398 FRANK W. BANCROFT 



other direction. But this does not mean that Euglena must 

 necessarily orient by ' trial and error ' and cannot orient directly 

 for, as we shall see, this organism does orient as directly as its 

 method of locomotion permits. 



5. THE FACTS OF HELIOTROPIC ORIENTATION 



Jennings maintains that the gradua^ orientation of Euglena 

 takes place by means of 'trial and error' and consists in a series of 

 shock-movements. If this is true then we should expect that under 

 various conditions the shock-movements and the gradual orien- 

 tation would vary together, and be influenced similarly by the 

 different factors of the environment. On the other hand, Torrey 

 maintains ('07, p. 317) that we have here a complex of differential 

 sensibility (Unterschiedsempfindlichkeit) and direct heliotropic 

 orientation. If Torrey is correct then it ought to be possible by 

 appropriate means to separate these two factors and to influence 

 one without influencing the other. Consequently my plan of work 

 consisted in sub j ecting Euglenae from various cultures to different 

 environments and noting whether the gradua^ heliotropic turning 

 and the shock-movements could be influenced independently of 

 each other. 



A. Correlation between the sign of the heliotropism and the sign of 

 the differential reaction^ 



Jennings found that positively heliotropic Euglenae always give 

 the motor reaction when suddenly shaded, and not when suddenly 

 illuminated. He maintains that in positive heliotropism the effec- 

 tive stimulus consists in the increased shading of some photo- 

 sensitive region while the infusorian is swerving away from the 

 light. The response is a shock-movement which manifests itself 

 as an increased swerving towards the light during the next half 

 of the spiral revolution. Similarly n negative heliotropism the 

 swerving away from the light is the shock-movement caused by 



^By diflferential reactions are meant reactions caused by temporal change in the 

 intensity of the stimulus. In this case they include reactions to both shading and 

 illuminating. 



