A pollen grain starts to grow if it lands on the stigma of a flower of the correct species. A pollen tube grows through the tissues of the flower until it reaches an ovule inside the ovary. The nucleus of the pollen grain (the male gamete) then passes along the pollen tube and joins with the nucleus of the ovule (the female gamete). This process is called fertilisation.
The slideshow explains what happens:
After fertilisation, the female parts of the flower develop into a fruit:
A seed has three main parts: