Bryophyte

From New World Encyclopedia

Bryophytes are members of the land plants that are non-vascular, meaning that they lack water- and food-conducting strands in their roots (xylem and phloem), or that they are poorly developed. They do not have roots, only filamentous rhizoids. Three of the twelve phyla of plants are found within the division Bryophyta. These are the mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta), and hornworts (phylum Anthocerotophyta). Bryophytes have tissues and enclosed reproductive systems, but they neither flower nor produce seeds, reproducing via spores. The ecologically persistent, photosynthetic phase of the life cycle is the haploid, gametophyte generation rather than the diploid sporophyte; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes and consist of only an unbranched stalk, or seta, and a single, terminal sporangium (Stotler and Stotler 2005). Bryophytes are widely distributed globally and exhibit significant ecological diversity. They are typically small, herbaceous plants, growing closely packed in mats or cushions on rocks, soil, or as epiphytes on the trunks of trees.

Bryophyte classification

Two hypotheses on the phylogeny of land plants.

About 18,000 species are classified into three groups, the Marchantiophyta (liverworts), Anthocerotophyta (hornworts), and Bryophyta (mosses). Mosses represent the phylum with the most diversity, wtih up to 15,000 recognized species. Modern studies generally show one of two patterns. In one of these patterns, the liverworts were the first to diverge, followed by the hornworts, while the mosses are the closest living relatives of the vascular plants. In the other pattern, the hornworts were the first to diverge, followed by the vascular plants, while the mosses are the closest living relatives of the liverworts. Originally the three groups were brought together as the three classes of division Bryophyta. However, since the three groups of bryophytes form a paraphyletic group, they now are placed in three separate divisions. Recent studies of cell ultrastructure and molecular analysis have determined that these three phyla come from three different evolutionary lineages.

Bryophyte Sexuality

The bryophytes are generally gametophyte-oriented; that is, the normal plant is the haploid gametophyte, with the only diploid structure being the sporangium in season. The bryophyte sporophyte remains attached to the gametophyte, and does not become a free-living plant, as in other land plants. As a result, bryophyte sexuality is very different from that of other plants. There are two basic categories of sexuality in bryophytes:

  • Bryophytes are said to be dioecious if both male and female sex organs are borne on separate gametophytes, namely the antheridia (male organs) or archegonia (female organs).
  • Bryophytes are said to be monoecious if both male and female sex organs, antheridia and archegonia, are borne on the same plant body.

Some bryophyte species may be either monoecious or dioecious depending on environmental conditions. Other species grow exclusively with one type of sexuality.

Bryophyte Life Cycle

Bryophytes undergo two distinct stages of their life cyle: the haploid and diploid generations. When a spore is released, it germinates, forming the protonema, which is a photosynthetic filament of cells. In mosses, the protonema can have multiple branches, but in hornworts and liverworts, the form is usually simpler. As the protonema matures, it forms leafy buds, which form the leafy gametophores, which produce the gametangia (antheridia or archegonia). These form at the apex of the stems. The male plants or plant parts produce antheridia, which produce thousands of sperm. Water is required for the sperm to be ejected, either in the form of rain drops or in thin films of water.

Stems that produce archegonia are identified by the pointed apices of the leaves, which enclose the archegonia. The sperm produced by the antheridia must swim to the archegonia and down the neck canal in order to fertilize the egg.

The start of the diploid (sporophyte) generation is marked by the formation of the zygote, which then develops into an embryo. The embyro grows a shoot apex, which occurs direcly out of the archegonial neck, which grows to enclose the developing sporophyte, forming the calyptra. A capsule, called the sporangium, develops at the apex of the sporophyte. The sporangia contain sporogenous cells, which undergo meiosis, producing meiospores. This marks the beginning of the haploid gametophyte generation.

Global Extent and Ecology

Bryophytes are found throughout the world in a variety of habitats, but they thrive in moist, humid environments. They grow on a number of substrates, including soil, parts of living and decomposing organisms, and rock surfaces. They flourish in both temperate and tropical rain forests, and are found extensively in wetlands in the Arctic and Antarctic.

Their ecological benefits include capturing and recycling nutrients and preventing erosion by binding the soil. Because of their ability to colonize rock surfaces, bryophytes are often the first to begin soil formation, eventually making it easier for other plants to colonize. For this reason, bryophytes are often thought of as pioneer species. They also provide seed beds for other larger plants within the ecosystem.

Bryophytes are instrumental in the formation and maintenance of wetlands, particularly high latitude bogs and fens. The moss genusSphagnum,or peat moss, is most prevalent in peat bogs. It creates a highly acidic environment in waterlogged areas by taking up cations such as calcium and magnesium and releasing hydrogen ions. Because of its ability to acidify its surroundings and its ability to hold up to 20 times its mass in water, Sphagnum is a key component of bog formation, where decomposition is minimal and peat accumulation is high. Because of its ability to retain water, Sphagnum controls the hydrology of many raised bogs.

Peat has been harvested and sold for both fertilizer and fuel, but conservationists question the sustainability of peat harvesting. Current debates also involve the ability of peatlands to sequester carbon by burying undecomposed organic matter. Under enough heat and pressure, peatlands will eventually turn to coal (though this process takes millions of years).

See also

References
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  • Chopra, R. N. & Kumra, P. K. (1988). Biology of Bryophytes. New York: John Wiley & Sons. ISBN 0-470-21359-0.
  • Crum, Howard (2001). Structural Diversity of Bryophytes. Ann Arbor: University of Michigan Herbarium. ISBN 0-9620733-4-2.
  • Goffinet, Bernard. (2000). Origin and phylogenetic relationships of bryophytes. In A. Jonathan Shaw & Bernard Goffinet (Eds.), Bryophyte Biology, pp. 124-149. Cambridge: Cambridge University Press. ISBN 0-521-66097-1.
  • Oostendorp, Cora (1987). The Bryophytes of the Palaeozoic and the Mesozoic. Bryophytorum Bibliotheca, Band 34. Berlin & Stuttgart: J. Cramer. ISBN 3-443-62006-X.
  • Prihar, N. S. (1961). An Introduction to Embryophyta: Volume I, Bryophyta (4th ed.). Allahabad: Central Book Depot.
  • Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. (2005). Biology of Plants (7th ed.). New York: W. H. Freeman and Company. ISBN 0-7167-1007-2.
  • Schofield, W. B. (1985). Introduction to Bryology. New York: Macmillan. ISBN 0-02-949660-8.
  • Watson, E. V. (1971). The Structure and Life of Bryophytes (3rd ed.). London: Hutchinson University Library. ISBN 0-09-109301-5.


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