234 SEMI-CENTENNIAL OF TORREY BoTANICAL CLUB 
this respect again I am inclined to regard it as a more specialized 
type even than P. asperum, though it is obvious that there is 
less differentiation between its cells than is found in P. Boryanum, 
in which the interior and the peripheral cells differ notably in 
their form. In P. clathratum and P. asperum, however, the 
hereditary cell form has become apparently so fixed that it comes 
to expression even under the difficult conditions of the interior 
cells. If, as I have suggested (718), the adaptive oblong four- 
lobed cell form originated and developed in direct response to the 
environmental limitations and stimuli imposed on sixteen cells 
adhering in a plate-formed colony and with a tendency owing to 
their partially fluid consistency to assume a surface tension form, 
P. clathratum certainly represents the most extreme expression of 
this evolutionary trend. The advance has been from such un- 
specialized and uniform cells as those of P. integrum through P. 
Boryanum with its cell differentiations to P. asperum and P. 
clathratum where all the cells are much alike again but vastly 
more specialized in form. 
The relations of P. simplex and P. triangulum, as 1 am recog- 
nizing. them, illustrate the same point. In the sixteen-celled 
colonies of P. simplex the interior cells differ regularly from the 
peripheral cells by the absence of the spine, though as shown in 
figure 4 an interior cell will develop a spine whenever it is so 
placed with reference to an intercellular space that this is possible. 
In P. triangulum both interior and peripheral cells develop spines 
and the configuration of the colony is altered accordingly by the 
achievement of symmetry relations which permit each cell to 
express much more fully its inherited form tendencies. That these 
form tendencies are really present equally in all the cells of P. 
simplex also is shown in the eight-celled ring-shaped colonies where 
every cell has an equal chance to achieve its full morphogenetic 
possibilities and the result is a remarkable uniformity in the size 
and shape of all the cells. 
The development of spines and the four-lobed cell form in 
P. asperum seems to have to do with the compactness and surface 
tension relations of the cells in the colony as a group, while in 
P. clathratum the length of the spines results in a light, open struc- 
ture of the colony perhaps adapted to life in the plankton. 
