We grow cacti in our homes and conservatories and know that they need special cultural requirements.  They require different care than, for example, a fern because the natural habitat and place of origin of these two types of plants are vastly different.  To better understand, as horticulturists, home gardeners, or enthusiasts, how to care for a specific group of plants, such as cacti, we should start by understanding the habitat in which they evolved and the special mechanisms they have developed in order to survive. 
This virtual tour is focusing on the cacti of the Chihuahuan Desert in the Southwest US, in west Texas, New Mexico, and southeast Arizona, and central Mexico. This desert is North America's largest Desert and one of the most barren places in the Western Hemisphere, but certain genera of cacti flourish there. 
The Chihuahuan Desert is 175,000 square miles most of which lies in Mexico between two mountain ranges, the Sierra Madre Occidental and the Sierra Madre Oriental (Figure 2).  Moisture from either the Gulf of Mexico or the Pacific Ocean is blocked from entering this region because of these two Mountain ranges.  The land is composed of limestone bedrock and the climate is harsh, with extreme arid temperatures and a regular, persistent wind that blows the soil to blur the horizon.  A desert climate is made by a low and irregular annual precipitation level, and a fluctuating daily temperature.  The moisture is not spread evenly throughout the year. It may not rain for months and then one summer day the sky will dump three inches of rain in an hour.  Although the annual average rainfall in the desert is nine inches, with the wind and high temperatures, the yearly evaporation potential is over thirteen times that of the rainfall at 120 inches.  The temperature variation, also, can change forty degrees over a day. 

The land is not pretty and not really favorable to life—or is it? How do plants survive in this environment?  They adapt.  Because they are stationary, desert plants have to tolerate the changes in daily temperature and the fluctuation in water availability. 
Some of the plants in the desert are annual wildflowers that go through their whole life cycle within a month from germination to reproduction to death.  They appear in the wetter months after the winter or summer rains.  By the time the soils dry out they are dead.  Plants such as the Mexican gold poppy blanket the desert with the color yellow for a short time.  And the key to their survival year after year is their ability to produce thousands and thousands of seeds.  These not only carry on the next generation of plants, but also feed the smaller desert wildlife such as birds, ants, and kangaroo rats. 
The other types of plants that grow in the Chihuahuan Desert are the perennial plants such as the cacti, yucca, mesquites, acacias and creosote bush that survive through the dry months with special adaptations. Other creatures that can be seen living in the desert are: black throated sparrows, cactus wrens, night flowering plants such as Datura sp., woodrats, hummingbirds, jackrabbits, sphinx moths, quail, grasshoppers, lizards, ants, termites, scorpions, centipedes, burrowing owls, night hawks, kit foxes, tarantulas and snakes. 

