Plants terrestrial. Stems branched, slender. Leaves often more than several meters, 2-pinnate or more divided, climbing by means of twining rachis; fertile pinnae borne toward apex of fertile leaves. Blades of short, alternate primary pinnae. Pinnules ± entire to palmately or pinnately lobed; fertile and sterile pinnae similar or fertile pinnae greatly contracted.
Rhizome creeping, slender, dichotomously branched; lvs distichous, remote, sympodially developed, one branch of each dichotomy elongate, the other short and bearing 2 terminal palmately lobed lfls, the whole lf thus resembling a stem with lvs; sterile pinnae basal, the fertile apical, with reduced lf-tissue; sporangia each borne on a short vein, covered by a membranous indusium-like outgrowth from the lf-surface, short-stalked, strongly curved, the annulus appearing lateral; spores tetrahedral; gametophyte thalloid. 3, warm reg.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.