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

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

s, with a subcosmopolitan distribution, but generally restricted to very damp places or to locations where they are wetted by spray from

waterfall A waterfall is any point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls can be formed in seve ...

s or springs. Fossil evidence shows that ferns of the family 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 Mosses are small, non-vascular plant, non-vascular flowerless plants in the taxonomic phylum, division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Wilhelm Philippe Schimper, Schimp. 1879) may also refer to the parent group bryo ...

liverwort Liverworts are a group of non-vascular land plants forming the division Marchantiophyta (). They may also be referred to as hepatics. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry ...

. The

rhizome In botany and dendrology, a rhizome ( ) is a modified subterranean plant stem that sends out roots and Shoot (botany), shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from ...

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

vascular tissue Vascular tissue is a complex transporting 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. T ...

. 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 (: stomata, from Greek language, Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the Epidermis (botany), epidermis of leaves, stems, and other organs, that controls the rate of gas exc ...

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

poikilohydric Poikilohydry is the lack of ability (structural or functional mechanism) to maintain and/or regulate water content to achieve homeostasis of cells and tissue connected with quick equilibration of cell/tissue water content to that of the environment ...

and 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. The ...

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. 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 spores. The spores grow into thread- or ribbon-like

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...

s; 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, the largest of these being the largest group of living ferns, including some 11,000 species worldwide. The group has also ...

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'' 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 In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

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. ***subg. – at least 1 species ***subg. – more than 5 species ***subg. – more than 4 species ***subg. – more than 30 species ** – about 4 species ** – about 5 species

Distribution and habitat

The great majority of the species are found in

tropical rainforest Tropical rainforests are dense and warm rainforests with high rainfall typically found between 10° north and south of the Equator. They are a subset of the tropical forest biome that occurs roughly within the 28° latitudes (in the torrid zo ...

s, but some also occur in

temperate rainforest Temperate rainforests are rainforests with coniferous or Broad-leaved tree, broadleaf forests that occur in the temperate zone and receive heavy rain. Temperate rainforests occur in oceanic moist regions around the world: the Pacific temperate ...

s (particularly

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

, with 25 species) and slightly drier forest regions. In

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

they are restricted to the

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

fringes of the continent, notably in the

Azores The Azores ( , , ; , ), officially the Autonomous Region of the Azores (), is one of the two autonomous regions of Portugal (along with Madeira). It is an archipelago composed of nine volcanic islands in the Macaronesia region of the North Atl ...

Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...

, and western

Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...

, but one species (''Hymenophyllum tunbrigense'') locally east to

Luxembourg Luxembourg, officially the Grand Duchy of Luxembourg, is a landlocked country in Western Europe. It is bordered by Belgium to the west and north, Germany to the east, and France on the south. Its capital and most populous city, Luxembour ...

, another (''H. wilsonii'') so far north as West

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

, Faeroes and South

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

, while in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

, they are restricted (often occurring solely as

s) 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. . {{Authority control Fern families