Life Movements in Plants, Volume I
by Jagadis Chandra Bose · English · Gutenberg #48280 · 402,952 chars · public domain (US)
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Title: Life Movements in Plants, Volume I
Author: Jagadis Chandra Bose
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Minor spelling inconsistencies are as in the original, e.g.
tetanising/tetanizing, Das/Dass, but the following overt spelling
errors have been corrected silently:
his —> this
despressing —> depressing
presistent —> persistent
actic —> lactic
excitabilitty —> excitability
Zephyanthes —> Zephyranthes
fal —> fall
tranmission —> transmission
substracting —> subtracting
issue —> tissue
conducing —> conducting
ummasking —> unmasking
be —> been
end —> and
flexture —> flexure
tentanising —> tetanising
anisotrophy —> anisotropy
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LIFE MOVEMENTS IN PLANTS
BY
SIR JAGADIS CHUNDER BOSE, Kt., M.A., D.Sc., C.S.I., C.I.E.,
PROFESSOR EMERITUS, PRESIDENCY COLLEGE,
DIRECTOR, BOSE RESEARCH INSTITUTE.
WITH 92 ILLUSTRATIONS
_B.R. Publishing Corp.
Delhi_
Cataloging in Publication Data-DK
Bose, Jagadish Chandra, 1858-1937.
Life movements in plants.
Reprint.
1. Plants--Irritability and movements. 2. Growth (Plants).
3. Plants--Development. 4. Botany. I. Title.
_First Published_ 1918
_Reprinted_ 1985
_Published in India by_
B.R. PUBLISHING CORPORATION
461, VIVEKANAND NAGAR,
DELHI-110052 (INDIA)
_Distributed by_
D.K. PUBLISHERS’ DISTRIBUTORS
1, ANSARI ROAD, DARYA GANJ,
NEW DELHI-110002 (INDIA)
PHONE: 27-8368
_Printed at_:
BRITE PRINTERS
NEW DELHI-110005 (INDIA)
CONTENTS
PART I.
RESPONSE OF PLANT ORGANS.
I.--THE PROBLEM OF MOVEMENT IN PLANTS.
PAGE
Complexity of the problem--Effects of different forms
of stimuli--Diverse responses under identical
stimulus--Modification of response determined by intensity
and point of application of stimulus, and tonic condition of
organ--Response of pulvinated and growing organs--Necessity
for shortening the period of experiment 1
II.--THE “PRAYING” PALM TREE.
Description of phenomenon--The Recording apparatus--Record
of diurnal movement of the tree--Universality of tree
movement--Cause of periodic movement--Periodic movement
of trees, and diurnal variation of moto-excitability
in _Mimosa pudica_--Relative effects of light
and temperature--Physiological character of the
movement--Transpiration and diurnal movement--Diurnal
movement in inverted position--Effect of variation of
temperature on geotropic curvature--Reversal of natural
rhythm by artificial variation of temperature 5
III.--ACTION OF STIMULUS ON VEGETABLE TISSUES.
Different types of Response Recorders--Response of a radial
organ--Response of an anisotropic organ--Response of pulvinus
of _Mimosa pudica_--Tabular statement of apex time and
period of recovery in different plants--Response of pulvinus
of _Mimosa_ to variation of turgor--Different modes of
stimulation 31
IV.--THE DIURNAL VARIATION OF EXCITABILITY IN _MIMOSA_.
Apparatus for study of variation of excitability--Uniform
periodic stimulation--The Response Recorder--Effects of
external condition on excitability--Effects of light
and darkness--Effect of excessive turgor--Influence of
temperature--Diurnal variation of excitability--Effect of
physiological inertia 43
V.--RESPONSE OF PETIOLE-PULVINUS PREPARATION OF _MIMOSA_.
Effect of wound or section in modification of normal
excitability--The change of excitability after immersion in
water--Quantitative determination of the rate of decay of
excitability in an isolated preparation--Effect of amputation
of upper half of the pulvinus--Effect of removal of the lower
half--Influence of weight of leaf on rapidity of responsive
fall--The action of chemical agents--Effect of “fatigue” on
response--The action of light and darkness on excitability 73
VI.--CONDUCTION OF EXCITATION IN PLANTS.
Hydro-dynamic _versus_ physiological theory of conduction
of excitation--Arrest of conductivity by physiological
blocks--Convection and conduction of excitation--Effect
of temperature on velocity--Effect of season--Effect of
age--Effect of dessication of conducting tissue--Influence
of tonic condition on conduction--Effect of intensity
of stimulus on velocity of transmission--Effect
of stimulus on sub-tonic tissues and tissues in
optimum condition--Canalisation of conducting path by
stimulus--Effect of injury on conductivity 97
VII.--ELECTRIC CONTROL OF EXCITATORY IMPULSE.
Method of conductivity-balance--Control of transmitted
excitation in _Averrhoa bilimbi_ by electric
current--‘Uphill’ transmission--Transmission
‘downhill’--Electric control of nervous impulse in
animal--Directive action of current on conduction of
excitation--Effects of direction of current on velocity
of transmission in _Mimosa_--Determination of variation
of conductivity by method of Minimal Stimulus and
Response--Influence of direction of current on conduction
of excitation in animal nerve--Variation of velocity of
transmission--After-effects on Heterodromous and Homodromous
currents--Laws of variation of nervous conduction under
electric current 107
VIII.--EFFECT OF INDIRECT STIMULUS ON PULVINATED ORGANS.
Conduction of excitation--Dual character of the transmitted
impulse--Effect of distance of application of
stimulus--Periods of transmission of positive and negative
impulses--Effects of Direct and Indirect stimulus 135
IX.--MODIFYING INFLUENCE OF TONIC CONDITION ON RESPONSE.
Theory of assimilation and dissimilation--Unmasking of positive
effect--Modification of response under artificial depression
of tonic condition--Positive response in sub-tonic specimen 141
PART II.
GROWTH AND ITS RESPONSIVE VARIATIONS.
X.--THE HIGH MAGNIFICATION CRESCOGRAPH FOR RESEARCHES ON GROWTH.
Method of high magnification--Automatic record
of the rate of growth--Determination of the
absolute rate of growth--Stationary method of
record--Moving plate method--Precaution against
physical disturbance--Determination of latent period
and time-relations of response--Advantages of the
Crescograph--Magnetic amplification--The Demonstration
Crescograph 151
XI.--EFFECT OF TEMPERATURE ON GROWTH.
Method of discontinuous observation--Method of continuous
observation--Determination of the cardinal points of
growth--The Thermocrescent curve--Relation between
temperature and growth 173
XXII.--EFFECT OF CHEMICAL AGENTS ON GROWTH.
Effect of stimulants--Effect of anæsthetics--Action of
different gases--Action of poisons 183
XIII.--EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH.
Response to positive variation of turgor--Method of
irrigation--Effect of artificial increase of internal
hydrostatic pressure--Response to negative variation of
turgor--Method of plasmolysis--Effect of alternative
variations of turgor on growth--Response of motile and
growing organs to variation of turgor--Effect of external
tension 188
XIV.--EFFECT OF ELECTRICAL STIMULUS ON GROWTH.
Effect of intensity--Effect of continuous
stimulation--Continuity between ‘incipient’ and actual
contraction--Immediate effect and after-effect 195
XV.--EFFECT OF MECHANICAL STIMULUS ON GROWTH.
Effect of mechanical irritation--Effect of wound 200
XVI.--ACTION OF LIGHT ON GROWING ORGANS.
Method of experiment--Normal effect of light--Determination of
the latent period--Effect of intensity of light--Effect of
continuous light--Effects of different rays of the spectrum 205
XVII.--EFFECT OF INDIRECT STIMULUS ON GROWTH.
Mechanical and electrical response to Indirect
Stimulus--Variation of growth under Indirect
Stimulus--Effects of Direct and Indirect Stimulus 213
XVIII.--RESPONSE OF GROWING ORGANS IN STATE OF SUB-TONICITY.
Theory of assimilation and dissimilation--Unmasking of
positive effect--Modification of response under artificial
depression of tonic condition--Positive response in
sub-tonic specimen--Abnormal acceleration of growth
under stimulus--Continuity between abnormal and normal
responses--Positive response to sub-minimal stimulus 219
XIX.--RESUMPTION OF AUTONOMOUS PULSATION AND OF GROWTH UNDER
STIMULUS.
Resumption of pulsatory activity of _Desmodium_ leaflet at
standstill--Renewal of growth under stimulus--General laws of
effects of Direct and Indirect Stimulus 227
XX.--ACTION OF LIGHT AND WARMTH ON AUTONOMOUS ACTIVITY.
The Oscillating Recorder--Record of pulsation of _Desmodium
gyrans_--Effect of diffuse light in diminution of amplitude
and reduction of diastolic limit of pulsation--Antagonistic
action of warmth in reduction of systolic limit 233
XXI.--A COMPARISON OF RESPONSES IN GROWING AND NON-GROWING
ORGANS.
Contractile response of growing and non-growing
organs--Time-relations of mechanical response of pulvinated
and growing organs--Similar modification of response under
condition of sub-tonicity--Opposite effects of Direct and
Indirect stimulus--Exhibition of negative electric response
under Direct, and positive electric response under Indirect
stimulus--Similar modification of autonomous activity in
_Desmodium gyrans_ and in growing organs under parallel
conditions--Similar excitatory effects of various stimuli
on pulvinated and growing organs--Similar discriminative
excitatory effects of various rays in excitation of motile
and growing organs--Action of white light--Action of
red and yellow lights--Action of blue light--Action of
ultra-violet rays--Action of infra-red rays--Diverse modes
of response to stimulus--Mechanical response--Electromotive
response--Response by variation of electric resistance 239
ILLUSTRATIONS.
FIGURE. PAGE.
1. Photographs of morning and evening positions of the
‘Praying Palm’ 7
2. The Recording Apparatus 9
3. Record of diurnal movement of the ‘Praying Palm’ 11
4. " " Sijbaria Palm 12
5. Curve of variation of moto-excitability in _Mimosa pudica_ 17
6. Effect of physiological depression on diurnal movement of
_Arenga saccharifera_ 19
7. Record of diurnal movements of young procumbent stem of
_Mimosa pudica_ 26
8. Erectile response of _Basella_ to gradual fall of
temperature 28
9. Responsive fall of _Basella_ to gradual rise of temperature "
10. Response of a straight tendril of _Passiflora_ 33
11. Response of a hooked tendril of _Passiflora_ 35
12. Response of pulvinus of _Mimosa pudica_ 36
13. " " _Mimosa_ to variations of turgor 40
14. Diagram of complete apparatus for record of diurnal
variation 46
15. The Oscillator 50
16. Effect of cloud on excitability of _Mimosa_ 52
17. Effect of sudden darkness 53
18. Effect of change from darkness to light 54
19. Effect of enhanced turgor 55
20. Effect of moderate cooling 56
21. Effect of application of intense cold 58
22. Effect of temperature above the optimum 58
23. Twenty-four hours’ record of excitability of _Mimosa_ 59
24. Midday record from noon to 3 P.M. 62
25. Evening record from 6 to 10 P.M. 63
26. Morning record from 8 A.M. to 12 noon 64
27. Diurnal variation of excitability showing marked
nyctitropic movement 65
28. Diurnal curves of temperature and of corresponding
variation of excitability of _Mimosa_ 68
29. Diurnal variation of excitability of a summer specimen 70
30. The Resonant Recorder 76
31. Variation of excitability after section 80
32. Effect of amputation of upper half of pulvinus of _Mimosa_ 84
33. Response of _Mimosa_ after amputation of lower half of
pulvinus 86
34. Effect of weight on rapidity of fall 87
35. Stimulating action of Hydrogen peroxide 88
36. Incomplete recovery under the action of BaCl_{2} and
transient restoration under tetanisation 89
37. Antagonistic action of alkali and acid 90
38. Fatigue due to shortening of recovery-period 91
39. Effect of constant current in removal of fatigue 92
40. Stimulating action of light and depressing action of
darkness 94
41. Action of glycerine in enhancing speed and intensity of
transmitted excitation in _Mimosa_ 102
42. Effect of injury in depressing conductivity in normal
specimen 104
43. Effect of injury in enhancing conductivity in a subtonic
specimen 105
44. Diagram of experimental arrangement for conductivity
control in _Averrhoa bilimbi_ 109
45. Diagram of complete experimental arrangement for
conductivity control in _Mimosa pudica_ 116
46. Record showing enhanced velocity in ‘up-hill’ and retarded
velocity in ‘down-hill’ transmission 121
47. Direct and after-effect of heterodromous and homodromous
currents 124
48. Diagram of experimental arrangement for variation of
conductivity of animal nerve 126
49. Effect of heterodromous and homodromous current in inducing
variation of conductivity in nerve 127
50. Record of ineffectively transmitted salt-tetanus becoming
effective under heterodromous current 129
51. Direct and after-effect of homodromous current 131
52. Effect of indirect electric stimulus on the responding
leaflet of _Averrhoa_ 136
53. Staircase responses of sub-tonic specimen of _Mimosa_ to
electric shock 145
54. Staircase responses of sub-tonic specimen of _Mimosa_ to
light 147
55. Positive, diphasic, and negative responses of extremely
sub-tonic specimen of _Mimosa_ to successive light stimuli 147
56. The compound Lever 154
57. The crank arrangement for oscillation 156
58. Photograph of the High Magnification Crescograph 157
59. Crescographic record of absolute rate of growth of
_Kysoor_, and of effects of cold and warmth on stationary
and moving plates 161
60. Record of physical change 164
61. Records of latent period and time relations of growth
response 165
62. Record of a single growth-pulse of _Zephyranthes_ 167
63. Records of growth-rate at different temperatures 175
64. Continuous record of growth, showing temperature minimum 178
65. Continuous record of growth, showing temperature maximum "
66. The Thermo-Crescent Curve 180
67. Curve showing the relation between growth and temperature 181
68. Effects of H_{2}O_{2}, NH_{3}, and ether on growth 184
69. Effect of CO_{2} on growth 185
70. Effect of irrigation on growth 189
71. Effect of plasmolysis on growth 191
72. Effect of increasing intensity of electric stimulus on
growth 196
73. Effect of continuous electric stimulation on growth 197
74. Immediate and after-effects of friction, and of wound on
growth 200
75. Normal retarding effect of light on growth 206
76. Record showing latent period of growth in response to light 207
77. Effect of light of increasing intensities 208
78. Continuous effect of light and of electric stimulus on
growth 209
79. Effects of different rays of the spectrum on growth 210
80. Photographic records of positive, diphasic and negative
electric responses of petiole of _Musa_ 214
81. Record of growth variation of _Crinum_ under Direct and
Indirect stimulus 216
82. Effect of electric stimulus on sub-tonic specimen of wheat
seedling 221
83. Acceleration of growth under sub-minimal stimulus of light 224
84. Revival by stimulus of light of autonomous pulsations of
_Desmodium gyrans_ at stand still 228
85. Renewal of growth in the mature style of a flower by the
action of stimulus 229
86. Effect of light in diminution of amplitude and reduction of
diastolic limit of pulsation of _Desmodium_ 236
87. Antagonistic effect of warmth in reduction of systolic
limit 237
88. Contractile response of a growing bud of _Crinum_ 241
89. Response of _Mimosa_ pulvinus to white light 245
90. Response of _Mimosa_ pulvinus to blue light 246
91. Response of _Mimosa_ pulvinus to ultra-violet rays 247
92. Response of _Mimosa_ pulvinus to thermal radiation 248
I.--THE PROBLEM OF MOVEMENT IN PLANTS
_By_
PROF. SIR J. C. BOSE.
The phenomenon of movement in plants under the action of external
stimuli presents innumerable difficulties and complications. The
responding organs are very different: they may be the pulvini of the
‘sensitive’ or those of the less excitable leguminous plants; the
petioles of leaves, which often act as pulvinoids; and organs of plants
in a state of active growth.
Taking first the case of the pulvinus of _Mimosa_, we find that it
responds to mechanical stimulation, to constant electric current, to
induction shock, to the action of chemical agents, to light, and to
warmth as differentiated from thermal radiation. The reactions induced
by these agents may be similar or dissimilar. An identical agent,
again, may give rise to movements which are not merely different, but
sometimes even of diametrically opposite characters. Certain organs,
for example, direct themselves towards light, others away from it.
Some plants close their leaflets on the approach of darkness, in the
so-called position of ‘sleep’; apparently similar ‘sleep’ movement is
induced in others by the action of the midday sun.
In _Mimosa_, the responsive movement is brought about by a sudden
diminution of turgor in the pulvinus. But very little is definitely
known about the responsive reaction in growing organs. Thus in a
tendril, one-sided contraction causes a shortening of the concave side
and a sudden increase of growth on the convex. No explanation of this
difference has hitherto been forthcoming. Under the action of light
of different intensities a growing organ may approach the source of
light, or place itself at right angles or move away from it. Again
under the identical stimulus of gravity, the root moves downwards,
and the shoot upwards. The sign of response in different organs thus
changes, apparently without any reason. It is thus seen, that there
is hardly any responsive movement that has been observed of which an
example directly to the contrary may not be found. For this reason it
has appeared hopeless to unify these very diverse phenomena, and there
has been a tendency towards a belief that it was not any definite
physiological reaction, but the individuality of the plant that
determines the choice of its movement.
The complexities which baffle us may, however, arise from the
combination of factors whose individual reactions are unknown to us. I
shall show, for example, how the movement of a pulvinus under a given
stimulus is determined by the point of application, direct stimulus
producing one effect, and indirect the diametrically opposite. The
normal reaction is again modified by the tonic condition of the plant.
There is again the likelihood of the presence of other modifying
factors. It is clear how very different the results would become by the
permutation and combination of these diverse factors.
For a comprehensive study of the phenomenon of plant movement,
it is therefore necessary to investigate in detail the effect
of a given stimulus under definite changes of the environmental
condition. With regard to a given stimulus we have to determine the
effects of intensity, of duration, and of the point of application.
The investigation has to include the effects exhibited not merely
by the pulvinated but also by growing organs. As a result of such
a comprehensive study, it may perhaps be possible to discover some
fundamental reaction operative in bringing about the responsive
movement in all plant organs.
I shall in the course of the following series of Papers, describe the
different apparatus by which the movement of pulvinated organ and its
time-relations are automatically recorded. In a growing organ the
induced movement under stimulus is brought about by the change in
its rate of growth. That the change is solely due to the particular
stimulus can only be assured by strict maintenance of constancy of
external conditions, during the period of experiment; this constancy
can, in practice, be secured only for a short time. The necessity
for shortening the period of experiment also arises from a different
consideration; for numerous and varied are the stimulating and
mechanical interactions between neighbouring organs. These effects,
however, come into play after a certain lapse of time. They may be
eliminated by reduction of the period of experiment.
In order to shorten the period of experiment for the study of growth
movements, the rate of growth has to be very highly magnified, so as
to determine the absolute rate and its variations in the course of a
minute or so. I shall in a subsequent Paper give full account of an
apparatus I have been able to devise, by which it is possible to record
automatically the rate of growth magnified many thousand times.
I stated that anomalies of plant movements would disappear, if we
succeeded in carrying out in detail investigations of effects of the
different individual factors in operation. In illustration of this
I shall, in the first Paper of the series, give an account of the
mysterious movement of the ‘Praying’ Palm of Faridpur, and describe the
investigations by which the problem found its solution.
II.--THE “PRAYING” PALM TREE
_By_
SIR J. BOSE,
_Assisted by_
NARENDRA NATH NEOGI, M.Sc.
Perhaps no phenomenon is so remarkable and shrouded with greater
mystery as the performances of a particular Date Palm near Faridpur
in Bengal. In the evening, while the temple bells ring calling upon
people to prayer, this tree bows down as if to prostrate itself. It
erects its head again in the morning, and this process is repeated
every day of the year. This extraordinary phenomenon has been regarded
as miraculous, and pilgrims have been attracted in large numbers.
It is alleged that offerings made to the tree have been the means
of effecting marvellous cures. It is not necessary to pronounce any
opinion on the subject; these cures may be taken as effective as other
faith-cures now prevalent in the West.
