140 LIGHT AND THE BEHAVIOR OF ORGANISMS 



cient to overcome the effect of gravity, and under such con- 

 ditions they appear to be hanging in the water motionless. 

 They are, however, rotating on their longitudinal axes. If 

 now the light intensity, to which these apparently motion- 

 less organisms are exposed, is increased they soon begin to 

 turn toward its source; but in so doing they swim upward, 

 as represented in the accompanying diagram (Fig. 20). 



Fig. 20. Diagram representing the reaction of a Volvox colony when the light 

 intensity is suddenly changed, a, outline of colony; b, longitudinal axis; c, light 

 rays; d, point in the course where the light is suddenly decreased; e, point where 

 it is suddenly increased; /, course taken by colony. In continuing from e, the 

 side of the colony facing the source of light travels over a shorter distance than 

 the shaded side. Consequently the backward stroke of the fiagella on the latter 

 side must be more effective than that of those on the former. 



"In thus swimming upward and horizontally toward the 

 source of light, it is clear that the effect of the backward 

 stroke of the fiagella increases both on the shaded side and 

 on the illuminated side, for both sides move forward. But 

 the shaded side moves farther than the illuminated side, 

 consequently the increase in the effect of the backward 

 stroke must be greater on the former than on the latter. 

 The difference in the effect of the stroke of the flagella on op- 

 posite sides, which results in orientation of positive Volvox 

 colonies, is, therefore, due to a greater increase in the back- 

 ward stroke of the flagella on the shaded side than of those 

 on the illuminated side. 



"If the light thrown upon apparently motionless colonies 

 is quite intense, they frequently may be seen to sink 4 or 



