Learning to confront our biases when it comes to plant intelligence.
“When we think of intelligence,” the philosopher Michael Marder explains via video call from his office at the University of the Basque Country, “we don’t take into account the layers that have been overlooked, neglected, and even repressed throughout the history of Western thought.” Marder, with a shock of espresso-colored hair combed behind his glasses, has the look of a dark-eyed, hipster-thin poet. “Overlooked forms of intelligence include emotional and embodied intelligence,” he continues. “The body has wisdom of its own, which it shares to a large extent with other living things, including plants. Consider our skin’s sensitivity to light. In this respect, plants are more intelligent, because they can register and differentiate many more waves of sunlight.”
I picture the living room window at my grandmother’s house, which was a lattice of leaves, a jungle in miniature through which only threads of sunlight could enter. The lacework curtain of philodendron spread from a single plant but seemed to be the product of dozens. The philodendron was not the only plant colonizing the parlor. My grandmother also made room for arrowhead vines, coleus, African violets, spider plants, and, in a terrarium in the corner, an array of succulents bright as tropical fish. As a child, I would peer through the beads of the steamy dome, filling my lungs with fresh oxygen. But were those oxygen producers smart?
I suggest to Marder that it may be an overstatement to consider plants’ receptivity to light on par with, say, calculus. “We need to confront our biases,” he responds. “When we preclude forms of intelligence that do not match our definition, we fail to see other models.”
Although Marder is not himself a scientist, he contemplates recent experiments in cellular and molecular botany because, he believes, philosophy begins where common sense ends. In his 2012 book Plant Thinking: A Philosophy of Vegetal Life, he approached question of ethics and ontology through a reflection on plant life, hoping to move metaphysics beyond the bounds of the human toward what he calls “vegetal existentiality.” Only by venturing beyond the edge of the known can we escape its limits, he says. But the cost of upending accepted knowledge can be quite high, as he learned when his 2012 New York Times op-ed piece “If Peas Can Talk, Should We Eat Them?” generated more than three hundred responses from neuroscientists, vegans, Christian fundamentalists, and other people, some of whom said they wished he would die.
Marder’s argument that plants communicate is based in part on a 2012 study conducted by Omer Falik and other ecologists at the Blaustein Institute for Desert Research in Israel. The research team subjected common pea plants, Pisum sativum, to drought conditions and learned that such plants will warn their neighbors of a potential water shortage through biochemical alerts. Both the parched and the watered plants initiated crucial coping mechanisms to deal with the drought. The “communication of stress cues between plants rooted in soil” revealed an underground network that Marder says should change the way we look at plant life.
“Intelligence is the optimization of consciousness,” the thirty-eight-year-old Russian-born philosopher says. He mentions plants’ well-known behavior of swiveling their leaves to maximize exposure to sunlight. Results of a 2017 study at the University of Bonn are even more suggestive of vegetal consciousness. The study found that a variety of plants appear to lose “both their autonomous and touch-induced movements after exposure to anesthetics” similar to those administered to humans before surgery. Although scientists continue to debate exactly how anesthetics operate on the nervous system (human or vegetal), the researchers who conducted the German study found that anesthetized Venus flytraps stopped snapping at flies until the drug wore off, after which they again became responsive.
Intelligence, however, is more than consciousness; it is also the ability to acquire and apply knowledge, to learn. Marder says that at least some plants can learn, citing a 1999 Dutch study in which lima bean plants were observed summoning insects as allies against organisms harmful to the plants. For instance, if spider mites started nibbling a lima bean, the plant could release a “volatile,” a biochemical substance that would alert mite predators to the nearby presence of prey. As if on cue, the predators would storm the area, rescuing the plant from the attack by devouring the mites. Similar tests on black mustard, wild cabbage, and various lettuces led researchers to conclude that such relationships are indicative of plant community dynamics, which are more active and interactive than we knew.
Another study of plant learning, conducted in 2014, involved Mimosa pudica, commonly known as touch-menot, whose leaves fold when touched. Researchers constructed a trapdoor-like platform with which they dropped the potted plants about two feet. At first the plants folded their leaves, assuming the fall to be threatening, but after several trials, they began to keep their leaves open. The touch-me-nots adapted quickly, and when the plants were retested, thirty days later, the lesson appeared to have stuck.
