In order for fertilization to take place in flowers, pollination has to occur. Pollination occurs when pollen, which is produced in the male parts of flowers, is transferred to the female parts of flowers. This pollen is produced in the anther of a flower's stamen. Within the pollen is sperm. Whether by wind, insects, mammals or other methods, when pollen gets transported to the stigma located at the top of a flower's pistil, pollination has occurred.
Besides a stigma, a pistil has a style that supports the stigma and connects it to an ovary at the bottom of the pistil. Within the ovary, at least one ovule waits for fertilization, along with two "polar nuclei." Once pollen attaches to the stigma, it grows a tube down through the style that eventually reaches the inside of the ovary. Two sperms travel down the tube. One sperm fuses with an egg while the other joins the two polar nuclei.
When the sperm and egg join, they become a zygote. This is an embryo destined to become a new plant. Meanwhile, the sperm and polar nuclei have formed an endosperm. This endosperm divides repeatedly, forming a structure around the embryo. The purpose of the endosperm is to provide food for the embryo, which has developed an embryonic root and leaf-like structures called cotyledons. Together, the embryo and endosperm form a seed. The ovary of the flower ripens to become the plant's fruit.