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 
A B 
Fia. 175. — Schizophycee. 
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.) 
adhering to fresh-water or ma- 
rine alge, 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 
