The position of the seed as regards the pericarp resembles that of the ovule in the ovary, and the same terms are applied—erect, ascending, pendulous, suspended, curved, etc. These terms have no reference to the mode in which the fruit is attached to the axis. Seeds exhibit great varieties of form. They may be flattened laterally (compressed), or from above downwards (depressed). They may be round, oval, triangular, polygonal, rolled up like a snail, as in Physostemon, or coiled up like a snake, as in Ophiocaryon paradoxum.
The endosperm formed in the embryo-sac of angiosperms after fertilization, and found previous to it in gymnosperms, consists of cells containing nitrogenous and starchy or oily material, destined for the nutriment of the embryo. It occupies the whole cavity of the embryo-sac, or is formed only at certain portions of it, at the apex, as in Rhinanthus, at the base, as in V accinium, or in the middle, as in Veronica. As the endosperm increases in size along with the embryo-sac and the embryo, the substance of the original nucellus of the ovule is gradually absorbed. Some times, however, as in Zingiberaceae, no endosperm is formed; the cells of the original nucellus, becoming filled with f ood materials for the embryo, are not absorbed, but remain surround ing the embryo-sac with the embryo, and constitute the perisperm. Again, in other plants, as Nymphaeaceae, both endosperm and perisperm are present. There is a large class of plants in which although at first after fertilization a mass of endosperm is formed, yet, as the embryo increases in size, the nutrient matter from the endospermic cells passes out from them, and is absorbed by the cells of the embryo plant. In the mature seed, in such cases, there is no separate mass of tissue containing nutrient food-material apart from the embryo itself. Such a seed is said to be exal buminous, as in most Leguminosae. When either endosperm or perisperm or both are present the seed is said to be albuminous.
The albumen varies much in its nature and consistence, and furnishes important characters. It may be farinaceous or mealy, consisting chiefly of cells filled with starch, as in cereal grains, where it is abundant ; fleshy or cartilaginous, consisting of thicker cells which are still soft, as in the coconut, and which sometimes contain oil, as in the oily albumen of Ricinus and poppy ; horny, when the cell-walls are slightly thickened and capable of swelling, as in date and cof fee; the cell-walls sometimes become greatly thickened, filling up the testa as a hard mass, as in vegetable ivory (Phytele phas). A cavity is sometimes left in the centre which is usually filled with fluid, as in the coconut. The relative size of the embryo and of the endosperm varies much. In monocotyledons the embryo is usually small, and the endosperm large, and the same is true in the case of coffee and many other dicotyledons. The opposite is the case in other plants, as in the Labiatae.
The embryo consists of an axis bearing the cotyledons or the first leaves of the plant. To that part of this axis immediately
beneath the cotyledons the term hypocotyl has been applied, and continuous backwards with it is the young root or radicle, the descending axis, their point of union being the collar or neck.
The terminal growing bud of the axis is called the plumule, and represents the ascending axis.
The radicular extremity points towards the micropyle, while the cotyledonary extremity is pointed towards the base of the ovule or the chalaza. Hence, by ascer taining the position of the micro pyle and chalaza, the two ex tremities of the embryo can in general be discovered. It is in many cases difficult to recognize the parts in an embryo ; thus in Cuscuta, the embryo appears as an elongated axis without di visions; and in Caryocar the mass of the embryo is made up by the radicular extremity and hypocotyl, in a groove of which the cotyledonary extremity lies embedded. In some monocotyledonous embryos, as in Orchidaceae, the embryo is undifferentiated, being a mere cellular mass showing no parts. When the embryo follows the direction of the axis of the seed, it is axile or axial; when it is not in the direction of the axis, it be comes abaxile or abaxial. In campylotro pous seeds the embryo is curved, and in place of being embedded in endosperm, is frequently external to it.
It has been already stated that the radi cle of the embryo is directed to the micro pyle, and the cotyledons to the chalaza. In some cases, by the growth of the integu ments, the former is turned round so as not to correspond with the apex of the nucellus, and then the embryo has the radicle directed to one side, and is called excentric, as is seen in Primulaceae and many palms, especially the date. The position of the embryo in different kinds of seeds varies. In an orthotropal seed the embryo is inverted or antitropal, the radicle pointing to the apex of the seed, or to the part opposite the hilum. Again, in an anatropal seed the embryo is erect or homotropal, the radicle being directed to the base of the seed. In curved or campylotropal seeds the embryo is folded so that its radicular and cotyledonary extremities are approximated, and it becomes amphitropal. In this instance the seed may be exalbuminous, and the embryo may be folded on itself ; or albuminous, the embryo surrounding more or less completely the endosperm and being peripherical. According to the mode in which the seed is attached to the pericarp, the radicle may be directed upwards or downwards, or laterally, as regards the ovary. In an orthotropal seed attached to the base of the pericarp it is superior as also in a suspended anatropal seed. In other anatropal seeds the radicle is inferior. When the seed is horizontal as regards the pericarp, the radicle is either centrifugal, when it points to the outer wall of the ovary; or cen tripetal, when it points to the axis or inner wall of the ovary. These characters are of value for classification, as they are often constant in large groups of genera.