Didymodon baicalensis 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 : 150-151

9. Didymodon baicalensis Ignatova & Ignatov , species nova. Fig. 15.

Type: Russia, Irkutsk Province, Irkutsk District, Listvyanka , ca. 51°50’N, 104°53’E, 500 m alt., Great Baikal Trail near its starting point at Listvyanka settlement, dry rock outcrops, 10 September 2018, Ignatov, Ignatova & Kolesnikova 18-4666 (holotype MHA9027319 , isotype MW9090865 ) .

Etymology: species name corresponds to its type locality, slopes to Baikal Lake where the Great Baikal Trail runs.

Plants small to medium-sized, in dense tufts, dark-green or brownish-green, dull. Stems to 3 cm long, erect, simple or branched, without hyalodermis, with central strand. Rhizoidal tubers absent. Leaves straight, appressed when dry, erectopatent to patent when wet, deltoid or deltoid-lanceolate, from ovate bases gradually or abruptly narrowed into short triangular acumina, acute, not decurrent, 1.0–2.0× 0.4–0.5 mm, canaliculate distally; margins narrowly revolute almost to apex or in proximal 3/4, unistratose, entire; costa strong, 40–50 µm wide at leaf base, weakly narrowing upwards, excurrent into short, stout mucro, strongly prominent dorsally, flat or weakly convex ventrally, on both surfaces with quadrate, papillose cells, without band of translucent cells on ventral side below apex, in transverse section semicircular, with 2–3 layers of guide cells, 3–7 cells in each layer, ventral stereids absent, occasionally 1 layer of substereids present, ventral epidermis differentiated, with flat or slightly bulging out- er walls, dorsal stereids in 2–3 layers, dorsal epidermis differentiated; lamina unistratose; upper and median laminal cells rounded-quadrate, elliptic and transversely elliptic, 7–10×7–10 µm, with moderately thickened walls, on both surfaces with 2–3 bifid papillae per cell; basal juxtacostal cells quadrate and short rectangular, 10–25×7– 10 µm, smooth, with moderately thickened, straight walls, chlorophyllose; basal marginal cells in 3–4 rows quadrate and transversely rectangular. KOH-reaction green or orange. Asexual reproduction by round multicellular gemmae 20–25 µm in diameter, born on branched stalks in leaf axils. Gametangia and sporophytes unknown.

Didymodon baicalensis occurs in southern Siberia (Irkutsk Province, Buryatia, Zabaikalsky Territory, Khakassia, and Altai Republic). It grows on rock outcrops and rocky soil on exposed xeric slopes and in larch forests. Some its specimens were stored in herbaria under the name D. cordatus . It shares with this species leaf shape, leaf margins revolute almost to apices, papillose laminal cells, and presence of multicellular axillary gemmae, but differs from it in costa structure: in D. baikalensis , there are 2–3 layers of guide cells but no ventral stereids, whereas in D. cordatus guide cells are 1-layered and ventral stereids present. Costa structure similar to D. baicalensis is characteristic of D. desertorum , but this species has smooth laminal cells.