LEAVES WITH BEGABD TO LIGHT. 447 



horizon in the normal position of the plant, the final result on 

 the klinostat would be the same. The observation which fig. 12 

 represents was as follows. A leaf stood, in the normal posi- 

 tion of the plant, 30° above the horizon or transverse plane ; 

 when the plant was placed on the klinostat, and the leaf was 

 in the position of f 3 it was 45° in front of the transverse plane 

 ( + 45°) ; when it was in the position f v it was again +30°. It 

 seems clear, therefore, that the weight of the leaf tends during 

 half a rotation to bring the leaf in front of the transverse plane, 

 while during the other half rotation it produces no alteration in 

 the normal position. The whole effect, therefore, of the weight of 

 the leaf is to bring the leaf in front of the transverse plane, a 

 result which is exactly the opposite of the effect which the weight 

 of the leaf has when the plant is in the normal position ; for then 

 the weight tends to bring the leaves below the horizon, which 

 would be equivalent to behind the transverse plane. From this 

 it follows that the epinastic curvature of cherry-leaves, when the 

 plant is on the klinostat, is work done against the effect of the 

 weight of the leaves, whereas in the normal position epinasty 

 works in conjunction with the effect of the weight of the leaves. 



These facts and considerations are given to show that the in- 

 fluence of the flexibility of the stem and unsymmetrical rigidity 

 of the petioles have been taken into consideration as elements in 

 the problem. 



Besides the experiments given with various kinds of leaves, a 

 number of experiments were made with the creeping stems of 

 Lysimachia Wummularia ; but as I was unable to arrive at a definite 

 general result, I have not thought them worth publishing at 

 present. 



Conclusion. 



The principle on which the facts above given are most explicable 

 is that given by my father in his recently published book* ; I 

 mean the principle that the chief movements of plants are modi- 

 fications of circumnutation. 



In the parts of plants which are capable of movement the 

 longitudinal tension is continually changing in such a way 

 that a circumnutating movement is produced essentially the 

 same as the revolving nutation of climbing-plants, though 

 much less in amplitude. In the case of leaves, the movements 

 are chiefly due to changes in the longitudinal tension of the 

 * ' The Power of Movement in Plants,' 1880. 



