The Genus Plagiothecium Schimp. (Plagiotheciaceae, Bryophyta) in Eurasia: An Annotated Checklist with Distribution and Ecological Data

An annotated checklist of the pleurocarpous moss genus Plagiothecium in Eurasia is presented for the first time based on a thorough review of the literature. Data have been compiled from previous relevant works conducted on the genus over more than 70 years and published up to the end of June 2020 for 107 Eurasian countries (and islands). Sectional classification is based on molecular phylogeny of the genus published recently. A total of 41 taxa are reported, including 29 species and 12 infraspecific taxa (nine varieties and three forms) belonging to eight sections. The highest numbers of taxa were found in China (20 taxa), the Russian Federation (20 taxa) and Japan (18 taxa), while the smallest numbers of taxa were recorded in the Middle East, Central Asia and the islands area. Not a single species of Plagiothecium was recorded in 26 regions, whereas P. denticulatum, P. nemorale and P. cavifolium turned out to be the most widespread species in the entire study area. They were recorded in most of the surveyed countries and islands. For each accepted taxon, information on relevant literature, synonyms, distribution within Eurasia and globally are provided. Comments on each taxon, ecological preferences, and notes on doubtful records are also included. Additionally, distribution maps for each recognised taxon are supplied. This checklist can enlighten and foster a better understanding of the distribution, diversity, and ecology of Plagiothecium in Eurasia and provides an incentive for future research on the genus.


Introduction
The pleurocarpous moss genus Plagiothecium Schimp. (Plagiotheciaceae M. Fleisch.) was erected by Schimper and initially described in "Bryologia Europaea" [1] to include plants with more or less flattened, asymmetrical leaves, well-developed decurrent alar cells, and double costae. Over time, the understanding of this genus changed, and many species were excluded from it to create new genera, viz. Herzogiella Broth., Isopterygiella Ignatov & Ignatova, Isopterygiopsis Z. Iwats., Isopterygium Mitt., Pseudotaxiphyllum Z. Iwats. and Taxiphyllum M. Fleisch. Our understanding of the family Plagiotheciaceae has also changed. It has been treated for a long time as monogeneric, with the single genus Plagiothecium [2][3][4], but further research using molecular methods [5][6][7] changed the view of the described genus and the whole family.
Biogeographically, Plagiothecium is a cosmopolitan genus represented in all continents, though varies in diversity and abundance, with largest concentrations in temperate, upland habitats of the Northern Hemisphere. Apart from Asia and Europe [8,9] (which will be discussed in detail later in this paper), the genus has been reported from, for example, Africa: Sub-Saharan Africa [10], tropical Africa [11], Northern Africa [12] and Western   Faroe Islands (Denmark) FRO [7,9,48,89,90]

Cyprus -
Islamic Republic of Iran Maldives -

Yemen
The conducted analysis of the frequency of occurrence of individual species showed that species with a limited range are a majority in the studied genus. Twenty-six taxa (60% of all) were reported only from one to five countries, and in the Eurasia they were recognised to be very rare. Wherein, 12 of them (e.g., P. argentatum, P. cochleatum, P. conostegium, P. decoratum, P. subglaucum) ( Figure S1) were listed only in Asia. The smallest number of taxa in this group (P. berggrenianum, P. enerve, P. fallax, P. obtusissimum and P. rossicum) was reported both in Asia and Europe ( Table 2). The frequency analysis also indicated that common species are the least numerous group. The species with the widest range were: P. denticulatum, listed in 65 countries and islands (65% of all analysed), P. nemorale (64, 62%), and P. cavifolium (59, 60%), they were considered as common in the study area ( Table 2, Figures S2 and S4).

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Annotations to the following list of Plagiothecium taxa include brief description for each taxon, with comments on nomenclature and taxonomic status (when necessary), ecological preferences, and geographic distribution inside and outside Eurasia (if available).

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Taxon distribution per country in Eurasia is given in detail in Appendix B, with all available literature sources.

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We did not make any broad attempt to provide an exhaustive list of records for all countries in each continent (outside Eurasia), but we gave a general overview of the geographic distribution outside the study area based on the selected references. As far as we know, none of the 41 taxa listed here have been reported from Antarctica [14] or Australia [17].

