Areole
Encyclopedia
Areoles are an important diagnostic feature of cacti
, and identify them as a family distinct from other succulent plant
s. The areoles on cacti are clearly visible; they generally appear as small light- to dark-colored bumps, out of which grow clusters of spine
s. The spines are not easily detachable, but on certain cacti, members of the subfamily Opuntioideae, smaller, detachable bristles, glochid
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 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
.
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
.
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.
Areoles have clearly contributed largely to the evolutionary success of the Cactaceae throughout the range
of the family, and account for the diversification seen in the many species
today.
Cactus
A cactus is a member of the plant family Cactaceae. Their distinctive appearance is a result of adaptations to conserve water in dry and/or hot environments. In most species, the stem has evolved to become photosynthetic and succulent, while the leaves have evolved into spines...
, and identify them as a family distinct from other succulent plant
Succulent plant
Succulent plants, also known as succulents or fat plants, are water-retaining plants adapted to arid climates or soil conditions. Succulent plants store water in their leaves, stems, and also in roots...
s. The areoles on cacti are clearly visible; they generally appear as small light- to dark-colored bumps, out of which grow clusters of spine
Thorns, spines, and prickles
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...
s. The spines are not easily detachable, but on certain cacti, members of the subfamily Opuntioideae, smaller, detachable bristles, glochid
Glochid
Glochids 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, causing irritation upon contact...
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 Saguaro
Saguaro
The saguaro is a large, tree-sized cactus species in the monotypic genus Carnegiea. It is native to the Sonoran Desert in the U.S. state of Arizona, the Mexican state of Sonora, a small part of Baja California in the San Felipe Desert and an extremely small area of California, U.S...
, 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
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
.
Evolution
The cactus family, the Cactaceae, 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 RhipsalisRhipsalis
Rhipsalis is a genus of epiphytic cacti. They are typically known as mistletoe cacti. The scientific name derives from the Ancient Greek term for wickerwork, referring to the plants' habitus. Rhipsalis is one is part of the tribe Rhipsalideae within the subfamily Cactoideae of the Cactaceae...
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
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...
and the Old World
Old World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....
.
Importance
For the cactus, areoles are an important evolutionary modification. They give rise to spines and glochids, which are their primary means of self-defence. In addition, the fact that these spines arise from areoles and not directly from the plant stemPlant stem
A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence , conifer cones, roots, other stems etc. The internodes distance one node from another...
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.
Areoles have clearly contributed largely to the evolutionary success of the Cactaceae throughout the range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...
of the family, and account for the diversification seen in the many species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
today.