But how do we know that this plant response isn’t involuntary, like the jerking of a knee when it’s tapped by a reflex hammer? As Marder explains, “A tapped knee will always respond the same way, because a reflex is independent of any accumulated past experience. Learning, on the other hand, requires applying past experience to future situations to create the best response possible. Therefore, memory, as well as the ability to anticipate future events, is crucial to plants.”
“Plants do not form an image of a remembered object,” Marder elaborates, “but they remember stimuli. Some are short-term, lasting perhaps only a single day. Others are long-term.” This phenomenon is closer to what we call muscle memory, the kind of cellular-level response evinced by athletes and musicians. Cherry trees, for example, take various environmental factors into account, including day lengths and daily temperature changes, in order to determine the best season to blossom. “Their leaves retain the memory of the lastinfrared rays from the setting sun, and compute the differences, day after day, between the lengths of light,” he says.
Plants can…compute? “There is a very complex kind of computing taking place,” Marder says. “My colleague Paco Calvo at the University of Murcia, in Spain, is working on plant intelligence from an analytic perspective, considering the sorts of algorithms they operate with.”
The word “algorithms” conjures up Facebook programmers rearranging our social networks. To imagine that plants employ similar rules may not be so far-fetched. If humans had invented photosynthesis, converting sunlight into sugar would be considered an engineering triumph, though it is unlikely that humans would be willing to share their discovery so freely with others of their species.
“There are those who suggest that plants are altruistic,” Marder says, citing research that shows that plants form alliances not only with insects but also with other plants. Neighboring plants send out signals to call for help even when they are in no immediate danger themselves. These plant communities operate over a biochemical network that appears to be more open than our nervous system, he explains, but whether plants intentionally warn each other of dangers is “hotly debated.” As with human neighbors who call the fire department to report nearby house fires, it is hard to tell if plants are being selfinterested or selfless. Either way, Marder sees plants as “perhaps the most ethical, least violent of living beings.”
What of parasitic plants or aggressively invasive species like kudzu and bamboo? “In most cases,” Marder says, “parasitism is relatively harmless and in fact can be used as a great example for plant coexistence, co-growth, or symbiosis.” He notes that people wrongly cite examples of what appears to be violence in the vegetal world to legitimate human violence. “Humans could learn a lot from the plant model,” he insists, but he acknowledges that what they have to teach us does not translate easily. “Plants provide such a radically different model that it conjures another kind of sense altogether.”
Plants force us to encounter an aspect of the physical world that we either fail to notice or attempt to explain with certain preconceived ideas. Like the best art, plants can help us encounter our surroundings and each other without so many presumptions. Plants don’t conform to the kind of concepts that define thought as we know it, but that doesn’t mean they aren’t highly engaged with their surroundings. They can even be more responsive than we are— something Marder discovered when he studied the vegetation near the site of the Chernobyl nuclear reactor explosion.
Marder was six in 1986 when Soviet physicians sent him and his family to live on the Black Sea coast to help him deal with severe allergies; in their new home in the Russian city of Anapa they were exposed to large quantities of radioactive fallout drifting south from Chernobyl. Being radiated informs all of his writing, especially in the illustrated Chernobyl Herbarium, a “testimony” to the flourishing plant life around the abandoned Ukrainian city of Pripyat, which the Soviets had built as a company town for the reactor workers. Unlike humans and animals, plants could not seek protection from the radioactive rain that fell for nine days after the reactor explosion. Yet many plant species thrived and went on to reclaim Pripyat, forsaken by humans the day after the explosion. Marder finds that response both intelligent and beautiful. “Plants maximized their exposure [to the fallout],” he says. “To behold that gesture, and all that it represents about plants as plants, which are not reptiles or great apes or fungi but singular living beings, is to at last encounter them on their own terms.” Such an encounter, he says, can help humans “recall something about our vitality that we have repressed and forgotten, which is the absolute necessity of living together, symbiotically, with others.”
In Marder’s view, our mutual vulnerability to circumstances has the potential to inspire a sense of solidarity that, if strange enough, can transcend nation, race, religion, gender, class, and even species.
“The systemic effects of violence stem from the same root,” he says. It is the illusion that we can harm another kingdom without affecting our own. Marder believes that when we look closer, we find small commonalities between ourselves and plants that reflect larger ones. Nor are such comparisons anthropomorphic, he says, as long as we resist projecting ourselves onto plants but instead open ourselves to what plants can teach us about us.