How a Cactus Saves Water
1Deserts receive very little rain, and summer days there can be extremely hot. Cactuses, however, have special ways to collect water and keep it for a long time, and the saguaro, a tall cactus with arms, is a good example.
2First, a cactus must catch water. Its roots do not grow deep but spread out just below the surface of the ground, so they can take in water from even a light shower. Next, the plant must store the water in its thick stem, which has folds along its sides, called pleats, that open wide after a heavy rain and slowly close again in dry months as the plant uses its supply. Because a large saguaro is mostly water, it can weigh two tons or more.
3A cactus must also protect the water that it has. Its spines are really changed leaves, and they shade the stem and slow down drying winds. In addition, its tiny openings for air, called pores, open only at night, when the air is cooler and wetter, so very little water escapes during the hot day. Together, these features allow a saguaro to live for 150 years or more in a place where few other plants can survive.