For centuries we have considered them as something not very different from rocks or furniture—a part of the landscape, or simple decorative elements that we tread on and uproot at our pleasure, since we never hear them complain. Except for their more or less slow growth or their seasonal cycles, plants always appear immobile and imperturbable, ignorant of what happens around them. Living beings, though simply passive.
But while we scarcely pay attention to them, plants are doing amazing things. For example, steering their roots towards water sources by listening to the vibrations of pipes, according to a study published in April 2017 in the journal Oecologia and led by the evolutionary ecologist Monica Gagliano of the University of Western Australia.
According to Gagliano and her collaborators, pea plants are capable of locating water at a distance in the absence of moisture thanks to this kind of vegetal sense of hearing. Only when they have located where the water flows, do they use the moisture itself as an additional clue to reach their target accurately. But before this they depend on the sound, to such an extent that background noise confuses them, and they are even able to distinguish the actual sound from a recording.
Plants are able to learn
The above would seem to be science fiction, were it not for it actually being one more milestone in understanding the unusual capabilities of plants, which have only been revealed when scientists like Gagliano and others have begun to probe beyond their apparent passivity. In another recent study published in Scientific Reports of the Nature group, the researcher revealed that plants are also able to learn in the style of Pavlov’s dogs, associating food with the sound of a bell until reaching the point of salivating simply by hearing the sound.
In Gagliano’s experiment, the food was a light source, and the bell was a current of air induced by a fan. When the plants were presented with a Y-shaped labyrinth, they grew along the illuminated path where the air also blew, but they continued to choose the same option in the absence of light; in other words, they had learned to associate the current of air with light. And they remembered.
In the case of plants, talking about concepts such as learning, memory, choice or, ultimately, cognition, can be shocking. This is well known by Gagliano and other researchers in this field, who for years have suffered the mistrust and scepticism of many of their colleagues. But if scepticism is essential for scientists, it is also necessary to remain open to a paradigm shift when experimental evidence so dictates. And in this case, there is enough evidence to warrant it.
See, smell and communicate with other plants
Leaving aside the terminology, this is a summary of the capabilities demonstrated by plants, according to researcher Simcha Lev-Yadun at the University of Haifa-Oranim (Israel): with their system of light sensitive pigments, plants can ‘see’ their neighbours thanks to the detection of infrared light emitted during photosynthesis; they can smell their neighbours and their enemies; they can communicate with other plants, warning them of dangers; they make decisions according to the environmental parameters; they recall past weather conditions and herbivorous attacks; they use animals for defensive purposes, pollination or to disperse their seeds; they deploy strategies to avoid aggression; and, of course, they can also hear, as Gagliano has shown. Most importantly, they make decisions.
Lev-Yadun, who recently published the book Defensive (anti-herbivory) Coloration in Land Plants (Springer, 2016), has no doubts about what to call all this: “I am certain that plants have intelligence,” he says to OpenMind. “It is clear that there are differences between plants and higher animals, but when we examine lower types of animals, the differences seem to be very small, and higher plants may be more intelligent than certain animals,” he adds.
Some of the mechanisms that govern these processes are not yet fully known. But Gagliano, who recently co-edited the upcoming book The Language of Plants (University of Minnesota Press, 2017), opts for the idea that evolution has followed convergent paths in large kingdoms such as animals and plants to reach similar goals with different tools. “Both ended up ‘inventing’ the same solutions to similar problems,” he sums up to OpenMind.
But although the need for the paradigm shift already seems undeniable, the question of terminology cannot be ignored. The problem is that we do not yet have adequate vocabulary, since traditionally behaviour and cognition have been considered exclusive faculties of beings with neurons—the animals. Nevertheless, some scientists speak of “plant neurobiology.” “Till a good term is found, neurobiology is fine,” says Lev-Yadun. On the contrary, Gagliano thinks that this word has been useful as a metaphor, but that it should be abandoned for being “zoocentric” and not very scientific.
However, on one thing the researchers in the new discipline of plant cognition do agree, and that is the need to refute the objections that all these capacities are nothing more than programmed molecular responses. For Gagliano, one cannot speak of behaviour when it comes to obligatory and irreversible actions, such as those that occur during the development of living beings; but the concept of behaviour is appropriate when there are optional choices that depend on stimuli. “Terms like ‘cognition’ or ‘learning’ or even ‘intelligence’ refer to aspects of the behavioural repertoire,” says the researcher.
The implications of all this go beyond the purely scientific, attracting the attention and reflection of philosophers, humanists and experts in ethics: if today we know that plants can also feel, can we continue to ignore this? As Gagliano wrote in a recent article, “as experimental evidence for the cognitive capacities of plants accrues, the controversial (or even taboo) topic regarding their welfare and moral standing and our ethical responsibility toward them can no longer be ignored.”