The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth

Prologue

I am walking along a dim path. Thick hillocks of moss undulate fuzzily around me. I look up, and am dwarfed by pillars of dank and slimy wood. The earth below me is damp, has give. A sign on the path tells me to be alert for aggressive elk in the area. I see no elk, I keep walking. Plumes emerge, sword ferns with their curled fiddleheads the size of a baby’s fist covered in velvety auburn hair, the unexpected prequel to the arching fronds that will fountain out above them like peacock feathers. Moss drips in long fingers from branches overhead. Fungi arc skyward from a downed tree. Everything seems to strain upward and downward and outward at once.

I intrude on all this, but no one notices. All things here are so thoroughly absorbed into their own living that I am like an ant slipping discreetly through a sponge. The lichens crawling up the base of trees curl the edges of their disklike bodies up, catching drops as they receive a new day and another chance to grow.

I am in the Hoh Rain Forest in the Pacific Northwest, and everywhere is a sense of secrets. And for good reason. For everything that science does know about what, biologically, is going on here, there is so much more it cannot yet explain. All around me are complex adaptive systems. Each creature is folded into layers of interrelationship with surrounding creatures that cascade from the largest to the smallest scale. The plants with the soil, the soil with its microbes, the microbes with the plants, the plants with the fungi, the fungi with the soil. The plants with the animals that graze on them and pollinate them. The plants with each other. The whole beautiful mess defies categorization.

Thinking about this, I am reminded of the concepts of yang and yin, the philosophy of opposing forces. We know that the forces that shape life are in constant flux. The moth that pollinates the flower of a plant is the same species that devours the plants’ leaves when it is still a caterpillar. It is not, then, in the plant’s interest to completely destroy the grazing caterpillars that will metamorphose into the very creatures it relies on to spread its pollen. But likewise the plant cannot bear total leaf destruction; without leaves, the plant can’t eat light, and it will die. So after a while the besieged plant, having already lost some limbs and therefore showing tremendous restraint, judiciously begins to fill its leaves with unappetizing chemicals. At least most of the caterpillars will have eaten enough to survive, metamorphose, pollinate. Everyone in this situation comes within a hair of death to ultimately flourish. This is the push and pull of interdependence and competition. At the grand scale, no one seems to have yet won. All parties are still here, animals, plants, fungi, bacteria. What we end up with is a sort of balance in constant motion. All of this pushing and pulling and coalescing, as I have come to understand, is a sign of tremendous biological creativity.

How to get our minds around all of that complexity is the shared professional problem of science and philosophy, but also of every person who’s stopped to wonder. All that roiling life that won’t stand still long enough for us to get a good look. Narrowing our focus to only plants seems at first sensical; that should be easier, being one thing. But that quickly proves naive. Complexity lives at every scale.

Journalists in my line of work tend to be focused on death. Or the harbingers of it: disease, disaster, decline. That is how climate journalists mark time as the earth passes benchmark after grim benchmark on its way into the foreseen crisis. There is only so much of this that one person can take. Or perhaps my tolerance was thin and easily worn out after years of focus on droughts and floods. In recent years I’d begun to feel numb and empty. I needed some of the opposite. What, I wondered, is the opposite of death? Creation, perhaps. A sense of becomings instead of endings. Plants are that, given as they are to continuous growth. They’d soothed me all my life, long before studies came out confirming what we already knew: that time spent among plants can ease the mind better than a long sleep. Living in a dense city, I’d walked in the park under a canopy of yews and elms when I needed to clear my head; I’d spent long minutes gazing at the new leaves forming on my potted philodendrons when my nerves were fried. Plants are the very definition of creative becoming: they are in constant motion, albeit slow motion, probing the air and soil in a relentless quest for a livable future.

In the city, they seemed to make a home in the least suitable places. They burst from cracks in crumbling pavement. They climbed the chain link fence at the edges of garbage-strewn lots. I privately delighted as I watched a tree of heaven—loathed as an invasive species in the Northeast—emerge out of a split on my stoop and grow nearly to the size of a two-story building in a single season. Privately, because I knew this was seen as a hellish species in New York, in part because it injects poisons into the ground around its roots to prevent anything else from growing nearby, thereby securing its patch of sun; delighted, because this seems devilishly brilliant. When my neighbor cut the tree down with a machete late in the season, I understood. Still, I regarded its stump with admiration on my way out of the house every morning. It was already sprouting new green protrusions. You have to respect a good hustle.

