About Mosses and Liverworts

Mosses, liverworts, and hornworts are non-vascular terrestrial plants that together are referred to as the bryophytes. Although they lack true veins, many species of mosses have long narrow cells in their stems, the midribs of their leaves, and their rhizoids (root-like plant tissue) that can be considered evolutionary precursors to true veins. Vascular plants with true veins include the clubmosses, ferns, and flowering plants. Because veins are a fast and effective means of transporting water, nutrients, and minerals from one area of a plant to another (for example, from the roots to the leaves and fruits), this enabled vascular plants to grow larger in size than the bryophytes and dominate most of the landscape, except for many Arctic regions, where mosses, especially Sphagnum mosses, are still the most common plants. The evolutionary development of woody tissue in vascular plants, resulting the development of woody vines, shrubs, and trees, further augmented the dominance of vascular plants.

Mosses, liverworts, and hornworts still persist in most terrestrial habitats to a greater or lesser degree, however. Liverworts first appeared on land at least 450 million years ago, while mosses appeared on land at least 380 million years ago, according to current scientific evidence from fossils. Fossilized hornworts up to 180 million years old have been found. However, various kinds of cladistic analysis suggest that they represent a much older lineage of plants dating back to the first appearance of mosses. With th exception of algae that can develop on land, liverworts are currently regarded as the oldest terrestrial plants on Earth (fossil evidence indicates that thalloid liverworts are probably older than leafy liverworts). The lack of true veins have restricted the size of bryophytes to only a few inches in height (inch = 2.5 centimeters). Today, these non-vascular plants occupy a variety of habitats on all continents, including Antarctica. These habitats include grassy meadows, lawns, muddy fields, the ground soil of woodlands, the bark and branches of living trees, rotting logs in woodlands, the slopes of woodland ravines, rocks exposed to sunlight, damp rocks in shaded areas, low ground along streams and ponds, rocks along streams and ponds, clay banks along rivers, sand dunes, north-facing hillsides, ledges along cliffs, crevices in cliffs, vertical walls of rock, stone and concrete walls, gravelly ground along roadsides and railroads, roof tops of buildings, moist ground and floating logs in swamps, peat bogs, calcareous fens, exposed garden soil, exposed ground along buildings, cracks in sidewalks and pavement, quarry floors and walls, and mine spoils. Some mosses and liverworts also occur as floating aquatic plants and submersed aquatic plants in streams, ponds, and swamps. Bryophytes are especially common in damp shaded habitats where they receive some protection from the heat and drying effects of the sun. One habitat where you will not find any bryophytes is the salt water of oceans; they only occupy terrestrial habitats or freshwater habitats.

