THE ALGA 237 
many miles in extent. It has been estimated that the 
diatomaceous earth of the Bohemian deposits contains 
forty-one million diatoms in every cubic inch. Similar 
deposits are to-day being formed in swamps, in lake-beds, 
and over large areas of sea-bottom. 
The conditions of life under which 
alge can exist are extremely various. 
One unicellular species, the red snow 
plant, Spherella nivalis, gives a pink 
color to many square miles of snow in 
the arctic regions. Some of the blue- 
green algz, on the other hand, live’in 
hot springs at a temperature of 145° 
Fahrenheit. It is usually stated that 
alge differ from fungi in being self- 
supporting, but at least one parasitic 
species is known. 
The green alge have a special inter- 
est for the scientific botanist from the 
fact that they are supposed to be the 
ancestors of the mosses. 
Few alge have much direct eco- 
nomic value as articles of human food 4) spores of cmnaleer 
. (greatly magnified) ; 
or as sources of manufactured prod- _ 8B, portion of thallus of 
ucts. Some, as the “Jrish moss,” the @ Florent Gees 
dulse, and “Ceylon moss,” are eaten. 
Rockweeds are considerably used as fertilizers. Diatoma- 
ceous earth is sold under various names for polishing silver 
and other metals. Algz, particularly of some minute float- 
ing marine groups, are of the greatest importance as food 
for shellfish and for many of the small organisms which 
form the diet of fishes. 
Fic. 168. Red Algz. 
