Argyrochosma is a genus of ferns in the subfamily Cheilanthoideae of the family Pteridaceae (order Polypodiales), comprising around nineteen to twenty species commonly known as false cloak ferns or silver ferns. Species in this genus were long classified within the related genera Notholaena or Pellaea before being formally recognized as a distinct genus by the pteridologist Michael D. Windham in 1987, following phylogenetic studies of the group.
The fronds of Argyrochosma are typically shorter than 40 cm (16 in) and bear rounded segments that are bluish or grayish green in color. A characteristic feature of most species is a coating of white powdery farina on the lower surface of the leaf segments — the silver dust that gives the genus both its scientific name and the common name "silver fern." The sporangia produce brown spores. Plants typically grow in crevices of rocks, an adaptation seen widely across their range.
The genus is distributed primarily throughout the Americas, from North America through Central America and the Caribbean to South America. A single outlying species, A. connectens, is known from Sichuan Province in China, where its placement in Argyrochosma was confirmed by DNA analysis in 2015. Well-known members of the genus include A. dealbata (powdery false cloak fern), A. fendleri (Fendler's false cloak fern), A. incana (hairy false cloak fern), A. limitanea (southwestern false cloak fern), and A. nivea, which served as the lectotype specimen anchoring the genus's early taxonomic history.
Etymology
The genus name Argyrochosma was coined by the botanist John Smith in 1842, derived from the Greek words ἄργυρος (argyros, meaning "silver") and χῶσμα (chosma, meaning "mound of earth"). The name refers to the white farinose (powdery) coating found on the undersides of leaves in most species. The common names "silver fern" and "false cloak fern" reflect, respectively, this silvery farina and the historical placement of these species within Notholaena, the "cloak ferns" (whose name itself means "false cloak" in Greek).
Distribution
Argyrochosma species are found predominantly throughout the Americas — from the southwestern United States south through Mexico, Central America, the Caribbean, and into South America. The sole exception is A. connectens, known only from Sichuan, China. Species typically inhabit rocky terrain, growing in crevices and on cliff faces.
Taxonomy
The species now placed in Argyrochosma were first described by John Smith in 1842 as Notholaena sect. Argyrochosma, distinguished by their white farinose leaf undersurfaces. Carl Christensen later argued the group belonged in Pellaea, designating Pellaea nivea as the lectotype. Through the mid-twentieth century, botanists including Edwin Copeland and C. A. Weatherby recognized the group as potentially distinct. The genus was formally established in 1987 when Michael D. Windham elevated the section to genus rank. In 1996 M. Mónica Ponce transferred two varieties of A. nivea to the genus; these were subsequently raised to species level in 2017 by Michael Kessler and Alan R. Smith. In 2015, A. connectens (previously placed in Pellaea) was confirmed as a member via DNA analysis.
