(2) Owing to the internal conduction of excitation the positive effect underwent neutralisation by the excitatory contraction of the distal side. This neutralisation depends on four factors: (_a_) on the intensity of the stimulus, (_b_) on the conductivity of the organ in a transverse direction, (_c_) on the thickness of the intervening tissue, and (_d_) on the relative excitability of the distal as compared to the proximal side. The extent of positive curvature also depends on the pliability of the organ.
(3) In anisotropic organs where the distal side is physiologically the more excitable than the proximal, the internally diffused excitation brings about a greater contraction of the distal, and the _positive_ phototropic curvature becomes reversed to a very p.r.o.nounced _negative_.
The effect of the internally diffused stimulus is thus the same as that of external diffuse stimulus.
(4) When the stimulus is applied on the more excitable half of the organ, the result is a predominant contraction of that half, which cannot be neutralised by the excitation conducted to the less excitable half of the organ. As the curvature is towards the stimulus, the phototropic curvature thus remains positive, even under continued stimulation.
The positive curvature is due to the differential action of unilateral stimulus on the proximal and distal sides. But when a strong light is made to act continuously on one side of an organ, the excitation becomes internally diffused, and the differential effect on the two sides is reduced in amount or vanishes altogether. Owing to the weak transverse conductivity of the tissue, while the effect of a feeble stimulus remains localised, that of a stronger stimulus is conducted across it.
Oltmanns found that the seedling of _Lepidium sativum_ a.s.sumed a transverse or dia-phototropic position under intense and long continued action of light of 600,000 Hefner lamps. He regards this as the indifferent position. But the neutralisation of curvature is not, as explained before, due to a condition of indifference, but to the antagonistic effects of the two opposite sides of the organ, the proximal being stimulated by the direct, and the distal by the transversely conducted excitation. I obtained such neutralisation with _Dregea volubilis_ under the prolonged unilateral action of arc-light.
The first effect was positive; this was gradually and continuously neutralised under exposure for two hours; even then the neutralisation was not complete. I shall presently adduce instances where the neutralisation was not merely complete, but the final effect was an actual reversal into negative response.
SUPPOSED PHOTOTROPIC INEFFECTIVENESS OF SUNLIGHT.
I may here consider the remarkable fact that has been observed, but for which no explanation has been forthcoming, that "direct sunlight is too bright to bring about heliotropic curvature, only diffuse, not direct sunlight has the power of inducing heliotropic movements."[13] But we cannot conceive of light suddenly losing its phototropic effect by an increase of intensity. The experiment just described will offer full explanation for this apparent anomaly. Feeble or moderate stimulus remains, as we have seen, localised, hence the contraction of the proximal side gives rise to positive curvature. But the intense excitation caused by sunlight would be transmitted to the distal side and thus bring about neutralisation. It is the observation of the final result that has misled observers as to the inefficiency of direct sunlight. A continuous record of the response of the organ shows, on the other hand, that the first effect of strong light is a positive curvature, and that under its continuous action the positive effect becomes neutralised (cf. Fig. 121). In the study of phototropic action, the employment of strong light has many advantages, since the period of experiment is, by this means, materially shortened. The continuous record then gives an epitome of the various phases of reaction.
[13] Jost--_Ibid_--p. 464.
NEGATIVE PHOTOTROPISM.
I shall next show the continuity of responsive phototropic effects, from the positive curvature to the negative, through the intermediate phase of neutralisation. I have in the preceding paragraph described an experiment where under a given intensity and duration of exposure the excitations of the proximal and distal sides bring about neutralisation, the organ a.s.suming a dia-phototropic position. If the intensity or duration of the stimulating light be further increased, it is easy to see that while excitation transmitted to the distal side is being increased, the excitatory contraction on the proximal side may, at the same time, be decreased owing to fatigue brought on by over-stimulation.
