By Emma Young
Comparative psychology is the study of animal behaviour, and its psychological underpinnings. But the term wasn’t always this restrictive. Until about 1935, plant behaviour also featured in texts in the field. Now Umberto Castiello at the University of Padua argues that it’s high time that plants regained their rightful place in the study of the psychology of non-human organisms.
In a paper published in the Journal of Comparative Psychology, Castiello gathers together a selection of recent evidence that plants can communicate, remember, recognise kin, decide and even count — “all abilities that one would normally call cognitive if they were observed in animals”. Far from being hard-wired, inflexible respondents to a changing world, they can adapt to change, benefit from classical conditioning, and even come to make predictions about the future.
There’s more than an improved understanding of plants at stake, writes Castiello: “As plants should be considered cognitive agents, as such, they offer us a unique opportunity for a comparative approach, which can potentially lead us to the ‘roots’ of cognition.”
Plants are clearly capable of all kinds of sensing — they can detect everything from changing levels of daylight to levels of the vital nutrient phosphorus in the soil, to signals from each other.
And when it comes to communication, plants have various methods at their disposal. Their sophisticated use of a suite of volatile organic chemicals (VOCs) could be considered a language, as varying groups of VOCs can be used flexibly in new interactions and new contexts, Castiello argues. A combination of different VOCs allows the plant to transmit information about attack by a herbivore, or a more general wound, for example. There’s even evidence for dialects among different species, and that plants that are more closely related understand each other better. This could mean that predator-attack signals would work to protect relatives more than unrelated plants.
Evidence for plant learning is also growing. For example, after being repeatedly dropped on the floor, Mimosa pudica (often known as “sensitive plant” or “touch-me-not”) stops responding with its normal leaf-folding response to a shock — it comes to “realise” that being dropped on the floor is “normal” — but remains sensitive to other types of assault. Even more remarkably, a lab study found that garden pea seedlings can learn that a sudden increase in airflow, caused by a fan, is followed by the occurrence of (desirable) blue light, and will then grow towards the fan, even in the absence of light. This work demonstrates training by classical conditioning.
In other research, pea plants have been found to make “decisions” that resemble those that a human would make. In this study, each plant’s roots were split between two different pots, one of which received a constant level of nutrients, the other a variable level. The overall level of nutrients in these pairs of pots was also varied, so that some pairs had high levels, and some low.
In overall low-nutrient situations, the plants went for the safe option of the pot with the stable (if unsatisfying) level of nutrients, focusing root growth in this pot. But in higher nutrient conditions, they took a punt on the variable-level pot. As Castiello writes: “The experiment showed that plants are able to respond to risk and to switch to risk-prone or risk-averse behaviour depending on resource availability.”
Castiello also outlines evidence that though plants will compete for resources, they can also support each other, exchanging nutrients via the mycorrhizal network of funghi, which connects tree root systems. As he reports, one study even found that Paper birch and Douglas fir trees growing together in a forest in British Columbia were aware when one was in need of help to dispose of excess carbon, and readily gave it. This kind of reciprocal generosity, based on need, to benefit all, looks very like the need-based giving observed in human societies.
In his paper, Castiello also considers research demonstrating that plants can learn the shape of a structure on which they’re growing and adapt to match, choose where to live (by directing their growth), cooperate with ants, and so much more.
“The question should no longer be if plants are cognitive organisms but how plants make use of their cognitive capacities,” Castiello writes. And as the work suggests that a complex, centralised brain is not necessary for cognitive behaviour, it has implications for how we understand cognition.
Over the past century or so, research on plants has focused on their physiology. Castiello, for one, would like that to change. As he says, psychologists have a suite of tools and techniques available to study plant behaviour — and they are also in an ideal position to theorise about the mechanisms that underlie them.