Plants That Are Predators

As a refined Victorian gentleman, Charles Darwin naturally gravitated toward the macabre, and few things fascinated him like those floral flouters of the conventional food chain: carnivorous plants. He experimented with them and wrote a major treatise about them. He called the Venus flytrap, with its elaborate hair-trigger snap trap and its lethal brew of digestive juices, “one of the most wonderful plants in the world.” He compared the glistening and gothically tentacled sundew plant, or Drosera, to a “most sagacious animal” and said, “I will stick up for Drosera to the day of my death.” To which a sagacious sundew might well have replied, Thanks, but I’ll take a damselfly instead. As a bounty of new research reveals, biologists are still sticking up for carnivorous plants, and still unearthing surprising details about the anatomy, evolution, biochemistry and hunting tactics of what Rainer Hedrich of the University of Würzburg calls “the green flesh-eaters.” One group lately has determined that a pitcher plant in Borneo supplements its insectivorous diet with regular helpings of bat guano, attracting the bats to roost — and void — in its slender goblet of a modified leaf by tuning its shape to precisely match the bats’ echolocating calls. Another team has nearly decoded the complete DNA sequence of the Venus flytrap — which is virtually the same size as the human genome — and has seen hints that, at some point in its evolutionary history, the plant may have imported from its insect prey nerve-related genes that in turn allowed the plant’s trapping mechanism to shut faster. Other researchers have compared proteins and hormones found in the digestive fluids of carnivorous plants with similar molecules active in noncarnivorous plants and concluded that a good offence was born of a good defence. Carnivorous plants, the researchers argue, gained the power to pulverise and absorb their insect prey by repurposing the defensive chemicals that ordinary plants use to deter herbivorous insects, effectively pounding shields into swords. Or maybe selfie sticks. Paulo Minatel Gonella of the University of São Paulo in Brazil and his colleagues recently reported in the journal Phytotaxa that they had identified a spectacular new species of sundew with the help of social media. After seeing photographs of the plant posted by an amateur naturalist on Facebook, the researchers travelled to the specified location, on a lone mountain in southeastern Brazil, and confirmed the sundew was new to science.


With stems reaching five feet long, Drosera magnifica practically qualifies for a turn on “Little Shop of Horrors” and is the largest sundew species in the Americas. With the bulk of its rosy, sticky tentacles enfolding trapped prey, the sundew stalks resemble nothing so much as giant insect kebabs. Yet while D. magnifica’s image has gone viral, researchers warn the real thing may soon go extinct, its habitat threatened by coffee and eucalyptus plantations. Researchers see in carnivorous plants a model for exploring a range of important questions, including how organisms adapt to extreme adversity and scarcity, and how sessile beings with neither muscles nor brains can outmaneuver mobile beings with both. Carnivorous plants may yield practical spinoffs, too. Dr. Hedrich pointed out that a number of enzymes in carnivorous plants remained exceptionally stable under conditions of high heat and blistering acidity that demolished most garden-variety enzymes. “Industry could learn a lot about how to make enzymes more tolerant to extreme conditions simply by studying the Venus flytrap,” he said. The kinkiness of their meat eating aside, the 590 or so known species of carnivorous plants are all legitimate, chlorophyll-carrying members of the kingdom Plantae. They photosynthesise, as other plants do, stitching together sugars from water, carbon dioxide and sunlight. Yet plants also need nutrients like nitrogen, phosphate and sulfur, which most species absorb from the ground. Carnivorous types, however, colonise marginal habitats with impoverished soil and must acquire their nutrients from alternative sources. Animal flesh happens to be a handy one. “In environments that are sunny and moist but nutrient-poor, the capture of prey can give plants a real competitive advantage,” said Thomas Givnish, a professor of botany at the University of Wisconsin-Madison. Carnivorous plants thrive in open bogs; in damp, fire-swept sand; by roadside puddles; in the leached mud of a mountainside — bright, sodden spots where competitors are negligible, the insects gullible, and nutrients alone limit plant growth. Through DNA analysis, researchers recently determined that carnivory has arisen in plants at least nine times, the oldest lineage dating back some 72 million years. Participants have evolved a battery of techniques for trapping and digesting their quarry, which generally means insects and other arthropods but can include frogs, fish and even small mammals. Some, like the pitcher plant, take the pitfall approach, impounding rainwater at the base of a brightly coloured cavity, adding a touch of tempting nectar and digestive enzymes to the mix, maybe lubricating the lip and sides of the tank with wax or dust. “The insects slide off, and it’s down into the abyss,” Dr. Givnish said. Others exude sticky droplets, or operate like a lobster trap, with spiralling grooves and pointing hairs that usher the prey through a nested set of ever-narrowing barriers and into the digestive centre. Perhaps the most impressive snare belongs to the rightfully legendary Venus flytrap, which can still be found in a scattering of swamps in the Southeast. Dr. Hedrich, a biophysicist who studies the plant as a kind of “serious hobby,” and his colleagues have determined that the trap — a highly modified leaf — relies on an action potential, or electrical pulse, to snap shut, a rare power in the floral community.


When a Venus flytrap is hungry, its two-lobed trap, now flushed with attractive red pigment, opens and exposes the plant’s sensory hairs. Should an insect land and jostle a hair, Dr. Hedrich said, “That triggers the first action potential.” And if, in the next 30 seconds, the luckless visitor touches another hair, wham, the trap snaps shut in a 10th of a second — three times faster than the blink of an eye. Naturally, the insect struggles to escape, Dr. Hedrich said, “but this is just more bad news.” The desperate motions provoke more action potentials, stimulating the plant to flood its trap with hydrochloric acid, pepsin- and trypsin-class enzymes, and chitinase to pierce through the insect’s exoskeleton and liquefy its meat. The green mouth becomes the green stomach becomes the green intestine, Dr. Hedrich said, and within seven to 10 days, prey and predator are as one. Carnivorous plants are not always up for the kill. By the looks of it, Nepenthes hemsleyana, a pitcher plant in the peat swamp forests of Borneo, is gradually shrugging off meat eating in favour of the nitrogen-rich offerings of an insectivorous bat. As reported in the July issue of Current Biology, Michael Schöner of the University of Greifswald in Germany and his colleagues found that the Hardwicke’s woolly bat and the pitcher have entered a mutually satisfactory partnership. The plant offers the bat a perfect place to roost, and the bat fertilises the plant with its faeces. Weighing barely more than a penny, the bat fits in the pitcher “like a cork in a bottle,” said Mr. Schöner, a doctoral student. The pitcher plant advertises its accommodations through a special concave structure along its orifice that reflects the bat’s sonar signal from many angles, making the roost easy to find; and it keeps its pool of digestive juices well below the space where the bat cocoons itself. Bats love their pitchers, and not just for sleeping. “We’ve found bats copulating inside,” Mr. Schöner said. “Also, mothers that have given birth to their babies.” Nepenthes: a plant to stick up for, from cradle to commode.

New York Times, 14 September 2015 ; ;