arbonic acid contained in the water, and
which causes it to dissolve, to some extent, substances otherwise
insoluble. It is in this way that lime, which occurs in the soil
principally as the insoluble carbonate, is dissolved and absorbed. And
phosphate of lime is also taken up by water containing carbonic acid, or
even common salt in solution. The amount of solubility produced by these
substances is extremely small; but it is sufficient for the purpose of
supplying to the plant as much of its mineral constituents as are
required, for the quantity of water which, as we have already seen,
passes through a plant is very large when compared with the amount of
inorganic matters absorbed. It has been shown by Lawes and Gilbert, that
about 2000 grains of water pass through a plant for every grain of
mineral matter fixed in it, so that there is no difficulty in
understanding how the absorption takes place.

It is worthy of notice, however, that the absorption of the elements of
plants takes place even though they may not be in solution in the soil,
the roots apparently possessing the power of directly acting on and
dissolving insoluble matters; but a distinction must be drawn between
this and the view entertained by Jethro Tull, who supposed that they
might be absorbed in the solid state, provided they were reduced to a
state of sufficient comminution. It is now no longer doubted that,
whatever action the roots may exert, the constituents of the plant must
be in solution before they can pass into it--experiment having
distinctly shown that the spongioles or apertures through which this
absorption takes place are too minute to admit even the smallest solid
particle.

CHAPTER II.

THE PROXIMATE CONSTITUENTS OF PLANTS.

The substances absorbed by the plant, which are of simple composition,
and contain only two elements, are elaborated within it, and converted
into the many complicated compounds of which its mass is composed. Some
of these, as, for example, the colouring matters of madder and indigo,
the narcotic principle of the poppy, &c., are confined to a single
species, or small group of plants, while others are found in all plants,
and form the main bulk of their tissues. The latter are the only
substances which claim notice in a treatise like the present. They have
been divided into three great classes, of widely different properties,
composition, and functions.

_1st. The Saccharine and Amylaceous Constituents