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Ceratophyllaceae (hornworts) A family of submerged, aquatic herbs, which have leaves in whorls of narrow, forked segments. The flowers are small, unisexual, sessile, and solitary in the leaf axils, with a tiny perianth of many narrow lobes. The male flowers have 10–20 stamens, the females a 1-celled ovary forming a 1-seeded nut. There is 1 genus (Ceratophyllum) and 2 species, cosmopolitan in distribution except for the Arctic.