Fertilisation in flowering plants

Fertilisation in flowering plantsSpec 3.13

After pollination (the transfer of pollen to a stigma), the events of fertilisation take place in the following order.

1. A compatible pollen grain absorbs water and nutrients from the stigma and germinates.
2. A pollen tube grows out of the grain, down through the style towards the ovary. It is guided by chemicals released by the ovule (chemotropism), and the tube nucleus directs its growth. The tip of the tube secretes enzymes that digest the tissues of the style.
3. The generative nucleus divides by mitosis to form two male nuclei, which travel down the tube.
4. The tube enters the ovule through a small pore, the micropyle, and enters the embryo sac.
5. The tip of the tube bursts and releases the two male nuclei.
6. One male nucleus fuses with the nucleus of the egg cell to form a diploid zygote, which develops into the embryo.
7. The other male nucleus fuses with the two polar nuclei in the centre of the embryo sac to form a triploid nucleus, which develops into the endosperm (food store). This is called double fertilisation.

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Common mistake:

The pollen tube carries nuclei, not sperm cells. Describe the fusion of nuclei: male nucleus with egg nucleus, and male nucleus with the polar nuclei.

Quick check

1. Define the term locus.

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   The location of a gene on a chromosome.

2. In which stage of meiosis does crossing over occur?

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   Prophase I.

3. Why is the zona pellucida hardened after fertilisation?

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   To prevent other sperm from entering, which would cause polyspermy.

4. What do the two male nuclei do in a flowering plant?

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   One fuses with the egg nucleus to form the zygote; the other fuses with the polar nuclei to form the endosperm.

5. Why does a sperm have many mitochondria?

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   To make ATP by aerobic respiration for the movement of the flagellum.