All subjects · Biology · WBI12 (Unit 2)

Fertilisation in flowering plants

In flowering plants, the male gametes are delivered to the ovule by a pollen tube. Two fertilisations happen, not one.

Growing the pollen tube

1. After pollination, a pollen grain lands on a compatible stigma and absorbs water.

2. A pollen tube grows out of the grain and down through the style, secreting hydrolytic enzymes that digest the surrounding tissue. It grows towards chemicals released by the ovule.

3. The tube nucleus is at the tip, controlling growth. The generative nucleus divides by mitosis to form two male gamete nuclei.

Double fertilisation

4. The tube enters the ovule through the micropyle, and the tube nucleus breaks down.

5. One male nucleus fuses with the egg cell nucleus, forming a diploid zygote, which develops into the embryo.

6. The other male nucleus fuses with the two polar nuclei, forming a triploid (3n) endosperm nucleus, which develops into the endosperm, a food store for the seed.

This is called double fertilisation.

Investigating pollen tubes

A recommended practical: put pollen grains in sucrose solutions of different concentrations (or with different temperatures) on cavity slides, then count how many have grown tubes, or measure tube length with a graticule, after a set time.

Worked example

A plant's leaf cells have 24 chromosomes. How many chromosomes are in its endosperm nuclei?

  1. Leaf cells are diploid: 2n = 24, so n = 12.
  2. Endosperm is triploid: 3n = 3 x 12.

Answer: 36

Key idea: The pollen tube grows down the style using enzymes; the generative nucleus gives two male nuclei. One fuses with the egg (diploid zygote), the other with two polar nuclei (triploid endosperm).

Check yourself

  1. A flowering plant has a diploid number of 14. How many chromosomes are in an endosperm nucleus?
    Show answer

    21. n = 7, so 3n = 21.

  2. The same plant (2n = 14) forms a zygote. How many chromosomes does it have?
    Show answer

    14. The zygote is diploid: 14.

  3. How does the pollen tube grow through the style?
    • It is pushed by water pressure from the stigma
    • Insects carry it down
    • It secretes hydrolytic enzymes that digest the tissue
    • It dissolves the ovary wall with acid
    Show answer

    It secretes hydrolytic enzymes that digest the tissue. Enzymes digest a path for the tube.

  4. What is the role of the generative nucleus?
    • It controls the growth of the tube
    • It divides by mitosis to form two male gamete nuclei
    • It forms the endosperm on its own
    • It digests the style
    Show answer

    It divides by mitosis to form two male gamete nuclei. The two male nuclei take part in double fertilisation.

  5. Through which opening does the pollen tube usually enter the ovule?
    • The stigma
    • The stoma
    • The anther
    • The micropyle
    Show answer

    The micropyle. The micropyle is a small gap in the ovule's coverings.

  6. What does double fertilisation produce?
    • Two diploid zygotes
    • A diploid zygote and a triploid endosperm nucleus
    • Two haploid embryos
    • A triploid zygote and a diploid endosperm
    Show answer

    A diploid zygote and a triploid endosperm nucleus. One male nucleus + egg; other + two polar nuclei.

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Sources

Last updated 7 November 2026. Spotted a mistake? Tell us.