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Writer's pictureAndré Geremia Parise

The extended cognition of plants

Updated: Aug 1, 2023


Floresta de pinheiros com nevoeiro
Plants may be extending their cognitive process to elements of the environment they modify. Where does a plant's cognition end and the environment begin?

An article has just come out in which we further develop the hypothesis of extended cognition in plants. In particular, we explore what it means for understanding what a plant is from a physiological, ecological and philosophical point of view. Furthermore, we discuss four case studies for the four ways of extending plant cognition proposed so far. This is a deepening of what was already proposed in 2020 and discussed in the article on Ariadne's web, published at the beginnig of the year.


First of all, by 'we' I mean Michael Marder and myself. Michael is a philosopher and professor at the University of the Basque Country in Spain, as well as a member of Ikerbaske, the Basque Foundation for Science, whose work encompasses ecological philosophy and ecological thinking, political theory and phenomenology. He is the author of several books and numerous articles, many on the philosophy of plant cognition. His most recent book, Time is a plant should be published this year by Brill Publishers.


And I... am I. PhD student at the University of Reading (UK) and someone trying to understand how plants sustain their cognitive and intelligent processes, being such fascinating and beautiful organisms, able to survive in harmony on this earth for over 400 million years.


In this work, we discuss a little the old question of the dualism between body and soul, an idea—or even, paradigm—that dominates Western science since the times of Plato. Over the centuries, the prevailing wisdom on this side of the world has been that the soul, the spirit, what gives life, what animates, was something separate from the body, being immanent and immaterial. This concept was perpetuated by the Catholic Church, as it helped to underpin much of Christian theology, from the contrast between the temptations and sins of the flesh (i.e., the body) as opposed to the virtues of the spirit, to explaining how souls would go to God after death while the physical body rots.


In the seventeenth century, inspired by these religious conceptions, the French philosopher René Descartes deepened this thesis even further by concluding that the body and the mind have completely different natures and that, therefore, one could even exist without the other (at least, the mind without the body). While the body could be studied analytically, scientifically, the mind would escape this condition, being more than anything a philosophical or theological problem. Descartes sedimented the dualism between body and mind by separating what makes up human beings in res extensa, that is, the extended thing, the body; and res cogitans, the thinking thing, the mind.


A detailed explanation of this Cartesian distinction and its implications for contemporary philosophy and science would require not only an entire post dedicated to it but books and books, and many doctoral theses, as it actually happens, so we will not delve further into this subject. Let us know, however, that Descartes established the paradigm that would guide all scientific research on the functioning of the mind practically until today. Currently, people no longer speak of the soul, but of the mind, or cognition. Nevertheless, the idea that it is made of something special, transcendental, remains ingrained in the way many study these phenomena, often without scientists even realising it.


A change began to occur in the 1970s (like so many others in those prolific times!), when some scientists proposed the abandonment of the dualistic conception of mind and body, with the mind being a “thing” (res, in Latin). Scientists such as Gregory Bateson, Humberto Maturana and Francisco Varela, among many others, have proposed that the mind, or cognition, is, on the contrary, a process that takes place in the body and by the body. The cognitive process is not something supernatural that dwells in us, implying a clear distinction between what is cognitive and what is not, but the product of the functioning of our bodies. Cognition is the observable effect of a very special kind of interaction (it is undeniable) between the matter and energy that make up our bodies. However, this is the very same matter and energy that we find around us but organised in other ways.


Before we go any further, I make a caveat: there is a lot of overlap between the terms mind and cognition, and even between those two terms and consciousness. Personally, I prefer to just use the term cognition as the process of perceiving the environment, processing information, learning, etc. Mind and/or consciousness are a product of the cognitive process when it becomes self-reflective, able to stimulate itself and generate perceptions about its own functioning. While cognition is a phenomenon present in all forms of life, mind and/or consciousness are probably more restricted to humans and some other animals. This does not mean that something supernatural remains in the mind, as we are always talking about physical phenomena, nor does it mean that other beings such as plants, fungi and insects cannot be conscious. We just don't have empirical evidence of this yet, although we are participating in a discussion about this concerning plants.


The developments in the science of cognition throughout the second half of the twentieth century until today have caused the cognitive process to lose, at least in part of the scientific community, its 'divine' status and become an 'earthly' product, something that happens here and in interaction with the matter and energy around us. An interesting consequence of this is that the brain, which in its extraordinary complexity was seen as the only possible reliquary to allow the manifestation of cognition, came to be seen as just another structure where this type of interaction could occur, but not necessarily the only one. In fact, any living body is cognitive, from a cell to a humpback whale, and cognition is understood by many as an inherent property of life.


