barrel cactus

“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.”

http://www.desertusa.com/mag98/may/stories/cactus.html