Stems compact, ascending, stout, 5--10 mm diam.; scales uniformly reddish brown (or tan), linear-subulate, 0.1--0.3 mm wide, thin, margins entire to denticulate. Leaves somewhat dimorphic, sterile leaves shorter and less divided than fertile leaves, clustered on stems, 5--50 cm; croziers villous. Petiole reddish purple to nearly black, lustrous, rounded adaxially, without prominent articulation lines. Blade elongate-deltate, usually 2-pinnate proximally, 2--18 cm wide; rachis reddish purple throughout, straight, rounded adaxially, densely pubescent adaxially with short, curly, appressed hairs. Pinnae perpendicular to rachis or ascending, not decurrent on rachis, usually with 3--15 ultimate segments; costae straight, 10--100 mm, often longer than ultimate segments. Ultimate segments linear-oblong, 10--75 mm, leathery, sparsely villous abaxially near midrib; margins weakly recurved to plane on fertile segments, usually covering less than 1/2 abaxial surface, borders whitish, crenulate; apex obtuse to slightly mucronate. Veins of ultimate segments obscure. Sporangia long-stalked, containing 32 spores, not intermixed with farina-producing glands. n = 2 n = 87, apogamous. Sporulating summer--fall. Calcareous cliffs and rocky slopes, usually on limestone; 100--2500 m; Ont., Que.; Ala., Ariz., Ark., Colo., Conn., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Miss., Mo., Nebr., Nev., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., W.Va., Wis., Wyo.; Mexico; Central America in Guatemala. Contrary to D. B. Lellinger's (1985) hypothesis, isozyme data indicate that neither Pellaea glabella nor P . ternifolia was involved in the origin of this apogamous triploid. Instead, it appears that P . atropurpurea is an autopolyploid derivative of a single diploid taxon that has not yet been located. A thorough survey of spore number per sporangium in this species should be undertaken to determine whether the diploid progenitor is still extant. Collections from western Canada identified as P . atropurpurea actually represent P . gastonyi , an apogamous tetraploid produced by hybridization between P . atropurpurea and diploid populations of P . glabella . Pellaea atropurpurea has also hybridized with P . wrightiana ; the hybrid is a rare apogamous pentaploid known only from western Oklahoma. Pellaea lyngholmii is the apogamous tetraploid hybrid between P . atropurpurea and P . truncata . Pellaea atropurpurea is distinguished from all these hybrids by having rachises that are densely pubescent adaxially, larger ultimate segments, and spores averaging less than 62 Ám in diameter.