Mast, Light Reactions in Loiver Organisms. IJJ 



6. The specific gravity of Volvox is greater than one. When 

 not active or when dead the colonies slowly sink with the longi- 

 tudinal axis vertical and the posterior end down. The vertical 

 orientation under such conditions is much more precise m speci- 

 mens containing large daughter-colonies than in others. 



7. Volvox tends to swim in the direction of its longitudinal axis. 

 Gravitation tends to cause this axis to take a vertical position. It 

 the colonies are not strongly positive the anterior end is directed 

 nearly straight up. If such colonies swim toward a source of 

 light, the rays of which are horizontal, they deflect upward. But 

 if the colonies are strongly positive the axis becomes nearly hori- 

 zontal, and they tend to swim parallel with the rays. Under these 

 conditions gravity causes them to sink gradually, so they deflect 

 downward. 



8. If the rays of light are parallel with the direction of gravi- 

 tation, i. e., vertical, and the source of light is above, the colonies 

 swim upward in a narrow spiral course nearly parallel with the 

 rays, but if the source of light is below and they swim downward, 

 there is a tendency to turn over, owing to the diff'erence in weight 

 of the two ends, and this causes them to swerve to the side 

 frequently. 



9. Deflection to the right or left as well as deflection upward 

 or downward, is caused by the eff^ect of gravitation on the direction 

 of the longitudinal axis in connection with rotation on this axis. 



10. Deflection in negative colonies is in all essentials like that 

 in positive. 



11. If a colony is swimming at a given angle to the light rays 

 and the direction of the rays is changed, the organism changes its 

 direction of motion until it again takes a course which makes an 

 angle with the rays equal to that it had before the ray direction 

 was changed, /. e., Volvox orients, but not necessarily so as to 

 swim parallel with the rays. 



12. If exposed to light of equal intensity from two sources, 

 Volvox swims toward a point nearly half way between the two 

 sources, provided it is strongly positive, but if the lights are un- 

 equal in intensity the colonies direct their course toward a point 

 nearer the more intense light than the other. 



13. If exposed to parallel rays such that one side of a colony, 

 swimming toward the source of light, is more strongly illuminated 

 than the other, it deflects toward the more strongly illuminated side. 



