Cardot & Thériot

Hypnum closteri Austin, Musci Appalach., 439. 1870; Amblystegium holzingeri Renauld & Cardot; H. malacocladum

Plants soft, spindly, dirty green. Stems to 15 cm, frequently denuded basally, irregularly branched; hyalodermis absent, epidermal cells small, walls thick, similar to subadjacent cortical cells, central strand well developed. Leaves somewhat erect-spreading, uniformly straight, markedly shrunken and twisted when dry, loosely patent when moist, narrowly ovate, ovate-lanceolate, or lanceolate, plane or rarely weakly concave, (0.3-)0.6-0.9(-1.7) × (0.2-)0.3-0.4(-0.8) mm; margins plane, entire; apex acute; costa usually single to 1/3-1/2 leaf length, much less frequently double and short, or single and 2-fid; alar region usually little differentiated, or cells few, quadrate or short-rectangular; basal laminal cells similar to medial cells or shorter, wider; medial cells fusiform or rhombic, straight or flexuose, sometimes linear-flexuose, (23-)30-50(-63) × (4-)6-7(-13) µm; apical cells shorter; marginal cells rarely longer than 55 µm. Sexual condition autoicous; ; perichaetial inner leaves triangular-lanceolate, . Seta yellowish or yellowish red, 0.7-1 cm. Capsule with endostome cilia 1-3.

Irrigated or submerged rock in streams; low to moderate elevations (20-1500 m); D.C., Maine, N.Y., N.C., Pa., Vt.

Hygrohypnum closteri is so unlike the main body of Hygrohypnum that it is surely ill-placed in the genus; the author and others have noted its Amblystegium-like appearance. The prostrate to somewhat ascending stems have distantly spaced leaves.