RESEARCHES ON IRRITABILITY OF PLANTS

WORKS BY THE SAME AUTHOR.

RESPONSE IN THE LIVING AND NON-LIVING.

With 117 Illustrations, 8vo., los. 6d. 1902.

PLANT RESPONSE: AS A MEANS OF PHYSIO-
LOGICAL INVESTIGATION. With 278 Illustra-
tions, 8vo., 21 s. 1906.

COMPARATIVE ELECTRO - PPIYS 10 LOGY :
A PHYSICO-PHYSIOLOGICAL STUDY. With
406 Illustrations, 8vo., 15^. 1907.

LONGMANS, GREEN, AND CO.
London, New York, Bonnbay, and Calcutta

Br

RESEARCHES ON
IRRITABILITY OF PLANTS

BY

Bvrj JAGADIS CHUNDER BOSI J.Sg. C.S.L

k PROFESSOR, PRESIDENCY COLLEGE, CA1.0UTTA

WITH ILLUSTRATIONS

.ce^^i

LONGMANS, GREEN, AND CO.

39 PATERNOSTER ROW, LONDON

NEW YORK, BOMBAY, AND CALCUTTA

1913

77/

-B4

DEDICATED TO

S. M.

PREFACE

I HAVE in this work dealing with my researches on the
irritabiUty of plants introduced new methods by which
the scope of investigation has been enlarged, and a very
high degree of accuracy secured. In my previous treatise
on Plant Response, the response recorder employed was a
modification of the optical lever, automatic records being
secured by the very inconvenient and tedious process of
photography. The delay thus imposed retarded seriously
the progress of the research. Those practically engaged in
investigations on plants can realise the difficulties that
arise from the too quick passage of the seasons. It thus
frequently happened that by the time new instrumental
appliances were rendered practicable the favourable season
for the plant was over, involving the postponement of the
experiment for another year. In spite of these difficulties,
the long series of investigations that I then carried out gave
many interesting results, which not only threw light on
many obscure problems, but also led to the discovery of
several important phenomena in plant physiology.

Some of these results, moreover, tended to cast doubt
on certain conclusions that had found universal acceptance.
It has, for example, been held that there was no trans-
mission of true excitation in Mimosa, the propagated
impulse being regarded as merely hydro-mechanical. The
question whether the transmitted impulse was physical or
physiological could only be satisfactorily decided if the
plant could itself be made to record the velocity of its
impulse and the changes induced in that velocity under
physiological variations. This is but one out of several

viii RESEARCHES ON IRRITABILITY OF PLANTS

ideal methods of attacking problems in the life of plants,
the realisation of which would make a great advance in
physiological investigation.

It would also be desirable to discard, if possible, the
troublesome method of obtaining record by photography,
which necessitates work in a dark room ; in this connection
it should be remembered that subjection of the plant to
darkness introduces compHcations by modifying its normal
excitability. For these reasons, another requirement which
it is necessary to fulfil is the devising of some simple and
direct method of obtaining the record. And in order that
the results obtained should not be influenced by any personal
factor, it would be further desirable that the plant attached
to the recording apparatus should be automatically excited
by stimulus absolutely constant, should make its own .
responsive record, going through its own period of recovery,
and embarking on the same cycle over again without
assistance at any point on the part of the observer.

The difficulties encountered in realising these ideal
requirements appeared at first to be insurmountable. In the
records of response serious errors occurred as regards ampli-
tude and time-relations, owing to the friction of the writing
lever against the recording surface. As an extreme instance
of this, in recording the rhythmic movement of the leaflets
of Desmodium the very slight friction which the smoked-
glass surface offered was enough to stop the pulse-record.

After many attempts, I was at last successful in
overcoming all obstacles by the device of the Resonant
and Oscillating Recorders. Taking the very difficult test
of direct record of the rhythmic movements of Desmodium
leaflets, it will be found that the pulsations recorded in this
book not only gave accurate measure of the amplitude and
period, but also the absolute rate of movement during any
phase of their autonomous response. Again, in the matter
of accurate measurement of short intervals of time required
for the determination of the latent period and velocity of
transmission of excitation, I have shown the possibility

PREFACE

IX

of recording time-intervals as short as a thousandth part of
a second. A brief account of this is given in my paper
"On an Automatic Method for the Investigation of the
Velocity of Transmission of Excitation in Mimosa," read
before the Royal Society. It will be recognised immediately
in how many directions our power of inquiry has become
extended by the elaboration of these new methods and*
the invention of several types of instrumental appliances
described in this work.

In presenting the results of these investigations, it will
be noted that the plant has been made to tell its own story,
by means of its self-made records. Each experiment has
been repeated at least a dozen times, in many cases as often
as a hundred times. The results may therefore be accepted
as fully attested. The establishment of the unity of re-
sponsive reactions in the plant and animal, which is the
subject of this work, will be found highly significant, since it
is only by the study of the simpler phenomena of irritability
in the vegetal organisms that we can ever expect to elucidate
the more complex physiological reactions in the animal
tissues.

I take this opportunity to thank my research assistants,
Messrs. Guruprasanna Das, L.M.S., and Surendra Chandra
Das, M.A., for the very efficient help rendered by them in
these researches.

J. C. BOSE.

Presidency College, Calcutta,
October 1912.

CONTENTS

CHAPTER I

PLANT SCRIPTS

Action of environment on plant — Revelation of internal condition ^
by character of response — Problems to be solved — Electrical |
response — Mechanical • response — Motile organ in Mimosa '>
pudica — Response in plant and animal — Different phases
of the responsive movement — Graphic record — Determination !
of absolute movement of leaf and its time-relations — *
Characteristic effects of different agencies on the response-
curve — Specific difficulties in recording plant-response . i \

CHAPTER II ]

THE RESONANT RECORDER i

Advantages of intermittent contact in record — Two types of ■]
apparatus : the Oscillating Recorder, and the Resonant
Recorder — Coercer and Vibrator — Perfect tuning — Recorders

with standardised frequencies — Slide and clockwork — The i
record its own chronogram — Smoked surface and its fixation —

Adjustments of the writer — Records with continuous and 'i

intermittent contacts ....... ii j

j

CHAPTER III ]

■\

METHODS OF STIMULATION \

Different methods of stimulating the plant : mechanical, chemical, |

thermal and electrical — Difficulties of securing quantitative ;

stimuli — Direct and indirect stimulation — Ideal modes of I
stimulation — Electro-thermic stimulation — Stimulation by
constant current — Stimulation by condenser-discharge — Non-

polarisable electrodes — Direct, extra-electrodal, and intra- ■

electrodal stimulation — Stimulation by induction-shock — :
Effects of make- and break-shock — Excitation by tetanising

shock .......... 23 \

xi

b a J

xii RESEARCHES ON IRRITABILITY OF PLANTS
CHAPTER IV

TIME-RELATIONS OF THE RESPONSIVE MOVEMENT AND
STANDARDISATION OF STIMULUS

PAGE

Latent period of Mimosa — Apex time — Rate of responsive move-
ment of leaf — Effect of intensity of stimulus, fatigue, and
temperature — Periodic dot marker — Time relations of response
and recovery — Effect of season — Response of Biophytum —
Response of Neptunia — Abitrary distinction between sensi-
tive and ordinary plants — Differential response in Mimosa —
Response of ordinary plants — Universal sensitiveness of
plants — Standardisation of stimulus — Maximal and Minimal
Stimuli — Extreme sensitiveness of Mimosa . . . . 35

CHAPTER V

THE ADDITIVE EFFECT ; INFLUENCE OF LOAD, TEM-
PERATURE, AND INTENSITY OF STIMULUS

Greater excitatory efficiency of the break-shock — Additive effect
of stimulus — Quantitative relation of additive effect — Effect
of load — Thermal chamber — Effect of temperature — Effect of
increasing intensity of stimulus on response . . . 52

CHAPTER VI

VARIOUS TYPES OF RESPONSE

Necessity of uniform stimulation — The Periodic Starter — The
Automatic Exciter — Electrolytic contact-maker — ^The com-
plete Response-recorder — The factor of tonicity — Uniform
responses — Fatigue under shortened period of rest — Growing
fatigue — ^Alternating fatigue — Staircase response — Explana-
tion of erection of leaf under continuous stimulation — Fatigue-
relaxation in plant and animal — Response under single stimulus
and under tetanisation ....... 64

CHAPTER VII

EFFECTS OF DIFFERENT GASES ON EXCITABILITY
OF MIMOSA

Induced change of excitability under sudden variation of light —
Abolition of excitability by absorption of water — Restoration
of excitability by application of glycerine — Stimulating,

CONTENTS xiii

depressing, and toxic agents — Phenomenon of accommodation

Stimulating action of ozone — Effects of carbonic-acid

gas, vapour of alcohol, ether, carbon disulphide, coal gas,
chloroform, ammonia, sulphuretted hydrogen, laughing-gas,
nitrogen dioxide, and sulphur dioxide 85

CHAPTER VIII

DEATH-SPASM IN PLANTS

Criterion of the death of plant — Abolition of electric response at
death — Mechanical spasm of death — ^Water-bath for uniform
rise of temperature — Excitatory effect of sudden cooling or
heating — Erection of leaf with rising, and depression of leaf
with falling, temperature — Thermo-mechanical inversion
at the death-point — Necessity for specification of rate of rise
of tfemperature — Death-record of Mimosa — Abolition
of response after death-spasm — Constancy of death-point
exhibited by different specimens — Death-records of
Desmodium gyrans and Vicia Fava — Death-spasm in ordinary
plants — ^The electric-spasm of death — Lowering of death-
point by fatigue and by poisonous solution .... 98

CHAPTER IX

DETERMINATION OF THE LATENT PERIOD

Difficulties of accurate determination of Latent Period — Advan-
tages of Resonant Recorder — Simultaneous tracings of tuning-
fork exciter and Resonant Recorder — Automatic stimulation
at a definite moment — Identical value of latent period in
successive determinations — Accurate measurement of time-
interval shorter than .005 second — Latent period little affected
by inertia of recorder — Tabular statement of value of different
specimens of Mimosa — Effect of season on latent period . 108

CHAPTER X

INFLUENCE OF INTENSITY OF STIMULUS, FATIGUE,
AND TEMPERATURE ON THE LATENT PERIOD

Diffuse stimulation under alternating-shock — Effect of intensity
of stimulus on Latent Period — Influence of optimum condition
— Effect of fatigue — Effect of temperature . . . . 122

xiv RESEARCHES ON IRRITABILITY OF PLANTS
CHAPTER XI

VELOCITY OF TRANSMITTED IMPULSE IN PLANTS

PAGE

Detection of transmitted excitation by means of electromotive
variation — Specific tissue for conduction of excitation — Hydro-
mechanical theory of transmission of stimulus — Propagation of
excitatory protoplasmic change — Physiological test — Auto-
matic record of transmission-period — Conditions for obtaining
constant velocity — Determination of velocity of transmission
in Mimosa — Differential method of determining velocity —
Constancy of results — Tabular statement of different deter-
minations of velocity — Effect of intensity of stimulus on
velocity of transmission — Effects on sub-tonic tissue and on
tissue in optimum condition — After-effect of stimulus in
enhancing conductivity — Effect of optimum condition —
Disturbing action of leakage of exciting current — Effect of
fatigue — Effect of temperature — Velocity of transmission in
Biophytum and Averrhoa — Direction of preferential conduction 132

CHAPTER XII

EXCITATORY CHARACTER OF TRANSMITTED
IMPULSE IN PLANTS

The hydro-mechanical theory — Inconclusive character of the
anaesthetic experiment of Pfeffer and scalding experiment of
Haberlandt — Kiihne's experiment showing transmission of
excitation under intense stimulation in a rigored nerve —
Error introduced by employment of excessive intensities of
stimulus — Discriminative polar effect of current in excitation
— Block of transmission of excitation by local application of
cold — Restoration of normal conductivity by tetanising shock
in tissue paralysed by cold — Electro tonic arrest of excitatory
impulse — Action of various poisons in inducing block of con-
duction .......... 154

CHAPTER XIII

THE POSITIVE RESPONSE

Two opposite kinds of responses, negative and positive — Excitatory
contraction, negative turgidity variation, fall of leaf, and
concomitant negative electric variation — Positive electric
response — Positive or erectile mechanical response — Dual
impulses under different forms of stimuli — Exhibition of
positive and negative impulses by different plants — Con-
ditions for obtaining positive response — Characteristics of
positive impulse — Masking and unmasking of positive effect —
Laws of Direct and Indirect effects of stimulus . . . 176

CONTENTS XV

CHAPTER XIV

POLAR EFFECTS OF ELECTRICAL CURRENT IN
EXCITATION OF PLANTS

PAGE

Polar excitation in animal tissues — Anomalous reactions in Pfo-
tozoa — Mono-polar method — Current Reverser — Excitatory-
polar action in plant — Method of record — Effects of ascending
and descending currents in animal and plant — Records giving
time-relations — Potential slide — Measurement of e.m.f. and
current — Potential keyboard . . . . . . 198

CHAPTER XV

POLAR EFFECTS OF FEEBLE AND MODERATE CURRENTS
ON VARIOUS SENSITIVE PLANTS

Polar effects of feeble and moderate currents on (i) leaflets of
Mimosa, (2) leaflets of Biophytum, (3) leaflets of Neptunia,
(4) leaflets of Averrhoa carambola, (5) leaflets of Averrhoa
hilimhiy and (6) primary leaf of Mimosa — Excitation with
feeble current only at kathode-make — Excitation at kathode-
make and anode-break, under moderate current — Tabular
statement of results . . . . . . . . 212

CHAPTER XVI

THE CONTRASTED EFFECTS OF ANODE AND KATHODE

Polar effects of currents on pulsation of Desmodium gyrans —
Reduction of systolic contraction by anodic action — Diminu-
tion of diastolic expansion by kathodic action — Arrest at
systole by make of kathode and diastolic expansion by break
of kathode — Arrest at diastole by make of anode, and systolic
contraction by break of anode — Effects of ascending and
descending currents of feeble and strong intensity in nerve-
and-muscle preparation — Parallel effects in petiole-and-pul-
vinus .......... 234

CHAPTER XVII

EFFECT OF TEMPERATURE ON POLAR EXCITATION, AND
MULTIPLE EXCITATION UNDER CONSTANT CURRENT

Excitability of nerve to induction-shock diminished by cooling —
Nerve excitation by constant current enhanced by cooling —
Excitation of conducting-tissue of Mimosa by constant
current enhanced by cooling and depressed by warming —
Ineffective stimulus becoming effective under cooling and vice

xvi RESEARCHES ON IRRITABILITY OF PLANTS

versa — Multiple response induced in Biophytum by the
passage of constant current — Comparison of sensitiveness of
plant and animal — Minimum current for excitation of human
tongue — Relatively higher sensitiveness of Biophytum . . 244

CHAPTER XVIII

POLAR EFFECTS UNDER STRONG CURRENTS

Abnormal polar reactions in Protozoa — Transformation of polar
reaction in leaf of Mimosa from Type II, to Type III. under
strong current — Further transformation to Type IV. under
stronger current — Exhibition of Type III. and Type IV. by
leaflets of Mimosa, Biophytum, Averrhoa carambola — Law
of polar action of strong currents . ... . . 253

CHAPTER XIX

VARIATION OF POLAR REACTION UNDER TISSUE
MODIFICATION

Modification of polar reaction under tissue-changes — Effect of
age — After-effect of moderate stimulation — Modified polar
effect ; excitation at kathode-make and anode-make ; excita-
tion at kathode-make, kathode-break, and anode-make —
General review of polar reactions . . . . . 266

CHAPTER XX

MULTIPLE AND AUTOMATIC RESPONSE

The Oscillating Recorder — Latent period of Biophytum — Refrac-
tory period — Response on ' all-or-none ' principle — Multiple
electrical response to a single strong stimulus — Multiple
mechanical response to strong stimulus in Biophytum and
Averrhoa — Continuity of multiple and automatic response —
Ordinarily responding Biophytum converted into automatically
responding condition by excess of stored energy — Automati-
cally responding Desmodium converted to ordinarily respond-
ing condition by depletion of stored energy . . . .278

CHAPTER XXI

THE AUTOMATIC PULSATIONS OF DESMODIUM GYRANS

Activity of detached leaflet of Desmodium — Pulsation maintained
uniform under constant internal hydrostatic pressure — The
plant-chamber — Time- relations of pulsating movement derived

CONTENTS xvii

PAGE

from dotted record — Significance of down and up movements
— Systole and diastole — Table showing rates of movement of
Desmodium leaflet at different phases .... 290

CHAPTER XXII

EFFECT OF HYDROSTATIC PRESSURE, LOAD, AND LIGATURE
ON THE PULSATION OF DESMODIUM

Effect of internal hydrostatic pressure on the pulsation of Desmo-
dium— Expansive erection of leaflet under increased pressure ;
diminution of the extent of systolic contraction — Effect of
load : diminution of period — Stannius' ligature on heart-beat
— Parallel effect of ligature on pulsation of Desmodium —
Arrest of pulsation by a cut and revival by electric shock . 300

CHAPTER XXIII

EFFECT OF STIMULUS ON LEAFLET OF DESMODIUM
AT STANDSTILL

Condition of standstill brought about by depletion of energy —
Renewal of pulsation by the stimulus of light — Response to
stimulus of induction-shock — Multiple response under tetani-
sation — Determination of the latent period and the apex time
— Refractory period — Effect of stimulus on leaflets in sub-
tonic condition — Effect of isolation on rhythmic activity —
Gradual arrest of pulsation resulting from run-down of stored
energy — Effect of fresh accession of energy . . . 306

CHAPTER XXIV

EFFECT OF ELECTRIC STIMULATION ON THE PULSATION
OF DESMODIUM GYRANS

Effect of electric shock on Desmodium leaflet — Incapability of
tetanus — Extra pulsation induced by electric shock — Relative
effectiveness of electric stimulus at diastolic phase — Effect of
transmitted excitation on normal pulsation of heart and
on pulsation of Desmodium leaflet — Effects of acceleration
and inhibition ........ 318

xviii RESEARCHES ON IRRITABILITY OF PLANTS
CHAPTER XXV

EFFECT OF TEMPERATURE ON RHYTHMIC PULSATION
OF DESMODIUM GYRANS

PAGE

Effect of lowering of temperature on rhythmic pulsation of cardiac
tissue — Similar effect on the pulsation of Desmodium —
Increase of systolic limit during cooling — Minimum tempera-
ture for arrest of pulsation — Arrest by cooling and subsequent
revival by warming — Increase of diastolic limit during
warming — Effect of rise of temperature on the pulsation of
frog's heart — Similar effect on the pulsation of Desmodium —
Effect of rise above and return to normal temperature —
Diminution of systolic contraction during rise of temperature
— Increase of systolic contraction during fall of temperature —
Permanent arrest due to heat-rigor . . . . . 323

CHAPTER XXVI

EFFECT OF CHEMICAL AGENTS ON THE AUTOMATIC
PULSATION OF DESMODIUM GYRANS

Application of gaseous or liquid reagents — Modifying influence of
tonic condition of specimen, strength, and duration of appli-
cation— Effect of sugar solution — Effect of alcohol — Action
of carbonic-acid gas — Effects of anaesthetics, ether, and chloro-
form— Action of carbon disulphide — Effect of copper sul-
phate solution — Effect of potassium cyanide solution — Anta-
gonistic actions of acids and alkalis on the pulsations of
the heart and of Desmodium — Similarities of reaction in
rhythmic tissues animal and vegetal . . . . . 332

CHAPTER XXVII

GENERAL SURVEY ... 342

Classified List of Experiments . . . . . 361

Index .......... 369

ILLUSTRATIONS

FIGURE.

I. Diagrammatic representation of Response Recorder
Response curve of leaf of Mimosa
Resonant Recorder, upper part ....

General view of the Resonant Recorder and accessories
Record showing advantage of intermittent contact
Electro-thermic stimulator .....

Response of Mimosa to indirect thermal stimulation

Response to stimulus of constant electric current .

Direct stimulation by condenser discharge .

Record of response to stimulation by condenser discharge

Arrangement for applying single make- or break-shock

Records giving apex time of response of Mimosa

The dot marker ......

Response of Mimosa giving time-relations
Record of response of leaflet of Biophytum .
Response of leaf of Neptunia

Erectile response of leaf of Mimosa, due to local stimulation
upper half of pulvinus ....

Responses of leaf of Desmodium gyrans

The electric signal .....

21. Records showing greater efficiency of break-shock

22, 23, 24, 25. Additive effects of stimulus

26. Effect of load on response of Mimosa .

28. Effect of temperature on response of Mimosa
Increasing response under increasing stimulation
Periodic starter, and automatic exciter
Electrolytic contact-maker ....

Photograph of duplex type of Resonant Recorder
Uniform responses of Mimosa
Fatigue under shortened period of rest
Growing fatigue ......

Periodic fatigue . ' .
Staircase response in frog's muscle

xix

2.
3-
4-
5-
6.

7-
8.

9-
10.
II.

12.

13-
14.

15-
16.

17-

18.
19.
20.

PAGF
6

7
17
20
21

25

26
27
28
30
32
38
40

41
43
43

45
46
52
53
54-56

57
60, 61
62
66
68
69
72
73
74
74
77

of

XX RESEARCHES ON IRRITABILITY OF PLANTS

FIGURE PAGE

38. Staircase response in Mimosa ...... 77

39. Effect of stimulus in modifying the tonic condition . . 79

40. Alternating response . . . . . . . .81

41. Fatigue-decline in frog's muscle . . . . . .81

42. Different phases in the fatigue reversal in plant ... 83

43. Fatigue reversal in Mimosa. ...... 83

44. Effect of sudden darkness on excitability of Mimosa

45. Abolition of motile excitability of pulvinus by absorption of 86

water . . . . . . . . . . 88

46. Stimulating action of ozone ...... 90

47. Effect of CO2 on excitability of Mimosa .... 90

48. Effect of vapour of alcohol ....... 91

49. Effect of ether ......... 92

50. Effect of carbon disulphide ...... 93

51. Effect of coal gas ........ 93

52. Abolition of excitability under chloroform .... 94

53. Effect of ammonia ........ 95

54. Effect of sulphuretted hydrogen ...... 95

55. Effect of nitrogen dioxide ....... 96

56. Effect of sulphur dioxide ....... 96

57. Excitatory effect on Mimosa by sudden cooling or warming . 100

58. Death-curve of Mimosa ....... 102

59. Abolition of response to warming or cooling after passing through

the death-point . . . . . . . .103

60. Death-curve of Desmodium gyrans . . . . .104

61. Thermo-mechanical inversion indicating death-point of leaf of

bean .......... 105

62. Lowering of death-point under fatigue . . . . .106

63. Effect of poison in lowering the death-point . . . .106

64. Latent period of hyoglossus muscle . . . . .109

65. Simultaneous record of vibrating recorder and exciting tuning

fork . . . . . . . . . .110

66. Apparatus for determination of latent period of Mimosa . 112

67. Two successive records exhibiting identity of latent period of

Mimosa . . . . . . . . -115

68. Record of latent period of highly excitable M^wosa . . 116

69. Record of latent period with a 200 D.V. recorder . . .117

70. The same record magnified . . . . . . .118

71. 72. Two records obtained with two different recorders . .119

73. Record of latent period of Neptunia . . . . .120

74. Simultaneous excitation in interposed tract under alternating-

shock .......... 124

75. 76. Effect of intensity of stimulus on latent period . . .126

ILLUSTRATIONS

XXI

77. Constancy of latent period under stimuli above the maximal .

78, 79. Effect of fatigue ........

80, 81. Effect of temperature .......

82. Determination of velocity of transmission of excitation in

Mimosa .........

83, 84. Determination of velocity by Differential Method . 139,

85. Effect of intensity of stimulus on velocity and after-effect

86. Effect of optimum condition on velocity ....

87. Effect of fatigue on velocity of transmission . .

88. Effect of temperature on velocity .....

89. Record giving transmission time in Biophytum . .

90. Effect of cold in the retardation and arrest of transmission .

91. Arrangement for electrotonic block .....

92. Record of effect of electrotonic block .....

93. Records of transmitted excitation with the block off and on .

94. Effect of CUSO4 in abolishing conduction ....

95. Abolition of conductivity by KCN . ....

96. Positive response followed by negative in Biophytum under

indirect thermal stimulus ......

97. Diphasic response in Biophytum under indirect chemical

stimulation . . . . . . .

98. 99. Positive followed by negative in ^z/eyMoa ....
100. Diphasic response in Mimosa due to indirect electric stimu-
lation of petiole ........

loi. Diphasic response in Mimosa due to indirect thermal stimu-
lation of stem ........

102. Effect of intensity of stimulus in modifying the diphasic response

103. Effect of diminishing distance in transforming positive into

diphasic response in Biophytum ....

104. Effect of diminishing distance on the response in Mimosa

105. Masking of the positive by the predominant negative

106. Pohl's commutator .......

107. Record of polar excitation under feeble current

108. Excitation by ascending and descending current .

109. Records of responses to ascending and descending current

in Mimosa ........

no. Records of responses to ascending and descending induction-
shock ........

111. The Potential Slide

112. The Potential Keyboard ......

113. The electrode holder . ......

114. Excitation induced in Biophytum by the make of kathode and

break of anode ....,,..

127
128
130

137
140

143
145
147
149

151

162

165
166
167
172
173

180

182

184

186
187

189
194
194
200
202
204

205

206
208
210

217

224

xxii RESEARCHES ON IRRITABILITY OF PLANTS

FIGURE. PACE

115. Record of kathode-make and anode-break in Mimosa . . 230

116. Effect of anode, opposing systolic contraction in Desmodium . 236

117. Effect of kathode, opposing diastoHc expansion in Desmodium 236

118. Alternate effect of anode and kathode .... 237

119. Arrest at systole by make of kathode, and diastolic expansion

at break of kathode ....... 237

120. Arrest at diastole by make of anode, and systolic contraction

at break of anode ....... 237

121. Effects of descending and ascending currents at make and

break on nerve-muscle and petiole-pulvinus . . . 239

122. Effect of cold on excitability to induction-shock . . . 246

123. Record showing abolition of polar excitation at high tem-

perature ......... 247

124. Multiple excitation in Biophytum under constant current . 249

125. Record of polar excitation of Type III. .... 255

126. Polar excitation Type IV. . . . . . . . 256

127. Transformation of Type II. to Type III. as after-effect of

previous stimulation ....... 268

128. Abrupt transition from Type I. to Type III. . . . 271

129. Polar reactions, Km, Km Am, and Km Am Ab under gradually

increasing current . . . . . . -273

130. Record exhibiting modified response Km Kb Am . . . 275

131. The Oscillating Recorder ....... 279

132. Record giving latent period of Biophytum .... 281

133. Record of responses of Biophytum to stimuli -i and i unit . 282

134. Responses of Biophytum to stimuli .1, .5, i, and 2 units . . 284

135. Multiple response in Biophytum under strong electric shock 284

136. Multiple response in Averrhoa under a single strong electric

shock 285

137. Multiple response under constant stimulus of light . . . 285

138. Multiple response under single strong thermal shock . . 286

139. Multiple response induced by strong chemical stimulation . 287

140. Leaf of Desmodium gyrans ....... 290

141. U-tube support and plant-chamber . . . . .292

142. Continuous record of pulsations of Desmodium leaflet for four

hours . . . . . . . . . 294

143. Record of a single pulsation of Desmodium giving time-relations 296

144. Record of two successive pulsations . . . . ' . 297

145. Series of automatic pulsations of Desmodium . . . 298

146. Effect of application of increased internal hydrostatic pressure

on the pulsation of Desmodium . . . . .301

147. Effect of increasing load on Desmodium pulsation . . . 302

148. Effect of ligature in inducing arrest of pulsation of Desmodium 303

ILLUSTRATIONS

XXlll

FIGURE. PAGE

149. Arrest of pulsation by cut and revival by electric shock . . 304

150. Action of light in renewing pulsation of Desmodium . . 307

151. Response of Desmodium leaflet, originally in a state of stand-

still

152. Multiple response under tetanisation .....

153. Response of Desmodium exhibiting apex time

154. Record exhibiting refractory period in Desmodium

155. After-effect of stimulation on pulsation of Desmodium in sub-

tonic condition ........

156. Effect of depletion of energy on pulsation of isolated

Desmodium .........

157. Gradual stoppage of pulsation in isolated leaflet of jDeswo^mw

158. Effect of stimulus in renewing pulsation of Desmodium,

brought to standstill .

159. Response of Desmodium leaflet to stimulus of light

160. Response of a depressed specimen of Desmodium .

161. Effects of strong tetanisation on Desmodium pulsation .
162, 163. Extra pulsation induced by induction -shock applied at

diastolic phase . . . . . . . .319

164. Inhibitory effect of transmitted excitation on the pulsation

of vigorous leaflet of Desmodium ....

165. Augmentation of pulsation induced by transmitted excitation

in less vigorous specimen of Desmodium

166. Effect of lowering of temperature on the pulsation of heart .

1 67. Effect of lowering of temperature on pulsation of Desmodium .

168. 169. Effect of rapid cooling on Desmodium pulsation
170. Effect of rise of temperature on amplitude and frequency of

pulsation of the heart of frog .....
171,172. Effect of rise of temperature on the pulsation of Desmodium

173. Effect of continuous rise of temperature from 30° C. to 38*5° C.

174. Effect of continuous rise from 30° C. to 42° C. and return to

30° C 330

175. Effect of rise of temperature and return . . . .330

176. Effect of dilute sugar solution . . . . . .333

177. Effect of alcohol on the pulsation of Desmodium . . .333

178. Effect of internal application of strong alcohol . . . 334

179. Effect of dilute carbonic-acid gas . . . . .334

180. Effect of strong carbonic -acid gas ..... 335

181. Effect of internal application of carbonic acid . . .335

182. Effect of vapour of ether ....... 336

183. Effect of vapour of chloroform . . . . . -337

184. Effect of carbon disulphide . . . . . .337

185. Effect of copper sulphate solution . . . . .338

308
308

309
310

312

313
313

315
316
316
318

320

321
324
325
326

327
328

329

xxiv RESEARCHES ON IRRITABILITY OF PLANTS

1 86. Effect of potassium cyanide . . . . . -339

187. Arrest of pulsation of the heart of frog at diastole by the

action of dilute lactic acid . . . . . . 339

188. Arrest of pulsation of Desmodium at diastole by application

of dilute lactic acid . . . . . . -340

189. Arrest of pulsation of heart at systole, by the action of dilute

sodium hydrate . . . . . . . .340

190. Arrest of pulsation of Desmodium at systole by the application

of dilute solution of sodium hydrate . . . -340

RESEARCHES ON IRRITABILITY
OF PLANTS

CHAPTER I

PLANT SCRIPTS

Action of environment on plant — Revelation of internal condition by
character of response — Problems to be solved — Electrical response —
Mechanical response — Motile organ in Mimosa pudica — Response
in plant and animal — Different phases of the responsive move-
ment— Graphic record — Determination of absolute movement of
leaf and its time-relations — Characteristic effects of different
agencies on the response-curve — Specific difficulties in recording
plant-response.

In strong contrast to the energetic animal, with its various
reflex movements and pulsating organs, stands the plant
in its apparent placidity and immobihty. Yet that same
environment, which with its changing influences so strikingly
affects the animal, is playing upon it also. Storm and
sunshine, the warmth of summer and the frost of winter,
drought and rain, all these and many more come and go
about it. What coercion do they exercise upon it ? What
subtle impress do they leave behind ? That they, in their
totahty, do leave the plant better or worse for their occur-
rence, we know. It is evident that internal changes are
effected by their agency which are entirely beyond our
visual scrutiny. Would it be possible to trace this general
action of the environment into some detail, and then follow
out the question of its particular effects upon the vegetal
organism ? Is there any means by which we might find out

2 RESEARCHES ON IRRITABILITY OF PLANTS

whether a given influence has contributed to the plant's
well-being or the reverse, whether it has left it more
or less excitable, whether it has rendered it more or less
energetic ?

It is conceivable that internal changes which eluded our
direct vision might nevertheless be brought within the
range of our observation if we could obtain any sort of
answer from the plant itself to a questioning shock. In
such a case, the feebleness or vigour of the reply would in
itself doubtless constitute a measure of the vitality of the
organism. It appears obvious that if any given influence had
rendered the plant more excitable, this fact would be mani-
fested by the greater intensity of its response. In a very
excitable condition we may suppose the slightest shock of
stimulus would evoke a very large responsive expression ;
in a state of depression, on the other hand, the strongest
stimulus would induce only a feeble reply. The relation
between the stimulus and the response would thus form a
gauge of the physiological condition of the organism. The
invisible fluctuating changes taking place in the plant, under
the changing conditions of the environment, might in this
way be made to reveal themselves.

All this presupposes, however, that the plant will answer
in some tangible way to the impinging testing stimulus,
and that it may be possible to obtain some record of this
answer. The possibility of this will be further discussed
presently. There are many important problems which wait
for their solution till some such means of inquiry is found.
What, for instance, are the various forms of stimulus which
evoke an answering reaction in the plant ? Again, has a
given plant-tissue, like animal muscle, any definite percep-
tion-period capable of exact measurement ? Is the respond-
ing tissue susceptible of fatigue ? Is the intensity of its
answer dependent on the intensity of the blow ? Is the
excitation that may be caused at one point transmissible
to a distance, as along animal nerve ? Is such transmission,
supposing it to occur, fundamentally of the same nature

PLANT SCRIPTS 3

as that in the animal ? Is there to be found in plants any
tissue that might twitch persistently, like the cardiac tissue
of the animal ? If so, are these rhythmic pulsations charac-
teristically similar ? Is there, again, any general resem-
blance between responsive actions in plant and animal ?
Going deeper, since the same protoplasmic basis underlies
them both, are these reactions to be regarded as essentially
the same, though different in degree ? If this last were
true, then since the simpler explains the more complex,
might not the physiological reactions of the plant be expected
to elucidate many of the obscurities in the similar reactions
of animal tissues ?

We return, then, to the question, Is the plant capable
of furnishing any such responsive indications as we have
supposed ? I have shown elsewhere that all plants give
response to impinging stimulus by a definite electrical
change,' which can be recorded by means of suitable
apparatus. For the purpose of the present work, however,
it will be convenient to employ the more conspicuous motile
indications afforded by certain plants, pre-eminent amongst
which is Mimosa ptidica.

The most prominent motile organ in Mimosa consists of
a mass of tissue known as the pulvinus, at the joint or arti-
culation of the primary leaf-stalk. The swollen mass on the
lower side of this organ is very conspicuous. Under excita-
tion the parenchyma, in this more effective lower half,
undergoes ' contraction,' in consequence of which there is
a fall of the leaf. This sudden movement constitutes
the mechanical response of the leaf to the impinging
stimulus, just as the contractile movement of a muscle in
similar circumstances forms its characteristic mechanical
response.

Digressing for a moment to consider the phenomenon
of excitatory contractions in general, it may be said that
our present knowledge is not complete as to the minutice

^ Bose : Friday Evening Discourse — Royal Institution, May 1901 ;
Comparative ElectrO'Physiology, Longman's, London, 1907.

4 RESEARCHES ON IRRITABILITY OF PLANTS

of the process by which these are brought about. In muscle
it is supposed that during the act of contraction there is a
transfer and redistribution of fluid material.^ In the case of
Mimosa there is known to be an escape of fluid from the
excited cells ; there is a diminution of turgor. It is sup-
posed that this may in some unknown way be connected
with a diminution of pressure within the cell.^

In the case of the stamens of Cynerece, Pf effer ^ observed
a contraction under excitation of as much as 30 per cent,
of the original length. There is an escape of water from
the cells into intercellular spaces. The mode in which the
fall of turgor takes place is uncertain, and various supposi-
tions have been made to account for it. It has been thought
that the escape of fluid is brought about by the elastic cell
wall which forces liquid out of the cell, when the protoplasm
lining it has become permeable under excitation. There
may in addition be an active contraction of protoplasm
which might force the liquid out of the cell-vacuole. This
latter supposition is regarded by many as improbable,
though the observations of Schiitt and Benecke indicate
that under stimulation the protoplasm of a diatom contracts
away from the cell wall. Similar withdrawal of protoplasm

^ * Schafer, working on the highly differentiated wing-muscle of the
wasp, concludes that each sarcomere contains a darker substance near
the centre, divided into two parts by Hensen's disc. At each end of the
sarcomere the contents are clear and hyaline. In the act of contraction
the clear material flows, according to Schafer, into tubular pores, in the
central dark material.' — Starling: Elements of Human Physiology,
8th edition, p. 91,

" * When the pressure in the cell decreases, we naturally assume this
to be due to decreasing osmotic pressure, a decrease which may well
amount to 2 1^ to 5 atmospheres, and may be due either to the transformation
of osmotically active substances into bodies with larger molecules, or to
alterations in the permeability of the plasma, and an excretion of materials
from the cell. As evidence of excretion of material we may quote the fact
that Pfeffer observed crystals of unknown nature appearing on evaporation
of the liquid expressed from the intercellular spaces. Still there are several
reasons for doubting this conclusion. It is a remarkable fact that plasmo-
lytic research affords no evidence of any decrease in osmotic pressure." —
Jost : Plant Physiology, English edition, 1907, p. 515.

^ Cf. Pfeffer : Physiology of Plants, vol. iii., English edition, p. 75.

PLANT SCRIPTS 5

has been observed in Spirogyra by Nageli and in Nitella
by Hofmeister.

Whatever theory may be held, the undoubted fact in
these two cases, of plant and animal alike, is the occurrence
of a fundamental excitatory protoplasmic change which finds
external expression in alteration of form. If we now
record the responsive movement, we shall be recording what
is an effect of excitatory change, either in plant or animal.
Whether or not this fundamental change is similar in the two
cases can only be decided by comparing the records due to
excitation, in plant and animal tissues, under all possible
variations of external conditions.

The fall of the leaf of Mimosa is brought about in
consequence of the contraction of cells in the lower half
of the pulvinus. I shall for convenience describe the fall
as the contractile movement, in contradistinction to the
erectile movement brought about by the recovery of cells
into normal turgid and expanded condition.

In studying the excitatory reactions of the plant, under
external stimulus, we have to determine, first, what time
elapses between the incidence of the shock and the initia-
tion of a perceptive responsive movement. This constitutes
the determination of the Latent Period. We have next
to find out at what rate this responsive movement of the
leaf takes place, and after what time the contractile phase
of the movement is exhausted. After a short pause
the plant gradually recovers from the effect of the shock,
and the leaf is re-erected to its former position. We there-
fore want to know the various rates at which recovery
gradually takes place. In order to secure these data, it
will be necessary to make a graphic record of the entire
responsive movement of the plant organ. This record,
further, must furnish us not only with the amount, but also
with the time-relations, of this movement. This would
involve the construction of a writing-lever which, deflected
by the pull of the falling leaf, would be capable of tracing
on a writing-surface, moving at a known uniform rate, the

6 RESEARCHES ON IRRITABILITY OF PLANTS

concomitant curve. For this there must be an axis, sup-
ported on frictionless jewelled bearings, and carrying two
arms of a horizontal lever and a thin vertical wire with a
bent tip, to serve as the writer. The different parts, as far
as possible, should be made of aluminium, to secure the
utmost lightness. A point of the petiole of the responding
leaf would be attached by a silk thread to one arm of the
lever, the other having on it a small weight, to act as counter-
poise. On the fall of the leaf, under ex-
citation, it would pull down with it the
attached arm of the lever. The vertical
writer would then also move, say, to
the left. If the finely pointed bent end
of the writer were to press lightly
against the smoked surface of a glass
plate, which was allowed to fall, at a
uniform rate, by means of clockwork,
a curve would then be traced which
would not only record the responsive
movement and recovery but also give
their time-relations (fig. i). To obtain
the latter, it would be necessary to
know the rate of movement of the
plate on which successive vertical lines
might be traced by a time-marker at
intervals of, say, one minute.

In order to find out the absolute
movement of the leaf we must know
the degree of magnification or reduction that has been
effected by the recording arrangement. This will depend upon
the relative lengths of the writer and the lever, and the
distance of the point of attachment on the leaf from the
pulvinus. When the lengths of the lever-arm and the
writer are equal, then the writer will describe a movement
which is equal to that of the point of leaf-attachment. By
shortening the arm of the lever to half the length of the
writer, we should obtain the magnification of two. This

Fig. I. — Diagrammatic
representation of Re-
sponse Recorder.

PLANT SCRIPTS 7

shortening might be accompHshed by attaching the thread
nearer to the fulcrum. Proceeding in the manner indicated,
any magnification, however high, can be obtained.

Reduction, again, may be effected with equal ease.
When the point of attachment is exactly midway between
the pulvinus and the tip of the leaf, then the movement
executed by it will be half that described by the extremity
of the leaf. By bringing the point of attachment nearer

Fig. 2. — Response-curve of primary leaf of Mimosa ; the vertical
lines below the record indicate intervals of one minute each.

to the pulvinus, we can obtain whatever reduction may be
required.

The resultant magnification or reduction of the record
will thus depend, in any given case, on two factors — namely,
the relation between the length of the writer and the length
of the lever-arm, on the one hand ; and, on the other, the
relation between the distance of the point of attachment
from the pulvinus and the entire length of the leaf.

Thus if the lever produce a magnification of four, and the
point of attachment cause a reduction to half, the resulting
magnification will be 4 X J = 2. As the movement of the
primary petiole of Mimosa is considerable, the records taken
are normally either equal or reduced to two-thirds. A
record taken in this manner is given in fig. 2. The height

8 RESEARCHES ON IRRITABILITY OF PLANTS

of this curve gives the ampHtude of the movement, and
the horizontal distance measures the corresponding time.
The up-hne here, a b, indicates the responsive fall, and the
descending line, b c, the gradual erection, due to recovery.
The responsive movement was initiated within an exceedingly-
short time after the application of stimulus — in this case an
electrical shock — and the fall was completed also within
a relatively short period. In a record which will be given
later, taken on a faster-moving plate, these characteristics
will be seen better. The recovery, however, is a slow process,
the earlier part being comparatively quick and becoming
slower towards the end. The entire recovery is here seen to
require 12 minutes.

If we were able to apply a stimulus of exactly identical
intensity at regular and suitable intervals, and if the physio-
logical condition of the responding tissue remained constant,
then we should obtain a series of responsive twitches which
would be practically identical. But if the physiological
condition were to undergo any change, under environmental
conditions, then the record would give us indications of that
internal change, otherwise entirely beyond our power of
scrutiny. Thus if the plant were to become depressed, the
amplitude of the pulse would undergo a diminution. If on
the other hand its excitability should be enhanced, that fact
would be indicated by an increase in the amplitude of the
response.

The mere amplitude of the twitch, however, affords
only a broad indication of the physiological condition of
the tissue. There are many factors the effects of which find
expression in subtler changes of the response-curve. One
agency, for instance, will make the plant more alert. This
is at once reflected at that part of the curve which corre-
sponds to the Latent Period. This becomes shorter. The
ascent of the curve will also be more abrupt. Another
agency will induce, let us say, a contrary change. Different
agencies, similarly, will bring about definite changes in
the contracting and relaxing portions of the curve. The

PLANT SCRIPTS 9

varying effects of freshness and fatigue, of stimulating or
depressing drugs, of heat or cold, of the environment, are
in this way clearly revealed by the characteristic flexures
of the curve. Thus, by means of testing-blows, we are
able to make the plant itself describe those obscure
internal changes which would otherwise have entirely
escaped us.

In fact, the phytographic records would, in the case
of plants, supply us with all the information that myo-
grams afford in the case of animal tissues. The experi-
mental difficulties which the plant offers are, however,
very great. In the case of muscle-contraction, the pull
exerted is considerable and the friction offered by the
recording-surface constitutes no essential difficulty, though
even here the time-relations of the curve are, I have reason
to think, rendered somewhat unreliable by this friction.
In the case of plants the contractile movement is relatively
feeble, and in the movement of the leaflet of Desmodium, for
instance, a weight so small as four-hundredths of a gram is
enough to arrest the pulsating leaflets. When employing
the very lightest lever, the extremely minute friction offered
by the smoked-glass surface of the recording-plate is sufficient
in this case to cause complete cessation of the record.
Even in the leaf of Mimosa, the friction offered is enough to
distort the curve to such a serious extent that errors are
introduced into the amplitude and time-relations amounting
to more than 50 per cent. These difficulties have been
overcome by the successful devising of my Resonant
Recorder, an account of which wiU form the subject of the
next chapter.

Summary

Obscure modifications of internal condition of plant,
resulting from changing factors of environment, may be
revealed by records of plant's response to testing-blows.

The relation between the impinging stimulus and the

10 RESEARCHES ON IRRITABILITY OF PLANTS

intensity of reply measures the physiological efficiency of
the plant tissue for the time being.

Automatic records of mechanical and electrical responses
of plant can be obtained with suitable apparatus.

For recording the mechanical responses of Mimosa and
other sensitive plants, a writing-lever is employed. Distor-
tion of record by friction is apt to introduce error, which
has to be eliminated.

CHAPTER II

THE RESONANT RECORDER

Advantages of intermittent contact in record — Two types of apparatus :
the Oscillating Recorder and the Resonant Recorder — Coercer and
Vibrator — Perfect tuning — Recorders with standardised frequencies —
Slide and clockwork — The record its own chronogram — Smoked
surface and its fixation — Adjustments of the writer — Records with
continuous and intermittent contacts.

It was stated in the last chapter that however Hght the
contact may be, and however smooth the glass recording-
surface, the record was still apt to be either arrested, or
seriously distorted, on account of friction. As long as I
employed the ordinary method of continuous contact of
the writing-point with the glass surface, it was impossible
to overcome this particular difficulty. It occurred to me at
last that the problem might find a solution if I could succeed
in making an intermittent instead of a continuous writing-
contact. I have solved this problem by devising two
different types of apparatus, which I have called respectively
the Oscillating Recorder and the Resonant Recorder. In
the former, the recording-surface itself is made by an
electro-magnetic device, to vibrate to and fro, thus bring-
ing it into periodic contact with the writing-point. ^ This
apparatus is extremely convenient for the general pur-
pose of recording responses in which the measurement of
excessively short intervals of time is not essential.

But there are many important problems, such as the
determination of the latent period, and accurate determina-
tion of the velocity of transmission of excitation, in which

^ Bose : British Association Report , Dublin, 1908, p. 903.

12 RESEARCHES ON IRRITABILITY OF PLANTS

we require time-measurements of the order of one-hundredth
of a second. It will be shown that these determinations
can be carried out with great precision, by means of the
intermittent dots themselves, when the periodicity of ^their
recurrence is rendered perfectly constant. For such pur-
poses, then, we require a frequency of intermittent contacts
amounting to something like a hundred times per second.

It was clearly impossible to make the heavy plate-
carrier oscillate with such a high frequency. There remained
only the theoretical alternative of causing the writing-
point to vibrate to and fro, at the required frequency, so
as to make the necessary intermittent contacts with the
surface of the recording-plate.

The advantage of this intermit fence may be understood
from a concrete example. It will be remembered that the
writing-point, under the action of the responsive fall,
moves parallel to the surface of the recording-plate. If
now, by means of some mechanism, the writing-point be
made to vibrate to and fro, say, ten times each second, at
right angles to the plate, this will in no way affect the record
beyond the fact that instead of a continuous line a dotted
line will be traced. The record will not now labour under
the defects inseparable from the friction of continuous
contact. Instead of this, we shall have the vibrating writer
tapping a record which is practically free from friction.
For it will be understood that, as in our concrete example, a
recording-point which is vibrating ten times each second will
execute one entire to-and-fro movement in one-tenth of a
second. The duration of contact, at the extreme forward
end of the swing, will represent only a small fraction, say
one-fifth, of the entire period of one vibration. Hence
after each contact, lasting only one-fiftieth of a second, the
recording-point is absolutely free to take up the movement
impressed upon it by the moving leaf. In a record lasting
for one second the sum of the intermittent contacts will then
amount to one-fifth, and the period of entire freedom to
four-fifths of a second. We can thus see the theoretical

THE RESONANT RECORDER 13

advantages of an intermittent over a continuous contact.
What has been said of the writer vibrating ten times in
a second holds good equally in those cases where the
vibration-frequency is much higher.

One great difficulty we encounter in carrying out
this idea hes in giving the recording-point an impulse
exactly perpendicular to the direction of its recording
movement. If the recording- writer be made of fine steel-
wire, and if we place behind it a small electro-magnet —
the pole consisting of a rectangular piece of soft iron, at
right angles to the wire — then, by sending a momentary
strong current through this electro-magnet, a pull will
be exerted on the wire which will make its recording-tip
strike for an instant against the glass recording-surface.
As the steel wire has to be made extremely fine, in order
to reduce to a minimum the inertia of the recorder, the
resulting pull exerted on it is very slight, unless an exces-
sively strong current be sent round the electro-magnet.
Again, unless the intermittent closures of the electric circuit
be properly timed, the writing-index may be subjected to
attraction in the course of its journey, now to the recording-
surface and again away from it. In the latter of these
cases its vibration, on which the intermittent contact
depends, is totally destroyed.

But the most serious difficulty of all is that introduced
by the edge of the attracting electro-magnet. It is known
that the magnetic intensity of a pole is strongest at its edges.
Should the writing-index by chance be placed exactly
symmetrically, as regards the right and left edges of the
pole, then the two lateral pulls, being equal, will neutralise
each other, and the index will vibrate to and fro perpendi-
cularly to the recording-surface. But should it be placed,
however slightly, nearer to one edge than to the other, then
one of the two pulls will be in excess, and the index will
be drawn to one side, thus producing a disturbance in the
record not due to the excitatory pull of the leaf. Even
if, at the beginning, the index had been placed in a

14 RESEARCHES ON IRRITABILITY OF PLANTS

strictly symmetrical position, the movement of the writer
caused by the excitation of the leaf would draw it into an
asymmetrical position, resulting in a one-sided pull which
would seriously interfere with the reliability of the record.

The Resonant Recorder

It is therefore absolutely necessary so to arrange matters
that the electro-magnet shall be without laterality. This
condition I was able to fulfil by making the pole of the
electro-magnet in the form of either a cylinder or a ring.
The axis, from which is suspended the writing-index, is
accurately supported, perpendicular to the plane of the
circular section of the magnetic pole at its centre. Every-
thing was thus made symmetrical, and as there was no
laterality there could be no tendency whatsoever for the
index to execute its to-and-fro vibrations in any other
direction than that which was perpendicular to the plane
of the terminal pole of the magnet. As this plane may be
adjusted parallel to the glass recording-surface, the tapping
movement of the writing-index can be made to take place
perpendicularly to the recording-surface.

Next, in order to overcome the difficulty of the irregular
timing of those electrical impulses which are to maintain
the recording-index or writer in a state of periodic vibra-
tion, I devised the Resonant Recorder. If we know the
natural frequency of vibration of the recording-index, and
if by means of some mechanism we can send periodic
currents of exactly the same frequency through the electro-
magnet, then the intermittent magnetic pulls will exactly
synchronise with the natural swings of the writing-index.
Owing to this perfect tuning the index will now resonate,
breaking out into a persistent and regular vibration of con-
siderable amplitude. In practice, all that is necessary in
order to secure this is to take a long steel reed, which in the
course of its regular vibration will periodically interrupt
the electro-magnet circuit of the vibrator coil. The reed

THE RESONANT RECORDER 15

itself is maintained in a state of persistent vibration by the
usual electro-magnetic arrangement. I shall for the sake
of convenience refer to this reed-interrupter as the Coercer ;
and the writing-index simply as the vibrating-recorder or
Vibrator. The reed was at first purposely selected of too
great a length, so that the natural frequency of the coercer
should be slower than that of the vibrator. The free end
of the coercing reed carries a platinum wire which, dipping
into a cup of mercury, completes the electric circuit. The
other end is clamped, and by shifting the clamping-point
the vibrating length of the reed is gradually and continuously
shortened. This has the effect of gradually raising the
vibration-frequency of the interrupting reed. A time soon
comes when the frequency of the coercer is exactly the
same as that of the vibrator. The latter, which has been
hitherto more or less inert, now suddenly breaks out, as
foreseen, into very regular and sustained vibrations of large
amplitude. For some purposes it is important that the
vibration-frequency of the recording-index should have a
definite value. The various frequencies most suitable for
these researches were 10, 20, 50, 100, and 200 vibrations
per second. The exciting reed was previously calibrated
by means of frequency-meters, or standard tuning-forks, to
give these values. Then, with great expenditure of time
and patience, different vibrating-recorders having various
standardised frequencies were constructed. For this we
have to select fine-steel wires of differing lengths and thick-
nesses. The final tuning is accomplished by careful filing
or hammering. Fihng the tip of the vibrator raises the
frequency ; and hammering of the wire, near the point of
suspension, lowers it. A general flattening, along the whole
length, tends to maintain the vibration in a definite plane,
otherwise the free tip is apt to execute an elliptical vibration.
The tuning of the vibrator with the coercer is not very
difficult when the vibration-frequency is low, say 10 per
second. But when the frequency is high, say 100 or 200 times
in a second, an exact tuning is essential. The slightest

i6 RESEARCHES ON IRRITABILITY OF PLANTS

variation of the length of the coercer will either bring about
full resonance or make it entirely ineffective. When the
tuning is nearly but not quite perfect, then we have the
phenomenon of beats. In this case, in the successive dots
of the record, there will be periodic blanks. For the purpose
of exact adjustment of length of the coercer I employ a
micrometer-screw, by means of which the most delicate
adjustment of length may be carried out.

For periodic interruption the coercing and vibrating
coils may be put in series, but I find it is much easier to
obtain a persistent vibration when the coercer coil is placed
in a multiple arc with the vibrator coil. An electro-motive
force of 4 volts should be sufficient for the purpose of maintain-
ing a steady vibration of both the coercer and the vibrator.

Having thus secured the requisite perfection of the
resonating-writer, it is necessary to describe the complete
apparatus by which to obtain records of responses in Mimosa
and other sensitive plants. For this purpose we require a
slide-carrier to hold the recording-plate, and this is to be
dropped at a definite speed, without jar ; also the clockwork
by which it is to be actuated. Besides these is needed some
special means by which the recording-point may be brought
to the proper distance from the recording-surface. It is
necessary, again, that the response-movement of the writer
should be absolutely parallel to the writing-surface, and that
its tip or contact-point should be capable of delicate adjust-
ment as regards distance. It should be possible, moreover,
to bring this writing-point to any position on the recording-
surface that may be required. I will now proceed to relate
the devices by means of which all these conditions have been
met. Some of these will be seen in fig. 3, whichJUustrates
only the upper part of the Resonant Recorder.

The Slide and Clockwork

A gunmetal upright, the upper part of which is of trian-
gular section, stands on a large disc of the same metal, which
is screwed to a larger wooden base-board. The slide-carrier.

THE RESONANT RECORDER

17

holding the glass recording-plate, moves up and down the
top part of the upright. It is essential to have this slide so
accurately fitted that the plate-carrier may be able to drop

Fig. 3. — Upper part of Resonant Recorder. (From a Photograph.)

Thread from clock, not shown, passes over pulley p, letting down
recording-plate ; s', screw for adjusting distance of writing-point
from plate; s, screw for vertical adjustment; t, tangent-screw
for exact adjustment of plane of movement of recorder, parallel to
writing-surface ; axis of writer supported perpendicularly at centre
of circular end of magnet ; c, coercer ; m, micrometer-screw for
adjustment of length of coercer; v, vibrating recorder ; g, smoked-
glass plate.

smoothly and uniformly, without any jerking whatsoever.
I have sometimes attained the same end by mounting the
plate-carrier on wheels and letting it slide down vertical rails.
The plate-carrier is allowed to drop by the running down of

i8 RESEARCHES ON IRRITABILITY OF PLANTS

a clock or a phonograph motor, according to what may
be the requirement of the speed. The quarter-plate size
(ii X 8 cm.) is convenient for record, as it is not too large
for book illustration. The suspending thread passing over
pulleys is wound round the winding- wheel of the clock. This
wheel is provided with click and ratchet, which allow it to
be wound without interfering with the axis of the clock.
Thus winding of the wheel in the left-handed direction pulls
up the recording-slide. The running-down of the clock then
allows the slide to fall at a uniform rate. The various
speeds found necessary for different records were such that
the entire length of the plate, ii cm., travelled past the
recording-point in .5 second, 6 seconds, 15 minutes, i hour,
or 3 hours. The first two of these rates were obtained from
a phonograph motor employing two different-sized wheels.
The last three were obtained by attaching three different-
sized wheels to the clock-axis which carries the minute-hand.
In these slow rates the movement of the plate is quite uniform
from the beginning, but when, as in the first two cases, this
has to be dropped at a relatively high speed, a short time
will elapse, equivalent at most to the first fourth of the
plate, before it becomes quite uniform. Should uniformity
of such movement be specially desired for the record, it must
be commenced after passing this first fourth. But on account
of the chronographic signals which accompany the record,
this uniformity is not absolutely essential, for they give
us the data from which the time-relations of the curve
may be derived.

I may here refer to a few practical points with regard
to the preparation of the glass for record and its subse-
quent fixing. In order to produce an even layer of smoke
on the recording-plate, it is moved over the gas-flame
from a bat's-wing burner ; and this deposit of smoke will
be improved if the gas has been previously passed through
a jar containing a small quantity of benzine. After the
record is taken, it is fixed by pouring carefully over
it a dilute solution of Canada balsam in xylol. It is

THE RESONANT RECORDER 19

afterwards easy to reproduce this record by contact-print
on a photographic paper.

Adjustment of the Writer

It is sometimes necessary to have the recording-point
brought two or more times to the same place, in order
that the successive records may be rendered the more
strictly comparable. This is accomplished by a rack and
pinion to adjust the height of the platform carrying the
plant. When the platform is lowered, the petiole, which
is attached to one arm of the recording-lever, pulls it down,
and the recording-point is moved to the left. When the
platform is raised, then by the action of the counterpoise
attached to the other arm of the lever the index is moved
in the opposite direction. In this way the recording-point
can be brought to any position that is desired. The same
end is secured through adjustments of a micrometer by
which the carrier of the writing-index is raised or lowered.

Another adjustment that is necessary is the bringing
of the recording-point near to the writing-surface without
actual contact ; so that, when the index is set in a state
of resonance, it may trace a dotted Hne. The necessary
adjustment is brought about by means of a micrometer-screw
at the top of the instrument, by which the lever can be made
to approach or recede from the writing-surface. When the
speed of the plate is slow, the successive dots may be so
close together as to appear hke a continuous Hne.

More troublesome is the adjustment necessary to render
the plane of movement of the index exactly parallel to the
writing-surface. If this be omitted, the writing-point in
one part of the record, say to the right, will be too far
away to strike the surface, whereas in another part, say
to the left, it will press against the plate and lose its freedom.
This difficulty I have been able to overcome by mounting
the vertical rod, carrying the writer, inside another tube. An
attached tangent-screw, t, then causes a very slow rotation

c 2

20 RESEARCHES ON IRRITABILITY OF PLANTS

of the vertical rod, either in the right-hand or the left-
hand direction. By this means it is possible to bring the

Fig. 4. — General view of the whole apparatus, and the electrical connec-
tions by means of which excitatory shock of a definite duration may
be given to the plant ; duration of shock determined by metronome
which completes electrical circuit. (From a Photograph.)

plane of movement of the writing-index exactly parallel to
that of the writing-surface. The complete apparatus for

THE RESONANT RECORDER

21

obtaining response of Mimosa is shown in the accompanying
illustration (fig. 4).

Having thus given an account, in some detail, of the
practical working of the Resonant Recorder, it will now
be well to show a pair of curves which demonstrate, in
a marked manner, the advantage of intermittent over
continuous contact in the making of these records (fig. 5).
These represent two successive experiments on the same
leaf, under identical stimulation of an electrical shock.
The recording-plate was
here moving at a mode-
rately high speed. The
lower record was taken
with continuous con-
tact, and the upper
with the same recorder
but in a state of vibra-
tion, giving intermittent
contact. The vibration-
frequency was 10 times
per second. Stimulus
was applied at the point
marked by the vertical
line. A comparison of
the two records will
show that owing to the
relative loss of freedom,
due to friction, in the

continuous contact, the latent period, or the interval
between stimulus and initiation of response, is prolonged
and the amplitude of the response itself reduced. In the
case of the intermittent contact, on the other hand, we see
that besides the freedom from this particular error we have
the further advantage that the record itself contains its
own time-marks, the successive dots being at intervals of
one-tenth of a second.

We have next to consider the practicability of devising.

Fig. 5. — Advantage of intermittent over
continuous contact in obtaining re-
cords.

22 RESEARCHES ON IRRITABILITY OF PLANTS

for the excitation of the plant tissue, perfect methods of
stimulation, the intensity of which can either be maintained
constant or varied in a perfectly known manner. We
have moreover to render the successive stimulations, and
consequent scripts of the plant, a perfectly automatic
process ; so that the experimenter may be comparatively
relieved of personal participation in the securing of the
records. This will have the incomparable advantage of
having no element of personal error in the results so
obtained. The question of the effects of the various forms
of stimulus will be dealt with in the next chapter.

Summary

In the response-records of plants, errors are introduced
on account of friction of the writing-point against the
recording-surface.

These errors are eliminated by the method of intermittent
instead of continuous contact for the record. By employ-
ing the principle of resonance, the writer is made to vibrate
to and fro at a known and definite rate. The record consists
of series of dots giving definite time-intervals. The record
is thus its own chronogram.

CHAPTER III

METHODS OF STIMULATION

Different methods of stimulating the plant : mechanical, chemical, thermal,
and electrical — Difficulties of securing quantitative stimuli — Direct
and indirect stimulation — Ideal modes of stimulation — Electro-
thermic stimulation — Stimulation by constant current — Stimulation
by condenser-discharge — Non-polarisable electrodes — Direct, extra-
electrodal, and intra-electrodal stimulation — Stimulation by induction
shock — Effects of make- and break-shock — Excitation by tetanising
shock.

In the case of contractile animal muscle, various stimuli
give rise to excitation, and it is a very remarkable fact that
the same stimuli exercise a similar excitatory influence
on the pulvinus of Mimosa. Classifying these stimuli,
we find that they are : —

1. Mechanical. — A blow will excite animal muscle and
cause mechanical response. A similar effect is induced by
a mechanical blow in the pulvinus of Mimosa. A prick or
cut also will cause contraction in either.

2. Chemical. — Various chemical agents are found to
induce excitation in both animal and vegetal contractile
tissues. Thus dilute hydrochloric acid or ammonia causes
excitation of both muscle and pulvinus.

3. Thermal. — The application of a hot wire will induce
responsive contraction in both cases.

4. Electrical. — The muscle may be excited by an induc-
tion-shock. The pulvinus of Mimosa is also excited by
such shocks. Other modes of electrical stimulation, such
as that of condenser-discharge and that of the applica-
tion of a constant electrical current, are found effective in
causing excitation of animal tissues. It will be seen in the

23

24 RESEARCHES ON IRRITABILITY OF PLANTS

course of the present chapter that plant tissues also may be
excited by similar methods.

In all these cases excitation may be either direct or
indirect. In the case of muscle, with its attached nerve,
we may cause excitation directly by applying the various
forms of stimulus on the muscle itself, or indirectly by
applying them on the nerve. In the latter case excitation
is transmitted by the conducting-tract — the nerve — and
reaching the muscle after a brief and definite interval,
induces there the usual contraction.

Taking the case of Mimosa, we may similarly have
either direct or indirect excitation. Excitation is direct
when it is applied, say, on the contractile pulvinus itself.
It is indirect when it is apphed on the petiole, at a distance
from the pulvinus. Certain tissues in the petiole conduct
this excitation, which, reaching the pulvinus after a definite
interval, induces a responsive contraction.

It is usually maintained that in the case of Mimosa
there is no true conduction of excitation, but that this con-
tention is not justified will be fully demonstrated in a sub-
sequent chapter. We have, then, in correspondence to the
nerve and muscle preparations of the animal, plant-speci-
mens, consisting of petiole and pulvinus. Indirect excitation
for specific experiments is effected in the animal through
the nerve, and in the plant through conducting-strands
embedded in a tissue, as in the petiole.

Although we thus have various forms of stimulus at our
disposal for inducing individual and isolated responsive
contractions in Mimosa, yet we are confronted with very
great difficulties when we wish to obtain a series of uniform
excitations for quantitative investigation. It is obvious
that chemical forms of stimulus would be impossible for
successive excitations. The objection to a mechanical
blow, as the stimulus to be employed, Hes in its Hability
to cause a mechanical jar and thus to disturb the record.

The ideal form of stimulation would be one the inten-
sity of which might be maintained uniform in successive

METHODS OF STIMULATION

25

experiments, or varied in a definite and known manner.
Another great obstacle to be overcome in practice is the
avoidance of injury which is caused by the stimulus itself.
The application of stimulus above a critical intensity induces
a depression or abolition of excitability of the tissue.

As the result of long investigation for the purpose of
securing various forms of quantitative stimulus, I find
that one mode of thermal and three modes of electrical
stimulation may be rendered practicable for our purpose.
These four different methods will be described in some
detail below.

Electro-thermic Stimulation

It is evident that touching the specimen with a hot
wire, though effective, is not a form of stimulus that is
capable of quantitative application or of repetition. It is
apt, moreover, unless
very great precautions
are taken, to injure the
tissue.

The thermal mode of
stimulation can, how-
ever, be rendered prac-
ticable by the electrical
mode of the generation
of heat. A loop of fine
platinum-wire is made
to clasp round the peti-
ole which is to be ex-
cited, and is connected

with an electrical circuit by means of fine flexible silver-wire
(fig. 6) . The circuit can be completed by a metronome inter-
rupter, the current from the battery flowing for a definite
length of time during, say, a single or definite number of beats
of the metronome. This produces a sudden thermal shock,
enough to cause excitation. Successive uniform stimuli can

Fig. 6. — Electro- thermic stimulator for
uniform stimulation ; metronome em-
ployed in place of key k, for closing
circuit for definite length of time.

