The roots grow close to the surface to collect as much water as possible for storage. After a heavy rain, they can absorb a large amount of water for the plant. The spines of a cactus protect the plant from being eaten by animals. Spines may be long or short and soft or sharp. They may have straight or hooked tips.
Spines grow in clusters out of small lumps or mounds, called areoles, on the stem.
Areoles occur in regular patterns on the stem. In some species, such as the saguaro and the barrel cactus, the areoles merge to form ribs that run along the length of the stem. The ribs give the plant shade and help the stem store water. All cactuses produce flowers. Like the spines, flowers grow out of the areoles.