In the pistil and stamen. Insects make it possible. Source: I am a bee farmer
Fertilization takes place when the male nucleus in the pollen tube joins the female nucleus of the ovule.
Fertilization is one of the key processes in a flower. Some flowers can self fertilize as they have stamens and pistils. Other flowers have to fertilize when the male and female flowers come in contact or the stamens of the flowers meet the pistils. For details on the fertilization process see the link below: