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Seeds (Unfertifised ovule see Figs)

In the unfertifised ovule (see Figs. 466 and 467), there are generally two coasts, surrounding a central cellular mass called the nucellus; but there is a passage (micropyle) at the true apex of the ovule, through both coats. In the middle of the nucellus lies a large cell, the embryo-sac, formed by the union of two or more cells. Within this sac are several cells (see Fig. 468), viz., at the end next the micropyle two oval or elongated cells, the "helper cells," with a round cell (two in Santalum album) at their inner end, called the embryonal vesicle. At the opposite end of the embryo-sac are three others, called antipodal cells, which are believed to be the last traces of a prothallus, such as is formed in Selaginella among Vascular Cryptogams. Among the protoplasm that lines the walls of the embryo-sac as a thin layer, two nuclei may generally be distinguished. After fertilisation, i.e., after the pollen tube has reached the embryonal vesicle through the micropyle, the helper cells and the antipodal cells disappear; the cell wall of the embryonal vesicle becomes thicker, and the cell grows, and becomes divided by cross-walls. At last it forms a cellular mass, the embryo, on the end of the suspensor, which part in most plants is a row, but in some is a mass, of cells. In the embryo of the ripe Seed, it is generally easy to make out radicle, stem, plumule, and one or two cotyledons. Thus, in it all the vegetative organs of the future plant are represented, except in a few families (e.g., Orchids), in which the embryo is only a mass of cells, without distinction of parts. In Coniferoe, the course of development of the ovule into the Seed is different in several respects; but the result is much the same as regards the structure of the mature Seed. While the embryo is in course of development, the nuclei of the embryo-sac give rise to the endosperm. At first, this consists of cells free from one another, but soon they come into contact, and, by their union, form a tissue that quite resembles those formed by ordinary cell-divisions, and that does itself increase in size in this way. The endosperm, also called the albumen, is formed in all Seeds; but in many it disappears, as it is used up in the growth of the embryo, e.g., in the Seed of an Apple, or of a Bean. Such Seeds are called "exalbuminous." In many Seeds a considerable, or even a large, albumen exists in the ripe Seed, with the embryo lying in it (Polygonum), or around it (Lychnis), or at the base (Carex), or on one side of it (Grasses). Besides the endosperm, another form of albumen, known as "perisperm," occurs in many plants. This is formed in the tissue of the nucleus, outside the embryo-sac. Perisperm is less common than endosperm, but co-exists with the latter in the ripe Seeds of a few plants (Nymphoea), and occasionally replaces it (Canna). The albumen may contain starch, oil, cellulose, or other materials for nourishment of the young plant; and its texture varies accordingly. The presence and nature of the albumen afford important characteristics of Natural Orders. Under Ovule, it was pointed out that the form of the ovule might remain orthotropous, or might become campylotropous or anatropous. The ripe Seeds show corresponding differences in form.

Seeds generally possess two coats, the outer called "testa," and the inner "tegmen." The latter is usually thin and membranous. The testa is very often thick and hard, e.g. in Brazil Nuts. It often bears outgrowths in the form of warts or ridges, hairs (Cotton), or wings, which are sometimes of large size (Pinus, Bignonia). All these outgrowths bear reference to the modes of distribution of Seeds, fitting them to adhere to the bodies of birds or quadrupeds, or to be distributed by wind. Occasionally, the testa has an outer layer of cells, which become mucilaginous on contact with water, and cause Seeds, when moist, to adhere to the bodies of animals. On the outer surface of the testa is the hilum, or scar, indicating the point of attachment of the Seed to its stalk. Many Seeds possess what resembles a third coat when ripe. In some, this coat, the "arillus," grows up from the placenta around the Seed; in others, it grows from the micropyle downwards, and it is then distinguished as the "arillode." An outgrowth is often seen in anatropous ovules (e.g., in Violets) over the funiculus (the adherent stalks); it is called a "caruncle." This third coat, whether arillus, arillode, or caruncle, in some Seeds is fleshy (e.g., mace of Nutmeg), serving as an inducement to animals to swallow the Seeds, and thus to secure wide distribution in their excrements; in others, it forms a tuft of hairs, and serves as a float for the Seed. In size and form, Seeds vary greatly in different plants, from the minute, dust-like Seeds of Orchids to the large Seed of the Cocoanut. The number produced by different plants is also very variable; but into these and similiar matters it is not possible to enter here.