Introduction
A succulent is a plant that stores water in fleshy tissues—usually leaves or stems, sometimes roots—to survive periodic drought. “Succulent” is not a taxonomic rank but a strategy that evolved repeatedly across flowering plants. Water-storage parenchyma (often mucilage-rich) swells and shrinks with supply, giving many species thick leaves, caudices, or ribbed stems. Many (not all) use CAM photosynthesis, opening stomata at night to reduce water loss. Thick cuticles, sunken stomata, and low surface-area-to-volume ratios further conserve moisture. Technically, cacti are succulents, but are generally not addressed in the term “succulent.” You will often hear the phrase “Cacti and Succulents” to indicate that cacti are a major subset of succulents.
The Details
Succulence spans diverse lineages. Cacti (Cactaceae) are stem-succulents with areoles that bear spines and flowers. Agave (Asparagaceae) and Aloe (Asphodelaceae) are mainly leaf-succulents forming rosettes. Euphorbia includes stem-succulents that mimic cacti by convergence. Aizoaceae ranges from pebble-like Lithops to shrubs; Crassulaceae includes sedums and jade plants; Haworthia and Gasteria have “windowed” leaves; caudiciforms such as Pachypodium and Adenium store water in swollen trunks. Note that geophytes storing primarily starch (e.g., many bulbs) are not, strictly, succulents.
Succulents are not confined to hot deserts. They occur in fog deserts, Mediterranean-type climates with summer drought, tropical dry forests, alpine zones with intense sun and thin soils, and as epiphytes where water arrives in pulses. Microhabitats—rock crevices, gypsum flats, saline shores—favor the buffering provided by stored water. Spines, hairs, and waxy bloom scatter light and wind; ribs and leaves expand and contract as reservoirs fill and empty. Succulence lies on a continuum: some plants are only modestly thickened, others extreme “water tanks.” Drought tolerance can exist without succulence, but all succulents integrate storage with conservation.
Additional Reading: Succulents
