Rachel Feltman: Can plants “think”? Do they “make plans”? Do they “talk” to one another? Believe it or not, these are questions that scientists are asking quite seriously—and it’s starting to look like the answer might be yes.
For Scientific American’s Science Quickly, I’m Rachel Feltman. I’m joined today by the author of a new book on the surprising science of plant behavior.
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Feltman: Would you mind introducing yourself?
Zoë Schlanger: I’m Zoë Schlanger. I’m a climate reporter at the Atlantic. And about five years ago I started researching the world of plant-intelligence research, which is really plant-behavior research, with a small cluster of botanists who are trying to push forward the idea that what we’re seeing plants be able to do is, in fact, signs of intelligence and possibly even consciousness. So I wrote a book called The Light Eaters that came out very recently, and it’s all about these incredible findings and what they might mean for our understanding of nonhuman minds.
Feltman: Zoë, thanks so much for sitting down to chat.
Schlanger: It’s so great to be here.
Feltman: So what initially drew you to writing about plants?
Schlanger: Well, like I said, I’m a climate reporter, and I, about five years ago, was feeling really burnt out. As you can imagine
Feltman: Yeah, I know what that’s like.
Schlanger: Covering wildfires and melting sea ice becomes really depressing. And I was feeling quite numb and detached from the material and decided I need to go seek out something that felt hopeful or awesome—in the literal sense. And I started kind of changing my filters for looking at journals to botany journals as opposed to only climate journals. And the timing was so incredible because I was seeing botanists argue in real time in these journals about whether or not plants might be intelligent. And it was accompanied with this kind of flurry of papers that was coming out asking questions like, “How is sound experienced by a plant?” Can plants, you know, quote, unquote, “hear”—with big air quotes around that? Of course, sound is, like, pure vibration to plants—it’s a physical stimulus—but they do respond to it. Or can plants experience touch, which is also this physical input? And about how plants were communicating and having—not just communicating but having, like, regional dialects: communicating differently with, like, their family group as opposed to strangers. And I just could not believe that this stuff was sort of, in a way, hiding in plain sight. So it felt like I just stumbled on the best story of my career.
Feltman: To back up a little bit I think [for] a lot of our listeners, this will probably be a very new concept for them. And they may even think it sounds kind of fringe. What is it that scientists think is going on with plants, and does this feel so surprising to folks who are paying attention to mainstream science?
Schlanger: Yeah, I totally know what you mean about this all sounding maybe a little New Age. And I came into this with a lot of skepticism, too, and the truth is: I don’t know where things will shake out in terms of intelligence and consciousness with plants. But I think what we’re really talking about is whether or not plants are far more active and responsive to their environments than we’ve kind of culturally given them credit for up to this point. I think a lot of us think of plants—in terms of, like, hierarchy of complexity and responsiveness—as somewhere, like, above rocks but below most animals, which is not our fault exactly. It’s the kind of, like, Western scientific cultural heritage. Most of us were raised within this scala naturae of, like, where things are on this, like, “ladder of life.” And it’s such an interesting moment for that, right? Like, we have collectively—I say “we,” I mean—what I mean is, like, biologists and philosophers have conferred now, very recently consciousness—or at least the possibility of consciousness—on to mammals and birds. That was in this 2012 declaration, this Cambridge Declaration. And then just, like, a month ago at [New York University] the same group got together and put out a declaration saying that insects and crustaceans and fish may also have the substrates for consciousness. So we keep widening this circle. And so now some people are stepping in—some people in the plant sciences and kind of the neuroscience world—and asking, “Huh, are we circumscribing the circle still too small?” Like, do you need a brain to have something like consciousness—or intelligence, which is something kind of higher order than consciousness in some ways. Consciousness is kind of this awareness of your situation, but then what happens when you make good decisions, have memory that you use to make wiser choices to thrive—all of which are things that we are finding plants absolutely can do, and they do it spontaneously. There’s, like, an incredible amount of behavioral flexibility with plants. So it might be time to kind of include them in that conversation.
Feltman: Yeah, well, and one thing that I love about this book is that, you know, obviously, the idea of plant intelligence is going to be very new and exciting for some people. But I think that the conversation about what consciousness and intelligence is is something that a lot of people are not clued in to, and that has really, really broad implications. What did you learn about that larger conversation?
