HELIOTROPISM IN EUGLENA 405 



The same is true for results obtained in thefew preparations from 

 Culture A that developed the shading reaction (p. 401). When 

 these preparations were brought into a weak light the shading 

 reaction was immediately observed but the heliotropism did not 

 become distinct enough so that I could be sure that it was negative 

 for five or ten minutes. Since there was no positive heliotropism 

 in these individuals the objections urged above do not hold, and 

 here we have another clear case of unequal sensitization. 



It was only occasionally that anything like fatigue was seen 

 for the heliotropic reactions. Thus in four or five cases some 

 preparations from Culture A that had been kept in the dark 

 developed positive heliotropism, while other preparations which, 

 so far as possible, had been treated in exactly the same way failed 

 to develop it.^" In all of these cases the positive heliotropism was 

 quite transient. It disappeared after five or ten minutes of illumi- 

 nation and could not be obtained again. 



When positi\'ely heliotropic Euglenae from Culture B are sub- 

 jected to stronger and stronger light the positive tropism disap- 

 pears before the negative heliotropism comes on. It may be 

 that this disappearance of the positive heliotropism should be 

 interpreted as fatigue. But as no especial attempt was made 

 to determine whether the usual positive and negative tropism are 

 separate phenomena or simply portions of the same phenomenon 

 it will be useless to discuss the matter here.^^ 

 . For the negative heliotropism in individuals from all the cultures 

 nothing of the nature of fatigue has been observed. The stronger 

 the Hght, the stronger the tropism until the organisms are killed 

 or become immobile as the result of the intense stimulation. But 

 the fatigue of the illuminating reaction, which usually accompanies 



i^In one of these cases the positive heliotropism appeared most distinctly in the 

 arc light and was changed to negative when the weaker incandescent light was 

 used ! This reversal was accomplished several times before the reaction became 

 indistinct. 



"In general I have written as if there were but a single mechanism for all kinds 

 of gradual heliotropic orientation, and another one for the differential reactions. 

 I should like to state, however,' that this is only done to prevent circumlocution. 

 I should not be at all surprised if it should turn out that there are different mechan- 

 isms for the shading and the illuminating reactions as well as for positive and 

 negative heliotropism, or even for the different kinds of positive heliotropism. For 



