Mniaceae. Genus Cinclidium

1. Costae ending well below leaf apices, not conflu­ent with the marginal border 1.C. minutifolium

A rare and poorly known species described from Yakutia and later synonymyzed with C. latifolium because both have suborbicular leaves with unistratose margins and a dioicous sexual condition. However, molecular evidence (K. Hassel, pers. comm.) indicates that C. minutifolium is closer to Rhizomnium than Cinclidium. Leaves of C. minutifolium have plane margins and are spreading when wet, while in C. latifolium they have widely recurved margins and are arcuate-recurved when wet. Leaf size in C. minutifolium is quite variable: 1.4–4.0 mm long. The species is sporadically distributed in northern regions of Asian Russia and NE European Russia.

— Costae percurrent, confluent with the marginal border 2

2. Leaves convex, strongly reflexed to arcuate-recurved when moist; leaf margins widely recurved 3. C. latifolium

Cinclidium latifolium is most common in the Arctic regions of Asian Russia. Among species of Cinclidium it is the most consistently found in the Arctic zone; however, its southernmost locality is in Kamchatka. The combination of dioicous sexual condition and reflexed to squarrose leaves is helpful in separating this species from other Cinclidium species. It grows on soil in moist habitats.

— Leaves ± plane, not strongly reflexed when moist; leaf margins narrowly recurved or plane 3

3. Leaves ovate, narrowly elliptic, or ovate-lanceo­late; leaf apices acuminate or acute; median leaf cells short-elongate or ± isodiametric, not arranged in clear oblique rows; plants dioicous 2. C. arcticum

In contrast to other species of the genus, C. arcticum often grows on cliffs, although in the very severe conditions of the Siberian Arctic it occurs on tundras as well. This species differs from other Cinclidium species in having more gradually acute leaves.

— Leaves ovate, broadly elliptic, spatulate, obovate, or ± orbicular; leaf apices obtuse or rounded, apiculate or cuspidate; median leaf cells elongate, arranged in clear oblique rows; plants synoicous 4

4. Leaf apices sharply apiculate or cuspidate, cells of apiculi elongate; leaf margins unistratose, narrowly recurved; capsules ovate-elliptic 4. C. stygium

This most widespread species of the genus is a characteristic component of rich fens and otherwise calcium-rich mires. It is especially common in the northern taiga and forest-tundra zones with relict localities in more southern regions (reaching Moscow Province in European Russia and the Altai in Asian Russia). Cinclidium stygium is extremely polymorphous, and this causes difficulties in separating it from C. latifolium and C. arcticum. But, C. stygium differs fundamentally from them in having synoicous rather than dioicous (in C. latifolium and C. arcticum) plants.

— Leaf apices usually bluntly apiculate, cells of apiculi short; leaf margins 2- or 3-stratose, plane; capsules subglobose 5. C. subrotundum

In Russia this species occurs mainly in the Arctic zone; in Asian Russia it extends southward to the northern taiga zone within permafrost areas as well as regions with oceanic climates, e.g., Kamchatka and the Commander Islands. It grows in minerotrophic mires, moist tundras, and occasionally in forsted mires, especially along streams. The combination of orbicular leaves and multistratose marginal leaf borders is unique in the genus and definitive in identifying the species.