Double Fertilization

Double fertilization is an essential characteristic in the sexual reproduction of angiosperms. In sexual reproduction, a haploid sperm cell fuses with a haploid egg cell, forming a diploid zygote that soon develops into an embryo. Following this fertilization process is a second event that involves another sperm cell and a second cell found in the female's reproductive tissue creating a triploid cell as a product. This product develops into what would be the embryo's food supply. Without double fertilization, angiosperms would be incapable of successfully reproducing due to an inability to fertilize and a lack of resources for the embryo.

Fertilization starts between sperm cells, which are transferred through pollen grains produced in the anthers, and two cells within an ovule, the reproductive organ of the female. The reproductive cell found in the ovule is a diploid (2n) megaspore mother cell, which eventually undergoes meiosis to produce four haploid (n) megaspores. Only one of the four megaspores remain in most species while the other three degenerate. The surviving megaspore goes through three rounds of mitosis to form eight haploid nuclei that share the same cytoplasm, forming the embryo sac. Cell walls form between the nuclei to form three antipodal cells opposite the micropyle and near the micropyle; the ones near the micropyle are distinguished as two synergids and an egg. The two remaining nuclei, called polar nuclei, remain together in a single large central cell.

Before the two polar nuclei can take place in double fertilization, the sperm must travel within the female's reproductive organs to the cells. A pollen grain lands on the stigma and begins to germinate, sending a long pollen tube through the style and ovary. The generative cell, a haploid, travels down the pollen tube behind the tube nucleus and divides by mitosis to form two haploid sperm cells.

After the pollen tube reaches the micropyle and makes its way into one of the synergids, the sperms cells are released, which degenerates the synergid and sends one of the two sperm cells to fertilize the egg cell. The second sperm cell fuses with both of the polar nuclei, forming a triploid cell. This cell develops into the endosperm and serves as the embryo's food supply as the zygote develops into an embryo.