1.
Plagiothecium angusticellum G.J. Wolski & P. Nowicka-Krawczyk 2020-a mediumsized plant, with asymmetrical leaves, not shrunken when dry; an acuminate, gently curved, not serrated apex; tight areolation, created with long and narrow leaf cells which do not form regular rows. This combination of features made it possible to distinguish P. angusticellum from similar and closely related species such as P. cavifolium, P. nemorale, and P. longisetum [27]. Ecology. From Poland (from where it was described), this species was recorded mainly in deciduous forests (e.g., eutrophic swamp forests Ribeso nigri-Alnetum Sol.  [191].

2.
Plagiothecium argentatum (Mitt.) Q. Zuo 2011-described as Hypnum argentatum by Mitten [192], transferred to the genus Struckia by Müller [193], and finally (based on DNA analysis) incorporated into the genus Plagiothecium [36], its integration in this genus was confirmed by other researchers (e.g., [7,23]). A pale green, julaceous plant, with symmetric, concave, plicate, long, non-decurrent leaves, with a serrulate or entire margin, and quadrate alars; it is characterised by quite an unusual (for a member of the genus Plagiothecium) set of features. Most of these features place P. argentatum well in the described genus, but serrulate margins, absence of decurrency and quadrate alars seem to exclude this species from the genus Plagiothecium. This large morphological separateness resulted in the placement of this species (as P. enerve) in a separate section of Struckia [23]. Ecology. The species was recorded in epiphytic and epixylic habitats [7]. Distribution. So far, recorded from East and South Asia.

3.
Plagiothecium berggrenianum Frisvoll 1981-a medium-sized to large, julaceous, crowded plant; leaves symmetrical, very concave, ovate, long decurrent, and an abruptly narrowed to hooked apex; leaf cells long and narrow, thick-walled; capsules straight and erect. This circumpolar species was described by Frisvoll [171], and is easily distinguishable from other species by leaves with recurved margins and by the shape of its apex, as well as well-developed alar regions. Plagiothecium berggrenianum is similar to P. svalbardense, but the former is longer, with elliptical, plicate leaves, and broadly recurved margins. Wynns [7] states that P. berggrenianum is a possible hybrid. Ecology. This species is recorded in epigeic, epilithic and epixylic habitats [26,171,172]. It is found in swales, tundra, and cliffs; low to moderate elevations [21]. Distribution. Apart from Eurasia, it is also reported from North America (Canada, Greenland, USA) [7,26,172].

5.
Plagiothecium cavifolium var. orthocladium (Schimp.) Z. Iwats. 1970-Iwatsuki [29] selected this name for the variety previously known as P. orthocladium [1], and also reported on a relationship between this taxon and P. nemorale fo. japonicum (currently P. japonicum) as well as P. succulentum. Wynns [7] mentioned difficulty in distinguishing this taxon, adding that he used this name for olivaceous, boreal specimens, with crispate and spreading leaves. Ecology. This taxon is recorded in epilithic habitat. Distribution. Apart from Europe (Nordic countries), this taxon is also reported from North America (Canada, Greenland) [7].

6.
Plagiothecium cochleatum Dixon 1938-a dark green plant; leaves loosely imbricated, concave, plicate, with rigid areolation, and quite well-developed alar decurrencies. Wynns [7] indicates that P. cochleatum is similar to P. cavifolium and may be confused with this species. Ecology. It is a rare species, present in disjunct Alpine and Himalayan habitats [23]. Distribution. So far, it is reported from Asia (India).

7.
Plagiothecium conostegium Herzog 1916-Suzuki [119] recorded this taxon from Japan. Considering the features and pictures published by Suzuki [119] in particular: Asymmetric, in dry condition shrunken leaves, long-hexagonal cells, we believe that the taxon described by Suzuki [119] looks more like P. longisetum. This requires checking herbarium materials, but at this stage, we consider the presence of P. conostegium doubtful in Eurasia. Ecology. In Central and South America, it is recorded as a forest species mostly growing on epigeic, epilithic, epiphytic and epixylic habitats [3,7]. Distribution. Apart from Asia (Japan) [119], the species is reported by Wynns [7] as a mountain species from Central America and Northern South America (Bolivia, Ecuador, Guatelama, Mexico, Peru) and also present at high elevations in North America, the Dominican Republic, Northern Andes and Tierra del Fuego [3]. 8.
this species is also reported from North America (Greenland) [7]. 41. Plagiothecium undulatum (Hedw.) Schimp. 1851-a large white to pale green, du plant; leaves large, imbricate, crispate, slightly asymmetric, acute and serrulate at th apex; rhizoids occur at the leaf insertion; leaf cells with papillae on abixal surfaces These mentioned features led Ireland [224] to create for this species (as well as P draytonii (Sull.) E.B. Bartram) a separate genus-Buckiella Ireland. Plagiothecium un dulatum is similar to P. neckeroideum, but it differs by size and colour of the plant a well as longer and broader median leaf cells. Ecology. Plagiothecium undulatum wa recorded in epigeic, epilithic, and epiphytic habitats (e.g., [25,28,30,79,197]). Distri bution. Apart from Eurasia, this species is also reported from North America (Can ada, USA) [7,18,79].

