Plants Warn, Defend, Scream, Remember, and Plan Ahead






Before social media, Charles Darwin relied on a global network of colleagues who corresponded with him. Before AI, he used photos to study facial expressions around the world. Before Buzzfeed, he created quizzes about what emotion was pictured. Before Fitbit, he walked daily and counted his steps by dragging a flint to the other side of the path after each lap. We obsess about evolution, but Darwin was ahead of his time in many ways. And one of the least talked about was his appreciation for the plants he observed on those walks.

“Is it not curious that a plant should be far more sensitive to a touch than any nerve in the human body!” he wrote to a friend in 1860. “Yet I am perfectly sure that this is true.”

Three years later, he wrote to another friend: “The more I look at plants the higher they rise in my mind: really the tendril-bearers are higher organized, as far as adapted sensitivity goes, than the lower animals.”

By 1873, he and his son were tracking the growth of tendrils on various climbing and twining plants, watching them corkscrew toward objects they could climb toward the sun. Measuring and diagramming their movement, he realized that all parts of a plant—tendrils, roots, blooms, leaves—moved in swaying circles.

Today, we have time-lapse photographs that show a vine circling aimlessly until it senses a nearby support, then pulling itself back, flinging itself toward the support, and grabbing hold. This is behavior, as surely as hunting is a wolf’s behavior. “To Darwin, a plant was basically an inverted animal,” writes Virginia Morell, “with its brains on the bottom, growing in the dark, while its sexual organs were on top, waving about in the sun.”

Until I gardened—sloppily, heedlessly, at first—I took the green of the world for granted. It seemed a passive landscape, there to be rearranged. I gave up at the first sign of wilt or scorch; handled the plants too roughly; bought exotic perennials without researching where and how they grew. Then I watched my patient husband and started paying closer attention, and with that came respect and often awe. Bold stalky plants shooting upward overnight; tangled, dead Medusa grasses greening with spring’s warmth; black bean hyacinth vines twining around our wrought iron fence like pole dancers looking for tips; tightly wrapped rosebuds slowly unfurling layers and layers of petals; daisies popping open like cartoons of the sun. They all had their own way of being in the world, and while they basically stayed put, they were anything but static. Observing a garden is like watching an elaborately choreographed and timed ballet, arms lifting and falling, tutus spinning, legs wrapped around other dancers, bodies arching, bending, turning.

Tempted as I am to lavish consciousness on everything around me, I was fascinated to learn that tobacco and tomato plants click when they are stressed. The frequency is too high for us to hear these distress calls, but mice and moths do. As a plant dehydrates, the clicking speeds up, as though they are nervously cracking their knuckles. The beach evening primrose reacts to the sound of bees buzzing by secreting a sweeter nectar. Deprived of water, plants avoid dehydration by tightening the pores in their leaves. Roots avoid salt in the soil by inching toward areas that are less salty. Grains of starch shift with gravity, telling a plant which way is up. Flowers plan ahead, turning toward the sun before it rises and timing their pollen production to be ready when a pollinator shows up. Plants release volatile chemicals when they are eaten or infected or mowed down (that summertime freshly-mown-grass smell we all rhapsodize about). Neighboring plants receive this communication and take, when possible, defensive measures.

Most astounding, plants learn. The “sensitive plant,” Mimosa pudica, folds up if you touch a single leaf—but if you keep touching, the stimulus ceases to frighten, and the plant stays open. The same thing happens in studies where the plant is dropped, an extreme stimulus that causes it to fold up instantly. Drop it again and again, though, and it will realize there is no danger and stop folding its leaves. A solid month later, you can drop the plant, and it will remember that it need not fold.

Paco Calvo, a philosopher of plant behavior, believes plants possess a form of intelligence that lets them remember, learn from the past, and anticipate the future. In Planta Sapiens, he notes that a plant “always has to make a compromise among different things. It needs some kind of valence, a higher-level perspective. And that’s the entry to sentience.”

If plants are that close to sentient, why would we not know this already? Because we focus all our attention on animals, especially the ones that are big and warm like us—and the ones that might attack us. Calvo calls this “plant blindness.” It explains why people focus better on photos of animals than plants, and why young children have trouble even realizing that plants are alive.

A plant’s vascular network can conduct electrical impulses, Calvo points out. Plants can sense and respond to light, color, brightness, touch, gravity, moisture, humidity, temperature, nutrients, microorganisms, salt, other chemicals, magnetic fields, viruses, competition, obstacles, and predators. In his opinion, plants’ behavior is “far too goal-directed and flexible” to be explained by genes or environmental adaptation; placed in complex and changeable situations with multiple variables, they manage to respond in novel ways that help them survive and grow. He is willing to call this cognition, pointing out that it just might be possible to “think” without neurons and a brain. Cognition is not a trait we possess inside ourselves, he maintains. “It is rather something created by the interaction between an organism and its environment.”

Temple Grandin, who has acute insight into other forms of intelligence, is not convinced: “There is little evidence that plants have a centralized area in which to digest information and make decisions based on multiple inputs and outputs, and the book does not persuasively suggest how consciousness could arise without it.” That said, she does acknowledge “evidence that some plants have structures that can certainly function like simple nervous systems,” and that plants have abilities we have yet to fathom. Which maybe makes us a wee bit uncomfortable?

Botanists argue about how automatic (or not) plant responses are. Some say epigenetics is sufficient explanation. New information gets encoded in the genes, no thinking required. Chemicals are released without any intent to communicate. Other scientists inch, slow as a creeping groundcover, toward Calvo’s point of view. Romantics like the mystery of plants that feel and think, and some pragmatists find the notion intriguing. Others envision a Little Shop of Horrors. Trample the grass and it winces, prune and the rose screams, yank a weed and its neighbors seethe with rage….

To what degree plants are sentient—whether there is any degree of consciousness, intent, and decision-making—I do not know, and I could not care less. What plants do is wondrous. Why use the human brain as the yardstick? Plants are incomparable. The green of the world is far more than a backdrop for our drama.


Read more by Jeannette Cooperman here.