[Sidenote: Action of plants upon carbon dioxid]
Carbon dioxid is an important factor in the life activity of the earth.
The leaves of plants absorb carbon dioxid from the air, and by means of
the chemical activity of the green coloring-matter or chlorophyl, the
plant has the power of combining the carbon dioxid with water, and with
the mineral salts which have been absorbed from the earth by the roots
of the plant. Sunlight is necessary to this action, especially in the
manufacture of starch.
This formation of food material in plants by the combination of simple
chemical substances, such as carbon dioxid and water, is one of the
fundamental life-processes. Animals do not possess this power of
utilizing simple or inorganic chemical compounds, therefore they must
take their food substances in the more complex forms which have been
created by the power of sunlight acting upon the plant.
[Sidenote: The wonderful carbon cycle]
I have already explained how carbon dioxid may enter the air. Thus we
see that the carbon dioxid which is withdrawn from the air, by the
growth of plants, is constantly replaced by combustion, and in this
way the "carbon cycle" is completed. This is one of the most beautiful
adaptations in nature. If the plant did not remove the carbon dioxid
from the air, it would soon accumulate in such quantities as to become
detrimental to life, and, on the other hand, if this gas were not
returned to the air by combustion, by the breathing of animals, and by
the decay of plants, the vegetable world would soon be without carbon
dioxid, which is as essential to plant life as is the oxygen of the air
to animal life.