DESMIDS 179 
family (Closteriaceae) the elongated cylindrical cells sepa- 
rate early and become more or less attenuated, as in 
Closterium. In a third family (Cosma- 
riaceae) the flattened, more or less con- 
stricted cells separate very early, and 
in many cases become terminally much 
lobed or otherwise modified. Of the 
less modified desmids the species of _Fic.71-—Desmids: 
° ° losterium, Cosma- 
Cosmarium are good examples, while rium, and Micra- 
those of Euastrum and Micrasterias are“ 
greatly modified, the cells of the latter being divided 
into many pointed lobes. 
254. In generation the desmid cells break open at the 
middle (where there is commonly a joint in the wall) and 
the two protoplasms (isogametes) unite into a zygote, 
which eventually becomes a thick-walled resting spore. 
After some time the resting spore germinates by ruptur- 
ing its wall and dividing the contents into two, four or 
eight new non-ciliated cells which eventually become like 
the parent cells. 
255. Desmids are fresh-water plants, floating free in 
the waters of quiet pools, or entangled with mosses or 
other aquatic plants. 
Crass 6. BACILLARIOIDEAE 
256. The plants of this class are the Diatoms, num- 
bering about 5700 species, or even as many as 10,000 
species in the opinion of some botanists. Some diatoms 
are filamentous, but in the greater number the filaments 
fragment early into single cells. The cells contain 
chlorophyll, which is commonly hidden by the addition 
of diatomin, a yellow-brown pigment. A few diatoms 
are colorless, and hysterophytic, and therefore are 
“ce fungi. ”? 
