7a. Woodsia obtusa subsp. obtusa (Sprengel) Torrey
Stems compact to short-creeping, individual branches usually 5--10 mm diam. Blade coarsely cut and evidently 2-pinnate. Proximal pinnules of lower pinnae usually shallowly lobed or merely dentate. Spores averaging 42--47 µm. 2 n = 152.
Sporulating summer--fall. Cliffs and rocky slopes (rarely terrestrial); found on a variety of substrates including both granite and limestone; 0--1000 m; Ont., Que.; Ala., Ark., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.
D. F. M. Brown (1964) hypothesized that tetraploid Woodsia obtusa might be an autopolyploid derived from W . oregana . Recent isozyme and spore ornamentation studies indicate, however, that these species are not closely related, and the discovery of a diploid cytotype of W . obtusa suggests a different (albeit autopolyploid) origin for this taxon (M. D. Windham 1993). Tetraploid subsp. obtusa crosses with diploid subsp. occidentalis ; the resulting triploids are sterile and have malformed spores. It also hybridizes with W . oregana subsp. cathcartiana to form the sterile tetraploid hybrid known as W . × kansana Brooks.