Do plants pay attention?
- André Geremia Parise
- Jan 22, 2023
- 10 min read
Aapaper has just come out that I hope will create some buzz in the scientific community—at least, the part of it that deals with plant cognition. The article, published in Progress in Biophysics & Molecular Biology, is a collective effort with the contribution of researchers from the most diverse fields (biology, physics, philosophy and psychology) to develop the hypothesis that plants are capable of paying attention. That's right: attention, literally!
But let's take it easy to understand better. It has been known for a long time that plants are intelligent and cognitvive. We know that they perceive and are aware of the environment around them, that they make decisions about what to do in different situations, that they can form memories and learn (even complex forms of learning have been observed, such as associative learnig!), that they communicate with each other and with other organisms and even that they act by anticipation, based on what they expect will happen in the future. A growing number of studies have revealed and consolidated this new understanding of plants as much more complex beings than previously thought.
Well, among all these studies on plant cognition, there is an important piece missing: the study of the phenomenon of attention in plants. Attention is a particularly important cognitive state because, as organisms have limited cognitive capacities, and since there are many stimuli received from the sense organs, it is necessary to prioritize what to perceive and what to respond to. In the same way, when we do something like driving or studying, we need to focus on what we are doing; we cannot be distracted by other stimuli that are unrelated to the task, under penalty of suffering an accident or not passing an exam—attention, therefore, is essential for the very survival of an organism.
Plants have an enormous amount of senses, according to some authors even more than animals. They can perceive light in its smallest variations of intensity and spectral quality, they feel chemical substances in the air and in the ground, they perceive touch, gravity, in a way they are able to hear—that is, perceiving and reacting to different sounds—, they can perceive gradients of moisture in the soil, are sensitive to magnetic fields, and so on. All this at the same time and in two completely different environments: the dark, cool and compact subterranean world, where the roots are, and the unstable, luminous and unpredictable aerial world, where the stem and leaves are. As you can see, plants receive and need to monitor an extraordinary amount of stimuli at the same time.
Imagine yourself as a plant: you cannot move and, at the same time, you feel that there are insects chewing your leaves, the wind blowing, the water ending up in the soil, the intensity of the sun changing every minute (especially when a cloud passes by), worms rubbing against the roots, the sound of cicadas in your branches, other plants outgrowing you that could be a threat in the future, the enticing "taste" of a nutrient patch a few feet away, but where you need to direct the growth roots, fungi and bacteria communicating with your roots and trying to form a symbiosis, and chemicals in the air that carry messages from other plants. It's a lot going on at once, isn't it? And remember: you don't have a brain, or even neurons, to help process all this information.
In order to deal with all these stimuli, part of the problem is solved with the structure of plant bodies, which are modular. This means that they are made up of several repeating, semi-autonomous parts, each capable of dealing reasonably independently with the stimuli it perceives. The problems are then divided, and the cognition of the plants is distributed through their bodies. Each party is responsible for one thing. So a root doesn't have to worry about the adjustments in photosynthesis that a leaf has to make every time a cloud passes in front of the sun, nor does a leaf have to decide whether the root will grow toward a wetter zone of soil or another one that appears to be rich in a particular nutrient. In addition, there could be a very important phenomenon, hitherto largely ignored in the literature: since the cognitive capacities of plants are limited, it is necessary to prioritise the stimuli to which the plant responds. In other words, you need to focus on some stimuli that are most important at the moment and ignore others. In other words still, one has to choose what one pays attention to.
Attention, then, can be understood as the phenomenon of prioritizing, in a flexible and dynamic way, which stimuli we perceive and which we ignore over time, and to which we disproportionately invest energy. This is something that is constantly changing and involves an important balance between what is happening inside the body, on a physiological level, and outside the body. For example, if we are focused on a task, we may not notice that someone has left a bag of biscuits next to us. However, if we are hungry, this otherwise irrelevant stimulus may reach our conscious awareness.
