SPECTRUM, HELIOTROPIC REACTIONS 25 



of it, or that they are turned away from it because they hate it; 

 or that the reactions are the result of 'Hrial and error. " 



We may, therefore, conclude that the hehotropic reactions of 

 animals and plants are due to photochemical reactions and that 

 the turning of the animal to (or from) the source of light is brought 

 about automatically if the velocity of photochemical reaction 

 is no longer the same in symmetrical areas of the photosensitive 

 surface. This automatic turning results when the mass of 

 photochemical reaction products on symmetrical points of the 

 surface of the anima (eyes or skin) exceeds a certain value; 

 and the variations of this value determine the relative sensitive- 

 ness of different heliotropic animals. Since this has been stated 

 more fully in former publications of Loeb we may refer the 

 reader to these publications.^ 



If the basis of heliotropic reactions is a photochemical process, 

 it follows that heliotropic animals must possess a photosensitive 

 substance, and the question arises: Is this substance identical 

 in all heliotropic organisms or do the photochemical substances 

 differ in different heliotropic organisms? Especially does this 

 question become of interest in respect to the question whether 

 there is a specific difference between these substances in animals 

 and plants. 



The method to decide this question consists in comparing the 

 relative heliotropic efficiency of different wave lengths in different 

 organisms. If we find that the optimal heliotropic effects occur 

 for one form of organisms in one kind of wave lengths, for another 

 in a widely different wave length of the same spectrum, we may 

 conclude that the photochemical substances in the two cases 

 are different, if deduction be made for the possible screen effect 

 of secondary substances contained in the sensitive organ. 



The older experiments of the botanists were mostly made with 

 colored screens, which yielded the result that behind red screens 

 only weak or no heliotropic reactions of plants occur, while behind 

 blue screens they occur as well as in mixed daylight. Loeb 

 was able to show in his earlier experiments that the same holds 



'The mechanistic conception of life. Chicago, 1912; article on Tropisms 

 in Winterstein's Handbuch der vergleichenden Physiologie, Bd. 4, p. 451, 1912. 



