Mosses are members of the green plant lineage, showing alternation of generations, with an unbranched persistent sporophyte generation dependent on the dominant leafy gametophyte generation. They are the third most speciose group of land plants, and are important members of most terrestrial ecosystems.
The mosses are one of the lineages descended from the earliest plants to colonize the land, and are still a very widespread and successful group of plants. There are more species of moss (10 000 +) than any other land-plant group except the angiosperms and ferns. Mosses are abundant in the natural environment and found in nearly all terrestrial and freshwater habitats. They are also found in urban environments, in lawns, on walls, and between cracks in paving stones. Despite their small size (most are between 0.1 and 5 cm tall) mosses have a complex morphology and biology. They are also very attractive plants, whether as velvety green carpets or when examined under the microscope.
Current treatments that provide a global classification of moss genera differ in the placement of some genera and in the recognition, circumscription and ranking of some higher level taxa. The most recent classification includes 25 orders and 116 families with 858 genera.
The orders are: Sphagnales; Ambuchananiales; Taka-kiales; Andreaeales; Andreaeobryales; Tetraphidales; Buxbaumiales; Oedipodiales; Polytrichales; Diphysciales; Timmiales; Funariales; Encalyptales; Grimmiales; Archi-diales; Seligeriales; Dicranales; Pottiales; Splachnales; Orthotrichales; Hedwigiales; Bryales; ‘Rhizogoniales’ (paraphyletic); Hookeriales; Hypnales (including Leuco-dontales).
The economic importance of mosses is considerable, although rarely appreciated. The Sphagnum peat mosses have traditionally been significant as a fuel source and as a horticultural product (see below). Fuel peat derived from Sphagnum is used extensively in many areas, especially northern Europe and Ireland. In addition, Sphagnum moss may be used in pharmaceutical applications and (historically) as field dressings on battlefields, due to the presence of natural antibiotic compounds. Peatlands in Scotland and Ireland provide the regional flavours of malt whiskies. In China, mosses are secondary hosts for aphids that form gallnuts on sumac (Rhus). Gallnuts are used to produce tannic and gallic acid for use in medicine, and the food, tanning, dying and ink-making industries. The gallnut harvest is valuable to local communities in China (£7.5 million per year in the 1980s), and would be lost without the moss host. Moss is an important non-timber forest product, and large quantities are extracted from forests for decorative uses, with values reaching as much as $ 16 per dry pound. This extraction is often carried out illegally and is a severe threat to the forest environment when it exceeds the natural regeneration capacity.
The ‘invisible’ economic importance of mosses includes watershed protection and function as a carbon sink. In high rainfall areas mosses (often together with liverworts) act as a sponge that both slows and regulates the throughput of water and mineral nutrients, preventing erosion and drought. Very large amounts of carbon have been deposited in peat and moss lands since the last Ice Age, but now may be released into the atmosphere as a result of global warming. The carbon dioxide in the atmosphere may be increased by as much as 50% as a result of the decomposition of peat as the permafrost melts, further accelerating global warming.
Dried Sphagnum plants and Sphagnum peat are used as soil modifiers and are major components of potting composts. This market is estimated at $100 million annually in Canada, which accounts for only 1 % of the global peat market. Dried forest moss is used for lining plant containers, and for growing epiphytic plants such as orchids in greenhouses. Mosses are valued in Japanese gardens, in the highly stylized Zen gardens and in the more relaxed woodland gardens, and moss gardening is becoming increasingly popular in Western countries.
Moss plants consist of two alternating generations that normally persist for weeks or months. The leafy green haploid gametophyte supports the spore-producing diploid sporophyte. The moss stem may be upright, creeping or hanging, is simple or branched, and normally bears leaves, rhizoids and gametangia. Different branching patterns produce dense cushions, velvety swards, loose mats, extensive carpets, or individual tufts, feather-like and treelike forms. Stems may have a central strand of thin-walled elongate cells (hydroids) that are analogous (or possibly homologous) to vascular tissue. Leaves usually consist of a lamina that is one cell thick, often with a central midrib (costa) that may be short or long, and single, forked or double. Leaf margins may be entire, have microscopic teeth, or be strongly toothed, and may have borders or be incurved or recurved. Rhizoids attach the plant to the substrate, and consist of uniseriate branching filaments along the stem, in leaf axils, or at leaf bases. Many mosses reproduce vegetatively, fragmenting or producing specialized structures, including elongate branches that root and develop new plants, detachable branchlets and leaves, and microscopic structures (gemmae) of one or a few cells. These propagules can start a new moss colony in favourable habitats.