There are many varieties of attention and most of them have been identified and studied in humans, of course. Never, however, in plants. As far as I am aware, the first person to address the topic of attention in plants was the philosopher from the University of the Basque Country Michael Marder, who in 2013 published an article in which he proposed that plants would be capable of having attention. However, since that year, little has been discussed on the subject until 2021, when my colleagues and I published the article resulting from my master's thesis. In it, when studying the electrical signals of a parasitic plant when it perceives prey close to itself, we found what seems to us to be empirical evidence of an attention process in plants. I explain the findings of this research in this post here.
The analysis of electrical signals from plants using techniques analogous to the electroencephalogram of brains to empirically study the phenomenon of attention in them seemed to be a promising idea, in addition to being an opportunity to combine theory and practice "in real time" to study an important aspect of plant biology. However, to study plant attention scientifically, a hypothesis is needed that justifies this idea. And new hypotheses are often born from the meeting of different minds and experiences.
This is how, combining the philosophical contributions of Michael Marder, the behavioral ecology knowledge of Monica Gagliano (Southern Cross University, Australia), the experience in experimental psychology of Umberto Castiello (University of Padua, Italy), the understanding of physics and electrophysiological analyses of Thiago F. C. Oliveira, and the knowledge of plant physiology and electrophysiology of Gustavo Maia Souza, Gabriel R. A. de Toledo and mine (all the latter from the Federal University of Pelotas, Brazil); and over many email and virtual conversations, given that we are still in times of a pandemic, the article Do plants pay attention? A possible phenomenological-empirical approach. In this article, we seek to develop a theoretical-practical hypothesis to explain what it would be, how it would work, and how one could study plant attention, which I will endeavour to explain below.
The (possible) attention of plants
Let's take two premises into account: first, attention is, very simply, the process of favoring certain stimuli and actions over others in order to be able to deal with the processing of these stimuli and the use of the information generated from there. In Marder's words, it is "a disproportionate investment of physical or mental energy by an organism, tissue, or cell into a particular activity or into the reception of a singled-out stimulus or set of stimuli." This is an interesting definition because it is not cerebrocentric: a brain is not necessary for attention to exist, just an organism, tissue or cell that meets the aforementioned requirements, which opens up the possibility of not only plants, but any organism being attentive, such as fungi and even bacteria.
And when we observe the behaviour of plants, we understand that many of the activities they do would require attention. For example, studies with climbing plants show that they actively look for a support to cling to, which requires identifying a goal and staying focused on that goal until you reach the desired support. Any goal-directed behaviour, such as growing towards a light source, supposedly requires attention. Think of an indoor plant that always grows towards the window. It maintains this growth without being "distracted" by the alternations between day and night, or the lights that are turned on in the house, especially when it is dark outside. Roots might also pay attention, as they grow towards a source of nutrients following that stimulus without being diverted by others, such as touch and sound. Another example is the carnivorous Venus flytrap, whose trap lobes contain six bristles that must be touched by an insect for it to close the trap. This plant counts the bumber of times the bristles have been touched to know when to trigger closure. When a bristle is touched once, the plant needs to pay attention to the next stimulus, which requires keeping the memory of the previous touch, in order to know whether or not to close the trap.
These examples allow us to assume that the plants are paying attention to what they do and feel, but how can we prove this empirically? How to know what is happening on a physiological level?
This is where electrical plant signalling comes in. Like animals, plants are able to generate many electrical signals in their cells that travel through their bodies carrying and bringing information. The set of all this electrical activity is called plant electrome, and the electrome can be studied using a technique analogous to the electroencephalogram, called electrophytogram (from the Greek: phytón, plant). I won't go into technical details, but by placing electrodes on plants, one can register how the electrical field between the electrodes varies over time, generating time series. Later on, time series are analysed in many ways, from mathematical analysis to artificial intelligence classifications. And by studying the complexity of these series and how the signals correlate, we can get some idea of what's going on inside the plant.
