Introduction to botany  © punam kumar

 

CHAPTER 16:

Bryophytes structure and reproduction

Sections --"A" Subsection "1"

16.1. What is bryophytes ?- Introduction

16.2. Habitats of Bryophytes

16.3. Classification of Bryophytes

Sections --"A" Subsections --"2"

16.4. General Characteristics of Bryophyta (Liverworts, Hornworts and Mosses)

16.5. General life cycle of Bryophyta (Liverworts, Hornworts and Mosses)

Sections --"A" Subsection "3"

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

Sections --"B" Subsection ---"1"

16.7. Characteristics of Mosses -

16.8. Gametophyte Characters of moss

16.9. Formation of bud apical cells of moss

16.10. Shoot Morphology and Habit of moss

Section " B" Subsection---"2"

16.11. Sexual reproduction of Moss

16.12. Sporophyte of Moss

Section "C" Subsection---"1"

16.13. Occurrence of Hepatophyta (Liverworts)

16.14. Size of Liverworts

16.15. General Characteristics of Liverworts

16.16. Difference between the gametophytes of thallose and leafy liverworts

Section "C" Subsection---"2"

16.17. Vegetative reproduction of Liverworts

16.18. Life Cycle of Liverworts

Section "D" Subsection---"1"

16.19. Introduction of Hornworts (Anthocerophyta)

16.20. Occurrence of Hornworts (Anthocerophyta)

16.21. The hornwort gametophyte

16.22. Internal Structures of Hornworts (Anthocerophyta)

Section "D" Subsection---"2"

16.23. Life Cycle of Hornworts (Anthocerophyta)

16.1. Introduction

Section " A" Subsection---"1"

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.

16.2. 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.

16.3. 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).

Section " A" Subsection---"2"

16.4. General Characteristics of Bryophyta (Liverworts, Hornworts and Mosses)

16.5. General life cycle of Bryophyta (Liverworts, Hornworts and Mosses:-

Section " A" Subsection---"3"

16.6. 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" Subsection----"1"

Division - Bryophyta (Mosses)

16.7. 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.

16.8. 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.

16.9. 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.

16.10. 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.

Section " B" Subsection---"2"

16.11. Sexual reproduction of Moss :-


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16.12. 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 :-

The gametophyte plant is produced by the germination of a haploid spore.

Section "C" Subsection---"1"

Division - Hepatophyta (Liverworts)

16.13. Occurrence :-Liverworts are odd little plants that appear as small, flat green patches attached to the ground, although they may form large masses in favorable habitats such as moist, shaded rocks or soil, tree trunks or branches and a few even grow directly in water.

16.14. Size :-Liverworts are the simplest of the living plants, and range in size from minuscule, leafy filaments less than 0.02 in (0.5 mm) in diameter, to plants exceeding 8 in (20 cm) in size.

16.15. General Characteristics of Liverworts :-

16.16. Difference between the gametophytes of thallose and leafy liverworts

Thallose liverworts Leafy liverworts
Liverworts consist of a prostrate, flattened, ribbon-like, dichotomously branching structure called a thallus (plant body); these liverworts are termed thallose liverworts . Not look like the mosses Liverworts produce flattened stems with overlapping scales or leaves and look very much like the mosses, these are called leafy liverworts or scale liverworts. They are most easily distinguished from the mosses by their leaf arrangement. Leafy liverworts have leaves that are arranged in two or three rows while the leaves in mosses are spirally arranged.
The tissue within the thallus can be quite differentiated, as in the Marchantiales, or can have little differentiation, as in the Metzgeriales. These relatively undifferentiated thalli are only one or two cell layers thick. Marchantiales typically have thalli that are composed of multiple cell layers.Thallose liverworts in which there is such a differentiation of cell function are called complex thallose liverworts. On the other hand, thallose liverworts in which there is no such differentiation of cell function are called simple thallose liverworts. Like the mosses, liverwort leaves are only one cell layer thick. These cells are usually isodiametric, meaning, the cell is as long as it is wide. .
Stem not present The stem of leafy liverworts is usually green. Unlike the mosses, the liverworts do not have any specialized tissue for internal water or nutrient conduction in the stem.

All liverworts produce mucilage, which helps liverworts absorb and retain water. The mucilage is produced by the gametophytes, either internally in slime cells or externally in slime papillae.Since liverworts are photosynthesizing plants, their cells contain chloroplasts. In addition to chloroplasts, the cells of about 90% of liverwort species contain oil bodies . These vary in size, shape and number per cell, depending on species and are therefore useful for identification.

Rhizoids Another feature common to virtually all liverworts is the presence of rhizoids. These are anchoring structures, superficially root-like, but without the absorptive functions of true roots. All liverworts rhizoids are single-celled, with just a few species having multi-celled rhizoids.Some float as aquatics.