9.
Plagiothecium curvifolium fo. julaceum Culm. & E. Bauer 1915-it is a forgotten taxon, which after being described in 1915 it was not later mentioned as a separate or at least as a synonym in any of the major bryological studies [28,30,32,[201][202][203][204][205]. Based on molecular analyses, Wynns [7] recognises it as separate, although the description of the gametophyte characteristics (ramis subjulaceis, foliis imbricatis saepe subhomomallis) indicates features similar to P. cavifolium. At this stage, this taxon definitely requires further in-depth research and detailed analysis. Ecology and Distribution. Detailed ecological data and distribution for this taxon are not known exactly and require specific investigation. However, according to Wynns [7], the isotype is found on fir roots near the upper tree line (epiphytic); from Switzerland (Burgfeld in Beatenberg, Canton of Bern). 10. Plagiothecium decoratum J.T. Wynns 2015-the species established by Wynns [23] and described as a julaceous to subcomplanate, slender plant, with concave, ovate, more or less symmetrical, plicate leaves, with recurved margins and a curled, denticulate, hyaline and recurved leaf apex. Ecology. It is listed in epiphytic habitat [7]. Distribution. The species is described as endemic of Bhutan and Nepal [7], present in evergreen forests around 3000 m. According to Wynns et al. [23], this taxon should be searched for elsewhere, as it could reasonably be expected to occur in Sikkim (Northeastern India)-which borders Bhutan in the east and Nepal in the west-and in Yunnan (China). 11. Plagiothecium denticulatum (Hedw.) Schimp 1851-a fairly large plant, green to yellowish green, often glossy ( Figure 2); the stem prostrate to ascending, densely foliate; concave, ovate to lanceolate, asymmetrical, acute to acuminate leaves, with recurved margins (Figure 3), an often denticulate apex, and loose cell areolation ( Figure 4); median cells linear-rhomboidal; alar regions well-developed, broadly decurrent, composed of large, inflated, round hyaline cells; sporophytes with inclined capsules. These features allow this species to be distinguished from others. Ecology. It is considered as a circumboreal species, where it is recorded in epigeic, epilithic, epixylic and epiphytic habitats; more abundant in alpine areas (e.g., [28][29][30]34,79,194,198,199,206]). Distribution. Apart from Eurasia, it is cited in the literature from North Africa, but considered doubtful by Ros et al. [12]; also the occurrence of this species in sub-Saharan Africa is considered doubtful by O'Shea [10]. It is also reported from North America (Canada, Greenland, USA) [7,18,25,79,207]. 12. Plagiothecium denticulatum var. affine Warnst. 1906-a species with small, flat, asymmetric leaves and well-developed alar regions [7]. Warnstorf [208] stated that P. denticulatum var. affine has a delicate form and resembles P. laetum, and that it could be an intermediate form between these species. Intermediate features of this taxon include its delicate structure as well as straight and erect capsules (which is characteristic for all species closely related to P. laetum). Wynns [7] supports this opinion that it may be a hybrid between these two species. Ecology. Warnstorf [208] did not provide details about the ecological preferences of this taxon, however, he indicated that it is a delicate form growing in flat turfs. Distribution. So far, it is reported only from Germany (Bärwalde, between Vietnitz and Nordhausen, Königsberg) [7,208]. 13. Plagiothecium denticulatum var. obtusifolium (Turner) Moore 1873-a small, subjulaceous, glossy, soft plant; leaves round, ovate or elliptical, with an obtuse apex, recurved margins and well-developed alar regions. These features distinguish this taxon from other species. Plagiothecium denticulatum var. obtusifolium was treated by some scientists as a synonym of P. denticulatum (e.g., [18,159]) or considered to be an ecotypic variety, but DNA sequence analyses indicate that P. denticulatum var. obtusifolium and P. denticulatum are not the same taxon [7,23]. Ecology. Wynns [7] reported on that P. denticulatum var. obtusifolium is restricted to mountains and cliffs, where it is recorded in epigeic, epilithic and epiphytic habitats. Distribution. Outside Eurasia, the species reported from North America (Canada, USA) [7,79]. 14. Plagiothecium enerve (Broth.) Q. Zuo 2011-due to a rather unusual combination of features: small plant with not tumid branches, narrowly lanceolate leaves bordered by hyaline, elongate, thin-walled cells, with an extremely long, often brownish piliferous apex, this plant was first described as Struckia enervis [209]. However, DNA analysis [36] indicates that it belongs to the genus Plagiothecium. Ecology. This species is recorded in epilithic and epiphytic habitats [7]. According to these authors, P. fallax is similar to P. denticulatum sensu lato, but it can be distinguished from this species by very small alar decurrencies. Ireland [18] and Iwatsuki [29] treated this species as a synonym or variety of P. cavifolium. Ecology. This species is recorded in epigeic habitat. Distribution. Apart from Eurasia (Russian Federation and Japan), it is also reported from North America (USA) and considered to be a typical North Pacific element [7]. 