Polytrichaceae. Genus Polytrichastrum

1. Margin of lamellae papillose; margin of leaf blade sharply serrate or, rarely, subentire; capsule terete — 2
—Margins of lamellae smooth; margin of leaf blade entire or subentire; capsule angular or terete — 5

2. Upper cell of lamellae with a crown of papillae crowded at the cell top; one collection from Chukotka — 5. P. papillatum

The species is known from a single collection from Chukotka; its distribution is poorly known, and the identity with Himalayan plants from the locus classicus needs confirmation. The main character of the species is the crown of papillae on the upper cells of lamellae.

—Upper cell of lamellae with papillae rather evenly distributed upon its distal part; widespread — 3

3. Leaf blade abruptly constricted at junction to sheathing base and leaves are strongly fragile at this junction; high Arctic — 2. P. fragile

A rare high Arctic species; it was often confused with the fragile-leaved plants of P. alpinum and P. septentrionale. The distinctive feature of P. fragile are leaves falling off in a specific place of constriction between sheathing base and limb, whereas in other species leaves break in variable places.

—Leaf blade gradually transiting to sheathing base, leaves not fragile or fragile at different levels, usually below the junction of blade and sheathing base; widespread — 4

4. Leaves incurved, entire to bluntly toothed at margin, seta 1-1.5 cm; capsule ovoid to subglobose; spores 17-22 µm — 3. P. septentrionale

TThe species was often considered as a poorly developed P. alpinum. However, subentire leaf margin and slightly larger spores are coinciding with genetic differentiation, supporting the separate status of the species (Ivanova et al. , 2014).

—Leaves straight, usually sharply serrate at margin (except strongly depressed arctic plants); seta (1-)3-5 cm; capsule cylindrical to ovoid; spores 14-20 µm — 1. P. alpinum

The species is widespread in mountain areas, where it grows in all altitudinal belts, being more dependent on the occurrence of rocky substrates than on Genusl climatic factors. When it grows in the forest belt, its plants are usually large, like large species of Polytrichum, but it differs from the latter in cylindrical capsules and is easily recognized in the field. However, in tundra its stems are often less than 5 mm, and microscopic study of leaf transverse section is needed for its identification. Papillose upper cell of lamellae is a main diagnostic character of P. alpinum, but attention is needed for its differentiation from Polytrichastrum septentrionale, P. fragile and Pogonatum urnigerum.

5. Plants small, growing on vertical walls with leaves pointed downwards; cells of stem cortex thick-walled; leaves with poorly developed sheathing base; seta 0.3-0.5 mm, curved; capsule subglobose to globose, sometimes very weakly 5-6-angular; peristome teeth 32; on volcanic rocks in North Pacific coastal area — 7. P. sphaerothecium

This species is characteristic for volcanic rocks in Kamchatka and Kuril Islands. Habitually it is similar to Oligotrichum falcatum by somewhat falcate and downward pointed leaves, but differs from the latter by the larger number of ventral lamellae. Cucullate leaves are similar to those in P. sexangulare, but plants of P. sphaerothecium are smaller, capsules are globose and not angular, on shorter seta, peristome teeth 32 (ca. 50 in all other species of the genus), and outer cells of the stem cortex are thick-walled. Molecular phylogenetic analysis demonstrated the most isolated position of this species in the genus (Ivanova et al. , 2014; Bell & Hyvönen, 2010a).

—Plants small to large, mostly orthotropic; cells of stem cortex thin-walled; leaves with well-developed sheathing base; seta 1-3 mm, straight; capsule subglobose to cylindrical, angular or smooth; peristome teeth 50 to 64; widespread — 6

6. Costa subpercurrent, leaf apex blunt and sub-cucullate; marginal cells of lamellae ovate to pyriform; lamellae entire at the upper edge from the side view, strongly incrassate; capsules (4-) 6-angular, often only obtusely; exothecial cells with diffuse thin spot on the outer wall — 4. P. sexangulare

TMainly high mountain species, often growing near late snow beds. It is distinct in cucullate leaves, 4-6-angular capsules and smooth upper cells of lamellae. Differentiation from P. sphaerothecium is discussed under that species.

—Costa shortly excurrent, leaf apex with an apiculate tip; marginal cells of lamellae gradually tapering to a knob-like tip; lamellae slightly crenulate at the upper edge from the side view, moderately incrassate; capsules round in transversal section, not angular; exothecial cells lacking thin spots — 4. P. altaicum

The species has a scattered distribution in the upper belts in the mountains of Kola Peninsula, Urals, Altai and Sayans. Smooth upper cells of lamellae in P. altaicum are similar to those of P. sexangulare, but it differs from the latter species in cylindrical (vs. angular) capsules, not cucullate leaf apex and more abruptly tapered upper cell of lamellae as seen in transverse leaf section. P. altaicum shares subentire leaf margins with P. septentrionale, but in the latter species upper cell of lamellae is papillose. Phylogenetic study indicates its affinity to Himalayan P. emodii, which, however, has distinctly serrulate leaves.