The Unseen World Of Plant Intelligence

ARIELLE DUHAIME-ROSS: This is Science Friday. I’m Arielle Duhaime-Ross.

Are plants intelligent? Maybe that’s not a question you ever thought to ask, and until recently, even botanists were a bit hesitant to ask the question, publicly at least. But that’s changing. In recent years, scientists have identified how plants communicate with each other, how they respond to touch, store memories, and even deceive animals for their own benefit, all bits of evidence that suggest that plants possess a unique form of intelligence that we’ve been overlooking.

Joining me now is Zoe Schlanger, author of the new book The Light Eaters– How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth. She’s also a climate reporter for The Atlantic based in Brooklyn, New York. Zoe, I loved your book. Welcome to Science Friday.

ZOE SCHLANGER: I’m so glad to hear that. So great to be here.

ARIELLE DUHAIME-ROSS: Yeah, Thanks for being here. So it’s not often that I read a popular science book and I am genuinely having my mind blown throughout. How did this book happen? What compelled you to write this?

ZOE SCHLANGER: Yeah, I was compelled to write this because of how mind blowing these findings were to me as well. I’m a climate reporter, and I’ve been covering climate change for something like seven years, and I was starting to get really burnt out. I’m sure you could relate to this. Climate news is a drag, and I was feeling really numb to the material, just sort of disconnected from all of the real impacts of climate.

And I was looking around for something else I could cover to re-enliven myself, and I sort of switched my lens from climate journals to botany journals. And to my immense surprise, botanists were fighting over whether or not plants could be considered intelligent or even conscious. And I realized right away if scientists were talking about this in public, in journals, that there must be a whole world of evidence and findings that were forcing this to bubble up to the surface.

And so I started calling botanists, and they were also the most delightful people I’d ever spoken to for work.

ARIELLE DUHAIME-ROSS: As you might imagine.

ZOE SCHLANGER: Yes. Plant people are great.

ARIELLE DUHAIME-ROSS: As I was reading your book, I was consistently surprised by the cleverness of plants, for lack of a better word, when it comes to their ability to protect themselves against harm. What’s your favorite example of a plant’s devious behavior to get insects to leave them alone?

ZOE SCHLANGER: Oh yes. So I love thinking about tomato plants because they’re everywhere. We all grow them in our gardens. What scientists didn’t realize until like the late ’90s was that tomato plants would sample the regurgitant and saliva from whatever caterpillars were eating them. So caterpillars are their predators in this situation. And from that information, they would synthesize chemical compounds in their bodies and then exude them through their pores. That would summon the exact parasitic wasp that would want to come inject their eggs into the caterpillar bodies. And then these wasps eggs would hatch, and the larva eats the caterpillar from the inside out and then glues its little Tic Tac-shaped cocoons to the outside of the caterpillar body. And that’s how the plant tries to save itself. It’s just an extremely metal approach to self-preservation.

ARIELLE DUHAIME-ROSS: And also playing the long game because this process doesn’t exactly happen super quickly.

ZOE SCHLANGER: It takes a while. Yeah, we see that a lot. There’s this interesting dance between all kind of predators and prey in nature, including the one between insects and plants, because if you think about it, in a lot of cases, the caterpillar stage of the insect is the one that’s eating the plant, and they don’t want to be destroyed. The plant doesn’t want to be completely destroyed. But yet in the later phase of its life, the winged insect that might come after the caterpillar phase might be its pollinator. So everybody’s sort of coming within this hair’s width of death, in a way, or this funny struggle to ultimately all survive.

ARIELLE DUHAIME-ROSS: The plant wants the caterpillar to make it to the butterfly stage, but it doesn’t want to get completely eaten so much so that it can’t benefit from that stage.

ZOE SCHLANGER: Exactly.

ARIELLE DUHAIME-ROSS: And by the way, I just want to be clear. When I say that a plant wants something– I’m sure, Zoe, you’ve had to do this the whole time that you’ve been talking about your book. We’re using these words that aren’t typically used for plants, and I understand if, for some listeners, this might be a stretch, but that’s something that you address in your book.

