The experiments described explains the reasons of complete recovery after moderate stimulation, and also the absence of recovery after strong stimulation. The immediate after-effect of moderate stimulation is shown to be an acceleration of rate above the normal. Returning to tropic curvature, the contraction at the proximal side induced by unilateral light is thus compensated by the accelerated rate of growth on the cessation of light. There is no such compensation in the case of strong and long continued action of light; for the after-effect of strong light shows no such acceleration as the immediate after-effect.

We may perhaps go a step further in explaining this difference. Stimulus was found to induce at the same time two physico-chemical reactions of opposite signs (p. 144). One is the 'up' or A-change, a.s.sociated with increase of potential energy of the system, and the other is a.s.sociated with 'down' or D-change, by which there is a run-down or depletion of energy. With moderate stimulation the A-and-D effects are more or less comparable to each other. But under strong stimulation the down-change is relatively greater. Hence on cessation of moderate stimulation the increase of potential energy, a.s.sociated with A-change, finds expression in enhancement of the rate of growth. The depletion of energy under strong stimulation is, however, too great to be compensated by the A-change.

LATENT PERIOD OF PHOTOTROPIC REACTION.

With reference to the latent period Jost thus summarizes the known results:[11] "The latent period of the heliotropic stimulus has already been determined. According to Czapek it amounts to 7 minutes in the cotyledons of _Avena_ and in _Phycomyces_; 10 minutes in hypocotyls of _Sinapis alba_ and _Beta vulgaris_, 20 minutes in the hypocotyl of _Helianthus_, and 50 minutes in the epicotyl of _Phaseolus_. If one of these organs be unilaterally illuminated for the specified time, heliotropic curvature ensues afterwards in the dark, that is to say, we meet with an after-effect in this case as in geotropism. We are quite ignorant, however, as to whether and how the latent period is dependent on the intensity of light."

[11] Jost--_Ibid_, p. 473.

With regard to the question of relation of the latent period to the intensity of stimulus I have shown (p. 166) that the latent period is shortened under increasing intensity of stimulus. In the case of tropic curvature induced by light, I find that the latent period is reduced under increasing intensity of light. The shortest latent period found by Czapek, as stated before, was 7 minutes. But by employing high magnification for record, I find that the latent period of phototropic action under strong light to be a question of seconds.

[Ill.u.s.tration: FIG. 116.--Latent period for photic stimulation at vertical line. Successive dots at intervals of 2 seconds. (_Erythrina indica_).]

_Determination of the latent period: Experiment 122._--I give a record of response (Fig. 116) of the terminal leaflet of _Erythrina inidca_ to light acting from above. The recording plate was made to move at a fast rate, the successive dots being at intervals of 2 seconds. The latent period in this case is seen to be 35 seconds. By the employment of stronger light I have obtained latent period which is very much shorter.

The term latent period is used in two different sense. It may mean the interval between the application of stimulus and the initiation of response. In the experiment described above, the latent period is to be understood in this sense. But in the extract given above, Jost uses the term latent period as the shortest period of exposure necessary to induce phototropic reaction as an after-effect. What then is the shortest exposure that will induce a r.e.t.a.r.dation of growth? For this investigation I employed the very sensitive method of the Balanced Crescograph.

[Ill.u.s.tration: FIG. 117.--Effect of a single electric spark on variation of growth. Record taken by Balanced Crescograph. Up-curve shows induced r.e.t.a.r.dation of growth; the after-effect is an acceleration (down-curve) followed by restoration to normal.]

GROWTH-VARIATION BY FLASH OF LIGHT FROM A SINGLE SPARK.

_Experiment 123._--I stated that the more intense is the light, the shorter is the latent period. The duration of a single spark discharge from a Leyden jar is almost instantaneous, the duration of discharge being of the order of 1/100,000th of a second. The single discharge was made to take place between two small steel spheres, the light given out by the spark being rich in effective ultra-violet rays. The plant used for the experiment was a seedling of wheat. It was mounted on the Balanced Crescograph, and its normal growth was exactly compensated as seen in the first part of the record. The spark gap was placed at a distance of 10 cm. from the plant; there was the usual arrangement of inclined mirrors for illumination of the plant. The flash of light from a single spark is seen to induce a sudden r.e.t.a.r.dation of rate of growth which lasted for one and half minutes. The record (Fig. 117) shows another interesting peculiarity of acceleration as an after-effect of moderate stimulation. After the r.e.t.a.r.dation which lasted for 90 seconds, there is an acceleration of growth above the normal, which persisted for 6 minutes, after which the rate of growth returned to the normal.

In order to show that the induced variation is due to the action of light and not to any other disturbance, I interposed a sheet of ebonite between the spark-gap and the plant. The production of spark produced no effect, but the removal of the ebonite screen was at once followed by the characteristic response.

MAXIMUM POSITIVE CURVATURE UNDER CONTINUED ACTION OF LIGHT.

The positive curvature is, as we have seen, due to the contraction of the proximal side and expansion of the distal side. The curvature will increase with growing contraction of the proximal side; a maximum curvature is however reached since:

(1) the contraction of the cells must have a limit,

(2) the bending organ offers increasing resistance to curvature, and

(3) the induced curvature tends to place the organ parallel to the direction of light when the tropic effect is reduced to a minimum.

