(a) : In $60\%$ of flowering plants, the pollen grains are shed at two­ celled stage (tube cell + generative cell). Further, development of male gametophyte (pollen grain) occurs on stigma. Pollen grain gives rise to pollen tube which absorbs nourishment from the cells of style for its growth. Generative cell divides to give rise to two male gametes. Out of these, one fuses with the egg to form diploid zygote (generative fertilization or syngamy) whereas the second male gamete fuses with the two haploid polar nuclei or diploid secondary nucleus of the central cell to form primary endosperm nucleus (vegetative fertilization or triple fusion). These two acts of fertilization occur in the same embryo sac and are referred to as double fertilization.