Revision of Gymnomitriaceae (Marchantiophyta) in the Korean Peninsula

Abstract This paper provides a revision of Gymnomitrion and Marsupella in the Korean Peninsula based on a study of the collections housed in the herbaria of Jeonbuk National University (JNU) and the Botanical Garden-Institute in Vladivostok (VBGI). In total, 12 species were recorded (six in Gymnomitrion and seven in Marsupella), including four taxa whose identity was not confirmed with the available materials and suspected to be recorded wrongly. Each confirmed species is annotated by morphological descriptions based on available Korean material, data on ecology, distribution, specimens examined as well as illustrations.


Introduction
Gymnomitriaceae in the recent World Liverwort Checklist (Söderström et al. 2016) include Nardia, which is clearly distinct from other genera in the family due to the presence of underleaves. In fact, as demonstrated by Shaw et al. (2015), who were the first to transfer Nardia to Gymnomitriaceae, Nardia formed a basal clade to all other genera traditionally included in the family (Gymnomitrion, Marsupella, Prasanthus, Poeltia). It is therefore questionable whether Nardia should be treated as a member of Gymnomitriaceae or as a representative of its own family which has not yet been described. Following the second possibility (although refraining from any taxonomical formalities), we treat Gymnomitriaceae in the Korean Peninsula as housing only Gymnomitrion and Marsupella, putting aside Nardia. This point of view corroborates with that accepted in the worldwide compendium of Gymnomitriaceae by Váňa et al. (2010). Gymnomitriaceae house 85 taxa that occur in all continents. This family is most common and diverse in hemiarctic and alpine areas of Holarctic, with other noticeable diversity centers in the Sino-Himalayas and East Asia (Váňa et al. 2010). The recent advances in the Marsupella taxonomy in East Asia revealed that some additional taxa need to be recognized, including one taxon described from the Korean Peninsula that at this stage of knowledge is endemic to the Korean liverwort flora . Temperate East Asia (including Northeast China, Japan, Korean Peninsula and South Kurils) is characterized by the occurrence of a number of remarkable Marsupella species with 'scapanioid' appearance due to strong distichous leaf arrangement and conduplicate leaves with commonly unequal leaf lobes and distinct keel. These 'scapanioid' Marsupella are absent outside East Asia. Three species of this group occur in the Korean Peninsula.
The main goal of the present paper was to revise Gymnomitriaceae for the Korean Peninsula as it was never substantially revised. Moreover, despite the peculiarity of Gymnomitriaceae flora in East Asia, the family was never revised in eastern China, adjacent to the Korean Peninsula, while the revision for Japan (Kitagawa 1963) is somewhat outdated. Recent advances in knowledge about Gymnomitriaceae have been summarized in the Liverwort and Hornwort Flora of Korea (Choi et al. 2017). However, the long history of the preparation of this flora has resulted in the fact that it is already out of date, despite being recently published. The flora was based on the data available until the spring of 2015 and does not include novelties published in 2016 and later. Due to these circumstances, it does not include the recently described Marsupella koreana Bakalin et Fedosov, which is mistakenly named as M. pseudofunckii S.Hatt. in the flora. It also does not include a record of M. vermiformis (R.M.Schust.) Bakalin et Fedosov identified by molecular methods ). Gymnomitrion faurianum (Steph.) Horik. was completely confused with G. concinnatum (Lightf.) Corda, while G. parvitextum (Steph.) Mamontov, Konstant. et Potemkin was misidentified as G. commutatum (Limpr.) Schiffn. It is worth mentioning that G. commutatum was nevertheless found in South Korea in Jeju-do, from where it was first reported by Mamontov et al. (2018), whereas other reports of G. commutatum occurring in the literature are based entirely on G. parvitextum. Therefore, about half of the information provided by Choi et al. (2017) is incorrect. Moreover, the flora provides the only identification keys (unreliable due to the aforementioned omissions) and morphological descriptions but does not include illustrations and taxonomical comments describing differentiation features. These facts inspired us to compile a new version of the Gymnomitriaceae revision.
