6. Pellaea ternifolia (Cavanilles) Link, Fil. Spec. 59. 1841.
Pteris ternifolia Cavanilles, Descr. Pl., 266. 1802
Stems compact, ascending, stout, 5--10 mm diam.; scales bicolored, linear-subulate, 0.1--0.3 mm wide, centers black, thick, margins brown, thin, erose-dentate. Leaves monomorphic, clustered on stem, 10--50 cm; croziers sparsely to densely villous. Petiole black or dark purple, lustrous, rounded or slightly flattened adaxially, without prominent articulation lines. Blade linear to ovate, deeply pinnate-pinnatifid proximally, 2.5--8 cm wide; rachis black or purple throughout, straight, often flattened adaxially, glabrous or villous. Pinnae perpendicular to rachis or slightly ascending, not decurrent on rachis, ternate at base of leaf; costae absent. Ultimate segments linear-oblong, 10--40 mm, leathery, glabrous to sparsely villous abaxially on midrib; margins recurved on fertile segments, rarely covering more than 1/2 abaxial surface, borders whitish, entire; apex mucronate. Veins of ultimate segments obscure. Sporangia long-stalked, containing 64 spores, not intermixed with farina-producing glands.
Pellaea ternifolia is represented in the flora by three morphologically and chromosomally distinct taxa. These discrete genetic entities also show a tendency toward geographic isolation and are treated here as subspecies. Diploid populations referred to P . ternifolia subsp. ternifolia are scattered from Texas through Mexico to South America. The pubescent tetraploid ( P . ternifolia subsp. villosa ) follows the Sierra Madre Oriental from Puebla, Mexico, north to Texas; the glabrous tetraploid ( P . ternifolia subsp. arizonica ) occurs in Arizona, Texas, and northern Mexico. Isozyme and chromosome studies suggest that both tetraploids are segmental allopolyploids produced by hybridization between subsp. ternifolia and other (as yet unidentified) diploid elements within P . ternifolia .