ely into the composition of vegetable substances. In
the soil, also, the part which ministers to the wants of vegetables is
relatively quite insignificant in amount.

Plants are unendowed with organs of locomotion, their food must
therefore be within easy reach. Every breeze wafts gaseous nutriment to
their expanded leaves, and their rootlets ramify throughout the soil in
search of appropriate mineral aliment. But no matter how abundant, or
however easy of reach may be the food of plants, the vegetable organism
is incapable of partaking of it unless under the influence of light.
Exposed to this potent stimulus, the plant collects the gaseous carbonic
acid and the vaporous water, solidifies them, decomposes them, and
combines their elements into new and organised forms. In effecting these
changes--in conferring vitality upon the atoms of lifeless matter--the
plant acts merely as the _mechanism_, the light is the _force_. As the
work performed by the steam-engine is proportionate to the amount of
force developed by the combustion of the fuel beneath its boiler,
so is the rapidity of the elaboration of organic substances by plants
proportionate to the amount of sunlight to which they are exposed. It is
an axiom that matter is indestructible; we may alter its form as often
as we please, but we cannot destroy a particle of it. It is the same
with _force_: we may convert one kind of it into another--heat into
light, or magnetism into electricity--but our power ends there; we can
only cause force, or _motion_, to pass from one of its conditions to
another, but its _quantity_ can never be diminished by the power of man.

The principle of the Conservation of the Forces gives us a clear
explanation of the fact that animals can obtain their food only through
the medium of the vegetable kingdom. Plants are stationary mechanisms;
they have no need to develop motive power, as animals have, in moving
themselves from place to place. Their temperature is, we may say, the
same as that of the medium in which they exist. Such beings as plants
do not, therefore, require the expenditure of force to maintain their
vitality; on the contrary, their mechanisms are, for a beneficent
purpose, constructed for the _accumulation_ of force. The growing
plant absorbs, together with carbonic acid, water, and ammonia, a
proportionate amount of light, heat, and the various other subtile
forces which have their abiding place in the sun-beam--