How do cacti survive in the desert?

A cactus stem is a water tank: its spongy tissue swells after rain, and many species have pleats or ribs that expand like an accordion to hold more water. Spines are modified leaves; they lose far less water than broad leaves would, shade the stem and protect it from thirsty animals. A waxy coating on the skin further reduces evaporation. Many cacti use a special form of photosynthesis called CAM, opening their pores (stomata) to take in carbon dioxide during the cool night and keeping them closed in the heat of the day. Their shallow, widespread roots can quickly soak up water from even a brief shower.