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Fern

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This article is about the family of pteridophyte plants. For the environmental organisation, see FERN (NGO). For the letter of the Ogham alphabet, see Fern (letter).

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. The term pteridophytes has traditionally been used to describe all seedless vascular plants so is synonymous with "ferns and fern allies". This can be confusing given that the fern phylum Pteridophyta is also sometimes referred to as pteridophytes.

A fern is a vascular plant that differs from the more primitive lycophytes in having true leaves (megaphylls), and from the more advanced seed plants (gymnosperms and angiosperms) in lacking seeds. Like all vascular plants, it has a life cycle, often referred to as alternation of generations, characterised by a diploid sporophytic and a haploid gametophytic phase. Unlike the gymnosperms and angiosperms, in ferns the 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).
Contents

Fern structure

Ferns at the Royal Melbourne Botanical Gardens
Enlarge
Ferns at the Royal Melbourne Botanical Gardens

Tree ferns, probably Dicksonia antarctica
Enlarge
Tree ferns, probably Dicksonia antarctica

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:

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: the adders-tongues, moonworts, and grape-ferns (Ophioglossophyta), the 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 onsidered "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 monophyletic 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 four groups as "fern allies".

The true ferns may be subdivided into six main groups, or classes (or, if the true ferns are considered as a class, then the last three would be orders):

The last group includes most plants familiarly known as ferns. Modern research indicates that the Osmundopsida diverged first from the common ancestor of the leptosporangiate ferns, followed by the Gleichenopsida.

A more complete classification scheme follows:

Economic uses

Ferns are not as important economically as seed plants but have considerable importance. 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). 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:

Religious 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.

Image:Fern.jpg|Fern leaf, probably Blechnum nudum Image:Tree_Fern.jpg|A tree fern unrolling a new frond Image:Fern02.jpg|Tree fern, probably Dicksonia antarctica Image:Ferns.jpg|Tree ferns, probably Dicksonia antarctica Image:Haeckel Filicinae 92.jpg|"Filicinae" from Ernst Haeckel's Artforms of Nature, 1904 Image:Oaxaca_fern.jpg|Unidentified tree fern in Oaxaca

Misunderstood names

Several non-fern plants are called "ferns" and are sometimes popularly believed to be ferns in error. These include: 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.

References and external links

 

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