1. Explain the process of pollination in angiosperms and its types.
Answer:
Pollination is the transfer of pollen grains from the anther to the stigma of a flower. It can be of two types:
- Self-pollination (Autogamy): Transfer of pollen occurs within the same flower or another flower on the same plant. Types include:
- Cleistogamy: Pollination within closed flowers.
- Chasmogamy: Pollination in open flowers.
- Cross-pollination (Xenogamy): Transfer of pollen from the anther of one flower to the stigma of another flower on a different plant. Types include:
- Anemophily: Pollination by wind.
- Hydrophily: Pollination by water.
- Zoophily: Pollination by animals.
2. Describe the structure of a mature pollen grain.
Answer:
A mature pollen grain consists of:
- Exine: The outer hard, protective layer made of sporopollenin, which is resistant to environmental factors.
- Intine: The inner layer made of cellulose and pectin.
- Cytoplasm: Contains the vegetative (tube) nucleus and generative nucleus.
The generative nucleus divides to form two sperm cells during pollen germination.
3. Explain the role of pollination in plant reproduction.
Answer:
Pollination is essential for the sexual reproduction of flowering plants. It enables:
- Transfer of genetic material: Ensures genetic recombination and diversity.
- Seed and fruit production: Pollination initiates fertilization, leading to the development of seeds and fruits.
- Adaptation and evolution: Facilitates the development of new traits through genetic variation.
4. What adaptations are found in wind-pollinated flowers?
Answer:
Adaptations include:
- Small, light, and dry pollen grains: Easily carried by wind.
- Feathery or sticky stigmas: Trap airborne pollen effectively.
- Reduced or absent petals: Avoid hindrance to pollen dispersal.
- Large numbers of pollen grains: Compensate for wastage during dispersal.
5. Outline the process of double fertilization in angiosperms.
Answer:
Double fertilization occurs as follows:
- The pollen tube grows through the style into the ovule via the micropyle.
- Two male gametes are released into the embryo sac.
- One gamete fuses with the egg cell to form a diploid zygote (syngamy).
- The other gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus.
This results in the formation of the embryo and endosperm.
6. Discuss the significance of double fertilization.
Answer:
Double fertilization ensures:
- Efficient use of resources: Endosperm develops only when fertilization occurs.
- Nutritional support: The triploid endosperm nourishes the developing embryo.
- Synchronization: Coordinates the development of the embryo and supporting tissues.
7. Describe the structure of an ovule in angiosperms.
Answer:
An ovule consists of:
- Integuments: Protective layers surrounding the nucellus.
- Micropyle: A small opening for pollen tube entry.
- Nucellus: Provides nutrition to the developing embryo sac.
- Embryo sac: Contains the egg cell, synergids, antipodals, and polar nuclei.
- Funicle: Attaches the ovule to the ovary wall.
8. How does the pollen tube grow towards the ovule?
Answer:
The pollen tube grows through chemotropism, guided by chemicals secreted by the synergids in the ovule. The tube penetrates the micropyle and releases the male gametes into the embryo sac.
9. Differentiate between self-pollination and cross-pollination.
Answer:
| Feature | Self-pollination | Cross-pollination |
|---|---|---|
| Genetic diversity | Low | High |
| Transfer of pollen | Within the same flower | Between different flowers |
| Dependency on agents | No | Yes |
| Examples | Pea, rice | Apple, mango |
10. What is the role of synergids in the embryo sac?
Answer:
Synergids assist in:
- Guiding the pollen tube to the egg cell using chemical signals.
- Supporting the fusion of male and female gametes.
11. Explain the events occurring after fertilization in angiosperms.
Answer:
- Zygote formation: The zygote develops into an embryo.
- Endosperm formation: Triploid endosperm provides nourishment.
- Ovule transformation: Becomes a seed.
- Ovary transformation: Develops into a fruit.
12. Describe the structure and functions of the embryo sac.
Answer:
The embryo sac contains:
- Egg cell: Participates in fertilization.
- Synergids: Guide the pollen tube.
- Polar nuclei: Fuse with a male gamete to form the endosperm.
- Antipodals: Have a supportive role.
13. What is geitonogamy? How is it different from xenogamy?
Answer:
Geitonogamy involves pollen transfer between flowers of the same plant, while xenogamy refers to pollen transfer between flowers of different plants, promoting higher genetic variation.
14. Explain the importance of pollinators in angiosperms.
Answer:
Pollinators facilitate:
- Successful transfer of pollen, ensuring fertilization.
- Genetic diversity through cross-pollination.
- Ecosystem stability by maintaining plant populations.
15. What are the characteristics of insect-pollinated flowers?
Answer:
- Brightly colored petals.
- Fragrance to attract insects.
- Nectar production as a reward.
- Sticky pollen grains for adherence.
16. Define and explain the significance of self-incompatibility.
Answer:
Self-incompatibility prevents self-pollination by rejecting pollen from the same plant. This promotes cross-pollination, ensuring genetic diversity and vigor.
17. Describe the formation of the seed and fruit in angiosperms.
Answer:
- The zygote develops into the embryo.
- The ovule transforms into a seed.
- The ovary wall becomes the fruit, protecting the seeds and aiding in their dispersal.
18. How does hydrophily occur, and what are its features?
Answer:
Hydrophily is pollination by water, as seen in plants like Vallisneria. Features include:
- Lightweight, waterproof pollen grains.
- Long, feathery stigmas to capture pollen.
19. Explain the role of vegetative and generative cells in the pollen grain.
Answer:
- Vegetative cell: Forms the pollen tube for sperm delivery.
- Generative cell: Divides to produce two sperm cells for fertilization.
20. Discuss the advantages and disadvantages of cross-pollination.
Answer:
Advantages:
- Promotes genetic diversity.
- Enhances adaptability and vigor.
Disadvantages:
- Requires external agents.
- Higher wastage of pollen grains.
These questions provide an in-depth understanding of key concepts related to reproduction in angiosperms, including pollination and fertilization processes.
