What a Plant Learns. A Curious Case of Mimosa pudica

by

Monica Gagliano and Michael Marder

 

Wandering through the Royal Botanic Gardens in Kew, renowned for housing one of the most diverse collection of living plants on the planet, you can be sure to fall under the spell of its notorious residents. There are the mighty oaks, thirteen-storey-tall redwoods, the sacred and endangered Pōhutukawa tree from Aotearoa, the striking spectacle of the giant Amazon waterlily. If lucky, you may even spot a flowering Titan arum with its unbearable corpse-like stench.

Among a treasure of distinct forms, colours and smells, you will also start discerning the relatively smaller, closer-to-the-ground dwellers, those whose feats are not displayed by virtue of their sheer size, but their actions. Action? A strange thought, perhaps, when talking of plants, given our misperception of them as stock-still and oblivious. However, it is precisely because her delicate fern-like leaves will suddenly curl up and droop to shy away from our touch, that many enjoy this intriguing plant and her weird performance.

Best known as touch-me-not or the sensitive plant, we are, of course, speaking of Mimosa pudica. Quietly, the highly touch-sensitive Mimosa recomposes herself by uncurling her leaves and pulling her stems upright again within a few minutes after the ordeal. This plant’s behaviour, especially her distinctive ability to ‘play dead’ in reaction to external disturbance such as touch, has captured human attention since ancient times and ignited the flames of imagination for centuries.

Now, the specimens of Mimosa residing in Kew Gardens no longer curl up to the nudging fingers of countless human visitors. As a colleague has recently pointed out, so many visitors of the Gardens have been touching these plants to see them perform their trick, that the plants cease to respond. Could it be that the Mimosa plants have learned that being touched repeatedly is a disturbance, yes, but one with no life-threatening consequences and therefore requiring no reaction?

The question underscores a phenomenon known as ‘habituation’, which is considered the simplest form of learning, one that scientists have observed pretty much wherever they have looked. And there is no reason to exclude plants from the effects of habituation merely on the basis of entrenched prejudices. Experimental evidence, combined with a sound theoretical framework that accounts for their behaviour, is required for us to make that call.

Like yawning, shivering, eye-blinking and knee-jerking in humans, the leaf-closing behaviour of Mimosa is an excellent example of an automatic response or reflex. Like all reflexes, Mimosa’s leaf folding trick is an evolutionary survival mechanism developed by members of the species through innumerable generations in the process of natural selection. It is part of the acquired habitus of the species, which has become deeply ingrained over its evolutionary history because it helped the specimens survive. How so?

Mimosa’s leaf folding allows the plant to respond quickly to perceived trouble, in order to protect her from harm. However, it does not come for free. When the plant folds her leaves shut, her capacity to forage for light suddenly plunges by half, meaning that the plant could face the risk of starvation. This risk may be a justifiable price to pay if the danger is real. But it is clearly a waste of precious opportunities to forage for light and thrive, when a perceived dangerous situation turns out to be not dangerous at all.

Mimosa is faced with a persistent emergency situation, urging her to keep evaluating the trade-off between the energetic gain of foraging and the risk of being eaten, constantly choosing between life and death. Wouldn’t it then be surprising to find that Mimosa has little or no control over her own fate? That the plant is incapable of assessing what the circumstances demand and what they offer? That she would be unable to learn from experience, unable to learn to ignore the harmless nuisance of, for example, being touched by yet another human finger visiting Kew Gardens, so as to spare herself the unnecessary trouble (and energy loss) of closing her leaves?

It would be surprising, indeed! Actually, this is not what these plants do: they learn. What we observe inside the controlled settings of a scientific laboratory[1], as well as outdoors in places like Kew Gardens confirms that plants can learn, remember, evaluate the choices they are faced with, and make decisions. So, the true surprise is our insistence on thinking that plants can respond only in pre-programmed and automatic ways already encoded to their DNA or that, devoid of agency, they are somehow being acted upon rather than acting in their own right[2].

Refusing to conforming to our expectations of how they should behave (or whether they are capable of behaving at all), plants are decomposing our obsolete ideas of what it means to be plant and more generally, living. Learning is a survival strategy, without which life would not have perdured. Because the circumstances of their existence and the environment are constantly shifting, it is important for organisms to act in new and creative ways, so as to rise to the occasion of unexpected challenges.

