Hypnum cavifolium Bridel, Bryol. Univ. 2: 556. 1827 (as cavifolius); Leskea flaccida Bridel; Plagiothecium acicularipungens Müller Hal. & Kindberg; P. attenuatirameum Kindberg; P. fallax Cardot & Thériot; P. roeseanum Schimper; P. sullivantiae (Sullivant) A. Jaeger

Plants in thin to dense mats, pale green to yellowish, glossy or rarely dull. Stems to 4 cm, 1-4 mm wide across leafy stem, erect or sometimes prostrate, julaceous or rarely somewhat complanate-foliate. Leaves erect or sometimes spreading, imbricate or rarely distant, ovate or oblong-ovate, symmetric, strongly concave, rarely nearly flat, 1-3 × 0.4-1.4 mm; margins plane or often narrowly recurved nearly to apex, entire or rarely serrulate at apex; apex abruptly acute or slender-acuminate, not abruptly contracted, ; costa with one branch often reaching mid leaf, sometimes appearing single with one branch poorly developed, or rarely ecostate; alar cells rectangular, 28-70 × 12-22 µm, in 1-5 vertical rows, terminating in 1 cell at base, region triangular; medial laminal cells 60-161 × 7-17 µm. Specialized asexual reproduction sometimes present as propagula, 36-110 × 9-17 µm, of 2-7 cells borne in leaf axils. Sexual condition dioicous, rarely fruiting. Seta light brown to red, 1-2.6 cm, straight or somewhat curved. Capsule erect to inclined, light brown to dark red when mature, straight or arcuate, 1-2.5 × 0.3-0.8 mm, smooth or often striate or wrinkled, strongly wrinkled at neck; operculum rostrate, 0.8-1 mm; endostome cilia 1-3. Spores 9-14µm.

Capsules mature spring-summer. Shaded soil or humus overlying boulders and cliffs, rotten logs, stumps, base of trees; low to high elevations (40-2000 m); Greenland; B.C., N.B., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Yukon; Ala., Alaska, Ark., Calif., Colo., Conn., D.C., Ga., Idaho, Ill., Ind., Iowa, Ky., Maine, Md., Mass., Mich., Minn., Mo., N.H., N.J., N.Y., N.C., Ohio, Oreg., Pa., S.C., S.Dak., Tenn., Vt., Va., Wash., W. Va., Wis.; Europe; Asia.

Plagiothecium cavifolium distinguished from other North American species of this genus by its concave, recurved leaf apices on julaceous stems. The stems are often flagelliform. The California record is from a report by K. McGrew (unpubl.). Plagiothecium cavifolium is very similar to P. sylvaticum (Bridel) Schimper of Europe; P. cavifolium is usually julaceous with concave leaves that have narrow medial cells, 7-17 µm wide, and P. sylvaticum is complanate-foliate with flat leaves that have broader medial cells, 12-22 µm wide.