A cactus is a perennial succulent plant that thrives in dry regions of the Western Hemisphere. Cacti are known for their thick stems containing chlorophyll and areoles, unique structures that produce plant hairs, spines, bristles, flowers, branches, and leaves. Most cacti lack leaves or have reduced leaves modified as spines to minimize water loss. Their shallow root systems efficiently absorb moisture. Cacti vary in size and shape, from small peyote to tall saguaros. While most grow in the ground, some are epiphytes that grow on other plants. People cultivate cacti as ornamentals, food sources, living fences, and medicine.

How do cacti survive in very dry environments?

Cacti are well-adapted to survive in dry environments through several unique features. As succulents , they have thick, fleshy tissues designed for water storage, primarily in their stems or leaves. To minimize water loss, cacti have reduced or absent leaves, often modified into spines, and their stems have taken over the photosynthetic functions. Many cacti also use a specialized process called crassulacean acid metabolism (CAM), where they open their stomata at night to take in carbon dioxide and close them during the day to prevent water loss. Their root systems are generally shallow and fibrous, allowing them to absorb superficial moisture over a wide area.

What are the main parts of a cactus?

Cacti are perennial plants with thick stems containing chlorophyll. They are known for their areoles, which are cushion-like structures that produce plant hairs and spines. While most cacti lack leaves to minimize water loss, some species have leaves that carry out photosynthesis. The root systems of cacti are thin and spread out to absorb moisture. Cacti vary in size and shape, from small peyote to tall saguaro. The primary method of reproduction is by seeds. Cacti flowers are often large and colorful with a floral tube. The stems of cacti have a cambium layer and storage cells that contain substances that prevent moisture loss, which also helps with food production and storage. Spines defend the juicy stems of cacti and also shade them from the sun.

Why do cacti have spines instead of leaves?

Cacti have spines instead of leaves to minimize the surface area from which water can be lost. In most cactus species, the stem has taken over the photosynthetic functions of the plant. Spines also defend the juicy stems of cacti against animals and shade them from the desert sun. Cacti can be distinguished from other succulent plants by the presence of spines that develop from areoles. These spines may be stiff, curved, hooked, feathery, bristly, flattened, sheathed, or needlelike.

How do people use cacti in their daily lives?

People use cacti in their daily lives in a variety of ways. They are widely cultivated as ornamentals and used in rock gardens. Various species, notably prickly pears and chollas, are cultivated as food . In Central and South America, species of Opuntia , Cereus , and others serve as living fences, while the wood from columnar cacti is used as fuel in some desert regions. During droughts, the spines are removed from cacti such as mandacaru ( Cereus jamacaru ) to use as fodder for livestock. Peyote, from Lophophora williamsii , has been used ceremonially since pre-Columbian times for its hallucinogenic properties, and many cactus species are of local importance in traditional medicine.

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Images & Videos

Large pipe cactus, Cereus thurberi, with multiple tall, ribbed stems growing upright among other desert plants and palm trees.

White flower of the Dutchman's pipe cactus, Epiphyllum oxypetalum, with long, narrow petals and yellow stamens, set against green leaves and a dark background.

Eastern prickly pear cactus, Opuntia humifusa, with flat green pads and several large yellow flowers blooming among grass and dry leaves.

At a Glance

Golden rainbow cactus (Echinocereus dasyacanthus), a hedgehog cactus, growing in the desert of southwestern Texas.

cactus summary

Quick Facts

Plural:: cacti or cactuses

Key People:: George Engelmann

Related Topics:: climbing cactus; cereus; Neolloydia; Thelocactus; Ferocactus

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saguaro cacti Saguaro cacti, Arizona, U.S.

cactus

plant

Also known as: Cactaceae, cacti, cactuses

Written and fact-checked by

Britannica Editors

Last updated

Oct. 9, 2026 •History

Top Questions

What is a cactus?
A cactus is a perennial succulent plant that thrives in dry regions of the Western Hemisphere. Cacti are known for their thick stems containing chlorophyll and areoles, unique structures that produce plant hairs, spines, bristles, flowers, branches, and leaves.

