Ovule

An ovule is the structure in seed plants that develops into a seed after fertilization. In flowering plants, ovules are located inside the ovary and are attached to it by a stalk called the funiculus. Each ovule is enclosed by protective layers called integuments, surrounding the nucellus (megasporangium). The opening at the end of the integuments, the micropyle, provides access for the pollen tube to the female gametophyte.

Inside the nucellus, a megaspores mother cell undergoes meiosis to produce megaspores, typically with only one surviving megaspore that develops into the embryo sac, the female gametophyte. In most angiosperms the embryo sac is seven-celled and eight-nucleate, containing an egg cell, two synergids, three antipodal cells, and a central cell with two polar nuclei. Fertilization usually involves two sperm cells: one fertilizes the egg to form a zygote, while the other fuses with the polar nuclei to form the triploid endosperm, providing nourishment to the developing embryo. Following fertilization, the ovule matures into a seed, the integuments become the seed coat, and the surrounding ovary develops into fruit in flowering plants. The micropyle often remains as a pore through which germination or pollen tube access occurs.

In gymnosperms, ovules are typically exposed on cone scales rather than enclosed in an ovary; they still contain a nucellus and one or more integuments, and after fertilization the ovule develops into a seed with nutritive tissue derived from the female gametophyte rather than a true endosperm. The ovule thus represents the essential structure that links female gametophyte development with seed formation.