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Sphagnum is a genus of between 151 and 350 species of mosses commonly called sphagnum, or peat moss, due to its prevalence in wet habitats where it contributes to the formation of peat bogs and mires. Sphagnum accumulations can store water, since both living and dead plants can hold large quantities of water inside their cells; plants may hold from 16-26 times as much water as their dry weight depending on the species.. The empty cells help retain water in drier conditions. Hence, as sphagnum moss grows, it can slowly spread into drier conditions, forming larger peatlands, both raised bogs and blanket bogs. 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. 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.
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.
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 gametophytes have no 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. Gametophytes have substantial asexual reproduction by fragmentation, producing much of the living material in sphagnum peatlands. Swimming sperm fertilize eggs contained in archegonia that remain attached to the female gametophyte. The sporophyte is relatively short-lived, and consists entirely of a shiny black, spherical spore capsule. 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 ruptures. The spores germinate to produce minute protonemae, which over time, develop the characteristic Sphagnum leaves.
Peat moss can be distinguished from other moss species by its unique branch clusters. The plant and stem color, the shape of the branch and stem leaves, and the shape of the green cells are all characteristics used to identify peat moss to species. Sphagnum taxonomy has been very contentious since the early 1900s; most species require microscopic dissection to be identified. In the field, most Sphagnum species can be identified to one of four major sections of the genus—classification and descriptions follow Andrus 2007 (Flora North America):
The reciprocal monophyly of these sections and two other minor ones (Rigida and Squarrosa) has been clarified using molecular phylogenetics. All but two species normally identified as Sphagnum reside in one clade; two other species have recently been separated into new families within the Sphagnaceae reflecting an ancestral relationship with the Tasmanian endemic Ambuchanania and long phylogenetic distance to the rest of Sphagnum. Within main clade of Sphagnum, phylogenetic distance is relatively short, and molecular dating methods suggest nearly all current Sphagnum species are descended from a radiation that occurred just 14 million years ago.
Sphagnum mosses occur mainly in the Northern Hemisphere in peat bogs, conifer forests and moist tundra areas. Their northernmost populations lie in the archipelago of Svalbard, Arctic Norway at 81° N.
In the Southern Hemisphere, the largest peat areas are in southern Chile and Argentina, part of the vast Magellanic Moorland (circa 44,000 square km). Peat areas are also found in New Zealand and Tasmania. In the Southern Hemisphere, however, peat landscapes may contain many moss species other than Sphagnum. Sphagnum species are also reported from "dripping rocks" in mountainous, subtropical Brazil.
As with many other mosses, Sphagnum species disperse spores through the wind. The tops of spore capsules are only about 1 cm above ground, and where wind is weak. As the spherical spore capsule dries, the operculum is forced off, followed by a cloud of spores. The exact mechanism has traditionally attributed to a "pop gun" method using air compressed in the capsule, reaching a maximum velocity of 3.6 meters per second, but alternative mechanisms have been recently proposed. High-speed photography has shown vortex rings are created during the discharge, which enable the spores to reach a height of 10 to 20 cm, further than would be expected by ballistics alone. The acceleration of the spores is about 36,000G. Spores are extremely important in establishment of new populations in disturbed habitats and on islands.
Decayed, compacted sphagnum moss has the name of peat or peat moss. This is used as a soil conditioner which increases the soil's capacity to hold water and nutrients by increasing capillary forces and cation exchange capacity. This is often necessary when dealing with very sandy soil, or plants that need increased or steady moisture content to flourish. A distinction is sometimes made between sphagnum moss, the live moss growing on top of a peat bog, and 'sphagnum peat moss' (North American usage) or 'sphagnum peat' (British usage), the latter being the decaying matter underneath.
Anaerobic acidic sphagnum bogs have low rates of decay, and hence preserve plant fragments and pollen to allow reconstruction of past environments. They even preserve human bodies for millennia; examples of these preserved specimens are Tollund Man, Haraldskær Woman, Clonycavan Man and Lindow Man. Such bogs can also preserve human hair and clothing, one of the most noteworthy examples being Egtved Girl, Denmark. Because of the acidity of peat, however, bones are dissolved rather than preserved. These bogs have also been used to preserve food.  Up to 2000-year-old containers of butter or lard have been found.
Sphagnum moss has also been used for centuries as a dressing for wounds. Since it is absorptive and extremely acidic, it inhibits growth of bacteria and fungi, so it is used for shipping seeds and live plants. However, see Health dangers below.
Peat moss is used to dispose of the clarified liquid output (effluent) from septic tanks in areas that lack the proper conditions for ordinary disposal means. It is also used as an environmentally friendly alternative to chlorine in swimming pool sanitation. The moss inhibits the growth of microbes and reduces the need for chlorine in swimming pools.
In New Zealand, both the species S. cristatum and S. subnitens are harvested by hand and exported worldwide for use as hanging basket liners, as a growing medium for young orchids, and mixed with other potting mixes to enhance their moisture retaining value.
Several of the world's largest wetlands are sphagnum-dominated bogs, including the West Siberian Lowland, the Hudson Bay Lowland and the Mackenzie River Valley. These areas provide habitat for common and for rare species. They also store large amounts of carbon, which helps reduce global warming.
Large-scale peat harvesting is not sustainable, as it takes thousands of years to form the peat that can be harvested in just a week. Coir has been touted as a sustainable alternative to peat moss in growing media. Another peat moss alternative is manufactured in California from sustainably harvested redwood fiber.
Europe has a long history of the exploitation of peatlands. The Netherlands, for example, once had large areas of peatland, both fen and bog. Between 100 AD and the present, they were drained and converted to agricultural land. The English broadlands have small lakes that originated as peat mines. More than 90% of the bogs in England have been damaged or destroyed. A handful of bogs have been preserved through government buyouts of peat-mining interests. Over longer time scales, however, some parts of England, Ireland and Scotland have seen expansion of bogs, particularly blanket bogs, in response to deforestation and abandonment of agricultural land.
New Zealand has, like the rest of the world, lost large areas of peatland; the latest estimates for wetland loss in New Zealand are 90% over 150 years. In some cases, better care is taken during the harvesting of Sphagnum to ensure enough moss is remaining to allow regrowth. This is commonly done using a three-year cycle. This "farming" is based on a sustainable management program approved by New Zealand's Department of Conservation; it ensures the regeneration of the moss, while protecting the wildlife and the environment. Most harvesting in New Zealand swamps is done only using pitchforks without the use of heavy machinery. During transportation, helicopters are commonly employed to transfer the newly harvested moss from the swamp to the nearest road.
Sphagnum moss can potentially harbour the fungi causing the chronic disease sporotrichosis. Sporothrix schenckii spores enter the skin via abrasions, scratches, and small puncture wounds as a result of unprotected contact exposure to Sphagnum moss.
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