This particular Date Palm, _Phœnix dactylifera_, is a full-grown rigid
tree, its trunk being 5 metres in length and 25 cm. in diameter. It
must have been displaced by storm from the vertical and is now at an
inclination of about 60° to the vertical. In consequence of the diurnal
movement, the trunk throughout its entire length is erected in the
morning, and depressed in the afternoon. The highest point of the trunk
thus moves up and down through one metre; the ‘neck,’ above the trunk,
is concave to the sky in the morning; in the afternoon the curvature
disappears, or is even slightly reversed. The large leaves which point
high up against the sky in the morning are thus swung round in the
afternoon through a vertical distance of about five metres. To the
popular imagination the tree appears like a living giant, more than
twice the height of a human being, which leans forward in the evening
from its towering height and bends its neck till the crown of leaves
press against the ground in an apparent attitude of devotion (Fig. 1).
Two vertical stakes, each one metre high, give a general idea of the
size of the tree and movements of the different parts of the trunk.
[Illustration: Fig. 1. The Faridpur ‘Praying’ Palm; the upper
photograph shows position in the morning; the lower, position in the
afternoon. The two fixed stakes are one metre in height. In front is
seen erect trunk of a different Palm.]
For an investigation in elucidation of this phenomenon it was
necessary:--
1. To obtain an accurate record of the movement of the tree day and
night, and determine the time of its maximum erection and fall.
2. To find whether this particular instance of movement was unique,
or whether the phenomenon was universal.
3. To discover the cause of the periodic movement of the tree.
4. To find the reason of the remarkable similarity between the
diurnal movement of the tree, and the diurnal variation of
moto-excitability in _Mimosa pudica_.
5. To determine the relative effects of light and temperature on
the movement.
6. To demonstrate the physiological character of the movement of
the tree.
7. To discover the physiological factor whose variation determines
the directive movement.
THE RECORDING APPARATUS.
I shall now describe the principle and construction of my recording
apparatus (Fig. 2) seen attached to a horizontally growing stem of
_Mimosa pudica_. When used to trace the movement of the palm tree,
a reducing device is employed to keep the record within the plate. A
lever, R′, records the movement of the attached tree or plant on
a moving plate of smoked glass. The plate is not in contact with the
tip of the recording lever, but separated from it by a distance of
about 3 mm. A special oscillating device, actuated by clock-work,
C, makes the plate move forwards and backwards. The forward movement
brings about a momentary contact of the recording tip with the smoked
plate inscribing a dot. These single dots are made at intervals of
15 minutes; at the expiration of the hour, however, contact is made
three times in rapid succession, printing a thick dot. It is thus
easy to determine the movement of the tree at all times of the day
and night. A second lever, R, placed above, gives on the same plate,
thermographic record of the diurnal variation of temperature. For
this I use a differential thermometer, T, made of a compound strip of
brass and steel. Curvature is induced by the differential expansion
of the two pieces of metal. The up or down movement of the free end
of the compound strip is further magnified by the recording lever.
This arrangement was extremely sensitive and gave accurate record of
variation of temperature. By the forward movement of the oscillating
plate two dots are made at the same time,--one for the temperature
and the other for the corresponding movement of the tree. As the two
recorders do not move vertically up or down, but describe a circle,
the dots vertically one above the other may not correspond as regards
time. Any possibility of error in calculation is obviated by the fact
that the thick dots in both the records are made every hour, and the
subsequent thin dots at intervals of 15 minutes.
[Illustration: Fig. 2. Apparatus for automatic record of movement of
trees and plants; T, differential metallic thermometer; R, recording
lever for temperature; R′, for recording plant movement; C,
clock-work for oscillation of recording plate. The same clock-work
moves plate laterally in 24 hours.]
A difficulty arose at the beginning in obtaining sanction of the
proprietor to attach the recorder to the tree. He was apprehensive
that its miraculous power might disappear by profane contact with
foreign-looking instruments. His misgivings were removed on the
assurance that the instrument was made in my laboratory in India, and
that it would be attached to the tree by one of my assistants, who was
the son of a priest.
From results of observation it is found that the tree moves through
its entire length; the fall of the highest point of the trunk is one
metre. The movement is not passive, but an active force is exerted;
the force necessary to counteract this movement is equivalent to the
weight of 47 kilograms: in other words, the force is sufficient to
lift a man off the ground. But far greater force would be required to
restrain the change of curvature of the neck of the hard and rigid tree.
[Illustration: Fig. 3. Record of diurnal movement of the ‘Praying’ Palm
(_Phœnix dactylifera_). Thermographic curve for 24 hours commencing
at 9 in the evening is given in the upper record; the corresponding
diurnal curve of movement of the tree is given in the lower. Successive
dots at intervals of 15 minutes; thick dots at intervals of an hour.]
Before entering into the investigation of the cause of periodic
movement I shall give a general account of its characteristics. A
casual observation would lead one to conclude that the tree lifted
itself at sunrise and prostrated at sunset. But continuous record
obtained with my recorder attached to the upper part of the trunk shows
that the tree was never at rest, but in a state of continuous movement,
which underwent periodic reversals (Fig. 3). The tree attained its
maximum erection at 7 in the morning, after which there is a rapid
movement of fall. The down movement reached its maximum at 3-15 P.M.,
after which it was reversed and the tree erected itself to its greatest
height at 7 next morning. This diurnal periodicity was maintained day
after day.
UNIVERSALITY OF TREE MOVEMENT.
The next question which I wished to investigate was whether the
movement of the particular Faridpur tree was a unique phenomenon.
It appeared more likely that similar movement would, under careful
observation, be detected in all trees. The particular palm tree was
growing at a considerable inclination to the vertical; the movement
of the tree and its leaves became easily noticeable, since the ground
afforded a fixed and striking object of reference. In a tree growing
more or less erect, the movement, if any, would escape notice, since
such movements would be executed with only the empty space as the
background.
[Illustration: Fig. 4. Record of the Sijbaria Palm from noon for 24
hours. Successive dots, at intervals of 15 minutes.]
_Experiment 1._--Believing the phenomenon to be universal I
experimented with a different Date Palm that was growing at my research
station at Sijbaria on the Ganges, situated at a distance of about 200
miles from Faridpur. The surrounding conditions were very different.
The tree was much younger; it was 2 metres in height and inclined
20° to the vertical. The curve obtained with this tree (Fig. 4) was
very similar to that of the Faridpur Palm, though this extent to the
movement was much reduced. The tree attained the highest erect position
at 7-15 A.M. and the lowest at 3-45 P.M. Hence the movement of the
Faridpur Palm is not a solitary phenomenon.
THE CAUSE OF PERIODIC MOVEMENT.
The recurrent daily movement of the tree must be due to some diurnal
changes in the environment,--either the recurrent changes of light
and darkness, or the diurnal changes of temperature. These changes
synchronise to a certain extent; for, as the sun rises, light appears
and the temperature begins to rise. It is therefore difficult to
discriminate the effect of light from that of temperature. The only
satisfactory method of discrimination would have been in the erection
of a large structure with screens to cut off light. The effect
of fluctuation of temperature under constant darkness would have
demonstrated the effect of one agent without complication arising from
the other. Unfortunately screening the tree was impracticable. I shall
presently describe other experiments where the action of light was
completely excluded.
The curve of movement of the tree, however, affords us material
for correct inference as regards the relative effects of light and
temperature. The experiment was commenced in March; light appeared
at about 5 A.M., the sunrise being at 6-15 A.M.; the sun set at 6-15
P.M., and it became dark by 7 P.M. The incident light would be the
most intense at about noon; after this it would decline continuously
till night time. If the movement was due to light, its climax, either
in up or down movement, would be reached at or about noon, and the
opposite climax at midnight. But instead of this we find (Fig. 3)
the up-movement reaching its highest point not at noon, but at 7 in
the morning; after this the fall is rapid and continuous, and the
lowest position was reached not in the evening but at 3-15 P.M. The
fluctuation of light has, therefore, little to do with the movement of
the tree.
Turning next to the element of variation of temperature we are at
once struck by the fact that the curve of movement of the tree is
practically a replica of the thermographic curve (Fig. 3). The _fall_
of temperature is seen to induce a _rise_ in the tree and _vice versâ_.
There is a lag in the turning points of the two curves; thus while
temperature began to rise at 6 A.M., the tree did not begin to fall
till 7 A.M. There is in this case a lag of an hour; but the _latent_
period may, sometimes, be as long as three hours. The delay is due
to two reasons; it must take some time for the thick trunk of the
tree to attain the temperature of the surrounding, and secondly, the
physiological inertia will delay the reaction. As a result of other
investigations, I find that the induced effect always lags behind the
inducing cause. It is interesting in this connection to draw attention
to the parallel phenomenon, which is described below, of lag in the
variation of sensibility of _Mimosa_ in response to variation of
temperature. In this case the lag was found to be about three hours.
Returning to the Palm, the tree continues to fall in the forenoon with
rising temperature. At about 2-30 P.M. the temperature was at its
maximum after which it began to decline; the movement of the tree was
not reversed into erection till after 3-15 P.M., the lag being now 45
minutes nearly.
I may state here that the movement of the tree is not primarily
affected by the periodicity of day and night, but by variation of
temperature. In spring and in early summer the rise of temperature
during the early part of the day and the fall of the temperature
from afternoon to next morning, are regular and continuous; the
corresponding movements of the tree are also regular. But at other
seasons, owing to the sudden change of direction of the wind, the
fluctuations of temperature are irregular. Thus at night there may
be a sudden rise, and in the earlier part of the day sudden fall of
temperature. And the record of movement of the tree is found to follow
these fluctuations with astonishing fidelity, the rise of temperature
being followed by a fall of the tree and _vice versâ_. That the
movement is determined by the temperature variation is exhibited in a
striking manner in Fig. 4, where, between 8 and 9 A.M., a common twitch
will be noticed in the two curves.
While trying to obtain some clue to the mysterious movement of
the tree, my attention was strongly attracted by certain striking
similarities which the record of the movement of the tree showed to the
curve of the diurnal variation of moto-excitability, of the pulvinus
of _Mimosa pudica_, an account of which will be found in a subsequent
Paper of the series.[A]
[A] _See_ also Bose--Diurnal Variation of Moto-Excitability in
_Mimosa_--Annals of Botany, Vol. XXVII, No. CVIII, October, 1918.
PERIODIC MOVEMENT OF TREES AND DIURNAL VARIATION OF MOTO-EXCITABILITY
IN _MIMOSA PUDICA_.
The excitability of the main pulvinus of _Mimosa pudica_ I find does
not remain constant during the 24 hours, but undergoes a striking
periodic change. At certain hours of the day, the excitability is at
its maximum; at a different period it practically disappears. The
period of insensibility is about 7 A.M., which, strangely enough, is
also the time when the palm tree attains its maximum height. At about
3 in the afternoon the excitability of _Mimosa_ reaches its climax,
and this is the time when the head of the palm tree bends down to its
lowest position. For the determination of the periodic variation of
excitability of _Mimosa_ I devised a special apparatus by which an
electric stimulus of constant intensity was automatically applied
to the plant every hour of the day and night, the responsive moment
being recorded at the same time. The amplitude of responsive fall of
leaf under uniform stimulus gave a measure of excitability of the leaf
at any particular moment. In the lower curve of Fig. 5 is given the
record of diurnal variation of excitability of _Mimosa_. Comparison of
this figure with Figs. 3 and 4, will show the remarkable resemblance
between the curves of diurnal movement of the Palm tree, and of
diurnal variation of moto-excitability of _Mimosa_. The excitability
of _Mimosa_ reached its maximum at about 3 in the afternoon, when
the Palm was at its lowest position. After this hour excitability
fell continuously till 7 or 8 next morning. Corresponding to this is
the continuous erection of the Palm from its lowest position at 3
P.M. to the highest between 7 and 8 A.M. Still more remarkable is the
modifying influence of variation of temperature on the diurnal curve
of excitability in _Mimosa_, and the diurnal curve of movement of the
Palm. This will be quite evident from the inspection of the temperature
curves in Figs. 4 and 5.
[Illustration: Fig. 5. Curve of variation of moto-excitability of
_Mimosa pudica_. The upper curve gives variation of temperature and the
lower, the corresponding variation of excitability.]
I have shown elsewhere[B] that the variation of moto-excitability of
the pulvinus of _Mimosa_ is a physiological function of temperature.
The remarkable similarity between the diurnal variation of
moto-excitability of _Mimosa_ and diurnal movement of the Palm is
due to the fact that both are determined by the physiological action
of temperature. I shall presently describe experiments, which will
establish the physiological character of the movement of the tree in
response to changes of temperature.
[B] Bose--“Irritability of Plants,” d. 60.
The records that have been given show that it is the diurnal variation
of temperature, and not of light that is effective in inducing the
periodic movement of the tree. Further experiments will be given in
support of this conclusion.
RELATIVE EFFECTS OF LIGHT AND TEMPERATURE.
As regards the possibility of light exerting any marked influence
on the movement of the Palm tree, I have shown from study of
time-relations of the movement, that this could not be the case.
Moreover, it is impossible for light to reach the living tissue through
the thick layer of bark that surrounds the tree. That the effect
of light is negligible will appear from the accounts of following
experiments, where the possibility of the effect of changing intensity
of light is excluded by maintaining the plant in constant darkness, or
in constant light.
The employment of the large Palm was obviously impracticable in these
investigations. I, therefore, searched for other plant-organs in which
the movement under variation of temperature was similar to that of the
Date Palm. I found that the horizontally spread leaves of vigorous
specimens of _Arenga saccharifera_ growing in a flower pot executed
movements which were practically the same as that of the Faridpur tree.
The leaf moved downwards with rise of temperature and _vice versâ_.
There are many practical advantages in working with a small specimen.
It can easily be placed under glass cover or taken to a glass house,
thus completely eliminating the troublesome disturbance caused by the
wind.
_Diurnal movement in continued darkness: Experiment 2._--The plant was
placed in a dark room and records taken continuously for three days.
These did not differ in any way from the normal records taken in a
glass house under daily variation of light and darkness. Exposure of
plant to darkness for the very prolonged period of a week or more,
undoubtedly interferes with the healthy photo-tonic condition of the
plant. But such unhealthy condition did not make its appearance in the
first few days.
PHYSIOLOGICAL CHARACTER OF THE MOVEMENT.
There may be a misgiving that the movement of the tree might be due to
physical effect of temperature. If the upper strip of a differential
thermometer be made of the more expansible brass and the lower of
iron, the compound strip bends down with the rise of temperature.
Similarly the movement of the tree might be due to the upper half being
physically more expansible. It would have been possible to discriminate
the physical from the physiological action by causing the death of
the tree; in that case physical movement would have persisted, while
the physiological action would have disappeared. As this test was not
practicable, I tried the effect of physiological depression on the
periodic movement of the leaf of _Arenga saccharifera_.
_Effect of Drought: Experiment 3._--In Fig. 6 is given a series of
records of movement of the leaf-stalk of _Arenga_, first under normal
condition, afterwards under increasing drought, brought about by
withholding water. The uppermost is the thermographic record which
remained practically the same for successive days. Below this are
records of movement of the leaf (_a_) under normal condition, (_b_)
after withholding water for three days, and (_c_) after deprivation for
seven days. It will be noticed how the extent of movement is diminished
under increasing physiological depression brought on by drought. On
the seventh day, the responsive movement disappeared, there being
now a mere fall of the leaf, which was slow and continuous. After
this I supplied the plant with water and the periodic movement was in
consequence nearly restored to its original vigour.
_Effect of poison: Experiment 4._--In another experiment the normal
diurnal record with the leaf was taken and the plant was afterwards
killed by application of poisonous solution of potassium cyanide. The
diurnal movement was found permanently abolished at the death of the
plant.
[Illustration: Fig. 6. Effect of physiological depression on diurnal
movement of the petiole of _Arenga saccharifera_. The uppermost curve
exhibits variation of temperature, (_a_), normal diurnal curve, (_b_),
modification after 3 days’ and (_c_) after 7 days’ withholding of
water.]
These experiments conclusively prove that the periodic movement of
the leaf-stalk induced by variation of temperature is a physiological
phenomenon, and from analogy we are justified in drawing the inference
that the movement of the Faridpur tree is also physiological. The
question, however, was finally settled by the unfortunate death of
the tree which occurred the other day, nearly a year after I commenced
my investigations. While presiding at my lecture on the subject, His
Excellency Lord Ronaldshay, the Governor of Bengal, announced that a
telegram had just reached him from his officer at Faridpur that “the
palm tree was dead, and that its movements had ceased.”
Since my investigation with the Faridpur ‘Praying’ Palm, I have
received information regarding other Palms, which exhibit movements
equally striking. One of the trees is growing by the side of a tank,
the trunk of the tree being inclined towards it. The up-lifted leaves
of this tree are swung round in the afternoon and dipped into the water
of the tank.
The movement of the tree has been shown to be brought about by the
physiological action of temperature variation; in other words the
diurnal movement of the ‘Praying’ Palm is a THERMONASTIC PHENOMENON. I
have found various creeping stems, branches and leaves of many trees,
exhibit this particular movement of fall with a rise of temperature,
and _vice versâ_. Such movements, I shall, for the sake of convenience,
distinguish as belonging to the _negative type_.
Having found that the temperature is the modifying cause, the next
point of inquiry relates to the discovery of the force, whose varying
effects under changing temperature induces the periodic movement. I
shall, in this connection, first discuss the various tentative theories
that may be advanced in explanation of the movement.
TRANSPIRATION AND DIURNAL MOVEMENT.
It may be thought that the fall of the tree during rise of temperature
may be due to passive yielding of the tree to its weight, there being
increased transpiration and general loss of turgor at high temperature.
I shall, however, show that the diurnal movement persists in the
absence of transpiration.
_Diurnal movement in absence of transpiration: Experiment 5._--In the
leaf of _Arenga saccharifera_, I found that the petiole was the organ
of movement. I cut off the transpiring lamina and covered the cut end
with collodion flexile. The plant was now placed in a chamber saturated
with moisture. The petiole continued to give records of its diurnal
movement in every way similar to the record of the intact leaf. In
another experiment with the water plant, _Ipoemia reptans_, immersed in
water, the normal diurnal movement was given by the plant, where there
could be no question of variation of turgor due to transpiration. (See
also _Expt._ 7.)
In the diurnal movement of the ‘Praying’ Palm the concave curvature of
the rigid neck in the morning, became flattened or slightly convex in
the afternoon. The force necessary to cause this is enormously great,
and could on no account result from the passive yielding to the weight
of the upper part of the tree.
From the facts given above it will be seen that the diurnal movement is
not brought about by variation in transpiration. I now turn to another
phenomenon which appeared at first to have some connection with the
movement of the tree. Kraus found that the tissue tensions of a shoot
exhibit a daily periodicity. He, however, found that between 10°C. and
30°C., variation of temperature had no effect on the daily period. But
as regards the diurnal movement of the tree, it is the temperature
which is the principal factor. Kraus also found a daily variation of
bulk in different plant-organs; this variation of bulk is connected
with transpiration, for the removal of the transpiring leaves arrested
this variation. But the periodic movement of the tree, as we have seen,
is independent of transpiration.
Millardet observed a daily periodicity of tension in _Mimosa pudica_.
He found that maximum tension occurs before dawn; the petiole becomes
erected, the movement being upwards or towards the tip of the stem.
Tension decreases during the day, and reaches a minimum early in the
evening; in correspondence with this is the fall of the petiole, the
movement being away from the tip of the stem.[C] If the plant were
placed upside down the periodic movement of the petiole in relation to
the stem will evidently remain the same, but become reversed in space.
Maximum tension in the morning will make the petiole approach the
tip of the stem, _i.e._, the movement will be _downwards_ instead of
upwards as in the normal position. The experiment described below will
show that the diurnal movement induced by variation of temperature is
not reversed by placing the plant in an inverted position.
[C] Vines.--‘Physiology of Plants,’ 1886, pp. 405 and 543.
