Polytrichaceae. Genus Polytrichum

1. Leaf margins serrate to indistinctly serrulate, flat to slightly incurved; laminal cells isodiametric, square to rounded, chlorophyllose — 2
—Leaf margins entire, broadly incurved and covering all or most lamellae; laminal cells transversely ovate to transversely elongate, hyaline — 8

2. Terminal lamellae cells ovate in transverse section, similar to cells below — 3
—Terminal lamellae cells broad, with flat to retuse distal walls, differ from cells below in transverse sectionl — 5

3. Leaf sheath cells short rectangular, 3-5:1; limb laminae with 4-9 cell rows (10-20 cell rows in var. anomalum); cells 15-18 µm wide; spores 16-28 µm — 1. P. longisetum

A widespread boreal species especially common on bare soil in forests (under upturned roots, on various soil banks, and stumps) and in swamps. It differs from similar species, especially P. densifolium, in having larger laminal cells and shorter sheath cells.

—Leaf sheath cells elongate-rectangular, 5-10:1; limb lamina with 2-4 cell rows; cells 10-14 µm wide; spores 10-15 µm — 4

4. Sheath cells 5-7:1; lamellae margins in profile weakly crenulate; capsules and apophyses separated by shallow constrictions; peristome teeth 220-305 µm long — 2. P. densifolium

This species is widespread in Russia in southern part of the boreal zone and the hemiboreal zone; it was previously reported as P. formosum. Both species grow in similar habitats, most commonly under upturned roots of fallen trees in mixed and conifer forests.

—Sheath cells 6-10:1; upper edge of lamellae margins in profile entire; capsules and apophyses separated by more or less abrupt, deep constrictions; peristome teeth 170-185(-250) µm long — 3. P. formosum

Although previously considered a widespread species in Russia, P. formosum occurs only in the Black Sea coastal area of the Caucasus, and in a few localities in the western part of the European Russia. In the Caucasus it grows on soil in beech forests.

5. Terminal lamellae cells in transverse section with flat distal walls — 6
—Terminal lamellae cells in transverse section with retuse distal walls — 7

6. Leaf sheath cells rectangular, 6-8:1; apophyses indistinctly delimited; mainly on soil and rotten wood in boreal and hemiboreal forests — 4. P. pallidisetum
In habit and habitat this species is similar to P. longisetum and P. densifolium; it can be separated from them by the presence of flat terminal lamellae cells in leaf transverse sections. Polytrichum swartzii also has flat terminal lamellae cells, but it occurs in different habitats and its morphology is more similar to P. commune. Color of plants of P. swartzii are not as dark green as those of P. pallidisetum and they also have a somewhat glaucous tint; in addtion, its leaves are abruptly constricted at the sheath/limb junction and its sporophytes are extremely rare (occasionally present in P. pallidisetum). Furthermore, the hypophysis in P. swartzii is sharply delimited from urn by deep groove (weakly delimited in P. pallidisetum).
—Leaf sheath cells linear, 10-15:1; apophyses sharply delimited; mire species — 6. P. swartzii

This species has a scattered distribution throughout the boreal zone in Russia. Although considered by some authors as an extreme variation of P. commune, it is treated here as a distinct species even though the shape of the terminal lamellae cells is almost the only distinction between it and P. commune. Moreover, P. swartzii has a peculiar ecology and is found only in mire environments.

7. Leaf margins serrate to serrulate; leaves not fragile; plants (2-)5-10(-70) cm high — 5. P. commune

A common, often dominating moss nearly throughout the territory of Russia. Many times the species can be recognized by its large plant size. Poorly developed plants can be recognized by the combination of retuse terminal lamellae cells and serrate leaf margins.

—Leaf margins subentire to weakly serrulate; leaves fragile; plants 2-12(-15) cm high — 7. P. jensenii

Northern species that also occurs in volcanic habitats in the Russian North Pacific. It c an be recognized by the combination of retuse terminal lamellae cells and subentire leaf margins.

8(1). —Leaves with long, hyaline hair-points; sheathing base short, 1.0-1.5 as long as wide — 11. P. piliferum

Common acidophilous species, found nearly throughout the territory of Russia. It can be recognized by the presence of leaves with incurved, hyaline margins and long, hyaline hair points.

—Leaves with short to moderately long, brown or brown at base hair-points; sheathing base elongate, 1.5-2.5 as long as wide — 9

9. Hair-point hyaline distally, brown proximally; terminal lamellae cells ovate, distal walls evenly thickened; spores 15-18 µm; northern species — 10.P. hyperboreum

Northern species that extends somewhat southwards. Its bicolored hair-point is a critical feature of the species, but other characters must be checked because in P. juniperinum plants sometimes have extensively developed hyaline hair-points.

—Hair-point brown nearly throughout; terminal lamellae cells pyriform or flask-shaped, distal walls strongly thickened (usually filling entire 'beak'); spores 6-10(-12) µm; widespread species — 10

10.Leaves 5-8 mm long, more or less appressed to spreading when dry; capsules elongate; stem naked to moderately tomentose — 8. P. juniperinum

Common species throughout Russia, it can sometimes be difficult to separate from P. strictum and, more rarely, from P. hyperboreum (see key for their distinctions).

—Leaves 3-5 mm long, evenly appressed when dry; capsules ±cubic; stem tomentose — 9. P. strictum

This species usually grows on hummocks in Sphagnum bogs; this habitat and the presence of plants with even, regularly arranged, small leaves is usually sufficient for its identification.