Despite the technical convenience, a potential difficulty in studying attention in plants is precisely their already mentioned modularity. If every part of a plant; leaves, branches and roots, is semiautonomous, how is an entire plant going to pay attention? What do we mean when we say a plant is paying attention? Well, precisely because of this, the phenomenon of attention in plants might be considerably rare. However, there are times when the entire plant needs to coordinate its behaviour. For example, when the roots detect that there is no more water in the soil, they send electrical signals to the leaves so that they close the stomata (small adjustable pores that allow the exchange of gases and the release of vapour), in order to avoid water loss. Afterwards, they begin to synthesise a hormone called ABA (not to be confused with the Swedish pop band), which encourages the leaves to keep their stomata closed. The leaves themselves begin to produce ABA on their own, and the whole plant then begins to accumulate sugars and produce proteins that increase resistance to drought. All of this requires collective, coordinated effort of all parts of the plant to solve a common problem.
If we use Anthony Trewavas' analogy that plants function as a democratic confederation, let's imagine that a plant in normal state is a country in normal times, where each inhabitant lives their own life, communicating with others when necessary. These citizens know that they are part of the same country: they have documents that prove it, they share a common culture and language, but this is a somewhat dispersed, weak belonging. Now let's imagine that the World Cup has arrived: in this case, everyone changes their behavior and the feeling of belonging to the country is exacerbated. When the national team is playing, everyone gathers in front of the televisions and, with perfect synchrony from north to south, they suffer every penalty and celebrate every goal. The whole country stops to pay attention to the game.
This is more or less how plant attention works. And since electrical activity is apparently involved in all plant behaviour, it is to be assumed that a plant paying attention would have the complexity of its signals reduced, becoming more regular, synchronised and predictable—just like the activity of the inhabitants of a country becomes more predictable during the Cup final: it's easy to assume that almost everyone will be in front of a television at the same time, that everyone will shout when the ball hits the opposing team's net, and everyone will jump, cheer and cry together when the referee whistles the end of the match announcing the victory of the team.
In the article, we reviewed the literature looking for evidence that plant activity would work like this when they are in situations that supposedly require attention. What the available information indicates is that, apparently, this hypothesis makes sense: in almost all the cases analysed, the electrical signalling of the different species studied behaved as expected, which paves the way for empirical research on plant attention. Therefore, we have a theoretical and practical starting point to investigate this possible phenomenon, which can fill an important gap in the study of plant cognition and intelligence.
Attention and consciousness
The article also adds fuel to an already quite hot discussion, which is the debate on whether plants could be conscious. The debate has gained momentum in recent years with some authors clearly demarcating their positions, for or against, and it is logical that it is still far from reaching any conclusion.
However, it is interesting to note that within the phenomenological tradition of philosophy, attention and consciousness are so related that they could be considered synonymous: attention is consciousness. This makes sense if we think that we are only aware of what we pay attention to. If at a party we are so absorbed in conversation with a beautiful person that we are interested, we are not aware of the horrible music that may be playing. Our attention and awareness is focused on the person who attracts us. When we sleep, we are not paying attention to anything—in fact, we are unconscious. So if there is such a thing as plant consciousness, studying its attention, both empirically and philosophically, can help shed light on this debate.
The interesting thing is that, being plants modular, they would rarely be in a full state of attention. The most likely is that each module, each part of the plant is dealing with local stimuli and actions, which are not perceived by the entire plant. This makes the plant as a whole closer to what we could call a state of distraction, with its awareness supposedly scattered and unfocused. However, when the plant as a whole needs to coordinate to achieve a goal or solve a problem, all the relevant modules synchronise and the plant "focuses" on its activity or problem, presumably becoming aware of what attracted its attention. And who knows, maybe becoming aware of itself as an organism—more or less like people who suddenly feel that they are part of the same nation during the World Cup.
These are, of course, uncommitted digressions. Nevertheless, perhaps the formulation of a theory of plant consciousness involves studying attention in plants, which—it is important to emphasise—is just a hypothesis. The idea of the article published in Progress in Biophysics & Molecular Biology is to demonstrate that the phenomenon of attention in plants is possible and that it can be tested empirically; is to pave the way for this line of research. Going forward, one should test the hypothesis with robust experiments and discuss them with due philosophical depth and appropriate levels of skepticism. In the future, we hope to have enough data to support the hypothesis that plants pay attention and, who knows, might be conscious. The work is just beginning.
This post was originally published in Portuguese on the 4th of July, 2022
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