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The beauty of the cactus

Scientists propose that a better understanding of the tricks the prickly plants apply to handling relentless heat may be relevant in a world of rising temperatures

  • A blooming yellow flower nestles among the pink spines of a barrel cactusImage Credit: NYT
  • A teddy bear cholla cactus in the Anza-Borrego Desert State ParkImage Credit: NYT
  • Cholla cacti in the Anza-Borrego Desert State Park near Borrego Springs, Calif., April 20, 2017. In a superbloImage Credit: NYT
  • A barrel cactus in the Anza-Borrego Desert State Park near Borrego Springs, CaliforniaImage Credit: NYT
Gulf News

A cactus in bloom is pure poetry — particularly that famous line by Walt Whitman: “Do I contradict myself? Very well, then I contradict myself.”

In the desert at Anza Borrego in California, the thick, spine-studded paddles of a beavertail cactus look as surly as always, ready to smack you into next week if you get within striking distance.

Yet now, in a superbloom spring that many judge the best in decades, the paddles are topped by dazzling fuchsia flowers the size of teacups, which beckon you closer to feast on the view.

The fish hook cactus lives up to its name, its surface covered with long, curved barbs and a snarl of fibrous hairs; but now it wears a festive garland of creamy white petals smartly trimmed in rouge.

“If somebody had taken me from rural Illinois, where I grew up, and dropped me here into this desert landscape to see all these fat succulent things,” said Jon P. Rebman, chief botanist at the San Diego Natural History Museum and a cactus taxonomist, “I would have thought I was on Mars.”

Rebman, 52, who is tall, fit, demonically ebullient, and has deep dimples on either side of his face, said he was “coerced” into studying cactuses as a graduate student, but the arranged marriage took. “Cacti are weird and attractive, and their giant, satiny flowers are stunning,” he said. “I fell in love, and I never looked back.”

For Rebman and other researchers who study the cactus family, Cactaceae, the 20-grit charm and mulish creativity of their subjects are always compelling, whether the plants are flowering wildly in response to rain after a sustained drought, as happened this year in California and parts of the Southwest, or simply doing what cactuses do best, which is persist in some of the world’s most parched and hostile environments for decades or longer.

“In some of the dry valleys of Mexico, they have giant columnar cacti that are hundreds and hundreds of years old,” said Erika Edwards, an associate professor of ecology and evolutionary biology at Brown University who studies photosynthesis in cactuses and other succulents.

While the basic contours of the cactus survival plan have been known for some time, researchers are still unearthing surprising details about how the plants adapt to adversity, and how they subtly manipulate the niches they inhabit and the other creatures they encounter to suit their defence and propagatory needs.

Recently, for instance, scientists have found that as many as 100 species of cactuses are essentially breasts for ants, exuding through tiny nipples in their flesh a minute but irresistible supply of sweet nectar that persuades the insects to nest at the cactal base.

The besotted ants in turn defend their green udder against potentially destructive insect predators; clean away pathogenic fungi and bacteria; fertilise the soil with their nitrogenous waste; and spread the cactus’s seed to new sites.

Other researchers have discovered that a cactus’s roots can operate like sensitive fingers, able to detect when the soil surface has grown dangerously hot and then contracting to yank the entire plant into a lower, slightly cooler position before it’s too late.

Scientists propose that a better understanding of the tricks cactuses apply to handling relentless heat and aridity could prove all too relevant in a world of rising temperatures and water scarcity.

Not that cactuses are immune to the effects of human avidity. In late 2015, an international group of researchers reported that nearly one-third of cactus species were at risk of extinction, making cactuses “among the most threatened taxonomic groups assessed to date.”

In addition to habitat loss and the conversion of cactus wilderness to agave plantations (to slake the rising demand for mezcal and tequila), the authors and other biologists cited excessive human affection as a driver of these extinctions.

“People can be fanatic about cacti,” said Gretchen North, a professor of biology at Occidental College. “Cactus rustling and illegal cactus collecting are real problems and a big business, and that’s one of the major causes of endangerment,” especially to rare species and lovable giants like the readily anthropomorphized saguaro.

Humans are not alone in their cactus love. Scientists have begun decoding the complex badinage between cactuses and pollinating bats.

