Roots not proliferous. Stems short-creeping, often branched; scales black throughout or with brown borders, lanceolate, 2--5 × 0.2--0.5 mm, margins entire to denticulate. Leaves monomorphic. Petiole reddish brown or blackish brown throughout, lustrous, 1--4(--7) cm, 1/6--1/4 length of blade; indument absent or of black, linear-lanceolate or filiform scales at base. Blade linear, 1-pinnate, 3--22 × 0.5--1.5 cm, thin, glabrous or sparsely pubescent; base gradually tapered; apex narrowly acute, not rooting. Rachis reddish brown throughout, lustrous, glabrous or nearly so. Pinnae in 15--35 pairs, oblong to oval; medial pinnae 2.5--8 × 2.5--4 mm; base broadly cuneate, with or without low, rounded acroscopic auricle; margins shallowly crenate to serrate or ± entire; apex obtuse. Veins free, evident. Sori 2--4 pairs per pinna, on both basiscopic and acroscopic sides. Spores 64 per sporangium. 2 n = 72, 144.
Subspecies 4 (2 in the flora): worldwide.
In North America, as in Europe, Asplenium trichomanes consists of diploid and tetraploid cytotypes, treated here as subspecies. Asplenium trichomanes subsp. trichomanes , the diploid, is found on noncalcareous rocks. In the southwestern United States it occurs at high elevations. Asplenium trichomanes subsp. quadrivalens , the tetraploid, grows on calcareous substrates and has a more northern distribution (R. C. Moran 1982). Triploid hybrids are known between the diploids and tetraploids (R. C. Moran 1982; W. H. Wagner Jr. and F. S. Wagner 1966).
