Cactus owe their success in the desert to their structural adaptations. While other desert plants may have similar features such as spines and succulent stems, these evolutionary traits reach a zenith in the cactus.
Cactus take advantage of the lightest rainfall by having roots close to the soil surface. The water is quickly collected by the roots and stored in thick, expandable stems for the long summer drought. The fleshy stems of the Barrel Cactus are pleated like an accordion and shrink as moisture is used up. These pleats also channel water to the base of the plant during rain showers.
When water is no longer available in the summer, many desert shrubs drop their leaves and become dormant. Cactus continue to photosynthesize because they have fixed spines instead of leaves. The green stems produce the plant's food, but lose less water than leaves because of their sunken pores and a waxy coating on the surface of the stem. The pores close during the head of the day and open at night to release a small amount of moisture.
The dense network of spines shades the stems, keeping them cooler than the surrounding air. Many Barrel Cactus lean to the south so that a minimum of body surface is exposed to the drying effect of the midday sun. Cactus pay a price for these water-saving adaptations -- slow growth. Growth may be as little as 1/4 inch per year in the Barrel Cactus, and most young sprouts never reach maturity.
Cacti have a special form of carbon fixation as part of their photosynthesis process: Crassulacean Acid Metabolism (CAM) after the plant family in which it was first found (Crassulaceae - cacti) and because the CO2 is stored in the form of an acid (malic acid/malate) before use in photosynthesis. Stomata open only at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to malic acid and stored during the night. During the day, the malate is broken down and the CO2 is released to RUBISCO for photosynthesis. Adaptive Value: Better water use efficiency than 'standard' C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.). When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells). CAM plants include many succulents other than cactuses, including agaves and some orchids and bromeliads