Before a flowering plant can reproduce sexually, it must make gametes. The male gametes develop inside pollen grains in the anther, and the female gamete develops inside the embryo sac in the ovule. Both processes begin with a cell dividing by meiosis.
Development of pollen grains
Inside each of the four pollen sacs of an anther are many pollen mother cells (microspore mother cells), which are diploid. Each pollen mother cell divides by meiosis to form four haploid microspores. Each microspore develops a tough wall and its nucleus divides by mitosis to form two nuclei:
- the tube nucleus (which later controls the growth of the pollen tube), and
- the generative nucleus (which later divides to form two male gametes).
Each such structure is a mature pollen grain.
Development of the embryo sac
Inside the ovule is a diploid cell, the megaspore mother cell (embryo sac mother cell). It divides by meiosis to form four haploid megaspores, but usually three degenerate and only one survives. The surviving megaspore enlarges and its nucleus divides by mitosis three times to give eight haploid nuclei arranged in the mature embryo sac. These include:
- the egg cell (the female gamete), with two cells beside it,
- two polar nuclei in the centre, and
- three other cells at the far end.
Key idea
Meiosis halves the chromosome number, so the gametes are haploid. When two haploid gametes fuse at fertilisation, the diploid number is restored in the zygote.
Example
One diploid pollen mother cell gives four haploid microspores, and therefore four pollen grains. One diploid megaspore mother cell gives four megaspores but only one embryo sac, because three megaspores die.
Remember
Pollen grain = tube nucleus + generative nucleus (male). Embryo sac = eight haploid nuclei, including the egg cell and two polar nuclei (female).