The story, in essence, is that plants do things that strike a lot of people as intelligent. Even people who are unwilling to use that word are willing to admit that there may be more to plant behavior than has previously been thought. The argument, gaining prominence in a 2006 paper in Trends in Plant Science that proposed a new field called plant neurobiology (according to Pollan, rather recklessly), is that plants respond to so many environmental conditions in coordinated ways that it looks suspiciously as though there is some sort of central processor that does the coordinating.
There is increasing evidence that these responses are more than simple, local chemical reactions. And that, indeed, some are homologous to responses in animals and even involve some of the same neurotransmitters. Many plant biologists are still reluctant to think in terms of plant neurobiology, however, primarily because plants don't have neurons. Or brains.
Indeed, even proponents don't argue that there's a heretofore unrecognized central nervous system or processor hidden somewhere inside the plant, but they do argue that the chemical signaling that goes on can be systemic, and that a plant's whole system of root tips may, as Pollan writes, be capable of "gathering and assessing data from the environment and responding in local but coördinated ways that benefit the entire organism."
Excitable cells just behind the root tip have been found to have high levels of electrical activity and oxygen consumption, and this may be in some way the equivalent of a brain in the sense of information-processing. Pollan writes,
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: “It could be the same kind of machinery, and figuring it out in plants may help us figure it out in humans.”
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.And, indeed, brains are just chemical and electrical responses to the environment, too. Somehow, in aggregate, this processing leads to 'mind' in at least some animals.
Here's a video, narrated by Pollan, showing an example of one of the behaviors that plant behavior people are trying to make sense of. The bean plants seem to be striving to find the pole that has been planted just outside their reach. Pollan makes the important point that plant behavior happens on a much slower time scale than we can observe, and that time lapse photography is what has made this new view of their behavior possible. Images of these growing plants were taken every 10 minutes, which makes it possible for us to see -- or imagine -- that the plant is striving to reach a goal.
Roots have also been found to seek out underground water pipes, even when the exterior of the pipe is dry, suggesting to researchers that the roots are somehow responding to the sound of flowing water.
The whole underground world of intertwined, communicating roots seems to be one that we've only just started to understand. Roots can distinguish self from other, and they know their own kind (species), they can share resources and information about insect attacks and deliver nutrients to trees in need. The preponderance of evidence does seem to suggest that plants are proactive in filling their own needs and the needs of others.
Pollan's beautifully written piece raises many intriguing questions and issues, and certainly not only about plants. Among them, what is intelligence? What is is a brain? What is an organism? We've long thought of plants in the same way we think about animals, as in competition for sun and soil nutrients, but now it seems that there's a lot of cooperation going on, and we even must ask whether plants, too, can be altruistic. And, the whole group selection question, whether an organism can evolve to behave for the good of the group rather than itself alone, will probably need to be revisited.
Plant intelligence is something we've touched on in the past. Of course, an over-riding issue for many biologists is the mysterious origin and nature of consciousness, which is not the same as neural information processing or problem-solving. We have in one of our books mused about whether there is some sort of organismal awareness in a plant or among plants, for example, whether the totality of a tree's leaves constitutes an analog of a retina in an animal eye, giving an overall 'image' of the light environment. But one has to be careful about terms and wording, and not to be suggesting (without a lot more evidence than we know of today) that plants have any sort of awareness that we would recognize as similar to our own experiences.
And finally, here are three odes to the carrot that we posted back in 2011. It seems like a good time to reprise them here. Again we thank Gary Greene, a poetically-inclined MT reader, for his poem, his permission to post it here, and the inspiration it gave us to write our own.
By Gary Greene
Carrots that feel, beets that pine,
Are they rooted in awareness,
These veggies of mine?
When arrayed at the store,
Is it dirt that they dish?
Can onions and potatoes
Make a vegetable wish?
Would they choose life in the ground,
Until they lay rotten?
Or to be served up in bowls,
Sauteed or au gratin?
Do they think deeper thoughts,
the deeper they grow?
Do they plan for the future,
or just wait for snow?
When the frost comes early,
do they think warmer things?
And at night, if they dream,
take flight on veggie wings?
We may never know
if veggies are aware,
if they think lofty thoughts,
or simply don't care.
It's so hard to tell,
there's nothing to do,
but pass the green peas,
and the turnip greens, too!
By Ken Weiss
A CARROT IS A THOUGHTFUL THING!
A carrot is a thoughtful thing,
That worries what the rain will bring.
With all its neighbors, sore it grieves,
The gruesome gnawing on its leaves.
Of peas we could the same relate,
And from the lettuce, no debate.
The Fall's last katydid doth moan,
Unanswered calls: it's all alone.
These plaintive cries unnoticed, all,
By humans: blinded to the pall,
And hearing naught, can naught believe.
So, voiceless, plants no pathos leave.
We spare no thoughts for 'thoughtless' being,
Assuming only we are seeing,
Yet trees observe the sun all day,
With leafy retinal display.
We grant no ‘self’ to fish nor fowl,
Hard-wired deem the lions growl.
Thus poems are writ by fools like me,
Who can’t converse with ant, or tree.
By Anne Buchanan
I just hope
carrots don't dream.
But if they do
I hope they don't
to run away
Legs and hips and feet,
in our image.
And arms to raise themselves
from the earth.
were made in
not only would
feel fear, but
have to confront
the ethical dilemma
to eat themselves.