1. Adiantum Linnaeus, Sp. Pl. 2: 1094. 1753; Gen. Pl. ed 5, 485. 1754.
Maidenhair fern [Greek adiantos, unwetted, for the glabrous leaves, which shed raindrops]
Cathy A. Paris
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.
Species ca. 150--200 (9 in the flora): 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).
Fernald, M. L. 1950b. Adiantum capillus-veneris in the United States. Rhodora 52: 201--208. Paris, C. A. 1991. Adiantum viridimontanum, a new maidenhair fern in eastern North America. Rhodora 93: 105--122. Paris, C. A. and M. D. Windham. 1988. A biosystematic investigation of the Adiantum pedatum complex in eastern North America. Syst. Bot. 13: 240--255. Wagner, W. H. Jr. 1956. A natural hybrid, ×Adiantum tracyi C. C. Hall. Madroño 13: 195--205.