The inner life of plants arouses the passions of even the mildest-mannered naturalists. A debate over plant consciousness and intelligence has raged in scientific circles for well over a century—at least since Charles Darwin observed in 1880 that stressed-out flora can’t rest.
There’s no doubt that plants are extremely complex. Biologists believe that plants communicate with one another, fungi, and animals by releasing chemicals via their roots, branches, and leaves. Plants also send seeds that supply information, working as data packets. They even sustain weak members of their own species by providing nutrients to their peers, which indicates a sense of kinship.
Plants have preferences—their roots move toward water, sensing its acoustic vibes—and defense mechanisms. They also have memories, and can learn from experience. One 2014 experiment, for example, involved dropping potted plants called Mimosa pudicas a short distance. At first, when the plants were dropped, they curled up their leaves defensively. But soon the plants learned that no harm would come to them, and they stopped protecting themselves.
But does any of this qualify as consciousness? The answer to that question seems to depend largely on linguistics, rather than science—how humans choose to define our conceptions of the self and intelligence.
Plant biotechnologist Devang Mehta, for one, says the answer to the question of whether plants are conscious “is unreservedly no.” In a February article for Massive Science entitled, “Plants are not conscious, whether or not you can sedate them,” he vehemently opposes the notion that plants can be conscious or intelligent.
Mehta was responding to a New York Times story (paywall) about a 2017 study in Annals of Botany. Researchers had arrested plant motion with anesthetics—a new take on a 1902 experiment by biologist and physicist Jagadish Chandra Bose, who used chloroform to put plants to sleep. The Times wrote that the vegetal response to anesthetics suggests that plants are intelligent. Basically, the article argued that to lose consciousness, one must have consciousness—so if plants seem to lose consciousness under anesthetics, they must, in some way, possess it.
The Grey Lady was making a major leap when it suggested that plants responding to anesthetics indicates intelligence, according to Mehta. He explains:
For one, definitions of consciousness and intelligence are hotly contested even when talking about humans and animals. Second, plants lack a nervous system, which has long seemed requisite for discussion of animal-like behavior. Third, while the way in which many anesthetics function in humans is still a mystery, there is no reason why they or other chemicals shouldn’t induce a response in any organism, let alone plants.
Mehta believes that plants deserve respect. He just thinks confusing their qualities and abilities with those of humans is unnecessary anthropomorphizing. Venturing into the territory of philosophers, he argues that in order to qualify as “conscious,” a thing must be aware of its self-awareness, or meta-aware.
Danny Chamovitz, director of the Manna Center for Plant Biosciences at Tel Aviv University in Israel, says that plants are neither conscious nor intelligent, though they are incredibly complex. Plant awareness shouldn’t be confused with the human experience of existence. He tells Gizmodo, ”All organisms, even bacteria, have to be able to find the exact niche that will enable them to survive. It’s not anything that’s unique to people. Are they self-aware? No. We care about plants, do plants care about us? No.”
The thing is, Chamovitz can’t prove that plants don’t care about us. No one can, really. We know that hugging trees, literally, makes us feel better. It has a medicinal effect. But we can’t test the reciprocity of this—whether plants love us back, or feel good when we care for them.
Philosopher Michael Marder, meanwhile, says we’re underestimating plants. The author of Plant Thinking: A Philosophy of Vegetal Life, Marder tells Gizmodo, “Plants are definitely conscious, though in a different way than we, humans, are.” He notes that plants are in tune with their surroundings and make many complex decisions, like when to bloom. Marder concludes, “If consciousness literally means being ‘with knowledge,’ then plants fit the bill perfectly.”
That said, Marder admits that we can’t know if plants are self-conscious, because we define both the self and consciousness based on our human selves and limitations. “Before dismissing the existence of this higher-level faculty in them outright, we should consider what a plant self might be,” he says.
Marder points out that plant cuttings can survive and grow independently. That suggests that if plants do have a self, it is likely dispersed and unconfined, unlike the human sense of self. It’s notable, too, that many scientists and mystics argue that the human feeling of individuality—of being a self within a particular body—is a necessary illusion.
He further argues that because plants communicate with one another, defend their health, and make decisions, among other things, they may well have some sense of self, too. He explains:
The project of an ongoing vegetal integration through feedback loops and other communication strategies and mechanisms may be considered analogous to what we, humans, define as self-consciousness. The trick is to let go of our fixed association of biological, if not psychological, structures and the functions they fulfill, imagining the possibilities of seeing and thinking otherwise than with the eye and the brain. Maybe once we manage to do so, we will finally become conscious of plant consciousness.
Because we are steeped in an ancient tradition of human-centrism, we believe that our experience of life is what defines consciousness, and that our brain’s processes are the height of intelligence. But there is some evidence that other modes of existence are equally complex, which suggests that other living things have arguably intelligent or conscious experiences.
Evolutionary ecologist Monica Gagliano insists that plants are intelligent, and she’s not speaking metaphorically. “My work is not about metaphors at all,” Gagliano tells Forbes. “When I talk about learning, I mean learning. When I talk about memory, I mean memory.”
Gagliano’s behavioral experiments on plants suggest that—while plants don’t have a central nervous system or a brain—they behave like intelligent beings. She says that if plants can summon knowledge about an experience repeatedly—as was the case with the potted plants that stopped curling their leaves after they learned they would come to no harm—then plants are clearly able to remember and learn from experience.
Gagliano, who began her career as a marine scientist, says her work with plants triggered a profound epiphany. “The main realization for me wasn’t the fact that plants themselves must be something more than we give them credit for, but what if everything around us is much more than we give it credit for, whether it’s animal, plant, bacteria, whatever.”
She’s aware of the criticisms of fellow scientists, who warn against anthropomorphizing vegetation. But she argues that there is no other doorway to understanding the inner life of all these other beings. Thinking about ourselves provides a subjective sense of a tree or a shrub’s inner life, but it doesn’t preclude the possibility that vegetations may be leading a rich existence in its own right. On the contrary, it propels us to explore the difficult questions about their lives. “To me, the role of science is to explore, and to explore especially what we don’t know. But the reality is that much research in academia tends to explore what we already know because it’s safe,” she argues.
Acknowledging plant intelligence could put us in an awkward position. Perhaps there is nothing we can eat that isn’t some form of murder, not even salad. Moreover, if we discover plant kinship relations are real, we’ll need to acknowledge that cutting trees down for furniture means splitting up families. More than that, expanding definitions of consciousness and intelligence could mean admitting we’ve been limited in our worldview altogether. What if everything around us is intelligent in its own way, and we’re just not smart enough to see it?
“I’ve been talking to people who work with amoebas and the slime molds and it’s the same all over,” Gagliano tells Forbes. “These guys, the critters, are amazing. They do stuff that we don’t even dream of. And by not dreaming of it, we assume that it does not exist.”