Atrichum

Atrichum is a genus of mosses belonging to the family Polytrichaceae. These plants are commonly known as hairy-cap mosses, though this name is more often associated with the genus Polytrichum itself. Atrichum species are typically found in moist to wet habitats, often on disturbed soil, roadsides, or open woodlands. They are characterized by their relatively simple leaf structure compared to other members of the Polytrichaceae.

The leaves of Atrichum mosses are usually lanceolate to ovate and have a distinct midrib. A key feature used in identification is the presence of lamellae, which are small, plate-like structures on the upper surface of the midrib. These lamellae play a role in water retention and photosynthesis. The sporophytes, which are the spore-producing structures, consist of a seta (stalk) and a capsule. The capsule is typically erect and cylindrical, covered by a calyptra (a cap-like structure that detaches when the spores mature).

Several species are recognized within the genus Atrichum, with variations in size, leaf morphology, and habitat preference. For example, Atrichum undulatum is a widespread species found in temperate regions worldwide. It is often encountered in damp, shaded areas. Another species, Atrichum angustatum, is also common in North America. The genus is ecologically important as a pioneer species in disturbed areas, helping to stabilize soil and initiate ecological succession.