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from NewYorker Website
may possess memory, and exhibit
brainy
behavior in the absence of brains.
Photograph by Grant Cornett
"The Secret Life of Plants," by Peter Tompkins and Christopher Bird, presented a beguiling mashup of legitimate plant science, quack experiments, and mystical nature worship that captured the public imagination at a time when New Age thinking was seeping into the mainstream.
The most memorable passages described the experiments of a former C.I.A. polygraph expert named Cleve Backster, who, in 1966, on a whim, hooked up a galvanometer to the leaf of a dracaena, a houseplant that he kept in his office (Evidence of a Primary Perception in Plant Life).
To his astonishment, Backster found that simply by imagining the dracaena being set on fire he could make it rouse the needle of the polygraph machine, registering a surge of electrical activity suggesting that the plant felt stress.
Backster and his collaborators went on to hook up polygraph machines to dozens of plants, including lettuces, onions, oranges, and bananas.
He claimed that plants reacted to the thoughts (good or ill) of humans in close proximity and, in the case of humans familiar to them, over a great distance.
In one experiment designed to test plant memory, Backster found that a plant that had witnessed the murder (by stomping) of another plant could pick out the killer from a lineup of six suspects, registering a surge of electrical activity when the murderer was brought before it.
Backster's plants also displayed a strong aversion to interspecies violence.
Some had a stressful
response when an egg was cracked in their presence, or when live
shrimp were dropped into boiling water, an experiment that Backster
wrote up for the International Journal of Parapsychology, in 1968.
Much of the science in "The Secret Life of Plants" has been discredited. But the book had made its mark on the culture. Americans began talking to their plants and playing Mozart for them, and no doubt many still do.
This might seem harmless enough; there will probably always be a strain of romanticism running through our thinking about plants. (Luther Burbank and George Washington Carver both reputedly talked to, and listened to, the plants they did such brilliant work with.)
But in the view of many plant scientists "The Secret Life of Plants" has done lasting damage to their field.
According to Daniel Chamovitz, an Israeli biologist who is the author of the recent book "What a Plant Knows," Tompkins and Bird,
Others contend that "The Secret Life of Plants" led to,
...that is, the
possibility that plants are much more intelligent and much more like
us than most people think - capable of cognition, communication,
information processing, computation, learning, and memory.
The six authors, among them,
...argued that the sophisticated behaviors observed in plants cannot at present be completely explained by familiar genetic and biochemical mechanisms.
Plants are able to sense and optimally respond to so many environmental variables - light, water, gravity, temperature, soil structure, nutrients, toxins, microbes, herbivores, 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.
The authors pointed out that electrical and chemical signaling systems have been identified in plants which are homologous to those found in the nervous systems of animals.
They also noted that
neurotransmitters such as serotonin, dopamine, and glutamate have
been found in plants, though their role remains unclear.
The article argued that plants exhibit intelligence, defined by the authors as,
Shortly before the
article's publication, the Society for Plant Neurobiology held its
first meeting, in Florence, in 2005. A new scientific journal, with
the less tendentious title
Plant Signaling & Behavior, appeared the
following year.
Its proponents believe that we must stop regarding plants as passive objects - the mute, immobile furniture of our world - and begin to treat them as protagonists in their own dramas, highly skilled in the ways of contending in nature.
They would challenge contemporary biology's reductive focus on cells and genes and return our attention to the organism and its behavior in the environment. It is only human arrogance, and the fact that the lives of plants unfold in what amounts to a much slower dimension of time, that keep us from appreciating their intelligence and consequent success.
Plants dominate every
terrestrial environment, composing ninety-nine per cent of the
biomass on earth. By comparison, humans and all the other animals
are, in the words of one plant neurobiologist, "just traces."
No such claim had actually been made - the manifesto had spoken only of "homologous" structures - but the use of the word "neurobiology" in the absence of actual neurons was apparently more than many scientists could bear.
