Plants can do some marvelous things — in addition to being “light eaters” they have their own ways of seeing, hearing and feeling. My guest this week, environmental reporter and author Zoë Schlanger, is here to discuss her new book on the concept of plant intelligence and how it changes our understanding of plant life.
Zoë reports on climate change for The Atlantic, and she recently published her first book, “The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth,” in which she delves into the science on plants’ ingenious methods of survival.
Zoë Schlanger is the author of “The Light Eaters” and an environmental journalist.
(Photo Credit: Heather Sten)
She says it’s been humbling to learn so much about evolutionary strategies and to understand that each of us is “just this incredibly specific and weird and messy outcropping of some evolutionary creativity, and every single species is this incredible manifestation of that, including us.” Her research for her book has shifted her sense of what it would mean to lose any one plant species, she says, and it brought to the fore why plant preservation is so crucial.
Each of these plants is the end of its genetic evolutionary environment that will go on forever and ever if we can allow them to,” she says.
How Zöe Came To Write About Plants
“I grew up always enjoying being around plants and spending a lot of time in family members’ gardens, and I grew up in the woods, so plants were always part of my life, but not in any sort of professional sense,” Zöe says.
Today, as a reporter, Zöe has been covering climate change for about a decade. She began to feel understandably burnt out on the topic about four or five years ago.
“It’s bleak out there,” she says of covering climate change, “and I started looking around for something else to cover — something else in the sciences that publication would be interested in, but that would also give me the sense of vitality and awe — these experiences that we can have with the natural sciences when they are wondrous.”
She started to read botany journals and became obsessed with ferns. She says ferns became her “gateway plant” because the first fern genomes had been sequenced around the time she started thinking about these topics.
“I was speaking to fern scientists about that, and they were just the most delightful people to speak to,” she says. “They were so enamored with their study subjects and had so much care and respect for plants as their own separate alien creatures with their incredible capabilities. And ferns are just so ancient and virtually unchanged for such a long time, and there’s something so calming about that fact.”
As she reviewed botany journals, she realized that scientists were debating among themselves — out in the open in published, peer-reviewed papers — whether plants could possibly be intelligent.
“Having spent a long time talking to researchers, I know they don’t say things publicly unless there’s really something worthwhile to sort of back that up,” she says. “I knew there must be a lot of evidence bubbling up.”
Incredible findings were coming out around plant communication, plant personality, interspecies communication — plants communicating with insects — and defending themselves in completely creative, unbelievable ways, she says.
“I realized this was one of the most compelling stories I’d come across in my career.”
She came up with the title “The Light Eaters” and began working on a book proposal.
“When I was thinking about what made plants so remarkable, photosynthesis is still the most remarkable thing they do,” she says of the book’s title. “They’re the only things in the known world that can make sugar out of non-living materials.”
Using air and water, catalyzed by sunlight, plants create the glucose that has powered all of our bodies into existence, she explains.
“Every single molecule of sugar you’ve ever consumed was made in a plant through photosynthesis,” she points out.
Zöe on Lummi Island, Washington, in 2022.
(Photo Credit: Sarah Sax)
The Debate Over ‘Plant Intelligence’
Zöe came across the ferocious debate among botanists over plant intelligence.
“There are plenty of very esteemed plant scientists who think it’s ridiculous to use terms like ‘plant intelligence’ because, why can’t we just leave these creatures to be their very own thing without putting human terms on them? And one of these critics was an ethnobotanist named Timothy Plowman, who was at some point quoted as saying, ‘They can eat light. Isn’t that enough?’”
The ‘Light Eaters’ Book Cover
The beautiful cover photo for “The Light Eaters” depicts a pea tendril illuminated in purple.
“I had asked for something as alien as possible,” Zöe explains. “I didn’t want a sort of botanical illustration or the sort of typical nature book look, which I respect. But my whole point of this was that plants are beyond our wildest imaginings. They’re not something we already know about really. And this photographer made a pea tendril look utterly alien, and like it was sort of grasping toward something. And I love that.”
Vines are some of her favorite plants because they’re some of the most “creaturely” plants, she says. “You can really watch them solve problems and navigate and clamber up things.”
A vining pea plant is the subject of the alien-looking cover of “The Light Eaters.”
Long-Form Reporting
As a journalist, Zöe is accustomed to tight deadlines. She usually has five days from the inception of an article idea to when she turns it in. At most, she’ll spend a couple of weeks or up to a month on an article. Book reporting demonstrated to her that having authority on a topic and a true understanding of it takes a lot of time.