Implicit or explicit, learning is a tool permitting them to do just that: to rely, with the help of memory, on a storehouse of accumulated past experiences in order to change their behavioural patterns into the future. In the age of climate change, the organismic learning capacity is more crucial than ever, seeing that environmental conditions undergo swifter alterations than before, posing a greater number of serious threats to life. Plants, in their turn, have collectively gone through many more climate catastrophes than Homo sapiens sapiens, which means that they are, perhaps, even better learners than we are. Obviously, such a comparison of a biological kingdom to a single species may look unbalanced, but it is wholly justified as a response to rampant anthropocentrism, which elevates our own species above all other forms of life on earth.

We might say that the living learn to go on living, and that their lives are pieced together from experiences that present myriads of learning opportunities. Plants have an added urgency to learn and to engage in flexible behaviours on that basis, because they are sessile organisms, unable leave their habitats and to flee from dangerous situations. With regard to catastrophic climate change, we learn that we, too, are sessile, bound to Earth, unless we are tempted by fantasies of permanent human colonies on other planets, the fantasies of humanity becoming an interplanetary species. This realization means that we are on the verge of developing a vegetal consciousness (of being a global species within the terrestrial fold) as a consequence of learning—including from plants.

But let us go back from the planetary, if not cosmic scale, to Kew Gardens and their diminutive inhabitants. When she learns not to close her leaves in response to thousands upon thousands of caressing fingers, Mimosa pudica teaches us a vital lesson about the nature of learning and intelligence. She shows that we share the faculties and processes once considered exclusively human not only with other animals but also with plants. No matter her small size, she puts us in our place: shoulder-to-shoulder (or shoulder-to-branch) with all those living.

 

Notes:

[1] Gagliano M, Renton M, Depczynski M & S Mancuso (2014) Experience teaches plants to learn faster and forget slower in environments where it matters. Oecologia 175:63-72. (DOI 10.1007/s00442-013-2873-7)

[2] Calvo P, Gagliano M, Souza GM & A Trewavas (in press) Plants are intelligent: here i show. Annals of Botany; Michael Marder (2013) Plant-Thinking: A Philosophy of Vegetal Life, NY: Columbia University Press; Michael Marder (2012) Plant Intentionality and the Phenomenological Framework of Plant Intelligence, Plant Signaling and Behavior 7: 1365-1372.

 

A version of this article has been published in Botany One.

 

Confessions of a Plant Philosopher

by

Michael Marder

 

More and more, I hear—or rather see in print—references to myself as a “plant philosopher.” In 2014, the American BOMB Magazine characterized me as “arguably the closest contemporary figure we have to a canonical plant philosopher.” The moniker has resurfaced many time since, most recently in Prudence Gibson’s book The Plant Contract and in the playbill of Manuela Infante’s theatre production Estado Vegetal, “The Vegetative State.”

This is not a term I invented. It has been, like a name one receives before one is able to form memories, given to me by others. Obviously, the fields of my philosophical investigations are not limited to plants; I write on politics, energy, ecological devastation, dust… But it is also true that the others know us better than we know ourselves, which means that there is a deeper truth in their assessment. So, I accept the designation, even if, like you, I am curious to learn precisely what it means.

Let’s start with the words themselves. “Plant philosopher” may refer to the one who philosophizes about plants. Or, it may be a shorthand for “a philosopher of plants,” an expression that needs further unpacking. On the one hand, its sense overlaps with philosophizing about plants, adding nothing new to the preceding interpretation. On the other hand, it becomes really strange, assuming that “of plants” implies belonging to them, being claimed by and for them. Heard in this sense, the expression vegetalizes me. To my mind, this ambiguity is as irresolvable as it is significant. In short, it integrates theory with practice, thinking with doing, knowledge production with ethical concerns.  

