Class 11 Biology – Bryophyta Notes
Complete notes on Bryophyta, general characteristics, Marchantia, reproduction, fertilization, sporophyte and alternation of generations.
Bryophyta
The term Bryophyta is derived from the Greek words bryon, meaning moss, and phyton, meaning plant.
Bryophyta is used as a collective name for a group of plants that includes liverworts, hornworts and mosses, which grow predominantly in amphibious environments.
According to the notes, about 180 genera and 893 species of bryophytes have been reported from Nepal.
- Hepaticopsida – Liverworts
- Anthocerotopsida – Hornworts
- Bryopsida – Mosses
General Characteristics of Bryophyta
- Bryophytes generally grow in moist and shady places.
- The plant body represents the gametophyte.
- The gametophyte may be thalloid or foliose.
- Vascular tissues such as xylem and phloem are absent.
- Bryophytes are called amphibian plants because they require water during fertilization to complete their life cycle.
- They show a distinct alternation between a haploid gametophytic generation and a diploid sporophytic generation.
- The haploid gametophytic generation is dominant in the life cycle.
- Reproduction takes place by vegetative and sexual methods.
- Sexual reproduction is of a highly advanced oogamous type.
Marchantia
Systematic Position
Occurrence
Marchantia includes about 65 species distributed throughout the world. It belongs to the liverworts.
It grows in moist and shady places such as rocks, soil, logs of wood, banks of streams and walls.
The notes mention three species of Marchantia reported in Nepal:
- Marchantia polymorpha
- Marchantia nepalensis
- Marchantia palmata
Structure of Marchantia
The plant body of Marchantia is a thallus.
The thallus is not differentiated into roots, stems and leaves. It is dorsoventrally flattened and dichotomously branched.
A distinct midrib is present on the dorsal surface. At the tip of each branch, a growing apex called the apical notch is present.
Along the line of the midrib, cup-like structures known as gemma cups are present.
Vegetative propagules called gemmae are produced inside the gemma cups and help in vegetative propagation.
On the ventral surface of the thallus, rhizoids and scales are present.
The rhizoids help in absorption and attachment, while the scales protect the thallus.
Types of Rhizoids
- Smooth-walled rhizoids
- Tuberculated rhizoids
Types of Scales
- Ligulate scales
- Appendiculate scales
Reproduction in Marchantia
Marchantia reproduces by:
- Vegetative reproduction
- Sexual reproduction
1. Vegetative Reproduction
A. Fragmentation
The thallus of Marchantia breaks into smaller fragments. Each fragment develops into a new thallus.
During fragmentation, the thallus begins to decay from its posterior end. When decomposition reaches the dichotomy, the thallus separates into segments.
Each separated fragment develops into a new thallus.
B. Adventitious Branches
Small branches develop on both lateral sides of the thallus. These are called adventitious branches.
Each adventitious branch can develop into a new thallus.
C. Gemmae
Gemmae are produced in gemma cups present on the dorsal surface of the thallus along the midrib.
Each gemma is a multicellular, bilaterally symmetrical, discoid body supported by a small stalk.
A gemma has a notch on each lateral side. Most of its cells contain chlorophyll.
At maturity, gemmae become detached and may be dispersed by agencies such as water currents, wind and animals.
On reaching a suitable substratum, the gemma germinates by developing rhizoids.
A small thallus develops from the growing notch in the opposite direction.
2. Sexual Reproduction
Sexual reproduction in Marchantia is a highly advanced oogamous type.
Marchantia plants are dioecious, meaning that male and female sex organs develop on separate thalli.
Alternation of Generations
The sequential alternation of the haploid gametophyte and diploid sporophyte generations is known as alternation of generations.
The notes describe the haploid gametophytic generation as beginning with the formation of spores through meiosis and ending with fertilization and the formation of a zygote.
The diploid sporophytic generation begins with fertilization and formation of the zygote and ends with meiosis and formation of spores.
Male Thallus
The male sex organ of Marchantia is called an antheridium.
An aerial outgrowth known as the antheridiophore develops on the male thallus.
At the tip of the antheridiophore is the male receptacle, which is star-shaped or disc-shaped.
According to the notes, the male receptacle contains eight lobes.
Antheridia are present on the dorsal surface of the male receptacle.
Each antheridium is globular in shape and is surrounded by a single-layered jacket with a short stalk.
The antheridium contains a mass of androgonial cells called androcytes.
At maturity, the jacket ruptures and the androcytes are released. They develop into biflagellate antherozoids, which act as male gametes.
