^ HISTORICAL REVIEW 53 



of tropisms in opposition to the anthropomorphic descrip- 

 tions current at that time. Animals go toward a source of 

 light neither because it is useful for them to do so nor 

 because they enjoy light or can see, but because they are 

 positively heliotropic. But what is the underlying cause 

 of tropisms? What are the mechanics involved in the 

 processes described by this term? Loeb applied the theo- 

 ries developed by botanists to answer these questions and 

 developed others (see p. 25). Verworn and other in- 

 vestigators added new ones or suggested modifications. 

 Thus it came about that the term tropism came to have 

 a multiplicity of meanings. 



(8) Some of the explanations of behavior offered under 

 the name tropism were founded on the idea that the external 

 agent acts directly or through a direct reflex mechanism on 

 the locomotor organs. This idea together with others 

 assuming unequivocal control of behavior by external 

 factors, Jennings and his followers found to be untenable 

 in their studies on the behavior of the lower organisms. 

 The new features introduced by this school have been 

 clearly set forth above ; it will therefore not be necessary to 

 emphasize them here. 



4. Various Definitions of Tropisms 



The term tropism was first used by De Candolle in 1832. 

 He called the bending of plants toward the light helio- 

 tropism, indicating merely the relation between the direc- 

 tion of bending and the source of stimulation. Later the 

 term tropism came to signify not only the bending or orient- 

 ing but also the explanation of the process. Thus for every 

 new explanation the term received a new signification, and 

 this has naturally led to much confusion. Let us point 

 out some of the different meanings which have been applied 

 to the term heliotropism. 



(i) Sachs in 1876 concluded, as stated above, that 

 orientation of plants is due not to difference in light inten- 



