The mode of reproduction in plants may be sexual (amphimixis), asexual (apomixis or agamospermy), or by specialized vegetative structures (vegetative reproduction). Sexual reproduction involves pollination, germination of pollen grains on the stigma, growth of pollen tubes down the style, the entrance of pollen tube to the ovule through the micropylar opening, the discharge of two sperm nuclei into the embryo sac and fusion of a sperm nucleus with the egg nucleus, producing an embryo, and conjugation of a second sperm nucleus with the two polar nuclei (secondary nucleus) forming a triploid endosperm. Thus, seeds develop as the result of double fertilization, and the majority of plants reproduce sexually (Figure 5.1). The disruption of the sexual process may lead to apomixis.