IRRITABILITY 39 



bending takes place further back. The 

 stimulus is evidently transmitted to the 

 spot where bending occurs. But while 

 this introduces an additional factor it 

 does not make a physico-chemical expla- 

 nation impossible. 



In some cases no mechanistic explana- 

 tion of tropism has as yet been reached 

 but since we can predict and control the 

 process we can not doubt that it is com- 

 pletely subject to physicochemical laws. 

 This may be illustrated by the gravita- 

 tional response of the root. If we place 

 the root on a horizontal wheel we can 

 subject it to a varying amount of centrif- 

 ugal force by causing the wheel to re- 

 volve at different rates. The centrifugal 

 force works horizontally, from the center 

 of the wheel outwards, while that of grav- 

 itation works vertically. If the response 

 of the root is wholly subject to physico- 

 chemical laws we should be able to deter- 

 mine its direction exactly by varying the 

 forces which act upon it. Since we can- 

 not change the vertical component we 



