Pollination and Fertilization

         Pollination occurs when pollen is transported to the surface of the flower’s stigma. After pollination takes place, the pollen grain produces a tube, which grows down the style and into the ovary to the ovule.  It is only capable of growing through the opening in the integument of the ovule to the embryo sac.  The pollen tube fuses with the embryo sac membrane and releases the two sperm nuclei into the sac.  At this point, another difference from the mosses and ferns occurs.  One sperm nucleus fuses with the egg nucleus to form a zygote.  The other sperm nucleus fuses with the two polar nuclei to form a triploid (3n) endosperm nucleus.  This process is termed double fertilization (Figure 17).  The triploid endosperm nucleus is capable of dividing quickly and governing the formation of food material to nourish the developing zygote.  The ovule with a zygote is now termed a seed.  During the whole time from gametophyte production through pollination to fertilization, the gametes have never been exposed to the environment.  The sperm nuclei were protected by the extremely resistant pollen grain wall and the egg cell was protected by the ovary and the integument of the ovule.

                                               

         Figure 17: Pollination and fertilization. Pollen is transferred from the anther to the stigma of the carpel. Two sperm resulting from division of the generative nucleus move from the pollen grain through the pollen tube to an opening in the ovule. One sperm fertilizes the egg, and the other fertilizes the polar nuclei.