Revisions by JRR, January 2008
Joseph R. Rohrer
Plants large to robust, forming loose wefts, growth monopodial. Stems procumbent to erect-ascending, irregularly and sometimes sparsely branched to regularly pinnate; paraphyllia absent; pseudoparaphyllia lanceolate to narrowly lanceolate; rhizoids sparse, at tips of branches and sometimes along stems. Leaves oblong-ovate, ovate-lanceolate, or lanceolate; margins narrowly revolute almost to apex, serrate to serrulate in distal 1/2, nearly entire proximally; apex acuminate; costa single, extending 1/3--2/3 leaf length, often ending in a spine; median cells prosenchymatous, narrowly elliptic to linear-flexuose; alar cells small, quadrate to short-rectangular. Sexual condition dioicous. Seta elongate, smooth. Capsule suberect to horizontal; peristome double, exostome cross-striolate on abaxial surface, papillose distally; endostome with high basal membrane, segments broad, perforated along the keel by narrow slits, cilia 1--3, often fused. Calyptra cucullate, smooth, naked. Spores 10--17 µm, very finely papillose.
Genus 1, species 1: widespread in temperate and boreal regions.
Rhytidium is an anomalous genus without clear affinities. As originally described, the Rhytidiaceae had eight genera, but as circumscribed by N. Nishimura et al. (1984), W. R. Buck and D. H. Vitt (1986), and W. R. Buck and B. Goffinet (2000), the family is monotypic. Although traditionally placed near Hylocomium, Rhytidiopsis, and Rhytidiadelphus, and included in the Hylocomiaceae by J. R. Rohrer (1985) and others, a cladistic analysis by W. R. Buck and D. H. Vitt (1986) placed it as the sister group of a large clade that includes such families as Amblystegiaceae and Brachytheciaceae, but not Hypnaceae or Hylocomiaceae.
1. RHYTIDIUM (Sullivant) Kindberg, Bih. Kongl. Svenska Vetensk.-Akad. Handl. 6(19): 8. 1882 * [Greek rhytis, wrinkle, alluding to the strongly rugose leaves]
Hypnum sect. Rhytidium Sullivant in A. Gray, Manual ed. 2, 675. 1856
Stem leaves crowded, imbricate, erect, falcate-secund, obscurely plicate, strongly rugose, not or shortly decurrent; cells coarsely prorate distally. Branch leaves smaller, less falcate-secund, otherwise similar to stem leaves. Capsule oblong-ellipsoid to cylindric, arcuate, not plicate but constricted below the mouth when dry; operculum high-conic to short-rostrate.
Species 1: North America, Mexico, Central America (Guatemala), South America (Bolivia), Eurasia, Africa.
1. Rhytidium rugosum (Hedwig) Kindberg, Bih. Kongl. Svenska Vetensk.-Akad. Handl. 7(9): 15. 1883
Hypnum rugosum Hedwig, Spec. Musc. Frond. 293. 1801
Plants 2--3 mm wide measured across leafy stem, to 10 cm long, yellowish-green to golden brown; stems and branches turgid because of crowded leaves; stems often hooked at tips; branches mostly short, up to 10 mm. Stem leaves 2.8--4.5 × 0.8--1.5 mm; costa sometimes forked; median cells 25--55 × 4--6 µ m; alar cells numerous, in a broad triangular area extending up leaf margin, 8--20 × 8--12 µ m. Branch leaves 1.2--2.3 × 0.4--0.8 mm. Seta 20--25 mm. Capsule 2--2.5 mm.
Mostly on rock or on thin layer of soil or humus overlying rock, especially of calcareous or mafic composition; commonly on exposed rock ledges, rocky slopes, or bluffs, or in semi-open dry forests or in tundra, much less common on moist sites; 100--3900 m; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Sask., Yukon; Alaska, Ariz., Colo., Conn., Ill., Ind., Iowa, Ky., Maine, Mass., Mich., Minn., Mo., Mont., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Oreg., Pa., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis., Wyo.; Mexico; Central America (Guatemala); South America (Bolivia); Eurasia; Africa (Kenya, Uganda, Morocco).
Even though widespread, Rhytidium rugosum is infrequent, presumably because of a preference for exposed calcareous or mafic bedrock in a cool habitat. The species is rarely found with sporophytes. Perhaps almost total reliance on asexual reproduction explains the strong morphological uniformity seen among specimens of R. rugosum collected from across its broad range.
Buck, W. R. and B. Goffinet. 2000. Morphology and classification of mosses.
In: A. J. Shaw and B. Goffinet, eds. 2000. Bryophyte Biology.
Buck, W. R. and D. H. Vitt. 1986. Suggestions for a new familial classification of pleurocarpous mosses. Taxon 35: 21--60.
Nishimura, N., M. Higuchi, T. Seki, and H. Ando. 1984. Delimitation and subdivision of the moss family Hypnaceae. J. Hattori Bot. Lab. 55: 227--234.
Rohrer, J. R. 1985. A generic revision of the Hylocomiaceae. J. Hattori Bot. Lab. 59: 241--278.