CYTOST IN THE PLANT WORLD 251 



surprising, since the human skin is constantly sub- 

 jected to mild friction from clothing, etc. In other 

 words, from Lewis's experiments it appears that the 

 degree of friction is important in determining the ex- 

 tent of the cellular irritation which leads to the dis- 

 charge of the endocellular substance responsible for 

 wheal formation. There seems to be no valid reason 

 why a similar state of affairs should not exist in the 

 plant, although of course the cells of the tendrils must 

 be considerably more sensitive to friction than are 

 those of the human skin. Indeed, it is recorded by 

 MacDougal (1896) that the tendrils of Echinocystis 

 will respond to the frictional forces incident to con- 

 tact with the delicate thread of a spider's web. 



Similar stereotropic responses in the roots of seed- 

 lings have been observed by Sachs (1873) and many 

 later investigators. This author found that when a 

 pin was placed in contact with the rapidly growing 

 -roots of various seedlings the growing region became 

 concave within about ten hours. More recent investi- 

 gations have disclosed that in such stereotropic re- 

 sponses the actual growth processes leading to the 

 tropic curvature takes place, not at the site of contact 

 between the stimulus and the growing root tip, but 

 at a short distance from it. In this respect the be- 

 havior is comparable to that which obtains in photo- 

 tropic bending. 



Curiously enough, however, it is only the delicate 

 growing root tip which is able upon contact to give 

 rise to the growth-promoting substance — a phenome- 

 non comparable to the observation that decapitated 

 coleoptile tips do not respond to light. In this con- 

 nection the experiments of Cholodny (1926) are il- 



