HELIOTROPISM 67 



side only, the stem on the side of the light contracts more 

 than on the other side, and this results in a bending of the 

 stem, whereby the polyp is put into the direction of the 

 rays of light. As soon as the axis of the polyp is in the 

 direction of the rays of light (provided there is only one 

 source of light), the tension of the contractile elements is 

 the same all around, and there is no more reason for the 

 organism to change its orientation. It, therefore, re- 

 mains in this orientation to the light. 



The muscle tension theory of animal heliotropism is, 

 therefore, proved for all classes of the animal kingdom, 

 infusorians, hydroids, annelids, crustaceans, etc. It would 

 bo wrong to state that the theorv holds only for insects. 