26 RESEARCHES ON IRRITABILITY OF PLANTS

thus be applied. By means of a variable resistance included
in the circuit, the intensity of the stimulus can be increased or
diminished. Care must, however, be taken that the heat
produced in the platinum loop shall not be such as to scorch
or otherwise injure the tissue.

I find that injury from scorching may be avoided by
adding a drop of water at the point of contact and after-
wards removing excess of water by blotting-paper. This
thin film of water protects the tissue from a bum. It is,
again, not absolutely necessary to place the platinum wire

in contact with the plant.
Excitation will take place
if the heating-wire is in
close proximity. How
practicable this form of
stimulus may be rendered
will be observed from the
record (fig. 7) of two suc-
cessive excitations by this
method, which are seen to
_ _ , . ,^. be uniform. For certain

Fig. 7. — Response records of Mimosa • i • • •

under indirect thermal stimulation. electrical mvestlgations it

is essential that stimulus
other than electrical should be employed. This require-
ment is admirably fulfilled by the thermal mode of stimu-
lation.

Another mode of stimulation — ^namely, that of thermal
radiation — can also be employed, though not so conveniently
as the former. A certain area may be rendered radiant by the
passage of an electrical current. A Nemst electrical lamp
can be conveniently utilised for the purpose' This, when
rendered incandescent, gives out radiation of constant
intensity. This radiation consists not only of light rays
but also of a large proportion of obscure heat-rays. The
excitatory value of the latter is more efficient than the
luminous rays. The radiant surface of the Nernst lamp
is suitably placed in front of a concave metal mirror, by

METHODS OF STIMULATION 27

means of which the rays can be focused upon any point that
is desired.

Stimulation by Constant Current

I have found that Mimosa and other sensitive plants show
certain very remarkable excitatory effects under the action
of a constant current. The characteristic feature of these
is that excitation is not induced during the passage of the
current but only at its initiation or cessation. The excita-
tory effect in this case is further conditioned by the point

Fig. 8. — Responses to stimulation by constant
electric current.

of entry, or anode, and that of exit, or kathode. The
specific characteristics of this mode of stimulation will be
found fully described in the chapter on the Polar Effects
of Currents in Excitation. It need only be mentioned here
that, in the matter of all these peculiar effects, the plant
tissue behaves in a manner exactly similar to the animal
tissue. A series of records obtained from Mimosa by the
stimulus of a constant current are shown in fig. 8.

Stimulation by Condenser Discharge

Another practical method of stimulation is that of
condenser discharge. The condenser consists essentially
of two conducting-plates — which may be two sheets of

28 RESEARCHES ON IRRITABILITY OF PLANTS

tin-foil — separated by a sheet of non-conducting material,
such as mica or paraffined paper. The capacity of the
condenser is increased by enlarging the effective area of
the plates. The diagram in fig. 9 illustrates this mode of
excitation. By increasing the number of cells, the charging
E.M.F. may be increased until a suitable value is obtained
which is efficient for excitation. This will depend on the
excitability of the plant-specimen. About 2 volts charging
•5 microfarad will in general be found sufficient, k is a
special spring-key by which the condenser may be charged

Fig. 9. — Direct stimulation by condenser discharge ; c, condenser, k,
key. (a) intra-electrodal and {b) indirect extra-electrodal mode of
stimulation.

or discharged. The plant to be excited is included in the
electrical circuit. When k is pressed down, the condenser
is charged, the instantaneous charging current passing in
one direction. The upper arrow in the diagram shows
the direction of this charging current. When the key is
released, it springs back and discharges the condenser.
The instantaneous discharge current now flows in a reverse
direction (fig. 9).

The electrical connections with the plant are diagra-
matically shown, in this and other figures, by two lines.
In practice the connections have to be made by means
of thread moistened in dilute saline solution. In certain
experiments it is necessary to avoid complications arising
from electrolytic polarisation. In these it is advisable to

METHODS OF STIMULATION 29

use non-polarisable electrodes for making the connections
with the plant tissue. Such non-polarisable electrodes
may be of the usual U-tube type. The shorter limb of the
glass U-tube is filled with kaolin paste in normal saline.
A cotton thread moistened in sahne protrudes from this
and makes the connections with the tissue. The longer
limb is filled with zinc sulphate solution, into which dips
a zinc rod. For ordinary purposes, however, a much simpler
contrivance is found effective. A narrow cork is partially
hollowed out and paraffined. The well thus formed is
filled with dilute saline solution. The bottom is pierced
for the entry of a cotton thread into the saline. A thick
silver-wire, whose surface has been covered electrolytically
with a film of chloride, pierces the side of the cork and
dips into the saline solution. The silver wire forms one
of the two electrodes, and the cotton thread makes the
necessary electrical connection with the tissue.

For the purpose of excitation we may make the two
electrical connections, one at or near the pulvinus itself, and
the other on the petiole at a short distance. It will be shown
later that when the electrical current leaves the tissue by
the pulvinus, that point becomes the seat of excitation.
Thus by making the pulvinus the point of exit of current,
or kathode, we may cause direct excitation. Or we may
have the pulvinus included between the two electrodes, so
that the electrical current passes through it (fig. 9, a). This
connection we may designate the intra-electrodal. Here, in
certain circumstances, the excitation throughput the tract
becomes diffuse and practically instantaneous. And lastly,
the two electrical connections may be made side by side,
say about i cm. apart on the petiole, at a moderate distance
from the pulvinus. The excitation thus caused in the
petiole reaches the pulvinus, as I have already said, by
conduction. This connection we may call extra-electrodal

(fig. 9. b).

I give below a series of records (fig. 10) of response to
stimulation by condenser discharge. The plant was highly

30 RESEARCHES ON IRRITABILITY OF PLANTS

excitable, and excitation was caused by the discharge of
•I microfarad condenser charged to -3 volt.

Stimulation by Induction-shock

Excitation may be induced by means of a single or
repeated shock from an induction coil. In my own expe-
rience I was at first under the impression that this mode of
stimulation was not suitable for repeated quantitative
experiments, as I found that the plant was liable to become
insensitive owing to the fatigue or injury caused by the shock.
Later, however, I was able to trace this difficulty to the

Fig. 10, — Record of responses to stimulation by
condenser discharge.

employment of an intensity of shock which was in excess
of a certain critical value. I had in fact been misled by
the prevaiHng belief that the excitabiHty of the plant
was considerably lower than that of the animal. Hence
I employed an intensity of current which was unnecessarily
high. This induced fatigue and consequent insensitiveness.
Afterwards I discovered that in so far as its sensitiveness
to electrical stimulation was concerned, Mimosa was in no
way inferior to the animal. Quantitative results will be
given later, in justification of this statement. Avoiding,
then, an intensity of stimulus which was too great, and
allowing proper resting-intervals, I found that the efficiency
of induction-shock as a mode of stimulation was all that
could be desired. It has also the great advantage of

METHODS OF STIMULATION 31

allowing successive stimuli to be maintained constant, or
to be increased in a known manner.

The induction coil consists of a primary made of a
few turns of thick wire enclosing a bundle of soft iron
wire — thus forming an electro-magnet — and of a secondary-
consisting of a larger number of turns of thin wire. The
secondary coil can be made to approach or recede from
the primary, by means of a slide. When a current is
suddenly started in the primary coil, by pressing a key an
instantaneous make-induction-current is induced in the
secondary. When, by releasing the key, this current
is broken, an instantaneous break-induction-current, whose
direction is opposite to that of make, is induced in the
secondary. Owing to the greater suddenness with which
the break is effected, the intensity of the break-shock is
greater and of shorter duration than that of the make-shock.
The intensity of either make- or break-shock may be in-
creased by bringing the secondary nearer the primary.
We can obtain successive uniform shocks, of either make
or break, by maintaining the distance between the two
coils constant.

We may subject the tissue to shock of either make or
break at will by employing an additional short-circuiting
key. When this key is down, the shock from the secondary
coil is practically diverted across the better conducting-
path provided by the key, so that for practical purposes
none passes through the plant tissue. If it is desired to
cause successive excitations by make-shock only, then the
short-circuit key is raised when the primary circuit is made,
and pressed down when this is broken. For exciting by
break-shock, the short-circuit key is pressed when the primary
is made and raised when the primary is broken (fig. 11).

Effects of Make- and Break-shock
In the response of animal tissue it is well known that
while single induction-shocks are effective in the case of
quickly reacting skeletal muscles, they induce hardly any

32 RESEARCHES ON IRRITABILITY OF PLANTS

contractile effect in the more sluggish smooth muscles.
Vegetal protoplasm also is commonly regarded as little
capable of excitation by these shocks, behaving in this
respect like the sluggish smooth muscles amongst animal
tissues. Again, while the break-induction-shock of higher
intensity and shorter duration is more effective in exciting
the quickly reacting skeletal muscle, in the case of the
sluggish smooth muscle it is the make-shock of low inten-
sity and long duration that proves more efficacious. That

Fig. II. — Arrangement for applying single make- or break-shock;
K, key in the primary circuit. The secondary circuit may be
short-circuited by the second key.

the inference commonly made about the reaction of vegetal
protoplasm to single induction-shocks is not of universal
application, is strikingly seen in the response of pulvinus
of Mimosa. Here, so far at least as single induction-
shocks are concerned, its reaction appears more analogous
to that of skeletal than of smooth muscle ; as a position
of the secondary in relation to the primary can be found
in which, while a single make-shock is ineffective, a single
break-shock is quite efficient. In order to render the make-
shock effective, the secondary has here to be pushed in
nearer to the primary, thus increasing the intensity of the
shock. A pair of records will be given in a later chapter

METHODS OF STIMULATION 33

(figs. 20, 21), showing the relative ineffectiveness of the
make-shock compared with the break-shock.

Excitation by Tetanising Shocks

It will be shown later that shocks individually ineffec-
tive become effective by repetition. It is thus possible to
excite a plant by subjecting it to a number of relatively
feeble make- and break-shocks. The advantage of this
mode of stimulation is that, owing to the low intensity of
these shocks, the liabihty of the tissue to injury is very
much reduced. Such alternating tetanising shocks can be
produced by means of an automatic spring-interrupter,
included in the primary circuits. This interrupter consists of
a steel spring carrying at its free end a soft-iron armature
which faces one pole of the electro-magnet of the primary.
An adjustable contact-rod touches the spring and com-
pletes the primary circuit. But the completion of the
circuit magnetises the electro-magnet, which, pulling the
armature, breaks the contact, thereby interrupting the
primary current. The electro-magnet is thus demagnetised,
the armature is released, and the spring returns suddenly,
re-establishing the circuit. By this automatic make-and-
break we obtain alternating induction-currents in the
secondary.

When we wish to subject the experimental tissue to the
additive effects of these shocks, of a given short duration,
this may be accomplished by including in the primary
circuit a metronome, which in the course of a single beat
closes the circuit for an approximately definite duration.
If the duration of closure be, say, one-fifth of a second, and
if the frequency of the spring-interrupter be 50 times per
second, the number of alternating double-shocks given to
the tissue would be ten.

A method of securing still greater accuracy in the
duration of the tetanising shock will be described in another
chapter, where also may be seen records obtained by that

34 RESEARCHES ON IRRITABILITY OF PLANTS

mode of excitation. In subsequent chapters we shall also
study in detail the various characteristics of response and
its time-relations.

Summary

The ideal method of stimulating a plant is one in which
the intensity might be maintained uniform or varied in a
definite and known manner.

If the stimulus exceeds a certain critical value, the
tissue is injured with concomitant diminution or abolition
of excitability.

One practical method of quantitative stimulation is
by electro-thermic stimulus, where the plant- tissue is sub-
jected to a sudden and definite thermal variation.

The plant may also be excited by the action of a constant
current.

Another method of excitation is by the discharge of a
condenser.

And lastly, excitation may be induced in the plant by a
single induction-shock or by a series. As in the skeletal
muscle of animals, so in the pulvinus of Mimosa, the break-
shock is more effective than the make-shock.

CHAPTER IV

TIME-RELATIONS OF THE RESPONSIVE MOVEMENT AND
STANDARDISATION OF STIMULUS

Latent period of Mimosa — Apex-time — Rate of responsive movement
of leaf — Effect of intensity of stimulus, fatigue, and temperature —
Periodic dot-marker — Time-relations of response and recovery —
Effect of season — Response of Biophytum — Response of Neptunia —
Arbitrary distinction between sensitive and ordinary plants — Differ-
ential response in Mimosa — Response of ordinary plants — Universal
sensitiveness of plants — Standardisation of stimulus — Maximal and
minimal stimuli — Extreme sensitiveness of Mimosa.

As already stated, when the pulvinus of Mimosa is sub-
jected to an instantaneous stimulus, say that caused by
an electric shock, a responsive movement is initiated after
the lapse of a very short interval. After the completion
of the fall of the leaf, the contracted pulvinus slowly recovers
its original expanded condition, with consequent re-erection
of the leaf. The movement of the leaf is thus a visible
indication of the responsive reaction and recovery of the
pulvinus under stimulus. In this entire process, we may
conveniently distinguish three separate phases : —

First, there is a brief period between the incidence of
stimulus and beginning of the responsive movement : the
contraction has not yet manifested itself. This lost time
is called the Latent Period.

Secondly, after the lapse of the latent period, the leaf
begins to fall, at first with increasing rapidity, which then
again diminishes, till it comes to a stop. The curve described
attains its maximum amplitude, corresponding to the
maximum fall of the leaf. The period required, up to this

35 D2

36 RESEARCHES ON IRRITABILITY OF PLANTS

point, we shall call the Apex Time. The pulvinus remains
for a short time in its contracted condition.

Third and lastly, recovery of the pulvinus from the
effect of stimulus begins to take place, with consequent
re-erection of the leaf. This process of recovery is
very much slower than the responsive fall. While the
responsive fall is a matter of a few seconds only, the
re-erection or recovery requires several minutes. This
recovery, again, is at first rapid and at the end rela-
tively slow.

Quantitative measurements of these different phases
may, as we shall see, be derived from the response-curve
itself. In obtaining these, there are two elements to be
measured — ^namely, the extent and the rate of movement.
The amplitude or height of the curve gives a measure of
the amount of movement. Magnification or reduction of
the record results, as we have seen, from two elements of
adjustment — namely, the ratio between the horizontal
arm of the lever and the length of the recorder, and the
ratio between the distance of thread-attachment from the
pulvinus and the entire length of the leaf.

In the record given in fig. 12 the length of the vibrating
recorder was 10 cm. and the thread-attachment with the
leaf was made with the horizontal arm of the lever at a
distance of 5 cm. from the fulcrum rod. The magnification
of the writing-lever was therefore 2. The total length of
the responding leaf was g cm. But the thread-attachment
to the horizontal lever was made at a point on the petiole
3 cm. from the pulvinus. The responsive movement of
that particular point on the petiole was therefore reduced
to one-third the movement of the tip of the leaf. Thus
we have a reduction to one-third brought about by the
selection of the point of attachment on the petiole, and a
magnification of two, due to the writing-lever. The record
obtained represents in this case the actual movement of the
tip of the leaf, reduced to two-thirds.

As regards the time-measurements of the responsive

TIME-RELATIONS OF RESPONSE 37

movement, we have seen that the successive dots in the
curve itself give time-intervals. When it is necessary to
measure short intervals, say of -i second, a resonant
vibrator accurately tuned to ten double vibrations per
second is employed. The successive dots in the curve then
represent intervals of a tenth of a second each.

For the correct determination of the first two phases
of the responsive movement in which the time involved
is short — ^namely, the latent period and the apex time — it is
necessary to have the recording-plate moving at a rapid
rate. But for determining the time-relations during the
period of recovery, which is a matter of several minutes,
the recording-plate has to be moved at a relatively slow
rate.

I give below records (fig. 12) which show these first two
elements in a typical manner. The record here, as explained
before, is reduced to two-thirds.

Latent Period. — It will be noticed that there is a short
interval between the application of stimulus, represented
by the vertical line, and the initiation of response. The
movement is here seen to begin before an interval of
•I second is completed. For more accurate determination
of the latent period a record must be taken on a faster-
moving plate. A detailed description will be found in a
later chapter, where it is shown that the average value
of the latent period may be taken as about "i second.
It will also there be observed that though in a given
specimen the latent period is constant, it varies slightly in
different specimens. It is also appropriately modified accord-
ing to the physiological changes induced by temperature,
fatigue, and the influence of the season.

The Apex Time. — It is seen from the upper of the two
curves in fig. 12 that the responsive fall practically attains
its maximum near the twentieth dot. This indicates that
the value of the apex time in this case is 2 seconds. As
regards the rate of this responsive fall, the spacing of the
successive dots, each representing an interval of -i second,

38 RESEARCHES ON IRRITABILITY OF PLANTS

shows in a striking manner how the speed first accelerates
and then slows down.

The maximum movement is generally attained about
•5 second after the shock. The actual rate of the maxi-
mum responsive fall is here 40 mm. per second. The rate
of the responsive fall is modified by various conditions : —

(i) The speed is greater under stronger stimulus. This
is well seen in fig. 12, where the lower one was taken under
stimulus intensity of i, and the upper one under stimulus
intensity of 4. The gentler slope of the lower curve, and

Fig. 12. — Records giving apex- time in the response of
Mimosa. Lower curve is in response to stimulus i
and upper to stimulus 4 units.

more abrupt rise of the upper, clearly show the greater speed
and vigour of the responsive movement under the stronger
stimulus. The curves show moreover that the amount
of this responsive movement is greater under stronger
stimulation.

(2) In a fatigued condition the rate of the responsive
fall under constant stimulus is relatively slow. Thus
in a certain experiment, where the maximum rate of fall,
when fresh, was 30 mm. per second, the rate was slowed
down to 20 mm. per second in consequence of fatigue.
In another case the rate when fresh was 50 mm. per

TIME-RELATIONS OF RESPONSE 39

second, which under pronounced fatigue was reduced to
8 mm.

(3) Temperature also modifies the rate of the responsive
movement. Thus in a given specimen the maximum rate
of fall at the relatively low temperature of 22° C. was 10 mm.
per second ; it became enhanced to 105 mm. per second at
25° C, and 115 mm. per second at 31° C.

The pulvinus after the attainment of the maximum
fall remains more or less persistently contracted for a
short time. This is shown by the horizontal portion of
the curve in fig. 12.

The Period of Recovery. — As this takes a relatively long
time, its record has to be made on a slowly moving plate.
The unit of time-measurement must therefore be relatively
long. If the successive dots were to be made at the
ordinary rate of 10 per second, they would become fused
and continuous in the record. For this reason I have
devised a contrivance by which the successive dots, in the
recovery-portion of the curve, are placed at such intervals
as to prevent overcrowding. A convenient interval is
either 5 or 10 seconds.

The device for producing periodic dots at intervals,
say of 10 seconds, consists of clockwork employed to interrupt
the current actuating the vibrating recorder at particular
intervals. A light six-rayed wheel is attached to the axis
of the seconds-hand, and during the course of a single com-
plete revolution, that is to say, in a minute, the projecting
rays press the spring-key six times at intervals of 10 seconds
each (fig. 13). It is only during the short interval when the
key is pressed that the circuit is completed and the recorder
set in vibration to make its dots. Each dot made, it should
be remembered, is the result of a succession of strokes inscribed
by the vibrating recorder 10 times in the second. But
as the movement of the plate is slow, these successive
strokes more or less superimposed make but a single large
dot. Ten seconds again elapse before the next pressure
of the key brings about another large dot, and in that

40 RESEARCHES ON IRRITABILITY OF PLANTS

interval the plate has moved a certain distance. There
is a second key used for short-circuiting by which the clock
interrupter can be put out of action. When this is done, we
obtain the usual series of dots at intervals of -i second.
It will be understood that if the whole response-record
were to be made by the vibrator, as actuated periodically at
intervals of lo seconds, we should have a very long gap in the

(:

Fig. 13. — Clockwork for the dot-marker; the six-rayed wheel
periodically completes electric circuit.

record of the contraction-portion, since the apex is reached
in 2 seconds. To obtain then a more or less continuous
inscription of the entire curve, the vibrator should at first
be allowed to make its normal dots 'i second apart. This is
done by taking care to commence the experiment with the
clock-interrupter short-circuited. As soon as the apex point
is reached the short-circuit is removed, and the succeeding
record, during the recovery of the leaf, consists of dots

TIME-RELATIONS OF RESPONSE

41

at intervals of 10 seconds. Fig. 14 gives us a record of
a response reduced to two-thirds taken in this way. It
will be seen that the recovery practically takes place in
16 minutes. Thus in the present case, while the pulvinus
took only 3 seconds to complete the contraction, it required

Fig. 14. — Response of Mimosa. Successive dots are at
intervals of yV second in the contractile portion, and lo
seconds in the recovery portion of curve. Vertical marks
below indicate intervals of i minute.

16 minutes to recover from it. It will be seen, further, that
the rate of recovery is quicker at the beginning and slower
at the end. The following is a tabular statement of the
time-relations of the different phases of response and
recovery : —

Tabular Statement showing Time-relations of Response and

Recovery in Leaf of Mimosa

Period of contraction

,, recovery
Maximum rate of contractile movement

„ „ movement of recovery

Average rate of contractile movement
„ „ movement of recovery . .

3 seconds

16 minutes

24 mm. per

second

.09

»

15

j>

.045 „

>>

We may now briefly recapitulate the sequence of events
in a typical specimen of Mimosa subjected, during the
summer season, to a moderate stimulus. Response does

42 RESEARCHES ON IRRITABILITY OF PLANTS

not commence immediately ; there is a latent period of
•I second. The responsive movement then begins and
proceeds for a time with increasing speed, the maximum
contraction being attained about 3 seconds after the shock.
The pulvinus remains in the contracted position for a short
period. After this the recovery is initiated. The rate of
recovery at the beginning is relatively rapid, and very
slow towards the end. The maximum rate of recovery is
•09 mm. per second, in contrast with the maximum rate of
contraction, which is 24 mm. per second. The movement
of recovery is thus about three hundred times slower than
the movement of contraction. The recovery is completed
in about 16 minutes.

A stronger stimulus, generally speaking, requires a
longer period for recovery. The influence of season is also
a factor to be taken into consideration. Under the physio-
logical depression induced by winter, the responsive process
is appropriately modified. The excitability of the tissue
becomes depressed. An intensity of stimulus which in
summer was effective, becomes in winter ineffective. To
evoke response much stronger stimulus has to be employed.
The latent period is prolonged and the amplitude of response
reduced. And lastly, in winter there is, generally speaking,
a great prolongation of the period of recovery. In summer,
with vigorous specimens, recovery may be practically
complete in as short a time as 8 minutes. But owing
to sluggishness induced in winter, on the other hand,
the recovery may be prolonged to 25 minutes or more.
In a severe winter response may even be abolished
altogether.

I have hitherto dealt in some detail with the responsive
movement of Mimosa. In contrast with this may be cited
other examples in which the excitatory reaction may be
either more rapid or extremely sluggish.

Response of Biophytum. — As an instance of relatively
quick reaction I give (fig. 15) a record of response of leaflet
of Biophytum, The maximum fall was here attained in the

TIME-RELATIONS OF RESPONSE 43

course of a second after the shock. The recovery was
completed in the course of only 3 minutes.

Response of Neptunia. — In marked contrast with the
quick reaction of Btophytum, I may cite the very slow action

Fig. 15. — Record of response of leaflet of Biophytum.
Vertical marks below record indicate intervals of 5
minute.

of the primary leaf of Neptunia oleracea. In fig. 16 is given
a record of its response under an exciting induction-shock of
moderate intensity. The clock-interrupter was so adjusted
that the successive dots should be at intervals of half a

Fig. 16. — Response of leaf of Neptunia. Successive dots are at
intervals of -5 minute in the contractile portion, and i minute
in the recovery portion of curve.

minute during contraction, and at intervals of a minute
during recovery. It will be seen that the maximum
fall was attained 3 minutes after stimulation, and the
recovery was not completed even after 40 minutes, which

44 RESEARCHES ON IRRITABILITY OF PLANTS

was the duration of this particular record. The recovery
was completed after a further period of 20 minutes, that is
to say, the total period of recovery was an hour. The apex
time or period of contraction is shortened by the application
of a stronger stimulus, but the period of recovery then
becomes very much prolonged. The following tabular
statement will display the range of variation in the speed of
reaction of these sensitive plants : —

Table showing Apex-times and Periods of Recovery in
Different Plants

Specimen

Apex time

Period of recovery

Biophytum sensitivum
Mimosa pudica
Neptunia oleracea ..

I second

3 seconds

180 seconds

3 minutes
16 minutes
60 minutes

Arbitrary Distinction of Sensitive and Ordinary

Plants

The arbitrary distinction that is generally drawn between
the so-called sensitive and ordinary plants may briefly be
referred to here. In the case of Mimosa, it is generally
supposed that the lower half of the pulvinus is alone sensi-
tive ; this however is not an accurate statement. By local
application of stimulus it can be shown that the upper
half also undergoes a feeble contraction, causing an * up '
movement of the leaf.

The localised stimulus may be applied by means of the
electro-thermic stimulator. Application of stimulus of
moderate intensity on the upper half of the pulvinus will
be found to give rise to an erectile response. Another prac-
tical method of local application of stimulus is by means of
sunlight. A narrow beam may thus be thrown on the
upper half of the pulvinus; this will be found to give
rise to erectile or * up ' response. Two such responses are

TIME-RELATIONS OF RESPONSE 45

shown in fig. 17, where the stimulus of sunhght was appHed
for 2 minutes followed by a period of recovery for 13 minutes.
If the stimulus apphed on the upper half be strong or
long continued, then the excitatory effect is transmitted
across the pulvinus to the more excitable lower half. In
these circumstances the * up ' is converted to ' down '
response, on account of the greater contraction of the lower
half of the pulvinus. Thus under any form of diffuse
stimulation the resultant response in Mimosa is brought
about by the differential excitabilities of the upper and
the lower halves of the pulvinus. We should also bear
in mind that the slight differential contraction-effect in
Mimosa leaf is very much magnified by the long petiolar

Fig. 17. — ' Up * response (represented by down curve) due to
local stimulation of upper half of pulvinus of Mimosa.

index. There are, again, numerous pulvinar organs whose
responsive movements have passed unnoticed. In Desmo-
dium gyrans there are two conspicuous pulvini ; the primary
pulvinus is at the junction of the petiole with the stem ;
there is a secondary pulvinus at the junction of the petiole
with the terminal leaflet. The primary pulvinus appears
at first sight to be insensitive. But on attaching the primary
petiole of Desmodium with the writing-lever, I obtained the
series of responses under a very feeble electric shock, as
seen in fig. 18. In this particular case the recovery is practi-
cally complete in 15 minutes. Other pulvini also exhibit
differential contraction under diffuse stimulation. Thus
the terminal leaflet of the bean plant {Vicia Fava) exhibits

46 RESEARCHES ON IRRITABILITY OF PLANTS

responsive down movement, though here recovery is very
protracted. But we have seen that the recovery of Neptunia
is also a very slow process.

The responsive movement of Mimosa is due, as has
been noted, to the unequal excitabilities of the upper and
lower halves of the pulvinus. The excitability of the tissue
is again modified by the state of turgor. In Mimosa there
is induced a periodic variation in the relative turgescence
of the two halves of the pulvinus. On account of this the
differential excitability, on which the motile response of

Fig. 1 8. — Series of responses of leaf of Desmodium gyrans,
under electrical stimulation.

Mimosa depends, undergoes great variation. The sensitive-
ness of this plant is in consequence often found to disappear
completely at certain hours of the day. I shall, moreover,
show in Chapter VII that the leaf of Mimosa becomes in-
sensitive when its pulvinus absorbs an excess of water. Thus
the mechanical movement of the sensitive plants on which
depended the assumption that * ordinary ' plants were
insensitive, rests on a basis which is very unreliable.

Responsive movements may, on the other hand, be
demonstrated in ordinary plants by the employment of a
suitable contrivance. In a radial organ diffuse stimulation
induces equal contractions on all sides, which balance each

STANDARDISATION OF STIMULUS 47

other. Hence lateral movement, dependent on differential
contraction, cannot take place. But if we take a hollow
tubular organ of some ordinary plant, say the peduncle of
daffodil, it is clear that the protected inner side of the tube
must be the more excitable. When this is cut in the form
of a spiral strip and excited by means of an electric shock,
we observe a responsive movement to take place by curling,
due to the greater contraction of the inside of the strip.
This mechanical response is at its maximum at that season
which is optimum for the plant. When the plant is killed
its response disappears.

It will be seen that the division of plants into sensitive
and insensitive is without any justification. Moreover, by
adopting the electric mode of investigation, I have shown
that every plant and every organ of the plant is sensitive
and responds to stimulus by a definite electric variation.

We have hitherto referred but vaguely to the question
of the intensity of the induction-shock employed as stimulus
to induce response. We have observed that on making
and breaking a current in the primary coil, instantaneous
currents are induced in the secondary. The intensity of
the induction-current employed for giving a shock depends
in the first place on the intensity of the primary current ;
secondly, on the suddenness with which the primary current
is made or broken ; and lastly, on the relative distance
separating the secondary from the primary coil. The in-
tensity of the current can be maintained uniform if we
always employ the same battery, say a 4-volt accumulator
or storage-cell. As the break of a current is accomplished
more quickly than make, the break-shock, as we have seen, is
more intense than the make-shock. The plant may, there-
fore, be excited by a single make-shock, or by a single
break-shock or a double make-and-break shock, or by a
sequence of make-and-break shocks, of definite duration,
according to the particular requirements of the experiment.

The intensity of the shock moreover may, as already

48 RESEARCHES ON IRRITABILITY OF PLANTS

shown, be increased by sliding the secondary nearer and
nearer to the primary coil. At a great distance the intensity
of the shock is very feeble, whereas in the nearest position
it is most intense. If a scale be placed to mark the relative
position of the secondary to the primary, we may be assured
of obtaining an identical intensity of shock whenever we place
the secondary at the same point on the scale ; or we can
obtain an increasing intensity of stimulus by progressive
movement along the scale towards the primary. There
is, however, no simple relation between the distance and the
intensity — that is to say, equal decrement of distance does
not mean equal increment of intensity. All that we are
sure of, is that the sliding in of the secondary coil secures an
increasing intensity of stimulation. In order to be certain
of obtaining quantitative values of intensity, the scale has to
be specially calibrated.

Standardisation of Stimulus

In subjecting the plant to the secondary shock, if we
begin with feeble intensity of stimulus, by placing the
secondary at a great distance, and gradually increase the
intensity by shding the secondary nearer and nearer, we
shall obtain that scale-reading at which the stimulus begins
to be effective. This particular intensity, the feeblest that
is effective, we designate the minimal stimulus. As we now
proceed to increase the stimulation by pushing the secondary
nearer to the primary, we find the amplitude of the response
is progressively enhanced, and ultimately we reach an
intensity beyond which there is no further increment of
amplitude. This intensity we designate the maximal stimulus.
When the plant is in an exceedingly vigorous condition, the
minimal intensity is low and the range between maximal and
minimal is narrow. But if the plant be in a less favourable
tonic condition, then the minimal stimulus is relatively high
and the range between minimal and maximal is wider.

As different induction-coils have different constants, the

STANDARDISATION OF STIMULUS 49

value of the intensity of stimulus as obtained from the scale-
reading of a particular coil gives us no idea of the absolute
intensity. It appeared desirable, nevertheless, in making
quantitative experiments, to adopt some unit of stimulus
in terms of which other intensities might be expressed. It
would be well, moreover, to select this unit in some way not
quite arbitrary, so that it might carry a significance more or
less universal. The unit intensity of exciting shock which
I have adopted for these reasons is that which barely induces
in ourselves a perceptible sensation. The observer dips two
fingers, one of each hand, into two troughs of saline solu-
tion, which are in series with the experimental Mimosa and
the secondary coil. The plant tissue is interposed so as to
ensure an identical current to pass through the experimental
individual and the plant. The resistance offered by the
plant tissue is very great ; in the case of Mimosa under the
usual mode of connection, it is about half a million ohms.
At the beginning the secondary is placed at a great distance
from the primary. The vibrating interrupter of the primary
is next started and the secondary gradually pushed in, till at a
certain scale-reading the observer, who is kept in ignorance
of the position of the secondary, just begins to perceive the
shock. This process is repeated several times in the case
of the individual observer, and the mean of various consecu-
tive readings, which ought not to differ from each other to
any extent, is taken as the unit for that particular individual.
The same observation is repeated with some ten different
individuals, and the mean of these ten readings is finally
adopted as that reading of the unit intensity which is to
serve as the standard.

Though this reading cannot be regarded as absolute
and invariable, yet, in the particular circumstances of the
case, it is fairly definite and on the whole satisfactory. It
gives us a general idea, moreover, of that intensity which will
be effective in stimulating the plant, in terms of the minimal
stimulus capable of evoking sensation in man. Having
thus obtained the scale-reading corresponding to this unit,

50 RESEARCHES ON IRRITABILITY OF PLANTS

we calibrate other positions of the scale in terms of this
unit. In this manner the scale is marked so as to indicate
intensities of 'i, '5, i, 2, 3, 4, 5, and so on. The calibration
is carried out by means of a ballistic galvanometer. In
subsequent chapters we shall employ these practical units,
which will thus have a definite significance.

Having shared the prevailing belief that the sensitive-
ness of the plant was very feeble compared with that of
the animal, I was considerably surprised to find that the
intensity of induction-shock which is barely sufficient to
induce sensation in man is quite enough to cause excitatory
faU in a Mimosa of moderate sensitiveness. Indeed, I found
that in the case of a highly excitable specimen an intensity
only one-tenth of this was sufficient to excite it. In other
words, under this particular test Mimosa may prove ten
times as sensitive as a human subject ! Later on I shall
give details of measurements which will show that, as far
as electric mode of stimulation is concerned, the plant is
in no way inferior to the animal in sensitiveness.

Summary

The extent of responsive fall in Mimosa increases with
increasing intensity of stimulus. The rate of movement
is also greater under stronger stimulus.

The rate of responsive movement becomes slower under
fatigue. In a given case the normal maximum rate of
movement of 50 mm. per second was reduced to 8 mm.
under fatigue.

Temperature enhances the rate of movement. A rate
of 10 mm. per second at a temperature of 22° C. was found
enhanced to 105 mm. per second when the temperature was
raised to 28° C.

In a typical case of Mimosa, in summer, the latent
period was found to be one-tenth of a second. The maxi-
mum contraction was attained in 3 seconds and the recovery
completed in 15 minutes. The rate of recovery was relatively

TIME-RELATIONS OF RESPONSE 51

rapid at the beginning and very slow towards the end.
The maximum rate of recovery was '09 mm. per second in
contrast with the maximum rate of contraction of 24 mm.
per second. The movement of recovery was about three
hundred times slower than the movement of excitatory
contraction.

A stronger stimulus, generally speaking, requires a longer
period for recovery.

Under the physiological depression induced by winter
the responsive reactions are modified. The latent period
is prolonged and amplitude of response reduced. The
period of recovery may also become protracted.

Different plants exhibit different characteristics of
response. Biophytum sensitivum may be taken as a type
of quickly reacting plant, while Neptunia oleracea is very
sluggish in its reactions. In Biophytum the apex time
is reached in a second and the recovery accomplished in
3 minutes. In Neptunia the apex time is reached in 180
seconds, and recovery completed in 60 minutes.

Mechanical response of Mimosa is due to differential
contraction of the upper and lower halves of pulvinus.

Erectile responses of Mimosa may be obtained by local
stimulation of the upper half of pulvinus.

Distinction of plants into sensitive and ordinary is
arbitrary. Under suitable conditions, ordinary plants, so-
called, may be made to exhibit motile response.

By means of electric response it may be shown that
every plant, and every organ of the plant, is sensitive and
responds to stimulation by a definite electric change.

The sensitiveness of Mimosa to electrical stimulus is
high and may even exceed that of a human subject.

E2

CHAPTER V

THE ADDITIVE EFFECT ; INFLUENCE OF LOAD, TEMPERATURE,
AND INTENSITY OF STIMULUS

Greater excitatory efficiency of the break-shock — Additive effect of
stimulus — Quantitative relation of additive effect — Effect of load —
Measurement of work under different loads — Rate of work — Thermal
chamber — Effect of temperature — Effect of increasing intensity
of stimulus on response.

In exciting Mimosa by means of induction-currents we
may employ either the make- or break-shock. It has already

been stated that the break-shock
is more efficient than the make-
shock. That is to say, as we
gradually push in the secondary
nearer the primary, excitation is
effected earlier with the break
than with the make. I will now
proceed to demonstrate this fact
by experiments.

For obtaining the record I
employed a writer which had a
vibration-frequency of 20 times
per second. The make and break
of the primary current was
effected by a metronome. In
the primary circuit an electrical
signal (fig. 19) was also included,
which marked at the base of the
figure the moments when the
current was made and broken. When the current is made,
an up-line is described by the writer attached to the

52

Fig. 19. — The electric signal.

THE ADDITIVE EFFECT

53

signal. So long as the current is flowing, the writer
remains in the up-position and draws a horizontal line
(fig. 20). At the time of make it will be noticed that,
owing to inertia, the writer was momentarily jerked some-
what above the level of this up-position. This jerked
line, therefore, always marks the moment of make, and
the horizontal line at the higher level the continuation
of the current. When the current is broken, the writer
falls suddenly to its original level. Thus a jerked up-line
indicates the moment of the application of the make-shock,

Figs. 20, 21. — Records showing greater efficiency of break-shock ;
frequency of vibrating recorder is 20, Signal below shows
by up movement ' make ' and by down movement ' break.'

and the down-line the application of the break-shock. In
the two accompanying figures are given records of the
effects of make- and break-shocks.

Greater Effectiveness of Break-shock

In the record (fig. 20) the secondary coil was placed at
the reading of 75 unit. It will be noticed that at ' make '
there was no response. But there was response at ' break,'
which took place 'i second later, the delay being due to
the latent period. In the next experiment, with the same
plant, the coil was pushed into the reading of i. It will
be seen (fig. 21) that excitation was here effective at ' make/

54 RESEARCHES ON IRRITABILITY OF PLANTS

a similar delay of 'i second being again due, as in the
previous case, to the latent period. Thus we see that
while the stimulus of the feebler intensity of 75 was effec-
tive at ' break,' it took the stronger stimulus of i to
induce response at ' make.'

Additive Effect of Stimulus

In the responsive tissue of the animal a single stimulus,
by itself ineffective, is found to become effective on repeti-
tion. In order to test whether this holds good in the case
of the plant also, I carried out the experiments which I

Fig. 22. — Stimulus of intensity '5 became effective
on being repeated four times.

shall now describe. With a given specimen I found that a
single make-and-break shock of intensity 75 was ineffective
in inducing excitation. I then adjusted the secondary for
intensity of '5, and made a reed-interrupter interposed in
the primary circuit give a series of make-and-break shocks
till the leaf responded by a fall. The interrupting reed was
adjusted to vibrate five times per second and the number of
interruptions is recorded below in the usual manner. It
will be seen in the record given in fig. 22 that the make-and-
break stimulus, which singly was ineffective, here became
effective on being repeated four times.

Desiring next to observe the effect of still further
reducing the intensity of stimulus with the same specimen,
I adjusted the secondary for an intensity of 'i. It must be
remembered that this is the intensity of tetanisation, which

THE ADDITIVE EFFECT 55

is only one-tenth of what is perceptible to the human subject.
Looking at fig. 23 it will be seen that even this very feeble
stimulus became effective on being repeated 20 times.

In carrying out this experiment I had expected in a
general way that a feeble stimulus, to be effective, must
be repeated a greater number of times. But I was not
prepared for so strictly quantitative a result as came out
in these two records. If the summated effect is to prove
strictly additive, then effective excitation must be equal to
the individual intensity multipHed by the number of repeti-
tions. From the record in fig. 22 the effective excitation

Fig. 23. — Stimulus of intensity "i became effective
on being repeated twenty times.

was seen to be '5 X 4 = 2. From the second record with
the same specimen, in fig. 23, it is seen to be 'i X 20 = 2.
In other words, for effective excitation the number of additive
stimuli varies inversely as the intensity of each. That this
is true, within certain limits, is borne out by another set of
results obtained from a different specimen, which was found
to be somewhat more excitable than the former.

In order to vary the condition of the experiment I ad-
justed the reed-interrupter to vibrate twice in a second.
There was thus an addition here of the effects of single
make-and-break shocks at intervals of half a second, in-
stead of one-fifth of a second as in the last case. In fig. 24
is seen the record of the additive effect, the intensity of
stimulus being -5. We find here that the stimulus became
effective on being repeated twice.

The experiment was again repeated with the same

56 RESEARCHES ON IRRITABILITY OF PLANTS

specimen, but using the reduced stimulus-intensity of '2.
The result given in fig. 25 shows that the stimulus had to be
repeated five times to become effective. We see once more
in this experiment that the additive effect is strictly quanti-
tative, and that the effective stimulation is constant under
varying intensity of stimulus, being equal to the individual
intensity multiplied by the number of repetition. In the

Fig. 24. — Stimulus of intensity *5 Fig. 25. — Stimulus of intensity '2
became effective after two became effective after five

repetitions. repetitions.

two cases here given we have a strict reaffirmation of this
quantitative relation — namely, '5 X 2 = *2 X 5 = constant.

Influence of Load

In the response of muscle it is found that the muscle-
curve is modified by the effect of the load which it has
to raise during contraction. With an increasing load the
height of response undergoes a progressive diminution,
but the period of recovery is at the same time correspond-
ingly shortened. In the contractile response of Mimosa
a similar phenomenon is observed. In carrying out this
experiment a load was placed on the arm of the horizontal
lever opposite to that of the leaf-attachment, and at an
equal distance from the fulcrum. The leaf, during its
contractile movement, has to lift this weight. In the first
experiment of the series a load of 100 milligrammes was
employed. In the second this was increased to 500 mgrms.,

INFLUENCE OF LOAD 57

and in the last it was made 2000 mgrms. Successive records
were made, for purposes of comparison, on the same part
of the plate. The vertical lines under the diagram (fig. 26)
are time-marks, indicating intervals of one minute. It
will be seen that the height of the record, with a load of
100 mgrms. is the greatest of the three, being 41 mm.
The recovery was completed in this case after the expiration

Fig. 26. — Effect of load ; the three records show responses under
varying loads of loo, 500, and 2000 mgrms.

of 9 minutes. The height of the second, under a load of
500 mgrms. is 28 mm., but recovery is nearly complete in
6.5 minutes. And, finally, with the load of 2000 mgrms. the
height is the least, being 13 mm., but the recovery is seen to
be completed in 5 minutes. Thus the effect of load in the
contractile response of the plant is shown to be strictly
parallel with its influence on the contractile response of
animal muscle.

Work performed by the Plant

The motile tissue of the plant, like that of the animal,
is capable of doing work during excitatory movement.
The influence of load on the height and period of response

58 RESEARCHES ON IRRITABILITY OF PLANTS

has been shown to be similar in the two cases. We may
next study the effect of load on the work performed. Work
is measured by the product of the weight raised and the
height of the lift. In the response of muscle, if the increas-
ing loads are represented by W^ Wg, Wg, and the correspond-
ing heights of response by h^ h^, hg, then it is found that up
to a certain limit W^hj < Wgh^ < Wghg ; in other words, the
work performed is increased under enhanced load and
increasing tension.

Turning to the effect of increasing load on the response
of Mimosa, we find that with a load of loo mgrms. the
height of response is 41 ; the value of W^h^ is therefore 4100 ;
under a load of 500 mgrms., Wgh^ = 500 x 28 = 14,000 ;
and, lastly, with a load of 2000 mgrms., Wghg = 2000 X 13
= 26,000. It is thus seen that as in the contractile response
of animal, so also in that of the plant, greater amount of
work is performed under increased load and higher tension.

We may now try to obtain some idea of the absolute
amount of work performed and the rate of work. We shall
take the tase where the plant had to lift a weight of 2000
mgrms. The following data are available from fig. 26. The
weight is seen to be lifted through 12 mm. in the course of
ten successive dots, each representing -i second. The mag-
nification of the lever was three times ; the absolute hft is
therefore 4 mm. The load to be lifted was 2000 mgrms. ; but
the weight of the leaf was 130 mgrms. and this helped the
fall. The actual work performed is therefore (2000 — 130)
X 4 miUimetre milligrams. This was accomplished in the
course of a second. Hence the absolute rate of work was
7480 mm. mgrms. per second.

Effect of Temperature

Our next inquiry is into the effect of temperature on
the response of the plant. For this we have to subject the
plant to different temperatures — some low, some high —
and to find means of maintaining it constant at any definite
temperature required. For this purpose a plant-chamber.

EFFECT OF TEMPERATURE 59

enclosing the plant and fulfilling these conditions, had to
be devised. It will be noticed that these investigations
involve two opposite sets of requirements — namely, in the
one case a definite lowering of the temperature of the
chamber below, and in the other a definite raising of it
above, the temperature of the environment.

The plant-chamber consists of a base-board with a
rectangular cover. This cover is made of a light wooden
framework, the sides being closed with sheets of mica.
The advantage of mica is its lightness, unbreakableness,
non-conductivity, and transparency. Transparency is
necessary because in darkness the sensitiveness of a plant
undergoes variation. The base-board consists of two
halves, with a small circular opening in the middle. When
these two halves of the base-board are slipped over the top
of the flower-pot they form one piece, fixed together by
means of suitable clasps. The base-board rests on the
flower-pot and the main stem of the plant passes through the
circular opening. The base-board thus forms the floor of
the thermal chamber. There are grooves cut in the base-
board for the reception of the wooden framework. The plant
is thus enclosed except on the top. After making the neces-
sary thread-connections of the lever with the responding leaf,
the top is closed by means of two sliding-pieces of mica, with
sHts for the passage of the thread. There are two side-tubes,
one near the top and the other near the base, for the passage
in and out of a stream of cold air, when the temperature of
the chamber is to be reduced. When the temperature is to
be raised, an electrical heating arrangement is employed.

The requirements of cooling are, first, a weighted air-
bag, provided with a stop-cock ; and second, a coiled copper-
pipe placed in an ice-box. By means of indiarubber tubing,
connections are made, first, between the stop-cock of the
air-bag and one end of the copper pipe ; and, second, between
the other end of the copper pipe and the upper tube of the
thermal chamber. Thus by more or less opening the stop-
cock of the air-bag a stream of cooled air is made to circulate

6o RESEARCHES ON IRRITABILITY OF PLANTS

through the plant-chamber, at varying rates. A steady low
temperature may thus be attained by adjusting the inflow
of cooled air, the degree of cooling being dependent on the
rate of flow. A thermometer placed inside the chamber
indicates the temperature attained.

In order to raise the temperature of the plant-chamber an
electrical device is employed. Inside the rectangular frame
there is a coil of wire of German silver, the ends of the wire

being led outside to two binding-
screws. An electrical current from
an outside battery is led through
this wire, a variable resistance
being also interposed in the circuit.
The heat generated inside the
chamber can be increased or de-
creased by changing the intensity
of the current ; this is accom-
plished by varying the adjustable
resistance. In this manner it is
quite easy to raise the temperature
inside the plant-chamber to any
degree that is desired, and to
maintain it constant as long as
necessary.

The temperature of the room,
at the time of the experiment I am
about to describe, was 27° C. I
desired to take three records, differ-
ing from each other by intervals of
5° C. For this purpose I reduced the temperature of the
plant-chamber to 22° C. and took the first record of the
series. Next, by stopping the inflow of the cooled air and
opening one of the side windows, I restored the temperature
of the chamber to 27° C, and after allowing a suitable
interval took the second record. Lastly, by means of
the electrical heating device described, the temperature of
the chamber was raised to 32° C. and the third record of

Fig. 27. — Effect of tem-
perature ; amplitude
of response seen to be
higher, and period of
recovery shorter, with
higher temperature.

EFFECT OF TEMPERATURE 6i

the series taken. It should be mentioned that in all these
cases the stimulus employed was of constant intensity —
namely, 2.

In fig. 27 are shown the responsive effects of an identical
stimulus at these three different temperatures. At 22° C.
it is seen that the height of response is small and the recovery
extremely prolonged. At 27° C. we find the ampHtude of
response enhanced and the rate of recovery increased. At

Fig. 28. — Response taken at three different temperatures,
the lowest below and the highest above, on a faster-
moving plate ; amplitude of response larger and
steepness of curve greater, at higher temperature.

32° C. the height of response is still more enhanced and the
rate of recovery, as seen in the steepness curve, still further
increased. In fig. 28 is given another set of records taken
on a faster-moving plate, exhibiting the effect of tempera-
ture on the amplitude of response. It will be shown in a
succeeding chapter that the latent period also is affected,
being progressively decreased with rising temperature.

Influence of Stimulus-intensity on the Response

It is usually supposed that in Mimosa every effective
stimulus causes the maximum response. That this is not
the case comes out very clearly in careful records taken

62 RESEARCHES ON IRRITABILITY OF PLANTS

with gradually increasing stimuli. We have already seen,
in fig. 12, the marked heightening of the response under
an increased intensity of stimulus. In muscle, in the
narrow range between minimal and maximal stimulation,
there is increasing amplitude of response with increasing
stimuli. But this soon attains a limit beyond which there
is no further increase of responsive contraction, whatever
be the stimulus-intensity employed.

In order to demonstrate a similar progressive increase
in the response of Mimosa, I first determined the minimal

Fig. 29. — Increasing response under increasing
intensity of stimulation.

stimulus that was barely effective in inducing a feeble
response. Starting from the particular position of the
secondary which gave this minimal intensity of stimulus,
I very gradually increased the intensity, by moving the
secondary only 5 mm. at a time nearer to the primary.
At each step I took a corresponding record.

In fig. 29 a series of seven such records is shown, the
successive responses being taken, as previously mentioned,
under slightly increasing stimuli and at intervals of 15
minutes. It will be seen how the height of response is
progressively increased. This increase is at first marked,
but towards the end we note that a limit is being ap-
proached, the difference between numbers 6 and 7 of the series

STIMULUS AND RESPONSE 63

being very slight. After the seventh, it was found that the
responses did not undergo any further increase.

The range within which the increasing effect is seen
is relatively extended in the case of plants in a somewhat
sub-tonic condition. But when the specimen is highly
excitable the range of variation is proportionately restricted.

Summary

The break-shock is more effective in inducing excita-
tion than the make-shock.

Stimulus, singly ineffective, becomes effective on repeti-
tion. The effective stimulation is equal to the individual
intensity of stimulus multiplied by the number of repetitions.

The effect of load on the response of Mimosa is similar
to that on the contractile response of muscle. With
increasing load the height of response undergoes a progres-
sive diminution with shortening of period of recovery.

Within Umits, the amount of work performed by a
muscle increases with the load. The same is true of work
performed by the pulvinus of Mimosa.

In a given case the rate of work performed by the pul-
vinus of Mimosa was 7480 mm. mgrms. per second.

The effect of rising temperature on response is to enhance
the amplitude and to shorten the period of recovery.

In Mimosa, increasing intensity of stimulus induces
increasing amplitude of response. This, however, soon
reaches a limit.

CHAPTER VI

VARIOUS TYPES OF RESPONSE

Necessity of uniform stimulation — ^The Periodic Starter — The Auto-
matic Exciter — Electrolytic contact-maker — The complete Response-
recorder — ^The factor of tonicity — Uniform responses — Fatigue under
shortened period of rest — Growing fatigue — Alternating fatigue —
Staircase response — Explanation of erection of leaf under continuous
stimulation — Fatigue-relaxation in plant and animal — Response under
single stimulus and under tetanisation.

Some of the effects brought about by varying external
conditions on the excitability of the plant have now been
noted. Certain other variations may, however, be induced
in the excitability, in consequence of the after-effect of the
stimulus itself, even when the external conditions are main-
tained constant. We may trace these induced internal
changes in the modification of the response-records.

It is clear that we can only be assured of the occurrence
of such internal changes from the observed variation of
response-record if we have been able in the first place to
keep the plant under unvarying external conditions. This,
taking certain special precautions as regards light, tempera-
ture, and so forth, presents no difficulty. But, in the second
place, we have to be specially careful that the testing-
stimulus itself shall be absolutely constant in successive
experiments. The problem then resolves itself into the
successful devising of some arrangement by which records
may be taken automatically at definite pre-determined
intervals of time. The stimulus of unvarying intensity
must also be made to act automatically upon the specimen.
Under these conditions any variation that may be observed

64

VARIOUS TYPES OF RESPONSE 65

in the record will be due to changes of excitability induced
as an after-effect of the stimulus itself.

Maintaining the stimulus-intensity absolutely constant
is not so easy to secure with a single make- or break-shock,
since the intensity of such a shock is liable to variation,
according to the degree of suddenness with which it is
effected ; but the total additive value of a group of such
shocks may be expected to be fairly constant. For this
reason, therefore, tetanising shocks caused by a vibrating
interrupter would be preferable, provided the duration of
these shocks, depending on the duration of closure of
current in the primary circuit, be maintained in successive
experiments rigorously equal. Such constancy cannot be
arrived at if the closure of the circuit be caried out by hand,
or even by metronome. Some special mechanical device
must therefore be adopted for this purpose.

It is further necessary, in order to maintain constancy of
conditions, that identical periods of recovery should be
allowed in successive records. For this the stimulus must
be applied at accurate and pre-determined intervals of
time. The ideal condition, then, for the final elimination
of all uncertainties due to the personal factor, is that the
plant attached to the recording apparatus should be auto-
matically excited by a stimulus absolutely constant, make
its own responsive records, go through its own period of
recovery, and embark on the same cycle over again without
assistance at any point on the part of the observer.

These demands have been fully met in the devices and
adjustments now to be described, consisting as they do
of two chief elements — namely, the Periodic Starter and
the Automatic Exciter. By the former the time-interval
between successive stimulations is regulated ; by the latter
the stimulation itself, of a definite duration, is applied.

The Periodic Starter

In the case of Mimosa recovery from excitation is
practically completed in a period of about 10 to 20 minutes.

66 RESEARCHES ON IRRITABILITY OF PLANTS

according to the season and the condition of the plant.
In practice, therefore, we require arrangements by which
successive stimulations can be automatically effected at
these intervals. As we require a slowly moving plate for
the purpose of these records, the plate-carrier is let down
by a thread which is wound round a wheel attached to the
minute-hand axis of the driving-clock. To the same axis
is also screwed one or other of the three separate discs,
bearing equidistant projecting rods, either 3, 4, or 6 in

Fig. 30. — Periodic Starter and Automatic Exciter.

number. During one complete revolution, which takes an
hour, these rods will press and release a spring at intervals
of 20, 15, or 10 minutes, as the case may be (fig. 30).

The Automatic Exciter

If the primary circuit of the induction coil provided with
a spring-interrupter be closed for a definite period of time,
say •! second, then the number of interruptions, with con-
sequent induction-shocks, will also be definite. What is
wanted is some contrivance for release, through which the
Periodic Starter can close the main circuit for a definite
length of time, say -i second. It might at first sight appear
that this could be secured by an electrical contact made
by the revolving radial-rods already referred to, but the

VARIOUS TYPES OF RESPONSE 67

period of such contact would be impracticably long —
more than 15 seconds.

So continuous a tetanisation would undoubtedly fatigue
or even injure the tissue. The contact made by a seconds-
hand, again, though sufficiently brief, would have the
serious defect that its movements were jerky and would
therefore make the duration of contact unequal.

I succeeded in overcoming these difficulties by using a
released revolving disc, which could be made to complete
an electrical circuit for any definite short period that was
required. For this I employed a phonograph motor, an axis
of which, carrying a disc, could be adjusted to revolve once
in a second.

This disc is usually held stationary by a lever-clutch,
and can be started only by the pressure on it of the re-
volving rod of the Periodic Starter. It is re-arrested after
one complete revolution and is not again released till the
next rod comes into position, after an interval of 10, 15, or
20 minutes, as the case may be. There is also attached
to the disc a sector whose arc is one-tenth the circum-
ference of the circle. This sector, during the revolution, will
press against a closing-key, the period of closure being then
•I second. By increasing or diminishing this arc the time of
closure, and with it the duration of the tetanising shock, can
be correspondingly changed. It is necessary that the sector
should, at the moment of the release, be at the greatest
possible distance from the closing-key. By the time it
reaches this key it will have acquired a constant and definite
velocity. Thus the periods of closure, and consequent
duration of the exciting-shock, will be identical in successive
experiments.

There is another possible source of variation which
must be guarded against. The electrodes of the secondary
coil are connected with the plant by means of moistened
threads. These threads, in long-continued experiments,
may become more or less dried up, the electrical resistance
being thus increased in an unknown manner. The intensity

F2

68 RESEARCHES ON IRRITABILITY OE PLANTS

of the exciting current may, under these conditions, undergo
a change. This difficulty has been overcome by a con-
trivance for keeping the thread uniformly moist. This will
be understood from the diagram (fig. 31) of the electrolytic
contact-maker : Two small cells are made of cork ; the upper
cell is filled with very dilute saline solution, a little of which
also lies in the bottom of the lower cell. A bent piece of
silver-wire coated with a deposit of chloride, and fixed to the
horizontal metalHc-rod, pricks through two cells and dips
into the solutions above and below. The moistened thread

coming through a hole near the
bottom of the upper chamber
makes one loop round that
portion of the plant speci-
men where electrical connec-
tion is desired, turns back
into the lower cell (which it
enters through the open aper-
ture), and dips into the saline
solution. It will be seen that
apart from capillary action,
owing to the upper cham-
ber being at a higher level,
the thread will be kept con-
stantly moist by the slow
streaming down of the solution. The current from the
coil, again, will have two entries by means of the
doubled thread, the resistance being thus halved. The
second electrode of the coil is connected with the other
contact-point on the plant in a similar manner. The
resistance offered by the plant tissue is relatively high,
being of the order of a million ohms. The resistance of
the electrolytic contacts, on the other hand, need be no
higher than a few thousand ohms. By thus making the
resistance of the moist contact relatively small, the total
resistance of the circuit remains practically the same,
especially since we guard against any variation that might

Fig. 31. — Electrolytic contact.

VARIOUS TYPES OF RESPONSE

69

Fig, 32, — Photograph of Duplex Resonant Recorder with plant and accessories.

70 RESEARCHES ON IRRITABILITY OF PLANTS

be induced in the moist thread by drying. The horizontal
rod holding the cork of the contact-maker is soldered to a
tube which can move up and down a vertical rod. These
adjustments enable the point of electrolytic contact to be
brought to a level with the point of connection on the
specimen. The rod is insulated on ebonite.

All the practical difficulties having thus been eliminated,
I shall now proceed to show the various records obtained
under this mode of periodic stimulation of uniform intensity.
In fig. 32 is given a photograph of the apparatus with its
accessories. The recorder is of duplex type, for taking two
sets of records at the same time.

Before entering upon the detailed consideration of the
results of these experiments, I may say that at the beginning
of this investigation my attention was roused by the
apparently capricious variations in the responses obtained
under conditions which were rigidly uniform.

A long-continued investigation through the different
seasons of the year has given me the clue to what at first
appeared to be so anomalous. The outward response, it is
obvious, is dependent on two factors — the intensity of the
impinging stimulus and the capacity for reply possessed by
the plant itself. It is easy to see that the second of these
factors must be dependent on the vigour of the plant, or
in other words, on its tonic condition, which in its turn is
modified by the environmental condition. Thus in unfavour-
able circumstances the plant may fall into an atonic or
sluggish condition. The absorption of energy from with-
out, by whatever form of stimulation, will improve the tonic
condition of the plant, with consequent enhancement of
excitability.

Taking a plant in a subtonic condition, then, we may
expect that any application of stimulus will increase its
excitability, a fact which will find expression in a growing
amplitude of response. This enhancement of excitability
will reach a limit at which the plant will be in an optimum
condition. After reaching this climax there may be a

VARIOUS TYPES OF RESPONSE 71

reversal, with decline of excitability, a state of things
which we associate with fatigue.

It must be remembered that in Nature, according to
the conditions of its environment, a plant may be found
in any of the three states. One specimen may be found in
the pre-optimum or subtonic condition ; another may be
near the optimum condition, and this we shall designate
as the normal ; a third may be found in the post-optimum
condition predisposed to fatigue. The first and third of
these conditions may be distinguished from each other by
means of testing blows or stimuH. If the plant be in the
former condition, these will evoke responses of increasing
amplitude ; in the latter, they will show a decline.

These three conditions modify not merely the amplitude
of response but also exhibit themselves appropriately in other
aspects of protoplasmic excitation. These will be seen in
the chapters on the Latent Period, and on the Transmission
of Excitation.

Uniform Responses

When — selecting a plant which is neither subtonic nor
yet at its optimum — we take a series of responses under
uniform stimulation of moderate intensity, allowing sufficient
intervals for complete recovery, we obtain uniform responses.
This may be accepted as the characteristic effect of a plant
in the normal condition.

In fig. 33 is seen a series of such responses taken at
intervals of 15 minutes. The ascending portion of each
response is here seen to be dotted. This is because of
the rapidity of the movement of fall. The successive dots
caused by the recorder vibrating ten times per second
are widely spaced. In the recovery or down part of the
curve, however, as that process is slow, the dots become
fused and make a thick continuous line. In the record of the
responsive fall, variations of rate of movement may be
noticed. At first the speed increases, then very gradually

72 RESEARCHES ON IRRITABILITY OF PLANTS

slows down, and the leaf becomes for a time stationary at
the apex. These varying rates of fall are seen in the
growing and then in the diminishing intervals between the
dots.

Fatigue

If instead of giving the full period of rest necessary
for complete protoplasmic recovery, the period of rest be

Fig. 33. — Uniform responses of Mimosa ; stimuli applied at
intervals of 15 minutes,

shortened, we obtain a diminution in the height of response
indicative of fatigue. This is well seen in fig. 34. The first
three uniform responses here — taken, as it is unnecessary
to repeat, under uniform stimulation — were recorded at
intervals of 15 minutes each. The intervals between succes-
sive stimulations were now shortened to 10 minutes, which
at once results in a fatigue-diminution of the height of
responses. The second three responses appear crowded
together, owing to the shortening of the time allowed for
record. The time of recovery, after the third of these
responses, was again restored to its first value of 15 minutes,
and we see at once the reversion of the response to its
original height. A similar exhibition of fatigue is also seen

VARIOUS TYPES OF RESPONSE

73

in muscle-records, in the same circumstances of diminished
interval of rest.

Under certain conditions we obtain an exhibition of
continuously growing fatigue. We have seen that when
the plant is intensely excited, it takes a longer time for
complete protoplasmic recovery. The specimen whose
responses are given in fig. 32 happened to be in an optimum
condition. A maximum ex-
citation was here induced,
even under a moderate stimu-
lus. The normal interval of
15 minutes, which was found
in the previous case to be
sufficient for complete proto-
plasmic recovery, here proved
to be insufficient. Hence we
have the exhibition of a
growing fatigue seen in the
diminishing heights of succes-
sive responses.

. Another very curious type
of response sometimes met
with, is that of alternating
fatigue. Here, while the first
response is very large, the
second is correspondingly
small, and this alternating
sequence is observed for a
longer or shorter time (fig. 36)
tions, however, the responses
An explanation of this interesting variation may be gathered
from careful observation of the record. The freshness of
the specimen and its high excitability account for the great
amplitude of the first response. An intense excitation
requires, as we have seen, a correspondingly longer time
than does a feeble one for complete recovery. Hence in the
present case the second stimulation is seen to have impinged

Fig. 34. — Fatigue under shortened
period of rest. First three
uniform responses obtained at
intervals of 15 minutes. The
second three, under shortened
period of rest of 10 minutes,
exhibit fatigue. On returning
to interval of 1 5 minutes, the
last record shows enhance-
ment.

After several such alterna-
tended to become uniform.

74 RESEARCHES ON IRRITABILITY OF PLANTS

on the organ before complete protoplasmic recovery has
had time to take place, during the usual resting-interval
of 15 minutes. The consequence of this is the diminished

■^^^- 35- — Record showing growing fatigue in Mimosa.

excitatory effect exhibited in the second response. As the
excitation in this case was relatively slight, the recovery
was very much more complete than in the first. The

Fig. 36. — Periodic fatigue ; alternate responses here tend
to become uniform.

to become uniform.

third response therefore was large, but not so large as the
first, when the organ was fresh. This excitation, however,
being less than the first, recovery is also somewhat more

VARIOUS TYPES OF RESPONSE

75

complete, and the subsequent fatigue is less than after
the first response. Therefore the fourth response, though
small, is not so small as the second. Thus while the first,
third, and odd series of responses are progressively diminish-
ing from a maximum, the even series — second, fourth, and
so on — are increasing from a minimum. In this way the
difference between the successive responses is tending to
disappear, a process which is practically complete in the
seventh and eighth, after which uniformity is attained.
It is very interesting to note that the sum of heights of
each pair of responses is approximately the same for succes-
sive pairs, and the height of a response in the uniform series
is not appreciably different from the mean of the maximum
and minimum of the preceding pairs, as will be seen from
the following table : —

Table giving Heights of Successive Responses

Number.

Height
of response.

Mean of
successive pair.

(I)

(2)

mm.

mm.
19

(3)
(4)

t,}

19.2

(5)
(6)

18.5)
i7-5f

18

(7)
(8)

i7-5|
17 ]

17.2

In this adjustment to uniformity we are able to watch
a tuning of the organ, as it were, its gradual accommodation
to the stimulus impinging upon it. Uniform responses may

76 RESEARCHES ON IRRITABILITY OF PLANTS

often be obtained in this way after a preliminary period of
variation.

The periodic variation seen in the above cases some-
times finds still more complex expression. This is the case
where waning and waxing occur in series instead of simple
alternation. That is to say, response may undergo a
continuous diminution in a sequence of three or more, to
be followed by a corresponding sequence in which the
amplitudes wax larger and larger, such serial alternations
being repeated.

