Northern wood fern, spreading wood fern, dryoptère dressée

Nephrodium expansum C. Presl, Reliq. Haenk. 1: 38. 1825; Dryopteris assimilis S. Walker; D. dilatata (Hoffmann) A. Gray subsp. americana (Fischer) Hultén

Leaves monomorphic, tardily dying back in winter, to 90 × 30 cm. Petiole 1/3 length of leaf, scaly at least at base; scales scattered, brown with dark brown stripe. Blade green, deltate-ovate, 3-pinnate-pinnatifid, herbaceous, usually not glandular, occasionally finely and densely glandular. Pinnae ± in plane of blade, lanceolate-oblong; basal pinnae deltate, slightly reduced, basal pinnules equal to or longer than adjacent pinnules, basal basiscopic pinnule longer than basal acroscopic pinnule; pinnule margins serrate. Sori midway between midvein and margin of segments. Indusia lacking glands or sparsely glandular. 2 n = 82.

Cool moist woods and rocky slopes; 50--1500 m; Greenland; Alta., B.C., Nfld., N.W.T., Ont., Que., Yukon; Alaska, Calif., Idaho, Mich., Minn., Mont., Oreg., Wash., Wis., Wyo.; Europe.

Dryopteris expansa is diploid and is one of the parents of D . campyloptera . Where their ranges overlap in eastern Canada, these two species are very difficult to distinguish except by chromosome number. The growth habit ( D . expansa leaves are more erect) is useful in the field. Three hybrids involving D . expansa are known; all are very rare.