By Katie L. Burke, an award-winning features editor and science journalist. She is a senior contributing editor at American Scientist. Originally published at Undark.
In 1973, the bestselling book “The Secret Life of Plants” was published, captivating the public with questions about plant sentience and communication. Even if you haven’t read the book, you’ve probably heard of the experiments it describes: playing classical music and rock and roll to plants, for instance, or hooking them to a polygraph. The book even inspired a film with a soundtrack by Stevie Wonder.
The experiments were fun ideas, but poorly designed. Scientists strongly rejected the book and distanced themselves from its views. “According to botanists working at the time, the damage that Secret Life caused to the field cannot be overstated,” writes Zoë Schlanger in her new book “The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth.” “Over the following years,” Schlanger reports, “the National Science Foundation became more reluctant to give grants to anyone studying plants’ responses to their environment.” And, she continues, “Scientists who had pioneered the field changed course or left the sciences altogether.”
It took about 40 years — a generation of scientists — for that chilling effect to lift. Over the past 15 years, funding for plant behavior research has returned, at least in small amounts. Schlanger acts as tour guide through this history and the pressing questions new research poses about the shared future of plants and humans.
Considering the history of research on plant intelligence, the book’s subtitle may elicit skepticism. Even wildly popular books like “The Hidden Life of Trees” have come under criticism for getting ahead of the evidence when it comes to plant communication. But “The Light Eaters” delivers: Schlanger’s thinking is rigorous and she describes these contentious intellectual debates with a sense of fairness and curiosity.
There is clear excitement in Schlanger’s endeavors to meet the few scientists who have been able to push the field forward. Her exploration takes her all over the world: to a Chilean rainforest to see a plant that mimics others like a chameleon; the Hawaiian island of Kaua’i, which is home to a staggering number of rare and endangered plants; and the University of Bonn in Germany, to meet one of the founders of the Society for Plant Neurobiology (now called the Society of Plant Signaling and Behavior). It hasn’t been easy for the scientists she meets along the way. Although a few of the lucky and intrepid have painstakingly carved out a niche, Schlanger comes across many who put their careers on the line to research plants’ uncanny abilities to sense their world; some sadly left the field entirely. Others put their research on hold for decades, turning to teaching or more fundable research questions.
Despite the challenges in the field, Schlanger finds a vibrancy in her subject matter that contrasts sharply with her job as a climate journalist, where she began to burn out from all the grim news she was processing daily. “Journalists in my line of work tend to be focused on death. Or the harbingers of it: disease, disaster, decline,” she writes. She wanted to be around life, celebrate it, in a way she rarely could in her day job. “In this ruined global moment, plants offer a window into a verdant way of thinking,” she writes. The world’s flora “suffuse our atmosphere with the oxygen we breathe, and they quite literally build our bodies out of sugars they spin from sunlight,” she continues. “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.”
“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.”
Indeed, Schlanger covers how plants sense and respond to their environment — or the evidence that they have such senses, even if scientists don’t know the underlying mechanisms. Plants communicate through not only chemicals in the air and soil, but also, potentially, through sound. Air bubbles pop as water travels from a plant’s roots up through their stems, emitting an ultrasonic click. Each type of plant that has been studied — wheat, corn, grapevine, and cactus, for example — has a unique frequency. Plants can perceive touch and transmit electrical signals, too, which poses another way they can communicate. And these beings sense light in sophisticated ways that invoke comparisons to vision; a vine that grows in the Chilean rainforest, Boquila trifoliolata, can mimic nearby plants down to the leaf shape, texture, and pattern of venation, though no one yet knows how it can “see” its neighbors. Plants also have memory and social behaviors. A plant in the nettle family, Nasa poissoniana, can anticipate when a pollinator will visit its star-shaped flowers, based on past time intervals between visits, and will raise its pollen-bearing stamen.
Yet plants do not have brains: Their intelligence is not centralized, but rather a distributed network. “How does information about the world get integrated, triaged by importance, and translated into action that benefits the plant?” Schlanger asks. That is the question at the forefront of research, and whether plants are conscious is an ongoing — and raging — debate. Schlanger seems partial to an idea posed by neuroscientist Giulio Tononi that the complexity and integration of wave patterns of electricity indicate the level of consciousness of an organism. Consciousness, in this view, is a spectrum, not a binary.
