Self-Incompatibility in Pollination

Steps:

• Examine the pollen grains: one germinates (from species 2), the other does not (from species 1).
• Identify the stigma as part of the carpel from species 1.
• Recognize self-incompatibility prevents germination of pollen from the same species on its stigma.
• Conclude the difference arises because the carpel's species identity blocks self-pollen.

Why C is correct:

• Self-incompatibility is a genetic mechanism in the carpel (stigma) of species 1 that rejects pollen from the same species, allowing only cross-species pollen to germinate.

Why the others are wrong:

• A: The difference is due to biochemical rejection, not superiority of cross-pollination.
• B: Self-pollination affects species 1 pollen, not species 2; species 2 germinates normally.
• D: Landing quality is irrelevant; the difference is species-specific germination control.

Final answer: C