Roots not proliferous. Stems short-creeping, unbranched; scales dark brown to black throughout, narrowly linear-deltate, 2--4 × 0.3--0.6 mm, margins entire. Leaves ± dimorphic; fertile leaves taller and more erect than sterile leaves. Petiole reddish brown throughout, lustrous, 1--10 cm, 1/4--1/3 length of blade; indument of dark brown to black, filiform scales at base. Blade lustrous, linear to narrowly oblanceolate, 1-pinnate throughout, 4--50 × 2--5(--7) cm, thin, glabrous, or occasionally sparsely pubescent; base gradually tapered; apex acute, not rooting. Rachis reddish or purplish brown throughout, lustrous, glabrous. Pinnae in 15--45 pairs, oblong to quadrangular; medial pinnae 1--2.5 × 0.3--0.5 cm; base with conspicuous acroscopic and sometimes basiscopic auricle, this overlapping rachis; margins crenate to serrulate, sometimes more deeply incised in robust specimens; apex acute to obtuse. Veins free, evident. Sori 1--12 pairs per pinna, on both basiscopic and acroscopic sides. Spores 64 per sporangium. 2 n = 72. Forest floor or on rocks, often invading masonry and disturbed soils; 0--1300 m; Ont., Que.; Ala., Ark., Ariz., Colo., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.Mex., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.; s Africa. The combining author for Asplenium platyneuron is often given as Oakes ex D. C. Eaton; see D. B. Lellinger (1981) for justification of the authorship employed here. Asplenium platyneuron is remarkable in that it occurs in southern Africa as well as in North America. No other North American fern has this distribution. Asplenium platyneuron is an ecological generalist and is particularly characteristic of disturbed woodlands. This species is migrating northward on the northern portions of its range in the upper Great Lake states (W. H. Wagner Jr. and D. M. Johnson 1981). Proliferous buds on the lowest pinnae allow formation of clumps with stems at several layers in the litter. Asplenium platyneuron hybridizes with A . rhizophyllum , A . trichomanes (producing A . × virginicum Maxon), A . pinnatifidum , A . ruta-muraria (producing A . × morganii W. H. Wagner & F. S. Wagner), A . bradleyi , and A . montanum (producing sterile A . bradleyi ).
Rhizome 2-3.5 mm thick, the few linear-subulate black scales 2.5-4 mm; lvs subdimorphic, 5-50 cm, with dark brown petiole and rachis; sterile blades spreading, 5-10 cm, very shortly petiolate, pinnate, with 15-22 pairs of sessile, alternate, crowded, oblong pinnae 7-10 נ3.5-4.5 mm, these rounded and remotely serrulate, tending to be auriculate at the upper base; fertile lvs few, erect, to 4.5 dm נ6 cm, short-petiolate, the blade linear-oblong, acute, narrowed basally, pinnate, the 30-50 pairs of alternate pinnae less crowded, more linear-oblong (except the reduced, more deltoid lowermost ones), to 3.5 cm נ6 mm, more prominently auriculate at the upper base and sometimes at the lower base as well, mostly serrate, seldom more deeply cleft; 2n=72. Woods, banks, and rocks in circumneutral soil; Que. and Ont. to se. Minn., Kans., and Colo., s. to Fla., Tex., and Ariz.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.