Life Sciences Icon Life Sciences
Neuroscience & Mind Icon Neuroscience & Mind

Are Plants Conscious? Science Writer Says Yes

Photo credit: Beatriz Moisset, CC BY-SA 4.0 , via Wikimedia Commons.

Annaka Harris, a science writer focusing on neuroscience and physics and the author of Conscious: A Brief Guide to the Fundamental Mystery of the Mind (2019), challenges us to reflect on two points:

1) In a system that we know has conscious experiences — the human brain — what evidence of consciousness can we detect from the outside?

2) Is consciousness essential to our behavior?

The editor notes, introducing an excerpt from the book, “But how sure can we be that plants aren’t conscious? And what if what we take to be behavior indicating consciousness can be replicated with no conscious agent involved? Annaka Harris invites us to consider the real possibility that our intuitions about consciousness might be mere illusions.”

Harris begins with a shoutout to natural selection (survival of the fittest), noting:

Our intuitions have been shaped by natural selection to quickly provide life-saving information, and these evolved intuitions can still serve us in modern life… But our guts can deceive us as well, and “false intuitions” can arise in any number of ways, especially in domains of understanding — like science and philosophy —that evolution could never have foreseen. An intuition is simply the powerful sense that something is true without having an awareness or understanding of the reasons behind this feeling — it may or may not represent something true about the world.


The problem with the “evolutionary” approach to thinking is this: If it’s true that we can’t trust the reasoning skills of our brains, which evolved merely in order to enable us to survive and reproduce (according to the theory) to arrive at a correct answer, we are in no position to evaluate Harris’s own thesis as either sound or unconvincing. Nor is she in a position to evaluate it herself.

The Blink of an Eye

She offers a look at locked-in syndrome — complete paralysis of the voluntary nervous system muscles except for those that control the eyes. The most famous example is probably Jean-Dominique Bauby (1952–1997) whose 1997 memoir of his post-stroke life The Diving Bell and the Butterfly, was written with about two hundred thousand blinks. He died two days after its publication in 1997. There’s also a film.

She also notes anesthesia awareness where, in rare cases, patients are aware of events and pain during surgery.

Yes, these rare events where people are conscious — but we don’t know it — do occur. But how do we generally notice consciousness in other human beings? By their conscious interactions with us in situations where no other explanation seems plausible. In social situations, sudden unconsciousness in a human is likely to result in calls to 9-11. Human consciousness remains mysterious but it is not ambiguous.

If Harris wants to introduce the idea that plants are conscious, efforts to denigrate the significance of human consciousness are simply not the best place to begin.

On Firmer Ground

She is on firmer ground when she observes that plants have been found in recent research to do many things that animals do. She cites plant geneticist Daniel Chamovitz, whose book What a Plant Knows: A Field Guide to the Senses (Farrar, Strauss & Giroux, 2017) describes plant responses to touch, light, heat, etc.:

Plants can sense their environments through touch and can detect many aspects of their surroundings, including temperature, by other modes. It’s actually quite common for plants to react to touch: a vine will increase its rate and direction of growth when it senses an object nearby that it can wrap itself around; and the infamous Venus flytrap can distinguish between heavy rain or strong gusts of wind, which do not cause its blades to close, and the tentative incursions of a nutritious beetle or frog, which will make them snap shut in one-tenth of a second. 


The electrical signals that stimulate nerve cells in plants are similar to those in animals and the genes that enable the plant to determine light or darkness are the ones humans use too. One might add to the list the fact that plants use glutamate to speed signal transmission — a technique also used by mammals.

In other words, given the physics and chemistry of our universe, a finite number of efficient communications systems is available. A variety of different life forms may be found using them. Those life forms may share nothing beyond the need to adopt one of the available systems.

