THALLOPHYTA: ALGAE 



23 



microscope. A good microscope will show that the striations on the silici- 

 fied cell wall generally consist of rows of very minute pores. They appear 

 as lines because the pores are very close together.^ 



Two views of a diatom are possible — girdle (side) view and valve 

 (top) view (Fig. 11). Many diatoms possess a raphe, which is a longitu- 

 dinal slit extending down the center of the valve. Such forms have the 

 power of locomotion, movement apparently being accomplished by a 

 streaming of protoplasm along the raphe. 



The Bacillariophyceae comprise two orders: (1) the Centrales, which 

 are radially symmetrical in valve view, often circular, and have no raphe 



Fig. 13. Triceratium, a large marine diatom, as seen in optical section, X400. The 

 nucleus lies in the center of the cell, while numerous small plastids lie just inside the cell wall. 



(Fig. 12A-C) and (2) the Pennales, which are usually bilaterally sym- 

 metrical, not circular, and generally have a raphe (Fig. \2D-G). The 

 difference in symmetry is clearly shown by the pattern of markings on 

 the valves, being radial in the Centrales and bilateral in the Pennales. 



In most diatoms the nucleus is suspended in the center of the cell by 

 slender strands or by a broad transverse band of cytoplasm connected 

 with a thin laj^er lying next to the cell wall (Figs. 13 and 14/1). Embed- 

 ded in the peripheral layer are one or more plastids that are usually 

 brown, frequently yellow, or rarely green. In the Centrales the plastids 

 are small and numerous. In the Pennales they are large and few in 

 number; commonly there are two. The plastids of diatoms vary greatly 

 in shape, being often irregular and sometimes elaborately lobed. The 

 cell contains no starch, food being stored mainly as oil. 



1 Some diatoms have striae so fine that they are used as test objects in determining 

 the efficiency of microscope lenses. A good oil immersion objective will resolve mark- 

 ings that are as fine as five striae to the micron. 



