HELIOTROPIC REACTIONS — ANIMALS AND PLANTS 



235 



DISCUSSION AND SUMMARY OF RESULTS 



1. As stated in the previous papers, the vaUdity of the Bun- 

 sen-Roscoe law for the heliotropie reactions of some (and possi- 

 bl}^ many or all) organisms suggests that these reactions are due 

 to a chemical action of the light. There seem to exist two types 

 of heliotropie substances (or elements), one with a maximum of 

 sensitiveness in the yellow-green region, and the second with a 

 maximum of sensitiveness in the blue. 



2. It would be wrong to state that the one type of photosensi- 

 tive substances is found exclusively in plants and the other ex- 

 clusively in animals. As a matter of fact, our experiments have 

 shown that the animals Eudendrium ramosum and (the larvae of) 

 Arenicola are most sensitive to blue light, which is also most effi- 



cient for the seedlings of the plant Avena (according to Blaauw) ; 

 while the larvae of Balanus, Daphnia, and probably many 

 other animals are most sensitive to the yellow-green or yellow 

 part of the spectrum. Of the two green flagellates, Euglena viri- 

 dis and Chlamydomonas pisiformis, the former is most sensitive 

 to blue, the latter to greenish-yellow. The two groups of photo- 

 sensitive substances (or elements) are, therefore, distributed in- 

 dependently of the boundaries between animals and plants. It 

 is quite possible, however, that plants are more generally sensi- 

 tive to the blue rays of the spectrum, while among animals those 

 may prevail that are more sensitive to yellowish-green or yellow. 

 3. Table I states the wave lengths in the carbon arc spectrum 

 for which the different organisms investigated by us are most 

 sensitive. 



THE JOURNAL OF EXPERIMENTAL ZOOLOUV. \ OL. 2(J, NO. 



