114 LIGHT AND THE BEHAVIOR OF ORGANISMS 



about by reactions which are induced by a change in the 

 effective intensity. This may be due to actual change of 

 the intensity of the field, to a movement from a region of 

 one intensity to that of another, or to a change of intensity 

 on the surface of the organism caused by changing the sur- 

 face turned toward the source of light. Direction of rays 

 and difference of intensity in the field are functional in the 

 process of orientation only in so far as they may influence 

 change of intensity on the organism. Orientation is not 

 induced by a constantly acting directive stimulus; it is the 

 result of a response to a time rate of change of intensity, a 

 shock-effect, U titer schieds em pfindlichkeit. 



Working independently of Jennings I obtained (1906) 

 results which were in all essentials in harmony with his. 

 Jennings assumed that the anterior end of Stentor is more 

 sensitive than the posterior. I proved that Stentors are 

 more sensitive to light when the anterior end is exposed 

 than they are when any other surface is exposed. The 

 minimum threshold in animals stimulated by rays perpen- 

 dicular to the long axis was found to be 1.2 ca. m., and 

 that in those stimulated by light striking the anterior end 

 only 0.25 ca. m. 



h. Orienting reactions. — I was of the opinion that while 

 the avoiding reactions no doubt play a large part in orien- 

 tation of Stentor, a direct effect of light as a constantly 

 acting stimulus in orientation might be discovered by a 

 careful investigation with this in mind. 



Three methods were used in this investigation: (i) Water 

 was removed from under the cover glass until the space 

 between it and the slide became so narrow that the Stentors 

 could no longer rotate on their axes. They were then illu- 

 minated so that various surfaces were successively exposed. 

 If light acts constantly as a directive stimulus one might 

 expect the cilia on the illuminated side to strike back more 

 vigorousl^^ than those on the shaded side regardless of the 

 surface exposed. I was however unable to observe any 

 relation between the rate of movement of the cilia and the 



