Organ Pipe Cactus National Monument

View from the visitors center trail at Organ Pipe Cactus National Monument.  In this photo jumping cholla, saguaro, and organ pipe cacti can all be seen with the Ajo mountains in the background.

Organ Pipe Cactus National Monument is located right on the boarder of Mexico as far south as you can go in Arizona. The monument is quite a ways off the beaten path and probably the most dangerous monument in the national park system. There are multiple boarder patrol checkpoints that are actually in or near the park and boarder patrols can be found all over the park. Tragically, a few years back a park ranger was killed in a boarder incident and things were quite dangerous within the park. Today however, a barrier fence has been put into place along the monuments boarder and dangerous sections of the park have been closed to visitors. The park is significantly safer today than it was several years ago. Unfortunately, Senita Basin, the only population of the columnar senita cactus is currently closed due to these issues. Fortunately, other sections of the park such as the Ajo Mountain Drive are open and offer spectacular views of the Sonoran Desert. For a desert, the monument is quite green and hope to a decent amount of vegetation. This unusually green desert is a result of this particular desert being one of the wettest deserts in the world. Organ Pipe receives about 10 inches of rain annually with significantly more falling at higher elevations. Both winter and summer rainfall seasons also contribute to the amount and diversity of vegetation here.

View along the Ajo drive in Organ Pipe Cactus National Monument

A total of 28 species of cacti can be found within the park. Several of these are at their northern most limits, and are prevented from migrating further north due increased number of days with freezing temperatures to the north. Organ pipe cacti are one of these species and can be found in abundance within the park. The amazing diversity of cacti can be found on the Ajo Mountain Drive, and it is spectacular for organ pipe viewing. Most commonly, the organ pipe is found on upper slopes facing south. The sun warms southern slopes just enough to prevent colder freezing temperatures that prevents the cacti from growing on colder northern slopes. Upper slopes also are slightly warmer due to warm air rising up these slopes. If the growing tissue of an organ pipe freezes for too long of a period of time or too many times in the winter it will kill the plant. So these slightly warmer areas give the cacti an added edge so they can become established. I also have a strong suspicion that organ pipes prefer soil types typically found on upper bajada slopes. Even with this added warmth however, conditions are not absolutely perfect. The organ pipe sill needs some help from what is called a nurse plant. Small shrubs, mesquites, ironwoods, and palo verdes all help protect young organ pipes from the intense summer heat and sun. The cooler temperatures and shady conditions also help hold the water in the soil for longer. On occasion, large rocks can even provide these added benefits of shadier, cooler, and slightly wetter conditions. Nurse plant associations can be found for several different species. The saguaro cactus has very similar nurse plant requirements. A different type of nurse plant association that can be found within the park is that of the jumping cholla and pincushion cactus. The jumping cholla is a rather large shrubby-tree like cactus that looses an abundance of spine dense joints. These joints naturally fall from the cholla and often will form a mat around the mother plant. Nothing really wants to go close to these piles of spiny cactus joints making it a perfect place for the small pincushion cactus to live.

Organ pipe cactus

Echinocereus sp.: The Hedgehog Cactus

Echinocereus engelmannii

The hedgehog cacti are probably my favorite group within the cactus family.  I find their forms, including shapes and spines very interesting.  Their flowers come in an assortment of beautiful colors ranging from pink to red to white to yellow to purple.  Flowers also survive quite a few more days than other cacti flowers.  The scientific name for hedgehogs is Echinocereus.  Fittingly, echino means "hedgehog", and cereus means candle.  I suppose the individual stems may look somewhat like a hedgehog or candle, but I consider them to look more like a cucumber standing upright.  The stems of hedgehog cacti most often are clumped together but with several species they are individual or forming mounds.  Hedgehog cacti are common throughout SW United States and NW Mexico surviving in the low deserts to the high mountains.  The full extent of hedgehogs ranges from South Dakota to Central Mexico.  Identifying these cacti can be a little tricky at times and knowing flower color as well as spine density, length, and coloration is essential.  Even within species there can be considerable variation of traits.  For this reason, botanists have been confused and arguing over classification of different hedgehog cacti for decades.  As time goes on, these botanists seem to discover more and more species, or identify new species out of already existing groups.  Hedgehog fruits are also typically very tasty and Native Americans would eat the fruit as they came across them.  The cacti do not seem to produce a lot of fruit though, so I suppose they were eaten more like a snack.   

