During the nineteen-seventies and eighties, a researcher at the University of Washington started noticing something strange in the college’s experimental forest. For years, a blight of caterpillars had been munching the trees to death. Then, suddenly, the caterpillars themselves started dying off. The forest was able to recover. But what had happened to the caterpillars? The researcher, David Rhoades, who had a background in chemistry and zoology, found that the trees in the forest had changed the chemistry of their leaves, to the detriment of the caterpillars. Even more surprising, trees that had been nibbled by caterpillars weren’t the only ones that had changed their chemistry. Some were changing their leaves before caterpillars reached them, as if they’d received a warning. A shocking possibility presented itself: the trees were signalling to one another.
Zoë Schlanger recounts Rhoades’s story in her new book, “The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth.” In a research paper that Rhoades published on his findings in the Journal of the American Chemical Society’s series “Plant Resistance to Insects,” he pointed out that the trees were too far apart to be communicating through their roots. This suggested a possibility so novel that Rhoades couldn’t resist an exclamation point in his otherwise cautious positing—the trees seemed to be using “airborne pheromonal substances!” That paper, Schlanger writes, “would change everything, and in a cruel twist, it would end his career. Because, back then, no one believed him.”
The contemporary world of botany that Schlanger explores in “The Light Eaters” is still divided over the matter of how plants sense the world and whether they can be said to communicate. But, in the past twenty years, the idea that plants communicate has gained broader acceptance. Research in recent decades has shown garden-variety lima beans protecting themselves by synthesizing and releasing chemicals to summon the predators of the insects that eat them; lab-grown pea shoots navigating mazes and responding to the sound of running water; and a chameleonic vine in the jungles of Chile mimicking the shape and color of nearby plants by a mechanism that’s not yet understood.
Schlanger acknowledges that some of the research yields as many questions as answers. It’s not clear how the vine gathers information about surrounding plants to perform its mimicry, or what exactly that ability says about plants’ ability to sense the world around them. And not all of the research is equally sound—the pea-shoot study, for example, performed in 2016 by the ecologist Monica Gagliano, who has written about communicating with plants while taking ayahuasca, is particularly controversial, and a replication effort was not successful. But an increasing number of scientists has begun to ask the question that animates her book: Are plants intelligent?
Schlanger’s own introduction to the notion that plants might be able to exhibit behavior at all came when she learned that male ferns release sperm that swim in rainwater; emerging research indicated that they emit a hormone to sabotage other nearby fern sperm in order to outcompete them. It’s already clear that plants have amazing biological capabilities: they respond to light, and react in sophisticated ways to seasonal changes, waiting for the right combination of warmth and water to grow or flower. Plants have also been shown to respond to sound—the beach evening primrose, a small yellow flower, makes its nectar sweeter when played a recording of a flying honeybee. A recurrent theme of Schlanger’s book is the challenge of categorizing such abilities in comparison with our own. Primroses may respond to sound—but that doesn’t mean that they “hear” the way that we do. As Schlanger writes, they have a version of “earless” hearing: “Sound, to them, is pure vibration.”
A pattern emerges in her survey of the field. The scientists Schlanger visits show her the plant versions of the building blocks of intelligence: sensation, communication, decision-making. Rhoades wasn’t able to convince the field that the trees were sending one another caterpillar warnings, but, not long after his paper, another study—this one in the controlled confines of a lab—had found that maple saplings changed their leaf composition when neighboring trees’ leaves were torn. In a lab in Wisconsin, Schlanger pinches the leaf of an Arabidopsis plant, and sees its veins light up under a microscope in “a bioluminescent ripple” of a wave of sensation. It seems to Schlanger that, by pinching the lab plant, she will cause it pain, and be partaking in “a vegetal version of the Milgram shock experiment.” One of the scientists running the experiment, Simon Gilroy, offers a nuanced description of the system; the ripple looks similar to an animal’s nervous system but is actually a set of “conduits of cells that could allow propagation of an electrical change that a plant uses for information.” Plants don’t have brains; they don’t have neurons. But they might have structures that function similarly.
In the field of botany, there’s a long shadow associated with questions of plant intelligence, cast by a book called “The Secret Life of Plants,” written by the writers Peter Tompkins and Christopher Bird. Published in 1973, the book contained a “mix of real science, flimsy experiments, and unscientific projection,” Schlanger writes. The authors claimed that plants could feel and hear, that they preferred classical music to rock, that they could—according to a polygraph test that a former C.I.A. agent administered to a houseplant—have a kind of memory. The book made it to the New York Times best-seller list and was turned into a documentary with a soundtrack by Stevie Wonder. It also made a laughingstock of the idea of plant intelligence, and for a long time it seriously hindered the possibility of serious study. “The twin gatekeepers of science funding boards and peer review boards—always conservative institutions—closed the doors,” Schlanger writes.