Lincoln Taiz says that the writings of the plant neurobiologists suffer from,
He is confident that eventually the plant behaviors we can't yet account for will be explained by the action of chemical or electrical pathways, without recourse to "animism."
Clifford Slayman, a professor of cellular and molecular physiology at Yale, who also signed the Alpi letter (and who helped discredit Tompkins and Bird), was even more blunt.
Slayman has referred to the Alpi letter as,
Scientists seldom use such language when talking about their colleagues to a journalist, but this issue generates strong feelings, perhaps because it smudges the sharp line separating the animal kingdom from the plant kingdom.
The controversy is less about the remarkable discoveries of recent plant science than about how to interpret and name them:
No one I spoke to in the loose, interdisciplinary group of scientists working on plant intelligence claims that plants have telekinetic powers or feel emotions.
Nor does anyone believe that we will locate a walnut-shaped organ somewhere in plants which processes sensory data and directs plant behavior.
More likely, in the scientists' view, intelligence in plants resembles that exhibited in insect colonies, where it is thought to be an emergent property of a great many mindless individuals organized in a network.
Much of the research on plant intelligence has been inspired by the new science of networks, distributed computing, and swarm behavior, which has demonstrated some of the ways in which remarkably brainy behavior can emerge in the absence of actual brains.
Mancuso is perhaps the field's most impassioned spokesman for the plant point of view.
A slight, bearded Calabrian in his late forties, he comes across more like a humanities professor than like a scientist. When I visited him earlier this year at the International Laboratory of Plant Neurobiology, at the University of Florence, he told me that his conviction that humans grossly underestimate plants has its origins in a science-fiction story he remembers reading as a teen-ager.
(Mancuso subsequently wrote to say that
the story he recounted was actually a mangled recollection of an
early "Star Trek" episode called "Wink
of an Eye.")
For instance, since plants can't run away and frequently get eaten, it serves them well not to have any irreplaceable organs.
Indeed, many of the most impressive capabilities of plants can be traced to their unique existential predicament as beings rooted to the ground and therefore unable to pick up and move when they need something or when conditions turn unfavorable.
The "sessile life style," as plant biologists term it, calls for an extensive and nuanced understanding of one's immediate environment, since the plant has to find everything it needs, and has to defend itself, while remaining fixed in place.
A highly developed sensory apparatus is required to locate food and identify threats.
Plants have evolved between fifteen and twenty distinct senses, including analogues of our five:
In a recent experiment, Heidi Appel, a chemical ecologist at the University of Missouri, found that, when she played a recording of a caterpillar chomping a leaf for a plant that hadn't been touched, the sound primed the plant's genetic machinery to produce defense chemicals.
Another experiment, done
in Mancuso's lab and not yet published, found that plant roots would
seek out a buried pipe through which water was flowing even if the
exterior of the pipe was dry, which suggested that plants somehow
"hear" the sound of flowing water.
Many involved the root, or radicle, of young plants, which the Darwins demonstrated could sense light, moisture, gravity, pressure, and several other environmental qualities, and then determine the optimal trajectory for the root's growth.
The last sentence of Darwin's 1880 book, "The Power of Movement in Plants," has assumed scriptural authority for some plant neurobiologists:
Darwin was asking us to
think of the plant as a kind of upside-down animal, with its main
sensory organs and "brain" on the bottom, underground, and its
sexual organs on top.
Roots about to encounter an impenetrable obstacle or a toxic substance change course before they make contact with it. Roots can tell whether nearby roots are self or other and, if other, kin or stranger.
Normally, plants compete
for root space with strangers, but, when researchers put four
closely related Great Lakes sea-rocket plants (Cakile
edentula) in the same pot, the plants restrained their
usual competitive behaviors and shared resources.
Once the definition of "behavior" expands to include such things as a shift in the trajectory of a root, a reallocation of resources, or the emission of a powerful chemical, plants begin to look like much more active agents, responding to environmental cues in ways more subtle or adaptive than the word "instinct" would suggest.
These are sophisticated
behaviors, but, like most plant behaviors, to an animal they're
either invisible or really, really slow.
