DIDYMODON MONGOLICUS ( BRYOPHYTA , POTTIACEAE ) , A NEW SPECIES FROM MONGOLIAN PLATEAU

Didymodon mongolicus D. P. Zhao &T. R. Zhang is described as a new species from northern China and Mongolia. The new taxon is distinguished from others species of the genus by its small, ovate leaves, weakly recurved margins, smooth laminal cells and costa ending several cells below the apex, with 1 layer of ventral stereids in the lower part of the leaf. Drawings and light microscope photographs of the main characters are given, and its relationships with some closely related species within the genus are discussed. Introduction The genus Didymodon Hedw. (Pottiaceae) currently consists of 126 species with distribution nearly throughout the world (Zander, 1993; 2007). The genus concept of Didymodon has been controversial, especially the morphological delimitation of Didymodon versus Barbula Hedw. The taxonomic differences between the two genera are mainly on gametophytic characteristics: Barbula has entirely hyaline axillary hairs, while in Didymodon the hairs have one or two brown basal cells. The leaves of Barbula are ovate to long-elliptical, the basal cells are hyaline, elongate and well differentiated, while those of Didymodon are mostly lanceolate to long-lanceolate, the basal cells are usually green, shortly rectangular and little differentiated (Saito, 1975; Zander, 1993, 2007; Jiménez et al., 2005; Werner et al., 2005; Jiménez, 2006). Recently, Jiménez et al. (2012) described Andina J.A. Jiménez & M.J. Cano as a new genus, which contained several species earlier recognized within Pseudocrossidium R.S. Williams and Didymodon. Andina and Didymodon both have brown basal cells in axillary hairs; however, Andina can be easily distinguished from the latter by the cucullate leaf apices, strongly recurved to revolute leaves with margins infolded in the apex, ventral costal outgrowths differentiated as pads of bulging and papillose cells, filaments or lamellae, and upper laminal cells usually bulging on both surfaces. Mongolian Plateau is divided politically and geographically by Mongolia in the north and the Inner Mongolia Autonomous Region of China in the south. Didymodon is the most species-rich genus in the Pottiaceae in this region and includes 30 species (Bai, 1997; Tsegmed, 2010; Zhao et al., 2014, 2015). Over the last several years we have been studying Didymodon of the Mongolian Plateau (e.g., Zhao 2010; Zhao et al. 2013, 2014, 2015). In the course of this work we found that several specimens fit into the generic concept of Didymodon based on the presence of stem sclerodermis, axillary hairs with one brownish basal cells, costa with ventral and dorsal stereids, upper laminal cells rounded, subquadrate or oval, and basal cells little differentiated. Nevertheless, they were not referable to any known Didymodon species. After studying the types deposited in H and JE herbaria and after comparison between the specimens with published information (Saito, 1975; Zander, 1993; Zander and Ochyra, 2001; Li et al. 2001; Jiménez et al., 2005; Jiménez, 1 Corresponding author. Email: topalizdp@aliyun.com


