1180 Wisconsin Academy of Sciences , Arts and Letters. 
my results on Scenedesmus acutus that this change really does 
not take place as Grintzesco supposed. Senn (35) does not re¬ 
port these irregularly branching colonies in Goelastrum, but he 
does describe a separation of the colonies into individual cells. 
This breaking down was especially abundant in cultures poor 
in oxygen in Senns cultures. I have also found isolated cells 
quite frequent in certain of my cultures. Possibly the varia¬ 
tion which I have described in; the form of the colony of Coel- 
astrum is due to some similar cause, hut I am of the opinion 
that neither a deficiency of oxygen nor a change in the osmotic 
pressure of the nutrient solution, is the chief factor involved. 
A more likely explanation is that of a mechanical rupture of 
the colony. It is well known that at times the oxygen formed 
in photosynthesis does not leave the vicinity of the plant but 
adheres it to the form of bubbles, large enough to be seen with 
the naked eye. This phenonomon may be observed both in 
filamentous algae and in aquatic seed plants such as Elodea. I 
have been able to demonstrate the presence of oxygen bubbles in 
connection -with Coelastrum, and believe that they are the cause 
for the rupture of the colony. The oxygen given off by the cells 
probably accumulates in the form of small bubbles both outside 
and inside the colony. Bubbles given off on the inside might 
very well coalesce and form a single large bubble, which when 
large enough would cause the rupture of the colony and the 
appearance of the “Palmella” condition above described. 
Several groups of four cells were observed in my cultures. 
Some of these are complete unbroken four-celled colonies (Pigs. 
11, 14, 15), others are undoubtedly fragments of larger colon¬ 
ies (Pigs. 5, 8, T6). In the four-celled groups, which seem to 
be complete colonies, the cells are arranged in the form either 
of a plate (Pigs. 11 & 15), or of a pyramid (Pig. 14). The 
latter arrangement is less frequent. This variation in arrange¬ 
ment, like that observed in the four-celled Ve l tracoccus colony, 
is due to variations in the cleavage planes by which the daught¬ 
er ceils were formed. 
