Another tree within approximately 50 feet that is also in its blooming period is required for pollination, such as Braeburn or Rome. Cross-pollination is required, when pollen of one apple comes in contact with the pollen of another flower, to initiate the growth of the pollen tube. Every pollen grain has two sperms and the classic, domestic apple has 17 chromosomes in both sperm and egg. Via double fertilization, one sperm fertilizes one egg in the ovule, and one sperm combines two haploid cells in the same ovule. The first fertilization produces the zygote, which develops into an embryo. The second fertilization forms the endosperm, which is used as food for the embryo as well as other animals. Seeds are formed when the endosperm and embryo combine, and they ovary becomes the fruit around the seeds. Seeds are also used for dispersal, reproduction, and genetic variation.