Mniaceae. Genus Mnium

1. Leaves not bordered or with indistinct borders below midleaf; leaf marginal teeth simple, rarely geminate, blunt 2. M. stellare

Mnium stellare is a common species almost throughout Russia, except the Siberian Arctic. Its unbordered leaves and glaucous color that turns to dark bluish with age make the plants conspicuous. Mnium stellare grows on soil banks, rock outcrops and occasionally tree (especially aspen) trunks. Mnium blyttii is widespread in the Siberian Arctic where it more or less replaces M. stellare, although the range of these species overlap, and they may grow together. Occasionally fertile plants of M. stellare and M. heterophyllum can be difficult to separate because both species have strongly modified, narrow upper leaves that may be bordered. However, in M. heterophyllum thevegetative leaves are bordered while the vegetative leaves of M. stellare are unbordered.

— Leaves distinctly bordered throughout; marginal teeth geminate, sharp or blunt 2

2. Plants turning blue after prolonged soaking in water 1. M. blyttii

Mnium blyttii is a northern species occurring in Arctic and permafrost regions. It is superficially similar to M. stellare, but differs in having a distinct leaf border. It grows most commonly among rocks and on various soil banks.

— Plants never blue colored 3

3. Median leaf cells 12–20 mm long, not collenchymatous 5. M. thomsonii

Mnium thomsonii has a scattered distribution throughout Russia; it is rare to totally absent in lowlands, but occurs in montane areas from the Arctic to the southern limits of Russia. It is similar to small phenotypes of M. lycopodioides, but differs in having smaller (12–17(–20) mm vs. 17–25(–30) mm long) leaf cells. The very regular cell areolation in M. thomsonii is a peculiar feature of the species: cells are small with only a slight difference in size from the juxtacostal cells to the marginal cells and conspicuously uniform in cell wall width.

— Median leaf cells 17–40 mm long, collenchymatous or not collenchymatous 4

4. Upper leaves length/width ratio 3–4(–6):1, costae subpercurrent 5

— Upper leaves length/width ratio 1.5–3(–4):1, costae percurrent 7

5. Uppermost leaves linear, other leaves ovate-lanceolate to ovate; costae smooth on dorsal surfaces or with a few small teeth 3. M. heterophyllum

This species is closely related to Mnium stellare and it is especially similar to female plants with well-developed perichaetial leaves. However, the stronger marginal leaf border of M. heterophyllum is helpful in its separating it from M. stellare. In Russia M. heterophyllum occurs in the southern part of the Russian Far East and the Caucasus. There is also one historical record of the species from an old growth, broad-leafed forest in Middle European Russia.

— Most leaves narrowly lanceolate, some lower leaves ovate-lanceolate; costae with large teeth on dorsal surfaces 6

6. Plants 1–5(–10) cm high; Europe 9. M. hornum

In Russia M. hornum has a very limited distribution in the western regions of the European part of the country; however, it is locally very common in deciduous and conifer forests, especially in Kaliningrad Province. Plants of M. hornum are usually much larger than plants of other species of the genus; they occasionally form small hummocks. The presence of leaves with large, sharp marginal teeth and costae that end well below the leaf apices are conspicuous features of the species.

— Plants 1–4 cm high; Far East 10. M. orientale

Mnium orientale has been considered an East Asian form of M. hornum,but Wyatt et al. (1997) on the basis of isosyme evidence found the two taxa were distinct at the species level. However, DNA sequence evidence indicates a very close relationship between these two entities (Ignatov et al., unpublished). Both species have leaf margins with large, sharp teeth and costae that are toothed on the dorsal surfaces. Plants of M. orientale are somewhat smaller than plants of M. hornum. In Russia M. orientale is an East Asian species found on the Russian Far East islands, and mainland south of the Amur River.

7(4). Leaf cells not collenchymatous 8

— Leaf cells distinctly collenchymatous 10

8. Plants synoicous; leaves not contorted when dry; costae smooth on dorsal surfaces 8. M. spinulosum

This mainly hemiboreal species is known in Russia from the Caucasus, South Siberia (especially in Abies forests), and the Far East. Mnium spinulosum is similar to M. spinosum in having crowded leaves, but differs in having a lighter color and synoicous sexual condition.

— Plants dioicous; leaves ± contorted when dry; cos­tae toothed or smooth on dorsal surfaces 9

9. Leaf cells oblong-hexagonal, (18–)25–44(–56) mm long, in distinct diagonal rows; upper leaves enlarged, forming a rosette 7. M. spinosum

This mainly boreal species can be recognized by its relatively large, dark plants. It has thin-walled leaf cells that are arranged in distinct rows, and a dioicous sexual condition. Mnium spinosum has leaf margins with sharp teeth that are often (but not invariably) red pigmented at the tips; this character is not unique for the species, but in combination with its broad, crow­ded leaves facilitates its recognition the field.

— Leaf cells rounded-hexagonal, 17–30(–40) mm long, not in rows; upper leaves not differentiated 4. M. lycopodioides

Mnium lycopodioides is one of the most variable members of this genus in Russia: plants large or small; leaf cells distinctly collenchymatous or fairly thin-walled thoughout; costae spiny or almost smooth on upper dorsal surfaces. Leaf cell size is also a variable feature of the species, and some forms closely approach M. thomsonii, which differs in having a very regular areolation and numerous cells smaller than 16 mm. Sterile plants of M. marginatum are somewhat similar to M. lycopodioides but M. lycopodioides has leaf margins with sharp teeth while M. marginatum has leaf margins with blunt teeth.

10(7). Plants synoicous; leaf cells with large corner thickenings and round lumina; leaf marginal teeth blunt 6. M. marginatum

The presence of synoicous plants greatly helps in recognizing this species. When sterile M. marginatum can sometimes be identified by its leaves that have blunt marginal teeth and conspicuously collenchymatous leaf cells. However, these distinguishing features are only moderately reliable, and often M. marginatum can be difficult to separate from M lycopodioides.

— Plants dioicous; leaf cells with small corner thickenings and polygonal lumina; leaf marginal teeth sharp 4. M. lycopodioides