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PLANT RESPONSE
WORKS BY THE SAME AUTHOR.
RESPONSE IN THE LIVING AND NON-
LIVING. With 117 Illustrations. Svo. 10s. 6^.
1902. .
ELECTRO-PHYSIOLOGY OF PLANTS.
{To be published shortly.)
LONGMANS, GREEN, \ CO., .59 Paternoster Row, Londc
N\w ^'ork and Bombay.
PLANT RESPONSE
AS A MEANS OF ^^ ^
PHYSIOLOGICAL INVESTIGATION
BY
JAGADIS CHUNDER ROSE, M.A., D.Sc.
PROFESSOR, PRESIDENCY COLLEGE, CALCUTTA
WITH ILLUSTRATIONS
LONGMANS, GREEN, AND CO.
3 9 PATERNOSTER ROW, LONDON
NEW YORK AND BOMBAY
1906
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PREFACE
The investigations described in the present volume have
been an outcome of my work on the Similarity of Responsive
Phenomena in Inorganic and Living Matter, first com-
municated as a Memoir to the Science Congress at Paris, in
August 1900,^ and subsequently expanded into greater detail
in my book on ' Response in the Living and Non-Living.'
The electrical responses described in the Memoir referred to,
had been obtained by the method of conductivity variation.
The same problem was next attacked by a different mode of
investigation, response being now obtained by electromotive
variation.- Believing in the continuity of responsive pheno-
mena in the inorganic and organic, I undertook on that
occasion to demonstrate by the same method the electrical
response of ordinary plants, and to show that every plant,
and every organ of every plant, was excitable. It was then
generally believed that so-called 'sensitive' plants alone ex-
hibited excitation by electrical response, and the proposition
that ordinary plants also showed excitatory electrical response
to mechanical stimulation, and that such response was appro-
priately modified under physiological changes, was much
controverted. I have to thank Professor Sidney H. Vines,
' ' De la Gencralite des Phenomenes Moleculaires produils par PElectricite
sur la Matiere Inorganique et sur la Matiere Vivante. ' (Travaux dtc Congr^s
International de Physique. Paris, 1900.)
- Paper read before Royal Society, June 6, 1901. Also P'riday Evening
Discourse, Royal Institution, May 10, 1901.
viii PLANT RKSrOXSK
at that time President, for the facilities which he then afforded
mc for the full publication of my results in the ' Journal ' of
the Linnean Society, and for the warm interest which he has
manifested in my work, both then and later.
I next undertook to demonstrate that all the important
characteristics of the responses exhibited by even the most
highly differentiated animal tissues, were also to be found in
those of the plant'
In m)' previous investigations I had shown that the tissues
even of ordinar)' plants gave electrical signs of excitatory
response. I now undertook an inquiry as to why they should
not also exhibit response by mechanical indications ; and I
was surprised to disco\er that ordinary plants, usually re-
garded as insensitive, ga\e motile responses, which had
hitherto passed unnoticed.
From the point of view of its movements a plant may be
regarded in either of two ways : in the first place as a
mysterious entity, with regard to whose working no law can
be definitely predicated, or in the second place, simply as a
machine, transforming the energy supplied to it, in ways
more or less capable of mechanical explanation. Its move-
ments are apparently so di\'erse that the former of these
hypotheses might well seem to be the only alternativ'e.
Light, for example, induces sometimes positi\e curvature,
sometimes negatixe. Gra\itation, again, induces one mo\e-
ment in the root, and the opposite in the shoot. From these
and other reactions it would appear as if the organism had
been endowed with various specific sensibilities for its own
advantage, and that a consistent mechanical explanation
of its movements was therefore out of the question. In spite
of this, however, I have attempted to show that the plant
may nevertheless be regarded as a machine, and that its
movements in response to external stimuli, though apparently
' Paper read hefure Renal .Society, I'eljniary 4. 1904.
TREFACE ix
SO various, are ultimately reducible to a fundamental unity
of reaction.
This demonstration has been the object of the present
work, and not that treatment of known aspects of plant-
movements which is to be found detailed together with the
history of the subject, in standard books of reference on
plant physiology, such as those of Sachs, Pfeffer, Strasburger,
Darwin, Francis Darwin, Vines, and Detmer.
In analysing plant-movements the greatest complexity
arises from the confusion of effects due to internal energy
and external stimulus respectively. I have, however, been
able to discriminate the characteristic expressions of these
two factors, and thus to disentangle the complex phenomena
which result from their combined action. Another very
obscure problem is found in the nature of so-called ' spon-
taneous or autonomous ' movements. By the discover}',
however, of multiple response, and by the continuity which I
have been able to establish, as existing between multiple and
autonomous responses, it has been found possible to demon-
strate that there are, strictly speaking, no ' spontaneous '
movements, those known by this name being really due to
external stimulus previously absorbed by the organism.
Thus all the experiments ha\e tended to show that the
phenomenon of life does not, as such, connote any intrusion
into the realm of the organic of a force which would interfere
with that law of the Conservation of Energy which is known
to hold good in the inorganic world.
The elucidation of the fact that such varied and obscure
phenomena in the life-processes of the plant, as, for instance,
growth and the ascent of sap, are fundamentally due to the
same excitatory reactions as are seen otherwise exemplified
in the simple mechanical response now familiar to us, con-
stituted a further result which, at the outset of the investiga-
tion, was little to be foreseen.
X PLANT RESPONSE
It has been shown finally that there is no physiological
response given by the most highly organised animal tissue
that is not also to be met with in the plant. This was proved
in detail in the case of the identical polar effects induced in
both by electrical currents ; in the conduction of the excitatory
impulse to a distance ; in the possibility of detecting the
excitatory wave in transit and measuring its rate ; and in the
appropriate modification of its velocity by different agencies,
even in the case of ordinary plants ; in the passing of multiple
into autonomous response in vegetable tissues ; in the light
thrown by this phenomenon on the causes of rhythmicity in
animal tissues ; in the similar effects of drugs on animal and
vegetable tissues, and in the modifications introduced into
these effects by the factor of individual ' constitution.' This
identity of effects, indeed, as between the responses of plant
and animal, is so deep and so extended, that it is to be
anticipated that as several of the obscure problems of animal
physiology have already been found elucidated by means of
these researches carried out on plants, so others will be
found capable of explanation by similar means in the near
future.
In conclusion, I wish to say that from my assistant,
Mr. J. Roy, and my pupils, Messrs. A. C. Basu, S. C. Acharya,
S. Chakravarty, N. Roy, and S. Gosvvami, I have received
able assistance at various periods during the course of these
long and extended investigations.
J. C. BOSE.
Presidency College, Calcutta :
July 1905.
CONTENTS
PART I.— SIMPLE RESPONSE
CHAPTER I
THE PLANT AS A MACHINE
Responsive movements in plants— Work done by plant — Plant as a machine
• — Indicator-diagrams — Physiological response-curves — Pulse-records —
Cardiagrams — Modification of pulse by poison and other agencies —
Automatic response in plants — Optical Lever Recorder — Effect of
external agencies on automatic pulse-beat in plants ....
CHAPTER n
MECHANICAL RESPONSE TO STIMULUS
Molecular derangement caused by stimukis — Expression in change of form,
contraction — Mechanical model — Myograph — Response by differential
contraction in pulvinated plant-organs — Longitudinal response in plants
— Response of plant to all forms of stimulus — Plant chamber — I'ractic-
able forms of graduated stimulus — Electro-thermic stimulator — Stimula-
tion by condenser discharge — Response-recorder — Advantage of counter-
poise— Response of Biophytitvi to thermal stimulation — Response to
condenser discharge — Absolute measurements of motile effect and of
work performed — Effect of load — Definite determination of threshold of
response — Determination of variation of excitability by measurement
of minimally effective stimulus ........
CHAPTER HI
ON THE UNIVERSALITY OF SENSITIVENESS IN PLANTS
AS DEMONSTRATED 15V MEANS OF ELECTRICAL RESPONSE
Arbitrary classification of plants into sensitive and ordinary — Method of
electro-motive variation for detecting state of excitation — Hydraulic
model — Excitation of vegetable tissue, like that of animal tissue, induces
galvanometric negativity — Methods of direct and transmitted excitation
— Electrical and mechanical response alike record molecular derange-
ment and recovery — Similarities in simultaneous record of mechanical
PLANT RESPONSE
and electrical response — True excitation has a concomitant negative
turgidity-variation, negative mechanical response or fall, and galvano-
metric negativity — These are true physiological responses, and are
abolished at death — Abnormal positive mechanical and electrical
responses brought about by positive turgidity-variation— Direct and
indirect effects of stimulation — Discrimination of differences of excit-
ability- by electric test — Excitability of plant-tissues in general — Re-
sponsive power characteristic of matter ...... 29
CHAPTER IV
ON CONDITIONS FAVOURABLE TO THE CONSPICUOUS
EXHIBITION OF MECHANICAL RESPONSE
Differences of degree of motile sensibility in sensitive plants so called —
Response of anisotropic organ brought about by differential contraction
— Production of response by artificial variation of turgidily — Variation
and counter-variation of turgescence, causing two opposite responsive
movements — Differences between hydrostatic and true excitatory effects
— Distinction of plants as ordinary and sensitive, arbitrary — Sensitive
plants may be excited, yet give no mechanical response — Certain con-
ditions necessary to exhibition of differential response — Balanced action
as result of diffuse stimulus on radial organ — Slight differential contraction
of pulvinus magnified by long petiolar index 43
CHAPTER V
MECHANICAL RESPONSE IN ORDINARY LEAVES
Pulvinoid and pulvinus — Demonstration of mechanical response in ordinary
leaves — Response of Ariocgt-pus similar to that of Biophyttim — Response
to stimulus, even in old tissues, by expulsion of water — Localisation of
motile organ in ordinary leaves — Conducting properties of various tissues
— Lamina is not the perceptive organ —Response in ordinary leaves,
though sluggish, yet comparable in extent to that of Mimosa —Peculiar
phenomenon of fatigue-reversal seen in Mimosa observed also in ordinary
plants — Periodic reversals ......... 53
CHAPTER VI
LONGITUDINAL RESPONSE OF RADIAL ORGANS
Absence of lateral response movements in radial organs due to mutually
antagonistic effects of equal contractions of diametrically opposite sides
— Lateral response in radial stem of Walnut under unilateral stimulation
— Also in pistil of i1///M— Diffuse stimulation of radial organ causes
longitudinal contraction — The ' Kunchangraph ' — Longitudinal contrac-
tion of stamens of Cyiiereie not unique — Similar longitudinal responses
CONTENTS
obtained with stems, roots, tendrils, petioles, stamens, and styles of
ordinary plants— Also in fungi — Responsive contraction in Pasxiflora,
comparable in extent with that in Cynerea — Longitudinal response in
plants modified by the physiological variations due to age, season, and
chemical agencies ....... . . 66
CHAPTER VII
RESPONSIVE CURVATURE OF MOLECULARLY ANISOTROPIC
ORGAN
Molecular anisotropy artificially induced by one-sided cooling — Cooled side
less responsive— Diffuse stimulation causes concavity of the uncooled,
that being relatively the more excitable— Local fatigue diminishes excit-
ability— Diffuse stimulation now causes concavity of the unstrained side
— Similar anisotropy induced in plagiotropic organs, by unilateral action
of light — The lower or shaded side of such organs relatively more excit-
able—Diffuse stimulation causes current of response from lower to upper,
and also concavity of lower half — Responses of plagiotropic Cttcurhita
and Convolvulus — Differences in excitabilities of outer and inner surfaces
of tubular organ — Complex response due to successive excitations of two
antagonistic halves of an anisotropic organ — Response of spiral tendrils
by uncurling— Response in certain cases by contraction of the spiral or
curling — Writhing movement in spiral tendril under strong stimulation . 82
CHAPTER VIII
RELATION BETWEEN STIMULUS AND RESPONSE
Ineffective stimulus becomes effective by repetition— Two types of response
in contractile animal tissues, cardiac and skeletal— Response of cardiac
muscle on 'all or none' principle; parallel case in Biophytum—\n
skeletal muscle, increasing stimulus causes increasing response, which
tends to reach a limit — Parallel results in longitudinal and electrical
response of plants — Effect of superposition of stimuli — Tetanus . . 94
PART II.— MODIFICATION OF RESPOx\SE
UNDER VARIOUS CONDITIONS
CHAPTER IX
ON THE UNIFORM, FATIGUE, AND STAIRCASE EFFECTS
IN RESPONSE
Uniform response in plants— Staircase effect — Fatigue due to molecular
strain — Fatigue in plant- responses — Periodic fatigue — Fatigue under
continuous stimulation — Explanation of anomalous erection of leaf of
PLANT RESPONSE
Mimosa under continuous stimulation— Conductivity and excitability of
tissue diminished through incomplete protoplasmic recovery — Relatively
greater fatigue in a motile than conducting organ — Disappearance of the
motile excitability earlier than conductivity — Refractory period — Absence
of responsive effect when stimulus falls williin refractory period . .103
CHAPTER X
THEORIES CONCERNING DIFFERENT TYPES OF RESPONSE
The chemical theory of response — Insufficiency of the theory of assimilation
and dissimilation to explain fatigue and staircase effects — Similar
responsive effects seen in inorganic substances — Molecular theory —
When molecular recovery is complete, responses uniform : when incom-
plete, fatigue brought about by residual strain — Fatigue under con-
tinuous stimulation, in inorganic substance, in plant, and in muscle —
Staircase effect brought about by increased molecular mobility : examples
seen in inorganic substance, and in living tissues— No sharp line of
demarcation in the borderland between physical and chemical pheno-
mena —Molecular changes attended by changes of chemical activity —
Uneijual molecular strain gives rise to a secondary series of chemical
actions — Volta-chemical effect and by-products — Supposition tliat re-
sponse always disproportionately larger than stimulus, not justified —
Existence of three types : (i) response proportionate to stimulus; (2)
response disproportionately greater than stimulus; (3) response dis-
proportionately less than stimulus — Instances of stimulus partially held
latent : staircase and additive effects ; multiple response ; renewed
growth ............
CHAPTER XI
EFFECT OF AN.ESTHETICS, POISONS AND OTHER CHEMICAL
REAGENTS ON LONGITUDINAL RESPONSE
Response modified by physiological change — Carbonic acid causes depression,
and transitory exaltation as after-effect — Gradual abolition of response in
hydrogen and restoration by access of air — Chemical agents cause con-
traction or relaxation of plant-tissue — Effect of alcohol causing temporary
exaltation of response followed by depression and protracted period of
recovery — Ether causes relaxation and temporary depression of response
— Explanation of anomalous action of ether on stimulated Mimosa leaf
— Abolition of response by hydrochloric acid — Response restored by
timely application of ammonia — Abolition of response by poisonous
reagent — Similarity of effect of chemical agents on the response of animal
and vegetable tissues
COxXTEXTS XV
CHAPTER XII
EFFECT OF TEMPERATURE
Temperatures optimum, maximum, and minimum — Diminution of electrical
response by cooling — Temporary or permanent abolition of response due
to cold — Characteristic differences exhibited by different species-
Mechanical response of Biophytinn and autonomous response of
Desmodmm arrested by cold — Prolongation of latent period — Diminution
of longitudinal mechanical response by cold— Diminution of electrical
response of plants by rise of temperature— Similar diminution seen in
longitudinal mechanical response — Increase of excitability due to cyclic
variation of temperature ......... 159
CHAPTER XIII
ON THE DEATH-SPASM IX PLANTS
Difficulty of determining exact moment of death — Various post-mortem
symptoms afford no immediate indication — Ideal methods for determina-
tion of death-point — Realised in four different ways : [a) Determination
by electrical method— (A) Determination by spasmodic lateral movement
at moment of death — Experiments with Mimosa — Death-contraction a
true physiological response— Continuity of fatigue and death— Death-
point earlier in young tissues — Composite spasmodic movement — (/'')
Determination of death-point in tendril of PassiJJora, by sudden move-
ment of uncurling— () by
rise of temperature — (c) by fatigue — {d) by anesthetics — Variation of
receptive excitability by ether — Conductivity versus excitability —
Abolition of motile excitability without abolition of conductivity —
Hydro-mechanical theory of transmission of stimulus untenable
CHAPTER XIX
ON EL.ECTROTONUS
The anode acts as a block to the transmission of stimulus— Opposite effect
of kathode —Experiments on Biophytum, showing variations of con-
ductivity by anode and kathode respectively — Experiments on I\Ii?nosa,
showing increase of motile excitability at or near the kathode, and
diminution of motile excitability at or near the anode - Curious ' develop-
ment ' of response, near the kathode .......
CHAPTER XX
ON THE VELOCITY OF TRANSMISSION OF EXCITATORY WAVES
IN PLANTS
Difficulties in accurate determination of velocity of transmission, due to
unknown variations of excitability arising from injury, and variations of
conductivity through fatigue — A perfect method of obtaining accurate
and consistent results — Relative advantages of studying conduction in
plants as compared with animals — Determinations of velocity of trans-
mission in centripetal and centrifugal directions — Preferential conductivity
in centrifugal direction — Diminution of conductivity and excitability by
fatigue — Within a certain critical interval, organ ' refractory ' to further
stimulus — Increased velocity of transmission with increasing stimulus —
Measurement of diminution of conductivity by cold — Fibro-vascular
elements the best conducting channels — Conductivity lengthwise greater
than crosswise — Electric mode of determination of velocity of trans-
mission— Indifferent parenchymatous tissues practically not transmitters
of excitation — Comparative tables showing velocity of transmission in
various plant and animal tissues 238
CHAPTER XXI
ON DETECTION OF EXCITATORY PULSE DURING TRANSIT BY
ELECTROTACTILE AND ELECTROMOTIVE METHODS
Pfeffer's experiment on expulsion of water from excited cells — Author's
experiment on a delicate method of detecting excitatory expulsion of cell-
sap^Chemical method of determining velocity of transmission of excita-
tion— Electrotactile detector — Demonstration of passage of excitatory
PLANT RESPONSE
I'AGE
contractile wave by means of electrotactile method — Determination of
velocity of transmission of excitation in ordinary plants by electromotive
method — Excitatory versus hydro-mechanical movement of water . -254
CHAPTER XXII
THE LATENT PERIOD AND REFRACTORY PERIOD
The determination of the latent period in Mimosa — Experimental arrange-
ments for obtaining automatic record — Prolongation of latent period by
cold— Spark-record for determination of latent period— Prolongation of
latent period by fatigue — Sluggishness of the response of Pliilanthus
urinaria, also long latent period and very protracted period of recovery
■ — Latent period reduced under strong stimulation — Response in
Biophytiim on the ' all or none ' principle — Definite value of effective
stimulus — Phenomenon of refractory period in Biophytiim — Parallelism
of responses in Biophytiim and in cardiac muscle — Additive effects —
Inappropriateness of term ' refractory period ' — Energy in excess of
effective stimulus held latent for subsequent manifestation . . . 264
PART IV.— MULTIPLE AND AUTONOMOUS
RESPONSE
CHAPTER XXIII
ON MULTIPLE RESPONSE
Multiple electromotive responses due to a single strong stimulus — Multiple
electrotactile responses — Multiple mechanical responses in Biophytiim —
Cyclic variations in multiple responses — Multiple retinal excitations —
Intermittent pulse in man and plant — Semi-automatism — Continuity of
multiple and automatic response — Conversion of Biophytiim into auto-
matically responding plant ; conversion of Desviodiiim into ordinarily
responding plant — Similar polar effects of current in Biophytiim and in
Desmodiiim leaflet at standstill — Moderate stimulus in Biophytiim and
in Desmodium at standstill produces single response ; and strong stimulus,
multiple response 279
CHAPTER XXIV
AN INQUIRY INTO THE CAUSES OF AUTONOMOUS MOVEMENTS
Production of pulsatory movements as after-effect of energy absorbed —
Physical analogue -Localisation of seat of automatic excitation in Des-
modium— Demonstration of multiple response to a constant stimulus :
(I) Chemical— (2) Electrical— (3) Stimulus of light— Multiple reiix»nse
CONTENTS
to constant stimulus of light, in : {a) retina — (/>) Biophytitiii—(c) Des-
modiuin — (4) Thermal — Induction of automatism in Biophytum at
favourable temperature — (5) Of internal hydrostatic pressure — Absorp-
tion of external energy and its absorption by the plant in latent form —
True meaning of ' tonic ' condition — Cause of rhythmicity — After-effect,
and its relative persistence ......... 295
CHAPTER XXV
INFLUENCE OF VARIOUS CHEMICAL REAGENTS ON THE
AUTONOMOUS RESPONSE OF DESMODIUM GYRANS
The recorder and experimental chamber — Absolute measurement of period
and amplitude of /5(,'jw£;f///^/;;.oscillation — Responsive significance of up
and down movements deduced from («) analogy with response of
Mimosa ; {b) test of increased internal hydrostatic pressure — ' Systolic '
contraction and 'diastolic' expansion oi DesmodhDw^wWixwx's, — Mode of
application of chemical reagents — Action of chemical reagents modified
by : tonic condition of plants ; strength of solution ; and duration of
application — Effect of anaesthetics — Effect of alcohol — Effect of carbonic
acid — Effects of ammonia and of carbon disulphide — Effect of copper
sulphate solution, either when applied externally, direct on the pulvinus,
or internally —Spark-record of Z?^^wc>rt'///w-pulsation .... 315
CHAPTER XXVI
EFFECTS OF TEMPERATURE ON AUTONOMOUS RESPONSES
Increase of frequency and diminution of amplitude of pulsation with rising
temperature— Converse effect of fall of temperature - Similar effect in
cardiac pulsation — Effect of the reduction of temperature to the thermo-
tonic minimum — Explanation of diminution of amplitude of pulsation
with rise of temperature — Anomalous use of the word ' relaxation ' —
Simple versus additive character of individual pulsation . . , 329
CHAPTER XXVH
SIMILARITIES OF RHYTHMIC RESPONSE IN VEGETABLE AND
ANIMAL TISSUES
The similarities, in their fundamental characteristics, of rhythmic tissues,
animal and vegetable: (i) In responses — (2) In possession of long re-
fractory periods — (3) In incapability of tetanus — Theories regarding the
causation of heart-beat — The similarities of rhythmic tissues, animal and
vegetable, as seen in : (i) The effects of internal hydrostatic pressure —
(2) The effects of variation of temperature — (3) The periodic groupings
PLANT RESPONSE
of response— (4) The effect of barium salt— (5) The antagonistic actions
of acid and alkali — Identity of rhythmic phenomena in animal and
vegetable tissues .......... 344
PART v.— ASCENT OF SAP
CHAPTER XXVIII
SUCTIONAL RESPONSE AND ASCENT OF SAP
Inadequacy of existing theories of ascent of sap — General considerations
regarding cellular activity and resultant propulsion of water — The
Shoshungraph — Balanced Shoshungraph for determining variations of
suction — Hydrostatic and Hydraulic Methods of Balance . . . 359
CHAPTER XXIX
MODIFICATION OF SUCTIONAL RESPONSE
Effect of temperature on suction by three methods of inquiry: (i) Un-
balanced method of Shoshungraph : {a) Action of cold — (d) Action of
moderate rise of temperature — (2) Method of Hydrostatic Balance : (a)
Action of cold — Reversal of normal direction of flow — {/>) Action of
warm water — (3) Method of Hydraulic Balance — {a) Action of cold —
(/>) Effect of warm water — Explanation of suction when the root is
killed by boiling water — Stimulation renews suctional activity in plant
whose suction has come to a standstill — Osmotic versus excitatory action
— Abolition of suction by poison — Suctional activity continued until
whole plant is killed by poison ■ . . . 372
CHAPTER XXX
THE PHENOMENON OF PROPULSION OF SAP
AND ITS VARIOUS EFFECTS r
The mechanics of the ascent of sap : {a) Uni-directioned flow — (/') Initiation
of multiple rhythmic excitations — Connection between conduction of
excitation and conduction of sap— Rapidity of ascent of sap accounted
for by stimulatory action — Positive and negative pressures due to one
cause — (i) Positive pressure — (2) Negative pressure — (3) Irregular
variations of pressure — Direct conduction and conduction by relays —
Excretion of water — Excretion of nectar — Translocation of organic food-
substances — Mechanical response to suctional activity — Effect of warmth
— E.ftect of cold — Explanation of the drooping of leaves during frost —
Explanation of response and recovery — Antagonistic actions of internal
energy and external stimulus
CONTENTS
PART VI.— GROWTH
CHAPTER XXXI
THE RECORD OF GROWTH-RESPONSE
I'AGE
The simple Crescograph— The Balanced Crescograph— Rhythmic growth-
response — Growth-response and excitatory response — Law of direct and
indirect effects of excitation — Positive turgidity- variation as indirect effect
of excitation — Mechanical test — Significance of ' inner stimuli ' . . 409
CHAPTER XXXH
THE EFFECTS ON GROWTH OF INTERNAL ENERGY
AND EXTERNAL STIMULUS
Characteristics common to growth and to other forms of rhythmic response :
(i) Periodic groupings — (2) Effect of external stimulus in renewal of
growth when at temporary standstill — (3) Renewal of growth-pulsation
by positive turgidity-variation — (4) Effect of increased internal hydro-
static pressure— (5) Effect of ascent of sap on growth — Effect of tempera-
ture on growth — Comparison of various types of multiple response —
Effect of external tension on growth — Effect of direct application of
stimulus on the growing region — Similarities between motile and growth
responses — Direct and indirect effects of stimulus, and laws of growth . 424
CHAPTER XXXni
ON THE RELATION BETWEEN TEMPERATURE AND GROWTH,
AND THE ACCURATE DETERMINATION OF OPTIMUM AND
MAXIMUM POINTS
General consideration of difficulties of accurate determination of effects of
temperature on growth— Four accurate methods : (i) Method of discon-
tinuous observations— Accurate regulation of temperature by electrolytic
rheostat — (2) Method of continuous observations — Thermo-crcscent
curve — Determination of the optimum point — (3) Method of balance -
(4) Method of excitatory response — Translocation of the optimum point 441
CHAPTER XXXIV
ON AN ATTEMPT TO DETECT AND MEASURE LATENT STIMULUS,
AND ON THE STUDY OF PERIODIC AFTER-EFFECTS
Positive and negative after-effects — Extreme delicacy of the Method of
Balance —Detection of absorbed stimulus by negative after-effect —
PLANT RESPONSE
Constancy of sum of direct and indirect after-effects — Latent component
almost vanishing above the optimum— Variation of receptivity — Direct
and indirect response of plant in sub-tonic condition — Table showing
direct and indirect effects at different temperatures — Is the change
induced by stimulus always of an explosive chemical character ? — Rela-
tion between stimulus and response in different tonic conditions— After-
effect— Factors which determine periodic after-effects: (i) Stimulus of
light — (2) Temperature— (3) Chemical stimulus — (4) Turgidity — Con-
tinuous photographic record of the pulsations of Desnwdiiim — Record of
periodic variation of rate of growth— Continuous photographic record
of periodic variations of transpiration — Continuous photographic record
of the variation of the rate of growth — Annual rings and seasonal
periodicity 456
CHAPTER XXXV
AN INVESTIGATION INTO THE DIFFERENT EFFECTS OF DRUGS
ON PLANTS OF DIFFERENT 'CONSTITUTIONS'
General consideration of the problem — ' Constitution ' and the elements
which determine it — Methods of investigation — Action of carbonic acid
gas — Action of ether — Effect of solution of sodium carbonate — Effect of
solution of sugar — Effect of alcohol — Effect of acids— PLffect of alkali —
Antagonistic action of alkalis and acids — Action of strong solution of
sodium chloride — Effect of poisonous solution of copper sulphate —
Opposite effects of the same dose on different constitutions— Opposite
effects of large and small doses .......'. 4
PART VII.— GEOTROPISM, CHEMOTROPISM,
AND GALVANOTROPISM
CHAPTER XXXVI
THE RESPONSIVE CURVATURES CAUSED BY GRAVITY.
NEGATIVE GEOTROPISM
Statement of the problem of apogeotropic response — Mode in which stimu-
lation is brought about : radial-pressure theory, and theory of statoliths
— Mechanics of responsive movement — Experiment demonstrating re-
sponsive curvature as brought ab.out by unilateral pressure of particles —
Record of curvature induced by gravitation — Record of different rates
of curvature when specimen is held at angles of 45° and 135° to the
vertical — Determination of the true character of apogeotropic response
— Responsive curvature of acellular organs— Curvature of grass haulm
under gravity — Growth of grass haulm on a klinostat .... 493
CONTENTS
CHAPTER XXXVII
THE RESPONSIVE PECULIARITIES OF THE TIPS OF
GROWING ORGANS
I'AGE
Difference between shoot and root in their response to stimulus of gravity
— Difference in character of response between tip and growing region of
root — Scope of the investigation — Electrical investigation— Responsive
results of: i. Longitudinal transmission of effect of stimulus from tip ;
(a) Moderate unilateral stimulation ; (/;) effect of stronger unilateral
stimulation — 2. Direct unilateral stimulation of growing region —
Moderately strong stimulus — 3. Transverse transmission of effect of
stimulus ; {a) moderate stimulation ; (6) stronger stimulation-
Mechanical response inferred from observed electrical response —
Tabular statement 512
CHAPTER XXXVIII
INQUIRY INTO THE LAWS OF RESPONSIVE GROWTH-
CURVATURES
Scope of the investigation : i. Mechanical response to unilateral stimula-
tionof the tips of shoot and root : (a) Moderate stimulus — (A) Stronger
stimulation — 2. Effect of unilateral stimulus, applied at the responding
growing region: (a) Moderate stimulus— (/6) Strong or long-continued
stimulus — Experiments on the direct and indirect effects of stimulus on
Mimosa : {a) Direct stimulation — (d) Indirect stimulation, longitudinal
transmission — (c) Indirect stimulation, transverse transmission — The
curious response of A riscswa — Table showing responsive effects common
to pulvini, pulvinoids, and growing organs — Laws of responsive growth-
curvature 524
CHAPTER XXXIX
INQUIRY INTO POSITIVE GEOTROPISM
No specific difference as regards their responses, between shoot and root —
Darwinian curvature — Localisation of geotropic sensibility at the root-
tip — Experiments as to whether amputation of root-tip abolishes
excitability — The tip of the root the organ of gravi-perception — The
perceptive z'ersi/s the responding organ — True perceptive region . . 536
CHAPTER XL
ON CHEMO-TROPISM AND GALVANO-TROPISM
General difficulties of the investigation — How to overcome these ditficuUies —
Three distinct methods of testing results : ( i ) by variation of longitudinal
dv PLANT RESPONSE
PAGE
growth ; (2) by responsive movement of pulvinus ; and (3) by growth-
curvature— Method of application of chemical reagent — Effect of alkali
— Effect of acid — Effect of copper sulphate — Action of sugar solution —
Chemo-tactic movements — Explanation of anomalous osmotic or plas-
molytic action— Excitatory versus plasmolytic reaction in pulvinus of
Mimosa : (i) Favourable tonic condition — (2) Ordinary tonic condition
— Polar effects of currents inducing growth-curvatures— Localised polar
effects on pulvinus — Anodic and kathodic effects on longitudinal growth
— Generalised law of polar excitation in plants — Gal vano-tropic response
— The indirect effect of polar excitation — The effect on growth of
' electrification ' of soil 546
PART VIII.— HELIOTROPISM
CHAPTER XLI
FUNDAMENTAL RESPONSIVE ACTION OF PLANT-TISSUES
TO STIMULUS OF LIGHT
Diversity of movements induced by light — Differentiation of responsive
movements — Action of light on tissues in sub-tonic condition — Effect of
light on pulvinated organs — Effect of diffuse stimulation of light on
non-growing radial organs — Retarding effect of light on longitudinal
growth — Phenomenon of oscillation under long-continued stimulation —
Similarity of responsive reaction under light and under other forms of
stimulation ........... 565
CHAPTER XLII
POSITIVE HELIOTROPISM
Introduction — Theory of de Candolle — Inadequacy of de Candolle's theory
— Definition of terms positive and negative — Darwin's theory of
modified circumnutation — Response of terminal leaflet ol Desmodium — ■
Extreme sensitiveness of some plant-organs to light — Merging of
multiple in continuous response — Orientation induced by light — The
perceptive region in the terminal leaflet of Desmodium — Heliotropic
response in radial organ — Magnetically controlled recorder — Heliotropic
response of hypocotyl of Sinapis — Recovery and theory of recti-petality. 579
CHAPTER XLHI
NEGATIVE HELIOTROPISM
Incomplete parallelism between actions of light and of gravitation —
Theoretical considerations — Recording microscope — Negative helio-
CONTENTS
tropic curvature induced by stimulation of the tips of root and shoot —
Intermediate phases between positive and negative heliotropic response :
{a) neutralisation by transverse transmission ; {6) neutralisation by
transverse transmission, with multiple response — Localised sensitiveness
to light and transmission of excitatory effect — Negative heliotropism of
a radial organ — Gradual transition from positive to negative, through
intermediate phase of neutrality — Apparent heliotropic insensitiveness of
certain tendrils — Negative heliotropism of tendril of V///s . . . 597
CHAPTER XLIV
EFFECT OF INVISIBLE RADIATION AND EMANATIONS
Effect of temperature and its variations — Demonstration of fundamental
effect of thermal radiation on growth — Response to successive uniform
stimuli of thermal radiation — Effect of continuous unilateral stimulation
— Effect of electrical waves on growth— Response of Mimosa to electric
radiation — Action of high frequency Tesla current .... 614
CHAPTER XLV
ON PHOTONASTIC PHENOMENA AND ON DIURNAL SLEEP
Photonasty and para-heliotropism — Response of Tropieolttm iiiajits — Re-
sponses of plagiotropic stems : {a) Mimosa — {b) Ipomaa — (c) Cuciirbiia —
Daily periodic movements of plagiotropic stems — Responsive movements
of pulvinated organs — Pulvinated organs showing positive heliotropic
movement: {a) Responseof terminal leaflet of Z)fj-w(7) Positive response —
Responsive action under stimulus of gravity — Heliotropic action in radial
organs — Heliotropic action in plagiotropic and dorsi-ventral organs —
Phototactic movements — Nyctitropic movements ..... 725
CHAPTER LII
ON PHYSIOLOGICAL RESPONSE,
AND ITS CONTINUITY IN PLANT AND ANIMAL
Vitalism -Fundamental unity of physiological response in plant and animal
— Theory of Darwin — Variation as induced by external forces . . 740
CLASSIFIED LIST OF EXPERIMENTS 755
INDEX 765
EBRATUM
Page 409, line i of analytical contents, y^r Growth-Recorder ?Ya<)' Crescograph.
ILLUSTRATIONS
FIG. I'AGE
1. Record of Healthy Adult and Senile Human Pulse (Broadbent) . . 3
2. Effect of Muscarin in arresting Pulsation of Frog's Ventricle (Cashing) 4
3. Record of Human Pulse (Broadbent) ...... 4
4. Demonstration Optical Pulse- Recorder 7
5. Death of Plant, and Arrest of Pulsation, by Poison .... 8
6. Death, and Arrest of Pulsation, in Leaflet of Dcsinodiuiii by Strong
Electric Shock 8
7. Response of India-rubber 12
8. Mechanical Lever Recorder 12
9. Series of Contractile Responses in Muscle 13
10. Photographic Record of Longitudinal Contractile Response in
ordinary Stamens {Browtiea ariza) . . . . . . . 13
11. Differential Lateral Response of Compound Strip .... 13
12. Plant Chamber and Recorder 16
13. The Electro-thermic Stimulator 17
14. Diagram of Connections for Stimulation by Condenser Discharge . 18
15. Photographic Record of Response and Recovery of Mimosa, taken on
a slowly moving drum 21
16. Photographic Record of Response in different specimen, taken on a
faster-moving drum 21
17. Response of .5///«y/'«;« to Thermal Stimulation .... 24
18. Response of -S/ij/Zy/Z/^w to Electric Stimulation . . . . . 24
19. Effect of Load ... 25
20. The Spiral Spring- Recorder ........ 26
21. Isometric Response of J//w(7j-rt; 26
22. Hydraulic Model for Explanation of Electric Response . . . 31
23. Electrical Response in Plant by Method of Block .... 32
24. Electric Response Recorder ........ 33
25. Method of Transmitted Stimulation 33
26. Simultaneous Mechanical and Electrical Responses in Biophytniii . . 34
27. The Abnormal Positive preceding the Normal Negative in Mechanical
and Electrical Responses in Biophytmn 37
28. Response of Selenium to the Stimulus of Light 39
XXX PLANT RESrONSE
FIG. I'AGE
29. Response of Metal abolished by the action of ' Poison ' (Oxalic Acid) . 40
30. Responses of quickly reacting Biophytitiii, and sluggish Philanthus
iirinajia, under moderate and under stronger Stimulation . . 44
31. Artificial Hydraulic Response of J//w(?j^tf 46
32. Response of Ordinary Leaf {Artocarpiis) 56
33. Response of Leaves of Ordinary Plants to Electric Stimulation . . 64
34. Alternate opposite-directioned Responses obtained by the successive
Unilateral Stimulations of opposite sides of Pistil of Miisa . . 68
35. The Kunchangraph 73
36. Diagrammatic Representation of Apparatus for Periodic Stimulation
of Plant 75
37. Response of Stem of C"m5-c«/i2 to Electric Stimulation . . • • 77
38. Photographic Record of Responses of Style of Datura alba to Thermal
Stimulation 78
39. Responses of Plagiotropic Stem of Ciicurbita 86
40. Responses of Plagiotropic Stem of Convolvulus .... 86
41. Response of Bifurcated ^////cw Tube by sudden Collapse . . . 87
42. Responses of Hooked Tendril of Passijlora 90
43. Response by Coiling of spirally-cut ^//?«w Peduncle . . . . 91
44. Ineffective Stimulus made Eftective by Repetition • • • • 95
45. Additive Effect in Electrical Response 95
46. Mechanical Responses to Stimuli increasing in Arithmetical Progression 96
47. Curve showing Relation between Stimulus and Response . . . 96
48. Increased Electrical Response with Increasing Vibrational Stimuli
(Cauliflower-stalk) 97
49. Genesis of Tetanus in Muscle 98
50. Photographic Record of Genesis of Tetanus in Mechanical Response of
Plants (Style of Datura alba) 99
51. Uniform Electrical Responses (Radish) 104
52. Staircase Effect in Longitudinal Mechanical Response of Plant (Style
of Eu char is) 104
53. Fatigue in Longitudinal Mechanical Response of Plant (Style of
Datura) 105
54. Fatigue shown in Electrical Response, when sufficient Time is not
allowed for Full Recovery 105
55. Alternate Fatigue in Electrical Responses of Petiole of Cauliflower ;
in Multiple Electric Responses of Peduncle of Biophytum ; in
Multiple Mechanical Responses of Leaflet of Biophytum ; and in
Autonomous Responses of Z?t;.vw^rt'///;« . . . . . . 106
56. Rapid Fatigue under Continuous Stimulation in Muscle, and in
Leafstalk of Celery 107
57. Fatigue under long-continued Stimulation in the Contractile Response
of Plants 108
58. Photographic Record of Periodic Fatigue under Continuous Stimula-
tion in Contractile Response (Filament of Uriilis Lily) . . .108
59. Photographic Records of Normal Response of Mimosa to Single
Stimulus (upper figure), and to Continuous Stimulation (lower
figure) ............ 109
ILLUSTRATIONS xxxi
■AGE
60. Inefi'ectiveness of Stimuli, owing to Increasing Fatigue, in yJ//w(7j-a . 113
61. Fatigue- Reversal in Arsenic, under Continuous Stimulation of Hertzian
Radiation . . . . . . . . . . . 119
62. Automatic Record of Fatigue in the Contractile Response of India-
rubber under Rapidly Succeeding Thermal Shocks . . .120
63. Preliminary Staircase Increase, followed by Fatigue, in the Response
of Ga/f«a to Hertzian Radiation 121
64. Preliminary Staircase Increase, followed by Fatigue, in the Response
of Style of ^//c/zar/j- . . . . . . . . .121
65. Preliminary Staircase, followed by Fatigue, in the Responses of
Muscle (Brodie) 122
66. Staircase Increase in Electrical Response of Petiole of BryophylliDit,
rendered sluggish by cooling 122
67. Mechanical Response in Tendril of Passiflora in which Growth
was originally at Standstill 127
68. Eftect of Carbonic Acid Gas on Longitudinal Contractile Response . 131
69. Efiect of Hydrogen Gas . . . . , . . ... 132
70. Photographic Record showing Effect of Carbon Bisulphide in Abolish-
ing Response . . . . . . . . . .132
71. Effect of .Vapour of Alcohol ........ 134
72. Efilect of Ether 135
73. Efilect of H CI Vapour 136
74. Action of Chlorine . . . . . . . . . -137
75. Diminution of Response in Eiicharis Lily by Lowering of Temperature 140
76. After-effect of Cold on Ivy, Holly, and Eucharis . . . .141
77. Effect of Cold on Longitudinal Response . . . . . . 143
78. Effect of Rise of Temperature on Electrical Response . . . 144
79. Effect of Rise of Temperature on Longitudinal Contractile Response
of Plant 144
80. Effect of Rising and Falling Temperature on the Electrical Response
of Scotch Kale (stimulus constant) 145
81. Effect of Cyclic Rise and Fall of Temperature on Longitudinal
Mechanical Response in Plant 146
82. Determination of Death-point in Allinin Tube by Observation of
Volumetric Contraction, causing sudden Expulsion of Water . . 157
83. The Thermometric Spiral and Optic Lever of the Morograph . . 165
84. The Morograph . . . . . . . . . .167
85. Thermo-mechanical Curve obtained Photographically (Coronal ]t'\\z.-
xacnt oi Pass ijlora) . . . . . . . . . . 168
86. Thermo-mechanical Curve of Two Different Specimens of Style of
Datura alba, obtained from Flowers of the same Plant . . .169
87. Thermo-mechanical Records of Young Specimen of Spirogyra ; Older
Specimen of same ; and Style of Z'a/z/ra a/^a . . . . . 170
88. Thermo-mechanical Record of Leaf of J//w^^a . . . .172
89. Diagrammatic Representation of Mono-polar Excitation . . . 193
90. Bi-polar Excitation of y)/i!V«(7M . . . . . . .194
91. Bi-polar Excitation of j^/i'/Z/j/z/w ....... 194
92. Make-kathode and Break -anode Effects in Biophytiiin . . -195
XXXll PLANT RESPONSE
KIG. PAGF.
93. Record of Responses of Leaflet of Biophytiini, showing Responses
occurring at Kathode at Malce and not at Break ; and at Anode at
Break and not at Make . . . . . . . . . 196
94. Effects of Ascending and Descending Currents, on Highly Excitable
Specimen of ^/?w'//cw 285
1 18. Multiple Response in i^/fj/z^j/z/w 285
119. Multiple Response in Biophylum, showing Cyclic Groupings of
Amplitude and Period ......... 286
ILLUSTRATIONS XXxiii
FIG. PAGE
1 20. Multiple Response in Biophyttim, showing Cyclic Groupings . . 286
121. Intermittent Human Pulse (Broadbent) 288
122. Intemiittence in Pulsation of j'?/(7//!j//?/;w 289
123. Multiple Response of Biophytum under the Continuous Action of
Light 303
124. Induction of Autonomous Response in Biophytum, at Moderately
High Temperature of 35° C. 305
125. Experimental Apparatus for Making Records of Pulsation q{ Desvio-
diwn . . . . . . . . . . . . 316
126. Photographic Record of Pulsation of Z)^j-;«^fl'/«w . . . -317
127. Photographic Record of Uniform Pulsations in Z?('.fW(?(^!'/«w . . . 318
128. Displacement of Mean Position of Vibration of DesinodiiiDi Leaflet
by Increased Internal Hydrostatic Pressure 320
129. Method of Application of Chemical Agent to Cut End of Petiole . 321
130. Photographic Record of Effect of Ether Vapour, Large Dose . . 324
131. Photographic Record of Effect of Alcohol 325
132. Photographic Record of Effect of Carbonic Acid Gas . . . 325
133. Photographic Record of Effect of Copper Sulphate Solution Applied
on the Pulvinus . . . . . . . . . . 326
134. Spark-record of Single Pulsation in Leaflet of Z>e.rw(?<^/«/;;/ . . 327
135. Photographic Records of Autonomous Pulsations in Desmodiuiii,
showing Increase of Amplitude and Decrease of Frequency, with
Lowering of Temperature 332
136. Effect of Lowering of Temperature in Producing Increase of Ampli-
tude and Decrease of Frequency in Pulsation of Frog's Heart . 333
137. Photographic Record of Pulsations of Desinodiitin during Continuous
Rise of Temperature 333
138. Record of Pulsations of Z>.fwc«/?V/w at Different Temperatures . 334
139. Record of Pulsations of Frog's Heart at Dift'erent Temperatures
(Pembrey and Phillips) 334
140. Effect of Cooling to Thermo-tonic Minimum on Pulsation of Desmo-
dittm 335
141. Effect of Rapid Cooling by Ice-cold Water 336
142. Photographic Record of Pulsation of Desmodiuin, showing Sub-pulses
during slower Up Movement, as Nodules ..... 340
143. Photographic Record of Cyclic Groupings in Autonomous Pulsations
of Desviodiuni, showing Sub-pulses ....... 341
144. Photographic Record of Autonomous Pulsation of Desniodiicin,
showing Hourly Period . . . . . . . .341
145. Record showing that Rhythmic Tissue of Desmodiiivi is Incapable of
being Tetanised 346
146. Curve showing Relation between Temperature and Period of
Pulsation in Desiiwdiitm ........ 349
147. Curve showing Relation between Temperature and Period of Pulsa-
tion in the Heart of a Frog 349
148. Simple Alternation of Pulsation in Z)^jw^/?/w .... 350
149. Periodic Groupings of Pulsation in Z>^jw(7^ww ..... 350
1 50. Simple Alternation of Pulsation in Frog's Heart (Pembrey and Phillips) 350
b
xxxiv PLANT RESPONSE
FIG. PAGE
151. Effect of Barium Chloride Solution on Z)^.w/^rf/«w . . . . 351
152. Arrest of Beat of Ventricle of Frog at Diastole by Application of
Acid 351
153. Systolic Arrest of Heart-beat by Dilute NaHO Solution (Gaskell) . 352
1 54. Arrest of Desinodium Pulsation at ' Diastole ' by Application of
Acid 352
155. Arrest of Pulsation of Desnioditiin at 'Systole' by Application of
Dilute Alkali 353
156. Diagrammatic Representation of the Shoshungraph . . . . 365
157. Photograph of Shoshungraph • . . . 370
158. Effect of Cold on Suction 374
159. Curve showing Normal Suction at 23° C, Increased Suction at
35° C, and the After-effect persisting on Return to Normal Tem-
perature ............ 375
160. Method of Hydrostatic Balance 375
161. Record obtained by Method of Hydrostatic Balance of Successive
Applications of Cold and Warm Water 378
162. Record, obtained by Method of Hydraulic Balance, of Successive
Effects of Cold and Warm Water 380
163. Effect of Strong KNO3 Solution 3S4
164. Effect of Strong NaCl Solution 384
165. Effect of Copper Sulphate Solution 386
166. Record showing Recovery to be Hastened by the Increase of Internal
Activity which is caused by Application of Warm Water to the
Roots ............ 401
167. Diagrammatic Representation of Balanced Crescograph . . . 414
168. Complete Apparatus for Crescographic Record under Ordinary and
Balanced Conditions 416
169. Multiple Growth-responses (Peduncle of Crocus) . . . . 417
1 70. Responses of Leaf of Artocarpits to Thermal Stimulation . . 420
171. Renewal of Growth-pulsation by Thermal Stimulus in Tamariudus
indica originally at Standstill ........ 425
172. Initiation of Erectile Response in Leaf by Supply of Water to partially
Drought-rigored Mimosa ........ 426
173. Initiation of Growth-pulsation by Small Supply of Water to
Drought-rigored Seedling of Citciirbita . . . . . . 426
174. Curve showing Relation between Internal Hydrostatic Pressure and
Rate of Growth ( Ci-imini Lily) ....... 429
175. Photographic Record showing the Slow Pulsations of Large Amplitude
of Dcsinodiujii Leaflet at 30° C. to become very much Quickened
and Reduced in Amphtude at 42° C 431
176. Growth-pulsation seen in Seedling of i^ir/j-rtw 432
177. Photographic Record of Responses of Mature Style of Datura alba
to External Thermal Stimulus 434
1 78. Photographic Record of Responses of Style of Datura alba in which
Growth had come to a Temporary Stop 435
179. Photographic Record of Response of Growing Style oi Datura alba
to External Stimulus , . . , 436
ILLUSTRATIONS XXXV
FIG. I-AGE
1 80. Balanced Record of Response in Growing Peduncle of Encharis
Lily to Electrical Stimulation 436
181. Semicircular Electrolytic Rheostat interposed in Heating Cir-
cuit 443
182. Record of Growth in Crinum at Temperature of 34° C. and
35° C 445
183. Thermo-crescent Curve of Growth in Criitiiin Lily under Continu-
ously Increasing Temperature 447
184. Curve showing Relation between Temperature and Rate of Growth,
as deduced from the Thermo-crescent Curve .... 449
185. Series of Responses of Growing Organ of Crinum Lily, taken under
Balanced Conditions at Three Different Temperatures . . . 460
186. Curve showing the Relation between Stimulus and Response in the
same Organ, under the two different Tonic Conditions of 30° C.
(upper curve) and 37° C. (lower curve) 467
187. Continuous Photographic Record of Autonomous Pulsation of Des-
modiion gyraiis from 6 p.m. to 6 a.m. . . . . . . 470
188. Hydrometric Apparatus for 'Recording Continuous Variation of Rate
of Growth 471
189. Photographic Record showing Variation of Rate of Transpiration in
Cticui-bita, from 3 p.m. to 12 p. m 472
190. Continuous Photographic Record of Variation of Rate of Growth in
Four Days' Old Seedling of Oryza saliva, from 3 p.m. till 9 a.m.,
that is during Eighteen Hours 473
191. Continuous Photographic Record of Variation of Rate of Growth in
Seedling of Tamarindiis indica, a Fortnight Old, from 3 p.m. to
3 A.M 474
192. Effect of Carbonic Acid Gas on Growth 480
193. Balanced Records of Effect of Ether on Growth . . . . . 481
194. Excitatory Effect of Dilute Solution of Sodium Carbonate on
Growth . . . . . . . . . . .481
195. Acceleration of Growth by Application of Solution of Sugar . . 482
196. Spasmodic Alternations of Growth under Alcohol .... 483
197. Unbalanced Record showing Action of Acid in Causing Relaxation
and Ultimate Arrest of Growth 483
198. Unbalanced Record showing the Action of Alkali, and the Antago-
nistic Action of Subsecjuent Application of Acid .... 484
199. Effect of Strong Solution of NaCl on Rate of Growth, as Modified by
Different Constitutions of Specimens 484
200. The Effect of Different Constitutions in Determining the Resistance
Offered to Poisons. The Action of 5 per Cent. Solution of Copper
Sulphate 487
201. The Effect of Favourable Induced Constitution in Enabling Plant to
Shake oft" Result of Toxic Dose of Copper Sulphate . . . . 487
202. Opposite Eftects of Large and Small Doses of Poison . . . 488
203. Diagrammatic Representation showing Difierential Eft'ect of Weight
on Lateral Walls of Cells 494
204. Diagrammatic Representation of a Multicellular Organ . . . 495
PLANT RESPONSE
FIG.
PAGE
205. Diagrammatic Representation of Experiment showing Curvature
Induced by Unilateral Pressure Exerted by Particles . . . 497
206. Record of Responsive Curvature induced in Crimim under Experi-
mental Conditions shown in fig. 205 498
207. Record of Apogeotropic Response in Scape of Uriclis Lily . . . 500
208. Response Records showing Differences in Rate of Curvature
according as Specimen is held at Angles of 45° and 135° . . 501
209. Diagrammatic Representation of Different Positions of a Single
Cell, according as the Specimen is held at an Angle of 45°
or 135°, showing Consequent Redistribution of Statoliths (after
F. Darwin) ........... 502
210. Effect on Apogeotropic Movement of Application of Ice-cold Water
to Upper and Lower Surfaces alternately of a Horizontally laid
Crinuvir Lily .......... 504
211. Effect on Apogeotropic Movement of Temporary Applications of
Cold alternately to Upper and Lower Surfaces of Horizontally
laid Scape of Uriclis Lily ........ 505
212. Experimental Connections for obtaining Electrical Response due to
Direct and Indirect Effects of Stimulation 515
213. Record of Positive Electrical Variation, indicating Positive Turgidity-
Variation (represented by Down Curve), induceci in Growing
Region by Moderate Stimulation on same side of Tip . . . 517
214. Record showing Galvanometric Positivity subsequently Neutralised
under Transmission of True Excitatory Effect, due to Continuance
of Mo.derate Stimulation of the Tip 517
215. Record showing Neutralisation and Reversal of Electrical Response
at Responding Region, under Strong Stimulation of Tip . . . 518
216. Record showing Negative Electrical Response represented by Up
Curve, indicating Negative Turgidity-Varialion due to Direct
Stimulation . , . . . . . , . .518
217. Record showing Positive Electrical Variation indicating Positive
Turgidity-Variation of Distal Point, under Moderate Stimulation of
Proximal 519
218. The Relative Electrical and Turgidity Variations of two Diametri-
ally Opposite Points when Strong and Long-continued Stimulus
is applied 521
219. Mechanical Responses of Shoot and of Root to Unilateral Stimulus
applied at the Tip 526
220. Mechanical Response of Root of Bindweed to very strong Unilateral
Stimulation applied at the Tip 527
221. Mechanical Responses of Peduncle of Crocus and Root of Bind-
weed to Unilateral Thermal Stimulation at the Growing Region . 527
222. Diagram showing the various Responsive Effects induced at the
Growing Region . ......... 529
223. Experimental Arrangement for obtaining Records on Smoked
Drum of Responses given to Direct and Indirect Stimulation by
'L^&i oi Mimosa 530
224. Mechanical Responses of Leaf of i1//wt?j-a .... 531
ILLUSTRATIONS XXXvU
KIG. PAGE
225. Erectile Response of Leaf of Mimosa due to Transmission of Indirect
Effect to Distal Side, when Proximal is Stimulated . ... 532
226. Curious Response of Ariscema ....... 533
227. Curves showing Effect of Amputation on Rate of Growth and
Response in Root of Bindweed 541
228. Response of Leaf of Mimosa in Favourable Tonic Condition to
Chemical Stimulus of 3 per Cent. Salt Solution .... 551
229. Response of Leaf of Mimosa in Ordinary Tonic Condition to the
Chemical Stimulus of 10 per Cent. Solution of Salt . . . . 552
230. Polar Effects of Currents due to Localised Application on Upper
WOii oiVvXvi'nyi's, ol Erythrina iitdica ...... 555
231. Effects of Anode and Kathode on Variation of Rate of Growth in
Root of Bindweed exhibited by Balanced Growth-record . . . 557
232. Responsive Curvature in Scape of Crinitm Lily by Unilateral
Application of Anode and Kathode 558
233. Effect in Acceleration of Rate of Growth of Seedling of Oryza sativa
of Current through the Soil ........ 560
234. Longitudinal Contraction and Retardation of Growth under Light in
Hypocotyl of Sinapis nigra ........ 572
235. Balanced Record of Variation of Growth in Flower-bud of Crinum
Lily under Diffuse Stimulation of Light . . . . . . 574
236. Oscillatory Response of Arsenic acted on Continuously by Hertzian
Radiation 575
237. Multiple Response to Light of Terminal Leaflet of Desmodiutn . . 584
238. Response of Terminal Leaflet of Desmodium to Strong Light from
Above 586
239. Response of Terminal Leaflet of Desmodium to Sunlight acting from
Below 587
240. Diagrammatic Representation oi the Magnetically Controlled
Recorder 389
241. Heliotropic Chamber and Magnetically Controlled Recorder . . 591
242. Heliotropic Response of Sinapis 593
243. Heliotropic Response of Sinapis to Sunlight 593
244. Microscope Recorder 599
245. Record of Response of Root of 5ma//j- «?^/-fl 601
246. Positive Heliotropic Movement of Terminal Leaflet of Desmodium
Converted by Strong and too Long-continued Stimulus of Light
into Oscillatory Movement ........ 604
247. Response of Hypocotyl of Sinapis nigra 609
248. Responses to Successive Uniform Stimuli of Thermal Radiation in
Pistil of yy?) Senile Human Pulse
(Broadbent)
The heart-record, however, has been still more directly
obtained, in the case of the lower animals, by attaching one
end of the lever to the apex of the heart itself Each con-
traction and subsequent recovery is now recorded, in the
manner which has been indicated. If we know the rate at
which the recording surface is travelling, or if we make time-
marks at regular intervals, we are able to determine the
frequency of pulsation. The record also gives us the amplitude
of each pulse.
If now these records are to furnish reliable indications
of the internal condition of the living machine, then, any
circumstance which affects this internal condition must reveal
itself in the external record. And this is found to be
the case. For example, the effects of age are seen in
the accompanying record (fig. i) ; and that of poison by
PLANT RESPONSE
the gradual waning of pulsation, culminating in arrest at
the moment of death (fig. 2). Similarly, changes of heat
and cold, and the influence of
various drugs (fig. 3), are all
discernible from the modifica-
tions which they induce in
the pulse-record.
For the purpose of studying
the actions by which the plant
responds to the various stimuli
of its environment, I have been
able to devise apparatus, by
means of which records of its
responsive pulsations may be
made. In the matter of automatic pulsations, we have in
plants many instances which have not hitherto been recog-
nised ; but in one case which is well known, that of Dcsmodiuvi
gyrans — Hedysarum gyrans, the telegraph-plant — we observe
pulsatory movements of its lateral leaflets, which, as I shall
Fig. 2. Effect of Muscarin in antst-
ing Pulsation of Frog's Ventricle
(Gushing)
The arrow indicates the moment of
application of reagent in this and
following.
Fig. 3. Record of Human Pulse
{a) before and (Zi) after Inhalation of Nitrite of Amyl. (Broadbent.)
elsewhere show, exhibit a resemblance to those of the animal
heart, a resemblance which is not merely superficial, but is
the result of causes fundamentally the same.
This telegraph-plant grows wild on the Gangetic plain,
where its Indian name is Bon Charal or 'outcast of the
forests,' and where the peasant belief is that it dances to the
clapping of the hand. It is a papilionaceous plant with
trifoliate leaves, of which the terminal leaflet is large, and the
two lateral very small. Each of the latter is inserted on the
petiole by means of a motile organ known as a pulvinus.
THE PLANT AS A MACHINE 5
These lateral leaflets, when in normal condition, go on con-
tinuously, and apparently spontaneously, executing approxi-
mately up and down movements, each of which takes from
two to four minutes to complete.
The great difficulty in recording the j)ulsatory movements
of Desmodiuui arises from the extreme slenderncss of these
lateral leaflets. This is such that in attaching to them a
recording lever, however light, its weight, and the friction of
the writing-point, are sufficient to bring their movements to
a stop. I have, however, succeeded in overcoming this
difficulty by devising a recording Optical Lever.
This lever consists of a very light aluminium wire, or,
which is still better, the stripped quill of a peacock's tail
feather, this being extremely light, and sufficiently rigid for
the purpose. The two arms of the lever are unequal.
The fulcrum rod rests on frictionless supports of glass or
agate. The same rod carries a light mirror. A thread of
cocoon silk is stuck to the motile leaflet by a minute drop
of shellac varnish. The far end of the thread is looped,
and fixed at any suitable notch on the arm B of the lever.
The other arm of the lever has a light sliding counterpoise.
It will thus be seen that by gradually shifting the silk loop
nearer the fulcrum, the magnification may be increased.
When the automatically moving leaflet executes a downward
movement, the arm B is pulled down, and there is a rotation
of the fulcrum rod with its attached mirror. A spot of light
reflected from the mirror is thus suddenly moved downwards
from its original position. It will be observed that by
moving the recording surface further away, the magnification
may be still more enhanced. A wide latitude of magnifica-
tion may thus be obtained, by changes in the effective length of
arm of the lever, and by variation of the distance of the record-
ing surface. Thus, for example, for purposes of demonstration,
with a screen at a distance of five metres, it is easy to exhibit
a pulsatory movement magnified to as much as one metre in
amplitude. But in the case of the illustrations in the present
book, it has not been found necessary to have any magnifica-
6 PLANT RESPONSE
tion whatever, since the movement of the leaflet is itself
considerable.
A very light counterpoise is used, as will be seen later, to
exert a slight pull on the leaflet in an upward direction when
necessary, the sliding arrangement enabling us to vary the
amount of this tension. It will thus be seen that the leaflet is
practically free from constraint, and any movement, however
slight, is easily detected. When the leaflet falls, the spot of
light moves, say downwards, and vice versa. The record of
the entire response — down movement followed by up — may
thus be made on a vertical revolving drum, whose speed is
regulated by clock-work. The magnification of the record
having been determined previously, and the speed of the
drum being known, the response- curve gives the absolute
movement and the time-relations of such movement.
Instead of using a vertical drum, it is more convenient to
record on the revolving surface of a horizontal drum. The
up and down movement of the spot of light may now be
converted into lateral, or left and right movement, by means
of a second mirror suitably inclined. The finer adjustment
of the reflected spot of light may be brought about by means
of a milled head with which the second mirror is provided.
A photographic record may be obtained by wrapping over
the drum surface a sensitised roll-film. But since these
movements are comparatively slow, it is easy to obtain the
record more simply by following the spot of light Avith a
recording pen, which slides on a horizontal guide-bar, parallel
with its movements.
These response-records can be traced on a large scale in
the presence of an audience, by the use of the Demonstration
Recorder, which consists of a twin-drum, over which is wrapped
an endless band of paper to serve as the recording surface
(fig. 4). Elastic bands pass over the two drums, one of which
is kept revolving by clock-work. The excursion of the spot of
light is now followed by means of a sliding ink-well, from
which projects the ink-sponge. By this means, the tracing
of the response-curve, and its various modifications under the
TIIK TLANT AS A MACHINK
7
action of different influences, can be made visible to the
whole audience. It is thus possible to obtain records of these
pulsatory movements, by attaching; the Optical Lever to the
leaflet of an intact plant. Or wc may detach a petiole,
Fig. 4. Demonstration Optica) Pulse- Recorder
B, Arm of Optical Lever, attached to moving leaflet ; L, Ray of light, which
after two reflections from the two mirrors falls on the recorder ; c, Clock,
which keeps twin-drum— on which is wrapped the recording paper —
revolving ; n. Horizontal guide-bar ; K, Ink-well, with projecting
sponge.
carrying the leaf, and place it in water, in which case it
will remain alive as long as a couple of days, executing its
accustomed pulsatory movements during a considerable time.
The effect of any given agency, say poison, on the living
PLANT RESPONSE
machinery may now be observed, as graphically indicated in
the waning and final arrest of the pulse-record (fig. 5). Or
the plant may be killed by passing through it excessively
strong electric shocks, after which the occurrence of death
will be indicated by the arrest of pulsation (fig. 6).
Thus we see not only the similarity between the pulsations
of Desmodiniii and those of cardiac muscle, but also how
similarly both are affected by external agencies, such as poison.
Later, we shall study the effects of other physiological in-
A/WU
0
10' 20' 30' 40
Fig.
5. Death of Plant, and Arrest of
Pulsation, by Poison
Fig. 6. Death, and Arrest oi
Pulsation, in Leaflet oiDesmo-
diuin by Strong Electric Shock
fluences on both. In the present chapter, however, it has been
my aim to show that these pulse-records give us a reliable
indication of the very obscure modifications of the life-processes
initiated in the living tissues by various external factors.
Speaking generally, we may say that an exciting reagent
exalts the pulse, a depressing reagent reduces the amplitude
of pulsation, and a poison arrests it permanently, this arrest
being death.
In the cases which we have chosen as examples, there is
the advantage of a store of latent energy, which maintains the
pulsation by providing an internal source of stimulus. This
internal stimulation, as will be shown later, is really derived
from external sources, the absorbed energy having been
held latent in the plant. We shall in the next chapter take
up a very much simpler case, in which the plant has no such
reserve, but responds immediately to external stimulus.
THE PLANT AS A MACHINE
Summary
A plant, like a machine, responds either to the impact of
external forces, or to energy that is latent within.
As the working efficiency of an engine is exhibited Uy
indicator-diagrams, so the physiological efficiency of a living
machine may be inferred from the character of its pulse-
records.
Agencies which depress the physiological condition of a
tissue, also depress its responsive pulsation. At the death
of a tissue there is a permanent arrest of pulsation.
CHAPTER II
MECHANICAL RESPONSE TO STIMULUS
Molecular derangement caused by stimulus — Expression in change of form,
contraction — Mechanical model — Myograph— Response by differential con-
traction in pulvinaled plant-organs— Longitudinal response in plants —
Response of plant to all forms of stimulus — Plant chamber — Practicable forms
of graduated stimulus — Electro-thermic stimulator —Stimulation by condenser
discharge — Response-recorder — Advantage of counterpoise — Response of
Biophytiim to thermal stimulation — Response to condenser discharge — Absolute
measurements of motile effect and of work performed — Effect of load— Definite
determination of threshold of response — Determination of variation of excita-
bility by measurement of minimally effective stimulus.
Few of the phenomena of plant-life are so striking as the
conspicuous mechanical movements of certain plants, like
Mimosa, commonly known as ' sensitive ' in contradistinction
to ' ordinary ' plants. These movements take place in
response to various forms of stimulation, such as is caused by
mechanical touch or application of heat. It will be shown,
however, in the course of the present book that this division
of plants into sensitive and ordinary is arbitrary, since all
plants are sensitive — that is to say, react to stimulus. The
plant, throughout its life, is constantly responding to stimuli,
external and internal. Some of its responses are manifested
in mechanical movements which are too striking to be over-
looked. Others, not so obvious, have passed hitherto un-
noticed. But in both these cases changes of form occur in
the tissue, in consequence of stimulation. In some instances,
owing to conditions which will be explained later, these
changes produce little visible effect. In others, the responsive
change of form is displayed in a striking manner, owing to
certain advantageous circumstances of structure, and to the
possession of a magnifying arrangement.
MECHANICAL RKSPONSE TO STIMULUS II
The shock of stimulus causes molecular derangement in
the tissue of the plant, and it is this fundamental mole-
cular change that finds expression in mechanical movement.
It finds independent expression also in electrical move-
ment. For the conspicuous display of mechanical response
certain peculiar structural arrangements are, as has been
said, advantageous ; but for the exhibition of electrical
response, the molecular change itself, which is concomitant
to excitation, is the only condition. This subject of the
electrical response of plants, however, I treat in detail else-
where.' For the present we are concerned only with the
question of mechanical response to stimulus. We have not
only to determine the existence of such response, but also to
ascertain under what conditions it occurs, and by what means
it is brought about.
The whole sequence of molecular events initiated by
stimulus and expressed as mechanical response, may be very
simply illustrated by means of an india-rubber model. We
take a piece of stretched india-rubber, attached to a recording
lever. The rubber is enclosed in a tube in which there is also
enclosed a spiral of thin German-silver wire, by which the india-
rubber may be subjected to the momentary action of heat.
The quantity of heat generated is regulated by the strength
and duration of an electrical current flowing through the
heating wire. This application may be uniform for successive
experiments, or increased at will.
Longitudinal response. — The thermal stimulus causes a
molecular rearrangement in the substance of the india-rubber,
in consequence of which the piece becomes shorter and
broader. This sudden longitudinal shortening is recorded by
the lever as the first half of the responsive movement. As
the substance gradually recovers from the effect of the
momentary stimulation, the molecules return to their normal
position, with a concomitant restoration of the india-rubber to
its original form. During this second half of the process, we
' Bose, Response in the Living and Non- Living (Messrs. Longmans, Green
&Co.). V,Q?,Q, Electro-Physiology of Plants.
PLANT RESPONSE
obtain the curve of recovery.' If we apply similar stimuli
buccessively, we obtain successive responses which are alike
(fig. 7). But if stronger stimulus be applied, by means of
stronger heating current, the amplitude of response will be
correspondingly increased.
In the simple instance which we have considered, the
response-record was obtained by taking advantage of the
sudden contraction of the india-
rubber. In the response of con-
tractile animal muscle, we obtain
response-records in exactly the
same manner (fig. 8), and such
records are known as myographs
(fig- 9).
Similar contraction in length, or
LONGITUDINAL RESPONSE under
the action of stimulus, has been
shown by( Pfeffer to occur in the
Fig. 8. Mechanical Lever
Recorder
The muscle M with the attached
bone is securely held at one
end, the other end being con-
nected with the writing lever.
Under the action of stimulus
the contracting muscle pulls
the lever, and moves the
tracing point to the right
over the travelling recording
surface P. Wlien the muscle
recovers from contraction the
tracing point returns to its
original position. See on p
the record of muscle-curve.
Fig. 7. Response of India-rubber
Thermal stimulus for i second at
intervals of two minutes.
filament of the sensitive stamens of Cynerece. I shall, how-
ever, show in Chapter IV. that such longitudinal contraction
under stimulus is not unique, but a phenomenon very exten-
sively exhibited by plant-tissues, as seen in the series of
uniform responses to stimulation, obtained from the stamen
of an ordinary plant, which is here given (fig. 10).
' Such models made of catgut and stretched caoutchouc have been used by
Engelmann for explaining muscle response.
MECHANICAL RESPONSE TO STIMULUS
13
Differential response. — But the responsive movement
in plants is more generally produced by differential contractile
movement, and a mechanical model again will clearly show
how such movements are brought about. We take two
equal strips of unequally contracting
substances, which are glued together
throughout their length. The two
strips consist of ebonite and the
relatively more contractile caoutchouc.
Fig. 9. Series of Contractile
Responses in Muscle
Fu;. 10. I'hotographic
Record of Longitudinal
Contractile Response
in ordinary Stamens
(Bro7C'/i/a ariza)
If such a compound strip be held horizontally, with the
more contractile element below, and if we subject it to
thermal stimulation in the manner described above, the
result will be a responsive curvature do\vnv\'ards, the more
contractile caoutchouc forming the concave surface. Thermal
Fig. II. Differential Lateral Response of Compound Strip
Thermal stinuili applied at intervals of three minutes.
stimulus may be applied, as in the last case, by sending a
momentary heating current through an enclosing spiral of
German-silver wire, the responses being recorded in the usual
manner (fig. 1 1).
In such cases, where the upper and lower elements are
14 PLANT RESPONSE
unequally contractile, we obtain a DIFFERENTIAL RESPONSE,
the more contractile becoming concave ; and it is evident
that such movements must take place in a direction perpen-
dicular to the plane of separation.
Typical cases of mechanical response in plants are
obtained from pulvinated organs. A good example of this
is found at the insertion of the petiole in Mimosa pudica.
When such an organ is stimulated, it is the lower half that
undergoes the greater contraction, and the leaf is depressed
by the concavity thus produced. It is generally assumed
that the upper half of the pulvinus is not excitable, but this,
as I shall show later, is an error. The responsive movement,
however, is due to the differential contraction of the two
halves, and, as already explained, takes place in a direction
perpendicular to the plane which separates them. Such
differential response will be found characteristic of all organs
possessing dorsi-ventral differentiation.
Whenever the plant is subjected to any sudden dis-
turbance, the sensitive leaf reacts by a fall, which is brought
about by the hinge-like mechanism at the pulvinus. The
sudden disturbance which induces the fall constitutes the
stimulus. The leaf responds when it is shaken, or cut, or
when a prick is applied to it, or when a sudden variation of
temperature is produced, as by touching it with a hot wire,
or with ice, or when an electrical shock is passed through it,
or if it be acted on by certain chemical reagents, or a beam
of strong light be thrown on it. i\ll these constitute the
various forms of stimuli — mechanical, thermal, electrical,
chemical, and photic.
We have next to study the relation between the intensity
of the stimulus and the extent of response under varying
conditions ; that is to say, we have to determine the
' threshold of response,' in other words, the minimum intensity
of stimulus that will be just sufficient to initiate reaction. We
have then to observe the repeated response of the plant to
repeated stimulation, whether uniform or gradually increasing.
We have to detect the signs of fatigue if there be any, and
MECHANICAL RESPONSE TO STIMULUS I 5
discover after what period of rest this disappears. We have
also to record the exact time-relations of these phenomena.
And further we have to study the effects of various external
agencies in modifying the response.
In order to carry out these investigations, it will be
necessary first to arrange for placing the plant under suitable
conditions for experiment. The next point is the devising of
facilities for applying a stimulus of known intensity, which
can be repeated, or increased by definite amounts, at will.
And, lastly, there must be some means of obtaining an exact
record of the response, from which the absolute movement
of the responding organ and its time-relations may be
deduced.
Experimental plant chamber. — As regards the first of
these, it is advisable to have a special plant chamber within
which the specimen can be subjected to the necessary con-
ditions. This chamber may consist of a base-board and a
movable cover. The framework of the latter is of wood,
with glass panes. In order to give easy access to the plant
during experiment, one side of the cover has a hinged
window. The recording Optical Lever is placed inside the
chamber, and the glass cover protects the recorder from any
accidental disturbance caused by air-currents.
In connection with this, it is also important to provide
arrangements for producing changes of temperature, and
maintaining the changed condition uniform, for the required
length of time. This is most satisfactorily accomplished by
means of a heating coil placed inside the chamber, the
temperature being regulated by suitable adjustment of the
electrical current, sent into the coil through proper electrodes.
Other necessary accessories consist of appliances for the
purpose of stimulation, and facilities by which a constant
current can be made to flow through the tissue, in experiments
on the effect of electric currents on the excitability of plants.
Details regarding these will be given later. The plant may
be maintained in favourable humid conditions by placing
wet blotting-paper inside the chamber (fig. 12).
i6
PLANT RESPONSE
The most important question with regard to the apph'cation
of suitable stimulus is, as has been said, that it should be
capable of exact measurement, of uniform repetition, and of
definite increase or decrease at will. Another point which
must be borne in mind is that the application of stimulus
should not, by causing injury, change the excitability of the
organ. As, moreover, a magnified record of the responsive
movement is to be made immediately after the application,
any stimulus which causes the slightest jar must necessarily
4
t
J
w
'.■^:s'^s^ym,i-;-^''^im€Ji£.'»s^':HM!i^i;s^n:?s[2a,ij:s2u.-
Fig. 12. Plant Chamber and Recorder
The glass cover is not shown.
be avoided. And for these reasons the mechanical form
of stimulation is inappropriate to the investigation. The
three most perfect modes of stimulation which I have
been able to render practicable are, then, the thermal, the
electrical, and the stimulus of light. The action of the last
will be described in detail in another chapter, and we shall
for the present confine our attention to the first two.
Electro-thermic stimulator. — Thermal stimulus may be
applied very easily by touching the plant with a hot wire, but
MECHANICAL RESPONSK TO STIMULUS 1/
it is difficult by this means to ensure the uniformity of
successive stimuli, inasmuch as the wire cannot be heated
repeatedly to the same temperature, or made to touch the
same point, many times in succession, with an equally effec-
tive contact. This difficulty is removed by means of what
I have named the ELECTRO-THERMIC STIMULATOR. This
consists of a thin M-shaped wire of platinum, with thick
copper leads. It is slipped over the petiole which carries the
sensitive leaflets. By now sending through it a current of
definite intensity and duration, we can raise its temperature
to any point we wish, and thus secure the application of a
known intensity of stimulus at will. The elasticity due to
the peculiar form of the thermal stimulator gives a definite
and constant pressure
of contact (fig. 13).
The observer ap-
plies the stimulus with
his left hand, by press-
ing a tapping- key
which is interposed in
an electric circuit, for
, ^ .. , ^ ,. Fig. n. The Electro-thermic Stimulator
a definite short time- ^
With his right^^hand (,he records on the revolving drum the
exact moment of this application. This mode of thermic
stimulation is, as will be shown presently, very efficient.
Electric stimulation. — I have been able, however, to
employ a mode of stimulus still more perfect, that, namely, of
the electrical discharge from a condenser. Other forms of elec-
trical stimulation may be used, such as those given by means
of constant or induction currents. But these are liable, not
only to cause more or less permanent internal changes by
polarisation, but also to induce fatigue of the tissue. It will
be shown in a later chapter that, on making the circuit, excita-
tion takes place at the point where the current leaves the
tissue — that is to say, at the kathode — and not at the anode,
or point of entrance. By appropriate connections shown in
the diagram (fig. 14), the point to be excited can be made
c
I 8 PLANT RESPONSE
kathode during ' charge,' when the key is pressed. When the
key is released, the circuit is ' discharged,' and the current
flows in the opposite direction.
The given point r, is, as has been said, excited by being
made kathode at the moment of charge. The immediately
succeeding discharge produces no exciting effect, but it wipes
off any residual polarisation effect caused by charge. The
plant-tissue is thus maintained in as completely normal a
Fig. 14. Diagram of Connections for Stimulation by Condenser Discharge
Pressure of key K charges the condenser c through the plant. Release of key
brings it in contact with M, discharging the condenser through the plant.
L, responding leaflet attached to recording lever by thread s.
condition as possible. The excitation produced in the plant
by current to or from the condenser, I shall, for simplicity,
designate as ' stimulation by condenser discharge.'
In the course of the present chapter we shall study the
response of the leaf of Mimosa^ shown by its fall, and also
that of other sensitive plants, exhibited by the closure of
the leaflets, as in the case of BiopJiytuin scjisitiviivi. One
difficulty encountered in obtaining successive responses, in
these latter cases, was due to the fact that the responding
MECHANICAL RESPONSE TO STIMULUS 19
leaflets, after each downward response, would sometimes
remain persistently closed, for an indefinite period, thus pre-
venting the continuation of the experiment. In cases where
the leaflets are completely closed, one naturally regards the
position as one of fatigue, or complete insensitiveness,
because no further mechanical response is then obtainable.
This depressed position, however, may not be indicative of
total want of sensibility, for the apparent absence of response
may really be due to the fact that further closure of the
leaflets is a mechanical impossibility. We may consider an
analogous instance in the case of animal tissues, muscle
floating in mercury for example. The tissue remains per-
sistently contracted after a single stimulus, and further
response is impossible. But if, again, the muscle be stimu-
lated while under tension, it responds to each stimulation,
the process of recovery being aided by the external tension.
Practical importance of counterpoise. — Acting on
this idea, it appeared to me that if we applied an external
tension, the restoration of the leaflet to the natural outspread
position might be helped, and the difficulty solved of ob-
taining the uniform repetition of effects of successive stimuli
at brief and regular intervals of time. I therefore placed
a small sliding counterpoise on that arm of the lever which
was not attached to the leaflet. This was found to fulfil its
purpose. For in observing the effects of successive stimuli
on different leaflets, I found that while neighbouring leaflets,
not under tension, closed up after a few stimulations, and
gave no further response, the leaflet which was attached to
the lever, and which was under some slight tension, recovered
its normal outspread position in the course of three to five
minutes, and continued to respond in a normal manner to a
long series of successive stimuli.
We shall now proceed to observe the actual responses
obtained. The object here is not to investigate the peculiar
or specific reaction of any one sensitive plant in particular,
but the effects found universally among motile plant-organs.
The occurrence of such effects in plants exhibiting all
c 2
20 PLANT RESPONSE
degrees of mechanical sensibility — from those in which it
is shown in an extreme degree, to others again in which it is
apparently almost non-existent — will be demonstrated in this
and succeeding chapters.
In order to study the responsive movements of plants,
we may take either the leaflets or the main petiole of
Mimosa pudica. The leaflets, however, in this case are so
excessively sensitive that even the contact for experimental
adjustment is sufficient to produce a closure from which they
do not recover for a considerable time. The pulvinus of the
main petiole, on the other hand, is considerably less sensitive.
Of intermediate sensibility are the leaflets of BiopJiytinn
sensitivum, which on the whole furnish the most suitable
specimens for the general purposes of these experiments.
This plant, which is known to be sensitive, grows in a wild
state near Calcutta, and is so common as to be considered
a weed. It is a low-growing herb, with simply pinnate
leaves, each bearing from ten to sixteen pairs of leaflets.
A better specimen could hardly perhaps be found for the
exhibition of some of the most important characteristics of
mechanical response. It is not, under ordinary conditions,
excessively sensitive. A gentle touch does not, as a rule,
produce the closing effect, but under specially favourable
circumstances its sensitiveness may equal, if not surpass, that
of the Mimosa leaflets. The closing of the leaflets takes place
not upwards as in Mimosa, but in the downward direction.
I shall presently give details of the response obtained with
Biophytum. But as this plant is not universally obtainable,
and as it flourishes only for a short season, during and after
our tropical rains, it may be best first to give an account of
experiments made on the more generally accessible leaf of
Mimosa.
Response of Mimosa. — As the responsive movement of
the leaf of Mimosa is of considerable extent, no magnifica-
tion is necessary for the record. Indeed, on the contrary,
for the illustrations in the present work, the records had fre-
quently to be taken on a reduced scale. This was accomplished
MECHANICAL RESFONSE TO STIMULUS
21
by attaching the leaf to the long arm of the recording Optic
Lever, and shortening the distance of the recording surface.
The records given in figs. 15 and 16 were automatically
obtained by the impression of
the moving spot of light on a
sensitive film wrapped about
the recording drum. The leaf
was excited by a single strong
induction-shock. In order to
obtain the complete curve of
response and recovery — the
double process being accom-
plished in the course of about
seven minutes— the first record
was taken on a slowly moving
drum. P'or the detailed study of the characteristic time-
relations of the first part of the curve, again, two more
records were obtained, one with a moderate (fig. 15) and the
;. 15. Photographic Record of
Response and Recovery o{ Mimosa,
taken on a slowly moving drum.
Record shows actual movement
reduced to one-third.
Fig. 16. Photographic Record of Response in different specimen, taken
on a faster-moving drum showing only first part of the curve. Each
division of time-scale = -5 second.
other with a rapid speed of drum (fig. no). The last of
these enables us to obtain time-measurements which are
accurate to less than -\,- of a second. The method by
22 PLANT RESPONSE
which these rapid records are obtained will be described in
Chapter XXII.
From records obtained on a fast-moving drum, with a
fairly average specimen of Miviosa, it is found that the
leaf does not respond to stimulus immediately, there
being a latent period of ^y^ of a second before it begins
to move. The maximum fall is attained in the course of
2 seconds after the shock. After attaining the position
of maximum depression, the leaf remains in its contracted
position for a further period of about thirty seconds. It then
begins slowly to recover, and perfect recovery takes place in
the further course of six minutes. The record given was
obtained from the leaf of a plant which was one year old, and
in the summer season. It will be remembered that these
responsive curves are modified by the physiological condition
of the plant ; thus, for example, the time taken by the leaf of
a vigorous }'oung plant for recovery may be as short as four
minutes, whereas an older specimen in winter may require as
long a period as eighteen minutes. We may thus obtain
from the record an idea of the physiological condition of the
specimen.
By means of photography the taking of the record is made
extremely simple, but there arc certain disadvantages insepar-
able from this method, which render the devising of other
means essential. For example, the motile sensibility of plants
like Mimosa and Biophyium is profoundly modified in dark-
ness. In the case of the records given, the plants have been
kept outside in the light, and brought in immediately before
experiment. But even then, after remaining in the dark for
half an hour or so, the leaves of Mimosa become abnormally
erected, till it can hardly be believed that the plant is sensi-
tive, for it often becomes irresponsive to the hardest blow.
Biophyttim leaflets, again, in the same circumstances undergo
closure. For these reasons, long-continued experiments in
a dark room are an impossibility. Various sensitive plants,
again, flourish only for a short-lived season, and during that
period some hundreds of experiments have to be carried out.
MECHANICAL RESPONSE TO STIMULUS 23
This necessitates some method of record more expeditious
than that of photography.
Fortunately, the responsive movements of these sensitive
organs are relatively slow, usually requiring several minutes
for completion. And it is quite easy to follow the excursion
of the responding spot of light, with the recording pen, on a
horizontal drum. There are some few special investigations,
such as those on exceedingly short latent periods, in which
automatic records by photography are a necessity, but for the
majority of the records the second method is all that is
required. By the latter means, moreover, we overcome the
serious difficulty occasioned by the variation of sensibility
which the plant undergoes when kept long in a dark chamber.
When the second method is employed, the specimen may be
placed in a well-lighted and well-ventilated room, and under
these conditions it is found to maintain its sensitiveness
unchanged for a considerable length of time. The fact that
the spot of light reflected on the drum becomes inconspicuous
in the surrounding daylight is overcome by placing in front
of the recording drum a special hood with a long horizontal
slit. The back of this hood curves over the head of the
observer, and the spot of light then appears very bright.
Response of Biophytum. — I shall next deal with the
responses obtained from BiopJiytmn. In fig. 17 are given two
successive responses to two successive thermal stimuli. It
will be noticed how uniform these responses are. The up-
curve represents the fall of the leaflets, and the subsequent
down- curve of the response exhibits its gradual return to
the normal outspread horizontal position. An abnormal
erectile twitch will be noticed at the beginning of each of
these responses. This effect is usually present when a
stimulus of whatever nature is applied at a distance from
the responding leaflet. Its cause will be explained later.
It should be stated here that stimulus was in this case
applied at a distance of 35 mm. from the responsive leaflet,
and that the true excitatory reaction, by the depression of
the leaflet, took place fifteen seconds after the application. In
24 PLANT RESPONSE
other words, the excitation travelled the intervening distance
with a speed of 2-3 mm. per second. The abnormal erectile
Fig. 17. Response oi Biophytum to Thermal Stimulalion
Stimulus was applied at some distance from the responding leaflet.
Thick dot represents moment of application ol .stimulus.
Fig. 18. Response oi BiophytiDii to Electric Stimulation
Stimulus was applied directly on the pulvinus. Ordinate represents absolute
movement m mm.
effect, however — due, as will be explained later, to hydrostatic
disturbance — took place almost instantaneously.
fltOPERTr UBRASr
N. C State CoUt^
MECHANICAL RESrONSE TO STIMULUS 25
The next figure (fig. 18) gives successive responses of
Biophyiuni to condenser discharge, the pulvinus of the leaflet
being directly excited. It will be noticed that in this case
of direct stimulation, no abnormal erectile twitch is present.
From the magnification of the record the absolute value of
the movement is known, and in the present case it was r88
mm. The force exerted by the leaflet during its responsive
movement was found equivalent to that exerted by the
weight of 17 milligrammes. The total work performed by
the leaflet during each responsive movement is therefore
nearly equal to 1,600 millimetre-milligrams.
Effect of load. — In order to observe the effect of load
on the response-curve, I added a slight additional counter-
poise to the other arm of the
lever. The record (fig. 19, a)
shows the response-curve when
the acting load is the weight of
the lever ; (^) shows the effect
of the additional load. It will
be seen that while the height of
the responses was diminished, yet
the period of recovery was very
much reduced, from five minutes to ^^^- i9-. Efteci of Load
(a) without, and {b) with,
less than three, under the increased additional load
load.
Isometric record. — The method of observing response
employed in the foregoing results was that of recording the
movement of the leaf. Similar methods are known in Animal
Physiology as isotonic. There is, however, an interesting
method corresponding to that known in Animal Physiology
as the isometric — where, in obtaining records of responses,
actual movement is almost abolished. The contraction of
the more excitable half of the pulvinus exerts a certain
tension, or pull. The object, under the isometric method of
experiment, is to obtain records of varying responsive ten-
sions of excited tissues, the physical movement being at the
same time restrained. This I have been able to accomplish,
26
PLANT RESVONSE
in the case of plant response, by the construction of a spiral
spring-recorder, the movement of whose index is approxi-
mately proportional to the tension.
J The recorder is constructed of a fine
*••' ■ — *? flattened spiral spring. Springs of
this description have the peculiarity
that, when they are stretched by
tension, the free end of the spiral
rotates round the axis of the
spring. A slight rotation may be
magnified by means of a mirror
and reflected spot of light (fig. 20).
This arrangement is specially
appropriate to the leaf-stalk of
Mimosa, where the pull exerted
by the excited leaf is consider-
able. Fig. 21 gives the isometric
response of Mimosa.
Minimally effective stimulus
in Biophytum.— It is well to
mention here that at least in the case of Biophytum
the minimal intensity of stimulus necessary to cause response
is very well defined. With
a certain specimen for
example, when the plant
was excited by the dis-
charge from a •01 micro-
farad condenser, charged
to seven volts, there was
no response. But when
the condenser was charged
to nine volts the discharge
always produced a large
and definite response.
Charging of the condenser to nine and seven volts alternately
would in the one case produce response, and in the other
none. If now, by the action of an external agency, the
The Spiral Spiing-
Kecoifler
The leaf is practically prevented
from moving. The tension
exerted by excited leaf causes
rotation of index or mirror.
Fig.
Isometric Response of Mimosa
MECHANICAL RESPONSE TO STIMULUS 27
excitability of the tissue be increased, the seven-volt con-
denser charge, before inadequate, will become adequate.
Conversely, if by the action of an external agent the excita-
bility of the tissue be depressed, the nine-volt charge, which
was formerly effective, will become now ineffective, I find,
for instance, that lowering of the temperature will, by in-
creasing molecular sluggishness, reduce excitability. Hence
a minimally effective stimulus becomes ineffective when
the tissue is cooled. Conversely, a rise of temperature
produces the reverse effect, namely, increase of excitability.
This was seen in a particular experiment with Biophytujii,
where the minimally effective stimulus necessary at 30° C.
was found to be reduced to two-thirds when the temperature
was raised to 35° C.
Having thus obtained a reliable stimulus, whose value may
be measured with precision, and which is capable of being
repeated, and having also discovered an arrangement by which
the effect of a given stimulus is invariably exhibited by a
uniform response-record, we are now in a position to attack
various physiological problems, as regards the influence of
given external agencies on the conductivity and excitability
of the plant-tissue.
Summary
Longitudinal responses are given by radial organs.
A differential response, causing lateral movement, is
given by an organ in which the excitability of one half is
different from that of the other ; and the movement takes
place in a direction perpendicular to the plane of separation
of the two halves. Such responses are characteristic of dorsi-
vcntral plant-organs.
In the responses of the sensitive organs of plants we
notice : a short latent period ; a period during which the
excitatory movement attains its maximum ; and a period of
slow recovery.
When the stimulus is applied at a distance, a preliminary
abnormal erectile twitch is occasionally observed, which is
28 PLANT RESPONSE
due to hydrostatic disturbance. The true excitatory response
takes place later.
Besides the isotonic response, obtained by recording the
actual movements of the excited leaf, it is also possible
to record the isometric response where the movement is
restrained, and the variation of tension caused by the con-
traction of tissue is alone recorded.
The intensity of a minimally effective stimulus in the
case of Biophytum is definite. This value undergoes appro-
priate variation with the variation of excitability of the
organ.
CHAPTER III
ON THE UNIVERSALITY OF SENSITIVENESS IN PLANTS
AS DEMONSTRATED BY MEANS OF ELECTRICAL RESPONSE
Arbitrary classification of plants into sensitive and ordinary — Method of electro-
motive variation for detecting state of exxitation — Hydraulic model— Exxita-
tion of vegetable tissue, like that of animal tissue, induces galvanometric
negativity — Methods of direct and transmitted excitation — Electrical and
mechanical response alike record molecular derangement and recovery —
Similarities in simultaneous record of mechanical and electrical response —
True excitation has a concomitant negative turgidity-variation, negative
mechanical response or fall, and galvanometric negativity — These are true
physiological responses, and are abolished at death — Abnormal positive
mechanical and electrical responses brought about by positive turgidity-
variation — Direct and indirect effects of stimulation— Discrimination of differ-
ences of excitability by electric test^Excitability of plant-tissues in general^
Responsive power characteristic of matter.
VVe have seen that when stimulus is applied to a sensitive
organ like the pulvinus of Mimosa there is a fall of the leaf,
which fall is due to the excitatory contraction of the more
excitable lower half of the pulvinus.
Ordinary plants are said to give no motile indications,
hence they are usually regarded as insensitive.^ It is difficult,
however, to conceive that while the protoplasm of certain
plants reacts to stimulus, that of others should not do so. On
the other hand, it may be that the absence of mechanical
response in these ordinary plants is not due to any want of
excitability, but rather to the fact that conditions favourable
to the conspicuous exhibition of motile effects do not in
' Vines has already drawn attention to the possibility of error here : We
must be careful not to assume that irritability is restricted to growing and to
motile organs. For all we know to the contrary, it is possessed by the proto-
plasm of all plant organs, and if in any case the action of a stimulus is not
followed by a responsive movement, we must, before we assume the absence of
irritability, assure ourselves that the structure of the organ is such that a move-
ment is a mechanical possibility.' — Vines, Physiology of Plants, 1886, p. 372.
30 PLANT RESPONSE .
such cases exist. What these conditions are will be detailed
in the next chapter, where it will also be shown that excita-
tion of an organ may take place, even where there is little
mechanical indication of the fact, owing to antagonistic and
balanced contractions.
Electrical response. — It is my intention, in the course of
the present work, to offer a complete demonstration of all the
phenomena of excitation in plants, by means of mechanical
response alone. But the conclusions to which we shall be led
by the study of this response will receive irrefragable support,
if they can also be established independently by some mode
of investigation altogether different. Such a mode of in-
quiry, namely the electrical, and the conclusions to which it
leads, will be fully described in the companion volume to this
work, on the Electro-Physiology of Plants. Meanwhile it is
convenient in this place to enter upon a short elucidation of
the principle of that method, in order that we may be able,
while considering the results of mechanical response, to make
casual references to confirmatory results of independent ob-
servations obtained by means of the electrical method.
It has been said that under the action of stimulus excited
cells undergo contraction, and that owing to the consequent
expulsion of water, the turgidity of the tissue is diminished.
Thus one expression of the molecular change induced by
stimulus is a negative variation of turgidity. But this
molecular change may also be detected by means of other
concomitant physical changes. For instance, the electrical
level or potential of a given point may, owing to the ex-
citatory molecular change, undergo variation, relatively to
another point which is unexcited. A hydraulic model will
serve to make this point clear (fig. 22).
Let us imagine a flexible pipe of india-rubber, with bent
ends of glass-tube, filled with water, and held in the middle
by a clamp c. It is also supported in the stable horizontal
position by spiral springs. If a single blow, say upwards,
be now given to the end A, the level of the pipe at that end
will be raised, and there will be a resultant flow of water from
UNIVERSALITY OF SENSITIVENESS IN PLANTS
31
A to B, or away from the struck end. The intensity of the
current is determined by the height to which the struck end A
has been raised, and this again depends on the intensity of
the blow. Hence the intensity of the current is a measure
of the intensity of the stimulus or disturbance. The flow
subsides with the return of the pipe to its equilibrium posi-
tion. If the pipe had been disturbed throughout, the level
would have been raised equally at both ends, and there would
have been no flow. The object of the clamp is, therefore, to
confine the disturbance to one side. If the blow had been
Fig. 22. Hydraulic Model for Explanation of Electric Response
When an upstroke is given to A, a responsive current flows from A to B,
and vice versa.
given on the B side, the direction of the responsive flow would
have been reversed.
The principle of electromotive response in plants is
exactly similar to this. The plant tissue is clamped at
c (fig. 23), and a stimulus is given at one end, say A. The
electrical level of that side is now found to be raised, it
becomes electro-positive, or like the copper in a voltaic com-
bination. The responsive current thus flows in the tissue
from A to 15, or away from the excited point. In the
external circuit containing the galvanometer, it flows, of
32
PLANT RESPONSE
course, in the opposite direction, that is from B to A.' The
excited point A is thus electro-positive, but in physiological
text-books it has been ambiguously termed negative. In
order therefore to keep touch with the older terminology and
yet avoid the implied error, I shall refer to the excited point
as ^ galvanometrically negative.'
If the B end of the specimen be now excited, the direction
of the responsive current will be reversed. With greater
intensity of stimulus the electrical response will be found
The record of such responses is
obtained on a revolving drum.
correspondingly increased
{a) (/')
by following the deflection of
a spot of light reflected from
the galvanometer mirror in a
manner precisely similar to
that employed with the Optic
Lever (fig. 24).
Method of transmitted
stimulation.— In the simple
23. Electrical Response in Plant case just described, the tissue
was stimulated directly and
locally at the end A, say by
Current of response %vlicn
A is stimulated ->
Current of response ivhen
B is stimulated <-
Fig.
by Method of Block
[a) The plant is clamped at c, between
A and B.
{h) Responses obtained by alternately torsional vibration. There is,
stnnulating the two ends, btnviula- '
tion of A produces upward response, howevcr, another method of
and that of B downward. transmitted stimulation, by
which a stimulus, — say by application of cut, or hot wire,
or electrical shock, — is given at a point X some distance
away, say to the right. The excitation now travels with a
velocity characteristic of the specimen, and when it reaches
the proximal electrode produces galvanometric negativity
of that point. The interval of time which elapses between
the application of stimulus and response will therefore depend
on the velocity of transmission and the distance of the point
' The failure to understand this point cleivrly has been the source of many
grave errors in some physiological text books. From the fact that the current in
the external circuit is seen to flow in the direction of A, it has been erroneously
supposed that that point is negative, or zinc-like. See Bose, Response in the
Living and Non- Living.
Universality of sensitiveness in plants
',3
of application. It will also be remembered that the state of
excitation is attended by an expulsion of water, or negative
turgidity-variation. After causing galvanometric negativity
of the proximal contact, the excitation may reach the distal,
and bring about reversal of response, thus constituting a
Fig. 24. Electric Response Recorder
diphasic variation. If, however, the distal point be very far,
the excitation may by transmission through the long tract
become so enfeebled as to produce practically no effect at
Method of Transmitted Stimulation
Stimulus applied to the right at x . Excitation reaches right contact first,
causing galvanometric negativity of the point.
that point, in which case we obtain only the monophasic
response of the proximal point.
Simultaneous mechanical and electrical events,
ensuing on excitation. — We may prove that these electrical
responses are undoubtedly signs of excitation, by choosing
34 PLANT RESPONSE
for the electrical experiment a plant in which the state of
excitation is independently manifested by mechanical response.
If now these electrical and mechanical responses be indeed
only two different expressions of the same thing — that is
to say, of a molecular disturbance and recovery which is
concomitant to excitation and recovery from excitation —
then we should expect that, on taking a simultaneous record,
the two responses would be shown to be initiated at the same
moment, and to bear some general resemblance to each other.
In the following record it will be seen that this is found to be
the case (fig. 26).
To recapitulate : let us take the concrete example of the
Mimosa leaf When the pulvinus is excited, owing to the
Fi(5. 26. Simultaneous Mechanical (m) and Electrical (e) Responses in Biophylmn
These responses are seen to take place at the same moment.
molecular change induced by stimulus, there is an expul-
sion of water, or negative turgidity-variation, and in the
absence of restraint this is attended by the normal negative
mechanical response, or fall of the leaf If, now, electrical
connections have been made, one with the pulvinus, and the
other with a distant point on the stem, it will be found that
the excitatory change is attended by a strictly concomitant
electrical change, the current of response flowing away from
the excited point, which in other words becomes galvano-
metrically negative.
. All these events will perhaps be more easily realised if
we remember that excitation, in the typical case of Mimosa
UNIVERSALITY OF SENSITIVENESS IN PLANTS 35
gives rise simultaneously to (a) contraction of the cells, with
concomitant negative turgidity-variation ; (/;) negative me-
chanical response, or fall of the leaf; and (c) galvanometric
negative variation. Had the leaf been physically restrained
by any means, the mechanical response would have been
prevented, although the negative turgidity-variation con-
comitant to excitation would have taken place just the same.
But this internal change would have been imperceptible, and
in that case we could still have detected the effect of excita-
tion by means of the electromotive response. As a matter
of fact it is found that the electrical response always takes
place in answer to effective stimulation, even in cases where
the mechanical response is rendered impossible. We thus see
that galvanometric negativity is a certain indication of the
excitatory contraction of a cell, whether or not the effect of
such contraction be outwardly manifested by mechanical
movement. The detection of the state of excitation by the
electric test is thus unfailing, and of universal application.
By the employment of this electric mode of investigation, I
have shown that not sensitive plants alone, but every plant,
and also every organ of every plant, is excitable.
True excitatory negative versus hydrostatic positive
variation. — It has been supposed that the galvanometric
negativity consequent on stimulation maybe due to mechani-
cal movement of water in the tissue. But I have shown that
this cannot be the case. For while it is true that the pro-
duction of water-movement by sudden forcing of water into
a tissue does cause electrical variation, yet it must be noted
that the sign of this electrical change is always one of
galvanometric positivity, which is opposite to that of the
true excitatory response. The intensity of the true negative
electrical response, moreover, varies with the physiological
activity of the tissue, and is abolished with its death. The
electrical variation due to mere water-movement, however,
may take place even in a dead tissue, and is, as has been
said, of positive sign.
If a piece of living tissue be subjected to direct stimula-
D 2
36 PLANT RESPONSE
tion — that is to say, be locally disturbed, say by torsional
vibration — two effects will be produced : first, the negative
turgidity-variation, which is the true excitatory effect, with
its attendant negative electrical variation ; and, second, the
electrical effect due to hydrostatic disturbance or water-
movement, which is positive. Of these two opposed electrical
effects, the first, or true excitatory variation, is generally
speaking much the stronger. It therefore completely masks
the second, or effect of water-movement, and the resultant
response is the normal negative variation. The water-move-
ment effect may, however, be unmasked by killing the tissue,
and then applying the same torsional vibration as before.
The result is now a positive electrical response.
Or the positive effect may be made to exhibit itself
separately, under favourable conditions in a living tissue, by
the method of indirect stimulation, that is to say, by the
application of stimulus at a distance. When such a distant
point is stimulated, there is a sudden expulsion of water
from that point, due to stimulation. This gives rise to a
wave of increased hydrostatic pressure (with its attendant
positive turgidity-variation), which travels with a relatively
great velocity. The true excitatory variation, travelling at
its slower rate, reaches any given distant point much later.
The two effects ought thus to be divided from each other by
some interval of time.
We should therefore expect on stimulation of a sensitive
plant to find the hydrostatic disturbance, with its attendant
positive turgidity-variation — reaching the distant motile
organ the earlier of the two. And since the negative
turgidity-variation due to excitation causes a fall of the leaf,
the positive turgidity-variation due to hydrostatic disturbance
should be expected to produce an abnormal positive or
erectile movement, and the same positive turgidity-varia-
tion should also find a simultaneous electrical expression
in the abnormal positive response. The true excitatory
response — with its attendant negative turgidity-variation —
should cause, later the normal negative mechanical response.
UNIVERSALITY OF SENSITIVENKSS IN PLANTS
37
and also the normal negative electrical response. We have
already seen, in fig. 17, an instance of this abnormal positive
followed by the normal negative mechanical response, in
experiments with BiopJiytuin. This abnormal positive re-
sponse, being a matter of the intensity of the blow delivered
by the water-movement, can only exhibit itself under favour-
able conditions. It is thus possible in mechanical response to
have either the normal response preceded by the abnormal, or
the normal response alone. But whenever we have an abnormal
mechanical response, due
to positive turgidity-
variation, we have also
simultaneously an ab-
normal positive electrical
response. In fig. 27 is
shown a simultaneous re-
cord of the two, in which
there is a preliminary ab-
normal positive mechani-
cal response, and a syn-
chronous positive elec-
trical response, followed
in both cases by the
normal responses.' It
should be stated here that
this positive turgidity-
variation, which is re-
ferred to as abnormal, is
of very great importance, and will be seen in Chapter XXX.
to be directly responsible for growth. It will be found
Fig. 27. The Abnormal Positive preceding
tiie Normal Negative in Mechanical and
Electrical Responses in Biophytiim
represents the moment of application of
stimulus. The upper is the mechanical
and the lower the electrical record. The
records downward indicate erection of the
leaf or galvanometric positivity.
' It must be understood that this positive electrical response, being dependent
on the excitatory expulsion of water at a distant point, is, in a certain sense, a
physiological response. For it is the excitability of that distant point which
determines the positive turgidity and concomitant positive electrical variation of
the point under examination. The contraction of the excited point gives rise to
a hydrostatic disturbance, by which a movement of water is brought about.
Such a disturbance, then, will be indifferently designated as the hydrostatic, or
hydraulic, wave.
38 PLANT RESPONSE
helpful in the future if we uniformly distinguish (i) the true
excitatory or normal as the direct, and (2) this positive or
abnormal as the indirect effects of stimulation.
Discrimination of differences of excitability by electric
test— Not only does the electrical response enable us to detect
the state of excitation, but I have been able further to devise
an electrical test by which differences in the natural excita-
bility of two points might be distinguished. The demonstra-
tion of the existence of two electrical responses, one of which
alone, namely the negative, constitutes the true excitatory
effect, is of much theoretical interest. For the hydraulic, or
positive electrical effect, has been mistaken for the true
excitatory response in plants. As the excitatory effect in
animal tissues, moreover, was known to be negative, this fact
was supposed to indicate a difference between the proto-
plasmic reactions of animal and vegetable. But the experi-
ments which I have just described conclusively prove that
such a difference does not exist, the sign of response in
animal and vegetable being the same. They offer us an
explanation, further, of the source of error. A more detailed
account of this subject will be found in my forthcoming work
on the Electro-Physiology of Plants.
Many of the motile phenomena of mechanical response
which we shall have to study in the course of the present
work are modified by differences of excitability at different
parts of a tissue. In the case of the primary pulvinus of
Mimosa for example, we can see how the responsive fall is
brought about by the evidently greater excitability of the
lower half of the pulvinus. But in organs which apparently
exhibit little motility, it is impossible from inspection to
know whether all parts of the tissue are equally excitable,
and, if not, which parts exhibit the greater excitability.
Such variation of excitability is often due to invisible molecular
differentiation, and eludes visual scrutiny. Fortunately, as
already said, I have been successful in devising a mode
of electrical investigation by which this differentiation is
detected with the greatest certainty. This method will be
UNIVERSALITY OF SENSITIVENESS IN PLANTS 39
found fully described elsewhere, but the results may be
summarised as follows : On simultaneous excitation of two
points, the current of response flows in the tissue from the
more excitable point A to the less excitable point i; ; con-
versely, if the direction of the responsive current is from A to
B, the point A may be taken as the more excitable. By
means of the unfailing discrimination of the differences of
excitability in a tissue which this method renders possible, it
will be shown in the course of the present work that many ot
the anomalies of growth-curvature receive a most complete
and satisfactory explanation.
Fig. 28. Response of Selenium to the Stimulus of Light
(Conductivity variation method.)
The electrical response given by plant-tissues in general,
as described in this chapter, is obtained by means of the
difference of electrical potential or electromotive variation,
induced as between the excited and unexcited portions of the
tissue.
There is another method of detection by means of
those changes of electrical conductivity which are concomi-
tant to excitation in the substance under experiment.^ It
should be borne in mind that the various responses, obtained
by the mechanical, by the electromotive, or by the conduc-
tivity-variation method, are merely different expressions of
' Bose, Molecular C/iatiges produced in Matter by Electric Waves, Roy.
Soc. Proc. 1 00.
40
PLANT RESPONSE
that fundamental molecular change which underlies excita-
tion, and which disappears on the restoration of molecular
equilibrium.
Universality of responsiveness in matter. — If we take
a plant-tissue and subject it to a sufficient degree of cold, its
responsive power will be found to disappear. It reappears,
however, on the return of the tissue to the normal tempera-
ture. The power of response is thus seen to depend on the
molecular condition of the substance.
Now this irritability, or power of responding to stimulus,
may be vaguely regarded as a characteristic property of
living substances ; and we
may evade the difficulty of
any attempt at a real ex-
planation by describing it
as a ' vital ' phenomenon.
But if we regard all such
phenomena as due ulti-
mately to physico-chemical
actions, we cannot rest
satisfied with what is, after
all, a mere descriptive
phrase. Progress can only
be made in scientific in-
quiry by attempting gradu-
ally to discard all such
assumptions of the working
of mystical forces in favour
of simpler and more rational
explanations.
By following the electrical method of inquiry which has
just been described, I have been able to prove that the power
of responding to stimulus, and, under certain conditions, the
arrest of this power, is the characteristic not of organic matter
only, but of all matter, both organic and inorganic ; ' and that
in general the various agencies which bring on the modifica-
' Bose, Ri'spoiisf in the Lh'ing and N'on-Li7>
•090 „
•100 ,,
•106 „
Tetanus. — Having now observed the effect produced by
single stimuli, we shall proceed to study the effects of similar
yWV— ^
j§0l«[^
J L
Fig. 49. Genesis of Tetanus in Muscle
Record to left shows incomplete tetanus, with moderate frequency of
stimulation. Record to right shows tetanus more complete, with
greater frequency of stimulation (Brodie).
stimuli when superposed. In muscle, we find that when
stimuli succeed each other with great rapidity, the effect of
the second stimulus becomes superposed on that of the first,
which has not had time"to disappear. The result is a fusion
of effects, more or less complete. With moderate frequency
of stimulation we thus obtain incomplete tetanus, which, with
Bose, Response in the Living and Non-Living, p. 53.
m^f\
RELATION BETWEEN STIMULUS AND RESPONSE 99
increasing frequency of stimulation, becomes more and more
complete (fig. 49).
As regards the mechanical response of plants, I obtained
similar tetanic effects with the longitudinal contractions of
the pistil of Datura alba (fig. 50). Stimuli were here applied
at intervals of ten seconds, which was too short an interval,
when compared with the natural period of recovery, lasting
about two minutes. Hence we obtained incomplete tetanus.
This incomplete tetanus became more complete when the
stimulation- frequency was increased,
successive stimuli being now applied at
intervals of five seconds. It may be
noted here that, in the tetanus both of
muscle and of plant, the effects of
individual stiniuli, when rapidly suc-
ceeding, become so merged as to appear yig. 50. Photographic
continuous. It is only after the maxi- Record of Genesis of
. Tetanus in Mechanical
mum effect has been attamed, that the Response of Plants
individual effects of stimuli sometimes (Style of /)«/«;■« a/^«)
become distinguishable by slight oscillatory movements of
the curve. In the case of rhythmic cardiac muscle, how-
ever, there is no tetanus ; and similarly, as described in
Chapter XXVII., we find no tetanus in the rhythmic
vegetable tissue of Desinodiuiu.
Summary
There is a minimal intensity of stimulus necessary to
initiate response.
A stimulus, singly ineffective, becomes effective on repe-
tition.
Increasing intensity of stimulus produces increasing
response, which, however, tends to approach a limit.
The effects of rapidly succeeding stimuli in plant-tissues,
as in animal, become fused, individual effects being then in-
distinguishable. A maximum contractile effect is then pro-
duced depending on the intensity of stimulus.
lOO PLANT RESPONSE
In all the above respects, we find that the responses of
plants in general exhibit the closest parallel to the responses
of skeletal muscle in animals.
But in the animal a different type of response is exhibited
by certain rhythmic tissues like cardiac muscle. The re-
sponse here is on the ' all or none ' principle, and such a tissue
cannot be tetanised. In the parallel instances of rhythmic
vegetable tissues, the same characteristics are present ; that
is to say, the responses are on the ' all or none ' principle, and
there is no tetanus.
PART II
MODIFICATION OF RESPONSE UNDER
VARIOUS CONDITIONS
CHAPTER IX
ON THE UNIFORM, FATIGUE, AND STAIRCASE EFFECTS
IN RESPONSE
Uniform response in plants — Staircase effect — Fatigue due to molecular strain
— Fatigue in plant-responses — Periodic fatigue — Fatigue under continuous
stimulation — Explanation of anomalous erection of \ez.i of M/'wosa under con-
tinuous stimulation— Conductivity and excitability of tissue diminished through
incomplete protoplasmic recovery— Relatively greater fatigue in a motile than
conducting organ — Disappearance of the motile excitability earlier than con-
ductivity— Refractory period — Absence of responsive effect when stimulus
falls within refractory period.
The mechanical response of plants is fundamentally due, as
we have seen, to those molecular changes which are the
result of stimulus. These changes bring about contractions
of the excited cells, in consequence of which water is ex-
pelled, and we obtain longitudinal response in radial organs,
or lateral movement in dorsi-ventral organs, the latter being
simply a special case of differential longitudinal contraction.
On the cessation of stimulus the expelled water is reabsorbed,
and the organ resumes its original position. In the case, for
example, of the leaves of Mimosa^ this position of equilibrium
is, approximately speaking, at an angle of 45° above the
horizon, and this, for convenience, may be called the erect
position. After a period of rest, then, molecular equilibrium
being re-established, the protoplasm recovers its original
properties, of which excitability is one, and response takes
place on stimulation as before. This resumption by the leaf
of its original position may thus be taken as a rough indica-
tion of the restoration of its original protoplasmic properties.
But this is only true in a general way, for there may be
cases, as we shall see, in which the apparent return of the leaf
I04
PLANT RESPONSE
to its original position is deceptive, and does not really indi-
cate a complete protoplasmic recovery.
Uniform responses. — If the motile organ, however, be
restored, by an appropriate period of rest, to exactly its
original molecular condition, and therefore to its original
condition of excitability, it is clear that we ought to be
Fig.
Uniform Electrical Responses (Radish)
able to obtain uniform responses to uniform stimuli. That
this is true has been shown to be the case, with regard to the
responses of the leaflet of Biophytuvi, and the longitudinal
contractions of various radial organs (figs. i8 and 38).
By taking electromotive instead of mechanical responses,
I obtained a similar result, of uniform responses to uniform
stimuli, from various species of plants
(fig. 51). In the case of muscle, also, the
responses are found to be uniform, if
intervening periods of rest be allowed^
sufficient for full recovery (fig. 9).
* Staircase' effect.— It is sometimes
found that a tissue falls into a sluggish
condition, and successive stimuli, by
increasing molecular mobility, have the
effect of gradually enhancing the re-
sponses, which are seen to increase in a ' staircase ' manner.
I give here an instance of this effect (fig. 52) in the case of
longitudinal response, obtained with a style of Eucharis Lily.
Fig. 52. Staircase Ef-
fect in Longitudinal
Mechanical Response
of Plant (Style of
Eucharis)
UNIFORM, FATIGUE, AND STAIRCASE EFFECTS 105
Fig. 53. Fatigue in Longitudinal
Mechanical Response of Plant
(Style oi Datura)
In the case of the first pair of re-
sponses, a sufficient interval for
recovery, namely one minutL',
was allowed. When the peri' ji 1
allowed for recovery was reduced
to half a minute, there was rapid
fatigue ; the second pair of re-
sponses shows this immediate
effect ; the third pair of responses
are to the tenth and eleventh
stimuli.
Fatigue. — It has been said that, when sufficient time is
allowed for protoplasmic recovery, the responses are uniform,
but that, if sufficient time be not
allowed, molecular recovery will
be incomplete, and the tissue
will remain in a strained con-
dition. Under these circum-
stances, it is obvious that there
will not be a complete restora-
tion of the original protoplasmic
excitability, hence successive re-
sponses will exhibit a diminution
or fatigue. The following record
(fig. 53) shows this in the case
of longitudinal response. Uni-
form stimuli were first applied
at intervals of one minute, by which time the recovery was
complete ; and these responses of twenty divisions are seen
to be large and uniform.
The stimuli were next
applied at intervals of half
a minute, and the response
at once fell to eleven divi-
sions. Now, owing to the
effect of cumulative strain,
the succeeding responses
at this interval underwent ~(^-^ ^-^ ^^^
continuous diminution, and Fig. 54. Fatigue shown in Electrical Re-
1 ] r .1 1 , sponse, when sufficient Time is not allowed
had fallen as low as to 4 Full Recovery
five divisions at the tenth in (,7) stimuli were applied at intervals of one
Stimulus. In the next minute ; in (/;) the intervals were reduced
to half a mmute ; this caused a dimmution
figure (fig. 54), fatigue is of response. In (<■) the original rhythm is
shown in the electrJ-l restored, and the response is found to be en-
m^^mu
hanced to nearly its original value (Radish).
m tne eiectncai
responses of plants under
the same conditions, sufficient time for recovery, that is to
say, not being allowed. Similar instances of fatigue are well
known in the case of muscle.
I06 PLANT RESPONSE
Fatigue being principally due to residual strain, it is to
be expected that, other things being equal, strain will be
more persistent with stronger stimulus, as has been shown in
the last chapter; It was there shown also that the period
required for recovery from a strong was more protracted
than from a moderate stimulus. From this it follows that
the stimulation-frequency which will exactly allow for com-
plete recovery, and so give rise to uniform responses, in the
case of a moderate stimulus, will not be sufficient for stronger
stimulus. Hence, keeping the intervals constant, we may
r>J^^
^KkKK
e f\r^
-J-
aAaA/
^ J
A A
JW^
"-t
A/<\Aa
Fig. 55. Alternate Fatigue {a) in Electrical Responses of Petiole of
Cauliflower ; {6) in Multiple Electric Responses of Peduncle of Bio-
phytuin ; (r) in Multiple Mechanical Responses of Leaflet of Bio-
phytum ; and ((/) in Autonomous Responses of Dcsmodiiim
obtain uniform responses to moderate, and diminished or
fatigue-responses to strong stimulus.
There is another curious phenomenon, of alternate or
periodic fatigue, which I have often observed in the response
of plants. The simplest type of such periodic fatigue is that
in which responses wax and wane, in regular alternation.
Such alternate fatigue is sometimes seen in the electrical
response of plants, the multiple response of Biophytum, and
also in the autonomous response of Desvwdmni (fig. 55).
Curiously enough, I have sometimes obtained similar alter-
nate responses with the compound strip of ebonite and india-
rubber, previously described. There are other cases of
response in plants, where the variations are cyclic in type, that
UNIFORM, FATIGUE, AND STAIRCASE EFFECTS
107
is to say, they consist of groups of responses, which wax and
wane alternately.
Fatigue under continuous stimulation. — In connection
with the induction of fatigue under long-continued stimu-
lation, I shall now anticipate certain results regarding the
character of the responses given by matter universally —
results to be described in detail in the next chapter. It will
there be shown that if we represent a given molecular change,
caused by stimulus, as positive, the continuation of stimulus
will at first increase that change, until it has attained a
maximum, after which, under
the still continued action of the
same stimulus, there will be a
reversal or change to the nega-
tive. In a living tissue, then,
where the incidence of stimulus
causes contraction, we may be
prepared to find that the same
stimulus, long continued, will
bring about a reversal of this
effect, or, that is to say, a re-
laxation.
In any case, such a reversal
is illustrated in the correspond-
ing phenomenon in contractile
muscle. It is there found
that, whereas the first effect of stimulus is contraction, the
same stimulus, when too long continued, brings about relaxa-
tion to the original form. In the electrical response of
plants under continuous stimulation, 1 also find this peculiar
fatigue-reversal (fig. 56J.
I have also obtained similar fatigue- reversals in the longi-
tudinal response of radial organs. In fig. 56, (a), a series of
tetanic electric shocks was continuously applied during a
period of four minutes. The maximum contraction was
attained in the course of one minute, after which there was
a reversal, or relaxation. Under this condition of fatigue-
FiG. 56. Rapid Fatigue under Con-
liuuous Stimulation in (a) Muscle ;
{/>) Leaf-stalk of Celery (Electrical
ixesponse)
io8
PLANT RESPONSE
reversal, the tissue is incapable of the normal excitatory con-
traction, but after a period of rest of about seven minutes it
Fig. 57. Fatigue under long-continued Stimulation in the Contractile
Response of Plants
{a) Stimulation by tetanising electric shocks ; {d) stimulation by rapidly
succeeding thermal shocks. Continuous lines represent action during
stimulation ; dotted lines represent after-effect (coronal filament of
Passiflora, magnification forty times).
again gives response, which is at first normal, and then, after
reaching a maximum, becomes reversed. The second re-
sponse is, however, seen to be smaller than the first.
I obtained parallel results
under the action of rapidly
succeeding thermal shocks
(fig- 57, b).
I have already described
curious instances of alter-
nating fatigue exhibited in
successive single responses
to single stimuli (fig. 54).
A very curious and interest-
ing effect of this nature
occurring under continu-
ous electric stimulation, is
shown in the accompanying photographic record (fig. 58)
of responses given by the filament of Uriclis Lily. It will
Fig. 58. Photographic Record of
Periodic Fatigue under Continuous
Stimulation in Contractile Response
(Filament of Uriclis Lily)
UNIFORM, FATIGUE, AND STAIRCASE EFFECTS IO9
there be noticed that the maximum contraction is attained in
tlie course of three minutes, after which there is a fatigue-
relaxation, which continues up to the eleventh minute.
There then occurs a second, though much feebler, response,
after which comes a slow and continuous reversal action.
So-called * anomalous ' response in Mimosa. — In con-
nection with the subject of fatigue, I shall here enter upon
the explanation of certain well-known responsive effects in
the case of Mimosa which
have hitherto been regarded
as anomalous. It is generally
found that an erect leaf of
this plant is sensitive, that is
to say, when stimulated it
becomes depressed. In this
depressed position it is ap-
parently insensitive, hence
we are apt to assume that the
erect posture is one of sensi-
tiveness, depression indicating
the reverse. It will be found,
however, that if a Mimosa leaf
be continuously stimulated
by successive blows or taps, ,,j^ ^^ photographic Records of
in the manner of Pfeffer's Normal Response of Mimosa to
, ^, I r -11 ^ Single Stimulus (upper figure), and
experiment, the leaf will at ^^ Continuous Stimulation (lower
first fall. But, though the (figure)
blows be continued, the I" the latter case the leaf is erected in
' spite 01 continuous stimulation,
petiole will, after a time,
return to its normal erect position. In this erect posture,
however, further blows prove to have no effect upon it, the
leaf being now insensitive.
1 give above a photographic record of this effect in
Mimosa (lower record, fig. 59), continuous stimulation in this
case having been produced by tetanic electric shocks. It
will be noticed that after its responsive fall the leaf returns
to the erect position, in spite of the fact that stimulus is
no PLANT RESPONSE
being continued. It is important to note that the two records
were taken with the same specimen, and in immediate suc-
cession to each other. The first, or upper, records the
response of the leaf to a single stimulus and its recovery ;
the lower gives the response to continuous stimulation. In
appearance the two records are singularly alike. But though
the leaf at the end of each response occupies the same
position, the molecular conditions in the two cases are, as
will be shown presently, entirely different, inasmuch as in
the first, renewed response was immediately obtained, thus
showing its sensitive condition ; while in the second, the
organ was insensitive, and could give no response until after
a period of rest of about ten minutes.
The explanation of this apparent anomaly is quite clear
from the experiments which have already been described,
showing that under continuous stimulation the normal longi-
tudinal contraction undergoes reversal and passes into relaxa-
tion, as is also the case with continuously excited muscle,
the motile response of Mimosa being only an instance of
differential longitudinal contraction. We obtain here also the
usual sequence of first, normal contraction, and second, the
fatigue-relaxation, or posture of erection.
To sum up, then, it is clear that our association of the
erect position with sensitiveness is not always correct, for the
leaf may assume this posture as the result of fatigue. That
its position in this case, however, though outwardly imitating
that of sensitiveness, is profoundly different, is at once
revealed on application of stimulus. The leaf is now irre-
sponsive. But if we allow it a period of rest — of some eight
or nine minutes in summer, or double that time in winter —
the internal molecular equilibrium is re-established. But of
this internal readjustment the leaf gives no visible indication.
It remains in the same unchangingly erect position. During
the course of the cycle, then, it has passed from the normal
erect to the normal depressed, and thence to the abnormal
erect position. It next passes, without any outward change,
from this abnormal erect to the normal erect position, after
UNIFORM, FATIGUE, AND STAIRCASE EFFECTS III
a period of repose. And this return of the leaf to its normal
condition is testified by its once more responding to stimulus.
Fatigue of conductivity and excitability.— It has already
been pointed out that the protoplasmic properties of a tissue
cannot be restored to their original condition after stimulation,
without the intervention of a suitable period for the re-
establishment of molecular equilibrium. This fact I shall
now demonstrate by additional experiments.
The restoration of the normal protoplasmic condition in
a tissue may be tested by observing the recovery of some of
those properties which are capable of measurement. One of
these is its conductivity, measured by determining the speed
with which excitation travels through the tissue, the method
of which determination will be fully described in Chapter XX.
Under normal conditions this velocity is constant. If we
excite the tissue, and measure the rate at which the excitation
travels, and if we then allow a sufficient interval of rest for
complete protoplasmic recovery and again determine this
velocity, we shall find that the two are the same. But, if the
necessary resting interval be not allowed, recovery being
incomplete, there will remain a residual strain. The velocity
of transmission of excitation will, under these circumstances,
be found to be reduced.
Another protoplasmic property which is capable of
measurement is the excitatory contraction seen in motile
organs, measurable by the amplitude of response. We
have seen that this amplitude is constant under normal
conditions, and when sufficient intervening periods of rest
are allowed. But diminution of the intervening resting period
produces diminution of the amplitude of response.
From what has been said it follows that, if the intervening
resting periods of a tissue be continuously diminished, there
will be a continuously increasing residual strain, and this
might be detected by the consequent continuous decrease
of conductivity and motile excitability. I have been able
to verify this deduction by an experiment on a leaf of
BiopJiytuin, the details of which will be found in Chapter XX.
112 PLANT RESPONSE
It will there be seen that, as the resting period was gradually
shortened to half a minute, the conductivity underwent di-
minution from the normal r88 mm. to 1*54 mm. per second,
that is to say, by 18 per cent. The reduction of motile
excitability was found, however, to be still more marked,
the height of response being reduced from the normal thirty-
four to one division, that is to say, by as much as 97 per cent.
On still further reducing the period of rest, from half a minute
to ten seconds, it was found that there was no motile response
whatever. The tissue is thus seen to be altogether incapable
of response during a certain refractory period. The time-
value of this refractory period not only differs in different
plants, but also varies with the physiological condition. In
BiopJiytum^ normally speaking, it is ten seconds, and in
Mimosa about one minute.
Earlier abolition of motile excitability than of con-
ductivity.^— We have seen in the last experiment, that while
the conductivity of the petiole was reduced by 18 per cent,
the motile excitability of the attached leaflet underwent a
diminution of 97 per cent. This shows that motile excitability
disappears earlier than conductivity. The reason will be
apparent if we consider the difference between the two
expressions of protoplasmic excitation. We have, in the
conduction of the state of excitation from point to point,
a direct expression of the transmission of the molecular
change initiated by stimulus. The motile response, however,
is a somewhat remote consequence of the series of events
which follows on the fundamental molecular change. Inter-
mediate occurrences are the permeability variation and the
contraction, in this case differential, which it produces. The
movement of the leaf is a result of all of these, and depends
for its complete fulfilment on certain favourable circumstances.
In any case, there are mechanical obstacles which have to
be overcome in forcing the expelled water through channels
of escape. That this must involve some degree of waste of
force is partly seen in the fact that all excitations do not
produce response, it being necessary that the stimulus should
UNIFORAr, FATIGUE, AND STAIRCASE EFFECTS II3
exceed a certain minimum value in order to produce any
movement at all. If, again, the escape of water should be
resisted, owing to the peculiar condition of the tissue, which
has already been described (p. 49), then even a strong
stimulus would be unable to bring about movement.
Analysis of the different phases in the response of
Mimosa under continuous stimulation. — Having now con-
sidered in detail some of those changes of protoplasmic pro-
perties which are brought about by the action of stimulus, we
are enabled to study the effect of long-continued stimulation
Fig. 60. Ineffectiveness of Stimuli, owing to Increasing Fatigue,
in Mimosa
In the left-hand figure stimuli were applied at intervals of 3-5 minutes.
These evoked feeble responses. In the right-hand figure stimuli were
applied at intervals of two minutes. Response now became incon-
spicuous. Where stimuli were applied at intervals of one minute no
effect was produced. The leaf was refractory.
from a somewhat different point of view. We saw that with
incomplete recovery, protoplasmic excitability was pro-
gressively diminished. In order to demonstrate this, in the
case of Mimosa, I obtained responses at intervals of 3*5
minutes, the uniform stimulus of condenser discharge being
employed. The responses, which had been uniform, when
stimulus was applied after complete recovery, at intervals of
about eight minutes, were now found to be very much reduced
(fig. 60). In a second series of experiments on the same
specimen, the intervening periods of rest were still further
I
114 PLANT RESPONSE
reduced, to two minutes ; and it will be noticed that, owing
to increasing incompleteness of recovery, the responses
were here reduced to the merest indications of twitches.
When the intervening periods were still further shortened
to less than one minute, the stimuli fell within the refractory
period of the tissue, and produced no indication whatsoever
of their effect. As an extreme instance, we have the effect
of continuous stimulation, already described on p. 109. To
be precise, we must remember that in this gradual abolition
of response, under quickening stimulation, we not only see
the action of diminished excitability, but also of diminished
conductivity.
The erection of the leaf of Mimosa, by the relaxing action
of fatigue, may also be assisted by the later contraction of
the upper half of the pulvinus. For we have seen that in an
anisotropic organ, the less excitable half responds subsequently
to the more excitable. The contraction of the upper half
of the pulvinus in Mimosa would produce erection of the
leaf, and that this might sometimes happen appears probable
from the fact that in the erection of the leaf under continuous
stimulation it is occasionally found to be lifted above its
normal position.
Summary
Stimulus, by causing molecular derangement, brings about
mechanical response ; and by molecular transmission of
disturbance from point to point, the excitation is conducted
to a distance. Excitatory mechanical response and conduction
of excitation are different expressions of the effect of stimulus
After a period of rest from the action of stimulus, there
is a restoration of molecular equilibrium. The protoplasmic
properties of excitability and conductivity are then completely
restored. Under such normal conditions respon.ses are uniform.
A tissue in a sluggish condition has its molecular mobility
increased by the action of successive stimuli. This produces
the * staircase ' effect, of gradually enhanced responses.
When sufficient time is not allowed, there is a residual
UNIFORM, FATIGUE, AND STAIRCASE EFFECTS II 5
molecular strain. The conductivity and excitability of an
organ are thus diminished, and the responses undergo
diminution, in consequence of cumulative residual strain.
Fatigue is greater in a motile than in a conducting organ.
Motile excitability disappears earlier than conductivity.
The anomalous erection of the Mimosa leaf is brought
about by the combined effects of diminution of conductivity,
abolition of excitability, and possibly, in some instances, by the
subsequent excitation of the upper half of the pulvinus also.
CHAPTER X
THEORIES CONCERNING DIFFERENT TYPES OF RESPONSE
The chemical theory of response— Insufficiency of the theory of assimilation and
dissimilation to explain fatigue and staircase effects— Similar responsive effects
seen in inorganic substances — Molecular theory — When molecular recovery is
complete, responses uniform : when incomplete, fatigue brought about by
residual strain — Fatigue under continuous stimulation, in inorganic substance,
in plant, and in muscle— Staircase effect brought about by increased molecular
mobility : examples seen in inorganic substance, and in living tissues — No sharp
line of demarcation in the borderland between physical and chemical phenomena
Molecular changes attended by changes of chemical activity — Unequal
molecular strain gives rise to a secondary series of chemical actions — Volta-
chemical effect and by-products - Supposition that response always dispropor-
tionately larger than stimulus, not justified— P:xistence of three types: (i)
response proportionate to stimulus ; (2) response disproportionately greater than
stimulus ; (3) response disproportionately less than stimulus — Instances of
stimulus partially held latent : staircase and additive effects ; multiple response ;
renewed growth.
It has already been shown, in previous chapters, that the
various types of response met with in animal tissues are
exactly paralleled, even in detail, in the response of plants ;
and numerous further instances of this fact will be met with
in the course of the present work. It would thus appear
that the theoretical explanation of either class of responses
must be applicable to the other also. Existing theories
regarding animal response, however, have not yet been found
sufficient to meet all the difficulties of the case, and it is
probable that the larger data now made available by the
inclusion of response in plants, may go far to throw light
on certain obscurities which are at present regarded as
perplexing.
Chemical theory of assimilation and dissimilation. —
The theory which is generally accepted at present may be
referred to briefly as chemical. According to it, living matter
DIFFERENT TYPES OF RESPONSE I 17
is maintained in a state of equilibrium by the two opposed
chemical processes of building up or assimilation, and break-
down or dissimilation. Stimulus causes a down or dissimi-
latory change, which is again compensated, during recovery,
by the building up, or assimilative change. In the case of
uniform responses, the two processes exactly balance each
other. But on occasions when the down change is the greater
of the two, the potential energy of the system falls below
par, for the building-up process cannot then sufficiently
repair the chemical depreciation caused by the downward
change. Hence occurs diminution of response, or fatigue,
which is supposed to be further accentuated by the produc-
tion and accumulation of deleterious ' fatigue-stuffs.' The
disappearance of fatigue, after a period of rest, is explained
by the renovating action of the blood supply, which is also
regarded as the means of carrying away the fatigue-stuffs.
A serious objection to these explanations, however, lies in
the fact that even excised and bloodless muscles exhibit
recovery from fatigue, after a period of rest. In isolated
vegetable tissues, again, where there is no active circulation
of renovating material, the same effect, and its removal after
a period of rest, are observed.
Thus the difficulties met with in explaining fatigue accord-
ing to a purely chemical theory are great enough. But still
greater are those which we encounter when we come to deal
with the staircase effect — typically shown in cardiac muscle
— in which successive responses to uniform stimuli exhibit a
gradual enhancement of amplitude. Here the result obtained
is in direct opposition to the theory described ; for in this
particular case, we have to assume that the same stimulus
which is usually supposed to cause a chemical break-down
becomes efficient to produce an effect exactly the reverse.
It is true that the heart, usually speaking, is charged with
blood ; but this particular staircase increment of response,
under uniform stimulation, is observed even in the initial
twitches of bloodless muscle (fig. 64), and here there can be
no question of a supply of renovating blood.
ri8 PLANT RESPONSE
Parallel types of response in living organic, and in
inorganic matter.— Such being the difficulties involved in
the explanation of a single class of phenomena, on the
chemical hypothesis of assimilation and dissimilation, it may
be well next to turn our attention to the conclusions suggested
by the observation of response in matter generally. And for
this purpose it is best to take the responses obtained from
inorganic matter in particular, the hypothetical assimilation
and dissimilation being in that case out of the question.
With regard to the mode of observation, I have already
explained how the molecular derangement consequent on
stimulus may be studied, either (i) by recording the change
of form ; or (2) by recording the variation of conductivity ;
or (3) by recording the electromotive variation.
As an example in the first place of responsive contraction
in inorganic matter, we may select for our investigation the
response of india-rubber, under thermal stimulation. In this
case, chemical changes, either up or down, are impossible.
The second, or conductivity variation method, may be used
in the case of metallic powders, the stimulus being that of
Hertzian radiation. In this case also chemical action
may be excluded, the experimental material being usually
placed in naphtha. In the third case, again, where response is
obtained by means of the electromotive variation, under
mechanical stimulus, the substance used is platinum, the
most chemically inactive of metals, electrolytic contacts being
made by water.^ The possibility of chemical action is thus
reduced to a minimum, and the assimilatory change entirely
excluded.
It will be found, however, that in all these cases of in-
organic response, in which substances, physically and chemi-
cally widely unlike, are subjected to diverse forms of stimula-
tion, and have their responses tested and recorded by absolutely
different methods, the results obtained are exactly parallel.
All alike, when sufficient intervening periods of rest are
' For details of these investigations and results, see Bose, ResJ^oitse in the
Living and Non-Living.
DIFFERENT TYPES OF RESPONSE II9
allowed, give uniform responses to uniform stimuli. And
when the period of rest is shortened, all alike exhibit
fatigue.
From the conditions of experiment it is clear that
these effects are physical or molecular. The molecular
derangement caused by stimulus is thus gauged by the
amplitude of response. Recovery is brought about by the
restoration of molecular equilibrium, and for this purpose it
has now become evident that the process of assimilation is
not essential. When sufficient time, however, is not allowed
for recovery, we have a residual molecular strain, and a
substance in this strained condition is less responsive, as
seen in the diminished height of its response. Fatigue is
thus due to molecular strain, and its
cumulative effects. But when the
fatigued substance is allowed sufficient
time for the strain to disappear, its
1 . 1 -1 -^ ii Fig. 61. Fatigue-Reversal
subsequent responses exhibit the nor- ;„ Arsenic, under Con-
mal amplitude. tinuous Stimulation of
Hertzian Radiation.
It was explained in the last chapter
. r -IT- -1 ^^^ horizontal line repre-
that in the case of Mimosa, as in that sents the duration ot
of muscle, a complete reversal of stimulation. Curve ob-
r- tamed by conductivity
response is brought about by extreme variation method.
fatigue, under continuous stimulation.
The following record shows a similar reversal in Arsenic,
under the continuous stimulation of electric radiation (fig. 61).
It was only after a sufficient interval of rest that this sub-
stance gave renewed normal response. It maybe added that
these fatigue-reversals, as in the longitudinal response of the
Uriclis Lily, are sometimes found to be recurrent.
This curve of fatigue-reversal in Arsenic under continuous
stimulation was obtained by recording the changes of electric
conductivity in the substance. A still more striking analogy
with the mechanical records of fatigue in plants and animals
is afforded, however, by the automatic record given in fig. 62
of contractile responses in india-rubber. When this substance
is excited by rapidly succeeding thermal shocks, we obtain
PLANT RESPONSE
first, the normal contractile effect, and secondly, the relaxation
due to fatigue, in a manner exactly similar to that which
characterises the fatigue-reversals of Mimosa and of skeletal
muscles. In the present case, the india-rubber attained its
maximum contraction in the course of two minutes, after
which there was a continuous relaxation.
In this responseof india-rubber, and in its fatigue-reversal,
we have an analogy with the response of living animal tissues,
such as muscle, so close as to
compel us to the conclusion
that both alike are phenomena
of molecular response. A
mere contraction of the india-
rubber might have been sup-
posed to be due to the specific
action of heat on that sub-
stance. And had this been
all, successive thermal shocks
would have had the effect of
continuously increasing the
contraction, till a limit was
reached. But if, on the other
hand, molecular excitability
be a factor in the process
of response, we might then
expect, under certain condi-
tions of fatigue, a loss of
molecular excitability. That
this is actually the case is shown by the fatigue-reversal,
attended with relaxation, which is seen in the figure.
A muscle, again, in this condition of fatigue-relaxation,
after a period of rest — during which the molecules have
time for recovery from their state of strain — becomes
once more excitable without any visible change. We found
a like phenomenon occurring in the case of fatigued and
relaxed Mimosa, under continuous stimulation. In that case
it was, as we saw, a period of rest of about eight or ten
Fig. 62. Automatic Record of Fatigue
in the Contractile Response of India-
rubber under Rapidly Succeeding
Thermal Shocks
The first effect of stimulus is contrac-
tion, but this passes into relaxation
under continued stimulation. The
time-marks represent intervals of
half a minute.
DIFFERENT TYPES OI' RESPONSE 121
minutes, which made the fatigued Mimosa once more con-
tractile. Returning to india-rubber, we find that here also
a period of rest of ten minutes enables it to recover
its excitability, and once more exhibit its responsive con-
traction.
From these facts it would appear that in order to explain
the phenomenon of response, and its various modifications by
fatigue and other factors, we have no option but to regard it
as an expression of the molecular responsiveness of matter
in general.
We next turn to the converse phenomenon of staircase
response. Since response is an expression of molecular
Fig. 63. Preliminary Staircase In'
crease, followed by Fatigue, in the
Response of Ga/cna to Hertzian
Radiation
(Conductivity variation method)
Fic;. 64. Preliminary
Staircase Increase, fol-
lowed by Fatigue, in
the Response of Style
of Eitcharis «
derangement, it follows that its extent will, other things being
equal, depend on the degree of molecular mobility. This
being so, it is conceivable that a substance, at first in a
sluggish condition, may, by the impact of successive stimuli,
have its molecular mobility gradually increased, with a corre-
sponding enhancement of its response. In fig. 63 we have an
example of this staircase effect, in the responses of Galena.
It is to be noticed that this effect occurs at the beginning of
the series of responses, as we should expect. After the
attainment of maximum mobility, the phenomenon of over-
strain is seen, with its accompanying diminution of response,
or fatigue. I have obtained similar results in the longitudinal
PLANT RESPONSE
response of plants (fig. 64). It need only be mentioned,
further, that exactly the same preliminary staircase effect,
reaching a maximum, and followed by fatigue, is found in
the responses of muscle (fig. 65).
The inference that the occurrence of the staircase effect
is due to the gradual removal of molecular sluggishness,
receives further support from the following experiment on
a plant where sluggishness is brought on artificially, and
made to disappear gradually.
Sluggishness may be induced
in a tissue by cooling, this con-
dition being made to disappear,
by the gradual return of the
substance to the ordinary tem-
perature of the room. On carry-
ing out such an experiment.
