Monographs Details: Adiantum
Authority: Mickel, John T. & Smith, Alan R. 2004. The pteridophytes of Mexico. Mem. New York Bot. Gard. 88: 1-1054.
Family:Pteridaceae
Scientific Name:Adiantum
Description:Genus Description - Terrestrial or epipetric; rhizomes short- to long-creeping or suberect, scaly; fronds small to large, usually monomorphic; stipes castaneous to atropurpureous, lustrous, glabrous to densely covered with narrow scales or hairs, brittle; blades simple to 5 times pinnate (or pedate); ultimate divisions (pinnae, pinnules, or pinnulets, depending on blade dissection) often rhomboidal, trapezoidal, or flabellate, sessile to short-petiolulate, never adnate, deciduous (articulate) in some species; laminae generally glabrous, sometimes glaucous, some species with stiff hairs or hairlike scales on the blades; veins free, forking, rarely anastomosing, sometimes with linear epidermal idioblasts (false veins) between the true veins; sporangia borne on the veins of recurved, membranous margins, indusia (and thus also sori) reniform to shortoblong shortoblong to linear, lunate, or arcuate, one to usually several per ultimate division; paraphyses absent; spores tetrahedral, usually tan to yellowish; x=29, 30.

Discussion:Lectotype (chosen by J. Smith, Hist. Fil. 274. 1875): Adiantum capillus-veneris L. Hewardia J. Sm., J. Bot. (Hooker) 3: 432, t. 16, 17. 1841. Type: Hewardia adiantoides J. Sm. [Adiantum adiantoides (J. Sm.) C. Chr.]. Worldwide, there are about 200 species of Adiantum, with more than half of them in tropical America; a few reach into temperate regions. About 35 species occur in Mexico, of which 13 are widespread into South America and the West Indies. Two species (A. aleuticum, A. jordanii) are found primarily in temperate North America and reach Mexico only in the extreme northwestern states. Five species are known so far only from Mexico (A. amblyopteridium, A. galeottianum, A. mcvaughii, A. oaxacanum, A. shepherdii), and two others are relatively rare outside of Mexico (A. feei, A. tricholepis). Adiantum species grow mostly at low to middle elevations in wet forests; one (A. apillusveneris) is pantropical and subtropical in moist microhabitats in otherwise dry regions. Adiantum andicola and A. poiretii generally occur at middle and high elevations, above the elevation of most other adiantums. Characters important in grouping and keying the species include the branching pattern of the blades (pinnate or imparipinnate; with or without a conform terminal pinna), degree of division of blades (pinnate, 2-pinnate, or more divided), presence or absence of venuloid idioblasts (epidermal false “veins”), shape of the ultimate divisions (whether dimidiate or flabellate), indument of blades and axes (whether of hairs, scales, or neither), scale margins (entire, denticulate, or ciliate), venation (free or anastomosing), vein ends (whether in teeth or sinuses), and presence or absence of articulation zones at the bases of the ultimate divisions. This last feature is presumably an adaptation to periodically drier habitats. The affinities of Adiantum are with genera of Pteridaceae, but the genus is clearly somewhat isolated and is often treated in a family of its own, Adiantaceae, or in its own subfamily, Adiantoideae, within Pteridaceae (e.g., by R. Tryon et al. in Kubitzki, 1990). In most published trees, Adiantum is sister to the vittarioid ferns, which are usually treated as a separate family Vittariaceae (e.g., Hasebe et al., 1995; Cranfill, unpubl. data). Sori are borne on the veins (and, reportedly, between the veins) of the strongly recurved and highly modified margins (false indusia), so that the sporangial capsules oppose the abaxial blade surfaces. This character is an autapomorphy for the genus. The indusia may be either round-reniform, lunate, oblong, or linear.