Ovule
The name given to that body which develops into the seed, after it has been acted on by the pollen. The Ovules are contained in the Ovary (which see) in all flowering plants, except the Coniferous trees and shrubs, the Cycads, so commonly grown in hot-houses, and a few other mostly tropical plants that form the natural order Gnetaceae. The ovary in almost all Phanerogams is closed, so as to entirely conceal the Ovules; but in a few it is open at the top from a very early period.
The Ovules are attached to the placentae to which they are fixed by a stalk, or "funiculus." They are usually very small, and are frequently so translucent as to allow the microscopic structure to be seen, either without special preparation, or, after laying them for a little time in some fluid (e.g., weak solution of caustic potash), to render them more transparent. Others require more elaborate treatment, and must be cut open before they are fit to be examined; but to enter into details would occupy too much space. The description here must, therefore, be restricted to one of the more simple forms of Ovules, afterwards pointing out in what respects, chiefly, other forms depart from this type. The Ovule selected for description is one that remains straight from its earliest appearance till its full maturity, such as is found in the Docks and the Buckwheat. The Ovule first appears like a small, rounded or egg-shaped swelling on the placenta, made up of cells alike in form and contents. This body is usually termed the "nucleus" of the Ovule; but the term "nucellus," now coming into use, is preferable, since "nucleus" should be restricted to the body in the protoplasm of living cells generally known under this name. Round the base of the nucellus a cellular ring soon begins to appear, and continues to grow up to form a thin covering around that body. But, while this has been growing, another covering forms over it also from below upwards, and the two grow till they leave only a minute opening through them at the upper end (i.e., farthest from the placenta), called the "micropyle," which allows the entrance of a slender tube from the pollen grain. The two coverings are named the inner and the outer coats. At the end next the placenta there is, generally, a rarrower neck, or there may be a well-formed stalk, or funiculus; though sometimes there is no visible stalk, the Ovule being sessile, or fixed by a broad base. From the woody, or fibrovascular, bundle in the placenta, a branch runs into the stalk of the Ovule. The part of the Ovule where the coasts are grown to the nucellus is the chalaza. At this part, separation takes place when the seed is ripe, and falls from the stalk; and a scar is left, known as the hilum. In the nucellus, a change goes on which results in the formation of a cell-the embryo-sac--very much larger than the others. It lies near the micropyle, from which, in most Ovules, it is separated by one or two layers of cells of the nucellus. It may be almost as long as the nucellus, and may, in course of growth, crush the latter, until all its tissue disappears.
This cell is full of protoplasm; and in the latter may be seen several bodies, as follows At the end next the micropyle are three cells, viz., two rather long ones, known as helper cells; and, at their lower end, a round cell--the embryonal vesicle--from which the embryo is afterwards developed. At the opposite end of the embryo-sac lie three small bodies--the antipodal cells--the use of which is unknown. They are believed to represent a structure that plays an important part in the development of Ferns and their allies, but that has become superseded in the higher plants. In the protoplasm between them lie one or two nuclei, which have to assist in the formation of the endosperm while the embryo, or young plant, is being formed. A brief account of the development of the Ovule into the seed will help to make these various structures more intelligible.
When the stigma is ready to receive the pollen grains, and one of these is placed upon it, the grain very soon, in most plants, pushes a slender pollen tube between the cells of the stigma, down the loose "conducting tissue" of the style, and into the cavity of the ovary, where it finds its way to the micropyle of an Ovule. Passing down this, it pushes the thin coat of the embryosac before it, presses between the helper cells, and reaches the embryonal vesicle. It is believed that a part of the protoplasm passes from the tube into one of the helper cells, which then acts in turn upon the vesicle. The helper cells, and, probably, the antipodal cells, disappear; and the embryonal vesicle begins to grow, and to be divided by cell-walls, so that a rounded mass of tissue is formed, attached above to a row of cells, or to a mass in some plants, called the suspensor. The lower rounded mass is the embryo, which increases in size, and shows beginnings of the axis and leaves, or cotyledons. These parts can easily be made out in most plants. In the meanwhile, after fertilisation has been effected, the nuclei in the protoplasm begin to divide, and to from new nuclei, around which cells from, and the embryo-sac becomes filled, more or less completely, with cellular endosperm. This may disappear in the ripening of the seed (e.g., in Beans); or it may remain, and may form a large part of the seed (e.g., in Wheat), in which case it is often called the albumen of the seed.
