228 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
and now widely diverging as shown in my figure of P. simplex 
(FIG. 11), would seem perhaps to be a factor in the wabbling, 
tipping, and trembling movements of the colonies. I have, 
however, seen no movement of the bristles. The species illus- 
trates the possibility that an orthogenetic tendency which is adap- 
tive in a specific particular when moderately developed may in its 
extreme development become adaptive in quite a different connec- 
tion. 
P. angulosum (Ehrenb.) Menegh. 
This form (FIG. 22) represents a type of the diactinial cell 
which seems quite remarkable for its constancy and the name 
and species have been less juggled with by descriptive writers 
than many others. The characteristically short oblique spines 
with the wide sinus between them show very clearly that the 
и 
Fic. 2 Pediastrum angulosum (Ehrenb.) Menegh. Irregular sixteen-celled 
colony, X about 325. 
morphogenetic tendency to the production of such projections 
involves other factors than merely those of length. Itisa widely 
distributed and fairly common form and yet apparently has 
developed no such series of fluctuating variants as have P. Bory- 
anum and P. pertusum. De-Toni (280) recognizes no varieties of 
it. Nitardy (714) has apparently never seen it and refers it with- 
out adequate evidence to P. Boryanum. The colonies tend to 
high cell numbers and in the 32- and 64-celled types have quite 
regularly a somewhat reniform outline which is suggested also 
in the sixteen-celled colony. That the cell form is in any way 
adapted to or determines this configuration of the colony is not 
obvious, and the form-determining factors are not so readily 
recognizable as in the other Diactinia. 
I shall discuss the species further in considering the general 
