Argyrochosma microphylla (Mett. ex Kuhn) Windham

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Argyrochosma microphylla, the small-leaf false cloak fern, is a species of fern native to New Mexico, Texas and northern Mexico. It grows on limestone rocks and cliffs, and has finely-divided leaves with small leaf segments, often folded in half when dry, which lack the white powder present on the leaf underside of many related species. First described as a species in 1869, it was transferred to the new genus Argyrochosma (the "false cloak ferns") in 1987, recognizing their distinctness from the "cloak ferns" (Notholaena sensu stricto).

The rhizome is short and horizontal, with leaves closely spaced. It bears thin, narrowly lanceolate or linear brown to dark orange, chestnut-brown, or reddish-brown scales 4 to 7 millimeters (0.2 to 0.3 in), of a uniform color and with entire (toothless) margins.

The leaves are 7 to 25 centimeters (2.8 to 9.8 in) long, arising in clumps. The stipe (the stalk of the leaf, below the blade) is rounded, somewhat flattened or grooved on the upper surface, reddish-brown to dark brown, or chestnut brown, usually darker at the base, without hairs or scales except for a few scales like those of the rhizome at the very base. It is 3 to 12 centimeters (1.2 to 4.7 in) long and 0.75 to 1.5 millimeters (0.030 to 0.059 in) in diameter, making up about one-half to one-third of the total length of the leaf. The leaf blades are 5 to 14 centimeters (2.0 to 5.5 in) long and 2 to 7 centimeters (0.8 to 3 in) wide. They vary from deltate (triangular) to ovate in shape, and range from tripinnate (cut into pinnae, pinnules and pinnulets) to quadripinnate at the base, where the leaf blade is most divided; it becomes merely bipinnate near the tip. The leaf blade is truncate (abruptly terminating), at the base and is acute (pointed) to acuminate at the tip. The rachis (leaf axis) is flattened or shallowly grooved above, straight or somewhat zig-zagging, and dark in color (though lighter than the stipe), as are the axes of the leaf segments. The color stops abruptly at a joint at the base of the leaf segment. There are 5 to 9 pairs of pinnae, alternating along the rachis or nearly opposite, and abruptly contracting in width near the tip. The costae (pinna axes) are straight to somewhat zig-zagging; when the latter, they typically do not branch at the angles. Leaf segments are orbicular (circular) to cordate (heart-shaped). They appear narrow or triangular when dry and curled, and are borne on small stalks. The leaf tissue is gray-green and leathery, obscuring the veins from the upper surface, and does not bear hairs or scales on either surface. Unlike many species in the genus, farina (powder) is not present on either surface of the leaf, although both surfaces are glaucous.

In fertile leaf segments, the sporangia are close to the margin, borne along the further third of the secondary veins branching from the midrib of the segment. They form a band about 1 millimeter (0.04 in) wide along the edge of each segment. Each sporangium contains 64 spores. The leaf segments are bent or curled under, often concealing the sporangia. The curled tissue retains the same texture as the rest of the leaf, and is not modified into a false indusium. Fertile segments often fold along their long axis. A. microphylla is a sexual diploid, with a chromosome count of 2n = 54.

 

Underside of a leaf, showing the lack of farina and slightly zig-zag axes.

The small leaf segments and shallow groove on the rachis make A. microphylla distinct from other species in the genus. The zig-zag character of the rachis in some specimens resembles A. fendleri, but the latter bears farina, and the smaller axes zig-zag as well. Of the other North American taxa lacking farina, A. formosa has somewhat larger leaf segments and a dark, straight rachis without a groove, while A. jonesii and A. lumholtzii lack the distinct joint at the base of the leaf, and do not have dramatically folded leaf segments when dry.

Argyrochosma microphylla - Wikipedia