Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Petiole mostly wiry, green to black, sparsely scaly, otherwise glabrous, with 2 bundles below, these united above into a single bundle mostly X- shaped in section; lvs clustered, the blade simple and entire to thrice pinnate; veins free except in A. rhizophyllum; sori on the lower lf-surface, ± elongate, the indusia attached at one side, all facing distally, thin and hyaline, glabrous, entire; mostly ± evergreen ferns with short-creeping to suberect rhizomes and membranous to coriaceous lvs. 600+, mostly warm reg. (Camptosorus)
Our spp. produce ± sterile hybrids and also enter into a large polyploid complex. The following names are believed to apply to sterile hybrids, as indicated: