Argyrochosma incana

Argyrochosma incana, the hoary 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).

Argyrochosma incana
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Class:	Polypodiopsida
Order:	Polypodiales
Family:	Pteridaceae
Genus:	Argyrochosma
Species:	A. incana
Binomial name
Argyrochosma incana (C.Presl) Windham
Synonyms
Cheilanthes incana (C.Presl) Mickel & Beitel Gymnogramma candida Mett. Hemionitis incana (C.Presl) Christenh. Notholaena incana C.Presl Pellaea candida (Mett.) Prantl

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 lance-shaped scales 5 to 6 millimeters (0.2 to 0.2 in) long and 0.8 millimeters (0.03 in) wide, without teeth at the margins,[1][2] of a uniform dark, shiny brown color[1] or yellowish-brown with some dark brown patches.[2] From the rhizome, the fronds arise in clumps. From base to tip of leaf, they are 5 to 20 centimeters (2.0 to 7.9 in) long,[3], sometimes up to 30 centimeters (12 in).[1] 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.[1][2] It is typically 4 to 12 centimeters (1.6 to 4.7 in) in length[2] and 0.75 to 2 millimeters (0.030 to 0.079 in) in diameter.[3]

The leaf blades are triangular[1] or lance-shaped,[2] tripinnate (cut into pinnae, pinnules and pinnulets) to almost quadripinnate at the bases. They are 3.5 to 8 centimeters (1.4 to 3.1 in) wide, occasionally as little as 2.5 centimeters (0.98 in).[1] They are blunt at the base and pointed to acuminate at the tip.[2] The rachis (leaf axis) is round, rather than 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.[1] Each blade bears 6 to 9 pairs of pinnae, borne oppositely on the rachis. The ultimate segments of the blade are elliptically shaped, 3 to 5 millimeters (0.1 to 0.2 in) long, occasionally as little as 2.5 millimeters (0.098 in). They are grayish-green in color[1] and leathery in texture.[3] The underside of the leaf is coated in white farina (powder), which may be sparsely scattered on the upper surface[1] or absent from it.[3][4] The leaf axes curl upwards when dry.[2]

The sori lie along the veins, forming a band 0.5 to 1.5 millimeters (0.02 to 0.06 in) wide.[1] The veins themselves tend to blend into the leaf tissue.[2] 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.[1][3]

Within the heart of its range, in Mexico, A. incana is both the most widespread member of the genus and the most variable.[5] Some of the characteristics observed to vary are the size of segments (reduced to 2 millimeters (0.08 in) wide), the density of adaxial farina, the shape and color of the blade axes (zigzag 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.[5]

In Mexico, it is quite similar to A. delicatula, from which it is distinguished by having white (rather than pale yellow) farina,[6] and A. formosa, which lacks farina.[1] Most specimens in the United States were originally mislabeled as A. limiteana, but this species lacks the joint and abrupt end of dark color at the base of its leaf segments.[7]

Taxonomy

It was first described in 1825 by Carl Borivoj Presl as Notholaena incana, based on material collected by Thaddäus Haenke in Mexico.[8][9] 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.[10] 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.[11] 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.[12]

William Ralph Maxon and Charles Alfred Weatherby placed N. incana within a group of ferns closely related to Notholaena nivea.[13] Both Edwin Copeland and Weatherby suggested in the 1940s that this group of ferns might represent a distinct genus of its own.[14] 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,[15] and transferred this species to that genus as A. incana.[16] Meanwhile, John Mickel and Joe Beitel had transferred the species to Cheilanthes as C. incana in their monograph on the ferns of Oaxaca, which was published in 1988;[17] Mickel and Alan R. Smith recognized Argyrochosma in 2004 when preparing a fern flora of Mexico.[1] In 2018, Maarten J. M. Christenhusz transferred the species to Hemionitis as H. incana, as part of a program to consolidate the cheilanthoid ferns into that genus.[18]

While the distinctness of A. delicatula from A. incana has been questioned in the past,[6] phylogenetic studies have upheld the separation of the two taxa.[19] It lies within a subclade of the genus that also contains A. delicatula, 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.[19]

