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

by Zoë Schlanger

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.

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.

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?

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.

Nature is chaos in motion. Biological life is a spiraling diffusion of possibilities, fractal in its profusion. Every organism, and certainly every plant, has ricocheted out of another fragment of the evolutionary web of green leafy things to variate further. These each are of course still morphing, because that sort of thing never ends, except in extinction.

Researchers had just found that plants could remember, but not where those memories were stored. They’d found kin recognition, but not how those kin are recognized. These discoveries were more like hints, fragments that pointed toward something larger, something whole.

If weighed, plants would amount to 80 percent of Earth’s living matter

A leaf is the only thing in our known world that can manufacture sugar out of materials—light and air—that have never been alive.

To be precise, it takes six molecules of carbon dioxide and six molecules of water, torn apart by power from the sun, to form six molecules of oxygen and—the true aim of this whole process—one precious molecule of glucose. The plant uses the glucose to build new leaves, which will be used to make more glucose. It also shuttles the glucose down through its body, passing it into its underground architecture, where it is used to grow more roots, which will pull more water back up through its body, which will be torn apart to make more glucose. In this way, life unfurls.

Think about it: every animal organ was built with sugar from plants. The meat of our bones and indeed the bones themselves carry the signature of their molecules. Our bodies are fabricated with the threads of material plants first spun. Likewise, every thought that has ever passed through your brain was made possible by plants.

All the glucose in the world, whether it arrives in your body packaged inside a banana or a slice of wheat bread, was manufactured out of thin air by a plant in the moment after photons from the sun fell upon it.

Each plant that dies that singularly lonely death marks the end of a multimillion-year evolutionary project. That species’ great genetic experiment is over; it’s the last in its line.

In many ways, Kaua‘i is the ultimate example of what a world would look like if plants were in charge. The whole island is covered in the surreal products of total floral freedom. When plants are allowed to evolve without fear, they get scrupulously and flamboyantly specific. Take the Hibiscadelphus genus, for example. Found only in Hawai‘i, these plants have long tubular flowers, custom-made to fit the hooked beak of the honeycreeper, the precise bird that pollinates them. Then there is the vulcan palm, Brighamia insignis, or ‘Ōlulu in Hawaiian, a short tree best described by its nickname, “cabbage on a stick.” Over tens of thousands of years, it has evolved to be pollinated only by the extremely rare fabulous green sphinx moth (its real name).

The first plants with seeds and flowers appeared some two hundred million years ago. Since then, they have split and evolved into hundreds of thousands of species that have had to adapt to threats of all kinds, which start the moment they sprout.

A plant is modular; snap off a leaf, and it can grow a new one. Without a central nervous system to protect, the plants’ vital organs are distributed and come in duplicates.

That also means a plant has evolved remarkable ways to coordinate its body and defend itself. They might grow thorns and spikes and stinging hairs, developed with remarkable precision, to pierce the flesh or exoskeleton of whatever mammal or bug might be its main threat. They might secrete sticky sugar to entice and then immobilize their antagonists, whose hungry mouths get stuck shut. Their flowers might be extra slippery, to deter nectar-thieving ants. Whatever the adaptation, it tends to be economical in its specificity. There’s a purpose to every tiny variation. This is true for all areas of plant physiology; every part of the architecture of a plant’s body is there for a reason, calibrated to fit its task. No more, no less.

Plants are themselves synthetic chemists, surpassing the best human technology in terms of the subtle complexity of the chemicals they can synthesize. A leaf, sensing that it has been nibbled, can produce a plume of airborne chemicals that tell a plant’s more distant branches to activate their immune systems, manufacturing yet more repellent chemicals to deter incoming aphids and other plant-eating bugs.

Several species of plants have been found to identify a caterpillar’s species by sensing the compounds in its saliva, and then synthesize the exact compounds to summon its predator. Parasitic wasps then obligingly arrive to take care of the caterpillars.

But vegetabilis came from the medieval Latin, meaning something that is growing or flourishing. Vegetāre, the verb, meant to animate or enliven. Vegēre was the very state of being alive, being active. Clearly, it hasn’t always been this way.

Glutamate and glycine, two of the most common neurotransmitters in animal brains, are present in plants also, and seem to be crucial to how they pass information through their stems and leaves. They have been found to form, store, and access memories, sense incredibly subtle changes in their environment, and send highly sophisticated chemicals aloft on the air in response. They send signals to different body parts to coordinate defenses.

Sometime after the age of the philosopher-naturalists, of Humboldt and Darwin, science became a pursuit of specialization. Despite relatively recent gestures toward interdisciplinary academics, we still live in an era of specialists, each of whom sees only their own narrow band within the larger problem of how life works. This has produced tremendous leaps in knowledge; with specialization comes depth. Yet for the most part, each specialist remains unaware of the larger picture.

It is no wonder then that zoologists and entomologists have been the ones to make some of the most groundbreaking discoveries about plants, often by viewing them through the lens of animals and insects. That’s not to knock botanists, but in an age where genetics dominate, many have ceased to see the plant as a pulsating whole, and instead see it as an amalgam of genetic switches and protein gates. Of course, a human could also be seen in such terms. But what is missed by looking that way?

