A Succulent Convergence
Thick succulent stems, adapted to conserve water, evolved independently in several lineages of desert flowering plants.
Both the familiar Latin American cacti and the African euphorbias lost their leaves and transferred photosynthesis to their enlarged stems, whose specialized anatomy provides water storage and support.
Although their stems are similar, their flowers (and fruits) are very different from one another.
Cacti (Cactaceae) belong within a major branch of flowering plants that includes carnations and ice-plants, while the euphorbs (Euphorbiaceae) are members of a very distantly related branch that includes violets and willows.
Leaf-bearing relatives of the stem-succulent forms have now been identified. Pereskia, with its large leaves, is actually a cactus! And, “Crown-of-Thorns” is a euphorb.