TROPISM IN PLANTS.
XXII.--THE BALANCED CRESCOGRAPH
_By_
SIR J. C. BOSE.
We shall in the succeeding series of papers deal with the subject of
tropism in general. Different plant organs undergo curvature or bending,
sometimes towards and at other times away from the stimulus which
induces it. The problem is very intricate; the possibility of its
solution will depend greatly on the accurate determination of the
immediate and after-effects of various stimuli on the responding organ.
The curvature induced in the growing organ is brought about by
variation, often extremely slight, of the rate of growth; the result,
moreover, is liable to be modified by the duration and point of
application of stimulus. The difficulties connected with the problem can
only be removed by the detection and measurement of the minutest
variation in growth, and by securing a continuous and automatic record
of the entire history of the change.
In the chapter on High Magnification Crescograph an account is given of
the apparatus which I have devised by which the rate of growth may be
magnified from ten thousand to ten millions times. It is thus possible
to measure the imperceptible growth of plants for a period shorter than
a single second. The variation of normal rate of growth is also found by
measuring successive growth records on a stationary plate at regular
intervals, say of ten seconds, or from the flexure in the growth-curve
taken on a moving plate (p. 163).
I was next desirous of exalting the sensitiveness to a still higher
degree by an independent method, which would not only reveal very
slight variation induced in the rate of growth, but also the latent
period and time-relations of the change. For this purpose I at first
devised the Optical Method of Balance[1] which was considered at the
time to be extremely sensitive. The spot of light from the Optical Lever
(which magnified the rate of growth) was made to fall upon a mirror to
which a compensating movement was imparted so that the light-spot after
double reflection remained stationary. Any change of rate of
growth--acceleration or retardation--was at once detected by the
movement of the hitherto stationary spot of light in one direction or
the other.
[1] "Plant Response"--p. 413.
A very careful manipulation was required for the adjustment of the
Optical Balance; the record moreover was not automatic. For these
reasons I have been engaged for several years past in perfecting a new
apparatus by which, (1) the balance could be directly obtained with the
utmost exactitude, (2) where an attached scale would indicate the exact
rate of growth, and (3) in which the upsetting of the balance by
external stimulus would be automatically recorded, the curve giving the
time relations of the change.
PRINCIPLE OF THE METHOD OF BALANCE.
I shall take a concrete example in explanation of the method of balance.
Taking the rate of growth per second of a plant to be 1/50,000 inch or
0·5 µ, per second (equal to the wave length of sodium light), the tip of
the plant will be maintained at the same point in space if we succeeded
in making the plant-holder subside exactly at the same rate. The
growth-elongation of the plant will then be exactly balanced by a
compensating movement downwards. The state of exact balance is indicated
when the recording lever of the Crescograph traces a horizontal line on
the moving plate. Overbalance or underbalance will deflect the record
below or above the horizontal line.
[Illustration: FIG. 93.--Arrangement for compensation of growth-movement
by equal subsidence of plant-holder; S, adjusting screw for regulation
of speed of rotation; G, governor; W, heavy weight; P, plant-holder.]
COMPENSATING MOVEMENT.
For securing exact balance the holder of the plant P, in the given
example, will have to subside at a rate of 1/50,000 inch per second.
This is accomplished by a system of reducing worm and pinion, also of
clock wheels (Fig. 93). The clock at first used for this purpose was
worked by the usual balance wheel. Though this secured an _average_
balance yet as each tick of the clock consisted of sudden movement and
stoppage, it caused minute variation in the rate of subsidence; this
became magnified by the Crescograph and appeared as a series of
oscillations about a mean position of equilibrium. This particular
defect was obviated by the substitution of a fan governor for the
balance wheel. But the speed of rotation slows down with the unwinding
of the main spring, and the balance obtained at the beginning was found
to be insufficient later on. The difficulty was finally overcome by the
use of a heavy weight W, in the place of coiled spring. The complete
apparatus is seen in figure 94.
[Illustration: FIG. 94.--Photographic reproduction of the Balanced
Crescograph. L, L', magnifying compound lever. R, recording plate.
P, plant. C, clock work for oscillation of the plate and lateral
movement. G, governor. M, circular growth-scale. V, plant-chamber.]
For purpose of simplicity of explanation, I assumed the growth rate to
have a definite value of 1/50,000 inch per second. But the rate varies
widely in different plants and even in the same plant at different days
and seasons. In practice the rate of growth for which compensation has
to be made varies from 1/150,000 to 1/25,000 inch, or from 0·17 µ to
1·0 µ per second. We have thus to secure some means of _continuous_
adjustment for growth, the rate of which could be continuously varied
from one to six times. This range of adjustment I have been able to
secure by the compound method of frictional resistance and of
centrifugal governor. As regards frictional resistance the two pointed
ends of a hinged fork rub against a horizontal circular plate not shown
in the figure. By means of the screw head S, the free ends of the fork
spread out and the circumference of the frictional circle continuously
increased. The centrifugal governor is also spread out by the action of
the adjusting screw. By the joint actions of the frictional control and
the centrifugal governor, the speed of rotation can be continuously
adjusted from 1 to 6 times. When the adjusting screw is set in a
particular position, the speed of rotation, and therefore the rate of
subsidence of plant-holder, remains absolutely constant for several
hours. The attainment of this constancy is a matter of fundamental
importance, and it was only by the employment or the compound system of
regulation that I was able to secure it.
The method of obtaining balance now becomes extremely simple. Before
starting the balancing movement by clock regulation, the plant is made
to record its magnified growth by the Crescograph. The compensation is
effected as follows: the speed of the clockwork is at the beginning
adjusted at its lowest value, and the pressure of a button starts the
balancing movement of the plant downwards. On account of partial balance
the record will be found to be less steep than before; the speed of the
clock is gradually increased till the record becomes perfectly
horizontal under exact balance. Overbalance makes the record slope
downwards. In figure 95 is seen records of underbalance (_a_) and
overbalance (_b_), to the extent of about 3 per cent.
[Illustration: FIG. 95.--Balanced Crescographic record: (_a_) showing
effect of underbalance and (_b_) overbalance of about 3 per cent.
(Magnification 2,000 times.)]
It will thus be seen that the effect of an external agent may be
detected by the upsetting of the balance; an up-movement indicates
(unless stated to the contrary) an enhancement of the rate of growth
above the normal; and a down-movement, on the other hand, a depression
of the normal rate.
_Calibration._--The calibration of the instrument is obtained in two
different ways. The rate of subsidence of the plant-holder, by which the
balance is obtained, is strictly proportional to the rate of rotation of
the vertical spindle and the attached train of clock-wheels. A striker
is attached to one of the wheels, and a bell is struck at each complete
revolution. The clockwork is adjusted at a medium speed, the bell
striking 35 times in a minute. A microscope micrometer is focussed on a
mark made on the plant-holder, and the amount of subsidence of the mark
determined after one minute; this was found to be 0·0525 mm. As this
fall occurred after 35 strokes of the bell the subsidence per stroke was
0·0015 mm.
_Determination of the absolute rate of growth._--If growth be found
balanced at N strokes of bell per minute, the rate of subsidence per
second
= N × ·0015/60 mm. per second
= N × ·000025 mm. per second
= N × ·025 µ per second
= N × 10^{-5} inch per second.
_Example._--The growth of a specimen of _Zea Mays_ was found balanced
when the number of strokes of the bell was 20 times in a minute.
Absolute rate of growth = 20 × ·025 µ = 0·5 µ per second
or = 20 × 10^{-5} inch "
or = 1/50,000 " "
If we take the wave length of sodium light [Greek: lambda] as our
standard, the growth in length per second is equal to [Greek: lambda].
This will give us some idea of the sensitiveness of the Crescograph
employed in recording the movement of growth.
GROWTH-SCALE.
The Balanced Crescograph enables us not merely to determine the absolute
rate of growth, but the slightest fluctuation in that rate.
_Indicator Scale._--All necessity of calculation is obviated by the
scale provided with the apparatus. The speed of clockwork which brings
about the balance of growth is determined by the position of the
adjusting screw S, the gradual lowering of which produces a continuous
diminution of speed. A particular position of the screw therefore
indicates a definite rate of subsidence for balancing growth. By a
simple mechanism the up or down movement of the screw causes rotation of
an index pivoted at the centre of a circular scale. Each division of the
scale is calibrated by counting the corresponding number of strokes of
the bell per minute at different positions of the adjusting screw. The
scale is calibrated in this manner to indicate different rates of growth
from 0·2 µ to 1·2 µ per second.
The determination of the rate of growth now becomes extremely simple.
Few turns of the screw bring about the balance of growth and the
resulting position of the index against the circular scale automatically
indicates the absolute rate. The procedure is even simpler and more
expeditious than the determination of the weight of a substance by means
of a balance.
SENSITIVENESS OF THE CRESCOGRAPHIC BALANCE.
Perhaps the most delicate method of measuring lengths is that afforded
indirectly by the spectrum of a light. A good spectroscope resolves
differences of wave lengths of D_{1} ( = 0·5896 µ) and D_{2} ( = 0·5890)
_i.e._ of 1 part in a thousand. The average rate of growth of _Zea Mays_
is of this order; being about 0·5 µ per second. Let us consider the
question of the possibility of detecting a fractional variation of the
ultra-microscopic length by means of the Balanced Crescograph. In
reality the problem before us is more intricate than simple measurement
of change of length; for we have to determine the _rate of variation_ of
length.
The sensitiveness of the balance will, it is obvious, depend on the
magnifying power of the Crescograph. By the Method of Magnetic
Amplification referred to in page 170, I have succeeded in obtaining a
magnification of ten million times. In this method a very delicate
astatic system of magnets undergoes deflection by the movement of a
magnetised lever in its neighbourhood. A spot of light reflected from a
small mirror attached to the astatic system, thus gives the highly
magnified movement of the rate of growth, which may easily be raised to
ten million times. I shall in the following describe the results
obtained with this easily managed magnification of ten million times.
