Bryophyte

Classification, characteristics, and habitats of bryophytes

Hepatophyta (division liverworts)

Hornworts (division Anthocerophyta)

Mosses (division Bryophyta)

Importance of mosses

Resources

Bryophytes include the mosses, liverworts, and hornworts. Bryophytes are the simplest of plants (excluding the algae, which are not considered plants by most botanists). Bryophytes are small, seldom exceeding 6-8 in (15-20 cm) in height, and usually much smaller. They are attached to the substrate (ground, rock, or bark) by rhizoids, which are one or a few-celled, root-like threads that serve only for anchoring and are not capable of absorbing water and nutrients from the substrate. Brypohytes lack vascular tissue (the specialized cells grouped together to pipe water and nutrients to various parts of the body), or in the rare cases when this tissue is present, it is not well differentiated. The leaves of bryophytes are technically not true leaves, because in most species they lack vascular tissue. However, they are functionally equivalent to leaves, containing chlorophylls a and b for photosynthesis. Leaves are usually one-cell thick, except for the midrib, which may be up to 15 cells thick. Bryophytes satisfy their nutritional requirements by absorbing minerals from dust, rainfall, and water running over their surface.

The life cycle of bryophytes is characterized by an alternation of generations, one of which is a multicellular, diploid individual called a sporophyte, having two of each type of chromosome per cell. This stage alternates with multicellular, haploid individual called the gametophyte, with only one of each type of chromosome per cell, as is also the case with animal sperm.

Bryophytes are unique among plants in that the dominant, conspicuous generation is the haploid gametophyte. In all other plants, the dominant stage is the diploid sporophyte.

Most reproduction of bryophytes is asexual, occurring by fragmentation of body parts, and by the production of specialized vegetative units called gemmae. Gemmae may be produced as microscopic plates (in the genus Tetraphis ), as bulbils in the axils of leaves (in Pohlia ), or as microscopic filaments (in Ulota ). When sexual reproduction occurs, it always involves a flagellated sperm (produced in a specialized organ called an antheridium) that must swim through water to reach an egg located in a specialized, flask-shaped organ (the archegonium). The antheridia and archegonia are surrounded by a layer of sterile cells, which protects the sex organs from mechanical damage and desiccation.

The union of the sperm and egg results in a diploid zygote, i.e., a new sporophyte. This is nourished by the gametophyte and grows on it in a parasitic fashion, although the sporophytes of some bryophytes photo-synthesize and make some contribution to their own growth. Initially, as the young sporophyte grows, the archegonium also enlarges. However, it ultimately fails to keep pace with the growth of the sporophyte and becomes detached from its base, forming a cap-like structure called a calyptra.

Classification, characteristics, and habitats of bryophytes

The classification of bryophytes has been controversial among botanists. Traditionally, the division Bryophyta has included the true mosses, liverworts, and hornworts. However, some scientists consider each of these groups sufficiently distinct to deserve their own division: Bryophyta for the mosses, Hepatophyta for the liverworts, and Anthoceratophyta for the hornworts. The latter view is followed here, although the bryophyte is used as a collective term for all of these.

About 15,000 species of bryophytes have been described. They are distributed throughout the world, and are especially abundant in arctic and boreal regions, where they often dominate the ground vegetation. Bryophytes also occur in humid tropical regions where they commonly grow on other plants, especially in higher-elevation forests. Bryophytes are considered the amphibians of the plant world, because they require abundant moisture to grow. This requirement for water results from a number of their characteristic features. Their stems and leaves are thin, and either lack a cuticle (that is, a waxy surface layer) or have a very thin one, making them prone to drying out. Because bryophytes lack roots and a vascular system, they cannot obtain water from the soil and transport it to above-ground tissues; for this same reason, bryophytes are necessarily small. In addition, their sperm require free water in order to swim from their parent plant to the egg on another plant.

Hepatophyta (division liverworts)

Hepatophyta means “liver plant” and refers to the body of some common species of liverworts, whose lobing is reminiscent of a liver. During Medieval and earlier times, many people followed the doctrine of signatures—a belief that the superficial resemblance of a plant to some part of the human anatomy indicated that the plant possessed medicinal properties related to the organ it resembled. Liverworts are the simplest of the living plants, and range in size from minuscule, leafy filaments less than 0.02 in (0.5 mm) in diameter, to plants exceeding 8 in (20 cm) in size. Liverworts lack specialized conducting tissues, cuticles, and stomates, and their rhizoids are always unicellular. The gametophytes arise directly from spores in most species. Most liverworts (75%) have nine chromosomes in their haploid cells. Based on body form, liverworts are categorized as either thallose or leafy.

Thallose liverworts have gametophytes with an undifferentiated body called a thallus which has a ribbon-like appearance. Marchantia is one of the most widely distributed thallose liverworts, especially in habitats that provide ideal conditions of light and high humidity. The body is typically 30 cells thick at the midrib, and only ten cells thick elsewhere. A thin, upper, green layer contains chlorophyll-rich cells, arranged in polygonal or diamond-shaped patterns each centered on a permanently open pore. Below each polygon is an airspace connected to the outside by the pore, and within the chamber are erect threads of photo-synthetic cells. Below the relatively thin, upper photo-synthetic layer is a lower layer that is colorless and stores the products of photosynthesis. Most reproduction is asexual by fragmentation, usually caused by wind or by animals breaking the plants apart while eating them or when stepping on them or when trampling them. Thallose liverworts also commonly reproduce asexually by producing small balls of cells called gemmae, within bowl-like structures called gemma cups. The balls become detached and are splashed out by raindrops, dispersing away to colonize favorable habitats.

Leafy liverworts grow in wet or humid habitats, and are especially common in the tropics and sub-tropics, although they also occur in temperate areas.

