HELIOTROPISM IN EUGLENA 427 



7. Jennings and Mast hold that Euglena keeps oriented because 

 "there is no longer any cause for turning." It was shown, how- 

 ever, that in an uneven environment the orientation disappears 

 within a few seconds after the light is cut off. As during illu- 

 mination the orientation in the same environment is precise, the 

 orientation must be maintained by the light. 



8. Jennings thinks that heliotropic orientation in Euglena is 

 not direct because among the ciliate infusoria galvanotropic orien- 

 tation is more direct than the heliotropic orientation. In Euglena, 

 however, it was found that the mechanism of galvanotropic orien- 

 tation is identical with that of heliotropic orientation. 



9. Hence it must be concluded that: 



a. The mechanism for producing heliotropic orientation is dif- 

 ferent from the mechanism foi* producing motor reactions, and one 

 does not depend upon the other. 



h. The gradual heliotropic orientation of Euglena does not 

 take place by ' trial and error ' but is as direct as the locomotor 

 mechanism of the organism will allow. 



B. The nature of the stimulus 



Jennings and Mast have come to the conclusion that changes 

 in the intensity of illumination are the stimulus that is responsible 

 for the heliotropic orientation of Euglena. It was shown, however, 

 that: 



1 . The criteria employed by Jennings and Mast are of no value 

 in deciding whether the stimulus consists in the changes or the 

 continuous action of the light. 



2. On the other hand, in favor of the view that the heliotropism 

 is due to the continuous action of the light there is the evidence 

 that: 



a. Continuous light may cause a gradual widening of the spiral 

 which lasts as long as the light does. 



h. The continuous action of the light is responsible for main- 

 taining the orientation of Euglena in an uneven environment. 



3. It is thus evident that there is no valid evidence against 

 the view that heliotropism in Euglena is due to the continuous 

 light action; and there is strong evidence in favor of it. 



