(Bryopsida) The largest and most widely distributed class of the *Bryophyta
, containing the mosses. It includes about 15 000 species in about 610 genera. The gametophyte is the dominant generation and exhibits two distinct morphological stages. The first, which arises on germination of the spore, is the filamentous protonema, which, except for its oblique cross walls, resembles a heterotrichous green algae. The protonema produces buds, from which the familiar leafy moss plant arises. In the Sphagnales and Andreaeales the protonoema is thalloid. The mature gametophyte, which is never thalloid, consists of a main axis (caulid) bearing delicate leaves (phyllids) usually only one cell thick, although a thickened central midrib is often seen. The leaves are generally inserted spirally on the stem. The stem also bears multicellular rhizoids, which distinguishes mosses from liverworts where the rhizoids are unicellular. The gametophyte may be *acrocarpous
. The sporophyte arises from an apical cell and exhibits complex spore dispersal mechanisms. The seta elongates gradually in contrast to liverworts where growth of the seta is rapid. There are no sterile elaters in the spore mass. The Musci is divided into three orders on the basis of differences in capsule structure and in formation of the protonema. The Bryales (commonly termed the true mosses) is the largest order and contains about 600 genera including the advanced Polytrichum, which shows some internal differentiation. The Sphagnales (bog or peat mosses) contains a single genus, Sphagnum, characteristic of waterlogged acid areas. The ability of sphagnum mosses to create vast areas of peat bog arises, in addition to their low pH and nutrient tolerance, from a peculiarity in their leaf structure, which contains many dead porous cells that act as water reservoirs. The third order is the Andreaeales, which again contains just one genus, Andreaea, the members of which are known as granite mosses. Certain classifications elevate these orders to class status, the Bryopsida, Sphagnopsida, and Andreaeopsida. The Bryopsida is then further subdivided into some 19 orders. The earliest fossil mosses are seen in rocks of the Permian but the group generally does not have a very rich fossil record.