Polytrichaceae. Genus Oligotrichum
1. Leaves 2.5-5 mm long, soft; ventral lamellae 2-5(-8), 3-6(-8) cells high —
1. O. parallelum
Russian Far East. When dry this species looks more like Atrichum because its large and broad leaves are strongly crisped and contorted, but its marginal teeth are not paired as in Atrichum. The species is more common in the upper mountain belt on soil banks along streams and near rock outcrops.
—Leaves 1. 0-2.5 mm long, rigid; ventral lamellae 5-16, 4-12 cells high —
2. Leaf margins coarsely serrate; dorsal lamellae numerous, (1-)3-5 cells high; Far East —
2. O. aligerum
This species has the most southern distribution within the genus; it grows mostly in disturbed places, especially at roadsides in mixed and coniferous forests. Bright yellow-green color helps to recognize it from a distance, especially because it usually forms large populations. Plants look quite rigid, due to coarsely serrate leaf margins and lamellae abundantly present on both leaf surfaces.
—Leaf margins subentire to weakly serrulate; dorsal lamellae absent or few, 1-3 cells high; northern and montane regions of European and/or Asian Russia —
3. Leaves falcate-secund to curved inwards when dry; dorsal lamellae absent or rare; upper edge of ventral lamellae serrate; spores 15-21 µm; calyptra naked —
3. O. falcatum
Sporadic or locally moderately common in the Arctic and permafrost areas of Siberia, westward to Taimyr, with isolated localities as far south as Altai and mountains of Transbaikalia and Khabarovsk Territory. When plants are well developed, its falcate-secund leaves are conspicuous, but in unfavorable conditions they often are shorter and can be confused with Psilopilum, because both have short, non-falcate leaves with cucullate apices and grow in similar habitats. However, P. laevigatum differs from O. falcatum in having rhombic cells in the transition zone between the sheathing base and limbidium, while P. cavifolium has larger laminal cells and more strongly concave, cucullate leaves.
—Leaves curved inwards when dry; dorsal lamellae 2-5; upper edge of ventral lamellae coarsely crenate; spores 10-15 µm; calyptra hairy —
4. O. hercynicum
A widespread and variable species. In the European North it can be recognizable by its low plants and succulent leaves, but in Asiatic Arctic separation from O. falcatum may require careful microscopic examination, because both have stressed, short-leafed phenotypes. The best way to separate the two species is by examining the upper edge of lamellae, see the key and Figs.