Cactus, (family Cactaceae), flowering plant family (order Caryophyllales) with nearly 2,000 species and 139 genera. Cacti are native through most of the length of North and South America, from British Columbia and Alberta southward; the southernmost limit of their range extends far into Chile and Argentina. Mexico has the greatest number and variety of species. The only cacti possibly native to the Old World are members of the genus Rhipsalis, occurring in East Africa, Madagascar, and Sri Lanka. Although a few cactus species inhabit tropical or subtropical areas, most live in and are well adapted to dry regions.