Most taxa in the family Cactaceae are succulents with large, leafless, long-living, fleshy stems of different shapes and sizes that often contain clusters of spines which arise from areoles (Benson 1979, 1982; Eggli 1993). Areoles—highly specialized axillary or lateral buds or short shoots or branches—are unique to the family (Mauseth 1983; Gibson and Nobel 1986). However, cacti come in a wide range of growth forms (Fig. 1). Succulent plants in other families are often mistakenly called ‘cacti’ on nursery labels and in popular publications. While it is usually easy to distinguish cacti from other succulents, some taxa look very cactus-like due to convergent evolution, e.g. many species in the genus Euphorbia of the family Euphorbiaceae