Mosses, liverworts, and hornworts can be observed throughout the year because their foliage is evergreen. However, in temperate areas, the spring is probably the best time of year to observe them before the deciduous vascular plants leaf out and obscure them. While a minority of bryophytes are annuals, the large majority of them are perennials; probably the majority of these perennials are short-lived. Bryophytes absorb the moisture and minerals through the walls of their cells that are exposed to the environment. The water and minerals are slowly transported from the exposed cells to the interior cells by passing through their membranes. Like other plants, carbohydrates are supplied through photosynthesis. Bryophytes are 'resurrection' plants in the sense that they become dried up and shriveled in the absence of water or adequate humidity levels, but with the return of moisture they fill out again in much the same state as they were before. Different species of bryophytes vary significantly in how often they can repeat this process, and for how long they can withstand dryness and heat before they die. The three groups of bryophytes are described in more detail in the next three sections.
After germinating from their spores, mosses start out life as protonemata (low fibrous mats) that are green; they are capable of photosynthesis. Protonemata superficially resemble a patch of filamentous green algae. Sphagnum mosses are exception to this, as their protonemata are thalloid, resembling a flattened green body of plant tissue. The protonemata of mosses persist much longer in the environment than the protonemata of liverworts. From the protonema of a moss, several leafy stems develop, and spore-bearing capsules are occasionally produced from the leafy stems to repeat the cycle again. The leafy stems are referred to as gametophytes because they produce inconspicuous antheridia (male sex organs) and archegonia (female sex organs). The antheridia of mosses are typically located in the lower axils of leaves (rarely at the tips of stems), while archegonia are located at either the tips of stems or from the upper axils of leaves along the stems. The archegonia of female or bisexual plants are partially hidden and surrounded by small leafy bracts. Mosses vary on whether or not antheridia and archegonia are located on the same plant (monoecious) or on different plants (dioecious). When environmental conditions are moist and a film of water develops on the leafy stems of a moss, two-tailed sperm are released from the antheridia to swim to the ovules of the archegonia. Chemicals may be released by the archegonia to attract the sperm, although to some extent the sperm swim around at random. After the sperm reach the ovules of the archegonia and fertilization occurs, the leafy stems of a moss develop sporophytes consisting of spore-bearing capsules on stalks (setae) of varying length (typically nearly 0 mm. to 50 mm., or 0–2 inches). The sporophyte is largely dependent on the gametophyte for its moisture and nutrients, although a limited amount of photosynthesis and moisture absorption can occur on the walls of the capsule body. The capsule body is typically ovoid, obovoid, or cylindrical in shape, but it usually terminates in a lid (operculum). Depending on the moss species, the lid of a capsule body varies in shape from a shallow dome to a long beak. Both the lid and upper body of a capsule are covered by a membranous hood (calyptra) that is usually early-deciduous. When the capsule body becomes mature, both it and its lid (if any) change color and the lid eventually falls off, releasing the tiny spores to the wind. In most moss species, the release of spores is regulated by one or two rings of teeth at the mouth of the capsule body. The ring of larger outer teeth is referred to as the peristome, while the ring of inner teeth (if present) is referred to as the endostome. The capsules of some moss species lack lids; their capsule bodies break open irregularly to release the spores. Some moss species have capsules with lids that lack significant teeth. The leaves of mosses are usually arranged along their stems in spirals, although a minority of mosses have their leaves on opposite sides of the stems (such mosses are 2-ranked). Usually the spirals of leaves are somewhat dense, causing the leaves along the stems to overlap each other. The leaves of mosses can assume various shapes, including linear, lanceolate, ovate, and orbicular, and they are variable in length (typically 0.5 mm. to 10 mm.). The leaves are never lobed, but they sometimes have teeth along their margins. The leaves also vary in whether or not they have midribs. Because petioles are absent in mosses, their leaves clasp the stems. Mosses are anchored to the underlying substrate by multicellular fibrous structures called rhizoids. Rhizoids are not true roots because they lack true veins; they exist primarily to anchor mosses to the substrate. There are two large categories of mosses: acrocarps and pleurocarps. Acrocarpic mosses have leafy stems that tend to stand erect; they often form dense colonies of such leafy stems from the underlying protonema. The stems of these mosses are usually unbranched, or they are at most sparingly branched. The sporophytes (spore-bearing capsules and setae) of acrocarpic mosses are almost always produced from the apex of each leafy stem. This terminates the growth of the leafy stem. However, some leafy stems may remain sterile because of a failure of fertilization or other reasons, especially if the acrocarpic moss is a perennial. Some acrocarpic mosses are able to reproduce asexually by forming gemmae-containing cups at the tips of their stems. These cups are formed from modified leafy bracts, while the gemmae are small clonal plant bodies that are capable of forming new plants. The gemmae are distributed from the cups by raindrop logistics. Pleurocarpic mosses produce a low mat of leafy stems that often overlap each other. The leafy stems of pleurocarpic mosses readily branch, producing new leafy stems. There is a tendency for old leafy stems to die out as new leafy stems are produced. The sporophytes of pleurocarpic mosses can be produced from the leaf axils anywhere along the stems; they do not develop at the tips of leafy stems and terminate their development. Pleurocarpic mosses can reproduce asexually when one or more leafy stems detach from the mother plant as a result of some disturbance. Such detached leafy stems are able to form rhizoids, anchoring a new clonal plant in a different location.
After germinating from spores, liverworts also begin life as green protonemata. The form of the protonemata among liverworts is highly variable, but they typically consist of fibrous strands, a flattened body, or ribbon of green plant issue. Unlike mosses, the protonemata of liverworts are very short-lived and they soon develop into the mature plant form. There are two broad categories of liverworts: thalloid and leafy.

