Tropisms — 'Relation to Modes of Behavior 55 



laws as heliotropism in plants. In both plants and 

 animals it is the direction of the rays that controls 

 the direction of movement. In both plants and ani- 

 mals it is the rays nearer the violet spectrum that 

 are the more potent in evoking the heliotropic re- 

 sponse. In both plants and animals temperature, 

 previous exposure to light and other external fac- 

 tors influence reactions to light in much the same 

 way. Back of all the differences of form and func- 

 tion of plants and animals, and notwithstanding the 

 higher organization of the animal world with its 

 specialized sense organs and complex nervous sys- 

 tems, the living substance of organisms possesses cer- 

 tain fundamental common properties of irritability 

 upon which the common and fundamental features 

 of behavior which we call tropisms depend. 



The theory of Loeb would sweep away all higher 

 psychic factors in the realm of tropisms, and re- 

 duce the phenomena to comparatively simple mani- 

 festations of reflex irritability. . Further he would 

 explain much of the so-called instincts of animals 

 as a result of these tropisms. Since the prospect of 

 finding a mechanical or causal explanation of any 

 feature of behavior is always an alluring one, it will 

 be of interest to pass in review some of these cases 

 of tropisms with the end of determining how far 

 the reflex theory will carry us. And then we shall 

 consider the relation of these tropisms to more com- 

 plex forms of behavior. 



An excellent illustration of a tropism is afforded by 



