any plant of the division Polypodiophyta. Fern species, numbering several thousand, are found throughout the world but are especially abundant in tropical rain forests. The ferns and their relatives (e.g., the club moss and horsetail) are the most primitive plants to have developed a true vascular system (see plant). The asparagus fern and shrub sweet fern (see bayberry) of florists are not true ferns.
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.
During the Carboniferous era, ancestors to modern ferns were the dominant vegetation of the earth; they contributed to the coal deposits then being formed. Ancient ferns were probably similar to the tree ferns, a declining race found today only in a few tropical areas. Their fronds are clustered at the top of a treelike trunk, sometimes 30 or 40 ft (9–12 m) in height, rather than growing directly from the rootstalk as do those of most temperate ferns.
Ferns reproduce by an alternation of generations (see reproduction), the fern itself being the sporophyte, which produces asexual spores. In most ferns the sporangia (spore-bearing sacs) are borne in clusters (called sori), which appear as brown dots or streaks on the underside of the leaves. Although no present-day ferns reproduce by seeds, there are fossils of some fernlike plants that were seed-producing, and it is believed that the seed plants (e.g., the gymnosperms and true flowering plants) evolved from fernlike ancestors.
The tree ferns (families Dicksoniaceae and Cyatheaceae) are the only living ferns of any commercial importance other than as ornamentals. In the tropics the trunks are employed in construction, and the starchy pith was formerly eaten by the Maoris and other native groups. The dense root systems are widely used as a substrate for growing orchids; many populations of tree ferns are destroyed for this purpose. Dense golden hair covers the base of the leaf stalks and buds in many species and is exported as "pulu" for mattress and pillow stuffing and for packing material. A large number of fern species are used medicinally by local populations, especially in the tropics.
Numerous superstitions have arisen about ferns. The mythical "fern seeds," believed to be produced by the male fern (Dryopteris filix-mas) and by the lady fern (formerly a name for the common bracken but now applied to Athyrium filix-femina), were reputed to create invisibility if eaten by a member of the appropriate sex. The bracken was also considered protection against goblins and witches because the broken stem and root appear to be marked with a C, symbolizing Christ.
Ferns are classified in the division Polypodiophyta, class Polypodiopsida.
- See G. M. Smith, Cryptogamic Botany, Vol. II (2d ed. 1955).
- A Field Guide to the Ferns (1956). ,
- Non-flowering Plants (1967). ; ,
- Introduction to the Lower Plants (1969). ,
- D. L. Jones Encyclopedia of Ferns (1987).
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