Staircase Response

We have seen the responses that characterise highly
excitable specimens, in which there is an exhibition of
growing fatigue. Taking a specimen in the contrasted
condition of more or less sub-tonicity, we obtain an equally
characteristic effect, which is the antithesis as it were of
that which we have been considering. In this, successive
responses undergo a gradual enhancement, or what is known
in muscle-response — with which it is exactly parallel — as a
staircase increase (figs. 37, 38). After attaining a maximum
excitabiUty, under successive stimulations, there generally
ensues a fatigue-decline.

Before entering on a detailed description of this parti-
cular response it would be well to discuss certain pheno-
mena characteristic of a relatively a-tonic condition of the
tissue. In a specimen in the normal condition there is a
certain amount of tonicity, accompanied by a moderate
degree of contraction. When deprived of the invigorating
influences of favourable external stimuli the plant becomes
sub-tonic, such relative a-tonicity being characterised by
relaxation or the absence of normal tonic contraction.
Under the action of successive stimuli the tonic condition
of the specimen will be improved. The loss of tone, with
its consequent relaxation, will gradually give place to a
better tone with increasing tonic contraction. Or the

VARIOUS TYPES OF RESPONSE 'j'j

same improvement of tone might take the form of a
gradually increasing excitability. Hence the gradual better-
ing the tonic condition, under successive stimulations, may
often find two simultaneous expressions. In the first
place the growing tone, with its increasing normal tonic
contraction, will be seen in the shifting of the base-line
upwards. Secondly, it will be exhibited in the growing
amplitude of successive responses. These two features will

Fig. 37. — Preliminary staircase Fig. 38. — Staircase response

followed by fatigue in the followed by fatigue in

response of frog's muscle- Mimosa,
(Brodie.)

both be noticed in the record depicted in fig. 38. Here,
as might be expected, in a specimen in sub-tonic condition
we find that the first stimulus gives rise to a relatively feeble
response. But in consequence of stimulation the tonic
condition itself is improved, as demonstrated by the fact
that the leaf remains in a slightly more contracted attitude
than at the beginning. The next stimulus finds it in a
better tonic condition, with accompanying higher excita-
bility. Hence the response is larger. In this way the tonic

78 RESEARCHES ON IRRITABILITY OF PLANTS

condition reaches an optimum, with the attainment of
highest degree of excitabiHty. Here impinging stimulus
has evoked the maximum response.

We see in a general way that in these responses the
accession of stimulus has given rise to two kinds of effects,
external and internal, whose relative values have been
progressively changing. At the beginning a portion of the
stimulus was utilised to improve the tonic condition, the
complementary portion inducing external response. Hence
at the beginning the response was small. At the end of
the series, however, where the maximum tonicity has been
attained, the whole blow of the stimulus is utilised in
giving external response, which now therefore is maximum.
After this attainment of maximum excitability the usual
fatigue-decline is seen to have taken place.

We must nevertheless be on our guard against drawing
too hasty a conclusion, as regards the tonic condition, from
the relaxation or contraction seen in the record ; we should
remember that a relaxed condition is not only indicative of
a-tonicity, but may also be brought about by fatigue due
to over-stimulation. The changing position of the leaf,
owing to daily periodicity, should also be taken into
account. Bearing in mind, however, the immediately pre-
ceding history of the given plant, the experimenter will not
find it difficult to guard himself against wrong inferences.

Being desirous of ascertaining how far the theoretical
considerations here advanced would be borne out in extreme
cases, I tested a specimen which from appearances was not
at all vigorous and likely to be a-tonic. The record it gave
at the beginning, of increasing relaxation, probably indicated
its growing a-tonicity (fig. 39) . That it was lacking in tone
at once became evident from the fact that the first stimulus
— applied at the point shown by the thick dot — did not
evoke any response. But that this nevertheless did cause
improved tonicity, is seen from the fact that the former
rate of relaxation underwent a ^diminution, the record
tending to become more horizontal. The second stimulus

VARIOUS TYPES OF RESPONSE

79

was then effective in evoking a feeble response. The most
striking fact, however, is that on the completion of recovery
the specimen actually exhibited a growing contraction as
an after-effect of stimulus. Thus, while at the beginning
a growing condition of a-tonicity gave rise to increasing
relaxation, afterwards in consequence of stimulation this
state of things became reversed, and we have a growing
condition of tonic contraction appearing as the after-effect
of stimulus. That the tonic condition in fact became
improved is shown by the large
response evoked as the immediate
effect of the usual stimulation. i

This after-effect of a single
stimulus in inducing a second
contraction is significant as show-
ing the possibility of holding inci-
dent stimulus latent for a time, to
find expression later. It heralds
the phenomenon of Multiple Re-
sponse, which we shall consider in
a subsequent chapter.

The curious phenomenon of
alternation sometimes observed in
a highly excitable specimen has
already been noticed (fig. 36).
The characteristic peculiarity ob-
served there was a large response
followed . by a small one, such
alternation continuing for a time,
successive responses, however, vanished after a time. With
plants in a sub-tonic condition the phenomenon of alterna-
tion is also found occasionally. The characteristics here
exhibited (fig. 40) are in sharp contrast to those seen in
fig. 36. Here the first response is small, and the second

Fig. 39. — Record showing
the effect of stimulus
modifying tonicity, and
producing staircase
effect.

The difference between

^ When the specimen is extremely sub-tonic the sign of response
may even be reversed into abnormal erectile movement. After a period
of stimulation, however, the response is converted into normal.

8o RESEARCHES ON IRRITABILITY OF PLANTS

large, and the difference between the pairs in the series goes
on increasing. The sum of heights of pairs of successive
responses remains however approximately the same. The
odd numbers in the series decline continuously (i) 18*5,
(3) 16; (5) ii» (7) ^ '> whereas the responses in the even
series grow in ampUtude (2) 25*5 (4) 30, (6) 33.

Table giving Heights of Successive Responses

Number.

Height
of response.

Sum.

(I)
(2)

mm.
25-5/

mm.
44

(3)
(4)

16 1
30 j

46

(5)
(6)

II \
33 J

44

Fatigue-reversal under Tetanisation

If the Mimosa leaf be subjected to continuous stimulation
it has been found that, after the preliminary fall, it re-erects
itself in spite of the stimuli which are still acting upon it.
This at first sight would appear to be very perplexing,
but the apparent anomaly would however disappear when we
recognise the essential unity of response in the plant and the
animal. A frog's muscle, under continued tetanising electric
shocks, at first exhibits the normal contraction, but after-
wards relaxes, in spite of the excitation to which it is being
subjected (fig. 41). The difference between the normal
relaxation of recovery (expansion) and this fatigue-relaxa-
tion induced under continuous stimulation, lies in the fact
that in the former case response takes place on renewed
stimulation, while in the latter the tissue has become

VARIOUS TYPES OF RESPONSE

8i

irresponsive, and only after a period of rest can it exhibit
excitation. The same phenomena are observed in the
case of the contractile organ of Mimosa. Here also, after

Fig. 40. — Alternating response ; differences between
alternate responses become accentuated.

erection (expansion) under continuous stimulation, the
leaf is irresponsive and only renews its excitability after
a definite period of rest.

As regards this particular reaction in Mimosa, we are

Fig. 41. — Fatigue-decline in frog's muscle under continuous
stimulation. (Brodie.)

in a position to trace out the various phases through which
contraction under single stimulus is reversed to expansion
under fatigue induced by continuous stimulation. It has

82 RESEARCHES ON IRRITABILITY OF PLANTS

to be borne in mind that the effect of continuous stimulation
is, after all, the effect of successive stimuli with the resting
interval shortened. On referring back to fig. 38 we notice
two phases in the response-series : in the first phase the ex-
citability is increasing ; in the second phase, it is decreasing.
In the first phase again, we notice that there is a residual
contraction, the recovery being incomplete. Owing to this,
the base-line is gradually shifting upwards. This, coupled
with the enhancing excitability and consequent staircase
increase in the individual responses, brings about a maximum
additive contraction, as will be understood, by joining the
tops of these contractile responses. The additive effect of
such contractions would be a responsive fall much greater
than could have taken place under any single stimulation.

If we were now to repeat this experiment, shortening
the intervals between the successive stimuli, we should
obtain a somewhat similar result, with the sole difference
that the successive component responses would appear
nearer each other and with their recoveries still further
reduced. The result of this would be shght notches in an
ascending curve. Carrying this process to a limit — that is
to say, when the successive stimuli follow each other
quickly, as in continuous tetanisation — the notches them-
selves will disappear and we shall have merely an ascending
curve.

Turning to the second phase in the response-series, where
the excitability has reached a maximum, we find these
phenomena reversed. The leaf having attained its maxi-
mum limit of fall, its capacity for further contraction is now
reduced. In sharp contrast to the first phase of the series,
however, successive contractions now grow smaller and
smaller, under growing fatigue, while the relaxations tend
to become increasingly large. In the extreme case of con-
tinuous tetanisation the resulting record in this phase would
be one of relaxation, appearing as a down-curve. Thus
under tetanisation we should have a response-curve, showing
first the normal contraction, followed in the second place by

VARIOUS TYPES OF RESPONSE

83

Fig. 42. — Different phases
in the fatigue- reversal
in plant.

relaxation, not at first sight very different from the response-
curve due to a single stimulus. There would nevertheless be
an actual difference, inasmuch as the resulting contraction
under tetanisation would, on account of additive effect, be
greater than that caused by a
single stimulus. After the ap-
parent recovery, due to fatigue-
reversal under tetanisation, how-
ever, the excitability, as already
shown, is temporarily abolished ;
whereas after the normal re-
covery from a single stimulus,
excitability is fully restored.

The typical case, the detailed
consideration of which led us to these conclusions, was
that of a plant which was in a somewhat sub-tonic con-
dition. Had the plant been in the optimum condition
to start with, then following the same line of reasoning
we should expect that the curve of tetanisation would be
modified in a definite way. Referring back to fig. 35, which

gives successive records

of a highly excitable
specimen, we find in
this instance that the
very first stimulus
evoked the maximum
response, and that the
subsequent responses ex-
hibited fatigue. There
is not here, to begin
with, any staircase effect,
nor are the contractions
additive, the initial response being the greatest possible.
On increasing the frequency of stimulation we should, after
the first maximum response, obtain the phasic variation due
to fatigue. The successive contractile responses would thus
appear smaller and smaller, their respective recoveries being

Fig. 43. — Fatigue-reversal in Mi-
mosa. Lower record shows
response under single stimulus ;
upper figure exhibits response
under continuous stimulation.

G 2

84 RESEARCHES ON IRRITABILITY OF PLANTS

correspondingly larger and larger. This is clearly seen in
fig. 42, where the successive stimulations are applied at
intervals of seven minutes.

Thus on subjecting a specimen in an optimum condition
to continuous stimulation, we should expect to find that
the extent of contraction due to tetanisation was but little
different from that due to a single stimulus. This is verified
by the following pair of records (fig. 43) showing the response
of a plant near optimum condition, under single stimulus
and under tetanisation.

Summary

The contractile response of the pulvinus of Mimosa
exhibits characteristics similar to those of the response of
muscle.

Under normal conditions of the plant, and with suffi-
cient intervening periods of rest, the responses are found
to be uniform.

The responses exhibit fatigue under conditions of incom-
plete recovery.

The excitability of the plant in a sub-tonic condition is
enhanced by the action of the stimulus itself. Under such
conditions the responses exhibit a staircase increase.

The anomalous erection, after a preliminary fall of the
leaf of Mimosa under continuous stimulation, is explic-
able on the common characteristics of response in plant
and animal tissues. In both, contraction is reversed to
relaxation under fatigue.

CHAPTER VII

EFFECTS OF DIFFERENT GASES ON EXCITABILITY OF MIMOSA

Induced change of excitability under sudden variation of light —
Abolition of excitability by absorption of water — Restoration
of excitability by application of glycerin — Stimulating, depressing,
and toxic agents — Phenomenon of accommodation — Stimulating
action of ozone — Effects of : carbonic-acid gas, vapour of
alcohol, ether, carbon disulphide, coal gas, chloroform, ammonia,
sulphuretted hydrogen, laughing-gas, nitrogen dioxide, and sulphur
dioxide.

In order to investigate the effects of various gases in modi-
fying the excitabiUty of Mimosa, a series of responses, more
or less uniform, is first obtained under uniform stimuH,
at intervals of 15 minutes. The given gas is now introduced
into the plant-chamber, and another series of responses are
once more obtained by the action of the same stimuli as
before. The variation of amplitude of responses then
gives an indication of the excitatory or depressing action
of the agent.

In carrying out the experimental investigation in this
manner, we proceed on the assumption that the stimuli
apphed are invariable, and that the external conditions are
maintained constant, with the sole exception of the change
induced by the introduction of the given gas. In order to
complete a single investigation a period of nearly two hours
is often necessary, which is the time required to take eight
responses at intervals of 15 minutes. Of these, the first two
give the normal responses, the next four the modified
responses under the influence of the gas, and the last two

85

86 RESEARCHES ON IRRITABILITY OF PLANTS

exhibit the after-effect on the removal of the gas. It will
thus be understood how important it is to maintain the
external conditions constant for so long a period as two
hours. The method of maintaining the testing stimulus
constant has already been explained. With special care
the temperature of the plant-chamber can also be kept
uniform. The other factor which is liable to variation
is the intensity of light. I have often noticed a fluctuation

Fig. 44. — Effect of sudden darkness on excitability of Mimosa.
First three responses, normal ; four succeeding responses
due to effect of darkness. Line below indicates period of
darkness. Vibration frequency of writer, five times per
second.

in the uniformity of responses which was traceable to a
passing cloud. I soon found that a sudden change in the
intensity of light induces a marked variation of motile
excitability in Mimosa. Thus on bringing a highly sensi-
tive plant to a dark room its excitability is found to dis-
appear. This aboHtion of excitability is generally speaking
temporary, since the plant often regains its sensitiveness
after about an hour, though still kept in the dark. The
fact that under normal conditions it is the sudden diminu-
tion of light rather than darkness that induces depression

EFFECTS OF DIFFERENT GASES 87

of excitability, is borne out by the fact that the plant is
fully sensitive at night.

Effect of Sudden Darkness

In order to demonstrate the variation of excitability
induced by sudden diminution of light, I first took a set
of three normal responses in diffuse daylight. The plant-
chamber was then suddenly darkened by means of an opaque
screen. It will be noticed (fig. 44) that the next two responses
were nearly abolished ; the excitability of the plant was
however beginning to be restored after 45 minutes' exposure
to darkness. After an hour in darkness the excitability was
fully restored, the response here being even larger than in
light.

In order to guard against the disturbing effect of varia-
tion of light it is advisable to carry out the following
experiments in an open veranda, the plant being kept in a
chamber with frames of ground glass. In this way the
plant is maintained under diffuse light of fairly uniform
intensity.

Effect of Absorption of Water

Another peculiarity I noticed in Mimosa was a depression
of excitability on rainy days. This effect I was afterwards
able to trace to the absorption of water by the pulvinus.
The variation of motile excitability by absorption of water
is very clearly exhibited in the accompanying record (fig.
45). A pair of normal uniform responses were first taken.
A drop of water was then applied on the pulvinus, when the
leaf was recovering from the second stimulus. It will be
noticed that the period of recovery became very much
protracted in consequence of absorption of water. The
usual time for complete recovery is about 15 minutes. In
the present case it was prolonged to 45 minutes. Testing
stimuU were applied at the usual intervals of 15 minutes,
the moments of application being represented by thick

88 RESEARCHES ON IRRITABILITY OF PLANTS

dots. It will be seen that there is an abolition of
excitability, stimuli which were formerly effective becoming
now quite ineffective.

I next tried to find out whether it were possible to restore
the lost excitability by artificial means. Guided by the
consideration that glycerin has the power of abstracting
water, I applied a drop of strong glycerin to the pulvinus.
It will be noted that this had the effect of quickly restoring
the motile excitability of the pulvinus. The two responses

Fig. 45. — Abolition of motile excitability of pulvinus by absorp-
tion of water. Note prolongation of period of recovery and
ineffectiveness of stimuli applied at moments marked with
thick dots. Subsequent restoration of excitability by applica-
tion of glycerin.

after the application of glycerin are practically similar
to the normal responses at the beginning of the series. I am
unable to say whether the restoration of excitability was
here due entirely to the abstraction of water. One might
think that continuous abstraction of water would induce
a continuous variation of excitability — ^probably an enhance-
ment reaching a maximum followed by a decline. I find,
however, that the appHcation of glycerin restores the normal
excitability, and that generally speaking this remains
constant even under the continued action of the reagent.
This is a fortunate circumstance for those particular investi-

EFFECTS OF DIFFERENT GASES 89

gallons where it is required to make an electrolytic contact
with the pulvinus without inducing any change in its motile
excitability.

I shall now proceed to describe the effects of various
gases and vapours on the excitability of Mimosa. The
plant is enclosed in a small glass chamber, the different
gases being made to stream in and out through entrance
and exit tubes. The various effects induced may be classi-
fied as (i) stimulating, (2) depressing, and (3) toxic. The
exaltation of excitability induced by stimulating agents
is exhibited by the enhancement of amplitude of response.
The effect of depressing agents is seen in the diminution of
amplitude of response ; in this class may be included agents
which have slight narcotic action. In all these cases the
removal of the gas is attended by the restoration of normal
excitability of the plant. A curious fact noticeable in this
connection is the phenomenon of accommodation. Under
the action of a slightly depressing agent, there is induced a
diminution of excitability. But the plant may accommo-
date itself to the change, in consequence of which the
excitability is more or less restored to the original condition.
It should also be borne in mind that the character of the
reaction is modified to a certain extent by the tonic con-
dition of the plant, a plant in a vigorous condition being
better able to withstand unfavourable circumstances than
one in a weak condition.

Lastly, there are gaseous agents which are toxic in
their action ; their application is attended by rapid loss of
excitability and death of the plant. I will now describe
in detail the effects of various gases, beginning with those
which stimulate and ending with others which cause the
death of the plant.

Ozone

The stimulating effect of this gas is clearly seen in
fig. 46. The particular leaf, before the application of

90 RESEARCHES ON IRRITABILITY OF PLANTS

ozone, was showing signs of fatigue, as evidenced by the
gradual diminution of the heights of successive responses.

Fig. 46. — Stimulating action of ozone.

Fig. 47. — Effect of carbonic-acid gas.

The introduction of ozone brought, however, an immediate
change ; the induced enhancement of excitabihty is seen

EFFECTS OF DIFFERENT GASES 91

in the growing amplitude of successive responses till a limit
was reached.

Carbonic-acid Gas

The effect of undiluted carbonic-acid gas is a depression
of excitability. This is seen in the present record (fig. 47),
where on the application of this gas the amplitudes of suc-
cessive responses are seen to undergo a decline. Another
noticeable fact is the incompleteness of recovery after each

Fig. 48. — Effect of vapour of alcohol ; note alternating
character of response after application.

excitation. The plant-chamber was next refilled with
fresh air, and we observe the restoration of normal
excitability.

Vapour of Alcohol

The immediate effect of dilute vapour of alcohol is
sometimes a transient enhancement of excitability. But
continued action of the vapour induces a depression. In
the accompanying record (fig. 48) there was little immediate
effect ; but after an application of 15 minutes there was
induced a depression of response ; another effect also

92 RESEARCHES ON IRRITABILITY OF PLANTS

noticeable is the alternating character of the response that
took place after the application of alcohol.

Ether

The vapour of ether induces a depression of excitability
as seen in the diminution of amplitude of response. The
first effect of dilute ether-vapour is often a short-lived
exaltation. The narcotic effect of this agent on Mimosa
is feeble compared with that induced by chloroform. The

Fig. 49. — Effect of ether,

depressing effect of ether passes off on readmission of
fresh air (fig. 49).

Carbon Bisulphide

The effect of vapour of carbon disulphide is similar to
that of ether. It induces a depression during the introduc-
tion of vapour into the plant-chamber ; the induced depres-
sion, however, passes off on restoration by means of fresh
air (fig. 50).

Coal Gas

Contrary to my anticipation, coal gas proved to be
but moderately depressing in its action. I have kept the
plant surrounded by this gas for more than two hours

EFFECTS OF DIFFERENT GASES 93

without the abolition of its excitabiUty. The effect of
the gas is very different when it contains impurities such
as sulphuretted hydrogen. I give a record (fig. 51) which

Fig. 50. — E£fect of carbon disulphide.

Fig. 51. — Effect of coal gas : note irregularity of response
after introduction.

exhibits the depressing effect of coal gas, and the gradual
restoration of normal excitability on admission of fresh air.

Chloroform

The vapour of chloroform acts as a very strong narcotic.
In the record here given (fig. 52) the response became very

94 RESEARCHES ON IRRITABILITY OF PLANTS

much reduced immediately after application ; the power
of recovery was also abolished. Subsequent application
of stimulus did not result in any sign of response. Even
on blowing off the vapour there was no restoration of

excitability for a very consider-
able period. In the present
case the period of total insen-
sibility lasted for six hours,
after which the excitability was
slowly restored.

Ammonia

The vapour of ammonia is
found to cause an abolition of
excitability in a very short
time. On the introduction of
ammonia there is produced an
excitatory fall. This may be
avoided, however, by introducing this vapour immediately
after the excitation induced by the testing stimulus.
In the record here given (fig. 53), the first two are the
normal responses. Introduction of ammonia is seen to
induce an abolition of excitability, three successive stimula-
tions, represented by thick dots, at the usual intervals of
15 minutes proving to be quite ineffective. On blowing
off the vapour the excitability is seen to be very gradually
restored. If stronger vapour of ammonia be employed,
then the loss of excitability lasts for several hours.

Fig. 52. — Abolition of excita
bility under chloroform.

Sulphuretted Hydrogen

The effect of this gas is not merely depressing but
extremely toxic. It can be seen from the record that the
introduction of this gas caused the period of recovery to be
very protracted. The abolition of excitability is evidenced
by the fact that successive stimulations at the usual interval

I

EFFECTS OF DIFFERENT GASES

95

of 15 minutes, proved to be quite ineffective (fig. 54). The
action of this gas was so poisonous that restoration of fresh
air did not bring about any revival. The plant was subse-

FiG. 53. — Abolition of excitability under the action
of ammonia.

Fig. 54. — Total abolition of excitability and death of plant
under the action of sulphuretted hydrogen,

quently found to have died under the poisonous effect of
this gas.

Nitrogen Dioxide

Nitrogen monoxide or laughing-gas has but little effect :
there may, however, be a slight excitatory action. Nitrogen

96 RESEARCHES ON IRRITABILITY OF PLANTS

dioxide, on the other hand, is extremely poisonous. Intro-
duction of this gas was attended by an immediate excitatory
fall, which was repeated twice. After this the plant became
perfectly insensitive (fig. 55) ; the gas had in reality killed it.

Sulphur Dioxide

Equally fatal is the effect of sulphur dioxide. Intro-
duction of the gas was attended by an immediate excitatory

F1G.55. — ^Toxic effect of
nitrogen dioxide : applica-
tion of gas at X induced
excitation, followed by loss
of excitability and death
of plant.

Fig. 56. — Abolition of ex-
citability and death of
plant by the action of
sulphur dioxide.

fall of the leaf, after which it became quite insensitive
(fig. 56). Restoration of fresh air did not revive the
plant, which succumbed completely to the toxic action of
the gas.

bUMMARY

There is in general a temporary depression of excitability
in Mimosa under sudden diminution of intensity of light.
Absorption of water induces a depression or abolition

EFFECTS OF DIFFERENT GASES 97

of the motile excitability of pulvinus. Excitability is
restored under application of glycerin.

Ozone enhances the excitability of Mimosa.

Carbonic-acid gas and vapour of alcohol induce a
moderate depression of excitabihty, which is fully restored
on admission of fresh air.

Depression of excitability is also induced under the
action of coal gas, and vapour of carbon disulphide.

The vapour of ether exerts a moderate narcotic action.
The effect of vapour of chloroform is very pronounced,
loss of excitability under its action being prolonged.

x\mmonia induces a marked abolition of excitability.

Sulphuretted hydrogen, nitrogen dioxide, and sulphur
dioxide aboHsh the excitability and bring about the death
of the plant.

CHAPTER VIII

DEATH-SPASM IN PLANTS

Criterion of the death of plant — Abolition of electric response at death —
Mechanical spasm of death — Water-bath for uniform rise of tempera-
ture— Excitatory effect of sudden cooling or heating — Erection of
leaf with rising, and depression of leaf with falling, temperature —
Thermo-mechanical inversion at the death-point — Necessity for
specification of rate of rise of temperature — Death record of Mimosa —
Abolition of response after death-spasm — Constancy of death-point
exhibited by different specimens — Death records of Desmodium
gyransdind. Vicia Fava — Death-spasm in ordinary plants — The electric-
spasm of death — Lowering of death-point by fatigue and by poisonous
solution.

A PLANT may be killed by subjecting it to a certain maxi-
mum temperature. The exact moment at which death is
initiated is difficult to determine, since there has been found
no certain and immediate criterion of death. One method
by which the occurrence of death may be determined is
by the abolition of that electric response which is characteris-
tic of the living condition. A plant as long as it is alive
gives in answer to a stimulus an electric response of galvano-
metric negativity. On the occurrence of death this parti-
cular response disappears. I find that the electric response
is abolished when the plant has been subjected for a time
to a temperature of about 60° C.

If the plant is subjected to a gradual rise of temperature,
there would arrive a time when the death-change will
begin to occur. In the animal an early symptom of death
is the setting in of rigor mortis. We shall find that in plants
also a death-spasm, analogous to the death-throe of the
animal^ occurs at a critical moment.

In order to obtain an automatic record which would

98

DEATH-SPASM IN PLANTS 99

indicate the beginning of death-change, I first took a specimen
of Mimosa and subjected it to a gradual rise of temperature
in a water-bath. The leaf was attached to the recording-
lever in the usual manner. The recording apparatus
employed was of the oscillating type, where the plate
oscillates to and fro by an electro-magnetic contrivance,
thus producing a series of dots in the response-curve. In
the present investigation the electro-magnetic circuit is
completed for a brief period, at every degree rise of tempera-
ture in the bath. Successive dots thus represent intervals
of temperature of 1° C. The ordinate of the curve indicates
expansive or contractile movement of the leaf : down-curve
representing the expansion, and up-curve the contraction.

The temperature of the bath is continuously raised by
the application of gas or spirit flame. For certain reasons,
to be presently explained, it is necessary to raise the tem-
perature gradually and continuously, without any sudden
variation. There should also be no mechanical disturb-
ance of water in the bath during heating, as that would
disturb the leaf and vitiate the record. These difficulties
are overcome by constructing the heating-bath of two
vessels, one placed within the other. Heating the water of
the outer vessel raises the temperature of the water in the
inner in a very even manner, and without any mechanical
disturbance.

It is necessary to subject the plant to gradual rise of
temperature in order to protect it from excitation. Any
sudden variation, due either to lowering or raising of tem-
perature, causes excitatory movement of the leaf. This is
seen in the following records (fig. 57), obtained with Mimosa.
The first response is of excitation due to application of a
drop of ice-cold water on the pulvinus ; the second response
is due to the very opposite treatment of application of a
drop of hot water. In both cases we obtain the excitatory
fall of the leaf.

The effect of temperature as such is, however, very
definite : gradual rise of temperature inducing progressive

H 3

100 RESEARCHES ON IRRITABILITY OF PLANTS

erection of the leaf ; gradual lowering of temperature, on
the other hand, inducing progressive depression of the leaf.
Thus the effect of temperature, as such, is expansion with
rise and contraction with fall. These opposite effects of
erection and fall are progressive and slow. Excitatory
reaction, on the other hand, is sudden and always attended
by the contractile fall of the leaf.

The Mimosa used for experiment may be an entire
plant ; or, if more convenient, a cut branch containing
a leaf may be employed. The result obtained is the same
in both cases. It is found that during continuous rise of

Fig. 57. — Excitatory response of Mimosa induced by sudden
application of either cold water (C) or hot water (H) .

temperature the leaf is erected till it reaches a critical
temperature at which the expansion is converted into
a spasmodic excitatory contraction. The curve is thus
V-shaped, the turning-point of the thermo-mechanical curve
being very sharp and definite. Under constant conditions,
the critical point of inversion is also very definite. The
sudden inversion marks the initiation of the death-change.
Here it is necessary to bear in mind certain conditions
for the securing of definite results. It is obvious that death
will ensue if a plant be placed in an unfavourable environ-
ment as regards temperature for a prolonged period. But
as such a temperature would only cause the death of the
plant by indirect and cumulative action, it cannot be said to

DEATH-SPASM IN PLANTS loi

constitute the death-point. For precision in such a deter-
mination it is necessary to discover a temperature which is
of itself efficient to initiate an abrupt death-change. On the
other hand, there must be a certain latent period after the
expiration of which the change would be outwardly mani-
fested. An interval will elapse, moreover, during which
the tissue is attaining the temperature of the bath. If the
rate of rise of temperature be too rapid, then, owing to the
lag caused by the two factors, by the time the death-spasm
commences the recorded temperature may have gone
beyond the actual death-point.

There are thus two points which are somewhat anta-
gonistic. In the first place, in order to obtain the immediate
point of death it is necessary that the plant should undergo
an exposure which is not too prolonged. Nevertheless, to
make due allowance for the latent period and for attainment
of the surrounding temperature, the rate of rise of tempera-
ture must be gradual. In the case of tissues which are
not too thick, the latter condition is sufficiently fulfilled by
a rate of rise of i"^ C. per minute. For the precise determina-
tion of the death-point the rate of rise of temperature must
be specified. It must also be borne in mind that after the
initiation of the death-change a certain time must elapse
before the whole mass of tissue in the interior is killed.
With a thick mass of tissue, owing to its inefficient thermal
conductivity, the attainment of the surrounding temperature
and occurrence of death throughout the tissue will be a
protracted process.

The definite rate of rise of temperature may be simply
secured by moving the heating flame nearer to, or further
from, the bath. With thin organs, such as the pulvinus of
Mimosa, I find that a spasmodic contraction takes place at
or very near 60° C, when the rate of rise of temperature is
approximately 1° C. per minute. This is seen in fig. 58 ;
the record was commenced at 25° C, and the successive dots
in the record are at intervals of 1° C. The down-curve
indicates the expansive erection of leaf. As soon as the

102 RESEARCHES ON IRRITABILITY OF PLANTS

temperature had reached 60° C. there was an abrupt
inversion, and the spasmodic contraction took place at a
very rapid rate. The successive dots in the up-portion of
the curve are at intervals of -2 of a degree. The point of
inversion, as we shall see, indicates the death-point, and
the curve giving the death-record we shall call the D£;\rH-
CURVE. It should be remembered that the particular

Fig. 58. — Death-curve of Mimosa. Successive dots in down
or expansive part of curve represent rise of temperature
of 1° C. Spasmodic contraction causing inversion of
curve takes place at 60° C.

electric response characteristic of living condition of the
tissue is found to disappear after the tissue had been
subjected to the temperature of 60° C.

If the sudden contraction that takes place at 60° C.
should prove to be the death-spasm, then this should
be the last response given by the plants. If we raise the
temperature of the plant short of the death-point, say to
45° C, we get a continuous responsive expansion ; when
cooled the leaf recovers, to a greater or less extent, its

DEATH-SPASM IN PLANTS 103

former position. If the temperature be raised again, there
is once more a growing erection, and when the death-
point is reached there is a sudden spasmodic contraction.

But if the specimen once passes through the tempera-
ture at which the spasm takes place, then there should be
an abolition of all further response, proving the sudden
contraction at 60° C. to be the spasm of death. Thus, after
obtaining the sudden inversion of the curve at 60° C. in
the last experiment, the plant was kept at that temperature
for 15 minutes. Cold water was now substituted in the
bath, and the record was taken once more of the effect of rise

Fig. 59. — Abolition of response to warming or cooling in specimen
which had passed the death-point.

and fall of temperature. A record is reproduced (fig. 59)
which exhibits the result. In the lower curve is shown the
record of effect of rise of temperature from 45° to 65° C,
and in the upper the effect of cooling from 60° to 45° C.
It is seen that while in the last experiment the plant
exhibited a spasmodic contraction at 60° C, there is no
trace of such an effect in the present case. The very slight
movement observable in the two curves is the physical
effect of heating and cooling, quite negligible compared
with the physiological erectile movement due to warm-
ing and the subsequent spasmodic contractile movement
heralding the initiation of death-change.

104 RESEARCHES ON IRRITABILITY OF PLANTS

In order to discover how constant is the death-point, I
repeated the experiment with numerous other specimens.
We have seen that the pulvinated organs present in the
leaves of Desmodium gyrans and the bean plant (Vicia
Faba) exhibit responsive movement under excitation. In
fig. 60 is depicted the death record taken under standard
conditions with the leaf of Desmodium. The record was
commenced at 35° C. ; it is seen that thermo-mechanical
inversion took place at 61° C.

Fig. 60. — Record showing death-point of Desmodium
at 61° C.

The next figure (fig. 61) shows the record with the leaf
of bean plant. Here the responsive movements are very
large. The inversion is seen to take place at 60° C.

The occurrence of death-spasm may also be shown by
means of ordinary plants. If we take a hollow tubular
organ, such as the hollow leaf-stalk of gourd or hollow
flower-stalk of any other plant, cut it in the form of a
spiral and subject it to the rising temperature of the bath,
there is noticed at first an expansive movement of uncurhng
of spiral. On reaching the death-point, however, the former
movement is suddenly reversed to one of curling.

Flowers like French marigold exhibit death-spasm by
sudden movement of opening or closure.

By employing the electric mode of investigation I

DEATH-SPASM IN PLANTS 105

found that there is induced an electric-spasm at the onset
of death. When the temperature is rising, a given point
of the plant-tissue exhibits increasing galvanometric posi-
tivity, till at the critical temperature there is a sudden
electric inversion into galvanometric negativity. The

Fig. 61. — ^Thermo-mechanical inversion indicating
death-point at 60° C. in leaf of bean plant.

electric-curve of death is of the same type as the thermo-
mechanical curve. With specimens of Musa and Amaranth
the death-point was found to be 59*5° C.^

As the death-spasm is a form of physiological response
we should expect the curve of death to undergo modi-
fication under physiological variation. One such modifica-
tion would lie in the translocation of the point of inversion,
or the displacement of the death-point. Thus age has
some influence, the death-point of very young specimens
being lower than that of mature ones. I shall demonstrate

^ Bose : Comparative Electro-Physiology , p. 202. Longmans, London,
1907.

io6 RESEARCHES ON IRRITABILITY OF PLANTS

the influences of other agencies, such as fatigue or poisonous
drugs, in the displacement of the death-point.

I have aheady given a record which showed the death-
point of the leaf of bean plant to be 60° C. under normal
conditions. Employing a similar specimen, fatigue was
induced in it by means of tetanising electric-shocks ; the
death record was then taken in the usual manner. It

Fig, 62. — Lowering of death-
point under fatigue ; death-
spasm took place at 37° C.

Fig. 63. — Effect of poison
in lowering the death-point.

will be seen (fig. 62) that in this particular case, on account
of fatigue, the death-point was lowered from the normal
60° C. to 37° C, that is to say, by as much as 23° C. The
lowering of the death-point, I find, is determined by the
extent of fatigue.

In order to discover the effect of poisonous solutions on
the death-point, I subjected a specimen of the bean leaf
to dilute copper-sulphate solution and took its thermo-
mechanical record (fig. 63). The effect of the poisonous
agent is clearly demonstrated by an appropriate lowering of
the death-point, in this case from the normal 60° C. to
42° C, or by 18° C.

DEATH-SPASM IN PLANTS 107

Summary

The electric response of galvanometric negativity is
characteristic of the living condition of the vegetable
tissue. Dead plants do not exhibit this characteristic
electric-response.

When a plant is subjected for a time to a temperature
of 60° C. its electric response disappears, such abolition
being indicative of the death of the plant.

A leaf of Mimosa subjected to abrupt variation of
temperature — either sudden cooling or sudden warming —
exhibits excitatory reaction. But if the temperature be
gradually raised, there is a progressive erectile movement
of the leaf ; gradual cooling induces a depression of the leaf.

When the leaf of Mimosa is continuously raised in
temperature, then at a critical point the erectile expansive
movement is suddenly converted into one of spasmodic
contraction. This inversion takes place under standard
conditions at or about 60° C. After this the response of
the plant is permanently abolished.

Various other plants, sensitive and ordinary, exhibit this
characteristic death-spasm at or about 60° C.

In taking an electric record it is found that an electric-
spasm also takes place at the critical temperature, which is
very near 60° C.

The death-point of the plant is lowered under physio-
logical depression. Thus under fatigue induced by tetanising
electric-shocks, the death-point was lowered from the
normal 60° C. to 37° C.

Poisonous reagents also lower the death-point. In a
particular case poisonous solution of copper sulphate
lowered the death-point by 18° C.

CHAPTER IX

DETERMINATION OF THE LATENT PERIOD

Difficulties of accurate determination of Latent Period — Advantages of
Resonant Recorder — Simultaneous tracings of tuning-fork exciter
and Resonant Recorder — Automatic stimulation at a definite
moment — Identical value of latent period in successive determina-
tions— Accurate measurement of time-interval shorter than '005
second — Latent period little affected by inertia of recorder — Tabular
statement of value of different specimens of Mimosa — Effect of
season on latent period.

When the motile pulvinus of Mimosa is subjected to an
exciting shock, a short time elapses between the incidence
of this shock and the initiation of the responsive movement.
This short interval is known as the Latent Period. In a
responding muscle, similarly, contraction does not occur
instantaneously on the application of stimulus. The latent
period in this case is determined from the record of the
muscle-twitch. When after the appHcation of stimulus the
muscle has not yet begun to contract, the record appears
as a straight line. Then on the commencement of con-
traction, the recording-lever is jerked up and the curve
likewise bends upwards. The length of the straight portion
of the record, between a mark that represents the incidence
of the shock and the flexure at the initiation of response,
gives us the duration of the latent period. We have,
however, to determine the time-value of this length. This
is done by means of a sinuous curve drawn below the record
by a tuning-fork, vibrating 100 or 200 times in a second.
Experimenting in this manner, the latent period for frog's
muscle has been determined at about "oi second.

There are still several difficulties to be encountered
in making this determination with any great exactness.

108

DETERMINATION OF THE LATENT PERIOD 109

As the muscle-record and the time-record are separate,
certain error is Hkely to be introduced in inferring the
time- value of any point on the muscle-curve (fig. 64).
This error becomes relatively serious when the total time
to be measured is very small. There is, again, the difficulty
of exactly determining the point of flexure which represents
the beginning of mechanical response. More troublesome
still is the error due to the inertia of the recording-lever.
On account of this and the mechanical inertia of the respond-
ing muscle itself, the latent
period thus obtained appears
somewhat in excess of the
true value.

In the apparatus which I
employed, these difhculties
have been reduced to a
minimum. In the first
place, the curve of response
or phytogram is at the same
time a chronogram. The
error which might arise
from an inference based on
a neighbouring time-record
is thus eliminated. I will later explain also the means
that make it possible to determine the point of flexure,
representing the beginning of the responsive movement,
with relative accuracy. And lastly, the error due to the
inertia of the recording part of the apparatus is reduced
to a minimum by making the writing-lever excessively
light. In the muscle recorders the weight of the recording-
lever is about 3-5 grams. The lever which I employ weighs
only *04 gram. The recording part of my apparatus is thus
nearly a hundred times lighter than that used for muscle
records.

The accuracy of the time-record when made by the
response recorder itself may be gauged from records giving
simultaneous tracings of the exciting standard tuning-fork

Fig. 64. — Latent period of hyoglos-
sus muscle : aa, moment of
stimulation ; ab, latent period.
Time tracing 200 per second.
(Brodie.)

no RESEARCHES ON IRRITABILITY OF PLANTS

(loo D.V.) and the resonant vibration in the recorder
induced by it. This latter had been previously tuned to
give exactly loo double vibrations in a second. A light
aluminium stylus attached to the tuning-fork traced a
sinuous line on a falling plate of smoked glass. The top
of the vibrating recorder was so adjusted as to make suc-
cessive dots during its vibration, simultaneously with the
tuning-fork tracings. It will be seen from the record
(fig. 65) that, corresponding to the crest of each tuning-
fork wave and slightly to its right, we have a dot. The
record given represents a period of fourteen one-hundredths
of a second, there being fourteen crests made by the tuning-

FiG, 65* — Simultaneous record of vibrating-recorder and
100 D.V. tuning-fork exciter.

fork time-marker, and exactly coincident with these are
the fourteen dots made by the vibrating recorder. The
interval between any two dots, therefore, is an accurate
measurement of one-hundredth part of a second. If the
plate be moving at a uniform rate, the interval between
these dots will be uniform. But the accuracy of the time-
measurements in the curve is independent of the rate of
movement of the plate, for we calculate not by the distance
but by the number of the dots. In the present figure the
record was made on a plate which had been released and
during its fall was acquiring increasing speed. The tuning-
fork waves are thus gradually broadening out, and in exact
correspondence to this the intervals between the dots are
lengthening. When the phonograph motor which lets
down the plate is just released, there is a short interval
during which both that and the dependent plate are

DETERMINATION OF THE LATENT PERIOD iii

acquiring increasing velocity. After this the velocity
becomes uniform. If this uniformity should be required
throughout the record, the tracing of response may be
taken during this later period only.

The mode of procedure, therefore, is first to make the
recording-writer vibrate at its own definite frequency of,
say, 100 times per second. The recording-plate is then
released and later, when its motion has become uniform,
we pass through the pulvinus an electrical stimulus of an
instantaneous break-shock. There should be a mark made
on the recording-plate corresponding exactly to the moment
of stimulation. The horizontal record, consisting of a
series of dotted points representing one-hundredth of a
second, is suddenly deflected upwards on the initiation
of the responsive fall of the leaf. The number of dots
intervening between the mark of stimulation and this point
gives us the value of the latent period for the specimen.

Stimulation cannot be effected by hand at any exact
predetermined point on the record. This must be done
automatically by the moving plate itself. We cannot again
give an instantaneous break-shock without previously com-
pleting the primary of the Ruhmkorff coil, which causes
a disturbing make-shock.

In order to avoid this the secondary electrodes, during
make, should be short-circuited by means of a thick con-
ducting-wire ; the secondary shock is thus practically
diverted from the plant through the path of least resistance,
which is the conducting wire. All these requirements
are provided for in practice by the special mechanical
devices of the apparatus.

^Device for Automatic Stimulation

The essential parts of the automatic arrangement by
which a break-shock is given, at a predetermined point on
the recording-plate, are shown in fig. 66.

The recording-plate is allowed to drop by pressing the

112 RESEARCHES ON IRRITABILITY OF PLANTS

handle K. The winding disc is attached to the revolving
axis of a phonograph motor. The disc is wound in a right-
handed direction, which at the same time winds the spring

i-

Fig. 66. — ^Apparatus for determination of^ latent period of Mimosa : m,
spring motor; w, winding disc; c, projecting catch; k, release-
handle, pressure of which also completes primary circuit of induction-
coil ; K^, short-circuit key. The automatic break consists of contact-
rod adjusted by micrometer-screw a.

of the phonograph motor. The circumference of the disc
is the same as the length of the recording-plate. One
complete turn pulls the recording-plate up to its highest
position. A projecting catch below the disc is caught by

DETERMINATION OF THE LATENT PERIOD 113

a pin attached to the spring-handle K, when a complete
turn has been made : the recording-plate is thus held
arrested at its highest position. When desired, a pressure
on the handle k releases the disc, the axis of the motor
begins to unwind, and the plate is allowed to fall. The
motor is fully wound at the beginning, and the partial
unwinding during one revolution is exactly compensated
before the next observation by the winding necessary to
pull up the plate. Owing to the constancy of this winding,
the rate of fall in successive experiments is kept the same.
The pressure of the handle K, which releases the plate,
also causes ' make ' of the current in the primary coil. This
circuit of the primary coil is completed in addition through
a contact-breaking device.

This consists of a long strip of ebonite, fixed along
one edge of the recording-plate carrier. On the lower
end of the ebonite a conducting-strip of platinum is sunk
in and provided with a binding-screw. In front of this
slides a rod with contact-point tipped with platinum.
This can be adjusted up or down by means of a fine micro-
meter-screw, A. When the recording-plate is released,
carrying with it the conducting-strip, the primary circuit
is broken as soon as the line of junction between platinum
and ebonite is reached. This sudden interruption of the
primary current gives rise in the secondary coil to an
instantaneous break-shock, which passes through the plant.
In order that shocks in successive experiments shall always
be given at the same definite predetermined position in
the fall of the plate, the following device is adopted : The
recording-plate, as we have seen in a previous chapter,
slides up and down a vertical support of triangular section.
A movable peg fixed in the support holds it temporarily
at a certain selected point chosen as that at which, during
the descent of the plate, the shock is to be given auto-
matically to the plant. For the purpose of adjustment a
galvanometer is interposed in the primary circuit. So
long as the point of contact-rod is in touch with the

114 RESEARCHES ON IRRITABILITY OF PLANTS

conducting-strip, so long there will be a deflection in the
galvanometer. By means of its screw-adjustment, the rod
is gradually raised till the line of junction between platinum
and ebonite is exactly reached ; the deflection in the
galvanometer will now cease suddenly. In this way the
point of interruption or * break * is determined with precision.
By puUing the thread in connection with one arm of the
recording-lever, we then trace a slightly curved line on the
smoked plate. This indicates the exact position in succeed-
ing records of the moment of application of stimulus. This
mark of stimulation is shown in the printed records as a
vertical line. After making this mark on the plate, the
peg is removed. It is easy to see that in successive experi-
ments stimulation will occur at that definite moment which
corresponds to this marked line of stimulation.

k' represents a key-device by which the make-shock
is prevented from exciting the plant. One end of a lever
carries a bent metal-rod of u-shape, which is partly immersed
in cups of mercury by means of a spring. During the
depressed position of this key, the secondary coil is short-
circuited. When the handle K is slightly pressed, there
is a ' make ' of the primary current. But the make-shock
is short-circuited as k' is still in the depressed position.
Further pressure of the handle k lifts k' up, removing the
short-circuit of the secondary coil. When the break-shock
is given by the contact-breaker of the falling plate, there
is no short-circuit to divert the shock, which now passes
through the plant and excites it.

The sequence of these events, then, is as follows : —
By turning the disc d the recording-plate is lifted and
held arrested in the up-position. The pressure of the
handle k releases the plate-carrier, which then begins to
fall. At the same time, the primary circuit is completed
and a make-shock is induced in the secondary. But this
make-shock is diverted by the short-circuit key k', which
is still in the depressed position. Further pressure of the
handle k removes the short-circuit by lifting the ends of k'.

DETERMINATION OF THE LATENT PERIOD 115

All this takes place during the continuance of one pressure
of the handle. During the descent of the plate, stimulation
due to instantaneous break-shock takes place at the definite
moment corresponding to the stimulation mark.

Electric connections are appropriately made with the
plant by means of threads moistened in dilute saline
solution. One electrode of the secondary coil is thus
connected with the stem of the specimen : the moistened
thread in connection with the other electrode is lightly
wound round the pulvinus. It is sometimes preferable,
for reasons previously explained, to make this contact with

Fig. 67. — Two successive records, exhibiting identity of latent
period. Recorder loo D.V. per second.

glycerin. The connections are so made that the current
of the break-shock enters by the stem and leaves by the
pulvinus, the latter being thus the kathode. We shall
understand later the reason of this, in as much as the kathode
is the point of excitation.

I now describe the record of an experiment (fig. 67) carried
out in summer for the purpose of determining the latent
period in a specimen of Mimosa. Stimulus was applied
at the point marked by the vertical line, and the upper of
the two records was the first taken. The vibrating-recorder
employed had been tuned to exactly 100 vibrations per
second ; successive dots therefore represent intervals of

ii6 RESEARCHES ON IRRITABILITY OF PLANTS

•01 second. It will be seen that the responsive movement
begins to occur between the tenth and eleventh dots, and
very near the latter. There are thus 10*9 spaces, each of
the value of *oi second, and the latent period is therefore
•109 second. In order to test to what extent successive
experiments might give concordant results, I took a second
record with the same specimen, which appears in fig. 67
as the lower of the two, having given the plant an interval
of rest of 20 minutes after the taking of the first record. It
will be seen that the second record is essentially a replica
of the first, thus demonstrating that with proper precautions

Fig. 68. — Record of highly excitable specimen, taken with
100 D.V. recorder on a slowly moving plate.

successive experiments on the value of the latent period will
give results which are of extraordinary constancy.

By making the travel of the recording-plate very rapid,
the successive dots become more widely spaced and the
minute time-intervals involved are made more conspicuous.
But this has the disadvantage of rendering the flexure of
the curve representing the responsive movement less abrupt,
making the exact point of initiation of response somewhat
more difficult to discriminate. Going, on the other hand, to
the opposite extreme of making the travel of the recording-
plate slow, the flexure of the curve becomes more abrupt,
enabling us the better to detect the point of initiation of
the responsive movement. The time-dots, however, are
now closer together. This can be seen in another record
(fig. 68) obtained with a vigorous specimen. Here the

DETERMINATION OF THE LATENT PERIOD 117

number of spaces before the initiation of response is eight,
the latent period being therefore 'oS second. The closeness
of the time-dots is not any great difficulty here, as with
the help of a magnifying glass it is quite easy to make the
necessary observation.

For the determination of the latent period in plants
this accuracy of an order higher than hundredths of a
second is more than ample. But such a limit is easily
exceeded. As an example of this, I give a record (fig. 69)
made with a different recorder, whose frequency was an
octave higher than the last — namely, 200 double vibrations

Fig. 69. — Record of L of Mimosa with a 200 D.V. recorder.

each second. The successive dots are therefore in this case
^^ part of a second apart. It has been said already that
by slowing the travel of the recording plate the abruptness
of the flexure of the curve would be increased, the spaces
between the dots being at the same time shortened.
But we may obtain wider spacings without losing this
sharpness of flexure, by making a magnified photo-
graphic reproduction of the curve, as shown in the next
figure (fig. 70), which is a reproduction of the first part of
the record in fig. 69 enlarged about three times by photo-
graphic means. In this way it is not difficult to measure,
say, one-fifth of the distance between two successive dots,
themselves representing an interval of ^-J ^ part of a second.
In other words, the calculation can be carried into
thousandths of a second. In the present case there are

ii8 RESEARCHES ON IRRITABILITY OF PLANTS

15*2 spaces between stimulus and initiation of response.
The latent period of the specimen is therefore '076 of a
second.

I have been able, moreover, to construct a vibrating-
recorder whose frequency is 500 times per second, a fact
which enables an easy determination of time-intervals of
less than a thousandth of a second to be made. These
recorders, owing to their excessive lightness, possess the
additional advantage of having a very small moment of
inertia. It is obvious, therefore, that the employment of
such recorders not only bears favourable comparison with
those at present used in animal physiology, but would also

Fig. 70. — The previous record magnified.

have the advantage of reducing the error due to inertia
to the lowest possible minimum, and of making the record
itself its own chronogram.

It has been said that owing to the extreme lightness
of the vibrating-recorder, the shght error usually due to
instrumental inertia is here negligible. To what extent
this is true may be judged by taking records from the same
leaf with two separate recorders of different sizes and
comparing the results. If the factors of inertia were promi-
nent, then two such determinations of an identical latent
period would give results varying somewhat from each
other. I therefore took two different records from the

DETERMINATION OF THE LATENT PERIOD 119

same specimen, using the same stimulus but varying the
mode of record — that is to say, the vibrator used in one
case had been tuned to 50 vibrations per second, the length
of the recorder being 12 cm. The speed of the recording-
plate was in this case relatively slow. The result is shown
in fig. 71. The other record in fig. 72 was taken immediately
afterwards from the same specimen with a vibrator tuned
to 100 double vibrations per second, the recording-plate

Figs. 71, 72. — Two successive records taken with the same leaf; upper
with 50 D.V. recorder and slow-moving plate ; lower with 100 D.V.
recorder and faster-moving plate. Latent period in either case is
•17 second, proving that value of L is unaffected by any peculiarity of
the recorder.

moving at a faster rate. It will be seen from fig. 71
that the time-interval in the first case is represented by
8*5 spaces, each representing -02 second, therefore proving
the latent period L to be -17 second. This, it should be
mentioned, was an autumn specimen, in which the latent
period is somewhat longer than in summer. In the second
record, fig. 72, under its different speed and with the vibrator
giving 100 vibrations per second — we find the intervening
spaces to be 17. This gives the latent period as again

120 RESEARCHES ON IRRITABILITY OF PLANTS

•17 second. The identity of these values shows that the
inertia of the recorder has but Httle effect on the results
obtained.

The latent period in any given specimen of the pulvinus
of Mimosa is, as we have seen, under uniform conditions
extremely constant. It differs, however, in different speci-
mens and from season to season. A thin specimen has in
general a shorter latent period than one which is stouter.
Perhaps this fact is illustrated, with a certain exaggeration,
in the case of Neptunia, the leaf and pulvinus of which are
comparatively thick. In any case, we have already seen

Fig. 73.— Record of latent period of Neptunia with 10 D.V. recorder.

that in its responsive movements, relatively to Mimosa
pudica, it is very sluggish. In order to determine the latent
period I employed a slow vibrator, that is to say, one which
vibrates with a frequency of 10 per second. It will be seen
by reference to fig. 73 that the responsive movement began
after the sixth dot, the latent period being thus '6 second,
or six times the value of the average latent period in
Mimosa.

With Mimosa pudica I have carried out more than a
hundred different determinations, and give below a tabular
statement of seventy of those values which occurred
most frequently amongst these. Specimens giving a latent

DETERMINATION OF THE LATENT PERIOD 121

period shorter than -08 second or longer than -12 second
in summer may be regarded as rather exceptional.

Tabular Statement of Values of Latent Period L in
Numbers of Different Specimens of Mimosa.

Number of

specimens

Value of L

10

•08"

9

•09"

26

•10"

10

•11''

15

•12"

The shortest latent period that I have come across
is -06 second obtained in summer and when the temperature
was specially high. The longest in summer was '14 second.
Most specimens have a latent period not appreciably
differing from -i second. This may be regarded as approxi-
mately the average value for summer. In winter and with
sluggish specimens the latent period may be prolonged to
a value of something like twice as much, that is to say,
•2 second, more or less.

Summary

Latent period of Mimosa may be determined with
great accuracy by means of Resonant Recorder. This
enables the measurement of time-interval shorter than
•005 second.

Error due to inertia is reduced to a minimum on account
of extreme lightness of plant-recorder, which is nearly a
hundred times lighter than muscle-recorder.

Successive values of latent period with the same speci-
men are found to be constant. The results are not modified
by employment of different recorders.

The shortest value of latent period given by a vigorous
Mimosa leaf in summer is -06 second, the average value
being i second.

The latent period of leaf of Neptunia oleracea is '6 second.

CHAPTER X

INFLUENCE OF INTENSITY OF STIMULUS, FATIGUE, AND
TEMPERATURE ON THE LATENT PERIOD

Diffuse stimulation under alternating shock — Effect of intensity of
stimulus on Latent Period — Influence of optimum condition — Effect
of fatigue — Effect of temperature,

I WILL now describe different experiments carried out
for the purpose of observing the effect of varying external
conditions — such as the intensity of stimulus, fatigue, and
temperature — on the latent period. The mode of pro-
cedure adopted was first to take a record giving the latent
period under standard conditions, and then to make further
records under conditions similar in all respects to the first,
except in regard to the one special factor whose influence
was to be determined.

As some of the experiments in question necessitated
a long period of observation, lasting sometimes over an
hour, it became necessary to eliminate one source of possible
uncertainty — namely, the effect of electrolytic contact on
the pulvinus. It has been shown that there is no variation
of excitability induced in the pulvinus where the contact
is made with glycerin. It would, however, be preferable
to effect direct stimulation without placing either of the
electrodes on the pulvinus. In connection with this it was
found that if one of the two electrodes were placed on the
petiole slightly to the right of the pulvinus, and the other
on the stem immediately below it, and a few rapidly alter-
nating shocks passed through, excitation was simultaneous
throughout the interposed tract.

This may be demonstrated in a striking manner by

INFLUENCE OF INTENSITY OF STIMULUS 123

experimenting with sub-petioles of Mimosa bearing numerous
pairs of sensitive leaflets. The two electrical connections
are made on the middle points of two neighbouring sub-
petioles. If a single induction-shock be passed, then it
will be found that the point by which the current leaves
the petiole — the kathode — becomes the seat of excitation,
which is transmitted serially to the neighbouring leaflets.
The characteristics of this phenomenon will be dealt with
in detail in a subsequent chapter. If instead of a single
shock a few alternating-shocks of moderately strong intensity
be next passed in rapid succession through the sub-petiole,
it will be found that the excitation has become diffuse,
the leaflets in the intrapolar tract exhibiting excitation
simultaneosly.

This fact of simultaneous excitation in an interposed
tract may be demonstrated by the following experiment
giving quantitative results : Two successive records are
taken of the response of the pulvinus of Mimosa, the
exciting electrode being first placed with the interposed
pulvinus 10 mm. apart and again 80 mm. apart. The
distance of the pulvinus, in the first case, would be 5 mm.
from either electrode, and in the second case, 40 mm. The
average velocity of transmission of excitation, as will be seen
in the next chapter, may be taken as approximately 16 mm.
per second in summer. If the excitation in the interposed
tract is not simultaneous, but locally initiated at the points
of application of the electrodes, we may expect to find
that the periods intervening between the beginning of
stimulation and the initiation of response will differ greatly
from each other in the two cases. In the first case, where
either electrode is distant from the pulvinus by 5 mm., the
delay in the response may be expected to be of the order
of -3 second. In the second case, where either electrode is
distant from the pulvinus by 40 mm., we may expect
the delay caused by transmission to be about 2*5
seconds.

If the excitation were to prove simultaneous, however.

124 RESEARCHES ON IRRITABILITY OF PLANTS

we should find no difference of time as between the two
cases ; and this stimulation would be equivalent to direct
stimulation and be expected to give us a latent period
of the order of -i second.

I give below (fig. 74) the record of this experiment,
which shows that under alternating shocks the excitation is
simultaneous, and that the value of the latent period is then
independent of the points of application of the electrodes,
provided the pulvinus be included in the tract. The upper
of the two records was taken when the electrodes were
10 mm. and the lower when they were 80 mm. apart. It

Fig. 74. — Simultaneous excitation in interposed tract under
alternating shock. In uppei: record two electrodes were
I cm. apart, in lower record, 8 cm. apart.

is seen that the latent periods obtained from the two experi-
ments are the same — namely, 'oS second — which is of the
order of other determinations already obtained. Had the
stimulation not been simultaneous this value would have
been increased to at least 3 second or to 2*5 seconds in the
respective instances. The identity of results in the two
cases shows, moreover, that we are measuring the effect
of a constant factor, which is the latent period.

For the purpose of applying alternating shocks I em-
ployed as usual a Ruhmkorff coil. In this the spring
vibrator in the primary was so adjusted as to cause
100 interruptions per second. This would give in the
secondary circuit 200 alternating shocks per second, of

INFLUENCE OF INTENSITY OF STIMULUS 125

which 100 would be due to make and the alternating 100 to
break.

In order to subject the plant to brief alternating shocks
applied at definite moments, and having definite duration,
the sliding interrupter and its connections already described
were appropriately modified. The vertical sliding contact-
plate now consists of a conducting platinum sheet, except
for a narrow non-conducting interruption made of a piece of
inlaid ebonite. The sliding-plate and the contact rod now
short-circuit the secondary, except during the brief interval
when the narrow strip of ebonite removes the short-circuit.
It is during this definite interval that the plant is subjected
to the alternating shock. The breadth of the strip is so
chosen that the duration of the shock is about '05 second.
During this time the plant will receive 10 alternating shocks,
5 of make and 5 of break. If desired, a simple device may
be introduced by which the duration of the shock can be
modified. This modification consists in inlaying a right-
angled triangle instead of a linear piece of ebonite on the
metallic plate. The base of the right-angled triangle is
kept horizontal. It follows that, by adjusting the rod
from right to left, the interval of the removal of short-
circuit can undergo continuous reduction of duration.

The mark which indicates the beginning of stimulation
is made in the usual manner, and in successive experiments
the stimulation is initiated at this precise moment. The
duration of stimulus is also constant. In the following
experiments, it is to be remembered, we are to determine
the effect of changing one factor whilst maintaining others
constant. Thus we have in each case to take one
record under standard and one or more under modified
conditions.

Effect of Intensity of Stimulus

The series of experiments to be described below have
in each case been repeated at least twelve times, with

126 RESEARCHES ON IRRITABILITY OF PLANTS

results that were invariably concordant. I content myself,
however, with giving two records in each case, obtained
from different specimens. In order to test still further the
reliability of these results I was careful, with each pair
of figures given for comparison, to employ two different
recorders, the vibration-frequency of the first being loo,
and of the second or companion-set 50 per second.

In fig. 75, with vibrating recorder of 100, are given
two records testing the effect of intensity of stimulus on
the latent period. The lower of the two was obtained
with the minimum stimulus of i ; and the latent period is
seen to be '155 second. In the upper of the two records we

Figs. 75, 76. — Effect of intensity of stimulus on latent period : the
upper record in each is due to stronger stimulus.

have the result of the maximal stimulus of 5. The latent
period is now found to be reduced to 'i second.

In fig. 76, with vibrating recorder of 50 and taking
a different specimen, we find a precisely similar result.
The lower of the two records, with the minimal stimulus
of I, shows a latent period of '14 second. The upper, with
stimulus 2, which in this individual case was maximal,
shows a latent period of '09 second.

It is interesting to note alike in figs. 75 and 76 the great
vigour of the responsive movement under higher intensity
of stimulus, as seen in the abruptness of the rise of the
curve and the wider spacing of the successive dots.

INFLUENCE OF INTENSITY OF STIMULUS 127

The following table shows the effect of intensity of
stimulus on the latent period : —

Table I. — Effect of Intensity of Stimulus on Latent Period.

Number.

Intensity of Stimulus.

Latent period.

■ ■■{

I

5

•155
•10

... (

I
2

•14
•09

3 ..

I
4

•15
•II

Fig. 77. — Constancy of latent period when stimulus is above maximal.
Lower record under stimulus 2 ; upper record under stimulus 5.
Recorder 100 D.V,

It is thus seen that with the increase of the intensity
of stimulus there is a corresponding reduction of the latent
period. But it would appear from further experiments
that a limit is soon reached, when the stimulus begins
to be maximal. A further increase of the intensity of
stimulation above this point will have little or no effect
in reducing the latent period. This is shown in fig. 77,
which gives a pair of records taken with a vibrator having

128 RESEARCHES ON IRRITABILITY OF PLANTS

a frequency of loo double vibrations per second. The
lower of these two records represents the effect of a stimulus
of 2, which was here maximal. The upper was taken
with the increased intensity of stimulus of 5. In the two
cases the latent period was practically the same — namely,
•12 second.

It should be mentioned here that in a plant in optimum
condition the latent period differs very little under strong
or feeble stimulus. We have also seen, it will be remem-
bered, that in an optimum condition of the specimen there
is very Httle difference in the amplitude of response under
strong and feeble stimulus respectively.

Effect of Fatigue

It has been shown that the successive values of the latent
period become constant provided a resting-interval be

Figs. 78, 79. — Effect of fatigue.

allowed for complete protoplasmic recovery. The period
required for full recovery I find to be about 20 to 25
minutes in summer, more or less. If this resting-interval
be shortened, the effect of fatigue is seen in the prolonga-
tion of the latent period ; if this shortening be carried too
far, then the motile excitability is temporarily abolished.
I give below a pair of records which exhibit the prolongation
of latent period on account of fatigue.

The mode of procedure is first to obtain the normal
record with a fresh specimen under a maximal stimulus of

EFFECT OF FATIGUE

129

intensity 3. This is the intensity which is always used
unless the contrary be stated. In order to exhibit the effect
of fatigue, the second record is taken after a period of rest
of only 15 minutes. The upper record (vibration-frequency
100) gives the normal value of L to be "ii second ; the
lower record shows that the latent period has been prolonged
to '16 second on account of fatigue (fig. y8).

The experiment was repeated with a different specimen
and with a vibrating recorder giving 50 vibrations per
second. The record (fig. 79) shows again that the latent
period is prolonged under fatigue, from the normal 'i second
to -14 second. The effect of fatigue is independently
seen in the record, in the relative sluggishness of the respon-
sive movement. The slope in the response of the fresh
specimen is almost vertical, with successive dots very widely
spaced. In the response of the fatigued specimen a great
contrast is observed in both these respects.

I give below a tabular statement showing results of
different experiments on the effect of fatigue : —

Table II. — Effect of Fatigue on the Latent Period.

Number.

L in fresh specimen.

L' when fatigued. |

j

I

11"

•16'

2

•lo'^

•14"

3

10"

•22"

4

•11"

•17"

5

•8"

•13"

6

11"

.,3.

130 RESEARCHES ON IRRITABILITY OF PLANTS

Effect of Temperature

The effect of temperature on the latent period is shown
in the next two sets of records (figs. 80, 81). In fig. 80
we have three sets of records, taken with a 100 D.V.
recorder at the three different temperatures of 23° C, 28° C,
and 33° C. respectively, under a uniform stimulus-intensity
of 2. These temperatures were maintained by means of the
thermal chamber, heated electrically. From the lowest
record at a temperature of 23° C. the latent period is seen to
be -165 second. At 28° C. in the middle record, it is found

Figs. 80,^81. — Effect of temperature.

to be reduced to '125 second. And at 33° C. it becomes
still further reduced to '065 second.

In fig. 81 these results are corroborated by records taken
with a different specimen, under stimulus-intensity of 2, the
vibration-frequency of recorder being 50 D.V. The three
records are for temperatures of 24° C, 29° C, and 33° C.
respectively. The shortening of the latent period with
rising temperature is also shown here in a very striking
manner. The lowest of the records, taken at 24° C, gives us
a latent period of -14 second. The next, at 29° C, shows a
reduction to -102 second. And the last and highest, at
33° C, gives us a latent period of only -07 second.

The increase of vigour in the responsive movement
under rising temperature is also very clearly apparent in the

EFFECT OF TEMPERATURE

131

record. It will thus be seen that the latent period decreases
with rising temperature.