_Determination of sensitiveness: Experiment 99._--A seedling of _Zea
Mays_ was placed on the Crescographic Balance; and the magnetic
amplification, as stated above, was ten million times. With 18 strokes
of the bell per minute the spot of light had a drift of + 266 cm. per
minute to the right; this is because the growth was underbalanced. With
faster rate of clock movement, _i.e._, 21 strokes in 68 seconds or 18·53
strokes per minute, the drift of the spot of light, owing to
overbalance, was to the left at the rate of -530 cm. per minute.
Thus
(1) 18 strokes per minute caused a drift of +266 cm. per minute.
(2) 18·53 strokes per minute caused a drift of -530 cm. per minute.
Hence by interpolation the exact balance is found to correspond to
18·177 strokes per minute.
Therefore the absolute rate of growth
= 18·177 × 0·025 µ per second.
= 0·45 µ per second.
= 0·000018 inch per second.
We learn further from (1) and (2) that a variation of
(18·53 - 18)/18·177 produces a change of drift of the spot of
light from +266 to -530 cm., _i.e._, of 796 cm. per minute.
As it is easy to detect a drift of 1 cm. per minute a variation of
0·53/(18·177 × 796), or 1 part in 27,000 may thus be detected by the
Method of Balance. The spectroscopic method enabled us, as we saw, to
detect change of wave length 1 part in a thousand. The sensibility of
the Balanced Crescograph is thus seen to rival, if not surpass that of
the spectroscope.
For obtaining a general idea of the sensitiveness, the absolute of
growth in the instance given above was 0·00018 inch per second, and the
Balanced Crescograph was shown capable of discriminating a variation
of 1 part in 27,000; hence it is possible to detect by this means a
variation of 1/1,500 millionth of an inch per second.
This method of unprecedented delicacy opens out a new field of
investigation on the effect of changes of environment in modification of
growth; instances of this will be found in subsequent chapters. I give
below accounts of certain demonstrations which will no doubt appear as
very striking.
After obtaining the exact balance a match was struck in the
neighbourhood of the plant. This produced a marked movement of the
hitherto quiescent spot of light, thus indicating the perception of such
an extremely feeble stimulus by the plant.
Breathing on the plant causes an enhancement of growth due to the joint
effects of warmth and carbonic acid gas. A more striking experiment is
to fill a small jar with carbonic acid and empty it over the plant. A
violent movement of the spot of light to the right demonstrates the
stimulating effect of this gas on growth.
The method described above is excessively sensitive; for general
purposes and for the method of direct record, a less sensitive
arrangement is sufficient. I give below accounts of several typical
experiments in which the recording form of Crescograph was employed, the
magnification being only 2,000 times.
[Illustration: FIG. 96.--Record showing the effect of CO_{2}. Horizontal
line at beginning indicates balanced growth. Application of CO_{2} at
arrow induces enhancement of growth shown by the up-curve followed by
depression, shown by the down-curve. Successive dots at intervals of 10
seconds. (Seedling of wheat.)]
_Effect of carbonic acid on Balanced growth: Experiment 100._--I have
already shown that carbonic acid diluted with air induces an enhancement
of the rate of growth, but its long continued action induces a
depression (p. 185). I shall now employ the Method of Balance in
studying the effect of CO_{2} on growth. It should be remembered in this
connection that the horizontal record indicates the balance of normal
rate of growth. An up-curve exhibits the induced enhancement and a
down-curve, a depression of growth. In the present experiment after
obtaining the exact balance, pure carbonic acid gas was made to fill up
the plant-chamber at the point marked with an arrow (Fig. 96). It will
be seen that this induced an almost immediate acceleration of the rate,
the latent period being less than five seconds. The acceleration
continued for two and half minutes; the accelerated rate then slowed
down, became enfeebled, and the growth returned for a short time to the
normal as indicated by the horizontal portion at the top of the record;
this proved to be the turning point of inversion from acceleration into
retardation of growth. The stronger is the concentration of the gas the
earlier is the point of inversion. With diluted carbonic acid the
acceleration may persist for an hour or more.
EFFECT OF ANÆSTHETICS.
_Effect of Ether: Experiment 101._--Dilute vapour of ether is found to
induce an acceleration of rate of growth which persist for a
considerable length of time. This is seen in the upsetting of the
balance upwards on the introduction of the vapour (Fig. 97a.).
[Illustration: FIG. 97.--(_a_) Effect of ether, acceleration of growth,
(_b_) effect of chloroform preliminary acceleration followed by
depression.]
_Effect of Chloroform: Experiment 102._--The effect of chloroform vapour
is relatively more depressing than ether. Application of chloroform is
seen to induce at first an acceleration which persisted for 50 seconds,
but after this depression set in (Fig. 97b). Prolonged application of
the anæsthetic is followed by the death of the plant.
SUMMARY.
In the Method of Balance the movement of growth upwards is compensated
by an equal movement of the plant downwards, with the result that the
record remains horizontal.
The effect of an external agent is immediately detected by the upsetting
of the balance, up-record representing acceleration above normal, a
down-record the opposite effect of depression below the normal rate.
The latent period and the after-effect of stimulus may thus be obtained
with the highest accuracy.
The sensitiveness of the Method of Balance may be raised so as to
indicate a variation of rate of growth smaller than 1/1000 millionth of
an inch per second.
XXIII.--ON TROPIC MOVEMENTS
_By_
SIR J. C. BOSE.
The diverse movements induced by external stimuli in different organs of
plants are extremely varied and complicated. The forces in operation are
manifold--the influence of changing temperature, the stimulus of
contact, of electric current, of gravity, and of light visible and
invisible. They act on organs which exhibit all degrees of physiological
differentiation, from the radial to the dorsiventral. An identical
stimulus may sometimes induce one effect, and at other times, the
precisely opposite. Thus under unilateral stimulation of light of
increasing intensity, a radial organ exhibits a positive, a
dia-phototropic, and finally a negative response. Strong sunlight brings
about para-heliotropic or 'midday sleep' movement, by which the apices
of leaves or leaflets turn towards or away from the source of
illumination. The teleological argument advanced, that in this position
the plant is protected from excessive transpiration, does not hold good
universally; for under the same reaction, the leaflets of _Cassia
montana_ assume positions by which the plant risks fatal loss of water.
In _Averrhoa carambola_ the movement is downwards, whichever side is
illuminated with strong light; in _Mimosa_ leaflet the movement, under
similar circumstances is precisely in the opposite direction. The
photonastic movement, apparently independent of the directive action of
light, has come to be regarded as a phenomenon unrelated to phototropic
reaction, and due to a different kind of irritability, and a different
mode of response. So very anomalous are these various effects that
Pfeffer, after showing the inadequacy of different theories that have
been advanced, came to the conclusion that "the precise character of the
stimulatory action of light has yet to be determined. When we say that
an organ curves towards a source of illumination because of its
heliotropic irritability, we are simply expressing an ascertained fact
in a conveniently abbreviated form, without explaining why such
curvature is possible or how it is produced."[2]
[2] Pfeffer--_Ibid_--Vol. II, p. 74.
The contradictory nature of the various responses is however not real;
the apparent anomaly had lain in the fact that two definite fundamental
reactions of opposite signs induced by stimulus had not hitherto been
recognised and distinguished from each other. The innumerable variations
in the resultant response are due to the summation of the effects of two
fluctuating factors, with further complications arising from: (1)
difference in the point of application of stimulus, (2) the differential
excitability of the different sides of the responding organ, and (3) the
effect of temperature in modifying tropic curvature. It is therefore
most important to have the means for automatic record of _continuous_
change in the response brought about by various factors, which act
sometimes in accord, and at other times in conflict. The autograph of
the plant itself, giving a history of the change in response and its
time-relations, is alone decisive in explanation of various difficulties
in connection with plant movements, as against the various tentative
theories that have been put forward. The analysis of the resulting
effect, thus rendered possible, casts new light on the phenomena of
response, proving that the anomalies which had so long perplexed us, are
more apparent than real.
One of the causes of uncertainty lay with the question, whether response
changed with the mode of stimulation. I have, however, been able to show
that _all forms of stimuli_ induce a definite excitatory reaction of
contraction (p. 218).
Tropic movements induced by unilateral action of stimulus may, broadly
speaking, be divided into two classes depending on the point of
application of stimulus:
In the first, the point of application of unilateral stimulus is not on
the responding organ itself, but at some distance from it. The question
therefore relates to LONGITUDINAL TRANSMISSION of effect of stimulus.
In the second, unilateral stimulus acts directly on the responding
organ. For the determination of the resultant movement, it is necessary
to take account of effects induced on the two sides of the organ. The
side adjacent to the stimulus I shall designate as the _proximal_, and
the diametrically opposite as the _distal_ side. The question to be
investigated in this case relates to TRANSVERSE TRANSMISSION of effect
of stimulus. It will be shown that the resulting movement depends on:--
(_a_) whether the tissue is a conductor or a non-conductor of
excitation in a transverse direction, and
(_b_) whether it is the proximal, or the distal side of the
organ that is the more excitable.
In connection with the response to environmental changes, a source of
uncertainty is traceable to the absence of sufficient knowledge of the
physiological effect of heat, which has been regarded as a form of
stimulus: it will be shown that heat induces two distinct effects
dependent on conduction and radiation. We shall in the succeeding
chapters, take up the study of the physiological effects induced by
changes in the environment.
