Leaves irregularly spaced along stem. Sterile leaves yellow-green, deltate, coarsely divided, 13--34 × 15--30 cm. Petiole of sterile leaf black, 22--58 cm, flattened at base; rachis winged, becoming broader toward apex. Pinnae 5--11 per side, lanceolate; proximal pinnae 9--18 cm, margins entire, sinuate, or laciniate. Sporophyll leaves green, becoming black at maturity, oblong, 7--17 × 1--4 cm. Petiole 19--40 cm, base sparsely scaly. Pinnae linear, 5--11 per side, 2.5--5 cm; ultimate segments revolute to form beadlike structures, 2--4 mm diam. Sori borne on free veins, enclosed by ultimate segments. 2 n = 74. Sporophylls produced May--October. Open swamps, thickets, marshes, or low woods, in sunny or shaded locations, often forming thick stands; 0--1500 m; St. Pierre and Miquelon; Man., N.B., Nfld., N.S., Ont., P.E.I., Que.; Ala., Ark., Colo., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis., Wyo.; e Asia. Onoclea sensibilis occurs in eastern North America, principally east of the Great Plains. Leaf forms with pinnae intermediate between those of sporophylls and sterile leaves, or with pinnae fertile only on one side of the blade, can occur on plants that also bear normal leaf forms. These do not merit taxonomic recognition (J. M. Beitel et al. 1981). Onoclea sensibilis resembles Woodwardia areolata (Linnaeus) T. Moore, with which it often grows. Onoclea has entire pinna margins and nearly opposite basal pinnae whereas Woodwardia areolata has serrate pinna margins and alternate pinnae.

As in Matteuccia struthiopteris (Linnaeus) Todaro, sporophylls of Onoclea sensibilis persist through the winter and release the green spores in spring before the sterile leaves expand (R. W. Hill and W. H. Wagner Jr. 1974; L. G. Labouriau 1958; R. M. Lloyd and E. J. Klekowski Jr. 1970). Onoclea sensibilis is occasionally cultivated; it has a tendency to spread rapidly and become weedy. The name 'sensitive fern' refers to the susceptibility of the leaves to even a light frost.

Sterile lvs to 1 m, the blade 18-40 נ15-35 cm, with a broadly winged rachis except near the base, the segments 8-12 pairs, opposite, 1.5-5 cm wide, with scattered white hairs on the midrib and veins beneath, glabrous above; fertile lvs smaller, the blade to 17 cm, the globular pinnules 3-4 mm wide; 2n=74. Swamps, open woods, and meadows in neutral to slightly acid soil; Nf. and Lab. to Fla., w. to Minn., Ark., Tex., and irregularly to Colo.; e. Asia.

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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