In double fertilisation, one male nucleus fuses with the egg cell to form a diploid zygote, and a second male nucleus fuses with the polar nuclei to form a triploid endosperm nucleus. Both fusions occur in the ovule.
This lesson follows comparing pollination mechanisms and leads to tracing seed and fruit development.
What is the sequence?
- A pollen grain lands on the stigma during pollination.
- The grain grows a pollen tube down the style toward an ovule.
- The tube carries two male nuclei, and it enters the ovule through the micropyle.
- One male nucleus fuses with the egg cell, forming a zygote.
- The other male nucleus fuses with the two polar nuclei, forming the endosperm nucleus.
Call step 4 the first fertilisation and step 5 the second. The word “double” counts these two fusions.
What are the chromosome numbers?
Here is a worked example with simple numbers. Suppose the plant’s body cells are diploid (2n) with 10 chromosomes, so each gamete nucleus is haploid (n) with 5.
| Product | Formed from | Chromosome number | Ploidy |
|---|---|---|---|
| Zygote | Egg (5) + male nucleus (5) | 10 | 2n |
| Endosperm nucleus | Two polar nuclei (5 + 5) + male nucleus (5) | 15 | 3n |
The arithmetic checks: 5 + 5 = 10, and 5 + 5 + 5 = 15. The zygote matches the parent, while the endosperm nucleus has three sets.
The mistake that costs marks
The common error is to say that both male nuclei fuse with the egg cell, or to confuse pollination with fertilisation. Pollination ends on the stigma, and fertilisation happens later in the ovule.
| Statement | Verdict |
|---|---|
| Pollen lands on the stigma, so fertilisation has happened | Wrong: that is pollination only |
| Two nuclei fuse with the egg | Wrong: one fuses with the egg, one with the polar nuclei |
| The zygote is triploid | Wrong: the zygote is diploid, the endosperm nucleus is triploid |
How do you write the explanation?
Keep the order in your answer and name each product. “The pollen tube carries two male nuclei into the ovule. One fuses with the egg cell to form a diploid zygote. The other fuses with the two polar nuclei to form a triploid endosperm nucleus.”
Finish with the roles: the zygote becomes the embryo, and the endosperm feeds it.
Check yourself
A plant has 14 chromosomes in its body cells. State the chromosome number in the zygote and in the endosperm nucleus.
Answer
Each gamete nucleus has 14 ÷ 2 = 7 chromosomes.
The zygote has 7 + 7 = 14 chromosomes (diploid). The endosperm nucleus has 7 + 7 + 7 = 21 chromosomes (triploid).
What to study next
Go on to tracing seed and fruit development. For the chromosome background, see cell division. The biological data interpretation trainer and the mistake log help you practise and track.
If you want a teacher to stop you at the step where your explanation breaks, see online one-to-one Biology tuition.