Introduction to botany  © punam kumar



Bryophytes structure and reproduction


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:-




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)

General life cycle :-

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.

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:-

Each protonema can spread over several centimeters, forming a fuzzy green film over its substrate. Usually this protonemal stage is short-lived, but in a few taxa, e.g., Buxbaumia it persists as the vegetative phase of the plant.

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:-

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.

They function primarily as anchoring structures . Rhizoids are not major sites of water and nutrient uptake, but can enhance capillary movement of water along the outer surface of the stem (M. C. F.Proctor 1984).

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.

  • Isophyllous:-The mature leaves of a given shoot are usually all similar in size and shape.
  • Anisophyllous:- but there are taxa that are anisophyllous, with either dorsal or ventral leaves decidedly smaller than the lateral leaves.
  • Except for a few taxa like Fissidens, leaves are attached to the stem along broad transverse lines.

    Sexual reproduction :-

    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.

    Sporophyte of Moss :- Syngamy of the egg and sperm produce a zygote within the archegonium. This zygote undergoes mitosis to produce an embryo, again retained within the archegonium. Finally, the embryo matures into a sporophyte. Diploid sporophyte is typically not photosynthetic and so is parasitic (dependent) on the gametophyte for its nutrition.The sporophyte consisting of :-