Mosses (Division Bryophyta) are generally seen as small plants confined to humid habitats, avoiding exposure to direct sunlight. Yet, an alert naturalist will quickly notice their presence in virtually every ecosystem. In parts of the world where short growing seasons limit plant growth, mosses may dominate the vegetation. Similarly, in temperate and tropical rain forests, mosses compose luxuriant epiphytic communities that play important ecological functions, especially in terms of water and nutrient flow. Approximately 12 000 species of mosses are currently recognized, reflecting a broad morphological diversity. In fact, a unifying macroscopic definition is impossible due to both fundamental architectural differences among major lineages and extensive reduction, and hence character loss, across the phylogenetic tree of mosses. A sporophyte composed of a robust stalk elevating the capsule diagnoses most mosses, but is lacking in peat mosses and various ephemeral taxa inhabiting seasonally dry ecosystems. In many cases, closer examination further reveals one or two rings of teeth lining the capsule mouth. The vegetative (gametophyte) body always consists of a terete axis with sessile leaves, rather than a flattened thallus as in hornworts and some liverworts. Although associations with fungi are common, evidence for a symbiotic nature of the relationship benefiting the moss is lacking.