Schlanger: The larger conversation about consciousness is so interesting because we don’t have definitions for consciousness in ourselves—or at least we don’t have a knowledge of the mechanical basis for our own consciousness, nor even how to definitively define something as conscious. It’s all this kind of wavy stuff around consciousness and many theories. Some people say consciousness can only be verified by the ability to knock someone unconscious—like, our ability to be put under, in a way. And remarkably that test also works on plants. You can etherize a plant—diethyl ether works on plants. So in general, plants are moving around all the time, slower than we mostly can see. But in time-lapse, like, a little pea plant will be, like, waving its tendrils around all the time and curling and uncurling them. And if you put a pea plant under a bell jar with diethyl ether, they will, like, grind to a halt when researchers removed ether, within 15 minutes, they kind of go back to dancing around again. And what’s happening there, they found in other plants, like Venus flytraps, is that their electrical pulses are reducing, which is also what we see in ourselves: the electrical activity in our bodies also goes down when we’re under the influence of anesthetics. And similarly we don’t really know the mechanism for why anesthetics works on us. So these mysteries of consciousness—even in ourselves—are so intriguing, especially when we’re trying to draw really hard lines in the sand between who has it and who doesn’t.
Feltman: It’s easy to see how some people have gotten carried away with the idea of plant intelligence and taken it too far, which is something you get into in the book.
Schlanger: I mean, the biggest danger in writing about this is that it gets flattened into some cartoonish version of itself, where, like, plants are little humanoid creatures, and that’s definitely not what I’m trying to do at all. And [it’s] in part because this whole book is haunted by a book that came before it in 1973 called The Secret Life of Plants, which many people probably remember. And it’s the reason people play classical music to their plants or think that plants like classical music more than rock and roll. There’s a famous chapter in this book about how a former CIA agent hooked up a polygraph test to a plant and then thought about setting this plant on fire and claimed that the polygraph went wild, which implied the plant was reading his mind.
Feltman: Lot of levels there—polygraphs: not accurate.
Schlanger: And then when that couldn’t be replicated, he said it was because you have to have a good rapport with your plant for it to work.
Feltman: It’s the perfect excuse.
Schlanger: The vibes have to be good. And the damage that book did to the field was immense. The funding agencies sort of closed the doors to research on plant behavior. It was a very embarrassing moment because you have this, like, unleashed on the popular imagination. It was very sticky. Everyone was talking about it. And there was so much in there that it was not reproducible science. And so we don’t want to jump that gun. This is a field in tremendous flux right now; we don’t have settled answers as to the full extent of what plants can do or what we should make of this. But there is a lot of incredible, fully peer-reviewed research coming out that is bringing up these questions of, “How should we regard the fact that they have ... like, plastic, responsive decision-making abilities or that they can communicate and things like that— they can detect their kin?”
Feltman: Thinking back to, you know, when you were just becoming aware of this field, do you remember what your initial reactions were to papers that got into the idea of plant intelligence and behavior?
Schlanger: I felt very skeptical toward them. I called several of the people behind these papers and several people who very strongly disagree with them. And I think people on all sides have really valid points. I mean, I think the most valid point is that, like, “Should we even evaluate plants according to human notions of intelligence?” It’s hard for us to dislocate our sense of intelligence away from the extremely human and, like, sometimes even academic version. But I was also intrigued because what happens if we can hold in mind the fact that plants are so incredibly other, that they diverged from our evolutionary branch so long ago as to be basically aliens to us, but to imagine how intelligence might have formed under the evolutionary pressures that they experienced and that it would be very different from what we’d expect of ourselves or even of mammals or in general—or animals in general but that that could still—whatever that was could still be a form of consciousness because they evolved under the same evolutionary pressures we did? And why should we we’re the only ones to have gotten it?
Feltman: It reminds me of animal-cognition researchers who say, “If you want to figure out how to communicate with aliens one day, the perfect place to start is studying how animals communicate—nonhuman animals—because it requires really figuring out how to peel away biases about how cognition works or what communication is that we might not even realize we have.” So I feel, like, with plants that it’s that times 10 at least.
Schlanger: Yes, doesn’t get really more other than plants.
Feltman: Totally.What are some of your favorite examples, you know, in introducing people to the concept of your book and this field?
