From a botanical perspective, the most critical point to understand is that all cacti (members of the plant family Cactaceae) are succulents, but not all succulents are cacti. "Succulent" is a descriptive term, not a formal taxonomic classification. It refers to any plant that has adapted to arid climates or soil conditions by developing thick, fleshy tissues to store water. This means the term encompasses a wide range of plant families from across the globe, including Crassulaceae (e.g., jade plants), Aizoaceae (e.g., living stones), and Asphodelaceae (e.g., aloes). Cacti are simply one specific, highly specialized family within this larger group of drought-tolerant plants.
The single most reliable botanical characteristic that distinguishes a cactus from other succulents is the presence of areoles. Areoles are highly specialized, condensed lateral shoots or branches that appear as small, cushion-like bumps on the plant's stem. From these areoles, all spines, hairs, flowers, and new stems (offsets or pups) grow. In the cactus family, spines are always clustered on and originate from an areole. Other spiny succulents, such as many Euphorbia species, may have sharp thorns, but these emerge directly from the stem tissue without the cushion-like areole as a point of origin. This unique morphological feature is the definitive marker of a true cactus.
Beyond areoles, cacti and other succulents exhibit different evolutionary adaptations. Most cacti are native to the Americas and have evolved to survive in some of the most extreme desert environments. Consequently, they have largely eliminated leaves to reduce surface area and minimize water loss through transpiration. Photosynthesis is performed by their thick, green stems. Many other succulents, like aloes and agaves, retain their leaves, which are themselves the water-storage organs. Furthermore, cacti possess a unique photosynthetic pathway called Crassulacean Acid Metabolism (CAM), which is shared by many other succulents. However, cacti have refined this process to an extreme degree, opening their stomata at night to take in carbon dioxide and closing them during the day to conserve water.
The reproductive structures of cacti also set them apart. Cactus flowers are typically perfect flowers, meaning they contain both male (stamens) and female (pistils) reproductive parts. They are also often solitary, showy, and emerge directly from an areole. The ovary of a cactus flower is inferior, meaning it is located below the petals and sepals. After fertilization, this develops into a unique type of berry that is often fleshy and brightly colored. While the flowers of other succulent families can be diverse, the consistent combination of an inferior ovary and the flower's emergence from an areole is a key identifier for the Cactaceae family.
