Quick answer: In flowering plants, the pollen tube carries two male gametes into the ovule. One fuses with the egg cell to form a zygote (this is syngamy). The other fuses with the two polar nuclei to form the primary endosperm nucleus; because three nuclei fuse, this is called triple fusion. Since fertilisation happens twice in the same ovule, the whole process is called double fertilisation. Afterwards, the zygote becomes the embryo, the ovule becomes the seed and the ovary becomes the fruit.
After pollination, a pollen grain on the stigma grows a pollen tube down through the style to an ovule in the ovary. The pollen tube carries two male gametes. In flowering plants, both of them take part in fusion inside the ovule.
Syllabus note: The CBSE Class 10 NCERT textbook (Chapter 7, How do Organisms Reproduce?) explains fertilisation as the fusion of the male germ cell with the egg cell, and what happens to the ovule and ovary afterwards. Double fertilisation, triple fusion and the embryo sac are covered in Class 12 Biology. They are explained here as background.
What You Will Learn
- The structure of the embryo sac inside the ovule (Class 12 background)
- How the pollen tube delivers the male gametes
- Syngamy: forming the zygote
- Triple fusion: forming the endosperm
- Why the process is called double fertilisation
- What each part of the flower becomes after fertilisation (Class 10)
1. Inside the Ovule: The Embryo Sac
Each ovule contains an embryo sac. In most flowering plants it has 7 cells with 8 nuclei:
| Part | Number | Role |
|---|---|---|
| Egg cell | 1 | Female gamete; fuses with a male gamete |
| Synergids | 2 | Next to the egg; help guide the pollen tube |
| Central cell | 1 (with 2 polar nuclei) | Its polar nuclei fuse with the second male gamete |
| Antipodal cells | 3 | At the opposite end from the egg |
2. The Two Fusions
Fusion 1: Syngamy
One male gamete fuses with the egg cell:
The zygote divides many times and develops into the embryo, the young plant inside the seed.
Fusion 2: Triple Fusion
The second male gamete fuses with the two polar nuclei in the central cell:
This is called triple fusion because three nuclei fuse. The primary endosperm nucleus develops into the endosperm, a tissue that stores food for the developing embryo.
3. Why Is It Called Double Fertilisation?
Definition: Double fertilisation is the process in flowering plants in which two fusions take place in the same embryo sac: syngamy (male gamete + egg) and triple fusion (male gamete + two polar nuclei).
It is called double fertilisation because fertilisation happens twice, using both male gametes from one pollen tube.
4. Changes After Fertilisation (Class 10)
| Before fertilisation | What it becomes |
|---|---|
| Zygote | Embryo |
| Primary endosperm nucleus | Endosperm (stored food) |
| Ovule | Seed |
| Ovule's outer layers (integuments) | Seed coat |
| Ovary | Fruit |
| Petals, sepals, stamens, style and stigma | May shrivel and fall off |
Remember: The ovule becomes the seed and the ovary becomes the fruit.
5. Summary Table
| Event | What fuses | Product |
|---|---|---|
| Syngamy | 1 male gamete + egg cell | Zygote (2n), which becomes the embryo |
| Triple fusion | 1 male gamete + 2 polar nuclei | Primary endosperm nucleus (3n), which becomes the endosperm |
| Double fertilisation | Both of the above, in the same embryo sac | Seed with an embryo and stored food |
6. Exam Tips and Common Mistakes
Exam Tip: For Class 10, focus on what happens after fertilisation: the zygote forms the embryo, the ovule forms the seed and the ovary forms the fruit.
Exam Tip: In Class 12, remember that the zygote is diploid (2n) and the endosperm is triploid (3n).
Common Mistake: Mixing up the ovule and the ovary. The ovule becomes the seed; the ovary becomes the fruit.
Common Mistake: Saying triple fusion involves three male gametes. It involves one male gamete and two polar nuclei.
Frequently Asked Questions
What is double fertilisation?
The process in flowering plants in which one male gamete fuses with the egg cell (syngamy) and the other fuses with the two polar nuclei (triple fusion), in the same embryo sac.
What is triple fusion?
The fusion of one male gamete with the two polar nuclei, forming the primary endosperm nucleus, which develops into the endosperm.
What does the ovule develop into after fertilisation?
The ovule develops into the seed.
What does the ovary develop into after fertilisation?
The ovary develops into the fruit.
Is double fertilisation in the Class 10 syllabus?
No. Class 10 covers fertilisation and the changes after it; double fertilisation and triple fusion are covered in Class 12 Biology.