Chap, xi Sensitivity and Stimulation 475 



existence of movable starch grains in the cells of the 

 perceptive zone of the root-tip, and attributed the stimula- 

 tion to the alteration in their position relatively to the cell 

 walls as the root is displaced from the vertical direction. 

 This theory, known as the statolith theory, has since the 

 century closed made its way into greater favour, if not 

 general acceptance. 



Czapek showed in 1898, that during and following the 

 sensory excitation, the processes of stimulation are accom- 

 panied by complex chemical changes which precede the 

 visible change in growth. These may be consequences of 

 the internal disturbance produced by the falling statoliths. 

 Czapek showed also that the perception of the stimulus 

 and the manifestations of the reaction are differently 

 affected by external conditions. 



Contact Perception 

 The possession of this form of sensitiveness had, prior 

 to i860, been observed and investigated principally in 

 connexion with the climbing of various plants by means 

 of tendrils, which were considered to be sharply separated 

 from twining stems by the manifestation of contact percep- 

 tion. Von Mohl, somewhat in advance of his contem- 

 poraries, claimed that the latter possess a certain irritability 

 of this kind, but the earlier writers of our period, especially 

 Darwin and De Vries, failed to find evidence of it. The 

 trend of thought during the later years of the century was 

 in favour of the view that the behaviour of stem twiners 

 is rather a response to geotropic stimulation, combined 

 with circumnutatory movement. It was ascribed, in 1873, 

 by De Vries to arrest of the latter movement as the twiner 

 comes into contact with its support, a view which had 

 previously been suggested by Darwin in 1865. In 

 1881 Schwendener suggested that the process is due to 

 circumnutation combined with antidromous torsion, while 



