Defining Consciousness in Non-neuronal Organisms: Limitations and Possibilities

Dakshitaa Babu
15 min readFeb 27, 2021

Materialism. Dualism. Idealism. There are one too many theories on consciousness in the world. Yet, they do not quite define consciousness in an objective way for other non-human, non-neuronal organisms to potentially be considered as conscious.

History of Consciousness Studies

For more than four centuries, questions on where consciousness originates and when- in a life cycle of a human- does such an advanced intellectual maturation occur, have persisted. Before the emergence of Neurophilosophy- a niche and relatively new stream of science that studies neurobiology as well as philosophical aspects of the mind, many philosophers held abstract views on the mind being an “intangible soul” that works independently from the brain (Descartes, 1641) while some scientists held a stark opposite view on the matter; the mind was purely a causation of the central nervous system in the body (Churchland, 2002). Some others have raised an epistemological (theory of knowledge with regards to its methods, validity) debate on whether the knowledge humans assume to know is legitimate. However, with more scientific discovery, philosophers started recognising that the brain might have a quintessential function for the mind to be working. More recent philosophers theorized that there is a crucial link between the two, yet no human may be able to unveil the link when everyone is limited by the capabilities of the human brain (McGinn, 1999). While different theories exist, a lot of them have an underlying assumption that the brain- consisting of neural networks- is necessary for the mind to function.

It is evident that most analysis of the mind has to do with the brain. Yet many organisms that do not have brains, can still process information. In the book “Other Minds” by Peter Godfrey-smith, Godfrey summarises and illustrates the evolution of all living beings over time. Importantly, he mentions that “even some single-celled organism has action potentials” (Godfrey-Smith, 2016). Scientifically, an action potential refers to the impulse passed from a neuron down an axon. Single-celled organisms and non-neuronal organisms do not have neurons. How then do they have action-reaction capabilities? On an evolutionary understanding of the world, all organisms arise from single-celled organisms that have a tool for transferring information; be it within in the cell or between cells. Over the years, this communication has been sighted in many other living beings- even those without a neuronal system- in terms of analogous structures. Plants too have such analogous structures. Neuroscientist Greg Gage exemplifies this using an electrocardiogram, with wire wrapped around the stem of a Mimosa plant and electrode inserted in the soil of the pot. A huge surge in action potential was recorded when the Mimosa plant was touched (Gage, 2017). Plants have a well-established system that processes collectable data… yet they do not have neurons or brains. They do, however, have action-reaction abilities.

There could be an evolutionary explanation of why plants did not develop an intricate neuronal system. Firstly, neurons require a lot of energy to work (Godfrey-Smith, 2016). Furthermore, evolution would not allow for wasteful use of resources (Clark, 1995). A neuronal system might be counter-productive to plants when all the energy required to function comes from a few hours of photosynthesis a day. Hence there must be a more effective and low energy consuming data-processing alternative inbuilt in plants.

It seems like there is no need for neurons per se for action-reaction in living things. What any living thing needs is a structure that allows for systematic delegation of information, be it synaptic nodes or action potential gradient. The real biology of these networks does not completely matter, as completely different network structures can give rise to similar consciousness. So, do plants really have consciousness?

Plants and History of Plant Consciousness Studies

“On the last day of the world, I would want to plant a tree.” — W.S Merwin

Plants are the primary source of energy that form the foundational tier of the food web. Despite their stark significance for all forms of life, humans often tend to overlook their importance in everyday sceneries. People do not give much recognition to plants because of how slowly they move, grow and respond to changes in contrast to the sensitivity of humans and their fast-paced lifestyle. Plants form the inconspicuous backdrop or ‘background noise’ of people’s day-to-day lives, especially in the context of ‘urban landscaping’. This familiarity of plants, and “the incapacity of humans to recognize elements of ourselves in the form of vegetal being”, (Marder, 2013) might be the fundamental reasons why plant cognition studies have been slow to progress over the years.

Charles Darwin was one of the first to explore plant cognition in the 1800s when he found a stark similarity between the action of plant root tips and brains of some animals. Since then, there has been much debate on plant cognition and consciousness, but the majority have been completely disapproving of plants having any semblance of consciousness. Lately, however, some researchers have challenged this view. They have raised uncertainties on the pre-existing definitions and understanding of consciousness, even those regarding human cognition.

