Desert plants have evolved to survive conditions of extreme heat and lack of water. Among numerous adaptations, desert plants store water, have spines rather than leaves, have long roots and they sometimes go dormant when there is no water. The evolution of desert plants depends on the degree of heat, cycles of rainfall and the elevation in a particular desert, but the characteristics of their survival mechanisms fall into broad categories.
Xerophytes have altered their physical structure to survive the lack of water and extreme heat. Cacti, also called succulents, have shallow root systems and no leaves. They store water in their spines and stems. Their waxy skin seals in moisture. Photosynthesis in cacti occurs in the outer tissue of their skins. Their shallow roots branch out to the side to collect as much water as possible when it does rain. DesertUSA.com lists the Yucca and Prickly Pear cactus of the Chihuahua Desert and the Saguaro cactus of the Sonora Desert as examples of this type of adaptation. Some desert shrubs and trees have replaced leaves with spines. By eliminating leaves, the plant reduces loss of water through the air. Smooth green bark both seals in moisture and produces food. Examples of adaptive desert plants listed by DesertUSA are Sagebrush, Saltbush, Burroweed and Joshua trees.
The Mesquite tree and other phreatophytes have developed extremely long roots to draw water from deep underground. The mesquite, abundant in the deserts of the American Southwest employs this strategy. The Encyclopedia of Earth tells us that the deepest recorded mesquite root went 200 feet deep. Since little rain soaks that deep, roots that long are unusual. The Creosote bush has an extensive root system that runs both deep and to the side to get both underground and surface water. Its tiny leaves close their pores during the daytime heat and open them at night to absorb moisture.
Desert perennials go dormant during dry seasons, springing to life when it rains. A good example is the spiny Ocotillo plant that has spiny, whip-like branches that can grow as high as 20 feet. It remains leafless in dry seasons. When it rains, leaves pop out in bunches above the spines.
Some desert plants have bulbs. The tops dry out during dormant periods. The bulbs, located deep below the surface, store moisture. The bulbs of the Desert Lily, also called the Ajo, are more than 18 inches under the surface. The Encyclopedia of Earth says a desert lily bulb can lie dormant for years waiting for a winter rain to bring it back to life.
Desert annuals are called ephemerals because many of them complete their life cycle in weeks or months. Environments unique to specific deserts have produced a diversity of ephemeral species adapted to different conditions.
When the heat, light and moisture tells ephemeral plants it's time to bloom, they act quickly. Some ephemerals send flower stalks above the ground in a few days. A bloom can last for days or weeks, depending on the elevation and weather. The Desert Sand Verbena, Mojave Aster and Desert Paintbrush are examples of ephemerals. In the spring after winter rains, they quickly germinate, grow, flower and produce seeds. The hardy seeds remain dormant through heat and drought. They can survive two or three years waiting for sufficient winter rains before they quickly repeat their rapid springtime cycle.