1. Leptodictyum riparium (Hedwig) Warnstorf, Krypt.-Fl. Brandenburg. 2: 878. 1906.
Hypnum riparium Hedwig, Sp. Musc. Frond., 241. 1801; Amblystegium brevipes Cardot & Thériot; A. riparium (Hedwig) Schimper; A. riparium var. flaccidum (Lesquereux & James) Renauld & Cardot; A. riparium var. fluitans (Lesquereux & James) Renauld & Cardot; A. riparium var. longifolium (Schultz) Schimper; Brachythecium pennellii E. B. Bartram; Campylium polygamum var. longinerve (Renauld & Cardot) Grout; Leptodictyum brevipes (Cardot & Thériot) Brotherus; L. laxirete (Cardot & Thériot) Brotherus; L. riparium var. abbreviatum (Schimper) Grout; L. riparium var. brachyphyllum (Cardot & Thériot) Grout; L. riparium var. elongatum (Schimper) Warnstorf; L. riparium var. flaccidum (Lesquereux & James) Grout; L. sipho (P. Beauvois) Brotherus; L. vacillans (Sullivant) Brotherus; Rhynchostegiella georgiana Dixon & Grout
Plants in flat, loose, long trailing mats, light green to yellow-brown. Stems 6-20 cm, flaccid, irregularly branched. Leaves erect- to wide-spreading or appearing distichous, somewhat complanate especially in branches, often obliquely attached to stem, slightly contorted when dry, 2.5-6 mm; margins entire; apex broadly short- to gradually long-acuminate; costa (45-)52-114 µm wide at base; alar region differentiated; basal laminal cell walls lax; distal cells (34-)42-117 × 7-12(-13) µm. Seta single, light to dark brown, 0.8-2.6 cm. Capsule brown to reddish, 2.2-2.5 mm; annulus deciduous, 2- or 3-seriate, cells large; operculum conic, apiculate; peristome yellow to brown. Calyptra naked.
Humus, logs, tree bases, swamps, wet depressions in forests, aquatic on rock in streams and rivers; low to moderate elevations; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., Fla., Ga., Ill., Ind., Iowa, Kans., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W. Va., Wis., Wyo.; Mexico; Central America; South America; Europe; Asia; Africa; Indian Ocean Islands; Pacific Islands; Australia.
As the specific epithet implies, Leptodictyum riparium occurs along rivers, often in flooded areas where the plants are stranded on tree bases in hardwood forests. The species is sometimes confused with another wetland species, Drepanocladus aduncus, which differs by its axillary hairs with 1-3 hyaline distal cells. There is tremendous morphological variability among the North American specimens of L. riparium, suggesting the existence of several ecotypes, which have been erroneously named as many varieties and forms in Leptodictyum, Amblystegium, and Campylium; these synonyms are summarized by A. J. Grout (1928-1940, vol. 3), H. S. Conard (1959), and H. A. Crum and L. E. Anderson (1981).