Academics, armed with hyperliteracy, can be vicious when they disagree.

When neuroscientists peer inside the brain, they find a distributed network. No discernible command post exists. Our own intelligence appears to emerge from a network of specialized brain cells exchanging information, but they do not appear to answer to some governing force. The intelligent decisions we make emanate not from one specific place but from a sort of network, a consolidated city of interconnected, communicative parts in our skulls.*

Plants will go on being plants, whatever we decide to think of them. But how we decide to think of them could change everything for us.

In the human body, electricity works like this: the membrane potential of our cells, when at rest, is ever so slightly negatively charged. Positively charged elements—sodium, magnesium, potassium, and calcium ions—are afloat in the plasma between those cells. These are your electrolytes. When touched, the cells open channels in their membranes and allow these ions to pass through them. Think of the sluice gates in canals that let water in and out.

Suddenly, with the influx of ions, the cell’s charge flips from negative to positive. This produces a burst of electricity known as an action potential. That sudden burst triggers the ion gates in the neighboring cell to open too, electrifying that cell in turn. This chain reaction travels fast, sending information via the electric current that the bestirred cells make from your finger (and cheek) to your brain and back again. Almost all our cells are capable of generating electricity. Muscles are electrically active every moment they’re contracting and releasing; it’s electricity that makes this movement possible. The same holds true for the smooth muscle around our veins, which contracts and releases to keep the blood flowing through our body. Our brains, of course, are fantastically electrical,

In our brain, electricity travels in waves. Information appears on colorized brain scans as pulses, like a wave traveling between two shores. The complexity and coherence of these waves are what neurologists routinely use to determine brain health and mental state.

They stroked the arabidopsis with soft paintbrushes, and then analyzed the plants’ genetic responses. Within thirty minutes of being touched, 10 percent of the plant’s genome was altered. Clearly, the plant was reorganizing its priorities to deal with the disturbance, and rerouting energy away from the hard work of getting taller. Touched multiple times, arabidopsis cut its upward growth rate by as much as 30 percent, just as Jaffe had found years before.

When touched, a plant will essentially activate its immune system. In this way, human touch has been shown to help plants ward off a future fungal infection, because the plants’ defenses are already up. Whatever the situation, touch a plant, and it will take note, most often by becoming incredibly stressed and defensive.

Botanist Barbara Pickard had been working on electricity in plants since the 1970s, and was known to rely as much on her intuition as her data, to the chagrin of her fellow researchers. But she discovered a channel that led straight through the membrane, with its own little gate; it was there to allow the flow of electric current—essentially, calcium ions—through the cells when something mechanically pushed on them; that is, when they were physically touched. Pickard and her team found the first definitive evidence of mechanosensitive ion channels in plants. For the first time, researchers had a way to look at how, at the cellular level, plants experience touch as a physical force from the inside.

Technology has evolved so dramatically that, with minimal investment, anyone can observe electricity in plants at home. You only need an electrode and something with which to read its output. If you attach the electrode to your wrist, a steady line of spikes and swoops will appear. If you attach that same electrode to your houseplant’s leaf, and touch it in any way, a spike and swoop will appear on the readout that looks remarkably similar. These are the action potentials—little bursts of electricity—produced, in your case, by the neurons in your heart that are firing at regular intervals to make it pump blood, and in the plant’s case—well, no one yet quite knows why they’re there, or what for.

In 2013, he and his colleague Masatsugu Toyota became the first people to witness electricity moving through a plant body in real time. To their delight, they saw that it moved in a wave.

Despite this, the plant is clearly processing the up-down information to decide how to grow—plant roots in general grow down, and shoots, in general, grow up. If you tip a plant over, it will eventually start growing upward again. They are clearly sensing gravity. Plus, they’re integrating that information into the information they’ve already gathered from various other aspects of their immediate surroundings—obstacles, neighbors, the direction of the light, the temperature of the soil. But how?

Of course a plant doesn’t have a brain—but what if the whole plant itself is something like a brain?

The flower, in this case, was definitely the part of the plant responsible for “hearing”—and it suggests that it had taken on the bowl shape for exactly the same reason satellite dishes are concave. “We found a potential hearing organ, which is the flower itself,” she said. When she looks at flowers now, she sees ears everywhere.

They discovered that these multicolor starburst-shaped flowers were able to remember the time intervals between bumblebee visits, and anticipate the next time their pollinator was likely to arrive.

The dissenting papers, he says, are all focused on the lack of brains—and no brains, they wrote, meant no intelligence. “Plants don’t have these structures, obviously. But look at what they do. I mean, they take information from the outside world. They process. They make decisions. And they perform. They take everything into account, and they transform it into a reaction. And this to me is the basic definition of intelligence.

