C
CraftExam
medium difficulty7 min

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.

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

Concept Practice Coming Soon

Understand it

  • 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.

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.

See it worked out

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

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 you can reuse

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

Sketch it in your answer

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

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.

Don't mix this up

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

Exam tip

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

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 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.

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    A pistil has stigma, style, and ovary.

Practice

Concept practice is coming soon

Login to access concept-level MCQs and weak-concept practice as they become available.

10 MCQs5 MinutesInstant Results
Login

Help improve this page

Found something confusing, incorrect, or missing?

Open Learning Map