Another important consequence of this change in conception is that there is no longer a concrete reason why cognition should be contained in the body, since it is the product of the interaction between matter and energy, as already mentioned. This opens the possibility for the cognitive process to involve other forms of matter and energy that are not necessarily in the body. Consequently, cognition can partially take place outside it, beyond the limits imposed by the skin.


This idea is known as the extended cognition thesis and was proposed by Andy Clark and David Chalmers in 1998, at the time to explain part of the human cognitive process. According to them, when we manipulate objects to help our cognitive process, as when making a mathematical account using paper and pen, what happens is that these objects external to our body become part of the structure that sustains the process, having importance akin (to a certain level) to that of neurons, for example. They are fundamental elements to arrive at the result of a calculation, at least for those who have never been good with maths.


The extended cognition thesis has been developed over the years and, recently, has found space in other areas that are not necessarily related to the study of the human mind. Some authors have proposed that spiders also extend their cognitive process to their webs, others that slime moulds do it as well, and our group has been working with the hypothesis that plants also extend their cognition. The possibility that the extension of cognition is relatively common in nature has already been discussed in this blog.


Three conceptions of extended cognition in plants


Before discussing how plants extend their cognitions, it is interesting to debate what extending cognition means, as there are three senses in which the noun 'extended cognition' can be interpreted: the first goes back to Descartes, again.


According to the philosopher, we are composed of a material that extends in space; which occupies a volume. That's the extended thing (res extensa). Furthermore, we would also be formed by the transcendent and mysterious thinking thing (res cogitans), which gives us life, volition, and spirit. As said, this 'thing' is immaterial, therefore, not extended. But if we abandon this dualism and understand the mind as an inextricable part of the body, then the mind necessarily extends in space as much as our body does, being, in this way, extended as well.


The second sense would be an extension beyond the epistemological limits of how cognition is understood by many scientists today. In particular, concerning the cognitivist idea (referring to the movement of US American psychologists in the mid-twentieth century). This view understands cognition as confined to the brain or central nervous system, being a process similar to a computer program that processes inputs in the form of symbols encoded in electrical impulses in our neurons to produce outputs. However, if we understand cognition as a phenomenon that involves a range of physiological processes and interactions between matter that go far beyond electrical signals in neurons, there is an extension in the very concept of cognition and in the understanding of what this phenomenon is.


The third meaning is the most radical and controversial, and it is precisely the one intended by Andy Clark and David Chalmers discussed above. That is, cognition extends outside the body, encompassing objects that are beyond our skin and processes beyond our neurons. However, in this case, we are talking about plants, which do not have neurons or skin, but the idea is equally valid. A plant's cognitive process need not be confined within its epidermis, but also occur outside it. Furthermore, one of the ways plants manifest their cognition is precisely by extending their bodies—through growth—and occupying spaces where they were not before.


Lessons from plant anatomy


If we pay close attention to how plant bodies are organised, perhaps the idea of extended cognition sounds natural. And for that, a comparison with the way animals are organised can facilitate understanding.


Animals evolved as heterotrophic beings. That is, they need to extract their food from the environment. Hence, over the course of evolution, they were forced to move around to find food and water. To do this, having a compact body helps a lot, as it is easier to move all your internal organs when they are 'packed', having the smallest possible contact surface with the environment. This locomotion also favoured the concentration of functions in certain areas of the body, including its senses. Generally, the sense organs are located in the head, the part of the body that explores the world before the rest of the body. In the human case, for example, if we exclude touch, which is a sense distributed throughout the skin, all other senses (sight, smell, taste, hearing, balance) are concentrated in the head. This means that there is a limitation of channels through which animals can engage with the world, as any interaction goes through the senses.


Plants, on the other hand, are autotrophic. This means that they produce their own food using the energy of the Sun and small molecules that reach them passively (sometimes not so passively, as we will see below). Thus, for plants, and unlike animals, it is more advantageous to have as much contact surface as possible with the environment. In this way, they can receive a greater amount of sunlight, carbon dioxide molecules, nutrients and water from the soil than if they had a more compact structure.


Furthermore, since they are rooted in the same place, that is, they are sessile, they cannot afford to centralise functions as animals did with their internal organs. After all, if a fictional plant had a heart to pump the sap and a herbivore ate the exact part of the body where its heart is, it would die immediately without being able to leave the place. An animal, if it feels threatened, flees, taking its organs with it.


(A clarification: I speak of animals carrying their organs around purely as a figure of speech to facilitate reasoning. After all, it is obvious that an animal does not carry its organs around as if they were a trinket. An animal is its organs, and therefore any decision to go anywhere involves its whole anatomy.)