Thalloid liverworts:
Mature thalloid liverworts assume the form of a flattened and undifferentiated body of greenish plant tissue that lacks either stems or leaves. The thalloid body of these liverworts grows by developing lobes up to 15 mm. across (rarely more). Because multiple lobes can develop from the same thalloid body, the larger thalloid liverworts can form bodies, or mats of ribbon-like bodies, spanning several inches in diameter (1 inch = 2.5 cm.). However, smaller thalloid liverworts may span only a few millimeters across. The lower (ventral) side of thalloid liverworts develop two kinds of rhizoids: short peg-like rhizoids in rows and long fibrous rhizoids. While rhizoids resemble the roots of vascular plants, they are not connected to a network of veins in the plant body. The rhizoids of liverworts are single-celled, unlike the multicellular rhizoids of mosses. The short peg-like rhizoids are thought to play a limited role in absorbing water and possibly nutrients from the substrate and transporting them to the thalloid body of the liverwort. Because the water and nutrients have to pass through cell membranes, this process is less efficient than what is encountered in vascular plants. The long fibrous rhizoids allow the thalloid body to adhere to the underlying substrate. Minute air bubbles are visible in the bodies of some thalloid liverworts; the cells of liverworts usually contain oil bodies that may protect them from herbivores, fungi, or bacteria. The thalloid bodies of these liverworts are gametophytes that can produce antheridia (male sexual organs) and archegonia (female sexual organs) on either the same plant (monoecious) or different plants (dioecious). This can vary with the liverwort species, although most thalloid liverworts are monoecious. Across different species of thalloid liverworts, there is considerable variety in the structures that are associated with the production of the sexual organs. In the simplest arrangement, both types of sexual organs are produced within the thalloid body of the liverwort; they are located at or near the upper (dorsal) surface. These sexual organs may, or may not, be associated with conspicuous tubercles, cavities, or flask-shaped structures on the upper surface. When environmental conditions are moist and a film of water is present on the thallus, 2-tailed sperm are released from the antheridia and they swim about until they either die or find the ovules of the archegonia. The sperm may be attracted by chemicals that are released by the archegonia, although to some extent this process is random. After fertilization occurs, the ovules of the archegonia develop into spores (sporophyte). These spores are not released until the thalloid body of the liverwort forms cavities above them, or until the thalloid body breaks apart and dies. The spores are then transported by water or animals. Other thalloid liverworts develop more elaborate structures for the archegonia and sometimes also the antheridia. For example, the thalloid liverwort, Marchantia polymorpha, forms an umbrella-like structure on a long stalk to facilitate the distribution of sperm from the antheridia. Because the antheridia develop on the dorsal surface of this structure, the sperm can be distributed in part by water currents and raindrop logistics. Similarly, a structure resembling a palm tree (several elliptic lobes at the apex of a stalk) develops in this liverwort to facilitate the spread of its spores. The archegonia are located along the lower sides of the lobes on this structure. After fertilization of the ovules occurs, the tiny spores are distributed primarily by wind. Other thalloid liverworts produce their antheridia near the upper surface of their thalloid bodies (sometimes indicated by tubercles), but their archegonia are produced on stalked structures resembling umbrellas or palm trees. Some thalloid liverworts are able to reproduce asexually by forming gemmae (small clonal plant bodies that resemble buds) along the margins of their lobed bodies. The gemmae eventually separate from the mother plant, forming new plants with rhizoids in other locations. Water currents can spread the gemmae for considerable distances. A very small number of thalloid liverworts, such as Marchantia polymorpha, also form gemmae cups on the upper surface of the thalloid body. The gemmae in these cups are surrounded by a short fringed membrane, and they are distributed to new locations by water currents and raindrop logistics.