_Diurnal movement in inverted position: Experiment 6._--I took a
vigorous specimen of _Arenga saccharifera_ growing in a pot, and took
its normal record, which as explained before exhibited down-movement
during rise, and an up-movement during fall of temperature. The plant
was now held inverted, the upper side of the petiole now facing the
earth. The diurnal curve of movement should now show an inversion, if
that movement was solely determined by the anisotropy of the organ.
But the record did not exhibit any such inversion. After being placed
upside down, the leaf did not, on the first day, show any diurnal
movement; there was, on the other hand, a continuous down-movement on
account of the fall of the leaf by its own weight. But in the course
of 24 hours the leaf readjusted itself to its unaccustomed position,
and became somewhat erected under the action of geotropic stimulus.
After the attainment of this new state of geotropic equilibrium, the
leaf gave a very pronounced record of its diurnal movement which did
not show any reversal; the inverted leaf continued to exhibit the
same characteristic movements as in the normal position, that is to
say, a down movement during rise, and an up-movement during fall of
temperature. As the plant in the inverted position did not show any
reversal of the periodic curve, it is clear that the diurnal movement
is determined by the modifying influence of temperature on the
physiological reaction of the plant to some external stimulus which is
constant in direction. I shall presently show that it is the constant
geotropic stimulus modified by the action of temperature, which
determines the diurnal movement of the tree.
This will be better understood if I refer once more to certain
characteristics in the movement of the “Praying” Palm. The neck of
the tree was seen to be concave in the morning. The physiological
effect of raising temperature is virtually to oppose or neutralise the
geotropic curvature as seen in the flattening or slight reversal of
curvature in the afternoon. Similarly, various plant organs, growing at
an inclination to the vertical, are subjected to geotropic action, and
thus assume different characteristic angles. This state of equilibrium
is not static but may better be described as dynamic; for it will be
shown that this state of geotropic balance is upset in a definite way,
by variation of temperature.
That geotropism is an important factor in the diurnal movement is
supported by the fact that the Sijbaria Palm with an inclination of 20°
to the vertical exhibited a daily movement which was only moderate
in extent. But the Faridpur Palm growing at an inclination of 60° was
subjected more effectively to geotropic action, and exhibited movements
which were far more pronounced. I shall now proceed to describe crucial
experiments which will demonstrate the effect of change of temperature
on geotropic curvature.
EFFECT OF VARIATION OF TEMPERATURE ON GEOTROPIC CURVATURE.
In the instances of diurnal movement already described the trees or
their leaves were already at an inclination to the vertical. I now took
a radial and erect shoot of _Basella cordifolia_ growing in a pot and
laid it horizontally for two weeks. The procumbent stem curved up and
attained a state of equilibrium under the action of geotropic stimulus.
_Diurnal curve of_ Basella cordifolia: _Experiment 7._--The plant
was completely immersed in a vessel of water, and its diurnal curve
recorded. This resembled in all essentials the diurnal curve of the
Palm; the slight deviation was due to the fact that owing to difference
in the season (August) the temperature maximum was attained at 12-25
P.M. and the minimum at 6 A.M. The geotropic curvature was reduced to
its minimum at the maximum temperature, and _vice versâ_. As in the
case of the Palm so also in the procumbent stem of _Basella_ there was
a physiological lag, which was 50 minutes in the morning and about
the same in the afternoon. The free end of the stem thus exhibited a
diurnal movement up and down. The temperature, as stated before, began
to rise from 6 A.M. and the down-movement commenced 50 minutes later,
_i.e._, at 6-50 A.M. The temperature, after reaching the maximum, began
to fall at 12-25 P.M., and the previous movement of fall of the stem
was arrested and reversed into an erectile movement shortly after 1
P.M. There are thus two “turning points,” one at 7 A.M., and the other
at about 1 P.M.; at these periods the movement of the plant remains
more or less arrested for more than half-an-hour.
[Illustration: Fig. 7. Diurnal curve of movement of procumbent young
stem of _Mimosa pudica_. Successive dots at intervals of 15 minutes.]
I obtained records of similar diurnal movements with various procumbent
or creeping stems. Figure 7 gives the diurnal record of the procumbent
stem of a young specimen of _Mimosa pudica_.
The experiment that has just been described shows clearly that
geotropic curvatures of stems is opposed, or neutralised to a greater
or less extent, during rise of temperature, and this antagonistic
reaction is removed during the fall of temperature. The diurnal
movement of the plant completely immersed under water shows once more
that transpiration has little to do with the diurnal movement.
REVERSAL OF NATURAL RHYTHM.
The diurnal rhythm of up and down movement in the particular specimen
_Basella_ had become established under the daily variation of
temperature. I now attempted to reverse this rhythm by artificial
variation of temperature. The plant was placed in water in a
rectangular metallic vessel which was placed within a second outer
vessel. The plant could thus be subjected, without any mechanical
disturbance, to variation of temperature, by circulating warm or
cold water in the outer vessel. In order to reverse the natural
rhythm I subjected the plant to the action of falling temperature
at the “turning” point at 7 A.M., at a time when the plant would
have undergone a down-movement under the daily rise of temperature.
Conversely the plant was subjected to the action of rising temperature
at the second “turning” point at 1 P.M. when the movement under diurnal
fall of temperature would have been one of erection.
_Effect of fall of temperature: Experiment 8._--As stated before
the experiment was carried out in the morning; ice cold water was
circulated in the outer chamber, the fall of temperature was in this
case sudden, and there was an almost immediate responsive movement.
This appeared anomalous, since the latent period of response to slow
variation of temperature was found from the diurnal curve to be as long
as 50 minutes.
As a result of further investigations I found that variation of
temperature produces two different effects which may be distinguished
as transient and persistent. Sudden variation of temperature affects
the superficial tissue, and gives rise to a transient reaction,
while it takes a long time for temperature variation to react on the
geotropically active tissue in the interior. The persistent effect
therefore takes place after a latent period from one to three hours
according to the thickness of the plant.
[Illustration: Fig. 8. Reversal of normal rhythm: Erectile response
_Basella_ to gradual fall of temperature.]
[Illustration: Fig. 9. Responsive fall of _Basella_ to gradual rise of
temperature. (Dots at intervals of 5 minutes).]
The persistent effect of rise of temperature is a movement downwards,
that of fall of temperature is a movement upwards. These definite
reactions will be seen exhibited in Figs. 8 and 9. The plant was
stationary at the turning point in the morning hence the curve at first
was horizontal. The temperature was gradually lowered through 5°C.,
from 29°C., to 24°C. in the course of five minutes and maintained
at the lower temperature. There was no immediate effect, but after
a latent period of 65 minutes the plant responded by a movement of
erection. The natural movement at this period of the day would have
been one of fall, but artificial change of temperature in the opposite
direction effectively reversed the normal diurnal movement. The latent
period for this reverse movement is, as stated before, 65 minutes as
against 50 minutes in the normal diurnal movement. The increase in the
latent period is probably due to the added physiological inertia in
reversing the normal rhythm.
_Effect of rise of temperature: Experiment 9._--The temperature was
raised through 5°C at the second turning point, at 1 P.M. After a
latent period of 50 minutes the plant began to rise steadily (Fig. 9)
thus exhibiting once more the reversal of its normal diurnal movement.
From the experiments described above it will be seen that the movement
of the Palm, and of other organs growing at an inclination to the
vertical, is brought about by the action of temperature in modifying
the geotropic curvature. The ever present tendency of geotropic
movement is opposed or helped by the physiological reaction induced by
rise and fall of temperature respectively. The state of equilibrium is
never permanent, but the dynamic balance is being constantly readjusted
under changing conditions of the environment.
The movement of the tree furnishes an example of the _negative_ type of
THERMONASTIC MOVEMENT. Parallel phenomena are found in floral organs,
where, in the well-known instance of _Crocus_, the perianth leaves
open outwards during rise of temperature and close inwards during the
onset of cold. Looked at from above, the opening outwards during rise
of temperature is a movement downwards, and therefore belongs to the
_negative type_. In such cases the changed rate of growth by variation
of temperature is the most important factor in the movement. It may be
asked whether all thermonastic movements must necessarily belong to the
_negative_ type, where rise of temperature is attended by a movement
downwards. I shall in my Paper on “Thermonastic Phenomena” show that
there is also a _positive type_ where rise of temperature induces an
up-movement or of closure.
SUMMARY.
The ‘Praying’ Palm of Faridpur, growing at an inclination of about
60° to the vertical, exhibited a diurnal movement by which its head
became erected in the morning and depressed towards the afternoon, the
outspread leaves pressing against the ground.
The record of the diurnal movement showed that the head was erected to
the highest position between 7 and 8 in the morning, after which there
was a continuous fall which reached its climax at 3-15 P.M.; after this
the movement was reversed and the maximum erection was again reached
next morning.
This phenomenon is not unique, but is found exhibited, more or less, by
all trees and their branches and leaves.
Diurnal records of temperature, and movement of the tree showed, that
the two curves closely resembled each other. Rise of temperature was
attended by a fall of the tree, and _vice versâ_.
The movement is brought about by the physiological action of
temperature; it may be arrested by artificially induced physiological
depression, and is permanently abolished at death.
The movement is primarily determined by the modifying influence of
temperature on geotropic curvature. Rise of temperature is found to
oppose or neutralise geotropic curvature, the fall of temperature
inducing the opposite effect. The ever present tendency of upwards
geotropic movement is opposed or helped by the effects of rise and fall
of temperature respectively.
The movement of the ‘Praying’ Palm is a thermonastic phenomenon. The
tree, apparently so rigid, responds as a gigantic pulvinoid to the
changes of its environment.
III.--ACTION OF STIMULUS ON VEGETABLE TISSUES
_By_
SIR J. C. BOSE,
_Assisted by_
NARENDRA NATH SEN GUPTA.
The leaf of _Mimosa pudica_ undergoes a rapid fall when subjected
to any kind of shock. This plant has, therefore, been regarded as
“sensitive,” in contradistinction to ordinary plants which remain
apparently immobile under external stimulus. I shall, however, show
in course of this Paper that there is no justification in regarding
ordinary plants as insensitive.
Let us first take any radial organ of a plant and subject it to an
electric shock. It will be found that the organ undergoes a contraction
in length in response to the stimulus. On the cessation of excitation
the specimen gradually recovers its original length. Different organs
of plant may be employed for the experiment, for example, the tendril
of _Cucurbita_, the pistil of _Datura_, or the flower bud of _Crinum_.
The shortening may be observed by means of a low power microscope.
Greater importance is, however, attached to the detailed study of
response and its time relations. The pull exerted by a delicate
organ during its excitatory contraction is slight; hence arises the
necessity of devising a very sensitive apparatus, which would give
records magnified from ten to a hundred times.
RESPONSE RECORDERS.
The magnification of movement is produced by a light lever, the short
arm of which is attached to the plant organ, the long arm tracing the
record on a moving smoked plate of glass. The axis of the lever is
supported by jewel bearings. The principal difficulty in obtaining
accurate record of response of plant lies in the friction of contact of
the recording point against the glass surface. This difficulty I have
been able to overcome by providing a device of intermittent instead
of continuous contact. For this, either the writer is made to vibrate
to and fro, or the recording plate is made to oscillate backwards and
forwards.
1. _The Resonant Recorder._--In this the writing lever is made of a
fine steel wire. One end of this wire is supported at the centre of a
circular electromagnet; this latter is periodically magnetised by a
coercing vibrator, which completes an electric circuit ten hundred,
or two hundred times in a second. The writing lever is exactly tuned
to the vibrating interrupter and is thus thrown into sympathetic
vibration. Successive dots in the record thus measure time from 0.1
to 0.05 second. The employment of the Resonant Recorder enables us to
measure extremely short periods of time for the determination of the
latent period or the velocity of transmission of excitation.[D]
[D] For detailed description _cf._ Bose.--“An Automatic Method
for Investigation of Velocity of Transmission of Excitation in
_Mimosa_.”--Phil. Trans., B. vol. 204, (1913).
2. _The Magnetic Tapper._--Measurement of very short intervals is not
necessary in ordinary records of response. In this type of recorders,
the circular magnet is therefore excited at longer intervals, from
several seconds to several minutes; this is done by completion of the
electric circuit at the required intervals, by means of a key operated
by a clock.
3. _The Mechanical Tapper._--In this, magnetic tapping is discarded in
favour of mechanical tapping. The hinged writing lever is periodically
pressed against the recording plate by a long arm, actuated by
clock-work.
4. _The Oscillating Recorder._--Here the plate itself is made to
oscillate to-and-fro by eccentric worked by a clock. The frame carrying
the plate moves on ball-bearings. The advantage of the Oscillating
Recorder lies in the fact that a long lever, made of fine glass fibre,
or of aluminium wire, may be employed for giving high magnification. A
magnification of a hundred times may be easily obtained by making the
short arm 2.5 mm. and the long arm 25 cm. in length.[E]
[E] Bose--“Researches on Irritability of Plants,” p. 279--Longmans,
Green & Co.
RESPONSE OF A RADIAL ORGAN.
[Illustration: Fig. 10. Response of a straight tendril of _Passiflora_
to electric shock. Successive dots at intervals of 5 seconds. The
vertical lines below are at intervals of a minute. In this and in all
following records (unless stated to the contrary) up-curve represents
contraction, and down-curve expansion or recovery.]
_Experiment 10._--As a typical example I shall describe the response of
a straight tendril of _Passiflora_. A cut specimen was mounted with its
lower end in water. Suitable electric connections were made for sending
a feeble induction shock of short duration through the specimen.
In this and all other records, unless contrary be stated, up-curve
represents contractile movement. On application of stimulus of electric
shock, an excitatory movement of contraction occurred which shortly
reached its maximum; the apex-time was one minute and forty seconds,
and recovery was completed after a further period of five minutes
(Fig. 10). Stronger shocks induce greater contraction with prolongation
of the period of recovery. The specimen was afterwards killed by
application of poisonous solution of potassium cyanide; this brought
about a permanent abolition of response. The experiment just described
may be taken as typical of response of radial organs.
In a radial organ contraction takes place equally in all directions;
it therefore shortens in length, there being no movement in a lateral
plane. But if any agency renders one side less excitable than its
opposite, diffuse stimulation will then induce greater contraction on
the more excitable side which will therefore become concave.
RESPONSE OF AN ANISOTROPIC ORGAN.
Excessive stimulation is found to reduce the excitability of an organ.
Under unilateral mechanical stimulation a tendril of _Passiflora_
becomes hooked or coiled, the concave being the excited side. From what
has been said, the unexcited convex side will relatively be the more
excitable.
[Illustration: Fig. 11. Response of a hooked tendril of _Passiflora_ to
electric shock. Successive dots at intervals of 5 seconds.]
_Experiment 11._--I took a specimen of hooked tendril, and excited it
by an electric shock. The response was by the greater contraction of
the more excitable convex side, on account of which the curved specimen
tended to open out. The record of this response is seen in Fig. 11; the
apex-time was nearly two minutes, and the recovery was completed in the
further course of 15 minutes.
From the responses of organs rendered anisotropic by the differential
action of the environment we pass to others which show certain amount
of anatomical and physiological differentiation between their upper
and lower sides. I find that many petioles of leaves show movement in
response to stimulus. Many pulvini, generally regarded as insensitive,
are also found to exhibit responsive movements.
RESPONSE OF THE PULVINUS OF _MIMOSA PUDICA_.
[Illustration: Fig. 12. Response of the main pulvinus of _Mimosa
pudica_.]
The most striking and familiar example of response is afforded by the
main pulvinus of _Mimosa pudica_ of which a record is given in Fig. 12.
It is generally assumed that sensibility is confined to the lower half
of the organ. It will be shown in a subsequent Paper that this is not
the case. The upper half of the pulvinus is also sensitive though in a
feeble degree, its excitability being about 80 times less than that of
the lower half. On diffuse stimulation the predominant contraction of
the lower half causes the fall of the leaf, the antagonistic reaction
of the upper half being, in practice, negligible. In order to avoid
unnecessary repetition, I shall ignore the feeble antagonistic reaction
of the less excitable half of the organ, and shall use the word
‘contraction’ for ‘relatively greater contraction.’
It is interesting in this connection to refer to the response of the
leaf of Water Mimosa (_Neptunia oleracea_). Here the reaction is
very sluggish in comparison with that of _Mimosa pudica_. A tabular
statement of contractile response of various radial, anisotropic and
pulvinated organs will show a continuity in the contractile reaction;
the difference exhibited is a question of degree and not of kind.
TABLE 1--PERIODS OF MAXIMUM CONTRACTION AND OF RECOVERY OF DIFFERENT
PLANTS.
+-------------------------------------+-------------+------------+
| | Period of | Period of |
| Specimen | maximum | recovery. |
| | contraction | |
+-------------------------------------+-------------+------------+
| Radial organ: | | |
| Tendril of _Passiflora_ | 100 seconds | 4 minutes.|
| | | |
| Anisotropic organ: | | |
| Hooked tendril of _Passiflora_ | 120 " | 13 " |
| | | |
| Pulvinated organ: | | |
| Pulvinus of _Neptunia Oleracea_ | 180 " | 57 " |
| | | |
| Pulvinus of _Mimosa pudica_ | 3 " | 16 " |
+-------------------------------------+-------------+------------+
As regards the excitatory fall of the leaf of _Mimosa pudica_, Pfeffer
and Haberlandt are of opinion that this is due to the sudden diminution
of turgor in the excited lower half of the pulvinus. The weight of
the leaf, no longer supported by the distended lower cells, causes it
to fall. This is accentuated by the expansion of the upper half of
the pulvinus which is normally in a state of compression. According
to this view the excitatory fall of the leaf is a passive, rather
than an active, movement. I have, however, found that in determining
the rapidity of the fall of _Mimosa_ leaf the factors of expansive
force of the upper half of the pulvinus and the weight of the leaf are
negligible compared to the active force of contraction exerted by the
lower half of the pulvinus (p. 87).
With regard to the fall of turgor, it is not definitely known whether
excitation causes a sudden diminution in the osmotic strength of the
cell-sap or an increase in the permeability of the ectoplast to the
osmotic constituents of the cell. Pfeffer favours the former view,
while others support the theory of variation of permeability.[F]
[F] With reference to the fall of _Mimosa_ leaf Jost says: “When the
pressure of the cell decreases we naturally assume this to be due to a
decreasing _osmotic pressure_ due to alterations in the permeability
of the plasma, and an excretion of materials from the cell. It is a
remarkable fact that plasmolytic research (Hilburg 1881) affords no
evidence of any decrease in osmotic pressure. No complete insight
into the mechanism of the stimulus movement in _Mimosa_ has yet been
obtained, although one thing is certain, that there is a decrease in
the expansive power on the under side of the articulation.”--Jost,
“Plant Physiology”--English Translation, p. 515. Clarendon Press
(1907). Blackman and Paine think that the loss of turgor on excitation
“is probably due to the disappearance or inactivation of a considerable
portion of the osmotic substances of the cells.”--Annals of Botany,
Vol. XXXII, No. CXXXV, Jan. 1918.
RESPONSE OF PULVINUS OF _MIMOSA_ TO VARIATION OF TURGOR.
Whatever difference of opinion there may be in regard to the theories
of osmotic and permeability variations, we have the indubitable fact
of diminution of turgor and contractile fall of the pulvinus of
_Mimosa_ under excitation. The restoration of the original turgor
brings about recovery and erection of the leaf. In connection with this
the following experiments on responsive movements of the leaf under
artificial variation of turgor will be found of interest:--
_Effect of Increased Turgor: Experiment 12._--A young _Mimosa_ plant
was carefully transplanted and the root embedded in soil placed in a
linen bag. This was held securely by a clamp, and one of the leaves
of the plant attached to the recorder. Withholding of water for a day
caused a general loss of turgor of the plant. A vessel full of water
was now raised from below so that the linen bag containing the roots
was now in water. The effect of increased turgor by suction of water
by the roots became apparent by the _upward_ movement of the leaf. The
distance between the immersed portion of the plant and the leaf was
2 cm. and the up-movement of the leaf was indicated within 10 seconds
of application of water (Fig. 13). The velocity with which the effect
of increased turgor travelled was thus 2 mm. per second. The leaf
exhibited increasing erection with absorption of water.