Unable to run away, plants deploy a complex molecular vocabulary to signal distress, deter or poison enemies, and recruit animals to perform various services for them.
A recent study in Science found that the caffeine produced by many plants may function not only as a defense chemical, as had previously been thought, but in some cases as a psychoactive drug in their nectar.
The caffeine encourages
bees to remember a particular plant and return to it, making them
more faithful and effective pollinators.
Sometimes this warning signal contains information about the identity of the insect, gleaned from the taste of its saliva.
Depending on the plant and the attacker, the defense might involve altering the leaf's flavor or texture, or producing toxins or other compounds that render the plant's flesh less digestible to herbivores.
When antelopes browse
acacia trees, the leaves produce tannins that make them unappetizing
and difficult to digest. When food is scarce and acacias are
overbrowsed, it has been reported, the trees produce sufficient
amounts of toxin to kill the animals.
Parasitic wasps some
distance away lock in on that scent, follow it to the afflicted
plant, and proceed to slowly destroy the caterpillars. Scientists
call these insects "plant bodyguards."
The first important discoveries in plant communication were made in the lab in the nineteen-eighties, by isolating plants and their chemical emissions in Plexiglas chambers, but Rick Karban, the U.C. Davis ecologist, and others have set themselves the messier task of studying how plants exchange chemical signals outdoors, in a natural setting.
Recently, I visited Karban's study plot at the University of California's Sagehen Creek Field Station, a few miles outside Truckee.
On a sun-flooded hillside
high in the Sierras, he introduced me to the ninety-nine
sagebrush
plants - low, slow-growing gray-green shrubs marked with plastic
flags - that he and his colleagues have kept under close
surveillance for more than a decade.
Karban believes that the plant is alerting all its leaves to the presence of a pest, but its neighbors pick up the signal, too, and gird themselves against attack.
He found that the more closely related the plants the more likely they are to respond to the chemical signal, suggesting that plants may display a form of kin recognition.
Helping out your
relatives is a good way to improve the odds that your genes will
survive.
Using clickers, they counted every trident-shaped leaf on every branch, and then counted and recorded every instance of leaf damage, one column for insect bites, another for disease.
At the top of the meadow, another collaborator, James Blande, a chemical ecologist from England, tied plastic bags around sagebrush stems and inflated the bags with filtered air. After waiting twenty minutes for the leaves to emit their volatiles, he pumped the air through a metal cylinder containing an absorbent material that collected the chemical emissions.
At the lab, a gas chromatograph-mass spectrometer would yield a list of the compounds collected - more than a hundred in all.
Blande offered to let me
put my nose in one of the bags; the air was powerfully aromatic,
with a scent closer to aftershave than to perfume. Gazing across the
meadow of sagebrush, I found it difficult to imagine the invisible
chemical chatter, including the calls of distress, going on all
around - or that these motionless plants were engaged in any kind of
"behavior" at all.
Jack Schultz, a chemical ecologist at the University of Missouri, who did some of the pioneering work on plant signaling in the early nineteen-eighties, is helping to develop a mechanical "nose" that, attached to a tractor and driven through a field, could help farmers identify plants under insect attack, allowing them to spray pesticides only when and where they are needed.
He added,
I first met Karban at a scientific meeting in Vancouver last July, when he presented a paper titled "Plant Communication and Kin Recognition in Sagebrush."
The meeting would have been the sixth gathering of the Society for Plant Neurobiology, if not for the fact that, under pressure from certain quarters of the 'scientific' establishment, the group's name had been changed four years earlier to the less provocative Society for Plant Signaling and Behavior.
The plant biologist Elizabeth Van Volkenburgh, of the University of Washington, who was one of the founders of the society, told me that the name had been changed after a lively internal debate; she felt that jettisoning "neurobiology" was probably for the best.
He said, and I quote,
Two of the society's
co-founders, Stefano Mancuso and František Baluška,
argued strenuously against the name change, and continue to use the
term "plant neurobiology" in their own work and in the names of
their labs.
