HOW TO KNOW THE FRESH-WATER ALGAE 



Diatoms occur in innumerable species both in the plankton, on the 

 bottom, and on submerged aquatics, etc. The cells should be cleaned 

 by boiling in acid so as to remove all organic content in order to make 

 the wall characters visible. Identification of genera and species is 

 based largely upon the shape of the cell and upon the manner of 

 decoration, the presence of septae in the cells, and upon other details 

 which cannot be seen unless the shells are clear and free from chroma- 

 tophores and oil droplets. Some are so characteristically shaped that 

 the genus can be determined without this treatment. The most com- 

 mon genera in the plankton are Stephanodiscus, Melosira, Fragilaria, 

 Asterionella, Navicula and Tabellaria. Stephenodiscus is shaped like 

 a drum; has a crown of short spines just within the margin. Melosira 

 is capsule-shaped, with the cells arranged end to end in filaments. 

 Fragilaria commonly has long, narrow rectangular cells placed side 

 by side to form flat ribbons. Tabellaria forms zig-zag chains. Navicula 

 is commonly 'boat-shaped' or cigar-shaped when the cell is seen in 

 valve (top) view. Asterionella has cells radiating from a common center 

 in one plane. 



Fig. 331. Tabellaria fenestrata (Lyngb.) Kuetz. 



Figure 331 



Fig. 332. Meridion circulare (Grev.) Ag. 



Fig. 333. Asterionella iormosa Hass. 



Figure 332 



Figure 333 



187 



