Argyrochosma incana, the hairy false cloak fern, is a fern known from the southwestern United States through Mexico to Guatemala, and from a disjunct population in the Dominican Republic. It grows on rocky slopes and steep banks, often in forests. Like many of the false cloak ferns, it bears white powder on the underside of its leaves. First described as a species in 1825, it was transferred to the new genus Argyrochosma (the "false cloak ferns") in 1987, recognizing their distinctness from the "cloak ferns" (Notholaena sensu stricto).
Description
Argyrochosma incana is a medium-sized epipetric fern. The rhizome is short, thick, and may be horizontal or somewhat upright. It bears linear to lanceolate or linear-ligulate (straplike) scales long and wide, without teeth at the margins and long-attenuate at the tip. They are of a uniform dark, shiny brown, chestnut-brown or yellowish-brown color with some dark brown patches. From the rhizome, the fronds arise in clumps. From base to tip of leaf, they are long, sometimes up to . Of this length, about half is made up by the stipe (the stalk of the leaf, below the blade), which is shiny and round, hairless, and dark purple to black in color, occasionally chestnut-brown. It is typically in length and in diameter.
The leaf blades are deltate (triangular) or lanceolate, tripinnate (cut into pinnae, pinnules and pinnulets) to almost quadripinnate at their bases. They are wide, occasionally as little as , obtuse (blunt) at the base and acute (pointed) to acuminate at the tip. The rachis (leaf axis) is round or slightly flattened. The rachis and the axes of the leaf segments are all dark in color; the color stops abruptly at a joint at the base of the leaf segment. Each blade bears 6 to 9 pairs of pinnae, borne oppositely, or nearly so, on the rachis. They are deltate to ovate in shape and borne on long stalks. The ultimate segments of the blade are broadly oblong to ovate or elliptic in shape, long, occasionally as little as , broadly obtuse at their tips and truncate or subcordate (nearly heart-shaped) at their bases. The leaf tissue is grayish-green in color and leathery in texture. The underside of the leaf is coated in white farina (powder), which may be sparsely scattered on the upper surface or absent from it. The leaf axes curl upwards when dry.
The sori lie along the veins, forming a band wide, occupying from two-thirds to the entire length of the veins. The veins themselves tend to blend into the leaf tissue. The leaf edges are not curved under or modified into false indusia. Each sporangium bears 64 spores. The plants are diploid, with a chromosome number of 2n = 54.
Within the heart of its range, in Mexico, A. incana is both the most widespread member of the genus and the most variable. Some of the characteristics observed to vary are the size of segments (reduced to wide), the density of farina on the upper surface, the shape and color of the blade axes (zig-zag rather than straight, and dark purplish rather than black), and the joint at the base of leaf segments (more sharply defined). Material collected in several states of northern Mexico, with small segments, somewhat zig-zag and dark purplish axes may represent an as yet undescribed species. Two specimens collected by C. H. Müller and his wife Mary in Nuevo Leon are anomalous in having chestnut-brown axes and rougher spores, similar to A. delicatula, but their size, abundance of farina below, and lack of it above, led Maxon and Weatherby to classify them as A. incana.
In Mexico, A. incana closely resembles A. delicatula, from which it is distinguished by having white (rather than pale yellow) farina, and A. formosa, which lacks farina. Most specimens in the United States were originally mislabeled as A. limitanea, but this species lacks the joint and abrupt end of dark color at the base of its leaf segments.
Taxonomy
It was first described in 1825 by Carl Borivoj Presl as Notholaena incana, based on material collected by Thaddäus Haenke in Mexico. He did not explain why he chose the species epithet incana, meaning "hoary" (grayish-white). In 1859, Mettenius recognized the genus Gymnogramma for species where sporangia were borne along the nerves and not densely clustered at the end of the nerves. He independently described the species based on material collected by Eugénio Schmitz in Mexico, naming it G. candida. The epithet refers to the chalk-white color of the farina beneath the leaves. Prantl expanded Pellaea to include several genera in which he perceived close affinities, including Gymnogramma. Accordingly, he transferred G. candida to Pellaea section Cincinalis as P. candida in 1882.
