Cactaceae can often times be easily confused with succulents. They both have leafless long lived fleshy stems of various shapes and sizes on which there are spines. A succulent is able to store water for survival in arid climates as are cacti. However, the dichotomy between the two has everything to do with one peculiar difference - Areols. Areols are highly modified lateral buds or dwarf shoots that produce spines and flowers in only cacti.
Though succulents may be distantly related to cacti, they are indeed separate species that occur in very different regions of the globe. What is interesting is though they both evolved on opposite sides of the world they share very similar evolutionary traits because of their similar latitudes. Cacti are native to the New World (South America). While succulents are only endemic to the Old World (Africa). The greatest confusion surrounds Euphorbia, of the family Euphorbiaceae.
Take a look at; Euphorbia Lactea, Euphorbia Obesa, Euphorbia Trigona or Euphorbia Abdelkuri and it becomes clear why it is so easy to get mixed with cacti and succulents.
The careful observer will note that though Euphorbia have spines that resemble cacti, they are paired in their dissimilar origin. It's the origin that set cacti apart from succulents. All true cacti have separate spines arising from the areole. Euphorbia lack the presence of areoles.
One sure way to tell is to cut the plant. Euphorbia will all secrete a milky white substance. Be warned! This sap, known as latex can be poisonous, causes blindness, and is highly irritating. Mucous membranes like the eyes,mouth and nose suffer severe inflammation from contact. If you pass on the surgery option flowers of Euphoria are another clue. They are not showy as in cacti, and can often go unnoticed. Small, clustered, and lacking pedals they only have unisexual flowers- only pistil or only stamens, while cacti display large, colourful flowers with many segments. Cacti flowers harbour pistils and stamens together.
Test yourself next time when staring at what may seem to be a cactus. Looks can be deceiving.
All cacti are succulents but not all succulents are cacti.