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Any of about 14,000 species of small, simple, non-flowering green plants of the Bryophyta division (see bryophyte), class Musci, which typically grow in colonies, often forming dense carpets. Each moss plant consists of an erect stalk to which primitive "leaves" are attached. The plants are anchored by root-like rhizoids. Mosses are distributed worldwide and are usually found growing on soil, rocks, and tree trunks in woods and other damp habitats. They are often early colonizers of bare soil and play an important role in preventing soil erosion. Sphagnum debris is an important constituent of peat.

Mosses reproduce by means of spores produced in a capsule on the stalk. The spores germinate into branching filaments, from which buds arise that grow into moss plants.

Mosses vary in growth and color according to species (see diagram). Fontinalis anti pyretica (A) is an aquatic moss, whose boat-shaped leaves have a sharp keel (1); the capsules are oblong or cylindrical (2, 3) and there is a pointed cap (4). Polytrichum commune (B) is extremely common and has a capsule (5) that looks like a four-sided box. It bears a long, golden brown cap (6) which is released before the spores are dispersed. Atrichum undulatum (C) is common on heaths and in woods, and has a capsule (7) with a long, pointed cap. Schistostega pennata (D) has flattened, translucent leaves.

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