The Endosperm serves as the food supplying tissue of a young and fertilized seedling. When sperm is discharged into the embryo sac, double fertilization occurs. Double Fertilization is the merging of two sperm with two different nuclei within the embryo sac. While the egg undergoes fertilization to become a diploid zygote, the already diploid central cell unites with a sperm to become a triploid nucleus. This triploid nucleus gives rise to the endosperm (food-storing tissue) that will nourish the zygote. The Key Lime, as pictured above, has an endosperm that provides nourishment to seeds within its skin.