body heat and the animal force or energy are supplied by the
combustion of organic food within the body, and here, too, carbon
dioxid is the chief product of combustion.

"Thus, as a general average, the amount of carbon removed from the
atmosphere by growing plants is no greater than the amount returned
to the air by these various forms of combustion or decay. In like
manner the supply of combined oxygen is maintained, both carbon and
oxygen being furnished to the plant m the carbon dioxid.

"As a matter of fact, the air consists very largely of oxygen and
nitrogen, both in the free state, but in this form these elements
cannot be utilized in the growth of agricultural plants. The only
apparent exception to this is in case of legume crops, such as
clover, alfalfa, peas, beans, and vetch, which have power to utilize
the free nitrogen by means of their symbiotic relationship with
certain nitrogen-fixing bacteria which live, or may live, in
tubercles on their roots.

"Carbon and oxygen constitute about ninety per cent. of the dry
matter of ordinary farm crops, and with the addition of hydrogen
very important plant constituents are produced; such as starch,
sugar, fiber, or cellulose, which constitute the carbohydrate group.
As the name indicates, this group contains carbon, hydrogen, and
oxygen, the last two being present in the same proportion as in
water.

"Water is composed of the two elements, hydrogen and oxygen, both of
which are gases in the free state. Water is taken into the plant
through the roots and decomposed in the leaves in contact with the