Part 10 (1/2)

The normal rate of growth of the specimen in the dark was 047 per second; this was reduced to 029 under an intensity of one unit, to 017 under two, and to 010 under three units. Growth became arrested when the intensity was raised to four units. Thus increasing intensity of light induces an increasing r.e.t.a.r.dation of growth at the proximal side of the organ. This aided by the effect of indirect stimulus at the distal side brings about an increasing positive curvature.

_Experiment 130._--The flower bud of _Crinum_ was used for the experiment, the source of light being a small arc lamp. The duration of exposure was one minute. Increasing intensity of light gave rise to increasing positive curvatures (Fig. 125) in the ratio of 1:25:5 under increasing intensities which varied as 1:2:3.

THE EFFECT OF INCREASING ANGLE.

The quant.i.ty of light which falls on an unit area of the responding organ varies as sin [Greek: th] where [Greek: th] is the directive angle _i.e._ the angle made by the rays with the surface. Some allowance has to be made for the amount of light reflected from the surface, this being greater at 45 than at 90.

[Ill.u.s.tration: FIG. 126.--The Collimator.]

_Tropic response of pulvinus of_ Desmodium gyrans: _Experiment 131._--For application of light at various angles an incandescent electric lamp was mounted at one end of a bra.s.s tube, a collimating lens being placed at the other (Fig. 126). The parallel beam of light from the collimator could be sent at various angles by rotating the collimator tube round an axis at right angles to the tube. The specimen employed was the terminal leaflet of _Desmodium gyrans_; light was applied for a minute in the two successive experiments for the two angles of 45 and 90. The record (Fig. 127) shows that the phototropic effect increases with the directive angle. In the present case the ratio of the two effects is 16:1, which is not very different from the ratio sin 90/sin 45 = 14.

[Ill.u.s.tration: FIG. 127.--Effect of angle of inclination of light on the tropic curvature of pulvinus. The first response is to light at 45 and the second, to 90. (_Desmodium gyrans_).]

[Ill.u.s.tration: FIG. 128.--Series of tropic curvatures of growing bud of _Crinum_ to alternate stimulation by light at 45 and 90.]

_Tropic response of growing organs: Experiment 132._--Similar experiment was carried out with the flower bud of _Crinum_, held vertical. Light was applied alternately at 45 and 90, in two successive series. The object of this was to make due allowance of possible variation of excitability of the organ during the course of the experiment. I have explained (p. 147), how the excitability of a tissue in a condition slightly below par, is increased by the action of previous stimulation.

Series of responses obtained under alternate stimulations at 45 and 90 enable us to find out, whether any variation of excitability occurred during the course of the experiment and make allowance for it.

The records show that stimulation did enhance the excitability of the organ to a small extent. Thus the first stimulation at 45 induced an amplitude of response of 5 mm.; the second stimulation at 45 _i.e._ the third response of the series, induced a slightly larger response 7 mm.

in amplitude. Similarly the two responses at 90 gave an amplitude of 9 mm. and 11 mm. respectively (Fig. 128). Taking the mean value of each pair, the ratio of tropic effects for 90 and 45 is = 10/6 = 17 nearly.

EFFECT OF DURATION OF EXPOSURE.

_Experiment 133._--The specimen employed for the experiment was a flower bud of _Crinum_ in a slightly sub-tonic condition. Successive responses exhibited on this account, a preliminary negative[15] before the normal positive curvature. The successive durations of exposure were for 1, 2, and 3 minutes. The amplitudes of responses (Fig. 129) are in the ratio of 1:25:5.

[15] An explanation of this preliminary effect will be found in the next chapter.

[Ill.u.s.tration: FIG. 129.--Effect of increasing duration of exposure 1:2:3 minutes, on phototropic curvature. Note preliminary negative response. (_Crinum_).]

We may now recapitulate the tropic effects of light of increasing intensity, directive angle, and duration of exposure. It has been shown that the tropic effect is enhanced under increasing intensity of light; it is also increased with the angle increasing from grazing to perpendicular incidence. And finally, the tropic effect is enhanced with the duration of exposure. Taking into consideration the effects of these different factors we arrive at the conclusion that _phototropic effect increases with the quant.i.ty of incident light_. It will be shown in the next chapter that strict proportionality of cause and effect holds good in the median range of stimulation, and the slight deviation from this, above and below the median range, is due to the fact that susceptibility for excitation is low at these two regions.

SUMMARY.

Increasing intensity of light induces increasing tropic curvature.

Tropic curvature increases with the directive angle, the effect being approximately proportional to sin [Greek: th], where [Greek: th] is the angle made by the rays with the responding surface.

Tropic curvature also increases with the duration of exposure.

The intensity of induced tropic effect is determined by the quant.i.ty of incident light.

x.x.xII.--THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS

_By_

SIR J. C. BOSE.