0Bryophytes are small, non-vascular plants, such as mosses, liverworts and hornworts. They play a vital role in regulating ecosystems because they provide an important buffer system for other plants, which live alongside and benefit from the water and nutrients that bryophytes collect.
Some bryophyte species are amongst the first to colonise open ground. Bryophytes are also very good indicators of habitat quality as many plant species in this group are sensitive to levels of moisture in the atmosphere, which are lower in disturbed habitats because there is less shade.
Bryophytes do not have seeds or flowers. Instead they reproduce via spores.
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.
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 alternates with diploid sporophyte .
- Gametophytes homothallic or heterothallic.
- The gametophyte generation is dominant, conspicuous and independent.
- The female is the archegonium.
- The male 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 .
- 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.