Echinocereus coccineus

Echinocereus engelmannii

Barrel Cactus Part 2

California barrel cactus, Ferocactus cylindraceus.

Barrel cacti are kind of as their names imply, barrels full of water.  The problem is, the water isn't just hanging out in the cactus like a big glass of water.  The water is stored inside of the cells that fill the interior of the cactus.  The best way to get this water is to eat the tissue, though it won't taste very good and probably will make you sick.  The thick layer of hooked spines will also deter any person or animal from easily accessing this water though.  In drought however, the barrel cacti is one of the best sources of water for desert animals there is, that is, if they can get through the spines.  Small animals like rats, chipmunks, or mice can avoid spines by burrowing underground slightly to where there are no spines and then eating up into the cactus.  I have actually found a few barrel cacti that have been entirely hollowed out by rodents, yet have there skin and spines fully intact.  Larger animals such as deer have no such luck though accessing moisture from a standing barrel cactus though.  The spines become just too big of a deterrent.

Red spines of the barrel cactus show up after being wet by rain.

Fortunately, for larger mammals the barrel cactus has a fatal flaw.  As a barrel cactus grows it generally leans towards the southwest, which is the direction from which the most intense sun comes from.  Nearly all barrels lean to the southwest, just as a compass always points north, thus the common name compass barrel.   It might seem that leaning in the direction of the brightest sunlight might mean the cactus is trying to gather as much sunlight as possible.  This is however the exact opposite of what it is doing.  With the top of the cactus pointing directly at the most intense sun, spines at the top actually shade out much of this light and all sides of the cactus actually avoid this direct sunlight.  The sides however gather the most sunlight from the sides, as the sun comes up or goes down, when the sun rays are less intense and therefore less damaging to the cactus.  Pointing tops towards the most intense sunlight is therefore actually a protection mechanism, rather than a gathering mechanism, against intense sunlight.

A barrel cactus that fell over due to leaning towards the southwest.  Even though this cactus fell over, it continues to grow.

Leaning is an important adaptive strategy of the cactus, but is this strength also lays a huge weakness.  As the barrel cacti grows and leans it becomes very off balance.  Older, large cacti will often simply fall over.  Oddly, even when the cactus falls over it will continue to live and grow as it lives laying on the ground.  Once the barrel cactus falls over, the underside of the cactus is exposed which is unprotected by spines.  Large mammals will often start eating the barrel from this unprotected portion during drought.  

Flower of the California barrel cactus Ferocactus cylindraceus.

Barrel Cactus Part 1

Compass Barrel cactus

The barrel cactus is one of the most common cacti in the Southwest.  There are four different species common to this area of the country, the most common of which are the compass barrel and the California barrel.  It can be extremely difficult to distinguish between these two common species of barrels.  In southern Arizona, such as around Tucson, the compass barrel is the most common of the two cacti.  In central Arizona such as around Phoenix, southern California, and even into the depths of the Grand Canyon the California barrel cactus is the most common.  Their ranges overlap in central Arizona and their similarities are pretty extensive.  Without closer investigation you may not be able to determine what specific species a particular barrel is, there are however a few differences that may help in identification.  First off is shape.  Of course, barrel cacti are all sort of barrel shaped.  The compass barrel is a little more wide and plump than the California barrel.  The California barrel  is a little skinnier.  The second way to distinguish between the two is by looking at the spines.  Both have very interesting spines which are often red colored.  This red coloration gives a sharp contrast to the dark green of the cacti's body, especially after a rare rainfall.  Both cacti also have flattened central spines that have a ribbing pattern on them.  The central spines are also hooked, giving both cacti another common name of fishhook barrels.  Compass barrel cacti spines are however considerably more hooked than California barrels.  Compass barrel central spines are a full "fishhook" shape and were in-fact used as fishhooks by some Native Americans.  California barrel central spines are closer to a 90 degree curve than an actual fishhook.  These are the best ways, though not necessarily foolproof ways of distinguishing the two while out in the desert.

In our next post we will talk about the leaning habit of barrel cacti.

California barrel cactus front left of picture.

How To Identify a Cactus

The columnar saguaro cactus.  Note the huge column like shape and ribs lined with spines traversing from the bottom to top.

