Bryophyte

Bryophytes are members of the land plants (embryophytes) that are non-vascular, meaning that they lack water- and food-conducting strands in their roots (xylem and phloem), or that they are poorly developed. They do not have roots, only filamentous rhizoids.

Bryophytes include mosses (division Bryophyta), hornworts (division Anthocerotophyta), and liverworts (division Marchantiophyta). Originally, the three groups were brought together as the three classes or three phyla within the division Bryophyta. However, since the three groups of bryophytes form a paraphyletic group, they now are placed in three separate divisions. They are grouped together as bryophytes because of their similarity as non-vascular, land plants. Algae are also non-vascular, but are not land plants.

Bryophytes reveal the diversity in creation, filling a unique niche. Being able to colonize rock surfaces, bryophytes are able to initiate the process of soil formation, making it easier for other plants to colonize. They can also grow on parts of living and decomposing organisms and on soil, among other substrates. Bryophytes role is an important one: they help to capture and recycle nutrients, provide a seed bed for larger plants, prevent erosion through binding the soil, and are instrumental in the formation and maintenance of wetlands. They can grow in many habitats: Mosses are even the most abundant and diverse plants in the Arctic and Antarctic.

Bryophytes have tissues and enclosed reproductive systems, but they neither flower nor produce seeds, reproducing via spores. 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 (Crandall-Stotler and Stotler 2000; Crandall-Stotler et al. 2005). Bryophytes are widely distributed globally and exhibit significant ecological diversity. They are typically small, herbaceous plants, growing closely packed in mats or cushions on rocks, soil, or as epiphytes on the trunks of trees.

Contents

Global Extent and Ecology

Bryophytes are found throughout the world in a variety of habitats, but they thrive in moist, humid environments. They grow on a number of substrates, including soil, parts of living and decomposing organisms, and rock surfaces. They flourish in both temperate and tropical rainforests, and are found extensively in wetlands in the Arctic and Antarctic.

Mosses are not only the most abundant plants in the Arctic and Antarctic, but also have the highest diversity. Mosses are able to live through extended drought, although they are not found in deserts. Bryophtyes require abundant water to reproduce and grow, and their sensitivity to pollution makes them rare near cities or other areas with high levels of pollution.

Their ecological benefits include capturing and recycling nutrients and preventing erosion by binding the soil. Because of their ability to colonize rock surfaces, bryophytes are often the first to begin soil formation, eventually making it easier for other plants to colonize. For this reason, bryophytes are often thought of as pioneer species. They also provide seed beds for other larger plants within the ecosystem.

Bryophytes are instrumental in the formation and maintenance of wetlands, particularly high latitude bogs and fens. The moss genus Sphagnum, or peat moss, is most prevalent in peat bogs. It creates a highly acidic environment in waterlogged areas by taking up cations such as calcium and magnesium and releasing hydrogen ions. Because of its ability to acidify its surroundings and its ability to hold up to 20 times its mass in water, Sphagnum is a key component of bog formation, where decomposition is minimal and peat accumulation is high. Because of its ability to retain water, Sphagnum controls the hydrology of many raised bogs.

Peat has been harvested and sold for both fertilizer and fuel, but conservationists question the sustainability of peat harvesting. Current debates also involve the ability of peatlands to sequester carbon by burying undecomposed organic matter. Under enough heat and pressure, peatlands will eventually turn to coal (though this process takes millions of years).

Mosses

Moss gametophyte generation plants with a single sporophyte.

Mosses are small, soft plants that are typically 1-10 cm tall, occasionally more. They commonly grow close together in clumps or mats in damp or shady locations. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems. At certain times mosses produces spore capsules, which may appear as beak-like capsules borne aloft on thin stalks.

Botanically, mosses are bryophytes, or non-vascular land plants. They can be distinguished from the apparently similar liverworts (Marchantiophyta or Hepaticae) by their multi-cellular rhizoids. Other differences are not universal for all mosses and all liverworts, but the presence of clearly differentiated "stem" and "leaves", the lack of deeply lobed or segmented leaves, and the absence of leaves arranged in three ranks, all point to the plant being a moss.