[Illustration: Fig. 13. Response of _Mimosa_ pulvinus to variation
of turgor. Increased turgor by application of water at point marked
with vertical arrow induced erectile movement. Diminution of turgor by
application of KNO_{3} solution at the point marked with the horizontal
arrow, brought about the fall of the leaf within 80 seconds. Successive
dots at intervals of 5 seconds (The down curve represents up-movement
and _vice versâ_.)]
_Effect of Diminution of Turgor: Experiment 13._--While the leaf in the
above experiment was in process of erection, a quick change was made by
substituting KNO_3 solution for the water of the vessel in which the
roots were immersed. The plasmolytic withdrawal of water at the roots
gave rise to a wave of diminished turgor, the effect of which became
perceptible within 40 seconds by the movement of _fall_ of the leaf.
(Fig. 13.)
DIFFERENT MODES OF STIMULATION.
In _Mimosa_ excitation is manifested by the contraction of the pulvinus
and the consequent movement of the leaf. But in most plants, excitatory
movement cannot be realized on account of the rigidity of the plant
structure, the thickness of the cell-wall and the want of facility
for escape of water from the excited cells. I shall show later how
excitation may be detected in the absence of mechanical movement.
As regards stimulation of vegetable tissues, there are various agencies
besides electric shock, which induce excitatory contraction; these
agencies I shall designate as stimuli. Excitation is detected in
_Mimosa_ by the downward movement of the leaf. It will be found that
such excitatory movement is caused by a mechanical blow, by a prick or
a cut, by the application of certain chemical agents, by the action of
electric current and by the action of strong light. The study of the
action of these stimuli will be given in greater detail in subsequent
Papers.
I shall give below a general classification of different stimuli which
cause excitation in vegetable tissues.
_Electric Stimulus._--Induction shock, condenser discharge, the make of
kathode and the break of anode.
_Mechanical Stimulus._--Mechanical blow, friction, prick or cut.
_Chemical Stimulus._--Effect of certain acids and of other chemical
substances.
_Thermal Stimulus._--Sudden variation of temperature; application of
heated wire.
_Radiation Stimulus._--Luminous radiation of the more refrangible
portion of the spectrum; ultra-violet rays; thermal radiation in the
infra-red region.
All these different forms of stimulus induce an excitatory contraction,
a diminution of turgor, and a negative mechanical response or fall of a
motile leaf.
SUMMARY.
A radial organ responds to stimulus by contraction in length; as all
its flanks are equally excitable there is no lateral movement under
diffuse stimulus.
Physiological anisotropy is induced in an organ, originally radial and
isotropic, by the unequal action of the environment on its different
sides. Diffuse stimulus induces a greater contraction of the more
excitable side.
In a curved tendril the concave side is less excitable than the convex.
Diffuse stimulus tends to straighten the curved tendril.
In the pulvinus of _Mimosa pudica_, the lower half is eighty times
more excitable than the upper, and the fall of the leaf is due to the
predominant contraction of the more excitable lower half.
A diminution of turgor takes place in the excited cells. Restoration of
turgor brings about recovery of the leaf to its normal erect position.
Independent experiments show that the fall of the leaf may be brought
about by an artificial diminution of turgor, and the erection of the
leaf by an increase of turgor.
IV.--DIURNAL VARIATION OF MOTO-EXCITABILITY IN _MIMOSA_
BY
SIR J. C. BOSE.
Several phenomena of daily periodicity are known, but the relations
between the recurrent external changes and the resulting periodic
variations are more or less obscure. As an example of this may be
cited the periodic variation of growth. Here the daily periodicity
exhibited by a plant is not only different in varying seasons, but
it also differs in diverse species of plants. The complexity of the
problem is very great, for not only are the direct effects of the
changing environment to be taken into consideration but also their
unknown after-effects. Even in the case of direct effect, different
factors, such as light, temperature, turgor, and so on, are undergoing
independent variations; it may thus happen that their reactions may
sometimes be concordant and at other times discordant. The nyctitropic
movement of plants affords another example of daily periodicity. The
fanciful name of ‘sleep’ is often given to the closure of the leaflets
of certain plants at night. The question whether plants sleep or not
may be put in the form of the definite inquiry: Is the plant equally
excitable throughout day and night? If not, is there any definite
period at which it practically loses its excitability? Is there, again,
another period at which the plant wakes up, as it were, to a condition
of maximum excitability?
In the course of my investigations on the irritability of _Mimosa
pudica_, I became aware of the existence of such a daily periodicity;
that is to say, the moto-excitability of the pulvinus was found to
be markedly diminished or even completely abolished at a certain
definite period of the day; at another equally definite period, the
excitability was observed to have attained its climax. The observations
on the periodic variation of excitability appeared at first to be
extremely puzzling. It might be thought, for example, that light would
prove to be favourable for moto-excitability; in actual experiment
the results apparently contradicted such a supposition: for the
excitability of the plant was found much higher in the evening than
in the morning. Favourable temperature, again, might be regarded as
an important factor for the enhancement of the moto-excitability;
it was, nevertheless, found that though the excitatory response was
only moderate at that period of night when the temperature was at its
minimum, yet the excitability was altogether abolished at another
period when the temperature was several degrees higher. The obscurities
which surrounded the subject were only removed as a result of
protracted investigation and comparison of continuous automatic records
made by the plant itself during several months, beginning with winter
and ending in summer.
The question whether a plant like _Mimosa_ exhibits diurnal variation
of excitability can be experimentally investigated by subjecting the
plant at every hour of the day and night to a test-stimulus of uniform
intensity, and obtaining the corresponding mechanical responses. Under
these circumstances the amplitude of response at any time will serve
as a measure of the excitability of the plant at the particular time.
Any periodic fluctuation of response will then demonstrate the periodic
character of variation of excitability.
The investigation thus resolves itself into:--
The successful construction of a Response Recorder which will
automatically record the response of the plant to uniform
periodic stimulation at all hours of day or night;
the study of the effects of various external conditions on
excitability;
the diurnal variation of excitability and its relation to the
changes of external conditions.
I will first give a diagrammatic view of the different parts of the
apparatus which I devised for this investigation.[G] The leaf of
_Mimosa_ is attached to one arm of a light aluminium lever, L, by means
of thread. At right angles to the lever is the writing index W, which
traces on a smoked glass plate allowed to fall at a definite rate
by clockwork the responsive movement of the leaf. Under a definite
stimulus of electric shock the leaf falls down, pulling the lever L,
and moving the writer towards the left. (Fig. 14.) The amplitude of
the response-curve measures the intensity of excitation. The leaf
re-erects itself after a time, the corresponding record exhibiting
recovery. A second stimulus is applied after a definite interval, say
an hour, and the corresponding response shows whether the excitability
of the plant has remained constant or undergone any variation.
[G] _See_ also Bose.--The Diurnal Variation of Moto-excitability in
_Mimosa_--Annals of Botany, Oct. 1913.
[Illustration: FIG. 14. Diagrammatic representation of the complete
apparatus for determination of diurnal variation of excitability.
Petiole of _Mimosa_, attached by thread to one arm of lever L;
writing index W traces on smoked glass plate G, the responsive fall
and recovery of leaf. A, primary, and S, secondary, of induction
coil. Exciting shock passes through the plant by electrodes E, E′. A,
accumulator. C, clockwork for regulating duration of tetanizing shock.
Primary circuit of coil completed by plunging rod, V, dipping into cup
of mercury M.]
UNIFORM PERIODIC STIMULATION.
_Electric mode of excitation._--I find that one of the best methods of
stimulating the plant is by means of tetanizing induction shock. The
sensitiveness of _Mimosa_ to electric stimulation is very great; the
plant often responds to a shock which is quite imperceptible to a human
subject. By the employment of a sliding induction coil, the intensity
of the shock can be regulated with great accuracy; the secondary if
gradually brought nearer the primary till a stimulus is found which is
minimally effective. The intensity of stimulus actually employed is
slightly higher than this, but within the sub-maximal range. When the
testing stimulus is maintained constant and of sub-maximal intensity,
then any variation of excitability is attended by a corresponding
variation in the amplitude of response.
The exciting value of a tetanizing electric shock depends (1) on the
intensity, (2) on the duration of shock. The intensity may be rendered
uniform by placing the secondary at a fixed distance from the primary,
and keeping the current in the primary circuit constant. The constancy
of the current in primary circuit is secured by the employment of an
accumulator or storage cell of definite electromotive force. It is
far more difficult to secure the constant duration of the tetanizing
shock in successive stimulations at intervals of, say, one hour during
twenty-four hours. The duration of the induction shock given by the
secondary coil depends on the length of time during which the primary
circuit is completed in successive excitations. I have succeeded in
overcoming the difficulty of securing uniformity of duration of shock
by the employment of a special clockwork device.
_The clockwork plunger._--The alarum clock can be so arranged that a
wheel is suddenly released and allowed to complete one rapid revolution
at intervals of, say, one hour. There is a fan-governor by which the
speed of the revolution can be regulated and maintained constant. This
will specially be the case when the alarum spring is long and fully
wound. The succession of short releases twenty-four times during the
day produce relatively little unwinding of the spring. On account of
this and the presence of the fan-governor, the period of a single
revolution of the wheel remains constant. By means of an eccentric
the circular movement is converted into an up and down movement. The
plunging rod R thus dips into a cup of mercury M, for a definite
short interval and is then lifted off. The duration of closure can be
regulated by raising or lowering the cup of mercury. In practice the
duration of tetanizing shock is about 0.2 second.
The same clock performs three functions. The axis which revolves once
in twelve hours has attached to it a wheel, and round this is wound
a thread which allows the recording glass plate to fall through six
inches in the course of twenty-four hours. A spoke attached to the
minute hand releases the alarum at regular and pre-determined intervals
of time, say once in an hour. The plunging rod R, actuated by the
eccentric, causes a tetanizing shock of uniform intensity and duration
to be given to the plant at specified times.
_Constancy of resistance in the secondary circuit._--In order that the
testing electric stimulus shall remain uniform, another condition has
to be fulfilled, namely, the maintenance of constancy of resistance in
the secondary circuit, including the plant. Electric connections have
to be made with the latter by means of cloth moistened with dilute
salt solution; drying of the salt solution, however, gives rise to a
variation of resistance in the electrolytic contact. This difficulty
is overcome by making the electrolytic resistance negligible compared
to the resistance offered by the plant. Thin and flexible spirals
of silver tinsel attached to the electrodes E, E′ are tied round
the petiole and the stem, respectively. In order to secure better
electric contact, a small strip of cloth moistened with dilute salt
and glycerine is wound round the tinsel. As the resistance of contact
is relatively small, and as drying is to a great extent retarded by
glycerine, the total resistance of the secondary circuit undergoes
practically no variation in the course of twenty-four hours. This will
be seen from the following data. An experiment was commenced one day at
1 P.M., when the resistance offered by 8 cm. length of stem and 2 cm.
length of petiole was found to be 1.5 million ohms. After twenty-four
hours’ record, the resistance was measured the next day and was found
unchanged. The fact that the stimulus remains perfectly uniform will be
quite apparent when the records given in the course of this paper are
examined in detail.
THE RESPONSE RECORDER.
The amplitude of response affords, as we have seen, a measure of the
excitability of the plant. In actual record friction of the writer
against the glass surface becomes a source of error. This difficulty I
have been able to overcome by the two independent devices, the Resonant
Recorder and the Oscillating Recorder. In the former the writer is
maintained by electric means in a state of continuous to and fro
vibration, about ten times in a second. There is thus no continuous
contact between the writer and the smoked glass surface, friction
being thereby practically eliminated. The writer in this case taps
a record, the successive dots occurring at intervals of 0.1 second.
The responsive fall of the leaf is rapid, hence the successive dots
in this part of the record are widely spaced; but the erection of the
leaf during recovery takes place slowly, hence the recovery part of
the curve appears continuous on account of the superposition of the
successive dots. The advantage of the Resonant Recorder is that the
curve exhibits both response and recovery. This apparatus is admirably
suited for experiments which last for a few hours. There is, however,
some drawback to its use in experiments which are continued for days
together. This will be understood when we remember that for the
maintenance of 10 vibrations of the writer in a second, 10 electric
contacts have to be made; in other words, 36,000 intermittent electric
currents have to be kept up per hour. This necessitates the employment
of an electric accumulator having a very large capacity.
In the Oscillating Recorder the recording plate itself moves to and
fro, making intermittent contact with the writer about once in a
minute. The recording smoked glass plate is allowed to fall at a
definite rate by the unwinding of a clock wheel. By an electromagnetic
arrangement the holder of the smoked glass plate is made to oscillate
to and fro, causing periodic contact with the writer.
[Illustration: FIG. 15. The Oscillator. Electromagnet M, M′,
periodically magnetized by completion of electric current by clockwork
C. Periodic attraction of soft iron armature A moves attached glass
plate G to left, making thereby electric contact with writer.]
The Oscillator is diagrammatically shown in Fig. 15. M, M′ are the two
electromagnetic coils, the free ends of the horseshoe being pointed.
Facing them are the conical holes of the soft iron armature A. This
armature carries two rods which slide through hollow tubes. The distal
ends of the rods support the holder H, carrying the smoked glass plate.
Under normal conditions, the plate-holder is held by suitable springs,
somewhat to the right of, and free from contact with, the writer. A
clockwork C carries a rotating arm, which makes periodic contact with
a pool of mercury contained in the vessel V, once in a minute. On the
completion of the electromagnetic circuit, the armature A is attracted,
the recording glass plate being thereby moved to the left making
contact with the writer. The successive dots in the record thus take
place at intervals of a minute. Only a moderate amount of electric
current is thus consumed in maintaining the oscillation of the plate. A
4-volt storage cell of 20 amperes capacity is quite sufficient to work
the apparatus for several days.
The responsive fall of the leaf of _Mimosa_ is completed in the course
of about two seconds. The leaf remains in the fallen or ‘contracted’
position for nearly fifteen seconds; it then begins to recover slowly.
As the successive dots of the Oscillating Recorder are at intervals
of a minute, the maximum fall of leaf is accomplished between two
successive dots. The dotted response record here obtained exhibits
the recovery from maximum fall under stimulation (_cf._ Fig. 23). The
recovery of the leaf in one minute is less than one-tenth the total
amplitude of the fall, and is proportionately the same in all the
response records. Hence the successive amplitudes of response curves
that are recorded at different hours of the day afford us measures
of the relative variations of excitability of the plant at different
times. This enables us to demonstrate the reality of diurnal variation
of excitability. In my experimental investigations on the subject I
have not been content to take my data from any particular method of
obtaining response, but have employed both types of recorders, the
Resonant and Oscillating. It will be shown that the results given by
the different instruments are in complete agreement with each other.
EFFECTS OF EXTERNAL CONDITIONS ON EXCITABILITY.
Before giving the daily records of periodic variation of excitability,
I will give my experimental results on the influence of various
external conditions in modifying excitability. The conditions which
are likely to affect excitability and induce periodicity are, first,
the effects of light and darkness: under natural conditions the plant
is subjected in the morning to the changing condition from darkness
to light; then to the action of continued light during the day; and in
the evening to the changing condition from light to darkness. A second
periodic factor is the change in the condition of turgidity, which
is at its maximum in the morning, as evidenced by the characteristic
erect position of the petiole. Finally, the plant in the course of day
and night is subjected to a great variation of temperature. I will
now describe the effects of these various factors on excitability. It
should be mentioned here that the experiments were carried out about
the middle of the day, when the excitability, generally speaking, is
found to remain constant.
EFFECTS OF LIGHT AND DARKNESS.
[Illustration: FIG. 16. Effect of cloud. Dotted up-curve indicates
responsive fall, and continuous down-line exhibits slow recovery. First
four responses normal; next three show depression due to diminution
of light brought on by cloud, the duration of which is indicated
by horizontal line below. Last three records show restoration of
excitability brought on by clearing of sky. All records read from left
to right.]
I have frequently noticed that a depression of excitability occurred
when the sky was darkened by passing clouds. This is clearly seen
in the above records obtained with the Resonant Recorder. Uniform
sub-maximal stimuli had been applied to a specimen of _Mimosa_ at
intervals of fifteen minutes. The dotted up-line represents the
responsive fall, and the continuous down-line, the slow recovery. The
first four are the normal uniform responses (Fig. 16). The next three
show the depressing effect of relative darkness due to cloudy weather.
The sky cleared after forty-five minutes, and we notice the consequent
restoration of normal excitability.
_Effect of sudden darkness and its continuation. Experiment 14._--In
the next record (Fig. 17) is shown the immediate and continued action
of darkness. The first two are the normal uniform responses in light.
By means of screens, the plant was next subjected to sudden darkness;
this brought about a marked depression of excitability. Subjection to
sudden darkness thus acts as a stimulus inducing a marked but transient
fall of excitability. Under the continuous action of darkness, however,
the excitability is at first restored and then undergoes a persistent
depression.
[Illustration: FIG. 17. Effect of sudden darkness. Plant subjected to
sudden darkness beyond horizontal line seen below. First two responses
normal. Note sudden depression of excitability, revival and final
depression under continued darkness.]
_Effect of transition from darkness to light: Experiment 15._--Here
we have to deal first with the immediate effect of sudden transition,
and then with the persistent effect of continuous light. In the record
given in Fig. 18 the plant had been kept in the dark and the responses
taken in the usual manner. It was then subjected to light; the sudden
change from darkness to light acted as a stimulus, inducing a transient
depression of excitability. In this connection it is interesting to
note that Godlewski found that in the phenomenon of growth, transition
from darkness to light acted as a stimulus, causing a transient
decrease in the normal rate. The effect of continued light on _Mimosa_
is an enhancement of excitability.
[Illustration: FIG. 18. Effect of change from darkness to light. The
first three records are normal under darkness. Horizontal line below
indicates exposure to light. Note preliminary depression followed by
enhancement of excitability.]
EFFECT OF EXCESSIVE TURGOR.
I have often found that the moto-excitability is depressed under
excessive turgor. Thus the “over-turgid” leaf of _Biophytum sensitivum_
does not exhibit any mechanical response on rainy days.
[Illustration: FIG. 19. Effect of enhanced turgor, artificially
induced. First two responses normal. Application of water, at arrow,
induces depression of moto-excitability.]
_Experiment 16._--The effect of excessive turgor on moto-excitability
may be demonstrated in the case of _Mimosa_ by allowing its main
pulvinus to absorb water. The result is seen in the above record
(Fig. 19), where water was applied on the pulvinus after the second
response. It is seen how a depression of moto-excitability results from
excessive turgor brought on by absorption of water. In such cases,
however, the plant is found to accommodate itself to the abnormal
condition and gradually regain its normal excitability in the course of
one or two hours.
INFLUENCE OF TEMPERATURE.
The moto-excitability of the pulvinus of _Mimosa_ is greatly
modified under the influence of temperature. For the purpose
of this investigation I enclosed the plant in a glass chamber,
raising the temperature to the desired degree by means of electric
heating. Responses to identical stimuli were then taken at different
temperatures. It was found that the effect of heightened temperature,
up to an optimum, was to enhance the amplitude of response. Thus with
a given specimen it was found that while at 22°C. the amplitude of
response was 2.5 mm., it became 22 mm. at 27°C., and 52 mm. at 32°C.
The excitability is enhanced under rising, and depressed under falling
temperature. The moto-excitability of _Mimosa_ is practically abolished
at the minimum temperature of about 19°C.
[Illustration: FIG. 20. Effect of moderate cooling during a period
shown by horizontal line below. Moderate depression followed by quick
restoration.]
