Mesophyll surface area drives transpiration differences

Steps:

• Transpiration rate depends on water evaporation from mesophyll cells to air spaces, then diffusion out via stomata.
• Greater mesophyll surface area provides more evaporation sites, increasing water vapor loss.
• Plant X's larger mesophyll area explains higher vapor loss compared to plant Y under identical conditions.
• Stomatal distribution affects diffusion but not the primary evaporation source.

Why C is correct:

• Transpiration rate is proportional to mesophyll surface area for evaporation, as defined by the pathway from cell walls to substomatal cavities.

Why the others are wrong:

• A: Stomatal position on lower surfaces reduces exposure to wind but equally affects both plants in identical conditions, not explaining the difference.
• B: Upper stomatal placement increases exposure but applies only to Y, without specifying X's configuration to cause disparity.
• D: Greater mesophyll area in Y would increase its loss, contradicting the observed difference favoring X.

Final answer: C