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Moss is a small green plant, rarely more than a few inches high, which grows anywhere not overshadowed by taller vegetation or perpetually dry. There are more than 600 species in the British Isles, growing on rocks and walls, in woodlands and on tree trunks and branches. They are important pioneers of rock surfaces, holding moisture in their leaves and building up humus in which the seeds of flowering plants can germinate.

There are two alternating generations in the life cycle of moss. The plant itself carries male and female organs. After fertilisation, these produce a capsule. Stalk and capsule together form an asexual, spore-bearing generation. The spores produced are capable of germinating into a new plant. Mosses also reproduce vegetatively, by small pieces breaking off and growing into a new plant.

As they are rootless and tend to get their nutrients from water running over or through them they are nearly always short.