I04 LIGHT AND THE BEHAVIOR OF ORGANISMS 



takes place just as Jennings represents (Fig. 12), with the 

 exception that if the direction of the rays is changed with- 

 out any change of the intensity, orientation may take place 

 without an increase in the diameter of the spiral course 

 represented in Fig. 12, a-c. The organisms may orient by 

 increasing the swerving only toward the source of light 

 after its position is changed, not in the opposite direction. 



The fact that these free-swimming specimens of Euglena 

 in certain physiological states do not respond at all after 

 the position of the source of light is changed from one in 

 which the anterior end is illuminated to one in which the 

 ventral surface is exposed, until the organism rotates so as 

 to expose the dorsal surface; that as soon as this surface 

 faces the light there is a sudden twitching turn toward the 

 source of light ; and that this reaction is repeated every time 

 the surface containing the eye-spot comes to be illuminated 

 in the course of the rotation on the axis, shows very clearly 

 that orientation in the free-swimming state as well as in 

 the crawling state is not due to a constantly acting stimulus, 

 as Torrey assumes. 



Unequal stimulation of symmetrically located points, as 

 an important factor in causing orientation in accord with 

 the theories of Verworn and Loeb, is of course out of the 

 question in this form. If in heliotropism the results are 

 a function of the constant intensity, as Loeb maintains 

 (1906, p. 135), it must be admitted that there is no evidence 

 indicating that Euglena is heliotropic. 



j. Threshold or sensitiveness when different surfaces are 

 exposed to light. — The difference in sensitiveness of the 

 organism with different parts of the surface illuminated 

 was measiared in the following way: positive specimens 

 were exposed in the small slide aquarium^ to light from the 

 glower on the track. After they had oriented, the intensity 

 was suddenly decreased without any change in the direction 

 of the rays, by sliding the glower away until it could be 



^ An aquarium made of glass slides glued together with balsam boiled 

 in linseed oil. 