Ecology. Acidophilic hygro-to hydrophyte, occupying various habitats, from very wet (and even submerged) shaded cliffs near running water to moist mineral substrata in full sun. In moist and sunny habitats, robust phases are formed (then commonly acquiring deep rusty-brown pigmentation), where it is associated with Anastrophyllum assimile, Trilophozia quinquedentata, and Diplophyllum taxifolium. As an extreme variant, the species may be intermixed with Gymnomitrion faurianum. In wet and shady habitats, its common association is Cephalozia otaruensis * .
Distribution. Montane temperate Kurils-Japanese-Korean endemic species is known in northern and middle Japan (until Shikoku), South Korea and South Kurils (Iturup Island), likely more widely distributed, at least to Kamchatka Peninsula in * The nomenclature of taxa mentioned in the 'Ecology' section follows Söderström et al. (2016). in the north. In Korea, Jeju-do, Chungcheongbuk-do, Chungchengnam-do, Gyeongsangnam-do, Gangwon-do, Jeollabuk-do and Jeollanam-do (Choi et al. 2017).

Marsupella koreana
Distribution. Montane temperate species, known from only the southern part of the Korean Peninsula, but probably spreading northward. In Korea, Jeju-do, Chungchengnam-do, Gyeongsangnam-do, Jeollabuk-do and Jeollanam-do .  Comments. This distinctive species is one of the most common Korean Marsupella members, completely misidentified with a couple of other taxa (most frequently with M. yakushimensis, M. apertifolia, M. pseudofunckii and M. tubulosa). It belongs to the peculiar group of East Asian Marsupella taxa with 'scapanioid' appearance. Tentatively, we suggest that M. koreana occurs in mainland China (we were unable to check whether specimens identified as M. pseudofunckii from China (Gao and Wu 2007) are in fact M. koreana), as well as in Japan. We were unable to find this taxon in areas adjacent to the Korean Peninsula northward, in the Primorsky Territory of Russia. The main distinctions between the mentioned morphologically related taxa are presented in Table 1.  Description. Plants in loose mats, more or less rigid, erect to ascending, strongly distichous, brownish green to deep green, rarely brown with rusty, purplish or reddish tint near apices, strongly dilated to the perianth, 500.0-600.0 μm wide in normally developed part, with common depauperate plants (in shady and dry habitats) starting from 150.0 μm wide, near perianth much wider and reaching 1100.0 μm, 10.0-25.0 mm long. Rhizoids virtually absent with exception of leafless geotropic stolons, where common, obliquely spreading, separated or united into unclear fascicles. Stem branching ventral and (more commonly) lateral, as subfloral innovations and ventral leafless geotropic stolons that sometimes transform into normal branches; stem cross section transversely elliptic, differentiated into strata, outer layer with walls unequally thickened (but external wall thin), 12.0-25.0 μm along margin, trigones moderate, concave, scleroderm cells 10.0-13.0 μm in diameter, with strongly thickened walls and moderate in size, concave trigones, gradually transformed to inner tissue, cells in inner part 10.0-18.0 μm in diameter, walls thickened to almost thin, trigones moderate, concave. Leaves transversely inserted, not or barely sheathing stem in base, obliquely spreading, transversely oriented, evidently keeled-conduplicate with plane margin, keel straight to slightly arched or suddenly turned downward near the end, leaves contiguous to distant, rarely enclosed one to another, 220.0-430.0 μm long and 300.0-600.0 μm wide, obliquely transversely elliptic, divided by γ-shaped sinus descending to 1/4-1/3 of leaf length into two strongly unequal gibbous lobes with acute to obtuse or very rarely rounded apes. Cells in the midleaf 10.0-18.0 × 7.0-15.0 μm, walls thin, trigones moderate in size to large, triangle to convex, cuticle smooth; cells along margin 5.0-8.0(-10.0) μm, thin to (more commonly) thickened, trigones small to large, mostly concave, cuticle smooth; cells in lobe middle 7.0-15.0 × 6.0-12.0 μm, walls thin to thick, trigones moderate to large, triangle to convex. Dioicous. Androecia intercalary, spicate, with 2-3 pairs of bracts, sequential generations divided by 10 and more pairs of sterile leaves, 1-3-androus, stalk biseriate, 125.0-220.0 μm long, body nearly spherical, ca. 150.0 μm in diameter; bracts cupped to spoon-shaped, commonly with recurved margin. Perianth hidden within bracts or barely exerted, couple-shaped, ca. 150.0 μm long and 400.0 μm wide, perigynium well developed, 500.0-700.0 μm long, with two pairs of bracts; bracts sheathing perigynium and above incurved inward to the perianth if archegonia were fertilized, or obliquely spreading, if archegonia not fertilized.