XXIV.--TROPIC CURVATURE WITH LONGITUDINAL TRANSMISSION OF EFFECT OF
STIMULUS
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
I have in previous chapters explained that the direct application of
stimulus gives rise in different organs to contraction, diminution of
turgor, fall of motile leaf, electro-motive change of galvanometric
negativity, and retardation of the rate of growth. I have also shown
that indirect stimulation (_i.e._ application of stimulus at some
distance from the responding organ) gives rise to a positive or erectile
response of the responding leaf or leaflet (indicative of an increase of
turgor), often followed by normal negative response. The positive
impulse travels quickly. The interval of time that elapses, between the
application of stimulus and the erectile response of the responding
leaf, depends on the distance of the point of application, and the
character of the transmitting tissue: it varies in different cases from
0·6 second to about 40 seconds. The positive is followed by a slower
wave of protoplasmic excitation, which causes the excitatory fall. The
velocity of this excitatory impulse is about 30 mm. per second in the
petiole of _Mimosa_, and about 3 mm. per second in _Biophytum_. The
positive followed by the negative thus gives rise to a diphasic
response. The excitatory impulse is much enfeebled during transit: the
negative impulse may thus fail to reach the responding organ, if the
stimulus be feeble or if the intervening distance be long or
semi-conducting. Hence moderate stimulus applied at a distance gives
rise only to positive response; direct application of strong stimulus
gives rise, on the other hand, to the normal negative. By employing the
electric method of investigation, I have obtained with ordinary tissues
the positive, the diphasic, and the negative electric response, in
correspondence with the responses given by a motile organ (p. 214). The
mechanics of propagation of the positive and the negative impulse are
different. It is therefore necessary to distinguish the quick
_transmission_ of the positive impulse from the slow _conduction_ of the
negative impulse due to the propagation of excitatory protoplasmic
change.
It should be borne in mind in this connection that all responsive
movements are ultimately due to protoplasmic changes which are beyond
our scrutiny. We can infer the nature of the change by the concomitant
outward manifestations, which are of two kinds: the _positive_,
associated with increase of turgor, expansion, and galvanometric
positivity, and the _negative_ with concomitant decrease of turgor,
contraction, and galvanometric negativity. Thus positive and negative
reactions indicate the fundamental protoplasmic changes of opposite
characters.
The movement and curvature induced by stimulus have, for convenience,
been distinguished as _positive curvature_, (movement towards stimulus),
and _negative curvature_ (movement away from stimulus). Though these
curvatures result from protoplasmic reactions, yet the _positive
curvature_ is not necessarily associated with _positive protoplasmic
reaction_. It will be shown that the curvature of an organ is determined
by the algebraical summation of effects induced at the proximal and
distal sides of the responding organ.
Physiologists have not been aware of the dual character of the impulse
generated by stimulus, and the term "transmission of stimulus" is thus
misleading since its effect may be an expansion, or its very opposite,
contraction. It is therefore necessary to discriminate the effect of one
from the other: the impulse which induces an increase of turgor,
expansion, and galvanometric positivity will be distinguished as
positive, in the sense that it causes an enhancement of turgor. The
other, which induces diminution of turgor and contraction, will be
termed as the excitatory impulse. Transmission of the latter is
dependent on conducting power of the tissue; the positive impulse is
practically independent of the conducting power.
In animal physiology again, there is no essential difference between the
effect of the direct and indirect stimulation. In a nerve-and-muscle
preparation, for example, indirect stimulation at the nerve induces the
same contraction as the direct stimulation of the muscle. The only
difference lies in the latent period, which is found to be longer under
indirect stimulation by the time interval necessary for the excitation
to travel along the conducting nerve. It is probable that stimulus gives
rise to dual impulses in the animal tissue as in the plant. But the
detection of the positive impulse in the animal nerve is rendered
exceedingly difficult on account of the high velocity of conduction of
excitation. I have explained that the separate effects of the two
impulses can only be detected if there is a sufficient lag of the
excitatory negative behind the positive, so that the relatively sluggish
responding organ may exhibit the two impulses one after the other. In a
highly conducting tissue the lag is very slight, and the negative will
therefore mask the positive by its predominant effect. In spite of the
difficulty involved in the problem, I have recently been successful in
demonstrating the dual impulses in the animal nerve.
In any case it is important to remember the following characteristic
effects of indirect stimulation.
TABLE XXII.--SHOWING THE EFFECT OF INDIRECT STIMULATION.
+-----------------------------------------------------------------+
| Intensity of | Character of intervening | Responsive effect. |
| Stimulus. | tissue. | |
+-----------------+--------------------------+--------------------+
|Moderate |Highly Conducting | Contraction. |
| " |Non-conducting | Expansion. |
| " |Semi-conducting | Expansion followed |
| | | by contraction. |
|Feeble | " " | Expansion. |
+-----------------------------------------------------------------+
These effects of indirect stimulation have been fully demonstrated in
the case of pulvinated organs (p. 136) and growing organs (p. 215).
Having demonstrated the fundamental reactions of direct and indirect
stimulation, we shall next study the tropic effects induced in growing
organs by the effect of unilateral application of indirect stimulus.
_Experiment 103._--I have already explained, how thermal _radiation_ is
almost as effective in inducing contraction and retardation of growth as
the more refrangible rays of the spectrum. The thermal radiation was
produced by the heating of a platinum spiral, short of incandescence, by
the passage of an electric current. The intensity of radiation is easily
varied by adjustment of the current by means of a rheostat. The
experimental specimen was a flower bud of _Crinum_. It was held by a
clamp, a little below the region of growth. Stimulus was applied below
the clamp so that the transmitted effect had to pass through S, the
securely held tissue (Fig. 98). A feeble stimulus was applied on one
side, at the indifferent point about 3 cm. below the region of growth.
The positive effect of indirect stimulus reached the region of growth on
the same side, bringing about an acceleration of growth with expansion
and convexity, the resulting movement being _negative_ or away from the
stimulus. The latent period was ten seconds, and maximum negative
movement was completed in the further course of ten seconds, after which
there was a recovery in the course of 75 seconds. A stronger stimulus S'
gave a larger response; but when the intensity was raised still higher
to S", the excitatory negative impulse overtook the positive within 15
seconds of its commencement; the convex was thus succeeded by the
concave curvature (Fig. 99). Direct application of stimulus at the
growing region gave rise to a positive curvature.
[Illustration: FIG. 98.--Diagrammatic representation of effects of
indirect and direct stimulation. Continuous arrow represents the
indirect stimulation, and the curved continuous arrow above, the induced
negative curvature: dotted arrow indicates the application of direct
stimulus, and the dotted curve above, the induced positive curvature.]
[Illustration: FIG. 99.--Tropic curvature of _Crinum_ to unilateral
indirect stimulation of increasing intensities: S, S' of moderate
intensity induced negative tropic effect (movement away from the
stimulated side); stronger stimulus S" gave rise to negative followed by
positive. Successive dots at intervals of 5 seconds Magnification 100
times.]
The effect of feeble stimulus transmitted longitudinally is thus found
always to induce convexity, a _negative curvature_ and movement away
from stimulus. I have obtained similar responsive movement of negative
sign with various plant organs, and under various forms of stimuli. Thus
in the stem of _Dregea volubilis_ the longitudinally transmitted effect
of light of moderate intensity was a negative curvature; direct
application of light on the growing region gave, on the other hand, a
positive curvature and movement towards light.
Thus while the effect of direct unilateral stimulation is a positive
curvature, the effect of indirect stimulation is a negative curvature.
The following table gives a summary of results of tropic effects under
unilateral application of indirect stimulus.
TABLE XXIII.--SHOWING TROPIC EFFECT OF UNILATERAL APPLICATION OF
INDIRECT STIMULUS.
+------------------------------------------------------------+
|Stimulus. | Character of intervening | Sign of tropic |
| | tissue. | response. |
+----------+---------------------------+---------------------+
| Moderate | Conducting | Positive curvature. |
| " | Semi-conducting | Negative followed |
| | | by positive. |
| Feeble | Semi-conducting | Negative Curvature. |
| Moderate | Non-conducting | " " |
+----------+---------------------------+---------------------+
|Direct application of unilateral stimulus induces a positive|
|curvature. |
+------------------------------------------------------------+
SUMMARY.
In sensitive plants stimulus applied at a distance induces in the
responding region an expansion indicative of increase of turgor.
The effect of indirect stimulation is also exhibited by an electric
change of galvanometric positivity, indicative of enhancement of turgor
and expansion.
Indirect stimulus induces in growing organs an enhancement of rate of
growth.
Unilateral application of stimulus causes an expansion higher up on the
same side to which the stimulus is applied; the result is an induced
convexity, a movement away from the stimulus, _i.e._, a negative
curvature. Direct stimulus applied unilaterally at the responding region
induces, on the other hand, a positive curvature.
XXV.--TROPIC CURVATURE WITH TRANSVERSE TRANSMISSION OF EFFECT OF
STIMULUS
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
We have next to consider a very large class of phenomena arising out of
the direct stimulation of one side and its transversely transmitted
effect on the opposite side. The unilateral stimuli to which the plant
is naturally exposed are those of contact, of light, of thermal
radiation, and of gravity. There is besides the stimulation by electric
current. I shall presently show that these tropic curvatures are
determined by the definite effects of direct and indirect stimulations.
Under unilateral stimulus, the proximal side is found to become concave
and the distal side convex; the organ thus moves towards stimulus,
exhibiting a positive curvature. This movement may be due: (1) to the
diminution of turgor, contraction or retardation of rate of growth of
the proximal side, (2) to the increase of turgor, expansion or
acceleration of rate of growth on the distal side, or (3) to the joint
effects of contraction of the proximal and expansion of the distal side.
As regards the reaction of the proximal side, it has been shown that
direct stimulation induces local contraction in a pulvinated organ, and
retardation of growth in a growing organ. The effect induced on the
distal side had hitherto remained a matter of uncertainty. In regard to
this we must bear in mind that it is the effect of indirect stimulus
that reaches the distal sided, inducing an enhancement of turgor and
expansion of that side.
