in mind the
anatomical and physiological characteristics of the two
halves of the pulvinus, the upper and the lower. The
pulvinus may summarily be described as consisting mainly
of two masses of cortex, separated by a thin flexible vascular
strand. The relative moto-excitabilities of the two halves
of such an anisotropic dorsiventral organ are easily demon-
strated by the application of diffuse stimulation, which causes
an impulsive fall of the leaf, proving the predominant
excitability of the lower half. The excitability of the upper
half of the pulvinus is, however, not altogether absent,
but relatively feeble, as will be presently demonstrated
by local application of stimulus on that half of the
organ.

I now describe the characteristic effects of local
application of :

1. Feeble stimulus on the upper half ;

2. Strong stimulus on the upper half ;

3. Feeble stimulus on the lower half ; and

4. Strong stimulus on the lower half.

LOCAL PHOTIC STIMULATION

159

My investigations relate to the effects not only of photic
but also of other modes of stimulation, the results being
essentially similar in all cases. I shall first deal with the
effect of photic stimulation, for which special means have
to be devised for the local application of stimulus of feeble
and of strong intensity.

Local Application of Photic Stimulus

Feeble or moderate stimulation. — -This is secured by a
device consisting of an 'incandescent 4-5-volt pea-lamp
placed at one end of a tube, the other end of which is pro-
vided with a focusing lens L. The incandescent filament
is adjusted horizontally, so that a sharp line of light can be

Fig. 87. The incandescent electric lamp-holder. A line of light
can be directed vertically downwards on to the upper half, or
vertically upwards on to the lower half, of the pulvinus.

thrown across the middle of either the upper or lower half
of the pulvinus. The lamp-holder can be raised or lowered ;
it can be inclined vertically downwards or upwards for the
specific purpose of the experiment (fig. d)y).

Strong stimulation. — The source of light is a self-feeding
small arc-lamp which maintains approximately constant
light. The focusing lens gives a horizontal and slightly
convergent beam of light ; this is thrown vertically either

i6o

CHAP. XV. PHOTONASTIC PHENOMENA

downwards or upwards by a totally reflecting prism
(fig. 88). The two devices just described ensure accurate

:;!l:

^ "" *" ~~-- —

- ^

^

^

Fig. 88. Arc-lamp with totally reflecting prism for application
of strong light vertically downwards or upwards.

investigation of the effects of local application of feeble or
strong light on the upper and lower halves of the organ.

Effect of Light on Upper Half of the

pulvinus

Experiment 93. Effect of feeble or moderate intensity
of light. — The record, with moderate magnification, w^as

Fig. 89. Records of effect of feeble light (left illustration) and
of strong light (illustration to right) on upper half of pulvinus
(Mimosa). Successive dots at intervals of i second. In this
and in the following records up-movement is represented by
down- curve and vice versa.

taken on a fast-moving plate, the successive dots being
at intervals of a second. The record to the left (fig. 89)

PULVINUS OF MIMOSA l6l

shows that the responsive movement was upwards towards
the Hght, and that this was initiated after a latent period
of 6 seconds. The positive curvature was continuously
increased during the whole period of illuminatiouy lasting
for 35 seconds or even longer. Hence moderate unilateral
photic stimulation of the less excitable upper half of a dorsi^
ventral organ gives rise only to a positive phototropic curvature,
The effect of stronger light is, however, strikingly different.

Experiment 94. Effect of stronger light. — Stronger light
from the arc-lamp induced at first an up-movement towards
the light after a latent period of 5 seconds. The positive
curvature increased continuously for 10 seconds, after which
a very striking change occurred. The erectile movement,
due to contraction of the upper half of the pulvinus,
became reversed into a down-movement, evidently by the
arrival of the excitatory impulse across the boundary line
between the upper and lower halves of the organ. The
slow fall of the leaf at the beginning was due to the suc-
cessive contraction of the cortical tissue of the lower half
of the organ ; a short while after this the contractile fall
of the leaf became very abrupt, the rapidity of fall becoming
so great that a scratch, and not a dot, was produced as
the writer was jerked off the plate (right record, fig. 89).

Moderate stimulation of the upper half of the pulvinus
is thus seen to give rise to a positive curvature towards the
Hght, while stronger stimulation gives rise to a positive
followed by a negative. What can be the reason for this
behaviour ? The probable explanation is, that under
moderate stimulation the downward percolation of exci-
tation along the vertical layers of cortical cells is a slow
process on account of the resistance offered by the semi-
conducting tissue. But under stronger stimulation the
partial block becomes forced, and the excitation not only
percolates through the upper half of the pulvinus but also
reaches the lower half. The moment of arrival of excitation
at the upper boundary of the lower half of the organ is, as
already stated, signahsed by the inversion of the curve.

l62 CHAP. XV. PHOTONASTIC PHENOMENA

The velocity of transmission of excitation through the
upper half of the pulvinus can be found from the data
given by the record. The thickness of the pulvinus was
2 mm., that of the upper half was i mm. approximately.
The time of vertical transmission can be found from the
record, being the interval between the application of
stimulus and the moment of reversal from up to down
movement of the leaf. In the present case it was found
to be i8 seconds. From this the rate of transmission across
the upper half of the organ can be ascertained :

Velocity of transmission = — = o -05 mm. per second.

This velocity I find to vary from o-oi to o-i mm. per
second, for the following reasons :

The velocity is increased {a) when the thickness of the
pulvinus is small ; {b) when the temperature is near the
optimum ; (c) when the season is favourable ; {d) when
the tissue is in a favourable tonic condition ; and (e) when
the stimulus is strong. The velocity is decreased under
opposite conditions.

Effect of Light on the Lower Half of the

Pulvinus

Experiment 95. Effect of moderate light. — After a
latent period of 7 seconds the leaf began to fall downwards

Fig. 90, Effect of feeble (left record) and of strong light (right record)
on lower half of pulvinus (Mimosa). Dot-intervals i second.

GRADATION OF EXCITABILITY 163

towards the light (positive curvature). This slow down-
movement continued for 40 seconds or more (left record,

fig- 90)-

Experiment 96. Effect of strong light. — The leaf

responded by a fall, that is, positive curvature, after
a latent period of 4 seconds. This fall was slow for about
7 seconds, after which it became very abrupt (right record,
fig. 90).

I would here draw special attention to the following
statement of results regarding the effect of strong light
applied on the upper and lower halves of the pulvinus :

1. Strong light applied above induces a preliminary and

short-lived up-response, detectable only by a sensitive
method of record. This up-response is followed by
a pronounced down-response, that is, with the fall
of the leaf.

2. Strong light applied below induces a fall of the leaf and

a down-response from the beginning, without any
other change.

3. The effect apparently is the same, whether light be

applied above or below, the direction of the responsive
movement being finally determined by the contraction
of the more excitable half of the organ.

Physiological Detection of Gradation of
Excitability of Different Layers

Some of the characteristics of the response led to the
important discovery of a gradation of excitability in the
different layers of tissue in the lower half of the pulvinus.
An inspection of the curve of response of strong Hght
applied on the lower half will make this clear {cf. right
record, fig. 90). The responsive fall of the leaf, due to
percolation of excitation upwards from layer to layer, was
at first slow. It became very abrupt as soon as the exci-
tation reached a certain tissue of the cortex intermediate
between the lowermost and uppermost layers in the lower

164 CHAP. XV. PHOTONASTIC PHENOMENA

half of the pulvinus. The same characteristic is observed
when strong hght acts from above. The arrival of the
excitatory impulse at the uppermost layer of the lower
half induces a reversal of response from up to down
movement. This excitatory fall is, however, slow at the
beginning, becoming very rapid as soon as the excitation
reaches the highly excitable layers lower down [cf. fig. 89).
The two results confirm the conclusion that the position of
the most excitable tissue is somewhere in the middle of the
lower half of the pulvinus. The results of experiments on
which this conclusion is based are by no means accidental,
but were obtained under all modes of stimulation applied
on the upper and lower halves of the organ.

To facilitate explanation, I give a diagrammatic repre-
sentation of the longitudinal section of the pulvinus, in
which the number of layers has been reduced. The vas-
cular bundle F divides the upper from the lower half of
the organ. The upper cortex extends from a to F ; b to d
represents the extent of the lower cortex ; c represents the
hypothetical intermediate layers, the moto-excitability of
which is exceptionally high (fig. 91).

Speaking generally, one half of the pulvinus, commonly
the lower half, is more excitable than the other ; it is this
more excitable half that has a determining influence on
the movement of response. The positive impulse due to
indirect stinmlation causes an enhancement of turgor and
expansion of this half, giving rise to a positive erectile
movement. The excitatory impulse reaching it later causes
contraction and fall of the leaf.

Now, if there is a gradation of excitability in the lower
half, then the most excitable as well as the most contractile
layer c will function as the essential responding layer.
The existence of a specially excitable layer in the lower
half of the pulvinus is indicated by the foregoing experi-
ments ; for percolation of excitation, either from above or
below, to such a layer would account for the abrupt change
from a slow to an abrupt fall.

GRADATION OF EXCITABILITY 165

When the stimuhis was apphed from above or from
below the pulvinus, careful observation of the recording
lever showed a curious erectile twitch preceding the normal

Fig. 91. Diagrammatic representation of the different layers
in a longitudinal section of pulvinus.

Cortex of upper half extends from a to vascular bundle [f ;
b, uppermost layer of lower half ; c, hypothetical most
sensitive layer ; d, lowest layer of cortex of lower half.
Vertical continuous arrow represents application of light
on upper, dotted arrow on lower, side.

response. This was especially the case when the tonic con-
dition of the specimen was not exceptionally high, so that
the rate of percolation of excitation was relatively slow.
A satisfactory record of the preliminary twitch, due to
positive impulse, was obtained by the employment of a
higher magnification.

i66

CHAP. XV. PHOTONASTIC PHENOMENA

Triphasic Response

Experiment 97. Effect of application of strong light on
upper half of pulvinus. — The record given in fig. 92 gives

Fig. 92.

Fig. 93.

Fig. 94.

Fig. 92. Triphasic response on application of light L above

(Mimosa).

Preliminary erectile twitch followed by erectile response {a — b)
due to contraction of upper cortex. Reversal of response
to negative fall, slow from 6 to c and rapid beyond c. (See text.)

Fig. 93. Parallel effect of radio-thermal stimulation T applied

above.

Fig. 94. Effect of application of strong light L on lower half

of pulvinus.
Erectile movements represented by down-curve and vice versa.

a striking illustration of the triphasic response. There
was at first a short-hved erectile twitch, evidently due to
the quick transmission of positive hydraulic impulse which,
reaching the most excitable region c, induced expansion and
erection of the leaf. This impulse was quickly exhausted.

LEAFLET OF AVERRHOA 167

as evidenced by partial recovery ; the contractile response
of the upper half of the pulvinus next induced an erectile
movement from atob. The excitatory impulse then reached
the uppermost layer of the lower half of pulvinus and in-
duced reversal of response, which was slow at the beginning
from b to c. But when the impulse reached the most
excitable layer c, there was an abrupt movement of fall
indicated by the scratch-line in the record.

I had been greatly puzzled by the triphasic electromotive
response which sometimes appeared in the records. The
presence of a highly excitable layer in the tissue would
appear to offer an explanation of the phenomenon.

Experiment 98. Effect of radio-thermal stimulation. —
The fact that triphasic response is universal is borne out by
the record taken under radio-thermal stimulation T, applied
on the upper half of the organ (fig. 93). The results are
essentially similar to those in the previous experiment.
The only difference is that on account of the stronger
intensity of the stimulus the reversal to negative occurred
somewhat earlier.

Experiment 99. Effect of application of strong light on
lower half of the pulvinus. — The preliminary twitch was still
erectile though the stimulus was applied below. Stimulation
of the outermost layer initiated the positive impulse which,
reaching the layer c, induced the erectile twitch. The
excitatory contraction of the layers from d io c caused
a slow fall of the leaf, which was next transformed into a
very rapid fall by the contraction of the highly excitable
and contractile layer c (fig. 94).

Thus, by the application of adequately sensitive physio-
logical tests, the gradations of excitability in the interior of
a tissue can be revealed.

Response of the Leaflet of Averrhoa

Diffuse stimulation induces a downward folding of the
leaflet, proving that the excitability of the pulvinule is
greater on the lower side.

1 68 CHAP. XV. PHOTONASTIC PHENOMENA

Experiment loo. Application of strong light from above. —
The response was a brief erectile movement, due to the
contraction of the directly stimulated upper side of the

Fig. 95. Response of leaflet of Averrhoa to strong light from

above.

organ ; it was positively phototropic, i.e, movement towards
the source of illumination. The response was subsequently
reversed to strong negative in consequence of transverse
conduction of excitation (fig. 95).

Fig. 96. Effect of strong light applied below (Averrhoa).

Experiment loi. Application of strong light from below. —
The response was positively phototropic, that is, towards the
source of illumination. As the more excitable lower side
was directly stimulated, its predominant contraction was
incapable of being neutralised by excitation transversely
transmitted to the less excitable upper side (fig. 96).

leaflet of mimosa 169

Response of the Leaflet of Mlmosa

Diffuse stimulation causes upward closure of the leaflet,
proving that the excitability of the pulvinule is greater
on the upper side.

Experiment 102. Effect of strong light on upper side. —
The result is a very pronounced up-response, due to direct
stimulation of the more excitable side, which was not re-
versed by transverse conduction of excitation to the lower
less excitable side of the pulvinule (fig. 97).

Fig. 97. Fig. 98.

Fig. 97. Effect of strong light on upper side of Mimosa leaflet.
Fig. 98. Effect of strong light on lower side of leaflet.

Experiment 103. Effect of strong light on lower side. — ■
A transitory down-response occurred, due to contraction of
the directly stimulated lower side ; this was subsequently
reversed into a strong up-response by transverse conduction
of excitation to the more excitable upper side (fig. 98).

These responsive manifestations find their fullest ex-
planation in the previous experiments on the main pulvinus
of Mimosa under the action of strong light, acting from
above or below. Had a delicate means of record not been
available, the gradual transition from positive to nega-
tive phototropic curvature would have passed unnoticed.

170

CHAP. XV. PHOTONASTIC PHENOMENA

A continuity is thus established between phototropic and
photonastic reactions, rendering the assumption of specific
sensibihty for each class of phenomena quite unnecessary.

The effects described are equally true of photic, thermal,
and electric stimulations. From these results of observation
and experiment on the movements of plants, the following
laws, which are of universal application, can be deduced :

1. All forms of moderate or strong stimulation of organs

in normal tonic condition induce contraction as
their direct, and expansion as their indirect, effect.

2. The response to unilateral stimulation is a positive

curvature, effected by contraction of the proximal
and expansion of the distal side.

3. Transverse conduction of excitation induces con-

traction of the opposite side, consequently neu-
tralising or reversing the positive responsive
curvature.

4. These effects are accentuated by the differential

excitability of the two halves of an anisotropic
organ.

The fundamental reactions of pulvinated and growing
organs to direct and indirect stimulation are summarised
in the following table :

Table XVIII. — Mechanical and Electric Responses in
Motile and in Growing Organs.

Inducing cause

Reaction

Mechanical response

Electric response

Direct stimula-
tion

Indirect stimu-
lation

Contraction
Expansion

Fall of leaf ; nega-
tive response

Erection of leaf ;
positive response

Negative response
Positive response

I give a classification of some of the principal types of
response to light that are met with in practice. In anisotropic
organs, stimulus is supposed to be applied on the less
excitable side.

TYPES OF RESPONSE I7I

1. Radial Organ :

{a) Thick shoot ; transverse conduction of excita-
tion neghgible ; positive phototropic response,
e.g. positive curvature.

(b) Thin shoot or root ; transverse conduction of
excitation possible ; sequence of responses :
positive, neutral, and negative, e.g. reversal of
positive into negative in stem of Oryza and in
root of Sinapis.

2. Pulvinated Organ :

(a) Pulvinus thick ; transverse conduction of excita-
tion negligible ; positive response ; pronounced
concavity of the excited side, e.g. midday sleep
or para-phototropism of Erythrina indica,
Clitoria Tcrnatea, and others.
(h) Pulvinus thin ; transverse conduction of ex-
citation pronounced ; transient and hitherto
unnoticed positive followed by predominant
negative ; application of stimulus on the opposite
and more excitable side produces movement
in the same direction, now positive response.
The result would thus appear to be independent
of the direction of light. Examples are found
in the photonastic movements of the leaflets
of Mimosa pudica and of Averrhoa and Bio-
phytum.
Owing to varying combinations of numerous unknown
factors the phenomena of growth and its responsive varia-
tions under stimulation present many perplexities. For
instance, take the effect of external stimulus on growth.
Here subminimal stimulus induces one effect, and moderate
stimulus the very opposite. Should the tonic condition
of the plant happen to be below par, the effect of stimulus
will be an abnormal acceleration of growth, but during the
course of the experiment (owing to the continued action of
stimulus) the effect will mysteriously revert to the normal
retardation. The point of application of stimulus will

172 CHAP. XV. PHOTONASTIC PHENOMENA

introduce further complication, indirect stimulation in-
ducing an effect precisely the opposite to that of direct
stimulation. The response to unilateral stimulation is
further modified by transverse conduction of impulse, by the
intensity of stimulation, and by the differential excitability
of the organ. In an actual experiment the permutation and
combination of these different factors will give rise to effects
which, no doubt, appear at first sight to be highly capricious.
A given modification of response can, however, be traced to
a corresponding definite variation in the intensity and point
of application of stimulus, or in the tonic condition of the
reacting organ.

The following is a diagrammatic representation of the
typical cases in which an arrow represents the direction of
incident light :

a

••" •. • • ••

I • • If -r

ft t !

+ +(+-)- +(+-)— + —

Fig. 99. Diagrammatic representation of different types of
response to unilateral stimulation. Presence of dots
represents possibility of transverse conduction.

+ represents positive curvature; + (H ) — represents sequence

of positive, neutralisation, and negative. Thick -^ repre-
sents predominant negative.

a, Radial thick organ. Transverse conduction absent.

Response positive.

b, Radial thin organ. Sequence of response : positive, neutral,

and negative.

c, Anisotropic thick organ. Thick line represents the more

excitable distal side. Sequence of response : positive,
neutral, and pronounced negative.

d, Anisotropic thin organ. High transverse conductivity.

Sequence of response : positive, quickly masked by negative.
When light strikes on the opposite side, the sign of response in a
and b will remain unchanged. In c and d the effect will be
only positive.

SUMMARY 173

Summary

By the application of sensitive physiological tests,
gradation of excitability has been discovered in the layers
of tissue in the pulvinus of Mimosa.

In an organ with pronounced physiological anisotropy,
in which the distal lower is far more excitable than the
upper or proximal side, stimulation of the proximal side is
followed by a transverse conduction of excitation which
brings about a greater contraction of the lower side. The
sequence of response is then positive, neutral, and very
pronounced negative. The terms proximal and distal are
used in the sense of stimulated and unstimulated sides.

When the stimulus is applied on the more excitable
lower side of the organ, the result is a predominant con-
traction of that half ; the resulting curvature cannot be
neutrahsed by transverse conduction of excitation to the
feebly excitable distal side.

The phototropic and photonastic movements are not
unrelated phenomena, but there is continuity between them.

. CHAPTER XVI

RADIO-THERMOTROPISM

Tropic curvature induced by different rays of light has
already been studied. It was found 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, the effect of which is complicated
by that of rise of temperature. The effects of these two
factors are antagonistic, and to this must be ascribed the
contradictory results that have been obtained by different
observers, of which Pfeffer ^ gives the following summary :

* 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
Lepidivim sativum and Zea Mays, as w^ell as sporangiophores
of Phycomyces, curved towards a hot iron plate emitting
dark heat-rays. Steyer has, however, shown that the
sporangiophore 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 investi-
gated the radicles of seedlings by growing them in boxes of
sawdust, 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

1 Pfeffer, ihid. vol. iii. p. 776.

RADIO-THERMAL STIMULATOR 175

same both under variation of temperature (as in experiments
with unequally heated sawdust) and under radiation from a
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.'

The effect of temperature, within optimum limits, is
physiological expansion and enhancement of the rate of
growth {cf. fig. 15). The effect of visible radiation is, on
the other hand, contraction and retardation of growth
(cf. fig. 35). 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 ascertain if thermal
radiation produces tropic curvature as does light, a crucial
experiment has to be devised in which the complicating
effect of rise of temperature on the responding organ is
eliminated. Before referring to that experiment, I will
describe the method of quantitative stimulation by thermal
radiation.

Radio-Thermal Stimulator

This consists of a V-shaped loop of wire, heated short of
incandescence by the passage of an electric current. The
intensity of incident radiation can thus be maintained
constant, and increased or decreased by approach or recession
of the radiating loop. A series of thermal shocks can also
be applied in rapid succession by means of a metronome,
which closes the electric circuit (fig. 100).

I referred to the crucial test by which the complicating
effect of rise of temperature on the responding organ can
be eliminated. This experiment has already been described
in Chapter XII {cf. fig. 67), in which radio-thermal stimula-
tion was applied on the stem of Mimosa at a point opposite
to the indicating leaf. The effect of unilateral stimulation
of Mimosa by heat-rays was found to be exactly the same as

176

CHAP. XVI. RADIO-THERMOTROPISM

that induced by stimulation by light, i.e. there was at first
an erectile movement due to indirect stimulation, followed
by the fall of the leaf due to transverse conduction of
excitation. The indicating leaf-organ was, in this case,
completely shielded from the effect of rise of temperature.

Fig. 100. Radio-Thermal Stimulator.

V-shaped loop heated short of incandescence by passage of
electric current. Successive thermal shocks can be given
by metronome.

Investigation has made it possible to estabhsh a strict
paralleHsm between the effects of luminous radiation and
those of heat-radiation in inducing tropic curvature of
plants. The experiments will be described in the following

order :

1. The effect of indirect stimulation.

2. Positive radio-thermotropism.

3. Dia-thermotropism.

4. Negative thermotropism.

5. The response of the root to unilateral thermal

radiation.

Effect on Growth of Indirect Stimulation by

Thermal Radiation

The appHcation of unilateral photic stimulation of
moderate intensity at a distance from the growing region.

INDIRECT STIMULATION

177

that is, indirect stimulation, has been shown to induce a
negative curvature away from the stimulus ; when the
effective stimulation was made stronger, a diphasic response
occurred, a negative followed by positive [cf. fig. 63).

Experiment 104. — A parallel experiment was carried
out with the stem of a seedling of Vicia Faha. Application
of unilateral thermal stimulation at a distance of 6 cm.
below the region of maximum growth evoked a negative
response, curvature away from the stimulus indicating

Fig. 1 01. Fig. 102.

Fig. ioi. Effect of indirect radio-thermal stimulation.

Duration of application of stimulus between vertical line and
horizontal arrow within circ'e. Negative response followed
bv complete recovery. (Vicia Faba.)

Fig. 102. Diphasic response of identical specimen under more
effective stimulation. Negative response followed by
positive. (Vicia Faba.)

acceleration of growth on the same side. This is seen in
the record (fig. loi) ; thermal radiation was applied at the
vertical line and withdrawn at the horizontal arrow in
a circle, the total duration of stimulation being 40 seconds.
The down-record represents negative curvature ; the move-
ment persisted for a further period of 40 seconds, after
which there was complete recovery.

N

178 CHAP. XVI. RADIO-THERMOTROPISM

Experiment 105. — The effective intensity of stimulation
was next made stronger by reducing the intervening distance
to 4 cm. The duration of stimulation was the same as
before, namely, 40 seconds. The response was, as in the
case of photic stimulation, diphasic negative curvature,
followed by positive (fig. 102).

Positive Radio-Thermotropism

Experiment 106. — The growing region of a stem was next
stimulated unilaterally for a short time by thermal radiation.

Fig. 103. Positive response to stimulation by short
exposures to thermal radiation. Successive dots
at intervals of 5 seconds (Dregea volubilis).

Fig. 103 gives a record of response of the stem of Dregea ;
the induced curvature is positive or towards the source
of heat. On the cessation of stimulation, there is a
recovery which is practically complete, and which takes
place at a slower rate than the excitatory positive curva-
ture. Repetition of stimulation gives rise to responses
similar to the first. Successive stimulations of moderate
intensity thus give rise to repeated responses of positive
growth-curvature.

DIA-THERMOTROPISM I79

Experiment 107. Response of pulvinated organ. — In
order to show that thermal radiation is an effective stimulus

Fig. 104. Response of pulvinus of Mimosa to thermal radiation.

for a motile organ, I reproduce the response of Mimosa to
the rays of heat (fig. 104).

Dia-Thermotropism

I shall henceforth use the shorter term thermotropism to
indicate tropic curvature under unilateral action of thermal
rays. In the case of photic stimulation, it has been
explained how the positive curvature is induced by retarda-
tion of growth at the proximal side and enhancement of
growth at the distal side, this latter being the effect of
indirect stimulation by transmitted positive impulse.

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 direction of orienting stimulus.

Experiment 108. — Neutralisation is seen in the record
given in fig. 105, where under continuous unilateral stimu-
lation the growing stem of Dregea exhibited its maximum

l8o CHAP. XVI. RADIO-THERMOTROPISM

positive curvature, after which the movement became
arrested by the arrival of the excitatory impulse at the

Fig. 105. Record of positive, neutral, and negative curvature
under continued unilateral action of thermal radiation.
The negative response went off the plate. Successive dots
at intervals of 5 seconds. (Dregea volubilis.)

distal side, on account of which the positive curvature be-
came neutrahsed. Further continuation of the stimulation
caused a reversal into negative in the course of 7 minutes.

Negative Thermotropism

Experiment 109. — The response does not stop at
neutralisation but proceeds further, ending in reversal ;
that is, to pronounced negative curvature by the contraction
of the distal side due to conduction of excitation, and to

RESPONSE OF THE ROOT

l8l

fatigue-relaxation of the proximal side. As the thermal
radiation is relatively more effective than light, the reversal,
generally speaking, occurs much earlier. I have obtained
numerous records in confirmation ; the second part of the
record (fig. 105) is, however, sufficient to illustrate this.
After neutralisation the curve is seen to undergo a reversal
indicating negative
thermotropic curva-
ture.

It is interesting
to note in this con-
nection that in the
phototropic curva-
ture induced by sun-
light the heat-rays
play as important a
part as the more re-
frangible rays of the
spectrum.

The Response of
THE Root

It has been shown
that under unilateral
action of light a root
exhibits first a posi-
tive, then a neutral,
and finally a negative
movement. This last

phase is exhibited under very prolonged exposure (p. 145).
The advantage of thermal radiation is, as already explained,
the relatively greater effectiveness of the stimulus, on
account of which the three stages can be observed within a
shorter period.

Experiment no. — I reproduce a record given by the
root of Ipomoea reptans under unilateral thermal radia-
tion. The latent period was 10 seconds, and the positive

Fig. 106. Response of root to unilateral
and continuous thermal radiation, T.
Positive, neutral, and negative curvature.
(Ipomoea reptans.)

iSz CHAP. XVI. RADIO-THERMOTROPISM

curvature continued for 40 seconds. Neutralisation then
occurred, after which the tropic curvature became reversed
into a very pronounced negative (fig. 106).

Summary

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

The response to unilateral stimulation by thermal
radiation is a positive curvature induced in both shoot
and root by the retardation of growth at the proximal, and
acceleration of growth at the distal, side of the organ.

There is complete recovery on the cessation of stimu-
lation of moderate intensity and short duration. Repeated
responses may therefore be obtained similar to the repeated
responses of pulvinated organs.

In certain cases the power of conduction in a transverse
direction is wanting ; excitation remains locaHsed 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 circum-
stances is dia-radio-thermotropic neutralisation, or even a
negative curvature.

The heat-rays in sunhght play as important a part in
inducing phototropic curvature as the more refrangible
rays of the spectrum.

The root, under unilateral action of thermal radiation,
exhibits positive, neutral, and negative curvature, as it does
under unilateral photic stimulation.

CHAPTER XVII

RESPONSE OF PLANTS TO WIRELESS STLMULATION

A GROWING plant bends towards the light, and this is true
not only of the main stem but also of its branches and
the attached leaves and leaflets. It has already been shown
how light affects growth, the effect being modified by the
intensity of radiation. Strong stimulus of light causes re-
tardation of the rate of growth, but a very feeble stimulus
induces acceleration. The tropic effect is very strong
in the more refrangible region of the spectrum with its
extremely short wave-length, but the effect declines
practically to zero towards the less refrangible rays — the
yellow and the red. Proceeding beyond the infra-red
region, there comes the vast range of electric radiation, the
wave-lengths of which vary from o-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 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 rays known to be the most effective are in the blue-
violet region with wave-length as short as 30 x lo"^ cm.,
whilst the electric waves used in wireless signalling are 50
million times as long. The perceptive power of the human
retina is confined within the very narrow range of a single
octave, the wave-lengths of which lie between 70 x io~^ cm.
and 35 X io~^ cm. It is difficult to imagine that plants
could respond to radiations so widely separated from each
other as those of visible light and those of invisible electric
waves.

184 CHAP. XVII. WIRELESS STIMULATION

It is the less refrangible rays which are most active in
the building-up process of photosynthesis. The dynamic
and potential manifestations are thus complementary to
each other, the rays which induce photosynthesis being
relatively ineffective for tropic reaction and vice versa.

The less refrangible yellow-red rays have been shown
to be phototropically ineffective. Proceeding further into
the infra-red region of the thermal rays, it has been shown
that they become suddenly effective in causing retardation
of growth and in inducing tropic curvature. A curve drawn
with the wave-length of light as abscissa, and the effective-
ness of the ray as ordinate, shows a fall towards zero in
proceeding from the violet to the red ; the curve, however,
shoots up in the region of the infra-red. Does the effective-
ness of the rays for inducing tropic reaction abruptly end
with the thermal rays, or does it persist, though in a lesser
degree, in the region of the electric radiation, the wave-
length of which is enormously greater ?

It should be borne in mind that the energy of the electric
waves which reach the plant from, a distance is relatively
feeble, and hence its effect is likely to be slight and detect-
able only by an extremely delicate method of record, and by
the employment of highly sensitive specimens.

Before proceeding further with the subject of the possible
effect of wireless waves on the plant, I thought it desirable
to find out whether or not a rapidly alternating electric
field of force has any effect on the plant.

Response of Plant to High-Frequency Alternating

Field of Electric Force

The investigation was undertaken to find whether the
plant, placed within the influence of the field generated
by high-frequency and high-tension alternating current,
exhibited any responsive reaction. The investigations were
carried out with Mimosa as well as with growing plants.

Experiment iii. Effect of alternating field on Mimosa.
For the high-frequency alternating current I employed a

EFFECT OF ELECTRIC FIELD 185

small portable set known as the ' violet-ray apparatus.'
It was not difficult to get a marked response when the so-
called violet ray emitted by one of the electrodes struck
the plant. But being desirous of obtaining the pure effect
of the high-frequency electric field, I had a large hollow
spiral placed in series with the secondary of the Tesla coil.
The pulvinus and petiole were placed inside the spiral,

Fig. 107. Effect of high-frequency alternating electric field

on response of pulvinus of Mimosa.

Note slow contractile fall with minute pulsations.

Arrow indicates moment of stimulation, arrow within circle

its cessation.

a portion of the petiole projecting out of it for connection

with a recorder.

With a very sensitive specimen of Mimosa the response
was an excitatory fall, the responsive reaction being induced
within 10 seconds of the incidence of the stimulus. The
contraction in this case was slow and gradual as under the
action of light, and not abrupt as under an electric shock.
The contraction persisted for a time after the cessation
of stimulation, after which there was a gradual recovery
(fig. 107).

l86 CHAP. XVII. WIRELESS STIMULATION

Effect of alternating electric field on growing organs. — The
experiments were carried out under two different methods
of producing the alternating field. In the first method
two insulated plates are connected with the terminals of
the secondary coil, the plant being placed between the two.
The intensity of the stimulation is in this case less than
when the plant is placed in the interior of a spiral.

Fig. io8. Effect of high-tension alternating field on growth.

Acceleration (up-curve) under moderate stimulation (Wheat).
Record under condition of balance. Successive dots at
intervals of lo seconds.

Experiment 112. — A seedling of Wheat was chosen for
this experiment, its rate of growth being relatively feeble
(0-19 ^ per second). The specimen was mounted on the
Balanced Crescograph, the horizontal line at the beginning
showing the record under condition of balance. On sub-
jecting the plant to the action of the alternating field of
moderate intensity at the point marked with arrow, the
balance was upset within a minute and a half of the applica-
tion of stimulus. There was an acceleration of growth
as exhibited by the up-curve (fig. 108). Another Wheat-
seedhng was so subtonic that growth was at a standstill,
the record being horizontal without balance. The high-

EFFECT OF STRONG STIMULATION 187

frequency electric field was found to initiate growth, which
persisted for a considerable length of time (fig. io8a).

Experiment 113. Effect of 'strong stimulation. — I next
took another seedling of Wheat which exhibited a more
active rate of growth, of 0-5 \x per second. The effective
intensity of stimulation was increased by enclosing the
plant within a spiral, as in the experiment with Mimosa.
The result was a marked retardation of growth exhibited

Fig. io8a. Revival of growth in Wheat-seedling^ in
condition of standstill.

Normal record, without balance.

by the down-curve, the balance being upset within 30 seconds
of the incidence of stimulus (fig. 109). The results prove
that the effect of high-tension alternating current on growth is
determined by the intensity of stimulation, and by the tonic
condition of the plant. A subtonic organ under feeble
stimulation exhibits, in general, acceleration of growth,
while an actively growing organ under strong stimulation
shows retardation. These characteristics are such as have
been found under other modes of stimulation.

The effects of high-tension alternating current on growth,
as noticed by different observers, have been found to be
very contradictory. The facts described above on the

l88 CHAP. XVII. WIRELESS STIMULATION

modifying influence of tonic condition and intensity of stimu-
lation will probably afford an explanation of the anomaly.
I now describe the effect of wireless waves on growth.

Fig. 109. Effect of strong intensity of field in
retarding growth (Wheat).

Record under condition of balance.

The Wireless System

For sending wireless waves, 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 an aerial 10 metres in height. At the receiving end
the aerial was connected by means of a thin wire with the
experimental plant growing in a pot, which was put in
electric connection with the earth (fig. no). The distance
between the transmitting and receiving aerial was about
200 metres, the maximum length permitted by the grounds
of the Institute.

I describe a typical experiment on the effect of wireless
waves on the growth of a seedling of Wheat. The specimen
was mounted on the Balanced Crescograph, and the growth

THE WIRELESS SYSTEM 189

exactly balanced. This gave a horizontal record ; an
acceleration of growth above the normal is represented in the
records by a down-curve, and a retardation by an iip-curve.
Experiment 114. Effect of feeble stimulation. — I first
studied the effect of feeble stimulation secured by decreasing
the energy of the sparks of the radiator. The response was

i

< '

Wlmrw^

;■« BQ) » m-

EARTH

EARTH

Fig. 1 10. 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.

an acceleration of rate of growth as seen in fig. 11 1, a. The
analogy of this with the accelerating effect of subminimal
intensity of light (p. 83) is very remarkable.

Experiment 115. Effect of strong stimulation.— 1\\g
maximum energy radiated by my transmitter, as stated
before, was only moderate. In spite of this, its effect was
very striking on plants in good tonic condition. The
balance was quickly upset, indicating retardation of the
rate of growth. The latent period, i.e. the interval between

igo

CHAP. XVII. WIRELESS STIMULATION

the incidence of the wave and the response, was only a
matter of a few seconds in very sensitive specimens
(fig. Ill, h). The record given in the figure was taken
with the moderate magnification of 2000 times only. But
with my Magnetic Crescograph the magnification can easily

Fig. III. Record of responses to electric wave with the
Balanced Crescograph.
a, Response to feeble stimulation by acceleration of growth

(down-curve),
fe. Response to strong stimulation by retardation (up-curve).
^ c', Response to medium stimulation — retardation followed by
'^ recovery. Note down-curve in this figure represents accelera-

tion, and up-curve retardation of growth. (Wheat.)

be raised to ten million times, and the response of the plant
to space-signalling can be increased in the same proportion.

Under an intensity of stimulation shghtly above the
minimal, the responses exhibit retardation of growth followed
by recovery, as seen in the series of records (fig. iii, c).

A remarkable peculiarity in the response was noticed
during the course of the experiment. Strong stimulation
by ether waves gives rise, as already stated, to marked
retardation of the rate of growth. Repeated stimulation

EFFECT OF WIRELESS WAVES IQI

induced fatigue and temporary insensitiveness of the organ.
The effect of moderate fatigue 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 5 minutes a marked response occurred to the wireless
stimulation that had been previously received. The latent
period was prolonged, on account of fatigue, from a few
seconds to as many minutes. In less sensitive specimens
the wireless stimulation has to be continued for several
minutes in order to evoke response.

Effect of Wireless Waves from a Portable

Generator

A short-wave four-valve set adjusted to send out wire-
less oscillations of definite wave-length was used. Just

Fig. 112. Acceleration of growth by wireless stimula-
tion in a subtonic specimen (Wheat).
Enhancement of growth in this and following records
represented by up-curve, retardation by down-curve.
Successive dots 20 seconds apart.
Record taken under condition of approximate balance.

before application to the plant the transmitter was tested by
the receiver to make sure that wireless waves of a particular

192

CHAP. XVII. WIRELESS STIMULATION

frequency were being produced, the reading dial having
been cahbrated beforehand. Employing the portable
generator for electric waves, I succeeded in repeating most
of the results given above. The energy of radiation emitted
was not very great ; the length of the wave was about a metre.

I give the following
typical results obtained
with the portable genera-
tor.

Experiment ii6. Ac-
celeration of growth in a
suhtonic specimen. — The
specimen of Wheat-
seedling exhibited the
feeble rate of growth of
o • 37 [jl per second. Wire-
less stimulation induced
an acceleration of the rate
of growth. The stimulus
was relatively feeble, and
the enhancement of re-
sponse, as exhibited by the
upsetting of the balance

Fig. 113. Retardation of rate of growth -^^ ^^ upward direction,
under strong wireless stimulation ^

(Crinum). occurred shortly after

Record taken under condition of balance, the application of stimu-
lus (fig. 112).

Experiment 117. Effect of moderately strong stimulation
on actively growing organ. — A peduncle of Crinum Lily
exhibited a moderately active rate of growth (0-40 [± per
second). It was subjected to wireless stimulation, which
induced a retardation of the rate of growth, shown by
the down-curve (fig. 113). The upsetting of the balance,
showing diminished rate of growth, occurred about 3 minutes
after the incidence of stimulus.

Experiment 118. Record of growth-variation without
balance. — An interesting and confirmatory experiment was

SUMMARY

193

carried out with a seedling of Zea Mays, the record being
taken without balance. The up-curve at the beginning
exhibits the normal rate of growth (0-49 [x per second).

Fig. 114. Effect of wireless stimulation; unbalanced growth.

Up-curve of growth reversed to contractile down-curve followed

by slow recovery (Zea Mays).

Application of stimulus at the horizontal arrow induced
a contraction exhibited by the reversal of the curve
80 seconds after the application of stimulus. On the
cessation of stimulation the normal rate of growth was
gradually restored (fig. 114).

Summary

Pulvinated and growing organs exhibit response to a
high-frequency alternating field of electric force.

194 CHAP. XVII. WIRELESS STIMULATION

The response is modified by the tonic condition of the
organ and the intensity of stimulation.

In response to wireless waves, growing plants exhibit
modification of their rate of growth. Feeble stimulation
induces an acceleration, while strong stimulation causes a
retardation of the rate of growth, as has already been shown
to be the case in response to other forms of stimulation.

CHAPTER XVIII

DIURNAL MOVEMENTS OF PLANTS

The most diverse and complicated movements of different
organs of plants occur in response to variations of the
environment, notably when the external conditions undergo
a periodic change. The plant is subjected, day and night,
to variation of illumination, to change of temperature, and
to change of turgor caused by the difference between the
accession of water by the root and loss by the transpiring
leaves. The organs of plants are, moreover, subjected to
the stimulus of gravity, the effectiveness of which varies
with the angle of inclination of the organ to the vertical.

The plant, as already stated, is affected by many forms
of stimulation acting simultaneously. The phenomena of
plant-movement have remained obscure on account of the
numerous factors which contribute to induce them. This
is sufficiently illustrated by the consideration of but two
out of the agents which affect the plant — the stimulus of
gravity and that of light. Certain organs are highly sen-
sitive to geotropic stimulation, while others are but feebly
sensitive to it. The stronger reaction may be represented
by G and the feeble by ^. In regard to light, there are two
distinct classes of reaction : positive phototropism, when the
organ turns towards the light ; and negative phototropism,
when the organ turns away from it. These reactions when
strong will be represented by + L and — L ; when feeble,
by + / and — /.

What will be the resulting effect when a horizontal
stem is exposed to combined geotropic and phototropic

196 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS

stimulation ? Under geotropic action the stem will tend to
curve upwards ; should it be positively phototropic, the curva-
ture under vertical light will also be upwards. Geotropism
and phototropism will thus conspire, the joint effect being
G + L ; but should the organ be negatively phototropic
the result would be G — L. If further account be taken
of the reactions of the organ to feeble and strong stimu-
lations of gravity and light, the following combinations
are possible :

G+L; G-L; G+/; G-/.

^ + L ; ^ - L ; g +l\ g ~l.

Eight different effects can thus be produced by the
combination of only two factors ; there are, however, other
factors active, such as the rise and fall of temperature.
Additional complications are introduced by the unequal
sensitiveness of the two sides of the organ ; in some it is
the upper side, in others it is the lower side that is the more
excitable and therefore reacts more effectively. There are
thus at least ten factors in operation, and the different
combinations possible would exceed a thousand.

It is therefore not surprising that the movements of
plants appear so extraordinarily complex. Efforts to dis-
cover a real explanation have long been baffled by the
fact that it has hitherto been impossible to isolate and
study the effect of each of the factors for the final analysis
of the complex result.

In the consideration of the diurnal movements of plants
in general, the following subjects will be treated in detail :

1. Daily movements in relation to light and darkness.

2. Diurnal movements due to variation of temperature

affecting growth.

3. The diurnal movements of the ' Praying ' Palm.

4. The effect of variation of temperature on fully grown

organs subjected to the stimulus of gravity.

5. After-effect of light.

6. The complex diurnal movement of the leaf of Mimosa.

SELF-RECORDING RADIOGRAPH I97

The diurnal movements of plants are generally due to
the recurrent changes of light and darkness and to variation
of temperature, the resulting movement being due to the
algebraical summation of their individual effects. In
regard to these two factors, the effect induced by the rise
of temperature is often antagonistic to that of increasing
intensity of light ; a rise of temperature enhances the rate
of growth up to an optimum, whereas light acts as a
stimulus, retarding the normal rate of growth. Variation
of temperature, moreover, affects the organ as a whole,
whereas light may act unilaterally, depressing the rate of
growth of the particular side subjected to light.

For a full analysis of the diurnal movements of plants,
it thus becomes necessary to obtain a continuous record
throughout every hour of the day and night of :

1. The movement of the plant-organ ;

2. The variation of temperature ; and

3. The change in the intensity of light.

In the next chapter I describe in detail the Auto-
matic Recorder for the diurnal movements of plants, the
variation of the temperature being also recorded by the
Thermograph on the same recording plate. The far more
difficult problem of the automatic record of variation of
light will be dealt with in the present chapter.

The Self-Recordixg Radiograph

The method adopted for obtaining a record of the
variation of intensity of light depends on the characteristic
property of the selenium cell, which exhibits a diminution
of its electric resistance under illumination. When a sele-
nium cell is placed in the dark, in series with a battery of
voltaic cells, it gives a small deflection of the galvanometer
in circuit ; illumination causes an increase of this deflection,
according to the intensity of hght. Several difficulties are,
however, encountered in the practical application of this

198 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS

method for obtaining a continuous record for the whole day.
The resistance of the selenium cell undergoes a change due
to polarisation under the continued action of the electric
current ; but this can be rendered neghgible by maintaining
a feeble current for a very short time. The variation of
temperature at different periods may also affect the resist-
ance of the selenium cell. This will be shown to be very
slight and practically neghgible. The most difficult problem
is the automatic record of the galvanometric deflection
under changing intensity of light.

The complete Radiograph consists of :

1. The Wheatstone Bridge for balancing the electric

resistance of the selenium cell in the dark, the
balance being upset on exposure to light.

2. The arrangement of three electric keys which are

automatically put on and off in regular sequence
and at predetermined intervals.

3. The Self-Recording Galvanograph.

Tlie Wheatstone Bridge. — This is diagrammatically repre-
sented in B (fig. 115). The resistance of the particular
selenium cell S is 76,000 ohms in the dark. An approxi-
mately equal resistance is placed in the second arm of the
bridge. A rheostat having a large number of turns of fine
wire with a sliding contact is used for the two variable arms
of the bridge, diagrammatically represented by a straight
line. An approximate balance is obtained when the shding
contact is in the middle ; a shght movement to the right
or to the left secures the exact balance, when the galvano-
meter deflection is reduced to zero. The balance is upset
when the selenium cell is exposed to light, and the resulting
deflection gives a measure of the intensity of the light.

