Final answer:
The polar nuclei are located in the center of the embryo sac of angiosperms, contained within the central cell and play a crucial role in forming the triploid endosperm after fertilization.
Step-by-step explanation:
In the center of the embryo sac are the polar nuclei. The embryo sac, which is the female gametophyte in angiosperms, develops within the ovule. It consists of seven cells: one egg cell, two synergids, three antipodal cells, and one central cell that contains the two polar nuclei. These nuclei play a critical role in fertilization as they fuse with one sperm cell to form the triploid endosperm, which functions as a food reserve for the developing embryo. Unlike the antipodal cells that eventually degenerate and synergids that assist in guiding the pollen tube, the polar nuclei are directly involved in the formation of a vital nourishing tissue within the seed.