Unlike humans, plants have to undergo double fertilization. Double fertilization is essential for fertility and seed production. Double fertilization involve two "twin" sperm cells and one egg cell. One sperm cell (haploid) join with the egg cell to produce the embryo (diploid) and the other join with a second cell in the ovary to produce the endosperm (triploid) inside the seed.
The ovule contains a reproductive cell called the megaspore cell (diploid), which undergoes meiosis to produce four haploid megaspores, but only one typically survive out of the four. The surviving megaspore undergoes three rounds of mitosis to produce eight haploid nuclei. The eight nuclei is in the embryo sac. Within the embryo sac, cell walls form between most of the nuclei. Three cells form opposite the micropyle opening of the ovule. Three also form near the micropyle (one of them is the egg). Two polar nuclei remain together in one large central cell.
After that step, the pollen (sperms) come into action. A pollen grain reach the stigma and begin to germinate. Then it sends a long pollen tube through the style and ovary. One of the two sperm cells fertilizes the egg cell. The second sperm cell fuses with both of the polar nuclei, fertilizing them to produce an unusual triploid cell. The endosperm serves as the embryo's food supply.
When the pollen grain reaches the stigma, the gene, FBL17 triggers the destruction of another protein that inhibits cell division. The gene acts as a switch to trigger precursor cells to divide into twin sperm cells.
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