Development of Pollen Grains and the Embryo Sac

Development of pollen grains

Inside each pollen sac of the anther, a diploid pollen mother cell undergoes meiosis to produce four haploid microspores. Each microspore develops a tough outer wall and divides once more by mitosis to form a pollen grain containing a tube nucleus and a generative nucleus.

The generative nucleus later divides to produce two male gametes (sperm nuclei), so a mature pollen grain is the structure that carries the male gametes to the female part of a flower.

Development of the embryo sac

Inside the ovule, a diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores, but only one usually survives while the others degenerate. The surviving megaspore divides by mitosis three times to form the embryo sac, a structure that typically contains an egg cell, two synergid cells beside it, three antipodal cells at the opposite end, and a large central cell with two polar nuclei.

Comparing the two processes

How it is examined

Questions may ask you to describe the stages from pollen mother cell to mature pollen grain, or from megaspore mother cell to embryo sac, using terms like meiosis, mitosis, haploid and diploid correctly, or to identify structures in a diagram of a pollen grain or embryo sac.

Meiosis then mitosis: keeping track of ploidy

Both structures begin with a diploid mother cell, and both use meiosis first and mitosis afterwards. The order matters: meiosis halves the chromosome number, turning one diploid mother cell into four haploid cells, and mitosis then copies those haploid nuclei without changing the chromosome number.

Every nucleus inside the finished pollen grain or embryo sac is therefore haploid.

The two structures differ in the number of products that survive. In the anther, all four microspores survive and each becomes a pollen grain, so one pollen mother cell yields four grains.

In the ovule, three of the four megaspores break down and only one continues, so one megaspore mother cell yields a single embryo sac. That difference in survival is why a flower releases pollen in large quantities while each ovule forms only one egg cell.

Common mistakes to avoid

Worked exam-style question

Question. A student examines a slide of an anther and a slide of an ovule. (a) Name the type of cell division that first occurs in the pollen mother cell and state its product.

(b) Explain why this division is described as a reduction division. (c) The ovule contains four haploid cells after the first division, but a mature embryo sac forms from only one of them.

Account for this.

Model answer. (a) The pollen mother cell divides by meiosis, producing four haploid microspores. (b) Meiosis is a reduction division because it halves the chromosome number, changing a diploid mother cell into haploid cells; this keeps the chromosome number constant from one generation to the next once fertilisation restores the diploid number.

(c) Of the four haploid megaspores, three degenerate and only one survives; the surviving megaspore then divides by mitosis to form the embryo sac that contains the egg cell and the polar nuclei.

Practice question

Try it. A flowering plant has a diploid number of 24 chromosomes in its leaf cells. State the chromosome number in (a) a microspore, (b) the egg cell in the embryo sac, and (c) the zygote formed after fertilisation, giving a reason for each answer.

Exam tip

Key terms

• Meiosis, halves the chromosome number to form haploid spores.
• Mitosis, copies nuclei without changing the ploidy.
• Haploid, a nucleus with one set of chromosomes, as in a spore or gamete.
• Diploid, a nucleus with two sets, as in the mother cell and the zygote.
• Fertilisation, fusion of a male and a female gamete to form a zygote.

Source:SRC-DSKP-EN

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