It is the process at the time of fertilization when two male gametes are brought to the oval sac one of them fuses with egg cell to form embryo and the other gamete fuses with the cells in the center that are called polar nuclie to form endosperm further to provide nourishment to the growing embryo. A single pollen grain has 2 sperm. Double fertilization occurs when 1 of these sperm combines with the egg, and the other sperm combines with a cell (endosperm). That is in order for the seed to keep being produced. Seed has to be germinated, and without the extra sperm (double fertilization) the plant couldn't germinate and would go extinct. 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.
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