138 LIGHT AND THE BEHAVIOR OF ORGANISMS 



toward the more strongly illuminated side as represented in 

 Fig. 19. 



Difference in light intensity on opposite sides of the 



c»- 



d 



\f 



mmmsm?m§^; 



a 



f 



Fig. 18. Representation of the movement of Volvox when subjected to light 

 from two sources, a, plate glass aquarium 8 cm. long and 8 cm. wide; b, 222-volt 

 Nernst glower, 66 cm. from aquarium (distance from aquarium constant); c, iio 

 volt glower (distance from aquarium variable); d, screen; e, point of introduction 

 of Volvox; /, direction of light rays; i, 2, 3, 4, courses of Volvox exposed to light 

 from both glowers: i, with no-volt glower igg cm. from aquarium; 2, with iio- 

 volt glower 99 cm. from aquarium; 3, with no-volt glower 49 cm. from aquarium; 

 4, with I ID- volt glower 24 cm. from aquarium; x-y, course of Volvox when exposed 

 to light from glower b only; y-z, course when exposed to light from glower c only. 



colonies, then, causes them to turn until the two sides are 

 equally illuminated. "The turning^ may be conceived to 

 be due to an increase in the backward phase of the stroke 



1 Mast, 1907, pp. 151-154. 