Ecology. Acidophilic meso-to hygrophyte taxon. The species occupies dry to moist cliffs, rarely wet rocks as well as stones near streams, in open to (more commonly) partly shaded places. It grows together with various Scapania and Cephaloziella or Marsupella tubulosa in wetter habitats. In dry to mesic and shady conditions, it is associated with Tetralophozia filiformis, Bazzania ovifolia, Cylindrocolea recurvifolia, Diplophyllum taxifolium, D. albicans, and Syzygiella autumnalis.
Distribution. Temperate Montane East Asian species, aside from Korea known from China (Taiwan, Zheijiang (the report from the latter is based on Zhu et al. 1998)), Russian Far East (southern Sikhote-Alin only), and Japan. In Korea, Jeju-do, Chungchengnam-do, Gyeongbuk-do, Gyeongnam-do, Gangwon-do, and Jeollabukdo (Kim and Hwang 1991;Yamada and Choe 1997). This species is here newly recorded from Gyeongsangbuk-do Province. Comments. This is a distinctive species belonging to the group of Marsupella with conduplicate leaves. It may be mistaken for M. koreana (distinguishing characters given under that species) or (less probably) for M. yakushimensis. The latter and M. pseudofunckii share conduplicate leaves with not revolute margins, although being different in: 1) lobes are subequal in M. yakushimensis, but strongly unequal in M. pseudofunckii, 2) leaf margin of M. yakushimensis is commonly undulate and lobe ends turned away of the stem, whereas leaf margin in M. pseudofunckii is always plane and not reflexed away from the stem, 3) shoots of M. pseudofunckii are strongly dilated to the perianth, versus almost stable shoot width in M. yakushimensis, 4) the width of sterile shoots of M. pseudofunckii generally less than 0.6 mm, but in M. yakushimensis it commonly is more than 1.5 mm.  Description. Plants merely rigid, forming loose patches, deep green-brown, purple-brown, purple-green, or rarely greenish (actually plants extracted from the patch yellowish brownish in general, but with purple-rusty coloration in apices and upper parts of insolated leaves that gives expression of purple-brown color of patch), or yellowish greenish, pale brownish with purple tint in apical parts, rarer brownish greenish without purple or rusty pigmentation; 0.6-1.2 mm wide (the largest lax plants up to 1.3-1.5 mm) and 5.0-15.0 mm long. Rhizoids nearly absent or few, colorless. Stem brownish, not branched or branched as ventral leafless stolons (rarely becoming to normal branch) or more commonly as subfloral ventral or lateral innovations; transversely elliptic in cross section, 160.0-180.0 μm high and 200.0-250.0 μm wide (depauperate shoots omitted), differentiated into strata; hyaloderm with external wall thin, radial walls thin to unequally thickened (becoming thicker inward), inner wall thick, 15.0-20.0 μm along margin; scleroderm in (1-)2 rows of cells, cells thick-walled, but not so strongly as in M. apertifolia; inner cells thin to slightly thickened, trigones moderate in size, concave, 12.0-18.0 μm in diameter. Leaves contiguous to distant, sometimes 'enclosed' one to another, concave to almost flattened in upper half, transversely inserted, evidently or very loosely sheathing stem in the base, obliquely (rarely erect) spreading, subtransversely to obliquely oriented, suborbicular to widely ovate, margin flat to loosely recurved near base (especially in the leaves sheathing the stem), divided into two unequal (rarely subequal) lobes by widely V-to γ-shaped sinus descending to 1/7-1/5(1/4) of leaf length, lobe apex couple-shaped, obtuse to acute. Cells in the midleaf shortly oblong, 10.0-20.0(-25.0) × 8.0-16.0(-18.0) μm, walls thin to slightly thickened, trigones large to (rarer) moderate in size, convex to bulging, cuticle smooth; cells along leaf margin 6.0-12.0 μm, walls thin to slightly (to strongly and unequally) thickened in tangential walls, trigones moderate to large, concave to slightly convex, tangentially sometimes confluent; cells in lobe middle oblong, 15.0-22.0 × 8.0-13.0 μm, walls thin to slightly thickened, trigones moderate to large, convex to triangle. Oil-bodies in the midleaf cells 2(-3) per cell, biconcentric (at least (30-)70%), finely to normally papillose, spherical, ca. 5.0 μm in diameter to oblong, 7.5-12.5 × 5.0-7.5 μm. Dioicous. Androecia intercalary, with 1-3 or 3-5 pairs of bracts, different generations divided by Ecology. Acidophilic meso-to hygrophyte. The species occupies sandy soils and mineral substrates, over wet to moist, and sometimes mesic cliffs, being most common along streams near running water. In drier habitats, it is commonly associated with Odontoschisma pseudogrossiverrucosum, Cheilolejeunea obtusifolia, and rarely with Microlejeunea punctiformis, Cephaloziella spp., Gymnomitrion faurianum. In wetter habitats M. tubulosa sometimes grows intermixed with Solenostoma minutissimum, Lophocolea horikowana, Marsupella pseudofunckii, and M. koreana.

