212 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
cells. There is a considerable number of species of Pediastrum 
in which the cell form is by no means so obviously adapted to a 
symmetrical configuration of the colony as a whole, species in 
which in some cases the lobing of the cells, as noted, is carried to 
extremes or is of a kind calculated to result in more unstable 
conditions of equilibrium and a greater tendency to asymmetry 
in the colonies. This increase in the length of the spines and 
the correlated large size of the intercellular spaces by favoring 
floating may be adaptive for species tending to assume the plank- 
ton habit of life. These species in their relations to P. asperum 
may be considered as extreme or aberrant, though representing 
natural and easily conceived modifications of the cell form shown 
in it and in P. Boryanum. Such types illustrate the operation of 
orthogenetic tendencies in the production of results which are 
non-adaptive from the standpoint of the organism as it was 
situated when the tendency first appeared, but may become adap- 
tive under new environmental conditions. 
P. asperum apparently represents a climax type viewed from 
the standpoint of the possibilities of developing a least surface 
configuration with unit cells derived by bipartition. The deeply 
four-lobed form of the cells permits the best possible approxima- 
tion to conformity with the circular outline and the intersection 
of all boundaries at 120° as found in the corresponding nineteen- 
unit least surface configuration. 
The two-spined series has by far the largest number of species 
and is unquestionably the most common type, though the single- 
spined species, P. simplex, is at times found in great abundance 
and in almost pure growths. In general, and in all its variations, 
the two-spined form of cell seems to be better adapted to the 
formation of the anomogenous cell group with the bipartition 
series of cell numbers as contrasted with normal least surface 
groups. There is general agreement that the delimitation of 
species must be based on cell form and we have quite a series of 
types differing primarily in the degree of development and the 
form of the two spines. 
We shall get a clearer understanding of the significance of 
cell form in such groups if we compare the conditions found in 
other species of the Diactinia with those in P. Boryanum and P. 
