Cleistocactus cactus (Cleistocactus sepium). The Cleistocactus cacti often form the spiny columns seen here. Cacti are found in arid environments, and have tissues adapted to minimise water loss. Cactus leaves are reduced to narrow spines, and the plant surface is covered in a thick, waxy outer layer. Many cacti also have a means of storing water. Photographed on Lanzarote, Canary Islands.