Cacti are one of the most diverse and interesting plant families in the plant kingdom.  Cacti are native only to North and South America but are prized worldwide by plant enthusiasts.  I once worked with a PH.D who was from England but came to the United States specifically to work with cacti.  While you can go to just about any botanical garden in the world to observe cacti, the Southwestern United States and Mexico are probably the best places to observe cacti in the wild (in North America at least).  Within the United States, cacti can be found in the wild in just about every state.  Where I grew-up in Iowa, every once in awhile I would come across plains prickly pears growing in a dry prairie.  Now, living in the Southwest I come across cacti every single day.  The Sonoran Desert is loaded with all kinds of different cacti ranging from the 50 foot Saguaro cactus to the six inch tall pincushion.  Cacti are really not that difficult to identify, at least to the genus or "group" level.  Just about anyone can learn the major groups of cacti simply by looking at three major traits; the shape, ribs, and spines.

Barrel cacti in foreground.  Named after their barrel like shape.  Barrel cacti also have ribs lined with spines.

Shape is possibly the easiest and best way to categorize a cactus.  The most common cactus group is the prickly pears.  These cacti have stems that are sectioned into flat, pear or pancake shaped pads.  The overall prickly pear plant is joined together by these pads typically forming a shrub shape.  Cholla cacti are similar in that the plant is made up of sections, but instead of these sections being flat and pear shaped, they are cylindrical, and the overall plant also is shrub shaped.  Barrel cacti are barrel shaped.  Columnar cacti such as saguaros form tall columns. Tiny pincushion cacti are small and often shaped like an actual pincushion.  Hedgehog cacti are sort of like small columnar cacti that only grow a few feet tall at most, with the small stems bunching together.

A cylindrical cholla cactus section.

Ribs are the next important way of identifying a cactus.  Saguaros and other columnar cacti have long ribs or pilleates that stretch from the bottom of the cactus to the top.  Hedgehogs and barrels also have ribs.  Pincushions, prickly pears, and chollas do not have ribs. 

Prickly pear cactus with flat pear shaped sections.

Lastly spines.  Spines don't always help us distinguish between different groups of cacti but are extremely useful in determining the actual species of cacti.  A few spines like the tiny hairlike glochid are only found on prickly pears.  Glochids are the tiny spines that get stuck in your skin and have to be taken out with a tweezers.  Pincushions typically have tons of white spins which helps give them a "pincushion" like appearance.  Spine color, number, and shape are essential in learning to distinguish specific species of cacti.

Hedgehog cactus

Pincushion cactus.

Life of a Cactus Part 10: Nurse Plants

The pulp within a cactus fruit contains hundreds to thousands of seeds.  Unfortunately, very few of these seeds will ever germinate, and probably less than 0.1 percent of these seed will ever grow to mature plants.   Life is just too dangerous and for the seed and young plants.  The first problem a cactus seed faces is also a blessing.  The succulent flesh of a cactus fruit is like a magnet during the bitterly dry and hot summer.  Birds and animals gorge themselves on these fruits as a rich source of moisture and nutrients.  As they eat the fruit though, many, if not most of the seeds are also eaten.  This isn't such a bad thing, as long as the seeds are not crushed by chewing.  The seeds are especially adapted to remaining intact and passing directly through the digestive tract unharmed.  With most cactus fruit being brightly colored and located high on the plant, these fruits are especially enticing to birds, which may be the only organisms capable of reaching the fruits on taller cacti.  For example, the saguaro cactus holds its fruits tens of feet off of the ground, only allowing birds to access it.  Being birds do not chew food, instead swallowing it whole by the beak-full, most of the seeds can pass through the digestive tract unharmed and be deposited in their fecal matter.  Kind of a disgusting start to life but true never-the-less. 

Young Saguaro Cactus growing under the canopy of a Palo Verde.

Being birds of course frequently perch and sleep in shrubs and trees, most of their fecal matter, and therefore cactus seeds, will be deposited below.  This is the ideal environment for a cactus seed to germinate and grow in.  The shrub or tree provides shade which creates a slightly cooler and moister environment for the seed to germinate and grow in, something much needed in the desert.  The shrub or tree also provides cover and protection from predators which might eat recently germinated seeds.  Soil also is slightly more rich in these locations also.  This environment under the shrub or tree is termed a microenvironment and the plant that makes it is called a nurse plant.  Of course, the nurse plant is termed such because it helps, or nurses, young plants such as cacti to maturity in the microenvironment they create.  A microenvironment is a small area, such as under a tree canopy, that has slightly different conditions than the surrounding environment.  Within the Sonoran Desert Triangle-Leaf Bursage is the most important nurse plant.  Very few plants are able to grow without the nursing aid of a Bursage microenvironment.  This is sort of odd considering Bursage is such a small desert shrub, usually only reaching 20 or so inches in height.  Bursage is however one of the few plants capable of establishing itself without a nurse plant and is extremely abundant across the desert.  Other plants such as Palo Verde trees are well known nurse plants but not as important as Bursage.  This probably is because Palo Verde can't become established without a nurse plant and are not as common as Bursage.  Interestingly, the most common nurse plant for Palo Verde is also Triangle-Leaf Bursage. 

