Bryophyte

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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".[1] 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.[2] 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".

Classification and phylogeny[edit]

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.[3][4] The three lineages are Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (hornworts).[5]

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.[6] Although a 2005 study supported the traditional view that the bryophytes form a monophyletic group,[7] 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.[6][8]

embryophytes

 liverworts 




 mosses 




 hornworts 



 vascular plants





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 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.[9][10] 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.[11] The contrast is shown in a slightly different cladogram:[12]

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[edit]

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.[13] 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.[14] 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.

Bryophyte sexuality[edit]

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.

Comparison[edit]

Resume of the differential characteristics of the gametophytes of the three groups of bryophytes:

Hepaticae Musci Anthocerotae
Structure Thalloid or foliose Foliose Thalloid
Symmetry Dorsiventral or radial Radial Dorsiventral
Rhizoids Unicellular Pluricellular Unicellular
Chloroplasts/cell Many Many One
Protonema Reduced Present Absent
Gametangia
(antheridia
and archegonia)
Superficial Superficial Immersed

Resume of the differential characteristics of the sporophytes of the three groups of bryophytes:

Hepaticae Musci Anthocerotae
Structure Small, without chlorophyll Large, with chlorophyll Large, with chlorophyll
Growth Defined Defined Continuous
Seta Present Present Absent
Capsule form Simple Differentiated
(operculum, peristome)
Alongated
Maturation of spores Simultaneous Simultaneous Graduate
Dispersion of spores Elater Peristome teeth Pseudo-elaters
Columella Absent Present Present
Dehiscence Longitudinal or irregular Transversal Longitudinal
Stomata Absent Present Present

See also[edit]

References[edit]

  1. ^ "Reviews glossary". Retrieved 2009-03-26. 
  2. ^ "What are Bryophytes". Retrieved 2009-05-31. 
  3. ^ Konrat, M., Shaw, A. J., Renzaglia, K. S. (2010). "A special issue of Phytotaxa dedicated to Bryophytes: The closest living relatives of early land plants". Phytotaxa 9: 5–10. 
  4. ^ Troitsky AV, Ignatov MS, Bobrova VK, Milyutina IA (December 2007). "Contribution of genosystematics to current concepts of phylogeny and classification of bryophytes". Biochemistry Mosc. 72 (12): 1368–76. doi:10.1134/S0006297907120115. PMID 18205621. 
  5. ^ "GLOSSARY B". Retrieved 2009-03-26. 
  6. ^ a b Knoop, Volker (2010). "Looking for sense in the nonsense: a short review of non-coding organellar DNA elucidating the phylogeny of bryophytes". Tropical Bryology 31: 51–60. 
  7. ^ Goremykin, V. V. & Hellwig, F. H. (2005). "Evidence for the most basal split in land plants dividing bryophyte and tracheophyte lineages". Plant Systematics and Evolution 254 (1–2): 93–103. doi:10.1007/s00606-005-0337-1. 
  8. ^ Qiu, Y. L., Li, L., Wang, B., et al. (October 2006). "The deepest divergences in land plants inferred from phylogenomic evidence". Proceedings of the National Academy of Sciences 103 (42): 15511–6. Bibcode:2006PNAS..10315511Q. doi:10.1073/pnas.0603335103. PMC 1622854. PMID 17030812. 
  9. ^ Kenrick, Paul & Crane, Peter R. (1997a). The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D.C.: Smithsonian Institution Press. ISBN 978-1-56098-730-7. 
  10. ^ Kenrick, P. & Crane, P.R. (1997b). "The origin and early evolution of plants on land". Nature 389 (6646): 33–39. Bibcode:1997Natur.389...33K. doi:10.1038/37918 
  11. ^ Bell, N. E. & Hyvönen, J. (2010). "Phylogeny of the moss class Polytrichopsida (BRYOPHYTA): Generic-level structure and incongruent gene trees". Molecular Phylogenetics and Evolution 55 (2): 381–398. doi:10.1016/j.ympev.2010.02.004. PMID 20152915. 
  12. ^ Crane, P. R.; Herendeen, P. & Friis, E. M. (2004). "Fossils and plant phylogeny". American Journal of Botany 91 (10): 1683–99. doi:10.3732/ajb.91.10.1683. PMID 21652317 
  13. ^ "Bryophytes - introduction". Retrieved 2009-05-31. 
  14. ^ Cronberg N, Natcheva R, Hedlund K (2006). "Microarthropods Mediate Sperm Transfer in Mosses". Science 313 (5791): 1255. doi:10.1126/science.1128707. PMID 16946062. 

Bibliography[edit]

  • Lesica, P., McCune, B., Cooper, S. V., Hong, W. S. (1991), Differences in lichen and bryophyte communities between old-growth and managed second-growth forests in the Swan Valley, Montana. Canadian Journal of Botany 69: 1745–1755

External links[edit]