II. 



THE DIATOMS. 



The Diatoms (J)iatomaceoR) are of very common occur- 

 rence. Not less than ten thousand species have been 



described. 



They occur in water, both still 

 and running, salt and fresh, 

 growing on the leaves and stems 

 of aquatic plants, or attached to 

 sticks and stones, or mingling 

 with the mud at the bottom. 

 They are brown in color, because 

 of the coloring matter called dia- 

 tomin, and are sometimes classed 

 with the Brown Algae for this 

 reason. They also contain chlo- 

 rophyl. Their exact position in 

 the classification of plants is not 

 easily determined, as they do not 

 seem to be connected even re- 

 motely with any other plants. 

 They are therefore given a place 

 by themselves in the classification. 

 The Diatoms occur as isolated cells, or connected in fila- 

 ments. The filaments sometimes partly separate, leaving 

 the cells strung together by their corners, forming striking 

 and characteristic patterns. Some forms grow on gelatine- 

 like stems. Diatoms vary in size from forms that require 

 a high power of the microscope to distinguish them to 

 forms that are plainly visible under the one-inch objective. 



54 



FIG. 26. Pinmtlaria viridis 

 Sm. A, valve view; B, 

 girdle view (diagrammat- 

 ic), m, raphe; g, central 

 node ; k, terminal nodules ; 

 a, outer and older valve ; 

 i, inner valve. (X 800.) 

 (From Bennett and Mur- 

 ray, after Pfitzer.) 



