THE LEAVES 57 



62. Composition of the air. — The leaves are 

 generally held aloft in the air, and since they come 

 into contact with no other medium, it is plain that 

 the composition of the air will be of great interest 

 to any one studjmig the activity of leaves. The air 

 is made up of nitrogen, carbonic acid gas, oxygen, 

 argon, and perhaps other rare gases in small quanti- 

 ties. Besides, watery vapor, and traces of acids are 

 present in a proportion which varies greatly with the 

 locality. In 10,000 gallons of ordinary air would be 

 found about 7795 gallons of nitrogen, 2061 gallons 

 of oxygen, 4 gallons of carbon dioxide, 140 gallons 

 of water, and small amounts of the other constitu- 

 ents. These gases and vapors are not united to form 

 a compound. Thus hydrogen and oxygen are united 

 to form the compound water, but the gases of the air 

 are simply mixed together without actually uniting. 



63. Gases of use to the plant. — The oxygen and 

 carbon dioxide are of the greatest importance to the 

 plant. Nitrogen of the air is of no use, except to 

 mix with and dilute the other gases. Only a few 

 species are capable of taking this gas and using it 

 as food. This element is usually gained from com- 

 pounds in the soil. 



