HELIOTEOPIC TEANSFOEMATION 117 



The attempt to explain all these reversals on the 

 assumption of a change in the central nervous system 

 meets with the difficulty that such reversals occur also in 

 unicellular organisms which have no central nervous 

 system. Thus the writer observed that Volvox, which 

 occurred in the same ponds in California from where 

 Daphnia came, could also be made positive by C0 2 . 296 

 In swarmspores of algae reversals of heliotropism are a 

 common phenomenon. While these unicellular organisms 

 have no central nervous system they may have synapses 

 such as exist between different neura of metazoa. The 

 writer is not sufficiently familiar with the behavior of 

 synapses in higher animals to suggest that this condition 

 is responsible for the changes in the sense of heliotropism. 



We may finally discuss briefly a possible solarization 

 effect. The writer found that it is possible to make ani- 

 mals generally negatively heliotropic with the aid of 

 ultraviolet light. 296 If once rendered negative such ani- 

 mals will be negative not only to ultraviolet rays but also 

 to the light of an incandescent lamp. A. E. Moore 366 

 found that the ultraviolet rays having such an effect have 

 a wave length shorter than 3341 A.U. Oltmanns had ob- 

 served that Phycomyces is positively heliotropic in weak 

 light, indifferent in somewhat stronger light, and nega- 

 tively heliotropic in still stronger light. Blaauw found 

 that when the illumination was strong the seedlings of 

 Avena became negatively heliotropic. 47 He suggests the 

 analogy with solarization effects in photography. The 

 discovery of photodynamic effects by v. Tappeiner 477 

 adds to the possibilities which should be considered in 

 this connection. 



While Drosophila is usually positively heliotropic, 

 McEwen has recently described a mutant of this species 



