Formation of Pollen

Pollen grains develop inside the anthers of the stamens. A young anther contains four pollen sacs, each filled with diploid pollen mother cells (2n). Around each sac is a layer of nutritive cells, the tapetum, which supplies the developing pollen with nutrients.The stages are:
• Each pollen mother cell divides by meiosis to form a group of four haploid cells, called a tetrad.
• The cells of the tetrad separate, and each becomes an immature pollen grain (a microspore, n).
• The nucleus of each immature pollen grain divides by mitosis, giving a tube nucleus and a generative nucleus.
• A tough two-layered wall forms: a thick outer exine, made of the very resistant substance sporopollenin, and a thin inner intine. The exine protects the grain from drying out and damage, and often has a pattern typical of the species.
• The generative nucleus divides by mitosis to form two male gamete nuclei. In some species this happens before the pollen is released; in others it happens later, inside the pollen tube.When the pollen is mature, the anther wall dries out and splits open (dehiscence), releasing the pollen grains.The tube nucleus is not a gamete: it controls the growth of the pollen tube after pollination. Only the two male nuclei take part in fertilisation.Worked example: an anther contains 500 pollen mother cells. Each gives four pollen grains by meiosis, so 500 × 4 = 2000 pollen grains are produced, carrying 2000 × 2 = 4000 male gamete nuclei.