Fig, 65. Preliminary Staircase,
followed by Fatigue, in the
Responses of Muscle (Brodie)
Fig. 66. Staircase Increase
in Electrical Response of
Petiole of Bryophylhtm,
rendered sluggish by
cooling
and recording the successive electrical responses to successive
uniform stimuli, I obtained, as I had expected, a marked
staircase effect (fig. 66).
Merging of physical into chemical phenomena.—
I have explained that responsive phenomena are primarily
due to the molecular distortion caused by stimulus, that is to
say, they are of a physical character. Fatigue has also been
shown to be primarily due to residual strain. But it must be
borne in mind that in the borderland between physics and
chemistry there is no sharp line of demarcation. For ex-
ample, yellow phosphorus under the stimulus of light is
DIFFERENT TYPES OF RESPONSE 1 23
converted into the molecular red, or allotropic variety. This
molecular change, however, is also attended by a concomi-
tant change in the chemical activity, phosphorus in the red
condition being less active chemically than in the yellow.
Under certain circumstances, further, it is possible to
have a secondary series of chemical events following upon a
condition of unequal molecular strain. We have seen that a
homogeneous living tissue, when unstimulated, is iso-electric.
When stimulated, however, an electromotive difference is
induced, as between the more and the less acted parts of
the tissue. The result is an electrical current attended by
chemical changes. As a consequence of such volta-chemical
action, when prolonged, by-products (fatigue-stuffs ?) may be
accumulated, and these may have a depressing effect on the
activity of the tissue. Hence, just as after very prolonged
activity of a voltaic element it is necessary to renew the
active element and change the electrolyte surcharged with
by-products, so, after sustained activity of a living tissue,
the process of renewal, or renovation, will be necessary.
Thus we see how, upon the fundamental molecular derange-
ment, a chain of very various chemical events may follow as
its after-effect. And it is only by going in this way to the
very root of the phenomenon, that we can avoid the many
contradictions with which we are confronted by the chemical
theory.
I have therefore aimed at demonstrating the universal
existence in matter of the property of responsiveness, and
by taking the simplest cases and excluding as many com-
plicating factors as possible, have attempted to .show further
how this power of response is modified by those conditions
which occasion fatigue, or its converse, the staircase effect.
Having thus cleared the ground, it is possible to take up
cases of greater complexity.
Different modes of transformation of stimulus. — It has
been assumed that response is brought about by a sudden
explosive chemical change, the stimulus acting as if on a
trigger, for the release and run-down of potential chemical
124 PLANT RESPONSE
energy. This implies that response is disproportionately
greater than stimulus, and that the responsive change is
attended by an evolution of heat and chemical by-products.
It would follow, however, from such rapid depreciation, that
fatigue must be the invariable consequence of any series of
responses. But it is notorious that in the responses of nerve,
not only is there no fatigue, but neither is there any evolution
of heat, nor occurrence of chemical change, that can be
detected.
As the simplest case, we have hitherto considered the
substance acted on to be neutral in its character, that is to
say. not active in the sense of being able to absorb stimulus
and hold it latent. But if we regard the living organism as
a machine, three different cases are conceivable. These are :
first, that in which the responding substance simply con-
verts the energy received as stimulus into response ; secondly,
that in which the responding substance possesses a large
amount of energy, some of which is set free by the action of
the stimulus ; and lastly, that in which the responding sub-
stance is capable of absorbing and holding latent, to a greater
or less extent, the stimulus which it receives.
(i) Response proportionate to stinmlns. — The first of these
types may be illustrated by a responding circuit which con-
tains a magnetic motor— say a galvanometer — translating cur-
rent into motion. The source of stimulus may be an external
battery periodically closed b\' a tapping key. The waste of
energy by the production of heat may be supposed to be
brought down to a minimum in this circuit by using a feeble
current and reducing the resistance. Uniform stimuli will
now cause uniform responses of the responder. Response
will be proportionate to stimulus, and there will be no chemi-
cal, and no appreciable thermal, change in the responder.
This is a state of things which may be said to approximate
closely to the responsive peculiarities of the nerve.
(2) Response disproportionately greater tJian stimulus: re-
sponding substance reduced below par. — As an example of the
second type, we have to imagine a responding system which
DIFFERENT TYPES OF RESPONSE I 25
contains a large amount of energy. We may suppose it to
consist of a storage battery and a galvanometer, the circuit
being normally incomplete. External stimulus may, by some
easily arranged mechanism, close the responding circuit
periodically. In this case, the responses might be made dis-
proportionately larger than the stimulus. There will then be
a progressive run-down of the latent energy of the system,
and the responses will show diminution, or true fatigue. The
system, at the end of the experiment, will be found to be
below par, This case may be paralleled by that of highly
excitable tissue which is wasted under excessive and long-
continued stimulation.
(3) Stimulns ivJiolly or partially absorbed. — The third type
of substance we have supposed to be one which is capable of
absorbing and holding latent, to a greater or less extent, the
stimulus which it receives, and under this we may have
several important sub-cases.
{ii) Staircase and additive effects. — The absorbed stimulus
may gradually enhance the molecular mobility, with gradual
enhancement of response. This is seen exemplified in the
staircase effect. Another example of the same thing is
probably to be found in the singly ineffective stimulus which
becomes effective on repetition. Evidently in this case, the
energy of the first few stimuli is held latent in the tissue and
added up until it reaches the threshold of response.
{b) Multiple response. — We can next see the possibility
of a very interesting case of stimulus becoming latent. A
spring which is immersed in a viscid fluid may, on receiving
a feeble blow, give a single vibrational response. But if the
blow be powerful, this single strong stimulus will give rise to
a multiple series of vibrational responses. Again, a phos-
phorescent substance acted on by light absorbs it, and, on
the cessation of incident stimulus, continues to give up the
excess of latent energy thus acquired in the form of luminous
vibration. A selenium cell again, when acted on by a single
strong flash of light, I have found to give what may be
regarded as two responses, one strong and the other feeble.
126 PLANT RESPONSE
From certain metallic particles, again, when exposed to a
single strong flash of electric radiation, I have obtained
pulsatory responses. From the consideration of all these
cases, I was led to investigate the question whether a plant-
tissue, when acted on by a single strong stimulus, could be
found to give similar repeated responses. Fromi this I was
led to the discovery of Multiple Response in plants, a pheno-
menon which I find to be very prevalent, and which I shall
describe fully in Chapter XX.
(c) Response disproportionately less than stimulus : respond-
ing substance raised above par. — It is the last of these sub-
cases, however, which we could least easily have foreseen. It
is as if here the active expression of the incident stimulus were
bifurcated. Returning once more to the mechanical model,
we may imagine the responding portion of the system to
contain, besides the mechanically responding galvanometer,
two plates of lead, immersed in dilute sulphuric acid, by
which the energy of the stimulating current is partially stored
up. The external stimulating current has now to do two
things, first, to cause mechanical response in the galvano-
meter, and secondly, to store up an increasing amount of
latent energy in the second part of the responding system.
It is evident here that, owing to increasing storage, the
mechanical response of the galvanometer may become pro-
gressively less. We are, perhaps, too apt to ascribe to
* fatigue ' all cases of diminution of responses. For in this
case we shall have an appearance of fatigue which is not due
to the run-down of energy, but to its actual increase, in the
responding system.
I have been able to discover a parallel case to this in the
response of plants, a case, that is to say, in which the response
is disproportionately smaller than stimulus, successive stimuli
nevertheless causing an increase of the energy of the system.
I took for my experiment a straight tendril of Passijlora in
which growth had undergone arrest. It may be pointed out
here that a certain tonic condition is necessary to the con-
tinuation of growth. This tonic condition is determined, as
DIFFERENT TYPES OF RESPONSE
127
but also in afterwards
will be shown later, by the sum total of energy latent in
the tissue. From the curve of response given in fig. 67, it
will be seen that at the beginning of the experiment, the
length of the tendril being constant, the first part of the
record was horizontal, instead of descending, as would have
been the case had the tendril been growing. The con-
tractile shortening brought about by stimulus is here
represented upwards. The stimulus of induction shock
was now applied at intervals of one minute, and the
record shows that the recovery, instead of stopping short at
the level of the horizontal line, has proceeded beneath,
thus indicating that stimulus has been effective not only
in producing contractile response
initiating growth ! (Cf fig. 178.)
And further, as the stimulus
goes on causing not only
mechanical response, but also
accelerated growth, we see that
the successive mechanical re-
sponses are undergoing dimi-
nution. The application of
stimulus ceased at the end of
the fourth response, and we
observe active growth proceed-
ing from this point as a result
of the energy absorbed from
the previous stimuli. We have
thus demonstrated the very
curious case in which the build-
ing up process is attended by an actual diminution of re-
sponse, and where, after stimulation, the energy of the
responding organ, instead of being reduced, is raised above
par. This experiment explains why an athlete in constant
practice does not waste away, but actually increases, muscle.
Fig. 67. IMechanical Response
shown upwards in Tendril of
Passijlora in which Growth was
originally at Standstill
Stimulation, besides producing
mechanical response, initiated
the opposite movement of
growth, as shown by the slope
of the base-line downwards.
128 plant response
Summary
The principal types of response seen in animal tissues are
found also in the responses of plants and of inorganic
substances.
Three types of responses are possible : (i) that in which
response is proportionate to stimulus ; (2) that in which
response is disproportionately greater than stimulus ; and (3)
that in which all or part of the stimulus is, for a longer or
shorter time, absorbed by the tissue and held latent.
The subsequent effect of stimulus which is held latent
may sometimes be seen in singly ineffective stimulus which
becomes effective on repetition ; or in the staircase response,
consequent on the enhancing of molecular mobility by the
partial absorption of previous stimulus.
The fact that stimulus may be held latent for a time, and
subsequently find expression, is strikingly shown in the
occurrence of multiple response, in answer to a single strong
stimulus.
It is also possible for the incident stimulus to become
divided in its expression, part of it finding an outlet directly
in mechanical response, and part becoming latent, and causing
accelerated growth. The gradual augmentation of the latter
of these may cause a corresponding diminution of the former.
An appearance of fatigue is thus brought about, which is
misleading, as the decrease of mechanical response is in this
case due. not to a diminution, but to an increase of latent
energy in the responding substance, as shown by its capacity
for renewed growth.
CHAPTER XI
EFFECT OF ANAESTHETICS, POISONS AND OTHER CHEMICAL
REAGENTS ON LONGITUDINAL RESPONSE
Response modilied l)y physiological change — Carbonic acid causes depression,
and transitory exaltation as after-effect— Gradual abolition of response in
hydrogen and restoration by access of air— Chemical agents cause contraction
or relaxation of plant-tissue — Effect of alcohol causing temporary ex.tltation of
response followed by depression and protracted period of recovery — Ether
causes relaxation and temporary depression of response — Explanation of
anomalous action of ether on stimulated Mimosa leaf — Abolition of response
Vjy hydrochloric acid — Response restored by timely application of ammonia —
Abolition of response by poisonous reagent — Similarity of effect of chemical
agents on the response of animal and vegetable tissues.
The subject of our inquiry, in this and succeeding chap-
ters, will be the determination of those influences which
bring about the variation of conductivity and excitability in
plant-tissues. The variation of excitability induced in a
tissue by chemical reagents, may be studied through the
consequent modifications of the responses. Various forms
of response which may be used for this investigation are
(i) electrical, (2) autonomous, (3) growth, and (4) the simple
mechanical response about to be described. A description
of the modifications induced in electrical response under the
action of chemical reagents will be found on pp. 73-80 of
my book, ' Response in the Living and Non-Living.' The
effects of chemical reagents on autonomous pulsation and
on growth are given in detail in Chapters XXV, XXVII,
and XXXV of the present work. But it may be said here,
that the results obtained by all these different methods are
concordant, and that the simp'e method of mechanical
response now to be detailed is to be regarded as an additional
corroboration. It should also be borne in mind that the
K
130 PLANT RESPONSE
effects of chemical reagents are subject to modification, as is
fully explained later (pp. 322 and 477), by the tonic con-
dition of the specimen.
The method of procedure consists in first obtaining a
series of responses to uniform stimuli under normal con-
ditions, and then a similar series after the application of the
reagent. The uniform individual stimuli in both series arc
applied at intervals which allow of complete recovery. The
intensity of stimulus, and the time-intervals, are kept constant
throughout the experiment. The chemical agent may take
the form of gas or vapour, or it may consist of a liquid. In
the former case, by the turning of the three-way stop-cock
described on p. 72, water-vapour is passed through the
plant chamber, and the responses thus obtained are taken as
the normal. By a quick manipulation of the stop-cock, the
gaseous or vaporous reagent is next introduced, and the
responses now obtained exhibit the physiological modifica-
tion induced by it. The effect produced by some agents is
permanent, by others transitory. This difference may be
demonstrated by turning the three-way cock once more, so as
to allow water-vapour torepiace the specific gaseous medium.
The responses now obtained exhibit the after-effect.
The difficulty in this investigation lies in the selection of
specimens which exhibit complete recovery, together with
uniformity of successive responses. For this purpose we
may take any radial organ, one of the most excitable, and
therefore suitable, of these being the filament of the corona
of Passifloi^a. In this case high magnification is not necessary,
and very moderate stimulus is sufficient. Failing Passiflora,
I have frequently used with success the staminal filaments of
the Uriclis Lily, and Broivnca ariza, and the styles of Datura
and Euchans Lily. It should be borne in mind that the
excitability of the tissue is to a certain extent influenced by
seasonal conditions, being different under different circum-
stances of time and weather.
It is best to choose specimens from flowers which are
already open, and in which growth has just ceased. Although,
CIIEMICAT. REAGENTS ON LONGITUDINAL RESPONSE 131
as already stated, this is not the most excitable period for the
tissue, yet it affords us the advantageof simplified conditions,
inasmuch as, owing to cessation of growth, the line of record
before stimulation, or the base-line, now remains horizontal.
Stimulation, producing contraction, is represented by up
movements of response, and the recovery brings the curve
back to the base-line.
Effect of carbonic acid gas. — I shall now proceed to
describe a few typical experiments, on the modification of
response by chemical reagents, out of a large number, which
were performed on the radial organs of plants of various
kinds, in the course of the season. And first we shall take
the effect of carbonic acid.
The effect of this gas, after
the lapse of about half an
hour, is one of considerable
depression. By this time
the responses are reduced
to about half their normal
value, and this depression,
though slow, is continuous.
This may be looked for as
the general effect, after a
certain length of time, of
the action of carbonic acid
gas. The immediate result of the sudden introduction of an
abnormal factor may, however, be slightly different in different
cases, according to the tonic condition of the tissue. This
immediate effect is sometimes one of brief depression followed
by equally brief exaltation, to be succeeded by the true
depression. Or there may be a short exaltation, followed by
the true depression. The restoration of the normal con-
dition, however, is generally followed in the case of carbonic
acid gas by a gradually increasing exaltation of the response,
which may culminate in double its ordinary height, and after
this it again attains the normal (fig. 68).
Effect of hydrogen gas. — We next investigate the
vvv\ wmvaN^^i^'v
Fig. 68. Effect of Carbonic Acid Gas
on Longitudinal Contractile Response
(rt) Normal response ; (d) after exposure
to carbonic acid ; {c) transient exalta-
tion after readmission of air.
32
PLANT RESPONSE
effect of hydrogen gas. The characteristic effect oi depres-
sion occurs in this case after a much longer interval than
is required by carbonic acid. The immediate result of
application is very erratic and various. There may some-
times be an exaltation, or even the contrary, a reversal, of
a
M
b
Tn nKK\
]W\
N
w\
N\/WM^\
Fig. 69. Effect of Hydrogen Gas
[a) Normal response ; {b) after twelve hours' exposure to H. ;
(r) slow revival of response after readmission of air.
the normal response. But after a twelve hours' exposure to
the action of this gas the responses of the tissue are so much
diminished as to approach very near abolition. On now,
however, allowing air charged with water-vapour to displace
the hydrogen gas, the responses undergo a steady revival
(fig. 69). There is here no sudden
exaltation, such as is the general after-
effect of carbonic acid gas.
Effect of carbon disulphide. —
We have seen that the depressing
effect of hydrogen takes place very
slowly. This is owing to the fact
that this gas acts here rather as an
agent for cutting off that supply of
oxygen that is necessary to the
Slf "SlpM? t maintenance of the normal life of the
plant, than as a direct poison. But
we have other gases which are actively
toxic, and in such cases the diminution
or abolition of response takes place
Such an agent may be found in the
Fig. 70. Photographic Re-
Abolishing Response
Normal response, seen to
left, abolished by intro-
duction of vapour of CSo.
with greater rapidity.
vapour of carbon disulphide (fig. 70).
CHEMICAL REAGENTS ON LONGITUDINAL RESPONSE 1 33
I may here draw attention to the great advantage offered
by the study of the variation of longitudinal response, in
determining the nature of the action of various chemical
agents. The modifications which these agents produce in
the lateral response of the pulvini of sensitive plants are
not so simple, inasmuch as we have to deal in these cases
with differential action. In radial organs, on the other
hand, the response-record gives us indications of the specific
action of each modifying agent. In the response itself there
are, it must be remembered, two factors which have to be
distinguished, namely, contraction in response to stimulus,
and the power of recovery from contraction, or relaxation.
The diminution and final abolition of response may be
brought about, then, in two different ways. The effect of a
given agent may be to diminish the normal relaxation which
brings on recovery. Succcosive stimuli will in that case
produce a cumulative residual contraction, which places the
tissue in a state of strain, in consequence of which subsequent
responses become enfeebled. We may, on the other hand,
have an agent whose effect is to produce abnormal relaxation.
The contractile impulse due to stimulus is in this case
opposed by the abnormal relaxation induced by the agent,
and we have, in this case also, an enfeeblement and abolition
of response.
The comparison of the time-relations of the normal and
modified curves, together with the trend of the base-line up
or down, will show the nature of the reaction, in an unmis-
takable manner. All these facts are clearly demonstrated
in the experiments and curves given below.
Effect of alcohol. — I shall next describe the action of
the vapour of alcohol. Generally speaking, the immediate
effect in this case is one of exaltation, though individual
idiosyncrasies may sometimes be present, which cause
depression from the very beginning. The general effect of
this reagent, however, appears to be a prolongation of the
period of recovery. So what is gained by brief exaltation
is lost again by induced sluggishness. Thus, from the result
134 PLANT RESPONSE
recorded on a fast-moving drum, using the style of Datura
alba, I find that the height of the normal response was eleven
divisions, and complete recovery took place in one minute
and a quarter. During the first period of exaltation, after
the application of alcohol, the height of the response was
increased to sixteen divisions, that is, practically half as
much again. But the period of recovery was protracted to
four and a half minutes, or nearly three times the period of
normal recovery. These considerations will fully explain
the series of responses under the continued action of alcohol-
vapour, given in fig. 71, where {ci) shows normal response,
{h) the immediate and transitory exaltation, and {c) — which
•
Fig. 71. Efiect of Vapour of Alcohol
{a) Normal response ; [h) immediate temporary exaltation on introduction
of alcohol ; {c) subsequent depression.
was taken after fifteen minutes' further application — the
diminished responses in which the contraction remainders
are a marked feature. On blowing off the alcohol-vapour,
however, and substituting fresh air, the tissue is found to
recover slowly its normal excitability. If, instead of alcohol-
vapour, dilute alcoholic solution be applied, the depressing
effect is immediate and very great.
Effect of ether. — We now pass on to the question of the
action of the anaesthetic agent, ether. This produces a relaxa-
tion so great as to be incapable of proper representations
within the limits of the diagram (fig. 72), where it is merely
indicated by the dotted line. It is to be remembered that
contraction is shown by lines upward, and recovery, or
CHEMICAL RKACKNTS ON LONCITUDINAL RESPONSE 1 35
relaxation, by lines downward. Owing to the predominance
of this relaxing tendency, it will be seen that true contrac-
tile movement is here very much diminished. Even after
the relaxation has attained its
maximum, the responses remain
insignificant. When the speci-
men is not too long etherised,
the blowing in of fresh air
brings on gradual restoration.
It may be mentioned here that
ether also produces relaxation
of animal tissue.
Explanation of anomalous
effect of ether on Mimosa. —
This experiment on the effect
of ether affords a very satis-
factory explanation of a phe-
nomenon in the recovery of
Mimosa, which has hitherto
been regarded as anomalous,
the leaflets is generally regarded as one of sensitiveness ; but,
when they have just closed, in consequence of stimulation,
if they be subjected to ether-vapour, they open out. Though
now, however, mimicking the appearance of sensitiveness,
they are in fact over- relaxed, a condition which is one of
relative insensitiveness. The explanation may be gathered
from the record in fig. Ji. It is necessary here to give
specific meanings to certain terms which have been used
somewhat indefinitely. We know that stimulation causes
the fall of the leaf, by differential contraction, and that the
organ recovers, or 'relaxes,' into its original form after a period
of rest. This term ' relaxation,' then, may be properly used
as the converse of ' contraction.' But, in consequence of the
expulsion of water from the organ after stimulation, it becomes
flaccid, and this condition also is sometimes vaguely described
as ' relaxed.' In my own use of the word, however, I shall
confine myself to denoting by it that process which is the
Fig. 72. Effect of Ether
Arrow marks moment of application.
This produced relaxation and
depression of response. Air
substituted at x , and there is
subsequent recovery of response.
The outspread position of
136
PLANT RESPONSE
opposite of contraction, and which therefore brings about a
position contrary to that effected by stimulus.
We have seen that ether produces a relaxing effect which
is more rapid than the process of relaxation that brings about
recovery. And we have seen how, in consequence of this
excessive relaxation, excitatory contraction is diminished or
abolished. Hence, we see how a stimulated Miinosa leaflet
under ether relaxes into an outspread position, which never-
theless is indicative of no re-
newed sensitiveness such as ac-
companies true recovery.
Effect of vapour of hydro-
chloric acid. — I shall now deal
with the case of strongly
poisonous agents, of which
hydrochloric acid may be taken
as typical. On passing the
vapour of hydrochloric acid into
the plant chamber there was
produced a great relaxation,
and the responses underwent a
rapid diminution which ended
in abolition. The effect of this
poison is so persistent that
the blowing-in of fresh air did
nothing to revive the response. But the timely application
of vapour of ammonia is found to act as an antidote, restoring
the response t'fig. 73).
Effect of chlorine gas. — This gas also produces a marked
depression of excitability, which, under long-continued action,
brings about the permanent abolition of response. The
accompanying photographic record (fig. 74) shows the effect
very clearly. The normal responses to the left are seen to
be very rapidly diminished after the application of this gas,
the response being reduced to one-eighth of its original value
in the course of nine minutes. There are other important
considerations in connection with this question, of relaxation
Fig. 73. Effect of HCl Vapour
Arrow indicates moment of ap-
plication. Depressing effect
neutralised by antagonistic
action of NH.. at y. .
CHEMICAL REAGENTS ON LONGITUDINAL RESPONSE 1 37
A\\\\
or contraction as the direct effects of chemical agents, which
it would be out of place to treat in detail here. It need only
be stated that these effects, which can be very accurately and
continuously recorded by the arrangement ot the Optic
Lever, are very suggestive. They are found to be modified
by the tonic condition of the tissue, the strength of the agent,
and the duration of application. Thus, an effect of relaxation
may, after a time, pass into the opposite, of contraction. And
since these relaxations or contractions of the tissue have a
modifying influence on the
response, much light on the
obscure subject of the effect
of drugs becomes possible
through this study. I have
been able already to obtain
several curious and interest-
ing results, of which I may
here refer to one, in which
two drugs, either of which
when applied singly would
abolish response, and produce death, are made, when
applied in succession, to act as antidotes to each other. It
is my intention to show in the course of this book that all
the physiological phenomena of the animal have the closest
correspondence with similar phenomena in the plant, and
this being so, an investigation carried out on the lines
indicated, with plants, is likely to be of very great import-
ance, practically as well as theoretically.
Fig. 74. Action of Chlorine
Photographic record, showing normal
effect to the left, depressed and
almost abolished after introduction
of the gas.
Summary
The responsive contractions of an organ afford a reliable
indication of the excitability of the tissue.
The physiological changes induced in plant-tissues by the
action of chemical reagents are outwardly manifested by
modification of response.
Certain agents, producing great relaxation, reduce the
138 PLANT RESPONSE
power of responsive contraction. Others produce the
opposite effect, thus protracting the natural period of
recovery.
H3'drogen gas produces a gradual diminution of response,
which is restored to its original value on the readmission
of air.
Carbonic acid causes depression, but the restoration to
normal conditions is generally followed by temporary exalta-
tion above the normal.
Vapour of alcohol causes gradual, and solution of alcohol
rapid, depression. This action may be preceded by tem-
porary exaltation. The recovery-period is very much pro-
tracted.
As in the animal, so also in the plant-tissue, ether causes
marked relaxation. The depression of response increases
progressively with the exposure ; on blowing off the vapour,
response is not only restored but may even show an exalta-
tion. The opening of the Mimosa leaflets under ether is not
indicative of true recovery but of over-relaxation.
A poisonous reagent, like hydrochloric acid or chlorine,
permanently abolishes the response.
Reagents which individually abolish response may, by
their antagonistic character, act as antidotes to one another.
CHAPTER XII
EFFECT OF TEMPERATURE
Temperatures oplimum, maximum, and minimum — Diminution of electrical
response by cooling — Temporary or permanent abolition of response due to
cold — Characteristic differences exhibited by different species — Mechanical
response of Biophytiim and autonomous response of Desniodium arrested by
cold — Prolongation of latent period — Diminution of longitudinal mechanical
response by cold — Diminution of electrical response of plants by rise ot
temperature — Similar diminution seen in longitudinal mechanical response —
Increase of excitabihty due to cyclic variation of temperature.
One of the factors which modify response in plants is
temperature. It is known in a general way that certain
temperatures are favourable, and others unfavourable, to
physiological activity. It is generally understood further
that there is a certain optimutn, in the case of each species,
above or below which the excitability of the plant undergoes
diininution. After this, on reaching a certain maximum or
minimum temperature, as the case may be, excitability is
abolished, and if these unfavourable conditions be long main-
tained the plant is apt to be killed. But the problem of the
precise determination of such points has hitherto offered
insuperable difficulties.
Effects of cold : {a) Diniiniition or abolition of electrical
response. — Already, however, by adopting the electrical mode
of investigation, I had been able to overcome these difficulties ;
for I had found that the amplitude of the electrical responses,
under different temperature-conditions, afforded a means of
measuring the excitability of a tissue at the respective points.
And now, by the use of mechanical response, I am enabled
again to investigate the same problem by new and indepen-
dent means. From the results obtained it will be seen that
I40
PLANT RESPONSE
each method furnishes a remarkable corroboration of the
other. I shall now proceed to describe these various results.
As an effect of low temperature I have found, by the use
of the electrical method, that response undergoes a very
great diminution. For example, the subjecting of a petiole
o{ Eucharis Lily to a temperature of —2° C. almost abolished
its excitability (fig. 75).
(^^) \ ^ When the specimen,
however, was restored to
the normal temperature
the original response
reappeared, and some-
times with even greater
amplitude than at first.
When the plant is
maintained at a very low
temperature for a con-
siderable length of time,
the normal electrical
response disappears al-
together, and the spe-
cimen undergoes per-
manent death. In this
respect, different species
of plants have charac-
teristic powers of resist-
ance. For example, the
tropical plant, EiicJiaris
Lily, after an exposure
of twenty-four hours to
a temperature of 0° C,
on being subsequently restored to its normal temperature,
gives no sign of revival by response ; whereas the hardier
Holly and Ivy, when subjected to the same treatment, do
exhibit signs of renewed life (fig. 76).
{b) Prolongation of latent period, or abolition of lateral
and autonomous responses. — Turning now to mechanical
ih)
Fig. 75. Diminution of Response in Eucharis
Lily by Lowering of Temperature
{a) Normal response at 17° C.
{b) The response almost disappears when plant
is subjected to -2° C. for fifteen minutes.
{c) Revival of response on warming to 20° C.
EFFECT OF TEMPERATURE
i4r
response, I find in the case of the plant Biophytum that the
effect of too great cold, or of long-continued exposure to low
temperature, is the abolition of its lateral response. But
when the temperature is, relatively speaking, only slightly
lowered, induced sluggishness is shown in a very interesting
manner. Whereas, at the normal temperature, say 23° C,
the response of the leaflet of Biopliytum takes place imme-
diately, after moderate cooling, on the contrary, the latent
CL
a
KK
Ivy Holly Eucharis
Fig. 76. After-effect of Cold on Ivy, Holly, and Eucharis
(n) Normal response ; {/') response after subjection to freezing temperature
for twenty-four hours.
period is increased, and response does not begin to take place
until from one to two seconds after the application of stimulus.
Lowering of temperature also abolishes the autonomous
response of Desniodium.
(c) Dimimition of longitudinal response. — With regard to
longitudinal response, I have been able to demonstrate the
effect of cold, by taking a specimen of the coronal filament
of Passiflora. The specimen was kept for fifteen minutes
in ice, after which electrical stimulus was applied with no im-
mediate contractile effect. Control specimens, on the other
hand, exhibited considerable contraction. These contractions
were measured in both cases by means of a micrometer. The
specimens taken for experiment were all 21 mm. in length.
The average contraction of the control specimens was
I '5 mm. The cooled specimen, as said before, exhibited
no im.mediate response. But on continuing stimulation for
two minutes, contraction began to take place slowly, reaching
a maximum of only -5 mm. It must be borne in mind
142
PLANT RESPONSE
that the specimen was all this time undergoing gradual
warming by the temperature of the room.
Another way of demonstrating the effect of cold on the
longitudinal response of this corona, is to take three filaments ;
of these {a) has been subjected to cold, {b) is normal, and {c)
has been killed by immersion in hot water at 60" C. The right-
hand ends of the filaments are so arranged as to lie perfectly
even, and a moist cotton thread touches these ends. A
brass spring presses against the left-hand ends. The thread
and the brass spring serve the purpose of two electrodes, by
which shocks can be sent through the three specimens at the
same time.
The right-hand end of the object, so arranged, is placed
within the field of a microscope of low magnifying power
which has a micrometer eye-piece. At the beginning the
ends of the specimens lie in a straight line, but after the
passage of a shock for a period of a few seconds it is found
that, while the normal {b) shows the maximum contraction,
the cooled {a) exhibits very little, and the killed {c) none
at all.
The difficulty in conducting experiments on cooling, lies
in the fact that the inertness due to cold is liable to disappear
with more or less rapidity when the specimen, for experi-
mental purposes, is exposed to the temperature of the room,
which is about 23° C. It cannot be kept immersed in ice-
cold water, as the exciting current will then to a great
extent pass through the conducting water itself The only
practicable way, therefore, is to subject the specimen to
prolonged cold, and make a rapid observation afterwards.
I next attempted to obtain a series of responses, when
the temperature of the space in which the specimen was
placed was very gradually lowered. As recovery from the
stimulus of electrical shock constitutes a very prolonged
process, I had to use thermal stimulation. But this intro-
duced the difficulty of itself raising to a certain extent the
temperature of the space. I had been fortunate enough,
however, to secure a few specimens of the style of Datura
EFFECT OF TEMPERATURE I43
which were extremely sensitive to cold, and thus, notwith-
standing the disadvantages incidental to the experiment,
I was able to obtain the very interesting records shown
below. The experiment was carried out as follows :
The specimen was mounted in the special plant chamber.
By pioper manipulation of the stop-cocks it was possible
to send at will through the chamber, first, air at the
ordinary temperature — under which conditions the normal re-
sponses were taken — and, secondly, air which had been cooled
by ice, reducing the temperature of the chamber by several
degrees. The responses then
obtained showed the effect of cool-
ing. And, lastly, ordinary air was
re-introduced, and the responses
at this temperature again re-
corded. Fig. 77 shows that while ^^i*- 77- . Effect of Cold on
r 1 Eongitudinal Response
the amplitude of the normal , ^ ^, , ,, ^
'■ . ... ('^) Normal response ; {0) alter
responses was eight divisions, cooling; (,-) during gradual
that of the responses at reduced Restoration to normal tempera-
temperature was only three, and
that, on the restoration of normal conditions, the responses
increased in a staircase manner tending to return to their
original value.
Effect of rise of temperature : (a) on electrical re-
sponse.— So much for the results of cooling ; we have now
to study the effect of rise of temperature. And, first, I
shall refer to observations made by means of electric
responses. It may be said that the optimum degree of
temperature for the excitability of the plant must be under-
stood to vary with different species. In several cases, how-
ever, I have found it to lie at about 22° C. But it must
be premised that this optimum of response refers to passive
tissues only, that is to say, to those in which there is no
growth. The optimum temperature for growth may be
different. In the ordinary response of passive tissues, heat
has only to bring about a condition favourable to mobility.
In the case of growing tissues, however, something addi-
144
PLANT RESPONSE
tional has to be efifected, namely, the acceleration of growth.
We may therefore expect that the optimum temperature
for growth will be relatively higher than that of simple
response. And this I generally find to be the case. When
the temperature is much raised, the electrical response of a
plant is seen to undergo diminution (fig. 78). It is to be noted
that the temperatures referred to are dry, or atmospheric only.
This explains why, even when the temperature was raised to
65° C, which is above the fatal point, response was still
^
20°C
30T ^gof, gg.fy gjc^ 55Tfn')
]lJLiUJUA_A_yv_A- —
1 min
Fio. 78. Effect of Rise of Temperature on Electrical Response
observed. Owing to the relative non conductivity of the
plant, and the evaporation from its surface, the tissue docs
not actually attain the temperature of the surrounding air.
When, however, it was subjected for some time to a water-
temperature of 55° C. response disappeared, by the death of
the specimen.
{b) On longitudinal mechanical response. — I give next
(fig. 79) the effect of rise of temperature on longitudinal mecha-
nical response. The specimen was a
filament of the corona of Passiflora,
the stimulus used being thermal. In
order to subject the pldnt to definite
temperature conditions, the adjust-
ments were made by means of a
subsidiary heating coil, placed inside
iven temperature above the normal
could thus be maintained constant, for the required length of
time, by simple adjustment of the strength of the heathig
current. The results thus obtained in mechanical response
Fig. 79. Effect of Rise of
Temperature on Longi-
tudinal Contractile Re-
sponse of Plant
the chamber. Any gi
EFFECT OF TEMPERATURE
H5
will be seen to be exactly parallel to those already given in
electrical response.
The responses are seen to be very much diminished at 35°C.,
and almost to disappear at 55° C, v/hich, as will be shown later,
is near the death-point. Heat-rigor began to manifest itself
a few degrees above this point, in the contracting of the speci-
men as a whole. This last phenomenon will be treated at
greater length in a subsequent chapter. I have already alluded
to the experiment in which, when the plant is killed by ex-
cessively high temperature, the response disappears altogether.
Effects of cyclic variation of temperature : (a) on
electrical response. — I detected another very curious result
19* C
Temperature
falUru/
70 C
Temperature rising ^
Effect of Rising and Falling Temperature on the Electrical
Response of Scotch Kale (stimulus constant)
in the course of my investigations on the effect of teinpcrature
by means of electrical response. This was, a marked increase
L
146 PLANT RESPONSE
of sensitiveness, which often appears as the after-effect of a
preceding cyclic variation of temperature. That is to say, if
we take a series of responses while the temperature is rising,
and afterwards a similar series while the temperature is falling,
it is found that during the process of cooling, the responses
are markedly enhanced in amplitude, as compared with those
given at the corresponding temperatures during heating.
This is seen in a very clear manner in fig. 80.
{b) On longitudinal mechanical response. — I have ob-
served a very similar phenomenon m the longitudinal
mechanical response of, for example, the coronal filaments of
Passiflora. An ascending series of responses was taken at
temperatures of 25°, 35°, and 45° C.
They exhibited a regular decrease,
as has already been explained. On
now, however, taking records of re-
sponses during cooling, it was found
that the response at 35° C. was 50 per
Fig. 81. Effect of Cyclic , , .v v u j u u .lu
Rise and Fall of Tempe- cent, greater than It had been when the
rature on Longitudinal temperature was ascending (fig. 8 1 ).
Mechanical Response in , • ,
Plant 1 have already explamed that
response is found to be abolished
when the plant is killed, by raising the temperature above
a certain maximum point. The exact determination of
this point has hitherto been a matter of great uncertainty.
I shall in the next two chapters, however, explain several
methods by which this investigation may be carried out with
great precision.
Summary
The electrical and mechanical responses of plants undergo
diminution under the influence of cold.
The latent period of response is prolonged by lowering of
temperature.
When the temperature is raised above an optimum, the
EFFECT OF TEMPERATURE 147
responses, both electrical and mechanical, undergo a diminu-
tion.
Prolonged exposure to excessive cold or heat brings on
abolition of response. Owing to cold or heat-rigor this
abolition may become permanent, indicating the death of the
plant.
CHAPTER XIII
ON THE DEATH-SPASM IN PLANTS
Difficulty of determining exact moment of death — Various post-mortem symploins
afford no immediate indication — Ideal methods for determination of death-
point — Realised in four different ways : {a) Determination by electrical method
— [b) Determination by spasmodic lateral movement at moment of death —
Experiments with A/iinosa — Death-contraction a true physiological response —
Continuity of fatigue and death— Death-point earlier in young tissues— Com-
posite spasmodic movement — (^') Determination of death-point in tendril of
Fassiflora, by sudden movement of uncurling— (c) Determination of death-
point by method of volumetric contraction of hollow organ, causing expulsion
of contained water.
It is known that when the temperature to which it is
subjected is raised above a certain maximum, a plant is
killed. But it is very difficult to determine at what exact
temperature this takes place. One reason of the difficulty
lies in the fact that hitherto a sure criterion of death, which
would <^ive an inunediate and reliable indication of its occur-
rence, has not been generally available. Its various symptoms,
such as drooping, withering, discoloration, and the escape of
coloured cell-sap, do not manifest themselves at the moment
of death, but at some time indeterminately later. Even
when a plant has been subjected to a temperature in excess
of the fatal degree, it continues to appear fresh and living,
and it is not till after some greater or less interval that the
death-symptoms are seen. Various investigators have taken
up different indications as the criteria of death, and this fact
accounts for discordance in the results, which would already
have been sufficiently uncertain even had a common standard
been decided upon.
Exact methods of determination of death-point. - A
good method for the determination of the death-point would
THE DEATII-SPASM IN PLANTS I49
consist in watching the waning of a given effect, characteristic
of the living condition. Still better would be the discovery
of some effect suddenly and strikingly manifested at death.
But the ideal method would be found, if some effect could be
detected which at the moment of death would undergo
sudden reversal to its opposite. In this last case, there
would not be even that minor degree of imcertainty which is
incidental to the determination of the exact vanishing-point
of a waning effect. I have been successful in devising four
distinct means, by which the death-point might be detected
with precision, and it will be shown that all these different
modes of investigation enable results to be obtained which
corroborate each other in a remarkable manner. The four
means are : (a) the method of electrical response ; (d) that
method by which the point of death is determined from the
occurrence of a spasmodic movement, in a dorsi-ventral or
anisotropic organ ; (c) that method which depends on the
sudden expulsion of water at the moment of death from a
hollow organ, previously filled with liquid ; and (d) the
method in which the death-point is determined from the
sudden reversal of a thermo-mechanical response-curve. I
shall, in the course of the present chapter, describe the first
three of these, leaving the fourth method to be treated in the
next chapter.
(a) Determination of the death-point by electrical response. —
As regards the electrical method, I have shown elsewhere '
that the response of normal galvanometric negativity is cha-
racteristic of the living condition of a plant-tissue. When the
plant is killed, by any means whatsoever, this normal response
disappears. At the moment of death from rise of temperature,
therefore, we shall have the abolition of the normal negative
excitatory response. But at or beyond this point, on the other
hand, we may have the positive response of hydrostatic dis-
turbance replacing the true excitatory effect. By this electrical
mode of investigation, I have been able to determine the
death-points of different plants. In the following table, for
' Bose, I\esponse in the Living and Non-Liz'iiii^; p. 62.
I50 PLANT RESPONSE
example, the specimens tested were radishes, and the experi-
ment was conducted during the winter season in England,
It would appear from the results given, that in these six cases
response begins to be abolished at temperatures varying from
35° to 55° C. It will be shown later that the death-point
depends on the season, being a few degrees lower in winter
than in summer.
Table showinc Effect of High Temperature in Abolition of
Response and Death of Plant
Galvanometric re- ' Galvanometric re-
Temperature sponse of specimen Temperature spon.se of specimen
at given temp. at given temp.
(17° C 70 dns. (17= C 80 dns.
(1) (4)
(53° C 4 „ (55° C o „
(i7°C 160 ,, (17° C 40 „
(2) (5)
53° C I ,, I60'' C o
'17° C 100 ,, (17° C.
(6)
.50-^ C 2 „ 1 (55" C.
{b) By observation of the spasmodic vwvciuent of lateral
response. — I now turn to the second of the four methods
which I have named, that in which a spasmodic lateral
movement is looked for, in a dorsi-ventral or anisotropic
organ, at the moment of death. It has been shown that
when an electric shock of moderate intensity is applied
to an anisotropic organ, say the leaf of Mimosa, response
occurs, in consequence of molecular derangement, and recovery
takes place on restoration of molecular equilibrium. If the
shock, however, be excessive, response occurs, it is true,
but there is no subsequent recovery, owing to the fact
that the molecular derangement has passed beyond the
point where restoration was possible. There is thus a per-
manent, irreversible ' set,' and the organ is now said to be
killed.
At death, then, a sudden irreversible molecular derange-
THE DEATH-SPASM IN PLANTS 151
mcnt is produced. It follows that if wc could bring on very
gradually those conditions which cause death, then, on arrival
at the critical point, we might expect the irreversible mole-
cular derangement to occur abruptly. If, further, throughout
this process, the organ could be protected from stimulation,
we might expect that this sudden molecular derangement
would also be attended by a correspondingly sudden evidence
of excitation, which would in this case, however, be at once
the first indication of excitation and the last sign of life.
This spasmodic movement we shall designate as the death-
response.
As regards the protection of the experimental organ from
accidental stimulation, it is to be remembered that excitation
under ordinary circumstances depends upon some sudden
variation of environmental conditions. A sudden change of
temperature may thus act as a stimulating agent and produce
depression of the leaf But a gradual change will not act as
a stimulus. The effect of such a gradual variation, on the
contrary, as will be shown presently, is to produce no excita-
tory contraction whatsoever.
If now we take a specimen of Mimosa and place it
suddenly in warm water, say at 35^" C, a responsive collapse
of the leaves will at once occur. But if the plant be placed
in water at the ordinary temperature of the room, and the tem-
perature gradually raised — say at a rate of 1° per one minute
and a half, or thereabouts — there will be no responsive
downward movement whatsoever. On the contrary, owing
to absorption of water by the organ, and also to the re-
laxing physiological action of heat, a delicate method of
record will show a slight and continuous movement upwards.
This proceeds till we reach a degree of temperature which
proves to be the death-point. For example, in the case of a
particular experiment in summer, with a young leaf of
Mimosa, when the temperature of 59° C. was reached, there
was a sudden spasmodic movement of the leaf downwards.
This was, in fact, the death-throe of the plant. In winter,
after a spell of cold weather, when the physiological condition
152 PLANT RESPONSE
of the tissue was somewhat depressed, this spasmodic move-
ment was found to take place at 53° C, which exactly agrees
with that of radish, under similar conditions. That this was
the true death-point of the Mimosa specimen was proved
when, on trying the electrical test, it was found that the
normal electrical response had disappeared. If, again, one
branch of Mimosa on the intact plant be bent over, and
subjected in the manner described to the death- temperature,
wc find, on examination after a considerable lapse of time,
that whereas the leaves of the rest of the plant are still fresh
and healthy, reacting to stimulus, those of this branch may
be seen, from their dried and shrivelled condition, to be
quite dead.
Death-response a true physiological response. — It will
be shown that this death-response is a true physiological
response. Under normal conditions, it will be found to be
extremely definite, even in different plants. But, under
physiological modification, it varies appropriately with the
season, age, condition of freshness or fatigue, and the action
of chemical reagents. I shall first, then, demonstrate the
effect of age on the death-point. Thus, on immersing a
branch of Mimosa in water whose temperature is raised con-
tinuously, we find that the spasmodic movement of death
occurs earlier in the young leaves than in the old. Young
seedlings, again, have a lower death-point than mature plants.
The following table gives results bearing on this fact. The
death-points of different plants in the same season and of the
same age are, however, so definite as to be almost like a
physical constant. This will appear from the following
tabular results, and also from results given in the next
chapter. A fact which will be explained later must be
stated here. In these experiments with continuously rising
temperature, it is found that the first spasmodic movement
downwards is succeeded by a later, upwards, by which time
the temperature has risen a few degrees. The temperatures
of both movements are given in the accompanying table,
corresponding to their occurrence in two leaves, one old and
THE DEATII-SPASM IN PLANTS
153
one young, of each plant tested. These experiments were
carried out at the beginning of spring.
Table showinc Death-points in Old and Young Leaves of
Different Specimens of Mimosa
Specimen
Young Leaf-organ Old Leaf-organ
Temperature
corresponding to
Temperature Temperature
corresponding to corresponding to
later erection fall
1
Temperature
corresponding to
later erection
L
IL
IIL
c. c.
61 -5° 60 -8'
61-50 1 61°
62 ' 61 -2°
C.
62°
63°
63-6°
Mean .
59-5° 1 617°
61°
62-9°
From these results it will be seen that there is a mean
difference of r5° C. between the death-responses of old and
young leaves. It would thus appear that the age of a cell
must be the occasion of a certain amount of protoplasmic
change, as manifested in the retardation of death-response.
We may also infer that sudden change to unfavourable
physiological circumstances — before the plant has accommo-
dated itself to the changed condition — will tend to lower the
death-point. This fact I found illustrated during the preva-
lence of an extraordinary wave of cold, which supervened
recently, during the progress of these experiments. I then
found that the mean death-point, in the case of various plants,
was reduced by several degrees. In Mimosa it fell from the
average of 59° C. to 55° C, i.e. as much as 4° C. We have
thus seen that the physiological differentiation concomitant
on protoplasmic change is attended by variation of death-
point.
Explanation of the subsequent erection, — In animal
tissues, the contraction produced by rigot^ mortis is succeeded
by a relaxation. The contractile death-spasm in a plant is,
similarly, followed by the relaxation seen in the subsequent
154 PLANT RESPONSE
erection of the leaf. There is another and very interesting
point of view, from which we may see in this phenomenon the
continuity of fatigue with death. In the curve of reversal,
due to fatigue, in Mimosa^ we saw that the first contraction,
induced by strong and long-continued stimulation, passed into
subsequent relaxation. In this latter state, the molecular
condition was such that responsiveness was abolished. It was
as if, in other words, the tissue had passed into a temporary
state of death. It is true that, if the stimulation had not
been excessive, the organ would recover its sensitiveness,
after a period of rest. But this transient would pass imper-
ceptibly into the permanent condition of death if, on the
other hand, stimulation had been excessive. In that case,
after the fatigue-reversal, the tissue would remain permanently
irresponsive.
I have already said that the death-spasm is an instance
of excitatory response to intense stimulation, and we should
therefore expect the same kind of effect to be produced as is
caused by excessive stimulation, that is to say, a preliminary
contraction, followed by relaxation, after which there is no
recovery. Again, we saw that in the fatigue-reversal of
Mimosa, the subsequent erection of the leaf, mainly due as it
was to relaxation, was possibly also aided by the later con-
traction of the less excitable upper half of the pulvinus.
Similarly, it might be expected that the death-contraction of
the less excitable upper, would take place slightly later than
that of the lower, half of the pulvinus. We have also seen
that the excitability of a tissue declines with age, and this
decline would naturally be greater in the more excitable half.
Thus the difference of excitability as between the two halves
would at the same time tend to disappear. As, then, this
spasmodic movement in dorsi-ventral organs is a true instance
of differential excitatory response, it would appear that the
younger the organ, the greater is the excitatory spasm caused
by death, and in experimenting with Mimosa I have found
that at the death-point hardly any spasmodic movement is
shown by old leaves. These considerations will also explain
THE DEATH-SPASM IN PLANTS 1 55
why older leaves give less motile indication than the young,
in response to stimulus in general.
{h') By observation of spasmodic movement of uncurling. —
I shall now proceed to demonstrate that the death- move-
ment which we have seen in Mimosa is in its essentials
characteristic of anisotropic response in general ; and this
may be shown by taking a spiral tendril of Passiflora which
has become anisotropic by curling. In order to detect and
measure with ease the responsive movements of uncurling
and curling the experiment is arranged as follows : a light
index is attached to the tip of the spiral, and the whole is
immersed co-axially in a glass cylinder filled with water. A
strip of paper, marked with degrees, is wrapped round the
outside of the cylinder on the plane of the index. The
temperature of the water is now raised very gradually, and
the responsive excursion of the index is read on the graduated
circle formed by the paper.
It is to be borne in mind that the true excitatory response
of the tendril is given by uncurling, which here corresponds
to the fall of the leaf of Mimosa ; thus the movement of
erection would be represented by that of curling. In the
case of Mimosa we saw that the first effect during rise of
temperature (due to absorption of water and relaxation) was
slow and gradual erection. On the arrival of the death-
point of the organ, however, this preliminary rise was
succeeded by a sudden responsive fall. The subsequent re-
laxation of death then produced an opposite movement, of
erection. Similarly, in the death-response of the spiral
tendril of Passiflora, we observe parallel phases. There was
first a slow and continuous movement of curling, during the
preliminary stages of warming. But this movement ceased
when a temperature of 57° C. was reached, and the tendril
remained stationary for a time. At 59° C, however, there
was produced a sudden excitatory response of death by an
uncurling, executed with great rapidity, an angular movement
of 360 degrees being described by the index during the course
of the next {q.\w degrees of rise in temperature. With regard
156 PLANT RESPONSE
to the short stationary period, it is to be borne in mind that
the death-point depends on the age of the tissue, and in the
tendril we have different parts in different stages of growth.
Hence while the uncurling movement of death was being
initiated in younger portions, older parts of the tendril were
still moving in an opposite direction. The outcome of these
antagonistic movements was a resultant pause, which only
lasted for a little while, and was followed by the vigorous
movement of uncurling, caused by the death-contraction of
the whole tissue. After the completion of the uncurling
movement, there followed the opposite, namely, the move-
ment of post viorteni relaxation. In a second experiment
with a younger specimen of tendril, I obtained results almost
identical. Here the uncurling response of death began at
one degree of temperature earlier, namely at 58° C, and the
index moved through 1 50 degrees of the circular scale.
On the subject of the death-contraction of the radial
organs of ordinary plants, I shall speak in some detail in
the next chapter, and shall there describe the perfected
apparatus by which the thermo-mechanical response can be
continuously recorded, the curve exhibiting the death point
with great precision.
{c) By observation of volumetric contraction, causing sudden
expulsion of water. — For the present I shall describe only
the third of those methods which I have enumerated for the
determination of the death-point, that namely which depends
on the sudden expulsion of water, at the moment of death,
from the hollow organ of an ordinary plant, previously filled
with liquid. The specimen used for the present demonstration
will be the peduncle of Allium, although there are many
tubular organs of various species of plants which are more
or less suitable for these experiments.
We cut a length of about 10 cm. from the middle of a
peduncle of Allium, rejecting the too young and too old
portions at top and bottom. As the presence of air-bubbles
is likely to be disturbing to the experiment, the water used
must have been previously boiled. The specimen is placed in
THE DEATH SPASM IN PLANTS
157
a vessel of this water, and as a further precaution against air-
bubbles clinging to the interior of the tube, the whole may
be put inside the receiver of an air-pump and subjected to
a repeated partial vacuum. The removal of air-bubbles may
also be effected by rinsing the tube of Allium in water
containing a small quantity of ether, and immediately after-
wards washing with ordinary water. This must be done,
however, with caution, as the presence of an appreciable
quantity of ether would be likely to affect the excitability of
the tissue. Before commencing the experiment, it is advis-
able to allow the specimen to
remain immersed in water for
about half an hour, by which
time it becomes fully turgid.
The lower end of the Allium
tube filled with water is closed
by a piece of solid glass rod,
and the upper end is also closed
with a piece of glass tube,
having a capillary bore. A
graduated scale is placed behind
this latter, so as to measure the
movement of the water- index, or
this movement may be con-
tinuously recorded on a revolving
drum (fig. 156). The Allium preparation is now placed in a
vessel of water, and subjected to a gradual rise of temperature
in the manner already described.
If, at the temperature corresponding to death, there
should now be a sudden excitatory contraction of the Allium
tissue, the volumetric change thus produced will force out
the contained water, and we shall observe a relatively rapid
expulsive movement of the water-index. From the curves
given above in fig. 82, it will be seen that this occurs at
a temperature of 59° C. in a younger, and at G^" C. in an
older specimen. Previous to this, there was an inward
movement of the water-column corresponding to the gradual
Fk;. 82. Determination of Death-
point in Allitiin Tube by Ob-
servation of Volumetric Con-
traction, causing sudden Expul-
sion of Water
A, record given by older specimen ;
death-point 63° C. B, record
of younger specimen ; death-
point 59° C.
158 PLANT RESPONSE
preliminary rise of the Mimosa leaf or curling of the Passiflora
tendril. But at the death-point this movement was at first
arrested, and then reversed with accelerated speed. The
condition of this particular specimen was afterwards tested
by the electrical method, when it was found that, while a
portion of the peduncle previously cut off gave the normal
response of living tissue, the specimen which had been
subjected to the death-temperature gave no response.
Summary
The death-point of a plant, under heat-rigor, is concomi-
tant to the disappearance of the true excitatory response of
galvanometric negativity. After this point is passed, hydro-
"static disturbance generally gives rise to the reverse positive
response.
When an anisotropic organ like the pulvinus oi Mimosa is
gradually raised in temperature, then, at a certain critical
temperature, a spasmodic movement is produced which
proves to be the death-response of the organ. This is a true
excitatory response. The critical death-points of similar
specimens are very definite and practically identical.
The death-point is modified by the physiological condition
of the tissue. Other things being equal, death occurs earlier
with a young than with a mature tissue.
The death-response of an anisotropic organ is composite.
In Mimosa it consists of a down, followed by an up, move-
ment. This is due to the death-contraction being followed
later by the. post-mortem relaxation of the organ.
In the spiral tendril of Passifiora, the death-response is
given at the critical point by a sudden uncurling.
In the case of the hollow peduncle of Allium, the death-
response is exhibited by volumetric contraction, producing
sudden expulsion of contained water.
CHAPTER XIV
THE DETERMINATION OF THE CRITICAL POINT OF DEATH
BY INVERSION OF THE THERMO-MECHANICAL CURVE
Death-spasm in anisotropic organ due to differential longitudinal contraction — In
radial organ the death-contraction is purely longitudinal — Death-point deter-
mined from point of inversion of a thermo-mechanical curve — The complete
record thus constitutes a curve of life-and-death, the two being separated hy
the death-point — Characteristic thermo-mechanical curve as resultant of varia-
tion of temperature and variation of length — The necessity of specifying the
rate of rise of temperature — Tlie thermo-mechanical curve characterised by
sharp and definite inversion at point of death — No inversion of thermo-
mechanical curve after death of plant — Death-contraction under heat-rigor in
plant analogous to similar phenomenon in animal — The Morograpli, a perfected
form of apparatus for determining critical point of death — Remarkable identity
of thermo-mechanical curves obtained with two similar specimens — Death-point
almost as definite as a physical constant — Vanishing of point of inversion with
age — Determination of death-point under cold-rigor — Constancy of death-point.
We have seen how the death-point can be determined in
anisotropic organs, by the occurrence of a spasmodic lateral
movement. We have also seen that this death- movement is
an excitatory response, at once initiated and terminated at
the point of death. It has been shown further that the
anisotropic is simply an instance of differential longitudinal
response. It follows that if the death-spasm in the aniso-
tropic organ be indeed caused by true excitation, then from
a radial organ, at the moment of death, we should obtain a
purely longitudinal contraction. We may look upon this phe-
nomenon, again, from a purely molecular standpoint. We can
then see that if death be brought about by a sudden mole-
cular change, such an event might be expected to exhibit
itself in a correspondingly sudden change of form. Let us
l6o PLANT RESPONSE
then take a radial tissue, and subject it to a gradual rise of
temperature, taking a continuous record of its variation in
length. From what has already been said it will be under-
stood that, the variation being gradual, no responsive con-
tractile effect will be induced during the process, but, on the
contrary, some relaxation. At the death-point, however, a
sudden inversion of the curve, due to death-contraction, may
be expected to appear, and thus the whole record will consti-
tute a curve of life-and-death, this point of inversion separating
the two. Should the inversion prove to be very abrupt, the
turning point will afford us a means of determining the
temperature at which death occurs, with very great accuracy.
In order to prove, further, that this specific response is
definitive, we may, after passing the death-point, bring the
tissue back to its original temperature once more, and repeat
the process. The record ought now to show no inversion
characteristic of a transition from life to death.
Means of obtaining thermo-mechanical record.— I shall
now proceed to describe the manner in which I have obtained
this thermo-mechanical record. I took a specimen of a radial
organ, in this case the long style oi Datm^a, and fixed it to a
small glass rod which in its turn was fastened to a weight,
the whole being placed in a vessel of water. The free end of
the style was attached to one arm of the Optic Lever. The
bath was now warmed gradually, a thermometer indicating
the rise of temperature. Variations of length corresponding
to the rise of temperature were progressively recorded, from
the movement of the spot of light. For this purpose the
mode of procedure was as follows : The vertical movement of
the spot of light— occasioned by the variation of length of
the specimen — was converted into lateral, by reflection from
a second mirror. The paper wrapped about the recording
drum was divided into millimetres. It was required that the
abscissa of the thermo-mechanical curve should represent
temperature, and the ordinate the corresponding length.
The position of the spot of light, at any given temperature,
was marked on the drum. At each rise of temperature of
DETERMINATION OF CRITICAL POINT OF DEATH l6l
1° C. the drum was rotated, say, through a distance of 2 mm.,
and the position of the spot recorded. In this way, by con-
necting the recorded points, a curve was obtained, in which
the length corresponding to each temperature was known.
In this curve an abrupt inversion, due to sudden death-
contraction, was found to occur at about 59° C. The curve
thus obtained, however, though the successive points recorded
were very near each other, is the result of intermittent
observations. Again, two observers were required, one to
read the temperature, and the other to take the record. It
was therefore subject to error of thermometric reading.
Means of obtaining automatic record. — For this reason
I was desirous of obtaining a curve which should be con-
tinuous and practically automatic, the plant itself being made
to record its own variations of length, and its own death-point.
The problem resolves itself into that of making the reflected
spot of light partake of two motions simultaneously, namely, a
horizontal movement proportional to the change of tempera-
ture, and a vertical movement proportional to change of length.
The horizontal, or thermometric, component of the movement
I secured as follows : I constructed a thermo-electric element
of iron and nickel, one junction of which was placed in melting
ice, and the other junction in the vessel of water containing
the specimen whose temperature was being subjected to change.
This element was placed in circuit with a resistance box and
a sensitive reflecting galvanometer. The amount of the
movement of the galvanometer spot of light could now, by
interposing suitable resistance, be brought to any appropriate
value. In my experiments, with a particular galvanometer,
the movement of this spot of light, for each degree of rise of
temperature, was 2-5 mm. — i.e. one-tenth of an inch — when
the recording surface was at a distance of 125 cm. from the
galvanometer. This extent of movement was quite sufficient
for the purposes of the experiment, as it enabled estimates to
be made with ease, correct to one fifth of a degree. By inter-
posing smaller resistances, however, one-twentieth of a degree
could easily be discriminated. The excursion of the spot of
M
1 62 PLANT RESPONSE
light was now found to be strictly proportional to the rise of
temperature.
In order to combine this horizontal thermometric move-
ment with that vertical movement occasioned by the variation
in length of the specimen, the vertically moving spot of light
from the Optic Lever was thrown on the galvanometer-
mirror. The apparatus, it should be mentioned, was so
arranged that the two mirrors were as close together as
possible. The spot of light now, having been reflected
from two mirrors, directly described a curve in which the
abscissa gave temperature-variation, and the ordinate, varia-
tion of length. When the source of light is a point, that is
to say, a pinhole with electric light behind — the excursion of
the reflected ray upon a photographic plate will produce an
automatic record. Or the movement of the light may be
followed continuously with a pen.
Conditions for securing accurate death-point. — Here
I must point out certain conditions which must be kept in
view if we are to obtain a very definite death-point. We
know that if a plant be placed in an unfavourable environ-
ment, or in a temperature much above the optimum, for a
prolonged period, death will ultimately ensue. But inas-
much as these temperatures would only cause the death of
the plant by indirect and cumulative action through pro-
gressive derangement of the several functions, they cannot
in themselves be said to constitute death-points. To be
scientifically precise in such a determination it is necessary
that we should discover a temperature which is of itself
efficient to initiate sudden death. On the other hand, again,
as the contraction of death is a phenomenon of response,
we see that it must have a certain latent period. Some
interval elapses, moreover, during which the tissue is attaining
the temperature of the bath in which it is placed. Now if
the rate of rise of temperature be too rapid, then, owing
to the lag caused by these last two factors, by the time the
death-response commences, the recorded temperature may
have gone beyond the actual death-point.
DETERMINATION OF CRITICAL POINT OF DEATH 163
We thus arrive at two conditions which must be regarded
as mutually somewhat antagonistic. In the first place, in
order to obtain the immediate point of death, it is essential
that the plant undergo an exposure which is not too pro-
longed. In order, on the other hand, to make due allowance
for the latent period and for attainment of the surrounding
temperature, the rate of rise must be gradual and definite.
In the case of tissues which are not too thick, the latter of
these conditions is amply fulfilled by a rate of 1° C. per
minute and a half. We see, therefore, that in precise deter-
minations of the death-point, the rate of rise must always be
specified.
With thick stems, however, owing to relative want of
thermal conductivity, the attainment of surrounding tem-
perature and occurrence of death throughout the whole of
the organ is a very protracted process. The experiments
which I shall describe were made with specimens which
were not too thick, death at the fatal point being ensured,
when the rate of rise of temperature was that prescribed,
namely, 1° C. per every minute and a half
This definite rate of rise may be secured by using an
electric heating apparatus, such as is commonly employed
for boiling a tea-kettle. The current from the street-mains,
which is 220 volts, heats water too rapidly. But the desired
rate may be obtained by interposing an electrolytic rheostat
of copper sulphate, the current being brought to a suitable
value, by separation of the two electrodes through which the
current enters and leaves the electrolyte.
In this case, when placing the specimen in the experi-
mental bath, it is advisable to secure it to a bent glass rod,
which rests outside. For if it is placed in the metallic vessel
itself, the record will be subject to a certain disturbance,
owing to the expansion of the supporting metal while heating.
The expansion of the glass rod is so small as to be negligible.
In this way, using for experiment a filament of the corona
of Passiflora, I obtained a record, showing a very abrupt
inversion, corresponding to the death-point, which was
M 2
1 64 PLANT RESPONSE
between 59° and 60° C, I shall presently have occasion
to describe in detail the various characteristics of this
curve.
Having described the apparatus with which these curves
were recorded, it is necessary to point out the difficulties
which were encountered in working with it. It must be
remembered that the excursion of the spot of light, in this
case, represented a high magnification of the actual move-
ments involved. The spot of light, moreover, was re-
flected from two separate instruments, and was liable to be
disturbed by the slightest jar or tremor in either of them.
Though the instruments were placed on a steady stone
pedestal, even this precaution could not be made wholly
effective, in the heavy traffic of a town. It was only, there-
fore, in intervals of quiet that approximately perfect results
could be obtained. This difficulty led me to the devising of a
much simpler and more perfect instrument, which I shall
designate as the Morograph} This is a small and portable
apparatus, self-contained, in which the necessity of a galvano-
meter is obviated. By its means, moreover, the record is
unaffected by any earth-vibration.
The Morograph. — The thermometric record is produced
by means of the curling and uncurling of a spiral compound
strip, of two metals, having different coefficients of expansion.
In order to give strength and steadiness, this helix, which is
about 2-5 cm. in diameter, is made of somewhat thick strips
of brass and tinned iron, soldered together. By increasing
the number of turns in this spiral, the extent of movement
per degree in the thermometric record may be increased
at will. In my own Morograph, a helix of three circles
was found to answer all requirements. The last half-circle
of the lower end of the spiral is fixed to a heavy circular
stand of brass, 3 cm. in diameter. The topmost half-circle,
on the other hand, has had the tinned-iron strip cut off, and
therefore consists of brass alone. It will thus be understood
that a line drawn diametrically across this last half-circle
' This word is derived from the Sanskrit root w;/, Latin niois, dcatli.
DETERMINATION OF CRITICAL POINT OF DEATH 165
would rotate round a vertical axis passing through the centre
of the spiral, under the influence of the differential expansion
or contraction produced in the compound strip of metal by
rise or fall of the temperature. When the outside of the
circumference of the spiral consists of the more expansible
metal, brass, then a rise of temperature will produce the
movement of curling. The difficulty in the construction of
this part of the apparatus lies in securing equal angular
rotation of the diameter about the axis, with every equal rise
of temperature. These indications
were at first extremely irregular.
I was able, however, to remove all
traces of irregularity by careful
and repeated annealing. In any
case the thermometric indications
of the compound helix may be
previously calibrated.
The axis of the Optic Lever —
one arm of which is attached to
the plant-specimen, and which is
to give the record of its variation
in length with rise of temperature
— is now supported on the diameter
of this last half-circle of the helix
and is thus rotated bodily, with rise
of temperature (fig. 83). And it
will thus be seen that the motion
of the spot of light, reflected from
the single mirror of the Optic Lever, is a resultant of two
movements, which take place at right angles to each other
— namely, the horizontal movement, due to thermometric
variation and the corresponding vertical movement, due to
the changes of length of the experimental plant-tissue.
Owing to the fact that the spot of light in this apparatus is
reflected only once, it is extremely bright.
I shall now proceed to describe the manner in which the
plant is mounted, and other accessories of the apparatus.
Fig. 83. The Thermometric
Spiral and Optic Lever of
the Morograph
1 66 PLANT RESPONSE
The circular brass stand on which the heHx is mounted
has at the centre a small tube, in which the lower part of
the specimen is clamped. The plant-organ thus occupies the
vertical axis of the spiral, its upper end being connected by
a thread with the short arm of the Optic Lever. It may be
pointed out here, as is better explained in the diagram, that
in order to give room to the specimen, the axis of the Lever
is made to rest upon T-pieces, which are erected at the two
ends of the diameter of the helix. The plant-organ being
thus placed at the centre, the inclosing spiral thermometer
gives an accurate indication of the temperature to which it is
exposed.
The circular stand, supporting both the specimen and the
recording apparatus, is placed in what I shall describe as the
inner thermal cylinder, within the circumference of which
the base fits exactly, while the helix is free, to the extent of
•25 cm. all round. This internal cylinder is made of copper,
coated with silver. It is filled with water and placed inside
an outer, or heating, cylinder of brass, which is also filled
with water. Heat is applied, by means of a spirit-lamp, to
the bottom of the outer cylinder ; thus the water in the inner
vessel is subjected to equal heat, on all .sides at the same
time. Had the heat been applied directly to the inner
cylinder, convection -currents would have caused great dis-
turbance of the recording spot. With these precautions,
however, there is no trace of such disturbance.
The whole apparatus is supported on a steady stand.
Below it is the spirit-lamp, which may be raised or lowered
till a distance is found which gives us the standard rate of
rise of temperature, that is to say, 1° C. per minute and a
half. Above the apparatus and on a sliding holder is the
electric lamp, with focussing lens ; the light from this falls
veitically on the mirror of the Optic Lever, which is inclined
at an angle of 45° to the horizon. The horizontally reflected
light is then thrown on a screen, which carries either semi-
transparent recording paper or a photographic plate. In
the former case the observer, standing behind the screen,
DETERMINATION OF CRITICAL POINT OF DEATH 167
traces the movement of the spot of light with a pencil. The
whole recording apparatus and the source of light being thus
placed on the same stand, any ordinary disturbance will
affect all equally, and cause no irregularity therefore in the
movement of the recording spot (fig. 84),
Fig. 84. The Morograph
Record may be taken by following excursion of spot of light on screen to the left.
For photographic record, plate-holder is substituted for screen.
I have given a great deal of space to the description of
these details, because on them depends the accuracy and
perfection of the results. The record may now be made
on any scale of magnification, without misgiving. In fact
l68 PLANT RESPONSE
I have obtained very perfect records even when the passing
traffic was at its thickest. How true this is may be seen
from the photographic record of a thermo-mechanical curve,
given in fig. 85.
It will be noticed from the curve that, as the temperature
rose, there was a continuous preliminary elongation, which
was suddenly reversed by the exci-
tatory contraction at the death-point,
found in this case to be 59"6° C.
If desired, the photographic curve
itself may be made to indicate the
different temperatures at different
parts of the curve. This is secured
by interrupting the light for a time
at, say, every half degree of rise of
temperature. As in the anisotropic
death-responses, described in the last
chapter, we have in this case also,
though not shown in the present
Fig. 85. Thermo-mechanical record, the post-inorteiii relaxation
Curve obtained Photo- ,• ^i . .• r •
graphically (Coronal Fiia- succeedmg the contraction of rigor
ment of Passiflora) mortis.
^"^.^^sh™" "rer/u™: In order to show that the mole-
but on reaching death- cular change which occurred at the
point, at 59-6° C, there is . ^ . . • j j i.u
a sudden inversion of the point of mversion was mdeed the
curve, due to spasmodic irreversible death-change, I took the
death-contraction. ,1 •
curve once more, after allowmg the
specimen to return to its original temperature. The curve
now obtained showed no reversal-point.
Remarkable agreement between thermo-mechanical
curves of similar specimens. — It was pointed out in the last
chapter that the death-point is almost as definite as a phy-
sical constant. And not only is this true of the death-point,
which I fird in different phanerogamous specimens, under
normal conditions, to be almost invariably close upon 60'^ C,
but it is also more or less true of the whole curve, those given
by similar specimens being almost identical. In this way
DETERMINATION OF CRITICAL POINT OF DEATH 169
the thermo-mechanical curve is, in a sense, characteristic of
the plant in a given condition. This is well seen in the two
records which I have obtained from the styles of two flowers —
both on the point of opening — of a single plant of Datura alba
(fig. 86). These two curves are so extraordinarily similar in
all their parts, that I was obliged,
in printing them, to raise the
origin of one slightly above that
of the other. If the point of
origin had been allowed to
remain the same in both, one
would have been superposed
upon the other, so as to prove
almost indistinguishable. On
minute examination, however,
I find that the death-point of
one differs from that of the
other by about -^V of ^ degree.
The possibility of securing
such uniformity of results, en-
ables us to attempt an investigation on the influence of
various agencies. For any deviation from the standard
characteristic curve will then form an indication of the action
of such agents.
Standardisation of curves. — Different plants, again, will
exhibit differences in their characteristic curves, and in order
to render these strictly comparable with one another, we
must know the absolute value of relaxation or contraction in
each part of the curve. By absolute value, is here meant the
amount of relaxation or contraction per unit-length of the
specimen. This is rendered simpler if we adopt a uniform
standard for all specimens ; that is to say, the horizontal dis-
tances representing temperature may in the standard curves
be T inch (2-5 mm.) per degree. Vertical distances, again,
of I inch maybe made to represent a relaxation or contraction
of one part in a thousand. The standardisation is carried out
in the following way : first, the recording surface is moved,
Fig. 86. Thermo-mechanical Curve
of Two Different Specimens of
Style of Datura a/ha, obtained
from Flowers of the same Plant
lyo
PLANT RESPONSE
till one degree of rise of temperature produces a horizontal
movement equal to -i inch. After this, keeping the distance
of the recording surface constant, the length of the short arm
of the Lever, to which the plant is attached, is so adjusted
that the vertical magnification is two hundred times. The
length of specimen used, unless the contrary is stated, is
always 5 cm. A movement of the
light-spot through a vertical distance
of one division ('i inch) will then
represent an expansion or contraction
of one part in two hundred of a
specimen whose length is 5 cm., that
is to say, one part in a thousand of
a specimen whose length is one
centimetre. In fig. 85, the original
record has been reduced to one-
fourth. The distance between two
horizontal lines represents a contrac-
tion or relaxation of i per cent.
In order to exhibit the differences
in the characteristic curves of different
specimens, or of the same specimen
at different ages, I append three re-
cords taken under the same standard
Fig. 87. Thermo-mechanical conditions : (d) that of the Style of
Records of s, Young Speci-
men of .S/Z/^r^o/r^ ; s' Older Datura alba ; (s) of a young specimen
or!l;L"ofliri.r' of Sprogyra- and (s') of an older
The distance between two specimen of the same (fig. 87). In
horizontal lines represents these three experiments, the rate of
a contraction or relaxation
of I per cent. rise of temperature and other circum-
stances having been the same, it is
instructive to compare the different parts of the different
curves.
Taking first the curve of Z>r?///r«, we find its death-point to
occur at 60° C. The relaxation undergone by the specimen
during the rise of temperature from 35° C. to the death- point,
was at the mean rate of 2- 1 parts per thousand per degree for the
DETERMINATION OF CRITICAL POINT OF DEATH 171
unit-length. This, for convenience, we shall call the coefficient
of relaxation. But after the death-point, the sign of response
undergoes an abrupt change to the negative, that is, contrac-
tion, the coefficient of contraction being fifty per thousand, or
nearly twenty-four times the coefficient of relaxation.
The next specimen whose curve (s) is given was young
Spirogyra of light-greenish colour. From the slight differen-
tiation of these simple algal forms, and from their lack at this
young stage of any efficient protecting envelope, we should
expect them to offer but feeble resistance to the effect of heat,
and we find the death-point lowered to 47°, that is to say,
13° below that of the phanerogam Datura. Along with this,
we find also a difference in the coefficients of relaxation and
contraction. The mean coefficient of relaxation was in this
case '001, and that of contraction 'OO/.
Vanishing of point of inversion with age. — The older
specimen of Spirogyra (S'), taken from the same place, had
its point of inversion raised by 4°, the death-point being
therefore at 51° C. There is a further and interesting
difference as between curves for young and old specimens-
In the younger specimen there was produced a very consider-
able contraction due to rigor, and this was followed after a
time by the usual post-mortem relaxation. But in the older
specimen the rigor was relatively slight and the subsequent
relaxation took place much earlier. We thus see that there
is a great loss of contractile power in old tissues. In still
older specimens the contraction tends to vanish altogether,
and we have no line of demarcation to mark the moment of
transition from life into death. In connection with this, it is
interesting to note that, whereas the death-spasm in young
leaves of Mimosa is very vigorous, old leaves exhibit little or
no spasmodic lateral movement at death.
Cold-rigor. — Turning from the effect caused by con-
tinuous rise of temperature, I shall now proceed to the con-
sideration of the effect produced by the reverse process of
continuous fall to the minimum temperature. Here also,
as in the case c^i the curve for rising temperature, there is a
172
PLANT RESPONSE
sudden inversion at a definite minimum point of temperature.
That is to say, just as we observe a sudden contraction
when the point of heat-rigor is reached, so also we obtain
a similar sudden contraction at a point corresponding to the
cold- rigor. For example, with the style of Eucharis Lily,
which is very susceptible of depression by cold, I found the
death-point to be at about i° C. The experimental diffi-
culties for the determination of cold-rigor are, however, very
great, owing to the fact that facilities do not exist for con-
tinuous lowering of temperature to zero or below.
Thermo-mechanical record of Mimosa. — At the begin-
ning of this chapter it was stated that the death-spasm in an
anisotropic organ, such as that of the
pulvinus of Mimosa, was an instance
of differential longitudinal excita-
tory contraction. The accompanying
curve (fig. 88) was obtained by means
of the Morograph. We must remem-
ber that in this case we are dealing
with a differential action. In the
first part of the curve, therefore, we
do not obtain such marked relaxation
as in radial organs, where we obtain
non antagonised and direct change
But when we reach a tem-
which corresponds to the
death-point, that is to say, 54° C,
there is a sudden downward move-
ment. It must be remembered that
this particular experiment was carried
out just after the spell of cold weather, when the death-points
of plant-organs were found to be lowered by several degrees.
After the downward movement, which commences at 54° C,
we see that there is an equally abrupt upward movement,
beginning at 59° C, due to post-mortem relaxation aided by
the later contraction of the upper half of the organ.
I
Fig. 88. Thermo-mechanical
Record of Leaf of Mimosa
The spasmodic contraction of form,
shown here as downward
movement took place at pGfS-ture
54° C, the normal death-
point being lowered by 5°
owing to prevailing cold.
.Subsequent erectile move-
ment took place at 60° C.
DETERMINATION OF CRITICAL POINT OF DEATH 1^3
A few words may be said here with regard to these
successive movements. As in animals the rigor mortis is
succeeded by relaxation, so also in radial organs, as has been
said, we see relaxation succeeding the death-contraction. It
may then be asked whether the second half of the present
curve, in fig. 87, giving the rise of the leaf, does not simply
represent a similar relaxation, in the case of the pulvinus of
Mimosa. But we have to notice that, in taking records with
the Lever, the weight of the Lever ensures the indication of
any passive relaxation of the specimen. If we inspect a
Mimosa leaf, however, during the death- spasm, the leaf
being free, i.e. unconnected with the Lever, we find that it,
after its first fall, becomes again almost vertically erected,
evidently in consequence, at least to some extent, of some
process of active contraction, which must be that of the upper
half of the organ. Had there merely been a general relaxa-
tion of the whole pulvinus, caused by death, then the weight
of the leaf might have caused it to fail.
The slope of the curve of relaxation, again, is, generally
speaking, relatively gentle. Its comparative steepness, in the
case o( Mimosa, after the passing of the death point, seems to
indicate that the movement of relaxation was partially aided
by later contraction of the upper half of the pulvinus.
Constancy of death-point. — Before concluding the present
chapter, I must refer to the remarkable fixity of the death-
point in all the phanerogamous plants which have come
under my observation in normal conditions. Thus, on re-
peating my experiments at the end of spring, by the perfected
method of morographic record, I invariably found that the
point of inversion was at, or within -^^ of a degree of, 60° C.
Other and less perfect modes of investigation, such as the
spasmodic lateral movement of a dorsi-ventral organ, the
movement of uncurling, the sudden expulsion of water, and
those opening and closing movements of flowers which are to
be described in the next chapter, enabled us to obtain death-
points which were not very different from this. I give below
174
PLANT RESPONSE
a tabular statement which makes it possible to see at a
glance how concordant these results are.
Table showing Death-points obtained by different Methods
Specimens
Method
Death-
point
g (Coronal filament oi Passi flora .
1 [Six specimens used. Each gave]
( Style of Hibiscus
' ° 1 [Four specimens. Each gave]
III ^"^y^^ °^ Datura . . .
'^ 1 [Four specimens. Each gave]
15. Vnh'inns of MiHiosa. Young .
16. Spiral tendril of /"rtrf/fw-rt
17. Peduncle of .4 //?«w. Young .
18. Flower of French Marigold
19. Flower of /j^^wa-a .
Morograph.
Spasmodic lateral movement.
Movement of uncurling.
Expulsion of contained water.
Opening or closing of flower.
C.
60°
60°
60°
59°-6o°
K
59°
59°
62°
It will thus be seen that, using very diverse methods and
specimens, we nevertheless always obtain a death-point which
is very near 60° C.
Summary
In . radial organs, the death-contraction due to heat-
rigor is abrupt, and takes place at a definite temperature,
which is to be regarded as the death-point.
In the thermo-mechanical curve given by the Morograph,
the point of inversion is the death-point.
When death has taken place, a repetition of the experi-
ment shows no inversion.
The death-point, due to heat-rigor, in phanerogamous
plants, under normal conditions, is found, though obtained
by various methods, to be very close on 60° C.
Under the action of continuous lowering of temperature
there is produced, at a definite minimum degree, a spasmodic
contraction, due to cold-rigor.
The death-contraction in plants is in every respect
similar to the same phenomenon in the animal, and is an
instance of true excitatory effect. As in the animal, so also
DETERMINATION OF CRITICAL POINT OF DEATH 1 75
in the plant, this rigor of death is succeeded, after a time,
by passive relaxation of the tissue.
The thermo-mcchanical curves of two similar specimens —
that is to say, two specimens of the same plant, having the
same previous history — are found to be practically identical.
Different plants have different characteristic curves. The
curve of the same plant also exhibits a variation of its
characteristics under changed conditions of age, experiment,
and previous history.
CHAPTER XV
EFFECT OF VARIOUS AGENCIES ON DEATH-RESPONSE :
THERMOGRAPHS OF REGIONAL DEATH
Lowering of death-point by fatigue — Modification of characteristic thernio-
mechanical curve by the action of chemical ngQWi^ — Coiiipan'soii'Morograph —
Duplication of rigor-point — Death- response a physiological response and not
due to coagulation — Death-movement of flowers — Approximate constancy of
death-point of florets in a capitulum — Definite interval between death-point
and discoloration-point — Translocation of discoloration-point by various
agencies — Thermographs of regional death — Thermograph of local fatigue —
Thermographic investigation of electrotonic excitation.
I HAVE shown that the death-contraction is a phenomenon
of excitatory response. We might expect from this that
various conditions which affect the excitabihty of a plant
would also have a modifying influence upon the characteristic
thermo-mechanical curve of death- response. One such
modification would lie in the translocation of the point of
inversion, or, in other words, in the displacement of the
death-point. In order to test this inference we might subject
the plant to the influence of various agents which modify the
physiological condition, and observe the consequent modi-
fication of the death-response. We have already seen how
the physiological modification induced by age causes dis-
placement of the death-point. We have seen, further, how
unfavourable seasonal conditions, such as sudden prevalence
of cold, will lower the death-point by several degrees. We
shall now study the effect of other agencies, such as fatigue,
and the action of chemical reagents, in producing displace-
ment of the death- point.
Effect of fatigue. — In the course of these experiments,
fatigue was produced by means of tetanising electric shocks,
VARIOUS AGENCIES ON DEATH-RESPONSE 1 77
care being taken that these should not be strong enough
to kill the plant. I have carried out experiments on this
subject with two different classes of specimens : first, with
anisotropic pulvinated organs, like that oi Mimosa^ where the
death-spasm is shown by lateral movement ; and secondly,
with radial organs, like the style of Datura, where the death -
point is determined by thermo-mechanical inversion. In
experimenting on Mimosa, I took a batch of young leaves of
the same age, whose death-point was found to be at, or close
upon, 59° C. After fatigue caused by moderate stimulation,
however, the death-point was found to be at 56° C, that is to
say, it had been lowered by 3°.
Working with Datura pistil, the death-point of which was
never normally lower than 60° C, it was found when fatigued
to be at 41° C. The lowering in this case was therefore
about 19°. It will thus be seen that fatigue does lower the
death-point of a plant, the degree to which it does so
depending on the extent of the fatigue. In the course of the
present chapter, I shall be able to demonstrate once more the
lowering of the death-point through fatigue, by means of an
altogether different m-ode of investigation.
Effect of chemical reagents. — Similarly I find that
death-response is modified by the action of various chemical
reagents. We have seen how characteristic is the thermo-
mechanical curve of each plant, under definite conditions
We found, for example, that two styles of two different flowers
on the same plant, having had the same previous history, gave
curves which were practically identical. Specimens thus re-
sembling each other are not difficult to obtain in spring, when
there is no sudden variation of weather conditions, or from
plants grown under glass, in an unchanging environment. The
characteristics of the thermo-mechanical curve being so con-
stant, the effect of a given agent will then be indicated by
certain variations from the normal. Thus, on taking a
thermo-mechanical curve — the specimen used being the style
oi Datura, subjected to the action of a 2-5 solution of copper
sulphate — I found that the form of the curve was much
N
1/8 PLANT RESPONSE
modified in contrast with the normal curve. The most
striking difference lay in the lowering of the death point by
3°C.
Comparison-Morograph. — In order to facilitate the
investigation into the modification of the curve, by various
agents, I have devised a Comparison-Morograph, by means of
which the thermo-mechanical curves of two similar speci-
mens, one under normal and the other under modified con-
ditions, can be taken simultaneously. We use here two
recording Optic Levers, supported on a single thermometric
helix. The normal specimen is placed in the internal
cylinder in the ordinary way and attached to one of the
Optic Levers. The second specimen, contained in a small
cylindrical tube, filled with the given chemical reagent, is also
placed inside the helix and the plant is attached to the
second Lever. The spots of light are so adjusted that one
lies immediately above the other. The two specimens are
thus subjected to the same temperature-variations, and the
variation of the second curve from the standard exhibits the
effect of the reagent.
I can here barely indicate the very extended line of
inquiry thus opened out. With regard to the general effect
of drugs on death-response, it may be said that the displace-
ment of the rigor- point varies with the tonic condition of the
tissue, the nature of the drug, and the strength of the solution.
Out of several possible cases, I shall here give only a few
simple instances.
Duplication of rigor-points. — One very curious effect
of certain chemical reagents, such as ether, lies in the ex-
hibition of two distinct points of rigor, instead, as normally,
of one. This effect is very easily seen, in the spasmodic
death- response oi Mimosa. We take a specimen, and subject
it to continuous rise of temperature, in water, which contains
a small quantity of ether. It will be remembered that, under
normal conditions, the first down movement of the single
composite spasm of death-response took place in young
leaves at an average temperature of 59-5° C, the second
VARIOUS AGENCIES ON DEATH-RESPONSE
179
upward movement being at 617° C. Under ether, however,
we have the peculiar phenomenon of two composite spasms
separated by an average interval of about 27° C. This
effect will be understood from the following table, which
gives the results obtained with two different specimens, B
and C, the specimen A being heated in water v*^ithout ether,
and thus constituted a standard.
Table showing Duplication of Rigor by Ether
Specimen
Preliminary Spasm
B'inal Spasm
Fall
Erection
Fall
Erection
Under normal conditions
( Leaf ( I ) young
^ I Leaf (2) older
Absent
Absent
Absent
Absent
59° C.
60° C.
61 -5° C.
62° C.
After application of ether
(Leaf (i) young
^ "I Leaf (2) older
31° C.
32° c.
37° C.
38° c.
59-5° C.
60° C.
61 -5° C.
62-5° C.
After application of ether
(Leaf (I) young
^ 1 Leaf (2) older
32° c.
33° C.
39° C.
40° c.
58-5° C.
59° C.
62-5° c.
64° c.
Another table is here given, showing the results obtained
when the water contained a small quantity of hydrochloric
acid.
Effect of Hydrochloric Acid
Specimen
Preliminary Spasm
Final Spasm
Fall
Erection
Fall
Erection
Leaf (I)
29-5° c.
31° c.
63° c.
66-2° C.
Leaf (2)
29-8° c.
31-5° C.
63° c.
66-8° C.
From these results several interesting observations arise,
the most striking of which is the occurrence of the pre-
liminary spasm itself, separated by so large an interval from
the final death-response. This duplication of the rigor-point
N 2
l8o PLANT RESPONSE
is not a distinctive effect of the action of anaesthetics as such,
since hydrochloric acid and various other chemical reagents
give a similar result. It would be premature to pronounce
on the significance of this very suggestive phenomenon.
A plausible .suggestion, which offers itself, is that the
approach of molecular rigidity concomitant with death,
which here appears imminent by the action of the reagent,
as seen in the preliminary spasm, is tided over, or counter-
acted, by the molecular mobility conferred on the tissue,
through the rising temperature of the bath. Should this
inference prove to be correct, these experiments might
throw an interesting light on an ancient practice, still current
in India amongst an old class of quack-doctors, by which
cases of snake-bite are said to be cured, under a treatment
whose essential feature is the application of hot water and
steam, with accessory incantations ! The same principle
may also be the basis of the alleged hot-water and steam
cures of more modern practitioners.
The duplication of the rigor-point by the action of ether
is also noticeable in the thermo-mechanical curve given
by a radial organ. Thus, in a curve given by the style of
Datura, the preliminary rigor-point was found to be at
36-5° C, the second being at 53° C.
As has been said, the effect produced by various poison-
ous reagents depends on the tonic condition of the tissue, as
well as on the nature of the drug. In those cases in which
the rigor is not duplicated, there is a translocation of the
death-point, which, as far as I have yet seen, is invariably
lowered. Thus, in an experiment already described with
the style of Datura, I found this translocation, under the
action of dilute copper sulphate solution, to be from the
normal 60° to 57° C,
Death-response not due to coagulation. — From the
experiments which have been described, it is evident that the
death-response, like other modes of excitatory response, is
appropriately modified by all those influences which affect
VARIOUS AGENCIES ON DEATH-RESPONSE l8l
the physiological condition of the tissue. The rigor of
spasmodic contraction at death is, therefore, not to be
regarded as due to any coagulative action. And, indeed, the
theory of a connection between rigor and coagulation is now
generally discredited.'
By the methods described, then, it is possible to study the
effect of various agencies in the modification of death-response :
in the case of anisotropic organs, by observation of their
lateral responsive movements, and in that of radial organs,
by the translocation of the point of inversion. I was next
desirous of discovering some still simpler means of determin-
ing the effects of various conditions in a qualitative manner.
This might be accomplished if we had a number of organs
exactly similar to one another, which would give some
unmistakable sign of death-response, at the point of occur-
rence, either immediately, or at some definite interval after-
wards. A certain number of these organs might then be
taken as standard, and the others subjected to the action
of various modifying influences. Any differences between
the temperatures concomitant with post-mortem symptoms
would now indicate the modifications produced by these
agents.
In a certain sense, such an experiment may be carried
out with a number of leaves on the same plant of Mimosa.
But in such a case the organs to be compared are not very
numerous, and different leaves of exactly the same age
cannot be secured.
Death-response in flowers. — This led me to investigate
whether, amongst flowers, specimens could be obtained which
would exhibit a death-movement at the critical temperature.
' ' The causes which determine the varying resistance of different plants to
heat are quite unknown. The fact that a temperature of from 20° C. to 40° C.
kills certain plants, shows that in their case death is not the result of coagulation
of the plant-albumin. Further, some plants grow at 75° C, i.e. above the
temperature at which egg-albumin coagulates. Coagulation need not always
occur, for we must remember that the acid and alkali albumins are not coagulated
by heat.' — Pfeffer, Physiology of Plants, English edition, 1903, vol. ii. p. 230.
I 82 PLANT RESPONSE
And I found that many flowers did so in a marked degree.
Thus, for example, in two different specimens of Convolvulus^
both full-blown, the flower being subjected to rising tempera-
ture, the corolla-bell folded up at exactly 62-5° C.
We thus see the possibility of obtaining flowers which,
having had the same previous history, are likely to exhibit the
death- movement more or less at the same point. I thought
that such a collection of similar specimens might probably be
obtained in a small space, from the capitulum of a composi-
taceous flower, and as a matter of fact I succeeded in finding
several. The nature of the movement, whether up or down,
and its more or less pronounced character, appeared to
depend in these cases on the age of the flower.
In connection with this question, we must remember that
in flowers, as in leaves, we may have in a single specimen
alternating hyponastic and epinastic growths. It is therefore
conceivable that the death-movements of old and young
flowers may take place in different directions, and that at
some stages there may be little or no motion of any kind.
However this may be, I have found that in the double Indian
marigold at the temperature of 625° C. the florets arranged
themselves in two groups, the outer and lower whorls turn-
ing down, and the younger or central whorls rising up,
at the critical temperature. In the case of .some of the
large garden daisies, yellow and white, I found the critical
temperatures to lie between 61-5° C. and 6^° C, the death-
movement consisting of a folding up in some cases, and a
curving down in others. If the flower have been subjected
to uniform illumination on all sides, then the movement of
all the florets will take place within a degree or so. In the
French marigold, grown in India, the florets of the ray fold
up, at from 59° C. to 60° C. From these experiments we
see that, the death-point for all the flowers on the same
capitulum being about the same, it might be possible to
treat one-half of the florets of a single flower-head as normal
or standard, while using the rest for comparative study on
the influence of various agencies.
THERMOGRAPHS OF REGIONAL DEATH 1 83
Method of thermographs of regional death.— But I
have found out another and distinct method for detect-
ing the effects of various agencies. And this method is
not only very interesting in itself, but it enables other
obscure problems to be attacked in a satisfactory manner.
It depends on the taking of Thermographs of Regional
Death.
It is known that amongst the symptoms which occur at
some indefinite interval after death is that of discoloration.
Although this phenomenon is not concomitant with death,
yet the temperature-interval between the two can in many
cases be rendered definite. Thus for example, when the blue
Convolvulus is subjected to rising temperature at the normal
rate, it shows death-movement at 62-5° C. But there is as
yet no sign of discoloration. When the temperature, however,
rises to 70° C. the heating water begins to undergo discolora-
tion from the escaping cell sap. It would appear probable,
from various experiments which I have carried out, that
discoloration does not begin at the point of death-contrac-
tion, but occurs at or about the point of the subsequent
relaxation. But in the case of Convolvulus there is no strik-
ing change seen in the flower itself, for the loss of colouring
matter is gradual. In the style oi Datura alba, however, we
have a more definite change of colour. This organ, from
being milk-white, becomes brown at a temperature of 64° C,
that is to say, 4° above the death-point, when the temperature
of the bath is rising at the ordinary rate. In the petals of
Sesbania coccineum, again, under similar conditions, the change
of colour is very striking. Rich crimson here turns into pale
blue, at a fairly definite temperature of 6"/° C. The most marked
and easily observed of all these changes is seen, however, in
the mauve petals of Passiflora quadraiigularis, which normally
becomes colourless at a temperature of 70° C. The filamentous
corona of the same flower again, in which the filaments are
barred by purple rings at intervals, loses its colour normally
at 68° C. The death-point of these filaments is, it should be
remembered, 60° C. We thus find on raising the temperature
1 84 PLANT RESPONSE
in each of these cases, at the standard rate of i° per 15
minute, that not only is there a definite death-point,
evidenced by sudden contraction, but that the discoloration-
point is separated from this, by a definite temperature-interval.
And since we have found the death-point to be translocated
by the influence of various agencies, we may expect the
discoloration-point also to be displaced, in a similar manner,
under the same conditions.
Development of thermographs. — This being so, it ought
to be possible to ' develop ' images of local death. We take
a coloured petal, say of Passiflora, and placing two circular
electrodes diametrically opposite to each other, with the petal
between, pass tetanising shocks, which are of sufficient
intensity to fatigue, but not to kill, the tissue. When the
electrodes are removed, there is nothing by which the eye
may distinguish the zone of fatigue. In order now to
develop this invisible picture, we have simply to subject the
specimen to the ordinary bath, with rising temperature. For
we have seen, from experiments already described, that the
power of the tissue to resist death is lowered by fatigue. In
the case of the present specimen, therefore, the fatigued area
will die, and undergo subsequent discoloration, earlier than
the rest of the petal. In carrying out this experiment, the
area of fatigue was found developed as a white image on a
purple background, at a temperature of 45° C. It should be
remembered that, as said before, the lowering of the death-
point varies with the amount of the fatigue ; hence the point
of discoloration may be found as low as 40° C. or as high as
50° C. and upwards. If the petal be removed from the bath
as soon as development begins, the image will remain. But
if it be maintained under the rising temperature, the thermo-
graph will vanish, with the death and discoloration of the
background.
Again, I took two similar styles of Datura. One of these
I kept as standard, and passed tetanising shocks through the
other. On subjecting both to rising temperature in a single
THERMOGRAPHS OF REGIONAL DEATH 1 85
bath, the fatigued style underwent the change from white to
brown at 56° C, whereas the test specimen was not discoloured
until 64" C.
Determination by thermographic method of relative
excitatory effects of anode and kathode. — This thermo-
graphic method also enables us to attempt the solution of
other recondite problems, such as that of the relative excita-
tory effects of the anode and kathode. It will be shown in
the next chapter that when the electromotive force is not too
excessive, it is the kathode which causes excitation when the
circuit is made or completed. This fact will be demonstrated
there by experiments undertaken with sensitive plants, in
which the excitatory effect is indicated by the mechanical
response of the motile leaflets. No such means is available,
however, in the case of ordinary, or so-called non-sensi-
tive, tissues. In such cases, therefore, I shall undertake
to demonstrate the same fact, but by means of death-
response.
We have seen that a tissue which has already been
excited, is more fatigued than one which has not, and a
fatigued tissue is, as we have seen, subject to death, and
subsequent discoloration, at lower temperatures than the
unfatigued. Hence, if excitation be caused in the kathodic
region at make, death-discoloration ought to occur there
earlier than in the anodic. This I have been able to
demonstrate in the following manner : I took two similar
petals, or two halves of the same petal, of Passiflora. The
two were held side by side in a glass vessel full of water,
at a distance of 3 cm. from each other, the temperature
being gradually raised. When the temperature is about
50° C. a current is sent from a battery so that it enters
by one petal, and leaves by the other. It is now found
that the discoloration of death takes place earlier at the
kathode than at the anode. The value of the difference is
about 4° C. I carried out this experiment on the petals of
the crimson Sesbania coccineum also.
1 86 PLANT RESPONSE
Table showing Effect of Electrotonus on Death-point
Specimen
Temperature of Discoloration
Anode
Kathode
1 . Petal of Passifora
63= c.
59° C.
2. Filament of Passiflora .
60° c.
56° C.
3. Petal of Sesbania ....
64° c.
61 °C.
The effect here described takes place, as has been said,
where the electromotive force is not excessive. Under these
conditions, it is the kathode u^hich is the more excitable. I
have, however, discovered a very curious case of inversion of
excitation which occurs when the E.M.F. exceeds a certain
value. With high electromotive force, then, it is the anode
which excites at make of the circuit. The demonstration of
this fact by means of mechanical response, and subsidiary
proof by means of death-response, will be given in detail in a
subsequent chapter.
Summary
The death-point is lowered by fatigue, the amount of
lowering depending on the intensity of fatigue.
The characteristic thermo-mechanical curve is modified
and the point of inversion translocated by the action of
chemical reagents.
Certain reagents produce a duplication of the rigor-point.
The death-point is translocated to a temperature lower
than normal by the action of poisonous reagents.
Under standard conditions, there is a definite interval be-
tween the death point and discoloration-point of vegetable
tissue.
Hence it is possible to obtain thermographs of localised
effects of various agents.
The excitatory effect of kathode is demonstrated by the
earlier discoloration produced there.
PART III
EXCITABILITY AND CONDUCTIVITY
CHAPTER XVI
ON EXCITATORY POLAR EFFECTS OF CURRENTS
Hydro-mechanical theory of excitation in plants —Theory of protoplasmic change
— Crucial tests applied by means of polar excitation — Mono-polar and Bi-polar
methods of excitation — Advantages of study of polar excitation in plant-tissues
as compared with animal — Effects of feeble E. M.F. — Effect of moderately high
E. M.F. — Experiments with highly excitable tissues.
Having observed, by means of mechanical responses, the
various excitatory effects which are caused in plants by
stimulation, and the influence of different agencies in modify-
ing these excitatory effects, it is now desirable to make an
inquir}' into the manner in which excitation takes place,
and into the method by which it is transmitted to a distance.
There has been a great deal of uncertainty regarding this
subject, and the prevailing view is that which holds the trans-
mission of excitation to be due to the propagation of hydro-
static disturbance.
Mechanical theory. — According to this theory, it is
supposed that stimulus causes a mechanical disturbance,
bringing about an alteration of the hydrostatic equilibrium.
The propagation of excitation in plants is thus regarded as
nothing more than the transmission of this hydro-mechanical
disturbance.
We know, however, that the transmission of hydrostatic
disturbance takes place with relatively great rapidity, while
these excitatory effects in the case of plants travel sometimes
as slowly as i mm. or less per second. I have shown, more-
over, that its responses, both mechanical and electrical,
are profoundly modified by the physiological condition of the
plant. There is, for example, an optimum temperature at
which response is at a maximum, any change, whether above
I90 PLANT RESPONSE
or below, inducing depression. Anaesthetics, moreover, tem-
porarily, and poisons permanently, abolish response. It will
be shown further, in Chapter XVIII, that the transmission
of excitation may be very much diminished, or even arrested,
by the application of cold or ether.
Theory of protoplasmic change. — It is thus seen that the
hydro-mechanical theory is incapable of explaining the facts
of the case. I shall now, therefore, proceed to demonstrate
that the excitatory change in plants is brought about in the
same manner as in animals, and that the transmission of
excitation depends upon the propagation of protoplasmic
changes, in the one case as in the other. This may be
determined by a crucial experiment as to whether vegetable
tissue exhibits those peculiar polar effects of the electric
current on excitability, which are seen in the protoplasm of
animal tissues. In the animal tissue, for example, it is the
kathode that, under normal conditions, produces excitation,
the effect of the anode being the reverse. In the case of animal
tissues, again, the anode will even act as a block to the trans-
mission of stimulus.
Crucial tests applied by means of polar excitation. —
Such effects are incapable of explanation by the hydro-
mechanical theory, and if we succeed in discovering similar
phenomena in the case of vegetable tissues, we shall establish
the existence of a fundamental property of protoplasm
common to the animal and vegetable alike. With this end
in view I have carried out numerous experiments on plants,
both sensitive and ordinary. As specimens of the former
class, I used Biophytum, Mimosa, and Averrhoa. The in-
vestigation resolves itself into the determination of the differ-
ences of excitatory effects, at the anode and kathode, both at
make when the circuit is completed, and at break when
it is interrupted. The presence of the excitatory effect is
indicated in the case of ' sensitive ' plants by the mechanical
responses of the motile organ. In order to separate the
effects of the anode and kathode, we may use the Mono-polar
method, i.e. have one electrode near a motile organ, and the
EXCITATORY POLAR EFFECTS OF CURRENTS 191
Other very distant from it (fig. 89). If the plant is not very
excitable the effect produced at the distant point will not
reach the motile organ, and we shall obtain the isolated effect
of a particular electrode. Again, if we wish to observe the
effects at both the electrodes simultaneously, we may employ
the Bi-polar method, in which both electrodes will be placed
at or near the motile organs. The most suitable means for
the application of electrical stimulus will be either a constant
electrical current from a voltaic battery, or the discharge from
a charged condenser.
We have again to study the respective effects of feeble,
moderate, and excessively strong electromotive forces.
In experimenting on polar excitation in animal tissues,
a nerve-and-muscle preparation is generally used, the ex-
citation of the nerve being studied by means of the indication
given by the terminal motile organ, the muscle. On the
other hand, experimenting on Biophytum for instance, the
petiole acts as the conductor of stimulus, and is provided
with - not a single terminal motile organ, but— a number of
lateral motile organs, viz. the 'sensitive' lateral leaflets.
The analogous case in animal tissue would be a hypothetical
nerve, provided with a hypothetical series of contractile
muscles attached to it laterally. The relative advantage
possessed by such a vegetable organ is, that the changes in
the excitabilities, throughout every portion of the excitable
conducting tissue, are visibly manifested.
I experimented altogether on some hundreds of specimens.
Some of these were very sensitive ; others only moderately
so. The results under normal conditions were perfectly
consistent. As it would entail much mere repetition to relate
every one of these experiments, I shall here give only typical
instances in detail. While I was studying the effect of the
establishment or cessation of a constant current I made a
practice — whenever the leaves or leaflets recovered within a
moderate time from the effects of the stimulus of a current
flowing in one direction — of trying a second experiment on
the same plant, by reversing the direction of the current, so
192 PLANT RESPONSE
that in the reversal experiment, what was formerly anode
became kathode, and vice versa.
In this way corroborative reversal effects were obtained.
In experiments with condensers it was not necessary, in order
to reverse the electrodes, to reverse the battery connection,
for owing to the special arrangements of the electric circuit
(fig. 14) the anode at 'charge' became kathode at ' discharge.'
In studying the effects of increasing intensity, in the case
of constant current, I simply add to the number of storage
cells, and in this way obtain increasing voltage. The strength
of the condenser discharge is increased by increasing the
voltage of the charging circuit. With the same tissue, where
the resistance is constant, the current increases with the
acting E.M.F. Hence, increasing E.M.F. here connotes also
increasing current. But we may have a very high E.M.F.
and, owing to high resistance of the tissue, only a feeble
current. From the trend of the various experiments that I
have carried out, it would appear that the characteristic polar
effects are determined more by the intensity of the E.M.F.
than by that of the current. In the case of the present
investigation, as we must also bear in mind, the experiments
were performed with many different specimens, the excitability
of some being greater than that of others.
Effect of feeble E.M.F. — The first experiments of this
series were carried out by the method of mono-polar ex-
citation. The first specimen employed was Mimosa, one
electrode, the kathode, being connected with the pulvinus,
and the anode, at some distance, with the main stem. The
electromotive force used was ten volts. The leaf-stalk fell
at make of the circuit. The leaf was found to recover,
after a due interval, during the continuation of the current.
The current was now broken, but this produced no responsive
effect whatsoever. The current was next reversed, the pulvinus
being made anode. But this anode-make did not produce
any excitatory effect, neither did the succeeding anode-break.
From these experiments we see {a) that a feeble E.M.F.
excites at the kathode at make ; {b) that the excitation takes
EXCITATORY POLAR EFFECTS OF CURRENTS
193
place during the variation of current, but not when the cur-
rent has attained a constant value ; and (c) that there is no
excitation at either kathode-break or anode-make or break.
I next used a specimen oi BiopJiytum, the E.M.F. employed
being eight volts. The kathode was at first at the lower end of
the leaflets, the anode being on the main stem (fig. 89). At
make there was an excitatory wave at the kathode. This
travelled outwards and produced depression of four pairs of
leaflets. On reversing the current, the new anode did not
produce any effect at make, nor did
its break produce any excitation. It
will be shown presently that it is
necessary to have a certain moderate
intensity of current in order that the
anode-break may cause excitation.
The effect on Biophytuvi of a
continuous current at the kathode is
to bring about a more or less pro-
longed 'contraction,' the period of
recovery being thereby much pro-
tracted. With the leaf-stalks of
Miiiiosa, however, the effect is not
so marked. With this plant, never-
theless, I have been able to observe certain antagonistic effects
of anodic and kathodic actions ; that is to say, while there is
slow recovery from kathodic contraction, on reversing the
current, there is often an impulse of relaxation, the recovery
being thereby suddenly hastened. But it must be under-
stood that these particular effects are liable to modification,
being dependent on the physiological condition of the tissue.
I next repeated these experiments on the effect of feeble
E.M.F. producing excitation by means of condenser dis-
charge. The results obtained were .precisely the same as
with constant current. That is to say, with relatively feeble
charge, the excitation took place at the kathode at make,
and not at the anode. The great advantage of excitation
by the method of condenser discharge is, that the total
O
Fig. 89. Diagrammatic re-
presentation of Mono-polar
Excitation
K was kathode, which at make
gave rise to excitatory
wave, causing depression
of four pairs of leaflets.
The electric connections
are always made with non-
polarisable electrodes.
194
PLANT RESPONSE
Fig. 90. Bi-polar
Excitation of
Mimosa
quantity of electricity passing through the tissue is very
small, and the changes produced in the substance of the
specimen are therefore slight.
The next group of experiments was carried out by the bi-
polar method of excitation, which enables us to make simul-
taneous observations of the effects at atiode and kathode. As
in the previous cases, the specimen used
>i;i| for the first experiment was Mimosa, the
I E.M.F. employed being twelve volts.
I , Connections were made with the pulvini
of two neighbouring leaves (fig. 90). On
make, the kathodic leaf-stalk fell ; there
was no action at the anode. At break,
there was no action. On now reversing
the electrodes and m.aking the anode
kathode, the leaf-stalk which had not
previously responded fell under kathodic excitation. There
was no effect on the anodic leaf-stalk, nor was there any
effect on either at break.
For the succeeding experiment 1 used a leaf of Bio-
phytiiin and an E.M.F. of eight volts, the electrical con-
nections being as shown in the diagram (fig. 91). On
completing the circuit, the
excitation was discharged at
the kathode, and the wave
"'^^'^T^^'iTS^ % % "n^S^ proceeded in both directions
Vv ^ ^ from the kathodic point, three
pairs of leaflets being de-
pressed towards the stem and
two in the interpolar region.
There was no effect at break
at either electrode. On reversal, the new kathode, formerly
the anode, became the point of excitation, as evidenced by
the depression of contiguous leaflets.
Similar results were obtained when excitation was
produced by condenser discharge. Thus with a condenser
having a capacity of 'Oi microfarad, charged to eight volts.
Fig. 91. Bi-polar Excitation of
Biophytum
At make an excitatory wave proceeded
in two directions from kathode,
but none from anode.
EXCITATORY POLAR EP^FECTS OK CURRENTS
195
response was observed at the kathode at charge. The
excitatory wave travelled in both directions, and five pairs
of leaflets were depressed. There was no effect at the anode.
At discharge, the former anode became kathode, and there
was a responsive movement of the leaflets near that point.
In the following table, the characteristic polar effects of
feeble E.M.F. may be seen at a glance.
Table showinc; the Polar Effects of Feeble E.M.F.
Kathode
All
Make
ode
Make
Break
Break
Response
No response
No response
No response
Effects of moderate E.M.F.— From this point onwards
it will be found sufficient to describe the results obtained
by means of the bi-polar method of excitation, this mode
of investigation being
more complete than
the'mono-polar.
With regard to
the first of these, a
number of experi-
ments were performed
on a single specimen
of Biophytuni, using
an E.M.F. of 24 volts.
The excitatory wave
at make was found
to be initiated at ka-
thode, and to travel
in both directions,
causing the depres-
sion of nine pairs of
leaflets. The forward
stopped at one pair of leaflets before the anode (fig. 92). This,
Fig. 92
Make-kathode and Break-am
Effects in Biophyluiii
{a) Sliows effect at make, excitalion being
produced at kathode ; (/') shows effect at
break, excitation being now produced at
anode.
half of this wave of excitation only
196 PLANT RESPONSE
as will be seen later, is due to the depressing action of a strong
anode. There was no action at the anode itself at make ; at
break, there was no action near the kathode, but there was
excitation at the anode, as was shown by the fall of three
contiguous pairs of leaflets. The direction of the current was
now changed, the poles being thus reversed, and eight pairs of
leaflets fell at the new kathode, in and out. There was, how-
ever, no effect at the new anode at make ; but at break, the
reverse was the case, leaflets falling near the anode, and no
response occurring at the kathode. This experiment was
repeated three times on the same specimen, and the results
were in every case similar. I give (fig. 93) a pair of records
Fig. 93. Records of Responses of Leaflet of Biophytuni, showing
Responses occurring at Kathode at Make and not at Break ; and at
Anode at Break and not at Make
in Biopliytuin leaflet, showing the opposite character of the
effects of make and break at the anode and kathode respec-
tively. The E.M.F. used in this particular experiment was
sixteen volts.
In the former series of experiments, it was seen that
there was no break-anode effect when the E.M.F. was
feeble. The present experiments show us that the break-
anode is effective when the E.M.F. is moderately strong.
I obtained similar effects when stimulation was produced
by means of condenser discharge, the experiments being
carried out on BiopliytMU.
From the investigation just described, it will be seen
that with moderate E M.F. we obtain response from the
EXCITATORY POLAR FFKECTS OF CURRENTS
197
kathode at make, and from anode at break. The following
tabular statement exhibits these various effects in a concise
form.
Table showing Efkects of Moderate E.M.F.
Kathode
Anode
Make Break
Make
Break
Response , No response
No response
Response
Experiments with highly excitable tissues.- In ex-
perimenting on the polar excitation of animal tissues, using
a nerve-and-muscle preparation, it is found that when the
proximal end of the nerve is made kathode, that is to say,
when the current is ascend-
ing, the indicating muscle Vfl-y
shows response. This is
due to the make action of
the kathode. At break
also response occurs ; but
this is due to the trans-
mitted action of the break
excitation of the distant
anode. When the current
is reversed, that is to say,
made to descend, there is
also response, due to the
make excitation of the
distant kathode. When the current is broken, response
takes place again, in consequence of the break of the
proximal anode. All these cases are rendered possible by
the high conducting power of the intervening tissue, the
nerve.
I have been able to obtain precisely similar results, by
selecting very highly excitable specimens of Mimosa. One
electrode was placed at the junction of stem and petiole, the
Fig. 94. Effects of Ascending and De-
scending Currents, on Highly Excitable
Specimen of Mimosa
In the figure to the left, the anode is Ijelow
and kathode above, and current ascend-
ing. In the figure to the right, the
kathode below, anode above, and cur-
rent descending.
I9o PLANT RESPONSE
second being at a distance of about 3 cm, lower on the stem.
In this case the stem, or certain of its elements, acted as the
conducting nerve, the leaf serving as the terminal indicator
(fig. 94). With such an arrangement, using a plant of
exceptionally high excitability, and E.M.F. of moderate
intensity, I have obtained the following results :
1. Current ascending {a). — At make, the leaf-stalk fell.
This was due to the direct make action at the kathode {U).
At break there was also a response. This was due to the
transmitted break-anode excitation reaching the leaf-stalk.
2. Ciirrent descending (a). — At make, the excitation of
the distant kathode reached the leaf-stalk, the current at
the anode not being sufficiently strong to act as an effective
block, {b) At the stoppage of the current, there was another
response of the leaf-stalk. This was due to the break effect
of the anode in the immediate vicinity.
The following tabular statement shows at a glance the
effects which are apparent at the terminal organ :
Table showing the Effect of AfoPERATE E.M.F. on Highly
Excitable Mimosa
Ascending
Descending
Make
Break
Make Break
Response Response
Response Response
The experiments described above show that the excitation
produced in plant-tissues by an electrical current is not
indiscriminate, but selective, or polar, in its action. The
effects seen here are of precisely the same nature as those
observed in animal tissues. The exhibition of such polar
effects completely disproves the hydro-mechanical theory of
excitation in plants. They point unmistakably, on the
other hand, to the existence of some fundamental property
of protoplasm, common to animal and vegetable alike, which
under normal conditions finds an identical expression in
the two, of kathodic excitation at make, and anodic at break.
excitatory i'olar effects of currents 1 99
Summary
Polar effects are observed in plants in every way similar
to those obtained from animal tissues. The laws of polar
excitation in plants are as follows :
A. With feeble E.M.F. the kathode excites at make, and not
at break. The anode excites at neither make nor break.
r>. With moderately strong E.M.F. the kathode excites at
make, and not at break. The anode excites at break, and not at
make.
CHAPTER XVII
ON CONDITIONS OF REVERSAL OF NORMAL POLAR
EFFECTS IN LIVING TISSUES
Effect of high E.M.F. — Effects at two stages, A and B — Experimental verifica-
tion of ^ stage effect — Similar effects seen in protozoa — Experimental verifi-
cation of complete reversal at ^ stage — Law of polar effects under high E.M.F.
— Investigation on polar effects by death-response — Reversal of polar effects as
due to fatigue, or tissue-modification — Investigation of polar effects by glow-
response of fireflies.
The phenomena ot polar excitation which have been
observed in animal tissues are summarised in the formula
which is known as Pfliiger's Law, viz. that excitation takes
place at the kathode at make, and the anode at break. It
has been found, however, by Kiihnc, Verworn, and others,
that in the case of the protozoa the polar effects are exactly
the opposite ; that is to say, in these instances, it is the
anode which excites at make. The inference has hence been
drawn, that Pfliiger's law was inapplicable in the case of un-
fibrillated protoplasm.
That this assumption, however, is incorrect, I have
already shown, by the fact that the undifferentiated proto-
plasm of the plant-body gives rise to polar effects which are
in every way identical with the normal polar effects seen in
animal tissues.
It occurred to me that the sttidy of polar effects in plants
might throw some light on this anomaly, and that I might
thus be able to trace out the stages by which the one effect
was gradually transformed into the other, determining further
the conditions which were effective in predisposing a tissue
towards this reversal.
Effect of high E.M.F. — In carrying out this investigation,
REVERSED POLAR EFFECTS IN LIVING TISSUES 201
I soon discovered that the value of the acting electromotive
force had an important influence on polar excitation. I
found that under an increasing E.M.F. the excitation pro-
duced at the kathode underwent, first an increase, and then,
on reaching a maximum, a decrease, which might even
become negative. The changes produced at the same time
at the anode were exactly the opposite. There was thus a
progressive variation, resulting in an exchange of the excita-
tory properties of the anode and kathode.
My first observation with regard to this question was
made in the course of my investigations on the determination
of the velocity of transmission of excitation (Chapter XX.),
and on the effect of increasing intensity of stimulation
en this velocity. I found that, using for instance thermal
stimulation, when this was strong, it was transmitted with a
greater velocity than when it was feeble. Hence the speed
with which the effect of stimulus travels in a given tissue
may be held to afford a measure of the effective intensity of
the stimulus. In order, however, to apply a stimulus which
might be increased by known amounts, I next tried the electric
mode of stimulation, expecting to produce an increasingly
effective intensity of stimulus by increasing the E.M.F., excita-
tion being produced at the kathode at make. In the course
of a particular experiment, on a leaf of Biophytum, I found
that as the E.M.F. was augmented from eight to thirty-two
volts, the excitatory value of kathode at make was also
increased, as shown by the fact that the velocity of the
transmission of excitation was raised from 3-27 mm. per
second, in the former case, to 3 83 mm. in the latter, an
increase, that is to say, of 17 per cent.
But on raising the E.M.F. still higher, I found that the
velocity of transmission from this point underwent a progres-
sive decrease. From this it would appear that the excitatory
effect of the kathode at make had an optimum value, in this
case of thirty-two volts, beyond which there was a decline.
This optimum value would naturally undergo a certain varia-
tion with the nature and condition of the tissue. Now, as the
202 PLANT RESPONSE
excitatory power of the kathode at make is seen to undergo
a gradual diminution, beyond this optimum, it follows that at
some certain high E.M.F. the excitation produced by it would
be zero. In other words, the kathode would cease to excite.
Possibility of two distinct stages of reversal, A and B.
Further, since the effects at anode and kathode are, generally
speaking, contrary in character, we might expect a corre-
sponding change, but of opposite nature, to make its appear-
ance progressively at the anode. In other words, it might
happen that at a certain stage in the raising of the E.M.F.
the exciting value of the kathode would be considerably
diminished, and that the anode would begin to show
excitatory effect. This might be designated as the A stage.
On still further raising the E.M.F. the same contrary-
directioned change might be expected to continue progres-
sively at the anode and kathode, and to reach a stage at
and beyond which it would be the anode which excited at
make, while the kathode produced either no excitation or
actual depression. This might be designated as the B stage.
We should then have a complete reversal of the normal
polar effects.
If we exhibit these inferences, as to the relative excitatory
powers of anode and kathode with increasing E.M.F., by
means of curve.s, whose abscissae represent the E.M.F. while
their ordinates give the corresponding excitator)^ values,
the kathode curve would first rise to a maximum, and
then fall continuously, till, reaching the zero line, it might
even proceed still further in the negative direction, thus
representing depression. The anodic curve, on the contrary,
would at first descend in the negative direction, thus indicat-
ing increasing depression of excitability, until it reached a
negative maximum, after which there would be a reversal,
and it would begin to ascend and reach the zero-line. Here
the anode would cease to depress. After this it would
proceed upwards in the positive direction, indicating a con-
tinuously increasing power of excitation. The anodic and
kathodic curves in the course of this ascent would cross at a
REVERSED ROLAR EFFECTS IN T.IVINC TISSUES 203
certain point, indicating that both of them now excited in
about equal degrees. This would constitute the A stage.
Beyond this would be the B stage, where the anode alone
would excite.
I now proceeded to subject these theoretical inferences to
the experimental test. The special difficulty of this investiga-
tion lies in the fact that it is necessary to discriminate the
direct from the transmitted effect at the two electrodes, with
absolute certainty. For if the two points be not at a sufficient
distance, and if the conductivity of the intervening tissue be
great, the true effect of one electrode may be rapidly trans-
mitted, and appear at the other. For these reasons, the
plant Biophytiiin is not in this case a very suitable subject for
experiment, its conductivity being great, and its opposite
leaves not at a sufficient distance from each other. Mimosa,
however, may be made to serve the purpose, for, though its
conductivity is great, it is possible to select two leaves on
different branches of the same plant which are very far
apart. The plant Averrhoa is also appropriate, its conduct-
ing power being relatively slight. Electrical connections
may, in this case, also be made with opposite leaves widely
apart. In the case of Mimosa the excitatory effect is made
visible by the fall of the leaf, in the case of Averrhoa by the
depression of the leaflets. It is thus possible to render the
effect of the two electrodes mutually distinct. It is also
possible to distinguish the transmitted excitation, if any, by the
serial depression of the intervening leaves or leaflets during
the passage of the wave of excitation.
As these excitatory effects are dependent on the physio-
logical condition of the tissue, we should expect that the
E.M.F. which produces reversal would vary with different
plants and their physiological conditions. The highest constant
E.M.F. available for my own investigations was 220 volts,
that being the pressure in the street-mains. I therefore
hoped that I might be fortunate enough to find plants in a
state to exhibit the expected reversal within this value. I shall
now proceed to describe actual experiments with various
204 PLANT RESPONSE
plants, and first I shall take those in which the A stage was
exhibited, that is to say, those in which the anode as well as
the kathode showed excitation at make.
Experimental verification of A stage effects. — With
the plant Biophytuin, I have alwa)'s found, without exception,
that up to thirty-two volts, or thereabouts, the polar effect was
normal ; that is to say, excitation was produced at the kathode
at make and not at the anode. On using an E.M.F. of forty-
eight volts, however, with a certain specimen, I obtained
excitatory response at make, at both anode and kathode.
That this anodic effect was not due to transmission of ex-
citation from the kathode, was seen in the fact that some
of the interpolar leaflets were not affected, as all would have
been had the wave of excitation passed from kathode to
anode.
I shall next describe experiments made on Mimosa, in
which, as has been said, the two electrodes can be separated
by a longer tract of tissue. In the case of this plant, the
value of the E.M.F. which is required to bring on the A stage
effect, is much higher than in Biophytu)ii. I have occasionally
obtained it with i lo, but more usually with 220 volts. In
order to show how at this stage the anodic and kathodic
effects tend to become interchangeable, 1 shall describe three
experiments.
In the first of these, an E.M.F. of 1 10 volts was used. At
make, the kathodic leaflets fell energetically, while the anodic
fell but slightly, and after a little delay. Here we see that
though reversal is setting in, yet the normal kathodic effect
is relatively predominant.
In the second of these experiments, I used 220 volts.
The anodic fall now took place slightly earlier than the
kathodic. The current was maintained till the leaflets
recovered. On now breaking the circuit, there was a slight
anode-break excitation, but none at the kathode. In this
case, though from the slight priority of the anodic excitation
we infer some predominance of the anode, yet the fact that
the effect at break is normal shows that we arc still in the
REVERSED POLAR EFFECTS IN LIVING TISSUES 205
transition stage. In the next experiment, the tendency to
reversal will be shown to have become predominant.
In this third experiment with another specimen, an
E.M.F. of 220 volts was again used. At make, there was
an immediate energetic fall of the anodic leaflet, while that
at the kathode was slight, and delayed for some time. At
break, moreover, there was no effect on the anode, and a
slight and delayed excitatory effect was distinctly perceptible
at the kathode. From this we see that the anode is now-
appropriating the normal action of the kathode, and vice versa,
reversal having set in unmistakably.
Reversed action in protozoa.— These experiments will
probably be found to explain the assumed anomaly in the
case of protozoa. In experimenting on ActinosphcErium, for
example, Verworn found that ' at closure of the current in
the first place, the pseudopodia, both on the anodic and
kathodic side of the globular body, become varicose and
begin to contract. If the circuit be opened the pseudopods
on the kathodic side become" varicose in about the same
degree as had taken place immediately after the closure of
the circuit.'^ In this experiment, where both anode and
kathode exhibit excitation at make and only the kathode at
break, we have a case exactly parallel to that of the third
experiment with Mimosa, which has just been described.
That, as in the case of plant-tissues, a fairly high E.M.F. was
instrumental in producing reversal appears probable, from
the fact that it is specially mentioned in the account of the
experiment, that' in consequence of the high resistance in the
circuit, a comparatively high E.M.F. had to be used.'
Experimental verification of B stage effect — I could
not obtain with Mimosa at 220 volts complete cessation of
excitation at the kathode at make, but I succeeded in doing
so with Averrhoa, in the autumn and winter seasons. With
Jthis plant, I observed all these A stage effects, which have
already been described in the case of Mimosa ; and that of
completed reversal, or the B stage effect, was obtained in
' Biedermann, Elcdro- Physiology , English edition, 1896, vol. i. p. 302.
206
PLANT RESPONSE
more than a dozen instances, out of which I shall give an
account of two.
In the first of these, the two electric contacts were made
on the same leaf, at a distance of 5 cm. from each other. At
make, using an E.M.F. of 220 volts, excitation was produced
at the anode only, and the depression of successive leaflets
proceeded towards the kathode, but was arrested at one pair
in advance of that point, the kathode apparently acting here
as a depressor.
In the second experiment, the electric contacts were made
at a great distance from each other, with middle points of two
opposite leaves. At make, excitation was produced at the
anode only. At break, however, it took place at the kathode
and not at the anode. We have here a complete reversal
of the normal polar effects under the action of very high
E.M.F.
The following tabular statements show at a glance the
polar effects at both A and B stages, under a high E.M.F. :
Table showing Effect of High E. M.F.— .^ Stai^e.
Make
Break
Make
Moderate response Occasional response Moderate response
Break
Occasional response
Table showing Effect of Excessively High E.M.F.— j5 Stage.
Kathode
Anode
Make
Break
Make
Break
No response
Response
Strong response
No response
Law of polar effects under high E.M.F. — We have now
traced out that process of continuous change by which under,
a gradually increasing E.M.F. there is produced a reversal of
normal polar effects, and we thus arrive at the following law
of polar excitation :
REVERSED POLAR EFFECTS IN LIVING TISSUES 20/
Under high E.M.F. — at the A stage — both anode and kathode
excite at make ; at break there is occasional excitation at
either anode or kathode. Under excessively high E.M.F. — at the
7) stage— the anode excites at make and the kathode at break.
Without this addition the law of polar excitation is in-
complete ; and I shall have occasion, in iny work on the
Electro-Physiology of Plants, to show its application in
explaining certain excitatory electromotive phenomena which
would otherwise have remained obscure.
Investigation on polar effects by death-response.^
I have already explained in Chapter XV. that the death-
point of an excited tissue is lowered below the normal. This
made it possible to devise a test by whose means it might be
determined which of the two electrodes produced excitation.
Thus, on taking two similar petals of Passiflora, and making
one anode and the other kathode, it was found with a
moderate E.M.F. that death-discoloration took place at the
kathode, at a temperature of 4° C. lower than at the anode
(p. 185), thus proving that under these conditions it was
the kathode which produced excitation. I was now desirous
of finding out whether- the same test could not be applied to
the demonstration of the reversed effect due to high E.M.F.,
and in this connection I shall give an account of an experi-
ment on the coloured petals of Sesbam'a coccineui/i, the death-
discoloration of which occurs normally at 65*5° C. Taking
two similar petals, and using the high E.M.F. of 220 volts,
I found on sending a current that death-discoloration took
place at the anode, at 60° C, that is at 6\° below the normal.
The discoloration point of the kathode was also lowered,
but only slightly, being 2^° below the normal. We thus see
that with a high E.M.P'. it is the anode which is more ex-
citable at make. It is clear from this that the reversal of the
normal polar effect has set in, the anodic excitation being
considerably predominant.
Reversal of polar effects as due to fatigue or tissue-
modification. — It has now been demonstrated that an exces-
sively strong E.M.F. is one of the conditions b)- which the
208 PLANT RESPONSE
reversal of normal polar effects may be brought about. We
shall next study other circumstances which may also be
efficient to induce this reversal. This subject assumes the
greater importance from the difference of opinion v^diich exists
among investigators in animal physiology as to the possi-
bility of such reversal. The question has not yet, as far as
I am au^are, been definitely settled. Thus, ' Aeby thought
he had proved that under certain conditions, more par-
ticularly with progressive fatigue of the preparation, the
normal reaction — in which the excitatory action of the
kathode far exceeds that of the anode — was exactly reversed.
Aeby's experiments, however, are by no means unimpeach-
able, as both Engelmann and Hering pointed out later.
Engelmann, also, came to the conclusion later, that such a
complete reversal of phenomena {i.e. of the law of polar ex-
citation) might take place. But until it has been determined
by unexceptional experiments, there must be great scepticism
in regard to such statements.' ^
We now turn our attention to that of the changed
condition of the tissue by which the normal polar response
may become reversed, and in this regard the experiments
which I shall describe are very instructive, as these changes
arc there seen to occur progressively. I took a specimen of
Mimosa and carried out on it five consecutive experiments.
The two electrodes were attached to the pulvini of different
leaves on the same stem, and the E.M.F. used was fifty volts ;
an interval of about seven minutes was allowed in each case
for recovery. For easy inspection, the results are given in
somewhat tabular form.
(i) At make — Leaves fell both at kathode and anode.
The kathodic fall was earlier and more energetic.
At break — No decisive effect observed at cither
electrode.
(2) At make — The kathodic leaf fell, and the anodic fall
was slight.
At break — No action at kathode, but energetic fall
' Biedermann, Electro- Physiology, English translation, 1898, vol. i. p. 271.
REVERSED POLAR EFFECTS IN LIVING TISSUES 209
at anode. The action of anode-break was here much
stronger than that of kathode-make.
(3) /i/ wa/ce—FaW of kathodic leaf; no action at anode.
At break — No action at kathode ; response at anode.
(4) At make — Kathodic action became feeble, and anode-
fall, though at make, the more pronounced of the two.
At break — No action at either electrode.
(5) At make — No action at kathode ; feeble action at
anode.
At break — No action at either electrode.
In tracing out the changes which are here taking place at
each electrode, we are struck by their progressive character.
If we fix our attention first on the kathode, we find that the
normal effect in the first of the series is gradually diminished,
till it disappears in the last. Again taking the anode, we
find a still more remarkable change, of a periodic character.
In the first experiment, we observe the most pronounced
abnormality, or reversal of the series, inasmuch as there was
response at make and none at break. In the second, the
response is tending towards normal, the anode-make effect
being feeble, and the break strong. In the third, the anodic
response has become normal, for there is no action at make,
but excitation at break. In the fourth, we again see a ten-
dency towards reversal, inasmuch as again there is response
at make and none at break. The same state of things,
though in a less degree, occurs in the fifth experiment.
We have thus observed two different conditions, each of
which may contribute to produce this reversal of polar effects.
These are, firstly, the influence of a high E.M.F., which, at or
beyond a certain critical value, will produce reversal ; and,
secondly, certain tissue-modifications similar to those which
we have observed during the progress of fatigue. It is clear
that with slight tissue-modification the critical value of the
E.M.F. at which, under normal conditions, reversal of polar
effects would take place, will be lowered. This, the experi-
ments on Mimosa just described clearly show ; for in them
we see that reversal has set in at the relatively low E.M.F. of
r
2IO PLANT RESPONSE
fifty volts, whereas normally in Mimosa the critical value is con-
siderably above a hundred volts. These tissue-modifications
sometimes proceed so far that I have occasionally observed
reversal in the case of this plant even with a moderate
E.M.F.
I was next desirous of determining whether these different
types of polar effects — normal, transitional, and reversed —
could not be demonstrated in some novel and striking manner,
in the case of animal tissues. It occurred to me that the
intermittent flashes of light emitted by the firefly might
be simple expressions of rhythmic excitation, a subject
which will be dealt with in detail in Chapter XXIII. The
emission of light, or an increased intensity of emission on
the part of the insect, would in that case be indicative of
the state of excitation, and this mode of excitatory expression
I shall designate as glow-response.
Investigation of polar excitation by glow- response —
I may here state in anticipation that I have succeeded in
demonstrating, by means of this glow-response, all the prin-
cipal characteristic effects of {a) normal response, due to
moderate electromotive force ; {b) the reversed effect due to
high electromotive force ; and {c) the reversed effect due to a
modified condition of the tissue. It may be pointed out
further, that some specimens gave the normal, and others,
owing to a modified condition of the tissue, the reversed
effect ; but that the results obtained from any given in-
dividual were always consistent and characteristic.
I shall first describe certain results which were frequently
observed, and which are entirely analogous to those described
in a previous chapter as given by a nerve-and-muscle pre-
paration, and highly excitable tissue of Mimosa (p. 197). We
there saw that while the current was ascending, the excira-
tion exhibited by the terminal organ at make was due to
direct action of the proximal kathode. Excitation was also
produced at break, and this was due to the transmission of
the distal anode-break effect. Again, when the current was
reversed, excitation was exhibited in a corresponding manner,
REVERSED POLAR EFFECTS IN LIVING TISSUES 2 11
through the action of the distal kathode make and the
proximal anode-bmak.
The firefly under natural conditions emits flashes of light
at intervals of about three seconds, from two discs, situated
on the ventral surface of its tail.
We select a specimen and make suitable electrical con-
nections, one with the head, and the other with the luminous
disc. The natural luminescence of the insect is moderate
and intermittent ; but on now passing through it a descending
current from a battery having an E.M.F. of twelve volts, the
light at once becomes persistent and very brilliant. We
must bear in mind that the luminous discs stand here in the
place of the terminal motile indicator, of the nerve-and-
muscle ox Mimosa preparation, and that the state of excitation
is indicated in them by the increase of luminescence instead
of by an excitatory movement. This glow-response, then, is
due to the action of the proximal kathode-make. The induced
brilliance slowly dies down, and in the course of a minute and
a half becomes very feeble. If the circuit be now broken,
a single intense flash is produced, due to the excitation of
the distal anode-break. The insect now recovers from the
state of induced excitation, and begins once more to exhibit
its natural intermittent flashes. We next pass the current
in the reverse, that is to say ascending, direction. The light
again becomes persistent and brilliant, owing to the excitatory
action of the distal kathode-make. During the continuation
of the current, the light wanes and becomes feeble. But when
the circuit is broken, there is once more seen a single flash of
intense light, due to the action of the proximal anode-break.
In order to ensure a simpler condition for experiment by
eliminating the nervous conduction of excitation, I next
isolated the double disc, and found that the detached organ
maintained its excitability for a couple of hours or more.
The discs now emitted a light which was somewhat feeble
but not intermittent. Electrical connections were then made
with the two discs, by means of fine cotton threads, moistened
with saline solution, and an E.M.F. of sixteen volts was used,
V 2
212 PLANT RESPONSE
At make the kathodic disc was found to become very
brilliant, and there was no effect on the anodic. In some
instances, indeed, the anode became dimmer than usual, thus
showing the depressing influence of the anode. At other
times, again, the luminous excitation of the kathode irradiated
and encroached upon the anodic region. At break it was
the anode which flashed out, showing excitation. These
results, as will be seen, are entirely normal. I shall next
describe experiments which illustrate the reversed effect
.sometimes observed with excessively high E.M.F., and at
other times due to a modified condition of the tissue.
With regard to the production of the reversed effect under
a high E.M.F., some difficulty is encountered owing to the
proximity of the two discs of the luminous organ. The effect
of one electrode is thus liable to encroach on the region of the
other. But specimens are occasionally obtained in which, the
conducting power of the tissue being feeble, each effect is
practically confined to its own area, though the excitatory
E.M.F. may be high.
In the following investigation it is to be noted that
successive experiments were carried out on the same speci-
men, without disturbing the electrodes. By proper manipula-
tion of the key, the current was made to flow now in one
direction, then in another, or the acting E.M.F. was changed
from low to high at will. The differences of the results
observed must therefore have been due, either in the first
case to the reversal of anode and kathode, or in the second
case to the difference in intensity of the E.M.F.
A specimen was taken of the detached luminous organ,
and electrical connections were made with the two discs, by
means of moistened threads. An E.M.F. of ten volts was
first used, and the effect at make was a brilliant illumination
of the kathode-disc. During the continuation of the current
this gradually waned, but at the break of the circuit a
brilliant flash appeared at the anode. Thus we have, in the
present case, the normal effect with moderate E.M.F. I next
used with the same specimen the high E.M.F. of fifty volts.
REVERSED POLAR EFEECTS IN LIVING TISSUES 213
The luminescence at make now took place at the anode,
and at break at the kathode. On reversing the current, the
new anode, formerly kathode, gave responsive illumination,
and at break the new kathode responded. In these results,
therefore, it will be seen that we have an instance of reversal
of polar effects, under excessively high E.M.F.
These reversed effects are usually observed with a high
E.M.F. ; but sometimes, as has been said, owing to a modified
condition of the tissue, they may be obtained, under the
action of even a moderate E.M.F. I shall now give a very
interesting example in which we can trace the process of
reversal owing to the modification induced by fatigue, in a
manner somewhat similar to the last experiment described in
the case of Mimosa (p. 209).
I took a fresh specimen of the detached organ, and carried
out four successive experiments on it, observing the effects
at both make and break, the E.M.F. used being twenty volts.
In order to present these results at a glance, I shall again put
them in a somewhat tabular form.
(i) At make — Luminous response at kathode, which
irradiates slightly towards anode.
At break — Little effect at anode, but natural luminosity
of the kathode falls below par. This shows the depressing
action of kathode-break.
(2) At make — Luminous response at both anode and
kathode.
At break — Luminous response at kathode only. These
effects, especially that of break-excitation at kathode, show
that the condition of reversal has set in. This will become
still more pronounced in the succeeding experiments.
(3) At make — Luminous response appears at anode and
irradiates slowly towards kathode.
(It will be seen that we have here a complete reversal of
the effects observed in (i) at make.)
At break — No immediate effect is at first observed ;
later, a flash passes from anode to kathode.
(4) At make — Luminous response at anode.
214 PLANT RESPONSE
At break — No effect at anode, but feeble augmentation
of luminosity at kathode.
These results afford us some insight into that obscure
phenomenon of the modified condition of tissue by which
reversal of response is brought about.
We have seen that in the case of Biophytniii, the polar
effects are alvvaj-s found to be normal, within rather a wide
range of E.M.F., that is to say, up to about thirty volts. The
kathode here excites at make, and the anode at break. I
have carried out several hundreds of experiments with this
plant, but have not once come across any deviation from this
normal action.
As the E.M.F. was progressively increased, however, we
found in this and other plants a tendency towards the
reversal of these normal polar effects. During the first, or
A, stage of this reversal, the excitatory value of the kathode
was seen to undergo a diminution, and the anode, which
normally had a depressing influence, was observed to have
its property reversed, and to produce excitation. The result
during this stage, therefore, was the exhibition of excitation
at both kathode and anode at make.
With still higher E.M.F. the B stage was reached, and
here there was a complete reversal of the normal polar effects.
It was then found that the anode produced excitation at
make, and the kathode at break. This reversal of polar
effects under a high E.M.F. was further demonstrated by
means of Death-response in plants, and Glow-response in
animals.
We have also seen that in consequence of progressive
molecular change induced by fatigue, the normal polar effect
tended to be reversed, and we have been able to trace the
successive stages of such a reversal, in experiments on the
plant Mimosa and on the firefly.
And, finally, specimens are occasionally found which,
owing to molecular modifications of their tissues— modi-
fications that a knowledge of their previous history could
reversf:d polar effects in living tissues 215
alone enable us to explain — tend to exhibit abnormal polar
effects.
Summary
Under high E.M.F. the normal polar excitation tends to
be reversed. In the A stage, both the anode and kathode
excite at make, and either kathode or anode at break ; in the
B stage — that is, with excessively high E.M.F. — it is the
anode which excites at make, and the kathode at break.
The firefly under excitation exhibits glow-response.
Under moderate E.M.F. it shows normal polar effects.
Under a high E.M.F. it, like the plant, exhibits a reversal
of these polar effects. Under fatigue, or other tissue-
modification, normal oolar effects tend to undergo reversal.
CHAPTER XVI 1 1
OM CONDUCTIVITY AND EXCITABILITY
Receptive excilahilily, conductivity, and motile excitability — Molecular model — ■
Modification of motile excitability : (a) by anesthetics— ((^) by cold — (c) by
fatigue — Variation of conductivity : {a) by cold — {6) by rise of temperature —
(r) by fatigue — {) by aneesthetics — Variation of receptive excitability by ether
— Conductivity versus excitability — Abolition of motile excitability without
abolition of conductivity — Hydro-mechanical theory of transmission of stimulus
untenable.
Hitherto we have been concerned mainly with the pecu-
liarities of the responding organ, by which the state of
excitation is outwardly manifested. Very often, the respond-
ing organ is not directly stimulated, but a distant point is
acted on by stimulus, and the state of excitation is trans-
mitted through the intervening distance, by the conducting
power of the tissue. In the actual life of a plant it is
frequently the case that the stimulus impinging on a recep-
tive area is transmitted along conducting channels, and is
manifested, on reaching some responsive organ. The whole
cycle of events is something like a telegraphic circuit, in
which the message taken at a transmitting station is sent to a
distance along conducting wires, and produces a signal at the
distant or responding station.
