Mniaceae. Genus Plagiomnium

1. Leaf margins toothed above mid-leaf, entire below; leaf apices acute, not cuspidate 2

— Leaf margins toothed from near base to apex; leaf apices rounded, occasionally widely acute, usually cuspidate 5

2. Leaf marginal teeth long, narrow, cilia-like 3

— Leaf marginal teeth short, ±broad 4

3. Plants pale-green to light-green, shiny; leaves scarcely contorted when dry, transparent, widest near mid-leaf; cell walls evenly thickened 4. P. drummondii

In European Russia this rare species is distributed mainly in the middle and southern taiga zones, where it is often known only from old records. In Asian Russia it is sporadic in the coniferous forests of Southern Siberia and the southern Far East, and extends northward to the middle course of the Ob River and southern Kamchatka. In conifer forestsP. drummondii grows on rotten wood, soil and tree bases. It is distinguished from other species of the genus with acute leaves by its light- or pale-green plants; non- or weakly contorted leaves; large leaf cells with evenly thickened cell walls; and polysetous (2–3 sporophytes) perichaetia.

— Plants green, slightly glossy; leaves strongly contorted when dry, not transparent, widest above mid-leaf; cell walls slightly collenchymatous 3. P. japonicum

This Japano-Himalayan species is very rare in Russia, known only from a few localities in Primorsky Territory. It is found in forests at low altitudes on wet soil in shady sites. Plagiomnium japonicum is similar to the more common P. cuspidatum and P. acutum in leaf shape, but differs in having larger plants; larger leaves and leaf cells; coarser leaf marginal teeth; costae subpercurrent; and polysetous (1–5 sporophytes) perichaetia.

4. Plants synoicous; median leaf cells 15–30(–45) mm in diameter, more or less homogeneous in size and shape; widespread throughout Russia 1. P. cuspidatum

This widespread circumholarctic species is common in the forest zone of European Russia, as well as the low forest zones in the Urals and Caucasus. In Asian Russia it is also common in Southern Siberia as well as the Southern Far East, and sporadically distributed northwards. It is rare in permafrost areas of Yakutia and absent in Taimyr and throughout Asian Arctic; in Chukotka P. cuspidatum was found once in a hot springs area. It grows on soil, rotten wood, tree bases and rocky substrates. Distinctive features of P. cuspidatum include its autoicous sexual condition; acute leaves with comparatively short marginal teeth above mid-leaf, and collenchymatous leaf cells, 15–30 mm in diameter.

— Plants dioicous; median leaf cells 8–20(–27) mm in diameter, heterogenous in size and shape (with admixture of small and large cells among medium-sized cells); East Asian species 2. P. acutum

In Russia this widespread Japano-Himalayan species is common in the southern Far East (Sakhalin, Kuril Islands, Primorsky and Khabarovsk Territories), northwards to Kamchatka and westwards to Lake Baikal and Transbaikalia. It is also known from the Altai Mts. in places with mild climates, and a few localities in Yakutia. It is found at low altitudes on forest floor litter, soil, rotten wood, bases of broadleafed trees and rocks. Plagiomnium acutum differs from P. cuspidatum in sexual condition (dioicous vs. autoicous); having smaller (8–15 vs. 15–30 mm) cells with less collenchymatous cell walls; and distribution (mainly temperate vs. boreal).

5(1). Leaves oblong-lingulate or lingulate, transversely undulate when moist; median leaf cells mostly less than 22 mm wide 6

— Leaves elliptic, smooth when moist; most median leaf cells wider than 22 mm (rarely 20 mm in P. rostratum, but then cells collenchimatous) 8

6. Leaves often unbordered at apices; marginal teeth small, blunt; juxtacostal cells larger than median leaf cells; subapical branching absent; plagiotropic stolons common; opercula rostrate 13. P. maximowiczii

In Russia this East-Asian species occurs primarily on the Kuril Islands and in Primorsky Territory. It is also known from scattered records westward: Khabarovsk Territory (upper course of Bureya River), Irkutsk Province (near Lake Baikal) and southern Yakutia (Tokinsky Stanovik Mt. Range). It is found at low altitudes (up to 600 m) growing in crevices and on cliff ledges, rocks, and rotten wood in oak and stone birch forests.

— Leaves bordered throughout; marginal teeth small or large, sharp; juxtacostal cells ± similar in size to median leaf cells; subapical branching common; plagiotropic stolons absent; opercula conic 7

7. Leaves oblong-lingulate; marginal teeth small, mainly 1-celled; leaf cells not porose 14. P. undulatum

In Russia this species occurs mainly in European Russia where it is rather frequent in the western lowland provinces, but abruptly declines northward and eastward. It grows on soils with basic and neutral bedrocks, usually in mesic habitats such as Alnus incana wooded ravines, along streams, and at forest edges. It is also rather common in the Caucasus where it grows in shady places in coastal broadleaved/boxwood forests and in the upper forest zone (to 1900 m a.s.l.) on soil, tree bases and rocks.

