Are Plants Smart?

Well, are they?

By Becky Rupp

Think about this much and it’s a downright creepy question. I mean, consider what we do to plants. We prune them, mow them, cage them, tie them to poles and fences, pick parts of them and eat them, and uproot them and toss them on the compost pile. We chop them up and burn them in the woodstove, hang things on them, and repeatedly stomp all over them, in spite of all those admonitory signs ordering us to Keep Off the Grass.

Then it turns out that plants might actually be…intelligent. Botanist Daniel Chamovitz, in What a Plant Knows (Farrar, Straus & Giroux, 2012), writes ominously, “Plants see if you come near them; they know when you stand over them. They even know if you’re wearing a blue or a red shirt. They know if you’ve painted your house or if you’ve moved their pots from one side of the liv-ing room to the other.”

Urk.

The plants are watching me. I haven’t been able to get this one out of my head. It’s the same sort of nervous-making feeling you get as a little kid when you find out that Santa sees you when you’re sleeping. Until now, I’d never viewed the vegetable patch as a lurking surveillance system.

The plant intelligence question, now hotly debated in botanical circles, isn’t as straightforward as it sounds—and revolves in large part around just what your definition of intelligence is. Plants obviously can’t solve quadratic equations, write Hamlet, or compose Beethoven’s Ninth; they don’t exhibit the sort of creative abstract thought that we equate with human intelligence. But for all that they seem to just sit there, they’re smarter than they look.

Prime mover in the modern field of plant intelligence is botanist Anthony Trewavas from the UK’s University of Edinburgh. Trewavas defines intelligence as the ability to sense the surrounding environment, analyze the results, and then, based on these, make decisions about how to behave. Plants, says Trewavas, have this ability in spades, but we tend not to notice it much since plants are generally slow responders. Anchored in place as they are, their most common response to input—sun, shade, water, rocks, other plants—is to change their growth pattern, which in the short term isn’t exactly exciting. There’s a reason that “like watching the grass grow” is a synonym for really, really boring.

Add time-lapse photography, however, and the plant world suddenly becomes a far more active and interactive place. Take, for example, the dodder vine. Dodder—a relative of the morn-ing glory—is something none of us want in our gardens, since it lives by wrapping itself around other plants and sucking the life out of them. (Among its nicknames are devil’s ringlets, hellbine, strangleweed, and witch’s shoelaces.) Its murderous behavior, however, isn’t a matter of random snatch and grab. A young dodder seedling literally sniffs out its prey, analyzing the organic volatiles released by neighboring plants, and then heading for the tastiest and most tempting of the bunch—choosing, for example, tomatoes in preference to wheat. Once attached, it then performs some version of a cost-benefit calculation to determine how many times to wrap around its victim: the more nutritious the host plant, the more coils generated by the climbing dodder vine.

Experiments by Australian ecologist Monica Gagliano seem to indicate that plants not only modify their behavior based on cues from the environment, but actively learn and form long-term memories. Gagliano works with Mimosa pudica, also known as the sensitive plant or touch-me-not because it rapidly folds up its fern-like leaves if bumped, shaken, poked, landed upon by a bug, or approached with a lit match. In Gagliano’s lab, potted mimosas were dropped from a height of about six inches onto a foam pad—a harmless jolt that nonetheless caused the plants to nervously close their leaves. After repeated drops, however, the plants ceased to respond, having apparently analyzed the situation and determined that dropping wasn’t dangerous. They did, however, continue to close their leaves when shaken or poked.

Furthermore, over a month later, the plants still remembered their experience. While untrained plants clapped their leaves shut in response to a drop, the trained plants—knowing they were perfectly safe—stayed calmly open.

It makes you think, doesn’t it? Every year Randy and I go out, armed with clippers, and prune our grapevines. What if they remember us? And if they do, what if they hold a grudge?

Some indications are that they may at least talk to each other about us. Plants communicate with themselves, each other, and the outer world via a sophisticated battery of chemicals. Sagebrush, for example, when it feels crowded, produces a volatile chemical called methyl jasmonate that keeps other plants from growing underneath it. Tobacco plants, attacked by bacteria or viruses, produce compounds that warn neighboring tobacco plants of trouble, causing them to ramp up their defense mechanisms. Corn, tobacco, and cotton, beset by caterpillars, release a chemical cry for help that attracts caterpillar-destroying parasitic wasps.

According to forest ecologist Suzanne Simard of the University of British Columbia, forests are vast interconnected webs, welded together by an underground system of mycorrhizal fungi that connect tree root to tree root, allowing the trees to share information and exchange needed nutrients. “Mother trees” use the network to nourish at-risk seedlings; evergreens use it to provide spare sugars to struggling deciduous trees to tide them through the winter.

However smart they may be, though, plants don’t have brains. (A group called the Society for Plant Neurobiology, repeatedly attacked by purists pointing out that plants don’t have neurons, recently caved and changed its name to the Society for Plant Signaling and Behavior.) The closest plants come to having anything brain-like may be a section of the root called the transition zone, located behind the root tip. The transition zone is electrically active—like human nerves—and it contains a hormone that regulates plant growth, which it ferries from cell to cell as needed in membrane-enclosed vesicles. It’s in the transition zone, scientists guess, that sensory cues picked up by the root tip—information about gravity, light, oxygen, water, and soil conditions—are translated into commands directing the root how to grow and bend.

Still, smart is as smart does. However bright plants may be, it’s clear that they don’t think like us. If anything, plants are an alien culture. The philosopher Ludwig Wittgenstein famously said, “If a lion could talk, we would not understand him,” because people and lions have no common ground. There’s even less between us and even the most vociferously communicating tomato plant, cucumber vine, or stalk of corn. But that doesn’t mean that there’s nothing going on there.

These days, while I potter about in the garden, I wonder if the plants like the red shirt or the blue shirt best. I worry about how they’re getting along with their neighbors. And when I pick up the pruning shears, I apologize.

Because now I know they’re watching me. ❖

By Becky Rupp, published originally in 2015, in GreenPrints Issue #101. Illustrated by Blanche Derby