THE REACTIONS OF ORGANISMS TO LIGHT 211 



symmetrical organisms since only such have " two symmetrical 

 parts." Stentor is neither radially nor bilaterally symmetrical 

 and consequently strictly speaking none of its reactions can fit 

 Parker's definition of tropisms. Even if we interpret this 

 definition very liberally and substitute opposite sides in place 

 of symmetrical parts there is no evidence that the gradual 

 turning of Stentor from the light is tropic as Parker maintains, 

 for it responds only when, in the process of turning the light 

 intensity on the oral side is increased, not when other surfaces 

 become more strongly illuminated. This I have fully described 

 in the text as well as in the legend, Fig. 14, p. 117. In spite of 

 much time and energy spent I was unable to obtain any evidence 

 whatsoever indicating that any reactions in Stentor are dependent 

 upon unequal stimulation of opposite sides, pp. 1 14-11 5. It 

 is consequently evident that as far as my experiments and 

 observations are concerned there is no evidence whatever indi- 

 cating that any features in the process of orientation in Stentor 

 are "tropic" in accord with Parker's definition of this term. 



4. It is maintained by my critic that I have not "made clear 

 how an organism, after it has once become oriented, can continue 

 to move in a straight line without involving the essential elements 

 of the tropism theory." I assume that in this connection he 

 means by "essential elements of the tropism theory" reactions 

 due to equal "stimulation of symmetrical parts of the body. 

 If my description of the process of orientation in Euglena, pp. 

 92-97, Stentor, pp. 11 4-1 19, fly larvae, pp. 189-193, and a num- 

 ber of other organisms is understood it will be clear without 

 further elucidation how organisms can take a fairly straight 

 course without equal stimulation of symmetrical parts. (The 

 idea that organisms move in a " straight line " is more imaginary 

 than real.) I have stated my position regarding this matter 

 referring to Euglena (p. 230) briefly but clearly as follows: 

 " The reactions 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 therefore con- 

 tinue in this direction." 



According to this explanation the organism retains a given 

 direction of locomotion not because of continuous action of light, 



