Plants: small to large, yellow-green, green, dark green, or blackish. Stems: unbranched or irregularly branched; hyalodermis rare, central strand well developed, rarely poorly so, or absent; paraphyllia absent; rhizoids or rhizoid initials on stem or abaxial costa insertion, rarely forming tomentum, slightly to strongly branched, smooth or rarely warty-papillose; axillary hairs well developed and many, or small, delicate, sparse, hyaline when young. Leaves: recurved, squarrose, or not, straight, falcate-secund, or both, ovate, broadly ovate, oval, orbicular, ovate-lanceolate, oblong-lanceolate, or lanceolate, not plicate, to or longer than 1 mm; base commonly decurrent; margins usually plane, sometimes recurved in apex, entire or weakly denticulate or serrulate, limbidia absent; apex broadly rounded, obtuse, or acute, occasionally apiculate from obtuse or rounded apex, sometimes blunt from acute apex, acumen not differentiated; costa usually double, short, ending before mid leaf, often single and slender or stout to 1/2–3/4 leaf length, occasionally double and long, rarely percurrent, terminal abaxial spine sometimes present; alar cells undifferentiated or little differentiated, quadrate, short-rectangular, or irregular, or cells enlarged, not or slightly inflated, hyaline, yellow-brown, brown, reddish brown, or rarely bright red, walls thin or incrassate, region mostly well delimited, variously small and irregular to large; medial laminal cells usually linear-flexuose, sometimes short-rhombic or fusiform, 27–66 µm (when costa is double); distal cells sometimes prorate distally on dorsal side; marginal cells short or very long. Sexual: condition autoicous or dioicous. Capsule: erect, symmetric, or inclined, ovoid to oblong-cylindric, slightly to strongly arcuate; peristome double; exostome margins almost entire to dentate; endostome cilia 1–3, absent or rudimentary to well developed, sometimes appendiculate. Spores: 10–24 µm. North America, Eurasia.
Species 16 (16 in the flora). Hygrohypnum has long been understood to be taxonomically difficult. As circumscribed here, it remains a repository for a number of coherent and disparate elements; there are few helpful cladistic and molecular analyses (M. S. Ignatov et al. 2006; G. Oliván et al. 2006). Difficulties in identification of Hygrohypnum are various, such as variability in leaf shape within a single specimen. Descriptive terms for leaf shape are imprecisely applied. Failure to remove intact leaves during dissection is problematic. Careful observation of alar cells in intact leaves is absolutely necessary. Alar cells in immature leaves are incompletely developed, and shape, wall thickness, wall color, and differentiation are regularly obscured by cytoplasmic contents. Hygrohypnum is usually found in or near running water or lakeshores. The stems have darkly pigmented cortical cells in several layers; the alar cells are sometimes excavated; and the capsules have conic or conic-apiculate opercula.