Chapter 2 — Sexual Reproduction in Flowering Plants — is one of the most important and high-scoring chapters in Class 12 Biology, carrying 8-10 marks in Board exams. It covers everything from flower structure to pollination, fertilisation, and seed formation. Diagrams of the embryo sac and microsporangium are frequently asked.
Key Concepts
Flower — The Reproductive Organ
A flower has four whorls:
| Whorl | Parts | Function |
|---|---|---|
| Calyx | Sepals (green) | Protection of bud |
| Corolla | Petals (coloured) | Attract pollinators |
| Androecium | Stamens (male) | Produce pollen grains (microsporangia) |
| Gynoecium | Carpels/Pistil (female) | Produce ovules (megasporangia) |
Microsporogenesis & Male Gametophyte
Occurs in the anther (part of stamen).
Pollen grain structure:
– Outer wall: Exine (made of sporopollenin — most resistant biological material; has apertures called germ pores)
– Inner wall: Intine (thin, made of cellulose and pectin)
– 2-celled stage: large vegetative cell + small generative cell
– 3-celled stage: generative cell divides → 2 male gametes (sperm cells)
Megasporogenesis & Female Gametophyte
Occurs in the ovule (inside the ovary).
Embryo sac (female gametophyte) structure:
– Micropylar end: 1 egg cell + 2 synergids (with filiform apparatus)
– Chalazal end: 3 antipodal cells
– Centre: 2 polar nuclei → fuse to form central cell (2n)
Total: 8 nuclei, 7 cells
Pollination
| Type | Description | Features |
|---|---|---|
| Self-pollination (Autogamy) | Pollen from same flower to stigma | Less variation, genetically uniform |
| Geitonogamy | Pollen from different flower of same plant | Genetically similar to self-pollination |
| Cross-pollination (Xenogamy) | Pollen from different plant | Genetic variation, requires agents |
Pollinating Agents
| Agent | Features of Flowers | Examples |
|---|---|---|
| Wind (Anemophily) | Light pollen, feathery stigma, no colour/scent/nectar | Grasses, corn, wheat |
| Water (Hydrophily) | Pollen water-resistant, mucilaginous coating | Vallisneria, Zostera, Hydrilla |
| Insects (Entomophily) | Colourful, fragrant, nectar, sticky pollen | Rose, sunflower, Salvia |
| Birds (Ornithophily) | Large, colourful, tubular, copious nectar | Bombax, Butea (Palash) |
Outbreeding Devices (Prevent Self-pollination)
- Dichogamy: Pollen and stigma mature at different times
- Herkogamy: Physical barrier between anther and stigma
- Self-incompatibility: Pollen of same genotype rejected by pistil
- Unisexuality (Dioecy): Male and female flowers on separate plants (papaya, date palm)
Double Fertilisation (Unique to Angiosperms!)
Syngamy: One male gamete + egg cell → Zygote (2n)
Triple fusion: One male gamete + 2 polar nuclei → Primary Endosperm Nucleus (PEN, 3n)
Since TWO fusions occur, it’s called double fertilisation.
Zygote → Embryo; PEN → Endosperm (nutritive tissue for developing embryo)
Post-fertilisation Events
Seed Development
| Part | Develops From | Ploidy |
|---|---|---|
| Embryo | Zygote | 2n |
| Endosperm | Primary endosperm nucleus (PEN) | 3n |
| Seed coat | Integuments of ovule | 2n (maternal) |
| Fruit wall (pericarp) | Ovary wall | 2n (maternal) |
Embryo Structure
- Dicot embryo: Radicle + Plumule + 2 cotyledons + Embryonal axis
- Monocot embryo: Radicle (covered by coleorhiza) + Plumule (covered by coleoptile) + 1 cotyledon (scutellum)
Endosperm
- Develops BEFORE the embryo (provides nutrition)
- Endospermic/Albuminous seeds: Endosperm persists (wheat, maize, castor, coconut)
- Non-endospermic/Exalbuminous seeds: Endosperm consumed by embryo (pea, bean, groundnut)
Special Phenomena
- Apomixis: Seed formation without fertilisation (e.g., some grasses, citrus). Produces clones.
