Describe double fertilization.

     Double fertilization is a process when a diploid zygote and a triploid endosperm form in flowering plants. Fertilization takes place between sperm cells and two cells within the ovule. The megaspore in the ovule is diploid and undergoes meiosis to produce four haploid megaspores. The surviving megaspore undergoes three rounds of mitosis to produce eight haploid nuclei. the eight nuclei share the same cytoplasm before cytoplasmic divisions start, this is called the embryo sac.

      In the embryo sac, cell walls form between the nuclei. Three cells form opposite the micropyle opening of the ovule and another three from near the micropyle. Two are called synergids and the other an egg. The polar nuclei (two nuclei) remain together in one large central cell. The egg, single cell, and two polar nuclei be in the process of double fertilization. But first the sperm travels to the cells by a pollen grain landing on the stigma, beginning to germinate, and sending a long pollen tube through the style and ovary. A haploid cell travels down the pollen tube, behind the tube nucleus, and divides by mitosis to produce two haploid sperm cells.

     The pollen tube reaches the micropyle of the ovule and gets into one of the synergids by digesting itself, releasing the sperm cells. The synergid degenerates and a sperm cell fertilizes the egg cell producing a diploid zygote. The other sperm cell fuses with polar nuclei fertilizing them to produce a triploid cell. The zygote will develop into an embryo, and the triploid cell into an endosperm. This will serve as the embryo's food supply.

source: http://bcs.whfreeman.com/thelifewire/content/chp39/3902001.html