Bryophyte

Marchantia, a liverwort

Bryophyte is a traditional name used to refer to all embryophytes (land plants) that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be true vascular tissue. Currently bryophytes are thought not to be a natural or monophyletic group; however the name is convenient and remains in use as a collective term for mosses, hornworts, and liverworts. Bryophytes produce enclosed reproductive structures (gametangia and sporangia), but they produce neither flowers nor seeds, reproducing via spores. The term bryophyte comes from Greek βρύον - bryon, "tree-moss, oyster-green" + φυτόν - fyton "plant".

Bryophyte classification and phylogeny

Mosses are one group of bryophytes.

Hornworts are the third group of bryophytes.

Traditionally, all living land plants without vascular tissues were classified in a single taxonomic group, often a division (or phylum). More recently, phylogenetic research has questioned whether the bryophytes form a monophyletic group and thus whether they should form a single taxon. A broad consensus among systematists has recently emerged that bryophytes as a whole are not a natural group (i.e. are paraphyletic), although each of the three extant (living) groups is monophyletic. The three lineages are Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (hornworts).

The vascular plants or tracheophytes are the fourth lineage of living land plants. Currently there is some uncertainty over the exact evolutionary relationships among these four lineages, although this may be nearing resolution as data on a variety of coding and non-coding DNA sequences from all three genomes (chloroplast, mitochondrion and nucleus) in addition to protein sequences are brought to bear on the problem. Although a 2005 study supported the traditional view that the bryophytes form a monophyletic group, the preponderance of currently available evidence suggests that the hornworts are sister to vascular plants and liverworts are sister to all other land plants, as shown in the cladogram below.

embryophytes liverworts  mosses  hornworts  tracheophytes

bryophytes

When extinct plants are taken into account, the picture is slightly altered. There are extinct land plants, such as the horneophytes, which are not bryophytes, but also are not vascular plants since like bryophytes they do not have true vascular tissue. A different distinction is needed. In bryophytes, the sporophyte is a simple unbranched structure with a single spore-forming organ (sporangium). In all other land plants, the polysporangiophytes, the sporophyte is branched and carries many sporangia. It has been argued that this contrast between bryophytes and other land plants is less misleading than the traditional one of non-vascular versus vascular plant, since many mosses have well-developed water-conducting vessels. The contrast is shown in a slightly different cladogram:

Land plants	Liverworts Mosses Hornworts Polysporangiophytes "Protracheophytes", such as Horneophyton or Aglaophyton Tracheophytes or Vascular plants

The term "bryophyte" thus refers to a grade of lineages defined primarily by what they lack: compared to other living land plants, they lack vascular tissue containing lignin; compared to all other land plants, they lack branched sporophytes bearing multiple sporangia. The prominence of the gametophyte in the life cycle is also a shared feature of the three bryophyte lineages (extant vascular plants are all sporophyte dominant).

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

Bryophyte sexuality

Individual liverwort, moss and hornwort plants can be unisexual or bisexual. The terms for this are as follows:

• Dioicous bryophytes produce only antheridia (sperm producing structures) or archegonia (egg producing structures) on a single plant body.
• Monoicous bryophytes produce both antheridia and archegonia on the same plant body.

Notice that these terms are not the same as monoecious and dioecious, which refer to whether or not a sporophyte plant bears megasporangia, microsporangia or both. Those terms generally apply only to seed plants.

Some bryophyte species may be either monoicous or dioicous depending on environmental conditions. Other species are exclusively unisexual or bisexual.

See also

• Embryophyte
• Marchantiophyta (liverworts)
• Anthocerotophyta (hornworts)
• Bryophyta (mosses)
• Plant sexuality

External links

• Glime, Janice M., 2007. Bryophyte Ecology, Volume 1. Physiological Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists.
• Andrew's Moss Site Photos of bryophytes