The Bryopsida constitute the largest class of mosses, containing 95% of all moss species. It consists of approximately 11,500 species, common throughout the whole world.
The group is distinguished by having spore capsules with teeth that are arthrodontous; the teeth are separate from each other and jointed at the base where they attach to the opening of the capsule.[2] Consequently, mosses in the Class Bryopsida are commonly known as the “joint-toothed” or “arthrodontous” mosses.[3] These teeth are exposed when the covering operculum falls off. In other groups of mosses, the capsule is either nematodontous with an attached operculum, or else splits open without operculum or teeth.
The Bryopsida can be simplified into three groups: the acrocarpous (pinnate), the pleurocarpous (side-fruited), and the cladocarpous (branching) mosses. This is based on the position of the perichaetia and sporophytes.
Acrocarps are generally characterized by an upright growth habit that is unbranched or only sparingly branched. Branching is usually sympodial with the branches similar to main shoot where they originate. Branches below the perichaetium are called subfloral innovations.
Pleurocarps are generally characterized by creeping shoot systems and extensive lateral branching. The main stem is indeterminant and offshooting branches may be dissimilar. The perichaetia in pleurocarps are produced at the tips of extremely reduced, basally swollen lateral branches that are morphologically distinct from the vegetative branches.
Cladocarps are mosses which produce perichaetia at the tips of unspecialized lateral branches. Such branches are themselves capable of branching.