Various analogies have been noticed between phototropic and geotropic reactions, and it has been supposed that the two phenomena are closely related to each other. This has even led to assumption that there are phototropic particles which function like statoliths in geotropic organs. There is, however, certain outstanding difference between the two classes of phenomena. In the case of light, the incident energy is entirely derived from the outside. But in geotropism, the force of gravity by itself is ineffective without the intervention of the weight of cell-contents to exert pressure on the sensitive ectoplasm, and thus induce stimulation. This aspect of the subject will be treated in greater detail in a subsequent chapter.
POSITIVE PHOTOTROPIC CURVATURE.
I shall now describe the phototropic effect of unilateral light in pulvinated, and in growing organs. From the explanation that has already been given, it will be understood that the side of the organ directly acted on by light undergoes contraction and concavity.
Tropic curvature of pulvinated organs: Experiment 117.—For this experiment I employed the terminal leaflet of the bean plant. The source of illumination was 32 c.p. electric lamp, enclosed in a metallic tube with circular aperture for passage of light. The leaflet was attached to an Oscillating Recorder. Light was applied on the upper half of the pulvinus for 20 seconds; this induced an up-movement of the leaflet, due to the contraction of the upper half of the organ. Recovery took place in course of 8 minutes (Fig. 112).
Fig. 112.—Successive positive responses of the terminal leaflet of bean. Light applied from above for 20 seconds; complete recovery in 8 minutes.
POSITIVE PHOTOTROPIC CURVATURE OF GROWING ORGANS.
Effect of moderate stimulation: Experiment 118.—I shall presently show that the intensity of phototropic reaction depends on the intensity and duration of the incident light. A moderate and effective stimulation may thus be produced by short exposure to strong light. For my present experiment I took a stem of Dregea volubilis, and applied light from a small arc lamp to one side of the organ for 1 minute; this induced a positive curvature followed by complete recovery on the cessation of light (Fig. 113).
