_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 = 1·7 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 1:2·5:5. [15] An explanation of this preliminary effect will be found in the next chapter. [Illustration: 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 quantity 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.