Plants terrestrial or on rock. Stems short- to long-creeping or suberect, branched; scales deep tawny yellow to dark reddish brown [black], concolored or bicolored, linear-lanceolate to lanceolate, margins entire, erose-ciliate, or minutely dentate. Leaves monomorphic to somewhat dimorphic, densely clustered to closely spaced [distant], 15–110 cm. Petiole chestnut brown to dark purple or blackish, with single groove adaxially, glabrous, hispid, or strigose, with 1 or 2 vascular bundles. Blade lanceolate, ovate, trowel-shaped, or fan-shaped, 1–4(–9)-pinnate proximally, membranaceous to papery, both surfaces commonly glabrous (2 species with scattered hairs), adaxially dull or shiny, not striate; rachis straight or flexuous. Ultimate segments subsessile to short-stalked (stalks terminating in cupulelike swelling at base of pinna in A. tenerum), round, fan-shaped, rhombic, or oblong, 3–29 mm wide; base truncate to cuneate, free from costa; stalk dark, often lustrous; fertile segments with marginal lobes recurved to form false indusia. Veins of ultimate segments conspicuous, free, ± dichotomously forking near base and well above segment base [anastomosing in a few tropical species], parallel distally. False indusia light gray-green or brown to dark brown, narrow, 0.6–1 mm wide, marginal, concealing sporangia until sporangia dehisce. Sporangia submarginal, borne along or sometimes also between veins on abaxial surface of false indusium, paraphyses and glands absent. Spores yellow or yellowish brown, tetrahedral-globose, trilete, rugulate to rugose or tuberculate, equatorial ridge absent. x = 29, 30.
Nearly worldwide except at latitudes greater than 60°.
Most diverse in Andean South America, Adiantum is primarily a tropical genus; of the nine species occurring in the flora, A. melanoleucum, A. tenerum, and A. tricholepis are strictly subtropical. Adiantum hispidulum occurs only as an escape from cultivation. The genus is absent from dry areas in the interior of the continent.
Adiantum is a very clearly circumscribed genus of ferns, the character state "sporangia borne on abaxial surface of false indusium" being both necessary and sufficient to define it. Within this large and widespread genus, however, species relationships are mostly unknown. An evolutionary classification of the group is indeed much needed (R. M. Tryon and A. F. Tryon 1982).
Species ca. 150–200 (9 in the flora).
|1||Segments at middle of penultimate divisions of blades ± fan-shaped, rhombic, transversely oblong, or nearly round, about as long as broad.||> 2|
|1||Segments at middle of penultimate divisions of blades ± oblong or long-triangular, at least 2 times as long as broad (rarely, reniform).||> 5|
|2||Dark color of stalks extending into base of ultimate segments.||Adiantum capillus-veneris|
|2||Dark color of stalks ending ± abruptly at base of ultimate segments.||> 3|
|3||Segment stalks terminating in small, cupulelike swelling at base of ultimate segments.||Adiantum tenerum|
|3||Segment stalks not terminating in small, cupulelike swelling at base of ultimate segments.||> 4|
|4||Ultimate segments glabrous.||Adiantum jordanii|
|4||Ultimate segments hirsute.||Adiantum tricholepis|
|5||Rachises hispid or strigose; blades pinnate (occasionally pseudopedate in Adiantum hispidulum).||> 6|
|5||Rachises glabrous; blades pseudopedate.||> 7|
|6||Ultimate segments with scattered multicelled hairs; rachises hispid; false indusia ± round.||Adiantum hispidulum|
|6||Ultimate segments glabrous; rachises strigose; false indusia crescent-shaped.||Adiantum melanoleucum|
|7||Segments at middle of penultimate divisions of blades ± oblong; leaves lax-arching, blades fan-shaped.||> 8|
|7||Segments at middle of penultimate divisions ± long-triangular or reniform; leaves arching to stiffly erect, blades fan-shaped to funnel-shaped.||> 9|
|8||Segments at middle of penultimate divisions of blades generally less than 3.2 times as long as broad, apices with rounded, crenulate or crenate-denticulate lobes, lobes separated by shallow sinuses 0.1–2(–3.7) mm, segment stalks ca. 0.6–0.9 mm.||Adiantum pedatum|
|8||Segments at middle of penultimate divisions usually more than 3.2 times as long as broad, apices with sharply denticulate, angular lobes, lobes separated by deep sinuses 0.6–4 mm, segment stalks to 0.6 mm.||Adiantum aleuticum|
|9||Central ultimate segments on stalks less than 0.9 mm; false indusia mostly less than 3.5 mm.||Adiantum aleuticum|
|9||Central ultimate segments on stalks generally greater than 0.9 mm; false indusia mostly exceeding 3.5 mm.||Adiantum viridimontanum|