DEVELOPMENT OF PLANTS 



159 



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

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

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

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



FiG. 95. Fig. 96. 



Fig. 95. Plankton forms of diatoms: A, Coscinodiscus, B, Planktoniella. 

 Below seen from side. — After Gran. 



Fig. 96. Structure of the diatom, Pinnularia: A, valve view. B, girdle 

 view. C, cross-section, the two dark bands showing the position of the chro- 

 moplasts in the diatom — p, pore in wall which appears as a line in A, running 

 from the ends towards the center of the valve. — After Lauterborn. 



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- 



