CHAPTER XI 



THE RELATIVE HELIOTROPIC EFFICIENCY OF 

 LIGHT OF DIFFERENT WAVE LENGTHS 



1. The validity of the Bunsen-Roscoe law for the helio- 

 tropic reactions of animals and plants leaves no doubt 

 that these reactions are determined by the rate of photo- 

 chemical processes. Heliotropic reactions depend, how- 

 ever, not only upon the intensity but also upon the wave 

 length of light. Photochemistry shows that the most 

 efficient wave length varies with the nature of the photo- 

 chemical substance and that comparatively slight changes 

 in the constitution of a molecule may bring about con- 

 siderable changes in the relative efficiency of different 

 wave lengths. The search for differences in the helio- 

 tropic effect of different wave lengths can be of service 

 in detecting the nature of the photochemical substances 

 responsible for heliotropic reactions. 



The investigations on the relative heliotropic efficiency 

 of different wave lengths have generally been undertaken 

 for a different purpose, namely, to get information con- 

 cerning the color sensations of animals. Graber gave 

 it as the result of his observations that all animals which 

 were fond of light were also fond of blue, and animals 

 which were fond of dark were also fond of red. 180 He put 

 animals into a box half of which was covered with trans- 

 parent glass and half with an opaque object, and then 

 counted the relative numbers of organisms in both halves 

 of the box. He then replaced these screens by colored 

 glasses and obtained the above-mentioned result. The 

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