Leafy Liverworts: The other major group, the leafy liverworts, superficially resemble mosses. Like mosses, leafy liverworts have leafy stems, rather than thalloid bodies. Most leafy liverworts are quite small in size – their leafy shoots typically span about 0.5–6 mm. across and 6-80 mm. in length. The leafy stems can be ascending to erect, or they sprawl across the substrate as a low mat of leafy stems (the latter is more common). Colonies of plants are often produced. Usually, the stems branch readily, but sometimes they are unbranched or sparingly branched (this varies with the species). The leaves of leafy liverworts are arranged very differently from the typical moss species. All leafy liverworts have pairs of opposite lateral leaves along their stems, and many species also have underleaves on the lower side of their stems. Unlike mosses, the leaves are often lobed. The leaves of these liverworts can assume various shapes, including linear, oval, orbicular, reniform (kidney-shaped), and more complex forms. The upper lateral leaves of some species are divided into distinct upper lobes and lower lobes (lobules), causing each leaf to appear like 2 leaves with one above the other. The leaf margins of leafy liverworts usually lack teeth, although a minority of species have leaf margins that are toothed or fringed to resemble teeth. Fibrous rhizoids are produced from the axils of the underleaves, if they are present, otherwise they are produced sparingly along the lower side of the stems. These rhizoids are used to anchor the plant to the underlying substrate. Like other bryophytes, leafy liverworts produce male sexual organs (antheridia) and female sexual organs (archegonia) on the same plants (monoecious) or different plants (dioecious). Thus, the leafy stems of these liverworts are gametophytes. The antheridia are usually produced on short lateral stems with modified lateral leaves (or leafy bracts); the antheridia are typically located on the lower axils of these leaves. The archegonia, in contrast, are produced at the tips of stems inside an erect perianth. The perianth consists of 3 modified leaves (or leafy bracts) that are joined together, forming a tubular-angular bud about 1-2 mm. high. The base of the perianth may be partially hidden by other modified leaves (leafy bracts). When environmental conditions are moist and a film of water covers the surface of plants, two-tailed sperm are released from the antheridia. The sperm swim around until they find the ovules of the archegonia and fertilize them, assuming they don't die first. Chemicals may be released by the archegonia to attract the sperm, although to some extent they swim around at random. After fertilization, a sporophyte develops from the perianth, consisting of a short-lived spore-bearing capsule on a stalk. The capsule is usually ovoid or globoid in shape and black or brown, while the stalk is typically translucent white. Because there is no green photosynthetic tissue nor pores in the sporophyte, it is dependent on the gametophyte for its source of water, carbohydrates, and minerals. At maturity, the capsule of the sporophyte divides into 4 parts to release the tiny spores to the wind. Some leafy liverworts are capable of reproducing asexually by forming gemmae along the margins of their leaves. These small bud-like gemmae eventually become detached from the leaves, after which they are capable of forming new clonal plants at different locations.

Hornworts are a third group of non-vascular plants that are considered to be bryophytes. There are fewer species of hornworts than either liverworts or mosses, and they tend to be less common than the other two groups of non-vascular plants. After germinating from a spore, a hornwort forms a very small and ribbon-like protonema; it is green and lies flat on the substrate (usually the ground). The protonema is soon replaced by a flattened thalloid body that lacks stems and leaves. This thalloid body is a mass of green or blue-green plant tissue that expands by forming short lobes of new growth along its margins. The thalloid body of a hornwort is usually small in size, typically no wider than 3 cm. (1" across). The thalloid body often has dark green spots that are visible from its upper surface. These dark green spots contain small colonies of cyanobacteria, usually Nostoc species, that are able to fix atmospheric nitrogen into a form that can be assimilated by the surrounding plant. The lower side of the thalloid body has fibrous rhizoids that anchor it to the underlying substrate. The photosynthetic cells of the thalloid body often contain a single large chloroplast. This contrasts with the mosses and liverworts, as the latter typically have several small chloroplasts per photosynthetic cell. The large chloroplasts of hornworts are often useful in identifying the species because their chloroplasts can vary considerably in their shape and texture. The thalloid bodies of  hornworts are gametophytes; male and female sexual organs are usually produced on the same plant (monoecious). The antheridia (male sexual organs) and archegonia (female sexual organs) are located near the upper surface of the thalloid body. The antheridia often occur underneath small tubercles, while the archegonia are often found in flask-shaped structures (involucres), that are found on the upper surface of the thalloid body. When these sexual organs become mature, the cells leading to the upper surface disintegrate, exposing the sperm chambers of the antheridia and the ovule chambers of the archegonia to open air. When environmental conditions are moist and a film of water occurs on the thallus body, two-tailed sperm swim from their antheridial chambers to the ovules of the archegonial chambers (assuming the sperm don't die first). The sperm may be attracted to chemicals that are released by the archegonia, however to some extent they swim around at random. After fertilization of an ovule occurs, a sporophyte develops that consists of a flask-like structure at its base and a more or less erect spore-bearing body. Unlike for mosses and most liverworts, the sporophytes of hornworts don't develop stalks (setae). The flask-like base transfers moisture and nutrients from the thalloid body of the gametophyte to the sporophyte; the latter contains cells at its base that are responsible for the continuous growth of the spore-bearing body. The spore-bearing body of the sporophyte is green, narrowly cylindrical to cylindrical in shape, and up to 7.5 cm. (3 inches) long; it is capable of absorbing moisture from the environment and contains photosynthetic cells. As a result, the sporophyte is only partially dependent for its survival on the thalloid body of the gametophyte. As the spore-bearing body matures, it gradually divides into two parts along its length, beginning at the top. This results in the gradual release of the tiny spores to the wind. The dispersal of the spores is assisted by coiled elaters; the latter resemble wire-like springs. Many hornworts are summer annuals that do not reproduce asexually. However, a few species of hornworts produce gemmae (bud-like bodies of clonal plant cells) on either the upper surface of the thalloid body or along its margins.

