What Plants Can Teach Us About Intelligence
Philosopher Michael Marder has a boyish frame with a shock of espresso-colored hair combed neatly behind his glasses. He has the look of an album cover drawing of a poet, dark-eyed and hipster-thin, buttoned into a deceptively conservative oxford. Jimmy Kimmel would have no trouble picking him out from a random sample of pedestrians as an academic or author, perhaps not as easily as a father, but his first child has only begun to toddle. His dress suggests nothing of his radical thoughts, but neither did Copernicus sport a mohawk when he upset his contemporaries’ belief in an Earth-centered solar system.
Marder is not a scientist like the Renaissance astronomer who replaced Ptolemy’s model, but he recognizes the potential for the scientific discovery of plant intelligence to force a shift in human perception as dramatic as the Copernican Revolution. He has taken on the role of puzzling out the ramifications of recent research in cellular and molecular botany because according to him philosophy begins where common sense ends, limited as our comfort level is to accepted knowledge rather than that which is only beginning to be known.
He grew up on the edge of a national forest in Moscow and writes toward the skyline of what we are coming to understand, sometimes at high emotional cost. His New York Times blog post, “If Peas Can Talk, Should We Eat Them?” for instance, generated hundreds of responses from a range of people, including neuroscientists, vegans, and Christian fundamentalists, some of whom wished he would die. For all the upset he created, we might recall that Copernicus did not publish De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) until just before his death, when it was banned as heretical. If admired in hindsight, revolutionary thinking often meets first with hostility. Marder’s tolerance for fallout from incendiary ideas is perhaps higher than most, though, because he survived Chernobyl.
In “If Peas Can Talk, Should We Eat Them?” he questions the need for human ethics to accommodate recent revelations that plants can process, remember, and communicate information. Specifically, he references a study conducted by Omer Falik and a team of ecologists at the Blaustein Institute for Desert Research. They learned that Pisum sativum, or common pea plants, subjected to drought conditions will warn their neighbors of the potential danger. The stressed plants emitted a biochemical alert through their roots that prompted nearby plants not experiencing the same water shortage to react as if they were. Both groups initiated crucial coping mechanisms, including decreased water intake, reduced flowering, and hormone synthesis to enhance drought-resistance. Not nearly as passive as they appear to us, these peas exposed an underground network that Marder says should change the way we look at plant life.
Readers who wanted to bury him were not convinced. After all, acknowledging plant intelligence crushes paradigms. To view the plants we eat, as well as those we wear or chop or burn, as conscious is as disruptive to the sensibilities as learning the ground beneath your feet is circling the sun rather than the other way around. Marder simplified the question to whether or not we should eat plants we now know are capable of communication, but the answer, and even the question, is far more complex and illuminating.
“When we think of intelligence,” he says in my sunroom via video call from his office at the University of Basque Country, “we don’t take into account the layers that have been overlooked, neglected, and even repressed throughout the history of at least Western thought.” These forms include emotional intelligence and that of the body itself. “The body has wisdom of its own,” he says, “which it shares to a large extent with other living things, including plants.” He gives as an example our skin’s sensitivity to light, which we share with plants who “in this respect, are more intelligent, because they can register and differentiate many more waves of sunlight.”
I suggest it may be an overstatement to refer to such plant capacities as intelligence. “We need to confront our biases,” he tells me. “When we preclude forms of intelligence that do not match our definition, we fail to see other models.”
I point to the first entry in my dictionary, which defines intelligence as “the ability to acquire and apply knowledge.” He assures me that many plants can in fact learn. A study led by Dutch ecologist Marcel Dicke revealed that lima bean plants determine which insects in their ecosystems can serve as allies. If spider mites, for instance, are nibbling its leaves or beans, the plant can release a bio-chemical into the air that cues mite predators that prey are nearby. Predators then storm the area like hungry teenagers, in effect rescuing it from the attack.
Lima beans are not alone in forming alliances in their ecosystems. Tests on black mustard, wild cabbage, and various lettuces led Dicke to conclude that such relationships are in fact indicative of plant community dynamics, which are more active and interactive than we realized.
Marder also cites research by Australian scientist, Monica Gagliano. She and her team conducted experiments on Mimosa pudica, commonly known as the touch-me-not because if you touch its leaves they close. For the experiment, they constructed a platform that dropped the plants in their pots for a controlled depth of sixty centimeters. At first the plants shuttered their leaves, assuming the fall to be threatening, but after several trials they learned their defensive behavior was not needed. Marder explains, “We have to remember that any sort of behavior by plants, animals, or humans, involves an expenditure of energy. If it isn’t necessary, it’s better to conserve that energy for future use.” In this case, learning helped the touch-me-nots adjust to the unexpected conditions as quickly as children learn that a stomach sinking from a swing set is not dangerous. The lesson seems to have stuck, too, since thirty days later the retested plants left their leaves open, unlike newly subjected plants.
I wondered aloud if their response was involuntary, like a knee tapped by a reflex hammer. Marder said, “What is in question here is memory. A tapped knee will always respond the same way, because a reflex is independent of any accumulated past experience.” In practice, he explains, learning requires “projecting past knowledge into the future in order to create the best response possible. Therefore, memory, as well as the ability to anticipate future events, is crucial to plants.”