ZOE SCHLANGER: Totally. You can’t see me, but I’m using big air quotes every time I’m using these words. And I think that’s really important to emphasize because I’m not suggesting plants are kind of little humanoid or like little animal creatures, just in a different form. They are this branch of life that are so entirely their own, and they’re so alien to us in so many ways. I mean, they diverged from us evolutionarily so long ago that all of their strategies are really, really so other, very fundamentally other from us.

So it’s useful to use these words that make sense to our human brains like wanting or communicating or thwarting predators, things like that. But you just have to hold in your mind at the same time that whatever’s going on here looks a lot different than it would to you or I.

ARIELLE DUHAIME-ROSS: Right. It might be worth diving into this a little bit more because I studied zoology for my undergrad, and it was there that I really understood how important it is not to project our own human values, desires, or compulsions onto other animals because you might miss something if you don’t put yourself fully in their shoes, so to speak. And as I was reading your book, I found myself really loving how some of these researchers were talking about plants. So I’ve got to ask, what’s the right way to talk about plants, and how should we think about using human terms for plants?

ZOE SCHLANGER: I love this question. I love that you have this zoology background because it’s so interesting. What’s happening now is that there are a lot of principles and methodologies from animal behavior being applied to plants that actually seem to pan out when you really take the plants’ point of view and adjust the methodology to make sense to something that evolved the way plants evolved. So it’s complicated, right?

On the one hand, these are not animals. We have to hold that in mind. On the other, there are parallel processes going on. And so the bridge between those two things I think is mostly metaphor, at least when it’s two laypeople talking to each other as opposed to two scientists.

I do think we have to use these metaphors, which are kind of near at hand to our own understanding, to understand plants. But I like to think of it as like a plant decision, not a kind of normal decision but a plant decision, just having this kind of–

ARIELLE DUHAIME-ROSS: I like that.

ZOE SCHLANGER: –prefix before the word. So your brain has a chance to be like, OK, wait. I’m going to hold back from completely anthropomorphizing.

ARIELLE DUHAIME-ROSS: Love that. OK, so let’s get back to some plant facts for a little bit here. I’ve got to ask you, keeping everything that we just said in mind, do plants have personalities, or are we able to say that yet?

ZOE SCHLANGER: So this is an interesting one. It is a field that’s just starting to poke through. There’s only been one serious assessment paper on this, but there’s a school of thought that plants could be seen to have personalities because of the way they consistently respond or the way that some of them consistently respond in similar ways throughout their lifetime. Animal personality is measured on this shyness to boldness continuum. That’s the most basic way we assess animal personality.

ARIELLE DUHAIME-ROSS: Right, risk averse versus risk taking, right?

ZOE SCHLANGER: Precisely. Exactly.

So we know that many plants emit volatile chemicals when they’re being attacked that then can be absorbed into the bodies of neighboring plants, and those neighboring plants can act on that information. They can boost their immune systems, start producing defensive compounds before the pests even reach them. So this is what we’re talking about, mostly, when we’re talking about communication in plants.

Some plants will signal wildly at the slightest disturbance.

ARIELLE DUHAIME-ROSS: So the risk averse ones.

ZOE SCHLANGER: Exactly. And then there’s this interesting kind of boy who cried wolf phenomenon where other plants in the area might not boost their immune systems in response to the calls from those individuals. But then other plants have a much bolder propensity, so to speak, in that they will only signal when the threat is really severe, and other plants appear to actually respond to those calls much more sensitively.

ARIELLE DUHAIME-ROSS: Wow. OK, again, just the stuff in your book is just so interesting. You also write about the dodder vine. What does its behavior tell us about what plants are capable of?

ZOE SCHLANGER: Oh, I love the dodder vine. And because this has been brought up before, it’s dodder, D-O-D-D-E-R, not daughter as in your offspring, before I explain the very metal things this plant can do.

So a dodder vine is parasitic, and it comes out of the ground as a baby vine. It looks like a little orange worm because it doesn’t have any need for chlorophyll because it doesn’t photosynthesize. It gets everything it needs from other plants’ bodies.

So this little dodder vine emerges, and it has about a day to find a host to parasitize. So it starts circling the air, and you can see this on time lapse. And presumably it’s sampling chemical compounds nearby, and researchers have found that it can assess from afar which plant nearby would be optimal to climb on. So researchers have put it between a tomato and a wheat, and wheat is a grass. It’s very hard to climb. And before even making contact, the dodder will swerve and go towards the tomato. And when researchers tried to figure out how it was doing this, they found it was actually because of the quality of the light passing through the bodies of other plants that it was able to sense the forms of plants near it.

