86 LIGHT AND THE BEHAVIOR OF ORGANISMS 



If Euglenae actually orient by local response to local 

 stimulation, as Torrey assumes, or if light acts constantly 

 as a directive stimulus in accord with Loeb's theory, one 

 should be able to find evidence of it in these organisms in 

 the crawling state. With this in mind, therefore, I took 

 up the study of specimens in this state. 



Before entering on the description of the reactions in 

 Euglena bearing directly on the problem just stated, I shall 

 however refer briefly to the question of orientation in 

 light from several sources, since the experimental results 

 obtained under these conditions throw some light on the 

 idea of Sachs, that the direction of the rays through the 

 organism regulates orientation, and on Loeb's idea that 

 symmetrically located points on the sensitive surface must 

 be struck by rays at the same angle when an organism is 

 oriented. 



c. Orientation in light from two sources. — In studying 

 the movement of Euglenae in light from two sources, 

 Nernst glowers in a dark room were so arranged and 

 screened as to produce two small horizontal beams of light 

 which crossed each other at right angles in the aquarium. 

 One glower was stationary. The other was mounted on a 

 horizontal track so that it could easily be pushed nearer 

 to or farther away from the aquarium. Thus the relative 

 intensity from the two glowers could be changed without 

 any change in the direction of the rays. Several species of 

 Euglena in the free swimming state, and two, Euglena deses 

 and Euglena x in the crawling state, were used in these 

 experiments. The results were the same in all. 



When the light from the two glowers was equal and the 

 Euglenae positive they moved in a general way toward a 

 point very nearly halfway between the glowers. But when 

 it was unequal, they moved toward a point nearer the 

 source from which the more intense light came. Negative 

 specimens take the same general course but in the opposite 

 direction. This experiment is particularly striking if the 

 glower on the track is gradually moved from a position in 