They are the simplest of the plants with leaflike structures. Their leaves lack vascular tissue, each is deeply cleft so as to appear two-lobed, and they are arranged in two rows along a much branched stem. Unlike the true mosses, which typically appear somewhat erect, leafy liverworts form small, flat mats. Asexual reproduction by fragmentation is common.

Hornworts (division Anthocerophyta)

The hornworts are the smallest of the three groups of bryophytes with only about 100 species in six genera. Hornworts are especially diverse in the tropics, although Anthoceros occurs in temperate regions.

The gametophyte of hornworts is saucer-shaped, with upturned edges, and only 0.4-0.8 in (1-2 cm) in diameter. These interesting little plants are more similar to algae than are any other plants, especially because they have only one, large chloroplast in association with a pyrenoid in each photosynthetic cell. Yet, the hornworts are more advanced in some ways than liverworts, for example, they possess stomates, which exchange gases between the plant and the air. They are also unique among plants in having stomates on their gametophytes. Unlike the liverworts in which internal spaces between cells are filled with air, in hornworts the cavities are filled with mucilage, a water-absorbing material within which the cyanobacterium Nostoc can be found. This symbiotic relationship greatly benefits the hornwort, because cyanobacteria are among the few organisms that can fix molecular nitrogen (as N₂) into nitrogen compounds that are in a form useable to plants as nutrients; no plants can produce these essential compounds on their own.

Hornworts derive their name from their sporophyte, which has the appearance of a tapered horn. The sporophyte has a mass of undifferentiated tissue called a meristem at its base. The meristem can actively grow, so that the sporophyte can continue to increase in height, especially if damaged at the top, and can reach a height of 0.4-1.6 in (1-4 cm). The sporophyte of hornworts possesses stomate-like openings and remains photosynthetic for several months. It is, therefore, only semi-dependent on the gametophyte to which it remains attached. The sporophyte of hornworts represents a transitional stage to more highly evolved plants such as ferns, in which the sporophyte is for the most part independent of the gametophyte.

Mosses (division Bryophyta)

Only members of the division Bryophyta are considered “true” mosses. Many other plants and some algae are commonly called mosses, because they superficially resemble the true mosses, but they are not in fact even closely related to them. For example, Spanish moss (Tillandia uneoides ) is a flowering plant in the pineapple family, Irish moss (Chondrus crispus ) is a red alga that is collected for the extraction of carageenan, a starch-like substance used in food preparation, reindeer moss (Cladina spp.) is a lichen, and club mosses (Lycopodium spp.) are advanced plants with well developed vascular systems.

A number of characteristics distinguish mosses from other bryophytes. Their gametophytes are leafy, whereas those of hornworts and thallose liverworts are not. The leaves of mosses occur in three ranks on the stem, but because the stem twists, they appear to have radial symmetry, the ability to be bisected into identical halves in more than one way. In contrast, leafy liverworts, whose leaves are two-ranked, only have one set of mirror images (bilateral symmetry). Furthermore, leaves of mosses are not lobed as in leafy liverworts. The rhizoids of mosses are multicellular, compared with single-celled in liverworts.

Mosses are distributed throughout the world, and are among the ecosystem dominants in boreal, arctic, and alpine environments. Mosses occur in a wide variety of habitats. They commonly grow on mineral and organic soils, and they can occur on volcanically heated soil that may reach temperatures of 131°F (55°C), on rocks in Antarctica where the temperature during the growing season does not exceed 14°F (-10°C), as epiphytes that grow on other plants, especially in tropical and subtropical regions, and in freshwater habitats. No mosses are truly marine, although some live within the spray zone of coastal habitats. The luminous moss Schistoste occurs within caves near the entrance where it concentrates the limited available light with the curved, lens-like surface of its leaves. The dung-loving species of the genus Splachnum are among the rarest and most beautiful of mosses. Their capsules occur on long, flimsy stalks, and the base of each capsule bears a thin, papery, brightly colored, umbrella-like structure-yellow in S. luteum and red in S. rubrum. The smallest mosses reach only 0.04-0.8 in (1-2 mm) in height, whereas the largest can grow to 20 in (50 cm). In temperate regions, mosses grow during cooler, wetter parts of the year, primarily during the autumn, mild spells in winter, and in early spring. The mosses naturally fall into three distinctive groups, taxonomically referred to as classes: Bryidae, the true mosses; Sphagnidae, the peat mosses; and Andreaeidae, the granite mosses.

True mosses

One of the most distinctive features of true mosses involves the development of their gametophytes. Spores germinate to produce a characteristic mass of algal-like threads, called protonema, which looks like a loose ball of wool. Bud-like structures develop later, and give rise to the familiar leafy gametophyte. Although mosses are considered non-vascular plants, many true mosses in fact have a primitive vascular system consisting of a central strand of water-conducting cells called hydroids. Some also have specialized cells around the column of hydroids called leptoids, which function in the transport of carbohydrates, the products of photosynthesis. The stems of true mosses are more-or-less uniformly leafy and erect. Their leaves usually have a midrib, and their sporophytes possess capsules that are borne on stalks that are made of sporophytic tissue. Also, the capsules contain one or two rows of toothlike appendages (peristome) over the opening of the capsule, which are exposed when the lid is shed. This is by far the largest group of mosses.

Peat mosses

This is a small, but extremely important group of mosses, numbering about 350 species. Their stems are branched at nodes, with the nodes closely spaced at the tips, giving the plants a tufted appearance. Their gametophytes develop from the margins of plate-like protonema, in contrast to the filamentous protonema of true mosses. The leaves of peat mosses lack a midrib, and the bulk of the leaf mass is composed of large, translucent cells that are dead. These hyaline cells contain pores, allowing them to readily take up water. Some peat mosses can absorb an amount of water equal to 26 times their dry weight. Narrower, living cells that photosynthesize occur in networks between the hyaline cells. The sporophytes of peat mosses are distinct in that the stalk on which the capsule sits is part of the gametophyte and not the sporophyte itself as in the true mosses. The capsule also characteristically disperses its spores by a minute explosion. At maturity, the globular capsules begin to dry so that the middle portion contracts inward. The contraction produces great internal pressure on the air trapped inside, which eventually increases enough to blow off the lid with an audible pop, shooting the spores into the air. The capsules lack a peristome.