Anthropomorphism in Plant Consciousness Studies

In National Bioneers Conference 2018, leading researchers and visionaries in the field of plant intelligence, Monica Gagliano and Michael Pollan spoke in the Plant Intelligence and Human Consciousness panel. Gagliano raises an important issue on anthropomorphism (the attribution of human characteristics or behaviour to non-human things) in plant studies. For example, the word intelligence literally means ‘to choose between’ as it is derived from the Latin word intellegere(intel (a variant of inter) + legere(to choose)). In this definition, many organisms are intelligent, because they make choices. However, humans tend to “colonize the words that we make, and we give them a very specific definition in a specific context”(Gagliano, 2018) which results in intelligence having a meaning that is almost as heavy as the word ‘consciousness’. The issue with this is that such loaded definitions of terms like consciousness might give no room for other non-neuronal organisms to even be considered to possess consciousness. Do researchers discredit the real self of non-neuronal living things by defining cognitive attributes based on human experience? To illustrate this phenomenon, consider an all-encompassing and balanced theory of consciousness by theorist Axel Cleermans. Three applications of Cleermans theory in the case of the plants as a non-neuronal living thing will be analysed.

Cleermans Theory of Consciousness and Applications

Belgian Researcher and Professor of cognitive science in Universite Libre de Bruxelles, Axel Cleeremans, theorises that consciousness is a multi-dimensional concept and the most complicated & difficult to evaluate aspect of consciousness is ‘phenomenal consciousness, or subjective experience, that is, the fact that information processing is accompanied by qualia- elements of conscious imagery, feelings or thoughts…’ (Cleeremans, 2001). In his paper “Implicit Learning: A graded, dynamic perspective”, Cleeremans rationalises various theories of consciousness and decides to use vehicle theory- that subjective experience is not something that could arise ‘in any representational vehicle’, but in a particular medium as a result of processes occurring in that medium — as a basis of his argument (Cleeremans,2001). He theorises that consciousness arises from learning and adaptation. Learning is the ability to assimilate knowledge and store this information to improve the chances of any living object surviving. He defines adaptation as a comprehensive link between an object and the environment through which the object can modify its states and constitutions as a result of previous sensitivity to the environment. The object then continuously modifies this sensitivity.

Application I: Plant Consciousness in Learning and Adaptability

Researcher Monica Gagliano used Mimosa Pudica that is known to be an extremely sensitive plant (commonly known as touch-me-not plant, that closes its leaves in a defensive curl when one touches it) to demonstrate plants ability to learn and remember. She dropped 56 potted mimosa plants 60 times each from a height of 6 inches onto a soft foam that prevents bouncing (as shown in Figure 1 below). Initially, the sudden drop motion causes the leaves to fold immediately as the speed was fast (despite height being a short distance). However, she observed that after a few drops, some plants did not close their leaves fully when dropped again, and some stopped responding. To test whether the lack of response was due to fatigue, Gagliano shook the plant horizontally and the leaves curled up as per normal. The plants had realised that the drop motion was no longer a threat to them and have disregarded the stimulus as anything relevant. Even when Gagliano retested them every week for a month, they continued to remember that this dropping motion can be ignored.

Figure 1: Mimosa Pudica leaves showing sensitivity to touch by folding its leaflets. (Photograph by Alamy Blickwinkel) Illustrations of Gagliano’s experimental set up (Krulwich, 2015)

The ability of the Mimosa plant to assimilate knowledge and learn is clearly visible. In terms of adaptations, while physical adaptations take a long time to show up on any living thing, behavioural adaptations to changes in stimulus can be supported by the dataset as a continuous physical change in the environment of the mimosa plant elicits a response in the leaves to stop closing, suggesting that continuous transfer of information results in an adaptation in a plant to be immune to the drop. Mimosa Pudica illustrates learning and adaptation clearly, which on hindsight makes them seem like they are conscious. However, it does not explicitly showcase Cleermans’ definition of subjective experience, as most of the mimosa plants responded similarly. The qualia aspect is also not clearly shown in the experiment, which is a necessary supposition to conclude plant consciousness by his theory.