Life of a Cactus Part 9: Cactus Fruit and Seed

Ripe Prickly Pear Cactus fruit.

Within a month or so after flowering, the cactus flower ripens into a mature fruit ready to eat.  Some cacti produce dry fruits that simply break apart and scatter the seeds.  These dry fruits are typically dull brown and shriveled up.  Not much exciting about this type of fruit.  A large portion of cactus species though produce a brightly colored fruit swollen with sweet juicy pulp.  Often these juicy fruits are ripe for harvesting during the hottest driest part of summer.  May and June in in the Sonoran Desert are extremely hot and dry.  It is very common for not even a trace of rain to fall during these months and temperatures will often be in excess of 110 degrees towards the end of June.  During these months however some of the most abundant cacti produce their ripe fruit.  Buckhorn and Teddy Bear Cholla fruit ripens in May along with many species of Hedgehog Cacti.  In June then, Prickly Pears and Saguaros produce their fruits.  Depending on the species these fruits are typically red, yellow, or green.  While green fruit is not extremely attractive, red and yellow fruit brightly advertise their presence to all that pass by.   These brightly colored fruits are plump full of moisture in the otherwise bone dry landscape, making them even more attractive to animals in search of moisture.  These fruits are also often located of the tops of cactus plants where they will be most obvious.  The cactus, which is otherwise extremely conservative with water liberally gives out water to any animal that desires to brave the spines.  Many cacti, such as Saguaros, have very few spines on their fruits making them even more accessible.  This fruit is meant to be eaten.  So to make sure the fruit is eaten, the cactus makes a sweet, water rich fruit that is easily visible and accessible.  Birds and animals will often gorge themselves on these fruits.  These fruits may in-fact be their only water source during the hot dry late spring and summer months.  The cactus liberally shares its water with these animals, but not simply because the cactus has a sharing heart.  The cactus wants something in return for its water.  Each of these cactus fruits is loaded with dozens to even thousands of tiny cactus seeds.  Prickly Pears and Chollas have BB sized seeds that are as hard as rocks.  Other cacti such as the Saguaro have tiny black seeds.  These seeds are so abundant and so small in the cactus fruit, anything eating the fruit cannot avoid also eating the seeds.  While the fruit pulp is full of nutrition and water, the small seeds cannot be digested and pass through the entire digestive tract of an animal unharmed.  The digestive juices within the animals stomach cannot penetrate the seed coat and actually can help the seed germinate after passing through the animal.  This is exactly what the cactus wants in return from the animal that eats its fruit.  The cactus wants the animal to help distribute and aid the germination of the seeds.  Most often, the eater of cactus fruits and distributor of the seeds are birds.  After eating the fruit and seeds these birds will rest on branches of trees where the seeds will pass out of the body and be deposited on the ground.  Under the canopy of Mesquites, Palo Verdes, and Ironwoods is the ideal location for a cactus seed to germinate and grow into an adult cactus. This strategy helps ensure the survival of the next generation of cacti.

Ripe Saguaro Cactus fruit.

Life of a Cactus Part 8: Flowers

Prickly Pear Cactus flower.

As you may deduct from above, if a cactus is ever to reach maturity, it must earn its way.  The cactus first puts years of effort into establishing itself in the harsh desert environment.  Drought, scorching sun, and relentless heat day after day are unforgiving.  Any violation of these harsh desert laws results in death.  Very few cacti survive much past the seedling stage and only those that are fortunate enough to land in ideal soil conditions during a good rainfall year will survive.  Those that do must grow and work to establish themselves as a strong young plant before even a single flower is produced.  Cacti that do flower are survivors to which few other plants are comparable.  Some cacti, such as prickly pears, can flower within a matter of several years after germination.  Others, such as the saguaro, require 35 to 40 years of growth and may be eight feet tall before a single flower is produced.  After the first year of flower production, those that continue to flower and produce seed year after year are even greater survivors.  It isn’t until young cacti grow from their seed however, that these cacti have truly beaten the brutal reality of natural selection in the desert environment.     

