archegonium (pl. archegonia) The multicellular flask-shaped female sex organ of bryophytes, clubmosses, horsetails, ferns, and many gymnosperms. Such plants are described as archegoniate to distinguish them from algae, which do not possess archegonia. The dilated base, the venter, contains the oosphere (female gamete). The cells of the narrow neck liquefy to allow the male gametes to swim towards the oosphere. The archegonium is thus an adaptation to the terrestrial environment as it provides a means for the male gametes to reach the female gamete. Compare antheridium.

archegonium A female sex organ of liverworts, mosses, ferns, and most gymnosperms. It is usually a flask-shaped organ, comprising a swollen base or venter containing a single egg-cell and a slender elongated neck containing one or more layers of cells.