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An archegonium (pl: archegonia), from the ancient Greek ἀρχή ("beginning") and γόνος ("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 canal and a swollen base. Archegonia are typically located on the surface of the plant thallus, although in the hornworts they are embedded.
In the moss Physcomitrella patens, archegonia are not embedded but are located on top of the leafy gametophore (s. Figure). The Polycomb protein FIE is expressed in the unfertilised egg cell (right) as the blue colour after GUS staining reveals. Soon after fertilisation the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo.
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. Gymnosperms have their archegonium formed after pollination inside female pine cones (megastrobili).
The corresponding male organ is called the antheridium.
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