Roots proliferous. Stems erect, unbranched; scales black with pale margins, linear, extremely narrow, 2--3 mm, only several cells wide. Leaves monomorphic. Petiole green in small leaves, black abaxially and green adaxially in large leaves, (1--)2--7(--16) cm, 1--2 times length of blade; indument of fine, nonglandular hairs on veins. Blade deltate, simple to 2-pinnate, 1---8(--12) × 1--6(--8) cm, thin, papery with scattered hairs on both surfaces; base truncate; margins crenate-dentate; apex pointed, not rooting. Rachis green, dull, glabrous. Pinnae in 0--5 pairs, ovate to deltate, simple to lobed to pinnate proximally, 1--6 × 1--3.5 cm, proximal pinna pair largest; base broadly cuneate to truncate; margins irregularly crenate; apex rounded to pointed. Veins free, evident. Sori 1--15(--35) per pinna, on both basiscopic and acroscopic sides. Spores 64 per sporangium. 2 n = 72.
Shaded limestone boulders; 0--50 m; Fla.; Mexico; West Indies; Central America; South America.
Asplenium pumilum is a widespread tropical American fern known only from a few spots in north central Florida. It is a very distinct species, readily recognized by its hairy blades and deltate leaves. Fertile forms vary from simple and only 2 cm to 2-pinnate and 28 cm, and all stages between the two extremes exist. Extreme forms are different enough to suggest that two species might be present.