Then researchers have also found that it can sense the nutritional quality of the plant it’s climbing. When researchers grew it between plants that had been grown in a nutrient-scarce environment or with nutrient supplements, it would almost always decide to climb onto and hook its fangs into the plant that was grown with nutrient supplements.

So then it grows these little vampiric spikes all along its edge. It’s a very vampire plant. And the amount of coils it makes represents how much energy it thinks, quote, unquote, it will need to parasitize it.

And then it keeps these plants hobbling along. It doesn’t want to kill its host. It’s a good parasite. It just sucks out enough sugar that it makes it very hard for these plants to grow fruit. So dodder is actually this extremely nasty agricultural pest in 55 countries.

ARIELLE DUHAIME-ROSS: Wow. OK, I want to go back to something you said, though. You mentioned the plant sensing light, which instantly my mind went to sight. Does this mean that some plants can, quote unquote, see?

ZOE SCHLANGER: You are touching on this very hot issue in botany at this very moment. There’s a vine I went to go see in Chile in the southern cold rain forests of Chile called Boquila trifoliolata. And it is a super-common vine in this forest, but just like 10 years ago, a researcher realized that this vine was transforming its body to look pretty much exactly like whatever it was growing beside, which is unheard of for plants. I mean, we have some plants that mimic other plants, but it’s like a one-to-one relationship.

Boquila can mimic, apparently, spontaneously. Whatever it’s growing beside, it will attempt to transform its leaf color, leaf shape, texture. Even the vein pattern of these leaves will change to match what it’s growing next to. And I saw that when I went down there. It was absolutely unbelievable to watch a plant that’s transforming in real time.

ARIELLE DUHAIME-ROSS: That’s incredible.

ZOE SCHLANGER: I know, right? This has provoked some speculation from one camp of botanists that this is evidence plants can see. They’re kind of like, plants have so many photoreceptors on their bodies. They have more types of photoreceptors than we have in our eyes. And we already know they’re very sensitive to light. They’re constantly sensing whether they’re being shaded by a cloud or by another plant, and they can, in some cases, tell whether that plant is their genetic kin or not and behave accordingly.

So there’s the plant vision hypothesis, but there’s also a huge camp of botanists who are going uh uh. That’s too far. Plants can’t see. There’s no brain. There’s nowhere to translate that information.

ARIELLE DUHAIME-ROSS: Right. So one of my favorite scenes in the book is this trip that you make to a lab where you describe really vividly being asked to pinch a plant really hard. You write about this beautifully. It’s actually quite funny because I believe the first time you don’t pinch quite hard enough, and you have to be sort of egged on to finally do it the right way.

ZOE SCHLANGER: Totally.

ARIELLE DUHAIME-ROSS: But I would love it if you could explain, What you were doing? Why were you asked to pinch a plant? and what did that help you and the scientists in the lab see that day?

ZOE SCHLANGER: This was at the lab of Simon Gilroy, who is at the University of Wisconsin, Madison, and he studies plant responses to touch. And in his lab, he has all these plants that have been grown to have these green fluorescent proteins from jellyfish imbued throughout them. So in every cell, there’s this green fluorescent protein expressing itself. And that protein is bound to calcium, which is thought of as a potential proxy for electricity. So when the plant is glowing, that means there’s electricity moving through the cell, which means maybe there’s an action potential. There might be electricity happening here.

So, as you said, I was in this dark microscope room, and I was given this pair of tweezers. And they were like, OK, pinch. And I, having just spent the last several years studying plant receptivity and how sensitive they are to everything, felt a little reluctant to pinch very hard, as silly as that sounds. And so I pinched it the first time, and the plant lit up a little bit.

But then they were like, you have to really go at it. So I pinch quite hard, and I pinched across the midrib of the leaf. That’s like the main vein that you can imagine on a leaf. All of a sudden, there was this green glow emanating from the place that I’d pinched this leaf that I then watched over the next two minutes travel down the vein system and spread through the veins to the entire rest of this plant. And this was a tiny little Arabidopsis plant, which is kind of the lab-rat plant of the botany world.

