Model Answer
0 min readIntroduction
*Marchantia polymorpha*, a common liverwort, exhibits a fascinating life cycle characterized by distinct vegetative and sexual reproductive structures. The plant body, or thallus, is a flattened, dichotomously branched structure responsible for vegetative propagation and nutrient absorption. Sexual reproduction in *Marchantia* is heterogamous, involving the production of antheridia (male reproductive structures) and archegonia (female reproductive structures). The archegoniophore, a specialized stalk bearing the archegonia, is a prominent feature of the female gametophyte. Understanding the morphology and development of the female thallus and archegoniophore is crucial to comprehending the reproductive strategies of this bryophyte.
The Female Thallus of *Marchantia*
The female thallus of *Marchantia* is morphologically similar to the male thallus, being a flattened, green, dichotomously branched structure. However, it differs in possessing archegoniophores, which are absent in the male plant. The thallus is approximately 3-10 cm long and 2-5 cm wide. It consists of a single layer of cells forming the cortex, with air chambers present for gaseous exchange. Rhizoids, unicellular root-like structures, anchor the thallus to the substrate and absorb water and nutrients. Gemma cups, involved in asexual reproduction, are also present on the dorsal surface of the thallus.
Development of the Archegoniophore
The archegoniophore develops from specific cells on the dorsal surface of the female thallus. The development proceeds in the following stages:
- Initiation: A group of cells on the dorsal surface becomes meristematic.
- Elongation: These cells divide rapidly, forming a stalk-like structure – the archegoniophore.
- Branching: The archegoniophore typically branches into two or more arms.
- Archegonial Development: Archegonia develop as flask-shaped structures at the tips of the branches of the archegoniophore.
Structure of the Archegoniophore
The archegoniophore is a stalk-like structure that can reach a height of 5-10 cm. It is typically reddish-brown in color, distinguishing it from the green thallus. The archegoniophore is composed of:
- Stalk: The supporting structure that elevates the archegonia.
- Branches: Usually two to three branches arising from the stalk.
- Involucre: A cup-like structure formed by bracts (leaf-like structures) surrounding the archegonia, providing protection.
- Archegonia: The female sex organs, flask-shaped structures containing the egg cell.
Structure of the Archegonium
The archegonium is the female sex organ responsible for producing the egg cell. It consists of the following parts:
- Neck: A slender, elongated portion extending from the venter.
- Venter: The swollen basal portion containing the egg cell and a sterile neck canal cell.
- Neck Canal Cells: Cells within the neck that degenerate before fertilization, forming a canal through which the sperm swims.
- Egg Cell: The female gamete located within the venter.
Fertilization and Development
Fertilization in *Marchantia* requires water. The motile sperm cells, released from antheridia, swim through the water to reach the archegonia. They pass through the neck canal and fertilize the egg cell within the venter, forming a zygote. The zygote develops into an embryo, which remains attached to the archegoniophore and eventually develops into a sporophyte. The sporophyte is dependent on the gametophyte for nutrition.
Comparison with other Bryophytes
| Feature | Marchantia | Other Bryophytes (e.g., Mosses) |
|---|---|---|
| Archegoniophore | Present, elevated stalk | Absent or less developed |
| Thallus Structure | Flattened, dichotomous | More erect, leafy |
| Gemmae Cups | Present for asexual reproduction | Generally absent |
Conclusion
The female thallus and archegoniophore of *Marchantia* represent a sophisticated adaptation for sexual reproduction in the bryophyte lineage. The archegoniophore provides an elevated platform for archegonia, facilitating fertilization by ensuring sperm access. The unique structural features of *Marchantia*, such as the flattened thallus and gemmae cups, distinguish it from other bryophytes and contribute to its ecological success. Further research into the genetic and developmental mechanisms governing archegoniophore formation could provide valuable insights into plant evolution and reproductive strategies.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.