 The Marsileaceae / <unk> / are 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 " <unk> family " or as the " water @-@ clover family " because the leaves of the genus Marsilea superficially resemble the leaves of a four @-@ leaf clover ( a flowering plant ) . Leaves of this fern have sometimes been used to substitute for clover leaves on Saint Patrick 's Day . In all , the family contains 3 genera and 50 to 80 species with most of those belonging to Marsilea . 
