:''This article defines the Gleicheniales in a loose sense. For the restricted definition, see

Gleicheniaceae The forked ferns are the family Gleicheniaceae, which includes six genera and about 160 known species. The formerly independent families Dicranopteridaceae and Stromatopteridaceae are generally included in the Gleicheniaceae, whereas the Dipteri ...

.'' Gleicheniales is an order of

fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...

s in the subclass Polypodiidae (the

leptosporangiate fern The Polypodiidae, commonly called leptosporangiate ferns, formerly Leptosporangiatae, are one of four subclasses of ferns, and the largest of these, being the largest group of living ferns, including some 11,000 species worldwide. The group has ...

s). The Gleicheniales have records potentially as early as the Carboniferous, but the oldest unambiguous records date to the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

Description

These ferns are characterized by root steles having 3–5

protoxylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

poles and

antheridia An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also ...

with 6–12 narrow, twisted or curved cells in their walls. Otherwise, their habitus is highly diverse, including plants with the typical fern

frond A frond is a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group. Other botanists allow the term frond to also apply to the lar ...

s, others whose leaves resemble those of

palm tree The Arecaceae is a family of perennial flowering plants in the monocot order Arecales. Their growth form can be climbers, shrubs, tree-like and stemless plants, all commonly known as palms. Those having a tree-like form are called palm ...

s, and yet others again which have undivided leaves. They are tropical ferns, most diverse in

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...

and the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

region.

Classification

In the molecular phylogenetic classification of Smith et al. in 2006, the Gleicheniales were placed in class

Polypodiopsida A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...

(the

s). Three families,

Dipteridaceae The Dipteridaceae is a family of ferns in the order Gleicheniales of the class Polypodiopsida. They are commonly known as umbrella ferns and contain two genera, ''Cheiropleuria'' and ''Dipteris'', with a total of nine species confined to Asia, N ...

, and

Matoniaceae Matoniaceae is one of the three families of ferns in the Gleicheniales order of the Polypodiopsida class. Fossil records reveal that Matoniaceae ferns were abundant during the Mesozoic era (about 250-million to 66-million years ago), during which ...

were recognized. The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009) which placed all land plants in Equisetopsida, reclassified Smith's Polypodiopsida as subclass

Polypodiidae Polypodiidae may refer to: * Polypodiidae (cnidarians), a zoological family that only includes ''Polypodium hydriforme'', a cnidarian parasite of fish eggs *Polypodiidae (plant), a botanical subclass of Equisetopsida sensu lato that includes the le ...

and placed the Gleicheniales there. The circumscription of the order and its families was not changed, and that circumscription and placement in Polypodiidae has subsequently been followed in the classifications of Christenhusz and Chase (2014) and PPG I (2016). The

form taxon Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of s ...

'' Microphyllopteris'' is used for some

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

Gleicheniales that cannot be reliably assigned to the present-day orders. The

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

'' Antarctipteris'' and '' Gleichenipteris'' are sometimes ascribed to the Gleicheniaceae, but are probably better considered Gleicheniales ''

incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...

''. The oldest possible members of the family are members of the genus '' Oligocarpia'' (reproductive organs) from the Carboniferous. However, these have alternatively been considered members of Sermayaceae. The oldest unambiguous records, belonging to the family Gleicheniaceae, are known from the Wuda Tuff flora of China, dating to the Asselian, approximately 298.34 million years ago. In historical treatments, the order has sometimes been treated as a subclass Gleicheniatae of the Pteridopsida, with the

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

treated as families here upranked to orders, so that a distinct subclass can be established for the

s. In other treatments, they were expanded to include the filmy ferns (order Hymenophyllales), as well as the similar-looking

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

'' Hymenophyllopsis'' (as order Hymenophyllopsidales). The resultant group was treated as class Gleichenopsida alongside the Pteridopsida, which would then be limited to the leptosporangiate ferns. However, this class is not monophyletic but rather a basal

grade Grade most commonly refers to: * Grade (education), a measurement of a student's performance * Grade, the number of the year a student has reached in a given educational stage * Grade (slope), the steepness of a slope Grade or grading may also ref ...

, retaining ancient traits among the living ferns. Irrespective of their modern taxonomic treatment, the Gleicheniales were formerly included in the order

Polypodiales The order Polypodiales encompasses the major lineages of polypod ferns, which comprise more than 80% of today's fern species. They are found in many parts of the world including tropical, semitropical and temperate areas. Description Polypodiale ...

. But the ferns in the loose sense are much too diverse a group to be shoehorned into one

taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

at such a low rank. The following diagram shows the phylogeny of Gleicheniales.

Footnotes

References

* (2006): A classification for extant ferns. ''Taxon'' 55(3): 705–731
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{{Taxonbar, from=Q1209668 Fern orders