By Katie L. Burke, an award-winning features editor and science journalist, and senior contributing editor at American Scientist magazine. Originally published in Undark.
The 1973 bestselling book The Secret Life of Plants captivated the public with questions about plant perception and communication. Even if you haven’t read the book, you’ve probably heard about the experiments that involved playing classical music or rock and roll to plants, or hooking them up to polygraphs. The book even inspired a film about plant perception and communication. soundtrack Written by Stevie Wonder.
The experiment was an interesting idea but badly designed. Scientists strongly rejected the book and distanced themselves from its views. “The damage that ‘The Secret Life’ did to the field is immeasurable, according to botanists who worked at the time,” Zoe Schlanger writes in her new book, “Light Eaters: The invisible world of plant intelligence brings new understanding to life on Earth“Over the next few years, the National Science Foundation became increasingly reluctant to give grants to people studying plant responses to the environment,” Schlanger reports. “And pioneering scientists in the field changed course or left science altogether.”
It took about 40 years—a generation of scientists—for that chilling effect to wear off. In the past 15 years, funding for plant behavior research has returned, at least in small amounts. Schlanger serves as a tour guide through this history and the pressing questions that new research raises about our shared future as plants and humans.
Given the history of research into plant intelligence, the book’s subtitle may raise some skepticism. Even a hugely popular book like The Hidden Life of Trees Advance When it comes to plant communication, evidence is scarce. But “The Light Eaters” delivers. Schlanger’s ideas are rigorous, and she explains these contentious intellectual debates with fairness and curiosity.
Schlanger is clearly excited by her efforts to meet the few scientists who have pushed the field forward. She has explored the globe, traveling to the Chilean rainforest to see plants that mimic other plants like chameleons, to the Hawaiian island of Kauai, home to an astonishing number of rare and endangered plants, and to the University of Bonn in Germany to meet one of the founders of the Society for Plant Neurobiology (now called the Society for Plant Signaling and Behavior). For the scientists she meets along the way, it hasn’t been easy. While a lucky and brave few have struggled to carve out a niche for themselves, Schlanger meets many scientists who have staked their careers on studying plants’ uncanny ability to sense the world. Sadly, some have left the field altogether; others have put research on hold for decades, turning to teaching positions or more-funded research projects.
Despite the challenges on the ground, Schlanger finds her subject energizing, in contrast to her work as a climate journalist, where she was beginning to feel burned out by the gloomy news she processes daily. “Journalists in my field tend to focus on death, or its precursors: disease, disaster, and decay,” she writes. She wanted to be around life and celebrate it in a way that her day job rarely allows. “In this moment of global devastation, plants offer a window into a greener mindset,” she writes. Plants around the world “fill the atmosphere with the oxygen we breathe and literally build our bodies from the sugars they produce from sunlight,” she continues. “Plants have complex, dynamic lives of their own, including social lives, sex lives, and a set of subtle sensory perceptions that we almost exclusively associate with animals.”
“Understanding plants opens new horizons in human understanding: the understanding that we share the Earth with and owe our existence to lifeforms that are at once foreign and familiar, uniquely cunning.”
In fact, Schlanger highlights evidence that plants have such senses even if scientists don’t know how they sense and respond to their environment, or the underlying mechanisms. Plants communicate not only through chemicals in the air and soil, but also possibly through sound. When water rises from the roots of a plant up its stem, air bubbles pop, emitting ultrasonic clicking sounds. Each type of plant that has been studied has its own unique frequency, for example wheat, corn, grapevines, and cacti. Plants can sense touch and also send electrical signals, which is another way to communicate. These organisms also sense light in a sophisticated way that can be compared to vision. The Chilean rainforest vine, Boquilla trifoliolata, can mimic the leaf shape, texture, and even vein patterns of nearby plants, but it is still unclear how it can “see” its neighbors. Plants also have memory and social behavior. Based on the interval between previous visits, Nasa poisoniana, a plant in the Urticaceae family, can predict when pollinators will visit its star-shaped flowers and erect pollen-laden stamens.
