Bryophyte

Bryophytes are an informal group consisting of three divisions of non-vascular land plants (embryophytes): the liverworts, hornworts and mosses.[1] They are characteristically limited in size and prefer moist habitats although they can survive in drier environments.[2] The bryophytes consist of about 20,000 plant species.[3][4] Bryophytes produce enclosed reproductive structures (gametangia and sporangia), but they do not produce flowers or seeds. They reproduce via spores.[5] Bryophytes are usually considered to be a paraphyletic group and not a monophyletic group, although some studies have produced contrary results. Regardless of their status, the name is convenient and remains in use as an informal collective term. The term "bryophyte" comes from Greek βρύον, bryon "tree-moss, oyster-green" and φυτόν, phyton "plant".

Marchantia, an example of a liverwort.

Terminology

The term "Bryophyta" was first suggested by Braun (1964). G.M. Smith placed this group between Algae and Pteridophyta.[6]

Features

The defining features of bryophytes are:

  • Their life cycles are dominated by the gametophyte stage
  • Their sporophytes are unbranched
  • They do not have a true vascular tissue containing lignin (although some have specialized tissues for the transport of water)[7]

Habitat

Bryophytes exist in a wide variety of habitats. They can be found growing in a range of temperatures (cold arctics and in hot deserts), elevations (sea-level to alpine), and moisture (dry deserts to wet rainforests).[8]

Bryophytes can grow where vascularized plants cannot because they do not depend on roots for an uptake of nutrients from soil. Bryophytes can survive on rocks and bare soil.[8]

Life cycle
See also: Alternation of generations
The life cycle of a dioicous bryophyte. The gametophyte (haploid) structures are shown in green, the sporophyte (diploid) in brown.

Like all land plants (embryophytes), bryophytes have life cycles with alternation of generations.[9] In each cycle, a haploid gametophyte, each of whose cells contains a fixed number of unpaired chromosomes, alternates with a diploid sporophyte, whose cell contain two sets of paired chromosomes. Gametophytes produce haploid sperm and eggs which fuse to form diploid zygotes that grow into sporophytes. Sporophytes produce haploid spores by meiosis, that grow into gametophytes.

Bryophytes are gametophyte dominant,[10] meaning that the more prominent, longer-lived plant is the haploid gametophyte. The diploid sporophytes appear only occasionally and remain attached to and nutritionally dependent on the gametophyte.[11] In bryophytes, the sporophytes are always unbranched and produce a single sporangium (spore producing capsule), but each gametophyte can give rise to several sporophytes at once.

The sporophyte develops differently in the three groups. Both mosses and hornworts have a meristem zone where cell division occur. In hornworts, the meristem starts at the base where the foot ends, and the division of cells is pushing the sporophyte body upwards. In mosses, the meristem is located between the capsule and the top of the stalk (seta), and produce cells downward, elongating the stalk and elevates the capsule. In liverworts the meristem is absent and the elongation of the sporophyte is caused almost exclusively by cell expansion.[12]

Liverworts, mosses and hornworts spend most of their lives as gametophytes. Gametangia (gamete-producing organs), archegonia and antheridia, are produced on the gametophytes, sometimes at the tips of shoots, in the axils of leaves or hidden under thalli. Some bryophytes, such as the liverwort Marchantia, create elaborate structures to bear the gametangia that are called gametangiophores. Sperm are flagellated and must swim from the antheridia that produce them to archegonia which may be on a different plant. Arthropods can assist in transfer of sperm.[13]

Fertilized eggs become zygotes, which develop into sporophyte embryos inside the archegonia. Mature sporophytes remain attached to the gametophyte. They consist of a stalk called a seta and a single sporangium or capsule. Inside the sporangium, haploid spores are produced by meiosis. These are dispersed, most commonly by wind, and if they land in a suitable environment can develop into a new gametophyte. Thus bryophytes disperse by a combination of swimming sperm and spores, in a manner similar to lycophytes, ferns and other cryptogams.