Female Thallus
The female sex organ of Marchantia is called the archegonium.
It develops on an aerial outgrowth known as the archegoniophore.
The archegoniophore bears a female receptacle that is umbrella-shaped or finger-like in structure.
Archegonia are present on the ventral surface of the female receptacle.
Each archegonium is flask-shaped and surrounded by a single-layered jacket.
The archegonium consists of two main regions:
- Venter
- Neck
The venter contains a large egg cell and a smaller ventral canal cell.
The neck contains about 4–6 neck canal cells.
At the tip of the neck, cover cells are present.
Fertilization
Water is essential for fertilization in Marchantia.
- The ventral canal cell and neck canal cells of the archegonium disintegrate and form mucilage.
- The mucilage absorbs water.
- The neck opens through the cover cells.
- Biflagellate antherozoids swim in water and enter the archegonium through the neck.
- An antherozoid reaches the venter and fuses with the egg.
- Fusion produces a diploid zygote.
Sporophyte
The zygote marks the beginning of the sporophyte.
It divides mitotically to form 2-celled, 4-celled and 8-celled embryos.
The embryo subsequently develops into the sporophyte.
The sporophyte is completely dependent upon the gametophyte and is also called the sporogonium.
The sporogonium is differentiated into:
- Foot
- Seta
- Capsule
Foot
The foot absorbs food and mineral nutrients from the gametophytic tissue.
Seta
The seta is a short connection between the foot and the capsule.
Capsule
The capsule is the fertile part of the sporophyte. It consists of a mass of sporogenous cells surrounded by a single-layered jacket.
According to the notes, the sporophyte is also surrounded by two coverings:
- Inner calyptra
- Outer perigynium
The sporogenous cells differentiate into:
- Fertile spore mother cells
- Sterile elater mother cells
Each spore mother cell undergoes meiosis and produces haploid spores.
The elater mother cells divide mitotically and form spindle-shaped elaters.
At maturity, the jacket ruptures and spores are released with the help of the elaters.
The haploid spores represent the beginning of the gametophytic generation. On germination, they give rise to male and female thalli.
Life Cycle of Marchantia
Gametophyte and Sporophyte
| Feature | Gametophyte | Sporophyte |
|---|---|---|
| Ploidy | Haploid (n) | Diploid (2n) |
| Nature | Dominant generation | Dependent generation |
| Main Role | Produces gametes | Produces spores |
| Beginning | Formation/germination of haploid spores | Formation of zygote after fertilization |
| End | Fertilization | Meiosis and spore formation |
Quick Revision
- Bryophyta includes liverworts, hornworts and mosses.
- Bryophytes are called amphibians of the plant kingdom because water is required for fertilization.
- Vascular tissues are absent.
- The gametophytic generation is dominant.
- Bryophyta is divided into Hepaticopsida, Anthocerotopsida and Bryopsida.
- Marchantia belongs to Hepaticopsida.
- The plant body of Marchantia is a dorsoventrally flattened, dichotomously branched thallus.
- Gemma cups occur on the dorsal surface.
- Rhizoids and scales occur on the ventral surface.
- Vegetative reproduction takes place by fragmentation, adventitious branches and gemmae.
- Marchantia is dioecious.
- The male reproductive organ is the antheridium.
- The female reproductive organ is the archegonium.
- Water is essential for fertilization.
- The zygote is the beginning of the sporophytic generation.
- The sporophyte consists of foot, seta and capsule.
- Spore mother cells undergo meiosis to form haploid spores.
- Elaters help in the release and dispersal of spores.
Important Questions
- What is Bryophyta?
- Why are bryophytes called amphibians of the plant kingdom?
- Write the general characteristics of Bryophyta.
- Name the three classes of Bryophyta.
- Write the systematic position of Marchantia.
- Describe the occurrence and habitat of Marchantia.
- Describe the external structure of Marchantia.
- Write the types and functions of rhizoids in Marchantia.
- Explain vegetative reproduction in Marchantia.
- What are gemmae and gemma cups?
- Explain sexual reproduction in Marchantia.
- Describe the structure of the male thallus of Marchantia.
- Describe the structure of the female thallus of Marchantia.
- Describe the structure of an antheridium.
- Describe the structure of an archegonium.
- Explain fertilization in Marchantia.
- Describe the sporophyte of Marchantia.
- What are elaters? State their role.
- Explain alternation of generations in Marchantia.
- Draw and explain the life cycle of Marchantia.
Discussion
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