Hymenophyllaceae, the filmy fern family (order Hymenophyllales), containing 7 or more genera and some 600 species. The family is distributed in tropical regions around the world, with only a few species extending into the temperate zone.
Members of Hymenophyllaceae are small delicate ferns and are most commonly epiphytes. The leaves vary from undivided to highly dissected and are characterized by a lamina (leaf blade) that is usually only one cell layer thick between the veins. The sori are positioned at the tips or along the margins of the leaf segments and are enclosed in a cup-shaped to narrowly conical protective covering of tissue (indusium) opening toward the margin, which is sometimes deeply lobed toward the mouth. The individual sporangia (spore-producing structures) are either attached to a short receptacle and enclosed by the indusium or are along a long stalklike receptacle that extends past the induisal mouth. The spores are globose (tetrahedral), and they are unusual in being green and physiologically active at the time they are shed.
The filmy ferns are considered an isolated, relatively primitive family of ferns. Their fossil record is long but very fragmentary because the delicate leaves generally do not preserve well as fossils. The oldest report of fossils definitely assignable to Hymenophyllaceae is that of Hopetedia praetermissa, from the Triassic Period (251.9 million to 201.3 million years ago), discovered in North Carolina. Traditionally, the two principal genera of modern filmy ferns have been recognized as Hymenophyllum and Trichomanes, which differ in details of the soral morphology. The other most commonly accepted genera in the family are Cardiomanes, Cephalomanes, Crepidomanes, Hymenoglossum, and Serpyllopsis, based on very subtle and inconsistent morphological differences. Modern molecular data suggest that some of these groups are not monophyletic (descended from a common ancestor), and their taxonomy is contentious.
In the eastern United States there is a species of filmy fern, Trichomanes intricatum, that is most unusual in that it exists only as isolated colonies of independent gametophytes. These gametophytes, which occupy cool, highly shaded microclimates underneath overhanging cliffs and boulders, apparently have lost the capacity to complete their life cycle and produce new sporophytes. In fact, sporophytes of this species are not known to exist anywhere on Earth. The plants survive as algaelike filaments and reproduce asexually through small vegetative offsets (gemmae) that are dispersed by air currents to new sites. Even more amazing is the fact that there exists a second unrelated species, the Appalachian shoestring fern (Vittaria appalachiana; family Pteridaceae), which occupies similar habitats in roughly the same range and is also apparently incapable of completing its sexual life cycle in the wild to form new sporophytes. The closest living relatives of these species grow in Asia and in the tropics, respectively.