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medium difficulty7 min

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.

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

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If a question asks for products of double fertilisation, write: syngamy produces zygote and triple fusion produces primary endosperm nucleus.
  2. 2Mention embryo and endosperm only as later developments.

Method you can reuse

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

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is fertilisation in angiosperms called double fertilisation?

Think first, then show answer
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.

Common wrong answer

Triple fusion forms the embryo.

Why it loses marks: Triple fusion forms the primary endosperm nucleus, while the zygote forms the embryo.

Don't mix this up

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.

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    After compatible pollination, the pollen tube enters the ovule, usually through the micropyle, and releases two male gametes into the embryo sac.

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