Cacti must have special mechanisms to minimize water loss and protect themselves from predators looking for water.  These adaptations which cacti evolved are: leafless succulent stems, spines in areoles, and shallow, wide spreading root systems. 
The specialized stems (the swollen body) of cacti have evolved to store water and take over the role of the reduced leaves.  Because the ratio of surface area to volume is low, there is minimum water loss.  Water collected during the wet period can be amassed in large amounts and stored to be used over the entire dry season.  Food manufacturing has been taken over by the stem.  The stems are green and photosynthetic and replace the function of the leaves. 
Cacti stems have many stomata that are closed in the day and open at night.  This limits transpiration and water loss during the day when temperatures are at their maximum and relative humidity is low.  Evaporation is much less likely at night.  This activity is termed Crassulacean Acid Metabolism or CAM. 
The stem is composed of lots of cortex and pith tissue, which provide a reservoir for food and water.  There is only a very little amount of woody tissue that acts as a support of the swollen stem, the rest of the tissue is used for storage.  This stored water is used sparingly during drought, most of the year.  Certain cacti that are termed barrel cacti, such as Echinocactus sp, Ferocactus sp., and Astrophytum sp., have pleated stems that swell when they take in a lot of water, and shrink during the dry months as they use up their supply.  The epidermal layer of the cacti stem is tough and can allow for this shrink and swell. 
A thick and waxy cuticle covers the surface of the stem.  Without this water retaining layer, the cacti would shrivel to a small volume within a day in the hot desert. In hot desert temperatures cacti cannot use evaporative cooling like other plants and wastewater.  Cacti have developed a thick corky layer to act as insulation, especially at the ground surface, where temperatures can reach 150 F.  But the main body of the stem around the spines must tolerate the heat for there are no leaves to provide shade and this cork cannot develop. 
Cacti, because they are filled with water are highly frost sensitive and are killed very easily by freezing temperatures. 
Stems can be simple, unbranched, and columnar like the barrel cacti or branched either at or above the ground surface.  They might be smooth like Opuntia sp., or may be covered with small mounds called tubercles, which are enlarged leaf bases.  Each tubercle bears an areole, a modified axillary bud where the spines, the modified leaves arise.  Tubercles are borne on ribs that run vertically or spirally along the stem. 
Cacti stems have also developed a mechanism to protect themselves from wounds.  If the plant is wounded, a slime is produced which covers the cut surface and quickly dries to become hard and impermeable.  This saves the plant from losing water and drying out.  Some other cacti produce cork around wounds.
The leaves of a cacti are vastly modified into spines.  Only in the Cactaceae family do stems bear spines in areoles.  These are arranged alternately like some leaves on other types of plants.  In some species spines may only be present in the juvenile stage.  There are many varieties of spines, long, short, thick, thin, straight and hooked.  Some spines are directed downward to concentrate any rainwater that may fall onto the soil around the roots. 
Along with spines, areoles usually bear glochids, which may be bristles, masses of hairs, or short barbed structures.  Some cultivated cacti just bear glochids.  These are what usually hurt a person who is handling a cacti. 
This adaptation of spines prevents the plant from losing their stored water through transpiration of large leaves in the prevalent desert winds.  Without large leaves the surface to volume ratio is low, which favors water retention.  The spines also act as a protection mechanism to keep predators who are seeking a water supply from feeding on cacti fleshy stems.  Some spines have also adapted to the harsh climate by becoming hairy.  This protects the stem from being bleached by the strong sunrays. 
When they are developing, cacti first penetrate the ground with an anchor root so they are securely fastened to the ground.  Later, with age, long lateral roots grow and develop. These have a cork layer to prevent water loss.  Lateral roots spread right beneath the soil.  The roots of a small plant 2 feet high, for example, can spread to eleven and a half feet in radius.  The outermost absorbing roots are only about 1.5 cm under the surface. 
Rapid growth of root hairs adds to the absorptive abilities of the cacti. These roots develop during the rainy months only and later during drought these die to prevent water loss through too much surface area. All of these shallow root cacti do not compete for water with other desert shrubs and plants such as Mendora sp. and Acacia sp. 
Segments of some cacti that fall off root easily.  This adaptation allows cacti not to depend solely on germination if no water is available for reproduction.
The Chihuahuan Desert contains a great diversity of cacti.  They are small cacti, but numerous and interesting species.  Most of the species can be found along the border of the U.S. and Mexico in the Big Bend Country of the Rio Grande River in the Guadalupe and Franklin Mountains of Texas, and in Southern New Mexico. 

This plant lives many years and dies after flowering only once.

The exposed portion of the living rock stem is flattened even with the ground surface and is usually about 3-5 inches in diameter.  During periods of drought the stems may shrink below ground level and become covered over with sand and gravel.  The tubercles are deeply fissured and warty, tightly overlapping each other.  Very young plants may have a few spines, but mature plants have no spines.
 The flowers vary from light pink to purple and are 11/2 to 2 inches in diameter.  They bloom in October and November.  Living rocks occur below 4,000 feet elevation and are more commonly found on limestone ridges.
The small genus Ariocarpus contains six species, only one of which occurs north of the Rio Grande and in Big Bend National Park.

The species get its name from a combination of the Greek words astron (star) and phyton( plant).

This plant owes its nickname to the long white "hairs" which cover its surface. These hairs are actually long white radial spines.

The name for this genus comes from the greek words corypha, meaning crown, and and anthos: "flower."

This plant produces fruits which taste like strawberries.

This plant is sometimes nicknamed the Giant Fishook Cactus because of its huge hooked central spines.

The name for this genus comes from the Latin word ferox, which means wild.

The species name for this plant comes ofrm the Greek words mikros, meaning hairy, and dasys, meaning small. This name refers to the small glochids.

The strange shape of this plants fruit gives it its species name, which comes from the Latin turbineus, meaning whipping top shape, and the Greek karpos, meaning fruit.