But the value of the seed lies not solely in the reproduction of its kind. There are many species, especially among cultivated plants, that are produced year after year without the agency of seed. A great part of the value of the seed lies in the fact that it is the product of a sexual union. Darwin and others have shown that the union of different individuals is advantageous to the species, and this union can occur only through the agency of the flower, and the effects of it can be propagated only by the seed. The union of different plants produces a progeny with a greater tendency to variation than is possessed by the product of inbreeding. Among wild plants these variations enable a species to adapt itself to new conditions, thus extending its range and increasing its chances of living; in cultivated plants they form the basis upon which the plant breeder works for the improvement of old and the development of new varieties.

Moreover, nearly all our field and garden crops are propagated by seed, and the production of good seed is as essential to continued success in agriculture as good soil or careful cultivation. The production of seed therefore becomes at once a matter of the first importance as well to the originator of new varieties as to him who aims to keep some standard variety true and of the best quality.

That the steps in the production of seed may be clearly understood, a brief description will be given of the parts of a flower and the growth of the seed both before and after fertilization. All seeds are produced by flowers. The flower usually contains two sets of organs, the sexual and the enveloping. Sometimes the latter are partially or wholly wanting, but the sexual organs - that is, the stamens and pistils - must be present either in the same or in different flowers in order that the plant may be fruitful.

The envelopes usually consist of two whorls of floral leaves, the outer or calyx, commonly green and more leafy than the inner whorl or corolla, which is often highly colored and of delicate texture. One or both of these whorls are found in nearly all flowers, and serve the double purpose of protecting the stamens and pistils while in the bud and, after opening, of attracting insects by their bright colors.

Frequently there is only one whorl, which is always the calyx; or both whorls may be absent, in which case the flower is said to be naked. The floral leaves forming the calyx are called sepals; those of the corolla are petals. The pistil or pistils, the female organs, occupy the center of the flower. They are commonly less numerous than the stamens, although this is often due to the union of several pistils into one, forming a compound pistil.

The pistil consists of the ovary at the base, the style, and the stigma at the end of the style. The stigma is the receptive surface on which the pollen falls, and is connected with the ovary by the style. In some flowers the style is suppressed or very short, while in others, as Indian corn, it is long and silky. The stamens, or male organs, are located between the pistil and the envelopes. They vary in number, but are commonly as numerous or twice as numerous as the sepals or petals. By suppression or multiplication these limits are frequently exceeded, and the different species possess from one to an indefinite number of stamens. A stamen consists of a stalk, or filament, and an anther. The anther, which is commonly two-lobed, produces the pollen, the fertilizing element. The mass of pollen, as seen in another consists of a countless number of pollen grains. These may be dry and dust-like, each grain being distinct from every other, or the grains may be sticky and adhere to each other in small, irregular, or sometimes regular, masses. When ripe, the anther opens and the pollen is exposed, to be carried away either by the wind or by insects and other animals.

In the majority of plants both sexes are present in the same flower. In a small number they are in different flowers, either on the same plant or on different plants. In the latter case the plants bearing only male flowers never produce fruit. For the production of seeds a union of both sexes is necessary. This takes place when the pollen grains are deposited on the stigma of the pistil, and grow out into tubes which traverse the style and fertilize the ovules in the ovary. The ovule, the future seed, begins as an outgrowth within a cell of the ovary. As it grows, one or two coats are developed about it. The coats do not entirely surround the ovule, but leave a narrow opening, the foramen or mieropyle, at one end. Through this opening the pollen tube finds its way to the embryo-sac of the ovule. After the union of one of the nuclei of the embryo-sac with that of the pollen tube the embryo begins to form. This is the future plantlet, and is the indispensable portion of every good seed.

After fertilization the ovary rapidly develops into the fruit. In albuminous seeds the embryo never entirely fills the seed, and it may be very small. Sometimes it is without differentiation of parts. In corn and wheat it lies outside of the albumen, which forms the bulk of the seed. The embryo and albumen are surrounded by a seed coat, which protects the delicate parts within, and in some seeds serves a useful purpose in germination.

These parts are readily distinguished in most embryos, viz: radicle, cotyledons, and plumule. The radicle becomes the primary root; the cotyledons or seed leaves either contain the reserve materials of the seed within themselves or they absorb the albumen after germination; the plumule becomes the stem of the plant.