The process of transfer of pollen grains from a mature anther to a mature stigma of the same flower or a flower of another plant is called pollination.
The following is the account of events between pollination and fertilization.
Germination of Pollen Grains on the stigma : The pollen grains are already two or three celled at the time of pollination. They absorb water and nutrients from stigmatic secretions through germ pores and germinate. The intine grows out through a germ pore present in the thick exine in the germ of a form tube. The germ tube grows through the stylar canal in the form of a pollen tube. The pollen tube secretes pectinases and other hydrolytic enzymes which digests the tissues of the stigma and style to form a passage. The tube or vegetative nucleus descends to the tip of the pollen tube.
The generative cells also moves down into the pollen tube and divides into two male gametes (sperms) if it is not already divided. Each male gamate is lenticular to spherical in outline. It has a large nucleus and is surrounded by a thin sheath of cytoplasm. The tube nucleus degenerates soon after. The fertilization by means of pollen tube is known as siphonogamy. If the pollen tube enters the ovule through the microphyle, the method is known as porogamy. If the pollen tube enters through the chalaza end of the ovule it is known as chalazogamy. When the pollen tube enters the ovule through integuments it is known as mesogamy. Sometimes the empty space above the micropyle has a plug of placental origin. The plug is formed by obturator. The obturator guides the growth of the pollen tube through the empty space.