The Automatic Keys. — After previous adjustment of
the balance in the dark, the electric circuit is completed by
the closure of key K^, after which the selenium cell is exposed
to light by an automatic electro-magnetic shutter. The
deflection of the galvanometer is recorded on a piece of

SELF-RECORDING RADIOGRAPH

199

moving paper by means of electric sparks. These different
operations are carried out in proper sequence by the auto-
matic devices described below.

Kj completes the battery circuit for about 10 seconds,
by which time the record is completed. The successive

Fig. 115. The Self-Recording Radiograph.

The selenium cell, s, is periodically exposed to light by the electro-
magnetic shutter, t. The selenium cell forms one arm of the
Wheatstone Bridge, b. The three keys, Kj, K,, K3, are
periodically closed and opened by clockwork, g, the
recording galvanometer with index, i, carrying double-
pointed sparking platinum wire, which moves between the
metal strip c and the plate m. r, sparking coil with its
electrode connected with c and m. The battery is not
shown in the figure {see text).

records of variation of light are taken at intervals of
15 minutes ; the periodic closures of the circuit are thus for
10 seconds at intervals of 15 minutes. In practice, this
short passage of the current is found to cause no polarisation.
The second key, K2, actuates an electro-magnetic device
by which the trap-door T is opened for the definite period
of I second; the selenium cell, S, inside the dark box is
thus exposed to light for this length of time. The trap-door

200 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS

is shown in the diagram immediately above the dark box.
In reahty it is at the upper end of a vertical tube, the
inside of which is coated with lamp-black to prevent side
reflection. The light that falls on the selenium cell is thus
from a definite area of the sky. The intensity of light
from the sky at different periods of the day causes deflection
of the galvanometer which is proportional to that intensity.
The maximum deflection of the galvanometer employed is
attained in the course of 3 seconds after exposure.

The third key, Kg, is for the completion of the spark-
circuit for record of the maximum galvanometric deflection,
3 seconds after the exposure of the selenium cell. This key
actuates a sparking coil, R, the vibrating interrupter of
which is not shown in the figure. The spark thus produced
punctures the maximum deflection of the galvanometer
index on a moving piece of paper attached to the plate M.
The successive closure and opening of the keys are made
automatically and in proper sequence by means of clock-
work, the whole process being repeated at intervals of
15 minutes.

The Galvanograph. — Now comes the most difficult
problem — the automatic record of the galvanometer de-
flections. A record may be secured without great difficulty
by means of photography. A spot of light reflected from
the galvanometer mirror may be allowed to fall on a photo-
graphic plate which descends at a uniform rate by clock-
work. This, however, entails the use of a dark room and
subsequent development of the plate. This trouble was
avoided by the device of direct record of the galvanometer
deflection by means of electric sparks.

A sparking method has been previously employed in
which the deflected index of the galvanometer in connection
with one electrode of an induction coil leaves a spark-record
on a moving piece of paper. Several difficulties are, how-
ever, encountered in the employment of this method with
a highly sensitive galvanometer. There is a liability of
leakage of the high-tension current into the galvanometer

THE GALVAXOGRAPH 201

circuit ; also the discharge of the spark gives a backward
kick to the index, by which the normal deflection undergoes
an unknown variation.

These difticulties were removed in the following manner:
The moving coil of the sensitive D'Arsonval galvanometer
has a long glass index I at right angles to the plane of the
coil, the index being coated with shellac varnish to render
it highly insulating. It projects for a short distance on
the opposite side for attachment of a counterpoise, which
takes the form of a vertical vane of mica acting as a damper.
The galvanometer itself is of an aperiodic type, and the
addition of the damper makes it perfectly dead-beat. The
sensitiveness of the galvanometer is such that a micro-
ampere current produces a deflection of lo mm. of the index.
The recording index has attached to it a short vertical piece
of thin platinum wire pointed at its two ends ; the index
moves between the sheet of metal M, covered with paper for
record, and a semicircular piece of narrow sheet-metal C.
The metal sheet M is mounted on wheels and moves at a
uniform rate by clockwork. One electrode of the sparking
coil is in connection with C, and the other with M. The
sparking thus takes place simultaneously, above and below
the vertical and double-pointed platinum wire carried at
the end of the index. There is thus no resultant kick, and
the index remains undisturbed. The sparking, as previously
stated, takes place 3 seconds after exposure of the selenium
cell to light, by which time the deflection reaches its
maximum. The record thus consists of successive dots
at intervals of 15 minutes, the dots representing the
maximum deflections of the galvanometer corresponding
to the intensity of light.

The record given in fig. 116 was taken about the end of
January ; the sun rose at about 6.45 a.m. and set at 5.30 p.m.
Twilight is very short in the tropics ; the sky is feebly
lighted about 6 A.M. The record shows the intensity of
light to be exceedingly feeble at 6 a.m. The rise in the
intensity was rapid, attaining the maximum at 12 midday.

202 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS

which will be designated as the light-noon. The intensity
of light then declined at a rate slower than the rise. But
after 5 p.m. the fall of intensity was extremel}^ rapid.

An important point arises in connection with the diurnal
variation of light and of temperature, and determination of

Fig. 116. Radiogram of variation of intensity of light from the
sky during 12 hours in winter. The upper record shows the
variation on a bright day, the maximum intensity being
attained at 12 midday. The lower record exhibits irregular
variation on a cloudy day. The horizontal record above the
base line shows that the electric resistance of the selenium
cell is practically unaffected by variation of temperature.
Successive thin dots at 15 minutes' interval, thick dots at
intervals of an hour.

the times of their maxima and minima. For this purpose
records of diurnal variation of temperature and of light were
taken on the same day in summer with the Thermograph
(described in the next chapter) and the Radiograph. The
two curves are given in fig. 117.

It will be seen that while the maximum intensity of
light is at 12 noon, the thermal maximum is at about 2 p.m.
The thermal noon is thus two hours later than the light-

LIGHT AND TEMPERATURE 203

noon. Light disappears at night from 6 p.m. to 6 a.m.,
that is to say, the period of minimum hght is prolonged for
12 hours. But the fall of temperature is gradual, and the
minimum is attained at or about 5 a.m., which is the thermal
dawn. The characteristic variation of these two important
factors should be borne in mind, since the diurnal move-
ments of plants are modified by the algebraical summation
of the effects of light and of temperature.

Fig. 117. Record of diurnal variation of light and of temperature

in summer.

It is sometimes desirable to carry out researches during
a period when the intensity of light remains approximately
constant. Such a period occurs between 11 a.m. and
I p.m., for a variation is only ± 5 per cent, of the mean.

The record given of the diurnal variation of light is
true of days when the sky is clear. But the passage of a
cloud causes change in the intensity which is accurately
recorded by the Radiograph. A record of such irregular
variation on a stormy day is given in the lower record of

fig. 116.

For facihty of treatment of the diurnal movement of

204 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS

plant-organs, I shall consider the three ideal types : (i) where
variation of light is the most important factor ; (2) where the
movement is principally due to differential growth under
variation of temperature ; and (3) where a fully grown tree
at an inclination to the vertical, and therefore subjected to
the stimulus of gravity, exhibits up or down movement
under variation of temperature.

The determination of the isolated effect of any one
individual factor, difficult as it is, can be arrived at by a
process of judicious elimination. Thus the predominant
effect of light, of variation of temperature, or of geotropic
action can be inferred more or less from the following
observations.

Predominant effect of light and darkness. — The obvious
method of observing the effect of keeping the plant in con-
tinuous darkness presents the difficulty that the tonic
condition of the plant is greatly depressed, resulting in the
abolition of its normal irritability under prolonged darkness.
The effect of withdrawal of light can therefore be satis-
factorily observed only for about two days in succession.

The characteristic effect of light is very marked at two
definite times of the day, when light appears and when it
disappears. The average dawn is approximately at 6 a.m.,
and the average sunset at 6 p.m. ; unlike the diurnal varia-
tion of temperature, which is gradual, the change from
light to darkness or from darkness to light is relatively
abrupt. Since the change of temperature and of light are
both connected with the appearance and the disappearance
of the sun, some difficulty arises in discriminating the effect
of one from that of the other.

Light and temperature effects, — Light appears in the
morning, say, at 6 a.m. ; it becomes most intense at noon ;
after 4 p.m. the hght 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 at about 5 A.M. in
summer, and at about 7 a.m. in winter. The maximum

SUMMARY 205

temperature is reached at about 3 p.m. in summer, and
about I P.M. in winter. The range of daily variation in
summer may be taken to be between 23° C. and 38° C. ; in
winter between 16° C. and about 29° C. These are the
normal variations 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 tempera-
ture, and vice versa. The average time of minimum tem-
perature may be taken to be at 6 a.m., which I distinguish
as the thermal dawn ; the maximum temperature, the
thermal noon, is attained at about 2 p.m. Variation from
these average times at different seasons does not amount
to more than an hour.

Light dawn and thermal dawn are more or less
coincident, while thermal noon is two hours later than
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 resembles 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.

The effect of geotropism can be detected, as will be
explained later, by taking the record of the plant in normal
and in inverted positions.

Summary

The phenomena of the diurnal movements of plants are
greatly complicated by the algebraical summation of the
effects of numerous factors. The most important of these
are the effects of light and darkness, of variation of tem-
perature, and of thermal variation on organs subjected to
the action of gravity.

In order to trace the effects of the more important indi-
vidual factors, it is necessary to obtain continuous record of
variation of temperature by the Thermograph, of variation

206 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS

of light by the Radiograph, and of the corresponding move-
ment of the plant-organs by the Plant-Recorder.

The Radiograph gives a record of the diurnal variation
of light. On a clear day in January the intensity was
found to increase rapidly from 6 a.m. to 12 noon, when it
reached its maximum. Light began to decline slowly up
to 5 P.M., the decline being less rapid than the rise in the
forenoon. The fall of intensity was extremely rapid after
5 p.m. Any fluctuation of light, due even to a passage of
a cloud, is accurately recorded by the Radiograph.

The individual effects of the main factors can be to some
extent discriminated from each other. The contrasted
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 predominant character of the
action of light. 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 2 hours
earlier than the thermal noon, at w^hich the temperature is
maximum.

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

CHAPTER XIX

EFFECT OF RECURRENT LIGHT AND DARKNESS ON
MOVEMENTS OF PLANTS

For the demonstration of the effect of recurrent change of
light and darkness in the course of day and night, the
experimental plants must be such as have organs which
readily respond to photic stimulation. One of these is
the leaf of Cassia alata, the petiole of which carries a

Fig. ii8. Leaflets of Cassia alata : open in daytime, and closed

in evening.

number of paired leaflets, each of which is about 5 cm.
long and 2-5 cm. broad. At night each pair of leaflets fold
themselves in a forward direction (fig. 118). 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. The
diurnal movements of the leaflets will be shown to be due
to the predominant effect of variation of light.

It will be convenient to begin with a general description
of the experimental methods employed, and of the apparatus
by which diurnal movements are recorded.

208 CHAP. XIX. RECURRENT LIGHT AND DARKNESS

Experimental Arrangements

When the diurnal record has to be taken continuously for
several days in succession, it is necessary to take precautions

Fig. 119. The Automatic Recorder of diurnal movements of
plants. Quadruplex form provided 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 movements
of a horizontally laid shoot ; the fourth lever, attached to a
differential thermometer, t, records diurnal variation of
temperature.

against the disturbing effect of watering the plants, and of
tropic curvature of the stem induced by one-sided illumina-
tion.

AUTOMATIC RECORD 209

Irrigation. — There is, as is well known, a periodic varia-
tion of turgor which is disturbed by watering the plant at
irregular intervals. Precaution against this was taken by
placing the flower-pots containing the plants in a long
trough filled with water (sec fig. 119). The height of water
in the trough is maintained constant by a syphon.

Vertical illumination. — The direction of sunlight changes
from morning to evening and the stem and leaves exhibit
appropriate movements under the changing direction of
hght. 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 light thereby becoming uniformly
diffused. The sides and the base of the chamber being
impervious to light, the plant was protected from side-
illumination. A narrow slit covered with red glass allowed
inspection of the curve during the process of record.

The Ventilator. — A revolving ventilator, acted on by
the wind, sucked the air aw^ay from the chamber, thus
ensuring a constant supply of fresh air without causing
any disturbance of the record.

Automatic Record of Diurnal Movement

The Recorder. — The Oscillating Recorder employed is
of the quadruplex type carrying four recording levers
(fig. 119). The function of the apparatus is to record
various types of diurnal movement. The fourth lever records
the daily variation of temperature : the other three are
attached to plants of either the same or different species.
In the former case three records are obtained of the same
species of plant under identical external conditions. If they
agree in all essentials, the periodic curve may be taken
as characteristic of that particular species. A very great
saving of time is thus ensured, and it is therefore possible
to obtain characteristic curves of numbers of different
species of plants within the short period of a season.
The quadruplex recorder also permits the obtaining of

210 CHAP. XIX. RECURRENT LIGHT AND DARKNESS

simultaneous records under identical external conditions
of leaves of different age of the same plant, or of leaves
of three different species. I have for several years past
taken records of numerous plants and at all seasons of the
year. The plant's autograph is often so characteristic that
it is possible to name it by mere inspection of its daily record.
The 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 an attached lever. The oscillation of the plate
takes place once in 15 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 sometimes employed is to make 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-organ corresponding to any particular variation
of temperature at any period may thus be easily deter-
mined. I shall now give a typical example of diurnal
movement induced by variation of light and darkness.

Response of the Leaflet of Cassia alata

These leaflets remain tightly closed during the night, but
from early morning onwards they begin to open and remain
widely spread out throughout the day. The problem 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 during the night. As
regards temperature there is a continuous rise from morning
till the thermal noon, after which the fall of temperature
is continuous till next morning. The opening of the leaflets
in the daytime will therefore be due to the summated effects
of rising temperature and increasing light ; the closure in

VARIATION OF LIGHT

211

the evening, on the other hand, will be due to falling tem-
perature and to darkness. The individual effect of each of
these factors is not known, and it is therefore necessary to
determine the relative effects of variation of temperature
and of light.

Effect of Variation of Temperature

Experiment 119. — The plant was enclosed in the glass
chamber and the experiment was commenced at midday,
when the leaflets were open, under uniformly diffused light.
The temperature was artificial^ raised by means of an
electric heater placed in the chamber, and lowered by the
introduction of cold air. One of the leaflets
was attached to the recording lever, and
its movement, up or down, indicated the
opening or closing movement of the leaflet.
The records showed that rise of tempera-
ture induced a movement of closure, while
fall of temperature brought about a move-
ment of opening.

Effect of Variation of Light

Experiment 120. — This experiment was
also carried out at midday, when the leaflets
were fully open. The horizontal part of the
record in fig. 120 represents the stationary
expanded condition of the leaflet ; a black
cloth was put over the glass chamber at
I P.M., and the effect of darkness was
recorded for one hour. Darkness is seen to
have initiated a movement of closure which
increased at a rapid rate ; the black cloth
was then removed, and the movement of opening under light
was completed in the course of five quarters of an hour.
A passing cloud causes an immediate movement of closure,
proving how very sensitive is the leaflet to variation of light-

Fig. 120. Effect of
sudden darkening
at arrow, produc-
ing movement
of closure (up-
curve). Restora-
tion of light in-
duces movement
of opening (down-
curve). Successive
dots at intervals of
1 5 minutes. (Leaf-
let of Cassia.)

212 CHAP. XIX. RFXURREXT LIGHT AND DARKNESS

The effects of rise of temperature and of light have been
shown to be antagonistic to each other. The opening
movement under Hght in the forenoon has to be carried out
against the closure-tendency due to rise of temperature.
Light, therefore, is the predominant factor in the diurnal
movement of the leaflet of Cassia. The closure-effect of
darkness at night, on the other hand, overpowers the
tendency to movement of opening due to fall of temperature.

Diurnal Movement of the Leaflet of Cassia alata

Experiment 121. — I next took the diurnal record of the
leaflet from 4 p.m. till i p.m. next day. As the leaflets were
open previously from i p.m. to 4 p.m., the record of this

w

^^^^^^^^^^m^^^^^^l

m
0

m
•

•
•

0

to . •

m

6 P.M.

1 1
"^ 6 A.M. ■■

1

m 12

MIDNIGHT :^£

NOON

Fig. 121. Diurnal movement of the leaflet of Cassia alata.
Closure-movement commenced at 5 p.m. and was completed
by 9 P.M. Leaflet began to open at 5 a.m.

period is omitted. In the diurnal record (fig. 121) the
first thick dot was made at 4 p.m. ; successive thick dots
are at intervals of an hour, the thin dots being at intervals
of 15 minutes. It will be seen that a rapid movement of

DIURNAL MOVEMENT OF DESMODIUM

213

closure was initiated at 5 p.m., when the hght was under-
going rapid diminution. The movement of closure was
completed at about 9 p.m. The leaflets remained closed
till 5 A.M. next morning, after which they began 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 of fall of temperature would be con-
cordant, and the opening in the early hours may possibly
be hastened by the low temperature in the morning. The
leaflets were open to their utmost by 9 a.m., and they
remained open till the afternoon.

Another interesting example of the diurnal movement
due to variation of light is found in the terminal leaflet of
Desmodium gyrans, an account of which is given below.

Diurnal Movement of the Terminal Leaflet of

Desmodium

Both the petiole and the terminal leaflet of this plant
exhibit a very marked nyctitropic movement. In the

Fig. 122. The day and night positions of the petiole and terminal

leaflet of Desmodium gyrans.

evening the petiole is raised and becomes almost erect,
while the terminal leaflet exhibits a sharp curvature
downwards (fig. 122).

214 CHAP. XIX. RECURRENT LIGHT AND DARKNESS

Experiment 122. — The petiole was held fixed, and the
terminal leaflet was attached to the recording lever. Under
natural conditions, daylight 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, so that the daily curve appears as a single
large pulse on which smaller autonomous pulsations may
be superposed.

The diurnal curve (fig. 123) exhibits a sudden fall at
about 5 P.M., resulting from the rapid waning of afternoon

Fig. 123. Record of diurnal movement of the terminal leaflet of
Desmodium gyrans. Up-curve represents down-movement.

light, till by 6.30 p.m. the leaflet becomes closely pressed
against the petiole.

The question arises whether or not variation of tempera-
ture has any marked effect on the diurnal movement of
the leaflet. It has been explained that when an organ of
a plant is sensitive to variation of temperature, the record
exhibits a flexure at about 2 p.m., when there is a change
from rise to fall of temperature. No such flexure was,
however, observable at that period. But the sensibility
of the leaflet to variation of light is 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

SUMMARY 215

early in the morning at about 5 A.M. This up-movement is
also very rapid, and the leaflet assumes its fullest outspread
position by 7 a.m. It remains in this position till the after-
noon, after which the cycle is repeated. As the leaflet is
very sensitive, the position of equilibrium of the leaflet is
hable to be disturbed by the slightest fluctuation of light
from the sky, which 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 move-
ments are determined by light rather than temperature, the
plant being more responsive to the former than to the latter.

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 movement of closure, re-
exposure to light brings about the opening of the leaflets.
These are so extremely sensitive to light that closure-
movement is 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 response to light overpowering
the response to rise of temperature.

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

The terminal leaflet of Desmodium exhibits a diurnal
movement which is very similar to that of the leaflet of
Cassia. It begins to open early in the morning and remains
outspread during the day ; it exhibits a rapid down-movement
after 5 p.m. and becomes closely pressed against the petiole
in the course of about 2 hours.

CHAPTER XX

THERMONASTIC MOVEMENT OF NYMPHAEA

The term ' nastic ' is convenient when employed in the
restricted sense as defined by Strasburger. * 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.' ^

In describing the direction of responsive movements,
confusion is likely to arise unless the observer's point of
view be carefully defined. An up-movement of the petal
in a flower 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 movement be nastic, then the closure or the opening
movement wdll remain the same, whether the organ be held
in normal position or upside down. If, on the other hand,
the direction of the movement be determined by the para-
tonic effect of an external stimulus, gravity for example,
then the responsive movement in relation to the plant will
be different. The closure-movement in the normal position
will, on inversion, be reversed into a movement of opening.
The reversal of closure into an opening movement or vice
versa will thus be a test of the paratonic effect of geotropic
stimulus.

Typical examples of thermonasty are afforded by the

1 strasburger, Text-Book of Botany (1912), p. 300.

RESPONSE OF ZEPHYRANTHES 217

flowers of Crocus vermis and Tidipa Gesneriana, specially
investigated by Pfeffer. He found that the flowers opened
under the action of a rise of temperature, and closed under
the action of a fall. He also established the important fact
that the opening and closing of flowers is a phenomenon of
differential growth under variation of temperature. The
dorsiventral perianth-leaf of the flower is affected unequally
at its two sides by rise or fall of temperature. In opening
under rise of temperature, the upper side grows relatively
the faster, the opposite effect being induced during fall of
temperature. I shall show that thermonastic movements
are not of one but of two distinct types : (i) the positive, in
which, as in Crocus, rise of temperature induces a movement
of opening ; and (2) the negative, where rise of temperature
induces a movement of closure, as in a number of the
Water-Lilies (Nymphaea) which grow in India.

Positive Thermonastic Response of Zephyranthes

I first give an account of the movement of the petal of
Zephyranthes, the reaction of which to variation of tempera-
ture is similar to that of Crocus. Viewed from above, the
inner side of the petal of a flower is the upper side ; under
rise of temperature the flow^er opens evidently by the
enhanced rate of growth of the relatively more active upper
side, as shown by the following experiments.

Experiment 123. Effect of variation of temperature. — •
In obtaining a record, all the floral leaves except one were
removed, this particular petal being attached to the record-
ing lever. There was an up-movement, or a movement of
closure, under fall of temperature, while rise of temperature
induced a movement of opening. The up-movement is
recorded by a down-curve and vice versa (fig. 124).

Experiment 124. Responsive movement under radiation.
I next determined the effect of radiation on the movement
of the petal. It has been shown that the effects of radiation,
whether visible or invisible, are similar, the effect of the

2l8 CHAP. XX. THERMONASTIC MOVEMENT

thermal rays being very pronounced. On subjecting the
leaf to thermal radiation, the movement of response was one
of closure (fig. 125), in sharp contrast with the movement of

Fig. 124. Fig. 125.

Fig. 124. Thermonastic responses of petal of Zephyranthes.

c, closing movement due to cooling ; h, opening movement

due to warming.

Fig. 125. R, closing movement due to heat-radiation. Note
opposite responses to rise of temperature and to radiation.

opening under rise of temperature. These experiments
clearly demonstrate the opposite effects of rise of temperature
and of radiation.

The following is an example of negative thermonasty.

Diurnal Movement of Nymphaea

The Indian White Water-Lily remains closed during the
greater part of the day and opens at night. Figs. 126
and 127 are photographic reproductions of the day and
night positions of the flower.

The question arises whether the diurnal movements of
the flower are predominantly tropic effects of photic or
geotropic stimulation or are due to thermonasty.

Effect of reciiYYent light and darkness. — It has sometimes
been supposed that the closure and opening of this flower
are mainly due to the alternation of light and darkness.

MOVEMENT OF NYMPHAEA

219

But there are definite facts which do not by any means
support this conclusion ; for if the movements of the petals

Fig. 126, Nymphaea closed in daytime.

Fig. 127. Nymphaea open at night.

were entirely dependent on light, two opposite effects
would be produced in the morning and evening respectively.
But the Water-Lily is open at both these times. Moreover,

220

CHAP. XX. THERMONASTIC MOVEMENT

the course of the opening and closing movements is, as will
presently be shown, not strictly coincident with the daily
changes of light and darkness. The movement of the petals

Fig. 128. The Thermonastic Recorder,

The specimen of Nymphaea has one of its perianth-leaves, n, attached
to the short arm of the lower lever by a thread, t, metallic
thermometer attached to the short arm of the upper lever ;
c, clockwork for oscillation of the plate.

of Nymphaea is therefore not essentially dependent on
variation of illumination.

Possible effect of geotropism. — It maybe asked, Does the
stimulus of gravity exert any influence on the movement
of the opening or closing of the flower ? The petals close

THE DIURNAL RECORD

221

up in the middle of the day, each of the petals standing
erect. If the flower were susceptible to the stimulus of
gravity, then, on turning the flower upside down, the closed
petals in their inverted position would curl upwards and
outwards, thus opening the flower. But no such effect

takes place.

There remains only one other operative factor, namely,
that of variation of temperature, the characteristic effect
of which will next be demonstrated.

The Thermonastic Recorder

The apparatus is illustrated in fig. 128. One of the
perianth-leaves, N, of the Lily is attached to the short arm
of the lower recording lever.
The metallic thermometer T is
connected with the upper lever.
Simultaneous records can thus
be obtained of the diurnal
variation of temperature and
of the movement of the
petal.

Experiment 125. Effect of
variation of temperature. —
Raising the temperature of the
chamber in which the flower
was placed induced a move-
ment of closure shown by the
down-curve (fig. 129).

The Diurnal Record

Fig. 129. Negative thermonastic
response of Nymphaea.

Application of heat at the vertical
mark induced up-movement of
closure seen as a down-curve.
Successive dots at intervals of a
second.

A continuous record of the
movement of the petal was

obtained from 6 p.m. to 12 noon next day. The flower
was tightly closed from the forenoon, the perianth-leaves
beginning to open out in the evening, at first slowly, then
very rapidly ; the flower became fully expanded by 10 p.m.

222 CHAP. XX. THERMONASTIC MOVEMENT

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 about 5 a.m., and the movement of
closure set in with the rise of temperature till the flower
became completely closed by 10 a.m. (fig. 130).

Fig. 130. Diurnal record of Nymphaea.

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.

The phenomenon of diurnal movement of the Water-
Lily is therefore thermonastic, the floral leaves exhibiting
a movement of opening at night owing to fall of temperature.
The outer side is the more active.

The Water-Lilies of Europe close at night and open in
the daytime. In searching for an Indian Water-Lily
resembling the European type, I was successful in dis-
covering a blue Lily, the diurnal record of which is exactly
the opposite. In such cases it is the inner side that is
relatively the more sensitive.

SUMMARY 223

The results described lead to the following generalisation :

1. Thermonastic movement occurs in organs which,
under variation of temperature, exhibit unequal growth of
their two sides.

2. When the inner or upper side is the more active, rise
of temperature induces a positive thermonastic movement,
that is, the opening of the flower.

3. When the outer or lower side is the more active, rise
of temperature induces negative thermonastic movement,
shown by the closure of the flower.

The following tabular statement describes the different

types of thermonastic organs :

Table XIX. — Showing the Effect of Rise of Temperature

ON Thermonastic Movement.

Type of reaction Specimen

/■ Zephyranthes

„ .^. , , . 1 Crocus

Positive ; movement of openmg -, European Nymphaea

i ( Indian blue variety of Nymphaea
Kegathe ; movement of closure I Indian White Water-Lily

Summary

Thermonastic movements are induced in growing
anisotropic organs, the two sides of w^hich exhibit different
rates of growth under variation of temperature.

Rise of temperature induces greater enhancement of the
rate of growth of the more active side ; fall of temperature
gives rise to 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 side of the organ is
relatively the more active. Examples of these are seen in
Crocus, Zephyranthes, and in European and certain Indian
Nymphaeas.

In the negative type, rise of temperature induces a move-
ment of closure. Here the outer side of the organ is the more
active. The Indian White Water-Lily belongs to this type.

CHAPTER XXI

THE DIURNAL MOVEMENT OF THE PRAYING PALM

The report of the performances of the ' Praying ' Palm of
Faridpore drew my attention to this most remarkable
phenomenon. This tree exhibited a regular up-and-down
movement through a considerable extent day after day.
It was a fully grown Date Palm [Phoenix sylvestris), the

Fig. 131. The ' Praying' Palm. The morning position.

length of the trunk being about 5 metres, with a diameter
of 25 cm. It must have been displaced by a storm, so that
the trunk was incHned to the vertical at about 60°. Fig. 131
shows the upper portion of the trunk. Two vertical stakes,
each I metre in length, give the relative positions of the
trunk in its two extreme excursions.' In the course of its

PHYSIOLOGICAL CAUSE 225

daily movement, the trunk throughout its entire length
was elevated in the morning and depressed towards the
evening ; the upper part of the rigid trunk was thus moved
through the distance of i metre. The ' neck,' the upper
end of the trunk bearing the leaves, was concave to the sky
in the morning ; in the afternoon the curvature disappeared

Fig. 132. The ' Praying' Palm. The afternoon position,

or was even slightly reversed. The large and long leaves,
which pointed high towards the sky at the beginning of the
day, were swung round towards the evening through a
vertical distance of 5 metres (figs. 131, 132). To the popular
imagination the tree appeared, at the time of evening prayer,
to bend its neck and press its head of leaves against the
ground in an attitude of devotion.

Physiological Cause of the Movement

What can be the underlying cause of this remarkable
periodic movement ? Is it due to mere physical expansion
and contraction by heat or cold, or to some specific reaction
of the living tree ? If physical, the movement would still

Q

226 CHAP. XXI. THE ' PRAYING ' PALM

persist after the death of the tree ; if physiological, the
movement would disappear.

The tree was old and died a natural death a year after
the commencement of my investigations. I then received
a communication from the Government officer in charge of
the district, that ' the palm tree is dead and its movements
have ceased.' This afforded conclusive evidence that the
movements had in some way been due to its vital activity.

The periodic movement of the tree must therefore be
attributable to the physiological response of the living cells
to the diurnal changes of the environment — either the
recurrent alternation of light and darkness, or the diurnal
changes of temperature. The only certain way of dis-
criminating the effect of the one from that of the other was
to obtain a continuous record of the movement of the tree,
and find whether light or temperature maxima coincide
with the maximal displacement of the tree.

The objects of the investigation resolved themselves
into the following :

1. A method of accurate and continuous record of the

tree day and night for the determination of the
exact times of maximum erection and of fall ;

2. Comparative determination of the effects of diurnal

variations of light and of temperature ;

3. To ascertain whether the characteristic movement of

the particular Palm was unique, or whether it was
of more or less universal occurrence ; and

4. Determination of the relatively more effective factors

in the production of the diurnal movement.

The problem is complicated, the movement of the tree
being modified by so many factors. This was realised
during the course of my investigations on the subject, now
extending over twelve years. I will first describe the effects
observed under diverse experimental conditions, which have
led to the disentanglement of the individual effects of the
several factors in operation.

AUTOMATIC RECORD

227

The Automatic Recorder for Trees

The record was obtained by the device illustrated in
fig- i33» for recording the diurnal movements of trees and
C

Fig. 133. Automatic Recorder of movements of trees and plants.

T, differential metallic thermometer ; r, recording lever for
temperature ; r', for recording plant-movement ; c, clock-
work for oscillation of recording plate. The same clock-
work moves plate laterally for 24 hours.

other plants. One of the two writing levers gives the
record of variation of temperature by the Thermograph,
and the other lever records the up or down movement
of the tree throughout 24 hours. As the extent of the

228

CHAP. XXI. THE ' PRAYING PALM

Fig. 134. Record of diurnal movement of the ' Praying ' Palm

(Phoenix sylvestris).

Thermographic curve for 2 4 hours, commencing at 9 in the evening,
is given in the upper record in which the fall of tempera-
ture is represented by an up-curve. The corresponding
diurnal curve of movement of the tree is given in the lower.
Successive dots at intervals of 1 5 minutes ; thick dots,
intervals of an hour.

EFFECT OF LIGHT 229

movement of the tree was considerable, means had to be
employed for the reduction of the movement recorded by
the lower writing lever. On the oscillating plate successive
dots were made at intervals of 15 minutes, the thicker
dots being inscribed at intervals of an hour. The erectile
movement of the tree is represented as an up-curve, while
rise of temperature is recorded as a down-curve.

The Diurnal Record of the Tree

Experiment 126. — Hasty observation led people to
beheve that the tree hfted itself at sunrise and prostrated
itself at sunset ; but the continuous record obtained with
my apparatus proved that the tree was never at rest, but
in a state of continuous movement which underwent
periodic reversal (fig. 134). The tree attained its maximum
erection at 7 in the morning, after which there was a rapid
fall. The downward movement reached its maximum at
3.15 P.M., after which it began to hft itself very slowly, then
more rapidly, until it hfted itself to the highest position at
7 next morning. This diurnal periodicity was maintained
day after day. TJie movement was by no means passive, hi.t
was effected with an active force sufficient to lift a man off the
ground.

The next point is the determination of the relative
effects of variations of light and of temperature in inducing
the diurnal movement.

The Relative Effect of Light

The following considerations will help in deciding
whether or not light had any perceptible influence in the
production of the periodic movement of the tree.

I. Since the movement has been shown to be a physio-
logical phenomenon, the stimulus of light, in order to be
effective, must act directly on the living tissue. In the
case of the Palm this is impossible, for the bark of the tree
is of considerable thickness, and the thick bases of dead

230 CHAP. XXI. THE ' PRAYING * PALM

leaves completely screen the deep-lying living tissue from

light.

2. The curve of the movement of the tree affords, how-
ever, data for correct inference as to the possible effect of
light. If the action of light is determined by its maximum
intensity, then there should be a climax at noon, and the
opposite at midnight. But the highest erection was reached
not at noon, but at 7 in the morning ; the lowest fall, on the
other hand, was attained not at midnight, but in the after-
noon. Again, if the movement was caused by the cumula-
tive action of light, then the maximum extent of movement,
either up or down, should be attained shortly before evening ;
but this was not the case, for it occurred several hours
earlier, at about 3 p.m.

It is thus probable that light had very little influence
on the movement of the tree. I will describe additional
experiments in support of this conclusion.

Effect of Variation of Temperature

Turning next to the factor of thermal change, the
record shows that the curve of the movement of the
tree is practically a replica of the curve of variation of
temperature [see fig. 134). The rise of the tree followed the
fall of the temperature and vice versa. A lag will be
noticed at the turning-points of the movement ; thus, while
temperature began to rise at about 6 a.m., the tree did not
begin to fall till an hour afterwards. Again the turning-
point from rise to fall of temperature was after 2.45 p.m. ; the
downward movement of the tree was, however, not reversed
into one of erection till after 3.15 p.m., the lag being about
30 minutes. The delay is attributable to two causes : it
took some time for the thick trunk of the tree to attain
the temperature of its surroundings ; in addition to this,
physiological inertia delayed the reaction.

In reference to the incidence of thermal noon, already
defined in the last chapter, it is modified (i) by the season ;

SIJBARIA PALM 23I

(2) by the locality, whether it is exposed or shaded ; (3) by
the condition of the weather ; and (4) by the radiation of heat
from the ground. It varies according to circumstances
from I to 3 P.M. The thermal dawn, generally speaking, is
shortly after sunrise. The movement of the tree lagged
somewhat behind the change of temperature ; erection
commenced after the thermal noon, and the fall began
after the thermal dawn.

The Periodic Movement of Sijbaria Palm

Another question which had to be investigated was
whether the movement of the Faridpore Palm was a unique
phenomenon, or whether other Date Palms exhibited similar
movements. With this end in view, I experimented with
a Date Palm that was growing at my Research Station at
Sijbaria on the Ganges, situated at a distance of 200 miles
from Faridpore.

Experiment 127. — The surrounding conditions wxre
very different ; the tree itself was much younger and w^as
at an inclination of 20° to the vertical instead of 60° as in
the previous case. The tree w^as enclosed in a dark tent to
exclude the action of hght. The diurnal curve of its move-
ment was found to be very similar to that of the Faridpore
Palm, though the extent of its movement was considerably
less. The tree attained its highest erect position at 7.15 a.m.
and the lowTst at 3.45 p.m. (fig. 135). In settled w^eather
the diurnal rise and fall of temperature is very regular, and
so was the movement of the tree. But under less settled
conditions, owing to change of direction of wind, the tem-
perature-curve exhibited fluctuation. It w^as a matter of
surprise that the plant-record should have repeated this
fluctuation with astonishing fidehty, as seen in the common
twitch in the two curves shortly after 8 a.m. There can,
therefore, be no doubt whatever about the movement being
principally due to variation of temperature.

The extent of the up and down movement of the Sijbaria

232

CHAP. XXI. THE ' PRAYING PALM

Palm was very much less than that of the Faridpore, presum-
ably because its angle of incUnation to the vertical was very
much smaller, and the tree was therefore less effectively
subjected to the stimulus of gravity.

Fig. 135. Record of the Sijbaria Palm from noon for 24 hours.
Successive dots at intervals of 15 minutes.

It will be shown in the next chapter that a unidirec-
tional movement cannot take place under diffuse stimula-
tion which acts on all sides of an organ, unless it has been
rendered anisotropic. There are reasons to beheve that
the stimulus of gravity is effective in inducing anisotropy.

The next question is — Is the diurnal movement confined
only to the Date Palm or do other Palms exhibit similar
movements ?

Diurnal Movement of Kentia Palm

In answer to this question I planted in the grounds of
my Institute a Kentia Palm at a considerable inchnation

KENTIA PALM 233

to the vertical. After a time it became curved upwards
under the stimulus of gravity, and showed a very marked
up and down movement similar to that of the Faridpore

Fig. 136. The Diurnal Indicator.

Movement up and down of tree indicated by anti-clock\\'ise or
clockwise movement of index on the circular scale (Kentia).
{See text.)

Palm. The apparatus described below made it easy to
demonstrate the periodic diurnal movement.

The Diurnal Indicator.— A small wheel is adequately
supported in a fork not shown in the figure. The upper end
of a string S is attached to the Palm where it curves up,

234 CHAP. XXI. THE * PRAYING ' PALM

the string itself making a loop round the wheel, the lower
end of the string being tied to the spiral spring T. When
the tree is erecting itself, the wheel is rotated anti-clockwise,
the angle of rotation being read by the index on a circular
scale C. Fall of the tree causes clockwise rotation of the
wheel (fig. 136). By making the diameter of the wheel
sufficiently small the Diurnal Indicator can be made
extremely sensitive.

From the results that have been obtained it would
appear that two conditions are necessary to ensure the
striking display of the periodic up and down movement of
the tree. The first is that the tree should have been rendered
anisotropic by the previous action of the stimulus of gravity ;
the second is that the tree should be subjected to the diurnal
variation of temperature. These, as well as other condi-
tions affecting the diurnal movement of plants, will be
treated in detail in the next chapter.

Summary

The ' Praying ' Palm of Faridpore, growing at an inclina-
tion of 60° to the vertical, exhibited a diurnal movement such
that its head became erected in the morning and lowered
towards the evening, the outspread leaves becoming pressed
against the ground.

Diurnal records of temperature and of movement of the
tree show that the two curves closely resemble each other.
The rise of temperature was followed by the fall of the tree
and vice versa.

The movement was not physical but physiological, as
was proved by cessation of all movement after the death of
the tree.

The influence of light is negligible in the diurnal move-
ment.

The movement is found to be essentially due to the
diurnal variation of temperature, a shght fluctuation of

SUMMARY 235

temperature being often quickly followed by a correspond-
ing movement of the tree.

Anisotropy induced by previous geotropic stimulation
appears to be a contributory factor in the striking mani-
festation of diurnal movement. For it appears that the
greater the incHnation of the tree to the vertical, the greater
is the extent of its periodic movement under variation of
temperature.

CHAPTER XXII

DIURNAL MOVEMENTS OF PLANTS RENDERED ANISOTROPIC

BY GRAVITY

Facts were given in the previous chapter which seemed to
indicate that one of the factors in the diurnal movement

Fig. 137. Diurnal curve of movement of procumbent young stem

of Mimosa pudica.

Successive dots at intervals of 15 minutes.

of the Palm under variation of temperature is the pre-
vious induction of anisotropy by the stimulus of gravity.
The question next arises whether these movements are

RECORD OF STEM AND LEAF 237

characteristic only of Palms, or whether they can also
be detected in other plants under parallel conditions. In
answer to this, I first experimented with the procumbent
stem of a young plant of Mimosa (fig. 137).

Diurnal Movement of Stem of Mimosa

Experiment 128. — The stem was nearly horizontal and
therefore subjected to the stimulus of gravity. The diurnal
records of the movement of the stem and of the variation
of temperature showed that while the temperature rose
from noon to 2.30 p.m., the Mimosa stem exhibited a fall.
The temperature fell after the thermal noon, this being
attended by rise of the stem, the lag of response being
about an hour. The erectile movement continued wdth the
fall of temperature till about 6 next morning. After this
the temperature began to rise and the stem responded by
a fall. As in all cases hitherto considered, the erectile
movement of the plant occurred from thermal noon to
thermal dawn, the fall taking place from thermal dawn to
thermal noon (fig. 137). The diurnal record of the pro-
cumbent stem of Mimosa is thus in every way similar to
that of the Sijbaria Palm Tree.

If it be true that induction of anisotropy by the geotropic
stimulus renders the stem of the Palm effective to respond
to variation of temperature by the characteristic up and
down movement, then ordinary stems curved under the
action of gravity, as well as dia-geotropically outspread
leaves of plants, might also be expected to exhibit such
movements.

Diurnal Records of Stem and Leaf

Experiment 129. — I took diurnal records of a geotropically
curved stem of Tropaeolum and of a dia-geotropic leaf
of Dahlia for two days in succession. The thermal record
shows the usual fall of temperature after thermal noon, from

238 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS

2.45 P.M. to thermal dawn next morning at 6 a.m., that is
to say, for nearly 16 hours. Rise of temperature occurred
through the same range in about 8 hours. The average

Fig. 138. Diurnal curve of the procumbent stem of Tropaeolum
majus and of the leaf of Dahlia for two successive days.
In the thermographic record the up-curve represents fall,
and down-curve rise of temperature.

rate of rise of temperature was thus about twice as quick
as the average fall, very clearly shown by the difference in
the slopes of the curve during thermal descent and ascent..

RECORD OF TROPAEOLUM 239

The records of the movements of the procumbent stem and
leaf exhibited a striking paralleHsm. They 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 also that of the thermal curve, relatively more
abrupt. The records on two successive days are seen to
be very similar (fig. 138).

Diurnal Record of Tropaeolum under Constant

Temperature

The fact that the diurnal movement was due to variation
of temperature was further proved by maintaining the
Tropaeolum stem, employed in the last experiment, at a
constant temperature.

This was secured by the construction of a special chamber,
in which the plant and the recording apparatus were placed.
This chamber was maintained at a constant temperature
by an electro-thermic regulator which interrupts the heating
current as soon as the temperature of the chamber is raised
a hundredth part of a degree above the predetermined
constant temperature. The automatic make-and-break
of the current takes place in rapid succession, so that
the temperature of the chamber is maintained constant
within one tenth of a degree Centigrade, throughout day
and night.

Experiment 130. — The records of the Thermograph and
of the plant's movements were now taken simultaneously.
On the first day the temperature was maintained constant,
with the result of abohtion of the periodic movement of the
plant. This is clearly seen from the horizontal curves
recorded during the first 24 hours. The thermal regulator
was then put out of operation, thus restoring the normal
diurnal variation of temperature ; the result of this was that
the stem exhibited once more its normal periodic movement

(fig. 139)'

240 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS

Experiment 131. — The diurnal records of adult leaves
of a number of plants, such as DahUa, Papaya, Croton and
others, exhibit the same characteristics as shown by the
Palms and by the geotropically curved stems. In all these,
the fall of temperature from thermal noon to thermal dawn

Fig. 139. Abolition of diurnal movement in Tropaeolum under
constant temperature, and its restoration under normal daily
variation. The upper record is of temperature and the lower
of plant-movement. The horizontal parts of the record
relate to the period of constant temperature.

induced an up-movement of the leaf, while rise of temperature
from thermal dawn to thermal noon caused a fall (fig. 140).
There is an individuality which characterises the record of
each species of plant, and thus makes it possible to identify
it from its autograph.

The facts given above appear to indicate that organs
previously subjected to gravitational stimulation exhibit
characteristic diurnal movements under variation of tem-

VARIATION OF TENSION

241

perature. The possibility of the influence of other factors
in the diurnal movement will now be considered.

Fig. 140. Diurnal record of adult leaves of Dahlia, Papaya,

and Croton.

Diurnal Variation of Tension and Bulk in

Different Organs

Kraus found that the tissue-tension of a shoot exhibits
a daily periodicity under normal alternation of day and
night. The tension diminishes throughout the day as the

242 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS

intensity of light increases, while it decreases in the early
afternoon as the intensity of light diminishes. Variation
of temperature within normal limits does not, according to
him, appear materially to affect the daily period. In other
words, it is the variation of light, and not of temperature,
that has a marked influence on the tension of the tissues.
In the case of diurnal movement of trees, however, the
effective factor is not light but variation of temperature.

Even if variation of tension were due to variation of
temperature, it is by no means clear how this could cause
a definite up and down movement of the tree. Rise or fall
of temperature acting on the tree as a whole cannot produce
any unidirectioned movement unless one side is relatively
more active than the other, as is the case only in anisotropic
organs.

In a young tree the geotropic effect is outwardly mani-
fested by the upward curvature induced in the growing
region. I have shown further that the cortex of an old
rigid stem, though incapable of outward movement, is still
sensitive to stimulus.^ The cortex of the tree throughout
its length may thus be rendered differentially excitable by
the prolonged action of geotropic stimulus which renders
it anisotropic. The fact that a condition of anisotropy is
essential for the production of diurnal movement under
diffuse stimulation will presently be demonstrated.