Slowly, researchers approached these questions again. In 2006, a group of plant scientists published a provocative article arguing that the field had been censoring itself, failing to ask the questions about the possible parallels between neurobiology and phytobiology. They wrote about the many “choices” that plants seemed to make, and about the emergent understanding of signalling. Plants can carry out signalling within their own bodies, producing electrical impulses that send information to their stems and leaves, even using two of the same neurotransmitters—glycine and glutamate—involved in animal brains. They also send information to one another, exchanging signals through chemicals that they emit in the air and underground, where long strands of fungi act as telephone wires between systems. The authors said it was time to “study plants as behavioral organisms with a capacity to receive, store, share, process and use information... and integrate this information into responsive behavior.”
In 2013, Michael Pollan wrote about the legacy of “The Secret Life” and the emerging field of plant neurobiology. “Depending on whom you talk to in the plant sciences today, the field of plant neurobiology represents either a radical new paradigm in our understanding of life or a slide back down into the murky scientific waters last stirred up by ‘The Secret Life of Plants,’” he concluded. Much of the research Schlanger writes about has been published in the past ten years, but that basic divide remains in place. The field is split between believers and doubters.
Schlanger writes about scientists who are studying how plants change their shape and respond to sound, how they use electricity to convey information, how they send one another chemical signals. Along the way, she becomes a sort of anthropologist of botanists. (“The Light Eaters” includes, briefly, an actual anthropologist, Kristi Onzik, who studies “the culture of the researchers who study plant behavior,” and who has herself followed and observed one of the researchers that Schlanger reports on.) Schlanger’s subjects like themed shirts (plant prints abound). They introduce one another by sharing their names and the systems that they study. They talk, with varying degrees of certainty, caution, and excitement, about how we should think about their individual discoveries and about whether there’s a revolution happening in their field.
Schlanger’s focus on the botanists themselves overcomes a challenge inherent to science writing: where to find drama. Changes in scientific understanding don’t often carry a big emotional charge, and writing that focusses on them can risk feeling dry, or without stakes. For those who are curious about a given system, the information alone is enough. Everyone else, though, needs a story. A character, ideally, who might face a dilemma and come up against her own flaws, might even do something unexpected, or change. Such arcs are in short supply in science reporting. Not because researchers aren’t full and fascinating individuals but because a good science story isn’t usually about a single person. Revolutionary discoveries that instantly transform our understanding are the stuff of great tales—Archimedes in the bathtub, Isaac Newton under the apple tree—but the process of a paradigm shift is usually much messier, more gradual. True leaps, undertaken by one heroic mind, are rare. Instead, knowledge accretes, mosslike, one study raising a question that another will try to answer, possibilities firming up gradually into new understanding. That process is important, even beautiful. But it doesn’t make for a ripping good yarn.
“The Light Eaters” is a special piece of science writing for the way it solves the genre’s bind; it doesn’t force its people or their findings into narrative engines. Instead, the field of botany itself functions like a character, one undergoing a potentially radical change, with all the excitement, discomfort, and uncertainty that transformation brings. The dilemma that many of Schlanger’s subjects are navigating is how to explore new observations and possibilities without losing important nuance—and without severing themselves from the existing consensus in their field, which is that plants are able to process information but don’t possess intelligence per se. One researcher, who studies the ways that plants turn mechanical stimuli into chemical signals—which is to say, how they respond to touch—replies to Schlanger’s question as to whether a plant’s body might function as a brain, with, “I think you’re right, I just don’t talk about it.” Another says that she doesn’t want to be quoted at all if she’s going to be lumped in with the “‘plant intelligence’ people” and proceeds only when Schlanger promises nuance. A third dives right in, using the boldest terms to describe plant intelligence—he’s “senior enough,” he tells her, that he doesn’t have to worry. Schlanger’s personal narrative gives the story its frame—we learn up front that when she started her research she was a jaded climate reporter, and through her immersion in plant science she rediscovers an appreciation of nature. But the book’s power comes from showing a field in flux and reminding us that ideas have their own life cycles: from crackpot theory to utter embarrassment to real possibility to the stuff of textbooks.
