Dissection of the style shows that numerous pollen tubes have grown down from the surface of the stigma and terminate well before reaching the ovules. Their failure to reach the ovules is most likely due to:

Question

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The fact that the pollen tubes fail to reach the ovules suggests the presence of a self-incompatibility system controlled by S-gene alleles. The self-incompatibility mechanism prevents self-fertilization by blocking the growth of pollen tubes that carry the same S-gene alleles as the pistil's S-genes. This ensures genetic diversity in the progeny and prevents self-fertilization, which can lead to inbreeding depression.

Dissection of the Style Shows That Numerous Pollen Tubes Have