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Areoles are the distinctive feature of cacti, and identify them as a separate family from other succulent plants. Areoles give rise to spines or, on certain cacti, small, detachable glochids which are an additional form of protection. The areoles on cacti are clearly visible. They generally appear as small light to dark colored bumps, out of which grow clusters of spines.

Areoles represent highly specialized branches on cacti. They are believed to have evolved as vestigial leaves of cacti were modified into spines over time. Thus, the branches became reduced to buds which give rise to the spines. This means that as cacti adapted and evolved to the desert climate, over time they got rid of branches and leaves, which were converted into areoles and spines to protect the plants, and to reduce water loss.

Some cacti lack spines on their areoles, but instead, (as said above) utilize small, detachable glochids which resemble small, sharp splinters and are very difficult to remove from the skin.

It is the areoles that signify the cactus family as separate from other succulent plants both in the New World and the Old World. The cactus family evolved 30-40 million years ago, in the Americas (known as the New World). They evolved there completely separate from Africa, Europe, and Asia (the Old World). One of the unique features that cacti developed in adaptation to their ambient climate was the areole.

For the cactus, areoles are an important evolutionary modification. They give rise to spines, which are the primary means of self-defense for the plants. In addition, the fact that these spines arise from areoles and not directly from the plant stem means that cacti can more effectively cover themselves with spines than other plants. The spines themselves can be of greater size and number.

In addition, areoles can produce spines of many different types to suit their needs. A typical areole may have one or a few long, sharp central spines, which serve as a defense. Beneath them there are often numerous (10 or more) smaller, radial spines produced around the edge of the areole. These serve to shade the plant and trap a layer of cool air next to it.

It is clear, then, that the evolution of areoles has played a huge role in the success of cacti. The advantages they offer have been numerous. They are why the cactus family has been evolutionary successful throughout its range, and account for the diversification seen in the many species today.