Marsileaceae, only family of the fern order Marsileales, comprising three genera and about 70 species of small aquatic and semi-aquatic ferns. The plants are found nearly worldwide, and root in mud or grow in shallow water. Members of the family do not resemble other ferns and are frequently confused with flowering plants such as clovers.
The family is typified by spore-bearing structures (sporangia) in hard cases (sporocarps) produced at or beneath ground level at the bases of the leaves. The sporocarps are extremely long-lived; Marsilea (water clover) sporocarps more than 100 years old have been grown successfully to give rise to new sporophytes.
Marsilea, with about 50 species, has cloverlike leaves with four leaflets and is widely distributed. Pilularia (pillwort), also nearly cosmopolitan, with six species, has threadlike (filiform) leaves without leaflets. Regnellidium, with one species, has leaves with two leaflets and is confined to southern portions of Brazil and Argentina.