For obtaining a complete explanation of tropic curvatures in general, it
is important that the induction of enhanced turgor at the distal side
(by the action of stimulus at the proximal side) should be corroborated
by independent methods of enquiry. One of the methods I employed for
this purpose was electrical. Two electric connections were made, one
with the distal point (diametrically opposite to the stimulated area),
and the other, with an indifferent point at a distance. On application
of stimulus of various kinds, the distal point was found to exhibit
galvanometric positivity, indicative of enhancement of turgor.[3]
[3] "Plant Response"--p. 519.
I have since been able to devise a new experiment by which the
enhancement of turgor on the distal side is demonstrated in a very
striking manner.
I have shown (p. 39) that the movement of the motile leaf of _Mimosa_ is
a reliable indicator of the state of turgor, increase of turgor inducing
erection, and diminution of turgor bringing about the fall of the leaf.
I shall employ the mechanical response of the leaf to demonstrate the
enhancement of turgor induced by transverse transmission of effect of
stimulus.
[Illustration: FIG. 100.--Increased turgor due to indirect stimulation,
inducing erection of Mimosa leaf: (_a_) diagram of the experiment, point
of application of stimulus indicated by arrow. (_b_) erectile response
(shown by down-curve) followed by rapid fall (up-curve) due to
transverse conduction of true excitation. (Successive dots at intervals
of 10 seconds.)]
TURGOR-VARIATION UNDER TRANSVERSE TRANSMISSION OF STIMULUS-EFFECT.
_Unilateral photic stimulation: Experiment 104._--A _Mimosa_ plant was
taken, and its stem was held vertical by means of a clamp. We apply a
stimulus at a point on one side of the stem, and observe the effect of
this on the state of turgor at the diametrically opposite side. In my
first experiment on the subject of detection of induced change of
turgor I employed the stimulus of light. A narrow beam from a small arc
lamp was made to fall on the stem, at a point diametrically opposite to
the motile leaf, which was to serve as a indicator for induced variation
of turgor at the distal side. The leaf was attached to the recording
lever, the successive dots in the record being at intervals of ten
seconds. Stimulation by light caused a positive or erectile movement
within 20 seconds of application. The positive response afforded a
conclusive proof of the induction of an increase of turgor at the distal
point. When the stimulus is moderate or of short duration, the response
remains positive. But with strong or prolonged stimulation, the slower
excitatory negative impulse is conducted to the distal point and brings
about the sudden fall of the leaf (Fig. 100). In the present case the
excitatory impulse reached the motile organ 200 seconds after the
initiation of the positive response. The stem was thin, only 2 mm. in
diameter. The velocity of excitatory impulse in a transverse direction
is thus 0·01 mm. per second; transverse transmission is, for obvious
reasons, a much slower process than longitudinal transmission of
excitation; in the _Mimosa_ stem this is about 4 mm. per second.
[Illustration: FIG. 101.--Response of leaf of _Mimosa_ under transverse
transmission of electric stimulus. (Compare this with fig. 100.)]
_Unilateral electric stimulation: Experiment 105._--In order to show
that the effects described above are not due to any particular mode of
stimulation but to stimuli in general, I carried out an additional
experiment, the stimulus employed being electrical. Two fine
pin-electrodes were pricked into the stem, opposite to the responding
leaf of _Mimosa_; these electrodes were placed vertically one above the
other, 5 mm. apart. After a suitable period, allowed for recovery from
mechanical irritation, feeble tetanising electric shock was passed
through the electrodes. The responsive effects at the distal side of the
stem is precisely similar to those induced under unilateral photic
stimulation; that is to say, the first effect was an erectile movement
of the leaf, indicative of an induced enhancement of turgor; the
excitatory negative impulse then reached the distal point and caused a
sudden fall of the leaf (Fig. 101).
The experiments that have just been described are of much significance.
An organ like the stem of _Mimosa_, since it exhibits no contraction,
may appear insensitive to stimulation; but its perception of stimulus is
shown by its power of transmitting two characteristic impulses, one of
which is the positive, giving rise to an enhancement of turgor, and the
other, the true excitatory negative, inducing the opposite reaction or
diminution of turgor. Unilateral stimulation gives rise to both these
effects in all organs: pulvinated, growing, and non-growing. It was the
fortunate circumstance of the insertion of the motile leaf on one side
of the _Mimosa_ stem that enabled us to demonstrate the important facts
given above.
The underlying reactions, which give rise to tropic curvature, could
have been foretold from the Laws of effects of Direct and Indirect
stimulation, established in previous chapters (pp. 136, 216). The
resulting curvature is thus brought about by the joint effects of direct
stimulation of the proximal, and indirect stimulation of the distal
side. We may now recapitulate some of the important facts relating to
tropic curvatures:
Indirect stimulation gives rise to dual impulses, positive and negative;
of these the positive impulse is practically independent of the
conducting power of the tissue; but the transmission of the excitatory
negative impulse is dependent on the conducting power. No tissue is a
perfect conductor, nor is any a perfect non-conductor of excitation, the
difference is a question of degree. In a petiole or a stem the
conducting power along the direction of length is considerable, but very
feeble in a transverse direction. In a semi-conducting tissue, a feeble
stimulus will transmit only the positive impulse; strong or long
continued stimulation will transmit both positive and negative impulses,
the positive preceding the negative. The transmitted positive gives rise
to increase of turgor, expansion, and acceleration of rate of growth;
the negative induces the opposite reaction of diminution of turgor, of
contraction, and of retardation of rate of growth. Transverse
transmission is only a particular instance of transmission in general;
the only difference is that the conducting power for _excitation_ is
very much less in the transverse than in the longitudinal direction.
Owing to feeble transverse conductivity, the transmitted impulse to the
distal side often remains positive; it is only under strong or continued
stimulation that the excitatory negative reaches the distal side and
neutralises or reverses the previous positive reaction. If the distal is
the more excitable side, the reversed response will appear as pronounced
negative. I give a table which will clearly exhibit the effects of
stimulus on the proximal and distal sides of the responding organ.
TABLE XXIV.--SHOWING RESPONSIVE EFFECTS COMMON TO PULVINI AND GROWING
ORGANS UNDER UNILATERAL STIMULATION.
+------------------------------------------------------------------+
|Effect of direct stimulation on | Effect of indirect stimulation |
| proximal side. | on distal side. |
+---------------------------------+--------------------------------+
|Diminution of turgor | Increase of turgor. |
|Galvanometric negativity | Galvanometric positivity. |
|Contraction and concavity | Expansion and convexity. |
+------------------------------------------------------------------+
| When stimulus is strong or long continued, the |
| true excitatory effect isconducted to the |
| distal side, neutralising or reversing the first response. |
+------------------------------------------------------------------+
The diagram which I have already given (Fig. 98) clearly explains the
different tropic effects induced by changing the point of application of
stimulus. We may thus have stimulus applied at the responding region
itself (Direct Stimulation) or at some distance from it (Indirect
Stimulation). The final effect will be modified by the conducting power
of the tissue.
DIRECT UNILATERAL STIMULATION.
_Type I._--The tissue has little or no power of transverse
conduction: stimulus remains localised, the proximal side
undergoes contraction, and the distal side expansion. The
result is a positive curvature.
_Type II._--The tissue is transversely conducting. Under
strong and long continued stimulation the excitatory impulse
reaches the distal side, neutralising or reversing the first
effect.
INDIRECT UNILATERAL STIMULATION.
_Type I._--The intervening tissue is an indifferent
conductor: transmitted positive impulse induces expansion and
convexity on the same side, thus giving rise to negative
curvature (_i.e._, away from stimulus).
_Type II._--Intervening tissue is a fairly good conductor:
the effect of positive impulse is over-powered by the
predominant excitatory negative impulse, the final result is
a concavity and positive curvature, with movement towards the
stimulus.
The following is a tabular statement of the different effects induced by
Direct and Indirect stimulation.
TABLE XXV.--SHOWING DIFFERENCE OF EFFECTS INDUCED BY DIRECT AND INDIRECT
STIMULATION.
+------------------------------------------------------------------+
|Stimulation. |Nature of the tissue. |Final effect. |
+---------------+----------------------------+---------------------+
|Direct (Feeble)|Semi-conducting tissue. |Positive curvature. |
|Indirect " | " " |Negative curvature. |
|Direct (Strong)|Better conducting tissue. |Neutral or negative |
| | |curvature. |
|Indirect " | " " " |Negative followed by |
| | |positive curvature. |
+------------------------------------------------------------------+
The results of investigations already described, enable us to formulate
the general laws of tropic curvature applicable to all forms of stimuli,
and to all types of responding organs, pulvinated or growing.
LAWS OF TROPIC CURVATURE.
1. (_a_) DIRECT APPLICATION OF UNILATERAL STIMULUS OF
MODERATE INTENSITY, INDUCES A POSITIVE OR CONCAVE CURVATURE,
BY THE CONTRACTION OF THE PROXIMAL AND EXPANSION OF THE
DISTAL SIDE.
(_b_) UNDER STRONG OR LONG-CONTINUED STIMULATION, THE
POSITIVE CURVATURE IS NEUTRALISED OR REVERSED, BY TRANSVERSE
CONDUCTION OF EXCITATION; THIS EFFECT IS ACCENTUATED BY THE
DIFFERENTIAL EXCITABILITY OF THE TWO SIDES OF THE ORGAN.
2. (_a_) INDIRECT APPLICATION OF UNILATERAL STIMULUS OF
FEEBLE INTENSITY INDUCES A NEGATIVE CURVATURE.
(_b_) IN A CONDUCTING TISSUE THE EXCITATORY EFFECT BEING
TRANSMITTED UNDER STRONG AND LONG CONTINUED STIMULATION,
INDUCES A POSITIVE CURVATURE.
It will thus be seen that the tropic effect is modified by:
(1) the point of application of stimulus,
(2) the intensity and duration of stimulus,
(3) the conducting power of tissue in the transverse
direction,
(4) the relative excitabilities of the proximal and distal
sides of the organ.
In the following series of Papers the tropic effects of various forms of
stimuli will be studied in detail.
SUMMARY.