Section "C" Subsection---"2"

Reproduction:- Bryophytes may reproduce both sexually and vegetatively and sexually.

16.17. Vegetative reproduction :-Liverwort gametophyte thalli may reproduce vegetatively by means of gemmae. These are produced in a more-or-less elaborated splash-cup on the upper epidermis. These give the appearance of a tiny nest. The "eggs" in the nest are the gemmae. Each gemma is a tiny gametophyte with two growing points from which new gametophyte tissue can arise mitotically. The splash cup works as expected. A drop of rain water falls into the cupule, some mucilage produced around the base of the gemmae swells with hydration and breaks off the connection between the thallus and the gemmae. The gemmae float freely into the drop of water in the cupule. The next drop of rain that falls into the cup will splash the gemmae out into the environment to either land in a suitable nearby environment, or be carried away in a film of flood water to a new location farther away.

16.18. Life Cycle:- Liverworts, like all plants, have a dibiontic life cycle. A dibiontic life cycle is one that displays two distinct phases that differ in ploidy: haploid and diploid. This phenomenon is known as alternation of generations, and the haploid and diploid phases are also called the gametophyte and the sporophyte, respectively. In liverworts, as in other bryophytes (mosses and hornworts), the dominant phase in the life cycle is the haploid gametophyte phase.The termination “-phyte” means “plant”, so the gametophyte is the “gamete plant” and the sporophyte is the “spore plant”.

Section "D" Hornworts (Anthocerophyta) Subsection---"1"

16.19. Introduction

Hornworts are a group of , or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.They are separated from the liverworts because of certain unusual characteristics of the sporophyte often considered to be advanced. These have an intercalary meristem at the base of the seta, are photosynthetic, and have guard cells.

16.20. OccurrenceHornworts may be found world-wide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees.

16.21. The hornwort gametophyte:-

16.22. Internal Structures:- In a hornwort thallus there are from one to a few chloroplasts per cell, most commonly just one, large chloroplast per cell (though there may be up to 12 per cell in the genus Megaceros).

  • Within photosynthesizing plants, chloroplasts are the sub-cellular structures that contain chlorophyll. In mosses and liverworts there are numerous chloroplasts per cell. Growing tips are covered by mucilage which is produced by the surface cells.
  • Hornworts don't have the slime papillae that are found in many liverworts.Many hornworts develop internal mucilage-filled cavities when groups of cells break down.
  • These cavities are invaded by photosynthetic cyanobacteria, especially species of Nostoc. Such colonies of bacteria growing inside the thallus give the hornwort a distinctive blue-green color.
  • There may also be small slime pores on the underside of the thallus. These pores superficially resemble the stomata of other plants.
  • Once inside a mucilage cavity the Nostoc multiplies and forms a symbiotic association with the hornwort.
  • Nostoc provides the hornwort with nitrogen and gets carbohydrates in exchange. All hornworts have these Nostoc cavities .
  • They are best developed in the genus Dendroceros, where they are readily visible to the naked eye as blackish dots.
  • The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. Hornworts sporophytes are unusual in that the sporophyte grows from a meristem near its base, instead of from its tip the way other plants do.
  • Unlike liverworts, most hornworts have true stomata on the sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros, which do not have stomata.
  • Section "D" Subsection---"2"

    16.23. Life Cycle:-

    Sex organsWhen the gametophyte has grown to its adult size, it produces the sex organs of the hornwort. Most plants are monoicous, with both sex organs on the same plant, but some plants (even within the same species) are dioicous, with separate male and female gametophytes. The female organs are known as archegonia (singular archegonium) and the male organs are known as antheridia (singular antheridium). Both kinds of organs develop just below the surface of the plant and are only later exposed by disintegration of the overlying cells.

    Fertilization:-The biflagellate sperm must swim from the antheridia, or else be splashed to the archegonia. When this happens, the sperm and egg cell fuse to form a zygote, the cell from which the sporophyte stage of the life cycle will develop.

    Development of Sporophyte:- Unlike all other bryophytes, the first cell division of the zygote is longitudinal.

  • Further divisions produce three basic regions of the sporophyte.At the bottom of the sporophyte (closest to the interior of the gametophyte), is a foot.
  • This is a globular group of cells that receives nutrients from the parent gametophyte, on which the sporophyte will spend its entire existence.
  • In the middle of the sporophyte (just above the foot), is a meristem that will continue to divide and produce new cells for the third region. This third region is the capsule.
  • Both the central and surface cells of the capsule are sterile, but between them is a layer of cells that will divide to produce pseudo-elaters and spores.
  • These are released from the capsule when it splits lengthwise from the tip.
  • Mature Sporophyte:-

     

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