Ball — Phototvopic Movements of Leaves. 



283 



The distance between the tops of the two petioles and also the distance 

 ■between the two leaf-tips were measured from day to day and the means of 

 these two distances are plotted in fig. 2, which shows the results obtained from 

 the two plants (Expts. I and II), both experiments being carried on 

 simultaneously. At the end of five days the shades were removed and replaced 

 by similar shades which covered the outer edges of the pairs of leaves, as shown 

 in fig. 1, h. The experiments were continued for a period of twenty days, the 

 shades being changed at intervals of five days. The curves in fig. 2 show clearly 

 that when the inner edges of the pairs of leaves were shaded the leaves moved 

 apart, and when the outer edges were shaded they came closer together. The 

 •slight irregularities in the curves seem to Ije due to iiiequalities in the 

 temperature of the surroundings, which varied from day to day during the 

 course of the experiments. The effect due to this cause was particularly marked 

 during the eighth day of the experiments, when, owing to failure of the 

 heating arrangements in the greenhouse, the temperature fell considerably, 

 with the result that movement of the leaves practically ceased. 



TIME IN DAYS 



Fig. 2. 

 Curves showing moTements of the leaves caused by shading portions of the leaf -blades. 

 During the periods marked a th? inner edges of the pairs of leaves were shaded, 

 and during those marked & the outer edges were shaded. (Cf. fig. 1, a and 6, 

 respectively.) 



The region of each petiole in which the greatest curvature took place was 

 about 1 cm. from the basal end. During the last period of five days 

 represented on the curves, the angles between the petioles were measured, and 

 in one plant the angle between the pair of petioles decreased by 13° and in 

 the other by 10°. 



In order to confirm these results a modification of the experiments was 

 carried out on another plant of the same species. As before the leaves were 

 horizontal, the plant being illuminated from above only. Strips of tinfoil 

 were attached to three of the leaves so as to cover one border of the lamina 

 to a distance of about 2 cm. from the edge, the breadth of the lamina being 

 about 10-12 cm. The tinfoil was attached by folding in the edges on the 

 under side of the leaves. In each of the three leaves a slow horizontal movement 



