122 LIGHT AND THE BEHAVIOR OF ORGANISMS 



increase of intensity causes them to respond with the avoid- 

 ing reaction. 



(2) An increase in the intensity of Ught in which Stentors 

 are oriented, without any variation in the direction of the 

 rays, causes them to respond either with the avoiding reac- 

 tion or by simply swerving farther toward the oral side, 

 making the spiral course wider. 



(3) Orientation in Stentor takes place essentially as it 

 does in Euglena. It is caused by changes in light intensity 

 on the sensitive tissue in the organisms. If, without a 

 change in the intensity, the direction of the rays is changed 

 so that the side of the organism instead of the end is exposed, 

 there is no reaction, provided the Stentors are not highly 

 sensitive, except when the oral side comes to face the light 

 in the process of rotation on the long axis. Then the reaction 

 may consist either of the avoiding reaction or merely of a 

 greater swerving from the source of light. Both result in 

 orientation. This shows that orientation is brought about 

 by reactions due to changes of intensity and not to light 

 acting constantly as a directive stimulus in accord with 

 the theories of Loeb and Verworn. 



(4) If the animals are highly sensitive or If the light 

 intensity is Increased when the direction of the rays is 

 changed, they respond no matter which side is exposed 

 after the change is made. In this response they turn in all 

 directions, toward the light as well as away from it. 



(5) Stentors may orient in a field of light which is uni- 

 form and constant In Intensity; but the orientation even 

 under such conditions Is due to a change of Intensity. This 

 change is caused by the movement of the animal which 

 results In alternately Illuminating and shading different 

 parts of the organism. 



(6) Orientation may be said to be duo to selection from 

 trial movements, just as In Euglena, ' en In those cases 

 where Stentor never errs by turning definitely toward the 

 light, for during every rotation the relatively highly sensi- 

 tive oral side is alternately shaded and Illuminated until 



