The quantity 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. [Illustration: 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 passes 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). [Illustration: 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._ 1:2·6. 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. [Illustration: FIG. 124.--Tropic effect of increasing intensity of light 1:2; on the response of terminal leaflet of _Desmodium gyrans_.] [Illustration: FIG. 125.--Tropic effect of increasing intensity of light 1:2:3 on growing organ (_Crinum_).]