132 DYNAMICS OF' LIVING MATTER 



positively heliotropic if i or 2 c.c. of a grammolecular solution of an 

 ester, e.g. ethylacetate, is added. Ethylbutyrate and methylacetate 

 act similarly, only they seem to be more toxic. The transformation is 

 rapid but not instantaneous, and by giving smaller dose? of ethylace- 

 tate the latent period may be prolonged. 



A second means of making them positively heliotropic 'v through 



the addition of acids. If, instead of an ester, i to 2 c.c. of — 



10 



solution of an acid, e.g. hydrochloric, oxahc, or acetic acid, is added,- 

 the animals also become positively heliotropic. It seems to me of im- 

 portance that CO2 is especially active in this respect. If COj is 

 allowed to bubble through the vessel in which the Gammarus are, or if 

 from 5 to 10 c.c. of soda water (or even beer !) is added to 50 c.c. of 

 water, they become also positively heliotropic. Boracic acid seems to be 

 ineffective. 



Of other substances which act in a similar way, alcohol, 

 paraldehyde, ammonium salts, and to a shght extent K-salts, may be 

 mentioned. But much higher concentrations of these substances are 

 needed than of the acids or esters. The positive heliotropism which 

 is produced in this way is only transitory. 



In a colony of Cyclops some individuals, as a rule, are outspokenly 

 positively heliotropic, others are rather indifferent to light, and a few 

 may gather at the room side of a glass dish. If, however, i to 2 c.c. 



of — HCl, or another not too weak acid is added, or CO. is admitted, 

 10 > ' . ' 



the animals, all gather in a narrow region at the window side of the 

 vessel. If the water is rendered weakly alkaline, they become less 

 outspokenly positively or even, in part at least, negatively heliotropic. 

 I have not been able to obtain this latter effect of alkaUes in Gammarus. 

 We see, therefore, that acid, especially CO,, not only makes negatively 

 heUotropic Cyclops positively heUotropic, but increases the intensity 

 of the positive heliotropism in those that were already positive at the 

 beginning of the experiment. 



In Daphnia it can be shown that when they are only weakly posi- 

 tively heliotropic, e.g. in weak light, the addition of acid in the above- 

 mentioned concentration makes them intensely positively heliotropic. 



I may perhaps call attention to the fact that acids, especially CO,, 

 are formed in organisms;. -that, moreover, esters are formed in the 

 stems of many plants. It may be that these substances play a r61e 

 in the production and variation of heliotropism in plants and animals. 



In my first publications on animal heliotropism I had already men- 

 tioned the fact that chemical changes in certain animals apparently 



