The carbon which forms over half of the
plant substance is gathered from the air by the leaves and it is
evident that the leaves are very active agents of plant growth. The
atmosphere consists chiefly of the gases oxygen and nitrogen in the
proportion of one to four, but associated with them are small
quantities of various other substances. Chief among the secondary
constituents of the atmosphere is the gas carbon dioxid, which is
formed when carbon burns, that is, when carbon unites with the
oxygen of the air. Whenever coal or wood or any carbonaceous
substance burns, carbon dioxid is formed. Leaves have the power of
absorbing the gas carbon dioxid from the air and separating the
carbon from the oxygen. The oxygen is returned to the atmosphere
while the carbon is retained to be used as the fundamental substance
in the construction by the plant of oils, fats, starches, sugars,
protein, and all the other products of plant growth.
This important process known as carbon assimilation is made possible
by the aid of countless small openings which exist chicfly on the
surfaces of leaves and known as "stomata.
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