Areoles are an important diagnostic feature of cacti, and identify them as a family distinct from other succulent plants. The areoles on cacti are clearly visible; they generally appear as small light- to dark-colored bumps, out of which grow clusters of spines. The spines are not easily detachable, but on certain cacti, members of the subfamily Opuntioideae, smaller, detachable bristles, glochids, also grow out of the areoles and afford additional protection.
Areoles represent highly specialized branches on cacti. Apparently they evolved as abortive branch buds while their spines evolved as vestigial leaves. In branched cacti, such as Opuntioidiae and Saguaro, new branches grow from areoles, because that is where the buds are. The development of the areole seems to have been an important element in the adaptation of cacti to niches in desert ecology.
Some of the Opuntioideae have spines as well as glochids on their areoles; some have only glochids. Structurally the glochids seem to be bristles rather than evolved leaves. They are detachable and resemble small, sharp splinters. Unlike the spines, glochids generally are barbed and are very difficult to remove from the skin.
In botanical morphology, thorns, spines, and prickles are hard structures with sharp, or at least pointed, ends. In spite of this common feature, they differ in their growth and development on the plant; they are modified versions of different plant organs, stems, stipules, leaf veins, or hairs. In nontechnical usage, the terms may be synonymous.