Stems compact, ascending, stout, 5--10 mm diam.; scales uniformly reddish brown, linear-subulate, 0.1--0.3 mm wide, thin, margins sinuous, entire to denticulate. Leaves monomorphic, clustered on stem, 2--40 cm; croziers sparsely villous. Petiole brown, lustrous, rounded adaxially, occasionally with prominent articulation lines near base. Blade linear-oblong to ovate-lanceolate, 1--2-pinnate proximally, 1--8 cm wide; rachis brown throughout, straight, rounded adaxially, nearly glabrous. Pinnae somewhat ascending, decurrent on rachis, usually with 3--7 lobes or ultimate segments; costae when present straight, 1--50 mm, often shorter than ultimate segments. Ultimate segments oblong-lanceolate, 5--20 mm, leathery to herbaceous, glabrous except for occasional hairlike scales abaxially near midrib; margins recurved on fertile segments, covering less than 1/2 abaxial surface, borders whitish, erose-denticulate; apex obtuse. Veins of ultimate segments usually obscure. Sporangia long-stalked, containing 32 or 64 spores, not intermixed with farina-producing glands. Pellaea glabella includes four geographically and genetically isolated taxa treated here as subspecies. D. B. Lellinger (1985) recognized three species in this difficult group, but isozyme analyses (G. J. Gastony 1988) showed that one of these ( P . suksdorfiana ) is an autotetraploid derivative of the diploid known as P . occidentalis . As a result, Gastony recognized just two species: P . glabella (with two varieties) and P . occidentalis (with two subspecies). The few morphologic features that distinguish these taxa, however, are subtle and environmentally plastic, and the isozyme data indicate that they are less divergent genetically than any other pair of Pellaea species in North America. Therefore, a more conservative taxonomic treatment seems warranted.
Similar to P. atropurpurea, but with monomorphic lvs mostly 10-20 cm, the pinnae somewhat decurrent, gradually departing at acute angles to the rachis; basal pinnae with 3-7 segments, less than twice as long as the upper; longest ultimate segments mostly 1-2 cm; petiole and rachis glabrous or slightly hairy; scales of the rhizome discrete, uniformly rust- brown, lustrous, the tips not entangled; margins of the fertile segments usually strongly reflexed, leaving but few sporangia exposed; ours mostly tetraploid (2n=116) and apogamous, the sporangia with 32 spores. Calcareous cliffs or bluffs; Vt. to Va. and Tenn., w. to Minn., Kans., and Tex., and in the western cordillera. Our pls are var. glabella. (P. atropurpurea var. bushii)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.