Bryophyte - Bryophyte Life Cycle

Bryophyte Life Cycle

See also: Alternation of generations

Like all land plants (embryophytes), bryophytes show 'alternation of generations'. A haploid gametophyte, each of whose cells contains a fixed number of unpaired chromosomes, gives rise to a diploid sporophyte, each of whose cells contains twice the number of paired chromosomes. Gametophytes produce sperm and eggs which fuse and grow into sporophytes. Sporophytes produce spores which grow into gametophytes.

Bryophytes are gametophyte dominant, meaning that the more prominent, longer-lived plant is the haploid gametophyte. The diploid sporophytes appear only occasionally and typically remain attached to and nutritionally dependent on the gametophyte. They produce a single sporangium (spore producing structure), with, in many cases, a complex capsule aiding in dispersal of the spores.

Liverworts, mosses and hornworts spend most of their lives as gametophytes. Archegonia and antheridia are produced on the gametophytes. These are sometimes at the tips of shoots, in the axils of leaves or hidden under thalli. Some bryophytes create elaborate structures that bear gametangia called gametangiophores. Sperm are flagellated and must swim from antheridia to archegonia. Arthropods may assist in transfer of sperm. Fertilized eggs become zygotes, which develop into sporophyte embryos inside the archegonia. Mature sporophytes do not branch and remain attached to the gametophyte. They consist of a stalk called a seta and a capsule enclosing a single sporangium. Inside the sporangium, spores are produced by meiosis. These are dispersed presumably by wind and if they land in a suitable environment can develop into a new gametophyte. Thus bryophytes disperse by a combination of swimming sperm and spores, in a manner similar to lycophytes and ferns.

Further information: Liverwort: Life cycle, Moss: Life cycle and Hornwort: Life cycle