Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Petiole stout, stramineous or green, with 3–7 free bundles, beset with chaffy scales, especially toward the base; blade pinnate-pinnatifid to tripinnate-pinnatifid, sometimes beset with flattened or hair-like, multicellular scales, otherwise glabrous or merely glandular, the minor axes decurrent on the major ones, the ridges on the upper side of the rachis not raised above the level of insertion of the costae; veins free, mostly ending a little short of the margin in elongate hydathodes; sori roundish, intramarginal (or nearly marginal) on the veins; indusium well developed, persistent, reniform or horseshoe-shaped, glabrous or sometimes glandular; mostly medium-sized to rather large mesophytic ferns with dark green, often evergreen lvs, the rhizomes short, stout, erect or shortly creeping, scaly and commonly covered with old petiole-bases, the lvs tufted, often in a vase-like configuration; x=41, at least for European and temperate American spp. 150, cosmop.
Our spp. produce frequent sterile hybrids and also enter into a polyploid complex. The following names are believed to apply to hybrids, as indicated: