Hymenophyllaceae

Hymenophyllaceae
Hymenophyllum tunbrigense in Luxembourg
Scientific classification
Kingdom:	Plantae
Division:	Pteridophyta
Class:	Polypodiopsida/Pteridopsida (disputed)
Order:	Hymenophyllales
Family:	Hymenophyllaceae Link
Genera
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The Hymenophyllaceae (filmy ferns and bristle ferns) is a family of two to nine genera (depending on classification system) and ca 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 great majority of the species are found in tropical rainforests, but some also occur in temperate rainforests (particularly New Zealand, with 25 species) and slightly drier forest regions. In Europe they are restricted to the Atlantic Ocean fringes of the continent, notably in the Azores, Ireland, and western Great Britain, but one species (Hymenophyllum tunbrigense) locally east to Luxembourg, another (H. wilsonii) so far north as West Norway, Faeroes 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.

They often appear as very dark green or even black clumps and may be mistaken for a robust moss or liverwort. The stem is thin and wiry and the fronds variously pinnate with a single strand ("nerve") of vascular tissue. In most species, the frond, apart from the vascular tissue, is only a single cell thickness, and they do not have any stomata; this makes the plants very susceptible to desiccation where a reliable water supply is not present. The sori are borne at the leaf margins at the end of the nerve. Individual plants may persist for many years.

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Wikipedia