When she spoke with botanists, they would each recommend five other people she should speak to. And each of those five people gave her five more. If she was writing an article under a tight deadline, she couldn’t possibly speak to everyone, but with her book deadline, she had the time she needed to contact many, many experts.
“Slowly this whole world of culture around these scientists started to come into view, and also just the understanding necessary of the history of the field and how humans have thought about plants for all time and how that’s shifted,” Zöe says. “That all took quite a long time to gather. So I suppose I could have been working on this forever if I wanted to.”
Zöe learned that botanists refrain from anthropomorphizing plants in papers and public discussions, but in the lab, they are comfortable talking in a way that anthropomorphizes plants, such as “the plant loves when I do this” or “they hate when I do that.”
She says the reason we’re cautioned, as lay people, not to anthropomorphize other organisms is because it’s dangerous to reduce any other creature into human notions of what we expect to happen. It can cause a scientist to see things that aren’t there or see the wrong thing. A better approach is to take the organism’s point of view, which can be hard to be when making human assumptions.
These scientists already knew they were thinking like plants, but they also knew that they don’t have the words to draw quick-to-understand images that explain what they mean when a plant “likes” something.
“We’re not talking about human preference. We’re not talking about thoughts and feelings. We’re talking about an organism ‘liking’ something,” Zöe says of anthropomorphizing language. “I’m doing it right now, but there’s just no other word for these things. So I’ve gotten pretty comfortable anthropomorphizing with caveats as a sort of bridge to understanding.”
Characteristics That Make Plants Appear Intelligent
Zöe visited some far-flung places, from Northern California to southern Chile, and a lab in Wisconsin.
“It’s one thing to read science and talk to scientists, but to see them doing it in the field is really something else,” she says.
She saw the ways that scientists have determined that plants can learn.
The story of Pavlov’s dog is an example of higher-order learning, Zöe explains. Ivan Pavlov’s dog heard a benign signal — a bell — and associated it with a meaningful outcome — getting fed.
“Plants learn in the sense that they appear to adjust their behavior according to what their environment requires,” Zöe says. “We’ve seen memory in plants, which is sort of an implied learning, where a plant can record the time between pollinators arriving, for example, in the case of this one Chilean flower, or a plant can count, as is in the case of a Venus fly trap, which can count how many times its trigger hairs have been flicked over a course of time,” erase that memory and start again.
One of the papers concerning higher-order learning in plants is by Australian researcher Monica Gagliano, and it has been controversial because no one has been able to replicate it, Zöe says.
“She was able to have pea seedlings associate a gentle breeze from a fan with access to sunlight, which is essentially their food. So it’s a bit of a Pavlovian dog setup.”
Gagliano put the seedlings in Y-mazes, which are used to test learning and intelligence in animals. She used PVC tubes in the shape of a Y and observed which direction the seedlings grew in after experiencing the fan with sunlight, and then just experiencing the fan.
“That paper will need to be redone probably,” Zöe says. “It was not able to be replicated when someone tried. And she’s a pretty controversial scientist herself. But I do have hope that we will get more answers about associated learning in the next couple of years.”
Zoë with a Moreton Bay fig tree at National Tropical Botanical Garden in Kauai, Hawaii.
(Photo Credit: Gloria Dickie)
Plant ‘Hearing’
“Plants do experience sound, and they experience it as pure vibration,” Zoë says. “They’re not hearing in the sense of hearing tones, necessarily, but if you think of tones as vibration, which they certainly are, they experience them — that’s a physical stimulus. It’s something that can literally vibrate their bodies.”
Research coming out in the world of phytoacoustics — the realm of questioning how plants experience sound — finds that plants are very in tune with sound information.
“So it’s not that plants can listen to music and enjoy it or like hear you talking and feel something about that, but they can pick up on noises, like the sound of their natural predator chewing, for example,” Zoë says. “There was a fabulous study from researchers who used guitar pickups — piezos — and clipped them to the leaves of a plant, and then played the exact frequency of their predator caterpillar chewing through these piezos. So that vibrated the leaf some 10,000th of an inch, and the plant reacted as though it were being eaten by a caterpillar. And then they try again with a different noise, a different frequency. And it was the sound of a leaf hopper insect mating call, which had the exact same amplitude, but different rhythmicity than the caterpillar chewing. And it didn’t react to that because leaf hoppers don’t eat plants.”
Another study looked at evening primrose, which has a cup-shaped flower.
“Researchers found that when a bee was buzzing nearby, or even just the audio of the bee being played nearby, the primrose would sweeten its nectar by three times within minutes.”