Granite mosses

This is the smallest group of mosses containing only about 100 species. Granite mosses are small, dark, tufted plants that grow on exposed rocks in alpine and arctic regions. Their leafy gametophytes arise from a lobed structure, rather than from a filamentous protonema. Their sporophytes generally are stalks that are derived from the gametophyte, as in the peat mosses. Their tiny capsules typically have four vertical sutures that split at maturity to release the spores. This method of spore dispersal is unique among the mosses.

Importance of mosses

Mosses are extremely important during the early stages of ecological succession. Succession begins with the generation of a new environment. This can occur, for example, by the formation of sand dunes, the exposure of land by deglaciation, or by the radical disturbance of a previously vegetated landscape as when an area is logged or burned by wildfire. In such cases, the ground becomes vegetated by the process of succession, during which various different plant communities dominate the site in turn. Because of their ability to reproduce asexually by fragmentation and gemmae combined with sexual reproduction, which produces enormous numbers of tiny, easily-dispersed spores, mosses play a vital role in being among the first colonizers of disturbed sites. They stabilize the soil surface, thereby reducing erosion, while at the same time reducing the evaporation of water, making more available for succeeding plants. Mosses are not an important source of food for vertebrate herbivores. Peat mosses are the dominant plants of extensive northern wetland areas, and are largely responsible for the development of bogs.

Most species of mosses are not of any direct economic importance, and none are a food source for humans. Peat mosses are economically the most important mosses. Peat mosses are an important source of fuel in some countries. Peat is abundant in northern regions and represents a vast reservoir of potential energy. In northern Europe, peat has historically been dried, and in some cases compressed into briquettes for use in fireplaces and stoves. In Ireland, peat is still extensively used for cooking. One great advantage of peat as a fuel is that it burns very cleanly. About 95% of peat harvested in Ireland is burned to generate electricity. Peat is also highly valued as a conditioner of inorganic soils. Because it absorbs large amounts of water readily, peat improves the water-holding capacity of soil. Peat mosses are characteristically acidic which prevents the growth of most bacteria. They have therefore been used by indigenous peoples for diapers, and during the World Wars, when

KEY TERMS

Antheridium— A sperm-producing organ consisting of sperm-producing tissue, surrounded by a sterile layer of cells.

Archegonium— An egg-producing organ, often flask-shaped, with an outer layer of sterile cells. Calyptra—An enlarged and modified archegonium that forms a cap around the capsule of the developing sporophyte.

Gametophyte— Individual plant containing only one set of chromosomes per cell that produces gametes i.e. reproductive cells that must fuse with other reproductive cells to produce a new individual.

Sporophyte— The diploid, spore-producing generation in a plant’s life cycle.

Zygote— The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

bandages were in short supply, peat mosses were a commonly used antiseptic dressing for wounds.

In recent years, mosses have become important in monitoring the health of ecosystems, especially in relation to atmospheric contamination. Because bryophytes lack roots, many of their nutritional requirements are met by nutrients deposited from the atmosphere. Thus, they are sensitive indicators of atmospheric pollutants. Changes in the distributions of mosses (and lichens) are therefore an early-warning signal of serious effects of atmospheric pollution.

See also Liverwort; Symbiosis; Wetlands.

Resources

BOOKS

Malcolm, Bill, and Nancy Malcolm. Mosses and Other Bryophytes: An Illustrated Glossary. Nelson, New Zealand: Micro-Optics Press, 2000.

Shaw, A. Jonathan, and Bernard Goffinet. Bryophyte Biology. Cambridge, U.K.: Cambridge University Press, 2000.

Les C. Cwynar

Bryophyte

Bryophytes include the mosses, liverworts, and hornworts. Bryophytes are the simplest of plants (excluding the algae , which are not considered plants by most botanists). Bryophytes are small, seldom exceeding 6-8 in (15-20 cm) in height, and usually much smaller. They are attached to the substrate (ground, rock, or bark ) by rhizoids, which are one or a few-celled, root-like threads that serve only for anchoring and are not capable of absorbing water and nutrients from the substrate. Brypohytes lack vascular tissue (the specialized cells grouped together to pipe water and nutrients to various parts of the body), or in the rare cases when this tissue is present, it is not well differentiated. The leaves of bryophytes are technically not true leaves, because in most species they lack vascular tissue. However, they are functionally equivalent to leaves, containing chlorophylls a and b for photosynthesis . Leaves are usually one-cell thick, except for the midrib, which may be up to 15 cells thick. Bryophytes satisfy their nutritional requirements by absorbing minerals from dust, rainfall, and water running over their surface.

The life cycle of bryophytes is characterized by an alternation of generations, one of which is a multicellular, diploid individual called a sporophyte, having two of each type of chromosome per cell . This stage alternates with multicellular, haploid individual called the gametophyte, with only one of each type of chromosome per cell, as is also the case with animal sperm. Bryophytes are unique among plants in that the dominant, conspicuous generation is the haploid gametophyte. In all other plants, the dominant stage is the diploid sporophyte.

Most reproduction of bryophytes is asexual, occurring by fragmentation of body parts, and by the production of specialized vegetative units called gemmae. Gemmae may be produced as microscopic plates (in the genus Tetraphis), as bulbils in the axils of leaves (in Pohlia), or as microscopic filaments (in Ulota). When sexual reproduction occurs, it always involves a flagellated sperm (produced in a specialized organ called an antheridium) that must swim through water to reach an egg located in a specialized, flask-shaped organ (the archegonium). The antheridia and archegonia are surrounded by a layer of sterile cells, which protects the sex organs from mechanical damage and desiccation.