Marsupella tubulosa
Distribution. The distribution of the species is confined to insular and peninsular areas in Amphi-Pacific Boreal and Temperate Eastern Asia, while the records provided by Bakalin, (2010), Cherdantseva and Gambaryan (1986), Gambaryan (1992Gambaryan ( , 2001, Konstantinova et al. (2002) for the Russian Far East continental mainland are likely incorrect. This species is strikingly characterized by biconcentric oil bodies -an uncommon feature in Marsupella that has never been reported from continental Asia or from North America. This suggests the reports of the species in Schuster (1974) for North America and Schljakov (1981) for Russian Asia are incorrect. We suggest the 'true' M. tubulosa (the type is from Honshu) occurs only in Japan, Kurils, Kamchatka and the Korean Peninsula, as well as probably in China (Anhui, Taiwan), from where, unfortunately, oil bodies were not studied. All specimens collected in the continental mainland of the Russian Far East and checked alive had non-biconcentric oil bodies, whereas specimens from peninsular and insular parts of the Far East possess biconcentric oil bodies. The species was recorded for nearly all provinces of the Korean Peninsula (Jeju-do, Gyeongsangnam-do, Gyeongsangbuk-do, Chungcheongnam-do, Chungcheongbuk-do, Gyeonggi-do, Gangwondo, Pyeonganbuk-do, Hamgyeongnam-do, Hamgyeongbuk-do: Yamada and Choe 1997;Kim and Hwang 1991) and was confirmed for most of the provinces in the southern part of the peninsula. Comments. This species may be confused with at least three other species: Marsupella koreana, M. apertifolia, and M. emarginata, though the last is not known from the Korean Peninsula. The distinctions between the former two are mentioned under those species. The species differs from M. emarginata (with which M. tubulosa was probably confused in parts outside of oceanic/suboceanic Eastern Asia) in biconcentric oil bodies (versus oil bodies without the central eye) and commonly obliquely oriented leaves that are not closely sheathing the stem in the base (versus transversely oriented leaves with leaves closely sheathing the stem near base). The latter feature is surely quantitative and variation in leaf orientation occurs within the two species. Description. Plants strongly vermicular, forming loose patches, brown to blackish brown, without red or purple pigmentation, orbicular in cross section, 100.0-140.0 μm in diameter, 3.0-6.0 mm long, freely ventrally branched, from leafless brownish to whitish in color densely ventrally branched rhizome. Rhizoids virtually absent, to solitary, colorless, obliquely spreading, short (less than 100.0 μm long). Stem 100.0-170.0 μm in diameter, orbicular in cross section, outer layer cells with external wall thin to obscurely thickened, tangential walls subequally thickened, trigones small, concave, walls brown in color, 6.0-10.0 μm in diameter, inner cells with walls unequally thickened, walls colorless, trigones moderate in size, concave. Leaves appressed to the stem (commonly lacerate into two segments when try to detach), transversely inserted and oriented, not decurrent, widely triangular, 65.0-110.0 μm long and 90.0-175.0 μm wide, divided by V-shaped sinus descending to 2/5-1/2 of leaf length into two subequal triangle lobes with acute apices. Cells in the midleaf 5.0-10.0 × 5.0-8.0 μm, walls moderately thickened, trigones small, concave; cuticle smooth; oil-bodies 1-2 per cell, spherical, 2.0-3.0 μm in diameter. Dioicous. Pants suddenly dilated to the perianth, to form club-shaped structure, perianth completely hidden within bracts, nearly conical, 75.0-100.0 μm long and 200.0-230.0 μm wide, smooth, perigynium 120.0-150.0 μm long, with one pair of bracts; bracts nearly orbicular to orbicular-triangular in shape, ca. 250.0 × 250.0 μm, covering perianth and then occlude one with another.