17. Plagiothecium japonicum Sakurai 1949-described by Sakurai [211], but Iwatsuki [29] treated this species as a form of P. nemorale (P. nemorale fo. japonicum), later even as a synonym to this species [33]. Plagiothecium japonicum can be easily recognised by large, broadly ovate, often concave leaves with stiff, extended cells and, as indicated by Wynns [7], it should be treated as a separate species, despite the fact that morphologically and genetically it shows intermediate features between P. nemorale and P. cavifolium. Ecology. This species is recorded in epigeic and epilithic habitats. Distribution. Apart from Asia (Japan), it is also reported from North America (USA) and considered a North Pacific element as P. fallax [7]. 18. Plagiothecium laetum Schimp. 1851-small plant, pale green to yellowish green, glossy, in loose mats; leaves asymmetrical, narrowly ovate-lanceolate, and narrowlydecurrent, gradually acuminate at apex; median leaf cells linear-rhomboidal; capsules more or less erect. Narrow alar decurrencies and sporophytes with erect capsules easily distinguish this species from closely related ones such as P. curvifolium. A recent taxonomic study of P. laetum complex allowed description of a new species, i.e., P. rossicum [26]. Ecology. The species recorded in epigeic, epilithic, epixylic, and epiphytic habitats (e.g., [25,28,30,66,79,171,172,195,212] [208] states that this taxon differs from P. laetum by longer, more lanceolate leaves, as well as narrow and long cells. Jedlička [202,203] characterised P. laetum var. tenellum as having small, narrow leaves, very short costae and narrow cells, often with propagules, and small erect capsules. Ecology and Distribution. Warnstorf [208] did not provide data about the ecological preferences of this taxon, therefore detailed ecological data and distribution for this taxon are not known exactly and require specific investigation. However, he indicated that var. tenellum plants are from all locations listed to P. laetum, which include Germany (Crossen, Lübeck, Hamburg and Altmark). The ecological data reported for these locations were: very rare in pine and deciduous forests at the bottom of trees or on forest floor, sometimes also in moors on the edge of old peat holes; often in crevices of lower and higher mountains. 20. Plagiothecium latebricola Wilson ex Schimp. 1851-a small, slender, bright green or yellowish green, glossy plant; leaves erect-spreading, symmetrical, narrowly ovatelanceolate, sometimes complanate, long acuminate, at times shrunken when dry, a margin narrowly recurved, entire or denticulate near the apex; costae very short; median leaf cells linear-rhomboidal; alar regions narrowly decurrent; fusiforme gemmae often present as well as rhizoids at the apex; capsules erect. Because of its small size, colour, short costae, narrow decurrent alar regions and erect capsules, this species can be confused with P. laetum, but even leaf symmetry in the latter helps to distinguish these two species. Ecology. Plagiothecium latebricola is found in swamps, fens, marshes and recorded in epigeic, epilithic, epixylic and epiphytic habitats (e.g., [21,25,28,30,79,101,199,206,213]). Distribution. This species has a circumboreal distribution [7], typically found in lowland, shaded locations [21,25]. Apart from Eurasia, it is also reported from North America (Canada, USA) [18,79]. 21. Plagiothecium longisetum Lindb. 1872-species described by Lindberg [214], synonymized with P. nemorale by Iwatsuki [29] and treated as such for about 50 years [33], recently resurrected and considered as a separate species [27]. A robust, green to yellow green, plant without metallic lustre ( Figure 2); leaves asymmetric to strongly asymmetric, shrunken when dry, ovate to lanceolate; a straight, not denticulate, acute to acuminate apex; leaf cells in regular rows, long and wide, areolation loose; long and burly costae; and very long seta (to 5.5 cm). This feature combination allows us to easily distinguish this species from P. nemorale and other closely related species. Ecology. It is recorded in epigeic, epilithic, epixylic and epiphytic habitats [27]. Distribution. Apart from Eurasia, this species is also reported from North America (Canada, USA) [191]. 22. Plagiothecium neckeroideum Schimp. 1851-a robust, green to yellowish green, strongly complanate plant; leaves domorphic, triangular, asymmetrical, ovate, undulate, con-cave, and a serrulate apex; median leaf cells linear-rhomboidal, narrow; alar decurrencies hyaline, thin-walled and well-developed. Apex cells are nematogenous, leaves often with differentiated apical cells, often seen as a longitudinal brown stripe at the leaf apex. Ecology. It is noted in epigeic, epilithic, epixylic and epiphytic habitats (e.g., [29,34,66,79]). Distribution. Wynns [7] reported P. neckeroideum from East and Southeastern Asia and in the Himalayas, from Europe (in the Alps). Deng-ke and Ireland [79] also gives this species from North America. 23. Plagiothecium neckeroideum fo. exile J.T. Wynns 2015-a taxon described by Wynns [23] as a small plant, with slender stems and branches, reddish stems; concave, not undulate, acuminate leaves; leaf cell areolation composed of short and narrow cells; decurrencies with enlarged, hyaline alar cells. Ecology. Reported from Quercus semecarpifolia forest, on tree trunk (epiphytic) [23]. Distribution. So far, the taxon is known only from Nepal. 24. Plagiothecium neckeroideum var. javense M. Fleisch. 1920-Fleischer [215] describes this taxon as a large, light green plant, with pale, symmetric, concave, undulate, long acuminate leaves, a denticulate apex, short costae, thin-walled leaf cells, enlarged in the basal area, with a vertical stripe of nematogenous cells at the apex. Ecology. This taxon was recorded in epigeic and epilithic habitats. Distribution. Apart from Southeast Asia (Indonesia, Philippines), it is also reported from Papua New Guinea [216] and from East Africa (Ethiopia) [10]. 