The union of the sperm and egg results in a diploid zygote, i.e., a new sporophyte. This is nourished by the gametophyte and grows on it in a parasitic fashion, although the sporophytes of some bryophytes photosynthesize and make some contribution to their own growth. Initially, as the young sporophyte grows, the archegonium also enlarges. However, it ultimately fails to keep pace with the growth of the sporophyte and becomes detached from its base, forming a cap-like structure called a calyptra.

Classification, characteristics, and habitats of bryophytes

The classification of bryophytes has been controversial among botanists. Traditionally, the division Bryophyta has included the true mosses, liverworts, and hornworts. However, some scientists consider each of these groups sufficiently distinct to deserve their own division: Bryophyta for the mosses, Hepatophyta for the liverworts, and Anthoceratophyta for the hornworts. The latter view is followed here, although the bryophyte is used as a collective term for all of these.

About 15,000 species of bryophytes have been described. They are distributed throughout the world, and are especially abundant in arctic and boreal regions, where they often dominate the ground vegetation. Bryophytes also occur in humid tropical regions where they commonly grow on other plants, especially in higher-elevation forests . Bryophytes are considered the amphibians of the plant world, because they require abundant moisture to grow. This requirement for water results from a number of their characteristic features. Their stems and leaves are thin, and either lack a cuticle (that is, a waxy surface layer) or have a very thin one, making them prone to drying out. Because bryophytes lack roots and a vascular system, they cannot obtain water from the soil and transport it to above-ground tissues; for this same reason, bryophytes are necessarily small. In addition, their sperm require free water in order to swim from their parent plant to the egg on another plant.

Hepatophyta (division liverworts)

Hepatophyta means "liver plant" and refers to the body of some common species of liverworts, whose lobing is reminiscent of a liver. During Medieval and earlier times, many people followed the doctrine of signatures—a belief that the superficial resemblance of a plant to some part of the human anatomy indicated that the plant possessed medicinal properties related to the organ it resembled. Liverworts are the simplest of the living plants, and range in size from minuscule, leafy filaments less than 0.02 in (0.5 mm) in diameter, to plants exceeding 8 in (20 cm) in size. Liverworts lack specialized conducting tissues, cuticles, and stomates, and their rhizoids are always unicellular. The gametophytes arise directly from spores in most species. Most liverworts (75%) have nine chromosomes in their haploid cells. There are two kinds of liverworts based on body form: thallose and leafy.

Thallose liverworts have gametophytes with an undifferentiated body called a thallus which has a ribbon-like appearance. Marchantia is one of the most widely distributed thallose liverworts, especially in habitats that provide ideal conditions of light and high humidity . The body is typically 30 cells thick at the midrib, and only 10 cells thick elsewhere. A thin, upper, green layer contains chlorophyll-rich cells, arranged in polygonal or diamond-shaped patterns each centered on a permanently open pore. Below each polygon is an airspace connected to the outside by the pore, and within the chamber are erect threads of photosynthetic cells. Below the relatively thin, upper photosynthetic layer is a lower layer that is colorless and stores the products of photosynthesis. Most reproduction is asexual by fragmentation, usually caused by wind or by animals breaking the plants apart while eating them or when stepping on them or when trampling them. Thallose liverworts also commonly reproduce asexually by producing small balls of cells called gemmae, within bowl-like structures called gemma cups. The balls become detached and are splashed out by raindrops, dispersing away to colonize favorable habitats.

Leafy liverworts grow in wet or humid habitats, and are especially common in the tropics and subtropics, although they also occur in temperate areas. They are the simplest of the plants with leaflike structures. Their leaves lack vascular tissue, each is deeply cleft so as to appear two-lobed, and they are arranged in two rows along a much branched stem. Unlike the true mosses, which typically appear somewhat erect, leafy liverworts form small, flat mats. Asexual reproduction by fragmentation is common.

Hornworts (division Anthocerophyta)

The hornworts are the smallest of the three groups of bryophytes with only about 100 species in six genera. Hornworts are especially diverse in the tropics, although Anthoceros occurs in temperate regions.

The gametophyte of hornworts is saucer-shaped, with upturned edges, and only 0.4-0.8 in (1-2 cm) in diameter. These interesting little plants are more similar to algae than are any other plants, especially because they have only one, large chloroplast in association with a pyrenoid in each photosynthetic cell. Yet, the hornworts are more advanced in some ways than liverworts, for example, they possess stomates, which exchange gases between the plant and the air. They are also unique among plants in having stomates on their gametophytes. Unlike the liverworts in which internal spaces between cells are filled with air, in hornworts the cavities are filled with mucilage, a water-absorbing material within which the cyanobacterium Nostoc can be found. This symbiotic relationship greatly benefits the hornwort, because cyanobacteria are among the few organisms that can fix molecular nitrogen (as N2) into nitrogen compounds that are in a form useable to plants as nutrients; no plants can produce these essential compounds on their own.

Hornworts derive their name from their sporophyte, which has the appearance of a tapered horn. The sporophyte has a mass of undifferentiated tissue called a meristem at its base. The meristem can actively grow, so that the sporophyte can continue to increase in height, especially if damaged at the top, and can reach a height of 0.4-1.6 in (1-4 cm). The sporophyte of hornworts possesses stomate-like openings and remains photosynthetic for several months. It is, therefore, only semi-dependent on the gametophyte to which it remains attached. The sporophyte of hornworts represents a transitional stage to more highly evolved plants such as ferns , in which the sporophyte is for the most part independent of the gametophyte.

Mosses (division Bryophyta)

Only members of the division Bryophyta are considered "true" mosses. Many other plants and some algae are commonly called mosses, because they superficially resemble the true mosses, but they are not in fact even closely related to them. For example, Spanish moss (Tillandia uneoides) is a flowering plant in the pineapple family, Irish moss (Chondrus crispus) is a red alga that is collected for the extraction of carageenan, a starch-like substance used in food preparation, reindeer moss (Cladina spp.) is a lichen, and club mosses (Lycopodium spp.) are advanced plants with well developed vascular systems.

