90 CHAP. X. EFFECT OF MECHANICAL STIMULATION 



The effect of mechanical stimulus on growth is thus 

 similar to those of other modes of stimulation, such as 

 electric and photic. Moderate frictional stimulation in- 

 duces incipient contraction, shown by retardation of growth, 

 recovery being completed within a short time ; but intense 

 stimulation, caused by a wound, gives rise to a greater and 

 more persistent retardation. 



I next describe the movement of curling induced in a 

 tendril bj^ unilateral mechanical stimulation. 



Mechanotropism : Twining of Tendrils 



In response to the stimulus of contact a tendril twines 

 round its support. Certain tendrils are uniformly sensitive 

 on all sides ; but in other cases, as in the tendril of Passiflora, 

 the sensitiveness is greater on the under side. A curvature 

 is induced when that side is rubbed with a splinter of wood, 

 the stimulated side becoming concave. This movement 

 may be distinguished as one of curling. There are, as wilJ 

 be presently shown, instances where the under side becomes 

 convex, the curvature being thereby reversed. 



As regards contrivances for enhanced perception of 

 mechanical stimulus by the tendril, Pfeffer discovered 

 tactile pits on the tendrils of Cucurbitaceae. These no 

 doubt facilitate sudden deformation of the sensitive proto- 

 plasm by frictional contact. No satisfactory explanation 

 has, however, been offered as regards the physiological 

 mechanism of the responsive movement. 



The twining must result from the modification of the 

 normal rate of growth under the stimulus of contact. 

 Further, this modification must be different on the two 

 sides of the organ : the proximal, on which the stimulus is 

 directly applied; and the distal, on which the stimulus can 

 only act indirectly. Investigations had therefore to be 

 undertaken to detect and record the effect of direct stimu- 

 lation, and also of indirect stimulation, that is, when the 

 stimulus acts at a distance. 