25. Plagiothecium neckeroideum var. myurum Molendo 1875-smaller than P. neckeroideum var. javense, other features that make it different from closely related species are that it is a julaceous plant, with strongly concave, not undulate leaves [7,217]. Ecology. It is a montane taxon, recorded in epigeic, epilithic, epixylic and epiphytic habitats [7].  [7], who describes it as a small, pale green, crispate when dry, with flat or undulate, very concave, cordate, short, broad and very asymmetrical leaves, with an acuminate, acute and denticulate apex, often with rhizoids; leaf cells narrow; this form is similar to P. subglaucum, but in P. neckeroideum fo. parvum the leaves are broader. Ecology. This taxon was recorded in epigeic habitats [7]. Distribution. So far, it is known only from Taiwan (East Asia). 28. Plagiothecium nemorale (Mitt.) A. Jaeger 1878-this species has been too widely described in the last few decades, a taxonomic review of P. nemorale sensu lato indicates that it is actually three separate species: P. nemorale sensu stricto, P. longisetum and P. angusticellum [27]. A medium to large, green to dark green plant, shrunken when dry ( Figure 2) and without metallic luster; leaves ovate, symmetric ( Figure 3); acute to acuminate, straight; a denticulate apex; leaf cells short and wide, loose areolation, symmetric, in regular rows (Figure 4). This feature combination makes it very easy to distinguish this species from other closely related species. Ecology. The species is recorded in epigeic, epilithic, epixylic, and epiphytic habitats (e.g., [25,[27][28][29]34,66,79,141,194,199,200,218]). Distribution. The species is quite common in Eurasia [27]. It is also reported by Ros et al. [12,47] from North Africa (Algeria, Tunisia) and reported from North America (Canada, USA) [27,191]. 29. Plagiothecium noricum Molendo ex Limpr. 1897-flaccid, not undulate, very concave leaves, with expended cell aerolation, denticulate; rhizoids at the apex. Ecology. Wynns [7] describes P. noricum as a still little-known Alpine species, where are listed from epigeic habitat. Distribution. Reported from the Southern part of Central Europe (Austria) and Southeast Asia (Myanmar). 30. Plagiothecium obtusissimum Broth. 1921-a yellowish green to pale green, glossy plant with metallic luster; leaves ovate, rounded-obtuse at the apex, asymmetrical, slightly concave, the margin often erect at one side; leaves with suddenly differentiated alars, composed by hyaline inflated cells; median cells linear-flexuose, very narrow and long, thin-walled. Iwatsuki [29] considered P. obtusissimum to be closely related to P. euryphyllum, due to similar alar cells, leaf cells and setae. Additionally, it is easily distinguished from this species by the plant size and shape of leaf apex. DNA analysis confirms the observations about the close relationship between these species [36]. Ecology. Plagiothecium obtusissimum is recorded in epigeic, epilithic, epixylic and epiphytic habitats (e.g., [29,34]). Distribution. Noguchi [34] considered this species to be endemic to Japan; however, it is later reported from Russia (e.g., [32]). 31. Plagiothecium piliferum (Sw.) Schimp. 1851-a small, slender, pale green and glossy plant; leaves ovate, deeply concave, almost symmetrical, abruptly contracted to a piliferous, sometimes flexuose apex, with recurved margins; median leaves linearrhomboidal, very narrow; alar regions narrowly decurrent. Ecology. Plagiothecium piliferum is recorded in epigeic, epilithic, and epixylic habitats (e.g., [25,28,30,79]). Distribution. Apart from Eurasia, this species is also reported from North America (Canada, Greenland, USA) in low to moderate elevations [7,21,30,79]. 32. Plagiothecium platyphyllum Mönk. 1927-a medium-sized to robust, green, glossy plant; leaves ovate-lanceolate, asymmetrical, complanate, facid, undulate; median leafcells linear-romboidal, apical cells often bearing rhizoids; alar cells hyaline to pale green. According to Ireland [18], it is an autopolyploid of P. denticulatum, while a DNA study [219] suggested that P. denticulatum var. obtusifolium is a haploid of P. platyphyllum. Ecology. Plagiothecium platyphyllum is recorded in epigeic, epilithic, epixylic, and epiphytic habitats (e.g., [25,28,30,79,128,194]). Distribution. Apart from Eurasia, it is also reported from North America (Canada, USA) [7,23]. 33. Plagiothecium rhizophyllum Sakuri 1932-described by Sakurai [220] as a small species, with not undulate, loose cell areolation and rhizoids at the apex. Iwatsuki [29,221] and researchers after him (e.g., [33]) consider it as a synonym of P. nemorale, but Wynns [7] treats P. rhizophyllum as a separate species. Ecology. It is recorded in epigeic habitat [7]. Distribution. So far, the species reported only from East Asia (China). 34. Plagiothecium rossicum Ignatov & Ignatova 2019-described on the basis of DNA analyses of the P. laetum complex by Ignatova et al. [26]. A small plant with distinctly complanate foliage; leaves asymmetrical, ovate-lanceolate, a narrowly acute to short acuminate apex, margins flat, entire or minutely denticulate at the apex; leaf cells long and very narrow; straight and erect capsules. In terms of many features, this species is similar to P. laetum, but a flat margin and strongly asymmetric leaves are very useful in distinguishing P. rossicum from this species. Many features allow this species to be distinguished also from P. svalbardense, including for example: A flat leaf margin, narrow cells, narrowly acute to short acuminate apex which characterise P. rossicum. Ecology. So far, this species has been recorded in epigeic, epilithic, epixylic and epiphytic habitats [26]. Distribution. The species in common in boreal and hemiboreal forests of Russia, one position is also given from Poland [26]. 