Embryo

Embryo, in cormophytic plants the result, direct or indirect, of the fertilisation of the oosphere. In the lower liverworts (q.v.), such as Biccia, this cell, which when fertilised is known as the oospore, divides into octants, periclinal walls afterwards dividing off a peripheral cell-layer, and the whole interior becoming the spore-mother-cells of the sporogonium. In higher forms the basal half of the embryo forms a stalk, which may be expanded below into a "foot," by which the sporogonium or sporophyte generation keeps up a quasi-parasitic connection with the oophyte or previous (sexual) generation. The central part of the upper, or apical, half of the embryo may be non-sporogenous, forming a "columella," over which the sporogenous tissue is developed like a bell. In some of the lowest mosses the development is much the same; but, though the successive cell-divisions occur in a variety of ways, there is generally a distinct wall, columella, and bell-shaped or cylindrical sporogenous layer (archesporium). In the Pteridophyta the oospore gives rise not merely to a sporogonium or fruit-like appendage, but also to the whole leafy plant, and the embryo, whilst still within the cavity (venter) of the arehegonium (q.v.), shows indications of a primary root, a leaf or "cotyledon," the apex of the stem or "plumule." and a foot. In ferns the embryo divides into octants, one of which forms the growing point of the stem, one at the other end the root, two others forming the cotyledon, and two the foot. In Selaginella (q.v.) and in almost all flowering-plants the upper half of the oospore goes to form a temporary chain of cells known as the pro-embryo or suspensor, the embryo arising from the lower half. In flowering plants the embryo has usually so far developed before the seed is ripe that cotyledons, plumule, and radicle are distinct; but some parasites, saprophytes, orchids, etc., have embryos, which remain rudimentary as mere undifferentiated masses of cells until after the dispersal of the seeds. In Gymnosperms the suspensor sometimes separates into parallel cell-chains, each terminated by an embryo, and more than one embryo may begin to develop in a single seed (polyembryony). In Angiosperms the two last cells of the suspensor contribute more or less to form the eenbryo, the development of which is very different in Monocotyledons from what it is among Dicotyledons.1 In the former there is generally one terminal cotyledon and a lateral plumule; in the latter, two cotyledons, one on either side of the apex and a terminal plumule between them. In both cases the root is terminal, being produced at the attached or suspensor end of the embryo and growing out of micropyle (q.v.) in germination (q.v.). In the dicotyledonous embryo it is generally possible at an early stage to distinguish three primitive tissues - viz. an outer layer, the dermatogen or primitive epidermis; a central axis of elongated cells, the plerome, from which pith and flbro-vascular tissue originate; and, between them, the periblem, or primitive cortex. The embryo is nourished by the surrounding tissue of the oophyte in mosses and pteridophytes, i.e. the prothallus in the latter; but in Selaginella and in flowering-plants there are special nutritive tissues, the perisperm (q.v.) and arckisperm, or endosperm (q.v.).