Final answer:
The delay in b. stigma receptiveness in perfect flowers after anther pollen release is a biological mechanism to prevent self-fertilization, thereby promoting cross-pollination and greater genetic diversity.
Step-by-step explanation:
In perfect flowers, where both male and female reproductive organs are present, the delay in the receptiveness of the stigma to outcross pollen until after the anthers have released pollen is a mechanism designed to prevent self-fertilization. This is an evolutionary adaptation to increase genetic diversity and improve the species' ability to adapt to environmental changes. The delayed stigma receptiveness ensures that by the time the stigma is ready to receive pollen, the anthers' own pollen will have been shed, therefore favoring cross-pollination. Plants have evolved several mechanisms to promote this, including physical barriers, biochemical reactions, and as mentioned, temporal separation - known as dichogamy. Self-fertilization, while immediately creating offspring identical to the parent, limits genetic diversity and may increase genetic defects due to inbreeding. Plants have thus developed self-incompatibility genes that prevent self-pollination at a biochemical level, including the S locus, which plays a critical role in this mechanism.