The Hymenophyllaceae, the filmy ferns and bristle ferns, are a family of two to nine genera (depending on classification system) and about 650 known species of ferns, with a subcosmopolitan distribution, but generally restricted to very damp places or to locations where they are wetted by spray from waterfalls or springs. A recent fossil find shows that ferns of Hymenophyllaceae have existed since at least the

Upper Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. T ...

Description

They often appear as very dark green or even black clumps and may be mistaken for a robust moss or

liverwort The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ge ...

. The rhizome is usually thin and wiry and the fronds variously pinnate with a single strand ("nerve") of

vascular tissue Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There ...

. As in most ferns, young fronds have circinate vernation. In most species, the frond, apart from the vascular tissue, is only a single cell thick, and they do not have any

stoma In botany, a stoma (from Greek ''στόμα'', "mouth", plural "stomata"), also called a stomate (plural "stomates"), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bor ...

ta. The

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

is also greatly reduced or absent, leaving filmy ferns very susceptible to

desiccation Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...

where a reliable water supply is not present. The leaves occasionally bear hairs, but scales are generally not present. The sori are borne at the leaf margins at the end of the nerve. They are protected by conical, bivalvate, or tubular

indusia A sorus (pl. sori) is a cluster of sporangia (structures producing and containing spores) in ferns and fungi. A coenosorus (plural coenosori) is a compound sorus composed of multiple, fused sori. Etymology This New Latin word is from Ancient G ...

. Within the sori, sporangia mature starting at the apex of the sorus and progressing to the base. They have a continuous, oblique annulus and release round, green

trilete In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

spores. The spores grow into thread- or ribbon-like gametophytes; in many species, the gametophyte has an extended, independent lifespan and can reproduce asexually by fragmenting or releasing gemmae. Individual plants may persist for many years.

Taxonomy

In the molecular phylogenetic classification of Smith et al. in 2006, the Hymenophyllales, containing the single family Hymenophyllaceae, were placed in class Polypodiopsida ''sensu stricto'' (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 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 and placed the Hymenophyllales 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 division of the family into genera was disputed, . Traditionally, only two genera of Hymenophyllaceae have been recognized: (1) ''

Hymenophyllum ''Hymenophyllum'' is a genus of ferns in the family Hymenophyllaceae. Its name means "membranous leaf", referring to the very thin translucent tissue of the fronds, which gives rise to the common name filmy fern for this and other thin-leaved fer ...

'' with bivalved involucres, and (2) ''Trichomanes'' s.l. with tubular involucres. Subsequent proposals have created 34 genera (Copeland 1938), 6 genera (Morton 1968), 47 genera (Sermolli 1977), and 8 genera (Iwatsuki 1984). These classifications all had only limited regional acceptance. Recent molecular phylogenic studies do show two distinct monophyletic clades of fairly equal size, but they are only roughly aligned with the two traditional genera. For example, the traditional ''Trichomanes'' subtaxa ''Pleuromanes'' and ''Cardiomanes'' were shown to belong to the "hymenophylloid" clade. To reflect these recent discoveries Atsushi Ebihara and Kunio Iwatsuki, in 2006, revised the taxonomy of Hymenophyllaceae to place all species of the "hymenophylloid" clade in a single genus ''Hymenophyllum'', and to place the eight clear "trichomanoid" subclades in eight corresponding genera. This subdivision was recognized by Smith et al. in 2006 and Christenhusz et al. in 2011, but Christenhusz and Chase, in 2014, reverted to combining the trichomanoid clades into ''Trichomanes''. The PPG I classification of 2016 again recognizes the segregate genera (and treats the two clades as subfamilies, Hymenophylloideae and Trichomanoideae), although the segregate genera are not always accepted by contemporary floras; e.g., as of 2016, the ''Flora of New Zealand'' preferred to recognize ''Trichomanes'' s.l. due to the difficulty of morphologically distinguishing the segregate genera.

Genera

The genera used in PPG I and the subgenera assigned by the system of Ebihara et al. are: *Hymenophylloideae (the "hymenophylloid" clade): ** – about 250 species ***subg. – about 100 species ***subg. – about 70 species ***subg. – more than 35 species ***subg. – about 25 species ***subg. – 5 species ***subg. – at least 8 species ***subg. – at least 3 species ***subg. – 2 species ***subg. – 1 species ***subg. – 1 species *Trichomanoideae (the "trichomanoid" clade) (sometimes all included in a single broad genus '' Trichomanes'' with about 400 species): ** – more than 30 species ***subg. – more than 20 species ***subg. – more than 10 species ** – more than 30 species ***subg. ***subg. ** – about 15 species ** – more than 15 species ***subg. – more than 15 species ***subg. – at least 2 species ** – about 25 species ***subg. – about 15 species ***subg. – more than 10 species ** – more than 60 species ***subg. – more than 30 species ***subg. – more than 5 species ***subg. – at least 1 species ***subg. – more than 4 species ** – about 4 species ** – about 5 species

Distribution and habitat

The great majority of the species are found in

tropical rainforest Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equatori ...

s, but some also occur in

temperate rainforest Temperate rainforests are coniferous or broadleaf forests that occur in the temperate zone and receive heavy rain. Temperate rain forests occur in oceanic moist regions around the world: the Pacific temperate rain forests of North American Pac ...

s (particularly New Zealand, with 25 species) and slightly drier forest regions. In Europe they are restricted to the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth#Surface, Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Afr ...

fringes of the continent, notably in the

Azores ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...

, Ireland, and western Great Britain, but one species (''Hymenophyllum tunbrigense'') locally east to Luxembourg, another (''H. wilsonii'') so far north as West Norway,

Faeroes The Faroe Islands ( ), or simply the Faroes ( fo, Føroyar ; da, Færøerne ), are a North Atlantic archipelago, island group and an autonomous territory of the Danish Realm, Kingdom of Denmark. They are located north-northwest of Scotlan ...

and South Iceland, while in North America, they are restricted (often occurring solely as gametophytes) to the humid eastern third of the continent and the rainforests of the Pacific Northwest.

References

USDA Plants Profile: Distribution in U.S.A.
*E.B.Copeland. 1947. ''Genera Filicum''. Waltham. *C.Michael Hogan. 2010
''Fern''. Encyclopedia of Earth. eds. Saikat Basu and C.Cleveland. National Council for Science and the Environment
Washington DC. *H.A.Hyde, A.E.Wade, & S.G.Harrison. 1978. ''Welsh Ferns''. National Museum of Wales. . {{Taxonbar, from=Q1066700 Fern families