A cactus (plural: cactii, cactuses, or cactus) is a member of the plant family Cactaceae, within the order Caryophyllales. Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north. There is only one exception, Rhipsalis baccifera, which is also found in Africa and Sri Lanka.
Most cacti live in habitats which are subject to at least some degree of drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines reduce air flow close to the cactus and provide some shade, both of which help to prevent water loss. Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch; areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multi-petaled.