Stems compact, erect to ascending, with cluster of persistent petiole bases of ± equal length; scales uniformly brown, lanceolate. Leaves 2.5--20 × 0.5--2.5 cm. Petiole reddish brown or dark purple when mature, articulate above base at swollen node, relatively brittle and easily shattered. Blade linear to narrowly lanceolate, usually pinnate-pinnatifid proximally, lacking glands, never viscid; rachis with widely scattered hairs and scales. Pinnae ovate-lanceolate to deltate, longer than wide, abruptly tapered to a rounded or broadly acute apex; largest pinnae with 1--3 pairs of pinnules; abaxial surface with isolated hairs and linear scales, adaxial surface glabrous. Pinnules entire or broadly crenate; margins nonlustrous, thin, with occasional isolated cilia, lacking translucent projections. Vein tips often enlarged to form whitish hydathodes visible adaxially . Indusia of narrow, hairlike segments, these uniseriate throughout, composed of cells many times longer than wide, usually surpassing mature sporangia. Spores averaging 46--53 µm. Sporulating summer--early fall. Crevices and ledges on cliffs (occasionally on rocky slopes); mostly slaty and calcareous rocks; 0--1500 m; Greenland; B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., Que., Sask., Yukon; Alaska, Maine, Mich., Minn., N.H., N.Y., Vt.; n Eurasia. Isozyme studies confirm the longstanding hypothesis that Woodsia alpina is an allotetraploid derived from hybridization between W . glabella and W . ilvensis (see reticulogram). Considerable disagreement exists concerning the chromosome number of W . alpina , but 2 n = 160 seems most likely, given the numbers reported for the two parental species. Hybrids between W . alpina and W . ilvensis have been reported from both Europe and North America. These morphologically intermediate triploids with malformed spores have been called W . × gracilis (Lawson) Butters.
Rhizome-scales lanceolate or lance-ovate, 4-6 נ1-2 mm, brown, denticulate, sparsely fimbriate; lvs 6-15 cm; petiole bright brown, sparingly scaly and hairy, articulate below the middle; blade linear, 1-2 cm wide, slightly narrowed below, pinnate-pinnatifid, sparsely hairy on the rachis and lower surface, the rachis also sparsely scaly; pinnae sessile, 8-15 pairs, ovate, obtuse, deeply pinnatifid, the segments 2-3(4) pairs, oblong to suborbicular, the lower sublobate, not ciliate; indusium a small disk with a few long septate marginal hairs; 2n=156, thought to be an alloploid derived from nos. 1 [Woodsia glabella R. Br.] and 3 [Woodsia ilvensis (L.) R. Br.]. Rock crevices in cool sites; circumboreal, s. to N.S., Vt., N.Y., Mich., and Minn. A sterile triploid hybrid with no. 3 [Woodsia ilvensis (L.) R. Br.] is W. اracilis (G. Lawson) Butters.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.