<P> A cactus ( plural : cacti , cactuses , or cactus ) is a member of the plant family Cactaceae , a family comprising about 127 genera with some 1750 known species of the order Caryophyllales . The word `` cactus '' derives , through Latin , from the Ancient Greek κάκτος , kaktos , a name originally used by Theophrastus for a spiny plant whose identity is not certain . Cacti occur in a wide range of shapes and sizes . Most cacti live in habitats subject to at least some 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 . Almost all cacti are succulents , meaning they have thickened , fleshy parts adapted to store water . Unlike many other succulents , the stem is the only part of most cacti where this vital process takes place . Most species of cacti have lost true leaves , retaining only spines , which are highly modified leaves . As well as defending against herbivores , spines help prevent water loss by reducing air flow close to the cactus and providing some shade . In the absence of leaves , enlarged stems carry out photosynthesis . Cacti are native to the Americas , ranging from Patagonia in the south to parts of western Canada in the north -- except for Rhipsalis baccifera , which also grows in Africa and Sri Lanka . </P>
why leaves of cactus are reduced to spines
defending against herbivores help prevent water loss by reducing air flow close to the cactus providing some shade