258 LIGHT AND THE BEHAVIOR OF ORGANISMS 



gation of Planaria to be due to Unterschiedsempfindlichkeit, 

 sensibility to changes of intensity, and classifies the reac- 

 tions with those of Serpula and Spirographis to shadows. 

 I can however see no similarity between the responses of 

 these organisms. 



c. Reactions of questionable cause. — There are many 

 responses in which it is as yet impossible to be certain as 

 to what characteristic of light causes them. The orienta- 

 tion of Amoeba for example is probably due to the direct 

 effect of the increase in light intensity on the protoplasm, 

 but for all that is known to the contrary it may be due to 

 continued illumination rather than to change in illumina- 

 tion. With regard to the orientation of the sessile plants 

 and animals, as well as all animals with well-developed 

 eyes, and many of the lower forms (entomostraca. Hydras 

 sea anemones, and the larvae of Arenicola, Limulus and 

 various crabs), experimental results do not as yet warrant 

 a definite conclusion as to whether the reaction is due to 

 the effect of change of intensity or continued illumination. 

 It should however be emphasized again that in no case has 

 it been demonstrated that orientation is " a function of the 

 constant intensity " as maintained by Loeb. Nor is there 

 any evidence indicating that the direction of the rays 

 through the tissue has any direct effect on this process. 



(2) On the basis of the fundamental cause of response, 

 the reactions to light may be classified as follows: a. Reac- 

 tions caused by the direct effect of light on the reacting 

 tissue; b. Reactions caused by an indirect effect of light; 

 c. Reactions due, not to any effect of light in itself, but to 

 what a given light condition or configuration may represent. 



a. Reactions caused by the direct effect of light on the 

 reacting tissue. — Examples: Inhibition of protoplasmic 

 streaming In the rhizopods and plasmodia and in numerous 

 different plant cells, probably reversal in the sense of reac- 

 tion and the change in sensitiveness, and the general activity 

 of some organisms at least. 



b. Reactions caused by an indirect effect of light. — 



