A xerophyte is a very special kind of plant that has adapted over many generations to be able to live in dry, desert-like conditions. Desert animal species rely on these plants for nutrition and hydration, and some of them even rely on the plants for their habitats or shelter. In order to thrive, though, the plants have to be able to tolerate the low water conditions and lack of rain, as well as the longer than average distance through the soil to reach ground water.
Some of the ways xerophytes have adapted have included oddly shaped or smaller leaves, massive root systems that are able to reach the water table below ground, or widespread root systems that grow parallel to the surface of the ground in order to trap as much of the rain that does fall.
Cactus species have adapted to the desert by developing a strange leaf structure. The spiny points of a cactus are technically the plant's leaves, although they certainly don't resemble a typical foliage. The thin, needle-like projections have a smaller surface area than "regular" leaves in order to halt as much evaporation as possible by reducing the amount of surrounding air that comes in contact with the leaf. In addition, the shape of the leaf means there are fewer stomata, or the holes through which water leaves the leaf by transpiration. The trunk of the cactus allows it to store as much water as possible within its succulent fibers, and its thick, rubbery surface helps to prevent water loss when it does manage to take up water.
Mesquite trees are famous for having deep roots that can extend as much as 200 feet below the surface of the soil. These deep root systems let the plant reach to the ground water far below the level of the ground and take advantage of the moist soil above the water table. In another unrelated adaptation, these deep root systems also help anchor the plant in high-wind conditions, a feature found in some deserts due to the lack of tree break to block high-speed winds.
Some plants, like the iconic multi-armed saguaro cactus, grow their roots in a wide circle around the plant, keeping them close to the surface to take advantage of the times when rain does fall. These plants' roots can extend two to three times the circumference of the plant's canopy, letting the roots remain near the surface when the soil absorbs rain water. These plants are best suited to low-wind desert environments since their roots do not run very deep in relation to their height above ground.