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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 - phyton "plant".
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 about the 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.
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 because, 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:
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).
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 remain attached to and nutritionally dependent on the gametophyte. They produce a single sporangium (spore producing capsule).
Liverworts, mosses and hornworts spend most of their lives as gametophytes. Gametangia (gamete-producing organs), archegonia and antheridia, are produced on the gametophytes, 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 single sporangium or capsule. Inside the sporangium, haploid spores are produced by meiosis. These are dispersed, most commonly 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, ferns and other cryptogams.
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Individual liverwort, moss and hornwort plants can be unisexual or bisexual. The terms for this are as follows:
Some bryophyte species may be either monoicous or dioicous depending on environmental conditions. Other species are exclusively unisexual or bisexual.
The terms monoicous and dioicous are not the same as monoecious and dioecious, which refer to whether or not a seed plant sporophyte plant bears megasporangia, microsporangia or both.
Bryophytes are the oldest of all lineages of land plants and are believed to be a vital link in the migration of plants from aquatic environments onto land. DNA sequencing of Bryophytes has elucidated the fact that they are a polyphyletic group and the three main clades (mosses, hornworts and liverworts) are believed to have evolved from green algae on three different occasions. A number of physical features link Bryophytes to both land plants and aquatic plants. Two distinct adaptations have helped to make the move from water to land possible and forged the way for plants to colonize the Earth's terrestrial environments. A waxy cuticle covering the soft tissue of the plant provides protection and prevents desiccation of the plant's tissues; and the development of gametangia provided further protection specifically for gametes. They also have embryonic development which is a significant adaptation seen in land plants and not green algae. Connections to their aquatic ancestry are also evident through their dependence on water for reproduction and survival. A thin layer of water is required on the surface of the plant to enable the movement of sperm between gametophytes and the fertilization of an egg.
Resume of the differential characteristics of the gametophytes of the three groups of bryophytes:
|Structure||Thalloid or foliose||Foliose||Thalloid|
|Symmetry||Dorsiventral or radial||Radial||Dorsiventral|
Resume of the differential characteristics of the sporophytes of the three groups of bryophytes:
|Structure||Small, without chlorophyll||Large, with chlorophyll||Large, with chlorophyll|
|Maturation of spores||Simultaneous||Simultaneous||Graduate|
|Dispersion of spores||Elater||Peristome teeth||Pseudo-elaters|
|Dehiscence||Longitudinal or irregular||Transversal||Longitudinal|
|Look up bryophyte in Wiktionary, the free dictionary.|
|Wikisource has the text of the 1911 Encyclopædia Britannica article Bryophyta.|