Final answer:
Only flowers with self-incompatibility mechanisms pollinated through cross-pollination will be fertilized, as this mechanism prevents self-fertilization to encourage genetic diversity. Incompatible pollen tubes are halted by the S locus, leading to their degradation or apoptosis.
Step-by-step explanation:
The flowers that possess self-incompatibility mechanisms are pollinated via cross-pollination rather than self-pollination. Self-incompatibility is a genetic mechanism that prevents self-fertilization and its associated inbreeding. The interaction between the pistil and incompatible pollen or pollen tubes involves the S locus, where incompatible pollen tubes are arrested in development either at the stigma surface or within the style, leading to their apoptosis or RNA degradation.
In compatible cross-pollination events, pollen from one flower is transported to another flower, often by pollinators like bees, butterflies, or physical agents like wind or water. This results in greater genetic diversity. When incompatible pollen is received, biochemical signals resulting from the S locus prevent the pollen tube from successfully growing and reaching the ovule, which ensures that the plant does not self-fertilize. The protein key to this process is often a ribonuclease secreted from the style that degrades RNA within the incompatible pollen tube.