From the memories of flowers to the sociability of trees, the cognitive capacities of our vegetal cousins are all around us.
At first glance, the Cornish mallow (Lavatera cretica) is little more than an unprepossessing weed. It has pinkish flowers and broad, flat leaves that track sunlight throughout the day. However, it’s what the mallow does at night that has propelled this humble plant into the scientific spotlight.
Hours before the dawn, it springs into action, turning its leaves to face the anticipated direction of the sunrise. The mallow seems to remember where and when the Sun has come up on previous days, and acts to make sure it can gather as much light energy as possible each morning.
When scientists try to confuse mallows in their laboratories by swapping the location of the light source, the plants simply learn the new orientation.
What does it even mean to say that a mallow can learn and remember the location of the sunrise? The idea that plants can behave intelligently, let alone learn or form memories, was a fringe notion until quite recently. Memories are thought to be so fundamentally cognitive that some theorists argue that they’re a necessary and sufficient marker of whether an organism can do the most basic kinds of thinking. Surely memory requires a brain, and plants lack even the rudimentary nervous systems of bugs and worms.
However, over the past decade or so this view has been forcefully challenged. The mallow isn’t an anomaly. Plants are not simply organic, passive automata. We now know that they can sense and integrate information about dozens of different environmental variables, and that they use this knowledge to guide flexible, adaptive behaviour.
For example, plants can recognise whether nearby plants are kin or unrelated, and adjust their foraging strategies accordingly. The flower Impatiens pallida, also known as pale jewelweed, is one of several species that tends to devote a greater share of resources to growing leaves rather than roots when put with strangers – a tactic apparently geared towards competing for sunlight, an imperative that is diminished when you are growing next to your siblings.
Plants also mount complex, targeted defences in response to recognising specific predators. The small, flowering Arabidopsis thaliana, also known as thale or mouse-ear cress, can detect the vibrations caused by caterpillars munching on it and so release oils and chemicals to repel the insects.
Plants also communicate with one another and other organisms, such as parasites and microbes, using a variety of channels – including ‘mycorrhizal networks’ of fungus that link up the root systems of multiple plants, like some kind of subterranean internet. Perhaps it’s not really so surprising, then, that plants learn and use memories for prediction and decision-making.
What does learning and memory involve for a plant? An example that’s front and centre of the debate is vernalisation, a process in which certain plants must be exposed to the cold before they can flower in the spring. The ‘memory of winter’ is what helps plants to distinguish between spring (when pollinators, such as bees, are busy) and autumn (when they are not, and when the decision to flower at the wrong time of year could be reproductively disastrous).
In the biologists’ favourite experimental plant, A thaliana, a gene called FLC produces a chemical that stops its little white blooms from opening. However, when the plant is exposed to a long winter, the by-products of other genes measure the length of time it has been cold, and close down or repress the FLC in an increasing number of cells as the cold persists.
When spring comes and the days start to lengthen, the plant, primed by the cold to have low FLC, can now flower. But to be effective, the anti-FLC mechanism needs an extended chilly spell, rather than shorter periods of fluctuating temperatures.
This involves what’s called epigenetic memory. Even after vernalised plants are returned to warm conditions, FLC is kept low via the remodelling of what are called chromatin marks. These are proteins and small chemical groups that attach to DNA within cells and influence gene activity. Chromatin remodelling can even be transmitted to subsequent generations of divided cells, such that these later produced cells ‘remember’ past winters.
If the cold period has been long enough, plants with some cells that never went through a cold period can still flower in spring, because the chromatin modification continues to inhibit the action of FLC.
But is this really memory? Plant scientists who study ‘epigenetic memory’ will be the first to admit that it’s fundamentally different from the sort of thing studied by cognitive scientists. Is this use of language just metaphorical shorthand, bridging the gap between the familiar world of memory and the unfamiliar domain of epigenetics? Or do the similarities between cellular changes and organism-level memories reveal something deeper about what memory really is?
Both epigenetic and ‘brainy’ memories have one thing in common: a persistent change in the behaviour or state of a system, caused by an environmental stimulus that’s no longer present. Yet this description seems too broad, since it would also capture processes such as tissue damage, wounding or metabolic changes.
Perhaps the interesting question isn’t really whether or not memories are needed for cognition, but rather which types of memories indicate the existence of underlying cognitive processes, and whether these processes exist in plants. In other words, rather than looking at ‘memory’ itself, it might be better to examine the more foundational question of how memories are acquired, formed or learned.
When the plant was dropped from a height, it learned that this was harmless and didn’t demand a folding response
‘The plants remember,’ said the behavioural ecologist Monica Gagliano in a recent radio interview, ‘they know exactly what’s going on.’ Gagliano is a researcher at the University of Western Australia, who studies plants by applying behavioural learning techniques developed for animals. She reasons that if plants can produce the results that lead us to believe other organisms can learn and remember, we should similarly conclude that plants share these cognitive capacities. One form of learning that’s been studied extensively is habituation, in which creatures exposed to an unexpected but harmless stimulus (a noise, a flash of light) will have a cautionary response that slowly diminishes over time.
Think of entering a room with a humming refrigerator: it’s initially annoying, but usually you’ll get used to it and perhaps not even notice after a while. True habituation is stimulus-specific, so with the introduction of a different and potentially dangerous stimulus, the animal will be re-triggered. Even in a humming room, you will probably startle at the sound of a loud bang. This is called dishabituation, and distinguishes genuine learning from other kinds of change, such as fatigue.
In 2014, Gagliano and her colleagues tested the learning capacities of a little plant called Mimosa pudica, a creeping annual also known as touch-me-not. Its name comes from the way its leaves snap shut defensively in response to a threat. When Gagliano and her colleagues dropped M pudica from a height (something the plant would never have encountered in its evolutionary history), the plants learned that this was harmless and didn’t demand a folding response. However, they maintained responsiveness when shaken suddenly.
Moreover, the researchers found that M pudica’s habitation was also context-sensitive. The plants learnt faster in low-lit environments, where it was more costly to close their leaves because of the scarcity of light and the attendant need to conserve energy. (Gagliano’s research group was not the first to apply behavioural learning approaches to plants such as M pudica, but earlier studies were not always well-controlled so findings were inconsistent.)
But what about more complex learning? Most animals are also capable of conditioned or associative learning, in which they figure out that two stimuli tend to go hand in hand. This is what allows you to train your dog to come when you whistle, since the dog comes to associate that behaviour with treats or affection. In another study, published in 2016, Gagliano and colleagues tested whether Pisum sativum, or the garden pea, could link the movement of air with the availability of light.
They placed seedlings at the base of a Y-maze, to be buffeted by air coming from only one of the forks – the brighter one. The plants were then allowed to grow into either fork of the Y-maze, to test whether they had learned the association. The results were positive – showing that the plants learned the conditioned response in a situationally relevant manner.
The evidence is mounting that plants share some of the treasured learning capacities of animals. Why has it taken so long to figure this out? We can start to understand the causes by running a little experiment. Take a look at this image. What does it depict?