One of the pitfalls of reaching for language to describe these phenomena is that it’s almost impossible to avoid some level of anthropomorphization. Describing how botanists have viewed the use of the word intelligence, Schlanger writes: “Measuring plants against human cognition made no sense; it just rendered plants as lesser humans, lesser animals.” Still, plants do “deploy several senses — or could one say, intelligences? — that far exceed anything humans can do in a similar category.” Scientists have wrapped this information in “layers of hedging, language that distances plants from ourselves at all costs,” ultimately making it challenging for their work to reach the public or other disciplines. Schlanger argues that people need understandable metaphors — ones that they can connect to but do not misinform them about how different plants are from humans. Or perhaps, she considers, we need to “vegetalize our language,” calling traits “plant-memory,” “plant-language,” or “plant-feeling.”
A cabbage caterpillar eats through a leaf of the mustard plant Arabidopsis, stimulating a wave of calcium across the plant that triggers defense responses in other leaves. The calcium is visualized by fluorescent light. Visual: Simon Gilroy/University of Wisconsin-Madison/YouTube
Schlanger explores why scientists missed such fundamental ideas about plants — even as many Indigenous traditions have treated them as kin, ancestors, or simply beings in their own right. Schlanger covers not only these Indigenous philosophies, but also how the influences on European thought of Aristotle and René Descartes led to treating living things as mechanistic and passive. Even though botanists use much more lively language in conversation, in their research papers they describe plant behaviors using passive voice. “A plant doesn’t ‘react,’ instead it ‘is affected,’” as Schlanger points out. “Articulating these processes without ascribing agency is actually quite difficult, fumbly, imprecise.”
Recognizing that plants are not simply passive, mechanistic groupings of cells, but rather intelligent beings, perhaps even worthy of personhood — meaning “one has agency and volition, and the right to exist for their own sake” — has tremendous moral, philosophical, and policy implications. Several legal arguments in recent years have grappled with the personhood of plants and ecosystems threatened by human activities. “At what point do plants enter the gates of our regard?” Schlanger asks. “Is it when they have language? When they have family structures? When they make allies and enemies, have preferences, plan ahead? When we find they can remember? They seem, indeed, to have all these characteristics. It’s now our choice whether we let that reality in.”
Schlanger repeatedly exposes the gaping distance between the public and scientists when confronted with the question of plant intelligence. For example, Monica Gagliano, a plant researcher in Australia, has become a “contested figure” in her field for her strong stands on studying plants’ ability to hear — and on using her intuition as well as evidence-based rigor. “She speaks to packed audiences at conferences on philosophy and at science events geared toward the general public,” writes Schlanger. At the same time, she is no longer funded through traditional federal grants, but instead by the Templeton World Charity Foundation.
Readers who loved “The Secret Life of Plants” may be crestfallen to find out that the book harmed exactly the scientists they would have wanted to have helped. “Science’s biggest flaw and biggest virtue is that it almost always mistakes agreement for truth,” Schlanger writes. Questions about plant intelligence may even invoke a spiritual and moral dilemma within science, a paradox on which historian Jessica Riskin at Stanford University has written: “The seventeenth-century banishment of agency, perception, consciousness, and will from nature and from natural science gave a monopoly on all of these attributes to an external god.” Early scientists avoided these topics because this view of nature fit with religious ideas at the time. “They bequeathed to their heirs a dilemma that remains active over three centuries later.”
Acknowledging plants’ agency could rid science of this vestige of the past, and, Schlanger wagers, bring about a new paradigm, one that integrates nature with humans and acknowledges the agency of all life. “Plants will go on being plants, whatever we decide to think of them,” notes Schlanger. “But how we decide to think of them could change everything for us.”
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Boquila trifoliolata triggered a memory
This is probably in the book, because it is so fascinating:
Boquila trifoliolata mimics leaves of an artificial plastic host plant
Jacob White and Felipe Yamashita
What other plants has plant vision? Does a wheat field see me walk by? If a tree falls in the forest, do the other trees see it?