The challenges the sessile way of life brings to plants have led to an important adaptation: modularity. That is, plants are composed of modules, semi-autonomous, repeated and redundant parts. Plants only have three organs that we would say essential: roots, stems, and leaves. And yet, sometimes they manage to survive even without some of these organs—anyone who has ever made a succulent seedling from a single leaf knows this very well. Modularity means that, in the body of a plant, there are more connections and interactions between the elements that make up the modules than between the modules among themselves, as you can see schematised in the figure below.


Likewise, the sense organs of plants are not centralised either: almost all leaves or roots are capable of perceiving the same stimuli, such as light, gravity, odours, sounds, humidity, etc. Putting this together with the modularity of the plants, we conclude that the modules will have more connections with the surrounding environment than with other modules. For a leaf at the top of a tree, interactions with the dry air and blazing sun are more intense than with another leaf at the base of the canopy being eaten by a caterpillar. Each leaf has its problems and its way of dealing with them, often without involving interaction with other modules. When, in rare moments, there is a more intense interaction between them, I dare to call this phenomenon as attention in plants, as discussed in this post.



From this reasoning, we can conclude that the life of a plant happens on the surface, in the greatest possible interaction with the environment. And what happens outside the plant is so effective in influencing its physiology almost immediately that it may be even more important than what happens inside the plant itself in distant modules. The life of the plant is the leaves, the bark, the tips of the roots.


And notice that this is so true that most of the mass of a woody plant is dead material that it will use as a support to grow on as if it were a coral that deposits rock under itself to get up towards the surface. When I remember the centuries-old sweetwood trees and the immense fig trees of my Florianópolis or contemplate the imposing oaks of England, where I am currently, I like to make a mental effort to remember that behind the thin bark of these immense trees is simply dead wood. What we value to build houses and furniture is, for plants, just a support created by this formidable biofilm that is the living part of trees to be able to have more contact with sunlight, air and water. The life of plants is on the surface.



The four possible channels for plant extended cognition


If plant life is so intrinsic to the surface, it is possible that this was a step towards the extension of their cognitions, especially because plants are very effective in modifying the environment where they live. No wonder, their shade favours the life of many organisms, their roots redistribute water in the soil, making it more humid, the aerosols they release through the leaves, together with water vapour, form clouds that will rain in distant places. In addition, plants actively modify their environment through the release of chemicals from roots, interaction with microorganisms, and the release of volatile chemicals from leaves. These latest environmental modifications can cause changes in the way they perceive and interact with their surroundings, serving as a means to extend their cognition.


Volatile organic compounds: plant cognition in the 'cloud'


There is no longer any doubt that plants communicate with each other and with animals through volatile organic compounds (VOCs). This phenomenon has been studied since the 1980s and much is already known about the compounds that plants release into the air and the effects they have on other plants. One of the most famous VOCs is ethylene, a gaseous hormone that induces the ripening of certain fruits, among other functions. Methyl jasmonate is also known to be released by plants when they are attacked by herbivores, which warns other plants of the attack and induces them to prepare their defences. There is also methyl salicylate, which is more related to fungal and bacterial infections.


However, plants never release one or two VOCs into the air, but always a veritable blend of several different molecules in which messages are encoded and other plants can understand. These are usually messages associated with herbivore attacks or disease—which is fair, considering that these are perhaps some of the most serious problems plants must face in their lifetimes. Thus, a plant under attack can warn others, which prepare their defences by increasing the production of substances that make the leaves more bitter and stimulating the production of extrafloral nectar, a nectar that attracts ants and other insects that can attack herbivores. It has already been proven that communication between plants makes them more resistant to herbivores and diseases.


Due to the obvious applications that plant-to-plant communication can have for agriculture, there has been a lot of emphasis on studying this phenomenon. However, this ended up ignoring an important fact: communication between plants probably arose not to talk to other individuals, but as an internal communication system of the plant itself.


Let's consider two branches that are physically close to each other: if a grasshopper started to eat the leaves of one of the branches and the plant does not produce VOCs, the only way for the other branch to know about this attack and prepare for possible predation would be if the branch under attack would send electrical or chemical signals along its entire length to the stem. The signals would then go up to the base of the other branch and from there to the tip. As you can imagine, not only is there a lot of room for noise and loss of information to disrupt the exchange of signals, but this process could take hours. The grasshopper would be much quicker to jump from one branch to another and eat more leaves off guard.


