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The union in flowering plants of two sperm nuclei with three nuclei in the embryo sac. One sperm nucleus unites with the egg to form a diploid zygote, and the other unites with two polar nuclei to form the triploid, primary endosperm nucleus.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.