Ecology. Acidophilic meso-xerophyte. In Korea, it occurs on dry well-exposed rocks in large block gravelly barrens in the crater rim of Halla Mt.  Comment. The very distinctive species, superficially quite similar to Gymnomitrion pacificum Grolle due to vermicular shoots and never spreading, but closely appressed leaves, forming in female branches, a club-like structure. It is clearly different from G. pacificum in having much smaller leaves, with normally developed cells along the margin and presence of distinct perianth. The species may be mistaken for dwarf forms of arcticalpine sub-circumpolar amphi-oceanic Marsupella boeckii (Austin) Lindb. ex Kaal. and  Type. Japan. Kagoshima Pref., Yakushima Island, Horikawa, 11895 (not seen). Description. Plants in loose patches, deep green-brown, yellow-brown, yellowish brownish, rarely with purple tint, (1.0)1.5-2.1 mm wide and 15.0-50.0 mm long, rigid. Rhizoids nearly absent to very sparse, colorless, obliquely spreading, however common in basal part of ventral branches and leafless stolons. Stem easily laterally and ventrally branched giving start to normal branches or geotropic leafless stolons; stem transversely elliptic in cross section 210.0-240.0 μm high and 250.0-320.0 μm wide, distinctly differentiated into strata, hyaloderm cell walls moderately thickened (but external wall thin), with small concave trigones, 17.0-25.0 μm along margin, scleroderm cells with very thick walls and visible median lamina, 12.0-17.0 μm in diameter, but with lumen disappearing or only 2.0-6.0 μm in diameter, inner cells with moderately thickened walls and moderate in size, concave trigones, 10.0-15.0 μm in diameter. Leaves strongly conduplicate and distichously arranged that gives 'scapanioid' appearance, contiguous to imbricate, as a rule enclosed one to another, obliquely spreading and transversely oriented, when flattened subquadrate, rectangular or obovate to suborbicular (mostly wider than long, but sometimes longer than wide), bistratose in lower 1/5-1/6 of the leaf length, 675.0-1250.0 μm long and 800.0-1500.0 μm wide, commonly dorsally secund, divided by γ-shaped sinus descending to 1/4-2/5 of leaf length into two equal to subequal lobes (either ventral or dorsal may be smaller), lobes gibbous, with obtuse to acute or rarely rounded apex. Cells in the midleaf subisodiametric to (mostly) oblong, 12.0-25.0 × 7.0-20.0 μm, strongly thick-walled, with moderate to small, concave trigones, cuticle smooth; cells along leaf margin 7.0-10.0 μm, thick-walled (but with much thinner external wall), with moderate in size, concave trigones; cells in lobe middle 10.0-17.0 × 8.0-15.0 μm, thick-walled, with small to moderate in size, concave trigones, cuticle smooth. Dioicous. Androecia intercalary, with 2-3 pairs of bracts, spoon-shaped, with revolute margin and commonly deflexed lobe ends. Perianth (only unfertilized were found) hidden within bracts, onion-shaped, perigynium the same length with perianth or slightly longer, with 2 pairs of bracts and 1-3 lateral and ventral subfloral innovations.
Ecology. Acidophilic hygro-to hydrophyte. The species occurs on wet cliffs at a distance from watercourses or on stones washed with sluggishly running water in partly shaded habitats in the middle elevation of mountains covered with evergreen to deciduous broadleaved forests. Commonly, the species forms pure patches or rarer, associated with Scapania undulata.