In our experiments, for example, on Biophytuni, as given
in the previous chapter, the stimulus was applied at the pe-
tiole, and was conducted along certain channels. On reaching
the specialised motile organ — the pulvinus — this transmitted
stimulus caused a responsive depression of the leaflet. The
petiole, during conduction of stimulus, was excited, but there
was no conspicuous external evidence of this, because, first,
the contractility of the tissue was relatively feeble, and
CONDUCTIVITY AND EXCITABILITY 21/
second, the differential excitability, on which responsive
curvature depends, was also slight. It is only at the pul-
vinated organ that the state of excitation is conspicuously
exhibited by motile response.
In the case of the plant, therefore, we have to study the
excitable property of the receptive area, the conducting
property of the transmitting tissue, and that property of the
responding organ by which the excitatory effect is outwardly
manifested. It will be convenient to distinguish the excit-
ability at the point of application of stimulus from that of
the motor region, by using a specific term for the former. I
shall therefore designate it as receptive excitability, or merely
as receptivity, whereas the excitability of the motor region
will be described simply as excitability. At the point ot
application, the stimulus comes from outside, and produces
internal changes. In the motile region, the internal excita-
tory disturbances are manifested outwards. In the physio-
logical study of excitation, some confusion is apt to arise
from the failure to discriminate between these three factors.
And this confusion becomes greater in those cases in which
the area of motile excitability coincides with that of
receptivity.
Molecular model. — The state of excitation being ulti-
mately due to molecular upset from the position of equi-
librium, we can understand that such a disturbance is
propagated from molecule to molecule, till it reaches the
responding organ. We may, perhaps, be enabled to
visualise this better by means of a mechanical model. The
individual molecules in our model should hold a position of
stable equilibrium. When disturbed from this stable poise,
they should return automatically to the equilibrium position ;
and further, the derangement of one molecule should cause a
subsequent disturbance of the next, and this disturbance
should be transmitted from point to point.
These conditions are realised in the case of the following
model, which consists of a row of small suspended spheres of
cork, within each of which is placed a magnetic needle. Each
2T8
PLANT RESPONSE
sphere is now in stable equilibrium, under the directive action
of the earth, and the mutual action of the needles ; hence the
north pole of each needle, represented by the arrow-head,
points to the north, which is, say, to the left. The disturb-
ance of any individual sphere, say E, brings about the
disturbance of its neighbour, and, owing to the mutual
magnetic action between contiguous north and south poles, a
derangement initiated in this way is transmitted onwards.
Such a disturbance may be initiated by means, for instance, of
Fig. 95. Molecular Model Exhibiting (a) Excitability at the Receptive
Area ; (/>) Conductivity of Intervening Region ; and (c) Mechanical
Response of Terminal Responder
Disturbance is initiated at the sphere connected with E, by the magnetic
action of the electro-magnet seen to the right. This disturbance is con-
ducted by the intervening spheres and reaches the terminal responder, R.
Molecular viscosity is increased Ijy immersion of attached dampers in
viscous fluid.
a small electro-magnet, placed at right angles to the molecular
magnet in E. This electro-magnet is magnetised for a short
time by the tapping of a key, which closes an electric current,
causing a rotation of the sphere E. The intensity of this
disturbing force, the stimulus, may be increased at will, by
appropriate exaltation of the strength of the magnetising
current (fig. 95).
In such a row of molecules, then, that to the extreme
right, E, is the point at which we shall initiate molecular
disturbance. That is to say, it corresponds to the receptive
CONDUCTIVITY AND EXCITABILITY 219
point. The intermediate row, C, is the conductor of disturb-
ance ; and the last molecule, R, which may be provided with
an index, or a reflecting mirror, by means of which the dis-
turbance can be made conspicuous, represents the motile
responder.
We shall next observe how the extent of the distortion of
each of the molecules from the position of equilibrium by a
given force — that is to say, the amplitude of its response —
is modified by the factor of molecular mobility. Under the
action of certain agencies the freedom of molecular movement
may be retarded, by variation of elasticity or of viscosity.
We may, with our model, imitate the resultant molecular
sluggishness, by means of dampers, which are seen in the
diagram, attached to each sphere. The extent of damping is
capable of increase by immersion of the damper in a viscous
fluid. The response-curve of this particular sphere may now
be taken by the usual method of a reflected spot of light.
The curves thus obtained will show, firstly, that, the disturb-
ing force remaining the same, diminished molecular mobility
is attended by diminution of amplitude of response ; secondly,
that this diminution may become so marked that visible
response may disappear ; thirdly, that though, with a given
moderate disturbance, response may thus be in abeyance, yet
it may be restored if the disturbing force be made sufficiently
strong ; and fourthly, that the sluggishness thus induced may
also be exhibited by delay in the initiation of response, that
is to say, by the prolongation of the latent period.
From such considerations, it is clear that if an agency
which reduces molecular mobility be applied on the receptive
area, then, inasmuch as the initiation of excitation is prevented,
there will be no response exhibited by the motile organ,
although the conducting power of the intervening tissue,
and the motility of the responding organ, remain unchanged.
Again, if the intervening conducting tissue be subjected to
loss of molecular mobility by any means, the power of
conduction will be either very much retarded, or abolished,
the receptivity and excitability of the terminal points
220 PLANT RESPONSE
remaining unaffected. And, finally, the excitability of the
motor region may be depressed by certain agencies, and
the stimulation, initiated at the receptive point, and trans-
mitted through the intervening conducting channels, will
nevertheless fail to find expression. We shall next proceed
to demonstrate experimentally the influence of various
agencies on the receptivity, on the conductivity, and on the
excitability of the tissue.
Variation of motile excitability : (a) Under ancestJietics. —
First we shall take the variation of excitability in the motor
region. Let us then select a leaf of Biophytum and apply
ether to the two terminal pairs of leaflets beyond D.
Thermal stimulus is then applied at x , by touching with a
hot wire (fig. 96). As the
^^^ receptivity of the point of
application, and the con-
ductivity of the intervening
Fig. 96. Effect of Ether in the Abolition ..;..„„ remain iini'mnaired
of Motile Excitability tissue, remam unimpaired.
Ether is applied to the two pairs of leaflets the excitatory disturbance
to the right of D ; stimulus is applied prOCCeds ill the normal
at X . Excitation travels up to D, and ,
cannot pass beyond. manner to the point D, a
fact seen by the successive
depressions of the leaflets. Owing, however, to the abolition
of their excitability, the last two pairs remain unaffected.
A similar loss of excitability, due to the action of ether,
may be demonstrated in Mimosa. On taking a stem pro-
vided with three motile leaves. A, B, and C, the pulvinus of B
is touched with ether, and thermal stimulus is applied
between A and B. The excitation is transmitted in both
directions, up and down, as seen by the fall of the leaves A
and C. But the intermediate leaf B fails to respond, showing
that its excitability has been abolished by the ether.
[b) By effect of cold. — The prolonged application of
cold, also, will produce, as would be expected, molecular
sluggishness, with consequent loss of motor excitability.
This may be shown by touching the small pulvinus of a
leaflet of Biophytum with ice- water. If stimulus now be
CONDUCTIVITY AND EXCITABILITY 221
applied on the petiole, it will be found that this particular
leaflet will not respond. This loss of excitability will, how-
ever, be temporary, disappearing as the leaflet returns to its
ordinary temperature, when it will be found to respond as
usual.
A moderate application of cold docs not altogether
abolish the response, but the molecular sluggishness induced
is shown in the prolongation of the latent period of response.
It was found, for example, in an experiment on Biophytitiii
that the latent period was sometimes prolonged by several
seconds (p. 268).
{c) By effect of fatigue. — We have already seen (p. 113)
how the motile excitability of the plant-tissue is diminished
by fatigue, as shown in the diminution of successive responses,
when the intervening periods of rest are not sufficient for
complete recovery. We have seen, too, that under strong
and long-continued excitation the motile excitability is
abolished ; and that it can be restored after the lapse of a
sufficiently long resting period.
Variation of conductivity — We shall next examine how
the transmission of stimulus from point to point is affected
by various external agencies. And, first, we shall refer back
to the mechanical model (fig. 95). We there saw how the
sluggishness, induced in the intermediate molecules by
plunging the dampers to a greater or less depth in a viscous
liquid, retarded the transmission of disturbance through them.
When this induced sluggishness is slight, the propagation will
merely be slowed below the normal ; but when the sluggish-
ness induced is great, the disturbance will not reach the
responder R.
{a) By effect of cold. — We shall now proceed to investi-
gate the effect of induced molecular sluggishness on the
conductivity of a plant-tissue ; and for this purpose we
shall first observe the influence of cold. In an experiment
on Biopkyt7iin, I found that the normal velocity of trans-
mission, depending on the conductivity, was 37 mm. per
second ; but on subjecting the tissue to moderate cold, the
222 PLANT RESPONSE
velocity of transmission was reduced to r3 mm. per
second, or nearly to one-third of its original value (p. 249) ;
a still greater application of cold produces a temporary
abolition of conductivity. This may be shown by touching
a given portion, E, of the pe-
tiole with ice, when moderate
stimulus applied below such
a point will not be transmitted
across the lethargic area, and
the motile leaflets beyond will
Fig. 97. Experimental Demonstration remain Unaffected. The nor-
of Effects cif Cold and Anaesthetics , j .• -^ -ii i
in Abolishing Conductivity ^^1 conductivity Will, how-
ever, be restored when the
tissue regains the temperature
Cold or ether applied at E ; stimulus
at X cannot be transmitted across E,
and there is no effect on the
motile leaflets. of the Surrounding atmo-
sphere, and a second similar
application of stimulus will then be found to be conducted
to the motile leaflets, producing successive depressions
(fig- 97).
{b) By rise of teuipcrature. — We have seen how, in
consequence of the molecular sluggishness induced by cold,
the conductivity of the tissue is lowered. A rise of tempera-
ture might therefore be expected, by increasing molecular
mobility, to enhance the conducting power. That this is
the case is shown in detail in Chapter XX. In a leaf of
Biophytuni, for instance, it was found that a velocity of 37
mm. per second at 30° C. was increased at 35° C. to 7-4 mm.,
and at 37° C. to 9-1 mm. per second. Thus, by a rise of
temperature of from 30" C. to 37" C. the conductivity of the
tissue was increased to nearly three times its initial value.
{c) By effect of fatigue. — We have already seen (p. iii)
that motile response, and the transmission of excitation, are
both alike expressions of the protoplasmic changes induced
by stimulus. We there saw also that just as fatigue of
motile excitability was exhibited by diminished motile
response, so too a diminished speed of transmission exhibits
fatigue of conductivity. An experiment will be described
CONDUCTIVITY AND EXCITAHILITV 223
later (p. 245), which shows that in that case, under moderate
fatigue, conductivity was diminished by 18 per cent, of its
normal value.
The following experiments give us a further and striking
demonstration of the diminution or abolition of conductivity
under fatigue. If we take a leaf of Mimosa, and excite it, by
snipping off a terminal leaflet, borne on one of the four sub-
petioles, the stimulus, transmitted along the narrow con-
ducting channel of that sub-petiole, and passing through the
large channel of the petiole, will, on reaching the pulvinus,
cause the fall of the leaf After a suitable period of rest,
the leaf will re-erect itself If now the operation be several
times repeated, by stimulating the same sub-petiole, it will
be found eventually that the leaf no longer responds. That
this is due to the fatigue in conductivity of the sub-petiole
may be proved, by snipping a leaflet off a second sub-
petiole, which will be found to conduct the stimulus, and
produce depression of the leaf, as did the first sub-petiole
when fresh. It will be noticed here that the excitation which
abolished the conductivity of the first sub-peciole, did not
abolish that of the main petiole. This is due to the fact that
the somewhat enfeebled stimulus on reaching the petiole is
spread over a larger channel, and therefore the strain-effect
which it produces there is relatively much less.
id) By effect of ancEsthetics. — We shall now study the effect
of anaesthetics on conductivity. This may be shown by the
local application of ether to the petiole, in the intermediate
portion of a BiopJiytmii leaf, beyond, say, the first three pairs
of leaflets. Stimulus applied below this area will be con-
ducted to it, as seen by the fall of intervening leaflets, but its
further passage will be blocked, and neither the leaflets of
the etherised area, nor those beyond, will show response.
That this abolition of conductivity, however, is only tempo-
rary, is seen when the stimulus is repeated after blowing off
the ether vapour. All the leaflets, from first to last, will now
be found to respond. If etherisation, however, be carried
too far, the abolition of conductivity persists for a long time.
224 PLANT RESPONSE
and its restoration may not take place for one or more
hours.
An interesting experiment, on the aboHtion of conduc-
tivity under ether, was performed with a specimen of Bio-
phytuni having eight leaves of fairly equal sensitiveness. Of
these, four, taken alternately, had ether applied on those
portions of their petioles which were next to the stem. On
now applying strong thermal stimulus on the stem, the state
I
Fig. 98. Diagrammatic Representation of Experiment on Biophytuiit
Ether was applied on the alternate petioles marked i, 2, 3, 4. Stimulus
at X is prevented from acting on the leaflets of these leaves.
The same diagram also represents the subsequent experiment on variation
of receptive excitability. Ether is applied at E instead of on the
petioles. Stimulus applied at E now produces no excitation.
of excitation radiated to all the leaves. But the passage of
stimulus through the four etherised petioles was blocked,
and no effect was produced on their leaflets. The leaflets of
the non-etherised leaves, however, promptly responded, falling
one after another from the centre outwards (fig. 9§^.
Variation of receptivity by anaesthetics. — Lastly, we
shall inquire into the variation of excitability at the point of
application of stimulus, that is to say, into the modification
of" the plant's receptivity, under the action of an external
agent. It is to be borne in mind that stimulus coming^ from
CONDUCTIVITY AND EXCITABILITY 22$
without directly affects the outer layer of the tissue, and the
excitation may then proceed inwards and in lateral directions,
by conduction.
The effect of ether in diminishing receptive excitability
may be demonstrated by taking, as in the last case, a specimen
of Biophytuiii. We first test the specimen by applying a
moderate stimulus on the stem at E. The excitation thus
initiated at the receptive area is transmitted to the leaves,
and causes depression of their leaflets. When these have
recovered, ether is applied locally on the area E. On now
repeating the stimulation, we find that none of the leaflets
respond. Since the conductivity of the intervening tissue
and the excitability of the motile organs have remained
unaffected, it is clear that the failure to respond is in this
case due to the depression of receptive excitability by ether.
A tissue, however, whose superficial excitabiiit}'is depressed
in this way, may still retain the power of conduction. This is
shown by applying stimulus on the stem, as in the last ex-
periment, but at X , below the etherised ring E. The stimulus
is now shown to be transmitted, by the fall of the motile
leaflets. The explanation of this difference probably lies in
the fact that the molecular torpidity induced by the etherisa-
tion does not extend very deep, unless it has been excessive
and long-continued. In that case, the internal layer of the
tissue, remaining unaffected, would serve as the channel of
conduction. This view is supported by the fact which I
have noticed, that it is much easier to produce a complete
block to the passage of stimulation, when a relatively thin
tissue, such as the petiole of a leaf, is etherised. It is much
more difficult, on the other hand, to do this with a thick
stem.
We saw from the molecular model (fig. 94) that though
when the molecules were sluggish no response could be
obtained to moderate stimulus, yet when the stimulus was
very strong response could be brought about. Similarly, in
experimenting on plants, I have found it possible, by careful
graduation of etherisation, to arrange matters in such a way
226 PLANT RESPONSE
that while moderate intensity of stimulus, applied on the
etherised area, failed to evoke a responsive movement of the
distant leaflet, a powerful stimulus was able to do so.
Receptivity versus motile excitability. — At the begin-
ning of the present chapter, I drew attention to the necessity
of discriminating between the functions of receptivity and
motile excitability. It is only by carefully distinguishing these
that we can possibly come to an understanding of certain
apparent contradictions. Let us suppose that stimulus is
applied on a motile organ, say the pulvinus of Mimosa. In
this particular case, the areas of receptivity and motile excit-
ability are coincident. By the reception of stimulus the motile
machinery is eventually set in motion. The mobility of the
superficial particles will thus determine the receptivity and
the inner mechanism of the organ, the motile excitability.
The motile excitability is measured by the amplitude of
response. Receptivity, on the other hand, may be partially
discriminated by (i) the length of the latent period, and (2)
the.value of the minimally effective stimulus.
When a tissue is cooled, say to 7° C. or lower, its recep-
tivity and motile excitability both undergo diminution. Hence
the latent period is prolonged (p. 268), and the stimulus
which was formerly effective becomes ineffective. In such a
case, where the two factors conspire, it is difficult to distin-
guish between the relative effects of receptivity and motile
excitability. But when, on the other hand, the temperature
is raised, say to 35° C, the amplitude of contractile response,
by which we are in the habit of gauging the motile excitability,
is generally speaking diminished (fig. 79). Hence we are
apt to infer that excitability in general is decreased at 35° C.
But if we test this question by means of the minimally
effective stimulus, we arrive at a very different conclusion.
For example, taking a specimen of Biophytum at 30° C,
I found that the minimally effective stimulus was given by
a condenser charged to twenty-two volts, whereas when the
temperature was raised to 35° C. the minimally effective
timulus was a charge of fourteen volts. It is clear from
CONDUCTIVITY AND EXCITABILITY 227
this that the excitability at 35" C. is higher than at 30" C.^
Hence we arrive at two conclusions directly opposed to each
other.
This apparent anomaly completely disappears, however,
in the light of the distinction between the receptive and
motile excitabilities ; for it was said that it was the
mobility of the superficial particles which determined the
receptivity, and this is evidently enhanced by rise of tempera-
ture. The amplitudeof mechanical response, however, by which
we measure the motile excitability, is not solely dependent on
molecular mobility. This mechanical response is, as we have
seen, brought about by diminution of turgor, and any agent
which produced increase of turgor would act antagonistically,
and thus diminish the motile expression of excitation. For
example, we have seen that a pulvinus of Mimosa, when
highly turgid, failed to show any motile response, though
excited (p. 49). Now, it will be shown (p. 400), that rise
of temperature has the effect of increasing turgor. Hence
the diminution of mechanical response with increasing
temperature does not indicate diminution of excitability in
general, but rather the setting in of an antagonistic force,
whose influence will be to increase the force of recovery from
molecular distortion. It should be mentioned, however,
that there is a limit to the enhancement of excitability by
rise of temperature ; for the molecular disturbance caused
by heat will when excessive be detrimental to response.
Excitability versus conductivity, — The same considera-
tions which have thus enabled us to distinguish between
receptivity and motile excitability, will also enable us to see
the difference between motile excitability and conductivity.
We have seen, for example, that at 35° C, the conductivity in
a given specimen of BiopJiytiim was almost three times as
great as at 30° C. in spite of the fact that, as just explained,
contractile response is considerably diminished at high
' It will be found in Chapter XXX. that growth, which is a phenomenon of
excitatory response, is, in the case of many plants, at its maximum at or near
35° C.
Q 2
228 PLANT RESPONSE
temperatures. This distinction between the effects of conduc-
tivity and of excitability is especially important, since by its
means we are enabled to explain certain facts apparently
anomalous, which seem at first sight to lend support to the
hydro-mechanical theory of excitation. I have shown that,
under normal conditions, the intensity of excitation must
exceed a certain value before it can be manifested as
mechanical response. I have also shown that under un-
favourable circumstances, motile excitability is abolished
earlier than conductivity. An excited tissue may thus
conduct stimulus, without itself exhibiting any motile indica-
tion. Numerous examples of such a state of things may be
cited. It must be borne in mind that the mechanical
indication of the state of excitation can be afforded by a
pulvinated organ, only when there is some difference of
excitability as between its upper and lower halves. If this
difference of excitability be in any manner reduced or
diminished, there will be a failure of the mechanical response.
In old leaves of BiophytJiuu for example, not only is the
general excitability diminished, but the differential excitability
also has disappeared. Hence, excitation of such leaves gives
rise to no local excitatory response of the leaflets. But that
the leaf is still nevertheless excitable, and can transmit that
state of excitation, is shown by the fact that on stimulating
it strongly, the leaflets of younger leaves at a distance are,
after a time, seen to be depressed in serial succession. This
proves that, though unable itself to give the motile indication,
the leaf was capable of receiving and transmitting the state
of excitation. Similarly, it may be shown that a tissue whose
motile excitability is temporarily abolished, by, say, the
application of ether, may, nevertheless, be the conductor of
stimulation.
In order to demonstrate this, let us take a plant of
Biophytum, and expose some of the leaflets of a particular
leaf to ether-vapour. Strong stimulation of that portion of
the petiole which bears them, will now fail to induce move-
ment of the leaflet in the etherised region ; but the excitation
CONDUCTIVITY AND EXCITABILITY 229
is found to be conducted through the anaesthetised area, and
to produce responsive depression, not only of the leaflets
beyond, but also of those of other leaves.
This experiment is important in its relation to the theory
of the mode of transmission of excitation. I have already
adduced conclusive proofs that the conduction of stimulus is
dependent, not on the mere mechanical transmission of
hydrostatic disturbance, but on the propagation of proto-
plasmic changes. Strong support has been lent to the
hydro-mechanical theory by a classical experiment in which
the pulvinus of a leaf of Mimosa was chloroformed. On
then strongly exciting the leaflets of this leaf, the ex-
citation was found to be conducted across the anaesthetised
pulvinus and to produce depression of leaves beyond. At first
sight it was natural to suppose that, as the motile excitability
of the pulvinus was abolished by chloroform, the conductivity
must also have been abolished. It was therefore inferred
that, unlike the conduction of stimulus in animal tissues,
where such transmission takes place by the propagation of
protoplasmic changes, the conduction of excitation in the
plant was purely mechanical. It will be seen, however, that
the assumption on which this conclusion is based — that con-
duction must necessarily be abolished, with the abolition of
motor excitability — has been invalidated by the experiments
which I have just described.
In the present chapter, then, it has been shown that those
agencies which, like cold, anaesthetics, and fatigue, diminish
molecular mobility, also diminish the excitability and conduc-
tivity of the plant-tissue. I shall in the next chapter describe
a series of experiments on the profound excitatory changes,
of opposite character, which are induced in the experimental
tissue, by the passage of an electrical current, the nature of
such changes being dependent on the question whether the
current enters or leaves the tissue at a given point. It must
be added that this series of observations will be found to
offer a further disproof of the hydro-mechanical theory of
conduction of stimulus.
230 PLANT RESPONSE
Summary
Motile excitability is temporarily abolished by anaes-
thetics.
Strong application of cold produces a temporary abolition
of motile excitability. Moderate application of cold prolongs
the latent period.
Similarly, fatigue produces a diminution or abolition of
motile excitability ; and this is restored, after a sufficient
period of rest.
Conductivity, similarly, undergoes diminution as the
effect of cold, anaesthetics, and fatigue.
Receptive excitability, again, undergoes diminution or
abolition by the action of similar agencies.
Conductivity may persist even after the abolition of
motile excitability. Hence a strong stimulus may be con-
ducted through a region which exhibits no motile excit-
abilit)'.
CHAPTER XIX
ON ELECTROTONUS
The anode acts as a -block to the transmission of stinuihis-;-Opposite effect of
kathode — Experiments on Biophytwn, showing variations of conductivity by
anode and kathode respectively — Experiments on Mimosa, showing increase of
motile excitability at or near the kathode, and diminution of motile excitability
at or near the anode — Curious ' development ' of response, near the kathode.
We have seen in the last chapter that on account of the
diminished molecular mobility caused by physical and
chemical agents, the response underwent a diminution. It
was also seen that this reduction of molecular mobility found
expression in the diminution of conductivity and excitability.
External agents, like cold and ether, produce a temporary
reduction of mobility, after which there is a revival to the
original condition on the removal of the depressing agents.
But certain other agents, such as poisons, produce permanent
immobility, from which there is no recovery of response. The
tissue is then said to be ' killed.'
Returning now to the molecular model, described in the
last chapter, we see that while stimulus causes molecular
upset, yet, at the same time, the force which restores the
molecule to its equilibrium position, or, in other words, that
which determines its stability, resists such an upset. Let us
then first imagine the molecular model to be under the
moderate directive action of the earth's magnetism. The
stability of the individual molecule will thus be neither too
great nor too small, and we shall call this, for convenience,
the normal stability. This stability may further be increased
by increasing the external directive force with the help of an
auxiliary magnet, arranged in a suitable manner. Or it may
232 PLANT RESPONSE
be decreased, below the normal, by the action of an external
magnet which reduces the earth's directive force.
On now obtaining responses to a uniform disturbing
force, under these three conditions of normal, increased, and
diminished stability, we shall find that while in the first case
we get moderate response, in the second the response is
very much diminished (and may even disappear entirely, when
the stability is very great), and in the third it becomes exalted.
An-electrotonus and kat-electrotonus. — 1 shall now
proceed to show the opposite effects of the anode and
kathode on molecular responsiveness, during the passage of
an electrical current through a plant-tissue. This change,
induced by an electrical current, is known as electrotonus,
and the effect due to the kathode is distinguished as kat-
electrotonus, while that due to the anode is known as an-
electrotonus. It is probable that here, also, the variation of
sensibility is brought about by the variation of molecular
mobility, and that this is induced by an increase or diminu-
tion in the conditions of stability, as in the model. These
opposite variations of the susceptibility to excitation, due to
the anode and kathode respectively, will be demonstrated by
the changes which they induce in the conductivity and
excitability of the tissue.
In the chapter on the Excitatory Polar Effects of Currents,
the intensity of the E.M.F. used was such that the excitation
caused by the kathode was visibly manifested in the motile
effects to which it gave rise. In the present chapter, how-
ever, we shall have to deal with latent excitatory effects, the
E.M.F. used not being sufficient to give rise to any imme-
diate external reaction. In the cases referred to, again, the
distinctive action of the anode could not be demonstrated,
inasmuch as under ordinary conditions it could not give
rise to any motile indication. It will now, however, be
shown that the effect of the anode is one of depression, or
the opposite of that of the kathode. In studying variations
of conductivity we have to remember that when the conduc-
tivity of a tissue is great, the state of excitation is transmitted
ELECTROTONUS
Fk;. 99. Efiect of Anode as Block.
A, Anode ; K, Kathode
The progressive wave of excitation,
initiated at x , stopped by anode,
one pair of leaflets lo its left.
at the kathode. Thermal
and the excitatory wave was
either with greater velocity or to a greater distance ; but if
conductivity be in any way diminished, the distance to which
the excitatory disturbance will be transmitted, will be corre-
spondingly reduced.
The anodic block. — In order to demonstrate the de-
pressing action of the anode, I took a leaf of Biophytuin, and
sent a current through portions of it, entering at A, the anode,
and leaving at K, or kathode
(fig. 99). The E.M.F. used was
two volts, and was thus insuffi-
cient to cause responsive action.
In this and the following expe-
riments, it will be understood,
unless the contrary is stated,
that the intensity of the elec-
trotonic currents was not such
as to create any direct action
stimulus was now applied at x ,
found to be stopped at a distance of one pair of leaflets to
the left of A. This shows that the depressing effect of the
anode acts as a block to the passage of stimulus, and that
such depressing action e.xtends to some distance beyond the
anode itself.
Experiments showing differences of anode and
kathode. — In order to show that the kathode acts differently
from the anode, not offering
a block, but rather facili-
tating the passage of stimu-
lation, I performed another
experiment on a leaf similar
to the last. In that case,
the anode was near the
point of application of
stimulus. I now made the
nearer electrode kathode.
On next applying the usual stimulus, the excitatory wave
passed on through the kathodic area, producing successive
-^^ft-rrny
Fu;. 100. Experiment showing the
Transmission of Excitatory Wave
through Kathodic Area, and its Slop-
page by the .\node
Stimulus was applied at x .
234 PLANT RESPONSE
fall of leaflets, and was only stopped by the depressing action
of the anode, which this time extended to a distance of two
pairs of leaflets to the left of A (fig. lOO).
The next experiment was devised to show the opposite
effects of anode and kathode simultaneously. For this, the
stimulus was applied in the
interpolar region, half-way
between the two. Two
.,*5s=**°r°^°^'WI^ ^^ ^ ^"W^ wave-systems were found
V) ^ ^ ^_ > to start from the excited
„ , . ,„. ,, pointin opposite directions.
Pig. ioi. Demonstration of bimultaneovis ^ '^'^
OppositeEffectsof Anode and Kathode That towards K not only
on Transmission of Excitation i j ^^ u i. j
reached K, but passed
Stimulus applied in interpolar region at x . j •*- • f I,
Excitation is transmitted through great DCyond it, Causmg the
distances in the kathodic region, but depression of all the leaf-
limited in the anodic. ^
lets, six pairs in number,
on that side. But the excitatory wave that travelled towards
A passed through only two pairs of leaflets, and was stopped
at a point one pair to the left of the anode (fig. loi).
Electrotonic variation of motile excitability. -We have
seen that protoplasmic excitability finds expression in,
among other things, the conductivit)'' and motile response
of the tissue. We have seen also how the former, that is to
say, the conductivity, is modified in opposite ways by the
influence of the anode and kathode. I shall now proceed to
describe experiments in which the opposite character of the
effects at anode and kathode is still more strikingly demon-
strated by the exaltation at kathode, and depression at anode,
of the motile excitability.
I used two pairs of electrodes, the first pair, KA, for the
purpose of producing stimulation ; and the second pair, K'a',
in order to produce variation of excitability, through electro-
tonus (fig. 102) ; or vice versa. The first pair was applied on
the stem, the kathode K being in contact with the pulvinus of
the lateral leaf, k'a' were applied on the petiole of that leaf.
The plant was very sensitive, and in order that there
should be no responsive fall, by the direct and unaided
ELECTROTONUS
235
excitatory action of K, the current due to an E.M.F. of two
volts was reduced, by separating A from K, and thus inter-
posing a greater resistance. A
distance was thus found — i.e.
10 cm. — such that, on complet-
ing the AK circuit, the excitation
was not sufficient to produce
response of the leaf.
The AK circuit was now
opened, and adjustments made
with the second circuit a'k',
with an E.M.F. of two volts, in
such a way that, on complet-
ing that circuit alone, there
was no response of the leaf
Owing to the shorter length of
petiole available — i.e. 3 cm. —
the current could not be reduced
to the requisite amount by
simply increasing the interpolar
distance. An external resist-
ance had therefore to be added,
in order to attain the desired
condition. Thus either circuit,
acting alone, was ineffective.
Kat-electrotonic increase of excitability. — Now, in order
to show the increase of excitability in the pulvinus at K, as
induced by the neighbourhood of kathode K', we first com-
plete the a'k' circuit. This, as has been said, is ineffective.
But now, on making the AK circuit, its previously ineffective
stimulus becomes effective, and the leaf responds. From
this it will be seen that during the passage of a current
through the a'k' circuit, a point in the neighbourhood of the
kathode k', that is to say, the pulvinus, is rendered more
excitable.
This experiment may be varied by first making the AK,
which is now the electrotonic, circuit, and then completing a'k'
Fig. 102. Diagrammatic Repre-
sentation of Electrical Connec-
tions in Mimosa to Exhibit
Variation of Motile Excitability,
induced by Anode and Kathode
In the first of the.se experiments,
the a'k' circuit is electrotonic
and the AK circuit excitatory.
In the second and third experi-
ments the AK circuit is made
electrotonic, and a'k' excitatory.
In the third experiment, A and K
are reversed.
236 PLANT RESPONSE
for the purpose of stimulation. It is then found that the
hitherto ineffective stimulus of a'k' is thus rendered effective.
An-electrotonic depression of excitability. — The de-
pressing action of the anode has been already demonstrated
in the case o{ Biophytuiii (p. 234). The following experiment
exhibits the same effect in a different manner in the case of
Mimosa. In this instance, I used an E.M.F. of four volts in
each of the two circuits ak and a'k'. When each circuit
was made separately, the leaf responded by depression.
At make, then, of one of the circuits the leaf responds, but as
the stimulus is only effective at make, the leaf recovers
during the continuation of the current. After this, on the
second circuit being completed, the excitement at make
again caused response.
The experiment was now modified in the following way.
The AK circuit was reversed, the pulvinus becoming anode.
The excitation of the distant kathode, however, was still
strong enough to cause response of the leaf The current
was kept on till the leaf recovered. On now making the
a'k' circuit, the leaf did not respond. Thus the stimulus of
a'k' at make, which was formerly effective, now became in-
effective, by the depressing action of A.
Developing action of kathode. — Another experiment,
showing the latent excitatory action of the kathode, is very
striking. This experiment,
however, is somewhat diffi-
cult, as it requires a very
delicate adjustment of the
stimulus. The specimen
' Developing ' Action of used was a leaf of Bioplivtuiii.
Kathode a , . rr • . ,'
,..,., ,. ^ A current msufficient to pro-
A suhminnnal stmuilus applied at x , .
ineffective to produce excitation of duce any direct excitation
r;"Xiorof":.h:o:'''' '- -«-- .k^p' fl°wing through
the circuit AK (fig. loi). The
point of special difficulty was to apply a stimulus of exactly
subminimal intensity at x , so as not to excite the adjacent
leaflet. I have sometimes succeeded in obtaining this condition.
ELECTROTONUS 237
The effect of this imperceptible stimulus, then, which passed
through the nearer pair of leaflets, without giving any sign of
its presence, became suddenly ' developed ' on reaching the
further pair of leaflets, R (fig. 103), which v^^ere rendered more
excitable by the neighbourhood of the kathode.
These peculiar variations of excitability, induced by the
action of the anode and kathode, as well as those caused by
other physical and chemical agencies, are exactly similar to
what are observed in animal tissues under the same influences.
They bring out, further, the essential unity of physiological
response, as seen in the highly differentiated protoplasm of
the animal and the undifferentiated protoplasm of plant
tissue.
Summary
The anode acts as a block to the transmission of stimulus.
The effect of the kathode is opposite to that of the
anode.
Motile excitability is diminished at or near the anode, so
that previously effective stimulus becomes ineffective.
Motile excitability is exalted at or near the kathode.
Stimulus previously ineffective here becomes effective.
CHAPTER XX
ON THE VELOCITY OF TRANSMISSION OF EXCITATORY
^VAVES IN PLANTS
Difficulties in accurate determination of velocity of transmission, due to unknown
variations of excitability arising from injury, and variations of conductivity
through fatigue— A perfect method of obtaining accurate and consistent results —
Relative advantages of studying conduction in plants as compared with animals
— Determinations of velocity of transmission in centripetal and centrifugal direc-
tions— Preferential conductivity in centrifugal direction -Diminution of con-
ductivity and excitability by fatigue — Within a certain critical interval, organ
'refractory' to further stimulus — Increased velocity of transmission with in-
creasing stimulus — Measurement of diminution of conductivity by cold — Fibro-
vascular elements the best conducting channels — Conductivity lengthwise greater
than crosswise - Electric mode of determination of velocity of transmission —
Indifferent parenchymatous tissu s do not transmit stimulation — Comparative
tables showing velocity of transmission in varitnis plant and animal tissues.
In the last two chapters the effects of various agencies on the
power of conduction were demonstrated qualitatively. It is
important, however, to obtain, if possible, the quantitative
values of this conductivity and its variations. The absolute
value of conduction can be obtained from, the determination
of the velocity of transmission of excitation through the
tissue. This determination of velocity may be made roughh',
by observing the time taken for the application of a stimu-
lus, say by cut or hot wire contact at a given point, to produce
motile effects on a leaflet at a known distance.
A result thus obtained, however, would, for reasons to be
given presently, prove very indefinite, and no two such results
in succession could be trusted to agree. In order to ascer-
tain the exact quantitative effects of various agencies on
conductivity, we must first be completely assured that our
determinations of velocity under normal conditions are trust-
worthy.
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 239
Difficulties in exact determination of velocity of trans-
mission of excitation.— In the course of the investigation
carried out on this subject, I have found that the discrepancies
between the velocities, determined in the way described, are
largely to be accounted for, first, by indefinite changes of
excitability at the point of application, due to the injury
caused by excessive stimulation ; and, second, to the changes
of conductivity, caused by fatigue, in the rest of the tissue.
I have also found that the velocity of transmission is only
a determinate quantity when the intensity of stimulus is
constant. It undergoes variation, with changes in the stimu-
lation-intensity.
These difficulties are met by using a stimulus which does
not cause injury, and which can be repeated at uniform
intensity. Such a stimulus is given by means of the con-
denser discharge. As regards the changes of conductivity
due to fatigue, I have found that fatigue is removed, and
conductivity fully restored, after a definite period of rest,
which, in the case of Biophytum, is about four to five minutes.
The next difficulty to be overcome is concerned with the
question of recording the exact moment of application of
stimulus, and that of the initiation of response at a distant
leaflet. A further source of uncertainty in the last respect,
lies in the existence of an unknown latent period of the
leaflet, which may delay the visible response, even after the
effect of stimulus has reached the point at the base of the
leaflet.
It is evident that the times of application of such rude
modes of stimulation as cut, or contact of hot wire, cannot be
accurately determined, and the exactmoment of the beginning
of the responsive movement of the motile leaflet is equally
difficult to ascertain by the unaided eye. These difficulties
are, however, removed, if we use the discharge from a con-
denser as our mode of stimulation, and the magnified move-
ment of the spot of light from the Optic Lever, as the
indicator of the commencement of response. The observer,
following the spot of light from the Optic Lever, makes two
240 PLANT RESPONSE
marks on the revolving drum — one when the discharge-key
is pressed, at the moment of appHcation of stimulus, and
another when the spot begins to move, that is to say, at the
commencement of response. It is then easy, knowing the
rate of movement of the drum and the distance between the
two marks, to determine the exact time-interval between
the two.
There then remains only the question of allowing for the
loss of time due to the latent period of the responding organ.
This is accomplished by means of a separate experiment, in
which the stimulus is directly applied at the base of the
motile organ. The latent period thus ascertained is sub-
tracted from the time-interval already determined, and we
have thus the true time of transmission of excitation through
the given distance ; from this the velocity, or rate of trans-
mission per second, may be deduced.
Exact determination of velocity.— I now give an
account of an actual experiment for the determination of
the velocity of transmission of excitation in the petiole of
Biophytuui. The two points A and B are connected in the
circuit of a condenser through the usual non-polarisable
electrodes (fig. 14). An indicating leaflet, L, is attached to
the Optic Lever, by which the exact moment of its response
may be recorded on the revolving drum. If now we make B
kathode, during the charge of condenser, an excitatory wave
will start from B and travel inwards towards L, in this par-
ticular case in a centripetal direction, i.e. towards the main
stem. A mark is made, as already explained, on the re-
volving drum, at the exact moment when the tapping-key
excites the plant. In this case, the capacity of the condenser
was -oi microfarad, and E.M.F. twelve volts. The time of
the stimulus reaching L, as indicated by the movement of the
spot of light, is also marked, as explained, on the revolving
drum. As has been said before, the time-interval is accu-
rately determined from the speed of the revolving drum. As
an additional precaution, the same time-interval is taken by
means of a stop-watch.
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 24 1
From a separate experiment, by direct stimulation of the
base of the petiolule, it was found that the latent period of
the leaflet was so small a fraction of a second, as, for our
present purpose, to be negligible. In this way, in my first
experiment, I found the time taken by the excitation to travel
the distance of 27 mm. between B and L to be 14-3 seconds.
I allowed the plant a period of rest of three minutes, and
again performed the experiment under similar conditions.
The time taken was found to be I4"5 seconds, which is
practically the same, within experimental error, as the result
first obtained. The slight difference was due to the residual
effect of fatigue. In any case, the extreme difference between
the two results amounts to less than 1-4 per cent. — or 7 per
cent, from the mean value of i4'4. From this we find that in
the particular plant under experiment the velocity of trans-
mission in a centripetal direction was i-88 mm. per second.
In order to show how consistent successive results are, I give
successive time-intervals taken by stimulus in two different
cases, to travel the intervening distances.
Case I. Time-interval in first experiment I2'6 seconds.
„ second „ 12-9 ,,
Case 2. „ first ,, 14-8 „
„ second „ 15 ,,
In all these cases, the second experiment was undertaken
after an interval of rest of three minutes. The slight re-
tardation uniformly observed is due, as already explained, to
residual fatigue. It is, however, so small as to be negligible.
At any rate, making allowance for all possible sources of
uncertainty, the variation of these determinations will be less
than 2 per cent.
We have to remember that, owing to the slow velocity of
transmission of impulses in plants, and also to the com-
paratively great length of tissue, that can, when necessary,
be brought under examination, the total interval of time that
has to be observed may be made as large as twenty to forty
seconds. In such periods, a mean error of even "2 second
K
TLANT RESPONSE
would hardly produce an inaccuracy of i per cent, in the result.
This would compare favourably with the determinations that
have been made of the velocity of transmission of nervous
impulses in animals. In a frog's nerve, for example, owing
to the high velocity and comparatively short length of nerve
available for experiment, the total interval of time which has
to be observed is of the order of some thousandths of a
second. To obtain an accuracy within i per cent here,
would mean the recording and measuring of an interval of
something like the Yirinro P^^t of a second.
The velocity of transmission in a given plant is found,
under normal conditions, to be constant. It varies in different
species, and even in the same species the value changes with
the season of the year and the physiological condition of the
specimen. A velocity determined in winter under less
favourable physiological conditions, is very much lower than
the velocity of transmission in the same plant in summer.
The exact determination of the velocity of nervous im-
pulses in animals has, therefore, been a matter of some
uncertainty. For example,
Helmholtz found this ve-
locity in man to be about
thirt}--three metres per
second. Some recent de-
terminations, again, give
a value twice as great.
Owing, moreover, to the
difficulty in exactly dis-
criminating the rising part
of the curve, the same re-
cord may be interpreted to
give results which differ
from each other by as much
as 20 per cent.^
Preferential conductivity. — I shall next pass to the
consideration of the very curious and interesting pheno-
' Nature, 1903, pp. 105, 151.
Fig. 104. Diagrammatic Representation
of Electrical Connections for Deter-
mination of Velocities of Centrifugal
and Centripetal Transmissions
A and B are the electrodes, and L the
indicating leaflet.
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 243
menon of preferential conductivity, by which it is seen that
the state of excitation travels through a tissue with greater
facility in one direction than in the opposite. For the
purpose of this demonstration, I took a fresh leaf of Bio-
phytuni. The two condenser connections (capacity 'Oi micro-
farad charged to twelve volts) were made at A and i;
(fig. 104). The indicating leaflet L was situated somewhere
between. On charge, B became the kathode, and an ex-
citatory wave was started in a centripetal direction. On
the abatement of this wave, the condenser was discharged ;
A now became the kathode, and an excitatory wave was
transmitted in a centrifugal direction. From these two
successive experiments, we are now able to determine the two
velocities in opposite directions in the same leaf
The following table exhibits the results obtained in this
manner with two different specimens :
Specimen I
Direction Distance | Time j Velocity
Centripetal (B ' 22-5 mm. 11 -2 seconds. \ 2 mm. per second.
Centrifugal (AL). 45 mm. 15-2 ,, [ 2-9 mm. ,,
Specimen il
Direction
Distance Time Velocity
Centripetal (bl).
Centrifugal (al).
28 mm. 15-2 seconds. 1-84 mm. per second.
39*5 mm. ! 17-5 ,, 2-2 mm. ,,
Excitatory discharge preferentially directed. — These
two observations show that the velocity is greater in the
centrifugal direction. In some instances I have found the
centrifugal velocity to be nearly twice as great as the centri-
petal. These experiments seem to indicate that under
certain conditions, excitatory discharges will take place
preferentially in one direction onl)-. We may imagine any
intermediate point in the midrib of the leaf, to be acted on
locally by a gradually increasing or accumulating stimulus from
244 PLANT RESPONSE
external sources. Immediately on this reaching the threshold
of response, it will give rise to an excitatory discharge. And
it is clear that this excitation will be transmitted preferentially
along the line of least resistance, that is to say, in the
direction of the greatest conductivity, or outwards. 1 have
been able to obtain further experimental verification of this
conclusion, by applying a gradually increasing stimulus of
condenser discharge to an intermediate point on a petiole of
Biophvtum. When this stimulus had reached a certain value,
it was found that while excitation, as indicated by the fall of
the leaflets, travelled through a great distance forwards, or
outwards, its transmission backwards was extremely limited.
Could we have adjusted the stimulus, so as to have been
slightly above the threshold of response, there would have been
no transmission backwards. When stimulus, on the other
hand, is excessive, the entire excitatory effect cannot be carried
forward ; there is an overflow backwards ; and under these
conditions excitatory movements take place in both directions.
Effect of fatigue on velocity of transmission. - We shall
next deal with the modification of the velocity of transmission
by fatigue. Specimens of Biophytuiii were used for the
purposes of this investigation, and experiments were per-
formed by ascertaining the times taken for the transmiission
of a repeated uniform stimulus, through the same distance,
under shortening periods of rest.
A stimulus was given to a leaf of Biopliytum, and the
record of time taken— the transmission being in a centripetal
direction. The plant was now given an interval of rest ot
three minutes. Stimulus was again applied, and the time-
records obtained in the usual manner. The next stimulus was
applied after a resting-interval of two minutes, the following
after one, and the last after half a minute, the time of trans-
mission and the records of response being taken throughout.
From the results given below, it will be seen how regular is the
decrease in velocity with the increase of fatigue. The
distance to be traversed, 27 mm., was kept the same in all
cases. The time taken at the beginning, when the plant was
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 245
fresh, to traverse this distance was I4'3 seconds. In the next
experiment, when the stimulus was applied after three minutes,
there was a slight residual fatigue, and this prolonged the
time to I4"5 seconds. On the third occasion, a still shorter
time, namely two minutes, was allowed for rest, and the rate
of transmission became slower, the time being now 157
seconds. The next interval of rest was still further shortened,
to one minute, and the time of transmission was correspond-
ingly increased to 1&4 seconds. And lastly, when the
stimulus was given after an interval of only half a minute, the
velocity was still further retarded, the time now taken being
1 75 seconds.
The following table gives the different velocities under
increasing fatigue, and the heights of the corresponding
responses. It has already been .said that the distance through
which the stimulus was transmitted was in all cases the same,
namely 27 mm.
Table showing Variations ok Velocity ok Transmission and ok
Amplitude of Response in Biophytum, with Increasing Fatigue
Intervals of rest
The plant fresh
3 minutes
2 minutes
I minute
^ minute
Time
I4"3 seconds
14-5 >,
157 ,.
i6-4 „
17-5 »
Height
of response
34 dns.
20 ,,
14-5 M
2-5 „
I'D ,,
Velocity
I -88 mm. per second
1-86 ,,
172 ,,
I "64 ,, ,,
1-54 .,
It will be seen from this table that, while the variation of
velocity due to the difference of conductivity between three
minutes' rest and an indefinitely longer period is slight, there
is a considerable diminution of this velocity when the resting-
periods are still further shortened. It will be noted, more-
over, that increasing fatigue is shown not only by a regular
decrement in the speed of transmission, but also in an inde-
pendent and still more striking manner by a steady diminution
in the heights of the responses themselves.
The following curve (fig. 105) shows the variation of
246
PLANT RESPONSE
FiCr. 105. Curve showing Decline in
Heights of Responses, with Diminish-
ing Periods of Rest
Abscissa represents resting-periods, and
ordinate heights of response
motile excitability under shortened periods of rest. This
curve when produced will cut the abscissa. Such a point
would mark the time-interval between two successive stimuli,
at which the response would
be zero, that is to say, the
motile excitability would be
abolished. In other words,
the leaf would, when the
resting-interval was short-
ened to this period, prove
refractory to stimulus. In
Chapter XXI I the existence
of this theoretical refractory
period will be demonstrated
by experiment.
It is thus seen that owing
to imperfect protoplasmic
recovery, or, in other words,
to residual molecular strain,
not only is the conductivity of the tissue gradually diminished,
but the excitability also. Thus we obtain some idea of the
processes by which fatigue is brought about.
Effect of intensity of stimulus on velocity. — We have
next to study the variation in the velocity of transmission of
the excitatory condition, with increasing strength of stimulus.
In the case of animal nerve, it has been ascertained by
different observers (Helmholtz, Vintschgau, and Fick) that
the velocity of transmission of the nervous impulse is not
independent of the strength of stimulus, but increases with
increasing intensity. The experimental verification of thi.s
with conducting animal tissue is, however, extremely difficult,
owing principally to the shortness of time involved.
1 have carried out an investigation on this subject with
vegetable tissues, which shows in an unmistakable manner
that velocity does undergo an increase, with increasing
stimulus. These experiments were carried out with leaves of
BiopJiytuvL I first demonstrated this in a qualitative manner.
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 247
by using- thermal stimulation. It was thus found that the
excitation caused by a strong, would travel with a greater
velocity than that due to feeble stimulus, the one being
sometimes double the other. I next tried to obtain quan-
titative results, by using a form of stimulus which was
measurable, and could be increased in a graduated manner.
For this T employed the method of stimulation by condenser
discharge, the stimulus being increased by increasing the
E.M.F. that charged the condenser (01 microfarad).
In one series, with a stimulus of eight-volt charge, the
velocity found was r8 mm. per second. When the stimulus was
increased by charging the condenser to twelve volts, there was
an increase of velocity to r9 mm. per second. And finally,
with a sixteen-volt charge, the velocity was found to be 2t mm.
per second. These velocities referred to centripetal trans-
mission. In the next series, the experiments were carried
out on a much more excitable leaf, and the velocity was
determined in a centrifugal direction ; with a charge of eight
volts, the velocity was 3 '27 mm. per second ; with sixteen volts,
it rose to 376 mm. ; and with thirty-two volts, it became
3-83 mm. per second. The two following tables exhibit these
results of increasing velocity with increasing stimulus :
Tables showinc; Incre.a.se ok Velocity with Increasing Stimulus
Sped men I. — Centripetal transmission.
The distance traversed hy stimulus was 27 mm.
Stimulus
Time
Velocity
•01 microfarad charged to 8 volts
. :: ;; \\ ;; 1
14-9 seconds
14-4 „ •
12-8 „
I -8 mm. per second
1-9 mm. ,,
2*1 mm. ,,
specimen II. — Cent
Distance traversed by
rifugal transmission,
stimulus was 38 vuii.
Stimulus
Time
Velocity
•OI microfarad charged to S volts
,, ,, i6 ,,
24 ,,
32 „
1 1 -6 seconds
IO-2 ,,
lo-i ,,
9-9 ..
3-27 mm. per second
372 mm. ,,
376 mm.
3-83 mm.
248 PLANT RESPONSE
The stimulus, it is to be remembered, is increased by
increasing the voltage. But, on an undue increase of this
charging voltage, to about forty volts or upwards, I have'
often found that the velocity undergoes an actual diminution.
This is to be ascribed to the fact, which was demonstrated in
the chapter on Polar Effects of Currents, that the excitatory
value of the kathode reaches a limit, with a certain E.M.F.,
and that if the E.M.F. be carried far beyond this, the excita-
tory effect is reversed, that is to say, it is now the anode
that excites (p. 206). We have then the curious case of a
negative direction, as it were, of transmission. For whereas,
with moderate voltage, the excitatory disturbance travels in the
interpolar region from kathode to anode, it now, with exces-
sive voltage, travels in the opposite direction, from the anode
towards the kathode.
We can now see with what great accuracy it is possible to
measure these changes of velocity, from which we can deduce
the variations of conductivity, not merely qualitatively, but
also quantitatively. This opens out to us the further possi-
bility of studying the quantitative effects. of various external
agencies in modifying conductivity. I shall here relate a
simple experiment which affords an example of the method
to be followed in such an investigation.
Effect of lowering of temperature on velocity of trans-
mission.— In order to study the effect of lowered tempera-
ture on conductivity, I applied ice-cold water over an area of
10 mm. of the conducting petiole in BiophytUDi. The length
of the conducting tissue experimented upon was 38 mm., and
the time taken for the stimulus of a condenser-discharge
(ten volts and 'Oi microfarad) under normal conditions,
i.e. before the application of ice-cold water, to traverse this
length, was lO'i .seconds, giving a velocity of 376 mm. per
second. But after the application of ice-cold water, the con-
ductivit)' was so diminished that the transmitted excitation
did not produce any response of the motile leaflet ; on
allowing the temperature of the cold water applied, however,
to rise a few degrees, the stimulus was found to be effective ;
TRANSMISSION OK EXCITATORY WAVES IN PLANTS 249
but the velocity of transmission was now found to be much
reduced. Instead of lO'i seconds being necessary for trans-
mission through the entire length, it was now found to take
148 seconds. The difference of 47 seconds here, represents
the additional time taken for transmission through the 10 mm.
length of cooled tissue. In other words, whereas transmis-
sion through 10 mm. of normal tissue had taken about
26 seconds, it now took about 2'6 -I- 47, or y^ seconds ; that
is to say, the conductivity was reduced by cooling to nearly
one-third.
Effect of rise of temperature on velocity. — It has been
proved that conductivity is reduced by lowering of tempera-
ture. We should therefore expect that a rise of temperature
would produce the opposite, namely, an increase of conduc-
tivity. That this is the case was shown by an experiment
on a leaf of BiopJiyiitin. C^are was taken that the leaf should
not be too young, since, as will be .shown later, the effect of a
rise of temperature on a young leaf is to initiate automatic
response. I found that in this specimen excitation travelled
a distance of 41 mm. in a centrifugal direction in 11 seconds,
the temperature being 30° C. The velocity at this tempera-
ture was therefore 37 mm. per second. On now raising the
temperature to 35° C. the time taken for transmi.ssion was
reduced to half, i.e. 5-5 seconds. The temperature was next
raised to 37° C, and the time was now found to be further
reduced to 4-5 seconds, the velocity being thus 9-1 mm. per
second, or nearly three times as great as at 30° C.
Tahle showing Efffx't of Rise of Temperature on Velocity
OF Transmission in Biophytum
Temperature
Distance
41 mm.
Tii.ie
II .seconds
Velocity
C.
30°
37 mm. per sec.
35°
41 mm.
5-5 seconds
7-4 mm. per sec.
37°
! 41 mm.
; 4-5 seconds
' 9-1 mm. per sec.
250 TLANT RESPONSE
Channels for conduction of effect of stimulus. -Before
concluding this chapter, it is important to consider the channels
through which stimulus is conducted with the greatest facility.
Since the conduction of stimulus is due to the transmission
of protoplasmic change, it is clear, as already said in a
previous chapter (p. 60), that such changes will be con-
ducted most easily along those paths in which there is least
interruption of protoplasmic continuity. It is evident, there-
fore, that certain elements in the fibro-vascular bundles will
furnish the best conducting medium for the transmission of
stimulus. It also follows that in the fibro-vascular tissue
itself, the conduction along the length would be more rapid
and complete than across.
On the other hand, the cells of indifferent tissue, such as
the parenchyma of the leaf, are divided from each other by
more or less complete septa, the fine filaments by which
neighbouring cells may be protoplasmically connected being
so minute that the conduction of stimulus through such im-
perfect channels must be exceedingly feeble.
These theoretical conclusions I have been able to verify
by direct measurement of conductivity in different kinds of
tissue. In this investigation, as motile tests of the state of
excitability were not available, I devised an electrical method
— to be referred to briefly in the next chapter, and described
more fully elsewhere— by which to attack the problem.
Using this method of investigation, I found that plant-
organs which contained fibro-vascular elements, such as the
stem, peduncle, and petiole, were the best conductors of the
state of excitation, and that conduction in such organs is
much greater along the length than . across it ; in the
peduncle of Mtisa, for example, the conductivity lengthwise
is three times as great as that crosswise ; and finally I found
that though indifferent tissues like the parenchyma are
directly excitable, yet there is practically no transmission of
that state of excitation through such tissue.
From anatomical and other considerations, Dutrochet and
Haberlandt came to the conclusion, that it was certain ele-
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 25 1
ments in the fibro- vascular bundle which were concerned in
transmitting the disturbance in Mimosa. This transmission
was, however, regarded rather as a hydro-mechanical than
as a true excitatory propagation. Such a conclusion,
as we have already seen, appeared at one time to be
probable in the light of the experiment on the trans-
mission of excitation through a narcotised area. I have,
however, already shown on p. 229, that abolition of motile
excitability need not always imply the abolition of conduc-
tivity. Haberlandt describes an experiment according to
which the excitation in Mimosa is said to have been propa-
gated over dead tracts of the petiole, these portions having
been destroyed by scalding. But it is extremely difficult to
ensure the death of interior tissue by such means as super-
ficial scalding. I have found that a portion of a plant-tissue
when subjected locally to the action of boiling water, after-
wards exhibited signs of true excitatory electric response.
It is only by prolonged immersion in boiling water that one
can be quite sure that the interior tissue is really killed by
scalding, and unless this is done thoroughly it is easy to see
that the inner cells may conduct the stimulus.
There is, moreover, another possibility, that of pseudo-
conduction, by which the effect of stimulus might appear to be
transmitted across dead areas. My meaning will perhaps be
clearer if we imagine two isolated muscle-preparations, one
of which is attached to a striking lever, under which is the
-second. Supposing stimulus to be applied to the first of
these, we can see that it would cause the lever to strike the
second muscle, thus causing excitation. In this way, the
effect of a stimulus applied to the first muscle would appear
to have been transmitted to the second, completely isolated
from it. In reality, however, this was not a case of true, but
of pseudo-conduction, the excitation of the second muscle
being started de novo by the blow of the lever, itself only a
secondary effect of the excitation of the first.
Similarly, we may have, in Haberlandt's experiment, two
living tissues isolated from each other by an intervening area
252 PLANT RESPONSE
of dead tissue. A strong stimulus applied to the first of these
will now cause an excitatory expulsion of water, which will
be transmitted across the dead area, and impart a mechanical
blow to the second living tissue, thus setting up excitation
de novo in that portion of the petiole.
Velocities of transmission in various plant and animal
tissues.— I give below a number of determinations of velo-
cities of transmission made with different plants, both
ordinary and sensitive, the electrical method of determination
having been used in the case of the former ; and with this,
for purposes of comparison, a series of values that have been
determined in the case of animal tissues. The respective
values given in the table refer to the maximum velocities
obtained. In .this connection, it should be remembered
that the velocity of transmission depends on the intensity of
stimulus. The intensity of stimulus, again, is diminished in
the course of transmission through a long tract. Hence the
velocity near the point of application of stimulus is relatively
great, and becomes less the further the stimulus travels. In
order, therefore, to make the different results comparable, my
experiments have been made on equal lengths of tissue,
namely, 7-5 cm. in each case, the stimulus applied being also
the same.
Tables giving Velocities of Transmission of Excitatory Wave
[a) Animal
Subject
Nerve of Anodou ....
Nerve oi Eledone (ol^seived by Uexki'ill) ,
(/') Sensitive Plants
Velocity
ro mm. per second
•5 to I mm. per second
Subject
Velocity
Mimosa pudica : Petiole ....
N'eptttnia olei-acea : Petiole
Biophytiim sensitiTum :
Petiole of direction centripetal
Petiole of direction centrifugal
Peduncle of
14 mm. per second
IT mm. ,,
2T mm. ,,
3-8 mm. ,,
37 mm-
TRANSMISSION OF EXCITATORY WAVES IN PLANTS 253
(c) OrdiJiary Plants
Subject
Velocity
Fiais religiosa : Stem ....
Cuairbita : Tendril ....
Jute : Stem
Artocarpiis : Petiole ....
9-4 mm. per second
5" mm. ,,
3-5 mxi-
•54 mm.
It will thus be seen that the velocity of transmission in
conducting plant-tissue is not very different from that in the
conducting tissue of certain animals.
Summary
Successive determinations of velocity of transmission are
consistent when the stimuli are uniform, and when inter-
vening periods of rest, sufficient for complete protoplasmic
recovery, are allowed.
Velocities of transmission are not the same in centripetal
and centrifugal directions. In Biophytum, for example, the
centrifugal velocity is greater than the centripetal.
When a given point in a plant-tissue is gradually raised
in excitability, the consequent excitatory discharge takes
place preferentially in one direction.
Conductivity and excitability are both diminished by the
increasing fatigue consequent on shortened intervals of rest.
When the resting-period is shortened below a certain critical
interval, the motile organ proves 'refractory' to further
stimulus.
The velocity of transmission is not the same for all
intensities, but increases with increasing stimulation.
The velocity of transmission is diininished by lowering,
and increased by raising, of temperature.
The fibro-vascular elements are the best channels for con-
duction of stimulus : in them, the transmission lengthwise is
greater than crosswise. Indifferent parenchymatous tissue
has little or no power of conducting stimulus.
The velocity of transmission in plants is not of an
altogether different order of magnitude from that in certain
animal tissues.
CHAPTER XXI
ON DETECTION OF EXCITATORY PULSE DURING TRANSIT
BY ELECTROTACTILE AND ELECTROMOTIVE METHODS
Pfcfter's experiment on expulsion of water from excited cells — Author's experiment
on a delicate method of detecting excitatory expulsion of cell-sap -Chemical
method of determining velocity of transmission of excitation — Electrotactile
detector — Demonstration of passage of excitatory contractile wave by means
of electrotactile method — Determination of velocity of transmission of exci-
tation in ordinary plants by electromotive method — Excitatory versus hydro-
mechanical movement of water.
The fact that the excitatory wave is propagated with a
constant and measurable velocity was demonstrated, with
regard to sensitive plants, in the last chapter, the arrival of
the wave from a distance at the motile organ being detected
by means of mechanical response. As ordinary plants, on
the other hand, do not possess such efficient motile indicators,
some other method, more universally applicable, is necessary
in order to show that in these also the state of excitation is
transmitted from point to point.
Before describing the two methods which I have devised
for this purpose, I shall give an account of a very interesting
experiment, depending on chemical reaction, by which I have
been able not only to demonstrate in a striking manner the
expulsion of water from excited cells, but also to make a
rough determination of the velocity of transmission. It was
shown by Pfeffer that on exciting the lower side of a pulvinus,
water oozes out from the cut end. On taking a detached
pulvinus, and stimulating the lower surface with a blunt
needle, he found that the organ curved downwards, and a
drop of water was seen to escape from the cut end.
ELECTROTACTII.E DETECTION OF EXCITATORY PULSE 255
Chemical method of detecting excitatory expulsion of
cell-sap. — My own experiment differs from this in several
particulars. Since there is a general impression that certain
specialised tissues in the pulvinus are alone excitable, it was
my object to show that cells which do not exhibit any motility
will also give rise to the expulsion of water by excitatory
contraction, and I desired further to utilise this effect for the
determination of the velocity of transmission of excitation in
that tissue. The essentials to this purpose were : some means
of detection of the excitatory expulsion at the exact moment
of its occurrence ; some means of marking accurately the
moment of application of stimulus ; and, lastly, the use of a
fairly long tract of tissue, in order that the interval between
application of stimulus at one end, and the manifestation of
its reaction at the other, might be of a duration capable of
exact measurement. For this purpose I took petioles of
Mimosa and the non-motile stems of the same plant, and
placed their cut ends in very dilute solution of sodium chloride.
So dilute a solution of salt does not, as 1 find, appreciably
affect the excitability of the tissue. Selecting one of the
specimens, say a petiole, I adjusted the electrothermic
stimulator at a distance of, say, 4 cm. from its lower or cut
end, the specimen being held vertical by means of a clamp.
The vessel of salt solution in which it had hitherto been
placed was now removed. The end of the petiole was
carefully rinsed, to remove all traces of salt from the outside,
and a small beaker of very dilute silver nitrate solution was
substituted. At this point it became necessary to finish the
experiment rapidly, as silver nitrate solution is likely after a
time to affect the excitability of the tissue. Momentary
thermal stimulus was given by brief closure of the electric
circuit. The excitation then travelled through the intervening
4 cm. of tissue, with a velocity characteristic of the plant.
When it reached the cut end, the excitatory contraction
produced an expulsion of cell-sap containing the salt
solution previously absorbed. This expulsion was instantly
made visible by the formation of a dense white precipitate
256 PLANT RESPONSE
of silver chloride. This was sometimes seen to be projected
as a white vortex ring. The interval between the application
of stimulus and this projection was found to be three seconds.
It is thus clear that the excitatory wave is not one of
hydrostatic disturbance, for such a disturbance would be
transmitted with very much greater velocity. The velocity
of transmission in this petiole is seen to have been 1 3 mm.
per second, or practically the same as that obtained by a
separate experiment on the fall of the leaf with similar
specimens.
In a similar manner I determined the velocity of trans-
mission through the stem. The stimulus was applied at a
distance of 5 cm. from the cut end, and the chemical pre-
cipitation was observed, after an interval of five seconds. The
velocity in the stem is thus seen to be 10 mm. per second.
A repetition of this experiment with another piece of stem
from the same plant gave similar results.
It is thus seen that cells through which excitation is
proceeding undergo excitatory contraction, in consequence of
which there is an expulsion of water forwards, in the direction
of propagation. And this effect is produced, not in pulvinated
organs alone, but in others which are not usually regarded as
motile. This result is also arrived at independently by the
method of electrical response.
The electrotactile method.— I shall next give an account
of the much more delicate methods which I have succeeded
in devising for the detection of the excitatory impulse during
transit, the first of which is the electrotactile method. It is
easy to understand that while the wave of excitation is passing
through any given section of tissue, it must produce there
certain form-changes, infinitesimal though they may be. Had
our sense of touch been more delicate, we might perhaps have
been able to perceive this pulse. It occurred to me, however,
that it might be possible, if it existed, to obtain its indication
by means of an electrical method of detection, the sensitiveness
of which could be exalted at will.
As regards the pulse of form-changes we can see that two
ELECTROTACTILE DETECTION OE EXCITATORY PULSE 257
different effects are possible. For example, in the case of
muscle with parallel fibres, the wave of excitation will travel
onwards from the excited point, the contraction produced
giving rise to expansion of the muscle in a direction at right
angles to that of propagation. In the intestinal muscle,
however, owing to a different distribution of the fibres, the
propagated wave is one of constriction. Now, it is clear that
these two kinds of muscle, placed within enclosing contacts,
will give rise, during the passage of excitatory waves, in the
one case to an increase of pressure, and in the other to its
diminution.
We are thus prepared to see that if similar contractile
waves pass through a vegetable tissue, they may be detected
by means of a concomitant variation of pressure — a variation
which may prove to be either an increase, or a diminution,
according to the particular disposition of the contractile
elements in the tissue. If the tissue, again, should happen to
be anisotropic, the same wave of contraction may appear to
give rise to a diminution of pressure in one direction, and an
increase in that at right angles. In any case the excitatory
wave, in the course of its transmission through any given area,
might be expected to produce variation of pressure, as between
two diametrically opposite leading-points.
I am now about to describe the electrical device which
I have used for the detection of such transient pressure-
variations, concomitant to the passage of excitation through
vegetable tissues. It is known that the electrical resistance
of contact varies with pressure, and on this principle depends
the construction of the microphone. But a loose contact,
such as would be favourable for microphonic use, is unsuitable
for our present purpose, by reason of the disturbance to which
it is subject from atmospheric vibration. The necessity to
be met, therefore, is that of the adjustment of an electric
contact, which shall not be subject to resistance-variation
from atmospheric disturbance, and which shall, at the same
time, be sensitive to the pressure-variation effected by the
excited tissue.
S
258 PLANT RESPONSE
I overcame the difficulty regarding extraneous dis-
turbance by using, instead of a loose, a steady pressure-
contact, capable of the finest adjustment, by means of a
micrometer screw. The next problem lay in choosing
contact material of great sensitiveness. For this purpose
I used different materials, the sensitiveness of some being
very great, while that of others was moderate. Carbon
contacts belonged to the latter class, but had the advantage
of being easily adjustable. Various metallic powders, how-
ever, such as that of bronze, were considerably more sen-
sitive, but at the same time required greater care for
adjustment.
The most important factor in the arrangements, by which
the sensitiveness of the'contact may be exalted to a very
high degree, lies in the proper adjustment of the electro-
motive force acting on the contact circuit. The sensitiveness
to pressure-variation increases with increasing electromotive
force, being greatest when this is just short of a certain
critical value, after which the electric contact is observed to
become unstable, and give rise to spontaneous oscillatory
variations. For the purpose of easy adjustment of the
electromotive force I use a sliding potentiometer. The
tissue, say a stem of Mimosa, is placed between points,
h and B, and by means of the micrometer screw, S, it is
pressed against the spring P.. The electric contact, say of
carbon points, is between B and c, which are in circuit with
a galvanometer, and the potentiometer. The pressure is
so adjusted that a feeble current flows through the gal-
vanometer. In order to increase the sensitiveness of the
detector, the electromotive force may be gradually increased,
by proper adjustment of the potentiometer, short of the
critical point (fig. 106). The deflected spot of light from
the galvanometer will remain steady, provided the adjust-
ments have been properly made.
The tissue is now stimulated at a distant point, care
being taken that it is not in any way jarred mechanically.
Stimulation without jar is effected, however, without difficulty
ELECTROTACTILE DETECTION OF EXCITATORY PULSE 259
by the use of the electrothermic stimulator. In any case, it
is easy to discriminate between the effect of any such ex-
traneous disturbance and the true excitatory effect in transit
for which we are lookinof ; for the effect of the former, if it
Vie. 106. Electiotactile Method for Deteclion of Excitatory Wave
during Transit
Stem placed under light pressure between H and sliding rod c. A, b, elec-
tjic contact points, the pressure of which is delicately adjusted by
micrometer screw, M. A moderate current flows round the circuit,
including the contact points, a t;, and the galvanometer, (;, the
E. M.F. being suitably adjusted by the potentiometer slide, P Stimu-
lation of stem effected by momentary closure of key, K, in circuit
with electrothermic stimulator. Excitatory wave reaching the zone B, c,
causes pressure- variation, with concomitant galvanometric response.
occurs, is immediate, whereas that due to excitation takes
place after a definite interval from the application of stimulus.
In the experiment of which the record is given in fig. 107,
stimulus was applied at a distance of 4 cm. The excitatory
wave reached the experimental zone after an interval of five
s 2
26o
PLANT RESPONSE
seconds, and indicated its presence by a diminution of
pressure on the contact (fig. 107), resulting in sudden
diminution of current. After the passage of the excitatory
wave the tissue was restored to its original condition, as
shown by its once more exerting its normal pressure. This
process of response and recovery by variation of pressure
is exhibited by the galvanometer record. The recovery is
usually found to be attained in the course of a half to one
minute, according to the intensity of stimulus. As the result
of this experiment, we find the velocity in this particular
stem to be 8 mm. per second, which
is very near the determination already
made by other methods, with different
specimens of stem of Mimosa.
Frequently as I have obtained this
response during transit by diminution
of pressure, its opposite, that is to say,
response by increase of pressure, is by
no means uncommon. I have already
explained how it is possible for excita-
tory contraction to give rise to two
such opposed effects, in consequence
either of different dispositions of the
contractile elements in the tissue, or
by the presence of anisotropy in the
organ. By means of the electrotactile
method, then, we are able to demon-
strate the passage of the excitatory wave, and also to measure
its velocity, in tissues which are not motile.
The electromotive method. — I shall now describe the
second, or electromotive, method which I have used for the
detection of the excitatory wave during its passage through
a vegetable tissue. I have already explained that when the
plant-tissue is directly excited, the state of excitation is
invariably accompanied by an electromotive variation, the
excited point becoming galvanometrically negative (p. 32).
Hence, when an excitatory wave is transmitted through the
Fig. 107. Electrotactile
Response in Stem of
Mimosa
Stimulus applied at the mo-
ment X , at a distance
of 4 cm. below the
detector.
ELECTROMOTIVE DETECTION OF EXCITATORY PULSE 26 1
tissue in any direction, from the stimulated point, there must
ahvays be an electromotive wave as its strict concomitant.
The moment, therefore, when the excitation reaches a given
point may be determined by observing the arrival of this
excitatory electrical disturbance of galvanometric negativity,
and for the detection of such an excitatory wave the gal-
vanometer takes the place of the motile leaflet.
In order to prove that the excitatory mechanical and
electrical effects are strictly concomitant, it is only necessary
Fig. ioS. Experimental Arrangements for Simultaneous Recording of
Mechanical and Electrical Responses
Stimulus applied by thermo-electric stimulator, s, at A. Excitation reach-
ing B causes mechanical response of leaflet, which is recorded by
optical lever on drum at M. Simultaneous galvanometric negativity
recorded at E.
to perform an experiment on a plant, such as BiopJivtiiiii,
which is provided with motile leaflets. We attach one of the
indicating leaflets to the Optic Lever, and connect its base B
with one of the electrodes of the galvanometer, the second
electrode of which is connected with a distant point of the
leaf (fig. 108). The two spots of light, one from the Optic
Lever indicating mechanical, and the other from the gal-
vanometer indicating electrical, response are adjusted to lie
262 PLANT RESPONSE
one above the other on the same revolving drum. On
applying a stimulus, say thermal, at A, it will be found,
after the lapse of a definite interval, that both spots of light
are deflected simultaneously, proving the concomitance of the
mechanical and electrical effects. Such a record has been
given already, in fig. 26. If we make a mark on the
revolving drum at the moment of the application of stimulus,
and a second mark when the electrical (and, in this particular
case, also the mechanical) response is initiated, we can, with
the previous knowledge of the speed of the drum, determine
the time taken by the excitation to travel from A to P., and
thus find the velocity of transmission for that specimen by
electrical means.
A further refinement of this rnethod lies in the use of two
galvanometers instead of one, the slight lag of response,
caused by galvanometric inertia, being in this way eliminated.
Particulars regarding this will be found elsewhere.
It will be seen, then, that in the electromotive method
we have a second means by which to determine the velocity
of transmission of excitation in what are known as ordinary
plants. I shall now describe experiments performed by this
method. The peduncle of Biophytiau is leafless. That it
does transmit stimulus is seen nevertheless when we excite
it at any point. Excitation will then be found transmitted
through it to the main stem, from which it travels outwards
to different leaves, a fact evidenced by the serial fall of
leaflets in a centrifugal order. In such a peduncle 1 have
determined the velbcity of transmission by the electromotive
method. The distance between the points A and B was
4*6 cm,; the time taken for transmission was 127 seconds.
The velocity is thus found to be 37 mm. per second.
Similarly, in the stem of Fiats religiosa, I found velocity of
transmission to be 9*4 mm. per second, which is almost the same
as that in the stem of the so-called ' sensitive ' plant Mimosa.
From the experiments carried out on the electrotactile
method it will be seen that excitation is conducted along
a plant-tissue from cell to cell, as a contractile wave. We
ELECTROMOTIVE DETECTION OF EXCITATORY PULSE 263
have also seen that water is expelled from an excited and
therefore contracted cell. It is further clear that when an
excitatory wave is proceeding in any direction, this cell-to-
cell passage of excitation will give rise to a cell-to-cell
contraction, the result of which will be a forward movement
of water, which will have the velocity, not of hydrostatic
transmission, but of the excitatory wave. The hydrostatic
disturbance is quite distinct, being transmitted with great
rapidity, and its presence has been shown in the preliminary
abnormal response of erection in leaflets of BiopJiytiiin
(p. 24). But that propulsion of water which is con-
comitant to the passage of true excitation is very much
slower, having the same speed as that of excitation itself
Summary
The direct effect of stimulus is not transmitted by means
of hydrostatic disturbance, but by a cell-to-cell propagation
of excitation.
This transmission of excitation from cell to cell is attended
by a cell-to-celi contraction.
The passage of such a contractile wave may be detected
by the electrotactile method, which thus enables us to
determine the velocity of transmission of excitation, even
in tissues which are not motile.
In consequence of the concomitance of the excitator}-
wave with cellular contraction, water is moved forward pro-
gressively, with a velocity and in a direction the same as that
of excitation.
This movement of water is not brought about by any
hydro-mechanical action, but is the direct effect of the con-
tractile wave due to excitation. The hydrostatic disturbance,
when present, is transmitted with very great velocity, and its
effect is seen in the abnormal preliminary response of erection,
exhibited, for example, by the leaflets oi Biophytuiii
The velocity of transmission of excitation in ordinary
plants may also be found by determining the velocity of the
concomitant electromotive wave.
CHAPTER XXII
THE LATENT PERIOD AND REFRACTORY PERIOD
The determination of the latent period in Mimosa — Experimental arrangements
for obtaining automatic record -Prolongation of latent period by cold — Spark-
record for determination of latent period — Prolongation of latent period by
fatigue - Sluggishness of the response of Philanthtis urinaria, also long latent
period and very protracted period of recovery — Latent period reduced under
strong stimulation — Response in Biophytum on the ' all or none ' principle —
Definite value of effective stimulus— Phenomenon of refractory period in
Biophytum — Parallelism of responses in Biophytum and in cardiac muscle —
Additive effects— Inappropriateness of term ' refractory period '— Energy in
excess of effective stimulus held latent for subsequent manifestation.
Having explained the means by which it is possible to
apply a quantitative stimulus of uniform or increasing
intensity, and also how the responsive effect and its time-
relations are accurately recorded, we shall next turn to the
study of the various characteristics of the response itself, as
given for example by the plant Mimosa or BiopJiyiinii. In
order that we may inspect different parts of the response-
curve in greater detail, it will be necessary to take the record
on a fast-moving drum, so that the curve may be drawn
out, and its several features more easily distinguished. As
we wish, moreover, to study the excitatory effect on the
motile organ, stimulus will be applied directly on the
pulvinus.
Automatic method of record. — As it will be necessary
in the course of the following investigation to measure the
times of reaction accurately, to small fractions of a second,
the record must be obtained automatically. It is to be
remembered that, as said before, response in vegetable tissues
is relatively more sluggish than in animal, and it is super-
fluous to arrange for measurements of time up to more than
LATENT PERIOD AND REFRACTORY PERIOD 265
hundredths of a second. The experimental method that I
am about to describe would, however, enable us, if necessary,
to make determination of time-intervals of one-tenth this
magnitude, the question being only one of using a recording
drum with the requisite increased speed. The drum used for
these experiments was one constructed by Verdin, and
provided with a very perfect governor. Some little time
elapses after starting the drum before it acquires uniform
speed, which it afterwards maintains, however — at least
during the short time required for the experiment — with
great perfection. The record is not taken until this uniform
condition is attained. The mirror of the Optic l.ever throws
a spot of light upon a sensitive photographic film, wrapped
round the revolving drum. In order to produce records of
the required rapidity, I employ sunlight, proceeding from a
pinhole, which after reflection from the mirror of the Optic
Lever falls on the drum, appropriately focussed by means of
a condensing lens placed at the end of a focussing tube, as
seen in the figure. It is understood that, the record to be
obtained being photographic, this experiment is carried out
in a dark room, the sunlight required for the record being
directed upon the pinhole by a heliostat outside.
The stimulus consists of a single strong break-shock,
from a Ruhmkorff's coil, one electrode of which, by means of
non-polarisable connections, is attached to the pulvinus of a
leaf, and the other to the main stem lower down, of a speci-
men of Mimosa. The shock is applied by the recording
drum itself, at a particular moment in the course of its
revolution ; and at the same instant the curve of response
begins to record itself automatically. These two acts — of
imparting stimulus, and of opening a shutter by which the
recording ray of light is allowed to fall upon the moving film
— are performed simultaneously ; and both alike are initiated
by the stroke of a rod which is fixed to the axis of the drum
underneath.
This rod, which I shall designate as the striker, at a certain
period in the revolution of the drum, impinges upon a balanced
266
PLANT RESPONSE
electric key, K,, thus closing an electro-magnetic circuit, and
so releasing the shutter S, which is immediately in front of
the pinhole, by which sunlight is admitted. This drop of the
shutter, simultaneously pi^oducing a break of the Ruhmkorff's
coil circuit, gives an excitatory electrical shock to the plant.
Fii". 109. Apparatus for Automatic Record
When the speed of the drum has become uniform, K., is closed, and the
striker, in connection with the drum, closes the electro-magnetic
shutter circuit. The dropping of shutter, s, interrupts by K., the primary
circuit of the induction coil, the secondary of which gives a shock to
the plant. The fall of the leaf pulls down the Optical Lever, o, pro-
ducing record on drum. K,, short-circuiting key of secondary.
The electro- magnetic circuit of the shutter is interrupted
by a key, K.,, and this is kept open till the speed of
the drum has become uniform. On closing k.^, the circuit
is still incomplete ; but the striker, impinging against the
LATENT PERIOD AND REFRACTORY PERIOD 267
balanced key, K,, completes the circuit, and actuates the
shutter.
A thread connects the shutter with one arm of a second
balanced key, K^. This arm is so overweighted that when
freed, it causes two prongs, at the opposite extremity of the
lever, which complete the primary circuit of the Ruhmkorff' s
coil, to be lifted out of their cups of mercury, and thus the
circuit is interrupted. But the thread is of such a length
that when the shutter is set, so as to close the pinhole,
the prongs, dipping into the cups of mercury, complete
the primary circuit. The overweighted arm of K3 falls,
with the drop of the shutter lifting up the prongs, and
thus suddenly interrupts the primary current, giving rise
to a break-shock in the secondary, which passes through
the plant. During the course of the preliminary adjustment,
when primary circuit of the coil is made, a make-shock is
produced, but this is prevented from affecting the plant by
a key, K^, which short-circuits the secondary. When the
adjustment has been made, this short-circuiting key is opened.
Briefly to recapitulate the procedure : The drum, carrying
the sensitive film, is released, and begins to revolve. The
key K2 of the shutter-circuit is kept open, until a uniform
speed is attained. It is then closed. The striker connected
with the drum now closes the balanced key, K, ; the shutter
drops, and simultaneously interrupts at K3 the primary current
of the induction coil, thus causing an excitatory shock to be
given to the plant.
The determination of the latent period. — It will be seen
from the upper of the two photographs, given in fig. i 10,
that for a period of yVV of a second the record remains hori-
zontal. This represents the latent period, after which the
tissue begins to respond. For a further period of half a
second the leaf is seen to fall with a considerable and approxi-
mately uniform speed. The rate of movement of the tip of
the leaf is now 71 mm. per second. After this the leaf con-
tinues to fall, but with a diminishing speed, till the maxi-
mum contraction fall is reached. From records obtained on
268
TLANT RESPONSE
slower-moving drums, I find that this is attained in different
specimens, in a period of I'S to 2-5 seconds after the shock.
This maximum contraction persists for a further period of
about thirty seconds. The leaf now begins to erect itself,
and full recovery is attained in the course of a further period
of about six minutes. These statements refer to reaction in
vigorous Mimosa, at a favourable season of the year, like
summer. In an unfavourable season, like winter, however,
the reaction becomes very sluggish, and recovery is not then
complete in less than
eighteen minutes, or
three times the normal
perio(5.
Effect of cold on
latent period. — I shall
next refer to the slug-
gishness induced by
cold, prolonging the
latent period. The ex-
treme instance of this is
seen when iced water is
applied to the pulvinus,
and too great cooling
being thereby effected,
the response is abolished.
With moderate cooling
the latent period is found
to be prolonged to
several seconds. This
effect cannot conveni-
ently be shown, however, within the limits of a fast record.
In order, therefore, to show the comparative effect of cold on
the latent period, in the case of the same specimen whose
record is seen in the upper of the two photographs in fig. i lo",
I was careful to cool the pulvinus very slightly. In the lower
of these photographs it will be seen that the latent period has
become prolonged, from the normal iirr. to ^ of a second.
Fig. 1 10. Photographic Record of Response
of Mimosa, Exhibiting the Latent Period
in its Variation
The upper curve was taken under normal
conditions, and the lower when the pul-
vinus was slightly cooled. Time-marks
= tenths of a second. Original record
reduced to half.
[The duplication, which will be observed in
each record, is due to the fact that the
heliostatic mirror was silvered behind,
thus producing two reflections, one from
the surface of the glass and the other from
that of the silver.]
LATENT PERIOD AND REFRACTORY PERIOD 269
The rate of responsive movement is also seen to have under-
gone considerable diminution. In the first, or normal, case,
during half a second after the commencement of response, the
rate of movement was, as said before, 71 mm. per second.
In the second case, however, after slight cooling, it is seen to
have been reduced to 26 mm. per second, or almost one-third
of the original rate.
Record by means of electric sparks : prolongation of
latent period by fatigue. — In order to overcome the difficulty
of the insensitiveness caused by keeping the plant in a photo-
graphic dark room, I have recently devised a method of
Fu;. III. Electric Spark Record, Showing Increase of Latent Period by
Fatigue, in Successive Responses of a Leaf of Mimosa
Latent period in normal topmost record seen to be ~ second ; this increased
in next — taken i minute before full recovery — to ■-— second ; latent
period further increased in last case — taken 3 minutes before full
recovery — to ~; second. Note also progressive change in slope of
record by means of a series of punctures produced by electric
sparks on a recording paper surface. The sparks occur at
the short gap between the end of the long arm of the record-
ing aluminium lever, and the drum, these being connected
respectively with the two electrodes of a Ruhmkorff's coil.
The electrical disturbance does not affect the plant, as the
leaf is attached to the lever by a long silk thread. One
great advantage of this method lies in the fact that the time-
intervals, which may be as short as desired, are indicated by
the distance between successive punctures, which are deter-
mined by the frequency of the vibrating interrupter of the
coil. In the case of which the record is given (fig. i 1 1) the
270 I'LANT RESPONSE
interval between successive sparks was -V of a second. By
this means, it was found that increasing fatigue induced a
corresponding increase in the latent period of a leaf of
Mimosa, from the normal fjS_ to jVtt of a second.
Response of Biophytum. I shall now proceed to de-
scribe effects essentially similar to the last, seen in Biophytuui.
In order to be able to observe in detail the various responsive
peculiarities of the curve, subsequent to stimulation, records
were taken in this case on a much slower-moving drum. The
record given in fig. 112 shows the mechanical response to
stimulation produced by discharge of condenser (capacity -oi
Fic. 112. Response of Biophytum ; Electrical Stimulus having been
Applied at the Pulvinus of the Motile Leaflet
The thick dot represents the moment of stimulation.
microfarad, charged to nine volts). The exact moment of
stimulation is marked on the record.
It will be seen that the leaflet begins to respond almost
instantaneously. The maximum contraction in this case,
being considerably more rapid than in that of Mimosa, is
almost attained in the course of half a second. An interest-
ing point to be noticed in the record is the flattening of the
top of the curve (fig 112). That is to say, the maximum
contraction persists for a considerable time before recovery
begins. In the present case this lasted for ten seconds. This
period varies in different specimens, from a maximum of ten to
about two seconds. But in this particular specimen the period
LATENT PERIOD AND REFRACTORY PERIOD 27 1
in question remained at least approximately constant, in suc-
cessive experiments. After this there was commencement of
recovery, which was completed — as seen by the return of the
spot of light to its exact original position — in the course of
five minutes. In order to form an idea of the consistency of
results which may be expected from a good specimen, I re-
peated this experiment six times in succession, commencing
the record each time at the same point on the recording
surface as before. The degree to which all these curves
coincided with each other in detail is almost incredible.
Their rising portions, their flat tops, and their gradual descent
during recovery, were all so coincident that the six successive
curves appeared as but one.
In specimens of plants which were not in good condition,
fatigue was shown by the gradual diminution in height of
successive responses. For accurate standard experiments it
is therefore necessary to have specimens which are vigorous.
We have already seen the prolongation of the latent
period which is induced by cold, in the case of Mimosa.
I have obtained similar results also in working with Bio-
phytuin. For example, in a certain experiment, moderate
cooling induced a prolongation of two seconds in the latent
period. When the plant was allowed to return to the sur-
rounding temperature of the room, however, the increase of
latent period disappeared.
Latent period diminished by increased intensity of
stimulus. — It has been said before that there are innumer-
able gradations between the extreme cases of motile sensi-
bility in plants. As regards motility, an extremely sensitive
leaflet was that of Mimosa pudica. Somewhat less quickly
reacting were those of BiopJiytum sensititmm, and we had in
the leaflets of Philanthus urinaria an instance of extreme
sluggishness.
The latent period of the leaflet of P/ii/ani/ius, under
moderate stimulus, is as long as three minutes, and the maxi-
mum contraction is not attained under forty minutes ; but
with a stronger stimulus the latent period is reduced to less
272 PLANT RESPONSE
than one minute, and the maximum contraction takes place
in a relatively short period of about fifteen minutes (fig. 30,
/; and c, p. 44).
Response of Biophytum on the * all or none ' principle.
It is well known that in the case of a contractile skeletal
muscle, there is a minimal intensity of stimulus which is
necessary in order to produce contraction. From this point
onwards, as the stimulus is gradually increased, the response
increases, till a maximum contraction is arrived at, beyond
which still further increase of stimulus produces no increase
in effect. In cardiac muscle, however, the range of stimulus
between minimal and maximal is practically narrowed to a
point, so that the minimally effective stimulation is also at
once maximal. It is to be remembered, at the same time,
that the differences between cardiac and skeletal response are
a question of degree, rather than of kind.
Curiously, the response of Biophytum is, in this respect,
somewhat similar to cardiac response. In an experiment
with a particular specimen of Biophytum, the intensity of
stimulus was increased by successive increments of the E.M.F.
u.sed for charging the condenser. With an E.M.F. of seven
volts there was no response. With a charge of nine volts
there was always a response, and this was maximal. A
charge of eight volts was almost on the threshold of response.
That is to say, when I started experimenting, the leaflet was
in a somewhat sluggish condition, and an eight-volt charge
was ineffective. But after obtaining response to a nine-volt
charge, I could obtain response also at eight volts. This was
due to the fact that molecular inertness had been removed by
the preceding effective shock. Thus we have two determi-
nate values of stimulation, giving respectively maximum
response and absence of response, the charges, namely, of
nine and seven volts. The effective stimulus is, of course,
constant for a given individual, but differs with the excita-
bility of different specimens. Here, then, we have an instance
of the ' all or none ' effect. The leaflet either responds to
the utmost, or not at all.
LATl'NT PERIOD AND REFRACTORN" PERIOD 273
Refractory period. — We shall next consider the pecu-
liarities of the refractory period which I have discovered in
the case of plant-tissues ; and for the material of this investi-
gation we shall use the plant Biophytuiii, taking, in fact, the
very specimen whose successive responses have already dis-
played such remarkable consistency (fig. 112). Such uni-
formity in successive responses is only possible when we
allow sufficient time of rest for complete protoplasmic re-
covery, by which the excitability is fully restored. Eut it has
been shown in Chapter XX. that if sufficient time of rest be
not allowed, the protoplasmic recovery is incomplete, and the
excitability is diminished. Hence the extent of response,
which is an outward indication of excitability, is diminished,
and this effect is known rs fatigue.
We also arrived, in the same chapter, at the theoretical
conclusion that there is a minimum resting interval, the dimi-
nution of which results in such a loss of excitability as to
abolish response, and this period we know as the Refractory
Period, because the leaf then apparently takes no account of
stimulus, or is 'refractory' to it (fig. 105 J. We shall now
enter into greater detail regarding the peculiarities of this
refractory period. After taking the six curves in response to
separate single stimuli which were so extraordinarily similar,
I proceeded to take a curve of response to two equal stimuli
of the same intensity as before — namely, nine volts, charging
•Qi microfarad — the two stimuli following each other at an
interval of one second. The application of the second stimu-
lus appeared to produce no effect, the extent and general
character of response being the same as in the case of single
stimulus, with only the difference of a slight elongation of the
flattened top of the curve. I next tried the effect of two
stimuli at an interval of five seconds. The leaflet was still
refractory to the second stimulation, but when I applied it at
an interval of ten seconds, the second stimulus became effec-
tive. It will thus be seen that Biophytiiui has rather a long
refractory period, during which, as far as can be seen, it takes
no account of the impact of a new stimulus. This refractory
T
274
PLANT RESPONSE
period is a matter of several seconds, but varies somewhat
with different specimens. I have, again, in some cases
observed a very curious phenomenon of two refractory
periods.
We thus find several very interesting parallelisms between
the response of BiopJiytum in plants and that of cardiac
muscle in animals. We find in both that the minimal
response is also the maximal, increasing stimulation pro-
ducing no increase of response. In the response-curve itself
the flattened top is common
to both ; both have a pro-
longed refractory period ;
and we shall see later that
in both there is a tendency
to the production of mul-
tiple rhythmic responses.
In all these respects the
responses of BiopJiytum
resemble the cardiac re-
sponses, rather than those
of skeletal muscle. But
they have one peculiarity
in which they share the
characteristics of the re-
sponses of skeletal muscle.
In the responses of cardiac
muscle, successive effects
are not additive, perhaps because that muscle undergoes
the maximum contraction possible. In Biopkytiun, however,
while any effective stimulus — whether minimal or largely
super-maximal — will produce response of the same extent,
yet this response, though the greatest possible for a single
stimulus, is not the utmost of which the leaflet is capable.
Hence, if we superpose successive stimuli, taking care that
they do not fall within refractory periods, we shall obtain
an extremely interesting response, showing the separate
additive effects. I give here two curves, exhibitine these
Fic;. 113. Additive Eftects seen in Re-
sponses o'i Biophytuni to Stimuli which
Fall outside the Refractory Period
The record to the left shows the effect of
stimuli, applied at intervals of one,
and that to the right of half a minute.
LATENT PERIOD AND REFRACTORY PERIOD 2/5
effects of superposition of stimuli. In the right-hand record
in fig. 113 the stimuli were applied at intervals of thirty
seconds. The successive responses, except the last, show
a regular decrease. In the left-hand record, successive
stimuli were applied at intervals of a minute, and appear
much more equal.
But even in these additive effects we find one peculiarity
which is also characteristic of cardiac response. In the latter
case, though on repetition of stimulus there is no summation
of height of response, yet the apex-time of the second
response is shorter than that of the first. We see this in
the case of BiopJiytiim^ in the right-hand curve. The first
response has a slightly rounded top, but this is reduced to an
acute angle in the second. The record, having been reduced
to one-eighth for reproduction, does not show this so plainly
as does the original.
Though each single response of Biopliytnin is maximal,
yet from fig. 113 v/e have seen that this maximal response
does not represent the utmost movement of which the leaflet
is capable. In the particular plant here used for experiment,
the maximal response to individual stimulus was always
about thirty-eight di\isions, but four superpositions produced
a total movement of ninety-four divisions. It must be
remembered that such effects can only be possible when
the second stimulus is timed to fall at the expiration of
the preceding refractory period.
If the absolute value of each individual minimally effective
stimulus be represented by s, and if the whole be added
together, or, in other words, if a stimulus of 4s be given
at once, we may regard such a stimulus as made up of one
minimally effective, plus three others which fall within the
refractory period, and arc thereby rendered totally ineffective.
In other words, we ma}' regard a very strong stimulus as
made up of so many minimally effective stimuli. It is as
if the first effective fraction alone acted, the succeeding
portions, which arrive within the refractory period, being
inoperative.
r 2
2/6 PLANT RESPONSE
In view of certain other phenomena not altogether dis-
connected, it seems unfortunate that the term ' refractory
period' should be used with its present significance. For
this term might be held to imply that the tissue refuses to
take any account of the superfluous energy that is impressed
upon it. It is more likely, however, that by some peculiar
mechanism, the superfluous stimulus — />. what is over and
above the amount necessary for producing maximal re-
sponse— is prevented from overflowing. This excess of
energy may, then, at least in some cases, remain latent,
to be manifested at a later period in the form of excitatory
impulses. Such impulses, again, attuned by some regulating
process, may give rise to periodic or rhythmic overflow.
That this is actually the case will be demonstrated in the
next chapter.
Summary
The latent period of response is protracted by cold.
It is also protracted by fatigue. It is shorter under strong,
than under moderate stimulation.
In vigorous specimens of BiopJiytmn leaflet, the mini-
mally effective stimulus is also maximal ; under normal
conditions, this minimally effective stimulus has a definite
value.
There is also a definite refractory period in the response
of BiopJiytiiJJi. If a second stimulus fall within this refractory
period, it appears to produce no effect.
The response of the leaflet of BiopJiytum resembles in
many respects that of cardiac muscle. In both, response is
on the 'all or none' principle, and both exhibit a relatively
long refractory period.
PART IV
MULTIPLE AND AUTONOMOUS
RESPONSE
CHAPTER XXIII
ON MULTIPLE RESPONSE
Multiple electromotive responses due to a single strong stimulus — Multiple
electrotactile responses — Multiple mechanical responses in Biophytiim —
Cyclic variations in multiple responses — Multiple retinal excitations— Inter-
mittent pulse in man and plant — Semi-automatism —Continuity of multiple
and automatic response —Conversion of Biophytuvi into automatically
responding plant ; conversion of Desmodiiim into ordinarily responding
plant — Similar polar effects of current in Biophytuin and in Desmodiiim
leaflet, at standstill — Moderate stimulus in ^/(7//^j///;« and in Desmodiiim at
standstill produces single response ; and strong stimulus, multiple response.
I PIAVE already explained in Chapter III. that the excitatory
wave initiated by a stimulus has a concomitant electro-
motive wave. If the plant experimented on is provided with
motile leaves or leaflets, the excitation is evidenced by the
simultaneous mechanical response of the motile organ, and
the electrical response of galvanometric negativity.
Multiple electromotive responses due to single strong
stimulus.— In my investigations on electrical response in
plants, I was surprised to find that whereas a single mode-
rate stimulus gave rise to a single electrical response, a very
strong stimulus very often initiated a multiple series of
responses. I have obtained such multiple responses to a
single stimulus with all kinds of plants, ordinary and ' sensi-
tive,' and under the action of various forms of stimulus —
chemical, thermal, and mechanical.
To obtain these multiple electrical responses in an unmis-
takable manner it is necessary that the excitation should
reach one electrical contact and not the other, for in the latter
case there would be complications arising from diphasic
variation. The necessary condition may be fulfilled by
2 8o
'LANT RESPONSE
applying stimulus near the proximal contact, the other
contact being at a relatively great distance. The periodicity
of these multiple electric responses I find to vary in different
Fig. 114. Multiple Electrical Responses due to Single Strong Stimulus
{a) In Mimosa due to thermal, and (/') to chemical stimulation ; (c) in
peduncle of Biopkytiim, due to thermal stimulus. [N.B. — This series
persisted for two hours.] (d), in hypocotyl of l\xiiiarindus indica, due
to stimulus of cut.
cases from about a half to five minutes. These multiple
electrical responses, resulting from a single strong stimulus,
sometimes persist for as long as
two hours (fig. 1 14).
Multiple electrotactile re-
sponses.— In the course of my
experiments on the electrotactile
method of detecting the excitatory
wave, I discovered multiple waves,
initiated by a single strong stimulus,
passing through the tissue. In
fig- 115 I gi^s ^ record of such
multiple responses in the stem of
Mimosa, in \\'hich it will be seen
that there are four such responses,
with an average period of one
minute each.
Now, as these pulsations are
signs of the presence of excitation,
it follows from the occurrence of such multiple responses
that a strong stimulus may give rise to a multiple series
Fig. 115. Multiple Electro-
tactile Response in Stem of
Mimosa, due to Single Strong
Thermal Stimulus
(Original record reduced to ^.)
MULTIPLE RESPONSE 28 I
of excitations. I was therefore led to expect that this
fact might be demonstrated in another and still more con-
vincing manner, if I should succeed in finding a sensitive
plant which exhibited these multiple excitatory waves by
repeated movements of the indicating motile leaflets.
Certain peculiarities of BiopJiytuiii which I had previously
discovered led me to think that I might find in this plant the
opportunity I sought ; for we have seen in the last chapter
that in Biophytuiu a certain minimal intensity of stimulus
induced the maximal mechanical response. With such
a stimulus we obtain only one response, and if we apply
a stimulus of very much more than minimal intensity it
produces no greater mechanical effect. What, then, happens
to the excess of stimulus ? This excess may be wasted as
heat, or it may continue to exist in some latent form,
and this latent stimulus may subsequently be given out
rhythmically.
Multiple mechanical responses. — In view of these facts
I expected a strong stimulus to give rise to those periodic
waves whose existence I had been led to suspect from the
observation of the recurrent electromotive and electrotactile
waves in various plants. As a matter of fact it has been
noticed that Biophytmn, when strongly excited, exhibits two
successive movements of mechanical response, the leaflets not
completing their closure at once, but in the course of two
twitches succeeding each other. But I expected to detect
a larger number of pulsatory movements in response to a
single strong stimulus.
In order to do this, however, it was necessary to prevent
the complete closure of the leaflets, by which the further
exhibition of mechanical response was made impossible.
For this purpose I used the Optic Lever, with the light
counterpoise (p. 19). In this way I succeeded in demon-
strating, through mechanical response, what had already
been demonstrated electrically, the fact that a single strong
stimulus, of whatever form — thermal, mechanical, electrical,
or photic — will induce, not one but a multiple series of
282 PLANT RESPONSE
rhythmic responses in a plant. Thus on applying a thermal
stimulus to the petiole of BiopJiytum near its insertion on the
stem, an excitatory wave was found, as usual, to produce
successive depressions of leaflets in a centrifugal order. But
after a while the existence of a second excitatory wave
became evident, by a second series of closures of leaflets.
That the wave was due — not to excitation reflected in some
way from the tip of the leaf, but — to a second wave starting
from the original point of stimulation, was made evident by
the fact that the successive fall of leaflets was again centrifugal,
from near the stem to the tip of the leaf From the records
now obtained by means of the Optic Lever, attached to one
of the leaflets, it was interesting to observe the very numerous
rhythmic pulsations, often as many as twenty, now made
visible.
As an instance of the simplest form of such multiple
response, I give the following record, which was obtained
under the thermal stimulation of the electrothermic stimu-
lator. The point of application was close to the responding
leaf The average period of these pulsations was about
thirty seconds (fig. ii6) It may be stated here that the
period of multiple responses of Biophytuni is found to vary
from about fifteen seconds to three minutes or so, depending
on the condition of the plant and the intensity of the
stimulus.
The question suggests itself, since multiple electromotive
and electrotactile excitations are observed in Mimosa, why
should not this plant also exhibit them mechanically ? The
answer to this is probably found in the fact that the Biophy-
tuni leaflet is light, and easily exhibits fluctuating impulses,
whereas the impulsive fall of the heavier Mimosa leaf persists,
owing to greater momentum, till it is more or less complete.
Again, unless an organ has at least partially recovered from
fatigue, it is not susceptible of fresh excitation. The period
of full recovery in the pulvinus of Mimosa is very long,
being about seven minutes. We saw, in studying fatigue in
MULTIPLE RESPONSE
^83
Mimosa, that when a second stimulus succeeds a first, after
an interval of less than one minute, it produces no responsive
effect. If, then, a second wave of excitation arrives within
the refractory period of the first, we must expect that it will
remain mechanically ineffective. In the multiple electro-
tactile responses seen in fig. 114, these successive excitations
are seen to have occurred at intervals of one minute, com-
plete recovery being accomplished in that time. In the
Fig. 116. Multiple Mechanical Response of Biophytiim, due to a Single
Thermal Stimulus
stem, then, the period of recovery is very much shorter than
in the pulvinus, and the same waves of excitation which
at intervals of one minute produce response in the one
case, prove ineffective in the other. Had these periodic
waves of multiple excitation been three or four times as
slow as they are, we might have been able to observe
multiple mechanical responses of the leaf of AFimosa. In
connection with this, I may state that I once observed
a second mechanical response to a single strong stimulus
284 TLA NT RESPONSE
in the leaf of Mimosa. In this case the second response
occurred four minutes after the first.
We have seen that multiple electrical response is obtained
when any form of strong stimulation is employed. I now
tried to find out whether multiple mechanical response could
be produced by chemical stimulation. One of the middle
leaflets of a Biophytuni leaf was attached to the recording
Optic Lever, and I applied a drop of sulphuric acid i cm.
away from the recording leaflet in the direction of the tip
of the leaf This was found to give rise to five vigorous
recurrent pulsations.
Thus, though a single moderate stimulus evokes but
a single response, yet under strong stimulation we obtain
not only the immediate consequent response, but also a
surplus of energy which remains over and is held latent in
the tissue, to be given out later, after a shorter or longer
interval, in the form of recurrent responses. From these
experiments it is clear that a rhythmic series of effects
need not have a periodic antecedent cause. As regards
these pulsatory movements, it was shown on page 47 that
the fall of the motile leaflet was due to a pulse of diminution,
and its erection to a pulse of restoration, or increase, of
turgidity. In these multiple responses, then, we have the
expression of rh3'thmic variations of turgidity initiated by
stimulus.
Among ordinary responses — that is to say, single response
to single stimulus — we have observed three types, depending
on the excitability of the tissue. When the excitability
remains uniform, the responses are uniform. When the
excitability undergoes a gradual diminution, there is a cor-
responding depression of response — that is to say, fatigue
supervenes. And when the excitability increases by degrees,
there is a correspondent enhancement of response known
as the 'staircase' effect. In addition to these, I have, as
explained before, also noticed some curious instances in which
the excitability of the tissue appeared to undergo periodic
fluctuations, in consequence of which successive responses to
MULTIPLE RESl'ONSK
285
uniform stimuli exhibited periodic fluctuations (p. 106). We
may thus have an alternate, periodic, or cyclic fluctuation of
excitability exhibited in the responses. In the first of these,
the responses are alternately large and small. In the last,
we may have either an ascending or descending series, which
is periodically repeated.
Cyclic variation in multiple response. — In multiple
responses also we find all these types. And if the seat of
origin of the multiple excita-
tion be at a distance from the
responding leaflet, we have
in this fact an added element
of variation. For we have
now to consider, not simply
the periodic variations of ex-
citability of the motile organ,
but also those of the excita-
bility of the intervening tissue,
causing periodic changes of
conductivity. Hence, in re-
cords obtained of multiple
responses, where the seat of excitation is at a distance, we
have a complex combination of variations of amplitude and
of period in the pulsations.
Confining our attention to the period alone, we find that
multiple responses may be classified under three headings :
Multiple Response in
Biophytiim
The rhythm, at first slow, becomes
quicker.
118. Multiple Response in Biophytuiit
Quick rhythm becoming slo\\.
those in which the successive periods are fairly uniform
(fig. 116); those in which the pulsations are at first slow and
later become quicker (fig. 117) ; and those which begin with
286
PLANT RESPONSE
rapid pulsations, and afterwards slow down (fig. ii8). And
finally we may have any combination of these (figs. 119 and
120).
F'lc. 119. Multiple Response in Biophytuni, showing Cyclic (jroupings
of Amplitude and Period
These cyclic changes of amplitude and period are seen in
all types of rhythmic multiple responses, of which I shall
show that the autonomous responses of Desmodiuin and the
growth-responses of all plants are only special cases. These
characteristics are seen not only in
the rhythmic responses of vegetable,
but also in those of animal, tissues.
The cause of the latter is still regarded
as very obscure. Hence the study
of similar phenomena in plants, under
simpler conditions, may be expected
to throw much light on the subject.
How striking, even in their more
intricate details, are the similarities
between multiple responses in plant
and animal, will be shown in the next
three chapters. For the present I
shall confine myself to the considera-
tion of two of the most obscure
instances in animal tissues, and
them to be paralleled in the case of the
Fk;. 120. Multiple Re-
sponse in Biophytitiit,
showing Cyclic Group-
ings
shall show
vegetable.
Recurrent visual impulses. — It is known that when we
look for some time at a strongly illuminated object and after-
wards close the eyes, we see the same image repeating itself
many times in succession ; and of this phenomenon, so far
MULTIPLE RESPONSE 287
as I am aware, no satisfactory explanation has hitherto been
offered. The phenomena which have been described in the
case of vegetable tissues show, however, that this is nothing
but an instance of multiple excitations in the retina caused
by strong stimulation. In the case of the plant these recurrent
excitations express themselves mechanically as twitches ; in
the case of the firefly as flashes of light ; and in the retina
as recurrent visual images. In these recurrent visual impulses
are found all the peculiarities which I have observed in
recurring excitatory impulses in Biophytuin. This is shown,
not only in simple cases, but also in the most complicated.
I shall presently give, in illustration of this, two parallel
instances of multiple excitation in Biopliytum and multiple
excitation in retina (see table, p. 288), in which the successive
intervals undergo similar cyclic changes.
In order to measure accurately the intervals between
successive visual images, I use a special stereoscope, based on
my discovery of the phenomenon of binocular alternation of
vision, and by this means I have found that the after-effects
of light in the two eyes are not simultaneous but alternate —
that is to say. when the after-image in one eye is most vivid,
that in the other eye has just vanished, and vice versa. For
accurate measurement of this periodicity, it is necessary to
have the effect on one eye distinguished from that on the
other. For this purpose I have been accustomed to use a
stereoscope containing, instead of photographs, incised plates
with two inclined cuts, the right eye seeing the slit inclined
to the right, and the left eye that inclined to the left. When
the observer looks through this stereoscope turned towards
a bright sky, his two eyes are acted on by strong stimulus
of light. On closing the eyes, periodic visual impulses are
sent from the strongly stimulated retina just as the stimu-
lated area of BiopJiytuin was found to send out periodic
impulses.'
' Bose, 'Binocular Altcination of W^xow,'' Respojise in the Liviu}; and Noii-
Lh'ing; p. 175.
>88
PLANT RESPONSE
Table showing Cyclic \'ariation ok Multiple Responses
IN BlOPIIYTUM AND IN ReTINA
Cyclic variat
multiple
on in the periodicity of the
response of Biophytuin
Cyclic N
ariation in the recurr
impulses
ent visual
Interval between ist & and responses 52"
,, ,, 2nd ,, 3rd ,, 37"
,, „ 3rd ,, 4th ,, 25"
,, ,, 4l-li ,, 5th ,, 22"
5th „ 6th ,, . 30''
„ „ 6th ,, 7lh ,, 52"
Interval between ist& 2nd responses 7"
,, ,, 2nd ,, 3rd ,, 5*25"
,, ,, 3rd ,, 4th ,, 6-5"
,, 4th ,, 5th ,, 7"
,, ,, 5th ,, 6th ,, 9"
„ „ 6th ,, 7th ,, 11"
In both these cases, therefore, we begin with a compara-
tively long interval, which grows shorter, and then, again, is
progressively lengthened.
Intermittent pulsation. — We have now dealt with one
of the two instances of correspondence between obscure pulsa-
tory phenomena in animal and vegetable. The second, which
remains to be considered, is that known to physicians as ' the
Fig. 121. Iiitermiltenl Human Pulse (Broadbent)
intermittent pulse' (fig. 121). ' The term " intermittent " is
employed to designate the pulse when a beat is occasionally
missing from time to time, while the pulse in the intervals is
perfectly regular. It is a remarkable variety of pulse, and is
perhaps the least capable of explanation of any. The inter-
mission may happen at regular and definite periods, every
four, six, or up to twenty beats ; or the number of interven-
ing pulsations may vary. The intermittent pulse may be
habitual and constant, and in this case is more likely to be
at definite intervals, or it may be occasional only, under the
influence of some disturbing cause. . . . Occasionally nervous-
ness and fatigue will render the intermissions more frequent' '
' Broadbent, The Pulse, p. 125.
MULTIPLE RESPONSE 289
In connection with this, I have observed, among the
types of cyclic variation in multiple response in plants, an
instance in which every third response was missing, the period
of each second beat being thus approximately twice as long
as that of the first. It was easy to see that this was an
instance of alternating fatigue, causing the particular record-
FlG. 122. Intermiltence in Pulsation of /«Vr;//?j'///w
ing leaflet to just miss the response every third time (p. 122).
That the excitatory wave arrived in regular sequence, was
seen by the fact that the neighbouring leaflets pulsated at
regular intervals.
Semi-automatism. — The plant Biophytuin growing in
the open, under favourable conditions of heat. and light, some-
times becomes so excessively sensitive that motile impulses
are generated, the stimulus causing which it is often difficult
to localise. A particular leaflet may have been moved by a
puff" of wind ; or the alighting of a small insect, or the accidental
grazing of an adjacent blade of grass, may have been the
original source of the impulse. But this is enough to set all
the leaflets of the plant quivering in an extraordinarily lively
manner. For from the excited leaflet, the impulse travels
inwards, the leaflets falling in centripetal succession. The
excitatory wave then reaches the stem and overflows to the
other leaves. But this time the progressive closure of the
leaflets proceeds in a centrifugal or outward direction.
Before the first discharge, however, going through the
numerous avenues, can exhaust itself, the second impulse of
the multiple response may begin ; and in this way the leaf-
lets exhibit most lively movements without any immediate
U
290 PLA^T RESPONSE
antecedent cause. In some of these cases the origin of the
impulse can be traced, but there are others in which no
external source of stimulus can be assigned. And here we
find ourselves passing imperceptibly into the obscure region
of automatism.
There is much resemblance between such semi-automatic
phenomena in BiopJiytuni and the well-known instance of
spontaneous movements exhibited by Desmodiuvi gyrans, the
successive mechanical responses of which exhibit all the
peculiarities of multiple response as seen in Biophytuvi. In
Desmodiiim as in Biophytuvi we have similar cyclic changes
of period and of amplitude. Just as in the case of Biophytiiin,
when we cannot determine the source of stimulus, we are
tempted to regard the phenomenon as automatic, so in the
case of Desmodiuui gyrans it is our own inability to trace out
the origin of stimulus that leads us to regard the periodic
movements of the plant as automatic.
Biophytuvi, then, under ordinary circumstances, exhibits
a single response to a single stimulus ; but when the stimulus
is strong, a single stimulation will produce multiple responses,
and these will persist long after the cessation of the primary
stimulus. Under exceptionally favourable circumstances
periodic movements occur, apparently without any exciting
cause. I have again found, as will be described more fully in
Chapter XXIV., that Biophytuvi itself under favourable cir-
cumstances of light and warmth exhibits persistent and long-
continued autonomous pulsations, which are in no way
distinguishable from those of Desviodiuvi. Even the periods
of vibration are, generally speaking, similar, inasmuch as in
both these plants, under different circumstances, I have
recorded pulsations, the periodicities of which vary from
one minute or less to four or five minutes. Thus Biophytuvi
forms a connecting link between those plants which exhibit
only ordinary response (single stimulus, single response) and
those in which mechanical movements appear to be auto-
matic. Biophytuvi is also particularly interesting, be-
cause in its case we find the same plant, under different
MULTIPLE RESPONSE 29 1
circumstances, behaving in either of these ways ; that is
to say, giving ordinary response, or exhibiting automatic
movements.
Continuity of multiple and automatic response. — If there
be thus no real breach of continuity between multiply and
automatically responding plants, it should be possible, under
suitable circumstances, to obtain from such a characteristi-
cally automatic plant as Desmodiuni all those peculiarities of
responsive indication which were found in Biophytum. The
latter, it will be remembered, under the condition of over-
excitability, consequent on the absorption of abundance of
energy of heat and light, became converted from an ordinarily
responding into an automatically responding plant. Con-
versely we might expect Desnwdium, under the opposite
circumstances, to pass from the condition of giving automatic
to that of giving ordinary response.
This inference I have been able to verify, for I succeeded
in obtaining the requisite conditions for converting the auto-
matically responding Desmodiuni into an ordinarily responding
plant, in three different ways : firstly, by artificially reducing
the absorbed energy of the plant, under cautious cooling, care
being taken that this did not produce permanent cold-rigor ;
secondly, by keeping a plant for some days in a dark room,
at uniform temperature ; and thirdly, by selecting a plant for
experiment in the most unfavourable season of the year — that
is to say, in the winter, when it was already exhausted by
flowering. It will be seen that the principle adopted in each
of these cases consisted in depriving the plant of its excess of
energy. The third method was the most satisfactory of all,
since the leaflets were in a state of natural standstill. In
every instance, the automatic movements of the leaflets came
to a stop, and the motile organ was reduced, as will be shown,
to the ordinarily responding condition. In choosing the
leaflets for experiment, it must be remembered that their
sensitiveness is liable to disappear with age. In Biophytiini,
for example, very old leaflets show no response. In experi-
menting, therefore, on leaflets of Dcsuiodium which have been
u 2
292 PLANT RESPONSE
brought to a state of standstill, care should be taken to select
those which have not permanently lost their motility, but in
which automatic movements have simply come to a stop for
want of a sufficient reserve of latent energy.
Polar excitation of Desmodium leaflet in a state of
standstill.— We have seen how the leaflets of Biophytiwi
showed kathodic excitation at make. I shall now show that
a Desuiodiuui leaflet, which has been brought to a state of
standstill, either naturally or artificially, will exhibit similar
effects of polar excitation ; and first, I shall take a case in
which arrest was produced by cautious application of cold.
I placed one electrode on the petiolule of the leaflet, and the
other on the main petiole of the leaf, and applied the stimu-
lus of condenser discharge, the petiole being made kathode.
In this way I obtained from the Desmodiiiin leaflet a series
of single responses to single moderate stimuli.
I next selected specimens which, in the winter season, had
come to a state of natural standstill, and tried on them the
polar effects of a constant current. Electrical connections
were now made at the bases of the petiolules of lateral
leaflets in two neighbouring leaves. In completing the elec-
trical circuit, one of these leaflets would be under anodic, and
the other under kathodic, action. I found that a consider-
able electromotive force was necessary to initiate the excita-
tory reaction. Thus, using thirty volts, I found that the
kathodic leaflet responded at make, and the anodic at break.
This shows that, as regards polar effects, Desuwdmin in a
state of standstill gives exactly the same responses as does
Biophytum.
In the case of Biophytuvi, moreover, we found that with
high E.M.F., we arrived at a phase of response, the A stage,
in which both the anode and the kathode caused excitation
at make ; and in Desviodiuni in a state of standstill
I obtained an exactly similar result, for, on now using a
higher E.M.F. of forty-eight volts, with the same specimens
as in the last experiment, I obtained excitation at make, at
both anode and kathode.
MULTIPLE RESPONSE 293
Multiple response caused by strong stimulation in
Desmodium. — I next tried to find out whether Desiiiodiiiin
in a state of standstill would give multiple responses to a
strong chemical stimulation, as I had found Biophytmn to do.
Remembering how successive twitches are produced in a
frog's muscle, in a nerve muscle preparation, when the nerve
is touched with salt, I applied a strong solution of the same
reagent to the petiolule of the arrested Desmodium leaflet.
This gave rise to a series of four vigorous mechanical
pulsations.
In order again to show that multiple response could be
initiated by strong thermal stimulus, in Desmodium as in
Biophytum, I selected a plant whose leaflets were in a state
of natural standstill. A fairly strong stimulus was applied
by means of the thermal stimulator, at a point on the petiole
half a centimetre above the insertion of the motile leaflet, in
precisely the same manner as was ordinarily done with
Biopliytum. The Desmodium leaflet now gave a multiple
series of responses, exactly similar to those obtained from
Biophytum. The first occurred three and a half minutes after
the application of stimulus. The successive rise and fall were
then uninterrupted. The average period of each response in
the series was approximately 4*5 minutes. The multiple
responses gradually declined in amplitude, and came to a
stop after the thirteenth oscillation. It is thus seen that
a strong stimulus will give rise to a multiple series of re-
sponses in the case of Desmodium^ precisely as in that of
Biophytum.
It is now clear that there is no rigid line of demarcation
between multiple and automatic responses. An ordinarily
responding plant like Biophytum, which gives a single response
to single moderate stimulus, and multiple response to strong
stimulus, will, under very favourable circumstances, that is to
say, when it has absorbed an excess of energy from without,
become automatically responding ; and, conversely, the pro-
nouncedly automatic Desmodium will, under unfavourable
circumstances, that is to say, when the sum total of its latent
294 PLANT RESPONSE
energy has fallen below par, be reduced to the condition of
an ordinarily responding plant, giving single response to
single moderate stimulus, and multiple response to strong
stimulus.
Summary
On application of a strong stimulus, of whatever nature,
to a vegetable tissue, a multiple series of electromotive re-
sponses is produced. These multiple responses may also be
observed by the electrotactile method.
These multiple excitations, in consequence of a single
strong stimulus, may be observed in BiopJiytuin as multiple
mechanical responses.
These multiple responses may be uniform in character, or
may exhibit cyclic variations, similar to those observed in the
rhythmic pulsations of animal tissues.
As, in the case of a plant-tissue, a strong stimulus causes
multiple excitations, so, in the retina, strong stimulus of
light causes multiple visual excitations, seen in recurrent after-
images, which have the same characteristics as the multiple
after-effects of stimulation in Biophytuni.
There is no strict line of demarcation between the
phenomena of multiple and of automatic response. Under
very favourable circumstances — that is to say, when it has
absorbed an excess of energy from without— an ordinarily
responding plant like BiopJiytuin will become converted into
an apparently automatically responding plant, like Desinodium.
Conversely, under unfavourable circumstances — that is
to say, when the sum total of its energy is below par — an
automatically responding plant like Desinodium^ will become
converted into an ordinarily responding plant like BiopJiytuin.
Its leaflets then come to a state of standstill.
Desinodium leaflets in a state of standstill respond to
stimulus in exactly the same way as do those of BiopJiytuin.
To moderate stimulus, both give single response ; the polar
effects of currents in both are the same ; and strong stimula-
tion causes a multiple series of responses in both.
CHAPTER XXIV
AN INQUIRY INTO THE CAUSES OF AUTONOMOUS
MOVEMENTS
Production of pulsatory movements as after-effect of energy absorbed — Physical
analogue— Localisation of seat of automatic excitation in Desniodiiim —
Demonstration of multiple response to a constant stimulus: (i) Chemical —
(2) Electrical — (3) Stimulus of light — Multiple response to constant stimulus
of light, in : [a) retina — (<^) Biophytitni — [c) Desmodiuiii — (4) Thermal —
Induction of automatism in Biophytiiiii at favourable temperature — (5) Of
internal hydrostatic pressure — Absorption of external energy and its absorption
by the plant in latent form — True meaning of ' tonic' condition — Cause of
rhythmicity — After-effect, and its relative persistence.
In living tissues, both animal and vegetable, we find
numerous cases of spontaneous periodic movements, to
which no direct exciting cause is apparently assignable.
We confess our inability to trace out the exciting cause
by classing such phenomena as automatic. Among well-
known examples of automatic movements in animal tissues
may be cited the pulsatory action of the heart. In the
vegetable kingdom, also, such movements are very numerous,
and are of various degrees of rapidity, from quick pulsations
of some few seconds in duration, to others which occupy
periods of several hours It is to be remembered that these
spontaneous movements take place in plants under favourable
circumstances, i.e. under that totality of the optimum degrees
of light, temperature, turgidity, and so on, which is vaguely
referred to in vegetable physiology as the tonic condition. I
intend to set forth presently experimental considerations
which will, I hope, serve to make clear the precise significance
of this term.
As already said, to call such movements automatic is
296 PLANT RESPONSE
only one way of evading the difficulty of finding out their
actual cause. When we see these responsive indications
given by a moving leaf or leaflet, we can but feel it
necessary to trace them to the impulses, internal or external,
by which they must have been occasioned. How are such
periodic impulses caused, and where is their seat? The
investigations which follow are intended to throw light on
this, one of the most obscure problems in Physiology. In
previous chapters I have demonstrated the fact that the
antecedent cause of a periodic effect need not itself be
periodic. A stimulus may remain long latent in a tissue,
and this latent stimulus may subsequently give rise to
periodic excitations.
Such periodic expression of the absorbed energy is not
without analogy in the world of physics. For example, we
take a glass tube of moderate diameter and push into it, to a
certain distance from the free end, a piece of wire gauze,
which is then heated over a Bunsen burner. On removal of
the heating flame, we observe the after-effect of this absorbed
thermal energy, in pulsating movements of the air-column,
which give rise to a musical note, whose pitch is determined
by their periodicity. We have here a physical analogue to
a case previously described, in which the thermal energy
absorbed by a tissue oi Biopliytmn was afterwards manifested
in long-continued periodic movements of the leaflets. In the
experiment with the glass tube, the periodicity of aerial
pulsation is determined by the size, shape, and temperature of
the pulsating column. Similarly, the periodicity of multiple
response in the Biophytinn leaflet is determined by the various
constants of the cell-complex which is the seat of the movement.
We saw in Biophytum that the region to which strong
external stimulus was applied, became the place in which it
was accumulated, and the source of subsequent excitation.
We further saw that there is no rigid line of demarcation
between plants which exhibit multiple responses and those
which show autonomous movements, the same plant passing
from one category to the other, according to circumstances.
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 297
Thus Biophytuni, which normally speaking gives ordinary
single or multiple response, may under favourable circum-
stances exhibit automatic response. On the other hand,
Dcsiitodiuiii, which normally speaking exhibits automatic
response, will under unfavourable circumstances become
converted into an ordinarily responding plant.
As already stated, in the case of Biophytitm, the point of
application of stimulus becomes also the source of subse-
quent multiple excitations. An observer, unacquainted with
the position of this point, might succeed in determining it, by
watching the order of pulsation of the leaflets. For if we
suppose the stimulus to have been applied at some point in
the middle of the leaf, the observer will in that case notice
periodic waves of excitation proceeding in opposite directions,
and giving rise to the closure of successive leaflets from a
common point outwards, thus clearly indicating the position
of the point from which successive excitations are initiated.
Had the stem, on the other hand, been stimulated, and thus
become the source of excitation, these successive impulses
would have begun by arriving at the first pair of leaflets,
and thence would have passed through all the leaflets to
the tip of the leaf, in a centrifugal order. Again, had the
stimulus remained latent at the tip of a particular leaf, the
successive excitatory waves would then have proceeded
through the leaf in a centripetal order, and on reaching the
stem would have radiated outwards, or in a centrifugal
sequence once more, throughout the other leaves. There
are other ways also, presently to be described, by whose
means we can obtain an idea of the direction in which the
excitatory wave is travelling.
Localisation of seat of origin of autonomous excitation
in Desmodium. — Our next inquiry is into the very obscure
question of the point of origin of the so-called autonomous
excitation of Dcsmodiiiiii. I have already shown that, if the
plant absorbs a certain amount of energy in excess of that
required for immediate response, the surplus is stored up, to
be given out subsequently in the form of pulsating waves of
298 PLANT RESPONSE
excitatory disturbance. But the difificulty is to determine
the point at which the latent energy is stored up, and from
which the excitatory disturbances subsequently proceed. As
we are accustomed to think that stimulus must be due to
some sudden variation^ one is tempted to suppose, as the
simplest explanation, that in the case of autonomous move-
ments the stimulation is caused by variations in the rate of
absorption of food material, which is probably being carried
on, in an intermittent manner only, by the roots and leaves.
In this case, if the stimulus proceed from the root, we may
expect the excitation to travel to the motile leaflets through
the stem, outwards, or in a centrifugal direction. If the
assimilating leaves, however, be the source of excitation, we
shall look to see the wave of excitation proceeding from
without inwards, or centripetally.
In order, therefore, to localise the source of stimulation
in Desiiiodium gyrans, for example, I first tried to find
out in which direction the excitatory impulse proceeded.
Unfortunately in this case, the leaf being provided with only
a single pair of motile leaflets, it is impossible to obtain the
indication of direction which is given by Biophytwn, by the
successive closure of leaflets in a definite order. I was
therefore obliged to have recourse to other expedients.
I have already shown how the transmission of stimulus
can be arrested by local application of ether, or by the anodic
block. If the pulsating stimulus, therefore, should be central,
that is to say, proceeding from the stem, we should then
expect the application of ether, or the anodic block, at the
junction of the leaf with the stem, to arrest the movement of
the leaflets. If, on the other hand, the source of the stimula-
tion should be peripheral, a block produced in the manner
described, at a point between the terminal expanded leaflet
and the small motile leaflets, would prevent the response. of
the latter. In carrying out these experiments, however, I
found that no such arrest took place in either case.
The fact that the source of stimulation is not central, is
also made evident by the following consideration. Had it
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 299
been so, the responding movements would have been syn-
chronous vi^ith that of the hypothetical source, and all the
leaflets of the plant would in that case have had the same
period of vibration. As a matter of fact, however, different
leaflets have different periodicities.
That the source of stimulation, again, is neither central
nor peripheral, may be shown by applying two tight liga-
tures, one behind and the other in front of the motile leaflets.
The passage of the excitatory impulse, should this be
transmitted from either direction, will by this means be
completely arrested. On carrying out this experiment, how-
ever, the periodic pulsation was not affected.
Again, the leaf may be completely detached from the
plant, and the terminal leaflet amputated. But the periodic
pulsation still proceeds as before, just like the persistent beat
of the isolated heart of a frog.
Thus we find that the isolated motile leaflet continues to
manifest an evolution of energy in the form of pulsating
movements, which cannot be derived from either of the
hypothetical sources, whether central or peripheral. It
maintains this activity for a long time, when kept under
favourable conditions. It thus becomes clear that in the case
of Desinodiuin the power of maintaining rhythmic pulsation
is local, and resides in the tissue of the motile leaflets.
This does not exclude the possibility of other periodically
moving organs obtaining the pulsating excitatory impulses
from a distant point. Such a case may well happen when,
the conductivity of the intervening tissue being very great,
and the excitability of the motile organ high, stimulation,
though enfeebled by transmission through a long tract, yet
remains above that critical intensity which would cause
effective response.
In the previous chapter, I showed that if Desuiodiiiiii be
kept in the dark for a sufficiently long period — but not so
long as to produce permanent rigor — or if a specimen be
taken in the unfavourable season of the year, its autonomic
movements would be found to have come to a stop for the
300 PLANT RESPONSE
time. This was because the energy stored up in the tissue
had become exhausted. If, then, a single stimulus were
given, say, by condenser discharge, a single response would
be found to ensue. This showed that in such a case the
arrest of the automatic movements was not due to the
abolition of motility in the leaflet, but only to exhaustion of
the energy stored up, which would have given rise to oscilla-
tion. It was also shown that if Desuiodiuni in a state of
standstill were subjected to a single strong thermal stimulus,
it exhibited a multiple series of rhythmic responses.
Multiple response to constant stimulus. — Having
observed the production of multiple responses as an after-
effect of a single strong stimulus, I shall now proceed to
demonstrate the production of similar multiple responses by
the action of a constant excitatory condition.
1. Chemical. — I have shown that in Biophytiiiii, and in
Desmodium at standstill, rhythmic pulsations are produced
by the constant action of a chemical stimulant. Analogous
phenomena are also known in animal tissues ; in the isolated
heart in a state of standstill, rhythmic pulsation can be renewed
by chemical stimulation.
2. Electrical. — I have often observed that when a strong
current is sent through Averrhoa carainbola, a number of
rhythmic pulsations are found to take place. In animal
tissues, again, similar rhythmic excitations have been ob-
served, not only in cardiac muscle, whose rhythmicity is
so marked a characteristic, but also in skeletal and other
muscles.
3. Stimulus of light : {a) On retina. — And now before I
describe the experimental demonstration of periodic excita-
tion in plants, as caused by constant stimulus of light, I shall
refer in detail to certain remarkable periodic effects which I
have observed in the retina, under the action of the same
stimulus. I showed in the last chapter that strong stimulus
of light gives rise in the retina to multiple responses, in the
form of recurrent after-images. I shall now prove that during
the continuance of constant light, pulsatory visual effects are
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 3OI
produced. These pulsations are not usually noticed in visual
sensation, owing firstly to the absence of a standard of com-
parison, and secondly to the fact that though the impressions
on the individual retina; undergo variation, the sum total of the
two remains constant. I have been able to provide the neces-
sary comparison-standards by having two distinguishable
images produced in the two eyes, the fluctuation in the visual
excitation in one eye being thus capable of detection by com-
parison with that in the other. It would have been impossible
to detect this fluctuation had the excitatory variation taken
place in the two eyes simultaneously, i.e. if the maximum
excitation in the one had occurred at the same moment as the
maximum excitation in the other. But I have found that, as
regards excitation, there is a relative difference of phase, of
half a period, between the two eyes, so that the maximum effect
at the given moment in one eye corresponds to the minimum
in the other. It is owing to this fact that the periodic excita-
tions in each retina are brought out in an unmistakable
manner by the following experiment, which consists in look-
ing at two slits through the modified stereoscope described in
the previous chapter. One of the two slits inclines to the
right and the other to the left, and on looking through these
at a bright sky, the right eye perceives a bar of light turned,
say to the right, and the left eye a bar turned to the left,
the resultant impression being that of an inclined cros.s.
When the stereoscope is turned to a bright sky, and the
cross looked at steadily for some time, it will be found, owino-
to pulsatory excitation in each eye, that when one arm of
the cross begins to be dim, the other becomes bright, and
vice versa. These periodic fluctuations are perceived con-
tinuously under the constant action of light. ^
I shall now proceed to the demonstration of the very
interesting rhythmic movements caused in plants under
constant light-stimulus. As we wish to prove that the
cause of automatic movements lies in the action of some
' The experiment will be found described in detail in my book, Kespoiise in
the Living and Non-Living, p. 175.
302 PLANT RESPONSE
continuous stimulation, it is necessary to satisfy ourselves
that this continuous source is the sole cause of the rhythmic
movements. This will appear conclusive, if it is made clear
that the plant does not possess any intrinsic energy of self-
movement, but only the power to regulate rh3'thmically the
overflow of energy supplied.
The demonstration will then be made rigorous, if we take
a plant which is not manifesting automatic movement, and
cause it to exhibit such action by the application of constant
external stimulus. For the purpose of this experiment, then,
we may take the leaflet of ■Desmodiuin in a state of standstill,
or that of Biophytuni under normal conditions, which, as we
have seen, may be regarded as practically equivalent to
Desmodiuin in a standstill condition. If therefore we can
succeed in making Biophytum exhibit rhythmic movements
under the continuous action of a given external energy, we
shall not need to look for the explanation of the so-called
automatic movements of Desmodiuin to some periodically
varying stimulus, any constant stimulus being then proved
fully competent to produce the rhythmic pulsations.
{b) On Biophytum. — I first subjected the motile leaflet of
Biophytum to light of comparatively short duration, i.e. two
minutes, by throwing sunlight upon it from a reflecting
mirror. There was no immediate response, but after a latent
period of one minute the leaflet gave a single response. I
next subjected the plant to the continuous action of light
during ten minutes. I obtained rhythmic pulsations, there
being two responses in the given time. I then cut off the
light, and the rhythmic action was stopped. After an
interval of several minutes I again applied light for ten
minutes, and this gave rise to similar rhythmic responses.
The procedure was repeated once more with the same results.
With another specimen, I applied light for a still longer
period, i.e. twelve minutes, and I obtained three pulsations.
It will thus be seen that we have here a regulated outflow of
energy ; the plant absorbs energy continuously, but gives it
out in a pulsating manner (fig. 123). In this way, a number
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 303
of autonomic pulsations may be obtained in Biophytum under
the continuous stimulation of light. The recovery not being
complete after each pulse, the result is a progressive folding
downwards of the leaflets, and this makes it impossible after
a time to observe further responses. I shall presently describe,
however, another method of supplying the plant with energy,
by thermal means, in which case
the autonomous pulsation in Bio-
phytum is prolonged indefinitely.
{c) On Desmodiuni leaflet at
standstill. — Turning now to the
leaflet of Desmodiuni in a state of
standstill, I find that application
of sunlight initiates rhythmic move-
ments. In a particular experiment,
h"ght was continuously applied for
half an hour, and there were pro-
duced in that time seven vibrations.
The responses were found to under-
go 'staircase' increase by increas-
ing absorption of energy, and the
resultant increase of molecular
mobility produced in the tissue. On the stoppage of light
the rhythmic pulsations persisted for some time, the ampli-
tude, however, undergoing diminution owing to the run-down
of absorbed energy.
4. Thermal : (a) Induction of autonomous response
in Biophytum. — I shall now describe an experiment of very
great theoretical importance, by which I have able to convert
a specimen of Biophytum from an ordinarily into an auto-
matically responding plant. Guided by the theoretical in-
ference which I have already stated, that it is an excess of
energy that brings about the condition of automatism, I
subjected a specimen of Biophytum to the constant stimulus
of a moderately high temperature. I first placed the speci-
men in a chamber whose temperature was 37° C, and it was
astonishing to see the younger (and therefore more excitable)
Fig. 123. Multiple Response
of Biophytum under the
Continuous Action of Light
304
PLANT RESPONSE
leaflets, at first quiescent, break into a sustained series of
uninterrupted pulsatory movements, which they kept up
throughout the maintenance of the condition of high tempera-
ture. It was interesting also to observe the quickening of
vibration by absorption of energy. The pulses were at first
slow, each having a period of four minutes, but they became
steadily more rapid, till they had reached a frequency of two
vibrations in a minute. A continuous record of these periods
was made during one hour, and the following tabular state-
ment exhibits the results :
Table showing Periods of Successive Pulsations in Biophytum
WHEN Temperature is Raised to 37° C.
No. of
pulse
Period
No. of
pulse
Period
No. of
pulse
Period
I
4 minutes
II
2-5 minutes
21
■5 minute
2
4
12
2-5 >,
22
3
4
13
. 2-5 „
23
! 4
4 >>
14
2-5 „
24
5
4 ,.
15
2
25
6
3
16
I '5 minute
26
7
3
17
I -5 „
27
8
3 >>
,8
I
28
9
3
19
I »
29
10
2-5 „
20
•5 „
30
Having thus found that a high temperature was favour-
able to the initiation of automatic movements in BiopJiytum,
I was next desirous of determining the minimum temperature
at which such responses could be induced. For this purpose
I took a fresh specimen of Biop/iytmn, and cautiously raised
the temperature of the chamber from 23° C. upwards. When
29° C. had been reached, I obtained the first pulse of auto-
nomous response, the period being rather slow — that is to say,
8'5 minutes — and by the time the temperature had gone up to
35° C. this period had become shortened to two minutes. We
find here a phenomenon identical with that of Desuiodium,
where the frequency of vibration is found to be increased with
rising temperature.
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 305
In Desniodiiiiii, the autonomous movement is initiated at
a certain more or less definite temperature, which is about
17° C. This we may call the critical thermo-tonic condition.
Below this critical degree, Desmodinm ceases to be autonomic,
and becomes an ordinarily responding plant. In BiopJiytuiii,
similarly, the critical thermo-tonic point is about 29° C.
Above this, the young leaflets are autonomic, and below it.
ordinarily responding. The difference between Desmodinm
and Biophytum in this respect lies, therefore, in the fact that
their critical thermo-tonic points are about twelve degrees
apart.
In the case of Biophytum, when the temperature is main-
tained at a uniform favourable degree, the periods of the
Fig. 124. Induction of Autonomous Response in Biophytum,
at Moderately High Temperature of 35^ C.
Note the diminution of amplitude of response with the gradual loss of
latent energy, consequent on falling temperature. The pulsations-
came to a stop below 29° C.
autonomous pulsations become veiy regular. It has been
said that these pulsatory movements are maintained by
means of energy absorbed, and with Biophytum I found an
added opportunity of demonstrating this fact. A particular
plant had been kept at a uniform temperature of 35° C,
under which the young leaflets gave autonomous responses.
The heating current, by which this temperature was main-
tained uniform, was now stopped, and the chamber gradually
X
306 PLANT RESPONSE
cooled down. One of the young leaflets had been attached
to the Optical Lever, and records were now taken con-
tinuously. It was very interesting to observe how, with the
gradual loss of absorbed energy, the pulsating movements of
the plant became diminished in amplitude (fig. 124) till they
came to a stop. We shall find exactly the same thing in the
case of DesuiodiHin.
I referred in the last chapter to certain observations of
my own, in which the leaflets of BiopJiytuni were found, under
favourable circumstances of light and temperature, to give
rise to apparently spontaneous movements, which could not
be traced to any definite varying external cause. From the
experiment just described, however, we see that it was the
continuous stimulus of favourable temperature and light com-
bined that caused this rhythmic movement, which appeared
as automatic.
ib) Resumption of automatic movements in Desmo-
dium. — Similarly in /}^jr;//c^/>/;//,if the isolated leaflet in which
all movement has been brought to a stop, but not to a condi-
tion of permanent rigor, be raised in temperature, and if the
tissue be maintained uniformly at this higher point, the
pulsatory movement will be found to commence and persist
for a long time. The initiation and maintenance of the
responses here, again, is undoubtedly due to the renewed
supply of energy.
A new difficulty now arises, however, from our habit
of regarding stimulation as dependent upon some sudden
variation of external conditions. For we are here confronted
with a case in which uniform and continuous application of
heat produces stimulation. It may be urged that there has
been some variation of environmental conditions, in the fact
of the rise of temperature before the constant point was
reached. But it has to be remembered, that this rise was
purposely made very gradual, and even if a single stimulation
had been caused by this preliminary thermal variation, it
would have evoked only a single response, or at most a few
multiple responses But h^w are we to account for these
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 307
long-continued periodic pulsations, which are kept up during
the whole time in which the leaflets are maintained at an
unvarying given temperature ?
We are thus led to see that stimulation is not in all cases
dependent upon the occurrence of a sudden change in the
exciting cause. On the contrary, excitation may be pro-
duced, as we have seen, under a constant and uniform
exciting condition. If we analyse the multiple responses
produced in BiopJiytuin as the after-effect of the application
of a strong thermal stimulus, we find, as said before, that
some part of this thermal energy remains latent, and after-
wards gives rise to recurring pulsations.
