In many plants, stomata are found only on the lower surface of the leaf. the most likely explanation for this fact is that
a. photosynthesis only occurs in the spongy mesophyll near the bottom of the leaf.
b. stomata are closer to vascular bundles that bring water into the leaf.
c. gravity plays a role in gas exchange.
d. water loss would be less on the shaded lower surface than in direct sun.

D. Plants have adapted to deal with the sun in many different ways, and this is one. The stomata are regulated by guard cells, which are activated by different things depending on the species of plant, and guard cells are supposed to keep the stomata from staying open all the time and losing too much water to evaporation. So, the solution is to have the stomata on the bottom of the leaf to prevent direct sunlight contact (and therefore more evaporation), and guard cells to protect the stomata. 

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