Chap. IT. SUMMARY. 181 



the stick, apparently by an undulatory movement. At 

 first I attributed this movement to the growth of the 

 outside ; black marks were therefore made, and the 

 interspaces measured, but I could not thus detect any 

 increase in length. Hence it seems probable in this 

 case and in others, that the curvature of the tendril 

 from a touch depends on the contraction of the cells 

 along the concave side. Sachs himself admits* that 

 " if the growth which takes place in the entire tendril 

 " at the time of contact with a support is small, a 

 " considerable acceleration occurs on the convex sur 

 " face, but in general there is no elongation on the 

 " concave surface, or there may even be a contraction ; 

 " in the case of a tendril of Cucurbita this contraction 

 " amounted to nearly one-third of the original length." 

 In a subsequent passage Sachs seems to feel some diffi- 

 culty in accounting for this kind of contraction. It 

 must not however be supposed from the foregoing 

 remarks that I entertain any doubt, after reading De 

 Vries' observations, about the outer and stretched 

 surfaces of attached tendrils afterwards increasing in 

 length by growth. Such increase seems to me quite 

 compatible with the first movement being independent 

 of growth. Why a delicate touch should cause one 

 side of a tendril to contract we know as little as why, 

 on the view held by Sachs, it should lead to extra- 

 ordinarily rapid growth of the opposite side. The 

 chief or sole reason for the belief that the curvature of 



* ' Text-Book of Botany, 1875, p. 779. 



