Hymenophyllaceae

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. Fossil evidence shows that ferns of the family Hymenophyllaceae have existed since at least the Upper Triassic.

==Description== 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, leaving filmy ferns poikilohydric and very susceptible to desiccation 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 gametophytes; in many species, the gametophyte has an extended, independent lifespan and can reproduce asexually by fragmenting or releasing gemmae.