5. Of internal hydrostatic pressure. — The isolated animal
heart, when in a state of standstill, is found to renew its
excitatory pulsation under an increase of internal hydrostatic
pressure. I shall show later (p. 349) that a Desmodiuin
leaflet, similarly, when in a state of standstill, can be made
to resume its excitatory rhythmic activity, by increasing the
internal hydrostatic pressure.
The true meaning of ' tonic ' condition. — We have thus
seen that under the continuous action of a constant source of
external stimulus, multiple responses are produced. We have
also seen that the excess of energy absorbed remains latent
in the tissue, in consequence of which, even on the cessation
of external stimulation, the pulsatory movements are main-
tained for a longer or shorter time. It is thus the excess of
stimulus absorbed which renders the tissue excitable, or
' tonic' Hence we may have tonicity imparted by light,
photo-tonus ; by favourable temperature, thenuo-tonus ; by
electric current, electro-tonus ; by internal hydrostatic pres-
sure, hydro-tonus ; or by the presence of favourable chemical
substances, chevio-tojius. We have seen that each one of
these, by itself, was competent to give rise to multiple
responses. It has been shown further that there is no hard-
and-fast line between such multiple and automatic responses,
the one passing imperceptibly into the other.
We have now seen that the tonic condition of the plant is
X 2
308 PLANT RESPONSE
determined by the sum total of the latent energy derived
from all the above-named factors. This internal factor,
of latent energy, will be shown to play a very important
part in all response-phenomena, and as it thus becomes
necessar}' to have some convenient means of referring to it,
we shall henceforth designate it as the Internal Energy of
the plant.
Automatic movements in plants are thus only exhibited
under favourable tonic conditions. It has been shown that
the plant displays rhythmic activity when subjected to different
forms of constant stimulus, and we have now investigated
separately the effects of such constant stimuli — chemical,
electrical, thermal, photic, and hydrostatic. As has been
explained before, if a given stimulus be not of sufficient
intensity to evoke visible response, yet the absorbed energy
may render the tissue capable of responding to another
subsequent stimulus, which by itself would have been in-
effectual ; in other words, the tissue is made excitable by the
presence of a stimulus which has not of itself been adequate
to cause response. When this latent excitability exceeds a
certain amount, any further increase may be expressed in a
visible manner by mechanical pulsation. Now, taking the
various forms of stimulus to which the plant is constantly
exposed — namely, warmth, light, moisture, and the action of
the various chemical reagents, organic and inorganic, present
in it or absorbed by it — it is clear that each of these exerts its
stimulating effect independently ; any one by itself may then
make the tissue excitable to the verge of response ; in this
condition, though there is no outward sign of the fact, there
is a considerable amount of latent energy ; and the incidence
of any second and additional form of stimulation is now
sufficient to precipitate the excitation in visible form. These
considerations will show how, by the cumulative and additive
effects of all the forms of constant stimulation mentioned
above, the plant may become so highly excitable as to
manifest the fact by giving rise to responses which appear as
automatic. The tonic condition is thus the latent excitatory
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 309
condition, which is determined by the sum total of all these
exciting factors.
In connection with these facts, it is well to bear in mind
that the excitability imparted by a stimulus does not always
increase continuously with the intensity. On the contrary,
there may be an optimum intensity beyond which excitability
may be diminished.
We have now seen that the energy which expresses itself
in pulsatory movements may be derived by the plant, either
directly from immediate external sources ; or from the excess
of such energy, already accumulated and held latent in the
tissue, aided by the incidence of external stimulation ; or from
an excessive accumulation of such latent energy alone. In
the last case, however, if the plant were kept isolated from all
external supply of energy, it is clear that its reserve would
become exhausted, and its automatic movements would cease.
I have already described an experiment in which this arrest
of pulsation took place in the case of a specimen of Desuiodiuui
kept in a dark room. We then saw that revival of response
was only brought about when fresh stimulus was applied.
And we have also seen the converse, namely, the ordinarily
responding Biophytum, when supplied with excess of energy,
become automatically responding.
Cause of rhythmicity. — Having, then, seen that it is a
constant source of energy, external or internal — the latter
being really derived from a previous absorption of external
stimulus — which maintains the so-called automatic movements
in plants, we have still to determine how it is that a latent
or constantly acting external stimulus can find only periodic
expression ? In connection with this we have seen how,
when the minimal factor of stimulating intensity is exceeded,
there is a manifestation of the fact by visible response. I
have also shown (p. 245) that after each excitatory discharge
there is a marked diminution of both conductivity and
excitability ; and a new stimulus, or existing excess of
stimulus, owing to the loss of these properties, is retarded
for a time from producing any effect on the motile tissue. It
3IO PLANT RESPONSE
is only after the lapse of an interval that the protoplasm
regains its original properties. There is thus an oscillatory
variation of conductivity and excitability. It will therefore
be seen how, under the circumstances, a constant stimulus,
or a stimulus which is latent in the tissue, can find an excitatory
expression only in a pulsating manner. Perhaps a physical
model will enable us to visualise this process. Imagine a
reservoir into which flows a constant supply of water. An
elastic conducting pipe is led from the reservoir, and this pipe
is constricted by a compressing spring. On the far open end
of this pipe abuts the flattened end of an indicating lever.
When water has been supplied for some time, its level is
gradually raised, producing an increasing pressure. At a
certain point, when the pressure becomes sufficiently great,
the spring which keeps the elastic tube constricted gives way,
and there is an impulsive discharge of water, which, impinging
against the lever, gives rise to a visible response. But the
yielding spring again closes, the tube is once more constricted,
and thus by the oscillation of the spring which regulates the
conduction of water, pulsating hydraulic impulses are kept
up. On account of the oscillating mechanism, the outflow,
and consequent mechanical response, are periodic, though the
supply is constant. In this model the first period, before the
pressure of water becomes sufficient to force open the spring,
corresponds to the latent period in plant response ; the
oscillation of the flow of water corresponds to the oscillation
of conductivity ; and the responding lever corresponds to the
motile leaflet. Similarly, an ill-fitting spring tap is thus often
thrown into a pulsating movement, by the constant pressure
of water from the main, and there is then seen a rhythmic
play of the water-jet.
'After-effect' and its relative persistence. — In the ex-
periment on Biophytum under the continued action of light, we
saw that for a period of one minute, during which the plant was
absorbing light, there- was no response. Then after this latent
period of one minute, the energy absorbed reached the verge
of response. The excitatory discharge was followed by a
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 3 II
retardation of conductivity and excitability, which was, how-
ever, gradually recovered, and there were produced a second
and then successive periodic responsive movements.
In the hydraulic model, if the capacity of the reservoir be
small, the cessation of the water-supply will cause an immediate
cessation of the rhythmic movement of the indicating lever.
In BiopJiytum, similarly, we found that the periodic response
continued as long as energy was supplied, and that the
movement soon stopped on the stoppage of the supply of
external energy. But if the capacity of the reservoir be great,
the accumulation may be sufficient to maintain the oscillation
for a considerable length of time, even after the main supply
is cut off. And we see that in Desnwdium the responsive
movements continue in a persistent manner, though the
immediate source of stimulation be interrupted. A similar
difference in the persistence of after-effects, depending on
relative capacities for storage of energy, is seen in the two
classes of inorganic substances, which are distinguished as
fluorescent and phosphorescent. In the first, the responsive
emission of light caused by a preceding excitation is
extremely short-lived ; but in the latter it continues long
after the light stimulus has ceased to act. Thus Desmodium
may continue to exhibit rhythmic movements, though not at
the moment exposed to the marked action of any special
source of stimulus. But for the display of long-continued
rhythmic movements it should previously have absorbed a
considerable amount of energy from an external source — in
other words, it must have been exposed to those circum-
stances which produce a favourable tonic condition.
We have thus obtained some insight into that very ob-
scure phenomenon which is known as the after-effect. By
the inertia of the organism there is a certain loss of time
before response begins to take place, and this determines the
latent period. But when the stimulus has already initiated
movement, the responding organ will, through the same inertia,
continue to show this movement even when the stimulus has
ceased to act. There is another factor, however, which
312 PLANT RESPONSE
determines the persistence of this after-effect, namely, the
larger or smaller capacity of the tissue itself to hold energy
latent within it.
In the attempt to investigate the cause of automatic
movements in Desniodhnn, there were three points of inquiry
which had to be determined. First, there was the question
of the seat of excitation; second, that of the nature of the
stimulus which maintained the rhythmic pulsation ; and third,
the determination of the process by which constant stimulus
found periodic expression.
I have shown that the seat of excitation in Desiiwdinni is
neither central nor peripheral, but is in, or in immediate con-
tiguity with, the motile tissue itself, which resembles in this
respect the animal heart, the seat of excitation there also
being in the cardiac tissue itself (Chapter XXVI.).
I have also shown that as the cause of excitation it is
not necessary to have any sudden variation. A constant
stimulus, of whatever nature, is found efficient to produce
excitation.
I have also demonstrated that periodic pulsation is pro-
duced in Desjiiodiinn at standstill, by a constant thermal stimu-
lus ; also that periodic pulsations are produced in BiopJiytuni
and in Desuiodiuin by the constant action of a chemical
stimulant. I have shown further that rhythmic excitation
was produced in Averrhoa carambola by the passage of a
constant electric current. And, finally, I have shown that,
just as in the retina, under constant stimulus of light we have
periodic visual excitations, so also in BiopJiytuni, under con-
stant stimulus of light we obtain periodic excitations which
give rise to rhythmic movements. In the animal tissue,
similar multiple rhythmic responses are met with under
constant stimulus.
Since we have found it to be a fundamental characteristic
of the tissue of a rhythmically-responding plant like Bio-
pJiytuni or Desniodiuni to give a response which cannot be
increased by any excess in the stimulus-intensity (maximal
response or none), we should expect that the excess over the
INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 313
minimally effective stimulus must remain latent in the tissue.
This is evidenced by the fact that on applying a very strong
stimulus we obtain, not a single but multiple responses.
Thus all the various forms of constant stimulus to which it is
exposed — warmth, light, moisture, and the different chemical
reagents, organic and inorganic, present in or absorbed by
"it — become latent, and the sum total of all these stimulating
factors determines its ' tonic ' condition.
When this accumulated latent energy exceeds a certain
value, it is visibly manifested in the form of the so-called
' automatic ' movements.
The periodicity of the excitatory discharges which give
rise to rhythmic movements in a plant that is under constant
latent stimulus, is brought about by the peculiarity which
has been demonstrated, that after each discharge the con-
ductivity and excitability of the tissue are diminished, and
are only gradually regained. This oscillation in the conduc-
tivity regulates the outflow of energy, and causes the
rhythmicity of the responsive movements.
Summary
In Desniodium, the seat of excitation of the lateral leaflets
lies in the motile organ itself.
Multiple response is produced in the plant, as in the ani-
mal, under constant thermal, chemical, or electrical stimulus.
The retina, under constant stimulus of light, exhibits
periodic visual pulsations.
Similarly, the leaflet of BiopJiytiiin, and that of Desinodiiuii
at standstill, under continuous stimulation of light, exhibit
rhythmic mechanical pulsations.
Biophytum, when raised to a temperature of about 29° C,
becomes automatically responding. Under these circum-
stances, the only difference between the so-called automatism
of BiopJiytuvi and Desiiiodiinn is that, in the latter case, the
critical thermo-tonic condition is arrived at about 12° C.
earlier.
3H PLANT RESPONSE
The energy which expresses itself in pulsatory movements
may be derived by the plant, either directly from immediate
external sources, or from the excess of such energy, already
accumulated and held latent in the tissue, aided by the
incidence of external stimulation, or from an excessive
accumulation of such latent energy alone.
By ' tonic ' condition is meant the latent excitatory con-
dition of the plant, as determined by the sum total of the
stimulating factors which are, or have been, derived from its
environment. In other words, the tonic condition depends
on the internal energy of the plant.
In rhythmic tissues, a constant stimulus, external or
internal, finds pulsatory expression in consequence of the
oscillatory variation of conductivity and excitability.
The duration of rhythmic movements, in the absence of
any external exciting cause, depends on the amount of
energy previously absorbed and held latent in the plant.
The persistence of this after-effect, therefore, depends also on
the greater or less capacity of the tissue for storage of energy.
These rhythmic movements thus appear to be automatic, but
when the reserve is exhausted, the so-called automatic move-
ments come to a stop. Renewal of pulsatory movements can
then take place only on the supply of fresh energy from
without.
CHAPTER XXV
INFLUENCE OF VARIOUS CHEMICAL REAGENTS ON THE
AUTONOMOUS RESPONSE OF DESMODIUM GYRANS
The recorder and experimental chamber— Absolute measurement of period and
amplitude of Z'^j-wo^f/z/w-oscillation— Responsive significance of up and down
movements deduced from (a) analogy with response of Mimosa ; (6) test of
increased internal hydrostatic pressure—' Systolic ' contraction and ' diastolic '
expansion of Destnodiiim pulvinus— Mode of application of chemical reagents
—Action of chemical reagents modified by : tonic condition of plants ; strength
of solution ; and duration of application— Effect of anesthetics- Effect of
alcohol— Effect of carbonic acid— Effects of ammonia and of carbon disul-
phide — Effect of copper sulphate solution, either when applied externally,
direct on the pulvinus, or internally — Spark-record of Z)t'5W(7f//V/w-pulsation.
Having thus, in the last chapter, traced the causes of
autonomous movements in plants, I shall now, taking Desnw-
diian gyrans as the type, describe the effect of various
agencies on the so-called ' spontaneous ' responses of its
lateral leaflets.
With regard to the experimental arrangements for
making the record, I have already in Chapter I. described
how records may be obtained, using the intact plant for
experiment. The automatic movements of the leaflets, how-
ever, persist, even after the petiole bearing them is detached,
the cut end being kept in water. Under proper conditions,
the rhythmic pulsations of the detached specimen will continue
for a couple of days. In order, therefore, to subject the motile
organ to various modifying conditions, it is much more con-
venient to use such a specimen than the whole plant.
The recorder and experimental chamber. —xA.s the
extent of the movement of the tip of the leaflet is consider-
able, no magnification is necessary for the record. A single
cocoon thread is attached to the middle of the leaflet by a
3i6
PLANT RESPONSE
drop of shellac varnish, the other end of the thread being tied
to the longer arm of the Optic Lever, which is 30 cm. long.
The distance of the recording drum from the mirror is also
30 cm. The length of the lever arm being thus equal to the
distance of the drum, and the extent of the angular move-
ment being by reflection doubled, it will be seen that the
movement is magnified twice. The thread is attached,
however, to the middle of the leaflet, and [the record there-
FlG. 125. Experimt'iUal Apparatus for Making Records of Pulsation
of Dcsinoditini
Leaflet, P, mounted in u-tube in pJant chamber, and attached to long arm
of Optical Lever, L ; M, mirror attached to fulcrum-rod ; D, recording
drum ; I, o, inlet and outlet pipes for gases and vapours introduced
into plant chamber ; C, electric heating coil.
fore gives the movement of the tip unmagnified. The records
given in some of the figures are thus without magnification.
In others, again, the records are on a reduced scale.
It will be understood here that the extent of movement
will vary with the length of the leaflet, some of these being
very small, and others relatively large.
In the apparatus shown in fig. 125, the smaller chamber
contains the motile leaflets mounted in a tube filled with
water. By means of an inlet-pipe, different gases may be
CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 317
introduced into the chamber for any desired length of time,
after which fresh air may be re-introduced. In order to
produce variations of temperature there is a coil of wire for
electric heating.
The automatic movements of the leaflet, both up and
down, take place in some cases as a number of jerks, which
may pass gradually into continuous movement. In others
they are continuous from the beginning. From the normal
or highest position, the leaflet, generally speaking, sinks some-
what rapidly. Having reached its maximum depressed position,
there is a pause, after which there is rather a slow return to
its original position. The up and down motion is in some
cases approximately straight. In other cases, the sub-petiole
of the leaflet is slightly twisted after its descent, and the
curve described becomes more circular. For the purpose of
the present investigation, specimens were selected in which
the movement of the leaflet took place gradually, without
jerks, the up and down move-
ments being approximately in a
straight line.
Absolute measurement of
period and amplitude of Des-
modium - oscillation. — The
period of a single oscillation
varies with the temperature and
the tonic condition of the plant.
In winter it may be as long as
five, in summer as short as two
minutes, and when the tempera-
ture is artificially raised, the
period may be even further re-
duced to one minute. As a con-
crete example, affording a clear
idea of the general characteristics
of the pulsatory movement of Desinodiuni, I shall give
the following results, obtained from a photographic record
which gives the extent of the absolute movement (fig. 126).
Fig. 126. Photographic Record of
Pulsation of Desmodium
The up movement of the record
means down movement of the
leaflet in this and all subsequent
records.
i8
PLANT RESPONSE
^ \A
ft i\ ft fi !|
The period of the complete vibration was in this case
3"5 minutes, of which the down movement was accom-
pHshed in the course of I'S minute. The up movement
was relatively slower, and was accomplished in two minutes.
The mean amplitude of pulsation — that is to say, the
vertical distance travelled by the tip of the leaf — was
25 mm. The fact that the down movement is, generally
speaking, relatively the quicker is seen visually demonstrated
in the photographic record of uniform pulsations obtained
with another specimen
(fig. 127). In connection
with this, certain pecu-
liarities of photographic
action should be borne
in mind. It is found
that a short exposure
gives an image which
in the case of a line is
very thin and sometimes
consists of only the
faintest impression; but
when the exposure is
prolonged the line is
much thickened. Hence,
in the records, the faitit or more sharply outlined portions
indicate responsive down movements which were relatively
rapid. In fig. 127, therefore, these differences of line afford
us a graphic representation of the various rates of movement,
and durations of pause, in the different parts of the curve. In
all the photographic records given here and elsewhere, we
are thus able to distinguish the down mov^ements by the
relative thinness of the recording line. In fig. 134, at the
end of this chapter, will be found a spark-record of the
pulsation of Desmodiuin in which the different rates of move-
ment at different stages of the response can be distinguished
at a glance.
In the multiple responses o'i BiopJiytuiii^ though, generally
i y y y ii y i y y ^
Fk
>7. Photographic Record of Uniform
Pulsations in Desmodiiim
Period of each'pulsation = 27 minutes.
CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 319
speaking, the down movement is more rapid, yet at times the
up and down movements approximate to each other in this
respect, I have occasionally found similar instances in the
pulsatory movement of Desuiodiiim, when there would be
only a slight difference between the rates of the up and down
movements. The normal rates again may even become
reversed under the influence of external agencies.
Responsive significance of up and down movements. —
A question of some difficulty and importance arises here as
to the significance, whether of contraction or relaxation, of
these up and down movements. VVe saw in the cases of
Mimosa and Biophytwu that the down movement was due
to the relatively greater contraction of the lower half of the
pulvinus, and the up movement to recovery or relaxation.
It must be borne in mind that the upper half of the pulvinus
of Mimosa, like the lower, is also excitable, though in a minor
degree, as will be shown in a subsequent chapter. We have
further seen that the more excitable half responds earlier to
stimulus. The depression of the leaf, then, is due to the
earlier and greater contraction of the lower half of the organ ;
and its subsequent erection, to a natural expansive recovery,
possibly aided by the later and feebler contraction of the
upper half of the pulvinus. Arguing from analogy, we may
regard the movement of the Desmodimn leaflet as essentially
similar to this. For here too we find, generally speaking,
that the down movement is the more energetic, and the up
movement relatively the slower, of the two. Hence we
may infer that in Desiuodium the down movement is due
to contraction and the up movement to relaxation.
Test by increased internal hydrostatic pressure. — It
will be shown on page 349 that when Desmodiitui in a sub-
tonic condition undergoes arrest of pulsation, an increase of
internal hydrostatic pressure is found to renew the rhythmic
activity. With fairly high internal pressure, the frequency of
the pulsation is increased, though the amplitude is decreased.
In order now to test the responsive significance of the up and
down movements respectively, I tried the effect of an increase
320
PLANT RESPONSE
of internal hydrostatic pressure in shifting the vibration-Hmits
of the Desjnodhun leaflet.
It will be remembered that in the case of Mimosa the
effect of this increased internal hydrostatic pressure was to
cause the erection of the leaf above its normal position. If,
then, the same result should follow in the case of Desuwdiujti,
we should be justified in inferring that the mechanics of the
motile organ were similar in the
two cases. The increase of in-
ternal hydrostatic pressure was
in this case effected by the same
method as was employed in that
of Mimosa — that is to say, the
cut end of the petiole of Des-
modiiim was placed in one limb
of a U-tube, filled with water, and
the pressure was increased by
adding water, so as to raise the
level at the free end of the tube.
As the leaflet of Desmodium, how-
ever, is in constant oscillation,
we must regard the mean of its
vibration-limits as the normal
mean, and the extent of the
leaflet's erection under increase
of hydrostatic pressure must be
measured by the shifting upwards
of this mean.
In carrying out this experi-
ment, I found, on applying the
increase of internal pressure exerted by a water-column of
lo cm., that, as will be seen in fig. 128, the lower limit
of oscillation was displaced upwards in the record by
11-5 mm. and the upper limit by 3'5 mm. The mean
position was thus raised by 7-5 mm. As the magnification
of the record was in this particular case five times, it will
be seen that the pressure exerted by a height of 10 cm. of
Fig. 128. Displacement of Mean
Position of Vibration of Des-
7nodium Leaflet by Increased
Internal Hydrostatic Pressure
U, upper limit ; 1,, lower limit ;
M, mean position under normal
conditions ; u', l', m', corre-
sponding positions under in-
creased internal hydrostatic
pressure ; M m' is the extent
of displacement upwards. In-
creased internal pressure, gene-
rally speaking, produces a
movement towards expansion,
and tends to diminish the
amplitude of the pulse, wliile
increasing the frequency.
CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 32 1
water had thus produced an absolute displacement of the
mean position of the leaflet, of 1-5 mm. Considering these
facts, it becomes reasonable to regard the motile indications
of Dcsviodiuui as similar to those of Mimosa ; hence the
down position of the Desmodiuni leaflet may be regarded
as one of contraction, the up position being one of relaxation.
Thus in Desmodiuin the down position of the leaflet corre-
sponds to the systolic contraction, and the up position to the
diastolic expansion of the heart.
Mode of application of chemical reagents.— Having
now to some extent determined the character of the move-
ments of the Desinodiniii
leaflet, we shall proceed
to observe in detail the
modification of their
movements by the action
of various chemical re-
agents. Three different
methods of application
may be employed. In
the first place, the chemi-
cal reagent may be dis-
solved in the water in
which the specimen is
placed. The solution will
thus reach the m.otile
organ by the same process as that which brings about the
ascent of sap. The characteristic action of the chemical
reagent will be demonstrated in the subsequent modification
of the responses.
This method of applying a reagent at one point — in this
instance the cut end of the petiole — and observing the sub-
sequent physiological effect on the distant motile organ, is of
special interest and importance in the case of poisons. For
it serves to elucidate the obscure question of the ascent of
sap through tissues that have been killed by poison (p. 385).
In order to observe and compare the responses of the
Y
Fig. 129. Method of Application of
Chemical Agent to Cut End of Petiole
322 PLANT RESPONSE
given plant, before and after the application of the solution,
it is necessary that the continuity of the record should be in
no way disturbed. For this purpose I insert the specimen
in the arrangement shown in the diagram (fig. 129). One
end of the tube is connected with a funnel, F, by means of an
india-rubber tubing ; the other end is provided with a .stop-
cock. The tube is first filled with water, and the stop-cock,
S, closed. The normal responses of the leaflet are now taken,
with the petiole in water. Next, by proper manipulation of the
stop-cock, the water is allowed to run out, and its place is taken
by the chemical solution which is passed in at the funnel. The
record which is now taken exhibits the effect of the drug.
The second of our three methods of experiment is that of
direct application — that is to say, touching the motile organ
itself with a drop of the solution. In this case, the modifi-
cation of response takes place rapidly. And, lastly, gaseous
substances may be introduced or withdrawn from the plant
chamber, by means of suitable inlet and outlet pipes.
Action of chemical reagents modified by ; tonic con-
dition of tissue ; strength of solution ; duration of appli-
cation.— There are certain general considerations of a very
significant kind which it will be well to specify at this point.
Though the fundamental effect of any given reagent is
definite, yet certain minor variations of this effect, due to
the constitution of the individual plant, are liable to occur,
and these are often extremely suggestive. Two successive
experiments, for example, were performed to determine the
action of the poison mercuric chloride on different specimens ;
one of these was extremely vigorous, and the other the
reverse. The effect of the poison on the robust specimen
consisted in depressing the pulsation, and reached its maximum
some fifteen minutes after application. But subsequently the
plant appeared to shake off the influence of the poison, and
the pulses slowly recovered, till in the course of an hour they
had once more become normal. The effect on the weakly
specimen, however, was somewhat different. Instead of
depression, the immediate result of application was a transi-
CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 323
tory exaltation of the amplitude of oscillation, which was
doubled, though at the same time the rhythm became slowed.
After ten minutes, however, these pulsatory movements grew
irregular and depressed, and the plant succumbed to the
action of the toxic agent, its pulsation undergoing complete
arrest forty-five minutes after application.. In the case of a
weakly specimen, again, on filling the chamber with an
atmosphere of carbonic acid gas, the pulsations soon come to
a stop, and unless fresh air be quickly introduced, this arrest
becomes permanent. But with a vigorous specimen, the depres-
sion produced by this gas is very slow, and the permanent
arrest does not take place till after a considerable lapse of time.
The effect of an agent, again, depends on the strength
of the solution, and the duration of application. A solution
which, in larger quantities, would produce depression, will
often, if given in very diluted form, cause the exaltation of
response. The sudden introduction of an agent which may
ultimately produce arrest, may act as a transitory stimulus,
bringing about a preliminary augmentation of response, to be
followed later by depression and arrest. The application of a
deleterious substance, again, for a short time, will cause a
temporary depression, from which there is revival ; but too
long-continued action of the same reagent will cause
permanent arrest. Besides all these, there is the interesting
phenomenon of accommodation, by which the plant becomes
gradually accustomed to the action of any adverse circum-
stance, and is thus rendered immune.
Effect of anaesthetics. — Taking a specimen of Desi>io-
diiuii, I passed ether-vapour into the plant chamber. The
pulsatory movement which had hitherto been fairly uniform
now showed a transitory exaltation, and then fell with
remarkable regularity of decrement. The response next
showed an equally regular tendency towards the gradual
recovery of its previous amplitude, but with longer period,
this being protracted, from the normal two to four minutes.
Finally, the pulsation of this specimen was abolished, the
leaflet remaining in a position of relative relaxation, thirty
Y 2
324
PLANT RESPONSE
JliklvL
minutes after the first introduction of ether into the
plant chamber. On blowing off the ether-vapour, there
was in this case no revival
of response. In those cases,
however, in which the ether-
vapour is more diluted with
air, or applied for a shorter
period, the depression is
temporar)^ only, and is fol-
lowed by revival. But if
a larger quantity of ether-
vapour be at once introduced,
a permanent arrest, in the
relaxed position, quickly en-
sues. Before this happens,
there may be one or two
spasmodic flutterings, but
these quickly subside, as will
be seen in the photographic
record given (fig. 130).
Effect of alcohol. — The effect of this reagent is much
modified by the tonic condition of the specimen. For
example, in the case of a weaklier plant, the application,
even of dilute solutions, induces rapid diminution and arrest
of pulsation. More vigorous specimens can, however, with-
stand the deleterious effect of this drug, and bear stronger
doses. In the photographic record here given (fig. 131) a
5 per cent, solution is seen to induce a greater regularit)-
and amplitude of pulsation. The subsequent application of a
10 per cent, solution causes a moderate, and i 5 per cent, a still
greater, depression. The application of a 5 per cent, solution
to a weakly specimen, however, is found, as said before, to
induce a depression so great as to cause a speedy arrest.
Effect of carbonic acid. — The first effect of the intro-
duction of this gas is sometimes one of exaltation. This is
however, brief, and is followed by depression and sudden
permanent arrest of pulsation in weaklier specimens. In
Fig. 130. Photographic Record of
Effect of Ether Vapour, Large Uose
Arrow marks moment of appHcation.
Pulsation arrested in up, or relaxed,
position. It has to be remembered
that the up movement of the record
corresponds to a down movement
of the leaf.
CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 325
other cases, again, an earlier re-introduction of fresh air into
the chamber is sufficient to restore the specimen to its
Fig. 131. Photographic Record of Effect of Alcohol
Pulsations to left show normal response, f Marks application of 5 per
cent, solution ; j-' application of 10 per cent. ; |'' application of 15 per
cent.
natural pulsatory activity. I give here (fig. 132) a photo
graphic record of the effect of this gas on a vigorous speci
men, in which its action
is seen to be somewhat
gradual. One curious phe-
nomenon which I have no-
ticed in connection with
the effect of this gas, is
that when it remains stag-
nant in the chamber its
depressing effect is much
more rapid than when a
current is allowed to stream
through.
In the manner which I
have just described, I have
carried out further a number
of experiments on the action of various gases and vapours,
such as ammonia, carbon disulphide, and others, all of which
Fi(
132. Photographic Record of Effect
of Carbonic Acid Gas
Arrow marks moment of application.
326
PLANT RESPONSE
are found to cause an arrest of the rhythmic movements of
Desmodiuni. It is not necessary to go into these in detail,
the experiments already given affording sufficient informa-
tion for their successful repetition.
I will only mention here the very interesting and impor-
tant fact, that I find acids and alkalies, generally speaking,
to produce effects which are in a certain physiological sense
antagonistic. These effects, together with the influence of
temperature on rhythm, and the action of tetanising electric
shocks on the autonomous movements of Desmodiuin, will be
found fully described in Chapters XXVI. and XXVII., where
the remarkable parallelism of their influence on rhythmic
animal and vegetable tissues will be demonstrated. I shall con-
clude the present chapter by describing the action of a strongly
poisonous reagent on the pulsatory movements of Desuiodiuiu.
Effect of copper sulphate solution. — I carried out two
experiments on similar specimens to test the effect of
this reagent. In the
first, the solution was
applied directly on the
pulvinus, and produced
a very quick arrest of
its rhythmic activity
(fig. 133). In the
second case, the ap-
plication was made at
the cut end of the
petiole, as already de-
scribed, which was at a
distance of 2 cm. from
the pulvinus. In this
case the arrest took
place much later, that
is to say, thirty minutes after the application. This delay was
due to the fact that the poisonous solution had to ascend
the intervening distance before it could afl'ect the rhythmic
activity of the tissue at the pulvinus. This experiment will
Fig. 133. Photographic Record of Effect of
Copper Sulphate SoUition Applied on the
Pulvinus
Arrow marks moment of application.
CHEMICAL REAGENTS ON AUTONOMOUS PULSATION 327
be found important, as touching a later investigation on
the ascent of sap. It is to be noticed that the petiole
allows the poisonous solution which kills it to pass upwards
through it.
Spark-record of pulsation of Desmodium. — Before
ending this chapter, I shall give a spark-record of a single
pulsation in a leaflet of Desmodium. The successive sparks
were produced at intervals of
5 seconds, and a glance at
the record affords a visual
demonstration of the peculiar
characteristics of the move-
ment of the leaflet. The
up line as usual indicates
down movement. It is thus
seen that, after a pause in
the highest position, a sudden
excitatory impulse is de-
veloped, which is gradually
exhausted, as the lowest
position is reached. The up movement takes place more
gradually, and at a much slower rate. The results are shown
in the following table.
ta15le showing rates of movement at different stages ol
Pulsation in Desmodium.
Fig. 134. Spark-record of Single Pul-
sation in Leaflet of Desmodium
Interval between successive sparks
= 5 seconds.
Down movement
Up movement
Total period
j Average rate
Maximum rate .
Duration of pause
45 seconds
■61 mm. per sec.
7 >, „ „
40 seconds
Total period
Average rate
Maximum rate .
Duration of pause
70 seconds
mm. per sec.
35 seconds
Summary
The effect of a chemical reagent on a plant is modified to
some extent by the tonic condition of the tissue. A vigorous
plant will, generally speaking, withstand for a considerable
time the action of deleterious agents ; a weakly specimen
succumbs more quickly.
328 PLANT RESPONSE
The effect of a reagent depends also on the strength of
the solution. A reagent which, in strong solution, induces
depression, may, if given in small quantities, cause exalta-
tion.
The effect of a reagent depends also on the duration of
application. The temporary depression produced by a short
application is overcome by the self-accommodation of the
plant. But it will succumb to too long or too strong an
application of the same reagent.
The effects of the various reagents on autonomous response
are, generally speaking, similar to their effects on simple
response.
The depressed position of the leaflet of Desniodiuni
represents a ' systolic ' contraction, and the up position a
' diastolic ' relaxation, of the motile organ. Increased internal
hydrostatic pressure increases the extent of the relaxed or
' diastolic ' limit.
The effect of too strong an application of ether is to
abolish response, the arrest of pulsation usually taking place
in a relaxed position. The immediate effect of application is
generally a transient exaltation of response.
The effect of vapour of alcohol is usually a transient
exaltation. If the application be prolonged, the result is a
permanent arrest of pulsation.
Carbonic acid sometimes produces a transient exaltation
of response, and always a subsequent depression, which,
under the long-continued action of this gas, may pass into
permanent arrest.
Copper sulphate solution, when applied directly on the
pulvinus, quickly causes arrest of pulsation ; but if the cut
petiole be allowed to absorb the solution, the final arrest
does not take place till after the lapse of a certain period,
required for the solution to ascend to the motile organ.
CHAPTER XXVI
EFFECTS OF TEMPERATURE ON AUTONOMOUS RESPONSES
Increase of frequency and diminution of amplitude of pulsation with rising
temperature — Converse effect of fall of temperature — Similar effect in cardiac
pulsation — Effect of the reduction of temperature to the thermo-tonic minimum
— Explanation of diminution of amplitude of pulsation with rise of temperature
— Anomalous use of the word ' relaxation ' — Simple 7'ersiis additive character
of individual pulsation.
It is my intention in the course of the next chapter to
make a comprehensive review of the similarities in all their
characteristics of rhythmically responding tissues, both vege-
table and animal. In the present chapter, then, we shall
confine our attention to a study in detail of the influence
of temperature in modifying the amplitude and period of
rhythmic autonomous responses, exemplified in the case of
plant-tissues by Dcsmodimn and in that of the animal by
cardiac muscle.
As then we are about to study the effect of temperature
on the period and amplitude of vibration of the Desniodinni
leaflet, it is clear that our first difficulty must be the .securing
of a specimen in which both are, to begin with, more or
less uniform ; for the pulsation of Desmodium, like that of
the isolated frog's heart used for experiments, is often
irregular. It is thus only by careful selection that one can
obtain suitable experimental subjects. A moderate increase
of the internal hydrostatic pressure, however, will often have
the effect of rendering the responses sufficiently uniform.
Regulation of temperature. — The second difficulty in
this investigation lies in subjecting the plant to the required
rise or fall of temperature. A rise of temperature may be
secured by any one of three different methods, (i) A spirit
330 PLANT RESPONSE
flame may be applied underneath a bath of water in which
the leaflets are placed. The temperature is thus gradually
and continuously raised, and the successive pulsations,
corresponding to different temperatures, are recorded in the
usual manner by means of the Optic Lever. (2) Water
at the required temperature may be syphoned into the bath,
and the responses taken in the ordinary manner. (3) The
air chamber in which the specimen is placed may be subjected
to electric heating. The use of temperature may now be
very accurately regulated by adjustment of the current, and
records of pulsations may be taken at different and deter-
minate temperatures.
This last is the most perfect method, the two former,
dependent as they are on the immersion of the specimen in
a bath of water, having as compared with it many disadvan-
tages. For the natural freedom of movement of the leaflet
is hampered by the water, and more troublesome still is the
difficulty which at times arises from capillary action in the
partially immersed cocoon-thread, by which the leaflet is
attached to the Optic Lever outside the bath.
But we have not the same perfect facilities for lowering
temperature in a gradual and continuous manner, as for
raising it. This may be accomplished, however, sufficiently
well for our purposes: (i) by placing fragments of ice in
the air chamber ; or (2) the pulvinus of the leaflet may
be touched with cold water which has been reduced to
the required temperature by means of ice. I find, however,
(3) that a much better method is that of placing in the air
chamber a coil of thin-walled metallic tubing, preferably
of highly conducting copper. When cooled brine is made
to circulate through this coil, the temperature of the chamber
is lowered, and by regulation of the flow, by means of stop-
cocks, it is possible to produce an adjustment of cooling.
Effect of temperatures maximum and minimum. —
Autonomous vibrations come to a stop when the temperature
is sufficiently lowered. The temperature minimum at which
this occurs depends, as we should expect, on the nature of
TEMPERATURE AND AUTONOMOUS RESPONSES 33 1
the specimen — whether Desniodiuvi, Biophytum, or cardiac
muscle — and also, with similar specimens, on the tonic condi-
tion. With regard to the first of these points, we have seen
that the autonomous vibration of Biophytum comes to a
stop below 29° C. The pulsation of Desinodiiim is said to
be arrested at 22° C, but I find that this is a matter which
is much modified by the tonic condition of the particular
plant. With vigorous specimens I have seen that the vibra-
tion may persist even at so low a temperature as 17° C.
The thermo-tonic minimum in BiopJiytuni and Desiiiodiuvi
thus shows a difference, as already said, of about 12° C. ;
and in the case of the frog's heart this is still lower, and is
said to be about 0° C.
When, again, the specimens are raised to a maximum
temperature, the pulsations come to a stop. The tempera-
ture at which this takes place depends in part on the
condition of the specimen. For example, with the frog's
heart, this maximum is sometimes at a temperature so low
as 38° C. In other cases, pulsation may be detected even at
so high a temperature as 44° C. Similarly, in Desmodiinii,
I have found that the maximum temperature at which arrest
took place was sometimes as low as 35° C. ; but in certain
specimens it did not occur till 45° C. A plant may,
again, be accustomed gradually to high temperatures, and
under these circumstances the maximum may be raised as
much as 3° C. or 4° C. higher.
Effect of temperature on period and amplitude of
response. — The most marked phenomenon of effect of
temperature on automatic pulsations, whether animal or
vegetable, lies, however, in the fact that the period and
amplitude are both affected. When the temperature is
lowered, the amplitude of pulsation is enhanced, while the
frequency is diminished. Conversely, with rise of tempera-
ture, the amplitude is diminished, and frequency augmented.
This is true not only of the pulsations of the rhythmic
tissue oi Desinodium, but also of those of the animal heart.
This is seen in the two following records, where the first
332 PLANT RESPONSE
set of responses in each series gives pulsations at the
temperature of the room, while the second set in each gives
pulsations of greater amplitude and smaller frequency,
due to the lowering of temperature by several degrees
(figs. 135, 136). Conversely, as already said, when the
temperature is raised, the frequency is increased and the
amplitude decreased. This is seen in a general way in the
following photographic record (fig. 137), which I took with
Fig. 135. Photographic P.ecoixls of Autonomous Pulsations in Des-
;;/^<«'///w, showing Increase of Amplitude and Decrease of I'^requency,
with Lowering of Temperature
The pulsations to the left were taken at the ordinary temperature of the
room, 29° C. Those to the right were taken when the temperature
had been lowered to 25° C.
Desinodiniii, the temperature being continuously raised, from
30° C. to 39° C. It will be seen how progressive in character
is the diminution of amplitude and increase of frequency in
these responses.
In another set of experiments in which I took records
of the responses of Desmodimu at various definite ascending
temperatures, I found that at \(f C. the period of a single
oscillation was 43 minutes. At 22° C. this was reduced to
TEMPERATURE AND AUTONOMOUS RESPONSES
133
3-2 minutes, or nearly to two-thirds of the period at 19° C.
At 28° C. it was found to be again reduced to 2'i minutes,
or half But at 40° C. it
was only 1-4 minute, or
one-third. Thus, while
in 4"3 minutes, at 19° C.
there is only a single
beat, there are two beats
at 28° C. and three beats
at 40° C. in the same
time. I give below (fig.
138) a record of these
responses at various
temperatures. The re-
cord given afterwards
(fig- 1 39) shows how
similar is the effect of
temperature on the
amplitude and rhythm
of the pulsation of the
animal heart. When
the temperature of Dcsniodium is raised above 40° C. there
appears to be an arrest of pulsation. But this need not be
Flc. 136. Effect of Lowering of Tempera-
lure in Producing Increase of Amplitude
and Decrease of Frequency in Pulsation
of Frog's Heart
The pulsations to the left represent normal
pulsations at the temperature of the
room. Those to the right were taken
at a temperature several degrees lower
(after Brodie).
57. Photographic Record of Pupations of Dcsnwdiitiii during
Continuous Rise of Temperature
regarded as due to heat-rigor. For in magnified record.s
I have often noticed that here we may have very much
334
PLANT RESPONSE
quicker pulsations, but of an amplitude so small as usually
to pass undetected. It is only under prolonged exposure
to this relatively high temperature, or after exposure to a
temperature above 50° C, that true
heat-rigor sets in. This maximum
temperature varies in individual cases
with the tonic condition of the plant.
Effect of the reduction of
temperature to the thermo-tonic
minimum. — It has been said that,
generally speaking, the amplitude of
response 'increases with the lowering
of temperature. It is evident, how-
ever, that this process must have a
limit. For we know that pulsation
vanishes at the thermo-tonic mini-
mum ; before this reduction of ampli-
tude to zero it is clear that there must
be some point where the increase of
amplitude due to continuous cooling
must undergo reversal, or diminution.
And this is what we should theoreti-
cally expect, for since it is the absorbed
thermal energy that maintains the pulsation, it follows that,
when this is diminished below par, the vibrational energy
Fig. 138. Record of Pulsa-
tions of Desniodiitin at
Different Temperatures
7^vVAyiAlAj\j\^^
Fig. 139.
Record of Pulsations of Frog's Heart at Different
Temperatures (Pembrey and Phillips)
should also undergo diminution. We had an illustration ot
this (p. 305), when the automatically vibrating BiopJiytidii
TEMPERATURE AND AUTONOMOUS RESPONSES S3S
at 35° C. was allowed to descend to the thenno-tonic
minimum. It was in that case found that there was a regular
diminution of amplitude, and the pulsation afterwards dis-
appeared below 29° C. (fig. 124).
I was next desirous of determining whether this theoretical
inference could be verified in the case of Desmodiiim. For
this purpose I rapidly cooled the plant, by means of the
cooling coil, through which ice-cold brine was passed, a
Fig. 140. Effect of Cooling to Thermo-tonic Minimum on Pulsation
of Desinoditini
The first two pulses to the left were taken at the normal temperature oi
29"^ C. ; those to the right during continuous cooling. The first
response of the latter series occurred at 22° C. ; the second at 20° C. ;
and the third at 17° C. Note that while in normal responses there
are no sub-pulses, these are seen with increasing distinctness as the
pulsation becomes continuously slower. They first make their appear-
ance during the up movement of the first pulsation under cooling. In
the two successive pulsations they are seen more and more clearly in
the down as well as the up movements.
photographic record being taken of the pulsations all the
time. It will be noticed that the first effect of cooling was
the normal increase of amplitude and prolongation of period.
This latter— which at the temperature of the room had had
a value of three minutes — was now prolonged to nearl}- six
minutes. But the most interesting fact was that, as the
thermo-tonic minimum was approached, the amplitude was
reduced (fig. 140).
^^6 PLANT RESPONSE
In the next experiment cooling was produced more
suddenly, by application of cold water at about 4° C. to
the pulvinus. We observe in this case how the quick
reduction to the thermo-tonic minimum reduced the ampli-
tude till the pulsation had come to a stop. I then allowed
the leaf to return gradually to the temperature of the room,
and it is very interesting to note the effect of increasing
Fig. 141. Efl'ect of Rapid Cooling by Ice-cold Water
Normal pulsations recorded to the left. Effect or application of ice-cold
water is seen in the production of diminished amplitude and abolition
of pulsation. Gradual return to the temperature of the room revives
the pulsation in a staircase manner, the period remaining approximately
constant. Note that cooling, in this and previous figure, displaced the
pulsation in a downward or contracted direction. Gradual warming,
conversely, is seen in this figure to produce the opposite displacement
towards relaxation.
absorption of thermal energy from its surroundings. The
increased energy thus absorbed is seen to give rise to
increased amplitude of oscillation, in a staircase manner,
which gradually approaches the original pulsation (fig. 141).
In both these figures it will be noticed that cooling dis-
places the pulsation in a downward or contracted direction.
And in the last series of fig. 141 we see that the raising of the
TEMPERATURE AND AUTONOMOUS RESPONSES 337
temperature displaces it upwards or towards relaxation.
These facts are of great importance, and should be borne in
mind in reference to the explanation of the cause of variation
of amplitude and period, which I shall bring forward.
Explanation of diminution of amplitude of pulsation
with rise of temperature. — We have thus seen, as we should
theoretically have expected, that with the increased absorp-
tion of energy at a higher temperature, the amplitude of
vibration is also increased. How is it, then, that with the
still further increase of absorption of energy, at still higher
temperatures, the amplitude should undergo a diminution ?
When approaching the maximum point, where heat-rigor
takes place, we can understand that the excitability of the
tissue would be very much decreased, with a consequent
reduction of amplitude of pulsation. But at temperatures of
25° C. to 30° C. excitability of the tissue could not be
diminished. Indeed, I shall in Chapter XXXIII. adduce
considerations to show that it must, at that temperature, be
highly excitable, and we should have expected that this
would have produced, in addition to the increased energy, an
augmentation of vibrational amplitude. But, instead of this,
we obtain the curious result which has been described, of
a diminution, in the cases both of cardiac muscle and of
Dcsviodium.
It might be suggested that if increased activity, due to
rise of temperature, increased the frequency of vibration,
then this fact would be sufficient in itself to account for a
diminution of amplitude ; for in this case, less time being
allowed for each single vibration, its extent must be cur-
tailed. But this consideration alone would not explain all the
facts of the case ; for we have seen that on approaching
the thermo-tonic minimum, though the frequency of vibra-
tion is reduced, and the period very much extended, yet
the amplitude is also at the same time decreased (fig. 140).
We thus see that the question of internal energy is important
in this connection. An increase of internal energy may be
expressed, as I shall show, in two different ways ; either, that
z
338 PLANT RESPONSE
is to say, by increase of amplitude or by increase of frequency
of pulsation.
Increased internal energy, shown by: {a) Increase of
amplitude, period remaining constant. — And we shall, as the
simpler of the two, consider that case in which the latent energy,
or tonic condition of the plant, is below par, that is to say, the
case in which it is near the thermo-tonic minimum. The effect
of increased absorption of energy with rising temperature
would here be indicated by increasing amplitude of pulsation,
the period remaining constant. Conversely the reduction of
latent energy with falling temperature would be indicated,
when the period is constant, by the fall of amplitude. This
we find fully illustrated in records obtained with Biophytum
and Desmodium. In the former, when nearing the thermo-tonic
minimum, it is found that while the period remains approxi-
mately constant, that is to say, two and a half minutes, the
amplitude of pulsation falls from 8 divisions at 35° C. to 5-5
divisions at 32° C. (fig. 124). In Desmodium, again, we find
a converse case. Here, while the plant is rising from the
thermo-tonic minimum to the normal condition, the amplitude
of pulsation is seen to increase progressively, while the period
of 2-4 minutes remains constant (fig. 141).
{b) Increase of frequency. — Taking next the case of a plant
in the ordinary tonic condition, we find that the increase of
internal activity, due to the greater absorption of energy
during a rise of temperature, is exhibited by a higher frequency
of vibration. The reason why, with this increase of frequency,
there is a diminution of amplitude of pulsation, is now to be
explained.
We have seen that an increase of internal energy, as
caused by rise of temperature, brings about an increase of
turgor, and that this increased turgor hastens the process
of recovery, and by acting antagonistically to the con-
tractile phase of responses diminishes its amplitude. In-
creased internal pressure also, generally speaking, increases
the frequency of vibration. If, then, the rise of temperature
increases the turgor of Desmodium, as we have found it to do
TEMPERATURE AND AUTONOMOUS RESPONSES 339
in plants in general, then the diminution of the amplitude of
its vibration with higher temperature is explained ; and I
shall be able to adduce independent proof that this is actually
the case in Dcsinodium, for we have seen that the external
indication of internal increase of turgor is the expansion of
the organ, which produces a movement upwards, the same as
that of relaxation. We found in the last series of responses
in fig. 141, moreover, that when the temperature was raised
gradually from the thermo-tonic minimum, the leaflet was
more and more erected, or ' relaxed.' Cooling, conversely,
produced diminution of turgor, and an opposite movement in
the direction of fall or contraction (figs. 140 and 141).
The anomalous use of the word ' relaxation.' — We have
seen that the motile organ of Desinodiuj/i, when anaesthetised,
is brought to a state of standstill in a position of relaxation.
Its tonic condition, by virtue of which it exhibits contractile
response, has thus been abolished. We may then regard
ether as having brought about a loss of tone, or as having
reduced the tissue to the a-tonic condition.
An apparently similar position of relaxation may, however,
be attained by the active process of expansion, which is the
result of an increase of internal turgor. It would thus appear
that we are liable to form many wrong inferences, as to the
tonic changes undergone by the organ, if we too hastil}'
conclude that expansion is always caused by loss of tone.
Simple versus additive character of individual pulsa-
tions.— One question, regarding which opposite views have
been put forward hitherto, is that of the simple or composite
nature of the individual pulsations of cardiac muscle. The
movement of systole may, for example, be regarded as con-
sisting either of a single or of the additive effect of several
constituent contractions. In the production of tetanus in the
ca.se of muscle and also in that of contractile vegetable tissue
we have seen several individual contractions, when following
each other with sufficient rapidity, become merged in one appa-
rently continuous contraction (figs. 49, 50). When less rapid,
however, they arc individually distinguishable. Similarly,
340
PLANT RESPONSE
in the case of cardiac pulsation, it is possible that one appa-
rently simple contraction may in reality consist of several,
which are rendered indistinguishable by the great rapidity
of their succession.
Now, since the pulsatory response of Desniodium is in
every way so similar to that of cardiac muscle, its analysis
might be expected to throw much light on this question ;
and this more especially since it possesses the added advantage
that its pulsation is executed in a period about one hundred
times as long as that of cardiac muscle. The constituent
elements of each pulsation, if such
exist, ought thus, owing to the
relative slowness of the movement,
to be much more easy of detection.
Now, it is often seen in watching
the pulsatory movements of Des-
iiiodiiini that they proceed some-
what discontinuously, or by jerks.
Under favourable circumstances,
nevertheless, the movements of
the leaflet up and down become
apparently continuous. When
such pulsations are recorded on a
rather rapidly moving drum, the
discrete nature of the movement
can be brought out more easily in
the case of the up movement, which is relatively slow. This
is shown in a very interesting manner in the accompanying
photographic record (fig. 142), where the subsidiary pulsations
make themselves visible as nodules ; places where the move-
ment is slow appear thicker, on account of the photographic
irradiation-effect. During the course of the single up move-
ment recorded in this photograph, we may count as many as
twenty-five of these sub-pulses. It has been said that these
subsidiary movements are more easily detected when the
general movement is relatively slow ; and in connection with
this, it is extremely interesting to note the record of pulsations
Fig. 142. Photographic Record
of Pulsation of Des/iiodiitiu,
showing Sub-pulses during
slower Up Movement, as
Nodules
TEMPERATURE AND AUTONOMOUS RESPONSES 34 1
Fig. 143. Photugraphic
Record of Cyclic Oroup-
ings in Autonomous Pul-
sations of Desniodiimt,
showing Sub- pulses
seen in fig. 140, where successive pulsations of the same
leaflet, at first apparently continuous, are made to exhibit the
sub-pulses with growing distinctness, as the period becomes
progressively slowed down by cooling.
First we are enabled to observe the
sub-pulses during the up movement,
and afterwards during the down move-
ment also. I give another photo-
graphic record also (fig. 143), in which
these subsidiary pulses are seen at both
the ' systole ' and ' diastole ' (see also
fig- 175)- Arguing from analogy,
therefore, it becomes highly probable
that any single pulsation of the heart
also may be made up of similar dis-
crete and constituent elements.
While dealing with this question, it must be borne in
mind that in each case the contractile organ as a whole is
made up of a mass of in-
dividually contractile ele-
ments, the sum of whose
separate and additive ac-
tions it is which is seen
as a single contraction and
expansion of the whole
organ. Thus, in Desnio-
diiim for example, a single
pulsation of the motile
organ is made up of sub-
sidiary pulselets. The
unit-pulses, again, may
themselves be grouped in
larger systems, either as
the alternate waxings and
wanings seen in periodic
groupings (figs. 143 and 149), or in those periodicities of
the order of an hour or so, which the plant sometimes
Fig. 144. Photographic Record of Auto-
nomous Pulsation of Desmodium,
showing Hourly Period
[Speed of drum = 5 cm. in one hour.]
342 PLANT RESPONSE
exhibits, under the influence of periodic variations of tem-
perature and other factors. An extremely interesting example
of such an hourly periodicity is given above (fig. 144), where
the mean plane of vibration is itself seen to exhibit a periodic
up-and-down oscillation. There is, again, the still larger
periodicity of diurnal variation of day and night. We thus
see how complex may be these wave-systems, in which,
superposed over large waves, are smaller waves, and on the
latter still smaller wavelets.
Summary
In Desinodiuin, as in cardiac muscle, rise of temperature
produces increased frequency with diminished amplitude of
pulsation. This is true within a certain normal range of
temperature. When the temperature of Desviodiuni is reduced
to a thermo-tonic minimum — which is about 17° C. but subject
to certain individual variations — the amplitude of pulsation,
owing to the loss of internal energy, is decreased till there is
an arrest. If now the temperature be gradually raised, the
pulsations, owing to the absorption of energy, become again
increased in a staircase manner, the period remaining approxi-
mately constant.
Under normal tonic conditions, the decrease of amplitude
of pulsation with rising temperature — when this is not
excessive — is not indicative of loss of excitability. It is due
to the increase of internal energy, which hastens recovery and
acts antagonistically to the responsive movement of contrac-
tion. The same explanation is probably applicable to the
diminished amplitude of pulsations with rise of temperature,
observable in cardiac response.
Rise of temperature, by increasing the internal energy
and consequent turgor of the plant, causes expansion of the
organ, thus bringing about a shifting of the pulsation towards
' diastole.'
A converse effect, or shifting towards 'systole,' is seen in
Desmodiutn, as the result of cooling.
TEMPERATURE AND AUTONOMOUS RESPONSES 343
The process of relaxation in the motile organ may be the
result of entirely different causes. It may be a consequence
of a loss of tone, such as results from narcotisation ; or it
may be brought about by excessive turgor, caused by increased
internal energy.
The apparently simple rhythmic pulsations oi Desuiodiuni
can be analysed and shown to consist of subsidiary minor
pulsations. The ordinary pulsations, again, may show hourly
and daily periodicities.
CHAPTER XXVII
SIMILARITIES OF RHYTHMIC RESPONSE IN VEGETABLE
AND ANIMAL TISSUES
The similarities, in their fundamental characteristics, of rhythmic tissues, animal
and vegetable : (i) In responses — (2) In possession of long refractory periods -
(3) In incapability of tetanus — Theories regarding the causation of heart-beat
— The similarities of rhythmic tissues, animal and vegetable, as seen in : (i)
The effects of internal hydrostatic pressure — (2) The effects of variation of tem-
perature— (3) The periodic groupings of response — (4) The effect of barium
salt— (5) The antagonistic actions of acid and alkali — Identity of rhythmic
phenomena in animal and vegetable tissues.
I HAVE, in the course of the previous chapters, shown the
remarkable general similarities, extending through numerous
details, between the responses in animal and vegetable tissues.
These similarities, however, become still more striking when
we compare the special characteristics of those plant and
animal tissues which exhibit the property of rhythmicity — that
is to say, those tissues which, under the action of a single
strong stimulus, give rise to a multiple and rhythmic series of
responses, such responses, under favourable circumstances,
passing into the so-called automatic movements.
In animal tissues, such rhythmic movements may be
observed in their perfection in the case of cardiac muscle.
The isolated heart, when brought to a state of temporary
standstill, will give, in answer to a single stimulus, a single
response, or to a sufficiently strong stimulus a multiple series of
rhythmic responses ; and under favourable circumstances it
will give automatic responses for a considerable length of time.
Similarly, the plant Biophytuvi, under exceptionally
favourable circumstances, exhibits what are apparently auto-
matic responses ; and again, under ordinary conditions it
RHYTHMIC RESPONSE IN PLANT AND ANIMAL 345
gives a single response to a single stimulus, or if the stimulus
be sufficiently strong, a multiple series of rhythmic responses.
In the plant Desmodium, under favourable tonic conditions
we observe automatic movements ; but when under less
favourable circumstances, as for instance owing to the
unfavourable season, it is brought to a state of standstill, it
gives a single response to a single stimulus. When the
stimulus, however, is strong, we have seen that it gives rise to
a multiple series of responses, in a manner precisely like that
of Biophytnm under similar circumstances. It will thus be
seen that Biophytnm in its ordinary condition may be
regarded as equivalent to Dcsuiodiuin in a state of standstill.
Similarities, in their fundamental characteristics, of
rhythmic tissues, animal and vegetable : {\) In responses.—
In the matter of response, we have found that the rhythmic
automatic movements of cardiac muscle are repeated in
Desmodiimi under ordinary tonic conditions, and in Biophytum
under exceptionally favourable circumstances. In a state of
standstill, all three give a single response to a single moderate
stimulus, and a multiple series of rhythmic responses to a
sufficiently strong stimulus. The following tabular statement
exhibits this parallelism in a concise form :
Tauular Statement showing Similartites i\ the Responses ok
Rhythmic Animal and Vegetable Tissues
At standstill
Under f:ivourable
conditions
Moderate stimulus
Sufficiently strong
stimulus
Cardiac muscle
Single stimulus,
single response
Single stimulus,
multiple series of
rhythmic
responses
Automatic
rhythmic
movements
Desmodium
Do.
Do.
Do.
Biophytum
Do.
Do.
Do.
346 PLANT RESPONSE
{2) In possession of lo7ig refractory period. — I n order to study
in detail the characteristics of response in Desmodiuin, I took
a plant in which the leaflets had come to a state of natural
standstill. To such a specimen I applied the stimulus of a
condenser discharge ; it was found, as stated already, that
a rather high electromotive charge (twenty-four volts) was
required to produce response. The first few responses were
somewhat feeble, owing to the sluggish condition of the tissue ;
they then increased in a ' staircase ' manner till they became
uniform, the period of a complete response being now about
six minutes. From this point on, the responses were the
maximal possible, and a higher E.M.F. produced no notice-
able increase. The most characteristic feature of these
responses was the possession of a long refractory period,
which we have also found to be characteristic of the response
of BiophytuDi. With this specimen of Desmodium 1 found
that when a second stimulus was given after three minutes,
there was no further response. But a stimulus given after
three and a half m.inutes was effective. It may be mentioned
here that the length of the refractory period varies somewhat
with the condition of the tissue, being relatively longer when
that is sluggish.
(3) In incapability of tetanus. — The rhythmic tissue of
Desmodium thus resembles cardiac tissue, in the possession
Fic. 145. Record showing that Rhythmic Tissue oi Desmodium is
Incapable of being Tetanised
After the first two pulsations, strong tetanising electric shocks were applied
continuously. No tetanic effect was produced, but the pulsation
became somewhat irregular.
of a marked refractory period. There is again another
interesting similarity. A rhythmically beating cardiac tissue
cannot be thrown into tetanus by quickly recurring electric
RHYTHMIC RESPONSE IN PLANT AND ANIMAL 347
shocks. An automatically moving Desinodinui leaflet is also
incapable of being thrown into a state of tetanus (fig. 145).
Rapidly succeeding shocks do not produce tetanic contraction,
though some irregularity may occur in the pulsation ; exces-
sively strong shocks kill the plant, and the pulsation is then
permanently arrested.
Theories regarding the causation of heart-beat. —
Having thus seen how similar are the phenomena of rhyth-
micity in cardiac muscle and in plants, we may proceed to
inquire into the theories which have been proposed to
account for the automatic pulsation of the heart. It has been
suggested :
(i) That discrete impulses are sent out from certain
motor nerve-centres in the heart to the muscular tissue, thus
causing the periodic heart-beat. Assuming the correctness
of this theory, however, the difficulty is merely transferred,
for we have still to account for the rhythmic excitation of the
nerve. But that the rhythmic heart-beat is not fundament-
ally due to rhythmic impulses from nerve-centres, has been
proved from facts discovered by various observers : (c?) that
the isolated ganglion-free apex of the frog's heart may be
thrown into rhythmic activity by stimulus ; it has also been
shown by Gaskell iU) that the apex of the tortoise-heart,
which is free from nerve-cells, is capable of rhythmic move-
ments ; and (c) it is found that even in the embryo, before
any connection with the central nervous system has been
established, there is a rhythmic heart-pulsation.
(2) That cardiac muscle may have the inherent property
of rhythmicity. This explanation, however, by itself, is incom-
plete, for it takes no account of the stimulus which must
exist, in order to give rise to rhythmic expression.
(3) That the pulsation of the heart is maintained by some
' inner stimuli,' its rhythmicity being brought about by the
long refractory period peculiar to cardiac muscle.
Independent light, however, may be expected to be
thrown on the question of the causation of spontaneous
348 PLANT RESPONSE
rhythmic action in cardiac muscle, by the consideration of
similar phenomena in plants, especially since it can be shown
that their similarity is manifested under numerous varying
conditions, and extends to fundamental characteristics.
We have already seen that there is a similarity of funda-
mental characteristics between the response of cardiac muscle
and that of rhythmic vegetable tissue.
Similarities of rhythmic tissues, animal and vege-
table.— We shall now, therefore, observe in detail those other
and more special similarities which are exhibited in a com-
mon modification of response under varying external condi-
tions, by Z)t'j-;//^<^/////^ and cardiac muscle alike, and I shall first
describe the remarkable effect produced on both by internal
pressure.
( I ) Hie effects of internal hydrostatic pressure. — It is found
that a heart which has come to a condition of standstill may
be set into rhythmic activity by filling the cavity of the heart
with liquid. Endo-cardiac pressure is thus found to act as a
stimulus.
In Desnwdium, when the season is favourable, the tissue
is in a turgid condition, and there is a considerable internal
hydrostatic pressure, which we have seen to be advantageous
to the maintenance of rhythm. This turgid condition
depends on the ascent of sap, which, as I shall show in
Chapter XXVIII., depends again on the rhythmic activity of
certain tissues. Thus, in the summer season, we have the
conditions most favourable for the maintenance of rhythmic
activity in Desmodiunt. But with the approach of winter
the vigour of the plant and its turgid condition undergo a
marked decline, and the autonomous movement of the leaflet
then comes to a stop.
It appeared to me that this cessation of movement might
to a great extent be due to the diminution of internal
hydrostatic pressure, and I undertook experiments to see
whether the pulsatory movement could be renewed by an
increase of this pressure, just as increased endo-cardiac pres-
sure was found to renew the beating of the heart. The
RHYTHMIC RESPONSE IN PLANT AND ANIMAL 349
Fig. 146. Curve showing Relation between
Temperature and Period of Pulsation in
Desmodiiivi
Abscissa represents temperature, and ordinate
time, in tenths of a minute.
experiment was carried out by mounting a detached petiole
containing the motile leaflets at one end of a limb of a
U-tube. Any desired
pressure could be ex-
erted by varying the
height of the second
limb, which was con-
nected with the first by
india-rubber tubing. By
exerting internal pres-
sure in this manner,
I was able to pro-
duce vigorous rhythmic
movements of leaflets
which were, before this,
in an absolutely qui-
escent condition. The
beneficial effect of this constant internal pressure was further
seen demonstrated by the extreme regularity and persistency
of the rhythmic beats.
Even in the best season
of the year, the pulsations
are irregular, and come to
an occasional stop ; but
under the action of internal
pressure, I have found the
detached leaflet to main-
tain its rhythmic activity
unimpaired for nearly one
hundred hours. In con-
nection with this question
of the increase of inter-
nal pressure, it should be
mentioned here that, after
the normal condition of
turgidity has been established, a further increase of internal
pressure is found to increase the frequency of pulsation.
Fig. 147. Curve showing Relation
between Temperature and Period of
Pulsation in the Heart of a Frog
Ordinate represents time, in tenths of a
second.
350
PLANT RESPONSE
Excessive pressure, however, brings on irregularity, or even
stoppage, of autonomous movement.
(2) The effects of variation of tej)iperature. — I have shown
in the last chapter how perfectly similar are the effects of tem-
perature in causing variations of the ampli-
tude and frequency of pulsation in rhythmic
tissues, both vegetable and animal. As
regards the effect of temperature on the
period, the similarity is strikingly exhibited
in the two curves given above, showing
the relation between temperature and
period in Desmodium and in cardiac muscle.
It will be seen that in both (figs. 146 and
147) the fall of period with increase of
temperature is at first rapid and then slow.
(3) Periodic groupings of response. —
Another very remarkable similarity be-
tween the pulsations of Desviodium and of
cardiac muscle lies in their exhibition of
periodic groupings, very simple types of which, given by
Desmodiuui, are seen in the photographic records (figs. 148,
149). In fig. 146 we have an alternate waxing and waning
of pulsation, and a remarkably similar record, given by frog's
Fig. 148. Simple
Alternation of
Pulsation in Des-
modiiiin
Fig. 149. Periodic Groupings
of Pulsation in Destnodium
Fig. 150. Simple Alternation
of Pulsation in Frog's
Heart (Penibrey and
Phillips)
heart, is seen in fig. ] 50. These groupings are ot various
degrees of complexity, one type of such heart-beats being
that known as Luciani's groups, where the successive groups
are separated by a long pause. Now, similar groups with
RHYTHMIC RESPONSE IN PLANT AND ANIMAL 351
intermediate pause are also seen in the pulsation of Des-
nwdiuDi. I have again shown that periodic rhythms of various
degrees of complexity are seen, not only in the pulsations of
Desmodiuni, but also in the multiple responses of Bwphytiau,
and even in the electrical responses in plants.
(4) Effects of hariHin salt. — There are agencies, such as
internal pressure and certain chemical reagents, which induce
regularity of pulsation in
irregularly pulsating rhyth-
mic tissues. Conversely,
there are others, which pro-
duce the opposite effect, that
is to say, a regular pulsation,
after such an application be-
comes irregular.
When the heart pulsations
are regular, it is found that
addition of Vemtrin disturbs
the uniformity. Remember-
ing the general similarity of the action of Veratrin and barium
salts, I applied a 5 per cent, solution of this substance to
the pulvinus of Desuwdium leaflet, which was executing very
regular vibrations. The application of this reagent disturbed
the regularity of the pulsation, and somewhat irregular
Fig. 151. Effect of Barium Chloride
Solution on Desi/iodiiiiii
The regular pulsation, seen to the left,
l;ecomes irregular after application
of this reagent.
152. Arrest of Beat of Ventricle of Frog at Diastole by Application
of Acid at Arrow
Record to be read from right to left (Gaskell).
groupings were at once established (fig. 151). In another ex-
periment with the same reagent, as the application of heat is
known to neutralise the action induced by Vemtrin and barium
salts, I raised the temperature of the plant chamber, with the
result that the beats became regular once more.
352
PLANT RESPONSE
(5) Antagonistic actions of acid and alkali. — But the most
remarkable of all the similarities seen in the pulsations of
rhythmic tissues of animal and vegetable, is that of the
antagonistic actions of acid and alkali. Acid induces in
the case of the heart a relaxed or diastolic standstill, whereas
Fig. 153. Systolic Arrest of Heart-beat by Dilute NaliO Solution (Gaskell)
alkali induces an effect exactly the opposite, the standstill
being brought about at systolic contraction (figs. 152 and 153).
It is also known that the standstill caused by one of these
reagents can be counteracted by the antagonistic action of
the other.
It is astonishing to find that exactly the same effects
are produced by these reagents on the tissue of Dcsmodiiun.
I first tried the effect of dilute hydro-
chloric acid, which, as will be seen,
produced an arrest of pulsation in
the ' diastolic ' or relaxed position
(fig. 154). I next tried the effect
of alkali — dilute solution of sodium
hydrate — and it will be seen that this
produced an arrest of pulsation in
the ' systolic ' or contracted position
(fig- 155)- In records of the effect of
this reagent on other specimens, in
which its action had not proceeded
so far, there was a continuous dimi-
nution of pulsation with a shifting
and when an acid was now applied,
the antagonistic character of its action to alkali was clearly
shown, by a gradual revival of response, with a shifting
towards the diastole. In the present record, the systolic
standstill caused by alkali was allowed to proceed far, and
Fig. 154. Arrest of Des-
modium Pulsation at
' Diastole ' by Applica-
tion \ of Acid
towards the systole
RHYTHMIC RESPONSE IN PLANT AND ANIMAL 353
yet on application of the acid there is seen to be a sh'ght
revival of pulsation and a final arrest towards diastole.
Identical nature of rhythmic phenomena in animal
and vegetable tissues. — We have thus found the responsive
phenomena of cardiac muscle to be in every respect similar
to those observed in the rhythmic tissues of Desniodiiivi and
Fl("r. 155. Arrest of Pulsation of Desmodiiiin at ' Systole ' by Application
of Dilute Alkali at t
Acid was next applied at j, and the record shows its antagonistic action.
Biophytiim, the response of the latter being regarded as
practically that of Desviodium in a state of standstill. We
have seen that in these rhythmic animal and vegetable
tissues the fundamental characteristics are identical :
(i) In all of them stimulus gives either maximum response
or none.
(2) They all exhibit a long refractory period, during
which additional stimulus produces apparently no effect.
(3) They are all incapable of being tetanised.
And further, as regards the influence of various external
agencies on the two classes of rhythmic tissues, animal and
vegetable, the effects are also remarkably similar. Internal
A A
354 PLANT RESPONSE
hydrostatic pressure renews in them rhythmic activity. All
of them exhibit under certain circumstances similar cyclic
groupings. The effects of chemical reagents are similar on
both classes. Rise of temperature quickens the rhythm and
reduces the amplitude of pulsation in a manner exactly
similar in both ; and, finally, the effects of chemical reagents,
even in the matter of antagonistic actions, are alike in the two
classes. From a consideration of all these, it would appear
that in studying response in rhythmic animal and vegetable
tissues, we are dealing, not with two distinct but with a
single class of phenomena. We are thus justified in
ascribing the rhythmic action of the heart to those same
causes which we have found to originate and maintain the
rhythm of Desmodium or BiopJiytnvi. We have seen that
these plants absorb energy continuously from the various
forms of stimulus — mechanical, thermal, chemical, and other
— to which they are subjected. This absorbed energy
remains latent in the tissue, and determines its tonic con-
dition, which is simply the sum total of these latent stimu-
lating factors. When the sum of these factors exceeds a
certain value, it will find expression outwardly in the form of
excitatory discharges. This discharge, however, is not single
and continuous, but intermittent. After each partial excita-
tory discharge, there is a diminution of conductivity and
excitability which are only restored gradually. The long
refractory period is merely an expression of this peculiar
property. Owing to this periodic oscillation of conductivity
and excitability, the constant latent stimulus finds expression
in a rhythmic manner. Under favourable circumstances,
there is a large surplus of accumulated energy, and long-
continued responses, apparently automatic, are thus produced.
Under less favourable conditions, when the stored-up energy
is not great, a single stimulus gives rise to a single response,
or, when the stimulus is stronger, to a multiple series of
rhythmic responses ; and this statement is true of ti-ssues
exhibiting ' spontaneous movements,' not only in the case of
the plant, but also in that of cardiac muscle.
RinTIIMIC RESPONSE IN PLANT AND ANIMAL 355
It is impossible to conceive that there could be movement
without an exciting cause. Automatism is said to be one of
the properties of protoplasm. It will be seen, however, from
the evidence which I have adduced in cases where experi-
mental investigation is possible, that, strictly speaking, there
is no such thing as automatism. Only under the action of a
stimulus can a living tissue give responsive indications. The
impact of an external stimulus may give rise to an imme-
diate expression, or it may partly or wholly be reserved in
latent form for subsequent manifestation. ' Inner stimuli ' are
simply external stimuli absorbed previously and held latent.
An animal or a plant is thus an accumulator which is constantly
storing up energy from external sources, and numerous mani-
festations of life — often periodic in their character— are but
responsive expressions of energy which has been derived
from external sources and held latent in the tissue.
Summary
The rhythmic tissue of Biophytutii may be regarded as
equivalent to that of Desniodiuni in a state of standstill.
Both alike, when at standstill, give a single response to a
single moderate stimulus, and multiple response to a strong
stimulus.
Both, when the sum total of latent energy is above par,
give apparently ' automatic ' responses.
The rhythmic tissues of both plants exhibit a long re-
fractory period.
The automatically responding leaflet of Desiiiodiuni is
incapable of being tetanised.
An artificial increase of internal hydrostatic pressure
renews pulsation in a Desmodium which was previously in a
state of standstill.
The effect of rise of temperature on Desmodiuni is to
produce a shortening of period and decrease of amplitude of
oscillation.
The automatic responses of Dcsmodiiivi often exhibit
periodic groupings, of various degrees of complexit)-.
A A 2
356 PLANT RESPONSE
Certain reagents tend to make irregular pulsations in
Desmodiuui regular. Conversely, other reagents, like barium
salts, induce irregularity in regular pulsations. The irre-
gularity induced by the latter reagent may, however, be
counteracted by a rise of temperature.
The effects of acids and alkalis on the pulsatory move-
ments of Desmodium are antagonistic, acids inducing arrest
of pulsation in a relaxed position, while alkalis induce
its arrest in a contracted position. The standstill induced
by one reagent may therefore be counteracted by the use of
the other.
By all these, the rhythmic phenomena of the plant are
seen to be identical with those of the animal.
The pulsation of the animal heart is thus to be ascribed
to the same causes as bring about and maintain the rhythmic
pulsations of Desmodium.
PART V
ASCENT OF SAP
CHAPTER XXVIII
SUCTIONAL RESPONSE AND ASCENT OF SAP
Inadequacy of existing theories of ascent of sap— General considerations regarding
cellular activity and resultant propulsion of water — The Shoshungraph —
Balanced Shoshungraph for determining variations of suction — Hydrostatic
and Hydraulic Methods of Balance.
There are few phenomena in plant life which have attracted
keener interest and inquiry than that process of transport by
which water is carried, from below the surface of the earth
to the tops of the tallest trees. The obscurity of the subject
is so great, and the secondary co-operating agencies so
numerous, that the inquirer is apt to be led into the error
of confining his attention to some one of them alone, imagin-
ing it to be the principal element in the problem. In
studying this subject, then, our first effort must be to distin-
guish between the essential factor and others which are
merely subsidiary.
For a statement of the inadequacy of these subsidiary
factors to the solution of the problem, it is only necessary to
refer to the summary of Strasburger and Pfeffer regarding
existing theories of the ascent of sap : ^
The theory of atmospheric pressure is discredited, inas-
much as water is known to be lifted, in certain cases, to many
times the height of the water-barometer.
The theory of capillarity is inadequate, inasmuch as
continuous capillaries are absent, and the height to which
liquids could be raised by such means would not, moreover,
approach that of an ordinary tree.
' Pfeffer, P/ivsio!ooy of Plants, English translation, 1903, vol. i. p. 222,
el seq. ; Strasburger, Text -book of Botany, English translation, 1903, p. 188.
300 PLANT RESPONSE
The theory of osmotic action cannot be considered satis-
factory, since such action is too slow ; besides which, there is
no fixed distribution of osmotic substances, such as would
account for the necessary transportation-current.
The theory of root-pressure, again, is open to the objection
that it cannot possibly account for the maintenance of a
sufficient force during the process of active transpiration, when
root-pressure is found to be negative. Moreover, this root-
pressure itself requires an explanation.
There is, however, another theory, due to Dixon, Joly,
and Askenasy, which has apparently more to support it than
any of those yet mentioned. According to this, the ascent is
brought about by transpiration from the leaves. The fluid
in the mesophyl cells of the leaves becomes concentrated by
evaporation ; thus osmotic attraction is set up by the leaves,
and the suction thereby exerted is supposed to be transmitted
backwards, as far as the roots, through cohering columns of
water. The difficulties in the way of this theory lie (i) in
explaining how a slow osmotic action could produce so rapid
a water-current ; (2) in the absence of any conclusive proof
that, under actual conditions within the plant, the water-
column could have sufficient tensile strength ; and, lastly,
(3) in the fact which I shall demonstrate, that, when evapo-
ration is not taking place in the leaves, the transport of water
is still very considerable, and that, besides, other related
phenomena, like exudation pressure, continue to take place even
in the complete absence of evaporative activity in the leaves.
I shall, moreover, be able to show that the movement of water
often takes place in the plant in a direction opposite to that
which would be the case if osmotic action were alone involved.
* Thus,' to quote Pfeffer, ' a satisfactory explanation of
the means by which the transpiration-current is maintained
has not yet been brought forward. If no vital actions take
part in it, then it is obvious that we have only an incomplete
knowledge of the causes at work, and of the relationship of
the different factors concerned.' '
' Pfefter, Physiology of liaiits, English Iranslalion, 1903, p. 224.
SUCTIONAL RESPONSE AND ASCENT OF SAP 36 1
There then remains the question as to whether Hving
cells by some unknown physiological activity might not be
instrumental in effecting this transport of water. But the
experiments of Hartig, Bohm, and Strasburger have been
held to contradict such a possibility. Thus, Strasburger
set the cut ends of trees in tubs of poison, such as copper
sulphate solution. The poison ascended to the leaves, a
distance, in the tallest trees, of twenty-one metres. Now, if
such violent protoplasmic poisons ascend the trunk, it is clear
that they must kill all the cells lying in their path. That
the living cells of the stems could not be necessary to the
rise of sap was taken to be a necessary inference from
this experiment. Strasburger also killed portions of the
stems of living trees by heat, and yet the upper living and
leafy portion was found to remain turgid for a few days.
Another well-known experiment which was held to negative
the theory of protoplasmic activity, was that in which
boiling water was poured on the roots, when the plant con-
tinued to transpire, in spite of the roots having been killed.
From such considerations, Strasburger was led to con-
clude that ' the supposition that the living elements in any
way co-operate in the ascent of the transpiration-current is
absolutely precluded.' '
I shall nevertheless show that the ascent of sap is funda-
mentally due to the physiological activity of living cells,
and that the experiments described above in no way negative
this, being, on the contrary, capable of a different, and very
satisfactory, explanation. Many difficulties connected with
the problem of the ascent of sap will be found to disappear,
when the physiological activity of living tissues is once
clearly established as the essential factor. But a vague
assumption of protoplasmic activity will not be sufficient for
the elucidation of the phenomenon. It will be necessary
to show further how this excitatory activity is initiated,
and by what means a definite-directioned flow is imparted to
the sap.
' Strasburger, Text-dook of Botany, English translation, 1903, p. 1S8.
362 PLANT RESPONSE
We have seen in Chapter XXI. that when a strong
stimulus is applied to the base of any organ, say a stem,
a multiple series of excitatory waves is propagated onwards,
such multiple responses being detected by electrotactile or
electromotive pulsations. It was also shown in Chapter
XXVI. that such multiple, passing into automatic, response,
may be induced by the action of a constant stimulus. It
was further demonstrated in Chapter XXI. that these excita-
tory waves, and the concomitant cell-to-cell contraction, would
produce a movement of water forwards, along the direction
of propagation. This series of excitatory waves, proceeding
from the base of the organ, and propelling water forwards,
must then cause a deficit of water behind. If, however, the
base of the organ be kept supplied with water, this deficit will
be made up by suction.
It is thus seen that by such rhythmic activity a one-
directioned movement of water may be produced. Just as
the various effects produced by multiple or autonomous
response — in, for example, the electromotive and electro-
tactile responses, and in the multiple mechanical responses of
Desniodiuni — give us an indication of the degree of rhythmic
activity exhibited by the tissue, so, in the rate of this water-
movement also, we have an additional means of measure-
ment. This would be analogous to the measurement of
the rhythmic activity of the heart by a determination of the
rate of flow of the circulating blood. In the case of the
plant this rate of movement may be measured, either by
means of the propulsion of water forwards or by the suction
exerted behind.
The ascent of sap in the plant, then, may be brought
about by the rhythmic activity of the tissue. How this
activity is initiated will be discussed later. Meanwhile it
is clear that if the movement of sap be really an expression
of protoplasmic activity, then any physiological modifica-
tion which tends to increase that activity will also tend to
increase the rate of movement ; and pari passu any physio-
logical condition which tends to depress the activity, will
SUCTIONAL RESPONSE AND ASCENT OF SAP 363
correspondingly express itself in a diminished rate of move-
ment. The movement of sap is thus taken to be another
expression of that autonomous activity (multiple response) of
the plant, which we have already seen exhibited locally by
the motile tissue of Desmodium. The conclusive test of this
would lie in proving that all those agencies which acted
in a given way on the autonomous response of Desmodium,
act also in the same way in modifying the rate of movement
of sap. In other words, just as an exciting reagent will in
the one case induce a greater amplitude, frequency, or
both, of oscillation, above the normal, so in the other the
excitatory nature of a given reagent may be expected to
exhibit itself by an increase above the normal, in the rate of
flow. A depressing reagent, on the contrary, should produce
the opposite effect in both. That is to say, just as we may
study the multiple or rhythmic excitability of a tissue
through mechanical, electromotive, or electrotactile response,
so here, in hydraulic response, or the determination of changes
in the rate of flow of sap, we have an independent mode of
investigating the same phenomenon.
The great difficulty of this investigation lies in the
absence of a method by which these changes can be imme-
diately recorded. In other words, we require some simple
means of making a direct record, which will show, in a
continuous manner, the changes produced by the various
agencies, enabling us to distinguish their immediate effects,
after effects, time-relations, and so on.
We have seen that the propulsion of water forward by the
tissue is attended by a suction behind.^ The quantity of
water sucked up in a definite period will therefore give us an
indication of the rate of movement of sap in the plant. Thus
from the readings afforded by the water-index of a potometer,
and the times at which such readings are taken, we may derive
' The forward movement of water, and the suction exerted, in the same tissue,
are not necessarily equal in all cases. Part of the water sucked up may be
deviated to increase the turgidity of the cells themselves. Suction may neverthe-
less be taken as a measure, other things being equal, of rhythmic activity.
364 PLANT RESPONSE *
the rate of movement. But these readings are necessarily
discontinuous, and important phases of change are thus apt
to be overlooked. They are, again, subject to error ; nor do
they give us, at sight, the rate of flow, nor the changes in that
rate, at any given moment. In order to determine such varia-
tions, a laborious process of construction of curves, from the
experimental data, must, generally speaking, be undertaken.
It was therefore necessary to devise an apparatus by
means of which curves might be obtained direct, so that a
mere inspection would be sufficient to inform us as to the
normal rate of suction, and the influence of external factors
on that rate. This suction will be shown to be a physio-
logical phenomenon ; its variation, therefore, will enable us
to measure the physiological influence of various external
agencies. It must be borne in mind that under ordinary
conditions there is a normal rate of suction. The incidence
of an external stimulus will change this rate, and the variation
of rate which results is thus a measure of the effect produced
by the stimulus. Similarly, we measure a force by noting
the variation which it produces in the rate of movement of a
uniformly moving body. The variation of the rate of suction
may thus be taken as constituting a form of response to
stimulus, which for the sake of simplicity we shall designate
as Siictional Response.
The Shoshungraph. — For the purpose of .subjecting the
plant to various conditions, and also in order to obtain the
record of the resultant suctional response, I have constructed
an apparatus to which I have given the name of the
Shoshnngraph} It consists of (i) an arrangement by which
the specimen may be rapidly subjected to the action of
different excitatory or depressing agents ; (2) a potometric
tube, by which the constant changes of suctional activity are
measured ; (3) a contrivance by means of which the move-
ments of the water-index, with their time-relations, are
recorded. The principal parts of this instrument are shown
diagrammatically in fig. 156. v is the plant-vessel, in which
' From Sanskrit, Shoshun = suction.
SUCTIONAL RESPONSE AND ASCENT OF SAP
565
the specimen is mounted by means of a water-tight india-
rubber cork. The vessel is closed at the bottom also by
means of a large cork through which enter four tubes. One
of these, controlled by the stop-cock A, is connected with
the capillary potometer-tube. The second, controlled by i\',
leads to the compensating vessel C, filled with water. The
interior ends of these two tubes reach almost to the top
of the plant-vessel. The third, with stop-cock P.', leads to
Diagrammatic Representation of the Shoshungraph
plant-vessel ; c, compensator ; R, reservoir. The potometer-tube is
controlled by stop-cock, A ; compensator by a' ; and reservoir by B'.
li is the stop-cock of the outflow pipe ; D, recording drum driven by
clockwork ; p, collar carrying recording pen, seen magnified above.
The water-index is followed by appropriate manipulation of the
wheel, w.
the reservoir R, from which water at various temperatures
or different chemical reagents may be introduced. The
fourth is an outlet-tube controlled by the stop-cock B.
Adjustment for Unbalanced Record. — The great
difficulty in connection with delicate experiments arises from
the presence of air-bubbles in the plant-vessel, which are not
easy to expel. This is done, however, by means of an escape-
tube, with a stop-cock, which runs through the upper cork,
^66 PLANT RESPONSE
and is not shown in the figure. The stop-cock A' is opened, A,
B, and b' being closed. Water from the compensator C thus
passes into the plant-vessel, and the air-bubbles which have
been accumulating at the top of the vessel escape, with the
water which is driven out at the escape-tube. When all are
expelled then the stop-cock of the escape-tube, and A', are
closed. The potometer stop-cock A is now opened, and the
water-index is adjusted at any point desired, by manipulation
of the stop-cocks B and A'. A temporary opening of the
stop-cock a' of the compensator c causes the water-index
to move to the left ; whereas, when the stop-cock B, of the
outflow pipe, is opened, it moves to the right. After this
preliminary adjustment the stop- cocks a', b, and b' are closed,
the potometer tap A being kept open ; the movement of the
water-index per unit-time now gives us the normal rate of
suction of the specimen.
We come next to the question of making a direct record
of the rate of movement. For this purpose, a writing pen is
fitted on the potometric tube, by means of a brass collar.
This brass collar has a rectangular opening, which enables
us to watch the water-index. It has also stretched across it
a fine wire, which is kept always coincident with the water-
index. This wire is parallel with, and placed vertically above,
the recording pen. The collar is attached to a thread which
passes round small pulleys. One end of this thread carries
a counterpoise and the other is wound round a wheel, w,
which can be so manipulated as to make the index-wire
follow the movements of the water-column. When the wheel
is wound, the index moves to the right ; when it is slightly
released, the weight of the counterpoise makes it move to
the left. The weighted recording pen rests with its point
on a revolving drum, D, covered with paper for the record ;
this drum is kept revolving by clockwork at a known and
adjustable speed. When the water-index is followed in the
way described, there is produced a direct record of water-
movement in the plant. A curve is thus traced, the ordinate
of which represents the quantity of water sucked up, and the
SUCTION AL RESPONSE AND ASCENT OK SAl" 36/
abscissa the time. The slope of the curve thus gives the
rate of movement. As long as the suction is uniform, the
slope remains constant. If any exciting agency increases
the rate of suction, there is an immediate flexure in the curve,
which thus becomes steeper. A depressing agent lessens
the slope of the curve. And when suction is abolished, the
record becomes horizontal.
By this arrangement, then, we are enabled, simply and
accurately, to obtain a direct and continuous record ; and as
the necessity for taking readings is obviated, a large number
of experiments can be performed very quickly, with little
trouble. The flexure in the curve affords immediate visible
indication of the effect of any particular agency. The value
of each division of the ordinate is found once for all by
determining the volume of unit-length of the potometer-tube.
A previous determination at the beginning of the experiment
of the rate of movement of the drum, gives us the time-
value of each division of the abscissa. Knowing these, we
can determine the absolute rate of suction at any period of
the curve required. Responsive variations of suction are
more easily detected when the normal curve is almost
equally inclined to the ordinate and abscissa — that is to say,
when it makes an angle of about 45° with either. This is
most easily accomplished if we keep the potometer-tube
always the same, and merely adjust the speed of the drum.
The Balanced Shoshungraph.— According to the simple
method of making records which has just been described, we
observe the responsive effect by means of flexures produced
in the curve under the action of various agencies. If the
effect of an agency be slight, the change in the slope of the
curve will be proportionately small and liable to escape
detection. In order to bring the sensitiveness of the instru-
ment to its highest, I have devised the Method of Balance,
by which the slightest responsive variation is made to exhibit
itself in a marked manner. For the purpose of many delicate
investigations, it is not so necessary to know the normal rate
of suction, as the variations positive and negative in that rate.
368 PLANT RESPONSE
An agency which induces the positive variation will then
be eNicitatory, while that which induces the negative is
depressing. In order to carry out my investigations along
these lines, I have employed two different methods of balance.
In one — the Hydrostatic Method of Balance— the natural
suction of the plant is arrested by a counter-hydrostatic
pressure suitably applied. The effect of an external agent
is now studied by the direction, positive or negative, and
the extent to which it disturbs the static equilibrium thus
established. Experiments carried out on this method will
be described in the next chapter.
The second — or Hydraulic Method of Balance — depends
upon an application of compensation, by means of which
under normal conditions the water-index is kept stationary,
though the suctional movement in the plant is in no way
disturbed. According to this hydraulic method, the normal
rate of suction, when balanced, gives rise to a neutral, or
horizontal, line in the record ; while an exciting agent
produces an inclination upwards ; and a depressing agent a
declination downwards. The balance, by which under normal
conditions the neutral line is secured, is obtained by allowing
water to enter the plant-vessel from the compensator c at a
rate exactly equal to that of its withdrawal by suction.
In practice, this adjustment is roughly made by opening
the stop-cock a' in connection with c, to a greater or less
extent. Over-balance causes movement of the water-index
to the left ; under-balance to the right ; and when the adjust-
ment is perfect, the water-index becomes stationary. After
making the preliminary adjustment, the final balance may be
obtained, by very careful and gradual movement of the com-
pensating reservoir up or down. When the reservoir is
raised the flow is increased, owing to the greater difference
of level established, as between the reservoir and the plant-
vessel. The stand on which the vessel C is placed is pro-
vided with a rack and pinion, by means of which the necessary
adjustment of height is made.
Introduction of changed conditions. — We have thus
SUCTIONAL RESPONSE AND ASCENT OF SAP 369
seen how the normal rate of suction and its variations may
be recorded accurately. The next difficulty to be overcome is
that of introducing the changed conditions without creating
an}' disturbance, thus practically maintaining the continuity
of record. It will be necessary to observe, among other
things, the immediate and after-effects of cold and heat, as
well as those of various chemical reagents. This is accom-
plished by the right manipulation of the four stop-cocks.
Let it be supposed that we wish to study the immediate
and after-effects of cold. Up to this time the stop-cocks
A and a' have been opened — B and B' being closed — and the
balanced horizontal record taken. For the sake of simplicity
I refer to the stop-cock A' as the only one which opens and
closes the communication with the compensator c. In reality,
however, there is a second stop-cock in its neighbourhood,
by which the balancing adjustment is first made, A' being
employed for opening or closing communication under such
an adjustment. The reservoir R is now filled with cold
water ; A and a' are next closed — thus arresting the water-
index— and B and b' opened. The water then leaves the
plant-vessel by the overflow pipe, and its place is taken by
cold water from R. After this, the stop-cocks B and P.' are
once more closed, and A and a' opened. The index, being
now released, indicates by its movement the excitatory
or depressing effect of cold. It must be remembered that
the index was previously adjusted to balance. Should the
effect of cold prove to be excitatory, the rate of suction
would be increased ; under-balanced by the supply of water
from c, the index would move to the right, thus giving
positive response. If, on the contrary, the effect should be
depressing, the rate of suction would be decreased, and the
water from the compensator would produce an over-balance,
causing a movement of the index to the left, or negative
response. By now filling the reservoir R with water at
the ordinary temperature, and repeating the operation, the
original condition is re-established, and the effect of re-esta-
blishment of old conditions observed. In a similar manner,
B B
370
PLANT RESPONSE
we may study the immediate and after-effects of rise of
temperature, and of the various chemical reagents.
Fig. 157. Photograph of Shoshungraph
V, plant-vessel ; R, reservoir ; c, compensator, whose balancing height is
adjusted by rack and pinion, s ; K, key for manipulation of four-way
stop-cock ; P, recording pen, |with counterpoise M, manipulated by
wheel, w. The drum is rotated by the clock at uniform speed.
SUCTIONAL RESPONSE AND ASCENT OF SAP 37 1
In order to make the explanation easier, I have described
all these stop-cocks and their serial opening and closing
separately ; but this arrangement, apart from its clumsiness,
would involve a certain loss of time. In many of these
experiments, it must be remembered, it is necessary to know
the immediate effect produced. I have therefore simplified
the procedure by the use of a special key, K, by turning
which, in one direction or another, the requisite alternate
openings and closings are accomplished.
Thus, on turning the key-handle to the left, A and a' are
opened, and B and b' closed. The balanced record is now
taken. As the handle is now being turned to the right, the
stop-cocks A and a' are first closed, arresting the index. Con-
tinued turning to the right opens B and B', by which the modi-
fying reagent is introduced into the plant-vessel. A sudden
turning of the key to the left now closes B and B', and opens
A and a', thus releasing the index and enabling the record
to be taken once more, under the changed conditions. The
whole process is thus made so rapid, that modified conditions
can be established, and the record renewed, within the short
interval of less than one minute. The photograph of the
completed apparatus is seen in fig. 157. Having now given
in detail all the experimental arrangements, I shall in the
next chapter describe the physiological modifications induced
by different agencies, as exhibited by the suctional response.
The periodic variation of ascent of sap may be recorded
continuously and automatically by photography. Another
method of recording transpiration will also be found described
on page 472.
Summary
This chapter gives a description of the Shoshungraph, by
means of which the rate of suction and its variations may be
indicated and recorded. The .sensitiveness of the apparatus
is very much increased by the Hydrostatic and Hydraulic
Methods of Balance.
CHAPTER XXIX
MODIFICATION OF SUCTIONAL RESPONSE
Effect of lemperature on suction by three methods of inquiry: (i) Unbalanced
method of Shoshungraph : (a) Action of cold — (<'^) Action of moderate rise of
temperature — (2) Method of Hydrostatic Balance : {a) Action of cold — Re-
versal of normal direction of flow — (/;) Action of warm water — (3) Method of
Hydraulic Balance : (a) Action of cold — {i) Effect of warm water — Explana-
tion of suction when the root is killed by boiling water — Stimulation renews
suctional activity in plant whose suction has come to a standstill— Osmotic
I'ersiis excitatory action —.Vbolition of suction by poison — Suctional activity
continued until whole plant is killed by poison.
I SHALL now proceed to prove that the movement of water
in plants is mainly due to rhythmic excitation of the tissue,
and that evaporation from the leaves, osmotic action, and
so on, are only co-operating factors, of subsidiary importance.
We have seen in Chapter XXI. that any part of a stem when
excited willjbecome the seat of rhythmic activity, and that
this pulsatory excitation causes movement of water. I shall
now demonstrate a similar phenomenon by means of suc-
tional response, eliminating from some of the typical ex-
periments all auxiliary factors, such as osmotic action and
evaporation from leaves. I have shown in the last chapter
how the effect of suctional activity may be continuously
recorded by means of the Shoshungraph. We saw also
that the effects of various agents, exciting or depressing, were
to be detected, under the unbalanced method, by appropriate
variation in the slope of the curve. By the balanced method,
whether hydrostatic or hydraulic, the derangement of the
balance upwards indicates an increase of activity, and its
derangement downwards a decrease. I shall first demon-
strate the fact that these observations, though obtained by
such various methods, are all reliable and mutually consis-
MODIl'ICATION OF SUcTIUNyVL RKSPONSK 373
tent, by subjecting- the plant to the action of an agent whose
general effect is well known, and recording the results by all
three methods. For this purpose we shall take the influence
of low and moderately high temperatures.
Modification of suctional response by various agencies.
We have seen that any sudden variation of temperature
acts as a stimulus in itself Thus, if we touch the pulvinus
of Mimosa or BiopJiytum with ice, there is a responsive
twitch. This may be taken as the preliminary effect. Pro-
longed application of cold, however, abolishes excitability.
If the ascent of sap be really a phenomenon of excitation,
we may expect to find a sudden application of cold, appro-
priately made, producing a preliminary augmentation, followed
by the depression and arrest of suction. An application of
hot water might be expected, on the other hand, to bring
about the contrary effect, that is to say, an increase in the
rate of suction. I shall now describe the experimental results
obtained by the three methods.
Effect of temperature on suction : (i) Unbalanced
Method, id) Action of cold. — As I did not know what might
be the effect of injury on the suctional activity of the plant,
I selected intact specimens for my first experiments. For
this purpose I took cuttings of Croton, a plant whose stem
when placed in water will develop roots in a {^.^n weeks'
time. When the roots were well developed, the specimen
was fitted in its place in the apparatus. In other cases,
I took pot-grown specimens and placed them in water, so
that the earth was dissolved away. 'Violence to the rootlets
was thus avoided. I may here state, however, that I found,
in the course of these experiments, chat there is no essential
difference between the effects exhibited in such intact plants
and those observed in' the case of cut branches. All that is
necessary in the latter case is that the specimen should be
mounted in the apparatus and left for some time, in order
that the effect of the disturbance caused by cut may pass.
The record afforded by a specimen thus mounted gives the
normal rate of suction. The attainment of constancy of
374
PLANT RESPONSE
external conditions is gauged by the uniform inclination to
the curve, and it nfiay be well to mention here that through-
out the investigation every experiment was begun with this
test.
The normal rate of suction, in the first experiment, in a
Croton at the temperature of the room (23° C), was eight
cubic mm. per minute. On now applying cold water at a
temperature of 4° C. to the root, by appropriate manipulation
of the stop-cock, the rate showed the preliminary excitatory
effect, due to sudden variation of temperature, by an increased
Fig. 158. Effect of Cold on Suction
The first part of the record shows the normal rate of suction at 23° C.
Asterisk marks moment of application of cold water at 4° C. , which is
seen to produce a preliminary exaltation, followed by arrest of suction.
Abscissa represents time in minutes ; ordinate, the quantity of water
sucked up in milligrammes or cubic mm.
rate during the first two minutes, of eighteen cubic mm', per
minute, or 2-25 times the normal value. But this temporary
exaltation gradually passed away, till there was an almost
complete arrest, ten minutes after the first application (fig.
158). This arrest by cold was not found to be permanent ;
for it disappeared on the return to a higher temperature, as
will be seen in the first part of the next figure (fig. 159),
which was taken after water at 23° C. had been substituted
for the cold water in the vessel. In this second curve, the
rate of suction is found to return almost to the normal
MODIFICATION OF SUCTIONAL RESPONSE
375
degree, being now about seven instead of eight cubic mm.
per minute.
{b) Action of moderate rise of teviperatiire. — I next tried
the effect of a rise of temperature. This experiment was per-
formed with the same specimen as the last, in which the
return normal rate of suction at 23° C. had already been
determined to be seven cubic ram. per minute. On now
I ir.. 159. Curve sliovving
Normal Suction at 23° C. , ,
Increased Suction at 35°
C, and the After-effect
persisting on Return to
Normal Temperature
Tliis experiment was carried
out on the same specimen
as the last.
A short length of stem, i', has its as-
censional water-movement ])alanced
hy superincumbent water-column of
variable heitrht.
applying water at 35° C. it will be seen that a very steep rise
was induced in the curve, indicating an increased suctional
rate of fifty-eight cubic mm. per minute, more than eight
times the rate at 23° C. On now once more substituting
water at 23° C. the rate became lowered, though not to the
original degree (fig. 159). It must be remembered, with
regard to this, that the movement of sap depends on the cell-
activity of the entire plant, and that the tissue has by this
17^ • PLANT RESPONSE
time absorbed some quantity of hot water, which cannot
immediately be displaced by the water of ordinary tempera-
ture which is applied at the roots. The rate of suction,
therefore, could not at once revert to the normal, but must be
expected for some short period to show a slight enhancement.
The curve shows that on the return to 23° C. the rate fell
from fifty-eight to fourteen instead of to the original eight
cubic mm. per minute.
(2) Method of Hydrostatic Balance : {a) Action of
cold. — For this experiment I took a Croton stem cut at both
ends, the lower end being placed in the plant-vessel of the
apparatus. There was now found to be a considerable
movement of water upwards. This movement was arrested
by suitable hydrostatic pressure, the upper end of the stem
being connected with an india-rubber tubing filled with
water and ending in a funnel (fig. 160). To prevent evapora-
tion, the surface of the water in the funnel was covered
with a thin film of oil. Rather a high hydrostatic pressure
was required to produce a balance. When the pressure of a
column of 75 cm. was applied, there was still a movement of
water upwards in the tissue, at so great a rate as ten cubic
mm. per minute.
In this experiment, then, the root having been cut off,
there is still a considerable propulsion of water upwards
through the stem. It is thus clear that root-pressure is not
the essential factor in the ascent of sap. As the leaves had
also been cut off, and the cut end of the stem covered by
water sealed with a film of oil, evaporation from the leaves,
and the osmotic action thereby produced, are also seen to be
excluded. These, like root-pressure, therefore, cannot consti-
tute the essential factors in the process of suction. But since,
on the contrary, any small length of the stem is competent
to show this water-movement, the required activity must
reside in the tissue of the stem.
A balance was finally obtained, by the pressure of a
water-column of 105 cm. This equilibrium is not to be re-
garded as merely the result of equality between the upward
MODIFICATION OF SUCTION AL KESPONSl'. 377
pressure, exerted by suction from below, and the downward
pressure of the superincumbent water-cokimn. There are
reasons for thinking that there may also be an important
additional factor, of opposed rhythmic activities, balancing
each other. For when a cut branch is placed in water, the
lower end becomes over-turgid, and this, as we know, is
a condition for the initiation of rhythmic activity. The
upper end of the branch not being so turgid, activity will
be greater below than above. We therefore obtain a
movement of water from the more to the less active. But
if the leafy end of the branch be immersed in water, and
the cut end held in the air, it is known that the direction of
the flow becomes reversed. This is evidently due to the fact
that it is now the upper end which is over-turgid, and there-
fore relatively the more active. Similarly, in the case of the
balance described, the activity of the lower end, which deter-
mines the flow upwards, was opposed and balanced by the
increased activity of the upper half, induced by increased
hydrostatic pressure.^ The experiment which I am about
to describe, besides demonstrating the effects of cold and
warmth, also lends strong support to the view that the direc-
tion of the resultant movement of sap is determined by the
relative activities of the two ends of the stem.
When the balance had been obtained, as already described,
with a pressure of water of 105 cm., the record on the drum
became horizontal, as has been explained. Cold water
was now applied to the specimen at its lower end. As
sudden cooling constitutes a stimulus, while its continued
Action produces depression, we should expect a transient
augmentation of the activity of the lower end of the specimen,
followed by its diminution and arrest. The record should
therefore show a preliminary movement of water upwards,
followed by the reversal of the current, which should now, as
the permanent effect of cold applied below, be from the
' Rhythmic activity is, in general, increased by an increase in the hydrostatic
pressure. But there is a limit to this. Excessive pressure, above a certain critical
point, is found to depress rhythmic activity (p. 350).
378 PLANT RESPONSE
more active upper, to the less active lower end, or downwards.
The record will be seen to verify this anticipation in every
particular (fig. i6i). Cold water was applied below — repre-
sented by X in the record — which before application was
horizontal. After this we observe a movement of water
upwards, at a rate of four cubic mm. per minute. The upward
flow continues for about seven minutes, by which time the
Fio. i6i. Record obtained by Method of Hydrostatic Balance of
Successive Applications of Cold and Warm Water
After obtaining static balance, with horizontal record not shown in the
figure, cold water was applied below at moment x . The transitory
excitation thus occasioned causes movement of water upwards, followed
by depression and reversal of flow downwards. Warm water next
applied at f causes second reversal and flow of water upwards.
activity of the lower end of the specimen has become so
depressed as to cause reversal of the flow, which is now from
above downwards at an average rate of about six cubic mm.
per minute. This record was taken continuously for some
time, when hot water was quickly substituted for cold, in the
plant-vessel. A cross marks this point in the record.
{b) Action of warm ivaten — The result was not only that
MODIFICATION OF SUCTIONAL RESPONSE 379
its sluggishness was now obviated, but that the lower end of
the specimen was actually rendered the more excitable of
the two, and we observe a second reversal of the direction
of current, which now flows upwards.
(3) Method of Hydraulic Balance. — This Hydraulic
Method of Balance is much easier to carry out than the
arrest of movement by hydrostatic pressure, it does not,
moreover, in any way interfere with the normal movement of
water through the plant. For the balance is obtained and
the index rendered stationary, by the simple device already
explained, of allowing a subsidiary flow of water to enter the
plant-vessel, at a rate exactly sufficient to compensate for the
loss by ascent of sap.
(a) Action of cold. — This experiment was performed on a
leafy branch of Croton. The balanced horizontal record was
first taken at 22° C, after which ice-cold water was passed
into the vessel. A record was made of the immediate or
preliminary excitatory effect of this cold water on the rate of
suction, and continued during the return of the water to the
temperature of the room. In this record, then, we shall find,
besides the immediate, the continued effect of cold, and
subsequently the effect of gradual restoration to an ordinary
temperature. In the present record (fig. 162) we see the
transient and permanent effects of cold exhibited as before.
According to the method of Hydraulic Balance, as has been
explained, an ascending line in the record means a positive
variation, or increase of the rate of suction over the normal
A descending line, on the contrary, denotes a negative
variation, or diminution of the rate of suction below the
normal. And a horizontal line shows return to the original
rate. In the first, or upper, of the two curves in fig. 162,
the immediate effect of cold is seen in a very marked positive
variation. After the lapse of about five minutes, the effect
of continued cold is seen in the depression, which shows
itself by the reversal of the curve. This depression continues,
till the temperature of the vessel has returned to that of the
room, which takes place in the course of about forty minutes.
380 I'LANT RESPONSE
The normal rate of suction is now re-established, as seen in
the fact that the record becomes horizontal.
{b) Effect of ivann water. — I next tried the effect on the
same specimen of water at a higher temperature, falling by
degrees to the temperature of the room. We should expect
in such a case, to obtain the excitatory effect of rise of
temperature, with subsequent approach to the normal, with-
out any reversal, indicative of the transition from exaltation
to depression, such as was observed in the case of application
of cold. From the second and lower of the two curves
KiG. 162. Record, obtained by Method of Hydraulic Balance, of Successive
Effects of Cold and Warm Water
shown in fig. 162, it will be seen that this is the case.
The curve at first rises abruptly, and it continues to rise,
though with decreased speed, till, when the temperature of
the vessel is once more normal, it becomes horizontal,
indicating the resumption of the original rate of suction.
We have thus seen, by three different methods of inquiry
—those namely of the Unbalanced Shoshungraph, the Hydro-
static Balance, and the Hydraulic Balance— that the re-
cords, though of different forms, exhibit effects of which the
interpretations are identical. For delicate experiments, the
MODIFICATION OF SUCTION AL RESPONSE 38 1
Hydraulic Method of Balance will be found most sensitive :
for ordinary purposes, however, the unbalanced Shoshun-
graph is simple and efficient ; and in the investigations which
follow I shall use this method only.
Explanation of suction, when the root is killed by
boiling water. I shall now take up the apparently anoma-
lous case in which, when the root has been killed, by pouring
boiling water over it, the suction of the plant is nevertheless
maintained. In such an experiment, the normal record was
first taken, and on allowing boiling water to enter the plant-
chamber there was a steep rise in the record, showing the
excitatory action due to the application of hot water. The
boiling water was now passed in continuously for several
minutes, so as to ensure the killing of that portion of the
plant which was immersed in the vessel. On allowing the
water in the vessel to return to the temperature of the room,
it was found that suction continued, at an even greater than
the original normal rate.
This result would at first appear to show that protoplas-
mic activity had nothing to do with the ascent of sap. And
the objection would have been fatal, if the rhythmic activity
which produces suction had been confined to the roots alone.
But such activity is present to a greater or less extent
throughout every zone of the plant, and it is by the com-
bined action of all these that the ascensional movement is
maintained (p. 376). Thus, when hot water is poured on the
root, its first effect is a sudden increase of the activity of that
organ, by which warm water is carried to the higher zones,
there as a stimulating agency to increase this rhythmic
activity. It must be remembered that on reaching the
stem above the vessel, the hot water itself is considerably
cooled. Hence the only portion of the plant which is killed
is that which is actually immersed in the boiling water,
or in immediate contiguity with it. The unkilled portions
above continue their suctional activity unabated.
I have said that the suction, on the return of the water
to its old temperature, continued to take place at a greater
382 PLANT RESPONSE
than the original rate. This was due to the fact that, instead
of the extremely attenuated channels of the root-hairs,
through which suction normally takes place, there was now
substituted the whole mass of the root, acting virtually as a
wet rag, tied round the base of the living stem ; and indeed
it was found that, whereas the stem outside the vessel was
turgid, the portion within was limp and soft. The mass of
water which it was thus possible to suck up directly, b)-
means of the broad-sectioned stem, was evidently much
greater than could have been the case through the interven-
tion of the resistant organically conducting channels of the
rootlets.
We must not forget the obvious fact that a plant is a
colony of more or less independent living cells, each of which
maintains its physiological activity as an individual. The
death of one group does not necessarily, therefore, arrest
the physiological activity of its neighbours. The plant is
finally killed only when every one of its cellular elements has
undergone death.
Further proof that suction is an excitatory response.—
We have seen in the case of rhythmic Desmodimii, when it is
kept for a long time under unfavourable circumstances, that its
activity comes to a stop owing to the run-down of stored-up
energy. We also saw how the application of thermal stimulus
would re-initiate this activit}^ Again, if we keep a cut branch
of any plant in water, after a few days its suctional activity,
as is well known, disappears. This abolition of suction is
attributed to the blocking of the cut end by mucilage and
bacterial growths, and the making of a fresh section is found
to renew the activity.
But, though the blocking of the cut end of the stem by
outgrowths does, no doubt, obstruct the passage of water,
yet the total abolition of suction may not be due to this
cause alone. It may be induced, in part at least, by depres-
sion of the rhythmic activity of the tissue, owing to the run-
down of its latent energy. The making of a fresh section
does not decide this question, for, in doing this, we apply
MODIFICATION OF SUCTIONAL RESPONSE 383
the strong mechanical stimulus of a cut. I therefore devised
an experiment which appears to show that this run-down
of energy does in such a case constitute a factor in the
abolition of suction. Without in any way disturbing the
mucilaginous end of the stem, which had ceased to exhibit
its suctional activity, I supplied it with water somewhat above
the ordinary temperature. This thermal stimulation at once
initiated renewed suctional activity with great vigour, just as
its rhythmic mechanical activity was renewed by Desmodiuni
on the application of similar stimulus.
Osmotic versus excitatory action.— Though, under the
co-operation of a favourable disposition of osmotic substances,
the suctional activity of the tissue may be increased, yet I
have shown that suction is normally maintained even with-
out the co-operation of this factor (p. 376). I shall now
proceed to show that this suction may increase even in
opposition to osmotic action. And such a demonstration will
further prove the excitatory physiological nature of the
processes which bring about the ascent of sap. Among
various solutions of salt, some are physiologically neutral in
their effects ; ' of these, potassium nitrate may be taken as
an example. Others, again, like strong solutions of sodium
chloride, act as excitatory agents. The application of this
last reagent is known to initiate rhythmic excitation in animal
tissues. Similar effects have been shown to be brought about
by this reagent, in the case of Biophytinn and Desniodiuvi.
Thus, in a strong solution of potassium nitrate, we have
a reagent whose physiological action is more or less neutral
while its osmotic action is pronounced, and in a strong
solution of common salt we have an agent which is both
excitatory and osmotic at the same time. If, then, we
apply KNO3 solution to the base of a cut stem, placed
in the Shoshungraph, water will be osmotically withdrawn
from the plant, in opposition to normal suction, and the
' It should, however, be remembered that solutions, even of inactive salts,
above a certain strength, will induce physiological depression, and thus bring
about diminution of transpiration.
384
PLANT RESPONSE
normal suctional rate will be somewhat reduced. This is
seen in the accompanying record (fig. 163), which I obtained
with a cut branch of Croton. The normal rate of suction
was in this case twenty-six cubic m.m. per minute. After the
application of potassium nitrate solution this was found
to be reduced to seventeen cubic mm. per minute. But if,
instead of this, we apply strong solution of sodium chloride,
two antagonistic effects will be produced. One, due to
Fu;. 163. Effect of Strong
KNO3 Solution
The first record shows the nor-
mal and the second the de-
pressed rate of suction caused
by the reagent.
IG. 164.
Effect of Strong NaCl
Solution
Tlie first record shows the nor-
mal and the second the ex-
alted rale of suction caused
by the reagent.
osmotic action, will oppose suction ; and the other, due
to the excitatory nature of the reagent, will accelerate
suction. The resultant effect will, then, be modified by the
excitability of the experimental plant itself In some cases
we should expect to find the excitatory reaction predomi-
nant, and in others the osmotic. In repeating the experi-
ment, on different specimens of Croton, I have found these
theoretical inferences to be fully verified. As the more
interesting of the two cases, I give a record (fig. 164) in
MODIFICATION OF SUCTIONAL RESPONSE 385
which the excitatory effect is shown by the very great
increase in the rate of suction induced after the application
of the reagent. We have here a very great enhancement of
the ascensional movement of water in spite of the osmotic
attraction of the solution, which alone would have retarded
the normal rate.
The action of poisonous reagents. — In studying the
effect of poison on the rhythmic activity of Desmodium^ we
found that this was modified by the tonic condition of the
plant. Thus a vigorous specimen was shown to be much
less affected by poison than one which was weakly. Certain
poisons, again, act more quickly than others in inducing
the death of the plant. We have also seen, in that experi-
ment in which the root was killed with hot water, that
upper and unkilled portions of a specimen will continue
to exhibit suctional activity when lower parts are killed ; and
also that, in general, the killed area offers no barrier to the
passage of water through its dead tissues.
That a poison can easily pass through killed tissues,
owing to the suctional activity of cells higher up, we have
seen in our experiments on Desmodium, when the cut end
of the petiole was placed in copper sulphate solution (p. 326),
It is fortunate that in this case, during the ascent of poison,
we have areas, the activity of which is indicated visibly
by the rhythmic motile indications of the pulvini of the
inserted lateral leaflets. That copper sulphate solution
arrests rhythmic activity and induces death, is seen by
the rapid stoppage of pulsation, when we apply it directly
on the pulvini of the pulsating leaflets. When it is applied,
however, at the cut end of the petiole, the arrest of
pulsation takes place much later, this delay being due to
the time taken for the poison to ascend through the
intervening distance. This shows clearly that successive
zones are killed one after another, and that the death of a
point below does not stop the suction above. From this
experiment it is evident that the application of poison at
the root, or the cut end of a stem, does not in general arrest
C C
386 PLANT RESPONSE
suction, until the whole plant is killed. And from the
Shoshungraphic records we find that the final arrest occurs
after an appropriately long period.
In this connection, I shall describe some very interesting
results of rapid arrest of suction, which I have often obtained
by the action of poison. I was already familiar with a fact
which I had come across while studying the effects of various
chemical reagents on the longitudinal response of radial
organs namely, that death was attended in such cases either
by an abnormal contraction or by an abnormal relaxation.
These two effects were liable, again, to be modified by the
Fig. 165. Effect of Copper Sulphate Solution
The first part of the record shows the normal rate 01 suction. The asterisk
denotes the time of application of the poisonous reagent.
tonic condition of the tissue. In now studying the effect
of solution of copper sulphate on the suctional activity of
Croion, I noticed certain peculiarities in the record, which
appeared to be related to the results just described. These
peculiarities, it should be mentioned, were specially notice-
able in those specimens which were experimented on during
the month of April, that is to say, at the end of the Indian
spring.
In a particular experiment the normal suctional rate
had been fifteen cubic mm. per minute. On the application
of copper sulphate, the suctional movement was quickly
MODIFICATION OF SUCTIONAL RESPONSE 387
arrested, and this was followed almost immediately by a
slight movement in the negative direction, showing that, by
some spasmodic contraction, water was being expelled from
the tissue. This phase was succeeded by an almost com-
plete arrest of suction, there being now only the feeblest
ascensional movement (fig. 165). Within a short period
after this, on washing off the poisonous reagent, it was
found that the arrest had been temporary only, suction being
renewed at the rate of eleven, instead of the normal fifteen,
cubic mm. per minute.
I applied the poison once more, and allowed it to act for
thirty-six hours. The arrest was then found to be permanent
— that is to say, the substitution of fresh water induced no
revival of response, the plant being killed throughout.
Strasburger, as we have seen, in his experiments on the
effect of poisonous reagents on plants, found that the reagent
is carried to the top of the tallest tree ; from this fact it was
inferred that since all the cells in the path of the poisonous
solution must necessarily be killed by its action, therefore
the activity of living cells was not the essential factor in the
ascent of sap. But I have proved that the ascent of sap
is brought about, not by any localised group of cells in a
particular region, but by cells which extend throughout the
length of the plant. Even after some of these have died,
therefore, by the access of poison, those above are still active,
and will continue to exhibit suction till they in their turn are
finally killed. It will thus be evident that the movement
of ascent cannot be completely abolished till the poison
has effectively reached the very top. As all the living cells
are actively concerned in the work of suction, this con-
veyance of poison to the top of the plant is what was
to be expected. Only after such conveyance, indeed, could
permanent arrest possibly take place, and, in fact, Strasburger
himself mentions that the movement of water did come to a
stop when the poison reached the top of the tree.
PLANT RESPONSE
Summary
Rhythmic activity being as a rule exalted by rise of
temperature, suctional response, as one of its effects, also
undergoes an increase.
Suction being an expression of excitatory response, the
direction of the resultant movement of sap is determined by
the relative excitabilities of the two ends of a tissue. Under
certain circumstances, the normal direction of movement of
sap may be reversed.
The application of cold produces a transient excitation,
and thus causes a preliminary enhancement of suction. Pro-
longed application of cold, causing a depression of excit-
ability, brings about arrest of suction.
As suction is produced by the rhythmic activity of the
tissue of the entire plant, local death, as by scalding or
application of poison, does not cause its arrest, until the
whole plant is killed.
When the sum total of the latent energy of the tissue —
that is to say, its tonic condition — is below par, its rhythmic
suctional activity comes to a stop ; fresh application of
stimulus, however, renews this activity.
Osmotic substances, as regards their stimulatory action,
may be either neutral or excitatory. If such a solution be
applied at the root, there will in the former case be a diminu-
tion, and in the latter, if the excitatory action be relatively
great, an increase of suction.
The application of poison abolishes local excitability and
power of suction. In some cases this arrest of suction may
occur quickly. But the total abolition of suction by poison
only takes place, for reasons already explained, on the death
of the plant as a whole.
CHAPTER XXX
THE PHENOMENON OF PROPULSION OF SAP
AND ITS VARIOUS EFFECTS
The mechanics of the ascent of sap : {a) Uni-directioned flow — (/;) Initiation of
multiple rhythmic excitations— Connection between conduction of excitation
and conduction of sap — Rapidity of ascent of sap accounted for by stimulatory
action —Positive and negative pressures due to one cause — ( i ) Positive pressure
— (2) Negative pressure— (3) Irregular variations of pressure — Direct con-
duction and conduction by relays-- Excretion of water — Excretion of nectar —
Translocation of organic food-substances— Mechanical response to suctional
activity — Effect of warmth — Effect of cold — Explanation of the drooping of
leaves during frost— Explanation of response and recovery — Antagonistic
actions of internal energy and external stimulus.
There are various phenomena connected with the transport
of water in plants, which are at present considered as entirely
distinct. Thus, for example, when a plant is cut above the
root, the exuding water exerts a considerable positive pressure
on a manometer, this being known as root or exudation
pressure. But when manometers are inserted in lateral holes
bored in the trunk of a tree, a negative pressure is observed.
These facts have led to the inference that there exist, in a
transpiring plant, two independent forces, one of suction, and
the other of pressure. The negative pressure, again, which
is, generally speaking, maximal at the top of a tree, falls to
a minimal value near the root. But this fall is characterised
by very irregular fluctuations, the negative pressure, in a
zone below, being sometimes greater than that at a given
distance above.
I shall now proceed to show, however, that these very
various results are not actually due, as supposed, to the
operation of distinct forces, but are, on the contrary, so many
different effects, under different conditions, of the rhythmic
390 PLANT RESPONSE
cellular activity of the plant-tissue. I shall show that this
activity is sufficient to account not only for the phenomena
of the excretion of solution from pores and nectaries, but also
for other phenomena, whose connection with it has been little
suspected.
The mechanics of the ascent of sap. — We have seen
that throughout the plant there are active rhythmic tissues,
which by their excitatory activity bring about the movement
of water. We have next to inquire, therefore, as to how this
excitatory action is initiated, and further in what manner so
many activities in different zones of the stem are correlated,
so as to give rise to a uni-directional flow, generally upwards ;
for the rhythmic activity which may cause any given group
of cells to act as a pump, would not alone be sufficient to
account for the regulated, one-directioned flow of sap, since,
while one such group propels water in one direction, there is
no obvious reason why another should not propel it in the
opposite.
{a) Uni-directioned flow. — In connection with the trans-
mission of multiple excitatory waves, such, for example, as
those which we have seen in Biophytum^ it is important to
note that these repeated excitations are all initiated at the
original point of stimulation, and are propagated in proper
sequence from point to point outwards. Thus, after each
wave of excitation is exhausted, it starts anew from the
original point. This sequence of multiple excitations is
exactly parallel to that which is observed in multi ciliated
ti.ssues in which the cilia repeatedly contract in sequence.
Thus a single cilium at one end gives, as it were, a signal
which is followed serially by the rest.
This being so, it is clear that if such a multi-ciliated
tissue take the form of a hollow tube, with the ciliated surface
inwards, and if the tube be filled with water, then, owing to
this peculiarity of the multiply-responding cilia, the water
will always be driven in one direction. A somewhat similar
phenomenon occurs in the blood- circulation of animals, where
the sinus giving the signal, the rhythmic contraction of the
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 39 1
heart proceeds towards the ventricle, and the pumping-action
thus initiated produces a one-directioned flow of fluid.
In the leaf of Biophytum, strongly excited at, say, the
inner end, we have similar rhythmic excitations, passing in
regular succession from leaflet to leaflet, the innermost leaflet,
which is near the seat of multiple excitation, giving the
signal to the rest. And just as rigid sequence is observable
in the movements of motile cilia, so it is also seen in the
plant, by the depression in orderly series of its lateral motile
leaflets.
{]}) Initiation of multiple rhythmic excitations. — We have
just seen that in multiple rhythmic response, the multiple
waves of excitation proceed serially outwards, from the point
of excitation. We also saw in Chapter XXI. that the wave
of excitatory contraction is attended by the propulsion of
water in the direction of propagation of excitation. It
remains to be determined, then, with regard to the ascent of
sap in a plant, how this one-directioned propagation of
excitation is initiated.
In the case of an intact plant, the root is acted on con-
stantly by various forms of stimulation, among which are
(i) contact with the soil; (2) friction of the growing organ
against rough surfaces ; (3) turgor of its own tissues, due to
absorption of water ; and there may also be in addition,
stimulation by (4), chemical substances of various kinds in
the soil, which possibly exert some excitatory influence. All
these factors, separately or in combination, serve to set up
multiple rhythmic excitation at the extremity of the plant,
and the excitatory effect is transmitted upwards, preferably
along certain better-conducting tissues. Similarly, in the
case of a cut branch placed in water, rhythmic excitation is
initiated at the lower end by excessive turgor, just as we
found rhythmic movements to be initiated in Desmodium
leaflets at standstill, by the artificial increase of internal hydro-
static pressure, that is to say, by excessive turgor (p. 348).
Connection between conduction of excitation and con-
duction of sap. — It was said in the case of the intact plant
392 PLANT RESPONSE
that excitation is transmitted upwards by conducting tissues
and we have already seen, in studying transmission of excita-
tion (p. 250), that the fibro- vascular elements are those which
conduct best. It is therefore to be expected that the move-
ment of water, which is itself an excitatory effect, should
also follow by preference the length of the fibro-vascular
elements ; and it is here worthy of note that the ascent of
sap is known to take place preferably along such channels.
Again, I have shown (p. 250) that the power of conducting
excitation is greater along the length of the plant than
across ; and we find also that the same is true as regards the
transport of water, which is known to be greater in the direc-
tion of length. Conduction of excitation takes place with
very great slowness across parenchymatous tissues, and the
same is the case as regards the conduction of water through
such a tissue. If, further, a plant were to be excited from
above, instead of below, the transmission of excitation would
be downwards, instead of upwards. We might then conceive
of the possibility of reversing the direction of the normal
transport of water. In such a case, rhythmic excitation would
need to be initiated at the top, instead of at the bottom, of the
plant. This may be seen in the experiment already referred
to, of placing a leafy branch upside down, with its leaves in
water. Rhythmic activity due to excessive turgor being
now initiated at the upper extremity of the branch, the water-
movement is reversed, and sap exudes from the cut end of
the stem. Turning back, however, to the subject of the trans-
mission of excitation, we have found, it will be remembered,
in the case of Biophytum that this takes place with greater
rapidity in a centrifugal than in a centripetal direction, that
is to say, in the direction of the normal transport of water.
It is therefore interesting to note that a reversed water-
movement is, in general, known to be somewhat less rapid
than the normal flow.
Rapidity of ascent of sap accounted for by stimula-
tory action. — One great difficulty with regard to the ascent
of sap has lain in its relatively great rapidity. No theory
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 393
hitherto suggested has been held to account for this. As,
however, we now find that the ascent of sap is due to the
propulsive energy of vigorous excitatory contraction pro-
ceeding from cell to cell, the rapidity of the movement is
easily understood.
We thus see how by the action of this cellular machinery,
set in motion by stimulus, an upward movement of water
takes place. We have in fact an active chain of pumps,
working throughout the length of the plant, partly carrying
water themselves, and partly pumping it into the better con-
ducting vessels of the xylem ; and there is no limit to the
height to which it may, by such means, be lifted.
Positive and negative pressures due to one cause. — Let
us suppose an india-rubber pipe,-open at its upper end, and
provided throughout its length with a series of pumps, one
above the other, each of these being independently engaged
in raising water upwards. The individual activity of these
several pumps may or may not be uniform, but, provided
that they are sufficiently numerous, when the pipe is placed
in connection with a supply of water, the result of their com-
bined action will be that water will be sucked in at the lower
end, and ejected at the upper, in a uniform stream.
If now we confined our attention to the lowermost pump,
it would appear to us to \iQ forcing water up ; if, on the other
hand, we observed the uppermost pump alone, it would
appear to be sucking water up ; and if, finally, we selected
some intermediate point for scrutiny, we should discover that
the pumps above were sucking, and those below pressing
water upwards. Thus, one single effect, namely, the rhythmic
activity of pumps, is made to appear various, by simply
changing the point of view. Again, certain peculiarities of
variation of pressure may appear in the pipe as a whole or
in particular parts of it, depending on the rates of supply and
removal of water.
(i) Positive pressure. — We may now suppose the
aperture of escape at the upper end to be narrowed. The
water pumped into the flexible pipe being thus in a state of
394 PLANT RESPONSE
compression, will produce a bulging or over-turgid ity, and a
manometric tube inserted laterally will indicate a positive
pressure. Or if, under these circumstances, we make a cut in
the pipe, we shall observe exudation under pressure. Now,
this corresponds to the exudation pressure, causing so-called
'bleeding,' which we find on making incisions in plants in
spring time, when the loss of water by transpiration is feeble,
the buds being still unfolded; or even in summer, if trans-
piration be by any means prevented.
In connection with this exudation of sap, we have to
remember that the whole question is one of income and ex-
penditure. Generally speaking, the loss by transpiration is
less at the end of winter or the beginning of spring than in
other seasons. Thus the wild date pci\m,OY Phcenixsylvestfis,
yields considerable quantities of sugary sap from incisions in
the stem, at the end of winter or the beginning of spring. In
the case of the Palmyra palm {Borassiis flabelliforinis) of
Bengal, however, the increase of cellular activity in summer
more than compensates for the loss by transpiration, and sugary
juice is collected in the height of summer at the top of the
tree by incisions in the peduncle. The pressure exerted by
the sap may be gathered from the fact that these trees are
often more than a hundred feet high.
(2) Negative pressure. — We may next suppose that in
the chain of pumps, those at the upper end are the most active,
and the aperture of escape wide open, the removal of water
being further aided by evaporation. The loss of water being
thus greater than the supply, it is clear that there will be a
negative pressure in the pipe, and the mercury in a testing
lateral manometer will be sucked in. This corresponds to
the negative pressure exhibited by an actively transpiring
plant. In such a case it will be noted that the activity of the
rhythmic cells, which is the fundamental cause of the ascent
of sap, is further aided by the evaporation from the leaves.
Concentration of cell-sap also, and the osmotic action
thereby produced, may then constitute an additional auxiliary
factor.
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 395
When the stem of an actively transpiring plant is cut across,
the stump of the plant does not always immediately show
bleeding, but often, on the contrary, will suck in water. This
is generally ascribed solely to the existence of negative pres-
sure in the stem. There is another element in the problem,
however, which is generally overlooked. By such stimulus
as that of amputation, excitation must be produced at the
cut end of the stem, which is propagated downwards, and
tends to induce a reversal of flow. But when the negative
tension and the excitatory effect, due to stimulus of cut, have
both subsided, exudation will begin to take place.
(3) Irregular variations of pressure.- It is evident
from what has been said that the hydrostatic pressure at any
given zone of the tissue will depend on the relative activities
of cells below and cells above. Cells immediately above, by
their activity, produce suction or negative pressure ; and
those immediately below, an increase or positive pressure.
The resultant pressure at any individual zone depends,
therefore, on the algebraical summation of these. Now,
though all the cells throughout the length of the plant may
be active, yet there will be some difference in their activities.
Or the same cells again, at different periods of life and
different conditions, may undergo variations of their activity.
There are, in nature, many disturbing influences which
produce local variations of excitability ; hence, by the dis-
tribution of cells whose excitabilities are irregular, we may
obtain a variation of internal pressure from the top to the
bottom of the plant, which is not uniform but fluctuating.
Direct conduction and conduction by relays. — I have
already said that the movement of water, being mainly
brought about by excitatory reactions, will take place
preferentially along conducting channels. We have seen that
every tissue possesses the power of conduction to a greater
or less degree ; in parenchymatous cells, however, owing to
the presence of numerous more or less complete septa, trans-
mission is enfeebled, whereas in prosenchymatous fibro-
vascular elements it is very much facilitated. Thus, for
396 PLANT RESPONSE
conduction through long tracts, the fibro-vascular tissues are
the most favourable. But even in this case the transmission
of the excitatory effect may be much enfeebled by distance.
Or an interposition of parenchymatous elements may offer a
relative obstruction to the transmission. In such cases there
might be pseudo-conduction by means of ' relays.' For as an
example of the last case we may imagine a mass of excitable
parenchymatous tissue, against which a conducting tissue
abuts. This mass, being supplied with water by the conduct-
ing elements, may become over-turgid, and thus rhythmic
activity may be initiated in it de novo.
That rhythmic activity may under favourable circum-
stances be started locally in a mass of excitable tissue, we
have seen in the case of the pulvinus of Desmodimn. For the
fact that activity in this case was not due to any transmitted
impulse was proved in the experiment on the localisation of
the excitable area, when it was found that an isolated leaflet,
if sufficiently turgid, could pulsate. Under natural condi-
tions, the necessary turgidity is maintained by the cellular
activity of the tissue below. It is worth while to remember,
in this regard, that the characteristic pulsation of the leaflet
has no immediate connection with that rhythmic activity of
the plant-tissue which brings about the ascent of sap. The
period of pulsation of the leaflet is determined by certain
constants of its cell-complex. We may in fact have various
vibration-periods in different organs of the same plant, the
different oscillations being brought about by the turgidity
caused by the ascent of the sap, just as the same electrical
current may give rise to various frequencies of vibration of
different electro-magnetic vibrators included in the same
circuit.
Excretion of water. — The rhythmic activity of a mass
of excitable cells is seen again in the case of such as ac-
tively excrete water. A very striking example is that of
Colocasia esciileiitian, in which the successive expulsions of
water-drops noticed by Musset were as many as eighty-five
in one minute.
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 397
A distinction is sometimes made between this excretion —
due to the special local activity of a certain group of cells —
and the somewhat passive excretion of water from water-
pores, which is said to be caused by the general pressure of
exudation. But this difference is really one of degree and
not of kind. All cells are excitable, and the exudation
pressure itself is produced by cellular activity. As regards
excitability, however, we may have a transition from
moderately to highly excitable cells distributed in a continu-
ous or discontinuous manner. Even in the stem we have
seen that there are cases of irregular distribution, bringing
about irregularities of water-pressure. Extreme instances of
these are found in Dcsmodium, and in the actively excreting
cells of Colocasia, where highly excitable cells are localised in
special areas. A test which is sometimes insisted on as a
means of distinguishing between the active and so-called pas-
sive excretions is, that in the latter case the flow ceases from
the excreting organ, as soon as the branch is cut off. But
this is not by any means a satisfactory proof of the absence
of active excreting cells in the latter. We saw that the only
distinction between the activities of the multiply-responding
tissues of Biophytuni and Desmodium, lay in the fact that the
latter had the capacity to hold latent a large amount of
energy by which the rhythmic activity was maintained even
on the cessation of a directly exciting cause. In the case of
Desinodiiun, indeed, if the tonic condition be above par, and
the leaf as a whole be cut off and isolated, without any supply
of water, the rhythmic activity will be maintained for a con-
siderable time, though the turgidity is undergoing constant
diminution. But when the tonic condition of the plant is
below par, its rhythmic activity comes to a stop, and can
only be maintained by an artificial increase of internal
hydrostatic pressure. Similarly in the case of water-excreting
organs, we have some which under a favourable condition can
maintain their activity for a considerable period, even when the
supply of water is cut off, while in other instances activity can
be maintained only under favourable conditions of turgidity. •
398 PLANT RESPONSE
Excretion of nectar. — It is often assumed that the
excretion of nectar is due to plasmolytic action. The
excreted solution of sugar dries up by evaporation, and this
by plasmolysis draws up more from behind. We have seen
that the ascent of sap in the plant is brought about mainly
by rhythmic activity, and that concentrated solutions in the
leaves may help this movement osmotically. In the case of
nectaries, the presence of concentrated sugar solution outside
may thus help continued excretion, but this does not explain
the initiation of excretion, which could only have been caused
by the rhythmic activity of cells.
Translocation of organic food-substances.— Though
the flow of organic food-materials towards places where there
is a deficit may be brought about by diosmosis, yet such a
mode of diffusion must be extremely slow. A more rapid
transport than could be produced by this means would appear
to be a necessity. There are certain considerations moreover
which may be brought forward, tending Jo show that this
translocation of food- materials receives considerable aid from
stimulatory actions. Czapek's observation, too, that there is a
cessation of translocation in a chloroformed leaf-stalk, points
to the inference that it is a physiological process.
We know that an excitatory action proceeds from the
more to the less stimulated. Now the accumulation of a
large quantity of organic food-material may of itself act as a
stimulating agent in a given case ; thus an excitatory move-
ment would proceed from cell to cell, from places where there
was excess to places where there was deficit. In this way
large quantities of food-materials may be rapidly transported
by excitatory reaction, through conducting channels. And
such transport is also possible by stimulatory action even
through unspecialised cells. These ordinary cells are known
to be connected with each other, by means of pores and
plasmic threads. Mr. Horace Brown has shown {Phil. Trans.
vol. cxciii.) that transport of fluid may take place through
such a 'multi-perforate septum' with almost as great rapidity
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 399
as if no closing membrane were present. It is clear
that by the contracting expulsive action of excited cells,
transport of materials might be brought about more rapidly
than by mere osmotic action.
Mechanical response to suctional activity. — The effect
of that internal rhythmic activity which, as we have seen, is
caused by the absorption of energy from various forms of
stimulus, is to induce an increase of turgidity throughout
the plant. I shall next proceed to describe a mechanical
means of obtaining some indication of this internal
activity.
I have shown that owing to the dorsi-ventral differentiation
in the pulvinus of Mimosa, when the turgidity of the organ is
increased, the leaf is erected ; and when the turgidity is
decreased, as by the direct action of an external stimulus, the
leaf is depressed. Similar effects occur not in pulvinated
organs alone, but in all dorsi-ventral organs, in which the
lower half is more excitable than the upper. Thus, for
example, the lower half of the petiole carrying the lateral
leaflets of Biophytiwi is more excitable than the upper half.
It is true that the leaf possesses a slightly developed pulvinus ;
the differential excitability on which the motile response
depends is not, however, confined to this pulvinus alone, but
extends throughout the length of the petiole. The petiole
thus acts like a diffuse pulvinoid.
The action of stimulus on the diffuse pulvinoid oi Biophytuvi
and on the pulvinus of Mimosa is to produce the fall of the
leaf. And just as the leaf of Mimosa was erected by increased
turgidity, so in Biophytum also we should expect to have, with
similar increase of turgidity, a similar responsive movement
of erection ; and this I find to be the case. The erection of
the leaf of Mimosa or Biophytum is therefore a mechanical
indication of an increase of turgidity or of positive turgidit)--
variation, by whatever means this maj^ have been induced.
Such an increase may be brought about by an augmentation
of suctional activity. And the up movements of dorsi-ventral
400 PLANT RESPONSE
petioles in which the lower surface is the more excitable may
thus be taken as mechanical indications of such added
activity.
Effect of warmth. — We have already seen that applica-
tion of warm water to the root of a plant, increases its suctional
activity. I was able to demonstrate this fact by means of the
mechanical response o^ Mimosa and BiopJiytuui. On making
the application, the leaves of both plants responded by erec-
tion to a position which in the case of Mimosa was 9 mm.
and in the case o{ Biophytum 12 mm. above normal. A rise
of temperature we have also seen to have the effect of enhanc-
ing the internal energy of the plant. By doing this, it brings
about an erection of the leaves.
Effect of cold. — We have seen, on the other hand, that
the effect of cold is to stop rhythmic activity, and thereby to
arrest the ascent of sap. The motile indic-ation given by the
leaf would in this case be the opposite to that of positive
turgidity-variation, that is to say, would consist of a fall or
droop. On applying ice-cold water to the root of Mimosa
I found that the effect of cooling was to induce a lowering or
drooping of the leaf, to a position of 3 mm. below the normal.
In Biophytum the corresponding fall was through 5 mm.
Lowering of temperature, by depressing the internal energy
of the plant, has the same effect.
Explanation of the drooping of leaves during frost. —
These experiments offer an explanation of that drooping of
leaves which is observed in frost, and of the disappearance of
this drooping when a plant is restored to a warmer atmosphere
indoors ; for we have seen that when the internal energy of
the plant is normal, or above par, its suctional activity and
consequent turgidity are high, this favourable internal condi-
tion being outwardly exhibited by the erection of the leaves.
But when the internal energy is below par, the reverse effect
is seen in their droop or fall.
Explanation of response and recovery. — It has been
stated, when describing the effect of stimulus in inducing a
fall of the leaf, and its subsequent erection, that the latter
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 4OI
movement, namely, that of recovery, was an active and not a
passive process. Certain experiments that I am about to
describe will enable us to analyse response and recovery
still more closely.
When an organ is locallycxcited, the contraction induced
results in an expulsion of water, thus reducing the turgidity
of the organ, and the consequence of this is a mechanical
fall of the leaf. Some internal activity now forces the water
back into the organ from
which it was expelled, and
we have the recovery or
erection of the leaf. That
it is the internal energy
of the plant to which this
recovery is due, will be
seen clearly from the
following experiment. A
leaf of BiopJiytiim falls, as
has been said before, when
it is stimulated, whatever
be the form of stimulus.
I shall explain in my
chapter on the effect of
stimulus of light, that the
effect of photic stimulus
is the same as that of
any other form, while its
special advantage is that
it may be applied without
causing any mechanical disturbance, and that its intensity can
be very easily regulated.
On subjecting a petiole of BiopJiytiim, therefore, to the
action of sunlight, the leaf responded by depression, the
average rate of fall of the tip of the leaf being '16 mm. per
minute. In half an hour it had passed through almost 5 mm.