Diurnal Variation of Tissue-Tension in Mimosa

It has just been stated that the characteristic response
under variation of temperature is best exhibited by an
organ which is anisotropic. Millardet found a daily
periodicity of tension in Mimosa pudica, shown by the up
and down movements of the leaf ; the organ of response is
here the pronouncedly anisotropic pulvinus. Millardet was
thus able to correlate the periodic movements of the leaf
with variation of temperature and of tension. The maximum

1 The Motor Mechanism of Plants (1928), p. 141.

EFFECT OF REVERSED ANISOTROPY 243

tension was found to occur before dawn, when the petiole
was erected towards the apex of the stem. Tension
decreased during the day and reached minimum early in
the evening, when the petiole fell, the movement being
away from the apex of the stem. The relation between
change of temperature and tension increased with the rise
and decreased with the fall of temperature.

The anisotropy of the responding pulvinus in Mimosa is
natural and permanent. This suggests the question, What
would follow if the Mimosa plant were placed upside down ?
The periodic movements of the petiole in relation to the
axis of the plant will obviously remain the same, but will
appear reversed in space. Maximum tension in the morning
will make the petiole approach the tip of the stem — that is
to say, the movement will be downwards, instead of upwards
as in the normal position.

Effect of Reversal of Induced Anisotropy on

Diurnal Movement

The next case to be considered is that in which the
anisotropy is not natural or permanent, but has been induced
by geotropic stimulation and is thus capable of becoming
reversed under appropriate conditions. For example, if
a stem, say, of Tropaeolum be held horizontal, it will curve
upwards ; one side, the upper (A) , will be contracted, and the
lower (B) expanded, the radial organ thus becoming aniso-
tropic. Next, if the stem be inverted by rotating it
through 180°, then the side A will have become the lower ;
the former geotropic curvature will shortly become reversed,
and A will undergo a change from contraction to expansion.
The induced anisotropy will thus undergo reversal.

If the periodic movement depends on the anisotropy
that is induced by geotropic stimulation, three stages of
transformation should be presented in the diurnal record
of the plant :

244 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS

1. The stage of normal anisotropy, when A is the

upper side and contracted ;

2 . The transitional, shortly after inversion (A below) ; and

3. The stage of reversed anisotropy, when the plant is

geotropically readjusted in the new position (A
below and expanded).

Fig. 141. Effect of inversion of the plant on diurnal movement.
a, normal record; b, record 24 hours after inversion, and
c, after 48 hours. (Tropaeolum.)

Experiment 132. — The normal, the transitional and the
reversed diurnal records were obtained with an identical
horizontal stem of Tropaeolum subjected to stimulus of
gravity (fig. 141). In a is seen the normal diurnal curve
when the surface A was uppermost ; the specimen was
then inverted, and it took nearly two days for the stem to
readjust itself to the new state of geotropic equilibrium.
The record b was recommenced 24 hours after inversion;
the persistence of the previous movement is seen in the
reversed curve during the first half of this record ; but in

ANALOGY WITH THERMONASTY 245

the second half the record became true ; on the third day
the inverted plant gave a record (c) which from an external
point of view was similar to that given by the plant in the
first or normal position.

There is another aspect of the subject which may be
of interest, namely, whether geotropic irritability itself
undergoes variation under high temperatures in the tropics.
The results of various investigations have shown (as
will be described in a later chapter) that geotropic
stimulation is effected by the fall of starch-grains in the
cells of the statolithic apparatus. Is the efficiency of this
apparatus modified by high temperature ? This problem
I investigated by the method of geo-electric response, in
which the excitatory reaction under geotropic stimulation
was detected by the concomitant electric response. I found,
for example, that while the intensity of geo-electric response
was, generally speaking, very marked in Calcutta during the
month of February with an average temperature of 20° C, it
disappeared by the middle of April, when the average tem-
perature had risen to about 30° C. With Tropaeolum majus
I could get no response even in March. On repeating the
experiments with the same plant three months later at my
Mayapuri Research Station, Darjeeling, I was considerably
surprised to find that the geo-electric response of Tropaeolum
was fully vigorous at the hill station, where the temperature
was lower than 20° C. This would appear to show that
geotropic irritability is accentuated within limits by a fall of
temperature and depressed by a rise. Though some plants
exhibit this change in a marked degree, yet it cannot be
asserted that all plants exhibit it. The particular periodic
movements under consideration can, however, be explained
by the effect of variation of temperature on an organ
rendered anisotropic by geotropic stimulation.

Analogy with Thermonasty

It has been shown that in an anisotropic growing organ,
rise of temperature, acting on it as a whole, induces a

246 CHAP. XXII. DIURNAL MOVEMENTS OF PLANTS

thermonastic movement in one direction, while fall of
temperature causes movement in the opposite direction. In
many ways the movement of the Palm appeared to be ther-
monastic. Thermonasty has, however, been defined as the
characteristic of differentially growing organs the anisotropy
of which is natural and ingrained. But since I have shown
(i) that there is a continuity of reaction between growing
and non-growing organs, and (2) that anisotropy may be
induced in an organ by the prolonged action of geotropic
stimulus, it follows that the phenomena of the diurnal move-
ments of trees and other organs may well be included under
the wider generalisation of Thermonasty.

It will be convenient to use the short term thermo-
geotropism as a descriptive phrase for the diurnal move-
ments under variation of temperature of plants rendered
anisotropic by geotropic stimulation.

Effect of Transpiration on Periodic Movement

A tree laid horizontally would exhibit a passive down-
ward droop due to loss of turgor, resulting from excessive
transpiration, this loss being very great at midday ; at
night there might be an opposite movement on account
of diminished transpiration. But such movements, more
or less passive, would not account for the active force which
enabled the Faridpore Palm to lift a man off the ground
(p. 229). An additional factor, not entirely dependent on
transpiration, thus appears to co-operate in the diurnal
movement. Whether this is so or not is demonstrated by
the following experiments.

Experiment 133. Periodic movement after abolition of
transpiration. — A Kentia Palm was placed horizontally in
my greenhouse under a canvas tent in semi-darkness ; the
tent also protected the plant from mechanical disturbance
caused by gusts of air. After taking these precautions it
was easy to obtain very accurate records of the diurnal
movement of the Palm by means of the Automatic Recorder.

EFFECT OF TRANSPIRATION

247

The record was taken on the first day under normal tran-
spiration (middle record, fig. 142). On the second day the

Fig. 142. Record of Kentia Palm before and' after abolition

of transpiration.

1. Record of diurnal variation of temperature, which was very

similar on two successive days.

2. Diurnal record of the Palm under normal transpiration.

3. Persistence of diurnal movement after abolition of transpira-

tion. {See text.)

Stem and all the leaves of the Palm were very thickly covered
with an impermeable coating of vasehne, which practically

248 CHAP. XXn. DIURNAL MOVEMENTS OF PLANTS

abolished the loss of water by transpiration. Any periodic
movement due to change in the rate of transpiration must
now have come to an end. The record (lowest illustration,
fig. 142) shows that there was no such arrest of movement ;

Fig. 143. Parallel record of a dia-geotropic leaf (Erythrina indica).

1. Record of diurnal variation of temperature.

2. Normal record of diurnal movement.

3. Diurnal record of movement after abolition of transpiration.

the vaselined tree executed the diurnal movement in much
the same way as under normal conditions ; and not only
on that day but even on the next day as well.

Experiment 134. Periodic movement of leaf after aboli-
tion of transpiration. — A parallel experiment was carried out
with the dia-geotropic leaf of Erythrina indica (fig. 143).
The results are essentially the same as those with the stem

SUMMARY 249

of the Kentia Palm. Record i is of diurnal variation of
temperature ; record 2 is the normal diurnal record of
the movement of the leaf ; record 3 was taken of the
same leaf after the plant and all its leaves had been thickly
coated with vaseline. The abolition of transpiration is
seen to have had little effect on its diurnal movement.

The facts described afford conclusive evidence that the
diurnal movement is not mainly dependent on changes
in transpiration ; that there is another factor, defined
as thermo-geotropism, which co-operates in the diurnal
movements of plants.

Summary

Continuity is shown to exist between the thermo-
geotropic response of rigid trees, young stems, and adult
leaves of plants.

In all these an erectile movement is exhibited from
thermal noon to thermal dawn, and a movement of fall
from thermal dawn to thermal noon.

The predominant effect of variation of temperature on
the diurnal movement is demonstrated by the absence of
any movement when the temperature is constant.

The effect of the stimulus of gravity in inducing aniso-
tropy, which determines the characteristic diurnal move-
ment, is proved by the effect of inversion of the plant on
the diurnal record.

The activity of thermo-geotropism as an independent
factor in the diurnal movement appears from its persist-
ence after the abolition of transpiration.

The thermo-geotropic movement is in many ways
analogous to the thermonastic movement. A wider
generalisation is reached by the inclusion under the head of
Thermonasty of the response of non-growing organs rendered
anisotropic by the stimulus of gravity.

CHAPTER XXIII

PHOTOTROPIC TORSION

In addition to positive or negative curvature induced by
light, a torsional response also occurs under certain con-
ditions. 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. It has been supposed
that such torsions were produced by the action of a number
of external factors, such as light, gravity, and weight of the
organ, which individually led to curvature but which in
combination induced torsion. Later investigations have,
however, shown that torsion actually occurs when light
alone is the external factor. No satisfactory explanation
has, however, been given of the mechanics of the torsional
movement.

The experiments here described were planned to throw
light on this obscure phenomenon. They show :

1. That the torsional response is not dependent on the

combination of two curvatures ;

2. That it is independent of the effect of weight ;

3. That it can be induced not merely by the stimulus

of light, but by all forms of stimulation ;

4. That the direction of the torsional response depends

on two factors : the direction of the incident stimulus,
and the differential excitability of the organ ; and

5. That there is a definite law which determines the

direction of the responsive movement of torsion.

pulvinus of mimosa 25i

Experimental Arrangements

I will first describe a typical experiment on torsional
response under the action of light. It has been shown, in
the case of the pulvinus of Mimosa, that light of moderate
intensity and of short duration applied on the upper half
induces a slow up-movement, while the stimulus of hght
apphed below induces a more rapid down-movement. The
difference is due to the fact that the lower half of the pul-
vinus is relatively the more excitable. Vertical light thus
induces a movement in a vertical plane. But an interesting
variation of the response occurs under the lateral action of
light. A stimulus will be termed lateral when it acts on
either the right or left flank of a dorsiventral organ.

The present series of experiments was carried out with
the leaf of Mimosa, and in order to eliminate the effect of
w^eight, and also to obtain a record of pure torsion, the
following device was employed : The petiole was held by
a hooked support made of a thin rod of glass, the points of
support being the concavity of a smooth surface. Friction
and the effect of weight are thus practically ehminated ;
the hooked support prevented up or down movement 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. 144). The end of the
aluminium wire is attached to the short arm of a recording
lever ; there is thus 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 can thus be
obtained from the dotted record.

The experimental device just described makes possible
the study of the effect of various stimuli apphed on the
flank of the pulvinus including 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

252

CHAP. XXIII. PHOTOTROPIC TORSION

against the hands of a clock. This responsive torsion, right-
handed or left-handed, will presently be shown to depend on
the direction of the incident stimulus.

Fig. 144. Diagrammatic representation of the Torsional

Recorder.

Lateral stimulation, r, applied on right flank (dotted arrow) induces
clockwise torsion c (dotted circle). Stimulus, l, applied on
left flank (full arrow) induces anti-clockwise torsion a (thick
circle).

Effect of Lateral Stimulation by Light

Experiment 135. — The pulvinus of the leaf was stimu-
lated by a horizontal beam of light thrown laterally upon
it ; the area contiguous to the line of junction of the
upper and lower halves of the anisotropic organ responded
by differential contraction. When light struck on the

DIRECTIVE ACTION OF STIMULUS 253

right flank, indicated by dotted arrow, the responsive tor-
sion was clockwise ; the responsive reaction thus made the
upper and less excitable half of the pulvinus face the stimulus.
Fig. 145 gives the record of the torsional response ; cessation
of stimulation is followed by recovery.

Fig. 145. Fig. 146.

Fig. 145. Record of torsional response of pulvinus of Mimosa
pudica. Clockwise response to stimulation by light applied
on the right flank (up-curve).

Fig. 146. Record of anti-clockwise torsional response to light
appHed on the left flank (down-curve). Duration of apphca-
tion between two thick dots. Successive dots at intervals
of 10 seconds.

Directive Action of Stimulus

Experiment 136. — If now the direction of stimulation
be changed so that the light strikes the left flank instead
of the right, the torsional response will be anti-clockwise
(fig. 146). 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 may therefore be concluded that the
direction of torsion, clockwise or anti-clockwise, depends on

254

CHAP. XXIII. PHOTOTROPIC TORSION

two factors — namely, the direction of incidence of the
stimulus, and the simultaneous but differential contraction
of both halves of the organ.

Effect of Different Modes of Lateral

Stimulation

I proceed to show that torsional response is induced
not merely by the action of light, but by other modes of
stimulation as well.

Fig. 147.

Fig. 148.

[Fig. 147. Clockwise torsion under stimulation of right flank^

by thermal radiation.

Fig. 148. Anti-clockwise torsion under stimulation by thermal
radiation applied to left flank. (Mimosa.)

Experiment 137. Effect of chemical stimulation. —
Dilute hydrochloric acid was at first applied on the left
flank of the pulvinus along the line of junction of the upper
and lower halves. This gave rise to a responsive torsion
against the hands of the clock. Chemical stimulation of the
right flank induced, on the other hand, a torsional movement

DIFFERENTIAL EXCITABILITY 255

with the hands of the clock. Here also the direction of
stimulus is found to determine the direction of responsive
torsion.

Experiment 138. Effect of lateral stimulation by thermal
radiation. — 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 shield
with a narrow horizontal slit, so as to localise the stimulus
at the junction of the upper and lower halves of the pulvinus.
The effectiveness of radio-thermal stimulus being great,
the response was very pronounced. Stimulus applied at
the right flank induced right-handed or clockwise torsion
(fig. 147) ; appHcation at the left flank gave rise to left-
handed or anti-clockwise torsion (fig. 148).

Geotropic stimulus. — The stimulus of gravity induces
a similar responsive torsion which is determined by the
direction of the incident stimulus. This will be fully
described in a subsequent chapter.

Effect of Differential Excitability on the
Direction of Torsion

Under normal conditions the torsional response to light
places the upper surface of the leaf or leaflet at right angles
to the incident 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 and others —
induce similar responsive torsion. The torsional response
is determined not only by the direction of the incident
stimulus, but also by 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 can be rendered relatively the less excitable.
Lateral stimulation by light is now found to induce a tor-
sional movement which is the reverse of the normal, so
that the upper surface of the leaf turns away from light.

256 CHAP. XXIII. PHOTOTROPIC TORSION

Teleological advantage cannot, therefore, be the determining
factor which causes the directive movement.

In all the instances given above, and under every mode
of stimulation, the responsive movement is such as to cause
the less excitable half of the pulvinus to face the stimulus.

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.

Advantage of the Method of Torsional Response

The experimental study of torsional response not only
opens a new line of inquiry into the reactions of the plant
to various stimuli, but it also possesses certain special
advantages. For instance, in investigations on the response
of the leaf of Mimosa to light by the ordinary method, the
responsive movements in a vertical plane are recorded.
The responsive up-movement, induced by light acting from
above, is, however, opposed by the weight of the leaf. But
in the torsional response, where the leaf is held by a hooked
glass support, the movement is free from the complicating
factor of the weight of the leaf. The pulvinus of Mimosa,
again, occasionally exhibits an autonomous pulsation ; in
the ordinary method of record the true response to external
stimulation may thus be modified by the 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 stimulation is unaffected by the
spontaneous movement of the leaf, The torsional method,

SUMMARY 257

moreover, opens out possibilities of inquiry in new directions,
such as the comparison of the excitatory effects of different
stimuU by the Method of Balance.

The Torsional Balance

A beam of light falling on the left flank of the pulvinus
of Mimosa initiates a torsion against the hands of the clock.
A second beam falling on the right flank initiates a contrary
movement ; the resultant effect is, therefore, determined
by the effective stimulation of the two flanks. The pulvinus
thus becomes a delicate indicator by which the effectiveness
of 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 139. — A parallel beam of light from a small
arc-lamp, passing through a blue glass, falls on the left
flank of the pulvinus ; a beam of blue light also strikes
the pulvinus on the right flank, the intensity of the latter
being so adjusted that the resultant torsion is zero. The
blue glass on the left side is now removed, the unobstructed
white light being allowed to fall on the left flank of the
pulvinus. This is found to upset the balance, the resultant
torsion being anti-clockwise, proving that white light
induces greater excitation than blue light. A red glass is
now interposed on the left side, with the result that the
balance is upset in the opposite direction, showing that the
phototropic effect of red light is comparatively feeble. It
is thus possible to compare the tropic effect of one form of
stimulation with that of another. It is enough here to draw
attention to the various investigations rendered possible by
the Method of Torsional Balance. Examples of some of
these will be given in a subsequent chapter.

Summary

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

258 CHAP. XXIII. PHOTOTROPIC TORSION

The responsive torsion is determined by the direction of
the incident stimulus, and by 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 in the demonstrated laws of
torsional response.

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

The Method of Torsional Balance permits of comparison
of the excitatory efficiency of two different stimuli which
act simultaneously on opposite flanks of the organ.

CHAPTER XXIV

THE AFTER-EFFECT OF LIGHT

Two types of diurnal movement have been considered : one
in response to the predominant effect of variation of hght,
and the other to that of changing temperature. There are,
however, certain other organs which are sensitive to varia-
tions both of hght and of temperature. The effect of hght is,
generally speaking, antagonistic to that of rise of tempera-
ture ; hence the movement which is the resultant of the
two effects requires careful analysis.

Still greater complexity is introduced by the conflict-
ing factors of the immediate and the after-effect of light.
Great obscurity surrounds this after-effect phenomenon,
and I endeavoured to determine its characteristics by
the electric method of investigation. A fuller account of
the after-effect of light on the response of various plant-
organs and of the animal retina will be found elsewhere.^
I here refer only to one or two characteristic results which
have immediate bearing on the present subject.

Direct stimulation by light induces an excitatory reac-
tion, which is exhibited mechanically by contraction and
electrically by induced galvanometric negativity. Under
continuous stimulation the excitatory effect, whether of
positive curvature or of induced galvanometric negativity,
is found to attain a maximum. The positive tropic curva-
ture and the induced galvanometric negativity exhibit,
on account of fatigue at the directly stimulated proximal
side, a decline and neutralisation. This neutralisation is

^ Comparative Electro-Physiology , p. 392.

26o CHAP. XXIV. THE AFTER-EFFECT OF LIGHT

further favoured by transverse conduction of excitation to
the distal side (p. 134).

The character of the after-effect will presently be shown
to be modified by the duration of the antecedent stimula-
tion, the different phases of which will, for convenience,
be distinguished as pre-maximum, maximum, and post-
maximum.

Determination of After-Effect by Electric

Response

Confining attention for the present to indications given
by 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. Fig. 149 gives the galvanographic record
of the electric response of the leaf-stalk of Bryophyllum
to light ; the up-curve represents increasing negativity
which, after attaining a maximum, undergoes neutralisa-
tion as seen in the down-curve. Fig. 150 exhibits the
various after-effects on sudden stoppage of light at three
different stages — before the attainment of maximum, at the
maximum, and after the maximum. Light is applied at

arrow.

Experiment 140. After-effect of pre-maximum stimula-
tion.— Continuous stimulation induces increasing galvano-
metric negativity. When stimulus is stopped at a, before
response reaches the maximum, the after-effect is a persist-
ence of excitatory galvanometric negativity, which carries
the response record higher up, after which 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 the response followed by
recovery (fig. 150).

Experiment 141. After-effect at maximum. — In this case
the photic stimulation was continued till the attainment of
maximum, when light was suddenly removed at h. The

ELECTRIC RESPONSE 261

after-effect was no longer a persistence of the negative
response, but disappearance of negativity and recovery to
zero-line of equilibrium at c (fig. 150).

Fig. 149. Fig. 150.

Fig. 149. Electric response of the petiole of Bryophyllum under
continuous photic stimulation. Increasing negativity repre-
sented by up-curve, neutralisation by down-curve.

Fig. 150. Semi-diagrammatic representation of electric after-
effects due to photic stimulation.

Pre-maximum stimulation, produced by stoppage of light at a,
gives rise to continuation of previous response followed
by recovery.
Stoppage of light at maximum, h, gives rise to recovery to equi-
librium position c.
Stoppage of light at post-maximum, c, gives rise to overshooting
below zero-line, as seen in the dotted record, c d.

262 CHAP. XXIV. THE AFTER-EFFECT OF LIGHT

Experiment 142. After-effect at post-maximum. — In this
case the light was continued till there was complete neu-
tralisation at c, the curve of response returning to zero-line ;
to all outer seeming the responsive indication of the organ is
the same as before excitation. But stoppage of stimulation
at c caused an overshooting at a rapid rate far below the
zero-line (fig. 150).

The condit'on 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 ' organ been exactly the same as when it was
fresh, cessation of stimulus would have kept the galvano-
metric spot of light at the zero-position.

The electric investigations described above indicate that
the after-effect is modified by the duration of stimulation,
and that :

1. The after-effect of pre-maximum stimulation is the

continuation of response in the original direction
(upwards, and away from the zero-line), followed
by recovery ;

2. The after-effect of maximum stimulation is a recovery

towards zero-position ; and

3. The after-effect of post-maximum stimulation is an

overshooting of response downwards, below the
zero-line.

Tropic Response to Light and its After-Effect

I next describe the after-effects of light exhibited by
mechanical response, the results of which will be found
to be parallel to those given by electric response. The
specimen employed was 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
the equilibrium of the leaf or leaflet undergoes a periodic
change. The equilibrium position, however, remains fairly

MECHANICAL RESPONSE 263

constant for nearly 2 hours about midday, the variation
of temperature at this period being slight. The pure effect
of light can be obtained by carrying out the experiment
during this period, and thus complications which may arise
from autonomous pulsation are also avoided.

The duration of the experiment may be shortened by the
choice of a 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 in the
following experiment w^as a 50-candle-power incandescent
lamp. The intensity was increased to a suitable value by
focusing the light on the upper half of the pulvinus by means
of a lens. The intensity was so adjusted that the maximum
positive curvature occurred in the course of about 6 minutes,
and complete neutralisation was attained after an exposure
of 17 minutes.

Experiment 143. After-effect atpre-maximum. — Light was
allowed to act on the upper half of the pulvinus for 2 minutes
and 20 seconds ; this induced an up-movement, i.e. a posi-
tive curvature. On the stoppage of light the up-movement
continued for i minute and 20 seconds, after which the
down-movement of recovery was completed in 6 minutes
and 20 seconds (fig. 151). The immediate after-effect is
thus a movement upward, away from the zero-line of
equilibrium. The result is seen to agree with the electric
after-effect of pre-maximum stimulation.

Experiment 144. After-effect at maximum. — Application
of light for 5 minutes and 20 seconds induced a maximum
positive curvature. Stoppage of light was followed at
once by recovery, which was completed in about 10 minutes

(fig- 152).

Experiment 145. After-effect at post-maximum. — 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 ; neutralisation took place
after application of light for 17 minutes. On the stoppage

264 CHAP. XXIV. THE AFTER-EFFECT OF LIGHT

of light there was a sudden overshooting below the zero-line
(fig. 153) ; the rate of the movement on the cessation of
light was nearly twice as quick as during the process of
neutralisation.

I also obtained a very interesting record of the post-
maximum after-effect of light with Cassia alata.

Fig, 151,

Fig. 152,

Fig. 153.

Fig. 151. Light applied at arrow, and stopped at the second
arrow within a circle. After-effect of pre-maximum stimula-
tion is continuation of positive curvature followed by
recovery.

Fig. 152. After-effect of stimulation at maximum ; recovery
towards zero-position of equilibrium.

Fig. 153. After-effect of post-maximum stimulation 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.

Experiment 146. — In Cassia, as in Mimosa, light acting
from above induces at first an erectile movement of the leaf
which reaches a maximum, after which there follow neutral-
isation and reversal. In the record given (fig. 154), light
from a small arc-lamp acting on the upper half of the
pulvinus for 48 minutes induced the maximum positive
curvature ; this was completely neutralised by further
exposure to light for 20 minutes ; cessation of light was
followed by a rapid fall of the leaf beyond the position of

SUMMARY 265

equilibrium, the responsive after-effect being more rapid
than under light. The after-effect of prolonged exposure

Fig. 154. Post- maximum after-effect of light on response of

leaf of Cassia.

There is an overshooting on cessation of light at arrow within

a circle.

is thus an ' overshooting ' beyond the normal position of
equilibrium.

Summary

The after-effect of light is modified by the duration of
the exposure.

Under continued action of light, the electric response
of galvanometric negativity in plants attains a maximum,
after which it undergoes decline and neutralisation.

The electric after-effect exhibits characteristic differ-
ences depending on the duration of the previous exposure
to light.

The pre-maximum after-effect is a temporary continua-
tion of the response induced by light followed by recovery.

The after-effect at maximum is an immediate recovery
to the normal equilibrium,

266 CHAP. XXIV. THE AFTER-EFFECT OF LIGHT

The after-effect at post-maximum is an overshooting
below the position of equiUbrium.

The mechanical effects and after-effects of light are similar
to the corresponding 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 position of equilibrium ; the post-
maximum after-effect is an overshooting, that is, a fall to
below the normal position.

CHAPTER XXV

THE DIURNAL MOVEMENT OF THE LEAF OF MIMOSA

The thermo-geotropic record of diurnal movement of plants
described in a previous chapter consists of an up-curve from
thermal noon to thermal dawn, and of a down-curve from
thermal dawn to thermal noon. The responding organ,
whether an inclined stem or a horizontally placed 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 ;
this apparent difference arises from the presence of fresh
complicating factors, inasmuch as the leaf is sensitive not
only to variation of temperature but also to changes of light.

Experiment 147. Diurnal record of Mimosa. — I took
the diurnal record of Mimosa (fig. 155) for 24 hours,
commencing at 2 p.m., which is the thermal noon. The
summer and winter records are essentially the same, the
only difference being the greater vigour of movement
exhibited by summer specimens. The diurnal movement
of the leaf is very definite and characteristic, for the curves
obtained for several years in succession are found to be
quite concordant. The record may conveniently be divided
into four phases.

First phase. — The leaf erects itself after thermal noon
until 5 or 5.30 p.m. The temperature, it should be re-
membered, 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.

268

CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA

Fourth phase. — There is a fall of the leaf during the
rise of temperature from thermal dawn to thermal noon.

Fig. 155. Diurna record of leaf 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 pulsation. The uppermost record
gives temperature variation, up-curve representing fall of
temperature and vice versa.

The uniformity of the fall is, however, interrupted by one
or more pulsations in the forenoon, which are more frequent
in summer than in winter.

It will thus be seen that the difference between the

CONSTITUENT FACTORS 269

typical thermo-geotropic curve and the curve of IMimosa is
not so great as appears at first sight, with the exception of
the spasmodic fall in the evening. I will presently explain
the reason of the sudden fall in the evening, and of the
multiple pulsations in the forenoon.

It is possible to trace a continuity between the typical
thermo-geotropic reaction and the characteristic diurnal
movement of the leaf of Mimosa which is affected by light.
The young leaves which expand 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 that of erection from thermal noon to thermal
dawn next morning, the record being in every way similar
to the typical thermo-geotropic curve. It is only at the
final stage of development of the leaf that there is the
spasmodic fall in the evening, which will be shown to be a
characteristic post-maximum after-effect of light.

The complexity of the diurnal movement of Mimosa
arises from the fact that there are three factors whose
fluctuating effects are different at various hours of the day.
The position at any particular hour results from the algebraical
summation of the effects of the following factors : (i) the
thermo-geotropic reaction ; (2) the autonomous pulsation of
the leaf ; (3) the immediate effect of light ; and (4) its
after-effect. I will take up the detailed consideration of
the subject in the following order :

1. The thermo-geotropic reaction. — A crucial experiment
is described later which demonstrates the thermo-geotropic
effect in the diurnal movement of the leaf.

2. Autonomous pulsation of Mimosa. — The natural pulsa-
tion of the leaf is obscured by the paratonic effect of
external stimulation. The occurrence of pulsatory response
in the morning record {see fig. 155) led me to search for

270 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA

multiple activity of the pulvinus. I found that the very
young leaves in spring exhibited automatic pulsation
throughout day and night ; in older leaves, tuned to diurnal
periodic movements, these natural pulsations are more or
less suppressed, but several pulsations may be exhibited in
the forenoon even by mature leaves (fig. 156).

3. The immediate effect of light. — This is not constant,
but will be shown to undergo a definite variation with the

Fig. 156. Record of automatic pulsation of Mimosa leaf in the
forenoon. Average period 25 minutes.

Successive dots at intervals of a minute.

intensity and duration of light. The great difficulty of
recording the change of intensity of light was overcome
by the construction of the Radiograph already described
(P- 197)- I reproduce a record (fig. 157) obtained in my
greenhouse on March 5, 1919, which gives a general idea
of the variation of light from morning to evening ; the
record shows that 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 fall of intensity was very abrupt after
5 P.M., the effect being reduced to zero at 6.30 p.m.

4. The after-effect of light. — The after-effect is greatly
modified by the intensity and duration of the illumination.

SPASMODIC FALL OF LEAF 27I

which give rise to the characteristic responses — the pre-
maximum, the maximum, and the post-maximum.

Fig. 157. Photometric record showing the variation of
intensity of Hght from morning to evening.

Successive dots are at intervals of 30 minutes.

The Spasmodic Fall of the Leaf

Pfeffer regarded the fall of the leaf which occurs abruptly
late in the afternoon as due to the increased mechanical
moment of the secondary petioles moving forward on with-
drawal of light. I proceed to show that this characteristic
movement occurs even after complete removal of the sub-
petioles, so that an increased mechanical moment cannot be
the true explanation of the fall.

Experiment 148. Diurnal movement of the petiole after
removal of sub-petioles.— In this experiment the possibihty
of any variation of mechanical moment 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 stimulation caused
by the amputation induced excitatory fall of the leaf, but

272 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA

it recovered its normal activity after a period of 3 hours
or so. The diurnal record of the leaf was commenced shortly
after i 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. 158). The fall of the leaf cannot

therefore be due to increased
mechanical moment. The
effect of weight was, more-
over, eliminated in another ex-
periment on torsional response
(Experiment 149), which also
exhibited a sudden movement
of the leaf after 5 p.m.

Pfeffer, in his * Entste-
hung der Schlafbewegung '
(1907), has offered another
explanation of the sudden fall
of the leaf of Mimosa. This,
according to him, is not the
direct effect of diminished in-
tensity 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. 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 (i) that the tension
continuously decreases till the evening ; and (2) that as soon
as the phototropic restraint which holds the leaf up is
removed, it falls down in accordance with the prevailing
diminished tension.

Fig. 158. Recordof leaf of Mimosa
after removal of sub-petioles.
The leaf fell up to 2 .30 p.m., and
then rose till 5 p.m., after which
there was a spasmodic fall.

Successive dots at intervals of
15 minutes.

GEOTROPIC TORSION 273

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 [cf. fig. 155). As the natural ten-
dency of the leaf was to erect itself, removal of phototropic
restraint cannot be the cause of the movement of fall.

The following experiment not only exhibits the diurnal
curve of an intact plant, but also clearly demonstrates
the thermo-geotropic effect, as well as the immediate and
the after-effect of light.

Diurnal Variation of Geotropic Torsion

I have already demonstrated the torsional response
under unilateral stimulation by light. I shall also show in a
later chapter that similar torsional response is obtained under
the stimulus of gravity (p. 307).

When a Mimosa plant is laid down sideways, so that the
plane of junction between the upper and the lower halves of
the pulvinus is vertical, geotropic stimulus acts laterally on
the two differentially excitable halves of the pulvinus. When
the less excitable upper half is to the left of the observer, the
responsive torsion under geotropic stimulus is clockwise,
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 is counter-clockwise. It has been shown that lateral
stimulation by light gives rise to torsion ; when light acts in
the same direction as the stimulus of gravity, i.e. from above,
there is an enhancement of the rate of torsion, the resulting
curve of response being due to the joint effects of light and
gravity.

Experiment 149. — I obtained a 24 hours' record of the
variation of torsional response in the leaf of Mimosa, com-
mencing with thermal noon at 2 p.m. It is to be borne in

274 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA

mind that increase of torsion corresponds with the erectile
movement of the leaf in the diurnal thermo-geotropic
curve. Inspection of fig. 159 shows that the fall of tempera-
ture after thermal noon at 2 p.m. was attended by an increase
of torsion. The curve went up till about 5 p.m., as in the
ordinary record of Mimosa. The torsion suddenly decreased
under rapid diminution of hght after 5 p.m. The torsion

Fig. 159. Record of diurnal variation of geotropic torsion in

Mimosa leaf.

Up-curve represents increase, and down-curve decrease of torsion.

then increased with falling temperature from 9 p.m. till
thermal dawn next morning. After 6 a.m. there was a
continuous diminution of torsion till 2 p.m.

The diurnal variation of geotropic torsion exhibited by
Mimosa may be summarised as follows : The torsion undergoes
periodic increase during the fall of temperature from after-
noon till next morning, and diminution during rising tem-
perature 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.

AFTER-EFFECTS OF LIGHT 275

a replica of the record of the periodic up and down move-
ments of the leaf.

This method of torsional response has a very important
advantage over that of the ordinary method, since, the
petiole being supported by a loop of wire, the weight of
the leaf can have no effect on the curve of response.

The Characteristic After-Effects of Light

In the previous chapter it was shown (p. 265) :

1. That the pre-maximum after-effect is a temporary

continuation of the response induced by light
follow^ed by recovery ;

2. That the after-effect at maximum is an immediate

recovery to the normal equilibrium ; and

3. That the after-effect at post-maximum is an over-

shooting below the position of equilibrium.

Consideration of these results will be found to explain
the various anomalies in the diurnal movement of the leaf of
Mimosa.

Effect of Artificial Darkness

Experiment 150. — Successive records w^ere taken of the
effect of artificial darkness for 2 hours, alternating with
exposure to light for the same time. The plant was sub-
jected to darkness by placing a piece of black cloth over
the glass case in which it was enclosed, from 12 to 2 p.m. ;
it was exposed to light from 2 to 4 p.m., and then subjected
to darkness once more from 4 to 6 p.m.

The record given in fig. 160 shows that the leaf had
been moving upwards under the action of light (positive
phototropism) ; the moment of exposure to darkness is
marked with a thick dot in the record. The after-effect of
the withdraiml of light is seen to be a movement in the same
direction as that under exposure to light ; this persisted for
10 minutes, followed by recovery which was completed by

276 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA

2 P.M., as shown in the horizontal character of the curve.
On restoration of Hght (at the point marked with the second
thick dot) the leaf moved upwards till the positive photo-
tropic movement attained a maximum in the course of
I hour and 20 minutes, after which neutralisation set in, and

Fig. 160. EfEect of periodic alternation of darkness d, light l,
and~of darkness d, on the response of Mimosa leaf. The
first' darkness causes the pre-maximum after-effect of slight
erection followed by recovery. The subsequent exposure
to 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.

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 a pre-maximum after-effect, whilst in
the second case it was a post-maximum after-effect.

The record of the responses of Mimosa just described
was obtained in the course of experiments which lasted

ARTIFICIAL DARKNESS 277

for more than 6 hours. In order to remove all misgivings
in regard to possible modification of the result by variation
of temperature I carried out the following experiments,
which were completed in a relatively short time. I have
already explained how the duration of experiment can be
shortened by suitable increase of the intensity of light. The

•

• •

•

■ ■

•

©• \

•

• •

• •

• •

• •
••
• •

• •

• •

• •

•

• •

•

•

•

•

•

•
•
• •

•

•

•

•••••

• •

•

-• •
•
•

• •

•

9HH^ *

Fig. 161. Fig. 162. Fig. 163.

Fig. 161. Pre-maximum after-effect of light on Mimosa.

Fig. 162. After-effect at maximum.

Fig. 163. Post-maximum after-effect exhibiting an overshooting

below position of equilibrium.

In the above record light was applied at arrow, and stopped at
the second arrow enclosed in a circle.

following experiments were commenced inside a room at
noon and completed by 2 p.m., so that the temperature
variation during this period did not exceed i° C.

Experiment 151. After-effect at pre-maximum. — Light
from a loo-candle-power incandescent lamp was focused
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
to normal (fig. 161).

278 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA

Experiment 152. After-effect at maximum. — Continued
exposure to light for 18 minutes induced maximum posi-
tive curvature, as seen in the upper part of the curve
becoming horizontal. On the withdrawal of light, there
was recovery to the original position of equilibrium

(fig. 162).

Experiment 153. After-effect at post-maximum. — A fresh
specimen of the plant was taken for this experiment ; it
exhibited maximum positive curvature after an exposure of
20 minutes ; continued exposure for a further period of
17 minutes produced complete neutralisation, as indicated
by return to normal position. Withdrawal of light at this
point gave rise to a rapid down-movement (fig. 163) below
the equilibrium position.

It is now possible to give a full explanation of the
different phases of diurnal movement of the leaf of Mimosa.
The fall of the leaf from its highest position commences
at thermal dawn at 6 a.m. in the morning and continues till
thermal noon at 2 p.m. ; this is the thermo-geotropic reaction
due to rise of temperature. In the forenoon the photo-
tropic reaction is positive, and the fall of the leaf, due to rise
of temperature, is effected in opposition to the response to
light. As the temperature begins to fall after 2 p.m., the
leaf begins to erect itself, and in the absence of any disturb-
ing factor would continue its up-movement till next morning.
But light undergoes rapid diminution after 5 p.m., the after-
effect of which is manifested as an ' overshooting ' of the
leaf in a downward direction. This fall continues till about
9 P.M., after which the leaf erects itself under the thermo-
geotropic action of falling temperature, the maximum
erection being attained at the thermal dawn at about 6 a.m.
next morning.

The peculiarity of the diurnal curve of Mimosa has been
shown to be due to the sensitiveness of the leaf to both
light and variation of temperature. This conclusion is veri-
fied by experiments with other plants similarly sensitive to
both photic and thermal variation. Nothing could be more

DIURNAL CURVE OF CASSIA

279

conclusive in this respect than the remarkable similarity
of the diurnal record of Cassia to that of Mimosa.

Diurnal Curve of the Petiole of Cassia alata

Experiment 154. — The leaf of Cassia exhibits, as does the
leaf of Mimosa, a slight erectile movement after thermal
noon at 2 p.m. ; there is then an abrupt fall, due to rapid
diminution of light after 5 p.m. ; the movement of fall

Fig. 1G4. Diurnal record of Cassia leaf. Note similarity
to diurnal record of Mimosa.

continues till about 9 p.m. The leaf then exhibits a con-
tinuous rise with the fall of temperature, till the chmax
is reached about 6 a.m. in the morning ; the leaf exhibits
later a movement of fall with rise of temperature, there
being a number of pulsatory movements in the forenoon
evidently due to unstable balance under the opposing re-
actions to light and to rise of temperature (fig. 164).

28o CHAP. XXV. DIURNAL MOVEMENT OE MiMOSA

Summary

The very complex type of diurnal movement of the
J^rimary petiole of Mimosa results from the combined effects
of thermo-geotropism and phototropism.

With the exception of a small part of the curve in the
•evening, the diurnal curve of the leaf is essentially similar
to the typical thermo-geotropic curve, exhibiting an erectile
movement from thermal noon to thermal dawn, and a fall
from thermal dawn to thermal noon.

The torsional response of the leaf of Mimosa exhibits
a diurnal variation similar to that exhibited when the leaf
is in the 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 of Mimosa 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 same sudden fall in the evening as does that of the
intact leaf.

The evening fall of the leaf is shown to be a post-
maximum after-effect of light, which causes an over-
shooting, the fall of the leaf carrying it below the position
of equilibrium.

CHAPTER XXVI

GEOTROPISM

No phenomenon of tropic movement appears so inexplicable
as that of geotropism. There are two diametrically opposite
responsive movements induced by the stimulus of gravity :
in the root a curvature downwards, and in the shoot
a curvature upw^ards. The seeming impossibility of ex-
plaining effects so divergent by a single fundamental reaction
to stimulation has led to the assumption that the irrita-
bilities of stem and root are of opposite character. I shall,
however, endeavour to show that this assumption is quite
gratuitous and unnecessary.

Beginning with the simple case of a horizontally laid
shoot, the geotropic up-curvature can be explained by one or
other of the two suppositions : either (i) that the stimulus of
gravity induces contraction of the upper side, or (2) that
it induces expansion of the low^er side. The second of these
two assumptions has found more general acceptance.

In the parallel phenomenon of phototropic curvature,
light incident on one side of the shoot induces local con-
traction and concavity of the directly stimulated proximal
side of the organ. Since light is visible there can be no
difficulty in ascertaining the exact direction of the incident
stimulus and the induced curvature by which the organ
tends to place itself parallel to the raj^s with its apex
towards the source of stimulation. But in geotropism
the stimulus is invisible, and there is no definite knowledge
available about its effective direction in induction of the
responsive curvature.

282 CHAP. XXVI. GEOTROPISM

In order to clear away obscurities connected with
geotropism, it will be necessary to elucidate the following

points :

1. The determination of the effective direction of stimulus
of gravity. This will be demonstrated by two independent
means of inquiry: (i) by the method of algebraical sum-
mation of the reaction to geotropic with that to photic
stimulation, and (2) by the method of geotropic torsion.

2. The sign of excitation is, as previously explained,
a contraction and concomitant galvanometric negativity.
Does the stimulus of gravity, like stimulus in general, induce
this characteristic excitatory reaction ?

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 the direc-
tion* of the stimulus makes with the responding surface.

4. Finally, it is necessary to investigate whether the
assumption of opposite irritabilities of the root and the
shoot is really justified. If not, the opposite curvatures
exhibited by the two organs have to be explained.

I propose in this and in the following chapters to describe
the investigations sketched above, employing two inde-
pendent methods of inquiry — namely, those of mechanical
and of electric response. I describe first the automatic
method that has been devised for an accurate and magnified
record of geotropic movement and its time-relations.

The Geotropic Recorder

The Recorder shown (fig. 165) is very convenient for
the 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 friction with the recording
surface. By an appropriate clockwork mechanism the
levers are pressed for a fraction of a second against the

GEOTROPIC REACTION 283

recording surfaces. The successive dots in the records
may, according to different requirements, be at intervals
varying from 5 to 20 seconds. The records therefore not

Fig. 165. The Quadruplex Geotropic Recorder.

only give the characteristic curves of the geotropic move-
ments of different plants, but also their time-relations.
For certain experiments I employ the high magnification
of 100 times ; a smaller magnification is, however, sufficient
for general purposes.

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 con-
traction of that side, or it may result from compression due

284 CHAP. XXVI. GEOTROPISM

to the active responsive expansion of the lower side. The
crucial test of excitatory reaction under geotropic stimula-
tion is furnished by the geo-electric response. When a shoot
is displaced from the vertical to the 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 (p. 315). The tropic
effect of geotropic stimulation is thus similar to that of any
other mode of stimulation, i.e. a contraction of the directly
stimulated side, which in the present case is the upper side.
The vertical lines of force of gravity impinge on the upper
side, and the effective direction of geotropic stimulus is
therefore the same as the vertical lines of force indicated by
the movement of falling bodies from above to the centre of
the earth. The effective direction of geotropic stimulus
inferred from the above considerations is fully confirmed
by experimental results (p. 308).

It has been explained that excitatory electric response is
manifested even in the absence of mechanical expression of
excitation ; and under geotropic stimulation a firmly held
shoot gave the response of a galvanometric negativity of
the upper side (p. 317). Hence the fundamental reaction to
geotropic stimulation is excitatory contraction, as under other
modes of stimulation.

Determination of the Latent Period

' As regards the interpretation of the record of geotropic
movement, it should be borne in mind that after the per-
ception 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 up-movement : the up-movement of
a shoot, in response to the stimulus of gravity, has to over-
come the opposition offered by its weight. On account
of the bending due to weight there is a greater tension on
the upper side, which, as already mentioned (p. 60), enhances

THE LATENT PERIOD 285

the rate of growth, and thus tends to make that side
convex. There is thus an undue delay in the exhibition
of geotropic response by induced contraction of the excited
upper side. 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 con-
siderablv reduced.

Experiment 155. Geotropic response of the peduncle of
Tuberose. — For this I took a short length of peduncle of

•
-■
•
•
•
•

0' 5' 10' 15'

1 -Mw^^ft JliMilffiTO . J

1 f ' 1 ■

0' 5' 10' 15'

Fig. 166.

Fig. 167.

Fig. 166. Geotropic response of peduncle of Tuberose ; pre-
liminary down-movement is due to weight.

Fig. 167. Geotropic response of petiole of Tropaeolum ; latent
period shorter than 20 seconds.

Tuberose in a state of active growth ; the flower 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. 166) after the tenth
dot, the latent period being thus 3 minutes and 20 seconds,
the greater part of which was spent in recovering the
down-movement caused by the weight of the organ.

