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 Fia.7i.— Desmids: 



^^ . , 1 1 •! Closterium, Cosma- 



Cosmarmm 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. 



Class 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 hysterophyliic, and therefore are 

 "fungi." 



