Sexual Reproduction In Plants

Reproduction in plants involves pollen grains and the embryo sac. The process of fertilisation in angiosperms is outlined below.

Formation of the Gametes

Specification Reference

"Understand how a pollen grain forms in the anther and the embryo sac forms in the ovule."

Pollen Grain Formation

Pollen grains are formed on the anthers - the male part of the plant.
  • Each anther contains 4 pollen sacs
  • These sacs contain many 2n microspore mother cells
  • These each divide twice by meiosis to for 4 microspores (n)
  • The nucleus in each pollen grain then divides by mitosis to produce a generative nucleus and a pollen tube nucleus.

Embryo Sac Formation

Embryo sacs are found the ovule - the female part of the plant.
  • Megaspore mother cells (2n) divide by meiosis to form 4 haploid megaspores (n)
  • One of these will then undergo three mitotic divisions to form 8 haploid nuclei
  • The remaining 4 haploid megaspores will degenerate
  • All of these nuclei remain in the embryo sac and the cytoplasm does not divide

Double Fertilisation

Specification Reference

"Understand how the male nuclei formed by division of the generative nucleus in the pollen grain reach the embryo sac, including the roles of the tube nucleus, pollen tube and enzymes."
"Understand the process of double fertilisation inside the embryo sac to form a triploid endosperm and a zygote."
The process of double fertilisation is outlined below and involves the male pollen grain and the female ovule.
  1. Insects or the wind transfer a pollen grain from the anther of one plant to the stigma of another (pollination).
  2. The pollen grain nucleus divides to form a generative nucleus and the pollen tube nucleus, both are haploid.
  3. The pollen tube nucleus then produces hydrolytic enzymes, providing nutrients for the formation of the pollen tube.
  4. As the generative nucleus moves down the pollen tub it divides once through mitosis.
  5. The pollen tube reaches the micropyle and breaks down.
  6. The two male nuclei are released into the ovule
  7. One fuses with the female gamete to form a diploid zygote
  8. The other fuses with the two polar bodies forming the endosprem (3n)
This process is known as double fertilisation.