Most cacti lack leaves or have reduced leaves modified as spines to minimize water loss. Their shallow root systems efficiently absorb moisture. Cacti vary in size and shape, from small peyote to tall saguaros. While most grow in the ground, some are epiphytes that grow on other plants.

People cultivate cacti as ornamentals, food sources, living fences, and medicine.
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Where do cacti usually grow?
Cacti are native to the Americas, mainly in dry regions, though some can be found in tropical or subtropical areas. Mexico exhibits the greatest diversity of cactus species. While most cacti thrive in soil, some species such as leaf cactus, Rhipsalis, and Schlumbergera are epiphytes, growing on other plants, rocks, or trees.

Desert plant communities are influenced by available moisture and local environmental factors. The Sonoran Desert in North America, for example, features large cacti like the saguaro, while the Mojave Desert is known for Joshua trees.
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How do cacti survive in very dry environments?
Cacti are well-adapted to survive in dry environments through several unique features. As succulents, they have thick, fleshy tissues designed for water storage, primarily in their stems or leaves.

To minimize water loss, cacti have reduced or absent leaves, often modified into spines, and their stems have taken over the photosynthetic functions. Many cacti also use a specialized process called crassulacean acid metabolism (CAM), where they open their stomata at night to take in carbon dioxide and close them during the day to prevent water loss. Their root systems are generally shallow and fibrous, allowing them to absorb superficial moisture over a wide area.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are the main parts of a cactus?
Cacti are perennial plants with thick stems containing chlorophyll. They are known for their areoles, which are cushion-like structures that produce plant hairs and spines. While most cacti lack leaves to minimize water loss, some species have leaves that carry out photosynthesis.

The root systems of cacti are thin and spread out to absorb moisture. Cacti vary in size and shape, from small peyote to tall saguaro.

The primary method of reproduction is by seeds. Cacti flowers are often large and colorful with a floral tube. The stems of cacti have a cambium layer and storage cells that contain substances that prevent moisture loss, which also helps with food production and storage. Spines defend the juicy stems of cacti and also shade them from the sun.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why do cacti have spines instead of leaves?
Cacti have spines instead of leaves to minimize the surface area from which water can be lost. In most cactus species, the stem has taken over the photosynthetic functions of the plant. Spines also defend the juicy stems of cacti against animals and shade them from the desert sun. Cacti can be distinguished from other succulent plants by the presence of spines that develop from areoles. These spines may be stiff, curved, hooked, feathery, bristly, flattened, sheathed, or needlelike.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do people use cacti in their daily lives?
People use cacti in their daily lives in a variety of ways. They are widely cultivated as ornamentals and used in rock gardens. Various species, notably prickly pears and chollas, are cultivated as food. In Central and South America, species of Opuntia, Cereus, and others serve as living fences, while the wood from columnar cacti is used as fuel in some desert regions.

During droughts, the spines are removed from cacti such as mandacaru (Cereus jamacaru) to use as fodder for livestock. Peyote, from Lophophora williamsii, has been used ceremonially since pre-Columbian times for its hallucinogenic properties, and many cactus species are of local importance in traditional medicine.
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coastal cactiCacti on Bonaire, Lesser Antilles.

cactus, (family Cactaceae), flowering plant family (order Caryophyllales) with nearly 2,000 species and 139 genera. Cacti are native through most of the length of North and South America, from British Columbia and Alberta southward; the southernmost limit of their range extends far into Chile and Argentina. Mexico has the greatest number and variety of species. The only cacti possibly native to the Old World are members of the genus Rhipsalis, occurring in East Africa, Madagascar, and Sri Lanka. Although a few cactus species inhabit tropical or subtropical areas, most live in and are well adapted to dry regions. See also list of plants in the family Cactaceae.

