Final answer:
In angiosperms, the primary endosperm nucleus is triploid (3n), resulting from double fertilization where one sperm fertilizes the central cell to form the endosperm, providing nutrients for the developing embryo.
Step-by-step explanation:
In most angiosperms, the primary endosperm nucleus is 3n. This results from a unique process called double fertilization, where one sperm cell fertilizes the egg to form a 2n zygote, and a second sperm cell fertilizes the central cell containing two polar nuclei to form the 3n endosperm. The endosperm serves as a nutritive tissue supporting the developing embryo of the angiosperm.
During the formative stages of seed development, this triploid endosperm nucleus undergoes a period of 'free-nuclear' development, where nuclear divisions are not accompanied by cell wall formation, resulting in a multinucleate cell. Eventually, cell walls form and a tissue develops that typically persists in the mature seed. However, in some angiosperms, the endosperm is absorbed as the cotyledons enlarge during the maturation process.