170 



STRUCTURE OF DIATOMS 



chains, Avhile others are imbedded in a gelatinous mass of a 

 more or less regular form (Fig. 94, D). The cells are covered 

 by two valves, one of which overlaps the other like the cover 

 of a box. Therefore a diatom presents two quite distinct ap- 

 pearances — the top or valve view and the side or girdle view 

 (Figs. 96, 97). This difference is further intensified by the 

 sculpturing of fine lines that appear upon the walls. An inter- 

 esting feature about these valves is the fact that they are com- 



FiG. 97. Structure of Cymbella: A, valve view. B, cross-section show- 

 ing the difference of the two sides of the diatom. C, the two girdle views. 

 —After Pfitzer. 



pletely infiltrated with silica, a substance resembling glass. If 

 a diatom is burned in a flame on a strip of platinum or placed 

 in acid to remove the organic substance the appearance of the 

 valves remains unchanged. 



The glass-like valves are quite transparent and it can be readily 

 observed that the cell contents is much more highly differenti- 

 ated than in the preceding group. The chlorophyll is deposited 

 in plastids of definite form, although this color is often masked 

 by a brown pigment which causes the characteristic appearance 

 of these plants when associated in masses. The oil drops seen 

 in the cells are the product of photosynthesis — starch not being 

 formed. Some species, however, can live upon decaying or- 

 ganic matter and in consequence contain colorless plastids. In 

 the free swimming forms the motion consists of an irregular 



