240 



FOUNDATIONS OF BOTANY 



THE STUDY OF DIATOMS 



270. Occurrence. Diatoms of different species may be found in 

 sediment in water in various kinds of places or mixed with or 



adhering to fresh-water or ma- 

 rine algse, in ponds and ditches 

 or on sand or earth at the 

 bottom of clear brooks. In the 

 last place they may be detected 

 with the eye, forming a yellow- 

 ish coloring. They may often 

 be obtained by straining hy- 

 drant water. Where diatoms 

 have been very abundant their 

 remains sometimes form beds 

 of rock, and fossil diatoms 

 compose some of the polishing 

 powders of commerce. 



271. Microscopical Examina- 

 tion of Diatoms. Place a drop 

 of water containing diatoms on 

 a slide and put a cover-glass 

 over it. Examine with a power 

 of ^200 or more diameters. Dia- 

 toms occur singly, resembling- 

 triangles, wheels, boats, rods, 

 and a great variety of other 

 forms (Fig. 176), or adhering 

 in long bands, as spokes of a 

 wheel, etc. The boat-shaped 

 kinds are among the common- 

 est. The color of the contents 

 is yellowish. The cell-wall is 

 encrusted with a shell of silica 

 whose surface is covered with beautiful markings, dots or lines, 

 which are conspicuous in some species, in others so minute that the 

 most powerful microscopes are required to detect them. By boiling 



FIG. 175. Schizophycese. 

 A, a filament of Calothrix, reproducing by 

 hormogonia, h, segmented portions which 

 escape from the sheath of the filament ; 

 B, Rivularia. (Both A and B greatly 

 magnified.) 



