Most of the angiospermic plants have a capability to interrupt germination of pollens and further growth of pollen tube so as to avoid inbreeding and help outbreeding. This chemical control of an individual plant is called self-incompatibility. It is achieved by the interaction of chemical substances secreted by male gametophyte and tissues of style. Normally, the functional pollen grains belonging to right mating type germinate on the stigma, develop pollen tube and bring about fertilization. The pollen grams of other mating types are discarded. If a pistil carrying functional gametophyte gets pollinated with right mating type of pollen grain but fails to set seeds due to inability of their gametes to fuse with each other, thephenomenon is known as Sexual incompatibility.

It is of two types : 
(i) Interspecific, between the individuals belonging to different species and 
(ii) Intraspecific, between the individuals of the same species. The intraspecific incompatibility also termed as self incompatibility or se lf-sterility. The incompatibility involves many complex physiological morphological and biochemical mechanisms associated with the interaction between pollen grain and tissues of stigma. The intraspecific self- incompatibility is controlled by more than one genes. It is achieved by different processes such as:

1. prevention of pollen germination
2. inability of pollen tube to penetrate into the stigma
3. inhibition of pollen tube growth in the style
4. failure of pollen tube to enter into the ovary or ovule
5. failure of the fusion of male and female gametes or
6. abortion of embryo following syngamy or development of embryo-endosperm incompatibility.

The self incompatibility is of two major types:

• Heteromorphic type, in which two or more morphologically distinct mating types esent within a species which can be recognized w;nout a breeding test.
• Homomorphic type in wnich two or more mating type within a species are morphologically indistinguishable. They require proper breeding test.

Based on the factors of pollen grain, the self-incompatibility is divided into two types:

1. If the incompatibility is controlled by the genotype of sporophytic tissue from which the pollen is derived or the sporophytic tissue of stigma, it is called sporophytic incompatibility.
2. If it is due to genotype of male gametophyte i.e., pollen grains, it is called gametophytic incompatibility. In nature the incompatibility is considered as the most widespread and effective device to prevent inbreeding and promote outbreeding.