Comments. This large and beautiful species is a rarity within the Korean flora and is known only from a few localities. Unlike Japanese populations, the Korean populations acquire purple to red pigmentation as an exception. The main characteristic of the species includes nearly equal lobes that do not have recurved margins, but commonly undulate and/or turned antically. Another characteristic feature is the absence of a distinctly sheathing leaf base. Dwarf plants of M. yakushimensis may be mistaken for M. koreana, and the distinctions are given under the latter. This species is regularly observed with androecia and rarely with archegonia. Androecious and gynoecious plants were intermixed within two specimens; however, we were unable to observe fertilized (in at least two descendant generations) and fully developed perianth. Whether this is the norm or not is not clear.
Description. Plants worm-shaped to ribbon-like, with densely imbricate leaves or similar to Marsupella and with loosely spreading leaves, rigid to soft, whitish to brownish, brown, rusty, and blackish brown, without red or purple pigmentation. There are two kinds of phenotypes: Gymnomitrion in the old sense and the former genus Apomarsupella R.M. Schust. nested within Gymnomitrion (cf. Shaw et al. 2015). The first phenotype, with imbricate leaves and stems creeping to ascending, subclavate, from rhizomatous base, dorsiventrally compressed, commonly immersed to the substrate and incrusted by soil particles. The second phenotype comprises plants with spreading leaves with shoots dorsiventrally not compressed, as well as having a not evident rhizomatous base. In both 'phenotypes' rhizoids are common in the rhizomatous shoot base and geotropic stolons, but rare in leafy parts of the shoot, soft, colorless to grayish or rarely and solitarily deep purple. Stem not evidently different in strata, rather it is monomorphic (outer layer cells slightly larger), cells with unequally thickened walls, and well-developed trigones. Leaves lobed to unlobed or shallowly emarginate, with plane or narrowly recurved margin. Leaf cells pachydermous, commonly with the rim of discolored cells. Dioicous (taxa known in Korea). Androecia intercalary, (1-)2(-3)-androus, stalk biseriate. Perigynium and perianth virtually absent or strongly reduced. Elaters bispiral.
Comment. This treatment follows the recent emendations e.g. transfer to Gymnomitrion of the taxa Marsupella commutata (Limpr.) Bernet and Apomarsupella revoluta (cf. Shaw et al. 2015). These transfers made Marsupella more monomorphic in the series of features, but resulted in greater polymorphism in vegetative characters of Gymnomitrion, which now includes many taxa of 'marsupelloid' habit, although in reproductive characters is characterized by an absence, or strong reduction of, perianth and perigynium. Comment. The species is very similar to G. parvitextum, and the distinctions between the two taxa are described below.
Perianth and perigynium absent, bracts similar to leaves, but larger, more deeply divided (up 15-1/4 of the length), with somewhat diverging and spreading, rarely erose-dentate lobes.
Ecology. Acidophilic mesophyte. The species occupies mesic to moist (rarely wet) cliffs in open places, and rarely occurs in partly shaded habitats, producing thinner forms with not densely appressed leaves. In drier habitats, it is commonly associated with Gymnomitrion parvitextum, dwarf forms of Sphenolobus saxicola, saxicolous unclear fascicles. Stem freely ventrally branched in the base of leaved part and variously and feely branched in rhizome; slightly transversely elliptic in cross section, ca. 100 μm high and 120 μm wide, without evident differentiation into layers, outer cells 12-15 μm along margin, with thick walls (become noticeable thinner to thin in ventral epidermis), with moderate in size, concave trigones, inner cells 7-13 μm in diameter, irregular in shape, with thickened walls and moderate to small in size concave trigones. Leaves densely imbricate, transversely inserted, not sheathing at base, transversely oriented, widely ovate-lingulate to widely triangular, cupped to spoon shaped, lacerate when flattened, 220-330 μm long and 270-500 μm wide, with rounded to emarginate apex and entire to crenulate margin. Cells in the midleaf subisodiametric (mostly quadrate) to oblong (mostly rectangular), 12-25 × 12-20(23) μm, walls thickened, trigones small to moderate in size, concave, cuticle smooth; cells along leaf margin 5-15 μm, mostly elongate perpendicularly to the margin, walls unequally thickened, trigones small to moderate, concave. Dioicous. Androecia intercalary, with cupped and loosely imbricate bracts. Perianth and perigynium absent, perichaetial area of the shoot distinctly wider than below.