Schlanger: There’s so many options. I think some of the most intriguing and, like, also rigorous work is happening now in the world of phytoacoustics—thinking about how a plant experiences sound. And this is work being done by researchers who are probably mostly not willing to take, like, an opinion on plant intelligence, but they’re certainly willing to think about how acoustic vibration, which is everything that sound is to plants, affects their lives. I spent a lot of time talking to a researcher named Heidi Appel at the University of Toledo in Ohio. We already know that plants—many plants will respond to caterpillars chewing on them as a form of, you know, “This is an attack.” They will boost their immune system. They might start making defensive compounds. Very cool thing I like to think about is the fact that, like, tomatoes and corn and cotton will all sample the saliva of the caterpillar eating them and then use that information to generate the exact chemical compounds needed to summon parasitic wasps that will then come and, like, inject their eggs into the caterpillar, and those larvae will hatch and eat the caterpillar from the inside out and then glue their little cocoons to the outside of the caterpillar husk. It’s all very metal.
Feltman: Really, really tempting to ascribe not just, you know, intention there but, like, some pretty, like, Game of Thrones–level intention.
Schlanger: One hundred percent. So knowing that plants respond to caterpillars, Heidi wanted to test whether they could be responding to merely the sound of their caterpillar predator chewing. And so she put real caterpillars on these plants, had them chew the plant. Then she took the real caterpillars off and clipped little guitar pickups onto these plants and essentially played the sound of the caterpillar chewing through the guitar pickups, which then vibrated the leaves this, like, infinitesimal amount, and she found that they responded as though they were being eaten by caterpillars. She tested it with a leafhopper mating song—so completely different insect—and it didn’t respond, and leafhoppers don’t eat the plant that was being tested. So it’s hyperspecific, and audio, or acoustics, do matter to plants. And that world of research is really blooming right now.
Feltman: And what about just, like, some of the most delightful or surprising things you learned in the course of writing the book?
Schlanger: I love the fact that plants in isolated geographic areas might develop regional dialects, which I think is so interesting. I’ve seen the work of two research groups on this: one on goldenrods and one on sagebrush. And these are both, like, plants probably most people can imagine if they live, at least, in North America. So basically we understand that plants release highly specific volatile chemicals that then alert other plants when there’s an attack happening in the area. And then the other plants have a chance to, like, boost their immune systems before, let’s say, the pest reaches them. When there’s a group of plants living in an isolated area, at least in the goldenrod and sagebrush, they will develop regional variations on the more common chemical compound language, very similar to how regional dialects develop in us.
And plants have also been found to communicate differently with their kin, as opposed to the, you know, kind of less related plants in their area, depending on how much, depending on how much herbivory is in the area. So when, when there’s a lot of pressure there, when there’s a lot of danger in the area, a plant will signal in a more universal way that can be kind of quote, unquote “understood”—air quotes—“understood” by everybody in the area, all the plants in the area, as opposed to when there’s hardly any danger, in the rare event that there is some, they’ll keep that signal only to their closest family members, which has clear evolutionary basis—there’s theory for this: that, like, when attacks are less frequent, you’re more concerned with saving your own little family.
Feltman: When we have these conversations about plant intelligence, I feel like there’s often this kind of reflexive response—people have to be a little freaked out about the potential implications, sort of like, “Are you going to tell me next that I can’t eat vegetables?” They’re, like, imagining the fruitarian character in the movie Notting Hill. But what do you think: Should or will actually change about the way we think and talk about plants as we come to understand that they have a lot going on—whatever we call that?
Schlanger: Yes, I have gotten the question many times of, like, what will the vegans do? And I’ve—it’s—I understand where that question comes from. And we are animals that need to eat plants.
They are the only thing that makes the glucose that we need to just keep our every organ running.
Feltman: And many of them have evolved to rely on us eating them.
