Double fertilization refers to a process in which two sperm cells fertilize cells in the ovary. If a pollen grain germinates, a pollen tube grows down the style toward the ovary. The pollen tube discharges two sperm into the female gametophyte (embryo sac) within an ovule. One sperm fertilizes the egg, forming the zygote. The other sperm combines with the two polar nuclei of the embryo sac's large central cell, forming a triploid cell that develops into the nutritive tissue called endosperm.