In a semi-conducting tissue Direct stimulation induces a diminution of
turgor and contraction, Indirect stimulation inducing the opposite
effect of increase of turgor and expansion.
Unilateral stimulation thus induces a positive curvature by the joint
effects of contraction at the proximal, and expansion at the distal
side.
Under strong and long continued unilateral stimulation, the excitation
at the proximal side is transmitted to the distal side. Transverse
conduction thus neutralises or reverses the normal positive curvature.
XXVI.--MECHANOTROPISM: TWINING OF TENDRILS
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
In response to the stimulus of contact a tendril twines round its
support. Certain tendrils are uniformly sensitive on all sides; but in
other cases, as in the tendril of _Passiflora_, the sensitiveness is
greater on the under side. A curvature is induced when this side is
rubbed with a splinter of wood, the stimulated under side becoming
concave. This movement may be distinguished as a movement of _curling_.
There is, as I shall presently show, a response where the under side
becomes convex, and the curvature becomes reversed.
As regards perception of mechanical stimulus, Pfeffer discovered tactile
pits in the tendrils _Cucurbitaceæ_. These pits no doubt facilitate
sudden deformation of the sensitive protoplasm by frictional contact. No
satisfactory explanation has however been offered as regards the
physiological machinery of responsive movement. The difficulty of
explanation of twining movements is accentuated by a peculiarity in the
response of tendrils which is extremely puzzling. This anomaly was
observed by Fitting in tendrils which are sensitive on the under side:
"If a small part of the upper side and at the same time the
whole of the under side be stimulated, curvature takes place
only at the places on the under side which lie opposite
to the unstimulated regions of the upper side. The
_sensitiveness_ to contact is thus as well developed on the
upper side as on the under side, and the difference between
the two sides lies in the fact that while stimulation of the
under side induces curvature, stimulation of the upper side
induces _no visible result_, or simply inhibits curvature on
the under side, according to circumstances."[4]
[4] Jost--_Ibid_--p. 490.
Here then we have the inexplicable phenomenon of a particular tissue,
itself incapable of response, yet arresting the movement in a
neighbouring tissue.
The problem before us may be thus stated: Is the movement of the tendril
due to certain specific sensibility of the organ, on account of which
its reactions are characteristically different from other tropic
movements? Or, does the twining of tendril come under the law of tropic
curvature that has been established, namely that it is brought about by
the contraction of the directly stimulated proximal side, and the
expansion of the indirectly stimulated distal side?
I shall now describe my investigations on the effects of direct and
indirect stimulus on the growth of tendril; I have in this
investigation studied the effect not merely of mechanical, but also of
other forms of stimuli. I shall also describe the diverse effects
induced by mechanical stimulus under different conditions. From the
results of these experiments I shall be able to show that the twining of
the tendril comes under the general law of tropic curvature; that the
curvature results from the contraction of the proximal and expansion of
the distal side. Finally I shall be able to offer a satisfactory
explanation of the inhibition of response of the tendril by the
stimulation of the opposite side of the organ.
GENERAL EFFECTS OF INDIRECT AND DIRECT ELECTRIC STIMULATION ON THE
GROWTH OF TENDRIL.
For this experiment I took a growing tendril of _Cucurbita_ in which the
sensitiveness is more or less uniform on all sides. The tendril was
suitably mounted on the Balanced Crescograph, which records the
variation of the rate of growth induced by immediate and after-effect of
stimulus. The specimen is held in a clamp as in the diagram (Fig. 102),
the tip being suitably attached to the recording lever. For indirect
stimulation feeble shock from an induction coil is applied at the two
electric connections below the clamp. Direct stimulus is applied by
means of electric connections one above and the other below the clamp.
[Illustration: FIG. 102.--Diagrammatic representation of indirect and
direct stimulation of tendril.]
_Effect of Indirect Stimulus: Experiment 106._--The growth of the
tendril was exactly balanced, and the record became horizontal. Indirect
stimulus was next applied below the clamp; this is seen to upset the
balance, with the resulting up-curve which indicates a sudden
acceleration of growth above the normal. This acceleration took place
within ten seconds of the application of stimulus, and persisted for
three minutes; after this the normal rate of growth became restored, as
seen by the balanced record once more becoming horizontal (Fig. 103).
[Illustration: FIG. 103.--Record by Method of Balance, showing
acceleration of growth of tendril (up-curve) induced by indirect
stimulation. (_Cucurbita._)]
_Effect of Direct Stimulus: Experiment 107._--The incipient contraction
induced by direct stimulation is so great that the record obtained by
the delicate method of balance cannot be kept within the plate. I,
therefore, took the ordinary growth-curve on a moving plate. The first
part of the curve represents normal growth; stimulus of feeble electric
shock was applied at the highest point of the curve. This is seen (Fig.
104) to induce an immediate contraction and reversal of the curve which
persisted for two and half minutes, after which growth was slowly
renewed. The most interesting fact regarding the after-effect of
stimulus is that the rate of growth became actually enhanced to three
times the normal. This is clearly seen in the record (upper half of the
figure) taken 20 minutes after stimulation, where the curve is far more
erect than that of the normal rate of growth before stimulation.
[Illustration: FIG. 104.--Variation of growth induced by direct
stimulation. First part of the curve shows normal rate of growth. Direct
stimulation induces contraction (reversal of curve). After-effect of
stimulus seen in highly erect curve in upper part of record, taken 20
minutes after.]
The effects of Indirect and Direct stimulation of the tendril are
summarised below:
(1) Indirect stimulation induces a sudden enhancement of rate
of growth, followed by a recovery of the normal rate.
(2) Direct stimulation induces a retardation of the rate of
growth which may culminate into an actual contraction. _The
after-effect of direct stimulus of moderate intensity is an
enhancement of the rate of growth._
The experiments described above demonstrate the effects of direct and
indirect electrical stimulus. I shall now proceed to show that
mechanical stimulus induces effects which are similar to those of
electric stimulus.
EFFECTS OF DIRECT AND INDIRECT MECHANICAL STIMULUS.
_Effect of Direct mechanical stimulus: Experiment 108._--In this case I
took a tendril of _Cucurbita_, and attached it to the ordinary High
Magnification Crescograph, the record of which gives the absolute rate
of its normal growth, and the induced variation of that rate. The
tendril was stimulated mechanically by simultaneous friction of its
different sides. The immediate effect was a retardation of growth, the
reduced rate being less than half the normal. There was a recovery on
the cessation of the stimulus; the rate of growth was even slightly
enhanced after an interval of 15 minutes. Table XXVI shows the immediate
and after-effects of mechanical stimulation on growth.
TABLE XXVI.--SHOWING THE IMMEDIATE AND AFTER-EFFECT OF MECHANICAL
STIMULATION ON TENDRIL (_Cucurbita_).
+--------------------------------------------------------------+
|Normal rate of growth 0·44 µ per sec. |
|Retarded rate immediately after stimulation 0·20 µ " " |
|Recovery and enhancement after 15 minutes 0·50 µ " " |
+--------------------------------------------------------------+
The immediate and after-effects of mechanical stimulus on the tendril
are therefore the same as that of electric stimulus. The incipient
contraction under direct mechanical stimulus, moreover, is not the
special characteristic of tendrils, but of growing plants in general.
For I have shown (page 203) that the growth of flower stalk of
_Zephyranthes_ is also retarded after mechanical friction, from the
normal rate 0·48 µ to 0·11 µ after stimulation. We shall find later that
different plant organs, after moderate stimulation, exhibit acceleration
of growth as an after-effect. The phenomenon of responsive reaction of
tendril is therefore not unique, but similar to that of other organs
under all forms of stimulation. The only speciality in tendril is that
owing to anatomical peculiarities, the perceptive power of the organ for
mechanical stimulus is highly developed.
We are now in a position to offer an explanation of the induced
concavity of the stimulated side of the tendril, and its recovery after
brief contact. The experiments that have been described show that:
(1) the proximal side contracts because it is directly stimulated,
and the distal side, being indirectly stimulated, expands; the
curvature is thus due to the joint effects of contraction of one
side, and expansion of the opposite side, and
(2) the recovery of the tendril after brief contact is hastened by
the after-effect of stimulus, which is expansion and acceleration
of growth.
The results given above will also be found to explain Fitting's
important observations[5] that (_a_) the stimulated side of the tendril
undergoes transient contraction with subsequent acceleration of growth,
and that (_b_) the distal or convex side undergoes an immediate
enhancement of growth.
[5] Pfeffer--_Ibid_--Vol. III, p. 57.
I give below a record given by a tendril of _Cucurbita_ in response to
unilateral contact of short duration (Fig. 105). Successive dots in the
record are at intervals of three seconds. The latent period was ten
seconds, and the maximum curvature was attained in the course of two and
a half minutes. The curvature persisted for a further period of two
minutes after which recovery was completed in the course of 12 minutes.
Feeble stimulation is attended by a recovery within a short period, but
under strong stimulus the induced curvature becomes more persistent.
[Illustration: FIG. 105.--Positive curvature of tendril of Cucurbita
under unilateral stimulus of contact at x.]
INHIBITORY ACTION OF STIMULUS.
I have referred to the remarkable observation of Fitting that though
the application of stimulus on the upper side of the tendril of
_Passiflora_ did not induce any response, yet it inhibited the normal
response of the under side.
The results of experiments which I have described will, however, afford
a satisfactory explanation of this curious inhibition. It has been
explained, that the curvature of the tendril is due to the joint effects
of diminished turgor and contraction at the directly stimulated side,
and an enhancement of turgor and expansion on the opposite side. In the
diagram seen in figure 106, the left is the more excitable side, and
contraction will induce concavity of the stimulated side. But if the
opposite or less excitable side of the tendril be stimulated at the same
time, then the transmitted effect of indirect stimulus will induce
enhancement of turgor and expansion on the left side, and thus
neutralise the previous effect of direct stimulus. An inhibition of
response will thus result from the stimulation of the opposite side.