The following table gives the results of several experi-
ments on the effect of temperature on the latent period : —

Table III. — Effect of Temperature on the Latent Period.

Number.

Temperature.

Latent period.

... {

23°

28°

33°

•165"
•125"
•065"

2 . .

24°
29°

33°

•140"
•102'
•070"

- 1

20°
25°
31°

•190"

•id''

•08'

Summary

Alternating electric shocks of moderate intensity induce
simultaneous excitation throughout the interposed tract.

Latent period is in general shorter under stronger
intensity of stimulus. There is no further variation above
a maximal.

In the optimum condition of the plant the latent period
is the same for feeble and strong stimulus.

Fatigue prolongs the latent period.

The latent period is shortened by a rise of temperature.

K 2

CHAPTER XI

VELOCITY OF TRANSMITTED IMPULSE IN PLANTS

Detection of transmitted excitation by means of electromotive variation —
Specific tissue for conduction of excitation — Hydro-mechanical
theory of transmission of stimulus — Propagation of excitatory
protoplasmic change — Physiological test — Automatic record of
transmission-period — Conditions for obtaining constant velocity —
Determination of velocity of transmission in Mimosa — Differential
method of determining velocity — Constancy of results — Tabular
statement of different determinations of velocity — Effect of intensity
of stimulus on velocity of transmission — Effects on sub-tonic tissue
and on tissue in optimum condition — After-effect of stimulus in
enhancing conductivity — Effect of optimum condition — Disturbing
action of leakage of exciting current — Effect of fatigue — Effect of
temperature — Velocity of transmission in Biophytum and Averrhoa —
Direction of preferential conduction.

We have hitherto dealt with the reaction of tissues which
exhibit the excitatory condition by motile response, as
in pulvinus in the case of the plant and muscle in the
case of animal. In the animal, again, we meet with certain
conducting-tissues in which the excitatory protoplasmic
change is transmitted to a distance, and, should one of these
nerves happen to lead to a contractile muscle, the trans-
mission of the excitatory change is conspicuously exhibited
by the contraction of the terminal organ.

We now come to the question whether there is a trans-
mission of a true excitatory change in the plant, and if so
whether there is in it any specific conducting-tissue, corre-
sponding to the nerve of the animal, for the conveyance
of excitation ? Since the transmission of excitation depends
on the propagation of a protoplasmic change, it follows that
a conducting-tissue must be characterised by a more or
less protoplasmic continuity. Should the plant possess

132

VELOCITY OF TRANSMITTED IMPULSE 133

any tissue analogous to the nerve, then it is in the fibro-
vascular bundle that we must look for it.

In a nerve-and-muscle preparation the transmitted
excitation is detected by the contraction of the terminal
muscle. Even in the absence of any terminal contractile
organ, we can detect the passage of excitation by an elec-
trical method. It is known that the excitation of a living
tissue is attended by a concomitant electrical change of
galvanometric negativity. If we make suitable galvano-
metric connections with two points on a nerve, and we
stimulate the nerve at a distant point, we shall find that
the arrival of excitation from the distant stimulated point
is at a proper moment signalised in the galvanometer by a
deflection of a definite sign.

Similarly, I have found that the excitatory change of
galvanometric negativity is transmitted to a distance
through certain plant-organs. Tissues containing fibro-
vascular elements, such as stems and petioles, are found
to be good conductors of excitation. Indifferent tissues in
leaves and tubers possess little power of conduction ; in
such cases excitation remains more or less localised. The
parenchyma in the leaf is thus an indifferent conductor,
whereas the midrib and veins are good conductors of exci-
tation. In stems also great difference is found, as regards
power of conduction, between the fibro- vascular strands and
the ground tissue. The results of electrical investigation
thus give strong support to the conclusion that plants
possess conducting-tissues by means of which the excitatory
state may be transmitted to a distance.

The prevailing opinion, however, up to the present
has been that in plants like Mimosa there is merely a trans-
mission of hydro-mechanical disturbance and no transmission
of true excitation comparable with that of animal nerve.
That this conclusion is erroneous will be shown from the
results of varous inquiries fully described in the next chapter.
In all these investigations it is necessary to determine the
velocity of transmission with the highest accuracy ; and in

134 RESEARCHES ON IRRITABILITY OF PLANTS

order to eliminate the errors that might be inherent in
personal observation, it is desirable that all the data
for this determination should be furnished automatically
in records made by the plant itself. Successive records,
therefore, should enable us to determine with equal accuracy
not only the normal velocity but also its variation under
given changed conditions.

And here the preliminary questions arise : With what
degree of accuracy can we determine the normal velocity
of transmission ? And how far may we depend on the
constancy of this velocity, in successive experiments, under
normal conditions ? As regards these points, some mis-
givings might naturally arise. For the factors calculated
to interfere with this constancy will in all probability
prove to be numerous. First, we may have the variation
of excitability at the point of application induced by the
stimulus itself. We have, therefore, to find out what is
the maximum intensity of stimulus that may be employed
without causing fatigue or other deleterious changes in
the tissue. Another point to be remembered is the question
already discussed in previous chapters of our ability to
apply stimuli, in successive experiments, of identical intensity
and duration. Unless this can be secured we cannot look for
consistent results, inasmuch as the velocity of transmission
may to some extent be dependent on the intensity of stimulus.
Likewise, if the transmission of excitation should prove to
be due to the transmission of a protoplasmic change, it is
easy to see that we must allow the tissue a definite time for
protoplasmic recovery after each application of stimulus,
without which interval consistency of results could hardly be
expected.

It was only after a long course of investigations — some
of which will be described in the course of the present
chapter — that I was able to analyse and provide against
these several sources of variation. But even after this,
I was by no means prepared for the very great consistency
of the results which it has been my good fortune to obtain.

VELOCITY OF TRANSMITTED IMPULSE 135

For successive determinations, with the same specimen,
of the periods required for the transmission of excitation
through a given length of conducting-tissue, did not differ
from each other by so much as one-twentieth of a second
and were often actually identical.

Determination of Velocity of Transmission

For the purpose of these experiments I used by prefer-
ence the petiole of Mimosa, for the reason that in this the
conducting-strands situated in the fibro-vascular bundle
would be more continuous and evenly distributed than in
a branching specimen. In order to determine the velocity
of transmission, the stimulus of induction-shock is applied
to the petiole at a distance d from the responding pulvinus.
Let us suppose t to be the true time taken by the excitation
to reach the pulvinus ; the initiation of the responsive
movement will however be further delayed by the latent
period of the pulvinus L. The total time-interval T observed
to elapse between the application of stimulus and the
initiation of response will therefore be the true time t plus
the latent period L. To obtain the true time we have to
subtract the latent period L from the observed interval T,
thus ^=T— L. The velocity of transmission is then found
by dividing the distance by the true time. The necessary
data are therefore the distance between the stimulated
point and the pulvinus, the time-interval between the
application of stimulus and the initiation of response, and
the latent period of the individual pulvinus.

In making these determinations the apparatus employed
is the same as that for the determination of the latent
period. As in these experiments we have to measure
time which may be several seconds in duration, the record-
ing-plate is made to travel at the relatively slow rate of
2 cm. each second or thereabouts. The vibrating recorder
must be selected according to the degree of accuracy that

136 RESEARCHES ON IRRITABILITY OF PLANTS

is required. For our present purpose a time-measurement
accurate to one-tenth or one-twentieth of a second is
ample.

We first obtain a series of records of indirect stimulation.
The two electrodes, e and e', in connection with the exciting
secondary coil, are applied on the petiole about lo mm.
apart, the proximal electrode E being at a distance d from
the pulvinus. The recording-plate during the course of its
descent completes the primary circuit of the induction coil
for a definite length of time, which is about one-twentieth
of a second. This gives rise to a definite number of alter-
nating shocks to the plant. The stimulus is always applied
at a definite instant in the descent of the plate ; hence
successive records on the same plate always commence on
the same level, the vertical line in the record indicating the
moment of the application of stimulus. After taking one
or more records of the effect of indirect stimulation, an
additional record is taken of the effect of direct stimu-
lation. This gives the latent period L of the particular
specimen.

Before proceeding further I must point out the neces-
sity of special precautions for the perfect insulation of the
electrodes in connection with the secondary coil. If one
of these should happen to touch the table, then, even with
connections made for indirect stimulation, a portion of the
current would pass through the flower-pot holding the
plant and the pulvinus would be directly stimulated by this
escaping current or current of leakage. In my own case, it
was some time before I discovered that certain anomalous
results were to be traced to this particular source of dis-
turbance, at first little suspected. To overcome this diffi-
culty the flower-pot should be placed on a block of insulating
ebonite, the electrodes also being carefully insulated on
ebonite rods.

I will now proceed to give the actual records obtained
with the arrangements detailed above. In the experiment
I am about to describe the specimen of Mimosa was very

VELOCITY OF TRANSMITTED IMPULSE 137

vigorous. The distance at which the stimulus was appHed
was 30 mm. from the responding pulvinus, and the inten-
sity of stimulus was 3 units. The frequency of the vibrat-
ing-recorder was 10 per second ; hence the distance between
any two successive dots in the record represents a time-
interval of one-tenth of a second, and from the record
itself it will not be found difficult to estimate intervals of
even one-fifth that amount. The lowest of the three records
in fig. 82 represents the results of the first experiment. It
will be seen that the interval between the stimulus and the

Fig. 82. — Determination of velocity of transmission of excitation
in Mimosa. Two lower records are in response to stimulus
applied at a distance of 30 mm. ; upper Tecord in response
to direct stimulation giving the latent period. Recorder
10 D.V.

beginning of response is i6'2 spaces, each of the value of 'i
second. Therefore the total time T is 1*62 second. After a
suitable interval necessary for complete recovery of con-
ductivity, a second record was taken, under the same con-
ditions, on the same plate. The minimum interval necessary
varies from 15 to 20 minutes, depending on the condition
of the specimen and the season. It will be seen that the
time-interval in this case is the same as before — namely,
1*62 second. The third record was taken with direct stimu-
lation, and from this it will be noted that the latent period

138 RESEARCHES ON IRRITABILITY OF PLANTS

is *I2 second. Thus the velocity of transmission as given
by both these experiments is identical — namely,

t 1-5

V = - = A_ = 20 mm. per second.

The Differential Method

In order to put the constancy of these results to a still
more rigorous test, I next modified the experiment in the
following way, employing the Differential Method. The
stimulus was first applied at a distance d from the respond-
ing pulvinus, and the total time T was found from this
record. In the next experiment the distance of the point
of stimulation was reduced to d^, and its corresponding
total time, T^, found in the usual manner. And lastly, a
record was taken under direct stimulation. This furnished
the value of the latent period L.

It will be seen that we have here three different sets
of data for the determination of the absolute value of the
velocity of transmission. In two of these we derive the
velocity in the usual manner from the distance, the total
time, and the latent period. In the third, knowledge of L is
not required, as it will be seen that the difference in the
times T — T^ of transmission observed in the first two cases
represents the time taken by the excitation to travel the
difference between the two distances d — d^. Hence three
separate determinations, Vi, Vo, and V3, obtained with the
same specimen, are

y ^ . Y — <^i y __d—di

T-L' ' Ti-L' ' T-Ti

The rigour of this test of constancy will be gauged by
the extent to which the different determinations of Vj, Vg,
and V3 are consistent with one another.

In an experiment carried out in this way the intensity of
stimulus appHed was 3 units, and the vibrating recorder
had a vibration-frequency of 10 per second. In the first

VELOCITY OF TRANSMITTED IMPULSE 139

experiment the point of application of stimulus was at
a distance of 30 mm. ; the total time was found to be
1*9 second. In the next experiment the distance was
reduced to half, that is to say, 15 mm., and the total time
was found to be i second. And lastly, the latent period
was determined, under direct stimulation, at -08 second
(fig. 83). Thus

Vj = - — ^-^-^ = 16-4 mm. per second

^2 = _^^ ^ ^^'3 ^^' P^^ second

V3 = — — = i6-6 mm. per second

1-9 -I

Fig, 83. — Determination of velocity by differential method. The records
from below to above are in response to stimuli applied at distances
of 30 mm., 15 mm., and directly. Recorder lo D.V.

The three results thus obtained from independent data
are here seen to be extremely consistent. They bear very
emphatic testimony not only to the accuracy of the method
but also to the constancy of the velocity in a given specimen
under unvarying external conditions.

It has been stated that the accuracy of these time-
measurements can be pushed to almost any extent. In
order to demonstrate this fact, and also to exhibit the high
mutual consistency of various determinations, I reproduce

140 RESEARCHES ON IRRITABILITY OF PLANTS

another set of records from a different specimen (fig. 84)
from which the velocity of transmission is to be determined
by the Differential Method. In this case a new recorder
was taken, with a vibration-frequency of 20 times per
second. Hence the distance between any two successive
dots represents a time-interval of one-twentieth of a second.
The stimulus intensity was again 3. The lowest record
gives us the result obtained when the point of application
of stimulus was 30 mm. away from the responding pulvinus.
The total time T is here seen to be 2*9 seconds. The
next record gives us the result when the point of stimulation
was at a distance of 20 mm., and the total time Ti is i'985

Fig. 84. — Determination of velocity by differential method. Uniform
stimuli applied at distances of 30 mm., 20 mm., and directly. Recorder
20D.V.

second. The third and highest gives the record of direct
stimulation, the latent period being shown as '085 second.
Thus

Vi = ^ ^ — =107 mm. per second

2'9 — -085 ^ ^

V2 = — Fi TT- = 10' 53 mm. per second

1.985 - -085 ^^ ^

V3 = ^r— = lO'Q mm. per second

2*9 - 1-985

It is thus seen that, taking the precautions described,
successive determinations of the velocity of transmission

VELOCITY OF TRANSMITTED IMPULSE 141

may be arrived at which are of great constancy. It may
be said of the velocity of transmission in the petiole of
Mimosa that it is constant with a given specimen, but
undergoes some variation with different individuals. It
is also subject to modifications induced by season. In
winter the velocity is much reduced. The highest velocity
which I have obtained with summer specimens of the
petiole of Mimosa is 30 mm. per second. The lowest, in
sluggish specimens, may be as little as 4 mm. per second.
The tabular statement below shows results obtained
in twenty-five determinations in different specimens.
Maximal stimulus was apphed in every case.

Table I. — Determination of Velocity of Transmission of
Excitation in various Specimens of Mimosa

!

No. Stimulus.

Distance in

i True time.

Velocity in mm.

mm.

per second.

I

3

30

i"

30

2

>j

40

1.4"

286

3 •

»>

20

0.76"

26-3

4 •

>>

20

0.8'

25

5 •

>>

40

lY

236

6 .

>»

30

^■3"

23

7 •

>>

30

^V

215

8 .

>>

15

oY

20

9 .

>>

30

1.6'

i8-8

10

>>

30

lY

176

II

»

40

2.4'

167

12

>>

15

0.9"

166

13 •

>>

30

1.9'

16-3

14 .

)>

25

1.6'

156

15 •

>>

40

2.7'

148

16 .

i>

40

2.8"

14-3

17 .

>>

40

3.0"

13-3

18 .

„

30

2.32"

130

19 .

>>

30

2.64"

II-4

20

M

30

2.96^

lo-

21

>>

20

2.1"

gs

22

„

20

2.4^

8-3

23 •

}>

40

5.4^

7*4

24 .

jy

20

3-3''

60

25 •

it

10

2.3"

43

142 RESEARCHES ON IRRITABILITY OF PLANTS

Having now obtained means for the accurate determina-
tion of the velocity of transmission, we next proceed to
study the effects of various agencies in inducing changes
in the normal rate. And first we must consider the im-
portant question of whether or not the velocity is in any
degree dependent on the intensity of stimulus. In the
corresponding case of conducting animal-nerve there is
considerable diversity in the results which have been
arrived at. It has been found by some investigators that
velocity is independent of the intensity of stimulus. Others
have found, on the contrary, that the velocity of trans-
mission increases with the intensity of the stimulus, till
with a very high intensity it becomes unmeasurable. The
results which I shall here describe will probably throw
light on this debatable question. It may be said, in anti-
cipation, that the effects are to some extent modifiable in
a definite way by the condition of the conduct ing-tissue.

If the specimen happens to be in a sluggish condition,
then increasing intensity of stimulus will be found to be
attended by increasing velocity of transmission. Again, a
moderately strong intensity of stimulus is often found to
leave, as an after-effect, increased conducting power. That
is to say, to a tissue which has been sluggish, stimulation
itself imparts a higher conductivity. In these facts there
is a remarkable parallelism to what has already been pointed
out in the matter of the amplitude of response of the sub-
tonic tissue. In that case we saw that increasing intensity
of stimulus gave rise to increased amplitude of response.
We saw, further, that following a given stimulus, increased
excitability appeared as an after-effect, so that the repeti-
tion of an identical stimulus evoked response of enhanced
amplitude.

Effect of Increasing Intensity of Stimulus

Returning now to the experimental inquiry into the
influence of intensity of stimulus on the velocity of trans-
mission, I reproduce a set of records (fig. 85) obtained with

VELOCITY OF TRANSMITTED IMPULSE 143

a tissue which was slightly sluggish. The distance of the
point of application of stimulus — namely, 20 mm. — was
maintained constant, the intensity being varied in the
successive experiments. The vibrating recorder had a
frequency of 10 per second. The lowest record is the
result of a stimulus-intensity of '5. The total time of
transmission is seen to have been 2'i seconds. The true
time is obtained by subtracting from this the latent period,
the average value of which is found to be about *i second.
No appreciable error will be introduced in practice by
adopting this average value for the latent period, for its

Fig. 85. — Effect of intensity of stimulus and its after-effect on velocity.
Lowest record under stimulus -5 ; the next under stimulus 4. Velocity
increased under stronger stimulus. Enhancement of conductivity by
previous stimulation seen in two upper records under -5 and 4 respec-
tively. Velocity high and practically the same in both cases.

variations are very slight, being of the order of hundredths
of a second. The actual time here taken for transmission
is thus 2 seconds, with a stimulus-intensity of '5.

The record next above gives the result when the stimulus
intensity was 4, that is to say, increased to eight times
its original value. The total time is now found to be
decreased to i'6 second, the true time after deducting the
latent period being thus i'5 second. The velocity under
this increasing intensity is thus enhanced in the proportion
of 2 : i'5, or 33 per cent. To find out if there had been
any after-effect of stimulation, a record was once more

144 RESEARCHES ON IRRITABILITY OF PLANTS

taken with the original feeble stimulus -intensity of -5.
It will be seen from the third record that the time taken
for the transmission of excitation was practically the same
as that with the previous strong stimulus, showing that
this has made the tissue better conducting and that this
property has reached a limit of uniformity. In order to
test this conclusion further, a fourth record was taken
with the second high stimulus-intensity of 4. It will be
observed that the time of transmission is now the same as
with the feebler intensity.

From these experiments it will be understood that when
the tissue is in a somewhat sluggish or sub-tonic con-
dition the velocity of transmission is enhanced under
increasing intensity of stimulation. This, however, reaches
a limit under a maximal stimulus the value of which is
about 3 units. The following table gives the results of two
sets of experiments on the effect of increased intensity of
stimulus on velocity : —

Table I. — Effect of Intensity of Stimulus on Velocity in
Sub-tonic Specimens.

No.

Stimulus.

Velocity.

■ (

05
4

25

mm. per sec.
10
13-3

'(

5-9

8-3

The fact that stimulus itself may enhance conductivity
in a sub-tonic tissue can be seen in a striking manner
in specimens of Mimosa which are in sluggish condition.
It will there be found that the application of stimulus on
the petiole will at first fail to be conducted. If, however,
we apply the same stimulus again after the usual interval
of, say, 15 minutes, the excitation which failed in the pre-
vious case to be conducted will then reach the pulvinus

VELOCITY OF TRANSMITTED IMPULSE 145

and induce the responsive fall. Stimulus had thus imparted
conductivity to the tissue.

Effect of Optimum Condition

We have seen that when the tissue is in a favourable
tonic condition the range between minimal and maximal
excitation tends to vanish— that is to say, a moderately
feeble stimulus induces the same amplitude of response
as the maximal. It appears probable that what was
found to be true in the case of motile excitability may be
equally appHcable to conductivity ; that velocity of trans-

FiG. 86. — Effect of optimum condition on velocity. Plant raised to tem-
perature 30° C. ; records taken under stimulus -5 (lowest), 2 (highest),
•5 once more (middle). Velocity practically the same.

mission will tend to be constant, even under varying in-
tensity of stimulus, when the tissue is in a favourable tonic
condition.

In order to test this induction I next tried the effect
of varying intensity of stimulus on a specimen which had
been brought to a favourable tonic condition. We have
already noticed how the excitability of the plant is enhanced
when the surrounding temperature is raised to 30° C. or
thereabouts. This was secured by enclosing the experimental
plant in a thermal chamber, which was maintained at the
uniform temperature of 30° C. The point of stimulation

146 RESEARCHES ON IRRITABILITY OF PLANTS

was at a distance of 30 mm. from the pulvinus, and the
frequency of the vibrating-recorder employed was 10 times
in a second.

In the lowest of the three records shown in fig. 86 a
stimulus-intensity of '5 was employed. The next record,
the highest in the figure, was taken with stimulus 2, that
is to say, four times the former intensity, and the time of
transmission was found to be practically the same as with
the feeble stimulus. The third, which is the intervening
record, was taken when the stimulus had been restored to
its original feeble intensity of -5. The results demonstrate
that these successive experiments, with varying intensities
of stimulus, gave practically uniform results in velocity of
transmission of 20 mm. per second.

These experiments confirm the conclusion that when
the plant is in an optimum condition its velocity of trans-
mission is practically constant, even under varying intensity
of stimulus. In ordinary circumstances the velocity in-
creases with increasing intensity of stimulus, till a limit is
reached under maximal stimulation.

Disturbing Action of Leakage of Current

On employing the very strong stimulus-intensity of 15
or 20 units, I have sometimes observed a sudden enhance-
ment of the normal velocity. In fact, the time elapsing
in these circumstances between stimulus and response was
tantamount to the duration of the latent period, the velocity
of transmission being thus practically infinite. One thing
that was noticeable in such experiments was that instead
of gradual and continuous enhancement of the velocity,
with increasing stimulus, the enhancement which occurred
was sudden and abrupt at a certain high intensity. This
justifies the conclusion that, in such a case, the stimulation
becomes virtually direct by leakage of currents of relatively
high tension.

VELOCITY OF TRANSMITTED IMPULSE 147

Effect of Fatigue

It should be stated here that very strong stimulation has
a tendency to induce fatigue, the result of which is seen
in the reduction of the rate of transmission in subsequent
experiments. The effect of fatigue can also be shown
under moderate stimulation by reducing the period allowed
for rest between two stimulations. In summer the period
of complete recovery of conductivity is about 15 minutes.
In winter the same process requires from 20 to 25 minutes.
In fig. 87 a pair of records is given showing the reduction
of the velocity under fatigue. The upper of the two records

Fig. 87. — Effect of fatigue. Upper record when plant fresh ;
lower record when fatigued,

was taken when the plant was fresh, the distance of the
point of stimulation was 10 mm., and the intensity of
stimulus was 2. The next and lower of the two records
was taken after allowing the incomplete resting-interval
of only 10 minutes. It will be seen that fatigue has here
prolonged the time taken for transmission of excitation.
This prolongation is due chiefly to fatigue of conductivity
and partly to the prolongation of the latent period. The
variation of the latter factor, however, is relatively insigni-
ficant, being only, as already stated, of the order of
hundredths of a second. Taking the approximate value of
the latent period to be 'i second, the period required for

l2

148 RESEARCHES ON IRRITABILITY OF PLANTS

transmission of excitation through a distance of lo mm.
was here 1*32 second when the plant was fresh ; when the
specimen was fatigued the period of transmission through
the same distance was prolonged to 17 second.

Table II. — Effect of Fatigue on Velocity of TRANSMissiaN.

No.

V. in fresh specimen.

V. under fatigue.

I

7-6 mm. per sec.

5-9 mm. per sec.

2

30 mm. per sec.

19 mm. per sec.

The effect of fatigue is also here depicted in an interesting
manner in the records of the responsive movements them-
selves. In the upper record of the fresh specimen the
movement is seen to have been vigorous, by the comparative
erectness of the curve and the distance between the succes-
sive dots, representing the amount of the excitatory fall
during periods of one-tenth of a second. The lower curve
offers a marked contrast in both these respects.

Effect of Temperature

If the phenomenon of transmission in the plant is
one of protoplasmic change, then any factor that causes
physiological variation must have a corresponding influence
on its velocity. One such cause of physiological variation
is found in change of temperature. If, on the other hand,
the propagation had been merely of a hydrostatic blow,
then a change of temperature would not have had any
marked effect upon it. Thus the accurate determina-
tion of the influence of temperature upon velocity of
transmission becomes an important consideration in dis-
criminating between the excitatory or mechanical nature
of the transmitted change.

I will now proceed to give quantitative results as to

VELOCITY OF TRANSMITTED IMPULSE 149

the variation of conductivity, under variation of tempera-
ture. The plant was maintained at the required tempera-
tures in the thermal chamber, either by the cooling device
or by the electrical appliances for heating, which have been
previously described. In fig. 88 time-records are given of
the transmission of excitation at the three temperatures of
22° C, 28° C, and 31° C. This experiment was carried out
in the Calcutta winter, when the temperature of the room
was 22° C. The normal velocity of transmission in the plant
was thus, owing to the season, somewhat low. Stimulus
of maximal intensity 2 was applied, at a distance of
10 mm. from the responding point. The lowest of the

Fig. 88. — Effect of temperature in enhancing velocity of transmission.
Three records, from below upwards, are for temperatures 22° C,
28° C, and 31° C. respectively.

three records gives us the period of transmission at
the temperature of 22° C. The next record was taken
at 28° C, and the third or topmost at 31° C. It is quite
evident from these figures that the velocity is continuously
increased under rising temperature. The period taken at
22° C. was 2*94 seconds ; at 28° C, 1-69 second ; and at
31° C, 1*2 second. We noted in a former experiment
that the latent period undergoes a variation with changes
of temperature. Thus in a given experiment, while the
latent period at 23° C. was -165 second, at 28° C. it was
•12 second, and at 33° C, -065 second. These variations are
very slight as compared with the total time required for the

150 RESEARCHES ON IRRITABILITY OF PLANTS

transmission of excitation. Hence if, in tHe results of the
last experiment, we make the small corrections representing
the variations of the latent period, the velocity of trans-
mission at 22° C. will be found to be 3*6 mm. per second,
at 28° C. it is increased to 6-3 mm. per second ; and at
31° C. it has become 9 mm. per second. Thus at 31° C. it
is two and a half times as great as the velocity at 22° C.

The results of this and a few of many other experi-
ments on the influence of temperature on velocity are given
in the following table. It need only be said that the effect
of rising temperature was always to induce an increase in the
velocity of transmission.

Effect of Temperature on Velocity of Transmission.

Number.

Temperature.

Distance.

True time.

Velocity.

I
(Winter speci-
men)

22° C.

28° C.

31° c.

10 mm.

2-93"
1-59"
II*

3-6 mm. per sec.
6-3 „
90 „

2 .

{

23° c.
33° C.

10 mm.

1-5"

•8"

66 mm. per sec.
12-5 „

3 .

(
\

29-5° C.

305° c.

20 mm.

2-2"
17"

91 mm. per sec.

-{

27<'C.

30° c.

10 mm.

10"

Y

100 mm. per sec.
143 »

'■■{

29° c.
32° c.

35° C.

20 mm.

20"

1-8"
1-5"

lo-omm. per sec.
Ill „
13-3 „

Velocity of Transmission in Biophytum and Averrhoa

Employing the method that has been described I have
determined the velocity of transmission of impulse in the

VELOCITY OF TRANSMITTED IMPULSE 151

petiole of Biophytum, the average value of which is about
2 mm. per second. Fig. 89 shows a record obtained with
a typical specimen. Stimulus was applied at a distance
of 50 mm. from the responding leaflet. The record was
taken by means of the Oscillating Recorder, the successive
dots being at an interval of a second. It will be seen
that the response took place 24*5 seconds after the appli-
cation of stimulus. Making allowance for the latent
period, the average value of which in Biophytum is

Fig. 89. — Record giving transmission-time in Biophytum :
Successive dots at intervals of a second.

•4 second, the velocity of transmission in this particular
case is 2'i mm. per second. The velocity in the petiole of
Averrhoa carambola varies from -5 to i mm. per second.

Direction of Preferential Conduction

In connection with the determination of velocity of
transmission of excitation in Biophytum, I made the discovery
of the curious phenomenon of preferential conductivity.
It was found that the state of excitation travelled through
the conducting petiole of Biophytum with greater facihty
in one direction than in the opposite. The experiment
was carried out by employing a leaflet, situated midway
in the petiole, as the motile indicator. Equal stimuli were

152 RESEARCHES ON IRRITABILITY OF PLANTS

alternately applied at equal distances to the right and to
the left of the leaflet. In one case the excitatory wave
was transmitted in a centripetal direction, that is to say.

Table showing Difference of Velocity in Two Directions.

Direction.

Specimen I.

Specimen II.

Specimen III.

Centripetal velo-
city

Centrifugal velo-
city-

2*5 mm.per sec.
4 mm. per sec.

2 mm. per sec.
2-9 mm.per sec.

1-84 mm. per sec.
2-2 mm. per sec.

towards the stem or main axis of the plant. In the other
case, excitation travelled outwards towards the tip of the
leaf or in a centrifugal direction. From a large number
of experiments carried out in this manner it was found that
the velocity is greater in the centrifugal direction.

Summary

By applying stimuli of constant intensity, and by
allowing proper intervals of rest, successive values of
velocity of transmission of excitation are obtained which
are constant.

Consistent results are also obtained by the employment
of Differential Method for the determination of velocity of
transmission. The automatic records afford measurement
of time as short as '05 second.

The highest velocity of transmission of excitation that
has been found in the petiole of Mimosa is 30 mm. per
second.

In a sub-tonic tissue the velocity of transmission of
excitation is enhanced under increased intensity of stimulus.
The tissue becomes a better conductor of excitation in
consequence of previous stimulation.

VELOCITY OF TRANSMITTED IMPULSE 153

In a tissue in optimum condition the velocity of trans-
mission is the same under feeble and strong stimulation.

Fatigue depresses the rate of conduction of excitation.

Velocity of transmission of excitation is enhanced at a
higher temperature.

Excitation is transmitted in both directions ; but the
velocity is not necessarily the same in the two cases.
In Biophytum the velocity in the centrifugal direction is
greater than in the centripetal.

CHAPTER XII

EXCITATORY CHARACTER OF TRANSMITTED IMPULSE IN

PLANTS

The hydro-mechanical theory — Inconclusive character of the anaesthetic
experiment of Pfeffer and scalding experiment of Haberlandt — •
Kiihne's experiment showing transmission of excitation under intense
stimulation in a rigored nerve — Error introduced by employment
of excessive intensities of stimulus — Discriminative polar effect of
current in excitation — Block of transmission of excitation by local
application of cold — Restoration of normal conductivity by tetanising
shock in tissue paralysed by cold — Electrotonic arrest of excitatory
impulse — Action of various poisons in inducing block of conduction.

In the previous chapter I referred to the prevaiUng behef
that the transmitted impulse in the plant was hydro-
mechanical, unlike the excitatory impulse in the animal
nerve. This view has been largely based on two well-known
experiments of Pfeffer and Haberlandt. In the former of
these, the effect of strong stimulus was found to travel over
chloroformed parts of the petiole. Pfeffer assumed that the
conductivity of this portion must have been abolished, since
chloroform is known to abolish motile excitability. In the
experiment of Haberlandt an intervening tissue was killed
by scalding ; in spite of this, stimulus was found to be
transmitted across the scalded area.

From these two experiments it was inferred that the
impulse which was transmitted could not have been of a
true excitatory nature. It was held that, instead of this,
a strong stimulus had given rise to a variation whether
of increase or diminution of hydrostatic pressure. This
variation of pressure, it was assumed, had been hydro-
mechanically transmitted, and on reaching the distant

^54

CHARACTER OF TRANSMITTED IMPULSE 155

pulvinus had inflicted on it a blow which had proved as
effective as if a mechanical stimulus had been applied
locally. It is thus held that in Mimosa there is a mere
transmission of stimulus but no transmission of excitation.

I shall, however, be able to show that the two experiments
referred to are not as conclusive as has been supposed. But
before doing this it is as well to examine the basis of the
hydro-mechanical theory. According to the Dutrochet-
Pfeffer theory, migration of water is the sole cause of pro-
pagation of stimulus, transmission being due to movement
of water in the vascular bundles. When a wound is made
in the stem causing an incision of a vascular bundle, fluid
exudes from the wound on account of which there is a
diminution of the hydrostatic equilibrium in the bundle.
There may, again, be a propagation of stimulus caused by
exciting the pulvinus, when a certain quantity of fluid
given out by the excited parenchyma is supposed to pass
into the vascular bundle.

According to Haberlandt, flaccidity ensues in the sensi-
tive parenchyma on direct stimulation of an articulation.
Owing to the deformation of the cells a pressure is induced
in the conducting-tissues which is propagated along them
and which, reaching a new pulvinus, stimulates it as if by a
blow from without. Haberlandt compares the transmission
of pressure in the plant with that in an indiarubber tube
filled with water, 1

But transmission through long, and more or less closed,
capillary tubes is not the same as that through uninterrupted

^ ' It is still harder to explain the mechanism by which a stimulus is
propagated from the relaxed parenchyma of the curving pulvinus to the
excitable parenchyma of an adjacent joint, after a single mechanical stimu-
lus or with chemical or thermic excitation. . . And when Haberlandt
compares the resulting movement of the sap " to that within an india-
rubber tube containing water at a given hydrostatic pressure in which
increase of pressure at any point is propagated in the form of an undulatory
wave from one end to the other," the anatomical relations of the con-
ducting-cells hardly seem to justify such a presumption. The experiments
on the conductivity of Mimosa would have to be scrupulously repeated
before forming any final judgment.' — Biedermann : Electro-physiology,
vol. ii. p. 16 (Macmillan). . .

156 RESEARCHES ON IRRITABILITY OF PLANTS

indiarubber tube having a large bore. In the former case
a considerable mechanical disturbance would be necessary
to start the hydrostatic wave which can effectively reach a
distant point. Haberlandt supposes such a mechanical
disturbance to be brought about by deformation of the mass
of parenchyma in the stimulated pulvinus or by injury of
the stem or petiole. But it is not at all necessary to
initiate the excitatory impulse in Mimosa by stimulating the
pulvinus ; such an impulse may be originated in the thin
petiole where there is no turgid mass of parenchyma to be
deformed. Excitation may be caused, moreover, by the
agency of a physiological stimulus which does not cause any
injury or give rise to any mechanical disturbance.

Again, as regards the question as to whether the trans-
mitted variation of pressure would always form an efficient
cause of excitation, it was found by Macdougal that sudden
artificial variation, whether by increase or diminution of
hydrostatic pressure, brought about no responsive fall of
the leaf of Mimosa.'^

We now return to the detailed consideration of Pfeffer's
experiment on anaesthetics and Haberlandt's on scalding.
As regards the former it has been assumed that the conduct-
ing-power was arrested under chloroform. It has, however,
been pointed out by Vines that though a narcotised pulvinus
certainly loses its motile excitability, it does not necessarily
follow that its conductivity likewise is completely abolished.
In fact, instances are known to physiologists in which a
tissue whose excitability has been abolished will still persist
in maintaining its conducting-power. This circumstance
may be demonstrated in the case of plants by taking a
specimen of Biophytum and applying a strong stimulus to
an old leaf the motility of whose leaflets has been abolished
on account of age. Though its own leaflets do not afford
any motile indication, the excitation is found conducted
through the petiole of the old leaf, inducing the fall of the
leaflets in a neighbouring young leaf.

* PfefifQr: Physiology of Plants, vol. iii. p. 95 (Cla,rendon Press),

CHARACTER OF TRANSMITTED IMPULSE 157

It is also extremely doubtful whether in the particular
experiment with Mimosa the conducting-tissue in the in-
terior could have been effectively narcotised by the external
application of the anaesthetic. The task would almost
be as difficult as narcotising a nerve-trunk lying between
muscles, by the application of chloroform on the skin out-
side ! In the case of the plant it is conceivable that after a
very long appUcation a small quantity of narcotic may, by
absorption, get access to the internal conducting-tissue ;
but narcotisation in these circumstances can only be partial.
In such a case the transmitted effect of a feeble or a moderate
stimulus will alone be arrested ; but the block will fail to
arrest the transmitted effect of intense stimulation. These
considerations will probably explain Pfeffer's observation
that, while the effect of strong injury stimulus was always
transmitted across the narcotised area, a moderate mecha-
nical stimulus was but occasionally transmitted.

In Haberlandt's experiment the conducting-tissue was
supposed to have been killed by scalding. If this had really
been the case, then it may be supposed that under an
exceptionally strong stimulus a hydrostatic disturbance had
been transmitted through the dead tissue and caused
stimulation of the distant leaf, as a mechanical blow de
novo. But excitatory transmission in a plant is usually
accomplished by a stimulus which is feeble. Strong doubt
may also be entertained as to whether the tissue had really
been killed. In my own experience I find it extremely
difficult to be sure of kiUing the interior of a tissue by
scalding the outside. This derives additional support from
certain experiments of Kiihne on conduction of excitation
in a nerve, the specimen employed being the sartorius of
a frog.

* The delicate nerve which enters the middle of the
sartorius by one side, divides within the muscle so that the
single fibres that constitute the bifurcation branch many
times dichotomously. When Kiihne threw the broad upper
end of the muscle into heat rigor by dipping it into warm

158 RESEARCHES ON IRRITABILITY OF PLANTS

oil, the half which remained normal twitched on cutting the
rigored portion with scissors, showing that excitable nerve-fibres
could still be mechanically excited between the rigored and dead
muscle-fibres, and thus carry the excitation centripetally
into branches which divide above the rigored portion of the
muscle.' 1

In this experiment we have an instance of transmission
of excitation through heat-rigored animal tissue parallel to
Haberlandt's experiment on transmission through scalded
plant-tissue. In both these cases it is evident that the
scalded tissues, though under heat rigor, were not really
killed ; and that the induced block or abolition of con-
ductivity (caused by heat rigor, electrotonus, and so on)
is after all relative. There may thus be an effective physio-
logical block for normal intensities of stimulation, which
would, however, fail under abnormal intensities of stimulus
such as that of a burn or of a cut. In Kuhne's experiment
the intense excitation of scissors-cut failed to be arrested,
though the conductivity of the nerve had been depressed
under heat-rigor. Similar considerations will explain how
the intense excitation caused by a burn or a cut may be
transmitted through the narcotised or scalded areas in
Mimosa.

The experiment of Kiihne shows further that the conduc-
tivity may persist even after the abolition of the motile
excitability. The rigored muscle is seen to have lost its
motility, though the embedded nerve retained a certain
amount 'of conductivity for excessively strong stimulus of
a cut.

In turning our attention to Kiihne's experiment we realise
the error involved . in ignoring the factor of intensity of
stimulus in the matter of the effectiveness of a given block to
the transmission of excitation. The necessity of discarding
crude and drastic methods of excitation in researches on
variation of conductivity will now have become obvious.
The object of our inquiry is not to find whether a violent
^ Biedermann : Electro-physiology, vol. ii. p. 57 (Macmillan) .

CHARACTER OF TRANSMITTED IMPULSE 159

mechanical disturbance is transmitted to a distance, but
the determination of propagation of physiological change,
under normal modes of stimulation. By employing stimulus
of graduated intensity, it should be easy to determine the
character of a given impulse by observing the effects of
various physiological depressors in modifying the power
of conduction.

In order to bring the question — whether in a plant
there is true transmission of excitation or mere passage of a
mechanical disturbance — to a satisfactory issue, it is clear
that we ought to proceed in the following way : First, we
have to inquire whether it is not possible to find modes of
excitation for the plant which would be purely physiological,
and in which there can be no element of physical disturbance.
Transmitted effect in such a case could only be due to propa-
gation of excitatory protoplasmic change. We will next sub-
ject the question to the final test of the physiological block
which would arrest an excitatory impulse, but could have
no effect on the passage of a hydro-mechanical disturbance.

I shall now describe four different lines of investigations
each of which furnishes an independent proof of the excita-
tory character of the transmitted impulse : —

(i) On methods of excitation by the discriminative polar
action of electric currents.

(2) On the block of transmission of excitation by local
application of cold.

(3) On the electrotonic arrest of excitatory impulse.

(4) On the action of poison in inducing block of conduc-
tion.

Characteristic Polar Excitation by Constant Current

If in a muscle-and-nerve preparation of frog two electrodes
are applied on the conducting-nerve, at a certain distance
from the responding muscle, it is found that on sending a
current through the included portion of the nerve excitation
is induced, which on reaching the responding muscle brings
about contraction. There is no excitatory action in the case

i6o RESEARCHES ON IRRITABILITY OF PLANTS

when the current is estabHshed very gradually. The
electrical current, as such, has generally speaking Httle or
no excitatory action. It is only at the moment of its sudden
initiation, or sudden cessation, that the excitatory effect is
most conspicuously induced. It is found, moreover, that
an excitatory effect is induced by the * make ' of the current
at the kathode, or the point where the current leaves the
nerve ; at the * break ' of the current, on the other hand,
excitation is induced at the anode or the point of entry.

Precisely parallel effects I find to take place when an
electrical current is sent through a portion of the conducting
petiole. A detailed description of the polar effects of currents
will be given in a subsequent chapter. I shall here only
give what is essential to my present purpose.

Two non-polarisable electrodes are applied, the proximal
on the conducting petiole at a distance of lo mm. from the
responding pulvinus, and the distal on the parenchyma of
one of the leaflets, such tissue being non-conducting. If the
current now be gradually applied by continuously increasing
the E.M.F. from zero to 3 volts by means of a suitable potential
slide, we shall find that there is no excitatory effect. But
if the E.M.F. of 3 volts be applied suddenly, the direction of
the current being such that the proximal electrode is kathode,
we shall find that the kathode now becomes the seat of
excitation and the leaf undergoes a responsive fall after the
short and definite period required for the transmission of
excitation through the intervening distance.

If the experiment be then repeated with the proximal
electrode as anode, and the distal indifferent parenchyma as
the kathode, we shall observe no excitatory effect. This is
because the effective proximal electrode, which is anode, does
not excite at ' make.' The excitatory effect will however be
found to take place at the anode, but only at the * break ' of
current.

Reverting to the hydro-mechanical theory, we are con-
fronted with great difficulties in accounting for the excita-
tory effects in the petiole initiated locally at the electrodes.

CHARACTER OF TRANSMITTED IMPULSE i6i

There is no turgid mass of parenchyma here which by its
deformation might cause mechanical disturbance. It may
be thought that in some way exudation of sap might cause
the necessary hydro-mechanical disturbance. The question
now arises : How did excretion occur at the kathode at
' make ' ? As the excitation takes place at the anode at
' break ' are we to suppose that exudation also takes place
at the anode ?

In the often cited instances of hydraulic transmission of
stimulus of a violent blow or a cut, mechanical disturb-
ance is necessarily present. But transmission of excitatory
impulse is found to take place under polar excitation, in the
absence of all such disturbing factors. This will be realised
when in Chapter XVII it is shown that in Biophytum
excitatory impulse is transmitted by the action of an electric
current which is so feeble as not to be perceived by the very
sensitive human tongue. This would indicate that the effect
transmitted here is physiological rather than physical.
In a nerve the protoplasmic change which is the basis of
excitation is initiated locally at the point of kathode at
' make ' and at the anode at ' break.' The protoplasmic
change is then propagated from point to point giving rise
to the excitatory impulse. In the petiole also, excitatory
protoplasmic change is initiated locally at the kathode at
* make ' and at the anode at * break.' And every circum-
stance indicates a point-to-point propagation of excitatory
protoplasmic change in the conducting petiole.

Block of Conduction by the Action of Cold

It has been shown that the velocity of transmission is en-
hanced by favourable physiological changes due to warmth.
Conversely the conductivity is depressed by lowering of
temperature, and this depression may become so great as to
induce an actual arrest of conduction. The object of the
investigation being the influence of cold on conductivity,
special care has to be taken that the lowering of tempera-
ture does not in any way affect either the excitability of the

i62 RESEARCHES ON IRRITABILITY OF PLANTS

point of application of stimulus or the motile sensibility of
the responding pulvinus. For this reason cold is applied
locally on the petiole, half-way between the point of appli-
cation of stimulus of induction-shock and the pulvinus.
The experimental plant was highly sensitive on account
of the favourable summer season. An intensity of stimulus
of '5 unit applied at a distance of 30 mm. from the pulvinus
was found to be effectively transmitted. The intensity of
stimulus actually employed was 2 units, which was maximal.
A strip of cloth 10 mm. in breadth was wrapped round the

Fig. 90. — EfEect of cold in inducing retardation and arrest of transmission :
(i) Normal record ; (2) Retardation due to slight cooling ; (3) Arrest

^ , of conduction brought about by intense cold ; {4) Record of direct
stimulation.

petiole midway between the point of stimulation and the
pulvinus. This was for the purpose of local application
of cold by means of cooled water or by means of small
fragments of ice.

Successive records were then taken at intervals of
20 minutes, which is more than sufficient for complete
recovery from previous stimulation. Record i in fig. 90
gives the time-interval between the application of
stimulus and response under normal conditions. There are
20 '5 seconds spaces, each space representing -i second. The
latent period is -15 second. The true time of transmission
is thus 1-9 second for 30 mm. ; the transmission time

CHARACTER OF TRANSMITTED IMPULSE 163

through 10 mm. is therefore '63 second. The next record
was taken when the intervening length of 10 mm. in the
petiole was moderately lowered in temperature by the
application of cold water. This cooling should be com-
menced immediately after the previous responsive fall of the
leaf. This not only gives sufficient time for the localised
cooling of the petiole but also avoids the excitatory disturb-
ance of the pulvinus caused by sudden application of cold to
the petiole. During the localised cooling of the petiole the
leaf erects itself and becomes fully sensitive when the time
arrives for the application of the next stimulus. Record 2
exhibits the effect of moderate cooling ; the transmission
period is now prolonged, the difference between the two
records being a time-interval of -8 second. On the assump-
tion that the effect of cooling had remained localised, it is
seen that the lowering of temperature had prolonged the
period of transmission through the 10 mm. of the petiole
from -63 second to 1*43 second. The conductivity has
thus been reduced by more than half.

In record 3 is seen the effect of further lowering of
temperature by placing small fragments of ice on the strip
of cloth. The excitatory impulse initiated by the maximal
stimulus of induction-shock had hitherto been unfailingly
transmitted. But under the action of intense cold the impulse
is arrested. In order to show that the abolition is not due
to the depression of motile excitabihty of the pulvinus,
record 4 is taken of the effect of direct stimulation. An
inspection of the record shows that the motile excitability
has undergone no change. It is thus clear that the impulse
initiated by the stimulus has been arrested by the physio-
logical depression of conductivity induced by cold.

Paralysis of Conductivity and Restoration by
Tetanising Shock

In connection with this subject I came across the interest-
ing phenomenon of paralysis of conductivity as an after-
effect of intense cooling. After obtaining the record of the

M 2

i64 RESEARCHES ON IRRITABILITY OF PLANTS

block under local application of cold, the fragments of ice
were removed and the cooled portion of the petiole allowed
to regain the temperature of the room, which must have
been accomplished in the course of 20 minutes. After this,
on taking the record of the transmitted effect of stimulus I
found that the block of conduction was still persistent. The
conducting-power of the benumbed tissue is thus paralysed
for a period which generally lasts for about 45 minutes.
I have, however, discovered the very suggestive fact that
the lost conducting-power can be very quickly restored by
subjecting the paralysed portion of the petiole to the action
of tetanising electric shocks.

The Electrotonic Arrest of Excitatory Impulse

If in a nerve-and-muscle preparation a constant current
be maintained in an intervening tract between the point of
stimulation and the responding muscle, this current is found
to act as a block to the passage of excitation. With moderate
intensity of current the block of conduction is due to the
depressing action of the anode. For demonstration of
electrotonic block of nerve-conduction, an intensity of
stimulus is found which is effective under normal conditions.
But during the continuation of the blocking current the
excitatory impulse due to the testing stimulus is found to
be arrested. The transmission is, however, renewed on the
stoppage of the current. It is of special interest to have
thus at our disposal a physiological block which can be put
' on ' and ' off ' at will and many times in succession. The
alternate transmission and its arrest then affords a very
striking demonstration of the excitatory character of the
propagated effect.

I have found a similar block of conduction induced in the
plant by electrotonus. Various forms of testing stimulus may
be employed. It is, however, more satisfactory to employ a
form of stimulus the intensity of which may be either gradu-
ally increased or maintained constant. These requirements

CHARACTER OF TRANSMITTED IMPULSE 165

are fulfilled by thermic and electric modes of stimulation.
I will now describe two typical experiments on electrotonic
block, in Biophytum under thermal stimulation and in
Mimosa under electric stimulation.

Biophytum. — In this plant we have a whorl of leaves
bearing sensitive leaflets. Stimulus is applied on the
stem by means of the electro-thermic stimulator. The
intensity of stimulus is so graduated as to cause an excitatory
impulse to traverse the petioles, effecting the fall of the
leaflets in a centrifugal order. Selecting a leaf, electrotonic
block is applied in the middle part of the petiole. When
the anode A is to the left, the
excitatory impulse is found ar-
rested at A ; when the current
is reversed, the arrest is found to
take place at the new anode a'
to the right. On the stoppage of
the blocking current the excitatory '^ ir p

impulse is observed to traverse the

I. 1 1 it, r 4.U 1 x Fig. 91.— Arrangement of

whole length of the leaf. the electrotonic block.

Mimosa. — In the next series Polarising circuit p inter-

of experiments a different species P°^^^^ ^"^^^'^ .r''*'"f

^ ^ secondary circuit s and

of plant and a different testing- responding puivinus.

stimulus is selected. The employ- The polarising current

, r 1 i • 1 T r acts as a block, whether

ment of an electrical mode of ascending or descending.

stimulation completely obviates

the difliculty of securing a stimulus the intensity of
which may be either maintained constant or increased
in a known manner. With the help of a sliding induc-
tion-coil it is easy to arrive at an intensity of stimulus
which is always effective in normal circumstances. The
proximal of the two exciting electrodes was placed at a
distance of 30 mm. from the primary puivinus. Half-way
between the point of excitation and the puivinus were
placed two polarising electrodes, 5 mm. apart, through which
a constant current could be maintained, for the purpose
of serving as a block (fig. 91). The first and uppermost

i66 RESEARCHES ON IRRITABILITY OF PLANTS

of the three records in the next diagram was taken without
this block and with a stimulus intensity of 2. It will be
seen (fig. 92) that excitation was transmitted as usual, the
velocity of transmission in this case being 29 mm. per second.
The specimen was exceptionally vigorous and the season
the height of summer, which facts account for the high
velocity. The blocking current was next introduced. In
order to prevent the excitation due to sudden make of

Fig. 92. — Record of effect of electrotonic block. Uppermost
record normal ; lowest record shows block of trans-
mission of excitation ; middle record shows restoration
of conductivity on removal of block.

current, the applied e.m.f. was gradually increased from
zero to 2 volts, by means of a potential slide. During the
passage of this constant current a second stimulus was
applied of the same value as before ; and it will be seen,
from the lowest record, that there was no response, the
transmission of excitation being effectively blocked. In
order to show that this block would only persist during
passage of the current, the latter was next reduced gradually
to zero by manipulation of the potential slide. This was
carefully done, to avoid the excitation due to sudden
cessation of current. On again repeating stimulation, the
block was found to be no longer operative (see intermediate
record) and response due to transmitted excitation took
place as usual.

In the last figure records were taken on a fast-moving

CHARACTER OF TRANSMITTED IMPULSE 167

plate. In the next record (fig. 93) a series of response-records
to transmitted excitation were taken on a slower-moving
plate. The testing stimulus was always the same, the
difference being that the electrotonic block was ' off ' and

Fig. 93. — Records of transmitted excitation with the block
off and on. Arrest of transmitted excitation under
electrotonic block at b, b.

' on ' alternately. It will be noted that the excitation was
invariably arrested whenever the block was appUed at b, b.

Block of Conduction by the Action of Poison

(i) Experiment with Biophytum sensitivum

Reference has been made to the inconclusive character
of Pfeffer's narcotisation experiment. The ineffectiveness
of the block, it was explained, might have been due to the
thickness of the tissue preventing free access of the anaesthetic
to the conducting-elements in the interior. It occurred to
me that the physiological block induced by a drug could be
rendered more effective in two different ways : first, by the
selection of a thin petiole in which access of the solution
to the interior by absorption would be less difficult ; and,
second, by the employment of strong toxic agents like

i68 RESEARCHES ON IRRITABILITY OF PLANTS

copper sulphate or potassium cyanide solutions. The
choice of a strong poison was deemed advisable because the
absorption of even a small quantity might in such a case
prove effective in inducing depression or abolition of con-
duction. Application of an anaesthetic, like chloroform, has
the drawback that the escaping vapour renders the motile
organ insensitive. There is no such disadvantage in the
employment of a non-volatile poison like copper sulphate,
which by local application would affect the conductivity of
the selected portion of petiole without modifying the motile
sensibility of the pulvinus. The petiole of Biophytum was
found to be very suitable for these experiments. Among
the whorl of leaves, about half a dozen are found the leaflets
of which are fairly sensitive. Graduated stimulus is applied
to the stem by means of an electro-thermic stimulator.
This latter consists, as explained in a previous chapter, of
a V-shaped piece of platinum wire, through which an electric
current of suitable intensity could be sent, the circuit being
completed for a definite length of time by means of a
metronome. The electric current is so adjusted as to give
rise to a thermal shock without causing a burn. The effective
intensity can be gradually increased by taking advantage
of the additive effect of stimulus. Thus in a given case while
the thermal shock was singly ineffective, it became minimally
effective when repeated four times and maximally effec-
tive when repeated eight times. The excitatory impulse
originated at the stem, radiated subsequently to the leaves,
where the progress of the excitatory waves was visually
demonstrated by the serial fall of leaflets in a centrifugal
order. It should be mentioned here that the excitation
caused by thermal stimulus is most intense ; the block needs
to be very perfect to arrest the conduction of such an
excitation.

Arrest of conduction hy copper sulphate solution. — After
determining the value of an effective stimulus, poison was
applied to the portion of petiole which is next to the stem.
This was done by wrapping a strip of cloth 5 mm. in breadth

CHARACTER OF TRANSMITTED IMPULSE 169

round the petiole and soaking it with strong copper-sulphate
solution. Warm solution was found to be more quickly
absorbed than cold. Out of six sensitive leaves, two were
subjected to the local action of poison. The selective and
local arrest of the excitatory wave in the poisoned leaves
would then afford a conclusive demonstration of the physio-
logical character of the transmitted impulse. It should be
remembered that the absorption of poison is likely to be
a slow process. Hence the physiological block of conduc-
tivity induced by poison will become increasingly effective
with the duration of application. We may therefore expect
the following sequence of events after the application of
poison on a narrow zone of the petiole : —

(i) There will be no noticeable variation of conductivity
at the beginning.

(ii) After the lapse of a certain length of time the quantity
of poisonous solution absorbed will be sufficient to induce a
certain depression of conductivity. The minimal excitation
which was formerly transmitted will now undergo an arrest.
But the blocking action will not be sufficiently great to arrest
maximal excitation.

(iii) After a still longer interval, the depression of
conductivity induced by poison will be very great. Even a
maximal excitation will now be practically arrested.

(iv) This arrest will be due to the abolition of conduc-
tivity in the localised poisoned zone. The conductivity of
the petiole beyond the poisoned area, and the motile
excitability of the leaflets will remain unaffected.

I shall now describe experiments on the effect of poison
on conductivity. The experiment was repeated with
twelve different specimens of Biophytum, all of which gave
similar results. The following experiment may be taken as
representative of the rest. Among the whorl of leaves in
the particular specimen of Biophytum there were six which
were fairly sensitive. The additive effect of four successive
thermal shocks applied on the stem was found to be sufficient
to cause excitatory fall of the leaflets in all the six leaves.

170 RESEARCHES ON IRRITABILITY OF PLANTS

Two out of the six leaves were subjected to the local action
of poison in the manner previously described : —

(i) The testing stimulus was applied 15 minutes after
the application of poison. Excitatory impulse was found to
traverse all the leaves. Absorption of the particular poison
in the course of 15 minutes was too slight to induce any
marked depression of conductivity.

(2) The experiment was repeated after half an hour
under the same stimulus as before — namely, the additive
effect of four shocks. The excitatory impulse was found
transmitted in the four normal, but arrested in the two
poisoned leaves. The block in the two leaves was, however,
forced by the application of a stronger stimulus due to the
additive effect of six shocks.

(3) After two hours the conductivity of the poisoned
portion of the petiole was found practically abolished in
the two leaves. In these there was no transmitted effect,
even under the maximal stimulus of sixteen additive shocks.
The untreated leaves exhibited vigorous conduction, the
leaflets undergoing their serial fall with great rapidity.

(4) In order to show that the absence of the transmitted
effect in the two leaves was due to local loss of conductivity
of the treated area, and not to the loss of motile sensibihty
of the leaflets, stimulus was applied on the petiole beyond
the poisoned zone. The leaflets exhibited their normal
excitatory fall.

Mercuric chloride solution. — ^After obtaining the block of
conduction by the action of copper sulphate, I tried the
effect of various other poisons, some of which were found
to be very virulent in their action. Mercuric chloride
solution, for example, abolished the power of conduction
in a much shorter time than copper sulphate. The only
drawback in the application of mercuric chloride is that it
exerts an excitatory action, the transmitted effect of which
induces fall of the sensitive leaflets, which often remain
persistently closed.

Solution of potassium cyanide. — There is no such exci-

CHARACTER OF TRANSMITTED IMPULSE 171

tatory action induced by this reagent ; its toxic power in
abolishing conductivity is however very great. A strong
solution is applied on a portion of the petiole in the usual
manner, the leaflets remaining open and fully sensitive all
the time. After an interval of an hour it is found that
the effect of a strong thermal stimulus applied in the stem
fails to be transmitted across the poisoned zone. The
sensibility of the leaflets is however found unaffected.
The leaflets in those petioles which have not been poisoned
exhibit vigorous response to transmitted excitation.

(2) Experiment with Mimosa

I was next desirous of testing the effect of poison on a
different species of plant and verifying the result by means of
automatic records. The effect of copper sulphate solution in
abolishing conductivity was not, as we saw, immediate ;
it required time to have its toxic effect fully developed.
Hence it appeared of much interest to test, by means of
successive records, the progressive diminution of conducting
power culminating in actual arrest. The specimen employed
for these series of investigations was the petiole of Mimosa.
Successive records of transmission-time were taken at
intervals of 20 minutes, before and after subjecting an inter-
mediate portion of the petiole to the action of poison. Effec-
tive stimulus of the induction-shock was applied on the
petiole, generally at a distance of 30 mm. from the responding
pulvinus. The intensity of stimulus employed was maximal,
being 2 units. The first record of the series gives the velocity
of normal conduction ; the second and the subsequent
records exhibit progressive action of the toxic agent. This
latter was applied on a strip of cloth 10 mm. wide, wrapped
round the petiole midway between the point of stimulation
and the responding pulvinus.

Copper sulphate solution. — ^The normal record (i) in
fig. 94 shows response to have taken place 27 spaces after
the application of the stimulus, the interval between the

172 RESEARCHES ON IRRITABILITY OF PLANTS

successive dots being "i second. The total time was there-
fore 27 seconds. Subtracting from this the latent period
•15 second, we obtain 2*5 seconds as the actual time for
transmission through 30 mm. The time of transmission
through 10 mm. is therefore '83 second.

Record 2 of the series shows the effect of application
of copper sulphate solution for 20 minutes on a por-
tion of petiole 10 mm. in breadth. It is noticed that the
transmission-time has been prolonged by ten spaces, i.e.
by I second. Assuming that the effect of poison was

Fig. 94. — Effect of copper sulphate solution in the retardation and
final arrest of conduction : (i) Normal record ; (2) Retardation
caused by 20 minutes' application ; (3) Arrest caused by application
for 40 minutes ; (4) Record of direct stimulation.

localised, it is seen that it had during 20 minutes' application
delayed transmission through 10 mm. from -83 second to
1*83 second. By the absorption of a small quantity of
poison the conductivity has thus been reduced by more
than 50 per cent.

Record 3 of the series was taken after a further period
of 20 minutes. The transmitted effect is seen to be com-
pletely blocked by the application of copper sulphate for
40 minutes. In order to show that the absence of response
was due not to the abolition of motile excitabiUty of

CHARACTER OF TRANSMITTED IMPULSE 173

pulvinus but to the block of conductivity of the petiole,
a fourth record was taken under direct stimulation, which
proves that the motile excitability of the leaf had remained
unimpaired.

Mercuric chloride solution. — A series of records was next
taken exhibiting the effect of mercuric chloride solution.
The power of conduction was found arrested after a period
of application so short as 10 minutes. The record obtained
was very similar to that given in the next figure.

Potassium cyanide solution. — I next took a series of
records which exhibited the effect of strong solution of

Fig. 95. — Abolition of conductivity by the action of potassium cyanide :
(i) Normal record ; (2) Arrest of conduction after application for
five minutes ; (3) Record showing arrest of impulse due to very
strong stimulus ; (4) Record of direct stimulation.

potassium cyanide. Record i, fig. 95, gives the normal
transmission period. Record 2 was taken, as stated before,
after allowing 20 minutes for recovery. The poisonous
solution was however applied 15 minutes after the pre-
vious record ; hence in the second record we see the effect of
application of potassium cyanide for a period of 5 minutes
only. The effect of this poison on the conductivity of
petiole of Mimosa was so great that even with such a
short application the transmission of excitation caused
by maximal stimulus of 2 units was completely blocked.

174 RESEARCHES ON IRRITABILITY OF PLANTS

Record 3 was taken with the secondary pushed close to the
primary, the stimulus intensity being thus raised to 15
units. Even under this intense stimulation the conduction
was found to be arrested. Record 4 was obtained under
direct stimulation. The response shows that the sensi-
bihty of the pulvinus had undergone no change. It is thus
clear that the abolition of response to indirect stimulation
was solely due to the abolition of conductivity induced by
the action of poison.

The arrest of the transmitted impulse in Mimosa by
the physiological block induced by cold, by electrotonus,
and by local application of poison, completely disproves
the hydro-mechanical theory. The results of the various
investigations that have been described lead on the other
hand to the conclusion that the transmission of excitation
in the plant is a process fundamentally similar to that which
takes place in the animal, being in the one case as in the
other a propagation of protoplasmic change.

Summary

Excitatory reaction is initiated in the petiole of various
sensitive plants by the discriminative polar action of an
electric current. Excitation is induced at the kathodic
point at ' make ' and at the anodic point at * break.'

Transmission of such an excitatory impulse takes place
in the absence of all mechanical disturbances.

The excitatory nature of the impulse in plants is further
demonstrated by the arrest of conduction brought about by
various physiological blocks.

Local apphcation of increasing cold retards and finally
abolishes the conducting-power.

Conductivity is for a time paralysed as an after-effect of
application of cold. The lost conducting-power may, how-
ever, be quickly restored by tetanising electric shocks.

The elect rot onic block induces an arrest of conduction

CHARACTER OF TRANSMITTED IMPULSE 175

during the passage of the blocking current, the conducting-
power being restored on the cessation of the current.

Conductivity of a selected portion of the petiole may be
abolished by local application of poison. The abolition of
conducting-power takes place slowly under the action of
copper sulphate and quickly under potassium cyanide.

CHAPTER XIII

THE POSITIVE RESPONSE

Two opposite kinds of responses, negative and positive — Excitatory
contraction, negative turgidity variation, fall of leaf, and concomitant
negative electric variation — Positive electric response — Positive or
erectile mechanical response — Dual impulses under different forms
of stimuli — Exhibition of positive and negative impulses by different
plants — Conditions for obtaining positive response — Characteristics
of positive impulse — Masking and unmasking of positive effect — Laws
of Direct and Indirect effects of stimulus.

When the leaf of Mimosa is excited, the lower half of the
pulvinus undergoes relatively greater contraction ; in conse-
quence of this differential action the leaf falls down. There
is a concomitant expulsion of water from the excited cells,
with diminution of turgor. I have shown elsewhere i that
the excitatory reaction of plant tissue may also be detected
by a method altogether different — namely, by means of
electric variation. As regards the sign of electrical change,
the excited point is found to become galvanometrically
negative ; a similar electrical change is also known to take
place in the excited animal tissue. The excitatory electro-
motive change of galvanometric negativity is therefore the
same in the plant and the animal.

Excitation in a plant tissue is thus characterised by
concomitant effects of contraction, diminution of turgor or
negative turgidity variation, mechanical fall of the leaf,
and by the electrical response of galvanometric negativity.
For the sake of clearness we shall designate this normal
excitatory effect as the negative response.

In recording electrical responses of plants to indirect

* Bose : Comparative Electro-physiology (Longmans, Green & Co.)*

176

THE POSITIVE RESPONSE 177

stimulation I was often surprised to find the occurrence —
which appeared at first as an abnormal response — of gal-
vanometric positivity preceding the true excitatory reaction
of galvanometric negativity. Thus it appeared as if, in
consequence of stimulation, there originated two distinct
impulses, positive and negative, which travelled with different
velocities. The former travelled faster and, reaching the
responding point earlier, induced there the response of
galvanometric positivity. The negative wave with its
slower velocity of transmission reached the responding-
point later and induced the true excitatory effect of
galvanometric negativity.

It was also found that the negative effect was much
the stronger of the two ; in consequence of this, if the two
impulses reached the responding-point about the same time,
the positive was completely masked by the predominant
negative. Hence in order to bring out the positive it was
necessary to apply the stimulus at a distance. The slow-
moving negative then lagged behind the positive to such an
extent as not to mask it.