On now shutting off the light, the after-effect persisted for
another five minutes, when there followed recovery by slow
D D
Fig. 166. Record showing Recovery to be
Hastened by the Increase of Internal
Activity which is caused by Application
of Warm Water to the Roots
The first part of the record shows the fall
of the leaf, due to direct stimulation by
sunlight during thirty minutes. Stimulus
is next cut off at point marked by
interruption of record. After-eflfect
persists for five minutes, and there is a
subsequent slow recovery. Warm water
applied to root at moment x , with the
result of quickening the rate of recovery
by fifteen times.
402 ' PLANT RESPONSE
erection of the leaf at a rate of "12 mm. per minute. Warm
water was now poured on the root, thus suddenly increasing
the internal activity of the plant. Now, if it be true that
recovery is brought about by this factor of internal energy,
then the increase of internal energy ought to produce a
sudden augmentation of the rate of the recovery. That this
is the case will be seen from the record (fig. i66), where it will
be noticed that the enhanced rate of recovery from this point
is r8 mm. per minute, that is to say, fifteen times the normal
rate.
Antagonistic actions of internal energy and external
stimulus.— VVe thus see that it is the internal energy of
the plant — vaguely known as a favourable tonic condition —
which actively determines the recovery of the organ. This
will explain the fact which I have mentioned elsewhere, that
in summer, when the internal energy is considerable, the leaf
of Mimosa recovers from the effect of stimulus in about six
minutes, whereas in winter, when the internal energy is lov/,
the same process may take as long as eighteen minutes. We
thus see that as regards mechanical response the external
stimulus and internal energy act antagonistically. Local
external stimulus induces a diminution of turgidity, while
internal energy causes an increase of turgidity. Thus when
the internal turgidity is very great it opposes the mechanical
response to external stimulus. This we saw in the case of
over-turgid leaves of Mimosa, and in those of Artocarpus
during the rainy season, which, though excited, did not exhibit
mechanical response to stimulation (pp. 49, 58).
This will be clearly understood also from an attentive
consideration of the experiment, the record of which is given
in fig. 166. In that case, had the warm water which in-
creased the internal activity been applied earlier at the root,
that is to say during the application of external stimulus,
the induced internal turgidity would then have been so great
as to arrest the responsive down movement of the leaf The
amplitude of the response to external stimulus would thus
have undergone diminution or even abolition.
PROPULSION OF SAP AND ITS VARIOUS EFFECTS 403
We must remember, however, that the internal energy
which maintains the normal turgid condition is itself the
result of energy previously absorbed from external sources ;
and if the plant be cut off from these sources of external
energy, then its own tonic condition will fall below par. Thus
the leaves of many plants are seen to droop when kept too
long in darkness, and exposure to light makes them recover
their natural position of normal turgidity. Hence in the case
of a plant whose condition is sub-tonic the leaves may be
made turgid by exposure to light ; but after the attainment of
the normal tonic condition, exposure to strong light will
bring about the proper contractile response to stimulus, with
the characteristic external motile indication of diminished
turgidity.
We have thus seen the various effects produced by the
internal or latent energy of the plant. We have seen it bring
about the ascent of sap by means of the increased activity of
the plant-cells. It was seen to produce exudation pressure,
and excretion of nectar from intact plants. We have traced
it out to its appropriate expression by the lateral movements
characteristic of positive turgidity-variation, in the case of
anisotropic or dorsi-ventral organs. We have seen, too, how
necessary it is to the production of recovery of an organ from
the action of an external stimulus, the effects of local ex-
ternal stimulation, and of this internal activity, being opposed
in character. Any increase of this internal energy is thus a
factor tending to hasten the recovery of the organ from
stimulation, and when it is sufficiently great, it may, by its
antagonistic action, reduce the amplitude or even abolish
response to external local stimulus. We have again seen
this internal activity finding motile expression in the auto-
nomous movements of leaflets of Desmodiuiii ; but, for its
mechanical exhibition, we need not confine our attention to
the sensitive plants so called, for we shall find the same
internal activity exhibited mechanically by all plants, in their
rhythmic growth-responses, to be described in the following
chapters.
404 PLANT RESPONSE
Summary
The ascent of sap is fundamentally due to excitatory
reaction, its uni-directioned flow being brought about by the
passage, from point to point, of the co-ordinated excitatory
reaction, propelling water forward.
This rhythmic excitation is initiated in the intact plant
at its root, by stimulus of contact with soil, the friction of
the growing organ against rough surfaces, the excessive tur-
gidity caused by the absorption of water, and possibly by
the chemical stimulus of substances present in the soil. In
the case of cut branches placed in water, the excessive
turgidity at the cut end initiates rhythmic activity. Again,
if stimulus be applied at the top instead of at the root, the
direction of water-conduction may be reversed, along with
the reversal of propagation of excitation.
The following facts show the intimate relation between
the conduction of stimulus and conduction of water :
(a) The movement of water takes place preferentially
through the fibro-vascular elements, these being also the
better conductors of excitation.
(d) The conduction of excitation along a plant is greater
than across. The same is true of its power of transport of
water.
(c) Though conduction of excitation may take place
either upwards or downwards, yet there is a preferential
direction for such conduction. The same is true of the trans-
port of water.
The same movement of water produced by the co-
ordinated rhythmic activity of cells throughout the plant
appears as either suctional or pressure movement, according
to the point of view.
When the removal of water from the plant is in any way
arrested, a positive pressure is produced, owing to the exces-
sive accumulation of water. When, on the other hand, the
TROrULSION OF SAP AND ITS VARTOUS EFFF.CTS 405
loss of water by transpiration is greater than the supply, a
negative pressure will be observed.
The ascent of sap primarily due to cellular activity, may
be secondarily aided by evaporation from the leaves, and the
osmotic action of the concentrated cell sap in the leaves.
Owing to the distribution of unequally active cells, an
irregular variation of pressure is induced in the stem.
The excitatory movement may be transmitted to a dis-
tance by conduction, or there may be conduction by ' relays.'
An isolated mass of highly excitable tissue may thus be
excited de novo.
The excretion of water and of nectar are phenomena of
cellular activity, analogous to that which brings about the
ascent of sap.
The translocation of food-material is also probably due,
at least in part, to excitatory reaction.
The internal activity of the plant, causing increase of
turgidity, may be detected mechanically by that erection of
the leaf which is characteristic of the positive turgidity-
variation.
Any increase of internal activity is exhibited in dorsi-
ventral organs, such as the petioles of Mimosa^ Biophytiini,
and Artocarpus, by the erection of the leaf. Thus, when the
internal energy of the plant is increased by a rise of tem-
perature, the leaves become erected. Conversely, under the
action of cold, on account of the diminution of the latent
energy, the opposite effect, or droop, is induced. This
explains the drooping of various leaves during frost, and
their subsequent erection when brought into a warmer atmo-
sphere.
This internal energy is also an important factor in
bringing about the recovery of an organ from the effect of
external local stimulus. The effect of external local stimulus
in causing the diminution of turgidity of an organ is thus
antagonised by the internal activity, which causes an increase
of turgidity.
406 PLANT RESPONSE
The internal energy, when sufficiently great, may thus
hasten the recovery of the organ from the effect of
stimulus.
This increased internal energy may also reduce the am-
plitude, culminating in the total abolition of mechanical
response, as seen in over-turgid Mimosa, or in Attocarpus
during the rains.
PART VI
GROWTH
CHAPTER XXXI
THE RECORD OF GROWTH-RESPONSE
The simple Growth-Recorder — The Balanced Growth- Recorder — Rhythmic
growth-response--Growth-response and excitatory response — Law of direct
and indirect effects of excitation — Positive turgidity-variation as indirect effect
of excitation — Mechanical test — Significance of ' inner stimuli.'
One of the most characteristic manifestations of Hfe is
growth. The question then arises, whether this particular
manifestation is to be regarded as a distinct and specific
phenomenon, unlike all others, or whether it may be possible
to trace a connection between it and those responsive re-
actions with which we are already familiar.
The occurrence, in response to stimulus, of numerous
growth-curvatures, sometimes positive and sometimes nega-
tive in character, offers us again a problem of very great
complexity. It is sometimes supposed that stimulus retards,
and sometimes that it accelerates, growth. But it is difficult
to understand how the same influence can produce opposite
effects. Then, again, there is intruded upon the problem
the unknown effect of ' inner stimuli.' From all these it will
be seen that the subject of growth and growth- movements is
one of extreme obscurity, and that the difficulties which
baffle us can only be met satisfactorily if we are able to
analyse and follow out, one by one, the various elements that
enter into the problem.
We have seen in the Desmodium leaflet at standstill, and
in that of Biophytiini under ordinary circumstances, that
when the latent energy is not excessive, we obtain a single
movement in response to a single stimulus. When the sum
total of the latent energy of the tissue, however, is above par.
4IO PLANT RESPONSE
it is manifested in a rhythmic manner, by periodic variations
of turgidity, bringing on responsive movements. It was
also stated that responsive movements under the action of
stimulus took place in ordinary young tissues, these move-
ments being lateral when the tissue was anisotropic, and
longitudinal when it was strictly radial. It would follow,
then, that when the sum total of the latent energy in such
tissues w^as above par, they might be expected — like the
leaflet of Biophytutn or Desniodimn under similar conditions —
to exhibit their rhythmic excitation by repeated lateral or
longitudinal movements. I shall now proceed to show that
in the case of these young tissues, under favourable circum-
stances, this multiple rhythmic excitation finds expression in
the responsive movement known as growth.
In the majority of instances an organ is not absolutely
radial ; hence, during growth, we obtain the lateral respon-
sive movements which are known as circumnutation. In
bilateral growing organs, these movements are to and fro,
in strict parallelism to the to and fro movements of the
leaflet of Biophytnm. In such instances the axis of bilate-
rality is fixed, and the responsive movement takes place in
a definite plane. In the leaflet of Desviodium also rectilinear
movements are often observed ; but as a rule, owing to the
revolution of the bilateral axis, the movement of the leaflet is
circular or elliptical, and in the case of growing organs, from
the same cause, circular or elliptical movements of nutation
are common. The ideally simple and most interesting
example of this multiple rhythmic activity is seen, however,
in the growth-movements of radial organs, these being longi-
tudinal ; for there is not in this case that complication which
arises from the gradual shifting of the bilateral axis seen in
the growth of anisotropic organs.
In order to prove the identity of these rhythmic growth-
movements with multiple response, we have to show
(i) that such rhythm is a characteristic of growth; (2) that
each pulsatory growth-movement of the series exhibits all
the characteristics of true response ; (3) that the series itself
THE RECORD OF GROWTH-RESPONSE 41 I
is characterised by the same cyclic variation which we have
observed in the case of multiple response ; (4) that just as the
application of appropriate stimulus renews pulsation in a
Desinodinni at standstill, so, in a plant with growth at stand-
still, appropriate stimulation renews pulsatory growth ; and,
lastly (5), that the modifying influence of external agents is
similar in both cases.
From a series of observations, taken at intervals of several
minutes, on Spyrogym princeps, Hofmeistcr found that growth
undergoes fluctuation, the first and second maximal points in
his series of observations being separated by an interval of
forty-four minutes, and the second and third by an interval
of ninety-five minutes. Such experiments, however, have
laboured under the great disadvantage of discontinuity, and
in order to overcome this I undertook to devise some appa-
ratus by whose means growth-pulsations might be recorded
continuously, in such a way as to give not only the period, but
also the individual peculiarities of each pulsation. And I may
here forestall matters to say that by such means I have been
able to detect longitudinal pulsations two hundred times as
quick as those observed by Hofmeister.
Conditions to be kept in view^ — Before attempting to
demonstrate the pulsatory character of growth-movements,
however, I shall point out certain facts which it is essential
to remember. We have seen that under rapidly succeeding
excitations, the separate responsive effects become merged,
and a response is produced, which is apparently continuous,
although the stimuli themselves were discontinuous. This
is seen, for instance, in the first part of the tetanic curve
(figs. 49, 50). But when once the maximum responsive
effect is produced, as there can be no further additive
effect, the subsequent responses show themselves in a series
of fluctuations of the top of the tetanic curve. In that
form of response, which we are now considering, as there is
no maximum limit, the additive effect of growth continues
indefinitely. It is thus clear that when rhythmic excitation
is very rapid, it may produce a growth-movement which
412 PLANT RESPONSE
appears to be continuous. But when this rapidity is not
excessive, it should be possible, by employing sufficient
magnification and a suitably quick rate of movement of
the recording surface, to display its actual pulsatory cha-
racter. The sensitiveness of this mode of detection becomes
again very much increased if we employ the method of
balance, or compensation, which will be described presently.
We thus require a high magnification, and some means of
continuous record.
A high magnification may be produced by microscopic
optical projection, but this labours under the great dis-
advantage that the specimen is subjected to the strong and
unilateral stimulus of light, by which its normal growth-
movements are greatly modified. The ordinary auxano-
metric method, again, cannot be employed, (i) because the
magnification produced is not sufficiently great ; and (2)
because the inertia of the wheel, and the unavoidable friction
of the apparatus, themselves combine to obliterate the quick
pulsations of the growth-response.
The Crescograph. — All these difficulties were over-
come by the use of my Optical Lever, for making growth-
records. The lever is made extremely light, and the fulcrum-
rod rests on agate planes. When the tip • of the growing
organ is attached by a thread to the short arm of the Lever,
the length of the latter being -5 cm., and when the record-
ing surface is at a distance of 2-5 metres, a magnification
of 1,000 times is obtained. This is in most cases more than
sufficient. But, when necessary, a magnification of 10,000
times can easily be secured. In order to avoid any dis-
turbance due to vibration in the room, the apparatus is
supported on a steady bracket, fixed on the wall. Using
these ordinary precautions, records are obtained with this
instrument which are absolutely free from external disturb-
ance.
The Balanced Crescograph. — In records of growth
we obtain a sloping curve whose abscissa represents time ;
and ordinate, the elongation produced in the organ during
THE RECORD OF GROWTH-RESPONSE 413
that time. A variation of growth will produce a variation
in the slope of the curve. When this variation of growth,
however, is slight, the variation of slope of the curve is so
small as not to be detected. It was in order to detect and
measure such small variations that I devised the Method of
Balance, in which the average rate of growth is represented
by the horizontal line of balance, any fluctuation, even the
slightest, appearing as a deviation from this horizontal. The
influence of various agencies, again, may be displayed in a
marked manner, by using this method ; for in such cases we
do not so much require the rate of growth itself, as the
variation — i.e. acceleration or retardation- in the normal
rate, which is induced by one agent or another.
The principle of the Method of Balance consists in
making the spot of light — which is moving in response to
growth — become stationary', by subjecting it to a compen-
sating movement. An example will make this clear. We
shall suppose the average rate of growth to be V2 mm.
per hour. This will cause an excursion of the moving spot
of light, from, say, left to right, through 1,200 mm. by the
end of the hour, in that case v/here the magnification is 1,000.
Had the growth been uniform, this would have meant a
movement of 20 mm. per minute. But if not uniform, the
rate might sometimes have risen above, and at others fallen
below, this average. If now we subject the spot of light to a
uniform compensating movement, such as by itself would
have made it move from right to left of the recording sur-
face, to the extent of 1,200 mm. by the end of the hour, we
shall find that, being acted on by these two opposite move-
ments, of growth and compensation, the spot will remain
approximately on a single base line of compensation. The
fluctuations, or variations, which have occurred in this average
rate of growth, will, however, be recorded as deviations
to one side or other of this mean neutral line. Thus it
will be seen that the slightest deviation from a uniform rate
of growth, will be found displayed by the record of the
moving spot of light. We are further enabled, from our
414
PLANT RESPONSE
knowledge of the speed of the recording-drum, and the
balancing rate, and from an inspection of the curve itself, to
determine not only the periodicities, but also the absolute
value of the rate of variation of growth, at any given
moment.
The compensating movement to which I have referred is
effected by means of an hydraulic device. The spot of light
from the Optical
I.ever falls upon a
mirror attached to a
second lever, or to a
rotating wheel. The
arm of the lever, or
a thread which is
passed round the
wheel, is attached to
a float on the surface
of a cylinder of water.
Water is escaping
from this cylinder, by
means of a syphon
arrangement, at a rate
which can be adjusted
with the greatest
nicety. The float can
thus be made to de-
scend at any speed
that is desired, this
descent producing a
rotation of the second
lever or of the wheel.
Fig. 167. Diagrammatic Representation of
Balanced Crescograph
r, plant attached to Optic Lever, L, with mirror,
M, attached to fulcrum-rod, resting on knife-
edges, A and a' ; l', lever attached to float,
F F, with second mirror, M, attached to its ful-
crum-rod ; B, balancing wheel adjusting difter-
ences of level of the two limbs of syphon, s.
We have, then, two mirrors, of which one is rotated in one
direction by the growth-movement of the plant, and the
second in the opposite direction by the descent of the float.
A spot of light reflected on the two mirrors will thus remain
stationary when the precise balance is effected, by proper
regulation of the escape of water from the cylinder (fig. 167).
THE RECORD OF GROWTH-RESPONSE 415
This outflow of water is roughly adjusted by opening the
stop-cock, to a greater or less extent. The finer adjustment
is then effected by the suitable variation of the difference of
level in the two limbs of the syphon. One end is con-
nected with a flexible india-rubber tube, which is attached to
a string passing over a pulley, and fixed to the adjustable
wheel B, attached to the observer's table. Rotation of the^
wheel in one direction will depress this end of the syphon, so
increasing the flow, and in the other direction will raise it, so
diminishing the escape of water. It will be noticed that the
rate of outflow of the water is not in any way affected by the
variation of level of the water in the cylinder. It simply
depends on the difference of level between the two ends of
the syphon. The rate of descent of the float is thus regulated
with the utmost nicety, till an absolute balance is obtained.
The observer recognises the condition of balance when there
is no drifting of the spot of light on the recording drum.
The adjusting wheel is now fixed at this position of balance.
If any agent should induce an acceleration of growth, the
balance is disturbed, and the spot of light moves, say, to the
right, or in a positive direction. Any agent which induces
retardation will, on the other hand, cause a deflection of the
spot of light in a negative direction. Should there be any
natural fluctuations in the growth, the oscillation of the spot
of light will give an indication of the fact.
The record is made in the usual manner on a revolving
drum. Fig. 168 illustrates the complete apparatus, which
enables us to obtain a record under balanced — or by closing
the stop-cock of the syphon, also under unbalanced — con-
ditions.
The wheel is graduated, and the absolute value of the
compensatory movement, at any position of the circular scale,
can be previously calibrated, by fixing the plant-mirror, and
observing the extent of movement of the spot of light on the
drum, due to the subsidence of the float in a given time.
For the exhibition of pure longitudinal growth-response,
the most perfect specimens are the growing radial styles and
4l6 PLANT RESPONSE
stamens of flowers. There are also other organs, which are
more or less strictly radial, such as the peduncle and the
hypocotyl. But in these latter cases care should be taken
that the specimen have been so grown, that its different sides
have been subjected to uniform conditions of light or dark-
ness ; for one-sided illumination tends to produce anisotropy,
Fig. 1 68. Complete Apparatus for Crescographic Record under Ordinary
and Balanced Conditions
With specimens of all the types mentioned, I have obtained
multiple growth-responses when the constituent pulsations
were not too rapid. In some cases, indeed, the effects
were so marked that they did not even require a balancing
arrangement to render them conspicuous. xA.s an instance
of this, I shall give a record of the growth-response of a
THE RECORD OF GROWTH-RESPONSE
417
vigorously growing peduncle of Crocus which brings out in
an interesting manner the mechanics of growth (fig. 169).
Rhythmic growth-response. — We saw that when the
sum total of energy is above par, a tissue becomes self-
excitatory in a multiple or rhythmic manner, giving rise
to periodic turgidity-variations. There may thus be respon-
sive pulsations of increased turgidity, each followed by
slow recovery from
such excess. With
each such pulse, a
transient elongation
of the growing tissue
will be produced, and
the succeeding slow
recovery will be more
or less incomplete.
This incompleteness
is due to the deposit
of material which
fixes growth. The
irreversible or per-
manent growth-effect
produced by each
pulsation, will thus
be measured by the
responsive elongation
minus the recovery.
This is well illustrated
in fig. 169, where
three separate sets of
responses are given,
taken from a single specimen in the course of the day.
Growth, as will be shown, is not uniform throughout the
day, but exhibits variation, in consequence of changing
conditions, such as that of temperature. But for a short
interval of time, the rate of growth under a constant en-
vironment, it may be taken as uniform. In the present
. J<: E
Fu;.
[69. Multiple Growlh-responses
(Peduncle of Crocus)
The ordinate represents the extent of responsive
elongations in mm. ; the abscissa, time in
seconds.
4i8
PLANT RESPONSE
instance the maximum rate was as high as 0035 and the
minimum as low as "ooio mm. per minute. Confining our
attention to the uppermost of these series (c), we find that
the responsive elongation is very quick, and the recovery
slow and incomplete. The average period of a single pulse
is twenty seconds. The results of series (c) are given in the
following table :
Table showing Pulsations of Growth
Number of
pulse
Responsive
elongation
Recovery
Permanent
growth
Time
I.
•0020 mm.
•GO 10 mm.
•0010 mm.
20 seconds
2.
•0023 ,,
•0008 „
•0015 ,,
22 ,,
3-
•0018 „
•0005 ,,
•0013 „
18 „
4-
•0021 ,,
•0012 „
•0009 „
20 „
Total
•0047 „
80 „
It will be seen from this table that the total growth
in eighty seconds is -0047 mm., giving an average rate of
growth of "0035 mm. per minute. Had a magnifying
arrangement not been used, this average rate of growth,
shown by the dotted base line, would have appeared as con-
tinuous growth. By the magnification of the responsive
curve, however, we are enabled to see that such a rate is in
reality made up of numerous fluctuating growths of which it
is an average.
Growth-response and excitatory response.— If we
compare these multiple growth-responses with the multiple
mechanical responses oi Biophytiim (fig. 116), their similarity
is at once evident. As in that case, so here also, recovery is
not complete, and the series of effects produced is therefore
additive in both cases. With Biophytum, however, when
the leaflet is depressed to the utmost, a limit is reached ;
but in growth there is no such limit, and the summation
of effects may go on indefinitely, until senility and death
supervene.
THE RECORD OF GROWTH-RESPONSE 419
Yet there is a certain difference between these responses.
In the case of Biophytnvi, the response is due to a sudden
diminution, but in growth, to a sudden increase, of turgidity.
This might at first sight appear anomalous, but I shall
presently show that both are expressions of an excitatory
reaction ; for we have seen that when the responsive organ
of Biophytuni is directly excited there is an expulsion of
water, and the response is brought about by negative turgidity-
variation. This variation we shall for the sake of con-
venience distinguish as tJie direct effect of stimulation. We
saw in the last chapter, however, that an increase of the
internal energy of a plant gives rise to an opposite re-
sponse, that is to say, one characteristic of positive turgidity-
variation. The two responsive turgidity-variations, then, both
negative and positive, are alike in being expressions of the
excitatory reaction, though the negative variation is the effect
of external stimulus applied directly to the responding organ,
while the positive variation is to be regarded as the effect
of the internal energy of the plant. This internal energy
may itself have been derived previously by the plant from
external sources of stimulation, or the internal energy of a
given point may result from the application of a stimulus at a
distance. The pumping-in of water by the stimulated root is
an example of the latter case ; the cells are thus made tense,
and the potential energy of the tissue is raised above par.
Again, we may conceive of another interesting instance as
follows. When stimulus is applied at a distance the excita-
tory expulsion of water gives rise to a wave of increased
turgidity, which produces an abnormal positive response, and
this positive turgidity-variation we shall designate as the
indirect effect of stimulation. The wave of true excitation
may in this case reach the organ after that of positive
turgidity-variation, and give rise to the direct effect of stimu-
lation, that is to say, depression of the leaflet. We may next
imagine that the seat of stimulus is at so great a distance
from the responding organ, that the transmitted excitation
becomes too much enfeebled, by the long tract which has to
420 PLANT RESPONSE
be traversed, to produce the excitatory negative response.
In this case, nevertheless, the sudden expulsion of water at
a distance will give rise to a wave of increased turgidity,
which will reach the responding organ, and produce there
only that response which is characteristic of positive turgidity-
variation.
Positive response as indirect effect of excitation. —
Numerous experiments have been described exhibiting the
negative turgidity-variation as the direct effect of stimulation.
I have already described the production of positive turgidity-
FiG. 170. Responses of Leaf c f Artocarpiis to Thermal Stimulation
Thick dots show points of application of stimulation. In a stimulus
was applied near the responsive pulvinoid and gave rise to normal
response of fall— here represented as up movement— preceded by pre-
liminary erectile twitch. In l> stimulus was applied at a greater
distance. The true excitatory effect did not reach the organ, and we
obtain positive erectile response of positive turgidity-variation, here
represented as down.
variation as the indirect effect of stimulation (p. 400). A
fuller demonstration of this, by the electrical method, will be
found in Chapter XXXVII. I shall here give an experiment
which establishes the fact by means of mechanical response.
In fig. 32 (reproduced in fig. 170, a) was shown a series of
normal mechanical responses of negative turgidity-variation
obtained from the leaf of Artocarpus. The conductivity
of the petiole in this case is relatively feeble, and these
normal responses, preceded by preliminary positive twitches,
were obtained when stimulus was applied at a distance ot
THE RECORD OF GROWTH-RESPONSE 42 1
3 mm. from the responsive pulvinoid. I then repeated the
experiment with the same leaf, but applying stimulus at the
greater distance of 5 mm. The responses now consisted of
a series of up movements of the leaf, indicative of positive
turgidity- variations (fig. 170, b), the direct effect of stimulus
not reaching the organ.
Now, turning our attention to the growing organ, we find
that the fibro-vascular element, which possesses the power of
conduction to a high degree, is not yet fully established in
the zone of growth. If, then, contiguous to the growing
zone there be a mass of active tissue thrown into a state of
rhythmic excitation, it is to be expected that the indirect
effect of such stimulation will alone act, and give rise in the
region of growth to pulsations of increased turgidity. It will
be remembered from the last chapter that water is conducted
by preference along the fibro-vascular elements ; and since
these strands end below the zone of growth, it is clear that
there must be in this region an accumulation of water, and
consequent over-turgidity of the tissue ; a condition which is,
as we know, sufficient to initiate rhythmic excitation.^ This
region, then, acts like the actively excitable tissue of Colocnsia,
which, as we saw, gives rise to spasmodic expulsions of water.
In the latter case there is, however, a channel by which the
water escapes, thus relieving the pressure on the tissue ; but
the growing organ offers only a ad de sac, and the constant
repetition of hydrostatic blows thus effects those positive
turgidity- variations that are to result in the responsive
elongations and incomplete recoveries of the tissue, bringing
about growth-movements.
* Inner stimuli.' — It is thus seen that growth represents
the indirect effect of stimulus ; its motive power residing
in the rhythmic activity of the internal tissues of the plant.
This rhythmic activity has been shown to be, in its turn,
the result of the tonic condition of the plant, that is to say,
of the sum total of energy previously absorbed, and held
' Or the ovei-lurgidily of the growing region may be .sufficient of itself to
initiate rhythmic activity.
422 PLANT RESPONSE
latent in the tissue (p. 314), which we have designated as
the internal energy. We have thus succeeded in defining
the actual nature of those ' inner stimuli ' to which the
phenomenon of growth is usually vaguely ascribed.
From what has been said, it is clear that the responsive
peculiarities of the growing region are not per se in any
way different from those of any other excitable tissue, the
apparent contrast between negative and positive turgidity-
variations — that is to say, between responsive contractions and
responsive expansions — having been shown to depend upon
the fact that in one case we see the direct, and in the other
the indirect, effects of excitation. If this be so, it follows that
the direct application of external stimulus to a growing tissue
ought to have the normal effect of excitatory contraction.
In other words, while the action of the so-called ' inner
stimuli,' or internal energy, gives rise, as explained above, to
responsive expansions, the direct effect of external local
stimulation must be the production of responsive contractions.
That this is the case, will be shown in the next chapter.
Summary
Growth is a phenomenon of multiple response.
Each of these multiple growth-responses consists of a
sudden elongation, due to a pulse of increased turgidity,
followed by an incomplete recovery. The difference between
elongation and recovery is the irreversible growth-effect.
Such responses, when very rapid, appear as continuous.
In ordinary excitatory response there is a pulse of
diminished, and in growth-response a pulse of increased,
turgidity. Both are, however, effects of excitatory reaction.
When a tissue is locally excited, it gives a response of
negative turgidity-variation, that is to say, of contraction.
This is the direct effect of stimulus.
When the source of stimulation is behind, and the inter-
vening tissue does not conduct excitation, then the excitatory
expulsion of water finds expression in a positive turgidity-
THE RECORD OF GROWTH-RESPONSE 423
variation, which produces an expansion of the responding
zone of growth. The growth-response is thus the indirect
effect of stimulation.
The rhythmic activity of internal tissue supplies its motive
power to the zone of growth. This activity, depending on
the tonic condition of the plant, constitutes the ' inner
stimuli ' to which growth is to be ascribed.
CHAPTER XXXII
THE EFFECTS ON GROWTH OF INTERNAL ENERGY
AND EXTERNAL STIMULUS
Characteristics common to growth and to other forms of rhythmic response :
(i) Periodic groupings— (2) Effect of external stimulus in renewal of growth
when at temporary standstill — (3) Renewal of growth- pulsation by positive
turgidity-variation — (4) Effect of increased internal hydrostatic pressure —
(5) Effect of ascent of sap on growth — Effect of temperature on growth
— Comparison of various types of multiple response — Effect of external tension
on growth — Effect of direct application of stimulus on the growing region
— Similarities between motile and growth responses — Direct and indirect
effects of stimulus, and laws of growth.
Having shown in the last chapter that growth is a form
of multiple or rhythmic response, I shall now proceed to
demonstrate in detail the fact that in it also are found various
phenomena which are characteristic of rhythmic response in
general.
(i) Periodic groupings. — In the multiple response of
Biophytum, and in the autonomous response of Desjuodium,
we have noticed the occurrence of various periodic groupings,
the simplest of which consisted of an alternate waxing and
waning of the pulses. In multiple growth-responses, similarly,
we are able to detect such groupings,, of which the simplest
was shown in fig. 169 {c). When a continuous series of
records is taken, extending over some time, these groupings
undergo various changes, as is illustrated in that figure, the
three series {a), ib), and {c) having been taken with the same
plant at different intervals. It will there be seen that the
pulse-records in ic) represent an alternate waxing and waning
of amplitude ; that in those of {b) the responses are small at
the beginning and large at the end ; and, finally, that in the
INTERNAL ENERGY AND EXTERNAL STIMULUS 425
responses of (a) this order is reversed, the amplitude being at
first great, and undergoing a steady decrease to the end.
(2) Effect of external stimulus on growth when at
standstill. — It will be remembered that the rhythmic excita-
tion of Desniodium comes to a standstill under unfavourable
circumstances — that is to say, when the sum total of internal
energy has fallen below par ; and when this has happened,
the application of fresh external stimulus is found to renew
the activity. Growth-response, similarly, comes to a stop
when the plant is in an unfavourable /
condition with regard to light, tem-
perature, or moisture. I shall now
show that under such circumstances
the application of external stimulus
is found to be competent to renew
growth. Taking a specimen of the
hypocotyl of Tamarindus indkiis, in
which growth had come to standstill,
I stimulated the plant by thermal
means, and this was found to renew
the multiple response of growth, as
is clearly seen in fig. 171. The pulses
are here characterised by interesting
periodic variations. The period of
each is relatively long, the average
value being about six minutes. In
some other cases the renewed pulsa-
tions were so rapid as almost to
appear continuous. When the tonic condition of the plant
was very low, the energy supplied by brief stimulation was
only sufficient to maintain the rhythmic growth-activity
for a short time, and after this the plant would again return
to the state of standstill. It will thus be seen that when the
tonic condition of the plant is below par, the applied external
stimulus is absorbed, and, becoming latent, serves as internal
energy for the production of growth-response (cf p. 462). I
shall adduce other instances of this in the course of the present
chapter.
Fig. 171. Renewal of
Growth-pulsation by Ther-
mal Stimulus in Tainatin-
litis iiid/iits originally at
Standstill
426
PLANT RESPONSE
Fig. 172. Initiation of
Erectile Response in leaf
by Supply of Water to
partially Drought-rigored
Mimosa
(3) Renewal of growth by positive turgidity-varia-
tion. — We have seen that in Dcsnwdiuui in a state of standstill,
increased internal hydrostatic pressure renewed the rhythmic
activity. It was also stated in the last
chapter, that growth is a responsive
expression of the positive turgidity-
variation. We have seen further that
the mechanical expression of the
positive turgidity-variation in a dorsi-
ventral organ takes the form of
erectile response. Thus this erectile
response and growth-elongation are
to be regarded as two different forms
of expression of the same internal
activity.
If we take for example a i^XzxsXoi Mimosa which is under-
turgid, for want of sufficient supply of water, but not to the
extent of drought-rigor, the leaves are found to assume a
certain horizontal position, corre-
sponding to the degree of tur-
gidity. If we now supply the
plant with water, poured on at
the roots, the consequent sudden
increase of suctional pumping
activity is seen in the positiveerec-
tile response of the leaf (fig. 172).
Similarly, when an ordinary
plant, under the same circum-
stances, has its growth brought
to a standstill, the growth-elonga-
tion is found to be renewed on
the application of a fresh supply
of water. This experiment was
carried out on a seedling of
Cncurbita 12 cm. in height, growing in a small pot, which
had come to growth-standstill for reasons described. Two cc.
of water was supplied to the dry soil about the roots, and
Fig. 173. Initiation of Growth-
pulsation by Small Supply of
Water to Drought-rigored Seed-
ling of Cucurhita
The first thick dot represents
application of water, which
induces growth for three minutes
only. A second application at
the second dot renews it for the
second time,
INTERNAL ENERCV AND EXTERNAL STIMULUS 427
growth-response was initiated after a latent period of eleven
seconds (fig. i73)-
It will be remembered that the positive turgidity-variation,
on which growth depends, is hydrostatically transmitted at a
much quicker rate than the state of excitation itself. In the
present case, the responsive elongation in the growing region at
a distance of about 12 cm. took .place eleven seconds after the
application of water at the root. There was a certain loss of
time before the cells in the growing region became of sufficient
turgidity to initiate growth. The small supply of water which
had been given was enough to maintain growth for three minutes
only, after which the plant came again to a standstill. Another
2 cc. of water was now applied, and the latent period was
found reduced, as we should have expected, being now three
seconds only. This was due to the fact that the cells had not
now to absorb water before they could be sufficiently turgid.
This renewed growth-activity was again, however, exhausted
after about three minutes ; and it was very interesting to
observe how the response of growth followed, for a little while,
after each such doling out of water.
(4) Effect of increased internal hydrostatic pressure. —
We found, in thecdiSQo'i Desmodiiun, when the cut petiole, carry-
ing the motile leaflets, was subjected to increased hydrostatic
pressure, applied by means of the U-tube, that the rhythmic
activity of the leaflet, as shown by its quickening, was thereby
increased (p. 320), We saw also, in that case that this increased
frequency, due to increased pressure, reached an optimum,
and that beyond this, under excessive pressure, the pulsations
became irregular, or even came to a stop.
In order, then, to study the effect of hydrostatic pressure
on growth, I mounted the specimens — in this case an entire
seedling of Balsam and a cut flower of Crinuui — in U -tubes
supplied with water, and proceeded to take records, first
under normal conditions — i.e. the level of water in the two
limbs being the same — and then under a gradually increasing
hydrostatic pressure. These records are made, it should be
said, only when the rate of growth under the changed
428
PLANT RESPONSE
conditions has become uniform. This occurs, generally
speaking, after a period of variation which does not exceed
two or three minutes. I give here a table embodying the
results of the experiments on Balsam, and on Crinmn, which
show how increase of internal hydrostatic pressure increases
the rate of growth up to an optimum, after which there is a
diminution of growth.
Table showing Effect on Growth of Increased Internal
Hydrostatic Pressure
Balsam seedling
1 Crinum Lily
Pressure Rate of growth
Pressure Rate of growth
■ Normal '0034 mm. per minute
5 cm. -0036 ,, ,, ,,
10 cm. -0060 ,, ,, ,,
15 cm. -0095 „ „
20 cm. -0130 ,, ,, „
Normal
10 cm.
20 cm.
30 cm.
•0038 mm. per minute
•0044 „ „
•0142 ,, ,, ,,
•0036 ,, ,, ,,
The curve seen in fig. 174 exhibits graphically the rela-
tion between these internal pressures and corresponding
growths in the case of Balsam seedling. It will be seen
that after a certain moderate rate of growth has been
attained by increase of pressure, the curve becomes a straight
line ; that is to say, after this point, equal variation of pres-
sure produces equal variation in the rate of growth. But in
the first part of the curve, where the rate of growth is feeble,
an equal increase of pressure causes a disproportionately
large increase in the rate of growth. This is still more
strikingly shown when the growth, to begin with, is zero —
that is to say, at standstill ; in such a case, by gradually
increasing the internal pressure, we arrive at a point where
growth begins abruptly, after which increasing pressure
causes an increasing rate of growth. But if the pressure be
now brought back to a point just below that at which growth
was initiated, it is found not to be arrested, but to persist.
Thus the curve does not here return upon itself
Since the various growth-curvatures are brought about by
the variations of internal hydrostatic pressure caused by the
INTERNAL ENERGY AND EXTERNAL STIMULUS 429
^^■R
action of external stimulus, this quantitative demonstration
of the effect of internal pressure on growth, is of much
theoretical importance.
I have already shown the connec-
tio.i between mechanical response and
growth-response, ^nd demonstrated
the fact that the erectile mechanical
response and growth-elongation are
but different expressions of increased
internal activity. I shall now show
how the suctional and growth re-
sponses are related to each other, and
in what manner the action of the
former affects the latter.
(5) Effect of ascent of sap on
growth. — We have already seen that
the positive turgidity-variation on
which growth depends is brought
about, under normal conditions, by
the ascent of sap. As regards the
latter, we have seen that when the
root is subjected to the stimulating
action of warm water there is a
sudden augmentation induced in the rate of suction. The
application of cold water, on the other hand, induces the
converse effect. The mechanical response of the plant to
warm or cold water, applied at the base, was shown to be
manifested in the erection or depression of the leaves of
Mimosa or Biophytuin (p. 400).
These agents are seen in the following experiments to
produce parallel effects on growth. A growing Crinum Lily
was taken, and its normal rate of growth ascertained to be
•005 mm. per minute. Ice-cold water was now applied at its
base, and this was found to cause an almost immediate arrest
of growth. As the temperature, however, was gradually
restored to that of the surroundings, the rate of growth was
also slowly recovered. Five minutes after, the rate was only
Fig. 174. Curve showing
Relation between Internal
Hydrostatic Pressure and
Rate of Growth {Crinum
Lily)
430 PLANT RESPONSE
•001 mm. per minute, or one-fifth of the original rate. It was
only after about half an hour that the original rate of growth
was once more attained.
I next applied warm water at the base, with the result
that the rate of growth was almost instantaneously enhanced
to "125 mm. per minute, or twenty-five times the normal!
That this effect was not due to the rise of temperature as
such, is shown by the fact that it was almost instantaneous,
and that, moreover, as will be shown in the next chapter, the
maximum rate of growth of Crmuni at the optimum tem-
perature is only about three or four times as great as the
normal.
From these experiments we see that the energy applied
at the root is transmitted hydraulically to the growing region
by the ascent of sap, where a certain amount of work is per-
formed in causing an increase of turgidity, and thus producing
in the cells a state of tension. Growth is now caused not
simply by the presence of water, but rather by the energy
conveyed by that water. It is well to bear in mind, at this
point, that the mobility or plasticity of the responding grow-
ing region is also an important factor in the production of
growth ; for if the molecular mobility of the zone of growth
be in any way reduced, the transmitted pressure, which was
formerly effective, will now become ineffective to bring about
growth.
(6) Effect of temperature on growth. — We have seen,
in studying the pulsatory movements of Dcsmodiuin, that the
rise of temperature, within certain moderate limits, increased
the rhythmic activity of the plant, as shown in the increased
frequency of pulsation. At a maximum temperature, again,
above 40° C. these movements almost disappeared, there
being now produced very rapid oscillations, of so small an
amplitude as to be almost incapable of detection. This will
be seen in fig. 175, where the normal pulsations, of a period
of 2'5' at 30° C, are seen reduced to a period of only 10" at
42° C. With this, the amplitude also is so far reduced as to
be visible only on very careful inspection. We meet with
INTERNAL ENERGY AND EXTERNAL STIMULUS 43 1
corresponding phenomena in growth -response. It will be
shown in the next chapter that the rate of growth increases
with the rise of the temperature up to a certain optimum.
At a determinate maximum, however, which is about 44° C,
growth is arrested, but this arrest does not, as we have just
seen in the corresponding instance of Desinodiuni, imply a
total cessation of internal activity. In making experiments
on a seedling of Balsam, I obtained the record shown in the
upper part of fig. 176, the temperature being 34° C, which is
below the optimum. The average period of a single pulse
Fig. 175. Photographic Record showing the Slow Pulsations of Large
Amplitude of Desmodiiim Leaflet at 30° C. to become very much
Quickened and Reduced in Amplitude at 42° C.
was in this case 125 seconds, and owing to the considerable
amplitude of each pulsation, combined with its incomplete
recovery, the average of the resultant rate of growth was as
much as "074 mm. per minute.
On now raising the temperature to 44° C. I obtained the
lower of the two records in fig. 176, showing no resultant
growth. It is interesting to observe the process by which
the cessation of growth comes about in this case. For it is
clearly seen from the record that there is no cessation of
activity. On the contrary, we find that the frequency of
oscillation has become increased from four pulsations to ten,
43^
PLANT RESPONSE
in the course of 50". The average period has thus fallen
from 12-5" at 34° C. to 5" at 44° C. The amplitude of pul-
sation at the same time is found to be decreased, and this,
with the fact that recovery
is now complete, accounts
for the resultant cessation
of growth.
Comparison of vari-
ous types of multiple
response. — At this point,
it is worth while to com-
pare two or three types
of multiple response. In
Biophytum we have seen
that, by reason of incom-
plete recoveries from nega-
tive turgidity- variation,
the multiply-responding
leaflet gradually becomes
depressed below its ori-
ginal level. In contrast to
this we have in growth-
response those incomplete
recoveries from positive
turgidity-variations which have the effect of gradual elonga-
tions. In the pulsation of Desinodinm again we have an
intermediate instance where, response and recovery being
equal, the responding organ is ultimately neither raised nor
depressed. It is interesting to note, therefore, that in raising
the temperature of a growing organ to the maximum, and
thus abolishing the resultant elongation, we bring on a con-
dition of equality of response and recovery which in so far
resembles the pulsation of Desniodiiuii.
Effect of external tension on growth. — Great advan-
tages are afforded by the method of magnified record, which
enables us to detect instantly the immediate and after effects
on the specimen of any changes of external conditions. It is
Fig. 176. Growth-pulsation seen in
Seedling of Balsam
Upper record shows slow pulsations with
incomplete recoveries at 34° C. Lower
record shows quickened pulsations with
complete recoveiies at 44° C. The
magnification employed is 500 times.
INTERNAL ENERGY AND EXTERNAL STLMULUS 433
thus easy to obtain exact records of the effect of tension on
growth. The normal record is first taken, with the very
sh'ght tension exerted by the recording lever itself The
short arm of the lever, '5 cm. in length, is, it should be
remembered, attached to the growing organ. A rider, half a
gramme in weight, can be placed on the longer arm of the
lever, at distances of "5, i, I'S, 2, 25, or 3 cm. from the ful-
crum. The effective tension may thus be gradually in-
creased, and the corresponding effects on growth recorded.
It may be stated here that, generally speaking, any sudden
change of external conditions, such as sudden cooling,
sudden warming, or sudden variation of tension, acts on the
organ as an external stimulus ; and I shall presently show
that an external stimulus always induces a contraction or
retardation of growth. When the organ is subjected to
sudden increase of tension, the preliminary effect of con-
traction occurs therefore, as we should expect. But after
this temporary disturbance has disappeared we are able to
observe the permanent effect of increase of tension on growth.
For these experiments I took different specimens of Criiiuui
Lily, and the results obtained show that increase of tension
enhances the rate of growth. This increase, however, appears
to reach a limit at a certain optimum point, beyond which
increase of tension would seem to retard growth. The follow-
ing table exhibits the results of two experiments on different
specimens of Criniini.
Taijle showing Effect of Tension on Rate of Growth
Specimen A 1
Specimen B
Tension
Weight of lever alone
1 gr. + lever
2 gr. + ,,
Sgr- + ,.
Rate of growth 1
Tension
Rate of growth
mm.
•0040 per min.
•QUO ,, ,,
•0140 ,, ,,
•0045 „ „
Weis^ht of lever
•Sgr- + „
1 gr. + „
i-Sgr- + „
2 gr. + „
2-5 gr. + „
mm.
•0036 per min.
•0070 ,, ,,
•0100 ,, ,,
•0130 ,, ,,
■0170 ,, ,,
•0050 „ „
434 PLANT RESPONSE
Effect of direct application of stimulus on the grow-
ing region. — Many of the phenomena of growth-curvature
are brought about, as we shall see, by means of the changes
induced by external stimulus in the rate of growth, and there
is much misconception as to whether the effect of stimulus is
to enhance or to retard growth. I shall be able to show that
this misconception is the result of the complexity of the
problem, depending (i) on the tonic condition of the tissue,
(2) on whether the stimulus is internal or external, and (3)
on the point of application of stimulus. The fundamental
effect of stimulus is, however, very definite.
But at the beginning of our investigation we are met by a
question of great importance, namely, as to whether the effect
of stimulus on a growing, is in any
way different from that on a stationary,
organ. We have seen, for example,
that a fully grown style of Datura, in
which growth has come to a stop,
Fig. 177. Photographic exhibits Oil the application of local
Record of Responses of stimulus the usual contractile response
Mature Style of Datura ^ ,
alba to External Thermal (fig. 1 77). On the completion of re-
^''™"'"^ covery again, the organ returns to its
original length. Hence the base-line of a series of these
responses is horizontal.
I shall now pass, by means of intermediate links, from
this to the response of growing organs. And first I shall take
the response of a style of Datura, in which, for want of a
sufficient supply of internal energy, growth has come to a
temporary stop. On applying thermal stimulus, at intervals
of a minute, the first five responses are seen to be practically
like those of the stationary style oi Datura (fig. 178, cf 177).
But a portion of the stimulus applied is being absorbed and
held latent in the organ, thus increasing the internal energy,
or tonic condition. The result of this is seen in the renewal
of growth at the sixth response. The stimulus now, therefore,
finds bifurcated expression in maintaining response, and in
renewing growth, as is seen by the trend downwards of the
INTERNAL ENERGY AND EXTERNAL STIMULUS
435
hitherto horizontal base-line. This bifurcation causes the
first contractile response of the now growing organ — that is
the sixth response of the record — to be smaller than usual.
But as the tonic condition is established, and the molecular
mobility of the responding organ is increased, the contractile
response becomes larger, and growth goes on at a certain
steady rate. From this intermediate link we pass on to the
case of response to stimulus
of a style of Datura which is
in a state of uniform growth
(fig. 179). Here also we find
that stimulus produces the
normal contractile effect. All
these clearly demonstrate that
the response of growing
organs is in no way different
from that of stationary organs.
In a stationary organ, sti-
mulation produces negative
turgidity-variation, resulting
in contraction. The same
contraction is seen in growing
organs, causing a temporary
retardation of the rate of
growth.
The retardation of growth
which is caused in a growing
organ by external stimulus
may be exhibited in a some-
what different way. We may use the Method of Balanced
Record, by which the normal rate of growth is made to appear as
a neutral horizontal line. For the purpose of this experiment
I took the growing peduncle of a Eucharis Lily. The balanced
record (fig. 180) is here seen to be horizontal, save for
minute autonomous oscillations about the neutral line. Sti-
mulation, in this case, was produced by tetanic electric shocks
from an induction coil, which were applied on the growing
F F 2
Fig. 178. Photographic Record of
Responses of Style of Daiura alba
in which Growth had come to a
Temporary Slop
The up curve shows contraction. As
long as the base-line is horizontal,
growth is seen to be at standstill.
Renewal of growth at sixth re-
sponse, after which growth-elon-
gation is shown by the trend of
the base-line downwards.
436
PLANT RESPONSE
organ, by means of non-polarisable electrodes, connected,
one a little above, and the other a little below, the growing
region. According to the conditions of the experiment, the
balanced horizontal base-line will represent uniform growth
under normal turgidity. An
up curve will here represent a
rate of growth below the normal,
or a retardation ; a down curve,
on the contrary, will represent
a rate of growth above the
normal, or an acceleration.
When stimulus is applied a
responsive negative turgidity-
variation is induced, in conse-
quence of which there is a
temporary disturbance of the
balanced growth-record. The
curve thus produced exhibits
the effect of stimulus, and recovery from that effect. It will
be observed from the figure that on the application of tetanic
electric shocks for two seconds, a responsive retardation of
Fig. 179. Photographic Record of
Response of Growing Style
of Datura alba to External
Stimulus
Fig. 180. Balanced Record of Response in Growing Peduncle of
Eucharis Lily to Electrical Stimulation
Up curve here represents retardation of growth. First response is to
stimulus of two seconds', the second to stimulus of three seconds',
duration.
growth was induced, as seen in the up curve, and that there
was a recovery after an interval of nine minutes from the time
of application of stimulus. It will also be noticed that
during the process of recovery in this particular case, the rate
INTERNAL ENERGY AND EXTERNAL STIMULUS 437
of growth is above the normal, which is due to the fact that
the internal energy has been augmented by the absorption of
the stimulus applied.^ A stimulus of longer duration, that is
to say of three seconds, was next applied, and the responsive
retardation had now a greater amplitude than before, the
period of restoration being also longer, that is to say, sixteen
minutes.
Similarities between motile and growth responses.—
We have, then, in growth-response an exact parallel to the
mechanical responses given by pulvinated or anisotropic
organs. The following tabular statement will show the
reason of this fundamental parallelism between responses
whose modes of indication are so widely different :
Tabular Statement showing Comparative Effects of Stimulus in
Pulvinated and Growing Organs
Mechanical response
Growth response
Effect of normal Uirgidity :
Effect of normal turgidity :
Normal horizontal position of leaf.
Uniform rate of growth.
Local action of external stimulus:
Local action of external stimulus :
Contraction ;
Contraction ;
Diminution of turgidity ;
Diminution of turgidity ;
and concomitant depression of leaf.
and concomitant depression of rate
of growth.
Action of internal energy exhibited by
Action of internal energy exhibited by
(a) Recovery :
(a) Recovery :
Re-establishment of turgidity and
Re-establishment of turgidity and
gradual return of leaf to normal
gradual return of organ to normal
horizontal position.
rate of growth.
(b) Increased hydrostatic pressure :
(6) Increased hydrostatic pressure :
Erection of leaf.
Increased rate of growth.
In studying the mechanical response of plants we found
that direct application of stimulus to the responding organ
always produced a response characterised by the negative
turgidity-variation, that is to say, a depression of the leaf.
We also saw that internal energy, inducing positive turgidity-
variation, caused the opposite response, that is to say, an
erection of the leaf ; and that an increase of the internal energy
' Other records showing the effect of external stimulus on growth will be found
in Chapter XXXIV.
438 PLANT RESPONSE
of the organ was, in some cases, brought about by that in-
creased suctional activity by which energy was transmitted,
by means of water forced into the responding organ. This
increased suctional activity was due in its turn to external
stimulation of the roots. Or a similar transmission of energy
to the responding organ might be the result of stimulation
applied at a distance on the stem, in consequence of which a
wave of positive turgidity-variation would travel towards the
responding organ from the excited point. For the exhibition
of this latter effect, however, it is necessary that the direct
excitatory effect of stimulation should not be conducted to
the organ. This condition is met if the point of stimulation
be at a sufficient distance, or when the intervening tissue is
not a good conductor of excitation. Again, the plant as a
whole, it must be remembered, has its internal energy raised,
as the after-effect of the absorption of stimulus from its sur-
roundings.
Direct and indirect effects of stimulus and laws of
growth. — In the phenomenon of growth-response we have
a case which is exactly parallel. Direct stimulation of a
growing organ always induces a negative turgidity-variation,
with a concomitant responsive retardation of growth. Any-
thing, on the other hand, which increases the internal energy,
brings about the opposite effect, of positive turgidity-variation,
with concomitant responsive increase in the rate of growth.
This may be induced by a favourable rise of temperature,
or by stimulating the root, and so increasing the ascent of
sap. It may also be brought about by stimulating a distant
point, and so causing a wave of positive turgidity-variation
to be transmitted to the responding organ, the stimulated
point being at a sufficient distance to prevent the direct effect
of stimulus from reaching it. And, finally, the energy ab-
sorbed from external stimulus may, as an after-effect, increase
the internal energy of the plant. The increase of internal
energy, under all these different conditions, we shall for the
sake of convenience designate as the indirect EFFECT OF
STIMULUS.
INTERNAL ENERGY AND EXTERNAL STIMULUS 439
The laws of growth are therefore :
(1) The response of a growing organ is the same as that of a
stationary organ. Direct application of stimulus, inducing con-
traction, retards the rate of growth.
(2) The effect of indirect stimulation is to increase the internal
energy, and thus augment the rate of growth.
Summary
The multiple response of growth is characterised by all
the peculiarities seen, for instance, in the autonomous response
of Desiiiodiiim.
The rhythmic responses of growth exhibit periodic
groupings.
External stimulus is found to renew growth in organs in
which, owing to the deficit of internal energy, it had come to
a temporary standstill.
The increase of internal hydrostatic pressure, up to an
optimum, increases the rate of growth.
The effect of increased internal hydrostatic pressure is
exhibited in the case of the leaf of Mimosa by an erectile
response. In a growing organ the effect of increased turgidity
is shown by growth-elongation. A drought-rigored Mimosa
on being supplied with water responds by erection of the
leaf; and similarly, a plant in which growth, owing to drought-
rigor, has come to a standstill, responds, on being supplied
with water, by renewed growth-elongation. When the root
of Mimosa is supplied with ice-cold water, the responding
leaf, owing to the consequent arrest of ascent of sap, becomes
depressed. The increased suctional activity, again, which is
caused by a supply of warm water to the root, induces an
erectile response of the leaf. Similarly, corresponding varia-
tions in the rate of ascent of sap, brought about in a growing
plant by the application of cold and warm water to the roots,
cause respective depressions and accelerations of the rate of
growth.
The energy supplied at the roots is hydraulically trans-
440 PLANT RESPONSE
mitted to the growing region, and finds expression in the
work of growth.
A rise of temperature, up to the optimum, enhances the
rate of growth. At a maximum temperature of about 44° C.
growth is apparently arrested. This is not, however, due to
any rigor or arrest of internal activity, but to the fact that in
each pulsation of growth the constituent response and recovery
are now equal. There is thus no resultant growth-elongation.
At such a temperature the amplitude of pulsation is re-
duced, and the frequency increased, as in Desniodiiim.
Longitudinal tension has the effect, up to an optimum, of
increasing the rate of growth.
The effect of external stimulus on a growing, is precisely
the same as on a stationary, organ, that is to say, a respon-
sive contraction. On account of this contraction, and con-
comitant negative turgidity-variation, growth is retarded, as
the direct effect of the action of external stimulus.
External stimulus, however, when absorbed and held
latent by the tissue, has the effect of increasing the internal
energy of the plant. This indirect effect of stimulus causes
acceleration of erowth.
CHAPTER XXXIII
ON THE RELATION BETWEEN TEMPERATURE AND GROWTH,
AND THE ACCURATE DETERMINATION OF OPTIMUM
AND MAXIMUM POINTS
General consideration of difficulties of accurate determination of effects of
temperature on growth — Four accurate methods: (i) Method of discon-
tinuous observations — Accurate regulation of temperature by electrolytic
rheostat— (2) Method of continuous observations — Thermo-crescent curve —
Determination of the optimum point— (3) Method of balance — (4) Method of
excitatory response — Translocation of the optimum point.
Determinations of the effect of temperature on growth are
usually carried out either by observing a single plant for
several hours in succession, or by obtaining the average
growth of various groups of plants, kept under different
temperatures. In the latter case the individual peculiarities
of different specimens cause them to give values which are
more or less discrepant, but this fact is to some extent
neutralised by taking the average of a large number. These
methods, however, are all very laborious, and the results not
highly consistent. Whichever method be adopted, we have
to remember that in all cases in which observations stretching
over periods of several hours are required, the growth of the
plant will be liable to spontaneous variations, resulting from
the periodicities impressed upon it by the changing conditions
of its environment. It is, perhaps, owing to this fact that the
results obtained by various authorities have been so widely
divergent. For example, the optimum temperature for
growth of Zea inais was found by Sachs to be 34° C. and by
Koppen to be 302° C.
Clearly, the perfect method would be one in which we
should be able to measure the effects of temperature alone on
442 PLANT RESPONSE
the growth of a particular plant, so that the varying factors
of age and constitution, or tonic condition, should remain
constant. And, further, it should be possible to carry out
this determination of growth at different temperatures in a
time so short that the spontaneous variations of the plant,
if any, would be insignificant. Only by such a method
could we hope to obtain results which would be reliable and
consistent.
Keeping these considerations in view, I have been fortunate
enough to be able to devise four distinct methods of determin-
ing the effect of temperature on the rate of growth, the
perfection of which may be gauged from the fact that the
results of all agree with and corroborate each other within a
fraction of a degree. With some of these, the entire experi-
ment on the different rates of growth at various temperatures,
ranging from ordinary through optimum to maximum, can
be carried out within about half an hour. The determination
of any single cardinal point, such as the optimum, can always
again be made within five minutes. The investigation thus
gains by simplicity and experimental accuracy, and observa-
tions may be made on many different specimens within a
very short period. I shall now proceed to describe these
different methods.
(i) The method of discontinuous observations. — We
shall first take the method by which rates of growth are
recorded at different temperatures, say one degree apart.
Some means of raising the temperature to exactly the
required point, and maintaining it unchanged during the
time of experiment, is an essential condition of all these
investigations. This I have been able to accomplish in the
following way. The electrical heating coil inside the plant
chamber is put in connection with an external battery. The
heat given out, and the consequent permanent rise of
temperature in the chamber, depend on the intensity of the
current that flows through the heating coil. This current
again may be progressively regulated by the interposition of
an electrolytic rheostat in the circuit, the resistance of which
RELATION BETWEEN TEMPERATURE AND (iROWTH 443
can be subjected to gradual variation. The electrolytic
rheostat consists of two semicircular troughs (fig. 181). By
turning the handle, say to the right, the electrolytic resistance
interposed is continuously increased, diminishing the current,
and hence diminishing the temperature inside the chamber.
Rotation of the handle in the opposite direction produces the
opposite effect, that is to say, raises the temperature inside
the chamber. Thus, by proper manipulation of the handle of
the rheostat, the chamber can be raised to any temperature.
Fig. 181. Semicircular Electrolytic Rheostat intei posed in Heating Coil
Current enters the first trough, filled with zinc sulphate solution, by the
electrode, z', and is led to the second trough by the diagonal metallic
connector, D d'. By turning the index-arm, i, clockwise, the interposed
resistance is increased, and the heating current thus diminished. Rota-
tion in the opposite direction diminishes the resistance and increases
the heating current.
which can then be maintained uniform for any length of time,
by keeping the rheostatic resistance constant. From a
previous experiment, the temperature-values of the position
of the index in connection with the handle can be ascertained
and marked. Thus by turning the handle to any given index
number, say of 31° C, the temperature of the chamber will be
found to be raised permanently to that value. A delicate
thermometer graduated in twentieths of degrees is placed in
the chamber, and affords an independent indication of the
444 PLANT RESPONSE
temperature thus attained. It is also necessary, for reasons
to be fully explained in a subsequent chapter, that the
specimen should not receive thermal radiation from the
heating coil, as such radiation, I find, retards growth. A
shield of mica, opaque to thermal radiation, is interposed
between the heating coil and the plant, which is thus subjected
only to the action of changes of temperature. For many of
my experiments I selected specimens of the Crimim Lily, on
account of its extreme regularity of growth, which is so
uniform that on adjusting the record under balance, the
external conditions being constant, the line of record remained
horizontal for a period of certainly over an hour.
The rate of growth at the temperature of the room, say
30° C, is first taken on the recording drum, which is covered
with paper divided into millimetres. The horizontal distance
or abscissa represents time, which, with the particular speed
of drum which I used was 6 mm. per minute. The ordinate
represents growth-elongation, and as the growth-recorder, or
cre.scograph, produces a magnification of 1,000 times, i mm.
distance of the ordinate is equal to an actual growth of
"ooi mm. The ordinate corresponding to an abscissa of
6 mm. would thus be equivalent to a growth in thousandths
of a millimetre per minute.
The rheostat handle is now turned to the index-number
corresponding to the raising of the temperature of the chamber
by I ° C. There is first a variable period of rise of temperature,
after which a permanent degree is attained. During this
preliminary stage, variation of temperature acts as a stimulus,
giving rise to lesponsive contraction or retardation of growth.
But after this transient disturbance, the growth attains a
constant rate, characteristic of the given temperature. These
peculiarities will be better understood on following the record
given in fig. 182 — reduced here to half the original size —
which was taken with a specimen of Criniim Lily, during ten
minutes. The record during the first five minutes is for the
temperature of 34° C. It will be seen that in two minutes
the growth-elongation is fifteen divisions, and as each division
RELATION BETWEEN TEMPERATURE AND GROWTH 445
represents "ooi mm., the rate of growth is thus '0075 mm.
per minute. The rheostatic handle was afterwards turned to
the mark 35° C. With the particular battery power used in
this case, the permanent rate of rise to 35° C. was attained
after a period of three minutes. It will be seen from the record
that the stimulus of sudden variation of temperature caused
a contractile twitch, after which
growth proceeded at a very
rapid rate during the variable
period. But as soon as the
temperature of the chamber had
attained a permanent condition
— i.e. 35° C. — the rate of growth
became constant. The attain-
ment of this constant rate was
practically simultaneous with
the attainment of the permanent
temperature condition. The
lag, in any case, if it existed,
could not be more than fifteen
seconds.
One curious and interesting
fact to be fully explained later,
which was noticed in the course
of the experiments, was that
the amount of contractile twitch
went on increasing during the
variable period, as the tempera-
ture was raised each time 1° C,
Fig. tS2. Record of Growth in
Crinujii at Temperature of 34° C.
and 35° C.
The dotted line represents the
varial)le period of temperature
change. Note the contractile
twitch and transient highly ac-
celerated growth which follows.
The rate of growth became con-
stant when the temperature be-
came permanent at 35° C.
from 30° to 35° C, but after
this point practically disappeared. The permanent rate of
growth, then, at a temperature of 35° C, i.s, as will be
seen from the figure, twenty-four divisions per two minutes,
or '012 mm. per minute. In this way, by taking successive
records at different temperatures, I obtained the following
rates of growth, in the cases of Crinuni Lily and the peduncle
of Crocus.
446
PLANT RESPONSE
Crinum Lily
Peduncle of Crocus
Temperature
Growth in mm. per minute
Temperature
Growth in mm. per minute
30° c.
•0036
3°:^-
•0059
31° c.
•0040
33° c.
•0100
32° c.
•0050
•0145
33° C.
•0055
36° c.
•0062
34° C.
•007 s
38° c.
•0030
35° C.
•0085
40° c.
•0010
36° c.
•0050
37° C.
•0035
(2) Method of continuous observations. — The method
which I have just described gives us results which, though
obtained at closely consecutive temperatures, are nevertheless
discontinuous. There is, besides, some loss of time involved
during the variable period, in addition to which there is the
factor of transient stimulation during sudden changes of
temperature. For these reasons I was anxious to perfect
some method by which the curve of growth should afford a
continuous means of obtaining the rate of growth at all
temperatures. I also wished to eliminate from this record
the preliminary disturbance caused by sudden change of
temperature.
I was enabled to do this in practice by bringing about a
gradual and continuous rise of temperature, instead of the
former sudden variations by steps. This was effected by
turning the handle of the electrolytic rheostat at a rate so
graduated that the rise of temperature within the chamber
was uniform. As it was necessary to complete the experiment
within not too long a period, I found that a rise of 1° C. per
2"S minutes was sufficient to meet the requirements of the case.
This means a rise of "1° C. in fifteen seconds. An observer
watches a delicate thermometer, which is placed in the plant
chamber, with his hand on the handle of the rheostat. By
means of this, and a chronometer beating seconds, he is able
to regulate the uniform rise of temperature with the greatest
jiicety. Should the rate be too quick, it may be reduced by
the slightest turn of the handle towards the increase of
RELATION BETWEEN TEMPERATURE AND GROWTH 447
resistance, or vice versa. After a little practice the process of
regulation becomes almost instinctive. The record of growth is
now taken continuously on the revolving drum, and the thermo-
crescent curve obtained under these conditions of continuous
variation of temperature is seen to be extremely regular, giving
data by which we may determine the rate of growth at any
point of the curve (fig. 183). The revolving drum gave, as
already said, a movement of the recording surface of 6 mm. per
3
•16
mm.
0';
il''32'33'34'35"3(
j" 37' 38° 39° 40° 41° 4
2^3° 44" 1
^-^
____
i
i
—
/
y
14
•121
•10
1
/
7^
y
r^
,'
/
/
■
1 /
/
•04
•02
i
— ^
V
—Z
/
/
/
1
.(
3' • 1
5' 10' 15' 20' 25' 30' 3
5'
Fig. 183. Thermo-crescent Curve of Growth in Crinuiii Lily
under Continuously Increasing Temperature
minute. That length of the abscissa would therefore repre-
sent one minute of time ; and since the rise of temperature
was regulated, at 1° C. of rise per 2-5 minutes, intervals of
15 mm. in the abscissa also represent 1° C. in temperature.
With the magnification used, i mm. of the ordinate represents
a growth-elongation of a thousandth part of a mm. In order,
therefore, to obtain from this curve the rate of growth at any
given temperature, say at 34° C, we have to find the elonga-
448
PLANT RESPONSE
tion per minute at that point. The rise of temperature in
one minute, then, under the experimental conditions described,
is through -4° C. The growth-elongation of the specimen,
therefore, while the temperature is rising from 338° C. to
34*2° C, gives us the rate of growth for the mean temperature
of 34° C. This is found in the magnified record to be
10 mm. The absolute value of the rate of growth is thus
•01 mm. per minute. In this way we can determine from the
curve the rate of growth corresponding to any temperature.
It will thus be seen how in the course of an experiment
lasting for thirty-five minutes only, we are able to obtain
data which give us the various rates of growth through a
wide range of temperatures.
This operation can, moreover, be made entirely automatic.
The breadth of the circular electrolytic trough may be appro-
priately varied at different parts of the circle, so that turning
the handle through equal arcs raises the temperature of the
plant chamber by equal degrees. The handle of the rheostat
may then be rotated by the recording drum itself. Hence
in the record, equal lengths of the abscissa will represent not
only equal times, but also equal rises of temperature. And
finally by taking the record photographically, the whole pro-
cess becomes automatic. From the data furnished by fig. 184
we obtain the following table :
Table showing Rates of Growth at Different Temperatures in
Flower of Crinum Lily.
Temperature
Growth in mm. per minute
Temperature
Growth in mm. per minute
30° c.
31° c.
32° c.
35° c.
35-5° C.
•0040
•0057
•0075
•0087
•0100
•Olio
•OII3
36° C.
37° C.
38° c.
39° C.
40° C.
42° c.
43° C.
•0070
•0045
•0030
•002Q
•0012
•0005
•0002
The curve shown in fig. 1 84 exhibits the relation between
these various temperatures and their corresponding rates of
growth. It is here seen that the rate gradually rises till we
RELATION BETWEEN TEMPERATURE AND GROWTH 449
approach the optimum point, which in the present case is
35"5° C. After this there is a steep fall, and growth is almost
abolished at a maximum temperature at or near 45" C. This
arrest of growth does not mean arrest of internal activity. I
have in the last chapter (fig. 176) explained why, in spite of
the persistence of internal activity, there is in this case no
resultant growth. It is to be borne in mind that as the curve
of growth in fig. 183 is continuous, the rate of growth at
m
■
1
^
1
B
B
Fig. 1S4. Curve showing Relation between Temperature and Rate of
Growth, as deduced from the Thermo-crescent Curve in fig. iSi
points intermediate to those specified may be determined
from it.
From a large number of experiments which I have carried
out on Crinum Lily, I find that the optimum temperature is
very constant, not varying by as much as one-tenth of a
degree from the mean value of 35"5° C. as the optimum tem-
perature. In the first portion of the curve, as the temperature
rises from 30° C. to 3 5 "5° C. the rate of growth is seen to
increase, from -004 to -01125 mm. per minute, or to nearly
three times its first value. The fall, beyond the optimum, is
steeper than this rise. At 37° C. the rate of growth 'has
G G
450 PLANT RESPONSE
fallen to "0045 mm. per minute, or almost the same as at
30° C. Thus, while 5 "5° C. of rise of temperature before the
optimum enhanced the rate of growth by three times, a
further rise of only i'^° C. beyond that point was sufficient to
bring it back to almost the same value as at 30° C. The
individual characteristics of each specimen are seen, not by
any perceptible variation of the optimum point, but rather by
differences in the steepness, during rise or fall, of the curve.
With some specimens, for example, the increase of rate of
growth during an equal rise of temperature from 30° C. to
35-5° C. is only half of that seen in the figure. The steepness
of fall, on the other hand, beyond the optimum may be
much greater; that is to say, a rise of 1° C. or less above
the optimum will sometimes reduce the rate of growth to its
value at 30° C.
(3) The Method of Balance.. — I shall now describe an
extremely delicate method of determining the rate of growth
at different temperatures, which is especially suited for the
exact determination of the optimum point. A balanced line
of record is first obtained by the turning of the balancing
wheel of the Crescograph (fig. 168). This regulates the
difference of level of the syphon tube, until the spot of light
is stationary at the temperature of the room. As the tem-
perature is now raised and the rate of growth increased, the
balancing wheel has to be rotated, say to the right, in order
to keep the spot of light stationary. The reading of the
circular scale at different temperatures thus gives the balanced
readings for the corresponding rates of growth at those
temperatures. A previous calibration of the value of the
circular scale enables us to determine the absolute growth-
movements at various temperatures. For the determina-
tion of the optimum point, however, this is not necessary.
All that has to be done in this case is to keep the spot,
which would otherwise drift to the right under a constantly
increasing rate of growth, on the point of balance, by the
right-handed rotation of the balancing wheel. This must be
done as long as the temperature and rate of growth are
RELATION BETWEEN TEMPERATURE AND CROWTIl 45 1
ascending towards the optimum. On reaching and passing
that point, however, it is found that the spot of light, which
has hitherto tended to move to the right, now has its move-
ment suddenly reversed to the left, thus necessitating a
corresponding reversal of the balancing rotation. This
turning point is extremely sharp and well defined, and enables
us to make an accurate determination of the optimum tem-
perature within less than a tenth of a degree. From a
previous knowledge that the optimum point lies, say, between
35° C. and '^6° C, the rise of temperature from 35° C. to 16° C.
within the chamber may be adjusted to take place in five
minutes, that is to say a rise of one-twentieth of a degree per
fifteen seconds. The second observer, watching the delicate
thermometer in the plant chamber, calls out at every twentieth
of a degree of rise of temperature. The first observer, at the
recording drum, notes the temperature of the turning point.
It has been said before that the permanent rate of growth
for any given temperature is always established in less than
fifteen seconds of reaching it. The possible error, owing to
this lag, could not therefore exceed one-twentieth of a degree.
Table showing Circular Readings of Balancing Wheel at
Different Temperatures.
Crinum Lily
Hypocotyl o{ Balsam
Temperature
Cir
cular reading
Temperature
Circular reading
30° c.
0°
30° c.
0°
31° c.
70°
31° c.
12°
32° c.
171°
32° c.
42°
33° C.
270°
33° C.
86°
34° C.
405°
34° C.
146°
35° C.
53«°
34-6° C.
170°
35'4° C.
600°
35° C.
112°
36° c.
494°
36° c.
85°
37° C.
354°
37° C.
37°
38° c.
140°
38° C.
5'
The turning point of another
specimen was
The turning point of another specimen was
found to be 35 "s"
C.
found to be 34-4° C.
I give above two sets of readings of the balancing wheel,
made during two experiments for the determination of the
G G 2
452 PLANT RESPONSE
optimum point for Crinuin Lily and the hypocotyl ot Balsam
The balanced reading at 30° C. is taken as zero. The ad-
justment of the stop-cocks for regulation of outflow was
different in the two cases.
(4) The method of excitatory response.— The method
which I am about to describe — and by which the relative
rates of growth at different temperatures are afforded in-
directly— is one of much theoretical importance, for it proves
what I have already suggested, that growth is a phenomenon
of excitatory response. This being so, it would follow that
the reason why growth is at its optimum at about 35° C. in
the case of most tropical plants, is that the excitability of
the tissue is greatest at that temperature. The different
excitabilities at different temperatures might further be
expected, this being true, to offer an independent indication
of the characteristic rate of growth of the tissue at those
temperatures.
The excitability of the tissue can be tested, in the case
of radial organs, by its longtitudinal contractile response to
external stimulus, which, as we have seen, will be represented
in growing organs by a retardation of growth, proportionate
to the excitability. We must bear in mind, at this point,
certain differences between responsive effects in mature and
in growing organs. In the former, owing to the increase of
internal energy brought about by rise of temperature, the
tissue becomes over-turgid and the internal hydrostatic
pressure is greatly increased. The contractile action of
external stimulus is thus strongly resisted by the tissue,
which in this way antagonises the normal extent of response
(P- 338)- Similarly, a closed india-rubber ball, fully distended
with water, will not yield to any great extent when struck.
But if we have, instead, a tube through which water is
running, the flexible pipe when struck will yield, and cause a
proportionate retardation or reversal of current behind. We
have a case somewhat analogous in a growing organ. For
here the tissue cannot be regarded as closed, since it is
constantly elongating. It therefore represents, not a static
RELATION BETWEEN TEMPERATURE AND GROWTH 453
condition of rest, but a dynamic condition of equilibrium,
and it will offer little effective resistance to excitatory con-
traction. We shall therefore expect that in growing organs
similar stimuli will induce responsive effects varying in pro-
portion to the changes of excitability in the tissue, under
different conditions.
We saw, in the case of Crinuui Lily, that the optimum
temperature was near 35° C, and that at this optimum the
rate of growth was something like one and a half to three
times as great as at 30° C. At 37° C. we saw, further, that
the rate of growth was again reduced, and had become equal
that, on recording the retardation of growth in response to
external stimulus at three definite temperatures, say 30° C,
35° C, and Z7° C., we should find it to be greatest at 35° C.
being in fact at that point about one and a half to three
times as great as at 30° C. The response at 37° C, on the
other hand, which is beyond the optimum, would be much
less than at 35° C, being equal to, or even less than, that at
30° C.
Table showing Variation of Excitatory Mechanical Response
AT Different Temperatures
S-— !te4rrrofio°c.
Response at
temperature of 35'' C.
Response at
temperature of 37° C.
Electrical I. ' 10-5 divisions
II. 22
37 divisions
31
iO'5 divisions
13-5 M
Thermal III.
IV.
32
18
46
28
24
12 ,,
I have made numerous experiments completely bearing
out these conclusions. The mode of experimental proce-
dure is as follows : a balanced record is taken at the given
temperature, and the growing organ is then subjected to a
definite intensity of stimulation, which may consist of tetanic
thermal or electrical shocks, lasting for twenty seconds.
Records are then made of the resulting contractile retarda-
454 PLANT RESPONSE
tion of growth at the different required temperatures. Three
responses were taken at each temperature, and were found to
be practically the same. Some of these records will be
given in the next chapter (fig. 185). I have given the
results of four such experiments, carried out on different
specimens.
The translocation of the optimum point. — We have
thus seen how constant is the optimum point in the same
species, under normal conditions ; but, since we found that
the otherwise constant death-point was liable to be shifted
under the disturbance caused by the sudden variation of
external conditions (p. 172), so it would appear probable that
the optimum point also would be liable to transposition under
the influence of similarly disturbing causes. The optimum
point of the Crinuni Lily has been seen to lie, normally
speaking, between 35*4° C. and 35-5° C. After a night of
heavy rain and gale, however, I found that the optimum
point of a specimen of this Lily had fallen to 34'6° C. Under
the action of a poison like copper sulphate, again, admi-
nistered in such dilution as not to kill, but only to retard
growth, I have observed the optimum point to be lowered
to 345° C. In the case of dilute solution of sugar, however,
which induces — as we shall sec in the next chapter — an
increase of growth-activity, I have found the optimum point
to be raised to 36"6° C.
Thus under normal conditions the optimum temperature
for each species is extremely definite. But circumstances
which increase or decrease the rate of growth abnormally,
operate also to transpose the optimum point, in the same
manner as the death -point was found to be translocated by
external influences.
Summary
The difficulties usually encountered in the accurate deter-
mination of the effect of temperature on growth have been
successfully overcome in the case of four distinct methods.
In taking records of growth at different temperatures, it
RELATION BETWEEN TEMPERATURE AND GROWTH 455
is found that the variation from one to another acts as a
stimulus, and induces a transient retardation of growth.
But this cause of disturbance is eliminated when the rise
of temperature is made gradual and continuous. In this
way, by taking a continuous record of growth under uniform
rise of temperature, a thermo-crescent curve is obtained, that
gives data from which the absolute values of growth at all
temperatures may be obtained. From this curve we are also
able to obtain an accurate determination of the optimum and
maximum points.
The Method of Balance also affords us, by means of a
sharply defined turning point, an exact indication of the
optimum point.
The optimum point is very definite, and under normal
conditions is always constant for a given species ; but just
as the death-point was found liable to be shifted under
abnormal external conditions, so the optimum point also is
apt to be transposed under similarly disturbing causes.
That growth is a phenomenon of excitatory response is
demonstrated by the fact that the growth-rate is increased or
decreased at different temperatures, in proportion to the
excitability of the tissue at the same points, as indicated by
its contractile response.
CHAPTER XXXIV
ON AN ATTEMPT TO DETECT AND MEASURE LATENT
STIMULUS, AND ON THE STUDY OF PERIODIC AFTER-
EFFECTS
Positive and negative after-effects — Extreme delicacy of the Method of Balance —
Detection of absorbed stimulus by negative after-eftect — Constancy of sum of
direct and indirect after-effects— Latent component almost vanishing above the
optimum — Variation of receptivity -Direct and indirect response of plant in
sub-tonic condition — Table showing direct and indirect effects at diflferent
temperatures— Is the change induced by stimulus always of an explosive
chemical character ? — Relation between stimulus and response in different
tonic conditions— After-effect — Factors which determine periodic after-effects :
(i) Stimulus of light — (2) Temperature — (3) Chemical stimulus— (4) Tur-
gidity— Continuous photographic record of the pulsations of Desmodiiim —
Record of periodic variation of rate of growth — Continuous photographic
record of periodic variations of transpiration — Continuous photographic record
of the variation of the rate of growth — Annual rings and seasonal periodicity.
By regarding the plant as a machine, as we did in the course of
the earlier chapters, we were enabled to understand the possi-
bility of its absorbing, and holding latent, more or less of the
incident stimulus (p. 124). The experimental demonstration
of this would, however, be difficult, in the case of the ordinary
response of motile organs ; for though we have seen that ex-
ternal stimulus and the absorbed internal energy are opposite
in their responsive effects, yet in the ordinary records of
mechanical response it is not easy to discriminate that part
of the effect which is due to the latter element ; for while
it is true that the presence of internal energy would tend to
hasten the recovery, it is still impossible to distinguish with
certainty a recovery so hastened from one which is natural.
The fact that excess of stimulus is transformed into latent
energy is demonstrated, however, by the occurrence of
multiple response.
DETECTION OF LATENT STIMULUS 457
Positive and negative after-effects. — We have seen that
when a moderately strong stimulus acts on a responding
organ, a short time elapses before the initiation of the re-
sponse, and this is known as the latent period ; but when
response has been initiated, it persists for some time, even on
the cessation of the stimulus, and this is known as the after-
effect. I shall, however, for important reasons, which will
appear later, further distinguish it as the positive after-effect.
By the term positive after-effect, then, is meant the continua-
tion of a response evoked by external stimulus, on the cessation
of that stimulus. We may, for example, imagine a heavy
elastic spring immersed in a viscous fluid. If this be sub-
jected to a sudden compressional blow, then, after a short
latent period, it will begin to undergo compression, and this
compressional movement will continue for some time, even
on the cessation of the blow that caused it, thus exhibiting a
positive after-effect. But a spring compressed in this manner
contains some amount of latent or potential energy, on
account of which it next begins to expand, exhibiting a
movement opposite to the first. This second movement, due
to the latent energy, we may distinguish as the negative after-
effect. This negative after-effect, it should further be stated,
may sometimes be separated from the direct effect by a con-
siderable interval of time. This may be seen in a viscous
wire subjected to a torsional impulse. After the twisting
has ceased, some time elapses before the wire is seen to
begin the contrary, or negative, movement of untwisting,
which is accomplished very slowly, and may even take hours
to complete.
Now, it occurred to me that, in the response of growth,
it was possible to find a means of detecting whether the
external stimulus in the case of living tissue might, or might
not, become partially latent, to be similarly manifested later,
in the form of the negative after-effect ; for if we take a
balanced horizontal record of growth, then the direct effect
of external stimulus will be seen in that retardation which
is shown in the shifting of the line — here, for convenience of
458 PLANT RESPONSE
inspection, represented upwards (fig. 185). On the cessation
of external stimulus, if the recovery of the excited region be
merely passive, it is evident that this ascending line will
gradually return to the horizontal, as in the third record of
fig. 185 ; that is to say, the retarded will be exchanged by
degrees for the normal rate of growth. But if some portion
of the external stimulus be held latent in the tissue, this will
go to increase the internal energy of the plant. Now, we
have already seen that the effect of augmented internal
energy is exhibited in an increase of the rate of growth
above the normal, shown in a balanced response-curve by an
opposite movement to that of retardation, constituting the
negative after-effect. Such a negative after-effect, consisting
of an enhanced rate of growth, will persist until the energy
thus held latent is exhausted, when the curve will again
return to the horizontal. Thus, the up curve will represent
the direct effect of external stimulus, and the down curve the
acceleration of growth due to absorbed stimulus, or the nega-
tive after-effect.
Extreme delicacy of the Method of Balance.— Such
transient variations in the rate of growth, occurring as the
expression of the absorbed fraction of incident stimulus,
would have been incapable of detection by the ordinary
auxonometric method of growth-record ; for here, owing to
the relatively slight magnification which is possible, it takes
nearly half an hour to obtain data from which the normal
rate of growth may be inferred. Another half-hour's obser-
vation would be necessary before we could infer the occur-
rence of variation under changed conditions, and it is clear
that, during a period relatively so long, the plant may
undergo spontaneous changes. The after-effects, however,
which we now wish to detect, are found to take place imme-
diately, and to last for a (ew minutes only, in the case of
moderate stimulation. Pwen with our crescographic arrange-
ment, though the usual magnification is a thousand times, the
variation constituting the after-effect is seen only in a slight
change of the slope of the curve ; but when the Method of
DETECTION OF LATENT STIMULUS 459
Balance is employed, the ordinary magnification is enough
to show, in a very marked manner, all the phases of these
transient variations. The records here reproduced have
been in fact reduced to one-third of the originals.
The sensitiveness of the arrangement can be very much
exalted by observing the balanced line of light with its devia-
tions, through a telescope placed at a distance. In this way,
I have been able to detect a variation from the normal rate of
growth, of so little as a two millionth part of r mm. per second,
within so short a period of observation as ten seconds.
Detection of absorbed stimulus by negative after-
effect.— We must now revert to the question of the detection
of latent stimulus by an increase in the rate of growth,
and we shall first take that simple case in which there is no
loss of energy from irreversible effects due to molecular
friction. The energy of external stimulus will here find
complete expression in doing external and internal work.
If the external stimulus remain constant, the sum of these
two— that is to say, the direct or immediate, and the indirect
effects — will also remain constant ; but if, under the same
circumstances, one of these factors, say the direct effect,
should for any reason be enhanced, we might then expect
that its complement, the indirect effect, would undergo a
corresponding diminution ; while, if the direct effect should
be small, the indirect effect would show augmentation.
These theoretical considerations are found very strikingly
verified in the experiment which I shall now describe. I
first took a balanced record of growth in a specimen of
Crhmiii Lily, at 30° C. This was then subjected to thermal
shocks for five seconds. The direct response, as will be seen
from the first record in fig. 185, which is reduced to one-third
of the original, was a retardation of growth represented by
33 divisions. On the cessation of stimulus, however, the rate
of growth did not at once return to the normal, but exhibited
the effect of absorbed energy by an acceleration shown in the
down curve, and represented by 13 divisions, after which it
became normal. The same stimulus was now repeated, and
460
TLANT RESPONSE
the direct effect was a retardation of 31, which was followed
by an augmentation of 14 divisions. Thus the sum of these
two effects is practically constant, being in one case 46, and
in the other 45, divisions.
Constancy of sum of direct effect and indirect after-
effect.— This constancy, however, becomes still more remark-
able when the same plant is raised to a temperature of 35° C.
and subjected once more to the same stimulation. The direct
effect is now shown by a retardation which may be repre-
sented as 39, and the in-
direct by 9, divisions. In
the second response of this
second series, we have a
direct effect of 37, and an
indirect effect of 8, divi-
sions. Thus the sum of
the first direct and indirect
effects is 48, and the sum
of the second 45, divisions,
the mean of the two at
35° C. being 46*5 divisions,
while the mean at 30° C.
was 45-5 divisions. We
have found, then, not only
that the sum of direct and
indirect effects at a given
temperature is practically
constant when stimulus is the same, but also that this sum
itself remains approximately constant at different tempera-
tures within the optimum ; and, further, we see that as the
excitability is increased in approaching the optimum, the
direct effect also increases at the expense of the indirect.
In other words, when the tissue is at its optimum tonic
condition, its capacity for the absorption of stimulus being
already fully satisfied, the external stimulus tends to be
immediately expended, in direct response, allowing relatively
little to become latent.
Fig. 185. Series of Responses of Growing
Organ of Crinum Lily, taken under
Balanced Conditions at Three Different
Temperatures
On comparing these records it will be seen
that the direct effect increases up to
the optimum, and that the indirect
effect of accelerated growth decreases.
Beyond the optimum, at 37° C, there
is no latent component, as shown by
recovery from direct effect to normal
rate of growth.
DETECTION OF LATENT STIMULUS 46 1
Latent component almost vanishing above the opti-
mum.— As an extreme instance of this, we may take the
response at a temperature beyond the optimum, say at 37° C.
Here, by its environmental conditions, the plant is already
supplied with an excess of energy. And besides this, there
is the fact which we have already noticed, that its general
excitability is diminished, so as to be equal to, or less than,
that at 30° C. At these two temperatures, then — of 30'^ C.
and 37° C. — we have two conditions of excitability more or
less the same, but with a different history.
Below the optimum there is an unsatisfied capacity for
absorption of stimulus, whereas above it this capacity has
been fully met. It would therefore appear that the power of
the tissue to hold stimulus latent is diminished progressively
up to the optimum, till beyond that point it practically
disappears. I obtained a remarkable confirmation of this
inference in the course of my experiments. In the experi-
ment just described, for example, the direct response was
a retardation of twenty-four divisions, and there was no
indirect effect, showing that little or no stimulus had become
latent (fig. 185).
Variation of receptivity. — In the experiments described,
the sum of the direct and the indirect effects, up to the optimum,
had been found to be approximately constant, that is to say,
a total response of about forty-six divisions. At 37° C, how-
ever, we see the total response reduced to twenty-four divisions
without any latent component. And since the total response
measures for us the amount of stimulus taken up by the
tissue, it would appear that at 37° C. not only is the
power to hold stimulus latent lost, but also that the general
receptivity of the tissue is very much reduced. It is thus
seen that the condition of a tissue modifies its receptive
power ; hence it is possible for different parts of the same
organ — say, for instance, the tip and the growing region —
being in different conditions, to possess different receptivities.
Direct and indirect response of plant in sub-tonic
condition. — Turning from this case of excess of energy to
462 PLANT RESPONSE
that in which it is below par, the plant being in a sub-tonic
condition of arrested growth, we find that external stimulus
gives rise to little expression in direct response. External
stimulus is found under such circumstances mainly to increase
the store of internal energy, in consequence of which we
obtain the indirect response of renewed growth. In such a
condition, the plant has a great capacity for the absorption of
external energy. After growth has commenced, the energy
of incident stimulus finds bifurcated expression, by inducing
direct or immediate response, and by the indirect or negative
after-effect, of enhanced rate of growth (p. 434).' The sum
total of these two — external stimulus being the same —
remains approximately constant, till the optimum tonic con-
dition is reached. The direct and indirect effects are thus,
up to this point, complementary to each other. After
passing this point, however, when the plant is possessed of
excess of energy, its power of absorbing stimulus appears to
undergo diminution. The direct effect of stimulus is then
found reduced, and there is no negative after-effect, due to
the absorbed component of the external stimulus.
From these considerations we are enabled to understand
the curious growth-response that was observed in varying the
temperature of the plant from 34° C. to 35° C. (fig. 182,
P- 445)-
The change of temperature was in that case accomplished,
as will be remembered, by changing the intensity of the
electrical heating current. The change from 34° C. to 35° C.
thus produced was not, however, brought about at once, but
took place in the course of a period of three minutes. We
had consequently the stimulating effect of variation of tem-
perature bringing about contraction, followed by the accele-
rated rate of growth which constituted the negative after-
effect of that stimulus, plus the accelerated rate of growth
due to rising temperature. That the first of these two factors
played a considerable part in this acceleration, is seen from the
fact that the permanent increase in the rate of growth charac-
teristic of the higher temperature of 35° C. is smaller than the
DETECTION OF LATENT STIMULUS
463
acceleration that precedes it. Incidentally, we see here the
difference between the temperature-effect per se, and the
stimulating effect of sudden variations of temperature, con-
stituting thermal shocks.
In the simple case we have just studied, where the whole
amount of incident stimulus was expressed in work external
and internal, without any loss from molecular friction, the
sum total of the two forms of response was found approxi-
mately the same under a constant stimulus ; but in other
cases, where a certain amount of energy is wasted in over-
coming molecular sluggishness, the results will be slightly
different, for at a temperature below the optimum a por-
tion of the stimulus will be wasted in overcoming such
sluggishness, whereas near the optimum temperature the
loss entailed on this account will be very slight. Hence, the
sum of direct and indirect responses, near the optimum, will
in such cases be somewhat greater than at a temperature
several degrees lower. I give below a table which shows at
a glance the direct effect and the indirect after-effect,
obtained at the three temperatures of 30° C, 35° C, and
37° C. respectively, with three different specimens, one of
which was a rice-seedling (0/y,':a sativa), and the two others
flower-buds of Crinum Lily. Each response given is the
mean of three.
Table showing Direct and Indirect Effects at Three Different
Temperatures
specimen
^»r"-| <2S|&
Indirect effect
(acceleration of
growth)
Total effect
I. Seedling of
Oryza saliva
37° C.
15 divisions
20 ,,
10 ,,
10 divisions
6-5 „
0 ,,
25 divisions
26-5 „
10 „
2. Crtniun Lily |
3. Crinwn Lily -j
30° c.
35° C.
37° C.
32 divisions
38 „
24
13-5 divisions
8-5
0 ,,
45-5 divisions
46-5 „
24
30° c.
35° C.
37° C.
18 divisions
28
12 ,,
17 divisions
13
0 ).
35 divisions
41
12 ,,
464 PLANT RESPONSE
It will be seen from the readings given by specimens i and
2, that up to the optimum the total response is approxi-
mately constant, while at '^'j° C. there is no expression of
energy held latent. The condition of specimen 3 having
been at starting somewhat sub-tonic, the total effect is
heightened at the optimum, for reasons which have been
explained. It has already been said that the proportion of
stimulus held latent will be greater with the degree of sub-
tonicity of the plant. That this was the condition of
specimen 3, then, is demonstrated, not only by the increase
of the total effect at the optimum temperature, but also by
the fact that at 30° C. so large a proportion of the stimulus
as almost exactly one-half is held latent. At -^y^ C. here, as
in the other cases, there was no latent component.
Is the change induced by stimulus always of an
explosive character? — It has generally been supposed that
stimulus causes response by an explosive chemical change.
According to this theory, the stimulus acts as upon a trigger,
to release suddenly a large amount of energy previously held
latent in the tissue. The response is thus assumed to be
always disproportionately larger than the stimulus, and to be
brought about by chemical degradation, or dissimilation of
the living tissue. The tissue, thus reduced below par, is sup-
posed to be restored by the process of assimilation.
In Chapter X., however — -on Theories concerning Different
Types of Response — I adduced considerations, showing that
there are cases of responsive phenomena in which this
description would not hold good ; that is to say, there are
instances in which the response cannot be due to a chemical
down change of explosive character. Nor is it always true
that response is disproportionally larger than stimulus. The
series of experiments which has just been described offers
conclusive evidence on this point, of a quantitative character.
Further, if the theory of an explosive down change had been
the real explanation of response in general, then it is clear
that in the case of the response induced by stimulus in grow-
ing organs, recovery would have taken place slowly, and
DETECTION OF LATENT STIMULUS 465
would have culminated in the restoration of the original rate
of growth ; but we actually find, on the contrary, that,
immediately following the responsive retardation, there is
an acceleration of growth above the normal, and the true
recovery, or restoration of the normal rate, takes place only
after this. Thus there is here, instead of a run down, an
actual increase, of the energy of the system.
Again, from this constancy of the sum of the immediate
and the after-effects of stimulus, we can see that, as in an
inorganic system, so also in a living organism, the law of the
Conservation of Energy holds good. For while at the
optimum point the entire stimulus finds expression in direct
response (stimulus being here equal to response), below the
optimum the direct response is less than the stimulus, the
missing fraction being left to find expression in the negative
after-effect.
Relation between stimulus and response in different
tonic conditions. — The experiments which I have described
were carried out under the thermal form of stimulation, which
is, as already explained, the most satisfactory in practice.
We shall hereafter come across instances of the after-effect of
accelerated growth, as caused by stimulus of light. And a
similar after-effect has already been seen to be caused by
electrical stimulus (p. 436). But for the clear demonstration
of this particular effect, electrical stimulus is not very suit-
able, inasmuch as it is apt to induce fatigue and, through
electrical polarisation, a certain amount of tissue-change. I
shall, however, describe an experiment, using this mode of
stimulation, which, by its responsive indications, will demon-
strate certain differences in the internal conditions of tissues,
according as they are sub- or super-tonic.
I have shown that the general excitability of the tissue at
30° C. is superficially the same as that at 2)7° C. or thereabouts ;
that is to say, the direct responses at these two temperatures
are in some cases approximately the same ; but we have seen
that there is a difference of molecular condition at these two
points, for at 30" C. the tissue is capable of holding a portion
II H
466 PLANT RESPONSE
of the incident stimulus latent, by which its molecular mobility
becomes enhanced, whereas at 37° C. there is already the
fullest molecular mobility. If now we apply at 30° C. stimuli
which increase, say, in arithmetical progression, we see that by
the very reception of the increasing stimuli the tissue is made
to approach more and more closely to the optimum condition,
at which point, as we have seen, the whole of the stimulus is
given up immediately, in the form of direct response. Hence
the curve showing the relation between stimulus and response
in a tissue that is in a condition below the optimum will be
steep, and somewhat convex to the abscissa which represents
the stimulus ; but at 37"" C, when no molecular sluggishness
has to be overcome, we may expect the response to increase
proportionately with the stimulus ; that is to say, the curve
showing the relation between stimulus and response will now
tend to be a straight line.
In order to apply electrical stimulation whose intensity
was increased by known amounts, I used an induction coil,
the primary coil of which was completely within the secondary.
The ordinary method of Du Bois-Reymond's sliding coil was
not very suitable in this case, because the increasing intensity
obtained by sliding the coil inwards is merely qualitative.
The required definite increase of induction-shock I secured
by suitable augmentations in the value of the current that
flowed round the primary coil. In order to determine these
values, a preliminary experiment was carried out. A storage
battery was in circuit with the primary coil, which had
interposed in it also an ammeter and a rheostat. By
increasing the resistance, a moderate current, say C, read by
the ammeter, was adjusted to flow round the circuit. The
secondary circuit contained a ballistic galvanometer, which
by its throw indicated the intensity of the induced current at
make or break of the primar}' circuit. When the first current,
C, had given an induced current, which caused a deflection of,
say, 30, it was increased to c^, when the deflection due to
induction was found to be 50 ; and lastly a third value, or C,^,
was found, whose induction-effect was 70. In this manner.
DETECTION OF LATENT STIMULUS
467
induction-shocks, increasing in arithmetical progression as 3
to 5 to 7, were determined. Tetanic shocks of the described
intensity — 3, 5, 7 — the total application of each group being
a period of 5", were now given, response being taken after
each. In this manner two series of responses were obtained at
the temperatures of 30° C. and 37° C. At both temperatures,
with the stimulus-intensity of 3, the
responses given by this particular
specimen were the same. But while
at 37° C. the lower of the two
curves, showing the relation between
stimulus and response, is a straight
line, that at 30° C, the upper of the
two, is seen to ascend more steeply
and to exhibit convexity to the
abscissa (fig. 186).
After-effect. — The phenomenon
which is generally referred to as the
after-effect is characterised by so
many complexities as to have been
regarded as highly perplexing. After
the foregoing analysis, however, by
which it has been resolved into two
elements, much of this obscurity
will be found to have disappeared.
We saw that the first effect induced
by a very strong or long-continued
external stimulus consisted in bring-
ing about the continuation of the
direct effect itself, its persistence
depending on the intensity and
duration of that stimulation. And
this we have distinguished as the positive after-effect. A com-
ponent part of the external stimulus, however, as we also saw,
becomes latent, thus increasing the internal energy ; and the
expression due to this element — the negative after-effect — is
opposite in sign to the effect of direct stimulus and its positive
H H 2
Fig. 186. Curve showing
the Relation between
Stimulus and Response
in the s.-ime Organ, under
the two different Tonic
Conditions of 30° C. (upper
curve) and 37° C. (lower
curve)
The abscissa represents the
intensity of stimulus, and
the ordinate the extent of
response. The response
was the same in both cases
when the stimulus-intensity
was 3 ; but later, the
curve for 30° C. is seen
to rise more steeply than
that at 37° C, exhibiting
at the same time convexity
to the abscissa.
468 PLANT RESPONSE
after-effect. Thus a plant under natural conditions, acted
upon by different stimuli, gives to each stimulus direct re-
sponse, direct after-response, and indirect response. But as
all these individual stimuli do not act, or cease to act,
simultaneously, we can easily understand the infinite com-
plexity of the combinations which take place between the
direct and indirect effects of stimuli of unlike forms, whose
maxima, instead of being coincident, are superposed on
each other, with various differences of phase.
Factors which determine periodic after-effects.— We
shall now proceed to enumerate some of the most important
S^timulating factors instrumental in modifying the response of
growth.
(i) Stimulus of light. — If we take the average rate of
growth during the twenty-four hours as the normal, then the
direct effect of this stimulus will appear as a retardation of that
normal rate of growth, and, after the long-continued action
of the whole day's illumination, this retardation may persist
for a time as the positive after-effect. Later, however, on
account of the stimulus which has been absorbed and held
latent, we shall observe an acceleration of growth above the
normal, or a negative after-effect. The persistence, again, of
this negative after-effect will depend on the amount of the
energy held latent by the tissue. The diurnal sequence of
light and darkness will, after long repetition, impress itself
upon the organism, and, other factors remaining constant,
will find expression as periodic retardation and acceleration
of growth during day and night ; such periodicity continuing
to show itself, for some time, even when the plant is kept in
continuous darkness.
(2) Temperature. — The effect of temperature up to the
optimum point will, by increasing the internal energy, prove
favourable to growth. Thus, the temperature during day-
light will usually be favourable, with the exception of the
tropical noon, when it may be excessive. A certain amount
of heat, again, may be stored up in the plant to give the after-
effect. At night, if the fall of temperature be very great,
there will be, relatively to this factor, a retardation of growth.
DETECTION OF LATENT STIMULUS 469
(3) Chemical Stimulus. — This we see supplied by the salts
taken up by the roots, and by the process of photo-synthesis
in the leaves. The amount of the former supply is depen-
dent not only on the richness of the soil, but also on the
suctional activity of the plant ; and the latter on the effective
intensity of light.
(4) Turgidity. — The turgid condition of the plant will
depend, firstly, on the supply of water ; secondly, on the
suctional activity ; and thirdly, on the relative absence of a
loss of water by transpiration. We have seen how watering
the roots of the plant will cause an immediate response of
enhanced growth, and the seasonal periodicity induced by
this cause may be observed in a very striking manner in
tropical countries.
The suctional activity, depending as it does on the
internal energy of the plant, will tend to be augmented by
an advantageous temperature, and by the after-effect of the
absorbed stimulus of light ; but the turgid condition will be
reduced by active transpiration, which is relatively greater
in the daytime.
We thus see how numerous are the factors which co-
operate to bring about periodic fluctuations in the rate of
growth during every twenty-four hours. Of all these factors,
the alternation of day and night is the most pronounced in
its action. The curve of growth, then, will exhibit not only
a large wave of alternation due to the diurnal period, but also
a number of sub-waves. And even beyond these, superposed
upon them, we may expect, when the magnification is suffi-
ciently great, to observe systems of still smaller wavelets,
caused by the rhythmicity of growth.
Continuous photographic record of the pulsations
of Desmodium. — Before describing the periodic diurnal
variation of that autonomous response which we know as
growth, it occurred to me that a continuous record of another
form of autonomous response — that is to say, of the rhythmic
pulsations of Desmodium — might prove interesting. I was
fortunate enough to succeed in obtaining a very good record
470
PLANT RESPONSE
of these pulsations during a period of twelve hours, by means
of photography. The record began at 6 p.m. and ended at
6 a.m. During this time there were no fewer than 1 80 con-
stituent pulses, and it will be noticed that these again fall
into groupings, whose average period is a little over an hour,
there being about ten such groups in the course of twelve
hours (fig. 187).
Returning now to the question of periodic growth-fluctua-
tions, Sachs and others have measured the different rates of
growth at various hours within the twenty-four, and from the
data thus obtained have constructed curves which showed
6 P.M.
9 I'.-M.
|||#%^L.A#V\%1M^^
2 3 A.M. 6 A.M.
Fig. 187. Continuous Photographic Record of Autonomous Pulsation of
Desmodiin/i gyraiis from 6 p.m. to 6 a.m.
The lower record is in continuation of the upper.
the periodicity of the rate of growth. These curves exhibit
the daily period in a marked manner ; but the subordinate
waves are more or less obliterated, in consequence of the
fact that the data from which the curves were constructed
were obtained from discontinuous observations. The curves
thus deduced show marked differences also, according as
their points were determined frequently or at long intervals.
Record of periodic variation of rate of growth. — For
this reason it appeared to me important to devise means by
which, not the growth, but the vanaiions, of its rate might
be automatically recorded, directly and continuously, for any
DETECTION OF LATENT STIMULUS
471
length of time. The curve thus obtained ought instanta-
neously to mark the fluctuations of rate throughout the
period in question. This I have been able to accomplish
by means of a modified Method of Balance. From the
description of that method already given, it will be under-
stood that after the establishment of the average balance, if
the rate increases, the curve will move upwards. When,
after this, the rate of growth returns to the normal, the
balance will be re-established, and the curve become again
horizontal ; but if the growth at any time should fall below
the average, the curve will
descend. In this way pe-
riodic fluctuations in the
rate of growth may be
recorded.
For the purposes of the
modified method of record,
the compensating arrange-
ment used for balance has
to be somewhat altered.
In recording these long
periodic changes, we have
fluctuations of larger am-
plitude than those of au-
tonomous pulsation. The
spot of light which is
thrown from the. mirror of
the experimental Optic
Lever upon the second, or
compensating, mirror is
thus apt to fall outside
the range of the latter. In
order to overcome this difficulty I mount the plant on the
float itself, and adjust the outflow of water from the
cylinder, so that the upward growth of the plant is, at a
given moment, exactly compensated by the descent of the
float supporting it. Deviations above or below the balanced
rate of growth may then be followed with a recording pen,
Fig. 188. Ilydrometric Apparatus for
Recording Continuous Variation of
Rate of Growth
float, supporting the plant, which is
attached by thread to optic lever, L.
w, wheel, for adjustment of balancing
overflow.
47^
PLANT RESPONSE
or will record themselves on a sensitised photographic film
wrapped round the revolving drum.
In carrying out such a continuous record, certain pre-
cautions are necessary. Owing to transpiration, the float on
which the plant is mounted will become lighter, causing an
ascensional movement of the record. In order to obviate
this, it is only necessary (i) to weight the float to such an
extent that the variation caused by transpiration is negli-
gible ; and (2), in addition to this, the diameter of the cylin-
drical float may be so increased as to reduce still further the
ascensional movement due to loss of weight by transpiration.
By these means the error from this source may be reduced
to any extent desired. In the
figure of the apparatus which is
here given (fig. 188), the specimen
was a young seedling of Oryaa
sativa, in v/hich transpiration was
relatively little. I shall presently
give photographic records obtained
in the manner described.
Continuous photographic re-
cord of periodic variations of
transpiration. — By a somewhat
similar method we are enabled to
determine the periodic variation of
the rate of transpiration. In this
case, the plant is mounted with its
roots in a test-tube, which acts like
a float, and is partially filled with
water, but not so full as to make it
sink. The test-tube containing the
plant is attached to one arm of the
Optic Lever, and the outflow from
the outer cylinder so adjusted that
the ascensional movement of the test-tube, due to its loss of
weight by transpiration, is exactly balanced by the subsidence
of the water-level of the water that buoys it up. In order
Fig. 1 89. Photographic Record
showing Variation of Rate
of Transpiration in Cucur-
bit a, from 3 P.M. to 12 P.M.
The rate is seen to undergo
enhancement till about
11.30 P.M., after which
there is a rapid fall of the
rate of transpiration.
DETECTION OF LATENT STIMULUS 473
to prevent the loss of water from the cyHnder by evaporation,
a fihn of oil covers the surface. In this way I obtained
the accompanying record of variation of rate of transpiration
in a young specimen of Cucurbita, from 3 I'.M. to 12 P.M.
(fig. 189). It will be noticed that in this case there is an
enhancement of transpiration which cof\tinues with a single
fluctuation till past 11 P.M., after which there is a sudden
depression of the rate.
Continuous photographic record of the diurnal varia-
tion of the rate of growth. — I shall first give a photographic
record (fig. 190) taken from a seedling of Oryza sativa, only
Fig. 190. Continuous Photographic Record of Variation of Rate of
Growth in Four Days' Old Seedling of Oryza sativa, from 3 p.m.
till 9 A.M., that is during Eighteen Hours
four days old. The diurnal periodicity has already, it will be
seen, become fairly impressed, though it is not yet sufficiently
powerful to mask, to any great extent, the subsidiary periodi-
cities induced by other factors. The record, it must be
remembered, was taken in continuous darkness, being com-
menced at 3 P.M., when it was balanced. From this time to
9 P.M. there were three pulsations. From 6 till after 8 P.M.
there was depression of the average rate of growth, after
which it rose somewhat rapidly till 12.30 A.M., exhibiting
during that period two groups of two pulsations each. There
was now a c[uick fall for the next half-iiour, and after this
474 PLANT RESPONSE
the growth-rate rose more or less continuously till 8 A.M. The
growth-rate then began to fall during the course of the day.
The second record was taken with a seedling of Tama-
rindus indica fourteen days old, the diurnal periodicity being
thus deeply impressed. It was placed in the dark room,
mounted on the float, and the balanced record begun at
3 P.M. It will be seen that, as the positive after-effect of
the day's illumination, there was a depression of the rate
Fig. 191. Continuous Photographic Record of Variation of Rate of
Growth in Seedling of Tamariiidiis indica, a Fortnight Old, from
3 P.M. to 3 A.M.
Owing to positive after-effect of dayhght, there is a depression of rate of
growth, although the seedling was now placed in a dark room. In tlie
evening, however, the rate began to rise.
of growth, though the plant was kept in the dark. This
persisted for two hours, till 5 P.M., after which the rate showed
increase, there being three pulsations before the end of the
record, at 3 A.M. (fig. 191).
Annual rings of wood and seasonal periodicity.— The
different growths of wood in spring and autumn, leading to
the formations known as ' annual rings,' constitute a pheno-
menon of growth not yet fully explained. I may here point
out an important factor in connection with this subject. It
DETECTION OF LATENT STIMULUS 475
has already been demonstrated that growth is a phenomenon
of excitatory reaction, being at its maximum when the
excitabihty of the tissue is greatest. Thus varying expres-
sions of growth at the two seasons, as seen in the production
of different sized' cells during spring and autumn, would
appear natural,' if they could be correlated to differences of
excitability, characteristic of those seasons. Now all modes
of testing degrees of excitability lead to the conclusion that
while it is very great in spring and summer, it is very much
enfeebled in autumn and winter. Thus, in the latter season,
contractility under stimulation, velocity of transmission of
excitation, and the electrical response of a tissue, are all
found to undergo a marked diminution, as compared with
spring and summer.
Summary
On the cessation of strong stimulus the responsive
movement continues for a time in the same direction. This
is the positive after-effect.
A portion of the incident stimulus is absorbed and held
latent, thus increasing the latent energy of the plant. On
the cessation of stimulus this latent component, either
immediately or after a time, finds expression in an opposite
responsive movement. This is the negative after-effect.
In the case of growth-response, the positive after-effect
consists in the persistence for a time of the retardation of
growth, and the negative after-effect exhibits itself as an
acceleration of the rate of growth above the normal.
With moderate stimulus and under normal conditions
the sum of the direct effect and the negative after-effect (due to
the latent component) remains constant up to the optimum ;
that is to say, the sum of the external work (direct effect)
and internal work (negative after-effect) done by the stimulus
is the same. The direct and the negative after-effect of
stimulus are thus complementary.
At the exact optimum almost the whole stimulus will
find expression in direct response, there being little or no
476 PLANT RESPONSE
latent component. In a sub-tonic condition, on the other
hand, a greater proportion of the stimulus is temporarily held
latent, and expresses itself as the negative after-effect, the
direct responses being here correspondingly diminished.
Above the optimum there is no latent component, and the
general receptivity of the organ shows great diminution.
From the constancy of the sum of the direct and indirect
effects it is demonstrated that, with regard to some forms of
response at least, response is not disproportionately greater
than stimulus. Thus the theory that response must always
be due to an explosive chemical change does not hold good.
The curve sho\ving the relation between stimulus and
response is appropriately modified by the tonic condition of
the tissue.
As each stimulus of every form thus finds expression in
direct response, direct after-effect, and indirect after-effect,
and as there are many forms of stimulus which under natural
conditions act on the plant, whose maxima, instead of being
coincident, are superposed on each other, in varying differ-
ences of phase, highly complex periodicities are induced, and
find expression in the various forms of plant response.
Among such varying factors of stimulation may be
mentioned the diurnal alternation of light, temperature,
chemical stimulus, and varying turgescence.
The induced periodicities which result from the conditions
described may be seen in the periodic groupings which
appear in a continuous record of the autonomous pulsations of
Desniodmm for example.
The autonomous response of growth, as the result of the
periodically acting stimuli mentioned, exhibits not only a
large wave of alternation, due to the diurnal period, but also
a number of sub-waves. But the impression made on the
organism by the diurnal period is the deepest of these, and
tends in an old plant to subordinate all others in a marked
degree. In the responses of a seedling a few days old, how-
ever, the minor waves are very distinct.
CHAPTER XXXV
AN INVESTIGATION INTO THE DIFFERENT EFFECTS OF
DRUGS ON PLANTS OF DIFFERENT ' CONSTITUTIONS '
General consideration of the problem —' Constitution ' and the elements which
determine it — Methods of investigation -Action of carbonic acid gas— Action
of ether — Effect of solution of sodium carbonate — Effect of solution of sugar —
Effect of alcohol — Effect of acids — Effect of alkali — Antagonistic action of
alkalis and acids — Action of strong solution of sodium chloride — Effect of
poisonous solution of copper sulphate— Opposite effects of the same dose on
different constitutions — Opposite effects of large and small doses.
We have already studied the effect of various chemical
agents on the physiological condition of plants, as seen
from the modifications induced by them in ordinary and in
autonomous responses. We have also noticed the remark-
able similarity between these effects in the two cases of
plant and animal tissues, and I have drawn attention to
the great practical utility of these investigations, inasmuch
as the experiments carried out on plants may be made
to throw light on many obscure phenomena regarding
the effects of drugs on the animal system. One puzzling
fact, however, which is encountered in medical practice, is,
that the same drug will often produce varying effects on
different individuals ; and this is vaguely ascribed to dis-
similarity of ' constitution.'
Similarly, we have sometimes seen different effects to
occur in plants under the action of a single given drug.
A large dose of some depressing agent, for example, though
it will ultimately produce the depressing effect, will not
always do so at once, for in some cases there will be a pre-
liminary period of exaltation of response. The same toxic
dose, again, which will in some instances' kill the plant, will
4/8 PLANT RESPONSE
in others fail to do so, the plant being ultimately able to
shake off the depressing influence after an interval of
struggle. In studying suctional response, again, we found
that copper sulphate applied at the roots induced, in some
cases, an immediate arrest of suction, while in other instances
this arrest did not take place till after a long time. The
depression of suction which was induced by the application
of strong sodium chloride, again, was in some cases imme-
diate, and in others preceded by a fairly long period of an
exalted rate of suction (pp. 384, 385). All these variations
of results we regarded as due to individual differences of
constitution, or of tonic condition.
Thus we can only hope to arrive at a complete knowledge
regarding the action of any given drug if we first obtain a
precise understanding of what is meant by constitution, and
of how, for experimental purposes, specimens of a definite
characteristic constitution can be secured, while others can be
subjected to an ascertained variation in a pre-determined
manner. It will thus be made possible to study the effect of
a drug, by applying it (i) in solutions of different strengths
to a number of plants of identical constitution ; and (2) in a
single strength of solution to specimens of definitely varying
constitutions.
' Constitution ' and the elements which determine it. —
The first factor in determining constitution will consist of those
properties which have been impressed upon the plant by its
heredity. The second will depend upon its environment.
The sum total of the energy absorbed by the plant from its
surroundings we have already designated as the tonic con-
dition. It is clear that we may secure the factor of a constant
heredity by taking either seedlings from the same batch of
seeds, or organs from the same plant. These, again, when
maintained under the same environmental conditions, in
respect of temperature and other circumstances, will give us
plants having practically the same constitution. In order,
next, to obtain specimens of different but well-ascertained
constitutions, it is only necessary to keep these under known
EFFECTS OF DRUGS ON PLANTS 479
differences of tonic condition. Thus, all other factors being
maintained constant, we may have three clearly defined
states, in the case, say, of Crinuvi Lily, according as it is
kept at a temperature of 30° C, 34° C, or 37° C. The first
of these we may regard as the normal ; the second as
near the optimum ; and the third as intermediate between
optimum and maximum. The excitability of the plant kept
at the optimum will be the greatest ; but though the excita-
bilities of those at 30° C. and 37° C. will be approximately
the same, yet, in the latter case, the plant will possess an
excess of latent energy which will be wanting in the former.
Having thus secured these definite artificial constitutions of
different values, some of the investigations given at the end
of this chapter will show how free from uncertainty the
action of drugs may be made, and how rational an expla-
nation can be given of the observed variations of effect.
Methods of investigation. — I shall now proceed to
describe the general methods of experiment, in studying the
effects of drugs, from the modifications which they induce in
growth-response. There are two ways of doing this. Accord-
ing to the first, we take a record of the growth before and
after the application of the reagent. From the variation
then seen in the rate of growth the excitatory or depressing
nature of the drug may be ascertained. The second, or
Method of the Balanced Crescograph, is much more delicate ;
it exhibits each transient variation of response, and its time-
relations, with perfect clearness. The balanced horizontal
record, which is first taken, indicates the normal rate of
growth. A deviation upwards from this horizontal line will
indicate accelerated growth ; a return to the horizontal will
mean a regaining of the normal rate ; and a deviation down-
wards will show responsive retardation. The specimens used
for this investigation were Crinuvi Lilies, and, unless stated to
the contrary, the experiments were carried out at the normal
temperature of 30° C.
Action of carbonic acid gas. — I first give a record of
the effect of carbonic acid gas on growth (fig. 192), taken by
48o
PLANT RESPONSE
Fig.
[92. Eftect of Carbonic Acid
Gas on Growth
the Unbalanced Method. The normal rate of growth was
•006 mm. per minute. On passing CO^ into the plant
chamber the immediate effect was an acceleration, the rate
for the next five minutes being -009 mm. or i^ times the
normal. Under the continued action of carbonic acid, how-
ever, the rate underwent a rapid diminution, and, as is seen
by the slope of the curve
becoming horizontal, growth
was arrested fifteen minutes
after the introduction of
carbonic acid into the plant
chamber. On the re-intro-
duction of fresh air the growth
was slowly renewed, and
gradually returned to its
original rate. The effect of
carbonic acid on growth-
response, then, is a prelimi-
nary exaltation, followed by
depression and arrest, which
arrest, if the action be not too
long continued, proves to be
only temporary.
Action of ether. — This experiment shows the curious
difference of results which occurs, according as an applica-
tion is external or internal. In this and in the following
cases, with the exception of the experiments on acids and
alkalis, I shall use the Balanced Method, as this brings out
even transient variations in a very striking manner. The
balanced horizontal record is seen to the left of each figure.
In curve a of fig. 193 is shown the effect of an external appli-
cation of this reagent, ether vapour being introduced into
the plant chamber. It will be seen that there was an imme-
diate retardation of growth, which lasted for more than a
minute. This was followed by an acceleration of growth,
which lasted for two minutes. There was then a depression,
which continued, aitd culminated in the arrest of growth.
The first arrow indicates introduction
of CO.^, which induces preliminary
enhancement of growth, followed
by subsequent arrest. The second
arrow indicates the introduction of
fresh air, followed by the revival
of growth. Record taken by Un-
balanced Method.
EFFECTS OF DRUGS ON PLANTS
481
In ^ is shown the effect of an internal application, made by
replacing the water, by which the cut end of the stem was
normally surrounded, with a 3 per cent, solution of ether.
It will be seen that, as the specimen sucked up the solution,
the immediate effect of this
internal application was an
enhancement of the rate of
growth, and that this was
followed by depression, lead-
ing to arrest of growth. The
preliminary depression seen
in a is thus wanting in the
case of the internal applica-
tion. It might be thought
that this first depression, as
seen in the case of the ex-
ternal application, was due to
a slight cooling, caused by
the introduction of ether. We
know, however, that while
lowering of the temperature, as long as that is below the
optimum, would suffice to retard growth, a similar lowering
of it when above the optimum would have the opposite effect
of acceleration. Now I find that
this preliminary retardation of growth,
seen in fig. 193, a, takes place in
exactly the same manner when the
experiment is repeated with the
specimen at 38° C. It cannot, there-
fore, be due to the suggested cooling,
which must in any case, under the
experimental conditions, have been
extremely slight. There is, however,
another explanation, which more nearly meets the require-
ments of the case. We know that any sudden variation of
environmental conditions is apt, generally speaking, to act as
a stimulus, and the effect of direct stimulation is always to
I I
Fig. 193. Balanced Records ol
Effect of Ether on Growth. Up
Curves represent Acceleration
Above, and Down Curves Re-
tardation Below, the Normal
a Effect of external application, and
b of internal application of ether.
Successive thick dots in the base-
line indicate time in minutes in
this and following records. Arrow
shows moment of application.
Fig. 194. Excitatory Effect
of Dilute Solution of So-
dium Carbonate on Growth
482 PLANT RESPONSE
induce contraction, which would in the present case take
the form of a transient retardation of growth.
Effect of solution of sodium carbonate. — In the course
of the following experiments the chemical reagents are
administered internally, by applying the solution at the cut
end of the specimen. A dilute solution of sodium carbonate
is known to increase excitability in the case of animal tissues.
I find that this holds good in the case of the growth-response
of plants, growth being accelerated, as will be seen in
fig. 194, where the balanced record suddenly gives place to
an ascending curve. But in the case of chemical reagents in
general, and of this especially, it must be remembered that
the strength of the solution, or dose, is an important element
in the result. A -5 per cent, solution of sodium carbonate
was always found in these experiments to be an excitant ;
but as the strength of the solution was increased, the excita-
tory effect was found to be gradually diminished, until at
2 per cent, it became neutral. If
now the strength were still further
increased, say to 5 per cent., the
effect was an actual depression of
the rate of growth.
Effect of solution of sugar.— I
next give a record (fig. 195) showing
the effect of the application of sugar
Fig. 195. Acceleration of jn a 2 per Cent, solution. This is
Growth by Application of
Solution of Sugar seen to mduce a responsive accelera-
tion of growth. In the case of the
Crinnm Lily, this acceleration is found to occur even under a
5 per cent, solution of sugar ; but very much stronger solu-
tions induce depression.
Effect of alcohol. — We have hitherto observed different
reagents- inducing a more or less uniform acceleration or
depression. In the case of alcohol, given in 5 per cent,
solution, however, we obtain a very curious instance of
alternating spasmodic effects (fig. 196) ; that is to say, the
growth at one moment exhibits a sudden acceleration, and
EFFECTS OF DRUGS ON PLANTS
483
196. Spasmodic Alternations
of Growth under Alcohol
in the case of the autono-
as of cardiac muscle, that
at the next a sudden depression, such alternations being
continued for a considerable length of time. On repeating
the experiment at the higher temperature of 34° C. I found
that these spasmodic alterna-
tions became still more violent —
that is to say, of greater ampli-
tude, though less frequent. At
a much higher temperature, how-
ever, the effect of alcohol was
an immediate depression. Still
stronger solutions caused arrest
of growth at all temperatures.
Effect of acids. — We found,
mous responses of Desviodiiini
acids induced relaxation, and that the long-continued action
of such a reagent, or a strong solution, would bring about
arrest in the relaxed position. Curiously enough, then, in
the case of growth, which I have shown
to be an instance of multiple or autono-
mous response, I find an effect exactly
parallel. In order to hasten the result
I used a somewhat strong solution,
namely 4 per cent., of hydrochloric acid.
This, as will be seen (fig. 197), caused
a marked relaxation, and growth came
to a standstill some six minutes after-
wards. In this experiment, and the
following on the effect of alkali, the
record was taken under unbalanced
conditions.
Effect of alkali. — The effect of alkali in the case of Des-
modinvi, and also of cardiac muscle, is to produce arrest in
the contracted position ; a similar effect is strikingly exhi-
bited in the growth-record given in fig. 198. The first part
of this record shows the normal rate of growth. A 3 per
cent, solution of sodium hydrate was then api)lied at the point
marked with the downward arrow (>!'). It will be noticed
1 1 2
'. 197. Unbalanced
Record showing Ac-
tion of Acid in Causing
Relaxation and Ulti-
mate Arrest of Growth
484
PLANT RESPONSE
that this induces a very great contraction, and that in the
course of seven minutes there occurs an arrest of growth, in
a contracted position. It will also be seen that the specimen,
after the application, actually be-
came shorter than it had been
before. The alkali therefore had
the effect not merely of arresting
growth, but also of causing an
active contraction of the tissue.
Antagonistic action of alkalis
and acids. — We saw that in the
autonomous responses of Des-
viodiuni and cardiac muscle, the
state of standstill induced by the
action of either acid or alkali was
neutralised and counteracted by the
antagonistic action of the other (cf
fig. 155). I have detected precisely
the same peculiarity in the case of
growth-response also. It was seen
in the course of the last experiment that growth was brought
to a state of standstill, in the contracted position, by the action
Fig. 198. Unbalanced Record
showing the Action of Al-
kali, and the Antagonistic
Action of Subsequent Appli-
cation of Acid
The downward arrow ( \ ) indi-
cates application of alkali,
which induces arrest of
growth in contracted posi-
tion. The upward arrow ( T )
indicates application of acid,
which by its antagonistic
action renews growth.
Fig. 199. Effect of Strong Solution of NaCl on Rate of Growth, as
Modified by Different Constitutions of Specimens
At a temperature of 30° C. there is an immediate depression. Near the
optimum there is a well-marked resistance, and preliminary acceleration
before depression sets in. The same is true to a less extent at 37° C.
These records were taken under balanced conditions. At 44'' C. there
was normally no growth, but this was temporarily initiated under the
action of strong NaCl solution. Record taken by Method of Balance.
of alkali (fig. 198). At this point, as marked by the upward
arrow (|), acid was applied. This reagent now had the effect
EFFECTS OF DRUGS ON PLANTS 485
as will be seen from the record, of neutralising the previous
contractile arrest, and in the course of two minutes it had
brought about the renewal of growth.
Action of strong solution of sodium chloride.— In the
course of the investigation on suctional response we noticed
that the effect of this reagent, when applied to the root, was
not always to cause a diminution of suction, as might have
been expected had osmotic action been the only factor. This
reagent, on the contrary, usually operated to bring about a
preliminary acceleration of suction. And I have already ex-
plained that this was due to its excitatory character, acting in
opposition to the osmotic action set up by the strong solu-
tion. It is usually supposed that the excitatory effect of
strong solution of salt on animal tissues is due to the osmotic
withdrawal of water ; but we have here seen that similar
excitation is induced in vegetable tissues without any with-
drawal of water. Hence the usual theory of the action of
salt solution in causing excitation is rendered very doubt-
ful. We found, in fact, that when the tonic condition of the
tissue was favourable, the excitatory reaction predominated,
and there was a consequent enhancement of suctional
activity. In other instances, where the tonic condition was
less favourable, osmotic action predominated, and there was
a consequent depression of the normal rate of suction (p. 385).
It is thus seen that the factor of variation in these two
different cases was the constitution, or tonic condition, of the
plant ; we are now able to study with precision the influence
exercised by constitution, in causing the plant to struggle
against, or succumb to, the action of adverse external cir-
cumstances. The action, in causing variation of suction, of a
strong solution of salt, applied internally through the cut end
of the stem, can also be studied by means of growth-response ;
for while increased suctional activity will give rise to a posi-
tive turgidity-variation, with concomitant enhancement of the
rate of growth, diminished suctional activity will have the
opposite effect, of retarding the rate of growth. We shall
next observe the effect of this reagent on different specimens
486 PLANT RESPONSE
of Crinuin Lily, which had been taken from the same flower-
head, and in which different constitutions were artificially
induced by carrying out the experiments at 30° C, 34° C,
37° C, and 44° C. The records were taken under balanced
conditions.
On making an application of a solution so strong as 10
per cent, to a specimen at the normal temperature of 30° C,
the result was always a depression which set in immediately ;
but when the tonic condition of the plant was exalted, by
raising the temperature to a point near the optimum — that is
to say, to 34° C. — the same reagent was found to induce an
excitatory effect, its depressing action being postponed for
a considerable length of time. The effect obtained at the
temperature of 37° C. was very instructive. Responsive
excitability at this temperature has been shown to be almost
the same, if not indeed slightly lower, than that at 30° C.
But we should remember that in these two cases we have very
different histories ; for if in the former — that is, at 30° C. —
we have a normal amount of latent energy, then it is clear
that in the latter — that is to say, at 37° C. — there must be an
excess of latent energy. And as the result of this we see
that, while at 30° C. the growth-response showed immediate
depression, at 37° C. it offered a considerable resistance, as
seen in the temporary exaltation of response. Still more
interesting, however, was the e^ect at 44° C. At this tem-
perature it will be remembered that there was an apparent
arrest of growth, often supposed to be due to the setting-in
of heat-rigor. I have shown, however, that, so far from this
being the case, there is still at 44° C. a good deal of rhythmic
activity, the cessation of growth being due to the fact that in
the multiple response of growth each constituent response and
recovery had become equal. The presence of this activity at
44° C. becomes quite clear, when we iind that the application of
salt at this temperature has the effect of renewing for a time
the resultant growth which had been in abeyance (fig. 199).
Effect of poisonous solution of copper sulphate. — The
influence of constitution in determining resistance to adverse
EFFECTS OF DRUGS ON PLANTS
487
Fig. 200. The Eftect of Different
Constitutions in Determining the
Resistance Offered to I'oisons.
The Action of 5 per Cent. Sohition
of Copper Sulphate
circumstances is made still more striking when we observe
the action on the plant of strongly poisonous reagents, such
as 5 per cent, solution of copper sulphate. At the normal
temperature of 30° C. its application, as will be seen in the
record (fig. 200 ), induces a _
very rapid depression, which
soon culminates in permanent
arrest But at 34° C. the
resistance offered is consider-
able, actually exhibiting it-
self in temporary exaltation.
At 44° C, again, we observe
the poisonous reagent actually
initiating growth, as in the
last case.
Opposite effects of the same dose on different consti-
tutions.— ^^In the case last considered, we studied the pheno-
menon of the resistance offered by the plant to a toxic dose
largely in excess of the fatal amount. In spite of the
excessive dose we found that, when a favourable constitution
was artificially induced, the plant
succumbed, it is true, but only
after considerable struggle. I shall
now proceed to show how, when
the artificial constitution is suf-
ficiently favourable, the plant,
instead of succumbing to an
ordinarily fatal dose, can shake off
the effect, and may even be stimu-
lated by it. Thus I found that a
I per cent, solution of copper
sulphate induces depression and
is ultimately fatal at 30° C. ; but
when the same dose is applied to a plant at 34" C. the effect
is seen in a marked exaltation (fig. 201), which continues for
a fairly long period, after which it shakes off the effect of the
poison altogether, and resumes its normal rate of growth.
;. 201. The Effect of Favour-
able Induced Constitution in
Enabling Plant to Shake off
Result of Toxic Dose of
Copper Sulphate
488 PLANT RESPONSE
Opposite effects of large and small doses. — I have
shown the toxic effect of a i per cent, solution of copper
sulphate at the normal temperature of 30° C. If, however,
the dose be reduced to -2 per cent, we shall find that its
action becomes stimulatory (ng. 202). This is an interesting
illustration of the general fact that a poisonous reagent, if
given in sufficiently minute doses, will act as an excitant.
In conclusion, a survey of the effects of drugs, both
stimulatory and poisonous, reveals the striking fact that the
difference between them is a question of quantity. Sugar,
for instance, which is stimulat-
ing when given in solutions of,
say, I to 5 per cent., becomes
depressing when the solution
is very strong. Copper sul-
FiG. 202. Opposite Effects of phate again, which is regarded
Large and Small Doses of • ■ i .
Poison as a poison, is only so at i per
A solution of I per cent, copper Cent, and Upwards, a solution of
sulphate produces depression -2 per cent, being actually a
and death, but -2 per cent. ^ ^, „
exercises stimulating action. Stimulant. The difference be-
tween sugar and copper sulphate
is here seen to lie in the fact that in the latter case the range
of safety is very narrow. Another fact which must be borne
in mind in this connection is that a substance like sugar is
used by the plant for general metabolic processes, and thus
removed from the sphere of action. Thus continuous absorp-
tion of sugar could not for a long time bring about sufficient
accumulation to cause depression. With copper sulphate,
however, the case is different. Here, the constant absorption
of the sub-toxic stimulatory dose would cause accumulation
in the system, and thus ultimately bring about the death of
the plant.
Summary
Chemical reagents are found to have the same effect on
the response of growth as on ordinary response, or as on the
autonomous response of Desniodiwn gyra?is.
EFFECTS OF DRUGS ON PLANTS 489
Carbonic acid gas induces a preliminary acceleration,
followed by retardation and arrest of growth. On re-
admission of fresh air growth is revived.
Ether, when applied internally, causes a preliminary
acceleration, followed by retardation and arrest of growth.
Minor differences of effect may be observed if the applica-
tion be made externally.
A very dilute solution of sodium carbonate is an exci-
tant, and accelerates growth ; but stronger solutions cause
retardation.
Solution of sugar also stimulates growth, if the strength
of solution be not excessive.
Alcohol causes spasmodic alternations of growth. Too
strong a solution arrests growth.
As in the case of autonomous response of Dcsviodium,
so in the response of growth also, acids and alkalis are
found to have antagonistic effects. Acid causes relaxa-
tion and ultimate arrest of growth ; alkali causes arrest
of growth in the contracted position. The arrest brought
about by one of these agents may be counteracted by the
other.
The reaction of a specimen to a chemical reagent is
determined by the strength of the dose, and by the duration
of application.
A stimulating agent if given in too strong a dose causes
depression. A poisonous reagent, again, if given in a suffi-
ciently small dose, acts as an excitant.
A clear insight into the nature of 'constitution,' so called,
as a factor in determining the reaction of a specimen to a
given drug, is afforded by the induction of definite arti-
ficial constitutions. This may be carried out by subject-
ing the plant to four different typical temperature con-
ditions— the ordinary, the optimum, post-optimum, and
maximum.
It is then found that a plant which has been made to
acquire excess of internal energy can struggle against, or
even overcome, the influence of such adverse circumstances as
490 PLANT RESPONSE
the action of poison, whereas under ordinary circumstances
it quickly succumbs.
The nature of the response is thus seen to be determined
in a definite manner by the constitution, or tonic condition,
of the plant, this factor being in its turn dependent on the
sum total of the energy which is latent in the organism.
PART VII
GEOTROPISM, CHEMOTROPISM AND
GALVANOTROPISM
CHAPTER XXXVI
THE RESPONSIVE CURVATURES CAUSED BY GRAVITV.
NEGATIVE GEOTROPISM
Statement of the problem of apogeotropic response— Mode in which stimuhition
is brought about : radial-pressure theory, and theory of statoliths — Mechanics
of responsive movement — Experiment demonstrating responsive curvature as
brought about by unilateral pressure of particles— Record of curvature induced
by gravitation— Record of different rates of curvature when specimen is held
at angles of 45° and 135° co the vertical— Determination of the true character
of apogeotropic response — Responsive curvature of acellular organs— Curvature
of grass haulm under gravity— Growth of grass haulm on a klinostat.
It is well known that growing organs exhibit certain
directive movements under the influence of gravitation.
Horizontally laid shoots, for example, bend upwards, or
against the direction of gravity ; while roots react in pre-
cisely the opposite way — that is to say, they bend so as to
lie in the direction of the force of gravity. In the case of
the various forms of stimulation hitherto studied, the action
of the plant is well defined and intelligible, consisting of con-
cavity of the excited side, in response to a stimulus which is
clearly understood. But in the case of geotropism the mode
in which stimulation is brought about is not quite evident, and
we find, moreover, two directly opposed effects brought about
by apparently the same force of gravity : in the root, as
already said, a positive movement, that is to say a movement
in the direction of gravity ; and in the shoot a negative move-
ment, away from the direction of gravity.
The seeming impossibility of explaining effects so divergent
as due to a single common cause, has led to the modern
idea that these responsive movements are ' executed at the
suggestion of changes in the environment, not as the direct
494
PLANT RESPONSE
and necessary result of such changes ; ' or in other words that
' light and gravitation could be classed together as external
agencies acting, not directly, but in some unknown indirect
manner.' ^
There are two distinct points to be borne in mind in
connection with the effect of gravity : first, the question as
to how gravity exercises stimulation ; and secondly, that of
how, in answer to this stimulus, a definite responsive curvature
is induced.
Mode in which stimulation is brought about. — Now it
is clear that, as regards the former of these points, the only
conceivable way in which gravity could produce stimulation
is by the effect of weight ;
not, that is to say, by the
weight of the plant as a whole,
but by the differential effect
of weight in the cells. Two
important theories have been
advanced which offer very
rational explanations of the
means by which gravi-percep-
tion may be induced. Accord-
ing to these, the necessary
differential weight-effect may
be due to the weight of the
cell-contents, whether of the
sap itself, or of those heavy particles like starch-grains which
are contained in it. When, therefore, the cell is laid horizon-
tally, it is the lower tangential wall which has to support the
relatively greater weight (fig. 203). This theory of hydrostatic
or radial pressure was suggested by Pfeffer, and supported by
Czapek. The other theory, of statoliths — advocated by Noll,
Haberlandt, and Nemec— substitutes for the weight of the
' A luminous resume of our present state of knowledge on this subject, with
all its difficulties and obscurities, is contained in the addresses of Francis Darwin
delivered before the British Association in 1891 and 1904. From these I have
made the quotations which appear in the text. Figs. 203, 204, and 209 appear
as illustrafions of the latter address in Nature of September 8, 1904.
Fig. 203. Diagrammatic Representa-
tion showing Differential Effect
of Weight on Lateral Walls of
Cells
In the figure to the right the cell is
laid horizontally, and the lateral
wall, D, has to bear greater weight
than c (after Francis Darwin).
RESPONSIVE CURVATURES — NEGATIVE GEOTROPISM 495
.1 1 1.
Fig. 204. Diagrammatic Representation
of a Multicellular Organ
On the upper side the statoliths act on the
inner, and on the lower side on the
outer, tangential wall (after Francis
Darwin).
water-column the weight of certain relatively heavy bodies,
such as starch-grains, differentially exercised upon the lower
tangential walls. In the case of multicellular plants, laid hori-
zontally (fig. 204), EE and E'E' may be regarded as regions in
which stimulation is caused by the weight of the particles.
The effects produced on
the upper and lower c
halves are evidently an-
tagonistic, and in spite of
this we obtain in the case
of shoots a resultant cur-
vature upwards. This
shows that the stimulation
of one half must be greater
than that of the other.
The inequality must be
due to this difference, that
the statoliths in EE rest
on the inner, and in E'E'
on the outer, tangential wall. It would thus appear that one
of these must be less excitable than the other.
Mechanics of responsive movement. — From these
hydrostatic or statolithic differences of weight, bearing on the
ectoplasm of the cell, it is understood that gravi- perception
arises, that is to say that the plant perceives the direction of
gravity. But there is no explanation as to how stimulation
is produced ; nor is there any satisfactory explanation as to
the mechanics of the responsive curvature. It is generally
supposed, as has been said, that this curvature is not the
direct and necessary result of some environmental change,
but that it is an instance ' of a plant reading a signal and
directing its growth' accordingly. It is supposed further
that in an apogeotropic cell the curvature takes place by the
development ' of relatively accelerated growth on the side on
which the pressure is greatest.'
This view, that the curvature induced in some unknown
way in a geotropic organ, by gravitation, depends upon the
496 PLANT RESPONSE
accelerated growth of the convex side, is apparently supported
by Elfving's experiments on grass haulms in which growth
had been at standstill. ' He found that the pulvini of grass
haulms placed on the klinostat increase in length. This
experiment shows incidentally that the klinostat does not
remove but merely distributes equally the geotropic stimulus ;
also that geotropic stimulus leads to increased, not to
diminished, growth. The same thing is proved by the simple
fact that a grass haulm shows no growth in its pulvinus while
it is vertical, so that when curvature begins (on its being
placed horizontally) it must be due to acceleration on the
convex, since there is no growth on the concave side in which
retardation could occur.' ^
In the cases of response to stimulation, hitherto studied,
however, it will be remembered that the fundamental effect
was always a contraction and concomitant retardation of
growth, the expansion and acceleration of growth being
always a secondary and indirect effect ; but in the case of
the response to stimulus of gravity, the interpretation of
results which has just been quoted would make it appear
that the responsive action brought about by gravity was
essentially distinct in its character from the responsive re-
action with which we have hitherto been familiar.
Experiment demonstrating responsive curvature as
brought about by unilateral pressure of particles.— My
own object in the course of the present chapter is, however,
to demonstrate the fact that the curvature of an apogeotropic
plant-organ in response to gravitation is the result of the
direct effect of stimulus, its reactive peculiarities being in no
way different from those other instances of the response of
the plant to stimulation, which we have already studied
And in order to simplify my explanation, I shall here
' B.A. Report, 1891, p. 671. It ought to be mentioned here that in other
plants when placed on the klinostat this increased rectilinear growth was not
observed, leading to the supposition that in such cases a simultaneous increase
and decrease of growth- rate on opposite sides of the rotating plant is produced.
Ibid. : Nature,
RESPONSIVE CURVATURES — NEGATIVE GEOTROPISM 497
describe a typical experiment showing the excitatory effect
of pressure in inducing responsive curvature. That unilateral
pressure or contact does induce curvature is known in the
case of tendrils ; but in the present experiment I shall show
that this responsive reaction is not peculiar to tendrils alone,
but is exhibited by all organs alike. I was also desirous of
making the action on my experimental specimens in every
way parallel to that of gravitationally excited tissue in which
the statolithic particles exert
their weight on a particular
side of the responsive organ.
