ne-third of the annual rain that falls in the
district watered by it, and the Rhine nearly four-fifths. Of course this
large exhalation must depend on the repeated absorption of the same
quantity of water, which, after being exhaled, is again deposited on the
soil in the form of dew, and passes repeatedly through the plant. This
constant percolation of water is of immense importance to the plant, as
it forms the channel through which some of its other constituents are
carried to it.

_Carbonic Acid._--While the larger part of the water which a plant
requires is absorbed by its roots, the reverse is the case with carbonic
acid. A certain proportion no doubt is carried up through the roots by
the water, which always contains a quantity of that gas in solution, but
by far the larger proportion is directly absorbed from the air by the
leaves. A simple experiment of Boussingault's illustrates this
absorption very strikingly. He took a large glass globe having three
apertures, through one of which he introduced the branch of a vine, with
twenty leaves on it. With one of the side apertures a tube was
connected, by means of which the air could be drawn slowly through the
globe, and into an apparatus in which its carbonic acid was accurately
determined. He found, in this way, that while the air which entered the
globe contained 0.0004 of carbonic acid, that which escaped contained
only 0.0001, so that three-fourths of the carbonic acid had been
absorbed.

_Ammonia and Nitric Acid._--Little is known regarding the mode in which
these substances enter the plant. It is usually supposed that they are
entirely absorbed by the roots, and no doubt the greater proportion is
taken up in this way, but it is very probable that they may also be
absorbed by the leaves, at least the addition of ammonia to the air in
which plants are grown, materially accelerates vegetation. It is
probable, however, that the rain carries down the ammonia to the roots,
and there is no doubt that that derived from the decomposition of the
nitrogenous matters in the soil is so absorbed.

_Inorganic Constituents._--The inorganic constituents of course are
entirely absorbed by the roots; and it is as a solvent for them that the
large quantity of water continually passing through the plants is so
important. They exist in the soil in particular states of combination,
in which they are scarcely soluble in water. But their solubility is
increased by the presence of c