524 S. O. MAST 



The second conclusion is based upon the contention that the 

 region in the spectrum of maximum stimulating efficiency is 

 either in the blue or in the green for all organisms. This is prob- 

 ably not true, but even if it were true it would be just as logical 

 to conclude, on the basis of the results obtained by these au- 

 thors, that there are three or four ty^es of photosensitive sub- 

 stances, for they maintain that the maximum is respectively at 

 460-490, 495, 535, 560-578 mm for Euglena, Arenicola, Chlamy- 

 domonas and Balanus. Our results and those of others also indi- 

 cate that the maximum is located in several different regions and 

 that some of these do not correspond with any of those given; 

 consequently if the argument of Loeb and Wasteneys is valid 

 there are at least four 'Hypes of photochemical substances." 

 This argument appears to be valid, however, only if the relative 

 absorption of the different colors is the same in those organisms 

 in which the location of the maximum differs. 



All that can be said, then, regarding the chemical reactions 

 associated with reactions to colors is that they differ in all organ- 

 isms for which the distribution of stimulating efficiency in the 

 spectrum differs, provided there is no difference in selective ab- 

 sorb tion. It can not be said, however, that the chemical re- 

 actions are the same in all organisms in which the spectral 

 distribution of stimulating efficiency is the same, for, judging 

 from our present knowledge of photochemistry, which is ad- 

 mittedly very inadequate, the relation between photochemical 

 reaction and wave-length is in many instances the same for 

 different substances. The fact that the relation between wave- 

 length and reaction is the same for individuals when they are 

 negative as it is when they are positive supports this con- 

 tention, for the chemical reactions associated with these differ- 

 ent responses are, in all probability, not the same. 



SUMMARY 



1. The distribution in the spectrum of the stimulating effect 

 and the stimulating efficiency in reference to energy was ascer- 

 tained for fifteen species, including. unicellular and colonial forms, 

 worms and fly larvae. 