A number of characteristics distinguish two mosses from other bryophytes. Their gametophytes are leafy, whereas those of hornworts and thallose liverworts are not. The leaves of mosses occur in three ranks on the stem, but because the stem twists, they appear to have radial symmetry, the ability to be bisected into identical halves in more than one way. In contrast, leafy liverworts, whose leaves are two-ranked, only have one set of mirror images (bilateral symmetry). Furthermore, leaves of mosses are not lobed as in leafy liverworts. The rhizoids of mosses are multicellular, compared with single-celled in liverworts.

Mosses are distributed throughout the world, and are among the ecosystem dominants in boreal, arctic, and alpine environments. Mosses occur in a wide variety of habitats. They commonly grow on mineral and organic soils, and they can occur on volcanically heated soil that may reach temperatures of 131°F (55°C), on rocks in Antarctica where the temperature during the growing season does not exceed 14°F (-10°C), as epiphytes that grow on other plants, especially in tropical and subtropical regions, and in freshwater habitats. No mosses are truly marine, although some live within the spray zone of coastal habitats. The luminous moss Schistoste occurs within caves near the entrance where it concentrates the limited available light with the curved, lens-like surface of its leaves. The dung-loving species of the genus Splachnum are among the rarest and most beautiful of mosses. Their capsules occur on long, flimsy stalks, and the base of each capsule bears a thin, papery, brightly colored, umbrella-like structure-yellow in S. luteum and red in S. rubrum. The smallest mosses reach only 0.04-0.8 in (1-2 mm) in height, whereas the largest can grow to 20 in (50 cm). In temperate regions, mosses grow during cooler, wetter parts of the year, primarily during the autumn, mild spells in winter, and in early spring. The mosses naturally fall into three distinctive groups, taxonomically referred to as classes: Bryidae, the true mosses; Sphagnidae, the peat mosses; and Andreaeidae, the granite mosses.

True mosses

One of the most distinctive features of true mosses involves the development of their gametophytes. Spores germinate to produce a characteristic mass of algal-like threads, called protonema, which looks like a loose ball of wool. Bud-like structures develop later, and give rise to the familiar leafy gametophyte. Although mosses are considered to be non-vascular plants, many true mosses in fact have a primitive vascular system consisting of a central strand of water-conducting cells called hydroids. Some also have specialized cells around the column of hydroids called leptoids, which function in the transport of carbohydrates, the products of photosynthesis. The stems of true mosses are more-or-less uniformly leafy and erect. Their leaves usually have a midrib, and their sporophytes possess capsules that are borne on stalks that are made of sporophytic tissue. Also, the capsules contain one or two rows of toothlike appendages (peristome) over the opening of the capsule, which are exposed when the lid is shed. This is by far the largest group of mosses.

Peat mosses

This is a small, but extremely important group of mosses, numbering about 350 species. Their stems are branched at nodes, with the nodes closely spaced at the tips, giving the plants a tufted appearance. Their gametophytes develop from the margins of plate-like protonema, in contrast to the filamentous protonema of true mosses. The leaves of peat mosses lack a midrib, and the bulk of the leaf mass is composed of large, translucent cells that are dead. These hyaline cells contain pores, allowing them to readily take up water. Some peat mosses can absorb an amount of water equal to 26 times their dry weight. Narrower, living cells that photosynthesize occur in networks between the hyaline cells. The sporophytes of peat mosses are distinct in that the stalk on which the capsule sits is part of the gametophyte and not the sporophyte itself as in the true mosses. The capsule also characteristically disperses its spores by a minute explosion. At maturity, the globular capsules begin to dry so that the middle portion contracts inward. The contraction produces great internal pressure on the air trapped inside, which eventually increases enough to blow off the lid with an audible pop, shooting the spores into the air. The capsules lack a peristome.

Granite mosses

This is the smallest group of mosses containing only about 100 species. Granite mosses are small, dark, tufted plants that grow on exposed rocks in alpine and arctic regions. Their leafy gametophytes arise from a lobed structure, rather than from a filamentous protonema. Their sporophytes generally are stalks that are derived from the gametophyte, as in the peat mosses. Their tiny capsules typically have four vertical sutures that split at maturity to release the spores. This method of spore dispersal is unique among the mosses.

Importance of mosses

Mosses are extremely important during the early stages of ecological succession . Succession begins with the generation of a new environment. This can occur, for example, by the formation of sand dunes, the exposure of land by deglaciation, or by the radical disturbance of a previously vegetated landscape as when an area is logged or burned by wildfire . In such cases, the ground becomes vegetated by the process of succession, during which various different plant communities dominate the site in turn. Because of their ability to reproduce asexually by fragmentation and gemmae combined with sexual reproduction, which produces enormous numbers of tiny, easily-dispersed spores, mosses play a vital role in being among the first colonizers of disturbed sites. They stabilize the soil surface, thereby reducing erosion , while at the same time reducing the evaporation of water, making more available for succeeding plants. Mosses are not an important source of food for vertebrate herbivores. Peat mosses are the dominant plants of extensive northern wetland areas, and are largely responsible for the development of bogs.

Most species of mosses are not of any direct economic importance, and none are a food source for humans. Peat mosses are economically the most important mosses. Peat mosses are an important source of fuel in some countries. Peat is abundant in northern regions and represents a vast reservoir of potential energy . In northern Europe , peat has historically been dried, and in some cases compressed into briquettes for use in fireplaces and stoves. In Ireland, peat is still extensively used for cooking. One great advantage of peat as a fuel is that it burns very cleanly. About 95% of peat harvested in Ireland is burned to generate electricity . Peat is also highly valued as a conditioner of inorganic soils. Because it absorbs large amounts of water readily, peat improves the water-holding capacity of soil. Peat mosses are characteristically acidic which prevents the growth of most bacteria . They have therefore been used by indigenous peoples for diapers, and during the World Wars, when bandages were in short supply, peat mosses were a commonly used antiseptic dressing for wounds.

