Question
Describe the process of double fertilisation in angiosperms. Name the two fusion events and the products formed from each.
This is a NEET 2024 favourite and appears almost every year in CBSE Class 12 boards too. High weightage — learn it cold.
Solution — Step by Step
The pollen grain germinates on the stigma and grows a pollen tube through the style. This tube carries two male gametes (sperm nuclei) — not one, two. That’s the whole point of what follows.
The tube enters the ovule through the micropyle and reaches the embryo sac.
One sperm nucleus fuses with the egg cell (n). This is standard fertilisation — called syngamy.
Product: Zygote (2n), which develops into the embryo.
The second sperm nucleus fuses with the two polar nuclei already present in the central cell. Two polar nuclei (n + n) combine with one sperm (n).
Product: Primary Endosperm Nucleus (3n), which develops into the endosperm — the food reserve for the developing seed.
Both events happen simultaneously, inside the same embryo sac, using both sperm nuclei. One sperm → one egg. One sperm → two polar nuclei. Neither sperm is wasted.
Double fertilisation = Syngamy + Triple Fusion.
After fertilisation:
- Zygote (2n) → Embryo
- Primary Endosperm Nucleus (3n) → Endosperm
- Ovule → Seed
- Ovary → Fruit
Why This Works
Angiosperms evolved double fertilisation as a highly efficient reproductive strategy. The endosperm only forms if fertilisation is successful — this means the plant doesn’t waste energy building food reserves for an unfertilised ovule. Gymnosperms don’t have this; their nutritive tissue forms before fertilisation happens, which is energetically costly.
The triploid (3n) nature of the endosperm matters because it gives the seed a nutrient-rich, genetically diverse food supply. Rice, wheat, maize — the part we eat is mostly endosperm. So double fertilisation isn’t just a biology concept; it literally feeds the world.
The key evolutionary insight is that angiosperms tie seed provisioning directly to fertilisation success. No fertilisation, no endosperm, no wasted resources.
Alternative Method — Mnemonic Approach
When a NEET question asks “what fuses with what”, students mix up the nuclei. Use this:
“1 egg + 1 sperm = baby (zygote)” → Syngamy
“2 polar + 1 sperm = food (endosperm)” → Triple Fusion
Triple fusion = 3 nuclei fusing, giving a 3n product. The name tells you the ploidy.
For NEET MCQs, the question sometimes gives you ploidy levels and asks you to identify the structure. Zygote = 2n. Primary Endosperm Nucleus = 3n. Endosperm cells = 3n. Embryo = 2n.
Common Mistake
Calling triple fusion “double fusion” — Students hear “double fertilisation” and then write “double fusion” for the second event. Triple fusion is named for the three nuclei involved (2 polar + 1 sperm), not for being the second event. Examiners specifically look for this distinction in 3-mark board questions.
Also: many students write that polar nuclei are diploid (2n). They’re not — each polar nucleus is haploid (n). It’s their combination that gives you 2n before the sperm arrives, and the sperm adds the third n.
| Event | Nuclei involved | Product | Ploidy |
|---|---|---|---|
| Syngamy | 1 sperm + egg | Zygote | 2n |
| Triple Fusion | 1 sperm + 2 polar nuclei | Primary Endosperm Nucleus | 3n |
Double fertilisation is unique to angiosperms.
In NEET 2024, the question asked students to identify which structure develops from the primary endosperm nucleus. The answer — endosperm — costs you marks if you write “embryo” out of reflex. Keep the two pathways separate in your head: sperm 1 builds the baby, sperm 2 builds the food.