_Effect of lowering of temperature: Experiment 17._--A simple way of
exhibiting the effect of lowering of temperature is by artificial
cooling of the pulvinus. This cannot very well be done by application
of a stream of cooled water, because, as we have seen, absorption of
water by the pulvinus is attended by a loss of excitability: diluted
glycerine has, however, no such drawback. This fluid at ordinary
temperature was first applied on the pulvinus, and after an interval of
half an hour records were taken in the usual manner. Cooled glycerine
was then applied and the record taken once more; the results are seen
in Figs. 20 and 21. In the former, the first response was normal at
the temperature of the room, which was 32°C.; the next two exhibit
depression of excitability under moderate cooling; the duration of
application of moderately cooled glycerine is there indicated by the
horizontal line below. On the cessation of application, the normal
temperature was quickly restored, with the restoration of normal
excitability.
In the next record (Fig. 21) is shown the effect of a more intense
cold. It will be noticed that the first effect was a depression, and
subsequently, a complete abolition of excitability. Thick dots in the
record represent applications of stimulus which proved ineffective. It
will also be noticed that even on the cessation of cooling, and the
return of the tissue to normal temperature the induced abolition of
excitability persisted as an after-effect for a considerable time. I
have likewise found that the after-effect of cold in abolishing the
conduction of excitation is also very persistent. These experiments
show that owing to physiological inertia, the variations of
excitability in the plant often lag considerably behind the external
changes which induce them.
[Illustration: FIG. 21. Effect of application of more intense cold.
Note sudden depression followed by abolition of excitability, also
persistent after-effect.]
_Effect of high temperature: Experiment 18_.--It has been shown that
the moto-excitability is enhanced by rising temperature; there is,
however, an optimum temperature above which the excitability undergoes
a depression. This is seen in the following record (Fig. 22), where the
normal response at 32°C. was depressed on raising the temperature to
42°C.; the excitability was, however, gradually restored when the plant
was allowed to regain the former temperature.
[Illustration: FIG. 22. Effect of temperature above optimum. Note
depression of excitability induced by high temperature, and gradual
restoration on return to normal.]
I may now briefly recapitulate some of the important results: darkness
depresses and light exalts the moto-excitability. Excessive turgor
depresses motility. Still more marked is the effect of temperature.
Lowering of temperature depresses and finally abolishes the
moto-excitability: rise of temperature enhances it up to an optimum
temperature, but beyond this point the excitability undergoes
depression. _The change in excitability induced by the variation of
external condition is not immediate; the induced effect, generally
speaking, lags behind the inducing cause._
DIURNAL VARIATION OF EXCITABILITY.
I will now give automatic records of responses taken once every hour
for twenty-four hours. They prove conclusively the diurnal variation
of excitability in _Mimosa_. After studying in detail the variations
characteristic of particular times of the day, I will endeavour to
correlate them with the effects brought on by the periodic changes of
the environment.
_Experiment 19._--As a typical example I will first give a record
obtained in the month of February, that is, say, in spring. From this
it will not be difficult to follow the variations which take place
earlier in winter or later in summer.
[Illustration: FIG. 23. Record for twenty-four hours, exhibiting
diurnal variation of excitability (spring specimen). The displacement
of base-line is due to nyctitropic movement.]
The record given in Fig. 23 was commenced at 5 P.M. and continued to
the same hour next day. The first thing noticeable is the periodic
displacement of the base-line. This is due to the nyctitropic movements
of the leaf. It should be remembered that the up-movement of the leaf
is represented by down-curve, and _vice versâ_. After the maximum fall
of the leaf, which in this case was attained at 9 P.M., there followed
a reverse movement: the highest erection, indicative of maximum turgor,
was reached at 6 A.M. The leaf then fell slowly and reached a middle
position at noon. The extent of the nyctitropic movement varies in
individual cases; in some it is slight, in others very large. The
erectile movement began, as stated before, at about 9 P.M.; in some
cases, however, it may occur as early as 6 P.M.
In following the characteristic variations of response occurring
throughout the day, we find that while they are practically uniform
between the hours of 5 and 6 P.M., a continuous decline is manifested
after setting in of darkness (7 P.M.); the fall of excitability
continues even after sunrise (6-30 A.M.), response being practically
abolished at 8 A.M. The excitability is then gradually restored in
a staircase manner, the maximum being reached after 12 noon. After
attaining this, the excitability remains more or less constant till the
evening. It will be noticed that the amplitude of response at 5 P.M.
on the second day was the same as the corresponding response on the
previous day.
The results of this and numerous other records taken in spring may be
summarized as:--
1. The maximum excitability of _Mimosa_ is attained between 1 and
3 P.M., and remains constant for several hours. In connection
with the constancy of response at this period, it should be
remembered that when the response is at its maximum a slight
increase of excitability cannot further enhance the amplitude of
response.
2. The excitability, generally speaking, undergoes a continuous
decline from evening to morning, the response being practically
abolished at or about 8 A.M.
3. From 8 A.M. to 12 noon, the excitability is gradually enhanced
in a staircase manner, till the maximum excitability is reached
after 1 P.M.
I have obtained numerous records in support of these conclusions,
some of which are reproduced in the following figures. In these cases
responses to uniform stimuli at intervals of half an hour were taken at
different parts of the day, the recorder employed being of the Resonant
type.
_Mid-day record: Experiment 20._--The record of daily periodicity
previously given shows that the excitability reaches its maximum after
12 noon, and that it remains constant at the maximum value for several
hours. This fact is fully borne out in the following record obtained
with a different specimen (Fig. 24). The responses were taken here from
noon to 3 P.M., once every half-hour.
[Illustration: FIG. 24. Mid-day record from noon to 3 P.M. exhibiting
uniform excitability. Responses taken once every half-hour.]
_Evening record: Experiment 21._--The record given in Fig. 23 shows
that the amplitude of response falls continuously after 6 P.M. It
might be thought that the diminished amplitude in the first part may
be due to the natural nyctitropic fall of the leaf. The range of the
pulvinar movement being limited, it is clear that the extent of the
responsive fall must become smaller on account of the natural fall of
the leaf during the first part of the night. That this is not the whole
explanation of the decline of response in the evening will be clear
from certain facts which I will presently adduce. It was stated that
the leaf of _Mimosa_ exhibits nyctitropic fall from 6 to 9 P.M., after
which there is a reverse movement of erection. In certain specimens,
however, the erectile movement commenced as early as 6 P.M. It is
obvious that in these latter cases diminution of amplitude of response
cannot be due to the reduction of the range of movement of the leaf.
In Fig. 25 is given a series of records from 6 to 10 P.M. obtained
with a leaf in which erectile movement had commenced early in the
evening. Though the full range of responsive movement was in this case
available, yet the amplitude of successive responses is seen to undergo
continuous diminution.
[Illustration: FIG. 25. Evening record from 6 to 10 P.M., showing
gradual depression of excitability.]
_Record in the morning: Experiment 22._--The excitability is, as we
have seen, nearly abolished about 8 A.M., after which there is a
gradual restoration. This gradual enhancement of excitability to a
maximum in the course of the forenoon is seen well illustrated in the
record below (Fig. 26).
[Illustration: FIG. 26. Morning record from 8 A.M. to 12 noon,
exhibiting gradual enhancement of excitability.]
The record of daily periodicity given in Fig. 23 may be regarded as
a typical example. Modifications may, however, be observed which are
traceable to individual peculiarities. As an example of this, I give
a record (Fig. 27) obtained with a specimen in which nyctitropic
movement was very pronounced. The periodic variation of excitability
exhibited here is practically the same as shown by other specimens.
The interesting variation is in the character of the recovery from
stimulus; the leaf was falling from 6 to 9 P.M.; owing to the shifting
of the base-line upwards the recovery appears to be incomplete. After 9
P.M. the leaf was erected, at first slowly, then at a very rapid rate.
The consequent fall of the base-line late at night is very abrupt;
hence there is an apparent over-shooting in the line of recovery.
[Illustration: FIG. 27. Record of diurnal variation of excitability; it
exhibits marked nyctitropic movement.]
EFFECT OF TEMPERATURE ON VARIATION OF EXCITABILITY.
So far I have merely described the observed diurnal variation of
excitability. We may next inquire whether there is any causal relation
between the change of external conditions and the observed variation of
excitability. It has been shown that the moto-excitability is greatly
influenced by temperature. In order to find in what manner the diurnal
variation of excitability was influenced by the daily variation of
temperature, I took special care to secure by means of the thermograph
a continuous record of temperature variations. The table which follows
shows the relation between the hours of the day, temperature, and
amplitude of response, in a typical case of diurnal variation of
excitability.
TABLE II.--SHOWING THE RELATION BETWEEN HOUR OF THE DAY, TEMPERATURE,
AND EXCITABILITY. (SPRING SPECIMEN.)
+--------------------------------+--------------------------------+
| _Hours _Tempera- _Amplitude | _Hours _Tempera- _Amplitude |
| of ture._ of | of ture._ of |
| day._ Response._ | day._ Response._ |
+--------------------------------+--------------------------------+
| 5 p.m. 28° C. 28 mm. | 5 a.m. 20° C. 5 mm. |
| 6 " 25.5° " 28 " | 6 " 20.5° " 4.2 " |
| 7 " 24.5° " 27 " | 7 " 21° " 3.5 " |
| 8 " 23° " 23.5 " | 8 " 22° " 2.5 " |
| 9 " 22° " 21.5 " | 9 " 24° " 0 " |
|10 " 21° " 18 " |10 " 26° " 6 " |
|11 " 20.5° " 15 " |11 " 26.5° " 15.5 " |
|12 " 20° " 13 " |12 " 28° " 22.5 " |
| 1 a.m. 20° " 10 " | 1 p.m. 28° " 26 " |
| 2 " 20° " 8 " | 2 " 28.5° " 28 " |
| 3 " 20° " 7.5 " | 3 " 28.5° " 28 " |
| 4 " 19.5° " 6 " | 4 " 29° " 28 " |
+--------------------------------+--------------------------------+
From the data given in the table, two curves have been obtained. One of
these shows the relation between the hours of the day and temperature;
the other exhibits the relation between the hours of the day and the
excitability as gauged by the amplitude of response (Fig. 28). It
will be seen that there is, broadly speaking, a marked resemblance
between the two curves, which demonstrate the predominant influence of
temperature on diurnal variation of excitability.
[Illustration: FIG. 28. The continuous curve shows the relation between
the hour of the day and temperature. The dotted curve exhibits relation
between the hour of the day and excitability.]
EFFECT OF PHYSIOLOGICAL INERTIA.
It has been shown (page 59) that owing to physiological inertia, the
change of excitability, generally speaking, lags behind the inducing
cause. This fact finds striking illustration in the lag exhibited by
the curve of excitability in reference to the temperature curve. The
minimum temperature was attained at about 4 A.M., but the excitability
was not reduced to a minimum till four hours later and again there is a
marked fall of temperature after 5 P.M., but the excitability did not
become depressed till two hours later.
There is again the factor of variation of light, the effect of which is
not so great as that of temperature. The periods of maximum of light
and temperature are, however, not coincident.
We may now discuss in greater detail the diurnal variation of
excitability in _Mimosa_, taking the typical case, the record of which
is given in Fig. 23. The temperature here is seen to remain almost
constant, and at an optimum, from 1 to 5 P.M., the condition of light
is also favourable. Hence the excitability is found to be constant,
and at its maximum between these hours. The temperature begins to fall
after 6 P.M., and there is, in addition, the depressing action of
gathering darkness. Owing to the time-lag, the fall of excitability
does not commence immediately at 6 P.M., but an hour afterwards,
and continues till the next morning. During this period we have the
cumulative effect of twelve hours’ darkness and the increasing
depression due to cold, the temperature minimum occurring at 4 A.M.
On account of the combined effects of these various factors, and
phenomenon of lag, the period of minimum excitability is in general
reached about 8 A.M. In certain other cases this may occur earlier.
After the attainment of this minimum, the excitability is gradually
and continuously increased, under the action of light and of rising
temperature, till the maximum is reached in the afternoon.
EFFECT OF SEASON.
It was said that temperature exerted a predominant influence in
inducing variation of excitability. We may, therefore, expect that
the diurnal period would be modified in a certain way according to
the season. In winter the night temperature falls very low; hence the
depression of excitability is correspondingly great, and results in
the complete abolition of excitability. The after-effect of intense
cold is seen in the condition of inexcitability persisting for a very
long period in the morning. In summer the prevailing high temperature
modifies the diurnal periodicity in a different manner. When the night
is warm, the fall of excitability is slight. In the day, on the other
hand, the temperature may rise above the optimum, bringing about a
depression. In such a case the excitability in the earlier part of the
evening may actually be greater than in the middle of the day. These
modifications are shown in a very interesting way in the following
record (Fig. 29) taken at the end of April. The temperature of Calcutta
at this season often rises above 100°F. or 38°C. Table III also
exhibits, in the case of the summer specimen, the relation between the
hours of the day, temperature, and excitability.
[Illustration: FIG. 29. Diurnal variation of excitability exhibited by
summer specimen.]
An inspection of the record given in Fig. 29 shows that the amplitude
of response was enhanced after 4 P.M. The temperature up to that time
was unusually high (38°C.), and there was in consequence a depression
of excitability. After that hour there was a mitigation of heat, the
temperature returning towards the optimum. Hence we find that the
maximum excitability was attained between the hours 4 and 6 P.M. The
minimum temperature at night was higher in the present case than that
of the experiment carried out in February; in the former the minimum
was 25.5°C., while in the latter it was 19.5°C. On account of this
difference the night record in summer shows a fall of excitability
which is far more gradual than that obtained in spring. The
excitability is here not totally abolished in the morning, but reaches
a minimum after 8 A.M.; the sensitiveness is then gradually enhanced
in a staircase manner.
TABLE III--SHOWING THE RELATION BETWEEN HOURS OF THE DAY, TEMPERATURE,
AND EXCITABILITY. (SUMMER SPECIMEN.)
+--------------------------------+--------------------------------+
| _Hours _Tempera- _Amplitude | _Hours _Tempera- _Amplitude |
| of ture._ of | of ture._ of |
| day._ Response._ | day._ Response._ |
+--------------------------------+--------------------------------+
| 1 p.m. 38° C. 22 mm | 1 a.m. 26° C. 21.5 mm. |
| 2 " 38° " 23 " | 2 " 26° " 20 " |
| 3 " 38° " 24.5 " | 3 " 25.5° " 18.5 " |
| 4 " 37° " 28 " | 4 " 25.5° " 17 " |
| 5 " 35.5° " 29 " | 5 " 25.5° " 16 " |
| 6 " 33° " 27 " | 6 " 26° " 15 " |
| 7 " 31° " 26 " | 7 " 27° " 14 " |
| 8 " 30° " 26 " | 8 " 29° " 13 " |
| 9 " 29° " 25 " | 9 " 30.5° " 11 " |
|10 " 27° " 24.5 " |10 " 33° " 16 " |
|11 " 27° " 24 " |11 " 35° " 17 " |
|12 " 26.5° " 22.5 " |12 " 37° " 21 " |
+--------------------------------+--------------------------------+
SUMMARY.
The moto-excitability of _Mimosa_ was gauged every hour of the day
and night, by the amplitude of the response to a testing stimulus.
This is effected by means of automatic devices which excite the plant
periodically by an absolutely constant stimulus, and record the
corresponding mechanical response.
From the record thus obtained, it was found that the excitability
of the plant is not the same throughout the day, but undergoes a
variation characteristically different at different times of the day.
In a typical case in spring the excitability attained its maximum
value after 1 P.M. and remained constant for several hours. There was
then a continuous fall of excitability, the minimum being reached at
about eight in the morning. The plant at this time was practically
insensitive. The moto-excitability was then gradually enhanced in a
staircase manner till it again reached a maximum next afternoon.
The effect of sudden darkness was found to induce a transient
depression, followed by revival of excitability. The effect of
persistent darkness was to induce a depression.
Exposure to light from darkness caused a transient depression, followed
by an enhancement of excitability.
Excessive turgor induced a diminished response.
Lowering of temperature induced a depression of excitability,
culminating in an abolition of response. The after-effect of excessive
cold was a prolonged depression of excitability.
Excitability was enhanced by rising temperature up to an optimum; above
this point a depression was induced.
Owing to physiological inertia the change of excitability induced by
variation of external condition lags behind the inducing cause.
The diurnal variation of excitability is primarily due to diurnal
variation of temperature. The effect is modified in a minor degree by
variation of light.
V.--RESPONSE OF PETIOLE-PULVINUS PREPARATION OF _MIMOSA PUDICA_
_By_
SIR J. C. BOSE,
_Assisted by_
SURENDRA CHANDRA DAS, M.A.
The most suitable plant for researches on irritability of plants is
_Mimosa pudica_, which can be obtained in all parts of the world.
An impression unfortunately prevails that the excitatory reaction
of the plant can be obtained only in summer and under favourable
circumstances; this has militated against its extensive use in
physiological experiments, but the misgiving is without any foundation;
for I found no difficulty in demonstrating even the most delicate
experiments on _Mimosa_ before the meeting of the American Association
for the Advancement of Science held during Christmas of 1914. The
prevailing outside temperature at the time was considerably below
the freezing point. With foresight and care it should not be at all
difficult to maintain in a hot-house a large number of these plants in
a sensitive condition all the year round.
In order to remove the drawback connected with the supply of sufficient
material, I commenced an investigation to find whether a detached leaf
preparation could be made as effective for the study of irritability
as the whole plant. Here we have at the central end of the leaf the
pulvinus, which acts as the contractile organ; the conducting strand
in the interior of the petiole, on the other hand, is the vehicle for
transmission of excitation. The problem to be solved is the rendering
of an isolated petiole-and-pulvinus of _Mimosa_ as efficient for
researches on irritability as the nerve-and-muscle preparation of a
frog. On the success of this attempt depended the practical opening
out of an extended field of physiological investigation which would be
unhampered by any scarcity of experimental material.
In connection with this it is well to note the surprising difference in
vegetative growth as exhibited by plants grown in soil and in pots. A
pot-specimen of _Mimosa_ produces relatively few leaves, but one grown
in the open ground is extremely luxuriant. As an instance in point,
I may state that for the last five months I have taken from a plant
grown in a field about 20 leaves a day for experiment, without making
any impression on it. A large box containing soil would be practically
as good as the open ground, and the slower rate of growth in a colder
climate could be easily made up by planting half a dozen specimens. The
protection of the plants from inclemencies of weather can be ensured by
means of a glass cover with simple heat-regulation by electric lamps,
in place of an expensive green-house.
Returning to the question of the employment of an isolated leaf, which
I shall designate as a petiole-pulvinus preparation, instead of the
entire plant, the first attempts which I made proved unsuccessful. The
cut leaf kept in water would sometimes exhibit very feeble response, at
other times all signs of excitability appeared to be totally abolished.
It was impossible to attempt an investigation on the effect of changing
environment on excitability when the normal sensitiveness itself
underwent so capricious a change
These difficulties were ultimately overcome from knowledge derived
through systematic investigation on the relative importance of
the different parts of the motor apparatus, on the immediate and
after-effect of section on the excitability of the leaf, and on the
rate of decay of this excitability on isolation from the plant. The
experience thus gained enabled me to secure long-continued and uniform
sensibility under normal conditions. It was thus possible to study the
physiological effects of changing external conditions by observing the
responsive variation in the isolated petiole-pulvinus preparation. I
propose to deal with the different aspects of the investigation in the
following order:--
1. The effect of wound or section in modification of normal
excitability.
2. The change of excitability after immersion in water.
3. Quantitative determination of the rate of decay of excitability
in an isolated preparation.
4. Effect of amputation of the upper half of pulvinus.
5. Effect of removal of the lower half.
6. Influence of the weight of leaf on rapidity of responsive fall.
7. The action of chemical agents.
8. Effect of “fatigue” on response.
9. The influence of constant electric current on recovery.
10. The action of light and darkness on excitability.
The isolated petiole-pulvinus preparation is made by cutting out a
portion of the stem bearing a single lateral leaf. The four diverging
sub-petioles may also be cut off. In order to prevent rapid drying
the specimen has to be kept in water. Preparations made in this way
often appeared to have lost their sensibility. I was, however, able to
trace this loss to two different factors: first, to the physiological
depression due to injury caused by section, and, second, to the sudden
increase of turgor brought on by excessive absorption of water. I shall
now proceed to show that the loss of sensibility is not permanent, but
is capable of restoration.
