Desert Cactus
Cacti are the iconic plants of the desert. They can survive months or years without rain, store huge amounts of water inside their thick stems, and have evolved special tricks like spines instead of leaves to reduce water loss. There are approx. 1,750 species of cactus, almost all native to the Americas. The giant saguaro of the American Sonoran Desert is the most famous, but there are also tiny cacti smaller than your thumb.
- Number of speciesapprox. 1,750Almost all native to the Americas
- TallestCardon (Mexico)Up to 19 m tall, weighs 25 tonnes
- Most famousSaguaro cactusThe "Western movie" cactus of Arizona
- Oldestapprox. 250 yearsA saguaro can live this long
- Water in one saguaroUp to 750 LStored inside its spongy stem
- Spines, not leavesCuts water lossSpines are modified leaves
How big can cacti get?
The Mexican cardon is the tallest cactus in the world. The Arizona saguaro is the second tallest. Most cacti are much smaller.
What is a cactus?
A cactus is a plant that has evolved special features for surviving extreme dryness. Most cacti have:
- Thick, fleshy stems that store water like a giant green sponge. The stems can swell up after rain and shrink between rains.
- Spines instead of leaves. Normal leaves lose huge amounts of water as they breathe in carbon dioxide for photosynthesis. Cacti do not have leaves at all; instead, they do photosynthesis in their thick green stems. Spines are modified leaves that have evolved to protect the plant from animals trying to eat its water.
- A waxy skin that seals water inside the plant.
- Shallow but wide roots that can quickly soak up rain when it does fall.
The saguaro
The saguaro is the most famous cactus in the world. It grows only in the Sonoran Desert of Arizona and northern Mexico. Saguaros can be 12 metres tall, weigh 2 tonnes, and live for over 200 years. They grow incredibly slowly. A typical saguaro might be 60 years old before it first sprouts an arm. The famous "arms up" silhouette of every Western film is actually quite rare; many saguaros never sprout arms at all.
After heavy summer rains, a mature saguaro can absorb up to 750 litres of water (about three quarters of a tonne) and visibly swell. The water is then released slowly over the following months to keep the cactus alive through the dry season.
How does a cactus do photosynthesis without losing water?
This is the cactus's cleverest trick. Most plants open tiny holes in their leaves called stomata during the day to take in carbon dioxide for photosynthesis. The problem is that water also escapes through the stomata, fast in hot dry conditions.
Cacti and a few other desert plants use a special variant called CAM photosynthesis. CAM stands for "Crassulacean Acid Metabolism", named after the Crassulaceae plant family where it was first discovered. CAM plants keep their stomata closed all day and open them only at night, when it is cooler and the air is more humid. They store the carbon dioxide they collect at night as an acid inside the plant, then use it during the day when the sun is shining. This trick lets cacti do photosynthesis with far less water loss than normal plants.
Cactus uses
People have used cacti for food, water and medicine for thousands of years. Prickly pear cactus fruit (called "tuna" in Spanish) is a popular food in Mexico and the Mediterranean. Prickly pear pads ("nopales") are eaten as a vegetable. The peyote cactus contains a hallucinogenic chemical called mescaline that has been used in Native American religious ceremonies for thousands of years. The dragon fruit sold in supermarkets comes from a climbing cactus.
Deeper dive: cactus evolution, why they are only in the Americas, and threats today
Cacti are members of the plant family Cactaceae, a relatively young family that probably evolved around 30 million years ago in South America. The original cactus ancestors were leafy plants similar to modern shrubs. Over millions of years, in increasingly dry environments, they gradually lost their leaves, developed water-storing stems, and evolved their characteristic spines (which are modified leaves and modified leaf-buds). Different lines of cacti then evolved into many different shapes: tall tree-like saguaros, ground-hugging barrel cacti, pad-shaped prickly pears, thin segmented chollas, and many others. About 1,750 species are recognised today.
Cacti are native only to the Americas, from southern Canada to Patagonia. Old-world plants that look like cacti (such as African euphorbias) are not closely related but are examples of convergent evolution: unrelated plant lines independently evolving the same features in response to similar dry environments. The only exception is one species of mistletoe-like cactus (Rhipsalis baccifera) that may have reached Africa naturally, possibly carried by migrating birds. All other cacti now growing outside the Americas (including the many prickly pears now common across the Mediterranean, Africa and Australia) were introduced by humans, often becoming invasive species.
Many cactus species are now under threat. The slow growth of large cacti like saguaros means populations recover very slowly from any damage. Cattle grazing, fires (which most cacti are not adapted to), urban expansion and especially illegal collection for the international plant trade have devastated many cactus species. About a third of all cactus species are now threatened with extinction, making cacti one of the most threatened plant groups in the world. Climate change is adding to the problem; many saguaro seedlings are failing to establish in the wild because hotter, drier conditions kill them before they can grow large enough to store enough water.
The cactus's home desert is the Sonoran Desert. Other desert wildlife includes desert animals generally.