“Plants have memories?” I ask.
“We think of memory as something our brains retain,” he says, “but even in humans memories exist at various levels.” The cells of our bodies remember physical impacts, he points out, and muscle memory helps athletes and musicians perfect their skills. “Plants do not form an image of a remembered object, but they remember stimuli. Some are short-term, lasting perhaps only a single day. Others are long-term,” he says, giving the example of a cherry tree. In order to determine the best season to blossom, cherry trees take into account various environmental factors, including day lengths and daily temperature changes. “Leaves retain the memory of the last infrared rays from the setting sun, and compute the differences, day after day, between the lengths of light,” he says.
I notice his use of the word computing. “There is a very complex kind of computing taking place,” he affirms, “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.”
His mention of algorithms reminds me of my preference for literary analysis, and he admits that popular culture celebrates different forms of human intelligence. Visual thinkers such as Temple Grandin raised awareness of non-linguistic thinking, and educators see the benefits of addressing diverse learning styles in and out of the classroom. Embodied intelligence in particular, he says, “dovetails with the intelligence of plants, sentient and plastic as they are in response to ever changing environmental conditions.” But the difference between human variants and plant intelligence is marked, he says, because “plant intelligence is a radically different model, one that makes us question the very notion of intelligence as human-centered.”
In fact, plants illustrate that no center holds when it comes to intelligence, any more than when we look beyond our galaxy to see the Sun is not the turning point of the Universe but a star among stars with multiple points of reference.
For starters, plants are not individual. Part of a plant can be removed to grow separately, as when a jade leaf falls to the soil to root into another plant, which can again be divided. Similarly, an Aspen grove that looks on the surface to be composed of a number of independent trees is conjoined below ground by a single genetically identical root structure. “It is very difficult to say what is individual and what is collective at that level,” Marder says, “and, by extension, whether intelligence belongs to the individual or to the collective.” Rather than finding this difficulty a drawback, his eyes shine with excitement.
I consider the medium we are using to have this conversation. His image streams into the room like sunlight through a web of connections I have consulted to answer countless practical and esoteric queries. This social network helps me at least begin to imagine intelligence that is neither entirely individual nor collective, but I have to ask, “Is it possible to fully comprehend a mode of thinking utterly alien to our own?”
He says we learn a lot by trying. The effort to understand plant intelligence has led him, for instance, to reexamine why we define intelligence the way we do. Categories of abstract thought and logic that privilege specific human abilities rose through thinkers like Aristotle, Plato, Avicenna, and others he traces in Through Vegetal Being: Two Philosophical Perspectives, which he coauthored with French philosopher Luce Irigaray. Inevitably, circumstances shift and with them the emphasis on specific skills, but Marder isn’t interested in just reprioritizing or revising these categories. Instead he invokes the plant model to conjure another kind of sense altogether.
We fail daily to encounter so much of our world, he says, because we hold an arsenal of preconceived ideas in front of us almost like shields. We shortchange the present when we view it through the veil of the past. When we look at a table, he explains, or a loved one, and fit them into a conceptual box labeled “Table” or “Joanne” with a check mark next to it, we deprive them and the moment of our attention to their uniqueness. He calls that tendency “pre-thinking that stops thinking in its tracks.”
I ask him how memory factors in, since it serves us as well as plants to remember. He says, “Of course we cannot (and should not) do away with past experience. Otherwise the world would become chaotic and unpredictable.” Still, he suggests we try on occasion to encounter objects and each other without preconceived notions. The best art can help us do that, he says. When a sculpture or book escapes our expectations, it enters our perception as “qualitatively new, and the same applies to thinking.” I think of the first cyclamen I ever saw, its petals shuttlecocked from its pistil so that it appeared to have shot its own bloom backward. It startled me into looking at it far longer and with greater curiosity than I would have had it met my expectations. How counterintuitive but promising it seems to offer that kind of interest to a spouse or sibling or friend who has met them.
Plants don’t form these concepts that intercept our experience and have defined thought as we know it, which is why Marder says some compare an unconscious person to being in a “vegetative state” when in truth plants are highly alert and engaged with their surroundings in ways we are only beginning to fathom. Just how engaged they are became clear to him after he was subjected alongside them to nuclear radiation.
He was six years old when the Chernobyl nuclear plant accident occurred in what was then the Ukrainian Soviet Socialist Republic of the Soviet Union. More than thirty years later we can pore over dramatic aerial footage of escaping lava, but the fallout was largely invisible to those on the ground as its spreading radius impacted distant towns and cities. Marder says the experience was “profound, perhaps one of the most profound of my life,” even as he couldn’t entirely register the experience. His youth was part of his inability to process it, he says, but even adults could not fully grasp the situation due to political circumstances in the Soviet Union. Plus, radiation is not perceptible to our senses, he adds, except when it stops them altogether.
