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.