[Illustration: FIG. 106.--Diagrammatic representation of effects of
Indirect and Direct unilateral stimulation of the tendril. Indirect
stimulation, I, induces movement away from stimulated side (negative
curvature) represented by continuous arrow. Direct stimulation, D,
induces movement towards stimulus (positive curvature) indicated by
dotted arrow.]
A difficulty arises here from the fact that the upper side of the
tendril (the right side in Fig. 106) is supposed to be inexcitable and
non-contractile. Hence there may be a misgiving that the stimulation of
the non-motile side may not induce the effect of indirect stimulus (an
increase of turgor and expansion) at the opposite side, which is to
inhibit the response. But I have shown that even a non-contractile organ
under stimulus generates both the impulses, positive and negative. This
is seen illustrated in figure 100, where the rigid stem of _Mimosa_ was
subjected to unilateral stimulation; the effect of indirect stimulus was
found to induce an enhancement of turgor at the diametrically opposite
side, and thus caused an erectile movement of the motile leaf. Electric
investigations which I have carried out also corroborate the results
given above. Here also stimulation of a non-motile organ at any point,
induces at a diametrically opposite point, a positive electric variation
indicative of enhanced turgor. It will thus be seen that inhibition is
possible even in the absence of contraction of the upper side of the
tendril; hence the contraction of the directly stimulated side is
neutralised by the effect of indirect stimulation of the distal side.
RESPONSE OF LESS EXCITABLE SIDE OF THE TENDRIL.
It is generally supposed that the upper side of the tendril of
_Passiflora_ is devoid of contractility. This is however not the case,
for my experiments show that stimulation of the upper side also induces
contraction and concavity of that side, though the actual movement is
relatively feeble.
_Experiment 109._--In order to subject the question to quantitative test
I applied feeble stimulus of the same intensity on upper and lower side
alternately. Successive stimuli were kept more or less uniform by
employing the following device. I took a flat strip of wood 1 cm. in
breadth, and coated 2 cm. of its length with shellac varnish mixed with
fine emery powder. On drying the surface became rough, the flat surface
was gently pressed against the area of the tendril to be stimulated, and
quickly drawn so that the rough surface 2 cm.×1 cm. was rubbed against
the tendril in each experiment. Stimulation, thus produced, induced a
responsive movement of each side of the organ. The extent of the
maximum movement was measured by the microscope micrometer. The
following results were obtained with four different specimens.
TABLE XXVII.--SHOWING THE RELATIVE INTENSITIES OF RESPONSES OF THE UPPER
AND UNDER SIDE OF TENDRIL (_Passiflora_).
+------------------------------------------------------------+
|Movement induced by | Movement induced by | B |
| stimulation of under | stimulation of upper | Ratio ---. |
| side, A. | side, B. | A |
+----------------------+-------------------------------------+
|(1) 85 divisions | 14 divisions | 1/6 |
|(2) 106 " | 15 " | 1/7 |
|(3) 60 " | 8 " | 1/7 |
|(4) 80 " | 10 " | 1/8 |
+------------------------------------------------------------+
It will thus be seen that the upper side of the tendril is not totally
inexcitable, its power of contraction being about one-seventh that of
the under side.
NEGATIVE CURVATURE OF THE TENDRIL.
I shall now describe certain remarkable results which show that under
certain definite conditions the tendril moves away from the stimulated
side. I have explained, how in growing organs the effect of unilateral
stimulus longitudinally transmitted, induces an expansion higher up on
the same side to which the stimulus is applied, resulting in convexity
and movement away from the stimulus (cf. Laws of Tropic Curvatures, p.
286). As the reaction of tendril is in no way different from that of
growing organs in general, it occurred to me that it would be possible
to induce in it a negative curvature by application of indirect
unilateral stimulus.
_Experiment 110._--A tendril of _Passiflora_ was held in a clamp, as in
the diagram (Fig. 106) in which the left is the more excitable side of
the organ. The responsive movement of the tendril is observed by
focussing a reading microscope on a mark on the upper part of the
tendril. Direct mechanical stimulation at the dotted arrow makes the
tendril move in the same direction, the response being _positive_. But
if stimulus be applied on the same side below the clamp the tendril is
found to move away from stimulus, the response being now _negative_.
This reversal of response, as previously stated, is due to the fact that
the transmitted effect of indirect stimulus induces an acceleration of
growth higher up on the same side, which now becomes convex. The result
though unexpected, is in every way parallel to the response of the
flower bud of _Crinum_, in which the normal positive response was
converted into negative by changing the point of application of
stimulus, so that it became indirect (p. 216).
SUMMARY.
The response of tendril is in no way different from that of growing
organs in general.
Direct stimulus, electrical or mechanical, induces an incipient
contraction; the after-effect of a feeble stimulus is an acceleration of
growth above the normal. Indirect stimulus induces an enhancement of the
rate of growth.
Under unilateral mechanical stimulus of short duration the directly
excited proximal side undergoes contraction, the indirectly stimulated
distal side exhibits the opposite effect of expansion. The induced
curvature is thus due to the joint effects of the contraction of one
side, and the expansion of the opposite side.
As the after-effect of direct stimulus is an acceleration of growth
above the normal, the stimulated side undergoes an expansion by which
the recovery is hastened.
Unilateral application of direct stimulus induces a _positive_
curvature, but the same stimulus applied at a distance from the
responding region induces a _negative_ curvature.
The tendril of _Passiflora_ is excitable both on the upper and under
sides: the excitability of the under side is about seven times greater
than that of the upper side.
Stimulation of one side of the tendril induces an expansion of the
opposite side, even in cases where the contractility of the stimulated
side is feeble.
The response to stimulation of the more excitable side of the tendril is
thus inhibited by the stimulation of the opposite side. This is because
of the neutralisation of the effect of direct by that of indirect
stimulation.
XXVII.--ON GALVANOTROPISM
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
Before describing the effect of unilateral application of an electrical
current in inducing tropic curvature, I shall give an account of the
polar effect of anode and cathode on the pulvinated and growing organs.
In my previous work[6] on the action of electrical current on sensitive
pulvini I have shown that:--
(1) at the 'make' of a current of moderate intensity a
contraction takes place at the cathode; the anode induces no
such contractile effect;
(2) at the 'make' of a stronger current both the anode and
cathode induce contraction.
[6] "Irritability of Plants," p. 212.
I have also carried out further investigations on the polar effect of
current on the autonomous activity of the leaflet of _Desmodium gyrans_.
These rhythmic pulsations can be recorded by my Oscillating Recorder.
Each pulsation consists of a sudden contractile movement downwards,
corresponding to the systole of a beating heart, and a slower up
movement of diastolic expansion. Application of cathode at the
pulvinule was found to exert a _contractile_ reaction, exhibited either
by the reduction of normal limit of diastolic expansion, or by an arrest
of movement at systole. The effect of anode was precisely the opposite;
the induced _expansion_ was exhibited either by reduction of normal
limit of systolic contraction, or by arrest of pulsation at diastole.
From the above results it is seen that with a feeble current:
(1) contraction is induced at the cathode, and
(2) expansion is brought about at the anode.
These effects take place under the action of a feeble current. Under
strong currents, contraction takes place both at the anode and the
cathode.
POLAR EFFECT OF ELECTRICAL CURRENT ON GROWTH.
The object of this investigation was to determine whether anode and
cathode exerted similar discriminative and opposite effects on growth.
For this experiment I took a specimen of _Kysoor_ and determined the
region where growth was maximum. A piece of moist cloth was wrapped
round this region to serve as one of the two electrodes. The second
electrode was placed in the neighbouring indifferent region where there
had been a cessation of growth.
_Effect of Cathode: Experiment 111._--The particular specimen of
_Kysoor_ had a normal rate of growth of 0·48 µ per second. On
application of the cathode the rate was reduced to 0·14 µ per second, or
to less than a third. This will be seen in record (Fig. 107), where N is
the normal rate of growth and K, retarded rate under the action of the
cathode.
[Illustration: FIG. 107.--Retardation of rate of growth under the action
of cathode (_Kysoor_).]
[Illustration: FIG. 108.--Acceleration of rate of growth under anode
(_Kysoor_).]
_Effect of anode: Experiment 112._--If the cathode induced a
retardation, the anode might be expected to induce an acceleration of
growth. But in my first experiment on the action of anode, I could
detect no perceptible variation of rate of growth. In trying to account
for this failure, I found that the specimen employed for the experiment
had normally a very rapid rate of growth. It appeared that an induced
acceleration would be brought out more conspicuously by choosing a
specimen in which the growth-rate was low, rather than in one in which
it was near its maximum. Acting on this idea, I took another specimen of
_Kysoor_ in which the normal rate was as slow as 0·10 µ per second. On
applying the anode to the growing region, there was an enhancement to
one and half times the normal rate (Fig. 108).
TABLE XXVIII.--EFFECT OF ANODE AND CATHODE ON GROWTH (_Kysoor_).
Specimen A Normal rate 0·10 µ per sec.
Acceleration under anode 0·155 µ per sec.
Specimen B Normal rate 0·48 µ per sec.
Retardation under cathode 0·14 µ per sec.
The effects given above take place under the action of a feeble current.
Strong current on the other hand induces a retardation or an arrest of
growth.
I have in the above experiments demonstrated the normal effect of anode
in inducing expansion and acceleration of rate of growth; the cathode
was shown to induce contraction and retardation of growth. Unilateral
application of anode and cathode thus induces appropriate curvatures in
pulvinated and in growing organs.
SUMMARY.
The effects of an electric current on growth is modified by the
direction of current. A feeble anodic current enhances the rate of
growth; a cathodic current on the other hand induces a retardation of
the rate. Strong current, both anodic and cathodic, induces a
retardation.
XXVIII.--ON THERMONASTIC PHENOMENA
_By_
SIR J. C. BOSE,
_Assisted by_
SURENDRA CHANDRA DAS.
