OBSERVATIONS ON UNICELLULAR FORMS 119 



any evidence that light acting constantly somewhat like a 

 constant electric current, has any effect on orientation as 

 Loeb's explanation of orientation demands. 



c. Difference in sensitiveness with different surfaces 

 illuminated. — The threshold of reaction in Stentors varies 

 so much in different individuals and In the same individual 

 in different conditions that quantitative results are of little 

 value unless they can be correlated with causes of variation. 

 A few measurements made may however be of interest 

 in showing the relative stability of these organisms with 

 different parts of the surface exposed. 



On February 12, specimens fresh from the culture jar 

 were put into the aquarium with the two Nernst glowers 

 arranged as described above. The intensity from the two 

 glowers was equal; it was 321 ca. m. When the ray direc- 

 tion was changed by intercepting alternately the beams of 

 light after the Stentors had become oriented, practically 

 all of them responded immediately with the avoiding reac- 

 tion regardless of their position when the change was made. 

 Some turned toward the light, others away from it, and the 

 remainder turned in various other directions. After these 

 specimens had been experimented upon for about fifteen 

 minutes only those responded immediately in which the 

 oral side faced the light when the direction of the rays 

 was changed. The rest did not respond until after they had 

 rotated sufficiently to expose the oral side. In responding 

 they gave either the avoiding reaction or merely swerved 

 farther from the source of light as they continued on their 

 spiral course. In both methods of reaction they always 

 turned directly from the source of light, never toward it. 

 In casually studying Stentors under these conditions only, 

 one might readily conclude that orientation is always direct 

 and that it is due to local response to a local stimulation. 

 This however is not the case. 



As soon as the observations described above were com- 

 pleted, I put the Stentors into darkness, left them for a 

 short time and then exposed them to light from the 