286 CHAP. XXVI. GEOTROPISM

Experiment 156. Geotropic response of petiole of
Tropaeoliim. — I expected to obtain a shorter latent period
by choosing thinner specimens with less weight. I therefore
took a cut specimen of the petiole of Tropaeolum 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-move-
ment, the geotropic up-movement being initiated within
a few seconds after placing the petiole in a horizontal
position (fig. 167). The successive dots in the record are
at intervals of 20 seconds and the second dot already ex-
hibited an up-movement ; the latent period is therefore
shorter than 20 seconds. It has been thought hitherto
that the latent period for geotropic reaction is a matter of
many minutes ; this very high value must have been due
to the crude methods employed for the determination.

The Complete Geotropic Curve

The characteristics of the geotropic curve are similar
to those of other tropic curves. That is to say, the sus-
ceptibility 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 reaches the state of geotropic equilibrium. The period
of completion of the curve varies in different specimens
from one to several hours.

Experiment 157. — 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
have been attained in this case after 3 hours and continued
for nearly 90 minutes. The final equilibrium was reached
after a period of 8 hours (fig. 168 V

For studying the effect of an external agent on geo-
tropic reaction, the period of uniform movement is the most

THE GEOTROPIC CURVE 287

suitable. Acceleration of the normal rate, with enhanced
steepness of curve, then indicates that the external agent

Fig. ie»8. The complete geotropic curve (Crinum).

co-operates with gravity ; depression of the rate with result-
ing flattening of the curve shows, on the other hand, the
antagonistic effect of the agent.

Determination of Direction of Geotropic

Stimulus

The experiments which have been described show that
it is the upper side (on which the vertical lines of force of
gravity impinge) that undergoes excitation. The direction
of the incident stimulus must therefore be the vertical lines
of gravity. This conclusion is supported by the results
of three independent lines of inquiry : (i) the algebraical

288 CHAP. XXVI. GEOTROPISM

summation of the effect of geotropic with that of photic
stimulation whose direction is known ; (2) the relation
between the directive angle and geotropic reaction ; and
(3) the torsional response under geotropic stimulation.

Effect of Algebraical Summation

Experiment 158. — A flower-bud of Crinum is laid hori-
zontally and a record taken of its geotropic movement. On
application of light on the upper side (at L) the responsive

C^

Fig. 169. Fig. 170.

Fig. 169. Stimulus of light or gravity, represented by arrow,
induces up-curvature as shown in dotted figure (Crinum).

Fig. 170. Additive efEect of photic stimulus.

The first, third, and fifth parts of the curve give the normal
record under geotropic stimulation. Rate of up-curvature
enhanced under superposition of stimulus of light, l, acting
from above.

upward curvature is enhanced, proving that gravity and
light are concordant in their effects. On the cessation of
light, the original rate of geotropic curvature is restored
(fig. 170). Apphcation of light of increasing intensity from
below induces, on the other hand, a diminution, neutrahsa-
tion, or reversal of the geotropic curvature,

PHOTOTROPISM AND GEOTROPISM 289

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 effect of
geotropic stimulation is precisely similar. In fig. 169 the
arrow represents the direction of stimulus w^hich may be
rays of light or vertical lines of force of gravity.

Analogy between Phototropic and Geotropic

Reactions

In geotropic curvature the direction of geotropic stimulus
may, for all practical purposes, be regarded as coinciding
with the vertical lines of gravity. The analogy between
the effects of light and of gravity is very close ; ^ in both
the induced curvature is such that the organ moves so as
to meet the stimulus. This will be made still more evident
in the investigations on torsional geotropic response
described in a subsequent chapter. The tropic curve under
geotropic is similar to that under photic stimulation. The
tropic reaction under the stimulus both of light and of
gravity increases similarly with the directive angle. These
real analogies are unfortunately obscured by the use of
arbitrary terminology in the description of the geotropic
curvature of the shoot. The record in fig. 170 gives the
response to vertical stimulation by light and by gravity of
a 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 have the effect

1 An exception to this will be found on p. 373, with an explanation.

u

290 CHAP. XXVI. GEOTROPISM

of obscuring analogies between phenomena. Until the exist-
ing terminology is revised, it would perhaps be advisable to
distinguish the geotropism of the shoot as Zenithotropism
and that of the root as Nadirotropism.

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 the lines of force
of gravity is zero, and there is no geotropic reaction. The
reaction increases with the directive angle ; theoretically,
the geotropic reaction should vary as the sine of the angle.
In Chapter XXXI, I will describe the very accurate electric
method which I have been able to devise for determining
the relative intensities of geotropic reaction at various angles.
Under perfect conditions of symmetry the intensity of
reaction is found actually to vary as the sine of the directive
angle.

Differential Geotropic Excitation

The geotropic excitability of a radial organ is the same on
all sides. It has been shown that when it is laid horizontal
it is the uppermost side that responds more effectively to the
geotropic stimulation.

The two sides of a dorsiventral organ are unequally
excitable to different forms of stimuli, the lower side of
the pulvinus of Mimosa being far more excitable than the
upper side. Since the reaction to geotropic stimulation
is similar to that of other forms of stimulation, the lower
side of the pulvinus should exhibit a more pronounced
geotropic movement than the upper.

Under ordinary circumstances the upper half of the
pulvinus is, on account of its favourable position, more
effectively stimulated by gravity ; in consequence of this,
the leaf assumes a more or less horizontal or ' dia-geotropic '
position of equilibrium. But when the plant is inverted.

SUMMARY 291

the more excitable lower half of the organ then occupies
the favourable position for geotropic excitation. The leaf
now erects itself till it becomes almost parallel to the stem
(r/. fig. 179). The response of the same pulvinus which
was formerly ' dia-geotropic ' now becomes ' negatively
geotropic ' ; but an identical organ cannot possess two
different specific sensibilities. The different effects in the two
positions are in reality due to differential excitability of the
two sides of the dorsi ventral organ.

It has also been explained that when the pulvinus of
Mimosa is subjected to lateral stimulation of any kind, it
undergoes torsion, in virtue of which the less excitable
half of the organ is made to face the stimulus. Experi-
ments \vill be described (Chapter XXVIII) which will show
that geotropic stimulation induces similar torsional response.
The results obtained from this method of inquiry give
independent proof (i) that the lower half of the pulvinus
is geotropically the more excitable, and (2) that the
direction of incident geotropic stimulus is that of the
vertical lines of force of gravity w^hich impinge on the upper
side of the organ.

Summary

The stimulus of gravity is shown to induce an excitatory
reaction w^hich is similar to that induced by other forms of
stimulation. The immediate effect of geotropic stimulation
on a horizontal growing organ is an incipient contraction
and a retardation of the rate of growth of the upper side
on which the stimulus is incident. This is followed by an
up-curvature.

That the upper side of the stem undergoes excitatory
contraction is demonstrated not only by the concave
curvature of that side, but also by the excitatory reaction
of galvanometric negativity of that side (p. 315).

Tropic reactions are said to be positive when the directly
stimulated side undergoes contraction, with the result that

292 CHAP. XXVI. GEOTROPISM

the organ curves towards the stimulus. According to this
definition, the geotropic response of the stem is positive.

The geotropic response is delayed by the bending due
to the weight of the horizontally laid shoot. Reduction of
weight consequently shortens the latent period ; in the case
of the petiole of Tropaeolum in a highly sensitive condition,
it is less than 20 seconds.

The complete geotropic curve shows characteristics
which are similar to those of tropic curves in general.

A dorsiventral organ is anisotropic ; the moto-excita-
bihties of the upper and lower sides are different. In the
pulvinus of Mimosa the excitabihty of the lower half is
greater than that of the upper half, and this differential
excitabihty of the dorsiventral organ determines its position
of geotropic equilibrium.

CHAPTER XXVII

EFFECT OF ANAESTHETICS ON GEOTROPIC RESPONSE

Geotropic response of growing organs has been shown to
be due to differential growth induced in the upper and lower
sides of the organ, there being a retardation of growth in
the directly stimulated upper side. The intensity of response
will therefore depend :

1. On the normal activity of growth ;

2. On the effect of season and of various external agents

which enhance or retard the normal rate of growth.

It has been shown, for instance, that growth is modified
by anaesthetics in a characteristic manner. Anaesthetic
agents may, according to their action, be divided into three
classes :

1. Strong anaesthetics, such as chloroform, induce a
preliminary acceleration of growth, followed by arrest.
Continued action causes a spasmodic death-contraction.

2. Ether is less toxic than chloroform ; a small dose of
ether enhances the rate of growth. It is only under pro-
longed apphcation that it induces retardation or arrest.

3. Carbon dioxide is a mild anaesthetic, the immediate
effect of which is an enhancement of growth. Its con-
tinuous action is followed by a decline and arrest of growth.
Prolonged apphcation often induces actual contraction.

I describe the effect of season and of various anaesthetics
in modifying geotropic response.

294 chap. xxvii. anaesthetics and geotropism

Experimental Method

In order to obtain a characteristic response within a short
time, it is desirable to choose a specimen which is sensitive
and in which unnecessary weight is reduced by cutting off
portions which are non-essential. These conditions are
fulfilled by an isolated petiole of Tropaeolum from which
the lamina has been removed. The cut ends are WTapped
in moist cotton, and after a rest of about half an hour the
irritability of the specimen is found to be fully restored.
The beginning of the geotropic response can now be easily
detected by the employment of a magnifying lever.

Tropaeolum grows in Calcutta during the winter months
from November to January, and also during the spring
season in February and March ; the plants begin to die
off from the beginning of April. The experiments described
below were carried out during two years in succession. The
records given by the spring- and the winter-specimens
exhibit certain characteristic differences. In the spring-
specimen the latent period — the time which elapses
between the application of the stimulus of gravity and the
commencement of the geotropic up-movement — varies from
20 seconds to 6 minutes ; the rate of movement afterwards
becomes uniform and remains so for about half an hour.
The slope of the curve and the distance between the suc-
cessive dots indicate the geotropic activity ; any induced
enhancement of the normal rate is, as already explained,
exhibited by the erection of the curve and greater separation
of the successive dots ; depression, on the other hand, is
indicated by the opposite change. In the winter-specimen,
owing to the general depressed rate of growth, the geotropic
response is very sluggish, as seen in the prolonged latent
period, which in the particular experiment was found to
be 48 minutes ; the sluggish character of the response is
also indicated by the gentle slope of the geotropic curve
(fig. 171).

The uniformity of the erectile response cannot be main-

EFFECT OF SEASON 295

tained for an indefinite period, since any further response
is impossible after the full erection of the organ. It is
therefore more practical to begin with it a few degrees
below the horizon, and continue the record till it rises to

Fig. 17T. Geotropic curve : (a) of a spring-, and (b) of a winter-
specimen. The latent period of the former is in the present
case 6 minutes, of the latter 48 minutes.

Note the relatively erect curve of the spring-specimen, indicating
a more intense geotropic reaction (Tropaeolum).

Time-interval between dots 3 minutes.

the same angle above the horizontal position. The slope
of the curve is then found to remain practically uniform
for about half an hour, which is more than sufiicient for
the completion of an experiment on the action of the
anaesthetic. The effect of anaesthetics will be described
not only on the growing but also on the pulvinated
organ.

296 chap. xxvii. anaesthetics and geotropism

Effect of Chloroform on Geotropic Response

OF Growing Organs

The following experiments on the action of chloroform
were carried out with two different species of plants with
the object of bringing out certain characteristic differences.

Fig. 172. Effect of chloroform on geotropic response, a, en-
hancement of geotropic reaction in Echpta; b, prehminary
enhancement followed by arrest in Tropaeolum {see text).

Experiment 159. Seedling of Eclipta. — The epidermis
of Eclipta is somewhat impervious to vapour ; hence a
relatively small quantity of chloroform vapour is absorbed
by the plant. The characteristic effect is seen to be a great
enhancement of geotropic reaction which persisted for a
considerable length of time (fig. 172, a). The moment of
application of the anaesthetic is indicated by an arrow.

Experiment 160. Petiole of Tropaeolum.- — In the last
experiment absorption of a small quantity of chloroform

EFFECT OF ETHER 297

gave rise to acceleration, which is characteristic of the first
stage. In Tropaeohmi the petiole absorbs the vapour
more readily, consequently the enhancement of geotropic
reaction at the first stage is followed by an arrest at the
second stage (fig. 172, b).

-Fig. 173. Effect of chloroform on geotropic response of the
terminal leaflet of Desmodium.

Note enhancement at the first and reversal at final stage.

Experiment 161. Effect on piilvinus of terminal leaflet
of Desmodium. — The leaflet was executing a slow up-
movement. Application of chloroform induced preliminary
enhancement of geotropic reaction in the course of 40 seconds.
Continued action induced a reversal of the geotropic move-
ment (fig. 173).

Effect of Ether on Geotropic Curvature

Experiment 162. Petiole of Tropaeolum. — After the
attainment of uniform geotropic up-movement, a speci-
men of Tropaeolum was subjected to the action of ether
vapour ; this induced a great enhancement of the move-
ment in the course of 3 minutes, as seen in the erection
of the curve and in wider spacing of the successive dots

(fig- 174).

Having thus obtained a definite proof of the enhancement

298 CHAP. XXVII. ANAESTHETICS AND GEOTROPISM

of the geotropic reaction under ether, two batches of
six similar petioles of Tropaeolum were taken and placed
horizontally ; of these, the first batch was placed in a
chamber containing air, the second batch being placed in
a chamber which contained a small quantity of ether
vapour. On examining the two batches after an hour, it

Fig. 174. Effect of ether in enhancing geotropic response

(petiole of Tropaeolum).

was found that while a slight curvature was produced in
the specimens under normal conditions, those subjected
to vapour of ether had become highly erected, the tips
being even bent backwards. The striking difference
between the two will be seen in the reproduction from a
photograph of the normal and the etherised specimens
(fig. 175). The experiments on pulvinated and on growing
organs thus exhibit similar results — namely, an enhancement
of geotropic reaction. under moderate application of ether

EFFECT OF CARBON DIOXIDE

299

vapour. I next describe the effect of the mild narcotic
carbonic acid gas on geotropic response.

h

Fig. 175. Effect of ether vapour on geotropism. n, the normal
reaction in air ; e, the reaction in an atmosphere of ether
vapour.

Effect of Carbonic Acid Gas on Geotropic
Response of Growing Organs

It has been explained that the intensity of geotropic
response is to a certain extent modified by the season.
I describe effects of application of CO2 to winter- and to
spring-specimens.

Experiment 163. Effect of CO 2 on winter- specimen of
Tropaeolum. — The record given (fig. 176) shows the sluggish-
ness of response in the cold season. The latent period was
found to be 40 minutes (first part of the record not shown
in the figure). A geotropic curvature was then initiated
at a slow rate, as seen in the shghtlyinchned curve of ascent.
Carbonic acid gas was next passed into the plant-chamber ;
this caused a great enhancement of the geotropic reaction
in the course of about 3 minutes. The induced en-
hancement is clearly seen in the very erect curve, and in
the separation of the successive dots ; the acceleration
persisted for 20 minutes, after which the movement became

300 CHAP. XXVII. ANAESTHETICS AND GEOTROPISM

arrested. This arrest was not permanent, for introduction
of fresh air was found to bring about the subsequent renewal
of the geotropic up-movement.

Experiment 164. Response of spring-specimen. — The
record (fig. 177) shows that the latent period is relatively

Fig. 176.

Fig. 177.

Fig. 1 76. Effect of CO2 on geotropic response of winter-specimen
of Tropaeolum. CO2 applied at thick dot induced preliminary
enhancement followed by arrest.

Successive dot-intervals 3 minutes.

Fig. 1 77. Effect of CO2 on geotropic response of spring-specimen
of Tropaeolum. CO2 applied at thick dot and fresh air
substituted at circle. The effect induced is a preliminary
enhancement followed by a reversal. Substitution of fresh
air renewed normal geotropic reaction.

short ; the erect curve, moreover, demonstrates greater
geotropic sensibility of the spring-specimen. After the
attainment of a uniform erectile movement, carbonic acid
gas was introduced into the plant-chamber ; this induced
a great enhancement of geotropic movement in the course
of 3 minutes, a result characteristic of the first stage. The
erectile movement was temporarily arrested in the course
of 12 minutes. There then followed the reversal of the

EFFECT OF CARBON DIOXIDE 3OI

normal geotropic movement which carried the tip of the specimen
below the horizontal position. Carbonic acid gas thus caused
an apparent reversal of normal geotropic response. Fresh
air was then introduced in the chamber, with the result that
the normal geotropic up-movement was renewed after an
interval of 5 minutes.

In another experiment a stream of carbonic acid gas
was maintained throughout, the experiment lasting for more

Fig. 178. Effect of CO2 on geotropic response of Tropaeolum.

Enhancement of geotropic response followed by persistent
reversal under continued action of the gas.

than an hour. It gave the same sequence of effects as
before — namely, enhancement at the first stage, a temporary
arrest at the second, and an actual reversal at the third
stage (fig. 178). The tip of the specimen under the con-
tinued action of gas persisted in its reversed position.

The results described above relate to the action of
carbonic acid gas on the geotropic response of Tropaeohim.
In order to demonstrate the universality of the phenomenon,
further investigations were undertaken with a large number
of growing and of pulvinated organs.

302 CHAP. XXVII. ANAESTHETICS AND GEOTROPISM

Experiment 165. Response of the peduncle of Tuberose. —
This specimen gave a normal geotropic response, though it
was relatively sluggish. The latent period was 45 minutes.
The effect of CO^ was a preliminary enhancement, followed
by arrest and reversal of normal geotropic response. On
the substitution of fresh air in the plant-chamber, the normal
up-movement was once more restored. Continued action of
carbon dioxide is thus seen to produce in the Tuberose
a reversal of geotropic action similar to that observed in
Tropaeolum.^

In regard to the reversal of geotropic movement under
carbon dioxide, it is to be borne in mind that geotropic
response is ultimately due to induced variation in the rate
of growth. It has been shown that all narcotics, including
carbonic acid gas, ether, and chloroform, induce variation in
the rate of growth, an acceleration at the first stage, an
arrest at the second, and a reversed contractile response at
the third stage. Corresponding to these are the accelera-
tion, arrest, and reversal of geotropic movement. The effect
of CO2 is therefore by no means unique, but the common
reaction under all narcotic agents.

Effect of Carbonic Acid Gas on Geotropic Response

OF PULVINATED OrGAN

The geotropic excitabihty of the upper half of the
pulvinus of Mimosa, as previously explained, is very much
less than that of the lower half. It thus happens that the
leaf of Mimosa is in a state of equilibrium in a horizontal,
the so-called dia-geotropic, position. But if the plant be
inverted so that the relatively more excitable lower half
is above, the geotropic excitation and the resulting curvature
are greatly enhanced, the leaf becoming continuously
erected in this inverted position. The experiment may be

1 J. Lynn observed {New Phytologist, Aug. 19, 1921) a reversal of
geotropic curvature in the hypocotyl of Helianihus annuus under the action
of carbonic acid gas. He suggests in explanation a far-fetched theory of
hydrion concentration, for which no experimental verification has been
adduced.

EFFECT OF CARBON DIOXIDE

303

carried out with a small piece of a shoot of Mimosa bearing
a lateral leaf ; the sub-petioles may also be cut off, thus
reducing the weight of the organ. In order to prevent
drying, the cut ends of the stem and of the petiole are
covered with small pieces of moist cloth. The sensibihty
of the pulvinus is fully restored in the course of an hour,
as shown by the normal fall of the petiole under mechanical
stimulation. The advantage of a cut specimen is that it
can be easily manipulated and held in the inverted position

(fig- 179)-

Fig. 179. Method of record of geotropic response of jSIimosa
held in an inverted position with the more excitable half of
pulvinus B facing upwards.

Experiment 166. Effect of CO2 on geotropic response
of Mimosa.- — The first part of the record in fig. 180 shows
the uniform erectile geotropic response in the inverted
position. The plant-chamber was next filled with carbon
dioxide. This caused an arrest and a subsequent reversal
of the geotropic response, which occurred 2 minutes after
the application of the gas. By this reversal the leaf was
brought below its original position and maintained there.
Substitution of fresh air brought about a restoration, and
normal geotropic response was renewed in the course of
4 minutes.

Experiment 167. Erythrina indica. — The pulvinus of
Erythrina is less sensitive than that of Mimosa ; the charac-
teristic effects are, in other respects, the same in the two

304 CHAP. XXVII. ANAESTHETICS AND GEOTROPISM

cases. The cut specimen was held in an inverted position,
and after the attainment of uniform up-movement, car-
bonic acid gas was apphed ; this caused a reversal of the
normal geotropic movement in the course of 4 minutes and
20 seconds ; the petiole was thus lowered below its original
position. Introduction of fresh air gave rise, in the course

Fig. 180.

Fig. 181,

Fig. 180. Effect of CO2 on geotropic response of Mimosa, applied
at arrow ; second arrow within a circle represents substitu-
tion of fresh air.

Successive dots at intervals of a minute.

Fig. 181. Effect of COg on geotropic response of Erythrina indica.
Note actual reversal of geotropic response under COg.

of 2 minutes, to the renewal of normal erectile movement

(fig. 181).

Results obtained with various growing and pulvinated
organs thus show that the normal geotropic response is
reversed under the continued action of carbonic acid gas.
The effect of CO2 in reversal of geotropic response is by
no means unique, for it occurs under the action of other
anaesthetics as well. The period of application of the
anaesthetic for reversal is relatively shorter in the case of
strong anaesthetic like chloroform, but the fundamental
reaction is similar in all cases.

SUMMARY 305

Summary

The geotropic reaction of a growing organ is dependent
on its growth-activity. In spring the latent period is
relatively short and the rate of erectile movement rapid.
In winter the latent period is prolonged and the geotropic
movement is very sluggish.

The effect of chloroform in moderate dose is an enhance-
ment of geotropic reaction followed by arrest. Stronger
application produces a reversal.

Ether, in moderate dose, induces a very marked enhance-
ment of geotropic reaction, the organ becoming fully erected
in a short time. The geotropic response of pulvinated
organs is also greatly enhanced under moderate application
of ether.

The immediate effect of carbonic acid gas on geotropic
response of both growing and pulvinated organs is an
enhancement of the erectile movement above the normal.
Continued action of COg gives rise to a reversal of the
normal geotropic movement.

CHAPTER XXVIII

GEOTROPIC TORSION

I HAVE explained that lateral application of certain stimuli
to the flanks of a dorsiventral organ induces a responsive
torsion by which the less excitable side is made to face the
stimulus (p. 256). I now show that the reaction to the

Fig. 182. Diagram of arrangement for recording torsional re-
sponse of leaf of Mimosa to geotropic stimulation. The less
excitable upper half of the pulvinus is to the left.

stimulus of gravity is in every respect similar to that to
other forms of stimulation.

As the direction of force of gravity is fixed, it is necessary
that the dorsiventral organ should be movable, that it may
be so placed that the geotropic stimulus may act upon its

TORSIONAL RESPONSE 307

flank. In the following experiments the pulvinus of Mimosa
was taken as the typical dorsi ventral 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. Two different positions, a and b, are distinguish-
able. 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 the b position they are to the right. The arrange-
ment for taking a record of the torsional response in the
a position is shown in fig. 182.

Torsional Response under Geotropic Stimulation

Experiment 168. Torsional response. — When the leaf is
in a position, the geotropic torsion is in the direction of the

Fig. 183. Fig. 184.

Fig. 183. Torsional response under geotropic stimulation.
Clockwise torsion in a position is shown in the record as an
up-curve. (Mimosa.)

Fig. 184. Anti-clockwise torsion in b position shown in the

record as down-curve.

In both these records there is a recovery on restoration of the
pulvinus to the normal position. The interval between two
thick dots represents duration of stimulation.

movement of the hands of the clock. The clockwise torsion
induced is indicated in the record (fig. 183) as an up-curve ;

3o8 CHAP, xxvni. geotropic torsion

on restoration of the pulvinus to the normal position there
is a recovery shown by the down-curve.

Experiment i6g. Anti-clockwise torsion in b position. —
The experiment was repeated with this difference, that the
opposite flank was now exposed to the vertical lines of force
of gravity. The record (fig. 184) shows by the down-curve
the anti-clockwise movement, and subsequent recovery on
removal of stimulation.

It has already been explained that the direction of
incident stimulus can be found from the responsive torsion
by which the less excitable side of the organ is made to face
the stimulus. The results of experiments just described
prove conclusively that the direction of geotropic stimulus
must be the vertical lines of force of gravity — a conclusion
which is of great theoretical importance.

Algebraical Summation of Geotropic and
Phototropic Effects

Experiment 170.— If the direction of the incident geo-
tropic stimulus is vertical, and should the leaf be in the
a position, then the stimulus of Hght acting from above
should enhance the previous torsional response due to
geotropism. In the above case the directions of the lines of
gravity and of the rays of light coincide. The effect of rays
of light acting from below should, on the other hand, oppose
that of gravity. The additive effect of stimulation by both
light and gravity is seen in the record (fig. 185). 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 have become greatly accelerated. On
cutting off the light the enhanced rate induced by it is found
to disappear, the response curve being now solely that of
geotropic reaction. The effect of phototropism in opposi-
tion to geotropism is demonstrated by the following experi-
ments, where the opposing action of light of different

PHOTO-GEOTROPIC BALANXE

309

intensities gives rise to a partial balance, to an exact balance,
or to an overbalance.

Fig. 185.

Fig. 186.

Fig. 185. Additive effect of stimulation by gravity g, and by
light L. Application of light at l increases torsional response.
Removal of light (l in circle) restores original geotropic
torsion.

Fig. 186. Algebraical summation of geotropic and phototropic
action. Light applied below at-L opposes geotropic action.
Cessation of light (-l within a circle) restores geotropic
torsion.

Balance of Geotropic by Phototropic Reaction

Experiment 171. PJwto-Geotropic Balance. — I 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 junction of
upper and lower halves of the pulvinus from below. An iris-
diaphragm regulates the intensity of incident light. The
first part of the curve is the record of normal geotropic
torsional movement. Light of a given intensity was apphed
from below at a point marked -L (fig. 186) ; this is seen
to produce an overbalance, the phototropic effect being
slightly in excess. The intensity of the incident light was

310

CHAP. XXVIII. GEOTROPIC TORSION

continuously diminished by regulation of the diaphragm till
exact balance was obtained, as shown by 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 entirely different, and
that the stimulus of gravity can be measured, as it were, in
candle-powers of light ! After securing the balance, light
was cut off, and geotropic torsion was renewed on the
cessation of the counteracting phototropic reaction.

Experiment 172. Comparative balancing effects of white
and red light. — ^White light was at first applied at— L, in

opposition to the geo-
tropic movement. The
intensity of light was
stronger than was necess-
ary for exact balance, and
its effect was at first to
retard and then to reverse
the torsional geotropic
response. When thus
overbalanced, a red glass
was interposed in the path
of the light at R. As the
phototropic effect of this
light is comparatively
feeble, the geotropic torsion became predominant, as seen in
the subsequent up-curve. The red glass was then removed,
white light being substituted at — L to stimulate once
more in opposition ; the result is seen in the final over-
balance, and reversal of torsion (fig. 187).

Experiment 173. Effect of coal-gas on the balance. — The
method of balance described above opens out new possi-
bilities in regard to investigations on the relative modifica-
tions of geotropic and phototropic excitability by any 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

Fig. 187. 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.

SUMMARY 311

1 am going to describe shows (i) the enhancement of
phototropic excitability on the introduction of coal-gas,
and (2) the depressing effect of excess of coal-gas in depriving
the organ of its supply of oxygen. After taking the norma]
curve of geotropic torsion,

light was applied below at
— L and an exact balance
obtained in the course of

2 minutes, as seen in the top
of the curve becoming hori-
zontal. Coal-gas was now
introduced into the plant-
chamber at C. This induced
an enhancement of photo-
tropic reaction with resulting
overbalance shown by the
reversal of torsion. This
enhancement persisted for
more than 3 minutes. By
this time the plant-chamber
was completely filled with
coal-gas, and the resulting

effect on the phototropic reaction is seen in the second upset
of the balance, this time to be depression (fig. 188). 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
geotropic.

Fig. 1 88. Effect of coal-gas on photo-
geotropic balance. Geotropic tor-
sion, G, is exactly balanced by
opposing action of light, — l. Appli-
cation of coal-gas at c at first
caused enhancement of phototropic
reaction with resulting reversal.
Prolonged application induced
depression of phototropic reaction,
the geotropic effect thus becoming
predominant.

Summary

Under lateral action of geotropic stimulus, a dorsi-
ventral organ exhibits a torsional response such that 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 the vertical lines of gravity.

312 CHAP. XXVIII. GEOTROPIC TORSION

It is the upper side of the organ that is directly
stimulated by geotropic action.

The effects of gravity and of light are algebraically
summated when in 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
can be exactly balanced.

This state of balance is upset by any slight variation in
the intensity of the opposing photic stimulation.

The relative modification of geotropic and phototropic
reaction by an external agent can be determined by the
resulting upset of the Photo-Geotropic Balance.

CHAPTER XXIX

THE GEO-ELECTRIC RESPONSE OF THE SHOOT

Having described the geotropic response and its modifica-
tion under variation of external conditions, the question
arises as to the underlying mechanism by which stimulation
is effected. The only conceivable way in which gravity
can produce stimulation in the higher plants is by the
pressure of weight acting on the sensitive ectoplasm of the
cells. The pressure of weight can be exerted by the cell-
contents, whether the sap, or the heavy particles, crystals or
starch-grains, contained in the cell. The former, the
Theory of Hydrostatic Pressure, was suggested by Pfeffer
and supported by Czapek ; the other, the Theory of Stato-
liths, has been advocated by Noll, Haberlandt, and Nemec.

In the case of a multicellular stem laid horizontally,
E and E' as indicated in the diagram (fig. 189) may be
regarded as the tissue in the cells of which stimulation is
caused by the pressure of particles. It is to be noted
that in the sensitive cells of the upper half the pressure is
exerted on the inner tangential protoplasmic layer, while in
the lower half the pressure is exerted on the outer ecto-
plasmic layer. Facts will be described which indicate that
the protoplasmic layer is not equally excitable on all sides of
the cell, which may possibly offer an explanation of the
opposite reactions on the two sides of the organ, the upper
exhibiting excitatory contraction, while the lower exhibits
the opposite reaction of expansion.

Having discovered that all excitatory contractions in
plant-tissues can be detected by the induced galvanometric

314 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT

negativity, I have been working for many years to find
an independent means of recording the effect of geotropic
stimulation by means of electric response. The character-
istic electric record is modified, as is the mechanical record,
by the bending down of the stem which precedes the geo-
tropic up-movement. I was able to eliminate this com-
phcating factor by restraining all movement of the shoot,
the responsive change of galvanometric negativity being

an independent expres-

M

E

C

excitatory re-
The following

E
M

E'
C

sion ol
action.

account is quoted from
my previous work pub-
lished in 1907 : ^

' The secondary effect,
due to mechanical dis-
turbance, which masks
for a time the excitatory
effect of gra\'itational
stimulus, may thus be
eliminated completely by
restraining all movement
of the shoots. The pro-
blem thus resolves itself
into the fixing of an ex-
perimental shoot — say, the peduncle of Eucharis Lily — in
such a way that mechanical response is completely restrained.
The next point is to subject the specimen at a given
moment to the stimulus of gravity, and to record the
consequent electric response.

' It is clear that when any two points are acted on
symmetrically by the force of gravity, there is no resultant
geotropic action. This is the case in regard to two diametric-
ally opposite points A and B situated laterally on an erect
shoot. When the shoot is laid horizontally, two lateral
points are like^^'ise acted on s}Tiimetrically by the force of

1 Comparathe Electro-Physiology , p. 442.

Fig. 1 89. Diagrammatic representation
of a multicellular organ laid hori-
zontally and exposed to geotropic
stimulation.

In the upper half, the statoliths act on
the protoplasm lining the inner side of
the tangential wall e ; in the lower
half they act on the protoplasm lining
the outer side of the tangential wall of
e'. (After Francis Darwin.)

EXPERIMENTAL ARRANGEMENT

315

gravity, and there is thus no differential action as between
the two. But if the shoot be now rotated on itself, so that
one of these points is diametrically above and the other
below, a differential effect will be induced between the
upper and lower sides, the upper being the more excited.
In the following experiment I took a specimen of Eucharis
Lily, and fixed the entire plant horizontally (fig. 190). The
pivoted support allowed the responsive points A and B to
be at first lateral.

' Owing to symmetry there w^as now no differential
action of gravity, nor any consequent electric variation

Fig. 190. Experimental arrangement for subjecting the shoot to
geotropic stimulation, mechanical response being restrained.

between the two. The specimen on its support was next
quickly rotated through 90°. An electric response was
perceived in about one minute, which went on augmenting
with time, the upper side being increasingly galvano-
metrically negative. By now rotating the specimen back
through 90°, the action of gravity is virtually removed.
The after-effect persists for two minutes, and after this the
response-curve show^s the usual recovery ' (fig. 191).

Experiment 174. — I repeated the experiment by the
above method with Polianthes tuberosa, first by rotating
through 90° so that A was above. This gave the up-response
of galvanometric negativity of that side, which disappeared
on return to 0°. The rotation was next carried through

3l6 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT

Fig. 191. Geo-electric response of the physically restrained scape

of Eucharis Lily.

Up-curve represents responsive current from upper to lower
surface during action of geotropic stimulus. Down-curve
represents recovery on cessation of stimulation. Response
commenced after latent period of i minute ; after-effect per-
sisted for 2 minutes. Breaks in curve are due to obscuration
of recording spot of light at brief intervals.

Fig. 192. Successive responses, first a above and secondly b above

(Polianthes tuberosa).

EXPERIMENTAL ARRANGEMENT 317

— 90°, the side B being now the upper ; the induced
galvanometric negativity of that side gave down-response,
which disappeared on return to the zero position (fig. 192).

Experimental Arrangement

I give a more detailed account of the experimental
method. The sensitiveness of the galvanometer for the
record should be such that a current of io~'° amp. gives a
deflection of i mm. at a distance of a metre.

Non-polarisahle electrodes. — -The electric connections
with the plant are usually made by means of non-polarisable
electrodes (amalgamated zinc rod in zinc-sulphate solution
and kaohn 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 w^ith liquid electrolyte is, for the present purpose,
extremely inconvenient in practice ; for the plant-holder
with the electrodes has to be rotated from vertical to hori-
zontal 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, seriously affect-
ing its excitability. The term ' non-polarisable electrode '
is in reality a misnomer ; for the action-current (whose
polarising effect is to be guarded against) is excessively
feeble, being of the order of a milhonth 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 thus not justified by the facts of
the case. The real reason for its use is very different.
The electric connection with the plant has to be made
ultimately by means of two metal contacts. If two pieces
of metal even from the same sheet be taken and put in con-
nection with the plant, a voltaic couple is produced owing
to slight physical differences between the two electrodes.

3l8 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT

Amalgamation of two zinc rods with mercury reduces the
electric difference, but cannot altogether ehminate it.

I have been able to wipe out the difference of potential
between two pieces of the same metal, say, of platinum,
making a voltaic couple by immersing them in dilute com-
mon salt solution. The circuit is kept complete for 24 hours,
and the potential of the two electrodes by this process is
nearly equahsed. Perfect equahty is secured by repeated
warming and cooHng of the solution and by sending through
the circuit an 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 con-
nection with the plant through kaolin paste moistened with
normal sahne solution. Care should be taken to put an
opaque cover over the plant-holder, so as to guard against
any possible photo-electric action ; moistened blotting-
paper maintains the closed chamber (not shown in fig. 190)
in a uniform humid condition.

Three Different Methods of Observation

For serving definite experimental purposes I have
employed three different methods : (i) The Method of
Contact with Sensitive and Indifferent Points ; (2) the
Method of Axial Rotation ; and (3) that of Vertical
Rotation.

Method of contact with sensitive and indifferent points. —
This is the most perfect method, for in it the indifferent
distant point is unaffected by geotropic action. An example
will make the matter clear. In the Water-Lily (Nymphaea)
it is the peduncle which is sensitive to geotropic stimulus.
One electric contact is made at an indifferent point on
a sepal, which is always kept vertical ; the other is made
at a point A on one side of the peduncle (fig. 193).

Experiment 175. Induced electric variation on upper
side of the organ. — The sepal being held vertical, the peduncle

METHODS OF OBSERVATION

319

is horizontal, 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 then exhibiting excitatory reaction
of galvanometric negativity (right-hand illustration, fig. 193).
When the peduncle is brought back to the vertical position,
geotropic stimulation disappears, and with it the responsive

current.

Experiment 176. Electric variation on the lower side. —
The peduncle is now displaced through — 90°, so that the

Fig. 193. 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 at a.
In the figure to the left, rotation is through — 90°, a being
below. The electric response is induced galvanometric
positivity at a. For simplification of the diagram, the
vertical position of the sepal n is not always shown.

point A, which under rotation through + 90° faced
upwards, is now made to face downwards. The direction
of the current of response is now found to have undergone
a reversal ; it now flows from A on the low^r side to the
indifferent point N ; thus under geotropic action the lower
side of the organ exhibits galvanometric positivity (left-
hand illustration, fig. 193).

It is obvious that when electric connections are made
on tw^o diametrically opposite sides A and B of the shoot,
inclination of the organ through 90° to the vertical makes
the upper side A galvanometrically negative whilst the
lower side B is rendered galvanometrically positive. The
resulting electromotive variation is therefore additive. Such

320 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT

connections are made in the two methods of Axial and
Vertical Rotation.

Method of Axial Rotation. — The principle of this method
has already been explained {cf. fig. 190). It is diagram-
matically represented in fig. 194, H.

Method of Vertical Rotation is diagrammatically repre-
sented in fig. 194, V. The specimen is held vertical and
electric contacts, A and B, made on two opposite sides ;
it is then rotated round a horizontal axis perpendicular to
the long axis of the specimen. For the purpose of simplicity

Fig. 194. Diagrammatic representation of the Method of Axial
Rotation h, and of Vertical Rotation v {see text).

I confine attention to the electric change induced at the upper
side of the organ. Rotation may be begun in a right-
handed direction, making an increasing angle with the
vertical, when the point A is subjected to increasing
geotropic stimulation and exhibits increasing electric
change of galvanometric negativity. Continuous decrease
of the 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 left-handed
direction, by which the point B is subjected to increasing
geotropic stimulation. B is now found to exhibit excita-
tory reaction, the current of response having undergone
a reversal. Rotation to the right and left may be dis-
tinguished by plus and minus signs.

characteristics 32 1

Characteristics of Geo-Electric Response

There are certain phenomena connected with the electric
response under geotropic stimulation which appear to be
highly significant. According to the 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 hori-
zontally, with the result that the intensity of stimulation
gradually increases, attaining its maximum value when all
the falling starch-grains have moved on to the lower region
of the ectoplast. The time required for the complete
rearrangement of the statoliths may be termed the period
of migration ; its average length varies from five to twenty
minutes in different organs.' ^

Stimulation, according to the statolithic theory, is
induced by the displacement of solid particles. The dia-
meter of the geotropically sensitive cells is considerably
less than o-i mm. ; and the stimulus will be perceived
after the very short interval taken by the statohths to fall
through a space shorter than 0 -i mm. This may be some-
what 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 few^ seconds, and
the latent period of perception of stimulation 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 incidence of stimulus.

Experiment 177. — The mechanical response is thus

1 Haberlandt, Physiological Plant Anatomy (1914), p. 598.

Y

322 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT

incapable of giving an accurate value of the latent period.
The electric method of investigation labours under no such
disadvantage : for, since the excitation is hereby detected
independently of movement, the incidence of stimulus is
followed by response without any undue delay. I will give
a record of the electric response of the quickly reacting

Fig. 195. Geo-electric response of Anthurium.
period shorter than 5 seconds.

Latent

peduncle of Anthurium, when the angle of inclination was
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 about 30 seconds. The angle was next reduced
to zero, and the deflection practically disappeared in the
further course of a minute and a half (fig. 195)- There
was a small ' excitation remainder ' : but with vigorous
specimens the recovery is completed in a short time.

The latent period of the mechanical response of the

ANGLE AND REACTION 323

petiole of Tropaeolum is also a few seconds, a value which is
quite consonant with the idea of particles inducing excita-
tion by their fall through an exceedingly short distance.
In less sensitive organs the latent period is protracted for
reasons that have already been given (p. 284).

Physiological Character of Geo-Electric Response

The intensity of the electromotive variation is found
to depend on the physiological vigour of the specimen.
The Tropaeolum plants often employed in my experiments
are in 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 178. — In February the intensity of electric
response was nearly double of that in March ; it was only
in March that I could construct an accurate potentiometer
for quantitative determination of the induced electromotive
force between the upper and lower contacts on rotation of
the specimen from zero to 90°. I give below the following
typical values obtained with two different specimens :

Specimen Induced E.M.F.

(i) . . . . . .12 millivolts

[2) ...... 15 ,,

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 electric reaction were found to be
abolished.

Relation between Angle of Inclination and
Geotropic Excitation

In the Method of Axial Rotation, experimental condi-
tions are 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

324 CHAP. XXIX. GEO-ELECTRIC RESPONSE OF THE SHOOT

the specimen is rotated on its long axis, the vertical com-
ponent of the force of gravity increases with the angle of
inclination. The hypothetical statolithic particles will
become displaced all along the cells, and the vertical
pressure exerted by them will also increase with the angle.
Experiment 179. — The specimen was rotated through
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 a measure of the
effective stimulation at the two angles. I give below a
table of the results obtained with twenty-four different
specimens.

Table XX.

-Relation between Angle of Inclination and
Geotropic Excitation.

Galvanometric deflection

Ratio -

No. of specimen

(«) at 45°

(b) at 90°

I

70 divisions

no 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

II

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 ra

tio =1-49

SUMMARY 325

The mean ratio 1-49 may thus be regarded as that

of the geotropic reactions at 90° and 45°; this is practically

. sin 00° ...

the same as -r— — 5 = 1-4, which may be stated as the
sm 45 ^ -^

following law :

The intensity of geotropic reaction varies as the sine of the
directive angle.

The results obtained with the Method of Vertical Rota-
tion will be described in a subsequent chapter.

Summary

The excitatory response to geotropic stimulation has
been discovered by the electrical method, the sign of ex-
citation being an electromotive change of galvanometric
negativity.

In addition to the method of contact with an indifferent
point, two other methods have been rendered practical,
the methods of Axial and of Vertical Rotation. The upper
side of a horizontally laid shoot is found to undergo an
excitatory change of galvanometric negativity.

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

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

CHAPTER XXX

LOCALISATION OF GEO-PERCEPTIVE LAYER BY ELECTRIC

PROBE

Having found that plant-organs are sensitive to geotropic
stimulation, the question arises as to the distribution of the
cells which perceive the stimulus, the impulse from which
causes neighbouring cells to carry out a movement of orienta-
tion in a definite direction. Are the perceptive cells diffusely
distributed in the plant or do they form a definite layer ?
Would it be at all possible to localise the sensitive cells
in the interior of the organ ?

It is true that post-mortem examination of sectioned
tissues under the microscope suggested the statolith-theory,
according to which the contents of certain cells cause
geotropic excitation. But for the clear understanding of
the physiological reaction which induces the orientating
movement, it is necessary to get hold, as it were, of the
sensory cells in a condition of full vital activity ; to detect
and follow the changes induced in the perceptive organ and
the irradiation of excitation to neighbouring cells, through
the entire cycle of reaction from the onset of geotropic
stimulation to its cessation.

The idea of obtaining access to the hypothetical geo-
perceptive cells in the interior of the organ would appear
at first sight to be almost hopeless ; the problem has been,
however, solved by the invention of the Electric Probe, by
which it has been possible to locate the excitatory electric
conditions in the interior of the plant.

THE GEO-PERCEPTIVE LAYER 327

The principle of the method will be readily understood
from the following. As every side of a radial organ is
geotropically excitable, the geo-perceptive cells must be
disposed in a cylindrical layer at some unknown depth from
the surface, which, in a longitudinal section of the shoot,
would appear as two straight lines G and G' (fig. 196). In a
vertical position the geo-perceptive layer will be electrically
neutral, but rotation through 90° will initiate an excitatory

Fig. 196. Diagrammatic representation of the geo-perceptive
layer in nnexcited vertical and in excited horizontal position
{see text).

reaction. This sensitive tissue will then respond to the
stimulus and become the focus of excitation, the state
of excitation being detectable by accompanying galvano-
metric negativity, which would be most intense at the
perceptive layer itself. The excitation of the perceptive
layer will irradiate into the neighbouring cells in radial
directions with diminishing intensity, reaching the cortical
cells which effect the curvature.

The distribution of the excitatory change, initiated at
the perceptive layer and irradiated in radial directions,
is represented by the depth of the shading, the darkest
shadow being on the perceptive layer itself (fig. 196). Were

328 CHAP. XXX. THE GEO-PERCEPTIVE LAYER

excitation attended with a change from hght to shade,
there would be witnessed the spectacle of a deep shadow
(vanishing tow^ards the edges) spreading over the different
layers of cells during displacement of the organ from the
vertical to the horizontal ; the shadow would disappear on
the restoration of the organ to the vertical position.

