106 TROPISMS 



The number of experiments was limited but they indi- 

 cate an optimum between 495 and 450 /*/*, in this respect 

 agreeing with the results on Avena. 



The fact then exists that for the heliotropic reactions 

 of certain plants the blue rays are most efficient, while 

 for the heliotropic reactions of a number of animals the 

 yellowish-green rays are most efficient. But this state- 

 ment cannot be generalized. 



Loeb and Wasteneys determined the most efficient 

 wave length of light for various lower organisms with 

 the result that there are heliotropic animals for which 

 the blue rays are as efficient as they are for plants ; and 

 that for different unicellular green organisms the opti- 

 mum lies in different parts of the spectrum. They found, 

 by a method similar to that used by Blaauw, that for the 

 heliotropic curvature of the animal Eudendrium the most 

 efficient part of the spectrum lies in the blue A = approxi- 

 mately 473 fj.fi. 311 The same was found by them for the 

 larvae of the marine worm Arenicola. 



On the other hand, on investigation of two closely 

 related forms of green flagellates, Euglena and Chlamy- 

 domonas, it was found 3U that they behave differently. 

 For Euglena viridis the blue rays A = 470 to 480 W are 

 especially efficient, while for Chlamydomonas pisiformis 

 the most efficient part was in the region of A = 534 /*/*, 

 in the yellowish-green. b For another green algae, Pan- 

 dorina, Loeb and Maxwell had already found the greatest 

 efficiency in the greenish-yellow. 



t> This would lead us, on the basis of the reasoning of Hess, to the 

 conclusion that the unicellular plant Chlamydomovas has sensations of 

 brightness, suffers from total color blindness (although it has no eyes), that 

 it is not heliotropic, and that it is an animal; while its unicellular cousin, 

 Euglena, has a highly developed color sense, has no sensations of brightness, 

 is heliotropic, and is a plant. 