Most of the papers were highly technical presentations on plant signaling - the kind of incremental science that takes place comfortably within the confines of an established scientific paradigm, which plant signaling has become.
But a handful of speakers presented work very much within
the new paradigm of plant intelligence, and they elicited strong
reactions.
Gagliano, who is tall, with long brown hair parted in the middle, based her experiment on a set of protocols commonly used to test learning in animals.
She focused on an elementary type of learning called "habituation," in which an experimental subject is taught to ignore an irrelevant stimulus.
How long does it take the animal to recognize that a stimulus is "rubbish," and then how long will it remember what it has learned?
Gagliano's experimental question was bracing:
Mimosa pudica, also called the "sensitive plant," is that rare plant species with a behavior so speedy and visible that animals can observe it; the Venus flytrap is another.
When the fernlike leaves of the mimosa are touched, they instantly fold up, presumably to frighten insects.
The mimosa also collapses its leaves when the plant is dropped or jostled. Gagliano potted fifty-six mimosa plants and rigged a system to drop them from a height of fifteen centimeters every five seconds. Each "training session" involved sixty drops.
She reported that some of the mimosas started to reopen their leaves after just four, five, or six drops, as if they had concluded that the stimulus could be safely ignored.
Was it just fatigue?
Apparently not: when the plants were shaken, they again closed up.
Gagliano reported that she retested her plants after a week and found that they continued to disregard the drop stimulus, indicating that they "remembered" what they had learned.
Even after twenty-eight days, the lesson had not been forgotten. She reminded her colleagues that, in similar experiments with bees, the insects forgot what they had learned after just forty-eight hours.
Gagliano concluded by suggesting that,
A lively exchange followed.
Someone objected that dropping a plant was not a relevant trigger, since that doesn't happen in nature. Gagliano pointed out that electric shock, an equally artificial trigger, is often used in animal-learning experiments.
Another
scientist suggested that perhaps her plants were not habituated,
just tuckered out. She argued that twenty-eight days would be plenty
of time to rebuild their energy reserves.
He explained that the word "learning" implied a brain and should be reserved for animals:
He was making a distinction between behavioral changes that occur within the lifetime of an organism and those which arise across generations.
At lunch, I sat with a Russian scientist, who was equally dismissive.
Later that afternoon, Gagliano seemed both stung by some of the reactions to her presentation and defiant.
Adaptation is far too slow a process to explain the behavior she had observed, she told me.
She noted that some of her plants learned faster than others, evidence that,
Many of the scientists in her audience were just getting used to the ideas of plant "behavior" and "memory" (terms that even Fred Sack said he was willing to accept).
Using words like "learning" and "intelligence" in plants struck them, in Sack's words, as "inappropriate" and "just weird."
When I described the experiment to Lincoln Taiz, he suggested the words "habituation" or "desensitization" would be more appropriate than "learning."
Gagliano said that her mimosa paper had been rejected by ten journals:
Instead, they balked at the language she used to describe the data.
But she didn't want to change it.
Rick Karban consoled Gagliano after her talk.
When I asked him what he thought of Gagliano's paper, he said,
Scientists are often uncomfortable talking about the role of metaphor and imagination in their work, yet scientific progress often depends on both.
"Plant neurobiology" is 'obviously' a metaphor - plants don't possess the type of excitable, communicative cells we call neurons.
Yet the introduction of the term has raised a series of questions and inspired a set of experiments that promise to deepen our understanding not only of plants but potentially also of brains.
If there are other ways of processing information, other kinds of cells and cell networks that can somehow give rise to intelligent behavior, then we may be more inclined to ask, with Mancuso,
Mancuso is the poet-philosopher of the movement, determined to win for plants the recognition they deserve and, perhaps, bring humans down a peg in the process.
His somewhat grandly named International Laboratory of Plant Neurobiology, a few miles outside Florence, occupies a modest suite of labs and offices in a low-slung modern building.
