Stereodon holmenii (Ando) Ignatov & Ignatova
Plants small, regularly to irregularly and densely pinnately branched, pale green to yellow-green, shiny, ascending to procumbent, 4--6 cm, branches to 1 cm, loosely affixed to substratum, turf or weft-forming. Stems yellowish, hyalodermous, with weak central strand, pseudoparaphyllia foliose, lanceolate, irregularly dentate. Leaves of stem falcate, secund broadly ovate-lanceolate, gradually narrowed to a slender acumen, curved to insertion (sometimes markedly), 1.4--1.6 × 0.5--0.6 mm, margins plane and serrulate distally, costa short and double; median cells linear (50--)60--80 × 3--4 µm, basal cells shorter and wider, yellowish or unpigmented; alar regions are distinct areas of subquadrate to rectangular cells, 9--20 µm wide, usually not decurrent, the most basal cells thinner walled and hyaline, often tearing away with the enlarged stem cortical cells; branch leaves similar. Sexual condition dioicous or phyllodioicous; perichaetial leaves erect, oblong-lanceolate, with slender serrulate acumen, plicate, costa obscure. Seta 1.5--2.2 cm, pale reddish brown. Capsule red-brown, inclined to horizontal, oblong-cylindric, curved, 1.5--1.8 mm, excluding conic operculum; annulus 1--2-seriate, cilia of endostome 2--3.
Sporophytes produced summer; capsules mature July--Aug. Mainly in moist heaths, but also shrub thickets and spruce forest, largely on calcareous substrata; 0--1500 m or more; Greenland; Alta., B.C., N.W.T., Nunavut, Yukon; Europe (Finland).
Hypnum holmenii occurs mainly at elevations above 1500 m. In size and general aspect this species often resembles H. plicatulum, but the leaves are generally not markedly auriculate, as in H. plicatulum. The alar cells are not differentiated in the latter species, while they usually are in H. holmenii. From Hypnum callichroum, it differs in the abruptly differentiated group of enlarged alar cells of the former, these gradually differentiated in H. holmenii. It is necessary to examine many leaves to achieve an accurate assessment. Hypnum hamulosum differs in its strongly hamate stem leaves and the less differentiated alar cells. Because H. holmenii often resembles related species, its distribution has been inadequately documented.