The evolutionary solution to this was VOCs. By releasing these VOCs into the atmosphere, the plant creates an invisible bridge between branches through which information can flow much more efficiently, thus ensuring the integration of the entire canopy and the correct functioning of the plant as an individual. But these networks for information transmission, so critical for survival, take place outside the plant's body in an atmosphere that has been modified by the plant itself in response to an attack by herbivores. These networks, in turn, modify the behaviour of the plant itself. Therefore, we can say that the cognitive process of plants extends to VOCs, as they have essentially the same functions as internal communication networks, such as hormones and electrical signals. Plant cognition is also in the air.


Root exudates and communities of microorganisms


Plants release chemicals all the time through their roots that serve a variety of functions, from lubricating the contact between root tips and the soil to solubilising nutrients to make them easier to absorb. These substances, known as exudates, can also be toxic to other plants, as a form of competition for underground space. In this case, they are called allelopathic exudates.


However, some allelopathic exudates can be toxic to the plant that produced them, which may sound counterintuitive. When investigating the role of this sensitivity of plants to their exudates, researchers in the United States and Israel discovered a surprising function for this autotoxicity: that of helping to detect obstacles at a distance, as if it were a 'chemical sonar.' For example, Israeli researchers in Ariel Novoplansky's group grew pea plants with nylon string around the roots. When the roots grew towards the threads, the exudates accumulated between the tip of the root and the strings until reaching a toxic concentration, which made the root stop growing and even wither in the vicinity of the obstacles. As a result, the plant ended up growing its roots away from the strings, organising its roots more efficiently. The accumulation of exudates and their interaction with the plant that produced them allows the solution of a problem, but the solution to the problem does not come from within the plant but from outside it. The cognitive process of perceiving obstacles and reorganising roots takes place partly outside the plant through, again, an environment which the plant has modified and which, in turn, modifies the plant itself.


More recently, a group at the University of Leeds in England published a paper showing that wheat possibly releases chemicals into the soil that signal how much room it has to grow. Based on the accumulation of these hypothetical and still unknown substances, wheat matches how much it can grow with the volume of soil available in the pot. Thus, it ensures that it will never grow more than the soil allows, making 'considered' use of the resources at its disposal. Again, we have an interaction between the environment modified by the plant to modify the behaviour of the plant itself to solve problems. The ability to solve problems is one of the most accepted definitions of intelligence. This is why we argue that root exudates are the second extension channel of plant cognition.


Still on the subject of exudates, another channel for the extension of cognition can be through modification of the communities of microorganisms that live in the soil. Depending on the chemical substances the plants exude through the roots, such as more or less complex organic acids, they can favour or inhibit the development of certain groups of microorganisms. These organisms, mainly bacteria, also modify the soil and release chemical substances that will alter the physiology and behaviour of plants—and plants use this to their advantage.


'Soil-borne legacy', or 'soil-borne memory', is a phenomenon increasingly studied that occurs when plants suffer some stress, usually a disease. In this case, the plant modifies the chemical composition of its exudates, which will modify the community of bacteria that lives near the roots. These bacteria will end up encoding a memory of that disease in the soil; outside the plant. If one day the plant gets sick again from the same pathogen, the 'immunological memory' that was stored in the soil bacteria will help it to suffer much less than the first time. This was proven when applying 'synthetic exudates' to the soil. It was observed that the plant became more resistant to diseases that it had never faced as if it had had an 'implanted' immunological memory. Furthermore, if naïve plants, which have never been sick before, are planted in soils conditioned by plants that had been sick, they become resistant to the same disease as well, which demonstrates that immunological memory in the soil potentially can be shared with other plants of the same species. And memory, as we know, is the basis of learning, a cognitive phenomenon. The figure below clearly shows a phenomenon of learning through memory in the soil.



Studies with soil-borne memory are showing that soil health is extremely important for the correct development of plants. The richer the soil is in microorganisms, the more material the plant has to work with and create these memories. As a result, the plant will be more resistant to pests and diseases. This is an alert for the way we do conventional agriculture, through monocultures that require the application of fertilisers that kill a large part of the soil microbiota. Without the support of these microorganisms, it is expected that crops will become more vulnerable to diseases, which will require the application of pesticides that will make the soil even sicker, in a vicious cycle of environmental degradation. Therefore, it is important to seek agroecological solutions that guarantee soil health and diversity, among other reasons, to give plants the tools they need to survive with less human intervention.


