Desert plants have evolved to survive harsh environments. Many desert plants store water in their trunks, stems, bulbs and roots. Some have roots that spread out and go deep to retrieve water. Others go dormant while they wait for water.
Cacti, also called Xerophytes, have evolved to store water in botanically creative ways. Cacti have no leaves and shallow root systems. They store water in their stems. Their waxy, smooth, green skin seals in moisture and conducts photosynthesis. Shallow cacti roots branch to the side to collect as much water as possible when it does rain. The Saguaro, Yucca, Prickly Pear and Barrel cacti of the Mojave and Sonora deserts are remarkable examples of desert adaptation.
Succulents are "fat plants." They store water in their roots, stems, or leaves. All cacti are succulents, but not all succulents are cacti. Cacti thorns emerge from pads; succulent thorns are extensions of the plant's body. The Agave, from which Mexicans make mescal and tequila, is not a cactus, as many think. It is a succulent. It stores water in its thick leaves. The aloe plant is a succulent that stores water in its long, thick leaves in a form of a yellowish sap that has many uses as a traditional medicine. The sap is used to treat wounds, sunburn, eczema and ringworm.
The Phreatophytes have extensive roots to draw water from the soil. Mesquite trees, abundant in southwestern deserts, have had roots measured as deep as 200 feet. The roots of the Creosote bush run both deep and to the side. Its yellow-green, pointed leaves are waxy and resinous, enabling them to dissipate heat and conserve water. The leaf pores close during the daytime heat and open at night.
Annuals bloom yearly; ephemerals are desert annuals. Many ephemerals complete their life cycle in weeks or months. In some deserts, plants only get rain for a few hours or days of the year--often in springtime cloudbursts. That's when ephemerals spring into action. They can shoot above the ground in days, flowering, producing seeds and dying in days or weeks. Their seeds resist heat and drought while they are dormant (sometimes for years), waiting for water so they can germinate. The Desert Paintbrush, Desert Sand Verbena and Mojave Aster are all ephemerals.
Desert perennials include plants that can endure long periods without water and plants that go dormant when there is no water. The spiny Ocotillo plant has whip-like, spiny branches that can grow as high as 20 feet. Leafless in the dry season, the leaves of the Ocotillo pop out in bunches when it rains.
Some plants store water in bulbs located deep beneath the surface. When there is no rain, the plants dry out and the bulb goes dormant, waiting for water. The bulbs of the Ajo or Desert Lily are located as much as 18 inches deep and can remain dormant for years. In the Gobi desert, where the cold plunges in the winter, the Saxaul tree stores water in its bark.
The Welwitschia plant has just two leaves that close in the heat and then open to get water from the fog that envelopes the Nabib desert in Africa. Locally called Two Leaves that Cannot Die, it lives from 400 to 1,500 years. Sagebrush has silvery-gray leaves covered with a fine hair that helps it resist the cold. Saltbush removes salt from the soil and deposits it on its leaves, where it attracts moisture from the air. When the leaves collect too much salt, they fall off.