 The project of plant-thinking, limited as its scope seems, has gradually drawn the philosopher (yours truly), philosophy as such, and the figure of the human into its orbit. Which is why, regardless of what I (or anyone, for that matter) think, we are all plant philosophers. We owe our thinking (all of it without remainder!) to plants—both at its source, at the inception of thought, and at its points of destination. I will explain myself on this in a moment, but, first, you might be musing: “What is his backstory? How did his affair with plants happen?”

 My relation to plants has never been easy. Since the age of three, I have been suffering from debilitating bouts of pollen allergy, so severe that the doctors in Moscow, where I was born, recommended that I spend much of the spring in another climate. Many years later, I would hypothesize that, the ideal of immutable being is a symptom of philosophy’s conceptual allergy to plants. After all, trees, flowers, bushes, grass are the exact opposite of this ideal: they are living beings in perpetual metamorphosis, growing, changing, shedding leaves or petals, adding new organs. While I could not do much about my physiological allergic reaction, I was determined to tackle the conceptual allergy, which I perceived to be incomparably more harmful to humans and plants alike.

Despite these early experiences, I have always been fascinated with plants. My nickname in elementary and middle school was botanik, “botanist.” For a few years running, I participated in city-wide and regional “Olympiads” on the subject. Living at the outskirts of the city, my geographical location was at the edge of an extensive forest “Elk Island,” which was granted the status of Russia’s first national park in 1983, the year of my initial allergy outbreak. The forest was an integral part of my existence, a place where I took endless walks, gathered berries in the summer, skied in the winter, imbibed the smells and sights of the fallen and already half-rotten leaves in the autumn.  

No wonder that the idea of plant-thinking came to me in another forest (which is, by the way, also a national park)—Paneda-Gerês, spanning the border between Northern Portugal and Spain. Though very different from the woods of my childhood, this enchanted and enchanting place with moss-covered trees and wild horses stimulated my body and mind, lungs, eyes, and cognition like few others before. I spent my days in the forest and, at nightfall, I huddled by the fireplace of the lodge where I was staying with a book on Aristotle by Italian philosopher Claudia Baracchi. It was then and there that I became aware of how plant life was a lacuna in contemporary thought, not to mention in ethical approaches to the world.

 For Aristotle, the soul (by which he means something like the principle of vitality that animates beings, putting them in motion) has three possible forms. A uniquely human principle of vitality is rationality, the capacity to reason; the animal soul revolves around sentience, the senses, feelings, emotions. And there is also the vegetal principle of vitality responsible for nourishment and reproduction.

Over the past fifty years or so, “the human” has been subject to unpicking through critiques of reason as a fake universality, one that spins absolute standards out of particular male, propertied, European experiences. In the last twenty-five years, traditional constructs of the animal as deficient in comparison to the human have been also questioned. At the heart of these constructs is a trick, a sleight-of-hand condensed in the assertion (whether or not implicit) that because animals neither think nor speak as we do, they neither think nor speak, period. Be this as it may, radical changes in the human self-conception and in the notion of the animal intervene in, unsettle, and reframe the upper crusts of the Aristotelian soul, or at least of the shape in which it has reached us.

It dawned on me one fine evening in Gerês that, for all their daring, current interrogations of the human and the animal have left the foundations of the entire philosophical edifice relatively untouched. They have not engaged with plants. Already Aristotle deemed vegetal vitality (or organismic capacity to obtain nourishment and reproduce) to be a shared faculty of the soul, presupposed in the animal and human modes of existence. But there is much more to this faculty than Aristotle is willing to admit. Unless one debunks the myths that plants cannot think and are not sentient, the rest of the exercise in what is known, somewhat pompously, as “the deconstruction of Western metaphysics” is going to be superficial.

 By taking an apparent detour, we have stumbled upon an answer to the question why the project of plant-thinking has drawn this philosopher, philosophy as a whole, and the human into its orbit. If vegetal vitality is the most basic, shared, common life, pulsing through plants, animals, and humans, then it replaces a particularly human brand of reason as the true home of universality. Further, if other modes of vitality, including sentience and thinking, are not external superimpositions but the often-times unrecognizable outgrowths of vegetality, then to really understand them we need to go back to their vegetal roots. It is for this reason that I claimed in Plant-Thinking that even in our highest endeavors we remain sublimated plants.