So plants seemed like the right place to land my weary apocalyptic attention. Surely they would refresh me. But I soon learned they would do more than that. Plants have, over the course of years of obsession, transformed my understanding of what life means, and what its possibilities are. Now as I gaze around the Hoh Rain Forest I see more than a soothing wash of green. I see a masterclass in living to one’s fullest, weirdest, most resourceful potential.

To start with, a life spent constantly growing yet rooted in a single spot comes with tremendous challenges. To meet them, plants have come up with some of the most creative methods for surviving of any living thing, us included. Many are so ingenious that they seem nearly impossible for an order of life we’ve mostly relegated to the margins of our own lives, the decoration that frames the theatrics of being an animal. Yet there they are all the same, these unbelievable abilities of plants, defying our anemic expectations. Their way of life is so astonishing, I will soon learn, that no one yet really knows the limits of what a plant can do. In fact, it seemed that no one quite knows what a plant really is.

This is of course a problem for the scientific field of botany. Or it’s the most exciting thing to happen to it in a generation, depending on how comfortable you feel with seismic shifts in what you once thought to be true. Now I was hopelessly intrigued. Controversy in a scientific field tends to be a harbinger of something new, some new understanding of its subject. In this case, the subject was all of green life itself. I began to direct my growing interest entirely toward emerging thought in plant science. The more botanists uncovered the complexity of forms and behaviors of plants, the less the traditional assumptions about plant life seemed to apply. The scientific field was eating itself alive with contradictions, points of contention multiplying as fast as the mysteries. But something in me was attracted to this lack of neat answers, as I suspect many of us are. Who doesn’t feel both drawn to and repulsed by the unknown?

This book will take up these new epiphanies in plant science, and the struggle along the way over how new scientific knowledge is made. Rarely does one get a glimpse of a field in true turmoil, debating the tenets of what it knows, about to birth a new conception of its subject. We will also consider a daring question currently being hotly debated in labs and academic journals: Are plants intelligent? Plants don’t have brains, as far as anyone can tell. But, some posit, they should be considered intelligent regardless, based on the remarkable things they can do. We determine intelligence in ourselves and certain other species through inference—by observing how something behaves, not by looking for some physiological signal. If plants can do things we consider indications of intelligence in animals, this group says, then it’s illogical, a sign of unreasonable zoocentric bias, not to use that language for them. Others go farther, suggesting that plants may be conscious. Consciousness is perhaps the least understood phenomenon in human beings, let alone other organisms. But a brain, this camp says, may be but one way to build a mind.

Other botanists are more circumspect, unwilling to apply what they see as distinctly animal-centric notions to plants. Plants, after all, are their own clade of life, with an evolutionary history that swerved away from our own long ago. Painting them with our concepts of intelligence and consciousness does a disservice to their essential plantness. We’ll meet this camp of scientists too. Yet no one I met—not a single botanist—was anything less than agape with wonder at what they were learning that plants are capable of. Thanks to new technology, scientists in the last two decades have gained incredible new powers of observation. Their findings are reshaping the meaning of plant before our very eyes.

Regardless of what we think of plants, they continue to surge upward, toward the sun. In this ruined global moment, plants offer a window into a verdant way of thinking. For us to truly be part of this world, to be awake to its roiling aliveness, we need to understand plants. They suffuse our atmosphere with the oxygen we breathe, and they quite literally build our bodies out of sugars they spin from sunlight. They made the ingredients that first allowed our lives to blink into existence at all. Yet they are not merely utilitarian supply machines. They have complex, dynamic lives of their own—social lives, sex lives, and a whole suite of subtle sensory appreciations we mostly assume to be only the domain of animals. What’s more, they sense things we can’t even imagine, and occupy a world of information we can’t see. Understanding plants will unlock a new horizon of understanding for humans: that we share our planet with and owe our lives to a form of life cunning in its own right, at once alien and familiar.

