21. Dicranum fragilifolium Lindberg, Bot. Not. 1857: 147. 1857.
Plants in compact tufts, light green to yellowish brown, glossy. Stems 1.5-6 cm, densely tomentose with dark brown to red rhizoids. Leaves straight, erect-spreading, rigid, appressed when dry, smooth, (5-)6-7(-7.5) × 0.4-0.6 mm, most of the leaf tips deciduous and absent, concave proximally, canaliculate distally, from a lanceolate base to a long subula formed by the excurrent costa, apex acute; margins entire to somewhat serrulate above; laminae 1-stratose or some 2-stratose regions near costa; costa excurrent, 1/4-1/3 the width of the leaves at base, smooth or slightly rough above on abaxial surface, abaxial ridges absent, with a row of guide cells, two thin stereid bands (2-3 cells thick), adaxial and abaxial epidermal layers not differentiated or sometimes with a few cells in both layers enlarged; cell walls between lamina cells slightly bulging; leaf cells smooth; alar cells 1-stratose, sometimes with some 2-stratose regions, well-differentiated, sometimes extending to costa; proximal laminal cells elongate-rectangular, usually pitted or indistinctly pitted, (25-)39-55(-84) × (5-)7-8(-10) µm; median laminal cells rectangular, not pitted, (11-)21-22(-37) × (4-)7-8(-10) µm; distal laminal cells nearly elliptic, incrassate. Sexual condition dioicous; male plants as tall as females, usually more slender; interior perichaetial leaves abruptly long-acuminate, convolute-sheathing. Seta 1.5-2.5 cm, solitary, yellowish to brown. Capsule 1.8-2 mm, arcuate to nearly straight, ± erect, smooth, sometimes striate when dry, yellowish brown; operculum 1-2 mm. Spores 16-28 µm.
Capsules mature in summer. Rotten logs and stumps, humic soil, humus over rocks, usually in coniferous woods, occasionally in mixed woods and bogs; 30-1900 m; Greenland; Alta., B.C., Man., Nfld. and Labr. (Nfld.), N.W.T., Ont., Que., Sask.,Yukon; Alaska, Mich.; Europe; Asia.
Dicranum fragilifolium is a rare boreal species with shiny, light green to yellowish brown, erect-spreading leaves. The distal portion of the leaves is fragile and often broken off, thereby giving the plants a distinctive appearance. The deciduous leaf apices presumably serve as a type of asexual reproduction by regenerating to produce new plants. The species is often confused with D. tauricum, another species with deciduous leaf tips, but the latter occurs only in western North America. Where their ranges overlap they frequently are difficult to tell apart. When sporophytes are present the straight capsules of D. tauricum are distinctive from the usually arcuate ones of D. fragilifolium, which unfortunately rarely produces them. When sterile, the best way to separate the two is by the costa cross section in the proximal half of the leaf: D. fragilifolium has stereid cells in two thin bands while D. tauricum has none. Also, in D. fragilifolium there are 2-3 layers of cells above and below the guide cells, while in D. tauricum there are 1 or rarely 2 layers of cells. One other species with deciduous leaf tips that has sometimes been confused with D. fragilifolium is D. viride of eastern North America. It has a straight capsule like D. tauricum but as in D. fragilifolium capsules are rarely produced. The broad costa of D. viride, covering 1/3 or more of the leaf base, will distinguish it from D. fragilifolium, the costa of which covers 1/4 or less of the leaf base.