The majority of the common living ferns are members of the polypody family (Polypodiaceae), usually characterized by the familiar triangular fronds subdivided into many leaflets (pinnae) and smaller pinnules. A popular house fern, a drooping-leaved variety of Nephrolepis exaltata, a tropical sword fern, is called the Boston fern (var. bostoniensis ) because it was first found in a shipment of sword ferns received in Boston. The maidenhair ferns ( Adiantum ), with a few species native to North America, were formerly used as a cure for respiratory ailments. The Brazilian A. cuneatum and its numerous varieties are now the major greenhouse ferns in North America. The most familiar of all woodland ferns, found the world over, is Pteridium aquilinum, the common bracken, or brake (names also applied to other similar ferns, especially species of Pteris ). Other North American woodland ferns include the Christmas fern ( Polystichum acrostichoides ), a dark-green evergreen plant; the walking fern ( Camptosorus rhizophyllus ), native to limestone areas and named for its characteristic vegetative reproduction, in which new plantlets root from the tips of the elongated fronds; and the common polypody ( Polypodium vulgare ), called also wall, or boulder, fern, a low, matted plant that is the most common of the rock-inhabiting ferns. Also included in the polypody family are many of the mostly tropical fern epiphytes. Some ferns of other families are aquatic. Among the better known aquatic genera are Marsilea and Salvinia, cultivated in aquariums; giant salvinia, S. molesta, native to South America, and common salvinia, S. minima, native to Central and South America, are prolific aquatic weeds in some S U.S. lakes. The adder's-tongue ferns ( Ophioglossum ) and rattlesnake ferns ( Botrychium ) belong to the most primitive fern family ( Ophioglossaceae ) and bear sporangia not in sori but in spikes arising from the leaves. Dicksonia, Cibotium, and Cyathea are the tree fern genera most frequently seen in greenhouses and conservatories.