A cactus (plural cacti) belongs to the plant family Cactaceae which comprises of about 127 genera with nearly 1750 known species of the order Caryophyllales. The word "cactus" is derived from Latin and from the Ancient Greek , kaktos which is a name that was originally used by Theophrastus for a spiny plant whose identity is not certain. Cacti can be found in a wide range of shapes and sizes. Most cacti live in habitats that are subjected to at least some drought. Many live in extremely dry environments such as the Atacama Desert which is one of the driest places on earth. Cacti exhibit many adaptations to conserve water. Almost all cacti are succulents which means that they have thickened, fleshy parts adapted to store water. The stem is the only part of most cacti where this vital process takes place, unlike many other succulents. Most species of cacti retain only spines which are highly modified leaves as they have already lost their true leaves. Spines help prevent water loss by reducing air flow close to the cactus and providing some shade as well as defending themselves against herbivores. Enlarged stems carry out photosynthesis in the absence of leaves. Cacti are native to the Americas and range throughout Patagonia in the south to parts of western Canada in the north, except for Rhipsalis baccifera, which also grows in Africa and Sri Lanka.
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