THE DIATOMS. 



55 



None are large enough to be seen with the unaided eye. 

 Some Diatoms resemble in shape certain Desmids, but the 

 brownish color of the Diatoms distinguishes them at once 

 from the bright green Desmids. Many species of the 

 Diatoms are capable of independent movement from place 

 to place. The manner in which the movements are accom- 

 plished is not well understood. 

 One theory is that the motions 

 are caused by currents of water, 

 which pass through small open- 

 ings in the organism, or perhaps 

 by osmosis through the thin par- 

 titions of the cells. Another 

 theory ascribes them to the con- 

 tractions of the protoplasm, which 

 protrudes through minute open- 

 ings in the cell wall. 



The Diatoms are one celled 

 plants. A plant cell may be 

 considered as a closed or nearly 

 closed box, whose walls are com- 

 posed essentially of a substance 

 called cellulose. All living cells 

 are filled or lined with proto- 

 plasm, in which resides the true 

 life of the plant. The proto- 

 plasm of a plant cell has been 

 compared to a living snail, and 

 the cell wall to the snail's shell, 

 secreted by the snail, so the cell wall is secreted by the 

 living protoplasm within. In the filamentous forms of 

 Diatoms the cells are x simply associated together, but are 

 without organic connection. The association of unicellu- 

 lar organisms in filaments will be seen in some forms of 

 Bacteria, in the Zygnemese, and the Desmids, 



FIG. 27. Gomphonema con- 

 strictum Ehrbg. s, view of 

 valves; g t , view of the girdle 

 corresponding to the right; 

 g,,, to the left margin of the 

 valves ; q, transverse section 

 through the middle of the 

 cell, showing with unusual 

 distinctness the composition 

 of the silicified cell wall of 

 two halves, one overlapping 

 the other (sa, the larger, si, 

 the smaller valve) ; k, the 

 nucleus ; p, dense proto- 

 plasm ; g t) (/,,, the two girdle 

 surfaces. (From Bennett 

 and Murray, after Pfitzer.) 



As the shell has been 



