Double fertilization

Double fertilization is the process in most flower plants where two male gametes enter the embryo sac and both get involve in the fertilization process. One male gamete mixes with the female gamete/egg nucleus to form a zygote , which turns into the embryo. The other male gamete mixes with either the polar nuclei (or the defenitive nucleus) to form a triploid primary endosperm nucleus, which will start the endosperm. 

Pictures are from Google Images

http://www.science20.com/news_releases/fbl17_discovery_gene_behind_%E2%80%98plant_sex_mystery%E2%80%99
http://www.gardenguides.com/87589-process-double-fertilization-flowering-plants.html