Reporting recently in the journal PLOS One, Tania P. Gonzalez-Terrazas of the University of Ulm in Germany and her colleagues showed that, whereas most echolocating bats use sonar to hunt moving targets like insects, the neotropical nectar feeding bat, Leptonycteris yerbabuenae, livestreams a volley of high-frequency clicks and cries as it approaches a flowering columnar cactus.

The bat’s goal: to pinpoint the exact spot on each tubular flower where it can insert its snout, lap up the pollen-salted nectar inside and then back off again. Sure, the flower may be stationary, but a mistaken approach, a random flit to the side, could prove fatal.

“The bat is flying in the middle of a windy desert, at night, and it’s feeding from plants with really big spines,” Gonzalez-Terrazas said. “It has to be superprecise.” She’s seen the impaled evidence to prove it.

For their part, cactuses like Pachycereus pringlei, the Mexican giant cactus, have adapted their blooms to suit their pollinators’ GPS. Its flowers are exceptionally hard and waxy, the better to bounce a bat’s call back to its ears, Gonzalez-Terrazas said.

The symmetrical arrangement of the petals makes it relatively easy for a bat to calculate the midpoint of an echo, and hence to find the floral opening.

Behind the success of the cactus family is its prodigious dry wit, its talent for maximising water uptake and minimising water loss.

Cactuses are succulents, which means their tissues are fleshy and designed to hold moisture, an essential trait for surviving in a place like the Atacama Desert of Chile, where annual rainfall averages half an inch.

Cactus roots spread wide and shallow, rather than deep, and are equipped with specialised nodules. On first exposure to moisture after a dry spell, the nodules quickly sprout a network of pale, spidery rain roots, allowing a cactus to suck up every possible droplet from a light desert sprinkle. When the showers are through, the rain roots are jettisoned but the nodules remain, poised to sprout anew.

Cactuses are shaped to minimise sun exposure. Rounded barrel cactuses have low surface area, relative to their succulent storage capacity, while columnar cactuses or prickly pears expose only their thin edges at the tops or sides to direct sunlight.

Because photosynthetic leaves are a serious source of water loss in most plants, cactuses have transferred their sugar production services to their bodies, and in many cases have transformed their leaves into spines.

Those spines serve assorted tasks, depending on species. But in general they are less about defence against desert animals, as is commonly believed, and more about water management. A mat of spines and hairs holds in moisture and slows the movement of evaporative air across the cactus surface.

“If you get out of your shower and run naked through your yard, what’s the last part of your body that dries?” Rebman said. “Anywhere you have hair. Spines and hair do the same for a cactus.”

Spines can also spear water droplets from fog and shunt them down to the cactus’s roots. With the aid of its insidious claws, a detachable segment of the aptly named jumping cholla can latch onto unsuspecting passers-by, with the hope of being delivered to fresh soil in which to take root.

Cactus ribs likewise play multiple roles, allowing the plant to expand in wet times and contract in dry periods, accordion-style, helping to trap humidity between the pleats.

Yet even a plumped-up cactus is not like a cartoon keg that can be tapped for free-flowing water; the water is absorbed into gluey tissue called mucilage that in most cases is not safe to eat.

Some traditional cultures, however, cook parts of the prickly pear cactus into nopales. Others tolerate the nausea and vomiting that comes from eating the dried crowns, or “buttons”, of the peyote cactus for the sake of experiencing the hallucinatory effects of its signature ingredient, mescaline.

Another key to cactus hydro-thrift is a willingness to work the graveyard shift.

Most plants photosynthesise in the daytime, opening pores in their leaves to allow carbon dioxide gas to diffuse in, and then using the ambient solar energy to stitch the harvested carbon and water into sugary fuel.

Opening a plant’s stomata in sunlight, however, means a lot of stored water ends up evaporating — which may be fine for the average temperate-zone bush, but not for a desert dweller.

So the cactus has evolved a multistage approach to photosynthesis. It waits until after dark to widen the stomata on its body and absorb carbon dioxide, stashing the gas as an acid until the next morning. At that point, the radiance of the sun can be exploited for sugar-making while the succulent’s stomata stay safely shut.

Some researchers are trying to engineer this nocturnal talent into standard crops, in order to allow cultivation of marginal lands using a fraction of the water currently devoted to agriculture.

“It’s an energetically more expensive form of photosynthesis,” Edwards said. “But it’s genius.” One could even say Whitmanesque.

–New York Times News Service

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