Schlanger: Exactly. Many people have brought up this kind of thought experiment. You think of agriculture as a form of domestication of these kinds of wild species, but have they domesticated us into tending them? And of course, when they produce fruit, that is something that’s very literally meant to be eaten. But beyond that, I think, you know, on some level, being an animal that relies on other living things to support our lives and our survival means we have to sort of acknowledge that we have to enact some form of violence, just keep going. It’s—I don’t think it’s useful to, like, segregate that fact away from your daily life. In some ways it’s kind of a beautiful thing to open yourself up to that because it underlines this sense of dependence. And that’s a very humbling thing. So if we acknowledge that we are, in some way, doing some violence to creatures that very much want to live, which is probably all biological life to some extent. Then what can we do with that feeling? Instead of backgrounding it, like, maybe we can transform it to have some greater sense of respect for how we cultivate these things or think about things more from a plant’s perspective in terms of “Can we grow them in, in ways that respect their inherent abilities to do things like communicate and defend themselves?” which gets into, like, growing crops maybe in more diverse settings, where they’re interacting a lot more in ways that might benefit everyone involved. Maybe including them sort of in our imagination as highly animate actors would not leave any more room for indiscriminate destruction. There’s, like, a degree to which we need them in every aspect of our lives, but then there’s a degree to which we’re, like, maybe ripping up a ton of mangroves to build a hotel is something we think twice about or even, you know, felling 30-year-old trees for toilet paper. There’s other ways to do these things, and maybe some cognizance of the incredible biological creativity of all plant life could change how we do those, ideally.
Feltman: Yeah, I definitely need to go home and hug my potted plants.
Schlanger: They may respond to that as an assault, so that’s up to you.
Feltman: I’ll just look at them lovingly—with appreciation.
Schlanger: They’ll read your thoughts.
Feltman: Yes. Think very pleasant, not setting-them-on-fire thoughts. So we talked about how you got started thinking about plants. How has your relationship with plants changed in the course of writing this book?
Schlanger: I see plants more as individuals rather than kind of the wash of green I think I, like, experienced in general before—I mean, I’ve always appreciated them. But now when I see, like, a bunch of plants growing together, I can’t help but think of, like, the intense plant drama that’s definitely happening. They sense the presence of other plants around them. They have all these complex dynamics depending on how related or unrelated or aggressive or whatever. You know, some of these words have way too much human feeling attached to them. But they are interacting aboveground, belowground. And I especially like walking around Brooklyn and see a ton of Japanese knotweed, for example, now that it’s spring. It’s a highly invasive plant. I’m so impressed by it in some ways. I mean, I get that it’s a problem, but I’ve learned a lot about how plants, particularly plants in the knotweed family—there’s a lab at Wesleyan [University] that studies this—smartweeds and knotweeds pass down skills generationally. So, like, if the parent plant is grown in a nutrient-scarce or, like, water-scarce environment, they will have offspring that is better suited—or some of them will have offspring that is better suited to handle those conditions. And then that happens every generation, and then you end up with, as this lab puts it, like, monsters of their own making—just, like, incredibly adapted plants. And that is so cool. So I can’t help but, like, commend plants for their resourcefulness when I see them around now, even the ones that pose problems for us.
Feltman: And has it changed how you think about other things in the natural world?
Schlanger: I think it’s really interesting how some of these findings about how amazing plants are are kind of resettling us back into this notion that plants are agentive organisms. And I feel like there’s a lot to learn from Indigenous science. Part of this whole process for me started out with reading Braiding Sweetgrass, by Robin Wall Kimmerer, in which she writes so much about how the questions Indigenous science can ask of the natural world are so different, in part because they see all of life as part of the same system—humans are not separate from nature.
One other thing I’m really interested in is how we kind of draw lines in the sand between species and groups of life. One thing I was really intrigued by in writing about plants was how those categories all start to falter. Like, I learned about this sea slug that I’m now obsessed with called—this brilliant-green sea slug; that’s literally what it’s called—and it eats—its first meals are chloroplasts from algae, and it’s shaped like a leaf. And so after these first few meals of chloroplasts, it starts running photosynthesis, and no one knows why—how it can genetically run photosynthesis, but that blurs this boundary between animals and plants.
Feltman: Sure, yeah.
And it makes me think of Why Fish Don’t Exist, by Lulu Miller, which is also—you know, she also came to that realization that maybe the whole category of fish as sort of separate from other animals is faltering, too. So I think it’s really important to remember that science is such an incredible form of knowledge generation, and, and it keeps changing its mind, so who knows what’s gonna come next.
Feltman: Zoë’s book, The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth, is now a New York Times bestseller. You can find it wherever books are sold, and I highly recommend you do so.
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