410 FRANK W. BANCROFT 



The shock-movements were in many cases not a mere widening 

 of the spiral, but the motion seemed to be in a plane. The shock- 

 movements also in most cases did not merge into the usual spiral 

 swimming, so that it was easy to tell when the shock-movement 

 stopped and the gradual swerving began. We see, then, that in 

 this light intensity which just sufficed to produce the shock-move- 

 ments the latter were incoordinated and variable, usually threw 

 the plant out of orientation and showed but little tendency to 

 merge into or resemble the usual spiral swerving. 



When the same experiment was repeated on the same plants 

 with a brighter light, the sudden action of which always produced 

 a marked shock-movement in all the Euglenae, quite a different 

 result was obtained. Here the orientation was more universal, 

 rapid and precise. After sudden illumination the shock-move- 

 ments of both oriented and unoriented individuals were of a more 

 orderly character, and in almost every case consisted in nothing 

 more than a pronounced widening of the spiral which rapidly 

 merged into the oriented spiral swmiming. Here then we have 

 with the brighter light the phenomenon which, according to Jen- 

 nings's point of view, was to be expected with the weaker light. 

 The explanation would appear to be that the mechanism which 

 underlies the direct gradual orientation responds more strongly to 

 a bright light, so that it can largely counteract the excessive swerv- 

 ing which the shock-movements might otherwise produce. This 

 masking of the shock-movements is probably also facilitated by 

 the more rapid tiring of the shock-movement mechanism described 

 above. 



G. How is Euglena kept oriented? 



Parker ('11, p. 462) in criticising Mast ('11) says ''Nor is it 

 anjrvi^here 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." Mast ('12, p. 211) 

 in reply to Parker says ''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 



