Figure 27.3: Self-incompatibility In a self-incompatible plant, pollen is rejected if it expresses an S allele that matches one of the S alleles of the recipient carpel. There are two mechanisms of rejection: (A) the pollen may fail to germinate, or (B) the pollen tube may die before reaching an ovule. Both mechanisms prevent the egg from being fertilized by a sperm from the same plant.