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); B. Cobb, A Field Guide to the Ferns (1956); F. S. Shuttleworth and H. S. Zim, Non-flowering Plants (1967); F. E. Round, Introduction to the Lower Plants (1969); D. L. Jones Encyclopedia of Ferns (1987).
A fern is any one of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. The group is also referred to as Polypodiophyta, or Polypodiopsida when treated as a subdivision of tracheophyta (vascular plants). The study of ferns and other pteridophytes is called pteridology, and one who studies ferns and other pteridophytes is called a pteridologist. The term "pteridophyte" has traditionally been used to describe all seedless vascular plants, making it synonymous with "ferns and fern allies". This can be confusing since members of the fern phylum Pteridophyta are also sometimes referred to as pteridophytes.
The stereotypic image of ferns growing in moist shady woodland nooks is far from being a complete picture of the habitats where ferns can be found growing. Fern species live in a wide variety of habitats, from remote mountain elevations, to dry desert rock faces, to bodies of water or in open fields. Ferns in general may be thought of as largely being specialists in marginal habitats, often succeeding in places where various environmental factors limit the success of flowering plants. Some ferns are among the world's most serious weed species, including the bracken fern growing in the British highlands, or the mosquito fern (Azolla) growing in tropical lakes, both species form large aggressively spreading colonies. There are four particular types of habitats that ferns are found in: moist, shady forests; crevices in rock faces, especially when sheltered from the full sun; acid wetlands including bogs and swamps; and tropical trees, where many species are epiphytes.
Many ferns depend on associations with mycorrhizal fungi. Many ferns only grow within specific pH ranges; for instance, the climbing fern (Lygodium) of eastern North America will only grow in moist, intensely acid soils, while the bulblet bladder fern (Cystopteris bulbifera), with an overlapping range, is only ever found on limestone.
Like the sporophytes of seed plants, those of ferns consist of:
The gametophytes of ferns, however, are very different from those of seed plants. They typically consist of:
One interesting difference between sporophytes and gametophytes might be summed up by the saying that "Nothing eats ferns, but everything eats gametophytes." This is an over-simplification, but it is true that gametophytes are often difficult to find in the field because they are far more likely to be food than are the sporophytes.
Ferns have traditionally been grouped in the Class Filices, but modern classifications assign them their own division in the plant kingdom, called Pteridophyta.
Traditionally, three discrete groups of plants have been considered ferns: two groups of eusporangiate ferns--families Ophioglossaceae (adders-tongues, moonworts, and grape-ferns) and Marattiaceae--and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with a large, fleshy rhizome, and are now thought to be a sibling taxon to the main group of ferns, the leptosporangiate ferns. Several other groups of plants were considered "fern allies": the clubmosses, spikemosses, and quillworts in the Lycopodiophyta, the whisk ferns in Psilotaceae, and the horsetails in the Equisetaceae. More recent genetic studies have shown that the Lycopodiophyta are only distantly related to any other vascular plants, having radiated evolutionarily at the base of the vascular plant clade, while both the whisk ferns and horsetails are as much "true" ferns as are the Ophioglossoids and Marattiaceae. In fact, the whisk ferns and Ophioglossoids are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade. Molecular data - which remain poorly constrained for many parts of the plants' phylogeny - have been supplemented by recent morphological observations supporting the inclusion of Equisetaceae within the ferns, notably relating to the construction of their sperm, and peculiarities of their roots (Smith et al 2006, and references therein).
One possible means of treating this situation is to consider only the leptosporangiate ferns as "true" ferns, while considering the other three groups as "fern allies". In practice, numerous classification schemes have been proposed for ferns and fern allies, and there has been little consensus among them. A new classification by Smith et al. (2006) is based on recent molecular systematic studies, in addition to morphological data. This classification divides ferns into four classes:
The last group includes most plants familiarly known as ferns. Modern research supports older ideas based on morphology that the Osmundaceae diverged early in the evolutionary history of the leptosporangiate ferns; in certain ways this family is intermediate between the eusporangiate ferns and the leptosporangiate ferns.
The complete classification scheme proposed by Smith et al. (2006; alternative names in brackets):
Ferns of the genus Azolla are very small, floating plants that do not look like ferns. Called mosquito fern, they are used as a biological fertilizer in the rice paddies of southeast Asia, taking advantage of their ability to fix nitrogen from the air into compounds that can then be used by other plants.
A great many ferns are grown in horticulture as landscape plants, for cut foliage and as houseplants, especially the Boston fern (Nephrolepis exaltata). The Bird's Nest Fern, Asplenium nidus, is also popular, and the staghorn ferns, genus Platycerium, have a considerable following.
Several ferns are noxious weeds or invasive species, including Japanese climbing fern (Lygodium japonicum), mosquito fern and sensitive fern (Onoclea sensibilis). Giant water fern (Salvinia molesta) is one of the world's worst aquatic weeds. The important fossil fuel coal consists of the remains of primitive plants, including ferns.
Ferns have been studied and found to be useful in the removal of heavy metals, especially arsenic, from the soil
Other ferns with some economic significance include:
In Slavic folklore, ferns are believed to bloom once a year, during the Ivan Kupala night. Although it's exceedingly difficult to find, anyone who takes a look of a fern flower will be happy and rich for the rest of his life. Similarly in Finland, the tradition holds that one who finds the seed of a fern in bloom on Midsummer night, will by the possession of it be able to travel under a glamour of invisibility and shall be guided to the locations where eternally blazing Will o' the wisps mark the spot of hidden treasure caches.
"Pteridomania"' is a term for the Victorian era craze of fern collecting and fern motifs in decorative art including pottery, glass, metals, textiles, wood, printed paper, and sculpture "appearing on everything from christening presents to gravestones and memorials." The fashion for growing ferns indoors led to the development of the Wardian case, a glazed cabinet that would exclude air pollutants and maintain the necessary humidity.
The dried form of ferns was also used in other arts, being used as a stencil or directly inked for use in a design. The botanical work, The Ferns of Great Britain and Ireland, is a notable example of this type of nature printing. The process, patented by the artist and publisher Henry Bradbury, impressed a specimen on to a soft lead plate. The first publication to demonstrate this was Alois Auer's The Discovery of the Nature Printing-Process.
In addition, the book Where the Red Fern Grows has elicited many questions about the mythical "red fern" named in the book. There is no such known plant, although there has been speculation that the oblique grape-fern, Sceptridium dissectum, could be referred to here, because it is known to appear on disturbed sites and its fronds may redden over the winter.