The Reversal of Tropisms 109 



of reaction up towards the optimal value; in the 

 latter case the sentient surfaces are turned away 

 from the light so as to decrease the velocity of re- 

 actions down toward the optimal value. As a result 

 of the orientation so caused there arises movement 

 of the organism towards or away from the source of 

 light, but such orientation is not a fixed orientation, 

 but rather a steering action; the animals as a result 

 do not remain in one fixed plane or direction of 

 movement, but the net result of the movement is 

 that the organisms move to or from the light." 



This explanation is one that is little more than a 

 statement of the facts to be explained. It is highly 

 probable a priori that the speed of reactions in the 

 stimulated cell would increase with the intensity of 

 the stimulus, and to say that when these reactions 

 become sufficiently rapid the sentient surface is turned 

 away tells us no more than that under these con- 

 ditions the organism becomes negatively phototac- 

 tic. In Moore's paper there is no recognition of 

 the fact that some organisms are negative when first 

 exposed and tend to become positive the more rapidly 

 in more intense light. 



Another interpretation of the reversal of trop- 

 isms has been given by Mast ( i ) in his paper on 

 the light reactions of Volvox. The endeavor is made 

 to correlate the reversal of tropisms with the rever- 

 sibility of chemical reactions. It is well known that 

 chemical reactions may proceed predominantly in 

 one or another direction according to the amount 



