Self sterility is also known as incompatibility. Failure of germination of pollen on the stigma of the same flower or on any flower of the same strain is called incompatibility or self sterility. It is on account of negative interaction between the pollen and pistol. The reason may be physiological or morphological. It may be determined by pollen or the male gametophyte or by the sporophytic tissue. Thus cross-pollination becomes inevitable in such cases.