Most cacti have huge masses of shallow, wide-spreading roots that can soak up large amounts of water very quickly. In contrast, mesquite, blue palo verde and ironwood trees have very deep roots that may tap into water deep under the ground. Special Leaves and Stems. All plants lose water from their leaf and stem surfaces to the air. This process is called transpiration.
Transpiration occurs mainly through a plant's stomates.
In many cacti the stomates lie deep in the plant's tissue. Many desert plants open their stomates only at night, since there is less water loss when it is cooler. More water evaporates from smaller leaves than from larger leaves due to the reduced surface area. Therefore, many desert plants have evolved smaller leaves. Some plants, such as the ocotillo and palo verde, drop their leaves during times of drought to further reduce water loss.