Areole

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In botany, areoles are small light- to dark-colored bumps on cacti out of which grow clusters of spines. Areoles are important diagnostic features of cacti, and identify them as a family distinct from other succulent plants.^[1][2] 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. They evolved as abortive branch buds while their spines evolved as vestigial leaves.^[2] In branched cacti, such as Opuntioidiae and the 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^{[citation needed]}. They are detachable and resemble small, sharp splinters. Unlike the spines, glochids generally are barbed and are very difficult to remove from the skin.