BRYOPHYTE

a division of green, seedless land plants numbering at least 18,000 species and divided into three classes: mosses (Bryopsida, or Musci), liverworts (Hepatopsida, or Hepaticae), and hornworts (Anthotopsida, or Anthocerotae). Most bryophytes are 2 to 5 cm (0.8 to 2 inches) tall, but a few are more than 30 cm (12 inches) long. Bryophytes are widely distributed throughout the world, from polar regions to the tropics. They are most abundant in humid environments, although none is marine. Bryophytes are extremely tolerant of extended periods of desiccation as well as of freezing conditions. The peat moss genus, Sphagnum, is of economic importance to humans in harvesting and horticulture and as an energy source. Some bryophytes are used ornamentally, as in moss gardens. In nature, bryophytes initiate soil formation on barren terrain and maintain soil moisture, and they recycle nutrients in forest vegetation. They are found on rocks, logs, and forest litter, and their growth forms are correlated with the available humidity and sunlight. Bryophytes are distinguished from vascular and seed plants by the production of a single sporangium, rather than many, in each sporophyte (spore-producing stage). In contrast to that of the vascular plants, the sexual gametophytic phase, rather than the asexual sporophytic stage, is most noticeable. The mature gametophyte phase of the life cycle often bears leaflike organs. In some liverworts and hornworts, the mature gametophyte is somewhat or much flattened (a thallus) and is often compressed against the substratum. The gametophyte of most bryophytes is often attached to the substratum by filamentous strands called rhizoids, which are not necessarily absorptive but do influence water and mineral uptake. The sporophyte, which is usually annual, is dependent to varying degrees on the associated gametophyte for nutrients and water. The stalk, or seta, of the sporophyte develops a sporangium, or spore-containing structure, from which asexual spores are shed. Upon germination, the spores develop either a leafy or thallose gametophyte, upon which the sporophyte is produced. Reproduction can also occur by asexual meansi.e., from fragments of the gametophyte or by specialized cells that are budded off and germinate as do spores. The earliest suspected bryophyte fossils are found in the Late Devonian epoch (374 to 360 million years ago), and they resemble the thallose gametophytes of the liverwort order Metzgeriales. division (Bryophyta) of green, seedless plants. Bryophytes are among the simplest of the terrestrial plants. Most representatives lack complex tissue organization, yet they show considerable diversity in form and ecology. They are widely distributed throughout the world and are relatively small compared with most seed-bearing plants. Most are 25 centimetres (0.82 inches) tall or, if reclining, generally less than 10 centimetres long. The phylum Bryophyta includes three main evolutionary lines: the mosses (class Bryopsida, or Musci), the liverworts (class Hepatopsida, or Hepaticae), and the hornworts (class Anthocerotopsida, or Anthocerotae). It is conservatively estimated that there are more than 1,000 genera and more than 18,000 species of bryophytes. The bryophytes show an alternation of generations between the independent gametophyte generation, which produces the sex organs and sperm and eggs, and the dependent sporophyte generation, which produces the spores. In contrast to vascular plants, the bryophyte sporophyte usually lacks a complex vascular system and produces only one spore-containing organ (sporangium) rather than many. Furthermore, the gametophyte generation of the bryophyte is usually perennial and photosynthetically independent of the sporophyte, which forms an intimate interconnection with the gametophytic tissue, especially at the base, or foot, of the sporophyte. In most vascular plants, however, the gametophyte is dependent on the sporophyte. In bryophytes the long-lived and conspicuous generation is the gametophyte, while in vascular plants it is the sporophyte. Structures resembling stems, roots, and leaves are found on the gametophore of bryophytes, while these structures are found on the sporophytes in the vascular plants. The sporophyte releases spores, from which the gametophytes ultimately develop. In some bryophytes, sporophytes are unknown. The gametophyte in these bryophytes often reproduces asexually, or vegetatively, by specialized masses of cells (gemmae) that are usually budded off and ultimately give rise to gametophytes. Fragmentation of the gametophyte also results in vegetative reproduction: each living fragment has the potential to grow into a complete gametophyte. The mature gametophyte of most bryophytes is leafy, but some liverworts and hornworts have a flattened gametophyte, called a thallus. The thallus tends to be ribbonlike in form and is often compressed against the substratum to which it is generally attached by threadlike structures called rhizoids. Rhizoids also influence water and mineral uptake. Additional reading W.B. Schofield, Introduction to Bryology (1985); and E.V. Watson, The Structure and Life of Bryophytes, 3rd ed. (1971), are comprehensive introductions with extensive bibliographies that lead to more detailed treatises. Thorough analyses are found in Rudolf M. Schuster (ed.), New Manual of Bryology, 2 vol. (198384); R.N. Chopra and P.K. Kumra, Biology of Bryophytes (1988); and Advances in Bryology (biennial), an official publication of the International Association of Bryologists. A comprehensive survey of ecology and habitats is presented in A.J.E. Smith (ed.), Bryophyte Ecology (1982). G.C.S. Clarke and J.G. Duckett (eds.), Bryophyte Systematics (1979), covers various aspects of classification. Works concerned with the botanical details of mosses include William T. Doyle, The Biology of Higher Cryptogams (1970); and N.S. Parihar, An Introduction to Embryophyta, 5th ed., vol. 1, Bryophyta (1965). References helpful in the identification of mosses include Henry S. Conard, How to Know the Mosses and Liverworts, 2nd rev. ed. (1979), a good manual for American mosses; and, still unsurpassed for its illustrations, Paul W. Richards, A Book of Mosses (1950). The following periodicals are devoted mainly to bryology: The Bryologist (quarterly); Journal of Bryology (semiannual); Hattori Botanical Laboratory, Journal (semiannual), published in Japan with text in European languages; and Lindbergia (irregular), published by the Nordic and Dutch Bryological Societies. Wilfred Borden Schofield