Marsileaceae is a small family of heterosporous aquatic and semi-aquatic ferns, though at first sight they do not physically resemble other ferns. The group is commonly known as the "pillwort family" after the genus Pilularia (pillwort), "pepperwort family" or as the "water-clover family" because the leaves of the genus Marsilea superficially resemble the leaves of a four-leaf clover. The family contains three genera; most of the species are in Marsilea. It is sister to the Salviniaceae, which is also aquatic and heterosporous. Fossils of the family are known as far back as the Triassic. The family is distinctive in producing its spores inside sporocarps, which are bean-shaped structures borne near the bases of the leaves. Some species of Marsilea are grown in garden pools or aquaria.

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