The Electric Probe

Different intensities of excitation in different layers
are capable of detection by means of the Electric Probe,
insulated except at the tip, which is gradually forced into
a horizontal stem from the surface (fig. 197). Increasing

Fig. 197. The Electric Probe.

Figure to the left represents one electric contact made with a
sepal of Nymphaea, and the other with the peduncle by
means of the Probe ; the included galvanometer is repre-
sented by a circle. Figure to the right is an enlarged view
of the Probe.

excitatory change of galvanometric negativity will be ex-
hibited as the probe approaches the perceptive layer at
which the electric excitation will be maximum. After
this, as the probe passes beyond the perceptive layer, the
electric indication of excitation undergoes dechne and final
abohtion. The characteristic excitation occurs only under

THE ELECTRIC PROBE 329

the action of gravitational stimulus ; it will disappear
when the organ is held in a vertical position and thus freed
from geotropic excitation.

Actual Observations

The degree of the induced galvanometric negativity of
the upper side of the horizontal stem was found to exhibit
variation at different depths, and to attain a maximum
value at a definite layer, beyond which there is a decline.
The geo-perceptive layer can thus be experimentally localised
by measuring the depth of the intrusion of the probe at
which the maximum galvanometric negativity is detected.

The electric response of the lower side of the organ to
gravitational stimulus is of an opposite sign to that of the
upper side, galvanometric positivity, indicative of expansion
and increase of turgor. The electric indications on the
lower side also exhibit variation in the different layers, the
maximum positivity occurring at the perceptive layer.

In animals it has been found that the weight of heavy
particles acting on sensitive protoplasm causes the percep-
tion of the direction of gravity. From histological con-
siderations Haberlandt and Nemec came to the conclusion
that heavy particles, such as starch-grains, perform a similar
function in many plants. The electro-physiological investi-
gation which I undertook had as its objects the exact
locahsation of the sensory cells in a living condition, and,
as already stated, the recording of the changes accompany-
ing their functional activity under geotropic stimulation.
The electric results obtained fully confirm the statolith-
theory in the conclusion that it is the ' starch-sheath,' con-
taining a number of large-sized starch-grains, which is the
geo-perceptive organ.

The Method of Experiment

In the practical apphcation of the probe the difiiculty
anticipated was the after-effect of the wound inducing

330 CHAP. XXX. THE GEO-PERCEPTIVE LAYER

variation of excitability in the different layers of tissue.
The wound-irritation is, however, reduced to a minimum
by making the probe exceedingly thin. A fine platinum
wire, o-o6 mm. in diameter, passes through a glass tube
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 its protruding sharp point. The length of the
capillary is, generally speaking, about 6 mm., just long
enough to traverse the experimental plant-organ trans-
versely from one side 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 end of the glass tube and
is led to one terminal of the galvanometer, the other being
connected with an indifferent point on the organ. The
probe can be gradually forced into the plant-organ by
rotation of a screw-head, one complete rotation causing a
forward movement through 0-2 mm.

Prick-reaction. — A prick acts as a mechanical stimulus,
and in normal excitable tissues induces an excitatory
change of galvanometric negativity; this prick-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 without making any
marked rupture ; the probe is, moreover, introduced very
gradually ; with these precautions the reaction due to
prick is found to be greatly reduced. The immediate
effect of the prick is a negative deflection of the galvano-
meter, which declines and attains a steady value in the
course of a few minutes. The depressing effect of the
passage of the probe on geotropic excitability disappears,
I find, in the course of about 10 minutes.

In the choice of experimental material it is necessary
to find specimens which are not merely geotropically
sensitive, but also give a large electric response under
stimulation. In these respects the petiole of Tropaeolum,
the peduncle of Nymphaea, and the shoot of BryophyUum,

PETIOLE OF TROPAEOLUM 33I

in their proper seasons, give good results. For the success
of the following experiments, it is essential that the plant
should be in an exceptionally vigorous condition.

Distribution of PZxcitabiuty ix the Petiole of

Tropaeolum

Detailed results of the experiments on the localisation
of the geo-perceptive layer in the petiole of Tropaeolum
are given below as typical. The petiole of Tropaeolum
has the following special advantages. Geotropically it is
very sensitive ; its latent period of response is very short,
the horizontally laid petiole beginning to bend upwards
in the course of a short time. The leaf may be isolated
from the plant, and the cut end of the petiole placed in
moist cotton. The normal geotropic irritability of the cut
specimen is found to be fully restored in the course of half
an hour. The manipulation of a cut specimen, placed
alternately in a vertical and in a horizontal position,
presents no difficulty. A very large number of specimens
can be, moreover, obtained from the same plant. As
regards geotropic reaction, the induced electromotive
variation of the petiole of Tropaeolum is considerable and
attains the maximum value within a few minutes ; the
recovery is practically complete after restoration to the
vertical.

The mode of experimental procedure (slightly different
from what has already been described) is as follows : the
probe is thrust into the petiole by successive steps of
0-05 mm., and the electric response observed on displace-
ment of the petiole from the vertical to the horizontal
position, in which latter position the organ is subjected to
geotropic stimulation. The induced electric variation, as
already stated, is of considerable intensity. The irritation
caused by the prick of the probe is very slight, the immediate
effect of the insertion of the probe being a negative deflection

332

CHAP. XXX. THE GEO-PERCEPTIVE LAYER

of the galvanometer which decHnes and practically dis-
appears in the course of about 5 minutes. The geotropic
irritability is fully restored in the course of about 10 minutes,
after which a record of the geotropic response may be
taken.

Experiment 180. Geo-electric response at different
depths. — I give below the photographic records of the

Fig. 198. Intensity of geo-electric response at different depths

in the petiole of Tropaeolum.

Note the maximum excitation at a depth of 0-20 mm.

response to the stimulus of gravity at different depths as
the probe was thrust in from outside by successive steps of
0-05 mm. (fig. 198). It will be seen that the geo-electric
response underwent a continuous increase till the maximum
excitation occurred at a depth of 0-20 mm. A rapid
decline occurred beyond this point, and the response dis-
appeared at a depth of 0-50 mm. The following table
gives the quantitative results of the experiment :

rETIOLE OF TROPAEOLUM

333

Table XXI. — Showing Geo-Electric Response at Different
Depths in the Petiole of Tropaeolum.

Distance from surface in mm.

Geo-electric response of galvanometric

negativity

o-oo

5 divisions

0-05

9

-

O'lO

18

0-15

29

0-20

42

0-25

20

0-30

II

•

0-35

7

0-40

5

0-45

2

1

0-50

0

Position of the starch- sheath was at a depth of o-2o mm.

Subsequent microscopic examination showed that the
maximally excited layer at the depth of 0-20 mm. was that
which contains the starch-grains. The geo-perceptive layer
is thus found to coincide with the starch-sheath.

Maximum excitability at the geo-perceptive layer. — The
maximum excitation induced at the perceptive layer
appears to be due to two factors : first, the direct stimula-
tion caused by the fall of the starch-grains ; secondly, the
greater general excitability of the geo-perceptive layer as
compared with that of the neighbouring ones. The rela-
tively greater excitability of the perceptive layer became
evident from the effects observed during the passage of
the probe with the specimen held vertical, when there was no
geotropic stimulation. The insertion of the probe then acts
as a mechanical stimulus, and the response of galvano-
metric negativity is found to be maximum at or near the
starch-sheath, proving that this is relatively the most ex-
citable. The response to prick takes place during the thrust
of the probe ; the resulting irritation disappears, however,
when the probe is left stationary. The normal excitability
of the cells is restored, as already stated, after a period of
rest of about 10 minutes.

334

CHAP. XXX. THE GEO-PERCEPTIVE LAYER

The following table gives the results obtained with
twelve different specimens of the petiole of Tropaeolum.
The specimens were unequally thick ; hence the sensitive
layer was found at a depth of 0-15 mm. in thin, and at
0-20 mm. in thick, specimens; the maximum electric
excitation was in all cases found to occur at the starch-
sheath.

Table XXII. — ^Geo-Electric Reaction at Various Depths in
Different Specimens (Petiole of Tropaeolum).

Responsive galvanometric negativity at

depths of

Position of

No. of
Specimen

starch-sheath

1

1

below surface

0 mm.
Divs.

o'lomm.

0*15 mm. o"2omm. 0*25 mm.

0-30 mm.
Divs.

0-40 ram
Divs.

Divs.

Divs. Divs. Divs.

Mm.

I

7

16

5<> 35 25

0

0

o-i6

2

10

24

47 84

05

57

7

0'20

3

0

6

10 25

8

2

0

0-20

4

5

35

57 30

19

13

0

0-15

5

6

15

22 30

24

15

0

o-i8

0

5

8

15 6

4

I

0

0-I5

7

3

5

14 5

3

0

0

0-15

8

0

16

31 48

14

12

8

0'22

9

3

12

15 II

8

6

0

0-I5

10

2

10

22 7

3

I

0

0-I5

II

3

18

32 43

35

23

7

0-2I

12

0

2

5 26

7

3

0

0-20

1
i

Decline of Geotropic Excitability at Distance
FROM THE Perceptive Layer

The experimental locahsation of the perceptive layer
is greatly facihtated by the abrupt enhancement of exci-
tation at that layer. This wdll be fully reahsed from the
resultant curve obtained from data derived from twelve
different specimens. Taking the perceptive layer itself
as the point of reference, successive distances, say, of
0-05 mm. are measured to the left and to the right of the
point of reference. The abscissa to the left is towards
the centre of the petiole, that to the right towards the
surface. The mean values of the excitatory reactions at
the different points are the ordinates for the curve. It

UPPER AND LOWER HALVES

335

will be seen how abruptly it rises to the maximum
at the perceptive layer and falls beyond it inwards
and outwards (fig. 199).

Fig. 199. Curve showing distribution of geotropic excitability.
Maximum excitation occurs at geo-perceptive layer o.
Excitatory reaction rapidly declined inwards and outwards.

The geo-perceptive layer may thus be experimentally
localised by measuring the depth of intrusion of the probe
at which maximum deflection of galvanometric negativity
is found to occur.

Opposite Reactions in Upper and Lower Halves

The experiments with Tropaeolum, Nymphaea, and
Bryophyllum brought out the striking fact that under the
stimulus of gravity the excitatory electric reaction in the
lower half is of opposite sign to that in the upper half,
a positive instead of a negative electric variation, the maxi-
mum positivity occurring at the lower starch-sheath. Since
galvanometric negativity is associated with contraction

336 CHAP. XXX. THE GEO-PERCEPTIVE LAYER

and galvanometric positivity with expansion, the geotropic
curvature of the stem or the petiole is thus due to the joint
effect of contraction in the upper and expansion in the lower
half. Some difficulty may be encountered in finding a satis-
factory explanation of the two opposite reactions, but the
following considerations will help to remove it. The facts
established are :

1. That the upper half of the horizontal shoot under-

goes contraction under geotropic stimulation.

2. That the lower half undergoes expansion.

3. That the pressure on the protoplasm in the cells of

the starch-sheath, exerted by the heavy particles, is
the cause of stimulation.

4. That the particles press on the inner tangential

ectoplasmic layer of the sensitive cells in the upper,
and on the outer tangential ectoplasmic layer in
the lower, half. A reasonable explanation of the
opposite reactions of the upper and lower halves
may probably be found in the unequal excitability
of the protoplasmic layer on different sides of the
sensitive cell. It will be shown (p. 356) that the
excitability of the ectoplasmic layer at the apical
end of the geo-perceptive cells is greater than at
the basal end. It will therefore be natural to
expect that a similar difference of excitability exists
between the ectoplasmic layers lining the inner and
the outer tangential walls of the ceUs (see fig. 189),
so that pressure of particles will induce maximal
stimulation in the former and subminimal stimula-
tion in the latter.
5. That while maximal stimulation induces retardation
of growth culminating in actual contraction, sub-
minimal stimulation causes expansion and enhance-
ment of the rate of growth (p. 83).

Hence the opposite electric responses given by the upper
and the lower sides of the organ can be explained on the not

UPPER AND LOWER HALVES

337

unjustifiable assumption that in the first case the stimulation
is maximal, while in the latter it is subminimal.

The following facts are of confirmatory value. In my
' Nervous Mechanism of Plants ' it has been shown that the

Fig. 200.

Fig. 201,

Fig. 200. Photo-micrograph of transverse section of upper half
of horizontally laid stem of Impatiens.

Note the starch-grains fallen on to the inner tangential wall of
statocysts s which abut on the phloem p.

Fig. 201. Longitudinal section of upper and lower halves of
geotropically curved stem of Eclipta.

Note contraction of the upper and expansion of the lower cortex
c. The starch-grains have fallen on the inner tangential wall
of the upper statocysts (abutting on the phloem p). The
starch-grains in the lower statocysts are lying against the
outer tangential wall.

phloem functions as a nervous tissue, and it may be of
significance that the statocysts are situated in closest
contiguity with the phloem.

Fig. 200 shows the transverse section of the upper half

338 CHAP. XXX. THE GEO-PERCEPTIVE LAYER

of the stem of Impatiens made shortly after it had been
placed in a horizontal position. The starch-grains are
seen to have fallen on to the inner tangential wall of the
statocysts which abut on the phloem. Geotropic curvature
has been explained to be due to the joint effect of con-
traction of the upper and expansion of the lower side.
What is the difference in the stimulating mechanism of
the geo-perceptive cells of the upper and lower halves ?
Taking a seedling of Eclipta which is highly sensitive to
geotropic stimulation, it is laid horizontally, and a vertical
longitudinal section made after the production of upward
curvature. The illustration (fig. 201) shows the physio-
logical changes induced in the two halves. The cortex in
the upper half shows a relative contraction, while that
in the lower half a relative expansion. The starch-grains
in the upper statocysts are seen to press against the inner
tangential wall of the cells nearest the phloem ; in the lower
half of the stem, the starch-grains in the cells are, on the
other hand, pressing on the outer tangential wall furthest
from the phloem. It is important to bear in mind this
characteristic difference in the stimulation of the two halves
of the organ.

•

Method of Transverse Perforation

Experiment 181. — I next carried out a complete explora-
tion of the interior of the peduncle of Nymphaea along its
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 response being observed at each
step. The successive readings were taken after right-
handed rotation from the vertical to 90° ; the rotation was
never carried out in the left-handed direction to — 90°.
The probe, entering from above, passed through a region
giving negative electric variation ; and then beyond the
central axis entered a region where the galvanometric
indication was positive.

LOCALISATION OF THE LAYER

339

Table XXIII. — Showing the Distribution of Geotropic

Excitability through the Peduncle of Nymphaea

(diameter = 6-8 MM.).

Position of

Galvanometer

Position of

Galvanometer

probe

deflection

probe

deflection

Surface

— TO divisions

3-6 mm.

0 divisions

0-2 mm.

- 26

3-8 „

0

0-4 ,.

- 40

4'0 ..

0

0'6

- 50

4-2 ..

2

0-8 „

- 62

4-4 ..

4

I -o ,,

- 72

4-f> ..

5

1*2

- 88

4-8 ,.

II

1-4 ..

- 108

5-0 „

22

1-6 ..

- 72

5-2 .,

38 „

1-8 „

- 44

5-4 ..

46 ..

2 -O ,,

- 30

5-6 ..

39

2-2 ,,

- 18

5-8 „

32

2-4 ..

— 10

6-0 ,,

24

2-6 ,,

~ 5

6-2 ..

18

2-8 ,.

— 2

6-4 ..

12

3-0 .,

0 ,,

6-6 ,.

6

3-2 ,.

0

6-8 ,,

3

3-4 ..

0 ,,

• • *

Localisation of Geo-Perceptive Layer

A curve constructed from the data given above is seen
in fig. 202. The diameter of the peduncle was 6-8 mm.
The negative geo-electric response is seen to increase till
it attains a climax at the depth of i -4 mm. It then shows
a continuous diminution till it becomes zero at the depth of
3 mm., where there is a neutral zone which extends through
I mm. When the probe reaches 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,
and I '4 mm. measured from the lower, surface. The points
of maximum positivity and negativity are situated sym-
metrically in the two halves of the organ. The electric
variation of maximum positivity in the lower half is

340

CHAP. XXX. THE GEO-PERCEPTIVE LAYER

comparatively feeble, less than half the corresponding maxi-
mum negativity in the upper half. Microscopic section
showed that the geo-perceptive layers were identical with
the groups of statocysts forming the starch-crescents.

m

-100

- .

-75

/I

-50

'n

-25

0

K

+ 25

1

V

+ 50

y(pac03<X»

S>5^?^^S!»!S^^5'^^^B

Fig. 202.

Fig. 203.

Fig. 202. Distribution of geotropic excitability in the peduncle
of Nymphaea. 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 in the two
halves.

Fig. 203. Distribution of geotropic excitability in the shoot

of Bryophyllum.

Experiment 182. — I carried out similar experiments with
the shoot of Bryophyllum. The results are given in
Table XXIV ; the curve of the electric distribution along
the diameter is given in fig. 203. The characteristics of
this curve are the same as that of Nymphaea. The maxi-
mum galvanometric negativity occurred at the depth of

METHOD OF PERFORATION

341

0-6 mm., and of positivity at a corresponding point on the
opposite side.

Table XXIV. — Showing Distribution of Geotropic Excitability

THROUGH the StEM OF BrYOPHYLLUM (DIAMETER =3-6 MM.).

Position of

Galvanometric

Position of

Galvanometric

probe

deflection

probe

deflection

Surface

0 divisions

2-0 mm.

0 divisions

0-2 mm.

-24

2-2

0

0-4 ,,

- 45

2-4 ..

3

o*6 ,,

-63 ,.

2-6 ,,

4

0-8 ,,

— 21

2-8 ,.

9

I 'O

- 9

3-0 ,,

36

I '2

- 6

3-2 ,,

21

1-4 -

- 3

3-4 ..

9

1-6 ,,

0

3-6 ..

0

1-8 ,,

0

• • •

After the electric test, transverse sections of the stem

were made at the radial line of the passage of the probe.

Thus, in a particular experiment

with Bryophyllum, the point of

maximum geo-electric response

was found to be at a distance of

0-8 mm. from the surface. By

means of the micrometer-slide on

the stage of the microscope and

the micrometer eye-piece, the layer

0-8 mm. from the surface was

examined ; this sensitive layer S

was recognised as the continuous

* starch-sheath ' or endodermis

containing unusually large-sized

starch-grains (fig. 204). These

often occurred in loosely cohering

groups of 8 to 10, and their

appearance is very different from the small-sized irregularly
distributed grains in other cells.

In all the specimens examined, the experimentally
located geo-perceptive layer was found to coincide with

Fig. 204. Transverse section
of the stem showing contin-
uous geo-perceptive layer
s ; enlarged view, s', of cell
of endodermis containing
group of large starch-grains
(Bryoph3'llum).

342 CHAP. XXX. THE GEO-PERCEPTIVE LAYER

the ' starch-sheath.' The statohth-theory thus obtains
strong support from an independent hne of experimental
investigation. The statohth-theory has been adversely
criticised because in simpler organs geotropic action takes
place in the absence of statoliths. There is no doubt that
the weight of other 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.' ^

As previously stated, the electric response in different
layers can be successfully detected in vigorous specimens at
the proper season. Under less favourable conditions the
sensitivity may be found to have disappeared.

Evidence of Insensitive Specimens

I describe the various physico-chemical concomitants
which accompany the condition of relative insensibility.
I employed three different tests — the electric, the geotropic,
and the microscopic — by which the sensitive could be dis-
tinguished from the insensitive condition.

Experiment 183. Electric test. — Later in the season the
geo-electric indications given by the various plants were
found to have almost disappeared. That the tonic con-
dition of the specimen was below par was independently
revealed by the response to the prick of the probe ; this, in
vigorous specimens, evokes an electric response of galvano-
metric negativity. But the response to prick in subtonic
specimens is quite different, being one of galvanometric
positivity. The prick-effect, in fact, often gave me an
indication as to the suitability of the particular specimen for
the observation of geo-electric response.

Experiment 184. Test of geotropic reaction.- — I took
four different insensitive specimens of Bryophyllum and

1 Haberlandtj ibid. p. ^gy.

IN VARIOUS PLANTS 343

Nymphaea, and held them horizontal. Shoots of these
plants had, earher in the season, exhibited very strong
geotropic reaction, the stem curving up through 90° in the
course of 10 hours or less. But these specimens, obtained
later in the season, exhibited very feeble curvature, which
hardly amounted to 10°, even after prolonged exposure to
geotropic stimulation for 24 hours.

Experiment 185. Test of microscopic examination. — I
next made sections of the insensitive specimens of Bryo-
phyllum and Nymphaea, and on examining them under the
microscope discovered a striking difference. A few weeks
before, the groups 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. This is evidence that the
presence of the starch-grains is associated with the sensitive-
ness of the perceptive layer.

Localisation of Geo-Perceptive Layer in

Various Plants

The geo-perceptive layer of a large number of plants
was similarly locahsed by the probe, a short account of
w^hich is given below.

Commelina.— The geotropic sensibihty of the stem of
this plant is shown by the erectile movement from a hori-
zontal position. The geo-electric response at the surface
was o. At o-io mm. it was —6, which increased to a
maximum of - 18 at a depth of 0 -20. After this the response
underwent a rapid decline. The maximally excited layer
was subsequently found to contain the starch-grains.

Myosotis.— The stem of Forget-me-not also gave strong
geo-electric response, the maximum excitation occurring
at a depth of 0-20 mm. In the microscopic section the
starch-containing layer was also found at the depth of

0-20 mm.

Centaurea.— The flower-stalk of the Cornflower was

344

CHAP. XXX. THE GEO-PERCEPTIVE LAYER

found to exhibit electric response of moderate intensity
under the stimulus of gravity. It is sensitive whilst the
flower-buds are still closed, but insensitive after the opening
of the flower. The maximally excited layer was at a depth
of 0-3 mm., which also contained the starch-grains.

Tiger Lily. — The flower-bud of this plant is strongly
geotropic. It gave the maximum geo-electric response at
a depth of 0-3 mm. The starch-grains occurred very near
this layer.

Convolvulus. — This gave the maximum geo-electric
response at a depth of 0-3 mm. from the surface, and the
starch-layer was found at 0-28 mm. below the surface.

The table given below embodies the results obtained
with different plants. Though the maximal excitation
occurs at unequal depths in different species of plants, the
maximally excited layer is found to coincide wdth the
starch-sheath.

Table XXV. — Geo-Electric Response at Various Depths in

Different Plants.

Galvanometric negativity at depths in mm.

Position of

Specimen

starch-
sheath at

O'O

0

O'l

6

0'2
18

0'3
2

0-4

0-5

0-6

0-7

0-8

1-4

1-6

depth of

Commelina

0

• • •

0-2I

Myosotis

15

27

80

31

7

5

0

0-20

Centaurea

0

10

22

40

8

7

0

. . .

0-30

Tiger Lily

7

II

25

54

7

5

3

0-30

Convolvulus .

0

3

13

33

20

5

0

0-28

Bryophyllum

0

• • •

24

• • •

45

• • •

63

21

3

0

o-6o

Nymphaea

10

• • •

26

• • •

40

• ■ •

50

62

108

72

1-40

Tropaeolum

(petiole)

0

6

25

2

0

• • •

■ • •

. . .

• • •

• • •

0-20

Duplication of Geo-Perceptive Layer

The experiments described above brought out the definite
fact that during the passage of the probe from the surface
to the centre, it encounters a particular starch-layer, at
which the geo-electric response is at its maximum. From

DUPLICATED LAYER 345

this it might at first appear that the geo-perceptive layer
must always be single. There is, however, an interesting
variation which is described below.

Experiment 186. — While experimenting with a specimen
which was supplied from a nursery garden as the Cape Mari-
gold (a species of Calendula), I was at first greatly puzzled
by the fact that this stem exhibited two definite electric
maxima during the passage of the probe from the surface
to the pith. Thus in a given specimen, while the geo-
electric response of galvanometric negativity at a depth
of o • 10 mm. was — 60 divisions, it increased abruptly to
— 115 divisions at 0-20 mm., and declined to — 15 at the
greater depth of 0-30 mm. The response continued to
decline till a depth of 0 • 60 was reached, when the response
exhibited a second maximum, this time of — 105 divisions.
Below this the excitatory reaction showed decline and
abolition. Detailed results are given in the following table :

Table XXVI. — Showing Duplication of Geo-Perceptive Layer.

Distance from surface

Geo-electric response

Surface 0 mm.

— 10 divisions

o-io mm.

- 60

0-20

-115

0-30 ,,

- 15

0-40

- 14

0*50

- 14

o-6o ,,

- 105

0-70 ,,

— 12

The two starch-sheaths occurred at depths of 0-17 and 0-58 mm.

Similar duplication of the geo-electric maximum was also
observed in a second specimen of the same plant. On
examining sections of the stem, it was a matter of great
surprise to find that there were two definite starch-layers
separated from each other by a distance of about 0-4 mm. ;
it was at these starch-sheaths that the maximum excita-
tions were observed.

346 CHAP. XXX. THE GEO-PERCEPTIVE LAYER

These results afford another striking demonstration of
the fact that the layer which contains the starch-grains
becomes the focus of excitation when the organ is geo-
tropically stimulated by change from the vertical to the
horizontal position.

Another significant fact was noticed in the case of
Calendula. Its geotropic excitability was very marked
at the beginning of its proper season, but disappeared later.
Microscopic sections showed that this insensitive condition
was associated with the disappearance of the starch-grains
from the two layers. Of these, the starch-grains in the layer
nearer the centre were the first to disappear.

Summary

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

The induced galvanometric negativity of the upper half
of an organ (indicative of excitation) shows 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 at which
maximum deflection of galvanometric negativity occurs.

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

In certain plants the distribution of geotropic excita-
bility exhibits two maxima ; that is, the focus of excitation
is not single but double. Microscopic sections showed that
the starch-sheath in these is double, and that the positions
of the two electric maxima coincide with those of the two
starch-sheaths.

The activity of geo-perception undergoes seasonal varia-
tion. It declines with the growing subtonicity of the

SUMMARY 347

plant ; such specimens exhibit positive electric response
under the stimulus of prick, and feeble curvature under
geotropic stimulation. The large-sized starch-grains, nor-
mally observed in the starch-layer, are found to have
disappeared in specimens which prove to be geotropically
insensitive.

The induced electric variation in a horizontal organ,
negative in the upper, positive in the lower, half, indicates
that the cortex contiguous to the upper perceptive layer
undergoes contraction, while that contiguous to the lower
perceptive layer undergoes expansion.

CHAPTER XXXI

RELATION BETWEEN ANGLE OF INCLINATION AND
GEOTROPIC EXCITATION

If the pressure of heavy particles on the sensitive ecto-

plasmic layer of the cell be the efficient cause of stimulation

by gravity, it would follow that the excitation caused by

them will increase with the angle of inclination. The

following experiments were carried out by the Method of

Vertical Rotation, the effect of which is somewhat different

from that of Axial Rotation described in a previous chapter.

There is no effective stimulation in the vertical position of

the plant, while it is most intense at an inclination of 90°.

The effective pressure on the protoplasm of the perceptive

cell which stimulates it, will evidently increase with the angle

of inchnation to the vertical. The problem to be solved is

the exact determination of the relation between the angle

of inclination and the associated excitation.

As regards the measurement of the induced excitatory
reaction, it is theoretically possible to determine it from either
mechanical or the electric response at various inclinations.
The practical difficulties in the measurement of the mechanical
response are, however, so numerous, that it is impossible
to obtain with it any sufficiently accurate result. No such
difficulties are encountered in the electric method, the
relative advantages of which are as foUows :

I. The latent period is very short and the maximum
excitatory reaction is attained in the course of
a minute or so ;

METHOD OF REVERSAL

349

2. The excitatory reaction of galvanometric negativity

disappears on the return of the specimen to the
vertical position ; and, finally,

3. The errors caused by the inaccurate reading of the

angular scale and the physiological asymmetry
of the organ may be eliminated by the Method of
Reversal. When the plant is inclined to the right
through + 90°, the current of response flows in
one direction ; when it is inclined to the left
through — 90°, the direction of the responsive
current is reversed ; the experimental error of a
single determination is eliminated by taking the
mean of the two galvanometric deflections.

(^

y

Fig. 205. Diagrammatic representation of inducing the geo-
tropic reaciion of the shoot by the Method of Reversal.
Rotation through + 90° (right) makes a negative, while
rotation through — 90° (left) renders a positive.

The details of the procedure will be understood from
the diagram given in fig. 205. The specimen is at first
vertical, with the two symmetrical contacts on its sides
A and B, the electrodes being connected in the usual manner
with the terminals T T' of the indicatinggalvanometer ; after
rotation through + 90° the upper side A becomes excited
and galvanometrically negative (right-hand figure). The
specimen is next rotated to — 90° ; A now becomes the
under, and B the upper and excited, side (left-hand figure).
The electromotive response is now reversed, B being
galvanometrically negative. The induced electromotive

350 CHAP. XXXI. INCLINATION AND EXCITATION

variation thus obtained is of considerable intensity, often
exceeding 15 millivolts.

Excitatory reaction at 45° and 90°. — The special difficulty
encountered is that of the accurate determination of the angle
of inclination. An index is attached to the plant, and a
stationary circular scale permits the determination of the
angle at which the plant is inclined to the vertical.* But

Fig. 206. Alternate geo-electric reactions at + 45° and — 45°,
also at -1- 90° and — 90° (Method of Vertical Rotation).

the vertical or zero-reading itself is subject to an error
of a few degrees, which is accentuated by the fact that the
perceptive layer inside the plant may not be exactly parallel
to the surface of the stem or the petiole. The only means
of eliminating the error is by taking two responses, say
at + 45° and — 45°, by the Method of Reversal. The
mean of the two responses obtained through successive
positive and negative rotations of the specimen will
reduce or eliminate all errors.

REACTIONS AT 45° AND 90°

351

Experiment 187. — I obtained a series of galvanometric
responses with a petiole of Tropaeolum when rotated
through the entire cycle of inclination from the vertical
to 45° (as read by the movement of the index), back to
zero and then to — 45°, and back once more to zero. The
same procedure was followed in the case of inclination at
90°. The records are given in fig. 206; the response to
inchnation of + 45° is seen as an up-curve, with subsequent
recovery on return to the vertical. Inchnation of — 45°
evoked a reverse response of down-curve, with subsequent
recovery. Inclinations of + 90° and - 90° evoked re-
sponses of larger amplitude. The ratio of reactions at
90° and 45°, as determined by the amphtude of responses,
came out in this case as i : i • 5. The ratio of sin 90° : sin 45°

is I : 1-41.

The following table embodies the reactions observed
at 45° and 90° in six different specimens of the petiole of
Tropaeolum :

Table XXVII. — Excitatory Reactions at Inclinations of
45° AND 90° (Petiole of Tropaeolum).

Electric

response

h

No.

Ratio ^

(0 at 45°

(b) at 90°

1-488

I

37 divisions

^^ divisions

2

28

40

1-428

3

192

274

I -426

4

22

32

1-454

5

31

44

I -420

6

37

53

1-432

Mean ratio of reactions

. a . *

• 1-44

Ratio of sines

. • • «

. 1-41

The ratio of excitatory reaction at the two angles is
I -44, while the ratio of the sines is i -41. It will be noted
that there is a persistent small difference between the
two ratios, the excitation at the larger angle being greater
than the value deduced from the ratio of the sines. The

352 CHAP. XXXI. INXLIXATION AND EXCITATION

relatively greater excitation at the larger angle may have a
physiological significance, to which reference will be presently
made.

Experiment i88. Excitatory reaction at 45°, 60°, and
90°. — The excitatory reactions of a different batch of
petioles of Tropaeolum were next obtained for the three
angles of inclination of 45°, 60°, and 90°.

Table XXVIII. — Excitatory Reactions at Angles of Inclina-
tion OF 45°, 60°, and 90° (Petiole of Tropaeolum).

No.

Electric response at

{'>) 45°

(6) 60-

1

I
2

3

4
5

6

39 divisions

32

31
22

14
19

49 divisions

40

40

29

19

22

54 divisions

47
46

34
25
25

Mean value

26

33

38

Ratio of reactions
Ratio of sines

I : I -28 : I '47
I : I -22 : I -41

The determinations given above again show that the
excitation is but approximately proportional to the sine
of the angle of inclination, the ratio of reactions at the
larger angles being relatively greater. Another series of
observations was made, the angle of inclination being
increased by steps of 10^.

Experiment 189. — The photographic records of the
reactions at the successive angles of inclination of 45*^, 55°,
65^, 75^, and 90^' are reproduced in fig. 207.

Experiment 190. — Further experiments were carried out
with twelve different specimens of the petiole of Tropaeolum
and the mean excitatory reaction at the different angles are
given in the following table :

REACTION AT 45° AND 90° 353

Table XXIX. — Excitatory Reaction at Various Angles.

Angles
Response

45*
. 40-8 divs.

55-

47 divs.

65'

55 divs.

75'
60 '5 divs.

90*
63-3 divs.

Ratio of reactions
Ratio of sines

I
I :

I • 1 5 :
I -ih :

I -34

I -28

: 1-48
: r-36

: I -55
: 1-41

The ratio of reactions, compared with the ratio of
sines, is hence seen to undergo a gradual increase with in-
creasing angle of inclination. Thus, to take the two extreme

Fig. 207. Records of geo electric response at various angles

( Iropaeolum).

cases, the divergence between the two ratios at 55^ is
2-6 per cent., whereas at 90° it is 9 per cent. This definite
divergence, which is persistent in all determinations, points
to some physiological difference.

In attempting to find an explanation of the relatively
greater reaction at the larger angles of inclination, it is
necessary to take account of two distinct factors in the

2 a

354 CHAP. XXXI. INCLINATION AND EXCITATION

sensitive cells ; first, the pressure exerted by the particles,
and, second, the irritability of the ectoplasmic layer pressed
upon by the particles. As regards the first, the effective
pressure exerted by the particles is proportional to the sine of
the angle of inclination ; but in regard to the second, the
irritabihty of the ectoplasmic layer may not be the same
throughout the length of the cell, but be greater towards the
apical end. At the smaller angle of incHnation, say to the
right, the statoUths, originally at the base of the cell, accumu-
late at the right-hand lower corner of the cell, a portion of the
basal end of the cell being thus subjected to pressure. When
the angle of inclination is increased, the statoliths come to lie
along the whole lateral length of the cell, extending to the
apical end. The relatively greater excitation with increasing
angle of inclination may therefore be explained on the
assumption that the excitability of the ectoplasm is greater
towards the apex ; facts will be given which appear to lend
support to this view.

Excitatory Reaction at 45° and 135°

Controversy has arisen over the question as to whether
the intensity of geotropic excitation is the same or different
at the angles of inclination 45° and 135°. The effective
pressure exerted by the stimulating particles in the cells is
the same at the two angles ; the only difference in the tw^o
cases is the collection of the particles at the basal end of the
cells at 45°, and at the apical end at 135°. Czapek found
that the effective stimulus of gravitation is greater when the
organ is held at 135° than when it is held at 45°, though
his results have not been accepted by others.

I carried out investigations on the subject, employing
the method of electric response. Allowance was made for
any possible change in excitability brought on by fatigue.
This was secured by conducting the experiments in the
following sequence of observation : (i) reaction at 45° ;
(2) reaction at 135° ; and (3) reaction once more at 45°.
The comparison of the first and the third responses showed

-o

REACTION AT 45 AND I35 355

whether any change in excitabihty had occurred on account
of fatigue, allowance for which was made by taking the
mean of the two responses for 45° at the beginning and at
the end of the series.

Experiment 191. Geo-electric response at 45° and 135°. —
The following observations were made with the petiole of
Tropaeolum : the first and the third responses at 45° were
found to be 47 and 45 mm. respectively, the mean of the
two being 46 mm. The second
or intermediate response, taken
at 135°, gave 55 mm. The
excitatory reactions at 45° and
135° are thus in the ratio of
46:55, or as 1 : 1-2. In a second
series with a different specimen,
the first and the third responses
at 45° were both 25 mm.
(fig. 208). There was in this case
no fatigue. The intermediate
response at 135° was 31 mm.
The ratio of the excitatory
reactions at the two angles is
thus in the proportion of 25 : 31,
or as i:i-2. The excitation
at 135° is thus about 20 per
cent, greater than at 45°.

The stem of Convolvulus was employed for the next
series of experiments. At 45° the excitatory deflection
was 26 divisions ; when the angle was increased to 135 °»
the deflection was enhanced to 31 divisions. That the
excitation at 135° was greater than at 45° was evidenced
in a convincing manner on return to the angle of 45°,
when the galvanometric spot of light was immediately
restored to almost the first deflection ; there was slight
fatigue, and the deflection was 25 divisions instead of the
former value of 26 divisions. The ratio of the two reac-
tions at 45° and 135° is thus 25-5 : 31, or i : 1-23. In the
second series of experiments the two excitatory deflections

Fig. 208. Geo-electric response
at 45°, 135°, and 45°in sequence.

356 CHAP. XXXI. INCLINATION AND EXCITATION

were as i8 : 21, or i : 1-31, the mean ratio from the two
experiments being i : 1-27. These various results all tend
to show that the excitation is greater at 135° than at 45°.
In the following table the readings at 45° are those at the
beginning and at the end of the series of observations, the
reading at 135° being the intermediate one.

Table XXX. —Excitatory Reactions at 45° and 135°.

Specimen

Electric response at

45"

135'

45°

Tropaeolum
Convolvulus

> 9

47 divisions

25
26
16

55 divisions

31

31
20

45 divisions

25
25
16

Ratio of reactions at 45° and 135° • • Tropaeolum" i : i -2

., . . Convolvulus I : I -27

These very definite results tend to prove that the ecto-
plasmic layer is not uniformly irritable at all points, but
that it is more excitable at the apical end than at the basal
end of the cells.

Summary

Geotropic reaction is found to vary approximately with
the sine of the angle of inclination.

The results of numerous experiments carried out by the
Method of Vertical Rotation tend to show that the excitatory
reaction at a large angle of inclination is relatively greater
than the value deduced from the law of sines. This suggests
the inference that the excitability of the ectoplasmic layer
is greater at the apical end of the geo-perceptive cells than
at the basal end.

This conclusion is confirmed by the fact that the excita-
tory reaction at 135° is about i-2 times greater than that
at 45°. At 45° the starch-grains accumulate at the basal,
and at 135° at the apical, end of the cells.

CHAPTER XXXII

THE CRITICAL ANGLE FOR GEOTROPIC EXCITATION

The results of investigations described in the present chapter
afford independent evidence that the falling starch-grains
are the effective agents in geotropic excitation. The facts
to be presently described will be better understood from the
following illustration. If some sand-grains be placed on
a flat board which is gradually tilted, the particles start
sHding down only after a certain critical angle has been
reached. If the board is rough, this critical angle will be
large ; if it is smooth, the angle will be small. Moreover,
by the scouring action of the sand, the rough surface may
become smoothed down after numerous repetitions of the
experiment, the result being a reduction of the critical angle.
If geotropic stimulation is effected by the fall of the
starch-grains, it would be expected that :

1. At a small angle of inchnation the grains will not be
immediately displaced ; therefore no excitation will ensue.

2. When the angle of inclination is gradually increased,
the grains will shde down as soon as the critical angle is
exceeded. This fall of particles and the resulting pressure
on the protoplasm will constitute a stimulus and give rise
to an excitatory response.

3. The critical angle will probably be lowered to a certain
extent, on repetition of the process, by the reduction of
frictional resistance to the fall of the particles.

4. Were the weight of the fluid contents of the cell in the
higher plants the only means for stimulation by gravity,
the excitatory reactions would be proportional to the sines

358 CHAP. XXXII. THE CRITICAL ANGLE

of all angles of inclination above zero. But if the fall
of solid particles be the efficient cause, there will be a
hiatus in this relation, for there will be no excitatory-
response at angles smaller than the critical. Even at a
slightly greater angle than the critical, some of the particles
may remain adherent, and the excitation will then be dis-
proportionately lower than what is demanded by the law
of sines. It is only after the critical angle has been con-
siderably exceeded that the relation of sines will be found
to hold good, at least approximately.

The above considerations will now be subjected to the
test of experiment to discover, in the first place, if there is
any discontinuity in the responsive reactions at angles
below 45°.

Excitatory Reaction at 35°, 45°, and 60°

From the results of experiments detailed in the previous
chapter it was found, for angles of 45° and 90°, that geo-
electric excitation is approximate^ proportional to the sine
of the angle of inclination. In order to observe whether
this relation holds good at other angles, a series of observa-
tions was made with six different specimens of the petiole
of Tropaeolum at 35°, 45°, and 60°. The results are given
in the following table :

Table XXXI. — -Excitatory Reactions at 35°, 45**, and 60°.

Specimen

Electric response

35°

45°

60°

I
2

3

4
5
6

19 divisions
19

7

7

5

5

39 divisions

32

31
22

14
19

49 divisions

40
40
29
19
22 ,,

Mean .

IO-3 „

26-1 „

33-1 »

REACTION AT 35^ 45^ AND 6o° 359

From these figures it appears that on increasing the
angle beyond 45°, the ratio of reactions at 60° and 45° is as
I • 27: 1, while the ratio of sines of 60° to 45° is as i -22 : i.

On decreasing the angle, on the other hand, the ratios
are the following :

Ratio of reactions at 45° and 35° is as i : 0-39

sines

of 45'

33

- O

o-8i

Fig. 209. Curves showing values of sines (dotted) and of
reactions (thick hne). The latter produced cuts abscissa at
31*5°, indicating absence of excitation at a critical angle.
Ordinate represents reaction, abscissa the angle of
inclination. (Tropaeolum.)

The reaction at smaller angles is thus found to be dis-
proportionately lower.

Multiplying the above ratios by 100, in order to avoid
decimals, they become :

35°

45''

60"

39

100

127

81

100

122

Reactions

Sines ....

The relation between angle of inclination and resulting

reaction above and below 45°, found from the results just

given, is shown in fig. 209 ; the dotted curve represents

360 CHAP. XXXII. THE CRITICAL ANGLE

the sines of the various angles of inclination ; the continuous
line represents the excitation at the corresponding angles.
It will be noticed that while the divergence between the sines
and the reactions is not excessive above 45°, it is very
pronounced at the smaller angle of 35° ; it indicates the
approach of some hiatus or discontinuity. On producing
the curve backwards it intersects the abscissa at or about
31-5°, at which angle the excitation would appear to be
reduced to zero. This is the critical point ; beyond this
angle the excitatory reaction should be abrupt.

It remains now to ascertain whether such a critical point
does actually exist.

Determination of the Critical Angle

The discovery of the critical angle was the outcome of
my investigations on the geo-electric response of Nymphaea
1919). The electric response recorded was that given on
inclining the specimen from the vertical to the hori-
zontal. This was done very gradually in order to avoid
any mechanical disturbance likely to disarrange the electric
contacts. There was at first no indication of geotropic exci-
tation as the angle was gradually increased from zero,
and it was a matter of astonishment to note the reaction
which occurred abruptly when the inclination reached the
approximate value of 33°. The excitatory reaction was
exhibited by the sudden deflection of the hitherto quiescent
galvanometer spot of light. On return to the vertical
position the excitatory deflection disappeared. Repetition
of the experiment gave practically the same result. The
only explanation for this unexpected phenomenon is that
geotropic excitation is caused by the abrupt fall of heavy
particles in the perceptive cells, when the inclination exceeds
the critical angle.

It is necessary at this point to define the critical angle ;
it is the angle of inclination at which the excitatory geo-
tropic reaction is abruptly manifested. This, as already

DETERMINATION OF THE ANGLE

361

explained, can be reasonably attributed to the sudden fall of
heavy particles on to the sensitive protoplasm of perceptive
cells. Displacement of the particles will no doubt occur
even at a smaller angle of inclination, but only after a
considerable length of time, the displacement being helped
by protoplasmic movement ; whereas at the critical angle
the excitatory reaction will be immediate. It is very re-
markable that the critical angle in the case of Nymphaea
should be so near the theo-
retical value of 31-5°. The
critical angle of 33° found
for Nymphaea was a rough
approximation. The follow-
ing experiments w'ere carried
out for the determination of
the critical angle in various
plants, every precaution being
taken for ensuring the highest
accuracy.

Experiment 192. Deter-
mination of the critical angle for
the petiole of Tropaeolum. — I
give a photographic record of
the electric response of the
petiole of Tropaeolum as its
inclination was gradually in-
creased from 25° to 31°, by successive steps of 2°. There was
no response at 25°, 27°, and 29°. When the inclination
reached 31°, response occurred abruptly (fig. 210). Restora-
tion of the organ to the vertical w^as attended by recovery.
The possible error in the exact setting of the index at
zero of the scale is ehminated by observing the effects of
alternate inchnations to the right and to the left. The mean
of the two effective angles of inchnation thus gives the true
value of the critical angle.

Experiment 193. Exact determination of the critical
angle for the petiole of Tropaeolum.— Ih^ angle of inchnation

Fig. 210. Abrupt geo-electric
response at an inclination of 31"
(Tropaeolum).

362 CHAP. XXXII. THE CRITICAL ANGLE

to the right was gradually increased, and the hitherto
quiescent spot of light exhibited a sudden deflection to the
right at 30°. On return to the vertical the excitatory reaction
disappeared. Inchnation to the left gave a sudden deflection
to the left at — 35°. The critical angle is therefore 32-5°.

