e soil. Manured ground, that is, soil containing
decomposing vegetable or animal matters, is constantly giving out this
gas into the interstices of the soil, whence the roots of the growing
crop absorb it. Carbonic acid thus supplied, primarily from the air, is
the source of the carbon which forms much the largest part of the
substance of every plant. The proportion of carbon may be roughly
estimated by charring some wood or foliage; that is, by heating it out
of contact with the air, so as to decompose and drive off all the other
constituents of the fabric, leaving the large bulk of charcoal or carbon
behind.

_Nitrogen_, the remaining plant-element, is a gas which makes up more
than two thirds of the atmosphere, is brought into the foliage and also
to the roots (being moderately soluble in water) in the same ways as is
carbonic acid. The nitrogen which, mixed with oxygen, a little carbonic
acid, and vapor of water, constitutes the air we breathe, is the source
of this fourth plant-element. But it is very doubtful if ordinary plants
can use any nitrogen gas directly as food; that is, if they can directly
cause it to combine with the other elements so as to form protoplasm.
But when combined with hydrogen (forming ammonia), or when combined with
oxygen (nitric acid and nitrates) plants appropriate it with avidity.
And several natural processes are going on in which nitrogen of the air
is so combined and supplied to the soil in forms directly available to
the plant. The most efficient is _nitrification_, the formation of nitre
(nitrate of potash) in the soil, especially in all fertile soils,
through the action of a bacterial ferment.

450. =Assimilation= in plants is the conversion of these inorganic
substances--essentially, water, carbonic acid, and some form of combined
or combinable nitrogen--into vegetable matter. This most dilute food the
living plant concentrates and assimilates to itself. Only plants are
capable of converting these mineral into organizable matters; and this
all-important work is done by them (so far as all ordinary vegetation is
concerned) only

451. _Under the light of the sun, acting upon green parts or foliage_,
that is, upon the chlorophyll, or upon what answers to chlorophyll,
which these parts contain. The sun in some way supplies a power which
enables the living plant to originate these peculiar chemical
combinations,--to organize matter into forms which are alone capable of
being endowe