The following conditions are found to favour the exhi-
bition of the positive effect : —

(i) The stimulus should be applied at a distance from
the responding point ; the response is then found to be
diphasic, positive followed by negative. A distance may
again be found where, owing to the enfeeblement of trans-
mitted excitatory effect, the negative impulse fails to reach
the responding-point ; in such a case there is an exhibition
of only the positive effect. Conversely, if the stimulus be
applied too near the responding-point, the negative effect
alone is exhibited, the positive being masked by the
predominant negative.

(2) From what has been said it is easy to understand
that, with a very highly conducting tissue, the negative or
true excitatory effect will be transmitted with great rapidity ;
it will therefore mask the positive effect. Hence the
positive effect is more easily obtained with plants in

178 RESEARCHES ON IRRITABILITY OF PLANTS

which the velocity of transmission of excitation is relatively
slow.

(3) As the velocity of transmission of true excitation is
greater under stronger stimulus, a feeble stimulation should
be employed for the exhibition of positive effect.

The above are the results obtained from the electric
mode of investigation. We have seen that the electric
variation of galvanometric negativity corresponds to the
excitatory reaction of negative turgidity variation, con-
traction, and concomitant motile-response by the fall of the
leaf. The positive electric response should connote effects
the very reverse of these — ^positive turgidity variation,
expansion, and concomitant motile-response of erection of
the leaf. Hence if in consequence of stimulation two
distinct impulses are sent out with different velocities, we
should be able to demonstrate their existence in an alto-
gether different manner — namely, by means of two distinct
mechanical responses of erection and of fall of the leaf.

The difficulty that confronts us in this demonstration
lies in the relatively small amplitude of the positive response,
which is liable to pass unnoticed unless a high magnification
be employed. It is easier to demonstrate the existence of
the positive impulse by the employment of a magnifying
optical lever. A very light mirror is fixed to the fulcrum
rod of a lever, one arm of which is attached to the motile
leaf or leaflet. Under excitatory fall there is produced
a rotation of the fulcrum rod with its attached mirror ;
a spot of Hght reflected from the mirror thus exhibits a
responsive down-movement, indicative of normal negative
response by contraction. An erectile positive response, on
the other hand, is recognised by the movement of the spot
of Hght upwards. Responsive movement may in this
manner be magnified from a hundred to a thousand times.
The optical method is simple and efficient. It is also
well suited for purposes of demonstration before a large
audience.

Great difficulties are, however, encountered in obtaining

THE POSITIVE RESPONSE 179

a record of the positive response by means of a writing-lever.
The weight of the lever stands in the way of obtaining any
high magnification, especially in the case of leaflets, where
the force of responsive expansion or contraction is very
slight. By the employment of an extremely light recording-
lever made of aluminium, I was able to secure a magnifi-
cation of twenty times, which was the highest that could be
obtained in the circumstances. With very good specimens
this magnification is often sufficient to exhibit the positive
effect. In order to reduce friction and obtain time-records,
the recording-plate was made to oscillate to and fro once
in a second or once in 2 seconds. The successive dots
in the record thus indicate intervals of i or 2 seconds.
It has sometimes been possible to employ the Resonant
Recorder with long writing-index for recording the positive
response of Mimosa. The vibration-frequency of the writer
in such a case was five times in a second.

Having thus secured two different methods of observa-
tion by means of optical and recording levers, I proceed to
demonstrate : —

(i) That whatever may be the form of stimulation em-
ployed, two impulses are transmitted, of which the positive
travels with a higher velocity than the negative.

(2) That such double impulses are exhibited not by any
particular plant but by various species of plants. The
phenomenon may therefore be regarded as universal.

Dual Impulses under Different Stimuli

The positive effect, as previously stated, may be separated
from the negative by taking advantage of the different rates
of transmission of the two impulses. The lag of one impulse
behind the other may be increased (i) by taking a specimen
in which the velocity of transmission of excitation is low,
and (2) by the appHcation of the stimulus at a distance
from the responding organ. It should be remembered
that the velocity of transmission is not only different in

N 2

i8o RESEARCHES ON IRRITABILITY OF PLANTS

different species of plants, but varies to a certain extent
in different individuals of the same species. Again, the
transmission under feeble stimulus is slower than under
strong stimulus. This accounts for the result frequently
obtained — that the propagation-time is much quicker under
the strong stimulus of thermal shock than under the
moderate stimulation by constant current.

Another interesting fact which has attracted my
attention is that the velocity of transmission is, generally

Fig. 96. — Positive followed by negative impulse in Biophytum, caused
by single indirect thermal stimulus. In this and in the following
records, * down ' curve represents positive, and ' up ' curve negative,
response. Frequency of oscillation once in a second.

Speaking, slower in the stem than in the petiole. There
also seems to be a loss of time when excitation has to
pass from the stem to the leaf. Taking advantage of
these facts we may, when desired, obtain long periods of
transmission by applying stimulus of moderate intensity
on the stem.

I will now describe experiments giving quantitative
results, which demonstrate the occurrence of two distinct
impulses under different forms of stimuli, the experimental
specimen being Biophytum sensitivum.

Thermal stimulus. — The electro-thermic stimulator was
employed for the appUcation of thermal shocks. The record

THE POSITIVE RESPONSE i8i

was taken by means of the Oscillating Recorder, the fre-
quency of oscillation being once in a second. Successive
dots are thus at intervals of a second. In the experiment,
the record of which is given in fig. 96, stimulus was applied
on the petiole at a distance of 50 mm. from the responding
leaflet. It is seen from the record that in answer to the
stimulus two distinct responses of opposite signs occurred
in succession. The positive or erectile response (repre-
sented by down curve) is seen to have occurred 1*5 second
after the application of the thermal shock ; the excitatory
or negative response took place much later, that is to say,
22 seconds after the shock. The velocity of the positive
impulse is here 33 mm. per second, that of the negative
being 2*3 mm. per second.

From the experiment just described it is clear that a
single stimulus gives rise to two impulses, positive and
negative. The positive travels at a faster rate and gives rise
at the responding-point to the erection of the leaflet indi-
cative of positive turgidity variation. The negative or the
excitatory impulse travels at a slower rate, inducing at the
responding organ negative turgidity variation, contraction,
and the fall of leaflet.

It was stated that the transmission-time is relatively
long when stimulus is applied on the stem instead of on the
petiole. This will be seen clearly from the results of experi-
ments which I shall now describe. Thermal stimulus was
first applied on the stem, the distance of the responding
leaflet being 10 mm. of stem and 20 mm. of petiole. The
positive response took place 3 seconds and the negative
21 seconds after the application of the thermal shock.
Stimulus was next applied on the petiole at a distance of
20 mm. Had the velocity of transmission in the stem and
the petiole been the same, then the negative impulse would
have reached the leaflet after an interval of f^ X 21 seconds
or 14 seconds. Instead of this, the transmission-period
in the petiole was found to be much shorter — namely.

i82 RESEARCHES ON IRRITABILITY OF PLANTS

6 seconds. A statement of the results of two different
experiments is shown in Table I.

Table I. — Showing Difference of Times of Transmission through
Stem and Petiole

No.

Distance.

Transmission-time
for + impulse.

Transmission- time
for— impulse.

I

(a) Stem lo mm.

and petiole 20 mm.
{b) 20 mm. of

petiole only.

seconds
3

2

seconds
21

6

2

{a) Stem 15 mm.

and petiole 20 mm.
(b) 20 mm. of

petiole only

3

2

27
5

Chemical stimulus. — The next record was taken under
the stimulus caused by the application of a drop of hydro-
chloric acid on the petiole, at a distance of 30 mm. from the

Fig. 97. — Positive response followed by negative in
Biophytum by the application of chemical stimulus.
Successive dots at intervals of i second.

responding leaflet. The record (fig. 97) shows that the
positive response occurred i second and the negative
14 seconds after the apphcation of the stimulus.

Induction shock. — Stimulus was applied on the petiole
at a distance of 30 mm. The positive response took place

THE POSITIVE RESPONSE

183

2 seconds and the negative 14 seconds after the appHcation
of the stimulus.

Constant current. — Stimulation was here induced by the
* make ' of kathode. The intervening distance was 20 mm. :
the positive impulse traversed this length in 3 seconds, the
negative requiring a longer period — namely, 9 seconds.

Condenser discharge. — The distance of the responding
leaflet from the point of application of stimulus was 25 mm. :
the positive impulse reached the leaflet 4 seconds after the
discharge-shock, but the transmission time of the negative
was much longer — namely, 15 seconds.

Table II. — Showing Transmission of Positive and Negative Impulses
IN Biophytum under different Forms of Stimuli

Organ
of

Distance

Transmission-
time for

Transmis-
sion-time

No.

Stimulus.

for

trans-
mission.

in mm.

+ impulse
in seconds.

— impulse
in seconds.

I

Petiole

Th'ermal

20

2

5

2

»

))

20

2

6

3

>>

>>

50

2

29

4

>>

>>

40

I

30

5

))

>»

65

25

50

6

>>

>>

50

I

22

7

>>

Chemical

30

1-5

14

8

>»

>>

60

7

16

9

>>

Induction-
shock

30

2

15

lO

>>

>>

60

9

23

II

>>

Condenser
discharge

25

4

15

12

a

Constant
current

20

3

9

13

Stem

and

petiole

Thermal

35

3

27

14

»

>>

35

3

30

15

>>

>>

30

2

45

16

»

>>

30

3

21

I give above a table showing the effect of various stimuli

i84 RESEARCHES ON IRRITABILITY OF PLANTS

on different specimens of Biophytum. In some cases the
stimulus applied was on the stem, the distance mentioned
being the sum of the length of stem and petiole through
which the two impulses were transmitted.

It has thus been shown that under the action of various
modes of stimulation two distinct impulses are transmitted,
of which the positive travels faster than the negative. The
specimen employed for these demonstrations was Biophytum
sensitivum. I next proceed to show that these results
are not confined to any particular plant but are universally
present.

Exhibition of Positive and Negative Impulses by
Different Plants

Averrhoa carambola. — The velocity of transmission of
excitation in the petiole of this plant is low, being i mm. per
second or even less. In the first experiment of the series

1 applied the stimulus
of induction-shock at
the moderate distance
of 10 mm. from the
responding leaflet, and
obtained automatic re-
cord by means of the
Oscillating Recorder.
The successive dots here
are at intervals of

2 seconds. It will be
noticed that a respon-
sive movement of erection took place after an interval of
three dots or 6 seconds ; the negative response occurred
after the much longer interval of 20 seconds (fig. 98).

In the next experiment with a different specimen stimulus
was applied at a distance twice as great as in the previous
case, that is to say, 20 mm. The positive response took

Figs. 98, 99. — Positive followed by nega-
tive response in Averrhoa carambola
due to indirect stimulation.

THE POSITIVE RESPONSE

185

place 14 seconds, and the true excitatory effect 48 seconds,
after the application of the stimulus (fig. 99).

In an experiment where the chemical mode of stimu-
lation was employed, the distance to be traversed was
40 mm. The positive impulse reached the responding leaflet
19 seconds, and the negative impulse 50 seconds, after the
application of the stimulus.

Thermal stimulus was applied in another experiment
at a distance of 70 mm. The transmission periods for
the positive and negative impulses were 22 seconds and
65 seconds respectively.
The negative impulse
thus lagged behind the
positive by as much as
43 seconds.

Mimosa pudica. — It
was stated that in speci-
mens like the petiole of
Mimosa, where the velo-
city of transmission of
excitation was high, the
positive response was
liable to be masked by
the predominant nega-
tive. It is therefore only
on rare occasions that

I obtained a positive response by stimulating the petiole
of Mimosa. This is seen in fig. 100, where stimulus of
induction-shock was applied at a point on the petiole
30 [mm. from the responding pulvinus. The Resonant
Recorder having a long writing-index was employed for
obtaining the record. The vibration-frequency of the writer
was five times in a second, hence successive dots represent
time-intervals of '2 second. It will be seen that the
positive response took place '6 second and the negative
3-2 seconds after the application of the stimulus.

It was stated that, by applying stimulus at a sufficient

Fig, 100. — Records showing positive
response in Mimosa followed by
negative. Stimulus was applied on
the petiole 30 mm. from pulvinus.
Vibration-frequency 5 per second.

i86 RESEARCHES ON IRRITABILITY OF PLANTS

distance on the stem, the transmission period could be made
as long as desired. This will be seen in fig. loi, where
thermal stimulus was applied on the stem of Mimosa at

Fig, ioi. — Positive response, preceding the negative in Mimosa.
Stimulus was applied on the stem. Frequency of oscilla-
tion I per second.

some distance from the pulvinus. The record was taken
on a slowly moving plate, by means of the Oscillating
Recorder, the successive dots being made at intervals of a

Table III. — Periods of Transmission of Positive and Negative
Impulses in the Petiole of Averrhoa and Stem of Mimosa

No.

Specimen.

Distance.

Stimulus.

Transmission-
period for

Transmis-
sion-period
for

+ impulse

— impulse.

mm.

seconds.

seconds.

I

A verrhoa

70

Thermal

22

65

2

))

130

>>

40

95

3

>>

10

Induction-
shock

6

20

4

}>

20

>)

14

48

5

>>

35

Chemical

21

50

6

Mimosa

5

Induction-
shock

•5

12

7

}}

10

)>

•6

9*4

8

a

20

>>

I

10

9

}>

60

>>

2

29

ID

»

35

Chemical

5

17

THE POSITIVE RESPONSE 187

second. The positive response is here seen to take place
4 seconds, and the negative 41 seconds, after the appHcation
of stimulus. The negative impulse in this case lagged as
much as 37 seconds behind the positive. In Table III will
be seen the transmission-periods of positive and negative
impulses in different specimens of Averrhoa and Mimosa.

Conditions Favourable for the Exhibition of Positive

Response

I will now describe experiments which bring out the
conditions which are favourable for the manifestation of
either the positive or the negative response : —

I. Positive response is more easily obtained under feeble
stimulus. — This is demonstrated by the following experiment

Fig. 102. — Effect of intensity of stimulus in the induction of positive or
negative response. Lowest record under stimulus-intensity of i,
middle record under 5, and the uppermost record under 8 units.
Vibration frequency 5 times per second.

on Mimosa, where successive stimuli were applied at the
same distance on the stem, the intensity being gradually
increased in a known manner. The distance of application
was 10 mm. The vibration-frequency of the writer was
5 times per second. The first and the lowest record of the
series (fig. 102) was taken under the stimulus intensity of i.
It will be seen that under this relatively feeble stimulus
a positive or erectile response, indicative of positive turgidity

i88 RESEARCHES ON IRRITABILITY OF PLANTS

variation, was alone induced '7 second after the application
of stimulus ; there was no indication whatsoever of the
later occurrence of the negative response. After the usual
interval of 20 minutes the next or middle record was taken
under the enhanced intensity of stimulus of 5 units. We
now observe the appearance of negative following the
positive. The positive took place '6 second and the negative
9'4 seconds after the application of stimulus. Finally,
when the stimulus-intensity was raised to 8 units, the
positive response took place at the same interval as
before, namely '6 second, but the negative or excitatory
response took place earlier than in the last case, that
is to say, after an interval of 4*6 seconds, instead of
9*4 seconds.

In another experiment with a different specimen, the
positive response took place '5 second after the application
of stimulus of intensity 2 ; there was no negative response.
The point of application of stimulus was kept always

Table IV. — Effect of Intensity of Stimulus on the Positive and
Negative Responses in Mimosa

Specimen.

Stimulus
intensity.

Transmission period
for + impulse.

Transmission
period for
— impulse.

I ..

I

5

8

second

•7
•6
•6

Negative

absent

9-4 seconds

4-6 seconds

"■•1

2
4

•5
•5

Negative

absent
12 seconds

the same — namely, 10 mm. When the intensity of stimulus
was increased to 4, the negative response was found to
occur after the positive. The former took place '5 second.

THE POSITIVE RESPONSE 189

and the latter 12 seconds, after the appHcation of the
stimulus.

From these experiments it is seen that while under feeble
stimulus we obtain only the positive response, on increasing
the intensity of stimulus the negative or excitatory response
makes its appearance in succession to the positive. Another
noteworthy fact is that while an increasing intensity of
stimulus enhances in a marked manner the velocity of
transmission of the negative or excitatory impulse, it has

Fig. 103. — Effect of diminishing the distance of point of application of
stimulus. Stimulus applied at a distance of 20 mm. gives rise only to
positive response (lower record) . Reduction of distance to half gives
rise to positive followed by negative response (upper record). Vibra-
tion frequency five times in a second.

little or no effect on the velocity of the positive impulse.
Thus in the first experiment of the series, while the trans-
mission-period of the negative impulse was shortened
from 9*4 seconds to 4*6 seconds by the stimulus increasing
from 5 to 8 units, the transmission period of the positive
impulse remained unchanged at "6 second.

2. Positive response is more easily obtained under a feeble
stimulus applied at a distance. — For the demonstration of
this, a feeble stimulus is applied on the stem of Mimosa
at a distance of 20 mm. from the responding pulvinus. The
response is only positive, occurring i second after the

190 RESEARCHES ON IRRITABILITY OF PLANTS

application of the stimulus. When the distance of application
is reduced to half, we find that the negative or excitatory
response makes its appearance in succession to the positive.
The positive is seen to take place "8 second and the negative
10 seconds after the application of the stimulus. It will be
noticed that the reduction of the distance to half causes
only a slight diminution in the transmission-period of
the positive impulse — from i second to -8 second. The
transmission-period of the negative impulse, on the other
hand, undergoes as we have seen a rapid diminution with
diminishing distance.

It has thus been shown that when a feeble or moderate
stimulus is applied at a relatively great distance there occurs
only the positive response. As the distance is reduced the
antagonistic negative response makes its appearance, there
being at first a considerable time-interval between the posi-
tive and the negative. On reducing the distance still further
the interval becomes reduced. The positive will then, as
we shall see later, become masked by the predominant
negative.

Characteristics of the Positive Impulse

I shall now pass under review the various characteristics
which distinguish the positive from the negative impulse.

(i) The positive impulse travels much faster than the
negative.

(2) The conditions for the exhibition of the positive
impulse are that the stimulus should be moderate or
feeble and applied at a distance. A semi-conducting tissue
again is more suitable for its exhibition than a highly
conducting tissue.

(3) While an increase of intensity of stimulus gives rise
to enhanced velocity of negative impulse, it produces
little or no change in the velocity of positive impulse.
Thus Table IV shows that while raising the intensity of
stimulus from 5 to 8 units shortened the transmission-

THE POSITIVE RESPONSE 191

period of the negative from 9*4 seconds to 4*6 seconds,
it induced no variation in the transmission-period of
the positive impulse which remained unchanged at
•6 second.

(4) When the distance to be traversed is reduced say
to half, the period of transmission of the negative impulse
is also reduced to half. But with the positive impulse
the diminution is very slight. Thus in fig. 103 we find
that with a distance of 20 mm. the period for the propa-
gation of the positive impulse was i second ; when the
distance was reduced to half, the transmission-period
was reduced not to half a second but to eight -tenths of a
second.

It is difficult from these data to arrive at any definite
conclusion as regards the nature of the positive impulse.
Is this impulse physical or physiological ? If the former,
could it be hydro-mechanical ? In consequence of a
feeble stimulus applied at a distance, we have at the
responding pulvinus, a positive turgidity variation, expan-
sion and erection of the leaf, evidently brought about by
the forcing in of water. This presupposes a forcing out of
water somewhere else, probably at the point of application
of stimulus. It may be supposed that an active contraction
occurred in the plant-cells under excitation, in consequence
of which the sap was forced out giving rise to a hydraulic
wave. On this supposition the positive impulse is to be
regarded as hydro-mechanical. There is, however, no
definite experimental proof in support of this theory. I
shall presently describe the experiment I devised to settle
the question. Unfortunately the results obtained were
not as decisive as I hoped they would be.

It should be clearly understood that even if the positive
impulse were found to be hydro-mechanical, it has nothing
whatever to do with the propagation of the excitatory or
negative impulse. For if the negative response took place
in consequence of the impact of the positive impulse,
then the negative should take place at a definite interval

192 RESEARCHES ON IRRITABILITY OF PLANTS

after the positive, the interval being equal to the latent
period of the responding pulvinus.

I have shown that the average latent period of the
primary pulvinus of Mimosa is 'i second. In unfavourable
circumstances even, I have never found it to exceed
*3 second. The latent period of the leaflet of Biophytum
or Averrhoa is about '4 second. The latent period of the
responding organs of the various sensitive plants mentioned
may therefore be taken as less than i second. If between
the positive and negative response there is any relation
of cause and effect, then the negative should occur within
a second of the positive. But this is by no means the case.
Referring to fig. loi we find that the positive impulse
reached the pulvinus 4 seconds after the application of
the stimulus, and instead of the excitatory response taking
place within a second, we find that it did not occur till
37 seconds after ! I give below a table which shows the
very lengthened periods which are often found to elapse
between the arrival of the positive impulse and the initiation
of the excitatory response : —

Table V. — Showing Difference of Time between the Arrival of
Positive Impulse and the Initiation of Excitatory Response

Transmission-

Transmission-

Difference
of time.

Specimen.

period of

period of

+ impulse.

—impulse.

seconds

seconds

seconds

Mimosa

4

41

37

Biophytum . .

2

29

27

„

25

50

47*5

A verrhoa

22

65

43

>»

40

95

55

The results just described, together with the fact that
we may have a positive response without the subsequent
negative (cf. lower record fig. 103), show conclusively that

THE POSITIVE RESPONSE 193

*

the excitatory response is independent of the positive
impulse. The physiological character of the negative
impulse has, moreover, been fully demonstrated by various
crucial experiments described in the last chapter.

For deciding the question as to the physical or physio-
logical character of the positive impulse, I employed the
test of local application of cold on the velocity of trans-
mission. It was found that while moderate application
of cold delayed the transmission of the negative impulse,
it had little effect in retarding the transmission of the
positive. Further test by the physiological block induced
by excessive cold was, however, less satisfactory. This
block not only arrested the negative impulse, but sometimes
brought about an apparent abolition of the positive also.
This latter result cannot, however, be taken as decisive.
The amplitude of positive response is naturally small ;
under unfavourable circumstances it may have become
so diminished as to pass unnoticed. In these circumstances
the question of the character of the positive impulse may
for the present be left an open one.

Masking and Unmasking of the Positive Effect

It has been shown that as the distance of the point of
application of stimulus is reduced, the transmission period
of the negative impulse is reduced at a greater rate than
that of the positive. When the point of application of
stimulus is at some distance, the negative impulse lags con-
siderably behind the positive ; but as the distance is reduced,
the lag tends to disappear. It thus happens that when the
distance of application is sufficiently reduced, the positive
effect is masked by the predominant negative.

In an experiment with Biophytum, electric stimulus was
applied at a distance of 30 mm. from the responding leaflet.
The positive response was initiated 2 seconds after the
appHcation of the stimulus and persisted for a further period

194 RESEARCHES ON IRRITABILITY OF PLANTS

of 12 seconds ; the negative impulse then reached the
leaflet, giving rise to the reversed negative response (fig. 104).
The transmission period of the negative impulse is thus
14 seconds for a distance of 30 mm. The distance was next
reduced to half, or 15 mm., and on repeating the stimulus

Fig. 104. — Effect of dimi-
nishing the distance of
application of stimulus.
The lower record shows
diphasic response in Bio-
phytum, positive followed
by negative. By applying
stimulus nearer, the posi-
tive is masked by the
predominant negative
(upper record) . Frequency
of oscillation once in a
second.

Fig, 105. — Positive (3), diphasic
(2), and negative response
(i) of Mimosa brought
about by gradual diminu-
tion of distance of appli-
cation of stimulus.

the positive impulse was found masked, the negative
occurring alone after an interval of 6*5 seconds.

I will next describe another experiment carried out with
Mimosa. Stimulus was applied on the stem at a distance
sufficiently great to give only the positive response. This
is shown in the lowest record in fig. 105. The point of
application of stimulus was then brought somewhat nearer,
with the result that a diphasic response took place, positive
followed by negative — as depicted in the middle record.
And finally, on applying stimulus still nearer, the positive
was completely masked by the predominant negative.

THE POSITIVE RESPONSE 195

We have hitherto dealt with the question of the suppres-
sion of the positive by the negative. It is sometimes possible
to unmask this suppressed positive by separation of the
two impulses brought about by the differential effect of cold.
Stimulus is at first applied on the stem at a distance near
enough to give rise to the resultant negative response. A
strip of cloth is then wound round the intermediate conduct-
ing portion of the stem, and the region gradually cooled.
The result of this cooling is to reduce the velocity of trans-
mission of true excitation.

I have in this manner often succeeded, by careful and
gradual cooling of the intermediate region, in converting
a resultant negative into a diphasic — positive followed by
negative ; further cooling gave rise to the positive alone.
In the case of the diphasic response, the result was brought
about by the lowering of the velocity of transmission of
excitation by cold, in consequence of which the negative
lagged behind the positive. In the case of the conversion
into positive, the effect was due to the arrest of the excitatory
negative impulse.

Direct and Indirect Effects of Stimulus

The effect induced by a given stimulus is modified, as
we have seen, by its point of application. A stimulus
may be applied directly on the responding organ, or it
may be applied at a distance from it. Following the usual
terminology we shall designate the former as Direct Stimu-
lation, and the latter as Indirect Stimulation. When
stimulus is applied directly on the responding organ, there
is induced an excitatory fall of the leaf concomitant with
contraction and negative turgidity variation. This parti-
cular reaction we shall designate as the Direct Effect of
stimulus. When, on the other hand, a feeble stimulus is
applied at a distance, there occurs only a positive response

02

196 RESEARCHES ON IRRITABILITY OF PLANTS

of erection of leaf with concomitant expansion and positive
turgidity variation. For simplicity we shall designate
this particular reaction as the Indirect Effect of stimulus.
If the intervening tissue be highly conducting and the
stimulus sufficiently strong, then the excitatory negative
effect masks the positive. In such a case the effect of
indirect stimulation is the same as that caused by direct
stimulation. But if the intervening tissue be semi-conduct-
ing, or if the stimulus be feeble or applied at too great a
distance, then we obtain the positive or Indirect Effect.

Two diametrically opposite effects may thus be induced
by an identical stimulus, depending on direct or indirect
application. The existence of the positive or indirect
effect of stimulus has hitherto been unsuspected. It
must be taken into full account in unravelling the com-
plexities of reaction in a responding organ.

The laws of Direct and Indirect Effects of stimulus
may thus be enunciated : —

The effect at the responding region of a strong excitation
transmitted through a short distance, or through a good con-
ducting channel, is negative, being the same as the effect under
direct stimulation.. The response is hy negative turgidity vari-
ation, contraction, fall of leaf, and electrical change of galvano-
metric negativity. This is the Direct Effect of Stimulus.

The effect of feeble stimulus transmitted through a great
distance, or through a semi-conducting channel, is positive.
The responsive reaction is by positive turgidity variation, expan-
sion, erection of leaf, and electrical change of galvanometric
positivity. This is the Indirect Effect of Stimulus.

Summary
A single stimulus gives rise in the plant to two impulses,
positive and negative. The positive travels at a faster rate
and induces positive or erectile response of the responding
leaf. The negative or excitatory impulse travels at a slower
rate and induces in the responding leaf a contractile fall.
The laws of Direct and Indirect Effects of stimulus are —

THE POSITIVE RESPONSE 197

The effect at the responding region of a strong excitation
transmitted through a short distance, or through a good con-
ducting channel, is negative ; the response is by negative tur-
gidity variation, contraction, fall of leaf, and electric change of
galvanometric negativity.

The effect of feeble stimulus transmitted through a great
distance or through a semi-conducting channel is positive. The
response is by positive turgidity variation, expansion, erection of
leaf, and electric change of galvanometric positivity.

CHAPTER XIV

POLAR EFFECTS OF ELECTRICAL CURRENT IN EXCITATION
OF PLANTS

Polar excitation in animal tissues — Anomalous reactions in Protozoa —
Mono-polar method — Current Reverser — Excitatory polar action in
plant — Method of record — Effects of ascending and descending
currents in animal and plant — Records giving time-relations —
Potential slide — Measurement of e.m.f. and current — Potential key-
board.

Towards the end of a previous chapter I made brief refer-
ence to certain remarkable excitatory effects which I have
observed in the plant tissue at the initiation or cessation of
a constant current. Electrical currents are well known
to have certain characteristic effects in the case of animal
tissues. On applying a current of feeble intensity to a
muscle-preparation by means of two suitable electrodes,
it is found that, at the moment of its sudden starting, an
excitatory contraction is initiated at the point of kathode —
that is to say, where the current leaves the tissue ; no such
effect takes place at the anode, the point at which it enters.
With current of feeble intensity there is, again, no excitatory
effect either at the anode or the kathode on the cessation or
break of current. It should be remembered that these
marked exhibitions of excitation are caused by sudden
variation of the current. They do not take place if the
current variation is made very gradual.

These excitatory effects occur not only when the appli-
cation of the current is direct but also when it is indirect.
That is to say, when the two electrodes are applied on
the conducting nerve in a nerve-and-muscle preparation,

POLAR EFFECTS OF ELECTRICAL CURRENT 199

excitation with a feeble current is initiated only at the
kathode at ' make,' this excitation being then conducted
along the nerve to cause the contraction of the terminal
muscle.

It has been ascertained that this particular type of effect
is by no means universal. In working on the polar effects of
currents on Protozoa, it was found by Kiihne, Verworn,
and others that the phenomena there displayed are more
or less the opposite. Hence it has been assumed by some
that the law of polar reaction in unfibrillated protoplasm
is entirely different from that which holds good in animal
tissues in general.

I shall, however, be able to show that the reactions in the
undifferentiated protoplasm of the plant body are identical
with those of highly differentiated animal tissues. Experi-
ments with plants, moreover, led me to the discovery that
what is known as Pfliiger's Law is not a complete statement
of the polar reactions that take place in living tissues. For
it will be shown that under high electromotive forces a new
class of phenomena comes into prominence, culminating in
a more or less complete reversal of the normal reactions.
From a consideration of these it would appear as if
the effects on Protozoa were perhaps to be regarded as less
anomalous than has hitherto been supposed.

In studying the polar effects of an electrical current on
the plant, I shall first take a simple case and employ the
pulvinus of Mimosa as the contractile organ by which the
excitatory action is to be detected. A straight form non-
polarising electrode is employed for making the necessary
electrical connections. This consists of a glass tube the
lower half of which is filled with kaolin paste moistened
with normal saHne. A wet thread hangs down from the
kaolin and makes contact with the plant. The upper half
of the tube, above the kaolin paste, is filled with a saturated
solution of zinc sulphate ; an amalgamated thin rod of zinc
dips into the zinc sulphate and serves as the electrode. In
the Mono-polar method of experiment one contact is now

200 RESEARCHES ON IRRITABILITY OF PLANTS

made at or near the pulvinus, and the other, with a distant
indifferent point, lower down on the main stem.

For suddenly starting or stopping a current, or for making
an electrode anodic or kathodic, we may use Pohl's Com-
mutator or Reverser (fig. io6). When the commutator is
in the middle position, the current is interrupted ; when
tilted to the left, the current enters the plant through the
stem and leaves it by the pulvinus, which thus becomes the
kathode. On partially tilting the commutator to the right,
the current is broken. On tilting further to the right, the

Fig. io6. — Pohl's commutator, for causing make and break of
kathode or anode.

current is suddenly reversed and the pulvinus becomes the
anode.

For experiment I took a leaf of Mimosa which was in
a fairly sensitive condition ; the electrical connections were
made in the manner already described. The electrical
resistance offered by the plant tissue between the two points
of contact was found to be half a million ohms. An e.m.f.
of 4 volts was found effective in inducing excitation at
make, when the pulvinus was the kathode. The intensity
of the exciting current was feeble, being 8 micro-amperes.
A micro-ampere, it should be noted, is one-milHonth part
of an ampere. When the commutator was turned to the

POLAR EFFECTS OF ELECTRICAL CURRENT 201

left, the pulvinus was suddenly made the kathode, and this
induced an excitatory fall of the leaf. It has been said
that the exciting action is in general induced by sudden
variation, and not during continuation of current. Hence,
if after the excitatory fall the current is continued, the leaf
is found to re-erect itself and to have its excitability restored.
The current is now broken ; this induces no excitation.
The commutator is then tilted to the right, the pulvinus
being made the anode ; this again induces no excitation,
nor is there any further excitation at the break of the anode.

It is thus seen that with a feehle current the polar effects
in Mimosa are precisely the same as in animal tissues —
namely, excitation only at the make of the kathode.

Having thus described the fundamental reaction of plant
tissues under feeble currents, we have still to study the
effects of stronger currents and the modifications that may be
induced in the normal reactions under changing conditions,
both internal and external. Indisputable evidence on
such points can only be obtained if it is possible to have
automatic records made by the plant itself. This I have
been able to secure by the method of record already described,
with the addition of a contrivance by which definite signals
are marked at the base of the record, at the moment of
application of either kathode or anode, its continuation,
break, renewal, and so forth. For this a rotating reversing
key has a disc attached to the axis. Semi-rotation of this
axis to the left or to the right, or its position half-way
between, renders the pulvinus kathode or anode, or suffices
to break the current. The disc has a thread wound round
its circumference, so that when the commutator-axis is
rotated in one direction the thread is wound, and in the
opposite direction unwound. This thread is ultimately
led to, and wound about, a second wheel with a long index,
which is to serve as the marker. This second wheel is
suitably fixed above the recording-plate, with the tip of the
index resting on it at the same vertical line but a little
lower than the response-recorder. When the commutator-

202 RESEARCHES ON IRRITABILITY OF PLANTS

key is turned to the left, the current is suddenly established,
the pulvinus being made kathode. The marker, which
has up to this moment been tracing a horizontal line, will
simultaneously mark an up-line, as shown in the record.
During the continuation of the current, the recorded signal
will appear as horizontal but at a higher level. The break
of kathode will be indicated by a down-line, stopping at
the original level. Anode-make is produced by turning
the key to the right, and this is indicated in the record as a

down-line passing below the
horizontal level. A horizontal
line below the general level
indicates the continuation of
the anode. An up-line reach-
ing the general level indicates
the break of anode.

Two records are reproduced
(fig. 107) exhibiting this polar
reaction under feeble current
in a second and more highly
sensitive specimen. The
effective e.m.f. was in this
case as low as 2 volts. It
will be seen that response
took place only at kathode-
make, there being no effect
either at kathode-break or
anode-make or break. The next response was taken under
the increased e.m.f. of 4 volts. The results depicted are
similar to those in the last case, the only difference being
the exhibition of an increased excitatory effect at kathode-
make due to the higher voltage.

Excitation by Ascending and Descending Currents

In the mono-polar method above described we have the

effect of one particular electrode, quite distinct and isolated

from the other. But when both the electrodes are placed

Fig. 107. — Record of polar exci-
tation under feeble current.
Response seen to take place
only at make of kathode.
The line of signal below indi-
cates make or break of kath-
ode or anode. Up-line above
normal denotes kathode-
make ; down-line indicates
anode-make.

POLAR EFFECTS OF ELECTRICAL CURRENT 203

on a conducting tissue, then there is no such distinctive
isolation and the excitatory effect initiated at either of the
electrodes may be transmitted to the terminal motile organ,
there to induce contractile response. We have an example
of this in the nerve-and-muscle preparation of a frog, where
under feeble current excitatory contraction of the muscle
is seen to take place at make of both ascending and descend-
ing currents in the nerve. The terms * ascending ' and
' descending ' may be found somewhat confusing, especially
in plant-experiments. The meaning will be clear if we
remember that the current is said to be descending when it
flows towards the responding motile organ. In the case of
descending current in a nerve-and-muscle preparation, the
kathode is proximal as regards the responding muscle ;
with the ascending current, the kathode is distal.

As already stated, excitation takes place in nerve-and-
muscle preparation at make of both ascending and descend-
ing currents. This is explained as due to make of the
proximal kathode in the case of descending, and make of the
distal kathode in the case of ascending, currents.

It is true that in the latter case the excitation has to
traverse the anodic area before it can reach the motile organ.
The anode induces, in general, a depression of conductivity
which may even block the passage of excitation. In the
present case, however, this blocking action is ineffective on
account of the feebleness of the current.

The fact that it is the proximal kathode of the descending,
and the distal kathode of the ascending, current which form
respectively the seats of excitation, may be proved by
taking time-measurements of the interval between * make '
of the current and the response of the muscle. With the
descending current the interval should be shorter, since in
such a case the kathode is proximal. This has been found
to be the case.

Characteristics in every way similar are found in experi-
ments with the petiole and pulvinus of Mimosa. In this
case the petiole contains the conducting tissue and the

204 RESEARCHES ON IRRITABILITY OF PLANTS

pulvinus is the indicating motile organ. The paralleHsm
of the two cases will be clearly understood by reference
to fig. io8.

With both ascending and descending currents of feeble
intensity I have obtained with Mimosa excitatory effects
at make exactly corresponding to the effects in a nerve-and-
muscle preparation. In order to prove that the excitation
is really due to make of the proximal kathode in the case of
descending, and of distal kathode in that of ascending,

A -^K

Fig. io8. — Excitation by descending or ascending currents in nerve-
muscle and petiole-pulvinus specimens. Motile organs are to the
left. Nerve-muscle (upper figure) ; petiole-pulvinus (lower figure).

currents, I took time-records of these, as seen in fig. 109.
The proximal electrode was placed at a distance of 2*5 mm.
and the distal at a distance of 15 mm. from the pulvinus.
The distance between the two electrodes was therefore
12 "5 mm. The time-record was taken in the usual manner,
the vibrating recorder employed having a frequency of
20 vibrations per second. The appHed e.m.f. was 6 volts.
The two records were obtained successively, the upper being
the one due to descending, and the lower to the ascending,
current. An inspection of the records in fig. 109 at once
shows that the response with descending current, the
kathode being proximal, took place the earlier of the two.
Theoretically the difference between the two time-

POLAR EFFECTS OF ELECTRICAL CURRENT 205

records should be equal to the time taken by excitation to
travel the intervening distance of 12 "5 mm. between the
electrodes. This time-interval should be equal to ^^,
where V is the velocity of transmission in the given specimen.
The velocity can easily be deduced from the two records
themselves. From the upper record we see that response
took place after 8 spaces, each representing one-twentieth
of a second. The true time taken by stimulus to traverse
the distance of 2*5 mm. is therefore /(j— L, where L is the

Fig. 109. — Records of responses to ascending and descending make
currents in Mimosa. In the upper record the current was
descending, kathode being proximal to pulvinus. In the
lower record the current was ascending, kathode being
distal. Frequency of vibrating recorder 20 D.V.

latent period of the pulvinus, the average value of which we
found to be -I second ; hence V = ^ = 8*3 mm. per second-
From the second record we find the true time taken by
excitation to traverse the distance of 15 mm. to be i-8— L
= 17 second. V = ^V^ = 8-8 mm. per second. The mean
velocity obtained from the two records is therefore 8*6 mm.
Hence the difference of time in the two cases of excitation by
ascending and descending current should be equal to the
time taken by the excitation to travel I2'5 mm. the distance
between the two electrodes. This difference should there-
fore be -|| =1-4 second. That this is really the case
can easily be tested by the simple process of counting the

2o6 RESEARCHES ON IRRITABILITY OF PLANTS

time-difference in the number of dots in the two records.
On counting we find the difference to be 28 spaces, each
representing one-twentieth of a second. That is to say, the
time difference is actually 1*4 second, precisely what was
inferred from theoretical considerations.

Effects of Ascending and Descending
Induction-Shocks

In the case of animal tissues, single induction -shocks of
moderate intensity are known, as regards polar action, to be

Fig. 1 10. — Records showing time-difference between re-
sponses due to ascending and descending induction-
shocks. Distance between two electrodes 5 mm. ;
upper record with kathode nearer the pulvinus.
Vibrating recorder 20 D.V.

effective at the commencement and not at the termination
of the current. Hence excitation here, as in the case of
constant current, takes place at the kathodic region. In
the case of plants also I find the same to hold good. The
exciting electrodes from an induction-coil were placed on
two points on the petiole, separated from each other by
5 mm. Two records were taken with single induction-
shocks of intensity 2, now in descending and afterwards in
ascending directions. In the first or upper of these records
(fig. no) the proximal electrode was the kathode ; in the
lower, the kathode was distal. It will be seen from these
two experiments that if it be the kathode which excited

POLAR EFFECTS OF ELECTRICAL CURRENT 207

then the arrival of excitation at the pulvinus will be quicker
when the kathode is near and later when the kathode is
distant. It is clear from the records that this is what
obtains ; excitation is earlier when the kathode is proximal
and later when the kathode is distal. We find moreover
that the difference of time between the moments of arrival
of excitation in the two cases is represented by six spaces,
each of '05 second. Hence the delay in the second case
is equal to '3 second, due to transmission through the
additional distance of 5 mm. The velocity of transmission
is therefore 16 mm. per second, which we have seen is the
average velocity of transmission through the petiole of
Mimosa.

After obtaining these characteristic effects with Mimosa
under feeble electric currents, we have still to find out
whether such effects are peculiar to this plant or whether
they are universally present. To answer this question we
have to experiment with every species of plant that happens
to be provided with a motile indicator. After this has been
done we have still to determine the effects of increasing
intensity of current brought about by the application of
increasing e.m.f.

The Potential Slide

For the application of a graduated increase of e.m.f.
up to 12 volts or thereabouts, a Potential Slide is very
convenient. This consists of two platinoid wires stretched
side by side. Two spring-contacts e and e' are carried by
a slide between the two wires. When a battery of six
storage-cells is applied at the terminals of these wires, at
A and B, the difference of electric potential at the two points
is 12 volts. When the slide is at the extreme end, to the
right, the difference of potential between E and e' will be
zero, whereas in being moved to the left the difference of
potential or e.m.f. between E and e' will continuously increase
till at the extreme left it is 12 volts, e being, say, positive,
and e' negative. The electrical current enters the plant

2o8 RESEARCHES ON IRRITABILITY OF PLANTS

by E, which is therefore the anode. It leaves by e', which
is therefore kathode. The advantage of the sHde is that
the E.M.F. appHed may be gradually increased from zero to
maximum, or decreased from maximum to zero. In certain
experiments, where we wish to apply a given e.m.f. without
causing excitation, this is essential. For we have seen
that a sufficiently graduated increase or decrease of current
causes no excitation. If excitation be desired, however,
it may be induced by a sudden variation of current brought
about by suddenly moving the slide either backwards or

Fig. III.— The Potential Slide.

forwards, or by suddenly completing or interrupting the
main circuit (fig. iii).

It is important also to know the e.m.f. that has been
appHed and the intensity of the current flowing through
the plant in a given experiment. For the former, a scale
fixed on the apparatus may be previously calibrated so as to
give the values of the e.m.f. at the different positions of the
sliding-contacts, it being understood that the same number
of storage-cells are always applied at the terminals. Or we
may place a voltmeter to measure the applied e.m.f. between
the two sliding-contacts.

For the measurement of the currents that flow through
the plant, a micro-amperemeter is used. Each division

POLAR EFFECTS OF ELECTRICAL CURRENT 209

of this instrument indicates one-millionth of an ampere.
The currents which usually have to be measured vary from
one to a hundred micro-amperes. A single instrument
provided with appropriate shunts enables us to measure all
these intensities.

Another point which should be borne in mind is the possi-
bility of induction-currents being generated in the measuring
instruments themselves, when the main current is suddenly
turned on and off. In order to guard against any disturbance
that might be caused in this way, it is necessary at the
moment of experimenting on the plant to put the voltmeter
and the micro-ammeter out of action for the time. On the
completion of each experiment the e.m.f. and the current
employed are measured by means of the voltmeter and the
micro-ammeter.

In experiments where still higher voltage may be
required this potential slide cannot be used, as the wires
become unduly heated. We may in such a case modify
the potential slide by replacing the platinoid wire with two
parallel troughs containing copper-sulphate solution. In
place of the spring-contacts two copper plates are here
employed, dipping into the solution. These, forming the
moving contacts E and e', are attached to the slide. With
this form of electrolytic potential slide a graduated e.m.f.
up to 100 volts may be easily obtained. The terminals A
and B are connected with a battery of the required number
of storage-cells from the installation in the laboratory.

A more convenient arrangement is a portable set of
storage-cells, with a Potential Keyboard, by means of
which any voltage up to 100 volts may be readily obtained
by steps of 2 volts at a time. Small storage-cells of fiat
form may be obtained 6 cm. broad, 2*5 cm. in depth, and
6 cm. in height. Fifty of these cells, giving a total e.m.f.
of 100 volts, may be packed in a small box 25 by 30 cm.,
having a height of 7-5 cm. Ten of these cells are arranged
in units, and forty in four groups of ten each.

As regards the former, the ten cells are arranged in

210 RESEARCHES ON IRRITABILITY OF PLANTS

series, the positive end of the first being connected with the
first projecting button. The negative end of each cell i, 2, 3,
and so on, is led to its own button. A key k carries a radial
arm, which during a rotation from left to right will make
successive connections with the projecting buttons, and
thus cause the e.m.f. between e and e' to rise by increments
of 2 volts at a time, the maximum e.m.f. thus obtained by
contact with the tenth button being 20 volts (fig. 112). A
second key, k', not shown in the diagram, may in a similar

Fig. 112. — The Potential keyboard,

manner make contact with other buttons, which are con-
nected with terminals of cells in groups of ten. Thus the
number of cells which may be included between the two
electrodes may be varied from 0 to 50, and the derived e.m.f.
may thus be raised from o to 100 volts by steps of 2 volts
at a time.

Having thus found suitable means of applying currents
of increasing intensity, we shall in the course of subsequent
chapters study in detail the effects of feeble, moderate, and
strong currents on various sensitive plants.

Summary

In Mimosa, under feeble intensity of current, excitation
takes place only at the make of the kathode. In this the
polar reaction in plant is the same as in animal tissues.

POLAR EFFECTS OF ELECTRICAL CURRENT 211

The effects of feeble ascending and descending currents
in the petiole of Mimosa are parallel to those in nerve-and-
muscle preparations. In both cases excitation takes place
earlier when the kathode is nearer the responding organ.

As in animal, so also in Mimosa, single induction-shocks
of moderate intensity are, as regards polar action, effective
at the commencement and not termination of the current.
In taking records with single ascending and descending
induction-shocks, response is found to take place earlier
when the kathode is proximal.

V 2

CHAPTER XV

POLAR EFFECTS OF FEEBLE AND MODERATE CURRENTS ON
VARIOUS SENSITIVE PLANTS

Polar effects of feeble and moderate currents on (i) leaflets of Mimosa,
(2) leaflets oiBiophytum, (3) leaflets of Neptunia, (4) leaflets otAverrhoa
carambola, (5) leaflets of Averrhoa hilimhi, and (6) primary leaf of
Mimosa — Excitation with feeble current only at kathode-make —
Excitation at kathode-make and anode-break, under moderate
current — ^Tabular statement of results.

In the previous chapter I described some typical experi-
ments showing the effects of feeble electrical currents on
the pulvinus of Mimosa. In order to demonstrate that
such effects are universal, it will be necessary to extend this
investigation to every kind of motile organ possessing
requisitive sensitiveness. I here give a list of the specimens
employed.

First, the leaflets of Mimosa form very sensitive speci-
mens, though there are certain experimental difliculties to
be overcome. These leaflets are borne on four sub-petioles
that radiate from the end of the main petiole. While the
excitatory effect is shown in the primary pulvinus by
causing the fall of the leaf, the same effect is seen in the
pulvinules by the folding of the leaflets upwards.

Second, the leaflets of Neptunia are moderately sensitive.
This plant flourishes in tanks, but can with care be grown
in pots. The excitatory effect is exhibited by the folding
upwards of the leaflets.

Third, the leaflets of Biophyttim sensitivum are very
sensitive and serve the purpose of the investigations in an
admirable manner. These leaflets show excitation by
folding downwards.

POLAR EFFECTS OF MODERATE CURRENTS 213

Fourth, the leaflets of Averrhoa caramhola are only
moderately sensitive. They close downwards on excitation.

Fifth, the leaflets of Averrhoa hilimhi possess more or
less the same kind of excitability as A. caramhola. Under
excitation these leaflets fold downwards.

Sixth, and lastly, we have the primary pulvinus of
Mimosa whose polar reactions have already been briefly
referred to. In this and a subsequent chapter I shall
give a more exhaustive series of experiments in connection
with the response of the leaf of this plant.

These practically exhaust the list of plant-organs possessed
of sensitiveness sufficient for our purpose. It will be my
object in this chapter to give a detailed description of the
reactions of these various specimens.

We shall find that the character of the reaction will
depend on the intensity of the electrical current. With a
feeble current a characteristic effect is induced which will
be designated as that of Type I. As the acting e.m.f. is
gradually increased, the effect is transformed into that of
Type II. The characteristic effects of feeble and moderate
currents on various sensitive plant-organs will now be dealt
with in some detail.

In these experiments, I shall generally speaking employ
the Bi-polar method, which possesses many advantages
over the Mono-polar method already described. In the
Bi-polar method the electrical contacts are made with two
points, at or near two different motile-organs. When the
commutator is turned to the right, the right contact is
suddenly made kathode and the left anode. We are now
able to note the excitatory effects, if any, at kathode-make,
Km, to the right, and at anode-make, A'm, to the left. The
dash affixed in this latter case will always be used to distin-
guish the contact on the left side. The circuit is now broken,
and we observe the effects of kathode-break, Kb, and anode-
break, A'b. The commutator is next tilted to the left,
making the right anode, and enabhng us to note the effects of
K'm and Am. The circuit is then broken, producing the

214 RESEARCHES ON IRRITABILITY OF PLANTS

results K'b and Ab. It will be noticed that in the second
half of this cycle of the operation we subject the experiment
to the further test of corroboration by reversal.

The make-effect may be observed simultaneously at both
anode and kathode at the beginning of the experiment.
The break-effect also may be observed immediately after-
wards, provided there has been no excitation during make.
But if such excitation has occurred, the current may be
maintained till the leaf or leaflet affected has re-erected
itself, with accompanying recovery of excitability. The
time required for this process is from lo to 15 minutes
in the leaf of Mimosa and about 3 minutes in the case of
the leaflet of Biophytum. It is the sudden variation of the
current, and not its continuance, that causes conspicuous
excitation. The current may now be interrupted and the
break-effect observed.

If it is desired to study the effect of break, apart from
a previous over-continued maintenance of current, con-
ceivably inducive of fatigue, we may proceed as follows :
By means of the potentiometer-slide already described the
current is initiated and brought to a maximum, so gradually
as to cause no excitation. The break can now be effected
suddenly, in order to note any excitatory effect that may
be characteristic. It will be seen that by adopting this
method the necessity for maintaining the current, while
waiting for the recovery of excitability, is avoided. In
order to avoid unnecessary repetition I may state here that
I have employed both methods in studying the break-
effect, and that as a matter of fact, in ordinary circum-
stances and within limits, there is but little modification of
normal results induced by the previous maintenance of
current, the intensity of which at its maximum is of the
order of millionths of an ampere.

The distinctive effects of Types I. and II. forming the
subject of the present chapter, are brought about the former
by relatively feeble and the latter by moderate currents.
With a given specimen, increasing the e.m.f. from zero,

POLAR EFFECTS OF MODERATE CURRENTS 215

a minimal value of current is reached at which the effect
characteristic of Type I. begins to be evident. On continuing
to increase the e.m.f., the same characteristic effect is still
for a time obtained, until a critical value of the current is
reached above which the effect observed is transformed into
that which is characteristic of Type 11. There is thus a
given range within which we obtain the characteristic effect
of a particular type.

The minimally effective current for the induction of any
given type of excitatory effect is modified by the condition
of the specimen, being relatively low when the plant is
highly excitable. Thus age, season, and temperature are
all modifying factors. Different species of plants, again,
exhibit different susceptibilities to excitation by a constant
current. Thus the leaflets of Mimosa pudica and Biophytum
sensitivum are highly susceptible, whereas the leaflets of
Averrhoa are much less so.

It must also be borne in mind that under the same e.m.f.
the intensity of the exciting current will depend on the
electrical resistance interposed between the two contacts
by the intervening tissue. This resistance, it is obvious,
will vary with the distance between the two contacts and the
character of the specimen. Thus the resistance offered by
a specimen of Mimosa, where one contact is at or near the
pulvinus and the other on the stem, 7 cm. below, will be
of the order of 0*5 million ohm ; whereas that interposed
by the same length of a thin petiole of Biophytum will be
something like 15 million ohms. Hence it will be seen that
with different plants the value of the e.m.f. applied does
not by itself give a correct idea of the intensity of the exciting
current. In a series of experiments with the same plant,
where the resistance is constant, an increasing e.m.f. does
actually connote an increasing current. But in different
specimens it should not be assumed that the higher e.m.f.
necessarily means a higher value of the exciting current.
With a high e.m.f. we may have a feeble current and vice

2i6 RESEARCHES ON IRRITABILITY OF PLANTS

versa, on account of the very different resistances offered
by different specimens.

To give some idea of the order of magnitude of
the current found efficient in causing excitation, I shall, in
typical cases, specify its value as read by an interposed micro-
ammeter. The unit of current found convenient for these
investigations is, as I have stated elsewhere, one-millionth
part of an ampere, designated as a micro-ampere or sym-
bolised as 10-6 ampere.

Having given a general description of the method
employed, we will now study the effects of feeble and
moderate currents in inducing excitation. I shall begin by de-
scribing the responsive effects seen in the leaflets of Mimosa.
The bi-polar method has the advantage, as already stated,
that the effects at anode and kathode are simultaneously
displayed ; moreover, the selection of the leaflets instead
of the primary pulvinus as indicators of the excitatory
effects enables us not only to observe the occurrence of
local excitation but also to watch its propagation, as shown
in a very striking manner by their serial closure.

Leaflets of Mimosa

Effect of feeble current. The sensibility of leaflets of this
plant is considerable, being even greater than that of the
leaf. The conducting power of the sub-petiole is of the same
order as that of the main petiole. Selecting a young leaf of
Mimosa, I made appropriate connections with two neigh-
bouring sub-petioles, the length interposed between the
two contacts being 4 cm. I shall distinguish the right of
these as R and the left as L.

In making repeated experiments on the secondary petioles
of Mimosa bearing the leaflets, much trouble is occasioned
by the travelling of the excitation to the primary pulvinus,
in consequence of which the leaf is apt to fall and thereby
interrupt the electric circuit. This is obviated by means of
a special electrode-holder. A lateral rod, carrying a piece

POLAR EFFECTS OF MODERATE CURRENTS 217

of ebonite, slides up and down on a vertical stand and can
be fixed at any height by a screw. The piece of ebonite
carries two small plates of brass, each supporting a bored
cork through which slides a straight -form non-polarisable
electrode. The two pieces of brass can also be adjusted
laterally, so as to make the electrical connections with the
two sub-petioles at varying distances apart. The main
petiole is also supported mechanically and thereby kept

Fig. 113. — ^The electrode-holder.

from falling, by a strut attachment underneath, as shown
in the diagram, which represents two out of four sub-petioles

(fig. 113).

Having thus supported the leaf, the next point is to
make effective electrical contacts, with middle points on
the two neighbouring sub-petioles. In doing this, consider-
able difficulty is encountered by reason of the waxy coating
on the surface of midrib and leaflets. On this account there
is apt to be no proper electrical contact between the moist
thread of the electrode and the specimen. This obstacle may
be overcome, however, by directing a jet of very dilute solution
of ether or alcohol — either of which will dissolve wax —

2i8 RESEARCHES ON IRRITABILITY OF PLANTS

upon the required point and immediately washing it off with
water. A minute speck of kaoHn paste is then attached to
each of the electrodal spots, with the point of a fine brush.
The electrical contact with the moistened thread is now
found to be perfect.

The E.M.F. is then applied and gradually increased, till
the minimally effective intensity is found. If the two sub-
petioles be equally excitable, then the same minimal stimulus
will excite both to the same extent. If their sensitiveness
be slightly different, then the extent of excitatory trans-
mission will be different in the two cases. In the present
case the excitabilities of the two sub-petioles were practically
the same, and the minimally effective e.m.f. was found to
be 4 volts. The exciting current was "j micro-ampere.
When the current was suddenly made by turning the Pohl's
commutator to the right, kathode-make, or Km, was effected
on R, .and anode-make, or A'm, on L. No effect was found
to be induced at A'm, but excitation was induced at Km.
This excitation not only caused closure of the pair of leaflets
at the kathodic point but was irradiated in both directions
from this point. Eleven consecutive pairs of leaflets under-
went successive closure in the intra-polar tract, and three
pairs in the extra-polar — a fact which may be indicated
by the symbol ii ^ ^ 3. In this convention it will al-
ways be understood that <- indicates intra-polar and ->
extra-polar transmission. Having observed the effects of
Km and A'm, the circuit was then broken, thus causing
kathode break, Kb, at R, and anode-break, A'b, at L. No
excitatory effect was induced.

In order to test these results further, the key was now
tilted to the left, making L kathode and R anode. This,
it will be noted, is the reverse of the previous arrangement.
The excitatory effects were now found to have changed
their places, the new kathode K' at L showing excitation at
make, with a similar overflow as before — namely, 11 -<- -> 7.
It is obvious that only by accident could the excitability
and conductivity of two different sub-petioles be exactly the

POLAR EFFECTS OF MODERATE CURRENTS 219

same ; hence the slight difference in the extent of trans-
mission in the two cases. As regards the sub-petiole R, the
anode-make, Am, showed no excitation. On break of the
circuit, again, no excitatory effects were observed at either
anode or kathode. This cycle of operations was repeated
once more, with precisely similar effects.

The results of these experiments will be seen, in proper
sequence, in the following tabular statement, which sum-
marises the effects caused by currents acting in a given
direction — the direct — and its reverse, for the purpose of
corroboration. It should be remembered that at any
given moment, when a point on the one sub-petiole is made
kathode, K, a point on the other is simultaneously made
anode, A'. Excitatory contraction or closure is denoted
by C,v and the direction of the arrow indicates the trans-
mission in intra- and extra-polar directions. O denotes
absence of excitation.

■ Table I. — Effect of Feeble Current on Leaflets of Mimosa

Direction of current.

1

R. Sub-petiole. L. Sub-petiole.

1

_ ("Make
Reverse ^^^^^

Km..Cii< — >i\ A'm O

Kb ..0 A'b ....0

Am , . 0 1 K'm C ii <--> 7

Ab .. 0 K'b ....0

The results given in the above table may be expressed in a
more condensed form as follows : —

Table II. — Leaflets of Mimosa under Feeble Current
E.M.F. =4 volts; current = "j micro-ampere

R. Sub-petiole.

L. Sub-petiole.

Km
C

Kb Am
0 0

Ab
0

K'm
C

K'b
0

A'm
0

A'b
0

Excitation formula — Km

220 RESEARCHES ON IRRITABILITY OF PLANTS

In another series the e.m.f. was gradually raised in
successive experiments. The specimen in this case was
older and less sensitive than the former. The minimally
effective e.m.f. was here found to be 8 volts. Successive
observations were taken with an e.m.f. of 8 volts, 12 volts,
and 14 volts. In all these the excitatory effect only took
place at kathode-make. The sole difference was in the
increased extent of transmission of excitation with higher
voltage. With 8 volts only one pair of leaflets underwent
closure ; with 12 volts this was increased to 7 pairs ; with
14 volts to 9 pairs. With still higher voltage the responsive
effects became those of Type II., which will be described
presently.

I carried out twenty-five sets of experiments with different
specimens of Mimosa on the effect of feeble current. The
minimally effective e.m.f. was found to vary from 4 to 12
volts, according to the age of the specimen. The young
leaflets were excited under a lower e.m.f. than the old. In
all these, without a single exception, excitation was found
to take place only at the kathode at make.

Hence as the result of these direct and reverse experi-
ments on the effect of feeble current, undertaken with various
specimens of leaflets of Mimosa, we find one invariable
result — namely, that kathode excites at make and that no
excitation is occasioned by the break of kathode or by the
make or break of anode. The excitation formula of the first
type is, to follow the parallel nomenclature in the case of
animal tissues, KCC, or, as I have preferred for the sake
of convenience, the simpler formula Km, which is a short
way of saying that excitation took place at the make of
kathode. The excitation formula will always be represented
by letters in thick type.

Effect of moderate current. — The characteristic effect of
the first type is obtained, normally speaking, from an
E.M.F. of 4 volts, and continues till the e.m.f. is raised to
about 8 volts. In the case of young and excitable specimens
we obtain here a transition from the effect of the first to

POLAR EFFECTS OF MODERATE CURRENTS 22t

that of the second type. With older specimens, the first
type may persist up to 12 volts.

Taking an excitable specimen of leaflets of Mimosa,
I first applied 4 volts and obtained with it Type I. effect,
that is to say, excitation took place only at the make of
the kathode. I then raised the e.m.f. to 8 volts. At make,
the kathodic point on the right became the seat of excitation,
four pairs of leaflets undergoing closure in the extra-polar
and six pairs in the intra-polar regions. There was no
effect at the anode. At break, however, excitation was
initiated at the anode, as evidenced by the serial closure
of three pairs of leaflets in the intra-polar and two pairs in
the extra-polar regions. This effect was corroborated by a
reversal experiment, when the new kathodic point to the
left exhibited excitation at make, and the new anodic point
to the right exhibited it at break. Thus with current of
moderate intensity, excitation took place in the sub-petiole
of Mimosa at the make of kathode and break of anode.
The polar excitation formula of the leaflets of Mimosa under
moderate current may therefore be expressed as KCC, AOC ;
or Km Ab.

Table III. — Leaflets of Mimosa under Moderate Current
E.M.F. = 8 volts ; current = i-6 micro-ampere

R. Sub-petiole.

L. Sub-petiole.

Km
C

Kb
0

Am
0

Ab
C

K'm
C

K'b
0

A'm
0

A'b
C

Excitation formula — ^Km Ab

I carried out this experiment on the polar effects of a
moderate current on leaflets of Mimosa with twenty-five
different specimens, and obtained invariably the charac-
teristic effects of excitation only at the make of kathode
and the break of anode.

222 RESEARCHES ON IRRITABILITY OF PLANTS

Leaflets of Bjophytum

No specimen appears better suited for experiments
on polar excitation than the leaflets of Biophytum. There is
but little difficulty here in securing good electrical con-
nections; the conductivity is moderate, the velocity of
transmission being about 3 mm. per second. The suscepti-
bility of Biophytum to polar excitation is considerable, an
extremely feeble current being sufficient to induce excitation.
Its greatest advantage, however, lies in the rapidity of its
recovery from excitation, which is practically effected in
the course of about 3 minutes. Thus employing the same
plant, experiments can be repeated under different conditions
without undue waste of time.

Young leaflets are highly excitable and the older speci-
mens less so. Thus we have a wide range of selection accord-
ing to the particular characteristics which we wish to bring into
prominence. When the plant is highly excitable the bi-polar
connections are made, on the middle points of opposite
leaves, at a distance of 7 cm. from each other. The electrical
resistance is high, being in the case described about 15 million
ohms. With older and less sensitive specimens the two con-
nections may be made on the same leaf, about 3*5 cm. apart.
There will then be about five pairs of leaflets in the intra-
polar and about two pairs in each of the extra-polar regions.

When the electrical connections are made with two
leaves of fairly equal sensitiveness the excitations induced
in the two will be equal. But if one leaf be old and the other
young, then a given excitatory effect will occur earlier,or under
feebler stimulus, in. the younger. The minimally effective
E.M.F. will thus be lower in this case than in the older.

Effect of feeble current. — In order to study the effect of
feeble current, I took a pair of leaves of Biophytum whose
sensitiveness was approximately equal. The minimally
effective e.m.f. was here found to be 4 volts, the current
being of the extremely feeble intensity of '5 micro-ampere.
The excitatory effect was induced at the right contact,
when that point was made kathode, six pairs of leaflet^

POLAR EFFECTS OF MODERATE CURRENTS 223

undergoing closure in the intra- and two pairs in the extra-
polar regions. No effect was induced to the left by anode-
make A'm. Nor was there any effect induced at break of
current, Kb or A'b. On reversal, the left contact showed
the excitatory reaction of K'm, two pairs of leaflets under-
going closure in the intra- and none in the extra-polar
regions. The difference in the extent of transmitted excita-
tion as given in Km and K'm is here due to the fact that
the left leaf was slightly the older and less sensitive of
the two. On break there was no effect induced at either
kathode or anode. I carried out in this way more than
fifty experiments regarding the excitatory effect of feeble
current on the leaflets of Biophytum. The minimally effec-
tive E.M.F. in these cases varied from 4 to 8 volts, and the
range of e.m.f. through which the effects of Type L persisted
was found to be between a minimum of 4 volts and a maxi-
mum of 20 volts or thereabouts. In all these experiments of
the first type excitation took place only at kathode-make
and not at kathode-break, or anode-make or break. This
result was without a single exception. The characteristic
excitatory formula for Biophytum, under feeble current, is
thus, as in leaflets of Mimosa, Km.

The following table gives the result with feeble
current : —

Table IV. — Effect of Feeble Current on Leaflets of Biophytum
E.M.F. = 4 volts ; current = -5 micro-ampere

R. Contact.

L. Contact.

Km
C

Kb
0

Am

Ab
0

K'm
C

K'b
0

A'm
0

A'b
0

Excitation formula—

-Km

Effect of moderate current. — ^Taking another specimen, I
applied an e.m.f. of 22 volts between two points in the same
leaf, the intensity of current being 2'i micro-amperes. In

224 RESEARCHES ON IRRITABILITY OF PLANTS

carrying out the experiments, I found that the excitation
took place at make of kathode and break of anode only.
I give a sketch showing these effects of kathode-make and
anode-break (fig. 114). The figure at the left shows the
depression of leaflets starting from the kathodic point
which was on the left. After their recovery the circuit was
broken and excitation is seen to have taken place at the
anode, on the right, and not at the kathode, on the left.
Thus the leaflets of Biophytum, with a moderate current,

K A

Fig. 114. — Illustration to the left shows excitation induced by the make
of kathode, that to the right the effect of break of anode.

gave the same reaction as the leaflets of Mimosa, the
excitatory formula in both being Km Ab. I carried out
more than fifty experiments on the effect of moderate
currents on the leaflets of Biophytum, and the results
without a single exception were as has been described.

