1. Amblystegium serpens (Hedwig) Schimper in P. Bruch and W. P. Schimper, Bryol. Europ. 6: 53. 1853.
Hypnum serpens Hedwig, Sp. Musc. Frond., 268. 1801; Amblystegium juratzkanum Schimper; A. serpens var. juratzkanum (Schimper) Rau & Hervey
Plants in slender, soft mats. Stems creeping; central strand cells small, walls thin; axillary hair distal cells elongate. Stem and branch leaves similar, soft, not complanate, slightly concave; costa 10-30 µm wide at base; alar region reaching from margin 65% distance to costa at insertion; medial laminal cells 12-55 × 7-12 µm, 3-5:1, walls firm, slightly incrassate, eporose. Perichaetia with inner leaves suddenly narrowed to apex, plicate, apex acuminate. Seta reddish, elongate, 1-2.5 cm. Capsule 1.5-2 mm, neck well developed; operculum conic. Spores spheric, smooth to minutely papillose.
Tree trunks, rotten wood, rock, soil, (sometimes salty) swamps to xeric habitats; low to high elevations (0-3000 m); Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Ark., Calif., Colo., Conn., Del., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., 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; n Africa; Pacific Islands (New Zealand); Australia.
Variable features in Amblystegium serpens include the serration of the leaf margin, which ranges from entire to serrulate or even serrate; length of the costa, from very short (less than 1/5 of leaf length) to subpercurrent; the subquadrate to transversely elongate alar cells; and leaf stance, which ranges from erect to spreading. These last two characters have sometimes been used to recognize a separate taxon, var. juratzkanum. However, the continuous range of variation of these characters precludes any infraspecific division within A. serpens, which is a fairly well-circumscribed species with comparatively low levels of plasticity with regard to species of Hygroamblystegium.