Application II: Plant Consciousness in Subjective Sound Processing

Researchers Cocroft and Appel played recordings of caterpillar feeding vibrations to a set of Arabidopsis thaliana plants and played only silence to another other set of plants. After 24 hours, the researchers found that the plants exposed to the feeding vibrations had changes in hormone secretion in plants, and they produced more defensive chemicals which are meant to detain the caterpillars from them (Body et al., 2019).

The sound waves emitted by insect crunching on leaves can be proposed to have a mechanical impact on cell membranes, causes dilation of pores and ion channels on the cell membrane, changing the ion concentration levels, hence eliciting a chemical response in plants. However, the most important finding in this data is that even sounds with similar acoustic features, such as gentle wind or different insect sounds can have varied effect in the release of chemical defences in plants. This seems to suggest that the plants can distinguish different vibrations, which gives insights into subjective experience in plants. The medium for information processing in plants is a chemical network system that is analogous to the neuronal system in humans. Just as humans can easily differentiate the sound of wind from the sound of approaching footsteps and react accordingly by the aid of message transmission in the central nervous system to motor neurons, plants also show signs of subjective experience as a result of chemical processes in their medium. However, Cleermans defined an important aspect of qualia as the ability to create an image which links emotions and thoughts to formulate an image of any situation. It is not so clear whether plants completely fulfil qualia as there isn’t enough information in the dataset to show that the plants can emote to the caterpillar crunching, as compared to the defensive mechanism being an adaptation that has arisen from evolution to increase the survival rate of the plant. In humans, any sound is reacted to emotionally, which leads to a motor response. For example, when hearing someone coming to attack a man, he feels fear, which propels him to run away from the situation. It is not explicit whether Arabidopsis Thaliana plant secretes defensive chemicals due to fear of the caterpillars eating their leaves.

Application III: Plant Consciousness and Awareness Through Communication

When Cakile edentula var. lacustris (Brassicaceae) plant was planted in a pot of sibling group and a pot of strangers, the sibling groups have shown much higher fitness than groups of strangers in the field (Donohue, 2003). In recent research papers on plant-plant communications, many researchers have identified plants ability to different a stranger plants roots and other plant parts compared to that of its own kind (Dudley, 2007).

The ability of plants to differentiate their own kind from others gives them a competitive advantage in increasing population of their species. This is a similar concept to tribal ethics of protecting their own people. Darwin’s theory of evolution also reinforces the need for species to protect their own kin in order to have a higher chance of survival of the species, lengthening their lifespan. However, it has always been assumed that adaptations that allow plants to compete less with their own kind have been a chance event, such as seed dispersal by wind, water, animal, whereby the outcome is not fully predictable or known to the plant. Donohue’s findings, however, suggests that plant actions to avoid crowding could be intentional to a certain extent. They seem to be able to identify and differentiate different plant roots (and other plant parts) as well as communicate with it accordingly. To be able to identify another organism’s structure to be like itself, one needs to be aware of itself in the first place, which might imply that plants have an idea of self. In the burgeoning field of bioacoustics, more information on plant-plant communications is being unveiled. Studies in plant communication by airborne signals have led to findings on allelopathy(releasing chemicals from roots to inhibit germination or development of another plant), cheaters (plants that exploit mutualistic interactions by absorbing information without returning) and eavesdropping (act of receiving benefit from an emitter by a receiver who is not in a mutual transaction) (Martein Heil, Richard Karban, 2010). All these findings show community and self in nature, suggesting consciousness. However in this case, the roots only partially fulfil Cleermans understanding of consciousness, as they show adaptation to competition in the environment, but the assimilation of knowledge and subjective experience is not exhibited as the experiment was done on different plants and not the same plant with siblings and the same plant with strangers(albeit using same species of plant in the experiment). This data set also suggests that Cleermans theory might be lacking some components such as the ability to identify self to be different from another, which also contributes to consciousness.

All three applications of Cleermans theory show the failure of plants to meet his criteria to be conscious. Yet, there is something that is not mechanical in the actions of plants exhibited above that suggests that they have consciousness to a certain extent. However, the definition of consciousness is so fixed that non-neuronal organisms like plants will never be able to fulfil it. The three applications illustrate that even the most balanced theory of consciousness does not give leeway for non-neuronal living things to fulfil the criteria. For instance, qualia, defined as the ability to emote and create mental imagery is in no way possible for a plant to fulfil because it does not have a ‘mental’ state to begin with. Yet, it might have an alternative state to that. As an extension of Pavlov’s classical conditioning for associative learning in dogs, Monica Gagliano did a similar experiment with pea plants (refer to Figure 2 below). Blue light triggers a phototrophic response in plants, which causes them to grow towards the light. When the plant was present with a fan instead, there was no response, the plant grew straight. But if the plant was presented with a fan and blue light, the plant eventually learnt that, just by the fan, it can prepare and bend towards the fan as the fan would tell it where the light is going to be (Gagliano, 2018).