“Science’s biggest flaw and biggest virtue is that it almost always mistakes agreement for truth,” Extremely well put, and sadly so valid. As to consciousness, it is not an easy subject to pursue, even when only looking at human consciousness. Anywys, thank you for bringing attention to this book.
“Plant intelligence has existed almost as long as the animal arrogance that denies it.”
The Thing From Another World – 1951 movie
The intellectual carrot – something to chew over.
Funny, I view this as the exact opposite: that it’s an exercise in anthropomorphizing. When are we going to figure out that we can consider things valuable without needlessly valorizing or exaggerating them? Sentience isn’t intelligence, and in fact doesn’t even imply it.
Perhaps adjacent to that, by the way, is that sentience and sapience are not at all the same thing (though it seems like every fantasy and scifi writer other than Terry Pratchett never understood the difference).
I’m reminded a bit of panpsychism, something that not only can’t muster a shred of evidence, but conceptually actually amounts to reductionism, just weirdly masquerading as a ‘deeper’ answer.
We have quite a few examples of assemblages of cells that exhibit what we generally seem to believe is consciousness. These would constitute “shreds of evidence” if we wanted to describe them so poetically.
I’d give power of attorney to a house plant before I would an algorithm.
The dearth of scientific rigour is a bit overemphasized I think. There have been organic farmers for a very long time, who see the positive natural results from plant decay, animal interaction with plants, as against harmful pesticide use,( the latter having been the ‘scientific’ approach), and who would rather let nature do its thing than interfere with those processes in order to understand them. Understand them, yes; make them rigorously conform to our latest science, no.
I love, for instance, Martin Buber’s reflections on having relationships with animals and with trees in “I and Thou.” Such a harmony is extremely Biblical. The plants are our forefathers in that they were created before us. And we first were gardeners.
All of nature, flora and fauna, deserves immense respect as we farm and harvest. My dairy farming uncle made sure the little bull calves that had to be culled from the herd were well looked after before they necessarily departed his custody. Had they been left in nature’s care one might think they’d have had a better life, but not necessarily so. The hard fact is we humans naturally care deeply, as do animals and plants in their different ways; nature really doesn’t.
But Something that looks after us all, as well as It does nature as a whole, does.
“Und doch ist Einer welcher dieses Fallen
Unendlich sanft in seinen Händen hält.”
Intelligence is arguably not the right word to use in this context. Sentient seems more appropriate.
Funny enough, I happen to be reading (slogging is probably the better word) through Kant right now. Never expected it to be relevant to something like this, but it weirdly is.
I agree most of the examples given don’t necessarily imply intelligence, just sensibility, though the ones suggesting some form of memory may be exceptions. I like the idea of staying open to some form of intelligence though, and at least in Kantian terms, you could arguably observe aspects of it experimentally.
If I understand right, experiments strongly suggesting that plants can discriminate between real stimuli in a contextual or rule-based way demonstrates that plants have judgment and understanding by definition. And if they show the ability to generate new rules that subsume previous ones, they’re also reasoning.
The real irony of our current moment is the way so many people expect certain AI/ML algorithms (LLMs, neural-networks, etc.) to have these things. At first glance they may seem to, but based on both how they work or if you just try to audit them, they essentially operate through anti-rules. Instead of unifying phenomena into a coherent whole, they use raw iterative power to descend to the muddle of experience, fine-tuning epicycles on top of epicycles.
Maybe ‘plantelligent’. Maybe ‘plantient’.
Vegans and clearcutters and rainforest arsonists will be the world’s very last bitter enders in denying all this.
Thank you for posting this book review! I hadn’t come across this book yet, and now I am excited to read it.
The issue of anthropomorphization – or, I would say, anthropocentrism – is a profound one. For many years now I have been contemplating exactly this problem when it comes to our understanding consciousness and intelligence in other organisms, especially non-animal species, and also in ecosystems. Consciousness and intelligence are temporal and spatial phenomena. That is, the processing of information which gives rise to intelligence and consciousness takes place across certain distances and within certain time-scales. The neurons in a human brain process information in milliseconds to a few seconds, across distances of tenths of a millimeter up to about 2 meters.
