Bryophytes are the most successful group of plants other than angiosperms in terms of their numbers of species, geographical distribution on all continents, and their habitat diversification. There are at least 10,000 species of mosses and over 6000 liverworts. All three groups of bryophytes, also including the hornworts, were the earliest green plants to move to the land; each group has had a very long evolutionary history, probably more than 400 million years. All three groups, derived from a green algal ancestor, evolved separately from one another and from vascular plants through this long period. Although the great diversity of tropical bryophytes is often cited, Rydin (2009) pointed out their important contribution to biodiversity in northern ecosystems: 7.5% of the world's bryophyte species are found in Sweden, whereas only 0.8% of vascular plant species are found there.
Bryophytes are unique among land plants in that their dominant stage is the haploid green gametophyte rather than the much shorter-lived diploid sporophyte. They differ from vascular plants in other ways as well, in aspects that make them excellent environmental monitors. They inhabit a very wide range of ecosystems, habitats, and specific microhabitats, including substrates on which vascular plants cannot live. Many species are able to live in nutrient-poor conditions, and are adapted to respond rapidly physiologically to intermittent periods favorable for photosynthesis.
Morphology and physiology
Bryophytes lack the roots, xylem, and phloem of vascular plants. The great majority are ectohydric, that is, without internal conducting tissues.