| Hymenophyllaceae | |
|---|---|
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| Hymenophyllum tunbrigense in Luxembourg | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Division: | Polypodiophyta |
| Class: | Polypodiopsida |
| Subclass: | Polypodiidae |
| Order: | Hymenophyllales A.B.Frank |
| Family: | Hymenophyllaceae Link |
| Genera | |
TheHymenophyllaceae, thefilmy ferns andbristle ferns, are a family of two to nine genera (depending on classification system) and about 650 known species[1] offerns, with asubcosmopolitan distribution, but generally restricted to very damp places or to locations where they are wetted by spray fromwaterfalls or springs. Fossil evidence shows that ferns of the family Hymenophyllaceae have existed since at least theUpper Triassic.[2]
They often appear as very dark green or even black clumps and may be mistaken for a robustmoss orliverwort. Therhizome is usually thin and wiry and the fronds variously pinnate with a single strand ("nerve") ofvascular tissue. As in most ferns, young fronds havecircinate vernation. In most species, the frond, apart from the vascular tissue, is only a single cell thick, and they do not have anystomata. Thecuticle is also greatly reduced or absent,[3] leaving filmy fernspoikilohydric and very susceptible todesiccation where a reliable water supply is not present.[4] The leaves occasionally bear hairs, but scales are generally not present.[3] Thesori are borne at the leaf margins[3] at the end of the nerve. They are protected by conical, bivalvate, or tubularindusia. Within the sori, sporangia mature starting at the apex of the sorus and progressing to the base. They have a continuous, obliqueannulus and release round, greentrilete spores. The spores grow into thread- or ribbon-likegametophytes; in many species, the gametophyte has an extended, independent lifespan and can reproduce asexually by fragmenting or releasinggemmae.[3]
Individual plants may persist for many years.
In the molecular phylogenetic classification of Smith et al. in 2006, the Hymenophyllales, containing the single family Hymenophyllaceae, were placed in class Polypodiopsidasensu stricto (theleptosporangiate ferns).[3] The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009)[5] which placed all land plants in Equisetopsida,[6] reclassified Smith's Polypodiopsida as subclass Polypodiidae and placed the Hymenophyllales there. The circumscription of the order and its families was not changed,[5] and that circumscription and placement in Polypodiidae has subsequently been followed in the classifications of Christenhusz and Chase (2014)[7] and PPG I (2016).[8]
The division of the family into genera was disputed, as of October 2019[update]. Traditionally, only two genera of Hymenophyllaceae have been recognized: (1)Hymenophyllum with bivalvedinvolucres, 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. Recentmolecular phylogenic studies do show two distinctmonophyletic clades of fairly equal size, but they are only roughly aligned with the two traditional genera. For example, the traditionalTrichomanes subtaxaPleuromanes andCardiomanes were shown to belong to the "hymenophylloid" clade. To reflect these recent discoveriesAtsushi Ebihara andKunio Iwatsuki, in 2006, revised the taxonomy of Hymenophyllaceae to place all species of the "hymenophylloid" clade in a single genusHymenophyllum, and to place the eight clear "trichomanoid" subclades in eight corresponding genera.[9]
This subdivision was recognized by Smith et al. in 2006[3] and Christenhusz et al. in 2011,[5] but Christenhusz and Chase, in 2014, reverted to combining the trichomanoid clades intoTrichomanes.[7] The PPG I classification of 2016 again recognizes the segregate genera (and treats the two clades as subfamilies, Hymenophylloideae and Trichomanoideae),[8] although the segregate genera are not always accepted by contemporary floras; e.g., as of 2016, theFlora of New Zealand preferred to recognizeTrichomanes s.l. due to the difficulty of morphologically distinguishing the segregate genera.[10]
The genera used in PPG I and the subgenera assigned by the system of Ebihara et al. are:
| Phylogeny of Hymenophyllaceae[9] | |||||||||
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| Phylogeny of Hymenophyllaceae[11][12] | |||||||||||||||
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The great majority of the species are found intropical rainforests, but some also occur intemperate rainforests (particularlyNew Zealand, with 25 species) and slightly drier forest regions. InEurope they are restricted to theAtlantic Ocean fringes of the continent, notably in theAzores,Ireland, and westernGreat Britain, but one species (Hymenophyllum tunbrigense) locally east toLuxembourg, another (H. wilsonii) so far north as WestNorway,Faeroes and SouthIceland, while inNorth America, they are restricted (often occurring solely asgametophytes) to the humid eastern third of the continent and the rainforests of the Pacific Northwest.
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