I took a specimen of
Crinuin Lily and mounted it
vertically, I next took a thin
strip of india-rubber, the inner
surface of which was studded
with iron particles, adhering
by means of shellac varnish.
This was adjusted laterally,
on one side of its zone of
growth, so as almost, but not
quite, to touch the specimen.
On the opposite side was
placed an electro-magnet,
which when excited attracted
the strip to which the iron
particles adhered, and thus
produced a unilateral pressure on the specimen, the magnetic
particles functioning as so many statoliths (fig. -205). A
recording microscope, which will be fully described in a later
chapter, was now focussed on the index, I, attached to the
specimen.
Before the application of pressure, the quiescent condition
of the specimen had been ascertained by noting the stationar)'
position of the index in the field of view of the microscope.
On now applying unilateral pressure by exciting the magnet,
we might expect to obtain two different effects. The first of
K K
Diagrammatic
Repre-
sentation of Experiment showing
Curvature Induced by Unilateral
Pressure Exerted by Particles
flower-bud of Criiiiim ; s, india-
rubber strip studded with iron
particles attracted by electro-
magnet, M, causing unilateral
pressure on growing region ; I,
index attached to flower.
498 PLANT RESPONSE
these would be the sudden magnetic pull which would cause
a movement of the flower aivay from the pressed side. This
effect would be instantaneous, and quickly come to a stop.
The second would be the excitatory effect on the specimen
of the unilateral irritation caused by the pressure of the
iron particles. If this response, like response in general,
were to take the form of a contraction, a curvature concave
to the source of stimulus would be produced, by which the
flower should be seen to bend toivards the stimulated side.
This stimulatory effect, unlike the mechanical disturbance,
would go on increasing with time. In this way it is easy to
demonstrate that unilateral irritation of pressure of particles
induces contraction, which in growing organs retards the
normal rate of growth,
the side acted upon
thus becoming concave
(fig. 206). It should
also be borne in mind
that in consequence of
T,-,^ ^^^ i> J f L. • n this responsive con-
1*IG. 206. Record of Responsive Curvalure _ '
induced in Crinuni under Experimental traction SOmC of the
Conditions shown in fig. 205. Magnifica- p.^neHpfl .^ater finds
tion twenty-five times expeuea Wdicr nnus
its way to the opposite
side, thus increasing its turgidity, with consequent acceleration
of rate of growth and convexity of that side. The experi-
mental proof of this latter phenomenon will be given in the next
chapter. Moreover, in consequence of the curvature induced
by this contraction of the excited side, the further side will
be stretched, and tension is, as we know, an agent tending to
increase the rate of growth. Thus we see that in this case
the contraction of the excited side is the active factor, and the
convexity of the further side is the secondary or subsidiary
eflfect in the growth-curvature.
If then such stimulation by unilateral weight-effect, whether
statolithic or hydrostatic, be the cause of the apogeotropic
curvature induced by gravitation, it follows that the active
factor in the process lies in the contraction of the upper side,
RESPONSIVE CURVATURES — NEGATIVE GEOTROPI-SM 499
and that the expansion and convexity of the lower must be
merely the subsidiary effect. But it is usually supposed, as
we have seen, that the predominatingly active factor in these
gravitation-curvatures is the accelerated growth of the convex
side. Hence the crucial experiment by which the correctness
of one hypothesis or the other may be determined will
evidently consist in demonstrating which of the two, contrac-
tion or expansion, is actually the essential element in the
responsive growth-curvature. And if, further, we should
succeed in proving that contraction was the essential element,
we should then have established a unity, as between the
phenomena of response to gravitation and those which are
the results of other forms of stimulation. But before I
describe this particular investigation I must explain the
method of continuous record, which I employ for observing
gravitational curvatures.
Record of curvature induced by gravitation. — We have
in the Optical Lever a means by which the responsive effect
of gravitation-curvature and its variations may be recorded
quickly and continuously, and with as great a magnification
as is desirable. The horizontally laid specimen, say the scape
of Uriclis Lily, has its terminal upper end attached to one arm of
the Lever, the other arm being weighted with a slight counter-
poise. A continuous record is then taken, on a revolving
drum, from v/hich we obtain the responsive curvature and its
time-relations. It will be seen from the record that the scape
first bent down during a period of forty minutes, after which
the effect of gravitation was seen by the reversal of the
curve, which indicated the curving up proper to gravitation
(fig. 207).
It is sometimes thought that this preliminary lapse of
time before the appearance of the gravitation-effect, some-
times known as the presentation time, is the interval necessary
for the statoliths to fall ; but I shall presently describe some
experiments which will show that the time taken for variation
in response to the varying action of gravitation is very much
shorter, probably less than a minute. In the present case we
500 PLANT RESPONSE
must remember that the movement in response to gravitation
has to overcome opposite mechanical movements before it
can be made perceptible at all ; for we have, firstly, the
weight of the organ, which tends to make it bend down ; and
secondly, on account of this bending, we have greater tension
on the upper surface, which, as we have seen, increases the
Fig. 207. Record of Apogeotropic Response in Scape of Uriclis Lily
The up curvature due to apogeotropic action proper commenced forty
minutes after the specimen was laid horizontally.
rate of growth, tending to make that side convex. The
differential effect due to gravity has to overcome all this, and
becomes visible only when it has done so. After this stage
has been reached, the rate of movement upwards goes on
increasing until a fairly constant rate is attained.
Record of different rates when specimen is held at
angles of 45° and I35°. — Czapek has found that the effective
stimulus of gravitation is greater when the organ is held at
135° to the vertical than when held at 45°. This difference
of effect can be obtained quantitatively with great accuracy
by taking successive records with the specimen in the two
positions ; and in order to eliminate the effect of any chance
disturbance, or of spontaneous variation, I took four alternate
records, first with a specimen at 45°, then at 135°, then back
once more at 45°, and then again at 135°, each position
being maintained just long enough for the attainment of
the permanent effect. The change from one position to the
other was made quickly, in the course of less than a minute.
The experiment was carried out with the unopened flower of
Crimim Lily, in which we have already found that the rate of
growth is very regular and considerable.
RESrONSIVE CURVATURES — NEGATIVE GEOTROPISM 50I
The first record was taken after the curvature movement,
due to gravitation at an angle of 45°, had attained a constant
vakie. The tip of the flower was then found to be moving
at a rate of four divisions per minute. On changing the
angle to 135°, the rate of movement showed an imme-
diate increase, and attained in the course of five minutes
a permanent rate of I5'S divisions per minute. It has been
said that the experimental adjustment of change of position
took only about a minute
to effect, yet on renewing
the record we find that the
effective increase thus esta-
blished in the gravitation-
stimulus was immediately
perceived and responded
to by the organ, in an
accelerated rate of curva-
ture. The permanent in-
creased rate at 135° is thus
found to be nearly four
times that at 45°. The
organ was now returned
to its position at 45°, and
the rate once more fell till
it became nearly equal to
what it had originally been
at 45°, being only very
slightly greater. The organ
was once more placed at 135°, and the rate again rose, till it
reached slightly beyond its former value at that angle. The
ratio of the second rates determined at 45° and 135° was also
found to be almost as i is to 4 (fig. 208).
With reference to the cause of this difference of gravita-
tional effect, Haberlandt .suggested that it may lie in the
fact that the weight of the statoliths in 45° position is on the
basal half, while at 135° it is on the apical half (fig. 209)
The next point is, to account for the greater reaction of the
Fig. 208. Response Records showing Dif-
ferences in Rate of Curvature according
as Specimen is held at Angles of 45°
and 135°
502
I'LANT RESPONSE
Fig, 209. Diap;rammatic Representation of
Different Positions of a Single Cell, accord-
ing as the Specimen is held at an Angle of
45" or 135°, showing Consequent Redistribu-
tion of Statoliths (after F. Darwin)
A, apical, B, basal, ends of cell.
apical half. Some light may, perhaps, be thrown on this
subject from the results of my experiments on BiophytJim.
I found that on subjecting this plant to the favourable tonic
condition of rise of tem-
perature, the younger
leaflets began to show
spontaneous excitatory
response much earlier
than the older leaflets.
This shows that in an
excitable tissue the
younger portions are,
generally speaking, the
more sensitive. This
will probably account
for the difference of
geotropic reaction in
the case under consideration, for the apical halves of the cells
are relatively younger than the basal halves.
Determination of the true character of apogeotropic
response. — I shall now proceed with the crucial determi-
nation of the true nature of gravitational response. In the
diagrammatic representation of the multicellular organ, we
have the upper and lower responding layers of cells repre-
sented by E and E' (fig. 204). Each of the.se may be one or
more layers in thickness. Since in apogeotropic organs the
curvature is upwards, this response may be due (i) to the
relative expansion or acceleration of growth of the loiver side,
or (2) to the relative contraction or retardation of growth of
the upper side. As regards the first hypothesis, the curva-
ture induced in grass haulms has been assumed, as stated
before, to have proved that the gravitational response is one
of accelerated growth. On the other hand, the curvature
may be due to the active contraction of the upper side, the
response then being of the same nature as was seen demon-
strated by the irritating pressure of the magnetic particles.
The crucial experiment in deciding between the two alter-
RESPONSIVE CURVATURES — NEGATIVE GEOTROPISM 503
native hypotheses will lie in determining which factor is
actively concerned in the production of responsive curvature.
If the lower should prove to be the side actively concerned,
then the response will be one of expansion ; the activity of the
upper, on the other hand, would demonstrate the contractile
nature of the response.
The principle of the mode of investigation which I adopted
in order to decide this question will be understood, if we
remember that motile response can be abolished temporarily
by application of cold. Thus, if we cool the pulvinus of
Mimosa, it ceases to exhibit any responsive contraction
under stimulation. Now, in a horizontally placed organ, if
the continued responsive curvature be due to the excitatory
contraction of the upper side, then local application of cold
on that side ought to arrest it. The application of cold
on the lower side, however, should produce little effect. But
if, on the other hand, the lower side should be actively con-
cerned in the production of response, then the application
of cold on that side would have the effect of arresting
the curvature, while its application on the upper would
have little or no effect. In connection with this, it should
be remembered that a twitch may sometimes be produced
by the transient excitation due to sudden variation of tem-
perature, but the effect will be short-lived. The permanent
arrest of hitherto continuous responsive movement due to
gravitational stimulus will only take place when the active
side has its power of response abolished by cold.
An experiment carried out in this manner would thus
decide the question as to whether it is the upper or the lower
side that is actively concerned, and also the question as to
whether the gravitational response, like all other forms of
response to direct external stimulus, is, or is not, funda-
mentally one of contraction.
In carrying out experiments on the principle described
above, I first took a record of the responsive curvature of a
Criniim Lily, which was lying horizontally. When a uniform
rate of upward movement had been attained, the tip of the
504 PLANT RESPONSE
bud moving at a rate of "13 mm. per minute, ice-cold water
was applied on the upper surface, by means of a strip of
cloth, at the point marked in the figure by a downward
arrow (i) (fig. 210). In consequence of this the movement
is seen to be retarded, and in the course of five minutes it
came almost to a stop. The cloth was now removed, at the
Fig. 210. Effect on Apogeotropic Movement of Application of Ice-cold
Water to Upper and Lower Surfaces alternately of a Horizontally laid
Crinum Lily
The first part of the curve shows normal up movement, which is arrested
in consequence of the application of cold on the upper surface, at the
moment marked with a downward arrow {\). On removal of the
application at point marked ;' 4. , the normal apogeotropic movement is
renewed, and continues unaffected by the application of cold below at
the moment marked with an upward arrow (t).
moment marked in the curve, and the movement of response
to gravitation recommenced and tended gradually to attain
its original value, with the return of the upper surface to the
normal temperature. Ice-cold water was next applied on
the lower surface, at the moment marked by an upward
arrow (t), and it will be seen that this produced no percep-
tible effect on the rate of responsive curvature. A similar
RESPONSIVE CURVATURES — NEGATIVE CEOTROl'IS.M 505
experiment was performed with a long flower-scape of
Uridis Lily laid horizontally. The attachment to the re-
cording lever was made with the upper end of the specimen,
the lower end being held in a clamp. The specimen was
30 cm. long, and the responsive movement was found to be
■23 mm. per minute (fig. 211). Here, too, ice-cold water was
applied to the upper and lower .surfaces alternately, four
times in succession, and it will be seen from the figure that
an application of cold to the upper surface caused arrest of
Fig. 21 r. Effect on Apogeotropic Movement of Temporary Applications
of Cold alternately to Upper (4) and Lower (|) Surfaces of Horizontally
laid Scape of Uridis Lily
Application above is seen to produce arrest of movement, while application
below has no perceptible effect.
the responsive movement, while a similar application below
produced no effect that could be detected.
These experiments conclusively prove that the funda-
mental responsive effect induced by .stimulus of gravitation
is not acceleration, but contraction, or retardation of growth,
precisely similar to the action of other forms of stimulus.
Though, under cooling, there cannot be any exhibition of
mechanical response to gravitation, yet it appears that the
effect of gravitation may be held latent in the organ, as will
5o6 PLANT RESPONSE
be seen from the following experiment. I took three long
scapes of Uriclis Lily and laid them horizontally, packed in
ice. As long as they were in the ice there was no responsive
curvature. After an hour they were taken out of the ice
and held erect, and by the time that they were restored to
the temperature of the room it was found that the top of the
scape in each case was bent from v^^ to i'5 cm. in the
direction of what had been the upper surface when they
were horizontal. This was evidently due to the fact that the
cold brought about an arrest of growth ; but the geotropic
stimulus remained latent, to express itself in a responsive
movement later, when growth was renewed, under the action
of a favourable temperature.
Responsive curvature of acellular organs. — There is
one point, in connection with the induction of gravitational
curvature, which might at first sight appear anomalous. In
the case of the negative geotropic curvatures of multicellular
organs, the fact that it is the upper side which is relatively
effective, and that the curvature is the result of its responsive
contraction and concavity, is evident from the experiments
already described on the local application of cold. When
we take an acellular apogeotropic organ, however, such as the
stalk of the sporangium of Mucor, we find that it is the
irritated lower side, differentially acted on by weight, whether
of sap or statoliths, that becomes convex. This looks at first
sight as if the effect of irritation were, as it is generally
supposed to be, to induce acceleration of growth, and
attendant convexity.
We must in this case, however, bear in mind the position
of the surface on which the stimulus acts. In our experi-
ment on the irritation produced by the pressure of magnetic
particles it was the outside surface of the organ that was
acted on by stimulus, and it was that side that became
concave. In the case of the organ with a single row of cells
as described, however, it is the internal surface which is so
irritated, and if we take as our object of observation that
internal surface, we shall find that, as in other cases, so here
RESrONSIVE CURVATURES— NEGATIVE GEOTROPISM 507
also, the response is by contraction and concavity of the
excited surface. The convexity of the outer is thus to be
taken as the inevitable result of the concavity of the inner.
The fact that in the acellular organ response actually
takes place by the concavity to stimulus of the surface acted
upon, is further seen in the response of such organs to stimulus
of light. In the case of geotropic stimulus it is the internal
surface of the lower side of the organ which is irritated by
the differential weight of the cell-contents, and becomes con-
cave to the stimulus thus acting upon it. In the case of light,
on the other hand, stimulus acts from the outside, and it is
thus the outer or external surface, say, of the same side,
which becomes concave. Thus stimulus, acting on the same
side in one case from within, and in the other from without,
induces responsive curvatures in opposite directions.'
' The explanation of the responsive curvatures of acellular organs has hitherto
oftered many difficulties. In a multicellular organ, acted on unilaterally by stimu-
lus, there is a difference induced in hydrostatic pressure as between the two
opposite sides. The diminished turgidity of the proximal, and increased turgidity
of the distal, explains the induced curvature. In an acellular organ, however,
there cannot be this difference of hydrostatic pressure on the two opposite sides.
But the considerations which I shall now offer may perhaps be found to meet the
difficulties of the case. The fundamental effect of stimulus is, as we know,, to
induce protoplasmic contraction. Hence unilateral stimulation acting on an
acellular organ may be expected to induce contraction and concavity of the
proximal side of the ectoplasmic layer, the result of which will be a curving over
of the organ. As a result of this, the ectoplasmic layer of the distal side will be
subjected to tension, which is, as we know, an influence that accelerates growth.
Hence the retardation of growth on the proximal, due to contraction, and its
acceleration on the distal under increased tension, will combine to produce growth-
curvature.
A problem of somewhat greater complexity arises in the case of stimulus of
light traversing a transparent acellular organ. Let us suppose such a vertical
organ to be acted upon horizontally by rays of light from the right-hand side.
We have in this case to consider the separate effects of stimulus of light on four
different surfaces: (i) the outer ectoplasmic layer of the proximal side Vo; (2)
the inner ectoplasmic layer of the proximal side Vi ; (3) the inner ectoplasmic
layer of the distal side D/ ; and (4) the outer ectoplasmic layer of the distal side
Do. The contractions of the outer surface of the proximal Vo, and the inner
surface of the distal D/, would induce a curvature to the right ; those of the inner
surface of the proximal Vi and the outer surface of the distal V>o a curvature to
the left. But it is evident that as light passes through the organ there must be
loss of intensity by absorption. Hence the sum of effective intensities at Vo and
508 PLANT RESPONSE
Curvature of grass haulms under gravity. — We shall
now take up the consideration of the curvatures induced in
grass haulms, laid horizontally, when growth had originally
been at standstill. It will be remembered that it was the
appearance of curvature under such circumstances in the
pulvinus of the grass haulm that gave the strongest support
to the theory, that negative gravitational curvature in general
was due to the increase in the rate of growth on the convex
side, rather than to its retardation from active contraction on
the concave. In the present case it was argued that since,
at the beginning of the experiment, the upper side of the
pulvinus was not undergoing growth, it was clear that growth
there could not be retarded. The curvature, therefore, must
be due to the induction of growth on the convex side, under
the stimulation of gravity.
This misconception has arisen from the supposition that
all curvatures must be induced by differential growth. I
have shown, however, (i) that contraction takes place in a
stationary organ in response to stimulus ; (2) that the
unilateral stimulation of such an organ induces concavity ;
and (3) that retardation of growth in a growing organ is
itself the result of the contractile effect of stimulus. Now, in
a horizontally laid grass haulm in which growth has ceased,
the upper side — which we have found to be relatively the
more effective — will contract under stimulus of gravity
That this is the case is seen from the fact that this upper
surface is found to become actually shorter than it was
before. But, as regards the convexity of the lower surface,
the water expelled from the actively contracting upper
side will reach the lower, and the increased turgidity thus
produced is sufficient, as we have already seen, to re-
initiate growth in a dormant tissue. This explains the
renewed growth and convexity of the lower side. Thus the
curvature of the grass haulm cannot be held to support the
at T)i will be greater than the corresponding sum of effects at P/ and D^. As
the result of this difference we shall have a right-handed or positive heliotropic
RESPONSIVE CURVATURES— NECxATIVE GEOTROPISM 509
view that the fundamental action of gravitational stimulus is
to increase the rate of growth ; it shows, on the contrary, that
contraction under stimulation is the active factor.
Growth of grass haulms on a klinostat. — I shall here
adduce certain considerations which may further serve to
explain the difference between the stationary or feebly grow-
ing grass haulms, and other normally growing organs, as
regards the effects induced in them by the rotation on the
klinostat. It is found that in grass haulms, when subjected
to the rotation of the klinostat, growth is recommenced, or
increased ; while in other normally growing plants there is
no such increase of rectilinear growth. For an explanation
of this difference we have to recall the effect on growth of
increased internal hydrostatic pressure, with the consequent
increase of turgidity, which has already been described
(p. 428). It was there shown that, when the plant was grow-
ing at a moderate rate, the curve of relation between increase
of turgidity and increase of growth was practically a straight
line ; that is to say, any increase or diminution of turgidity
would then produce a proportionate increase or decrease of
growth. But this relation did not hold good when the
natural rate of growth was feeble or absent. In the latter
cases, increase of pressure induced an effect that was dis-
proportionately large. And this was specially the case when
the growth of an organ had come to a temporary condition
of standstill. In that case, when the pressure was gradually
increased, growth was found at a certain point to be abruptly
renewed, and to go on increasing with the increase of pres-
sure, at a rate disproportionately large. If, now, the pressure
be once more brought down to what it was just before the
point was reached at which growth was started, we find that
growth is not arrested, but persists. Thus the net result
of these alternations of pressure is a positive resultant
growth. We have, thus, two distinct cases: (i) that in
which the normal rate of growth is moderate, and in which
alternate increase and diminution of pressure, acting for
equal lengths of time, will induce equal increase and decrease
510 PLANT RESPONSE
of growth alternately, the total growth during the whole of
the time being the same as it would have been without alter-
nation ; and (2) that in which, the original rate of growth
having been feeble or absent, equal alternations of pressure
have the net effect of causing a positive increase of rectilinear
growth. . Now, in an ordinary growing plant, rotating on the
klinostat, we have an instance of the first of these two cases ;
for if at any given moment the side A be below, and the
side B above, then B under stimulus of gravity will undergo
contraction ; hence there will be a diminution of local tur-
gidity, and the expelled water will produce an increase of
turgidity on the opposite side A. At the end of a semi-
revolution of the klinostat, however, this state of things will
be exactly reversed, and the alternating effects being thus
equal and opposite, there will be no resultant increase of
rectilinear growth ; but in the second case — that is to say, of
stationary or feebly growing grass haulms — the resultant
effect is not nil, but a positive increase of rectilinear growth.
Summary
It has been shown that the effect of gravitational stimulus
on an apogeotropic organ is fundamentally the same as
that of any other form of stimulus, namely, a responsive
contraction.
A rational explanation of the mode in which geotropic
stimulus acts, is afforded by the radial-pressure theory of
Pfeffer and Czapek, or by the statolithic theory of Noll,
Haberlandt, and Nemec — the essential element of both lying
in the hypothesis that stimulus is caused by means of the
weight of the cell-contents acting differentially on the inner
wall of the horizontally placed cell.
That the unilateral- pressure of particles is competent to
induce responsive curvature of the organ, has been shown
experimentally by pressure resulting from the magnetic
attraction of iron particles.
RESPONSIVE CURVATURES— NEGATIVE (lEOTROPISM 5 II
It has generally been supposed that the active factor in
apogeotropic curvature was the accelerated rate of growth on
the convex side of the organ ; but it has here been shown
by crucial experiments on the unilateral application of cold,
that the active factor is really the responsive contraction
and retardation of growth on the concave side.
CHAPTER XXXVII
THE RESPONSIVE PECULIARITIES OF THE TIPS OF
GROWING ORGANS
Difference between shoot and root in their response to stimuhis of gravity —
Difference in character of response between tip and growing region of root —
Scope of the investigation — Electrical investigation — Responsive results of: i.
Longitudinal transmission of effect of stimulus from tip ; (a) Moderate uni-
lateral stimulation ; (/') effect of stronger unilateral stimulation — 2. Direct
unilateral stimulation of growing region— Moderately strong stimulus — 3.
Transverse transmission of effect of stimulus ; (a) moderate stimulation ; (/>)
stronger stimulation — Mechanical response inferred from observed electrical
response — Tabular statement.
We have seen in the last chapter that the responsive effect
of gravitational stimulus in an apogeotropic organ is of the
same nature as that of any other form of stimulation. In
positively geotropic organs like roots, however, this would
seem not to be the case, for here the responsive curvature
is in the opposite direction. Thus, a root placed horizontally
bends in the direction of gravity, and not away from it.
It may be urged that there is some polar difference
between shoot and root, on account of which, if the response
of the one be regarded as positive, that of the other must be
negative ; but I have shown that, so far from this being the
case, the response of the root to stimulus is precisely the
same as that of all other organs, the shoot included, for all
alike under stimulation exhibit contraction (p. y6).
We shall first see, then, whether in the root there is any
difference, as regards the action of gravity, from, for instance,
the stem. We know that the growing stem with regard to
the gravitational stimulus is both the perceptive and respond-
ing organ ; for we may cut and isolate any portion of it,
RESPONSE OF TirS OF SHOOT y\ND ROOT 513
and it will still show an apogeotropic curvature. But the
case is quite different with the root. Here the perceptive
organ and the responding organ are, as will be seen in the
next chapter, distinct and separate from one another.
Difference in character of response between tip and
growing region of root. — We shall next turn our attention
to the peculiar characteristics of the tip of the root as dis-
tinguished from the responding region of growth. Darwin, on
applying moderate artificial stimulus unilaterally to the tip
of the root, found that it moved away from the source of
stimulus ; whereas Sachs, on applying similar unilateral stimu-
lus to the responding growing region directly, found that it
moved towards it. Thus we see that two different effects are
induced, according as stimulation is applied on the responsive
zone itself, or transmitted to it from the distant tip ; and in
this fact we may perhaps find a clue to the explanation of
the opposite effects produced by stimulus of gravity on
positive and negative geotropic organs.
No explanation has yet been offered of the opposite
characters of these responsive effects induced by similar
stimuli, according as they are applied on the responsive zone
or on the tip of the root. It was suggested by Darwin that
*the tip of the radicle is endowed with diverse kinds of
sensitiveness ; and that the tip directs the adjoining growing
parts to bend to or from the exciting cause, according to the
needs of the plant.' ^ These diverse kinds of sensitiveness
have in his view been acquired by the tip of the root, for the
final advantage of the plant.
The question, then, which we must investigate is, as to
whether the peculiar sensitiveness of the tip has been specially
evolved by the burrowing root, or is characteristic of the tips
of growing organs in general. Should the latter prove to be
the case, we have next to account for this characteristic itself.
I have already suggested, as a possible explanation of the
difference between the responsive actions in apogeotropic
and geotropic organs, that in the one case stimulus acts
' MoTcnients of r/aii/s, pp. 552 and 573.
L L
514 PLANT RESPONSE
directly, producing a movement tozvards, while in the other
the effect of stimulus is transmitted from a distance, produc-
ing a responsive movement in the opposite direction. As
against this assumption, however, we are confronted with the
fact, which we shall find later, that in some instances of
transmitted stimulus of light, the responding organ bends
towards, and not away from, the source of stimulation.
Scope of the investigation. — We have therefore to
determine (i) whether or not the tips of all growing organs
behave alike ; (2) why the behaviour of the tip is different
from that of the responsive zone of growth ; and (3) under
what circumstances the transmitted effect of stimulus causes
an organ to move towards, and under what, to move away
from, the source of stimulation, at the same time ascertaining
clearly the mechanics of such movements. For the purpose
of this investigation I shall first use the electrical method of
inquiry, which I have already fully described, since it has
the unique advantage of offering unerring indications under
the most difficult experimental conditions ; and shall study by
its means the characteristic differences of response as between
the tip and growing region of a single organ, in this case the
shoot.
Electrical investigation. — It will be shown that growth-
curvature is produced by unequal variations of turgidity on
the two sides of the responding organ. This variation can
be detected with great certainty by electrical means. I have
already explained how a positive turgidity-variation gives a
concomitant electrical variation of galvanometric positivity ;
and that the true excitatory effect of negative turgidity-
variation gives rise to a concomitant electrical change of
galvanometric negativity. For the present experiments
I took specimens of various growing plants — such as Biyo-
phyllum, Cucurbit a, and others — and the results obtained
from all were alike. In order to obtain unmistakable indica-
tions of the effect produced in the responding zone, one
electrical connection was made at a point in the growing
region, A (fig. 212, a), and the other with a point so distant
RESPONSE OF TIPS OF SHOOT AND ROOT 515
that the effect of stimulation could not reach it. As
parenchymatous tissues offer great resistance to the conduc-
tion of stimulus, it is an advantage to make this second
contact with a leaf In the present investigation we have to
study the effect of unilateral stimulation of varying intensity
and duration, when applied either at the tip T, or at C, near
the responding point A, the latter being in the same longi-
tudinal line as the excited points T and C. We shall also
study the effect of stimulation of A, on the transverse point
Fig. 212. Experimental Connections for obtaining Electrical Response
due to Direct and Indirect Effects of Stimulation
(a) electrical connections for detection of electrical changes at A, caused
by indirect effect of stimulus, from excitation of same side of distant
tip, T, and direct effect of stimulus from excitation of adjacent point, c ;
{d) electrical connections for detection of electrical changes at B, caused
by excitation of diametrically opposite point, A ; (f) electrical connec-
tions for detection of relative electric variations of diametrically opposite
points, A and 15, due to excitation of c, adjacent to A.
B (fig. 212, /;), and the resultant effects on A and 11, when
the point c, near A, is stimulated (fig. 212, c).
Before proceeding further, it will be well to consider the
theoretical conclusions to which we are led from the demon-
strations already made of the turgidity-variations caused by
stimulation. The tip T consists, as we know, of undifferen-
tiated tissue, which is a relatively bad conductor of stimula-
tion. Moderate stimulus, then, at T, might be expected to
induce local excitatory contraction, and the water thereby
expelled would originate a wave of positive turgidity-varia-
tion ; this would reach A, a point on its own line, with greater
effect than the transversely placed P. The moderate stimula-
tion of T would thus produce a positive turgidity-variation
L L 2
5l6 PLANT RESPONSE
at A on its own side. This transmitted effect of increased
turgidity we have already distinguished as the indirect effect
of stimulation.
But all cells conduct stimulus more or less efficiently, the
difference being one only of degree. Hence an indifferently
conducting tissue, such as that of the tip, will conduct the
true excitatory state only if the stimulation be sufficiently
strong and long continued. The transmitted effect of this
true excitation may then be expected to produce negative
turgidity-variation, and concomitant galvanometric nega-
tivity, at A.
When the stimulus is applied, however, at or near the
responding point A, we may expect to obtain the direct effect
of stimulation — that is to say, a negative turgidity-variation
and the concomitant galvanometric negativity.
To sum up, then, it may be expected that moderate
unilateral stimulus applied at the tip T will give rise on the
same side of the responding region to the indirect effect of
stimulation, which is an increase of turgidity exhibited by a
positive electrical variation. The direct effect of stimulus,
whether immediate or transmitted, always produces a negative
turgidity-variation, evidenced by galvanometric negativity.
This effect may be obtained either by the local application
of moderate, or by the distant application of strong, stimulus.
In making the electrical investigation which is now to be
described, I employed various forms of stimulation — thermal,
mechanical, chemical, and the stimulus of light. Mechanical
stimulus may be applied by friction of emery paper, or by
means of a pin-prick. Chemical stimulation is applied by
touching the point with a brush which has been moistened in
hydrochloric or sulphuric acid. Very dilute acid produces
moderate, strong acid a more intense, stimulation. The most
perfect mode of stimulation is by means of incident light, the
intensity of which may be varied at will. The effect ot
stimulus of light, however, will be fully described in the
chapter on heliotropism. Another form of stimulation which is
also very suitable is the thermal. A short piece of platinum
RESPONSE OF TIPS OF SHOOT AND ROOT
517
wire, heated by electrical current, is placed in the neighbour-
hood of the point to be stimulated. The intensity of stimulus
in this case is regulated by varying either the strength of the
heating current, or' the distance of the heating wire from the
point to be excited. All the different forms of stimulation
will be shown to produce the same results.
Responsive results of : i. Longitudinal transmission of
effect of stimulus from tip : {a) .Moderate unilateral stivmla-
tioii. — Using the thermal mode of stimulation applied at T (fig.
212, a\ I obtained positive electric-variation at A, the record
of which is given in fig. 213. A similar result was obtained
Fig. 213. Record of Positive
Electrical Variation, indicating
Positive Turgidity- Variation
(represented by Down Curve),
induced in Growing Region
by Moderate Stimulation on
same side of Tip. Time-
marks = minutes
Fig. 214. Record showing
Galvanometric Positivity
subsequently Neutralised
under Transmission of
True Excitatory Effect,
due to Continuance of
Moderate Stimulation of
the Tip
with the mechanical stimulation oi a pin-prick. The same
was again observed on effecting stimulation by dilute acid.
{Jj) Effect of stronger unilateral stimulation. — I next pro-
duced a somewhat stronger thermal stimulation by suitably in-
creasing the heating current. This gave rise to the electrical
indication of the preliminary positive turgidity- variation, as the
immediate effect. But the long-continued action of the stimulus
caused the transmission of the true excitatory, which, reaching
A, caused the neutralisation of the previous effect (fig. 214).
With another specimen, I next applied to the tip a still stronger
unilateral stimulus. This caused a brief positive electrical
and turgidity variation, followed by a reversed response of
5i8
PLANT RESPONSE
galvanometric negativity, thus proving that the strong
excitatory effect transmitted to the organ had not only
neutralised, but also reversed, the previous effect (fig. 215).
2. Direct unilateral stimulation of • growing region :
Moderate/y strong stimulus. — When stin:iulus is applied near
the responding organ, say at C (fig. 212, a\ there is always
Fig. 215. Record showing Neutralisalion
and Reversal of Electrical Response
at Responding Region, under Strong
Stimulation of Tip
This indicates that the first positive turgidity-
variation, due to indirect stimulation, is
converted into negative turgidity-varia-
tion of transmitted true excitation.
Fig. 216. Record showing
Negative Electrical Re-
sponse represented by Up
Curve, indicating Negative
Turgidity - Variation due
to Direct Stimulation
produced at A a negative electrical, indicating a negative
turgidity, variation (fig. 216).
Thus we have obtained, using the same moderate stimulus,
two opposite effects, of positive and negative turgidity-varia-
tions, according as the point of application was at the distant
tip or in the vicinity of the responding organ. It is evident,
moreover, that there is a continuity between these two
extreme effects, for, as we gradually shift the point of applica-
tion from the distant point nearer to the responding organ
we observe corresponding intermediate changes, from pure
positive, through the neutral, due to equal positive and
negative effects, to pure negative.
RESPONSE OF TIPS OF SHOOT ANT) ROOT
519
3. Transverse transmission of effect of stimulus :
{a) Moderate stimulation. — We have seen that the direct appli-
cation of stimulus at a point has the local effect of a nega-
tive turgidity-variation. It is of great theoretical importance
that the effect of this stimulus on the diametrically opposite
point should be clearly demonstrated. As conductivity has
already been shown to be very feeble across a tissue (p. 250),
we might expect that it would be
the indirect effect, that is to say the
positive turgidity-variation, which on
stimulation of A would reach the
diametrically opposite, or distal,
point B. The experiment is carried
out, by making galvanometric con-
nections with B and L, and applying
stimulus at A (fig. 212, b).
On now applying moderate sti-
mulus at A, we obtain a positive elec-
trical response, indicating positive
turgidity-variation, at P> (fig. 217) —
a result precisely the same as was
obtained by stimulating the distant
tip. The effect, then, of stimulating
any point is to induce a negative
turgidity-variation of the point itself,
and a positive turgidity-variation of the diametrically opposite
point.
{b) Stronger stimulation. — When stronger stimulus is
applied, however, at A, the true excitatory effect is gradually
transmitted across the tissue, and we obtain at l> the neu-
tralisation of the preliminary positive effect, as in fig. 214.
And, lastly, when the point A is very strongly stimulated, the
responsive effect on the diametrically opposite point P. is a
transient positive, followed by a strong negative variation,
as in fig. 215.
We have thus studied the separate effects produced at
A and P by stimulation of a point near A. The differential
Fig. 217. Record showing
Positive Electrical Varia-
tion indicating Positive
Turgidity-Variation of Dis-
tal Point, under Moderate
Stimulation of Proximal
520 PLANT RESPONSE
effect as between A and B can be inferred by the algebraical
summation of these separate effects. Or it can be directly
obtained by making electrical connections with the dia-
metrically opposite points A and B, as in fig. 212, c, and
applying gradually increasing stimulus at C near A.
It will be remembered that these opposite effects of
direct and indirect stimulation have received independent
demonstration, in both mechanical and growth responses.
In Biophytuin and in Artocarpus the positive turgidity-
variation was proved to be the indirect effect of stimulation,
by the erectile responses of the motile organs (pp. 24, 420).
In the case of growth- response, again, the indirect effect of
stimulation, with the concomitant positive turgidity-variation,
was shown in the increased rate of growth (p. 430). The
effect of direct stimulation in inducing a negative turgidity-
variation, again, was exhibited in Biophytuin and Artocarpus,
by the responsive depression of the motile organs. In the
case of growth-response, it was exhibited by contraction and
concomitant retardation of growth.
Mechanical response inferred from observed electrical
response. — From the results of the electrical investigation on
these turgidity-variations, induced by direct and indirect
effects of stimulus, as just described, we are led to conclude
that :
I. (a) Moderate unilateral stimulation of the tip gives
rise by longitudinal transmission to the indirect effect of
stimulus, namely a positive turgidity-variation, on the same
side of the distant responding-organ. This will give rise to
acceleration of growth and convexity of that side, and by
the consequent responsive movement the tip will be carried
■ciiuay from the source of stimulation.
{b) When the unilateral stimulation of the tip is a little
stronger, the indirect effect of stimulus is neutralised by the
subsequently transmitted true excitatory effect, and there is
no resultant action.
Very strong unilateral stimulation of the tip gives rise by
longitudinal transmission to the direct effect of stimulus,
namely a negative turgidity-variation, on the same side of
RESPONSE OF TIPS OF SHOOT AND ROOT
521
the distant responding organ. This will give rise to retarda-
tion of growth and consequent concavity of that side, the tip
being carried, by the responsive
movement, towards the source of
stimulation.
2. {a) Stimulation of a point
at or near the responding region
of growth will induce a negative
turgidity-variation as the direct
effect of stimulus, and at the
diametrically opposite point a
positive turgidity-variation as the
indirect effect of stimulus. This
will give rise to concavity of the
proximal with convexity of the
distal sides. The mechanical
effects of negative turgidity-
variation on one side and posi-
tive turgidity-variation on the
other side are thus additive. In
this way, the concavity of the
proximal and convexity of the
distal conspire to bring about the
resultant curvature.
{b) When the stimulus applied
at A is stronger, the effect is a
negative turgidity-variation at A, and later, owing to the
transmission of stimulus across the tissue, the positive is
succeeded by a negative turgidity-variation of B. The
resultant effect obtained by algebraical summation thus tends
to become zero. When the stimulus at A, however, is still
stronger and of longer continuance, the negative response of
A will be found to undergo a gradual diminution owing to
fatigue, while the transmitted effect at the diametrically
opposite point undergoes an increase. Under these circum-
stances the responsive negative change at B will predominate
over that at A. The final result may thus be a relative
negativity of 15, that is to say an effect opposite to that seen
Fig. 2 1 8. The Relative Elec-
trical and Turgidity Variations
of two Diametrically Opposite
Points, A and H, when Strong
and Long-continued Stimulus
is applied near A (cf. fig.
212, C).
We see here (i), in the up curve,
the negative variation of the
proximal, followed by (2)
neutralisation, followed by (3)
reversal, that is to say, rela-
tive negativity of B. Note the
multiple response which here
makes its appearance.
522
PLANT RESPONSE
in case (a). These different phases of the effect induced by
long-continued application of strong stimulus at A — namely
the relative negativity of A, followed by neutralisation,
followed by reversal or relative negativity of B — are well
seen in the record given in fig. 218. Translated into terms
of the resulting mechanical response, this would mean : (i) a
movement of the organ towards the stimulus ; (2) neutralisa-
tion of this movement ; and (3) a pronounced movement
away. I give here a tabular statement which shows at a
glance the various electrical effects and the corresponding
mechanical responses which are theoretically to be inferred
from them, the experimental verification of these inferences
being given in the next chapter.
Tabular Statement of Electrical Effects and Inferred
Mechanical Responses
Stimulation
I. Unilateral, of tip, on
side A :
(a) Moderate stimulus.
(d) Stronger stimulus.
Electrical and turgidity effects
in the responding region
(c) Very strong and long-
continued stimulus.
2. Direct action of
stimulus on respond-
ing region :
(a) jVloderate stimulus.
Mechanical response
theoretically inferred
Positive electrical and
positive turgidity vari-
ation of same side A.
Convexity of A, • and
movement of tip away
from stimulus.
Positive and subsequent i No resultant effect,
negative effects neu- i
tralise each other. '
Positive twitch, followed
by strong negative elec-
trical and turgidity
variations, of same
side, A.
Negative variation of side
acted upon, and posi-
tive variation of dia-
metrically opposite
side.
(/>) Stronger stimulus. | Positive and subsequent
I negative effects neu-
tralise each other.
(c) Very strong and long-
continued stimulus.
Positive twitch, followed
by strong negative
electrical and turgidity
variations of the op-
posite side, 1:.
Transient twitch away
from, followed by
strong movement to-
wards, stimulus.
Concavity of the excited,
with convexity of the
opposite side.
Responsive movement of
organ towards source
of stimulus.
No resultant effect.
Resultant mechanical
response opposite to
that in 2 (a) ; i.e.
movement away from
the stimulus.
RESPONSE OF TIPS OF SHOOT AND ROOT 523
The verification of these theoretical inferences- will be
given in full in the following chapter.
Summary
From electrical investigation we find that the responsive
peculiarities of the tip of the root are not characteristic of
that organ alone, but of the tip of the shoot also.
The different effect between stimulation applied to the tip
and to the growing region of an organ lies in the fact that
from the former — it not being a good conductor of excitation
— only the indirect effect, that is to say the positive turgidity-
variation, is transmitted to the responding organ. In the
latter case, however, direct excitation gives rise to negative
turgidity- variation.
Moderate unilateral stimulation of the tip, therefore, gives
rise on the same side of the responsive region to increased
turgidity, which, translated into mechanical response, means
a negative curvature or movement away from stimulus.
Stronger and long-continued stimulation of the tip causes
transmission of the direct effect. This will gradually neu-
tralise or even reverse the first effect, the positive turgid ity-
variation giving place to negative. These events translated
into terms of mechanical response mean a change from
negative to positive response, or movement towards stimulus.
Direct unilateral stimulation of the responsive region
causes negative turgidity-variation of the proximal and
positive turgidity-variation of the distal. The mechanical
expression of these will be a movement towards stimulus.
By strong and long-continued action the stimulus ceases
to be unilateral and becomes internally diffused. The excita-
tion thus reaches the distal side. The difference of turgidity-
variation on the proximal and distal is thus gradually
abolished, or even reversed. The corresponding mechanical
response will be a neutralisation or reversal into negative,
that is to say a movement away from the stimulus.
CHAPTER XXXVIII
INQUIRY INTO THE LAWS OF RESPONSIVE GROWTH-
CURVATURES
Scope of the investigation : i. Mechanical response to unilateral stimulation of the
tips of shoot and root : {a) Moderate stimulus — (//) Stronger stimulation —
2. Effect of unilateral stimulus, applied at the responding growing region :
{a) Moderate stimulus — {d) Strong or long-continued stimulus — Experiments
on the direct and indirect effects of stimulus on Mimosa : {a) Direct stimula-
tion— {b) Indirect stimulation, longitudinal transmission — (c) Indirect stimula-
tion, transverse transmission — The curious response of an Aroid — Table showing
responsive effects common to pulvini, pulvinoids, and growing organs — Laws
of responsive growtli-curvature.
We were able, at the end of the last chapter, to pass in
review the theoretical conclusions to which we had been led
by the electrical mode of investigation, as to the responsive
movements which might be expected to follow on the
unilateral application of stimulus to the tip and the growing
region respectively. I shall now proceed to submit these
theoretical conclusions to experimental verification, by taking
records of the mechanical movements actually induced.
It will be understood here that my object is (i) to show
that the peculiar response given by the tip of the root is
characteristic of the tip of the shoot also ; and (2) to demon-
strate the effect of unilateral stimulation on the growing
region. We have therefore to study the effects which are
induced at the growing region by the action of stimuli of
different intensities, according as they are applied unilaterally
at the distant tip, or, locally, on the growing region itself As
a specimen of the shoot-tip, we may employ either the tip of
a stem or that of an unopened flower-bud. This latter,
composed mainly as it is of indifferently conducting elements
LAWS OF RESPONSIVE GROWTH-CURVATURES 525
has all the characteristics of the tip of the organ, while the
peduncle below often represents the area of quickest growth,
and functions as the responding region. Another advantage
of the unopened flower-bud, again, lies in the fact that its
upper end is not covered over with appendages like that of
the stem. The unopened buds, with peduncles, of Crocus^
then, will be found suitable for this investigation. As
specimens of the roots, again, the long straight water-roots of
Bindweed are very suitable for these experiments.
The records of the responsive movements are taken by
means of the recording microscope fully described in a
subsequent chapter. The quiescence of the organ, before the
application of stimulus, is tested by the fact that the record
is a horizontal line. The occurrence of an up curve in the
record represents responsive movement towards, while a
down curve means movement away from, the stimulus. In
this investigation, as in the preceding, various forms of
stimulation have been employed. Mechanical stimulus is
applied by friction of emery-paper. The jar produced by
this causes a temporary disturbance of the image in the field
of view of the microscope ; but this soon subsides, and the
excitatory movement commences after a short latent period,
increasing steadily until the effect of stimulus is exhausted.
The chemical form of stimulation has the advantage of
producing no mechanical jar. The stimulation produced by
light, which is the most perfect, will be described in the
subsequent chapter on heliotropism. Thermal stimulation is
effected by holding a platinum wire, heated by the electrical
current, in more or less proximity to the point in the tissue
which is to be excited.
I. Mechanical response to unilateral stimulation of the
tips of shoot and root : {a) Moderate j-Z/w/^/z/j.— Applying a
single mechanical stimulus of emery-paper friction, of mode-
rate intensity, unilaterally to the bud of Crocus, a movement
was induced in the responding organ, which carried the tip
away from the source of stimulation. This movement
persisted for four minutes, after the application of this single
526
PLANT RESPONSE
Stimulus. I performed a similar experiment on the tip of
the root of Bindweed, the response being precisely the same.
The rate of movement was in this case somewhat slower, but
persi-sted for eight minutes. The application unilaterally of
dilute sulphuric acid brought about exactly similar results in
both cases. The unilateral application of thermal stimulus of
moderate intensity, again, to the tip, induced movement away
from the source of stimulation in
both shoot and root, as will be seen
from the record given below (fig. 219).
{b) Stronger stimulation. — A
somewhat stronger stimulation of the
same character caused a movement
away, followed by movement towards,
the source of stimulus, the resultant
effect being neutral. We now pass
on to the effects induced by still
stronger stimulation. I have already
explained how, when very strong
stimulus is applied unilaterally at the
tip of an organ, its first and transitory
effect is to induce a positive turgidity-
variation on the same side of the
growing region. From this we in-
ferred the occurrence of a convexity on that side, which
would carry the tip away from the source of stimulus. The
direct effect of stimulus next reaches the responding region,
reversing the first effects and causing a negative turgidity-
variation, which would, pari passu, induce a concavity, and
carry the tip towards the source of stimulation.
In carrying out the experimental verification of these
mechanical movements on a bud of Crocus, I found that an
application of strong sulphuric acid on one side of the bud
caused it to move first away from, and then very energetically
towards, the direction in which the application was made. I
next tried to determine the effect of a strong unilateral
application of thermal stimulus on the root-tip of Bindweed.
Fig. 219. Mechanical Re-
sponses of Shoot, s, and
of Root, R, to Unilateral
Stimulus applied at the
Tip
In this and following records
the down curve indicates
negative movement or away
from source of stimulus.
The up curve indicates
positive or movement to-
wards source of stimulus.
The time-marks represent
minutes (cf. fig. 213).
LAWS OF RESPONSIVE GROWTH-CURVATURES
527
Here, too, after a transient movement away, there was an
energetic movement towards the stimulating heated wire
(fig. 220).
2, Effect of unilateral stimulus applied at the respond-
ing growing region,— I shall now show that when stimulus
is applied near the growing region, it induces effects which
are opposite to those resulting from stimulation of the tip.
Fig. 220. Mechanical Response
of Root of Bindweed to very
strong Unilateral Stimulation
applied at the Tip
This causes a preliminary negative,
followed by a positive, move-
ment, that is to say towards the
source of stimulus (cf. fig. 215).
Fig. 221. Mechanical Re-
sponses of Peduncle of
Crocus, s, and Root of
Bindweed, R, to Unilate-
ral Thermal Stimulation
at the Growing Region
The responses are positive
and towards the source of
stimulus (cf fig. 216).
{a) Moderate stimulus. — When moderate stimulation of
any kind is applied unilaterally in the growing region, the
consequent negative turgidity-variation of the side directly
excited makes it concave ; and a positive turgidity-variation
due to the indirect effect of stimulation occurs at the distal
side, making that side convex. Thus the induced concavity
of the proximal, and convexity of the distal, both conspire to
cause a movement of the organ towards the source of stimula-
tion. This is seen in the following records obtained with the
peduncle of Crocus and the root of Bindweed, the stimulus
used having been thermal (fig. 221).
528 PLANT RESPONSE
{b) Strong or long-continued stimulus. — I have already
explained that with moderate stimulation the negative
turgidity-variation and concomitant contraction of the proxi-
mal, and the positive turgidity-variation and concomitant
expansion of the distal, conspire to induce a positive respon-
sive curvature ; but when the stimulus is strong or long-
continued the excitation is conducted across the tissue to
the distal side, which, now contracting, antagonises and
reverses the action of the proximal. We have seen this ex-
emplified in the electrical response given in fig. 218, where the
first positive electrical response of the distal, indicating positive
turgidity-variation, was afterwards neutralised and converted
into negative by the transverse conduction of excitation.
In the case of responsive growth-curvature we. obtain
results precisely similar. Long-continued unilateral stimula-
tion is here often found to neutralise the first or normal
effect. Or if, again, the unilateral stimulation be very strong,
the proximal side is liable to become fatigued, and the
response of the distal to transmitted stimulus being thus
predominant, a responsive movement occurs which is reversed
or negative, that is to say, away from the source of stimula-
tion. Examples of this will be seen in Chapter XLII.
I shall also presently give a demonstration of similar
preliminary effects with subsequent transverse conduction,
giving rise to reversed effects, in the case of the motile
response of Mimosa.
We have thus seen that growth-curvature is induced by
unequal variations of turgidity, on the diametrically opposite
sides of the growing region. We have seen that the effect
of indirect stimulation is a positive turgidity-variation. When
a feebly conducting tissue is unilaterally subjected to moderate
stimulation, the direct excitatory effect cannot be transmitted
far, and it is the indirect effect which reaches the responding
region, R (fig. 222), and induces convexity. Hence we obtain
the typical examples of this effect by stimulating the tip, T,
of either root or shoot. The sensibility of these regions is
itself in no way different from that of any other portion of
LAWS OF RESPONSIVE GROWTH-CURVATURES
529
the plant-tissue, for they respond to direct external stimula-
tion by contraction. But their power of transmitting stimulus
is relatively feeble. For this reason, under ordinary circum-
stances, they transmit only the indirect effect of stimulus,
and it is only when the unilateral stimulus is very strong
that the direct excitatory effect is transmitted, inducing the
opposite to the usual result, in the
responsive concavity of the same side
of the growing region.
The fact that the sensitiveness of
the tip is not fundamentally different
from that of the growing region may
be demonstrated by applying stimulus
to a given point D in the growing
region, and observing the responsive
effect induced at R, diametrically
opposite. The power of the tissue to
conduct stimulus transversely being
feeble, the result is in this case
the same as in that of the ordinary
longitudinal transmission from the
tip ; that is to say, it is the indirect
effect that reaches the diametrically
opposite point, R, and induces con-
vexity there, this effect being aided,
as it happens, by the concavity of
the proximal side, due to the direct
effect of stimulation. Here again, as before, a stronger or
long-continued stimulus may later transmit the direct effect,
and neutralise or reverse this first responsive curvature. A
third case arises when unilateral stimulus is applied at L,
lower down on the stem, at some distance from the respond-
ing region, and if this be sufficiently feeble, it will be the
indirect effect which will reach the same side of the respond-
ing region, and produce convexity there. Stronger or long-
continued stimulation will in this case, as before, neutralise or
reverse the first effect.
M M
L—
Fig. 222. Diagram showing
the various Responsive
Effects induced at the
Growing Region, R
When moderate stimulus is
applied unilaterally at
the tip, '1', or distant
point, I,, it is the indirect
effect that reaches r, and
produces convexity. The
same convexity of r is
induced by stimulation
of the transverse point, D;
but here the induced
curvature is aided by the
concavity of the directly
excited D.
530
PLANT RESPONSE
I have been at some pains to make these examples of
the direct and indirect effects of stimulus clear ; for all the
complex curvatures of growth, which appear at first sight so
anomalous, are ultimately resolvable into these. And since
the subject is so important, I shall add still another demon-
stration, which is capable of easy repetition, and will be
found to be striking and convincing.
Experiments on the direct and indirect effects of
stimulus on Mimosa.— We have now seen that all growth-
curvatures may be analysed
^^ into, (i) that contraction, with
BIS— ^— i[i^..=__ consequent concavity, which is
^^P concomitant to the negative
ll ^ f^ turgidity-variation that consti-
tutes the direct, or transmitted
direct, effect of stimulus ; and (2)
that expansion, with consequent
convexity, that is concomitant to
the positive turgidity-variation,
which constitutes the indirect
effect of stimulus. Now, the
negative turgidity-variation is
exhibited in the case of the
motile leaf of Mimosa by depres-
sion, and the positive turgidity-
variation by erection.
I shall now explain the
experimental arrangements by
which the plant itself may be
made to record these opposite effects. The indicating leaf is
attached to the short arm of a long writing lever. This lever
consists of the quill of a long tail-feather of a peacock. Its
short arm, i cm. in length, is tied by a thread to the petiole
of the leaf. A fine needle is passed through the quill, and
rests on frictionless supports which may be glass tubes.
The longer arm of the lever, 10 cm. in length, has a piece
of bent aluminium, with a sharp point, tied to the end,
Fig. 223. Experimental Arrange-
ment for obtaining Records
on Smoked Drum of Responses
given to Direct and In-
direct Stimulation by Leaf of
Mimosa
Thermal stimulator at s produces
.direct stimulation, and conse-
quent fall of leaf. Moderate
stimulation, at a distant point,
s,,, gives rise to indirect effect
of erection.
LAWS OF RESPONSIVE GROWTH-CURVATURES
531
to serve as a writer. This writing-point is, by the elasticity
of the feather, pressed lightly against the smoked-paper
surface of a vertical revolving drum (fig. 223).
When the leaf is under no stimulation, either direct or
transmitted, the record is a horizontal line. But true excita-
tion produces a depression of the leaf, causing an up curve.
The erectile response, on the
other hand, which is due to
indirect stimulation, produces
a down curve. The records
given in the following figures
are accurate reproductions of
some which were taken in this
way (fig. 224).
(rt) Direct stimulation. —
Stimulus is applied by the close
proximity of a V-shaped plati-
num wire, heated electrically.
Its intensity is varied by varying
either the distance of the stimu-
lating wire or the intensity of the
heating current. When stimula-
tion is now applied directly at S,
that is to say, near the respond-
ing organ, response takes place
by a negative turgidity-varia-
tion, producing a fall of the leaf
This is seen in the up curve
(fig. 224, a).
(b) Indirect stimulation, lon-
gitudinal transmission. — Sti-
mulus of moderate intensity is
now applied lower down on the same side of the stem, at
S,,. This is observed first to induce the positive turgidity-
variation, causing erectile response, which is due to the indirect
effect of stimulus, and is shown in the preliminary down
twitch of the curve (fig. 224, b). Later, the direct effect is trans-
Fig. 224. Mechanical Responses
of Leaf of Mimosa
(a) record of responsive fall when
stimulus applied near the re-
sponding organ (cf. fig. 221);
(b) response when stimulus is
applied on same side, but at
greater distance, s^,. A pre-
liminary erectile response is here
followed by the true excitatory
depression. This is due to the
indirect effect Hrst transmitted
being succeeded by the direct.
Had the stimulus applied been
feebler, or more distant, there
would have been only the first,
or indirect erectile effect, similar
to fig. 225 (cf. fig. 220).
53-
PLANT RESPONSE
mitted, causing the fall of the leaf, as shown in the up curve.
When the stimulus is feebler, or applied at a still greater
distance, the indirect effect alone reaches the organ, and only
the erectile response, due to posi-
tive turgidity-variation, results, being
similar to that shown in the next
record (fig. 225).
(c) Indirect stiuinlation, transverse
transmission. — We next obtain the
very interesting case in which feeble
stimulus is applied at the transverse
points^. The record (fig. 225) shows
that we have here an erectile response
due to the positive turgidity-variation
of indirect stimulation. But if this
transverse stimulus be made strong
or be long continued, the direct effect
is transmitted somewhat later, and,
in that case, we obtain a fall of the
leaf, preceded by the positive erectile
twitch, which is similar to that shown
in the previous record (fig. 224, b).
The curious response of an
Arisaema. — This fact will explain a
very remarkable phenomenon which I
have noticed in certain species of Ariscsnia, that grow on the
mountains round Darjeeling, at a height of about 7,000 feet.
This plant, before flowering, consists of a long petiole bearing
a terminal whorl of leaflets, which are arranged like rays in a
strictly horizontal plane. Later, however, the inflorescence,
borne on a peduncle enclosed within its spathe, breaks out
from one side of this petiole. Unilateral mechanical stimu-
lation is thus undoubtedly brought about, and gives rise to
indirect stimulation on the distal side, which, as we have just
seen, causes an erectile mechanical response. In the case of
this Arisesnia, it is a striking fact that immediately after flower-
ing, the most distal leaflet of the whorl — that is to say, the
Fig. 225. Erectile Re-
sponse of Leaf of J//w^.fa
due to Transmission of
Indirect Effect to Distal
Side, when Proximal, s,,
is Stimulated
If stimulus were stronger,
this would be followed
by the fall of the leaf
due to the later trans-
mission of true excitation.
The response would then
become like that of fig.
224 {b) (cf. fig. 219).
LAWS OF RESPONSIVE GROWTH-CURVATURES
533
leaflet which is situated in the diametrically opposite line-
hitherto horizontal, becomes abruptly vertical (fig. 226).
This is the only leaflet which stands out from its fellows,
and it is invariably found to be situated on the line diametri-
cally opposite to the flower, such differentiation being induced
only after flowering.
All the variations exhibited by diverse forms of response
— electrical, mechanical, responsive acceleration or retarda-
tion of growth, and growth-curvatures - are only so many
Fig. 226. Curious Response of Ariscema
IJefore flowciing the leaflets lie in a horizontal plane, as seen in tlic figure
to the left ; but when inflorescence breaks through unilaterally, the
indirect stimulation of the distal side causes erection of tlie diametrically
opposite leaflet, as seen in figure to the right.
expressions of these two fundamental phenomena, the effects
of direct and indirect stimulation. It is these two variables
which, conjoined with stimulus unilateral or diffuse, give
us all those manifold effects that at first sight would appear
to belong to different classes of phenomena. That such a
unity does actually underlie them all may be seen at a
glance from the following concise statement of responsive
effects, induced in pulvini, pulvinoids, and in growing regions
which act as pulvinoids.
534
PLANT RESPONSE
Table showing Responsive Effects Common to Pu
pulvinoids, and growing organs
Stimulation diffuse
-
I. Direct effect.
Stimulation unilateral
Effect on proximal side
Effect on distal side
3. Direct effect.
4-
Corresponding
effect.
Negative Uirgidity-
Negative turgidity-
Positive turgidity-
variation.
variation.
variation.
Galvanometric nega-
Galvanometric nega-
Galvanometric posi-
tivity.
tivity.
tivity.
Depression of motile
Contraction and
Expansion and con-
leaf.
concavity. '
vexity.
Retardation of
growth.
When stimulus is strong or
long continued, the
true excitatory effect passes
to the distal side,
neutralising or reversing the first response. 1
5. Transmitted
6.
Corresponding
direct effect.
EFFECT.
Negative turgidity-
Positive turgidity-
variation.
variation.
Galvanometric nega-
Galvanometric posi- 1
tivity.
tivity.
Contraction and
Expansion and con-
concavity, in re-
vexity in respon-
sponsive region.
sive region.
2. Indirect effect.
7. Indirect effect.
8.
Corresponding
EFFECT.
Positive turgidity-
Positive turgidity-
Negligible.
variation.
variation.
Galvanometric posi-
Galvanometric posi-
tivity.
tivity.
Erection of motile
Expansion and con-
leaf.
vexity in respon-
1
Acceleration of
sive region.
growth.
Remarks. — It will be remembered that the indirect effect is a secondary con-
sequence of the direct contr
actile effect of stimulus on the
excited point. Thus
the motive power is the ac
ive contraction of that point.
The indirect effect,
described as No. 7, is exem
plified by the moderate unilateral stimulation of the
tip of shoot or root. Wf
en this stimulus is stronger
or sufficiently lone
continued, we have a transn
lission of the direct effect of stimulus, and case No. 7
is displaced by case No. 5.
' The induction of concavity, of either upper or lower side of the pulvinus,
by local stimulation will be found demonstrated in a subsequent chapter.
Confining our attention to the effects induced at the
growing region, we arrive at the following laws of growth-
curvature.
LAWS OF RESPONSIVE GROWTH-CURVATURES 535
Laws of responsive growth-curvature.— It must be
remembered here that the effect of indirect stimulation is to
cause an increase in the rate of growth, and that of direct
stimulation a retardation of the rate. By a positive effect is
meant a responsive mechanical movement towards, and by
negative, away from, the source of stimulation.
1 . Unilateral stimulus of moderate intensity, applied at the tip
of root or shoot, gives rise to a negative effect, the tip being moved
away.
2. Stronger or long-continued stimulus, applied unilaterally
at the tip of root or shoot — being conducted gradually to the
growing region — results in a neutralisation of the first negative
by a subsequent positive effect. Or there may be a resultant
positive, due to the predominance of the transmitted effect.
3. Direct unilateral stimulation of moderate intensity on the
growing region causes a positive response or movement towards
stimulus.
4. Strong or long-continued unilateral stimulation of the
growing region, on account of the transmission of effect to the
distal side, may give rise either to neutralisation of the normal,
or to a reversed or negative effect, that is to say, to movement
away from stimulus.
Summary
All the mechanical effects induced at the responsive
growing region of either root or shoot, by unilateral stimula-
tion, may be summarised as follows, it being understood that
positive response means movement towards, and negative,
movement away from, stimulus :
(i) Positive response is induced, first, by direct unilateral
application of stimulus on the growing region ; second, by the
long-continued unilateral application of moderately strong
stimulus at the tip.
(2) Negative response is induced, first, by the unilateral
application of feeble stimulus at the tip ; and second, by long-
continued unilateral application of strong stimulus at the
growing region, causing fatigue of the proximal, and trans-
mission of true excitation to the distal, side.
CHAPTER XXXIX
INQUIRY INTO POSITIVE GEOTROPISM
?), and (2) rom again in the opposite
right to left (<-). Dotted line shows i- ^- ^, ,
effect on cessation of current. Record direction. 1 he growth-
of growth taken under balanced con- record of a single seed-
dilions. °
ling was first taken under
balanced conditions (fig. 233). The electrical current was
next sent through the soil from left to right, and during
the continuation of this current, we see from the up curve
the acceleration of growth above the normal that took
place. The current was now stopped, and the former
induced acceleration was replaced by a brief retardation,
as seen in the down curve, after which the normal rate
was restored. The current was next reversed, and yet
the response was one of accelerated growth, and on the
stoppage of the current after a brief retardation there waix-
again a restoration of the normal rate. This experime-^'-
CHEMO-TROPISM AND GALVANO-TROl'ISM 561
was carried out using an E.M.F, of ten volts, the two elec-
trodes being applied at a distance of 10 cm. from each other.
This meant a potential gradient of one volt per cm. This
relatively strong voltage was applied for the sake of obtaining
measurable response within a short time. But a much
smaller E.M.F. is found to produce similar effects, though
the action is slower. The application of a strong E.M.F. has
the disadvantage of inducing fatigue, which is a drawback
not present in the use of a feeble E.M.F. It is, however,
shown that, during the continuation of a current through the
soil, the rate of growth of a plant is enhanced.
Summary
The effects of unilateral chemical and galvanic excita-
tion may be studied, both from pulvinar and growth
movements.
The unilateral application of alkali gives rise to positive
curvature, that is to say, a movement towards the stimulat-
ing agent.
Acids, under similar circumstances, give rise to negative
curvature.
The unilateral application of copper sulphate gives rise, by
retardation of growth, to a concavity or positive responsive
movement.
The unilateral action of dilute solution of sugar, by
enhancing the rate of growth, induces negative response. A
strong solution, however, gives the reversed or positive
response.
Effects fundamentally similar are expressed by swimming
organisms in appropriate movements to and from sources of
chemical stimulation.
The assumption that the variation of curvature which
occurs when an already curved organ is placed in strong
solutions of salt is due to the action of plasmolysis is not
always justified, for such solutions also c.xert characteristic
ex._itatory effects.
-The pulvinus of Mimosa in a favourable tonic condition
O o
562 PLANT RESPONSE
reacts to the action of salt, at least for a time, by an erectile
responsive movement. Under a less favourable tonic con-
dition, it responds by depression. That this latter is not
entirely due to plasmolytic action is seen from the fact that
the leaflet subsequently becomes erected, as is the case under
continuous stimulation.
TJie localised and imilateral action of anode and kathode
on a pulvinated organ is as follows :
The anode-make causes expansion, inducing convexity.
This effect attains its maximum in a short time.
The anode-break induces contraction and concavity.
The kathode-make induces contraction and concavity.
This effect is stronger and more persistent than the opposite
effect of expansion at anode-make.
The kathode-break induces expansion and convexity.
The polar effects on longitudinal growtJi are as follows :
The anode-make causes expansion and acceleration of
growth. This effect attains its maximum in a short time.
The anode-break causes contraction and transient retarda-
tion of growth.
The kathode-make causes contraction and retardation of
growth. This effect is stronger and more persistent than
the opposite effect of acceleration at anode-make.
The kathode-break induces an expansion and transient
acceleration of growth.
The localised and unilateral action of anode and kathode
on a growing organ, results from the unilateral exhibition of
those growth-variations which have just been described.
Since a growing organ is virtually a diffuse pulvinoid, the
effects are exactly similar to those seem in a pulvinated
organ, already described.
Owing to the fact that kathodic action is relatively
stronger than anodic, a feeble or moderate current flowing
through the soil exerts an excitatory action on roots, by
which the suctional activity of the plant is increased. The
result is an increased rate of growth, which is independent
of the direction of flow of the current through the soil.
PART VIII
HELIOTROPISM
CHAPTER XLI
FUNDAMENTAL RESPONSIVE ACTION OF PLANT-TISSUES
TO STIMULUS OF LIGHT
Diversity of movements induced by light — Differentiation of responsive movements
— Action of light on tissues in sub-tonic conditions— Effect of light on pulvinated
organs — Effect of diffuse stimulation of light on non-growing radial organs- —
Retarding effect of light on longitudinal growth— Phenomenon of oscillation
under long-continued stimulation — Similarity of responsive reaction under
light and under other forms of stimulation.
There is perhaps no other phenomenon in the plant-world
which is at once so striking and so universal as that of the
response which is evoked from plants by the stimulus of
light. Under this influence the plant as a whole, and every
part of it, is tremulous. Not only does the growing stem
curve towards or away from the incident rays, but every
leaf under their action is thrown into a state of periodic daily
rhythm. By the absorption of light, again, the plant is
energised. Thus the two factors, of external stimulus and
internal energy, whose manifestations are so opposite in
character, are brought into play
Diversity of movements induced by light. —Light, as a
form of external stimulus, evokes responsive movements, which
appear to be extremely diverse in their nature. Radial organs,
for example, such as stems, under certain conditions, direct
themselves towards the light, and under others, again, away
from it. Leaves, however, behave quite differently. These
are said to possess the peculiarity of placing themselves with
their surfaces at right angles to the light ; but even this
is not a property universally exhibited, for there are certain
leaves which place themselves in a direction coinciding with
566 PLANT RESPONSE
that of light, either towards or away from it, Some plants,
again, close their leaves or leaflets on the approach of night,
in the so-called ' position of sleep ; ' while a further complica-
tion arises from the fact that an apparently similar ' sleep '
movement is produced, in these or others, by the action of
the noonday sun.
Light, again, appears in some cases to initiate movement,
as was seen in Desmodium at standstill, and in others to arrest
it, as is said to happen with the spontaneous movements of
Trifoliwn pratense. Certain swarm-spores, moreover, appear
to be attracted by light, swimming towards it, with energetic
beats of their cilia ; while others, on the contrary, are affected
in the opposite manner, and swim away. Or the same
specimen may be found to swim, now towards, and again
away from, light, swinging backwards and forwards like a
pendulum.
We thus see that not a single responsive effect of light
has been observed in the case of plant-organs of which an
example directly to the contrary may not be found. For
this reason it has appeared hopeless to attempt to unify these
phenomena, and this fact has left investigators with little
option but to tend towards a ' belief in the individuality of
the plant in deciding what shall be the effect on it of external
conditions.' ^
So far we have been considering only the diversity of the
responsive effects which are induced by light in plants.
When we come, however, to the further question of the
responsive mechanics by which the stimulus of light evokes
these movements, we are confronted at the outset of our
inquiry by the fact that, as Pfeffer says, ' the precise character
of the stimulatory action of light has yet to be determined.' ^
Differentiation of responsive movements. — It is cus-
tomary, in treating of plant physiology, to draw sharp lines of
demarcation between the different classes of movements which
are to be attributed to the action of light, ascribing each to
' Francis Darwin, B.A. Report.
' Pfeffer, Physiology of Plants, English translation, 1903, vol. ii. p. loi.
RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 567
some unknown specific sensibility. Thus, Sachs differentiates
some of the principal effects as follows :
' In the case of that stimulation of light which produces
waking and sleeping, the stimulus lies in the variations of the
intensity of light ; it is not the light as a constant force which
effects these movements, but the varying intensity. A further
great difference between the heliotropic curvatures and those
which bring about the sleep movements, lies in the fact that
the organ can make heliotropic curvatures in all directions.
The movement of waking and sleeping, on the contrary, only
takes place in one plane, which divides the leaf and motile
organ S3'mmetrically, and it is thus unimportant here in what
direction the rays of light fall upon the motile organ, but only
important that light is present at all, or increases or decreases
in intensity. The above will suffice for the distinction of the
movement of waking and sleeping from the heliotropic
curvatures.' '
Finally, he summarises the differences of motile effects as
follows :
'We may thus say shortly, the movements of waking and
sleeping are called forth by paratonic light stimulus, whereas
the spontaneous movements of the same leaves are in-
dependent of any light stimuli, but probably dependent on
phototonus. Heliotropic curvatures, on the contrary, have
nothing to do with phototonus.'
It will be found, however, that all these effects, sharply
differentiated as they are, may be seen in one and the same
organ. We may take for example the terminal leaflet of
Desmodiiim gyrans. This exhibits under favourable condi-
tions autonomous movements, whose period is short. It
exhibits also daily periodic movements, with the very long
period of twenty-four hours. It further, as I shall show,
exhibits positive heliotropic curvature when exposed to one-
sided illumination. Of these effects, it is supposed that the
autonomous movement is independent of the paratonic action
of light, but probably dependent on phototonus. The daily
' Sachs, Physiology of Plants, English translation, p. 628.
S68 PLANT RESPONSE
periodic movement is held to be dependent on paratonic
effects and phototonus. The heliotropic effect is ascribed to
the continuous action of h'ght, independent of phototonus.
Thus in the same organ we have to postulate various
irritabilities and mechanisms, in order to account for its
multifarious movements. Are there,' then, independent or
different irritabilities, coexisting simultaneously in the same
organ ? Such a state of things is so difficult to imagine, that
it prompts us to try to look at the problem in a fresh light,
divested of all assumptions which are incapable of experi-
mental proof
Approaching the matter thus directly, then, we see that,
instead of so many different irritabilities, there may possibly
be, fundamentally, but a single phenomenon of irritability,
finding expressions apparently diverse, in consequence of the
anatomical or physiological differentiations of the responding
organ. If this should be so, the question will resolve itself
into three separate inquiries. First, what is that responsive
action which constitutes the characteristic effect of stimulus
of light ? Second, is such response to light unique in
character, or is it a single instance of that universal
phenomenon of contraction which we have seen to be the
response of all excitable cells to stimulus in general ? And,
lastly, in what manner do the various anatomical and physio-
logical differentiations of responding organs operate to modify
the expression of this response ?
Action of light on tissues in sub -tonic condition. — ^ Before
proceeding to a decisive demonstration of the nature of the
effect of light on excitable tissues in a normal condition,
however, I shall briefly refer to the suggestion which has
been offered, that light in some unknown way induces a
lessening of turgor, which brings about diminution of growth.
This theory could not hitherto find acceptance for want of a
precise knowledge of the exact nature of the stimulatory
action of light, and of the relative significance of absorbed
energy in promoting growth. Against the assumption that
light diminished turgor^ it was urged that the pileus of
RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 569
Coprinus drooped in darkness, and became turgid on restora-
tion to light ; and the further supposition that light dimi-
nished growth was held to be negatived by the instance
of a dark-rigored plant, in which growth, so far from being
retarded, was accelerated or renewed by simple exposure to
light.
In both these instances we notice the abnormal condition
induced in the plant. We must bear in mind that that
rhythmic activity which is essential to growth depends, like
all other rhythmic activities, not only on the turgor of the
tissue, but also on the energy which it has absorbed. We
have seen that when the sum total of independent stimulating
factors present in the plant is adequate to raise its tonic condi-
tion above par, then rhythmic activity is initiated or renewed.
Among these factors are, as has been shown : {a) a proper
condition of turgidity ; {b) favourable temperature ; and (<:)
that previous absorption of energy of light which determines
what we may distinguish as the phototonic condition.
The fact that, besides turgor, a certain amount of energy
is also necessary to initiate growth has been fully demon-
strated in a previous chapter. Taking now the case of the
dark-rigored plant, we see that the arrest of its growth is due
to a deficit of absorbed energy — in this instance, phototonus.
Under these conditions, the renewed exposure to light would
be sufficient, by supplying the missing factor, to re-initiate
rhythmic activity and consequent growth. The drooping of
the pileus of Coprinus in darkness is another expression of
the sub-tonic condition of the plant. It must be remembered
that the suctional activity which determines turgor is itself
dependent on the rhythmic activity, and therefore on the
tonic condition, of the plant. The critical element of this
tonic condition may in certain cases be the absorption of
light. I have noticed a similar drooping in flowering plants
kept in the dark for a longtime. Exposure to light, restoring
the tonic condition, is in such cases, as also in Coprinus^
sufficient to restore the normal turgidity of the plant.
In connection with this, I may draw attention to the fact
570 PLANT RESPONSE
that the after-effect of absorbed stimulus in maintaining a
favourable tonic condition is more prolonged in some cases
than in others. In Biophytwti, for example, the rhythmic
activity by which the leaflets are thrown into pulsation is
maintained only as long as the stimulating factors are acting.
In Desniodmni, on the other hand, there is a considerable
capacity for storage of energy, and rhythmic activity persists
for a long time, even on the removal of external stimulating
agencies. "
We thus see that the question of turgidity-variation alone
is not sufficient to explain the action of light on a plant.
We have also to take into account the important element of
energy. We have briefly considered the effect of light on
tissues in an abnormal condition ; but our main inquiry
concerns itself with the precise nature of the stimulatory
effect of light on tissues which are in a normal tonic condition.
I propose to demonstrate the character of this action by
three independent lines of investigation. First, we shall
study the action of light on pulvinated organs, in order to see
whether or not this stimulus produces the same kind of
differential contractile effect as other forms of stimulation.
Secondly, discarding those complications which inevitably
result from the differentiated structure of the pulvinated
organ, I shall proceed to determine the precise nature of the
fundamental form-change undergone by a tissue when
excited by light. For this purpose I shall subject a radial
organ to diffuse stimulation of light, observing whether or not
under these circumstances it exhibits longitudinal contraction.
And, lastly, I shall study the effect of light in inducing
changes in the rate of growth, and shall also try to find out
whether the fundamental responsive action discovered in the
case of stationary, that is to say of non-growing, organs,
might not be capable of explaining the observed variations of
growth under the action of light.
Effect of light on pulvinated organs.— If a strong
beam of light be applied to the palvinus of the \&^.{o{ Mimosa
RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 57 1
or to the pulvini of the leaflets of Biophytuni, it is known that
a responsive movement, similar to that evoked by any other
form of stimulus, is induced. Now, we have seen that the
response of such pulvini is given by means of differential
longitudinal contraction. If, then, the effect of stimulus of
light in this case is to produce a differential longitudinal
contraction, we ought to be able to obtain from a radial
organ, subjected to stimulus of light from all sides, a respon-
sive longitudinal contraction.
Effect of diffuse stimulation of light on non-growing
radial organs. — In order to demonstrate this, I took a radial
style o{ Datura, and subjected it to the stimulus of light from
all sides, by throwing a beam of sunlight which struck it on
one side directly, and on others by reflection from properly
inclined mirrors. On exposing the specimen to this stimulus
for a period of four minutes, a responsive contraction of seven
divisions was induced. On the cessation of light, there was
recovery in the further course of a period of nine minutes. I
next applied stimulus of light for six minutes, and a respon-
sive contraction of eleven divisions was then induced, with
a subsequent recovery on the stoppage of light, which was
completed in fifteen minutes. We thus see that the effect
of stimulus of light in producing responsive contraction is
precisely the same as that which is the result of any other
form of stimulation ; that a feeble or short-lived stimulus
induces a corresponding effect, from which recovery takes
place, in a comparatively short time ; and that a strong
stimulus — unless it induce fatigue— will bring about a
considerable contractile effect, from which the recovery is
accomplished in a proportionately longer time. Again, I find
that continuous stimulation of light produces a maximum
tetanic effect, and that too long-continued action, bringing
about fatigue, may induce fatigue-reversal — contraction
passing into relaxation— as we found in the case of JMimosa,
and of various radial organs which were subjected to too
long continued action of stimulus.
5/2 PLANT RESPONSE
Retarding effect of light on longitudinal growth.— We
thus see that the fundamental contractile effect of stimulus is
precisely the same in the case of light as in that of any other
form of stimulation. We know also that the response of a
growing organ is the same as that of one which is not
growing. It has been shown further, that in a growing
organ the contraction due to direct action of stimulus had
the effect of retarding growth. Now, from the fact that the
effect of light is the same as that of other forms of stimu-
lation, it follows that the result of its direct action on a
growing organ should be to produce contraction, and
Fig. 234. Longitudinal Contraction and Retardation of Growth under
Light in Hypocotyl of Sinapis nigra
The first part of the curve shows the normal rate of growth. Arrow (|)
indicates moment of application of diffuse light, which is seen not only
to retard growth, but also to induce a marked contraction. The
second arrow indicates moment of withdrawal of light, and dotted
portion of the curve shows recovery.
resultant retardation of growth. Though this conclusion,
however, was thus clearly established in theory and by other
experiments, using different forms of stimulus, I yet thought
it important to test the matter with regard to the specific
action of light, under conditions so simple as to bring out the
fundamental phenomenon unmistakably.
In order to do this, I took a seedling ot Sinapis nigra in
which the hypocotyl was strictly radial, and made a record
of its longitudinal growth (fig. 234). Its normal rate of
growth, seen in the curve as movement upwards, was at the
rate of -015 mm. per minute. On now applying light to
the specimen on all sides at once, the growth is seen to
RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 573
undergo rapid diminution till arrested, as seen by the curve
becoming horizontal at its highest point. The continued
application of light now proceeds to cause a marked contrac-
tion, the maximum rate of which is -02 mm. per minute. It
is therefore to be noticed that, in this particular experiment,
light not only retarded growth, but also produced an actual
shortening of the plant. On the cessation of light, growth was
slowly recommenced, but the average rate of growth during
the sixteen minutes following the stoppage of light was only
•001 mm, instead of the normal rate of 015 mm. per minute.
This rate was, however, gradually increased in the absence
of light, and after a certain interval became normal again.
It must be borne in mind, in connection with this, that
though the immediate and after-effects of incident light are
here seen as retardation of growth, nevertheless the rate,
after the lapse of a certain interval from the cessation of
stimulus, may be again increased above the normal in con-
sequence of the enhancement of the tonic condition of the
plant, by its absorption of energy of light.
In the last experiment we saw that responsive contraction
not only arrested growth, but made the tissue actually
shorter. In other instances the contraction and resulting
retardation are not so great. Thus in a second experiment
with Sinapis, the normal rate of growth was -02 mm. per
minute, which during the continuance of light fell to one-
tenth of this, or -002 mm. per minute." Thus the effect of light
on the growing organ is always a contraction, which may
in some cases induce a mere retardation, but in others
culminates not even in cessation of growth, but in an actual
shortening of the responding tissue.
The effects thus described occur in plants which arc in
normal tonic condition. But we have seen that when the
specimen is, on the other hand, in a sub-tonic condition,
absorption of enegy in any form from outside will, at first,
by increasing the internal energy, serve to accelerate growth ;
and that afterwards, when the normal tonic condition has
been attained, external stimulus will have the normal effect
574
PLANT RESPONSE
of retarding growth. In order to verify this inference I took a
growing flower-bud of Crinuvi Lily, which had been previously
kept in the dark. And for the further purpose of detecting
even the transitory variations, I used the delicate method of
balanced record. On now subjecting the specimen to the
stimulation of sunlight, acting on it from all sides, I observed
a preliminary acceleration of growth, which lasted one
minute (fig. 235). By
this time the plant had
evidently attained its nor-
mal tonic condition, and
the continued action of
light resulted in a retarda-
tion of growth, as seen in
the rapid descent of the
curve. The light was
next shut off, and the
after-effect of absorbed
energy is seen in the con-
sequent acceleration of
the rate of growth above
the normal. This ac-
celeration lasted for four
minutes, after which the
plant returned to almost
its normal growth, as seen
by the record approximat-
ing to the horizontal. The
plant may at this point
be regarded as in ordinary
tonic condition. Light was again applied, and retardation of
growth is immediately shown by the descent of the curve.
There is now no preliminary acceleration of growth, as in the
case when the plant was sub-tonic. Under the long-continued
action of light, there is now seen the very interesting
phenomenon of the induction of autonomous pulsations of
the rate of growth, whose period is about ten minutes.
Fig. 235. Balanced Record of \'ariation
of Growth in Flower-bud of Criniini
Lily under Difiuse Stimulation of Light
Continuous lines represent the effect during
application of light, the dotted line on
withdrawal of light. The plant was
originally in a sub-tonic condition, and
application of light at x , after short
latent period, induces preliminary ac-
celeration of growth. After this follows
the normal retardation. On withdrawal
of light, in the dotted portion of the curve
is seen the negative after-effect, followed
l^y return to the normal rate of growth.
A second and long-continued application
of light induces retardation, followed by
oscillatory response.
RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 575
Phenomenon of oscillation under long-continued
stimulation. — We have seen, in Chapter XXIV., that
when a tissue is subjected to continuous stimulation, so
that it becomes possessed of excess of energy, its response
becomes pulsatory in consequence of periodic variations of
its excitability. The exhibition of this variation of excita-
bility we have, for the sake of convenience, designated as
periodic fatigue. The occurrence of autonomous response is
intimately connected with this periodic variation. In some
tissues it may be exhibited only once, while in others it may
be repeated indefinitely. For example, in Mimosa, under
continuous stimulation, we see a single complete pulsation,
consisting of the contractile response,
and the subsequent fatigue-relaxation,
which looks like recovery (fig. 59).
But, in the longitudinal response of
the style of Uriclis Lily to continuous
stimulus, we observed two pulsations
(fig. 58). And in Biophytum and
Desniodium we see these repeated
indefinitely, constituting what we Fig. 236. Oscillatory Re-
, ii- 1 i. sponse of Arsenic acted
know as multiple or autonomous ^^ Continuously by
response. Hertzian Radiation
It is very interesting to note, in Taken by method of con-
. ductivity variation.
connection with this, that even in-
organic substances, under continuous stimulation, exhibit
this pulsatory response. I give here a record (fig. 236)
of the oscillatory response of arsenic when acted on con-
tinuously by Hertzian radiation.
Or we may view this induction of multiple response in a
radial vegetable organ, again, from a different standpoint.
By the action of stimulus, it is easy to see that antagonistic
hydrostatic actions may be set up as between the excited
organ and the rest of the plant. '•Thus, the* direct effect of
continuous stimulation is contraction, with increasing expul-
sion of water from the excited organ into the rest of the
plant. The hydrostatic reaction which this will induce in the
5/6 PLANT RESPONSE
rest of the plant will also constantly increase. The occur-
rence of oscillatory action, under these balanced and opposed
forces, is what might be expected. Moreover, in the re-
sponding organ itself, we see the action of opposed forces
to be induced ; for while the direct effect of local stimulus is
to cause contraction, the absorbed stimulus is meanwhile
increasing the internal energy, the result of which is the
opposite expression in expansion. Thus, from what has
been said, it would appear that in all these — namely, the
hydrostatic action and reaction between the excited organ
and the rest of the plant, the opposed effects of external
stimulus and internal energy, and the presence of an excess
of latent energy, with periodic variations of excitability — -we
have so many factors, which would all contribute to a
common result, in the oscillatory character of the responsive
expression.
But when we come to the case of anisotropic or dorsi-
ventral organs, we find an additional element making for
alternation of effects, for when such an organ is diffusely
stimulated, we obtain a differential response. But as the
constitutions of the two anisotropic halves are different, the
fatigue produced on opposite sides will not be simultaneous
but alternate. Similar effects will also appear under uni-
lateral stimulation of a radial organ. For here, too, the
organ, owing to the relatively greater fatigue of one side,
becomes molecularly anisotropic, and the stimulus by its long-
continued action becomes internally diffused.
Similarity of responsive reaction under light and other
forms of stimulation. — It has thus been fully demonstrated
that the fundamental response of the plant to the stimulus of
light takes place, like that to all other forms of stimulation,
by contraction, leading, in the case of growing organs, to
retardation of growth. Hence the unilateral stimulus of
light may be expected to induce curvatures similar to those
observed under other forms of stimulation, that is to
say :
I. The direct effect of moderate unilateral stimulus of
RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 577
light on the growing region will be a responsive con-
cavity.
2. The effect of such sttmukis of light, acting on the tip
of either shoot or root, will be a convexity of the same side of
the responding region.
3. Strong or long-continued stimulation of light acting
on the growing region will induce a neutral or reversed
effect.
4. The response, under certain conditions of continuous
stimulation, may be characterised by pulsations.
It has been shown in previous chapters that the responsive
action of anisotropic or dorsi-ventral organs is not funda-
mentally different from that of radial organs, the seeming
differences being accounted for by differential action. In
explaining the various effects of stimulus of light, then, the
assumption of various sensibilities in the plant is unjustified.
I shall attempt, therefore, to trace out the manner in
which one fundamental effect of responsive contraction is
made to find diverse expressions, owing to the anatomico-
physiological differentiation of the responding organ. And as
the supposed different specific sensibilities do not exist, I
shall designate all movements and curvatures induced. by
light as heliotropic, the signs positive, negative, and dia- being
used only for descriptive purposes. An investigation into the
action of stimulus of light, then, must apply itself to the
following points :
1. The effect of unilateral stimulus, of varying intensity
and duration, on the tips and growing regions respectively of
radial organs.
2. The induction of autonomous movement by the absorp-
tion of energy of light.
3. The action of stimulus of light on molecularly aniso-
tropic and on dorsi-ventral organs.
4. The direct and after-effects of light, in inducing move-
ments of daily periodicity.
These are the questions which will be specially dealt with
in the course of the following chapters.
V 1'
578 PLANT RESPONSE
Summary
The action of the diffused stimuhis of light on a radial
organ is, like that of other forms of stimulation, to induce a
longitudinal contraction. The action on mature does not
differ from that on growing organs. In a growing organ the
induced contraction has the effect of retarding growth.
• The response to light may thus consist not merely of a
retardation of growth, but sometimes also of an actual
shortening of the responding organ.
When an organ is in a sub-tonic condition, absorption of
energy of light may give rise to a transient acceleration of
the rate of growth ; but when the plant has attained the
normal tonic condition, response is by the usual retardation of
growth.
The after-effect of stimulus of light may consist of a
simple continuation of the characteristic responsive contrac-
tion or retardation of growth. This constitutes the positive
after-effect. There may also be a negative after-effect, con-
sisting of expansion or acceleration of the rate of growth.
Under the long-continued action of light, fatigue is
induced. There is sometimes an exhibition of periodic fatigue
with oscillatory response.
The response to light is not different from that evoked by
any other form of stimulation. The various responsive
movements which occur under the action of light are thus
explicable without the assumption of the possession by
different organs of different specific sensibilities to light.
CHAPTER XLII
positivj: heliotropism
Intrnduclion— Theory of de Candolle — Inadequacy of de Candolle's theory —
Definition of terms positive and negative— Darwin's theory of modified circum-
nutation— Response of terminal leaflet oi Desinodiiivi—'ExixQmt sensitiveness
of some plant-organs to light — Merging of multiple in contiiiuous response —
Orientation induced by light — The perceptive region in the terminal leaflet of
Desmodium — Heliotropic response in radial organ — Magnetically controlled
recorder^Heliotropic response of hypocotyl of Siiiapis — Recovery and theory
of recti-petality.
Having demonstrated the fact that the stimuhis of h'ght
induces contraction in mature organs, and retardation of
growth in growing organs, in precisely the same manner as
any other form of stimulus, we shall now proceed to study
in detail the various effects produced by the unilateral
application of light. Such application, long continued, is
seen to induce movement of the organ, in some cases towards
the light, in others away from it, and in still other organs to
induce a position at right angles to it. While it is doubtless
convenient to distinguish such external effects as positive,
negative, and dia-heliotropic, it is nevertheless unfortunate
that these terms carry svith them an assumption that the
movements in question are due to as many distinct sensi-
bilities on the part of the plant-organs. I shall, however,
endeavour to show that there is in the plant but one funda-
mental sensibility to light, as to other forms of stimula-
tion, which finds expression in contraction under its direct
action ; and that the resulting movements of the organs are
dependent on the question as to whether stimulus acts
directly or indirectly, unilaterally or diffusely, and also,
on anatomical or physiological peculiarities of structure
The complexity of the subject being very great, it has often
58o PLANT RESPONSE
been found difficult to resolve a given movement into its
components, and this has led to the abandonment of the
attempt to relate these various movements to any single
basic reaction. So far was this carried that, in spite of the
well-known observation that a radial organ, illuminated with
different intensities on two different sides simultaneously,
bends in the direction of the more intense illumination,
Sachs found himself compelled to believe that it was the
direction and not the intensity of light that determined the
responsive movement.
Theory of de Candolle. — It is appropriate to make here
a brief mention of the theory of de Candolle, which has
hitherto met with unmerited neglect. De Candolle started
froin the known fact that light retards growth, and explained
growth-curvature as due to the relatively greater growth of
the shaded, inducing concavity of the lighted, side. This
explanation of the mechanics of such movements constitutes
an important advance, though it does not take full account
of all the factors of the problem. This theory of de Candolle
has, however, been discarded, in consequence of the difficulty
which it presented of explaining the action of the negatively
heliotropic organs, in which the lighted side is found to be
convex. Extending this theory to cases of negative helio-
tropism, it was regarded as a logical inference that light
should here accelerate growth. It is doubtful, however,
whether such an inference is justifiable. In any case this
was negatived by the researches of Miiller-Thurgau, F.
Darwin, and Wiesner, who showed that light retarded general
growth in negative as well as in positive heliotropic organs.
It will be shown, however, in the course of succeeding
chapters, that though the circumstances which modify the
response of the plant, so that it is exhibited as negative
heliotropism, are somewhat complicated, yet they in no way
detract from the theory of de Candolle as applied to positive
heliotropism.
Inadequacy of de Candolle's theory. — The flaw in his
theory lies rather in the fact that he regarded the normal
POSITIVE IIELIOTROI'ISM 58 1
rate of growth under shade as the active factor in growth-
curvature, elongation on the h'ghted side being retarded, where-
as in the case of positive Hght-curvatures the motivc-pov/er
really lies in the active responsive contraction of the lighted
side, the expelled water from which, reaching the opposite,
may further cause an increase of growth above the normal.
This fact, that it is the active contraction of the lighted, and
not the passive growth of the unlighted, side that is actually
the efficient cause of heliotropic curvature, will be made clear
by taking an extreme case in which there is no growth.
Here, if heliotropic curvature had been due simply to differ-
ential growth, the occurrence of curvature would have been
an impossibility. But heliotropic curvatures are observed in
organs which have come to growth-standstill. Again, grass
haulms, in which growth is arrested, exhibit curvatures due
to the contraction of the lighted, and the renewal of growth
(due to the increased turgescence caused by expelled water)
on the unlighted, side, in precisely the same manner as they
were observed to do under the action of gravitational
stimulus.
I shall now proceed to describe some typical experiments
on heliotropic effects as induced by the unilateral application
of light. And since it has been shown that the responses of
growing organs are not essentially different from those of
pulvinated organs, it will be helpful to begin by observing the
effect of the unilateral application of light on the latter,
especially as these have the advantage of showing relatively
rapid reactions. In order to obtain a pulvinus which
approximates in character to that of a radial organ, a speci-
men must be selected in which the difference of excitability,
as between the upper and lower halves, is as small as possible.
This may be found in the pulvinus of the large terminal
leaflet of Dcsniodiuui.
Definition of terms ' positive' and ' negative.' — It will
be demonstrated, in this and succeeding chapters, that how-
ever various in type may be the responsive movements
induced by light, they are not due to the different specific
582 PLANT RESPONSE
sensibilities of different organs, but can all be shown to form
only special instances of a single fundamental effect. And this
fundamental effect is the same in growing, in stationary, in
radial, and in anisotropic organs. In describing the respon-
sive actions induced by light, it will, however, be necessary to
distinguish the direction of movement, in relation to the
stimulating light, by clearly defined terms. I shall therefore
designate all movements, of whatever organs, towards light
as positively helioti'opic, and all movements away from light
of whatever organs, as negatively heliotropic.
Darwin's theory of modified circumnutation.— Before
describing the response, it is necessary to say a few words
regarding Darwin's view of heliotropic movements as a
modified form of circumnutation. According to this, the
already existing movement (of circumnutation) had only to
be increased in some one direction, and lessened or stopped
in others, in order to become heliotropic or ap-heliotropic, as
the case might be ; ^ but, in order to prove conclusively that
heliotropic curvatures were caused by the modification of the
pre-existing movements, it would be necessary to show that
they did not take place in those organs from which circum-
nutation was absent, and this would constitute the crucial
test of the theory. The difficulty of obtaining such a speci-
men is, however, so great, that Darwin, although he noted
the point, was unable to apply the test.
I have shown elsewhere that circumnutation is only a
particular manifestation of that multiple or autonomous
response of plants which is due to an excess of energy,
previously absorbed. In order to obtain a plant-organ com-
pletely at standstill, therefore, it would be necessary to find
a specimen in which there was no such excess of latent
energy. The fact that circumnutation was absent could then
be ascertained by means of the high magnification obtainable
from the recording apparatus which I have alread}' employed.
Another difficulty lay in the fact that the use of light, how-
ever feeble, for purposes of observation, would be apt of itself
' Darwin, The Movements of Planls, p. 449.
POSITIVE HELIOTROPISM 583
to give sufficient stimulus to initiate multiple responses
which had not been present before. This might have
appeared incredible had I not, in the course of my experi-
ments, had reason to know how extraordinarily sensitive
plants may become to the influence of light, instances of
which will be found in the investigations presently to be
described. In view, indeed, of the vitiation of results which
might be caused in this way, I was compelled to devise
special means for obviating the use of any light whatsoever
for the observation of responsive curvature.
Response of the terminal leaflet of Desmodium. — In
order, then, to determine the important question of whether or
not light will produce heliotropic movement in a plant devoid
of circumnutation, I acted on the idea that the withdrawal of
superfluous energy was the essential preliminary condition,
and chose for my purpose, as already said, the large terminal
leaflet of a specimen of Desmodium gyrans in which all
movement had come to a stop — the plant having been
exhausted by flowering, and by the unfavourableness of the
season, which was winter. And further, in order that there
should be no storage of energy derived from light, I kept
this plant for one day in a dark room. To eliminate the
necessity of using light for purposes of observation, I attached
the leaflet by cocoon fibre to the arm of the Optic Lever
Recorder. The plant itself was enclosed in a dark box, and
thus protected from any access of light. Through a trap-
door in the box, light for stimulation could be thrown down
on the leaf at the desired moment. The preliminary absence
of any autonomous movement in the plant was seen by the
quiescence of the spot of light reflected from the mirror
attached to the recording lever.
I now subjected the terminal leaflet of the selected speci-
men to light from a candle, this being thrown down on the
leaflet vertically by means of a mirror, the effective distance
of the candle from the leaf being twenty centimetres. The
leaflet, which had previously been quiescent, began to respond
after a latent period of ten seconds, and during the course of
584
PLANT RESPONSE
an exposure of twenty minutes executed five complete
oscillations (fig. 237). The notable points in this record are :
(i) that a perfectly quiescent organ is made to give multiple
response by the stimulus of light ; (2) that molecular
sluggishness appears to be gradually removed by the con-
tinuous absorption of energy, and the successive responses
exhibit an enhanced or ' staircase ' effect ; and (3) that from
the tendency of the series of curves to tilt towards the light,
the organ is seen to exhibit a resultant positive movement.
It will thus be seen that we have here 7iot a modifica-
tion of an existing movement, but a series of multiple
responses, with a trend in
a particular direction, that
is to say towards the
light, constituting a posi-
tive heliotropic movement
In a growing organ also,
which was previously de-
void of circumnutation, we
shall be able to observe
the induction of a simi-
lar movement. As in the
case of the movements of
growth, so also in those of
heliotropism, we are often
able to detect multiple
constituent pulsations, especially at the commencement of
response when stimulus is moderate. It will be understood
here that, under the unilateral contraction induced in the
excited side of the organ by the stimulus of light, a hydrostatic
disturbance is set up, the expelled water being forced to the
opposite side — a state of things calculated to show pulsation
in a marked degree. The final resultant curvature, then, as we
have seen under the action of other forms of unilateral stimulus
also (p. 521), represents the joint effects of the concavity of
the proximal and the convexity of the distal sides. When
light acts on the organ continuously for some time, the
Fig. 237. Multiple Response to Light
of Terminal Leaflet of Desmodiiun
The moment of application is marked
by X . The arrow shows the direction
of light, i.e. from above. The
numbers in the abscissa represent
time in minutes.
POSITIVE HELIOTROPISM 585
multiple responses become more frequent by the increased
absorption of energy, and their individuality is lost.
The extreme sensitiveness of some plant-organs to
light. — I shall now say a few words about the extraordinary
sensitiveness of some plants to the stimulus of light. Darwin
gives a striking example of this in a case where the cotyledons
Q>{ Phalaris canariensis.dSx&x three hours of continuous exposure
to a small lamp at a distance oftwelve feet, became doubtfully
curved towards the light, and after seven hours and forty
minutes from the first exposure were plainly, though slightly,
curved towards the lamp. The candle-power of the lamp is not
given, but it may be taken to be about four candles. Reducing
this to the standard distance of one metre, we find four candles
at a distance of twelve feet (four metres approximately) to
be equal to a quarter candle at a distance of one metre. If
three hours' exposure induced a doubtful curvature, then the
smallest amount of light to be effective must have been
I or 75 candle-hour ; the candle-hour giving an indication
of the quantity of light that had to be absorbed by the plant
in order to induce a movement that was just perceptible.
Now, with the terminal leaflet of Desjiiodiuin, exposure to
the light of a candle at a distance of 20 cm. for ten seconds
was sufficient to initiate responsive movement. This when
reduced to standard conditions is equivalent to "07 candle-
hour. In other words, we find, as far as these two experi-
ments can determine the point, that the terminal leaflet of
Desmodium in this experiment was at least ten times more
sensitive than Darwin's cotyledons of Phalaris canariensis.
Darwin, however, mentions another instance which is more
like the sensitiveness of which I have just given an example.
He has been using a small wax taper, in order to observe the
cotyledons of PJialaris ; he used this light for one or two
minutes at each observation, and observed the seedlings
seventeen times in the course of the day, in consequence of
which he found that zigzag responsive movements had been
induced.
I must again point out here that the specimen of
586
PLANT RESPONSE
Desinodiimi which exhibited such remarkable sensitiveness
had been specially chosen, as being in the least favourable
tonic condition. And yet I could not even strike a match
near this plant without inducing responsive movements.
This will indicate the extreme sensitiveness of certain plants
to light, and show that it is necessary to make our
observations of induced movements without its aid. The
manner in which this was done will be described presently.
Merging of multiple in continuous response. — We
found that light acting from above on the terminal leaflet of
Desinodium gyrans gave rise to multiple
responses. As the light was acting con-
stantly on the upper half, there was a
cumulative contractile effect on that half
The consequence of this was a trend of
the series of curves towards the light, or
an incipient positive heliotropic move-
ment. I shall now proceed to show how
these constituent multiple movements
may often, if not always, merge into one
continuous movement.
For this experiment I used the same
leaflet as in the last, the only difference
being that I now applied the strong
stimulus of sunlight from above. The
response induced is seen in fig. 238.
During the first impact of the stimulus a
pulsatory movement may sometimes be
observed. But the response soon becomes
a continuous movement upwards. Generally speaking, the
constituent multiple marks are to be seen under feeble
or moderate stimulation, and a continuous movement when
the stimulus is strong. In the present case, the average
rate of movement of the tip of the leaf was almost
I'S mm. per minute (fig. 238). On the stoppage of light
there was persistence for some time of the after-effect of
light. This was succeeded by recovery. The persistence
Fig. 238. Jvesponse
of Terminal Leaflet
of Desmodimn to
Strong Light from
Above
Abscissa gives time in
minutes, and or-
dinate movement in
■ millimetres.
POSITIVE HELIOTROPISM
587
of the after-effect varies widely, depending on the condition
of the tissue as well as the intensity of stimulus.
Orientation induced by light. — When the leaflet, with
its sensitive motile organ, is exposed to strong sunlight, the
heliotropic movement continues till the organ becomes par-
allel to the direction of light. The question now arises,
in this as in other cases of heliotropic movement, why should
the movement come to a stop when the organ reaches this
parallel position ?
A partial answer to this question may be found in the
fact that such movements depend upon the effective intensity
of light which is absorbed, and this
effective intensity is greatest at perpen-
dicular incidence, and becomes reduced
to nearly zero as the rays of light are
rendered more and more oblique by the
responsive movement of the organ. This
consideration alone, however, would not
wholly explain the orientation of the
organ, parallel to the direction of light,
for we know that the directive impulse
caused by light persists for some time,
and this would cause the organ to over-
shoot the parallel position. In order,
therefore, to obtain a satisfactory explana-
tion, I undertook the following experi-
ment, which, as will be seen, completely
meets the difficulties of the case.
I now took the same leaflet of Dcs-
inodiuin as was used in the previous ex-
periments, and caused sunlight to strike
the pulvinus vertically, from below upwards, by means of a
suitably inclined mirror. I obtained, as will be seen (fig. 239), a
continuous responsive movement downwards — i.e. towards the
direction of the light. The average rate of movement of the
tip of the leaf was in this case about 2'5 mm. per minute.
This is somewhat greater than the upward rate of movement.
Fir.. 239. Response
of Terminal Leaflet
of Dcsmodiitin to
Sunlight acting from
Below
The dotted portion of
tlie curve represents
the after- effect and
recovery on the
cessation of light.
588 PLANT RESPONSE
and is probably due to the fact that the excitability of the
lower half of the pulvinus is slightly greater than that of the
upper.
We are now in a position to understand the reason of
orientation ; for if we suppose light to be incident from a
position slightly above the pulvinus, it will curve upwards,
owing to positive heliotropism, till it has become parallel
with the rays of light. Should there be any over-shooting of
this position owing to after-effect, the pulvinus will then
begin to curve downwards, because the light will now be
acting from below. Permanent equilibrium can thus only be
attained when the plant-organ has become parallel to the
direction of light.
The perceptive region in the terminal leaflet of Des-
modium. — In connection with the response of the Desmodiwn
leaflet to light, it is interesting to note that the pulvinus is
not only the responding, but also the perceptive region ;
for, throwing the light on the leaflet alone, and protecting
the pulvinus with an opaque shield of black paper, we
find that no responsive movement takes place ; conversely,
if the pulvinus alone be exposed, and the rest of the leaflet
shaded, we observe the normal action.
Heliotropic response in radial organs. — Having observed
the peculiarities of heliotropic movement in a pulvinated
organ, I shall now describe the experimental arrangements
for studying the same problem in non-pulvinated growing
organs. It is understood that there is no essential difference
between the two movements. They are both caused by the
same contractile effect, due to the stimulus of light, the only
difference being that, whereas in the pulvinated organ the
recovery is complete, in the growing organ it remains more
or less incomplete, the curvature being fixed by growth.
Magnetically controlled recorder.^The great difficulty
which stands in the way of accurate investigation is the
question of how to take a continuous time-record of the
heliotropic curvature of the growing organ without being
under the necessity of using light for purposes of observation.
POSITIVE HELIOTROPISM 589
a procedure which, as we have seen, causes disturbance.
The free end of a normally growing organ, when acted on,
say by horizontal light, bends towards it. Thus the problem
is to obtain a continuous time-record of this movement, from
which the latent period, the actual rate of movement, the
after-effect, and other related effects might be ascertained.
I have been able to solve this difficulty by devising a mag-
i<"iG. 240. Diagrammatic Representation of the Magnetically Controlled
Recorder
.l', the Optic Lever, to one arm of which the plant is attached by a
thread ; M, the mirror, with small magnet, Ns, attached behind. The
lever is rotated to dotted position by heliotropic curvature of the plant,
diagrammatically represented disproportionately magnified.
netically controlled recorder, the principle of which will be
understood from the accompanying diagram (fig. 240).
The principal part of this recording instrument consists
of a magneto-metric arrangement. Attached to a long
aluminium lever, ll', is a vertical T-piece, v. This T-piece,
V, carries a reflecting mirror, behind which is a short magnet,
NS. The whole arrangement is freely suspended by a silk
thread. By means of a controlling magnet not shown in the
590 PLANT RESPONSE
figure, the suspended lever may be adjusted in any con-
venient azimuth.
The free end of the growing plant is attached to one end
of the lever, L, being on its own level, at a distance of, say,
20 cm. from the line of suspension, the attached thread being
at right angles to the lever. There is a slight tension of the
thread, due to the magnetic force of the controlling magnet,
which tends to draw the lever away from the plant. This
tension, however, is very slight. When the grov/ing organ is
acted on by light in a horizontal direction, parallel to the
attached thread, and therefore at right angles to the lever,
then if the action of light be to induce a positive heliotropic
effect, the rotation of the suspended magnetic system, owing
to the pull exerted by the curving organ, will be, when seen
from the top, in the direction opposite to that of the hands of
a watch.
If, however, the heliotropic effect be negative, the top of
the growing organ will move in the opposite direction. But
it has been said that there was a slight tension of the thread,
owing to the action of the controlling magnet. This being
now released, the lever will move to a proportionate extent
in the same direction as the hands of a watch. A spot of
light reflected from the mirror magnifies these movements,
and a record of this moving spot of light on a revolving
drum gives the response-curve. Instead of this magnetic
control, it would also be possible to use, as the controlling
force, the torsion of a fine metallic wire.
By using a shorter lever arm and increasing the distance
of the revolving recording drum from the mirror, a wide
range of magnification up to 500 times may easily be
obtained. For ordinary purposes a magnification of ten
times is all that is necessary.
The movement of the spot of light is proportionate to
the heliotropic movement of the plant, at least when the
amount of that movement is not excessive. Thus, know-
ing the magnification produced by the system, and the
rate of the revolving drum, we can determine from the
POSITIVE HELIOTROPISM 591
response-curves the absolute movement, and the rate of such
movement.
I now give a description of the complete apparatus
actually used (fig. 241), consisting of the Recorder and the
Heliotropic Chamber. In a dark chamber is placed the
plant, attached to the Lever as explained before. A portion
of the vertical piece with attached mirror, M, projects outside
the chamber. It will be seen that by this arrangement the
Fig. 241. Heliotropic Chamber and Magnetically Controlled | Recorder!
Heliotropic chamber seen in the middle of the apparatus. Guide-bars to
right and left carry sliding lamps, L and i/. Exposure is given by
pressing key, k, which raises shutter, s. The corresponding shutter
to the right is not shown in the figure. M, mirror of the Optic Lever ;
c, controlling magnet.
plant can be completely protected from light, while its move-
ments are at the same time recorded by the spot of light
thrown from the mirror, M, upon the recording drum, without
the possibility of its reaching the plant within the chamber.
The chamber carries two projecting graduated arms or
guide-bars, one to the right and the other to the left, over
which slides the holder for a candle, or incandescent, or
Nernst's electric lamp. Li/ ; but if a still stronger light be
592 PLANT RESPONSE
desired, sunlight may be reflected in the required direction
by a mirror. From whatever source, the Hght can be made
to strike the plant horizontally through the slit, which is
usually covered with a sliding shutter, S. By manipulating
a key, K, the shutter is raised. Thus exposure may be
made at any moment, and continued for the length of time
desired.
That part of the record which is made on the revolving
drum before exposure begins, gives an indication of the
quiescent condition of the plant. The moment and duration
of exposure are found from corresponding marks made on
the recording surface, at the instants of opening and closing
the shutter.
It will be seen that we have means of controlling the
intensity of illumination within wide limits by (i) the use of
different sources of light as enumerated above, and (2) varia-
tion of the distance of the source of light, at least when this
is artificial.
Thus we are able to illuminate the plant from the right
or left flanks, or from both simultaneously, and by lights of
equal or of different intensities, at will. Again, by covering
the slit with a second plate, provided with suitable apertures,
we are enabled to subject any part of the plant, whether tip
or growing region, or both, as desired, to the stimulus of
light, and thus to determine the characteristic response of
each. All these considerations will show the facilities
afforded by this apparatus for carrying out a great number
of diverse experiments. I shall, however, content myself
here with the description of a few necessary examples.
Heliotropic response of hypocotyl of Sinapis.— As
examples of radial organs, exhibiting the positive helio-
tropic effect, I took seedlings of Sinapis nigra. They were
attached to the lever, as indicated above, and at least half an
hour was permitted to elapse, in order to remove the last
possible trace of excitation due to contact. The attainment
of the quiescent condition was ascertained from the stationary
position of the spot of light.
POSITIVE HELIOTROnSM
593
In this experiment, on a seedling of Sinapis, light was
allowed to strike the growing organ horizontally. The
specimen was very sensitive, and the source of light employed
at first was a candle placed at a distance of 20 cm. acting on
the plant for three minutes. The responsive movement began
within five seconds, and though the light was cut off, there
were produced three multiple responses, after which the plant
underwent a complete recovery and resumed its former
position. Sunlight was next applied, and a continuous move-
ment towards the light was induced (fig. 242).
Fig. 242. Heliotropic Response of
Smapis
X Application of candle-light for three
minutes gave three multiple re-
sponses ; f application of sunlight
gave rise to continuous response.
P'iG. 243. Heliotropic Response of
Sitiapis to Sunlight
Dotted line shows after-effect and
[recovery on cessation of light.
In another case sunlight was applied for twelve minutes.
The response commenced almost immediately on application,
and the average rate of movement was i mm. per minute.
In this particular case, the positive after-effect persisted for
five minutes, even on the stoppage of light, after which
there was a gradual recovery (fig. 243).
Recovery and theory of recti-petality. On the cessation
of stimulus, there is a more or less perfect recovery of the
radial organ from its induced curvature. I have already
demonstrated the fact that the growing organ acts as a
diffused pulvinoid. We have just seen that the primary
action of light is to induce similar motile effects in both
O I.)
594 PLANT RESPONSE
pulvinated and growing organs. We have also seen that in
both, on the cessation of stimulus, there is a tendency towards
recovery. In the case of the growing organ, when stimulus
is moderate, recovery is fairly complete ; but when the
stimulus is very strong and long-continued, some part of the
induced curvature is rendered permanent through fixation by
growth.
An attempt has been made in the case of Vochting's
Theory of Recti-petality to account for this recovery, by
assuming the action of an unknown regulating power which
would tend always to bring the organ back to a straight line ;
but, beyond the assumption of an unknown specific power,
this theory affords no explanation of the mechanism by
which recovery is brought about, and I am able to adduce
considerations which obviate the necessity for thus assuming
the existence of any such specific agency as that of recti-
petality.
In a growing organ which is radial, the tip of the growing
region being free, the vertical direction is that in which there
is least obstruction to growth, and as long as all the lateral
tensions are the same in all directions, there is no reason why
the organ in the course of its upward growth should bend
permanently on any one side more than on another. We
have therefore the norrfial growth of radial organs in a
straight line ; but when stimulus acts unilaterally on the
growing region, a sequence of events ensues, which has already
been fully explained :
(i) Active contraction is induced during the continuance
of stimulus, on the proximal or excited side, with concomitant
diminished turgidity, and retardation of growth.
(2) The water thus expelled is forced, against tension,
into the growing cells of the distal side, raising their
turgescence and consequent rate of growth above par.
(3) The curvature thus induced is maintained as long as
the difference of hydrostatic pressure on the two sides is
continued, by the persistent contraction of the proximal,
under the action of stimulus.
POSITIVE HELIOTROPISM 595
(4) When the stimulus ceases to act, the active contraction
which forced the water against tension to the distal side comes
to an end, and there is a rebound of the expelled water to
the proximal side. Thus the increased growth of the distal
falls, and the decreased growth of the proximal rises, to the
normal rate of growth. The unequal tensions on the two
sides, which previously maintained the curvature, being now
equalised, the organ shows a tendency to straighten itself
(5) I have also shown that a tissue which has been
subjected to stimulus, having absorbed energy and held it
latent, exhibits it on the cessation of external stimulus, in
the form of a temporary negative after-effect, that is to say
an acceleration of growth above the normal. As a result o
this fact, the stimulated side of the organ will show an active
tendency to neutralise the previous curvature, and return to
the straight line.
It is thus seen, from facts which I had already established
regarding the nature of the after-effect of stimulus, that the
recovery of the organ is fully explained, without postulating
the existence of any specific power, such as that of recti-
petality.
Summary
The responsive movement of the plant-organ towards
light is due to the excitatory contraction of the side acted
upon. The curvature of a growing organ towards light is
brought about by the joint action of the induced concavity
of the proximal and the convexity of the distal sides. The
former is the result of the contraction, negative turgidity-
variation, and retardation of growth caused by the stimulus.
The latter comes about by the positive turgidity-variation
due to forcing-in of expelled water, expansion and accelera-
tion of growth of the distal side.
Such responsive movements take place in organs previously
devoid of circumnutation.
The sensitiveness of certain plant-organs to hcliotropic
stimulus is very great. The terminal leaflet of Dcsuiodium
QQ2
596 PLANT RESPONSE
responds under the briefest exposure to feeble candle-
light. This necessitates the making of observations on
heliotropic effects without the aid of light.
The perceptive region for the stimulus of light in the
case of the terminal leaflet oi Desmodium is the pulvinus.
There is no essential difference between the heliotropic
response of a growing and a pulvinated organ.
On the cessation of stimulus the recovery of a pulvinated
organ is com.plete ; and this is more or less true also of the
recovery of a growing organ from response, if the stimulus
have not been excessive.
For the explanation of this recovery in a growing organ,
it is not necessary to assume the existence of any specific
power such as recti-petality. The cessation of the difference
of hydrostatic pressure on the two sides — such difference
being only maintained during the action of stimulus —
together with the accelerated rate of growth on the proximal
side, which constitutes the negative after-effect, are quite
sufficient to explain the recovery from induced curvature.
CHAPTER XLIII
NEGATIVE HELIOTROPISM
Incomplete parallelism lietween actions of light and of gravitation — Theoretical
considerations — Recording microscope — Negative heliotropic curvature
induced by stimulation of the tips of root and shoot — Intermediate phases
between positive and negative heliotropic response : (a) neutralisation by
transverse transmission ; (d) neutralisation by transverse transmission, with
multiple response — Localised sensitiveness to light and transmission of
excitatory effect — Negative heliotropism of a radial organ— Gradual transition
from positive to negative, through intermediate phase of neutrality — Apparent
heliotropic insensitiveness of certain tendrils — Negative heliotropism of tendril
of Ftizs.
We have seen in the chapter on the response due to gravita-
tion, that the responsive curvature of the root is opposite in
character to that of the stem, this fact having led to the
assumption of specific sensibiHties as characteristic of the
root-tip. It was there shown, however, that the responsive
characteristics of the root were not actually different from
those of the shoot, and that the differences in their observed
responses were simply a consequence of the fact that in the
one case the stimulus of gravity acted indirectly, and in the
other directly, upon the responding growing organ. This
assumption that the root possessed a definite sensitiveness
characteristically different from that of the stem, was ap-
parently supported by certain differences in heliotropic action
also, as between shoot and root ; for example, while the
hypocotyl of Sinapis bends towards the light, the root is
found to bend away from it.
Incomplete analogy between action of light and
gravitation. — But I have already explained the fact that the
supposed analogy is false ; for while the stimulus of gravity
acts, in the case of the root, only on a restricted area of
598 PLANT RESPONSE
the tip, the stimulus of light is not necessarily restricted
in the area of its action. Again, whereas the stimulation
caused by statolithic particles is moderate, that caused by
light may be of any degree of intensity. The fact that
there is no true extended analogy between the action of
light and that of gravitation, is seen from the fact that, while
gravitation in the case of the root induces a movement
opposite to that induced in the stem, in the case of light this
is not always so ; for though a few roots turn away from
light, in others there is either no resultant movement, or
movement towards the light. Again, while the shoot makes
a definite curvature with reference to the direction of gravity,
in the case of light we shall observe that though under
moderate stimulation it turns towards it, yet it will sometimes
under stronger stimulation be found to move away. The idea
that positive and negative heliotropic curvatures are due to
two distinct sensibilities could not be better disproved than
by the fact, which will be demonstrated shortly, that the same
organ can be made under different conditions of illumination
to exhibit the two opposite effects.
Discarding, then, the theory of any specific sensibility, we
shall now proceed to show how the movement away from the
stimulating light, the so-called negative heliotropic curvature,
is brought about.
Theoretical considerations. — From the movements al-
ready demonstrated (p. 535) as taking place in plant-organs
in response to stimulus unilaterally applied, we can see the
possibility of such movement becoming negative, or away
from stimulus, under three different conditions :
(i) Under longitudinal transmission of the indirect effect
of stimulus, when, for example, moderate stimulus is applied
to the tip of either shoot or root.
(2) Under transverse conduction of the direct excitatory
effect of stimulus to the distal side of a radial organ, the
proximal side being fatigued by excessive stimulation ; and
(3) Under the transverse transmission of excitation to the
distal side of an anisotropic organ, the distal side being the
NEGATIVE HELIOTROl'ISM
599
more excitable. In this last case, we may obtain a very
pronounced negative response in consequence of the relatively
greater natural excitability of the distal side.
We shall see in the course of this and the following
chapters how heliotropic movements other than positive are
actually brought about under the.se different conditions, and
in the present chapter we shall study cases which are illustrative
of the first two.
Fig. 244. Microscope Recorder
Plant mounted in cubical glass trough with root in water. Light strikes
root, R, unilaterally from the right side. Movement of root observed
by microscope, M, the inclined transparent disc of glass, G, giving at
the same time the reflected image of the recording pen, r.
Recording microscope. — Since the growing root has to
be kept in water, for the purpose of studying the phases of its
responsive curvature, the method hitherto employed of obtain-
ing records by the Optic Lever is inapplicable. I therefore
devised a different method of observation— that of the Record-
ing Microscope (fig. 244). The method of record will be
understood from the figure, where in a cubical glass trough
a piece of the stem of Bindweed, with its water root, R, is
securely fixed on the surface of the water. Light is made to
600 PLANT RESPONSE
strike the root unilaterally, say from the right side. This
pencil of light may be so thrown as to act locally on the root-
tip, or on the growing region, or on both at the same time.
The movement of the root towards or away from light is
observed through the microscope focusscd on the tip. The
eye-piece end of the microscope has a disc of glass adjusted
at an angle of 45° to the vertical. The observer sees the tip
of the root directly through the transparent disc, and at the
same time the reflected image of the recording point of the
pen, lying against the revolving drum below. The two
images are at the beginning of the experiment coincident,
and the responsive movement of the tip of the root, which
takes place afterwards, is easily followed by the observer with
the recording pen. Thus we obtain the response-record on
the moving surface. This method of the recording microscope
can always be used when attachment to the Optic Lever is not
possible or not desired.
Negative heliotropic curvature induced by stimulation
of the tips of root and shoot. — I have by this method ob-
tained various records of the responses of the root and shoot
to the unilateral stimulus of light applied at the tip. Of these
I shall give, as a typical example, the record of the root of a
seedling of Sinapis nigra, suitably mounted in the cubical
trough by means of a cork. The curve seen to the left of
fig. 245 represents the negative movement, or movement
away from light, of this root, when the tip alone was
unilaterally stimulated. This movement was due therefore
to the indirect action of the stimulus on the growing
responding region. After a period of rest in darkness I next
took a record of its movement resulting from the direct
unilateral illumination of the growing region. I now ob-
tained a positive responsive curvature, as seen to the right of
fig. 245. It will be noticed that this particular movement
was relatively smaller than the preceding. We must here
remember that the receptivity of an organ is not the same in
all its different parts, and the greater negative response
induced in this case by the indirect action of stimulus on the
NEGATIVE HELIOTROPISM
60 1
tip is probably due to the higher degree of receptivity
possessed by that part of the organ. In taking a third record
in a case in which both tip and growing region were simul-
taneously subjected to unilateral stimulation of light, I found
that a resultant responsive movement was induced, which was
away from light.
That this negative movement, induced by stimulation of
the root-tip, is not due to any specific sensitiveness of the
root as such, is seen from the fact that on local stimulation of
the tip of the shoot, e.^. the flower-bud of Crocus, I obtained
a responsive movement
away from, whereas uni-
lateral stimulation of the
growing region of the pe-
duncle induced a move-
ment towards, light.
Apart from this pos-
sible factor, however, of
the greater receptivity of
the tip, there is another,
which tends to make the
positive curvature of the
growing region of the
root relatively ineffec-
tive. This region, being
acted on unilaterally by
light, the proximal excitation often passes by conduction
to the distal side, thus neutralising the first positive action.
Instances of this will be given in greater detail presently.
The negative curvature induced by the action of the tip,
depending as it does on the indirect transmission of stimula-
tion, requires as a condition the relative non-conductivity of
the intervening tissue to the passage of true excitation.
Hence, if the conductivity of such a tissue be not sufficiently
feeble, or if the intensity of stimulus be too great, we shall
find that the direct effect of stimulus is transmitted to the
growing organ, and a positive curvature is induced. This
Fig. 245. Record of Response of Root of
Sinapis nigra
The curve seen to the left shows the negative
response due to stimulation of root-tip.
The curve to the right exhibits positive
response on stimulation of growing region.
602 PLANT RESPONSE
explains the positive heliotropic curvature exhibited by many
roots.
Intermediate phases between positive and negative
heliotropic response. — I shall next proceed to demonstrate
the induction of negative heliotropic movements in radial
shoots, a phenomenon which, for reasons already explained,
has no parallel in the case of geotropic action (p. 544). As it
has already been said that there is no specific sensibility which
determines the positive or negative character of the heliotropic
response, it would be interesting to trace out the transitions
by which the normal positive is gradually transformed into
the negative movement. We have seen that when stimulus
is applied unilaterally to a growing region, the positive
curvature at first induced is jointly due to the contraction
caused by direct stimulation of the proximal and the expan-
sion caused by the indirect stimulation of the distal ; but
when the stimulus is strong or long-continued, excitation is
transmitted from the proximal to the distal, the contraction
of which latter now neutralises the first effect. Hence the
normal positive curvature disappears.
{a) Neutralisation by transverse transmission. — The con-
siderations just related explain the curious anomaly that has
been observed, by which, while feeble or moderate stimulus of
light, or interrupted light, gives rise to well-marked positive
heliotropic curvature, the continuous application of stronger
light induces a relatively feeble effect. Thus under moderate
lighting we often observe strong heliotropic curvature, which
disappears under strong sunlight. The curvature induced
is, as we have seen, due to the differential action of unilateral
stimulus, on the proximal and distal sides ; but when a strong
light is used the stimulus becomes internally diffused, and the
differential effect on the two sides is reduced in amount or
vanishes altogether. Such internal diffusion is due to the fact
that, owing to the weak transverse conductivity of the tissue,
while a feeble stimulus is not conducted across it, a stronger
stimulus is. This consideration, together with the fact that
the conductivity of a tissue undergoes seasonal variation,
NEGATIVE HELIOTROnSM 603
will be found to offer a satisfactory explanation of various
anomalies in heliotropic response. Sinapis, for example,
exhibits a strong positive effect in winter, while in hot
weather its action is very feeble. It may be supposed that
this is due, in some unknown way, to a greater rapidity of
growth in warm than in cold seasons. That this, however,
cannot be the reason, will be seen from the fact which I have
demonstrated, that that contractile response of the plant to
external stimulus on which curvature depends is greatest
when the rate of growth is at its optimum. The real
explanation lies in the fact that the neutralisation, or reversal
of normal positive response, caused by transverse conduction,
takes place more easily in warmer seasons, the general con-
ducting power being then great. This accounts for the
feebler positive response in summer, which culminates in
certain instances in an actual reversal into negative (p. 623).
ib) Neutralisation by transverse transmission, with umltiple
response. — Thus if stimulus be sufficiently strong or long-
continued, the positive curvature will become neutralised,
and the organ will return to its original position. I have,
however, observed an interesting modification of this neu-
tralisation, in which it is attended by oscillatory move-
ments to and fro about the mean position. We have seen
that unilateral stimulus, when its action is long continued,
becomes diffused, and thus both sides of the organ become
excited. The tissue, moreover, is now possessed of an excess
of energy — a condition conducive to the production of
multiple response. This fact, together with the periodic
and alternate variation of excitability on the two sides, is
then found to give rise to oscillatory movements of the kind
described. I give below a record which shows the initiation
of these oscillatory movements when the organ had been too
long subjected to unilateral stimulus. It will be remembered
that the pulvinus of the terminal leaflet of Destnodium
executes a positive heliotropic movement, the record of
which has been given in fig. 238. In winter, when the
conductivity of the tissue is feeble, the leaflet curves towards
6o4
PLANT RESPONSE
the light to the maximum extent possible, and remains in
that position as long as the light acts. But we have seen
that in summer the stimulus is more likely to be internally
transmitted to the distal side, the positive effect being thus
gradually neutralised. Thus, in the course of an experiment
during the summer on the pulvinus of the terminal leaflet of
Desjjiodmm, I found, on subjecting it to sunlight from above,
that for the first forty minutes the leaflet rose continuously,
its tip having moved during that time through a little more
than 4 cm. After this there was induced, instead of the
continuous movement upwards, a pulsatory movement up
and down (fig. 246). After a series
of such movements the leaflet was
gradually depressed, the former
positive curvature being thus
neutralised.
The supposed localisation of
sensitiveness to light, and the
transmission of excitatory effect.
I have fully explained the man-
ner in which the effect of stimulus
of light applied at a given point is
transmitted to the distant growing
organ, and the mechanics by which
the curvature is induced. In con-
nection with this, a peculiar phe-
nomenon has been observed, which has led to the belief
that in seedlings, like that of Avena sativa, the zone for
the perception of heliotropic stimulus is confined to the
upper region, or tip of the shoot. This conclusion is based
on Darwin's observations on the unilateral effect of light
on these seedlings. It was found that, generally speaking,
when the lower part of the cotyledon was alone exposed to
the unilateral light — the upper part being covered with a cap
of tinfoil or with an opaque glass tube — there was little
curvature induced ; but when such light was allowed to act
on the upper part of the seedlings the curvature was con-
FiG. 246. Positive Heliotropic
Movement of Terminal Leaf-
let of Desniodinm Converted
by Strong and too Long-
continued Stimulus of Light
into Oscillatory Movement
NEGATIVE HELIOTROl'LSM 605
siderablc. From this it was concluded that sensitiveness to
Hght was mainly confined to the upper part of the plant, and
that this determined the curvature ; but to this conclusion,
that it was the upper rather than the lower part that was
sensitive to light, Darwin found and recorded several excep-
tions, which he regarded as inexplicable. In the case of six
seedlings, for instance, of which the upper parts were covered
with opaque shields, there was as much curvature induced as
in seedlings which were unshielded. In these, therefore, there
must have been sensitiveness in the lower parts also, thus
negativing the conclusion, drawn from other and more
numerous experiments, that it was characteristic of the upper
alone.
In order to see if these discrepancies were not capable of
explanation, I undertook an investigation into the heliotropic
action of Hght on the seedlings of A vena sativa. The method
of screening the upper part of the seedlings from light, which
has usually been employed by Darwin and others, labours
under the disadvantage that the weight and contact of the
tinfoil or the blackened glass tube are not unlikely themselves
to set up a certain mechanical irritation, which may have the
effect of causing an unknown disturbance in the result. I
was therefore desirous of keeping the delicate seedling free
from the irritating contact of caps in the course of my own
experiments. For this reason I arranged for the localised
application of light on upper or lower or both parts of the
organ at will, by the method which has already been described
of throwing a pencil of light on the required spot in the plant
placed in the heliotropic chamber (p. 592). The resultant
movement of the organ was now continuously observed and
recorded, by means of the Recording Microscope which has
been described.
For the sake of clearness I may here forestall matters,
by saying that the observed results fall under two types,
according to whether the given specimen possesses feeble or
high conductivity — that is to say, power of transmitting
stimulus to a distance. Taking first the case in which the
6o6 PLANT RESPONSE
organ possesses feeble conductivity, I have found that when
the extreme tip was stimulated unilaterally, by using a pencil
of light from a sixteen candle-power incandescent electric lamp,
the result was a negative movement — that is to say, a move-
ment away from light. This is exactly parallel to the
response to stimulus of the unopened flower-bud of Crocus
and the root-tip of Sinapis, in both of which the indirect
effect at the growing region, of stimulus applied on the tip,
was seen to be a convexity of the side acted upon, with
consequent negative movement of the tip.
I next applied unilateral stimulus to the same specimen
a little lower down, and now, owing to the better conductivity
of this part of the tissue, the excitation itself was transmitted
to the growing region, inducing concavity and positive
heliotropic movement. The same effect was found to be
produced when stimulus was applied on the growing region
itself It must be remembered that in this case the con-
ducting power of the tissue is not high, hence there is no
transmission of stimulus to the distal side, by which, as
we have seen, the positive curvature would be neutralised.
The long-continued action of light on one side here tends
only to increase the positive curvature to a maximum. Thus
when one side of the entire seedling is acted upon by light,
while the response of the extreme tip tends to induce a slight
negative, all the other parts, from immediately below it to
the growing region, conspire together to exhibit a much
stronger positive heliotropic action. The result is therefore a
movement towards the light. Thus we see that in the case
of seedlings having feeble conductivity the curvature will
be positive, whether it is the lower part only or the entire
plant which is exposed to the one-sided action of light. In
this fact we find the explanation of those exceptional cases
observed by Darwin, in which the seedling was found to bend
towards the light, in spite of the upper part being covered.
We shall next take up that type of response in which the
tissue of the specimen is rather better conducting. In this
case, when the upper part of the organ is locally stimulated,
NEGATIVE HELIOTROPISM 607
the excitation is longitudinally transmitted to the growing
region lower down, and induces a concavity there which
increases with the duration of the stimulus ; but if the
stimulus of light be applied directly on the growing region
itself, instead of on the upper part of the specimen, then, by
reason of the transverse transmission of excitation to the
distal side, we obtain a state of things in which there is no
resultant curvature at all. In this case, then, the direct effect
of stimulus on the proximal side of the growing region is
balanced or neutralised by the transmitted effect on the distal
side ; but this condition of balance will be upset if the uni-
lateral stimulus of light, hitherto acting on the growing
region alone, be allowed to act simultaneously on the upper
part of the specimen also. The longitudinally transmitted
stimulus from the upper part being now added to the direct
excitation of the proximal side of the growing region, causes
an over-balance of responsive effect on that side, resulting in
a positive heliotropic curvature. The fact that there is no
heliotropic movement, when only the lower part of the
seedling is unilaterally acted on by stimulus, is thus not due
to any absence in that region of heliotropic sensibility, but to
the neutralisation of the proximal effect by the equal excita-
tion of the distal. Such transmission of stimulation along
the length of the organ is observed to take place in a specially
marked manner in the cotyledon of graminaceous plants.
We may account for this by the fact that such organs are
parallel-veined— that is to say, the fibro-vascular elements,
which we already know as good conductors of excitation, run
along their length. This is no doubt the reason of their
ready transmission of stimulus to a distance.
Negative heliotropism of a radial organ. — We have
seen that when moderate stimulus acts unilaterally on a
growing organ a positive curvature is induced, and that, with
stronger or long-continued stimulation, this reaches the distal
side, producing neutralisation. We shall now proceed to
trace out the continuity of responsive heliotropic effects, from
the positive curvature to the negative, through the inter-
6o8 PLANT RESPONSE
mediate phase of neutralisation. We have seen that at a
certain definite intensity of illumination the excitations
of proximal and distal sides, balancing each other, cause
neutralisation.- If now the intensity of stimulating light be
further increased, it is easy to see that while the stimulation
transmitted to the distal side, with the concomitant contrac-
tion, is being increased, the excitatory contraction of the
proximal will be at the same time decreased, owing to the
fatigue brought on by over-stimulation. The result then
will be the greater contraction of the distal side, with a
consequent negative heliotropic curvature of the organ.
These considerations show the mechanical action of
heliotropic stimulus in causing : (i) positive heliotropic curva-
ture under moderate illumination ; (2) the neutralisation of this
action under stronger illumination ; and (3) the conversion
of the normal positive into negative when illumination is
excessive. Thus the observation made by Oltmanns, on
young seedlings of Lepidium, subjected to varying intensities
of light, an observation of which there has been hitherto no
satisfactory explanation, is fully accounted for. Oltmanns
subjected a row of seedlings of Lepidiujn to the action of
sunlight, diverging from the focus of a lens. The seedlings
nearest the focus were thus subjected to the strongest stimu-
lation, those further from this point being under gradually
decreasing intensities of light. It was then found that
the seedlings nearest the focus, which were subjected to the
strongest degree of light, exhibited negative heliotropic
curvature, while others, further away, and therefore subjected
to less intense illumination, did not .show any effect at all
(neutralisation), and others again, which were still further
away, and therefore under only moderate intensity of
illumination, exhibited positive curvature.
Gradual transition from positive to negative, through
intermediate phase of neutrality. — I shall, however, give a
still more conclusive verification of the theoretical inferences
which I have just set forth regarding the gradual transition
of positive heliotropic response into negative, through the
NEGATIVE HELIOTROPISM
609
intermediate neutral, in consequence of the increasing internal
diffusion of stimulus with increasing intensity of stimulating
light. This I shall do by a continuous record taken from a
single plant under changing conditions of increasing illumina-
tion, in which record, further, we shall be able to follow all
the phases of responsive change, from positive to negative.
Taking a hypocotyl of Sinapis tugra, I subjected it to the
unilateral action of light from a sixteen-candle-power incan-
descent electric lamp, placed at a distance of 10 cm. from
the specimen. The plant, hitherto quiescent, began to move
towards the light, as shown
by the up curve in the
record (fig. 247), the maxi-
mum being attained in
the course of fifty minutes.
The intensity of incident
stimulus was now in-
creased, by bringing the
lamp to a distance of 6 cm.
from the specimen, at the
moment marked with the
downward arrow. It will
be seen that this resulted
in a process of neutralisa-
tion of the preceding re-
sponse, and that this
became complete in the
course of a further exposure of seventy minutes, the hypo-
cotyl being then erect and free from curvature, having thus
placed itself at right angles to the incident light. Still
stronger illumination of sunlight was now applied, at the
point marked x . This induced, as is seen in the down curve,
a very marked reversed or negative heliotropic response.
Thus, in other words, in an identical organ, under different
conditions of illumination, the plant turning towards the
light exhibits the positive licliotiopic, at right angles to the
light the dia-licliotropic, and away from the light the negative
i< R
Fig. 247.
Response of Hypocotyl of
Sinapis nigra
The first curve shows positive response
induced by incandescent, ^electric lamp
at distance of lo cm. Increased in-
tensity of light applied at arrow (|)
hy bringing lamp to distance of 6 cm.
causes neutralisation. Reversal, ornega-
tive response, when sunlight applied
at X .
6lO PLANT RESPONSE
Jieliotropic effect, thus conclusively proving that the induction
of these three effects is not due to the presence of three
distinct and characteristic sensibilities. These different
responsive movements are thus traced to a single pheno-
menon of contractile response.
This may be made equally clear from another point of
view, as illustrations namely of the general law that response
takes place by the contraction of the more excited, or the
relatively more excited. With moderate stimulation it is the
proximal side of the organ that is excited, and, becoming
concave, gives rise to a positive heliotropic curvature. On
the application of somewhat stronger stimulation, however,
when excitation is transmitted to the distal side, there is a
case in which the excitation of the proximal and distal are
equal, and the differential excitation being thus zero, there is
no resultant response. The organ, standing thus at right
angles to the light, and moving neither towards nor away
from it, will now appear to be either dia-heliotropic or else
irresponsive to heliotropic stimulus.
But under still stronger stimulation two effects are in-
duced simultaneously: (i) a physiological anisotropy, by the
fatigue and loss of excitability of the proximal, in conse-
quence of which the distal becomes the more excitable ; and
(2) the internal diffusion of the stimulus, which, now acting
on the physiologically anisotropic organ, induces concavity of
the distal, that is to say a negative heliotropic curvature.
1 shall now proceed to give further examples of that
transverse transmission of excitation, in consequence of which
an organ appears either to be irresponsive to heliotropic
stimulus or to give negative response.
Organs apparently insensitive to light. — Many vegetable
organs exhibit no resultant movement under the action of
light, and are therefore supposed to be insensitive to it ; but
this inference could be accepted only on the theory of a
specific sensibility, in which case some organs would be
without it, while in others it would be characterised by
certain inherent positive or negative properties. We have
NEGATIVE IIELIOTROITSM ClI
seen, however, that such a theory is untenable. The apparent
absence in some organs of sensibility to light may perhaps,
then, be explicable, not as want of sensitiveness, but as the
neutralisation of effect, by the equal excitation of the two
opposite sides.
In the case of certain seedlings of Avena, it has already
been shown that the so-called insensitivenessof the lower part
of the organ was due to this cause. Many tendrils, again,
according to Mohl and others, are heliotropically insensitive.
Thus, for example, on subjecting the tendril of Passifiora to
lateral sunlight, there is "practically no responsive movement ;
but as the tendril is a highly conducting organ, we might
expect that its responsive movement would be neutralised by
the transverse transmission of excitation. It occurred to me
that this question, as between a characteristic insensitiveness,
and a sensitiveness with equal excitation of two sides, might
be tested by artificial reduction of the conducting power by
cooling. Under such circumstances, if any sensitiveness
existed, a one-sided excitation by light would remain
localised, and induce concavity, or positive heliotropic move-
ment. On carrying out this experiment, I found that the
selected tendril of Passiflcna now exhibited a marked positive
heliotropic movement by the induced concavity of the side
acted upon.
Negative heliotropism of tendril of Vitis. — The tendril
of Vitis is adduced as the type of those organs which exhibit
the negative heliotropic effect. 1 therefore undertook an
investigation on this organ, to determine whether it would
not be possible to explain its negative movement without
postulating the existence, in its case, of a specific heliotropic
sensibility of negative sign. When sunlight strikes it on one
side it is found that it moves away from the light. If, now,
this movement be really due to the intensity of stimulus,
causing it to be rapidly conducted to the distal side, and at
the same time giving rise to the fatigue of the proximal, then
we should expect that the application of moderate unilateral
illumination would induce the positive heliotropic movement.
R K 2
6l2 PLANT RESPONSE
The crucial test would thus lie in the observation of the
responsive movement under moderate unilateral stimulation.
I placed a tendril in a dark room, and subjected it to light
of moderate intensity from a sixteen-candle-power electric
lamp, placed at a distance of 15 cm. This induced an active
movement of the tendril towards the light, or positive helio-
tropic response. I then brought the lamp nearer, to a distance
of 5 cm., thus increasing the intensity of light. The active
positive movement was now quickly reversed into a move-
ment away, or negative heliotropic response. This experiment
once more demonstrates the fact that positive and negative
heliotropic responses are not due to two specific sensibilities of
opposite sign.
The negative heliotropic curvature in an organ originally
radial, which we have just studied, was due to the physio-
logical anisotropy induced by the stimulus itself; but there
are organs in which anisotropy is already developed in
various degrees of perfection, and in them we shall be able to
observe varying intensities of this negative heliotropic effect.
This will be discussed in detail in succeeding chapters.
