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.]