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)

• 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 :-
  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.

Distinguishing Characters of Division- Bryophyta(Mosses) , Division- Marchantiophyta or Hepatophyta(Liverworts) and Division - Anthocerotophyta(Hornworts)

Character	Bryophyta	Marchantiophyta	Anthocerotophyta
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.

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.

Gametophyte Characters:-

Spore Germination and Protonemata :- Moss life cycle begins when haploid spores are released from a sporophyte capsule and begin to germinate. In the majority of mosses, germination is exosporic, i.e., the spore wall is ruptured by the expanding spore protoplast after its release from the capsule and prior to any cell division. However, in some mosses, e.g. Andreaea, Drummondia, and Leucodon, germination is precocious and endosporic, meaning that cell divisions occur prior to spore release and spore wall rupture, respectively. There are variations in patterns of germination of moss ( K. Nehira 1983). In most mosses, a highly branched filamentous, uniseriate protonema are formed.

Cell specialization occurs within the protonema as a result two types of filaments are formed:-

• a horizontal system of reddish brown,anchoring filaments (rhizoids), called the caulonema
• upright, green filaments, the chloronema.

Formation of bud apical cells:- As the protonema grows, target cells usually on the caulonema generate bud initials that will ultimately divide by sequential oblique divisions to form bud apical cells. This initiates the growth of the leafy gametophore or shoot stage of the moss.

Shoot Morphology and Habit:-

• The leafy shoot continues to grow by mitotic division of its obovoidal to fusiform apical cell and surrounding meristem.
• Divisions occurring in the apical cell form spirally arranged derivatives, each of which will give rise to a single leaf and a portion of the stem.
• The angle of divergence between successive derivatives is responsible for the spatial arrangement of the leaves or phyllotaxy of the shoot.
• Mature leaves of few mosses are clearly ranked; e.g., the leaves of Fissidens and Bryoxiphium are in two rows, a 1/2 phyllotaxy
• Fontinalis and Tetraphis have leaves aligned in three rows, a 1/3 phyllotaxy.
• In most mosses, however, the leaves are spirally distributed, with 2/5 and 3/8 phyllotaxies being most common (W. Frey 1971; B. Crandall-Stotler 1984).
• The peristomate or true mosses (Superclass V) on the basis of position of the perichaetia and subsequent sporophytes have traditionally been divided into two broad morphological groups:-
  1. Acrocarps :-Acrocarps are characterized by erect or ascending shoot systems that are either unbranched or only sparingly branched. Branching is typically sympodial with the branches morphologically comparable to the determinant main shoot from which they arise. Perichaetia are differentiated at the tip of the main or primary shoot and terminate its growth, so further plant growth occurs only if a branch is produced below the perichaetium; such branches are called subfloral innovations
  2. Pleurocarps :-. Pleurocarps are generally characterized by creeping shoot systems, with extensive lateral branching.In such systems, the indeterminant main stem may be morphologically distinct from the secondary and tertiary level branches that arise from it (C. La Farge 1996). Perichaetia in pleurocarps are produced at the tips of very short, basally swollen lateral branches that are very short, morphologically distinct from the vegetative branches.
• Cladocarpic mosses produce perichaetia at the tips of unspecialized lateral branches that display the same heteroblastic leaf series as the vegetative branches. Such branches are themselves capable of branching, and these mosses are neither acrocarpic nor pleurocarpic.
• Pleurocarps form a natural, monophyletic lineage of true mosses (B. Goffinet and W. R. Buck 2004), but cladocarpy has evolved in several different lineages.
• The main stems of Sphagnum (Superclass II) display a furcate or dichotomous branch architecture (H. A. Crum 1984).

Rhizoids:-Mosses are anchored to their substrates by filamentous, often branched, reddish brown rhizoids, except Takakia and Sphagnum. The rhizoids (As in caulonemata) are multicellular with oblique cross walls; their walls are smooth or roughened with papillae.

• Most rhizoids are slender and only sparingly branched (micronematal type) arise from any of the epidermal cells of the stem.
• But others are larger in diameter and extensively branched (macronematal type) and is associated only with branch primordia.

Stem Anatomy:-. In many mosses, the stem is anatomically complex, consisting of a differentiated epidermal layer, a cortex, and a central strand of thin-walled, hydrolyzed water conducting cells, called hydroids.

Leaves:-Considerable variation in the arrangement and structure of moss leaves provides some of the most morphologically useful characters for species identification. Leaves typically arise from all sides of the stem, most commonly exhibiting a spiral phyllotaxy, but distichous and tristichous arrangements can also be found.