As with all bryophytes (including hornworts and liverworts), in addition to lacking a vascular system, mosses have a gametophyte-dominant life cycle, i.e., the plant's cells are haploid for most of its life cycle. Sporophytes (i.e. the diploid body) are short-lived and dependent on the gametophyte. This is in contrast to the pattern exhibited by most "higher" plants and by most animals. In vascular plants, for example, the haploid generation is represented by the pollen and the ovule, while the diploid generation is the familiar flowering plant.

Hornworts

Phaeoceros laevis

Hornworts are a group of bryophytes, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.

Hornworts may be found world-wide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees.

The plant body of a hornwort is a haploid gametophyte stage. This stage usually grows as a thin rosette or ribbon-like thallus between one and five cm in diameter. Each cell of the thallus usually contains just one chloroplast per cell. In most species, this chloroplast is fused with other organelles to form a large pyrenoid that both manufactures and stores food. This particular feature is very unusual in land plants, but is common among algae.

Spores of Phaeoceros sp. showing the Y-shaped triradiate ridge.

Many hornworts develop internal mucilage-filled cavities when groups of cells break down. These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color. There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants.

The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. Hornworts sporophytes are unusual in that the sporophyte grows from a meristem near its base, instead of from its tip the way other plants do. Unlike liverworts, most hornworts have true stomata on the sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros, which do not have stomata.

When the sporophyte is mature, it has a multicellular outer layer, a central rod-like columella running up the center, and a layer of tissue in between that produces spores and pseudo-elaters. The pseudo-elaters are multi-cellular, unlike the elaters of liverworts. They have helical thickenings that change shape in response to drying out, and thereby twist in and thereby help to disperse the spores. Hornwort spores are relatively large for bryophytes, measuring between 30 and 80 um in diameter or more. The spores are polar, usually with a distinctive Y-shaped tri-radiate ridge on the proximal surface, and with a distal surface ornamented with bumps or spines.

Liverworts

A thallose liverwort, Lunularia cruciata

In ancient times, it was believed that liverworts cured diseases of the liver, hence the name. In Old English, the word liverwort literally means liver plant. This probably stemmed from the superficial appearance of some thalloid liverworts (which resemble a liver in outline), and led to the common name of the group as hepatics, from the Latin word for liver. An unrelated flowering plant, Hepatica, is sometimes also referred to as liverwort because it was once also used in treating diseases of the liver.

The most familiar liverworts consist of a prostrate, flattened, branching structure called a thallus (plant body). These liverworts are termed thallose liverworts. However, most liverworts produce flattened stems with overlapping scales or leaves in three or more ranks, the middle rank being conspicuously different from the outer ranks. These are called leafy liverworts or scale liverworts.

They can be distinguished from the apparently similar mosses by their single celled rhizoids. Other differences are not universal for all mosses and all liverworts, as noted above, but the lack of clearly differentiated stem and leaves, the presence of deeply lobed or segmented leaves, and the presence of leaves arranged in three ranks all point to the plant being a liverwort. Confirmation of the identification of a moss or a leafy liverwort can only be performed with certainty by microscopical investigation.

Aside from lacking a vascular system, liverworts have a gametophyte-dominant life cycle, as noted above for mosses.

Bryophyte classification

Two hypotheses on the phylogeny of land plants.

About 18,000 species are classified into three groups, the Marchantiophyta (liverworts), Anthocerotophyta (hornworts), and Bryophyta (mosses). Mosses represent the division or phylum with the most diversity, with up to 15,000 recognized species.

Modern studies generally show one of two patterns. In one of these patterns, the liverworts were the first to diverge, followed by the hornworts, while the mosses are the closest living relatives of the vascular plants. In the other pattern, the hornworts were the first to diverge, followed by the vascular plants, while the mosses are the closest living relatives of the liverworts.

Although originally the three groups were placed together in the division Bryophyta, they are generally classified now in the three separate divisions. This is because it was determined by studies of cell ultrastructure and molecular analysis that these three groups come from three different evolutionary lineages.