Presumably, the flower sweetens its nectar to better entice the nearby bee.
“I love that study because they then plucked some petals off of this bowl-shaped flower and tried it again, and it didn’t work,” Zoë says.
The researchers realized that with the petals removed, the bowl shape is broken, and the sound does not resonate the same.
That researcher told Zoë she is perfectly willing to look at a field of flowers now as a field of ears.
Plant ‘Sight’
Plants are incredibly sensitive to light and have more types of photoreceptors on their leaves than we have in our eyes.
“They can sense light with such incredible subtlety,” Zoë says. “For example, they know if they’re being shaded by a cloud or by a fellow plant. And if it’s a plant, they can detect whether or not that plant is a genetic relation, not just the same species, but whether or not it’s literally a sibling. And we know that they can detect color — or at least have the photoreceptors to detect blue and red, which is important for them.”
Though plants experience light, it’s not the same thing as having vision.
“They don’t have brains,” Zoë says, explaining that means plants have no place to route the light they detect into an image.
In southern Chile, there is a vine that can spontaneously mimic whatever it grows beside. It is the first plant of its kind to be discovered.
There are other plants that can mimic one other species. For example, Zoë says, the sheoak mistletoe (Amyema cambagei) can perfectly mimic sheoak (Casuarinaceae). “That is a long evolutionary relationship, and mistletoe is a parasitic plant, so it’s hooked into the body of what it’s mimicking,” she explains. “So it’s getting a lot of information. This vine in Chile, which is called Boquila trifoliolata, has no such long evolutionary history with some of the plants it’s mimicking.”
The count of plants that Boquila trifoliolata has been observed mimicking is now up to 20 or 21. It’s being called the “chameleon vine.”
“There are now researchers in Germany who are very convinced that this proves that plants can legitimately see — that they have vision — because how else would they recreate these leaf shape, leaf color, leaf vein pattern, texture of whatever it’s growing beside?”
But there is another theory that has nothing to do with vision that will take a few years to shake out.
A Chile-based researcher and ecologist named Ernesto Gianoli noticed the vine by accident in 2014. He was in a forest to study something entirely different when he noticed a Boquila vine growing next to and into a bush, then disappearing. Upon closer inspection, he realized that the vine did not, in fact, disappear, but it had become indistinguishable from the bush because it copied the bush’s leaf shape.
“Of course, this vine has been there for far longer than humans have been probably anywhere near Chile. But yet it took us this long to notice, which really begs the question of, how much other plant behavior has been overlooked?” Zoë says.
Plant ‘Communication’
Plants communicate through chemical signals.
“We’re actually talking about this entirely other sense that plants have that we simply do not, which is the capacity to synthesize very complex chemical compounds in their bodies and then kind of push them out through their pores, where they then float along in the air and reach other plants who take them into their pores and use those chemical compounds as information,” Zoë says.
As gas-sensing technology improves, researchers are gaining a greater understanding of how subtle and diverse these compounds are.
Signs of plant communication occur when a plant is under attack by a pest. The first thing a plant under attack does is boost its immune system.
“Maybe it’ll make its leaves bitter or summon the parasitic wasp that can come eat the caterpillars,” Zoë says.
We see this when tomato hornworms feed on tomato plants and the tomato plants put out a signal to parasitoid Cotesia wasps to come take care of the hornworms.
Then the plant pushes out chemical signals that unaffected plants can receive and boost their own immune systems before the pests show up.
A paper in the early 1980s reported that alders and willows that were being defoliated by caterpillars were communicating with unaffected plants.
The researcher noticed that when the caterpillar blight subsided, certain trees that had never even been under attack had tannins in their leaves to make the leaves unappetizing to caterpillars. The researcher wondered how that was possible and discovered it was because a signal was being transmitted long distances.
“We now know that plants can communicate differently with their kin than they would with stranger plants, and they can develop regional dialects of a more common ‘plant language,’ quote-unquote,” Zoë says.
Plants also communicate with insects through scent. Their natural “perfume” attracts pollinators. Zoë notes there is a group of orchids in Australia that produce a replica of female wasp and fly pheromones to attract male wasps and flies — essentially tricking the males into expecting copulation when all they actually get to do is pollinate the orchid flowers.
Monkey flowers use a chemical compound to lie to bees about how much pollen they contain, to trick the bees into thinking there is a lot of pollen that they can’t pass up.