But the best plant videos on the internet are of the dodder vine, a parasitic plant that takes being a vine to extremes: it grows no leaves, so it must find a host from which to suck out sugars immediately after emerging from the dirt. When it finds a host, it will detach from the ground entirely, relying on its new benefactor for everything it needs. It has no use for chlorophyll, since it does not photosynthesize, so is instead an intriguing shade of orange, and without leaves it gives the impression of a sleek little worm. Watching it grow in time-lapse is a marvel. When the seedling of a parasitic dodder vine emerges, its tip circles the air slowly. It’s impossible not to understand that it is looking for something. In fact the gesture looks unmistakably like sniffing. Indeed, the dodder is sampling the air for the emanations of a suitable plant to parasitize. Then, before making physical contact, it begins to move more purposefully in one direction.

We may not know where plants store their memories—somewhere, it could be said, in their brainless mind—but the knowledge that they evidently have them is enough to change our world.

The bumblebees, she realized, were starving. They’d been flying around the closed buds of mustard flowers for days, but no bloom had yet opened. If they didn’t slip their tongues in a pool of sugary flower water soon, they would begin to slow down, their bodies desperately trying to conserve calories. Eventually they would land on the dirt, crawl a while, and die. The poor bees; their timing was all wrong. The flowers weren’t due to open for another month. It wouldn’t do. The bees, she saw, began to bite the plants’ leaves. The next day the flowers bloomed. The bees drank the nectar and survived.

Common bumblebees, she’s found, can tell at a distance whether a yellow monkey flower will have lots of pollen for them or not, by the scent of a certain floral volatile compound that translates, in a bee brain, to “heaps of pollen.” But making heaps of pollen takes a lot of resources. So the monkey flower has developed a shortcut. It recognizes the rules of this pre-screening process, and instead of making extra pollen, will exude the volatile anyway—in essence, it will lie. The bumblebee, now duped, will arrive to disappointment. Either way, the monkey flower got what it wanted: a bumblebee to dust with pollen. The monkey flower is an excellent liar.

So it makes sense that the goldenrod would have evolved a way to figure out if gall-forming flies were nearby. But the flies, knowing the plants might sense their arrival, also assess the goldenrod. If the goldenrod is exuding volatiles that indicate it has put up anti-fly defenses, the female flies that carry the eggs take notice and avoid it. It’s better, for the fly, to move on in search of a less-defended specimen.

Bittersweet nightshade, a plant in the same family as tomatoes, potatoes, and tobacco, secretes sugary nectar in order to recruit ants as bodyguards. The ants, hooked on the sticky syrup the plant oozes for them, dutifully pluck off the larvae of the bittersweet’s mortal enemy, the flea beetle, which are clinging to the plant’s stem. They must be quick, before the wriggling flea beetle babies have a chance to bore themselves into the bittersweet’s body and wreak havoc. The ants march the larvae deep into their ant nest. The larvae are never seen again.

But back to the orchids. In Australia, evolutionary biologist Rod Peakall spent more than thirty years studying the ways several groups of orchids convinced wasps to try to have sex with them. The point of this was to coat the wasps in their pollen. Which is to say, in order to have plant sex, the plant pantomimed wasp sex, and the wasp unwittingly had plant sex. The mechanics are a little complicated, but perhaps nothing else we know of shows just how intimately involved plants can get in other species’ lives. So we will try our best to imagine it.

WE KNOW PLANTS’ biochemical genius makes them resilient, defends them well against predators, and gets their needs met in subtle and overt ways. And they don’t always hew to what a European sensibility might call the “natural order” of things. They don’t stick to their own species, or even to some clearly definable gender. After all, the orchid is reproducing through sex with a wasp. Some plants almost exclusively clone themselves, like aspens or dandelions, and still others clone themselves sometimes and have sex other times, like the strawberry. Many plants are bisexual, with male and female genitalia occurring together on the same flower (in plant anatomy these are called, intriguingly, “perfect” flowers). The ancient ginkgo tree can spontaneously switch the sex of a section of its body, producing a female branch on an otherwise male tree. Ginkgo are one of the oldest lineages of trees we live alongside, having persisted for hundreds of millions of years, stubbornly surviving since the time of the dinosaurs. Their sexual fluidity may make them remarkably resilient to whatever the eons have thrown at them.

The pollution steadily filling the air appears to sabotage plants’ ability to send and interpret each other’s signals. Just as plants’ communication can cross the species divide, so can ours. And we’re speaking to them in smog.

Globally, as many as 11,000 farmworkers are fatally poisoned by pesticides each year, and another 385 million are severely poisoned but don’t die, to say nothing of the birth defects, breathing disorders, and other long-term health impacts of constant exposure to regular doses of the stuff.*

Knowing that cyanobacteria have the capacity to see opens the possibility that perhaps the plant kingdom, which evolved from cyanobacteria, never actually discarded it.

To the researchers’ surprise, the dodder’s assessment of red light ratios appeared to be exquisitely fine-grain. In the lab, they used a combination of far-red LED arrays and real plants to set up tests; when given the option of LEDs arranged to resemble light passing through a grass-shaped plant and another resembling the body of a branched plant, the seedlings chose the direction of the “branched” one (dodders can’t grow on grasses). They also chose to grow toward the nearer of any two same-sized plants, even if the difference in distance was only four centimeters. It’s not a stretch to say that this parasitic plant can, in this basic way, see its host—or at least the size and shape of it.