In the Hoh Rain Forest, a bigleaf maple stretches out above me. Its trunk is sheathed entirely in licorice ferns and lungwort and spike moss, giving the impression of a tree wearing a Grinch suit. Only a few ridges of the tree’s bark are visible, rising through the green fuzz like a mountain range above a mat of thick woods, like the Olympic peaks that pierce the evergreen forests just east of here. I lean in, looking closer. The green suit is a world within a world, the little tufts and fronds replicating the structure of a forest at tiny scale. Three-leafed oxalis and feathery stairstep moss coat the ground. I am lost in their world, taken into it. Then again, we’ve all been lost in it a long time, unaware of its true machinations. This seems imprudent. I wanted to know, so I went out and looked.

Chapter 1

The Question of Plant Consciousness

What is a plant? You likely have an idea. You might be picturing a fat sunflower with its hubcap face and thick felted stalk, or the bean vine looped around a trellis in your grandmother’s yard. Maybe, like me, you’re eyeing the golden pothos hanging in your kitchen window that you should probably water. A known entity; the green of every single day.

Of course you’re right, in the same way humans have been able to point to an octopus for all of history and call it an octopus. But we didn’t know, until recently, that octopuses could taste with their arms, use tools, remember human faces, see their world far more sensitively than we can ours; that they have neurons distributed throughout their bodies like a multitude of disparate miniature brains. So, then, what is an octopus? Something much more than we ever imagined.

The answer is only beginning to dawn on us, and has already revolutionized our understanding of nonhuman intelligence in one crucial aspect: the octopus’s branch on the evolutionary tree diverged from ours very early in the history of animal life. Our last common ancestor was likely a flatworm that trailed the ocean floor more than five hundred million years ago.* Up until this point, we’ve located intelligence in animals much evolutionarily closer to ourselves, like dolphins, dogs, and primates, our much more recent cousins. But we now know that powerful cunning can evolve completely independently from our own. A similar tectonic shift is happening with plants, only—for now—more quietly, in the labs and field sites of one of the least flashy disciplines within the life sciences. But the weight of this new knowledge is threatening to burst the walls of the container in which we place plants in our minds. Ultimately it may change how we think about life altogether.

So what is a plant? I certainly thought I knew. And then I began to talk to botanists.

SEVERAL YEARS AGO, I was an environmental reporter with a problem. Most of my work focused on two things: the steady advance of climate change, and the health effects of polluted air and water. In other words, I was writing about humanity’s unrelenting death plod. After five or six years on that beat, a crawling sense of dread threatened to eclipse me. In response, I began to act strangely. I’d explain the latest IPCC report—the ones telling us we had very few years to stave off catastrophe—to my colleagues with an eerie sort of glee, awaiting their paled faces. I’d often spend a morning ingesting news about record-breaking wildfires and hurricanes and pivot seamlessly to office gossip by lunch. The compartmentalization became so total that I could no longer muster any emotional response to environmental cataclysm. Melting ice sheets in Greenland started to look like just another good story.

It was around then that I began to search, without realizing it, for something in the natural sciences that felt wonderful and alive. I liked plants; I loved watching my night-blooming jasmine clamber up my window frame and my fiddle-leaf fig burst open three new leaves in a sudden growth spurt after months of no visible change. My apartment was a refuge of satisfying plant drama, better than the drama unspooling inside my computer. So, I thought, why not turn my reporter brain toward them? I began searching botany journals on my lunch breaks, using the same online portals I used to find climate papers, a system that lets journalists see the latest research before it is available to the public, on the condition that they not publish stories about it until its stipulated release date. The journals were full of fundamental discoveries about plants; uncovering the evolutionary origins of bananas and understanding, at last, why some flowers are slippery (to deter nectar-thieving ants). I felt like I was spying on a version of science from an earlier time; were there really this many fundamentals left to be uncovered? Two weeks into my new fascination, I learned that a complete fern genome had been sequenced for the first time, and a paper on it would be coming out soon. I didn’t yet know how remarkable that was—ferns, being extremely ancient, can have up to 720 pairs of chromosomes, versus humans’ mere 23, which explained why the genomic revolution took so long to reach them. I was immediately taken by the image of the fern that accompanied the embargoed scientific paper. It was a photograph of a tiny, scalloped plant atop a researcher’s thumbnail, an azolla. It was so green it looked lit from within. I was in love.

