led to inquire into the means by which they are enabled to arrive at
such strength and maturity; and whether it may be considered as a
humiliation we will not determine, but, with all the ingenious
mechanical contrivances of man, we are still unable to define the limits
of the animal and vegetable kingdoms. "Plants have been described by
naturalists, who would determine the limits of the two kingdoms, as
organized living bodies, without volition or locomotion, destitute of a
mouth or intestinal cavity, which, when detached from their place of
growth, die, and, in decay, ferment, but do not putrefy, and which, on
being subjected to analysis, furnish an excess of carbon and no
nitrogen. The powers of chemistry, and of the microscope, however,
instead of confirming these views, tend more and more to show that a
still closer affinity exists between plants and animals; for it is now
ascertained that nitrogen, which was believed to be present only in
animals, enters largely into the composition of plants also. When the
microscope is brought to aid our powers of observation, we find that
there are organized bodies belonging to the vegetable kingdom which
possess very evident powers of locomotion, and which change about in so
very remarkable a manner, that no other cause than that of volition can
be assigned to it." Thus it would seem that, in this particular at
least, some vegetables bear a very close resemblance to animal life; and
when we consider the manner in which they are supplied with nourishment,
and perform the functions of their existence, the resemblance would seem
still closer. If, for example, we take a thin transverse slice of the
stem of any plant, or a slice cut across its stem, and immerse it in a
little pure water, and place it under a microscope, we will find that it
consists principally of cells, more or less regular, and resembling
those of a honeycomb or a network of cobweb. The size of these varies in
different plants, as it does in different parts of the same plant, and
they are sometimes so minute as to require a million to cover a square
inch of surface. This singular structure, besides containing water and
air, is the repository or storehouse of various secretions. Through it,
the sap, when produced, is diffused sideways through the plant, and by
it numerous changes are effected in the juices which fill its cells. The
forms of the cells are various; they are also subject to various
transformations. Sometimes a number of cylindrical cells are laid end to
end, and, by the absorption of the transverse partitions, form a
continuous tube, as in the sap-vessels of plants, or in muscular and
nervous fibre; and when cells are thus woven together, they are called
cellular tissue, which, in the human body, forms a fine net-like
membrane, enveloping or connecting most of its structures. In pulpy
fruits, the cells may be easily separated one from the other; and within
the cells are smaller cells, commonly known as pulp. Among the
cell-contents of some plants are beautiful crystals, called _raphides_.
The term is derived from [Greek: rhaphis] a _needle_, on account of the
resemblance of the crystal to a needle. They are composed of the
phosphate and oxalate of lime; but there is great difference of opinion
as to their use in the economy of the plant, and one of the French
philosophers endeavoured to prove that crystals are the possible
transition of the inorganic to organic matter. The differences, however,
between the highest form of crystal and the lowest form of organic life
known, viz., a simple reproductive cell, are so manifold and striking,
that the attempt to make crystals the bridge over which inorganic matter
passes into organic, is almost totally regarded as futile. In a layer of
an onion, a fig, a section of garden rhubarb, in some species of aloe,
in the bark of many trees, and in portions of the cuticle of the
medicinal squill, bundles of these needle-shaped crystals are to be
found. Some of them are as large as 1-40th of an inch, others are as
small as the 1-1000th. They are found in all parts of the plant,--in the
stem, bark, leaves, stipules, petals, fruit, roots, and even in the
pollen, with some few exceptions, and they are always situated in the
interior of cells. Some plants, as many of the _cactus_ tribe, are made
up almost entirely of these needle-crystals; in some instances, every
cell of the cuticle contains a stellate mass of crystals; in others, the
whole interior is full of them, rendering the plant so exceedingly
brittle, that the least touch will occasion a fracture; so much so, that
some specimens of _Cactus senilis_, said to be a thousand years old,
which were sent a few years since to Kew, from South America, were
obliged to be packed in cotton, with all the care of the most delicate
jewellery, to preserve them during transport.
[Illustration: SILICEOUS CUTICLE FROM UNDER-SIDE OF LEAF OF DEUTZIA
SCABRA.]
[Illustration: SILICEOUS CUTICLE OF GRASS.]