How Do Cacti Survive the Desert? Storing Water and Breathing at Night

Most plants open their stomata during the day, admit carbon dioxide and fix it directly into sugar using light. In a desert that would be fatal, because midday air is hot and dry and the water lost through open stomata would far exceed what the roots could replace. Cacti and many other succulents use crassulacean acid metabolism, named after the plant family in which it was first described. At night, when the air is cooler and more humid, the stomata open and carbon dioxide is taken in and fixed into an organic acid, which accumulates in the cell vacuoles. By morning the stomata close completely and stay closed all day, while the stored acid is broken down to release carbon dioxide inside the leaf for the light reactions of photosynthesis to use. The cost is a considerably lower rate of growth, since the amount of carbon that can be stored overnight is limited, which is exactly why cacti grow slowly. The saving is enormous, with water loss per unit of carbon fixed a small fraction of that in an ordinary plant.

Almost every visible feature of a cactus is a water measure:

• •Spines are modified leaves, which removes the large evaporating surface a leaf represents and transfers photosynthesis to the stem, which is why a cactus stem is green
• •The spines also shade the surface, trap a layer of still air that reduces evaporation, collect dew in some species and deter animals that would eat the stored water
• •A thick waxy cuticle over the stem drastically reduces water loss through the surface itself
• •Ribs and pleats allow the stem to expand when water is available and contract as it is used without the skin splitting, so a saguaro can swell measurably after rain
• •A shallow and very wide root system captures light rainfall across a large area before it evaporates, since desert rain frequently does not penetrate deeply
• •A globular or columnar shape minimises surface area relative to volume, which reduces both water loss and heat gain
• •Areoles, the small cushioned structures from which spines and flowers emerge, are unique to cacti and are the feature botanists use to identify the family

Plants that look like cacti are not necessarily cacti, and the resemblance is one of the clearest examples of convergent evolution in biology. Cacti are a New World family, native to the Americas, with a single genus occurring naturally in Africa. Many African and Asian succulents in the Euphorbia family look strikingly similar, with thick ribbed water-storing stems and spines, because the same desert conditions selected the same solutions independently. The reliable distinctions are that cacti have areoles and Euphorbias do not, and that Euphorbias exude a milky latex when cut which is frequently toxic and irritating, while cacti do not. Other unrelated families including the Apocynaceae have produced similar forms. The pattern matters beyond identification, since it demonstrates that the set of workable solutions to a severe environmental problem is small, so unrelated lineages arrive at the same answers repeatedly.

Cacti have been used extensively and are now under pressure. Prickly pear provides both fruit and edible pads and is a significant crop in Mexico and elsewhere, and it also became one of the world's worst invasive plants after introduction to Australia, where it covered enormous areas until controlled by an introduced moth in one of the most successful biological control operations ever conducted. Cochineal insects farmed on cactus pads supplied the red dye that was among the most valuable exports of colonial Mexico. Peyote and San Pedro have long ceremonial use in indigenous traditions and are legally protected in some contexts and restricted in others. The conservation position is serious: a global assessment found a substantial proportion of cactus species threatened, with illegal collection for the ornamental trade a leading cause alongside agricultural conversion, since slow growth and restricted ranges make wild populations very slow to recover from removal. Saguaro, which may take decades to produce its first arm and over a century to reach full size, illustrates how little margin those populations have.

Cacti open their pores only at night, fixing carbon dioxide into an acid that is stored until daylight, which lets them keep the surface sealed through the hot day at the cost of slow growth. Spines are modified leaves that remove evaporating surface, shade the stem and trap still air, while ribs allow the stem to swell after rain and shallow wide roots catch light rainfall. African Euphorbias resemble cacti through convergent evolution and lack areoles.