The gametangia consist of antheridia (elongate sac-like structures, with a short stalk and a cap that breaks apart to release numerous motile antherozoids) and archegonia, which consist of a short stalk, a ventral region containing a single egg cell, and an elongate neck opening at the tip. The haploid egg cell, produced by mitotic division, is fertilized by a haploid antherozoid, resulting in a diploid embryo. This develops into a diploid sporophyte, which consists of a foot embedded in the gametophyte, and a spore-producing capsule on a stalk (seta). The young sporophyte is protected by the remains of the archegonium and some of the stem tissues (calyptra). The capsule mouth is closed by a lid (operculum). Around the mouth is the peristome, which is thought to regulate the dispersal of spores. The peristome consists of either a ring of solid multicelled teeth (in nematodontous mosses) or one or two rings of flexible peristome teeth formed from cell wall remnants (in arthrodontous mosses). In arthrodontous mosses the outer ring (exostome) is composed of 8 or 16 (sometimes more) exostome teeth, often brownish and ornamented with papillae or striations. The inner ring (endostome) is usually thin and membranous, and when complete consists of a basal membrane surmounted by tooth-like segments interspersed with wisp-like cilia. In the haplolepidous mosses only the endostome is present, with 16 or 32 deeply divided segments that resemble an exostome. In the capsule diploid spore mother cells undergo meiosis, resulting in haploid spores (thousands from a single capsule). These germinate in suitable conditions to form a thread-like protonema, on which buds form and develop into the leafy gametophyte shoots.
Mosses grow in virtually every terrestrial ecosystem, in high montane and polar regions, deserts, and tropical, temperate and boreal forests. In cold or dry deserts there may be high species diversity although the plants are often very small and obscure. Together with algae, cyanobacteria and fungi, the mosses form a ‘cryptogamic crust’ that protects the fragile soil from erosion by wind and rain, and enriches the soil by trapping nutrients and fixing atmospheric nitrogen. The highest biomass of mosses is found in very wet cool regions, such as the west coast of America, Western Europe, and montane and cloud forests in the tropics. In such regions the ground and tree trunks may be covered in mosses (and liverworts) several inches to a foot or more thick, and hanging moss curtains several feet long. Peatlands, with Sphagnum and many other mosses, cover very large areas of the northern hemisphere. Sphagnum can obtain minerals from rainwater by exchanging them for hydrogen ions, producing a highly acid environment that prevents decomposition and results in peat accumulation.
Mosses depend on surface water for sexual reproduction, and only grow actively when wet. However, many tolerate short dry periods by becoming inactive, or long dry periods by dying back and then regrowing from desiccation resistant spores, tubers or other propagules. Some desert mosses (e.g. Tortula species) have cellular repair mechanisms that allow them to return rapidly to active growth after severe drought. Different growth strategies allow mosses to colonize a wide range of habitats. Ephemeral and colonist plants grow rapidly in favourable conditions, produce large numbers of spores or other propagules, and then die back as conditions deteriorate, whereas perennials persist for long periods in a single location, growing only when conditions are suitable.
Intact moss communities retain water and trap nutrients. Within the communities live large numbers of insects and spiders, microscopic invertebrates such as rotifers, tardigrades (moss piglets) and nematodes, unicellular protozoans, algae, fungi and bacteria. Some of these organisms live in the moss as larvae, or use it as a pupation site, but many live entirely within the moss community. Rotifers trap smaller organisms and particles using rotating filters, and fungi and bacteria are saprophytic or parasitic. Birds and small mammals feed on the invertebrates, and use moss for nesting or bedding material. Mosses provide a germination-bed for seeds of trees and other plants.
