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[1] 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.[2]
They often appear as very dark green or even black clumps and may be mistaken for a robust moss or liverwort. The rhizome is usually thin and wiry and the fronds variously pinnate with a single strand ("nerve") of vascular tissue. 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 stomata. The cuticle is also greatly reduced or absent,[3] leaving filmy ferns very susceptible to desiccation where a reliable water supply is not present. The leaves occasionally bear hairs, but scales are generally not present.[3] The sori are borne at the leaf margins[3] 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 gametophytes; in many species, the gametophyte has an extended, independent lifespan and can reproduce asexually by fragmenting or releasing gemmae.[3]
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 ferns).[3] The linear sequence of Christenhusz et al. (2011), intended for compatibility with the classification of Chase and Reveal (2009)[4] which placed all land plants in Equisetopsida,[5] reclassified Smith's Polypodiopsida as subclass Polypodiidae and placed the Hymenophyllales there. The circumscription of the order and its families was not changed,[4] and that circumscription and placement in Polypodiidae has subsequently been followed in the classifications of Christenhusz and Chase (2014)[6] and PPG I (2016).[7]
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 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.[8]