In recent years, mosses have become important in monitoring the health of ecosystems, especially in relation to atmospheric contamination . Because bryophytes lack roots, many of their nutritional requirements are met by nutrients deposited from the atmosphere. Thus, they are sensitive indicators of atmospheric pollutants. Changes in the distributions of mosses (and lichens ) are therefore an early-warning signal of serious effects of atmospheric pollution .

See also Liverwort; Symbiosis; Wetlands.

Resources

books

Longton, R.E. The Biology of Polar Bryophytes and Lichens. U.K.: Cambridge University Press, 1988.

Richardson, D.H.S. The Biology of Mosses. New York: John Wiley and Sons, Inc., 1981.

Schofield, W.B. Introduction to Bryology. New York: Macmillan Publishing Co., 1985.

Les C. Cwynar

KEY TERMS

Antheridium

—A sperm-producing organ consisting of sperm-producing tissue, surrounded by a sterile layer of cells.

Archegonium

—An egg-producing organ, often flask-shaped, with an outer layer of sterile cells.

Calyptra

—An enlarged and modified archegonium that forms a cap around the capsule of the developing sporophyte.

Gametophyte

—Individual plant containing only one set of chromosomes per cell that produces gametes i.e. reproductive cells that must fuse with other reproductive cells to produce a new individual.

Sporophyte

—The diploid, spore-producing generation in a plant's life cycle.

Zygote

—The cell resulting from the fusion of male sperm and the female egg. Normally the zygote has double the chromosome number of either gamete, and gives rise to a new embryo.

Bryophytes

Plant scientists recognize two kinds of land plants: bryophytes (nonvascular land plants) and tracheophytes (vascular land plants). Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks or soil or as epiphytes on the trunks and leaves of forest trees. Bryophytes are distinguished from tracheophytes by two important characteristics. First, in all bryophytes the ecologically persistent, photosynthetic phase of the life cycle is the haploid , gametophyte generation rather than the diploid sporophyte ; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes, and consist of only an unbranched stalk, or seta, and a single, terminal sporangium. Second, bryophytes never form xylem tissue, the special lignin-containing, water-conducting tissue that is found in the sporophytes of all vascular plants. At one time, all bryophytes were placed in a single phylum, intermediate in position between algae and vascular plants. Modern studies of cell ultra-structure and molecular biology, however, confirm that bryophytes comprise three separate evolutionary lineages , today recognized as mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta), and hornworts (phylum Anthocerotophyta). Following a detailed analysis of land plant relationships, Paul Kenrick and Peter R. Crane proposed that the three groups of bryophytes represent a structural level in plant evolution, identified by their monosporangiate life cycle. Within the bryophytes, liverworts are the geologically oldest group, sharing a fossil record with the oldest vascular plants (Rhyniophytes) in the Devonian era.

Mosses

Of the three phyla of bryophytes, greatest species diversity is found in the mosses, with up to fifteen thousand species recognized. A moss begins its life cycle when haploid spores, which are produced in the sporophyte capsule, land on a moist substrate and begin to germinate. From the one-celled spore a highly branched system of filaments , called the protonema, develops. Cell specialization occurs within the protonema to form a horizontal system of reddish-brown anchoring filaments and upright green filaments. Each protonema, which superficially resembles a filamentous alga, can spread over several centimeters to form a fuzzy green film over its substrate. As the protonema grows, some cells of the specialized green filaments form leafy buds that will ultimately form the adult gametophyte shoots. Numerous shoots typically develop from each protonema so that, in fact, a single spore can give rise to a whole clump of moss plants. Each leafy shoot continues to grow apically , producing leaves in spiral arrangement on an elongating stem. In many mosses the stem is differentiated into a central strand of thin-walled water-conducting cells, called hydroids, surrounded by a parenchymatous cortex and a thick-walled epidermis. The leaves taper from a broad base to a pointed apex and have lamina that are only one-cell-layer thick. A hydroid-containing midvein often extends from the stem into the leaf. Near the base of the shoot, reddish-brown multicellular rhizoids emerge from the stem to anchor the moss to its substrate. Water and mineral nutrients required for the moss to grow are absorbed, not by the rhizoids, but rather by the thin leaves of the plant as rain water washes through the moss cushion.

As is typical of bryophytes, mosses produce large, multicellular sex organs for reproduction. Many bryophytes are unisexual, or sexually dioicous . In mosses male sex organs, called antheridia, are produced in clusters at the tips of shoots or branches on the male plants; female sex organs, the archegonia, are produced in similar fashion on female plants. Numerous motile sperm are produced by mitosis inside the brightly colored, club-shaped antheridia

while a single egg develops in the base of each vase-shaped archegonium. As the sperm mature, the antheridium swells and bursts open. Drops of rainwater falling into the cluster of open antheridia splash the sperm to nearby females. Beating their two whiplash flagellae , the sperm are able to move short distances in the water film that covers the plants to the open necks of the archegonia. Slimy mucilage secretions in the archegonial neck help pull the sperm downward to the egg. The closely packed arrangement of the individual moss plants greatly facilitates fertilization. Rain forest bryophytes that hang in long festoons from the trees rely on torrential winds with the rain to transport their sperm from tree to tree, while the small pygmy mosses of exposed, ephemeral habitats depend on the drops of morning dew to move their sperm. Regardless of where they grow, all bryophytes require water for sperm dispersal and subsequent fertilization.

