Bryophyte

Only members of the division Bryophyta are considered "true" mosses. Many other plants and some algae are commonly called mosses, because they superficially resemble the true mosses, but they are not in fact even closely related to them. For example, Spanish moss (Tillandia uneoides) is a flowering plant in the pineapple family, Irish moss (Chondrus crispus) is a red alga that is collected for the extraction of carageenan, a starch-like substance used in food preparation, reindeer moss (Cladina spp.) is a lichen, and club mosses (Lycopodium spp.) are advanced plants with well developed vascular systems.

A number of characteristics distinguish two mosses from other bryophytes. Their gametophytes are leafy, whereas those of hornworts and thallose liverworts are not. The leaves of mosses occur in three ranks on the stem, but because the stem twists, they appear to have radial symmetry, the ability to be bisected into identical halves in more than one way. In contrast, leafy liverworts, whose leaves are two-ranked, only have one set of mirror images (bilateral symmetry). Furthermore, leaves of mosses are not lobed as in leafy liverworts. The rhizoids of mosses are multicellular, compared with single-celled in liverworts.

Mosses are distributed throughout the world, and are among the ecosystem dominants in boreal, arctic, and alpine environments. Mosses occur in a wide variety of habitats. They commonly grow on mineral and organic soils, and they can occur on volcanically heated soil that may reach temperatures of 131°F (55°C), on rocks in Antarctica where the temperature during the growing season does not exceed 14°F (-10°C), as epiphytes that grow on other plants, especially in tropical and subtropical regions, and in freshwater habitats. No mosses are truly marine, although some live within the spray zone of coastal habitats. The luminous moss Schistoste occurs within caves near the entrance where it concentrates the limited available light with the curved, lens-like surface of its leaves. The dung-loving species of the genus Splachnum are among the rarest and most beautiful of mosses. Their capsules occur on long, flimsy stalks, and the base of each capsule bears a thin, papery, brightly colored, umbrella-like structure-yellow in S. luteum and red in S. rubrum. The smallest mosses reach only 0.04-0.8 in (1-2 mm) in height, whereas the largest can grow to 20 in (50 cm). In temperate regions, mosses grow during cooler, wetter parts of the year, primarily during the autumn, mild spells in winter, and in early spring. The mosses naturally fall into three distinctive groups, taxonomically referred to as classes: Bryidae, the true mosses; Sphagnidae, the peat mosses; and Andreaeidae, the granite mosses.