Bryophytes (nonvascular Plants) are the only embryophytes (plants that produce an embryo) whose life history includes a dominant gametophyte (haploid) stage.They are an ancient and diverse group of non-vascular plants.They comprise three main taxonomic groups: mosses (Bryophyta), liverworts (Marchantiophyta or Hepatophyta) and hornworts (Anthocerotophyta) which have evolved quite separately.They are not considered to have given rise to the vascular plants but they probably were the earliest land plants (Qui & Palmer, 1999). Like the rest of the land plants, they evolved from green algal ancestors, closely related to the Charophytes.
Most bryophytes have erect or creeping stems and tiny leaves, but hornworts and some liverworts have only a flat thallus and no leaves.Worldwide there are possibly 10,000 species of mosses, 7000 liverworts and 200 hornworts.
Habitats :- Small in size, but they can be very conspicuous growing as extensive mats in woodland, as cushions on walls, rocks and tree trunks, and as pioneer colonists of disturbed habitats.
Classification of Bryophytes:-
Although the bryophyte is used as a collective term for all of these -Bryophyta(mosses), Hepatophyta (liverworts), and Anthoceratophyta (hornworts).
General Characteristics of Bryophyta (Liverworts, Hornworts and Mosses)
- All of these are land plants (terrestrial) with some aquatic forms.
- They are very small. The sporophyte and gametophyte have very different morphologies (heteromorphic generations) and the sporophyte is usually partly dependent on the gametophyte.
- Photosynthetic, non-vascular plants
- Plant body is either :-
- Thalloid and attached to the substratum by hair-like structures called rhizoids (true roots are absent) or
- is differentiated into stem-like (caulalia) and leaf-like structures (phyllids), true stems and leaves lacking.
- Cuticle and stomata are absent.
- The bryophytes show alternation of generations - the haploid gametophyte (producing gametes for sexual reproduction) alternates with diploid sporophyte (producing spores for asexual reproduction).
- Gametophytes homothallic or heterothallic.
- The gametophyte generation is dominant, conspicuous and independent.
- The female sex organ is the archegonium.
- The male sex organs are antheridia.
- The ovum remains in the archegonium and spermatozoids swim to it by chemotaxis.
- Although bryophytes are land plants, they are still dependent upon water for fertilization, as the sperm swim in a water film.
- The sporophyte is attached and dependent upon the gametophyte for nutrition i.e. is parasitic on the gametophyte
- The diploid sporophyte usually consists of a basal foot, an elevating seta and a terminal sporangium - the capsule
- Spores are produced as a direct result of meiosis.
- Spores dispersed by a mechanism which ensures dispersal in dry weather only.
- These plants (in either generation) lack specialized cells for the transport of materials (vascular tissue). Absence of vascular tissue limits bryophytes to moist habitats and small size.
General life cycle :-
- Archegonia ;- Archegonia are stalked, multicellular, flask-shaped female sex organs.
- Archegonia are consisting of an elongated upper portion called neck and lower swollen portion -venter. The neck consists of an axial row of cells called neck canal cells surrounded by a sterile jacket.The venter also made up of a 1-2 layer-thick wall of sterile cells which encloses a larger egg cell or the ovum and the smaller ventral canal cell just above the egg.
- At maturity, the tip of the archegonium opens and the neck canal cells as well as the ventral canal cells disintegrate, opening the neck for the entrance of the antherozoids.
- Antheridia consist of rounded structure consisting of a single layered jacket surrounding a central mass of cells - androcytes.
- Each changes into slender biflagellated antherozoids.
- The antherozoids are released when the antheridium ruptures, thus allowing them to swim freely in a water film.The antherozoids enter through the open necks and fuses with egg to form diploid zygote.
- After, divisions of zygote a multicellular embryo is formed, which is nourished by the gametophyte.
- The embryo grows & forms a mature sporophyte, within which sporogenous tissue will form spore tetrads, which in turn are released as the spores, forming either the gametophyte, or the protonema, which in turn forms the typical gametophyte.
Distinguishing Characters of Division- Bryophyta(Mosses) , Division- Marchantiophyta or Hepatophyta(Liverworts) and Division - Anthocerotophyta(Hornworts)
|Protonema||Filamentous, forming many buds||Globose, forming one bud||Globose, forming one bud|
|Gametophyte form||Leafy shoot||Leafy shoot or thallus; thallus simple or with air chambers||Simple thallus|
|Leaf arrangement||Leaves in spirals||Leaves in three rows||Not Applicable|
|Leaf form||Leaves undivided, midvein present.||Leaves divided into 2+ lobes, no midvein||Not Applicable.|
|Special organelles||None||Oil bodies||Single plastids with pyrenoids.|
|Water conducting cells||Present in both gametophytes and sporophytes||Present only in a few simple thalloid forms||Absent.|
|Rhizoids||Brown, multicellular||Hyaline, one-celled.||Hyaline, one-celled|
|Gametangial position||Apical clusters||Apical clusters (leafy forms) or on upper surface of thallus||Sunken in thallus, scattered|
|Stomates||Present on sporophyte capsule.||Absent in both generations||Present in both sporophyte and gametophyte.|
|Seta||Photosynthetic, emergent from gametophyte early in development||Hyaline, elongating just prior to spore release||Absent.|
|Capsule||Complex with operculum, theca and neck; of fixed size||Undifferentiated, spherical or elongate; of fixed size||Undifferentiated, horn-shaped; growing continuously from a basal meristem.|
|Sterile cells in capsule||Columella.||Spirally thickened elaters||Columella and pseudoelaters.|
|Capsule dehiscence||At operculum and peristome teeth||Into 4 valves||Into 2 valves.|