HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 215 
can be no doubt that in different localities and in different seasons 
one or another of them may be found in almost pure growths and 
to the exclusion of the others. It seems clear that a particular 
type of cell form tends to perpetuate itself, but with much fluctua- 
tion. Nitardy (r4) includes all the forms under the name P. 
triangulum (Ehrenb.) A. Br., dropping the name P. simplex 
because it has been used so variously by different authors. 
I have photographs of both eight- and sixteen-celled colonies. 
A comparison of the sixteen-celled colonies of P. simplex, as I 
find it, with those of any species with well-developed two-spined 
cells suggests at once the greater capacity of the latter to form 
symmetrical colonies with the bipartition series of cells 4, 8, 16, 
32, 64, etc. The two common arrangements in my observation 
seem to be, first, an irregular group of five in the center and eleven 
around them (Fic. 2), and, second, an irregular group of four in 
the center with twelve around them (FIG. 3). 
The peripheral cells show their polarity by the шану 
with which the spine is turned outward. Whether they аге 
flattened enough to indicate the existence of a second differentia- 
tion in a transverse axis, I have not been able to determine. 
The arrangement of the inner group of cells is, as noted, quite 
irregular in all the colonies I have seen. Several intercellular 
spaces are commonly present, but show no constancy as to size, 
shape, or position. They may be from three- to six-sided. It 
is generally quite impossible to determine which side of any one 
of these interior cells tends to be produced into the spine. In two 
cases, however, I have observed an interior cell with a well- 
developed spine projecting into an intercellular space. One of 
these is shown in FiGURE 4. Whether the tendency to form the 
one-spined form is as fully fixed in P. simplex as is the correspond- 
ing tendency to form two spines in the Diactinia is not clear. 
It is possible that the lack of adaptation in the one-spined cell 
form to the production of symmetrical colonies is correlated with 
a failure to fix this cell form so firmly in heredity and that with 
an increased fixity of cell type such symmetrical forms as Nitardy’s 
P. triangulum (Ehrenb.) A. Br. become possible. De Wildeman 
(93), however, has observed both irregular and symmetrical 
colonies and includes them all in P. simplex. De Wildeman 
