_After-effect at post-maximum: Experiment 219._--As the plant was fatigued by previous experiments, a fresh specimen was taken and light was applied continuously on the upper half of the pulvinus. This gave rise first to a maximum positive curvature, subsequently diminished by transverse transmission of excitation. Neutralisation took place after application of light for 17 minutes. On the stoppage of light, there was a sudden overshooting _below_ the zero line (Fig. 209), and the rate of the movement on the cessation of light was nearly twice as quick as during the process of neutralisation.

SUMMARY.

The after-effect of light is modified by the duration of exposure to light.

Under continued action of light, the electric response of galvanometric negativity in plants attains a maximum after which it undergoes decline, and neutralisation.

The electrical after-effect exhibits characteristic differences depending on the duration of previous exposure to light.

The pre-maximal after-effect is a temporary continuation of response under light followed by recovery.

The after-effect at the maximum is a recovery to the normal equilibrium.

The after-effect at post-maximum is an 'overshooting' below the position of equilibrium.

The immediate and after-tropic response of light are similar to the corresponding photo-electric effects.

The pre-maximum after-effect is a continuation of positive tropic movement followed by recovery; the after-effect at maximum is a recovery to the normal equilibrium position of the organ. The post-maximum after-effect is an overshooting below the position of normal equilibrium.

LI.--THE DIURNAL MOVEMENT OF THE LEAF OF _MIMOSA_

_By_

SIR J. C. BOSE.

In the standard curve of nyct.i.tropic movement under thermo-geotropism described in a previous paper, the diurnal record consisted of an up-curve from thermal-noon to thermal-dawn, and a down-curve from the thermal-dawn to thermal-noon. The responding organ, which may be an inclined stem or a horizontally spread petiole, underwent an erection during the decline of temperature, and a fall with the rise of temperature. The diurnal record of the _Mimosa_ leaf appears, however, to be totally different.

[Ill.u.s.tration: FIG. 210.--Diurnal record of _Mimosa_ in summer, and in winter. Leaf rises from 2 to 5 p.m., when there is a spasmodic fall.

Leaf re-erects itself from 9 p.m. to 6 a.m. after which there is a gradual fall till 2 p.m. with pulsations. The upper-most record gives temperature variation, up-curve representing fall of temperature and _vice versa_.]

_Experiment 220._--I obtained the diurnal record of _Mimosa_ (Fig. 210) for twenty-four hours commencing at 2 p.m. which is the thermal-noon.

The summer and winter records are essentially the same; the only difference is in the greater vigour of movement exhibited by summer specimens. The diurnal movement of the leaf is very definite and characteristic; for the curves taken five years ago do not differ in any way from those obtained this year. The record may conveniently be divided into four phases.

_First phase._--The leaf erects itself after the thermal-noon up to 5 or 5-30 p.m. The temperature, it should be remembered, is undergoing a fall during this period.

_Second phase._--There is a sudden fall of the leaf in the evening which continues till 9 p.m. or thereabout.

_Third phase._--The leaf erects itself till thermal-dawn at about 6 a.m.

next morning.

_Fourth phase._--There is a fall of the leaf during the rise of temperature from thermal-dawn to thermal-noon. The uniformity of the fall is, however, interrupted by one or more pulsations in the forenoon.

These pulsations are more frequent in summer than in winter.

It will thus be seen that the difference between the normal thermo-geotropic curve, and the curve of _Mimosa_ is not so great as appears at first sight. With the exception of the spasmodic fall in the evening, the diurnal curve shows an erectile movement during lowering of temperature, and a movement of fall during rise of temperature. I shall presently explain the reason of the sudden fall in the evening, and of the multiple pulsations in the forenoon.

I have, moreover, been able to trace a continuity in _Mimosa_ itself, between the standard thermo-geotropic reactions and the modification of it by the action of light. The young leaves which sprout out at the beginning of spring take some time to become adjusted to the diurnal variation. There are two intermediate stages through which the leaves pa.s.s before they exhibit their characteristic diurnal curve. Slow rhythmic pulsations are at first seen to occur during day and night. At the next stage the leaves exhibit the diurnal movement of fall from thermal-dawn to thermal-noon, and movement of erection from thermal-noon to thermal-dawn next morning, the record being in every way similar to the standard thermo-geotropic curve. It is only at the final stage that there is a spasmodic fall in the evening which we shall find is the characteristic after-effect of light.

