2.5

• Pollen formation


  1. Diploid microspore mother cells undergo meiosis to form haploid microspores

  2. Haploid microspores undergo mitosis to form pollen grains
• Ovum formation


  1. Diploid megaspore mother cells undergo mitosis to form ootid and polar bodies

  2. Ootid undergoes mitotic divisions to form embryo sac

  
• Plant fertilisation


  1. Pollen grain adheres to stigma and germinates

  2. Pollen tube grows down style

  3. Pollen tube enters embryo sac

  4. Generative nucleus divides to form two sperm cells

  5. One sperm cell fuses with egg nucleus to form zygote

  6. Other sperm cell fuses with polar nuclei to form endosperm nucleus

  
• How does a pollen grain form in the anther?

  1. Meiosis of diploid microspore cells in the anther forms 4 haploid microspores

  2. Haploid microspores mature into pollen grains via mitosis
• Describe the structure of pollen grains

  • Generative cell with haploid nucleus produces 2 male gametes via mitosis
  • Pollen tube cell (also has its own nucleus) elongates to penetrate ovule
  • Outer protective coating
• How does the embryo sac form in the ovule?

  1. Mitosis of diploid megaspore cells in ovule forms 1 ootid & 3 polar bodies. Polar bodies degenerate then are reabsorbed

  2. After 3 mitotic divisions, ootid forms embryo sac
• Describe the structure of the embryo sac

  • 2 polar nuclei form endosperm
  • Ovum forms zygote
  • 2 synergids help generative nucleus of pollen grain to reach ovum
  • 3 antipodal cells
  • Outer protective coating
• How do male nuclei reach the embryo sac?

  1. Pollen grain germinates on stigma

  2. Enzymes digest surrounding tissue, providing nutrient source. Enables pollen tube to grow down style then through micropyle

  3. Mitosis of pollen grain generative cell produces 2 male gametes which enter embryo sac
• What happens during double fertilisation?

  1. Diploid zygote forms when 1 male gamete and ovum nucleus fuse

  2. Triploid endosperm forms when 1 male gamete and 2 polar nuclei fuse
• What is the purpose of a triploid endosperm?

  Nutrient source for embryo
• Where does double fertilisation occur?

  Embryo sac
• At what sucrose concentration do pollen tubes grow best?

  Near isotonic, prevents osmotic damage