Physical characteristics

Cacti are succulent perennial plants. Cacti generally have thick herbaceous or woody chlorophyll-containing stems. Cacti can be distinguished from other succulent plants by the presence of areoles, small cushionlike structures with trichomes (plant hairs) and, in almost all species, spines or barbed bristles (glochids). Areoles are modified branches, from which flowers, more branches, and leaves (when present) may grow.

rose cactusRose cactus (*Pereskia grandifolia*).

In most species, leaves are absent, greatly reduced, or modified as spines, minimizing the amount of surface area from which water can be lost, and the stem has taken over the photosynthetic functions of the plant. Only the tropical genera Pereskia and Pereskopsis, both vines, have conventional-looking functional leaves, while the leaves of the Andean Maihuenia are rounded, not flattened. The root systems are generally thin, fibrous, and shallow, ranging widely to absorb superficial moisture.

Venus's-flytrap. Venus's-flytrap (Dionaea muscipula) one of the best known of the meat-eating plants. Carnivorous plant, Venus flytrap, Venus fly trap

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Plants: From Cute to Carnivorous

Cacti vary greatly in size and general appearance, from buttonlike peyote (Lophophora) and low clumps of prickly pear (Opuntia) and hedgehog cactus (Echinocereus) to the upright columns of barrel cacti (Ferocactus and Echinocactus) and the imposing saguaro (Carnegiea gigantea). Most cacti grow in the ground, but several tropical species—including leaf cactus (Epiphyllum), Rhipsalis, and Schlumbergera—are epiphytes, growing on other plants; others live on hard substrates such as rocks, while yet others climb far up trees. Epiphytic species tend to have thin, almost leaflike flattened stems. The appearance of the plant varies also according to whether the stem surface is smooth or ornamented with protruding tubercles, ridges, or grooves.

Thanksgiving cactusThanksgiving, or crab, cactus (*Schlumbergera truncata*) in bloom.

The primary method of reproduction is by seeds. Flowers, often large and colourful, are usually solitary. All genera have a floral tube, often with many petal-like structures, and other less colourful and almost leaflike structures; the tube grows above a one-chambered ovary. A style topped by many pollen-receptive stigmas also arises from the top of the ovary. The fruit is usually a berry and contains many seeds. Soon after pollination, which may be effected by wind, birds, insects, or bats, the entire floral tube detaches from the top of the ovary to leave a prominent scar.

Cluster of round ball cacti (Parodia magnifica) with ribbed green bodies, yellow spines, and several yellow flowers blooming on top, growing in a desert garden. — ball cactusBall cactus (*Parodia magnifica*) surrounded by clonal pups.

chin cactusChin cactus (*Gymnocalycium mihanovichii*).

Several cacti develop plantlets at ground level that, as offsets, reproduce the species vegetatively. Many others can reproduce by fragmentation, whereby segments broken from the main plant will readily root to form clonal individuals. Tissues of cacti are broadly compatible so that terminal portions of one species may be grafted on top of another.

The internal structure of cacti stems conforms to the pattern of broad-leaved angiosperms; a cambium layer of dividing cells, located between the woody inner tissues and those near the outside of the stem, is present. The bulk of the stem, however, consists of thin-walled storage cells that contain mucilaginous substances that prevent the loss of moisture. The stem of cacti is the main food-manufacturing and food-storage organ for most species.

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Uses

Cacti are widely cultivated as ornamentals. In addition, various species, notably prickly pears and chollas (Opuntia and Cylindopuntia, respectively), are cultivated as food. In Central and South America, species of Opuntia, Cereus, and others are used as living fences, and wood from columnar cacti is used as fuel in some desert regions. In times of drought, the spines are removed from cacti such as mandacaru (Cereus jamacaru) to use as fodder for livestock. Peyote, from Lophophora williamsii, has been used ceremonially since pre-Columbian times for its hallucinogenic properties, and many cactus species are of local importance in traditional medicine.

The Editors of Encyclopaedia Britannica

This article was most recently revised and updated by Melissa Petruzzello.

cactus

Physical characteristics

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Uses