CBSE Class 10 Answered

This is an adaptation to prevent excess water loss. Dicot leaves are held horizontally and hence upper epidermis is directly illuminated. The stomata must be open during the daylight hours to let oxygen and carbon dioxide pass through. While they are open, water vapor escapes into the atmosphere (transpiration). If the stomata are on the lower surface, they are not directly exposed to the sun, and they are a little more protected from breeze - both of which lessen the amount of transpiration (water evaporation) that occurs.

(Note that this is true of dicot leaves, but not monocot leaves. Their leaves are held vertically, they will have the same number of stomata on the two epidermes.)