154 TROPISMS 



Harper for the heliotropism of certain worms, namely 

 that in strong light the animals move by heliotropism, 

 in weak light by "trial and error." These statements 

 are as erroneous as the assertion that while a stone falls 

 under the influence of gravity a feather finds its way down 

 by the method of ' ' trial and error. ' ' 



Barrows and Harper overlook the role of mass action 

 and reaction velocity. When an animal is struck on one 

 side only by light or by a chemically active substance 

 emanating from a center of diffusion, the mass of this 

 substance or of the photochemical reaction product in- 

 creases on this side. These substances react with some 

 substance of the nerve endings and as soon as the mass 

 of the reaction product reaches a certain quantity the 

 automatic turning, the tropistic reaction, occurs. When 

 the light is strong or when the animal is near the center 

 of diffusion, this happens in a short time and the tropistic 

 character of the reaction is striking, since the animal is 

 quickly put back into its proper orientation if it deviates 

 from it. When the light is weak or when the animal is at 

 some distance from the center of diffusion it will take a 

 longer time before this critical value of the reaction prod- 

 uct is reached, and in this case the animal can deviate 

 considerably out of the correct orientation before it is 

 brought back into the right orientation. 



