Embryo sac

In seed plants, the ovule ("small egg") is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument(s) forming its outer layer(s), the nucellus (or remnant of the megasporangium), and female gametophyte (formed from haploid megaspore) in its center. The female gametophyte—specifically termed a megagametophyte—is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell (or several egg cells in some groups) for the purpose of fertilization. After fertilization, the ovule develops into a seed.

In flowering plants, the ovule is located inside the portion of the flower called the gynoecium. The ovary of the gynoecium produces one or more ovules and ultimately becomes the fruit wall. Ovules are attached to the placenta in the ovary through a stalk-like structure known as a funiculus (plural, funiculi). Different patterns of ovule attachment, or placentation, can be found among plant species, these include:

Ovule orientation may be anatropous, such that when inverted the micropyle faces the placenta (this is the most common ovule orientation in flowering plants), amphitropous, campylotropous, or orthotropous.

In gymnosperms such as conifers, ovules are borne on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate cone (also called megastrobilus). In some extinct plants (e.g. Pteridosperms), megasporangia and perhaps ovules were borne on the surface of leaves. In other extinct taxa, a cupule (a modified leaf or part of a leaf) surrounds the ovule (e.g. Caytonia or Glossopteris).

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