Azolla filiculoides, or just azolla for short, is one of the world’s smallest ferns, and has grown in wet places for millennia. As is generally the rule with plants, it is unwise to mistake size for complexity. Roughly fifty million years ago, when the earth was a much warmer place, azolla began growing over the Arctic Ocean in vast fern blankets. For the next million years they absorbed so much CO2 that paleobotanists believe they played a crucial role in cooling the planet, and some researchers are looking into whether they could help do that again.

The azolla performs another miraculous trick still; some one hundred million years ago, it evolved a specialized pocket in its body to house a packet of cyanobacterium that fixes nitrogen. The air around us is nearly 80 percent nitrogen, and every life form, including ours, needs it to manufacture nucleic acids, the building blocks of all life. But in its atmospheric form, it’s entirely out of our reach. Nitrogen, nitrogen everywhere, and not a single molecule that we can use. In a humbling twist, plants rely wholly upon bacteria that know how to recombine nitrogen into forms the plant—and all of us who get our nitrogen from plants—can use. And so azolla morphed itself into a hotel for this bacterium. The tiny fern feeds the cyanobacteria the sugars it needs, and the cyanobacteria busy themselves with transforming nitrogen. Farmers in China and Vietnam took note of this and have been grinding azolla into their rice paddies for centuries.

I sought out fern guidebooks and fern lore. I was charmed by my own rapaciousness, which had been activated like this only a few times in my life. I was so enamored that I got the tiny azolla tattooed on my left arm. Journalists are notorious generalists, interested briefly and drenchingly in many things they soon leave behind. But this was, I thought, what it must be like to be taken with something. I had questions, suddenly, about this commonest group of plants that had sprouted seemingly without fanfare. They changed the world. What else did I not know?

As part of this inquiry I bought and devoured Oaxaca Journal, a slender volume of Oliver Sacks’s observations from a fern expedition to southwestern Mexico he took with a bus full of dedicated amateur pteridologists, all of them from the New York chapter of the American Fern Society. The expedition was co-led by Robbin C. Moran, a forty-four-year-old curator of ferns at the New York Botanical Garden, who brought them all around the state of Oaxaca. At one point, after days of visiting villages and landscapes, marveling at produce in the markets and dye vats of red cochineal, and of course all manner of liverworts and ferns, Sacks has a moment that can only be described as rapture. The afternoon sun is falling strong and slant against high stalks of corn. An older gentleman, a botanist and specialist on Oaxacan agriculture, is standing beside the corn. Sacks acknowledges the numinous moment—the briefest flicker—with barely half a sentence, but it stung me immediately as true.

. . . the tall corn, the strong sun, the old man, become one. This is one of those moments, indescribable, when there is a sense of intense reality, an almost preternatural reality—and then we are descending the trail to the gate, reboarding the bus, all in a sort of trance or daze, as if we had had a sudden vision of the sacred, but were now back in the secular, everyday world.

The experience of flashes of the eternal, the real, the gestalt, runs like a thread throughout naturalist literature. I wasn’t the only one who had been taken like this before. In Pilgrim on Tinker Creek, the writer Annie Dillard has a similar moment in front of a tree, watching light pour through its branches. A flash of the real. Almost as soon as she realizes she is having it, it is gone, but it leaves her with the awareness of a sort of open-plan attentiveness that can be accessed in snippets, and which might be a more direct observation of the world than the usual everyday version.

As I read more books about plants and their enraptured naturalists after work and into the early morning, I began to find these moments sprinkled everywhere. In The Invention of Nature, Andrea Wulf’s biography of the famous nineteenth-century naturalist Alexander von Humboldt, I learned he’d had it too. Von Humboldt wondered aloud why being in the outdoors evoked something existential and true. Nature everywhere speaks to man in a voice that is familiar to his soul, he wrote; Everything is interaction and reciprocal, and therefore nature gives the impression of the whole. Humboldt went on to introduce the European intellectual world to the concept of the planet as a living whole, with climatic systems and interlocking biological and geological patterns bound up as a net-like, intricate fabric. This was Western science’s earliest glimmer of ecological thinking, where the natural world became a series of biotic communities, each acting upon the others.

