OBSERVATIONS ON UNICELLULAR FORMS lOi 



is dependent more upon the time of exposure to light than 

 upon the increase of intensity. 



The experimental difficulty of course lies in the fact 

 pointed out above, that the movement of the organism 

 itself causes change of intensity on different structures 

 in it. The fact however that Volvox, e.g., in swimming 

 toward a source of light into regions of higher intensity 

 without changing its orientation, swims more slowly as it ap- 

 proaches the region of optimum intensity and finally stops 

 altogether, seems to show very clearly that the intensity 

 affects the rate of movement. But the results of Holmes 

 also show that the rate bears no definite relation to the 

 absolute intensity. There is much need of more experi- 

 mental data on this subject. 



Does Euglena always turn toward or from the side 

 stimulated? Is orientation due to differential response to 

 localized stimulation? Or is the organism stimulated as 

 a whole with a reaction dependent more or less upon the 

 structure of the organism? If we are correct in our assump- 

 tion that there is a highly sensitive protoplasmic structure 

 located in the anterior end of Euglena, it is likely that it 

 is always stimulated by light in the same place regardless 

 of the portion of the surface exposed. Judging from this 

 alone one might conclude that the stimulus acts as a local 

 sign. But the fact that Euglena in the crawling state 

 always bends toward the ventral surface when stimulated 

 by light, while specimens in the free-swimming state always 

 turn toward the dorsal surface when stimulated, contra- 

 dicts this conclusion and supports the idea that the stimulus 

 acts upon the organism as a whole. This is again in direct 

 opposition to Torrey's statement (1907, p. 319), " The 

 interpretation of the behavior of heliotropic organisms on 

 the basis of general changes concerning the whole organism, 

 not only does not accord with the known facts, but is 

 rather psychical than physiological in character." It is 

 difficult to see how the fact that an organism always turns 

 in the same direction when it is stimulated as a whole can 



