Savvy in the Grass

Of the many books published by Charles Darwin, his second to last, coauthored with his son Francis, ranks among the most tedious. The Power of Movement in Plants (1880) chronicles a series of painstaking botanical experiments Darwin père and fils conducted in the late 1870s. One involved attaching little squares of cardboard to seedlings’ roots to see how they responded. For another, they affixed needles to various plant parts with tiny drops of wax, then traced the plants’ movements on a pane of glass.

The Darwins performed the same experiment many times on different species. The little-squares-of-cardboard routine, for instance, was carried out on bean, pea, nasturtium, cotton, pumpkin, and horse chestnut seedlings; the needle procedure on, among other plants, wild cabbage, candytufts, fuschias, geraniums, raspberries, ivy, azaleas, and Japanese snowflowers. Each iteration received its own write-up in the book.

Darwin himself seems to have found the work dull. “I have written a rather big book—more is the pity,” he wrote to the Harvard botanist Asa Gray on October 24, 1879. At the time, Darwin was in the process of reviewing the manuscript, a task he described as a “horrid bore.”^*

But even at his most monotonous, Darwin still managed to provoke. In the last chapter he and Francis argued that potted plants should not be regarded as, well, potted plants. Rather, all plants should be viewed as active agents, capable of sensing stimuli and reacting accordingly. Radicles—essentially plants’ first roots—are, the Darwins argued, particularly significant in this regard; as they probe the soil, they gather information on conditions like moisture, which the emerging seedling uses to direct its growth. “It is hardly an exaggeration to say,” they wrote, that the tip of the radicle “acts like the brain of one of the lower animals.”

For most of the twentieth century, the Darwins’ notion that plants possess something analogous to a brain was seen as unscientific or just plain kooky. But in the twenty-first century it has swept back into vogue. How plants process information is an active area of research. There are botanists who maintain that peas are capable of associative learning, à la Pavlov’s dogs, and scientists who think that tropical vines possess a sort of vision. In Italy there’s a laboratory devoted to “plant neurobiology.” (“Plants are as sophisticated in behaviour as animals,” the lab’s website declares.) Several recent books contend that the latest discoveries in plant cognition are so significant they force us to rethink our view of life itself. Depending on how you look at things, this prospect is thrilling to contemplate or nothing less than tragic.

Zoë Schlanger is a journalist who began researching plant behavior as a form of escape from her day job covering global warming. Reading papers on ferns and banana trees gave her the sense, she writes in The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth, that she was peering into a “parallel universe.” It relieved a “crawling sense of dread.”

As she read more deeply, Schlanger came upon one intriguing botanical discovery after the next. Plants communicate with one another. When sagebrush, for example, is attacked by insects, it releases chemicals that prompt its neighbors to beef up their defenses. The signals appear to be picked up by other plant species as well; wild tobacco responds to sagebrush’s warnings by producing chemicals of its own. Plants are also able to confer privately, with just their kin, by emitting chemicals only relatives can interpret.

Plants communicate with animals as well, sometimes honestly, sometimes not. Corn plants that are being nibbled by caterpillars release chemicals that attract parasitic wasps. The wasps lay their eggs inside the caterpillars, killing them—a process that benefits both participants in the colloquy. Spider orchids synthesize chemicals that very precisely mimic the pheromones emitted by female wasps. These chemicals attract male wasps, which try to mate with the orchids. In this case, the wasp is frustrated, while the plant gets pollinated.

Arabidopsis, or thale cress, is the lab rat of the botanical world. Researchers have found that Arabidopsis can, in a manner of speaking, hear. The plant responds to the vibrations produced by munching caterpillars by synthesizing insect-repelling compounds. Many other plants sport tiny hairlike structures, known as trichomes, on their leaves; these, it is speculated, may act like tiny antennae.

What do these structures and stratagems reveal about the inner lives of plants? Many biologists say not much. From asters to zinnias, there is, this group insists, no inner life to analyze. Others argue that this is a form of prejudice—call it antibotanical bias.

