224 JACQUES LOEB AND HARDOLPH WASTENEYS 



From the viewpoint of objective science we accept the fact 

 that in some heliotropic organisms the place of highest efficiency 

 is in that region of the spectrum which for the totally color blind 

 is the brightest, but on this we put a different interpretation, 

 namely the following. The sensations of brightness in the totally 

 color blind human are determined by the rapidity with which 

 visual purple is bleached by the light. The region in the yellow- 

 ish-green in the carbon arc spectrum appears brightest to the 

 totally color blind human because this region X = 526 mm has 

 according to Trendelenburg the greatest bleaching effect. As- 

 suming that heliotropic reactions are also due to a photochemical 

 effect, the fact that in certain organisms the region not far from 

 X = 526 MM is the most efficient in calling forth heliotropism 

 means simply that the photosensitive substance responsible for 

 the heliotropic reaction in these organisms has one peculiarity 

 in common with the visual purple, namely that it is also most 

 sensitive to a region not too remote from X ^ 526 ijlijl; the two 

 substances may possibly be identical, but this would require a 

 definite proof. The fact that the optimal effect for other organ- 

 isms lies in the region of blue would indicate that the photo- 

 sensitive substance in these animals is in all probability different 

 from visual purple. If the effect of light in causing heliotropic 

 reactions were other than chemical we still should be compelled 

 to find a physicochemical and not a psychological explanation for 

 the different heliotropic efficiency of different wave lengths. 



The question to which we intend to confine ourselves in this 

 paper is a very simple one, namely : Is it true that a sharp line of 

 demarcation exists between animals and plants in that sense 

 that for the heliotropic reactions of plants the blue is most 

 effective, while for the heliotropic reactions of all animals a 

 region in the yellowish-green is the most efficient, as Hess claims? 

 In this paper we shall deal with motile organisms, namely first 

 the two unicellular green organisms Euglena and Chalmydomonas 

 and the larvae of two animal forms, of the annelid Arenicola 

 and of the crustacean Balanus eburneus. 