35. Plagiothecium ruthei Limpr. 1897-a medium-sized to large plant; leaves strongly complanate on the stem, transversely undulate when moist, sometimes shrunken when dry, flaccid, acuminate, strongly asymmetrical, on side almost straight, leaves with narrowly recurved margins and well-developed alar regions. These features distinguish this species from other closely related species. It is recognised as a separate species throughout Eurasia, despite the fact that DNA data places this plant closer to P. denticulatum, even closer than P. denticulatum var. obtusifolium [7,23]. Ecology. Plagiothecium ruthei is typical of wetlands species, recorded in epigeic, epixylic, and epiphytic habitats (e.g., [25,30,194]). Distribution. Apart from Eurasia, this species is also reported from North Africa but considered doubtful by Ros et al. [12]; and from North America (Canada, USA) [7]. 36. Plagiothecium ruthei var. rupincola Limp. 1897-Limpricht [222] described this taxon as similar to P. ruthei due to the size and cell areolation, but different due to closer foliage; symmetrical leaves, lacking recurved margin. Ecology. Limpricht [222] described it as an epilithic, alpine taxon. Distribution. According to the protologue and Wynns [7], this taxon is reported from Central, Northern and Western Europe (Asutria, Czech Republic, France, Germany, Norway and Sweden). 37. Plagiothecium subglaucum Thwaites & Mitt. 1873-Mitten [223] described this species as a plant with ovate, flat leaves, with an acute to acuminate apex. Plagiothecium subglaucum is similar to and can be confused with P. neckeroideum. Both species require further in-depth research [7]. Ecology. It is recorded in epigeic and epiphytic habitats. Distribution. So far, known only from Sri Lanka (South Asia) and Myanmar (Southeast Asia). 38. Plagiothecium succulentum (Wilson) Lindb. 1865-a robust, yellowish green to golden green, very glossy plant, leaves symmetric, lanceolate, not shrunken when dry, with an entire apex; median leaf cells very long. Plagiothecium succulentum differs from P. nemorale by lanceolate leaves, longer cells and a smooth apex; and from P. longisetum by lanceolate, symmetrical leaves; from other closely related species (e.g., P. angusticellum), it is very easy to distinguish, for example, by leaf symmetries and loose cells areolation [27]. Wynns [7] considered P. succulentum as problematic and described this species as polyphyletic or intermediate between P. nemorale and P. cavifolium. Plagiothecium succulentum in some countries is indicated as doubtful (Table 2, Figure S4). In our opinion, the relationship between these above-mentioned species requires a detailed analysis. Ecology. Plagiothecium succulentum is recorded in epigeic, epilithic, and epiphytic habitats (e.g., [25,28,30,62,79,194]). Distribution.
Apart from Eurasia, this species is also reported from North Africa (a single record from Tunisia) [47]; and from North America (Canada, USA) [191]. 39. Plagiothecium succulentum fo. propaguliferum E. Bauer 1902-a very dark, small plant, with shrunken leaves when dry. These are the features that distinguish this form from P. succulentum. Wynns [7] commented that this taxon can be frequently found in herbaria under the name P. succulentum. Ecology. This taxon is recorded in epilithic and epiphytic habitats [7]. Distribution. Currently, P. succulentum fo. propaguliferum is recorded in Western, Northern and Western Europe [7], and from North America (Canada, USA) [7,191]. 40. Plagiothecium svalbardense Frisvoll 1996-a small, growing erect plant, crispy when dry; leaves small, weakly undulate, concave, symmetrical to slightly asymmetrical, short, ovate, gradually tapered to the apex; margins narrowly recurved, entire or minutely denticulate at the apex; a subpiliferous apex; capsules straight and erect. Plagiothecium svalbardense is different from P. laetum by leaf shape and apex shape, the described species is also similar to P. piliferum due to its apex, but the latter has narrower leaf cells and a longer apex. The shape and arrangement of capsules is similar to P. laetum and P. berggrenianum but different from P. curvifolium. Ecology. Wynns [7] described P. svalbardense as an arctic species, where it is recorded in epilithic and epixylic habitats [26]. Distribution. Apart from Eurasia (Russia, Svalbard, Sweden), this species is also reported from North America (Greenland) [7]. 41. Plagiothecium undulatum (Hedw.) Schimp. 1851-a large white to pale green, dull plant; leaves large, imbricate, crispate, slightly asymmetric, acute and serrulate at the apex; rhizoids occur at the leaf insertion; leaf cells with papillae on abixal surfaces. These mentioned features led Ireland [224] to create for this species (as well as P. draytonii (Sull.) E.B. Bartram) a separate genus-Buckiella Ireland. Plagiothecium undulatum is similar to P. neckeroideum, but it differs by size and colour of the plant as well as longer and broader median leaf cells. Ecology. Plagiothecium undulatum was recorded in epigeic, epilithic, and epiphytic habitats (e.g., [25,28,30,79,197]). Distribution. Apart from Eurasia, this species is also reported from North America (Canada, USA) [7,18,79].

