3.13 Fertilisation - Flowering Plants

• Sexual reproduction in flowering plants requires the transfer of pollen between male and female parts of flowers
• The development of flowers occurs in the reproductive stage of the plant life cycle
• Flowers contain all the necessary organs and tissues required for sexual reproduction by pollination
• Key structures of the flower include
  • The anther - where pollen is produced
  • The stigma - part of the female reproductive organ which receives the pollen
  • The ovary - where the female gametes are located
• Flowers usually contain both male and female reproductive parts
• The male reproductive parts produce pollen
• The transferal of pollen from the anther to the stigma is known as pollination

Double fertilisation

• After pollination has occurred, the pollen grain begins to 'grow' or 'germinate' and a pollen tube grows from the pollen grain down the style to the ovary of the plant
• As the pollen tube grows towards the ovary, two haploid male nuclei move down the tube
  • These are known as the pollen tube nucleus and the generative nucleus
• At some point, as it travels down the pollen tube, the generative nucleus divides by mitosis to form a further two haploid male nuclei
  • These are the male gametes
  • The two haploid male nuclei travel down the pollen tube towards the female ovule
• As the pollen tube reaches the ovule, the pollen tube nucleus breaks down and the two haploid male nuclei pass into the ovule so that fertilisation can occur
• In fact, a process known as double fertilisation occurs:
  • One haploid male nucleus fuses with the nucleus of the egg cell to form a diploid zygote
  • The other haploid male nucleus fuses with two polar nuclei present in the ovule to form a triploid endosperm nucleus, which will form the endosperm (the food supply for the embryo plant when it begins to germinate)