38 THE NATURE OF LIFE 



wheel in such a position that the centrif- 

 ugal force acts on the tip in one direction 

 and on the rest of the root in the opposite 

 direction the root bends as if only the tip 

 were acted on. The stimulus is therefore 

 perceived at the tip and transmitted to 

 the region where the bending takes place. 

 The bending is due merely to the fact that 

 the side of the root which is stimulated 

 grows more slowly than the opposite side. 



The root bends under the influence of a 

 variety of stimuli such as light, heat, air, 

 water and various other chemical sub- 

 stances, currents of water, contact, etc. 

 These are all cases of tropism. 



The tropisms of plants seem to be sus- 

 ceptible of a physicochemical explana- 

 tion. Where, for example, a plant bends 

 toward the light it is evident that growth 

 is slower on the illuminated side. As it 

 is well known that light checks growth 

 the explanation seems to be fairly clear. 

 It is, however, complicated by the fact 

 that in many cases the stimulus is per- 

 ceived at the tip of the plant and the 



