HARPER: CELL TYPES AND RESPONSES IN PEDIASTRUM 235 
Schroeter (’97) and many others have noted that various species 
of Pediastrum may be plankton organisms. 
The gradual appearance of the bifid spine in the Tetractinia 
is certainly a further development of the tendency to lobing 
of the cells and the group forms an obviously orthogenetic series, 
but here the bifid spine is quite unadapted to the development of 
cell groups with the bipartition cell numbers. In cases where 
symmetry, either bilateral or merely concentric, in the arrange- 
ment of the interior cells of the eight- or sixteen-celled groups is 
achieved it is at the expense of the bifid tips which appear, if at 
all, only as a broadening of the ends of the spine which hinders 
rather than helps the achievement of equal contact and pressure 
relations among the cells. 
The whole Pediastrum group seems well calculated to show that 
fixed trends in development do not necessarily imply adaptation, 
though frequently resulting in highly specialized structural differ- 
entiations which are plainly adaptive from the standpoint of the 
life habits of the organism. Further, openness and lightness 
with increased surface in the colony as a whole is the same thing 
as deep lobing of its body for the single cell. But the develop- 
ment of a rounded least surface contour for a group of cells made 
up of the bipartition numbers 4, 8, 16, 32, etc., requiring an oblong 
form and perhaps favoring the lobing of the cells, is thus quite a 
different thing for them from the same tendency to round up 
expressed in their individual masses. In order to make a surface 
tension group under the given conditions the cells must lose in 
some degree their own tendency to assume the surface tension 
form and yet, as I have pointed out elsewhere (718), this anomoge- 
nous condition imposed upon the cells in achieving their inter- 
relations in the colony becomes then fixed in heredity so that the 
cell develops the characteristically lobed form even when as a 
result of accident it develops in almost complete freedom from 
contact and pressure relations with its sister cells. 
I have referred to the interactions by which the type pattern 
of the colony is achieved as based on the polarities of the swarm- 
spores and their sensitiveness to contact and pressure stimuli. 
That there can be no mosaic inheritance in the case of these 
colonies formed by groups of free-swimming zoóspores is, as I 