But plants don’t have brains. Their intelligence is not centralized, but rather a distributed network. “How is information about the world integrated, sorted by importance, and transformed into actions that benefit the plant?” Schlanger asks. This is a question at the forefront of research, and whether plants are conscious is the subject of ongoing and fierce debate. Schlanger seems to subscribe to the idea, proposed by neuroscientist Giulio Tononi, that the complexity and integration of electrical waveform patterns indicates an organism’s level of consciousness. In this view, consciousness is a spectrum, not a duality.
One of the pitfalls of relying on language to explain these phenomena is that it is nearly impossible to avoid a certain degree of anthropomorphism. Regarding how botanists have viewed the use of the word intelligence, Schlanger writes: “It makes no sense to compare plants to human cognition; it simply renders plants inferior humans, inferior animals.” And yet, plants “deploy several senses—or should we say intelligence—that go far beyond what humans can do in a similar category.” Scientists wrap this information in “layers of ambiguity, a language that distances plants from us at all costs,” ultimately making it difficult for their research to reach the general public and other disciplines. Schlanger argues that we need metaphors that people can understand—metaphors that people can connect to but that don’t misinform people about how plants are different from humans. Or, she thinks, perhaps we need to “botanize language” and call these traits “plant memory,” “plant language,” or “plant emotion.”
Cabbage caterpillars feed on Arabidopsis leaves, stimulating waves of calcium throughout the plant and triggering a defense response in other leaves. The calcium is made visible under fluorescent light. Image: Simon Gilroy/University of Wisconsin-Madison/YouTube
Schlanger explores why scientists have missed such fundamental ideas about plants, even though many indigenous traditions treat plants as kin, ancestors, or simply entities in their own right. Schlanger covers how the influence of these indigenous philosophies, as well as Aristotle and René Descartes, in European thought led to treating living organisms as mechanistic and passive. While botanists use much more vivid language in conversation, in research papers they use the passive voice to describe plant behavior. “Plants don’t ‘react,’ they ‘are affected,’” Schlanger points out. “It’s actually very difficult, clumsy, and imprecise to articulate these processes without attributing them to agents.”
Recognizing that plants are not merely passive, mechanical collections of cells, but that they are sentient beings and may be worthy of personhood (i.e., that “people have the capacity and will to act, and the right to exist for themselves”) has significant moral, philosophical, and policy implications. In recent years, several legal discussions have focused on the notion of plant personhood. Ecosystem Plants are threatened by human activity. “At what point do plants enter the gates of our interest?” asks Schlanger. “When they have language? When they have family structure? When they make friends and enemies, have preferences, and make plans? When we discover that they have memory? Plants certainly seem to have all of these traits. It is now our choice whether to accept that reality.”
Schlanger repeatedly reveals the huge gulf between laypeople and scientists when it comes to the question of plant intelligence. For example, Australian plant researcher Monica Gagliano has become a “controversial figure” in the field for her strong stance on plant hearing and her willingness to use intuition as well as evidence-based rigor. “She speaks to capacity audiences at philosophy conferences and public science events,” Schlanger writes. At the same time, she is no longer funded by traditional federal grants but by the Templeton World Philanthropy Foundation.
Readers who loved The Secret Life of Plants may be disappointed to learn that the book has hurt the very scientists they wanted to help. “The greatest flaw and greatest virtue of science is that it almost always mistakes consensus for truth,” Schlanger writes. Questions about plant intelligence may even raise spiritual and moral dilemmas in science. Stanford historian Jessica Riskin has this to say about this paradox: written“The banishment of action, perception, consciousness, and will from nature and natural science in the seventeenth century led to the monopoly of all these attributes on an external deity.” Early scientists avoided these topics because this view of nature was consistent with the religious ideas of the time. “They left their successors with dilemmas that are still troubling them more than three centuries later.”
Schlanger hopes that recognizing plant agency will help rid science of its past vestiges and give rise to a new paradigm that integrates nature and humans and recognizes the agency of all life. “Plants will remain plants no matter what we think about them,” Schlanger points out, “but how we think about plants can change everything for us.”
