Hygrohypnum Lindberg, 1872.

Hygrohypnum are aquatic pleurocarps mostly growing tightly adhering to rocks in flowing water. They have costae of various lengths that usually end below mid-leaf. The immediate leaf apex is blunt although many species seem acute to even acuminate when viewed in more gross detail. One should verify that the cells in the apex of the leaves are shortened. That apex can overall be bluntly rounded or even acuminate.

Key to Hygrohypnum

Species included are in Campyliaceae:
Hygrohypnum alpinum (Lindberg) Loeske
Hygrohypnum bestii (Renauld & Bryhn) Brotherus
Hygrohypnum cochlearifolium (Venturi in De Notaris) Brotherus
Hygrohypnum duriusculum (De Notaris) Jamieson
Hygrohypnum luridum (Hedwig) Jennings
Hygrohypnum molle (Hedwig) Loeske
Hygrohypnum ochraceum (Turner ex Wilson) Loeske
Hygrohypnum smithii (Swartz in Liljeblad) Brotherus
Hygrohypnum styriacum (Limpricht) Brotherus

Hygrohypnum is a genus almost restricted to rocks in rapidly flowing streams. The majority of our species will be found in alpine to montane elevation streams, but occurrences in lower elevation are possible in stenotrophic streams. When one examines this aquatic, petricolous, pleurocarpous moss, one should look for a distally or even basally forked costa. It should be noted here that a number of leaves often must be examined for the forking of the costa. It should further be noted that Hygrohypnum smithii will only occasionally show costal forking, and often the single costa will extend to 3/4. This species of the genus is, however, easily determined in the field by its orbicular and spreading leaves. Platyhypnidium riparioides is somewhat similar in habitat, leaf shape and costal length, but this latter moss has strongly serrate to denticulate leaf margins from base to apex compared with the mostly entire margins of H. smithii.

The key below opens with Hygrohypnum ochraceum, a plant that can have straight or strongly falcate-secund leaves; a plant that can be complanate or almost julaceous. Its diagnosis under the compound microscope is, however, easy: the presence of a hyaloderm can be determined by a stem cross-section, or one can even rely simply on the fact that portions of the hyaloderm can usually be seen on leaves scraped from the stem for viewing.

A serious problems with the collection of Hygrohypnum is relate to the close adherence of the plant to rocks under rapidly flowing water. This often results in small and badly fragmented collections. We find removal of the plant with a putty-knife or plastic picnic knife to be fairly successful.

A. Stems and branches with a hyalodermis .....Hygrohypnum ochraceum
A. Outer cortex of stems and branches thick-walled .....B

B. Leaves falcate or some leaves on the same or different stems within the same specimen falcate .....C
B. Leaves all straight, sometimes secund .....D

C. Alar cells undifferentiated from adjacent cells; leaf apex abruptly acuminate .....Hygrohypnum styriacum
C. Alar cells small, quadrate to short-rectangular, incrassate; leaf apex acute .....Hygrohypnum luridum

D. Leaves with a single marginal, sometimes incomplete, row of marginal cells broader and thinner walled than more interior cells; these marginal leaf cells 60 µm long or longer; leaves mostly more than 1.5 mm long .....Hygrohypnum bestii
D. Marginal leaf cells not so differentiated; leaves mostly smaller .....E

E. Costa on most leaves single, stout, extending to mid-leaf or slightly beyond, forked or short and double on smaller leaves; alar cells quadrate and incrassate; vegetative leaves less than 1 mm long .....F
E. Costa short and double, or longer and distally forked; alar cells variation; leaves mostly longer .....G

F. Leaves broadly ovate, about 1.5:1, somewhat concave .....Hygrohypnum smithii
F. Leaves orbicular, so concave as to describe a hemisphere .....Hygrohypnum cochlearifolium

G. Leaves orbicular, to 1.2:1 .....H
G. Leaves mostly about 1.5:1 .....J

H. Alar cells undifferentiated or with the few quadrate or short-rectangular cells incrassate or thin-walled .....Hygrohypnum molle
H. Alar cells clearly differentiated, either thin-walled or incrassate .....I

I. Alar cells thin-walled, usually hyaline, enlarged, rounded rectangular, forming a rectangular group whose long axis parallels the leaf margins; median leaf cells not at all prorate; inner perichaetial leaves prorate on the abaxial surface near the apex; dry plant soft to the touch .....Hygrohypnum alpinum
I. Alar cells incrassate, clearly pigmented in older leaves, quadrate, short rectangular or irregular, forming an irregular group, median leaf cells uniformly prorate on distal ends; inner perichaetial leaves smooth; dry plant harsh to the touch .....Hygrohypnum duriusculum

J. Alar cells clearly differentiated, smaller, incrassate and quadrate to short rectangular; leaf apex gradually contracted to an obtuse to bluntly rounded tip; plants autoicous .....Hygrohypnum luridum
J. Alar cells undifferentiated or not localized in a well-demarcated group; leaf apex abruptly acuminate, tapering to a slender tip; plants with dwarf males .....Hygrohypnum styriacum