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Describe double fertilization.
Fertilization takes place between sperm cells and two cells within an ovule. The ovule contains the reproductive cell, megaspore. The megaspore undergoes meiosis to produce 4 haploid megaspores. In most species, three degenerate leaving one megaspore. The remaining megaspore undergoes mitosis to produce 8 haploid nuclei who share the same cytoplasm. These 8 haploid nucleic structure is called the embryo sac. Cell walls form between most of the embryo sac. Three cells form near the micropyle, two are called the synergids and one of the cells is the egg. The remaining two nuclei stay together in one large main cell. The egg cell and the single cell will soon take part in double fertilization. But before this event can happen, the sperm has to enter the female reproductive organs. To reach the embryo sac, a pollen grain begins to germinate, sending a long pollen tube the the style and the ovary of the female flower. The generative cell(one of the haploid cells) goes down the pollen tube. The generative cells undergoes mitosis to make 2 haploid sperm cells. Once the pollen tube reaches the micropyle of the ovule, it digests its way into the synergids, releasing the sperm cells. The synergid degenerates, and one sperm cell fertilizes the egg cell, producing a diploid zygote. The remaining sperm cell, fuses with the polar nuclei and makes triploid(3) cells. When the zygote turns into an embryo, the triploid cells develops into the endosperm( food for embryo).
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