Racomitrium is a genus of mosses belonging to the family Grimmiaceae, in the order Grimmiales. It was established in 1818 by the Swiss botanist Samuel Elisee Bridel-Brideri, with Racomitrium lanuginosum later designated as its type species by Wilhelm Philippe Schimper in 1860. The genus name derives from the Greek racos ("rag") and mitra ("cap"), a reference to the torn or ragged calyptra characteristic of the group.
Members of Racomitrium are small to medium-sized acrocarpous mosses that typically form dense mats or cushions on exposed rock surfaces, boulder scree, and montane or arctic tundra. The leaves are lanceolate with a strong midrib and, in many species, end in a long, translucent hair-point that is toothed along both margins. These hair-points, which are hyaline and often decurrent along the leaf margins, give the plants a silvery or woolly appearance — particularly when dry — and are responsible for common names such as "woolly moss," "hoary rock-moss," and "woolly fringe-moss." Sporophytes are produced infrequently in most species.
The genus has a wide distribution spanning both hemispheres, with species occurring across arctic and alpine zones of the Northern Hemisphere as well as disjunct populations in tropical mountain ranges, the Southern Hemisphere, and sub-Antarctic islands. Most species show a strong preference for acidic substrates such as siliceous rock, and are frequently encountered in heath and bog communities as well as on exposed upland terrain.
The most widespread and ecologically significant member is Racomitrium lanuginosum, which forms extensive mat communities on mountain heaths and blanket bogs in oceanic climates and is one of the few bryophytes familiar to many non-bryologists. Other species in the genus include R. canescens, R. fasciculare, and R. elongatum, all of which inhabit similar rock and montane habitats.
Etymology
The genus name Racomitrium is derived from the Greek words racos ("rag") and mitra ("cap"), alluding to the torn or ragged calyptra that characterises the group. The genus was established in 1818 by Samuel Elisee Bridel-Brideri, with R. lanuginosum as the type species.
Distribution
Racomitrium species occur across the Northern and Southern hemispheres, with the greatest diversity in arctic, subarctic, and montane zones. R. lanuginosum, the most widespread member, has a circumpolar distribution in the Northern Hemisphere and disjunct occurrences in the tropical mountains of Central America, South America, Africa, and Asia, as well as in New Zealand, Hawaii, Tasmania, and several sub-Antarctic islands. Most species favour acidic siliceous rock; calcareous substrates are largely avoided.
Ecology
Racomitrium mosses grow predominantly on exposed rock, boulder scree, tundra, and blanket bog, from sea level to alpine elevations. They are well adapted to desiccation, recovering quickly after drought. R. lanuginosum in particular forms extensive heath communities on oceanic mountain landscapes and can dominate ground cover on acidic upland rocks and peats.
Taxonomy Notes
Racomitrium belongs to the family Grimmiaceae (order Grimmiales, class Bryopsida, phylum Bryophyta). Within the family, it sits alongside eight other genera including Grimmia, Schistidium, and Bucklandiella. Racomitrium lanuginosum is classified in sect. Racomitrium, which also includes R. geronticum and R. pruinosum. The genus was first described in 1818 by Samuel Elisee Bridel-Brideri; Johann Hedwig had earlier described the type species in 1801 under the name Trichostomum lanuginosum.
