114 TROPISMS 



window side of the dish. Stronger acids act in the same 

 way but the animals are liable to die quickly. Esters, 

 e.g., ethylacetate, act also like acids and the addition of 

 1 c.c. of a grammolecular solution of ethylacetate to 50 c.c. 

 fresh water also makes all the organisms positively helio- 

 tropic. Alcohols act in the same way. In the case of 

 Gammarus the positive heliotropism lasts only a few 

 seconds, while in Daphnia it lasts from 10 to 50 minutes 

 and can be renewed by the further careful addition of 

 some C0 2 . The following table gives the minimal con- 

 centration of various acids and alcohols for the production 

 of positive heliotropism in certain California species of 

 fresh water copepods, and Daphnia: 



For Copepods For Daphnia 



Formic acid 0.006 N 



Acetic acid 0.006 N 



Propionic acid 0.005. N 



Butyric acid 0.004 N 



Valerianic acid 0.004 N 



Capronic acid 0.002 N 0.0 N 



Ethyl alcohol 0.19 N 0.2 N 



Propyl alcohol 0.054 N 0.05 to 0.1 N 



Normal butyl alcohol 0.019 N 



Isobutyl alcohol 0.04 N 



Amyl alcohol 0.011 N 



As far as alcohols are concerned each higher alcohol is 

 about three times as efficient as the previous one, with 

 the exception of amyl alcohol. This order of relative 

 efficiency is also characteristic for the surface tension 

 effects of these alcohols. 299 



It was of importance to find means of making these 

 organisms negatively heliotropic. Moore 368 found that 

 caffein makes the heliotropically indifferent fresh water 

 crustacean Diaptomus intensely negatively heliotropic. 

 It required the addition of 1.2 c.c. of a 1 per cent, solution 

 of caffein to 50 c.c. of water to bring about this intense 





