On the underside of leaves there are little holes to let water and gasses in and out. These are stomata. Obviously they would not be found on the topside since that side recieves light and water evaporates faster from that sort of hot environment. Also, the leaf can be specialised into two surfaces - one with waxy covering and cells that absorb light and the other with internal spaces and stomata.

The opening and closing of a stomatal pore is controlled by guard cells on either side which expand and contract to block or widen the hole.