Argyrochosma delicatula

Argyrochosma delicatula is a fern known from northeastern Mexico (and from one collection in Arizona). It grows in rocky habitats, either in sun or in shade, and is distinguished from similar species by the presence of pale yellow (rather than white) powder on the underside of its leaves. First described as a species in 1939, it was transferred to the new genus Argyrochosma (the "false cloak ferns") in 1987, recognizing their distinctness from the "cloak ferns" (Notholaena sensu stricto).

Argyrochosma delicatula
Scientific classification
Kingdom: Plantae
Clade: Tracheophytes
Class: Polypodiopsida
Order: Polypodiales
Family: Pteridaceae
Genus: Argyrochosma
Species:
A. delicatula
Binomial name
Argyrochosma delicatula
(Maxon & Weath.) Windham
Synonyms
  • Hemionitis delicatula (Maxon & Weath.) Christenh.
  • Notholaena delicatula Maxon & Weath.

Description

Argyrochosma delicatula is a medium-sized epipetric fern. The rhizome is compact, and may be horizontal or upright. It bears linear scales 4 to 6 millimeters (0.2 to 0.2 in) long and 0.2 to 0.3 millimeters (0.008 to 0.01 in) wide, of a uniform orange-brown to dark brown color, without teeth at the margins.[1] From it, the fronds arise in clumps. From base to tip of leaf, they are 10 to 25 centimeters (3.9 to 9.8 in) long, occasionally as short as 6 centimeters (2 in) or as long as 35 centimeters (14 in). Of this length, nearly half is made up by the stipe (the stalk of the leaf, below the blade), which is shiny and round, hairless, and chestnut-brown to dark purple in color.[1]

The leaf blades are triangular in shape, tripinnate (cut into pinnae, pinnules and pinnulets) to almost quadripinnate, and leafy, rather than leathery, in texture.[1] The rachis (leaf axis) is round, rather than flattened, and dark in color, as are the axes of the leaf segments. These are all dark in color; the color stops abruptly at a joint at the base of the leaf segment.[1] Each blade bears 4 to 8 pairs of pinnae. These are divided into pinnules which range from diamond-shaped to round, and are typically 1 to 2 millimeters (0.04 to 0.08 in) across, occasionally to 3 millimeters (0.1 in).[1] The underside of the leaf is coated in pale yellow farina (powder), which is sparsely scattered on the upper surface.[1]

The sori lie along the veins, in the portions closest to the edge of the leaf. The leaf edges are not modified into false indusia. Each sporangium bears 64 spores. The plants are diploid, with a chromosome number of 2n = 54.[1]

It is most similar to Argyrochosma incana and can be somewhat difficult to distinguish from that species. The yellow (rather than white) color of the farina, the brown to purple (rather than black) color of the axes, the narrow segments (typically 3 to 5 millimeters (0.1 to 0.2 in) in A. incana) and the presence of sparse farina on the upper surface serve to distinguish A. delicatula.[1]

Taxonomy

It was first described by William Ralph Maxon and Charles Alfred Weatherby in 1939 as Notholaena delicatula, based on material collected by Edward Palmer in Coahuila. They placed it within a group of ferns closely related to Notholaena nivea.[2] Both Edwin Copeland and Weatherby suggested in the 1940s that this group of ferns might represent a distinct genus of its own.[3] 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,[4] and transferred this species to that genus as A. delicatula.[5] In 2018, Maarten J. M. Christenhusz transferred the species to Hemionitis as H. delicatula, as part of a program to consolidate the cheilanthoid ferns into that genus.[6]

While the distinctness of A. delicatula from A. incana has been questioned in the past,[1] phylogenetic studies have upheld the separation of the two taxa.[7] It lies within a subclade of the genus that also contains A. incana, A. pallens, A. peninsularis, A. palmeri, and A. pilifera. All members of the clade share pale farina principally composed of terpenoid compounds, unlike the dihydrostilbenoids found in the farina of other members of the genus.[7]

Distribution and habitat

Argyrochosma delicatula is principally known from the states of Chihuahua, Coahuila, and Nuevo Leon, in Mexico. One specimen collected in southeastern Arizona by John Gill Lemmon in 1880 has also been identified as this species.[1]

It grows in rocky places in either sun or shade, especially on limestone ledges, and on shady slopes.[1]

Citations
  1. Mickel & Smith 2004, p. 65.
  2. Maxon & Weatherby 1939, pp. 7–8.
  3. Windham 1987, p. 37.
  4. Windham 1987, p. 38.
  5. Windham 1987, p. 40.
  6. Christenhusz, Fay & Byng, p. 13.
  7. Sigel et al. 2011, p. 561.

Bibliography
  • Christenhusz, Maarten J. M.; Fay, Michael F.; Byng, James W. Plant Gateway's the Global Flora: A practical flora to vascular plant species of the world. 4. ISBN 978-0-9929993-9-1.
  • Maxon, W. R.; Weatherby, C. A. (1939). "Some species of Notholaena, new and old". Contributions from the Gray Herbarium. 127: 3–17. JSTOR 41764097.
  • Mickel, John T.; Smith, Alan R. (2004). The Pteridophytes of Mexico. Memoirs of the New York Botanical Garden. 88. Bronx, New York: New York Botanical Garden. ISBN 978-0-89327-488-7.
  • Sigel, Erin M.; Windham, Michael D.; Huiet, Layne; Yatskievych, George; Pryer, Kathleen M. (2011). "Species Relationships and Farina Evolution in the Cheilanthoid Fern Genus Argyrochosma (Pteridaceae)". Systematic Botany. 36 (3): 554–564. doi:10.1600/036364411X583547. JSTOR 23028975.
  • Windham, Michael D. (1987). "Argyrochosma, a new genus of cheilanthoid ferns". American Fern Journal. 77 (2): 37–41. doi:10.2307/1547438. JSTOR 1547438.
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