Model Answer
0 min readIntroduction
Double fertilization is a complex and unique characteristic feature of flowering plants (angiosperms). Discovered by Sergei Nawaschin and later elaborated by Edströem, it involves two separate fertilization events occurring simultaneously within the embryo sac. Unlike other forms of sexual reproduction where fertilization leads directly to zygote formation, double fertilization results in the formation of both a zygote and the endosperm, a nutritive tissue essential for embryo development. This process is pivotal for seed development and plant propagation, representing a significant evolutionary advancement in plant reproduction.
Understanding Double Fertilization
Double fertilization is the process where two sperm cells from a pollen grain fertilize two different cells within the female gametophyte (embryo sac) of an angiosperm.
The Process – A Step-by-Step Explanation
- Pollen Germination and Pollen Tube Growth: Following pollination, the pollen grain germinates on the stigma and develops a pollen tube. This tube grows down through the style, guided by chemical signals, towards the ovule within the ovary.
- Entry into the Embryo Sac: The pollen tube enters the embryo sac typically through the micropyle, an opening in the ovule.
- Discharge of Sperm Cells: The pollen tube releases two sperm cells into the embryo sac.
- Syngamy (First Fertilization): One sperm cell fuses with the egg cell (female gamete) located in the egg apparatus. This fusion results in the formation of a diploid zygote (2n), which will eventually develop into the embryo.
- Triple Fusion (Second Fertilization): The second sperm cell fuses with the central cell, which typically contains two polar nuclei. This fusion results in the formation of a triploid primary endosperm nucleus (3n). This nucleus divides repeatedly to form the endosperm, a nutritive tissue that provides nourishment to the developing embryo.
Structures Involved
- Pollen Grain: Contains the male gametes (sperm cells).
- Pollen Tube: Facilitates the transport of sperm cells to the ovule.
- Ovule: Contains the embryo sac.
- Embryo Sac: The female gametophyte containing the egg cell and central cell.
- Egg Cell: The female gamete.
- Central Cell: Contains the polar nuclei.
- Zygote: Formed by the fusion of sperm and egg.
- Endosperm: Formed by the fusion of sperm and central cell.
Significance of Double Fertilization
- Endosperm Formation: Ensures that nutritive tissue is available only when fertilization occurs, preventing wastage of resources.
- Seed Development: Both zygote and endosperm development are crucial for seed formation.
- Hybrid Vigor: Contributes to the genetic diversity and vigor of offspring.
- Evolutionary Advantage: Represents a significant evolutionary adaptation in angiosperms, contributing to their dominance in terrestrial ecosystems.
Comparison with other fertilization types
| Feature | Double Fertilization (Angiosperms) | Normal Fertilization (Animals/Gymnosperms) |
|---|---|---|
| Number of sperm involved | Two | One |
| Number of fertilization events | Two (Syngamy & Triple Fusion) | One (Syngamy) |
| Products of fertilization | Zygote & Primary Endosperm Nucleus | Zygote |
| Endosperm formation | Post-fertilization | Pre-fertilization or absent |
Conclusion
Double fertilization is a defining characteristic of angiosperms, representing a remarkable evolutionary innovation. The simultaneous formation of the zygote and endosperm ensures efficient resource allocation and successful seed development. This process is fundamental to the reproductive success and ecological dominance of flowering plants, highlighting its importance in plant biology and agricultural practices. Further research into the molecular mechanisms governing double fertilization continues to reveal its intricacies and potential for crop improvement.
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.