Bryophyta(redirected from Bryophytes)
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Bryophyta(brī`əfī'tə, brī'əfī`tə), division of green land plants that includes the mossesmoss,
any species of the class Bryopsida, in which the liverworts are sometimes included. Mosses and liverworts together comprise the division Bryophyta, the first green land plants to develop in the process of evolution.
..... Click the link for more information. (class Bryopsida), the liverwortsliverwort,
any plant of the class Marchantiopsida. Mosses and liverworts together comprise the division Bryophyta, primitive green land plants (see moss; plant); some of the earliest land plants resembled modern liverworts.
..... Click the link for more information. (Marchantiopsida), and the hornworts (Anthocerotopsida). The liverworts and hornworts are generally inconspicuous plants; common liverworts include species of the genera Porella and Marchantia. Anthoceros is the most familiar temperate-zone hornwort genus. Bryophytes differ from ferns, cone-bearing plants, and flowering plants in that they lack a vascular system for the transportation of water. Since their cells must absorb water directly from the air or the ground, nearly all bryophytes grow in moist places.
The conspicuous green plant body of a bryophyte is the haploid, or gametophytegametophyte
, phase of plant life cycles in which the gametes, i.e., egg and sperm, are produced. The gametophyte is haploid, that is, each cell contains a single complete set of chromosomes, and arises from the germination of a haploid spore.
..... Click the link for more information. , generation of the plant life cycle. It consists of a small stem with leaflike projections, as in all mosses and most liverworts, or a leafless, flattened body (thallus), as in some liverworts and all hornworts. The plant is anchored by means of threadlike structures called rhizoids. The leaflike structures and the rhizoids lack the complex internal anatomy found in the leaves and roots of plants with vascular systems. The gametophyte reproduces sexually, giving rise to a diploid, or sporophyte, generation; the sporophyte is a structure that grows directly out of the gametophyte and is at least partly dependent on the gametophyte for nourishment.
In mosses, germinating spores (haploid) produce a green filamentous structure on the surface, called a protonema, the first stage of the gametophyte. Erect branches arise out of the protonema. After the branches produce rhizoids, the protonema dies. Antheridia (or sperm-producing structures) and archegonia (egg-producing structures) are borne in clusters on the tips of the branches of the gametophytes; these structures are usually microscopic. The different sex organs may be in a single cluster, in separate clusters on the same branch, or on separate branches, depending on the species. In the hornworts, antheridia and archegonia are borne either on the same thallus or, in some species, on separate thalli; the antheridia are borne either singly or in small groups, and the archegonia are borne singly. In the liverworts, the gametophyte may be a thallus or may be leafy; the antheridia and archegonia are borne on special branches that arise from the leafy stem.
Fertilization and Reproduction
In all bryophytes fertilization is dependent on water—usually a film of water or the splashing of raindrops—for the transfer of sperm to the egg. Chemical stimuli direct the motile flagellate sperm to the archegonium. The fertilized egg (zygote) grows out of the gametophyte, which is also the source of its nourishment. Typically the sporophyte is a slender stalk from 1 to 2 in. (2.5–5 cm) long, with a capsule at the tip; in some species it may be green and manufacture some of its own food. Cells within the capsule undergo meiosis (reduction division) to produce haploid spores. In many mosses the capsule has a lid, the operculum, which is shed, releasing spores. In other bryophytes the mature capsule ruptures in other ways to release spores.
Classification and Importance
The mosses are generally divided into three orders, with the order Bryales most prominent. It is now believed that the bryophytes descended from green algae by way of now extinct ancestors (the RhyniophytaRhyniophyta
, division of plants known only from fossils, of which the genus Rhynia was perhaps the most important. These plants date from the Silurian and Devonian age.
..... Click the link for more information. ). The bryophytes are important because they are pioneer plants and soil builders on surfaces lacking other vegetation. Sphagnumsphagnum
or peat moss,
any species of the large and widely distributed genus Sphagnum, economically the most valuable moss. Sphagnums, the principal constituent of peat, typically grow as a floating mat on freshwater bogs.
..... Click the link for more information. moss (order Sphagnales) has been economically important as packing material and as peatpeat,
soil material consisting of partially decomposed organic matter, found mainly in swamps and bogs in various parts of the northern temperate zone but also in some semitropical and tropical regions.
..... Click the link for more information. .
