No plant could live in a true desert, where it never rains. But the places where cacti and other desert plants grow do get rain only every couple of years or so. When it rains, desert plants quickly take up the water through their roots, storing it in fleshy leaves and stems. That stored water allows them to survive until the next rain comes.
The roots of desert plants are very deep, drawing from hidden underground water, or widespread and shallow to best gather the water that hits the ground. Their leaves and stems usually have a heavy, waxy coating to help keep moisture in. The tiny pores, or stomata, on their leaves through which carbon dioxide, oxygen, and water pass are located in pits instead of on the surface to further minimize moisture loss. Some desert plants even keep their stomata closed during the blistering heat of the day and open them in the cool of the night.
Cactus plants are particularly well adapted to their hot, dry surroundings. Over millions of years the leaves of cacti became so small to reduce water loss that on many only sharp spines remain. Their thick stems or branches now do the work that leaves ordinarily do making food through photosynthesis. Often the stems of a cactus are ribbed, allowing any morning dew to run down and be absorbed by the plant’s roots. The ribs also allow the plant to expand and contract according to the amount of water it has stored inside. After a good rain, up to 90 percent of a cactus’s weight may be water.