Cacti shine brighter than ever

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, California, USA, 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. Keep away. Come closer. You got a problem with that?

“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.” Jon said he was “coerced” into studying cacti as a graduate student, but the arranged marriage took place. “Cacti are weird and attractive, and their giant, satiny flowers are stunning,” he said. “I fell in love, and I never looked back.”

For Jon 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, USA and parts of the Southwest US, or simply doing what cacti do best, which is persist in some of the world’s most parched and hostile environments for decades or longer.

How cacti adapt
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 cacti  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’ seed to new sites.

Other researchers have discovered that a cactus’ 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 cacti apply to handling relentless heat and aridity could prove all too relevant in a world of rising temperatures and water scarcity. Not that cacti 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 the risk of extinction, making cacti “among the most threatened taxonomic groups assessed to date.”

In addition to habitat loss and the conversion of cactus wilderness to agave plantations, 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, USA. “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 anthropomorphised saguaro. Humans are not alone in their cactus love. Scientists have begun decoding the complex badinage between cacti 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,” Tania said. “It has to be superprecise.” She’s seen the impaled evidence to prove it. For their part, cacti 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, Tania 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.

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

Cacti are shaped to minimise sun exposure. Rounded barrel cacti have low surface area, relative to their succulent storage capacity, while columnar cacti 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, cacti 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?” Jon said. “Anywhere you have hair. Spines and hair do the same for a cactus.”