But plant communication can be quite complex as well, as Suzanne Simard, has shown:

She was studying the levels of carbon in two species of tree, Douglas fir and paper birch carbon in two species of tree, Douglas fir and paper birch, when she discovered that the two species were engaged “in a lively two-way conversation.” In the summer months, when the fir needs more carbon, the birch sent more carbon to the fir; and and at other times when the fir was still growing but the birch needed more carbon because it was leafless, the fir sent more carbon to the birch — revealing that the two species were in fact interdependent. Equally surprising were the results of further research led by Simard in the Canadian National Forest, showing that the Douglas fir “mother trees” were able to distinguish between their own kin and a neighboring stranger’s seedlings. Simard found that the mother trees colonized their kin with bigger mycorrhizal networks, sending them more carbon below ground. The mother trees also “reduced their own root competition to make room for their kids,” and, when injured or dying, sent messages through carbon and other defense signals to their kin seedlings, increasing the seedlings’ resistance to local environmental stresses. Likewise, by spreading toxins through underground fungal networks, plants are also able to ravage threatening species. Because of the vast interconnections and functions of these mycorrhizal networks, they have been referred to as ‘Earth’s natural Internet.’”


Evaluating Plant Interactions

It’s possible that plant interactions are as complex as those of social insects, but that does not, in itself, establish consciousness. Ants, for example, might be best understood as thinking like computers, which implies efficacy but not consciousness. Harris acknowledges that fact: “Still, we can easily imagine plants exhibiting the behaviors described above without there being something it is like to be a plant, so complex behavior doesn’t necessarily shed light on whether a system is conscious or not.”

But then, in pursuit of plant consciousness, she cites artificial vs. human intelligence: “The problem is that both conscious and non-conscious states seem to be compatible with any behavior, even those associated with emotion, so the behavior itself doesn’t necessarily signal the presence of consciousness.”

No, wait. With AI, we humans are insiders. We invented AI. We know how it’s done. No one is sure what human consciousness even is but we are pretty sure what computers are and do. Even at their best, chatbots — to take one example — are simply scarfing up and reprocessing what humans say on the Internet. AI could only be conscious if somehow consciousness arises naturally from large scale computations. We do not, at present, have a reason to believe that it does.

The Philosopher’s Zombie

She then brings up the philosopher’s zombie, the zombie that might act exactly like a close friend but has no consciousness:

Let’s say your “zombie friend” witnesses a car accident, looks appropriately concerned, and takes out his phone to call for an ambulance. Could he possibly be going through these motions without an experience of anxiety and concern, or a conscious thought process that leads him to make a call and describe what happened? Or could this all take place even if he were a robot, without a felt experience prompting the behavior at all. Again, ask yourself what, if anything, would constitute conclusive evidence of consciousness in another person?

I have discovered that the zombie thought experiment is also capable of influencing our thinking beyond its intended function in the following way: Once we imagine human behavior around us existing without consciousness, that behavior begins to look more like many behaviors we see in the natural world which we’ve always assumed were non-conscious, such as the obstacle-avoiding behavior of a starfish, which has no central nervous system [7]. In other words, when we trick ourselves into imagining a person who lacks consciousness, then we can begin to wonder if we’re in fact tricking ourselves all the time when we deem other living systems — climbing ivy, say, or stinging sea anemones — to be without it. We have a deeply ingrained intuition, and therefore a strongly held belief, that systems that act like us are conscious, and those that don’t are not. But what the zombie thought experiment makes vivid to me is that the conclusion we draw from this intuition has no real foundation. Like a 3D image, it collapses the moment we take our glasses off.


Again, wait. Every human beings knows about human consciousness in the first person. But not one of us can ever be absolutely sure that another human being is conscious. Our minds are, perhaps by design, accessible to others only by what we say and do. Yes, the consciousness of others could be an illusion but then the whole universe around us could be an illusion — in theory.

We assume conscious human behavior in other human beings when they behave like conscious human beings. That makes sense because the alternative — that you or I are the only conscious one — requires a much greater stretch of belief.

As for “climbing ivy, say, or stinging sea anemones,” we don’t think they are conscious because nothing in their behavior prompts such an interpretation. It’s not a matter of intuition or prejudice; we are just not seeing evidence. 

A Comparison with Chimpanzees

Harris’s argument here is similar to the one we encounter in claims that chimpanzees think like humans. If they do, why don’t we see anything like a human culture growing up among them, just occasional flashes of intelligent behavior?

Harris would do well to stick to the point that plant behavior is turning out to be as complex as animal behavior. The question of consciousness is a separate one and there is no reason or need to assume that plants are conscious.

You may also wish to read: Do ants think? Yes, they do — but they think like computers. Computer programmers have adapted some ant problem-solving methods to software programs (but without the need for complex chemical scents). Navigation expert Eric Cassell points out that algorithms have made the ant one of the most successful insects ever, both in numbers and complexity.