Bryophytes are the second largest group of land plants, after the flowering plants, with about 20,000 described species. Because of their small size and often delicate structure, bryophytes have a poor fossil record, dating back only about 290 million years. Because bryophyte species are often closely tied to specific environmental conditions, it is considered likely that bryophyte diversity is currently at its greatest level: the group evolved in conjunction with the evolution of flowering plant communities. Bryophytes are characterized by the dominant, leafy, or thallose (that is, a more or less flattened plant without differentiation into stem and leaves) plants being the gametophyte generation (the generation with a single set of chromosomes, which produces the male and female gametes). The reduced sporophyte generation (with two sets of chromosomes) is dependent upon the photosyn-thetic gametophyte for part of its existence, if not all of it. Typically, the sporophyte generation consists of a spore-producing capsule that usually is elevated on a stalk (seta). The leaves of bryophytes are mostly a single cell thick and either lack any sort of midrib or, especially in the mosses, have a single, unbranched midrib or a double midrib, forked from the base of the leaf. The midrib is never branched. Some bryophytes, such as the hornworts and some groups of liverworts, have no leaves but rather are characterized by a flat thallus, usually closely appressed to the substrate, and often several cell layers thick. The stomata found on leaves in flowering plants, ferns, and the like to allow oxygen and carbon dioxide to get into and out of the cells are found only in the capsules of mosses and hornworts, but they are lacking in liverworts. Both in flowering plants and bryophytes the stomata are found on the part of the plant with two sets of chromosomes.
Throughout the bryophytes, evolution has proceeded mostly toward the simplification of plants rather than toward more complex structures. Ecologically, although individual plants are small, bryophytes frequently play significant roles in the environment because they often occur in large populations. They are very important in maintaining humidity levels in ecosystems and in slowing soil erosion, as well as in mineral recycling. Because of their delicate structure, they are particularly sensitive to pollution and have been used to monitor air and water quality. Conservation of bryophytes depends upon conservation of their habitats. Unlike larger plants, individual bryophyte plants are often unable to survive outside of their natural habitats. Because of dissimilarities between the various groups of bryophytes, as well as recent evidence from DNA sequencing, it is now thought that bryophytes are not a natural group, and rather that the various components are not closely related. However, because of the dominant gametophyte generation, which is unique among land plants, they are often studied as a group. Following are the major groups of bryophytes.
Phylum Bryophta (mosses). These are bryophytes with leafy gametophytes and long-lived sporophytes, typically with the capsule elevated on a seta. The rootlike structures (rhizoids), which are primarily for adhering the plants to the substrate and have no transport capabilities, are multicellular, with the numerous cells in a single row. The remnants of the archegonium (which surrounds the egg cell) expand after fertilization and form a protective cap (calyptra) over the capsule. The capsules are typically dehiscent, with a lid (operculum), allowing spore release and dispersal. Most species have specialized hygroscopic structures (peristome teeth) around the mouth of the capsule to aid in spore dispersal.
Class Bryopsida (true mosses): True mosses are characterized by capsules that open because of an operculum and with peristome teeth (two-layered toothlike structures, usually sixteen in number, around the mouth of the capsule, composed of dead cell walls only at maturity, which are hygroscopic because of the different thicknesses of the two layers, thus aiding in spore dispersal). The capsule also has stomata, and internally the spores are formed around a central column (columella). This is the largest group of bryophytes, and the most conspicuous, with approximately 13,000 species. They are often the dominant component of the landscape in the Arctic and the Antarctic, as well as at very high elevations. Some forests in particularly humid areas are designated as "mossy forests" because mosses (and liverworts) form great sheaths around the tree trunks. Mosses range in size from minute plants no more than a millimeter tall, growing on soil and sometimes completing their life cycle (from spore germination to spore production) in only a matter of months, to long-lived perennial plants more than a meter in length. They occur in almost all habitats except those in direct contact with salt water. They are especially prominent in extreme habitats. For example, on the Antarctic continent, there are thirty to forty species of mosses, but only two of flowering plants. The true mosses are standardly divided into two
groups based on the habit of the plants. The presumably primitive acrocarps are mostly erect, unbranched plants growing in dense tufts, with their capsules arising from the stem tip. The more specialized pleurocarps are mostly prostrate, branched plants forming mats, with their capsules arising from along the sides of the stems. In the tropics the pleuro-carpous mosses often form long, pendent masses cascading from tree branches. This is also the group to which "sheet moss" in the horticultural trade belongs.
Class Andreaeopsida (granite mosses): Characterized by capsules that dehisce by longitudinal slits, and with no peristome or stom-ata, this relatively small group of mosses (approximately 120 species) occurs worldwide, and, as the common name suggests, usually on acidic rocks. The capsules are partic ularly unusual within the mosses, and instead are more like those of some liverworts.