The Area of the Checklist
The present study covers Eurasia ( Figure 5), which comprises all of Europe and Asia, excluding Papua New Guinea. It also includes archipelagos of Northern Macaronesia, i.e., the Azores, Madeira and the Canary Islands but excl. the Cape Verde (Cabo Verde) archipelago. From the political and administrative perspective, Eurasia includes Europe and Asia (excl. the Sinai Peninsula, which is usually linked to Africa; being part of Egypt, though geographically it is located in Asia; anyhow Plagiothecium had never been reported from Sinai, see [225]). The area covered by the present checklist includes 107 countries of Europe and Asia, some well-separated islands, and archipelagos. According to the "World Geographical Scheme for Recording Plant Distributions" by Brummitt [226], it includes: A list of Eurasian countries and islands, along with their abbreviations used throughout the text, is presented in alphabetical order in Table 1. Abbreviations of countries and regions follow TDWG geographical codes [226] with some exceptions. The resulting distribution maps of the accepted taxa are provided as Supplementary Materials (Figures S1-S4).

Data Collection and Presentation
This study is based primarily on information garnered from the literature available to the authors up to the end of June 2020. More than two hundred publications have been consulted and analysed, including important regional and local floras, checklists, floristic reports, various studies and revisions of the family Plagiotheciaceae and the genus Plagiothecium in all European and Asian biogeographic regions or countries. Information on species names, occurrence, distribution, ecology and taxonomy have been compiled from these publications, which covered almost all the work published on Plagiothecium over a period of more than 70 years by several authors and written in different languages. In addition, the authors' own observations and online databases/checklists available on bryophytes of several regions/countries under consideration were used. All synonyms appearing in the collected literature were compiled for each taxon (see Appendix A).
The Eurasian regions are arranged alphabetically and a regional distribution is reported for each individual taxon. Data are presented in tabular format (Table 2) and distribution maps are provided for all reported taxa (see Figures S1-S4). Sections are arranged phylogenetically. Within sections, taxa are arranged alphabetically, first by the generic name, then by the specific epithet and infraspecific name-without any consideration of their phylogenetic relationships. Entries within each section follow the order: Taxon name, authority, publication data (pertaining to taxon first valid publication), then followed by distribution data (literature references for each country from which the taxon is reported), see Appendix B. Abbreviations of countries are printed in boldface. Author abbreviations follow [227].
The frequency of occurrence of Eurasian species has been presented on a five-point scale: very rare species (from 1% to 5% of all countries/islands in which a given taxon occurred), rare (6-10%), frequent (11-25%), very frequent (from 26-50%) and common (more than 50% of all countries/islands in which the taxon occurs).