Chemical ecologist and entomologist Consuelo De Moraes learned that at least three species of bumblebees bite the leaves of plants that haven’t opened their blooms yet, and the bites cause the plants to bloom days or weeks earlier than they would otherwise.
When a tomato plants is being eaten by a hornworm, it lets out a chemical signal that attracts the Cotesia wasp, which lays its eggs in the body of hormworms. This hornworm is covered in wasp cocoons.
Plant Agency
The word “agency” gets around a lot of the fights botanists have over the idea of plant intelligence, Zoë says.
“It’s less purely humanoid than intelligence or even consciousness,” Zoë says.
Humans have been reluctant to even bestow consciousness upon animal species, and that’s even more true when it comes to plants.
“Agency implies more of a sense of having a stake in your own future and having an active role in shaping the trajectory of your life,” Zoë says. “And we can’t deny plants that. They simply do that. They’re constantly, constantly creating conditions for themselves so that they may better survive even so that their offspring may better survive. There’s a whole world of research around maternal care in plants, and that’s not my stylistic choice. That’s simply what scientists call it, because plants will deliberately adjust the situation for their offspring so that they may better survive.”
For example, plants can design the seed coat of their seeds to be applicable to whatever moisture environment they are in
“And if that’s not agency, what is?” Zoë says.
Applying agency to plants turns them from objects into subjects, she says. “That transition from object to subject just brings in a whole world of awe and respect and curiosity that we might not otherwise have.”
Plant ‘Brains’
“Disparate parts of plants sense their environment and then communicate each of those parts, or have a way to kind of integrate that information, communicating across the whole plant,” Zoë says.
Zoë shyly suggested to University of Wisconsin Professor Emerita Dr. Elizabeth Van Volkenburgh, what if the entire plant is like a brain? Zoë felt gratified when Dr. Van Volkenburgh leaned in and whispered, “I think that too. I just don’t talk about it.”
Zoë says, culturally, people are becoming more comfortable with the idea of network intelligence, or a diffuse system-wide intelligence that doesn’t require central processing. An example of this is the mycelium network, through which fungi communicate.
Plant ‘Touch’
Zoë went to a lab to see plants that had been imbued with fluorescent green proteins from jellyfish. The lab studied how plants experience touch and how that signal travels through a plant.
She pinched a plant’s midrib with tweezers then observed the green fluorescence move through the plant. The fluorescence was tied to what is a proxy for electricity moving through a cell, bound to calcium.
“Within two minutes, the whole plant appeared to get the signal of this pinch. And presumably what then happens, it would boost its immune system — its defense process,” Zoë says.
There have been studies where researchers stroked plants and found the plants’ growth changes because the plants braced for attack rather than putting their energy into growth.
Plants and Climate Change
Plants, of course, are linked to climate because they sequester carbon. Zoë says the first plant she fell in love with in the process of writing her book is Azolla filiculoides, a tiny fern the size of a fingernail that excels at sucking up carbon dioxide.
“A long time ago, this plant helped change the climatic regime on the planet because it grew so prolifically up in the Arctic over water,” she says.
It absorbed so much CO2 that it helped tip the climatic regime from a very hot one to a cooler one. It also houses blue-green algae as its own nitrogen fertilizer factory. It could potentially be used in fighting climate change.
What Zoë’s book has done for her, and what she hopes it will do for others, is induce “re-enchantment of the world.” She wants people to “see plants as within the realm of our living, deliberate, active, sensing creatures that we ourselves are just one of.”
If you haven’t listened to my conversation with Zoë Schlanger on the Light Eaters, you can do so now by clicking the Play button on the green bar near the top of this post.
What strange plant behaviors have you observed? Let us know your experience in the comments below.
Links & Resources
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Episode 048: The Simple Science Behind Great Gardening, with Lee Reich
Episode 083: Gardening Indoors: The Science of Light, with Leslie Halleck
Episode 179: Plant Partners: The Science-based Benefits of Companion Planting, with Jessica Walliser
Episode 270: Plant Science for Gardeners, with Robert Pavlis, Part I
Episode 272: Plant Science for Gardeners, Part II, with Robert Pavlis
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joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
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joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
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Zoë Schlanger on X | @zoeschlanger
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One Response to “376-The Light Eaters: The Unseen World of Plant Intelligence”
Thank you for the great podcast. I am looking forward to reading her book. I have one question though. She said leaf hoppers don’t eat plants, which is not true. (She was referring to a study) Leafhoppers bite through leaves, stems and bits of tree trunk to suck up the delicious and nutritious plant sap, particularly Eucalyptus trees.