Figure 27.4: Double Fertilization Two sperm are involved in two nuclear fusion events, hence the term “double fertilization.” One sperm is involved in the formation of the diploid zygote and the other in the formation of the triploid endosperm. The events shown here are for two-celled pollen grains; for three-celled pollen grains, the generative cell divides before the pollen is released from the anther.