i6 



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 algse. 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 ; 3, Cosmartum ; c, Xanthidium ; 

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

 — After Kerner. 



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

 projections (£, 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. 