Schlanger’s reporting eventually takes her to the labs of several scientists at the vanguard of what’s been called the “plant neurobiology revolution.” In Bonn she talks to František Baluška, a cell biologist who’s originally from Slovakia. A few years ago, in a paper that appeared in the Annals of Botany, Baluška and some colleagues reported that several species of plants, including peas and Venus flytraps, respond to anesthetics “in a similar manner to animals and humans.” (The online version of a New York Times article about the paper was accompanied by time-lapse photos of pea plants waving their tendrils and then suddenly going still, like a patient etherized upon a table.)

Baluška is of the opinion that plants feel pain. “If you don’t feel pain, you ignore danger and you don’t survive,” he has said. He also believes that plants are conscious. If they weren’t, how could they be rendered unconscious? “I think consciousness is a very basic phenomenon which started with the first cell,” he tells Schlanger. “If you are not conscious, you are not aware of your environment and you cannot act.”

In Berlin Schlanger meets with Tilo Henning, a researcher who studies Nasa poissoniana, a plant native to the Peruvian Andes. Nasa poissoniana produces spectacular star-shaped blossoms and is capable of moving unusually quickly, at least for a flower: in a matter of minutes it can raise one of its pollen-bearing stamens from a horizontal to a vertical position. Henning and his colleagues have shown that the plant alters the timing of this move depending on how often it expects a pollinator to stop by. If bees are visiting frequently, Nasa poissoniana raises its stamens at short intervals; if the visits are rare, the plant waits longer. Should the interval between apian callers change, Nasa poissoniana adjusts its pace.

“They obviously are able to count the time between the visits and keep that memory,” Henning observes. Although plants don’t possess brains or even, as far as anyone can tell, any structures that resemble neurons, still they take in information and respond to it. “This to me is the basic definition of intelligence,” Henning says.

Together with Baluška, Stefano Mancuso, an Italian botanist, is a founding member of the Society for Plant Neurobiology, which held its first meeting in 2005. (The group has since changed its name to the Society of Plant Signaling and Behavior.) Mancuso teaches at the University of Florence, where he directs the Laboratorio Internazionale di Neurobiologia Vegetale. In a recent interview with The Guardian, he recalled how he came to see plants as, if not brained, at least brainy. “One of my tasks during my doctorate was to understand how a root growing in the soil was able to move around an obstacle,” he said. What he found was that “the root was changing direction well before touching the obstacle. It was able to sense the obstacle and to find a more convenient direction. That was my first eureka moment.”

In The Nation of Plants, Mancuso imagines himself as the spokesman for the vegetable kingdom. His green brethren have drafted a constitution reflecting the insights they have gained over hundreds of millions of years and have deputized him to deliver it to Homo sapiens, a species that’s been around for only a small fraction of that time. “Learn from those who have more experience than you,” Mancuso’s fictional photosynthesizers exhort. (The writer Michael Pollan has described Mancuso as the “poet-philosopher” of the plant intelligence movement.)

According to these sagacious shrubs, people are dangerous precisely because of the way they think. They rely on their brains to organize their ideas and also to regulate their other organs. In their institutions, they replicate this top-down structure, with a few leaders or bosses empowered to make decisions.

Plants, by contrast, have a more diffuse body plan. If a leaf or root or branch is damaged, other leaves or roots or branches take up the slack. They thus model a form of organization that’s inherently tougher and more equitable. “Plants are masters of cooperation, and through alliances and communities, they have succeeded in building mutualistic societies in any and all earthly environments,” Mancuso observes.

Not surprisingly, the “nation of plants” has strong opinions about the way it has been represented or, really, misrepresented. It’s disgusted by depictions of the “food pyramid,” which put plant products at the bottom and animal products above them. Plants make animals possible, so shouldn’t the order be reversed? And it scoffs at the notion that being able to move somehow endows fauna with special abilities. In fact just the opposite is the case: “It is precisely because they do not have the possibility to change places and run away if something in their environment changes that plants must necessarily be more sensitive than animals,” Mancuso maintains.

