ALGjE 



65 



A second group of unicellular plants, resembling the 

 Phaeophycese in color, but otherwise more like some of 

 the green algae, are the 

 Diatoms (Fig. 15, C,D), a 

 group including many- 

 thousand species, which 

 often occur in enormous 

 masses. Although these 

 are unicellular, they are 

 often united into colonies 

 of definite form, but more 

 commonly are free. The 

 chromatophores are usu- 

 ally two in number and 

 flattened in shape, but may ^^%^lf^ 

 be numerous and of the 

 round or oval form com- 

 monly found in the higher 

 Phseophycese. As in the 

 latter there is present a 

 brown pigment (diatomin) which quite conceals the 

 chlorophyll. A further peculiarity of these plants is 

 the presence of a silicious shell, composed of two valves, 

 one fitting into the other (Fig. 15, C, II). This glassy 

 case is often sculptured in a most beautiful manner, and 

 the fine markings are favorite tests for microscopic 

 lenses. The diatoms often exhibit creeping move- 

 ments, but are never ciliated. The multiplication of 

 the diatoms is either by fission, or by the formation of 

 so-called " auxospores," which may be formed either 

 sexually or asexually. 



While diatoms are common in fresh water, it is in 



A, B, Peridineae; C, D, 

 Diatomaceae. A, Hemidiniiiin na- 

 sutum (after Stein) ; B, Peridi- 

 niitm divergens (alter Schiitt) ; C, 

 Pinnulariaviridis: i.from above, 

 II, from the side, showing the over- 

 lapping valves of which the shell is 

 composed ; D, Navicula sp. ? show- 

 ing the two chromatophores, cl. 



