How Cacti Survive: Surprising Strategies Quench Thirst

Corey Binns

Date: 23 July 2006 Time: 08:10 PM ET

Cacti have can be found in rain forests and as far north as Canada. But it is their ability to thrive in the desert, where rain falls infrequently and unpredictably, that is their most remarkable trait.

How do they do it?

By working nights, using alternative methods to generate energy and keeping some prickly tricks up their arms.

"The cacti evolved a whole suite of adaptations to survive living in the desert," said plant evolutionary biologist Erika Edwards.

Standing Tall

Few cacti are as recognizable as the saguaro, or Carnegiea gigantea. They grow only grow in the Sonoran Desert, however, standing tall in parts of southern Arizona, northern Mexico and a tiny patch of southeastern California.

They grow only about an inch in their first eight years, typically under a tree for protection. Branches do not sprout until they're at least a half-century old. After 125 years, a saguaro can be 50 feet tall and weigh 6 tons. A few live beyond 200 years.

The pleated ribs of a saguaro expand like an accordion when water is available and contract during dry periods.

In June, a mature saguaro produces a crop of flowers that look like bonnets atop the main stems and arms. Each flower lasts less than 24 hours and must be pollinated. Each flower can produce 2,000 seeds, spread by the defecation of coyotes or birds.

LiveScience
Source and images: Saguaro National Park

Edwards and Michael Donoghue of Yale University recently determined that the Pereskia genus of leafy shrubs and trees were the first plants to exhibit some of these water-saving traits, about 20 million years ago.

The findings were published in the June issue of the journal American Naturalist.

Risky business

All plants have stomata, little pores in their skin that open and close to collect carbon dioxide. During photosynthesis, plants turn the collected carbon dioxide into food in the form of sugars. The process is troublesome in the desert because water escapes from the pores each time they open.

"If you're trying to conserve water, it's risky business to open the pores and lose water," Edwards told LiveScience.

While most plants open up their stomata during the day, cacti and other nocturnal plants such as the agaves and aloes open their pores at night.

The cooler temperatures, lack of sun, and calmer breezes help cacti retain water.
But in the dark, cacti can't use the sun's energy needed to make sugars out of carbon dioxide, so the hardy plants must store the carbon dioxide for the next day.

Once the sun rises, the plant goes to work making sugars.

Other useful traits

Cacti have also developed succulent tissue, waxy skin, prickly spines, and a specialized root system to take every advantage in their harsh ecosystems.

The stem acts as a reservoir; the plant will expand and contract depending on the amount of water it holds.
The skin's waxy coating helps retain moisture.
The pointy spines protect against thirsty animals looking for a free drink.

In some cacti, spines also collect rainwater and funnel precious drops to the plant's roots.

Surprising tactic

You might think cacti would grow deep roots to search for a constant supply of groundwater. Instead, they often develop extensive, shallow root systems that sit just under the surface of the Earth and can extend several feet away from the plant, ready to absorb as much water as possible.

When it rains, cacti shoot out more roots. During dry periods, roots will shrivel up and break off to conserve the plant's water supply.

"The cactus becomes more hydrated than the soil it's growing in," Edwards said. "It runs the risk of losing water to the soil, so it has to disconnect itself from the soil."

Leafy cacti, such as the Pereskia, and other plants have developed similar water-conserving traits and make their home in the desert, even without the anatomical specializations of the familiar leafless cacti.

"It's good evidence that it's a successful strategy," Edwards said. "The plants do really well in these environments."

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