214 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
tal conditions which check the vigor of the swarm-spores just as it 
is checked in agar cultures of P. Boryanum with resulting irregu- 
larity of the colonies and reduction of the spinous projections. It 
seems hardly possible that P. integrum is only a habitat form of P. 
Boryanum but it would be interesting to grow it in water cultures 
along with P. Boryanum and observe its behavior as to cell shape 
and the symmetry of the colonies. I have not had the species in 
numbers sufficient for such experiments. The specimen shown in 
FIGURE I shows the thirty-two cells in one plane but the arrange- 
ment is quite irregular and asymmetrical, though there is a fair ap- 
proach to the concentric circles. The short papillae seem to be quite 
reularly directed radially outward in the colonies so far figured, but 
the tendency to have certain cells out of the plane of the colony is 
marked and either indicates that the poles of the transverse axes 
of the cells and the affinities which they represent are relatively 
less strongly developed or that the cells are unable for environ- 
mental reasons to achieve their normal orientation and inter- 
relations in the colony. Р. integrum may be a species whose 
colonies regularly fail to achieve their typical development and 
yet are able to maintain themselves. I am more inclined to 
believe, however, that it is a primitive type in which the polarities 
and cell differentiations characteristic of the Monactinia and 
Diactinia are not fully achieved. Whether or not P. integrum 
is a good species, it may certainly be regarded as representing in 
the form of its cells a primitive type out of which the better 
adapted cell forms of the Diactinia have been developed. 
B. Monactinium.—Pediastrum simplex Meyen. 
The species of Pediastrum whose cells show a single spine are 
common and widely distributed. The evidence of their varia- 
bility is well shown in the fact that De-Toni (89) believes they 
can all be included in one polymorphic species, P. simplex, while 
various other authors have recognized P. duodenarium (Bailey) 
Rabenh., P. clathratum Lemm., P. triangulum (Ehrenb.) A. Br., 
P. Sturmii Reinsch, etc. P. simplex has been described as in- 
cluding forms both with and without intercellular spaces. 
It is impossible to determine from the evidence in the litera- 
ture as to the independence of these various forms and yet there 
