SECTION XII.GENERAL STRUCTURE OF FLOWERING PLANTS.
Insome 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 thefructifying 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 alwaysafter 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 Bignoniais 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.