Experiment 194. Determination of the critical angle
for the stem of Tropaeohim. — The critical angle for the stem
was next determined, the procedure adopted being the same
as in the last case. The minimum angle at which response
occurred to right-handed rotation was 33° : rotation to the
left elicited reaction at —30°. Hence the true critical
angle for the specimen was 31*5°. Five other determina-
tions were made with other specimens, and the mean critical
angle obtained was 32-7°, which is very nearly the same as
the critical angle for the petiole.

Experiment 195. Critical angle for the stem of Com-
melina bengalensis. — With this plant an inchnation of 30°
to the right induced the excitatory reaction ; inchnation in
the opposite direction induced reaction at — 33°. The true
critical angle is thus 31-5°. In a second specimen the mean
value of the critical angle was found to be 31°.

Table XXXII. — The Critical Angle for Various Plants.

Specimen

No.

Inclination to
right and left

Critical angle
in degrees

Mean

I I-

30

- 35

32-5 ^

II.

30

- 34-5

32-25

Petiole of Tropaeolum

J III.

3.5

- 30

32-5 \

31-8

IV.

29

- 32

30-5

\ V.

31

-32

31 -5 '

/ I-

33

- 30

31 -5 \

II.

35

- 30

32-5

Stem of Tropaeolum .

J III.

1 IV.

36
35

- 30

- 32

33 -o 1
33-5 (

32-6

V.

34

- 33

33-5

I VI.

33

- 30

31 -5 /

Stem of Commelina . i

M;

30
32

- 33
~ 30

31-5 )
31 -o /

31 -25

EFFECT OF REPETITION 363

The mean critical value for all the various plants
examined is 31-8°, the maximum variation from this is
less than 1°. It is very remarkable that the critical angles
for different plants should exhibit so close an agreement.

The Effect of Repetition

Experiment 196. — It was stated at the outset that
repetition of the experiment might reduce the friction and
diminish the critical angle. It is very interesting that this
should have been found to be actually the case. I took
three different specimens of the petiole of Tropaeolum, the
experiments being carried out three times in succession. In
every case it was found that the effect of repetition was to
produce a not inconsiderable lowering of the critical angle.
In the first specimen the critical angle was lowered from
32-5° to 28-5°, in the second from 31° to 22-5°, and in the
third from 30° to 22-5°.

Table XXXIII. — The Effect of Repetition ox the Critical

Angle.

Specimen

Sequence of
repetition

Direction o

Right

■ inclination
Left

Mean critical
angle

I. Petiole of Tropaeolum

- 2

i3

30
30

27

35
32
30

3^ -5

31

28-5

II. Petiole of Tropaeolum

(I
■2

.3

30
25
20

32

28

25

31
2(> -5

22-5

III. Petiole of Tropaeolum

3

29
20

31
30
25

30
27
2 2 * S

These experiments offer a very strong confirmation of the
statolithic theory. The fact that when the organ is gradually
inclined from the vertical there is no excitation till the critical

364 CHAP. XXXII. THE CRITICAL ANGLE

angle is reached, and that then there is an abrupt excitatory
reaction, can only be satisfactorily explained on the theory
of the sudden fall of heavy particles from the base on to the
side of the perceptive cells.

Summary

The excitatory reaction under the stimulus of gravity is
reduced disproportionately with the diminution of the angle
of inclination. This indicates the approach of some hiatus
or discontinuity. By producing the curve of excitation
backwards, it cuts the abscissa at about 31-5°, at which
angle the abrupt excitatory reaction should be reduced
to zero.

The critical angle for geotropic excitation has been
found in a large number of plants to be about 31-8°.

The effect of repetition of inclination is found to lower
the critical angle.

The abrupt excitatory reaction induced beyond the
critical angle can only be attributed to the sudden fall of
heavy particles from the base on to the side of the sensitive
cells.

CHAPTER XXXIII

THE RESPONSE OF THE ROOT TO DIFFERENT

STIMULI

The electric response of the shoot to the stimulus of gravity
has been described in previous chapters ; it was shown
that its response to geotropic stimulation is similar to that
to other modes of stimulation.

The response of the root to stimulation of various kinds
will be described in the present chapter. 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. Stimulation is there-
fore direct only when the stimulus is appHed at the respond-
ing growing region ; it is indirect when applied at the tip.
The intervening distance between the root-tip and the
responsive zone of growth is semi-conducting.

I may briefly recapitulate the effects of indirect and
direct stimulation as exhibited by mechanical response of
the root described in greater detail in a previous chapter.

Effect of unilateral stimulation of the root-tip. — Stimula-
tion of the tip induces indirect stimulation of the growing
region higher up on the same side. The resulting expansion
produces a negative curvature away from the stimulated side.
This occurs under modes of stimulation as diverse as photic
and thermal. It has been shown that this effect is not
pecuhar to the root but also occurs in the shoot as the conse-
quence of indirect stimulation of the growing region (p. 142).

Effect of direct unilateral stimulation of the growing
y^aion.^ln contrast with the negative curvature induced

366 CHAP. XXXIII. THE RESPONSE OF THE ROOT

by indirect stimulation, direct unilateral stimulation of the
growing region gives rise to a positive curvature (p. 143).

I now go on to describe the electric response resulting
from (i) indirect stimulation of the root-tip, and (2) direct
stimulation of the growing region.

Electric Response to Indirect Stimulation

Experiment 197. — One of the two electric connections
with the galvanometer is made at a point A {see fig. 211) on

one side of the growing region

^ Y\ of the root of the Bean (Vicia

I I Faha), the other connection

being made at the diametrically
opposite point B. Unilateral
stimulation was applied at the
root-tip a, and on the same side
as A. The tip was subjected to
various modes of unilateral
stimulation ; mechanical stimu-
lation was effected by friction
with emery-paper, or by pin-
prick ; chemical stimulation was
produced by application of
dilute hydrochloric acid ; ther-
mal stimulation was effected by
the proximity of an electrically
heated platinum wire. In
every case the response was that
of induced galvanometric posi-
tivity at A. The electric varia-
tion took place within about 10 seconds of the application
of stimulus; the time-interval obviously depends on the
length of path to be traversed by the transmitted impulse
causing indirect stimulation.

The galvanometric positivity at A gave indication that
there was an increase of turgor and expansion induced at

Fig. 211. Diagrammatic repre-
sentation of mechanical and
electric response of root to
indirect stimulation, the
stimulus having been applied
at the tip a. Figure to the left
shows responsive mechanical
movement away from the
stimulus. The electric response
to indirect stimulation is in-
dicated in the figure to the
right, the point on the same
side exhibiting galvanometric
positivity at a. The shaded
part indicates the responsive
region of growth at some
distance from the tip. (Vicia
Faba.)

RESPONSE OF THE ROOT-TIP 367

that point, in consequence of which the organ would curve
away from the stimulus. Thus the mechanical and elec-
tric methods of investigation both lead to the identical
conclusion that unilateral stimulation of the tip of the root
gives rise to a movement such that the organ recoils from
the source of stimulation ; since tropic movement towards
the stimulus is termed positive, this opposite response must
be termed negative.

Table XXXIV. — Effect of Unilateral Stimulation of the

Root-Tip,

Effect at the proximal side A in the growing region

Effect at the distal side B

Galvanometric positivity, indicative of increase
of turgor and expansion

Negligible

The corresponding tropic curvature is negative, i.e. a movement awav
from stimulus.

A similar electric method was employed for the detection
of geotropic excitation in the root, both at the root-tip and
at the zone of growth in which geotropic curvature is
effected. The two diametrically opposite contacts at the
tip will be distinguished as a and h, the corresponding points
higher up in the growing region being A and B [cf. figs. 212,
213). When the root is vertical the electric conditions at
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 is found to take place ;
the responsive current disappears when the root is brought
back to the vertical. Rotation through — 90° gives rise to
a responsive current in the reverse direction.

Geo-Electric Response of the Root-Tip

Experiment 198. — I took the root of a Bean {Vicia
Faha) and made two electric contacts at two diametrically
opposite points, a and h (see fig. 212), on the root-tip at
a distance of about i -5 mm. from the extreme end. Owing

368 CHAP. XXXIII. THE RESPONSE OE THE ROOT

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 points, without exerting undue
pressure. The root has to be laid horizontal for geotropic
stimulation, and as the root of the Bean is somewhat long
and limp, displacement from the vertical position is apt to
cause a break of the electric contact. This is avoided by

Fig. 212. Diagrammatic representation of geo -electric response
of root-tip. The middle figure shows root in vertical position.
Rotation through -f 90° places a above, which becomes
galvanometrically negative. Rotation through — 90° places
h above and makes it negative.

supporting the root from the top and also from the sides
with padding of cotton-wool.

After due observance of these precautions, the electric
response obtained is found to be very definite. When the
root is made horizontal by rotation through + 90°, the
point a is above, and the responsive current is found to flow
from h to a, the upper side of the tip becoming galvano-
metrically negative ; when the root is brought back to
the vertical, the responsive current disappears. Rotation
through — 90° now makes the point h occupy the upper
position, and the responsive current is from a to h. Hence
the upper side exhibits in every case an excitatory electric
change of galvanometric negativity (fig. 212). The root-
tip thus gives the characteristic response to direct stimu-
lation. Experiments carried out with twelve different

RESPONSE IN THE CxROWING REGION 369

specimens gave concordant results. The following table
gives the absolute values of electromotive force induced at
the tip under geotropic stimulus in three different specimens.

Table XXXV — Geo-Electric Response of the Root-Tip

{Vicia Fab a).

Specimen

I
2

3

Induced E.M.F.

O-OOII volt
O-OOIO ,,
0-0015 ,,

Electric Response in the Growing Region

Experiment 199. — Investigation was next undertaken
on the electric variation induced in the growing region under
the stimulus of gravity. The experimental difficulties are
here less serious, since the available area of contact for
galvanometric connection is not so restricted as in the case
of the root-tip. The specimen was securely mounted so that
the root was vertical. It was then rotated through + 90°
into the horizontal plane, so that the point A {see fig. 213)
in the growing region occupied the upper position. The
electric response in the growing region was very definite
and took place in a short time. The induced electric
change at A was one of galvanometric positivity indicative
of increase of turgor and expansion.

The next series of experiments was carried out in the

following order. The root was first rotated through + 90°

so that A was above. The responsive electric variation, as

already stated, was galvanometrically positive. The root was

then rotated back to the vertical position when the current

disappeared. The root was next rotated through —90°

and the responsive current became reversed, the now

upper point B having become electro-positive. Alternate

rotations through + 90° and — 90° were carried out six

times in succession with consistent results. The interval

allowed bet\veen one stimulation and the next was determined

2 3

370

CHAP. XXXIII. THE RESPONSE OF THE ROOT

by the period of complete recovery. Growing fatigue
was found to increase this period ; at first it was 7 minutes,
at the second repetition it was 10 minutes, and at the third
time it was prolonged to 15 minutes.

A.B

1

B

L0J

Fig. 213. Diagrammatic representation of geo-electric response

in growing region of root.

a, rotation through — 90° makes b galvanometrically positive.
b, vertical and neutral position, c, rotation through
-f- 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 rotation 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 XXXVI. — Geo-Electric Response of Root in the

Region of Growth.

Odd series

Galvanometer

deflection A,

positive

Even series

Galvanometer

deflection B,

positive

I

3

5

20 divisions

16

10

2

4
6

18 divisions

18

12

Additive Action-Current at the Tip and at the

Growing Region

It has been shown that under geotropic stimulation
the upper side of the tip a, being directly stimulated, be-
comes galvanometrically negative ; while the indirectly

ADDITIVE ACTION-CURRENT 37I

stimulated point A, in the growing region higher up,
becomes galvanometrically positive. If now two galvano-
metric connections be made at the points a and A, the in-
duced electric difference is increased and the galvanometric
response becomes enhanced.

Experiment 200. — The root was at first held vertical,
and two electric contacts made at a and A. In this neutral
position there is little or no current. But as soon as the
root was laid horizontal, an electromotive response was
exhibited which showed that a was galvanometrically
negative and A galvanometrically positive (see fig. 213, d).
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 invariably 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 XXXVII. — Induced Electromotive Variation between the
Tip and the Growing Region (a Negative and A Positive).

Geotropic stimulation

Resulting electric response

First stimulation

Second

Third

Fourth

Fifth

Sixth

140 divisions

130

130

123

127

115

These results lead to the conclusion that under geotropic
stimulation :

(i) the induced galvanometric negativity at the upper
half of the root-tip is due to direct stimulation ; and

(2) the induced galvanometric posit ivity of the growing
region on the same side is due to indirect stimulation
by transmitted impulse.

372 chap. xxxiii. the response of the root

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 ; he found that removal of the tip abolished the
geotropic response of the root. The objection has been
raised that the shock-effect of the operation is itself the
cause of abolition of response. But subsequent observa-
tions have shown that Darwin's conclusions are in the main
correct.

The experiments which have been described on the geo-
electric response of the root-tip and of the growing region
offer convincing proof of the perception of stimulation at
the tip, and of the indirect stimulation of the growing
region. These experiments exhibit, in one and the same
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 is the effect of indirect
stimulation.

Difference in Geotropic Response of Shoot

AND Root

The next step is to endeavour to form some idea of the
difference in the conditions of geotropic stimulation of the
shoot and of the root, to account for the opposite responses
in the two organs. The reason for this difference lies in the
fact that in the shoot the perceptive and responding regions
are one and the same ; every piece of growing stem exhibits
the characteristic geotropic curvature. In the root the case
is different, since the perceptive and the responding regions
are separated from each other. When the perceptive root-
tip is removed the geotropic movement is either reduced or
abohshed. It must be borne in mind that this holds good

GEOTROPIC AND PHOTIC STIMULATION 373

only in regard to gravitational stimulation, the seat of
which is at the root-tip ; for the decapitated root continues
to respond to other forms of stimulation, such as chemical
or photic.

Difference between Effects of Geotropic and

Photic Stimulation

In the case of light, the source of stimulation is outside
the plant : but in geotropism the force of gravity is by
itself inoperative ; it is only, as already explained, through
the weight of the starch-grains contained in the geo-
perceptive cells that the gravitational stimulus becomes
effective. Want of recognition of this fundamental difference
has led many observers to attempt to establish an identity
of reaction of the root to geotropic and to photic stimula-
tion, in spite of facts which plainly contradict it. The
impulse under geotropic stimulation is originated at the
root-tip, and transmitted to the growing region at a distance.
This indirect stimulation makes the root curve away from
the incident vertical lines of force of gravity. There is,
however, no necessary restriction in regard to the point of
application of light, which can be directly applied at the
growing region, causing movement of the root towards the
incident light.

The distribution of the cells (statocysts) containing the
starch-grains in the shoot and in the root, furnishes material
for an explanation of the different geotropic response of
the two organs. In this connection the results of the
investigations of Haberlandt and of Nemec are highly
suggestive. Haberlandt finds statocysts present in the
responding region of the stem ; in fact, every section of the
growing stem not only possesses the responding tissue, but
also the apparatus for initiating excitation — namely, the
statocysts. The geotropic stimulation of the stem is
therefore direct. Nemec's investigations on the distribu-
tion of statocysts in the root show, on the other hand, that
it is in the central portion of the root-cap that the cells

374 CHAP. XXXIII. THE RESPONSE OF THE ROOT

containing the starch-grains are situated; and this would
account for the necessarily indirect geotropic stimulation
of the growing region of the root.

The anatomical fact that the perceptive region in the
root is separate from the responding region, explains the
physiological difference between the two organs. Through
whatever means the stimulus of gravity may act, it is
inevitable, inasmuch as the stimulation of the shoot is
direct and that of the root indirect, that an identical
stimulation should induce responsive reactions of opposite
sign in the two cases.

It will thus be seen that the postulation of two different
irritabilities in the shoot and in the root is wholly unneces-
sary and unwarranted by facts. Experiment has shown
that the irritability of the root is in no way different from
that of other organs.

Summary

When the tip of the root is subjected to the stimulus of
gravity, the upper half exhibits an excitatory reaction of
galvanometric negativity indicative of direct stimulation.

The consequent electric response in the growing region
above the stimulated root-tip is positive, indicative of
increase of turgor and expansion. This is the effect of an
impulse transmitted from the stimulated tip. The stimulus
of gravity is perceived at the root-tip, whilst the responsive
movement takes place in the distant growing region.

In contrast with the root, the growing region of the shoot
both perceives and responds to geotropic stimulation.

As the effects of direct and indirect stimulation on growth
are antithetic, the geotropic responses of shoot and of root
cannot but be of opposite signs, since the stimulation is direct
in the one case and indirect in the other.

There is no necessity for postulating different irrita-
bilities for the shoot and the root, since it has been demon-
strated that positive or negative curvature is dependent
upon whether the stimulation is direct or indirect.

CHAPTER XXXIV

ft

THE MECHANISM OF THE TWINING STEM

An unsupported twining stem bends over and its apex
circumnutates in a path more or less circular. When in
the course of circumnutation it comes across a support, it
twines round it. The direction of movement is in a large
number of cases against the hands of the clock, which I will
distinguish by a plus sign ; in a few cases it is negative or
clockwise. It is, however, often difficult to say what the
sign of normal movement actually is ; for the same plant is
found sometimes to move in an anti-clockwise and at other
times in a clockwise direction.

No satisfactory explanation has yet been given of these
movements. To quote Pfeffer : ' The factors which deter-
mine the permanent homodromous curvature of the apex
are uncertain. . . . The homodromous curvature of the
apex is probably due to autonomic variations of tone, in
which the external world and the progress of twining act
as directive stimuH. Baranetzky and Noll on insufficient
grounds assume the existence of a dia-geotropic irritability
in the apex inducing paranasty. Ambron ascribes the
homodromous curvature to the conjoint action of circum-
nutation and negative geotropism, a conclusion which
Schwendener disputes. The latter erroneously regards
circumnutation and geotropism as factors of constant
magnitude, and forgets that the circumnutation and the
klinotropic position of the shoot caused by it are themselves
the result of regulated geotropic reactions. De Vries sup-
posed the curvature to be due to the torsion produced by

376 chap; XXXIV. THE MECHANISM OF THE TWINING STEM

the weight of the free portion of the apex, but this has been
shown to be untrue by various investigators. The causes
of twining are therefore unknown.' ^ There has been much
discussion in regard to the question whether circumnutation
is autonomous or whether it results from the action of some
external stimulus.

Characteristic Response of Anisotropic Organs

The torsion, or twisting growth, characteristic of the
twining stem is the result of the unequal growth of two
opposite sides. The stem is, in fact, anisotropic ; it may
be regarded as consisting, at any given moment, of two
diverse longitudinal halves, which differ not only in their
rate of growth but also in their degree of excitability so
that they are differently affected by an identical change in
the environment. The differentially growing stem may be
compared with the anisotropic pulvinus of Mimosa. The
upper and the low^er halves of the pulvinus are excitable, but
in a different degree. Local stimulation of the upper half
produces a contraction of that half, causing an up-movement,
while local stimulation of the lower causes a more vigorous
down-movement. Diffuse stimulation causes antagonistic
reactions in the two halves, but since the contraction
of the lower half is greater, the predominant effect is a
resultant down-movement. For the sake of simplicity the
antagonistic action of the upper half may be ignored, and
the responsive movement attributed mainly to the action
of the more active half of the organ. The essential difference
between the anisotropic pulvinus of Mimosa and the aniso-
tropic stem of twining plants is that the plane which de-
marcates the two diverse halves in the former is fixed ;
whereas in the latter it regularly travels from segment to
segment of the circumference in a slow revolving motion.

Whilst the action of the less active half of a twisting
organ may, in general, be ignored, some complications may

* Pfeffer, ihid. vol. iii. p. 37.

CIKCUMNUTATION 377

arise from the fact that the optimum point of some variation,
such as a change of turgor or a change of temperature,
may be reached earher in the more active half than in
the less active.

•

Preliminary Adjustments

In working with cut specimens it is found that the
.torsion is at first clockwise (negative), instead of being
anti-clockwise (positive). I was able to trace this reversal
to the effect of the strong stimulation caused by section.
If, after mounting, the cut specimen be left undisturbed, the
normal torsion will be found to be fully restored after a
period of rest of about an hour. In obstinate cases, recovery
is hastened by immersion of the cut stem in tepid water.
It should be borne in mind that any rough handling acts
as a stimulus, and may thus retard or even reverse the
direction of the normal torsion.

Autonomous Torsional Circumnutation

Experimental investigations have been carried out with
a number of species of twining plants, such as Thun-
bergia gigantea, Thunhergia coccinea, P or ana paniculata,
Ipomoea, Clitoria Ternatea, and the common Phaseolus.
The researches were carried out under widely varied climatic
conditions. Tropical conditions prevailed in my Institute
in Calcutta, the mean temperature for 24 hours being about
30° C. in summer. Colder conditions existed at my Research
Station at Mayapuri, Darjeeling, situated at a height of
7000 feet, the mean temperature in summer being 18° C.

Experiment 201. Rate of circumnutation. — I will first
describe the movement of the horizontally inclined apex of
Thunhergia coccinea growing wild on the hill-side at Darjeeling.
This movement of circumnutation is to be distinguished
from the torsional movement of the erect stem. The method
of observation of circumnutation will be understood from

378 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM

fig. 214. The successive positions of the tip of the circum-
nutating plant are read at different hours of the day against
a circular scale placed below, in order to determine the
changing rates of movement due to external variations of
intensity of light and of temperature. The rates during
the hours of the day are given in the table.

Table XXXVIII. — Rates of Circumnutation during Different

Hours of the Day (Thunbergia coccinea).

Hours of the
day

Rotation

Difference

Hours of the
day

Rotation

Difference

5 A.M.

0°

• « •

2 P.M.

405°

15°

6 „

15°

15°

3 >.

315°

-90°

7 ..

37°

22°

4 ..

340°

25°

8 .,

130°

93°

5 ..

380'

40"

9 ,,

230°

100°

6 ., '

400°

20°

10 ,,

280°

50°

7 ..

412°

12°,

II

325°

45°

8 .,

424°

12°

12 Noon

370°

45°

9 „

435°

11°

I P.M.

390°

20°

As regards external conditions, the plant was in the
shadow of surrounding trees up to 9 A.M., when sunlight
fell on it directly and continuously till 3 p.m. ; after this
the rays of the sun were obstructed by trees on the other
side. The temperature rose from 12° C. at 5 A.M. to the
maximum of 20° C. at 2 p.m. Very strong sunlight and
high temperature at 2 p.m. brought about a condition of
drought which caused a reversal from anti- clockwise to
clockwise movement. The incidental effect of drought will
be described later in detail. The rise of temperature in the
morning enhanced the rate of circumnutation up to 9 a.m.,
but sunlight caused increasing retardation ; the effect of
drought referred to above caused a reversal of movement
after 2 p.m. As the sun disappeared behind the trees there
was a recovery to 25° between 3 and 4 p.m. This increased
to 40° between 4 and 5 p.m. After 5 p.m. the temperature
fell rapidly, causing diminution in the rate of movement.
The above example shows in a general way how the con-
joint effects of changes of intensity of hght, of temperature.

TORSIONAL ROTATION 379

and of condition of turgor modify the circumnutation of the
twining shoot. The effect of these different factors on the
movement of the shoot will be presently considered in
greater detail.

Fig. 214.

Fig. 215.

Fig. 214. Method of measurement of Circumnutation of a

twining shoot.

Fig. 215. Measurement of relative activities of successive
internodes. i', i", i'", indices fixed at different nodes.

Experiment 202. Torsional rotation round the vertical
axis. — The observation was continued the next day with
the same plant with this difference, that the horizontally
inclined portion of the stem was now held erect by means of
a long and thin piece of string passing over a pulley, the
end of the string being attached to a suitable counterpoise ;
the string itself was practically torsionless. A fibre of

380 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM

glass was attached to the apex of the stem, to serve as an
index. The object of holding the stem erect was to eliminate
the action of lateral geotropism, so called. In spite of this,
the erected stem exhibited at rotational movement similar
to that of the stemwhen inclined.

Torsional Activity of Different Internodes

Experiment 203. — For the determination of the relative
activities of different internodes, suitable indices were
attached to the plant at the corresponding nodes. The fixed
circular scale serves for the measurement of the torsional
rotation. The index at the tip of the stem indicates the
total rotation. The difference between the angles described
by V and 1" measures the torsional rotation of the first
large internode. Similarly, the different activities of
successive internodes from top to bottom may be measured
(fig. 215). The results are given in Table XXXIX.

Table XXXIX. — Rates of Rotation of Successive

Internodes.

Different nodes

Rotation

Difference

Rotation of tip .....
,, second internode

third internode .
,, fourth internode

fifth internode

55°

40°

8°

3"
0°

• • •

15°

32°
5°
3°

Taking the position of the lowest index, which did not
move, as zero, the torsion of the first or highest internode
is 15°. Torsion rises to a maximum of 32° at the second
internode, where the rate of growth is also at its greatest.
The rate dechnes to 5° at the third, and to 3° at the fourth.
There is no rotation in the fifth internode, which was the
oldest. Old internodes occasionally exhibit a slight rota-
tional movement which is negative, i.e. opposite to that
of the normal.

The results of the experiments just described prove that

^f^

CONTACT STIMULUS AND TWINING 381

the stems of twining plants have an autonomous torsional
activity which is not dependent upon geotropism. This
will be clear when it is realised that it is differential growth
which causes the torsional movement, growth itself
being a phenomenon
of autonomous activity.
The stimulus of gravity,
it is true, modifies
growth, but does not
initiate it. Examina-
tion of the ribbed stem
of a twining plant before
it has bent over shows
that the erect stem
had undergone a twist
(fig. 216). Circumnuta-
tion of the bending stem
is thus principally due
to the torsional activity
of the organ. When the
circumnutating stem
encounters a more or
less vertical support, it
coils round it.

V

Contact-Stimulus
AND Twining

Fig. 216. The twist of the ridges of the stem
(left figure) indicates the natural torsion of
the stem. The right figure exhibits shoot
IT- i_ twining round support in anti-clockwise

Kohl lound that the direction (Thunbergia coccinea).

stem of Calystegia

twined round a loose string, the stem being concave at all
points of contact. In spite of this, there is a prevalent
opinion that twining stems are not sensitive to the stimulus
of mechanical contact, a view which raises difficulties in
explaining the primary cause of twining round a support.
In the case of tendrils which are highly sensitive to contact,
twining is produced by the retardation of growth at the
points of contact, aided by acceleration of growth of the

382 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM

distal side (p. 97). I subjected the matter to the following
experimental test.

Experiment 204.— My High Magnification Crescograph
enabled me to obtain a record of the normal rate of
growth of the stem of Thunbergia ; this was found to
be 0*7 [Jt per second. The stem was next subjected for
5 minutes to mechanical stimulation by friction applied to
all sides of the organ. The growth was now found to be
arrested, and it was not till after an hour that growth was
renewed. It is evident that continuous stimulation of one
side of the stem by friction against a rough support retards
or arrests the growth, with induced concavity, of the
stimulated side. The twining stem is therefore sensitive
to mechanical stimulation.

The following may be taken as the sequence of events
which lead to twining. The autonomous torsion of the
stem gives rise to circumnutation of the overhanging part
of it, in the course of which movement the stem comes in
contact with a support. Continued autonomous torsion and
the contact-sensitiveness of the stem then co-operate to effect

the twining.

Having now indicated what is the essential mechanism
of twining, I may proceed to explain several sensitive
methods for the measurement of the torsional movement
and its induced variations.

Method of Optical Magnification

In order to study the effect of the change of anyone of the
external factors, it is essential to maintain all other factors
absolutely constant, a condition which can be secured only
for a short time. Hence necessity arises for rapid observa-
tion of the rate of normal torsion, and of the changes induced
in it by external agents. This I secured by the Optical
Method which I devised for my earher investigations on
autonomous torsion (' Plant Response,' 1906). The tip
of the plant is attached to a thin, torsionless string which
passes over a pulley with a suitable counterweight at the
other end ; the plant is thus held erect. A light concave

METHOD OF OPTICAL MAGNIFICATION 383

mirror, fixed at the upper end of the stem, reflects a spot of
light on to a scale which may be placed at a distance
of 3 metres, the scale itself being divided into millimetres.
The magnification thus produced is then 6000 times, which
is sufficient to measure the rate of torsion in the course

Fig. 217. Method of Optical Alagnification.

P, stem, upper end of which is held in clamp c (shown in the side
figure); M, plant-mirror; m', fixed mirror; s', scale for
measurement of torsion. Plant held at lower end in a
small phial filled with water.

of a minute. Certain precautions have to be taken in attach-
ing the string to the tip of the plant. The very crude method
of tying a knot gives rise to unequal pressure on different
sides of the organ, resulting in the transmission of unequal
excitations down the stem. To avoid this, cotton-wool is
wrapped round the tip, to act as a soft cushion. A light
three-pronged aluminium clamp now holds the tip of the
plant. A spring clip attaches the plant-mirror to the clamp
(fig. 217).

384 CHAP. XXXTV. THE MECHANISM OF THE TWINING STEM

I find that it is more convenient to work with a cut
specimen than with an intact plant ; the effect of wound
disappears after a time, when the stem regains its normal
torsional activity. The cut end of the stem is fixed by
means of a cork in a small phial filled with water. This
plant-holder H passes through a circular scale ; by rotation
of H, different flanks of the plant can be subjected to uni-
lateral action of light L. The light from a pea-lamp is
reflected from M and re-reflected on the millimetre scale
S' by means of a second stationary mirror M' at a distance
of 75 cm., the magnification produced being 3000 times.
The rate of movement is observed every 5 minutes, the
intervals being signalled by clockwork. It may be said in
general that the rate of autonomous torsion depends on the
species of the plant, on its physiological vigour, and on
external condition, such as season and temperature.

*

Automatic Recorder

Method of Magnification. — I also succeeded in the
difficult task of obtaining an automatic record of the normal
rate of torsion and its induced variations. The method
will be understood from the diagrammatic representation
given in fig. 218. The clamp bears a light aluminium
index I, about 10 cm. in length, which presses against V,
a vertical projection from the short arm / of a lever, the
vertical fulcrum-rod of which rests on a conical jewel-
support p. The short arm is 0-5 cm., while the longer
arm L is 15 cm. The total magnification of the system of
levers is therefore about 300 times. By slightly tilting
the apparatus, the recording end of L on the left tends to
rest on the smoked-glass recording plate G. The vertical
projection V at the other end of the lever then presses against
the index L This latter is pushed to the left by the anti-
clockwise rotation of the shoot, while the recording end of
L is moved towards the right. It must be remembered
that the recording index is moving in a horizontal plane ;
hence, in order to obtain a dotted record, the recording

AUTOMATIC RECORD

385

plate has to be moved up and down by means of clockwork.
The recording plate has a movement from right to left,
and an oscillating movement up and down, at intervals of
2 minutes. The slope of the dotted record shows the rate
of natural torsion. An enhancement induced in the rate

Fig. 218. Automatic Recorder of Torsion.
M, method of magnification; r, method of reduction. (See text.)

causes an erection of the curve, while a diminution causes

a flattening. If the natural torsion becomes reversed, the

index I moves in clockwise direction, and the record is

correspondingly reversed from an ascending to a descending

curve.

Method of reduction. — A continuous record for 24 hours

was necessary for my investigation of the diurnal variation

of torsional movement described in the next chapter. The

total rotation during 24 hours was too great to be recorded

2 c

386 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM

in the plate. The problem to be solved was therefore not
one of magnification but of reduction. This was secured
by the employment of a device which produced a suitable
reduction by a system of aluminium toothed wheels T
and T' (fig. 218, R). The necessary condition for perfect
working of the arrangement is that the centre of curvature
of the toothed wheel T' should be at the fulcrum of the
recording lever.

Having explained the method of observation and of
record of torsional movement, I describe in this chapter the
results of the investigations of the following subjects :

1. Modification of torsional movement by variation in

the rate of ascent of sap.

2. Torsional movement under variation of temperature.

3. The critical fatal temperature.

4. Effect of chemical agents on autonomous torsion.

Effect of Variation in the Rate of Ascent of Sap

I have shown ^ that the rate of ascent is increased, within
limits, by a rise of temperature of the water applied at the cut
end of the stem. Diminution of the rate of ascent is pro-
duced under drought or by withdrawal of water. Artificial
drought may also be caused by the application of a plasmo-
lytic solution of KNO3 at the cut end of the stem. Increase
in the rate of ascent causes an increase, while diminution in
the rate gives rise to diminution of turgor in the stem. The
experiments show that an increase of turgor, however produced,
causes an increase in the rate of normal torsion. Diminution
of turgor, on the other hand, causes an arrest or even a reversal
of normal torsion.

Experiment 205. — The normal rate of the anti-clockwise
torsion of the stem of Phaseolus was 10 mm. per 5 minutes.
The vessel which supplied water at the cut end of the stem
was removed ; the effect of the resulting drought was a
continuous diminution in the rate of the anti-clockwise

^ Physiology of the Ascent of Sap, p. 62.

EFFECT OF ASCENT OF SAP 387

torsion. This culminated, in the course of half an hour,
in an actual reversal to negative or clockwise torsion of
— 25 mm. per 5 minutes.

I next studied the effect of application of warm water
to the cut end of the stem. The resulting increased rate
of ascent of sap converted the negative — 25 to positive 20,
which is greater than the original rate. Water was with-
drawn once again, and the torsion became reversed to — 5.
Warm instead of tepid water was next applied at the cut
end, with the result that the negative became replaced by
an enhanced positive of 50. The above is typical of
numerous experiments.

Experiment 206. Effect of plasmolytic withdrawal of
water. — Artificial drought was produced by the application
of a plasmolytic solution of KNO3 at the cut end of the
stem. The normal rate in a specimen of Phaseolus was
+ 14 ; application of KNO3 solution at the cut end caused
a reversal to — 6 in the course of half an hour.

The effect of variation in the rate of ascent of sap on
torsional response will now be related with the effect induced
in growth itself. I have already shown (p. 53) that normal
growth is accelerated by enhancement of turgor caused
by increased rate of ascent of sap, drought producing
the opposite effect. The relation between changes in the
rate of growth and of torsion will be seen by comparison of
the two following tables :

Table XL.— -Effect of Alter-
nate Variation of Turgor
ON Longitudinal Growth
(Zephyranthes).

Table XLI. — Effect of Alter-
nate Variation of Turgor
ON Torsional Growth
.'Phaseolus).

Condition of
experiment

Rate of
growth

Dry soil

Application of
warm water

Application of
KNO3 solu-
tion .

0 -04 /i per sec.

0-20_« ,,

0-03//

Condition of

Rate of torsion

experiment

per 5 minutes

Normal .

ID

\Mthdrawal of water

- 25

'J epici water .

20

Withdrawal of water

— 5

Warm water .

50

Normal

M

KNO3 solution

— 20

388 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM

Effect of Variation of Temperature

For studying the effect of variation of temperature,
either fall or rise, it is necessary that the variation should
be gradual and not abrupt : for sudden variation acts as a
shock, causing excitatory reaction.

Method of gradual variation of temperature. — This is
accomplished by enclosing the plant within a double-
walled cylindrical chamber made of highly conducting
copper. Warm or cold water from a reservoir enters the
hollow cylinder, and leaves it by a pipe provided with a stop-
cock. By careful manipulation of the stopcock it is possible
to change the temperature of the enclosure very gradually,
the rate of variation being approximately i° C. per i J minutes.

Effect of lowering the temperature. — The following results
were obtained with tw^o different species of plants, Porana
and Thunbergia.

Experiment 207. — With falling temperature, the normal
rate gradually diminished in both till the torsional move-
ment was practically arrested at or about 9° C. There
was a revival of growth on raising the temperature above
this critical point. My other investigations have shown that
various physiological activities are also arrested at a critical
temperature. For example, the photosynthetic activity of
HydriUa is arrested at 9*5° C.

Table XLII. — Effect of
Lowering the Tem-
perature (Porana
Paniculata).

Table XLIII. — Effect of Cyclic
Variation of TExMPErature
(Thunbergia).

Temperature.

Rate of torsion

°C.

mm. per minute

23

54

20

31

18

22

15-5

18

II

10

9

0

Temperature.
°C.

Rate of
torsion (through

Rate of
torsion (through

descent)

ascent)

20

21

<

' 21

17

12-5

12

15

10

9-5

12

8

7-5

9

^

r 2

2

Table XLII gives the diminution of the rate of torsional
movement in Porana by falling temperature. Table XLIII

RISE OF TEMPERATURE 389

gives the changes induced in Thunbergia by a cycHc varia-
tion of temperature, first of thermometric fall to the
critical point, and then of rise to normal temperature.

Effect of rise of temperature. — The torsional activity
increased with rise of temperature till an optimum was
reached. Two different t^^pes of effect were observed, as
described below.

Experiment 208. — The first type is exemplified by
Porana. The activity increased till the optimum tempera-
ture was reached, which is between 32° and 33° C. ; above
this point it underwent a decline. It may be stated here
that my other investigations show that the optimum tem-
perature for growth, for photosynthesis, for ascent of sap,
for transpiration, and for maximum sensibility of Mimosa,
is also about 33° C. This characteristic, it should be
remembered, relates to plants in the tropics.

Table XLIV. — Showing Variatiox of Torsional Activity
UNDER Rise of Temperature (Porana).

Temperature.
°C.

Rate of torsion per

minute

1

23

14

I

2t

24

31

52

32

54

35

28

!

3S

t8

-12

10

43

5

1

Experiment 209. — Thunbergia represents the second
type in which there are two optima instead of one. The
rate of torsion increased up to 32° C, which is the first
optimum. There was then a continuous decline till 40° C,
after which the rate showed a sudden increase, reaching
its maximum at 47° C, which is the second optimum. This
double optimum will be noted in the record given in fig. 219,
where the violent contraction at 60° C. is the death-spasm
which will now be explained.

390 chap. xxxiv. the mechanism of the twining stem

The Critical Fatal Temperature

As the temperature is raised above the optimum,
a very abrupt and striking change occurs at a critical
temperature of about 60° C.

Fig. 219. Death-spasm at fatal temperature of 60°,
double optima before reversal. (Thunbergia.)

Note

Experiment 210. — The results are graphically repre-
sented in the automatic record obtained with Thunbergia,
during rise of temperature between 23° and 60° C.
(fig. 219). It will be seen that the rate of torsion con-
tinuously increased till a diminution of the rate occurred
above the first optimum at 32° ; with further rise of tem-
perature to about 40°, a second optimum was attained. At

ACTION OF CHEMICAL AGENTS 39I

60° C, however, a spasmodic reversal of torsional movement
occurred, which w^as so violent that the record went down
off the plate in a very short time.

Explanation. — I have shown that an erectile movement is
produced in the leaf of Mimosa, under rising temperature,
by the expansion of the more excitable lower half of the
pulvinus. But a sudden spasmodic fall takes place at
the critical temperature of 60° C, which is the spasm of
death. This critical temperature is more or less definite
in fresh and vigorous specimens. Growing organs like-
wise exhibit a sudden contraction at the fatal tempera-
ture (p. 37). It may fairly be inferred that the sudden
reversal of normal torsion at this critical temperature is
due to the violent death-contraction of the more active
longitudinal half of the stem.

Action of Chemical Agents on Torsion

A chemical solution applied to the cut end of the stem
ascends with the rising sap and affects the torsional

Fig. 220. Effect of Formaldehyde applied at arrow, showing
prehminary acceleration followed by reversal. A prelimi-
nary portion of the record is omitted. Successive dots at
intervals of 3 minutes.

activity. A certain interval necessarily elapses betw^een the
application and the responsive reaction. In an experiment

392 CHAP. XXXIV. THE MECHANISM OF THE TWINING STEM

with gas or vapour, they are blown into the cylindrical
chamber surrounding the plant.

Experiment 211. Formaldehyde. — The typical action of
poison is exemplified by the effect of the application of 7
per cent, solution of formaldehyde to the cut end of the

Fig. 221. Effects of Chloroform and Ether on autonomous

torsion.

The record to left shows the effect of chloroform applied at
arrow. There is a preliminary acceleration followed by-
reversal.

The record to right shows marked acceleration under dilute

vapour of ether.

stem. My investigations on the effect of poison on growth
has shown that the preliminary effect is an enhancement of
growth, followed by arrest and by the death of the plant, often
signalised by a spasmodic contraction (p. 46). In the case
of the twisting growing stem, the automatic record (fig. 220)
shows the different phases of reaction. The immediate effect
of poison is an enhancement of the rate of normal torsion,
seen in the wider spacing of the successive dots. This

SUMMARY 393

continued for 12 minutes, after which the rate of movement
diminished and became arrested after a further period of
24 minutes ; the response now became reversed from
positive to negative ; all responsive movement disappeared
in the course of an hour after application of the toxic agent,
indicating complete poisoning of the stem.

Aimnonia vapour abolishes the torsional activity. The
vapour of hydrochloric acid is also toxic, producing an arrest
of movement.

Experiment 212. Chlorofonn. — I give in the record to the
left (fig. 221) the effect of chloroform vapour. It is seen to
produce a preliminary enhancement of the rate, followed by
reversal and arrest in the course of half an hour.

Experiment 213. Ether. — Ether is less toxic and its
effect in a moderate dose is an enhancement of the rate of
torsion which persists for a considerable length of time
(record to right, fig. 221). Very prolonged application,
however, induces depression in the rate.

Summary

The growing twisting stem is anisotropic, two longi-
tudinal halves at any given moment being unequally ex-
citable, like the two halves of the pulvinus of Mimosa.
The plane of demarcation between the two halves in
Mimosa is fixed ; in the twisting stem the plane slowly
travels round the axis.

The torsion of the stem is autonomous : it is the result
of the unequal growth of the diverse longitudinal halves.

The twining of the stem is effected by its autonomous
torsional growth and by its sensitiveness to contact.

By the Method of Optical Magnification the rate of tor-
sional growth can be accurately observed. The Automatic
Method records the actual rate of torsional movement.

The rate of torsional growth is modified by the rate of
ascent of sap. Enhancement of the rate of ascent induces

394 CHAP. XXXIV. THE MECHANISIM OF THE TWINING STEM

an increase in the rate of torsion ; while depression of ascent,
as well as plasmolytic withdrawal of water, induces retarda-
tion of the rate or reversal of the direction of torsion.

Rise of temperature up to an optimum induces an
enhancement of the rate of torsion ; lowering of tempera-
ture brings about a depression of the rate, or an arrest of
the movement.

At the critical temperature of about 60° C. there is a
reversal of the direction of torsion, which is the spasm of
death.

A feeble dose of chloroform enhances the rate, but a
strong dose causes arrest or reversal.

Ether in moderate dose greatly enhances the rate of
torsional growth.

CHAPTER XXXV

EFFECT OF DIFFUSE STIMULATION ON AUTONOMOUS

TORSION

The effects of different modes of diffuse stimulation on auto-
nomous torsion will be specially studied in this chapter.
I begin with the effect of electric stimulation.

Response to Electric Stimulation

The great advantage of the electric mode of stimulation
is that (i) it causes no mechanical disturbance in the record,
and that (2) the intensity can be increased from minimal
to maximal by the gradual approach of the secondary to
the primary coil. The two electrodes from the secondary
coil are applied one above and the other lower down on the
stem, so that the induction current passes along the length
of the organ. Since the effect of stimulus is additive, the
effectiveness of stimulation depends on the product of
intensity and duration. A minimal stimulus may thus
become maximal by prolonging the duration of application.

As in other responding organs, a certain period elapses
between the reception of stimulus, and the response, which
continues for a time even after the cessation of stimulation.

Effect of Strong Stimulation

Experiment 214. — I will first describe the effect of
electric stimulation applied for 2 minutes on a vigorous
specimen of Thunbergia. The normal rate was 28 mm. per
minute. In response to stimulation the normal rate in the
positive direction was diminished, culminating in reversal
of direction to negative 3 minutes after the application

39^ CHAP. XXXV. TORSION UNDER DIFFUSE STIMULATION

of stimulus. The negative attained its maximum of — 55
after a further period of 5 minutes. The specimen then
exhibited recovery, which was completed in the course of
about an hour. The results are given in the following
tabular form :

Table XLV. — Effect of Maximal Stimulus on Torsional
Response of Vigorous Thunbergia.

Effect of stimulation

i
Rate of torsion ■

Effect of stimulation

Rate of torsion

per minute

per minute

Normal

28

1 After 6th min.

— 30

After ist min.

25

7th ,,

- 50

,, 2nd „

10

8th „

— 55

.. 3rd „

- 3

Qth ,,

- 52

„ 4th „

- 7

loth ,,

- 47

., 5th ..

- 15

53rd ,,

25 "

I give the record of another experiment (iig. 222), in
which the up-curve represents the normal torsion, the

Fig. 222. Direction of torsion reversed under electric stimulation
applied at arrow. Note gradual recovery. (Thunbergia.)

negative being represented by the down-curve. A reversal
from positive to negative torsion after stimulation will be
noticed, after which the response exhibited recovery.

MINIMAL STIMULATION 397

The following table gives typical results obtained with
different plants :

Table XLVI. — Effect of Strong Electric Stimulation on

Torsional Response.

