"Muscinae" from Ernst Haeckel's Kunstformen der Natur, 1904
Moss is the common name for any of the small, green, non-vascular land plants of the division Bryophyta, characterized by multi-cellular rhizoids, a gametophyte-dominant life cycle, and typically the presence of clearly differentiated stems and leaves. The division Bryophyta used to include the hornworts and liverworts, but they are now placed in their own divisions. These three groups are still grouped together as "bryophytes" because of their similarity as non-vascular, land plants.
Small, soft plants, mosses typically are one to ten centimeters (0.4-4 inches) tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems. At certain times mosses produce spore capsules that may appear as beak-like capsules borne aloft on thin stalks.
There are other non-bryophyte organisms that use the common name of moss, such as Irish moss (Chondrus crispus), a species of red algae; reindeer moss (Cladonia rangiferina) and Iceland moss (Cetraria islandica), which are lichens; Spanish moss (Tillandsia usneoides), which is flowering plant in the family Bromeliaceae; and club mosses of the class Lycopodiopsida and division Lycopodiophyta. This article will be restricted to members of the Division Bryophyta.
Mosses provide important ecological functions, including soil formation, erosion prevention, release of nutrients from substrates, and providing food for some animals, such as insects. For humans, some moss is commercially valuable, such as peat from Sphagnum, which is used as fuel, as a soil additive, and for other uses. While moss in some contexts is considered as a weed, such as growing in grass lawns, it also is popular for a number of aesthetic purposes, such as in Japanese gardens, in old temple gardens, and for some home decoration and floral display purposes. Moss is thought to add a sense of calm, age, and stillness to a garden scene.
Mosses comprise a division of bryophyte plants, which are non-vascular land plants (embryophytes), meaning that they lack water- and food-conducting strands in their roots (xylem and phloem), or that they are poorly developed. Bryophytes do not have roots, only filamentous rhizoids. Bryophytes do require water to propagate, and thus live in water or moist habitats.
Mosses are one of three main groups of bryophytes, the others being liverworts (division Marchantiophyta) and hornworts (division Anthocerotophyta). Originally these three groups were placed together as three separate classes or phyla within the division Bryophyta, with mosses comprising the taxon Musci. However, it was determined that these three groups together form a paraphyletic group, and thus they now are placed in three separate divisions. Together, they are still labeled bryophytes because of their similarity as non-vascular, land plants, but the Division Bryophyta now typically refers only to the mosses. (Algae are also non-vascular, but are not land plants. Those land plants that are so familiar to us—flowering plants, conifers, ferns, and so forth—have a vascular system and true roots.)
Like the vascular plants, bryophytes (mosses, liverworts, and hornworts) do have differentiated stems, and although these are generally only a few millimeters tall, they do provide mechanical support. They also have leaves, although these typically are one cell thick and lack veins. However, they lack true roots, with their filamentous rhizomes having a primary function of mechanical attachment rather than extracting soil nutrients (Palaeos 2008).
Mosses can most reliably be distinguished from the apparently similar liverworts because the mosses have multi-cellular rhizoids, while the liverworts have single-celled rhizoids (Nehira 1983). Other differences are not universal for all mosses and all liverworts (Schofield 1985); however, the presence of clearly differentiated "stems" and "leaves," the lack of deeply lobed or segmented leaves, and the absence of leaves arranged in three ranks, all point to the plant being a moss (Allison and Child 1975). In addition, ninety percent of liverworts contain oil bodies in at least some of their cells, and these cellular structures are absent from most other bryophytes (and from all vascular plants) (Bold et al. 1987). The overall physical similarity of some mosses and leafy liverworts means that confirmation of the identification of some groups can be performed with certainty only with the aid of microscopy or an experienced bryologist.
There are approximately 12,000 species of moss classified in the Bryophyta (Goffinet and Buck 2004). They typically are quite small plants, from one to ten centimeters (0.4-4 inches) tall, but there are species that can reach one meter (40 inches) in height. Such aquatic or semi-aquatic mosses can greatly exceed the normal range of lengths seen in terrestrial mosses. Individual plants 20 to 30 centimeters (8-12 inches) or more long are common in Sphagnum species, for example.
