Myriopteris rufa, commonly known as Eaton's lip fern, is a small to medium-sized fern of Mexico and the southwestern United States, with outlying populations in Costa Rica and the Appalachian Mountains. The leaf is about three times divided into oblong or round bead-like segments, which are covered in a thick mat of reddish-brown hair below and are variably hairy above. Broad scales on the leaf axis distinguish it from woolly lip fern (M. tomentosa). One of the cheilanthoid lip ferns, it was usually classified in the genus Cheilanthes, as Cheilanthes eatonii, until 2013, when the genus Myriopteris was again recognized as separate from Cheilanthes. It typically grows in rocky habitats, most frequently on limestone but also sometimes on basalt or shale. The common name honors Daniel Cady Eaton, who helped distinguish it from other lip ferns.
Description The rhizome is compact, horizontal and about 3 millimeters (0.1 in) or 4 to 8 millimeters (0.2 to 0.3 in) in diameter, with leaf blades closely spaced along its length. and branching. It bears persistent scales 3 to 4 millimeters (0.1 to 0.2 in) long, which are linear to slightly lanceolate, straight or slightly contorted, and loosely pressed against the surface of the rhizome. They have a broad, dark reddish-brown or black central stripe which is sharply differentiated from the narrow, light brown margins. The margins are entire (toothless). The fronds arise from the rhizome in clusters. Unlike many ferns, they do not emerge as coiled fiddleheads (noncircinate vernation). When mature, they are 6 to 38 centimeters (2.4 to 15.0 in) long and 1.5 to 5 centimeters (0.59 to 1.97 in) wide. The stipe (the stalk of the leaf, below the blade) is 3 to 16 centimeters (1.2 to 6.3 in) long and typically about one-quarter to one-third of the total length of the frond. It is dark brown, chestnut brown, or reddish-brown to purplish-black in color, round in cross-section, and bears abundant linear tan or white scales 1.5 to 2 millimeters (0.059 to 0.079 in) long, and sometimes some narrowly lanceolate scales and a few hairs. The leaf blades are narrowly lanceolate to oblong-lanceolate (broadest near but not at the base, but not very much broader than the rest of the leaf). The blade is bi- to tripinnate, or tri- to tetrapinnate (cut into pinnae, pinnules, pinnulets, and divisions of pinnulets) at the base. Each blade is cut into about 12 to 18 pairs of pinnae. The blades are acute (pointed) at the tip and obtuse (blunt) to truncate at the base. The rachis (leaf axis) is rounded on the upper side, dark brown in color, and bears soft hairs of uniform shape, and linear-lanceolate scales. The pinnae are not jointed at the base, and the dark pigmentation of the rachis enters the base of the pinnae. The pinnae at the base of the leaf are about the same size as the pinnae immediately above them, and the pinnae are more or less symmetric about the costa (pinna axis). The upper sides of the costae are green for most of their length. On their underside, they bear multiple rows of lanceolate to linear scales, not more than 0.4 to 0.7 mm wide, loosely overlapping, but not concealing the leaf surface, with a truncate or subcordate base, lacking lobes that overlap with other scales. The margins are erose-dentate (jagged to toothed) and occasionally have one or two cilia (hairlike projections) at their base and without cilia at their edges. (This character distinguishes the species from the very similar Myriopteris tomentosa, where the scales of the costa are linear and appear hairlike except on close examination.) The smallest segments of the leaf are oblong and obtuse to round and bead-like, as in many species of Myriopteris, up to 1 to 3 millimeters (0.039 to 0.118 in) in length.The upper surface of the leaf may bear abundant fine, white to rusty, unbranched curly hairs 0.5 to 1 millimeter (0.020 to 0.039 in) long, or they may be sparse to almost entirely absent. The lower surface is covered with a mat of dense, woolly, curly reddish-brown hairs.
On fertile fronds, the edge of the leaf curls over somewhat to protect the sori on the underside, but the leaf tissue is not differentiated into false indusia or only slightly so. When distinct, the false indusia are somewhat different in appearance and texture of the leaf tissue, and are 0.05 to 0.25 mm wide. Beneath them, the sori are more or less continuous around the margin of the bead-like leaf segments. Each sporangium in a sorus carries 32 brown spores. The triploid sporophyte has a chromosome number of 90. Reproduction is apogamous: triploid spores are formed by mitosis, rather than meiosis, and grow into gametophytes, which sprout a genetically identical sporophyte without fertilization. Specimens intermediate between M. rufa and M. windhamii (Cheilanthes villosa) are known from Texas and New Mexico and may be hybrids. However, both taxa are apogamous in North America, so recent sexual hybridization is unlikely. The most similar species to M. rufa, which overlaps it in much of its range, is M. tomentosa, from which M. rufa may consistently be distinguished by the presence of distinctly lanceolate, rather than hair-like, scales beneath the rachis, as described above. Material placed in C. castanea (with a relatively hairless upper surface) has been confused with M. gracillima, but the latter is smaller and has narrower scales underneath with long cilia.
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