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There are approximately 14,500 species of mosses. They are world-wide in distribution and can be found at sea level as well as the highest altitudes occupied by plants. Although they can occur in deserts or be submerged in water, most mosses occupy moist, shaded habitats.

Moss gametophytes are either erect or extensively branched prostrate plants that consist of an axis (commonly called a stem) bearing spirally arranged leaf-like appendages (usually referred to as leaves); more often than not, they are anchored to the substratum by branched multicellular filaments called rhizoids.

Although variable in shape, moss leaves usually consist of a single cell layer and are traversed by a midrib that is always more than one cell in thickness; it may extend from the base of the leaf to the tip or beyond, or may terminate some distance from the tip. The margins of the leaves are often toothed, the teeth pointed or rounded.

The haploid gametophyte is the long-lived or dominant generation and is, therefore, the plant most likely to be encountered in the field. The smallest gametophytes are only a few millimeters in length whereas the largest attain lengths of 70 centimeters or more. Although morphologically diverse, moss gametophytes are not as variable in form as are those of the leafy liverworts.

The sporophytes of mosses, like those of the liverworts, consist of a foot, seta and capsule and remain permanently attached to the gametophytes. They are borne at the tips of erect gametophytes (a) or the tips of the short lateral branches of prostrate gametophytes (b).

During the early stages of development, the sporophyte is completely surrounded by a tough protective covering called the calyptra. As the sporophyte grows and enlarges, the calyptra is carried up into the air and eventually comes to sit atop the capsule like a tiny hood (a). When the sporophyte is mature, the calyptra is shed and the capsule is revealed (b). At the tip of the capsule is a lid (operculum) which, prior to spore dissemination, falls away exposing the so-called peristome teeth, a set of structures, often delicate and thread-like in appearance, that form a ring around the mouth (rim) of the capsule (c). The peristome teeth are perhaps the most characteristic feature of the mosses; usually composed of cell wall remnants, they respond to changes in the humidity of the atmosphere. Under conditions of low humidity, the teeth dry out and splay away from the mouth of the capsule, thus, allowing the commonly more than 50,000 spores within to be gradually released.

Under suitable conditions, a spore will germinate and give rise to a microscopic chlorophyllous branched filament from which, eventually, the leafy gametophytes will arise. The gametophyte generation of the mosses is, thus, dimorphic.

The moss usually illustrated in introductory botany textbooks is Polytrichum. This relatively large (25-30 cm in height), rather widely distributed and, therefore, readily available moss is structurally more complex than other mosses. An unusually broad midrib, bearing unistratose parallel plates (lamellae) of chlorophyllous tissue, extends from the base of the leaf to the tip. The gametophytes and sporophytes, although devoid of xylem and phloem, contain water- and food-conducting cells analogous to those of vascular plants. The mouth of the capsule is covered by a membrane that is overarched by massive multicellular peristome teeth the tips of which are joined to the membrane.

Included in the mosses is the rather common and widely distributed Sphagnum, a genus of more than 300 species. It grows best in sites where moisture is abundant.

Although the gametophytes of Sphagnum, like those of other mosses, are comprised of a stem and leaves, Sphagnum differs from all other mosses in a number of important ways. The erect, 5-10 cm long main stem terminates in a dense cluster of young branches. Below the tip are whorls of branches; the branches of the topmost whorl grow out horizontally whereas the branches of the whorls farther down are pendant, some twisting around the main stem. The mature plants are devoid of rhizoids. The unistratose leaves are made up of two cell types: larger, hyaline, dead cells (a) with spiral thickenings and circular pores (c) and smaller, relatively narrow, elongated cells that contain chloroplasts (b) and surround the hyaline cells; a midrib is lacking.

The sporophyte of Sphagnum consists of a nearly globose capsule and a foot; a seta is absent. At maturity, the sporophyte is elevated into the air by a stalk of gametophytic origin termed the pseudopodium. The capsule, like that of other mosses, is provided with an operculum, but a peristome is lacking and spore discharge, rather than being gradual, is explosive. Upon germination, each spore gives rise to a unistratose sheet of cells that forms multicellular masses (buds) from which the leafy gametophytes arise.

Sphagnum has a remarkable capacity to absorb and retain water; it can hold 20-30 times its weight of water. The unusual water-holding capacity of Sphagnum derives from the large hyaline cells in the leaves and, in some species, the dead porous cells of the stem, the closely overlapping, spirally arranged leaves of the branches which hold water by capillarity, and the twisting of some of the pendant branches around the main stem to form a structure that functions like a wick.

Sphagnum has the distinction of being the only economically important bryophyte. Along with other plants growing among it, Sphagnum becomes compacted into peat which, in places such as Scandinavia and Ireland, is dried and used as fuel. Thus Sphagnum is commonly known as peat moss. Due to its tremendous water-holding capacity, Sphagnum is used in packing live plants for shipment, is added to soil to improve its water-holding capacity and has been employed as an absorbing material for oil spills. Because of its absorptive capacity as well as its antiseptic properties which result from the capacity of Sphagnum to increase the acidity of its environment, during World War I and the Franco-Prussian and Russo-Japanese wars, Sphagnum was used for the manufacture of surgical dressings; the Eskimo used Sphagnum to curb diaper rash.


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