Archegonium

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An archegonium (pl: archegonia), from the ancient Greek αρχη (beginning) and γονος (offspring), is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. The archegonium has a long neck and a swollen base. Archegonia are typically located on the surface of the plant thallus, although in the horned liverworts they are embedded.

They are also much-reduced and embedded in the megagametophytes of gymnosperms. The term is not used for angiosperms or the gnetophytes Gnetum and Welwitschia because the megagametophyte is reduced to just a few cells, one of which differentiates into the egg cell. The function of surrounding the gamete is assumed in large part by diploid cells of the megasporangium (nucellus) inside the ovule.

The corresponding male organ is called the antheridium.