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The female gametophyte of a seed plant, within which the embryo develops.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
embryo sacthe female GAMETOPHYTE of flowering plants, consisting of a sac-like structure inside the OVULE in which are found six HAPLOID cells (without cell walls) and two haploid nuclei. See Fig. 147 . All cells have originated from one haploid nucleus and so are all genetically identical. The name ‘embryo sac’ comes from the fact that the plant embryo develops from within the sac. Two MALE GAMETE NUCLEI enter the embryo sac from the POLLEN TUBE and carry out a ‘double’ fertilization. One male nucleus fuses with the egg cell to give a DIPLOID zygote (the future plant), the other male nucleus fuses with two polar nuclei giving a triploid fusion nucleus (3n) which develops into the ENDOSPERM (the future food supply in some species).
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005