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 ), as bulbils in the axils of leaves (in ), or as microscopic filaments (in ). 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.

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 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. 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.

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 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 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.

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 ( ) is a flowering plant in the pineapple family, Irish moss ( ) is a red alga that is collected for the extraction of carageenan, a starch-like substance used in food preparation, reindeer moss ( spp.) is a lichen, and club mosses ( 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 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 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 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 and red in 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.

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.

Liverwort; Symbiosis; Wetlands.

BOOKS

Malcolm, Bill, and Nancy Malcolm. Nelson, New Zealand: Micro-Optics Press, 2000.

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

Les C. Cwynar

The term Bryophyta came up from the word ‘Bryon’  which means mosses and phyton meaning plants. Bryophyta includes embryophytes like mosses, hornworts and liverworts. These are the plants that grow in shady and damp areas and are small in size. They lack vascular tissues. They reproduce through spores instead of producing flowers and seeds.The study of bryophytes is called Bryology.

Why are Bryophytes Called Amphibians of Plants?

Bryophytes are called “amphibians of the plant kingdom” because they’re the terrestrial plants but require water to finish their life cycle at the time of sexual reproduction.

General Characteristics of Bryophytes:

• Plants occur in damp and shaded areas.

• The plant body is thallus like, i.e. prostrate or erect.

• It is attached to the substratum by rhizoids, which are unicellular or multicellular.

• They have a root-like, stem-like and leaf-like structure and lack true vegetative structure.

• Plants lack the vascular system (xylem, phloem).

• The dominant, a part of the plant body is gametophyte which is haploid.

• The thalloid gametophyte is divided into rhizoids, axis and leaves.

• The gametophyte bears multicellular sex organs and photosynthetic.

• The antheridium produces antherozoids, which are flagellated.

• The shape of an archegonium is a sort of a flask and produces one egg.

• The antherozoids fuse with egg to make a zygote.

• The zygote develops into a multicellular sporophyte.

• The sporophyte is semi-parasitic and dependent on the gametophyte for its nutrition.

• Cells of sporophyte undergo meiosis to form haploid gametes which form a gametophyte.

• The juvenile gametophyte is known as protonema.

• The sporophyte is differentiated into foot seta and capsule.

Classification of Bryophytes

According to the newest classification, Bryophyta is split into three classes:

1. Hepaticopsida (Liverworts)

2. Anthocerotopsida (Hornworts)

3. Bryopsida (Mosses)

A. Hepaticopsida (Liverworts): The name hepaticopsida comes from the word “hepatic” which means liver. Liverworts come under this class.

Hepaticopsida is Further Divided into 4 Orders:

1. Marchantiales (e.g. Riccia, Marchantia)

2. Sphaerocarpales (e.g. Sphaerocarpos)

3. Calobryales (e.g. Calobryum)

4. Jungermanniales (e.g. Pellia)

The Main Characteristics of the Class Hepaticopsida are:

• Gametophyte plant is either thalloid or foliose.

• Thalloid forms are dorsiventral, lobed and dichotomously branched.

• Rhizoids are unicellular, branched and septate.

• Sex organs are borne dorsally embedded in gametophytic tissues.

• The sporophyte is a compilation of only capsule (in Riccia) or foot, seta and capsule (in Marchantia).

• The columella is absent in the capsule.

• Sporogenous tissues develop from endothecium.

Reproduction:

1. Asexual Reproduction: It takes placer by the formation of gemmae or by the process of fragmentation. Gemmae are produced inside gemma cups. Gemmae are green and multicellular and are also are asexual in nature. The gemma cup develops into a newly born plant after detaching from the parent plant.

2. Sexual Reproduction: Sexual reproduction: Antheridium (male organ) and archegonium (female organ) could also be present on an equivalent thalli or different thalli. They produce sperm and egg respectively. After fertilisation zygote is formed. The zygote develops into a diploid sporophyte, a couple of cells of the sporophyte undergo meiosis to make haploid spores. These spores become haploid gametophytes, which are free-living and photosynthetic.

B. Anthocerotopsida (Hornworts): There are around 300 species present during this class. They are commonly known as hornworts. It has only one order i.e. Anthocerotales. Examples: Anthoceros, Megaceros, Notothylas.

The Main Features are:

• The gametophytic body is flat, dorsiventral, simple thalloid and has no internal differentiation.

• Rhizoids are smooth-walled.

• Each cell has one chloroplast with a pyrenoid.

• Sex organs are present dorsally embedded in the thallus.

• Sporogenous tissues develop from amphithecium.

• Pseudoelaters are present in the capsule.

• The columella is present within the capsule, which originates from endothecium.