The tabular statement below shows the results due to
a current of moderate intensity on leaflets of Biophytum
as given in the typical experiment. Each experiment, it
should be remembered, was carried through the usual cycle
of direct and reverse currents.

Table V. — Effect of Moderate Current on Leaflets of Biophytum
E.M.F. = 22 volts; current = 2"i micro-amperes

R. Contact.

L. Contact.

Km
C

Kb
0

Am
0

Ab
C

K'm
C

K'b
0

A'm
0

A'b
C

Excitation formula — ^Km Ab

Experiments made on leaflets of other plants, in which

POLAR EFFECTS OF MODERATE CURRENTS 225

the sensibility is much less than that of the leaflets of
Mimosa and Biophytum, will next be described. Amongst
these the first plant dealt with is Neptunia.

Leaflets of Neptunia

This plant is somewhat like Mimosa pudica in appear-
ance, except that under natural conditions in Bengal it
grows in water, being an inhabitant of the pools amongst
the ricefields. In these circumstances it develops curious
growths on the stem, which serve as floats. The leaves are
much longer than those of Mimosa, but considerably less
sensitive. There are three pairs of sub-petioles, which are
articulated to the main petiole. Electrical connections are
made with two points at the middle of two sub-petioles which
carry numerous leaflets. The sensitiveness of the leaflets
'of this plant is much less than in Mimosa. The conducting-
power is also very moderate, the velocity of transmission of
excitation in one of the sub-petioles being 1*5 mm. per second.

Efect of feeble current. — The minimum e.m.f. which will
induce excitation is higher in the case of Neptunia than
in Mimosa, being 20 volts in the typical instance now to
be described. The resulting current was 7 micro-amperes.
On going through the usual cycle of direct and reverse
experiments, it was found that the kathode alone excited
during make. From experiments with five other specimens
I obtained the same result without exception.

Table VI. — Effect of Feeble Current on Leaflets of Neptunia
E.M.F. = 20 volts ; current = 7 micro-amperes

R. Contact.

L. Contact.

Km
C

Kb
0

Am
0

Ab
0

K'm
C

K'b
0

A'm
0

A'b
0

Excitation formula-

-Km

Effect of moderate current. — Continuing to increase the

9

226 RESEARCHES ON IRRITABILITY OF PLANTS

E.M.F. still further, I found the characteristic effects of
Type II. to take place under an e.m.f. of 30 volts, the
resulting current being 11 micro-amperes. Excitation thus
takes place, with moderate currents, at the make of kathode
and the break of anode. This result was borne out without
any exception by a series of experiments on five different
specimens.

Table VII. — Effect of Moderate Current on Leaflets of Neptunia
E.M.F. = 30 volts ; current = 11 micro-amperes

R. Contact.

L. Contact.

Km
C

Kb
0

Am

0

Ab

K'm
C

KO)
0

A'm A'b
0 C

Excitation formula — Km Ab

It will be seen that, in this case as in others, with feeble
current kathode alone excites at make, and with moderate
current excitation takes place at make of kathode and
break of anode.

Leaflets of Averrhoa c a ram bo la

These leaflets are as a general rule but slightly sensitive,
and require a comparatively high e.m.f. to induce excitation.
The bi-polar connections are here made on two points of the
same leaf, separated from each other by a distance of 7 cm.
The electrical resistance of the petiole is high, being about
5 million ohms when the connections are as described. The
velocity of transmission of excitation, also, is relatively
feeble, being in a representative case '5 mm. per second.
Under feeble stimulus the excitation travels through a
short distance only ; still feebler excitation remains more
or less localised.

Effect of feeble current. — With young and excitable
specimens the minimally effective e.m.f. is 10 volts, the
resulting current being 2 micro-amperes. Generally speak-
ing an E.M.F. of 22 volts and a current of 4 micro-amperes

POLAR EFFECTS OF MODERATE CURRENTS 227

are necessary. On starting the current, a pair of leaflets
at the kathode underwent an excitatory fall. No effect
was induced at the left anodic contact. At break there
was no effect at either. On reversal, the left contact became
kathode, resulting in the excitatory fall of a pair of leaflets.
This local character of excitation was due to feeble con-
ductivity. It is only under much stronger polar excitation
that a moderate amount of transmission takes place. At the
anode there was on this occasion no effect at make — nor
was there any excitation at break of kathode or anode.

This experiment was repeated in the case of ten different
specimens, the result being invariably the same.

Table VIII. — Effect of Feeble Current on Leaflets of

Averrhoa carambola

e.m.f. = 22 volts ; current = 4 micro-amperes

R. Contact.

L. Contact.

Km
C

Kb
0

Am
0

Ab
0

K'm
C

K'b
0

A'm
0

A'b
0

E

.xcitation

formula-

-Km

Effect of moderate current. — In order to obtain the
characteristic reactions of Type II., the e.m.f. has generally,

Table IX. — Effect of Moderate Current on Leaflets of

Averrhoa carambola

E.M.F. = 50 volts; current = ii micro-amperes

Km
C

R. Contact.

Kb
O

Am
O

Ab
C

L. Contact.

K'm
C

K'b
O

A'm
O

Excitation formula — Km Ab

A'b
C

in the case of Averrhoa carambola, to be raised to 50 volts or
thereabouts. Thus in a given experiment, under the action
of 50 volts and with a current intensity of 11 micro-amperes,

Q 2

228 RESEARCHES ON IRRITABILITY OF PLANTS

excitation took place at kathode at make and at anode at
break. The experiment was repeated with ten different
specimens, the result being always the same.

The excitatory reaction in this plant is thus charac-
teristically the same as in others already described.

Leaflets of A verrhoa bilimbi

The excitability of these leaflets is perhaps slightly
greater than that of A. carambola. I carried out the usual
cycle of observations on the effects of feeble and moderate
currents, employing in each case live different specimens.
The two bi-polar contacts were made in the case of the
same leaf 7 cm. apart.

Effect of feeble current. — The excitatory effect of kathode-
make characteristic of Type I. was exhibited with an e.m.f. of
10 volts, the current being 6 micro-amperes. The electrical
resistance offered by the petiole of this plant was much
less than that of A. carambola. All the specimens, without
exception, gave the effects characteristic of Type I. under
feeble current.

Table X. — Effect of Feeble Current on Leaflets of

Averrhoa bilimbi

e.m.f. = 10 volts ; current = 6 micro-amperes

R. Contact.

L. Contact.

Km
C

Kb
0

Am
0

Ab
0

K'm
C

K'b
0

A'm
0

A'b
0

Excitation formula-

-Km

Effect of moderate current. — When the e.m.f. was raised to
28 volts, with a resulting current of 16 micro-amperes, the
effects induced were characteristic of Type II. That is
to say, excitation was shown at the kathode at make and

POLAR EFFECTS OF MODERATE CURRENTS 229

at the anode at break. Repetition of the experiment on
other specimens gave similar results.

Table XI. — Effect of Moderate Current on Leaflets of
Averrhoa hilimhi
E.M.F. = 28 volts ; current = i6 micro-amperes

R. Contact.

Km Kb Am Ab

CO 0 C

' 1

L. Contact.

i .
K'm ' K'b ! A'm A'b
c 0 0 1 C

1 1 i

Excitation formula — Km Ab

Primary Leaf of Mimosa

I have already described in the previous chapter the polar
effects of feeble currents on the pulvinus of Mimosa. In that
case the mono-polar method of experiment was employed.
The bi-polar method will now be taken up, enabling us to
observe the make-effect of anode and kathode in two
different organs simultaneously, and, in the same way,
the break-effect ; all these to be further corroborated by
reversal experiments.

Effect of feeble current. — The bi-polar connections are
made either directly with, or in the close vicinity of, two
different pulvini, at varying heights on a single main stem
of Mimosa. With highly excitable specimens the polar
effects characteristic of Type I. may here be obtained with
as low an e.m.f. as 2 volts. In another case the applied
E.M.F. was 4 volts, and the resulting current 4 micro-amperes.
On going through the usual experimental cycle — with direct
and reverse currents — it was found that the kathode alone
exhibited excitation at make. There was no excitation
either at kathode-break or anode-make or break. Similar
experiments were carried out on some fifteen different plants
of Mimosa, the results without exception being as described.
The only variation seen in the case of different individuals

230 RESEARCHES ON IRRITABILITY OF PLANTS

lay in the value of the minimally effective e.m.f., which in
less excitable specimens was sometimes as high as 8 volts.

Table XII. — Effect of Feeble Current on Pulvinus of Mimosa
E.M.F. = 4 volts ; current = 4 micro-amperes

R. Leaf

L.

Leaf

Km
C

Kb
0

Am
0

Ab
0 '

K'm
C

K'b
0

A'm
0

A'b
0

Excitation formula-

-Km

Effect of moderate current. — Under an increasing e.m.f.
the characteristic polar effects seen in the pulvinus of
Mimosa are, as in other cases, transformed into those of

Fig. 115. — Under 8 volts excitation at kathode-make; under
12 volts excitation at kathode-make and anode-break.

Type II. Thus in a particular case the e.m.f. applied was
10 volts, the resulting current being 8 micro-amperes. In
carrying the experiment through the usual cycle of direct
and reverse currents it was found that the kathode excited
at make, and the anode at break. In fig. 115 are shown
a series of records given by a different leaf under kathode-
make, kathode-break, anode-make, and anode-break. It
will be seen that under a feeble current, due to an e.m.f.

POLAR EFFECTS OF MODERATE CURRENTS 231

of 8 volts, the reactions are those characteristic of Type I.

But when the acting current was increased by applying the

higher e.m.f. of 12 volts, the responsive characteristics, as

was to be expected, changed to those of Type 11. Whereas

under feeble current the kathode alone had excited at

make, now under moderate current excitation occurred

at both kathode-make and anode-break.

These distinctive effects of the moderate current were

obtained with no fewer than fifteen different specimens in

spring, and were thus shown to be characteristic of plants in

normal condition. Later in the year, however, seasonal

changes were seen to induce certain modifications of the

polar effects ; these will be fully described in another

chapter.

Table XIII. — Effect of Moderate Currents on Mimosa
E.M.F. = 10 volts ; current = 8 micro-amperes

R. Leaf.

L. Leaf.

Km
C

Kb
O

1
Am ! Ab

o i C

K'm
C

K'b
O

A'm
O

A'b
C

Excitation formula — Km Ab

It will now be advisable to recapitulate the results
described in the present chapter, in order to show how
universal these phenomena are. I will first give a tabular
statement of the effect of feeble current : —

Table XIV. — Effect of Feeble Current on Various Specimens
of Sensitive Plants

Type I. — Excitation formula Km

Specimen.

Number of
Experiments.

Minimum
Current.

Leaflet of Mimosa

„ Biophytum . .
„ Neptunia
„ A. carambola
„ A . bilimbi . .

Leaf of Mimosa

25
50

5
10

5
15

micro-ampere
0-7
o"5
7
4
6
2

232 RESEARCHES ON IRRITABILITY OF PLANTS

I next give a table showing results obtained with
moderate current : —

Table XV. — Effect of Moderate Current on Various Specimens
OF Sensitive Plants

Type II. — Excitation formula Km Ab

Number of

Minimum

Specimen.

Experiments.

Current.

micro-amperes

Leaflet of Mimosa

25

1-6

,, Biophytum . . . .

50

21

,, Neptunia

5

12

„ A. carambola

10

II

„ A. bilimhi . .

5

16

Leaf of Mimosa

15

4

It is here shown that we have examined not one species
only but all the species that were available for experiment.
More than a hundred experiments have moreover been
carried out on different specimens. The result of all these
investigations has been to demonstrate clearly the fact that
in plants, under a feeble current, it is the kathode alone that
excites at make, while under a moderate current excitation
takes place by the make of kathode and the break of anode.

Thus, so far from there being any necessary divergence in
the excitatory polar reactions of fibrillated and unfibrillated
protoplasm, we find on the contrary that they are identical
in the plant and in the animal. In other words, the respon-
sive characteristics of Type 1. and Type II. in plants corre-
spond in every respect to those in animal tissues which are
classed under Pfluger's Law as Stage I. and Stage II.

This is not by any means to be taken as a complete
statement of the law of polar effects. We shall find that
if the current be increased still further, certain new types
of effects make their appearance which will be described in
detail in a subsequent chapter. In the meantime I have to
describe certain further characteristics of these polar effects

POLAR EFFECTS OF MODERATE CURRENTS 233

and the relative sensitiveness of plant and animal to this
form of stimulation. These will be dealt with in the follow-
ing chapters.

Summary

The polar effects observed in diverse sensitive plants are
in every way similar to those in animal tissues. The Laws
OF Polar Excitation in Plants are : —

L With feeble current the kathode excites at make and
not at break. The anode excites at neither make nor break.

IL With current of moderate intensity the kathode
excites at make and not at break. The anode excites at
break and not at make.

CHAPTER XVI

THE CONTRASTED EFFECTS OF ANODE AND KATHODE

Polar effects of currents on pulsation of Desmodium gyrans — Reduction
of systolic contraction by anodic action — Diminution of diastolic
expansion by kathodic action — Arrest at systole by make of kathode
and diastolic expansion by break of kathode — Arrest at diastole by
make of anode, and systolic contraction by break of anode — Effects
of ascending and descending currents of feeble and strong intensity in
nerve-and-muscle preparation — Parallel effects in petiole-and-pulvinus.

We have seen that under currents of moderate intensity the
polar effects of anode and kathode are more or less antithetic.
The kathode excites at make, whereas the anode excites at
break. If the kathode at make induces contraction, then
it would appear probable that at the make of anode there
may be produced an expansion. At break, again, supposing
the same antithesis to be maintained, the contractile effect
at the anode will have a contrasted effect of expansion at
the kathode.

Biedermann has shown that in a beating heart the point
of appHcation of the anode, during make, remains expanded
as a dark red blistered swelling, even when the rest of the
heart, during contraction, is becoming pallid. This shows
that during the continuation of the anode an expansive
reaction is induced in the tissue. An effect similar to this
is induced at the kathodic point at break.

I have been able to demonstrate the contrasted effects
of anode and kathode, in a still more striking manner, in
the case of rhythmic pulsation of the leaflet of Desmodium
gyrans.

In a future chapter it will be shown how remarkable
are the similarities of the rhythmic tissue of the plant and

234

EFFFXTS OF ANODE AND KATHODE 235

animal. In the case of the Desmodium leaflets the pulsations
are seen to take place in a very regular manner, the period
of one complete pulsation being about 3 minutes. Of the
up and down movements of the leaflet, the down movement
takes place more quickly, corresponding to the systolic con-
traction of the rhythmic cardiac tissue. The systolic move-
ment in Desmodium takes place in the course of about
I minute and 10 seconds, the slower diastolic expansion being
accomplished in the course of about i minute and 50 seconds.
In the record, the quicker systolic movement is represented
by the up-curve. The extent of contraction is thus repre-
sented by the upper limit of the curve of response ; the lower
limit indicates the extent of diastolic expansion. If the
leaflet is executing its greatest possible amplitude of pulsa-
tion, then an external agent will be unable to increase it
any further ; but the extent of contraction or expansion
can be individually reduced under agencies which have an
opposing tendency. Thus if the continuation of anode tends
to induce expansion, then during a cycle of pulsating
activity it would oppose the contraction at the systolic
phase. The effect would be a diminution of contraction ;
in the record this would appear as progressive diminution
of heights of responses.

If the application of kathode, on the other hand, induces
contraction, this would oppose the diastolic expansion ; the
amplitude of pulsation will be progressively diminished,
with continuously diminishing relaxations, and the base-line
would be shifted upwards.

In order to demonstrate these contrasted polar reactions
the experiment was carried out by making the pulvinule of the
leaflet alternately anode or kathode. One electric connection
is made with the pulvinule by means of a thin thread,
special care being taken that this in no way interfered with
the free pulsation of the leaflet ; the second connection is made
with an indifferent point lower down in the petiole.

I shall first describe the expansive effect of anode-make
and the contractile effect of kathode-make ; these particular

236 RESEARCHES ON IRRITABILITY OF PLANTS

effects are easy to demonstrate, the current applied being of
moderate intensity. In fig. ii6 is seen the anodic effect ; the
first two pulsations are normal, after which the anode was
applied. This is seen to result in a continuous lessening
of the height of successive contractions.

The application of kathode, on the other hand, is found
to induce a precisely opposite effect (fig. 117) ; here the
diastolic expansion is opposed, which results in a con-
tinuously diminishing relaxation. In fig. 116 a line joining

Fig. 1 16. — Effect of anode, opposing
contraction in pulsation of
Desmodium gyrans. Note the
gradual diminution of systolic
contraction.

A
\ \

Fig. 117. — Effect of kathode, op-
posing expansion. Note the
gradual diminution of diastolic
expansion.

the apices of successive contractions is seen to descend,
while in fig. 117, under the action of kathode, the line joining
the extreme points of the diastolic excursion is seen to
ascend. The same fact is seen again in fig. 118, where a
single specimen is subjected first to anode and then to
kathode. The contrasted effects of anode and kathode
in this case are very obvious.

By employing a suitable intensity of current it is some-
times possible to exhibit the contrasted effects of the
kathode and anode by the actual arrest of pulsation. By
the make of kathode the arrest takes place towards systole,
and by the make of anode towards diastole. In such cases

EFFECTS OF ANODE AND KATHODE 237

it is possible to demonstrate further the remarkable effects
of the break of kathode and break of anode. The difficulty
in this experiment lies in the fact that the application
of an intensity of current, greater than is exactly sufficient
to induce the arrest, is apt to bring about fatigue, with the
abolition of pulsation. I have several times succeeded in

Fig. 118. — Alternate effects of anode and kathode in dimin-
ishing systoHc contraction and diastolic expansion.

inducing an arrest which was not followed by a permanent
abolition of excitability. In these cases it is possible to
exhibit the very interesting effects of break of kathode
and of anode. In fig. 119 is seen the arrest at systole induced

Fig. 119. — Arrest at systole by Fig. 120. — Arrest at diastole
the make of kathode and by the make of anode and
diastolic expansion as im- systolic contraction as im-
mediate effect of break of mediate effect of break
kathode. of anode.

by the make of kathode. The current was then broken, and
as an immediate result an expansive or diastolic phase of
pulsation was obtained, followed by several other pulses
of somewhat diminished ampUtude. The effect of break of
anode, on the other hand, is exactly opposite. In fig. 120

238 RESEARCHES ON IRRITABILITY OF PLANTS

the pulsation was arrested towards diastole by the make
of anode. The break of anode was immediately attended
by an abrupt termination of the standstill. A pulse of
systolic contraction followed, and was succeeded by the
renewal of ordinary pulsation. It is thus seen that while
the make of kathode and break of anode induce contraction,
the make of anode and break of kathode induce the reverse
effect of expansion.

The contrasted effects of anode and kathode, and of
make and break, are also exhibited by induced variations
of excitability. These are well seen in characteristic
differences of effects induced by a constant current in a
muscle-and-nerve preparation, these being modified by
the strength of current. Thus employing a feeble current
it is found that both ascending and descending currents in the
nerve induce excitation of the terminal muscle at make but
not at break. With strong currents, on the other hand,
excitation only takes place at the make of the descending
current and break of the ascending current. We shall
presently see how these different effects are explicable on
the assumption of the contrasted effects of anode and
kathode, and of make and break.

The obscurity of the subject lies in the fact that there
is no visible indication of an excitatory change in the nerve.
The case would have been different had the conducting-
tissue been provided with indicators which could signal the
passage of excitation. Such a conducting-tissue is provided
by the petiole of Biophytum, where the successive closures
of the lateral leaflets indicate the transmission of excitation
through the central conduct ing-strand. We shall see how
the effects visually manifested in Biophytum elucidate
the characteristic effects observed in the nerve-and-muscle
preparation.

Fig. 121 shows arrangements of parallel experiments
in nerve of frog and conducting petiole of Biophytum. The
polarising electrodes are applied to the right, the terminal
motile organ, muscle or leaflet, being to the left. The series

EFFECTS OF ANODE AND KATHODE 239

to the left represent the effect of descending, those to the
right the effect of ascending, current. In the case of the
descending current the terminal indicator is to the left of K,
and the conducting tissue between K and A. In the case
of the ascending current the terminal indicator is to the
left of A. Transmitted excitation can be detected by the

Descending

--iS>+^=i-=

Feeble

St

rang

«<^

•m.

make

break

Fig. 121. — Effects of Descending and Ascending currents at make and
break on nerve-muscle and petiole-pulvinus. Excited leaflets shaded
dark. Vertical series to left represent effects of descending currents ;
series to right effects of ascending currents. With feeble current,
terminal leaflets excited by both descending and ascending currents
at make and not at break. With strong currents, excitation of
terminal leaflets takes place only at make of descending and break
of ascending currents. Two upper pairs of leaves exhibit effects of
feeble current at make and break. The two lower pairs of leaves
show effects of strong current.

contraction of muscle or leaflet, to the left of k in the case of
descending, and to the left of A in the case of ascending
current. But the characteristic effect at the electrodal
points, K and A themselves, or the overflow of the effect
between the two points, cannot be detected in the case of
frog's nerve. It can, however, be detected in the case of

240 RESEARCHES ON IRRITABILITY OF PLANTS

conducting-tissue of Biophytum on account of the presence
of the lateral motile-leaflets.

I will now describe experiments carried out on Biophy-
tum with feeble and strong currents, and show how they
throw light on parallel experiments with nerve-and-muscle
preparation of animal. In Biophytum the excited leaflets
are indicated by dark shading, which will also clearly
exhibit the extent of excitatory overflow. The point of
initiation of excitation, and direction of transmission, are
indicated by arrows.

Effect of Feeble Current

In the nerve-and-muscle preparation it is known that (i)
excitation takes place at the make of descending current ;

(2) excitation also occurs at the make of ascending current ;

(3) no excitation occurs at the break of descending, (4) or
ascending, currents. Accounts of parallel experiments with
Biophytum will now be described : —

(i) Make of descending current. — The kathode is proximal
to the extra-polar or terminal leaflets to the left. At
make, excitation is seen to be initiated only at K, the two
waves proceeding in opposite directions. Five pairs of
leaflets thus fall in the extra-polar and three pairs in the
intra-polar regions. This excitatory overflow in the intra-
polar region cannot be observed in the animal nerve on
account of absence of a visible indicator.

(2) Make of ascending current. — In fixing our attention to
the terminal or extra-polar leaflets to the left of A, we find
occurrence of excitation corresponding to the excitation of
muscle at the make of the ascending current. That the
excitation was really initiated at the distal kathode, and
traversed without hindrance through the feeble anode, will
clearly be seen from the serial closure of the leaflets initiated
at K. Eight pairs of leaflets underwent excitatory fall to
the left, and one pair to the right of K.

(3 and 4) Break of ascending and descending currents. —

EFFECTS OF ANODE AND KATHODE 241

There is no excitation, since feeble anode does not excite at
break.

Effect of Strong Current

We now take up the question of the effect of strong
currents : —

(i) Make of descending current. — In the nerve-and-muscle
preparation there is excitation of muscle at the make.
Here the kathode is proximal, and we obtain the normal
excitatory effect of kathode-make.

In the corresponding experiment with Biophytum we
find excitation transmitted to the terminal or extra-polar
region to the left, five pairs of leaflets undergoing closure.
Only two pairs of leaflets closed in the intra-polar region,
further progress of excitation being arrested by the
depressing action of the anode.

(2) Make of ascending current. — Unlike the action of
feeble current, there is no excitatory effect in the nerve-
and-muscle preparation at the make of strong ascending
current. This is explained on the supposition that the
excitation at the distal kathode cannot traverse the region
of the strong anode with its depressed excitability. The
proof of this assumption is strikingly afforded by the
corresponding experiment with Biophytum. We observe
the initiation of excitation at the kathode, but the pro-
gress of excitation is arrested near the region of anode.
Hence, in spite of the occurrence of excitation, there
could be no transmitted effect in the terminal extra-polar
region.

(3) Break of descending current. — In a nerve-and-muscle
preparation there is no excitation at break of a descending
current. Here, though the distal anode excites at break,
the intervening kathodic region is assumed to undergo
depression at break. Hence a block occurs to the trans-
mission of excitation.

This is made quite clear in the corresponding experiment

242 RESEARCHES ON IRRITABILITY OF PLANTS

with Biophytum, We here observe excitation initiated
at the anode at break, two pairs of leaflets undergoing
closure, and the further progress of the excitatory wave is
arrested before reaching the depressed region of kathode-
break.

(4) Break of ascending current. — Excitation occurs in a
nerve-and-muscle preparation at the break of strong ascend-
ing current. Here the anode is proximal, and excitation
induced at break reaches the terminal organ without
hindrance.

In the corresponding experiment with Biophytum we
observe five pairs of leaflets undergoing closure in the
terminal extra-polar region, the excitation being initiated
at the anode at break. Excitation also traversed the
intra-polar region, two pairs of leaflets undergoing closure.
Further progress in this direction was, however, arrested
by the depressing action of kathode-break.

Summary

The contrasted effects of anode and kathode are exhibited
by appropriate modification in the pulsating activity of
Desmodium gyrans.

The anodic effect of expansion is seen in the reduction
of normal limit of systolic contraction.

The kathodic effect of contraction is observed in the
reduction of normal limit of diastolic expansion.

The immediate effect of break is the reverse of that at
make or continuation of current.

The diastolic arrest by anode is followed at its break by
systolic contraction.

The systolic arrest by kathode is succeeded at its break
by diastolic expansion.

In nerve-and-muscle preparation the effects of ascending
and descending currents are found modified by the intensity

EFFECTS OF ANODE AND KATHODE 243

of the current. Effects in every way parallel are observed
in experimenting with petiole-pulvinus of Biophytum.

These characteristic modifications are easily traceable in
Biophytum to the contrasted effects of anode and kathode,
and of make and break. Excitability is enhanced by the
make of kathode and break of anode. It is depressed by
the make of anode and break of kathode.

R 2

CHAPTER XVII

EFFECT OF TEMPERATURE ON POLAR EXCITATION, AND
MULTIPLE EXCITATION UNDER CONSTANT CURRENT

Excitability of conducting tissue to induction-shock diminished by cooling —
Nerve-excitation by constant current enhanced by cooling — Excitation
of conducting-tissue of Mimosa by constant current enhanced by
cooling and depressed by warming — Ineffective stimulus becoming
effective under cooling and vice versa — ^Multiple response induced
in Biophytum by the passage of constant current — Comparison of
sensitiveness of plant and animal — Minimum current for excitation
of human tongue — Relatively higher sensitiveness of Biophytum.

In continuing the investigation on polar reaction, certain
modifications of effect were noticed as the season advanced
from spring to summer. Some of these will be described in
a succeeding chapter. But one modifying effect — namely,
that due to temperature — appeared at first very puzzling.

The temperature of Calcutta in summer is high. More-
over experiments with plants had to be carried out in a
glass-house. Thus the temperature to which the plants were
subjected in summer was often as high as 35° C. In these
circumstances it was found that the indirect stimulation of
the pulvinus of Mimosa by the action of constant current
often became ineffective. It was quite easy in spring to
excite the leaf of Mimosa by the transmitted excitation due
to the make of kathode when the kathodic point was at a
distance of several centimetres from the pulvinus. But in
summer there was hardly any transmitted excitation even
when the exciting kathode was at a comparatively short
distance from the pulvinus.

This ineffectiveness might be due to the impairment of
conductivity or excitability. It could not be due to the

244

TEMPERATURE ON POLAR EXCITATION 245

loss of conductivity, for we have seen in a previous chapter
that the conductivity is enhanced by a rise of temperature.
The pulvinus, again, was found extremely sensitive. Yet,
in spite of the high conductivity and motile excitability, the
transmitted effect of excitation was often found ineffective
at a high temperature.

Thus the only remaining factor to which the change
might be attributed was the excitability of the conducting
tissue itself. Taking the parallel case of the animal nerve,
it is known that the excitability of a nerve is diminished by
local cooling, the stimulus being that due to break induction-
shock. But Gotch and Macdonald have made the very
interesting observation that the effect is reversed in the case
of stimulation by constant current ; here the nerve excita-
bility is enhanced by lowering of temperature. By employ-
ing a descending current — with the kathode proximal to the
contractile muscle — they found that the make-excitation
which was ineffective when the nerve was locally warmed
to 30° C. became effective when locally cooled to 5° C.

It occurred to me that the failure of indirect stimulation
by the closure of constant current in Mimosa might be due
to the depression of excitability of the conducting petiole,
in consequence of the high temperature. Should this prove
to be the case, then this specific reaction would afford a
very striking demonstration of the characteristic similarities
in the conducting-tissues of the animal and plant.

The effect of cold in modifying the excitability of the
conducting animal nerve is, as said before, dependent on the
mode of stimulation. We shall now see whether this holds
good in the case of the plants also. First, in order to
determine the effect of cold on the exciting efficiency of the
break induction-shock, the two electrodes from the secondary
coil were placed on the petiole, one 20 mm. from the pulvinus
and the other further away. It has been shown that with
a single induction-shock it is the kathode which causes
excitation. The electrodes of the secondary coil are so
connected with the petiole as to render the proximal contact

246 RESEARCHES ON IRRITABILITY OF PLANTS

the kathode, and therefore the point of excitation. Records
are then taken under the same stimulus alternately, (i)
with the excited point as warm as the general temperature
of the room, which was 30° C, and (2) with the tempera-
ture lowered to about 5° C. by the application of cooled
water. It is essential that the cooling should be effected
gradually, for sudden variation of temperature of itself
causes excitation.

The series of records in fig. 122 shows the result. It is
seen that while the excitation is effective at H, h, h, when

Fig. 122. — Effect of cold on excitability to induction-shock : h, c,
alternate effective and ineffective excitation at moderately
warm and low temperatures respectively. Testing stimulus,
a single break induction-shock was maintained constant.

the stimulated point is warm, it becomes ineffective at c, c,
when the excited point is cooled.

This proves that in the conducting-tissue of the plant
lowering of temperature depresses the excitatory efficiency
of a break induction-shock.

For studying the effect of cold on the exciting efficiency
of the constant current, I next made suitable electrical
connection with two points on the petiole, the proximal
kathode, k, being at a distance of 10 mm. from the pulvinus.
Excitation was produced by the make of the descending
current. An e,m,f, was appUed which caused minimal

TEMPERATURE ON POLAR EXCITATION 247

response ; this was found to be 2 volts, the temperature
of the room being 30° C.

The point k was now alternately raised and lowered in
temperature. This was effected by means of a stream
of hot or cold water applied at the point. It is essential
that the warming or cooling should be effected gradually,
for, as stated above, sudden variation of temperature of
itself causes excitation.

Fig. 123. — Record showing abolition of polar excitation at high
temperature : n, normal response ; h, c, alternate ineffec-
tive and effective excitations at high and low temperatures
respectively. Testing stimulus of kathode-make was kept
constant.

A series of automatic records are shown in fig. 123,
made by the plant. The first record of the series, N, gives
the response to the descending make-current ; the amplitude
of this is only moderate. The petiole was locally warmed
to about 37° C, and an identical stimulus of the make of
descending current was applied at a moment marked H. It
will be noticed that the stimulus which was formerly effective
has now become ineffective.

The petiole was next locally cooled to about 10° C. and

248 RESEARCHES ON IRRITABILITY OF PLANTS

stimulus of descending make current applied as before at c.
It will be seen that not only is the stimulus now effective,
but the amplitude of response is much greater than that of
normal response at 30° C. In order to test these results
further, the experiment was repeated under alternate heat-
ing and cooling. It will be noticed that under rising tem-
perature there is an invariable failure of excitation, while
under lowered temperature excitation occurred always and
became maximal. Therefore one of the features which
characterise the animal nerve is found also in the conduct-
ing-t issue of the plant.

Multiple Excitation by Constant Current

A very striking result of the passage of a constant current
is the production of multiple responses in certain rhythmic
tissues. Thus the non-ganglionated preparation of the
apex of the frog's heart is under ordinary conditions
quiescent, but on the passage of a constant current it
breaks forth into a rhythmic series of excitations.

In a subsequent chapter it will be shown that certain
vegetable tissues exhibit multiple responses. The pulvinus
of the leaflet of Biophytum is multiple-responding and
behaves in many respects like the cardiac tissue. Now
on maintaining a constant current of certain intensity
through the petiole, the kathode being placed on a
pulvinus, the particular leaflet will be found to execute
a series of pulsatory movements.

Of still greater interest is the indirect effect of such
stimulation. A particular leaflet of Biophytum is attached
to the recording lever. A constant current is sent for a
short time through the petiole, the kathode being at a
distance of 10 mm. from the leaflet. The leaflet responded
to the excitation caused by kathode-make ; the response
here was due to the excitation transmitted through the
distance of 10 mm. The first part of fig, 124 gives four

MULTIPLE EXCITATION BY CURRENT 249

sets of responses to individual stimuli, applied at intervals
of 3 minutes. After this the petiole was subjected to the
action of a continuous current, represented below the
record as a continuous up-line. It will be seen that under
the continuous current, rhythmic excitations were initiated
at the kathode, which, reaching the responding leaflet,
caused multiple responses.

There were five such rhythmic responses in the course
of 10 minutes, the period of/each being 2 minutes. The

Fig. 124. — Multiple excitation in Biophytum under constant current.
First four records are responses to individual stimuli of electrical
current of short duration, applied at intervals of 3 minutes. The
last five are multiple responses due to continuous action of current.

production of such multiple excitations in a plant under
constant current cannot be explained by the theory of
hydro-mechanical blow. On the other hand, their simi-
larity to corresponding phenomena in animal tissues is
sufficiently obvious.

Relative Sensitiveness of Plant and Animal to
Electrical Current

There is a general assumption that the sensitiveness of
plant tissue is very much lower than the animal tissue.
The question then arises : How sensitive is the plant as
compared with the animal in the matter of excitation by
a constant current ?

There can probably be no specimen more susceptible

250 RESEARCHES ON IRRITABILITY OF PLANTS

of such excitation than the nerve-and-muscle preparation
of a frog. This preparation is an exceedingly sensitive
detector of induction-current. A shock too feeble to be
felt by the intact human subject when passed across the
body through two fingers, will, when applied to the exposed
nerve of the frog, provoke vigorous movement in the
attached muscle. The intact plant, like the human
subject, is not so sensitive to an induction-shock as the
bare nerve.

But the case is different when we come to the question
of excitation by a constant current. In order to compare
the relative excitatory effects here, in plant and animal,
I carried out two different investigations, comparing the
sensitiveness of the plant on the one hand with that of
frog's nerve-and-muscle preparation, and on the other,
with that of intact human subject.

In the first of these I placed an intact petiole of
Biophytum in series with a nerve-and-muscle preparation of
frog. A gradually increasing e.m.f. was applied by means
of a potentiometer-slide arrangement, till one of the two
specimens showed excitation at make. The intensity of
the exciting current was now measured by means of micro-
ammeter. The E.M.F. was then further increased till the
second of the two specimens gave excitation at make, and
the current again measured. From the value of these two
effective currents the relative sensitiveness of the two
specimens can be gauged. Instead of measuring the
current, we may take, if we wish, the readings of the
slide-wire for the two effective values.

These experiments were carried out in August, during
the rainy season. As regards the specimen of Biophytum,
it should be mentioned that the vitiated atmosphere of a
town is particularly inimical to the sensitiveness of this
plant. I have succeeded in raising Mimosa, Desmodium,
and other sensitive plants in Calcutta, without much loss
to their sensitiveness ; but I have invariably failed to do
this in the case of Biophytum. It always becomes stunted

SENSITIVENESS OF PLANT AND ANIMAL 251

and discoloured, and ceases to exhibit its normal motility.
The only alternative has been to grow the plant in the
suburbs and bring it to the laboratory. Here, with one or
two days' rest after the disturbing effect of transport, it
regains a fair degree of sensitiveness, but the effect of the
air of the town is to bring about a daily deterioration, and
in the course of a week or ten days, except in rare cases, it
becomes practically insensitive. It will thus be seen that
the experimental plant employed for our comparison may be
taken as having had its sensitiveness reduced certainly to
half.

In proceeding with the experiment, according to the
method already described, it was found that the nerve-and-
muscle preparation responded at the make of a current
which was as feeble as -25 micro-ampere. The Biophytum
at this time had not yet responded, but when the current
intensity was increased to -5 micro-ampere excitation was
seen to occur. From this it would appear that Biophytum
had in this case fully half the susceptibility of the nerve-and-
muscle preparation of frog. Bearing in mind the peculiarly
unfavourable character of the circumstances to which the
plant was in this case subjected, it does not seem too much
to infer that the sensitiveness of the two specimens was
not naturally of a different order.

It might perhaps be still more interesting to institute a
comparison between the intact plant and the intact human
subject. The tongue has always been considered a very
sensitive detector of electrical current. According to
Laserstein, the acid effect of anode is appreciated by it
when the current is only yi^ of a milliampere. This is
equal to 6-4 micro-amperes.

But I have found amongst my pupils some who could
perceive by the tongue a current as feeble as 1*5 micro-
ampere, whereas the Biophytum in the same circuit responded
to the much smaller current of '5 micro-ampere only. This
demonstrates that the plant was in this case three times as
sensitive as the human tongue. A more excitable specimen

252 RESEARCHES ON IRRITABILITY OF PLANTS

of Biophytum would doubtless prove to be about ten times
as sensitive.

The improbability of the theory of hydro-mechanical
disturbance becomes evident when we realise that an
excitatory impulse is initiated and transmitted in the plant
under a stimulus that cannot even be perceived by the
extremely sensitive human tongue.

Summary

Nerve-excitation by constant current is enhanced by
cooling and depressed by warming. This is one of the
characteristics of polar excitation by constant current.

Similarly the excitability of conducting petiole of Mimosa
to polar excitation is enhanced by cold and depressed by
warmth. Minimal excitation becomes maximal under
cold, and ineffective under warmth.

The passage of a constant current through the petiole of
Biophytum gives rise to series of multiple excitations.

The sensitiveness of Biophytum to an electrical current
is remarkably high. Compared with the very sensitive
human tongue, the sensitiveness of Biophytum is about ten
times as great.

CHAPTER XVIII

POLAR EFFECTS UNDER STRONG CURRENTS

Abnormal polar reactions in Protozoa — Transformation of polar reaction
in leaf of Mimosa from Type II. to Type III. under strong current —
Further transformation to Type IV. under stronger current — Ex-
hibition of Type III. and Type IV. by leaflets of Mimosa, Biophytum,
and Averrhoa carambola — Law of polar action of strong currents.

It has been shown in a preceding chapter that the charac-
teristic effect of a feeble current in various plants was
excitation at make of kathode, but that with a moderate
current there was excitation not only at the make of kathode
but also at the break of anode. These two types of effect
corresponded to those recognised in the case of animal
tissues under Pfluger's Law as Stage I. and Stage 11. It
will be remembered that in the case of Protozoa the polar
reactions have been found to be very different from these.
Thus Kiihne found that in Actinosphaerium excitatory con-
traction took place at the make of both kathode and anode,
and that excitation again took place at the break of kathode.
The excitatory formula here, then, may be expressed as
Km Kb Am.

It will be noticed that we have here the abnormal
reactions of anode-make and kathode-break. Verworn has
corroborated and extended these observations. He further
finds that Amphistigma is excited by both anode and
kathode at make. There was apparently no effect at break.
The excitation formula in this case is therefore Km Am.

From these abnormal effects it has been suggested
either that the polar effects in fibrillated and unfibrillated

253

254 RESEARCHES ON IRRITABILITY OF PLANTS

protoplasm are more or less opposed, or that there is no
law of polar action which is of universal application.

But we have seen that the unfibrillated protoplasm of
the plant exhibits polar effects which are identical with
those of animal tissues. It is not impossible that normal
polar effects may be subject to modification from the
influence of diverse factors, such for example as strength of
current, physiological condition of specimen, its age, and
the season of the year. When an exhaustive survey has
been made, it may be found that the responses of the
Protozoa were not, after all, anomalous, but have a place of
their own in some transitional type of reaction given by
specimens whose characteristics are definitely recognised as
normal. With the object of completing such a survey, we
may continue our detailed study of the polar reactions of
sensitive plants under widely different conditions. And
first we shall study the effects pf a still further progressive
increase of current, in order to see whether any new pheno-
menon comes into the field of observation, taking the
reactions of the pulvinus of Mimosa first in the series.

Primary Leaf of Mimosa

I have employed both the bi-polar and mono-polar
methods. The mode of procedure was to increase the current
step by step from minimum to maximum, and note the
changing types of reaction at different critical points. With
vigorous specimens this can be done without any danger
of the onset of fatigue. After the value of certain critical
points has been determined, experiments were repeated with
fresh specimens, at and about these particular critical
points. I shall first describe a typical experiment with a
very sensitive and vigorous specimen, and for the sake
of simplicity I give in detail the effects observed at one
pulvinus only, leaving it to be understood that similar
effects were also induced at the other.

The specimen, as said before, was very sensitive, and

POLAR EFFECTS UNDER STRONG CURRENTS 255

when the exciting current applied reached the value of 3*5
micro-amperes the characteristic effect of Type I. — ^namely,
excitation at make of kathode Km — ^made its appearance.
When the current was still further raised to 5*6 micro-
amperes, the excitatory effect was of Type II. — ^namely.
Km Ab, that is to say, excitation at the make of kathode
and break of anode. In these two cases we have Types I.
and II., with which we are already familiar.

Polar Reaction Type III., Km Am Ab.

When the progressively increasing current had reached
a value of 6*3 micro-amperes, however, a new and unexpected

Km A

\\\ I ^

\

Fig. 125. — Effect of increasing intensity of current in trans-
forming response Km Ab of Type II. to response Km Am
Ab of Type III.

type of reaction made its appearance. With the bi-polar
connections it was found that while one leaf, the kathode,
underwent an excitatory fall, the other, or anode, also
showed a simultaneous fall at make. After the re-erection of
the leaves the current was broken, and now the anodic leaf
showed excitation once more, whereas the kathodic exhibited
no excitation whatever. In this new type of effect, then,
we have excitation both at anode and kathode at make and

256 RESEARCHES ON IRRITABILITY OF PLANTS

at the anode at break only. This therefore constitutes
Type III., with the excitatory formula Km Am Ab.

This type of excitation will be seen very clearly in the
automatic record which is given in fig. 125, taken with a
different specimen, where a single pulvinus was carried
through a complete cycle of kathode-make, kathode-break,
anode-make, and anode-break. It will be seen from the
record that while under 6 volts excitation took place at
kathode-make and anode-break, under a higher e.m.f. of
10 volts excitation not only took place at kathode-make
and anode-break but also at anode-make. Under a stronger
current Type II. has thus been transformed to Type III.

Polar Reaction Type IV., Km Kb Am Ab

Returning to the first specimen, the current was further
increased till an intensity was reached of 127 micro-amperes.

Fig. 126. — Polar excitation Type IV., Km Kb Am Ab.

and here was obtained another new type of reaction —
namely, excitation at both kathode and anode at both make
and break. The excitatory formula for this fourth type
is thus Km Kb Am Ab.

In fig. 126 is exhibited an automatic record of this type

POLAR EFFECTS UNDER STRONG CURRENTS 257

obtained with a given pulvinus when it was carried through
the usual cycle of kathode-make, kathode-break, anode-
make, and anode-break. It will be noticed that excitation
took place here at each of these phasic changes.

In the case of the particular specimen whose consecutive
changes have been traced, from the first type to the fourth,
it will be noticed that it was subjected to a long-continued
series of experiments. After this, however, I took up fresh
specimens, with a view to the immediate observation in their
case of Type III. and Type IV. There could here be no
possible suspicion of changes induced by previous currents
too protracted. Thus in a certain fresh specimen the
third-stage effect was obtained with a current of 9*1 micro-
amperes. When this was further increased to I2'6 micro-
amperes the excitatory reaction was transformed into that
of Type IV. It will be remembered that with less sensitive
specimens a higher intensity of current is necessary for the
induction of any particular type of effect. Thus with a
certain less sensitive Mimosa a current of 14 micro-amperes
was required to induce responses of Type III. The current
had in this case to be increased to 32 micro-amperes before
the effects were transformed to those of Type IV.

The particular excitations that appear extraordinary in
these higher types are those at anode-make and kathode-
break. The anode-make effect, as has been shown, can be
induced in the initial response of a fresh specimen. But to
obtain that at kathode-break — since this presupposes a
previous kathode-make which, if sudden, has caused excita-
tion— a certain time must be allowed to elapse after make,
in order that the leaf under the continuous action of the
current may re-erect itself, with restoration of its sensitive-
ness. Lest the long-continued action of the current should
here have induced some unknown change to which the
excitation at kathode-break might be attributable, I have
carried out a number of test-experiments in which the
kathodic increase of current was made gradually, though
in a comparatively short time, without allowing any

258 RESEARCHES ON IRRITABILITY OF PLANTS

excitation to occur at make. The kathode was then broken
suddenly, with the result that excitation was immediately
exhibited. Hence the kathode-break effect under strong
current must be taken as a normal phenomenon.

I have carried out more than fifty different sets of experi-
ments on the characteristic effects of relatively strong
currents, which have fully confirmed the results described.
It was uniformly found that as the current was increased
step by step, the transformation took place, as here laid
down, to a higher type, and never consisted of reversion
to a lower.

Taking these facts, then, as well established, the question
arises whether the new, and apparently anomalous, effects
cannot be explained in a simple way as a special case of these
reactions with which we are already familiar ? Thus the first
explanation that occurred to me was that the fall of the
leaf at anode-make might be due to the arrival of excitation
at the anode from the distant kathode. Against this sup-
position, however, was the fact that a strong anode is known
to be so depressing as to act as a block to the arrival of any
excitation from outside. Overlooking this difficulty, the
arrival of excitation from a very distant kathode on a
different branch of the plant might be expected to take a
certain interval of time. But in the cases given, the excita-
tion at make of anode was, to all seeming, instantaneous.
Finally, I killed a point on the distant branch by severe
scorching, and made it kathode. By this means the possible
excitatory effect at kathode was completely eliminated.
In spite of this abolition of kathodic action, however, the
excitation at anode-make continued to take place as before.
Another modification of the experiment lay in previously
killing the end of the petiole whose pulvinus was being
experimented on. This was done, as before, by scorching.
The intense excitation caused by this injury induced an
excitatory action at the pulvinus, and it was only after
the expiration of about an hour that the organ regained its
excitability, and not even then to the fullest extent. The

POLAR EFFECTS UNDER STRONG CURRENTS 259

kathode was placed on the injured point, and the anode
at or near the pulvinus ; the exciting effect was again
obtained at the make of anode. The only difference in this
case lay in the fact that, owing to the partial loss of sensi-
bility at the pulvinus, the intensity of current for inducing
one of the higher types of response had to be increased.

From these experiments the new types of effects. Type
III. and Type IV., appear to be normal. A further con-
sideration of this question will be postponed till I have given
in detail the reactions of strong currents with other species
of sensitive plants.

Leaflets of Mimosa

I have already mentioned the misgiving I at first enter-
tained that the somewhat unexpected appearance of excita-
tion at anode-make should in reality be due to transmission
from the distant kathodic-point. I have also described
the several means by which these doubts were finally set at
rest, one of those being previous injury of the kathodal-point
by scorching, in order to eliminate it as a possible source of
excitation. I was still desirous, however, of finding some
independent means of demonstrating conclusively the
independent character of the excitation under a strong
current at anode-make.

For this purpose I found the leaflets of Mimosa eminently
suitable. The two bi-polar connections are made at about
the middle points of two different sub-petioles, care being
taken that the contact of each electrode is made on the
midrib at the insertion-points of two opposite pulvinules.
With this arrangement the character of excitation at anode-
make, whether independent or transmitted effect from the
distant kathode, can quite easily be discriminated, for it is
impossible that any excitation should be transmitted along
one sub-petiole to another without causing the fall of the
intervening leaflets. In the case of independent excita-
tions the fact will be displayed by the immediate fall of

s 2

26o RESEARCHES ON IRRITABILITY OF PLANTS

leaflets at the particular points, in this case the two elec-
trodes. As regards the transmitted effects of excitation,
these can easily be followed by watching the serial fall of
leaflets.

I shall now describe an experiment in which, the con-
nections being as already described, the current was increased
step by step to a maximum. Beginning with a current
of 75 micro-ampere, it was found that excitation took place
only at the kathode-make. When the current had been
increased to 2*5 micro-amperes, excitation was initiated
at the kathode at make and at the anode at break. In
these two cases we have the familiar effects of Type I. and
Type 11. With the same identical specimen, however, when
the current was raised to 5 micro-amperes it was found that
excitation was simultaneously initiated at the bi-polar
contacts, both kathode and anode, at make. The fact that
the anodic fall had nothing to do with any transmission of
excitation from the kathode was evident by the slow march
of the excitatory waves, sent out from the two points inde-
pendently, towards each other. It may be mentioned here
that this phenomenon can be made still more prominent
by selecting for experiment leaflets which are a little older
and not excessively sensitive. In such a case the march
of the two waves can be made as slow as may be desired.

We have seen that with strong current there is excitation
at both kathode and anode ; on the break of the current,
excitation was now found to take place only at the anode.
Thus we have the excitatory effect of Type III. — namely
Km Am Ab — established independently by an experiment
on the leaflets of Mimosa.

Employing the minimal current essential to the mani-
festation of Type III. we find the excitation at anode-
make somewhat localised, whereas that at kathode-make is
transmitted to a considerable distance. But on increasing
the current the power of excitatory transmission of anode-
make is greatly enhanced.

Returning once more to the experiment, I found that

POLAR EFFECTS UNDER STRONG CURRENTS 261

when the current had been increased to 12 micro-amperes,
excitation took place at both kathode-make and kathode-
break and also at anode-make and anode-break. Here we
have the characteristic effects of Type IV. — namely, Km
Kb Am Ab. The following table gives a synopsis of these
results : —

Table I. — Effects of Currents of Increasing Intensity
ON Leaflets of Mimosa

Intensity of current.

Excitation induced.

Characteristic type.

micro-amperes
•75

5
12

Km

Km Ab
Km Am Ab
Km Kb Am Ab

Type I.
Type II.
Type III.
Type IV.

These conclusions were uniformly supported by the
results of no fewer than thirty different sets of experiments
on the leaflets of Mimosa. The only variations between
different experiments lay in the fact that with less sensitive
specimens a relatively higher current was required. Thus
in a leaf which was somewhat older, and therefore less
sensitive, the intensity of the current necessary for Type I.
was I micro-ampere ; for Type II. 3 micro-amperes ; for
Type III. 7'4 micro-amperes ; and for Type IV., 15 micro-
amperes.

Leaflets of Biophytum

With leaflets of Biophytum I obtained results which were
practically the same as those given by leaflets of Mimosa,
Thus with a given experimental specimen, the characteristic
effect, Km of Type I. was exhibited under a current-intensity
of '5 micro-ampere. When the current was increased to 2
micro-amperes the response was transformed to Km Ab,
that of Type II. At 3*5 micro-amperes the characteristic
effects of Type III. — Km Am Ab — made their appearance.

262 RESEARCHES ON IRRITABILITY OF PLANTS

And finally, with a current of lo micro-amperes the typical
response of Type IV. was obtained — namely, Km Kb Am Ab.
These results are shown in the following tabular state-
ment : —

Table II.

-Effects of Currents of Increasing Intensity
ON Leaflets of Biophytum

Intensity of current.

Excitation induced.

Characteristic type.

micro-amperes

•5
2

35
10

Km

Km Ab
Km Am Ab
Km Kb Am Ab

Type I.
Type II.
Type III.
Type IV.

These results were confirmed by fifty sets of experiments
on different specimens of Biophytum.

Leaflets of Averrhoa carambola

These leaflets are, as already explained, relatively
insensitive and therefore require a higher e.m.f. with higher
current. With these specimens also I obtained, as before,
the four types of effects in their usual sequence. Thus in a
given experiment a current of 4 micro-amperes gave Km,

Table III. — Effects of Currents of Increasing Intensity
ON Leaflets of Averrhoa carambola

Intensity of current.

Excitation induced.

Characteristic type.

micro-amperes

4
II
20
30

Km

Km Ab
Km Am Ab
Km Kb Am Ab

Type I.
Type II.
Type III.
Type IV.

the effect indicative of Type I. ; when the current was now
raised to 11 micro-amperes excitation at Km Ab, constituting

POLAR EFFECTS UNDER STRONG CURRENTS 263

Type II., was obtained. With a current of 20 micro-
amperes the result was Km Am Ab, or Type III. And
finally, with 30 micro-amperes the characteristic reaction of
Type IV. — ^namely. Km Kb Am Ab — ^was observed.

These results were confirmed with ten different speci-
mens.

I have thus described about a hundred experiments
with different species of sensitive plants which conclusively
demonstrate the existence of Types III. and IV. of polar
reaction.

There remains an alternative hypothesis in regarding the
effects seen in Types III. and IV., as in some way due to
the production of secondary poles. The exact physical
conditions under which the formation of the secondary pole
is possible so as to cause complications in the excitatory
phenomena are clearly shown in the experiments of Engel-
mann and Biedermann on the ureter of rabbit. In an
insulated specimen, under a moderate current, it was found
that excitation took place only at the kathode at make
and at anode at break. But when the specimen was laid
on a good conducting support, such as salt clay, then the
polar reactions were found to be exactly reversed — that
is to say, the anode excited at make and the kathode at
break. This opposition of effects under differing circum-
stances is explained by the fact that in the second of these
cases we have a conducting-sheet which gives rise to diffusion
of the current and a rich development of secondary poles.
Thus, opposite to the kathode, there are produced numerous
secondary anodes, and conversely, opposite to the anode,
there are numerous secondary kathodes. It is these
secondary kathodes which are effective in causing excitation
in the neighbourhood of the anode at make. In the neigh-
bourhood of the primary kathode, on the other hand, excita-
tion is prevented by the depressing influence of the secon-
dary anodic points. The presence of secondary poles thus
induces an apparent reversal of the normal effects, which
is simultaneous at the two electrodes. The simultaneity

264 RESEARCHES ON IRRITABILITY OF PLANTS

of the reversal is therefore a presumptive evidence for the
occurrence of secondary poles.

It may be asked next whether the effects seen in Mimosa
as Types HI. and IV., opposed as these are to Pfluger's
Laws, could be explained away by any of the physical
conditions of the experiment. In answer to this it may
be pointed out that in the experiments described, which
were carried out according to the bi-polar method, the
electrical connections were made directly on the two
contractile pulvini themselves, the thread forming a
complete loop round each organ, which was thus equally
and throughout its circumference anode or kathode, as
the case might be. The organ in this case, moreover,
was isolated and free, like the insulated ureter. Hence
there was an absence of all those conditions which might
favour the formation of secondary poles. . Had there
been any such possibility we should have expected to
see the reversal of normal effects, more or less, from the
beginning, and the reversal should have occurred at the
two bi-polar contacts simultaneously.

Instead of this, we found on the contrary that in the
first two stages, with feeble and moderate currents, the
reactions of the organ were absolutely normal. It was only
after this, with the same specimen and with identical
connections, that by merely increasing the current we
obtained the effect characteristic of the third type —
namely simultaneous excitation of kathode and anode at
make, and excitation of anode at break. Had there been
any induction of secondary pole, the result would have been
excitation at anode at make and at kathode at break.

From these considerations, in addition to others already
given, it appears that the normal effect of strong currents
on Mimosa and other sensitive plants is to cause excitation
at both kathode and anode at make, and anode only at break.
These results, and those of Type IV. which immediately
succeed, are definite and different from either of the two
types that had previously occurred. These characteristic

POLAR EFFECTS UNDER STRONG CURRENTS 265

excitations, moreover, cannot very well be explained on
the assumption of hypothetical secondary poles. The
view that these . effects are physiological will become
strengthened when in the course of the next chapter
we shall observe the modification of polar effects under
physiological changes.

Summary

As moderate increase of intensity of current transforms
the polar reaction from Type I. to Type II., so also further
increase in the intensity of current gives rise to reactions
indicative of other types.

Pfluger's Law is not a complete statement of the polar
action of currents. Under strong intensities of current,
two additional types of reaction — Type III. and Type IV. —
make their appearance. These are included in the
following supplementary Law of Polar Effect of Strong
Currents : —

Under the action of strong current, excitation takes
place at the make of kathode and make and break of
anode.

Under still stronger currents, excitation takes place at
the make and break of both kathode and anode.

These four characteristic types of reactions are serially
observed in all sensitive plants under increasing intensities
of current.

CHAPTER XIX

VARIATION OF POLAR REACTION UNDER TISSUE MODIFICATION

Modification of polar reaction under tissue-changes — Effect of age —
After-effect of moderate stimulation — Modified polar effect : excitation,
at kathode-make and anode-make ; excitation at kathode-make,
kathode-break, and anode-make — General review of polar reactions.

In the course of my investigations into the effects of currents
on the pulvini of different leaves of Mimosa, I found that
during the beginning of the summer season the normal
effects which have been described occurred uniformly, in
their proper sequence. But later in the year, at the end of
the rainy season, when the plants had begun to seed, I was
puzzled by the appearance of certain new and unexpected
types of response with which I had not hitherto been
familiar. Thus in the case of certain leaves, with a moder-
ate current the excitatory reaction took place only at
make of kathode and make of anode. It will be remem-
bered that in Amphistigma also this was the characteristic
response observed by Verworn. With certain other leaves,
again, under fairly strong current, the polar excitations
took place at the make of kathode, at the break of
kathode, and at the make of anode. This again was like
the responsive reaction in Actinosphaerium observed by
Kiihne. I found a very large number of leaves which gave
these specific reactions, which precluded the idea of their
being accidental. These were most frequently exhibited
late in the season and also in winter.

I made many experiments with a view to solve these
anomalies, and found that the normal polar reactions of
plants are modifiable, to a greater or less extent, by various

266

VARIATION OF POLAR REACTION 267

physiological factors. Among the most important of thesfe
may be mentioned the influence of age and of season.
Experiments on the effect of age in the modification of
response will first be described.

The Effect of Age

It is impossible to dissociate from the consideration of
the age of a given leaf its past history as regards the stimulus
of sunlight. If we examine a plant we find that the youngest
leaf is quite green, not having as yet been long exposed to
the action of light. The next below it will be older, and
owing to the longer action of the sunlight, reddish-brown in
colour. The leaves lower down will be older again and still
more russet in tint. In this way the sequence of the leaves
in point of age, from above downwards, corresponds to the
other sequence of duration of exposure to stimulus of light.

It will be remembered that we may broadly classify the
condition of a tissue under three different phases : first, the
pre-optimum or sub-tonic state ; second, the optimum ;
and third, the post-optimum condition, in which we see
an approach towards the condition of fatigue. Moderate
stimulation, as we have seen, will carry a tissue out of the
pre-optimum towards the optimum condition, with con-
sequent enhancement of excitability. Excessive or too-
prolonged stimulation, on the other hand, will carry it
to the post-optimum condition with the characteristic
depression of its excitability.

From this point of view alone, then, we might expect that
the uppermost or youngest leaf of Mimosa would be in the
pre-optimum and therefore less sensitive condition ; that
the sensitiveness of the leaves should attain a maximum as
we descend lower in the plant ; and that after this has been
reached, continuing to descend, the excitability of the
different leaves will be progressively decreased. Represent-
ing these gradations by means of a curve, there would be at
first an ascent, then a climax, and after this a sharp turn and

268 RESEARCHES ON IRRITABILITY OF PLANTS

descent. Independent of or concomitant with this will be
the changes of excitability, more or less obscure, which are
brought about by increasing age. Before subjecting this
inference to the test, I will describe an experiment which
shows that the after-effect of moderate stimulation on
a sub-tonic tissue is to raise its susceptibility to polar
excitation.

After-Effect of Moderate Stimulation

For this purpose I used a specimen which appeared to
be in a pre-optimum condition, and took its records under

Fig. 127. — ^Transformation of Type II. to Type III. as after-effect of
previous stimulation. First two responses taken before, and last
three after, tetanisation.

polar excitation by an identical current, before and after
the application of tetanising shocks of moderate intensity
and duration.

Under an e.m.f. of 16 volts, the records obtained show
excitation (fig. 127) at kathode-make and anode-break,
characteristic of Type II. The record was then stopped
and tetanising electric shocks of moderate intensity applied
for 2 minutes. After a period of rest of 15 minutes, the
record of polar excitation t was taken once more, the applied

VARIATION OF POLAR REACTION

269

E.M.F. being the same as before — namely, 16 volts. It
will be remembered that before tetanisation 16 volts, though
effective for Ab excitation, had been ineffective for Am
excitation. After tetanisation, however, the Am excitation
became effective. The after-effect of moderate stimulation
had thus been to transform Km Ab, the polar reaction of
Type II., to Km Am Ab of Type III.

As already mentioned, from the point of view of stimulus
the youngest green leaf at the top of the plant, say Li,
may be regarded as being in a sub-tonic condition. A leaf
lower down, L2, must be taken as having been previously

Table IV. — Simultaneous Action of Currents of Increasing
Intensity on a very young Li, and a slightly older Leaf L2

Effective inten-
sity of current

Resulting types of reaction.

I.
Km

11.
Km Ab

III.
Km Am Ab

IV.
Km Kb Am Ab

ForLi ..
For L2 . .

micro-
amperes
4 to 8
4 to 5

micro-
amperes
127
5-6

micro-
amperes
20
6-3

micro-
amperes

20

subjected to moderate stimulation. From the experiment
just described, then, we should expect that one identical
current would evoke response of a higher type from L2
than from Li.

In order to test this inference I took a plant and made
bi-polar connections with the second and fourth leaves from
the top, the former of these being very young. Besides
these, a second pair of bi-polar connections was also made,
between the fifth and seventh leaves in descent, and these
will be referred to as L3, which was old, and L4 which was
very old. The results obtained from the second pair will
be dealt with separately.

Above I give a tabular statement of the results obtained

270 RESEARCHES ON IRRITABILITY OF PLANTS

with Li and L2. The mode of procedure was to apply a
current which was gradually increased till successive types
of responsive reactions were observed in one or other of
the two leaves : —

It is apparent that while a maximum current of 8 micro-
amperes induced responses of Type I. in the very young
leaf Li, a very much feebler current, of 5*6 micro-amperes
was sufficient in the case of L2 to induce Type II. A
similar relative exaltation of effect in slightly older
specimens occurs in subsequent types also. Thus while
127 micro-amperes gives rise in Li to Type II., half that
intensity is enough to induce Type III. in L2. And lastly,
while 20 micro-amperes gives Type III. in Li, the same
current gives Type IV. in L2.

Having thus verified our inference with regard to the
relative excitabilities of two young leaves, we may expect
contrasted results with older leaves, where the excitability
will be on the wane. Table V. shows the comparative
effects of increasing currents on leaves of the aame plant,
L3 and L4, which are old and very old respectively. For
greater facility of comparison the effects on Li and L2 are
repeated.

Table V.

-Polar EffecIs of Currents on Leaves of
Different Age

I.

II.

III.

IV.

.. :

Km

Km Ab

Km Am Ab

Km Kb Am Ab

micro-amperes

micro-amperes

micro-amperes

micro-amperes

Li

4 to 8

12-7

20

—

L2

4 to 5

5-6

6-3

20

L3

4 to 16

—

20

23

L4

4 to 21

23

—

From this experiment it will be seen, (i) that the general
excitability, having reached a maximum in L2, has under-
gone a progressive decline with age ; (2) that while the
excitatory efficiency of Km has undergone but slight decline.

VARIATION OF POLAR REACTION 271

that of Ab is very marked. This is seen in the fact that while
Ab excitation was exhibited by the youngest leaf Li under
12*7 micro-amperes, and by L2 under 5-6 micro-amperes, no
such excitation was exhibited by L3 and L4 under a current
as strong as 19 micro-amperes. The decline in excitatory
efficiency of Am, on the other hand, was but slight ; in Li
anode-make excitation was induced by 20 micro-amperes,
in L2 by 6-3 micro-amperes, in L3 by 20 micro-amperes, and
in L4 by 23 micro-amperes. It will be noticed that in the
two leaves L3 and L4 there was no excitation of type Km Ab.

Apparent Vanishing of Type II, Km Ab

We will now consider the possibility of the appearance
of new types of polar reaction by the progressive diminution
of Ab excitation. A reference to Table V. will show that by

Fig. 128. — Abrupt transition from Type I. to Type III., with the vanishing

of Type II.

its greater loss of excitatory efficiency Ab has been reduced
to the level of the excitatory reaction of Am. Normally
speaking, the excitation induced by anode-break Ab takes
place earlier than that caused by anode-make Am. And this
is the reason why Km Ab, or Type II., precedes Km Am Ab,

272 RESEARCHES ON IRRITABILITY OF PLANTS

or Type III. But now when Am is equal to Ab the latter
cannot precede, and the two occur at the same minimal
intensity of current. Hence the sequence of excitation
with increasing current is L, Km ; II., absent ; and III.,
Km Am Ab. A series of records are shown in fig. 128, in
which Type I., Km., passes abruptly to Type III., Km Am Ab,
as the acting e.m.f. is slightly increased from 5 to 6 volts.
It may be stated here that late in the season, and with
somewhat old leaves, I was frequently puzzled by this
vanishing of Type II. ; but the quantitative explanation
which has just been given will adequately account for this.
We have seen that under certain conditions the
Ab excitation declines at a greater rate than that of
Am, and have considered the resulting effect when the
two become equal. In certain circumstances it is quite
conceivable that this relative decline of susceptibility to
excitation by anode-break might proceed further. It
may then happen that Am becomes more effective than

Table VI. — Modifications of Normal Types by Decline of Ab in
Excitatory Efficiency

Normal types
Ab > Am

Modification (a)
Ab=Am

Modification (b)
Ab< Am

I., Km
II., Km Ab
III., Km Am Ab

I., Km
II., 0
III., Km Am Ab

I., Km
ir.. Km Am
III., Km Am Ab

Ab. The sequences of excitatory effects would then be
I., Km; IF., Km Am; III., Km Am Ab. It will here
be noticed that to avoid adding to the number of types,
I have designated Km Am — which takes the place of
Km Ab — as IV. or transitional II. In the accompanying table
these theoretical modifications of type, due to the decline
in excitatory efficiency of Ab, will be seen displayed in a
convenient form. Under normal conditions the excitatory

VARIATION OF POLAR REACTION 273

efficiency of Ab is greater than Am ; under modification it
may become (a) equal to, or (b) less efficient than, Am.