Further information: Liverwort § Life cycle, Moss § Life cycle, and Hornwort § Life cycle

Sexuality

The arrangement of antheridia and archegonia on an individual bryophyte plant is usually constant within a species, although in some species it may depend on environmental conditions. The main division is between species in which the antheridia and archegonia occur on the same plant and those in which they occur on different plants. The term monoicous may be used where antheridia and archegonia occur on the same gametophyte and the term dioicous where they occur on different gametophytes.[14]

In seed plants, "monoecious" is used where flowers with anthers (microsporangia) and flowers with ovules (megasporangia) occur on the same sporophyte and "dioecious" where they occur on different sporophytes. These terms occasionally may be used instead of "monoicous" and "dioicous" to describe bryophyte gametophytes. "Monoecious" and "monoicous" are both derived from the Greek for "one house", "dioecious" and "dioicous" from the Greek for two houses. The use of the "oicy" terminology refers to the gametophyte sexuality of bryophytes as distinct from the sporophyte sexuality of seed plants.[14]

Monoicous plants are necessarily hermaphroditic, meaning that the same plant has both sexes.[14] The exact arrangement of the antheridia and archegonia in monoicous plants varies. They may be borne on different shoots (autoicous or autoecious), on the same shoot but not together in a common structure (paroicous or paroecious), or together in a common "inflorescence" (synoicous or synoecious).[14][15] Dioicous plants are unisexual, meaning that the same plant has only one sex.[14] All four patterns (autoicous, paroicous, synoicous and dioicous) occur in species of the moss genus Bryum.[15]

Classification and phylogeny
Hornworts include those bryophytes that are believed to be the closest living relatives of the vascular plants.
Mosses are one group of bryophytes.

Traditionally, all living land plants without vascular tissues were classified in a single taxonomic group, often a division (or phylum). More recently, phylogenetic research has questioned whether the bryophytes form a monophyletic group and thus whether they should form a single taxon. Although a 2005 study supported the traditional view that the bryophytes form a monophyletic group,[16] by 2010 a broad consensus had emerged among systematists that bryophytes as a whole are not a natural group (i.e., are paraphyletic), although each of the three extant (living) groups is monophyletic.[17][18][19]

The three bryophyte clades (which may be treated as divisions) are the Marchantiophyta (liverworts), Bryophyta (mosses) and Anthocerotophyta (hornworts).[20] The vascular plants or tracheophytes form a fourth, unranked clade of land plants called the "Polysporangiophyta". In this analysis, hornworts are sister to vascular plants and liverworts are sister to all other land plants, including the hornworts and mosses.[19][21] Phylogenetic studies continue to produce conflicting results. In particular those based on gene sequences suggest the bryophytes are paraphyletic, whereas those based on the amino acid translations of the same genes suggest they are monophyletic. A 2014 study concluded that composition biases were responsible for these differences and that the bryophytes are monophyletic.[22] The issue remains unresolved.

Paraphyletic view
embryophytes
 

liverworts

 
 
 

mosses

 
 
 

hornworts

 
 

vascular plants

 
 
 
 
bryophytes
Liverworts are included in the bryophyte group

When extinct plants are taken into account, the picture is slightly altered. Some extinct land plants, such as the horneophytes, are not bryophytes, but also are not vascular plants because, like bryophytes, they do not have true vascular tissue. A different distinction is needed. In bryophytes, the sporophyte is a simple unbranched structure with a single spore-forming organ (sporangium). In all other land plants, the polysporangiophytes, the sporophyte is branched and carries many sporangia.[23][24] It has been argued that this contrast between bryophytes and other land plants is less misleading than the traditional one of non-vascular versus vascular plant, since many mosses have well-developed water-conducting vessels.[25] The contrast is shown in a slightly different cladogram:[26]

land plants
 

liverworts

 
 
 

mosses

 
 
 

hornworts

 
 
polysporangiophytes
 

"protracheophytes", such as Horneophyton or Aglaophyton

 
 

tracheophytes or vascular plants

 
 
 
 
 
 

The term "bryophyte" thus refers to a grade of lineages defined primarily by what they lack. Compared to other living land plants, they lack vascular tissue containing lignin and branched sporophytes bearing multiple sporangia. The prominence of the gametophyte in the life cycle is also a shared feature of the three bryophyte lineages (extant vascular plants are all sporophyte dominant).

