230 LIGHT AND THE BEHAVIOR OF ORGANISMS 



Euglena and Trachelomonas the most highly sensitive part 

 is probably restricted to a relatively small mass of proto- 

 plasm in close proximity with the concave surface of the 

 eye-spot. The eye-spot probably functions in shading this 

 structure when its convex surface faces the light. The 

 eye-spot may also function by absorbing light in a manner 

 similar to that in the choroid coat in the eye. In Chlamy- 

 domonas and the volvocineae, judging from its location, the 

 eye-spot cannot function by shading the sensitive part of 

 the organism, as it appears to in Euglena. In these forms 

 it can only function by absorbing light, if it functions in 

 light reactions at all. 



The difference in sensitiveness with different surfaces 

 exposed is surprisingly great in Euglena and Stentor, and 

 probably in all the other ciliates and flagellates. In Stentor, 

 under the conditions of the experiment it requires an in- 

 crease in intensity from 150 ca. m. to 444 ca. m. to induce 

 a reaction when the posterior end faces the light, while a 

 change from a position in which the aboral to one in which 

 the oral surface is exposed causes a reaction without any 

 change of intensity in the field, showing the marked effect 

 of the shadow of the former surface on the latter. There is 

 no evidence that the direction of the rays functions except 

 in so far as it may affect change of intensity. Nor is there 

 any evidence that light acts constantly as a directive stimu- 

 lus similar to the effect of the constant current in accord 

 with Loeb's theory of tropisms. 



In Euglena the avoiding reaction is not a differential re- 

 sponse to a local stimulus, for in the crawling state it bends 

 toward the ventral surface, while in the free-swimming 

 state it bends toward the dorsal surface. The direction 

 of bending is governed by internal factors. The reac- 

 tions caused by changes of intensity result in directing the 

 organisms toward various points of the compass. As soon 

 as they reach a position in which the rotation on the long 

 axis no longer causes a change of intensity on the sensitive 

 region there is no longer any cause for turning; they there- 