— Leaves lingulate or oblong-elliptic; marginal teeth large, often 2-celled; leaf cells usually porose 15. P. confertidens

In Russia this mainly boreal Asian species is widespread in the southern Far East and southern Siberia, extending northwards to Yamal, southern Taimyr and Central Yakutia. It is also known from scattered localities throughout the Ural Mts. and by single records from Tatarstan and Vologda Province. It is found in lowlands as well as montane forest zones (to 1800 m a.s.l. in Altai), in spruce, fir/spruce and larch forests, alder stands, birch/poplar woods, shrubs, and shaded cliff ledges on soil and rotten wood.

8(5). Leaf decurrencies short or absent 9

— Leaf decurrencies ± long 12

9. Plants dioicous 10

— Plants synoicous 11

10. Median leaf cells in oblique rows; leaf marginal teeth sharp; widespread 6. P. ellipticum

This species is sporadically distributed throughout Russia, but generally absent from most xeric areas. It grows in wet meadows, grassy swamps, along water courses/lakes, and other wet habitats.

— Median leaf cells in moderately apparent oblique rows; leaf marginal teeth blunt, not projecting; Russian Far East 12. P. vesicatum

This East-Asian species is known in Russia from Sakhalin, Kuril Islands and Primorsky Territory. It grows at low altitudes (up to 500 m), in wet, shady places on cliff ledges, on rocks along streams and brooks, near waterfalls, occasionally also on soil and rotten wood in wet forests.

11. Leaf marginal teeth sharp or blunt, directed upwards; median leaf cells 27–9225–52 mm; marginal leaf borders not colored; opercula conic 10. P. curvatulum

A northern species known in European Russia from Murmansk Province, Karelia and NE regions. In Asian Russia it is sporadically distributed in the Arctic regions and permafrost zones, Kamchatka and upper mountain zones in the Altai. It is found in Arctic/ mountain tundras, and boreal forests. It often grows in wet habitats on soil, peat, wet cliff ledges covered with fine earth/soil and occasionally on calcareous substrates.

— Leaf marginal teeth blunt, not projecting, occasionally absent; median leaf cells 27–35(–50) 20–25(–32) mm; marginal leaf borders often colored; opercula rostrate 11. P. rostratum

This species is sporadically distributed throughout Russia, but most common in the forest and forest-steppe zones mainly in areas with limestone outcrops or calcareous bedrocks. In Asian Russia it is most frequent in regions with mild climate, but curiously also present in permafrost areas. It grows on calcareous substrates, both rock surfaces and soil; it is occasionally found in spruce and mixed forests on soil and at tree bases.

12(8). Leaf apices often widely acute; median leaf cells ± isodiametric or shortly oblong; plants synoicous 13

— Leaf apices rounded or obtuse; median leaf cells elongate; plants dioicous 14

13. Leaves ± long-decurrent, 3–72–5 mm; leaf marginal teeth large, closely spaced, sharp; setae reddish-brown in lower 1/2–2/3 or throughout 9. P. medium

This species is widespread in the southern boreal to northern taiga zones of European Russia and in the forest zones of the Urals and Caucasus, becoming rarer in the north and in the steppe zone. It is also common in Southern Siberia and the Russian Far East (including Kamchatka), and sporadically distributed north­wards in taiga zone. It has been reported from Yakutia, the Arctic regions, and Chukotka, but all specimens from these regions previously named P. medium are P. curvatulum. It is found in conifer and mixed forests growing on litter, soil, rotten wood and rocks.

— Leaves short-decurrent, 2.5–60.7–3.5 mm; leaf marginal teeth small, ± moderately spaced, often obtuse; setae reddish-brown in lowermost part, yellow above 10. P. curvatulum

14. Leaf decurrencies long and narrow 5. P. affine

In Russia P. affine is widespread in the western areas of European Russia with a few records from the Urals, and the Caucasus. In lowland areas this species is common in spruce forests with Oxalis acetosella where it grows on litter and rotten wood; it sporadically occurs also in mixed/broadleaved woods. In the Caucasus it is found from sea level to timberline in various types of forests growing on soil, litter and rocks.

— Leaf decurrencies long or short, wide 14

15. Costae narrow, subpercurrent; leaf decurrencies moderately wide; median leaf cells 50–11020–55 mm; Far East 8. P. tezukae

This rare East Asian species is known in Russia from only a single record in the Primorsky Territory. It was collected at 900 m a.s.l. on soil along a stream.

— Costae broad, percurrent; leaf decurrencies wide; median leaf cells 40–75(–100)17–32(–37); Europe 7. P. elatum

This mainly European species is frequent in the northern and western regions of lowland European Russia; in Murmansk Province it occurs only in the southern areas. It is known from scattered records in the steppe zones, the Southern Urals and the Caucasus. All records of P. elatum from Asian Russia are based on erroneous identifications. There are specimens from Central Yakutia (“Lena Pillars”) that are morphologically similar to P. elatum but molecular-phylogenetic data indicates they are different taxa. Plagiomnium elatum is found mainly in areas with calcareous bedrocks, minerothrophic mires, wet forests (spruce, fir, alder, willow stands), along springs and in wet meadows.