The archegonium is a flask-shaped female sex organ of all plants except algae. Bryophytes (liverworts, hornworts, and mosses), pteridophytes (club mosses, ferns, and horsetails), and many gymnosperms (seed-bearing plants) have archegonia. Female gametes (ova) develop in the base of the "flask." At fertilization the cells forming the "neck" of the flask disintegrate, allowing male gametes (antherozoids) to enter.