Bryophytes

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Bryophytes, collective term applied to about 22,000 species of small plants that usually grow in moist areas on soil, tree trunks, and rocks. Bryophytes are nonvascular embryo-bearing plants of three plant divisions: the Bryophyta (mosses), the Hepatophyta (liverwort), and the Anthocerophyta (hornworts). Historically, these phyla were considered to be a monophyletic group and were thus placed as classes within an inclusive Bryophyta. In their level of organization, the bryophytes lie between the green algae, Chlorophyta, from which they most likely have evolved, and the simpler lower vascular plants such as the Lycopodiophyta (see Lycopsid). Bryophytes are similar to higher plants in that the fertilized egg develops into an embryo, a cell mass dependent on the gametophyte (the sexual plant). Bryophyte embryos, however, develop into sporophytes (asexual plants), which, unlike those of higher plants, remain almost entirely dependent on the gametophytes and have no leaves, stems, or roots. True conductive tissues, such as those in ferns and higher vascular plants, are not found in bryophytes.

Some species of bryophyte are aquatic, and some can survive in dry, arid areas. Although they range in size from microscopic to 30 cm (12 in) in length, the average bryophyte is between 1.2 and 5 cm (0.5 and 2 in) long and varies from green to black to almost colorless. The most primitive bryophytes, the liverworts, have flat bodies sometimes only one cell thick. The mosses have a central plant body resembling a stem from which tiny leaves radiate and rootlike structures, called rhizoids, extend. Bryophytes, however, absorb water directly from the base on which they grow or from the air, rather than through these structures.

All species of bryophytes are characterized by the alternation of generations. The embryo from the sexual plant matures into a small asexual plant that remains attached to and dependent on the sexual plant. The asexual plant produces spores similar to the spores produced by lower plants, and they are spread by wind and other carriers to produce new sexual plants. The sexual organs of bryophytes are multicellular.



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