VERTICAL DISTRIBUTION OF VOLVOX 
183 
concerned with an attempt to explain the mechanics of orientation. 
He notes that Volvox is negatively phototactic in strong light, while 
in very weak light colonies exhibit no pronounced phototactic move- 
ment, lying quietly or rolling about sluggishly. The most extensive 
studies are those of Mast, whose "light grader" for producing a graded 
intensity of the illumination overcomes the experimental errors of 
Oltmanns' apparatus. His investigations center about the orientation 
of the colony with respect to the direction of the rays of light, and he 
concludes that direction of movement in a colony is determined by the 
difference in intensity of illumination of the two sides of the colony 
rather than by the colony placing itself in a specific position with 
respect to the rays of light. 
The explanation of the formation of the stratum at a depth of three 
meters on July 6 seems to me to rest on a heliotactic basis. Several 
investigators have noted that Volvox is positively heliotactic in weak 
light and negatively heliotactic in strong light. The formation of this 
stratum was probably caused by the colonies moving into the region 
of optimum illumination in the same way that Oltmanns (7) found in 
his experiments. This condition remained fairly constant during the 
day. The fact that the stratum did not move upwards when the illu- 
mination decreased at sunset is, however, not in accordance with this 
view. The dropping of the colonies from the three-meter level after 
sunset has already been ascribed to geotaxis. 
The condition found at sunrise on the following day has been ex- 
plained by the tendency of colonies to swim upwards in darkness after 
the greater weight of the posterior end orients them into a vertical 
position. There is the possibility that heliotaxis had been operating 
for some time before the collection was made at 4:25 A.M., since there 
is considerable illumination before the sun actually appears. Three 
quarters of an hour before sunrise a man could not be distinguished at a 
distance of seventy-five yards. If, then, we ascribe this movement of 
the colonies to the surface to heliotaxis they must have traveled three 
meters or more during the half hour of faint illumination. It would be 
reasonable to assume that this rate of movement continued during 
the hours after sunrise; so, when we find that the colonies did not rise 
more than a meter between 4:30 and 6:30, it seems illogical to assume 
that the light before sunrise caused the change of position. This is 
especially true when we remember that Holmes (2) finds no pronounced 
phototactic movement of Volvox in weak light. 
