The process by which two gametes (reproductive cells having a single, haploid set of chromosomes) fuse to become a zygote, which develops into a new organism. The resultant zygote is diploid (it has two sets of chromosomes). In cross-fertilization, the two gametes come from two different individual organisms. In self-fertilization, the gametes come from the same individual. Fertilization includes the union of the cytoplasm of the gametes (called plasmogamy) followed by the union of the nuclei of the two gametes (called karyogamy). gymnosperms and angiosperms.