An endosperm is a tissue produced inside the seeds of most flowering plants around the time of fertilization. In the seed, it surrounds the embryo and provided nutrition, mainly in starch but sometimes also in oils and protein. Flour comes from the ground up endosperm of wheat. The endosperm forms when two sperm nuclei reach the interior of the embryo sac. One fertilizes the egg, and the other fuses with the two polar nuclei at the center of the embryo sac, forming the primary endosperm cell. This develops into the endosperm.