XXXI.—THE RELATION BETWEEN THE QUANTITY OF
LIGHT AND THE INDUCED PHOTOTROPIC CURVATURE
By
Sir J. C. Bose,
Assisted by
Surendra Chandra Das, M.A.
I shall in this chapter describe experiments in support of the important proposition that the intensity of phototropic action is dependent on the quantity of incident light. The proportionality of the tropic effect to the quantity of light will be found to hold good for the median range of stimulation; the deviation from this proportionality at the two ends of the range of stimulation—the sub-minimal and supramaximal—is, as we shall find, capable of explanation, and will be fully dealt with in the next chapter.
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.
EFFECT OF INCREASING INTENSITY OF LIGHT ON TROPIC CURVATURE.
Fig. 122.—Leaf of Desmodium gyrans, with the terminal large, and two lateral small leaflets. These latter exhibit automatic pulsations.
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.