Didymodon truncatus Ignatova & Ignatov, 2024

Ignatova, E. A., Fedosov, V. E., Kuznetsova, O. I., Fedorova, A. V. & Ignatov, M. S., 2024, On the genus Didymodon s. str. (Pottiaceae, Bryophyta) in Russia, Arctoa 33 (2), pp. 129-155 : 152

11. Didymodon truncatus Ignatova & Ignatov , species nova. Fig. 17.

Type: Russia, Altai Republic, Kosh-Agach District, Yuzhno-Chuisky Range , valley of Tarkhata River , along small creek – its right tributary, 49°37’32”N, 88°27’24”E, 2240 m alt., dry cliffs, in small cracks, 21 June 2021, Ignatov & Ignatova 21-450 (holotype MHA9131861 ) .

Etymology: name of this species reflects its often truncate leaf apices.

Plants small, in dense, soft tusfts, yellowish-green above, light brownish below, dull. Stems 3–5 mm long, erect, simple, without hyalodermis, with central strand. Rhizoidal tubers absent. Leaves slighly incurved when dry, erectopatent when wet, ovate or ovate-lanceolate, with weakly differentiated bases, gradually tapered into acumina, widely rounded or truncate at apices, not decurrent, 0.55–1.0× 0.22–0.28 mm, concave distally; margins narrowly recurved from proximal 1/3 almost to apex, unistratose, entire; costa 30–50 µm wide at leaf base, slightly narrowing upwards, percurrent or ending few cells below apex, weakly prominent dorsally, flat or convex ventrally, sharply delimited from laminal cells, on both surfaces with elongate rectangular, smooth cells, without band of translucent cells on ventral side below apex, in transverse section semicircular or round, with 2–3 guide cells in 1 layer, ventral stereids absent, ventral epidermis differentiated, with outer walls not bulging, dorsal substereids in 1 layer, dorsal epidermis differentiated; lamina unistratose; upper and median laminal cells irregularly polygonal, 8–17×9– 12 µm, thick-walled, smooth; basal juxtacostal cells rectangular, 11–30×6–9 µm, smooth, with moderately thickened, straight walls, hyaline; basal marginal cells in 3–4 rows shorter, short rectangular. KOH-reaction yellowish-green. Specialized asexual reproduction absent. Gametangia and sporophytes unknown.

This species is currently known only from type locality. There is some similarity between D. truncatus and Geheebia tophacea in elongate, smooth ventral cells of costa and leaves with rounded apices. However, plants of G. tophacea are larger, with leaves 1.0–2.2×0.3–0.4 (–0.8) mm vs 0.55–1.0× 0.22–0.28 mm in D. truncatus , decurrent at base, with narrower acumina, and laminal cells with low papillae (smooth in D. truncatus ).