Something about reading botany papers gave me snatches of that feeling, glimpses of a sort of whole I couldn’t yet fully articulate. I had the sense I was uncovering large gulfs in my knowledge. How long had I spent in the presence of plants while knowing next to nothing about them? I felt a curtain drawing back little by little onto a parallel universe. I knew it was there now, but not yet what it held.

I signed up for a fern-science class at the New York Botanical Garden, taught by none other than the Moran from Sacks’s expedition, no longer forty-four but youthful just the same. (I would come to learn that the botany world comprises a cast of recurring characters with storylines between them, some amiable, some not.) We learned how to identify ferns, about their basic structure, and about the more idiosyncratic species; a resurrection fern grows on the branches of oak trees, and can almost completely dehydrate in times of drought, shrinking down into a dead-looking crisp. It can remain in its dried state for more than a hundred years and still fully rehydrate. Tree ferns can grow more than sixty-five feet high, and other ferns are minuscule fertilizer factories, like the tiny azolla. And then there’s bracken fern, which makes cows who dare to graze it bleed internally to death. Just a totally cruel fern, Moran said.

I learned that ferns are much, much older evolutionarily than flowering plants. They appeared on the scene even before evolution dreamed the concept of seeds; they reproduce without them. Days later, during lunch hours spent reading about ferns, falling desperately into true obsession, I learned that this lack of seeds boggled Europeans for centuries. All plants had seeds; it was key to their sexual reproduction, or so medieval people thought. If they couldn’t find the fern’s seeds, contemporary logic followed that they simply must be invisible. And because another dominant theory at the time suggested that the physical characteristics of plants were clues about what they could be used for, people believed finding these invisible seeds would grant humans the power of invisibility.

Actual fern sex turned out to be much weirder. First of all, they reproduce using spores, not seeds. But here’s the kicker: they have swimming sperm. Before they grow into the leafy fronds we all know, they have a completely separate life as a gametophyte fern, a tiny lobed plant just one cell thick—not remotely recognizable as the fern it will later become. You’d miss them on the forest floor. The male gametophyte fern releases sperm that swim in water collected on the ground after a rain, looking for female gametophyte fern eggs to fertilize. Fern sperm are shaped like tiny corkscrews and are endurance athletes—they can swim for up to sixty minutes. You can watch them squiggle under a microscope.

The sperm alone isn’t the most amazing thing about fern reproduction. In 2018, at the beginning of my infatuation, research was emerging to suggest that ferns compete with other ferns for resources by emitting a hormone that causes the sperm of neighboring fern species to slow down. Slower sperm means less of that species survives, so the sabotaging fern can enjoy more of whatever is scarce, be it water, sunlight, or soil.

Scientists were just beginning to wrap their heads around this fact. This is brand-new, Eric Schuettpelz, a research botanist at the Smithsonian’s National Museum of Natural History in Washington, DC, told me over the phone. Sabotaging sperm was evidently the cutting edge of fern science. We know it’s the plant hormone but no idea how it works, he said. How did a fern know it was beside some fern competition? How did it time its malevolent release? A fern researcher at Colgate University had just presented an early paper on the phenomenon at a botany conference that same month.

I let that sink in a minute: ferns can remotely interfere with other ferns’ sperm. This was incredibly salty plant activity. I began to see where Moran was coming from. This also seemed remarkably brilliant. What else could plants do?

With that question in mind I began to focus my newfound lens on a relatively young area within plant science: plant behavior. The announcements of emerging research, I found, were peppered with plant behavior papers. This represented a new mental gate for me to walk through; that plants could be thought to behave at all was still an enchanting possibility. But several papers I found pushed the farthest edge of that concept farther: they suggested that plants might have a form of intelligence. I was intrigued, and skeptical. I wasn’t the only one. As it turned out, the suggestion of plant intelligence had recently started an all-out war.

I’d come across this corner of the scientific world at a remarkably exciting time. In the last decade and a half a revival of plant behavior research had brought countless new realizations to botany, more than forty years after an irresponsible best-selling book nearly snuffed out the field for good. The Secret Life of Plants, published in 1973, captured the public imagination on a global scale. Written by Peter Tompkins and Christopher Bird, the book was a mix of real science, flimsy experiments, and unscientific projection. In one chapter, Tompkins and Bird suggested that plants could feel and hear—and that they preferred Beethoven to rock and roll. In another, a former CIA agent named Cleve Backster hooked up a polygraph test to his houseplant and imagined the plant being set on fire. The polygraph needle went wild, which would mean the plant was experiencing a surge in electrical activity. In humans, a reading like that was believed to denote a surge of stress. The plant, according to Backster, was responding to his malevolent thoughts. The implication was that there existed not only a sort of plant consciousness but also plant mind-reading.

