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THE AMEBIC AN NATURALIST [Vol.XLIV 



known to occur in plants seems to me to illustrate fairly 

 well the so-called direct response of the zoologists: The 

 leaves of some plants roll up when illuminated by the hot 

 sun, due to the loss of water on the illuminated side. 

 This is certainly direct, but most biologists would call it 

 a mechanical, and not a sensitive response. 



It has already been said that botanists accept the in- 

 direct method as a] (plying in all cases to plants; and 

 almost all zoologists take the same view with regard to 

 animal reactions. We may conclude, therefore, that the 

 term "tropism" may be used in the same sense for both 

 plants and animals. The "tropism" resulting from 

 direct action has not yet been demonstrated for either 

 plants or animals. This conception of the significance of 

 the term "tropism," as presented in the foregoing para- 

 graphs, is not simple, but very complex ; and in attempt- 

 ing to analyze the behavior of organisms, we meet with 

 several conditioning phenomena, some of which may now 

 be described. The terminology used in the following 

 paragraphs is taken mostly from the zoologists, and bot- 

 anists will note that the same terms are applicable to 

 plant response. 



One of the most widely extended of these observed phe- 

 nomena is the variability of the response when the same 

 external conditions are operating. Thus, Strasburger 

 found the swarms-cells of Ulothrix and other alga-, during 

 their early active period, positively heliotropic, but neg- 

 atively heliotropic during their later active period. The 

 change in disposition of the peduncle of the poppy, of the 

 Narcissus, and of the members of various other plants, 

 is familiar to all botanists. All of the foregoing changes 

 in disposition can be referred, for want of a clearer under- 

 standing, to internal changes incident to age. These 

 internal conditions which influence the response to ex- 

 ternal stimulus are called also the physiological state. 

 The foregoing examples are illustrations of the influence 

 of physiological state on response. 



The physiological state may also be altered, and hence 



