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Many smaller cacti have globe-shaped stems, combining the highest possible volume for water storage, with the lowest possible surface area for water loss from transpiration.  Many cacti have short growing seasons and long dormancies, and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system that quickly absorbs any water reaching the ground surface.  Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" (CAM) as part of photosynthesis.  Transpiration, during which carbon dioxide enters the plant and water escapes, does not take place during the day at the same time as photosynthesis, but instead occurs at night.  Their areoles identify them as cacti, and in spite of their appearance, they, too, have many adaptations for water conservation.  Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water.  Cactus stems are often ribbed or fluted, which allows them to expand and contract easily for quick water absorption after rain, followed by long drought periods.  As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade.  Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced.  Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place.  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.  Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves.  Cacti show many adaptations to conserve water.  The plant stores the carbon dioxide it takes in as malic acid, retaining it until daylight returns, and only then using it in photosynthesis.  A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 200 U.S. gallons (760 l; 170 imp gal) of water during a rainstorm.  Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch.  Areoles are an identifying feature of cacti.  Most cacti live in habitats subject to at least some drought.  Cacti occur in a wide range of shapes and sizes.  The tallest free-standing cactus is Pachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft), and the smallest is Blossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at maturity.  In the absence of leaves, enlarged stems carry out photosynthesis.  At least superficially, plants of the genus Pereskia resemble other trees and shrubs growing around them.  As well as spines, areoles give rise to flowers, which are usually tubular and multipetaled.  They have persistent leaves, and when older, bark-covered stems.  Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth.  A few species differ significantly in appearance from most of the family.