Ecology. Acidophilic meso-xerophyte. The species occupies dry to mesic fine soils in well-exposed places at higher altitudes. It is commonly associated with dwarf xeric forms of Marsupella tubulosa, Gymnomitrion parvitextum, and Cephaloziella divaricata.
Distribution. Temperate Montane Eastern Asian endemic species with distribution confined to Japanese Honshu and Kushu as well as the southern tip of Korea (Jeju Island).
Comment. Due to the presence of entire to emarginate leaves, Gymnomitrion noguchianum is unlikely to be mistaken for other members of this genus. However, it may be mistaken for Cryptocoleopsis imbricata, but that species is not yet known from the Korean Peninsula, though it should be expected to occur there. Both taxa are similar in prostrate growth, occurring in well-exposed places, and entire imbricate leaves. However, two taxa may be easily separated by 1) leaf cell walls are thickened and smaller in G. noguchianum, versus larger (more than 20 μm wide) and thin-walled in Cryptocoleopsis imbricata, 2) total absence of brown pigmentation in G. noguchianum versus almost constant presence in Cryptocoleopsis; and 3) presence of calyptral perigynium in Cryptocoleopsis -the structure does not occur in Gymnomitrion. Comment. Easily recognizable species in most cases, although confused several times with dwarf plants of Marsupella tubulosa from which it differs in commonly present subquadrate to quadrate trigones in leaf lobe cells, almost constantly recurved leaf margin, and in the absence of perianth. Gymnomitrion parvitextum is morphologically similar to G. alpinum. The two taxa are distinct in the presence of subquadrate trigones in leaf lobe cells in G. parvitextum (absent in G. alpinum), recurved margin (versus margin plane), shortly or barely decurrent leaf base (versus long decurent leaf base on both sides in G. alpinum) and inability to develop red or purple pigmentation (versus such coloration common in G. alpinum).
The bigger problem is the delimitation of Gymnomitrion parvitextum from morphologically similar and 'semicryptic' (Mamontov et al. 2018) G. commutatum (Limpr.) Schiffn., disjunct, mostly northern hemiarctic species. Gymnomitrion parvitextum differs from G. commutatum in the following features: 1) pale yellowish to brownish and even yellowish-greenish coloration, but never blackish brown and rusty-brown coloration so common in G. commutatum, 2) leaves semi-distichously arranged versus leaves subimbricate, 3) leaves in the vast majority of cases with widely recurved margins, versus margin recurved narrowly and commonly near the leaf base only.
Remarks. The taxon was recorded for North Korea (Gangwon-do, Hamgyeongnamdo, cf. Kim and Hwang 1991), but was not found in specimens examined. We suggest that all reports regarding the occurrence of this species in Korea and adjacent regions are erroneous and probably belong to M. apertifolia to which the species are similar in coloration and rounded leaf lobes, but differ in leaf margin characteristics, leaf sinus deepness, and stem cross section features, all of which are included in the identification key. In general, M. sphacelata is a rarity in East Asia and probably absent southward of 53°N (southern part of Kamchatka Peninsula). The records for Honshu Island in Japan are doubtful due to reported red pigmentation (Kitagawa 1963) which is not known for this species from other parts of its range (cf. Schuster 1974).
Gymnomitrion corallioides Nees, Naturgesch. Eur. Leberm. 1: 118, 1833. Remarks. This taxon was recorded for Jeju-do (Yamada and Choe 1997) and identified many times in unpublished collections; however, all so-named specimens checked by us are characterized by acute lobes with persistent cells with moderate size trigones along the leaf margin. These features are characteristics of Gymnommitrion faurianum. Everywhere southward of 48°N. in East Asia G. faurianum was commonly misidentified as G. corallioides (Bakalin 2016 Remarks. This species was recorded on the Korean Peninsula from North Korea (Yanggang-do, cf. Kim and Hwang 1991) but the vouchers were unavailable for the present study. The species resembles G. parvitextum, and the main differences are included in the key. The occurrence of G. revolutum in the Korean Peninsula is somewhat probable. In adjacent countries, it was recorded in China, in areas remotely located from the Korean Peninsula (Xizang, Taiwan) and in Japan.