In connection with this it should be borne in mind that the pulvinus of _Mimosa_ exhibits under continuous stimulation, a fatigue relaxation instead of normal contraction. Similar effects are known to take place in animal muscles. The effect of relatively greater excitation will thus give rise to negative phototropic curvature. The transverse conductivity of organs of diverse plants will necessarily be different. The neutralisation and reversal into negative will thus depend on three factors: the transverse conductivity of the organ, the intensity, and duration of stimulus.
_Neutralisation and reversal under increased intensity of light: Experiment 127._--It is advisable to employ thin specimens (in which the transverse distance is small) for the exhibition of reversal effect. I took a hypocotyl of _Sinapis nigra_ and subjected it to unilateral action of light from a 16 candle-power incandescent electric lamp placed at a distance of 10 cm. A maximum positive curvature was induced in the course of 50 minutes. The intensity of light was afterwards increased by bringing the lamp nearer to a distance of 6 cm. This resulted in a process of neutralisation of the preceding response; after an exposure of 70 minutes the specimen a.s.sumed a dia-phototropic position in which it remained in equilibrium. Sunlight was next applied, and in the further course of 30 minutes there was a p.r.o.nounced reversal into negative phototropic curvature.
[Ill.u.s.tration: FIG. 121.--Positive and negative phototropic responses of _Oryza_ under continued unilateral stimulus of intense light from arc lamp.]
_Neutralisation and reversal under continuous stimulation: Experiment 128._--In the last experiment the different changes in the response were brought about by successive increase in the intensity of light. In the present experiment, very strong light was applied from the beginning, and continuous record was taken of the change in the response. In order to reduce the period of experiment I employed a mercury vapour lamp which emits the most effective violet and ultra-violet rays. The specimen used was a seedling of the rice plant (_Oryza sativa_). The first effect of light was a positive curvature which attained its maximum; after this there was a neutralisation in less than six minutes after the application of light. The further continuation of light induced a p.r.o.nounced negative curvature (Fig. 121).
I shall in the next chapter give other instances which will show that all organs (pulvinated and growing) possessed of power of transverse conduction, exhibit a transformation of response from positive to negative under continued action of strong light.
Thus an identical organ, under different conditions of intensity and duration of stimulus, exhibits _positive_ phototropic, _dia_-phototropic, and _negative_ phototropic curvatures, proving conclusively that the three effects are not due to three distinct irritabilities. The responsive movements are, on the other hand, traced to a fundamental excitatory reaction, remaining either localised or increasingly transmitted to the distal side.
NEGATIVE PHOTOTROPISM OF ROOTS.
From the a.n.a.logy of opposite responses of shoot and root to stimulus of gravity, it was surmised that the root would respond to light by a negative curvature. This was apparently confirmed by the negative phototropic curvature of the root of _Sinapis_. The supposed a.n.a.logy is however false; for while the stimulus of gravity acts, in the case of root, only on a restricted area of the tip, the stimulus of light is not necessarily restricted in the area of its action. That there is no true a.n.a.logy between the action of light and gravitation is seen from the fact that while gravitation induces in the root a movement opposite to that in the stem, in the case of light, this is not always so; for though a few roots turn away from light, others move towards the light.
As regards negative phototropic response of the root of _Sinapis_, it will be shown (p. 376) to be brought about by algebraical summation of the effects of direct and indirect photic stimulus.
SUMMARY.
The normal positive phototropic curvature is modified by transverse conduction of true excitation to the distal side of the organ.
The extent of neutralisation or reversal due to internal conduction of excitation from the proximal to the distal side of the organ depends: (_a_) on the intensity of the incident stimulus, (_b_) on the conductivity of the organ in a transverse direction, (_c_) on the thickness of the intervening tissue, and (_d_) on the relative excitability of the distal as compared to the proximal side.
The dia-phototropic position is not one of indifference, but of balanced antagonistic reactions of two opposite sides of the organ.
The supposition that direct sunlight is phototropically ineffective is unfounded. The response is fully vigorous, but the first positive curvature may in certain cases be neutralised by the transmission of excitation to the distal side.
