104 TROPISMS 



bees the yellowish-green part of the spectrum is most 

 efficient, and he concluded that bees are totally color blind. 

 To a totally color blind person the blue cardboard appears 

 only like a shade of gray, and such a person is unable to 

 learn to discriminate between a blue and gray piece of 

 cardboard, v. Frisch's experiments support the con- 

 clusion that it is unjustifiable to use experiments on 

 heliotropism to draw conclusions concerning light or 

 color sensations, v. Frisch and Kupelwieser 169 have 

 also demonstrated selective effects of different light 

 waves for Daphnia which differ from those found for the 

 eye of a totally color blind person, and their observations 

 have been confirmed by Ewald. 147 



2. Hess's conclusions are in conflict with another 

 group of facts. For many plants the blue region of the 

 solar spectrum is the most efficient. Hess is, therefore, 

 compelled to conclude that the heliotropism of such plants 

 is different from that of animals, since it would seem 

 preposterous to assume that swarmspores of plants should 

 go to the light because they have sensations of color. He, 

 therefore, assumes that plants are heliotropic in the 

 sense of the mechanistic theory but that positively helio- 

 tropic animals go to the light on account of their love for 

 brightness which is exactly the old viewpoint of Graber. 

 It can be shown, however, that the difference in the helio- 

 tropism of animals and plants, which Hess assumes, is 

 contrary to the facts, since there are heliotropic animals 

 for which the blue rays are the most efficient, as for most 

 plants ; and there are green algae for which the yellowish- 

 green rays are most efficient, as for animals. 



For many if not most plants the blue rays are the most 

 efficient for inducing heliotropic curvatures. Blaauw 47 

 proved this in the following way: He exposed a row of 