Other views

An alternative phylogeny, based on amino acids rather than genes, shows bryophytes as a monophyletic group:[22]

embryophytes
bryophytes
 

hornworts

 
 
 

liverworts

 
 

mosses

 
 
 
 

vascular plants

 
 

If this phylogeny proves correct, then the complex sporophyte of living vascular plants might have evolved independently of the simpler unbranched sporophyte present in bryophytes.[22] Other studies suggest a monophyletic group comprising liverworts and mosses, with hornworts being sister to vascular plants.[27]

Evolution

There have probably been several different terrestrialization events, in which originally aquatic organisms colonized the land, just within the lineage of the Viridiplantae.[28] Between 510 - 630 million years ago, however, land plants evolved from aquatic plants, specifically green algae. Molecular phylogenetic studies conclude that bryophytes are the earliest diverging lineages of the extant land plants.[29][1][30][31] They provide insights into the migration of plants from aquatic environments to land. A number of physical features link bryophytes to both land plants and aquatic plants.

Similarities to algae and vascular plants

Green algae, bryophytes and vascular plants all have chlorophyll a and b, and the chloroplast structures are similar.[32] Like green algae and land plants, bryophytes also produce starch stored in the plastids and contain cellulose in their walls.[32] Distinct adaptations observed in bryophytes have allowed plants to colonize Earth's terrestrial environments. To prevent desiccation of plant tissues in a terrestrial environment, a waxy cuticle covering the soft tissue of the plant may be present, providing protection. In hornworts and mosses, stomata provide gas exchange between the atmosphere and an internal intercellular space system. The development of gametangia provided further protection specifically for gametes, the zygote and the developing sporophyte.[33] The bryophytes and vascular plants (embryophytes) also have embryonic development which is not seen in green algae.[32] While bryophytes have no truly vascularized tissue, they do have organs that are specialized for specific functions, analogous for example to the functions of leaves and stems in vascular land plants.[32]

Bryophytes depend on water for reproduction and survival. In common with ferns and lycophytes, a thin layer of water is required on the surface of the plant to enable the movement of the flagellated sperm between gametophytes and the fertilization of an egg.[33]

Comparative morphology

Summary of the morphological characteristics of the gametophytes of the three groups of bryophytes:

Liverworts Mosses Hornworts
Structure Thalloid or foliose Foliose Thalloid
Symmetry Dorsiventral or radial Radial Dorsiventral
Rhizoids Unicellular Pluricellular Unicellular
Chloroplasts/cell Many Many One
Protonemata Reduced Present Absent
Gametangia
(antheridia
and archegonia)		 Superficial Superficial Immersed

Summary of the morphological characteristics of the sporophytes of the three groups of bryophytes:

Liverworts Mosses Hornworts
Stomata Absent Present Present
Structure Small, without chlorophyll Large, with chlorophyll Large, with chlorophyll
Persistence Ephemeral Persistent Persistent
Growth Defined Defined Continuous
Seta Present Present Absent
Capsule form Simple Differentiated
(operculum, peristome)		 Elongated
Maturation of spores Simultaneous Simultaneous Graduate
Dispersion of spores Elaters Peristome teeth Pseudo-elaters
Columella Absent Present Present
Dehiscence Longitudinal or irregular Transverse Longitudinal

Uses

Environmental

  • Soil Conditioning
  • Bioindicators
  • Moss gardens
  • Pesticides

Characteristics of bryophytes make them useful to the environment. Depending on the specific plant texture, bryophytes have been shown to help improve the water retention and air space within soil.[34] Bryophytes are used in pollution studies to indicate soil pollution (such as the presence of heavy metals), air pollution, and UV-B radiation.[34] Gardens in Japan are designed with moss to create peaceful sanctuaries.[34] Some bryophytes have been found to produce natural pesticides. The liverwort, Plagiochila, produces a chemical that is poisonous to mice.[34] Other bryophytes produce chemicals that are antifeedants which protect them from being eaten by slugs.[34] When Phythium sphagnum is sprinkled on the soil of germinating seeds, it inhibits growth of "damping off fungus" which would otherwise kill young seedlings.[35]

Moss peat is made from Sphagnum

Commercial

  • Fuel
  • Packaging
  • Wound Dressing

Peat is a fuel produced from dried bryophytes, typically Sphagnum. Bryophytes' antibiotic properties and ability to retain water make them a useful packaging material for vegetables, flowers, and bulbs.[34] Also, because of its antibiotic properties, Sphagnum was used as a surgical dressing in World War I.[34]

See also

References
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