Animal Instincts and Tropisms 277 



the assumption be correct that the red light is con- 

 siderably less efficient than light which goes through 

 blue glass (such glass also allows green rays to go 

 through). The botanists had already shown that red 

 glass is impermeable for the rays which cause helio- 

 tropic reactions of plants, and the writer was able to 

 show the same for the heliotropic reactions of animals. 

 Red glass acts, therefore, almost like an opaque body 

 for these animals. 



A closer examination of the most efficient rays for 

 the heliotropic reactions of different organisms has 

 revealed the fact that for some organisms a region in 

 the blue X = 46o 490 pt.^, for others a region in the 

 yellowish-green, near about X = 520 530^ is the most 

 efficient. 1 For many plants and for some animals, 

 like Eudendrium and the larvae of the worm Arenicola, 

 a region in the blue is most efficient; for certain, if not 

 most, animals a region in the yellow-green is most 

 efficient. Among unicellular green algae, Chlamydomo- 

 nas, has its maximal efficiency in the yellowish-green 

 and Euglena in the blue. According to observations 

 by Mast, some green unicellular organisms like Pan- 

 dorina, Eudorina, and Spondylomorum seem to behave 

 more like Chlamydomonas, while certain others behave 

 more like Euglena. 2 Wasteneys and the writer suggested 



1 Loeb, J., and Maxwell, S. S., Univ. Cal. Pub., 1910, PhysioL, Hi., 

 195; Loeb and Wasteneys, Proc. Nat. Acad. Sc., 1915, i. f 44; Science, 

 1915, xli., 328; Jour. Exper. Zool., 1915, xix., 23; 1916, xx., 217. 



3 Mast, S. O., Proc. Nat. Acad. Sc., 1915, i., 622. 



