Moss
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- This is an article about the plant. For other uses, see Moss (disambiguation)
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 A moss-covered log | ||||||
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Mosses are a type of simple or non-vascular plant. They can be distinguished from the apparently similar liverworts or Marchantiopsida (formerley the Hepaticae) by their multi-cellular rhizoids. Other differences are not universal for all mosses and all liverworts but the presence of clearly differentiated stem and leaves, the lack of deeply lobes or segmented leaves,and the absence of leaves arranged in three ranks all point to the plant being a moss.
Aside from lacking a vascular system, they have a gametophyte-dominant life cycle, i.e. the plant's cells are haploid for most of its life cycle. Sporophytes (i.e. the diploid body) are short-lived and dependent on the gametophyte. This is in contrast to the pattern exibited by most higher plants and animals. In higher plants, for example, the haploid generation is represented by the pollen and the ovule whilst the diplod generation is the familiar flowering plant.
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Life cycle
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 ('microphylls'). From the tips of stems or branches develop the sex organs of the mosses. The female organs are known as archegonia (singular 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) or monoicous (compare monoecious). In dioicous mosses, both male and female sex organs are borne on different 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 fertilisation occurs, leading to the production of a diploid sporophyte. The sperm of mosses is biflagellate, i.e. they have two flagella that aid in propulsion. Without water, fertilisation cannot occur. 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, 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.
Classification of mosses
Three_mosses_and_a_tree.jpg
Mosses were traditionally grouped with the liverworts and hornworts in the Division Bryophyta (bryophytes), within which the mosses made up the class Musci. This group, however, is paraphyletic and now tends to be split up. In such system, the Division Bryophyta refers specifically to mosses. They appear to be the closest living relatives of the vascular plants.
The mosses are grouped as a single class, now named Bryopsida, and divided into seven subclasses:
- Andreaeidae 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 Sphagnidae, the peat-mosses, comprise the single genus Sphagnum. These 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.
- The Tetraphidae are unique as their name implies, in having only four large peristome teeth surrounding the opening of the capsule.
- Polytrichidae have leaves with lamellae, which are flaps on the leaves that look like the fins on a heat sink. These help it retain moisture. They differ from other mosses in other details of their development and anatomy too.
- The Buxbaumiidae are called 'bug mosses' because they usually have a very small and reduced gametophore and the whole plant is mostly the sporophyte capsule. The shape reminds one of a bug, which is the reason for its common name.
Most (>95%) mosses belong to the Bryidae.
The Archidiidae are distinguished by their extremely large spores and the way the sporangium develops.
Habitat
Mossforest.jpg
Mosses are found chiefly in areas of low light and damp; any area of the world. Mosses are common in wooded areas and at the edges of streams. They are also found in cracks between paving stones in damp city streets. They 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 fertilisation. Some mosses can survive desiccation, returning to life within a few hours of rehydration.
In northern latitudes, the north side of trees generally will have more moss on average than other sides. South of the equator the reverse is true.
Cultivation
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 site (with its unique combination of light, humidity, shelter from wind, etc).
Growing moss from spore is even less controlled. Moss spores fall in a constant rain on exposed surfaces - those surfaces which 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 which 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 pureed mixtures of moss samples, water and ericaceous compost.
Mossery
A passing fad for moss collecting in the late 19th 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.