Under light of strong intensity and long duration, the transmitted excitation to the distal side neutralises, and finally reverses the positive into negative curvature.
The _positive_-phototropic, the _dia_-phototropic, and the _negative_ phototropic curvatures are not due to three distinct irritabilities but are brought about by a fundamental excitatory reaction remaining localised or increasingly transmitted to the distal side.
x.x.xI.--THE RELATION BETWEEN THE QUANt.i.tY OF LIGHT AND THE INDUCED PHOTOTROPIC CURVATURE
_By_
SIR J. C. BOSE,
_a.s.sisted by_
SURENDRA CHANDRA DAS, M.A.
I shall in this chapter describe experiments in support of the important proposition that _the intensity of phototropic action is dependent on the quant.i.ty of incident light_. The proportionality of the tropic effect to the quant.i.ty of light will be found to hold good for the median range of stimulation; the deviation from this proportionality at the two ends of the range of stimulation--the sub-minimal and supramaximal--is, as we shall find, capable of explanation, and will be fully dealt with in the next chapter.
The quant.i.ty of light incident on the responding organ depends: (1) on the intensity of light, (2) on the angle of inclination or _the directive angle_,[14] and (3) on the duration of exposure. I shall give a detailed account of the investigation relating to the individual effects of each of these factors on the tropic reactions not merely in pulvinated but also in growing organs.
[14] The directive angle [Greek: th] is the angle of inclination of the rays of light to the responding surface. The angle [Greek: th] is complementary to the angle of incidence _i_ in optics. Sin [Greek: th] = Cos i.
EFFECT OF INCREASING INTENSITY OF LIGHT ON TROPIC CURVATURE.
The intensity of light was increased in successive experiments, in arithmetical progression 1:2:3 by suitably diminishing the distance between the plant and the source of light, and the resulting tropic curvatures recorded.
[Ill.u.s.tration: FIG. 122.--Leaf of _Desmodium gyrans_, with the terminal large, and two lateral small leaflets. These latter exhibit automatic pulsations.]
_Effect of increasing intensity of light on the pulvinus of_ Desmodium gyrans: _Experiment 129._--The source of light was a 50 candle-power incandescent lamp, and the duration of exposure was 1 minute. The specimen employed was a terminal leaflet of _Desmodium gyrans_ (Fig.
122) the pulvinus of which is very sensitive to light. It is more convenient to manipulate a cut specimen of the leaf, instead of the whole plant. The petiole is placed in water contained in a U-tube; the depressing effect of wound pa.s.ses off in the course of an hour or so.
Light of increasing intensity is applied from above; this induces a contraction of the upper half of the pulvinus, and the resulting response is recorded by means of the Oscillating Recorder (Fig. 123).
[Ill.u.s.tration: FIG. 123.--The Oscillating Recorder (From a photograph).]
The first record was obtained under a given intensity, and the second, under an intensity twice as great. The tropic effects are seen to increase with the intensity (Fig. 124). If the tropic curvature increased proportionately to the intensity, the two responses should have been in the ratio of 1:2; the actual ratio was however slightly greater, _viz._ 1:26. In this connection it will be shown in the next chapter, that strict proportionality holds good only in the median range, and that the susceptibility for excitation undergoes an increase at the beginning of the phototropic curve.
[Ill.u.s.tration: FIG. 124.--Tropic effect of increasing intensity of light 1:2; on the response of terminal leaflet of _Desmodium gyrans_.]
[Ill.u.s.tration: FIG. 125.--Tropic effect of increasing intensity of light 1:2:3 on growing organ (_Crinum_).]
_Effect of increasing intensity of light on the tropic curvature of growing organs._--As the tropic curvature is primarily due to the r.e.t.a.r.dation of growth induced by light at the proximal side of the organ, it will be of interest to recapitulate the results I obtained (p.
208) on the effects of increasing intensity of light on growth itself.