Ultimately the lesson the nation of plants wants to deliver is: wise up! At the rate people are running through resources, mowing down forests, heating up the globe, and driving other species to extinction, there isn’t much time before the entire biosphere starts to unravel. “I do not believe that the gravity of the situation is clear to most people,” Mancuso writes. “At least I hope that is the case. If not, it would mean that humanity has lost all sense of its own future.”

If Mancuso is the poet-philosopher of plant intelligence, Paco Calvo is the actual philosopher. Calvo, a professor at Spain’s University of Murcia, runs the Minimal Intelligence Lab, or MINT, which bills itself as “the world’s first laboratory in the philosophy of plant signaling and behavior.” In 2016 Calvo published a thirty-five-page “manifesto” arguing that the study of plant intelligence demands the same kind of multidisciplinary research effort that has in recent decades advanced our understanding of human cognition.

In Planta Sapiens: The New Science of Plant Intelligence, Calvo invokes many of the same studies that feature in The Light Eaters. Plants, he notes, respond to anesthetics. They recruit allies and communicate with one another by emitting volatile organic compounds. They seem to be able to anticipate the future. For example, they will keep sending their roots into salty soil if the salt concentration is decreasing, indicating better circumstances ahead, but if the salt content of the soil remains high, they stop growing in that direction. Plants, Calvo writes, “are prediction machines with the ability to self-correct.”

Calvo argues that people have undervalued vegetable intelligence because plants are so different from us: “We can’t look at their faces to understand what is going on internally.” Still, he believes, it should be possible for us to sympathetically engage with them, to imagine—to paraphrase Thomas Nagel—what it is like to be a bamboo. He points to octopuses, which, like plants, lack a centralized brain but clearly are quite clever, and cites the example of Craig Foster, the South African filmmaker who chronicled his relationship with a tender eight-legged creature in his award-winning documentary, My Octopus Teacher.

“Perhaps we can learn something from this communing between human and cephalopod,” Calvo observes. “In many ways, the plural brain and hydrostatic form of the octopus is not so different from the fluid body plan of a vine, through which its particular consciousness diffuses.”

Toward the end of Planta Sapiens, Calvo says that he considers plants to be akin to people suffering from locked-in syndrome. Locked-in patients are almost completely paralyzed, but nevertheless aware and able to communicate through eye movements. “This is just where we are with plants: they may have the ability to suffer as part of an internal experience,” he writes.

It is here that things grow thorny. As Calvo notes, “if plants are intelligent and aware of their surroundings, we cannot turn a blind eye to ethical considerations.” But what would it mean actually to face these considerations?

People who consider it their ethical obligation to reduce animal suffering can—and often do—become vegans. (Any kind of agriculture, it’s worth noting, inflicts death and suffering on the animals who are displaced, so even vegans shouldn’t get a bye here.) But no human or, for that matter, animal of any kind can survive without consuming plants, either directly or indirectly. As the nation of plants, which is to say Mancuso, observes, everything people eat traces its origins back to photosynthetic organisms. If cucumber vines and banana trees possess sentience, what is the morally appropriate response? Perhaps it would still be okay to eat seeds and grains, but harvesting them would be tough. Could people in good conscience mow down a field of wheat or rice plants? Could we even sow a field of wheat or rice, knowing that the plants might be suffering from a sense of overcrowding, say, or panic, but be locked into silence, unable to express themselves?

Calvo seems reluctant to follow his logic to its logical conclusion. The furthest he’s willing to go is to ask: “Were plants to be given the status of ‘sentient,’ would this give them rights that might encumber our exploitation of them?” People have, he acknowledges, “been slow to consider these issues.”

The idea of plant consciousness or intelligence stops being cool and fun as soon as you start thinking through the implications. Yes, a worldview less hierarchical than our own would have to allow that sentience is widely distributed across the phylogenetic tree. It would also accept a wider distribution of suffering. Evolution has no particular stake in ethics; plants may understand this even if we don’t.

I contemplated this possibility the other day as I was weeding my garden. One green shoot after another got tossed into the wheelbarrow and left to die. I confess that, after reading the books by Schlanger, Mancuso, and Calvo, I felt a twinge of guilt about this. But I kept right on weeding.