conformation, in colour, in size, in duration, in
every apparent particular; it appeared to
him altogether hopeless to bring these
marvellously different structures under one
general law of production and of growth; or
to trace the harmony of their functions. But
the microscope has brought new eyes to man;
and, after years of patient investigation, the
great result was obtained which was expressed
in a former paper;* that the basis of all
the vegetation of the world is a little closed
vesicle, composed of a membrane usually
transparent and colourless as water; —the
vegetable cell. At first, perhaps, this idea, so
novel to the botanist of the old school and
apparently so opposed to the evidence of the
unassisted vision, is difficult to grasp; but
when we have satisfied ourselves, as we
easily may, that even the hardest portions
of vegetables —such as wood —are capable
of being resolved into cells no less than
the softest vegetable slime, and that the
processes of production and nutrition are
regulated in both by the same great laws,
we begin to comprehend how marvellously
this aphorism of the universality of the cell
simplifies botanical research.
* See page 354 of the current volume.

The simple relation thus established
throughout the vegetable kingdom, enables
us to reduce our investigation, to the simplest
form, at the same time that we include in
them the whole vegetable world. As the
bulk of every plant, whether great or small,
is only an aggregation of the separate cells;
so the life of the whole plant is but the sum
of the vitality of each individual cell. Every
cell being, in itself, a distinct structure, carrying
on independent vital processes, possesses,
necessarily, an independent vitality; and
thus in studying the life of a plant cell
individually, we shall also be contemplating the
life of the whole plant. The first necessity
of cell-life is, of course, nutrition, and
before the cells can be agglutinated together or
increased in size, they must receive nourishment
from without. The materials for this
nourishment are chiefly gases; —carbon,
hydrogen, oxygen, and nitrogen, of which the
philosophical Schiller sings—
        Four elements in one firm band
        Give form to life, build sea and land.

These four great organic elements the plantcell
receives in the form of carbonic acid gas,
atmospheric air, water, and ammonia;
together with these it takes up certain salts and
metals. The question which here presents
itself is, how does this globular vesicle, which
has no aperture, obtain these materials of
nutrition; or, in other words, how do they
arrive at the interior of the cell? The first
fact to be observed in solving this important
problem is, that the cell receives no food
which is not dissolved in water. All its
nourishment is obtained by the absorption of a
nutritive fluid —an aqueous solution of the
materials mentioned. This function cannot be too
strongly impressed on the mind; the passage of
nutritive fluid through the walls of the cell is
the universal means of growth in both animal
and vegetable kingdom; it is a process with
the due performance of which the existence
of the whole animal and vegetable creation
is intimately connected. It depends upon a
physical law, with examples of which every
one is familiar. If one end of a piece of
sponge be immersed in water, the fluid will
ascend throughout the cells of the sponge,
and will moisten that part which is not so
immersed. The same operation may be seen
still more rapidly exemplified on dipping a
lump of white sugar into water at one
extremity. This law holds true of gases;
and it explains the process by which the
plant receives its nourishment.

The nutritive fluid, being brought in
contact with the external wall of the cell,
passes in by a process precisely similar to that
which was seen in the sponge and the sugar
— travelling from one cell to the other until
it permeates the whole plant. And, since the
same holds true of gases, the aqueous vapour
in the atmosphere is no less active in aiding
in the nutrition of the plant than the liquid
water which is absorbed by the roots. The
plant cell is acted upon by the sun, and we
know that it rapidly and largely exhales
watery vapour. The process of nutrition is,
consequently, continually renewed; heat drawing
off a great part of the water, and leaving
in the cell the substances which it brought
with it. So that the cell-membrane being
kept dry by the action of heat while the
atmosphere and earth are charged with moisture,
it is perpetually absorbing fresh nutritive
fluid. This is the reason why the life of most
plants is only active during the summer,
when, the heat being greatest, evaporation
is also greatest, the exhaling organs of the
plant are put forth, and the processes of
nutrition are vigorously carried on. It has
been shown that for every grain of the salts
deposited in the plant, two thousand grains of
water must be exhaled; and for every grain
of other substances two hundred grains of
water must be driven off. Now, as this is
effected by the agency of heat and light, it is
easy to comprehend that in summer the plant
is actively nourished, old cells perfected, the
sections of the cell produced, and new cells
formed. These new cells spring up between
the cortex or bark and the first layer of cells
internal to this cortex. It is by their agency
that the process of absorption is so rapidly
carried on. They receive the raw nutritive
fluid, and exert such a chemical influence
over it, that whatever remains in the cell is
converted into a more highly organised fluid
— the sap of the tree —and is absorbed by the
inner and dry cells, which form out of this
the secretions of the plant. It is this fresh
layer of cells which springs up every summer