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Are plants intelligent?
Contrary to popular belief, plants can move, communicate and feel.
M
imosa pudica is a commonly grown plant used as an ornament in many parts of the world.
It is native to America but has reached cultures as far as India, Congo and Senegal where it has
been used for its medicinal properties. It is most famous, however, for the unusual way it reacts to
touch. Nicknamed the “humble plant” or “touch-me-not”, Mimosa‟s leaves fold up when brushed,
rapidly drawing back to protect itself from predators.
The English scientist Robert Hooke is one of the first people known to have investigated the
movements of Mimosa pudica in the 17th century, suggesting that plants had nerves and tissues
similar to those in animals. It was later discovered that the leaves of Mimosa pudica fold as a
result of the internal movement of water, and the mechanics of the process are now welldocumented. Stimuli, such as touch or air movement, trigger certain areas of the stem to release
chemicals, which cause water to move out of the leaf cells, causing them to crumple up in the
space of a second.
Plants‟ responses to stimulation have since captured the imagination of numerous scientists
attempting to discover whether or not plants are truly intelligent. It goes without saying that
however responsive or sensitive plants are, they lack brains and neurons, the specialised cells in
the brain designed to transmit information to other nerve cells, muscles and gland cells. But the
range of plant behaviour scientists have observed over the past five centuries suggests that
members of the plant kingdom are somehow able to feel. Although many scientists remain
sceptical, others are determined to prove that there is a link between plant behaviour and
intelligence.
To many people, the notion that plants exhibit behaviour of any kind is entirely foreign. People
who become brain dead after illness or injury are sometimes referred to colloquially as
vegetables. Despite Charles Darwin towards the end of his life in 1880 writing a book entitled
„The Power of Movements of Plants‟, the subject didn‟t gain immediate traction and Darwin‟s work
on the plant kingdom became largely outshadowed by the popularity of his masterpiece on
evolution, „The Origins of Species‟.
Intelligence is based on how efficient a species becomes at doing the things that they
need to do to survive.” Charles Darwin , 1871
Years of observation made Darwin highly aware of the fact that plants move. „The Power of
Movement of Plants‟ details in 570 pages numerous examples of stem movements in different
plant species. He demonstrated that shoot movements were often the result of stimulation to
motor tissue in a process that resembles that of neurons. But beyond this, he lacked the tools to
assert that plants possess certain characteristics that his theories pointed to.
It was not until the start of the 20th Century that time-lapse photography provided the means to
test almost 400 years-worth of theories. Researchers have since proved an array of behaviours,
from movement to altruism, revolutionising the way that “still” life is perceived.
Mimosa Pudica has been renamed humble plant or touch-me-not because of its highly-sensitive behavior.
Plants move
Plants may not be able to go in search of food and they can‟t escape a hostile environment. But
plants are not as immobile as we think; their movement is not limited to growth.
Some plants move faster than others, of course. Mimosa is one example of a visibly moving
plant; the Venus flytrap, famously, is another. A carnivorous plant native to North America, it
catches its prey, mainly flies and spiders, with traps triggered by tiny hairs. When an insect or
spider crawling along the leaves contacts a hair twice, the trap closes in 100 milliseconds. The
Venus flytrap is so fast that slow-motion video is needed for the human eye to pick out details.
But such cases are exceptions in the plant kingdom, in which the majority of species move
imperceptibly.
Environmental journalist Michael Pollan has observed in time-lapse the behaviour of runner bean
plants over the space of six weeks. Mr. Pollan noticed with surprise the extent to which the plants
would throw themselves in order to reach a far-away pole. The behaviour couldn‟t be observed
without the help of a mechanism that would speed up movement, which, according to Mr. Pollan,
tells us how much in the plant kingdom went unnoticed over the centuries. Mr. Pollan added that
despite the rigorous studies carried out since the 1660s, our perception of plants has always
been hindered by the gulf in the time-frame in which plants react, and our own.
But what does it mean if a plant moves, considering that it lacks a brain? Darwin was fascinated
by the reactions of plants to external stimuli, in particular with carnivorous plants such as the
Venus flytrap. He believed its almost instantaneous response and the way the plant closed its
leaves around an insect indicated the presence of a central nervous system comparable to that of
an animal.
Between 1870 and 1880, British physiologist John Scott Burdon-Sanderson conducted a series of
experiments on the Venus flytrap. The first and most revealing of all was to attach electrodes to
the surface of the trap lobes in the hope of recording electrical activity. Burdon-Sanderson was
amazed to find that each time a trigger hair was touched, it provoked a wave of electrical activity
almost identical to the nerve impulses produced by animal neurons.
In a controversial 2006 article in the journal Trends in Plant Science, a group of scientists,
including American plant molecular biologist Eric Brenner, Italian plant physiologist Stefano
Mancuso, Slovak cell biologist František Baluška and American plant biologist Elizabeth Van
Volkenburgh drew from these theories to propose a new field of study called “plant neurobiology”.
According to the authors, plants are able to sense and optimally respond to so many
environmental variables - light, water, gravity, temperature and chemical signals from other plants
- that there may exist some “brainlike information-processing system” to integrate the data and
coordinate a plant‟s behavioral response. As well as pointing out that electrical and chemical
signalling systems have been identified in plants similar to those in the nervous systems of
animals, the authors noted that some neurotransmitters present in humans such as serotonin and
dopamine have been found in plants, although their role remains unclear.
COUNTERPOINT
A group of biologists including Yale Professor Clifford Slayman wrote a letter to journal Trends in
Plant Science in 2007, dismissing plant neurobiology as "a foolish distraction, not a new
paradigm.” Urging proponents of the new field to reevaluate the concept critically, the signatories
added that “although at the molecular level the same general principles apply and some
important parallels can be drawn between the two major organismal groups [in plants and
animals], this does not imply a priori that comparable structures for signal propagation exist at the
cellular, tissue and organ levels”.
The ability to feel is not the only remarkable property of plants that scientists are studying. They
may also be able to communicate their feelings.
Plants communicate
The evidence for plant communication is only a few decades old. Two 1983 experiments by
scientists Jack Schultz and Ian Baldwin analysed in an essay published in Science appeared to
show that injured poplar and maple trees release chemical signals that are picked up by healthy
neighbouring trees.
Wilde maize plants have the ability to release chemicals that attract wasps in order to kill parasitic moths .
The injured trees were alerting neighbours to the presence of a predator by releasing chemical
signals into the air. The plant research community criticised the experiments themselves as
flawed and the results too difficult to replicate. Since then, rigorous experiments have overcome
those early criticisms with repeated testing in labs, forests and fields. It‟s now well established
that when insects chew leaves, plants respond by releasing volatile organic compounds (VOCs)
into the air, in the hope of warning other plants of possible predators.
Nearly every plant researched releases its own cocktail of volatile chemicals to which other plants
and other plant species respond. The tobacco plant is a classic example.
Professor of Entomology, Richard Karban and other researchers from the University of California
showed that a cut or eaten sagebrush plant "told" nearby wild tobacco plants about its injury, and
the tobacco plants apparently responded to protect themselves from damage. Over three
seasons, researchers cut leaves of sagebrush plants to mimic the work of insects. The cut
sagebrush released a particular type of VOC - methyl jasmonate - which the wind carried to
nearby plants. The tobacco plants seemed to sense the chemicals and they increased the
production of nicotine, a defensive agent that caused their leaves to taste bad to insects.
The possibility that plants share information has prompted scientists to study crop resistance to
pests in an effort to improve it. A 2011 study published in Ecology Letters pointed out that
commercial corn hybrids seem to have lost the wild maize plant‟s ability to release chemicals that
attract wasps in order to kill parasitic moths.
The parasite or stem borer, Chilo partellus, is a major pest for maize in eastern and southern
Africa and South Asia, causing yield losses of up to 88%. As much as it likes to chew leaves, it
also lays eggs on them. When wild maize touches the eggs, it releases chemical compounds
called herbivore-induced plant volatiles that attract parasitic wasps that lay into the eggs of Chilo
partellus. Researcher Amanuel Tamiru and his colleagues found that while "land-races", locally
available varieties that haven‟t been subjected to intensive commercial breeding programmes,
were able to attract wasps following an attack by Chilo partellus, the commercial varieties were
not able to attract parasitoids at all.
Plant communication research of this kind has prompted scientists to find alternatives to the use
of detrimental pesticides. Particularly sensitive plants, or ones with enhanced self-defence
systems, could be planted next to crops at risk.
Plant species hold the records among living organisms for both size and longevity. The giant
sequoia, Sequoiadendron giganteum, in Sequoia National Park, California, is considered to be
the largest living plant in the world, with a measured trunk girth of 31.3 meters. The oldest living
plant is the Creosote bush, estimated to be 11,700 years old.
Plants care
Plants‟ ability to interact and communicate is not the only revealing finding of recent botanical
research. Plants are also able to recognise kin and act “altruistically” towards their relatives.
Yellow Jewelweed of the Balsaminaceae family native to the US .
Evolutionary biologists have recently joined the longstanding philosophical debate on the concept
of altruism. Social behaviour, kin recognition and altruism are well researched in the animal
kingdom, the latter exhibited through behaviours such as food sharing and warning calls, and
have been explained as key elements to enhance the chances of reproduction and the survival of
particular DNA.
In a paper published in November 2009 in the American Journal of Botany, Ph.D. student
Guillermo Murphy and Dr. Susan Dudley explored kin recognition in Impatiens pallida, commonly
known as yellow jewelweed.
Yellow jewelweed plants are often found growing in close proximity to related individuals and are
known to respond strongly to above-ground competition, especially for sunlight, making this
species a likely candidate for kin recognition. By investing all of their resources into their leaves,
the plants can grow quickly to cover their competitors' leaves, awarding themselves preferential
access to sunlight. But their behaviour is not only competitive above the ground: yellow
jewelweed can stimulate their root growth and drive away the root systems of neighbouring
plants.
What Mr. Murphy and Dr. Dudley revealed, however, is that competitive behaviour of yellow
jewelweed fades when the neighbouring plant is one of their kin. Among close relatives, the
plants did not increase resource allocation to roots or leaves. Rather, they altered their
morphology by increasing stem elongation and branching.
“One of the most fundamental laws of nature is that if you are going to be an altruist, give it up to
your closest relatives,” evolutionary biologist Professor William Friedman at Harvard University
stated when carrying out similar experiments on corn plants. Plants, as much as humans and
other animals, are social organisms.
Neurobiology without neurons
At first glance, altruistic behaviour, as well as plant communication and movement, are
impossible to detect. It took nearly five centuries of research and the use of modern technology,
such as electrical sensors, time-lapse photography and infrared cameras, to offer visible and
tangible proof of plants‟ highly developed behaviour.
Many among botanists and biologists still argue that referring to these behaviours as intelligence is an exaggeration and
essentially an inaccuracy. Plants lack brains, neurons and synapses and their sensitive qualities, say the sceptics cannot be
equated to those of humans or other animals.
Jainism, one of the oldest religions in the world, prescribes a path to non-violence towards all
living creatures. Jains, who today form a small community in India, recognise growing plants as
living forms and therefore abstain from eating them. Vegetables and fruits that grow underground
are prohibited as a general rule, though there are exceptions. Fresh fruits and vegetables that
grow above the ground should be plucked only when ripe and ready to fall off, or ideally after they
have fallen off the plant. Grains such as wheat and rice, maize and beans are obtained when the
plants or the pods are dry and dead. Cutting down green trees for wood or any other use is
strictly prohibited in Jainism.
Humans, or human animals as cognitive ethologists prefer to call them, show a degree of
reluctance in acknowledging forms of intelligence in other species. Progress in recognising
animal intelligence has only been made in the past 30 years, when experiments on dolphins,
chimpanzees and pigs, among others, have revealed consciousness, self-awareness and
intelligence in a way that humans had never previously understood.
Australian philosopher Peter Singer in his groundbreaking 1975 work Animal Liberation advanced
the concept of “speciesism”, the belief in the inherent superiority of humans over all other species
due to humans' alleged unique cognitive capacity, in turn justifying the exploitative treatment of
other animal species. This attitude must be discarded, Singer argues, in favour of a more equal
environment for human and non-human animals.
The roots of the Creosote Bush are the oldest in the world and they date back to over 11,000 years ago .
Thirty years later, the Society for Plant Neurobiology held its first meeting in Florence where this
new field, “aimed at understanding how plants perceive their circumstances and respond to
environmental input”, was formally recognised. Italian physiologist and founder of the Society for
Plant Neurobiology Professor Stefano Mancuso rejected two main assumptions that had to do
with the importance of neurons and the equation of intelligence to mobility when studying the
plant kingdom. “A plant has a modular design, so it can lose up to 90% of its body without being
killed,” Professor Mancuso said. “There‟s nothing like that in the animal world.”
While Professor Mancuso‟s arguments were dismissed as extreme by neurobiology sceptics, he
is far from alone in criticising mankind‟s anthropocentric approach to nature. Professor Anthony
Trewavas and author of the book „Plant Behaviour and Intelligence‟, writes that one of the biggest
obstacles in recognising plant behaviour is people's preconceptions in considering intelligence.
“It is a common error to impose human expectation and restrictions on the behaviour of other
organisms. As human beings, we identify visible movement with behaviour because that is how
we recognise human behaviour. If is does not appear nor move, then it is not behaving,”
Professor Trewavas explained. “The problem of anthropomorphic attitudes was supposed to have
been eliminated centuries ago from science, but it is very difficult to disentangle in this area of
work. Many scientists commonly adopt public (i.e. unqualified) attitudes to behaviour and
intelligence and to these capabilities in plants.”
Analysing the responses of those distinctively mobile plants, Mimosa Pudica and the Venus
flytrap, biologists at the University of Missouri have found that despite not having hearing
structures, plants can discern the sound of predators through the vibrations on their leaves. After
“hearing” warning sounds, plants can enhance their defences against predators and the Missouri
study showed that this process can be triggered by sound alone. But the question science is still
struggling to answer is how do plants interpret information exchanged with each other, despite
having no brain? And how can they integrate or organise information without a nervous system?
Dr. James Cahill, experimental plant ecologist at the University of Alberta, maintains that plants
are living proof that there is a system of intelligence that does not rely on neurons - humans just
don‟t know how to find it yet.