Fertilization begins when a pollen grain lands on the stigma of a flower. A pollen tube then grows and carries two male gametes down to the ovule. One male gamete fuses with the egg cell to form a zygote, while the other fuses with the polar nuclei to form the endosperm. This double fertilization process results in the formation of a seed containing an embryo and nourishment in the form of endosperm. After fertilization, the ovary of many flowers swells to become the fruit, protecting the developing seeds inside.