DEVIATIONS FROM THE ABOVE TYPE OF STRUCTURE AND DEVELOPMENT. Structure. In many plants (e.g., Actaea, Compositae, &c.) there is only one coat on the Ovule in a few (e.g., Mistletoe) there is no coat, the nucellus being exposed in the cavity of the ovary. The funiculus varies much; in some Ovules it is very long while others do not possess it. Most Ovules have it closely adherent to one side, the Ovule being inverted, or anatropous, as in Actaea, so as to bring the micropyle close to the placenta, while the Ovule itself remains straight other Ovules are curved or campylotropous as in the Pea and in Lychnis; so that in them also the micropyle is brought near the placenta, but the funiculus is only adherent slightly to one side of the Ovule. The form selected for description is called erect, or orthotropous The Ovules of orchids have no fibrovascular bundle in the funiculus.
Development. This shows peculiarities in certain plants. In the Ovules of Santalum album, and occasionally in those of some orchids, there are two embryonal vesicles, so that two embryos may form in a seed. In seeds of Oranges, of the Spindle-tree, of Funkias, and of certain other plants, it is usual to find two, or even three or four, embryos. But this results from budding from the tissues of the nucellus of several small cellular growths, which push the wall of the embryo-sac before them, and occupy the place of the true embryo: the latter is usually crushed by them, and perishes. This process has been called Polyembryony. It is a case of vegetative replacing true sexual reproduction, and, by its discovery two or three years ago, various apparent anomalies in hybridisation have been explained.
The processes of fertilisation, and of subsequent development, in the Conifers and other Gymnosperms, are so different in many respects from those above described, as to call for a brief notice. The Ovules are not inclosed in carpels, as in other flowering plants; but in general appearance and structure they do not show any marked peculiarities as contrasted with the Ovules of these plants. They are orthotropous in some (Yew), and anatropous in others (Pines, &c.). They have only one coat. The contents of the embryo-sac, and the development of the embryo, are markedly different from what we find in other Phanerogams. At an early stage, the sac is filled with protoplasm, in which lies a single nucleus; but this very soon divides repeatedly (as in other Phanerogams to form the endosperm), and the sac may thus become more or less full of new cells, the temporary endosperm, ranged along the wall, or occupying most of the cavity. Certain of these cells near the micropyle do not divide like the others, but remain larger than them. Each of these large cells after a time divides into a small cell above and a larger one below. The small cell may remain undivided, but usually divides into a rosette of four (e.g., Juniper), or into more, arranged in two or three layers (e.g., Pines), forming the neck, or rosette cells. In each case, an opening is left between them down to the larger central cell. The whole body formed by the group of cells is known as the corpusculum. The number of corpuscula varies from three to fifteen. The pollen grain lies directly upon the micropyle, with no apparent effect, in most cases, for a long time; but at last it pushes a tube down to the embryo-sac, which is pierced, allowing the end of the tube to lie directly on the rosette cells, and to push between them into the upper end of the large central cell. Some of the substance passes into the latter cell, part becoming diffused, and part remaining visible for a time as an upper nucleus; but this nucleus afterwards becomes united with the true nucleus of the cell, to form a new nucleus. Then follows a series of cell divisions at the lower end of the central cell, which result, in general, in the formation of four rows of cells, side by side, lengthwise. The lowest cell of each row divides, to from an embryo. Thus, it is the rule for Gymnosperms to produce several embryos in a seed; but in most seeds only one embryo is fully developed, the others being destroyed by pressure of the one upon them. This one developed is nourished largely at the expense of the endosperm.
Much use is made in Systematic Botany of the differences in structure, form, and development of and in Ovules.
The meaning of the lettering in the sections of Ovules is as follows; a, outer coat; b, inner coat; c, nucellus; d, embryo sac; e, micropyle; f, stalk, or funiculus; g, fibrovascular bundle of stalks; h, chalaza; i, placenta.