226 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
axis of symmetry of the colony. No such configuration is found, 
so far as I have observed, in any other species of the Diactinia 
so far described, and yet the cell form shown in FIGURE 14 connects 
P. clathratum with P. asperum very closely. This figure is from 
material collected in Wisconsin and I have quite a series of photo- 
graphs showing cells of this form in colonies with 16, 32 and 64 
cells, but I have never seen one of these colonies with the 5 + 11 
cell configuration of P. clathratum. Му figures of P. clathratum 
are from material collected at Woods Hole, where the typical 
form is common as well as the less developed types of P. asperum, 
but I have not found with these forms colonies exactly like those 
from Wisconsin. 
We have here two types, which, as the confusion in the litera- 
ture shows, can be connected very closely by all possible inter- 
gradations in cell form and yet it seems clear that either when 
the modification of the cell form passes a certain point or as a 
result of modifications of the cell polarities a change in the type 
configuration of the colony results. There is no good evidence 
in the literature that colonies with the 5 + 11 configuration of 
P. clathratum and those with the 1 + 5 + 10 configuration of 
P. asperum can arise from the same parent colony and, as noted 
above, it must be left to further culture work to show whether 
this is possible. 
Fics. 17, 18, and 19. Pediastrum clathratum A. Br., irregular colonies. 17, 
circular colony, X about 175; figure 18 showing quite clearly the pear-shaped 
bodies at ends of spines, X about 200. Тһе colony shown in figure то is about 
ready for reproduction, X 175. 
The configuration of such colonies as those shown in FIGURES 
15-17, involving the absence of a central cell, the increased variety 
of form in the intercellular spaces, and the increased number of 
paired cell contacts is certainly more complex and further removed 
from that of a simple 1 + 6 + 12 least surface group than is the 
type of sixteen-celled colony of P. asperum (ғіс. 13). Greater 