Moss spores are very small and light and resistant to desiccation, and if they reach the upper airstreams can be carried very long distances. Some species (for example, Bryum argenteum Hedw., Funaria hygrometrica Hedw.)are found worldwide in suitable habitats (on walls and between paving stones, and on old bonfire sites), and many other species have very wide distributions. However, species that occur in very specific habitats are restricted to the range of those habitats. Many large groups of mosses are restricted to, or most abundant and speciose in, broad climatic zones. For example, the families Pterobryaceae and Meteoriaceae are found primarily in tropical regions, whereas the Amblystegiaceae occurs mostly in temperate and boreal regions. Several groups are largely restricted to the southern hemisphere (e.g. Rhizogoniaceae, Dicnemon), a distribution that may reflect origin prior to the break-up of Gondwanaland.
The mosses, with radially symmetric leafy stems, multicellular rhizoids, and persistent sporophytes dependent on the gametophyte, form a monophyletic lineage supported by both morphological and molecular data. However, the relationship of the moss lineage to other ‘bryophyte’ lineages (hepatics, hornworts, fossils) and to the vascular plants is as yet unresolved. It is possible that some or all of the liverworts plus the mosses are sister to the vascular plants, and that hornworts are basal in the land-plant phylogeny, but this question is still subject to active research. The peat moss (Sphagnum) lineage appears to have very ancient origins, but the extant taxa may represent a very rapid and recent divergence. A curious small genus, Takakia, originally considered a hepatic until sporophytes were found in 1993, is often placed as sister to Sphagnum in a position basal relative to all the other moss lineages. The relationships of the rock-mosses (Andreaea, Andreao-bryum) and Oedipodium, are still unclear, but all the other mosses form a monophyletic group marked by the presence of a peristome. In the Polytrichales (hair-cap mosses) the peristome consists of small solid teeth fused to a membrane (epiphragm) that covers the capsule mouth, and there are ridges of photosynthetic cells covering the leaf surface.
Mosses in the arthrodontous lineage share flexible peristome teeth and show enormous diversity. The relationships of the major groups (haplolepidous mosses with one row of peristome teeth, diplolepidous-opposite mosses with two rows of teeth in opposite ranks, and diplolepidous mosses with two rows of teeth in alternating positions) are not yet fully resolved. The diplolepidous-opposite mosses (e.g. Encalyptales, Funariales, Timmiales) may form a paraphyletic grade from which the other two groups are derived, or may be placed with either of the other lineages. The haplolepidous mosses (e.g. Dicranales, Pottiales) include many taxa characteristic of exposed and extreme habitats. Many of the diplolepidous-alternate mosses occupy similar habitats, but the large group of pleurocarpous mosses are characteristic of mesic and tropical forests. The pleurocarpous mosses consist of two or more lineages (the monophyletic Hookeriales + Hypnales + Leucodontales lineage and the bryalean pleurocarps) that may not be closely related, have sporophytes on reduced lateral branches rather than on stem apices, and a great diversity of branching patterns.
Mosses seem to be poorly represented in the fossil record. Spores are more resistant and fossilize more readily than macrofossils, but most show little morphological variation. Macrofossils (leaves, stems, sporophytes) are usually fragmentary and intact plants are rare. Recent and Quaternary material (sub-fossil and fossil) is abundant in peat bogs and postglacial soil horizons, and is used in palaeoecological studies. Mosses in amber (e.g. from the Baltic and Dominican Republic) mostly date from the Miocene and Oligocene (to 35 million years ago), and in many cases can be identified as extant species or genera. There is then an enormous gap in the fossil record until the Permian (260 Ma and older) with only a few fossils from the late Cretaceous (85 Ma) and the early Jurassic (180- 210 Ma). The late Cretaceous fossils have been assigned to extant genera and families, whereas the Jurassic and Permian fossils have usually been placed in the fossil formgenus Muscites, with possible affinities to extant taxa noted. Very early fossils, from the Devonian and Silurian (360-440 Ma), may represent bryophytes, possibly hepatics, but there are none that can be confidently placed in the moss lineage.
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