Argyrochosma limitanea

Argyrochosma limitanea (Maxon) Windham subsp. mexicana (Maxon) Windham
(sp.: Latin: limitatus, bounded, limited, or bordered, in reference to its occurrence along the U.S.-Mexican border; subsp.: of Mexico, where the type specimen was collected)

Local names: Mexican silver fern, southwestern false cloak fern.

Leaves to 30 cm long, evergreen; leaf blades 4–5 pinnate, leathery, the upper surface bluish green, glabrous or with a few glands, the lower surface with very conspicuous whitish powdery or wax-like material; n = 2n = 81 (Windham 1993b). Limestone cliffs or rocky slopes; Brewster (O’Kennon 10952, BRIT, E.G. Marsh 245a, Chisos Mts., TEX-LL, R.T.Clausen & H. Trapido 5305, TAMU, and Dead Horse Mts.), El Paso (R.D. Worthington 3463.5, Franklin Mts., TEX-LL), and Culberson (W.D. Pohl & J.L. Blassingame 3518, BRIT; Sierra Diablo and Guadalupe Mts.—Yarborough & Powell 2002) cos.; AZ, NM, and TX; also n Mexico. Sporulating summer– fall. [Cheilanthes limitanea (Maxon) Mickel var. Mexicana (Maxon) Mickel, Notholaena limitanea Maxon subsp. Mexicana Maxon, N. limitanea var. Mexicana (Maxon) M. Broun] Although this taxon is similar to A. dealbata, according to Windham (1993b) the “distinctions between Argyrochosma dealbata and A. limitanea are subtle but apparently absolute. Although both occur in TX, their ranges do not overlap. Despite the morphologic similarities, isozyme analyses indicate that A. dealbata, a diploid species, was not involved in the origin of polyploid A. limitanea.” Windham (1993b) also noted that the species is an asexual triploid that reproduces via apogamy (= a type of asexual reproduction that does not involve fertilization; the sporophyte is formed directly from the gametophyte without gamete production). The taxonomy of this plant is not yet settled—although it and A. limitanea subsp. limitanea (AZ, CA, NM, UT, and n Mexico) are similar and are both triploids, molecular studies show they have different polyploid origins (Windham 1993b); changes in nomenclature may be necessary in the future if their evolutionary relationships are clarified. As with A. dealbata, the leaf blades are bluish green above and with obvious whitish farina below; however, the two can be distinguished by the key to species. Because of its rareness and limited distribution in the state, we consider this species to be of conservation concern in TX.

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