Fertilisation in Plants (Edexcel IGCSE Biology: Double Science)

• In plants, the ovary contains one or more ovules
• The ovules are the structures that eventually develop into seeds
• Each ovule contains an ovum
• An ovum is an egg cell that contains the female nucleus that a male pollen nucleus can fuse with
• In plants, fertilisation occurs when the pollen grain nucleus fuses with the ovum nucleus

The growth of the pollen tube

• Unlike the male gametes in animals (sperm), the pollen grain has no ‘tail’ to swim to the ovary of the flower
• Instead, in order to reach the ovum nucleus, the pollen grain grows a pollen tube
  • This only happens if the pollen grain has landed on the right kind of stigma (i.e. of the same species as the flower the pollen came from)

• The nucleus inside the pollen grain moves down the tube as the tube grows down the style towards the ovary (which contains the ovule that, in turn, contains the ovum)
• Once the nucleus of the pollen grain and the nucleus of the ovum have fused (joined together), that particular ovule has been fertilised and a zygote has been formed
  • The zygote will then start to divide (it is the structure that eventually develops into an embryo plant)

The process of fertilisation in a typical plant

Seed and fruit formation

• After fertilisation, the ovule (that contains the zygote) develops into the seed
• The wall of the ovule develops into the seed coat, known as the testa
• The parts of the flower surrounding the ovule (mainly the ovary walls) develop into the fruit, which contains the seeds
  • The fruit provides a mechanism for seed dispersal (getting the seeds away from the parent plant)
  • Some fruits are eaten by animals, which then disperse the seeds in their droppings (the tough outer coat of seeds stops them from being digested)
  • Some other fruits have sticky hooks that get caught in the fur of passing animals

• As different plants have different numbers of ovules, this explains why different fruits have different numbers of seeds (which develop from the ovules)