The pulvinus of _Erythrina_ exemplifies the type of reaction in which the positive curvature reaches a maximum, (see below Fig. 132) beyond which there is no further change. This is due to absence of transverse conductivity in the organ. The modifying effect of transverse conductivity on response will be dealt with in the next chapter.

SUMMARY.

The positive phototropic curvature is brought about by the joint effects of the directly stimulated proximal, and indirectly stimulated distal side.

The phototropically curved organ undergoes recovery after brief stimulation.

The recovery after moderate stimulation is hastened by the previously stimulated side exhibiting an acceleration of the rate of growth above the normal. The after-effects of photic and mechanical stimulation are similar.

The latent period of photic reaction is shortened with the increasing intensity of light. The seedling of wheat responds to a flash of light from an electric spark, the duration of which is about a hundred thousandth part of a second.

Tissues in which the power of transverse conduction is negligible, the positive phototropic curvature under continued action of light attains a maximum without subsequent neutralisation or reversal.

x.x.x.--DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM

_By_

SIR J. C. BOSE,

_a.s.sisted by_

GURUPRASANNA DAS.

I have explained how under the action of unilateral light the positive curvature attains a maximum. There are, however, cases where under the continued action of strong light the tropic movement undergoes a reversal. Thus to quote Jost: "Each organism may be found in one of the three different conditions determined by the light intensity, _viz._ (1) a condition of positive heliotropism, (2) a condition of indifference, (3) a condition of negative heliotropism"[12]. No explanation has however been offered as to why the same organ should exhibit at different times, a positive, a neutral, and a negative irritability.

These changing effects exhibited by an identical organ is thus incompatible with the theory of specific sensibility, a.s.sumed in explanation of characteristic differences in phototropic response.

[12] Jost--_Ibid_--p. 462.

In regard to this I would draw attention to an important factor which modifies the tropic response, namely, the effect of transverse conduction of excitation. I shall presently describe in detail a typical experiment of the effect of unilateral stimulus of light on the responsive movement of main pulvinus of _Mimosa pudica_. The results will be found of much theoretical interest, since a single experiment will give an insight to all possible types of phototropic response.

Before describing the experiment I shall demonstrate the tropic reactions of the two halves of the pulvinus of _Mimosa_.

UNEQUAL EXCITABILITY OF UPPER AND LOWER HALVES OF PULVINUS TO PHOTIC STIMULATION.

I have by method of selective amputation shown that as regards electric stimulation the excitability of the upper half of the pulvinus is very much less than that of the lower half (p. 85). I have obtained similar results with photic stimulation.

_Tropic effect of light acting from above: Experiment 124._--Light of moderate intensity from an incandescent electric lamp was applied on the upper half of the pulvinus of _Mimosa_ for 4 minutes; this induced a contraction of the stimulated upper half and gave rise to an up or erectile response. On the stoppage of light recovery took place in the course of ten minutes. The phototropic curvature is thus seen to be positive. A series of such positive responses of the upper half of the pulvinus is given in figure 118.

_Effect of light acting from below: Experiment 125._--Light was now applied from below; this also induced a contraction of the lower half of the pulvinus, causing a down-movement (Fig. 119). As the responsive movement is towards light, the phototropic effect must be regarded as positive. The greater excitability of the lower half of the pulvinus is shown by the fact that the response of the lower half of the pulvinus to ten seconds' exposure is even larger than that given by the upper half under the prolonged exposure of 240 seconds.

[Ill.u.s.tration: FIG. 118.--Series of up-responses of _Mimosa_ leaf to light applied on upper half of pulvinus.]

[Ill.u.s.tration: FIG. 119.--Down-responses given by the same plant on application of light from below.]

TRANSFORMATION OF POSITIVE TO NEGATIVE PHOTOTROPIC CURVATURE.

_Experiment 126._--A beam of light from a small arc lamp was thrown on the upper half of the pulvinus. After a latent period of 5 seconds, a positive curvature was initiated, by the contraction of the upper and expansion of the lower side of the organ. But under continued action of light, the excitatory impulse reached the lower half of the organ, causing a rapid fall of the leaf, and a _negative_ curvature. The arrival of transmitted excitation at the more excitable distal half of the organ is clearly demonstrated by the very rapid down-movement, seen as the up-curve in the record (Fig. 120). In sensitive specimens this movement is so abrupt and rapid, that the writing lever is jerked off above the recording plate before making a dot on it. The thickness of the pulvinus was 15 mm., the distance which the excitatory impulse has to traverse to reach the lower half would thus be about 075 mm. The period for transverse transmission of excitation under strong light was found to vary in different cases from 50 to 80 seconds. The velocity of transmission of excitation in a transverse direction through the pulvinus is about 0011 mm. per second, which is not very different from 0010 mm. per second in the stem (p. 282).

[Ill.u.s.tration: FIG. 120.--Record of effect of continuous application of light on upper half of pulvinus of _Mimosa_ leaf. Note erectile response (positive curvature) followed by neutralisation and p.r.o.nounced reversal into negative due to transverse conduction of excitation. Up-movement shown by down curve, and _vice versa_.]

Returning to the main experiment we find that:

(1) As a result of unilateral action of light, there was positive phototropic curvature which lasted for 50 seconds.