Introduction

Section " A"

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:-

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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)

• Photosynthetic, non-vascular plants
• Plant body is either :-
  1. Thalloid and attached to the substratum by hair-like structures called rhizoids (true roots are absent) or
  2. 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.

Section " B"

Division - Bryophyta (Mosses)

Characteristics of Mosses :-Mosses are mostly-terrestrial bryophytes.Mosses are found in a range of habitats, although moist and shady habitats are more common. Mosses are often epiphytes.

• The dominant phase of the moss life cycle is the gametophyte (haploid).
• The plant is called a thallus, they may be erect or prostrate (axis along the ground).
• Mosses have radial symmetry, in that a cut down the long axis of an individual gives two similar halves.
• The gametophyte has a stem like axis with spirally arranged leaves, which are known as phyllids . Mosses attach to their substrate with multicellular rhizoids .
• Moss leaves are variable in shape.Leaves usually consist of a single cell layer and are traversed by a midrib that is always more than one cell in thickness.The phyllids of mosses such as Mnium may be a single cell thick, but with a midrib with hydroids and leptoids. Polytrichum have a pad of cells and filamentous strands of photosynthetic cells. The margins of the leaves are often toothed, the teeth pointed or rounded.
• It lacks xylem and phloem.The plant body may have conducting tissue.
• The xylem-like water-and-mineral-conducting tissue is called hydroid. The phloem-like sugar-and-amino-acid-conducting tissue is called leptoid.
• All mosses have a sporic (diplohaplontic) life cycle that is oogamous.

Life cycle of Mosses :-

Asexual reproduction :- The gametophyte may reproduce asexually via bulbils or fragmentation of secondary protonema.

Sexual reproduction :-

• For sexual reproduction, the moss gametophyte produces gametangia. The male and female gametangia may be on the same thallus (homothallic or monoecious) or on separate gametophytes (heterothallic or dioecious).
• Both the antheridium and archegonium have a sterile jacket of cells, which better protects the gametes against desiccation in the terrestrial environment.
• Antheridium :- The antheridium consists of a stalk, a sterile jacket, and spermatogenic tissue. The antheridium sterile jacket has a cap cell which disintegrates when turgor pressure rises.By mitotic division of haploid spermatogenic tissue inside the sterile jacket haploid flagellated sperms are formed. Water is required for transfer of the motile sperm to egg.Most antheridia are in terminal disk-shaped clusters to facilitate water capture for sperm transfer. Sperms are chemotactic and swim through free-water up a concentration gradient of the chemotactic agent to find the open archegonium.The first drop of water landing in the cup causes the cap cell of the anteridium to burst providing an opening for sperm into the drop of water. Filaments of cells found between the antheridia, called paraphyses, swell up with water and squeeze the antheridia to help expel sperm into the water of the splash cup. The next raindrop to land in the splashcup will splash out a solution containing sperm. These will swim through a film of rainwater to fuse with the egg.
• Archegonium:-The archegonium consists of a stalk, a venter surround the egg, and a long neck. The neck is filled with canal cells. The sterile jacket has a cap cell which disintegrates when turgor pressure rises. All cells of the archegonium, including the egg cell, are produced by mitosis of haploid gametophyte cells. The disintegrating neck and ventral canal cells provide chemicals involved in sperm chemotaxis to fuse with the egg. After fusion of egg and sperm zygote is formed which diploid.
• After fertilization, the sporophyte grows out of the archegonium, and nutrients for the developing sporophyte are provided by the gametophyte.
• Meiosis in the capsule produces haploid spores. When spores are mature, the lid of the capsule, called the operculum, opens. Due to changes of humidity a row or rows of hygroscopic teeth, the operculum, open and release spores.

The gametophyte plant is produced by the germination of a haploid spore. As a spore germinates, it produces a branched filament of photosynthetic cells called a protonema. This branching filament is similar to a green alga.The protonema produces a caulonema filament which can produce either a leafy moss gametophyte or a hard, dry bulbil for asexual reproduction.The moss gametophyte produces male and female gametangia. The sperm and egg fuse in syngamy. The resulting diploid zygote and subsequently-developing diploid sporophyte is typically not photosynthetic and so is parasitic (dependent) on the gametophyte for its nutrition.

Sporophyte of Moss :-The resulting diploid zygote and subsequently-developing diploid sporophyte is typically not photosynthetic and so is parasitic (dependent) on the gametophyte for its nutrition.The sporophyte consists of a foot , anchored in the archegonium, a seta, or stalk, which elevates the sporangium, or capsule.