Tortella cuspidatissima (Cardot & Thér.)

Tortella cuspidatissima (Cardot & Thér.) View in CoL O. Wern- er, Köckinger & Ros, Nova Hedwigia 98(3–4): 287. 2014. — Trichostomum cuspidatissimum Cardot & Thér., Proc. Wash. Acad. Sci. 4: 302. 13 f. 4. 1902. Fig. 4, 6 C, H .

Plants medium-sized to large, in loose or dense tufts or cushions, yellowish-green or brownish. Stems to 6 cm long, irregularly branching, densely foliate, weakly tomentose, round in transverse section, with weak, occasionally indistinct or, rarely, absent central strand. Leaves 2.5–3.5(–4.0)× 0.7–0.9 mm, linear-lanceolate, with length:width ratio 4–6:1, flexuose, occasionally slightly twisted when dry, erect-spreading or patent when wet, sigmoid, limb gradually tapered, narrowly acuminate, not cucullate, keeled above; leaf bases short rectangular or ovate, slightly widened, with indistinct or distinct shoulders; margins plane in mid-leaf, widely incurved above, entire, slightly wavy; costa moderately strong, 50–100 (–120) µm wide at leaf base, gradually narrowing distally, on ventral and dorsal surfaces with elongate, smooth cells, excurrent into moderately long, smooth or weakly denticulate mucro, in transverse section with one layer of guide cells, two stereid bands, ventral epidermis differentiated, dorsal epidermis weakly differentiated; lamina unistratose; upper and median laminal cells rounded-quadrate, (7–)11– 14(–16) µm wide, thick-walled, densely papillose, papillae low, obscuring cell walls; basal laminal cells elongate-rectangular, moderately thick-walled, smooth, yellowish, reaching into the limb at equal height near costa and at margins, gradually transiting into limb cells; basal marginal cells wider or narrower than central basal cells. Dioicous, sporophytes unknown.

Distribution and ecology. Tortella cuspidatissima was described from Hall Island near NW coast of North America, which remains its only known locality in this continent. Werner et al. (2014) considered it as a species of hybrid origin with parent species T. arctica (= T. splendida ) and T. spitzbergensis . They also found it in Sweden. Köckinger & Hedenäs (2023) referred to T. cuspidatissima type specimens of T. arctica from the lower course of Lena River. We also found it in herbarium collections from Chukotka and Vrangel Island. It grows on soil in wet arctic tundra, on moist calcareous rocks with soil layer, along streams, and on pebbly sea shore terrace.

Differentiation. Tortella cuspidatissima differs from T. tortuosa and T. splendida by presence of stem central strand (though occasionally indistinct) and smooth costa on ventral surface, and from the former species also by gradual transition between basal and limb cells. Small central strand, 2–4 cells wide, differentiates T. cuspidatissima from T. spitzbergensis (in the latter species central strand is usually very conspicuous, large, 7–14 cells wide, only very rarely +small). In T. cuspidatissima stems are round in transverse section, while in T. spitzbergensis they are triangular in outline. Basal cells reach into the limb at equal height at margins and near costa in T. cuspidatissima , but higher near costa in T. spitzbergensis . In T. cuspidatissima basal marginal cells are narrower or lightly wider than central cells, whereas in T. spitzbergensis several marginal cell rows at leaf base are distinctly wider and shorter, forming pellucid area. An additional character is the width of upper and median laminal cells: (7–)11–14(–16) µm in T. cuspidatissima vs. 8–11(–13) µm in T. spitzbergensis . Tortella fleischeri , with also posesses weak central strand and costae smooth ventrally, can be distinguished by abrupt transition between basal and limb cells ( Fig. 6 View Fig ).