As humans, we experience our world in a very narrow scale of time and space, and we have a great difficulty in sensing and understanding things in our world that exist and behave outside of these “human scales”. We also tend to see systems as discrete (both spatially and temporally), rather than part of a continuum. As a result, there is a real difficulty related to coming up with definitions for consciousness and intelligence (something we are anyway still far from reaching consensus), especially ones that are applicable beyond the narrow confines of anthropocentric conception. This anthropocentric perspective poses a few problems:
1. Suppose there exists an organism that does indeed possess intelligence and consciousness, but its information processing occurs in a time scale of months and years; would we even be able to recognize it? If so, how?
2. Suppose there exists an ecosystem of species which is capable of processing information such that it, as a system, possesses intelligence and consciousness. Moreover, that the associated phenomena occur within time and space scales outside of the narrow human band. And finally, that this information processing occurs across the boundaries of species in this ecosystem, such that it must be understood as a continuum rather than discrete units. Would we be capable of recognizing that this ecosystem is intelligent and conscious? If so, how would we achieve that?
Nature already offers us some examples to contemplate for 1 and 2 above. For 1, plants and fungi are obvious examples, and are interesting due to the difference in their scales of time and space.
For 2, we can already see many ecological relationships that are potential candidates, such as plant-plant, plant-fungi, or plant-insect relationships, the human microbiome, and many parasite-host relationships. A fun set relates to cordyceps, parasitic fungi that literally turn their hosts into zombies. But that’s thinking small: how about ecosystems such as a forest, in which information processing occurs across both distances that can vary between a few micrometers up to many kilometers, and within timescales as low as a few milliseconds up to decades, centuries, or millenia? Would we be able to recognize intelligence or consciousness in such cases, if it was in fact present?
I think we ought to examine a bit this problem of anthropocentrism, its origins, and also how to break away from it. I recently began to dip my toes into the culture and world view of the Guarani, an indigenous people from South America. Their conception of time and the spatial and temporal boundaries of self, of others, of species and ecosystems, is so entirely different to my own (having received a western liberal upbringing) that it can be accurately described as distinctly alien. It feels to me that, in the case of understanding intelligence and consciousness outside of human scales, the Guarani might have a much more capable intellectual toolkit than western liberal scientists. Alas, perhaps it may be appropriate to narrow down from an “anthropocentric” to “western-liberal-centric” world view.
Last weekend I was in a large grove of bamboo, formerly all cedar, hunting for new shoots of madake with a few others. One of the older gentlemen in the group pointed out how a cluster of bamboo will surround a single cedar tree, to take over its sunlight and nutrients.
He mentioned a Japanese botanist, who hypothesizes that the bamboo will actually “elect” one of their group to grow larger, to block and rob light from the cedar, so that the entire group of bamboo can benefit by prevailing over their competitor. I was struck that he framed this by saying “if I were to anthropomorphize the bamboo and the cedar…”.
Regarding the problem of anthropocentrism, if we look even at the origins of the “western-liberal-centric” world view returning to the sixth century before Christ, we find that the pre-Socratics were interested in the root principle of all things (arché, e.g., water, fire, infinity, atoms, etc.), and questioned anthropocentric thought, e.g., Xenophanes: “mortals suppose that the gods are born (as they themselves are), and that they wear man’s clothing and have human voice and body. […] But if cattle or lions had hands, so as to paint with their hands and produce works of art as men do, they would paint their gods and give them bodies in form like their own — horses like horses, cattle like cattle.”
However, this mode of thinking evidently did not become widespread in the ancient world. The mode that eventually did become widespread, though, was Christianity, which substitutes genesis for arché, and anthropomorphizes the world.
Thank you for the excellent review.
Now I feel guilty eating my veggies.
Also just want to say that I’ll have to check this book out. I don’t normally jump on new books or media, but beyond the topic and review, the title “The Light Eaters” is just too gloriously metal for me to turn down.
Goethe had some interesting ideas about plants in the late eighteenth century which, as Steven Jay Gould pointed out, were partly wrong-headed but nevertheless serious, and forty years after Gould was writing Goethe’s ideas about the “urpflanz”, the primordial leaf from which all other elements of the plant develop, stands up relatively well.
Of course Goethe didn’t have the right credentials, being a poet.