re is more
earthy matter (i. e. more ashes) in the leaves than in any other part
(sometimes as much as seven per cent, when the wood contains only two
per cent); because it is through the leaves that most of the water
escapes from the plant. Some of this earthy matter incrusts the
cell-walls, some goes to form crystals or rhaphides, which abound in
many plants (422), some enters into certain special vegetable products,
and some appears to be necessary to the well-being of the higher orders
of plants, although forming no necessary part of the proper vegetable
structure.

_The essential constituents_ of the organic fabric are those which are
dissipated into air and vapor in complete burning. They make up from 88
to 99 per cent of the leaf or stem, and essentially the whole both of
the cellulose of the walls and the protoplasm of the contents. Burning
gives these materials of the plant's structure back to the air, mainly
in the same condition in which the plant took them, the same condition
which is reached more slowly in natural decay. The chemical elements of
the cell-walls (or cellulose, 402), as also of starch, sugar, and all
that class of organizable cell-material, are carbon, hydrogen, and
oxygen (399). The same, with nitrogen, are the constituents of
protoplasm, or the truly vital part of vegetation.

449. These chemical elements out of which organic matters are composed
are supplied to the plant by water, carbonic acid, and some combinations
of nitrogen.

_Water_, far more largely than anything else, is imbibed by the roots;
also more or less by the foliage in the form of vapor. Water consists of
oxygen and hydrogen; and cellulose or plant-wall, starch, sugar, etc.,
however different in their qualities, agree in containing these two
elements in the same relative proportions as in water.

_Carbonic acid_ gas (Carbon dioxide) is one of the components of the
atmosphere,--a small one, ordinarily only about 1/2500 of its
bulk,--sufficient for the supply of vegetation, but not enough to be
injurious to animals, as it would be if accumulated. Every current or
breeze of air brings to the leaves expanded in it a succession of fresh
atoms of carbonic acid, which it absorbs through its multitudinous
breathing-pores. This gas is also taken up by water. So it is brought to
the ground by rain, and is absorbed by the roots of plants, either as
dissolved in the water they imbibe, or in the form of gas in the
interstices of th