Summary
Negative curvature is induced under the action of light in
two different ways : (i) by the indirect effect of the moderate
stimulation of the tip of root or shoot ; and (2) by the
transversely transmitted effect of strong or long-continued
stimulus acting on the distal side of an organ, when the
proximal has become fatigued. The parallelism between
geotropic and heliotropic effects is thus incomplete.
The direct effect of stimulus, unilaterally applied, may be
longitudinally transmitted, and cause responsive movement
in the distant growing region.
The unilateral effect of stimulating light on the growing
region varies with its intensity, as follows :
I. Moderate intensity induces the normal positive re-
sponsive movement.
NEGATIVE HELIOTROPISM 613
2. (a) Stronger or long-continued stimulus, on account
of its internal diffusion by transverse transmission, causes
neutralisation of effect. In other words, the organ, after
first moving towards the light, returns to its original position
at right angles to it, or assumes the dia-heliotropic attitude.
This neutralisation, depending as it does on transverse con-
duction of stimulus, can only take place when the stimulus is
very strong, or when the organ is highly conducting. The
first of these considerations explains the fact that while
moderate stimulation causes positive curvature, stronger
stimulation has no resultant effect. The effect of the second
factor (the higher conductivity of the tissue), which is brought
about by a warmer season, is seen when the transversely
transmitted effect causes neutralisation. The apparent
absence of heliotropic effect in various tendrils is due not
to any want of sensibility, but to this neutralisation by
transverse conduction of the effect of stimulus,
{d) The transverse transmission of stimulus to the distal
side is sometimes attended by oscillatory responsive move-
ments, owing to periodic or alternate fatigue.
3. With still stronger unilateral stimulation, the organ
becomes for the time being anisotropic, owing to the fatigue
of the proximal side. The internally diffused stimulus then
induces negative curvature, through the relatively greater
excitability and contraction of the distal half of the organ.
The negative heliotropic movement of the tendril of Vi^t's
is explained by these considerations. This tendril, under
moderate unilateral stimulus, exhibits positive heliotropic
movement ; but stronger stimulation, in con.sequence of
transverse transmission and unilateral fatigue, gives rise to
negative heliotropic movement.
The statement that the different responsive curvatures
brought about by light are not due to different sensibilities
possessed by different organs, is proved by the fact that the
same organ exhibits continuous changes, from positive to
negative through neutral, under different intensities of
stimulation.
CHAPTER XLIV
EFFECT OF INVISIBLE RADIATION AND EMANATIONS
Effect of temperature and its variations — Demonstration of fundamental effect of
thermal radiation on growth — Response to successive uniform stimuli of
thermal radiation — Effect of continuous unilateral stimulation — Effect of
electrical waves on growth — Response of Alintosa to electric radiation —
Action of high frequenc}' Tesla current.
We have now studied the curvature effects induced in plants
by those ethereal vibrations that lie within narrow limits, and
are known as visible light. There are, however, other vibra-
tions outside this range, the ultra-violet and the infra-red.
The excessively quick vibrations beyond violet are known to
produce very marked heliotropic effects. But below the red,
again, we have comparatively long waves which give rise to
thermal, and others, still longer, to electrical radiation.
In studying the curvature-effects on plants of invisible
radiations of low frequency, it is necessary to distinguish
carefully between the action of radiation as such and the
subsidiary effect of temperature.
Effect of temperature and its variation. — In this in-
vestigation it becomes especially important to distinguish
the temperature from the radiation-effect, and I shall pre-
sently describe a very decisive experiment by which the
effects of the two may be clearly distinguished. The effect
of temperature up to the optimum is, as we have seen, to
increase the internal energy of the plant, in consequence of
which there is an acceleration of the rate of growth ; but
variation of temperature acts as an external stimulus, and
would thus be effective in inducing a transient retardation of
growth. A part of the stimulus, however, is held latent, as
EFFECT OF INVISIBLE RADIATION AND EMANATIONS 615
we have seen, in the tissue, so long as the temperature is
below the optimum, to give rise later to an acceleration of
the rate of growth. Hence, frequent variations of tempera-
ture below the optimum will, by reason of these alternate
retardations and accelerations, produce little total effect on
the rate of growth ; but above the optimum, the stimu-
lating action of variation of temperature will retard growth,
and as there is here no latent factor, this will not be made
up by any subsequent acceleration (p. 461). Hence, the
total effect of such variation will be a retardation. This
consideration explains the different conclusions to which
observers have been led as to the effect of frequent variation
of temperature on growth.
Demonstration of fundamental effect of thermal radia-
tion on growth. — In the usual experiments on the effect
of thermal radiation on the induction of growth-variation,
a difficulty arises in distinguishing between the effect of
thermal radiation and that of temperature. In order, then,
to determine the fundamental effect of thermal radiation,
the experiment must be so arranged that the radiation whose
effect is to be observed causes no change of temperature.
I have been able to accomplish this by mounting the growing
organ in a plant chamber surrounded by a wide heating coil
of platinum wire. Between the coil and the specimen there
is a cylinder of mica, which is opaque to thermal radiation.
By means of a string attached to it from above, this cylinder
may be alternately lifted and lowered. The plant is attached
to the Crescograph, and a balanced record is taken of its
growth when the shield is down, and when, by maintaining a
current through the heating coil, the chamber has already
been brought to a steady temperature of 34° C. As every-
thing inside the chamber has now attained a steady tempera-
ture, the movement of the shield up and down will produce
no change in this condition. By now raising the mica
shield, we can subject the specimen to the action of thermal
radiation, which proceeds from the heated spiral wire, with-
out producing any variation of temperature. When the
6l6 PLANT RESrONSE
mica shield is again dropped the action of radiation on the
organ is cut off.
Experimenting in this manner, and obtaining a horizontal
record with the shield down, we find, when the shield is
lifted, that there is at once an upsetting of the balance, which
indicates a retardation of growth. When the shield is once
more allowed to drop, the deflected record becomes again
horizontal, indicating the restoration of the original rate of
growth. This experiment conclusively shows that radiation
by itself acts as an external stimulus, retarding the rate of
growth. It will be remembered that this is quite distinct
from the effect of temperature, which, up to the optimum,
always induces the opposite result, namely, an enhancement
of the rate of growth. The same distinction is also to be
borne in mind in dealing with the excitatory effect of light,
for incident light also has the twofold effect of causing
external stimulation and at the same time raising the tem-
perature of the tissue.
Response to successive uniform stimuli of thermal
radiation. — Having thus demonstrated the fundamental
action of thermal radiation, we shall now study the effect of
the unilateral appli-
cation of this form
of stimulus, which
will be found similar
to that caused by
visible light. The
responsive effect of
successive uniform
Fig. 248. Responses to Successive Uniform c-f;»^..lof;^»ic- r^^-,, K^
Stimuli of Thermal Radiation in Pistil of Musa Slimuianons may DC
studied by placing a
V-shaped platinum wire with its point opposite to the
region of growth, and heating it periodically by definite
currents of short duration ; the flashes of thermal radiation
thus produced bring about the usual responsive concavity.
These responses and recoveries are recorded in the usual
manner. I give in fig. 248 a series of such responses
EFFECT OF INVISIBLE RADIATION AND EMANATIONS 617
of the pistil of Miisa, in some specimens of which the
growing region is found to be narrow and sharply defined.
In this case the V-shaped radiator was placed opposite to
this region. In these responses we see that there is a con-
siderable amount of recovery after each transient stimulation,
and also a certain degree of fatigue.
Effect of continuous unilateral stimulation, — We shall
next study the effect of continuous unilateral stimulation.
For this experiment I took the hypocotyl of Taiiiariudus
indica. As the growing region in this case is extended,
Fig. 249. Response of Hypocotyl of Tamarindtis indica to Continuous
Stimulation of Thermal Radiation
I used a thermal radiator, which consisted of a linear plati-
num wire, 5 cm. in length, placed parallel to one side of
the growing region. Under the action of continuous uni-
lateral radiation, an increasing positive curvature — that is to
say, towards the stimulus — was induced, till a maximum
effect had been attained (fig. 249). With specimens of other
plants I obtained either the neutralisation or the reversal of
this curvature, as it might happen, in consequence of the
internal diffusion of stimulus to the distal side. This neutra-
lisation is brought about by equal excitation of the proximal
and distal sides, in precisely the same manner as in the
6l8 PLANT RESPONSE
case of light It is seen in the return of the organ to its
original position, or by oscillations about that position as the
mean ; but in cases where the proximal becomes fatigued,
the responsive contraction of the distal gives rise to a nega-
tive curvature. In all these cases we see thermal radiation
inducing the same curvature effects — positive, neutral, and
negative — as were found to be induced by visible light.
Effect of electric waves on growth. — I shall next describe
the effect of Hertzian waves in inducing responsive curvatures ;
and first, in order to obtain the fundamental effect on growth
itself, I took an electric radiator consisting of a rod 5 cm.
Fig. 250. Effect of Electric Waves on Growth
Connection with electric vibrator at downward arrow { 4 ) is seen to arrest
growth and to induce contraction, by which the specimen undergoes
an actual shortening.
in length, excited by oscillatory discharge from a Ruhmkorff's
coil. The specimen was a flower-bud of Crinum Lily, whose
upper and lower ends were connected by means of thin wires
with the two ends of the electric radiator. The natural
growth-record under unbalanced conditions was first taken
(fig. 250), and the specimen was now subjected to the action
of electric waves at the point marked in the record with a
downward arrow. It will be seen that this gradually
diminished the rate of growth, till at the end of five minutes
it was completely arrested ; and as the action of the electric
waves continued, the contractile effect by which growth is
arrested is seen to be carried further, actually bringing about
a shortening in the length of the specimen.
EFFECT OF INVISIBLE RADIATION AND EMANATIONS 619
Response of Mimosa to electric radiation. — In the last
case, in order to subject the specimen to the intense action of
electric waves, the radiator was electrically connected with
the responding organ, which was diffusely stimulated by it,
and exhibited a longitudinal response of contraction. If,
however, we wish to observe the effect of the unilateral action,
it will be necessary that the radiation shall act on one side
only. A difficulty is here met with, however, owing to the
relatively greater length of these electric waves, which, like
those of sound, do not cast shadows, but curl round corners.
In order to produce unilateral action, then, the length of
these waves has to be reduced to a minimum. By using
small spheres of platinum as the source of radiation, I
succeeded in obtaining short electric waves of about i cm. in
length ; but the intensity of such radiation is somewhat
feeble, and with them I could only occasionally obtain
responsive curvatures of growth. I was, however, more
successful in obtaining responsive effects from the pulvinus
of Mimosa. When the small radiator which I have described
was placed at a distance of a few centimetres from the lower
half of a pulvinus, and when the radiator was allowed to act,
a depression of the leaf of this plant occurred after an
exposure of nearly half a minute to the continuous action.
It should be mentioned that for the demonstration of this
effect it is the younger leaves which are suitable, the older
not being sufficiently sensitive. With regard to the electric
waves, it is to be borne in mind that to them water is opaque,
while ebonite is highly transparent. Hence the interposition
of a sheet of ebonite, between the radiator and the plant,
does not stop the responsive action, while a parallel-sided
trough of water in the same place would effectually prevent
the passage of the rays, and thereby put an end to the
action.
Action of high frequency Tesla current. Ifoncelcctrode
of a Tesla coil be put in connection with the growing organ,
this electric variation of high frequency is found to cause
an arrest of growth. Even the contiguity of wires carr)-ing
620 PLANT RESPONSE
these high frequency currents is often found to retard growth.
It is probable that this is due to certain material emanations
that proceed from the wire. A coil of iron wire was made to
surround the growing organ, and during the excitation of
this coil the normal growth-rate was found to be retarded ;
but there was no such retardation when a glass cylinder was
interposed between the specimen and the coil. During the
short period of the experiment there was but little rise of
temperature within the coil, and the retardation of growth
could not have been due to any thermal radiation.
Summary
Variation of temperature acts as a stimulus. Below the
optimum the direct effect of variation of temperature is a
retardation of growth, and the negative after-effect, owing to
absorption of stimulus, is an acceleration of growth. Hence
repeated variations of temperature below the optimum have
little resultant effect on growth.
Above the optimum, stimulus is not held latent, and there
is no negative after-effect of accelerated growth. Hence,
repeated variations of temperature above the optimum will
retard growth.
While rise of temperature up to the optimum accelerates
growth, thermal radiation, as such, acts as a stimulus, and
retards growth.
The unilateral action of thermal radiation is similar to
that of luminous radiation — that is to say, thermal radiation,
when of moderate intensity, gives rise to positive, and when
strong, to neutral, or negative movement.
Electric waves induce retardation of growth. The uni-
lateral application of electric radiation is also found to induce
responsive movements.
High frequency Tesla currents retard growth.
CHAPTER XLV
ON PHOTONASTIC PHENOMENA AND ON DIURNAL SLEEP
Photonasty and para-heliotropism — Response of Tropicohim majits — Responses
of plagiotropic stems : {a) Mimosa — {b) Ipoviiva—{c) C//<-«;-(5//(i— Daily periodic
movements of plagiotropic stems — Responsive movements of piilvinated organs
— Pulvinated organs showing positive heliotropic movement : {a) Response of
terminal leaflet of Dcs)uodinm—{b) Response of leaflet of Roliiuia^{c) Re-
sponsive movements of leaflets of Erythrina indica and Clitoria ternatea —The
negative heliotropic type of response : («) Response of pulvinus of Mimosa —
(/') Diurnal sleep of f^rrt/zV— (c) Diurnal sleep of /)'^^//y////w— Directive rwj-//j-
non-directive action of light — General view of responsive curvatures induced
in different organs by unilateral application of light.
I SHALL now enter upon the investigation of a very large
class of phenomena, brought about by the action of Hght,
which have hitherto been regarded not only as obscure, but
also as totally unrelated to each other. These phenomena
may be described in a general way as the exhibition of the
differential effects of light on anisotropic or dorsi-ventral
organs. Such effects may again be divided for convenience
into two classes, according as they are exhibited either by
growing or by mature and pulvinated organs. The respon-
sive curvature in the former of these cases, due to the
differential growth induced by light, we shall designate as
pJwtojiastic. It must be understood that there is in reality,
as we shall see, no fundamental difference between the
responsive curvatures induced in growing organs, whether
radial or dorsi-ventral, and those in pulvinated organs ; but
it is nevertheless suitable for the purposes of this investigation
to treat them under separate headings.
Photonasty and para-heliotropism. — In experimenting
with dorsi-ventral shoots, Dc Vries found that when light
was applied to the lower or normally shaded surface of, for
622 PLANT RESPONSE
example, runners of Lysimachia Numimilaria, the result was
a concavity of that side ; and when strong light was applied
on the upper surface, the result was still the concavity of the
normally shaded, and now distal, side. He obtained similar
results also with midribs of leaves. Again, in the case of
Marchantia thallus, it was found by Frank and Sachs that it
was the normally shaded side which became concave, whether
light was applied above or below.
De Vries explained the curvature away from light, when
the dorsal surface was illuminated, by the assumption that
the organ was negatively heliotropic ; but the concave
curvature induced when the lower surface of the same organ
was illuminated necessitates the description of that surface,
at least, as positively heliotropic. Thus we are driven to
assume that the two surfaces of the organ are endowed at the
same time with two opposite heliotropic properties, the obvious
impossibility of which has led to the idea that in this class of
phenomena we have not to deal with the directive action of
light at all.
In the case of certain pulvinated organs I shall be able to
show an exact parallel to these phenomena — that is to say, an
induction under light of concavity in the lower surface, whether
light be applied from above or below. This being so, it is
clear that the only theory of the phenomena which could be
satisfactory would be one which would apply equally to both.
Closely connected with the same inquiry is the pheno-
menon of para-heliotropism, or the so-called diurnal sleep.
Under the intense illumination of midday, the leaves or
leaflets of certain plants take up positions which outwardly
resemble those which they adopt at night. In some cases
again, in which the normal daylight position is outspread, and
the nocturnal one of downward folding, the effect of light at
midday on the leaflets is to induce a folding upwards.
These phenomena have not yet been satisfactorily explained.
Darwin suggested that such habits had been acquired for the
special purpose of avoiding too intense an illumination. I
shall be able, hov/ever, to offer a simple and inclusive ex-
PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 623
planation, applicable to the phenomena both of photonasty
and of para-heliotropism. It will be noticed that in the first
case we have to deal with the effect of light on all groxving
dorsi-ventral organs, and in the second with its effect on
mature dorsi-ventral pulvini ; and in dealing with both
these classes I shall pass step by step from the consideration
of simple to that of more complex types.
The response of Tropaeolum majus.— Sachs found that
when the stem of Tropccoluni majus is exposed to intense
and long-continued unilateral illumination, a negative helio-
tropic curvature is induced ; but when the plant is exposed
to moderate unilateral illumination it exhibits positive helio-
tropic movement. The explanation of this difference is
made quite clear from the experiments which I have already
described, with reference to seedlings of Sinapis nigra and to
the tendril of Vitis ; for it has been shown that in these
cases moderate unilateral stimulus of light caused a positive
heliotropic effect ; whereas, under the action of intense stimu-
lation, a transient anisotropy was induced, on account of
which the excitability of the over-stimulated proximal side
was diminished. Hence the distal side was rendered the
relatively more excitable, and the intense stimulus becoming
internally diffused caused a concavity of the distal side, and
gave rise to a negative heliotropic effect (p. 609).
The negative effect is thus due to anisotropy, and internal
diffusion of excitation from the proximal to the distal sides,
this latter factor evidently tending to be facilitated by any
agency that increases the conductivity of the tissue (p. 603).
We have also seen elsewhere that this power of conductivity
is very much augmented in summer (p. 475). In connection
with this, it is interesting to note that the negative heliotropic
curvature of TropcEoliun majus, due to transmitted excita-
tion, has only been observed by Sachs during summer, while
in autumn the effect observed by him was always positive,
owing to stimulus remaining localised. Another instance of
the same kind is furnished by the hypocotyl of Ivy {Hedera
helix),wh.\ch. from the normal positive heliotropic curvature, in
624 PLANT RESrONSE
autumn, as observed by Darwin, passes into the exhibition of
a strongly negative heliotropic condition in summer.
Responses of plagiotropic stems. — It is found that
under feeble diffuse illumination the Gourd-plant iCuciirbitd)
for example, grows erect. In the open, however, if, on
account of its own weight, or by the action of the wind, it
is once made to bend, the stem is brought into a position
where its upper surface is constantly acted on by strong
light, whilst its lower is shaded. By the continuous action
of the stimulus of light the upper surface is now rendered
less excitable, and a permanent anisotropy (plagiotropism)
is induced, such as we saw transiently exhibited in the
hypocotyl of Sinapis (p. 609) under the short experimental
exposure of a specimen to intense unilateral illumination. It
has already been shown in Chapter VII. that such plagio-
tropic stems are more excitable on their lower or shaded
side, and that under diffuse stimulation response is by con-
cavity of that side. Hence a recumbent plagiotropic stem
of this description, acted on by strong vertical light, which
becomes internally diffused, will always exhibit concavity of
the lower surface, in consequence of which it will be closely
pressed against the ground.
Such a plagiotropic organ, then, acted upon dorsally,
shows a concavity of the ventral surface, or a negative helio-
tropic effect ; but if the ventral, or normally shaded, side
be itself acted upon directly by light, the result will still be
the concavity of the lower or more sensitive surface — that is
to say, a positive heliotropic effect. In the former case, then,
we have an example of the differential response of an aniso-
tropic organ to diffuse stimulation, by concavity of the more
excitable ; and in the latter, the direct contraction and con-
cavity of the surface acted upon, which happens in this case
to be also the more excitable. This will explain why, in the
midribs of leaves, in plagiotropic shoots of Lysiniachia, and
in the thallus of Marchantia, we always observe the concavity
of the shaded and more excitable side, in response to the
action of strong light, whether it is applied from above or below.
FHOTONASTIC THENOiMENA AND DIURNAL SLEEP 625
'^fiM.
(a) Mimosa. — This fact, that in plagiotropic stems under
the diffuse stimulation of light it is the more excitable
shaded side which becomes concave, I have been able to
demonstrate by numerous other experiments ; for example,
taking four creeping stems of Mimosa, I tied them in such a
manner that their free ends should be vertical. The shaded
sides of the four specimens were turned so as to face each a
different point of the compass
— east, west, north, and south.
Subjected thus to the diffuse
stimulation of light from the
sky, they all executed curva-
tures. The specimen whose
under side faced the east be-
came bent towards the east.
The same happened to those
which faced north, south, and
west — that is to say, they
became curved towards the
north, south, and west re-
spectively. The fundamental
responsive action by which
all these were determined
was the induced concavity of
the under, or normally shaded,
side, which is the more excit-
able.
{b) Ipouuva. — Another ex-
ample was that of the creeping
stem of Ipoimva. This I tied
up vertically with its end free, at 10 A.M., the normally
shaded side being represented in the diagram to the left
(fig. 249). For purposes of the record I placed behind
it a piece of paper, on which its different positions were
traced from time to time. It will be seen that by i r.M
it had become considerably curved, the normally shaded
side being concave. This concavity had become still more
s s
EiG. 251. Response to Diurnal Light
and Darkness of Plagiotropic Stem
of IpoiiHva held vertical
The curvature is seen to increase with
the progress of day, the shaded side
to the left becoming more and more
concave. The dotted figures repre-
sent positions of gradual recovery
at night.
626
PLANT RESrONSE
marked by 4 P.M. The next record was taken at 7 P.M.,
when the sun had gone down, and the source of stimulus
was therefore removed. The dotted portion of the record
shows the partial recovery which had now taken place,
and whose extent was still further increased by 10 P.M.
This partial undoing of the
induced curvature is mainly
due to the recovery which
is always observed on ces-
sation of stimulus. It
might, however, be urged
that geotropic action, ab-
sent when the plant was
vertical, and coming into
increasing effectiveness
with its growing horizon-
tality, played a large part in
bringing it about. In order,
then, to eliminate this ele-
ment, I next experimented
on a stem of Ciiciirbita.
Response to Daily Periodic (0 Cucuvbita. — This
Light and Darkness of P'lagiotropic plagiotropic stem waS held
Stem of Citciirbita vmxiina
Fig. 252
The specimen held horizontally, with plane
of dorsi-ventral division vertical, the
ventral surface being here represented
to the right. The action of gravitation
is thus practically eliminated. Cumula-
tive efiect of day's illumination is seen
in progressive concavity of normally
lower or shaded surface. Dotted figures
show positions of recovery after dark.
horizontally and sideways,
in such a way that the
plane which divided its
dorsal and ventral halves
was vertical. The shaded
side is here represented in
the diagram to the right
(fig. 250). Owing to this particular arrangement of the
plant it will be seen that the responsive heliotropic move-
ment must take place in a horizontal plane, and that on it
geotropism will have no influence. The free end of the
plagiotropic stem had been naturally somewhat curved up-
wards, and this appeared, when placed sideways, as a slight
curve to the left, in its position at 7 A.M. Under the stimulus
PHOTONASTIC
PHKNOMENA AND DIURNAL SLEEP 627
of daylight by 10 A.M. the stem is seen to have taken up a
strongly curved position, with concavity of the shaded side.
This process is seen from the figure to have been progressive,
but on the approach of darkness there was recovery, the
return being considerable at 7 P.M., and still greater at
10 P.M. It may be stated here that such recoveries are never
quite complete, part of the curvature being fixed by growth.
Thus in a naturally growing plagiotropic stem, the portion
which was one day slightly lifted above the ground is on the
next, owing to this residual effect, made to lie closely against
it, so that by the action of light the growing stem is pressed
progressively closer to the earth.
Daily periodic movement of plagiotropic stem. — From
this demonstration of light-curvature and recovery, it will be
seen that the free organ has a daily periodicity in virtue of
which, in the daytime, it moves gradually downwards, and
at night, owing to recovery, upwards. In this plagiotropic
stem, then, we see, as will be shown later, the first induction
of that nyctitropic movement which is more strikingly dis-
played in dorsi-ventral leaves.
Responsive movement of pulvinated organs. — The
demonstration which I have just made of the peculiar
responses of anisotropic organs to the stimulus of light, as
exemplified in the case of plagiotropic stems, will be found
still more strikingly applicable to pronouncedly dorsi-ventral
organs like pulvini. In .the last-named instance, indeed, as
motility is great, the investigation has the advantage of con-
cerning itself with responsive effects which take place very
quickly.
From what has already been said, we shall be prepared
to meet with two different types of responses, according as
the transverse conductivity of the organ is feeble or consider-
able. In the former of these cases, under unilateral stimu-
lation, there is no internal diffusion of stimulus, and the side
acted on, whether above or below, will respond by concavity ;
but in the latter case — that is to say, where the transverse
conductivity of the tissue is great — it will be the more excit-
628 PLANT RESPONSE
able side which, under strong unilateral stimulation of either,
will become concave. Supposing the lower side to be the
more excitable, then, strong stimulation, whether of dorsal
or of ventral, will bring about concavity of the lower ; but
here we must bear in mind the possibility of an effect which
will be the result of moderate stimulus. The differential
effect, which brings about the concavity of the lower, even
when it is the dorsal surface that is excited, depends on
the internal diffusion of stimulation, and this in turn is
dependent not only on the conductivity of the tissue, but
also on the intensity of the stimulus. If, then, a feeble or
moderate stimulus be applied on the dorsal surface, even of
a highly conducting organ, the result will be a concavity of
that surface. The long-continued action of moderate stimu-
lus will, however, bring about a gradual percolation, and
the first response due to localised, will give place to the
differential effect of diffuse, stimulus. If the stimulus again
be very much stronger, this reversal will take place much
more quickly. We thus see that responsive movements may
be positive, neutral, or negative, according to the strength
and duration of the stimulus.
Pulvinated organ showing positive heliotropic move-
ment : (n) Terminal leaflet of Desinodimn. — We shall now
take the case of a pulvinus in which the conductivity is
feeble, and in which we should therefore expect to obtain
positive response. I have already, given response-curves in
illustration of this positive response, whether it be the upper
or lower surface of the pulvinus which is subjected to light
(p. 586). It was also shown that the leaflet moved towards
the light, and that in such cases it was the pulvinus, and not
the lamina, which was both the perceptive and responding
region.
{b) Response of leaflet of Rohinia. — The leaflets oi Robinia^
under the vertical light of the noonday sun, exhibit what is
known as diurnal sleep — that is to say, they fold themselves
upwards. This is simply an instance of the positive helio-
tropic effect common to all those pulvinated organs in which
PIIOTONASTIC PHENOMENA AND DIURNAL SEEK
629
transverse conductivity is feeble. I give here (fig. 253) a
response-record obtained with a leaflet of Robinia under the
action of sunlight from above. A similar response, but in a
downward direction, is obtained when
light is made to act from below ;
but as, under natural conditions,
light always acts from above, the
leaflets are found, when the acting
daylight is sufficiently strong, to
exhibit what is known as diurnal
sleep, or the para-heliotropic effect,
in an upward direction. I have
obtained numerous similar records
with other leaflets, which fold upwards
under the action of sunlight.
(c) Responsive movements of leaflets
of Erythrina indico. and of Clitoria
ternatea. — For the sake of simplicity
I described the movement of Robinia
leaflet as upward ; but the actual direction is one which
more or less accurately coincides with that of incident
sunlight. As further examples of this particular type of
diurnal sleep movement, I may mention the leaflets of
Erythrina indica and Clitoria ternatea
(Indian name Aparajita). Both of
these are so remarkably sensitive that
they follow the course of the sun, in
such a way that the axis of the cup
formed by the folding leaflets at the
end of the petiole is coincident with
the rays of light, and continues so
from about 1 1 A.M. till about 3 P.^L
The negative heliotropic type
of response. — We shall now pass
on to the second, or negative, heliotropic type of response
due to the internal diffusion of stimulus ; and in order
to show that there is a continuity between these and the
. 253. Positive lldio-
Iropic Response of Leaf-
let of Robinia to Sunlight
Acting from Above
Dotted line represents re-
covery on cessation of
stimulus. Time - marks
represent intervals of five
minutes.
Fig. 254. Positive Helio-
tropic Response of Leaf-
lets of Erythrina indica
The leaflets form a hollow
cup with direction of
incident sunlight as axis.
630 PLANT RESrONSE
former instances, I shall here refer once more to the
case, already mentioned, of the heliotropic response of the
terminal leaflet of Dcsniodiuni, which in winter is always
positive, but in a given experiment, under the conditions of
greater transverse conductivity which are brought on in
summer, exhibited, after two hours of continuous exposure to
vertical sunlight, a neutralisation of the previous positive
effect (p. 604).
{a) Response of pulvinus of Mimosa. — The second type of
response will be the better understood if we first study in detail
all its characteristics. As I have already said, these negative
heliotropic responses result from the transverse diffusion of
stimulus across the tissue, which brings about the concavity of
the more excitable lower half of the organ. We can easily,
in the continuous response-record to be given presently,
detect this gradual process of the percolation of stimulus
through the tissue, when light is applied from above. As
there is in the case of Mimosa a considerable mass of inter-
vening tissue between the upper and lower surfaces of the
pulvinus, it follows that unless the stimulus applied be
excessive, there will be a certain interval of time required
for its passage. We should therefore expect that on the
application of light to the dorsal surface there would be a
local contraction and concavity of that side, raising the
leaf up, and causing a preliminary positive response ; but
this movement will be arrested, and gradually reversed, so
soon as the stimulus reaches the lower side, and begins to
induce antagonistic contraction there ; and after this, the
greater excitability of the lower will be manifested by its
greater contraction, as seen in the negative response, or
depression of the leaf All this will be clearly understood
from the series of records given below.
It is necessary, however, before describing the effect of
the continuous application of light from above, to analyse
the responsive sensibilities of the two sides of this organ ;
and this is the more desirable since, in the case of Mimosa,
it is commonly assumed that excitability characterises only
PHOTONASTIC PHENOMENA AND DIURNAE SEEEP 63 1
the lower half of the organ, the responsive movement of the
pulvinus being due to that factor alone. In the course of the
present work, however, I have frequently stated that the
upper half also was excitable, and that the usual responsive
movement was due to the differential Q.y.cXV?i}a\\\\.y of the two.
It is now easy, using localised stimulus of light, to submit
this question to a crucial test.
First I took a record of the responsive movement of the
leaf of Mimosa, when the upper half of the pulvinus alone
was subjected to stimulus of sunlight. Fig. 255, a, shows
the moderate positive,
or in this case upward,
movement, which was
the result of this sti-
mulation. By means
of a properly inclined
mirror light was now
thrown vertically up-
wards, so as to strike
the lower half of the
pulvinus. The con-
sequent positive re-
sponsive movement,
in this case down-
wards, is seen to be
much stronger (fig.
255, h), on account of
the greater excitability of the lower half of the organ. The
differential character of the responsive movement under
externally diffused stimulation is shown in fig. 255, r, which
is a record of the response given b)' the pulvinus when both
its upper and lower sides were simultaneously acted upon by
light.
It will be seen that in this record, then, we have a case of
response to externally diffused stimulus. We shall next
observe the effect of stimulus which has become intenially
diffused, owing to conduction from the upper to the lower
Eic. 255.
Responses of Mimosa to Sunlii^ht
not too long Duration
() Light acting on pulvinus from above ; {/<) light
acting on pulvinus from below ; (r) light acting
simultaneously from above and below. Dotted
line represents recovery on cessation of light.
PLANT RESrONSE
half of the pulvinus. We have just seen that in the case of
externally diffused stimulation both sides of the organ are
acted upon at the same moment, and the differential response
is downward from the beginning ; but when continuous
stimulus is applied on the dorsal surface, it is at first
unilateral, and only afterwards becomes internally diffused.
We therefore obtain in this case (fig. 256), as was theoretically
inferred, all the phases of re-
sponse, positive, passing through
neutral, into negative.
A very interesting feature of
this record is the after-effect, on
the cessation of stimulus, which
is represented by a dotted line.
In Chapter XXXIV., while deal-
ing with the detection of the
latent factor, it was explained
that there are two distinct after-
effects, positive and negative
(p, 457). In the former, the
movement is simply a continua-
tion of the effect seen when
stimulus was acting ; but the
latter, or negative, after-effect,
being due to the increase of
latent energy by absorption of
incident stimulus, finds expres-
sion in an opposite movement. In the case of the response
of growth we saw that the positive after-effect consisted of a
persistent retardation, and the negative of an acceleration,
of growth. In the case of pulvinated organs, however, the
negative after effect is generally indistinguishable from the
movement of recovery ; but in the present instance we find
it clearly exhibited in the fact that the after-effect not only
causes recovery to the original position, but carries the leaf
to a distance beyond.
The positive heliotropic response of the leaf, then, persists
Fig. 256. Response of Pulvinus
of JMimosa to Action of Con-
tinuous Light from Above ap-
plied at Moment marked with \
Positive heliotropic movement
caused by excitation of upper
half neutralised by transmission
to distal side, and ultimately
reversed owing to greater ex-
citability of lower half. Dotted
line represents recovery on ces-
sation of light. Note final
erection of leaf above original
position as negative after-eft'ect
by absorbed stimulus.
PIIOTONASTK" PHKNOMENA AND DIURNAI, SI-KKI' 633
only as long as it takes the stimulus to reach the distal side
of the pulvinus. When strong concentrated sunlight is
applied from above, therefore, the duration of this upward
movement is reduced, and it appears only as a slight twitch.
The same effect is produced under less intense light when,
as in thinner pulvinated organs, the distance to be traversed
by stimulus is not so great.
{b) The diurnal sleep of Oxalis. — In consequence of strong
vertical ilkimination, as at noonday, negative heliotropic
response, with downward
folding of the leaves, takes
place in those pulvinated
organs in which there is
internal diffusion of sti-
mulus, and in which the
lower side of the pulvinus
is much more excitable
than the upper. As ex-
amples of this, we shall
take the cases of Oxalis
and Biophytuni. In the
former, if light of moderate
intensity, say from a lamp,
be applied on the dorsal
or upper surface, an up-
ward, or positive, heliotro-
pic curvature is induced.
If the same light be
applied below, a positive
heliotropic movement, in this case downwards, is again
induced ; but the fact that the lower side is much the
more excitable is seen when the upper and lower surfaces
are excited simultaneously, for there is now a downward
movement (fig. 257, a). Thus external diffuse stimulation
induces downward movement, and we shall also find that
stimulus internally diffused has the same effect. This is seen
by throwing a strong beam of sunlight on the upper surface.
Fit'.. 257. Responses of Oxalis to .Sun-
light
(rt) Shows greater excitation of lower half
and downward response when both
upper and lower are simultaneously
subjected to stimulus of light ; (/') shows
downward or negative response owing
to internal diffusion of stimulus when
upper surface only is acted on by
strong light. Arrows indicate the di-
rections of incident light.
634
PLANT RESPONSE
when the leaflet is found to respond by depression (fig. 255, b ).
Here, then, we have a repetition of those movements which we
have already seen in the case of plagiotropic stems, where
strong illumination of whichever surface always had the same
effect, that is to say the induced concavity in the lower.
{c) Diurnal sleep of Biophytiun. — In the case of Oxalis
and in that of Mimosa the responsive movement is more
or less continuous ; but in such a pronouncedly multiply-
responding organ as that of Biophytuni, the effect of internally
diffused stimulus on the more excitable lower half of the
pulvinus is to induce depression by a series of multiple
responses. This is well seen in
the following record (fig. 258),
where under the action of strong
sunlight from above the leaflet is
undergoing a progressive fall. A
record of a similar effect in the
case of Biflphytuvi is seen in fig.
123, the fall being in that case
represented by an up curve. The
specimen there was somewhat
sluggish, and there were three
pulsations in the course of ten
minutes, making an average of 3-3
minutes to each. In the present
instance, however, there were six
pulsations in the course of fifteen
minutes — each, that is to say, having an average period of 2*5
minutes. Similarly in Averrhoa the fall of the leaflet occurs
in a pulsating manner, an instance of which is given by
Darwin.^
The directive versus non-directive action of light. — It
will be well at this point to enter into the question of arbitrary
distinction which is commonly made as between heliotropic
action in radial organs and the same action in the anisotropic
or dorsi-ventral. It is clear, however, that such a distinction
' Darwin, Movements of Plants, p. 333.
Fig. 258. Negative Multiple
Response in Biophytiun
when Acted on from Above
by Strong Sunlight
niOTONASTIC PIIKNOMKNA AND DIURNAL SLEEP 635
is no longer possible when the fundamental unity of effects
in the two cases has been perceived.
But it is customary to make a further distinction between
these effects, on the ground that the movement of the radial
is determined by, while that of the anisotropic is independent
of, the directive action of light. In this matter again I shall
be able to show further that no such line of demarcation can
be drawn. It will then be seen that the differences between
the two classes of phenomena are only apparent.
In the case of radial organs, we saw that the direct action
of moderate stimulus of light, inducing concavity of the
proximal, gave rise to positive heliotropic movement. The
same is true of the similar direct action of a moderate
intensity of light on, say, the pulvinus of Mimosa, where the
upper and lower halves of the organ both exhibit positive
response. With stronger light, again, the radial organ
develops an induced anisotropy, by which the proximal
side, owing to fatigue, becomes the relatively less excitable.
Under this condition, the proximal side of the radial organ
corresponds to the less excitable upper half of the pulvinus
of Mimosa, and the distal to the more excitable lower half.
This differentiation is .seen to be continuous throughout the
three cases of the radial organ, the plagiotropic stem, and the
pulvinus, although in the first of these it is transient, lasting
only during the action of stimulus. In all these cases alike,
strong unilateral stimulus, acting on the less excitable
proximal or upper surface, becomes internally diffused, and
causes movement away from stimulus, or negative heliotropic
response, in a direction which is perpendicular to the plane
of separation of the two anisotropic halves. Thus there is
no difference between the negative heliotropic responses of
a radial and a pulvinated organ.
The distinction between the two cases of the permanent
and temporary differentiation of the organ has been illustrated
by Sachs by a reference to the induction of polarity in steel
and in soft iron respectively. According to this analogy,
MarcJiantia behaves to intense liq.ht like steel to a magnet,
636 PLANT RESPONSE
retaining permanently the induced polarity, Tropcsoluin
majus, on the other hand, behaves like soft iron, assuming a
definite but temporary polarity, which disappears when the
influence of light, like that of the magnet, is withdrawn. This
illustration, however, though vivid, is likely to be misleading,
for it suggests that, under strong stimulus, the normal pro-
perties of the organ pass into a polar or opposite condition.
But there is no such change. Light merely induces a
difference of excitability in the two sides. That there is no
reversal of heliotropic sensibility is shown by the fact that
either side, when excited by moderate intensity of stimulus,
gives positive response. The necessary condition for the
exhibition of negative heliotropic response is not only the
differential excitability of the two sides of the organ, but also
the internal diffusion of stimulus.
Owing, however, to the permanence of this differentiation
in a pulvinated organ, there will be certain conditions under
which the action of light will appear to be non-directive,
for it is the diffuse stimulus, however produced, that brings
about the responsive concavity of the more excitable lower
half of the pulvinus, and this internal diffusion will take
place just the same, whatever be the flank of the organ on
which the strong external stimulus may have been applied.
In the case of the radial organ, on the other hand, the
differentiation between the less excitable proximal and more
excitable distal is not fixed, but changes according to the
side acted upon at the time by light.
The different responsive movements induced by light —
positive, negative, dia-heliotropic, and para-heliotropic— have
hitherto been ascribed, as I have already had occasion to
point out, to as many specific sensibilities possessed by the
plant. They have now, however, been demonstrated to be
but so many examples of the general law that plant-organs
respond to stimulus, by the induced concavity of the relatively
more excited.
There is thus no fundamental discontinuity between the
responses of radial and of dorsi-ventral organs, or between
PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 637
positive heliotropic and negative heliotropic response. This
will be seen still more clearly in the following statement,
applying to the two extreme cases of radial and of dorsi-
ventral organs, the plagiotropic constituting an intermediate
link between the two.
General view of responsive curvatures induced in
different organs by unilateral application of light
1. Radial organs : (a) Positive irspousc -. stimulus localised,
the proximal side more excited and concave ; e.g. Sinapis
under moderate unilateral light (p. 609).
{b) Intermediate or neutral response : stimulus internally
diffused, causing equal excitation of proximal and distal sides ;
the organ takes up a neutral (so-called dia-heliotropic) posi-
tion, at right angles to light ; e.g. Sinapis under moderately
strong unilateral light (p. 609) ; certain tendrils apparently
insensitive.
{c) Negative response : strong stimulus internally diffused,
which also induces anisotropy, the distal being then the more
excitable ; concavity of the more excited distal ; eg. seedling of
Sinapis, and tendril of Vitis under strong unilateral sunlight
(pp. 609, 611).
2. Dorsi-ventral organs : {a) Positive response : stimulus
localised by relative non-conductivity of organ : the more
excited proximal side concave ; e.g. positive movement of
terminal leaflet of Desniodiiun ; the diurnal sleep movements
of Robinia, Erythrina indica, and Clitoria ternatea (p. 629).
{b^ Intermediate or neutral response : stimulus internally
diffused ; upper and lower halves equally excited ; e.g.
terminal leaflet of Desuiodiuni, under several hours of
vertically acting sunlight (p. 604).
{e) Negative response : strong stimulus acting from above
on highly conducting organ, of which lower half is con-
siderably more excitable ; concavity of the more excitable
lower half ; e.g. fall of Mimosa leaf under strong illumination
from above ; diurnal sleep movements of Oxalis, Biophytum,
and Averrhoa.
638 plant response
Summary
The responsive action of growing organs being not
essentially different from that of pulvinated organs, a
similar explanation must be applicable to the two classes of
phenomena.
Owing to long-continued unilateral excitation by light, the
upper side of a plagiotropic stem is rendered the relatively
less excitable. Hence such organs may be regarded as
equivalent to diffuse pulvinoids, of which the upper side is
the less excitable.
In both these cases, of pulvini and diffuse pulvinoids, the
local application of moderate stimulus on either side induces
normal positive response.
But with long-continued application of strong stimulus
two types of response may be obtained, according as the
organ is characterised by a feeble or high power of trans-
verse conductivity. In the former of these cases the stimu-
lus will remain localised, and the response will be positive.
In the second, the stimulus will become internally diffused,
and if the lower side be the more excitable, stimulation of
the upper will give rise to the concavity of the lower, con-
stituting negative response.
As examples of the first of these classes may be
mentioned the diurnal sleep movements of Robinia, Erythrina
indica, and Clitoria teniatea, in which vertical illumination
induces upward folding of the leaflets.
As representing the second class, we have both plagio-
tropic and pulvinated organs.
The plagiotropic stems of Mimosa, Ipomcca, and Cucur-
bita exhibit increasing concavity of the lower side with the
duration of the day's illumination.
A periodic downward movement is thus induced in
them, which reaches its maximum at the end of the day.
The reverse movement, of gradual erection, occurs during the
night.
In pulvinated organs, negative heliotropic response, with
PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 639
downward folding of the leaflets, is exhibited in the diurnal
sleep movements of Oxalis and BiopJiytiiin.
In Mimosa the application of moderate intensity of light
from above induces upward or positive heliotropic move-
ment. Strong and long-continued application of light on
the same upper surface, however, owing to the internal
diffusion of excitation, induces concavity of the lower and
more excitable side, or negative heliotropic movement. Both
these responses may be regarded as cases of the directive
action of light ; but if the organ be excited by stimulus
which is externally diffuse, the responsive movement is still
downwards. The directive action of light has thus passed
into non-directive.
CHAPTER XLVI
ON DIA-HELIOTROPISM AND DIA-CxEOTROPISM
Difficulty of distinguishing between effect of light and other reactions— Theories
of f^rank and De Vries— Subsidiary factors: (i) Epinasty and hyponasty ;
- (2) Effect of gravity; (3) Effect of suctional activity and of turgescence ;
(4) Modification of effect by characteristic Hmits of flexibiUty — Discrimination
of the part played by heliotropism in the movement of the leaf — Proof of
absence of any specific dia-heliotropic tendency in leaves— The lamina not
the perceptive organ — Principal types of the response of leaves to stimulus of
light — Positive type of response : mango leaf — Negative type of response :
leaf of Artocarpus.
We shall now take up the question of the effects of
illumination on ordinary leaves, which are generally supposed,
on account of sonne special dia-heliotropic sensibility, to have
the habit of placing themselves at right angles to incident
light. We have seen in the last chapter that the movement
of anisotropic organs under heliotropic stimulation is not the
result of any specific sensitiveness, but constitutes a simple
instance of the response of plant-organs to all forms of
stimulus, the response being appropriately modified in this
particular case by the anatomical and physiological pecu-
liarities of the responding organ. We have been able to
demonstrate the continuity of this responsive phenomenon
by analysing it in two extreme cases of the anisotropic
differentiation, those namely of the plagiotropic stem, in
which we see one of its earlier phases, and of dorsi-ventral
pulvinated organs, in which it attains its highest development.
In the movements of ordinary leaves we have what is merely
an intermediate stage between these, hence any explanation
which would elucidate their action must be one which is
applicable to both the extremes.
1)IA-1I1;LI()TK()I'ISM and DIA-GKOTKOI'IS.M 641
Difficulty of distinguishing- between effect of light and
other reactions. — In the case of the response of pulvinated
organs, we had the advantage, owing to their great motility,
of being able to refer each particular responsive movement
to the immediate stimulating action of incident light. In
ordinary leaves, however, the movements being very sluggish,
it takes so long a time for any given responsive action to
attain the requisite magnitude for ordinary observation, that
other factors of variation intervene,. and it becomes difficult
to know how much of the resultant response is due to
heliotropic stimulus as such. It is this great difficulty of
disentangling the response due to light, which is the proper
subject of the inquiry, from numerous other subsidiary factors
that has led to the existing divergence of views among
observers.
In the course of the present chapter, therefore, I shall
shortly enumerate those various agencies which are subsidiarily
instrumental in bringing about the ultimate position assumed
by the leaf I shall then describe a method by which helio-
tropic action proper can be discriminated with certainty from
other influences. The relation between the fundamental action
in response to light, which has already been demonstrated, and
the heliotropic response of the leaves will thus be made
apparent. But before entering upon these questions I shall
briefly allude to the principal existing theories on this
subject.
Theory of Frank. — There are at present two main types
of opinion with regard to the question of the effect of light on
leaves. Frank and his school account for that action of leaves
by which they place themselves with their flat surfaces perpen-
dicular to the direction of incidence of the rays of light, or of
the action of gravity, by assuming that dorsi-ventral organs
possess a peculiar propert)' of sensitiveness to the directive
action of light and gravity. This they designate as Transverse
or Dia-heliotropism, and Transverse or Dia-geotropism. It is
supposed that the habit has been acquired for the advantage
of the. plant, inasmuch as the leaves, by placing themselves
T T
642 PLANT RESPONSE
in this particular position, are enabled to absorb the largest
amount of sunlight. Such arguments, however, do not throw
any light on the mechanism by which the movement is
brought about. It is even somewhat difficult to understand
how this generalisation, that leaves place themselves at right
angles to light, has come to be accepted as a universal fact ;
for it is only necessary to make a visit to the open forest in
order to see that, so far from this being the case, many leaves
place themselves vertically upwards, others downwards, and
the rest in all possible intermediate angles between the two.
All that can be claimed on behalf of the dia-heliotropic
position is that no deviation from it is greater than plus or
minus 90''.
Theory of De Vries. — De Vries, however, is in disagree-
ment with this theory. He establishes the importance of the
unequal rates of growth in the upper and lower halves of the
leaves — i.e. epinasty or hyponasty — as a factor in bringing
about their ultimate attitude. He next assumes, in accordance
with the generally accepted view, the existence of two opposite
responsive reactions, positive and negative, in regard to light
and gravity. He then proceeds to show that there can be no
necessity for the further assumption of a third or dia-heliotropic
tendency in the leaves, since epinasty and hyponasty, their
different combinations with either of the opposite actions
of positive and negative heliotropism and geotropism, and
considerations of the weight and balance of parts, are all
factors which take a share in determining the position ulti-
mately to be assumed by the dorsi- ventral organ with regard
to light.
I shall now attempt to demonstrate the fact that the
responses of ordinary leaves are in every way similar to
those of the pulvinated, the mechanics of whose movements
have already been fully described. It will further be shown
how and under what circumstances the normal positive passes
into negative response, through certain intermediate phases.
Subsidiary factors.— But before undertaking either of
these inquiries, it will be well to enumerate briefly the
DIA-lIELIOTROPISiM AND DIA-GEOTROPISM 643
subsidiary factors which combine with the heliotropic action
proper in determining the final position of the leaf.
I. Epinasty and hyponasty. — These unequal growths of
one side or the other, De Vries believed to be brought about
by some spontaneous unknown cause in the plant itself,
Detmer, however, came to the conclusion that, as regarded
epinasty, it was not spontaneous, but induced by the action
of light. This conclusion was based on the observations
(i) that cotyledons of Cncurbita remained closed up, in
continuous darkness, but opened out when subjected to light ;
and (2) that the primordial leaves of Phascoliis, kept in
darkness, remained folded, and only opened out on illumina-
tion. The investigations of Vines, however, though he
supports the contention of Detmer with regard to Phaseolns,
have led him to a different view in the case of Cucurbita,
where he finds the epinastic movement to be induced even
in the absence of light. He has therefore come to the
conclusion that the phenomena of epinasty and hyponasty
are spontaneous, and not directly due to the action of
illumination.
These differences of opinion, however, have arisen from
the obscurity in which autonomous or spontaneous movement
has been involved, and they may be expected to disappear
when its true nature is clearly perceived. These alternate
movements of growth are in fact only another example of
multiple or autonomous response, the difference between
them and those other forms with which we are already
acquainted lying in the greater slowness of their period, and
in the relative fewness of the pulsations that can be exhibited.
In the case of autonomous pulsation or circumnutation of
stems, since the growth on which they depend is indefinitely
prolonged, we have an indefinite continuance of these pulsa-
tions. In the case of lea\es, however, the organ usually
completes only half its swing, whether epinastic or hyponastic,
and by that time further pulsation is arrested, owing to the
cessation of growth ; yet in some cases there is more than
a semi-pulsation, as where hyponastic movement is followed
r T 2
644 PLANT RESPONSE
by epinastic. It has already been shown that autonomous
response in general can take place only when the internal
energy or the sum total of the latent stimulating factors is
above par. This is equally true of the autonomous response
of growth itself In the case of autonomous epi- or hypo-
nastic pulsations, therefore, a certain amount of internal
energy is essential for their initiation. This is in some cases
supplied by other forms of stimulus, but there may be others
in which light is the critical factor. In order, then, to study
the directive action of light on dorsi-ventral organs under
normal tonic conditions, we must be able to determine the
characteristic influence which will be exerted by the stimulus
of incident light in modification of the already existing
movement of the organ. This inquiry is therefore exactly
parallel to our previous study of the action of light in
modifying the existing growth-movements of radial organs.
In that case the variations induced in the ordinary rate of
movement afforded us a measure of the effect of the stimulus
of light. In the case of dorsi-ventral organs such as leaves,
similarly, the effect of light can be correctly inferred only by
observing the variations which it induces in their existing
movements. The manner in which this is done will be
described presently.
2. Effect of gravity. — In long-continued experiments on
the curvatures induced by light, the observed movement is
also modified in part by the influence of geotropism. This
geotropic action in leaves is by some investigators believed
to be of two types, negative and positive. Others, again,
regard it as dia-geotropic. I shall, however, adduce con-
siderations which will show that the upper and lower halves
of dorsi-ventral organs exhibit differential excitability to geo-
tropic as to other forms of stimulus. In order to neutralise
the geotropic action, and thus study the heliotropic effect
alone, Francis Darwin mounted the plant on a rotating
klinostat. It is true that in a strictly radial organ the
geotropic effect is successfully neutralised by rotation on the
klinostat, since in this case the geotropic sensitiveness of the
DIA-IIELIOTROriSM AND DIA-GEOTROPISM 645
different flanks is the same ; but this, as will appear from
certain experiments to be described later, cannot be said of
dorsi-ventral organs, for in these geotropic sensitiveness is
different in the upper and lower halves. In my own experi-
ments on the action of light, however, I shall be able to give
results which are but little affected by geotropism.
3. Effect of suctional activity and of turgescence. — There
are two other factors not hitherto taken into account which
exert considerable influence in determining the attitude
finally assumed by the leaf These are the general condition
of turgescence of the plant, and the limits of flexibility
which characterise the particular responding organ. The
first of these is, as we have seen, dependent on the suctional
activity. Now, the mechanical response of a plant-organ is
the result, as we already know, of the expulsion of water
from the excited tissue ; but if the tissue be over-turgid
expulsion of water is opposed, and the responsive movement
is thereby reduced or abolished, as we have seen in the case
of Mimosa in a condition of excessive turgor. The same
phenomenon I have again seen manifested, in a remarkable
manner, in the difference of the movements made by the
leaves in response to light according as they were normally-
or super- turgid. Thus, in a small plant of Artocarpits,
grown in a pot, and in the autumn season, when the suctional
activity was not great, the leaves responded to light by
making a progressive angle with the vertical, so that, under
the long-continued action of light, they first reached the
horizontal position and then fell many degrees below it ; but
in the rainy season, when they held them.selves abruptly
vertical in consequence of excessive turgor, the action of
light produced little or no responsive movement. In the
autumn season, again, the limited system of roots and rootlets
possessed by this plant, when grown in a pot, allows it only
a moderate degree of turgescence, and in this condition it
readily responds to light ; but a large tree of the same
species, growing in the open and possessed of a highly
ramified and extensive root-system, will during the same
646 PLANT RESPONSE
season, its turgescence being great, maintain its leaves in a
vertical position, but little affected by the action of light.
From this it will also become clear that any influence, such
as long maintenance under darkness, which modifies the
suctional activity, is liable to render the response of the plant
abnormal.
4. Modification of effect by characteristic limits of flexi-
bility.—\ shall here deal with another important factor in
the determination of the final position assumed by the
leaf — that is to say, with the anatomical peculiarities which
determine the limits of flexibility. Let us take the petiole
bearing the terminal leaflet of a leaf of Desniodium. We
now suppose this leaflet to be outspread, in such a position
that its midrib is in a continuous straight line with the
petiole. This upper line we shall know as the dorsal line.
It consists of two parts or components, the laminal and the
petiolar. In this particular position they form a straight
line ; but the movement of the leaflet takes place with the
point of junction as the hinge. We shall then distinguish
that particular position of the dorsal line in which the laminal
and petiolar halves are continuous and straight as neutral,
and angular movements above or below will be measured
accordingly. In the case of Desniodium, when the terminal
leaflet has reached the neutral position, it cannot, owing to
the anatomical peculiarity of the joint, be bent further up-
wards ; but it can be bent in the opposite direction, that is
downwards, until the leaflet lies along the under side of the
petiole, the curvature being then through 180°. Thus the
limits of flexibility of this leaflet may be expressed by the
formula : that is to say, its flexibility above the
180
neutral position is 0°, and below, 180°. Now, the petiole
itself, in the case of Desniodium, for example, may, and does,
occupy many different positions with regard to the stem.
Let us suppose it at a given moment to subtend an acute
angle, being thus more or less vertical, and suppose the
terminal leaflet to be horizontal. If light now acts from
DIA-HKLIOTROPISM AND DIA-GKOTROPISM 647
above, the leaflet will move continuously upwards till the
lamina has reached the neutral position— that is to say, till the
midrib constitutes a straight prolongation of the petiole ; but
if light act on the leaflet from below, it will bend downwards
(fig. 259, a). If the petiole, however, at the beginning of the
observation be horizontal instead of vertical, and the lamina
be in the neutral position, then vertical light cannot, owing
to the anatomical peculiarities of the pulvinar joint, carry the
leaflet further above the dorsal line (fig. 259, /;;. Or we may
=:i-:::?^r ^ -' ~::^
a
Flc. 259. Difl'erent Limits of Flexibility
Vertical light on terminal leaflet of Desmodiiim causes (a) up movement
till the dorsal line is a continuous straight line ; light applied below
causes movement downwards below this neutral line through iSo°.
In /' is shown neutral position, after reaching which there is no further
movement upwards. In c is shown movement of terminal leaflet of
Erythrina iitdica upwards, under vertical illumination, through 180°
above the neutral line.
again take as an example the terminal leaflet of Erythrina
indica (fig. 259, c). The limit of flexibility is in this case
represented by an angle of almost 180° above the neutral,
whereas downwards its limit is about 90°. The formula is
1 80°
thus . Now, when this leaflet is acted upon bv light
90°
from above, it may become almost doubled upon the petiole
upwards, just as we found the terminal leaflet of Desmodiuiii
to be almost doubled downwards. Hence we see that
though the heliotropic eflect of light is always the same, \et
648 PLANT RESPONSE
the position of the leaflet in space — that is to say, its relation
to the vertical line — is largely modified not only by the limits
which the anatomical structure of the laminal pulvinus or
pulvinoid imposes upon its flexibility, but also by the angle
which the petiole makes with the stem ; for there is,
generally speaking, a second pulvinus or pulvinoid at the
junction of the petiole and the stem, and this petiolar
pulvinus is, in its turn, more or less sensitive to stimulation
(p. 59). We have thus obtained some idea of the anatomical
elements, regarding the petiole and its joints, which enter
into the complex question of the final position assumed by
the leaf in space.
Discrimination of the part played by heliotropism in
the movement of the leaf. — In studying the heliotropic
effect we are concerned only with the action of light itself,
and not with the resultant effect, due to various co-operating
factors. When the action of each of these is definitely
understood, it becomes a simple problem to understand the
effects due to their combination. The difficulty of this
investigation has hitherto lain, as already said, in the fact
that, owing to the generally sluggish nature of respon-
sive movements in ordinary leaves, a long time must be
allowed to elapse before they become measurable, and during
this long period other factors may become operative in
unknown ways. The effect of light, however, can easily be
discriminated by the use of the Optic Lever for record, for this
allows us any degree of magnification which may be desired.
Thus the natural curve gives us the resultant effect of all the
pre-existing factors under normal tonic conditions, and its
subsequent variations under the incidence of light at once
exhibits the distinctive action of that stimulus. On the
withdrawal of light, again, the recovery from the induced
variation affords an additional corroboration of the inference
that the variation itself had been due to the action of light.
In consequence, moreover, of the delicacy of this means of
continuous record, the characteristic effect can be detected
in the course of a few minutes, thus eliminating the unknown
DIA-HELI()TR()I'ISM AND DIA-GEOTROI'ISM
649
changes which are Hable to occur during the lapse of long
intervals of time. The action of light is, generally speaking,
so predominant over that of the subsidiary factors, that a
high magnification of record is not necessary. Under these
circumstances, that is to say under low magnification, the
record before the application of light is practically a hori-
zontal line. Under vertical illumination of the upper side of
the leaf, then, deviation above this horizontal represents
a positive heliotropic movement, while deflection in the
opposite direction signifies the
negative.
But before I proceed to
give details of my experi-
ments, I shall adduce facts
which will show that there is
no inherent tendency in the
leaves to move in such a
manner as to place themselves
at right angles to the light.
Proof of absence of any
specific dia-heliotropic ten-
dency in leaves. — A Des-
inodium plant was taken, and
the petiole fixed horizontally,
Fig. 261. Curve sliowiiig Time-
relations of the Responsive
Angular Movement of Terminal
Leaflet of Desinodiiini under
Light, as represented in the
last figure
The original angle of 45° between
the surface of the leatlel and the
direction of incident light was
reduced to 13" in the course ot
25 minutes.
iG. 260. Movement of Ter-
minal Leaflet of Destnodiui/i
placing itself Parallel to
Incident Horizontal Light
the terminal leaflet being at an angle of 45° with the horizon.
Sunlight was then made to strike it horizontally (fig. 260). It
now the leaflet had a dia-heliotropic tendency, it is clear that
its movement would be such as to increase its angle with the
horizon to 90°, thus bringing the light to strike it at right
650 PLANT RESPONSE
angles. Movement in the other direction — that is to say,
towards the decrease of the angle — would, on the other hand,
indicate that the effect of light was to induce the normal
positive heliotropic response. From the record (fig. 261) it
will be seen that the latter was the case — that is to say,
the leaflet moved continuously, tending to become parallel
to the light, having in the course of twenty-five minutes
moved from its position at an angle of 45° to one at 13° to
the horizon — that is to say, at an average rate of angular
movement of about 1-3° per minute.
The lamina not the perceptive organ.— If, again, the
object of the so-called dia-heliotropic movement had been
the absorption by the upper surface of the lamina of the
largest possible amount of light, it would have been neces-
sary for the lamina to be the perceptive organ, determining
its movement according to the direction of stimulus ; but
that this is not the case may be demonstrated by subjecting
the lamina alone to the action of light, and covering the
pulvinus with a small opaque shield of black paper. On
doing so all movement is found to be arrested. If now, on
the other hand, the leaflet be covered with opaque paper, and
the pulvinus be left exposed, the usual heliotropic movement
is found to take place. This conclusively proves that, as
regards the respon.se of the leaf to light, the lamina is not the
perceptive organ, and therefore that a supposed advantage to
itself cannot be the important factor in determining its move-
ment. That the lamina is not the perceptive organ has
indeed been shown already in the case of ordinary leaves,
when subjected to forms of stimulus other than light (p. 60).
We saw this, for example, in the case of a \ea.( of Ar^oc■arp^ls
when subjected to electrical and thermal stimulation. It was
in that case shown that since the lamina consisted of a mass
of non-conducting parenchymatous tissue, local excitation
might be caused by the incidence of stimulus, but could not
be effectively transmitted to the distant pulvinus or pulvinoid
by which the movement of the leaf was brought about. The
only ca.se in which such transmission is possible to some
DIA-HELIOTROPISM AND DIA-GEOTROl'ISM 65 I
extent in leaves is in the parallel-veined leaves of monocoty-
ledons. This fact, that the lamina is not in general the per-
ceptive organ with regard to stimulus, say of light, is still
further demonstrated in the following experiment, performed
on the leaflet of Erythrina indica. Sunlight was first applied
locally to the lamina of this leaflet by means of a suitably
inclined mirror, at the point in the record which is marked x
(fig. 262). It will be noticed that during ten minutes of such
application not the slightest responsive movement was in-
duced. Keeping everything else the same, the light was
Fig. 262. Record showini; thai Lamina is not the Perceptive Organ
Light applied vertically at x on lamina t)f Erythrina indica, and con-
tinued for ten minutes, produces no response ; but when applied on
the puh inus at the arrow { \ ) there is immediate positive response.
Dotted line shows positive after-effect and recovery on cessation of
light.
now shifted by a movement of the mirror, and thrown
directly on the pulvinus, at the point in the record which is
marked with an arrow (t). It will be observed that the leaf
began to respond immediately by a movement towards the
light, and in the course of seventeen minutes' exposure its
tip moved through a distance of 20 mm. or at an average
rate of a little over i mm. per minute. On the withdrawal
of light the movement persisted, owing to the positive after-
effect, for a period of seven minutes, after which recovery
began. The marked difference between the quiescent con-
652 PLANT RESPONSE
dition of the leaflet during the ten minutes' exposure of the
lamina to the action of light, and its energetic movement
immediately on the application of light to the pulvinus,
shows once more, in a striking manner, that it is the latter
organ and not the lamina whose perception of light is effec-
tive in initiating the responsive action.
Principal types of the response of leaves to stimulus
of light. — I shall now proceed to show that the directive
effect of light on leaves is very definite. In studying the
response of anisotropic organs — that is to say, plagiotropic
shoots and dorsi-ventral pulvinated leaves — we have seen
that there are two extreme types of response, of which the
first is exhibited by organs possessing feeble transverse con-
ductivity, and the second by those in which the transverse
conductivity is great, and the lower side the more excit-
able. In the first of these types, light acting on the organ
from above gives rise to a positive heliotropic response. In
the latter, long-continued application of strong light from
above gives rise to internal diffusion of stimulus, causing
concavity of the more excitable lower half, with the result of
negative heliotropic response. Intermediate between these
we have seen that there are cases of the equal excitation ot
the upper and lower halves of the organ, bringing about the
so-called dia-heliotropic position. As examples of the two
extreme types — within which lie the responses of all ordinary
leaves — I shall here give records of the movements of leaves
of Mango (^JMangifera indica) and o{ Artocarpiis.
Positive type of response : Mango. — The leaves of
this plant when young are bent abruptly downwards by the
sharp curvature of the short petiole. In the course of a week
or so, however, they rise, and gradually attain a position at
or above the horizontal, a process in which the action of
light is an important agent. This will be seen from the
following record of the movement of a Mango leaf when
acted upon by sunlight from above (fig. 263). The record
before the application of light was practically horizontal,
showing that there was little natural movement ; but the
DIA-IIKLIOTROI'ISM AND DIA-(;i:()TK()l'IS.M
^^53
application of light induced an energetic movement upwards —
that is to say, a positive heliotropic response, the tip of the
leaf moving through a distance of 35 mm in the course
of sixty-five minutes —
that is to say, at an
average rate of -53 mm.
per minute. On the
cessation of light there
was a slow recovery,
which was found to be
only partial.
Negative type of
response : Artocar-
pus. — As an example
of negative response by
the internal diffusion
of stimulus, due to the
high transverse con-
ductivity of the tissue,
and the greater excita-
bility of the lower half
of the pulvinoid, I give
here two records, from
leaves of different ages, borne on the same plant, under the
action of strong sunlight from above (fig. 264). The upper of
these two records was taken from a young leaf, which was
second in order from the top of the shoot, and the lower from
the fourth. It is generally found that motile sensitiveness is at
its greatest in leaves which are neither too young nor too old.
In the present experiment the upper of the two leaves,
which was very young, gave a negative heliotropic response,
the tip moving through 15 mm. in the course of eighty
minutes, or at an average rate of almost '2 mm. per minute.
In the case of the lower leaf the rate of the responsive move-
ment was more rapid, being on an average -5 mm. per minute.
As regards leaves, then, their responsive movements under
light have been shown to be very definite, and simply
Tnm
35
/ N
30
-
/
Z5\
/
20
/
15
10.
: A
5.
, 10' 20' 30' 4C
)' 50' 60' 70' 80' 90'
;. 263. Positive Heliotropic Movement of
Leaf of Maiigife7-a indica under Sunlight
acting from Above
Dotted line shows
recovery on cessation of
light.
654
PLANT RESPONSE
determined by the fact that it is always the more excited
side of the motile organ that undergoes contraction and
concavity. That various types of this effect arise is due only
to the unequal excitabilities of the two halves, and to the fact
that the stimulus remains
in some cases localised,
while in others, owing to
the better transverse con-
ductivity of the tissue, it
becomes internally diffused.
The position ultimately
taken up by the leaves is
thus determined primarily
by the action of light, and
secondarily by that of
various subsidiary factors,
which are : (i) the natural
movement of the organ,
due to hyponasty or epi-
nasty ; (2) the differen-
tial excitability of the
two halves of the dorsi-
ventral organ under the
stimulus of gravity ; (3) the turgescent condition of the plant,
as determined by its suctional activity ; and (4) the limits of
flexibility of the organ, as determined by its anatomical
peculiarities. It is out of the innumerable possible combina-
tions of these factors that the variety of attitudes ultimately
assumed by the leaves directly arises.
With regard to the nature of geotropic action on leaves,
it will be shown in the next chapter that here also, as in the
case of dia-heliotropism, the assumption of a dia-geotropic
property is unnecessary ; for the observed effects are ex-
plained by the fact, which I shall demonstrate, that a dorsi-
ventral organ possesses differential geotropic sensibility.
Fig. 264. Negative Heliotropic Response
of Leaves of Arfocar^ns \mder Sunlight
acting from Above
Tlie upper record exhibits the response of
a very young leaf, second in order from
the top of the shoot. The lower record
shows that of an older leaf, fourth in
order on the same shoot.
DIA-lIELIOTKOl'ISM AND DIA-(;EOTROl'ISM 655
SUMMAR\
The position ultimately assumed by a leaf is determined
by heliotropic action and other subsidiary factors.
These subsidiary factors are: (i) the natural movement
of the organ, due to epinasty or hyponasty ; (2) the differ-
ential excitability to gravitational stimulus of the two halves
of the organ ; (3) the effect of suctional activity and of
turgescence ; and (4) the modification of effect by the
characteristic limits of flexibility of the organ.
As regards the action of light, the lamina is not the
perceptive organ, and there is no specific dia-heliotropic
sensitiveness possessed by the leaves.
The sensibility of the pulvinoid of a leaf is essentially
similar to that of a pulvinus. There are two main types of
response given by pulvinoids : (i) that exhibited when the
conductivity of the organ is feeble, and vertical light induces
movement upwards, ox positive response ; and (2) that exhibited
when conductivity is great, and the lower half is the more
excitable, inducing movement downwards, or negative response.
The various attitudes assumed by the leaves are the joint
effects of these responsive actions, and their modification by
epinasty or hyponasty, the differential action of gravity, the
turgescent condition of the plant, and the limits of flexibility
of the pulvinus or pulvinoid.
CHAPTER XLVII
TORSIONAL RESPONSE TO HELIOTROnC AND GEOTROPIC
STIMULUS: AUTONOMOUS TORSION AND ITS VARIATIONS
Torsional effect — Method of recording torsional response — Torsional response
under the lateral action of light — Torsional response to other forms of lateral
stimulation — Torsional response of compound strip of ebonite and stretched
india-rubher — Modification of torsional response by artificial variation of the
relative excitabilities of the two halves — Laws of torsional response — Demon-
stration of differential geotropic excitation in a dorsi ventral organ - Torsional
response to lateral geotropic stimulation — Modification of torsional geotropic
response by artificial variation of differential excitability — Autonomous torsion:
effect of temperature— Effect of light — Effect of electrical current — Effect of
gravity — The twining of stems.
It has been shown in the course of the last chapter that the
movement of the leaf in response to light constitutes a
simple instance of that general reaction of plant tissues to
stimulus with which we have now become familiar, and that
no specific sensibility requires to be postulated in order to
account for it.
Torsional effect. — There is one effect, however, which
has hitherto appeared to be inexplicable, except on the
supposition that some such specific sensibility had actually
been acquired by the leaf for the definite purpose of sub-
serving the advantage of the plant by placing the upper
surface of the lamina at right angles to incident light. A
leaf when struck laterally by light undergoes a torsion, which
carries the upper surface of the lamina into such a position
that it faces the light. No result could seem at first sight
more conclusively to support the theory of dia-heliotropic
sensibility. Before going further into this question, I shall
give records of the actual effect observed. In fig. 265 is
shown a curve which exhibits the increasing torsion induced
HELIOTROPIC TORSIONAL RESPONSE 657
by lateral application of sunlight in the terminal leaflet of
Desmodium. A light index was attached transversely to
the lamina, by means of which, and with the help of a pro-
tractor, the gradually increasing torsion was measured at
definite intervals of time. It will be seen from the curve
Fig. 265. Torsional Response of Terminal Leaflet of Desnwdiitm under
the Action of Lateral Sunlight
Abscissa represents lime in minutes, and ordinate angular movement
in degrees.
thus obtained, of which the abscissa represents time, and
the ordinate the induced angular torsion, that in this
case, within twenty-five minutes, a torsion of 14° had been
induced.
Method of recording- torsional response. — For the
purpose of certain investigations, presently to be described,
in connection with this torsion, it became necessary to devise
special experimental arrangements by which a continuous
record of the rate of torsion and its variations could be
obtained. The mode in which this was accomplished will
be understood from the illustration in fig. 266. A mirror,
carried on a light spring-clip made of aluminium, is slipped
over and attached to the petiole, at a short distance from the
pulvinus or pulvinoid, which is the seat of the torsional
movement. As the object is to record only the torsion, the
U U
658
PLANT RESPONSE
vertical up and down movement, if there be any, is prevented
by a lateral support, which has at its end a smooth bent rod
of glass, in the concavity of which the free petiole rests. If
there be any responsive torsional movement, a spot of light
reflected from the mirror will now undergo a vertical deflec-
tion. This is, for convenience of the record, converted into
lateral by reflection from a second mirror suitably inclined.
The response record is then taken in the usual manner on
Fig. 266. Torsion-recorder
M, mirror slipped over petiole by light aluminium clip behind ; G, bent
glass piece supporting petiole to prevent vertical movement.
a revolving drum. The absolute value of the angular
movement shown in the record can be determined from a
knowledge of the distance from the mirror of the recording
drum.
Torsional response under the lateral action of
light. — The pulvinus of the leaf, say of Mimosa, is now
stimulated by throwing upon it a horizontal beam of light,
which strikes it laterally. By the word lateral is here meant
either the right or left flank of the pulvinus or pulvinoid,
consisting of part of the upper and part of the lower aniso-
tropic halves. This causes a responsive torsion, by which the
petiole is rotated, the tendency being for the upper, or less
excitable, half to face the stimulating agent. The up curve,
then, in this and the following records will represent a torsional
movement by which the less excitable upper side is made to
IlELIOTROPIC TORSIONAL KKSI'ONSK
659
face the source of stimulus. In the present case (fig. 267)
we see that the responsive torsion took place during the
application of light, and that on the cessation of stimulus
there was a positive after-effect, followed by recovery. If
light were applied laterally on the
opposite flank, the torsion would be
found to take place in the opposite
direction, the law which determines
such movement being, as said before,
that the less excitable is always
turned towards the stimulus. In
connection with this, we have to
notice, in the first place, that the
responsive torsion takes place not
when the lamina, but when the
pulvinus, is acted on laterally by
light ; and, secondly, the particular
torsion in question results from the
differential action of stimulus, by its
lateral application to a complex
organ, the lower half of which is the
more excitable.
Torsional response to other forms of lateral stimu-
lation.— The supposition that this torsional response is due
to a specific sensibility to light, evolved for the advantage of
the plant, will be found entirely untenable if it can be shown
that the same movement is manifested under the same
conditions in response to other forms of stimulus. Thus, on
bringing a heated wire near that side which was previously
excited by light, I obtained exactly the same torsional
response of the same sign. Still more interesting is the
excitation of the same lateral side by chemical stimulus, say
a strong solution of common salt. In the record given
(fig. 268) we see how similar in every way are this response
and that evoked by light. The upward arrow (t) indicates
the application of chemical stimulus to the same flank as had
previously been stimulated by light, of which the record is
Fu;. 267. Torsional Re-
sponse of Leaf of Miinosa
when Laterally Stimu-
lated by Sunlight
The ordinate gives the ab-
solute angular movement
in degrees. The dotted
line shows positive after-
effect and recovery on
stoppage of light.
66o
PLANT RESPONSE
given in fig. 267. The downward arrow (i) indicates the
application of the chemical solution to the opposite flank. It
will be noticed that we have in consequence, in the dotted
portion of the record, a reversal of the first torsional response.
Torsional response of compound strip of ebonite and
stretched india-rubber. — Enough has already been said to
demonstrate the fact that the torsion induced in leaves by the
lateral application of light is not due to any specific sensibility
to light as such. I shall next therefore proceed to show that
the same torsion is the mechanical result of the differential
Fk;. 268. Torsional Response of Leaf
of Mimosa to Lateral Chemical
Stimulus
Upward arrow (f) indicates moment
of application of stimulus to one
flank ; ( J, ) indicates application of
stimulus to opposite flank, with effect
of reversing torsional movement.
Fig. 269. Torsional Re-
sponse of Complex Strip
of Ebonite and India-
rubber under Lateral
Action of Strong Radia-
tion
contraction of a complex organ, which is fixed at one end,
and subjected to lateral stimulation. I have in a former
chapter described an artificial model of the pulvinus of
Mimosa, which consisted of a compound strip, the upper half
of which was ebonite, and the lower the more contractile
stretched india-rubber. If such a strip be held securely at
one end in a clamp, and if the lateral flank, consisting half
of ebonite and half of india-rubber, be subjected to the
strong action of light, records being taken in the usual
manner, it will be found that a torsional response takes place
which is in every respect similar to that of the pulvinus of a
HELIOTUOl'IC TORSIONAL RKSPONSK 66 1
leaf, the less contractile ebonite being turned so as to face
the light (fig. 269).
Modification of torsional response by artificial varia-
tion of the relative excitabilities of the two halves. —
I find it necessary to go still further into this subject of
torsional response, as by its means 1 have been enabled to
solve a question of very great importance, that, namely, of the
different excitabilities of the two halves of the anisotropic
organ to geotropic stimulus. The fact that it is the differential
character of the excitabilities of these two halves that brings
about the torsion of a dorsi-ventral organ under lateral stimu-
lation may be still further established in a very interesting
manner by inducing artificial variation in the existing excita-
bilities. The torsion depends, as said before, on the difference
of excitability as between the two. If, then, we could render
the lower and more excitable half gradually less and less
excitable, till its differential excitability had disappeared, the
organ would thus have been rendered virtually radial. The
torsional effect might then be expected to vanish, and a
simple curvature towards stimulus to result, without torsion,
as in the case of other radial organs. Let us next suppose
the reduction of excitability to be carried still further, till the
lower half of the pulvinus have been rendered less excitable
than the upper. On the theory of torsional response which
has just been advanced, it is the less excitable lower half
which should now twist round to face the stimulating light.
In other words, there would then be a reversal of the original
torsion. Thus, as the excitability of the lower half becomes
gradually reduced, the intensity of the normal positive
torsional response by which the upper half was made to face
the stimulus would be gradually first dccrea.sed to zero, and
then reversed to negative, as the excitability of the lower
became first equal to, and then less than, that of the upper half
If, on the other hand, the excitability of the upper half
be reduced, the existing differential excitability as between
upper and lower would then be still further increased, and
the intensity of the positive torsional rciponse would be
662
PLANT RESPONSE
enhanced. All these conclusions will be found exactly
verified in the records given in fig. 270. In the first part of
the left-hand figure we see the normal positive torsional
response of J//;//(?j-(3: under the lateral action of light. The
excitability of the lower half of the pulvinus was then
reduced by the local appli-
cation of chloroform, at the
moment represented by the
arrow from below (t). It
will be seen that the response
now undergoes reversal,
owing to the fact that it
is the upper side that is
the relatively more excit-
able. In the figure to the
right, again, is shown the
effect on the torsional re-
sponse of the leaf, of increas-
ing the already existing
difference as between the ex-
citabilities of the two halves,
when that of the upper is
reduced by the local applica-
tion of chloroform. It will
be seen that this application as marked by the arrow from
above (|) caused an enhancement of torsional response, as
seen in the greater steepness of the curve.
Laws of torsional response. — From these experiments
we arrive at the following laws :
1. An anisotropic organ, when laterally excited, under-
goes torsion, by which the less excitable side is made to face
the stimulus.
2. Stimulus remaining constant, the intensity of torsional
response increases with the differential excitability. But when
the original difference is in any way reduced or reversed,
the torsional response undergoes concomitant diminution or
reversal.
Fig. 270. Records showing Modilica-
tion of Torsional Response under
Induced Variations of Differential
Excitabilities in Pulvinus of Mimosa
In left-hand figure the normal torsion is
seen to be reversed by application of
chloroform to the lower half (f),
reversing the differential excitability
of the organ. In the right-hand
record the normal responsive torsion
is enhanced by application of chloro-
form to the upper half ( j ), increasing
the natural differential excitability of
the organ.
GEOTROPIC TORSIONAL RESPONSE 663
3. An organ which, under lateral stimulation, responds by
torsion, is always physiologically anisotropic, and the side
which is made to face the stimulus is the less excitable.
Demonstration of differential geotropic excitation in
a dorsi-ventral organ.— From this experimental demon-
stration, then, we have obtained a new means of discrimi-
nating the relative excitabilities of the two halves of an
organ, since that side which is turned by the responsive
movement to face the given stimulus is relatively the less
excitable to it. It will be remembered that we found that
the reason why certain dorsi-ventral organs showed a tendency
to assume a horizontal position under the action of vertical
light was not that they possessed dia-heliotropic sensibility,
but was due to the differential excitability of the two
halves under stimulus in general, including that of light.
Again, in the case of the action of gravity, it is found that
such organs exhibit a similar tendency to place themselves
horizontally ; and the assumption of a specific dia-geotropic
sensibility is not necessary, if it can be shown that the upper
and lower sides are unequally sensitive to this stimulus
also. We may first take an ideally simple case. We have
seen that in a radial apogeotropic stem, when laid hori-
zontally, it is only the upper half that is effectively
stimulated by geotropic action ; and there was reason to
believe that this was due to the fact that it was the inner
tangential wall of the upper side — in contrast to the less
excitable outer wall of the lower side — that was excited by
the statolithic or other influence of weight (pp. 495, 503). If
now, for any reason, the excitability of the upper half of the
horizontally placed radial organ be abolished, the geotropic
response of that effective half will disappear, and the organ
will remain horizontal, as if unaffected by stimulus of gra-
vity. This state of things we have already realised, when
the excitability of the upper half was artificially diminished
by the local application of cold, and geotropic response was
seen to be arrested (p. 504). Now, a horizontally placed
radial organ which has been rendered unequally excitable
664 PLANT RESPONSE
in the two halves, by the reduction of excitability of the
upper, is virtually a plagiotropic or dorsi-ventral organ, and
we can thus see why a true plagiotropic organ, laid hori-
zontally, has a tendency to remain in that position. It
would remain in that position absolutely if the excitability
of the outer tangential wall of the lower side were actually
zero, and the general excitability of the upper half of the
dorsi-ventral organ had also completely vanished ; but if
these two values were not both zero, various effects would
occur, according to the relative differential excitabilities of
the two halves.
I shall now describe the line of investigation by which
it is possible to demonstrate the fact that the two halves of
an anisotropic organ possess different excitabilities with
regard to geotropic stimulus ; but before entering upon this,
it is necessary to obtain a clear idea of the direction of
response, in relation to direction of the force which causes
stimulation of gravity. In the case of an apogeotropic stem,
laid horizontally, we have vertical lines of force striking the
organ from above, and the curvature induced makes it turn
upwards to meet, as it were, these lines of force. We have
here an example which is analogous to the response of the
organ to the rays of light, in which the responsive move-
ment consists in a similar curvature upwards to meet them.
The fact that the upper and lower halves of an aniso-
tropic organ are unequally excited by light has just been
demonstrated by means of torsional response, where it was
shown that lateral excitation induced a twisting movement
by which the less excitable was made to face the incident rays.
By a similar torsional response of the anisotropic organ we
are able not only to demonstrate the unequal geotropic
excitability of the upper and lower halves, but also, by
noting which of the two is made to face the lines of gravita-
tional force, to determine which it is that is the less excitable
to this particular form of stimulus.
Torsional response to lateral geotropic stimulation.—
For this experiment I took a leaf of Erythrina indica, whose
GEOTkOPIC TORSIONAL RtSl'ONSE 665
pulvinus I find to be very sensitive to geotropic action.
Under normal conditions the leaflets of this plant place
themselves horizontally. I now took one of these, say the
terminal, and adjusted the plant so that its lateral edge was
vertical. The dorsal and ventral halves of its pulvinus were
thus equally subjected to geotropic action. This experiment,
it is to be noted, was carried out in a dark room, where
the only stimulus acting was geotropic. It will be seen, from
the first part of the left-hand record in fig. 271, that under
geotropic stimulus a torsional response was here obtained,
which was exactly similar to that given under the action of
light in fig. 270. The angular movement was in this case
at the rate of about -11° per minute, and it was the dorsal
surface of the organ that was eventually turned upwards —
that is to say, so as to face the lines of force of gravity.
This experiment, then, conclusively demonstrates the fact
that the two halves of the dorsi-ventral organ are unequally
sensitive to geotropic stimulus, and that it is the upper which
is the less sensitive.
Modification of torsional geotropic response by arti-
ficial variation of differential excitability. — The fact that
the upper half is, under normal conditions, the less excitable
to geotropic stimulus is capable of further and striking
demonstration, by the tests of reversal, and acceleration of
torsional response. I have already explained, as will be
remembered, that if the existing difference of excitability
were reversed, the direction of torsional response would also
be reversed ; and that if, on the contrary, the difference were
by any means increased, the rate of torsional movement would
be correspondingly accelerated. In the first part of fig. 271
is seen the natural torsional response of the terminal leaflet
of Eiythrina indica. The excitability of the lower half of
the pulvinus was now abolished, by the local applica-
tion of chloroform, at the point marked with an upward
arrow, and the torsional response to geotropic stimulation
was thus found to be reversed, from plus -11° to minus -05°
per minute. Hence it was now the lower half of the pul-
666
PLANT RESPONSE
vinus — artificially rendered the less excitable of the two —
that was found turning upwards, to face the rays of incident
force of gravity.
The converse of this experiment would consist in still
further reducing the excitability of the upper half of the
pulvinus, in which case the normal differential excitability
of the two halves would be increased, and the geotropic
response enhanced. The right-hand record in fig. 271 shows
that this is what actually
occurs. The first part of
the record displays the
normal torsional response,
and that which follows,
after the local application
of chloroform on the
upper surface — indicated
by the arrow from above
— exhibits, by its in-
creased steepness, the en-
hanced character of this
response.
We have thus seen,
by means of torsional re-
sponse, that the differ-
ential character of the
excitability of the two
halves of an anisotropic
organ under the stimulus
of light is in every way paralleled by their differential ex-
citability to the stimulus of gravity. The geotropic response
of the organ, as a whole, is neither simple positive nor
negative, but may be more fittingly described as differential.
And as we saw with regard to photic stimulus that there was
no need for the assumption of any specific dia-heliotropic
sensibility in plagiotropic stems or in leaves — their move-
ments being fully accounted for by the mechanical con-
siderations arising out of their differential excitability to
Fig, 271. Records showing Torsional
Response to Geotropic Stimulus and
Induced Modifications in Terminal Leaf-
let of Erythriiia indica
In the figure to the left is seen the normal
torsion under lateral geotropic action,
and its reversal when the differential
excitability of the organ is reversed by
the application of chloroform to the
lower half ('f). In the figure to the
right is seen the enhancement of the
torsional response when the natural
differential excitability is increased by
the application of chloroform to the
upper half of the organ {],).
AUTONOMOUS TORSION (>67
stimulus in general — so similarly with regard to geotropic
stimulus, there is no necessity for the assumption of any
specific dia-gcotropic sensibility.
Autonomous torsion. — We have now seen how torsional
response is induced in an anisotropic organ. We have next
to deal with that interesting class of phenomena which
consists of the natural torsional movements of growing
organs. It is to be remembered that, in referring to these
torsional movements, we mean the torsional growth-move-
ments of certain stems themselves about their own axes.
The term positive torsion in such cases means a movement
which appears, when looked at from above, to take place in
the same direction as the movement of the hands of a watch,
the term negative being used in the contrary sense.
Such torsional movements are found to occur in the stems
of climbing plants. In some of these they are of a positive,
and in others of a negative character, while in still others
again they alternate, the natural torsion being, say, positive
at one period, and negative at another.
Now, we have seen in the case of leaves that, owing to
different rates of growth in the antagonistic upper and lower
halves of the petiole, autonomous epinastic or hyponastic
movements occur in a rectilinear direction. When the
growth of the upper half is predominant, the movement of
the organ is downwards, and vice versa. More complex,
however, is that case in which the line of maximum growth
is a spiral revolving about an axis, thus bringing about a
growth-movement which causes a torsion of the organ round
that axis. In the case of epinasty and hyponasty, it was
said that the rectilinear movement induced was due, not to a
total absence of growth on either side, but to the relatively
greater growth of one of the antagonistic halves. Similarly,
in these torsional movements we have to remember the
existence of antagonistic elements in the stem, and it is the
predominant growth of one of these, the right-handed or left-
handed, that brings about the resultant positive or negative
torsion.
668 PLANT RESPONSE
Effect of temperature on autonomous torsion. — In
order to take a record of these torsional movements, a stem
is taken and held securely fixed at one end— say the lower —
the other being left free. On this free end, the mirror,
by the reflected light from which the record is to be made,
is attached ; and a long thread from above is tied to the tip
of the specimen, in order to prevent its falling to one side
by its own weight. The limpness of this supporting thread
allows the torsional movements of the specimen to occur
without hindrance.
In taking records of the natural torsional response of
plants at various temperatures, I have obtained results which
are in a general way similar to the records made of longi-
tudinal growth-response (p. 446) — that is to say, the response
is enhanced up to the optimum, which is at or near 35° C.
Above this, response is diminished. These facts will be seen
from the following table, which gives the absolute rate of
the angular movements of the torsional response at different
temperatures.
The specimens employed in these and the following
experiments were climbing stems of Porana paniciilata, a
plant belonging to the Campanulacea, which normally ex-
hibits a strong negative torsion.
Table giving the Absolute Rate of Angular Movements
OF Torsional Response at various Temperatures
Temperature
Torsional movements
28° c.
35° C.
37° C.
■09° per minute
•18°
•282° „
•18°
An interesting effect was observed, however, in this case
of torsional response, when the temperature was raised
above 43° C, which was apparently different from what
occurred in ordinary longitudinal growth-response at the
same temperature. It was found in the latter case that at
44° C. or thereabouts growth underwent an apparent arrest ;
AUTONOMOUS TORSION 669
but this was shown not to be due to any actual arrest
brought about by heat-rigor, for the rhythmic activity
induced at this high temperature was even greater than
before (p. 432). Now, in the case of torsional response, I find
that though the rate of torsion undergoes a continuous
decrease up to 42° C. or 43° C, yet beyond this an unex-
pected increase is induced. Thus in the experiment just
described, the torsional movement at 45° C. was at least
temporarily enhanced to -8° per minute. This phenomenon
may be due to abnormal relaxation at these high temperatures.
Or there is another possible explanation. It has been said
that the resultant torsion is due to the differential growth-
activities of two antagonistic elements. Now, it may be that
at 44° C. or thereabouts the growth of the less excitable of
these antagonistic elements may undergo the same kind of
arrest as we have seen to occur in the longitudinal growth of
a radial organ. The sudden increase observed at and above
44° C. in the rate of natural torsion may then be due to the
withdrawal of the resistance previously offered by the growth-
activity of the antagonistic element.
Effect of increased suctional activity. — Another in-
teresting point, in connection with the occurrence of auto-
nomous torsion, lies in the fact that its rate is enhanced by
any means which tends to increase internal energy. One
example of this has just been seen in the effect induced by
rise of temperature. Another is found in the application of
warm water to the base of the specimen, a process which has
already been shown, in the case of radial organs, to cause, by
means of the consequent increase of suctional activity, a
sudden enhancement of the rate of autonomous growth
(p. 430). I find, similarly, that the application of warm
water to its ba.se enhances the torsional response of a torsion-
ing plant.
Effect on natural torsion of unilateral application of
light. — We have seen that when an anisotropic organ is
laterally excited by external stimulus, a response torsion
takes place, by which the less excitable side is made to face
670 PLANT RESPONSE
the stimulus. Now, in a naturally torsioning organ, such an
induced torsional movement must obviously be opposite in
direction to the natural movement caused by internal energy.
This will be found to be illustrated in the modification in-
duced in autonomous torsion by the unilateral application of
light, as shown in fig. 272, where the first part of the curve
shows the normal negative torsion of the plant. At the
point marked with the upward arrow, light from a thirty-two
candle-power electric lamp was allowed to strike the stem
from one side. It will be seen that the excitatory effect of
this external stimulus is first exhibited in the retardation of
Fig. 272. Retardation and Reversal of Normal Torsional Movement
by the unilateral Action of Light in Parana paniculaia
Light applied at t retards, and in the course of two minutes reverses, the
normal movement of negative torsion, making it positive. On cessation
of light at / ^ ': there is recovery of the original negative torsion.
the normal rate of torsion, culminating in its abolition, suc-
ceeded by actual reversal of direction. The normal negative
was thus converted into positive torsion. On the cessation
of the external stimulus the normal torsion was gradually
resumed. The effect described is best observed in vigorous
specimens in which the natural torsional movement is fairly
strong.
In studying the action of light on autonomous longi-
tudinal growth, we found that when a series of responses to
the action of light were recorded, the first effect of incident
AUTONOMOUS TORSION 67 I
light, if the specimen were in a sub-tonic condition, would
be, by increasing the internal energy, to give rise to an
enhanced rate of growth ; but when the normal tonic
condition had been attained by absorption of light, the
subsequent responses would be by a movement opposite to
that of growth-elongation — that is to say, a contraction or
retardation of growth. In experimenting on the effect ot
light on the autonomous torsional movements of the stems
of certain specimens of IpoiiiKa, I have met with results
exactly parallel — ^that is to say, the natural rate of torsional
movement, in this case negative, was transitorily enhanced
by the first incidence of light, but declined and underwent
reversal after continued action of stimulus. In the second
and subsequent responses this preliminary enhancement was
found to have disappeared, the effect of light now being a
retardation and subsequent reversal of the natural torsional
movement.
Effect of electric current.— I have often observed a
very interesting effect, as induced by a constant current
flowing along the length of the organ. During the continu-
ance of the current the normal torsional response is first
decreased and then reversed. On the cessation of current
there is recovery and restoration of the normal direction of
torsion. If the current maintained be strong or long con-
tinued, the induced reversal may become more or less
persistent. Such an induced reversal of torsion, moreover,
is independent of the direction of current.
Effect of gravity. — I shall next describe a series of
effects which I have only been able to obtain with any
degree of certainty under favourable conditions. Where
natural torsion is feeble the retarding effect of gravity which
I am about to describe is such as to arrest torsion, and it is
difficult to decide whether such arrest is accidental or in-
duced. When the specimen, on the other hand, is too
vigorous, the retardation is not easily observed, probably
because it tends to be masked by the natural movement. In
practice, therefore, the best specimens are those characterised
672 PLANT RESPONSE
by moderate vigour and uniformity of natural torsion during
a considerable length of time.
In studying the effect of geotropic stimulus on radial
organs, we have seen that its effective intensity was greater
when the specimen was inclined at an angle of 135° to the
vertical than when at 90° (p. 501). It occurred to me,
then, that the effect of geotropic stimulus might possibly be
detected, by means of variations induced in natural torsional
response at different angles of inclination. Thus, we might
first take a record of the normal rate of torsion, occurring
when the tip of the organ was in its natural position upwards.
We might next take a record of torsion, the stem being held
horizontal ; and we might finally take a record with the tip
held vertically downwards.' The effect of geotropic stimulus
might then be seen in the retardation induced in the normal
torsion. I give here a summary of results obtained from two
different specimens oi Porana paniadata, and from a specimen
of Ipoiiioea.
In the first case, the record taken in the normal upright
position gave a rate of movement of negative torsion through
minus twenty-three divisions of the scale per minute. The
record of torsion in the horizontal position was found to give
a very much diminished rate, being now only minus seven divi-
sions per minute ; and finally, with the tip held downwards,
the torsional movement of the specimen was found to have
undergone an absolute reversal to positive — that is to say, the
tip now moved ivith the hands of a watch at a rate of plus
five divisions of the scale per minute.
In the second case, the normal negative torsion in the
erect position was at the rate of minus twenty divisions per
' Here we must bear in mind, with regard to the effective angle of inclination,
a certain difference as between radial and torsioning organs. In the former, the
line of growth may be taken as vertical, and coincident with the axis, whereas in
the latter it is spiral, or inclined at some undetermined angle to the axis of the
organ. Hence any inclination of the growth-line of a radial organ is measured
by the angle between the organ and the vertical. In the case of the torsioning
organ, however, when it is held vertically downwards, its growth-line makes an
angle — not of i8o° to the vertical, but — of l8o° less by the degree of its inclination
to the axis.
ANTONOMOUS TORSION 673
minute. This Was reduced in the horizontal position to
minus fourteen divisions per minute, and in the vertically
downward position it was found to have undergone reversal
\.o plus nine divisions per minute.
In the next series of experiments' I took records from
specimens held alternately up and down and up again.
This was done in order to eliminate the effect of any chance
variation. The specimen employed was Jpoi/uva. The tor-
sional response in the first up position was at the rate of
minus sixteen divisions per minute. When held in the in-
verted position, the rate was found reduced to minus ten
divisions per minute, and when once more placed in its
normal vertical position, the Ipomoea exhibited an increased
rate of movement — that is to say, the torsion now took place
at the rate of minus twenty divisions of the scale per minute.
All these results tend to show that the action of stimulus
of gravity is to retard the autonomous torsion, this effect
being at a maximum when the specimen is in a position at
180° from the vertically upright.
The twining of stems. — The twining movements of
certain stems are the result of various contributing factors,
the relative values of which may differ in different cases.
One such factor, suggested by Von Mohl and denied by
others, may lie in the irritability of the stem to the contact
of its support. Such response to unilateral pressure was found
to occur in various organs (p. 497), and probably plays an
important part, in some cases, in the phenomenon of twining.
Some connection would also seem to exist, in many in-
stances, between autonomous torsion and twining. In the
first place, most of the twining plants also exhibit autonomous
torsion. Again, just as we have various types of torsion ing
organs — some characterised by positive, others negative, and
others again by positive alternating with negative, or negative
with positive, torsions — so in twining stems also, we see some
which exhibit positive-directioned twining, others negative,
and others again alternately one and the other. A plant,
moreover, which has twined, shows, if inverted, a tendency
674 PLANT RESPONSE
towards reversal or untwining. This is analogous to the
effect of inversion on autonomous torsion, in which, as we
have seen, the original torsion tends to be reversed.
The effect of gravity is known to be a very important
factor in the induction of twining. It is believed that we
have here a third type of geotropic action, which is neither
positive nor negative, but lateral in its character. Under
lateral geotropism, a curvature is induced in the twining stem
in a horizontal plane. Since we have found, however, that
even such diverse effects as positive and negative geotropic
actions are not to be regarded as due to different specific
sensibilities, it would be interesting to inquire whether lateral
geotropism also is not simply a case of ordinary apogeo-
tropism in combination with some other tendency, say that
of autonomous torsion.
At any rate, that this erectile or apogeotropic action is
always an element in the process, is shown by the fact that the
coils in the older portion of the stem become drawn out, show-
ing that the ascending movement predominates in that region.
The whole subject is, however, extremely complicated,
owing to the presence of many factors and their different
relative intensities. And each of these may again be subject
to modification, under the action of external stimulus, as was
seen to be the case with autonomous torsion under the in-
fluence of light, gravity, and the variation of internal energy.
Summary
The observed effect of torsion, by which the upper
surface of the leaf is turned to face the light, is not due to
any specific dia-heliotropic sensibility.
Similar torsional response is induced by the lateral
application of any other form of stimulus, such as thermal
or chemical.
Such torsional response is also obtained from a com-
pound strip made of two unequally contracting inorganic
materials, such as ebonite and stretched india-rubber, when
stimulus is applied to one of the lateral flanks.
TORSIONAL RESPONSE TO STIMULUS 675
The general law of induced torsional response is that the
less excitable side of the organ is made to face the incident
stimulus. If the excitability of the lower half of the pulvinus
be artificially abolished by the local application of chloro-
form, the torsional responsive movement is reversed. The
leaf now executes a movement by which its lower surface is
made to face the light.
On artificially increasing the natural difference, as between
the two halves, by altogether abolishing the excitability of
the upper, the normal torsional response is accelerated.
That the upper and lower halves of a dorsi-ventral organ
are unequally sensitive to geotropic stimulus is shown by the
fact that they give torsional response, under which the
less excitable half is carried so as to face the incident lines
of gravitational force. An artificial increase of this natural
difference of excitability has the effect of accelerating the
rate of torsional response. The artificial reversal, on the
other hand, of these relative excitabilities has the effect
of reversing the direction of the natural responsive move-
ment.
From these considerations it is seen that none of the
responsive movements caused in dorsi-ventral organs by
light or gravitation are due to any specific dia-heliotropic or
dia-geotropic sensibility, and that they are in reality due
to the differential excitability of the two halves of the organ.
The autonomous torsional movement of growth increases
in rate up to an optimum temperature, after which it begins
to decrease. There may be a second acceleration after the
attainment of the first minimum.
This autonomous torsional movement is retarded or even
reversed in sign by unilateral stimulus of light. A constant
current, flowing along the length of the organ, often retards
or reverses the normal autonomous torsion. The stimulus
of gravity also has the effect of retarding or reversing normal
torsional movement. The effective intensity of this stimulus
appears to be greatest when the organ acted upon is in a
position at 180° from the vertically upright.
CHAPTER XLVIIl
NYCTITROFIC MOVEMENTS
Comparison between nyclitropic and autonomous pulsations — Diurnal movement
of plagiotropic stem— Supposed distinction between nyctitropic and other
movements of response to stimulus of light — Diurnal response of leaf of
Biophytum — Diurnal response of primary petiole of Mimosa — I'eriodic
impulses acting on the leaf — Periodic impulses contrilnited by the plant as a
whole— Other modes of exhibition of diurnal periodicity of hydrostatic tension
— Forced vibration and its periodic after-effects — Physical analogue — Impressed
periodic vibrations in organ originally radial.
In addition to the other effects of light which we have been
studying, there is also a periodic movement, induced by the
alternation of day and night, which consists of a pulsation
executed by the responding organs in the very long period
of twenty-four hours. In its pulsatory character this move-
ment resembles the so-called autonomous pulsations of such
• leaflets as those of Desinodiuin gyrans. But besides the fact
that it has the long and definite duration of twenty-four
hours, whereas the autonomous pulsations of the leaflets of
Desmodium are short and variable in period, there are other
differences between the two.
Comparison between nyctitropic and autonomous
pulsations. — In the first place it will be noticed with regard
to the daily periodic movement that at any given time all the
motile organs of the same plant are going through the same
phase. This rhythm therefore is in a manner controlled by
the plant as a whole. In the case of autonomous movements
so called, on the other hand, the seat of rhythm is localised
in the motile organ of that particular leaflet whose pulsations
are being observed, and there is no necessary coincidence of
period, as between any two leaflets on the same plant. The
NYCTITROPIC MOVEMENTS 6^7
period of autonomous vibration is much modified, again, by
rise or fall of temperature, and other conditions ; whereas the
daily period is unaffected by these circumstances. This is
due to the fact that the autonomous pulsations are akin to
free or natural vibrations, whereas the daily periodic move-
ments are, as we shall see, of the nature of forced vibra-
tions.
As the nyctitropic movement in the primary petiole of
Mimosa is of a simple character, free from those other
complicating considerations which have to be taken into
account in the case of the leaflets, this organ has been chosen
as the typical specimen by many investigators. We shall
therefore consider in detail all its peculiarities in regard to
this movement, but I shall at the same time endeavour by
the use of the comparative method to trace the evolution of
the movement, as first seen in plagiotropic stems, more clearly
manifested in certain dorsi-ventral organs like ordinary leaves,
and strongly exhibited in pulvinated leaves, such as those of
]\Iiinosa.
Supposed distinction between nyctitropic and other
movements of response to stimulus of light— This nycti-
tropic movement has been sharply distinguished from ordinary
response to the stimulation of light, by the statement that
while the latter depends on the direction, this is determined
only by the varying intensity of illumination. As a further
distinction, it is also insisted that, whereas the responsive
curvature induced by light takes place in all directions,
nyctitropic movement always occurs in a single definite plane.
But whenever we subject the pulvinus of Mimosa to the
action of strong light, we obtain response by a greater or
smaller depression or fall, which occurs in a definite vertical
plane, and is precisely the same in character as that which is
exhibited slowly during the course of the whole day, and
attains its maximum in the evening. This response to strong
light, moreover, is always the same, on whichever side of
the pulvinus the stimulus may act — that is to say, it is
independent of the direction of light. In analysing this case
678 PLANT RESPONSE
further (p. 635) we saw that it constituted a true instance of
phototropic reaction, and that this particular movement of
fall in a definite direction was due to the stimulus being
diffused, whether internally or externally, and by differential
action inducing concavity of the lower and more excitable
half of the pulvinus.
In the assumption by the leaf at evening, then, of its
lowest position, we have nothing which is specifically different
from this responsive movement, as caused by the action of
light on the anisotropic organ ; but this same fall, when it
attains its maximum in the evening, as a phase of the
nyctitropic movement, is usually said to be due, not to the
stimulus of light, but to the variation in that stimulus, or to
the effect of on-coming darkness. Now, if it had been true
that the diminution of light, or on-coming of darkness in the
evening, had acted in some unknown manner as a stimulus
to bring about the fall, then we should have found that such
a fall was at that time extremely rapid, and that it persisted
on the withdrawal of light during the night. As a matter of
fact, however, it is found that this movement of the petiole
downwards has been taking place progressively throughout
the day, and that the fall of the leaf at evening is merely a
continuation of this previous movement, and not markedly
more rapid at that time than before, if the increased
mechanical moment due to the closing of the secondary
petioles and their leaflets be eliminated by their removal.
Nor does the petiole remain in this depressed position, but
begins after an interval to erect itself, till in the morning, or
earlier, it has attained its highest degree of erection. So far,
again, from darkness being efficient to cause the depression
of the leaf, it is well known that on artificially darkening the
Mimosa plant the leaves respond by erection.
It is thus clear that we must seek some other explanation
of the nyctitropic movement, and as it is known that such
periodic movement persists for a time, even when the plant
is kept for days in continuous darkness, the inquiry resolves
itself into the two questions: (i) how are these diurnal
NVrTITKOPir MOVEMENTS 679
motile impulses generated ? and (2) how is it that the move-
ment will still persist in the absence of a periodically exciting
cause ?
Diurnal movement of plagiotropic stem.— In order to
understand clearly the periodic action of light, in induc-
ing diurnal movements, it is best to take as our starting-
point the simplest type of anisotropic organ, in which this
effect may be observed, and for this we may select the
plagiotropic stem of, say, Cncurhita. This creeping stem is
acted on, under natural conditions, by vertical light from
above, the excitatory effect of which, by long-continued
action, reaches the lower and more excitable side. The
stimulus thus becomes internally diffused, and there is also a
certain amount of externally diffused light from the environ-
ment. Hence it happens that, by the contraction of the more
excitable lower half, the free end of the stem becomes progres-
sively depressed, under the continued action of light, during
the course of the day (p. 627). We have seen that in conse-
quence of this the greatest depression occurs about the time
of evening, and that on the cessation of the stimulus of light
there is a recovery, the stem erecting itself more and more as
the night advances (fig. 252). Thus this nyctitropic depression
at nightfall is not dne to the on-coming of darkness, but repre-
sents the cumulative responsive effect oj the day's illumina-
tion. In fact we may regard the responsive movement
down, and recovery up, which are executed in the course of
the diurnal period, as parallel to that phenomenon with which
we are already familiar, 'of a^single response to a single
transient stimulus. The only difference lies in the fact that,
while in this latter case the whole process is completed in a
few minutes, in the former the stimulus acts continuously
during something like twelve hours, and the recovery is
allowed to take place during the course of the night. Thus,
in the ordinary case of records of uniform responses to
uniform stimuli, the successive stimuli, each lasting for a {^\^
seconds, are given at intervals of some minutes, but in the
case of the diurnal responses we have a series of stimuli.
68o
PLANT RESPONSE
each lasting for twelve hours, and the beginning of each
separated from the next by a period of twenty-four hours.
Diurnal responses of leaf of Biophytum. — From this
simple instance of anisotropy we pass on to a more highly
differentiated case of dorsi-ventrality, as seen in leaves. We
have already seen that the petiole of Biophytum is only
provided with a moderately developed pulvinus, and that the
petiole also, as a whole, acts as a diffuse pulvinoid. In this
case also, by the in-
ternally and externally
diffused action of light
during the course of
the day, the petiole is
progressively depressed,
from its position of
highest erection in the
morning to its lowest
depression in the even-
ing. Recovery takes
place again in the ab-
sence of the stimulus of
light, and the leaf once
more assumes its highest
position by morning. If,
however, in the course
of the afternoon, say, at
five o'clock, the plant be
taken to a dark room,
then, owing to the positive after-effect, the leaf will still con-
tinue to fall for about an hour, and then begin to erect itself.
Owing to the daily periodicity impressed on the plant, of which
we shall presently speak in greater detail, the leaves continue
to exhibit the diurnal movement even when kept in continuous
darkness. I give here (fig. 273) an interesting photographic
record of the diurnal movement of the leaf of BiopJiytnui
from 5 P.M., when it was brought into the dark room, till
9 A.M. next morning. It will here be seen that, owing to the
Fig. 273. Photographic Record of Diurnal
Movement of Petiole of Biophytum from
5 P.M. till 9 A.M.
First part of record shows maximum depres-
sion to be reached at 6 p.m. and maximum
erection at 7 a.m. After the latter hour
the leaf again begins to exhibit its usual
daytime depression.
NVCTITROPIC MOVEMENTS
68 I
positive after-effect, the leaf continued to fall till 6 P.M., and
that after this it erected itself by a series of five pulsations,
until its highest position was attained at about 7 a.m. It
next began to exhibit the impressed effect of day, and the
leaf then fell rapidly.
Diurnal response of primary petiole of Mimosa. -In
Mii/wsa we merely find the repetition of those movements
whose evolution we have already traced through the plagio-
tropic stem and the dorsi-ventral petiole. This is made very
I
1
1
1
Fig. 274. Continuous, Records of the Diurnal Movement during Thirty-
six Hours in Two Specimens of Mimosa
The continuous line represents the movement of the one-year old, and the
dotted line that of the six-months old specimen. The maximum
depression is seen to take place at six in the evening, and the recovery
nearly completed by midnight. Tliis maximum erection is slightly
augmented at dawn.
apparent in the continuous records which I took of the
diurnal movement in two specimens of Mimosa, the records
being continued during a period of thirty-six hours (fig. 274).
The two plants were placed in an open verandah, and
long light indices made, as already described, of peacock's
quills, were so attached to the petiole as to form jjrolonga-
tions. Records were taken on a vertical revolving drum.
A vertical thread was suspended in front of the drum, and
the point at which the moving index cut this vertical line
was marked at every fifteen minutes. Thus the record gives
682 PLANT RESPONSE
a measure of the angular movement at different parts of the
day. The dotted line shows the diurnal movement of the
younger of the two plants, which was six months old, and the
continuous line that of the other, which was one year old. The
two, as will be seen, are practically the same. It should be
mentioned here that, when this record was taken, 6 A.M. and
6 P.M. were the hours of sunrise and sunset, there being no
twilight. The records show that the leaves exhibited the
erectile or recovery effect with great rapidity during the first
part of the night, this movement being almost completed by
I A.M. After this there was but slight upward movement,
until about 7 A.M., from which time onwards, during twelve
hours, they fell continuously. The maximum depression was
reached at almost exactly 6 P.M., and then the leaves again
rose, repeating their former movement of recovery.
Periodic impulses acting on the leaf. — The periodic
movements down during the day, and up during the night,
are thus seen to be due to periodic impulses of stimulus
and recovery acting on the responding leaves. It must be
pointed out here that the process of recovery is not alto-
gether passive. We saw in the records of Mimosa under
light (fig. 256) that, on the cessation of light, the responding
leaf continued to move down for a while owing to the
positive after-effect. Later, however, owing to the latent
energy which it had acquired by the absorption of light, it
exhibited the negative after-effect in an erection which
carried it beyond the original position. Hence we can see
that the increased internal energy due to previous absorption
of light plays an important part in that movement of erection
which is initiated shortly after nightfall. We may therefore
say that the diurnal movements of the leaves are brought
about by two alternate periodic impulses, those, namely,
of external stimulus and internal energy. The persist-
ence of the effect of each of these impulses will depend on
the intensity of the two factors respectively, and also on
the capacity of the tissue for absorbing stimulus. For
example, the positive after-effect by which the leaf continues
NYCTITROPIC MOVEMKNTS 683
to be depressed, even on the cessation of the stimulating day-
light, may, in some cases, be short-lived, and in others maj-
continue for a considerable period. The return movement
will in the latter case be somewhat delayed. Again, the
internal energy which hastens recovery may, in certain cases,
bring about the utmost erection of which the leaf is capable
at "some time earlier than the ensuing morning. Thus, for
example, in the response of Biophytniii, the highest position
(fig. 273) was. attained at about 7 A.M., while in the records of
these particular Mimosa leaves the corresponding point was
reached very much earlier, that is to say at i P.M.
Periodic impulses contributed by the plant as a
whole. — In addition, however, to those periodic impulses
whose seat is in the responding leaf, there is another concor-
dant impulse contributed by the plant as a whole which
intensifies the diurnal movement. The plant is subjected
during the day to the stimulus of light, which causes con-
traction of the exterior tissues, and thereby tends to drive
the water inwards. The turgidity of the cortex is thus
progressively diminished during daylight. But at night the
water which has thus been driven into the central reservoir of
the plant will flow outwards. In this also the absorbed
energy will play an important part. The rhythmic activity
of the cortical tissues being thereby increased, they will suck-
water outwards from the central reservoir, just as the rootlets
suck it from the ground. The result of this alternation of
external stimulus during the day, and internal stimulus during
the night, will be a periodic inflow and outflow — a diminu-
tion and increase of tension— the first half of the cycle being
completed in the daytime, and the second half in the night.
I have shown (p. 46) that the leaf of Mimosa is
erected whenever the internal hydrostatic tension is arti-
ficially increased, and depressed when it is diminished ; and
we have seen how such variations are induced by the alter-
nation of day and night, not only in individual petioles, but
throughout the plant as a whole. The alternate ebb and
flow of the water, from the central reservoir, will thus be
684 PLANT RESPONSE
indicated by the periodic erection and depression of all the
leaves synchronously, which will thus act, so to speak, as
signal-flags.
Other modes of exhibition of diurnal periodicity of
hydrostatic tension. — There are also other modes besides
that of mechanical response of the leaves by which this
diurnal periodicity of tension can be detected ; and Millardet
has shown that the maximum tension in Mimosa occurs at
dawn, when the primary petiole is in its most erect position.
The minimum tension, again, occurs in the evening, when the
leaf occupies its most depressed position. Kraus, further, has
found the organs of the plant diminish in bulk from morning
till afternoon, and that the reverse process takes place from
afternoon till morning. Growth itself, again, is well known
to exhibit a diurnal periodicity. It is interesting, however,
to realise that this is simply the mode by which a radial
organ exhibits, in a form of longitudinal response, what was
otherwise exhibited in Mimosa as lateral response. The
periodic variation of tension induced by the diurnal period
has been seen to manifest itself in Mimosa in two alternate
movements, positive and negative. Similarly we shall find,
in growth- response, positive and negative variations above
and below the normal or average rate. If we take a
balanced record of growth, representing its average rate, a
downward line will indicate retardation or negative response,
while an upward line will indicate an acceleration of growth,
or positive response ; and in thus recording variations of the
rate of growth, by the balanced method, for a period of
twenty-four hours, we obtain, as we have seen (fig. 191),
a curve which closely resembles that of the nyctitropic
movement.
Forced vibration and its periodic after-effects. — We
have thus traced out the two alternating impulses — the direct
effect of external stimulus and its direct after-effect, and the
internal stimulus with the negative after-effect — which induce
the forced vibration of the responding organ. The periodic
effects of protoplasmic contraction and expansion, induced
NYCTITROPIC MOVEMENTS 685
alternately in the plant under alternating light and dark-
ness, thus leave a molecular impress, and this impression is
deepened by repetition, finding subsequent expression even
when the primary alternating cause is absent. The length
of time during which such after-oscillation persists will
depend, amongst other things, on the depth of the molecular
impression.
Physical analogue. — An analogous physical phenome-
non will make the point clearer. If a wire be taken and
subjected to alternating molecular strains, say by giving it
alternate positive and negative twists, the wire being held in
each of these twisted positions for a time, then, even after
stoppage of such alternate twisting movements, the released
wire will continue to vibrate to and fro in expression of the
release of impressed latent strains, consequent on previous
forced vibrations.
That such forced vibrations may persist in a plant has
been shown by F. Darwin and D. Pertz, who subjected a
plant to alternating geotropic stimuli, by which a rhythmic
movement was found to persist for a time, even on the
stoppage of stimulation. Czapek and Wiesner obtained
similar after-vibrations with alternating phototropic stimuli.
Impressed periodic vibrations in organ originally
radial. — I shall here give a very remarkable instance of such
forced periodic vibrations, as induced in an originally radial
organ. I had a row of sunflowers planted, in a line which
ran accurately east and west, at the season when the sun
moved daily in an almost vertical plane. The plants were
thus stimulated in the morning from the east, and in the
afternoon from the west. All these plants, then, by helio-
tropic action, followed the path of the sun during the course
of the day, and during the course of the night, again, there
was recovery. At first the diurnal swing east and west was
only through a small angle, but under the action of these
repeated periodic impulses it became larger and larger, like
that of a pendulum under regular and repeated blows.
When the plants had grown to a height of one metre, it was
686 PLANT RESPONSE
a remarkable sight in the early mornings to see all the six,
with their upper halves bent over equally to the east, and in
the evenings equally to the west. One curious phenomenon
connected with this consisted in the fact that not only was
the nightly recovery completed by i A.M., but the upper part
of the shoot was already carried over to the east, just as we
found the Mimosa leaf to be erected to the highest position by
midnight in consequence of the presence of internal energy.
Analysis of constituent impulses causing nyctitropic
movement. — Nyctitropic movements are thus brought about
by two different periodic factors, themselves induced by the
periodic action of light and darkness. These periodic forces
acting on the pulvinus are :
1. The differential heliotropic effect on the pulvinus
itself By the stimulus of light, externally and internally
diffused, the dorsi-ventral leaf is progressively depressed
during the day. The reverse process takes place at night,
by means of natural recovery, aided by internal energy,
which gives an impulse opposite to that of external stimulus.
2. A periodic inflow and outflow of water taking place
in the plant as a whole, by the recurrent action of light and
darkness. This, acting on the dorsi-ventral pulvinus, causes
periodic movement of the petiole.
3. Both these periodic forces are concordant in their
action on the pulvinated organ, and give rise to periodic
movements of large amplitude.
Summary
The nyctitropic movement of such leaves as that of
Mimosa is believed to be distinguished from heliotropic
action proper by the facts that (i) it takes place in a definite
plane, and (2) it is caused, not by light as a constant force,
but by its variation of intensity, the fall of the petiole in the
evening being thus ascribed to the on-coming of darkness.
With regard to the first of these points, I have shown
that, even under heliotropic action proper, all dorsi-ventral
organs move in a definite plane ; and as regards the second.
NYCTITROPIC MOVEMENTS 687
it has been shown that the fall of the leaf in the evening is
not due to the action of on-coming darkness, but to the
cumulative stimulation of the day's illumination — that is to
say, to the action of light as a constant force.
It is possible to trace out the gradual evolution of this
periodic diurnal movement, taking as the simplest type that
of the plagiotropic stem. Under the action of stimulus,
which is internally or externally diffused, the lower and more
excitable side of such a stem undergoes progressive con-
cavity, the lowest position being attained in the evening.
At night, however, on removal of the stimulus of light, re-
covery takes place by erection of the stem. A diurnal up-
and-down movement is thus induced.
A similar effect is observed in the petiole of Biophytuni.
In the leaf of Mimosa the action is precisely the same.
Here, owing to the direct action, and the positive after-
effect of light, the leaf is depressed progressively till evening.
At night, however, recovery takes place by an erectile move-
ment. This is not due to a passive recovery merely, but is
aided by the negative after-effect, consequent on the storage
of internal energy by the day's illumination. By this active
internal impulse the leaf attains its highest position some
time before dawn. The alternate impulses acting periodi-
cally on the leaf are thus : (i) the direct effect of continuous
stimulation of light, and (2) the opposite impulse due to
internal energy.
In addition to these alternate impulses the plant as a
whole contributes periodic impulses, which are concordant
with the periodic impulses in the leaf. The external tissues
of the plant, acted on by light stimulus, contract and drive the
water inwards, into the central reservoir. At night a reverse
movement of water takes place. As a result of this alter-
nation of external stimulus during the day, and internal
stimulus during the night, there is a periodic inflow and out-
flow, a diminution and increase of tension, and these varia-
tions of tension are indicated by the periodic depression and
erection of all motile leaves synchronously.
688 PLANT RESPONSE
The fact that these periodic forces in leaves and in plant
act concordantly causes the periodic movements to be of large
amplitude.
The periodic diurnal variations of internal hydrostatic
tension are also exhibited by periodic variations in the rate
of growth. The curves of the diurnal periodicity of growth
and of the nyctitropic movement of the leaf are therefore
similar.
This forced diurnal vibration, being often repeated, gives
rise to periodic after-effects, which persist for a time even on
the cessation of the periodically exciting cause.
CHAPTER XLIX
ON PULSATORY RESPONSE AND SWIMMING MON'KMKNTS
AS INITIATED AND MODIFIED BY LICIIIT AND OTHER
FORMS OF STIMULUS
Investigations on the influence of light on the lateral leaflet of Dcsmodiiini gyraiis :
(a) in sub-tonic condition ; (/') in normal tonic condition - Changes induced in
existing anisotropy of Desiiiodiuiii leaflets— Reversal under intense stimulation
seen in all forms of response — The swimming movements of ciliated organisms
— Fundamental resemblance between the swimming responses and the ordinary
heliotropic responses in radial organs — Phototactic movements : {a) Two
natural types of responsive movements ; [b) Responsive movement positive,
negative, or intermediate, according to intensity of stimulation — Directive
action of light— Thermotaxis — Galvanotaxis — Chemotaxis.
We have already studied in detail the autonomous responses
of the leaflets of Desmodiuni ; but there are also other
leaves which execute similar pulsatory movements, and we
have now to investigate the effect of the stimulus of light on
such multiply-responding organs, either in initiating these
movements or in modifying them. We have also another
instance of such multiple pulsations, in the swimming move-
ments of ciliated organisms, of which the causes are at
present regarded as very obscure. The treatment of these I
include within the present chapter, because I hope to show
that they really belong to a class of phenomena essentially
similar to the autonomous movements of Desniodium, and
that from this point of view all their seeming peculiarities
may be very satisfactorily explained.
In the first place, I shall take up the question of multiply-
responding leaves or leaflets. Concerning the effect of light
on these, there is a certain amount of discrepancy of observa-
tion. Strasburger, for example, referring to such movements
V \
690 PLANT RESPONSE
in the lateral leaflets of Desmodium gyrans, says they are in
no way disturbed b}^ variation in the intensity of light.
According to Pfefifer, similarly, the autonomous movements
of the leaflets of Trifolmvi pratense are not affected by
illumination. But Strasburger again states that the leaflets
of this plant, on exposure to light, cease their oscilla-
tions.
Investigations on the influence of light on the lateral
leaflet of Desmodium gyrans. — {a) In sub-tonic condition. —
A clearer insight into this subject will, however, be obtained
when I describe my experiments on Desmodium gyrans. I
have shown that when this plant is in a sub-tonic condition it
behaves like an ordinarily responding plant, such as Biopliy-
tum ; a single moderate stimulus then evokes a single response,
and strong stimulus multiple responses. Thus a leaflet of Des-
moditmi in a state of standstill has multiple response initiated
by the incident stimulus of light. The following record
Fig. 275. Initiation of Multiple Response in Lateral Leaflet of
Dcsntodiiim originally at Standstill
Light applied at x and continued till the end of the sixth response, as
shown by the thick line. The responses show a staircase increase
with increase of absorbed energy. Pulsations persist for a short time
even on the cessation of stimulus.
(fig. 275) shows this clearly. The leaflet was in a quiescent
condition, but during the application of light it exhibited
multiple responses, which, owing to the increasing absorption
of energy, showed a staircase enhancement of amplitude. On
the cessation of light the energy absorbed maintained the
pulsation for some time.
ib) In normal tonic condition. — When the specimen, how-
ever, is in a favourable tonic condition, its pulsations being
PULSATORY RESPONSE 69I
apparently autonomous, the application of excessive stimulus
of strong light often brings about fatigue, and under these cir-
cumstances I have obtained two different types of response,
according to the particular condition of the tissue. In the
first of these, progressive fatigue, under the continued action
of light, is shown by the gradual diminution of arnplitude of
pulsation. If the strong stimulus of light, which causes this
fatigue, be applied for a short time only, the pulsation whose
amplitude was diminished, again on the cessation of stimulus
recovers its first amplitude in a staircase manner ; but if the
light be long continued, fatigue becomes so great that the
pulsatory movements are brought to a stop, at least for a
considerable period. We have seen again that an organ
sometimes exhibits fatigue somewhat differently from this,
that is to say in a periodic manner. In accordance with
this fact the leaflets of Dcsniodiuni are sometimes seen
to display periodic fatigue — that is to say, the pulses wax
and wane in groups, an example of which is seen in
fig. 276.
Thus we see that in a leaflet at standstill, light, by
supplying energy, initiates autonomous movement ; while in
an actively pulsating leaflet, strong illumination may bring
about such fatigue as to cause cessation of movement At
still other times, again, owing to the favourable condition of
the tissue, fatigue is slight, and there is no arrest of activity.
These facts are sufficient to explain the discrepancy alreadj-
mentioned, in the observations made by Strasburger and b)'
Pfeffer, on the action of the leaflets of Trifolinm pratoise
under light.
Changes induced in existing anisotropy of Desmodium
leaflets. — A very interesting consideration arises at this
point as to the differential fatigue caused in the anisotropic
organ by strong stimulus. We have seen that the lower half
of such an organ is the more excitable, hence we can see the
probability of fatigue being relatively greater there. Now, it
is the naturally greater excitatory contraction of the lower
half of the organ which causes the downstroke of the leaflet
692 . PLANT RESPONSE
to be the quicker ; but if stimulus produce greater fatigue of
the lower half, \vc can see that its movement will be made
somewhat slower, and the upstroke will then become the
relatively quicker of the two.
This anticipation finds remarkable verification in the
photographic record given in fig. 276. We see there, in the
first or normal record, from the fineness and steepness of the
upward line, representing the dovvnstroke, that this is the
quicker movement of the two ; but after the application of
strong stimulus of light, at the point marked with an upward
EiG. 276. Pholographic Record of Autonomous Pulsations in Lateral
Leaflet of Desinodiitm gyraiis under Action of Sunlight, showing
I'eriodic Reversals
Light applied continuously from arrow onwards. In the first two
responses, representing the normal, the downstroke of the leaflet,
represented by the up curve, is relatively the quicker. After the
application of light, this relation is gradually reversed, till in the
fourth and fifth pulses after application it is the upstroke, represented
by the down curve, which is pronouncedly the quicker. This reversal
is in its turn reversed at the eighth pulsation.
arrow, we observe a gradual change, by which the existing
normal difference between up and down strokesis first abolished
and then reversed. In the next complete response, after the
application, we see that the two strokes have become equally
rapid. In the next, the downstroke has become distinctly
the slower, and this goes on progressively till we see in the
fifth of this series a very remarkable degree of difference
between the two, the upstroke being now much the quicker.
These reversals are found to be recurrent, further on in the
record. Such alternate changes of excitability on the two
sides we have noticed even in the case of radial organs ; for
PULSATORY KKSl'ONSK 693
we have seen that such an organ, when acted on unilaterally
by strong light, often exhibits to-and-fro oscillations, due to
alternate fatigue of the two sides.
We thus find, starting with a Dcsniodinin leaflet in a
state of standstill, that moderate intensity of light initiates
normal pulsatory movements, in which the downstroke is
quicker than the upstroke ; but, in a pulsating leaflet, under
intense or long-continued light, these beats are reversed, that
is to say the upstroke becomes the quicker. Under the
action of continuous light, again, these reversals themselves
may become periodic or recurrent.
Reversals under intense stimulation seen in all forms
of response. — VVe have already seen that autonomous pulsa-
tion is simply ordinary response repeated, owing to excess of
energy ; and that the Dcsmodhnn leaflet is an ordinary aniso-
tropic organ, in which response may be initiated by any forna
of stimulation — -thermal, photic, chemical, or electrical. We
have just seen, further, in the case of photic stimulus, that
the character of the response may be reversed by the in-
tensity of the stimulation. Thus the normal response, in
which the downstroke is more energetic than the upstroke,
may be exchanged for a type of response in which the up-
stroke is quicker than the down. This law of rc\ersal of
response with varying intensity of stimulation has already
been shown to be illustrated in different types of response.
For example, in the case of chemical stimulation, we found
that the leaf of Mimosa when subjected to the action of
a dilute solution of sodium chloride gave an erectile response,
whereas when the solution was stronger it responded by
depression (pp. 551, 552). In the response of growth, again,
very dilute and very strong solutions of a given reagent were
shown to produce opposite effects. Thus, while dilute solu-
tion of sugar accelerated, a very strong solution retarded, the
rate of growth (p. 488).
In the case, then, of an organ which is capable of multiple
response, we find that an)- form of stimulus is competent to
initiate such response, and that, with regard to certain charac-
694 PLANT RESPONSE
teristics, such as the relative quickness of the ' up or down
movement, it may be exhibited in opposite ways, according
to the intensity of stimulus.
The swimming movements of ciliated organisms. — It
is the automatic character of ciliary movements which has
hitherto rendered them a subject of such great perplexity,
for these movements are independent of any nervous system
for their initiation or control. The impulses that lead to
ciliary motion arise in the cilia themselves. There is, again,
some difficulty in understanding the mechanics of these
movements.
Their automatism may, however, be explained by those
considerations which are now familiar to us in similar in-
stances— that is to say, initiation of movement by external
stimulus, and its maintenance by excess of latent energy.
There is, again, the closest resemblance, from a' mechanical
point of view, between the movements of the cilia and the
movements of Desviodiujii leaflets. In both cases alike, one
of the component strokes is quicker than the other. Again,
though in both cases we meet with instances in which the up
and down movements take place in the same plane, yet there
are also others in which more complicated paths, whether
circular or elliptical, are described. The fact that one move-
ment is quicker than the other, and that the strokes are
lateral, shows that we have in the cilium an anisotropic
organ, essentially similar to the pulsating pulvini of the
lateral leaflets of Desniodiuiu. The only difference between
the two cases lies in the fact that we have in the pulvinus a
multicellular, and in the cilium a unicellular, organ.
Let us suppose a detached petiole of Desmodinut, bearing
the lateral leaflets, to be thrown into water contained in
a glass trough ; let us further suppose these leaflets to be in
a sub-tonic condition, that is to say in a state of standstill.
If this vegetable organism be now stimulated by sunlight
of moderate intensity, striking it horizontally from one side
of the vessel, it will be found that rhythmic excitation is
initiated, and that the stroke backwards is much quicker and
PllOTpTAXIS 695
more energetic than that forwards. The consequence of this
will be, in the vegetable organism as in the case of a man
swimming, its forward propulsion. This result depends upon
the fact that the lower half of the anisotropic organ is in this
case the more excitable. Reversal of the relative activities
of the two halves of the dorsi-ventral organ was, however,
seen to occur in the case of the leaflets of Desviodiuui
when the intensity of stimulation was very great. Such a
reversal, under excessive stimulation, would give rise then
to a swimming movement in the opposite direction. We also
saw such reversals under continuous stimulation of light
becoming periodic (fig. 276). The corresponding swimming
response would thus consist of a movement to and fro.
Supposing, however, that the excitability of the upper
half of the motile organ had been the greater, it is clear that
the normal excitatory response would have taken the form of
a backward or negative swimming movement. We thus see
the possibility of normal responsive swimming movements of
two different types, according to the particular half of the
anisotropic motile organ which is the more excitable. Taking,
again, that type of swimming in which the response is posi-
tive or forward, a stronger intensity of stimulation may give
rise to a reversal, or negative movement. And from what
has already been said, it will be seen that similar responsive
movements may also be expected under forms of stimulation
other than light.
Similarity between swimming responses and the
ordinary heliotropic responses of radial organs. — Though
at first sight it would appear as if there were no con-
nection between the simple responsive curvatures of radial
organs and the apparently complicated responsive move-
ments of swimming, yet on a closer analysis we shall find
that there is little essential difference between the two ;
for we have seen that growth itself, or growth-curvature,
is simply a phenomenon of multiple responsive movements,
which, owing to the rapidity of the individual responses,
appears continuous. Hence, when, under moderate stimula-
696 PLANT RESPONSE
tion, the organ moves towards the Hght, or exhibits a positive
response, this means that the resultant of its multiple move-
ments is towards the stimulus, like the resultant movement
of the ciliated organism towards light. Similarly, under
strong photic stimulation, the negative heliotropic movement
of the organ corresponds to the swimming of the ciliated
organism away from light. In the intermediate case, again,
where the stimulated organ shows no resultant curvature, but
oscillates about a mean position, we have an instance which
is paralleled, in the case of the ciliated organism, by alter-
nate swimming backwards and forwards.
Again, just as in the heliotropic response of a radial
organ, the minor pulsations by which it is brought about are
too rapid and minute to be easily detected, and we can per-
ceive only the resultant movement of the organ as a whole,
.so in the case of the ciliated organism the individual beats
cannot easily be perceived, and we infer their presence from
the resultant motion of the organism as a whole.
Phototactic movements. — From the fundamental de-
monstrations which I have already given of the character-
istics of multiple response, and its modification by relative
variations of contractility, as between the upper and lower
halves of the responding organ, it will be found that the
multifarious responsive movements of ciliated organisms,
under various forms of stimulus of differing intensities, will
have been elucidated.
(a) Two natural types of responsive iiiovenients.~-T\v& two
opposite types of movement, positive and negative, which we
have now theoretically anticipated, are found to be com-
pletely illustrated in the case of swarm-spores under the
action of light. Thus, for example, in the case of Botrydiurn
grannlatiiin, they respond by a positive swimming move-
ment, or motion towards the light. Again, while certain
varieties of Ulothrix exhibit the positive effect, by movement
towards light, there is another variety which gives the nega-
tive response, by swimming away from it. This opposition
of effects is obviously due, as we have anticipated, to a
PHOTOTAXIS 697
natural difference of relative excitabilities, as between the
upper and lower halves of the anisotropic swimming organ
(p. 695).
{h) Responsive iiioveiiieiits positive, negative, or intermediate,
according to intensity of stimulation. — Turning now to the
question of different movements in the same organism as
modified by the varying intensity of illumination, examples
are furnished by the observations of Stahl and Strasburger.
These investigators find that the swarm- spores, generally
speaking, when the intensity of light is moderate, move
towards it, and when stronger, away.
In the lateral leaflets of Desmodium, again, under con-
tinuous illumination, we have observed recurrent reversals of
the direction of the more rapid of the two strokes which con-
stitute each individual pulsation. Even this phenomenon
finds a curiously exact parallel in the movements of certain
swarm-spores of Ulotlirix under the continuous action of
light, as noticed by Strasburger. These he finds first to
retire from the light, then to remain stationary, and again to
return towards the light, only then to begin the whole
process over again, thus moving to and fro for some time
like a pendulum.
Directive action of light. — The movement of the ciliated
organism, then, whether towards or away from it, is parallel
to the direction of incident light. But a difficult question
arises here as to how the organ perceives this direction as it
were, and by what mechanism it determines its own course
accordingly. At first sight, there appears no reason why
rhythmic beats caused by stimulus should not produce
propulsive movement in any direction. In this connection,
turning to the case of Ulothrix, we find the organism pro-
vided with symmetrical pairs of cilia. We also know that
the excitatory effect of stimulus of light depends upon its
angle of incidence. If, then, light strike the two cilia of a
given pair asymmetrically, they will undergo unequal excita-
tion, causing them to execute a turning movement. Thus a
stable condition can onlv be arrived at when excitation is
698 PLANT RESPONSE
equal in the two cilia, and this can clearly happen only when
the organism has so orientated itself that its axis is parallel
to the direction of the incident rays ; and it is evident that
in this position the equal beats of both cilia must result either
in progressive or in retrogressive motion, parallel to the direc-
tion of the incident rays.
Thermotaxis. — Thermal stimulation also causes respon-
sive movements towards or away from the source of stimula-
tion ; and these reactions, again, are found to change their
signs, with varying intensities of stimulus. Thus, Paramcecia
swim towards the warmer side of a vessel which is unequally
heated, provided the temperature of the warmer side does not
exceed 24° C. The response is in this case, then, seen to be
positive ; but when the temperature of the heated side is
higher than 28° C. the Paramcecia are found to swim away,
thus exhibiting a negative response.
Galvanotaxis. — Similar multiple response finds expres-
sion in certain Infusoria, in swimming movements of posi-
tive and negative character. Thus Verworn finds, for
example, that under the excitation of an electrical current,
Polytonia swims away from the kathode, and Pleuronema
towards it.
In these galvanotactic movements, also, we meet with
the same recurrent reversals with which we are already
familiar in the case of the leaflets of Dcsvwdium. In work-
ing with Parania:ciuin, Arthur W. Greely ^ found that after
being subjected for about half an hour to a moderate current,
the organisms which had previously gathered round the
kathode reversed their action, and moved towards the anode,
only to dart back immediately, again, towards the kathode,
I have demonstrated in Chapter XXXVI. the opposite
responsive changes which occur under the action of acids
and alkalis respectively. This explains the appropriate
modification of galvanotactic response in Parainarium,
according as the organism has been reared in an acid or in
' Science has lost a very promising worker in the early death of this investi-
gator. His experiments on Paraiiiwchim are very valuable and suggestive.
CHEMOTAXIS 699
an alkaline culture. Thus Greely has found that while
alkali-reared Paramcecia invariably give the initial response
by swimming towards the kathode, the reverse is generally
the case with those which have been subjected to acid — that
is to say, the latter as a rule begin by swimming towards the
anode.
Chemotactic movements.— ^Similar effects are, again,
observed under chemical stimulation. The opposite reactions
of acids and alkalis which have already been described are the
occasion of opposite chemotactic responses. Thus Jennings
found Paramaxia moving towards, or showing positive reaction
to, acids, whereas to alkalis they exhibit negative response,
or movement away. Chemotactic reaction, again, is modified
by the strength of the solution, that is to say by the intensity
of stimulation. The opposite effects of strong and feeble
doses (p. 488) are here illustrated by the fact that in many
organisms positive response is observed under the action of a
weak .solution, and negative under a strong. Connected with
the subject of chemotaxis is the interesting phenomenon of
the response of the antherozoids of ferns and of Sclagijiella
to malic acid, as discovered by Pfeffer. In the response of
growth we found that dilute solutions of sugar gave rise to
one kind of response, that is to say to acceleration of growth,
and that with very strong solutions, say about 10 per cent.,
the opposite occurred — that is to say, retardation (p. 482).
Now, it is found by Pfeffer that whereas the dilute solution of
a malate exerted an attractive influence on the antherozoids,
a 10 per cent, solution had a repellent effect.
Summary
A rhythmic vegetable organ, in a state of standstill, has
its autonomous movement renewed by a supply of energy
from incident light.
Too strong an intensity of light may, by causing fatigue,
arrest such movement. Or the greater fatigue of the more
excitable half of the organ may cause a reversal of the relative
rapidities of the up and down beats.
yoo PLANT RESPONSE
111 Desinodiui/i,Vir\der the continuous stimulation of strong
light, these reversals are often recurrent. The dovvnstroke,
which is at first quicker, becomes less quick than the upstroke,
and this may be again and again reversed.
In a ciliated organism the swimming movements are
explicable by similar unequal up and down strokes of the
anisotropic cilia. When the downstroke is quicker, the
organism propels itself forward. When the upstroke is quicker,
there is a movement backwards. Such swimming movements,
due to multiple response, are initiated by stimuli of various
forms. There are two natural types of these responses :
positive movement, or swimming towards, and negative, or
away from, stimulus. These are determined by the relative
excitabilities of the upper and lower halves of the cilium.
Unilateral light exerts a directive action on responsive
swimming movements, because the only stable position is one
in which pairs of cilia are equally excited. The axis of the
organism is thus orientated till parallel with incident light.
Strong stimulation of light, for reasons described, causes a
reversal of the normal movement. Alternate periodic fatigue
causes a responsive movement of the organism to and fro.
Under moderate unilateral thermal stimulus the organism
exhibits a positive swimming movement, and under stronger
a negative movement.
Similar responses to stimulus are exhibited under galvanic
excitation. The normal response here also undergoes reversal
under long-continued stimulation ; since the actions of acids
and alkalis are antagonistic, organisms reared under acid
and alkali cultures tend to exhibit opposite reactions under
galvanotaxis.
Under chemical stimulation, swimming organisms exhibit
multiple responses. Acid and alkali, owing to their an-
tagonistic actions, bring about opposite responses. The.
same reagent again occasions opposite responses, according
to the amount of the dose. Thus antherozoids of ferns are
attracted by the dilute solution of a malate, but stronger
solutions exert a repellent action.
PART IX
GENERAL SURVEY AND CONCLUSION
CHAPTER L
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE
Responsive contraction — Kunchangraphic records — Direct and indirect effects uf
stimulation — Various forms of responsive expression ; (a) Lateral motile
response by differential contraction ; (d) Suctional response ; (c) Growth-
response ; (d) Torsional response ; {e) Death response ; {/) Thermographs of
regional death ; (^) PZlectrical response— Different types of response : {a)
Uniform response ; (d) Fatigue ; (<:) Staircase response— Excitability-
Conductivity — Polar effects of currents — Multiple response — Continuity of
multiple and autonomous responses — The ascent of sap.
