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USES OF ALG 
ATER covers two thirds of the surface of the earth, and 
alge, with a very few exceptions, constitute the whole 
vegetation which exists in that enormous area. They have, 
therefore, an important part to perform in the economy of 
nature. Alge do not, like land plants, derive their nourishment 
from the soil to which they are attached, but from substances 
held in solution by water. In their growth they effect changes 
in the water analogous to those effected by land plants in the 
air; that is, they change so-called impurities in the water into 
materials essential to animal life. The function of plants is that 
of transforming or manufacturing inorganic matter, which they 
assimilate, into organic matter (such as starch, albumen, sugar), 
which forms their own structure and which is the food essential 
to animals. In this process, plants inhale carbonic acid gas 
which animals breathe out, and exhale oxygen which animals 
breathe in. Plants feed on mineral substances and furnish vege- 
table food, thus keeping up the balance of life. 
Fresh-water alge have a like economic value. The green sur- 
face on stagnant pools is a vegetable growth whose function is 
to assimilate the matter which makes the pool offensive. A 
submerged district soon becomes covered with scum, or minute 
plants (Spheoplea annulina), which grow with great rapidity, using 
up the materials of the decaying vegetation, and in great measure 
counteracting the ill effects, in the atmosphere, of such decay. 
When the waters subside, the plants shrivel up and appear like 
thin paper covering the ground. This ephemeral substance soon 
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