﻿IRRITABILITY 25 



force causes the movement. The leaf then is 

 a living machine the movements of which are 

 directed by two forces— light and heat (some- 

 times gravity), while the root is directed by 

 many forces. 



Although the motor zones of the shoots do 

 not include as large proportions of the plant 

 as the sensory zones, yet the distribution is 

 fairly general throughout growing regions. 

 It is possible to induce curvatures in some 

 stems in which growth has almost entirely 

 ceased. The curvature is accompanied by a 

 revival of the growth activity, however. 



Having followed the shoot to its ultimate Development 

 and present form we may retrace our steps to r 

 the beginnings of the root-system. The primi- 

 tive function of the root of the plant emerging 

 from an aquatic habit was of course purely 

 mechanical and consisted in holding the or- 

 ganism in place. With the recession of the 

 water the plaut no longer found solutions 

 of mineral salts bathing its surface. Its rudi- 

 mentary anchoring organs were, it is true, 

 left in contact with small quantities of fluid, 

 but the amount of surface was by no means 

 adequate or proportional to the now rapidly 

 enlarging shoot. Under such circumstances 

 it might do but one thing— extend the root- 

 system and thereby increase its absorbing 

 surface. 



