Plants small to robust, 0.5-- -15 cm, 1--2-pinnate or irregularly branched; pseudoparaphyllia filamentous to foliose, toothed or blunt; axillary hairs 3--4-celled. Stems with or without hyalodermis, with or without central strand. Leaves of stem and branch similar but branch leaves tend to be smaller and narrower and with alar cells less well differentiated; broadly to narrowly ovate; margins sinuate to entire proximally, toothed to entire distally, sometimes recurved near base and usually plane distally; apex acuminate or acute; costa double or obscure, usually confined to proximal 1/4 of leaf; leaf cells usually smooth, usually elongate and somewhat vermicular, alar cells often differentiated as a distinct group of enlarged or diminished cells, often shorter than those of the rest of the leaf, indentation just above the alar region sometimes present. Sexual condition autoicous, dioicous or phyllodioicous; inner perichaetial leaves erect, ovate to lanceolate or subulate, abruptly narrowed to a slender acumen, serrate or entire, plicate or not, outer leaves reflexed, costa single, double, or absent. Seta smooth, yellowish to reddish. Capsule erect, inclined or horizontal, varying from long-cylindric to ovoid, usually curved, annulus 1--3 seriate to scarcely differentiated; operculum conic to rounded-mammillate; peristome double, exostome teeth subulate-acuminate, yellowish to brownish, outer surface with distinct zigzag line and lamellae, finely cross-striolate basally, hyaline and papillose distally, inner face trabeculate; endostomial segments about as high as exostomial teeth, pale and yellowish, carinate, weakly to strongly split between articulations, minutely papillose, cilia 1--3 or sometimes rudimentary. Calyptra cucullate, naked.

Species ca. 50 (22 in the flora): widely distributed in all continents but Antarctica, but especially temperate areas.

The genus Hypnum remains a repository for a number of discordant elements, some apparently belonging to other families. It once included a high proportion of the pleurocarpous mosses. The generic concept accepted here contains some species probably not Hypnum, but it is a reasonably “natural” genus that can be recognized in the field, especially when examined with a hand-lens. I have included Pseudostereodon, Breidleria and Stereodon within Hypnum, and remain unconvinced of arguments that they should be segregated. Gametophytic features are supreme in separation of species although the presence of sporophytes provides additional features that strengthen the present species concept.

Size, branching patterns, leaf form and arrangement, areolation, especially alar cell differentiation, leaf margins, pseudoparaphyllium morphology, and stem anatomy are all features used here for taxonomic distinctions. Unfortunately many of these features are altered by environmental extremes, especially in moisture and light. In spite of this plasticity, most specimens can be named with reasonable confidence; there are, however, some specimens that cannot. The problems in discriminating among the species have been considerably reduced through the elegant studies of H. Ando, but he treated only part of the genus in detail. Fortuitously, however, he studied most of the species that occur in North America.

To use the key, a number of recommendations, if followed, are likely to yield more satisfactory determinations. It must be noted again that some plants may be impossible to name satisfactorily. Leafy plants of vegetative specimens may consist almost entirely of branches, with different but characteristic stem leaves unrepresented; this can happen when the plants form turf-like colonies or when the stem disintegrates, leaving only branches. Plants that grow in extreme environments, especially with respect to moisture, may be unusual; for instance, plants of deeply shaded or wet sites may be markedly attenuated. Plants in highly illuminated and well-drained sites are also subject to morphological alteration that can “mimic” another taxon. Immature specimens can lack critical characters or are inconsistent in form. Intermixed material can contain several taxa, and it is necessary to tease out as complete a plant as possible for determination.

Thoroughly wet leafy shoots before removing many leaves and retaining the stripped shoot for determining form of the pseudoparaphyllia. Observations of leaf morphology should be based on many leaves. Remove leaves from the shoot by stripping them toward the stem base using fine-tipped forceps as aided by low magnification of a dissecting microscope. It is important to get complete leaves, including the cells at the point of attachment to the stem. In hyalodermous stems, the thin-walled cortical cells usually remain attached to the leaf base when the leaves are stripped off the stem, in which case cross-sections can be made of the stem to confirm whether the stem is hyalodermous or not. Use sporangium-bearing specimens when possible to provide additional characters. In this key, the following features define differences in size of gametophores: small refers to stem leaves 1.2--1.5 mm and stems usually 1--2 cm and slender (related to the leaf size); medium refers to stem leaves 1.5--3 mm and stems 2--4 cm; large or robust refers to stem leaves more than 3 mm and stem more than 4 cm. When length and width of leaves are given, length is cited first, and the width is that of the widest part of the leaf, usually the base.

EXCLUDED TAXA

Hypnum geminum (Mitten) Lesquereux & James does not appear to be a species of Hypnum as treated here. The type specimen is extremely small and difficult to interpret.

Hypnum fertile Sendtner as treated previously in North America has been shown to be H. fauriei Cardot See discussion of that species.

Hypnum canariense (Mitten) A. Jaeger (Hypnum uncinatulum Juratzka), though noted by A. J. Grout (1932) presumably from Newfoundland, is not in North America. Grout appears to have based the record on H. waghornei Kindberg, in which the type contains a mixture, none of it Hypnum. Grout did not see authentic material, thus his noting also New Brunswick and Nova Scotia (Cape Breton Island) in its distribution remains puzzling.

Hypnum obsoletinerve Kindberg, is clearly not Hypnum.

OTHER REFERENCES

Ando, H. 1989. Studies on the genus Hypnum Hedw. (VI). Hikobia 10: 269--291.
Grout, A. J. 1932. Moss Flora of North America 3: 127. Newfane. 3 vols.

SELECTED REFERENCES

Ando, H. 1972. Studies on the Genus Hypnum. I. J. Sci. Hiroshima University, Series B, Div. 2 (Botany) 14: 52--73. Ando, H. 1973. Studies on the Genus Hypnum II. J. Sci. Hiroshima University, Series B, Div. 2 (Botany) 14: 165--207.