Introduction
The genus Didymodon Hedw.(Pottiaceae) currently consists of 126 species with distribution nearly throughout the world (Zander, 1993;2007).The genus concept of Didymodon has been controversial, especially the morphological delimitation of Didymodon versus Barbula Hedw.The taxonomic differences between the two genera are mainly on gametophytic characteristics: Barbula has entirely hyaline axillary hairs, while in Didymodon the hairs have one or two brown basal cells.The leaves of Barbula are ovate to long-elliptical, the basal cells are hyaline, elongate and well differentiated, while those of Didymodon are mostly lanceolate to long-lanceolate, the basal cells are usually green, shortly rectangular and little differentiated (Saito, 1975;Zander, 1993Zander, , 2007;;Jiménez et al., 2005;Werner et al., 2005;Jiménez, 2006).Recently, Jiménez et al. (2012) described Andina J.A. Jiménez & M.J. Cano as a new genus, which contained several species earlier recognized within Pseudocrossidium R.S. Williams and Didymodon.Andina and Didymodon both have brown basal cells in axillary hairs; however, Andina can be easily distinguished from the latter by the cucullate leaf apices, strongly recurved to revolute leaves with margins infolded in the apex, ventral costal outgrowths differentiated as pads of bulging and papillose cells, filaments or lamellae, and upper laminal cells usually bulging on both surfaces.
Mongolian Plateau is divided politically and geographically by Mongolia in the north and the Inner Mongolia Autonomous Region of China in the south.Didymodon is the most species-rich genus in the Pottiaceae in this region and includes 30 species (Bai, 1997;Tsegmed, 2010;Zhao et al., 2014Zhao et al., , 2015)).Over the last several years we have been studying Didymodon of the Mongolian Plateau (e.g., Zhao 2010;Zhao et al. 2013Zhao et al. , 2014Zhao et al. , 2015)).In the course of this work we found that several specimens fit into the generic concept of Didymodon based on the presence of stem sclerodermis, axillary hairs with one brownish basal cells, costa with ventral and dorsal stereids, upper laminal cells rounded, subquadrate or oval, and basal cells little differentiated.Nevertheless, they were not referable to any known Didymodon species.After studying the types deposited in H and JE herbaria and after comparison between the specimens with published information (Saito, 1975;Zander, 1993;Zander and Ochyra, 2001;Li et al. 2001;Jiménez et al., 2005;Jiménez, 2006;Zander, 2007;Kučera and Ignatov, 2015), we conclude that our samples belong to an undescribed species of Didymodon, which is described, illustrated and compared with the most closely related species in the study.
( Figs 1 & 2).Diagnosis: Didymodon mongolicus is differentiated from other species in the genus by the following combination of characters: small sized, ovate leaves, weakly recurved margins up to 3/4 of the apex, smooth laminal cells, costa ending below the apex and cross-section of the costa with1 layer of ventral stereids in the lower part of the leaf.
Etymology: Named after the type locality, Mongolian Plateau.
Habitat and Distribution: Didymodon mongolicus grows on soil and rocks in Juniperus communis forests.It forms small and mostly monospecific patches.Besides the type locality, we discovered four additional populations from Inner Mongolia and ones from Mongolia.The new species seems to be restricted to the Mongolian Plateau, but it is likely to be found elsewhere in this large plateau with progress in field studies.Note: Didymodon mongolicus is most closely related to D. tectorum (Müll.Hal.) K. Saito and D. cordatus Jur., sharing such charateristics as the shape of the leaves, yellowish green colour of the lamina with KOH reaction, rectangular to subquadrate ventral cells of costa above midleaf, and growing in dry soil and rock.In fact the small forms of D. tectorum, which lack gemmae cannot be easily distinguished from the new species.Didymodon mongolicus can be separated readily from D. tectorum by its leaf margins weakly recurved from base to 3/4 of the leaf (Fig. 2: A, F), cells smooth above midleaf (Fig. 2: C, D, H, I), and transverse section of costa with 1 layer of ventral stereids (Fig. 2: E, J).Although the type of D. tectorum does not contain gemmae,  numerous specimens of D. tectorum from China and Mongolia have copious gemmae.D. cordatus differs from D. mongolicus in having a stout costa that is shortly excurrent, ending in a wide mucro (Fig. 2: L) and with 2-5 layers of ventral steroids (Fig. 2: N, O), and margins revolute up to the apex.Small forms of Didymodon acutus (Brid.)K. Saito may be confused with D. mongolicus, as both species have appressed leaves when dry, smooth laminal cells, slightly recurved margins, and absence of gemmae in the axils of the leaves, but D. mongolicus differs in its smaller size, more ovate leaf shape and never having lanceolate leaves, and the costa ending before the apex.D. validus Limpr., a species known from Central Asia and Europe (Jiménez, 2006), resembles D. mongolicus in the smooth laminal cells.It differs from the new species by its recurved or incurved leaves when dry, shape of leaves ovate-lanceolate or linear-lanceolate, costa long-excurrent and transverse section of the costa with 1-3 layers of ventral stereids while D. mongolicus has appressed leaves when dry, ovate leaves, costa ending several cells below the apex and transverse section of the costa with 1 layer of ventral stereids.

Fig. 1 .
Fig. 1.Didymodon mongolicus sp.nov.: A. Plants, B. Leaves, C. Transverse section of stem, D. Leaf apex, E. Middle part of leaves,F.Basal part of leaves, G-J.Transverse section of leaves from apex to base, K. Axillary hair.(All drawn from the holotype).