Classification of mosses

Two different types of mosses (and a lichen, in the smallest box) surround this tree trunk.
Ant on mosshill

Mosses were traditionally considered the class Musci in the Division Bryophyta, grouped with the two classes of liverworts and hornworts. Currently, however, the Division Bryophyta refers specifically to mosses.

The mosses are grouped as a single class, now named Bryopsida, and divided into seven subclasses:

  • Andreaeidae
  • Sphagnidae
  • Tetraphidae
  • Polytrichidae
  • Buxbaumiidae
  • Bryidae
  • Archidiidae

Andreaeidae are distinguished by the biseriate (two rows of cells) rhizoids, multiseriate (many rows of cells) protonema, and sporangium that splits along longitudinal lines. Most mosses have capsules that open at the top.

The Sphagnidae, the peat-mosses, comprise the single genus Sphagnum. These large mosses form extensive acidic bogs in peat swamps. The leaves of Sphagnum have large dead cells alternating with living photosynthetic cells. The dead cells help to store water. Aside from this characteristic, the unique branching, thallose (flat and expanded) protonema, and explosively rupturing sporangium place it apart from other mosses.

The Tetraphidae are unique (as their name implies) in having only four large peristome teeth surrounding the opening of the capsule.

Polytrichidae have leaves with lamellae, which are flaps on the leaves that look like the fins on a heat sink. These help it retain moisture. They differ from other mosses in other details of their development and anatomy too, and can also become larger than most other mosses, with e.g., Polytrichum commune forming cushions up to 40 cm high.

The Buxbaumiidae are called 'bug mosses' because they usually have a very small and reduced gametophore and the whole plant is mostly the sporophyte capsule. The shape reminds one of a bug, which is the reason for its common name.

The Bryidae are the most diverse group; over 95 percent of moss species belong to this subclass.

The Archidiidae are distinguished by their extremely large spores and the way the sporangium develops.

Classification of Hornworts

Traditionally a class within the Division Bryophyta, Hornworts are now considered their own division, Anthocerotophyta. There is a single class of hornworts, called Anthocerotopsida, or traditionally Anthocerotae. This class includes a single order of hornworts (Anthocerotales) in this classification scheme. In some other classification schemes, a second order Notothyladales (containing only the genus Notothylas) is recognized because of the unique and unusual features present in that group.

Among land plants, hornworts appear to be one of the oldest surviving groups. There are only about 100 species known, but new species are still being discovered. The number and names of genera are a current matter of investigation, and several competing classification schemes have been published since 1988.

Families and genera

Anthocerotaceae

  • Anthoceros
  • Folioceros
  • Leiosporoceros
  • Phaeoceros
  • Sphaerosporoceros

Dendrocerotaceae

  • Dendroceros
  • Megaceros
  • Notoceros

Notothyladaceae

  • Notothylas

Classification of Liverworts

Bryologists classify liverworts in the division Marchantiophyta. This divisional name is based on the name of the type species Marchantia polymorpha. In addition to this taxon-based name, the liverworts are often called Hepaticophyta. This name is derived from their common Latin name as Latin was the language in which botanists published their descriptions of species. This name has led to some confusion, partly because it appears to be a taxon-based name derived from the genus Hepatica which is actually a flowering plant of the buttercup family Ranunculaceae. In addition, the name Hepaticophyta is frequently misspelled in textbooks as Hepatophyta, which only adds to the confusion.

Traditionally, the liverworts were grouped together with other bryophytes (mosses and hornworts) in the Division Bryophyta, within which the liverworts made up the class Hepaticae (also called Marchantiopsida). However, since this grouping makes the Bryophyta paraphyletic, the liverworts are now usually given their own division. The use of the division name Bryophyta sensu latu is still found in the literature, but more frequently the Bryophyta now is used in a restricted sense to include only the mosses.

Another reason that liverworts are now classified separately is that liverworts appear to have diverged from all other embryophyte plants (land plants) near the beginning of their evolution. The strongest line of supporting evidence is that liverworts are the only living group of land plants that do not have stomata on the sporophyte generation. The earliest fossils believed to be liverworts are compression fossils of Pallaviciniites from the Upper Devonian of New York. These fossils resemble modern species in the Metzgeriales. Another Devonian fossil called Protosalvinia also looks like a liverwort, but its relationship to other plants is still uncertain, so it may not belong to the Marchantiophyta.

