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.

Distribution

North America, Eurasia.

Discussion

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.

Selected References

None.

Lower Taxa

Hygrohypnum alpestre, Hygrohypnum alpinum, Hygrohypnum bestii, Hygrohypnum closteri, Hygrohypnum cochleariifolium, Hygrohypnum duriusculum, Hygrohypnum eugyrium, Hygrohypnum luridum, Hygrohypnum molle, Hygrohypnum montanum, Hygrohypnum norvegicum, Hygrohypnum ochraceum, Hygrohypnum polare, Hygrohypnum smithii, Hygrohypnum styriacum, Hygrohypnum subeugyrium

Key

1	Stems with hyalodermis present or at least some cells with walls slightly thinner	> 2
1	Stems with hyalodermis absent, epidermal cells small, walls thick, similar to subadjacent cortical cells	> 4
2	Costae single, percurrent.	Hygrohypnum polare
2	Costae usually double, short or to mid leaf, if single or 2-fid, ending well before apex	> 3
3	Alar cells inflated, regions clearly defined, often red or reddish brown with age; leaf apices acute.	Hygrohypnum eugyrium
3	Alar cells not inflated, regions not distinct, hyaline or yellowish, never red; leaf apices acute to long-tapering acuminate, obtuse, or blunt.	Hygrohypnum ochraceum
4	Leaves falcate-secund, or some falcate-secund or straight within same plant	> 5
4	Leaves straight, sometimes secund	> 9
5	Alar regions not or scarcely differentiated	> 6
5	Alar regions well differentiated	> 7
6	Leaf margins finely to coarsely serrulate, especially in apex, narrowly recurved proximally.	Hygrohypnum montanum
6	Leaf margins entire, plane.	Hygrohypnum styriacum
7	Alar cells inflated, walls thin.	Hygrohypnum eugyrium
7	Alar cells small, walls incrassate	> 8
8	Leaf apices acute; margins entire.	Hygrohypnum luridum
8	Some leaf apices obtuse; margins distinctly denticulate, or apices acute with few fine teeth.	Hygrohypnum subeugyrium
9	Leaves usually broadly ovate, oval, or orbicular, length usually less than 1.5 times width	> 10
9	Leaves ovate, oblong-ovate, ovate-lanceolate, elliptic, or oblong-lanceolate, length usually more than 1.5 times width	> 15
10	Marginal laminal cells 60 µm or longer.	Hygrohypnum bestii
10	Marginal laminal cells rarely longer than 55 µm	> 11
11	Alar regions clearly differentiated	> 12
11	Alar regions undifferentiated or cells few	> 13
12	Alar cell walls thin, usually hyaline, irregular to rectangular, regions ± rectangular, long axis parallel to margins; inner perichaetial leaves with some apical laminal cells papillose abaxially.	Hygrohypnum alpinum
12	Alar cell walls incrassate, clearly pigmented in older leaves, quadrate, short-rectangular, or irregular, regions irregular; inner perichaetial leaves with apical laminal cells smooth abaxially.	Hygrohypnum duriusculum
13	Costae usually single and stout, to mid leaf or slightly beyond, sometimes 2-fid, or short and double; plants very coarse, stiff.	Hygrohypnum smithii
13	Costae usually short and double, if single, then costae slender; plants soft	> 14
14	Leaves deeply concave to cochleariform, usually 0.8-1.2 mm; apices obtuse to broadly rounded; endostome cilia 2 or 3.	Hygrohypnum cochleariifolium
14	Leaves concave, never cochleariform, usually 1-1.7 mm; apices tapering to bluntly acute point; endostome cilia rudimentary or.	Hygrohypnum molle
15	Alar regions clearly differentiated	> 16
15	Alar regions undifferentiated or cells few	> 18
16	Leaf margins at apex narrowly recurved, forming reflexed apiculus; leaves deeply concave.	Hygrohypnum alpestre
16	Leaf margins at apex not recurved, never forming reflexed apiculus; leaves usually not deeply concave	> 17
17	Leaf apices acute; margins entire at apex.	Hygrohypnum luridum
17	Leaf apices acute with few fine teeth or obtuse and margins distinctly denticulate.	Hygrohypnum subeugyrium
18	Leaf apices abruptly acuminate; perichaetia and perigonia in bracted complex.	Hygrohypnum styriacum
18	Leaf apices obtuse or acute, sometimes blunt; perichaetia and perigonia not in bracted complex	> 19
19	Costae usually single to mid leaf or beyond, sometimes double and short	> 20
19	Costae usually double and short, rarely to mid leaf or beyond	> 21
20	Leaf apices acute; plants soft, spindly; inner perichaetial leaves not plicate, margins plane.	Hygrohypnum closteri
20	Leaf apices obtuse; plants coarse; inner perichaetial leaves plicate, margins recurved.	Hygrohypnum smithii
21	Leaf margins undulating to denticulate at apex; leaves 1-1.7 mm.	Hygrohypnum molle
21	Leaf margins entire at apex; leaves 0.5-0.8 mm.	Hygrohypnum norvegicum