Animals adapt to their habitat over the course of thousands of years. The same phenomena happens with plants. Plants in arid regions such as the desert must adapt to lack of water. These adaptations are not behavioral like an animal's adaptations, but rather are physical and chemical.
Water and sunlight are essential to a plant's growth, and in the desert, there is plenty of the latter and too little of the former. Plants that depend on a root system to suck water out of the ground must adapt to the barren conditions. Many desert plants have a dual root system. One set of roots acts much as any other plant's root system: It's shallow and searches the immediate surface for any water, as well as keeping the plant grounded. The second set of roots goes deeper, trying to tap into an underground aquifer, which is present in some of the driest desert conditions.
The leaves of a plant are the most common area in which water can be lost. Desert plants have equipped their leaves with a kind of waterproofing that stops water molecules from dissipating or being absorbed into the air. This waxy substance, however, takes a tremendous metabolic toll on the plant, usually meaning that these plants do not grow quickly.
Microscopic pores on a plant, called stomata, allow carbon dioxide to enter. However, closing stomata is one of the best ways in which a plant can conserve water. This presents a dichotomy for a desert plant: How to conserve water while still getting enough carbon dioxide for photosynthesis. The answer usually entails opening the stomata during the coolest times of the day to inhale carbon dioxide and then closing it during the hotter parts when water evaporation is likely to occur.