Before proceeding further I shall refer briefly to the theory of Millardet in explanation of the diurnal movement of the leaf of _Mimosa_. He found that the tension in stems, and presumably its turgor, is increased with rise and decreased with fall of temperature. The movement of the lateral leaf may, therefore, be due to the induced variation of tension in the main axis. Had this been the case the minimum tension would have occurred at the minimum temperature in the morning, and the leaf should have undergone a maximum fall. The maximum temperature attained in the afternoon should have, on the other hand, brought about the maximum erection. The observed facts are, however, the very opposite to these. Kraus and Millardet also found that light and darkness had great influence on the tension, which increases in darkness and diminishes in light. The tension at dawn may therefore be a resultant of the depressing effect of low temperature opposed by the promoting effect of darkness, the latter being the predominant factor.

The erect position of _Mimosa_ leaf in the morning may thus be accounted for by the resultant increase of tension of the stem. The explanation of the movements of the leaves is thus to be attributed to the variation of tension in the main axis to which the leaves are attached; this leads to the conclusion that the leaf movement should be determined in relation to the plant, and not in relation to the external stimulus. I shall, however, describe a crucial experiment in the course of this paper, which will show that the direction of stimulus of gravity has a determining influence on the periodic movement. The sudden fall of the leaf before evening is again inexplicable from the theory of periodic variation of tension.

The complexity in the diurnal movement in _Mimosa_ arises from the fact that there are three factors whose fluctuating effects are different at different parts of the day. The effect at any particular hour results from the algebraical summation of the following factors: (1) the thermo-geotropic action, (2) the immediate effect of photic stimulus and (3) the after-effect of light. The leaf of _Mimosa_ has, moreover, as I shall show, an autonomous movement of its own. I shall take up the full consideration of the subject in the following order:

1. _The thermo-geotropic reaction._--A crucial experiment will be described which demonstrates the effect of thermo-geotropism in the diurnal movement of the leaf of _Mimosa_.

2. _Autonomous pulsation of Mimosa._--The natural pulsation of the plant is obscured by the paratonic effect of external stimuli. I shall explain the method by which the natural pulsation of the leaf becomes fully revealed.

3. _The immediate effect of light._--This is not constant, but will be shown to undergo a definite variation with the intensity and duration of light. A very great difficulty in the study of effect of daylight at different parts of the day is introduced on account of the absence of any reliable recorder for measurement of fluctuation of light. I shall describe a device which gives a continuous record of photic variation for the whole day.

4. _The after-effect of light._--The spasmodic fall of the leaf of _Mimosa_ towards the evening presents the most difficult problem for solution. I shall first describe the diurnal movement of another plant which presents characteristics similar to those of _Mimosa_. I shall also demonstrate the various after-effects of light at different parts of the day. These results will offer the fullest explanation of the sudden fall of the leaf towards evening.

As regards the sudden fall of the leaf about evening, Pfeffer regarded it as due to increased mechanical moment of the secondary petioles moving forward on the withdrawal of light. I shall, however, in the course of this paper show, that the characteristic movements occur even after complete removal of the sub-petioles. In the following experiment, carried out with the intact plant, the effect of possible variation of weight is completely eliminated. In spite of this, the diurnal movement exhibited its characteristic phases including sudden movement in the evening.

The experiment I am going to describe will exhibit the diurnal curve obtained by an entirely different method, and will clearly exhibit the thermo-geotropic effect, as well as the immediate and after-effect of light.

DIURNAL VARIATION OF GEOTROPIC TORSION.

I have shown that the pulvinus of _Mimosa_, subjected laterally to the action of stimulus of gravity, exhibits a torsional response. When the _Mimosa_ plant is laid sideways, so that the plane of separation of the upper and lower halves of the pulvinus is vertical, geotropic stimulus acts laterally on the two halves of the differentially excitable pulvinus. When the less excitable upper half is to the left of the observer (see Fig. 179), the responsive torsion under geotropic stimulus will be clock-wise, the less excitable upper half of the pulvinus being thereby made to face the vertical lines of gravity. When the plant is turned over to the other side (the less excitable upper half being now to the right of the observer) the induced torsion will be counter clock-wise. The response is therefore determined by the directive action of stimulus of gravity. Light has also been shown to give rise to torsion (p. 400). Light acting in the same direction as the stimulus of gravity, _i.e._, from above, enhances the rate of torsion, the curve of response being due to the joint effects of light and gravity.

[Ill.u.s.tration: FIG. 211.--Record of diurnal variation of torsion in _Mimosa_ leaf. Up-curve represents increase and down-curve decrease of geotropic torsion.]

