Bryophyte

The classification of bryophytes has been controversial among botanists. Traditionally, the division Bryophyta has included the true mosses, liverworts, and hornworts. However, some scientists consider each of these groups sufficiently distinct to deserve their own division: Bryophyta for the mosses, Hepatophyta for the liverworts, and Anthoceratophyta for the hornworts. The latter view is followed here, although the bryophyte is used as a collective term for all of these.

About 15,000 species of bryophytes have been described. They are distributed throughout the world, and are especially abundant in arctic and boreal regions, where they often dominate the ground vegetation. Bryophytes also occur in humid tropical regions where they commonly grow on other plants, especially in higher-elevation forests. Bryophytes are considered the amphibians of the plant world, because they require abundant moisture to grow. This requirement for water results from a number of their characteristic features. Their stems and leaves are thin, and either lack a cuticle (that is, a waxy surface layer) or have a very thin one, making them prone to drying out. Because bryophytes lack roots and a vascular system, they cannot obtain water from the soil and transport it to above-ground tissues; for this same reason, bryophytes are necessarily small. In addition, their sperm require free water in order to swim from their parent plant to the egg on another plant.