262 
PLANT STUDIES 
The chief peculiarity is that the wall is composed of two 
valves, one of which fits into the other like the two parts of 
a pill box. This wall is so impregnated with silica that it 
is practically indestructible, and siliceous skeletons of dia- 
toms are preserved abundantly in certain rock deposits. 
They multiply by cell division in a peculiar way, and some 
of them have been observed to con- 
jugate. 
They occur in such numbers in the 
ocean that they form a large part of 
the free-swimming forms on the sur- 
face of the sea, and doubtless showers 
of the siliceous skeletons are constant- 
ly falling on the sea bottom. There 
are certain deposits known as "si- 
liceous earths," which are simply 
masses of fossil diatoms. 
Diatoms have been variously placed 
in schemes of classification. Some 
have put them among the Brown 
Algae because they contain a brown 
coloring matter; others have placed 
them in the Conjugate forms among 
the Green Algae on account of the 
occasional conjugation that has been 
observed. They are so different from 
other forms, however, that it seems 
best to keep them separate from all 
other Algae. 
176. Characeae. These are common- 
ly called " stoneworts," and are often 
included as a group of Green Algae, 
as they seem to be Thallophytes, and 
have no other coloring matter than 
chlorophyll. However, they are so peculiar that they are 
better kept by themselves among the Algae. They are such 
FIG. 231. A common Chara, 
showing tip of main axis. 
After STRASBURGER. 
