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THE AMERICAN NATURALIST [Vol. XLTY 



stimuli, such as gravitation, light and chemicals; and 

 both kinds vary their response according to their physio- 

 logical state. In the various published observations 

 which treat of the reactions of fixed and free-moving or- 

 ganisms, the authors regard the responses as belonging 

 to the same class ; and we may so regard them here. It 

 might be said also, that the majority of reactions in the 

 middle and higher metazoa are regarded by some as the 

 same in kind, differing only in degree from the reactions 

 of the protozoa. With the more complex reactions of 

 the higher metazoa, botanists have little to do. 



"What now are the characteristics of these sensitive 

 phenomena in plants and animals by which we class such 

 phenomena together? 



1. On both the plant and the protozoan side, the organ- 

 isms possess no nerve cells. We used to be taught that 

 for a reflex action there must be an afferent nerve, a 

 ganglion and an efferent nerve. Such a chain is no 

 longer necessary for a reflex according to definition. 

 Pfeffer speaks of reflex acts in plants, and Massart, 

 Jennings, Bohn and others expressly state that the 

 tropisms of plants and nerveless animals are to be denom- 

 inated reflexes, or, more specifically, non-nervous re- 

 flexes. 



2. By the term "tropisms'' (including "taxims"), 

 the botanist has understood those direction movements 

 of plants due to external stimuli, such as gravitation, 

 light and chemicals. But this use of the term is now too 

 broad. 



The zoologists apply the same term to similar move- 

 ments of the lower animals. But not all zoologists adopt 

 the definition of a tropism as the definition is understood 

 by the botanists. To the botanist, < ' tropism " always im- 



tion of a stimulus; that is, the stimulus produces first an 

 excitation, the excitation sets up a wave, or impulse, 

 which is transmitted to the reaction protoplasm, whose 

 action gives the sensitive response. The controversy 