In concrete terms, is it at all possible to obtain nourishment, reproduce, and finally live without thinking? Would any organism invest large amounts of energy into movement or growth in a haphazard manner, regardless of the whereabouts of what it is seeking? Evidently, such a scenario would be absurd. In fact, plant scientists have shown that roots grow intelligently toward patches of soil that have the most mineral and water resources. Along the way, they utilize biochemical substances to communicate with the roots of other plants. In the so-called transition zones that surround them, they exchange information with fungi and bacteria, with which they live in a symbiotic relation. They can adjust the direction of their growth, navigating underground labyrinths, filled with obstacles and rewards, no less proficiently than rodents placed in above-ground labyrinthine structures. 

This is just a small piece of the puzzle in the rapidly expanding field of plant intelligence studies. I have great respect for scientists (including, among others, Monica Gagliano, Stefano Mancuso, Frantisek Baluska, Richard Karban) who are spearheading this field. They are still working in a state of siege, under attack by their more traditionally-minded colleagues who deny that plants are intelligent and who prefer to see in them green, photosynthesizing machines. But I do not think that intelligence is the whole story here. For me, intelligence and thinking are not synonymous, not interchangeable. Plant-thinking is emphatically not plant-intelligence. So, what’s the difference between them?

Intelligence is basically instrumental. It is a sophisticated tool—using calculations, algorithmic functions, and so on—for getting what one needs or wants in an efficient way. Although different kinds of beings have different types of intelligence at their disposal, the form of intelligent conduct is suspiciously homogeneous: it boils down to utility-maximizing subjects acting to avoid pain (or environmental stress) and to procure pleasure (or ensure optimal conditions for flourishing).

Thinking will have none of that. I see it more as a fragile balancing act happening in the interfaces of the organism and its environment, an act looking, Janus-faced, both ways at the same time. Thinking is not utility-maximizing; it is, precisely, the opposite—leaving one’s comfort zone, one’s enclosure in oneself. It catapults the thinker into the world, for nothing in particular, for no apparent purpose or end. Thinking is existence. (You will have recognized that I am inverting Descartes’ dictum into “I am, therefore, I think.”) Arguably, plants think with unrivalled intensity, because their mode of existence constantly takes them outside of themselves into leafing, blossoming, extending branches and roots. Not only do they have transition zones around some of their organs, but they also are transition zones between themselves and their environments. That is the place of thinking, the site where it happens.

As you can imagine, these and related arguments have landed me in hot water over the years. To give you an example, exactly seven years ago I penned an op-ed titled “If Peas Can Talk, Should We Eat Them?” for “The Stone” philosophy section of The New York Times. I did not expect at that time that it would stir as much controversy (peppered with online death threats) as it did in the following weeks. I was attacked by everyone from Christian fundamentalists to vegans and from neuroscientists to humanist rationalists. I see in the scandal that ensued a symptom of psychological resistance. For centuries, if not millennia, plenty of psychic energy has been poured into creating and maintaining a certain image of humanity, accorded a place at the pinnacle of existence, with lower positions assigned to non-human lifeforms. These ideational structures cannot collapse overnight (and the spans of ten, twenty-five, or fifty years amount to “overnight” compared to the history of metaphysics). When its walls are breached, defenses are mobilized and everything is done to patch up the damaged section and to repel a perceived menace.

Another possibility is that, in my capacity of a plant philosopher, I have been unconsciously conflated with the plants themselves. And so, I have received a fraction of the negative and nihilistic discharge our culture releases against life and, above all, against vegetal life.

Whimsical as it may sound, to be a plant philosopher is to assume the utmost responsibility, which goes beyond the tasks of representing the interests of plants or speaking for them. Deeply engrained in the fabric of existence, of my thinking-breathing body, this ethical function cannot be limited to that of a press secretary or even a legal representative. An ethical stance, which is quite different from a moral position, motivates the unfolding of one’s life and thought in excess of a coherently formulated commitment, a decision one makes. At that level, none of us really knows her- or himself, just as we have no memories of ourselves roughly before the age of three. Others know us better than we, ourselves, do. It is only fitting, then, that they would name us, that they would name me, for example, “plant philosopher.”