Hymenophyllum Sm., Mém. Acad. Roy. Sci. (Turin)

Kessler, Michael & Smith, Alan R., 2017, Prodromus of a fern flora for Bolivia. X. Hymenophyllaceae, Phytotaxa 328 (3), pp. 201-226 : 206

Hymenophyllum Sm., Mém. Acad. Roy. Sci. (Turin) 5: 418, pl. 9, f. 8. 1793.

Hymenophyllum is easily recognized by thin blades, long-creeping, filiform rhizomes, and bivalvate involucres with the receptacles usually not exserted. In the Neotropics it can be conveniently divided into 3 subgenera: Hymenophyllum , Mecodium, and Sphaerocionium ( Ebihara et al. 2006; Hennequin et al. 2006). Further studies may show that these groups are better treated as distinct genera, but the delimitation of these, especially among Old World taxa, is still uncertain. Species limits in some groups of Hymenophyllum are frequently unclear and identification is often based on suites of characters. Hymenophyllum has over 300 species, mostly in tropical areas, but with a few in subtropical and temperate regions, and about 100 species in the Neotropics.

One of the important characters distinguishing species relates to whether the involucres are displayed at an angle to the plane of the blade or they are in the same plane as the blade. This character is most important in distinguishing species within subg. Hymenophyllum ; within subgenera Mecodium and Sphaerocionium , the involucres are in the same plane as the blade. Hair types – simple, forked, stellate, or bistellate – are very important in subg. Sphaerocionium ; the two other subgenera essentially lack hairs, or hairs are obscure. Blade margins, whether toothed or not, may also be important, especially in subg. Mecodium.

Copeland (1937) monographed all of the Old World species of Hymenophyllum , which included a few neotropical species with subcosmopolitan ( H. tunbrigense (L.) Sm.; the type of the genus and now considered to comprise a species complex: see Larsen et al. 2017), pantropical ( H. polyanthos ), or amphioceanic ( H. hirsutum ) distributions. Diem & Lichtenstein (1959), Larsen et al. (2013, 2017) and Arana et al. (2016) gave detailed accounts of the species in Argentina and Chile.