EFFECT OF WOUND OR SECTION IN MODIFICATION OF NORMAL EXCITABILITY.
In connection with the question of effect of injury, it is to be borne
in mind that after each excitation the plant becomes temporarily
irresponsive and that the excitability is fully restored after the
completion of protoplasmic recovery. A cut or a section acts as a very
intense stimulus, from the effect of which the recovery is very slow.
If the stem be cut very near the leaf, the excitation of the pulvinus
is very intense, and the consequent loss of excitability becomes more
or less persistent. But if the stem be cut at a greater distance, the
transmitted excitation is less intense, and the cut specimen recovers
its excitability within a moderate time. I have also succeeded in
reducing the excitatory depression by previously benumbing the tissue
by physiological means. The isolated specimen can be made still more
compact by cutting off the sub-petioles bearing the leaflets; the
preparation now consists of a short length of stem of about 2 cm. and
an equally short length of primary petiole, the motile pulvinus being
at the junction of the two.
[Illustration: FIG. 30--The Resonant Recorder, with petiole-pulvinus
preparation. (From a photograph.)]
For the restoration of sensitiveness, and to meet working conditions,
the lower end of the cut stem is mounted on a T-tube, with
funnel-attachment and exit-tube, as shown in Fig. 30. The other two cut
ends--of the stem and of the petiole--may be covered with moist cloth
or may be closed with collodion flexile to prevent rapid evaporation
and drying up of the specimen. A slight hydrostatic pressure maintains
the specimen in a moderately turgid condition. A preparation thus
made is insensitive at the beginning, but if left undisturbed it
slowly recovers its excitability. The history of the depression of
excitability after shock of preparation and its gradual restoration
is graphically illustrated by a series of records made by the plant
(Fig. 31).
The petiole-pulvinus preparation thus made offers all facilities for
experiment. Owing to its small size it can be easily manipulated; it
can be enclosed in a small chamber and subjected to varying conditions
of temperature and to the action of different vapours and gases. Drugs
are easily absorbed at the cut end, and poison and its antidote can
be successively applied through the funnel without any disturbance
of the continuity of record. In fact, many experiments which would
be impossible with the entire plant are quite practicable with the
isolated leaf.
The arrangement for taking records of response is seen in Fig. 30,
which is reproduced from a photograph of the actual apparatus. For
recording the response and recovery of the leaf under stimulation,
I use my Resonant Recorder fully described in the ‘Philosophical
Transactions’ (1913). The petiole is attached to one arm of the
horizontal lever. The writer, made of fine steel wire with a bent tip,
is at right angles to the lever, and is maintained by electromagnetic
means in a state of to-and-fro vibration, say, ten times in a second.
The record, consisting of a series of dots, is free from errors arising
from friction of continuous contact of the writer with the recording
surface. The successive dots in the record at definite intervals of a
tenth of a second also give the time-relations of the response curve.
On account of its small size, the petiole-pulvinus preparation offers
great facilities for mounting in different ways suitable for special
investigations. Ordinarily, the cut stem with its lower end enclosed
in moist cloth is supported below. A very suitable form of stimulus is
that of induction shock from a secondary coil, the intensity of which
is capable of variation in the usual manner by adjusting the distance
between the primary and the secondary coils. The motile pulvinus, P,
may be excited directly. For investigations on velocity of transmission
of excitation, stimulus is applied on the petiole at some distance
from the pulvinus, by means of suitable electrodes. Excitation is now
transmitted along the intervening length of petiole, the conducting
power of which will be found appropriately modified under the action
of chemical and other agents. In this normal method of mounting, the
more excitable lower half of the pulvinus is below; excitatory reaction
produces the fall of the petiole, gravity helping the movement. The
preparation may, however, be mounted in the inverted position, with
the more excitable lower half of the pulvinus facing upwards. The
excitatory movement will now be the erection of the petiole, against
gravity.
Under natural conditions the stem is fixed, and it is the petiole which
moves under excitation. But a very interesting case presents itself
when the petiole is fixed and the stem free. Here is presented the
unusual spectacle of the plant or the stem “wagging” in response to
excitation.
THE CHANGE OF EXCITABILITY AFTER IMMERSION IN WATER.
The isolated specimen can be kept alive for several days immersed in
water. The excitability of the pulvinus, however, undergoes great
depression, or even abolition, by the sudden change of turgor brought
on by excessive absorption of water. The plant gradually accommodates
itself to the changed condition, and the excitability is restored in a
staircase manner from zero to a maximum.
In studying the action of a chemical solution on excitability,
the solution may be applied through the cut end or directly on
the pulvinus. The sudden variation of turgor, due to the liquid,
always induces a depression, irrespective of the stimulating or
the depressing action of the drug. The difficulty may be eliminated
by previous long-continued application of water on the pulvinus and
waiting till the attainment of uniform excitability which generally
takes place in the course of about three hours. Subsequent application
of a chemical solution gives rise to characteristic variation in the
response.
QUANTITATIVE DETERMINATION OF THE RATE OF DECAY OF EXCITABILITY IN AN
ISOLATED PREPARATION.
[Illustration: FIG. 31--Variation of excitability after section. (1)
Immediate effect; (2) variation of excitability in a second specimen
during 50 hours: (_a_) response 4 hours after section; (_b_) response
after 24 hours; (_c_) after 49 hours. Up-line of record represents
responsive fall of the leaf, down-line indicates recovery from
excitation.]
_Variation of excitability after section: Experiment 23._--In order
to test the history of the change of excitability resulting from the
immediate and after-effect of section, I took an intact plant and
fixed the upper half of the stem in a clamp. The response of a given
leaf was now taken to the stimulus of an induction shock of 0.1 unit
intensity, the unit chosen being that which causes a bare perception
of shock in a human being. The specimen was vigorous and the response
obtained was found to be a maximum. The stem bearing the leaf was cut
at the moment marked in the record with a cross, and water was applied
at the cut end. The effect of section was to cause the maximum fall of
the leaf, with subsequent recovery. After this, successive responses
to uniform stimuli at intervals of 15 minutes show, in (1) of Fig. 31,
that a depression of excitability has been induced owing to the shock
caused by section. In course of an hour, however, the excitability
had been restored almost to its original value before the section.
This was the case with a vigorous specimen, but with less vigorous
ones a longer period of about three hours is required for restoration.
In certain other cases the response after section exhibits alternate
fatigue; that is to say, one response is large and the next feeble,
and this alternation goes on for a length of time. The isolated
specimen, generally speaking, attains a uniform sensibility after a few
hours, which is maintained, with very slight decline under constant
external conditions, for about 24 hours. On the third day the fall
of excitability is very rapid, and the sensibility declines to zero
in about 50 hours after isolation [Fig. 31 (2)]. We may describe
the whole cycle of change as follows: by the shock of operation the
isolated preparation is rendered insensitive for nearly an hour, the
excitability is then gradually restored almost to its normal value
before operation. Under constant external conditions, this excitability
remains fairly constant for about 24 hours after which depression sets
in. The rate of fall of excitability becomes very rapid 40 hours after
the operation, being finally abolished after the fiftieth hour. It
is probable that in a colder climate the fall of excitability would
be much slower. The most important outcome of this inquiry is the
demonstration of the possibility of obtaining persistent and uniform
sensibility in isolated preparations. On account of this, not only is
the difficulty of supply of material entirely removed but a very high
degree of accuracy secured for the investigation itself.
EFFECT OF AMPUTATION OF UPPER HALF OF PULVINUS.
_Experiment 24._--The determination of the _rôle_ played by different
parts of the pulvinus in response and recovery is of much theoretical
importance. Our knowledge on this subject is unfortunately very
scanty. The generally accepted view is that on excitation “the actual
downward curvature of the pulvinus is partly due to a contraction of
the walls of the motor cells consequent upon the decrease of turgor,
but is accentuated by expansion of the insensitive adaxial half of the
pulvinus--which was strongly compressed in the unstimulated condition
of the organ--and also by the weight of the leaf.”[H] According to
Pfeffer, after excitation of the organ, “the original condition of
turgor is gradually reproduced in the lower half of the pulvinus, which
expands, raising the leaf and producing compression of the upper half
of the pulvinus, which aids in the rapid curvature of the stimulated
pulvinus.”[I]
[H] Haberlandt, ‘Physiological Plant Anatomy,’ 1914, p. 570. English
Translation, Macmillan & Co.
[I] PFEFFER--‘Physiology of Plants,’ vol. 3, pp. 75 and 76. English
Translation, Clarendon Press.
It was held, then, that the rapidity of the fall of leaf under stimulus
is materially aided (1) by the expansion of the upper half of the
pulvinus, which is normally in a state of compression, and (2) by the
weight of the leaf. So much for theory. The experimental evidence
available regarding the relative importance of the upper and lower
halves of the pulvinus is not very conclusive. Lindsay attempted to
decide the question by his amputation experiments. He showed that when
the upper half was removed the leaf carried out the response, but rigor
set in when the lower half was amputated. Pfeffer’s experiments on the
subject, however, contradicted the above results. He found that “after
the upper half of the pulvinus was carefully removed, no movement
was produced by stimulation, whereas when the lower half is absent a
weakened power of movement is retained.” Pfeffer, however, adds, “since
the operation undoubtedly affects the irritability, it is impossible
to determine from such experiments the exact part played by the active
contraction of the lower half of the pulvinus.”[I]
The cause of uncertainty in this investigation is twofold. First,
it arises from the unknown change in irritability consequent on
amputation; and, secondly, from absence of any quantitative standard
by which the effect of selective amputation of the pulvinus may
be measured. As regards the first, I have been able to reduce the
depressing action caused by injury to a minimum by benumbing the tissue
before operation, through local application of cold, and also allowing
the shock-effect to disappear after a rest of several hours. As regards
the physiological gauge of efficiency of the motor mechanism, such
a measure is afforded by the relation between a definite testing
stimulus and the resulting response with its time-relations, which
is secured by my Resonant Recorder with the standardised electrical
stimulator.
[Illustration: FIG. 32--Effect of amputation of upper half of pulvinus.
Upper record gives normal response before amputation, and the lower,
response after amputation. (Successive dots at intervals of 0.1 sec).
Apex-time 11 sec, in both.]
In carrying out this investigation I first took the record of normal
response of an intact leaf on a fast moving plate. A second record,
with the same stimulus, was taken after the removal of the upper half
of the pulvinus, having taken the necessary precautions that have been
described. Comparison of the two records (Fig. 32) shows that the only
difference between them is in the exhibition of slight diminution of
excitability due to operation. But, as regards the latent period and
the quickness of attaining maximum fall, there is no difference between
the two records before and after the amputation of the upper half. The
upper part of the pulvinus is thus seen practically to have little
influence in hastening the fall.
EFFECT OF REMOVAL OF THE LOWER HALF.
_Experiment 25._--The shock-effect caused by the amputation of the
lower half was found to be very great, and it required a long period of
rest before the upper half regained its excitability. The excitatory
reaction of the upper half is by contraction, and the response is,
therefore, the lifting of the petiole. Thus, in an intact specimen,
excitation causes antagonistic reactions of the two halves. But the
sensibility of the upper half is very feeble and the rate of its
contractile movement, relatively speaking, very slow. The record of
the response of the upper half of the pulvinus, seen in Fig. 33, was
taken with an Oscillating Recorder, where the successive dots are at
intervals of 1 sec.: the magnification employed was about five times
greater than in recording the response of the lower half (Fig. 32). The
intensity of stimulus to evoke response had also to be considerably
increased. Taking into account the factors of magnification and the
intensity of stimulus for effective response, the lower half I find to
be about 80 times more sensitive than the upper. Thus, under feeble
stimulus the upper half exerts practically no antagonistic reaction.
The excitatory response of the upper half is also seen to be very
sluggish.
[Illustration: FIG. 33.--Response after amputation of lower half of
pulvinus. (Successive dots at intervals of a second; vertical lines
mark minutes.) Apex-time, 40 secs.]
INFLUENCE OF THE WEIGHT OF LEAF ON RAPIDITY OF RESPONSIVE FALL.
_Experiment 26._--It is obvious that the mechanical moment exerted by
the weight of the leaf must help its responsive fall under excitation.
But the relative importance of the factors of active contraction
of the lower half of the pulvinus and of the weight, in the rate
of the responsive down-movement, still remains to be determined. A
satisfactory way of solving the problem would lie in the study of
the characteristics of response-records taken under three different
conditions: (1) When the leaf is helped in its fall by its weight; (2)
when the action of the weight is eliminated; and (3) when the fall
has to be executed against an equivalent weight. An approximation to
these conditions was made in the following manner. We may regard the
mechanical moment to be principally due to the weight of the four
sub-petioles applied at the end of the main petiole. In a given case
these sub-petioles were cut off, and their weight found to be 0.5 grm.
The main petiole was now attached to the right arm of the lever, and
three successive records were taken: (1) With no weight attached to
the petiole; (2) with 0.5 grm. attached to its end; and (3) with 0.5
grm. attached to left arm of the lever at an equal distance from the
fulcrum. In the first case, the fall due to the excitatory contraction
will practically have little weight to help it; in the second case,
it will be helped by a weight equivalent to those of the sub-petioles
with their attached leaflets; and in the third case, the fall will be
opposed by an equivalent weight. We find that in these three cases
there is very little difference in the time taken by the leaf to
complete the fall (Fig. 34).
[Illustration: FIG. 34.--Effect of weight on rapidity of fall. N,
without action of weight; W, with weight helping; and A, with weight
opposing.]
It has been shown that the presence or absence of the upper half of
the pulvinus makes practically no difference in the period of fall;
it is now seen that the weight exerts comparatively little effect. We
are thus led to conclude that in determining the rapidity of fall,
the factors of expansive force of the upper half of the pulvinus and
the weight of the leaf are negligible compared to the active force of
contraction exerted by the lower half of the pulvinus.
ACTION OF CHEMICAL AGENTS.
In connection with this subject it need hardly be said that the various
experiments which I had previously carried out with the intact plant
can also be repeated with the isolated preparation. I will only give
here accounts of experiments which are entirely new.
The chemical solution may be applied directly to the pulvinus, or it
may be absorbed through the cut end, the absorption being hastened
by hydrostatic pressure. The normal record is taken after observing
precautions which have already been mentioned. The reaction of a
given chemical agent is demonstrated by the changed character of the
record. The effect of the drug is found to depend not merely on its
chemical nature, but also on the dose. There is another very important
factor--that of the tonic condition of the tissue--which is found to
modify the result. The influence of this will be realised from the
account of an experiment to be given presently, where an identical
agent is shown to produce diametrically opposite effects on two
specimens, one of which was in a normal, and the other in a sub-tonic,
condition. The experiments described below relate to reactions of
specimens in a normal condition.
[Illustration: FIG. 35.--Stimulating action of hydrogen peroxide.]
_Hydrogen Peroxide: Experiment 27._--This reagent in dilute solution
exerts a stimulating action. Normal records, were taken after
long-continued application of water on the pulvinus. The peroxide, as
supplied by Messrs. Parke Davis & Co., was diluted to 1 per cent., and
applied to the pulvinus; this gave rise to an enhancement of response.
Re-application of water reduced the amplitude to the old normal value
(Fig. 35).
[Illustration: FIG. 36.--Incomplete recovery under the action BaCl_{2}
and transient restoration under tetanisation at T.]
_Barium Chloride: Experiment 28._--The action of this agent is
very characteristic, inducing great sluggishness in recovery. The
preparation had been kept in 1-per cent. solution of this substance for
two hours. After this the first response to a given test-stimulus was
taken; the response was only moderate, and the recovery incomplete. The
sluggishness was so great that the next stimulation, represented by a
thick dot (Fig. 36), was ineffective. Tetanising electric shock at T,
not only brought about response, but removed for the time being the
induced sluggishness. This is seen in the next two records, which were
taken under the old test-stimulus. There is now an enhanced response
and a complete recovery. Beneficial effect of tetanisation disappeared,
however, on the cessation of stimulus. This is seen in the next two
records which were taken after two hours. The amplitude of response was
not only diminished, but the recovery also was incomplete.
[Illustration: FIG. 37.--Antagonistic action of alkali and acid. Arrest
of response in contraction under NaOH (↑), restoration and final arrest
in expansion under lactic acid. (↑)]
_Antagonistic actions of Alkali and Acid: Experiment 29._--Alkali and
acid are known to exert antagonistic actions on the spontaneous beat
of the heart; dilute solution of NaOH arrests the beat of the heart
in systolic contraction, while dilute lactic acid arrests the beat in
diastolic expansion. I have found identical antagonistic reactions
in the pulsating tissue of _Desmodium gyrans_, the telegraph plant.
It is very interesting to find that these agents also exert their
characteristic effects on the response of _Mimosa_ in a manner which
is precisely the same. This is seen illustrated in Fig. 37, where the
application of NaOH arrested the response in a contracted state; after
this, the antagonistic effect of dilute lactic acid is seen first, in
its power of restoring the excitability; its continued application,
however, causes a second arrest, but this time in a state of relaxed
expansion.
_CuSO_4 Solution._--This agent acts as a poison, causing a gradual
diminution of amplitude of response, culminating in actual arrest at
death. Certain poisons, again, exhibit another striking symptom at the
moment of death, an account of which will be given in a separate paper.
EFFECT OF “FATIGUE” ON RESPONSE.
[Illustration: FIG. 38.--“Fatigue” induced by shortening intervening
period of rest.]
With _Mimosa_, after each excitation the recovery becomes complete
after a resting period of about 15 min. With this interval of rest the
successive responses for a given stimulus are equal, and are at their
maximum.
_Experiment 30._--When the resting interval is diminished the recovery
becomes incomplete, and there is a consequent diminution of amplitude
of response. There is thus an increased fatigue with diminished period
of rest. This is illustrated in Fig. 38, where the first two responses
are at intervals of 15 min.; the resting interval was then reduced to
10 min., the response undergoing a marked diminution. Conversely, by
increasing the resting interval, first to 12 and then to 15 min., the
extent of fatigue was reduced and then abolished.
THE INFLUENCE OF CONSTANT ELECTRIC CURRENT ON RECOVERY.
[Illustration: FIG. 39.--Action of constant current in removal of
fatigue by hastening recovery; N, curve of response in fatigued
specimen; C, after passage of current.]
_Experiment 31._--From the above experiment it would appear that
since the incompleteness of recovery induces fatigue, hastening of
recovery would remove it. With this idea I tried various methods for
quickening the recovery of the excited leaf. The application of a
constant electric current was found to have the desired effect. Two
electrodes for introduction of current were applied, one on the stem
and the other on the petiole, at some distance from the pulvinus. In
order to avoid the excitatory effect of sudden application, the applied
current should be increased gradually; this was secured by means of a
potentiometer slide. In my experiment a current having an intensity of
1.4 micro-ampère was found to be effective. Responses at intervals of
10 min., as we have seen, exhibit marked fatigue. Two responses were
recorded on a fast-moving plate, N before, and C after, the application
of the current. It will be seen (Fig. 39) how the application of
current has, by hastening the recovery, enhanced the amplitude of
response and brought about a diminution of fatigue. In connection with
this, I may state that the tonic condition is, in general, improved as
an after-effect of the passage of current. This is seen in some cases
by a slight increase in excitability; in others, where the responses
had been irregular, the previous passage of a current tends to make the
responses more uniform.
ACTION OF LIGHT AND DARKNESS ON EXCITABILITY.
In taking continuous records of responses I was struck by the marked
change of excitability exhibited by the intact plant under variation
of light. Thus the appearance of a cloud was quickly followed by
an induced depression, and its disappearance by an equally quick
restoration of excitability. This may be explained on the theory that
certain explosive chemical compounds are built up by the photosynthetic
processes in green leaves, and that the intensity of response depends
on the presence of these compounds. But the building up of a chemical
compound must necessarily be a slow process, and it is difficult on the
above hypothesis to connect the rapid variation of excitability with
the production of a chemical compound, or its cessation, concomitant
with changes in the incident light.