The gametophytes may consist of leafy stems or flat thalli. They have no roots but are anchored to the substrate by hairlike rhizoids. Vascular tissue is at best poorly differentiated, with no lignification of cells. Growth results from the divisions of single cells (rather than meristematic tissues) located at stem tips or in notches at the margins of thalli. The sex organs are multicellular and have a jacket of sterile cells surrounding either the single egg produced in flask-shaped archegonia or the vast number of sperms produced in globose to cylindric, stalked antheridia. The sperms swim by means of two flagella. The sporophyte commonly consists of a capsule that produces a large number of spores, a stalklike seta, and a swollen foot anchored in the gametophyte. The spores, nearly always single-celled, are dispersed in the air, except in the case of a small number of aquatics. They germinate directly or produce a juvenile stage called a protonema. See Reproduction (plant)
A division that consists of some 23,000 species of small and relatively simple plants commonly known as mosses, granite mosses, peat mosses, liverworts, and hornworts (see illustration). The bryophytes display a distinct alternation of sexual and asexual generations; the sexual gametophyte, with a haploid chromosome number, is the more diversified. The sporebearing, diploid sporophyte is reduced in size and structure, attached to the gametophyte, and partially or almost completely dependent on it.
The division can be divided into five classes: Sphagnopsida (peat mosses), Andreaeopsida (granite mosses), Bryopsida (true mosses), Hepaticopsida (liverworts), and Anthocerotopsida (hornworts). The mosses have radially organized leafy gametophytes that develop from a protonema and have multicellular rhizoids with slanted crosswalls. The liverworts and hornworts are mostly flat and dorsiventrally organized and have no protonematal stage; the rhizoids are unicellular. Though obviously related, as evidenced by similar sex organs and attachment of a simplified sporophyte to a more complex and independent gametophyte, the classes differ greatly in structural detail. See Plant Kingdom
a division of higher autotrophic terrestrial and, less frequently, freshwater plants. Bryophytes are characterized by a relatively simple structure. They have multicellular sporangia with sporebearing tissue (archesporium) shielded by walls and by two types of multicellular gametangia: male (antheridia) and female (archegonia). Each archegonium has one nonmotile gamete—an egg cell.
The structure of the sporophyte and gametophyte resembles that of other higher plants. For example, in Anthocerotales and many Bryaceae, the epidermis covering the sporogonium has characteristic stomata. Bryophytes are terrestrial plants that usually avoid chloride and sulfate salinity; their chloroplasts contain only chlorophyll. The kinship of the Bryophyta to algae is indicated by the presence in Anthocerotales of pyrenoids and spermatozoids, as well as by the need for enough water to form drops for the sexual process.
The autotrophic gametophyte, which is capable of prolonged independent existence and distribution as a result of vegetative reproduction, dominates the developmental cycle of bryophytes. The sporophyte has been converted into a sporogonium, an organ of asexual reproduction that exists mainly heterotrophically on the gametophore and that only slightly uses its capacity for autotrophic feeding.
In the course of evolution the gametophyte of the Bryophyta changed very little. This probably accounts for its small size and minor tissue segmentation, as well as for the slow rate of species formation in mosses.
The Bryophyta, an isolated branch in the evolution of higher plants, are apparently descended from hypothetical green algae with a segmented thallus and with gametangia similar to a type of multicellular gametangia characteristic of several extant brown algae. The genetic connections of Bryophyta with Pteropsida are not clear. A close relationship between bryophytes and psilophytes has been noted; a number of scientists consider psilophytes to be direct ancestors of the Bryophyta.
Paleobotanical data attest to the antiquity of the Bryophyta. Verified fossil remains of Hepaticae (belonging to the Metzgerialiales) are known from the Devonian; Marchantiales and Musci have been traced back to the Carboniferous, and leafy Jungermanniales to the Jurassic. The richer finds of the Tertiary reveal close similarities with present-day bryophytic flora. Bryophytes of the Quaternary do not differ from extant representatives; only their distribution has essentially changed.
REFERENCESTakhtadzhian, A. L. Vysshie rasteniia, vol. 1. Moscow-Leningrad, 1956.
Abramov, 1.1., and L. I. Savich-Liubitskaia. “Tip Bryopsida: Mokhoobraznye.” In Osnovy paleontologii. Moscow, 1963.
Zerov, D. K. Ocherk filogentii bessosudistykh rastenii. Kiev, 1972.
I. I. ABRAMOV