Hedgehog Cactus in bloom.

When it comes to cacti and reproduction, water again is central.  This time it’s a little different though.  The normally water conservative cactus becomes quite liberal with water use in flower and fruit production.  Many cacti produce very succulent, tender, and beautiful flowers containing lots of water.  If you were to touch the petals you would notice a slight succulence.  Then, touching the inside of the flower you would notice a slightly sticky dampness.   This is odd when compared to other typical desert plants that produce much drier and water conservative flowers.  So why would the cactus put so much water into flower production?  Why is it wasting this water?  If a cactus could talk it would probably say it is not wasting the water at all.  Rather, the wetness of the flower displays its heavy dependence as well as contribution to the animal world around it.  The cactus gives a little and takes a little.  In the form of nectar, the cactus flower provides both water and food for animals.  Bats, bees and other insects, as well as birds look to cacti as both a food and water source in the hot dry desert.  Often, the moisture found in a cactus flower is the most available source of hydration smaller desert animals can find at certain times of the year in the desert.  This water and food source often becomes a magnet for activity during flowering time.  The cactus flower can become a small swarm of buzzing insects looking for nourishment.  The cactus doesn’t just give out nectar just because it’s nice though, it does expect something in return.  When bugs, birds, and bats feed on cacti nectar, they also inadvertently pick-up pollen.  The pollen is then carried to the next cactus flower as the animal searches for more nectar.  In this way pollen is distributed from one flower to the next and pollination is accomplished.  Without bugs, birds, or bats carrying out this pollination cactus fruits and seeds would never form.  So the cactus gives a little and takes a little. 

 

Life of a Cactus Part 7: The Cactus and Freezing Temperatures

All that water stored inside the cactus can be quite a problem certain times of the year.  Freezing temperatures can create ice crystals that burst cells causing tissue damage.  Depending on the amount of tissue that freezes and how well a cactus is adapted to freezing temperatures, freezing can actually kill a cactus.  Most species of cacti are not well adapted to freezing and for that reason most are tropical and subtropical.  Tropical climates never freeze while subtropical climates occasionally freeze.  The extreme southern United States, such as southern Arizona, Florida, and California are all subtropical.  The subtropical Sonoran Desert of Southern Arizona and Sonora, Mexico typically average freezing temperatures a few nights every year or fewer.  Freezing temperatures rarely last more than a few hours and results in the abundance of cacti found in the Sonoran Desert.   Saguaro cacti are the largest cacti that can tolerate freezing, but only as long as freezing temperatures last less than 24 hours.  Areas that receive freezes lasting longer than 24 hours have no saguaro cacti.  Organ Pipe cacti are much more frost sensitive and only survives along the U.S.-Mexico border in Arizona and further south.  Some species such as the Cordon can survive no freezing whatsoever and therefore only survive further south in Mexico.  A handful of prickly pears are able to survive very deep freezes for long durations of time.  Several species, such as Plains Prickly Pear, grow in grasslands of the central United States up to the Canadian border.  One species, Brittle Prickly Pear, grows nearly to the Arctic Circle surviving temperatures as cold as -40 degrees Fahrenheit.  Species able to endure freezing temperatures actually remove water from their cells, essentially dehydrating themselves, so ice crystals will not burst and kill cells.

Brittle Prickly Pear, able to survive temperatures of -40 degrees F by pumping water out of its cells so ice crystals do not burst cells.

Life of a Cactus Part 6: Callus

Woodpecker entering a Saguaro "boot" used for nesting.

The last water conserving feature of cacti we will discuss is the abundance of mucilage all cacti produce internally.  If you were to cut open a cactus and touch the internal tissues you would notice the cut surface as being wet.  It would not be a watery wet though, it would be a slightly sticky and slimy wet.  This sliminess, known as mucilage, is a result of polysaccharides, or carbohydrates, produced by the cactus.  This mucilage helps hold on to water, slowing evaporation.  It also functions to form a callus or scab once it is exposed to the air.  This callus functions similar to how scabs function for us.  When we bleed a scab forms to prevent further loss of blood and to prevent infections from entering the body.  When a cactus is damaged it also forms a scab to prevent bleeding of water as well as to prevent infections from entering the cactus.  The scab also hinders predation of the cactus by being a barrier between the cactus flesh and potential predators.  Often the callus can become several millimeters thick.  If you are around cacti at anytime, look for damaged sections of the plant.  The callus will be a tan to black coloration on the damaged portion.