Nomenclature and Taxonomy
The revised sectional classification of Plagiothecium established by Wynns [7] based on phylogenetic analysis of DNA sequence data, is adopted in the present study with a few exceptions. Consequently, the following taxa have been excluded from the present study: P. handelii Broth. and P. paleaceum (Mitt.) A. Jaeger, which after Wynns and Schröck [37] were sanctioned as Ortholimnobium handelii (Broth.) C. Schröck & J.T. Wynns and O. paleaceum (Mitt.) C. Schröck & J.T. Wynns, respectively. Whereas taxa considered in this study and not included in [7] are: P. rossicum Ignatov & Ignatova, which was established by Ignatova et al. [26] based on phylogenetic analysis of the P. laetum complex; as well as P. angusticellum G.J. Wolski & P. Nowicka-Krawczyk and P. longisetum Lindb., which were erected and resurrected, respectively, based on application of polyphasic approach to investigate the P. nemorale sensu lato [27].

Conspectus of Classification of Plagiothecium
All species are classified into the sections proposed by Wynns [7] and Wynns et al. [23], while for species that were described/accepted after 2015, we suggested that they can be provisionally classified under Plagiothecium sect. Leptophyllum (for P. rossicum) and Plagiothecium sect. Orthophyllum (for P. angusticellum and P. longisetum) and so this was adopted and included below (based on the literature cited in the two sections above).
All This section consists of one species: P. piliferum (Sw.) Schimp.
Note: All sections proposed by Wynns [7] and Wynns et al. [23] are represented in Eurasia except Plagiothecium sect. Ortholimnobium. It should be noted, however, that this section includes the two species P. handelii and P. paleaceum, which are not considered in the present study in light of the new evidence published by Wynns and Schröck [37], as explained above.

Data Availability Statement:
No new data were created or analysed in this study. Data sharing is not applicable to this article.

Acknowledgments:
We are grateful to Anders Hagborg from Field Museum of Natural History, Chicago, U.S.A, and Samantha D 'Acunto reference librarian from LuEsther T. Mertz Library, New York Botanical Garden, New York, U.S.A. We are also deeply indebted to all scholars who have shared their own research or contributed useful publications and/or comments and to those who have contributed directly or indirectly to this checklist. Thanks are also due to the academic editor and three anonymous reviewers for their constructive comments and suggestions which improved the manuscript a great deal.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
List of synonyms and names used in the studied area literature, with reference to the currently accepted names (in bold).