Bryophyte

Bryophytes (/ˈbraɪ.əˌfaɪts/)^[1] are a group of land plants (embryophytes), sometimes treated as a taxonomic division, that contains three groups of non-vascular land plants: the liverworts, hornworts, and mosses.^[2] In the strict sense, the division Bryophyta consists of the mosses only. Bryophytes are characteristically limited in size and prefer moist habitats although some species can survive in drier environments.^[3] The bryophytes consist of about 20,000 plant species.^[4][5] Bryophytes produce enclosed reproductive structures (gametangia and sporangia), but they do not produce flowers or seeds. They reproduce sexually by spores and asexually by fragmentation or the production of gemmae.^[6]

Though bryophytes were considered a paraphyletic group in recent years, almost all of the most recent phylogenetic evidence supports the monophyly of this group, as originally classified by Wilhelm Schimper in 1879.^[7]

The term bryophyte comes from Ancient Greek βρύον (brúon) 'tree moss, liverwort' and φυτόν (phutón) 'plant'.

The defining features of bryophytes are:

• Their life cycles are dominated by a multicellular haploid gametophyte stage
• Their sporophytes are diploid and unbranched
• They do not have a true vascular tissue containing lignin (although some have specialized tissues for the transport of water)^[8]

Bryophytes exist in a wide variety of habitats. They can be found growing in a range of temperatures (cold arctics and in hot deserts), elevations (sea-level to alpine), and moisture (dry deserts to wet rain forests). Bryophytes can grow where vascularized plants cannot because they do not depend on roots for uptake of nutrients from soil. Bryophytes can survive on rocks and bare soil.^[9]

Like all land plants (embryophytes), bryophytes have life cycles with alternation of generations.^[10] In each cycle, a haploid gametophyte, each of whose cells contains a fixed number of unpaired chromosomes, alternates with a diploid sporophyte, whose cells contain two sets of paired chromosomes. Gametophytes produce haploid sperm and eggs which fuse to form diploid zygotes that grow into sporophytes. Sporophytes produce haploid spores by meiosis, that grow into gametophytes.

Bryophytes are gametophyte dominant,^[11] 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.^[12] In bryophytes, the sporophytes are always unbranched and produce a single sporangium (spore producing capsule), but each gametophyte can give rise to several sporophytes at once.

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, such as the liverwort Marchantia, create elaborate structures to bear the gametangia that are called gametangiophores. Sperm are flagellated and must swim from the antheridia that produce them to archegonia which may be on a different plant. Arthropods can assist in transfer of sperm.^[13]

Fertilized eggs become zygotes, which develop into sporophyte embryos inside the archegonia. Mature sporophytes 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.

The sporophyte develops differently in the three groups. Both mosses and hornworts have a meristem zone where cell division occurs. In hornworts, the meristem starts at the base where the foot ends, and the division of cells pushes the sporophyte body upwards. In mosses, the meristem is located between the capsule and the top of the stalk (seta), and produces cells downward, elongating the stalk and elevating the capsule. In liverworts the meristem is absent and the elongation of the sporophyte is caused almost exclusively by cell expansion.^[14]

Traditionally, all living land plants without vascular tissues were classified in a single taxonomic group, often a division (or phylum). The term "Bryophyta" was first suggested by Braun in 1864.^[17] As early as 1879, the term Bryophyta was used by German bryologist Wilhelm Schimper to describe a group containing all three bryophyte clades (though at the time, hornworts were considered part of the liverworts).^[18][7] G.M. Smith placed this group between Algae and Pteridophyta.^[19] Although a 2005 study supported this traditional monophyletic view,^[20] by 2010 a broad consensus had emerged among systematists that bryophytes as a whole are not a natural group (i.e., are paraphyletic).^[21][22][23] However, a 2014 study concluded that these previous phylogenies, which were based on nucleic acid sequences, were subject to composition biases, and that, furthermore, phylogenies based on amino acid sequences suggested that the bryophytes are monophyletic after all.^[24] Since then, partially thanks to a proliferation of genomic and transcriptomic datasets, almost all phylogenetics studies based on nuclear and chloroplastic sequences have concluded that the bryophytes form a monophyletic group.^{[24][25][18][26][27][28][29][30][31]} Nevertheless, phylogenies based on mitochondrial sequences fail to support the monophyletic view.^[32]

The three bryophyte clades are the Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (hornworts).^[33] However, it has been proposed that these clades are de-ranked to the classes Marchantiopsida, Bryopsida, and Anthocerotopsida, respectively.^[18] There is now strong evidence that the liverworts and mosses belong to a monophyletic clade, called Setaphyta.^[25][32][34]

Paraphyletic view

embryophytes vascular plants hornworts setaphytes mosses liverworts

bryophytes

In this alternative view, the Setaphyta grouping is retained, but hornworts instead are sister to vascular plants.^[34] (Another paraphyletic view involves hornworts branching out first.)^[32]

There have probably been several different terrestrialization events, in which originally aquatic organisms colonized the land, just within the lineage of the Viridiplantae.^[40] Between 510–630 million years ago, however, land plants emerged within the green algae.^[41] Molecular phylogenetic studies conclude that bryophytes are the earliest diverging lineages of the extant land plants.^{[21][2][42][43]} They provide insights into the migration of plants from aquatic environments to land. A number of physical features link bryophytes to both land plants and aquatic plants.^[44]

Summary of the morphological characteristics of the gametophytes of the three groups of bryophytes:

Summary of the morphological characteristics of the sporophytes of the three groups of bryophytes:

This article is in list format but may read better as prose. (April 2017)

Environmental

• Soil Conditioning
• Bioindicators
• Moss gardens
• Pesticides

Characteristics of bryophytes make them useful to the environment. Depending on the specific plant texture, bryophytes have been shown to help improve the water retention and air space within soil.^[48] Bryophytes are used in pollution studies to indicate soil pollution (such as the presence of heavy metals), air pollution, and UV-B radiation.^[48] Gardens in Japan are designed with moss to create peaceful sanctuaries.^[48] Some bryophytes have been found to produce natural pesticides. The liverwort, Plagiochila, produces a chemical that is poisonous to mice.^[48] Other bryophytes produce chemicals that are antifeedants which protect them from being eaten by slugs.^[48] When Phythium sphagnum is sprinkled on the soil of germinating seeds, it inhibits growth of "damping off fungus" which would otherwise kill young seedlings.^[49]

Commercial

• Fuel
• Packaging
• Wound Dressing

Peat is a fuel produced from dried bryophytes, typically Sphagnum. Bryophytes' antibiotic properties and ability to retain water make them a useful packaging material for vegetables, flowers, and bulbs.^[48] Also, because of its antibiotic properties, Sphagnum was used as a surgical dressing in World War I.^[48]

• Plant sexuality
• List of British county and local bryophyte floras
• Thallophyta