[6 



PLANT LIFE. 



gliding movements of some free diatoms are due to the pro- 

 trusion of strands of cytoplasm through slits in the valves. 



22. The desmids. — These form another group of one- 

 celled green alga;. They have neither the brownish color 

 nor siliceous wall characteristic of diatoms, but are bright 

 green cells of remarkably diverse and often beautiful forms. 

 As a rule the cell is flattened and is divided almost into two 

 by a deep constriction near the middle (a, b, c, e, fig. 22). 



Fig. 22. — Various desmids. a, Micrasterias ; 6, Cosmarium ; c, Xanthidium ; 

 d, Closterium ; f, Staurastrum ; J, Aptogonum. Magnified about 200 diam. 

 — After Kerncr. 



Often the body of the cell is covered with warts or spine-like 

 projections (b, c, fig. 22), or is prolonged into horn-like or 

 hair-like lobes. These plants also frequently cohere into 

 colonies (_/", fig. 22). In that case tooth-like projections of 

 the cell-wall may interlock. 



