ALG^E 



65 



A second group of unicellular plants, resembling the 

 Phgeophycese 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 

 chro mat oph ores are usu- 

 ally two in number and 



V 



flattened in shape, but may FlG - 15 - -- A - B Peridinefe; C, D, 



L J Diatomacese. A, Hemzdinwim na- 



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 



sutunt (after Stein) ; B, Peridi- 

 nium diverr/ens (after Sc-hiitt) ; C, 

 Pinnularia viridis : i, from above, 

 n, from the side, showing the over- 

 lapping valves of which the shell is 

 composed ; D, Xavicula sp. ? show- 

 ing the two chromatophores, cl. 



r 



