218 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
eight-celled colonies of P. Boryanum. The markedly oblong form 
of the body of the central cells in P. Boryanum is plainly quite 
impossible for P. simplex. 
I have seen a number of colonies in which (FIG. 7) all eight 
cells were arranged in a very perfect circle about a central rounded 
space. Meyen (720) and other authors since have figured such 
forms. Nitardy has found it in his P. triangulum (FIG. 276). 
There evidently is the tendency here to achieve a symmetrical 
arrangement of one sort or another—a tendency which is quite 
independent of the presence or absence of any adaptation in the 
form of the cells to the production of such symmetrical inter- 
relations. We may assume, as in the other species, that this 
tendency is based on the effort of the cells in the swarming period 
to achieve a position in which their contact and pressure relations 
will be equal and balanced in as many directions as possible or 
that at least such pressure relations as are achieved shall be as 
nearly as possible mutually compensatory, as in the ring-shaped 
colony. The significance of occasionally achieved chance con- 
figurations is well illustrated in these cases. The symmetry of the 
circle is here very perfectly illustrated in FIGURE 7, but the chance 
Fics. 5, 6, апа 7. Eight-celled colonies of Pediastrum simplex Meyen. 5, 
central cell does not fill space enclosed by peripheral cells. 6, no intercellular space. 
7, ring-shaped colony. X about 150. 
that out of a swarm of eight free-swimming cells attempting to 
achieve interrelations of equal or balanced contact and pressure 
such a circle will be achieved would seem very remote. The 
chance for seven about one in a free-swimming group is much 
greater and affords a sufficiently close approximation to symmetry 
to make unlikely any very radically different configuration when 
once it is achieved. A statistical study of the relative abundance 
of such colonies as are shown in FIGURES 5, 6, and 7 might throw 
