embryosak

Embryo sac, sometimes written embryo-sac or in some languages embryosak, is the female gametophyte of flowering plants (angiosperms). It develops inside the ovule from a megaspore after meiosis of the megaspore mother cell.

In the common Polygonum-type embryo sac, the megaspore mother cell yields four megaspores, typically only one surviving. This megaspore divides by three mitotic rounds to produce eight haploid nuclei arranged as seven cells: an egg cell, two synergids, three antipodal cells, and a central cell containing two polar nuclei. Other species exhibit alternative patterns of megagametogenesis.

The embryo sac functions as the site of fertilization. Pollen tubes deliver two sperm cells; one fertilizes the egg to form a zygote, the other fuses with the central cell to form endosperm, usually triploid, which nourishes the developing embryo.

The embryo sac is central to seed development and reproductive biology of flowering plants and varies among taxa; its structure and development are used in taxonomic studies and in assisted breeding to understand compatibility and fertilization timing.

See also: Megagametogenesis, Double fertilization, Angiosperm reproduction, Ovule.