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Ovule (Form selected for description)

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.