Final answer:
Two sperm nuclei travel down the pollen tube to ensure double fertilization, where one sperm fertilizes the egg forming a diploid zygote and the other one combines with two polar nuclei, creating a triploid endosperm, crucial for seed development.
Step-by-step explanation:
The process by which two sperm nuclei travel down the pollen tube is crucial for a unique event called double fertilization, a distinctive feature of angiosperms. When pollen lands on the stigma, the pollen grain germinates, and the pollen tube cell grows a tube through the style to reach the ovule. Meanwhile, the generative cell divides to form two sperm cells within the tube.
These sperm cells are then delivered to the embryo sac where one sperm fertilizes the egg, forming a diploid zygote, and the other sperm combines with two polar nuclei to produce a triploid endosperm. The latter serves as a food source for the developing embryo. This simultaneous fertilization of the egg and the polar nuclei ensures the proper development of the seed, thus maximizing the plant's reproductive success.