Orthotrichiaceae. Genus Nyholmiella

1.Leaves strongly concave, margins involute throughout (including apices); leaf cells pluripapillose; peristome absent; rare species 1. N. gymnostoma

This species has an Amphiatlantic distribution: North America (Newfoundland only) and Europe (widespread). It has also been reported from south-western Asia (Turkey). In Europe it is common in Fennoscandia and known from scattered localities in Central Europe and the Balkans. There are many collections of it from the boreal zone of European Russia (Leningrad, Pskov, Vladimir and Kostroma Provinces, Kare­lia and Tatarstan Republics). In addition, the species is found in areas dominated by Abies and Picea forests at 1700–1900 m elev. in the Western Caucasus. Almost all Russian collections of N. gymnostoma grew on Populus tremula trunks. Nyholmiella gymnostoma has been reported from Asian Russia, but these collections are misidentified. Although N. gymnostoma can be confused with N. obtusifolia in the field, because they are similar in overall aspect, the presence in N. gymnostoma of strongly concave leaves with margins involute throughout (including the apices) will serve to distinguish the two even with a handlens.

— Leaves moderately concave, margins incurved to plane; leaf cells unipapillose; peristome present; widespread species 2. N. obtusifolia

This is a widespread and common species throughout the Holarctic. In Russia, N. obtusifolia occurs in nearly all regions except the Arctic. It is common in the forest zone on aspen and poplar where it extends to 71°N in Siberia, almost the northernmost limit of the forest zone. In addition, N. obtusifolia is widespread in the steppe zone in fragmented forested areas and common in cities and along highways on trunks of cultivated trees (mostly poplar). Plants of N. obtusifolia have a distinctive aspect because of their broadly rounded leaf apices. In addition, the presence of numerous gemmae on both sides of the leaves is a distinguishing and characteristic feature of the species. Its differences from the closely related N. gymnostoma are given in the key. Orthotrichum furcatum Otnyukova was described from dry, sunny habitats in Tyva (South Siberia) (Otnyukova, 2001); its distinctive feature is occasionally having high, forked papillae. In contrast, N. obtusifolia usually has entire papillae. However, transitional phenotypes between O. furcatum and typical N. obtusifolia were seen in several collections. Furthermore, there appears to be considerable variation in the size and shape of papillae in collections from different localities. As a result O. furcatum is here treated as a synonym of N. obtusifolia. Orthotrichum crenulatum also has broadly rounded, ovate leaves but differs from N. obtusifolia in having broadly revolute rather than incurved to plane leaf margins.