The union in flowering plants of two sperm nuclei. One sperm nucleus unites with the egg to form the diploid zygote, from which the embryo develops, and the other unites with two polar nuclei to form the triploid, primary endosperm nucleus.双受精：一个精核与卵结合构成双重合子，由此长成胚胎，另一个精核与两个偶极核结合，形成三重基本胚乳核