A cactus (plural cacti, cactuses, or less often, cactus) is a plant belonging to the Cactaceae family, which includes around 127 genera and 1750 species belonging to the order Caryophyllales. The term “cactus” comes from the Ancient Greek o, Kaktus, which was first used by Theophrastus to describe a spiky plant whose identification is now unknown. Cacti come in a variety of sizes and forms. The majority of cacti reside in areas where there is at least some drought. Many live in extremely arid areas, like the Atacama Desert, which is one of the world’s driest locations. Cacti have a variety of water-saving adaptations. Succulents, or cactus with enlarged, fleshy portions designed to hold water, account for nearly all cacti. Unlike many other succulents, most cacti have only one component where this crucial process occurs: the stem. The majority of cactus species have lost their genuine leaves, leaving just spines, which are highly modified leaves, behind. Spines assist in minimize water loss by limiting air movement around the cactus and giving some shade, in addition to fighting against herbivores. In the lack of leaves, photosynthesis is carried out by larger stems. Except for Rhipsalis baccifera, which grows in Africa and Sri Lanka, cacti are endemic to the Americas, extending from Patagonia in the south to portions of western Canada in the north.

Cactus spines are made up of areoles, which are specialized structures that resemble a very shortened branch. Cacti have areoles, which distinguish them from other plants. Areoles produce blooms that are tubular and multipetaled, in addition to spines. Many cactus have short growing seasons and extended dormancy, allowing them to respond rapidly to any rainfall. This is aided by a large but shallow root system that absorbs any water that reaches the ground surface. Cactus stems are frequently ribbed or fluted, allowing them to expand and contract quickly in response to rain, followed by lengthy periods of dryness. Most cacti, like other succulent plants, have a unique photosynthetic system known as “crassulacean acid metabolism” (CAM). Transpiration, in which carbon dioxide enters the plant and water exits, happens at night rather than during the day when photosynthesis is taking place. The carbon dioxide that the plant absorbs is stored as malic acid, which it retains until daylight returns, at which point it is used in photosynthesis. Water loss is considerably decreased since transpiration occurs during the colder, more humid night hours.

Many smaller cacti have globe-shaped stems, which combine the most volume for water storage with the smallest surface area for water loss through transpiration. Pachycereus pringlei is the tallest free-standing cactus, reaching a maximum height of 19.2 m (63 ft), whereas Blossfeldia lilliputian is the smallest, reaching just around 1 cm (0.4 in) in diameter at maturity. During a rainfall, a fully grown saguaro (Carnegiea gigantea) is believed to be able to absorb up to 200 US gallons (760 l; 170 imp gal) of water. A few species stand out from the rest of the family in terms of appearance. Plants in the genera Leuenbergeria, Rhodocactus, and Pereskia resemble other trees and shrubs growing nearby, at least on the surface. They have persistent leaves and bark-covered stems as they get older. Their areoles distinguish them as cacti, and despite their look, they, too, have several water-saving adaptations. Leuenberger is thought to be closely related to the ancestor of all cacti. Other cacti thrive as forest climbers and epiphytes in tropical climates (plants that grow on trees). The Christmas cactus and the Thanksgiving cactus, for example, have flattened, almost leaf-like stems with few or no spines (in the genus Schlumbergera).

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