Specimen

Normal rate per minute

After electric stimulation

Convolvulus
Tliunbergia

> f • • •

Poiana ....

25

12

10

— 4

— 20

— 2
-25

The results given in previous chapters have established
the generalisation that all forms of maximal stimulation — ■
electric, mechanical, or photic — induce a retardation of
the rate of growth ; that under increasing intensity or
duration of the stimulus the retardation culminates in an
actual contraction (p. 67) . On this principle, it is retardation
of the rate of growth and contraction of the more active
half of the differentially growing stem that cause the dimi-
nution in the rate of torsion, culminating in actual reversal
of direction, which follows strong stimulation.

Effect of Minimal Stimulation

In investigations on growth, I found that while a
strong stimulus induces retardation, a feeble stimulus
causes enhancement of the rate of growth. In the range
of stimulation between minimal and maximal there is a
critical intensity, above which there is retardation, and
below which there is acceleration. The critical point is
modified by the tonic condition of the organ, being rela-
tively high in a subtonic specimen. It is therefore easier
to obtain the enhancement of the rate of growth under
stimulation in subtonic specimens. I found, moreover, that
in such specimens the tonicity is improved by the stimula-
tion, as manifested by a permanent increase in the rate of
growth. In torsional response the rate of normal movement

398 CHAP. XXXV. TORSION UNDER DIFFUSE STIMULATION

may therefore be expected to be enhanced under the action
of feeble stimulation.

Experiment 215. — A specimen of Thunbergia was in a
slightly subtonic condition, as evidenced by its slow rate
of torsion, which was 3 mm. per minute. The effect of
minimal stimulation for 4 minutes is given in the following
table :

Table XLVII. — Effect of Minimal Electric Stimulus in
Enhancing the Rate of Torsion.

Effect of stimulation

Rate of torsion
per minute

Effect of stimulation

Rate of torsion
per minute

N'ormal

After ist min.
,, 2nd „
„ 3rd ,,
,, 4th ,,

3
14
19
32
36

After 5th min.
6th „

7th ,.
,; 25tli ,,

43

42

38

5

The results show that feeble stimulation enhanced the
rate from 3 to the maximum positive of 43 in the course
of 6 minutes, after which there was the commencement
of recovery. The improvement of the tonic condition is
shown by the permanent enhancement of the rate from
the original 3 to 5.

Other examples are given in the following table, in
which the maximum enhancement and permanent after-
effect are given in different columns.

Table XLVIII. — Effect of Minimal Stimulation in Enhancing
THE Rate of Torsion in Different Plants.

Specimen

Normal rate
per minute

Maximum
acceleration

After-effect

Porana ....

Thunbergia

Convolvulus

6

4

7

22
26
20

8

8

10

It has been stated that the effect of stimulation is
cumulative. Hence, with a stimulus of moderate intensity,
the effective stimulation is minimal at the onset and only

INDIRECT STIMULATION

399

becomes maximal after a certain duration of application.
This explains the most frequently observed effect of stimu-
lation— namely, a preliminary increase followed by a
diminution in the rate of torsion.

Effect of Indirect Stimulation

Another important result which I have obtained is the
effect of indirect stimulation on growth. Stimulation is
said to be indirect when it is applied at some distance from
the responding growing region. I have been able to estab-
lish the generalisation (p. 131) that while direct stimulation
induces retardation, indirect stimulation causes enhance-
ment, of the rate of growth.

The effect of indirect stimulation on autonomous
torsion proves to be an increase in the rate of movement.
Stimulation was effected indirectly by application of electric
shock for 4 minutes to the cut stem below the clamp which
supported it.

Experiment 216. — The specimen employed was Thun-
bergia, the normal rate being 8. As the result of indirect
stimulation, the rate was increased to a maximum of 95
in the course of 6 minutes. The rate was restored to nearly
the normal in the course of 40 minutes. Other examples are
given in the following table :

Table XLIX. — Effect of Indirect Stimulation in Enhancing

THE Rate of Torsion.

Specimen

Normal rate per minute

Maximum increase

Porana ....

t > • • • •

Thunbergia

10

7
10

36

42
37

Effect of Thermal Shock

It may be said in general that any sudden variation of
external conditions constitutes a stimulus. Electric stimu-
lation by an induction current is due to abrupt change of

400 CHAP. XXXV. TORSION UNDER DIFFUSE STIMULATION

the electric potential. I have found that a sudden variation
of temperature also acts as a stimulus.

Experiment 217. — I have already explained that a
steady rise of temperature up to an optimum enhances the
rate of torsion. In the present experiment a specimen of
Clitoria was mounted within a double cylinder through
which a flow of water at the constant temperature of 25° C.
was maintained. The rate of normal anti-clockwise move-
ment was 12. Warmer water at 30° C. was now made to
flow through the cylinder, thus subjecting the plant to a
sudden variation of temperature. The result was an abrupt
reversal in the torsional movement from 12 to — 75- This
persisted for 2 minutes. As the temperature became steady,
the movement was gradually converted from negative to
positive. After half an hour it had become 14, slightly
greater than at the beginning. This is due to the fact that
the steady temperature inside the cylinder was now about
28° C.

Effect of Mechanical Stimulation

Experiment 218. Feeble stimulation. — The normal rate
of Thunbergia was 20, which was increased to 40 under the
application of a feeble mechanical stimulus. The recovery
to the normal rate of 20 was attained in 5 minutes.

Experiment 219. Effect of stronger stimulation. —
Stimulus of friction was applied by rubbing the specimen
lengthwise with a piece of fine emery paper for about
3 minutes. The normal rate in Porana was 35 ; after the
frictional stimulation the rate was diminished to 3. The
specimen exhibited a recovery of the normal rate in
the course of 20 minutes.

It has been shown (Experiment 50) that frictional
stimulation induces a general retardation of growth. In a
twisting stem the retardation is the greater in the more
actively growing half, which is also the more excitable. This
differential effect causes a diminution of the rate, or even
a reversal of the direction, of normal torsion.

STIMULATION BY LIGHT 4OI

Experiment 220. — The next experiment was undertaken
with Thunbergia, and stronger and more prolonged friction
was appHed. The normal rate was 38, which became
reversed into — 22 after stimulation. The recovery was
nearly complete in the course of an hour.

Effect of Direct and Indirect Stimulation

BY Light

\\ ith regard to the unilateral action of light, Pfeffer
summarises the results as follows : ' According to Mohl,
Dutrochet, Darwin, and Baranetzky, the circumnutating
shoots of climbers are positively heliotropic, but this
irritability is so weak as merely to somewhat accelerate
circumnutation when the stimulus is applied so as to aid
the autonomic movement, and shghtly to retard the latter
when acting against it.' ^ The relation to light is, how-
ever, more complex than has been supposed. I will first
describe the effect of diffuse light acting on all sides of a
twisting stem. Two mirrors, suitably inclined, were placed
behind the stem, so that the incident sunlight acted on all
sides of it. The plants had been previously kept in dark-
ness for a short while.

Experiment 221. Effect of diffuse sunlight. — I describe
experiments with two different plants — Porana and Thun-
bergia. The former was highly excitable, and the latter
slightly subtonic ; Porana was subjected to the stimulus
of light for 5 minutes, and Thunbergia for 4 minutes.

The normal rate of Porana was 41 ; it exhibited an
immediate diminution, which reached a minimum of 4,
8 minutes after the cessation of exposure to light. The
recovery was practically complete after 50 minutes.

In Thunbergia the normal rate was 34 ; the rate ex-
hibited a preliminary enhancement which persisted till
the fourth minute, after which retardation set in, the
maximum retardation being at the tenth minute when the
rate was reduced to 2. After this there was slow recovery,

^ Pfefier, ibid. p. 41.

2 D

402 CHAP. XXXV. TORSION UNDER DIFFUSE STIMULATION

and the rate was 24 at the twenty-fifth minute. The re-
sults are given in detail in the following tabular statements :

Table L. — Effect of Diffuse
Sunlight on Porana.

Table LI. — Effect of Diffuse
Sunlight on Thunbergia.

Effect of stimulation

Rate of torsion
per minute

Normal

41

After ist minute

34

,, 2nd ,,

24

3rd

12

„ 4th .,

10

.. 5th „

8

6th

7

7th

6

8th

4

„ 50th „

39

Effect of stimulation

Rate of torsion
per minute

Normal

34

After ist minute

42

2nd ,,

3rd
„ 4th „
„ 5th „

6th

40

36
28
20
iS

7th „
8th

12

2

,. 25th

24

The results given above show that the effect of the
stimulus of light is the same as that of other modes of
stimulation. The effect of minimal stimulation is seen in
the preliminary enhancement of the rate of torsion, while
maximal stimulation caused a retardation of the rate.
When exposure to light is continued for a longer period,
the retardation culminates in actual reversal.

Effect of red and of blue light. — In my investigation of
growth under the action of light of different colours, I found
that blue light was very effective in the retardation of
growth. Red light induced practically no retardation
(p. 78). I could not be quite certain if it did not induce
the opposite effect of enhancement.

The contrasted effects of red and of blue light on
autonomous torsion are, however, very marked. The
different lights were obtained by means of colour-filters
placed in the path of strong white light.

Experiment 222. — The normal rate of torsion in Thun-
bergia was 14 ; this was enhanced to 38 under continuous
exposure to diffuse red light. On the cessation of light
there was a complete recovery to the normal rate of 14
in the course of 15 minutes. The same specimen was next
subjected to the action of blue light. The retardation was

GEOTROPIC STIMULATION 403

immediate and the rate was reduced from 14 to 3 in the
course of 8 minutes. On the cessation of exposure to Hght,
recovery was found to be complete after an interval of half
an hour. A second specimen gave very similar results.

Experiment 223. Effect of indirect stimulation by light. —
In Thunbergia the normal rate of 20 was enhanced to 96
by indirect stimulation by light applied for I minute.
The recovery was complete in the course of half an hour.
In Porana, similarly, indirect stimulation by light enhanced
the rate from 8 to 38. The effect of indirect stimulation
by light is similar to that of indirect electric stimulation.

Effect of Geotropic Stimulation

In order to obtain a satisfactory explanation of the
effect of geotropic stimulation on autonomous torsion, it is
necessary to obtain a clear idea of the means by which that
stimulation is effected. The experiments that have been
described in previous chapters fully confirm the theory
that it is the statolithic particles in the statocysts which
cause geotropic stimulation {see p. 342).

I have already shown that the apical ends of the stato-
cysts are more sensitive than the basal ends (p. 356). In
that case, a maximum variation in the geotropic action
should occur in two positions, erect and inverted — i.e. at
inclinations of 0° and 180°. The question now arises
whether this would be confirmed by observations made on
an organ performing torsional growth.

Since in such organs the effect of more intense stimula-
tion is manifested by a greater retardation of the rate of
torsion (which may culminate in reversal), it is quite possible
to observe the effect of geotropic stimulation in the two
positions, erect and inverted.

Experiment 224. — The mode of procedure is as follows :
The rate of torsion is observed by the optical method, first
when the stem is erect, and afterwards when it is upside
down. It is to be understood that the observer is looking
at the rotating apex of the stem from above in the erect,

404 CHAP. XXXV. TORSION UNDER DIFFUSE STIMULATION

and at the base in the inverted, position. The tabulated
results show that in the inverted position the rate of rotation
invariably underwent a marked diminution ; the statolithic
particles now press against the apical ends of the cells,
which are thus shown to be the more excitable.

Table LII. — Effect of Geotropism in Inverted Position in

Retardation of Torsion.

Specimen

Normal rate
per minute

Rate in inverted
position

1. Momordica monadelpha

2. Clitoria Tematea .

3. Vitis quadvartgularis

4. Phaseolus ....

21 -5

8-0

5-5
(>-o

•

" -5

2-5
O-S

3-0

Experiment 225. Reversal of torsion. — In highly sen-
sitive specimens, inversion of the plant did not merely
retard the rate of torsion, but induced an actual reversal of
its direction. Thus, in a specimen of Porana the rate in the
erect position was 23. When inverted, the torsional move-
ment was found to have undergone an actual reversal to
negative — that is to say, the direction of torsion was now
with the hands of the clock, at a rate of — 5.

In the next series of experiments, observations were
taken with specimens erect, then inverted, then erect again,
in order to eliminate the effect of any chance variation.
These repetitions induce fatigue, so that the response on re-
erection of the stem is sometimes less than at the beginning.
In vigorous specimens, however, there is but little decline.
The following table gives results obtained with a number
of different plants :

Table LIII. — Geotropic Reaction in Different Positions :
Erect, Inverted, and Re-erected.

Specimen

Erect
position

Inverted
position

Re-erected
position

Thunbergia

, , • . •

Porana ....

14
35
10

- 35

- 9

- 11

14
21

9

SUMMARY 405

Summary

Direct electric stimulation of adequate intensity
induces a retardation of torsional response which may
culminate in actual reversal.

Minimal electric stimulation, as well as moderate
stimulation of a subtonic specimen, induces enhancement
of the rate of torsion.

Indirect stimulation causes an acceleration of the rate
of torsion.

Sudden thermal variation acts as a stimulus and causes
retardation or even a reversal of direction of torsion.

Mechanical stimulation induces retardation or reversal
of torsion according to the intensity of stimulation.

Under diffuse stimulation by strong light the rate of
torsion undergoes diminution.

Red light induces an enhancement of the rate of torsion,
while blue light induces a marked retardation.

Geotropic stimulation has a definite influence on the
rate of normal torsion. When the stem is held inverted,
the rate of normal torsion undergoes a retardation which
may even culminate in an actual reversal of the direction
of torsion, proving that geotropic stimulation is more
effective in the inverted position. Facts have already been
adduced which show that the excitabihty of the ectoplasmic
layer at the apical end of the geo-perceptive cells is greater
than at the basal end. In the inverted position the heavy
particles, which are supposed to cause geotropic excitation,
press against the apical ends. The retardation of torsion
in the inverted position thus offers a further confirmation
of the statolith-theory.

CHAPTER XXXVI

EFFECT OF UNILATERAL STIMULATION ON AUTONOMOUS

TORSION

The experiments already described show how diverse are
the influences which modify the autonomous torsional
movement of plants. I describe some of the important
factors in operation during the course of the day and their
individual effects.

Effect of variation of turgor. — An increase of turgor
induces an enhancement of the normal rate of torsion,
while a diminution causes a retardation culminating in a
reversal. Turgor may be increased {a) by enhanced rate
of ascent of sap, or [h) by increased moisture in the air
(dependent on the direction of the wind) ; diminished turgor
is, on the other hand, produced by drought due to tran-
spiration increased by dry air, by high temperature, and
by strong sunlight.

Effect of variation of temperature. — Rise of temperature
up to an optimum enhances the rate, while increasing cold
induces a retardation. A rapid variation of temperature
acts as a stimulus and induces a reversal of direction.

Effect of stimulation. — The following is true of all modes
of stimulation. Feeble stimulation causes an enhancement
in the normal rate, while strong stimulation causes a
retardation culminating in a reversal. The effect of stimu-
lation is modified by its point of application, direct stimu-
lation producing one effect, indirect stimulation giving rise
to an effect precisely the opposite.

Effect of light.— The intensity of light during the day
undergoes continuous variation. It increases from dawn
to noon and dechnes from noon to evening. The effect of

DIURNAL VARIATION OF TORSION 407

increasing intensity of light is retardation of the rate of
torsional movement, which may culminate in reversal. The
retarding effect ceases under diminished intensity of light.

Effect of gravitational stimulus. — Gravity also has a
pronounced effect on torsional movement, which is depen-
dent on the angle of inclination.

A continuous fluctuation of torsion occurs as the resultant
of these different factors ; some of them are concordant
in their effect, thus increasing the normal movement, while
others diminish or even reverse it. The record of torsional
movement taken for a long period thus appears to be highly
erratic, a sudden increase being followed by an equally
abrupt reversal.

Diurnal Variation of Torsion

I give an automatic record (fig. 223) taken continuously
for 24 hours, the movement being reduced to keep the trac-
ings within the limits of the smoked glass plate. The experi-
ment was carried out under relatively simple conditions.
A young internode of a stem of Thunbergia was clamped
at the node below, and held erect in the manner already
described (p. 385). The shoot was introduced, through a
hole in a table, into a glass chamber which protected the
recording apparatus from disturbance caused by air-currents.
Ground glass was employed for the cover, to give a more
uniform illumination during the day. The record was com-
menced at 4 P.M. and continued for 24 hours. Light was at
that time disappearing, the retarding effect of light being
thus removed ; the temperature, very high at thermal noon,
had fallen to the optimum at about 4 p.m. Owing to these
two concordant factors, the direction of torsion was positive,
that is, anti-clockwise. After 9 p.m. the fall of temperature
was rapid and the torsional movement underwent a slight
reversal up to early morning. From 6 a.m. onward, the
temperature rose above 20° C. and light increased. The
effect of rise of temperature was predominant, so that

IluILIBRARY^^

408 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION

rate of positive torsion increased rapidly. At noon the
temperature rose to 42°, very much above the optimum, and
the retarding effect of the strong light was very great. The

MMBSI

•

•

*

• •

•

•

^^limamk^

•
• ••

• .

'•...*

1 j TJiii^B^^Br—

mr'\ ,v-''«'v^: ' . t^^S^TT^^^^^^^^^

•

r^^^^^^/

• •

• • •

• •

KaiB'

m/'''.y •'SIHI^^H^^^^^H^^I

B ' >-#*"'*^^^^^^^^^^^^^^^^^^^B^^^^^H

''^"^■■^^H^^QBLt .

- ^ff^H^^^^^^^Hi^^l^BB

-

H^^B ■. ->'-'-— 7 - " .^' ^^^^^H

•

^^H '.- -'' -■WM

1 1 1 1

4 8 12. 4

8 \Z

4

PM MIDNIGHT

NOON

RM.

Fig. 223. Automatic record of diurnal variation of

torsion.

condition of drought caused by excessive transpiration also
conspired to induce a reversal of direction from positive
anti-clockwise to negative clockwise movement. This con-
tinued till 3 P.M., after which time the record shows a
repetition of what occurred 24 hours before.

Effect of Unilateral Stimulation by Light

In the previous chapter I described the effect of stimulation
by light applied simultaneously on all sides of an anisotropic

STIMULATION BY LIGHT 409

twisting organ. The modification of torsion produced was
due to the differential excitability of the two longitudinal
halves of the organ. In the case of the stimulus of gravity,
stimulation is also diffuse in the erect and in the inverted
positions. But a new class of phenomena makes its
appearance when the two flanks of the anisotropic organ
are successively excited by lateral stimulation. The plane
of junction of the two halves of the twisting stem, it is
to be remembered, is always undergoing slow revolution
{see p. 376). The results are of high significance, though
at first sight they appear to be contradictory. I will first
describe the effects of unilateral application of light to the
different sides of the organ.

Method of Procedure

The normal rate of torsion is first taken in the dark ;
the effect is then observed of subjecting the different
sides of the organ to a narrow line of light produced by a
cylindrical lens interposed in the path of a parallel beam
of light from an arc-lamp. The different sides of the organ
are successively exposed to light by rotating the small glass
bottle in which the specimen is mounted ; the angle through
which the plant is rotated can be read on the horizontal
scale (cf. fig. 215).

Experiment 226. — I give detailed results obtained with
Clitoria which may be taken as typical. The rate of torsion,
when one of the sides was exposed to light, was ascer-
tained ; readings of the rate of torsion were next taken
when light acted on another side. In the first position the
natural rate of torsion in the dark was 30 mm. per 5 minutes.
Exposure to light increased it to 35. When the plant
was rotated through 22-5°, the rate increased to 45. This
increase continued till a maximum rate of 61 was attained.
Further rotation of the plant in the same direction now
induced an increasing retardation which culminated in

410 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION

actual reversal to — 4. Further rotation beyond this point
restored the direction of torsion to positive and the rate to
nearly the same value as when the plant was in the previous
position. The results are made clear by the following
tabular statement of the rates of torsion in four different
positions 90° apart— at 67°, 157°, 247°, and 337°.

Table LIV. — Effects of Exposure of Different Sides (90° apart)

TO Light (Porana).

[Normal rate in dark — 30.]

Different sides .
Rate of rotation

67° (A)
61

157' (N)
29

247° (R)
- 4

337° (NO
27

A careful examination of the above results leads to the
following conclusions :

1. The action of light on the different sides of the stem

induced responsive torsional variations which are
very characteristic.

2. Four different sides can be distinguished by their

respective induced variations. Two of these, N
and N', at 157° and 337° in Table LIV, are neutral,
the rate of torsion having remained practically
the same as when in the dark.

3. The two flanks, at right angles to the neutral sides,

exhibited marked changes : the one (A), at 67°,
showed a maximum acceleration of 61 ; the other
(R), at 247°, showed a maximum retardation
culminating in actual reversal to — 4.

I obtained numerous other results in confirmation of
the above : the following table gives a synopsis of the
effects induced in four other specimens of Clitoria. It is to
be remembered that N, N', and A, R, are at right angles to
each other.

RESPONSE OF ANISOTROPIC ORGANS

Table LV. — Effects of Exposure of the Different

Sides to Light

411

Spscimen

Maximum

acceleration.

A

First
neutral.

N

Reveisei.
R

Second

neutral.

N'

I

12

5

- 4

3

2

32

20

— 2

18

3

II

8

~ 4

6

4

24-5

9-5

-6-5

7-5

The above results, which fully confirm the previous
conclusions, were totally unexpected, and appear at first
sight to be quite inexplicable.

Torsional Response of Anisotropic Organs

The difficulty is completely overcome on reference to
the Laws of Torsional Response of Anisotropic Organs
which have already been established (p. 256), that —

1. An anisotropic organ, when laterally excited by any

stimulus, undergoes a torsion by which the less excitable
side is made to face the stimulus.

My present results extend this generalisation
to differentially growing organs also, and the wider
inclusive law is enunciated as follows :

2. A differentially growing organ, when laterally stimulated,

undergoes a torsion by which the less excitable side is
made to face the stimulus. The induced torsional
movement is algebraically summated with that of the
existing autonomous torsion.

The parallel results in differentially excitable pulvinated
and in differentially growing organs will be clearly under-
stood from the following illustrative examples.

Torsional Response of the Pulvinated Organ

The diagram (fig. 224) will explain the principal effects.
The shaded lower half of the pulvinus is the more excitable,
and the plane of junction of the two halves is marked

412 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION

on the flanks by A and R. l^he neutral sides are N, N'.
If a stimulus, light for example, acts from above on N, an up-
movement due to positive phototropism will occur ; if from
below, a more pronounced down-movement due to the
greater excitability of the lower half. No torsional move-
ment will, however, occur on stimulation of N or N'. But
the results will be different when stimulation is applied

L L

i 1

Diagrammatic representation of torsional response of anisotropic
organs to unilateral stimulation by light l.

Ideal transverse sections of the organs: n, n', ventral and dorsal
sides ; A, R, flanks ; a-r, plane of junction of the two diverse
longitudinal halves, of which the more excitable is shaded.

Fig. 224. Pulvinus of Mimosa: Stimulation of sides n, n',
evokes no torsional response ; stimulation of flank a induces
positive (anti-clockwise) torsion (left curved arrow) ; stimu-
lation of flank R induces negative (clockwise) torsion (right
curved arrow).

Fig. 225. Twisting Stem (supposed erect) : Normal torsion, t, un-
affected by stimulation of sides n, n' ; increased on stimulation
of flank A, decreased on stimulation of flank r.

laterally to either flank A or R. Lateral stimulation of
flank A induces a torsional response against the hands of
the clock, the less excitable upper half being moved so as
to face the stimulus. This will be distinguished by a
positive sign. Lateral stimulation of the opposite flank R
induces a negative or clockwise torsion.

t

Torsional Response of the Twisting Stem

The effects of unilateral stimulation by light on the
autonomous torsion of the twisting stem are much the

RESPONSE OF THE TWISTING STEM 413

same as those observed with the anisotropic pulvinus, but
with the difference that the torsion induced by the unilateral
action of light on the different sides of the stem is algebraically
summated with its autonomous torsion. This is made clear
on inspection of lig. 225, in which the shading indicates
the more excitable half. The straight arrows indicate the
direction of the light incident on the four sides, N, A, N',
and R. The arrow T indicates the natural torsion. Light
incident on the neutral sides N or N' induces no torsional
effect, but there may be a slight rectilinear movement up or
down due to positive phototropism. The torsion induced
by light falling upon either flank, A or R, is maximum, and
is algebraically summated with the natural torsion. That
induced by light falling on flank A co-operates with the
natural torsion : that induced by light falling on flank R
opposes the natural torsion and may even overpower it, so
that it is not only retarded but the direction is reversed.

The results obtained show that there is no ground for
the assumption that twining plants possess a specific sensi-
bility to light different from that of ordinary plants. The
variation of torsional movement due to the action of uni-
lateral light on different sides of the organ results from the
differential excitability of the organ, the characteristics of
which conform to the Law of Torsional Response in all
anisotropic organs.

A few words may be added in regard to the conditions
for securing the best result. These are : (i) quick determina-
tion of the maximum position A, and (2) completion of the
whole series of observations for N, R, and N' within a short
time. For the plane of junction is not stationary but
slowly revolving, and can only be regarded as stationary
for a relatively short time. It is also desirable to take only
a short length of the stem for the observation of torsional
movement, so that the plane of junction of the two halves
of the organ is vertical and straight.

I shall next show that the effect of unilateral stimulation
by gravity is essentially similar to that induced by light.

414 ch. xxxvi. torsion under unilateral stimulation

Effect of Unilateral Stimulation by Gravity

As the direction of stimulus of gravity is fixed, the differ-
ent sides of the twisting stem can only be exposed to it by

Fig. 226. Optical Method of observing rate of torsional move-
ment under unilateral stimulation by gravity. The different
sides of the horizontally laid stem are exposed to vertical
lines of force of gravity by rotating the plant-holder with
attached index, the angle being measured by the circular
scale.

rotating the horizontally laid stem round its axis. The
diagrammatic representation of the apparatus explains
the method of observing the rate of torsional movement.
The different sides of the organ are subjected to the stimulus
of gravity G, by rotating the small cylindrical vessel of

STIMULATION BY GRAVITY 415

water in which the lower end of the stem is fixed. This
w^ater-vessel is shown to pass through the fixed circular
scale, the angle of rotation being measured by means of
the index attached to the vessel (fig. 226).

The torsional response to unilateral stimulation by
gravity is, as in the case of light, not the same for all the
sides, but shows characteristic differences, as will be seen
from the results of the following experiment.

Experiment 227. — The stem of Clitoria was rotated till
the maximum rate of positive torsion was obtained, indi-
cating the A position. Representing the natural rate by T,

Fig. 227. Torsional response of the stem to stimulation of
its different sides by gravity in four positions, n, a, n', r.

No effect at n, n'; effect at a, additive; at r, in opposition.
T, normal autonomous torsion; g, gravity.

and by G the torsion induced by geotropic stimulation, the
result is the summated effects T + G, which in the present
case was 42. The plant was next rotated through 90"",
bringing it to N position, in which the torsional response
induced by G declines to zero. The response was now 7.
Further rotation through 90° brought the plant to the
R position ; now the induced torsion was negative, and it
overpowered and reversed the natural torsion, the resultant
T — G being — 24. Further rotation through 90° brought
the stem to the second neutral position N', the rate being 5,
which is not very different from 7, which was the rate at
the first neutral position N.

A large number of other experiments carried out with
different species of plants gave very similar results, of which

4l6 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION

a diagrammatic representation is given (fig. 227). The
following tabular statement gives a synopsis of results with
four specimens, two of Porana and the other two of
Thunbergia.

Table LVl. — Effect of Unilateral Geotropic Stimulation.
''T = auto- torsion ; G = induced torsion under gravity.)

Specimen

A

NorN'
T

R
T - G

1. Porana

2. Porana

3. Thunbergia .

4. Thunbergia .

74
80

135
25

6

33

35

7

- 60

- 13

-28

- 12

The results of the experiments on the effect of unilateral
stimulation by gravity show that there is nothing to support
the assumption of three types of geotropic reaction in twining
stems, negative, dia-geotropic, and lateral. The results, on
the other hand, demonstrate the characteristic effects of
diffuse and of unilateral stimulation on anisotropic organs.
In the vertical and erect position, the natural rate of torsion
is slightly retarded by the diffuse action of geotropic stimu-
lation, the statolithic particles pressing on the basal end
of each statocyst. In the vertical but inverted position,
the effective stimulation is intensified on account, pre-
sumably, of the greater excitability of the apical ends of the
cells. The result is an increased retardation which some-
times culminates in an actual reversal of torsion from
positive to negative.

The share of geotropic reaction in the movement of a
young stem about to twine, of which the lower portion is
erect whilst the upper portion overhangs, requires careful
analysis. The erect portion twists round its own axis,
a movement not affected by gravity, which causes the
circumnutation of the overhanging portion. Of this the
terminal internode twists but slightly : it tends to curve
upwards in response to gravity. In the succeeding inter-
nodes torsion is most active, and it is affected by gravity

GEOTROPIC AND PHOTIC STIMULATION 417

in a characteristic manner depending upon the rate of
revolution of the plane of junction of the two diverse longi-
tudinal halves of the anisotropic stem which necessarily
accompanies torsion. If the rate of revolution be such that
the flank A is always uppermost, then the autonomous torsion
will be accelerated by gravitational reaction. The movement
of the stem as a whole will then be composed of (i) the cir-
cumnutation, say, anti-clockwise, due to the torsion of the
erect portion of the stem, and (2) the circumnutation of the
overhanging portion round a more or less inclined axis.
This compound movement is not unlike planetary motion
in which, in addition to the revolution round its own axis,
the planet describes an elliptical path round the sun.

But if the revolution of the plane of junction does not
keep pace with the torsional movement of the stem, a lag
will occur, the result of which will be an alternate accelera-
tion and retardation of autonomous torsion by gravi-
tational reaction, according as the A or the R flank of the
stem faces upwards. An oscillatory torsional movement,
positive and negative, will then take place.

Combined Effects of Geotropic and Photic

Stimulation

Having ascertained the individual effects of stimulation
by light and by gravity of the different sides of the organ, it
is possible to predict their effects in different combinations,
taking examples which exhibit strongly contrasted effects.
Since the stimulus of gravity is unchangeable in direction,
the stimulus of light is superposed on it either from above
or from below. The horizontal stem is adjusted in darkness
till the rate of torsion is maximum, that is, in the A position
in which autonomous torsion and geotropic reaction are con-
cordant, represented by the symbol T -f G. By means of
two inclined mirrors, a beam of light from an arc-lamp is
then thrown upon it alternately from above and from

below. In the first case, the action of light co-operates

2 E

4l8 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION

with that of T + G, the combined action of T + G + L
giving rise to a large positive or anti-clockwise torsion. In
the second case, light acts in opposition, the resultant action
being T + G — L. Now, if the light be sufficiently intense,
it will overpower T + G, and the resultant will be a negative
or clockwise torsion. The theoretical result is diagram-
matically represented in fig. 228, where, in the A position,

Fig. 228. Combined effects of simultaneous geotropic and
photic stimulation on autonomous torsion t.

Light, L, alternately applied above or below (straight arrow).
Induced torsion (curved arrow) under light l and gravity g.

the unshaded half to the right is the less excitable. In the
figure to the left, light L and gravity G act from above ;
their additive effect co-operates with autonomous torsion T.
In the figure to the right, G acts from above on the flank A,
while light acts from below on the opposite flank R. The
torsion induced by L is therefore in the opposite direction
to that induced by G.

Experiment 228. — Thunbergia placed in A position.

1. Rate of torsion without light, T + G =

2. ,, ,, with light from above, T + G + L =
Light withdrawn ; complete recovery under T + G =

3. Rate of torsion with light from below, T + G — L =

Experiment 229. — Porana placed in A position.

1. Rate of torsion without light, T + G =

2. ,, ,, with light from above, T + G + L =

3. ,, „ with light from below, T + G — L =

16

40

16

-8

28

58
-3

SUMMARY 419

The results of the above experiments prove that the
effect of light is algebraically summated with that of auto-
nomous torsion, and also with that induced by geotropic
stimulation.

Summary

It has been shown that the twining of stems is brought
about by circumnutation, which is the expression of auto-
nomous torsion.

The stem being also sensitive to the stimulus of contact,
twining takes place when the circumnutating stem comes
into contact with a support.

The twisting stem is alike sensitive to all modes of
stimulation, electric, thermal, photic, and geotropic.

All forms of diffuse stimulation have a retarding effect
on growth.

The twisting stem is an anisotropic organ, the two
diverse longitudinal halves of which are unequally excitable.
Diffuse stimulation induces greater retardation of growth
in the more active half, which reduces the normal rate of
torsion even to reversal of direction.

Under lateral stimulation by light or gravity, a tor-
sional response is evoked in all anisotropic organs. When
the stimulus acts on the A flank, a maximum anti-clock-
wise movement is induced. The effect of stimulation of
the diametrically opposite flank R is a clockwise move-
ment. No responsive torsion occurs on stimulation of the
neutral sides N, N'.

The responsive reactions, algebraically summated with
the autonomous torsion, explain the varied movements of
twisting organs under the simultaneous action of different
stimuli.

The reactions to light and gravity are algebraically
summated with the autonomous torsion. Light, when
sufficiently strong, acting in opposition to gravitational
stimulus, induces a reversal of the direction of torsion.

CHAPTER XXXVII

GENERAL REVIEW

The different chapters of this book describe and discuss
observations on the movements of growing organs under
the changing conditions of the environment. Some of
these, such as variation of temperature and the alternation
of light and darkness, affect the rate of growth ; others,
such as the action of gravity, of unilateral illumination, and
of contact with foreign bodies, induce changes in the direction
of growth, these changes being generally described as
Tropisms. They are of many kinds and are more or less
complex, so that it is no easy matter to analyse them in
the hope of arriving at some general principles which would
explain them all.

The attempt has been made to account for the
movements teleologically, to regard them as determined
by the advantage they may contribute to the well-being
of the plant, rather than to study them physiologically as
manifestations of stimulation and response. It is the latter
experimental method which has been pursued in the work
here recorded, with results summarised in this chapter.

All movements of growing organs, whether spontaneous
or induced, are effected by change in the rate of growth ;
this change is subject to modification according to the
vigorous or feeble tonic condition of the growing organ. It
is therefore necessary to have means of immediate measure-
ment of the actual rate of growth and its induced variation.

The Method of Magnified Record

The High Magnificatioii Cresco graph permits of the deter-
mination of the absolute rate of grow^th and its induced

AUTONOMOUS ACTIVITY OF GROWTH 42 1

variation in the course of less than a minute, during which
time other conditions can be maintained absolutely constant.
The record shows that growth is a pulsatory phenomenon
(p. 18). The Magnetic Crescograph gives an amplification
of more than ten million times, thus making possible the
measurement of the smallest movement of growth and its
slightest variation. The sensitiveness of the method of
record has further been increased by the use of the Method
of Balance, in which the movement of growth is exactly
compensated by an equal movement of the plant down-
wards, the upsetting of the balance upwards or downwards
being due to induced acceleration or retardation of growth.
The Method of Balance offers a unique opportunity for
determination of the influence of the Time-Factor — that
is to say, of the effect on the reaction of the duration of
the action of an external agent.

Autonomous Activity of Growth

All autonomous movements are dependent on the
following conditions :

1. Sufficiency of internal hydrostatic pressure in the

active cells;

2. Supply of energy for the maintenance of the normal

tonic condition of the active cells ; and

3. An adequate temperature.

Ejfed of variation in the rate of ascent of sap. — It has been
observed in both motile and growing organs that increase
or positive variation of turgor, due to enhanced rate of ascent
of sap, induces an erection or positive response of a motile
organ, and a positive variation or enhancement of the rate
of growth. A diminution or negative variation of turgor,
due to withdrawal of water, induces a negative response of
a motile leaf, and a negative variation or retardation of the
rate of growth (p. 55).

Effect of suhtonic condition. — When the energy stored
for the maintenance of the autonomous activity becomes

422 CHAP. XXXVII. GENERAL REVIEW

depleted, so that the growing organ has become subtonic,
growth comes to a standstill. Stimulation is found to
revive the activity of the arrested growth (p. 86).

Effect of variation of temperature. — Sudden variation of
temperature acts as a stimulus and retards growth ; in
order to prevent this, experimental thermal change should
be gradual. In a number of tropical plants the minimum
temperature for arrest of growth is about 22°, the optimum
being 34° C. A continuous record of change of growth
during uniform rise of temperature gives the Thermo-
CRESCENT Curve from which the absolute rate of growth
at any temperature can be obtained (p. 39).

Effect of Anaesthetics on Growth

The effect on growing organs of exposure to COg, to
ether vapour, or to chloroform, is at first expansion and
increase in the rate of growth, followed by retardation,
even active contraction (pp. 44, 46, 293). Growth in
a state of standstill is revived by ether and chloroform,
w^hich offers an explanation of the action of anaesthetics
in forcing growth of dormant buds in winter. The accelera-
tion of growth by CO2 in the first stage of its action gives
rise to an enhancement of geotropic response, but continued
action causes contraction and brings about a reversal of
geotropic response, from an up to a down curvature.

Response to Diverse Modes of Stimulation

It has been generally assumed that the effects of diverse
modes of stimulation are specifically different. In reality
there is no such difference. Perception of stimulus and
the consequent reaction arise in all cases from excitation
of the sensitive protoplasm. Though in certain cases ana-
tomical structures, such as tactile hairs and pits, and others,
facilitate the perception of a particular form of stimulation
by causing deformation of the ectoplasm in an effective
manner, the normal results of stimulation are essentially the
same whatever the stimulus employed. The first effect on

RESPONSE TO STIMULATION 423

growing organs is retardation of the rate of growth ; if the
stimulus be intensified or the stimulation prolonged, the
contractile reaction of retardation is more marked, and, at
a critical point, causes complete arrest.

The independent method of electrical investigation fully
confirms the results obtained by the mechanical method.
The closest parallelism has been established between the
mechanical and electric responses in both non-growing and
growing organs under stimulation. Conditions which evoke
negative mechanical and electric response in a non-growing
organ, also give rise to negative variation and retardation
of the rate of growth. Other conditions which cause
positive mechanical and electric response in a non-growing
organ bring about positive variation or enhancement of
the rate of growth. The physiological machinery is the
same in pulvinated and in growing organs.

Effects of feeble and strong stimulation. — It may be said
in general that two opposite effects are induced in growth
under feeble and strong stimulation. There is a critical
intensity below which there is an acceleration and above
which there is a retardation. This critical intensity varies
in different species of plants. These opposite effects are
exhibited in all modes of response to diverse methods of
stimulation.

Effect of electric stimulation. — In normal conditions,
direct stimulation induces incipient contraction exhibited
by a retardation of the rate of growth ; this is sometimes
effected by an intensity of stimulus below the range of human
perception. Under increasing intensity of stimulation the
contractile reaction becomes more and more pronounced.
At a critical intensity of stimulus, growth becomes arrested ;
under a still stronger intensity there is an actual shortening
of the organ. A continuity thus exists between incipient
and actual contraction.

Effect of mechanical stimulation. — Moderate frictional
stimulation induces incipient contraction, manifested as
retardation of growth, recovery being completed within a

424 CHAP. XXXVII. GENERAL REVIEW

short time ; but intense stimulation caused by a wound
gives rise to a greater and more persistent retardation of

growth (p. 89).

Effect of photic stimulation. — The normal effect of light
is an incipient contraction or retardation of growth. The
after-effect of brief and moderate stimulation by light is
an acceleration of growth above the normal rate. The
effectiveness of the different rays of the visible spectrum in
retarding growth undergoes a decline from the blue to the
yellow-red (p. 78).

Radio-thermal stimulation. — The effectiveness of the
infra-red rays in retarding growth undergoes sudden aug-
mentation when the rays of heat are reached (p. 79).

Response to wireless stimulation. — The effect is found to
be essentially similar to that under the action of visible
light. Under strong intensity of stimulation the response
is a retardation of growth. Under feeble intensity of
electric waves the response is an acceleration of growth,
which is also the characteristic effect of feeble light (p. 189).

Effect of high-frequency alternating field of electric force. —
The effect of this is analogous to that of wireless electric
waves. The response is modified, as in the case of visible
light, by the intensity of stimulation and by the tonic
condition of the tissue ; feeble stimulation induces an
acceleration, while strong stimulation causes a retardation,
of growth. These facts offer a satisfactory explanation of
the anomalous results obtained by different observers on
the effect of high-tension alternating current on growth.

Effect of Tonic Condition on Response

The sign of response, negative or positive, is dependent
on the tonic condition. Mimosa in a subtonic condition
responds to stimulation by an abnormal positive or erectile
movement instead of by the normal negative fall of the leaf.
Under continuous stimulation the tonicity is raised to a
condition of par, the abnormal positive response being now
replaced by the normal negative. The effect of stimulation

DIRECT AND INDIRECT STIMULATION 425

on growth is modified in a similar manner ; the organ in a
subtonic condition responds to stimulation by enhancement
of its feeble rate of growth ; in extreme cases growth at
standstill becomes revived under stimulation. Continuous
stimulation transforms the response from abnormal ac-
celeration into normal retardation. This is equally true
of photic and of electric stimulation (p. 86).

Opposite Effects of Direct and Indirect

Stimulation

Stimulation gives rise to dual impulses : the positive,
which is of a hydraulic nature, not so dependent on the
conductivity of the tissue as is the excitatory negative,
is transmitted quickly ; the negative, which is the propa-
gation of protoplasmic excitation, is conducted slowly.
The positive impulse gives rise to expansion, positive
electric response, and acceleration of the rate of growth.
The excitatory negative impulse causes contraction, negative
electric response, and retardation of the rate of growth.
The negative reaction is more intense than the positive,
so that when the intervening distance between the point
of application of stimulus and the responding organ is
sufficiently small, the positive response is masked by the
predominant negative. When the intervening distance is
considerable, the negative impulse lags behind the positive,
and the response is diphasic, positive followed by negative.
When the distance to be traversed is still greater, the
excitatory negative impulse becomes weakened to the
point of extinction ; consequently, the hydraulic impulse
alone is effective and the response is positive, as show^n by
the movements of various motile leaves and by an enhanced
rate of grow'th in growing organs (pp. 124, 131).

Tropic Movement under All Modes of Unilateral

Stimulation

The difficulty in arriving at an explanation of the diverse
effects induced by unilateral stimulation is attributable to

426 CHAP. XXXVII. GENERAL REVIEW

the want of knowledge of the important part played by
direct and indirect stimulation, and by the conduction of
excitation across the organ. The directly stimulated
proximal side of the organ exhibits excitatory contraction,
electromotive change of galvanometric negativity, diminu-
tion of turgor and retardation of rate of growth ; the distal
side which is indirectly stimulated exhibits, on the other
hand, expansion, electromotive change of galvanometric
positivity, increase of turgor, and enhancement of the rate of
growth. Positive curvature towards the stimulus is thus
caused by the joint effects of the contraction of the proximal
and expansion of the distal side. The fact that stimulation
of one side of the organ causes an increase of turgor at the
diametrically opposite distal side has been demonstrated by
stimulation of one side of the stem of Mimosa, which caused
the erectile movement of the motile leaf on the opposite
side, indicative of an enhancement of turgor (p. 121).

The tropic movement has been vaguely ascribed to
change of turgor ; but this cannot take place without a
definite and active force which determines the direction of
flow of sap, causing differential turgor at the two sides of
the organ. I have shown that the law which governs the
directive movement of sap is that it follows the stimulation
gradient from the stimulated to the unstimulated region. The
turgor is thus diminished at the directly stimulated proximal
side from which the sap is expelled, and increased at the
distal side where it is accumulated (p. 57).

Transverse conduction of excitation across the organ in-
duces further modification of the normal positive curvature.

Mechanotropism and Twining of Tendrils

Under unilateral mechanical stimulation the directly
stimulated side of a tendril undergoes contraction, while
the indirectly stimulated distal side exhibits expansion ;
a positive curvature is thus produced with a movement
towards the stimulus, which results in the twining of the

PHOTOTROPISM 427

tendril round a support. The after-effect of stimulation
of short duration is an acceleration of growth of the stimu-
lated side above the normal, in consequence of which the
recovery becomes hastened. Stimulation of one side of the
tendril induces expansion of the indirectly stimulated distal
side, even in cases where the contractility of the stimulated
side is feeble. Hence the response to stimulation of the
more excitable side of the tendril may be inhibited by
stimulation of the opposite side (p. 99).

Phototropism

Quantitative relation. — It has been shown that the
amount of phototropic curvature depends (i) on the intensity
of light ; (2) on the duration of exposure ; and (3) on the
sine of the directive angle. The intensity of phototropic
action is therefore determined by the quantity of the inci-
dent light (p. 114).

Dia-phototropism and negative phototropism. — Transverse
conduction of excitation to the distal side induces
(i) a neutralisation or dia-phototropic response, and sub-
sequently (2) a negative phototropic curvature. An im-
portant contributory factor in the reversal of response is
the fatigue-relaxation of the proximal side (p. 137).