In describing thermonastic curvatures Pfeffer says that "a special power
of thermonastic response has been developed by various flowers, in which
low temperatures produce closing movements, and high temperatures,
opening ones. The flowers of _Crocus vernus_ and _Crocus luteus_ are
specially responsive, as also those of _Tulipa Gesneriana_ for these
flowers perceptibly respond to a change of temperature of half a degree
centigrade."[7]
[7] Pfeffer--_Ibid_, Vol. III, p. 112.
We have hitherto studied the response of various organs to _stimulus_;
we have now to deal with the effect of thermal variation. Does rise of
temperature act like other forms of stimuli or is its action different?
We have therefore to find:
(1) The physiological effect of variation of temperature.
(2) Whether thermonastic irritability is confined only to
certain classes of organs, or is it a phenomenon of very wide
occurrence?
(3) Whether variation of temperature induces in anisotropic
organs only one type of response, or two types, positive and
negative.
(4) The law which determines the direction of responsive
movement.
EFFECT OF RISE OF TEMPERATURE.
As regards the effect of rise of temperature we have seen that, within
normal limits, it induces expansion and acceleration of the rate of
growth. Stimulus, on the other hand, induces precisely the opposite
effect. Hence the physiological reaction of steady rise of temperature
is, generally speaking, antagonistic to that of stimulus. This
conclusion is supported by numerous experiments which I have carried out
with various plant organs. Example of this will be found in the present
and subsequent chapters.
DIFFERENT THERMONASTIC ORGANS.
The only condition requisite for the exhibition of response is the
differential excitability of an anisotropic organ. It is therefore
likely to be exhibited by a large variety of plant organs, such as
pulvini, petioles, leaves, and flowers, and my results show that this is
actually the case. This particular sensibility, moreover, is not
confined to delicate structures, but is extended to rigid trees and
their branches.
Before proceeding further, it is necessary to draw attention to the
confusion which arises from the use of the common prefix '_thermo_' in
thermonasty and thermo-tropism. With regard to this Pfeffer says "It is
not known whether radiated and conducted heat exercise a similar
thermotropic reaction."[8] I shall show that the reactions to radiant
heat, and to conducted heat (rise of temperature) are of opposite
character, radiation inducing contraction, and rise of temperature,
expansion. It is therefore advisable to distinguish the thermal, or
temperature effect, from the radio-thermal effect of infra-red
radiation.
[8] Pfeffer--_Ibid_, Vol. III, p. 177.
TWO TYPES OF RESPONSE.
As regards the effect of variation of temperature I shall proceed to
show that there are two distinct types, which I shall, for convenience,
distinguish as the _Positive_ and _Negative_.
Positive thermonastic reaction is exhibited by organs in which the upper
half is the more excitable. Response to rise of temperature is by
_downward_ or _outward_ movement. In floral organs this finds expression
by a movement of opening. In illustration of this may be cited the
examples of the well known Crocus and also of _Zephyranthes_.
Negative thermonastic movement is shown by organs in which the lower
half is the more excitable. Here the response to rise of temperature is
by an _upward_ or _inward_ movement. I shall show that an example of
this is furnished by the flower of _Nymphæa_ which closes under rise,
and opens during fall of temperature.
POSITIVE THERMONASTIC RESPONSE.
_Response of Zephyranthes: Experiment 113._--Viewed from the top, the
inner side of the petal of a flower is the upper side. The _Crocus_
flower under rise of temperature opens outwards by expansion of the
inner side, which must be the more excitable. As _Crocus_ was not
available in Calcutta, I found the flower of _Zephyranthes_ (sometimes
called the Indian _Crocus_) reacting to variation of temperature in a
manner similar to that of _Crocus_, that is to say, the flower opens
under rise and closes with a fall of temperature. For obtaining record
all the perianth segments but one was removed. This segment was attached
to the recording lever. On lowering of temperature through 5°C. there
was an up-movement, or a movement of closure. Rise of temperature
induced, on the other hand, a movement of opening.
[Illustration: FIG. 109.--Thermonastic and radionastic responses of
petal of _Zephyranthes_ C, closing movement due to cooling, and H,
opening movement due to warming; R, closing movement due to
heat-radiation. Note opposite responses to rise of temperature and to
thermal radiation.]
_Effect of thermal radiation: Experiment 114._--I stated that the effect
of thermal radiation acts as a stimulus, inducing a reaction which is
antagonistic to that of rise of temperature. In verification of this, I
subjected the specimen to the action of infra-red radiation acting from
all sides. The result is seen in the responsive movement of closure
(Fig. 109 R). These experiments demonstrate clearly that the responses
to rise of temperature and thermal radiation are of opposite signs.
As a movement of closure was induced by the diffuse stimulus of thermal
radiation, it is evident that this must have been brought about by the
greater contraction of the inner half of the perianth; hence the inner
half of the organ is relatively the more excitable.
[Illustration: FIG. 110.--The Thermonastic Recorder. T, metallic
thermometer attached to the short arm of the upper lever; the specimen
of _Nymphæa_, N, has one of its perianth leaves attached to the short
arm of the second lever by a thread. C, clockwork for oscillation of the
plate.]
NEGATIVE THERMONASTIC RESPONSE.
_Response of_ Nymphæa: _Experiment 115._--Many of the Indian
_Nymphæaceæ_ have their sepals and petals closed during the day, and
open at night. I find that the perianth leaves of this flower are
markedly sensitive to variation of temperature. The Thermonastic
Recorder employed in this investigation is shown in figure 110. The
record given in figure 111 shows that the perianth segment, subjected to
a few degrees' rise of temperature, responded by an up-movement of
closure, due to greater expansion of the outer half. The latent period
was 6 seconds, and the maximum effect was attained in the further course
of 21 seconds. This experiment shows that the thermonastic response of
this flower is of the negative type.
[Illustration: FIG. 111.--Negative thermonastic response of _Nymphæa_.
Application of warmth at the vertical mark induced up-movement of
closure, but stimulus of electric shock at arrow induced rapid
excitatory down movement of opening. Successive dots at intervals of a
second.]
_Effect of stimulus: Experiment 116._--In the positive type of
thermonastic organs, where rise of temperature induced a movement of
opening, stimulus induced the opposite movement of closure (Expt. 114).
We shall now study the effect of stimulus on the movement of _Nymphæa_,
which undergoes closure during rise of temperature, as seen in the first
part of the record in figure 111. Stimulus of electric shock was applied
at the point marked with an arrow; the response is seen to be by a
movement of opening. Here also we find the effects of rise of
temperature and of stimulus to be antagonistic to each other. This will
be clearly seen in the following tabular statement.
TABLE XXIX.--SHOWING THE EFFECT OF RISE OF TEMPERATURE AND OF STIMULUS
ON THERMONASTIC ORGANS.
+---------------------------------------------------------------+
| Specimen. | Effect of rise | Effect of stimulus. |
| | of temperature. | |
+------------------+---------------------+----------------------+
| _Zephyranthes_ | Movement of opening | Movement of closure. |
| (positive type). | | |
| | | |
| _Nymphæa_ | Movement of closure | Movement of opening. |
| (negative type). | | |
+---------------------------------------------------------------+
In _Nymphæa_ it is the outer side of the perianth that is relatively the
more excitable since diffuse electric stimulus induces a movement of
opening due to the greater contraction of the outer side. It is by the
greater expansion of this more excitable side that the movement of
closure is effected during rise of temperature.
From the results of experiments given above we arrive at the
following:--
LAW OF THERMONASTIC REACTION
RISE OF TEMPERATURE INDUCES A GREATER EXPANSION OF THE MORE EXCITABLE
HALF OF AN ANISOTROPIC ORGAN.
SUMMARY.
Thermonastic movements are induced by the differential physiological
effect of variation of temperature on the two halves of an anisotropic
organ.
Rise of temperature induces greater expansion, and enhancement of rate
of growth of the more excitable half of the organ; lowering of
temperature induces the opposite effect.
Two types of thermonastic movements are met with, the _positive_
exhibiting a movement of opening during rise of temperature; in these
the inner half of the organ is relatively the more excitable. Example of
this is seen in the _Crocus_ and in _Zephyranthes_.
In the _negative_ type, rise of temperature induces a movement of
closure. Here the outer half of the organ is the more excitable. The
flower of _Nymphæa_ belongs to this type.
The effect of stimulus is antagonistic to that of rise of temperature.
In positive thermonastic organs stimulus induces a movement of closure;
in the negative type it induces a movement of opening.
XXIX.--ON PHOTOTROPISM
_By_
SIR J. C. BOSE.
In different organs of plants the stimulus of light induces movements of
an extremely varied character. Radial organs exhibit tropic movements in
which the position of equilibrium is definitely related to the direction
of incident stimulus. Nastic movements under the action of light are, on
the other hand, regarded as curvatures of the organ which show "no
relation to the stimulus but is determined by the activity of the plant
itself".[9] There are thus two classes of response to light which seem
to be unrelated to each other. Returning to the directive action of
light, radial stems often bend towards the light, while certain roots
bend away from it. It may be thought that this difference is due to
specific difference of irritability between shoot and root, the
irritability of the former being of a positive, and of the latter, of a
negative character. But there are numerous exceptions to this
generalisation. Certain roots bend towards the light, while a stem,
under different circumstances, moves towards light or away from it.
Again an identical organ may exhibit a positive or a negative curvature.
Thus the leaflets of _Mimosa pudica_ acted on by light from above fold
upwards, the phototropic effect being _positive_. But the same leaflets
acted on by light from below exhibit a folding upwards, the phototropic
effect being now _negative_. Effects precisely the opposite are found
with the leaflets of _Biophytum_ and _Averrhoa_. They fold downwards
whether light acts from above or below. Finally, a radial organ in
found to exhibit under light of increasing intensity or duration, a
positive, a dia-phototropic, or a negative phototropic curvature.