Here a handful of collaborators and graduate students work on the experiments Mancuso devises to test the intelligence of plants.
Giving a tour of the labs, he showed me maize plants, grown under lights, that were being taught to ignore shadows; a poplar sapling hooked up to a galvanometer to measure its response to air pollution; and a chamber in which a PTR-TOF machine - an advanced kind of mass spectrometer - continuously read all the volatiles emitted by a succession of plants, from poplars and tobacco plants to peppers and olive trees.
He estimates that a plant has three thousand chemicals in its vocabulary, while, he said with a smile,
Mancuso is fiercely devoted to plants - a scientist needs to "love" his subject in order to do it justice, he says.
He is also gentle and unassuming, even when what he is saying is outrageous. In the corner of his office sits a forlorn Ficus benjamina, or weeping fig, and on the walls are photographs of Mancuso in an astronaut's jumpsuit floating in the cabin of a zero-gravity aircraft.
He has collaborated with the European Space Agency, which has supported his research on plant behavior in micro- and hyper-gravity. (One of his experiments was carried on board the last flight of the space shuttle Endeavor, in May of 2011.)
A decade ago, Mancuso persuaded a
Florentine bank foundation to underwrite much of his research and
help launch the Society for Plant Neurobiology; his lab also
receives grants from the European Union.
Spending so much time with the plant neurobiologists, I could feel my grasp on the word getting less sure. It turns out that I am not alone:
Most definitions of intelligence fall into one of two categories.
Not surprisingly, the plant neurobiologists jump into this second camp.
In place of a brain,
In a flock, each bird has only to follow a few simple rules, such as maintaining a prescribed distance from its neighbor, yet the collective effect of a great many birds executing a simple algorithm is a complex and supremely well-coordinated behavior.
Mancuso's hypothesis is that something similar is at work in plants, with their thousands of root tips playing the role of the individual birds - gathering and assessing data from the environment and responding in local but coordinated ways that benefit the entire organism.
Plants have their own excitable cells, many of them in a region just behind the root tip.
Here Mancuso and his frequent collaborator, František Baluška, have detected unusually high levels of electrical activity and oxygen consumption. They've hypothesized in a series of papers that this so-called "transition zone" may be the locus of the "root brain" first proposed by Darwin.
The idea remains unproved and controversial.
How plants do what they do without a brain - what Anthony Trewavas has called their "mindless mastery" - raises questions about how our brains do what they do.
When I asked Mancuso about the function and location of memory in plants, he speculated about the possible role of calcium channels and other mechanisms, but then he reminded me that mystery still surrounds where and how our memories are stored:
The hypothesis that intelligent behavior in plants may be an emergent property of cells exchanging signals in a network might sound far-fetched, yet the way that intelligence emerges from a network of neurons may not be very different.
Most neuroscientists would agree that, while brains considered as a whole function as centralized command centers for most animals, within the brain there doesn't appear to be any command post; rather, one finds a leaderless network.
That sense we get when we think about what might
govern a plant - that there is no there there, no wizard behind the
curtain pulling the levers - may apply equally well to our brains.
Next came Darwin, who brought the humbling news that we are the product of the same natural laws that created animals.
In the last century, the formerly
sharp lines separating humans from animals - our monopolies on
language, reason, toolmaking, culture, even self-consciousness -
have been blurred, one after another, as science has granted these
capabilities to other animals.
Their project entails breaking down the walls between the kingdoms of plants and animals, and it is proceeding not only experiment by experiment but also word by word. Start with that slippery word "intelligence."
Particularly when there is no dominant definition (and when measurements of intelligence, such as I.Q., have been shown to be culturally biased), it is possible to define intelligence in a way that either reinforces the boundary between animals and plants (say, one that entails abstract thought) or undermines it.
Plant neurobiologists have chosen to define intelligence democratically, as an ability to solve problems or, more precisely, to respond adaptively to circumstances, including ones unforeseen in the genome.
We exist on a continuum with the acacia, the radish, and the bacterium.
