Bryophyte

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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 within the division Bryophyta 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 hte haploid gametophyte generation.

Ecology

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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