16. Hypnum plicatulum (Lindberg) A. Jaeger, Ber. Thatigk. St. Gallischen Naturwiss. Ges. 1877-1878: 316. 1880.
Stereodon plicatulus Lindberg, Contr. Fl. Crypt. As., 254. 1873; Hypnum implexum Renauld & Cardot; H. subplicatile Limpricht
Plants small to medium-sized, pale yellow-green to golden green. Stems 2-5 cm, yellow-green or green, occasionally brown tinged, procumbent to suberect, often regularly pinnate, branches 0.2-0.5 cm; hyalodermis present, central strand weak or absent; pseudoparaphyllia foliose, often 2-fid. Stem leaves curved to falcate-secund, ovate-lanceolate to narrowly triangular, , gradually narrowed to apex, 1-1.5 × 0.5-0.8 mm; base auriculate; margins plane, often toothed on auriculate portion, entire or toothed distally; apex long-attenuate; costa double and short or obscure; alar cells , region usually well defined, distalmost and outermost cells pitted, sometimes reduced or absent, ; basal laminal cells shorter, broader than medial cells, pigmented or walls pitted; medial cells (50-)60-70(-80) × 4-5 µm. Sexual condition dioicous (phyllodioicous). Seta red to red-brown, 2-2.3 cm. Capsule inclined, brown, cylindric, 1.5-2 × 0.7-0.8 mm; annulus 1- or 2-seriate; operculum conic; endostome cilia 2.
Capsules mature Jul-Aug. Base and trunks of trees, logs, humus on cliff shelves and rock, forest floors, bog margins, tundra; low to high elevations (0-2000 m); Greenland; Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Yukon; Alaska, Ga., Maine, Mich., Minn., N.Y., N.C., Tenn., Wis.; Europe; Asia.
Hypnum plicatulum is a predominantly boreal to Arctic species, scattered in the Northern Hemisphere and producing sporophytes infrequently in summer. Useful distinguishing features include the frequently close-pinnate stems, yellowish green to golden green leaves with green to yellow stems, stems with hyalodermis, and auriculate leaves with a few differentiated alar cells that are sometimes excavated. See also discussions under 3. H. callichroum, 10. H. hamulosum, and 11. H. holmenii. Plants of H. plicatulum have branches 0.5-1 mm wide; the leaves have five or six pigmented, thick-walled alar cells, 6-15 µm; and the endostome cilia are as long as the segments.