I asked him why he thinks people have an easier time granting intelligence to computers than to plants.
(Fred Sack told me that he can abide the term "artificial intelligence," because the intelligence in this case is modified by the word "artificial," but not "plant intelligence." He offered no argument, except to say, "I'm in the majority in saying it's a little weird.")
Mancuso thinks we're willing to accept artificial intelligence because computers are our creations, and so reflect our own intelligence back at us.
They are also our dependents, unlike plants:
Our
dependence on plants breeds a contempt for them, Mancuso believes.
In his somewhat topsy-turvy view, plants "remind us of our
weakness."
We tend to think of memories as immaterial, but in animal brains some forms of memory involve the laying down of new connections in a network of neurons. Yet there are ways to store information biologically that don't require neurons.
Immune cells "remember" their experience of pathogens, and call on that memory in subsequent encounters. In plants, it has long been known that experiences such as stress can alter the molecular wrapping around the chromosomes; this, in turn, determines which genes will be silenced and which expressed.
This so-called "epigenetic" effect can persist and sometimes be passed down to offspring.
More recently, scientists have found that life
events such as trauma or starvation produce epigenetic changes in
animal brains (coding for high levels of cortisol, for example) that
are long-lasting and can also be passed down to offspring, a form of
memory much like that observed in plants.
At one point, he told me about the dodder vine, Cuscuta europaea, a parasitic white vine that winds itself around the stalk of another plant and sucks nourishment from it. A dodder vine will "choose" among several potential hosts, assessing, by scent, which offers the best potential nourishment.
Having selected a target, the vine then performs a kind of cost-benefit calculation before deciding exactly how many coils it should invest - the more nutrients in the victim, the more coils it deploys.
I asked Mancuso whether he was being literal or metaphorical in attributing intention to plants.
He swiveled his computer monitor around and clicked open a video.
But Mancuso's video seems to show that this bean plant "knows" exactly where the metal pole is long before it makes contact with it.
Mancuso speculates that the plant could be employing a form of
echolocation. There is some evidence that plants make low clicking
sounds as their cells elongate; it's possible that they can sense
the reflection of those sound waves bouncing off the metal pole.
As soon as contact is made, the plant appears to relax; its clenched leaves begin to flutter mildly. All this may be nothing more than an illusion of time-lapse photography.
Yet to
watch the video is to feel, momentarily, like one of the aliens in
Mancuso's formative science-fiction story, shown a window onto a
dimension of time in which these formerly inert beings come
astonishingly to life, seemingly conscious individuals with
intentions.
He began by questioning its value as scientific data:
The bean's behavior was, in other words, an anecdote, not a phenomenon.
Taiz also pointed out that the bean in the video was leaning toward the pole in the first frame. Mancuso then sent me another video with two perfectly upright bean plants that exhibited very similar behavior.
Taiz was now intrigued.
In this case, the stimulus remains unknown, but tropisms,
Perhaps the most troublesome and troubling word of all in thinking about plants is "consciousness."
If consciousness is defined as inward awareness of oneself experiencing reality - "the feeling of what happens," in the words of the neuroscientist Antonio Damasio - then we can (probably) safely conclude that plants don't possess it.
But if we define the term simply as the state of being awake and aware of one's environment - "online," as the neuroscientists say - then plants may qualify as conscious beings, at least according to Mancuso and Baluška.
In support of their contention that plants are conscious of their environment, Mancuso and Baluška point out that plants can be rendered unconscious by the same anesthetics that put animals out: drugs can induce in plants an unresponsive state resembling sleep. (A snoozing Venus flytrap won't notice an insect crossing its threshold.)
What's more, when plants are injured or stressed, they produce a chemical - ethylene - that works as an anesthetic on animals.
When I learned this startling fact from Baluška in Vancouver, I asked him, gingerly, if he meant to suggest that plants could feel pain. Baluška, who has a gruff mien and a large bullet-shaped head, raised one eyebrow and shot me a look that I took to mean he deemed my question impertinent or absurd.
