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Plants communicate through their network of roots and can even create and respond to sounds. But experts

are now asking – can they think, too?

Story by Peter Meredith

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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 informa-tion. 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.

They are known to compete for limited resources, can dis-tinguish 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?

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 Evolu-

tionary Biology, and her passion for her scientific work suffuses her every word and movement. I met her in one of the univer-sity’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 fre-quency, 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. Hav-ing 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

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.”

IN LEWIS CARROLL’S

Plant posse. Dr Monica Gagliano (top) is passionate about plants. In a temperature-controlled lab (above), she and research assistant Mavra Grimonprez (at right) pot peas with undergraduates (l–r) Nicholas Clairs, Hannah Etchells and Trent Betts. They are testing whether seedlings (left) communicate through their roots via sound.

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Some plants, such as tomatoes and blueberries, can ‘hear’ the buzz of an approaching bee.

afraid that they might compromise our careers,” she said. Monica’s work in what has come to be known as ‘plant bio-

acoustics’ 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 cater-pillars– 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 echo-locate – 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.

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 tech-nique 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

Flower power. By sending signals that lure bees to their

nectar, flowers trick the insects into helping them reproduce.

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Sensitive botanicalsPLANTS PERCEIVE GRAVITY, touch, light, heat, magnetic fields and moisture, as well as concentrations of oxygen,

carbon dioxide and soil chemicals. This information helps them compete for resources, distinguish friend from foe and repel rivals. They’re ingenious at tricking animals into helping them reproduce, and they communicate with other

plants and animals on many levels and in many ways.

aboveground6 Like many animals,

plants sense magnetic fields with proteins called cryptochromes, which also have a role in maintaining their internal clocks.

7 Flowers communicate with pollinators through smell, colour and shape. Some add a little caffeine to their sugary nectar to make feeding memorable and addictive.

8 Some flowers communicate through electricity. A bee landing on a petunia, for example, experiences a small and pleasurable burst of static.

9 Tomatoes and blueberries ‘hear’ the buzz of a bee and will release a puff of pollen at the right moment.

1 Plants sense light with photosensitive chemicals. This enables them to distinguish night and day, light and shade, and also detect nearby plants (by the light reflected or shade generated). Plants position their leaves and flowers in response to light.

2 Plants are able to ‘hear’ – that is, sense acoustic vibrations. The audio recording of a caterpillar chomping a leaf, for example, causes some plants to produce defence chemicals.

3 Plants under attack from herbivores or pathogens produce defensive chemicals such as tannin and caffeine.

4 Plants can warn each other of danger by emitting volatile chemicals. When they sense these, they boost their own defences.

5 Some volatile chemicals, such as those emitted by corn and lima beans, attract predators of plant pests, such as parasitic wasps.

10 In Central American rainforests, a night-flowering vine shapes its flowers to echo the calls of nectar-feeding bats, luring them towards its nectar.

11 Root tips are highly sensitive and electrically active. They are mobile and can cover large distances, gathering information on soil quality, moisture, chemicals, microbes and temperature. This suggests that large numbers of roots acting in unison exhibit ‘swarm intelligence’.

12 Plants are able to sense the roots of competitor plants and may then direct themselves away. Alternatively, they can become aggressively territorial, competing by rapidly growing more roots in order to steal territory and nutrients away from rivals.

13 Roots can ‘hear’ water flowing in pipes, and direct growth towards the sound.

14 Some roots can generate sounds. Corn seedling roots click and react to similar sounds, bending towards them. This helps plants assess if neighbours are friends or foes.

15 Roots sense when they are approaching an obstacle and change course to avoid it.

16 The roots of broad beans, tomatoes and even some trees plug into fungal networks and use them as a kind of ‘internet’ to share nutrients, water and information.

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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.

All this is impressive enough, but does any of it imply con-sciousness 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 (see page 72). 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 gain-ing 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.

THIS STILL DOESN’T imply intelligence or conscious-ness, or even a brain-like control 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 ecosys-tem”. 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.

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 František Baluška, a cell biologist at the University of Bonn, Germany, who is also a member of the society. František 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

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.

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Touchy type. Mimosa pudica folds its leaves when touched or shaken. It has

been found capable of learning and remembering new tricks.

Nervous network. Professor Sergey Shabala says the electrochemical signalling system within plants is remarkably similar to the nervous system of animals. But although plants are highly complex organisms, he doubts that they think or have consciousness.

WATCH Use the free viewa app to scan this page and see an interview with Dr Monica Gagliano. You can also watch it on our website.

N o v e m b e r – D e c e m b e r 2 0 1 4 73

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 organ-isms in the soil using their chemical language,” František 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 František and his coworkers to con-clude that the roots of a plant constitute a kind of brain. In an essay entitled The ubiquity of consciousness, František and Professor Tony Trewavas, a molecular plant scientist at the University of Edinburgh, Scotland, concluded: “Plant behaviour is active, pur-pose-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 condi-tions 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 sim-plest sense, consciousness is an awareness…of the outside world.”

František 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.”

According to Tony Trewavas, plants are ‘prototypical intelli-gent 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 scien-tists 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 contro-versial 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.

DR ELIZABETH VAN VOLKENBURGH is a plant physi-ologist 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 simi-larities, 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 under-ground ‘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 extraor-dinary 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.” AG

Critics maintain that the words ‘intelligence’, ‘consciousness’, ‘brain’, ‘think’ or ‘thought’ must be used purely as metaphors or analogies in reference to plants.

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IN THE EARLY 1900s, European scientists either loved or snubbed Jagadish Chandra Bose, Indian physi-

cist, botanist, biologist and polymath.He was loved for his pioneering

work on microwaves, so much so that it earned him a knighthood and member-ship to the Royal Society, the UK’s elite national science academy. But he was spurned for the conclusions he drew from his plant experiments – that plants were intelligent and had feelings.

Born in Bengal, India, in 1858, Bose studied at universities in Calcutta, London and Cambridge. After 1900 he turned his attention from microwaves and radio receivers to the tiny elec-trical impulses that plants generate. He designed and built extraordinarily sensitive instruments that could detect not only this electrical activity but also growth and movement in plants.

With this gear he detected and measured plants’ responses to stim-uli such as touch, temperature, light or radio waves. He found that plants quivered when injured and concluded that a plant’s electrical signalling system amounted to a nervous system and that it could learn, remember and feel pain.

By the time Bose died in 1937, his ideas on signalling were mostly forgot-ten. Today, nearly 80 years later, they are becoming part of mainstream science.

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Sir Jagadish Chandra Bose

A SCIENTIST AHEAD OF HIS TIME

Looking ahead. The Cornish mallow or

Cretan hollyhock (right) reorientates

its leaves at night in anticipation of

sunrise. US botanist Dr Elizabeth van

Volkenburgh (below), says

that, although plants lack brains,

investigations are underway to

discover if there is a “non-brain way” to process information

and communicate.

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