GENERAL STRUCTURE OF FLOWERING PLANTS.
IN some of the Cryptogamic families fertilization takes place before the
plant itself is developed. In the two highest classes, those containing
the great groups of Flowering Plants, on the contrary, it is the
ultimate result of the inflorescence, which consists of calyx, corolla,
stamens, and pistils, all which are the later expansions of the cellular
tissues and groups of vessels which have in earlier stages of
development formed the leaves. They contain the same materials, and in
fact they are leaves modified in form, structure, and function.
Although the almost innumerable diversities in the form of leaves must
produce corresponding diversities in the inflorescence, yet the general
characteristics are the same or similar in both of the great botanical
classes. The structure of the calyx and corolla which form the floral
envelope of the fructification, is similar to that of leaves. The calyx
consists, in its early stage, of several parts called sepals, which have
all the characters of leaves; subsequently they are sometimes united by
their edges, so as to form a tube, or are otherwise modified. The
corolla only differs from a leaf by greater delicacy of organization; it
has fewer stomates or breathing pores, and the veins have less woody
fibre and sometimes many spiral vessels; the veins in the petals of
chickweed and some other plants are entirely composed of spiral vessels.
The calyx and corolla protect the fructifying organs consisting of
stamens with their anthers, and the pistil or pistils.
The stamens are formed of very fine filaments, and the anthers, when
young and still enclosed in the unexpanded flower, are full of a liquid
which is afterwards changed into a delicate homogeneous, cellular
tissue. Then the internal part of that tissue becomes divided into two
kinds of cells, one of which goes to form the walls of two lateral and
parallel chambers or loculi, separated by a part more or less continuous
with the filament. The other kind of cells are developed into pollen
grains within the chambers. They gradually form a cylindrical assemblage
of mother cells. Within each of these, four cells are ultimately formed,
each containing a single pollen cell. In all plants, except the aquatic,
the pollen cells are soon clothed by the deposition of one or more
layers of cells, which form the outer membrane, on which are impressed
figures or markings peculiar to each species of plant, such as slits,
points, papillæ, sharply defined circles, pores, &c. The colour of the
external layer is generally yellow, rarely green, blue, or red; the
thread is usually white, except in the fuchsia and some others. When the
pollen is ripe the chambers dehisce to emit it. The opening is usually a
slit between the valves which close the chambers. The length of the slit
is variable, and the form of the pollen grains is equally so; they are
commonly ellipsoidal, and attenuated at the extremities; but in all
cases they are beautiful microscopic objects.
The contents of the pollen grains consist of a liquid containing
mucilaginous granules, which is in circulation. The granules increase in
number towards the period of maturation, mingled with oil globules, and
frequently starch. The circulation of the fluid ceases long before
maturation in all cases, except the Zostera; but the granules of the
contents exhibit an active molecular motion often within the pollen
cell, and always after expulsion even in pollen taken from dried
specimens. After the liquid ceases to circulate, it becomes concentrated
and contracted; and when the pollen is only enclosed in one delicate
membrane, it simply bursts the vesicle; but, as most pollen grains have
a double coat, they burst open at the slits, or pores, on their surface,
and through these the internal membrane, in the form of a tube, is
protruded, and emits its contents; but it may be projected to a
considerable distance without bursting.
The pistil in a very young state is a greenish concave body scarcely to
be distinguished from a nascent leaf; it becomes more concave; and
finally the borders meet and unite, so that a hollow body is formed,
which ultimately becomes a perfect pistil consisting of three parts, the
apex or stigma, the style, and the ovary, in which the ovules, or unripe
seeds, are formed and fertilized, the latter afterwards becoming the
fruit.
The ovary is composed of an outward skin containing cellular tissue and
vascular bundles of spiral vessels, which, running upwards, converge
towards the style; they vary in number, and are sometimes ramified. A
single ovule, which is an unripe seed, may be produced within the ovary,
or the ovary may be divided into two or more compartments, in each of
which an ovule may be formed and attached to the ovary by a mass of
cellular tissue, and not unfrequently by a single thread.
The style, when examined with a microscope, is a hollow tube, or canal,
extending from the cavity of the ovary to the stigma. It consists of
cellular tissue with vascular bundles near the circumference, which pass
upwards in straight lines, and end below the summit. In certain cases
the canal is open; in others it is obstructed by lax cellular tissue
having many gaps. The stigma is an expansion of that lax tissue at the
point where the canal opens externally. At the time of fructification,
the canal of the style is occupied by soft humid elongated cells mixed
with a viscous fluid, which exudes upon the surface of the stigma,
rendering it moist and glutinous. When the pollen grains adhere to that
humid substance, the tubular extension of their inner lining, with its
contents, passes down the style and fertilizes the unripe seeds.
After fructification the anthers, stigma, and conducting tissues wither,
yet, in many fruits, additions are made to the ovary and its contents by
the remains of some of the other parts of the inflorescence. In the
apple, which is a simple fruit, the skin of the calyx forms the skin of
the apple, and the flesh or edible part is developed out of the ovary
and the remainder of the calyx, while the inner layer of the ovary forms
the horny cells containing the ripe seeds. In the strawberry, which is a
compound fruit, the pulp is the enlarged fleshy receptacle bearing the
simple fruit on its surface. In the orange, the membranous partitions of
its segments are the linings of the ovary, and the pulp is formed of lax
large-celled cellular tissue developed within it.[77] All fruits have
the mark of the style; it is very evident in the apple and orange.
Simple fruits are formed by a single flower; multiple fruits, like the
ananas, or pine-apple, and fir cone, are formed of masses of
inflorescence in a state of adhesion.
Spiral vessels are frequently found passing from the fleshy part of
fruits into the seeds; they are very numerous in the seeds of the
Collomia grandiflora and others, coiled up and compressed by the outer
skin; but they start to their full length as soon as released. The coats
of the seeds of various plants, when seen through a microscope, are
beautifully marked. That of the Bignonia is surrounded by a fringe of
elongated spiral vessels, the seed of the poppy has a reticulated
surface, and many have wings like those of the ash tree, or down like
the thistle, that they may be dispersed by the wind. The tendency of
roots to strike downwards is so great, that seeds right themselves
whichever way they may fall.