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More than 95% of moss species belong to the Bryopsida. Diversity in this group has been classified into 90 to 110 families and 11 to 16 orders. The Dicranales and Hypnales are the most diverse groups with 13 and 44 families, respectively.
The most important characteristic of the Bryopsida is the architecture of the ring of teeth (peristome) surrounding the mouth of the sporophyte capsule (Figure 1). Bryopsida are the only mosses that have an arthrodontous peristome, i. e., a peristome in which the teeth are structured by articulated cell wall remnants (see Characteristics).
Figure 1. Closeup of the peristome of the capsule of Sematophyllum (Hypnales). This arthrodontous peristome is composed of a double ring of teeth. The external teeth (exostome) are shorter and thicker than the segments of the internal row (endostome). Note that each tooth in the exostome is alternated with a segment of the endostome.
Image copyright © 2000, Efrain De Luna.
Despite all orders sharing a basic arthrodontous peristome, the Bryopsida comprises a diverse set of species of various gametophytic morphologies. Branching systems combine pinnate (monopodial) and stepwise extension of new branches (sympodial architectures) with apical (acrocarpous) or lateral (cladocarpous and pleurocarpous) sporophyte locations. Leaf cells are variously differentiated forming specialized groups at allar, basal, medial, upper and apical leaf zones. Leaves in some taxa are multilayered stratose, at least in the apex and margins. The sporangia also show a wide range of variation in shape and size of the capsule, operculum, and peristome organization. The outer surface of teeth can be variously ornamented with papillae, striations, or both.