NEGATIVE PHOTOTROPISM IN DAPHNIA 



575 



collection is allowed to form, and then to the 20 cc. of water con- 

 taining the animals there be added 2 cc. of carbonated water, 

 a complete positive collection of the Daphnia at once occurs 

 and remains for ten to thirty minutes, after which they may 

 again become negative. The same effect may be produced b}^ 



MERCURY ARC 



MERCURY ARC 

 WITH GLASS 

 INTERPOSED 



Text figure 1 



substituting 1 cc. x^ HCl for the carbonated water. Alcohols 

 and ether are not effective in producing this result, since narcosis 

 sets in before any significant movement takes place. It is evi- 

 dent, however, that the acids which Loeb found would cause 

 Daphnia to become positive to visible light, are effective in mak- 

 ing these animals positive to the ultra-violet light. 



SUMMARY 



1. Ultra-violet light of wave length shorter than 3341 A. u. 

 is specific for causing negative phototropism in Daphnia pulex. 



2. Negative phototropism so produced is reversed when small 

 quantities of COo or of HCl are added to the water containing 

 the animals. 