Mosses have a gametophyte-dominant life cycle. In other words, for most of its life cycle, the plant's cells are haploid. The diploid body, or sporophyte, is short-lived and dependent on the haploid body, or gametophyte. This is in contrast to the pattern exhibited by most "higher" plants and by most animals. In seed plants, for example, the haploid generation is represented by the pollen and the ovule, while the diploid generation is the familiar flowering plant.
In other words, most kinds of plants, excluding algae and bryophytes, have two sets of chromosomes in their vegetative cells and are said to be diploid—in other words, each chromosome has a partner that contains the same, or similar, genetic information. Mosses (and other bryophytes) have only a single set of chromosomes (haploid—in other words, each chromosome exists in a unique copy within the cell). There are periods in the moss lifecycle when they do have a full, double set of paired chromosomes but this is only during the sporophyte stage.
The life of a moss starts from a haploid spore, which germinates to produce a protonema, which is either a mass of filaments or thalloid (flat and thallus-like). This is a transitory stage in the life of a moss. From the protonema grows the gametophore ("gamete-bearer") that is differentiated into stems and leaves. From the tips of stems or branches develop the sex organs of the mosses. The female organs are known as archegonia (sing. archegonium) and are protected by a group of modified leaves known as the perichaetum (plural, perichaeta). The archegonia have necks called venters, which the male sperm swim down. The male organs are known as antheridia (singular antheridium) and are enclosed by modified leaves called the perigonium (plural, perigonia).
Mosses can be either dioicous (compare dioecious in seed plants) or monoicous (compare monoecious). In dioicous mosses, both male and female sex organs are borne on different gametophyte plants. In monoicous (also called autoicous) mosses, they are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilization occurs, leading to the production of a diploid sporophyte. The sperm of mosses is biflagellate, in other words, they have two flagellae that aid in propulsion. Since the sperm must swim to the archegonium, fertilization cannot occur without water. After fertilization, the immature sporophyte pushes its way out of the archegonial venter. It takes about a quarter to half a year for the sporophyte to mature. The sporophyte body comprises a long stalk, called a seta, and a capsule capped by a cap called the operculum. The capsule and operculum are in turn sheathed by a haploid calyptra, which is the remains of the archegonial venter. The calyptra usually falls off when the capsule is mature. Within the capsule, spore-producing cells undergo meiosis to form haploid spores, upon which the cycle can start again. The mouth of the capsule is usually ringed by a set of teeth called peristome. This may be absent in some mosses.
In some mosses, such as Ulota phyllantha, green vegetative structures called gemmae are produced on leaves or branches, which can break off and form new plants without the need to go through the cycle of fertilization. This is a means of asexual reproduction, and the genetically identical units can lead to the formation of clonal populations.
Mosses are found chiefly in areas of dampness and low light. Mosses are common in wooded areas and at the edges of streams. Mosses are also found in cracks between paving stones in damp city streets. Some types have adapted to urban conditions and are found commonly only in cities. A few species are wholly aquatic, such as Fontinalis antipyretica, and others such as Sphagnum inhabit bogs, marshes, and very slow-moving waterways.
Wherever they occur, mosses require moisture to survive because of the small size and thinness of tissues, lack of cuticle (waxy covering to prevent water loss), and the need for liquid water to complete fertilization. Some mosses can survive desiccation, returning to life within a few hours of rehydration.
In northern latitudes, the north side of trees and rocks will generally have more moss on average than other sides (though south-side outcroppings are not unknown). This is assumed to be because of the lack of sufficient water for reproduction on the sun-facing side of trees. South of the equator the reverse is true. In deep forests where sunlight does not penetrate, mosses grow equally well on all sides of the tree trunk.
The mosses are grouped as a single division, now named Bryophyta, and typically divided into six classes (Buck and Goffinet 2000):
Andreaeopsida and Andreaeobryopsida are distinguished by the biseriate (two rows of cells) rhizoids, multiseriate (many rows of cells) protonema, and sporangium that splits along longitudinal lines. Most mosses have capsules that open at the top.
The Sphagnopsida, the peat mosses, comprise the two living genera Ambuchanania and Sphagnum, as well as fossil taxa. These large mosses form extensive acidic bogs in peat swamps. The leaves of Sphagnum have large dead cells alternating with living photosynthetic cells. The dead cells help to store water. Aside from this character, the unique branching, thallose (flat and expanded) protonema, and explosively rupturing sporangium place it apart from other mosses.