Embryonic growth of the sporophyte begins within the archegonium soon after fertilization. At its base, or foot, the growing embryo forms a nutrient transfer zone, or placenta, with the gametophyte. Both organic nutrients and water move from the gametophyte into the sporophyte as it continues to grow. In mosses the sporophyte stalk, or seta, tears the archegonial enclosure early in development, leaving only the foot and the very base of the seta embedded in the gametophyte. The upper part of the archegonium remains over the tip of the sporophyte as a caplike calyptra. Sporophyte growth ends with the formation of a sporangium (the capsule) at the tip of the seta. Within the capsule, water-resistant haploid spores are formed by meiosis. As the mature capsule swells, the calyptra falls away. This allows the capsule to dry and break open at its tip when the spores are mature. Special membranous structures, called peristome teeth, that are folded down into the spore mass now bend outward, flinging the spores into the drying winds. Moss spores can travel great distances on the winds, even moving between continents on the jet streams. Their walls are highly protective, allowing some spores to remain viable for up to forty years. Of course, if the spore lands in a suitable, moist habitat, germination will begin the cycle all over again.

Liverworts and Hornworts

Liverworts and hornworts are like mosses in the fundamental features of their life cycle, but differ greatly in organization of their mature game-tophytes and sporophytes. Liverwort gametophytes can be either leafy shoots or flattened thalli . In the leafy forms the leaves are arranged on the stem in one ventral and two lateral rows or ranks, rather than in spirals like the mosses. The leaves are one cell-layer thick throughout, never have a mid-vein, and are usually divided into two or more parts called lobes. The ventral leaves, which actually lie against the substrate (soil or other support), are usually much smaller than the lateral leaves and are hidden by the stem. Anchoring rhizoids, which arise near the ventral leaves, are colorless and unicellular. The flattened ribbonlike to leaflike thallus of the thallose liver-worts can be either simple or structurally differentiated into a system of dorsal air chambers and ventral storage tissues. In the latter type the dorsal epidermis of the thallus is punctuated with scattered pores that open into the air chambers. Liverworts synthesize a vast array of volatile oils, which they store in unique organelles called oil bodies. These compounds impart an often spicy aroma to the plants and seem to discourage animals from feeding on them. Many of these compounds have potential as antimicrobial or anticancer pharmaceuticals.

Liverworts.

Liverwort sporophytes develop completely enclosed within gametophyte tissues until their capsules are ready to open. The seta, which is initially very short, consists of small, thin-walled hyaline cells. Just prior to capsule opening, the seta cells lengthen, thereby increasing the length of the seta up to twenty times its original dimensions. This rapid elongation pushes the darkly pigmented capsule and upper part of the whitish seta out of the gametophytic tissues. With drying, the capsule opens by splitting into four segments, or valves. The spores are dispersed into the winds by the twisting motions of numerous intermixed sterile cells called elaters. In contrast to mosses, which disperse their spores over several days, liverworts disperse the entire spore mass of a single capsule in just a few minutes.

Hornworts.

Hornworts resemble some liverworts in having simple, unspecialized thalloid gametophytes, but they differ in many other characters. For example, colonies of the symbiotic cyanobacterium Nostoc fill small cavities that are scattered throughout the ventral part of the hornwort thallus. When the thallus is viewed from above, these colonies appear as scattered blue-green dots. The cyanobacterium converts nitrogen gas from the air into ammonium, which the hornwort requires in its metabolism, and the hornwort secretes carbohydrate-containing mucilage, which supports the growth of the cyanobacterium. Hornworts also differ from all other land plants in having only one large, algal-like chloroplast in each thallus cell. Hornworts get their name from their long, horn-shaped sporophytes. As in other bryophytes, the sporophyte is anchored in the gametophyte by a foot through which nutrient transfer from gametophyte to sporophyte occurs. The rest of the sporophyte, however, is actually an elongate sporangium in which meiosis and spore development take place. At the base of the sporangium, just above the foot, is a mitotically active meristem , which adds new cells to the spore-producing zone throughout the life span of the sporophyte. In fact, the sporangium can be releasing spores at its apex at the same time that new spores are being produced by meiosis at its base. Spore release in hornworts takes place gradually over a long period of time, and the spores are mostly dispersed by water movements rather than by wind.

Mosses, liverworts, and hornworts are found throughout the world in a variety of habitats. They flourish particularly well in moist, humid forests like the fog forests of the Pacific Northwest or the montane rain forests of the Southern Hemisphere. Their ecological roles are many. They provide seed beds for the larger plants of the community , they capture and recycle nutrients that are washed with rainwater from the canopy, and they bind the soil to keep it from eroding. In the Northern Hemisphere peatlands, wetlands often dominated by the moss Sphagnum, are particularly important bryophyte communities. This moss has exceptional water-holding capacity, and when dried and compressed forms a coal-like fuel. Throughout northern Europe, Asia, and North America, peat has been harvested for centuries for both fuel consumption and horticultural uses, and today peat lands are managed as a sustainable resource.

see also Evolution of Plants; Gametophyte; Nitrogen Fixation; Peat Bogs; Reproduction, Alternation of Generations and; Sporophyte.

Barbara Crandall-Stotler

Bibliography

Crandall-Stotler, Barbara. "Morphogenetic Designs and a Theory of Bryophyte Origins and Divergence." BioScience 30 (1980): 580-85.

Hébant, Charles. The Conducting Tissues of Bryophytes. Vaduz: J. Cramer, 1977.

Kenrick, Paul, and Peter R. Crane. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, DC: Smithsonian Institution Press, 1997.

Miller, Norton G. "Bogs, Bales and BTUs: A Primer on Peat." Horticulture 59 (1981):38-45.

Schofield, W. B. Introduction to Bryology. New York: Macmillan, 1985.

Shaw, Jonathon A., and Bernard Goffinet, eds. The Biology of Bryophytes. Cambridge, England: Cambridge University Press, 2000.

Bryophytes

Bryophytes are seedless plants without specialized water-conducting tissues. Bryophytes include mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta Hepatophyta), and hornworts (phylum Anthocerophyta). They are plants that virtually everyone has seen, but many have ignored. The most commonly encountered group is the green mosses that cover rotting logs, anchor to the bark of trees, and grow in the spray of waterfalls, along streams and in bogs. Even though mosses often thrive in wet habitats, many mosses and some liverworts can survive in relatively dry environments such as sandy soils and exposed rock outcrops.