We have now studied in detail the manner in which a plant
reacts to the varied forces of its environment, and although
its response to them finds modes of expression which appear
highly diverse, yet we have found that on analysis these are
all reducible to two very simple and well-defined factors, of
responsive contraction and expansion.
Responsive contraction. — In vegetable cells, as in other
protoplasmic bodies, it has been shown that the impact of all
external stimulus evokes responsive contraction ; and this
we found to be true, not in the so-called sensitive plants
alone, but in all plants. Taking the simple case of a radial
vegetable organ, such as a stem, style, stamen, or other
filamentous structure, we find that it undergoes longitudinal
contraction under stimulation. Here, then, we have a phe-
nomenon which is analogous to the contraction of muscle,
The amount of contraction of these ordinary vegetable
organs, further, is sometimes very considerable, as we saw in
the case of the coronal filament of Passiflora, where it was
as great as 20 per cent, of the original length. Such
responsive contraction takes place, moreover, under all forms
of stimulation, mechanical, thermal, electrical, photic, and
chemical ; and we have found that all the various move-
704
PLANT RESPONSE
ments of plants which are seen in nature, under the action of
external stimulus, are but different expressions of a single
fundamental response by contraction.
Kunchangraphic records. — By taking advantage of this
responsive contraction, we are able to study all the physio-
logical modifications induced in the vegetable tissue by
various reagents, with as great ease and certainty as similar
phenomena can be studied in animal muscle, using myographic
records. By such study, again, we are once more led to
see how misleading has been the superficial distinction be-
tween sensitive and non-sensitive plants, since the latter,
or so-called ordinary, plants also exhibit contraction under
stimulation.
Direct and indirect effects of stimulation.— The living
organism is thus a delicately responding machine, whose
responsive movements are brought about by external stimulus ;
but this complex machinery has also the power of holding
part of the energy of the external stimulating shock latent,
for a longer or shorter time, so that part only may find
immediate expression, while the rest is stored up as internal
energy to be given out after the lapse of an intervening
period. These two factors, of external stimulus and internal
energy, again, induce opposite effects, of contraction and
expansion respectively. And the infinite multiplicity of
responsive processes in the life-cycle of the plant is brought
about by their mutual play. That the combination of these
two elements in varying degrees of each, finding expression in
different ways, creates a tangle which would at first sight appear
inextricable, can be easily understood. And it was the bewilder-
ment which this fact imposed upon the observer, that drove
us to postulate the existence of an unknown and indefinable
vital force, whose mysterious working was to be held to
account for the occurrence of all those phenomena that we
were otherwise unable to explain.
It is possible, however, as we have found, going back step
by step, to trace out the different expressions of these two
distinct factors, of external stimulus and internal energy, and
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 705
to show, moreover, how the latter may be derived from the
former. This may be clearly and easily seen by taking a simple
case, in which we excite the stem by the application of external
stimulus, on a zone a few centimetres below its upper end.
As the direct effect of this stimulation, a contraction takes
place in that zone. By this contraction an active expulsion
of water, with local negative turgidity-variation, is brought
about, and the expelled water is driven outwards in both
directions from the excited zone. Following the course
of the water which is thus forced upwards, we have found
that it produces an increase of turgidity, or positive turgidity-
variation, with consequent distension or expansion of all the
cells above the stimulated area. Work is thus performed on
these cells, in consequence of external stimulus, by which
their latent energy is increased.
The energy of the stimulus applied in one has thus been
conveyed to another region, hydraulically, there to give rise
to an effect of increased turgidity and expansion, which was
designated as the indirect effect. Thus the effect of external
stimulus is seen to be twofold — namely, first by its direct
action to induce local contraction, and secondly by its indirect
effect, of increasing the internal energy, to bring about an
expansion. The expressions of direct and indirect stimula-
tion are thus seen to be opposite in character, and we have
seen how they find opposite modes of expression in the case
of each form of response.
Various forms of responsive expression. — The next
question to be passed in review is that of the various modes
in which the response of the plant was seen to find expression ;
and here we found various responsive phenomena which arc
characteristic of life, and apparently entirely unrelated, to be
ultimately dependent upon this fundamental inter-action
between, on the one hand, the contraction due to external
stimulus, and on the other the expansion due to internal
energy.
(ir) Lateral iiioiilc irspo)isc by diffcirutial contractioti. —
Taking first the responsive mechanical nio\cment of motile
Z Z
7o6 PLANT RESPONSE
organs, we find that its gradual evolution can be traced from
the simplest case, namely, the longitudinal response of a
radial organ. In such a radial organ, if one side be rendered
in any way the less excitable — say by the unilateral applica-
tion of cold or anaesthetics, or by the fatigue induced by
long-continued unilateral stimulation— we have an induced
anisotropy. On now subjecting the organ to diffuse stimula-
tion, the relatively more excited side will undergo the
greater contraction, and the response will thus be by that
movement which results from the concavity of the more
excited. We find many instances again of the various stages
through which this anisotropy passes before it culminates in
the dorsi-ventral pulvinus ; thus, for example, in a spirally
formed tendril, where concavity has been induced by the
unilateral excitation of that side, the concave surface is less
excitable than the convex ; and such a tendril, when
diffusely excited by strong electrical stimulus, exhibits the
excitatory effect by extraordinary writhing movements due
to the relatively greater excitation and concavity of the
originally convex side (p. 92). A plagiotropic stem, again,
whose upper side is fatigued by the action of sunlight,
exhibits on diffuse stimulation a downward movement, due
to the greater excitatory contraction of the more excitable
lower half (p. 86). The phenomenon of differential response,
then, whose various preliminary stages we have thus traced,
comes to its greatest perfection in the dorsi-ventral pulvinus
of such plants as Mimosa, for we have found that in the
last-named organ the characteristic responsive movement is
not brought about by the action of excitable cells restricted
to the lower half That the upper half also is excitable is
shown by the fact that on applying to it localised stimulus,
say of light, its cells contract and raise the leaf (p. 63 1). The
fall of the leaf, on the application of diffuse stimulation, is
thus the result of the greater excitability of the lower. This
fall, again, is not due to mere flaccidity caused by the ex-
pulsion of water from the excited organ, but to the actual
differential contraction of the lower half, whose activity may
REVIEW OF RESl'OXSl-,, SIMPLE AM) MULTIPLE 707
be gauged by the tension it can be made to exert on a
spiral spring (p. 26). As regards mechanical response, then,
a single law — that response is always by concavity of the
more excited side— will be found universally applicable. In
a dorsi-ventral organ, like the pulvinus of Mimosa, we have
seen that response to diffuse stimulation is always by the
contraction of the more excitable lower half The same
law, however, is also applicable even in the case of a radial
organ excited unilaterally, for here the side acted on is
relatively the more excited, and response is by concavity of
that side.
A radial organ, acted upon from one side, undergoes
concavity of that side, and consequent movement towards
the stimulus. The same is true of pulvinated organs, when
either the upper or lower half is acted upon locally by
stimulus. As in radial organs, then, so also here, under these
circumstances, we obtain instances of the directive action of
stimulus ; but when stimulus is either internally or externally
diffused, we obtain from a dorsi-ventral pulvinus the greater
contraction of the more excitable half, and movement is
thus made, for anatomical reasons, to occur in the direction
at right angles to the plane which separates the two aniso-
tropic halves.
Even in Mij/iosa, the contraction of the puKinus itself is
not very great, but the contractile mo\ement is highl}-
magnified by the attached petiolar index. When a radial
organ is diffusely stimulated there is no lateral mo\cment at
all, and from this fact it has been erroneously inferred that
ordinary plants are not sensitive. In fact, however, every
plant is sensitive, and exhibits contractile response, which
is shown, in the case of radial organs, by longitudinal con-
traction under diffuse stimulus, and by lateral response under
unilateral .stimulus.
The direct effect of stimulus on the puKinus of Mimosa,
causing a negati\e turgidity-variation of the organ, finds
appropriate expression in the responsive movement of fall.
But we have seen that the expression of the indirect effect of
■i.Z2
708 PLANT RESPONSE
stimulus is opposite in character to that of the direct effect.
The indirect effect of stimulus, or the increase of internal
energy, thus results in an erectile response, as seen in the
erection of the leaf in MiJiiosa, or Biophytuni^ when their
internal energy is in any way enhanced, say by rise of
temperature (p. 400). Taking all kinds of response, it will be
found universally true thit if the increase of internal energy
give rise to one form of expression, the impact of external
stimulus evokes the opposite.
ib) Sitctional response. — It has been shown that con-
tractile response gives rise to a forward impulsion of water ;
hence by the excitatory contraction of the root-cells the
movement of water upwards is initiated. When such con-
tractile movement is not single, but repeated or multiple,
continuous propulsion of water is maintained. The rate of
this propulsion therefore affords us a means of measuring the
rhythmic activity of the plant-tissue.
(^) Growth response. — This pumping-in of water causes
the transmission of energy to the distant growing region, and
as by this the internal energy of the plant is increased, it
finds expression there in a pulsatory expansion, which is the
movement of growth. Following each pulse of expansion,
there is a recovery which is incomplete, owing to fixation of
o-rowth-material The resultant growth is thus the irreversible
effect of the entire process. This growth-movement is another
expression of that indirect effect of stimulation which we
have already considered, by which a distant excited point
gives rise to a progressive train of waves of positive turgidity-
variation. The action of these waves may be seen not only
in the movement of expansion at the growing point, but also
by the erectile response of an intervening motile organ. This
will be understood from the following diagram of an artificial
plant (fig. 277), which shows how contractile action at the
base, giving rise to an hydraulic wave, causes two different
expressions of {a) motile response of the lateral organ, or leaf,
and (b) growth-expansion of the terminal growing point.
The pulvinus of this artificial motile organ consists of an
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 709
india-rubber tubing, the lower half of which is thinner, and
therefore more expanding or responsive, than the upper.
The upper end of the tube, representing the end of the plant,
is closed over with a thin elastic strip of india-rubber. When
now there is contractile action at the base, the pulse of
increased hydraulic pressure thus induced is found to bring
about a practically simultaneous erection of the artificial
lateral leaf L, and an expansion or bulging outwards of the
terminal yielding body G. The ..^
analogy in the case of the latter
would be still more perfect if
we imagined it covered, instead
of with india-rubber, with some
plastic substance which would set
quickly on elongation or expansion,
thus representing the permanent
growth-effect. From this we see
the connection between growth and
those other responses with which
we are already familiar. The only
difference between the responsive
expansion of pulvinated organs and
this responsive expansion of growth
lies in the fact that in the former
recovery is perfect, and in the latter
imperfect. It must also be borne
in mind that growth may be
initiated locally at the growing region if the sum total
of energy, directly or indirectly supplied to it, be above
par.
We further see in the case given that growth-response is not,
strictly speaking, ' of its own accord ' or spontaneous ; for the
energy of a definite stimulus, causing contractile movement
at the base, is transmitted hydraulically, and performs the
work of growth. It would be as accurate to describe the
work done by hydraulic machinery as spontaneous, ignoring
the energy that had set the pump in action, as to call growth
277. Model of an Arti-
ficial Plant
Sudden compression of india-
rubber bull), R, causes
hydraulic wave upwards,
downward movement being
prevented by valve V.
This wave of increased
pressure causes erection of
the leaf, l., and the expan-
sion of the terminal se[)lum
into c.
710 PLANT RESPONSE
spontaneous, without recognising or tracing that energy
which in the form of stimukis must have been supplied to
some part of the plant machinery.
We have seen that a plant in a state of growth-standstill
has its activity renewed when a stimulus is applied to the
distant root, and that when the amount thus supplied is
exhausted, the actixity ceases. In growth, then, which is
regarded as so characteristic a phenomenon of vital action,
we see the law of the conservation of energy holding
good, as in an ordinary inorganic system. The plant thus
expresses the absorbed energy, by a responsive expan-
sion, either of erection or of increased rate of growth,
according to the particular organ of response which is
concerned, both of these constituting cases of work done
by internal energy, or indirect effect of stimulus ; but where
the responding organ is directly excited by external stimulus,
we obtain a contractile response of the organ, with expulsion
of water, or negative turgidity-variation. The hydraulic cur-
rent in our model is now reversed, and opposite responsive
movements take place, by the fall of growth below the
normal rate, and by depression of the leaf If now we take
a balanced record of growth, the impact of a uniform series
of external stimuli will be found to give rise to a series of
responses by depression of the rate, followed by recovery,
exactly similar to those records of depressions of the leaf,
with recoveries, which are obtained from pulvinated organs.
We may also see the expression of external stimulus and
cessation of stimulus, in the appropriate periodic variations
of growth, and movements of motile organs, which occur
under the stimulating action of daylight, and the withdrawal
of such stimulus at night. Thus in the daytime we see
a response consisting of depression of the rate of growth,
which corresponds to the depression of the leaf, say oi Mimosa,
and at night a recovery, or enhancement of growth, and
erection of the motile organ. The daily periodic curves
obtained of this growth-variation and responsive mechanical
movement are very similar.
REVIEW OF RESPONSE, SIMPLE AND MULTII'EE 71 I
{d) Torsional response. — Another interesting type of
response occurs when an anisotropic or dorsi-ventral organ
is stimulated laterally. Under such conditions, a responsive
torsion is induced, by which the less excitable side is made
to face the external stimulus. The extent of the response
depends on the intensity of stimulus, and the differential
excitability of the anisotropic organ. In dorsi-ventral organs,
the plane of anisotropy is fixed. Rut in certain climbing
plants, this plane revolves in a positive or negative direction,
and under the internal activity of growth, an autonomous
torsional movement is thus observed. The opposition of the
effects of internal energy and external stimulus is here seen,
when such an organ is uni-laterally acted on, say by light.
The autonomous torsional movement is then found to be
retarded, or even reversed.
{e) Death response. — There is another curious phenomenon
of response, which takes place at a certain definite point.
When an organ is gradually raised in temperature, the
internal energy is increased, and the organ exhibits a re-
sponsive expansion, if radial by elongation, or if pulvinated
by erection ; but when the death-point is reached, a sudden
and irreversible molecular change takes place, attended by
an excitatory contraction. In the curve of thermo-mechanical
response we here find a sharply defined point of reversal,
which affords us an exact index of the death-point. This
death-point is very definite in plant-organs under normal
conditions ; in phanerogamous plants it is very near 60^" C.
Physiological modification of the tissue, moreover, ma}' be
gauged by the transposition of the otherwise definite death-
point (p. 185).
(/) Thermographs of regional death. — Just as there is a
definite point of reversal in the thermo-mechanical curve, so
there is also a point of discoloration which is, under standard
conditions, at a determinate interval from the death-point.
A particular region, physiologically changed, may thus be
thermally * developed,' and made to exhibit as a thermo-
graph, a picture of localised ph)'si()logical \ariation ^p. 184).
712 PLANT RESPONSE
(£■) Electrical response. — We have, lastly, to consider
briefly the electrical mode of responsive expression. The
excitatory contraction, with negative turgidity-variation, of
a vegetable, as of an animal tissue, is accompanied by an
electrical variation of galvanometric negativity. The in-
direct effect of stimulus with its positive turgidity-variation,
moreover, has also a concomitant electrical expression of
galvanometric positivity (p. "i^j^.
Different types of response : {a) Unifonn response. —
These various expressions of response are brought about by
the fundamental molecular change induced by stimulus.
When stimulus is moderate, and sufficient time allowed for
recovery from molecular distoition to the original condition,
a series of responses to uniform stimuli will be uniform ;
but if very strong stimulus be applied, recovery will only be
completed after a long interval.
{b) Fatigue. — If successive stimuli be applied before
complete recovery has taken place, the successive responses
will exhibit diminution, or fatigue. Under strong and long-
continued stimulation the plant-tissue exhibits, as in the
case of tetanised muscle, a fatigue-reversal — that is to say,
the contracted tissue passes into what is apparently its
original expanded condition ; but the difference between
the normal condition and the condition of fatigue-reversal
is seen in the fact that, while the former is sensitive to fresh
stimulation, the latter is insensitive. The fatigued tissue,
however, resumes its original excitability after a period of
rest. This fatigue-reversal explains the erection of the
Mimosa leaf under continuous stimulation (p. iio). We
observe similar fatigue-reversals, even in inorganic substances
like india-rubber, where the normal contraction under thermal
stimulation passes into relaxation under the long-continued
action of such stimulation ; and the india-rubber becomes
sensitive again only after a sufficient period of rest (p. 120).
In connection with this, we sometimes meet with the very
curious case of alternate or periodic fatigue, both in living
and in inorganic substances. The simplest type of this
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 713
occurs when, under uniform stimuli, responses are alternate!}'
large and small. These alternations sometimes show them-
selves in groups. Under continuous stimulation, again, this
periodic fatigue exhibits itself by response of an oscillatory
character.
(6-) Staircase response. — The tissue is sometimes in a
relatively sluggish condition, and by the absorption of
stimulus the molecular mobility is gradually increased. The
effect of this is seen in the amplitude of successive responses,
increasing in a staircase manner.
It is usually supposed that response is brought about by
a chemical run-down of energy, of an explosi\e character.
The external stimulus is thus supposed to act as it were
on a trigger, to release the latent energy. The response
is hence assumed to be disproportionately larger than the
stimulus. That this cannot, however, hold good in all
cases is clear ; for the tissue is often found to absorb a
certain proportion of the incident stimulus, the immediate
expression of response being thus disproportionately smaller
than stimulus. The energy of the system is now found,
instead of being lowered, to be raised above par. The
internal energy thus held latent is sometimes seen, as
in the case of strongly stimulated Biophytuvi, to find
expression later by multiple response. In the case of
growth-response, again, it is a variable fraction of incident
stimulus that finds immediate expression in the direct effect
of retardation of growth, whereas the absorbed component
gives rise to the subsequent responsive effect of an enhanced
rate of growth. Referring to the former of the.se as the
direct, and to the latter as the indirect, effect of stimulus, it
is found that the sum of the two remains approximately
constant. Below the optimum tonic condition it is found
that the indirect effect is relatively the larger, but near the
optimum this relation is reversed, and the direct effect is
the larger, in a sub-tonic condition stimulus produces little
or no direct effect, it being utilised to produce the indirect
effect of enhanced growth. At the optimum, on the other
714 PLANT RESPONSE
hand, practically the whole of the incident energy is expressed
in direct response, there being little or no absorbed element
(p. 460). A similar series of considerations may be applied
to the response of mature pulvinated organs. In this case,
however, the indirect effect of stimulus will find expression
in an enhancement of the rate of recovery of the organ. It
is, however, difficult always to discriminate with certainty
between the natural and an enhanced rate of recovery ;
but on turning to growth-response we find that, by using
the balanced method of record, it is easy to distinguish
between the direct and indirect effects of stimulus, since
these are here shown by curves in opposite directions.
This method moreover affords us some means of measuring
the relative magnitudes of the two factors in the response.
Excitability. — It is interesting to find that an agency
which induces a variation of excitability produces a similar
modification of all the different forms of response. In this
way the long-continued application of cold has the effect of
lessening excitability, and the response of a motile organ is
thus found to be temporarily diminished or abolished. At
the moment of application, however, owing to the fact that
any sudden variation of environment acts as a stimulus, its
effect is the induction of an excitatory movement. These
phenomena are repeated with curious exactness in the case
of suctional response. On the application of ice-cold water
to the root, the immediate effect is a transient exaltation of
suction, followed later by depression and arrest (p. 375). In
growth-response, also, growth is diminished or arrested by
this agency.
Another effect of the moderate application of cold is to
induce a molecular sluggishness by which the latent period is
increased. A moderate rise of temperature, on the other
hand, increasing the molecular mobility, has the contrary
effect, of reducing the latent period.
Anaesthetics, again, induce a diminution or abolition of
excitability, as is seen by their effect on the various forms of
response.
REVIF.W OF RESPONSE, SIMPLE AND MULTIPLE 715
The excitability of a tissue which has not recovered fully
from previous strong stimulation is found to be impaired.
The fatigue-effect only disappears after the lapse of a period
of rest. If, then, the resting intervals between successive
stimuli be gradually shortened, the motile responses will be
found to be progressively diminished, and a time arrives
when the succeeding stimulus evokes no response, the tissue
having become as it were refractory. The minimum interval
during which the tissue remains thus irresponsive is known
as the refractory period. In BiopJiytiim, under normal condi-
tions, this period is about ten seconds in duration (p. 273).
Conductivity. — It is usually supposed that the trans-
mission of the excitatory effect, as seen in sensitive plants
like Mimosa^ is merely the transmission of a hydro-
mechanical disturbance, and therefore unlike the transmis-
sion of the excitatory effect in animal tissues. It has,
however, been shown that this is not the case, for here, as in
the animal, transmi.ssion of excitation takes place by the
propagation of protoplasmic changes. It was shown further
that the hydrostatic disturbance was transmitted with a
relatively great rapidity, whereas the true excitatory effect
has a slower and definite velocity, characteristic of the
particular specimen. This velocity, moreover, is found to be
modified, and that in a manner precisely similar, by all those
agencies which modify the velocity of transmission of excita-
tion in animal tissues. Thus, cold, anesthetics, and fatigue
are all influences which reduce the velocity of transmission.
As an example of this, we saw, in a certain specimen of
Biophytiuii, in which the normal velocity was 3-8 mm. per
second, that slight cooling reduced it to 1-3 mm. per second,
or almost to one-third. Conversely, the raising of the
temperature from 30° C. to 37'' C. increased the velocity from
37 to 9-1 mm. per second. Strong stimulus is found to be
conducted further and more rapidly than feeble or moderate.
It is found in the case of animal tissues, again, that on
account of its physiologically depressing effect, the anode
acts as a block to the transmission of excitation ; and the
7l6 PLANT RESPONSE
same statement holds good in the case of the plant. The
passage of mere hydro-mechanical disturbance could not
have been affected in any way by the anode.
Nor is this transmission of excitation confined to sensi-
tive plants alone. The fact that it occurs in all plants alike
I have been able to demonstrate by various other methods,
in using which we are rendered independent of the mo-
tile indications afforded by lateral leaflets. Thus by the
Electrotactile Method we are enabled to detect, in any
zone of the plant, the moment of arrival of the state of
excitation from a distant point (p. 259). The Electro-
motive Method, again, displays the moment of arrival of the
wave by the induced galvanometric negativity of the point
(p. 261). The Chemical Method, again, shows the arrival
of the wave of excitation by the projection of a dense
precipitate, produced in a suitable solution (p. 255). All
these different methods give us results which are in mutual
agreement.
As showing how ill founded is the common distinction as
between sensitive and ordinary plants, it was demonstrated
that the velocity of transmission in some of the latter is
greater than in the former. Thus in Ficiis rcligiosa the
velocity was determined at 9*4 mm. per second, whereas in
the 'sensitive' Neptttnia oUracca it was only ri mm. per
second. The velocity of transmission of the excitatory
impulse in plants is found, again, to be of the same order
of magnitude as in the nerves of lower animals (p. 252).
I have shown further that excitation is transmitted in
the plant in both directions. It is, however, interesting to
note that, generally speaking, the facility of this trans-
mission is greater centrifugally — that is to say, in the
direction of the ascent of sap— than centripetally. Thus, for
example, in a petiole of BiopJiytuniy while the centripetal
velocity was i'8 mm. per second, the centrifugal velocity was
3 27 mm. per second.
Since conduction of stimulus takes place by transmission
of protoplasmic change, such change is naturally conducted
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 717
most easily along- those paths in which there is most proto-
plasmic continuity. Parenchymatous tissues, in which the
cells are divided from each other by more or les5 complete
septa, are thus relatively inefficient as conductors of the
excitatory effect to a distance. Hence, the lamina of the
leaf does not transmit its local excitation to any distance ;
but tissues which contain fibro-vascular elements, and
which are thus characterised by a greater protoplasmic
continuity, are therefore batter conductors of excitation.
Thus stems, petioles, and peduncles are better conductors
than the laminre of leaves, and parallel-veined leaves, again,
are better in this respect than reticulated. As regards stems
petioles, and peduncles, moreover, the conducting power is
greater longitudinally than transversely. In the peduncle
of Musa, for example, I find the conductivity lengthwise to
be three times as great as that crosswise. In consequence
of this difference, it is found that the transmitted excitatory
effect of a stimulus unilaterally applied is greater on the
same than on the opposite side. This explains the appear-
ance of responsive concavity at a distance from the point of
stimulation, but on the same side.
It is to be remembered that, owing to the fact that this
conduction of the effect of stimulus is an excitatory process,
we find that in autumn and winter, when the physiological
excitability is low, the conducting power of the tissue is also
very much reduced.
Various degrees of conductivit}' are possessed by different
tissues, and the distance to which the excitator\' effect is
conducted depends not only on the conducting power, but
also on the strength and duration of stimulus. Thus, while
in a feebly conducting tissue the effect of moderate stimu-
lation is not transmitted to any distance, strong and long-
continued stimulation is transmitted to a certain extent.
Even in a better conducting tissue the excitatory effect of a
moderate stimulus, on account of gradual cnfecblcment, can
only reach up to a certain distance. It is to be borne in
mind that, while no tissue is absolutely non-conducting.
7l8 PLANT RESPONSE
neither is any a perfect conductor, the difference between
extreme examples being one only of degree.
The true excitatory effect, whether due to direct or trans-
mitted excitation, consists, as has been shown, of contraction,
with concomitant negative turgidity-variation. The result
of this contraction and concomitant expulsion of water is,
however, the sending out of a wave of positive turgidity-
variation. Thus, up to the point reached by the true
excitatory effect, we obtain contraction, with negative
turgidity-variation; and beyond this point, a positive
turgidity-variation, with consequent expansion. This latter
effect we have designated as the indirect effect of stimulus.
It is thus seen that, whereas the direct effect of unilateral
stimulus is a concavity, its indirect effect is a convexity.
We have also seen that it is possible by electrical means to
determine whether it is the direct or indirect effect of stimu-
lus which has in any given instance reached a point from a
distance ; for the indirect effect of stimulus, with its positive
turgidity-variation, is always attended by galvanometric posi-
tivity, whereas the true excitatory effect with its negative tur-
gidity-variation is characterised by galvanometric negativity.
A tissue may conduct without exhibiting any motile
indication of its state of excitation. With reference to this it
is to be borne in mind that certain advantageous circum-
stances are necessary for the display of motile response ; for
since the fall of an excited leaf, such as that of Mimosa,
takes place in consequence of the expulsion of water, it
follows that when this is in any way impeded, as by over-
turgidity of the tissue, there may be excitation without any
responsive movement ; for this reason, the leaflets of Bio-
phytum, in the morning, when they are most tense, are not
so sensitive as later in the day. Motile excitability is as a
rule found to be abolished earlier than conductivity ; hence
a strong stimulus may be conducted through a region which
exhibits, through narcotisation, no motile excitability (p. 229).
Polar effects of currents.— Another observation by
which the fundamental identity of excitatory phenomena in
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 719
the animal and vegetable may be seen, lies in the respective
effects induced at anode and kathode ; for example, em-
plo}ing the leaflets of BiopJiytuni as experimental specimens,
and using a moderate E.M.F., wc find that the excitatory
depression of the leaflets takes place at the kathode at make,
and at the anode at break. The antagonistic effects of
anode and kathode are further seen in the fact that while the
kathode-make excites, the anode-make depresses. It is
owing to this latter fact that an excitatory wa\e is blocked
during transit at an anodic area (p. 233).
Another very interesting difference between anode and
kathode, both at make and break, is seen in the fact that
at make, while an induced contraction takes place at the
kathode, an induced expansion occurs at the anode ; at break
both these effects are reversed, there being now an expan-
sion at the kathode, and contraction at the anode. Itxpansion
at make, moreover, attains its maximum in a short time,
while the kathodic contraction is relatively strong and per-
sistent. These fundamental effects find appropriate expression
in the response of growth. Thus, the unilateral application
of the anode induces expansion, acceleration of growth, and
resultant convexity, while the effect of the kathode is to
induce contraction, retardation of growth, and resultant con-
cavity (p. 558).
Owing to the fact that kathodic action is .stronger than
anodic, a feeble or moderate current flowing through the
soil exerts a predominant excitatory action on the roots, by
which the suctional activity of the plant is increased. The
result is an increased rate of growth of the plant, which is
independent of the direction of the current through the soil
(p. 560).
The normal polar effects which have been described take
place under the action of a moderate electromotive force.
When this is excessively high, however, the normal effects
are, or tend to be, reversed. In this reversal there a])pear to
be two stages— the A stage and the B stage. In the .\ stage
both anode and kathode excite at make ; but in the B stage.
720 PLANT RESPONSE
under a still higher E.M.F., there is a complete reversal,
inasmuch as the anode here excites at make, and the kathode
at break. This reversal, further, is facilitated by fatigue of
the tissue (p. 215).
These polar effects may also, as I have shown, be demon-
strated in the case of animal tissues by means of the glow-
response of the firefly. An excitatory reaction is here shown
by an increase of the intensity of luminescence, and a
depressing reaction by its diminution.
Multiple response. — When response is observed by
means of the electromotive or electrotactile method, v/e
obtain a single response to a single moderate stimulus ; but
on the application of strong stimulus a multiple series of
responses is found to be evoked. In the case of the retina,
similarly, a single intense stimulation by light gives rise to
recurrent visual impulses.
In the same way, in the leaf of Biophytum, while a single
moderate stimulus gives rise to a single mechanical response,
a .strong stimulus gives rise to a multiple series of responses.
In this case certain other peculiarities may also be observed ;
for instance, a certain minimal intensity of stimulus induces
the maximal mechanical response, which is not increased by
any increase of intensity of the stimulus. The excess of such
stimulus is held latent by the tissue for the time being, to find
subsequent expression as rhythmic multiple response. These
multiple responses are evoked by all forms of stimulation,
mechanical, thermal, chemical, photic, and electrical. In
this respect, of the minimally effective stimulus inducing
maximal response, we have an important point of resem-
blance between the actions of a rhythmic plant-tissue and
the cardiac muscle of the animal. Both, again, are charac-
terised by the exhibition of a long refractory period, which is
an expression of fatigue, or temporary loss of excitability
after excitatory discharge. The periodic oscillation of excit-
ability which is thus induced, imparts a rhythmic character
to the mechanical expression of the excess of stimulus which
is held latent in the tissue.
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 72 1
The sum total of the energy derived by the plant from
the various stimuli of its environment determines what is
known as its tonic condition.
Continuity of multiple and autonomous responses. —
There is no line of demarcation between the phenomena of
multiple and autonomous response. When the latent or
internal energy of the plant is above par, it finds expression
in the form of multiple response, which is apparently auto-
matic. Taking the typical case of a multiply-responding
plant which is furnished by Biophytiivi, we find, on supplying
it with excess of energy, by maintaining it at the tempera-
ture of say 35° C, and thus exalting its tonic condition, that
it displays autonomous response. Conversely, when the
tonic condition of an autonomously responding plant, such
as Desmodium^ is in any way reduced, by reason of low tem-
perature, unfavourable season, or other circumstances, it
becomes converted into an ordinarily-responding plant like
Biophytuni. A single moderate stimulus now gives rise to a
single response, and a strong stimulus to multiple responses.
It is in accordance with this, that a Desmodinin leaflet in
a state of temporary standstill has its multiple or autonomous
response renewed by any circumstance, or combination of
circumstances, which sufficiently enhances the internal energy
of the plant. Amongst such circumstances are: (i) The
action of light ; (2) favourable temperature ; (3) the presence
of stimulating chemical substances ; (4) an increase of internal
hydrostatic pressure.
The energy which expresses itself in pulsatory move-
ments, then, may be derived by the plant either directly
from immediate external sources ; or from the excess of such
energy already accumulated and held latent in the tissue,
aided by the incidence of external stimiulation ; or from an
excessive accumulation of such latent energy alone. I'hus
there is, strictly speaking, no such thing as automatism, for
only when acted upon by stimulus can a living tissue give
responsive indications. The impact of an external stimulus
may give rise to immediate response, or it may be held
3 A
722 PLANT RESPONSE
latent, in whole or in part, for subsequent expression. ' Inner
stimuli ' are simply external stimuli absorbed previously, and
held latent. A plant or an animal is thus an accumulator,
which is constantly storing up energy from external sources ;
and in the case of the plant, its suctional activity, determining
the ascent of sap, its growth, and its spontaneous motile
indications, are some of the principal forms in which this
accumulated energy finds expression.
The ascent of sap. — The ascent of sap has been shown
to be due to a multiple excitatory reaction of the plant-
tissue, the movement of the water being a secondary effect
of the rhythmic activity. The excitatory nature of the
phenomenon has been demonstrated by the fact that various
agencies which induce increase or diminution of excitability,
have also the effect of bringing about an enhanced or
diminished rate of suction, above or below the normal. The
effects induced by these agencies, together with their time-
relations, can be easily and accurately recorded, as has been
shown, by means of the Balanced Shoshungi'apJi. The transient
excitation due to a sudden application of cold, and the
abolition of excitation under its prolonged application, are
seen in a transient enhancement of suction, followed by
arrest. The excitatory effect of the application of hot water,
again, is shown by an enhanced rate of suction. Poisonous
chemical reagents arrest suction quickly in specimens where,
owing to a less favourable tonic condition, the power of
resistance is low, and slowly in other cases. As the tissue of
the plant exhibits this suctional activity throughout its length,
the local death of a given portion, by scalding or by poison,
would not necessarily arrest the suction of the entire plant.
Such an arrest can only occur definitely when the entire
plant is killed.
The internal energy, on which the activity of suction
depends, may fall so much below par as to bring it to a
standstill ; but the activity is renewed on the application of
fresh stimulus.
That the ascent of sap is not fundamentally due to
REVIEW OF RESPONSE, SIMPLE AND MULTIPLE 723
transpiration from the leaves is seen from the fact that in a
saturated atmosphere it continues to take place. That it
is not, again, fundamentally due to the osmotic action of the
concentrated cell sap in the leaves is seen from the fact that
the ascent continues to take place on the removal of leaves.
It is seen again from the further fact that under favourable
circumstances, on the application of an osmotically strong
solution of sodium chloride to the root, the cell sap, instead of
being withdrawn by osmotic action, is made, by the excita-
tory effect of the salt, to ascend more vigorously.
The ascent of sap is thus an excitatory phenomenon, and
its uni-directioned flow is due to the graduated passage from
point to point of the co-ordinated excitatory reaction,
propelling water forward. This rhythmic excitation is ini-
tiated in the intact plant at its root, by the stimulus of
contact with soil, the friction of the growing organ against
rough surfaces, the excessive turgidity caused by the absorp-
tion of water, and possibly by the chemical stimulus of
substances present in the soil. In the case of cut branches
placed in water, the excessive turgidity at the cut end
initiates rhythmic activity, which drives the water upwards ;
but if such a branch be placed upside down, with its foliage
in water, the now turgid anatomically upper end becomes
the seat of excitation, and the direction of the flow of sap
is reversed.
The connection between the conduction of stimulus and
conduction of water is seen from the fact that the movement
of water takes place preferentially along those channels
which are also good conductors of excitation. Hence it is
transported more easily along the plant than across it ; and
while the movement is possible either upwards or down-
wards, yet it is quicker in the upward direction, which is
also preferentially the direction of conduction of stimulus.
The same movement of water which is produced by the
co-ordinated rhythmic activity of cells throughout the plant
appears either as suctional or as pressure movement, accord-
ing to the point of view which we adopt. When the removal
3 A 2
724 PLANT RESPONSE
of water from the plant is in any way arrested, a positive
pressure is produced, owing to its excessive accumulation.
Similarly, when loss is greater than supply, the pressure will
be negative. The ascent of sap, primarily due to cellular
activity, may be secondarily aided by evaporation from the
leaves, and by the osmotic action of the concentrated cell sap
there. Owing to the distribution of unequally active cells, an
irregular variation of pressure may be induced in the stem.
The excitatory movement may be transmitted to a distance
by conduction, or there may be conduction by ' relays.' An
isolated mass of highly excitable tissue may thus be excited
de novo. The excretion of water and of nectar are phe-
nomena of cellular activity, analogous to that which brings
about the ascent of sap. The translocation of food-material
is also probably due, at least in part, to excitatory reaction.
The internal activity of the plant, causing increase of
turgidity, may be detected mechanically by that erection of
the leaf which is characteristic of the positive turgidity-
variation. Any increase of internal activity is exhibited in
dorsi-ventral organs, such as the petioles of Mimosa, Biophy-
tu7n, and Artocarptis, by the erection of the leaf. Thus, when
the internal energy of the plant is increased by a rise of
temperature, the leaves become erected. Conversely, under
the action of cold, on account of the diminution of the latent
energy, the opposite effect, or droop, is induced. This ex-
plains the drooping of various leaves during frost, and their
subsequent erection, when brought into a warmer atmosphere.
CHAPTER LI
RESPONSIVE GROWTH-CURVATURES IN PLANTS
Longitudinal growth and its variations— Efilect of temperature on growth — Re-
sponsive growth-curvature under unilateral stimulation : — I. Direct unilateral
stimulus on the responding organ : {a) Positive response under moderate stimu-
lation ; (/>) Intermediate or neutral response ; (r) Negative response ; {d) Dorsi-
ventral positive response ; {e) Dorsi-ventral response which may become
negative— 2. Indirect effect of unilateral stimulation : {a) Negative response ;
{/>) Positive response — Responsive action under stimulus of gravity— Helio-
tropic action in radial organs — Heliotropic action in plagiotropic and dorsi-
ventral organs — Phototactic movements— Nyctitropic movements.
We shall next pass in review the responsive growth-
curvatures induced in plants by various agencies, and shall
then in the following chapter consider at some length the
extended range of those similarities which exist as between
the physiological responses of plant and of animal tissues.
Longitudinal growth and its variations. — It was shown
by means of the highly magnified continuous record which
was obtained with the ordinary and Balanced Crescographs,
that growth was a phenomenon of multiple response ; and
it was further shown that these multiple responses of growth
exhibited the same characteristics as had previously been
observed in the multiple motile responses of BiopJiytum and
Desmodiuni. Each of the constituent responses consisted of
a sudden elongation due to a pulse of increased turgidity,
followed by an incomplete recovery. The irreversible
growth-effect consisted of the difference between this elon-
gation and its recovery. These pulses of positive turgidity-
variation were mainly due to excitatoiy reactions occurring
about the zone of growth, which delivered from within, upon
726 PLANT RESPONSE
the plastic material of that zone, repeated hydrostatic blows.
The consequent expansive response was thus the indirect
effect of stimulation.
It is thus the internal energy, ultimately derived from
external stimulus, that gives rise to those rhythmic activities
by which the pulsations of growth are maintained. When
the sum total of the latent stimulating factors that determine
the tonic condition is below par, there is an arrest of the
multiple response of growth, corresponding to the similar
arrest of multiple motile response in Desmodiiim. In a plant
in which growth is at standstill, it may be renewed by a fresh
supply of energy. Thus, if hot water be applied to the root
of such a plant, energy is hydraulically transmitted to the
growing region, and there re-initiates growth.
If moderate stimulus be thus imparted, the responsive
growth-movement persists for a short time, and then comes
to a standstill, to be again renewed by a fresh supply.
Again, the movement of growth being due to the indirect
effect of stimulus, we might renew or accelerate it by apply-
ing stimulus, say, on the stem or its top, at such a distance
from the growing region that the direct excitatory effect
would not be transmitted to it. Stimulus applied directly on
the growing region would, however, by its true excitatory
effect, induce contraction and retardation of growth.
The longitudinal growth thus described takes place in a
strictly radial organ. If the organ, however, be bilateral,
instead of radial, it will exhibit lateral oscillation, owing to
the alternate growth of the two sides. Or growth may pro-
ceed in a spiral line, giving rise to circular or elliptical move-
ments. A very good example of the last is afforded by the
torsional growth-movements of climbing plants. These
various circummutating autonomous movements of growth,
passing from regular movements in a circle, through ellipses,
to a straight line, are exactly paralleled by different
examples of autonomous mechanical responses in Desnio-
dium, where also we find circular, elliptical, and rectilinear
movements.
RESPONSIVE GROWTH-^UR^■ATURES IN PLANTS 727
Effect of temperature on growth. — That growth is an
excitatory phenomenon is seen, again, in the fact that it is
increased by any circumstance that tends to increase excit-
abiHty. Thus, for example, in the case of most tropical
phanerogamous plants, it is found that responsive excitatory
contraction is greatest at a temperature of about 35° C. ; and
this is also found to be the optimum temperature, at which
the natural rate of growth is at its maximum.
I have described a method of obtaining a TlIERMO-
CRESCENT Curve for the determination of the various rates
of growth which correspond to different temperatures. The
continuous record thus obtained in the course of about half
an hour affords us not only the rate of growth at any tem-
perature, but also a means of determining its optimum and
maximum points. The optimum temperature may also be
determined, with an accuracy within one-tenth of a degree,
by means of the Balanced Crescographic record. The
results obtained by all the different methods employed are
found to concur. The optimum point is thus shown, under
normal conditions, to be very constant (p. 451). It may be
said here that in the case also of plants which exhibit tor-
sional growth-response, the rate of torsional movement is
greatest at this optimum point.
The arrest of growth which occurs at the maximum tem-
perature does not appear to be due to any cessation of
activity as brought on by rigor ; for we found in a record
taken from a seedling of Balsam at 44° C. that at this
temperature the constituent growth-pulsations had actually
become more frequent than before, the resultant abolition of
growth being due to the fact that response and recovery
were now equal. It was likewise shown that the apparent
arrest of the pulsatory movements of Dcsinodiiivt at certain
high temperatures was not due to the cessation of activity,
but that at such temperatures the pulsations had become
more frequent and very minute (p. 431). The fact that at
the maximum temperature growth is not arrested by rigor
receives curious illustration, again, when the application of
728 PLANT RESPONSE
doses of poison at such a temperature brings about, at least
temporarily, a renewal of resultant growth (p. 487).
Another important point in the effect of temperature has
already been alluded to. It has been shown that a plant
below the optimum temperature, being in proportionately
sub-tonic condition, will to a very great extent, or even
entirely, hold the incident stimulus latent, thus increasing
its own latent energy. In this sub-tonic condition, then,
the stimulus induces little direct contractile effect, but is
utilised to induce the indirect acceleration of growth. At
the optimum temperature, however, almost the whole of
the incident stimulus finds expression in direct contractile
response, there being now little or no absorbed com-
ponent ; and beyond the optimum, the tissue not only
possesses little or no power of holding stimulus latent, but
its receptivity also appears to undergo great diminution
(p. 461).
Responsive growth-curvature under unilateral stimu-
lation.— I have shown that the response of a growing is not
essentially different from that of a pulvinated organ. The
direct effect of unilateral stimulation gives rise in both cases
alike to negative turgidity-variation, with consequent con-
cavity of the side acted upon ; and the indirect effect, on
the other hand, consequent on the unilateral stimulation of a
distant point, gives rise, in both cases alike, to a positive
turgidity-variation, or convexity of the same side of the
responding region. This fact was demonstrated in the case
of Mimosa by applying stimulus : (i) near the motile organ,
in which case we obtained the direct effect by fall of the
leaf; and (2) at a considerable distance, when the indirect
effect gave rise to the erection of the leaf (p. 531). It was
found, however, that when the stimulus applied at a distance
was very strong and long-continued, true excitation was
ultimately transmitted by conduction, inducing excitatory
contraction, with fall of the leaf
In growth-curvatures, similarly, we obtain responsive
movements appropriately due either to the direct or indirect
RESPONSIVE f.ROWTH-CURVATURES IN PLANTS 729
effect of stimulus. These have been shown to be classified
as follows :
I. Direct unilateral stimulus on the responding
organ : (a) Positive response under moderate stimulation. — -
The proximal, by the direct action of stimulus, contracted ;
and the distal, by the indirect action of stimulus, expanded.
The result was a concavity of the proximal, and convexity
of the distal, conspiring to bring about movement towards
stimulus.
(J)) Intermediate or neutral response. — Though the trans-
verse conductivity of a tissue may be feeble, yet under
somewhat 'strong .stimulation the true excitatory effect
is transversely conducted from proximal to distal. The
result is that when the two opposite sides are equally excited,
there is a neutralisation, or disappearance of responsive
curvature ; or, by alternate fatigue of the two sides again,
the organ may be made to. oscillate to and fro about a more
or less mean position.
{c) Negative response. — When stimulus is \e.\-y strong and
long continued, we obtain not only the transverse conduction
of effect, but also temporary induction of anisotropy of the
organ. The proximal side is now, owing to fatigue brought
about by the direct impact of excessive stimulus, the less
excitable ; and the internally diffused stimulus, causing
greater contraction of the more excitable, induces concavity
of the distal, or a negative responsive movement.
Besides this we have organs which are characterised b}-
a permanent anisotropy or dorsi-ventrality, and we then
obtain two classes of effects, according as the transverse
conductivity is very feeble or moderately strong. Owing to
the dorsi-ventral structure, the responsive movement can
only take place at right angles to the plane which .separates
the anisotropic halves of the organ. These effects are the
same in growing organs, such as plagiotropic shoots and
dorsi-ventral petioles, and in mature dorsi-ventral organs,
such as pulvini.
{d') Dorsi-ventral positive response. — When the transverse
730 PLANT RESPONSE
conductivity is feeble, the stimulus remains localised on the
side of the organ acted upon. Thus the stimulation of either
upper or lower side induces a positive response, or movement
towards stimulus.
(e) Dorsi-ventral resp07ise which may become negative. —
When the transverse conductivity of the organ is con-
siderable, and the excitability of the lower half relatively
great, then the strong stimulation of the upper side will, by
internal diffusion, cause contraction in, and concavity of, the
more excitable lower. The responsive movement will then
be negative, or away from stimulus ; but feeble or moderate
stimulation of the upper half, not being transmitted to the
lower half, causes a positive response. Direct excitation of
the more excitable lower half will always give rise to a
movement towards stimulus, or positive response.
2. Indirect effect of unilateral stimulation : {a)
Negative response. — When moderate stimulus is unilaterally
applied at a distance from the responding organ, it is the
indirect effect that is transmitted to that region, causing
convexity of the same side, with consequent movement
away from stimulus, or negative response. This is very
well illustrated when the tip of either shoot or root is sub-
jected to moderate unilateral stimulation.
{b) Positive response. — But when the unilateral stimulus
at the distant point is strong or long continued, the excitatory
effect is transmitted by conduction, and induces a contraction
and concavity of the same side, resulting in a movement
towards stimulus, or positive response.
From what has been said it will be understood that
moderate unilateral stimulation of the tip of root or shoot
induces negative, and excessive stimulation positive, while
between these two extreme cases there may be intermediate
or neutral, response of the responding region.
These effects are induced by stimulation of all forms, and
it is thus clear that there is no specific sensitiveness of the
dorsi-ventral as distinguished from the radial organ, nor is
there any polar difference between the response of root or
RESPONSIVE GROWTH-CURVATURES IN PLANTS 73 1
shoot, the tips of both organs behaving ahke. The one
universal law which applies in every case is, that the direct
effect of stimulus is to induce contraction, and its indirect
effect to cause expansion.
On taking a general survey of the responsive movements
which are induced by the unilateral action of stimulus, we
find that moderate stimulation of the growing region induces
a positive movement. Or negative movement, again, may
be induced in either of two ways — that is to say, by moderate
stimulation of the tip, or by very strong stimulation of the
growing region.
Responsive action under stimulus of gravity. — I have
shown that the unilateral application of pressure of particles
is efficient to cause responsive contraction. An experiment
was described in which it was shown that the unilateral
pressure of magnetically attracted particles would induce
concavity of the side acted on (p. 497). The weight of
statolithic particles may thus be the efficient cause of
stimulation by gravity. It is to be borne in mind, however,
that stimulation caused by such means as the weight-effect
of these minute particles can only be moderate. We have
therefore in the case of geotropic stimulation to deal onh-
with the direct and indirect effects of unilateral stimulus of
moderate intensity. In the case of the stem the growing
region is directly stimulated. A horizontally laid stem
thus curves upwards to meet the lines of force, or rays of
gravity, just as it would bend upwards under heliotropic
action to meet the rays of incident light. It is supposed
that the curvature of the stem under gravity is mainly due
to an active growth of the convex side ; but I have shown
that it is due, on the contrary, to an excitatory response,
which consists, like all other forms of response to external
stimulus, of a contraction. The active element in the in-
duced responsive curvature is thus the contraction of the
upper side of the organ, aided subsidiarily by that expansion
of the under side which is brought about by the indirect
effect of stimulus on the distal. That this is the case is seen
732 PLANT RESrONSE
from the fact that, on localised cooling of the upper side, the
movement of the organ in response to gravity is abolished,
whereas cooling of the lower side has little or no effect on
the responsive movement. This experiment incidentally sup-
ports the view that it is. the inner tangential wall of the
cells which is relatively effective in responding to the stimulus
of gravity. In turning to the geotropic response of the root,
on the other hand, we find that it is the distant tip which
is the perceptive region for gravitational stimulus. Hence
it is only the indirect effect of stimulus which acts on the
responding growing region. But we have seen that moderate
stimulation of the tip, by any form of stimulus whatsoever,
always induces a movement at the responding region, of
opposite sign to that which is the result of direct stimulation,
and from this the opposite geotropic responses of shoot and
root follow as a matter of course. This fact entirely negatives
the assumption that shoot and root are possessed of any
polar difference of sensibility, or that any specific geotropic
sensibility has been evolved in the radicle for the advantage
of the plant.
Heliotropic action in radial organs. — We shall find
similarly, in studying the various movements of the plant in
response to heliotropic stimulus, that, diverse as they seem,
they are characterised by an underlying unity, being in fact but
so many expressions of the universal law that response takes
place by the contraction and concavity of the more excited.
The fundamental effect of light was demonstrated by
showing that, in a growing organ, diffuse stimulation induces
a contraction and retardation of the rate of growth. This
was also shown to be true of all other forms of stimulation,
including those of thermal and electrical radiation. The
incidence of radiation may, it is true, cause a rise of tem-
perature ; and this would, as we know, have the effect of
enhancing the rate of growth. In order, therefore, to
discriminate the effect of radiation as such from that of
temperature, an experiment was described in which the
circumstances were so arranged that no rise of temperature
RESPONSIVE GROWTH-CURVATURES IN PLANTS 733
could take place while the effect induced by radiation as
such was being observed. Under these crucial conditions it
was demonstrated that the effect of radiation is to induce
responsive contraction.
Under moderate unilateral stimulus of light, as in the
case of gravity, two definite and distinct effects were observed,
according as stimulus was applied directly on the responding
region or on the distant tip. In the former case we obtained
a positive, and in the latter a negative, responsive movement.
Up to this point, then, the actions of light and of gravitation
are parallel in their effects — that is to say, the positive
heliotropic movement of the stem corresponds to the so-called
negative geotropic movement of the same organ ; and tlie
negative heliotropic movement of the root to its so-called
positive geotropic. Looked at in relation to the direction of
stimulus, however, it may be said that the response which is
commonly known as ' negative geotropic ' is actually positive,
and vice versa ; for, accepting the theory of statolithic or
hydrostatic pressure as to the effective cause of stimulation,
the direction of the excitatory pressure is in the direction of
the lines of force of gravity. In a stem laid horizontally,
then, and acted on by vertical lines of gravitational force, or by
vertical rays of light, we obtain the same directive response
to these similar directive stimuli, by the bending upwards of
the organ to meet the rays, or the lines of force. Some
confusion is therefore inevitable when one of these responses
is designated as positive, and the other as negative, for the
essential similarity of the two is here masked by the use
of directly opposite terms. This difficulty might perhaps be
overcome by naming the normal responsive movement of
the stem as positive phototropic and positive gravitropie, or
pro-gravi tropic, and that of the root as fiegative phototropic
and negative gravitropie or a nti-gravi tropic.
We next turn to the differences between the effects of helio-
tropic and geotropic action. Such differences arise from the
two facts that : (i) only in the root is the region of the percep-
tion of gravitational stimulus separated from that of response ;
734 PLANT RESPONSE
and that (2) geotropic stimulus is always of moderate in-
tensity. As regards the first of these two differences, it has
been shown that, on applying unilateral heliotropic stimulus
of moderate intensity to the tip of the shoot, we obtained the
same negative response of indirect stimulation as is given by
the root-tip. In the case of geotropic stimulus, however,
there can be no phenomenon corresponding to this, inasmuch
as in the stem the statolithic particles appear to be diffused,
instead of being localised at the tip. The second point of
difference between the two responses arises from the fact that
heliotropic stimulus may be of any degree of intensity.
Hence the direct excitatory effect of strong unilateral stimula-
tion of the root-tip may in the case of light be transmitted to
the growing region, and there induce a positive response, or
movement towards stimulus. This accounts for the fact that
while roots in general give one kind of gravitational response
of so called positive sign (but really negative), some roots
give negative response to light, and others positive.
Turning next to the direct action of unilateral heliotropic
stimulus on the growing region, we find, as explained in
the summary of responsive action in general (p. 535), that
the effect is modified by the intensity of stimulus, by the
transverse conductivity of the organ, and by its existing
anisotropy. Thus in the case of a radial organ, such as the
hypocotyl of Sinapis, moderate stimulus, its effect remaining
localised on the proximal side, has been shown to evoke a
positive responsive movement. Stronger or long-continued
stimulus, reaching the distal side by transverse conduction,
neutralises this first effect, and the organ thus remains at
right angles to the incident light, or in a dia-heliotropic
position, apparently unaffected by it. In other instances,
again, owing to the alternate excitation of the two sides, the
organ may oscillate to and fro about a mean position. With
still stronger stimulus, however, an anisotropy is induced, by
which the proximal side becomes, through fatigue, the less
excitable, and the internally diffused stimulus causes greater
contraction and resultant concavity of the distal side ; that is
RESPONSIVE GROWTH-CURVATURES IN PLANTS 735
to say, a negative response. It was thus made clear that the
three types of response — positive, negative, and dia-hcHo-
tropic — are not due to three different specific sensibiHties.
It has been pointed out, further, that these considerations
explain why it happens in many cases that, while moderate
stimulation induces a considerable responsive movement,
stronger stimulation, instead of increasing this, actually
neutralises it. It is due, as we have seen, to the transverse
conduction of stimulus by the tissue, that the positive effect is
counteracted or reversed. This explanation has been shown to
account satisfactorily for various cases apparently anomalous.
Certain tendrils are regarded as heliotropically insensi-
tive. For example, the tendril of Passifiora when acted on
by sunlight shows little or no responsive movement. On
artificially diminishing the transverse conduction, however,
by the application of cold, I have shown that it exhibits
the ordinary positive responsive movement. The tendril
of Vitis, again, which is supposed to be endowed with
a specific sensibility of negative character, has also been
shown to exhibit the normal positive response under light of
moderate intensity. The modifications of transverse con-
ductivity which are brought about by age and season, with
their consequent appropriate variations of response, are seen
in Tropceolum. A very young tissue, as a general rule, owing
to the fact that the fibro-vascular elements are not fully
developed, is a bad conductor of stimulus, which therefore
remains localised at the point of application. Hence young
plants exhibit movements of positive response, whereas older
plants, owing to transverse conduction, with its effect of
neutralisation, appear to be little affected b}' light. In con-
nection with this it must also be borne in mind that the
power of contraction declines with age. The characteristic
effect of season, again, results from the fact that the con-
ducting power of a tissue is at its feeblest in autumn and
winter, and correspondingly greater in spring and summer.
In autumn, therefore, stimulus remains localised, and 7^/7^-
pceoluui and Ivy during that season respond to heliotropic
736 PLANT RESPONSE
Stimulus by positive curvature ; whereas in summer strong
unilateral stimulation is transversely conducted, and induces
negative responsive curvature.
Heliotropic response in plagiotropic and dorsi-ventral
organs. — We have seen that negative response is brought
about in a radial organ by induced anisotropy, and transverse
conduction of stimulus. Effects fundamentally similar are
seen in organs which are characterised by a natural aniso-
tropy. A connecting link between this transient induced
anisotropy of radial organs, and the permanent anisotropy of
a dorsi-ventral pulvinus, is afforded by plagiotropic stems, in
which anisotropy has become more or less permanent, owing
to the long-continued unilateral action of vertical light.
Two different types of response are exhibited by anisotropic
organs, depending on their transverse conductivity and on
the relative excitabilities of their two sides. In the first of
these, transverse conductivity being feeble, vertical illumina-
tion remains localised, and induces positive response. This
is the true explanation of the so-called diurnal sleep, with
upward folding of the leaflets, of Robinia, Erythrina indica,
and Clitoria ternatea (p. 629). In the second type, the
stimulus of vertical illumination is transmitted to the more
excitable distal side, inducing concavity of that side, and
consequent negative response. The different stages of this
effect are well seen in Aliniosa, when the stimulus of light
acts on the dorsal or upper side of the pulvinus. Here the
immediate effect is a positive response or erection of the leaf;
and as the stimulus percolates to the distal side, this effect is
neutralised and converted into an increasingly negative
response. The greatest degree of negativity or fall in nature
is thus attained by the cumulative action of the whole day's
illumination. Such is the response which is characteristic of
the second type. The action of strong vertical light in such
cases induces movement downwards. And this is seen in
plagiotropic stems like those of Cuau-bita and Ipotncea ; in the
thallus of Marchantia, and the midribs of various leaves ; and
in the so-called diurnal sleep, with downward folding of the
RESPONSIVE GROWTH-CURVATURES IN PLANTS 737
leaflets, of such leaves as those of Oxalis, Biophytuiii, and
AverrJioa. In the heliotropic responses of ordinary leaxes,
again, we have exactly similar classes of phenomena. In
these cases it has been shown that there is no specific dia-
heliotropic sensitiveness, such as that by which the upper
surface of the leaf was supposed to place itself at right angles
to the light, for the purpose of absorbing the largest possible
amount of stimulus. It was shown, moreover, that with
regard to that response by which the ultimate position of the
leaf is determined, the lamina was not the perceptive organ.
In ordinary, as in pulvinated, leaves we find response to be
of two extreme types, connected by innumerable gradations.
First, we have leaves like that of Mangifera indica, in which
on account of the feeble conductivity of the pulvinoid
vertical illumination induces a positive response, or move-
ment upwards ; and as an example of the second type we
saw that negative response, or movement downwards, was
given by the leaf oi Artocarpns wwdi^x xQ.x\\c-a\X\^\.. These
responsive movements induced by light, although, generally
speaking, predominant, are modified by the presence of
other subsidiary factors, which all contribute in various
degrees to bring about the variety of attitudes ultimately
assumed by the leaves. These subsidiary factors were
enumerated as: (i) the epinastic or hyponastic tendency;
(2) the general turgescent condition of the plant ; (3) the
characteristic limits of flexibility of the motile organ ; and
(4) the differential geotropic excitability of the organ. When
a petiole is acted on laterally by light a torsion is induced,
by which the upper surface of the leaf is made to face the
incident stimulus. It has been shown that this movement is
not due to any specific dia-heliotropic sensibility ; for any
form of lateral stimulation, say chemical or thermal, will
induce a similar response by torsion, the result being always
that the less excitable surface is made to face the stimulus.
Similar effects arc also observed in compound strips made of
such unecjually contractile substances as ebonite and india-
rubber.
738 PLANT RESPONSE
The fact that it is the differential excitability of the
organ which under lateral stimulation causes this torsional
movement was further demonstrated, when the difference
was artificially increased by the local application of chloro-
form to the upper half of the pulvinus. The torsional
response was then found to take place, with a corresponding
enhancement of rate, in the same direction as before. But
when this natural difference of excitability was reversed, by
the abolition through local application of chloroform of the
excitability of the lower half of the pulvinus, the direction
of the responsive torsion was found to undergo reversal.
By carrying out a similar series of experiments, with
special reference to the lateral action of gravitational
stimulus on a dorsi-ventral organ, it was shown that such
an organ as a whole exhibited neither a positive nor a
negative, but a differential geotropic action. The investiga-
tion showed that the upper half of a pulvinus was less
excitable than the lower half under geotropic stimulus. An
artificial increase of the existing difference between the
excitabilities of the two halves enhanced the rate of the
normal torsional response, and the reversal of these natural
excitabilities reversed the direction of the torsional response
to geotropic stimulus (p. 664).
Phototactic movements. — A leaflet of Desniodium in a
state of standstill resumes its pulsatory beats when stimu-
lated by light. Owing to the anisotropy of the motile
organs, one half of the beat is more rapid than the other.
Too strong an intensity of light, however, by causing greater
fatigue of the more excitable half of the organ, may cause a
reversal of the relative rapidities of the up and down beats.
In Desniodium, under the continuous stimulation of strong
light, these reversals are often recurrent. The downstroke,
which was at first the quicker, becomes less quick than the
upstroke, and this reversal may take place again and again,
in alternation with its opposite. These effects, seen in a
pair of anisotropic motile organs in Desniodimn, afford an
explanation of the swimming movements of certain ciliated
RESPONSIVK GROWTH-CURVATURES IN PLANTS 739
organisms. These swimming movements are brought about
by the rhythmic beats of the anisotropic cilia under unilateral
stimulus of light, either the up or down stroke of each such
beat being relatively quicker than the other. For reasons
which have been explained, moderate stimulation, initiating
these rhythmic responses, causes movement in one direction,
and stronger stimulation movement in the opposite direction ;
or, as in the case of Desntodiuin leaflets, there may be
recurrent reversals, causing alternate progressions or retro-
gressions to and fro. Similar forms of response with similar
variations are brought about by forms of stimulation other
than light ; there are thus thermotactic, galvanotactic, and
chemotactic swimming movements.
Nyctitropic movement. — The nyctitropic movement has
been shown to be the result of heliotropic action, the fall of
the leaf of Mimosa at evening being due not to the action of
on-coming darkness, but to the cumulative stimulus of the
whole day's illumination. Taking this plant as the type, it
was shown that the diurnal movement was caused by the
action of two different periodic factors, namely: (i) the
differential effect of light on the pulvinus itself during the
day, alternating with the cessation of stimulus at night ; and
(2) a periodic inflow and outflow of water, which takes place
in the plant as a whole by the recurrent action of light and
darkness. By the first of these factors the leaf is progres-
sively depressed during the day, the reverse process taking
place during the night, as a result of natural recovery, aided
by the conserved internal energy which gives an impulse
opposite to that of external stimulus. These two periodic
factors, of the effect on the pulvinus itself, and of that on the
plant as a whole, act concordantly, and give rise to periodic
movements of the leaf which are of large amplitude. Such
forced diurnal vibrations, by long repetition, give ri.se to
periodic after-effects which persist for a time, even on the
cessation of the periodically exciting cause.
CHAPTER LII
ON PHYSIOLOGICAL RESPONSE,
AND ITS CONTINUITY IN PLANT AND ANIMAL
Vitalism— Fundamental unity of physiological response in plant and animal-
Theory of Darwin — \'ariation as induced by external forces.
We have reviewed, in the last two chapters, the various
phenomena of plant response. We shall now turn our atten-
tion to the consideration of irritability, or the capacity of
responding to stimulus, in general.
Vitalism. — We have seen that when a tissue is rendered
molecularly sluggish by any physical means, such as cooling,
its irritability is found to be temporarily abolished. Irritability
is thus ultimately due to molecular responsiveness, and ex-
citatory response is brought about by the molecular derange-
ment consequent on stimulus, with the subsequent self-
recovery. We have seen further that the state of excitation
is exhibited, either by a mechanical or by the electrical mode
of response, and that even where mechanical indications are
not available, the electrical sign of excitation is unfailing.
We have also seen briefly in the course of the present work,
and I have demonstrated in full elsewhere,^ the fact that
.similar excitatory response is given, even by inorganic matter,
under stimulation ; and in such cases also we have been able
to observe and record not only the phenomenon of response
itself, but also its numerous appropriate modifications under
varying conditions. Thus fatigue brings about diminution
of inorganic, as of organic response. Amongst chemical
reagents, again, some induce exaltation and others depres-
sion ; and many so-called poisons act here, as in the case
' Bose, Rcspoise in the Living and Noil-Living, 1902.
niVSIOLOGICAL RESPONSI-: IN 1>LANT AND ANnrAI, 741
of the plant or animal, by indiicinoj the abolition of response.
Irritability or molecular responsiveness, therefore, must be
regarded not as characteristic of organic substances alone,
but as the universal property of matter. In the case of what
is commonly known as the living, we have merely higher
complexities, with greater instabilities, of molecular structure.
External stimulus is here liable to induce greater derange-
ment, and the irreversible molecular change known as death
takes place the more easily, the more highly organised the
complexus may be. Bacteria, for example, will survive con-
ditions which would immediately prove' fatal to more com-
plex organisms.
In studying the responsive phenomena of living organisms,
therefore, we must fix our attention on their molecular aspect,
and try to follow out the physico-chemical changes which are
consequent on the molecular derangement induced by stimu-
lus ; and we are more likely to succeed in obtaining a grow-
ing insight into the various phenomena of life, when we
approach the subject from this point of view, than when we
permit ourselves to evade each difficulty as it arises by
referring it to the inexplicable action of a mystical vital
force. Physical and mechanical considerations at first appear
to us to be inadequate to the explanation of the complex
movements of the living machine, just as the similarly com-
plex movements of a wind-motor, connected with a hidden
electrical apparatus, would at first sight be inexplicable
to an inexperienced observer (fig. 278). Let such an
observer be brought face to face for the first time with such
a windmill. Its movements under the action of wind will
arouse his wonder, and this will be increased when some-
times even in the absence of wind he sees the vanes revolv-
ing still, but now in an opposite direction. He may again
notice oscillatory rotations, now in one way and then in the
other. Failing to find any rational explanation of these
movements in this first stage of his inquir}-, he will be
driven to attribute them to an unknown power, whose
characteristic it is to manifest itself by such erratic actions,
742
PLANT RESPONSE
now in one direction and again in the opposite, and whose
mystery lies mainly in this caprice.
But the observer, in the course of his further inquiry, finds
that the vanes, whose rotation under the impact of the
external stimulus of wind first attracted his attention, are
but a part of a complex machine, the interior of which had
been hidden from his view.
He finds that the energy
supplied from outside is
being transformed by a
dynamo inside, and stored
up in an accumulator. When
the external force is not
acting, the reverse move-
ment is caused by the
internal energy thus stored
up. This very movement,
being apparently without a
cause, he would formerly
have designated as auto-
matic. When the stored-
up energy is exhausted,
the seemingly autonomous
movement comes to a stand-
still, and only by the
accession of fresh external
stimulus, causing renewed
storage, can it be resumed.
At a given moment, more-
over, the responsive move-
ment of the vanes is determined by the opposing actions
of the external and internal factors. As long as the wind
is sufficiently strong, movement takes place in one direction,
and when there is a pause the internal energy begins to
find expression, by causing movement in the opposite
direction. If the circumstances were such that the rise of
the wind were synchronous with day, and its fall with night,
Fig. 278. Diagrammatic Representa-
tion of a Windmill with Attached
Dynamo, D, and Accumulator, s
Wind acting on vanes, v, from right,
represented by arrow {<-), causes
responsive rotation in direction
opposite to that taken by the hands
of a watch. This external energy
also causes electrical storage. On
the cessation of the wind the
accumulator begins to part with its
stored-up energy, and, the dynamo
now acting as a motor, causes a
responsive rotation of the vanes in
the other direction, as shown by the
arrow (■■>).
PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 743
the windmill, with its alternate movements, would afford a
very excellent illustration of the alternate day and night
phases in the nyctitropic movement of the plant.
Now, with regard to the living machine, similarly, a full
insight into its action can only be obtained if we are able
to disentangle the two opposite factors, of internal energy
and external stimulus, and follow them into their responsive
expressions, at the same time recognising that the principle
of the conservation of energy must hold good in the living
system as in the non-living. External energy acting on the
plant performs work on it, a part of this incident energy being
taken and held latent ; and, in virtue of the latent energy so
conserved, work is performed by the plant. Of this internal
work, besides the potential chemical energy which is accumu-
lated, the maintenance of suction, growth, and autonomous
movements may be cited as examples.
Fundamental unity of physiological response in plant
and animal. — Having now seen the intimate connection
which exists between the physical and the physiological, and
having also seen that the molecular response of the inorganic
is not altogether different from that of the plant, it remains
only to glance at the physiological continuity of response as
between plant and animal ; and here we shall find that
there is hardly any phenomenon of irritability, observed in
the case of animal tissues, which is not also to be discovered
in some simple form in the case of the plant. These resem-
blances, moreover, are so numerous and so detailed as to lead
us inevitably to the conclusion that we have to deal in the
two cases with a single identical phenomenon.
In the longitudinal response of a radial vegetable organ
we have seen how similar is responsive contraction in animal
and plant ; and that this similarity extends even to charac-
teristic details is seen when we compare the records of the
Kunchangraph in the case of the plant, with those of the
Myograph in that of the animal. The two are alike in the
exhibition of a latent period, which is prolonged by cold and
reduced by warmth. The staircase increase of the respon-
744 PLANT RESPONSE
sive effect, its diminution by fatigue, and the induction of
tetanus under rapidly succeeding stimuli, in either case
corresponds to a like phenomenon in the other. Fatigue-
relaxations, moreover, under strong and long-continued
stimulation, are the same in both. And, turning to the
electrical mode of response, we find that the excitatory
condition of a tissue is indicated by its induced galvano-
metric negativity, whether the tissue be animal or vegetable ;
and further, similar physiological modifications, as induced
by the action of various external agents, are manifested by
similar changes in the two cases in the electrical response.
As by the nerves of the animal, so also by certain con-
ducting channels in the plant-tissue, the state of excitation
is, in the two cases alike, transmitted to a distance ; and
this conduction takes place in both by propagation of proto-
plasmic changes. In both alike, cold reduces, and warmth
accelerates, the velocity of the transmission. In both alike,
the stronger the stimulus, the greater is the velocity with
which it is transmitted. In both, this velocity is diminished
with fatigue. Anaesthetics cause the temporary abolition
of conduction in both. The anode, again, blocks the trans-
mission of the excitatory wave in both. And, lastly, the
velocity of the transmission of excitation in the plant is com-
parable to that of its transmission in the nerves of some of
the lower animals. Thus, in a certain specimen of the sensi-
tive Mimosa, excitation was found to be transmitted with a
velocity of 14 mm. per second ; and in the ordinary plant
Ficus religiosa this velocity was determined at 9*4 mm. per
second ; while in the nerve of Anodon a value of 10 mm.
per second has been recorded. If, then, the characteristic
of nerve be to conduct excitation, it must be admitted
that the plant, like the animal, is provided with a nervous
system.
In the matter of the excitation induced by the electrical
current, there is an equally remarkable similarity of effects in
animal and vegetable. Under normal conditions in both
cases, the kathode excites at make and the anode at break ;
I'lIYSIOr.OGICAT. RKSPONSK IN PLANT AND ANHFAL 745
and that reversal of these normal effects which is liable to
occur under fatigue, or under excessively strong electro-
motive force, is the same in the one case as in the other. In
addition, moreover, to the contractile effects of kathode-make
and anode-break, the plant exhibits a responsive expansion
at anode-make and kathode-break, which is also seen in
animal tissues.
Turning next to such tissues as are characterised by the
property of rhythmicity in a marked manner, we have seen
that this effect as exhibited by the plant cannot be distin-
guished from that in the animal. The rhythmic tissues of the
plants Biophytuui and Desntodiuui are characterised by the
possession of relatively long refractory periods, a peculiarity
which also marks the rhythmic cardiac muscle of the animal.
The response of rhythmic tissues in both plant and animal is
found to be on the ' all or none ' principle. In both alike the
rhythmic tissue is incapable of tetanus ; and in both, when
at standstill, a single moderate stimulus gives rise to a single
response, and stronger stimulus to a multiple series of
responses. In Desmodium, again, as in the cardiac muscle,
increased internal hydrostatic pressure renews pulsation in
a tissue at standstill. In rhythmic tissues, again, whether
animal or vegetable, under favourable tonic conditions, per-
sistent pulsatory movements take place which are apparently
automatic ; and these rhythmic pulses are found to exhibit the
same types of cyclic variation in the plant as in the animal.
The effects of temperature on both are exactly the same—
that is to say, its rise increases the frequency and diminishes
the amplitude of pulsation. And still more striking, finally,
is the identity of the modifications induced by drugs in the
rhythmic respon.ses of animal and vegetable.
But this unity of rhythm.ic responses in plant and animal
is not merely a question of their fundamental characteristics.
They sometimes appear also to subserve functions somewhat
similar. Thus the rhythmic cardiac tissue of the animal
maintains the circulation, and we have seen that the rhyth-
mic tissue of the i)lant maintains the ascent of the sap.
746 PLANT RESPONSE
From such considerations it may perhaps appear not very
far-fetched to regard a plant as possessed of a diffuse heart.
With special reference to the effect of drugs on plant and
animal tissues, we find the identity of phenomena similarly
impressive. This is exemplified in both cases by their action
not only on ordinary contractile tissues, but on rhythmic
tissues also. Thus it is only necessary to mention such
facts as that anesthetics, like the vapours of ether and
chloroform, induce a transient abolition of excitability, with
abolition of response, in both animal and vegetable ; that
this excitability, with its concomitant response, is gradually
restored in either case on blowing off the applied vapour ; and
that poisonous reagents, on the other hand, induce a permanent
abolition of all response. The action of these and other
chemical agents has already been described in some detail.
In the case of rhythmic response, again, a like parallelism
was found to exist, as between the effects of drugs on plant
and animal tissues respectively ; and this parallelism was
further shown to extend through a wide range of phenomena.
A remarkable instance was seen in the antagonistic effects
on responsive rhythmic tissues of the actions of acid and
alkali. Acid, when applied to cardiac muscle, induces a
diastolic standstill, whereas the effect of alkali is exactly the
opposite, a standstill, namely, of systolic contraction ; and
the standstill induced by either of these is found to be
counteracted by the application of the other. Now, I have
shown that the effects of acid and alkali on the rhythmic
tissues of Desmodium are similarly antagonistic. Thus, dilute
hydrochloric acid induced arrest of pulsation in the diastolic,
or relaxed, position, in the motile organ of Desmodium ;
whereas, with solution of sodium hydrate, the induced arrest
was of systolic contraction. Moreover, when arrest in its
own particular position had been brought about by either of
these reagents, its effect was neutralised by the application
of the other (p. 353). The same effects were found again
curiously reproduced in the case of the autonomous response
of growth. Here, the application of acid induced an arrest
PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 747
of growth, but only after an abnormal relaxation or expan-
sion. Alkali, on the other hand, induced arrest, but aftet
contraction. And, finally, the arrest induced by one was
counteracted by the effect of the application of the other
(p. 484). These facts, and others which have already been
fully described, afford a conclusive demonstration of the
essential unity of the physiological effects of drugs on plant
and animal tissues.
The existence of such a unity having been established, it
is evident that much may be gathered from investigations
carried out on plants, as to the obscure question known to
medical practice as the modification of the effect of drugs
by individual constitutions ; for while a given individual will
succumb quickly to the action of a certain poison, another,
as is well known, will throw off its influence and survive.
Again, a particular dose of a given drug may have the effect
of producing excitation in one case, and in another profound
depression. The effect on a tissue of any given reagent, then,
does not merely represent the action of that reagent as such,
but is further determined also by the reacting power of the
responding tissue itself. And this reacting power is modified
by what is known as the individual constitution of the
organism. Thus no result can be definitely predicted of a
reagent, unless we have a precise knowledge of the action
of the same drug on various definite constitutions. This
problem, of the variations induced in the effects of drugs by
the different reacting powers of different constitutions, may
now be attacked, therefore, through the study of the plant, in
which, as I have shown, it is possible to induce known
differences of constitution by artificial means.
It was shown, for example, in one case that, while a 5
per cent, solution of the poisonous reagent, copper sulphate,
produced an immediate depression, quickly followed by death,
another similar plant, whose tonic condition had been raised
to the optimum, was found to withstand the action of this
poison for a considerable time, the immediate effect being an
actual exaltation of its response. The opposite effects of the
748 PLANT RESPONSE
same dose on different constitutions were shown, again, in the
fact that, while a i per cent, solution of copper sulphate
caused depression, and subsequent death, of a plant under
normal tonic conditions, the same dose in the case of a similar
specimen, which had been raised artificially to the optimum
condition, brought about exaltation of response, which was
found to continue for a fairly long period, after which the
effect of the poison was completely overcome (p. 487).
The generalisation which has thus been established will
be found to be of great significance. A unity of phenomena,
as between animal and plant, so fundamental, so detailed,
as has been shown to exist, points unmistakably to a basic
property of responsiveness common to the two, and mani-
fested in both alike by the same effects and modification
of effects under stimulation. Nor is the power of response
something which makes its appearance suddenly in organic
substances only, for it has been demonstrated as existing
even in the inorganic. Thus inorganic and organic are
held together in a linked continuity. All are responsive,
all are depressed by fatigue, all are made excitable by
stimulants and rendered irresponsive by ' poisons.' Again,
with regard to plants, it may be said that there is hardly
a responsive physiological peculiarity in the highest animal
that may not be found foreshadowed here. Thus the serial
development of the physiological functions in these two
cases has been more or less parallel.
In the establishment of this generalisation, further, it
has been made possible to solve many of the most obscure
and difficult problems of Animal Physiology, by studying
them under the simple and more manageable conditions of
vegetable life. That this is the case has already been seen
in many instances, such as that of the polar effects of
currents and their reversal under given conditions ; in the
light shed on the nature of automatism ; in the different
parts played by external stimulus and internal energy, and
their mutual relation ; in the bifurcated expression of incident
stimulus as external and internal work ; and finally, in the
PHYSIOLOGICAL RESPONSE IN PLANT AND AKISIAL 749
similar action of chemical reagents on plant and animal. It
must be remembered, moreover, that one of the greatest
advantages to be derived from such a use of the plant as
a means of physiological investigation, lies in the fact that
this study can be carried on with intact and growing
specimens under normal circumstances. The experimental
conditions are in this case, therefore, better than those which
correspond to them in regard to animal tissues, since the
latter specimen will generally be found to be suffering from
the effects of injury, and may therefore have been rendered
abnormal to an unknown degree. The study of the re-
sponsive phenomena in plants must thus form an integral part
of physiological in\estigation into the various problems relat-
ing to the irritability of living tissues, and without such study
that investigation must in future be regarded as incomplete.
In thus reviewing the movements of plants as a whole, in
the light of the investigations which have been described in
the course of the present work, we cannot have failed to be
struck by the fact that all alike represent, under changing
conditions, a single fundamental responsive phenomenon ;
and this response of the plant offers us, as we have seen,
a means of tracing the process by which physiological
differentiation from simple to complex has taken place, under
the action of stimulus itself. It also enables us to refer a
specific differentiation back to definite forces, which have
acted asymmetrically upon the organism and induced such a
change. We are thus able to see that responsive movements
apparently opposite in kind, are nevertheless traceable, not
to different specific sensibilities, but to a single universal
sensibility, finding different expressions by reason of these
induced physiological differentiations.
Theory of Darwin. — It will be remembered that, accord-
ing to the theory of Darwin, a given individual variation
which might be in any way advantageous to the organism
was perpetuated by the process of natural selection. Thus, in
the struggle for existence, only those could continue to live
750 PLANT RESPONSE
that were best fitted to their external conditions. Going
back a step, however, to the question of the origin of these
variations themselves, we find that by some writers they
are held to be due to spontaneous unknown causes, in-
herent in the organism. Now it would obviously be more
satisfactory, since no effect can occur without a cause, if we
could assign at least some of these variations to something
more definite than this. And Darwin himself was of opinion
that variability of every kind was due, directly or indirectly,
to changes in the conditions of life. It was difficult, however,
to distinguish clearly how much of any given variation was
due ' to the accumulative action of natural selection, and
how much to the definite action of the conditions of life.' ^
It was this difficulty which, for example, compelled him
to ascribe the movements of plants to specific heliotropic
or geotropic sensibilities, acquired as the result of natural
selection. The particular reaction or reply to stimulus,
manifested by the plant in any special case, was thus to be
regarded, not as the direct and necessary result of changes
in the environment, but as an adaptive act forced on the
species by the struggle for life.
Variation as induced by external forces. - With the
delicate modes of investigation, however, which are now at
the disposal of observers, it has become possible to demon-
strate a direct connection between some of the differentia-
tions induced in plant-organs and the conditions of the
environment. The factors which are conceivable as bringing
about variation may be classed under two heads, first,
internal or spontaneous, and second, those which arise from
external stimulus ; but with regard to the first of these we
have seen that, as far as experiment has carried us, there is
no such thing as spontaneity, in the sense of an effect which
occurs without antecedent cause, for the internal energy to
which such seeming actions have hitherto been vaguely
ascribed has been shown to be itself traceable to external
stimulus. And as regards the effect of external stimulus, on
' Darwin, Origin of Species, p. 107.
PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 75 1
the Other hand, we have seen that under its action in nature,
heterogeneity is evolved out of homogeneity. Thus it is the
unequal action of an external force which, for example,
causes a radial organ to become anisotropic, with corre-
sponding physiological complexity, culminating in dorsi-
ventrality. One instance of an organ in which, owing to
this differentiation, a movement of apparent advantage to the
plant has been induced, is found in the pulvinus of Oxalis.
Here, by the greater excitability of the lower half, the leaf-
lets are made to fold downwards, with the consequence of
avoiding too intense illumination, in the responsive move-
ment known as diurnal sleep. But the differentiation which
we find here is not unique or suddenly evolved, for we
find a similar anisotropy even in the pulvinoids of ordinary
leaves such as those of Artocarpus. And such physiological
differentiation can be traced still further back, to the case of
organs which were originally radial. Thus, a long stem,
such as that of Cucurbita, happening to become recumbent,
becomes also dorsi-ventral, by the unequal action of sunlight
on the two sides, the too long excited upper side being now
the relatively less excitable. There is again no fixed line of
demarcation between this more or less permanent and a
transient differentiation ; for when a radial stem of Cucur-
bita is acted on by a transient unilateral stimulus, a tempo-
rary anisotropy is induced as between the excited and the
unexcited sides, the latter, which is fresh, being now the
more excitable ; but this anisotropy immediately disappears
on the recovery from excitation of the excited side. We
may thus have in the same organ at different times a
transient anisotropy, lasting for a minute or so, under
moderate unilateral stimulation ; a more prolonged aniso-
tropy, lasting for an hour or so, under stronger stimulation ;
and a permanent differentiation under still stronger and
longer continued unilateral stimulation. The difference
between the first and the last of these is simply a question
of whether the limit of elasticity has been exceeded or not.
In a torsioned wire, similarly, on the cessation of moderate
752 PLANT RESPONSE
stress, there is complete recovery ; and the same wire,
when torsioned beyond the hmits of elasticity, does not
recover, but remains permanently strained. In the same
way, we have seen that the effect of stimulus on a living
tissue is to induce a molecular derangement, from which
there is complete recovery, with a concomitant recovery
of all the physiological properties, if the derangement have
been not too great ; but, with excessive or long-continued
stimulus, the limit of physiological elasticity is exceeded, and
a permanent physiological differentiation is thus induced.
The same causes, moreover, which initiated the primitive
differentiation may now act to induce further a series of com-
plex movements. Thus we first see plagiotropic dorsi-
ventrality induced in the creeping stem of Cticurbita, by the
unilateral action of sunlight ; and then the diurnal periodicity
of light and darkness acting on this already differentiated
organ to cause a periodic swing, which increases with repeti-
tion. In this we have, as has been pointed out, the first
stage in the evolution of the nyctitropic movement. Now, it
is quite possible that this nyctitropic movement may be
found, at leastMn some cases, to subserve the advantage of
the plant ; yet it would not be true to say that it was
evolved for the purpose of such advantage. Indeed, we have
to guard ourselves carefully against being led, by this theory
of the final advantage of the plant, into an argument in a
circle. Assuming any given movement to be advantageous
to the plant, we have first to determine the nature of the
mechanism by which it is produced, and secondly to find out
what was the exciting cause, and what the character of the
conditions under which it first arose. If we are not guided
in our inquiry by such considerations, we are liable to be
misled in our inferences. One such example was seen in the
general belief that a certain specific sensibility resides in the
root-tip, by which it is endowed with the faculty of moving
away from rough surfaces, which might have been injurious
to it. So far from its having any such peculiar faculty, how-
ever, we found that when a red-hot wire was presented to it
PHYSIOLOGICAL RESPONSE IN PLANT AND ANBLVL 753
the tip moved towards it, and was thus destroyed. The
sensitive reaction of the root-tip is thus seen to be, not an
adaptive act evolved for the advantage of the plant, but
merely an example of the general law, that under moderate
stimulation it is the indirect effect of stimulus that reaches
the growing organ and causes movement away from, while
under strong stimulation the direct effect determines move-
ment towards, the source of stimulus. Now, it is perfectly
true that had any given reaction been such as, under normal
conditions, to bring about the self-destruction of large numbers
of organisms, we should not have witnessed the survival of
organisms characterised by that particular movement. The
plant lives because the physiological differentiations induced
in it under natural conditions, and the movements induced
under periodic changes of those conditions, are in harmony
with the fluctuating forces of the environment. Thus the
forced rhythm becomes more deeply impressed with repeti-
tion, and the greater is the harmony between this rhythm
and the environment, the greater will be its stability under
given conditions ; the plant persists, that is to say, because
it is perpetually in tune, instead of perpetually at war, with
its surroundings. We may take it, therefore, in the case of
any particular movement, that it constitutes an expression of
this stable relation of the plant to its environment, but not
that it represents any deliberate adaptation to such an end.
Reverting to the .nyctitropic movement in particular, we
find it adduced by Uarwin as to a certain extent furnishing
an example of the influence of heredity on the individual
organism, a view which has been questioned. But if we arc
prepared to give a sufficiently extended and consistent mean-
ing to the word, we must accept this, for heredity is essentiall}-
the repetition of a past cycle, the persistence of after-effects ;
and there are innumerable degress of such persistence, be-
tween that of a transitory after-effect and the phenomenon
of absolute persistence, if such occurs. The diurnal periodi-
city of Mimosa, for example, is maintained for several days
under unchanging conditions of illumination or of darkness,
3 ^
754 PLANT RESPONSE
after which the impressed periodicity is lost. The autum-
nal periodicity of trees which grow in temperate climates,
where the leaves are habitually shed at the approach of
winter, persists for a few or for numerous years when the
plant is transferred to a warm climate. Certain species of
bacteria, again, can be made to exhibit an artificial change of
characteristics which will persist during many generations,
even on the return of the organism to normal conditions ;
but even after this degree of hereditary persistence of effect
has been exhibited, these induced varieties are liable to
undergo reversion to their original type. It is thus seen
that the persistence of hereditary characteristics is merely
relative. Had it been absolute, the species itself would have
been immutable. A given rhythm only persists so long as
those circumstances which originally occasioned it persist,
and during a certain period afterwards ; but this after- or
hereditary-effect must, in the non-continuance of the original
periodically exciting cause, prove, however long maintained,
to be but transitory, like the ultimate arrest of a pendulum
when its vibrational energy is exhausted.
The phenomenon of life, then, introduces no mystical
power, such as would in any way thwart, or place in
abeyance, the action of forces already operative. In the
machinery of the living, as in that of the non-living, we
merely see their transformation, in obedience to the same
principle of conservation of energy as obtains elsewhere ;
and it may be expected that, in proportion as our power of
investigation grows, the origin of each variation of the living
organism will be found more and more traceable to the
direct or indirect action upon it of external forces, the
element of chance being thus progressively eliminated, as the
definite sequence of cause and effect comes to be perceived
with an increasing clearness ; and only, I venture to think,
as this is worked out, can we learn to apprehend fully the
true significance Ot the great Theory of Evolution.
CLASSIFIED LIST OF EXPERIMENTS
Mimosa :
Differential Mechanical Response
1. Response to induction shock
2. Isometric response .......
3. Determination of latent period and its variation by cold .
4. Variation of latent period by fatigue ....
5. Response to artificial turgidity variation ....
6. Effect of excessive turgor on sensibility
7. Erectile response to suction ......
8. Response to transmitted direct stimulation
9. Response to indirect stimulation longitudinally transmitted
10. Response to indirect stimulation transversely transmitted
11. Response under favourable tonic condition when chemically stimulated
by salt solution ........
12. Response to salt solution in ordinary tonic condition .
Biophytum :
13. Response to transmitted stimulation with preliminary erectile twitch
14. Response to direct stimulation
15. Effect of load
16. Determination of minimally effective stimulus . . . . .
17. Effect of cold on latent period ........
18. Response to increased internal energy ......
19. Demonstration of factor of internal energy in the process of recover)-
20. Response on ' all or none ' principle .......
21. Determination of refractory period ......
22. Additive eflect ...........
I'AGK
21
26
268
269
46
49
426
531
532
532
551
552
24
24
25
27
141
400
401
272
273
274
Philanthus urinaria :
23. Latent period diminished by intensity of stimulus
Ordinary leaves :
24. Response of Cilrus deciiniaiia ....
44, 271
25. Response of Artocarpiis ....
26. Response of Artocarpns to indirect stimulation
27. Localisation of motile area in Artocarpiis
64
55
420
58
3C2
756 PLANT RESPONSE
Anisotropic organs :
28. Inorganic response of compound strip
29. Response of plagiotropic stem of Cucurbita
30. Response of plagiotropic stem of Convolvulus
31. Response by collapse of bifurcated Allium peduncle
32. Response by uncurling of Passijlora tendril
33. Response of spirally cut Alliuiu peduncle by uncurling
34. Writhing response of spiral tendril of /'rt.w?/?c;-rt;
Longitudinal Mechanical Response
■AGE
^5
86
87
89
91
92
35. Response of stem of Cuscuta 76
36. Response of root of Bindweed 77
37. Response of style of Datura alba 78
38. Response of coronal filament of Passijlora . . . . . . 78
39. Effect of age on excitatory contraction ...... 79
40. Effect of season on responsive contraction 80
Electric Response
41. Response to direct stimulation 32
42. Response to transmitted direct stimulation . . . . . . 34
43. Indirect hydrostatic and true excitatory response .... 37
44. Electric test of differential excitability 38
45. Response to moderate stimulation of tip of growing organ . -517
46. Response to strong stimulation of tip . . . . 517,518
47. Response to direct stimulation of growing region .... 518
48. Response to moderate transverse stimulation . . . . . 519
49. Response to strong transverse stimulation . . . -519. 522
Elecj roj'actii.e Response
50. Electrotactik response in jMintosa stem 260
Death-response
51. Determination of death-point by electrical method . . . 149
52. Determination of death-point by spasmodic lateral response . . 150
53. Determination of death-point by spasmodic movement of uncurling . 155
54. Determination of death-point by sudden volumetric contraction . 156
55. Determination of death-point from critical point of thermo-mechanical
inversion ........... 168
56. Constancy of critical point of inversion . . . . . . 169
57. Modification of thermo-mechanical curve showing effect of age . . 170
58. Thermo-mechanical curve of Mimosa 172
59. Duplication of rigor-point in Mimosa by chemical ag(?nts . . 1 79
60. Translocation of death-point by unfavourable weather . .172, 176
61. Translocation of death-point by poisonous solution .... 180
62. Translocation of death-point by fatigue 185
CLASSIFIED LIST OF KXl'KRIMKNTS
757
63. l)calli-ves[i()nse of flowers
64. Theimoi^rapli of regional clcalh
. 182
1S3, 184
MuLiiPLE AND Autonomous Restonsf.
65. Multiple electromotive response 279
66. Multiple electrotactile response 2S0
67. ^Multiple mechanical response in Biophyliiiii 283
68. Cyclic variation of multiple response in Biophytiini . . . 285
69. Cyclic variation of multiple visual response 2S7
70. Multiple response in Desinodiuin originally at standstill . . 293
71. Multiple response to constant chemical stimulus in Biophytitm 300
72. Multiple response to constant current in Averrhoa . ... 300
73. Multiple response to constant light in retina 301
74. Multiple response to constant light in Biophytiini . . . . 302
75. Multiple response to constant light in Desinodiinii .... 303
76. Initiation of autonomous response in j9/(7///j///w above tliernio-lonic
minimum ........... 305
77. Resumption of autonomous response in Desinodiinii by accession of
energy ........... 306
78. Resumption of autonomous response in Desinodiinii under increased
internal hydrostatic pressure ........ 307
79. Localisation of seat of autonomous excitation in /desinodiinii . . 297
80. ' Systolic ' and ' diastolic ' positions in Desinodiinn pulsation . . 320
81. Incapability of tetanus in Z'fj'/;/0(2'/V///( ...... 346
82. Measurement of period and amplitude of Desniodiinn pulsation from
photographic record . . . . . . . . . 317
83. Measurement of period and amplitude of Z)t'J-wwf'//cw pulsation from
spark record .......... 3^7
84. Periodic variation in Desinodiinii pulsation . . . 34 1' 47°
See also Periodic variation of groioth and Siiclioiial response 472, 473, 474
Suction A [. Response
85. Record of suction, Unbalanced Method .
86. Shoshungraphic record under Hydrostatic Balance
87. Shoshungraphic record under Hydraulic Balance
88. Renewal of suctional response under stimulus
89. Periodic variation of transpiration in Ciicurbita
See also Alechanical, Growth, and Torsional respons
activity ........
suctional
400, 426,
669
Growth-response
90. Balanced Method of Crescographic record 412
91. Multiple growth-pulsation 4i7
92. Cyclic groupings of multiple growth-pulsation 417,425
93. Resumption of growth l)y stimulus ....... 425
94. Resumption of growth in droui;lil-rigored C/C(7/;7'//(f after supply ol
water 4^6
758 PLANT RESPONSE
95. Growth in response to hydraulic transmission of energy
430
429
428
428
433
96. Effect of decreased suctional activity on growth
97. Effect of increased hydrostatic pressure on growth of Balsam
98. Effect of increased hydrostatic pressure on growth of Crimini Lily
99. Effect of tension on growth .......
100. Bifurcated expression of stimulus in contractile response and resump
tion of growth .......... 435
loi. Similarity of response in stationary and growing organs . . . 436
102. Retardation of growth by external stimulus 436
103. Detection of latent stimulus by negative after-effect . . . . 459
104. Constancy of sum of direct effect and negative after-effect . . 460
105. Vanishing of latent component above the optimum . . . . 461
106. Variation of receptivity with temperature ..... 461
107. Direct and indirect effect of stimulus on plant in different tonic
conditions 462, 463
108. Diurnal variation of rate of growth in 0;;j'c^ ^a/Zz/a .... 473
109. Diurnal variation of rate of growth in seedling of Tainarindiis indica 474
no. Effect of high frequency Tesla current on growth .... 619
111. Effect of Hertzian waves on growth . . . . . . . 618
Responsive Growth-curvatures
112. Response of pistil of yl/>cj-« to unilateral thermal stimulation . . 68
113. Response of Cr^i"«5 to moderate unilateral stimulation of tip . . 526
114. Response of C;-(Jc«i- to stronger stimulation 526
115. Response of Crocus to direct stimulation of growing region . . . 527
116. Response of root of Bindweed to unilateral stimulus of tip . . 526
117. Response of root of Bindweed to stronger stimulation of tip . . 527
118. Response of root of Bindweed to unilateral stimulation of growing
region ........... 527
Torsional Response
119. Torsional response of pulvinus of Mimosa under lateral stimulus of
light 659
120. Torsional response of pulvinus of Mimosa under lateral chemical
stimulus ........... 659
121. Torsional response of inorganic compound strip ..... 660
1 22. Modification of torsional response in Mimosa by reversal of differential
excitability of pulvinus . . . . . . . . 66 1
123. Modification of torsional response in Mimosa by increase of
differential excitability of pulvinus 662
124. Torsional response in Erythrina indica under lateral geotropic
stimulus ........... 664
125. Modification of geotropic torsion by reversal of differential excita-
bility 665
126. Modification of geotropic torsion l)y increase of differential excitability 665
127. Autonomous torsion of stems . ....... 668
CLASSIFIED LIST OF EXPERIMENTS 759
I'AOK
128. Enhancement of rate uf autonomous torsion Ijy increased suctional
activity ........... 669
129. Modification of rate of autonomous torsion by electrical current . . 670
Uniform Response, Fatigue, and Staircase Efi-ect
130. Uniform electrical response in Radish ...... 104
See also Uniform mechanical responses in Biophytitm and Datura . 24
131. Staircase effect in mechanical response of ^w/^arw . . . . 104
132. Staircase effect, followed by fatigue, in the response oi Eiicfiaris . 121
133. Staircase effect, followed by fatigue, in the response o{ Bryophylluni 122
134. Staircase effect in inorganic response of Galena . . . . . 121
135. Fatigue in mechanical response of ZJa/z/ra 105
136. Fatigue-reversal under continuous stimulation in Passijlora . . 108
137. Fatigue-reversal under continuous stimulation in Mimosa . 109, 113, 552
138. Periodic fatigue in Uriclis . . . . . . . .108
139. Fatigue in electrical response of Radish 105
140. Fatigue-revetsal under continuous stimulation in electrical response of
Celery 107
141. Alternate fatigue in electrical and mechanical response . . . 106
142. Fatigue in inorganic response : arsenic . . . . . .119
143. Fatigue in inorganic response : india-rubber . . . . . 120
144. Alternating fatigue in arsenic ........ 575
145. Apparent fatigue in response of plant, due to bifurcation of energ)- . 127
Additive Effect and Relation i;et\veen Stimulus and Response
146. Additive effect in mechanical response ...... 94
147. Additive effect in electrical response ... " • • • 95
148. Genesis of tetanus in mechanical response ..... 99
149. Relation between intensity of stimulus and amplitude of mechanical
response ........... 96
150. Relation Ijetween intensity of stimulus and amplitude of electrical
response ........... 97
151. Influence of tonic condition on relation between stimulus and respon-
sive growth-retardation ......... 467
Polar Effects of Currents
Pulvinar response :
152. Action of feel)le E. ALE. on J/////£ii'rt, Mono-polar Metliod . . 192
153. Action of feeble E.M.F. on J//wo.fa, Bi-polar Method . . . 194
154. Action of feeble E.M.F. on Biophylum, Mono-polar Method . 193
155. Action of feeble E.M.F. on Biophytnm, Bi-polar Method . . . 194
156. Effect of moderate E.M.F. on /^/i7///j'//^w 195
157. Effect of moderate E.M.F. on highly excitable tissues . . . 197
158. Effect of moderate E.M.F. on Z)6'j-;«<7^j'/>/'w 292
76o PLANT RESPONSE
Reversing effect of high E.M.F. :
159. ^1 stage, BiophyUim 204
160. Alimosa ......... 204
161. >9 stage, Averrkoa . . . . . . ... 205
162. and fatigue, Mimosa ........ 208
163. Localised effect of Anode and Kathode on puhinus of Erytliriiia
iiidica ........... 555
Death-response :
164. Polar effects of moderate E.M.F. on death-response . . . 1S5
165. Polar effects of high E.M.F. on death-response . . . . 207
Glow-response :
166. Polar action of moderate E.M.F. on glow-response of firefly . . 212
167. Reversal of effects by high E.ALF. or by tissue-modification . . 213
Growth-response :
168. Anodic and kathodic effects on growth of root of Bindweed . . 557
169. Galvanotropic response of Crinum Lily . . . . • • 55^^
170. Indirect effect of polar excitation on C/zflw/rtcrt . . . • S59
171. Effect of electrification of soil on growth of C?7;a j-a/?z'a . . . 560
See also Galvanotaxis ......... 69S
ExCITAr.II.ITY AND CONDUCTIVITY
172. Diminution or abolition of motile excitability by anesthetics
173. Diminution or abolition of motile excitability by cold
174. Diminution or abolition of motile excitability by fatigue .
175. Diminution or abolition of motile excitability by An-electroto
176. Increase of e.xcitability by Kat-electrotonus
177. ' Developing ' action of Kathode
178. Diminution or abolition of conductivity by ana-sthelics
179. Diminution or abolition of conductivity by cold
180. Diminution or abolition of conductivity by fatigue
181. Diminution or abolition of conductivity by anodic block
182. An-electrotonic and Kat-electrotonic action on conductivity
183. Depression of receptive excitability by anaesthetics
184. Receptivity y hot water . . . . 3S1
Growth :
217. Determination of various rates at different lemperaun-es by Discon-
tinuous observations 442
218. Determination of various rates at different temperatures from Thermo-
crescent curve 447
219. Determination of various rates at different temperatures by Method
of Balance .......... 45°
220. Determination of various rates at different temperatures by Method
of Excitatory Response 45-
221. Determination of oplinnim point by Method nf Balance . . . 451
762 PLANT RESPONSE
PAGE
222. Translocation of optimum point 454
223. Persistence of rhythmic activity at maximum ..... 432
Autonomous torsion :
224. Effect of temperature on rate of autonomous torsion . . . . 668
Effect of Chemical Agents
Mechanical response :
225. Effect of carbonic acid . . . . . . . . -131
226. Effect of hydrogen 132
227. Effect of carbon disulphide 132
22S. Effect of alcohol 133
229. Effect of ether on longitudinal response. . . . . .134
230. Effect of ether on motile response of yl//w^M . . . . . 135
231. Effect of acid 136
232. Effect of chlorine gas 137
Autonomous pulsation of Desmodium :
233. Effect of ether 323
234. Effect of alcohol 324
235. Effect of carbonic acid gas ........ 325
236. Effect of copper sulphate solution 326
237. Effect of barium salt solution ....... 351
238. Arrest at ' diastole ' by acid 352
239. Arrest at ' systole ' by alkali 353
240. Antagonistic actions of acid and alkali 353
Suctional response :
241. Action of KNO3 solution 384
242. Osmotic versus excitatory action of NaCl solution . . . . 384
Growth :
243. Effect of carbonic acid 479
244. Action of ether, external application 480
245. Action of ether, internal application 481
246. Excitatory effect of dilute solution of sodium carbonate . . . 482
247. Effect of solution of sugar 482
248. Effect of alcohol 483
249. Effect of acid inducing relaxation and arrest ..... 483
250. Effect of alkali inducing contraction and arrest 484
251. Antagonistic actions of alkali and acid 484
252. Action of strong sodium chloride solution under different thermo-
tonic conditions .......... 485
253. Differing effects of poisons on different ' constitutions ' . . . 487
254. Opposite effects of same poisonous dose on different ' constitutions ' 487
255. Opposite effects of large and small doses . . . . . . 4S8
CLASSIFIED LIST OF EXPERIMENTS 763
Growth-curvature :
i'A<;i-'.
256. Chc-nio-tropic effect of alkali 548
257. Antagonistic chemo-tropic effect of acid ..... 548
258. Effect of copper sulphate solution ....... 548
259. Effect of sugar solution 549
See also Chemo-tactic response ....... 699
Geotropism
260. Apo-geotropic response of scape of Uric/is ..... 499
261. Response of Criniitn Lily under unilateral pressure of particles . . 498
262. Apo-geotropic response at inclinations of 45^ and 135° . . . 501
263. Demonstration of apo-geotropic curvature as induced by responsive
contraction ........... 504
264. Effect of alternate applications of cold on upper and lower surfaces
on apo-geotropic response 505
265. Darwinian curvature and its reversal under different intensities of
stimulation 536
266. Determination of effect of amputation of root-tip on excitability . 541
267. Effect of gravitational stimulus on autonomous torsion . . .671
See also Response to stimulation of tips of root attd shoot . . . 526
Ki?^o Differential geotropic sensibility of dorsi-ventral organs . 662, 666
Efkfxt ok Visible and Invisible Radiations
268. Response of Mimosa to electric radiation .....
269. Discrimination of efl'ects of thermal radiation and of temperature
270. Response of Musa to unilateral thermal radiation .
271. Response of Taniarindus to continuous unilateral thermal radiatio
272. Effect of diffuse stimulation of light on growth of sub-tonic tissue
273. Effect of diffuse stimulation of light on stationary radial organ
274. Retarding effect of light on longitudinal growth
275. Oscillatory growth-response under continuous action of liglit .
276. Effect of light on autonomous torsion .....
619
615
616
617
574
571
572
574
669
Heliotropism
In radial growing organs :
277. Positive responses of 5///a//j- to moderate light . . . 593,609
278. Positive responses of cooled tendril of /"ajj-Z/^ra . . . . 611
279. Positive responses of tendril of F/V/j- to feeble light . . .611
280. Neutral response of Sinapis under strong stimulation . . . . 609
281. Neutral response of tendril of /'(z.fj;yf.ra//.s- (diurnal movement) . . 633
298. Multiple heliotropic response . . . . . 303, 584, 604, 634
299. Multiple merging into continuous heliotropic response . . . 586
303. Localisation of heliotropic sensibility in pulvinus . . . 588, 650
301. Lamina not perceptive organ to stimulus of light . ... 651
302. Absence of specific dia-heliotropic tendency in leaves . . . 649
In ordinary leaves :
303. Positive response of leaf of Mangifera indica 653
304. Negative response of ^;-/'i?(rtr/// J- ....... 653
305. Torsional response to lateral stimulation of terminal leaflet of
Desmodiitm . . . . . . . . . ..657
See also Torsiona/ response 0/ dorsi-z'e/i/ra/ organs . . . .661
306. Limits of flexibility in various leaves 647
Nyctitropic movement :
307. Diurnal movement of plagiotropic stem ...... 679
308. Diurnal movement of leaf of Biophytitm . . . . . . 680
309. Nyctitropic movement of Mimosa leaf, due to cumulative heliotropic
action 678
310. Nyctitropic movement of primary petiole of J//w^jra . . . . 681
311. Impressed heliotropic periodicity in Sunflower .... 685
Heliotropic action on autonomous response :
312. Initiation of autonomous response by light in /Jfj-w^rt'/z/w . . . 690
313. Changes of amplitude in autonomous pulsation of Z>^5w /«;// . 691
314. Periodic reversals of relative rapidities of up and down beats in
Desmodiiim . . . . . . . . . . . 692
315. Modification of autonomous torsion ...... 669
See also Photolaxis .......... 696
INDEX
AcELLULAR Organ, responsive curvature of, 506
Acid, chemo-tropic action of, 548
,, effect on mechanical response, 136
Acid and alkali, antagonistic action of, on cardiac pulsation, 351
,, ,, ,, ,, on chemo-tropic response, 548
,, ,, ,, ,, on Desmodium pulsation, 352
,, ,, ,, ,, on growth, 484
Additive effect, 95
After-effect, detection of al)S[)rl)ed stimulus by negative, 459
,, factors modifying, 468
,, persistence of, 310
., positive and negative, 457
,, ,, ,, constancy of sum of, 460
Age, effect of, on contractile response, 79
,, ,, on thermo-mechanical inversion, 171
Alcohol, eft'ect of, on Desmodium pulsation, 325
,, on growth, 482
,. ., on mechanical response, 133
All or none ' principle in response of Biophytiim, 272
,, ,, ,, cardiac tissue, 353
Allium, death-response of, 157
,, mechanical response of, 87
Anivsthetics, action of, on autonomous response, 324
,, ,, on conductivity, 220, 229
,, ,, on excitability, 220
,, ,, on mechanical response, 134, 135
,, (jn receptivit)', 224
An-electrotonus, 232
Anisotropy, induction of, by cold, 82
,, ,, by fatigue, 83
,, ,, by light, 84
,, necessary condition for lateral response, 44
Annual rings, 474
Anode, effect of, on growth, 556
Anodic block, 233
Ariscema, curious response of, 533
Arsenic, fatigue-reversal in, 119, 575, 653
7^ PLANT RESPONSE
Artocarpiis, effect of over-turgidily, 58
,, localisation of motile areas in, 58
,, response of, 55, 420, 653
Ascent of sap, mechanics of, 390
,. rapidity of, 392
,, theories of, 359
,, uni-directioned flow of, 390
Autonomous excitation, seat of, in Desiiiodiitm, 297
,, response, continuity with multiple, 290, 291
,, ,, induction of, by hydrostatic pressure in cardiac tissue, 307
,, ,, ,, ,, ,, in Desmoditu/i, 307
,, ,, ,, by accession of energy in Biophytiini, 305
,, ., ,, ,, ,, in Desinodiiint, 306
,, ,, periodic groupings of, 291, 350
Avena sativa, heliolropic response of, 604
Balsam, effect of liydrostatic pressure on growth of, 428
„ growth -pulsation of, 432
Barium chloride, effect on Destnodiiiiii pulsation, 351
Bindweed, effect of section of root-tip on excitability of, 541
,, polar effect of current in the growth of root of, 556
,, response of root of, to thermal stimulus, 76, 526, 527
Biophytuin, additive effect in, 274
,, autonomous response of, 305
„ diurnal sleep movement of, 634
,, effect of anaesthetics, 223, 225
,, ,, an- and kat-electrotonus, 232-236
,, ,, cold on excitability of, 222, 224
,, ,, ,, latent period of, 141
,, ,, load on response of, 25
,, electric response of, 34, 37 *
,, erectile response of, 24, 37, 400
,, minimally effective stimulus in, 27
,, multiple response of, 282 et seq.
,, normal mechanical response of, 24, 37
,, nyctitropic movement of, 680
,, polar effect of current on, 195, 197
,, refractory period of, 273
,, response on 'all or none ' principle, 272
Bi-polar method, 191
BotrydiiDii granulatniii, phototaxis of, 497
Brown, H., on diffusion through multi-perforate septum, 399
Bryophylliiiii calcynitui, response of, 62
Carbon disulphide, efilect of, on mechanical response, 132
Carbonic acid, effect of, on Desmodiiivi pulsation, 325
,, ,, ,, on growth, 480
,, ,, ,, on mechanical response, 131
Chemical method, determination of velocity by, 255
INDEX yej
Chemical reagents, efiect on autonomous response, 323-326
,, ,, ,, growth, 479-488
,, ,, ,, mechanical response, 131-137
,, ,, ,, suctional response, 383, 384, 385
,, ,, modification of effect of, 322
Chemo-taxis, 698
Chemo-tropic response : effect of acid, 548
,, ,, ,, alkali, 548
,, ,, ,, copper sulphate solution, 54S
,, ,, ,, sugar solution, 549
Chlorine, effect of, on mechanical response, 137
Ciesielski on effect of amputation of root-tip, 539
Citrus decumana, response of leaf of, 64
Clitoria ternatea, diurnal sleep movement of, 629
Cold, abolition of autonomous response by, 141, 331
,, ,, mechanical response by, 141
,, ,, suctional response by, 374
,, diminution of conductivity by, 221, 249
,, prolongation of latent period by, 141, 264
Cold-rigor, 171
Colocasia, excretion of water in, 397
Conduction, channels of, 60, 250
,, of excitation and of sap, 391
,, pseudo-, 251
Conductivity, difterence between longitudinal and transverse, 250
,, preferential direction of, 242
,, seasonal variation of, 602
,, variation of, by aniesthetics, 223
,, ,, by cold, 221, 249
,, ,, by electrotonus, 233
,, ,, by fatigue, 222
,, ,, by rise of temperature, 222, 249
Conductivity-variation, response by, 39
' Constitution,' elements determining, 478
,, modifying effect of, on action of drugs, 487
Convolvulus, death-response of flt)werof, 182, 183
,, response of plagiolropic stem to light, 625
,, ,, ,, ,, to thermal shocks, 86
Copriiius, action of light on, 569
Counterpoise, importance of, 19
Crescograph, 412-416
Criiniin Lily, apo-geotropic response of, 498, 500, 504
,, ,, direct and indirect after-effect of stimulus in, 460
,, ,, effect of increased hydrostatic pressure on growth of, 428
,, ,, ,, tension on growth of, 433
,, ,, ,, unilateral pressure of particles on, 497
,, ,, excitatory response at different temperatures of, 453
,, ,, optimum point of growth in, 448, 451
,, ,, ihermo-crescent curve of growth in, 447
76^ PLANT RESPONSE
Crocus, growlh-curvature by slimulalion of tip and growing region of shoot,
526, 527
Citciirbita, diurnal variation of rate of transpiration in, 472
,, initiation of growth-pulsation in, 426
,, response of plagiotropic stem of, to thermal shocks, S6
)> >> ,, ,, to light, 626
Cusatta, response of stem of, 77
Cynerece, response of filaments of, 69
Czapek on localisation of gravi-perception at root-tip, 542
, , on radial pressure theory, 494
Darkness, modification of sensitiveness in, 22, 50
Darwin, Francis, on effect of alternating stimulation, 685
,, ,, on growth-curvature under light and gravitation, 494
Darwin on localised geotropic sensibility at root-tip, 539
,, on sensitiveness of radicle, 513
Darwinian curvature, 536
,, theory, 749
Dal lira alba, style of: death-discoloration of, 183
,, ,, ,, death- response of, 160
,, ,, ,, effect of light on growth, 571
,, ,, ,, lowenng of death-point by fatigue, 1 85
,, ,, ,, response of growing and stationary, 434, 435, 436
,, ,, ,, response to thermal stimulus, 78
,, ,, ,, tetanus in, 99
Death-discoloration, 183
Death-point, constancy of, 173
,, determination of, by electrical response, 149
,. ,, by movement of uncurling, 165
,j ,, by spasmodic lateral response, 150
,, ,, by thermo-mechanical inversion, 161
,, ,, by volumetric contraction, 156
,, electrotonic effect on, 185, 186, 199, 207
,, translocation of, by age, 170
,, ., by fatigue, 185
,, ,, by poison, 180
,, ,, by unfavourable weather, 172
Death-response, effect of chemical agents on, 179, 180
,, in flowers, 181
,, physiological nature of, 152
Death-spasm in plants, 150, 155, 156
De Candolle, theory of heliotropism, 580
Desmodium gyrans, multiple response of, 293
,, ,, polar excitation of, 292
,, ,, seat of autonomous excitation in, 297
,, ,, significance of up and down movement in, 330
Desinodiiiin pulsation, diurnal record of, 470
,, ,, incapability of tetanus, 346
,, ,, effect of ana'sthetics on, 32;
INDEX 769
Desitiodiuin pulsation, eflect of acid on, 352
,, ,, ,, alcohol on, 324
,, ,, ,, alkali on, 352 '
,, ,, ,, barium salt on, 351
,, ,, ,, carbonic acid on, 325
,, ,, ,, copper sulphate solution on, 326
,, ,, „ light on, 691, 692
,, ,, ,, temperature on, 332-33S
,, ,, period and amplitude of, 317
,, ,, periodic variation of, 341
,, ,, similarity between Biophytuni pulsation and, 345
,, ,, ,, ,, cardiac pulsation and, 345-354
,, ,, spark- record of, 327
,, ,, temperature, maximum of, 331
,, ,, thermo-tonic minimum of, 331
Detmer on photonasty, 643
Development, thermographic, 184
De Vries on epi- and hypo-nasty, 643
,, on heliotropic action on leaves, 622
,, on plasmolytic method, 549
Dia-geotropism, so-called, 640, 654
Dia-heliotropism, so-called, 640-654
Differential response in plant, 14
,, ,, magnification of, 51
,, ,, of compound strip, 13
Directive versus non-directive action of light, 634
Diurnal sleep, 629, 633, 634
Dorsi-ventral organ, differential geotropic action inducing torsion in, 662
,, ., ,, heliotropic action inducing torsion in, 658
,, J, no specific sensibility in, 577
Dutrochet on conducting elements in Mimosa, 251
Electric radiation, effect of, on Mimosa, 619
,, response, difference of excitability detected l)y, 38
,, ,, laws of, 42
,, ,, multiple, 279
,, ,, of inorganic substance, 40
,, ,, simultaneous mechanical and, 34
,, ,, to direct stimulation, 32, 518
,, ,, to indirect stimulation, 37, 517
,, ,, true excitatory negative and hjdrostatic ]X)sitive, 35, 37
,, waves, effect of, on growth, 618
' Electrification ' of soil, effect of, on growth, 560
Electromotive Method, detection of excitatory wave by, 261
Electrotactile Method of detection of excitation, 256, 280
Electrothermic stimulator, 1 7
Elfectrotonus, variation of excitabilit)' \>y, 234, 235, 236
Elfving on geotropic action on grass-haulm, 496
Eiytkriita iiu/iia, differential geolrupic response of, 664, 665
3D
770 PLANT RESPONSE
Erythrina indica, heliotropic response of, 629
,, ,, polar effect of current on, 555
Eucharis Lily, cold-rigor in, 141, 172
Excitability, seasonal variation of, 80, 474
,, variation of, by anesthetics, 220
,, ,, by cold, 220
,, ,, by electrotonus, 234
,, ,, by fatigue, 221
,, versus conductivity, 227
,, ,, receptivity, 226
Excitation, longitudinal conduction of, 516, 517, 526, 532, 604
,, mechanical versus protoplasmic theory of, 189, 229
,, transverse conduction of, 519, 528, 529, 532
Excitatory contraction, physiological modification of, 79
,, discharge, preferential direction of, 243
,, versus hydrostatic effect, 35, 47
,, wave, centrifugal velocity of, 243
,, ,, centripetal velocity of, 243
,, ,, determination of velocity of, by Chemical Method, 255
,, ,, ,, ,, by Electromotive Method, 260
,, ,, ,, ,, by Electrotactile Method, 25
,, ,, ,, ,, by Mechanical Method, 240
,, ,, modification of velocity of, by cold, 248
,, ,, ,, ,, by fatigue, 245
,, ,, ,, ,, by intensity of stimulus, 246
,, ,, ,, ,, by rise of temperature, 249
,, ,, velocity of, in plant and animal, 252
Fatigue, alternating, 106
,, effect of, on conductivity and excitability, in
,, ,, on electrical response, 105
,, ,, on mechanical response, 105
,, ,, on velocity of transmission, 244
,, periodic, 65, 108
,, reversal of normal polar effect by, 207
Fatigue-reversal under continuous stimulation, in arsenic, 119
,, ,, ,, ,, in india-rubber, 120
,, ,, ,, .. in plant, 63, 107, 108, 109
Fick on velocity of nervous impulse, 246
Firefly, glow-response of, 211
Flexibility, characteristic limits of, 646
Flowers, death-response of, 181
Food-substance, translocation of, 398
Forced vibration, 684
Frank on heliotropic response of Marchaniia, 622
,, on transverse heliotropism and geotropisni, 641
Frost, effect (m leaves, 400
Galena, staircase response in, 121
Galvanotaxis, 698
INDEX 771
Galvanotropic response, 558
Geolropic response, at inclinations of 45° and 135°, 500
,, ,, effect of unilateral application of cold on, 504, 505
,, ,, mechanics of, 495
,, ,, method of recording, 499
,, ,, of shoot and root, 544
,, ,, torsional, 664
,, ,, true character of, 502
,, sensibility, localisation at root-tip, 539
Geotropism, differential, 663
,, radial pressure and statolitliic theories of, 494
Glow-response, 211
Grass-haulms, geotropic response of, 496, 508
,, growth of, on klinostat, 509
Greely, A. W. , galvanotaxis of /a;-a/;/rtr?a, 698
Growing organ and pulvinus, identical nature of responses in, 534
Growth, anodic and kathodic effect on, 556
,, antagonistic action of acid and alkali on, 4S4
,, apparent arrest of, at high temperature, 432
,, balanced record of, 413
,, daily periodicity in, 473, 474
,, direct and indirect stimulus on, 438
,, effect of acid on, 483
,, ,, alcohol on, 482
,, ,, alkali on, 483
,, ,, carbonic acid on, 479
,, ,, electric waves on, 618
,, ,, ' electrification' of soil on, 560
,, ,, ether on, 480
,, ,, NaCl solution on, 485
,, ,, Na.^Co^ solution on, 482
,, ,, poisonous solution on, 486
,, ,, sugar solution on, 482
,, external stimulus on, 434, 435, 436
,, laws of, 439
,, o})posite effects on, of chemical agents on different ' constitutions,' 487
,, ,, ,, of large and small doses, 488
,, optimum temperature of, 448, 451
,, ,, translocation of, 454
,, periodic variation of rate of, 470, 473, 474
,, polar action of current on, 557, 558, 559
,, relation of, to hydrostatic pressure, 428
,, ,, to tension, 432
Growth-curvature, induction of, by electrical current, 553
,, ,, by unilateral stimulus, 525, 527, 528, 529
Growth-pulsation, multiple, 417, 432
,, periodic groupings of, 417, 425
Grow th, renewal of, by external stimulus, 425
,, ,, by positive turgidity-variation, 426
3 D2
772 PLANT RESPONSE
Growth-response and excitatory response, 418
,, similarities between pulvinar and, 437
Haberlandt on transmission of excitation in Mimosa, 251
,, on statolithic theory, 494
Heart-beat, theories of, 347
Hedera helix, seasonal variation of heliotropic response in, 623
Ileliotropic excitation, transmission of, 604
,, negative response of dorsi-ventral organs : Xtai oi Artocarpus, 653
,, ,, ,, ,, ,, ,, Mimosa, 631, 632
,, ,, ,, ,, ,, leaflet of ^/i?//y///w, 634
,, ,, „ ,, ,, ,, Oxalis, 633
,, ,, ,, ,, ,, plagiotropicstem of C//tv/;--
f)ita, 626
,, ,, ,, ,, ,, plagiotropic stem of
Ipomcca, 625
,, ,, ,, ,, ,, plagiotropic stem of
Mimosa, 625
,, ,, ,, radial organs : hypocotyl of J5V;?a//V, 609
,, ,, ,, ,, ,, Lepidiitm, 60S
,, ,, „ ,, ,, xoo\. oi Si napis,(iO\
,, ,, ,, ,, ,, shoot of Croats, 601
,, ,, ,, ,, ,, tendril of F?V?V, 611
,, neutral response: leaflet oi Desmodium, 604
,, ,, ,, Lepiditim, 608
,, ,, ,, Siiiapis, 609
, , , , , , tendril of Passi flora, 6 1 1
,, perception, absence of, in lamina, 588, 650, 651
,, positive response of dorsi-ventral organs : Xtzioi Mangiferai^idica, 653
,, ,, ,, ,, ,, leaflet of Clitoria ternatea,
639
,, ,, ,, ,, ,, leaflet of Desmodium, 586,
587, 628
,1 }, ,, ,, ,, leaflet of ^;j/>^;-ma, 629
,, ,, ,, ,, ,, leaflet of i?(?(^/«z'a, 629
,, ,, ,, radial organs : Lepidiitm, 60^
,, ,, ,, ,, ,, Si/iapis, 593, 609
5, ,, ,, ,, ,, iendril o[ Passijiora, 611
,, ,, ,, „ ,, ,, J'ifis, 611
,, Recorder, 588-591
,, response, definition of positive and negative, 581
,, ,, intensity of stimulus on, 608, 609
,, ,, modifyingactionofanisotropy and conductivity on, 608, 613,
628
,, ,, multiple, 303, 584, 604, 634
J, ,, ,, merging into continuous, 586
,) ,, positive, negative, and dia-, 609
,, ,, seasonal variation of, 602, 623
,, ,, .torsional, 659, 661
INDEX yyi
Ileliotropic sensitiveness, localisation of, 604, 650
,, ,, supposed absence of, in organs, 610
Heliotropism, theory of Darwin, 582
,, ,, de CandoUe, 580
Hydraulic model, 31
,, response, 363
Hydrogen, effect of, on mechanical response, 131 '
Hydrostatic disturbance, effect on electric response, 35
,, ,, ,, mechanical response, 24
,, versus true excitatory efitect, 35, 47
India-rubber, contractile response of, 12
,, fatigue-reversal of, 120
Indicator diagram, 2
Inner stimuli cause of growth, 421
,, ,, derived from external, 355
Inorganic response, 12, 13, 40, 118
,, ,, abolition of, by poison, 40
,, ,, fatigue in, 119, 120
,, ,, staircase effect in, 121
Internal energy, 308
,, „ antagonistic action of, and external stimulus, 403
,, ,, effect of, on amplitude of pulsation, 338
,, ,, ,, on frequency, 338
,, ,, a factor of recovery, 401
,, ,, spontaneous movement initiated by excess of, 305, 306
,, ,, tonic condition dependent on, 308, 314
Ipomaa, torsional response of, 670
Isometric record, 25
Isotonic record, 25
Ivy, seasonal variation of heliotropic sensibility of, 623
,, variation of electrical response by temperature, 141
JiiN'NiNGS on chemotaxis of protozoa, 699
Kat-electrotonus, 235
Kathode, effect of, on growth, 556
,, excitatory effect of, 192- 197
Kuhne on polar excitation of protozoa, 200
Kunchangraph, 71
Lamina not perceptive organ, 60, 588, 650, 651
Latent period, diminution of, by strong stimulus, 271
,, ,, in Mimosa, 22, 268
,, ,, prolongation of, by cold, 141, 268
,, ,, ,, by fatigue, 269
Lateral geotropism, 672
,, response, general absence of, in radial organs, 50
,, ,, necessary conditions for, 44, 48
774 PLANT RESPONSE
Laws of electrical response, 42
,, growth, 439
,, growth-curvature, 535
,, mechanical response, 51
,, polar action of currents, 199, 207, 558
,, torsional response, 662
Leaves, dia-heliotropism of, 640
,, drooping of, by frost, 400
,, response in ordinary, 61
Lepidium, heliotropic response of, 608
Light, action of, on autonomous torsion, 669
,, ,, on longitudinal growth, 572, 573, 574
,, ,, on non-growing radial organ, 571
,, ,, on pulsation of Z>(?jw^fl'?Mw, 690, 691
,, ,, on pulvinated organ, 570
,, ,, on subtonic tissue, 569
„ anisotropy induced by, 84
,, directive versus non-directive action of, 634
,, diverse movements induced by, 565
,, efiect on periodic reversals, 692
,, induction of torsion by, 659
,, multiple response induced by, 303, 5S4, 604, 634
,, orientating action of, 587, 697
,, supposed specific differences in the action of, 567
,, transmitted action of, 606
Light and gravitation, incomplete analogy between the actions of, 597
See Heliotropic response
Longitudinal response in india-rubber, 12
,, ,, in muscle, 12
,, in plants, 13, 76-80
Lysimachia Nuiininilaria, heliotropic response of, 622, 624
Matigifera ndica, response of, 653
Marchantia hallus, action of light on, 622
Mechanical response, by curling, 91
,, ,, by uncurling, 89
,, ,, effect of chemical agents on, 131-137
,, ,, ,, temperature on, 142-146
,, ,, favourable conditions for exhibition of, 48
,, ,, o\ Allium peduncle, 87
,, ,, oi Arfocarpus, 55
,, ,, oi Biophytitiii, ?,ee Biophy/iiiii.
, , , , of Mimosa, see JMimosa.
,, ,, of ordinary leaves, 61, 653, 654
,, ,, similarity of, in pulvinus, pulvinoid and growing organ, 534
,, ,, to artificial turgidity-variation, 45
,, ,, to suctional activity, 399
,, ,, two types of, 46
Michelia Champaca, chemotropic response of, 548
INDEX 775
Alichelia Chainpaca, galvanotropic response of, 559
Microscope Recorder, 599
Mimosa, death-point of, 153
,, duplication of rigor-point in, 179
,, effect of anaesthetics on excitability of, 135
,, ,, darkness on the sensibility of, 22, 50
,, ,, electrotonus on excitability of, 235
,, ,, ether on conductivity of, 229
,, ,, excessive turgor on the sensibility of, 49
,, ,, fatigue on response of, 109
,, erectile response of, to suction, 426
,, excitatory fe;-j-/« plasmolytic reaction, 551, 552
,, fatigue-reversal in, 109, 113, 114
,, magnification of response of, 51
,, mechanical response of, 21, 22, 268
,, nyctitropic movement of, 681
,, polar effects on, 194, 197, 204, 208, 210
,, response of, to artificial variation of turgor, 46
,, ,, to direct stimulation, 531
,, ,, to electric radiation, 619
,, ,, to indirect stimulation, 532
,, ,, to light, 631, 632
,, ,, to salt solution, 551, 552
,, thermc-mechanical record of, 172
,, torsional response of, 658, 659
Mohl on heliotropic insensitiveness of certain tendrils, 611
Molecular model, 217
Mono-polar method, 193
Morograph, 167
Motile areas in Artocarptis, 59
,, excitability, earlier abolition than conductivity, 112
,j . ,, effect of anaesthetics on, 229
,, ,, versus conductivity, 227, 228, 229
,, ,, ,, receptivity, 226
Multiple response to constant stimulus : in inorganic substance, 39, 575
,, ,, ,, in plant, 300, 301, 302, 303, 574, 604
,, ,, ,, in retina, 301
,, to strong stimulus : electrical, 279
,, ,, ,, electrotactile, 280
,, ,, ,, mechanical, 283
,, ,, ,, visual, 286
,, comparison of various types of, 432
,, continuity of, with autonomous, 291
,, cyclic variation of, in BiophytuDi, 285
,, ,, ,, in frog's heart, 351
,, ,, ,, in growth, 417, 424
,, ,, ,, in visual impulse, a88
,, induction of, by light, 303, 584, 604, 634
,, merging of, into continuous, 586
77^ PLANT RESPONSE
Musa paradisiaca, response of, 68, 6i6
Muscarin, effect of, on cardiac pulsation, 4
Nectar, excretion of, 398
Nemec on statolithic theory, 494
Noll on anomalous plasmolytic movement, 550
Nyctitropic movement, of Biophytiitn petiole, 680
,, ■ ,, of il/Zw^ija' petiole, 681
5) )! supposed distinction between heliotropic and, 677
Old tissues, excitatory reaction in, 57
Oltmanns on heliotropic response of Lepiditini, 608
Optic Lever Recorder, 5
Oryza saliva, direct and indirect effects of stimulus on, 463
,, ,, diurnal variation of growth of, 473
,, ,, effect of electrification of soil on growth of, 560
Osmotic versus excitatory action : on ascent of sap, 383
,, ,, ,, ,, on growth, 485
Oxalis, diurnal sleep of, 63
on mechanical response, 549, 550, 551, 552
Para-heliotropism, 621
Paramacia, chemotaxis of, 699
,, galvanotaxis of, 698
,, thermotaxis of, 69S
Particles, response to unilateral pressure of, 496
Passifiora, death-discoloration of petals of, 183
,, effect of age on response, 79
,, ,, season on response, 80
,, response of filament, 78
,, ,, tendril, 89, 611
,, writhing movement of tendril of, 92
Perceptive region, oi Artocarpus leaf, 60
,, ,, oi Desiitodiuin leaflet, 588, 650
1, ,, of leaflet of £^j'Mr/«rt /«/£V7, 651
,, ,, true, 544
, versus responding organ, 542, 588
Periodic after-effect, 683, 685
,, ,, factors deterniiniog, 468
,, fatigue, 108
,, groupings: of growth-pulsation, 417, 425
,, ,, of mechanical response, 286
,, reversals in Desmodiiini under light, 692
,, variation oi Destnodmm pulsation, 341, 470
,, ,, of growth -rate, 470, 473, 474
,, ,, of transpiration, 472
Pertz, D., on influence of alternating stimulation, 684
Pfeffer on action of light, 566
,, on chemo-taxis, 599
INDI':X y-jy
Pfetier on effect of light on autonomous resjionse, 6go
,, on excitatory contraction in CynerecE, 69
,, on expulsion of water from excited pulvinus of Mimosa, 254
,, on geotropic sensitiveness of root-tip, 542
,, on radial pressure theory of geotropism, 494
Pfliiger on law of polar excitation in animal tissue, 200
rhilanthus urinaria, response of, 44, 271
Photonasty, 621
Phototaxis, 696
Physical phenomena, merging of, into chemical, 122
Plagiotropic stem, daily periodic movement of, 627
,, ,, response of, to light, 624, 625, 626
,, ,, ,, to thermal shocks, 85
Plant-chamber, 16
Plasmolytic action, anomalous, 549
Poison, abolition of inorganic response by, 40
,, effect of, on autonomous pulsation, 326
,, ,, on growth, 486
,, ,, on mechanical response, 132
,, ,, on suctional response, 386
,, modifying action of constitution on the effect of, 322, 487
,, opposite effects of dose on action of, 488
Polar action of currents : effect of fatigue on, 208
,, ,, high E.M.F., 207
,, ,, ,, ,, low E.M.F., 195
,, ,, ,, ,, moderate E.M.F., 197
,, ,, ,, indirect effect of, 558
,, ,, ,, laws of, in plant, 199, 207, 558
,, ,, ,, localised effect of, on pulvinus, 554
,, effects of currents : localised action on pulvinus, 554
,, ,, ,, on death-response, 186, 207
,, ,, ,, on glow-response, 211
,, ,, ,, on growth, 557, 558, 559, 560
,, ,, ,, on mechanical response, 193-206
f'orana panitu/afa, autonomous torsional response of, 670
Pressure, internal hydrostatic : irregular variation of, 395
,, ,, ,, positive and negative, 393
Protozoa, polar effects in, 205
Pseudo-conduction, 251
Pulsation, discrete nature of, in Dcsniodiiiiii, 340, 341, 431
„ intermittent human, 2S8
,, ,, in Biophytum, 289
Pulse, healthy adult and senile, 3
,, human, nitrite of amyl on, 4
Pulvinus and pulvinoid, 54
Radiation, discrimination from temperature effect, 615
,, effect of electric, on Mimosa, 619
,, ,, thermal, 615-617
778 PLANT RESPONSE
Receptivity, variation of, 461
,, ,, by anesthetics, 224
,, versus conductivity, 226
Recorder, Crescographic, 414, 416
,, Demonstration, 6
,, Electrical, 33
,, Magnetically controlled, 568-5S8
,, Microscopic, 519
,, Optic Lever, 6, 16
,, Spiral-Spring, 26
,, Suctional, 340
Recti-petality, theory of, 593
Refractory period, 246, 273
Regional death, thermographs of, 183- 186
Relaxation, anomalous meaning of, 339
Response, different types of, 123
,, identity of, in mature and growing organ, 435
,, relation of, to stimulus, 95, 96, 97
,, similarities of, between motile and growing organs, 437
See Cheiiiotactic-, Cheinotropic-, Death-, Differential-, Galvanotadic-, Gal-
vanotropic-, Geotropic-, Groivth-, Heliotropic-, Inorganic-, Mechanical-,
Phototropic-, and Suctional-.
Responsiveness in matter, universality of, 40
Retina, multiple excitation of, 286
Rhythmic tissue, similarity of response in plant and animal, 345, 346, 348, 353
Rhythmicity, cause of, 309
Rigor, duplication of, 178
Rigor inortis, 153
Robinia, diurnal sleep movement of, 628
Root and shoot, no polar difference between, 512
,, difference of effect between excitation at tip or growing region, 513
,, electric response, on direct stimulation of growing region, 518
,, ,, ,, on stimulation of tip, 517
Root-tip, effect of amputation of, on excitability, 540
,, localisation of geotropic sensitiveness at, 539, 542
Sachs on action of light on Marchaiitia thallus, 622
,, on characteristic effects of light, 567
,, on heliotropic response of Trop^roluin niajus, 623
Seasonal variation of conductivity and excitability, 474
,, ,, of contractility, 79
,, ,, of growth, 474
,, ,, of heliotropic response, 623
Selenium, response of, 39
Semi-automatism, 289
Sensitiveness, abolition by over-turgidity, 58
,, modification in darkness, 22, 50
,, wide range in plants, 43
Sesbania coccineum, death-discoloration of, 183
INDEX 779
Shoshungraph, 364^371
,, record by H)draulic Balance Method, 368
,, ,, Hydrostatic Balance Method, 368, 375
Simultaneous mechanical and electrical records, 34
Sinapis nigra, effect of diffuse light on growth of, 572
,, ,, negative heliotropic response of root of, 601
,, ,, positive, dia-, and negative heliotropic response of, 609
,, ,, positive heliotropic response of seedling of, 592
Sodium carbonate, efliect of solution of, on growth, 481
,, chloride, effect of solution of, on growth, 484
,, ,, ,, ,, on mechanical response, 551
,, ,, ,, ,, on suction, 384
Sphygmograph, 3
Spirogyra, thermo-mechanical curve of, 170
Stahl on phototaxis, 697
•Staircase ' response in galena, 121
,, .,, in muscle, 122
,, ,, in plant, 104, 121, 122
Stimulation, direct effect of, on growth, 434, 435, 436, 438
,, ,, ,, on mechanical response, 47, 420
,, effect of unilateral direct, on growth-curvature, 527, 529
,, ,, ,, indirect, on growth -curvature, 525, 528, 529
,, indirect effect of, on growth, 438
,, ,, ,, on mechanical response, 47, 420
,, minimally effective, 26
,, ,, ,, variation of, at different temperatures, 27
Stimulus, additive effect of, 94
,, and response, relation between, 95, 96, 97
,, ,, ,, ,, modified by tonic condition, 465
,, bifurcation of effect of, 127
,, detection of latent, 459
,, different modes of transformation of, 123 -126
,, supposed explosive effect of, 454
Strasburger on action of light on autonomous movements, 690
,, ,, of poison on ascent of sap, 311
,, ,, on phototaxis, 697
Suction, automatic record of variation in the rate of, 371
,, internal pressure due to, 393, 394, 395
,, mechanical response to, 399
Suctional response, effect of cold water on, 373, 376, 379
,, ,, ,, KNO3 solution on, 383
,, ,, ,, NaCl solution on, 384
,, ,, ,, poisonous solution on, 386
,, ,, ,, warm water on, 375, 378, 380
,, ,, excitatory nature of, 382
,, ,, ,, versus plasmolytic action on, 383
,, ,, persistence of, when root killed, 381
Sugar, chemotactic effect of, 699
,, chemotropic effect of, 549
78o PLANT RESPONSE
Sugar, efiect of solution on growth, 482
Sunflower, impressed periodic movement in, 685
l^amarindits ittdica, diurnal variation in the rate of growth of, 474
,, ,, initiation of growth in, by stimulus, 425
,, ,, response to thermal radiation, 617
Temperature and growth, relation between : determination by Continuous
Method, 446
,, ,, ,, ,, determination by Discon-
tinuous Method, 442
,, ,, ,, ,, determination by Method of
Balance, 450
,, ,, ,, ,, determination by Method of
Excitatory Response, 452
,, and rhythmic frequency, relation between, 349
,, effect of maximum, on arrest o^ Desviodium pulsation, 432
,, ,, ,, on growth-arrest, 431
,, ,, on cardiac pulsation, 333, 334
,, ,, on Desmodiitm pulsation, 332-338
,, ,, on intensity of minimally effective stimulus, 27
,, ,, on latent period, 141, 268
,, ,, on response, 140-146
,, optimum, of growth, 449, 451
,, variation, effect on growth, 615
Tesia current, eff"ect of, on growth, 619
Tetanus in muscle, 98
,, in plant, 99
,, incapability of, in rhythmic tissue, 340
Thermal radiation, discrimination between effect of, and temperature, 615
,, ,, growth-curvature induced by, 616, 617
Thermo-crescent curve, 447
Thermographs, development of, 184
,, of regional death, 183-186
Thermo-mechanical curve, 160
Thermo-taxis, 698
Thermo-tonic minimum in Biophytuiii, 305
,, ,, in Desmodium, 305
Tip of njot or shoot, electric response on stimulation of, 517, 518
,, ,, mechanical response on stimulation of, 525, 526, 601, 606
,, efiect on excitability on amputation of, 541
Tonic condition, modifying effect of, on relation between stimulus and response, 465
,, ,, true meaning of, 307, 313
Torsion, autonomous, 666
,, ' ,, effect of electric current on, 670
,, ,, ,, gravity on, 671
,, ,, ,, light on, 669
,, ,, ,, tenlperature on, 667
Torsional response, laws of, 662
,, ,, of compound strip, 660
INDEX 781
Torsional response, of Desiiiodiiaii, 657
,, ,, o[ Erythriiia iitdica, 6G1
,, ,, of Mimosa, 658, 659
Transmission of heliotropic excitation, 604
Transpiration, automatic record of variation of, 371, 472
,, effect of dissolved salt on, 383
Transverse conduction of stimulation, 519, 528, 529, 532
TropaoUim niajits, heliotropic response in : seasonal variation of, 623
,, ,, ,, ,, variation of, with intensity of stimulus,
623, 626
Turgidity, positive and negative variation : electrical indication of, 37, 41
,, ,, ,, ,, induced changes of rate of growth by,
426, 435
,, ,, ,, ,, mechanical indication of, 37, 41
,, response by artificial, 45
Ulothrix, phototaxis of, 697
Uniform response, 104
Uriclis Lily, apogeotropic response of scape of, 500, 505
,, periodic fatigue in the response of style of, loS
Variation, induction of, by external forces, 750
Verworn on polar excitation of protozoa, 200
Vines on autonomous epi- and hypo-nasty, 643
,, on protoplasmic irritability, 53
Vintschgau on velocity of nervous impulse, 246
Visual impulse, multiple, 286, 300
Vitalism, 740
Vitis, heliotropic response of tendril of, 611
Vochting on theory of recti-petality, 594
Water, excretion of, 396
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