Stems long-creeping, 1.5--3 mm diam. Leaves monomorphic, dying back in winter, often 1 cm or more apart, fertile leaves often somewhat longer than sterile leaves, 25--80 cm. Petiole straw-colored above base, 12--45 cm × 1--3 mm, at base sparsely set with tan, ovate, glabrous scales. Blade lanceolate, 14--35 cm, proximal pinnae largest, or lowest pair slightly smaller, blade tapered gradually to pinnatifid apex. Pinnae deeply pinnatifid to ca. 1 mm from costa, 3--10 × 1--2 cm; segments oblong, somewhat oblique, entire; proximal pair of veins from adjacent segments meeting margin above sinus. Indument abaxially of sparsely set hairs 0.2--0.4 mm primarily on costae, also of yellowish short-stalked glands less than 0.1 mm, blade tissue with numerous reddish to orangish, resinous, shiny, sessile, hemispheric glands; blades adaxially with scattered hairs on veins. Sori round, medial; indusia tan, often glandular; sporangia glabrous. 2 n = 128. Terrestrial in acid soils of shaded swamps and bogs, frequently associated with sphagnum; 0--100 m; N.B., N.S., Ont., Que.; Conn., Del., Maine, Md., Mass., N.H., N.J., N.Y., Pa., R.I., Vt., Va., W.Va., Wis.
Resembling no. 5 [Thelypteris palustris Schott]; petioles of the sterile blades to 2 dm, of the fertile to 3.5 dm, mostly a little shorter than the blades; blades a little narrower, to 5 נ1.5 dm; costae and veins only sparsely or scarcely hairy; veins simple, those of the sterile segments 4-5 pairs, of the fertile ones 6-7 pairs; indusia margined with minute glands; 2n=128. Swamps and moist woods, in acid soil; N.S. to Va. and W.Va.; disjunct in the driftless area of Wis. (Aspidium s.; Dryopteris s.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.