Mimosa pudica. Photograph by M. Martin Vicente.
by Monica Gagliano and Michael Marder
MARCH 18, 2014
When we consider the propensity to learn, plants are probably the last living beings that come to mind. They seem to be so passive, immobile, and completely determined by their attachment to the place, in which they are rooted, that any of the so-called “cognitive abilities” appear to be inapplicable to them. And this is not even to mention the holy grail of cognition—the central nervous system with extensive neural networks—that plants do not have.
At the same time, the suspicion that plants are capable of learning is not entirely new. While Sir J. C. Bose (1858-1937) proposed the idea just over a century ago, it has truly entered the arena of scientific enquiry only in recent years. The reason for the delay is obvious: one widespread assumption in the twentieth and even the twenty-first centuries has been that learning is reliant either on neuronal processes or, in the case of machines, on artificial neural networks modeled on their biological counterparts. With this assumption in the background, conducting experiments on or contemplating plant learning, memory, and decision-making have been interpreted as acts of sheer madness. But, in all fairness, although we have pondered human and animal learning since antiquity, we are still asking the basic questions about how learning really works because finding the answers is not an easy task even in these kinds of organisms.
So, how would we ask those same questions in plants? Do plants have motivations and preferences? Do they have expectations and, if so, how can we assess them? Do they make choices and if so, what are the implications for how we regard them? The creative challenge here is to develop suitable experimental and theoretical approaches that enable plants to show us what and how they learn, while avoiding the temptation to turn human or animal abilities into the standard template for these investigations.
If we consider the issue from the ground up, we realize that everything we know about the world, we learn through repetition. Learning from the things that we experience again and again is in fact one of the most effective ways of acquiring new behaviors, or of adjusting and refining old ones, in order to survive and flourish in a range of ecological and social settings. Why would plants be excluded from such a useful evolutionary process? Wouldn’t being barred from learning harm their chances for survival? Wouldn’t it be utterly wasteful, not to say dangerous, for them to register stimuli from their environments each time anew, without the accumulated memory of past experience that would enable them to respond appropriately in the future?
Like humans and many animal species, the sensitive plant Mimosa pudica, for example, also learns through repeated practice. Usually, when subjected to a disturbance for the first time, it instinctively closes its leaves, a mechanism designed to defend it against predators. But, as recent experiments have shown, Mimosa quickly learns that to continue closing its leaves when a repeated disturbance proves to have no negative consequences is a waste of energy. By considering the experience no longer threatening, this plant stops closing its leaves, a behavioral change motivated by the energetic reward that keeping its leaves open brings. After all, the opportunity for ‘feeding on sunlight’ drops considerably every time Mimosa keeps its leaves closed, which is something that can cost it dearly. That is where the learning processes steps in, helping the plant optimize its behavior.
Interestingly, the extent to which Mimosa is willing to keep its leaves open despite the disturbance depends on the environmental context. Experimental data clearly show that this plant does not simply react to the immediate stimuli available from the environment. Instead, it assesses a given situation and preferentially engages in behaviors that pay off from its perspective. The tendency of individual Mimosa plants to take the specific action of keeping the leaves open in response to a known disturbance is greater in limited light environments where the consequences of leaf closure can be dire. And, remarkably, the strength of Mimosa’s motivation to keep the leaves open does not loosen, as one may expect, when light conditions improve. These plants do not ‘relax’ when moved from a light-limited environment to one where light is abundant. They remain highly responsive and in a state of alertness, as if anticipating that the environment is likely to deteriorate again.
At every moment, plants, like humans and other animals, perceive a variety of things simultaneously, but they learn to focus their attention on whatever they need to perceive and exhibit a behavioral tendency to approach or avoid situations. In this process, repetition is the learning platform upon which motivation galvanizes and steers behavior toward specific goals and expected rewards. It is not by chance that, on the basis of plant learning, we comfortably use words such as motivation, tendencies, preferences, expectations and choices, which in everyday speech we often equate to feelings, desires, and conscious tendencies. As we learn about plant learning, the questions may not be about whether plants have motivations and preferences, expectations or choices, but what it means for us to know that they do.