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Pistil, Megasporangium and Embryo Sac

The pistil is the female reproductive part of a flower, and the ovule inside its ovary acts as the megasporangium where megasporogenesis and embryo sac formation occur.

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Student-friendly explanation

A pistil has stigma, style, and ovary. The ovary contains ovules, each with nucellus, integuments, micropyle, and chalaza. In the nucellus, a megaspore mother cell undergoes meiosis to produce four haploid megaspores, usually only one of which remains functional. This functional megaspore undergoes three mitotic divisions to form an embryo sac with seven cells and eight nuclei: egg apparatus at the micropylar end, three antipodals at the chalazal end, and a central cell with two polar nuclei. Exams often test both the developmental sequence and diagram orientation.

How to write this in exams

  1. 1

    Start with the exact idea

    The pistil is the female reproductive part of a flower, and the ovule inside its ovary acts as the megasporangium where megasporogenesis and embryo sac formation occur.

  2. 2

    Then show how to use it

    1. Locate the ovule inside the ovary. 2. Identify nucellus and the megaspore mother cell. 3. Megaspore mother cell undergoes meiosis to form four haploid megaspores. 4. Usually one megaspore remains functional. 5. The functional megaspore undergoes three mitotic divisions to form eight nuclei. 6. Nuclei arrange as egg apparatus, polar nuclei, and antipodals. 7. Cell walls form around nuclei to produce the typical 7-celled embryo sac.

  3. 3

    Add one concrete example

    The common angiosperm embryo sac is described as 7-celled and 8-nucleate because the central cell contains two polar nuclei while the other six cells usually contain one nucleus each.

  4. 4

    Avoid this incomplete answer

    A frequent wrong answer is that all four megaspores form four embryo sacs; usually only one megaspore forms the functional embryo sac.

Definition

The pistil is the female reproductive part of a flower, and the ovule inside its ovary acts as the megasporangium where megasporogenesis and embryo sac formation occur.

Example

The common angiosperm embryo sac is described as 7-celled and 8-nucleate because the central cell contains two polar nuclei while the other six cells usually contain one nucleus each.

Rule to remember

Key sequence: megaspore mother cell undergoes meiosis to form four megaspores; one functional megaspore undergoes three mitotic divisions to form an 8-nucleate embryo sac arranged into 7 cells.

Memory hook

Three at micropylar, two in centre, three at chalazal: 3-2-3 nuclei arrangement.

Examples and method

Worked example

If an ovule diagram shows three cells at one end, two nuclei in the centre, and three cells at the opposite end, identify the micropylar end by locating the egg apparatus with two synergids and one egg cell.

Method to apply

1. Locate the ovule inside the ovary. 2. Identify nucellus and the megaspore mother cell. 3. Megaspore mother cell undergoes meiosis to form four haploid megaspores. 4. Usually one megaspore remains functional. 5. The functional megaspore undergoes three mitotic divisions to form eight nuclei. 6. Nuclei arrange as egg apparatus, polar nuclei, and antipodals. 7. Cell walls form around nuclei to produce the typical 7-celled embryo sac.

Diagram support

Label funicle, hilum, integuments, micropyle, nucellus, embryo sac, chalaza, egg cell, synergids, polar nuclei, central cell, and antipodal cells. Notice that egg apparatus lies near the micropyle.

How CBSE asks it

This is commonly asked as a labelled ovule diagram, embryo sac arrangement, short note on megasporogenesis, or long-answer sequence from megaspore mother cell to mature embryo sac.

Avoid common mistakes

Common confusion

Students often write 8-celled embryo sac. The correct description is usually 7-celled, 8-nucleate in the typical angiosperm condition.

Common wrong answer

A frequent wrong answer is that all four megaspores form four embryo sacs; usually only one megaspore forms the functional embryo sac.

Exam tip

In embryo sac answers, always mention the micropylar end and chalazal end because orientation helps score diagram and explanation marks.

Quick check

Why is the typical embryo sac called 7-celled and 8-nucleate?

It is called 7-celled and 8-nucleate because it has three antipodal cells, two synergids, one egg cell, and one central cell containing two polar nuclei.

Answer writing and exam use

1-mark answer

Define pistil and ovule, name nucellus and integuments, describe meiosis of megaspore mother cell, state that one megaspore is functional, then give the 7-celled 8-nucleate arrangement with micropylar and chalazal ends.

2-mark answer

Define pistil and ovule, name nucellus and integuments, describe meiosis of megaspore mother cell, state that one megaspore is functional, then give the 7-celled 8-nucleate arrangement with micropylar and chalazal ends.

3-mark answer

Define pistil and ovule, name nucellus and integuments, describe meiosis of megaspore mother cell, state that one megaspore is functional, then give the 7-celled 8-nucleate arrangement with micropylar and chalazal ends.
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