52 Studies in Animal Behavior 



may look forward to a rapid increase in our knowl- 

 edge and insight. 



From the standpoint of analysis the subject of 

 tropisms is one of great import. Certain stimuli 

 exercise a directive effect upon the movements of 

 animals, causing them to go toward or away from 

 the source of stimulation. The moth flies toward 

 a candle; infusorians gather in regions of dilute 

 acids and avoid regions of too great heat or cold; 

 certain caterpillars tend to crawl opposite the di- 

 rection of the force of gravity. These directed 

 movements are commonly called tropisms but there 

 is a variety of opinions regarding the kinds of be- 

 havior to which the term tropism may be applied 

 and usage has not settled authoritatively upon any 

 rigid definition of the word. We shall therefore 

 use the word in a somewhat broad and indefinite 

 sense. 



Tropisms have long been recognized in plants. 

 The familiar phenomenon of the turning of plants 

 to the sun was termed heliotropism by De Can- 

 dolle in 1835, and he, in common with several other 

 botanists in the early and middle parts of the nine- 

 teenth century, regarded this turning as a direct and 

 more or less mechanical effect of sunlight upon 

 the tissues of the plant. Sachs on the other hand 

 emphasized the aspect of irritability in tropisms, 

 and maintained that it is the direction in which the 

 rays of light penetrate the tissues of the plant and 

 not merely the different degrees of illumination on 



