Mechanism of pollination

       Pollination mechanism can be defined by cross-pollination or by self-pollination.

Cross-pollination is also known as allogamy which occurs only when pollen is delivered to a flower from a different plant. Plants adapted to cross-pollinate often these plants have taller stamens than carpels.

Self-pollination occurs when pollen from one flower pollinates the same flower or other flowers of the same individual. It is thought to have evolved under conditions when pollinators were not reliable vectors for pollen transport, and is most often seen in short-lived annual species and plants that colonize new locations. Self-pollination may include autogamy, where pollen moves to the female part of the same flower; or geitonogamy, when pollen is transferred to another flower on the same plant. Plants adapted to self-fertilize often have similar stamen and carpel lengths. Plants that can pollinate themselves and produce viable offspring are called self-fertile. Plants that cannot fertilize themselves are called self-sterile, a condition which mandates cross pollination for the production of offspring. Cleistogamy is self-pollination that occurs before the flower opens. The pollen is released from the anther within the flower or the pollen on the anther grows a tube down the style to the ovules.