Effect of increasing intensity of light on the tropic curvature of growing organs.—As the tropic curvature is primarily due to the retardation 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. The normal rate of growth of the specimen in the dark was 0·47 µ per second; this was reduced to 0·29 µ under an intensity of one unit, to 0·17 µ under two, and to 0·10 µ under three units. Growth became arrested when the intensity was raised to four units. Thus increasing intensity of light induces an increasing retardation 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 1:2·5:5 under increasing intensities which varied as 1:2:3.
THE EFFECT OF INCREASING ANGLE.
The quantity of light which falls on an unit area of the responding organ varies as sin θ where θ 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°.
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 brass 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 1·6:1, which is not very different from the ratio sin 90°⁄sin 45° = 1·4.
| Fig. 127. | Fig. 128. |