Double Fetilization

In Fertilization, two male gametes are brought to the oval sac. One of the gametes fertilize with the egg cell to form an embryo and the other gamete fertilized with the polar nuclei cells to form endosperm, which is used to provide nourishment for the embryo.
In Double Fertilization, one sperm combines with the egg whole the other sperm combines with the endosperm. Double fertilization is required because seeds needs to be germinated and it needs the extra sperm in order for it to germinate.

http://bcs.whfreeman.com/thelifewire/content/chp39/3902001.html
http://www.science20.com/news_releases/fbl17_discovery_gene_behind_%E2%80%98plant_sex_mystery%E2%80%99