Significance of Tropisms for Psychology 43 



alcohol acts in the same manner, only more weakly and much 

 more slowly. Animals which were previously indifferent to 

 light become, under carbonic acid treatment, complete slaves 

 of the light.i 



How does the acid produce this result? We will assume 

 that it acts as a sensitizer. The light produces chemical 

 changes, for instance, oxidation, on the surface of the animal, 

 especially in the eye, as was suggested in the case of the aphids. 

 The mass of photochemical substance which is acted upon by 

 the light is often relatively small, so that even when the light 

 strikes the crustacean (copepod) on one side only, the difference 

 in the chemical changes on the two sides of the body remains 

 still too small to call forth a difference in tension or action in the 

 muscles of the two sides of the body, sufficient to turn the ani- 

 mal toward the source of light. But if we add an acid this could 

 act as a catalyzer, as, for instance, in the catalysis of esters. 

 In the catalysis of esters, the acid acts, according to Stieglitz, 

 only to the extent of increasing the active mass of the substance 

 which undergoes a chemical change. In order to fix our ideas 

 provisionally we will assume that the acid makes the animal 

 more strongly positively heliotropic by increasing the active 

 mass of the photosensitive substance. In this way the same 

 intensity of light which before produced no heliotropic reaction 

 now may cause a very pronounced positively heliotropic reac- 

 tion; because if now the animal is struck on one side only by 

 the light, the difference in the reaction products in both retinae 

 becomes rapidly large enough to cause automatically a differ- 

 ence in the action of the muscles of both sides of the body and a 

 turning of the head toward the source of light. 



In certain forms, for instance, in Daphnia and in certain 

 marine copepods, a decrease in temperature also increases the 

 tendency to positive heliotropism. If the mere addition of 

 acid is not sufficient to make Daphniae positively heliotropic, 



1 Loeb, Pflugers Archiv, CXV, 564, 1906. 



