DEVELOPMENT OF PLANTS 



169 



the plankton (Fig. 95) . This is especially abundant in northern 

 waters and has been compared to great pasture lands, since it 

 furnishes the principal food of surface feeding fishes and other 

 marine life. The Diatoms are unicellular plants and exhibit 



Fig. 95. Fig. 96. 



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

 BeloTs^ sten from side. — ^After Gran. 



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

 vjew. 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 ceijter of the valve. — ^After Lauterborn. 



a great variety of forms as circular, elliptical, rod or wedge- 

 shaped, curved or straight. They are equally variable as regards 

 their association. Some are solitary and free swimming, others 

 are attached by stalks; some form bands or ribbons or zigzag 



