6. Trichostomum arcticum Kaalaas, Bot. Not. 1900: 257. 1900.
Didymodon arcticus (Kaalaas) Brotherus; Trichostomum cuspidatissimum Cardot & Thériot
Stem triangular in section. Leaves flattened, narrowly lanceolate, distal margins plane, minutely crenulate, not bordered; apex narrowly acute, plane or keeled; basal cells differentiated across leaf base as a W, commonly running up margins, distinctly enlarged submarginally submarginally just proximal to the lowest point the quadrate medial cells extend in the leaf; distal laminal cells pluripapillose with low papillae; mucro narrowly conic, of 6-9 cells. Not fruiting in the flora area.
Gravel, fen, mire, calcareous bog, sedge meadow, low-center polygon, tundra, wet or occasionally dry areas; often associated with snow melt runnels; moderate elevations (400-700 m); Greenland; B.C., Nfld. and Labr. (Labr.), N.W.T., Nunavut, Ont., Que., Yukon; Alaska; n Eurasia.
The triangular stem cross section with a strong central strand will easily distinguish Trichostomum arcticum from the disconcertingly similar Tortella tortuosa var. arctica. Additionally, the former has leaves usually strongly recurved when wet and the basal cells extend upwards both juxtacostally and marginally, while the latter has leaves weakly recurved when wet and basal cells extending upwards only towards the leaf margins. Trichostomum arcticum also has a distinctive area of enlarged basal cells submarginally just below the lowest point to which the quadrate distal cells extend in the leaf, and its laminal papillae are more coarse. Most species of the genus have leaf margins to some extent minutely crenulate by projecting laminal cells and their papillae, but this feature seems distinctively pronounced in T. arcticum, which is found generally worldwide across the arctic.