Micranthes is a genus of herbaceous, perennial flowering plants in the family Saxifragaceae, often informally called the alpine saxifrages. The plants are usually rosette-forming and sometimes rhizomatous or stoloniferous, with herbage that is typically hairy. From a basal cluster of leaves rises a mostly leafless, erect flowering stem that can range from a few centimetres to well over a metre tall, depending on the species. Basal leaf shape varies considerably across the genus — obtrullate, spatulate, fan-shaped, or obovate to oblanceolate — with margins that may be entire, crenate, serrate, or dentate.
The inflorescence is a cyme or thyrse bearing small flowers whose petals are usually white or cream but can be variably coloured in some species. Flowers have a two-carpellate ovary that is superior or semi-inferior, and the seeds bear distinctive longitudinal ribs. A base chromosome number of x = 8 underlies a group in which polyploidy and dysploidy are widespread, contributing to the morphological complexity of the genus.
The genus was described by Adrian Hardy Haworth in 1812 (Synopsis Plantarum Succulentarum, p. 320). For most of the next two centuries its species were treated as part of a broadly defined Saxifraga, but molecular phylogenetic work — most influentially Soltis and colleagues' 1996 analysis of matK and rbcL sequences — showed that Saxifraga in the broad sense is not monophyletic. The taxa now placed in Micranthes turned out to be more closely related to Boykinia and Heuchera than to the rest of Saxifraga, and the segregate genus has been increasingly accepted since.
Kew's Plants of the World Online currently lists 89 accepted species, with other databases reporting figures from roughly 34 to 186 depending on which infrageneric treatment is followed. The genus is distributed across the temperate, subarctic, subalpine, arctic, and alpine zones of the Northern Hemisphere, occurring from Alaska and Canada southward through the United States, across Scandinavia, the Alps, and parts of the Mediterranean, and eastward through Russia, Mongolia, the Himalayas, and into Japan. Many species are circumboreal, and some show notably greater morphological variability in their North American populations than in their European ones.
Etymology
The genus name Micranthes derives from Greek roots meaning "small-flowered" (mikros, small, plus anthos, flower), and refers to the modest size of the individual flowers borne in the cymose or thyrsoid inflorescences typical of the group. The name was published by the English botanist Adrian Hardy Haworth in his 1812 Synopsis Plantarum Succulentarum (p. 320).
Distribution
Micranthes is broadly distributed across the Northern Hemisphere, with a centre of diversity in the temperate, subarctic, subalpine, and alpine zones of North America and Eurasia. In North America the genus reaches from Alaska and the Canadian Arctic south through the western mountains and into the eastern United States; three species — M. foliolosa, M. pensylvanica, and M. virginiensis — occur in New England alone. In Eurasia it extends from Scandinavia through the Alps and Mediterranean Europe and eastward across Russia and Mongolia through the Himalayas to Japan.
Ecology
The genus is largely a cold-climate group, characteristic of arctic tundra, alpine meadows, montane wet rocks and seeps, and boreal forest openings. Several species are circumboreal, occurring across both Eurasian and North American Arctic regions; among these, populations in North America have been observed to show greater morphological variability than their European counterparts, hinting at active divergence within the genus.
History
For most of botanical history, the plants now placed in Micranthes were treated as a section within the large genus Saxifraga. Haworth had already proposed Micranthes as a distinct genus in 1812, but the name was largely overlooked for the next 180 years. The decisive shift came with molecular phylogenetic studies in the 1990s, most notably Soltis et al. (1996) in the American Journal of Botany, which used the matK and rbcL chloroplast genes to demonstrate that Saxifraga in the broad sense was not monophyletic. The Micranthes lineage proved to be more closely related to Boykinia and Heuchera than to the rest of Saxifraga, providing a phylogenetic justification for re-elevating Haworth's old genus. Since then Micranthes has been progressively adopted by major checklists including POWO and GBIF.
Taxonomy
Micranthes Haw. (1812) is currently accepted by Plants of the World Online, GBIF, and other major checklists as a genus of family Saxifragaceae, with POWO recognising 89 species. Other datasets list anywhere from about 34 to 186 descendants depending on synonymy and infrageneric treatment. The type species is M. semipubescens. Nine heterotypic synonyms are recorded at genus level — Aulaxis Haw., Dermasea Haw., Hermesia Hoppe, Heterisia Raf., Hexaphoma Raf., Hydatica Neck., Ocrearia Small, Spatularia Haw., and Steiranisia Raf. — along with the sectional names Saxifraga sect. Micranthes Tausch (1823) and Saxifraga sect. Micranthes (Haw.) D.Don (1822). A later homonym, Micranthes Bertol. (Lamiaceae), is now treated as a synonym of Hoslundia Vahl and should not be confused with Haworth's saxifrage genus. The group has a base chromosome number of x = 8 and shows extensive polyploidy and dysploidy.
Conservation
The genus as a whole is not listed in the Global Invasive Species Database, so Micranthes is not considered invasive anywhere in its range. Conservation status is best assessed at the species level: many alpine and arctic members occupy restricted, climate-sensitive habitats that are vulnerable to warming and habitat disturbance, but no genus-wide listing applies.
