Stereodon imponens (Hedwig) Mitten
Plants golden to yellow-green or brownish, medium-sized to large, creeping, 3--10 cm. Stems reddish brown, central strand weak, hyalodermis not differentiated from the cortical, usually regularly pinnately branched, occasionally partly 2-pinnate, or irregularly branched; branches usually in a single horizontal plane, 0.3--1.2 cm; pseudoparaphyllia frequent, lanceolate or foliose, mostly with incised margins. Leaves of stem falcate-secund, triangular-ovate to oblong-lanceolate, gradually narrowed to a slender acumen, 1.8--2 × 0.6--0.8 mm, weakly to not plicate, margins serrulate in distal half of leaf or rarely nearly entire, plane or weakly recurved near base; costa short and double or indistinct; median cells 50--70(--80) × 3--4(--5) µm, sometimes porose; basal cells broader and colored yellow-orange to orange-brown, especially in inner portion of alar region; alar portion not to weakly excavate, somewhat decurrent, of subquadrate cells, the basal ones often heavily pigmented orange-brown, the outer cells hyaline with thinner outermost marginal wall, 5--10 cells high on margin. Sexual condition dioicous; inner perichaetial leaves erect, oblong-lanceolate, with long, serrate acumen, often strongly revolute margined in proximal half, plicate, costa indistinct. Seta red-brown, 1--3 cm. Capsule red-brown erect to slightly inclined and somewhat curved below mouth, cylindric, 1.5--3 mm, excluding the conic to rostellate operculum; annulus scarcely differentiated, cilia of endostome usually single, sometimes rudimentary or double.
Sporophytes produced, often abundantly, in summer, capsules mature July--Sept. Commonly on decaying logs, but also rock and soil; 0--2000 m; s Greenland; N.B., Nfld., N.S., Ont., P.E.I., Que.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Ky., Me., Md., Mass., Mich., Minn., Miss., Mo., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.; Europe.
Hypnum imponens is an amphi-Atlantic species. The often colored, heterogeneous alar cells, usually reddish stem, long-toothed foliose, usually numerous, pseudoparaphyllia, and the nearly erect, cylindric capsules are characteristic. The lanceolate pseudoparaphyllia and generally homogeneous alar cells of H. cupressiforme and its near relatives are usually sufficient to separate them from this species.