Are plants talking to each other?
Plants communicate through their roots and can even create and respond to sounds. Experts are now asking – can they think, too?
IN LEWIS CARROLL'S Through the Looking-Glass and What Alice Found There, Alice loses her way in a magical flower garden. Exasperated, she turns to a tiger lily and says, "I wish you could talk." To her astonishment, it retorts, "We can talk, when there's anybody worth talking to."
Mere Carrollian fantasy? Maybe. But given recent scientific findings, perhaps not. The notion that plants are far smarter than we've thought is catching on in the scientific community. No longer can plants be seen as passive organisms, incapable of determining their fate.
It's been shown that they are tireless collectors of information. They can sense gravity, light, temperature, soil quality and moisture content, the presence of micro-organisms, and signals from other plants. They combine this information with data about their own internal situation, assess it all and then act on it to maximise their chances of survival.
- Young roots of corn made regular clicking sounds
- Plants can learn to change their behaviour
- GALLERY: Secrets of plant sex
- VIDEO: Plants use raindrops to spread seeds
They are known to compete for limited resources, can distinguish kin from stranger, repel enemies, avoid rivals, and are territorial and far from static (most are so slow that they only appear to be static, but a few are known to make some of the fastest biological movements on the planet – the Venus flytrap can snap its trap shut in one-tenth of a second). And they're ingenious at tricking animals into helping them reproduce.
As for communicating, it's known that plants – from broad beans to forest trees – do it with one another (their own kind and other species), and with creatures such as insects and other animals, on many levels and in many ways. They can do it using sound, smell, the emission of volatile chemicals, magnetism, electricity and light; and through their leaves as well as their roots.
All of which is leading some scientists to wonder whether plants are capable of doing something akin to thinking. Are they conscious? Might they be intelligent? In short, do they possess mechanisms comparable to the human brain and nervous system.
Plants: are they making sounds?
Such questions often cross the mind of Dr Monica Gagliano. This tall, vibrant 38-year-old is the lead researcher at the University of Western Australia's Centre for Evolutionary Biology, and her passion for her scientific work suffuses her every word and movement. I met her in one of the university's glasshouses, where she was checking pots of seedlings that were helping her with one of her latest projects.
Monica and her fellow researchers have shown that plants can not only generate sound but can also perceive it. In recent experiments they discovered that the roots of corn seedlings make clicking noises and also react to the sounds of similar frequency, bolstering the theory that plants can 'talk' to each other.
In other experiments Monica found that chilli seedlings grow better next to basil plants but worse next to fennel. Having blocked other potential communication channels, such as those facilitated by light and airborne chemicals, she concluded that acoustic vibrations were telling the chilli plants whether their neighbours were friendly or not (basil is friendly because it inhibits weeds and pests).
Plants that can talk or appreciate music are the stuff of popular myth – and often the butt of ridicule. But Monica cautioned against scoffing at such ideas. "People laugh, but do they know?" she said. "Laughing without knowing, that is stupidity. I am a scientist and my job is to test such things without prejudice."
Ultimately, Monica does her testing to answer the questions that other scientists may be reluctant to ask. "The questions I want to ask are the ones that we haven't asked, probably because they appear a bit crazy and even inappropriate and we are afraid that they might compromise our careers," she said.
Plants and bees: an electric attaction
Monica's work in what has come to be known as 'plant bioacoustics' has added to the growing catalogue of facts about plant communication discovered in recent years. For example, when attacked by insects or browsing animals, plants give off volatile chemicals that act as alarm signals to others of their kind, prompting them to take pre-emptive defensive measures.
One tactic is to produce unpalatable or even lethal toxins. Caffeine is one toxin that can be lethal to some pests. Another tactic in the event of insect infestation – for instance by caterpillars – is to emit a volatile chemical that attracts carnivorous predators, such as parasitic wasps, to attack the pests.
Flowers also communicate with pollinators through fragrance, colour, pattern and shape. In 2013 researchers at the University of Bristol in the UK added electricity to the list. They discovered that flowers are negatively charged and bumblebees are positively charged. When a bee lands on the flower, there's a tiny exchange of energy, creating an electric field that the bee feels. This is thought to imprint the flower on the bee's memory, encouraging it to return.
In the tropical forests of Central America, a species of night-flowering vine shapes its flower to echo the locating calls of nectar-feeding bats. This makes the bats' foraging work easier and thus improves the flower's pollination chances.
Some plants, such as tomatoes and blueberries, can 'hear' the buzz of an approaching bee and will release pollen at just the right moment. It's also thought that, like bats, plants can echolocate – make a sound and detect its echo and in that way 'see' things – enabling climbers to home in on something to climb.
Using their roots, plants can plug into the vast underground networks of certain fungi and use them as a kind of internet to communicate with others of their kind. Through the network, plants can send warnings about aphid infestations, prompting their neighbours to take evasive action. In one experiment, trees in a Canadian forest used a fungal network to direct nutrients to where they were most needed – to seedlings.