Why should We have any Interest in Bryophytes?

Like other photosynthetic plants, bryophytes take in carbon dioxide and release oxygen into the atmosphere, making it possible for life forms like ourselves to breathe. Because some bryophytes don't decay readily in some environments, they can sequester surprisingly large amounts of organic carbon and prevent it from being released into the atmosphere. For example, sphagnum mosses (Sphagnum spp.) are a dominant form of plant life in the vast boreal and arctic areas of North America and Eurasia. Because sphagnum mosses don't decay readily in the highly acidic peat bogs and cool to cold areas where they occur, layer after layer of older sphagnum mosses accumulate underneath the surface layer of sphagnum. As a result, huge quantities of organic carbon are sequestered by this single family of mosses, lowering the amount of carbon dioxide and methane gases that are released into the atmosphere. This has prevented the climate of Earth from becoming even warmer than it is. It is thought that the amount of sequestered carbon of sphagnum mosses greatly exceeds the amount of sequestered carbon in all of the tropical forests of the world. No other plants on Earth currently exceed the impact of sphagnum mosses on the Earth's climate!

Bryophytes also have aesthetic value and they are worthy of cultivation in ornamental gardens. Moss gardens have existed in China and Japan for a long time, especially around Buddhist temples. Unfortunately, in North America and Europe, using mosses in gardens is still under-appreciated. Mosses can be used as substitutes for lawns, especially in areas that are shaded and moist. Because of their small size, mosses and other bryophytes never need mowing, although accumulated leaves and other debris should be gently raked away occasionally. Mosses and other bryophytes also have few problems with mammalian herbivores, insect pests, and disease organisms. These plants are also evergreen, so they are able to remain attractive all year long. During warm dry spells, some water from sprinklers can be helpful in keeping mosses healthy and attractive, although they are able to survive severe dessication until the next rainfall. Because of their small size, mosses and other bryophytes don't compete well against larger ground vegetation (for example, vascular plants), although a mixture of mosses and other vegetation can be highly attractive. Some mosses are well-adapted to water, therefore they are useful additions to indoor aquariums and outdoor pools as aquatic plants. A small number of aquatic mosses are readily available through commercial sources. Mosses and other bryophytes are sometimes used as potted plants and inside terrariums. These plants help to beautify and diversify natural areas; they should be used more often in habitat restoration projects.

Mosses and other bryophytes are potentially useful as a source of medicine, insecticides, herbicides, and fungicides. Extracts of some mosses have been found to control potentially harmful bacteria in the human mouth and gastrointestinal tract. Both mosses and liverworts are being studied as a possible source of chemicals that fight various kinds of cancer in humans. Sphagnum mosses are important commercially because they are used to improve the texture of both outdoor soil and potting soil, as an ornamental ground cover around the bases of plants, as a source of insulation material, as an antibacterial dressing for wounds, and as a source of fuel in some parts of the world. In the past, Sphagnum mosses and other mosses have been used to fill chinks in wooden cabins and to stuff pillows and mattresses. The smoke of burnt peat moss is still used to flavor Scotch whiskey during the malting process.

Mosses and other bryophytes have an important role in the environment. Because of their short life-spans, they create high quality soil underneath themselves relatively quickly. Because these small plants often colonize areas with exposed open ground, they help to prevent soil erosion and reduce run-off from heavy rainfall. A cover of mosses also protects the underlying soil from drying out and it moderates the surface temperature of the landscape. Mosses and other bryophytes are able to colonize degraded land that has been left behind by strip mines and other industrial waste areas; this enables higher vascular plants to colonize such land at a later time. Many of these plants are able to tolerate and sequester toxic heavy metals and other contaminants in their tissues, which makes possible the rehabilitation of polluted ground and water. Mosses and other bryophytes growing on exposed ground, trees, roofs of buildings, pavement, and walls are able to buffer harsh urban sounds. Some mosses are used for 'green roofs' in urban and residential landscapes; the insulation that green roofs provide during the winter can reduce heating bills, while the moderation of temperatures that they provide during the summer can reduce air-conditioning bills. While bryophytes are not considered a major source of food for vertebrate fauna and most invertebrate fauna, their colonies help to provide habitat and cover for many insects and other small organisms that dwell near the ground. They also provide habitat and cover for minnows, insect larvae, and other small organisms in some aquatic habitats, such as streams and ponds. Certain kinds of mosses and liverworts often provide nesting sites for the eggs of salamanders, and their foliage and stalked capsules are common components of bird nests.