From a botanical perspective, the most critical distinction lies in taxonomy, the scientific classification of plants. "Succulent" is not a formal taxonomic group but a descriptive term for plants that have adapted to store water in their leaves, stems, or roots to survive arid conditions. This category includes thousands of species from over 60 different plant families. Cacti, on the other hand, belong to one specific botanical family: Cactaceae. Therefore, all members of the Cactaceae family are, by definition, succulents. However, the term "succulent" also encompasses a vast array of other plants from families like Crassulaceae (e.g., jade plants), Aizoaceae (e.g., living stones), and Asphodelaceae (e.g., aloes).
The single most reliable botanical characteristic that separates cacti from other succulents is the presence of areoles. Areoles are highly specialized, cushion-like structures on a cactus from which spines, branches, hairs, and flowers grow. They are unique to the Cactaceae family. In contrast, other succulents that may have spines or thorns (such as many species of Euphorbia) produce them directly from the plant tissue without the intermediary of an areole. If a plant does not have areoles, it is botanically not a cactus, regardless of how much it may resemble one.
The evolutionary strategy for water storage and photosynthesis differs significantly. Most cacti have evolved to have minimal or no true leaves. Their primary photosynthetic organ is the stem, which is typically thick, fleshy, and green. This adaptation reduces surface area and minimizes water loss through transpiration. The spines of a cactus, which are modified leaves, primarily serve for protection and shading. Other succulents, however, often store water in their leaves. Plants like Echeveria, Aloe, or Haworthia have prominent, fleshy leaves arranged in rosettes or other formations. Their stems may be less pronounced or even subterranean.
While less obvious to the casual observer, the chemical makeup and flower structure of cacti are distinct. Cacti do not produce the milky sap (latex) that is common in many other succulent families, most notably the Euphorbiaceae. Furthermore, cactus flowers are typically complex, with numerous petals and stamens, and the ovary is usually embedded within the stem tissue (inferior ovary). The flowers of other succulents vary enormously by family. For instance, flowers from the Crassulaceae family are often star-shaped with a set number of petals, while Euphorbia flowers are highly reduced and grouped into specialized structures called cyathia.
There is a strong geographical pattern to their distribution. With only one known exception (Rhipsalis baccifera), all cacti are native to the Americas, ranging from Patagonia to Canada. Their diversity is greatest in the desert and arid regions of Mexico and the southwestern United States. Succulents as a whole are globally distributed. They are native to Africa (home to aloes, euphorbias, and lithops), Europe (such as Sempervivum), and Asia. This separate evolutionary history in different continents is a primary reason for their distinct anatomical and physiological adaptations.
