In desert environments, water is a resource of primary importance, so plants tend to take up as much water as possible and lose as little water as possible from day to day. Most water in plants is lost through transpiration, a process that cools the plant.
One type of desert plant keeps cool by having not only its roots underground, but most of its leaves as well. This may seem counterproductive to the purpose of leaves, but in this plant the tips of the leaves are heavily coated with wax and emerge just above the ground's surface. These wax tips serve as little windows for sunlight to reach the rest of the photosynthetic tissue in the underground leaves.