[Illustration: FIG. 40.--Stimulating action of light, and depressing
action of darkness. Horizontal line below represents period of
darkness.]
_Experiment 32._--In order to find out whether photo-synthesis had
any effect on excitability, I placed an intact plant in a dark
room and obtained from it a long series of responses under uniform
test-stimulus. While this was being done the green leaflets were
alternately subjected to strong light and to darkness, care being taken
that the pulvinus was shaded all the time. The alternate action of
light and darkness on leaflets induced no variation in the uniformity
of response. This shows that the observed variation of excitability
in _Mimosa_ under the alternate action of light and darkness is not
attributable to the photo-synthetic processes.
I next took a petiole-pulvinus preparation from which the sub-petioles
bearing the leaflets had been cut off, and placed it in a room
illuminated by diffused daylight. The normal responses were taken,
the temperature of the room being 30°C. The room was darkened by
pulling down the blinds, and records were continued in darkness. The
temperature of the room remained unchanged at 30°C. It will be seen
from records given in Fig. 40, that in darkness there is a great
depression of excitability. Blinds were next pulled up and the records
now obtained exhibit the normal excitability under light. The sky
had by this time become brighter, and this accounts for the slight
enhancement of excitability. This experiment proves conclusively that
light has a direct stimulating action on the pulvinus, independent of
photo-synthesis.[J]
[J] See also Bose and Das--‘Physiological Investigations with
Petiole-Pulvinus preparations of _Mimosa pudica_.’ Proc. Roy. Soc. B.
Vol. 89, 1916.
SUMMARY.
On isolation of a petiole-pulvinus preparation, the shock of operation
is found to paralyse its sensibility. After suitable mounting the
excitability is restored, and remains practically uniform for nearly
24 hours. After this a depression sets in, the rate of fall of
excitability becomes rapid 40 hours after the operation, sensibility
being finally abolished after the fiftieth hour.
Experiments carried out on the effect of weight, and the influence of
selective amputation of the upper and lower halves of the pulvinus,
show that in determining the rapidity of fall of leaf, the assumed
factors of the expansive force of the upper half of the pulvinus and
the weight of the leaf are negligible compared to the force of active
contraction exerted by the lower half of the pulvinus. The excitability
of the lower half is eighty times greater than that of the upper.
Chemical agents induce characteristic changes in excitability.
Hydrogen peroxide acts as a stimulant. Barium chloride renders the
recovery incomplete: but tetanisation temporarily removes the induced
sluggishness. Acids and alkalis induce antagonistic reactions,
abolition of excitability with alkali taking place in a contracted, and
with acid in an expanded condition of the pulvinus.
The responses exhibit fatigue when the period of rest is diminished.
The passage of constant current is found to remove the fatigue.
Response is enhanced on exposure to light, and diminished in darkness.
Light is shown to exert a direct stimulating action on the pulvinus,
independent of photo-synthesis.
VI.--ON CONDUCTION OF EXCITATION IN PLANTS
_By_
SIR J. C. BOSE.
The plant _Mimosa_ offers the best material for investigation on
conduction of excitation. With regard to this question the prevailing
opinion had been that in plants like _Mimosa_, there is merely a
transmission of hydro-mechanical disturbance and no transmission of
true excitation comparable with the animal nerve. I have, however, been
able to show that the transmission in the plant is not a mechanical
phenomenon, but a propagation of excitatory protoplasmic change. This
has been proved by the arrest of conduction by the application of
various physiological blocks. Thus local application of increasing cold
retards, and finally abolishes the conducting power. The conducting
tissue becomes paralysed for a time as an after-effect of application
of cold; the lost conducting power may, however, be quickly restored
by tetanising electric shocks. The conducting power of an animal nerve
is arrested by an electrotonic block, the conductivity being restored
on the cessation of the current. I have succeeded in inducing similar
electrotonic block of conduction in _Mimosa_. Conductivity of a
selective portion of petiole may also be permanently abolished by local
action, of poisonous solution of potassium cyanide.[K]
[K] BOSE--“An Automatic Method for the Investigation of Velocity of
Transmission of Excitation in _Mimosa_.” ‘Phil. Trans.’ ‘B, Vol. 204
(1913) and also “Irritability of Plants.” Longman’s Green & Co. (1913),
p. 132.
Having thus established the physiological character of the transmitted
impulse in plants I shall now proceed to give some of the principal
results of my earlier and recent investigations on the effects of
various agencies on conduction of excitation in plants.
Apart from any question of hydro-mechanical transmission, it is
important to distinguish two different modes of transmission of
excitation. In a motile tissue contraction of a cell causes a physical
deformation and stimulation of the neighbouring cell. Examples of this
are furnished by the cardiac muscle of the animal, the pulvinus of
_Mimosa_, and the stamen of _Berberis_. This mode of propagation may
better be described as a _convection_ of excitation.
The _conduction_ of excitation, as in a nerve, is a different process
of transmission of protoplasmic change. The conducting tissue in this
case does not itself exhibit any visible change of form. In the plant
the necessary condition for transmission of excitation to a distance
is that the conducting tissue should be possessed of protoplasmic
continuity in a greater or less degree. This condition is fulfilled
by vascular bundles. There being greater facility of transmission
along the bundles than across them, the velocity in the longitudinal
direction is very much greater than in the transverse.
For accurate determination of velocity of transmission the testing
stimulus should be quantitative and capable of repetition. Abnormal
high velocity has been observed in _Mimosa_ by applying crude and
drastic methods of stimulation, by a transverse cut or a burn. This
is apt to give rise to a very strong hydro-dynamic disturbance,
which travelling with great speed, delivers a mechanical blow on the
responding pulvinus. Such hydro-dynamic transmission is not the same as
physiological conduction.
In the primary petiole of _Mimosa_ the highest velocity under electric
stimulation I find to be about 30 mm. per second. This velocity is
considerably lower than the velocity in the nerve of higher animals,
but higher than in the lower animals. As an example of the latter,
mention may be made of the velocity of 10 mm. per second in the nerve
of _Anodon_ and 1 mm. per second in the nerve of _Eledone_.
PREFERENTIAL DIRECTION OF CONDUCTION.
_Experiment 33._--The conduction of excitatory impulse takes place
in both directions. This can be demonstrated by taking a petiole of
_Biophytum sensitivum_ or of _Averrhoa carambola_. These petioles are
provided with a series of motile leaflets. Stimulation at the middle
point of the petiole gives rise to two waves of excitation, one of
which travels towards the central axis of the plant, and the other away
from it. The centrifugal velocity is greater than the centripetal as
will be seen from the following results:
+---------------+---------------------------+--------------------+
| | Velocity in | |
| _Biophytum_ | centrifugal direction | 2.9 mm per second.|
| | " centripetal " | 2 mm " " |
| _Averrhoa_ | " centrifugal " | 0.5 mm " " |
| | " centripetal " | 0.26 mm " " |
+---------------+---------------------------+--------------------+
EFFECT OF TEMPERATURE.
Variation of temperature has a marked effect on the velocity of
transmission of excitation. Lowering of temperature diminishes the
velocity, culminating in an arrest. Rise of temperature, on the
other hand, enhances the velocity. This enhancement is considerable
in specimens in which the normal velocity is low, but in plants in
optimum condition, the velocity being already high, cannot be further
enhanced. The following tabular statement gives results of effects of
temperature on velocity of transmission in _Mimosa_ and _Biophytum_:--
TABLE IV.--EFFECT OF TEMPERATURE ON VELOCITY OF TRANSMISSION.
+---------------------------+--------------+--------------------+
| Specimen. | Temperature. | Velocity. |
+---------------------------+--------------+--------------------+
|_Mimosa_ (winter specimen) | 22°C | 3.6 mm. per second.|
| | 28°C | 6.3 mm. " " |
| | 31°C | 9.0 mm. " " |
| | | |
|_Biophytum_ | 30°C | 3.7 mm. " " |
| | 35°C | 7.4 mm. " " |
| | 37°C | 9.1 mm. " " |
+---------------------------+--------------+--------------------+
EFFECT OF SEASON.
The velocity of transmission is very much lower in winter than in
summer. In the petiole of _Mimosa_, the velocity in summer is as high
as 30 mm. per second; in winter it is reduced to about 4 mm. The
lowering of velocity in winter is partly due to the prevailing low
temperature and also to the depressed state of physiological activity.
EFFECT OF AGE.
In a _Mimosa_ plant, different leaves will be found of different age.
Of these the youngest will be at the top. Lower down, we obtain a fully
grown young leaf, and near the base, leaves which are very old. The
investigation deals with the effect of age on the conducting power of
the petiole.
_Comparison of conducting power in different leaves: Experiment
34._--Selecting three leaves from the same plant we apply an identical
electric stimulus at points 2 cm. from the three responding pulvini.
The electric connections are so made that the same tetanising shock
is applied on the three petioles, very young, fully grown, and very
old. The secondary coil is gradually pushed in till the leaves exhibit
responsive fall. The fully grown leaf was the first to respond, the
velocity of transmission being 23 mm. per second. The secondary coil
had to be pushed nearer the primary through 6 cm. before excitation
could be effectively transmitted through the young petiole; for the
oldest leaf still stronger stimulus was necessary, since in this
case the secondary had to be pushed through an additional distance
of 4 cm. for effective transmission of excitation. I also determined
the relative values of the minimal intensity of stimulus, effective
in causing transmission of excitation in the three cases. Adopting as
before the intensity of electric stimulus which causes bare perception
in a human being as the unit, I find that the effective stimulus for a
fully grown young petiole is 0.3 unit, while the very young required
2.5 units, and the very old 5 units. Hence it may be said that the
conducting power of a very young is an eighth, and of the very old
one-sixteenth of the conductivity of the fully grown young specimen.
It will thus be seen that the conducting power of a very young petiole
is feebler than in a fully grown specimen. The conducting tissue, it
is true, is present, but the power of conduction has not become fully
developed. This power is, as we shall see later, conferred by the
stimulus of the environment. In a very old specimen the diminution of
conducting power is due to the general physiological decline.
EFFECT OF DESICCATION ON CONDUCTING TISSUES.
I have already shown that transmission in the plant is a process
fundamentally similar to that taking place in the animal nerve; it
has also been shown that the effects of various physical and chemical
agents are the same in the conducting tissues of plant and of animal.
[Illustration: FIG. 41--Action of glycerine in enhancing the speed and
intensity of transmitted excitation. Stimulus applied at the vertical
line. Successive dots in record are at intervals of 0.1 sec.]
_Effect of application of glycerine: Experiment 35._--It is known that
desiccation, generally speaking, enhances the excitability of the
animal nerve. As glycerine, by absorption of water, causes partial
desiccation, I tried its effect on conduction of excitation in the
petiole of _Mimosa_. Enhancement of conducting power may be exhibited
in two ways: first, by an increase of velocity of transmission; and,
secondly, by an enhancement of the intensity of the transmitted
excitation, which would give rise to a greater amplitude of response
of the motile indicator. In Fig. 41 are given two records, N, before,
and the other after the application of glycerine on a length of petiole
through which excitation was being transmitted. The time-records
demonstrate conclusively the enhanced rate of transmission after the
application of glycerine. The increased intensity of transmitted
excitation is also seen in the enhanced amplitude of response seen in
the more erect curve in the upper record.
INFLUENCE OF TONIC CONDITION ON CONDUCTIVITY.
Different specimens of _Mimosa_ are found to exhibit differences in
physiological vigour. Some are in an optimum condition, others in an
unfavourable or sub-tonic condition. I shall now describe certain
characteristic differences of conductivity exhibited by tissues in
different conditions.
_Effect of intensity of stimulus on velocity of transmission._--In a
specimen at optimum condition, the velocity remains constant under
varying intensities of stimulus. Thus the velocity of transmission in
a specimen was determined under a stimulus intensity of 0.5 unit; the
next determination was made with a stimulus of four times the previous
intensity, _i.e._, 2 units. In both these cases the velocity remained
constant. But when the specimen is in a sub-tonic condition, the
velocity is found to increase with the intensity of the stimulus. Thus
the velocity of conduction of a specimen of _Mimosa_ in a sub-tonic
condition was found to be 5.9 mm. per second under a stimulus of 0.5
unit; with the intensity raised to 2.5 units, the velocity was enhanced
to 8.3 mm. per second.
_After-effect of stimulus._--In experimenting with a particular
specimen of _Mimosa_ I found that on account of its sub-tonic
condition, the conducting power of the petiole was practically absent.
Previous stimulation was, however, found to confer the power of
conduction as an after-effect. It is thus seen that stimulus canalises
a path for conduction.
The effect of excessive stimulus in a specimen in an optimum condition
is to induce a temporary depression of conductivity; the effect of
strong stimulus on a sub-tonic specimen is precisely the opposite,
namely, an enhancement of conductivity. I give below accounts of two
typical experiments carried out with petiole-pulvinus preparation of
_Mimosa_. Excessive stimulation in these cases was caused by injury.
[Illustration: FIG. 42.--Effect of injury, depressing rate of
conduction in normal specimen; (1) record before, and (2) after injury.
(Dot-intervals, 0.1 sec.).]
_Action of Injury on Normal Specimens: Experiment 36._--A cut stem
with entire leaf was taken, and stimulus applied at a distance of
15 mm. from the pulvinus. From the normal record (1) in Fig. 42 the
velocity of transmission was found to be 18.7 mm. per sec. The end of
the petiole beyond the point of application of the testing stimulus was
now cut off, and record of velocity of transmission taken once more.
It will be seen from record (2) that the excessive stimulus caused by
injury had induced a depression in the conducting power, the velocity
being reduced to 10.7 mm. per sec. Excessive stimulation of normal
specimens is thus seen to depress temporarily the conducting power.
_Action of Injury on Sub-tonic Specimens: Experiment 37._--I will now
describe a very interesting experiment which shows how an identical
agent may, on account of difference in the tonic condition of the
tissue, give rise to diametrically opposite effects. In demonstrating
this, I took a specimen in a sub-tonic condition, in which the
conducting power of the tissue was so far below par, that the
test-stimulus applied at a distance of 15 mm. failed to be transmitted
(Fig. 43). The end of the petiole at a distance of 1 cm. beyond the
point of application of test-stimulus was now cut off. The after-effect
of this injury was found to enhance the conducting power so that the
stimulus previously arrested was now effectively transmitted, the
velocity being 25 mm. per sec. This enhanced conducting power began
slowly to decline, and after half an hour the velocity had declined
to 4.1 mm. per sec. The end of the petiole was cut once more, and the
effect of injury was again found to enhance the conducting power, the
velocity of transmission being restored to 25 mm. per sec.
[Illustration: FIG. 43.--Effect of injury in enhancing the conducting
power of a sub-normal specimen; (1) Ineffective transmission becoming
effective at (2) after section; (3) decline after half an hour, and (4)
increased conductivity after a fresh cut.]
SUMMARY.
There are two different types of propagation of excitation: by
convection, and by conduction. In the former the excited cell undergoes
deformation and causes mechanical stimulation of the next; example
of this type is seen in the stamen of _Berberis_. The conduction of
excitation consists, on the other hand, of propagation of excitatory
protoplasmic change. The transmission in the petiole of _Mimosa_ is a
phenomenon of conduction.
This conduction takes place along vascular elements. The conductivity
is very much greater in the longitudinal than in the transverse
direction.
Rise of temperature enhances, and fall of temperature lowers, the
rate of conduction. Excitation is transmitted in both directions; the
centrifugal velocity is greater than the centripetal.
Dessication of conducting tissue by glycerine enhances the conducting
power. Local application of cold depresses or arrests the conduction.
Application of poison permanently abolishes the power of conduction.
Conductivity is modified by the effect of season, being higher in
summer than in winter.
The power of conduction is also modified by age. In young specimens the
conducting power is low, the conductivity is at its maximum in fully
grown organs; but a decline of conductivity sets in with age.
The tonic condition of a tissue has an influence on conductivity. In
an optimum condition, the velocity is the same for feeble or strong
stimulus. Excessive stimulation induces a temporary depression of the
conducting power.
The effects are different in a sub-tonic tissue: velocity of
transmission increases with intensity of stimulus; after-effect of
stimulus is to initiate or enhance the conducting power. The conducting
path is canalised by stimulus.
VII.--ON ELECTRIC CONTROL OF EXCITATORY IMPULSE
_By_
SIR J. C. BOSE.
I have in my previous works[L] described investigations on the
conduction of excitation in _Mimosa pudica_. It was there shown that
the various characteristics of the propagation of excitation in the
conducting tissue of the plant are in every way similar to those in
the animal nerve. Hence it appeared probable that any newly found
phenomenon in the one case was likely to lead to discovery of a similar
phenomenon in the other.
[L] BOSE--“Comparative Electro-Physiology” (1907). Longmans, Green and
Co.
As the transmission of excitation is a phenomenon of propagation of
molecular disturbance in the conducting vehicle, it appeared that the
excitatory impulse could be controlled by inducing in the conducting
tissue two opposite ‘molecular dispositions’, using that term in the
widest sense. The possibility of accomplishing this by the directive
action of an electric current had attracted my attention for many years.
METHOD OF CONDUCTIVITY BALANCE.
I have previously carried out an electric method of investigation,
dealing with the influence of electric current on conductivity. The
method of Conductivity Balance which I devised for this purpose[M] was
found very suitable. Isolated conducting tissues of certain plants
were found to exhibit transmitted effect of excitatory electric
change of galvanometric negativity, which at the favourable season of
the year was of sufficient intensity to be recorded by a sensitive
galvanometer. A long strand of the conducting tissue was taken and two
electric connections were made with a galvanometer, a few centimetres
from the free ends. Thermal stimulus was applied at the middle, when
two excitatory waves with their concomitant electric changes were
transmitted outwards. By suitably moving the point of application of
stimulus nearer or further away from one of the two electric contacts,
an exact balance was obtained. This was the case when the resultant
galvanometer deflection was reduced to zero. If now an electrical
current be sent along the length of the conducting tissue, the two
excitatory waves sent outwards from the central stimulated point will
encounter the electric current in different ways; one of the excitatory
waves will travel with, and the other against the direction of the
current. If the power of transmitting excitation is modified by the
direction of an electric current then the magnitudes of transmitted
excitations will be different in the two cases, with the result of the
upsetting of the Conductivity Balance. From the results of experiments
carried out by this method on the effect of feeble current on
conductivity, the conclusion was arrived at that _excitation is better
conducted against the direction of the current than with it_. In other
words, the influence of an electric current is to confer a preferential
or selective direction of conductivity for excitation, the tissue
becoming a better conductor in an electric up-hill direction compared
with a down-hill.
[M] _Ibid_, p. 478.
The results were so unexpected that I have for long been desirous
of testing the validity of this conclusion by independent method of
inquiry. I shall presently give full account of the perfected method,
and the various difficulties which had to be overcome to render it
practical. Before doing this I shall describe a simple method which I
have devised for demonstrating the principal results.
CONTROL OF TRANSMITTED EXCITATION IN _AVERRHOA BILIMBI_.
The petiole of _Averrhoa bilimbi_ has a large number of paired
leaflets, which, on excitation, undergo downward closure. Feeble
stimulus is applied at a point in the petiole, and the transmission of
excitation is visibly manifested by the serial fall of the leaflets.
The distance to which the excitation reaches is a measure of normal
power of conduction. Any variation of conductivity, by the passage of
an electric current in one direction or the other is detected by the
enhancement or diminution of the distance through which excitation is
transmitted. I shall describe the special precautions to be taken in
carrying out this investigation.
[Illustration: FIG. 44.--Diagram of experimental arrangement for
control of transmitted excitation in _Averrhoa bilimbi_. For
explanation see text.]
Electric stimulus of induction shock of definite intensity and
duration is supplied at the middle of the petiole at EE′ (Fig. 44).