Saguaro boots like this one will often survive long after the cactus decays away.

All cacti form calluses, which may be helpful to the cactus but an annoyance to any animal that may want to feed on cactus flesh.  A lot of birds though find cactus calluses quite useful.  In large columnar cacti such as saguaros, woodpeckers will remove the spines and peck a hole.  Gila woodpeckers and flickers are the most common birds that do this in the Sonoran Desert.  These woodpeckers will excavate a hole large enough to nest in.  Of course, excavating a hole in a cactus will cause it to bleed but a callus will in short time line the hole.   Typically, this causes little harm to the plant itself in the long-run.  These callused holes are called “boots” and will remain for the entire lifespan of the cactus.  Once the boot is abandoned by the woodpecker, an assortment of other birds will also use it for nesting being it is warmer in the winter and cooler in the summer.  Birds using saguaro boots for nesting include elf owls, kestrels, ash-throated flycatchers, and purple martins.  In-fact several species of birds can be found nesting in the same large cactus if it has many different boots.  This is why saguaros have been called by some a “cactus hotel”.  Oddly, these boots are so durable that, years after a saguaro dies and most all remnants of the cactus have decayed, the boot still remains lying on the ground.

A callused hole in a Saguaro cactus.

Life of a Cactus Part 5: CAM Photosynthesis

This is a microscopic image of the surface of a cactus stem.  The oval shaped objects with dark areas in the middle are stomata, the pores through which cacti and other plants breath.

Once water is stored inside of the cactus water conservation does not stop.  Microscopic pores called stomata cover the green tissue of all plants, allowing them to “breathe” and carry out photosynthesis.  Through these stomata, all plants “exhale” water vapor and oxygen and “inhale” carbon dioxide.  The exhaled oxygen is a waste product of photosynthesis.  Inhaled carbon dioxide, along with water stored in the plant, are converted into sugar and starches by use the sun’s rays through photosynthesis.  Water vapor is passively lost through the stomata whenever they are open to inhale and exhale carbon dioxide and oxygen.  Even when the stomata are closed a small amount of water vapor is lost.  Nearly all plants open their stomata to carry out this breathing process during the day.  As a result, most plants are opening their stomata when it is hottest and are therefore releasing huge amounts of water through evaporation.  In areas where there is plenty of water this really isn’t much of a problem, if you have water to spare you have water to waste and can afford to use it quite liberally.

Crested Saguaro Cactus.

 In the desert, there is no water to spare and to waste.  The cactus therefore does the exact opposite of what almost all plants do; it opens its stomata to “breath” during the night and closes them during the day.  By closing stomata when it is hottest and opening them when it is cooler the cactus conserves huge amounts of water.  The problem with this is cacti still need a constant input of carbon dioxide in order to carry out photosynthesis during the day, and without the stomata open there is no direct supply.  To overcome this, cacti absorb huge amounts of carbon dioxide at night time when their stomata are open.  They do not absorb this carbon dioxide like a balloon would though.  Instead, they convert it to a chemical called malic acid, in which form the carbon dioxide can be stored until day light.  Once daylight appears the, malic acid is then converted back to carbon dioxide needed to carry out photosynthesis.  Plants only need to breathe when it is daylight and photosynthesis is being carried out.  So in the dark no photosynthesis is being carried out and there is no reason for the plant to breathe.  This is why most plants only open their stomata and breathe during the day, and because day temperatures are warmer, more water is lost through the stomata.  As said before, cacti open their stomata at night to breath, storing the carbon dioxide as malic acid for later use when daylight appears.  Once daylight appears, the cactus closes its stomata and sort of holds its breath, converting the malic acid back into carbon dioxide for use in photosynthesis.  By only opening their stomata in the cool of night, far less water is lost.

Cacti stomata not only open and close in an ideal fashion to conserve water, they are also specifically designed to conserve water.  Most plants have large amounts of relatively small stomata all over their green tissues.  Relatively small cells also open and close these stomata.  Cacti however have far fewer stomata but the stomata they do have are much larger.  Overall though, this decreases the amount of water that can evaporate through the stomata.  Cacti also locate their stomata is a shallow pit as opposed to directly on the surface of green tissue.  This protects the stomata from drying winds.  Lastly, the cells that open and close the stomata are huge in comparison to the typical plant.  This allows the cacti to firmly close their stomata so water vapor does not accidentally leak out.