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Sphagnum
''Sphagnum'' is a genus of approximately 120 species of mosses, commonly known as peat moss. Accumulations of ''Sphagnum'' can store water, since both living and dead plants can hold large quantities of water inside their cells; plants may hold 16–26 times as much water as their dry weight, depending on the species.〔Bold, H.C. 1967. Morphology of Plants. second ed. Harper and Row, New York. p. 225-229.〕 The empty cells help retain water in drier conditions. Hence, as sphagnum moss grows, it can slowly spread into drier conditions, forming larger mires, both raised bogs and blanket bogs.〔Gorham, E. (1957). The development of peatlands. Quarterly Review of Biology, 32, 145–66.〕 These peat accumulations then provide habitat for a wide array of peatland plants, including sedges and ericaceous shrubs, as well as orchids and carnivorous plants.〔Keddy, P.A. (2010). Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 pp.〕 ''Sphagnum'' and the peat formed from it do not decay readily because of the phenolic compounds embedded in the moss's cell walls. In addition, bogs, like all wetlands, develop anaerobic soil conditions, which produces slower anaerobic decay rather than aerobic microbial action. Peat moss can also acidify its surroundings by taking up cations, such as calcium and magnesium, and releasing hydrogen ions. Under the right conditions, peat can accumulate to a depth of many meters. Different species of ''Sphagnum'' have different tolerance limits for flooding and pH, so any one peatland may have a number of different ''Sphagnum'' species.〔Vitt, D. H. and Slack, N. G. (1984). Niche diversification of ''Sphagnum'' relative to environmental factors in northern Minnesota peatlands. Canadian Journal of Botany, 62, 1409–30.〕
Individual peat moss plants consist of a main stem, with tightly arranged clusters of branch fascicles usually consisting of two or three spreading branches and two to four hanging branches. The top of the plant, or capitulum, has compact clusters of young branches. Along the stem are scattered leaves of various shapes, named stem leaves; the shape varies according to species. The leaves consist of two kinds of cells; small, green, living cells (chlorophyllose cells), and large, clear, structural, dead cells (hyaline cells). The latter have the large water-holding capacity.
==Life cycle==
''Sphagnum'', like all other land plants, has an alternation of generations; like other bryophytes, the haploid gametophyte generation is dominant and persistent. Unlike other mosses, the long-lived gametophytes do not rely upon rhizoids to assist in water uptake.〔 ''Sphagnum'' species can be unisexual (male or female, dioecious) or bisexual (male and female gametes produced from the same plant; monoecious); In North America, 80% of ''Sphagnum'' species are unisexual.〔Andrus, Richard. (''Sphagnum''. ) ''Flora of North America.'' 2007〕 Gametophytes have substantial asexual reproduction by fragmentation, producing much of the living material in sphagnum peatlands.〔Rydin, Hakan and Jeglum, John K. 2006. Biology of Peatlands. Oxford University Press, Oxford.〕 Swimming sperm fertilize eggs contained in archegonia that remain attached to the female gametophyte. The sporophyte is relatively short-lived, and consists almost entirely of a shiny green, spherical spore capsule that becomes black with spores. Sporophytes are raised on stalks to facilitate spore dispersal, but unlike other mosses, ''Sphagnum'' stalks are produced by the maternal gametophyte. Tetrahedral haploid spores are produced in the sporophyte by meiosis, which are then dispersed when the capsule explosively discharges its cap, called an operculum, and shoots the spores some distance. The spores germinate to produce minute protonemae, which start as filaments, can become thalloid, and can produce a few rhizoids. Soon afterwards the protonema develops buds and these differentiate into its characteristic, erect, leafy, branched gametophyte with chlorophyllose cells and hyaline cells.〔Schofield, W. B. 1985. Introduction to Bryology. Macmillan Publ. Co., N.Y. & London〕 This stage dominates the environment where ''Sphagnum'' grows, obliterating and burying the protonema and eventually building up into layers of dead moss called peat. Carpets of living ''Sphagnum'' may be attacked by various fungi, and one fungus that is also a mushroom, ''Sphagnurus paluster'', produces conspicuous dead patches. When this fungus and other agarics attack the protonema, ''Sphagnum'' is induced to produce non-photosynthetic gemmae that can survive the fungal attack and months later germinate to produce new protonema and leafy gametophytes. It is unknown whether the leafy stage can produce such gemmae.
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