Argyrochosma dealbata

Argyrochosma dealbata is a small, epipetric fern. Its leaves are dusted with white powder on the underside.

The rhizome is compact, more or less upright, and usually unbranching,[1] and bears brown scales.[2] The fronds of A. dealbata spring from the rhizome in clusters[1] and are 3 to 15 centimeters (1 to 6 in) long.[2] The stipe (the stalk of the leaf, below the blade) is a shiny[1] chestnut brown[2] or reddish-brown in color,[1] measuring 0.50 to 0.75 millimeters (0.020 to 0.030 in) in diameter.[2] The shiny brown color of the stipe continues into the rachis and the axis of the leaf segments, and blends with the leaf tissue at the base of each segment, without an abrupt end point.[1][2] The rachis (leaf axis) is rounded or slightly flattened on the upper surface. The costae (pinna axes) are more or less straight.[2]

The leaf blades are triangular in shape, broadest at the base, and are most highly divided there, ranging from tripinnate (cut into pinnae, pinnules, and pinnulets) to pentapinnate (five times divided).[2] The blade tissue is bluish green[1] and somewhat leafy in texture (rather than densely leathery); veins are often visible on the upper surface of the leaf.[2] and a whitish farina (powder) covers the underside, while the upper surface is free of hairs, powder, etc., although it may bear a few glands.[2] The edges of leaf segments curl back under and partially protect the sori, which are borne along veins close to the edge. The sporangia contain 64 spores, released in summer and fall. Plants are sexual diploids, with a chromosome number of 2n = 54.[2]

Like many cheilanthoid ferns, this species has been placed in many genera at different times. It was first described by Pursh in 1814 as Cheilanthes dealbata, based on material collected on the banks of the Missouri River. The epithet dealbata refers to the white color of the underside of the leaves, due to the coating of farina.[3] Gustav Kunze independently described the species as Notholaena pulchella in 1843. His specimen was collected for the Berlin Botanical Garden from rocks in Missouri. Initially misidentified as Notholaena nivea, he noted that the Missouri material was smaller and more highly divided, and commented on the white powder on the underside of the leaves, lacking in Pursh's description.[4] He subsequently recognized it as synonymous with C. dealbata and transferred the older name to Notholaena as N. dealbata in 1848, noting that he "would not have looked for it under Cheilanthes.[5][lower-alpha 1]

Delineating natural genera in the cheilanthoids has proven to be extremely difficult, and other placements of the species were subsequently put forward. Fée transferred it to Cincinalis as Cincinalis dealbata in 1852, breaking with most other authorities in recognizing and re-circumscribing that genus.[6] In 1859, Mettenius rejected Fée's attempt to delineate Cincinalis as a separate genus, but recognized the genus Gymnogramma for species where sporangia were borne along the nerves and not densely clustered at the end of the nerves.[7] He transferred the species there as G. dealbata,[8] but this name had already been used by Presl for a different species in 1825, rendering it invalid. Prantl expanded Pellaea to include several genera in which he perceived close affinities, including Cincinalis. Accordingly, he transferred C. dealbata to Pellaea section Cincinalis as P. dealbata in 1882.[9]

By the late 20th century, most authorities favored placement of Notholaena nivea and closely related ferns, including N. dealbata, either in Notholaena or Pellaea. Both Edwin Copeland and C. A. Weatherby suggested in the 1940s that this group of ferns might represent a distinct genus of its own.[10] This was finally addressed in 1987 by Michael D. Windham, who was carrying out phylogenetic studies of these genera. He elevated Notholaena sect. Argyrochosma to become the genus Argyrochosma,[11] and transferred this species to that genus as A. dealbata.[12] In 2018, Maarten J. M. Christenhusz transferred the species to Hemionitis as H. artax (the epithet dealbata was preoccupied), as part of a program to consolidate the cheilanthoid ferns into that genus. The epithet refers to the horse of Atreyu in the novel The Neverending Story.[13]

Species of Argyrochosma are commonly referred to as "false cloak ferns"; the name apparently refers to their resemblance to Notholaena proper, known as "cloak ferns" for the reflexed leaf margins, which cloak the sporangia.

Argyrochosma dealbata is endemic to the United States. It is found from the western edge of Illinois south and west through Arkansas and Missouri to southeastern Nebraska and thence south through Kansas and Oklahoma to Texas, with a disjunct station in south-central Kentucky.[14][15] NatureServe assigns the an overall conservation ranking of apparently secure (G4), but it is considered imperiled in Arkansas, and the single occurrences in Nebraska and Kentucky are considered extirpated and historical, respectively.[15]

Kunze suggested the species was suitable for cultivation, noting that it was grown in Berlin in a well-lit part of the greenhouses.[16] It has been described as preferring medium light, and relatively dry, well-drained soil mixed with sand, perhaps of high pH.[1]

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• Pictures and description of an A. dealbata occurrence