Didymodon sibiricus Ignatova & Fedosov, 2025

Ignatova, E. A., Fedosov, V. E., Fedorova, A. V., Kuznetsova, O. I. & Ignatov, M. S., 2025, Two new species of Didymodon (Pottiaceae, Bryophyta) from Asian Russia, Arctoa 34 (1), pp. 24-30 : 26-27

Didymodon sibiricus Ignatova & Fedosov , species nova. Fig. 2 .

TYPE: Russia, Republic Sakha / Yakutia, Tomponsky District , Verkhoyansky Mountain system, Suntar-Khayata Range , ca. 63°07’N, 139°00’E, 980 m a.s.l., Sukhaya Creek , right bank, larch forest with Chosenia on low terrace, cliffs at slope base, on inclined rock surface, 23.VII.2015, Ignatov & Ignatova 15-11 (Holotype MHA9108739 !, isotype MW9035965 !) .

Diagnosis: the new species is similar to Didymodon daqingi J. Kou, R.H. Zander & C. Feng in having leaves with long, narrow acumina and long excurrent costae, occasionally fragile, and costae lacking ventral stereids, but differs from it in smaller leaves, partially bistratose lamina not only at margins, and smooth laminal cells.

Etymology: the species name corresponds to Siberia, a region of northern Asia where it is distributed.

Description. Plants small or medium-sized, in dense tufts, dark green or brownish-green, not glossy. Stems 1.0– 1.5 cm long, simple or irregularly branched, with moderately large central strand. Leaves loosely appressed and slightly flexuose when dry, widely spreading when wet, 0.8–1.8× 0.3–0.4 mm, moderately fragile or, rarer, not fragile, from ovate bases gradually or +abruptly tapered into long, linear-lanceolate acumina, canaliculate distally, not decurrent at bases; margins slightly recurved on both sides at proximal 1/2–3/4 of leaf, entire; costa moderately strong, 50–75 µm wide at leaf base, gradually narrowing upwards, long excurrent, strongly prominent dorsally, flat ventrally, with quadrate, smooth cells on both surfaces, without a pad of large, translucent cells on ventral surface below leaf apex, in transverse section semicircular, guide cells in 1–2(–3) layers, dorsal stereids in 1–3 layers, ventral stereids lacking or few substereids present in proximal part, ventral and dorsal epidermis differentiated; lamina partially bistratose distally at margins or near costa, or between costa and margins, unistratose in proximal part of leaf; cells in distal part of leaf rounded-quadrate and oblate, with angular lumina, 5–10×4–8 µm, smooth; basal leaf cells weakly differentiated, in few rows short rectangular, 15–25×6–10 µm, basal marginal cells quadrate and oblate. KOH reaction yellowish-green. Brood bodies absent. Dioicous, sporophytes unknown.

Distribution and ecology. Didymodon sibiricus is currently known from two close localities on Suntar-Khayata Mountain Range in Yakutia, on Byrranga Range in northern Taimyr (this is the northernmost known locality, at 74.47°N), and in several distant localities in southern Siberia: Republics Altai, Tyva, and Buryatia, and Zabaikalsky Territory. It was collected on outcrops on dry, open or forested slopes, in cliff niches, on siltstone and dolerite outcrops, at elevations 400–2100 m a.s.l.

Differentiation. Didymodon sibiricus has a considerable similarity to the described from China D. daqingi , and its specimens were previously referred to this species ( Ignatova et al., 2024). They are similar in leaf shape, with short, ovate bases and long, linear lanceolate acumina, long excurrent costa, and partially bistratose lamina. However, they differ in plant size and size of leaves: in D. sibiricus , leaves are shorter and narrower [0.8–1.8 (–2.0)× 0.3–0.4 mm vs 2.0–2.8× 0.4–0.5 mm], costa is more slender (50–75 vs 75–105 µm wide at leaf base), leaf margins are only slightly recurved in midleaf (much stronger recurved to revolute in proximal 1/2–3/4 of leaf in D. daqingi ; lamina cells are smooth in D. sibiricus (vs weakly papillose in D. daqingi ), and it possesses bistratose patches in distal part of leaf not only at margins but also near costa, or only in midleaf (leaf lamina is described as bistratose only at margins in D. daqingi ). Another species described from Inner Mongolia, China, D. manhanensis C. Feng & J. Kou , has also a certain similarity with D. daqingi and D. sibiricus in leaf shape,

Figures

Fig. 2. Didymodon sibiricus Ignatova & Fedosov (from holotype). A: habit, dry; B: habit, wet; C: upper leaf cells; D: stem transverse section; E: median leaf cells; F–H: leaf transverse sections; I–M: leaves; N: basal leaf cells. Scale bars: 5 mm for B; 2 mm for A; 1 mm for I–M; 100 µm for C–H, N.