But apparently not.
I must have shown some alarm.
Unprepared to consider the ethical implications of plant intelligence, I could feel my resistance to the whole idea stiffen.
Descartes, who believed that 'only humans' possessed self-consciousness, was unable to credit the idea that other animals could suffer from pain.
So he dismissed their screams and howls as mere reflexes, as meaningless physiological noise.
Lincoln Taiz has little patience for the notion of plant pain, questioning what, in the absence of a brain, would be doing the feeling.
He puts it succinctly:
Mancuso is more circumspect.
We can never determine with certainty whether plants feel pain or whether their perception of injury is sufficiently like that of animals to be called by the same word. (He and Baluška are careful to write of "plant-specific pain perception.")
Mancuso believes that, because plants are sensitive and intelligent beings, we are obliged to treat them with some degree of respect.
That means protecting their habitats from destruction and avoiding practices such as genetic manipulation, growing plants in monocultures, and training them in bonsai.
But it does not prevent us from eating them.
He cited their modular
structure and lack of irreplaceable organs in support of this view.
The question is as much philosophical as it is scientific, since the answer depends on how these terms get defined.
The proponents of plant intelligence argue that the traditional definitions of these terms are anthropocentric - a clever reply to the charges of anthropomorphism frequently thrown at them. Their attempt to broaden these definitions is made easier by the fact that the meanings of so many of these terms are up for grabs.
At the same time, since these words were originally created to describe animal attributes, we shouldn't be surprised at the awkward fit with plants.
It seems likely that,
if the plant neurobiologists were willing to add the prefix
"plant-specific" to intelligence and learning and memory and
consciousness (as Mancuso and Baluška are prepared to do in the case
of pain), then at least some of this "scientific controversy" might
evaporate.
Even Clifford Slayman, the Yale biologist who signed the 2007 letter dismissing plant neurobiology, is willing to acknowledge that, although he doesn't think plants possess intelligence, he does believe they are capable of "intelligent behavior," in the same way that bees and ants are.
In an e-mail exchange, Slayman made a point of underlining this distinction:
He defined "intelligent behavior" as,
Humans may or may
not be intrinsically more intelligent than cats, he wrote, but when
a cat is confronted with a mouse its behavior is likely to be
demonstrably more intelligent.
Seen that way, he added, the outlook of Mancuso and Trewavas is,
He pointed out that while it is an understandable human prejudice to favor the "nerve center" model, we also have a second, autonomic nervous system governing our digestive processes, which,
Brains are just one of nature's ways of getting complex jobs done, for dealing intelligently with the challenges presented by the environment.
But they are not the only way:
To define certain words in such a way as to bring plants and animals beneath the same semantic umbrella - whether of intelligence or intention or learning - is a philosophical choice with important consequences for how we see ourselves in nature.
Since "The Origin of Species," we have understood, at least intellectually, the continuities among life's kingdoms - that we are all cut from the same fabric of nature.
Yet our big brains, and perhaps our experience of inwardness, allow us to feel that we must be fundamentally different - suspended above nature and other species as if by some metaphysical "skyhook," to borrow a phrase from the philosopher Daniel Dennett.
Plant neurobiologists are intent on taking away our skyhook, completing the revolution that Darwin started but which remains - psychologically, at least - incomplete.
Upon a foundation of the simplest competences - such as the on-off switch in a computer, or the electrical and chemical signaling of a cell - can be built higher and higher competences until you wind up with something that looks very much like intelligence.
To say that higher competences such as intelligence, learning, and memory,
All species face the same existential challenges - obtaining food, defending themselves, reproducing - but under wildly varying circumstances, and so they have evolved wildly different tools in order to survive.
Brains come in handy for creatures that move around a lot; but they're a disadvantage for ones that are rooted in place. Impressive as it is to us, self-consciousness is just another tool for living, good for some jobs, unhelpful for others.