The Marchantiophyta is subdivided into two classes. The Jungermanniopsida includes primarily the two orders Metzgeriales (simple thalloids) and Jungermanniales (leafy liverworts), as well as a smaller order Haplomitriales. The Marchantiopsida includes primarily the orders Marchantiales (complex-thallus liverworts) and Sphaerocarpales (bottle hepatics), as well as the problematic genus Monoclea, which is sometimes placed in its own order Monocleales.

Form and Function

Most bryophytes do not exceed 2 centimeters in length. Their gametophytes are green and produce their own food through photosynthesis and nutrient uptake from the water. The gametophytes are larger than the sporophytes, which obtain food from the gametophyte.

Food conducting tissue similar to that of vascular plants occurs in some mosses, but this feature is lacking in all hornworts and liverworts. For this reason, it is believed that mosses have a separate lineage from that of hornworts and liverworts.

Mosses have a distinct stem and leaves, but hornworts and liverworts do not. With hornworts and liverworts, there is little distinction between the above ground and below ground parts of the gametophyte, even when stem and leaves are distinct. All bryophytes have distinct, strand-like filaments called rhizoids, which are comprised of a few cells, acting to anchor these plants to the substrate and rarely take part in nutrient or water uptake. Water and minerals are usually absorbed directly through the stems and leaves.

Bryophyte life cycle

The bryophytes are generally gametophyte-oriented; that is, the normal plant is the haploid gametophyte, with the only diploid structure being the sporangium in season. The bryophyte sporophyte remains attached to the gametophyte, and does not become a free-living plant, as in other land plants. As a result, bryophyte sexuality is very different from that of other plants. There are two basic categories of sexuality in bryophytes:

  • Bryophytes are said to be dioecious if both male and female sex organs are borne on separate gametophytes, namely the antheridia (male organs) or archegonia (female organs).
  • Bryophytes are said to be monoecious if both male and female sex organs, antheridia and archegonia, are borne on the same plant body.

Some bryophyte species may be either monoecious or dioecious depending on environmental conditions. Other species grow exclusively with one type of sexuality.

Bryophytes undergo two distinct stages of their life cycle: the haploid and diploid generations. When a spore is released, it germinates, forming the protonema, which is a photosynthetic filament of cells. In mosses, the protonema can have multiple branches, but in hornworts and liverworts, the form is usually simpler. As the protonema matures, it forms leafy buds, which form the leafy gametophores that produce the gametangia (antheridia or archegonia). These form at the apex of the stems. The male plants or plant parts produce antheridia, which produce thousands of sperm. Water is required for the sperm to be ejected, either in the form of rain drops or in thin films of water.

Stems that produce archegonia are identified by the pointed apices of the leaves, which enclose the archegonia. The sperm produced by the antheridia must swim to the archegonia and down the neck canal in order to fertilize the egg.

The start of the diploid (sporophyte) generation is marked by the formation of the zygote, which then develops into an embryo. The embryo grows a shoot apex, directly out of the archegonial neck, which grows to enclose the developing sporophyte, forming the calyptra. A capsule, called the sporangium, develops at the apex of the sporophyte. The sporangia contain sporogenous cells, which undergo meiosis, producing meiospores. This marks the beginning of the haploid gametophyte generation.

Moss life cycle

Most kinds of plants have a double portion of chromosomes in their cells (diploid, i.e., each chromosome exists with a partner that contains the same genetic information). Mosses (and other bryophytes) have only a single set of chromosomes (haploid, i.e., each chromosome exists in a unique copy within the cell). There are periods in the moss life cycle when they do have a full, paired set of chromosomes but this is only during the sporophyte stage.

Life cycle of a typical moss (Polytrichum commune)

The life of a moss starts from a haploid spore, which germinates to produce a protonema, which is either a mass of filaments or thalloid (flat and thallus-like). This is a transitory stage in the life of a moss. From the protonema grows the gametophore ("gamete-bearer") that is differentiated into stems and leaves ('microphylls'). From the tips of stems or branches develop the sex organs of the mosses. The female organs are known as archegonia (sing. archegonium) and are protected by a group of modified leaves known as the perichaetum (plural, perichaeta). The archegonia have necks called venters which the male sperm swim down. The male organs are known as antheridia (singular antheridium) and are enclosed by modified leaves called the perigonium (plural, perigonia).

