Double Fertilization

Angiosperms, also known as flowering plants, are capable of undergoing both sexual and asexual reproduction. In sexual reproduction, angiosperms are capable of double fertilization. In the process of double fertilization, sperm cells are delivered by pollen grain to the ovule, the female reproductive organ. The ovule contains a reproductive cell called the megaspore. One megaspore undergoes three rounds of mitosis, producing eight haploid nuclei, which share the same cytoplasm, the embryo sac. These eight nuclei are then separated near the micropyle and the synergids. The egg is near the synergid while two polar nuclei are located together in the middle. In order to start double fertilization, the pollen grain starts to germinate, creating a long pollen tube through the style and ovary in which the sperm is able to reach the embryo sac. The haploid sperm cell undergoes mitosis and produces two haploid sperm cells which is then released into the embryo sac. One of the sperm cells fertilizes the egg cell which produces a diploid zygote. The other sperm cell combines and fertilizes the two polar nuclei which produces a triploid (3n) cell which then develops into an endosperm. The endosperm is then known as the embryo’s food supply.

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