A cactus is a type of succulent plant adapted to living in the desert. Cacti (meaning more than one cactus) have even lower watering requirements than their smooth succulent cousins and are often covered in prickly hairs or spines. These hairs are actually highly modified leaves that help with water retention by reducing airflow to the surface of the cactus while the spines protect cacti from predators. The body of a cactus is actually an engorged stem filled with water, giving the plant a source of water for many weeks. These special cylindrical, flat, or round stems give cacti their distinctive silhouette that so many people love.