Angiosperms (flowering plants) contain male and female structures responsible for producing sex cells in order to carry out sexual reproduction. The female sex cell of a plant is known as an egg cell.
The female sex organ of the flower is the pistil and located at the bottom of the pistil is the ovary containing one or more ovules. Within the ovule, female gametophytes are produced. In the alternation of generations that occurs in flowering plants, this gametophyte is both multi-cellular and haploid and gives rise to seeds when double fertilization occurs inside this embryo sac.
A pollen grain will land on the top of the female pistil and begins to germinate downward towards the ovule forming a pollen tube. Once the tube reaches the ovule inside the ovary, the pollen tube will release its two sperm cells. Inside the ovule are one egg cell and two polar bodies--all three are haploid cells. The egg cell fuses with a sperm cell to produce a diploid zygote while the other two polar cells combine with the second sperm nuclei to form triploid endosperm material.
This process is called a double fertilization because of the formation of both the embryo plant along with its food supply (endosperm) that occurs within the ovule which ultimately develops a hard covering and becomes the plant's seed. These seeds were housed in the plant's ovary, which enlarges to become the fruit of the plant.