Petiole (7--)15--55 cm; scales brown to dark brown, linear-lanceolate, 8(--10) × 2 mm. Blade elliptic, 2-pinnatifid to 2-pinnate, (20--)30--75 × (5--)10--35 cm, moderately narrowed proximally, broadest near or just below middle, apex acuminate. Pinnae short-stalked or sessile, oblong-lanceolate. Pinnules decurrent onto costal wing or sessile, linear to oblong, base unequally cuneate, apex obtuse to acute or in larger leaves acuminate, lobed halfway or more to costules. Rachis , costae, and costules glabrous or with pale glands. Sori straight, less frequently hooked at distal end or horseshoe-shaped; sporangial stalks bearing glandular hairs; indusia irregularly dentate, ± ciliate. Spores yellow or brown. 2 n = 80. Moist woods, swamps, thickets; 0--1100 m; Greenland; Man., N.B., Nfld., N.S., Ont., P.E.I., Que., Sask.; Conn., Del., Ill., Ind., Iowa, Maine, Md., Mass., Mich., Minn., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Pa., R.I., S.Dak., Vt., Va., W.Va., Wis.

Petiole commonly ca half as long as the blade, the basal scales often blackish; blade widest near the middle, the fourth or fifth pair of pinnae the largest; cilia of the indusia eglandular; spores yellowish to brownish. Nf. and Que. to Ont. and S.D., s. to Md., Va., Ky., and Mo. (A. angustum)

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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