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The microspores which are released from the pollen tetrad are known as the pollen grains. This pollen grain represents the first cell of gametophytic generation. A mature pollen grain has two layered wall; the outer layer is thick and the inner layer is very thin and smooth they are known respectively as the exine and intine. The exine is formed of complex substances known as the sporopollenin. The region where the exine is absent is known as the germ pores.