The leaflets to the left of E, are not necessary for the purpose of
this experiment and therefore removed. The intensity of the induction
shock may be varied in the usual manner by removing the secondary coil
nearer or farther from the primary. The duration of the shock is always
maintained constant. On application of electric stimulus excitation
is transmitted along the petiole, the distance of transmission
depending on the intensity of stimulus. With feeble stimulus two pairs
of leaflets may undergo an excitatory fall; with stronger stimulus
the transmission is extended to the end of the petiole, and all the
leaflets exhibit movements of closure. We shall now study the modifying
influence of a constant current on conduction of excitation. C is
an electric cell, R the reversing key by which the electric current
could be sent from right to left or in the opposite direction. When
the current is sent from right to the left, the excitatory impulse
initiated at EE′ travels against the direction of the current in an
‘up-hill’ direction. When the current is reversed it flows in the
petiole from left to right and the transmitted impulse travels with the
current or in a ‘down-hill’ direction.
Two complications are introduced on the completion of the electric
circuit of the constant current: the first, is the distributing effect
of leakage of the induction current used for excitation, and second,
the polar variation of excitation induced by the constant current.
_Leakage of induction current._--Before completing the constant current
circuit, the alternating induction current goes only through the path
EE′. On completion of the constant current circuit, the alternating
induction current not only passes through the shorter path EE′ but also
by the circuitous path of the constant current circuit. The escaping
induction current would thus excite all the leaflets directly and
not by its transmitted action. This difficulty is fully overcome by
the interposition of a choking coil which will be described below.
A simpler, though less perfect, device may be employed to reduce and
practically eliminate the leakage. This consists of a loop, L, of
silver wire placed outside EE′. The leakage of induction current is
thus diverted along this path of negligible resistance in preference to
the longer circuit through the entire petiole, which has a resistance
of several million ohms.
_Polar action of current on excitability._--It is well known that an
electric current induces a local depression of excitability at the
point of entrance to the tissue, or at the anode, and an enhancement
of excitability at the point of exit, or at the cathode. But the
excitability is unaffected at a point equally distant from anode
and cathode. This is known as the indifferent point. The exciting
electrodes EE′ are placed at the indifferent point. But when the
current enters on the right side, the terminal leaflets to the right
have their excitability depressed by the proximity of anode, but
the leaflets near the electrodes EE′, being at a distance from the
anode are not affected by it. Moreover it will be shown that the
enhanced conductivity conferred by the directive action of the current
overpowers any depression of excitability in the terminal leaflets due
to the proximity of the anode. I shall, for convenience, designate
the transmission as ‘up-hill’, when excitation is propagated against
the direction of the constant electric current, and ‘down-hill’ when
transmitted with the direction of the current.
_Transmission of excitation ‘Up-hill’: Experiment 38._--I shall give
here an account of an experiment which may be taken as typical. I took
a vigorous specimen of _Averrhoa bilimbi_, and applied a stimulus whose
intensity was so adjusted that the propagated impulse brought about
a fall of only two pairs of leaflets. This gave a measure of normal
conduction without the passage of the current. The constant electric
current was now sent from right to left. A necessary precaution is to
increase the current gradually by means of a suitable potentiometer
slide, to its full value. The reason for this will be given later. The
intensity of the constant current employed was 1.4 micro-ampères. Now
on exciting the petiole by the previous stimulus, the conducting power
was found to be greatly enhanced. The excitatory impulse now reached
the end of the petiole, and caused six pairs of leaflets to fall.
_Transmission of excitation ‘Down-hill’: Experiment 39._--In
continuation of the previous experiment, the constant electric current
was reversed, its directions being now from left to right. Transmission
of excitation was now in a down-hill direction. On applying the
induction shock stimulus of the same intensity as before, the
conducting power of the petiole was found to be abolished, none of the
leaflets exhibiting any sign of excitation. This modification of the
conducting power persists during the passage of the constant current.
On cessation of the current the original conducting power is found
to be restored. It will thus be seen that the power of conduction is
capable of modification, and that the passage of an electric current of
moderate intensity induces enhanced power of conduction in an ‘up-hill’
and diminished conductivity in a ‘down-hill’ direction.
ELECTRIC CONTROL OF NERVOUS IMPULSE IN ANIMALS.
In my ‘Researches on Irritability of Plants’ I have shown how
intimately connected are the various physiological reactions in the
plant and in the animal, and I ventured to predict that the recognition
of this unity of response in plant and animal will lead to further
discoveries in physiology in general. This surmise has been fully
justified, as will be seen in the following experiments carried out on
the nerve-and-muscle preparation of a frog. It is best to carry out
the experiments with vigorous specimens; this ensures success, even in
long continued experiments, which can then be repeated with unfailing
certainty for hours. It is also an advantage to use a large frog for
its relatively great length of the nerve.
_Directive action of current on conduction of excitation in a
nerve-and-muscle preparation: Experiment 40._--A preparation was made
with a length of the spine and two nerves leading to the muscles. The
specimen is supported in a suitable manner, and electric connections
made with the toes, one for the entrance and the other for exit of
the constant current. The current thus entered, say, by the left
toe ascended the muscle and went up the nerve on the left side, and
descended through the other nerve on the right side along the muscle
and thence to the right toe. Before the passage of the constant
electric current the spinal nerve was stimulated by an induction
shock of definite intensity. The nervous impulse was conducted by the
two nerves, one to the left and the other to the right, and caused
a feeble twitch of the respective muscles. A feeble current of 1.5
micro-ampère was sent along the nerve-and-muscle circuit, ascending
by the left and descending by the right side. It will be seen that
excitation initiated at the spine is propagated ‘against’ the electric
current on the left side, and ‘with’ the current on the right side.
On repetition of previous electric stimulus the effect of directive
action of current was at once manifested by the left limb being
thrown into a state of strong tetanic contraction, whereas the right
limb remained quiescent. By changing the direction of the constant
current the induced enhancement of conductivity of the nerve was
quickly transferred from the left to the right side, the depression
or arrest of conduction being simultaneously transferred to the left
side. Turning the reversing key one way or the other brought about
_supra_ or _non_-conducting state of the nerve, and this condition was
maintained throughout the duration of the current.
I shall next describe a more perfect method for obtaining quantitative
results both with plant and animal. In order to demonstrate the
universality of the phenomenon, I next used _Mimosa pudica_ instead of
_Averrhoa_, for experiments on plants.
For determination of normal velocity of transmission of excitation
and the induced variation of that velocity, I employed the automatic
method of recording the velocity of transmission of excitation in
_Mimosa_, where the excitatory fall of the motile leaf gave a signal
for the arrival of the excitation initiated at a distant point. In this
method the responding leaf is attached to a light lever, the writer
being placed at right angles to it. The record is taken on a smoked
glass plate, which during its descent makes an instantaneous electric
contact, in consequence of which a stimulating shock is applied at a
given point of the petiole. A mark in the recording plate indicates
the moment of application of stimulus. After a definite interval the
excitation is conducted to the responding pulvinus, when the excitatory
fall of the leaf pulls the writer suddenly to the left. From the
curve traced in this manner the time-interval between the application
of stimulus and the initiation of response can be found, and the
normal rate of transmission of excitation through a given length of
the conducting tissue deduced. The experiment is then repeated with
an electric current flowing along the petiole with or against the
direction of transmission of excitation. The records thus obtained
enable us to determine the influence of the direction of the current
on the rate of transmission. I shall presently describe the various
difficulties which have to be overcome before the method just indicated
can be rendered practical.
The scope of investigation will be best described according to the
following plan[N]:--
[N] For fuller account _see_ Bose--‘The influence of Homodromous and
Heterodromous Electric Current on Transmission of Excitation in Plant
and Animal.’ Proc. R. S. B., Vol. 88, 1915.
PART I.--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF
EXCITATION IN PLANTS.
General method of experiment.
Effect of feeble current on velocity of transmission of excitation
‘up-hill’ or ‘down-hill.’
Determination of variation of conductivity by the method of minimal
stimulus and response.
The after-effect of current.
PART II.--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF
EXCITATION IN ANIMAL NERVE.
The method of experiment.
Variation of velocity of transmission under the action of current.
Variation in the intensity of transmitted excitation.
PART I.--INFLUENCE OF DIRECTION OF CURRENT ON TRANSMISSION OF
EXCITATION IN PLANT.
THE METHOD OF EXPERIMENT.
I may here say a few words of the manner in which the period of
transmission can be found from the record given by my Resonant
Recorder, fully described in my previous paper. The writer attached to
the recording lever of this instrument is maintained by electromagnetic
means in a state of to-and-fro vibration. The record thus consists
of a series of dots made by the tapping writer, which is tuned to
vibrate at a definite rate, say, 10 times per second. In a particular
case whose record is given in Curve 1 (Fig. 46), indirect stimulus
of electric shock was applied at a distance of 15 mm. from the
responding pulvinus. There are 15 intervening dots between the
moment of application of stimulus and the beginning of response; the
time-interval is therefore 1.5 seconds. The latent period of the
motile pulvinus is obtained from a record of direct stimulation; the
average value of this in summer is 0.1 second. Hence the true period
of transmission is 1.4 seconds for a distance of 15 mm. The velocity
determined in this particular case is therefore 10.7 mm. per second.
Precaution has to be taken against another source of disturbance,
namely, the excitation caused by the sudden commencement or the
cessation of the constant current. I have shown elsewhere[O] that the
sudden initiation or cessation of the current induces an excitatory
reaction in the plant-tissue similar to that in the animal tissue. This
difficulty is removed by the introduction of a sliding potentiometer,
which allows the applied electromotive force to be gradually increased
from zero to the maximum or decreased from the maximum to zero.
[O] BOSE--‘Plant Response’ (1906); ‘Irritability of Plants’ (1913).
[Illustration: FIG. 45.--Complete apparatus for investigation of
the variation of conducting power in _Mimosa_. A, storage cell; S,
potentiometer slide, which, by alternate movement to right or left,
continuously increases or decreases the applied E. M. F.; K, switch key
for putting current “on” and “off” without variation of resistance; E,
E′, electrodes of induction coil for stimulation; C, choking coil; G,
micro-ammeter.]
The experimental arrangement is diagrammatically shown in Fig. 45.
After attaching the petiole to the recording lever, indirect stimulus
is applied, generally speaking, at a distance of 15 mm. from the
responding pulvinus. Stimulus of electric shock is applied in the
usual manner, by means of a sliding induction coil. The intensity of
the induction shock is adjusted by gradually changing the distance
between the secondary and the primary, till a minimally effective
stimulus is found. In the study of the effect of direction of constant
current on conductivity, non-polarisable electrodes make suitable
electric connections, one with the stem and the other with the tip of
a sub-petiole at a distance from each other of about 95 mm. The point
of stimulation and the responding pulvinus are thus situated at a
considerable distance from the anode or the cathode, in the indifferent
region in which there is no polar variation of excitability. By
means of a Pohl’s commutator or reverser, the constant current can be
maintained either “with” or “against” the direction of transmission of
excitation. The transmission in the former case is “down-hill,” and
in the latter case “up-hill.” Electrical connections are so arranged
that when the commutator is tilted to the right, the transmission is
down-hill, when tilted to the left, up-hill.
The electrical resistance offered by the 95 mm. length of stem and
petiole will be from two to three million ohms. The intensity of the
constant current flowing through the plant can be read by unplugging
the key which short-circuits the micro-ammeter G. The choking coil
C prevents the alternating induction current from flowing into the
polarising circuit and causing direct stimulation of the pulvinus.
Before describing the experimental results, it is as well to enter
briefly into the question of the external indication by which the
conducting power may be gauged. Change of conductivity may be
expected to give rise to a variation in the rate of propagation
or to a variation in the magnitude of the excitatory impulse that
is transmitted. Thus we have several methods at our disposal for
determining the induced variation of conductivity. In the first place
the variation of conductivity may be measured by the induced change
in the velocity of transmission of excitation. In the second place,
the transmitted effect of a sub-maximal stimulus will give rise to
enhanced or diminished amplitude of mechanical response, depending on
the increase or decrease of conductivity brought about by the directive
action of the current. And, finally, the enhancement or depression
of conductivity may be demonstrated by the ineffectively transmitted
stimulus becoming effective, or the effectively transmitted stimulus
becoming ineffective.
_Exclusion of the factor of Excitability._--The object of the enquiry
being the pure effect of variation of conductivity, we have to assure
ourselves that under the particular conditions of the experiment the
complicating factor of polar variation of excitability is eliminated.
It is to be remembered that excitatory transmission in _Mimosa_ takes
place by means of a certain conducting strand of tissue which runs
through the stem and the petiole. In the experiment to be described,
the constant current enters by the tip of the petiole and leaves by
the stem, or _vice versâ_, the length of the intrapolar region being
95 mm. The point of application of stimulus on the petiole is 40 mm.
from the electrode at the tip of the leaf. The responding pulvinus is
also at the same distance from the electrode on the stem. The point of
stimulation and region of response are thus at the relatively great
distance of 40 mm. from either the anode or the cathode, and may
therefore be regarded as situated in the indifferent region. This is
found to be verified in actual experiments.
EFFECTS OF DIRECTION OF CURRENT ON VELOCITY OF TRANSMISSION.
A very convincing method of demonstrating the influence of electric
current on conductivity consists in the determination of changes
induced in the velocity of transmission by the directive action of the
current. For this purpose we have to find out the true time required
by the excitation to travel through a given length of the conducting
tissue (1) in the absence of the current, (2) ‘against’ and (3) ‘with’
the direction of the current. The true time is obtained by subtracting
the latent period of the pulvinus from the observed interval between
the stimulus and response. Now the latent period may not remain
constant, but undergo change under the action of the polarising
current. It has been shown that the excitability of the pulvinus does
not undergo any change when it is situated in the middle or indifferent
region. The following results show that under parallel conditions the
latent period also remains unaffected:--
TABLE V.--SHOWING THE EFFECT OF ELECTRIC CURRENT ON THE LATENT PERIOD.
+------------------------------------------------+-------+-------+
| Specimens | I. | II. |
|------------------------------------------------+-------|-------|
| | sec. | sec. |
| Latent period under normal condition | 0.10 | 0.09 |
| " " current from right to left | 0.11 | 0.10 |
| " " current from left to right | 0.09 | 0.09 |
+------------------------------------------------+-------+-------+
The results of experiments with two different specimens given above
show that a current applied under the given conditions has practically
no effect on the latent period, the slight variation being of the order
of one-hundredth part of a second. This is quite negligible when the
total period observed for transmission is, as in the following cases,
equal to nearly 2 seconds.
_Induced changes in the Velocity of Transmission._--Having found that
the average value of the latent period in summer is 0.1 second, we
next proceed to determine the influence of the direction of current on
velocity.
_Experiment 41._--As a rule, stimulus of induction shock was applied
in this and in the following experiments on the petiole at a distance
of 15 mm. from the responding pulvinus. The recording writer was
tuned to 10 vibrations per second; the space between two succeeding
dots, therefore, represents a time-interval of 0.1 second. The middle
record, N in Fig. 46, is the normal. There are 17 spaces between the
application of stimulus and the beginning of response. The total time
is therefore 1.7 seconds, and by subtracting from it the latent period
of 0.1 second we obtain the true time, 1.6 seconds. The normal velocity
is found by dividing the distance 15 mm. by the true interval 1.6
seconds. Thus V = 15/1.6 = 9.4 mm. per second. We shall next consider
the effect of current in modifying the normal velocity. The uppermost
record (1) in Fig. 46 was taken under the action of an ‘up-hill,’ or
‘against’ current of the intensity of 1.4 microampères. It will be seen
that the time interval is reduced from 1.7 seconds to 1.4 seconds;
making allowance for the latent period, the velocity of transmission
under ‘up-hill’ current V_{1} = 15/1.3 = 11.5 mm. per second. In the
lowest record (3) we note the effect of ‘down-hill’ current, the
time-interval between stimulus and response being prolonged to 1.95
seconds and the velocity reduced to 8.1 mm. per second. The conclusion
arrived at from this mechanical mode of investigation is thus identical
with that derived from the electric method of conductivity balance
referred to previously.
[Illustration: FIG. 46.--Record showing enhancement of velocity of
transmission “up-hill” or against the current (uppermost curve) and
retardation of velocity “down-hill” or with the current (lowest curve).
N, normal record in the absence of current; ← indicates “up-hill” and →
“down-hill” transmission.]
That is to say, _the passage of a feeble current modifies conductivity
for excitation in a selective manner. Conductivity is enhanced_
against_, and diminished_ with_, the direction of the current._
The minimum current which induces a perceptible change of conductivity
varies somewhat in different specimens. The average value of this
minimal current in autumn is 1.4 microampères. The effect of even a
feebler current may be detected by employing a test stimulus which is
barely effective.
TABLE VI.--SHOWING EFFECTS OF UP-HILL AND DOWN-HILL CURRENTS OF FEEBLE
INTENSITY ON PERIOD OF TRANSMISSION THROUGH 15 MM.
+-------+-------------+---------------------+---------------------+
|Number.|Intensity of | Period for | Period for |
| | current in | up-hill | down-hill |
| |microampères.| transmission. | transmission. |
+-------+-------------+---------------------+---------------------+
| 1 | 1.4 |14 tenths of a second|16 tenths of a second|
| 2 | 1.4 |13 " " |15 " " |
| 3 | 1.6 |19 " " |Arrest. |
| 4 | 1.7 |12 " " |14 tenths of a second|
+-------+-------------+---------------------+---------------------+
Having demonstrated the effect of direction of current on the velocity
of transmission, I shall next describe other methods by which induced
variations of conductivity may be exhibited.
DETERMINATION OF VARIATION OF CONDUCTIVITY BY METHOD OF MINIMAL
STIMULUS AND RESPONSE.
In this method we employ a minimal stimulus, the transmitted effect of
which under normal conditions gives rise to a feeble response. If the
passage of a current in a given direction enhances conductivity, then
the intensity of transmitted excitation will also be enhanced; the
minimal response will tend to become maximal. Or excitation which had
hitherto been ineffectively transmitted will now become effectively
transmitted. Conversely, depression of conductivity will result in a
diminution or abolition of response. We may use a single break-shock
of sufficient intensity as the test stimulus. It is, however,
better to employ the additive effect of a definite number of feeble
make-and-break shocks.
We may again employ additive effect of a definite number of induction
shocks, the alternating elements of which are exactly equal and
opposite. This is secured by causing rapid reversals of the primary
current by means of a rotating commutator. The successive induction
shocks of the secondary coil can thus be rendered exactly equal and
opposite.
_Experiment 42._--Working in this way, it is found that the transmitted
excitation against the direction of current becomes effective or
enhanced under ‘up-hill’ current. A current, flowing with the direction
of transmission, on the other hand, diminishes the intensity of
transmitted excitation or blocks it altogether.
Henceforth it would be convenient to distinguish currents in the
two directions: the current in the direction of transmission will
be distinguished as _Homodromous_, and against the direction of
transmission as _Heterodromous_.
AFTER-EFFECTS OF HOMODROMOUS AND HETERODROMOUS CURRENTS.
The passage of a current through a conducting tissue in a given
direction causes, as we have seen, an enhanced conductivity in an
opposite direction. We may suppose this to be brought about by a
particular molecular arrangement induced by the current, which
assisted the propagation of the excitatory disturbance in a selected
direction. On the cessation of this inducing force, there may be a
rebound and a temporary reversal of previous molecular arrangement,
with concomitant reversal of the conductivity variation. The immediate
after-effect of a current flowing in a particular direction on
conductivity is likely to be a transient change, the sign of which
would be opposite to that of the direct effect. The after-effect of a
heterodromous current may thus be a temporary depression, that of a
homodromous current, a temporary enhancement of conductivity.
[Illustration: FIG. 47.--Direct and after-effect of heterodromous and
homodromous currents. First two records, N, N, normal. ↓, enhanced
transmission under heterodromous current; ⇣ arrest of conduction is an
after-effect of heterodromous current. Next record ↑ shows arrest under
homodromous current. Last record ⇡ shows enhancement of conduction
greater than normal, as an after-effect of homodromous current. (Dotted
arrow indicates the after-effect on cessation of a given current. ↑
homodromous and ↓ heterodromous current.)]
_Experiment 43._--This infer
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