HELIOTROPISM IN EUGLENA 407 



I then tried to determine whether it was possible to obtain orien- 

 tation without the widened spiral in the arc light. It was found 

 that when Euglenae that were oriented in the arc light were 

 subjected to a gradual change of direction by rotating the stage 

 of the microscope on which the slide was resting, the gradual orien- 

 tation was frequently seen without a trace of the initial widening 

 of the spiral. In this way the most convincing demonstrations 

 of gradual orientation without any shock-movements or widen- 

 ing of the spiral were possible. With the bright light the orien- 

 tation is much more prompt and precise; so that in these cases 

 80 per cent or 90 per cent of the organisms were oriented. When 

 the stage was turned through an angle of 90 degrees in three or 

 four seconds the Euglenae would respond to the changed direction 

 so quickly that they would remain oriented throughout the 

 turning, or would become oriented within a few seconds after the 

 turning stopped. In this way hundreds of Euglenae were seen 

 to orient with only a very occasional shock-movement and in a 

 light intensity which was capable of producing shock-movements 

 in all of them when it was allowed to shine suddenly on weakly 

 illuminated organisms. Thus, we see that also in bright light, 

 which is capable of producing shock-movements, the mechanism 

 underlying the gradual orientation is more active than in weaker 

 light ; and if sudden changes of intensity and direction are avoided 

 this mechanism may cause prompt and precise orientation without 

 the appearance of any shock-movements. We thus get additional 

 evidence for considering the mechanism just discussed different 

 from that producing the shock-movements. 



E. Time required for stimulation 



As we have seen, Jennings supposes that the stimulus for the 

 heliotropic orientation consists in the changes of light intensity 

 which take place as Euglena swerves towards or away from the 

 light in each half of a spiral revolution. Each of these stimuli is 

 then supposed to cause a short shock-movement lasting during 

 the next half of the spiral revolution. If this view is correct then 

 it should be possible to produce shock-movements by sudden il- 