- Polyembryony: More than one embryo in a seed (e.g., citrus → nucellar embryos)
- Parthenocarpy: Fruit development without fertilisation → seedless fruits (banana, grape)
Important Definitions
| Term | Definition |
|---|---|
| Microsporogenesis | Formation of microspores (pollen) from microspore mother cells by meiosis |
| Megasporogenesis | Formation of megaspores from megaspore mother cell by meiosis |
| Pollination | Transfer of pollen from anther to stigma |
| Double Fertilisation | Two fusions in embryo sac: syngamy + triple fusion |
| Endosperm | Triploid nutritive tissue that nourishes the developing embryo |
| Apomixis | Asexual seed production without fertilisation |
| Parthenocarpy | Fruit development without fertilisation (seedless fruit) |
Solved Examples — NCERT Based
Example 1
Q: What is the significance of double fertilisation?
Solution: Double fertilisation ensures: (1) Formation of diploid zygote that develops into embryo, and (2) Formation of triploid endosperm that provides nutrition to the developing embryo. The endosperm develops only when fertilisation occurs — this is an energy-efficient mechanism as the plant doesn’t invest resources in endosperm formation unless fertilisation is successful.
Example 2
Q: Describe the structure of a mature embryo sac. Why is it called 7-celled and 8-nucleate?
Solution: The mature embryo sac contains:
Micropylar end: 1 egg cell + 2 synergids = 3 cells
Centre: 2 polar nuclei in one large central cell = 1 cell
Chalazal end: 3 antipodal cells = 3 cells
Total: 7 cells, 8 nuclei (the central cell has 2 nuclei but is counted as 1 cell).
Example 3
Q: Distinguish between geitonogamy and xenogamy.
Solution: Geitonogamy: Transfer of pollen from one flower to another flower on the SAME plant. Functionally cross-pollination but genetically similar to self-pollination (no new genes introduced). Xenogamy: Transfer of pollen from a flower on one plant to a flower on a DIFFERENT plant. True cross-pollination — introduces genetic variation.
Example 4
Q: What are the differences between apomixis and parthenocarpy?
Solution: Apomixis: Seeds develop without fertilisation → seeds are present but genetically identical to parent. Parthenocarpy: Fruits develop without fertilisation → fruits are seedless. Apomixis produces seeds; parthenocarpy produces seedless fruits.
Important Questions for Board Exams
1 Mark Questions
- What is sporopollenin? Why is it significant?
- What is the function of filiform apparatus in synergids?
- Define double fertilisation.
- What is apomixis?
2 Mark Questions
- Draw a labelled diagram of a mature pollen grain.
- What are outbreeding devices? Name any two.
- Differentiate between endospermic and non-endospermic seeds.
- What is self-incompatibility and what is its significance?
3 Mark Questions
- Describe the development of the female gametophyte in angiosperms.
- Explain the process of double fertilisation and its significance.
- What are the adaptations of wind-pollinated flowers?
- Draw a labelled diagram of a mature embryo sac and describe its structure.
5 Mark Questions
- Describe microsporogenesis. Draw a labelled diagram of a T.S. of anther showing microsporangia.
- Explain post-fertilisation changes in a flower. How does the ovule become a seed and the ovary become a fruit?
Quick Revision Points
- Flower: 4 whorls — calyx, corolla, androecium (male), gynoecium (female)
- Microsporogenesis: MMC (2n) → meiosis → 4 microspores (n) → pollen
- Pollen: exine (sporopollenin) + intine (cellulose); 2 or 3 cells
- Megasporogenesis: MMC → 4 megaspores → 1 functional → 7-celled, 8-nucleate embryo sac
- Embryo sac: 3 (egg+2 synergids) + 1 (central cell, 2 polar nuclei) + 3 (antipodals)
- Double fertilisation: sperm + egg → zygote (2n); sperm + polar nuclei → PEN (3n)
- Ovule → seed; Ovary → fruit
- Endospermic seeds: wheat, maize; Non-endospermic: pea, bean
- Apomixis: seeds without fertilisation; Parthenocarpy: seedless fruits
- Sporopollenin: most resistant biological material
Chapter Navigation
Previous: Reproduction in Organisms Class 12 Notes
Next: Human Reproduction Class 12 Notes
Related Chapters in Class 12 Biology
- Reproduction in Organisms Class 12 Notes
- Human Reproduction Class 12 Notes
- Reproductive Health Class 12 Notes
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