Modified Response Km Am

The existence of modification (a) with the vanishing of
Km Ab has already been demonstrated. We next come
to the theoretical possibility of a modified type Km Am.
As already stated, in certain circumstances, especially late

Fig. 129 — Polar reactions, Km, Km Am, Km Am Ab, under gradually
increasing current.

in the season and in older specimens, I have not infrequently
met with this type. A series of records are reproduced in
fig. 129, in three cycles of effects of increasing current. In
these the acting e.m.f. was increased from 6 to 8 volts and
then to 10 volts. It will be seen that the stage Km Ab has
apparently vanished, its place being taken by Km Am.
An account of results obtained with four different specimens,
under gradually increasing e.m.f., is given in the following
table, the reaction under a particular type being indicated
by an inclined cross.

It is clear therefore that we have in all these cases a

274 RESEARCHES ON IRRITABILITY OF PLANTS

vanishing of Type Km Ab and a substitution of the transi-
tional type Km Am. It may be mentioned here that in
some cases this characteristic excitation, at make only of
both kathode and anode, was found to persist through an
extended range of current-intensities. The resemblance of
this type to the response of Amphistigma is obvious.

Table VII. — Experimental Results with Different Specimens
Exhibiting Km Am Effect

Specimen

E.M.F.

Km

KmAb

Km Am

Km Am Ab

■■■!

volts
6
8

lO

X

—

X

X

2 ••

4
6

X

—

X

—

(

3-.|

2

4
6

X

—

X

X

4 •• ■

12

i8

22

X

—

X

X

Modified Response Km Kb Am

If the excitatory efficiency of Ab were to fall still further,
it might be reduced even lower than the excitatory effective-
ness of kathode-break. In such a case, with a given current,
excitation would be less for Ab than for Am or Kb. Hence
the anode-break effect would vanish and the excitatory
formula for this type under a certain intensity of current
would be Km Kb Am, and this we may call III., Transitional.
It is interesting to find that this particular responsive
modification is not seldom obtained, especially with leaves
the petioles of which are very thin.

VARIATION OF POLAR REACTION

275

The following table shows such results obtained from
four different specimens : —

Table VIII. — Exhibition of Km Kb Am Effect

Specimen

E.M.F.

Km

KmAb

Km Am

Km Am Ab

KmKbAb

■■■{

volts

10
12
16

X
X

—

X

—

X

(

2

4
6 to 10

—

X

—

X

'■■{

4 to 9

12 to 18
20

X

X

E

X

'■■{

4
16

X

—

—

X

A record which exhibits the Km Kb Am excitation is
given in fig. 130.

We may conclude by briefly reviewing the polar reactions

Fig. 130. — Record exhibiting modified response Km Kb Am.

of living tissues with the theories and inferences that are
current on the subject.

In the first place there are many who hold the universality

X 2

276 RESEARCHES ON IRRITABILITY OF PLANTS

of Pfluger's Law. Against their views is brought the fact that
in the unfibrillated protoplasm of Protozoa the reactions
observed are more or less opposed to those of Pfluger's
generalisation.

Next are those who hold with Verworn that, in view of
the anomalous reactions of unfibrillated protoplasm in
P;'o/o^o^, there could not possibly be any law of polar reactions
of universal applicability.

There is, however, a third view still possible. I have
shown that the reactions of fibrillated and unfibrillated
protoplasm are not necessarily opposed, since the plant,
within certain limiting values of current, shows reactions
identical with those which are normal in animal tissues.

If then we find that the generally recognised reactions
of stage I. — ^namely, response at kathode-make only — can
be transformed into those of stage 11. — response at kathode-
make and anode-break — by merely increasing the intensity
of current, then there can be no inherent impossibility in
further transformations into Types III. and IV. under still
further increase of the current -intensity.

The possibility of such transformations has now I think
been demonstrated by the experiments described in this and
previous chapters on numerous species of sensitive plants.
It has also been shown, further, that these polar reactions
were subject to variation under physiological modification
of the tissues. And finally, in plants whose reactions were
already established as identical with those of animal tissues,
it has been shown that — under the separate or joint action
of an increasing current and physiological modification —
responses could be obtained which were similar to the
so-called anomalous responses characteristic of Protozoa.

Summary

The after-effect of moderate stimulation converts the
polar reaction of a given type to one of higher type.

Under the same exciting current the resulting type of

VARIATION OF POLAR REACTION 277

reaction depends on the age of the specimen. A moderately
young specimen gives higher type of reaction than one
which is very young or one which is old.

On account of modification of the tissue, its excitability
to anode-break undergoes progressive diminution. In
consequence of this, modified types of reaction Km Am
and Km Kb Am are sometimes found to occur in Mimosa.

The polar reactions in Protozoa are not exceptional.
Similar effects are observed in Mimosa under specific
conditions.

CHAPTER XX

MULTIPLE AND AUTOMATIC RESPONSE

The Oscillating Recorder — Latent period of Biophytum — Refractory
period — Response on ' all-or-none ' principle — Multiple electrical
response to a single strong stimulus — Multiple mechanical response
to strong stimulus in Biophytum and A verrhoa — Continuity of multiple
and automatic response — Ordinarily responding Biophytum con-
verted into automatically responding condition by excess of stored
energy — Automatically responding Desmodium converted to ordinarily
responding condition by depletion of stored energy.

We have studied in detail the responsive characteristic of
the leaf of Mimosa. We next take up the study of the
responses of the leaflet of Biophytum. In doing this, we
shall observe a certain new class of phenomena of great
theoretical importance come into play.

The difficulties encountered here, however, in the taking
of automatic records are extremely great. The leaflets are
very slender, and the pull exerted in the course of the
excitatory fall is very slight. I overcame the difficulties in
the taking of the record by the use of the Optical Lever, in
which a moving spot of light either traced the response-
curve on a travelling photographic-plate, or was itself
followed by the pen of the observer on a moving drum
covered with paper. These devices have their disad-
vantages, and I was desirous of contriving means to
secure records at once simple, effective, and perfectly
automatic.

The Resonant Recorder was not found specially suit-
able for the tracing of Biophytum movements, inasmuch
as the Hghtest steel wire was still too heavy for the slight pull
exerted by the leaflet ; the only lever that was sufficiently

278

MULTIPLE AND AUTOMATIC RESPONSE 279

light to give a slightly magnified record of these movements
was some special kind of dry grass haulm, which combined
rigidity with excessive lightness. I have tried the finest
feathers from small birds, but these were not so efficient

Fig. 1 3 1. ^Photograph of the Oscillating Recorder, reduced to one-
fourth the natural size.

as the selected specimen of grass which I was so fortunate as
to obtain later. Though in this I was successful in obtaining
a recording-lever, there yet followed the difiiculty that owing
to its being non-magnetic it could not be thrown into
resonant vibration by the electro-magnet. I had therefore

28o RESEARCHES ON IRRITABILITY OF PLANTS

to devise some arrangement by which the recording-plate
could be maintained in a state of oscillation. The recording-
plate was consequently made as light as possible, using the
glass plate employed for covering magic -lantern slides ; the
carrying-frame was made of aluminium. By means of an
electric motor and an eccentric device, the plate-carrier
was made to oscillate to and fro, the frequency of oscillation
being regulated by an adjustment of the electrical current

(fig. 131).

The plate-carrier has small wheels which run between
horizontal rails, above and below. The recording-plate
thus travels in a horizontal instead of vertical direction.
There is also a knock-over key or trigger arrangement,
not shown in the figure. During a particular part of the
travel of the plate the trigger is released, causing a single
break-induction-shock to pass through the plant. The
moment of stimulation is marked in the usual manner on the
travelling-plate, and the number of dots intervening between
this mark and the beginning of response enables us to deter-
mine the latent period when the stimulation is direct, or
the velocity of transmission when the stimulation is indirect.
In the latter case it is of course necessary to know the
distance between the point of application of stimulus on
the midrib and the responding leaflet.

For the determination of the latent period, and of the
velocity of transmission, the oscillation-frequency of the
plate should be about 10 times in a second. But for the
mere obtaining of response-records the frequency of oscilla-
tion need not be high.

Determination of the Latent Period

Taking a record of Biophytum on a fast-moving plate,
with a recorder having a frequency of oscillation of 10 times
a second, it is seen (fig. 132) that there are four spaces between
the incidence of stimulus and the initiation of response ; it
would thus appear that the latent period of the leaflet is -4 of

MULTIPLE AND AUTOMATIC RESPONSE 281

a second. This seems very high compared with the latent
period of Mimosa, which is *! second. Perhaps this difference
may be due to certain characteristics that mark the response
of Biophytum. In animal tissues it is found that while
the singly responding skeletal tissue of the frog has a latent
period of about 'oi second, its multiple responding cardiac
tissue has a latent period of about *! second, or ten times as
long. We shall presently see that the leaflet of Biophytum
exhibits multiple response.

If a second stimulus be applied a short time after the first,
it is found that it is ineffective unless a certain minimum

Fig. 132. — Record giving the latent period of the leaflet of Biophytum.
Frequency of vibration of recorder lo times per second.

interval of time elapses between the two. In these circum-
stances the leaflet takes no account of the second stimulus,
becoming apparently refractory to it. The minimum
interval that must elapse before the second stimulus can
be effective — the refractory period — varies somewhat in
different specimens. In Biophytum it is usuafly 10 seconds.

Response on All-or-none Principle

In a previous chapter I have given the record of a
response in Biophytum under a single stimulus (fig. 15).
We will now study the effect of increasing intensities of
stimulus on the amplitude of response. We have seen
that in the case of Mimosa increasing intensities of stimulus
induce, generally speaking, increasing amplitudes of response.

282 RESEARCHES ON IRRITABILITY OF PLANTS

which however reach a limit. The same is true of the
responses of a skeletal muscle of frog.

In carrying out experiments on Biophytum, I first
determined the minimal intensity of induction-shock that
was effective in inducing response. This happened when
the intensity of stimulus was -i unit. The record of this
response under minimal stimulation was then taken. After
this, a second response to stimulus which was ten times as
strong, was recorded. It will be seen that both minimal
and maximal stimuli induced practically the same effect (fig.
133). In other words, we have here an example of what is

Fig. 133. — Record of responses of Biophytum to stimuli 'i and
I unit, respectively.

known as response on the ' all-or-none ' principle. The leaflet
either responds to its fullest or not at all.

In the various characteristics which have just been
described, the responses of the vegetal organism bear a
curious resemblance to those of the cardiac tissue of the
animal. In the response of the animal heart the latent
period is relatively long, and it exhibits a similar prolonged
refractory period. Its responses are also on the * all-or-none '
principle — it either responds to the utmost or not at all.

Returning to Biophytum, we have seen that in order to
induce any response a certain minimal intensity of stimulus
was necessary. But when the intensity of this stimulus
was further increased, the outward expression, or response
of the leaflet, remained apparently as before. What then
became of the excess of energy that impinged upon it in
the form of stimulus ? It is not necessary to suppose that

MULTIPLE AND AUTOMATIC RESPONSE 283

in every instance the whole energy imparted by stimulus
gives rise to useful work. Some of it may be wasted as
heat. But, on the other hand, it is conceivable that the
excess of this energy may be stored up, for the time beings
to find subsequent expression. To take a physical illustra-
tion, the energy stored up in a compressed spring may,
on release, give rise to long-continued and rhythmic oscilla-
tions. The question arises, then, whether in the leaflet of
Biophytum, impressed by an excess of stimulation, there may
be any analogous storage of energy. Supposing this to take
place, the superfluous energy might be utilised to do some
internal work not discernible by the observer ; or it might, in
favourable circumstances and in the presence of suitable
motile indicators, find expression in rhythmic movements.

I have referred elsewhere to another mode of recording
excitatory response in plants — namely, the electrical.
Using this, I have frequently observed that although under
a moderate stimulus a single stimulus gives rise to a single
electric response, yet under a strong stimulus there would
arise a series of responses. Thus, while a feeble stimulus
induced a single response, a strong stimulus gave rise to
multiple responses.

Having this in mind and the peculiar characteristics of
Biophytum response, I expected to demonstrate the occur-
rence of multiple responses by means of mechanical move-
ments. It had already been noticed that Biophytum when
strongly excited closed its leaflets, not by one but by two
successive twitches. It appeared to me that this curious
phenomenon was parallel to the multiple response of rhythmic
animal tissues ; and I expected, if this were so, that instead
of two it would be possible to obtain from it a long series of
rhythmic responses comparable with the multiple rhythmic
responses in animal tissues. In order to obtain this in the
case of Biophytum it was necessary to prevent the complete
closure of the leaflet, in consequence of which further
exhibition of mechanical response is rendered impossible.
This was secured by applying a light counterpoise in the

284 RESEARCHES ON IRRITABILITY OF PLANTS

second arm of the lever. The effect of this was by exerting
a tension to hasten recovery, and thus oppose the complete
closure of the leaflet.

I then took records of responses of Biophytum to quan-

FiG. 134. — Response of Biophytum leaflet to stimuli *i, "5,
I, and 2. Response is seen to be multiple with the last.

Fig. 135. — Multiple response in Biophytum under a single
strong electric shock.

titative stimuli of increasing intensity, the stimuli being
applied at intervals of 3 minutes (fig. 134). The successive
stimuli were of intensities -i, *5, i, and 2. Owing to incom-
plete recovery during the intervening resting-intervals.

MULTIPLE AND AUTOMATIC RESPONSE 285

the base-line is seen to be shifted upwards. The ampHtude
of successive responses is about the same, though the
stimuU are increasing. At the application of the fourth

Fig. 136. — Multiple response in Averrhoa under a single strong
electrical stimulus. Vertical marks below indicate time-
interval of I minute in this and in the following records.

stimulus, of intensity 2, we find that the response becomes
multiple. Thus we see that while a single stimulus of
moderate intensity gives rise to a single response, a strong

Fig. 137. — Multiple excitation in Biophytum under the
action of constant Hght.

stimulus gives rise to a multiple series of responses. In
fig. 135 are depicted multiple responses in Biophytum to a
very strong electrical stimulus, there being four multiple
responses in the course of 6 minutes. In fig. 136 are

286 RESEARCHES ON IRRITABILITY OF PLANTS

shown the multiple responses obtained in Averrhoa carambola
under a single stimulation caused by strong induction-shock.
Here there are six responses in the course of 15 minutes,
the average period of a single pulsation being 2*5 minutes.
We have also seen multiple responses induced in Bio-
phytum under the action of constant current (cf. fig. 124).
Multiple responses also take place in Biophytum and in
Averrhoa under the action of constant light. I give a

Fig. 138. — Multiple response under the action of single strong
thermal shock.

record (fig. 137) in which is seen the occurrence of five
multiple-responses in the course of 8 minutes under the
continued action of light from an arc-lamp.

I have also obtained multiple responses under other
forms of strong stimulation. In fig. 138 is given a record
of multiple responses induced by the action of a strong
thermal stimulus. In certain other cases I obtained with
Biophytum as many as sixteen recurrent responses under a
single thermal shock.

In fig. 139 is seen a series of multiple responses induced
by strong chemical stimulation. This was caused by

MULTIPLE AND AUTOMATIC RESPONSE 287

the application of a drop of hydrochloric acid on the
petiole.

From these experiments it is clear that a rhythmic
series of effects need not have a periodic antecedent cause.
We see on the other hand that under strong stimulation
there is not only an immediate response but that the surplus
of energy remains over and is held latent by the tissue to
be given out later in the form of recurrent responses.

It is the excess of latent or Internal Energy that gives
rise to phenomenon of multiple response. Sometimes we
may not have noticed the antecedent external stimuli the

Fig. 139. — Multiple response induced by strong chemical
stimulation,

absorption of which had contributed to that storage of
internal energy which gave rise to the rhythmic activity.
In these circumstances the pulsations appear to us as
spontaneous or automatic.

Under natural conditions, the plant is exposed to the
action of various stimuli supplied by its environment. It
is exposed to warmth, to the action of light, to internal
hydrostatic pressure, to the action of various chemical
agents — present in it or absorbed by it. We have seen
that each of these factors exerts its stimulating action
independently. From the joint action of these external
sources of stimulation, the energy stored up by the plant
may become sufficiently great to cause an excitatory
overflow. It will thus be seen how, by the cumulative
effects of these various stimuli, the excitability of the

288 RESEARCHES ON IRRITABILITY OF PLANTS

plant may become so excessive as to manifest itself by
outward response, apparently automatic.

Thus I have obtained from Biophytum seemingly auto-
matic pulsation by subjecting a vigorous plant to the
favourable conditions of light and warmth. In a particular
case the favourable temperature was found to be 35° C.
When this was lowered to 29° C. the pulsations came to
a stop.

It is thus clearly seen that there is no strict line of demar-
cation between multiple and automatic responses so called.
An ordinarily responding plant like Biophytum, which gives a
single response to a single moderate stimulus, and multiple
responses to a strong stimulus, will in very favourable circum-
stances, that is to say, when it has absorbed an excess of
energy from without, become automatically responding.

Conversely, an automatically responding plant in un-
favourable circumstances is found to be converted to the
condition of an ordinarily responding plant. The leaflet
of Desmodium gyrans under favourable conditions is found
to execute pulsatory movements which appear to be spon-
taneous. But when this plant is subjected to unfavourable
conditions, then its spontaneous rhythmic activity comes
to a stop. In this condition of standstill, the reaction of the
leaflet is like that of Biophyiiim. It then gives rise to a
single response to a single moderate stimulus, and multiple
responses to a strong stimulus.

There is thus seen a continuity in the multiple and auto-
matic responses. Biophytum is equivalent to Desmodium
when brought to a state of standstill by depletion of storage
of energy. Pulsating leaflets of Desmodium may, on the
other hand, be regarded as equivalent to Biophytum with
an overflow of energy.

Summary

In plant a single moderate stimulus is found to give rise
to a single electric response. A strong stimulus, on the

MULTIPLE AND AUTOMATIC RESPONSE 289

other hand, often gives rise to a multiple series of electric
responses.

In Biophytum, similarly, while a moderate stimulus gives
rise to a single mechanical response, a strong stimidus
gives rise to a multiple series of responses.

These multiple responses are induced by various modes
of strong stimulation such as induction-shock, constant
current, strong light, thermal shock, and chemical excita-
tion.

Certain plant tissues have the power of holding the
excess of stimulus latent, to be given out later in the form
of recurrent responses.

The characteristics of the response of Biophytum are like
those of cardiac tissue of the animal. Both are characterised
by long refractory period and response on ' all-or-none '
principle. A single moderate stimulus gives rise to a single
response in both, and a strong stimulus gives rise to a multiple
series of responses.

There is no strict Hne of demarcation between the
phenomena of multiple and of automatic response. In very
favourable circumstances of absorption of excess of energy
from without, an ordinarily responding plant like Biophytum
will become converted into an apparently automatically
responding plant like Desmodium.

Conversely, under unfavourable conditions, that is to
say, when the sum-total of its energy is below par, an
automatically responding plant like Desmodium will
become converted into an ordinarily responding plant like
Biophytum. Its leaflets then come to a state of standstill.

CHAPTER XXI

THE AUTOMATIC PULSATIONS OF DESMODIUM GYRANS

Activity of detached leaflet of Desmodium — Pulsation maintained uniform
under constant internal hydrostatic pressure — The plant chamber—
Time-relations of pulsating movement derived from dotted record —
Significance of down and up movements — Systole and diastole —
Table showing rates of movement of Desmodium leaflet at different
phases.

We have hitherto studied the responsive movement in
sensitive plants, where such movement was initiated by
a directly exciting stimulus. We shall now take up the
consideration of another class of move-
ments, which are apparently auto-
matic or without any immediately
preceding cause. In certain plants we
observe what are known as spontane-
ous movements, of which Desmodium
gyrans or the telegraph-plant fur-
nishes an example.

This telegraph-plant grows wild
in the Gangetic plain ; its Indian
name is Bon Charal, or * forest churl,'
the popular belief being that it
dances to the clapping of the hands.
There is, however, no foundation
for this belief. It is a papilionaceous
plant -with trifoliate leaves, of which the terminal leaflet
is large, and the two lateral, very small (fig. 140).
Each of these is inserted on the petiole by means of
pulvinule. The lateral leaflets are seen to execute pulsating
movements which are apparently uncaused, and are not

290

Fig. 140. — Leaf of Des-
modium gyrans. The
two small lateral
leaflets exhibit spon-
taneous movements.

PULSATIONS OF DESMODIUM GYRANS 291

unlike the rhythmic movement of the heart. The extra-
ordinary similarity imder various conditions of the rhythmic
reactions in the plant and the animal will be seen in the
experiments which will be presently described.

It will, moreover, be shown that, strictly speaking, there
is no such thing as spontaneous movement. The energy
which expresses itself in pulsating activity is 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. When the storage is exhausted,
the spontaneous movement, so called, is found to cease.
In this condition of standstill the rhythmic activity can be
renewed by an accession of fresh stimulus.

In the intact plant, under favourable conditions, these
spontaneous movements are observed to take place more or
less continuously ; but there are times when they come to a
standstill. For this reason and because of the fact that a
large plant cannot easily be manipulated as a whole and
subjected to the various changing conditions which the
purposes of investigation demand, it is desirable if possible
to experiment with the detached petiole carrying the pul-
sating leaflet. The required amputation, however, may be
followed by arrest of the pulsating movements. But, as in
the case of the isolated heart in a state of standstill, I find
that the movement of the leaflet can be renewed in the
detached specimen by the application of internal hydrostatic
pressure. Under these conditions, the rhythmic pulsations
are easily maintained uniform for many hours. This is a
great advantage, inasmuch as in the undetached specimens
the pulsations are not usually found to be so regular as they
now become. So small a specimen, again, can easily be
subjected to changing experimental conditions, such as
variations of internal hydrostatic pressure and temperature,
application of different drugs, vapours, and gases.

The petiole after detachment should be put in water
immediately, to prevent complications arising from drying
of the cut end. It is then mounted water-tight, in the

u 2

292 RESEARCHES ON IRRITABILITY OF PLANTS

shorter open end of a narrow u-tube filled with water. For
this purpose an indiarubber cork is taken, and a slit made
from circumference to centre. The petiole is then slipped
into the centre of the cork, which is gently forced on to the
short open end of the u-tube. The longer end of the u-tube
partly consists of indiarubber tubing. By raising or lower-

FiG. 141. — U-tube support for leaflet, and the plant chamber.

ing this longer limb of the u-tube the hydrostatic pressure
to which the specimen is being subjected can be varied ;
different chemical solutions can also be applied internally by
its means ; a stopcock allows the water to run out of the
u-tube, making way for the particular solution poured in
at the open end of the tube.

The u-tube (fig. 141) is hinged on a rod which slides
up and down inside an upright tubular support. This
rod can also be rotated inside its support and clamped in
any desired position. Facilities are thus secured for three

PULSATIONS OF DESMODIUM GYRANS 293

different modes of adjustment. One up and down, the
second lateral, and the third, by means of the hinged support,
for the inclining of the specimen. The movement of the
leaflets, it must be remembered, does not always take
place in a vertical plane. The object of these mechanical
adjustments therefore is to enable us to place the specimen
at such an angle that its to-and-fro vibration when straight
shall be vertical, or have its long axis vertical when the
movement is elHptical.

A light cover with mica windows can be made to enclose
the specimen. By means of electric current sent through a
spiral of German silver, the inside of the chamber may be
heated to any desired degree. The temperature can be
lowered, on the other hand, by sending through the chamber
a stream of cooled air : different vapours or gases could be
passed into the chamber for studying their effects on the
automatic pulsation.

The arm of the recording-lever is attached by means of a
cocoon thread to a point about the middle of the leaflet, by
means of a drop of shellac-varnish. As the pull exerted
by these leaflets is very feeble, the writer has to be made
extremely light. The vibration of the Resonant Recorder
being about 10 times per second, the record taken with it
appears as continuous. In certain experiments it is desirable
to obtain data for accurate time-measurements of different
phasic movements of the leaflet. This I have been able
to secure by the employment of the Oscillating Recorder,
where the recording-plate, by means of an electric motor
provided with an eccentric, is made to execute a to-and-
fro movement. The intermittent dots thus produced may be
once each second, or once in 2 seconds. As the oscillating
recorder permits the employment of light grass haulm for
the recorder, we may easily obtain a fair magnification pro-
duced in the record. I have used both the methods —
resonant and oscillating — for obtaining the records : in the
former they appear continuous ; in the latter, dotted.

As an example of the extreme regularity which can be

294 RESEARCHES ON IRRITABILITY OF PLANTS

secured in the pulsating movement of the cut specimens,
I reproduce here a continuous record lasting for four hours
(fig. 142), the movements themselves being maintained
uniform for more than seven hours. The run of the breadth
of the plate was accomplished in one hour and twenty
minutes, successive series of records being taken on the
same plate from below to above. It will be seen how
uniform are the successive pulsations, not only as regards

Fig. 142. — Continuous record of pulsations of D^smo^mm leaflet
for four hours ; the series to be read from below to above.

the ampUtude, but also the period. It is only after securing
such uniformity, under normal standard conditions, that
the experimenter is justified in drawing correct inferences,
from variations induced in the record, on the influence of
changed conditions which he has introduced.

Time-Relations of the Pulsating Movements of

Desmodium Leaflet
The up-and-down movement of the Desmodium leaflet
is characterised by certain pecuHarities. In its usual form

PULSATIONS OF DESMODIUM GYRANS 295

the ' up ' movement is executed somewhat slowly, and
comes to what may be called a pause at the extreme position
for a certain length of time. There is in reality no cessation
of movement ; but the rate becomes extremely slow at the
turning-point where * up ' is reversed to * down ' movement
and vice versa. After reaching the extreme ' up ' position the

* down ' movement is commenced, and this is accomplished
in a much shorter time. After reaching the lowest position
there is again a pause, when the cycle is again repeated.
These normal movements and their rates are, however,
subject to modification under the influence of external
conditions.

These movements of Desmodium leaflet are brought
about, as in the case of Mimosa, by the contraction or
expansion of the pulvinus. Here, also, it is the lower half
of the motile organ that is predominant in its action. A
question now arises as to the significance, whether of
contraction or relaxation, of the up and down movements.
The question may be settled in three different ways : The
contractile movement, generally speaking, is quicker ;
hence the quicker down movement of the leaflet may be
regarded as that due to contraction. Again, I have found
that a leaflet in a state of standstill exhibits under stimu-
lation an excitatory contractile movement which is down-
wards. And lastly, by means of internal hydrostatic
pressure, we may induce expansion of tissue. This has the
effect, as we shall see in a later chapter, of shifting the
pulsatory movements upwards. All these different con-
siderations point to the conclusion that in Desmodium the

* down ' position of the leaflet represents contraction and
the ' up ' position denotes expansion of the more effective
lower half of the motile organ. The up-and-down move-
ments of the leaflet thus correspond to the diastolic and
systolic movements of the animal heart. In the records
of the pulsation of Desmodium, up-curve represents down
movement and vice versa.

It would be interesting to know the absolute rate of

296 RESEARCHES ON IRRITABILITY OF PLANTS

these movements in their various phases. The movements
themselves are relatively slow, much slower than the con-
tractile movement of Mimosa. In that case we saw that
the movement of the fall was accomplished in about 2 seconds
but with Desmodium the down movement may occupy as
long as 40 seconds. I was able to determine the different
rates of movement in Desmodium by making the recording

Fig. 143. — Record of a single pulsation of Desmodium', magnification
25 times. Successive dots at intervals of a second.

plate oscillate to and fro once in a second or once in two
seconds.

In fig. 143 is given a record of a single pulsation, mag-
nified 2*5 times and taken on a fast-moving plate, the
successive dots being at intervals of a second. The period
of an entire pulsation was 10 1 seconds, of which the down
movement was accomphshed in 41 seconds and the up
movement in 60 seconds. The spacing of the successive
dots at once gives a visual representation of the changing
rate. It is noticed that the leaflet attains its maximum
rate during the fourteenth second of its downward journey.
The maximum rate of the down-movement is '9 mm., the

PULSATIONS OF DESMODIUM GYRANS 297

average rate being -44 mm. per second. The maximum rate
of the up movement was -56, and the average rate -3 mm.
per second.

In fig. 144 is shown a record of two successive pulsations
obtained with a different specimen. The amphtude is

Fig, 144. — Records of two pulsations of a different specimen of Desmodium
reduced to f . Successive dots are at intervals of 2 seconds.

reduced to two-thirds in the diagram and the successive dots
are at intervals of 2 seconds. A detailed account is given
in the accompanying table of the rates of movement
exhibited by the two specimens : —

Table showing Rates of Movement at Different Phases of
Pulsation of Desmodium

Specimen i

Specimen 2

Down movement

Total period . .

Average rate . .

Maximum rate
Up movement

Total period . .

Average rate . .

Maximum rate

41 seconds
•44 mm. per sec.
*9 mm. per sec.

60 seconds
•3 mm. per sec.
•56 mm. per sec.

42 seconds
•46 mm. per sec.
10 mm. per sec.

92 seconds
•21 mm. per sec.
•5 mm. per sec.

I give another series of records obtained with a vigorou.s

298 RESEARCHES ON IRRITABILITY OF PLANTS

specimen taken on a slower-moving plate (fig. 145). This
record shows the extreme uniformity of these pulsations.
How uniform the time-relations are will appear from the
tabular statement below, where the intervals of time are

Fig. 145. — Series of automatic pulsations of Desmodium.
Successive dots at intervals of 2 seconds.

measured between the attainment of highest and lowest
positions : —

Table showing Periods of Down and Up Movement in
Desmodium Pulsation

No.

Period of down

Period of up

Total period

movement

movement

seconds

seconds

seconds

I

32

56

88

2

32

56

88

3

34

54

88

4

36

54

90

5

32

54

86

6

32

54

86

These figures are typical of the movement of Desmodium
leaflet under normal conditions in the summer season. In
winter the rate is very much slower, the period of an entire
pulse being then as long as 4 minutes.

Various other factors modify either the amplitude or
frequency of the pulsatory movement. One factor which
induces a pronounced change is that of temperature. The

PULSATIONS OF DESMODIUM GYRANS 299

detailed consideration of this and of other factors will be
given at some length in the following chapters.

Summary

The pulsating activity of the detached leaflet of Desmo-
dium gyrans can be maintained uniform for a long time by
subjecting it to internal hydrostatic pressure.

The application of a shock to a leaflet in a state of
standstill induces a down movement. The phase of
down movement is in general quicker. Enforced expansion
by increased internal hydrostatic pressure induces move-
ment of the leaflet upwards. These facts indicate that the
down position of the leaflet represents a * systolic ' contrac-
tion, and up position a * diastolic ' relaxation of the motile
organ.

In a typical example of the rhythmic pulsation of
Desmodium leaflet the down movement is accomplished
in 41 seconds. The maximum rate of down movement is
•9 mm. per second, the average rate being -44 mm. per
second. The period of up movement is longer, being
60 seconds ; maximum rate of up movement is "56 mm.
per second, the average rate being -3 mm. per second.

CHAPTER XXII

EFFECT OF HYDROSTATIC PRESSURE, LOAD, AND LIGATURE
ON THE PULSATION OF DESMODIUM

Effect of internal hydrostatic pressure on the pulsation of Desmodium —
— Expansive erection of leaflet under increased pressure : diminution
of the extent of systolic contraction — Effect of load : diminution of
period — Stannius' ligature on heart-beat — Parallel effect of ligature
on pulsation of Desmodium — Arrest of pulsation by a cut and revival
by electric shock.

We have already seen that after the appHcation of a suitable
internal hydrostatic pressure the pulsation of the detached
leaflet of Desmodium becomes extremely regular. Some-
times a leaflet is found which has come to a state of standstill.
Internal hydrostatic pressure is often found in such cases to
renew the pulsatory activity.

Effect of Hydrostatic Pressure

The application of internal hydrostatic pressure enables
us, moreover, to understand the significance of the up-and-
down movements. Such movements may be the results
of contractions and expansions, as in a pulsating heart. In
the case of Desmodium we can induce expansion artificially
by internal hydrostatic pressure ; the shifting of the base-
line up or down will then distinguish for us the result of
expansion. Of the up-or-down movement again, the one
corresponding to expansion will be helped by increased
pressure ; the other, on the contrary, will be opposed.

In subjecting the detached leaflet of Desmodium to

300

EFFECT OF HYDROSTATIC PRESSURE 301

increased hydrostatic pressure it is found that the base-Hne is
shifted downwards, which in the leaflet it must be remem-
bered means erection. The contractile effect is also opposed,
hence we observe the extent of contraction gradually

Fig, 146. — Application of increased internal hydrostatic
pressure at arrow. Displacement of base-line downwards
in the record indicates expansive erection of leaflet.
Enforced expansion also causes decline in the extent
of systolic contraction.

decreased, with the result that a line joining the apices of
the successive pulsation slopes downwards (fig. 146).

Effect of Load

We will next study the effect of load. Different weights
are placed on the second arm of the lever. The effect of
the weight is to pull the leaflet slightly upwards. This
pull will have the effect of opposing the contractile
movement.

I took a series of records of the pulsation of Desmodium,
first without any weight, then with increasing loads of ^m,
y^, and -i-Q grm. (fig. 147). It will be seen that the am-
plitude of pulsations is in consequence progressively
diminished. Without load, the amplitude is 19 mm. ;
under a load of ^Jugrm. it has become reduced to 11 mm. ;

302 RESEARCHES ON IRRITABILITY OF PLANTS

under j-fogrni. to 8 mm., and finally, under -^V grm. to 4 mm.
When the load is increased to 4V grm. the pulsation is
arrested. The pulsation is also seen to become slower with
increasing load.

Arrest of Pulsation by Ligature

A very interesting phenomenon, often observed in a
pulsating heart, is the effect of cut or ligature. Thus by the
application of what is known as * Stannius' ligature ' above

Fig. 147. — Effect of increasing load of 3^0, tuo, and -^\ grm. Amplitude
of pulsation decreased, and period increased, with increasing load.

the heart, its beat may be arrested. This arrest takes place
in a relaxed condition, that is to say, at diastole. The
effect, however, is liable to be modified by the condition of
the heart. Sometimes there is a failure of arrest. At other
times, on the application of ligature there are a few vigorous
pulsations followed by arrest. Effects similar to these are
also induced by cut. The various explanations hitherto
offered of these peculiar effects have all been pronounced
untenable for one reason or another.

Seeing the remarkable parallelism which obtains be-
tween the pulsation of the cardiac tissue and that of the
leaflets of Desmodium, I was curious to discover whether in
the case of * Stannius' ligature,' also, there was a similar
correspondence. It was a great surprise to me, on applying

EFFECT OF LIGATURE 303

a cut or a ligature to the petiolule of the leaflet, about 3 mm.
below the pulsating pulvinule, to find the various peculiarities
recorded of the action of the heart repeated here, with
striking similarity.

The most convenient way to apply what was equivalent
to the ligature, was to hold the petiolule in a very small
clamp and proceed to record the normal pulsation. After
doing this for some time, the clamp was suddenly tightened.

Fig. 148. — Effect of ligature in inducing an arrest of pulsation
of Desmodium at diastole.

This induced an arrest of pulsation, either at once or after
one or two vigorous beats. Similar effects were also obtained
by making a cut, after suitably supporting the leaflet.
These results depend somewhat on the condition of the
specimen. It is generally found that the nearer it is made to
the pulvinule the more effective is the cut or ligature in
inducing the arrest.

In fig. 148 is seen the arrest induced by ligature : it is
very remarkable that here> as in the case of the pulsating
heart, the arrest by ligature took place at diastole.

In fig. 149 is seen the arrest induced by a cut. While
the leaflet is still in the condition of arrest it is often possible
to renew the pulsation by an induction-shock. These

304 RESEARCHES ON IRRITABILITY OF PLANTS

phenomena of arrest and revival are easily observable in
the record.

The question next arises whether any theory can be
suggested as to the cause of this arrest. A possible explana-
tion might lie in regarding the cut or sudden ligature as
intense forms of stimulation, whose effect is somewhat
persistent rather than instantaneous. We have further
to suppose that this intense stimulation is conducted by
the intervening petiolule. We have seen, moreover, that
the leaf of Mimosa under prolonged stimulation, after a
preliminary excitatory contractile movement, assumes a

Fig. 149. — Arrest of pulsation of Desmodium by a cut applied at
moment marked by first arrow. Pulsation was revived by
electric shock applied at moment marked by second arrow.

relaxed position indicative of over-stimulation and fatigue.
After the lapse of a requisite time, however, the leaf
regains its sensitiveness. In the case of the Desmodium
leaflet a similar effect might conceivably be induced
by the transmission of intense excitation from the stim-
ulated cut or ligatured end. This question of the effect
of ligature is admittedly one of great difficulty, even
in the case of the cardiac tissue. The explanation here
offered, as applying to the Desmodium leaflet, may be taken
as suggestive and more or less tentative. Such an idea
presupposes that the intervening petiolule contains some
conducting-tissue by which excitation may be transmitted.
In any case, the question as to the power of the petiolule to

EFFECT OF LIGATURE 305

conduct excitation applied at a distance, and thereby induce
a modification in the pulsating activity of the attached
leaflet, is one of very great importance. This question will
be taken up in a succeeding chapter.

Summary

Increased hydrostatic pressure induces an expansive
erection of the leaflet of Desmodium gyrans. The enforced
expansion induces a diminution in the extent of systolic
contraction.

Increasing load induces a diminution of amplitude and
prolongation of period of pulsation. A load of -^ grm. is
enough to arrest the pulsation.

A ligature appHed 3 mm. below the motile organ arrests
the pulsation of Desmodium leaflet at diastole. This cor-
responds to the action of Stannius' ligature on the heart.

A cut also arrests the pulsation of Desmodium. After the
arrest, pulsation may sometimes be revived by the action
of an electrical shock.

CHAPTER XXIII

EFFECT OF STIMULUS ON LEAFLET OF DESMODIUM AT
STANDSTILL

Condition of standstill brought about by depletion of energy — Renewal
of pulsation by the stimulus of light — Response to stimulus of
induction-shock — Multiple response under tetanisation — Determina-
tion of the latent period and the apex time — Refractory period —
Effect of stimulus on leaflets in sub-tonic condition — Effect of isolation
on rhythmic activity — Gradual arrest of pulsation resulting from
run-down of stored energy — Effect of fresh accession of energy.

The leaflets of Desmodium exhibit in favourable circum-
stances a more or less persistent rhythmic activity. But
this activity may cease under conditions which are less
favourable. The arrest may be due to either of two causes,
of which the first is the exhaustion of that reserve of energy
without which the pulsation cannot be maintained, and
the second is the loss of motility of the pulvinule brought
on by age or other factors. As a parallel instance to this
we have the case of Biophytum, the old leaflets of which
become quite insensitive, while the plant as a whole remains
sensitive.

Renewal of Pulsating Activity under the Action
OF Stimulus

In those cases where the arrest is due to run-down of
energy, the pulsation of Desmodium leaflets may often be
renewed by the application of appropriate stimulus. This
may be seen in the following record on the action of the
stimulus of light (fig. 150). The leaflet had been reduced
by extreme sub-tonicity to a state of standstill. The

306

RENEWAL OF PULSATION 307

quiescent leaflet was then subjected to the continued action
of hght from a Nernst lamp. It will be noted that by the
absorption of the energy of light the leaflet regained its
so-called spontaneous activity.

It should be mentioned here that the effect induced by
the stimulus is, to a certain extent, modified by the tonic
condition of the plant. In a sub-tonic specimen, with the
leaflet in a state of standstill, the action of stimulus is to
renew the pulsating activity. In a different specimen,

Fig. 150. — Action of light in renewing the pulsation of Des-
modium leaflet at standstill. Light was applied at the point
indicated by the arrow and continued afterwards.

where the rhythmic activity is feeble, the incidence of
stimulus enhances the amplitude of pulsation. If the
leaflet should be in a vigorous condition, excessive stimu^
lation is apt to bring on fatigue, in consequence of which
the pulsations become either irregular or diminished in
amplitude. These various effects are found to take place
not only under the action of stimulus of light but, as we
shall see, under electric stimulus also.

Effect of Electric Stimulus

The leaflet of Desmodium in a state of standstill may
have its activity revived by other modes of stimulation,

X2

308 RESEARCHES ON IRRITABILITY OF PLANTS

such as that of induction-shock. This can be seen in the
following record (fig. 151). The leaflet had been reduced by

extreme sub-tonicity to a state
of standstill. A single electrical
stimulus of moderate intensity is
seen to give rise to a single re-
sponsive pulsation. Repetition of
the same stimulus gave rise once
more to a second response.

In another instance, direct
tetanising electric shock of short
duration was applied to a leaflet in
a state of standstill. It is seen
(fig. 152) that this application of moderately strong stimulus
gave rise to renewed pulsation which persisted for a certain
length of time, even on the cessation of stimulus.

Fig. 151. — Response of
Desmodium leaflet in
a state of standstill.
Single response to
single stimulus.

The Latent Period

Having thus found that a leaflet in a state of standstill
can be made to give response to instantaneous stimulation,
it is possible to determine the latent period. We have seen
that the latent period of the rhythmic cardiac tissue is,

Fig. 152. — Multiple responses, under moderate tetanisation,
in leaflet originally at standstill.

generally speaking, longer than that of the ordinary skeletal
tissue. In the plant, again, while the average value of the
latent period of ordinarily responding Mimosa is -i second
that of the multiple responding Biophytum is -4 second.

APEX TIME OF DESM ODIUM 309

The latent period of motile leaflet of Desmodium is of the
same order. It varies from -4 second to -5 second.

The Apex Time

In order to study in detail the time-relations, I took on a
fast-moving plate the response of Desmodium leaflet, origin-
ally in a state of standstill, to the action of induction-shock.
The frequency of oscillation of the recording-plate was once
in a second, hence successive dots represent intervals of

Fig. 153. — Response of Desmodium leaflet originally at standstill.
Successive dots at intervals of i second.

I second. The entire response, consisting of contractile
down-movement and subsequent recovery, was accom-
plished in the course of 2 minutes and 45 seconds ; the
leaflet attained its maximum down or contractile position
45 seconds after the application of stimulus. The top of
the response-curve is seen to be somewhat flattened, indi-
cating a persistent contraction from which recovery takes
place slowly. The period of relaxation is much longer,
being 120 seconds ; the record in fig. 153 shows recovery
which is not quite complete.

Taking thus a typical case we find that in Desmodium,

(i) The latent period is -5 second.

(2) The apex time is 45 seconds.

(3) The period of relaxation is 120 seconds.

310 RESEARCHES ON IRRITABILITY OF PLANTS

The Refractory Period

A striking characteristic of the rhythmic cardiac tissue
of the animal is its long refractory period. It is found that
by applying successive stimuli there is a minimum resting-
interval, the diminution of which brings about such a loss
of excitability as to abolish response. This minimum
interval is known as the Refractory Period, because short
of this period the tissue takes no account of stimulus or is
refractory to it. In the cardiac muscle the refractory period
lasts during the entire period of the contraction.

In the response of the leaflet of Desmodium also we
observe in this respect a very striking similarity, in its

Fig. 154. — Record demonstrating refractory period in Desmodium.
In the lower record the second stimulus, applied after 45
seconds, is seen to be ineffective, having fallen within the
refractory period. In the upper record the second stimulus,
applied after an interval of 90 seconds, is seen to be effective.

possession of a long refractory period. This will be seen
in fig. 154, where the leaflet in a state of standstill was sub-
jected to two successive stimuli of induction-shocks. In
the lower record of the two, after the responsive movement
due to the first stimulus, a second was applied after an
interval of 45 seconds. The leaflet, however, took no
account of it, there being induced no second response. The
refractory period in this case is seen to be longer than
45 seconds. After a suitable interval the same leaflet was
subjected to two successive stimuli, and at an interval of
90 seconds. It is seen from the upper record that the
second stimulus was effective, having fallen on the leaflet
beyond the refractory limit.

EFFECT OF DEPLETION OF ENERGY 311

I stated in a previous chapter that, strictly speaking,
there is no such thing as spontaneous movement ; that
the energy which expresses itself in the pulsating activity of
the plant is in reality derived from external sources. It is
thus the stimulus supplied by the environment which is
held latent by the plant-tissue to find expression later in
rhythmic pulsations. For the experimental demonstration of
this theory we may carry out the following investigations :

(i) The effect of stimulus on the pulsating activity of a
specimen in a moderately sub-tonic condition.

(2) The effect of stimulus on a leaflet in a state of stand-
still. The renewal of pulsation by absorption of energy has
already been shown in the records given in figs. 150, 151, 152,
and 153.

(3) Observation of the effect of gradual depletion of
energy on the pulsation of the leaflets.

(4) Effect of fresh stimulation on specimens in which
the stored energy has been allowed to run down.

Effect of Stimulus on Sub-tonic Specimens

We take a specimen in which the pulsating activity is
moderate or feeble, and subject it to stimulation. If the
rhythmic activity is the result or after-effect of stimulus
previously absorbed, then a more vigorous pulsation may
be expected from the greater accession of energy.

This inference is found fully verified in the record given
in fig. 155. The specimen had been kept in the dark, and
its pulsating activity was only moderate as seen in the first
part of the record. The leaflet was then subjected to the
stimulus of light from a Nernst electric lamp for half an
hour. The record taken after this interval shows a marked
enhancement in the amplitude of pulsation.

Effect of Gradual Depletion of Energy

Having observed the effect of accession of energy, it is
still more interesting to note the effect of the converse process
of gradual depletion of energy. I had hitherto been relying on

312 RESEARCHES ON IRRITABILITY OF PLANTS

chance specimens for exhibition of the condition of standstill.
As the previous history of the particular specimen was not
known from moment to moment, no definite information was
available as to the cause of the stoppage of pulsation, except
the very natural inference that it must have been due to
the run-down of the stored energy. In order to verify this
inference, I now proceeded deliberately to isolate an active
specimen from all accession of energy from outside, and
observe the effect of gradual depletion of energy that had
been stored up. The cut specimen, mounted in the usual

Fig. 155. — After-effect of stimulation on pulsation of Desmodium
leaflet in sub-tonic condition.

manner, was kept in a dark room, and a continuous record
of its pulsations was taken all the time.

In taking a series of such records under isolation, it was
found that the persistence of the rhythmic activity depended
on the vigour of the specimen — that is to say, on the storage
of energy in the tissue. Thus a vigorous specimen exhibited
persistent activity for more than twenty hours ; its pulsations
showed great uniformity for the first twelve hours, after
which the amplitude began to decline and the rhythmic
beat came to a stop at the twenty-first hour.

In another specimen, less vigorous, the pulsations of the
isolated specimen came to an end at the ninth hour. A

EFFECT OF DEPLETION OF ENERGY 313

continuous record from the third to the sixth hour is seen
in fig. 142, given on page 294 ; the series of records are to
be read from below to above each series, lasting for one hour
and twenty minutes. It is seen that the amplitude is

Fig. 156. — Effect of depletion of energy on the pulsation of isolated
specimen of Desmodium gyrans,

practically constant ; the period is also constant in the first
two series, there being twenty-seven pulsations in each
in the course of eighty minutes. The period is, however,

Fig. 157. — Gradual stoppage of pulsation in isolated leaflet
of Desmodium.

slightly lengthened in the third series, there being twenty-six
pulsations in the given time. The record was continued
(fig. 156) during the seventh, eighth, and part of the ninth
hour. In these two series, the effect of the depletion of

314 RESEARCHES ON IRRITABILITY OF PLANTS

stored energy is well seen in the regular diminution of the
amplitude and the prolongation of the period ; there are
now, in the lower or earlier series, twenty-three pulsations
in the course of eighty minutes, instead of twenty-seven,
which was found at the beginning. The period becomes
still more prolonged in the next series, where there are only
twenty pulsations. The pulsating activity of this specimen
was found completely arrested at the end of the ninth hour.
In another specimen less vigorous, the arrest took place
at the seventh hour. I give a record taken during the
seventh hour (fig. 157) which shows the process of arrest
in a very interesting manner.

Renewal of Pulsation under Fresh Stimulus

The arrest of pulsation seen here is not due to death
or loss of sensibility, but merely to the run down of stored
energy. I shall presently show that the activity of the
leaflets can in these cases be renewed by the incidence of an
external stimulus. Before describing the experiments, it is
well to anticipate the modifying results brought on by the
varying loss of stored energy. The extent of depletion will,
it is obvious, depend on the length of time during which
the specimen had been kept isolated from the external
supply. At the moment of arrest of pulsation the specimen
will still have a certain reserve, but not enough to cause an
overflow. At this stage a stimulus of even short duration
is likely to give rise to multiple responses. If we allow a
longer time to elapse after the stoppage of pulsation, then
the storage-level will be much lowered. A more intense
stimulus or one of longer duration will now be effective ; the
response is more hkely to be single rather than multiple.
And lastly, if we allow a very long time to elapse after the
cessation of pulsation, the vitality will be found to have
so far declined as to bring about the condition of death.
To recapitulate : the effect of fresh incidence of stimulus on
a leaflet brought to a state of standstill by isolation will
be modified according to the period which had been allowed

EFFECT OF STIMULUS ON LEAFLET 315

to elapse after the cessation of pulsation. Stimulus applied
within a short time of stoppage will give rise to multiple
responses ; after a long period, an identical stimulus will
be found far less effective. After a still longer interval,
which is critical, even a strong stimulus will fail to evoke
any response.

These theoretical considerations will be seen verified in
the following series of records. In fig. 158, stimulus of light
of two seconds' duration was apphed after half an hour of
the stoppage of the pulsation of the leaflet. The response is

Fig. 158. — Effect of stimulus in renewing pulsation of Desmodium
brought to standstill through depletion of energy. Note two series
of multiple responses due to stimulus of light .of two seconds'
duration.

seen to be multiple even under a stimulus of this short
duration. The stimulus was applied once more, with similar
results.

In the next series of records, stimulus was applied on a
specimen which had been in a state of standstill for five
hours. The store of energy here had undergone a consider-
able decline. Stimulus of light of two seconds' duration was
found ineffective ; it was only after an appUcation lasting
for half a minute that a response took place (fig. 159).
The response was single, instead of multiple as in the last
case. Stimulus was applied a second time, but of still

3i6 RESEARCHES ON IRRITABILITY OF PLANTS

longer duration of one minute. The response is now found
to consist of a large, followed by two small, pulsations.

I next took another series of records, with a specimen
which had been in a state of standstill for seven hours.

Fig. 159.— Response of Desmodium leaflet at standstill to stimulus of
light of '5 and i minute's duration.

Stimulus of light was successively applied for one, two,
three and four minutes. The general result is somewhat
similar to those that have been seen in the previous record.
The noticeable differences are, first the diminished amplitude

Fig. 160. — Response of a depressed specimen of Desmodium to stimulus
of light applied successively for one, two, three, and four minutes.

of response, and the exhibition of decline in the successive
responses. The specimen was nearing the critical condition
of death, and after a while there was a cessation of all
response. In other specimens the critical period was found

EFFECT OF STIMULUS ON LEAFLET 317

to be exceeded when the state of standstill had been allowed
to persist for nine or ten hours. In these, stimulus failed
completely to evoke any response.

Summary

The leaflet of Desmodium comes to a state of standstill
by depletion of its store of energy.

The pulsation may be revived by the action of various
stimuli, such as that of light or of induction-shock.

Desmodium leaflet in a state of standstill gives a single
response to a single stimulus of induction-shock of moderate
intensity. In a typical case the latent period is '4 second,
the apex time 45 seconds, and the period of relaxation 120
seconds. The response-curve exhibits a flattened top.

The response of Desmodium, like that of the cardiac
tissue, is characterised by a long refractory period.

The rhythmic activity of the leaflet of Desmodium comes
to an end when its store of energy is depleted. A leaflet
isolated from external sources of stimulation is thus gradually
brought to a state of standstill.

In this condition, response occurs under fresh stimulation.
If the depletion of energy had not been excessive, then a
moderate stimulus gives rise to multiple responses. But
under greater depletion even a very strong stimulus induces
only a single response.

CHAPTER XXIV

EFFECT OF ELECTRIC STIMULATION ON THE PULSATION OF
DESMODIUM GYRANS

Effect of electric shock on Desmodium leaflet — Incapability of tetanus
— Extra pulsation induced by electric shock — Relative effectiveness
of electric stimulus at diastolic phase — Effect of transmitted excita-
tion on normal pulsation of heart and on pulsation of Desmodium
leaflet — Effects of acceleration and inhibition.

In the previous chapter we studied the action of electric
shock on the leaflet of Desmodium in a state of standstill.
We will now inquire into the effect of induction-shock on
the pulsating leaflet.

The cardiac tissue is known to be incapable of tetanus.

Fig. i6i. — Application of strong tetanising shock at arrow brings
about diminished amplitude of pulsation.

This is also true of the rhythmic tissue of Desmodium.
Under tetanising shocks there is no continuous contraction.
Under feeble stimulus the ampHtude of pulsation may in
certain circumstances be enhanced. Under excessive
tetanisation the natural pulsations, generally speaking,
become irregular or diminished in amplitude (fig. i6i).

318

EFFECT OF ELECTRIC STIMULATION 319

Extra Pulsation due to Induction-Shock

In the case of cardiac tissue it is known that the excita-
biHty is least at the commencement of systole. A moderate
stimulus applied at this period induces hardly any effect.
During the occurrence of diastole, however, excitability is
relatively great. Hence, if stimulus be applied at diastole,
it is followed by an extra contraction.

Turning to Desmodium, I find that similar characteristics
obtain in this case also. The application of stimulus at the
beginning of the contractile movement, which corresponds to
systole, has little or no effect. But during the movement of

Figs, 162, 163. — Extra pulsation induced by induction-shock applied
at diastolic phase.

relaxation, which corresponds to diastole, the application of
an induction-shock induces the arrest of relaxation and
gives rise to an extra or interpolated pulsation. I give
here two records, out of many, obtained from two different
specimens, in both of which a momentary electrical shock
was applied about the middle of the phase of diastolic
relaxation. The arrest of relaxation, and reversal to the
opposite phase of contraction, giving rise to an extra pulse,
are here clearly seen in both cases (figs. 162, 163).

Effect of Transmitted Excitation on Pulsation
OF Desmodium

On exciting the nerve that goes to the heart, two opposite
effects have been observed. In some cases there is induced

320 RESEARCHES ON IRRITABILITY OF PLANTS

a diminished amplitude of pulsation : in other cases an
augmentation. It has been supposed that this nerve
contains two different kinds of fibres, the accelerators and
the inhibitors. In the case of lower animals, such as the
tortoise, these have been isolated by Gaskell. But the
accelerator fibres have not been distinguished in the mam-
malian vagus. It has been found that, generally speaking,
in a vigorously beating heart the effect of vagus-stimulation
is to induce depression, whereas in a sluggish heart the same
stimulation is apt to induce an augmentation.

To return to the case of the pulsating pulvinule of

Fig, 164. — Inhibitory effect of transmitted excitation on the pulsa-
tion of vigorous leaflet of Desmodium. The line below indicates
duration of transmitted excitation ; note the gradual removal
of inhibitory effect on cessation of stimulation.

Desmodium, the question arises whether the organ is in
communication with any conducting channel by which
distant excitation might be transmitted to it. The fact
of such transmission, if it occurred, would be tested by its
modifying influence upon the normal pulsation. In order,
then, to subject this question to the test of experiment, I made
suitable electrical connections — one contact being on the
petiolule, 5 mm. below the contractile pulvinule, and the
other still lower down on the petiole. Normal responses
were first taken, after which indirect stimulation was applied
by means of tetanising electrical shocks of moderate intensity.
The first specimen employed was very vigorous, as will be
observed from the amplitude of its pulsations (fig. 164). It

EFFECT OF ELECTRIC STIMULATION 321

will be noticed that stimulation, thus applied at a distance,
was transmitted and induced an inhibitory effect in diminish-
ing the amplitude of normal pulsation. On the cessation
of excitation the pulsations are seen gradually to regain
their normal amplitude. This inhibitory effect is what
takes place more frequently, and may be regarded as typical.
Somewhat exceptional is the converse effect of augmentation,
seen in the next record (fig. 165). The particular specimen
was less vigorous ; this fact is seen in the smaller amplitude
of its normal pulsations, the magnification being the same
in the two cases. After these normal pulses had been

. ^ ^ \ vi V i \i

Fig. 165. — Augmented pulsation induced by transmitted
excitation in less vigorous specimen of Desmodium.

recorded, indirect stimulation of moderate intensity was
applied, as in the previous case. It will be noticed that, in
consequence of this, there occurred a marked enhancement
of the ampUtude of pulsation. Even on considerably
raising the intensity of the stimulation this enhancement of
pulsation still persisted.

A remarkable parallelism has thus been shown to exist
between the responsive characteristics of rh5rthmic animal
and vegetal tissues. This is seen in their incapability of
tetanus, in their prolonged refractory period, in the extra
pulsation induced by electric shock at the diastolic phase,
and in the transmitted excitation causing in different
circumstances effects either of inhibition or acceleration.
Other similarities, equally remarkable, will be found in the

322 RESEARCHES ON IRRITABILITY OF PLANTS

effect of temperature and of drugs on the pulsating activity
of animal and vegetal tissues.

Summary

The rhythmic tissue of Desmodium, like the rhythmic
cardiac tissue, is incapable of tetanus.

Pulsating leaflet of Desmodium, like the pulsating heart,
is more susceptible to excitation at diastole than at systole.
An extra pulsation is induced by an electric shock applied
during the diastolic phase.

Transmitted excitation affects the normal pulsations of
rhythmic tissues — animal or vegetal — in a similar manner.
In certain circumstances the effect is one of inhibition ;
in other circumstances the effect is one of acceleration.

CHAPTER XXV

EFFECT OF TEMPERATURE ON RHYTHMIC PULSATION OF
DESMODIUM GYRANS

Effect of lowering of temperature on rhythmic pulsation of cardiac tissue — •
Similar effect on the pulsation of Desmodium — Increase of systolic
limit during cooling — Minimum temperature for arrest of pulsation —
Arrest by cooling and subsequent revival by warming — Increase of
diastolic limit during warming — Effect of rise of temperature on the pul-
sation of frog's heart — Similar effect on the pulsation of Desmodium —
Effect of rise above and return to normal temperature — Diminution of
systolic contraction during rise of temperature — Increase of systolic
contraction during fall of temperature — Permanent arrest due to
heat-rigor.

In the course of our study of automatic pulsation of Des-
modium we shall find striking similarities between the
rhythmic activities of the plant and animal tissues. In the
present chapter we will study in detail the effect of tem-
perature in modifying the amplitude and period of the
spontaneous movements.

I have already described the thermal chamber by means
of which the temperature of the plant can be regulated.
Temperature, as we have seen, may be raised to any degree
by the adjusting of the heating current which passes through
a coil of German silver. Lowering of temperature, on the
other hand, is effected by allowing a stream of cooled air to
pass through the chamber containing the specimen.

Effect of Lowering of Temperature

In the rhjrthmic pulsation of frog's heart the marked
effect of variation of temperature is to change the period and
modify the amplitude of pulsation. Lowering of tempera-
ture has the ef ect of lengthening the period and enhancing

323 Y 2

3 24 RESEARCHES ON IRRITABILITY OF PLANTS

the amplitude. This is seen in the following record (fig.
i66), where the normal pulsations are modified in
consequence of lowering the temperature through a few
degrees.

The following experiment, carried out on the leaflet of
Desmodium, shows that the effect of lowering of temperature
on it is precisely the same. The temperature of the room
at the time of the experiment was 30° C. A record of three
normal pulsations was taken at this normal temperature.

Fig. 166. — Efifect of lowering of temperature in increasing
amplitude, and decrease of frequency of pulsation of
frog's heart. Series to the left represent normal pulsa-
tions at the temperature of room ; series to the right
were recorded at a temperature several degrees lower.
(Brodie.)

The specimen was then gradually cooled by sending through
the chamber a stream of cold air, the record being taken
all the time. The successive dots in the diagram are at
intervals of 2 seconds. Hence the period of complete
pulsation can be accurately determined.

It will be seen from fig. 167 that at 30° C. the period
of a complete pulsation was 80 seconds, the amplitude
being 25 mm. On lowering of temperature to 29° C. the
period became lengthened to 88 seconds, the amplitude
being enhanced to 30 mm. At 28° C. the period was

EFFECT OF TEMPERATURE ON PULSATION 325

protracted to 96 seconds, the amplitude being enhanced
to 35 mm. And finally at 27° C. there was further
lengthening of the period to no seconds, the amplitude
of pulsation being enhanced to 40 mm.

In addition to the changes of period and amplitude there
is another noticeable effect induced by variation of tempera-
ture. We have seen in the case of Mimosa that the lowering
of temperature induces a depression of the leaf, and the rise
of temperature an erection of the leaf. Effects similar to

Fig. 167. — Effect of lowering of temperature on the amplitude and
frequency of Desmodium pulsation. Note the shifting of base-
line upwards, indicating fall of leaflet during cooling ; observe
also the enhancement of systolic contraction.

these are also induced in the leaflet of Desmodium. The
result of cooHng is thus a slight fall of the leaflet, in conse-
quence of which we observe the shifting of the base-line
upwards. A still more striking effect is the increase in
the extent of contraction, by which the systolic limit is
enhanced. This will clearly be noticed in the series of
records in fig. 167.

The rhythmic pulsation of the cardiac tissue is arrested
when subjected to a certain low temperature. Similarly the
pulsation of Desmodium gyrans is arrested at a sufiiciently

326 RESEARCHES ON IRRITABILITY OF PLANTS

low temperature. The critical point is somewhat modified
by the tonic condition of the specimen. With vigorous
specimens the temperature at which arrest takes place
may be as low as 17° C.

Fig. 168. — Effect of rapid cooling by ice water applied at
the moment marked by arrow. Note arrest at systole
and gradual revival on warming, the pulsations exhibiting
staircase increase.

There are certain interesting points in connection with
the arrest brought about by cooling. Two series of records
are here given in which the arrest was quickly brought about

Fig. 169. — Magnified record given by a different specimen of
arrest of pulsation by cooling and subsequent revival by
warming. Note extension of diastole during warming,

by the application of cold water to the pulvinule. The
leaflet was then allowed to return to the temperature of the
room, with the revival of pulsatory activity. The entire

EFFECT OF TEMPERATURE ON PULSATION 327

process of arrest and subsequent revival is clearly seen in
fig. 168. It will be noticed that sudden cooling arrested
the pulsation at systole and that on gradual warming the
rhythmic activity was revived with the gradual restoration
of the original amplitude.

This is still better seen in fig. 169, which gives a mag-
nified record of the arrest due to cold, and subsequent
restoration of pulsation on return to the temperature of
the room. Ice-cold water was applied after the second
pulsation. The arrest at systolic contraction is clearly

Fig. 170. — Effect of rise of temperature on the amplitude and frequency
of pulsations of the heart of frog. (Peinbrey and Phillips.)

demonstrated. As the temperature was raised there was
increasing expansion, in consequence of which the diastolic
excursion was continuously increased. The effect of
cooUng is thus to increase the force of contraction and
diminish that of expansion. The effect of warming is the
reverse of these.

Effect of Rise of Temperature

We have seen that both in the rhythmic animal and
vegetable tissues, the period is increased and amplitude
enhanced under the lowering of temperature. The converse
is the case with rise of temperature. Fig. 170 shows the
effect of rising temperature on the pulsation of the heart
of the frog. Similar effects are induced in the pulsation of
Desmodium, Fig. 171 gives a series of pulsations of the
leaflet at the temperatures of 19° C, 23-5° C, and 28-5° C,

328 RESEARCHES ON IRRITABILITY OF PLANTS

the record in each case being continued for a period of
20 minutes. At a low temperature the pulsation is apt to be
somewhat irregular, hence the unequal amplitude in the
successive pulsations at 19° C.

It is apparent that while at 19° C. there were 3I pulsa-
tions, at 23*5° C. the number had been increased to 4 J,
and at 28*5° C. to 6 pulsations.

Fig. 171. — Effect of rise of tem-
perature on the pulsation of Des-
modium gyrans. Time-marks
below indicate intervals of i
minute.

Fig. 172. — Effect of rise of
temperature on a different
leaflet.

Taking a more vigorous specimen, I obtained records
for 12 minutes each at temperatures of 28*5° C, 3i'5° C, and
34*5° C. It will be seen that while at 28*5° there were only
4 pulsations, these had become increased to 6 J pulsations at
31*5° and to 10 pulsations at 34*5° C. The other noticeable
feature is the marked diminution of amplitude with the
rise of temperature (fig. 172).

Instead of taking isolated records at different tempera-
tures, I next raised the temperature of the plant-chamber
very gradually, by careful manipulation of the heating
current, and obtained a record with a different specimen.
In this way the temperature was raised continuously from

EFFECT OF TEMPERATURE ON PULSATION 329

30° C. to 38-5° C. How regularly the frequency of pulsation
is increased, and the amplitude diminished, with the rise
of temperature is shown in fig. 173.

In the record just given there is observable an arrest of
pulsation when the specimen was subjected to as high a
temperature as 38° C. But by accustoming it to warmth,
the plant can resist even higher temperatures. Thus I kept

Fig. 173. — Effect of continuous rise of temperature from
30° C. to 38 -5° C.

several specimens in a glass-house, the temperature in
which at midday was 37° C. These specimens could be
exposed to a temperature as high as 45° C. without the
arrest of the pulsation. I reproduce here a record (fig. 174)
where the specimen was gradually raised from 36° C. to
42° C, and then allowed to cool and return to the tempera-
ture of 30° C. It will be seen that the amplitude of pulsa-
tion was continuously decreased, yet there was no arrest
even at 42° C. On cooHng, the amplitude was restored to
the original value.