And I was literally watching it become aware of my touch, so to speak. This is entirely new information to botany, this idea that there may be an electrical signal moving through the plant body from the place it’s touched, and that could be one way it’s understanding its world, that it has this ability to respond very quickly to being wounded.

ARIELLE DUHAIME-ROSS: Yeah, I mean, I feel like there are two immediate questions that come to mind for me. The first one being, Is this a pain signal? and the second one which comes right out of that, which is, does this mean that, if the plant has an ability to transmit this information via something like a pain signal, does that mean that plants have a nervous system?

ZOE SCHLANGER: So your first question about pain, I would say definitively no, not at the moment. We have no reason to believe plants are experiencing pain. They don’t have a brain, which is one of these things that we think of as quite necessary for any experience of pain.

ARIELLE DUHAIME-ROSS: OK, got it.

ZOE SCHLANGER: But there’s something very nervous system like about how this all works, and those aren’t my words. Those are the words of other researchers that have looked at what Gilroy is doing and gone, does this mean plants should be considered to have a type of nervous system, in a sense? And it sparked this conversation around whether or not nervous systems could be considered the product of convergent evolution, where a system arises across different types of forms of life separately multiple times. Like flight evolved separately in bats and bees and birds and to very similar effect. Could this be another case of that?

And what’s also very interesting is that we are now understanding plants use neurotransmitters, including glutamate. So the tweezers that I was pinching this plant with were dipped in glutamate, which is an important neurotransmitter in our own bodies and appears to also be important to signaling in plant bodies.

ARIELLE DUHAIME-ROSS: We have to take a quick break, and when we come back, I’ll continue my conversation with science journalist Zoe Schlanger about her book The Light Eaters. We’ll tackle the big questions. Are plants conscious, or are they intelligent? Stay with us.

This is Science Friday. I’m Arielle Duhaime-Ross. If you’re just joining us, I’m talking with science journalist Zoe Schlanger about her new book Light Eaters– How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth.

So we’ve talked about chemical seeing. We’ve talked about photoreceptor, maybe, seeing of some sort. We’ve talked about summoning animals to kill a predator in some ways. We’ve talked about a nervous system. All of these findings, does that point to plant consciousness?

ZOE SCHLANGER: This is the big, sticky question. I think one has to back up and remember that we still don’t know the mechanical basis for consciousness, even in ourselves. What is consciousness? is an open question for humans.

And we also are just starting to ascribe consciousness to other beings. I’m sure you saw this news a couple of weeks ago where consciousness has now been conferred to fish and insects and crustaceans. So we are constantly widening this circle of what we think contains consciousness.

And there’s also a school of thought that suggests there’s consciousness all the way down. There’s some prominent consciousness researchers who suggest that even something as basic as a cell contains some modicum of consciousness to function. I think about consciousness as a sort of awareness of self. And then intelligence, which is maybe even higher than consciousness in some ways, is the ability to make good decisions based on information that you acquire from your environment and then kind of sift through that and assess it and then produce a response that is matching that information.

ARIELLE DUHAIME-ROSS: Which is a little bit what you’ve described in some of these findings, right?

ZOE SCHLANGER: Exactly. This is my very long and smushy way of saying, we do not know, but it really depends on how we choose to think about intelligence and consciousness. We may never come to a moment where we’re like, this is definitively what consciousness is, and plants have it. But I think there’s a lot of evidence to suggest these are good questions to ask.

ARIELLE DUHAIME-ROSS: I do have to ask, we’ve mentioned multiple times that plants don’t have brains. I guess the next logical question is, can a plant be intelligent without a brain?

ZOE SCHLANGER: I think we’re all kind of culturally coming around to this idea that intelligence might exist in things that are brainless. I know that probably a lot of listeners have thought about fungi and the new findings about these mats of mycelium under the ground that are able to communicate across their entire body but are very disparate and have no centralized kind of command center. And I’m thinking we’re starting to get more comfortable imagining intelligence as a network phenomenon, that there’s forms of intelligence that are sort of systemwide rather than centralized.

And I like to think about the fact that we evolved with brains for very good reason. I mean, we have to run around. We have to flee from threats. We have to go find our food. So it made sense we have this very portable command center.

