During the process of fertilization, when the pollen tube enters the embryo sac the tube nucleus disintegrate.
While the pollen tube is growing towards the ovary of the plant, the generative nucleus divides into two sperm nuclei. Of these, one sperm nucleus enters the egg nucleus and fuses with it's nucleus forming the ZYGOTE. The other sperm nucleus moves towards the two polar nuclei in the central cell and fuses with them forming the ENDOSPERM NUCLEUS.
The fate of tube nucleus is that it disintegrates and the generative nucleus forms the ZYGOTE and ENDOSPERM NUCLEUS.