The liverworts can take leafy forms, which are very similar superficially to mosses, but differ in the details of leaf size and arrangement. Other liverwort genera are characterized by a thallus made up of relatively small, flattened, ribbonlike segments of photosynthetic tissue, which have the general appearance of short, branched pieces of rich dark green egg noodles or linguini.

The leafy liverworts and the mosses differ in the appearance of their spore-forming structures. The mosses have thin stalks called seta extending from the ends of leafy branches. Seta bear capsules, which produce spores. The leafy and thalloid liverworts have very small, balloon-shaped spore-producing stages that remain virtually hidden within, and totally dependent upon, the photosynthetic plant tissues. The third major group of bryophytes is the hornworts. They received this common name because their spore producing structures, called sporangia, are generally long, slender, hornlike, and without capsules. More than eighteen thousand different bryophyte species have been identified throughout the world, and there are perhaps ten thousand species of moss, approximately eight thousand liverwort species, and only a little more than one hundred species of hornworts.

Characteristics of Bryophytes

There are several characteristic features of bryophytes. First, the green tissue that makes up most of the plant body is not vascularized; it does not have xylem and phloem cells. This absence of specialized tissues for transporting water and dissolved food throughout the organism limits terrestrial forms to being very short plants, since the only way to move substances through the plant body is by osmosis and diffusion from surface moisture.

Second, bryophytes do not have roots, but have rhizoids, which are relatively simple, sometimes multicellular filaments of thin-walled cells that extend from the photosynthetic tissue into the soil or other substrate . They anchor the plant somewhat and in some cases facilitate water and nutrient uptake.

Sexual Reproduction

The third characteristic of bryophytes is something that one could not guess by just looking at the conspicuous green tissue. Unlike other plants (and indeed most other multicellular organisms), the conspicuous portion of bryophytes is composed of haploid cells, containing only one set of chromosomes .

Sexual reproduction in animals involves the union of an egg and a sperm to form a fertilized egg (zygote). This diploid (2n) cell divides mitotically to produce an embryo, and ultimately a mature adult organism. These adults have specialized cells, which divide meiotically to produce haploid (n) sperm or eggs depending on the sex of the individual. In the plant kingdom, this cycle of fertilization and meiosis involves an alternation of generations between the haploid gamete -producing stage (gametophyte) and the diploid organism (sporophyte).

Vascular plants, including flowering plants, conifers, and many, such as ferns, that do not produce seeds, have life cycles with the diploid sporophyte being the predominant generation. In the bryophytes, it is the haploid gametophyte that produces the leaves and thali and therefore predominates. This change from predominant gametophyte to sporophyte was a major evolutionary advancement, which along with the development of vascular tissue facilitated the ultimate success of plants in a diversity of terrestrial habitats.

In order to accomplish sexual reproduction, bryophyte gametophytes produce eggs (n) in the archegonium, a vase-shaped structure that is the female reproductive organ. The sperm (n) are produced in antheridia, which may occur on the same gametophyte, but are often located on separate male plants. Water is generally required for them to swim to the eggs for fertilization. The resulting zygote (2n) develops into the sporophyte (2n). The sporophytes remain attached to and dependent on the female gametophyte. These parasitic sporophytes produce spores (n) by meiosis that then divide mitotically to produce the obvious multicellular gametophyte.

see also Alternation of Generations; Angiosperms; Plant; Pteridophytes; Seedless Vascular Plants; Translocation; Water Movement in Plants

Dean Cocking

Bibliography

Conard, Henry Shoemaker, et al. How to Know the Mosses and Liverworts, 2nd ed. New York: McGraw-Hill, 1980.

Malcolm, Bill, and Nancy Malcolm. Mosses and Bryophytes: An Illustrated Glossary. Portland, OR: Timber Press/Micro-Optics Press, 2000.

Shaw, A. Jonathan, and Bernard Goffinet, eds. Bryophyte Biology. New York: Cambridge University Press, 2000.

Bryophytes

Bryophytes are nonflowering plants that make up one of the major divisions of the plant kingdom. They are composed of moss, liverwort, and hornwort, all of which are small, simple land plants that live in damp places.

The most widely accepted method of classifying the many types of plants in the kingdom Plantae divides it into ten divisions, one of which is called Bryophyta. Plants in this division are unlike plants in any of the other nine since they make up the nonvascular plants. Unlike typical plants, nonvascular plants lack a transport system made up of tubelike vessels or internal pipelines to move their water and food about. Because of this, these plants live very close to the ground and do not have stems, leaves, or roots.

Bryophytes live in almost any part of the world, and are found in the Arctic as well as the tropics. They may have been among the first land plants, since fossils of bryophytes have been found that date to about 400,000,000 years ago. Since bryophytes do not have a specialized method of moving its food and water from one part of the plant to another, all parts of the plant can absorb water and nutrients directly from the environment. Bryophytes also reproduce differently from most plants by using spores rather than seeds. A spore is similar to a seed in that it has an outer coat that protects its inner reproductive cells. Spores are usually released into the air by fungi (such as mushrooms) and are light enough to travel great distances. If a spore lands in a suitable place, it will germinate (begin to grow or sprout) and produce a new fungus. Bryophytes cannot reproduce without water, which is why most are found in moist places.

Mosses, part of the bryophyte group, are soft and leafy, usually never more than 2 inches (5.08 centimeters) tall. They grow by anchoring themselves by rootlike growths called rhizoids. There are nearly 10,000 species of moss. Liverwort and hornwort also are both bryophytes and have a ribbonlike or flat, leafy shape that grows low to the ground and is anchored like the mosses. Bryophytes do not produce flowers since they use spores to reproduce.

[See alsoPlants ]