The cactus is a family of succulent plants native almost entirely to the Americas, storing water in swollen stems and reduced to spines the leaves that would lose it, so as to survive in desert and drought. That endurance in the harshest country is the root of its symbolism across the New World: the cactus stands for survival, protection and the persistence of life where little else can live, its spines a natural emblem of defence and its rare, brief flowering a sign of beauty wrung from hardship.
The cactus family's desert survival rests on a specific and genuinely remarkable metabolic adaptation, Crassulacean acid metabolism or CAM photosynthesis, in which the plant opens the pores of its skin to absorb carbon dioxide only at night, when the desert air is cool and moist, and keeps them sealed through the heat of the day, drastically cutting water loss compared with an ordinary plant, real plant physiology underlying the folk reading of the cactus as a plant that seems to live on nothing. Many cactus species are also extraordinarily long-lived, the giant saguaro of the Sonoran Desert commonly reaching well over a century and sometimes two, a genuine record of endurance behind the plant's symbolism of persistence.
Sources Plants of the World OnlineRoyal Botanic Gardens, Kew with Cunningham's Encyclopedia of Magical HerbsScott Cunningham