But plants evolved rooted in place, so there’s something to be said for the fact that evolution wouldn’t have produced a compact command center. Plants are modular. They can lose a limb at any time and be pretty much fine. They also have to sense their environment from this one spot, so they have to be able to sort of spread out, both above and below ground, and then have a way to kind of coordinate their findings, so to speak, about what’s going on around them. Would that necessarily require a brain? It’s not clear.

ARIELLE DUHAIME-ROSS: That’s an excellent question. To me, what this points to is the fact that your book is sort of a snapshot in time of a field that is changing. And we’re going to find out more over the years, which is really exciting. At the same time, I can’t help but think back to a previous book that also had a huge impact on the world of plants. And your book, The Light Eaters, is basically in dialogue with it sort of throughout.

I’m talking about a book that was largely incorrect, published in 1973, called The Secret Life of Plants. Tell me about that book and its relationship to yours.

ZOE SCHLANGER: Absolutely. So I was sort of haunted by this book the entire time reporting out this, The Light Eaters, because botanists were telling me that when that book came out, it was such a phenomenal popular success that it was incredibly embarrassing for the field when it turned out that about half that book, or maybe more, was just pseudoscience. I mean, there was stuff in there about plants reading people’s minds. There’s this whole scene with a CIA agent who hooked up a polygraph test to his houseplants and then thought about setting them on fire, and he said that the test went wild, which was suggesting they were reading his malevolent thoughts. And then also, if you ever hear people talk about how their plants enjoy Mozart more than rock and roll, that’s all from this book.

And the effect of all of that was that the very conservative funding bodies of science kind of stopped funding studies that had any whiff of plant behavior or kind of veered into this what was then beginning to be seen as a kind of woo woo, new-age concept. And it’s taken 30, 40 years for that taboo to finally wear off. I think we’re there now, and that, combined with all of this incredible new technology we have for sensing gases and all the chemical compounds plants make and the genomic revolution, has all made it possible to ask these questions again with much more rigor. And I’m not talking about the mind-reading questions but plant-behavior questions that are genuinely rigorous.

So in some ways, this book is an answer to that. We saw that kind of hit the field really hard in a negative way, and I’m hoping this will do the opposite.

ARIELLE DUHAIME-ROSS: It certainly seems likely. I want to ask, though, from your perspective, in writing this book, The Light Eaters, how have you shifted your understanding of the role of plants in the world? How has this affected your relationship with your houseplants?

ZOE SCHLANGER: Oh, my house plants? Oh my goodness, yes. I definitely walk through the world with a different awareness that all the plants around me are engaging these very complex plant dramas all the time.

[LAUGHTER]

Like, I know that they are potentially fighting underground or communicating underground or rearranging their leaves to either shade their neighbors or not shade their neighbors or summoning insects and forming collaborations with insects. And that has totally re-enchanted my life. I mean, it’s such a delight now to just look around and see a climbing vine on a chain-link fence and kind of internally commend it for its total resourcefulness now that I know so much more about vines and what it takes for them to seek good structures and clamber over obstacles. So I love that about it.

And it’s also couched in this general awareness now that plants are totally the primary beings out here. I mean, they’re photosynthesizing. They’re eating light. That’s still the coolest thing they can do. And that means they have made every single molecule of sugar through that process that we have ever consumed, and our whole bodies run on plant sugar. I mean, glucose, you can’t do without it. You will shut down very quickly without glucose, and we get it from nowhere else. I mean, any other form of sugar we eat was recycled from sugar plants first made.

And holding that kind of preeminence of plants in mind really kind of unsettles this very human notion that we are on top of some evolutionary heap. It kind of settles us back into the mess of biology rather than something on some perceived hierarchy.

ARIELLE DUHAIME-ROSS: I love when new findings or a set of new findings have this humbling effect on me, and your book certainly did that for me and also introduced so much wonder into my life. So I really, really appreciate the book itself and also appreciate you taking the time to talk to us about it today. Thank you so much.

ZOE SCHLANGER: I’m so glad. I’m so glad it had that effect. It’s great talking with you.

ARIELLE DUHAIME-ROSS: Zoe Schlanger is the author of The Light Eaters– How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth. She’s also a climate-change reporter for The Atlantic based in Brooklyn, New York.

If you want to read an excerpt of the book, go to sciencefriday.com/lighteaters. That’s sciencefriday.com/lighteaters.

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