I have shown that under excessive cooling the force
of expansion is reduced, in consequence of which there is an
arrest towards systole. With excess of heat, on the other

330 RESEARCHES ON IRRITABILITY OF PLANTS

hand, the reverse effect takes place. On account of increase
of force of expansion, or diminution of force of contraction,
the systolic limit is progressively diminished. When the

Fig. 174- — Effect of continuous rise from 30° C. to 42° C. and return
to 30° C. Note progressive diminution of systolic contraction
during warming, and its increase during cooling.

specimen is allowed to cool, the reverse effect is exhibited
by gradual enhancement of systolic contraction. Similar

Fig. 175. — Effect of continuous rise from 32° C. to 43° C, and
return to 33° C. in a different specimen.

effects are seen to take place in a record (fig. 175) obtained
with a different specimen.

The temperature may sometimes be raised to 45° C.
without inducing any arrest of pulsation. A tendency is now
observed towards contraction, as shown by the general

EFFECT OF TEMPERATURE ON PULSATION 331

shifting of the pulsations upwards. This movement of
contraction goes on till there is a complete arrest brought
about by heat rigor.

Summary

The effect of lowering of temperature on the rhythmic
pulsation of Desmodium gyrans is similar to that on the
pulsation of frog's heart. Lowering of temperature enhances
the amplitude but reduces the frequency of pulsation of
both.

The pulsation of Desmodium leaflet is arrested at the
minimum temperature of about 17° C. Arrest takes place
at systole ; gradual warming revives the pulsation, which
undergoes a staircase increase with enhancing diastolic
expansion.

Rise of temperature induces enhanced frequency and
diminished amplitude of pulsation.

During rise of temperature to about 43° C. there is a
tendency of arrest towards diastole. The systolic con-
traction undergoes continuous diminution during rise of
temperature.

During the fall of temperature there is a gradual enhance-
ment of systolic contraction.

The temperature maximum at which arrest of pulsation
takes place may be as high as 45° C. Above this temperature
there is a tendency to contraction and permanent arrest
under heat-rigor.

CHAPTER XXVI

EFFECT OF CHEMICAL AGENTS ON THE AUTOMATIC PULSATION
OF DESMODIUM GYRANS

Application of gaseous or liquid reagents — Modifying influence of, tonic
condition of specimen, strength, and duration of application — Effect
of sugar solution — Effect of alcohol — Action of carbonic-acid gas —
Effects of anaesthetics, ether, and chloroform — Action of carbon
disulphide — Effect of copper sulphate solution — Effect of potassium
cyanide solution — Antagonistic actions of acids and alkalis on the
pulsations of the heart and of Desmodium — Similarities of reaction in
rhythmic tissues, animal and vegetal.

We will now study the effects of various chemical agents
on the pulsating activity of the Desmodium leaflet. These
may be applied either externally or internally. As regards
external application, a liquid reagent may be applied
directly on the pulvinule ; gases and vapours, on the other
hand, are made to circulate in the plant-chamber. We may
secure internal application by forcing in the solution at the
cut end of the petiole, by means of hydrostatic pressure
(cf. fig. 141).

The characteristic effects of different reagents are seen
exhibited in the induced change of period or of amplitude
of pulsation. Another noticeable effect often observed is
the transposition of the systolic or diastolic limit of pulsa-
tion. Certain agents may thus induce an arrest at systole,
others at diastole.

The effect of a given agent may be modified by various
conditions, such as internal or external application, the
strength of the solution, and the duration of application.

Thus the same agent which in a strong solution induces
depression, may in a very dilute solution cause an exaltation.

332

EFFECT OF CHEMICAL AGENTS

333

The result also is dependent on the tonic condition of the
tissue. A specimen in which the amplitude of pulsation is
the maximum possible cannot exhibit any further increase
under a stimulating agent. But with a less vigorous spe-
cimen the effect of the same agent is manifested by a marked

Fig. 176. — Stimulating action of dilute sugar solution applied
at the moment marked by an arrow.

enhancement of amplitude of pulsation. Again, a vigorous
plant may survive a given dose of a toxic agent ; while a
less vigorous specimen will succumb under the same treat-
ment. Finally we have the interesting phenomenon in

Fig. 177. — Effect of external application of alcohol. Preliminary
enhancement followed by depression, the period of pulsation
being prolonged.

virtue of which the plant accommodates itself to a changed
unfavourable condition.

Certain chemical agents act as stimulants, inducing an
enhancement of amplitude of pulsation. As an example of

334 RESEARCHES ON IRRITABILITY OF PLANTS

this I may mention dilute sugar solution. The specimen
experimented on was but moderately vigorous. After
taking four normal pulsations, which are seen to be uniform
(fig. 176), a 2 per cent, solution of sugar was applied inter-
nally at the moment marked by an arrow. No immediate
effect was noticeable, but after an interval of a single

Fig. 178.-

-Effect of internal application of 15 per cent, solution
of alcohol.

Fig. 179. — Effect of dilute carbonic-acid gas ; enhancement of
amplitude and slowing of the period.

pulsation the stimulating character of the agent became
evident by the resulting staircase enhancement of pulsation.

Effect of Alcohol

The effect of this reagent in dilute solutions is in general
to induce an enhancement of response. Stronger solu-
tion, however, induces a depression which may culminate

EFFECT OF CHEMICAL AGENTS

335

in arrest of pulsation. In fig. 177 a slight preliminary
exaltation of amplitude, followed by depression, may be
noticed. The period of succeeding pulsations is found to
become prolonged.

The effect of internal application of alcohol is very
similar to that of external application. In fig. 178 is seen a

WW

VA_y

Fig. 180. — Strong carbonic-acid gas, inducing arrest. Line
below indicates duration of application. Slow revival
of pulsation on substitution of fresh air.

Fig. 181. — Effect of internal application of water charged
with carbonic acid.

record exhibiting the action of a 15 per cent solution of
alcohol.

Effect of Carbonic-acid Gas

This gas when diluted with air causes an enhancement
of amplitude, though the period becomes longer (fig. 179).

336 RESEARCHES ON IRRITABILITY OF PLANTS

Application of undiluted gas, however, induces an arrest.
If fresh air be now substituted, there is produced a slow
revival of pulsation (fig. i8o).

In another experiment I obtained a record of the effect
of internal application of water charged with carbonic acid.
The pulsating activity was found slowed down, the ampli-
tude of the pulsation also undergoing a diminution. The
depressing effect is, however, seen to pass away gradually
(fig. i8i).

Effect of Ether

This agent was applied in the form of vapour, which was
slowly blown into the plant-chamber. If the vapour be

Fig. 182. — Effect of vapour of ether; depression and subse-
quent arrest. Pulsations revived on blowing off the vapour.

much diluted with air, then the first effect of ether is to
induce a transient exaltation, followed by depression and
arrest of pulsation. If the leaflet be subjected to strong
vapour, or if the application be prolonged, then the arrest
of pulsation proves to be permanent. But if diluted vapour
is employed and fresh air substituted immediately after
the arrest, then there is a slow revival of pulsation.
This can be seen in fig. 182, where after the application of
ether an arrest took place after three rapidly diminishing
pulsations. On blowing off the ether vapour, the pulsation
is seen to revive slowly after a period of 20 minutes.

Effect of Chloroform

The effect of this reagent on the pulsation of Desmodium
is similar to that of ether. It is, however, far more toxic in
its reaction, a slight excess in the application being attended

EFFECT OF CHEMICAL AGENTS 337

by permanent arrest of pulsation. In the experiment of
which a record is given (fig. 183), diluted chloroform vapour
was introduced into the chamber. This is observed at

Fig. 183. — Effect of vapour of chloroform. Immediate excitatory
effect followed by depression and arrest. On blowing off the
anaesthetic, pulsations were revived after half an hour, repre-
sented by a gap in the record.

first to have an excitatory effect in the first two pulsations.
The amplitude of the third pulsation became much reduced,

Fig. 184. — Arrest of pulsation by vapour of carbon disulphide,
and revival after readmission of fresh air. Duration of
application indicated by the horizontal line under the
record.

and an arrest ensued at the fourth pulsation after the appli-
cation of the anaesthetic. The vapour was then quickly

338 RESEARCHES ON IRRITABILITY OF PLANTS

blown away and fresh air substituted in the chamber. But
the arrest persisted for half an hour. After this interval
there was a revival, and the pulsation attained for a time
an amplitude even greater than the normal.

Effect of Carbon Bisulphide

The vapour of this reagent also arrests the pulsating
activity of the leaflet. The record (fig. 184) shows the
quick arrest after a single pulsation. Substitution of fresh
air is seen again to revive the pulsation.

Effect of Copper Sulphate Solution

As an example of toxic agents we may take solutions of
substances like copper sulphate. When a strong solution of

Fig. 185. — Effect of internal application of CUSO4 solution
in inducing arrest of pulsation.

this substance is applied directly on the pulvinule, an arrest
takes place within a short time. The effect is delayed when
the solution is applied at the cut end of the petiole. A record
of the effect of internal application of copper sulphate
solution is given in fig. 185.

Effect of Potassium Cyanide Solution

We saw in Chapter XII. that the toxic effect of potassium
cyanide in abolishing the conductivity of the tissue was far

EFFECT OF CHEMICAL AGENTS

339

more pronounced than that induced by copper sulphate.
The poisonous action of cyanide is equally powerful in
abolishing the rhythmic activity of the Desmodium leaflet.

Fig. 1 86. — Quick arrest of pulsation at systole by
the action of KCN solution.

This is well observed in fig. i86, where the pulsation is
seen to be quickly arrested at systole.

Antagonistic Actions of Acid and Alkali

We have hitherto seen the remarkable similarities of the
effect of various chemical agents on the rhythmic activi-
ties of the animal and vegetal tissues. A very striking

Fig. 187. — Arrest of pulsation of the heart of frog in diastole by the action
of dilute lactic acid. (Gaskell.) Record to be read from right to left
in this and following figures.

characteristic is the antagonistic reactions of acid and alkaH
on the animal heart. Apphcation of very dilute acid
induces in the heart an atonic reaction, in consequence of
which there is induced an arrest of pulsation in the relaxed
or diastohc condition (fig. 187) . The action of dilute alkahne

Z 2

340 RESEARCHES ON IRRITABILITY OF PLANTS

solution is the very reverse, inducing tonic contraction and
arrest in systole (fig. 189).

I find these effects repeated in an astonishing manner in

Fig. 188. — Arrest of pulsation of Desmodium in diastole, by
the action of dilute lactic acid.

Fig. 189.— Arrest of pulsation of heart in systole, by the action
of dilute NaHO. (Gaskell.)

Fig. 190. — Arrest of pulsation of Desmodium in systole, by the action of

dilute NaHO.

the pulsation of Desmodium. The internal application of
dilute solution of lactic acid is seen to induce an arrest in a
state of diastolic relaxation (fig. 188). The application of

EFFECT OF CHEMICAL AGENTS 341

dilute NaHO solution (fig. 190), on the other hand, induces
exactly the opposite effect — namely, an arrest at systolic
contraction.

Summary

The effect of chemical reagent on the pulsating activity
of Desmodium is to a certain extent modified by the tonic
condition of the specimen.

The effect is also dependent on the strength of the
reagent. Very dilute application often induces an effect
opposite to strong.

Alcohol induces a transient exaltation followed by
depression. The period of pulsation is lengthened.

Carbonic acid when dilute induces an exaltation of
amplitude, with prolongation of period of each pulsation.
Long application of undiluted gas induces an arrest : revival
takes place on substituting fresh air.

Vapour of ether induces a temporary arrest of pulsation ;
the pulsation may be revived on blowing off the vapour.

The effect of chloroform is similar to that of ether, but
its toxic effect is greater, any excess causing permanent
arrest. Diluted vapour causes a preliminary exaltation
followed by arrest. Revival may take place after quick
substitution of fresh air.

Vapour of carbon disulphide applied for a short time
also induces a transient arrest.

Copper sulphate solution when applied directly on the
pulvinule causes an almost immediate arrest. But when
applied at the cut end of petiole, the arrest does not take
place till the lapse of a period, required for the solution
to ascend to the motile organ.

The poisonous reaction of potassium cyanide solution is
more powerful than that of copper sulphate.

The effect of acids and alkalis on the rhythmic move-
ments of Desmodium are, as on the animal heart, antagonistic.
In both, acids induce a standstill in diastole, while alkalis
induce arrest in systole.

CHAPTER XXVII

General Survey

For the study of phenomenon of irritabiHty, the so-called
' sensitive ' plants have been selected for the purpose of
experimental investigation. From this it must not be
inferred that the fundamental reactions that have been
demonstrated are different in ordinary plants. By the
employment of the electrical mode of investigation, I have
shown elsewhere that not sensitive plants alone, but every
plant, and also every organ of every plant, is excitable.
Even in the matter of mechanical response, longitudinal
contraction under excitation may be demonstrated in the
case of various organs of ordinary plants ; but the extent
of movement in such cases is not very great. A * sensitive
plant * differs from others only in the possession of mobile
mechanism by which excitatory change of form is mani-
fested by a conspicuous movement.

In the case of Mimosa, the responsive down move-
ment or the fall of the leaf is brought about by the
differential excitabilities of the upper and lower halves
of the pulvinus, the movement being magnified by the
long petiolar index. It is these advantageous circum-
stances that render the motile apparatus of Mimosa
a good indicator of excitation. The leaf may thus by its
responsive movement indicate the passage of an excitatory
impulse originated at a distance just as the excitation
transmitted through a nerve is detected by the mechanical
response of the attached muscle. In connection with this
it should be remembered that the plant indicator may
become inefficient under unfavourable circumstances.
Absorption of excess of water, for example, will annul the

342

GENERAL SURVEY 343

motility of the pulvinus of Mimosa. Under these circum-
stances, we may have an excitatory impulse without any
external manifestation of its passage through the plant.
It will thus be understood how an excitatory impulse
in an ordinary plant may pass unnoticed on account
of the absence of an efficient motile indicator. The exist-
ence of such an impulse can, however, be detected by means
of electric response.

In the present work the various excitatory phenomena
of the plant have been investigated by means of mechanical
response under the action of a testing stimulus. The
different responses of plants may be included under three
classifications : (i) Simple response, where a single stimulus
evokes a single response ; (2) Multiple response, where a single
strong stimulus gives rise to multiple series of responses ;
and (3) Automatic response so called, where the pulsations
appear to be spontaneous. In reviewing these in their
proper sequence we shall be struck by the extraordinary
similarities which are reveaJed between the response of the
plant and the animal.

The Response Recorder

In the course of this work it has been shown that physio-
logical changes induced in the plant owing to the action of
the environment may be revealed by the characteristic
variation of the amplitude and time relations of the
normal curve of response. In obtaining records of response
errors are, however, introduced on account of friction of the
writing point against the recording surface. In recording
the pulsations of Desmodium leaflet it is found that a
weight as small as '03 gram is enough to arrest the
pulsatory movement. The difficulty of friction has been
overcome by the method of intermittent instead of con-
tinuous contact for record. In the Resonant Recorder
the writer is made to vibrate to and fro at a known and
definite rate. The record consists of series of dots giving

344 RESEARCHES ON IRRITABILITY OF PLANTS

definite time intervals. In this manner time interval
shorter than a hundredth part of a second can be measured.
Owing to the extreme lightness of the recorder the error
due to inertia is reduced to a minimum (p. 14).

Methods of Stimulation

The various stimuli which evoke motile response in the
animal are also found effective in giving rise to excitatory
response in the plant. Thus the plant may be excited by
mechanical, chemical, thermal, and electrical modes of
stimulation. Electric stimulation may be caused by the
polar action of a constant current, by the discharge of a
condenser, or by the induction current. The electrical
method allows the intensity of stimulus to be maintained
constant, or varied in a quantitative manner. As in the
skeletal muscle of animal, so also in the pulvinus of Mimosa,
the break induction shock is more effective than the make
shock. The sensitiveness of Mimosa to an electric shock
is very great. It often reacts to an intensity of shock
which is only one-tenth of the minimum perceived by a
human subject (p. 23).

Time Relations of the Responsive Movement

Different plants exhibit different characteristics of
response. The reactions are relatively quick in some and
slow in others. In a typical case of Mimosa in summer
the latent period was one-tenth of a second. The maximum
fall of the leaf was attained in three seconds, and the recovery
completed in 15 minutes. The rate of recovery was rapid
at the beginning and very slow towards the end. The
maximum rate of recovery was '09 mm. per second in
contrast to the maximum rate of fall of 24 mm. per second.
The movement of recovery was about three hundred times
slower than the movement of the excitatory fall. The
extent of responsive fall in Mimosa increases with the

GENERAL SURVEY 345

increasing intensity of stimulus. The rate of movement is
also increased under the action of stronger stimulus and
higher temperature ; it is decreased under fatigue. A
stronger stimulus, generally speaking, requires a longer
period for recovery. Under the physiological depression
induced by winter the responsive reactions are modified,
the latent period prolonged, and the amplitude reduced.

Biophytum sensitivum may be taken as typical of a
quickly reacting plant. The leaflets undergo closure within
a second after receiving the excitatory shock ; recovery is
accomplished in the course of three minutes. In marked
contrast with this is the extremely sluggish reaction of the
leaf of Neptunia oleracea, where the apex time is reached
only after an interval of 180 seconds, and recovery com-
pleted in 60 minutes (p. 44).

Additive Effect of Stimulus

In the response of animal tissue it is found that a single
stimulus by itself ineffective becomes effective on repetition.
The same is found to be the case with plant tissues. Thus,
with a given specimen of Mimosa, it was found that, while
an electrical stimulus of intensity 'i was singly ineffective,
it became effective after being repeated twenty times. It is
found, moreover, that this additive effect is, within limits,
strictly quantitative. Thus with the identical specimen of
Mimosa, when the intensity of individual stimulus was
increased from "i to '5, the number of repetitions necessary
to cause effective excitation was reduced from 20 to 4. Here
•I X 20 = '5 X 4. The effective stimulation is thus found
constant, being equal to individual intensity multiplied by
the number of repetitions (p. 54).

Effect of Temperature and of Intensity of Stimulus

The response of Mimosa is abolished at a low tempera-
ture. With the rising temperature the amplitude of re-
sponse is increased and the period of recovery shortened.

346 RESEARCHES ON IRRITABILITY OF PLANTS

As in the response of animal muscle, so also in Mimosa
there is a range between minimal and maximal stimulations
where increasing amplitude of response occurs under
increasing intensity of stimulus. The range within which
the increasing effect is observed to take place is relatively
extended in the case of plants in a somewhat sub-tonic
condition. The range of variation is, however, restricted
in specimens which are highly excitable (p. 6i).

Work Performed by the Plant

During the responsive movement the pulvinus of
Mimosa can do the work of lifting a weight. The effect of
load on the response of Mimosa is similar to that on the
contractile response of muscle. In both, under increasing
load, the height of response undergoes a progressive
diminution with shortening of the period of recovery.
Within limits, the amount of work performed by the muscle
increases with the load. The same is true of the work
performed by the pulvinus of Mimosa. Thus it is found
that, while under a load of loo mg. the work performed was
1340 mm. mg., it became enhanced to 8666 mm. mg. under
a load of 2000 mg.

The rate of work was found to be 7480 mm. mg. per
second (p. 57).

Various Types of Response

The response of the pulvinus of Mimosa exhibits charac-
teristics which are similar to those of the response of muscle.
Under normal conditions, and with sufficient intervening
periods of rest, the successive responses are found to be
of uniform height. Under conditions of incomplete re-
covery, however, the responses exhibit signs of fatigue.
The excitability of the plant in a sub-tonic condition is
enhanced by the action of stimulus itself. Under such
conditions the response exhibits a staircase increase.

GENERAL SURVEY 347

Under extreme sub-tonicity the normal negative response
may even be converted into abnormal positive.

The anomalous erection, after a preliminary fall of the
leaf of Mimosa under continuous stimulation, is explicable
on the common characteristics of response in plant and
animal tissue. In both contraction is reversed to relaxa-
tion under fatigue (p. 84).

Variation of Motile Excitability under
Changes of External Condition

The motile excitability of Mimosa undergoes abolition
under sudden change from light to darkness. After a
certain time the plant accommodates itself to the changed
condition, with the restoration of normal excitability.

There is again a variation of motile excitability depending
on the time of the day. The motility is at its maximum at
a certain hour, and minimum at a different hour. This
peculiarity is probably connected with the question of
variation of turgor. The excess of turgor in the pulvinus
caused by absorption of water is attended by a reduction
or abolition of motile response. The lost motility may,
however, be restored by the application of glycerin.

Very characteristic are the effects exerted by the different
gases. Some induce a stimulating action, others give rise
to depressing or toxic effects. Ozone enhances the excita-
bility. Carbonic-acid gas and vapour of alcohol induce a
moderate depression of excitability from which the plant
recovers on readmission of fresh air. Coal gas and vapour
of carbon disulphide also induce a depressing effect. The
vapour of ether exerts a moderate narcotic action. The
effect of chloroform is far more pronounced, the loss of
excitability under its action being more complete and
persistent. Ammonia has marked effect in the abolition
of excitability. Sulphuretted hydrogen, nitrogen dioxide,
and sulphur dioxide are very toxic in their action ; their

348 RESEARCHES ON IRRITABILITY OF PLANTS

application is attended by quick abolition of excitability
followed by the death of the plant (p. 94).

Death-spasm in Plant

One test by which a dead plant may be distinguished
from a living one is that of electric response. The response
of galvanometric negativity characteristic of living condi-
tion is abolished at death. When the plant is subjected
for a time to a temperature of 60^ C. its electric response,
generally speaking, disappears. This temperature is, there-
fore, fatal for most plants.

When the leaf of Mimosa is continuously raised in tem-
perature there is produced a progressive erectile movement ;
but at a critical temperature the erectile movement is
suddenly reversed into a spasmodic contraction. This
inversion takes place under standard conditions at or about
60° C, after this the response of the plant is permanently
abolished. The death record is a V-shaped curve, the
point of inversion being the death-point. After death
a repetition of experiment shows no further inversion.
Various other plants, sensitive and ordinary, exhibit this
characteristic death-spasm at or about 60° C. In radial
organ this movement consists of an abrupt longitudinal
contraction. In taking an electrical record it is found
that an electric-spasm also takes place at the critical tem-
perature which is very near 60° C.

The death contraction in plants is similar to that seen
in the animal. The death-point is found lowered under
physiological depression. Thus, in a certain case, fatigue
lowered the death-point of the plant from the normal
60° C. to 37° C. In another case dilute solution of poison
lowered the death-point by 18° C. (p. 106).

Polar Effects of Electrical Current in Excitation

The fundamental unity of excitatory phenomena in

the animal and plant finds a striking illustration in the

characteristic effects induced at the kathode and anode.

GENERAL SURVEY. 349

In animal tissues, with feeble current it is found that
excitation takes place only at the kathode at make. On
moderately increasing the current excitation is found to
take place at the make of kathode and break of anode.
These effects are included in Pfiuger's law of polar excita-
tion in animal tissues. I have shown that effects precisely
similar to these take place in the vegetal tissues. That is
to say, the laws of polar excitation in plant under feeble
and moderate currents are :

I. With feeble current the kathode excites at make
and not at break. The anode excites at neither make nor
break.

II. With moderately strong current, the kathode excites
at make and not at break. The anode excites at break and
not at make.

The polar reactions in the undifferentiated protoplasm
of the plant body are thus identical with those ofhighly
differentiated animal tissues (p. 233).

The effect of feeble ascending and descending currents
in the petiole of Mimosa are parallel to those in nerve-and-
muscle preparations. In both cases excitation takes place
earlier when the kathode is nearer to the responding organ.
As in animal, so also in Mimosa, single induction shock of
moderate intensity is, as regards polar actions, effective at
the commencement and not termination of the current
(p. 206).

The sensitiveness of Biophytum to an electrical current
is remarkably high ; compared to the very sensitive human
tongue, the sensitiveness of Biophytum is about ten times
as great (p. 251).

Pfiuger's law cannot be taken as a complete statement
of the polar action of currents (p. 265). For strong currents
there are induced two additional types of reaction:

III. Under the action of strong current, excitation
takes place at the make of kathode and make and break
of anode.

IV. Under still stronger currents, excitation takes

350 RESEARCHES ON IRRITABILITY OF PLANTS

place at the make and break of both kathode and
anode.

Physiological changes are also found to modify the
polar effects of current. Under the separate or joint action
of an increasing current and physiological modification,
types of reaction are observed in plants which are similar
to the so-called anomalous response in Protozoa (p. 276).

Contrasted Effects of Anode and Kathode

In the animal heart the contrasted effects of anode and
kathode are exhibited by characteristic modification of its
pulsating activity. Effects precisely similar are shown in
the rhythmic tissue of Desmodium gyrans. The effect of
the make of anode is to induce an expansion ; this is shown
in the record of pulsation of Desmodium by a reduction of
normal limit of systolic contraction. The opposite effect
of contraction due to the make of kathode is seen in the
reduction of normal limit of diastolic expansion (p. 236).

In nerve- and-muscle preparation the effects of ascending
and descending currents are found to be modified by the
intensity of the current. Effects in every way parallel are
observed in experimenting with petiole-pulvinus of Biophy-
tum. These characteristic modifications are easily trace-
able in Biophytum to the contrasted effects of anode and
kathode and of make and break. Excitability is enhanced
by the make of kathode and break of anode. It is
depressed by the make of anode and break of kathode
(P- 239)-

A very important problem in plant physiology relates
to the question as to whether in plants there is any trans-
mission of true excitation. The prevailing opinion has
been that in plants like Mimosa pudica there is merely a
passage of hydro-mechanical disturbance, unlike the trans-
mission of excitatory protoplasmic change, which takes
place in an animal nerve. This view has been based on

GENERAL SURVEY 351

the experiments of Pfeffer and Haberlandt, who found
transmission of stimulus to take place in spite of narcotisa-
tion or scalding of the intervening tissue. It is shown that
those experiments are not conclusive, inasmuch as super-
ficial narcotisation or scalding is not effective in abolish-
ing the conducting power in the interior of the tissue.

For the settling of the question whether the transmission
is physical or physiological it was necessary to have quanti-
tative measurements of the highest accuracy in order to
determine whether physiological changes affect in a definite
manner the velocity of transmission. For the accurate
determination of velocity it is essential to allow for the
latent period of the responding pulvinus and its variations
under different conditions.

Determination of Latent Period

By the employment of the Resonant Recorder the value
of latent period can be accurately determined within a
hundredth part of a second. Successive determinations of
the latent period under constant external conditions are
found to give identical results. The shortest value of the
latent period in vigorous Mimosa is -06 second, the average
value in summer being 'i second. The latent period is in
general shorter under stronger intensity of stimulus ; but
the value becomes constant above a maximal stimulus.
In the optimum condition of the plant the latent period
is the same for feeble or strong stimulus. Fatigue prolongs
the latent period ; a rise of temperature, on the other hand,
shortens the latent period (p. 130). The latent period of the
pulvinus of Neptunia oleracea is '6 second.

Determination of Velocity of Transmission of
Excitation

Successive values of velocity of transmission are found
constant when the applied stimulus is constant, and when
the intervening period of rest allows complete protoplasmic
recovery. Consistent results have been obtained by the

352 RESEARCHES ON IRRITABILITY OF PLANTS

employment of the Direct and the Differential Methods.
The automatic records afford measurement of time as
short as '05 second. The highest velocity of transmission
of excitation that has been found in the petiole of Mimosa
is 30 mm. per second. In a sub-tonic tissue the velocity
of transmission of excitation is enhanced under increased
intensity of stimulus. The tissue became a better conductor
of excitation in consequence of previous stimulation.
Fatigue depresses the rate of conduction of excitation.
Velocity of transmission becomes markedly enhanced at a
higher temperature (p. 149).

The transmission of excitation takes place in both
directions ; but the velocity is not necessarily the same in
the two directions. In Biophytum the velocity in the
centrifugal direction is quicker than in the centripetal.

Excitatory Character of the Transmitted Impulse

' The enhancement of velocity under favourable condition
of rising temperature proves the physiological character of
the transmitted effect. In support of this there are various
confirmatory proofs, some of which may be regarded as
crucial.

Transmission of excitatory electric impulse. — The excita-
tory change in the plant is accompanied by a concomitant
electric change of galvanometric negativity. Electrical
investigation shows that this excitatory electric impulse is
transmitted to a distance through certain plant organs.

Excitatory impulse in absence of mechanical disturbance. —
The hydro-mechanical theory presupposes the occurrence
of a strong mechanical disturbance to give rise to the
transmitted impulse. But initiation of excitatory impulse
is found to take place under the polar action of an electrical
current in the absence of any mechanical disturbance.
This is realised when we find that an excitatory impulse is
initiated and transmitted by the action of a current which
is so feeble as not to be perceived by the very sensitive

GENERAL SURVEY 353

human tongue. The further fact that the excitation occurs
only at the point of kathode at make and at anode at
break shows that the effect transmitted is not physical
but physiological.

Multiple excitation by constant current. — According to
the mechanical theory multiple excitation can only occur
under separate hydro-mechanical disturbances caused by
multiple blows. It is, however, found that as in the rhythmic
tissue of the animal, so also in that of the plant, multiple
excitations are induced by the action of a constant current
(p. 249).

Characteristic effect of temperature on polar excitation. —
Excitation of animal nerve by induction shock is enhanced
by warmth and depressed by cold. The reverse is the case
when the stimulation is caused by constant current. The
excitatory effect here is depressed by warmth and exalted
by cold. These specific effects are found repeated in the
conducting petiole of Mimosa. The excitation caused by
induction shock is depressed by cold and enhanced by
warmth. But as in the animal nerve, so also in the petiole
of Mimosa, these effects are reversed in excitation under the
polar action of a constant current. The excitation is now
enhanced by cold and depressed by warmth. Minimal
excitation becomes maximal imder cold and ineffective
under warmth (p. 247).

The crucial test of the excitatory character of the
transmitted impulse is furnished by the action of various
physiological blocks which arrest the transmission of
excitation.

Paralysis of conduction hy cold. — The local application
of increasing cold on the conducting petiole retards and
finally arrests conduction of excitation. As an after-effect
of the application of cold the conducting power is paralysed
for a considerable length of time. The lost conducting
power may, however, be quickly restored by tetanising
electric shocks (p. 164).

The electrotonic block. — The excitatory impulse may

354 RESEARCHES ON IRRITABILITY OF PLANTS

also be arrested by the action of electrotonic block. This
arrest persists during the continuation of the blocking
current, the conductivity being restored on its cessation
(p. 167).

Block of conduction by action of poison. — Finally, the
conductivity of a selected portion of a petiole may be
abolished by the local application of poison. The abolition
of conducting power proceeds slowly under the action of
copper sulphate solution and quickly under potassium
cyanide (p. 173).

These results prove conclusively that the transmission
of excitation in plant is a process fundamentally similar
to that which takes place in the animal, being in the one
case as in the other a propagation of protoplasmic change.

Direct and Indirect Effects of Stimulus

When stimulus is applied directly on the responding
organ there is induced an excitatory fall of the leaf, con-
comitant with contraction and negative turgidity-variation.
This particular reaction is designated as the Direct Effect
OF Stimulus. When, on the other hand, a feeble stimulus
is applied at a distance there occurs only a positive or
erectile response of leaf with concomitant expansion and
positive turgidity-variation. This particular reaction is
designated as the Indirect Effect of Stimulus. If the
intervening tissue be highly conducting and the stimulus
sufficiently strong, then the excitatory negative effect
masks the positive. In such a case the response to indirect
application of stimulus is negative — that is to say, the same
as caused by direct stimulation. But, if the intervening
tissue be semi-conducting, or if the stimulus be feeble or
applied at too great a distance, then there is induced the
positive or Indirect Effect (p. 196).

These two opposite reactions are found to take place
in various plants under definite conditions and under diverse
forms of stimuli. The two opposite effects are demonstrated

GENERAL SURVEY 355

independently by means of mechanical and electric
responses.

Two diametrically opposite effects are thus induced by
an identical stimulus, depending on direct or indirect appli-
cation. The existence of the positive or the indirect effect
of stimulus has hitherto been unsuspected. It must be
taken into full consideration in unravelling the complexities
of reaction in a responding organ.

The laws of direct and indirect effects of stimulus
are :

The effect at the responding region of a strong excitation
transmitted through a short distance or through a good
conducting channel, is negative, being the same as the
effect under direct stimulation. The response is by negative
turgidity-variation, contraction, fall of leaf, and electrical
change of galvanometric negativity. This is the direct
effect of stimulus.

The effect of feeble stimulus transmitted through a
great distance or through a semi-conducting channel, is
positive. The responsive reaction is by positive turgidity-
variation, expansion, erection of leaf, and electrical change
of galvanometric positivity. This is the indirect effect of
stimulus.

Multiple Response

In taking records of electric response it is often found
that, while a single moderate stimulus gives rise to a single
response, a strong stimulus gives rise to a multiple series
of responses. Similarly in Biophytuni and Averrhoa, while
a moderate stimulus gives rise to a single mechanical
response, a strong stimulus gives rise to a multiple series
of responses. These multiple responses are induced by
various modes of strong stimulation, such as induction
shock, constant current, strong light, thermal shock, and
chemical excitation (p. 285).

When Biophytuni is subjected to successive stimuli

356 RESEARCHES ON IRRITABILITY OF PLANTS

of increasing intensity, the amplitudes of the successive
responses remain the same. The response is, therefore, on
' all-or-none principle.' After the stimulus intensity has
reached a certain limit the excess of absorbed energy finds
expression in multiple responses. Certain plant tissues
have thus the power of holding the excess of stimulus latent,
to be given out later in the form of recurrent responses
(p. 282).

The characteristics of the response of Biophytum are
like those of cardiac tissue of the animal. Both are charac-
terised by a long refractory period and response on ' all-or-
none principle.' In both a single moderate stimulus gives
rise to a single response and strong stimulus to a multiple
series of responses.

There is no strict line of demarcation between the
phenomenon of multiple and of spontaneous response so
called. Under very favourable circumstances of absorption
of excess of energy from without an ordinary responding
plant like Biophytum will become converted to an auto-
matically responding plant like Desmodium gyrans.

Automatic Pulsations of Desmodium gyrans

No satisfactory theory has been offered in explanation
of the so-called spontaneous movement. It has, however^
been shown in this and in my previous work that there
is no such thing as an absolutely spontaneous movement^
but that every movement is the result of the action of
stimulus which has been stored up. That this is the case
may be demonstrated in the case of Desmodium by isolating
the leaflet from external sources of stimulation. The effect
of run down of stored up energy is then seen in the gradual
stoppage of the pulsatory movement. In this condition of
standstill response occurs under fresh stimulation. If the
depletion of energy has not been excessive, then a moderate
stimulus gives rise to a multiple series of responses. But,

GENERAL SURVEY 357

under greater depletion, a strong stimulus evokes only a
single response (p. 316).

Desmodium leaflets in a state of standstill give a single
response to a single induction shock of moderate intensity.
In a typical case the latent period was found to be -4 second,
the apex time 45 seconds, and the period of relaxation
120 seconds. The response curve exhibits a flattened top.

In summer a single pulsation of a vigorous leaflet of
Desmodium is accomplished in the course of about 100
seconds. The quicker down movement is completed in
41 seconds, the maximum rate being -9 mm. and average
rate '44 mm. per second. The period of up movement is
slower, being 60 seconds ; maximum rate of up movement
is '56 mm. per second, the average rate being -3 mm. per
second.

The pulsating activity of the detached leaflet of Des-
modium can be maintained uniform for several hours by
subjecting it to a moderate internal hydrostatic pressure.
When the internal hydrostatic pressure is increased, the
limit of diastolic or up movement is increased ; the con-
tractile movement being opposed, the systolic limit is
decreased. Under increasing external load the pulsation
is decreased in amplitude and is finally arrested (p. 301).

Similar Characteristics of Rhythmic Pulsation
IN Animal and Plant

The rhythmic tissues of the plant exhibit characteristics
which are extraordinarily similar to those of the rhythmic
tissue of the animal. The cardiac tissue of the animal has
a long refractory period ; the tissue takes no account of
a stimulus which falls within the refractory period. This
is also characteristic of the response of the rhythmic tissue
of Desmodium (p. 310). The rhythmic tissues, animal and
vegetal alike, are incapable of tetanus.

By the application of Stannius' ligature the pulsation
of the heart is arrested at diastole. A similar arrest at

358 RESEARCHES ON IRRITABILITY OF PLANTS

diastole is found to take place in the pulsation of Desmo-
dium by the application of ligature below the motile organ

(p. 303).

The pulsating leaflet of Desmodium, like the pulsating
heart, is more susceptible to excitation at diastole than at
systole. An extra pulsation is induced by an induction
shock applied during the diastolic phase (p. 319). Trans-
mitted excitation affects the normal pulsations of rhythmic
tissues — animal and vegetal — in a similar manner. In
certain circumstances the effect is one of inhibition ; in
other circumstances the effect is one of acceleration (p. 320) .

Still more remarkable are the similarities of effect of
temperature and of chemical reagents on the rhythmic
pulsations in animal and plant.

Effect of Temperature

The effect of lowering of temperature on the rhythmic
pulsation of Desmodium gyrans is similar to that on the
pulsation of a frog's heart. Lowering of temperature
enhances the amplitude, but reduces the frequency of
pulsation of both. The pulsation of Desmodium leaflet is
arrested at the minimum temperature of about 17° C.
Arrest takes place at systole ; gradual warming revives
the pulsation, which undergoes a staircase increase with
enhancing diastolic expansion (p. 326).

Rise of temperature induces enhanced frequency and
diminished amplitude of pulsation. During rise of tempera-
ture to about 43° C. there is a tendency of arrest towards
diastole. The systolic contraction undergoes continuous
diminution during rise of temperature. During the fall of
temperature there is a gradual enhancement of systolic
contraction (p. 330). The temperature maximum at which
arrest of pulsation takes place may be as high as 45° C.
Above this temperature there is a tendency to contraction
and permanent arrest under heat-rigor.

GENERAL SURVEY 359

Effect of Chemical Agents

The effect of drugs on the rhythmic pulsation is modified
by the tonic condition of the plant, the strength of the
reagent, and the duration of application.

Vapour of alcohol and dilute carbonic acid induce a
transient enhancement of amplitude with prolongation of
period. Stronger application induces an arrest of pulsation.
Dilute vapour of ether and carbon disulphide induce a
temporary arrest, revival taking place after quick substitu-
tion of fresh air. The action of chloroform is more intense
than that of ether.

Copper-sulphate solution causes a permanent arrest of
pulsation. The poisonous reaction of potassium cyanide
is more powerful than that of copper sulphate.

A very striking characteristic modification in the
rhythmic activity of animal tissue is found in the antagon-
istic action of acid and alkali on the pulsation. Application
of dilute acid induces in the heart an atonic reaction with
arrest of pulsation in the relaxed or diastolic condition.
The effect of alkali is the very reverse of this, the arrest
taking place in systole. These specific effects are repro-
duced in an astonishing manner in the rhythmic pulsation
of Desmodium. Dilute solution of lactic acid induces in
the pulsating leaflet an arrest at diastolic relaxation. The
application of dilute sodium hydrate induces, on the other
hand, exactly the opposite effect of arrest at systole (p. 339) .

At the beginning of this work we took up the question
of the possibility of detecting internal changes in a plant
by subjecting it to a questioning shock. It has been shown
how the plant can be made to record its answer to an
impinging testing stimulus, and how the effects of environ-
mental changes may be read in the script made by the
plant itself.

It has been shown that the variations in the plant's,
physiological activity, under changing external conditions

36o RESEARCHES ON IRRITABILITY OF PLANTS

may be gauged by the waxing or waning of its response.
It has been shown also how numerous and varied are the
factors that go to make up the complexity of the responses
in the plant. It has been shown that stimulus may be
modified in its effect, according as it is direct or indirect,
according as it is feeble or strong. The modifying influence
of the tonic condition of the tissue has also been shown,
depending on whether it was normal, sub-tonic, or fatigued.
In the numberless permutations and combinations of these
varied factors lies the infinite complexity of the responsive
phenomena of life.

In surveying the response of living tissues we find that
there is hardly any phenomenon of irritability observed in
the animal which is not also found in the plant. The
various manifestations of irritability in the plant have been
shown to be identical with those in the animal. From the
standpoint of the theory of evolution this will be found
highly significant. It may be confidently predicted that
the recognition of this unity of response in animal and plant
will in no small degree further the progress of plant
physiology. Many difficult problems in animal physiology,
moreover, will find their solution in the experimental study
of corresponding problems under simpler conditions of
vegetable life. The study of the responsive reactions in
])lants must, therefore, be regarded as of fundamental im-
portance in the elucidation of various phenomena relating
to the irritability of living tissues.

CLASSIFIED LIST OF EXPERIMENTS

Simple Mechanical Response

Mimosa

1. Response to indirect thermal stimulus .... 26

2. Response to constant current . . . . • 27

3. Response to condenser discharge ..... 30

4. Excitatory response induced by sudden application of

cold 100

5. Greater efficiency of break induction shock ... 53

6. Response due to stimulation of upper half of pulvinus . 45

7. Determination of apex time ..... 37

8. Determination of time relations of response ... 41

9. Additive effect of stimulus ...... 54

10. Influence of temperature ...... 60

11. Relation between stimulus and response ... 62

12. Determination of work performed by Mimosa . . 57

13. Effect of load on work performed .... 56

14. Determination of rate of work ..... 58

Neptunia oleracea

15. Response of leaf of Neptunia to electric stimulus . . 43

Biophytum sensitivum

16. Response of leaflet of Biophytum ..... 42

Desmodium gyrans

17. Response of terminal leaflet of Desmodium to electrical

stimulation . . . . . . . .45

Ordinary plant

18. Response of ordinary plants to stimulus ... 47

Different Types of Response

19. Uniform response in Mimosa . . . . . ♦. 71

20. Induction of fatigue under shortened period of rest . 73

21. Rapid fatigue ........ 74

22. Periodic fatigue ........ 74

361

362 RESEARCHES ON IRRITABILITY OF PLANTS

PAGE

23. Staircase response in Mimosa ..... 76

24. Modification of tonicity by stimulus . . . -79

25. Alternating response ....... 80

26. Fatigue reversal in Mimosa . . . . .81

Effect of External Conditions on Simple Response

27. Effect of sudden darkness .....

28. Abolition of motile response by absorption of water

29. Restoration of motile excitability by action of glycerin

30. Stimulating effect of ozone .

31. Effect of carbonic-acid gas .

32. Effect of alcohol

33. Effect of ether ....

34. Effect of carbon disulphide .

35. Effect of coal gas

36. Effect of chloroform .

37. Action of ammonia in abolition of excitability

38. Abolition of excitability and death of plant under sul

phuretted hydrogen ......

39. Toxic effect of nitrogen dioxide .

40. Poisonous action of sulphur dioxide .

Determination of the Death Point

41. Death record of Mimosa ....

42. Abolition of mechanical response after death

43. Determination of death point of Desmodium .

44. Death point of bean plant ....

45. Determination of death point in ordinary plant

46. Lowering of death point under fatigue

47. Translocation of death point under the action of dilute

poison .........

Determination of Latent Period

Mimosa

87

89
91
91
92
92
93
93
94

95
96
96

102
103
104
104
104
106

106

48. Identity of latent period in successive experiments . 115

49. Determination of latent period in highly excitable Mimosa 116

50. Value of latent period unaffected by peculiarity of

recorder . . . . . . . .118

51. Simultaneous excitation in intra-electrodal tract . .124

52. Effect of intensity of stimulus on latent period . .125

53. Constancy of latent period under maximal stimulus . 127

54. Effect of optimum condition on latent period . . 128

55. Effect of fatigue 129

56. Effect of temperature . . . • . .130

Biophytum sensitivum

57. Determination of latent period of Biophytum . .281

CLASSIFIED LIST OF EXPERIMENTS 363

PAGE

Neptunia oleracea

58. Determination of latent period of Neptunia . . .120

Transmission of Excitation

59. Retardation of conduction under action of cold . .162

60. Block of conduction under excessive localised cooling . 163

61. Paralysis of conduction as after-effect of cold . .164

62. Restoration of conductivity under tetanising shock . 164

63. Time record showing arrest of conduction by electrotonus 166

64. Effect of electrotonic block ' on ' and ' off ' . . . 167

65. Block of conduction by local application of copper

sulphate in petiole of Biophytum . . . .170

66. Effect of mercuric chloride in inducing block of con-

duction . . . . . . . . .170

67. Effect of KCN solution on block of conduction in

Biophytum . . . . . . . -171

68. Time record showing block of conduction by CUSO4 in

petiole of Mimosa . . . . . . ,172

69. Effect of mercuric chloride in arresting conduction . 173

70. Quick abolition of conduction in petiole of Mimosa by

action of KCN solution . . . . . -173

Determination of Velocity of Transmission

Mimosa

thod .

137

138

143

144

145

147

148

71. Determination of velocity by the direct method

72. Determination by the differential method

73. Effect of intensity of stimulus on velocity

74. After-effect of stimulus on conductivity

75. Effect of optimum condition on velocity

76. Effect of fatigue ....

77. Effect of temperature

Biophytum sensitivum

78. Determination of velocity of transmission of excitation

in Biophytum sensitivum . . . . . .150

79. Determination of direction of preferential conductivity . 151

Dual Impulses and Positive Response
Biophytum

80. Dual impulse under thermal stimulation . . .180

81. Effect of chemical stimulus . . . . .182

82. Action of induction shock . . . . . .182

83. Diphasic response under constant current . . .183

84. Effect of condenser discharge . . . , .183

Averrhoa carambola

85. Dual impulse under electrical stimulation . . .184

364 RESEARCHES ON IRRITABILITY OF PLANTS

Mimosa

86.
87.

89.
90.
91.

Dual impulse under induction shock . . . .185

Double response under thermal shock . . . .186

Effect of intensity of stimulus in inducing positive or
negative response . . . . . . .187

Effect of distance of point of application of stimulus . 189
Masking of positive impulse by predominant negative . 194
Unmasking of positive by suppression of negative by cold 195

Mimosa
92.
93.

94.

Polar Action of Feeble and Moderate Currents

Effect of make of kathode on the leaf of Mimosa .

Time difference between excitation by ascending and
descending currents ......

Time difference between effects of ascending and descend-
ing induction shock ......

95. Effect of moderate current at make of kathode and

break of anode .......

96. Effect of feeble current on leaflet of Mimosa .

97. Effect of moderate current on leaflet of Mimosa .

Biophytum sensitivum

98. Effect of feeble current

99. Effect of moderate current .

Neptunia oleracea

100. Excitatory effect of feeble current
loi. Excitatory effect of moderate current .

Averrhoa carambola

102. Polar effect of feeble current

103. Polar effect of moderate current .

Averrhoa bilimbi

104. Effect of feeble current at make of kathode

105.

Effect of moderate
break of anode

current at make of kathode and

204

206

230
219
221

223
224

225
226

226
227

228
229

106
107
108
109
no
III
112
"3

Polar Reaction under Strong CurrexNts

Effect of strong current on Mimosa . . . -255

Effect of strong current on leaflets of Mimosa . . 260

Effect of strong current on Biophytum sensitivum . 261

Effect of strong current on Averrhoa carambola . . 263

Effect of very strong current on leaf of Mimosa . . 256

Effect of very strong current on leaflets of Mimosa . 261

Effect of very strong current on leaflets of Biophytum . 262

Effect of very strong current on Averrhoa carambola . 263

CLASSIFIED LIST OF EXPERIMENTS 365

Modification of Polar Reaction under Tissue
Modification

PAGF

114. After-effect of moderate stimulation .... 268

115. Effect of age ........ 270

116. Modified response Km Am ...... 273

117. Modified response KmKbAm ..... 274

Characteristics of Polar Excitation

118. Comparison of relative sensitiveness of plant and animal

to polar excitation . . . . . . .249

119. Depression of excitability to induction shock by the

action of cold ........ 246

120. Enhancement of excitability to polar action of current

by the action of cold . . . . . -247

121. Multiple excitation in Biophytum by constant current . 249

Contrasted Effects of Anode and Kathode

122. Effect of application of anode on pulsation of Desmo-

dium ......... 236

123. Effect of application of kathode on pulsation of Des-

modium ......... 236

124. Arrest at systole by the make of kathode, and diastolic

expansion by the break of kathode . . . .237

125. Arrest at diastole by the make of anode, and systolic

contraction by the break of anode . . . .237

126. Extrapolar effect of feeble ascending current at make . 240

127. Extrapolar effect of feeble ascending current at break . 240

128. Extrapolar effect of feeble descending current at make . 240

129. Extrapolar effect of feeble descending current at break 240

130. Extrapolar effect of strong ascending current at make . 241

131. Extrapolar effect of strong ascending current at break . 242

132. Extrapolar effect of strong descending current at make . 241

133. Extrapolar effect of strong descending current at break . 241

Multiple Response

134. Response of leaflet of Biophytum on ' all-or-none '

principle ........ 281

135. Response of Biophytum under increasing intensity of

stimulus . . . . . . . . 284

136. Multiple response of Biophytum under a single strong

electric shock . . . . . . . .285

137. Multiple response in Averrhoa ..... 286

138. Multiple response under strong thermal shock . . 286

139. Multiple response under strong chemical stimulation . 286

140. Multiple response under constant light . . . 286

366 RESEARCHES ON IRRITABILITY OF PLANTS

Automatic Pulsation of Desmodium

141
142

143
144

145

Uniform pulsations of Desmodium .... 294

Determination of rate of movement at different phases 297

Determination of significance of up-and-down movement 295

Effect of internal hydrostatic pressure . . . 300

Effect of load ........ 301

Condition of Standstill and Revival of Pulsation

146
147,
148

149
150
151
152
153
154
155

Gradual diminution of amplitude under isolation . .313

Stoppage of pulsation brought on by depletion of energy 314
Single response to moderate induction shock, in leaflet at
standstill ........ 308

Multiple excitation under tetanisation .... 308

Single response to stimulus of light of brief duration . 316
Restoration of pulsation under continued action of light 307
Arrest of pulsation by ligature ..... 303

Arrest of pulsation by the action of a cut . . . 304

Determination of apex time ..... 309

Refractory period of Desmodium . . . . .310

Effect of Electric Stimulation on Automatic Pulsation of
Desmodium

156. Incapability of tetanus . . . . . .318

157. Extra pulsation induced by application of electric stimu-

lation during diastole . . . . . -319

158. Inhibitory effect of transmitted excitation on rhythmic

pulsation ........ 320

159. Augmentation of pulsation induced by transmitted ,

excitation ........ 321

Influence of Temperature on Rhythmic Pulsation of
Desmodium

160. Effect of lowering of temperature . . . . 325

161. Effect of rapid cooling ...... 326

162. Arrest of pulsation at low temperature . . . 326

163. Effect of rise of temperature on amplitude and period . 327

164. Effect of continuous rise of temperature . . . 329

165. Effect of continuous rise and return to original tem-

perature . . . . . . . . 330

166. Effect of variation of temperature on systolic and diastolic

limits ......... 330

167. Arrest of pulsation at high temperature . . . 331

CLASSIFIED LIST OF EXPERIMENTS

367

Action of Chemical Agents on Rhythmic Pulsation of
Desmodium

168
169
170,
171
172
173
174
175
176
177
178
179

180.

Stimulating action of dilute sugar solution . . . 333

Effect of dilute alcohol ...... 334

Effect of strong alcohol ...... 334

Effect of dilute COo ....... 335

Arrest of pulsation by the action of strong COo . . 336

Effect of internal application of water charged with CO2 336

Effect of ether ........ 336

Effect of chloroform ....... 336

Effect of carbon disulphide ..... 338

Arrest of pulsation by CUSO4 solution .... 338

Quick arrest of pulsation by the action of KCN solution 339
Arrest of pulsation at diastole by the action of dilute

lactic acid ........ 340

Arrest of pulsation at systole by the action of sodium

hydrate ......... 340

INDEX

Acid and alkali, antagonistic action of, on cardiac pulsation, 339

„ ,, „ ,, ,, ,, ,, pulsation of Desmodium, 340

Additive effect, 54

,, „ quantitative relation of, 55
Age, effect of, on polar excitation, 267
Alcohol, effect of, on Mimosa response, 91

„ „ „ ,, Desmodium pulsation, 333
* All-or-none ' principle, in response of Biophytum, 281

„ ,, ,, ,, „ ,, cardiac tissue, 282

Ammonia, effect of, on Mimosa response, 94
Anaesthetics, action of, on conductivity, 154

,, » „ >, response of Mimosa, 92

,, ,, „ >) rhythmic response, 336

Anode and kathode, contrasted effects of, 234
Anode, break effect of, on Desmodium pulsation, 237

„ excitatory effect of break on plant-tissue, 221, 224, 226, 228, 229,

231
,, make effect of, on Desmodium pulsation, 236
Apex time, determination of, in Mimosa, 37

„ „ „ ,, Desmodium, 309

Ascending and descending currents, time difference in the actions of, 204

„ „ „ electrotonic effects of, 165, 239

Ascending and descending induction shock, time -difference in the

actions of, 206
Automatic exciter, 66

Automatic response, continuity with multiple, 288
,, „ due to storage of energy, 287

„ „ cessation of, by depletion of energy, 312

see also Desmodium pulsation
Autonomous response, see Automatic response
Averrhoa hilimhi, polar action of current on, 228
Averrhoa carambola, multiple response in, 286

„ „ polar action of current in, 226, 262

„ „ velocity of transmission of excitation in, 151

Biophytum sensitivum, arrest of excitatory impulse in, by poison, 169
„ ,, comparative sensitiveness of animal and, 249

„ „ direction of preferential conduction in, 152

„ „ dual impulse in, 181

369

2 B

370 RESEARCHES ON IRRITABILITY OF PLANTS

Biophytum sensiiivum, effect of increasing intensity of stimulus on, 284
latent period of, 281

multiple response in, under chemical stimulus, 287
„ „ ,, ,, constant current, 249

,, ,, ,, ,, electrical stimulus, 285

„ „ light, 286
,, ,, ,, ,, thermal stimulus, 286

polar effects of current in, 221, 239, 261
positive response of, 180
response on ' all-or-none ' principle, 282
velocity of transmission in, 151
Bi-polar method, 113

Carbon disulphide, effect of, on Desmodium pulsation, 338

,, ,, ,, ,, ,, Mimosa response, 92

Carbonic acid, effect of, on pulsation of Desmodium, 335

„ „ „ ,, ,, response in Mimosa, 91

Cardiac pulsation, similarity between Desmodium pulsation and, 321, 323,

339
Chemical agents, effect of, on rhythmic response, 332

„ ,, ,, ,, ,, mechanical response of Mimosa, 85

Chloroform, effect of, on Desmodium pulsation, 336

,, ,, ,, response of Mimosa, 93

Coal gas, effect of, on Mimosa response, 92
Coercer, 15

Cold, arrest of Desmodium pulsation by, 326
,, diminished velocity of transmission by, 162
,, excitatory action by sudden application of, 100
„ paralysis of conductivity by, 163
„ prolongation of latent period by, 130
,, unmasking of positive impulse by, 195
Condenser discharge, stimulation by, 27
Conduction of excitation, channels of, 133

„ „ determination of velocity of, 136

,, ,, effect of fatigue on. velocity of, 147

,, ,, ,, ,, intensity of stimulus on velocity of, 142

„ ,, ,, ,, optimum condition on velocity of, 145

„ ,, ,, ,, temperature on velocity of, 148

„ ,, ,, ,, preferential direction of, 151

,, velocity of, in Averrhoa, 151
„ „ „ Biophytum, 151

,, ,, ,, Mimosa, 141

Constant current, polar effect of, 198
Contact, electrolytic, 68

Darkness, effect of sudden, on response of Mimosa, 87
Death-point, determination of, by electrical response, 98, 105

„ ,, ,, ,, thermomechanical inversion, 104

,, „ >, of bean plant, 104

,, ,, )f ,, Desmodium, 104

,, ,, ,, „ Mimosa, 102

INDEX

371

Death-point, translocation of, by fatigue, 106
,, ,, „ „ poison, 106

Death-spasm in plants, 98
Desmodium, extra pulsation by induction shock, 319

„ response of terminal leaflet of, 45

Desmodium pulsation, continuous record of, 294

determination of apex time, 309

,, ,, latent period, 308

,, ,, refractory period, 310

effect of acid on, 340

,, alcohol on, 334
,, alkali on, 340
„ anaesthetics on, 336
,, anode and kathode break on, 237
„ anode and kathode make on, 237
„ carbon disulphide on, 338
,, carbonic acid on, 335
,, chloroform on, 336
,, copper sulphate on, 338
„ cut on, 303
„ ether on, 336

„ internal hydrostatic pressure on, 300
„ ligature on, 302
„ load on, 301

,, lowering of temperature on, 325
„ potassium cyanide on, 338
„ rise of temperature on, 327
„ sugar solution on, 333
„ transmitted excitation on, 320
gradual stoppage of, by depletion of energy, 313
incapability of tetanus, 318
measurement of amplitude and period of, 296
similarity between Biophytum pulsation and, 288
„ ,, cardiac pulsation and, 317, 323,

339
temperature maximum in, 330
,, minimum in, 326

time relations of, 297
Desmodium at standstill, by cut, 303

depletion of energy, 313
ligature, 302

revival of pulsation in, by electric shock, 308
,, ,, „ stimulus of

light, 315
Differential excitability, existence of, in Mimosa pulvinus, 45

„ method, velocity of transmission by, 138

Direct and indirect effect of stimulus, laws of, 196

„ „ „ „ exhibition of, under different

stimuli, 179
Dual impulses in Averrhoa carambola, 184
,, ,, ,, Biophytum sensitivum, 180

372 RESEARCHES ON IRRITABILITY OF PLANTS

Dual impulses in Mimosa, 185
Duplex recorder, 69

Electric current, polar effects of, in excitation, 198

„ „ multiple excitation induced in Biophytum by, 249

„ response, detection of excitation by, 98
„ „ „ „ excitatory wave by, 133

„ ,, determination of death-point by, 105

,, „ multiple, 283

Electro- thermic stimulator, 25

Electro-tonic block, arrest of excitatory impulse by, 164, 238
Electrotonus, modification of excitability by, 164
Energy, effect of run-down of, on rhythmic pulsation, 313

„ storage of, from external stimulus, 287
Ether, effect of, on Desmodium pulsation, 336

„ „ „ Mimosa, 92

Excitability, diurnal variation of, 46

„ effect of temperature on, 60

„ electro-tonic variation of, 238

„ variation of, by chemical agents, S$

Excitatory impulse, arrest of, by action of poison, 167
„ „ „ „ cold, 161

„ „ „ „ electrotonic block, 164

„ ,, determination of velocity of, 135

„ „ effect of fatigue on, 147

„ ,, ,, ,, intensity of stimulation, 142

„ ,, ,, ,, temperature, 148

„ „ versus hydro-mechanical, 155

Exciter, automatic, 66

Fatigue, alternating, 80

„ effect of, on death point, 106

„ „ „ „ latent period, 128

,, ,, ,, ,, response of Mimosa, 72

„ „ „ ,, velocity of transmission of excitation, 147

„ reversal, under continuous stimulation in plant and animal, 81

Friction, errors in record introduced by, 9

Haberlandt on theory of mechanical transmission, 155

Hydro-mechanical theory, 154

Hydrostatic pressure, effect on Desmodium pulsation, 300

Intermittent contact, advantage of, in record, 21
Inversion, thermo-mechanical, 104

Kathode, depression of excitability at break of, 242
„ diastolic expansion by break of, 237
„ excitatory effect of, at make, 201

„ reduction of diastolic expansion in Desmodium by make of, 236
„ systolic arrest by make of, 237

INDEX 373

Latent period, apparatus for determination of, iii
,, „ determination of, in Biophytum, 280

„ „ Mimosa, 120
„ „ „ „ „ Neptunia, 120

„ „ effect of fatigue on, 128

„ ,, „ ,, optimum condition on, 128

,, „ ,, ,, strength of stimulus on, 125

„ ,, ,, „ teinperature on, 130

Laws of direct and indirect effects of stimulus, 196

„ „ polar action of current on plant tissues, 233, 265
Ligature, effect of, on Desmodium pulsation, 303

„ Stannius', on cardiac pulsation, 302
Light, initiation of pulsatory movement in Desmodium by the

action of, 315
Light, multiple response induced by, in Biophytum, 286
Load, effect of, on mechanical response, 56

„ „ ,, „ amount of work performed, 58

Mimosa, abolition of motile response by absorption of water in, 87
„ block of conduction of excitation in, 161, 164, 167
,, conduction of excitation in, 174
,, death-point of, 102
„ dual impulses in, 185
„ effect of alcohol on, 91

ammonia on, 94

carbon disulphide on, 92

carbonic acid gas on, 91

chloroform on, 93

coal gas on, 92

ether on, 92

nitrogen dioxide on, 95

ozone on, 89

sudden darkness on sensitiveness of, 87

sulphur dioxide on, 96

sulphuretted hydrogen on, 94
„ erectile response of, 44, 185
„ fatigue in response of, 72
„ fatigue reversal in, 80
,, latent period of, 121
„ polar effects of current on, 229, 254
„ positive response in, 185
„ sensitiveness of, to electric stimulus, 50
„ staircase response of, 76
„ time relations of response of, 41
„ velocity of transmission of excitation in, 141
Multiple response, continuity between automatic and, 288
» „ to light, 315

„ ,, ,, induction shock, 284

„ ,, ,, thermal shock, 286

„ ,, ,, strong stimulus in Averrhoa, 285

374 RESEARCHES ON IRRITABILITY OF PLANTS

Multiple response to strong stimulus in Biophytum, 284
„ „ under chemical stimulus, 286

„ ,, „ constant current, 249

light, 286, 307

Neptunia oleracea, latent period of, 120

„ ,, polar effects of current on, 225

„ „ response of, 43

Nitrogen dioxide, abolition of response by the action of, 95

Periodic starter, 66

Pfefier on hydro-mechanical theory, 155

Pfliiger on law of polar excitation in animal tissue, 232, 276

Plant, on relative sensitiveness of animal and, 249

Poison, block of conduction of excitation by local application of, 167

„ abolition of pulsation in Desmodium by, 338
Polar action of currents, effect of tissue modification on, 266
„ „ „ laws of, in plants, 233, 265

„ „ feeble currents, 216

,, ,, moderate currents, 220

,, „ strong currents, 253

Polar effects of ascending and descending currents, 203, 204, 206
„ currents on Averrhoa bilimbi, 228
,, ,, „ Averrhoa carambola, 227, 262

,, „ ,, Biophytum, 223, 224, 262

,, ,, ,, Desmodium, 236

,, ,, „ Mimosa, 217, 221, 229, 255, 256, 260

,, ,, „ Neptunia, 225, 226

,, „ ,, Protozoa, 199, 266, 276

Positive response, characteristics of, 190

„ „ conditions favourable for, 187

„ „ electrical, 177

„ „ unmasking of, 195

Potassium cyanide, abolition of rhythmic pulsation by the action of, 33

„ ,, arrest of excitatory impulse by, 173

Potential keyboard, 210

„ slide, 208
Pulvinus of Mimosa, differential excitability in, 45

Record, fixation of, 18
Recorder, optical lever, 178

„ oscillating, 279

„ resonant, 14
Response, different types of, 64

„ effect of fatigue on, 72

„ „ „ intensity of stimulus on, 61

„ „ „ optimum condition on, 63

INDEX 375

Response, electrical, 98, 177

„ positive and negative, 177, 181

„ relation of stimulus to, 62
Rhythmic tissue, similarity of response in animal ana plant, 321

Sensitiveness, comparison between animal and plant, 249

' Sensitive ' plant, arbitrary distinction between ordinary and, 44

Staircase response in Mimosa, 77

Stannius' ligature on cardiac pulsation, 302

Stimulation, automatic, 66

„ chemical method of, 23

,, electrical methods of, 27, 28, 30

,, thermal method of, 25

Stimulus, direct effect of, 195

,, indirect effect of, 196

„ relation between response and, 61

,, standardisation of, 48
Sugar solution, stimulating action of, on Desmodium pulsation, 333
Sulphur dioxide, effect of, on response of Mimosa, 96
Sulphuretted hydrogen, toxic action of, 94

Temperature, arrest of Desmodium pulsation by lowering of, 326

„ ,, ,, transmission of excitation by lowering of, 161

,, effect of, on cardiac pulsation, 323

„ ,, ,, „ Desmodium pulsation, 325

„ „ ,, „ excitatory efficiency of induction shock, 246

„ „ ,, ,, ,, „ ,, polar action of

current, 247
„ „ ,, „ latent period, 130

„ ,, „ „ velocity of transmission of excitation, 148

„ maximum for Desmodium pulsation, 331

,, minimum for Desmodium pulsation, 326

Tetanus, incapability of, in rhythmic tissue, 318

Thermo-mechanical curve, 104

Time relation of response of Mimosa, 41

Tissue modification, effect on polar reaction, 266

Tonicity, modification of, by stimulus, 79

Uniform response, in Desmodium, 294
„ „ „ Mimosa, 71

Velocity of excitatory impulse, determination of, by direct method, 135
„ ,, ,, „ „ „ differential

method, 138
„ „ „ effect of cold on, 161

376 RESEARCHES ON IRRITABILITY OF PLANTS

Velocity of excitatory impulse, effect of fatigue on, 147

,, ,, ,, intensity of stimulus on, 142
,, ,, „ temperature on, 148
,, in Averrhoa, 151
,, ,, Biophytum, 151
„ ,, Mimosa, 141

Work, effect of load on, in Mimosa; 56
,, measurement of, in Mimosa, 57
„ rate of, in Mimosa, 58

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