Are Plants Conscious?

By Daniel Kolitz on at

Animal rights activists have done stellar work in foregrounding the question of creature-consciousness: no meat-eater is now ignorant of the fact that their food once lived, breathed, maybe nuzzled its kin in a blood-soaked slaughterhouse. Environmentalists have a harder go of it. Fracking footage will always be less upsetting than your average fast food expose: Plants, after all, can’t wail frantically as they’re mowed down by the millions. But does that mean they’re not conscious? Is it sensible, or desirable, to start anthropomorphising crabgrass and dandelions, or are plants really as insensitive as we all instinctively assume?

For this week’s Giz Asks, we posed those questions to a number of environmental scientists and philosophers — including a professor on the vanguard of something called “plant neurobiology.” Plants may not be able to wistfully reflect on their childhood, or hear/see anything in a conventional sense, but they do, as it turns out, retain information, and make “decisions” based on past experiences. Whether this constitutes “consciousness” depends, as always, on how you define the term.

Michael Marder

Ikerbasque Research Professor, Philosophy, University of the Basque Country, Vitoria-Gasteiz and author of Plant-Thinking: A Philosophy of Vegetal Life, among other works

Plants are definitely conscious, though in a different way than we, humans, are.

To find the resources they need for living and thriving, they need to orient themselves in above- and below-ground environments. And so, plant roots navigate the subterranean labyrinths of soil, rock, water, bacteria and roots of other plants no less proficiently than mice in search of food. They must be aware of dangers — the onset of drought, or invading herbivore insects — in order to carry out the most essential life activities, or to activate their defences (for instance, by releasing biochemical cues that call upon the predators who will devour the threatening herbivores). They have to make complex decisions on the best time to blossom, juggling up to twenty environmental factors, such as the length of day or the warmth of the air, comparing the evolution of these conditions over a span of at least a month. In other words, plants gather as much information about the world they live in as possible and, attentive to changes in it, act with discernment.

If consciousness literally means being “with knowledge,” then plants fit the bill perfectly. Of course, they do not have the sense organs we are used to, such as the eyes and the ears, to receive stimuli from the environment. But they do have cells and tissues (say, photosensitive receptor cells) that do the trick as well as — and sometimes better than — an animal or human eye or ear would. The data they receive from the constantly changing world is essential for their survival. In fact, they change in tune with the world and with the seasons, growing when the conditions are optimal, or shedding leaves and bringing life to a minimum in the cold of winter. We might say that plant consciousness is saddled with tonnes of knowledge, because plants live with an extreme sensitivity to the places where they grow.

It is another question whether plants are self-conscious. Before dismissing the existence of this higher-level faculty in them outright, we should consider what a plant self might be. Plants are only loosely integrated into a unity (take a cutting from a magnolia stem, and it will grow independently!). It stands to reason that their sense of self would be equally dispersed. Quite often, in fact, parts of a plant subject to danger (e.g., the leaves invaded by unwelcome insects) will communicate the threat by releasing airborne biochemical substances to other parts of the same plant. 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 fulfil, 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.

Heidi Appel

Professor, Environmental Sciences, University of Toledo, whose research focuses on how plants recognise and respond to insect herbivores with chemical defences

Are plants conscious? My view is that they are not, even though they are aware of many aspects of the environment in which they live. My answer is shaped by the common definitions of consciousness in the English language, which all include the concept of mind and self-awareness, in addition to being aware of one’s environment.

The ability to sense things in the environment and to integrate those sensations into a beneficial response is not in itself consciousness. Plants are not exceptional in being able to do this — it’s a trait of all life forms. Plants are just usually underestimated because they lack the specialised organs that vertebrates possess to sense their environment.

Some have suggested that since plants can form ‘memories’ they are therefore conscious beings. Plants do retain information about what they experience, in that their response to changes in their environment can depend on what they’ve experienced previously. Even offspring may exhibit some traits that are influenced by what their parents experienced. Retaining information within and between generations of organisms is a trait of all living things, with an increasingly well-understood genetic basis. Whether these constitute ‘memories’ depends on whether you define ‘memory’ as ‘recall’ or something more. If we return to common definitions in the English language, memory as commonly defined does not require self-awareness, even though our personal experience of memory as humans is certainly integrated with the notion of self-awareness.

