Didymodon borealis 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 : 141-142

2. Didymodon borealis Ignatova & Ignatov , species nova. Fig. 7 .

Didymodon borealis differs from otherwise similar Didymodon acutus in smaller size of plants, leaves 0.7– 0.85× 0.25–0.3 mm versus 0.8–1.8× 0.3–0.65 mm, and thinner-walled leaf cells.

Type : Russia, Yakutia , Tomponsky District , small Creek – left tributary of Dyby River, 62°44’51”N, 139°04’46”E, 765 m a.s.l., on verticall cliff wall on right bank of the creek, 27.VIII.2017, Ignatov & Ignatova 17- 426 (holotype MHA9025679 , isotype MW9092501 ) .

Etymology: species name reflects a peculiarity of its distribution area, as all its currently known localities are within boreal areas of Eurasia.

Plants tiny, in dense, often easily separating tufts, dull brownish-green or olivaceous-green. Stems to 1 cm long, erect, simple or branched, without hyalodermis, with central strand. Rhizoidal tubers absent. Leaves straight, appressed when dry, erectopatent when wet, ovate-lanceolate, from ovate bases abruptly or gradually narrowed into triangular acumina as lonf as the bases or slighly longer, narrowly acute or acuminate, not decurrent, 0.6– 0.85× 0.25–0.30 mm, canaliculate distally; margins narrovly recurved at proximal 1/2–2/3, unistratose, entire; costa 50–60 µm wide at base, slightly narrowing upwards, percurrent, moderately prominent distally, slightly convex ventrally, with quadrate, smooth cells on both surfaces, without band of translucent cells on ventral side below apex, in transverse secrion elliptic, weakly differentiated in the uppermost portion, below with 3–5 guide cells in 1 layer, ventral stereids in 0–1 layer, ventral epidermis differentiated, with outer walls not bulging, dorsal stereids in 1–2 layers, dorsal epidermis present or absent; lamina unistratose; upper and median laminal cells rounded-quadrate, elliptic and transversely elliptic, 6–14×9–12 µm, with moderately thickened walls, smooth; basal cells weakly differentiated, basal juxtacostal cells short rectangular and quadrate, 12–20×11–12 µm, smooth, with moderately thickened walls, chlorophyllose; basal marginal cells in several rows short rectangular, quadrate and travsversely elliptic. KOH-reaction yellowish-green. Specialized asexual reproduction absent. Apparently dioicous, only male plants seen. Perigonia terminal. Perichaetia and sporophytes unknown.

Didymodon borealis is currently known from Yakutia, Urals (Perm Territory and Sverdlovsk Province) and SW Finland. It grows on wet, rarer moderately dry cliff walls in the forest, along streams and rivers. It can be confused with D. acutus and forms of D. mongolicus with smooth cells; however, both these species have larger leaves (see key) and grow in xeric habitats.

Figures

Fig. 7. Didymodon borealis (from holotype). A–C: leaf transverse sections; D: mid-leaf cells; E: habit, dry; F: habit, wet; G–I: leaves; J: stem transverse section; K: basal leaf cells; L: upper leaf cells. Scale bars: 2 mm for E–F; 1 mm for G–I; 100 µm for A– D, J–L.