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Germination of Pollen Grain. Germination of the pollen grain starts within the pollen sac. The cell divides to form two cells. One of these is smaller in size and called generative cell and bigger one the tube cell. At this two celled stage the pollen grains are transferred to the stigma of the carpel and the pollen germinates further. The surface of the stigma is moist. The pollen grains swell due to absorption of moisture. The intine grows out through the germ pore in the form of a tube called pollen tube. It grows into the tissue of the style towards the ovary. As the pollen tube grows in length, the tube nucleus and generative cell moves into it. The tube nucleus keeps ahead of the generative nucleus. After reaching the ovary, the pollen tube makes its way towards the ovule and enters it through the micropyle. In certain cases it may enter from the chalaza or even from the sides. Ultimately, it passes through nucellus and reaches the egg apparatus. By this time the tube nucleus, has degenerated and the generative nucleus divides to form two male gametes which now move to the tip of the tube. The pollen tube bursts open either due to absorption of moisture from the synergid cell or by the tip of these cells causing a mechanical injury to the tube. The male gametes are released and also synergid disintegrate. Thus fusion takes place between one of the gametes with egg nucleus. It is termed fertilization. The cell thus formed is called oospore.

 
Germination of pollen grain and formation of male gametophyte in an angiosperm.