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Cacti (plural of Cactus) are a subgroup of Succulents. Cacti occur naturally only in the New World (The Americas). However, they have been cultivated all over the world and Opuntia now can be found growing wild in many Mediterranean locations. Cacti are unique from other Succulents in that they have curly wool at the base of the spine clusters. Other Succulents may or may not have spines, but those that do have spines do not have the wool. Succulents are one of the most recently emerging plant groups. They are all flowering plants (Angiosperms), but variants such as crests often cannot flower. Just about every flower color can occur in Succulents except blue.
Click on the graphic below to see a diagram of the parts of a Cactus: