Plants are generally considered to be more basic forms of life than animals. They cannot speak, or walk; they do not have powers of reasoning or conscious thought.
Or do they?
Scientists are now coming around to the idea that plants are sentient living beings, with a sophisticated awareness of their surroundings. Not only that, but research shows that they can communicate with one another, can pay attention to stimuli, have the capacity to memorize information. Charles Darwin was ahead of his time when he wrote his book "The Power of Movement in Plants", though at that time he was alone amongst his peers with this viewpoint.
Dedicated research on "plant neurobiology" began in 1900 when Indian biophysicist Jagdish Chandra Bose paved the way for other free-thinking scientists to explore the possibility of plant intelligence. In his early work, he claimed that plants are not only aware but actively explore their environments, and are therefore capable of adapting their behaviour to suit their location. Bose indentified that these capabilities were down to a complex plant nervous system that allowed information to travel around the organism via electrical signals.
A decade later, Anthony Trewavas from the University of Edinburgh, UK, focused his research specifically on determining plant intelligence. The criteria to establish this was defined as the ability to sense one’s environment, to process and integrate such sensory perceptions, and decide on how to behave.
"The great problem of plant behaviour has always been that you can’t see it going on," he says. There are a few exceptions, such as the snap of the Venus flytrap. "But the most visible plant behaviour is simply growth, and growth is a very slow business," he said.
But time lapse photography has opened up the possibility for scientists to watch plant movements in detail and identify if a specific motivation governed their movements. When the parasitic vine Cuscuta was filmed by time lapse, its seedlings appeared to sniff the air looking to spy an available host; when found, the seedlings lunged at their victims and wound themselves around them like a snake.
"It is remarkably snakelike in the way it behaves," says Trewavas. "You’ll stop doubting that plants aren’t intelligent organisms, because they are behaving in ways that you expect animals to behave."
Trewavas’ research appeared to be so convincing that he began to win approval from his peers, and in 2005, The Society for Plant Neurobiology was formed so that the subject could be fully explored and discussed.Stefano Mancuso, based at the University of Florence in Italy, was one of the founders.
"There is a kind of brain chauvinism," Mancuso said. "We think that a brain is something that is absolutely needed to have intelligence."
Despite a lack of neurons and an animal-like nervous system, plants can process and utilise information to result in apparently intelligent behaviour that can be called intelligent. Mancuso and society co-founder Frantisek Baluska at the University of Bonn, Germany, believe that the powerhouse of a plant is to be found in its roots, which can sense stimuli such as gravity, humidity, light, oxygen and nutrients. Baluska and Mancuso have homed in on an area known as the transition zone as being the most significant. It was originally thought to have no purpose, but it is actually the nerve centre of the plant. The transition zone is electrically active and fulfils functions similar to those of our own brains, such as the consumption of oxygen, and transporting protein-holding vesicles in a similar way to human neurotransmitters.
In a full circle sweep, this finding correlates with Darwin’s "root brain" hypothesis. In the last paragraph of The Power of Movement in Plants, he stated that the root was the intelligent end of a plant. Referring to a plant’s primary root, or radicle, he wrote: "It is hardly an exaggeration to say that the tip of the radicle… acts like the brain of one of the lower animals."
"He was right once more," says Mancuso. "If we need to find an integrative processing part of the plant, we need to look at the roots."
But are plants actually conscious; can they really "think?"
It seems that many plant cells are capable of neuron-like activity.
"In plants, almost every cell is able to produce and propagate electric signals. In roots, every single living cell is able to," says Mancuso. Likewise, the phloem is extremely electrically active, and capable of fast electrical signalling.
"It is some kind of huge axon, running from the shoot tip to the root tip," says Baluska. "This suggests a lot of conscious activity."
There is evidence that plants can even become depressed: plants produce chemicals that in animal brains act as hormones and neurotransmitters, such as serotonin, GABA and melatonin, but the significance of these chemicals in plants is not clear. Susan Murch of the University of British Columbia in Kelowna, Canada, has shown that drugs like Prozac, Ritalin and methamphetamines, which disrupt neurotransmitters in our brains, can do the same in plants.
"If you really mess with a plant’s ability to either transport or make melatonin or serotonin, root development is very strange – they are malformed and disjointed," she reports.
Yet there are still those who find it difficult to conceive that plants have such complex physiology, and that "plant neurobiology" is a contradiction in terms. Daniel Chamovitz at Tel Aviv University is scornful:
"Plants just don’t have neurons," he says. "It’s like saying ‘human floral biology’."
It is this attitude amongst their peers that have forced the Society for Plant Neurobiology to change its name to the Society of Plant Signaling and Behavior.
But Baluska defiantly ignores the critics and pushes the boundaries, maintaining that plants are not only conscious but may even feel pain. They produce ethylene in response to stressors, a gas that can anaesthetise animals.
"If you consider ethylene as an anaesthetic, and if some organism is producing an anaesthetic under stress then you could get ideas that plants maybe feel some pain," he explains.
"Ethylene is the plant equivalent of a scream," agrees Murch. "Inevitably, there is a vegan in the audience who goes, ‘Then what will I eat?’" she says.
Critics like Chamovitz cannot dispute that plants have an evolved sense of awareness, however, as the evidence to support this is too great. In some ways they have been shown to be more highly sensitive to certain stimuli than animals; animals have just a handful of photoreceptors to sense light, whereas plants have around fifteen.
"Plants are acutely aware of their environment," admits Chamovitz. "They are aware of the direction of the light and quality of the light. They communicate with each other with chemicals, whether we want to call this taste, or smell, or pheromones. Plants ‘know’ when they are being touched, or when they are being shook by the wind. They integrate all of this information precisely. And they do all of this integration in the absence of a neural system."
Plants have even been shown to have a memory, if memory can be defined as "recording an event, storing that event and recalling it at a later time in order to do something," says Chamovitz. Certain species, such as Mimosa pudica, appear to have long-term memories: during their research, Mancuso and team dropped potted mimosas on to foam from a height of 15 centimetres. Initially, the plants closed their leaves in response to the fall, but after just four to six drops they stopped doing this, apparently realising that the fall posed no danger to them. The same response still continued to occur in response to other stimuli, however:
"Even after one month, they were able to discriminate and be able to understand whether the stimulus was dangerous or not," says Mancuso.
But does all of this evidence really indicate that plants are "intelligent?"
It appear to be very clever, but it’s not intelligence, says Chamovitz: "I don’t like the term plant intelligence. We don’t even know what intelligence is for humans. If you get five psychologists together you will get 20 different definitions."
Despite years of studying and being awestruck by giant redwood trees, Steve Sillett agrees. "I wouldn’t call it intelligence, but awareness. These trees are keenly aware of their environment, and they respond to it in many ways that we can measure as performance."
Yet it is possible that our own intelligence is too limited to comprehend plant capabilities; when we measure intelligence purely by comparison to human levels of cognisance, then we may overlook other, more incredible potentials in other living beings. Michael Marder of the University of the Basque Country in Vitoria-Gasteiz, Spain, suggests that we need a different framework to evaluate intelligence and consciousness, and ask: what does the world look like from the standpoint of plant life?
"Our task is to think about these concepts of attention, consciousness and intelligence in a way that becomes somehow decoupled from the figure of the human," he says. "I want [us] to rethink the concept of intelligence in such a way that human intelligence, plant intelligence and animal intelligence are different sub-species of that broader concept, which can somehow encompass these different life forms."
Certainly plants live a more symbiotic existence with their environment, sensing how to live in harmony with its surroundings, whereas humans are often totally ignorant of the damage they inflict on their environment. Is that "intelligence?"
Marder suggests that we can all learn from something from the plant life that we share this earth with:
"Maybe we can use that model for ourselves, to temper a little bit the excessive separation from our environment that has led in large part to the profound environmental crisis we find ourselves in."