Unlike humans and animals, plants could not seek shelter or other protection from the radioactive rain that fell from the plumes the burst reactor lofted for nine days. Yet many plant species thrived and reclaimed the city of Pripyat, which was evacuated by humans the day after the disaster. Marder finds their response both intelligent and beautiful, as he describes in Chernobyl Herbarium, illustrated by Anaïs Tondeur. “They maximized their exposure,” he says with wonder that plants reacted the exact opposite way that humans and animals do. “To behold that gesture,” he says, and all that it represents about plants as plants, which are not reptiles or great apes or fungi but singular living beings, he says, is to at last encounter them on their own terms. Those who meet them there “can perhaps recall something about our vitality that we have repressed and forgotten, which is the absolute necessity of living together, symbiotically, with others.”
Plant terms are defined by plant experiences and desires borne of them, as they are with all intelligent life forms, he says. In their case in particular, being rooted and relatively immobile has forced plants to adapt inventive means to address environmental threats that birds, fish, and mammals, etc. can flee.
I suggest that climate change puts humanity in a similar predicament, since we are bound for the foreseeable future to a planet that is already forcing adaptation across species. He agrees and says that recognizing our mutual vulnerability to such circumstances has the potential to reach further than any national or international movement yet. It may even generate a sense of solidarity that transcends differences of class, race, religion, gender, nation, and species. In lieu of death as the great equalizer, he proposes radiation exposure, or any other threat far-reaching enough to embody the force that binds us to one common, mortal coil. “The systemic effects of violence stem from the same root,” he says.
His use of the word “root” reminds me that Darwin proposed in 1880, in The Power of Movement in Plants, that the tip of a plant’s root “acts like the brain of one of the lower animals.” I ask if there is any currency to his theory.
“Yes,” he says, “and it speaks to how far ahead of his time Darwin was even in that respect, since contemporary scientists have proven him right.” He cites research by František Baluška, which has shown that the root tips are highly intentional in navigating the soil to find moisture and mineral nutrients. “Plant roots do not grow haphazardly,” Marder says. Although we cannot watch the roots wend toward the richest patches of soil as we can see leaves turning to face the sun, Baluška’s research shows that they do. Roots also moderate their growth depending on whether neighboring roots belong to their own or a rival species. If they detect competing species he says, “they go all out to capture resources,” whereas they are more likely to limit their growth when the species is the same.
He adds that Baluška also found that plant root tips have some cells that resemble the neurons in humans and animals. Since neurons are the basic working units of the brain, transmitting signals from nerve cells, muscles, and glands that plants do not have, I ask him how these cells function without a central nervous system.
He admits that he may not be the best person to ask, since the question is “hotly debated” among botanists. Plants’ method for processing environmental information appears to be more open than our nervous system though, since neighboring plants can “eavesdrop” on each other’s signals, as when the lima bean plants emitted airborne chemicals to summon mite predators to them. Whether plants are intentionally warning their community of dangers is one of the topics of contention, he says, since “there are those who suggest plants are altruistic.”
In Through Vegetal Being, as well as several of his other books, Marder suggests that humans could learn from the plant model, because he calls them “perhaps the most ethical, least violent of living beings.”
Admittedly, I find this idea easier to accept with regard to stately magnolias than strangler figs, a non-carnivorous plant that squeezes to death the host tree it climbs, or dodder, another parasitic plant that entangles its host in knots of orange spaghetti vines. In most cases though, he argues, “parasitism is relatively harmless and in fact can be used as a great example for plant coexistence, co-growth, or symbiosis.” Even in cases such as the fig, which reverts to an either/or, Marder says he “does not detect the least aggression.” Rather he finds that people invoke these examples of what appears to be violence in the vegetal world to justify or naturalize human violence. Unlike these plant outliers, he says, our own species depletes the planet without contributing to the balance of life-sustaining elements. “What is most disturbing,” he adds, “is that we do so knowingly, and then we enlist the examples of select plant species to show that there are ‘natural’ precedents for this conduct.” In fact, if we were to truly emulate their model, he says, we would reclaim an underlying sense of connectedness that would correct current destructive environmental practices.
The closer he says that we look at the material and genetic realms, the more small commonalities we find between ourselves and plants that reflect larger ones. We might scoff at such attempts at comparison as anthropomorphic, but he knows we do best not by projecting our assumptions onto plants but by opening our receptors to what they might teach us. As our evolving astronomical models demonstrate, neither the Milky Way nor the Universe can be reduced to what orbits us, but a new lodestar can periodically help us look beyond our own lights toward a larger framework in which we are one of many incomparable but interrelated systems.
 Falik, Omer, et al. “Plant responsiveness to root–root communication of stress cues.” Annals of Botany, vol. 110, 2012, pp. 271-280, www.aob.oxfordjournals.org. Accessed 3 March 2017.
 Dicke, Marcel, et al. “Jasmonic acid and herbivory differentially induce carnivore-attracting plant volatiles in lima bean plants.” Journal of Chemical Ecology, vol. 25, issue 8, 1 August 1999, pp. 1907-1922.
 Gagliano, Monica, et al. “Experience teaches plants to learn faster and forget slower in environments where it matters.” Ocelogia, vol. 175, issue 1, 2014, pp. 63-72, link.springer.com/journal/442. Accessed 7 February 2017.