nt of carbon in plants
comes from the air. All the carbon that a plant gets is taken in by the
leaves of the plant; not a particle is gathered by the roots. A large
tree, weighing perhaps 11,000 pounds, requires in its growth carbon from
16,000,000 cubic yards of air.

Perhaps, after these statements, you may think there is danger that the
carbon of the air may sometime become exhausted. The air of the whole
world contains about 1,760,000,000,000 pounds of carbon. Moreover, this
is continually being added to by our fires and by the breath of animals.
When wood or coal is used for fuel the carbon of the burning substance
is returned to the air in the form of gas. Some large factories burn
great quantities of coal and thus turn much carbon back to the air. A
single factory in Germany is estimated to give back to the air daily
about 5,280,000 pounds of carbon. You see, then, that carbon is
constantly being put back into the air to replace that which is used by
growing plants.

The carbon of the air can be used by none but green plants, and by them
only in the sunlight. We may compare the green coloring matter of the
leaf to a machine, and the sunlight to the power, or energy, which keeps
the machine in motion. By means, then, of sunlight and the green
coloring matter of the leaves, the plant secures carbon. The carbon
passes into the plant and is there made into two foods very necessary to
the plant; namely, starch and sugar.

Sometimes the plant uses the starch and sugar immediately. At other
times it stores both away, as it does in the Irish and the sweet potato
and in beets, cabbage, peas, and beans. These plants are used as food by
man because they contain so much nourishment; that is, starch and sugar
which were stored away by the plant for its own future use.

=EXERCISE=

Examine some charcoal. Can you see the rings of growth? Slightly
char paper, cloth, meat, sugar, starch, etc. What does the turning
black prove? What per cent of these substances do you think is pure
carbon?

SECTION XIII. THE SAP CURRENT

The root-hairs take nourishment from the soil. The leaves manufacture
starch and sugar. These manufactured foods must be carried to all parts
of the plant. There are two currents to carry them. One passes from the
roots through the young wood to the leaves, and one, a downward current,
passes through the bark, carrying needed food to the roots (see Fig.
28).

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