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Double Fertilisation

Double fertilisation is the angiosperm process in which one male gamete fuses with the egg to form the zygote and the other fuses with two polar nuclei to form the primary endosperm nucleus.

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Student-friendly explanation

After compatible pollination, the pollen tube enters the ovule, usually through the micropyle, and releases two male gametes into the embryo sac. One male gamete fuses with the egg cell; this is syngamy and produces a diploid zygote. The second male gamete fuses with the two polar nuclei in the central cell; this is triple fusion and produces the triploid primary endosperm nucleus. Because two fusions occur in the same embryo sac, the process is called double fertilisation. It is a defining feature of angiosperms and links fertilisation with nourishment of the developing embryo.

How to write this in exams

  1. 1

    Start with the exact idea

    Double fertilisation is the angiosperm process in which one male gamete fuses with the egg to form the zygote and the other fuses with two polar nuclei to form the primary endosperm nucleus.

  2. 2

    Then show how to use it

    1. Compatible pollen germinates on stigma. 2. Pollen tube grows through style and enters ovule. 3. Pollen tube releases two male gametes into embryo sac. 4. One male gamete fuses with egg cell to form zygote. 5. Second male gamete fuses with two polar nuclei in the central cell. 6. Primary endosperm nucleus forms. 7. Zygote develops into embryo and endosperm begins nourishing tissue formation.

  3. 3

    Add one concrete example

    In a typical angiosperm embryo sac, syngamy forms the future embryo, while triple fusion initiates endosperm development that nourishes the embryo.

  4. 4

    Avoid this incomplete answer

    A common wrong answer is that triple fusion forms the embryo. Triple fusion forms the primary endosperm nucleus, while the zygote forms the embryo.

Definition

Double fertilisation is the angiosperm process in which one male gamete fuses with the egg to form the zygote and the other fuses with two polar nuclei to form the primary endosperm nucleus.

Example

In a typical angiosperm embryo sac, syngamy forms the future embryo, while triple fusion initiates endosperm development that nourishes the embryo.

Rule to remember

Fusion rule: male gamete plus egg gives zygote; male gamete plus two polar nuclei gives primary endosperm nucleus, usually triploid.

Memory hook

One sperm makes the baby plant, one sperm starts the food tissue.

Examples and method

Worked example

If a question asks for products of double fertilisation, write: syngamy produces zygote and triple fusion produces primary endosperm nucleus. Mention embryo and endosperm only as later developments.

Method to apply

1. Compatible pollen germinates on stigma. 2. Pollen tube grows through style and enters ovule. 3. Pollen tube releases two male gametes into embryo sac. 4. One male gamete fuses with egg cell to form zygote. 5. Second male gamete fuses with two polar nuclei in the central cell. 6. Primary endosperm nucleus forms. 7. Zygote develops into embryo and endosperm begins nourishing tissue formation.

Diagram support

Label pollen tube, micropyle, synergids, egg cell, male gametes, central cell, polar nuclei, zygote, and primary endosperm nucleus. Notice the two separate fusion sites.

How CBSE asks it

This is asked as a short answer, assertion-reason, labelled embryo sac diagram, or explanation of why double fertilisation is unique to flowering plants.

Avoid common mistakes

Common confusion

Students often write that both male gametes fuse with the egg. Only one fuses with the egg; the other participates in triple fusion with polar nuclei.

Common wrong answer

A common wrong answer is that triple fusion forms the embryo. Triple fusion forms the primary endosperm nucleus, while the zygote forms the embryo.

Exam tip

Write syngamy and triple fusion separately, with the fusion partners and products for each.

Quick check

Why is fertilisation in angiosperms called double fertilisation?

It is called double fertilisation because two fusion events occur: one male gamete fuses with the egg to form the zygote, and the other fuses with the two polar nuclei to form the primary endosperm nucleus.

Answer writing and exam use

1-mark answer

Define double fertilisation, describe pollen tube entry, write syngamy with partners and product, write triple fusion with partners and product, then state its significance in forming embryo and nutritive endosperm.

2-mark answer

Define double fertilisation, describe pollen tube entry, write syngamy with partners and product, write triple fusion with partners and product, then state its significance in forming embryo and nutritive endosperm.

3-mark answer

Define double fertilisation, describe pollen tube entry, write syngamy with partners and product, write triple fusion with partners and product, then state its significance in forming embryo and nutritive endosperm.
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