Polytrichopsida have leaves with sets of parallel lamellae, flaps of chloroplast-containing cells that look like the fins on a heat sink. These carry out photosynthesis and may help to conserve moisture by partially enclosing the gas exchange surfaces. The Polytrichopsida differ from other mosses in other details of their development and anatomy too, and can become larger than most other mosses, with Polytrichum commune forming cushions up to 40 centimeters (16 inches) high. The tallest land moss, a member of the Polytrichidae, is probably Dawsonia superba, a native to New Zealand and other parts of Australasia.
The Bryopsida are the most diverse group; over ninety-five percent of moss species belong to this class.
The Archidiidae are distinguished by their extremely large spores and the way the sporangium develops.
The fossil record of moss is sparse, due to their soft-walled and fragile nature. Unambiguous moss fossils have been recovered from as early as the Permian of Antarctica and Russia, and a case is put forwards for Carboniferous mosses (Thomas 1972). It has further been claimed that tube-like fossils from the Silurian are the macerated remains of moss calyptræ (Kodner and Graham 2001).
Moss is considered a weed in grass lawns, but is deliberately encouraged to grow under aesthetic principles exemplified by Japanese gardening. In old temple gardens, moss can carpet a forest scene. Moss is thought to add a sense of calm, age, and stillness to a garden scene. Rules of cultivation are not widely established. Moss collections are quite often begun using samples transplanted from the wild in a water-retaining bag. However, specific species of moss can be extremely difficult to maintain away from their natural sites with their unique combinations of light, humidity, shelter from wind, and so forth.
Growing moss from spores is even less controlled. Moss spores fall in a constant rain on exposed surfaces; those surfaces that are hospitable to a certain species of moss will typically be colonized by that moss within a few years of exposure to wind and rain. Materials that are porous and moisture retentive, such as brick, wood, and certain coarse concrete mixtures are hospitable to moss. Surfaces can also be prepared with acidic substances, including buttermilk, yogurt, urine, and gently puréed mixtures of moss samples, water and ericaceous compost.
Moss growth can be inhibited by a number of methods:
Heavy traffic or manually disturbing the moss bed with a rake will also inhibit moss growth.
The application of products containing ferrous sulfate or ferrous ammonium sulfate will kill moss; these ingredients are typically in commercial moss control products and fertilizers. Sulfur and Iron are essential nutrients for some competing plants like grasses. Killing moss will not prevent regrowth unless conditions favorable to their growth are changed (Whitcher 1996).
A passing fad for moss-collecting in the late nineteenth century led to the establishment of mosseries in many British and American gardens. The mossery is typically constructed out of slatted wood, with a flat roof, open to the north side (maintaining shade). Samples of moss were installed in the cracks between wood slats. The whole mossery would then be regularly moistened to maintain growth.
There is a substantial market in mosses gathered from the wild. The uses for intact moss are principally in the florist trade and for home decoration. Decaying moss in the genus Sphagnum is also the major component of peat. Peat is a dark, fibrous accumulation of partially decomposed and disintegrated organic matter found in wet areas, usually comprising residues of plants such as mosses. Peat formed from decayed, compacted Sphagnum moss may sometimes be labeled as sphagnum peat; however, peat can form from a wide variety of plants, as well as include partially decayed organic matter of animals. Peat is "mined" for use as a fuel, as a horticultural soil additive, and in smoking malt in the production of Scotch whisky. In its natural setting, peat can help in flood mitigation. Longer term, peat is a early transition stage in the formation of coal.
Sphagnum moss, generally the species cristatum and subnitens, is harvested while still growing and is dried out to be used in nurseries and horticulture as a plant growing medium.
In World War II, Sphagnum mosses were used as first-aid dressings on soldiers' wounds, as these mosses are highly absorbent and have mild antibacterial properties. Some early people used it as a diaper due to its high absorbency.
In rural UK, Fontinalis antipyretica was traditionally used to extinguish fires as it could be found in substantial quantities in slow-moving rivers and the moss retained large volumes of water, which helped extinguish the flames. This historical use is reflected in its specific Latin/Greek name, the approximate meaning of which is "against fire."
Physcomitrella patens is increasingly used in biotechnology. Prominent examples are the identification of moss genes with implications for crop improvement or human health (Reski and Frank 2005), and the safe production of complex biopharmaceuticals in the moss bioreactor, developed by Ralf Reski and his co-workers (Decker and Reski 2007).
All links retrieved November 24, 2014.
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