The book was an immediate and meteoric success on the popular market, surprising for a book about plant science. Paramount put out a feature film about it. Stevie Wonder wrote the soundtrack. The first pressings of the album version were sent out scented with floral perfume. To its many astonished readers, the book offered a new way to view the plants all around them, which up until then had seemed ornamental, passive, more akin to the world of rocks than animals. It also aligned with the advent of New Age culture, which was ready to inhale stories about how plants were as alive as we are. People began talking to their houseplants, and leaving classical music playing for their ficus when they went out.

But it was a beautiful collection of myths. Many scientists tried to reproduce the most tantalizing research the book presented, to no avail. Cellular and molecular physiologist Clifford Slayman and plant physiologist Arthur Galston, writing in American Scientist in 1979, called it a corpus of fallacious or unprovable claims. It didn’t help that former CIA agent Backster, as well as Marcel Vogel, an IBM researcher who claimed to be able to reproduce the Backster effect, believed that one had to develop an emotional rapport with a plant before any effect was possible. In their view, that explained away any inability of another lab to replicate the results. "Empathy between plant and human is the key, Vogel said, and spiritual development is indispensable."

According to botanists working at the time, the damage that Secret Life caused to the field cannot be overstated. The twin gatekeepers of science funding boards and peer review boards—always conservative institutions—closed the doors. Over the following years, according to several researchers I spoke with, the National Science Foundation became more reluctant to give grants to anyone studying plants’ responses to their environment. Proposals with so much as a whiff of inquiry into plant behavior were turned down. The money, whatever little there had been, dried up. Scientists who had pioneered the field changed course or left the sciences altogether.

But a select few held on, biding their time with other lines of inquiry, waiting for the tide to turn.

In the last decade and a half, it finally has. Funding for some plant behavior research began flowing again, though the grants were still hard to come by at first. Botany journals, though many were still edited by opponents of the plant intelligence field, began letting a trickle of these papers through. The change was likely the result of new technology like genetic sequencing and more advanced microscopes, which made it possible to come to previously outlandish conclusions with real rigor. Or perhaps the political mockery that followed the Secret Life debacle was just far enough in the rearview mirror. Many of the authors didn’t use words like intelligence to describe what they found, but the results nonetheless suggested that plants were much more sophisticated than anyone had dared think.

Recently, as I came across in my reading, researchers had found promising indicators of memory in plants. Others found that a wide variety of plants are able to distinguish themselves from others, and can tell whether or not those others are genetic kin. When such plants find themselves beside their siblings, they rearrange their leaves within two days to avoid shading them. Pea shoot roots appeared to be able to hear water flowing through sealed pipes and grow toward them, and several plants, including lima beans and tobacco, can react to an attack of munching insects by summoning those insects’ specific predators to come pick them off. (Other plants—including a particular tomato—secrete a chemical that causes hungry caterpillars to turn away from devouring their leaves to eat each other instead.) Papers probing other remarkable behaviors were growing from a trickle to a fairly robust stream. It seemed like botany was on the verge of something new. I wanted to stick around and watch.

BACK AT MY desk in the air-conditioned newsroom, I savored these small tears in the fabric of my day. Something about this renaissance in the study of plant behavior spoke to an earlier me. I was an only child for the first nine years of my life, until my brother was born. Even then a newborn wasn’t much use to a nine-year-old girl, especially not one who truly believed she was an adult trapped in a child’s body. Which is to say I was alone and predisposed to fantasy. Girls of that nature tend to build complex internal worlds that they proceed to drape like a blanket over the world around them. Adults who don’t understand this disposition tend to call it melodramatic. But I resented that word, which implied that my version of reality couldn’t be trusted. I was sure I was simply seeing things around me for what they actually were. In most cases, those things were trees and squirrels and sometimes rocks, and they were very much alive, alert to the world. Children are known to be inborn animists.

Noticing things that other people, namely adults,