Distribution and habitat

Argyrochosma incana is known from New Mexico and Arizona[3][20] south through most of the states of Mexico to Guatemala,[5] and also as a disjunct in the Dominican Republic.[21] David Lellinger reports a specimen (Gómez 7156, CR) collected in Costa Rica, although this occurrence is not mentioned in other floras.[22]

In Mexico, it grows on rocky slopes, banks, and ledges, often shaded or in woods, especially pine-oak forests. It is found at an altitude from 800 to 2,950 meters (2,620 to 9,680 ft).[5] Irving Knobloch reported that material collected from rock crevices in Chihuahua was growing in neutral soils within the crevices.[23] At the northern edge of its range, in the United States, it is found growing from canyon walls, exclusively on igneous rock.[3] In Guatemala, it is found on both shaded and sunny rocks, and growing from rock walls and cliff crevices.[4]

Citations

1. Mickel & Smith 2004, p. 66.
2. Lellinger 1985, p. 160.
3. Windham 1993.
4. Stolze 1981, p. 323.
5. Mickel & Smith 2004, p. 67.
6. Mickel & Smith 2004, p. 65.
7. Mickel & Smith 2004, p. 68.
8. Presl 1825, p. 19.
9. Maxon & Weatherby 1939, pp. 6–7.
10. Mettenius 1859, pp. 49–51.
11. Mettenius 1859, p. 50.
12. Prantl 1882, p. 417.
13. Maxon & Weatherby 1939, pp. 3–4.
14. Windham 1987, p. 37.
15. Windham 1987, p. 38.
16. Windham 1987, p. 40.
17. Mickel & Beitel 1988, p. 115.
18. Christenhusz, Fay & Byng, p. 16.
19. Sigel et al. 2011, p. 561.
20. Kartesz 2014.
21. Tryon & Weatherby 1956, pp. 92-93.
22. Lellinger 1989, p. 121.
23. Knobloch 1942, p. 132.

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.
• Kartesz, John T. (2014). "Argyrochosma". Biota of North America Program.
• Knobloch, Irving W. (1942). "A fern collection from Chihuahua". American Fern Journal. 32 (4): 127–138. doi:10.2307/1545217. JSTOR 1545217.
• Lellinger, David B. (1985). A Field Manual of the Ferns & Fern-Allies of the United States & Canada. Washington, DC: Smithsonian Institution Press. ISBN 0874746035.
• Lellinger, David B. (1989). The Ferns and Fern-Allies of Costa Rica, Panama, and the Choco (Part I: Psilotaceae through Dicksoniaceae). Pteridologia. 2A. The American Fern Society. ISBN 0-933500-01-7.
• Maxon, W. R.; Weatherby, C. A. (1939). "Some species of Notholaena, new and old". Contributions from the Gray Herbarium. 127: 3–17. JSTOR 41764097.
• Mettenius, G. (1859). "Uber einige Farngattungen: Cheilanthes". Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft. 3 (1): 47–99.
• Mickel, John T.; Beitel, Joseph M. (1988). Pteridophyte Flora of Oaxaca, Mexico. Memoirs of the New York Botanical Garden. 46. Bronx, New York: New York Botanical Barden. ISBN 978-0-89327-323-1.
• 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.
• Prantl, K. (1882). "Die Farngattungen Cryptogramme und Pellaea". Botanische Jahrbücher für Systematik, Pflanzengeschichte und Pflanzengeographie. 3 (5): 403–430.
• Presl, Carl Borivoj (1825). Reliquiae Haenkeanae. 1. Prague: J.G. Calve.
• 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.
• Stolze, Robert G. (1981). "Ferns and Fern Allies of Guatemala Part II Polypodiaceae". Fieldiana. Botany New Series, No. 6. ISSN 0015-0746.
• Tryon, Rolla M.; Weatherby, Una F. (1956). "A revision of the American species of Notholaena". Contributions from the Gray Herbarium of Harvard University (179): 1–106. JSTOR 41764632.
• Windham, Michael D. (1987). "Argyrochosma, a new genus of cheilanthoid ferns". American Fern Journal. 77 (2): 37–41. doi:10.2307/1547438. JSTOR 1547438.
• Windham, Michael D. (1993). "Argyrochosma incana". In Flora of North America Editorial Committee (ed.). Flora of North America North of Mexico. 2: Pteridophytes and Gymnosperms. New York and Oxford: Oxford University Press.