fern

 

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.

Common Species

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.

Ancient Ferns

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.

Reproduction

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.

Uses and Lore

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.

Classification

Ferns are classified in the division Polypodiophyta, class Polypodiopsida.

Bibliography

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).

Ferns (Pteridophyta) Fossil range: Late Devonian - Recent
Polystichum setiferum unrolling young frond
Scientific classification
Kingdom: Plantae Division: Pteridophyta
Classes
†Cladoxylopsida Psilotopsida Equisetopsida Marattiopsida Pteridopsida (Polypodiopsida)

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 is called pteridology, and one who studies ferns 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.

Life cycle

Ferns are vascular plants differing from the more primitive lycophytes by having true leaves (megaphylls), and they differ from seed plants (gymnosperms and angiosperms) in their mode of reproduction - lacking flowers and seeds. Like all vascular plants, they have a life cycle, referred to as alternation of generations, characterized by a diploid sporophytic and a haploid gametophytic phase. Unlike the gymnosperms and angiosperms, the ferns gametophyte is a free-living organism. The life cycle of a typical fern is as follows:

1. A sporophyte (diploid) phase produces haploid spores by meiosis;
2. A spore grows by cell division into a gametophyte, which typically consists of a photosynthetic prothallus
3. The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis
4. A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus
5. The fertilized egg is now a diploid zygote and grows by mitosis into a sporophyte (the typical "fern" plant).

Fern ecology

The popular 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.

Fern structure

Like the sporophytes of seed plants, those of ferns consist of:

• Stems: Most often an underground creeping rhizome, but sometimes an above-ground creeping stolon (e.g., Polypodiaceae), or an above-ground erect semi-woody trunk (e.g., Cyatheaceae) reaching up to 20 m in a few species (e.g., Cyathea brownii on Norfolk Island and Cyathea medullaris in New Zealand).
• Leaf: The green, photosynthetic part of the plant. In ferns, it is often referred to as a frond, but this is because of the historical division between people who study ferns and people who study seed plants, rather than because of differences in structure. New leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead. This uncurling of the leaf is termed circinate vernation. Leaves are divided into two types:
  • Trophophyll: A leaf that does not produce spores, instead only producing sugars by photosynthesis. Analogous to the typical green leaves of seed plants.
  • Sporophyll: A leaf that produces spores. These leaves are analogous to the scales of pine cones or to stamens and pistil in gymnosperms and angiosperms, respectively. Unlike the seed plants, however, the sporophylls of ferns are typically not very specialized, looking similar to trophophylls and producing sugars by photosynthesis as the trophophylls do.
• Roots: The underground non-photosynthetic structures that take up water and nutrients from soil. They are always fibrous and are structurally very similar to the roots of seed plants.

The gametophytes of ferns, however, are very different from those of seed plants. They typically consist of:

• Prothallus: A green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3-10 mm long and 2-8 mm broad. The prothallus produces gametes by means of:
  • Antheridia: Small spherical structures that produce flagellate sperm.
  • Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck.
• Rhizoids: root-like structures (not true roots) that consist of single greatly-elongated cells, water and mineral salts are absorbed over the whole structure. Rhizoids anchor the prothallus to the soil.

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.

Evolution and classification

Ferns first appear in the fossil record in the early-Carboniferous period. By the Triassic, the first evidence of ferns related to several modern families appeared. The "great fern radiation" occurred in the late-Cretaceous, when many modern families of ferns first appeared.

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.

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:

• Psilotopsida
• Equisetopsida
• Marattiopsida
• Polypodiopsida

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):

• Class Psilotopsida
  • Order Ophioglossales
    • Family Ophioglossaceae (incl. Botrychiaceae, Helminthostachyaceae)
  • Order Psilotales
    • Family Psilotaceae (incl. Tmesipteridaceae)
• Class Equisetopsida [=Sphenopsida]
  • Order Equisetales
    • Family Equisetaceae
• Class Marattiopsida
  • Order Marattiales
    • Family Marattiaceae (incl. Angiopteridaceae, Christenseniaceae, Danaeaceae, Kaulfussiaceae)
• Class Pteridopsida [=Filicopsida, Polypodiopsida]
  • Order Osmundales
    • Family Osmundaceae
  • Order Hymenophyllales
    • Family Hymenophyllaceae (incl. Trichomanaceae)
  • Order Gleicheniales
    • Family Gleicheniaceae (incl. Dicranopteridaceae, Stromatopteridaceae)
    • Family Dipteridaceae (incl. Cheiropleuriaceae)
    • Family Matoniaceae
  • Order Schizaeales
    • Family Lygodiaceae
    • Family Anemiaceae (incl. Mohriaceae)
    • Family Schizaeaceae
  • Order Salviniales
    • Family Marsileaceae (incl. Pilulariaceae)
    • Family Salviniaceae (incl. Azollaceae)
  • Order Cyatheales
    • Family Thyrsopteridaceae
    • Family Loxomataceae
    • Family Culcitaceae
    • Family Plagiogyriaceae
    • Family Cibotiaceae
    • Family Cyatheaceae (incl. Alsophilaceae, Hymenophyllopsidaceae)
    • Family Dicksoniaceae (incl. Lophosoriaceae)
    • Family Metaxyaceae
  • Order Polypodiales
    • Family Lindsaeaceae (incl. Cystodiaceae, Lonchitidaceae)
    • Family Saccolomataceae
    • Family Dennstaedtiaceae (incl. Hypolepidaceae, Monachosoraceae, Pteridiaceae)
    • Family Pteridaceae (incl. Acrostichaceae, Actiniopteridaceae, Adiantaceae, Anopteraceae, Antrophyaceae, Ceratopteridaceae, Cheilanthaceae, Cryptogrammaceae, Hemionitidaceae, Negripteridaceae, Parkeriaceae, Platyzomataceae, Sinopteridaceae, Taenitidaceae, Vittariaceae)
    • Family Aspleniaceae
    • Family Thelypteridaceae
    • Family Woodsiaceae (incl. Athyriaceae, Cystopteridaceae)
    • Family Blechnaceae (incl. Stenochlaenaceae)
    • Family Onocleaceae
    • Family Dryopteridaceae (incl. Aspidiaceae, Bolbitidaceae, Elaphoglossaceae, Hypodematiaceae, Peranemataceae)
    • Family Oleandraceae
    • Family Davalliaceae
    • Family Polypodiaceae (incl. Drynariaceae, Grammitidaceae, Gymnogrammitidaceae, Loxogrammaceae, Platyceriaceae, Pleurisoriopsidaceae)