Figure 2. Pavlov’s classical conditioning in pea plants, experiment by Monica Gagliano (Gagliano, 2019)

The plant exhibits the ability to learn through forming associations, which requires the plant to be able to detect, distinguish and compartmentalise cues according to its own internal value system. As all plants are different, these associations are formed based on subjective system of feelings and experiences that connect to the cues. The experiment also shows Pea plants ability to extend its perception beyond its immediate environment, increasing the information available to its internal system beyond what is present. The presence of the fan itself would not result in a phototrophic response unless the plant is imagining and expecting blue light to follow it. This experiment is meant to disrupt linear thinking in humans, that if an organism does not have brain and neurons, it cannot make associations, or imagine. The organisms may work in other ways; hence it is vital for us to relinquish the linear thinking or expand it. The first step to change the linear thinking is to open the definitions of the terms used to explain phenomenon. Even in this case, the definition of qualia does not allow for pea plant to be deemed to possess it, although it very clearly does.

Gagliano too does not believe that these definitions are doing justice to the true self of the beings. She says, “We’re using words to describe the world, and if the words are failing us, well the world is still there, maybe we just need to choose different words, or remove the loading…Open those words so that words are a medium to explore and investigate and learn and understand, appreciate, connect more rather than disconnect and being a world of separation.” (Gagliano, 2018) Indeed, there seems to be a world of separation between humans and other living things. Yet the world is never separate, humans rely on all other living things as much as they rely on the ecosystem which humans are also a part of. Michael Pollan’s The Botany of Desire illustrates the many instances in which plants manipulate human taste and preferences, contrary to popular beliefs that humans manipulate agriculture. The separation is completely constructed, and it is important that the gap is bridged as nothing truly is separate.

More discovery of plant behaviour might push the scientific industry to rethink the existing definition of qualia, to one that is not limited to how the 5 human senses and mental imagery allow them to react to any situation. Other living things and animals might have other means of response to a situation that does not depend on this very mechanism, but mediums and structures of their own. This essay argues that key terms in cognition studies, should be defined based on the ‘self’ of the subject being observed instead of human’s understanding of self, especially for living things that do not have neuronal systems like humans.

It is also about time that humans leave the Plato’s cave and consider plants as subjects instead of objects. When questioned “What it is like to be a mimosa?”, Gagliano beautifully answered, “If you’re asking the scientist, then the answer would be: I have no data to answer the question… If you’re asking the person that has experienced plant in an intimate way, then…there is no plant consciousness or human consciousness, there is only one consciousness, and what is residing here is residing there.” Anthropomorphism gives no justice to what a non-human living thing truly is. Instrumentalising plants based on their uses only obscures us of their real being. The faster we quieten our identifying mind to see the spirit in the plants, the faster we dissolve the false separation between people and the natural world.

Works Cited

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Body MJA, Neer WC, Vore C, Lin C-H, Vu DC, Schultz JC, Cocroft RB and Appel HM. Caterpillar Chewing Vibrations Cause Changes in Plant Hormones and Volatile Emissions in Arabidopsis thaliana. Front. Plant Sci. 2019 (10:810)

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Gagliano, Monica. Abramson, Charles I. Depczynski, Martial. Plants Learn and Remember: Let’s Get Used To It. 2017

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Gagliano, Monica. National Bioneers Conference 2019: Plant Intelligence and the Importance of Imagination In Science. Youtube: Bioneers Channel. 2019

Godfrey-Smith, Peter. Other Minds: The Octopus, The Sea, and The Deep Origins of Consciousness. New York: Farrar, Straus and Giroux. 2016.

Krulwich, Robert. “Can a Plant Remember? This One Seems to — Here’s the Evidence”. National Geographic. 2015

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Pollan, Michael. “The Intelligent Plant”. The New Yorker. 2013