Mosses are small flowerless plants that typically grow in dense green clumps or mats, often in damp or shady locations. The individual plants are usually composed of simple leaves that are generally only one cell thick, attached to a stem that may be branched or unbranched and has only a limited role in conducting water and nutrients. Although some species have conducting tissues, these are generally poorly developed and structurally different from similar tissue found in vascular plants.

Mosses are now classified on their own as the division Bryophyta. There are approximately 12,000 species.

Mosses are commonly confused with lichens, hornworts, and liverworts. Lichens may superficially look like mosses, and have common names that include the word "moss" (e.g., "reindeer moss" or "iceland moss"), but are not related to mosses. Mosses used to be grouped together with the hornworts and liverworts as "non-vascular" plants in the former division "bryophytes", all of them having the haploid gametophyte generation as the dominant phase of the life cycle.

The main commercial significance of mosses is as the main constituent of peat (mostly the genus Sphagnum), although they are also used for decorative purposes, such as in gardens and in the florist trade. Traditional uses of mosses included as insulation and for the ability to absorb liquids up to 20 times their weight.

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