hat of absorbing carbonic acid from the atmosphere--that
of assisting in the chemical preparation of the sap--and that of
evaporating its water. If we examine leaves with a microscope we shall
find that some have as many as 170,000 openings, or mouths, in a square
inch; others have a much less number. Usually, the pores on the under
side of the leaf absorb the carbonic acid. This absorptive power is
illustrated when we apply the lower side of a cabbage leaf to a wound,
as it draws strongly--the other side of the leaf has no such action.
Young sprouts may have the power of absorbing and decomposing carbonic
acid.

[What parts of roots absorb food?

How much of their carbon may plants receive through their roots?

What change does carbonic acid undergo after entering the plant?

In what parts of the plant, and under what influence, is carbonic acid
decomposed?]

The roots of plants terminate at their ends in minute spongioles, or
mouths for the absorption of fluids containing nutriment. In these
fluids there exist greater or less quantities of carbonic acid, and a
considerable amount of this gas enters into the circulation of the
plants and is carried to those parts where it is required for
decomposition. Plants, under favorable circumstances, may thus obtain
about one-third of their carbon.

Carbonic acid, it will be recollected, consists of _carbon and oxygen_,
while it supplies only _carbon_ to the plant. It is therefore necessary
that it be divided, or decomposed, and that the carbon be retained while
the oxygen is sent off again into the atmosphere, to reperform its
office of uniting with carbon. This decomposition takes place in the
_green_ parts of plants and only under the influence of daylight. It is
not necessary that the sun shine directly on the leaf or green shoot,
but this causes a _more rapid_ decomposition of carbonic acid, and
consequently we find that plants which are well exposed to the sun's
rays make the most rapid growth.

[Explain the condition of different latitudes.

Does the proportion of carbonic acid in the atmosphere remain about the
same?]

The fact that light is essential to vegetation explains the conditions
of different latitudes, which, so far as the assimilation of carbon is
concerned, are much the same. At the Equator the days are but about
twelve hours long. Still, as the growth of plants is extended over eight
or nine months of the year, the duration of daylight is sufficie