Fern

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 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.

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.

One problem with fern classification is the problem of cryptic species. Cryptic species are those which are morphologically similar to another species, but which differ genetically in ways that prevent fertile interbreeding. A good example of this can be seen in the currently-designated species Asplenium trichomanes, the maidenhair spleenwort. This is actually a species complex which includes distinct diploid and tetraploid races. There are minor but unclear morphological differences between the two groups, which prefer distinctly differing habitats. In many cases such as this, the species complexes have been separated into separate species, thus raising the number of overall fern species. Possibly many more cryptic species are yet to be discovered and designated.

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). However, there are still differences of opinion about the placement of the Equisetum species (see Equisetopsida for further discussion).

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)