Animal Instincts and Tropisms 283 



of green plants could not exist if their stems were not 

 positively, their roots negatively, heliotropic. It is 

 the positive heliotropism which makes the top grow 

 toward the light, which enables the leaves to get the 

 light necessary for assimilation, and the roots to grow 

 into the soil where they find the water and nutritive 

 salts. 



10. While we do not wish to deal here with the 

 different tropisms it should be stated that aside from 

 heliotropism, chemotropism as well as stereotropism 

 play the most essential role in the so-called instinctive 

 actions of animals. It is a problem of orientation by 

 the diffusion of molecules from a centre when a male 

 butterfly is deviated from its flight and alights on the 

 wooden box in which is enclosed a female of the same 

 species. We have already alluded to certain phenomena 

 of chemotropism in Chapter IV. Certain organisms 

 have a tendency to bring their bodies as much as 

 possible on all sides in contact with solid bodies; thus 

 the butterfly AmpMpyra, which is a fast runner, 

 will come to rest under a glass plate when the plate 

 is put high enough above the ground so that it touches 

 the back of the butterfly. The animals which live 

 under stones or underground or in caves are as a rule 

 both negatively heliotropic and positively stereotropic. 

 Their tropisms predestine or force them into the life 

 they lead. 



The sensitive area which forms the basis of tropisms 



