Barriers to Fertilisation
Incompatibility can occur anywhere from pollination to syngamy and consequently obstructing fertilisation. The pollen fails to germinate or the pollen tube is inhibited to penetrate the stigma.
The progamic barriers to fertilisation may be on the stigma or in the style at any level from pollen germination to the discharge of male gamete in the vicinity of egg.
On the other hand, syngamy barriers include the inhibition of the entry of pollen tube to the ovary, ovule or within embryo sac. At the stage of pollination the barriers to fertilisation are of morphological and ecological types, whereas, at progamic phase and syngamy physiological barriers occur.