Plants that have developed in the desert, have adaptations that help them survive. Some endure the heat and drought, while others simply evade it by shutting down and going into a state of dormancy. 

The purist example of plants which evade drought are the desert wild flowers. Plants such as Mexican gold poppy begin their growth in autumn. Taking advantage of cool, moist conditions, they mature and produce seed prior to the onset of dry weather. Plants die, but seeds remain to begin growth anew when favorable conditions return. 

The majority of arid plants endure, not evade periods of drought. They find ways to remain alive during droughts, by lessening the amount of water they use. Plants slow or cease growth to reduce water consumption. Some plants even reduce their size, by shedding leaves and stems to reduce moisture loss. 

A great amount of moisture can be lost through leaves. This is why most desert plants have small leaves; or no leaves at all! Palo verde trees typically have very small leaves. When dry conditions prevail, they shed their leaves, and even drop small stems to reduce the area exposed to moisture loss. Green bark serves the function of leaves in manufacturing food for the plant, through the process of photosynthesis. In this way, palo verdes can continue to function in the absence of leaves. When rains return, stems and leaves regrow. 

Cacti have no true leaves, but rather green stems of pads. These function as leaves to manufacture food, but the bulk of the plant serves as a water storage unit. During wet weather cacti "bulk up" with water. Saguaro can even become bloated, with excess watering. During drought periods, this stored water can be utilized to maintain the cactus. Because it's outer surface is small compared to it's inner structure, a relatively small ratio of the plant is exposed to the drying effects of sun and air. A thick, waxy cuticle covering the cactus further protects it from drying out. 

Have you ever noticed the number of desert plants with light colored, silvery leaves? These also serve a survival function. The silvery foliage of plants like salt bush, Texas ranger, indigobush, and desert sage reflect light and heat, keeping the plants cooler. In addition, hairs on the leaf surface of many desert plants, help to shade the leaf from the sun and hold in moisture. 

A final adaptation to desert life is the ability of plants to efficiently harvest water. Trees like the palo verde and mesquite have open, spreading branches which allow rain and dew to be directed in towards the tree. Roots radiate out far beyond the edge of the branches to collect all available moisture. Many cacti are ribbed to help direct even the smallest amount of rain and dew, down towards the plant base. 

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona 626-5161.
Material originally appeared in Arizona Daily Star gardening column, on June 8, 1997.
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