Some plants have memories
To these many plant skills, Monica has recently added one more memory. She has shown that one plant, a mimosa (Mimosa pudica, known as sleepy plant or touch-me-not), can learn and remember new behaviours. Using a technique commonly used to test learning in animals, she submitted potted mimosas to an experience they would never have in the wild – being dropped repeatedly.
This particular mimosa closes up its leaves when touched or otherwise treated roughly. During Monica's drop tests (which involved 60 drops per plant), the plants folded their leaves in the first 4-6 drops but after that began to reopen them, as though concluding that the experience was not threatening and could be ignored.
To test whether this might be because the plants were simply getting tired, she shook them, at which they instantly closed their leaves again. But when she went back to dropping them, they did not respond. Retesting up to 28 days later, she found the plants still did not close their leaves.
It was a finding that sent ripples across the world of plant science. "It showed that the mimosa can learn, in the same way we talk about an animal learning, acquiring a new skill; and they have an amazing memory," Monica said.
The questions that continue to divide scientists
All this is impressive enough, but does any of it imply consciousness or intelligence in plants, or the existence of an internal control mechanism resembling a brain? Charles Darwin was one of the first of the modern thinkers to ask such questions, and they continue to divide scientists today.
Darwin was passionate about plants and focused his researches on them during the last 30 years of his life. With his botanist son Francis he showed that plant roots could sense moisture, gravity, light, pressure and other environmental characteristics, and would pick the best route for growth based on this information.
In his 1880 book The Power of Movement in Plants, he theorised that a plant's intelligence network reached greatest complexity in the roots, specifically the tips. He concluded: "It is hardly an exaggeration to say that the tip of the radicle [rudimentary root];having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals."
Sir Jagadish Chandra Bose (1858-1937), an Indian botanist, was another scientist who challenged the prevailing view of plants as mindless. He deduced from his research that plants and animals are physiologically similar, and that plants are intelligent and have feelings.
Darwin's root-brain concept and Bose's ideas have been gaining fresh currency of late. Which brings us back to the issue of intelligence and consciousness and whether plants have internal mechanisms that act like an animal brain or nervous system.
Part of the answer must lie in how the various organs of the plant body communicate with one another, or, to use the technical term, how they signal to one another internally. How does a plant's root signal to a leaf?
Sergey Shabala, professor of crop physiology and plant nutrition at the University of Tasmania, specialises in electrical signalling in plants. He says that in plants, as in animals, there are both chemical and electrical signalling systems and they are remarkably alike. In most plants, the chemical pathways are slow, with signals sometimes taking hours to reach their destination. In some, however, they are almost as fast as in animals.
"But in most plants these electrical signals decay rapidly over short distances. So it is communication between a dozen or so connected cells rather than the entire body, as in humans or animals," Sergey said. "So that is the difference. Nevertheless, it is clear that electrical signalling is highly important to plants."
In other words, a plant does possess something that has the elements of a nervous system. And, coincidentally, scientists have discovered neurotransmitters such as glutamate, dopamine and serotonin – chemicals that are the basis of messaging in the human brain – in plants, though their function is not yet clear.
Plants may have the 'swarm intelligence' of insects
This still doesn't imply intelligence or consciousness, or even a brain-likecontrol centre. Nevertheless, building on Darwin's root-brain theory, some scientists suggest vast numbers of roots beneath a plant might exhibit some kind of cerebral analytical ability when acting in concert, similar to the 'swarm intelligence' seen in insect colonies.
Stefano Mancuso, a plant scientist at the University of Florence, Italy, likes the swarm intelligence idea. Stefano has collaborated with Monica and is a leading light in the Society of Plant Signaling and Behavior (SPSB), a body that he and other scientists set up in the USA in 2005. Initially called the Society for Plant Neurobiology, the group changed its name in 2009 after opposition from the scientific community against the idea that plants might possess mechanisms or organs meriting the term 'neuro', with its connotation of nervous systems.
The society sees plants as "information-processing organisms with complex, long-distance communication systems within the plant body and extending into the surrounding ecosystem". Its view is that plant intelligence, rather than implying abstract thought and reasoning, can be defined more simply as the ability to gather information and act on it. As it says, "plants accurately compute inputs from the environment, use sophisticated cost-benefit analysis and take action to mitigate diverse environmental insults". Put more succinctly, plants have the capacity to solve problems.
Roots similar to brain neurons
In further support of the swarm intelligence idea and Darwin's root-brain theory, Stefano points out that an area near the tip of a plant root generates intense electrical signalling. Even a small plant has myriad roots and all may be networking in much the same way as the neurons of a brain do, signalling and labouring in unison for the benefit of the entire living organism but without a central command post.
Stefano has researched plant swarm intelligence with Professor Frantisek, a cell biologist at the University of Bonn, Germany, who is also a member of the society. Frantisek points out that, unlike the rest of a plant, its root tips are highly mobile. They cover relatively vast distances through a complex environment crammed with microbes, minerals and varying degrees of moisture, gathering information as they go.