Mosses can be either dioicous (compare with dioecious in seed plants) or monoicous (compare monoecious). In dioicous mosses, both male and female sex organs are borne on different gametophyte plants. In monoicous (also called autoicous) mosses, they are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilization occurs, leading to the production of a diploid sporophyte. The sperm of mosses is biflagellate, i.e. they have two flagella that aid in propulsion. Without water, fertilization cannot occur. After fertilization, the immature sporophyte pushes its way out of the archegonial venter. It takes about a quarter to half a year for the sporophyte to mature. The sporophyte body comprises a long stalk, called a seta, and a capsule capped by a cap called the operculum. The capsule and operculum are in turn sheathed by a haploid calyptra which is the remains of the archegonial venter. The calyptra usually falls off when the capsule is mature. Within the capsule, spore-producing cells undergo meiosis to form haploid spores, upon which the cycle can start again. The mouth of the capsule is usually ringed by a set of teeth called peristome. This may be absent in some mosses.

In some mosses, green vegetative structures called gemmae are produced on leaves or branches, which can break off and form new plants without the need to go through the cycle of fertilization. This is a means of asexual reproduction.

Hornwort life cycle

The life of a hornwort starts from a haploid spore. In most species, there is a single cell inside the spore, and a slender extension of this cell called the germ tube germinates from the proximal side of the spore. The tip of the germ tube divides to form an octant of cells, and the first rhizoid grows as an extension of the original germ cell. The tip continues to divide new cells, which produces a thalloid protonema. By contrast, species of the family Dendrocerotaceae may begin dividing within the spore, becoming multicellular and even photosynthetic before the spore germinates. In either case, the protonema is a transitory stage in the life of a hornwort.

From the protonema grows the adult gametophyte, which is the persistent and independent stage in the life cycle. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter, and several layers of cells in thickness. It is green or yellow-green from the chlorophyll in its cells, or bluish-green when colonies of cyanobacteria grow inside the plant.

When the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoicous, with both sex organs on the same plant, but some plants (even within the same species) are dioicous, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells.

The biflagellate sperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form a zygote, the cell from which the sporophyte stage of the life cycle will develop. Unlike all other bryophytes, the first cell division of the zygote is longitudinal. Further divisions produce three basic regions of the sporophyte.

At the bottom of the sporophyte (closest to the interior of the gametophyte), is a foot. This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence. In the middle of the sporophyte (just above the foot), is a meristem that will continue to divide and produce new cells for the third region. This third region is the capsule. Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores. These are released from the capsule when it splits lengthwise from the tip.

Liverwort Life Cycle

The life of a liverwort starts from a haploid spore, which germinates to produce a protonema, which is either a mass of filaments or thalloid (flat and thallus-like). This is a transitory stage in the life of a liverwort. From the protonema grows the gametophore ("gamete-bearer") that produces the sex organs of the liverworts. The female organs are known as archegonia (singular archegonium) and are protected by the perichaetum (plural perichaeta). The archegonia have necks called venters which the male sperm swim down. The male organs are known as antheridia (singular antheridium) and are enclosed by the perigonium (plural perigonia).

Liverworts can be either dioicous or monoicous. In dioecious liverworts, male and female sex organs are borne on different plants. In monoecious liverworts, they are borne on the same plant. In the presence of water, sperm from the antheridia swim to the archegonia and fertilization occurs, leading to the production of a diploid sporophyte. The sperm of liverworts is biflagellate, i.e., they have two flagellae that aid in propulsion. Without water, fertilisation cannot occur. After fertilization, the immature sporophyte elongates, pushing its way out of the archegonial venter. The sporophyte body comprises a long stalk, called a seta, and a spherical or ellipsoidal capsule. Within the capsule, cells divide to produce elater cells and spore-producing cells that will undergo meiosis to form haploid spores, upon which the cycle can start again.

References

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