megasporangium

Meagasporangium is the sporangial structure that produces megaspores in heterosporous plants. In seed plants, the megasporangium is the nucellus within the ovule, usually surrounded by integuments (in gymnosperms typically a single integument; in many angiosperms two). A diploid cell called the megasporocyte undergoes meiosis to produce four haploid megaspores. Typically only one megaspore remains functional; the others degenerate. The functional megaspore develops into the female gametophyte (megagametophyte), which contains the egg-producing apparatus.

In gymnosperms, the megagametophyte is nourished within the ovule and provides nutrients to the developing embryo until fertilization; in angiosperms, the embryo sac is the reduced megagametophyte within the ovule. The integuments surrounding the nucellus form the seed coat after fertilization, and the micropyle is the opening through which pollen enters in many species. The megasporangium, together with integuments and nucellus, thus forms the ovule, which can develop into a seed following fertilization.

In heterosporous ferns and some lycophytes, megasporangia occur on sporophylls and produce megaspores that germinate into female gametophytes; these lineages illustrate the ancestral origin of megasporangia in seed plants.