Primary endosperm cell ( $3 n$ ) formed by fusion of male and polar nuclei.After entering the ovule the pollen tube is attracted toward the micropylar end. The attractant is secreted by filiform apparatus. The pollen tube pierce one of the two synergid and releases its gametes. Out of the two male gametes one fuses with egg to perform generative fertilization (syngamy)It gives rise to the diploid zygote. The nucleus of the second male gametes fuses with the two haploid polar nuclei to form triploid endosperm nucleus. This second fertilization is called vegetative fertilization