[9] Jost--_Ibid_, p. 428.
In these circumstances the theory of specific positive and negative
irritabilities is untenable; in any case, it throws no light on the
phenomenon of movement. The difficulties of the problem are thus clearly
stated by Pfeffer: "When we say that an organ curves towards a source of
illumination, because of its heliotropic irritability and we are simply
expressing an ascertained fact in a conveniently abbreviated form,
without explaining why such curvature is possible or how it is
produced.... Many observers have unfortunately devoted their attention
to artificially classifying the phenomenon observed, and have entirely
neglected the explanation of causes underlying them."[10]
[10] Pfeffer--_Ibid_, Vol. II, p. 74.
COMPLEXITY OF PROBLEM OF PHOTOTROPIC REACTION.
The complexity of phototropic reaction arises from the summated effects
of numerous factors; for explanation of the resultant response it is
therefore necessary to take full account of the individual effect of
each of them.
Among these operative factors in phototropic reaction may be
mentioned:--
(1) The difference of effects induced by light at the proximal
and distal sides of the organ.
(2) The modification of the latent period with the intensity of
stimulus.
(3) The after-effect of stimulus.
(4) The modifying influence of tonic condition on response.
(5) The effect of direction of light.
(6) The effect of intensity of light.
(7) The effect of duration of stimulation.
(8) The transmitted effect of light.
(9) The effect of unequal excitability in different zones of
the organ.
(10) The effect of transverse conduction in modification of
the sign of response.
(11) The effect of temperature on phototropic action.
(12) The modification of response due to differential
excitability of the organ.
(13) Nastic and tropic reactions.
(14) The torsional effect of light.
The sketch given above will give us some idea of the complexity of the
problem. In this and in the following papers I shall describe the
investigations I have carried out on the subjects detailed above.
ACTION OF LIGHT.
I have shown that there is no essential difference between the responses
of pulvinated and growing organs, that diminution of turgor induced by
stimulus brings about contraction in the one, and retardation of the
rate of growth in the other. Indirect stimulation, on the other hand,
induces an expansion and acceleration of the rate of growth. The
experimental investigation on the tropic effect of light may therefore
be carried out both with pulvinated and growing organs.
As regards the effect of direct stimulus of light on growing organs we
found (p. 208) that it induces an incipient contraction, seen in
diminution of the rate of growth; this incipient contraction culminates
in an actual contraction under increasing intensity of light. The
contraction under direct stimulation is also observed in pulvinated
organs. When light acts from above the upper half of the pulvinus
undergoes contraction, resulting in erection of the motile leaf or
leaflets. As regards the effect of indirect unilateral stimulus of light
on the distal side of the organ, we found that its effect is an
enhancement of turgor (p. 281). Hence the positive tropic curvature
under light is brought about, as in the case of other forms of stimuli,
by the contraction of the proximal, and expansion of the distal sides of
the organ.
Various analogies have been noticed between phototropic and geotropic
reactions, and it has been supposed that the two phenomena are closely
related to each other. This has even led to assumption that there are
phototropic particles which function like statoliths in geotropic
organs. There is, however, certain outstanding difference between the
two classes of phenomena. In the case of light, the incident energy is
entirely derived from the outside. But in geotropism, the force of
gravity by itself is ineffective without the intervention of the weight
of cell-contents to exert pressure on the sensitive ectoplasm, and thus
induce stimulation. This aspect of the subject will be treated in
greater detail in a subsequent chapter.
POSITIVE PHOTOTROPIC CURVATURE.
I shall now describe the phototropic effect of unilateral light in
pulvinated, and in growing organs. From the explanation that has already
been given, it will be understood that the side of the organ directly
acted on by light undergoes contraction and concavity.
_Tropic curvature of pulvinated organs: Experiment 117._--For this
experiment I employed the terminal leaflet of the bean plant. The
source of illumination was 32 c.p. electric lamp, enclosed in a metallic
tube with circular aperture for passage of light. The leaflet was
attached to an Oscillating Recorder. Light was applied on the upper half
of the pulvinus for 20 seconds; this induced an up-movement of the
leaflet, due to the contraction of the upper half of the organ. Recovery
took place in course of 8 minutes (Fig. 112).
[Illustration: FIG. 112.--Successive positive responses of the terminal
leaflet of bean. Light applied from above for 20 seconds; complete
recovery in 8 minutes.]
POSITIVE PHOTOTROPIC CURVATURE OF GROWING ORGANS.
_Effect of moderate stimulation: Experiment 118._--I shall presently
show that the intensity of phototropic reaction depends on the
intensity and duration of the incident light. A moderate and effective
stimulation may thus be produced by short exposure to strong light. For
my present experiment I took a stem of _Dregea volubilis_, and applied
light from a small arc lamp to one side of the organ for 1 minute; this
induced a positive curvature followed by complete recovery on the
cessation of light (Fig. 113).
[Illustration: FIG. 113.--Positive curvature under moderate phototropic
stimulation. Note complete recovery (_Dregea_).]
[Illustration: FIG. 114.--Persistent positive curvature under stronger
stimulation (_Dregea_).]
_Effect of strong stimulation: Experiment 119._--After recovery of the
stem of the last experiment, the same light was applied for 5 minutes.
It is seen that the curvature is greatly increased (Fig. 114). Thus the
phototropic curvature increases, within limits, with the duration of
stimulation. The curvature induced under stronger stimulation remained
more or less persistent. In certain instances there was a partial
recovery after a considerable length of time; in others curvature was
fixed by growth.
PHENOMENON OF RECOVERY.
On the cessation of stimulus of moderate intensity the heliotropically
curved organ straightens itself; similar effects are also found in other
tropic curvatures. Thus a tendril straightens itself after curvature
induced by contact of short duration. The theory of rectipitality has
been proposed to account for the recovery, which assumes the action of
an unknown regulating power by which the organ is brought back to a
straight line; but beyond the assumption of an unknown specific power,
the theory affords no explanation of the mechanism by which this is
brought about.
The problem before us is to find out the means by which the organ
straightens itself after brief stimulation. It will also be necessary to
find out why there is no recovery after prolonged stimulation. We have
thus to investigate the after-effect of stimulus of various intensities
on growth, and the Balanced Method of recording Growth offers us an
unique opportunity of studying the characteristic after-effects.
IMMEDIATE AND AFTER-EFFECT OF LIGHT ON GROWTH.
As regards the effect of light I have already shown:
(1) that a sub-minimal stimulus induces an acceleration of
growth, but under long continued action the acceleration is
converted into normal retardation (p. 225),
(2) that a stimulus of moderate intensity induces the normal
retardation of the rate of growth.
It is evident that there is a _critical intensity_ of stimulus, above
which there is a retardation, and below which there is the opposite
reaction of acceleration. This critical intensity, I have found to be
low in vigorous specimens, and high in sub-tonic specimens. Thus the
same intensity of stimulus may induce a retardation of growth in
specimens the tonic condition of which is _above par_, and an
acceleration in others, in which it is _below par_. The following
experiments will demonstrate the immediate and after-effect of light of
increasing intensity and duration.
[Illustration: FIG. 115.--Immediate and after-effect of stimulus of
light on growth. (_a_) shows immediate effect of moderate light to be a
transitory acceleration (down-curve) followed by retardation (up-curve).
The after-effect on cessation of light is an acceleration (down-curve)
followed by restoration to normal. (_b_) Immediate and after-effect of
stronger light: immediate effect, a retardation; after-effect, recovery
to normal rate without acceleration.]
_Effect of light of moderate intensity: Experiment 120._--The source of
light was a small arc lamp placed at a distance of 50 cm., the intensity
of incident light was increased or decreased by bringing the source of
light nearer or further away from the plant. Two inclined mirrors were
placed behind the plant so that the specimen was acted on by light from
all sides. A seedling of wheat was mounted on the Balanced Crescograph,
and record was first taken under exact balance; this gives a horizontal
record. The up-curve represents retardation, and down-curve acceleration
of rate of growth. The source of light was at first placed at a distance
of 50 cm. from the plant, and exposure was given for 4 minutes at the
point marked with an arrow (Fig. 115a). We shall find in the next
chapter that the _intensity of phototropic effect is proportional to the
quantity of incident light_. This quantity at the beginning proved to be
sub-minimal, and hence there was an acceleration at the beginning.
Continued action induced the normal effect of retardation, as seen in
the subsequent resulting up-curve. On the cessation of light, the
balance was upset in an opposite direction, the resulting down-curve
showing an acceleration of the rate of growth above the normal. This
acceleration persisted for a time, after which the normal rate of growth
was restored, as seen in the curve becoming once more horizontal. _The
after-effect of light of moderate intensity is thus a temporary
acceleration of rate of growth above the normal._
_Effect of strong light: Experiment 121._--The same specimen was used as
in the last experiment. By bringing the source of light to a distance of
25 cm. the intensity of light was increased fourfold; the duration of
exposure was kept the same as before. The record (Fig. 115b) shows that
a retardation of rate of growth occurred from the very beginning without
the preliminary acceleration. This is for two reasons: (1) the increased
intensity was now above the critical minimum, and (2) the tone of the
organ had become improved by previous stimulation. On the cessation of
light, the after-effect showed no enhancement of rate of growth, the
recovery from retardation to the normal rate being gradual. In the next
experiment (the result of which is not given in the record) the
intensity of light was increased still further; the retardation now
became very marked, and it persisted for a long time even on the
cessation of light.
We thus find that:
(1) The immediate effect of light of moderate intensity is a
preliminary acceleration, followed by normal retardation. The
acceleration is the effect of sub-minimal stimulation. The
immediate after-effect is an acceleration above the normal.
(2) The immediate effect of strong light is a retardation
from the beginning; the immediate after-effect shows no
acceleration, the growth rate being gradually restored to the
normal.
(3) Under very strong light the induced retardation is very
great, and this persists for a long time even on the removal
of light.