Barrel cactus (Ferocactus sp.). This cactus is barrel-shaped, but becomes columnar with age and reaches around 4 metres in height. Cacti live in arid environments, and have adapted their tissues in order to minimise water loss. Cactus leaves are reduced to narrow spines, as seen here, and the plant surface is covered in a thick, waxy outer layer. Many cacti also have water-storing stems. Photographed in the Grand Canyon, Colorado, USA.