Reproduction:

1. Asexual Reproduction: Vegetative propagation takes place by the process of fragmentation of thallus and by tubers, which are formed under unfavourable conditions.

2. Sexual Reproduction: They reproduce sexually with the help of  waterborne sperm that travels from antheridium to archegonium. A fertilised egg develops into sporophyte. Sporophyte splits lengthwise to release spores which become a gametophyte.

C. Bryopsida (Mosses): It’s the important class of Bryophyta with around 1400 species. They are commonly called mosses. Examples: Funaria, Polytrichum, Sphagnum.

Bryopsida is Further Divided Into 5 Classes:

1. Bryales

2. Andriales

3. Sphagnales

4. Polytrichales

5. Buxbaumiales

The Main Features are:

• The gametophyte is divided into protonema and foliose gametophore.

• Foliose is formed of stem as an axis and leaves without midrib.

• Rhizoids are multicellular with oblique septa.

• Sex organs are borne apically on stem.

• Elaters are absent.

• The sporophyte is differentiated into foot, seta and capsule.

• Sporogenous tissues develop from endothecium.

• Columella is present.

• Seperation of the lid is the result of Dehiscence of the capsule.

Reproduction:

1. Asexual Reproduction: Asexual reproduction takes place by budding and fragmentation of the secondary protonema.

2. Sexual Reproduction: Antheridia and archegonia are present at the apical part of leafy shoots. After fertilization sporophyte is produced, which is more differentiated than liverworts. The gametophyte develops from the spores.

Examples of Bryophytes

Bryophytes consist of around 20,000 plant species. Bryophytes are divided into liverworts, mosses and hornworts. Some common examples are:

Liverworts:

• Marchantia

• Riccia

• Pellia

• Porella

• Sphaerocarpos

• Calobryum

Mosses:

• Funaria

• Polytrichum

• Sphagnum

Hornworts:

• Anthoceros

• Notothylas

• Megaceros

Ecological Importance of Bryophytes

Bryophytes have great ecological importance. Mosses and lichens are the first organisms to colonise rocks.

• They decompose the rock making it suitable for the expansion of the higher plants. Soil formation takes place by the acidic secretion that causes due to the death and decay of mosses. 

• Bryophytes grow densely so act as soil binders.

• Mosses play an important role in bog succession. The thick mat formed of mosses forms suitable substratum for germination of hydrophilic seeds due in the presence of water and humus. In the meanwhile, the dead and decayed mosses and hydrophilic plants form a solid soil for mesophytic development.

• They prevent erosion of soil by reducing the impact of the falling rain.

• They reduce the quantity of run-off water because of their water holding capacity. 

• They help in recycling of the nutrients. 

• They act as a rock builder. These plants decompose bicarbonate ions resulting in the precipitation of insoluble calcium carbonate. The mineral deposit continues to increase and therefore extends over several hundred square feet.

Economic Importance of Bryophytes

1. Medicinal Uses:

• Sphagnum is employed in dressing as it has high absorptive power and a few antiseptic property for filling absorptive bandages in replacement of cotton for the treatment of boils and discharging wounds

• Marchantia has been employed to cure pulmonary tuberculosis and affliction of liver

• The decoction of dried sphagnum is used in the treatment of acute haemorrhage and eye infections

• Peat-tar is antiseptic and used as a preservative. Polytrichum species has shown to dissolve stone in kidney and gallbladder

• Antibiotic substances are often extracted from certain bryophytes having antibiotic properties

In Research: Mosses and liverworts are used in research in the field of genetics. The mechanism of sex determination within the plant is discovered in liverworts

Packing Material: Dried mosses make superb packing for fragile goods like glassware, bulbs. For trans-shipment of living material like cuttings and seedlings as they need water retention capacity

Food: Some mosses provide food for herbaceous mammals, birds and other mammals

As Indicator Plants: Some bryophytes grow during a specialised area and may be used as an indicator for acidity and basicity of the soil. E.g. Polytrichum indicated the acidity of the soil, Tortella species grow well within the soil rich in lime or other bases and occur as calcicoles

In seedbeds: Because of its water retention capacity, it is used in seedbeds, greenhouses, nurseries to root cuttings. Sphagnum is additionally wont to maintain high soil acidity required by certain plants

Peat Formation: Sphagnum is additionally referred to as sphagnum. Peat is formed by slowing down the decaying process. The gradual compression and carbonisation of partially decomposed vegetative matter in bogs produce a dark coloured substance called Peat.

It is used as a fuel.

Lower layers of peat form coal.

Peat is also used in the production of ethyl alcohol, ammonium sulphate, ammonia, dye, paraffin, tannins etc.

It improves soil texture in horticulture.

Formation of Stone: The travertine rock deposits are extensively used as a building stone.