That
humans would rate this particular adaptation so highly is not
surprising, since it has been the shining destination of our long
evolutionary journey, along with the epiphenomenon of
self-consciousness that we call "free will."
I asked Taiz about the question of "choice," or decision-making, in plants, as when they must decide between two conflicting environmental signals - water and gravity, for example.
I asked Mancuso if he thought that a plant decides in the same way we might choose at a deli between a Reuben or lox and bagels.
But isn't the root responding simply to the net flow of certain chemicals?
One way to exalt plants is by demonstrating their animal-like capabilities.
But another way is to focus on all the things plants can do that we cannot.
Some scientists working on plant intelligence have questioned whether the "animal-centric" emphasis, along with the obsession with the term "neurobiology," has been a mistake and possibly an insult to the plants.
When I met Mancuso for dinner during the conference in Vancouver, he sounded very much like a plant scientist getting over a case of "brain envy" - what Taiz had suggested was motivating the plant neurologists.
If we could begin to understand plants on their own terms, he said,
How do plants do all the amazing things they do without brains? Without locomotion?
By focusing on the otherness of plants rather than on their likeness, Mancuso suggested, we stand to learn valuable things and develop important new technologies.
This was to be the theme of his presentation to the conference, the following morning, on what he called "bioinspiration."
Mancuso was about to begin a collaboration with a prominent computer scientist to design a plant-based computer, modeled on the distributed computing performed by thousands of roots processing a vast number of environmental variables.
His collaborator, Andrew Adamatzky, the director of the International Center of Unconventional Computing, at the University of the West of England, has worked extensively with slime molds, harnessing their maze-navigating and computational abilities.
(Adamatzky's slime molds, which are a kind of amoeba, grow in the direction of multiple food sources simultaneously, usually oat flakes, in the process computing and remembering the shortest distance between any two of them; he has used these organisms to model transportation networks.)
In an e-mail, Adamatzky said that, as a substrate for biological computing, plants offered both advantages and disadvantages over slime molds.
But because plants are already "analog electrical computers," trafficking in electrical inputs and outputs, he is hopeful that he and Mancuso will be able to harness them for computational tasks.
With a grant from the European Union's Future and Emerging Technologies program, their team is developing a "robotic root" that, using plastics that can elongate and then harden, will be able to slowly penetrate the soil, sense conditions, and alter its trajectory accordingly.
The most bracing part of Mancuso's talk on bioinspiration came when he discussed underground plant networks.
Citing the research of Suzanne Simard, a forest ecologist at the University of British Columbia, and her colleagues, Mancuso showed a slide depicting how trees in a forest organize themselves into far-flung networks, using the underground web of mycorrhizal fungi which connects their roots to exchange information and even goods.
This "wood-wide web," as the
title of one paper put it, allows scores of trees in a forest to
convey warnings of insect attacks, and also to deliver carbon,
nitrogen, and water to trees in need.
They injected fir trees with radioactive carbon isotopes, then followed the spread of the isotopes through the forest community using a variety of sensing methods, including a Geiger counter.
Within a few days, stores of radioactive carbon had been routed from tree to tree.
Every tree in
a plot thirty meters square was connected to the network; the oldest
trees functioned as hubs, some with as many as forty-seven
connections. The diagram of the forest network resembled an airline
route map.
The evergreen species will tide over the deciduous one when it has sugars to spare, and then call in the debt later in the season.
For the forest community, the value of this
cooperative underground economy appears to be better over-all
health, more total photosynthesis, and greater resilience in the
face of disturbance.
As I listened to Mancuso limn the marvels unfolding beneath our feet, it occurred to me that plants do have a secret life, and it is even stranger and more wonderful than the one described by Tompkins and Bird.
When most of us think of plants, to the extent that we think about plants at all, we think of them as old - holdovers from a simpler, pre-human evolutionary past.
But for Mancuso plants hold the key to a future that will be organized around systems and technologies that are networked, decentralized, modular, reiterated, redundant - and green, able to nourish themselves on light.
Or should be:
At dinner in Vancouver, Mancuso said,
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