Dr. François Bouteau

Assistant Professor, Plant Biology, Université Paris-Diderot

The main problem with this question is the definition of what we call consciousness. If we consider the definition of consciousness in a psychological sense, i.e. made to describe different aspects of human life that would be related to notions of knowledge, emotion, existence, intuition, thought, psyche, subjectivity, sensation, reflexivity.... it is obviously difficult to answer the question for a plant.

However, doctors working on coma patients know that consciousness is not binary and that there is a wide range of conscious states in humans between the total loss and awakening state of a healthy subject. If we consider a more general definition of consciousness as the ability to perceive our own existence and the world around us, and accept that we do not need a brain to have a consciousness, the matter begins to be simpler. Many studies have shown that plants perceive and interact with the world around them, using complex behaviours.

As regards the perception of their own existence, no one can say, but there is growing evidence that plants are capable of a recognition of kinship that could argue in favour of a capacity for self-recognition.

Another way to approach the issue is to seek operational evidence of this consciousness. A classic way to make a human being lose consciousness is to anaesthetise him, the very practical consequence during surgery being the loss of perception of our existence and the world around us. The effectiveness on plants of anaesthetics used during such operations has been demonstrated for a very long time. Although we do not yet really know how these anaesthetics work in plants or animals, it seems, in a very interesting way, that the same cellular mechanisms, including the functioning of ion channels allowing the genesis of the action potential, are inhibited. Another point of argument is that plants, when injured, synthesise molecules with anaesthetic power. That plants like all living beings on earth have a form of consciousness is very likely since it could correspond to an adaptive necessity for survival.

Richard Karban

Professor, Entomology and Nematology, UC Davis and author of Plant Sensing and Communication

The answer to this question depends entirely upon how you define consciousness. Most definitions include awareness of one’s environment. By this definition there can be little doubt that plants are conscious organisms. Other definitions require the operation of a person’s brain. Plants obviously fail to meet these requirements as they are not persons and lack brains.

But the evidence that plants are aware of their environments is overwhelming. Anyone who has ever had a houseplant in a window has observed that the plant grows towards the light. To achieve this response it must perceive the direction of the light and allocate resources preferentially. Indeed, plants perceive self and non-self and allocate resources differently when they encounter tissues of these two types. Plants differentiate the quality of shade cast by a green competitor and respond more strongly to this threat than to an inanimate object. Plants anticipate future conditions and respond to the light cast by competitors before actually being shaded.

Plants not only perceive light but many other qualities of their environments that affect their survival and ability to reproduce. However, there is no credible scientific evidence that they perceive music or prefer classical music rather than rock. Plants forage for nutrients in the soil, growing dense roots in rich areas and abandoning poor ones. They evaluate microbes that they come in contact with and reward those associations that are beneficial to them while actively defending against those that are harmful.

Plants also actively defend themselves against insects and other larger herbivores which make their own living feeding on plants. They respond to actual damage, as well as a variety of reliable cues that predict future risk by elevating sophisticated and often costly defences. These cues include airborne chemicals that are emitted by their own damaged tissues or those of their neighbours. They respond to chemicals associated with mating and egg-laying of insects as well as insect footsteps, insect saliva, and vibrations caused by insect chewing. Plants that have experienced damage remember those experiences and respond more rapidly and more strongly to subsequent attacks. In some cases, this memory persists for several plant generations.

Danny Chamovitz

Dean, George S. Wise Faculty of Life Sciences, Director, Manna Centre for Plant Biosciences, Tel Aviv University

Plants are obviously aware of their visual environment, and of smells in the air; they know if they’re being touched; they have different types of memories; they can differentiate between up and down — but that doesn’t mean that plants are conscious.

I can use the word awareness: Plants are aware of their environment. But all organisms are aware of their environment — you have to be in order to survive. 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.

Of course, this also falls into the question of intelligence. Plants are incredibly complex — does that make them intelligent? Can we even define what intelligence is? Psychologists can’t even agree on a definition of intelligence for people.

The bottom line as I see it is that plants are incredibly complex organisms that have evolved for the past two billion years completely differently than animals have, and we don’t need to anthropomorphise them in order to appreciate their complexity. Plants don’t have nerves, and plants don’t have brains, but they still integrate signals from their roots and their leaves and their flowers, and know how much light there is and what the temperature is and how many bugs are in the area, and they integrate all of this information to yield a plant that is exquisitely adapted to its environment, and they do it all without a brain. So what’s that say about the need for a brain?

In other words: you can eat your plants without feeling guilty.

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