A small order of heterosporous, leptosporangiate ferns (division Polypodiophyta) which grow in water or wet places and are rooted to the substrate. The leaves arise on long stalks from the rhizome and typically have floating blades with four leaflets, suggesting a four-leaved clover. The sporangia are enclosed in modified folded leaves or leaf segments called sporocarps. Megasporangia and microsporangia are borne in the same sporocarp. The order contains a single family and only 3 genera, about 50 species in all, most of them belonging to the widespread genus Marsilea (water clover).