Acting on this input, they direct their growth to greatest advantage, seeking out water and minerals, while avoiding patches that lack water or oxygen, are toxic or have predators, pathogens or parasites. By means of electrical signals they communicate internally, and, by secreting chemicals, they communicate externally with other roots of the same plant or with those of other plants. Plant roots can distinguish friend from foe.
"Of course, they communicate extensively with most organisms in the soil using their chemical language," Frantisek told me. "They even show swarming behaviour and can kill roots from competing plants. And they enter into symbiotic alliances with mycorrhiza fungi or symbiotic bacteria."
All of which has led Frantisek and his coworkers to conclude that the roots of a plant constitute a kind of brain. In an essay entitled The ubiquity of consciousness, Frantisek and Professor Tony Trewavas, a molecular plant scientist at the University of Edinburgh, Scotland, concluded: "Plant behaviour is active, purpose-driven and intentional. In its capability for self-recognition and problem-solving, similarly to other organisms described in this article, it is thus adaptive, intelligent and cognitive."
Associate Professor Paco Calvo, of the University of Murcia, Spain, says plants not only respond to environmental conditions around them (that is, they are reactive) but also anticipate events. He cites the Cornish mallow, also known as the Cretan hollyhock, as an example, pointing out that its leaves reorient themselves towards the east at night, in readiness for sunrise. This, he told me, is a clear sign of what he terms anticipatory behaviour, which in his view is "the very mark of intelligence; Perceiving the world amounts to successfully predicting it".
But even the most passionate supporters of the idea of plant consciousness are careful to qualify the terms. None equate them to human or animal consciousness or intelligence, even though, as American biologist Dr Lynn Margulis said, "Not just animals are conscious but every organised being is conscious. In the simplest sense, consciousness is an awareness of the outside world."
Frantisek explained: "Plants must have plant-specific intelligence...in order to survive outside in nature. Of course, we should not consider these attributes similar to our human--specific intelligence, cognition, and consciousness. Plants live in different environments and solve their own plant-specific challenges and problems using their plant-specific behaviour."
Brainy behaviour without brains?
According to Tony Trewavas, plants are 'prototypical intelligent organisms' that exhibit simple forms of behaviour, which neuroscientists describe as basic intelligence. Paco Calvo has used the term 'minimally cognitive' in relation to plants.
So instead of 'brain' we could think 'network', or what Tony has termed a 'democratic confederation' of organs and cells that interact in ways akin to thought. US author Michael Pollan sums it up this way: "Intelligence in plants resembles that exhibited in insect colonies, where it is thought to be an emergent property of a great many mindless individuals organised in a network."
The result, he says, is that "brainy behaviour can emerge in the absence of actual brains", a phenomenon known as 'distributed minimal cognition'. Tony calls it 'mindless mastery'. Some scientists point out that the human brain itself is not so very different, being a network of mindless neurons working systematically without a control centre.
The idea that plants may be intelligent is hugely controversial in the scientific world. There are dissenting voices and much scepticism about plant intelligence as a whole and the root-brain notion in particular. Critics maintain that the words 'intelligence', 'consciousness', 'brain', 'think' or 'thought' must be used purely as metaphors or analogies in reference to plants.
Plants are the dominant multicellular life form
Dr Elizabeth Van Volkenburgh is a plant physiologist at the University of Washington and chairs the SPSB. When I asked her about this, she replied categorically that she would choose not to use these kinds of words for plants. "These words have to do with brains, and plants don't have brains," she said.
For Sergey Shabala, 'signalling' is a more appropriate term to use when describing what plants do. "When it comes to similarities, indeed there are many. There is a 95 per cent overlap in how signalling in plants and animals occurs," he said. "But we need to be careful we don't say more than we see."
Dr David Johnson, a biologist at the University of Aberdeen, Scotland, proved last year that broad-bean plants use the hyphae, the filaments that make up the body of fungi, as a kind of underground 'internet' to link with others of their kind and warn them of insect attacks. Despite the ingenuity of this stratagem, David was dubious about the broader idea of plant intelligence.
"It's a long way from consciousness or intelligence," he said. "Plants lack the vital organ that generates these properties. I'm not aware of any credible evidence to suggest plants are intelligent."
But research in this field continues to come up with extraordinary findings. Whether they will ever answer the intelligence question is probably immaterial. Scientists keep probing deeper because they're convinced their discoveries will one day benefit agriculture, horticulture and perhaps even our entire relationship with the botanical kingdom.
In this respect, we shouldn't forget that plants remain, in terms of sheer bulk, the dominant multicellular life form on this planet. They are the foundation of our food chain and produce the world's oxygen, thereby keeping us alive.
"We don't know if they're intelligent, have feelings, experience pain or have different levels of consciousness," Monica said. "I'm not saying they do, but we don't know. And until we know we should give them the benefit of the doubt and treat them as if they do. Maybe if we can see them as living companions and appreciate the many things they do for us, we'll think twice before knocking down the forests." And and what have we done to his people."