Incompatibility means inability of male gametes to fuse with egg nuclei. Certain pollen grains even fail to germinate on the stigma of the same (lower. This is because of self sterility factor. It is also called intraspecific incompatibility. It depends endosperm on the pollen-pistil interaction and or some genetic factor. This helps to prevent self pollination.
(a) Homomorphic incompatibility:
Here, compatible and incompatible plants cannot be differentiated. Hence, the plants bear only one type of flower but there is occurrance of two types of incompatibility. First one occurs due to the genotypes of sporophyic or stigmatic tissues. The other one is gametophyte incompatibility due to the genotype of the pollen grains.
(b) In heteromorphic condition there are two or three types of morphologically different flowers. They differ in the length of styles and anthers. Each type is self sterile but cross fertile. Therefore, pollen grain from any other plant of the same species bearing other type of flowers is required for effective fertilization.