botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...

, areoles are small light- to dark-colored bumps on

cacti A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Gree ...

out of which grow clusters of

spine Spine or spinal may refer to: Science Biology * Vertebral column, also known as the backbone * Dendritic spine, a small membranous protrusion from a neuron's dendrite * Thorns, spines, and prickles, needle-like structures in plants * Spine (zoolo ...

s. Areoles are important diagnostic features of

, and identify them as a family distinct from other succulent plants. Gordon Rowley - What is an Areole

The spines are not easily detachable, but on certain cacti, members of the subfamily

Opuntioideae Opuntioideae is a subfamily of the cactus family, Cactaceae. It contains 15 genera divided into five tribes. The subfamily encompasses roughly 220–250 species, and is geographically distributed throughout the New World from Canada, to Argentina ...

, smaller, detachable bristles,

glochid Glochids or glochidia (singular "glochidium") are hair-like spines or short prickles, generally barbed, found on the areoles of cacti in the sub-family ''Opuntioideae''. Cactus glochids easily detach from the plant and lodge in the skin, causin ...

s, 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 the

saguaro The saguaro (, ) (''Carnegiea gigantea'') is a tree-like cactus species in the monotypic genus ''Carnegiea'' that can grow to be over tall. It is native to the Sonoran Desert in Arizona, the Mexican state of Sonora, and the Whipple Mountains ...

, 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.

Evolution

The cactus family, the

Cactaceae A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Gre ...

, evolved 30–40 million years ago in the Americas, originally completely separately from Africa, Europe, and Asia, although, probably within the last few million years, some species of ''

Rhipsalis ''Rhipsalis'' is a genus of epiphytic flowering plants in the cactus family, typically known as mistletoe cacti. They are found in parts of Central America, the Caribbean and northern regions of South America. They also inhabit isolated location ...

'' appear to have been carried to parts of Asia and Africa, most likely by birds. The areole was one of the unique features that cacti developed, possibly in adaptation to the ambient climate in the area in which they emerged. Accordingly, the areoles distinguish the family Cactaceae from other succulent plants both in the New World and the

Old World The "Old World" is a term for Afro-Eurasia that originated in Europe , after Europeans became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia, which were previously thought of by th ...

Importance

For the cacti, areoles are an important evolutionary modification. They give rise to spines and glochids, which are their primary means of self-defense. In addition, because these spines arise from areoles and not directly from the plant stem, 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 the primary defense. Beneath them are often numerous (10 or more) smaller, radial spines produced around the edge of the areole. These may serve as a secondary defense against small creatures that can get past the central spines. In some species, though, such as ''

Cephalocereus senilis ''Cephalocereus senilis'', the old man cactus, is a species of cactus native to Hidalgo and Veracruz in central Mexico. It is threatened in the wild, but widespread propagation and popularity in cultivation have reduced the demand on wild popula ...

'', the tangle of hairy radial spines serves to shade the plant and retain a layer of cool, humid air next to it. Areoles contributed largely to the evolutionary success of the

throughout the

range Range may refer to: Geography * Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra) ** Mountain range, a group of mountains bordered by lowlands * Range, a term used to i ...

of the family, and account for the diversification seen in the many species today.

Gallery

Image:Echinocactus grusonii ies.jpg, Close-up of the ''

Echinocactus grusonii ''Echinocactus grusonii'' or ''Kroenleinia grusonii'', popularly known as the golden barrel cactus, golden ball or mother-in-law's cushion, is a species of barrel cactus which is endemic to east-central Mexico. It is rare and endangered in th ...

'' areole Image:Myrtillocatus geometrizans closeup.jpg, Close-up of ''

Myrtillocactus geometrizans ''Myrtillocactus geometrizans'' (bilberry cactus, whortleberry cactus or blue candle) is a species of cactus in the genus '' Myrtillocactus'', native to central and northern Mexico.Germplasm Resources Information Network''Myrtillocactus geometriz ...

'' areoles with spines

References

{{Commonscat, Areoles Plant morphology Cacti