Argyrochosma peninsularis

Argyrochosma peninsularis is a medium-sized epipetric fern. The rhizome is short and compact, and may be horizontal or upright. It bears linear or lance-shaped, orange-brown scales 3 to 5 millimeters (0.1 to 0.2 in), without teeth at the margins.[1] Fronds arise from the rhizome in clumps; they measure 18 to 37 centimeters (7.1 to 15 in) in total length, about 40% of which is made up by the stipe (the stalk of the leaf, below the blade).[1] The stipe is a glossy dark purple in color, rounded, and bears scales near the base.[1]

The leaf blades are roughly triangular, widest at the base or somewhat widely lance-shaped, with the widest point a little above the base. They are bipinnate-pinnatifid (divided into pinnae and deeply lobed pinnules).[1] The rachis (leaf axis) has sparse farina (powder) scattered on its surface and occasional narrow scales about 1 millimeter (0.04 in) long.[1] Each blade has 7 to 12 pairs of pinnae, which are narrowly oblong, with a short stalk at the base, or none at all, connecting them to the rachis.[1] The ultimate segments are widest near the base or the middle, and the dark color of their stalks usually passes into the leaf, rather than halting at a joint at the segment base. Hairs are not present on the leaf blade.[1]

The sori occur along the veins, starting from halfway to two-thirds of the way out from axis to edge. The edges of the leaf segments do not curl or fold to protect them. Each sporangium contains 64 spores.[1] The spores are nearly smooth, unlike many members of the genus.[2]

The lack of joints at the leaf segment bases, very dark leaf and segment axes, and the presence of scales and farina on the rachis help distinguish it from other species in the genus,[3] particularly A. incana.[2]

It was first described by William Ralph Maxon and Charles Alfred Weatherby in 1939 as Notholaena peninsularis, based on material collected in Baja California. The epithet presumably refers to its range being limited to the Baja California Peninsula. They placed it within a group of ferns closely related to Notholaena nivea.[4] Both Edwin Copeland and Weatherby suggested in the 1940s that this group of ferns might represent a distinct genus of its own.[5] 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,[6] and transferred this species to that genus as A. peninsularis.[7] In 2018, Maarten J. M. Christenhusz transferred the species to Hemionitis as H. cairon (the epithet peninsularis was preoccupied), as part of a program to consolidate the cheilanthoid ferns into that genus. The epithet refers to the centaur Cairon in the novel The Neverending Story.[8]

It lies within a subclade of the genus that also contains A. delicatula, A. incana, A. pallens, 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.[9]

Argyrochosma delicatula is only known from the state of Baja California Sur, in Mexico.[3]

It grows in dry, rocky places, especially on granite, at an altitude of 1,200 to 2,150 meters (3,940 to 7,050 ft).[10]