_Experiment 221._--I obtained 24 hours' record of variation of torsional response of _Mimosa_, commencing with thermal-noon at 2 p.m. It is to be borne in mind that increase of torsion indicates increase of geotropic action, just as the erectile movement of the leaf in the normal position indicates the enhanced geotropic effect. Inspection of figure 211 shows that the fall of temperature after thermal-noon was attended by increase of torsion. The curve went up till about 5 p.m., as in the ordinary record of _Mimosa_. The torsion suddenly decreased with the rapid diminution of light after 5 p.m. The torsion then increased with falling temperature from 9 p.m. till thermal-dawn next morning. After 6 a.m.

there is a continuous diminution of torsion till 5 p.m.

We may now summarise the diurnal variation of torsion exhibited by _Mimosa_. The torsion undergoes a periodic increase during the fall of temperature from afternoon till next morning, and a diminution during rising temperature from morning till afternoon. A sudden diminution of torsion occurs at about 5 p.m. due to the disappearance of light. The torsional record is, to all intents and purposes, a replica of the record of periodic up and down movements of the leaf.

This method of torsion has several advantages over the ordinary method.

First, the petiole being supported by the loop of wire, the weight of the leaf has no effect on the curve of response. In the second place, the periodic variation of turgor of the stem, as suggested by Millardet, will not in any way affect the record. Variation of turgor can only cause a swing to and fro, in a direction perpendicular to the plane which divides the pulvinus into upper and lower halves; it can in no way induce a torsional movement, or a variation of the rate of that movement.

_The automatic pulsation of the leaf of Mimosa._--The occurrence of the pulsatory response in the morning record of _Mimosa_ led me to search for multiple activity in the response of the pulvinus. I have in my previous investigation on the electric response of _Mimosa_ obtained multiple series of responses to a single strong stimulus. Blackman and Paine have recently shown that an isolated pulvinus of _Mimosa_ exhibit multiple mechanical twitches under excitation.[44]

[44] Blackman and Paine--"Annals of Botany" January 1918.

Even under normal conditions, the sprouting young leaves in March, as already stated, exhibit automatic pulsations throughout the day and night; in older leaves tuned to diurnal periodic movements, these natural pulsations are more or less suppressed. But in the forenoon, several pulsations are exhibited even by the old leaves.

The question may now be asked: Why should the pulsations occur preferably in the morning? In connection with this I shall refer to the suppression of the pulsatory activity of _Desmodium gyrans_ when the leaflet was pulled up by the action of light (cf. Fig. 188). The leaf of _Mimosa_ executes a very rapid movement of erection at night, and the natural pulsations are thereby rendered very inconspicuous. These pulsations may, however, be found in the night record of young leaves.

The general occurrence of pulsations in the forenoon is probably due to the fact that the resultant force which causes the down-movement is at the time relatively feeble--the operative factors being: (1) the action of the rising temperature which induces down-movement, and (2) the action of light which in the forenoon opposes this movement. It will thus be seen that the forces in operation in the forenoon are more or less in a state of balance, hence conditions for exhibition of natural pulsations are more favourable in the morning than in other parts of the day.

_Experiment 222._--I next tried to discover conditions under which the plant would exhibit its normal rhythmic activity during the whole course of 24 hours. The external stimuli which may interfere with the exhibition of its automatic pulsations are those due to gravity and light. They act most effectively on the pulvinus, when that organ is more or less horizontal and therefore at right angles to the direction of the incident stimulus; they act least effectively on the pulvinus when the organ is parallel to the direction of the external force. This latter condition may be secured by holding the plant upside down, when the pulvinus bends up and the leaf becomes erect and almost parallel to the vertical lines of gravity and to vertical light from above. The leaf, now relatively free from the effects of external stimulus, was found to exhibit its autonomous pulsations for more than seven days. I reproduce two sets of records (Fig. 212) for 24 hours each, obtained on the first and the third day. The average period of a single pulsation is slightly less than six hours; but this is likely to be modified by the age of the specimen and the temperature of the environment.

One of the factors that determines the diurnal movement of the leaf is the immediate and after-effect of light. The movement under the action of light, is modified by the intensity and duration of illumination. The experimental investigation of the subject offers many difficulties, princ.i.p.ally owing to the absence of any reliable indicator for the varying intensity of light during the course of the day.

[Ill.u.s.tration: FIG. 212.--Continuous record of automatic pulsation of _Mimosa_ leaf. The two series are for the first and the third day.]