HELIOTROPISM IN EUGLENA 411 



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 continue in this 

 direction. ' ' ' The first point then to be determined about Euglena 

 in this connection is whether it stays oriented because it has got 

 started that way and there is no cause for turning, or whether it 

 is kept oriented by the light. 



It is easy to devise experiments which seem to answer this 

 question in favor of Mast. If a light of medium intensity be used 

 and a hanging drop free from debris then it can frequently be seen 

 that, when the orienting light is cut out after the Euglenae are 

 half way across the drop, the plants continue on their course and 

 reach the farther side of the drop nearly as directly as if the orient- 

 ing light were still shining. 



But it is just as easy to devise experiments that prove that 

 the light does keep the Euglenae oriented even in the presence of 

 stimuli which are constantly tending to destroy the orientation. 

 If a small open trough be filled with tap water and a drop of cul- 

 ture fluid containing debris be mixed with the tap water then a 

 very uneven en^'ironment is produced in which the Euglenae 

 swim about haltingly giving a constant series of shock-move- 

 ments as they encounter the particles of debris and the dissolved 

 substances diffusing from them. If now a strong enough arc 

 light is directed along the trough the organisms are neverthe- 

 less compelled to orient themselves very accurately and to move 

 away from the light. The foreign particles are so small that, 

 when the Euglenae fail to respond to the mechanical and chemi- 

 cal stimuli, the particles are brushed aside and the organisms 

 are not thrown out of orientation. When, however, the orienting 

 light is now cut off, the orientation is rapidly lost on account of 

 the shock-movements produced by the uneven environment. 

 There is no reason for thinking that while the orienting light 

 shines, the stimuli that cause these reactions both before and 

 after suddenly cease. We must conclude, therefore, that the 

 mechanism for heliotropism does actively maintain the orienta- 

 tion in the face of disturbing stimuli. Since the plants remain 

 well oriented there are' no temporal changes in illumination, and 