The effect of light on the root shows that its irritability
is in no way different from that of the shoot. In a thick
root, in which there is no transmission of excitation to the
distal side, the response is positively phototropic, but in a
thin root transverse conduction of excitation transforms the
positive curvature into negative. Thus, in the root of
Sinapis, the sequence of response is positive, dia-phototropic,
and finally negative. It was w^ant of knowledge of the pre-
liminary positive curvature that led to the wrong inference
that the root possessed an irritability specifically different
from that of the shoot (p. 145).

The complete phototropic curve of leaf and of stem consists
of four parts : (i) the stage of subminimal stimulation ;

428 CHAP. XXXVII. GENERAL REVIEW

(2) the stage of increasing positive curvature reaching a
maximum ; (3) the stage of neutrahsation ; and (4) the
stage of reversal to negative. The first part of the curve is
negative, due to physiological expansion induced by sub-
minimal stimulation. The curve then crosses the abscissa
upwards ; in the second stage, the susceptibility for excita-
tion, feeble at the beginning, increases very rapidly with
increasing intensity or duration of stimulation. The
positive phototropic curvature then attains its maximum.
At the third stage neutralisation occurs. And, finally, at
the fourth stage the curve crosses the zero-line downwards,
the phototropic curvature being reversed to negative.

Phototropic torsion. — Lateral stimulation of any kind
induces a torsional response in a dorsiventral organ, the
direction of torsion being such that the less excitable half
of the organ is made to face the stimulus. Conversely, the
direction of an incident stimulus can be determined from
the observed direction of torsional movement. The torsional
movements of leaves and leaflets of many plants are ex-
plicable on the above general principle. The excitatory
efficiency of two different stimulations can be compared by
the Torsional Balance, by observing the resulting effect
when the two stimuli act simultaneously on opposite flanks
in the plane of junction of the more and less excitable
halves of the anisotropic organ (p. 257).

Photonastic curvature. — There is no line of demarcation
between phototropic and photonastic movements ; con-
tinuity exists between them. By the application of a
method of recording the effect of percolation of excitation
through the pulvinus of Mimosa, a gradation of excitability
has been discovered in the different tissues of the lower half
of the organ (p. 163). In an organ with pronounced physio-
logical anisotropy, in which the distal side is far more
excitable than the proximal, the transverse conduction of
excitation brings about a greater contraction of the distal
side. The sequence of response is then positive, neutral,
and very pronounced negative. Application of stimulus

DIURNAL MOVEMENTS 429

to the more excitable side of the organ causes predominant
contraction of that half, which cannot be neutralised by the
transverse conduction of excitation to the feebly excitable
other half of the organ. These facts explain the effect of
strong light from above causing downward folding of the
leaflets of the Averrhoa and upward folding of the leaflets
of Mimosa.

Diurnal Movements of Plants

The diurnal movements are the outcome of the co-
operation of numerous factors, the most important of which
are : (i) thermonastic movement caused by differential
growth of two opposite sides of an anisotropic organ under
the hourly variation of temperature ; (2) the movement
due to change in the transpiration-current ; (3) the thermo-
geotropic response, under variation of temperature, of
organs rendered anisotropic by gravitational stimulation ;
(4) the response of organs sensitive to light under recurrent
alternation of light and darkness ; and (5) the movements
of plants like Mimosa which are sensitive to variation of
both light and temperature, and which are also affected by
both the direct and the after-effects of light.

Thermonasty. — Thermonastic movements are of two
types. The positive is a movement of opening during rise
of temperature, the inner side of the organ growing the
faster on account of its being the more sensitive. Examples
of this are found in Crocus, Zephyranthes, and in European
and a few Indian Nymphaeas. In the negative type, rise
of temperature induces a movement of closure by the
acceleration of the growth of the more sensitive outer side
of the organ. The flower of a blue Nymphaea growing
in India belongs to this type.

Thermo-geotropism. — This phenomenon was discovered
in the investigation of the remarkable periodic up and
down movement exhibited by the ' Praying ' Palm of
Faridpore (p. 224). It was afterwards found to be of wide
occurrence, being exhibited by rigid trees as well as by young

430 CHAP. XXXVII. GENERAL REVIEW

stems and by adult leaves of plants. In all these cases an
erectile movement is exhibited from thermal noon to thermal
dawn, and a movement of fall from thermal dawn to thermal
noon. The predominant effect of variation of temperature
on the movement is demonstrated by the fact of the abolition
of the movement under constant temperature (p. 240). The
effect of the stimulus of gravity in inducing the anisotropy
which determines the characteristic movement, is proved by
the effect on the diurnal record of inversion of the plant
(p. 244). The action of thermo-geotropism as an inde-
pendent factor is proved by its persistence even after the
abolition of transpiration (p. 248). The thermo-geotropic
movement is in many ways analogous to the thermonastic
movement, (p. 249). A wider generalisation of thermonasty
is reached by the inclusion under it of the movements of
full-grown organs which have been rendered anisotropic
by the stimulus of gravity.

Diurnal movement due to alternation of light and darkness.
The effect of light on the leaflet of Cassia alata is predomi-
nant as compared with that of temperature. The leaflets
begin to close when light is undergoing rapid diminution
after 5 p.m., the closure being completed by 9 p.m. This is
also partially due to the after-effect of light. The leaflet
remains closed till 5 a.m. next morning, after which it begins
to open with the dawning light and becomes fully expanded
by 9 a.m. The large terminal leaflet of Desmodium exhibits
diurnal movements similar to those of Cassia (p. 214).

Diurnal movement of leaf of Mimosa.—The complexity
of the diurnal movement of the leaf arises from the fact
that it represents the algebraical sum of the effects of three
fluctuating factors : (i) the thermo-geotropic action ;
(2) the immediate effect of light ; and (3) its after-effect.
With the exception of a small part of the curve in the
evening, the diurnal curve of the plant is essentially similar
to the typical thermo-geotropic curve, the leaf exhibiting
an erectile movement from thermal noon to thermal dawn,
and a fall from thermal dawn to thermal noon (p. 269).

GEOTROPISM 431

The spasmodic fall of the leaf towards evening is not due,
as has been suggested, to the increased mechanical moment
caused by the forward position of the sub-petioles, for after
removal of the sub-petioles the same abrupt fall of the
petiole occurs in the evening (p. 272). The evening fall
of the leaf is shown to be a post-maximum after-effect of
light, which causes an ' overshooting,' so that the leaf falls
to below the position of equilibrium (p. 278).

Geotropism

The obscurity surrounding the reaction to geotropic
stimulation arises from lack of definite knowledge in regard
to the exact direction of the incident stimulus, and as to
the character of the response, whether it is contraction or
expansion. The direction of the incident stimulus has been
definitely determined in two different ways : (i) by ob-
serving the effect of superposition of the stimulus of visible
light on the geotropically stimulated shoot ; and (2) by
observing the effect of geotropic stimulation in inducing
torsion in the pulvinus of Mimosa, the less excitable side of
the organ moving so as to face the stimulus. The results of
both lines of inquiry are fully concordant, proving that the
direction of incident geotropic stimulus coincides with the
vertical lines of force of gravity. According to the first
method, the geotropic up-movement is enhanced by light
acting from above, geotropic and phototropic effects being
concordant. Light acting from below induces diminution,
neutralisation, or reversal of the geotropic movement, the
two forces being now in opposition (p. 288). A similar
conclusion is arrived at by the method of geotropic torsion
(p. 308). This evidence, that in a horizontal shoot the upper
side becomes stimulated by gravity, is confirmed by the
excitatory reaction of that side which is an induced electro-
motive variation of galvanometric negativity (p. 315).
The electric response of the lower side is one of galvano-
metric positivity indicative of enhancement of turgor and

432 CHAP. XXXVII. GENERAL REVIEW

increased rate of growth. The method of geo-electric
response is more sensitive in the detection and quantitative
determination of the effect of stimulation by gravity than
that of mechanical response.

Localisation of geo-perceptive layer. — The induced gal-
vanometric negativity of the upper side of the shoot when
stimulated by gravity is not uniform in the different tissues
of the organ. The excitatory reaction attains a maximum
value at a definite layer, beyond which there is a decline.
This is the geo-perceptive layer which has been localised
by measuring the depth of intrusion of the exploring Electric
Probe at which maximum deflection of galvanometric
negativity is detected. The geo-perceptive layer thus
localised is found to be at or near the starch-sheath which
contains a number of large-sized starch-grains (p. 341). In
the stem of certain plants the distribution of excitability ex-
hibits two maxima, the focus of excitation being not single
but double. Microscopic examination showed that the
starch-sheath in these is double and that the positions of
the two electric maxima coincide with those of the two
starch-sheaths (p. 345). These results afford strong support
to the statolith-theory that it is the weight of heavy particles
which induces geotropic excitation in the higher plants.

Critical angle .for immediate geotropic excitation. — The
critical angle has been found in a large number of plants
to be about 31 "8°. This is additional evidence in favour
of the statolith-theory, for the abrupt reaction beyond the
critical angle can be most satisfactorily attributed to the
sudden fall of particles from the base to the side of the
sensitive cells (p. 360).

Geotropism of root. — On subjecting the tip of the root
to the stimulus of gravity, the upper side exhibits excitatory
reaction of galvanometric negativity (p. 368). This shows
that the root-tip, which contains starch-grains, becomes
directly stimulated. The electric response in the growing
region, above the stimulated point at the root-tip, is positive,
indicative of increase of turgor and expansion, showing that

AUTONOMOUS TORSION 433

this region has been indirectly stimulated by a transmitted
impulse (p. 369). The stimulus of gravity is perceived at the
root-tip and the responsive movement takes place at the
distant growing region. In contrast with this is the fact
that the growing region of the shoot is both sensitive and
responsive to geotropic stimulation. As the effects of
direct and indirect stimulation on growth are antithetic,
the responses of shoot and root cannot but be of opposite
sign. There is thus no necessity for postulating two different
irritabilities for the shoot and the root ; the difference of
response is not due to any inherent quality, but to the mode
of stimulation, direct in the one case, indirect in the other.
Difference between effects of geotropic and photic stimula-
tion of the root. — In phototropism, the energy of light
directly stimulates the responding cells of the growing
region, causing a curvature towards the stimulus. In
geotropism, the force of gravity is in itself inoperative ;
it is only through the weight of the cell-contents of the
perceptive tissue that stimulation is effected at the root-tip.
The impulse there initiated travels to and indirectly stimu-
lates the growing region which responds by a curvature
away from the incident stimulus.

Autonomous Torsion

It has been shown that the twisting growing stem is
anisotropic, two longitudinal halves at any given moment
being unequally active, like the two halves of the pulvinus
of Mimosa (p. 376). The plane of demarcation separating
the two physiologically diverse halves in Mimosa is fixed ;
but in the stem the plane slowly revolves, passing from
segment to segment (p. 393). The resulting torsional growth
brings about the movement of circumnutation (p. 381).

Effect of variation in the rate of ascent of sap. — Enhance-
ment of the rate of ascent induces an increase in the rate
of torsion, while depression of ascent or plasmolytic with-
drawal of water induces retardation of the rate, culminating
in actual reversal of the direction of torsion (p. 386).

2 F

434 CHAP. XXXVII. GENERAL REVIEW

Effect of variation of temperature. — Rise of temperature
up to an optimum induces an enhancement of the rate of
torsion ; lowering of temperature, on the other hand, brings
about a depression or an arrest of movement (p. 388).
At the critical temperature of about 60° C. there is a sudden
reversal of torsional movement indicative of the spasm of
death (p. 390).

Effect of chemical agents. — The effect of a moderate dose
of ether is a very marked enhancement of the rate of
torsion. Chloroform causes a preliminary enhancement of
the rate of torsion, followed by reversal and arrest of the
movement (p. 393). The preliminary effect of poisonous
solutions is an enhancement of the rate of torsion, followed
by reversal and final abolition of movement (p. 392).

Effect of Diffuse Stimulation on Autonomous

Torsion

Feeble stimulation, whether electric, mechanical or
photic, enhances the rate of torsion, whereas strong stimu-
lation retards it even to actual reversal (p. 396).

The effect of indirect stimulation is an enhancement of
the rate of torsion (p. 399).

In regard to photic stimulation, the effect of red light
is an enhancement of the rate of torsion, while blue light
causes a marked retardation (p. 402).

Geotropic stimulation. — When the organ is held upside
down, the rate of normal torsion undergoes retardation,
which may even culminate in the actual reversal of the
torsional movement, proving that geotropic stimulation
is more effective in the inverted position. Facts have
already been adduced which appear to show that the
excitability of the ectoplasmic layer is greater at the apical
end of the geo-perceptive cell than at the basal end (p. 356).
In an inverted position the heavy particles, which are
with good reason supposed to cause geotropic excitation,
press against the apical ends. The retardation of torsion

EFFECT OF STIMULATION ON TORSION 435

in an inverted position thus offers an important confirmation
of the statoHth-theory.

Effect of Unilateral Stimulation on Autonomous

Torsion

Action of light. — The effects of hght on different sides
of the twisting organ are unequal. There are two sides,
N, N', which are neutral, exhibiting no responsive variation
of torsion under stimulation. The two flanks A and R, at
right angles to N, N^ are, however, highly sensitive, stimula-
tion of A inducing maximum acceleration, while stimulation
of R causes maximum retardation (p. 410). All the varied
results are included under the Law of Torsional Response
that has been established : A differentially growing organ
laterally stimulated undergoes a torsion by ivhich the less
excitable side is made to face the stimulus. The i^iduced
torsional movement is algebraically summated with that of the
existing autonomous torsion.

Action of stimulus of gravity. — The effects under geotropic
stimulation are precisely similar to those of photic stimulation
— that is to say, there are two sides, N, N', which are neutral,
whereas the effect of stimulation of flank A causes a maxi-
mum enhancement of the rate of torsion, and stimulation of
flank R induces a maximum retardation culminating in
reversal.

Combined effects of geotropic and photic stimulation. —
The experimental results prove that the effect of geotropic
stimulation is algebraically summated with that of photic
stimulation, thus inducing a variation in the rate of torsion
(p. 415). Light, when sufficiently strong, acting in op-
position to gravitational stimulus, may thus induce a
reversal of the direction of torsion.

In conclusion it may be stated summarily that the
experimental evidence adduced in the foregoing chapters,
as to the nature and conditions of the growth-movements,
makes it possible to explain their mechanism in a simple
and comprehensive manner. The growth-movements are

436 CHAP. XXXVII. GENERAL REVIEW

often complex, owing either to the organisation of the organ,
as, for instance, when it is anisotropic, or to the fact that
they are the resultants of simultaneous action of two or
more stimulating agents. But it has been shown that such
complex movements are susceptible of analysis, and can
then be adequately accounted for on general principles.

The fundamental principle is that growth is retarded by
strong and accelerated by weak stimulation of whatever
kind. Closely connected with it is the further principle
that direct stimulation retards and indirect stimulation
accelerates the rate of growth : this is the essential feature
of the mechanism of tropisms. There is no longer any
ground for assuming distinct irritabilities, such as the
phototropic and the geotropic, or negative and positive
phototropism and geotropism : these terms may remain as
merely descriptive of the visible response. There is but
one irritability of the growing organ which responds to all
stimuli that may act upon it, and in essentially the same
manner.

INDEX

A- AND D-REACTIONS, 152

Acceleration of growth : under subminimal stimulation, 82 ; in subtonic
plants under normal stimulation, 85

Algebraical summation of effects of geotropic and photic stimulation,
288, 308

Allium, growth of : effect of carbon dioxide on, 44 ; direct and after-
effect of light on, 76 ; revival of, under stimulation, 84

Alternating field of electric force : response of Mimosa to, 185 ; accelera-
tion of growth under moderate intensity of, 186 ; revival of growth
under, 187, 424 ; retardation of growth under strong intensity of,
187, 188, 424. See also Wireless stimulation

Ammonium sulphide, effect of, on growth, 48

Anaesthetics, effect of, on growth, 43-7, 293, 422 ; on geotropic response,
293 ; on autonomous torsion, 393. See also Carbon dioxide, Chloro-
form, and Ether,

Angle of inclination, relation between, and excitation : incident light
and phototropic curvature, 113 ; incident geotropic stimulation and
excitation, 324, 348, 353, 355, 358

Angle of inclination, critical, for geotropic excitation, 360, 432

Anisotropic organs, characteristic response of, 376

Anisotropic organ, torsional response of, under lateral stimulation :
photic, 252 ; radio-thermal, 255 ; geotropic, 307

Anisotropy, effect of reversal of geotropic, on diurnal movement, 244

Ascent of sap, effect of variation of rate of : on growth, 52, 54, 55, 58,
421 ; on autonomous torsion, 386, 433

Automatic Torsional Recorder, Mechanical, 385 ; Optical, 383

Autonomous pulsation : in growth, 18 ; of Mimosa leaf, 269

Autonomous torsion : rates of different internodes, 380 ; variation of,
under changed rate of ascent of sap, 386, 433 ; effect of temperature
variation on, 388, 434 ; of formaldehyde on, 392 ; of ether and
chloroform on, 393, 434 ; of feeble and strong intensity of electric
stimulation on, 395, 397 ; of indirect stimulation on, 399, 403 ; of
mechanical stimulation on, 400 ; diurnal variation of, 407 ; effect
of unilateral light on, 409, 412 ; combined effect of geotropic and
photic stimulation on, 417, 435.
Averrhoa Carambola, anomalous response of leaflet to light, 2 ; diffuse
stimulation on, 167 ; response to light from above and below, 102,
157, 168

438 INDEX

BioPHVTUM SENSITJVUM, anomalous response of leaflet of, to light, 102
Bryophyllum calycinum : positive, diphasic, and negative electric re-
sponse of, under indirect stimulation of increasing intensity, 129 ;
electric after-effect of light on, 260 ; geo-electric response of, 340 ;
localisation of geo-perceptive layer in, 341

Calendula, duplication of geo-perceptive layer and starch-sheath in, 345

Carbon dioxide, effect of : on balanced growth, 32 ; on growth, 44, 293,

422 ; on geotropic response of growing and of pulvinated organs, 299,

303
Cassia alata, diurnal movement : of leaflet, 210, 215, 430 ; of petiole, 279

Cassia alata, response of : to alternate light and darkness, 212 ; post-
maximum after-effect of light on, 264 ; diurnal record of, 279

Centaurea, effect of ether vapour and chloroform on growth of, 44, 46,
47 ; geo-electric response of, 343

Chemical agents, effect of, on growth, 43

Chloroform, variation of growth under, 46, 293, 422 ; revival of growth
by, 46 ; arrest of growth and death-spasm under continued applica-
tion of, 46, 293 ; effect of, on geotropic response, 296 ; effect of, on
autonomous torsion, 393, 434

Circumnutation, autonomous torsional, 377, 381 ; determination of rate

of, 377
Clitoria Ternatea, variation of autonomous torsion of stem of : under uni-
lateral light, 409, 410, 411 ; under unilateral geotropic stimulation,

415.
Clitoria Ternatea, positive phototropic curvature of leaflet of, 104

Coal-gas, effect of, on photo-geotropic balance, 310

Collimator, 112

Commelina bengalensis, localisation of geo-perceptive layer in, 343 ;

critical angle for geotropic excitation of, 362
Contact-stimulus, effect of, on tendril, '94 ; on twining stem, 381-2
Convolvulus, geo-electric response of, 344

Copper sulphate, effect of small and large doses of, on growth, 49
Cosmos, effect of prolonged indirect stimulation on growth of, 128
Crescograph, High Magnification, 6, 420 ; Magnetic, 19, 421
Crescograph, Balanced Method of : by Falling Weight, 23 ; by Inclined

Plane, 27
Crinum Lily, contractile response of, under electric stimulation : latent

period and time relations of, 65 ; continuity between incipient and

actual contraction in, 67
Crinum Lily, growth of : effect of COg on, 44 ; of ether vapour on, 45 ;

of chloroform on, 46 ; of external tension on, 60 ; of increasing

intensity of light on, 73 ; of longitudinally transmitted impulse on,

126 ; of direct and indirect stimulation on, 127 ; responsive variation

in, under high-frequency alternating current, 192
Crinum Lily, phototropic curvature of : under increasing intensity of

light. III ; under increasing duration of exposure, 112 ; under

different angles of incidence of light, 113 ; algebraical summation of

geotropic and phototropic curvature of, 288
Crocus vernus, thermonastic response of, 217
Croton, diurnal record of leaf of, 240

INDEX 439

Cucurbita, eifect of ether vapour on growth of, 44 ; of drought and
irrigation on growth of, 51 ; latent period of responsive variation
of growth to stimulus of light, 73

Cucurbita tendril : tactile pits in, 90 ; effect of indirect stimulation in
accelerating growth of, 91 ; after-effect of stimulation on growth of,
92 ; twining of, under unilateral stimulus of contact, 94

Daffodil, variation of growth under ether, 44

Dahlia, diurnal movement of dia-geotropic leaf, 237, 238 ; diurnal record

of leaf, 240
Datura, variation of growth under : ether vapour, 44 ; chloroform, 46
Death-spasm, at fatal temperature, 37, 390 ; under chloroform, 46, 47 ;

test of, 47
Desmodium gyrans, terminal leaflet of : effect of angle of inclination of

incident light on tropic curvature of, 113 ; complete phototropic curve

of, 153 ; diurnal movement of, 213-15, 430 ; effect of chloroform on

geotropic response of, 297
Dia-phototropism, 133, 134, 427
Dia-radiothermotropism, 1 79

Differential excitability, effect of, on the direction of torsion, 255
Diurnal Indicator, 233
Diurnal movement of Mimosa leaf : four phases of, 267 ; spasmodic fall

in the evening, 271, 431 ; variation of geotropic torsion of, 273 ;

characteristic after-effect of light on, at pre-maximum, maximum,

and post-maximum, 277, 278, 431
Diurnal movement, thermo-geotropic : of procumbent stem of Mimosa,

237 ; of Tropaeolum and of Dahlia, 238 ; of dia-geotropic leaves,

240 ; effect of constant temperature on, 239, 240 ; effect of reversal

of geotropic anisotropy on, 244 ; effect of abolition of transpiration

on, 247, 248 ; analogy with thermonasty, 246, 249. See also ' Praying '

Palm
Diurnal movement, thermonastic : positive and negative types of, 217,

223 ; of Zephyranthes, 217, 223 ; of Nymphaea, 221, 222, 223 ; of

Sijbaria Palm, 231
Diurnal movement, under recurrent light and darkness : of leaflet of

Cassia, 212, 430 ; of terminal leaflet of Desmodium, 213, 430
Diurnal movements of plants, phenomena of, 195, 429
Diurnal Recorder, 208, 209, 227
Diurnal variation : of tension and bulk, 241 ; of tissue-tension in Mimosa,

242
Dorsiventral organ : geotropic response of Mimosa in normal and in

inverted position, 290
Dregea volubilis : positive radio-thermotropic curvature of stem of, 178 ;

neutralisation of curvature and negative response under prolonged

exposure, 180

EcLiPTA, enhanced geotropic curvature of, under chloroform, 296
Electric Probe, 328 ; localisation of geo-perceptive layer by, 335
Electric spark, effect of light from, on growth, 107

440 INDEX

Electric stimulation, effect of, on growth, 62, 423 ; latent period of
response to, 64 ; effect of increasing intensity and duration of, 65,
68 ; effect of continuous application of, 66, 68 ; continuity of effect
between incipient and actual contraction under, 67 ; effect of direct
and indirect, on tendril, 91 ; after-effect of, 92

Electrodes, non-polarisable, 317

Electromotive response, under unilateral stimulation at proximal and
distal sides : of tendril, 95 ; of stem, 130

Electromotive response, positive, diphasic, and negative, under increasing
intensity of indirect stimulation, 128. See also Geo-electric response
of shoot and of root

Erythrina indica : latent period for photic stimulation, 106 ; positive
phototropic curvature of, 104 ; characteristics of phototropic curve
of, 149 ; effect of COg on geotropic response of, 303

Ether, effect of vapour of : on growth, 44, 293, 422 ; on geotropic
response, 298 ; on autonomous torsion, 393, 434

Fatigue-relaxation : of growth under prolonged stimulation, 136 ;

reversal of tropic curvature from positive to negative by, 137, 138
Fitting, cited on response of tendril, 97

Force, active nature of force in diurnal movement of ' Praying ' Palm, 229
Formaldehyde, effect of, on autonomous torsion, 392
Frictional stimulation, effect of, on growth, 87, 93

Galvanograph, 200

Geo-electric response of shoot : discovery of, 314 ; latent period of, 322 ;
method of contact with sensitive and indifferent points, 318 ; electro-
motive variation at upper and at lower side, 319 ; methods of Axial
and of Vertical rotation, 320 ; characteristics of, 321 ; physiological
character of, 323 ; relation between angle of inclination and, 324 ;
law of sines, 325 ; the Electric Probe, 328 ; localisation of geo-
perceptive layer in, 329 et seq., 432 ; distribution of excitability at
different depths, 332 ; curve of geotropic excitability, 334, 335 ;
duplication of geo-perceptive layer and of starch-sheath, 345, 432 ;
intensity of response at various angles of inclination, 353 ; critical
angle for, 360, 432 ; effect of repetition on critical angle for, 363

Geo-electric response of root : at the region of tip, 368, 433 ; at the
region of growth, 369, 370, 433 ; additive action-current between
tip and growing region, 370

Geo-perception by tip of the root, 372

Geotropic curve : complete record of, 286 ; effect of season on, 294 ;
effect of chloroform on, 296 ; effect of ether on, 297 ; effect of COg
on, 299

Geotropic excitation, differential, in upper and lower halves of dorsi-
ventral organ, 290, 302

Geotropic stimulation, 281-4 ; determination of direction of, 284, 287,
308, 431 ; latent period of effect of, 294 ; algebraical summation of
effects of photic and, 288, 308 ; certain analogy between effect of
photic and, 288, 289 ; difference between response to photic and, 373,
433 ; difference of response to, in shoot and in root, 372, 433

INDEX 441

Geotropic torsion, diurnal variation of, 273, 274 ; induction of, under
lateral stimulus of gravity, 307 ; determination of direction of
geotropic stimulus by, 308 ; algebraical summation of phototropic
and, 308 ; balance of phototropic and, 309

Growth : determination of absolute rate of, by High Magnification
Crescograph, 15, and by Balanced Crescograph, 26, 30 ; pulsation
in, 18 ; variation of temperature on, 18, 422 ; determination of
cardinal points of temperature on, 36 ; Thermocrescent Curve
of, 39 ; effect of carbonic acid gas on, 32, 44, 293, 422 ; ether vapour
on, 44 ; chloroform on, 46 ; revival of, under ether and chloroform,
44, 46 ; effect of sulphuretted hydrogen on, 48 ; copper sulphate
solution on, 49 ; modifying effect of dose on, 49 ; revival of, under
irrigation, 51 ; variation of rate of ascent of sap on, 52, 421 ; acces-
sion or withdrawal of water on, 55 ; parallel effect in response of
motile organ, 56 ; effect of hydrostatic pressure on, 59 ; external
tension on, 60

Growth recorders : the Optical Lever, 5 ; the High Magnification
Crescograph, 6, 420 ; precautions against physical disturbance in
employment of, 16 ; the Magnetic Crescograph, 19

Growth, responsive variation of : identical reaction under diverse modes
of stimulation, 62 ; latent period of, under electric stimulation, 64 ;
incipient contraction or retardation of growth under stimulation, 67 ;
continuity between incipient and actual contraction, 67 ; similarity
of response of pulvinated and of growing organs, 68 ; effect of photic
stimulation on, 72 ; latent period of response, 73 ; effect of increasing
intensity of light on, 74 ; after-effect of light on, 75 ; effect of different
rays of visible spectrum on, 78 ; of subminimal stimulation in
enhancement of, 82 ; enhancement of, in subtonic plants under
stimulation, 85 ; revival of growth under stimulation, 84 ; effect of
mechanical stimulation on, 87 ; of wound on, 89 ; of direct and indirect
stimulation on, 91, 124

Haberlandt, quoted, 321, 342 ; cited, 329, 373

Helianthus annuus : effect of ether vapour on growth of, 44 ; electric

response at proximal and distal sides of stem under unilateral photic

stimulation, 130
Hibiscus, growth of : normal rate of, 12 ; ether vapour on, 44
High-Frequency Alternating Current, see Alternating field of electric

force and Wireless stimulation
Hydrion concentration theory, 302 n.
Hydrogen peroxide, effect of, on growth, 43
Hydrostatic pressure, effect of, on growth, 59

Impulse : initiation of positive and negative, under stimulation, 124 ;

velocity of, 120; longitudinal transmission of, 118; transverse

transmission of, under photic, electric, and radio-thermal stimulation,

121, 122, 123
Indirect stimulation, effect of : on growth, 93, 125, 126, 127 ; on tropic

curvature of shoot and root, 125, 141, 142, 365 ; on autonomous

torsion, 399, 403
Indirect stimulation, inhibitory action of, on tropic response of tendril, 99

442 INDEX

Ipomoea reptans, response of root : negative phototropic curvature under
stimulation of root-tip, 141 ; effect of simultaneous photic stimulation
of root-tip and growing region, 143 ; positive, dia-, and negative
phototropism of, 144 ; positive, dia-, and negative radio-thermo-
tropism of, 181

Irrigation: revival of growth, under, 51 ; effect of, with cold and warm
water, 52, 54

JosT, quoted, 98, 133, 139

Kentia, diurnal movement of, 232

Kraus, on diurnal variation of tension and bulk of organs, 241
Kysoor (Scirpus Kysoor), growth of : rate of, 12, 13 ; determination of
absolute rate, 14 ; optimum and minimum temperature for, 36, 37 ;
Thermocrescent Curve of, 39 ; effect of irrigation with cold and
warm water on, 53 ; retardation of, under light, 72 ; acceleration of,
under subminimal photic stimulation, 83 ; acceleration under light
in subtonic specimen, 86

Latent period of phototropic response, 106 ; of geotropic response, 284-6

Latent period of responsive variation of growth : under electric stimula-
tion, 64 ; under photic stimulation, 73 ; effect of intensity of stimula-
tion on, 65

Laws of Direct and Indirect Stimulation, 131

Laws of Torsional Response of anisotropic organs : pulvinated organ,
256, 411 ; twisting stem, 411

Light and temperature, diurnal variation of, 202-5

Light on growth : retarding effect of, 72 ; latent period of response to,
73 ; effect of increasing intensity of, 74 ; direct and after-effect of, 75,
259, 260-5, 275, 277 ; effect of different colours of, 78 ; accelerating
effect of subminimal stimulation by, 82 ; accelerating effect of, on
subtonic plants, 86 ; effect of a single spark of, 107

Light, relation between quantity of, and tropic curvature, 109, 114;
effect of, on autonomous torsion, 401, 402, 409, 418

Light, tropic curvature under: effect of angle of inclination on, 113, 114;
of increasing intensity and duration of exposure on, 109, iii, 114

Magnetic Crescograph, 19, 421

Mechanical stimulation : effect of friction on growth, 87, 93, 423 ; effect
of wound 89 ; effect of, on autonomous torsion, 400

Mechanotropism of tendril, 90, 426 ; effect of direct and indirect unilateral
mechanical stimulation on, 94 ; electric response of proximal and
distal sides, 95 ; response of less excitable side, 97 ; inhibitory action
of stimulus on, 98

Millardet, on periodic variation of tension in Mimosa, 242

Mimosa pudica, responsive movement on accession and withdrawal of
water, 56 ; dual impulses, positive and negative, generated under
stimulation, 117 ; velocity of impulse, 120 ; effect of longitudinal
transmission of impulse, 119 ; effect of transverse transmission of im-
pulse under photic, radio-thermal, and electric stimulation, 121, 122,

INDEX 443

123, 124, 175 ; response of leaf to direct radio-thermal stimulation, 179 ;
response to high-frequency alternating field of electric force, 185 ;
autonomous pulsation of, 269

Mimosa pudica, diurnal movement of : four phases in, 267 ; summer and
winter curves of, 268 ; combined effects of thermo-geotropism and
photo-geotropism, 269, 430 ; spasmodic fall of leaf in the evening,
271, 431 ; after-effect of light at pre-maximum, maximum, and
post-maximum, 277, 278, 431

Mimosa pudica, pulvinus of : characteristics of, 158, 164, 165 ; detection
of gradation of moto-excitability in lower half, 163 ; velocity of trans-
verse conduction, 162 ; response to Ught acting from above and below,
160-3 '> triphasic response of, 166 ; response to radio-thermal stimula-
tion, 167; diurnal variation of geotropic torsion, 273; effect of CO2
on geotropic response of, 303

Mimosa pudica, photonastic response of leaflet of, 157, 158, 169 ; effect of
light applied above and below, 169

Nadirotropism, 290

Nastic movement, definition of, 216

Negative phototropism : of shoot, 137 ; continuity between positive and
negative, 138, 139 ; of root, 139 ; transformation from positive to, 146

Negative radio-thermotropism : in shoot, 180 ; in root, 181

Nemec, cited, 329, 373

Non-polarisable electrodes, 317

Noon, difference between light, and thermal, 202

Nymphaea, diurnal movement of, 218 ; two types of thermonastic move-
ments of, 222, 223 ; distribution of geotropic excitability in peduncle
of, 338, 339 ; localisation of geo-perceptive layer, 339 ; critical angle
for geotropic excitation, 360, 361

Optical Lever, for measurement of rate of growth, 5 ; for measurement
of rate of autonomous torsion, 382

Optimum temperature for growth, 36

Oryza, fatigue-relaxation under continuous stimulation, 136 ; trans-
formation from positive to negative phototropic curvature under
prolonged unilateral exposure to light, 138

Oscillating Recorder, the quadruplex type, 209

Palm, diurnal movement : of ' Praying,' 224 ; of Sijbaria, 231 ; of Kentia,

232
Papaya, diurnal record of leaf, 240
Passiflora, response of tendril : effect of indirect mechanical stimulation,

95 ; relative intensities of upper and under sides, 98 ; inhibitory action

of stimulus on, 99
Perceptive organ, root- tip the geotropic, 372
FieSer quoted, 2, 97, 174, 401 ; cited, 217, 271, 272
Phaseolus, variation in the rate of torsion under changed rate of ascent

of sap, 386
Photic and geotropic stimulation, certain resemblance between reactions

to, 289 ; difference between effects of, 373

444 INDEX

Photic stimulator, 159, i6o

Photo-Geotropic Balance, 309 ; effect of coal-gas on, 310

Photometric Recorder, see Radiograph

Photonastic phenomena under light applied above and below, 157, 429;
of leaflet of Averrhoa, 157, 168; of leaflet of Mimosa, 157, 158, 169;
continuity between, and phototropic reactions, 170, 428

Phototropic curvature, mechanism of, 116 ; effect of direct and indirect
stimulation, 117 ; effect of longitudinally transmitted impulse, 119 ;
effect of transversely transmitted impulse under photic, electric, and
radio-thermal stimulation, 121, 122, 123

Phototropic curve : characteristics of, 150, 428 ; definition of susceptibility,
149 ; variation of susceptibility at different parts of, 151 ; complete
curve of pulvinated organ, 154 ; complete curve of growing organ, 155

Phototropic Recorder : for shoot, 103 ; for root, 140

Phototropism of shoot : positive response, 104, 105 ; the latent period,
106 ; effect of increasing intensity of light, 109 ; effect of increasing
duration of exposure, m ; effect of angle of inclination of light, 112 ;
relation between quantity of light and tropic response, 114 ; partial
or complete neutralisation under long exposure, 133, 134; fatigue-
relaxation as contributory factor in transformation of positive to
negative curvature, 135, 136 ; positive, dia-, and negative, 137

Phototropism of root : reaction to unilateral stimulation of root-tip, 141 ;
effect of simultaneous unilateral stimulation of root-tip and growing
region, 143 ; positive reaction of Sinapis transformed into negative
under continuous unilateral stimulation, 145. See also Radio-thermo-
tropism of shoot and of root

Porana paniculata, autonomous torsion of : effect of variation of tempera-
ture on, 388, 3S9 ; of indirect stimulation on, 399, 403 ; of diffuse
sunlight on, 401 ; of geotropic stimulation on, 404 ; combined effect of
unilateral geotropic and photic stimulation on, 418

' Praying ' Palm, the diurnal movement of, 224, 228, 229 ; physiological
cause of, 225 ; active not passive, 229 ; relative effect of light on,
229 ; of variation of temperature on, 230. See also Thermo-geotropism

Pulsation, of growth, 18 ; of Mimosa leaf, 270

Pulvinated organ, contractile response of, 68 ; relation between quantity
of incident light and phototropic curvature, 114 ; complete photo-
tropic curve of, 153 ; principal types of response in, 171 ; response
to direct radio-thermal stimulation, 179 ; differential geotropic
excitation of, 290, 291, 303 ; effect of carbon dioxide on geotropic
response of, 303

Pulvinated organ, torsional response to lateral stimulation: by light, 253,
411 ; by thermal radiation, 255 ; by geotropic stimulation, 307

Pulvinus and growing organ, similarity of response of, 55, 56, 68

QuADRUPLEX Recorder, Oscillating, 209 ; Geotropic, 282

Radial organs, response in, 171
Radiograph, Self -Recording, 1 97,^206
Radio-Thermal Stimulator, 175
Radio-thermotropism, 1 74

INDEX 445

Radio-thermotropism of shoot : response to longitudinally transmitted
impulse, 177 ; positive curvature under moderate unilateral stimu-
lation, 178 ; neutralisation, 179, 180 ; gradual transformation from
positive to negative curvature under prolonged stimulation, 180, 181

Radio-thermotropism of root : continuity between positive and negative
curvature under continued stimulation, 181

Recorder, High Magnification Crescograph, 6, 420 ; Magnetic Crescograph,
19, 421; Balancing by Falling Weight, 23; Balancing by Inclined
Plane, 29; Phototropic, for Shoot, 103 ; Phototropic, for Root, 140 ;
Radiographic, 197,206; Galvanographic, 200; Thermographic, 210;
Thermonastic, 220, 221; for diurnal movement: of plants, 209; of
trees, 227 ; Torsional, 251, 252, 306 ; Ouadruplex Geotropic, 282 ;
Optical, for torsion, 382 ; Mechanical, for torsion, 384

Root, electric response of, at growing region : to direct and indirect
stimulation, 366 ; geo-electric response of. 367-70, 432

Root, tropic response of : to direct and indirect photic stimulation, 141,
365 ; gradual phototropic reversal from positive to negative in
Sinapis, 145 ; to indirect radio-thermal stimulation, 177 ; to direct
unilateral stimulation, 178 ; gradual reversal of positive to negative
under continued unilateral stimulation, 179, 180

Root-tip, effect of unilateral stimulation of, 365, 366 ; geo-electric response
at, 367 ; additive action-current between tip and growing region, 370,

371

Sap, effect of variation of rate of ascent of : on growth, 52, 54, 55, 58 ;

on torsion, 386, 433
Sijbaria Palm, diurnal movement of, 231
Sinapis, tropic response of root of, under unilateral light, 145 ; gradual

transformation from positive to negative curvature under continuous

illumination, 145, 146
Spasmodic death-contraction, in growing organs : under chloroform, 46 ;

at critical fatal temperature, 37 ; reversal of direction of torsion at

critical temperature, 390
Spectrum, effect of different rays of visible spectrum on growth, 78
Starch-sheath, geo-perceptive layer and, 341 ; duplication of, in Calendula,

345
Statolithic theory of geotropic stimulation, 313

Stimulants, effect of, on growth, 43

Stimulation, identical reactions under diverse modes of, 62, 170, 422 ;

retardation of growth under electric, 64 ; under photic, 72 ; under

mechanical, 87; under radio- thermal, 178; under wireless, 189, 192 ;

under geotropic, 284
Stimulation, laws of reaction under direct and indirect, 131, 170 ; dual

impulses generated under, 124, 425 ; electric response to, 128
Stimulation, opposite effects of feeble and strong : on growth, 83, 187, 189,

190, 423 ; on autonomous torsion, 395, 397
Strasburger, quoted, 216
Subtonic plant, acceleration or revival of growth under photic stimulation,

84, 86 ; electric stimulation, 85 ; high-frequency alternating current,

186, 1S7, 18S ; wireless stimulation, 191, 192

446 INDEX

Sulphuretted Hydrogen, effect of. on growth, 48

Sunlight, supposed ineffectiveness of, 1 39 ; phototropic curvature under,

181, 182
Susceptibility : definition of, 149 : variation of, at different parts of tropic

curve, 151

Tactile pits in tendril of Cucurbita, 90

Temperature and light, diurnal variation, 202-5

Temperature, optimum and minimum, for growth, 36, 37 ; double optima,
389, 390 ; Thermocrescent Curve, 39 ; death-spasm at fatal point,
37. 390 ; effect of variation of, in diurnal movement, 230, 237; on
autonomous torsion, 388, 434

Tendril, twining of, 90, 97, 426 ; effect of electric stimulation on growth
of, 91 ; of mechanical stimulation on growth of, 93 > oi unilateral
stimulation on response of, 94; electric response at proximal and
distal sides, 95 ; inhibitory action of stimulus on response of, 98

Tension, effect of, on growth, 60

Thermal dawn and thermal noon, 202, 203, 205, 230, 231 ; light noon and
thermal noon, 202, 205

Thermocrescent Curve, 39, 422

Thermo-geotropism, 246, 429

Thermograph, 210

Thermonastic Recorder, 220, 221

Thermonasty, 216, 429 ; positive and negative types of, 217, 222, 223

Thermotropism, 179

Thunbergia, autonomous torsion of : cyclic variation of temperature on,
388 ; death-spasm, at fatal temperature, 390 ; effect of feeble and
strong electric stimulation on, 395, 397 > oi indirect stimulation on,

399 ; of mechanical stimulation on, 400 ; of photic stimulation on,

401 ;' of red and blue light on, 402 ; of geotropic stimulation on, 404 ;
of unilateral geotropic stimulation on, 416 ; combined effects of
simultaneous unilateral geotropic and photic stimulation on, 418

Time-Factor, modification of responsive variation of growth by, 30
Torsion, autonomous : effect of variation of rate of ascent of sap on, 386,
433 ; of change of temperature on, 388, 434 ; of poisonous solution
on, 391, 392; of anaesthetics on, 393, 434; of feeble and strong
electric stimulation on, 395, 397 ; of indirect stimulation on, 399,
403 ; of thermal shock on, 399, 400 ; of mechanical stimulation on,

400 ; of photic stimulation on, 401, 434 ; of red and blue light on,

402 ; of geotropic stimulation on, 403, 434 ; diurnal variation of,

407
Torsion, autonomous : laws of modification of, under unilateral stimulation,

411 ; effect of unilateral stimulation by light on, 409, 412 ; effect of

unilateral stimulation by gravity on, 414, 415; combined effects of

geotropic and photic stimulation on, 417, 435
Torsion, induction of, under lateral stimulation, 252, 255, 307 ; directive

action of stimulus on, 253 ; of differential excitability on direction of,

255 ; laws of torsional response, 256, 411, 435
Torsional Balance, comparison of reactions to diverse stimulations by, 257
Torsional Recorder, 251, 252, 306, 383, 385
Transpiration, diurnal movement in absence of, 246

INDEX 447

Transverse transmission of impulse : under photic stimulation, 121, 134 ;
under electric stimulation, 121, 122 ; under radio-thermal stimula-
tion, 123

Trees, recorder for diurnal movements of, 227

Tropaeolum, effect of ether vapour and chloroform on growth of, 44, 46 ;
diurnal movement of procumbent stem, 238; constant temperature
on diurnal movement, 239; geotropic response of, 286, 294; under
chloroform, 296 ; under ether, 297 ; under carbon dioxide, 299 ;
geo-electric response at different angles of inclination, 350-3, 355 ;
critical angle for geotropic excitation, 361, 362

Tuberose, geotropic response of , 285 ; effect of carbon dioxide on geotropic
response of, 302

Tulipa, Pfeffer on thermonasty of, 217

Twining, mechanism of : in tendril, 97 ; in stem, 373, 381, 382, 393 ;
effect of unilateral stimulation by light on torsion of stem, 412, 435

ViciA Faba, revival of growth on irrigation, 52 ; gradual transformation
of positive into negative phototropic curvature, 138 ; effect of longi-
tudinal transmission of impulse under unilateral stimulation, 177;
geo-electric response of root-tip, 367, 369

Vigna Catjang, contractile response and recovery after brief stimulation,
135 ; fatigue-relaxation after prolonged stimulation, 136

Water-Lily, diurnal record of thermonastic movement in, 221, 222
Wheat, effect of ether vapour on growth of, 44 ; acceleration of growth

in subtonic plant under electric stimulation, 85 ; effect of light from

a single spark on growth, 107 ; of high-frequency alternating current

on growth, 186, 187
Wireless stimulation, response of plants to feeble and strong intensity of,

189-194, 424
Wound, effect of, on growth, 89

Zea Mays, complete phototropic curve of, 154 ; effect of wireless stimu-
lation on growth of, 193

Zenithotropism, 290

Zephyranthes sulphurea, pulsation of growth in, 18; effect of alternate
supply and withdrawal of water on growth of, 55 ; of mechanical
stimulation and wound on growth, 87, 89 ; positive thermonastic
response of, 217, 223

Printed in England at The Ballantyne Press

Spottiswoode, Ballantyne & Co. Ltd.

Colchester, London & Eton

t