476 The Physiology of Plants BOOK in 



Baranetzky a year later held that asymmetric circumnutation 

 will explain it. 



The ordinary regular movement of circumnutation, 

 depending on rhythmic and coordinated variation of turgor, 

 failed to satisfy the minds of investigators as an explana- 

 tion of many of the observed phenomena of tendril twining, 

 but there was no general agreement as to what modifica- 

 tions of the circumnutatory processes are brought about. 

 Sachs held that the curvature is due to a disturbance of 

 the growth of the tendril in consequence of the stimulation, 

 both concave and convex sides growing, but at different 

 rates. De Vries, writing in 1873, suggested that the 

 stimulus of contact produces an increase of turgor in the 

 cells of the side which becomes convex, causing a stretching 

 of that side, which is followed and fixed by growth. The 

 old view of Knight, that there is an actual contraction of 

 the concave side, was reasserted by Darwin in 1865 ; he 

 held that the rapidity with which the curvature sets in in 

 certain cases forbids the idea of growth as its cause. Careful 

 measurements which he made gave in many instances no 

 evidence that the convex side had grown. Earlier measure- 

 ments made by both De Vries and Sachs were somewhat 

 conflicting, and led those authors to the views already 

 attributed to them. In 1896 McDougal found that con- 

 traction is certainly shown by the tendrils of Passiflora 

 gracilis, but that its occurrence is not universal. He sug- 

 gested that the movement is due to a diminution of turgidity 

 on the concave side and to the curve being rendered per- 

 manent by subsequent growth, a view which has much in 

 common with that of De Vries. The latest researches of 

 the century on the subject, which were not published till 

 1903, were those of Fitting, which were carried out by 

 means of microscopic measurements of the distances of 

 lines marked along the sides which became respectively 

 convex and concave in response to the contact stimulus. 



