a) The given diagram shows generating pollen. b) 1-Exine     2-Intine     3-Pollen Tube     4-Vegetative nucleus     5-Generative nucleus c) The stage is found when the pollen reaches a stigma of a flower and the chemical reaction stimulate the tube elongation and subsequent fertilisation. d) The vegetative nucleus synthesizes the pollen tube. e) The Generative nucleus bifurcates into two male gametes out of which one fuses with the polar nuclei in triple fusion to form endosperm and the other male nuclei fuse with the egg cell to form the zygote and subsequent embryo.