One-third of the Earth's land surface is desert, a biome that receives 10 or fewer inches of rain per year. Although the desert has a reputation as a barren region, too inhospitable for life to thrive, many plants make their home in the desert, having developed adaptations that allow them to survive a challenging environment.
Unlike all other life on Earth, plants do not consume other organisms but produce their own food using sunlight, water and carbon dioxide that they take from the air. Plant leaves are equipped with pores called stomata that open and close, taking in carbon dioxide but also releasing some of the plant's water. Because of the extremely arid conditions in the desert, plants that thrive there must develop adaptations that help them to conserve water.
The classic desert inhabitant is the saguaro cactus, a tall, thick plant covered in spines. Desert plants have developed many adaptations to prevent water loss, including spines on cacti that offer a painful disincentive to any animal trying to get at the water in the plant's core. Other plants grow fine hairs that help to reflect the sun and minimize evaporation, while some have fewer stomata or only open them at night, when evaporation is not a concern. Still other species have waxy leaves that prevent water loss.
The shapes of many cacti suggest barrels or drums, and that observation reveals their purpose. Cacti and other desert succulents store water in their thick, fleshy stems, which also have expandable ribs that allow the plant to make more room for water when it is available. The round shapes of most cacti, furthermore, provide maximum storage capacity with minimal surface area for losing water.
Because deserts receive little rainfall--and much of that often comes as brief rain showers--some desert plants have developed adaptations that allow them to acquire water, even in times of severe drought. For example, the salt-cedar, an invasive desert plant, grows roots so deep and powerful that it can drain and damage rivers and aquifers. Deep-reaching root systems let plants tap into water supplies far enough from the surface that there is no threat of evaporation. Other plants try the opposite tactic and grow far-reaching roots close to the surface. When a rare rain shower occurs, these species suck up the water before it evaporates.
Many plants lose hundreds of gallons of water each year through their stomata, a loss that desert plants cannot afford. While other adaptations limit water loss, plants must open their stomata sometimes to obtain the carbon dioxide needed for photosynthesis, or they will starve. Many desert species use a special kind of photosynthesis called the C4 pathway. During this process, carbon dioxide produced during respiration is recycled within the plant and used for photosynthesis, allowing the plant to keep its stomata closed.