Class Sphagnopsida (peat mosses): Characterized by branches occurring in groups (fascicles) along the stems, the leaves of the peat mosses are composed of two types of cells forming a reticulum of a single cell layer: small green cells and large, hollow, dead cells. The capsules lack a peristome but have an opercu-lum and dehisce explosively because of an increase of internal air pressure. The internal pressure can reach four to six atmospheres, a pressure similar to that in the tires of tractor-trailers, and the plants can be heard when they dehisce. Although a relatively small group (there are approximately 250 species in the single genus Sphagnum), the peat mosses are probably the best known bryophytes because of their use as horticultural peat and also as a source of fuel. However, as living plants they are very important in nature. In Arctic areas they are largely responsible for drainage patterns. The peat mosses are also able to absorb large quantities of water because of the dead hollow cells of the leaves, and as a result of physiological reactions they can increase the acidity of the water in which they live. Because of the absorptive properties of the peat mosses, they were traditionally used as diapers and even socks. During World War I the peat mosses were harvested and used as surgical bandages, not only because of their ability to absorb large quantities of blood but also because of natural antiseptic qualities. Even now one major manufacturer uses milled peat moss as the absorbent portion of their "all natural" menstrual pads.
Phylum Marchantiophyta (liverworts): These are bryophytes with either leaves or just a flattened thallus; when leafy, the leaves are often deeply lobed and never have a midrib. The cells of the gametophyte have a unique organelle found in no other group of organisms: the oil body. Liverworts are well known for their complex chemistry, whereas mosses usually have a very simple chemistry. Many of the complex chemical compounds are contained within the oil bodies. These structures must be viewed in living plants, or those recently collected, because they disintegrate with age. The rhizoids, which primarily hold the plant to the substrate, are composed of a single cell. The capsules are often elevated on a very short-lived seta, sometimes lasting no more than a couple of hours. The capsules typically dehisce by splitting into several valves. The spores have sterile threads (elaters) among them that are hygroscopic and aid in spore dispersal. The capsules lack both stomata and a columella. The liverworts are mostly plants of moist environments, with about 6,000 to 8,000 species worldwide. They are of little economic value but are often of significant ecological value in forest habitats.
Class Marchantiopsida (thalloid liverworts): The Marchantiopsida are characterized by thalloid plants several cell layers thick. Often there are pores in the surface of the thallus that allow gas exchange for the internal cells. The sporophytes are often elevated on a complex structure of gametophytic tissue. The capsule walls are only one cell layer thick. This group includes the relatively large, coarse thalloid liverworts often seen growing along streams and in other areas of high humidity. Because of their size and their use in biology classes as a liverwort example, this group of liverworts is the best known, although it has only about 300 species.
Class Jungermanniopsida (leafy liverworts): This class is characterized by both leafy forms and thalloid forms. The capsules are typically elevated at maturity on a short-lived seta. The capsule walls are two or more cell layers thick. When thalloid the thalli are relatively thin and structurally simple. The leafy forms are the largest group of hepatics, with almost 300
genera and at least 5,000 to 6,000 species, mostly in the tropics. There are typically two kinds of leaves, larger lateral leaves that are either entire or lobed, and smaller leaves on the underside of the stems. Sometimes the under-leaves are lacking. Like other groups of liverworts, the Jungermanniopsida have little economic value. However, in areas that have high humidity and are major logging areas, such as the Pacific Northwest of North America, some liverworts, especially Frullania, are a significant cause of dermatitis because of the chemicals contained within the oil bodies.
Phylum Anthocerophyta (hornworts): Hornworts are bryophytes with a thin, mostly flattened thallus growing on bare mineral soil (rarely epiphytic in the tropics), and with slender, hornlike sporophytes. The cells of the thallus have only a single chloroplast, like many algae, but otherwise they are unique among land plants. Often there are colonies of Nostoc (cyanobacteria) embedded in the thalli. The sporophyte is unique in having a basal meristem, so that it continues to grow from the base throughout the life of the plant, with spores maturing toward the tip. The sporophyte typically splits along two longitudinal lines to release the spores. Intermixed with the spores are sterile, hygroscopic, spirally twisted structures (pseudoelaters) that aid in spore dispersal. The sporophyte has a columella (like mosses), and the wall has stomata. This small group of plants (with approximately six genera and 150 species) often grows on bare, disturbed, wet soil. Because of the thalloid gametophyte and unicellular rhizoids, the hornworts have been associated with the liverworts, but they are quite distinct. Recent research suggests that the hornworts may be the oldest living lineage of land plants.
See also: Angiosperms; Gymnosperms; Pteridophytes
Bates, Jeffrey W., Neil W. Ashton, and Jeffrey G. Duckett, eds. 1998. Bryology for the Twenty-first Century. Leeds, England: Maney; Crum, Howard. 1988. A Focus on Peatlands and Peat Mosses. Ann Arbor: University of Michigan Press; Crum, Howard. 2001. Structural Diversity of Bryophytes. Ann Arbor: University of Michigan Herbarium; Schuster, Rudolf M., ed. 1983-1984. New Manual of Bryology. 2 vols. Nichi-nan, Japan: Hattori Botanical Laboratory; Shaw, A. Jonathan, and Bernard Goffinet, eds. 2000. Bryophyte Biology. Cambridge: Cambridge University Press.
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