USES OF ALGLE 



WATER covers two thirds of the surface of the earth, and 

 algae, 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. Algae 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 algae 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 (Sphceoplea anmilina), 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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