The many similarities between euphorbia and cacti result from convergent evolution. For example, both euphorbias and cacti are stem succulents, a type of plant that stores water in stem tissues. This adaptation, along with lack of leaves, allows these plants to survive in highly arid conditions. Similarly, presence of spines or thorns protects the juicy stems from attack by thirsty desert animals. Once you know what to look for, however, euphorbias and cactus are quite distinct.
One defining characteristic of the family Cactaceae is presence of external structures known as areoles. Areoles are easily visible on the surface of a cactus, where they look like small, often fuzzy dots. These structures typically occur in a linear or geometric pattern over the body of the cactus. Areoles are regions of highly condensed growth nodes, and they give rise to spines, flowers and offsets. No other plants, including euphorbias, possess areoles.
Spines and Thorns
Cactus spines are different from euphorbia thorns in one very noticeable way: cactus spines grow out of areoles. In contrast, euphorbia thorns appear to grow directly out of the stem. In addition, euphorbia thorns typically occur in pairs. This is due to the fact that euphorbia thorns are highly modified stipules, a form of shoot that comes in pairs. Cactus spines are highly modified leaves, and can occur singly or in groups, depending on the species.
The flowers of cacti, like their spines, grow out of areoles. Cactus flowers are similar to common garden flowers in that they have easily identifiable petals and stamens, and they are often large and highly colored. Euphorbia flowers, on the other hand, are less prototypical. The true flowers of euphorbia are highly modified, consisting of a single nubbin of tissue corresponding to a solitary stamen or ovary. These reduced flowers occur inside a cuplike structure, known as a cyathium. What may look like colorful flowers in euphorbias are actually pigmented leaves, known as bracts.
Type of Sap
Euphorbias possess thick, milky sap, known as latex. In many species, the latex is poisonous and causes a painful rash. For this reason, protect euphorbia houseplants from breaking. Caution is also needed when pruning euphorbias or taking cuttings from them. Be especially careful to keep euphorbia sap away from your eyes and mouth. Cacti generally have watery sap, although a few, such as certain members of the genus Mammillaria, have milky sap.