Double Fertilisation in Flowering Plants: Process Explained

Double fertilisation is a unique process in flowering plants where two male gametes from one pollen grain take part in two separate fusion events. One fusion produces the embryo, and the other produces the endosperm that nourishes it. It is found only in angiosperms.

This process matters because it links seed development to a food supply that forms only when fertilisation succeeds. It explains why seeds have both an embryo and stored food, and why cereal grains are such important human foods.

Key Concepts

Pollen tube: A tube grown from pollen grain that carries two male gametes to the embryo sac.

Syngamy: Fusion of one male gamete with the egg cell to form a diploid zygote, 2n.

Triple fusion: Fusion of the second male gamete with the diploid central cell to form a triploid primary endosperm cell, 3n.

Zygote: Diploid cell that divides to form the embryo.

Endosperm: Triploid nutritive tissue that feeds the developing embryo.

Embryo sac: Female gametophyte containing egg, synergids, antipodals, and central cell.

How It Works

1. Pollen lands on stigma, germinates, and grows a pollen tube through the style.
2. The tube enters the ovule through the micropyle and reaches the embryo sac.
3. It releases two male gametes near the egg apparatus.
4. One male gamete fuses with the egg. This is syngamy and forms the 2n zygote.
5. The second male gamete fuses with the central cell containing two polar nuclei. This triple fusion forms the 3n primary endosperm cell.
6. The zygote develops into embryo and the primary endosperm cell divides to form endosperm.

Worked Example

A plant has haploid chromosome number n equals 10. The egg has 10 chromosomes and one male gamete has 10, so the zygote has 10 plus 10 equals 20 chromosomes, which is 2n. The central cell has two polar nuclei, each with 10 chromosomes, total 20. Adding the second male gamete with 10 gives 30 chromosomes in the primary endosperm cell, which is 3n. If the mother sporophyte is 2n equals 20, all values match correctly and show why endosperm is triploid.

Common Misconceptions

• Double fertilisation means two eggs are fertilised. There is one egg. The second fusion involves the central cell.
• Endosperm is diploid like the embryo. Endosperm is usually triploid because of triple fusion.
• All seeds store food only in endosperm. In beans and peas, endosperm is used up and food is stored in cotyledons.

Real-World Applications

Cereal grains such as rice, wheat, and maize are valued because their endosperm stores starch for human nutrition. Seed companies monitor fertilisation success because endosperm failure causes seed abortion. In breeding, understanding triple fusion helps explain incompatibility in wide crosses. Coconut water is an example of liquid endosperm in early development.

Double fertilisation ensures that food tissue forms only when an embryo is also formed. This elegant pairing is a key reason flowering plants dominate land ecosystems and agriculture.