Stems short-creeping; scales bicolored, centers dark reddish brown, margins pale tan, erose-ciliate. Leaves arching or sometimes pendent, closely spaced, 20–110 cm. Petiole 1–3 mm diam., glabrous, occasionally glaucous. Blade trowel-shaped, pinnate, 12–60 × 12–60 cm, gradually reduced distally, glabrous; proximal pinnae 3-pinnate; rachis straight, glabrous, not glaucous. Segment stalks 1–5 mm, with dark color ending abruptly at segment base, terminating in cupulelike swelling at base of segment (unlike any other species of Adiantum in the flora). Ultimate segments fan-shaped or rhombic, about as long as broad; base cuneate; apex rounded or acute, lobed, lobes separated by narrow incisions 0.5 mm wide. Indusia transversely oblong to crescent-shaped, 0.5–2 mm, glabrous. Spores mostly 40–58 µm diam. 2n = 60.
Phenology: Sporulating throughout the year.
Habitat: Restricted to moist, shaded, limestone ledges, sink walls, and grottoes in the flora
Elevation: 0–50 m
Fla., e, s Mexico, Central America in Guatemala, Honduras, Nicaragua, Costa Rica, South America in Venezuela.
Adiantum tenerum is readily distinguished from other species in the flora by the ultimate segments conspicuously articulate to the stalks.