Economic uses

Ferns are not as important economically as seed plants but have considerable importance. Some ferns are used for food, including the fiddleheads of bracken, Pteridium aquilinum, ostrich fern, Matteuccia struthiopteris, and cinnamon fern, Osmunda cinnamomea]. Diplazium esculentum is also used by some tropical peoples as food.

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 birds-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.

Other ferns with some economic significance include:

• Dryopteris filix-mas (male fern), used as a vermifuge, and formerly in the US Pharmacopeia; also, this fern accidentally sprouting in a bottle resulted in Nathaniel Bagshaw Ward's 1829 invention of the terrarium or Wardian case
• Rumohra adiantoides (floral fern), extensively used in the florist trade
• Osmunda regalis (royal fern) and Osmunda cinnamomea (cinnamon fern), the root fiber being used horticulturally; the fiddleheads of O. cinnamomea are also used as a cooked vegetable
• Matteuccia struthiopteris (ostrich fern), the fiddleheads used as a cooked vegetable in North America
• Pteridium aquilinum (bracken), the fiddleheads used as a cooked vegetable in Japan and are believed to be responsible for the high rate of stomach cancer in Japan. It is also one of the world's most important agricultural weeds, especially in the British highlands, and often poisons cattle and horses.
• Diplazium esculentum (vegetable fern), a source of food for some native societies
• Pteris vittata (brake fern), used to absorb arsenic from the soil
• Polypodium glycyrrhiza (licorice fern), roots chewed for their pleasant flavor
• Tree ferns, used as building material in some tropical areas
• Cyathea cooperi (Australian tree fern), an important invasive species in Hawaii
• Ceratopteris richardii, a model plant for teaching and research, often called C-fern

Cultural connotations

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^{[citation needed]}.

Ferns were popular as a decorative motif in Victorian England, the designs frequently appeared on crockery, glassware, cast iron objects, and textiles. 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 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.

Misunderstood names

Several non-fern plants are called "ferns" and are sometimes confused with true ferns. These include:

• "Asparagus fern" - This may apply to one of several species of the monocot genus Asparagus, which are flowering plants.
• "Sweetfern" - A flowering shrub of the genus Comptonia.
• "Air fern" - A group of animals called hydrozoan that are distantly related to jellyfish and corals. They are harvested, dried, dyed green, and then sold as a "plant" that can "live on air". While it may look like a fern, it is merely the skeleton of this colonial animal.

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.

Gallery

Fern leaf, probably Blechnum nudum	A tree fern unrolling a new frond	Tree fern, probably Dicksonia antarctica	Tree ferns, probably Dicksonia antarctica
"Filicinae" from Ernst Haeckel's Kunstformen der Natur, 1904	Unidentified tree fern in Oaxaca	Tree Fern Spores San Diego, CA	Leaf of fern
Unidentified fern with spores showing in Rotorua, NZ.	Ferns in one of many natural Coast Redwood undergrowth settings Santa Cruz, CA.	Nature prints in The Ferns of Great Britain and Ireland used fronds to produce the plates

See also

• Fern spike

References

• Pryer, Kathleen M., Harald Schneider, Alan R. Smith, Raymond Cranfill, Paul G. Wolf, Jeffrey S. Hunt and Sedonia D. Sipes. 2001. Horsetails and ferns are a monophyletic group and the closest living relatives to seed plants. Nature 409: 618-622 (abstract here).
• Pryer, Kathleen M., Eric Schuettpelz, Paul G. Wolf, Harald Schneider, Alan R. Smith and Raymond Cranfill. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany 91:1582-1598 (online abstract here).
• Moran, Robbin C. (2004). A Natural History of Ferns. Portland, OR: Timber Press. ISBN 0-88192-667-1.
• Lord, Thomas R. (2006). Ferns and Fern Allies of Pennsylvania. Indiana, PA: Pinelands Press. [1]
• Smith, A. R., K. M. Pryer, E. Schuettpelz, P. Korall, H. Schneider & P. G. Wolf. 2006. A classification for extant ferns. Taxon 55(3):705–731.online available
• Boyd, Peter D. A. (2002-01-02). "Pteridomania - the Victorian passion for ferns". Revised: web version. Antique Collecting 28, 6, 9-12.. Retrieved on 2007-10-02.

External links

• Tree of Life Web Project: Filicopsida
• A classification of the ferns and their allies
• A fern book bibliography
• Register of fossil Pteridophyta
• L. Watson and M.J. Dallwitz (2004 onwards). The Ferns (Filicopsida) of the British Isles.
• Non-seed plant images at bioimages.vanderbilt.edu
• Ferns in detail, an online book

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