SEED. The seed is formed from the ovule as the result of fertilization (q.v.). It is contained in a seed vessel (see FRUIT) which eventually forms merely a thin layer around the sac, or completely disappears. The remainder of the nucellus and the integuments of the ovules form the seed-coats. In some cases a delicate inner coat or tegmen can be distinguished from a tougher outer coat or testa; often, however, the layers are not thus separable. The consistency of the seed-coat, its thickness, the character of its surface, etc., vary widely, the variations being often closely associated with-the means ofseed-dispersal. An account of the develop ment of the seed from the ovule will be found in the article ANGIOSPERMS. When the pericarp is dehiscent the seed-covering is of a strong and often rough character ; but when the pericarp is indehiscent and encloses the seed for a long period, the outer seed-coat is thin and soft. The cells of the testa are often coloured, and have projections and appendages of various kinds. Thus in Abrus Precatorius, it is a bright red ; in French beans it is beautifully mottled ; in Asclepias it has hairs attached to it ; and in Bignonia and the pines and firs it is expanded- in the form of wing-like appendages. In Collomia and other seeds, it contains spiral cells, from which, when moistened with water, the fibres uncoil in a beautiful manner; and in flax (Linum) and others the cells swell up and become mucilaginous. These structural peculiarities of the testa in different plants have relation to the scattering of the seed. But in some plants the pericarps subserve the same purpose ; this especially occurs in small pericarps enclosing single seeds, as achenes, caryopsides, etc. Thus in Compositae, the pappose limb of the calyx forms a parachute to the pericarp ; and the epicarp is prolonged as a wing in Fraxinus and Acer.
Sometimes there is an additional covering to the seed, formed after fertilization, to which the name arillus has been given. This is seen in the passion-flower. In the nutmeg this additional coat constitutes a laciniated scarlet covering called mace. The fleshy formed from the ovaries in the plants called angiospermous; while in gymnospermous plants, such as Coniferae and Cycadaceae, it is naked, or, in other words, has no true pericarp. It sometimes
happens in angiosperms, that the seed vessel is ruptured at an early period of growth, so that the seeds become more or less exposed during their devel opment; this occurs in mignonette, where the capsule opens at the apex, and in Cuphea, where the placenta bursts through the ovary and floral envelopes, and appears as an erect process bearing the young seeds. After fertilization the ovule is greatly changed as a result of the formation of the embryo. In the embryo-sac of most angiosperms (q.v.) there is a development of cellular tissue, the endosperm, more or less filling the embryo-sac. In gymnosperms (q.v.) the endo sperm or embryo sac tissue is formed preparatory to fertilization. The fertilized egg enlarges and forms the embryo. The embryo scarlet covering formed around the naked seed in the yew is by some considered of the nature of an aril. On the testa, at various points, there are produced at times other small outgrowths to which the name caruncles has been given. The funicles of the ovules frequently attain a great length in the seed, and in some magnolias, when the fruit dehisces, they appear as long scarlet cords suspending the seeds. The hilum or umbilicus of the seed is usually well marked, as a scar of varying size ; in the calabar bean it extends along a large portion of the edge of the seed; it frequently exhibits marked colours, being black in the bean, white in many species of Phaseolus, etc. The micropyle of the seed may be recognizable by the naked eye, as in the pea and bean tribe, etc., or it may be minute or microscopic. It indicates the true apex of the seed, and is important as marking the point to which the root of the embryo is directed. At the micropyle in the bean is observed a small process of integument, which, when the young plant sprouts, is pushed up like a lid. The chalaza is often of a different colour from the rest of the seed. In the orange it is reddish-brown, and is easily recognized at one end of the seed when the integuments are carefully removed.