THE REVERSAL OF TROPISMS 



very prevalent and noteworthy peculiarity 

 the tropic movements of animals and plants 

 is the phenomenon of reversal. The same organ- 

 ism may show a positive or a negative reaction to 

 temperature, light, gravity or the electric current 

 according to the strength of the stimulus, the or- 

 ganism's own condition or the coincident influence 

 of other agencies. Through this power of reversal 

 behavior is rendered more plastic. Granting that 

 the tropisms are involuntary reactions to external 

 stimuli, the fact that the organism may go either 

 toward or away from a stimulus according to how 

 it is influenced by various internal and external con- 

 ditions makes its behavior much more varied, and 

 affords an opportunity for a closer adaptation to 

 the numerous environmental agencies that affect it. 

 It is not to be inferred that all reversals of trop- 

 isms are adapted to meet some organic need. In 

 many cases nature does not seem to have conferred 

 upon the organism any power of reversal whatso- 

 ever. The pomace-fly Drosophila is positively pho- 

 totactic in very strong light, but Carpenter has shown 

 that when an intensity of 480 candle power is 



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