2.3 Fertilisation: The Fusion of Gametes

Introduction: What is Fertilisation?

• Definition: Fusion of male gamete (from pollen) with female gamete (egg cell) to form a zygote.
• In angiosperms, it involves two fusion events: Double Fertilization.

Events from Pollination to Fertilisation

1. Germination of Pollen Grain: Pollen absorbs nutrients from stigma, forms pollen tube through a germ pore.
2. Growth of Pollen Tube: Pollen tube (with male gametes and tube nucleus) grows down style, guided by chemical signals (chemotropic).
3. Entry into Ovule/Embryo Sac: Pollen tube enters ovule (usually via micropyle) and then the embryo sac.
4. Release of Male Gametes: Pollen tube ruptures, releasing two male gametes into embryo sac.

Double Fertilization and Triple Fusion

• Syngamy: Fusion of one male gamete with the egg cell → diploid zygote (2n).
• Triple Fusion: Fusion of the second male gamete with the two polar nuclei → triploid Primary Endosperm Nucleus (PEN) (3n).
• Double Fertilization: The two simultaneous fusion events (syngamy + triple fusion).

Significance of Double Fertilization

• Ensures nutritive tissue (endosperm) forms only if fertilization is successful.
• Zygote develops into embryo.
• PEN develops into endosperm (provides nourishment to embryo).

Fruit and Seed: The Products of Fertilisation

• Fruit: Mature or ripened ovary, develops after fertilization.
  • Pericarp: Fruit wall.
  • Significance: Protects seeds, aids seed dispersal, source of food.
• Seed: Mature ovule, contains embryo and food store, enclosed in seed coat.
  • Significance: Contains dormant embryo, provides nourishment, aids dispersal, allows dormancy for survival.

Summary Table: Double Fertilization

| Event | Fusion Of | Product | Ploidy | Fate of Product |
| :--- | :--- | :--- | :--- |
| Syngamy | Male Gamete + Egg Cell | Zygote | Diploid (2n) | Develops into the Embryo |
| Triple Fusion | Male Gamete + 2 Polar Nuclei | Primary Endosperm Nucleus (PEN) | Triploid (3n) | Develops into the Endosperm |