Smith—The Organization of the Colony . 
1181 
The, Axially Differentiated Series. 
COPLANAR FORMS. 
Pediastrum, A Honparallel Concurrent Colony. 
In Pediastrum the number of cells in the colony is normally 
•a multiple of two. Coenobia containing 8-32 cells are com¬ 
monest in Pediastrum Boryanum, while in Pediastrum tetras 
the number varies from 4-16. In P. Boryanum, however, the 
formation of coenobia that contain only four cells is by no 
means an abnormal condition. The number of cells in the 
eoenobium is largely dependant on the vitality of the mother 
cell which produced it. When the alga is growing under favor¬ 
able conditions, the coenobia formed contain comparatively 
large numbers of cells; when environmental conditions are not 
so favorable, the number of cells is smaller. This same vari¬ 
ation in the number of cells in the coenobe, coincident with 
changes in external condition, has been observed in Scenedes- 
mus. In cultures of Pediastrum Boryanum that have been 
running for two weeks four-celled colonies are of rare occur¬ 
rence, but in the same cultures two months later four-celled 
colonies are comparatively abundant. When the alga is grown 
on a firm substratum, as an agar slant, many four-celled coeno¬ 
bia are present. The occurrence of four-celled colonies in na¬ 
ture is quite rare. In Pediastrum tetras, which I have ob¬ 
served only as it occurred in nature, four-celled colonies are 
quite abundant. 
Chodat and Huber (12) have cultivated P. Boryanum in 
different concentrations of Haegeli’s solution. They find that 
the more concentrated solutions inhibit the formation of swarm 
spores, and that the cells are apt to be arranged in rounded 
masses resembling Coelastrum. The swarm-spores become 
rounded forming cells which resemble hypnospores, the hypno- 
spores having a wall which, at times, may bear horns. 
Figures 27-30, Plate LXXXYI, show variations in arrange¬ 
ment in four-celled coenobia of Pediastrum tetras . The normal 