The networks of mycorrhizal fungi


The fourth channel of extension of plant cognition proposed so far is the celebrated network of mycorrhizal fungi that connect plants underground. These fungi are much smaller than roots, grow much faster and are more efficient at finding nutrients in the soil. Plants, in turn, are the only ones that can produce what every living being wants: glucose, sugar. Thus, plants and certain fungi began a partnership in which the fungi seek the nutrients and water the plant needs from the soil and exchange for the sugars produced by photosynthesis. They do this from a very intimate interface called mycorrhiza, where fungi grow into the roots and make connections with plant cells.


Mycorrhizas are so advantageous for both partners that almost all plants today make this symbiosis. A symbiosis that lasts for more than 400 million years, and likely is what made it possible for plants to leave the seas and colonise the surface of the Earth. In addition to finding nutrients, mycorrhizal fungi also help plants to resist droughts, activate their immune system making them more resistant to diseases, isolate the root tip from contact with harmful microorganisms, as is the case of some mycorrhizas, and even protect plants against excess nutrients in the soil.


This symbiosis is so intense that some plants have begun to delegate much of their search for nutrients to these fungi, like many people order food through apps instead of going to the supermarket or restaurant. Some studies have shown that plants with thick roots are more likely to make this sort of deal, possibly because thick roots are much more expensive than fungal hyphae in terms of glucose investment. Some authors comment that these fungi are practically 'extensions' of the roots, but we have every reason to suspect that this word can be employed beyond the metaphor.


Grupo de cogumelos numa floresta
Mycorrhizal fungi like this Amanita muscaria could be extending the cognition of the trees with whom they form symbioses

If fungi find nutrients for plants, make decisions, guide roots, and make problem-solving and exploring the world much easier—if not possible—then these fungi could perhaps be considered part of the cognitive apparatus of plants. The fungus' behaviour is regulated by the plant through the supply of glucose and also the 'permission' to be associated with the plant. This is so because the plant needs to lower its immunity locally to allow the entry of the fungus. But this behaviour is reversible if the plant needs to decrease the number of mycorrhizas. However, the behaviour of the plant is also altered by the fungus. For example, there is evidence that it can provide more or less resources, or even mobilise nutrients from one side of the mycorrhizal network where they are more abundant to another with lesser amounts to increase the cost of its 'product'. If there is demand for nutrients, the price of sugar ends up rising, and the fungus exploits it. Although very surprising, it is good to make it clear that there is no evidence that this behaviour is a conscious choice of the fungus. It's more likely a product of the dynamics of this symbiosis.


However, foraging requires cognitive skills, and it's not just the plant or the fungus that has those skills, but the plants and the fungus together. They are both looking for nutrients by regulating each other's growth and behaviour, in such an intimate connection that it is difficult to tell apart plant and fungus, as one ends up being the continuation of the other.


A plant is much more than what we see


Something that the extended cognition hypothesis teaches us is that plants are much more than what meets the eye. Plants are immersed in an environment profoundly altered by them, which feeds back by influencing the plants themselves. Their crowns are not alone in the air but immersed in an invisible cloud of VOCs that take and bring information from everywhere, all the time. Their roots are not just in the earth, but in a matrix created by the plants for themselves and all beings that live with them. This matrix helps them to obtain information about what is around them and how to react to obstacles and solve problems. Rather than looking like the picture above, a plant should be seen like the picture below. Plants are much larger and more dispersed than we imagine. They are the nodes of an intricate network of relationships in the environment that condenses into the shape of a plant, but which does not have a defined border and extends in all directions. And if cognition happens in the body, in the matter, there are no limits for it either.


Esquema de planta como rede em várias cores diferentes
The cognitive structure of a plant is very different if we take into account its extended dimension. Plants are the focal point, nodes of wide and complex relationships between the world's elements. In green, a representation of VOCs. In red, representation of all forms of extension of cognition underground, such as exudates from roots, mycorrhizae and other microorganisms | Parise and Marder (2023)

In this case, another interesting question arises: up until now, we have been talking, in general, about a plant and its environment, but plants are never alone. They grow close to each other, interacting through all these channels and more. If VOCs and mycorrhizal networks connect different plants and allow communication between them, where does one plant's cognitive structure end and another's begin? How fluid is this cognitive process? How would we see a forest, or even a garden, in a case like this? Once we recognise the extended cognition of plants, these and many other questions about their cognitive process and as a general phenomenon will have to be addressed.


This work was published in the special issue Advances in Philosophical and Theoretical Plant Biology of the scientific journal Theoerical and Experimental Plant Physiology, maintained by the Brazilian Society for Plant Physiology, which proposed to reflect, philosophically and ecophysiologically, on the nature of what we call plant and that we so easily take for granted. Our article can be downloaded for free at this link, and it is worth checking out the other articles published in the same issue.


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