Sexual reproduction in plants

Formation of pollen

• Meiosis occurs in the anthers of flowering plants, which results in the formation of pollen grains that carry the male gametes.

  • Each anther carries four pollen sacs.

    • Pollen sac formation starts in these sacs with diploid mother cells.

      • These cells divide to form haploid microsphore cells.

• The gametes themselves are formed from the microsphores during mitosis.

  • They contain 2 haploid nuclei.

    • One is the tube nucleus, the other one is the generative nucleus.

      • The tube nucleus produces the pollen tube. This penetrates the stigma, style and ovary to enter the ovule.

      • The generative nucleus fuses with the ovule to form a new individual.

Formation of egg cells

• Within the ovary of the plant, meiosis occurs to form a relatively small number of ova.

• The ovule is attached to the ovary’s wall by a pad of special tissue called the placenta.

  • A complex structure around this tissue forms called the nucellus.

    • In the centre of this forms the gametophyte generation.

• Diploid megaspore mother cells divide to create four haploid megaspore daughter cells.

  • Three of these cells degenerate and one continues to develop.

    • The megaspore then undergoes three divisions via mitosis.

      • This results in the embryo sac containing one egg cell, two polar nuclei and some other small cells.

Fertilisation

• The pollen grain lands on the surface of the stigma during pollination.

• The molecules on the surface of the pollen grain and stigma start to interact.

  • If they are the same species, the pollen grain begins to germinate.

  • A pollen tube grows out of the pollen grain, through the stigma and into the style.

  • The tip of the pollen tube produces hydrolytic enzymes that hydrolyse the style tissue, so the pollen tube begins to grow down towards the ovary. This involves the hydrolysis of the protein in the cells.

  • The breakdown of the products will supply a source of ATP through cellular respiration for pollen tube growth.

    • As this happens, the generative nucleus travels down the pollen tube. It divides via mitosis to from 2 male nuclei.

    • The pollen tube continues to grow, passes through the ovary and then through the microphyte of the ovule.

    • The two male nuclei pass through the ovary and double fertilisation occurs.

    • One male nucleus fuses with the two polar nuclei to form the endosperm nucleus (triploid), while the other fuses with the egg cell to form a diploid zygote.

• The role of the endosperm is to provide the embryo plant with food when it begins to germinate.