GILBERT MORGAN SMITH 
tion of the alga in any particular place in response to a chemotactic 
stimulus. 
Geotaxis, however, plays an important part in the distribution of 
Volvox. Mast (5) finds that, when a horizontal beam of light is 
thrown across an aquarium, the colonies tend to move horizontally 
towards the source of light, but the force of gravity pulls them down- 
wards so that they sink very rapidly until, as a result of the difference 
in weight between the anterior and posterior ends, they become ver- 
tically oriented and begin to swim upwards. He also finds that after 
an aquarium has been in darkness some four or five hours many colonies 
are at the surface of the water. This probably explains the condition 
noted at 9 :2o P.M. After sunset gravity is the only stimulus operating 
upon the colonies. Since they are heavier than water they tended to 
settle, and sank from one to three meters below their position at 7:45 
P.M. Some time between 9:20 and sunrise the next morning the 
colonies became vertically oriented and swam upwards, so that at 
sunrise they were in the upper three meters of the lake. The physio- 
logical condition of the colony affects this upward swimming. Mast 
(5) observed that the colonies would be lying motionless on the bottom 
of the aquarium after standing in darkness for some hours. When 
illuminated they did not respond immediately, but after a while 
slowly began to move in all directions, later becoming normally active 
and moving towards the light. He applied the term "dark rigor" to 
describe their condition when lying on the bottom. Apparently there 
was no ''dark rigor" in the Volvox colonies in Lake Monona since 
they were found in the upper part of the lake at sunrise. 
Light is by far the most important factor governing the distri- 
bution of Volvox. The response to light by Volvox has been noted by 
many naturalists, but Oltmanns (7), Holmes (2) and Mast (5,6) are 
the only ones to have made quantitative studies. Oltmanns produces 
a graded intensity in the illumination of an aquarium by passing a 
beam of light through a hollow glass prism filled with a mixture of 
India ink and gelatine. The narrow end of the prism permits most 
of the light to pass through while the broad end of the prism absorbs 
the greater portion of it. He finds that the colonies collect at a given 
light intensity, but if the general illumination of the prism is lowered, 
as when a cloud passes over the sun, the colonies move toward a region 
of formerly greater illumination. The experimental errors in this 
method have been analyzed by Mast (6). Holmes's studies are largely 
