Xo. 5-2] 



PLANT RESPONSES 



341 



Another descriptive for a group of phenomena in be- 

 havior, used in both botany and zoology, but in quite dif- 

 ferent senses, is differential sensibility. Pfeffer applies 

 this term to all of the tropistic processes of plants, in- 

 cluding the typical tropisms and the shock-movements; 

 and it is, as he says, true that it is only by a perception 

 of difference in the stimulus on the two sides of an or- 

 ganism that we obtain any tropism. On the other hand, 

 Xagel uses the same term— rntcrsehiedungsempfindung 

 —in application to the shock-movements alone. Jen- 

 nings, I believe, uses the term in Pfeffer 's sense, Loeb 

 and Bohn with nearly the same meaning as Jennings's 

 trial and error idea, or, at least, they would claim that 



tropisms and responses to differential sensibility. 



To Summarize : In eonsidering the foregoing phenom- 

 ena, only the so-called direction movements of fixed or- 

 ganisms and the locomotory movements of free-swim- 

 ming organisms have been referred to. 



The organisms considered— plants and protozoa— have 

 no nerve-cells, but nevertheless, their reactions are, and 

 may be, spoken of as non-nervous reflexes. 



While the responses in fixed and free-moving organ- 

 isms are different in their external manifestations, the 

 relations of stimulus and response and the conditions of 

 operation of stimulus and response are much the same in 

 both classes of organisms. 



The terminology as introduced and generally used by 

 botanists has been chosen as mere designation of the ob- 

 vious stimulus and the direction of movement of the 

 organism. Hence tropism or tarism is used to designate 

 all the responses which animal biologists, with closer at- 

 tention to details of behavior, have subdivided into a 

 larger number. In botany, therefore, the term tropism 

 should be used in a more restricted sense than at present. 



In both plant and animal behavior, this te 

 restricted to those simple movement- whir! 



