This is related to the surface area of the leaf with the amount of water lost being directly proportional to the surface area. A tree such as a sycamore, which is usually found along a stream, will have large broad leaves while a sage, usually found in the arid hills of Southern California, will have narrow leaves. Evergreen trees are an interesting case where leaf size is also an important factor as the needles have very little surface area. The ultimate adaptation of this type is, of course, the cacti which have reduced their leaves completely until they are no more than thorns. In this case, the process of photosynthesis now takes place in the green stem.
Some plants reduce water loss by reducing heating by the sun.
The less sunlight they absorb, the less they heat up and the less water evaporates from them. Some plants have their leaves arranged so that they do not directly face the sun, either by being permanently positioned that way or by rotating throughout the day to present only their edges to the sun. Other plants reduce heating by having shiny or light colored leaves that reflect most of the heat producing sun while others have surface coatings that decrease heat loss. Then examine the microscope slides showing cross sections of these leaves. Diagram the microscopic view with particular reference to the thickness of the epidermis and, if visible, the cuticle.