Areoli

Areoli are small, cushion-like growths on the stems of cacti (family Cactaceae). They are the functional shoot-like structures from which spines, hairs, branches, and, in many species, flowers develop. As a defining feature of cacti, areoli are key in identifying and distinguishing cacti from other succulents. They are typically arranged in regular patterns along the ribs or at the nodes of the plant.

Structure and development: An areolus contains meristematic tissue and of ten a bud that can give rise to new growth. Areoli may be woolly or bare and can bear spines, bristles, or hairs. They serve as the sites where spines emerge and, in flowering periods, where flowers or short shoots may originate. The arrangement of areoli along the stem often reflects the plant’s overall morphology, such as ribbed or columnar forms.

Variation and function: Areoli vary widely in size, density, and appearance across cactus species. Some are densely covered with spines and wool, while others are nearly naked. Spines from areoli provide defense against herbivores and can reduce air flow over the plant surface, aiding in water conservation. Areoli also give rise to new lateral shoots, contributing to the growth and shape of the plant over time.

Taxonomic significance: Because areoli are unique to cacti, their pattern—spacing, density, and the presence or absence of spines and wool—is a valuable diagnostic trait for cactus identification and classification. The study of areoli helps distinguish different genera and species within the diverse Cactaceae.