274 FIRST FORMS OF VEGETATION. 
ceivable rapidity ; and hence it is not difficult to 
account for their almost universal diffusion, and 
the enormous accumulation of strata which they 
form in certain places. 
Closely connected with the Diatoms are the 
Desmidias, which have attracted almost as large 
a share of attention among microscopic observers. 
They are equally remarkable for their universal 
presence and abundance, and for the variety and 
beauty of their forms. Microscopic in their in- 
dividual state, when adhering in large quantities 
to Potamogetons and other aquatic plants, or lying 
at rest at the bottom of pools, they form a green 
perceptible film or coating. Unlike Diatoms, which 
are found in salt as well as fresh water, the Des- 
mids are exclusively denizens of fresh water, pre- 
ferring that which is pure and limpid, and always 
most abundant not in shady places or woods, but 
in open pools in exposed situations. Owing to 
their isolated, unattached condition, they love to 
dwell in quiet shallow waters, never growing in 
rapidly running or very deep water; the larger 
species being generally found nearest the bottom. 
Under the microscope they are remarkable for 
their singular shapes and their external markings 
and appendages. As a rule they are devoid of 
the silicious envelope which characterizes the 
Diatoms, and their markings are always elevations 
