Polytrichales

Polytrichales is an order of mosses in the class Bryopsida. It is best known for the family Polytrichaceae, which includes the large haircap mosses. Genera such as Polytrichum, Atrichum, Pogonatum, and Dawsonia illustrate the group's diversity. Species occur worldwide, with many in boreal and temperate regions, in habitats ranging from acidic soils and humic litter to exposed rock and disturbed sites.

Mosses in Polytrichales are typically acrocarpous, forming tufted or cushiony clumps with upright shoots. Leaves are relatively large and stiff, with a prominent costa and a pointed apex, arranged in several ranks around the stem to give a bristly appearance. The sporophyte, when present, typically rises on a slender seta and bears a capsule protected by a calyptra early in development.

Ecologically, Polytrichales mosses prefer moist, acidic substrates but tolerate a range of moisture conditions. They contribute to soil stabilization and litter formation in mossy ecosystems and can serve as indicators of acidic, nutrient-poor substrates. Reproduction is sexual, producing spores via sporophytes; many propagate vegetatively by fragmentation or remaining rhizoids.

Systematically, Polytrichales lies within Bryopsida. The order's composition is centered on Polytrichaceae, and relationships among taxa have varied with molecular studies. The name derives from Greek poly- "many" and thrix "hair", alluding to the dense, hairlike leaves common in many species.