HELIOTROPISM IN EUGLENA 425 



Since no one has yet been able to show that the Bunsen and Ros- 

 coe law holds for the gradual heliotropic orientation of Euglena, or 

 that this reaction is abolished when the changes in illumination 

 which accompany it are made more gradual, it cannot be said that 

 the nature of the stimulus has been conclusively demonstrated. 

 It may be said, however, that at present we have no evidence 

 against the view that in Euglena the gradual orientation is a 

 function of the continuous action of the light, and that we have 

 strong evidence in favor of this view. 



In Euglena we are not familiar with the anatomical relations 

 that allow light coming from one direction to stimulate more effec- 

 tively than light coming from another. But most eyes, including 

 our own, are so arranged that the light produces its maximum 

 effect when coming from some one particular direction. As a 

 result of this fact, in both ourselves and Euglena, revolution about 

 the long axis in light coming from one side will subject the photo- 

 sensitive substances to changes in light intensity (Jennings, '09, 

 p. 318). But it does not follow from this that it is the changes 

 in light intensity which constitute the stimulus for light percep- 

 tion or for reactions to light. In ourselves we know from the 

 researches of Bloch and Charpentier that the law of Bunsen and 

 Roscoe holds good for the threshold of the perception of light of 

 very short duration. Each case must be independently investi- 

 gated ; and in the case of Euglena we have seen that what evidence 

 we have is all in favor of the view advocated by Loeb that the 

 heliotropism is a function of the continuous action of the light. 



