Plant Brains

Some of you might have seen Michael Pollan’s recent piece on Plant Neurobiology in the New Yorker. If not, you might be able to read it here:

Here is a response I sent him:

Dear Michael Pollan,

I read zealously your recent article in The New Yorker on plant neurobiology. Hardly an issue goes by without a medical dispatch and yours is the only article I can remember reading about plant science. This disparity is perfectly reflected in funding where the summed budget of NSF, USDA, and DOE for basic research on plants is but a grain of barley compared to the silo of the NIH for research on animals. And yet, as you wrote, the world is covered by plants, we depend on plants for our survival, and they are fascinating. Any attention your article brings to plant science is thus welcome.

Further, I commend your command of the discipline. My intent here is not to carp. Rather the opposite, because of your understanding, I thought it would be fun to discuss these matters a little further or, at least, to put to you a couple of points.

I start with the mandrake root. This root resembles a person and the resemblance is uncanny. Ages ago, this resemblance was taken as demonstrating the root’s beneficial power, and mandrake was prized in drug stores of the day. (Actually, this could be a fable; I have never seen a mandrake root, for all I know it looks like a lobster.) Today, a scientist would consider that resemblance a coincidence, nothing more, because there is no connection known between the shape of the root and the substance of the person. Analogies are attractive but we need to be careful. I suppose, the condition of many an ill squire was aggravated by munching mandrake.

In your article, you describe the view that an organism’s ability to integrate a diverse set of inputs and pick an output is to be called intelligence. Instead, I see this as being required for any thing to live. What is a functional definition of a living thing? Life, like obscenity, is easy to recognize but difficult to define. Still, we can identify attributes that seem relevant. Growth is one, replication is another. But a crystal can grow, only to split in half and then continue to grow. Few would consider such a proliferating mineral alive. In addition, there needs to be something like the aforesaid intelligence.

Both animals and refrigerators can maintain a constant internal temperature but if the door comes off the fridge the contents are going to spoil. An animal does (or tries to do) the equivalent of building a new door. All living organisms constantly sense their environment and adjust their conditions to maintain some kind of optimality. When done on a time scale of one generation, it is called acclimation; on a longer time scales, adaptation. It is not only multi-cellular organisms that do this as you discussed, single-celled organisms do it too. For lack of a better phrase, I will call this “the ability to restore homeostasis”. Not only is the restoration of homeostasis typical of living things, it is impossible for me to imagine a living thing that lacks this property. This is why, in the popular science fiction story, we know the computer has come alive exactly at the moment when it prevents the human from pulling out its plug.

We could call this restoration of homeostasis ‘intelligence’. It would be easier to say and it seems like a smart thing to do. Indeed, homeostasis is restored superlatively by our mental processes, our consciousness and cogitations. Human intelligence allows us to anticipate what the world is going to do and thereby to prevent the door’s coming off the fridge at all. This tends to be easier than rebuilding. But to suggest that this is all one happy continuum from Einstein to euglena is to fall for the mandrake root: What looks like a person has nothing do with a person. The output (restoration of homeostasis) is similar but the mechanisms differ.

Intelligence in humans is the product of nerves. Plants have nothing remotely like nerve cells. Most nerve cells have hugely convoluted shapes, formed from miles and miles (well, millimeters and millimeters) of axon and dendrite projecting out from the cell body. These projections allow one cell to contact myriad others, some nearby, others far away, and build a network. By contrast, plant cells contact their own immediate neighbors, period. In ganglia of flies and squid (organisms with an economy of nerve cells), one can identify a neuron with some baroque shape and find that same neuron with that same shape in every individual of that species. Neighbor neurons have their own elaborate but specific and reproducible shapes. These cells embody the connections (and internal chemistry) required for the pathway to function; and in addition, they exemplify differentiation down to the level of the single cell. We know of no comparable deep differentiation among cells in plants.

Human intelligence arose not only from making decisions among sensory inputs but also from activating locomotion. Animals have hundreds of muscles and even a simple task like taking a walk requires a nearly fabulous organization of outputs. Human intelligence cut its eye teeth on the demands of managing musculature. Even though today our intelligence dwells in the aerie realm of abstraction and the choreography of our movement has been outsourced within the brain, the history of being a locomotive engineer permeates the workings of mind. That is how it evolved. Plants have no system of dozens or hundreds of interconnected muscles. They have no requirement for a central coordinator of dizzying motion. Plants do move, and powerfully so, but on a scale that allows management by distributed systems. The ends might be similar but the means differ; but it is the means that define biological phenomena.

For these reasons I think that intelligence is unique to brains. If we call something similar in a machine ‘artificial intelligence’, I suppose we could call the capacity to restore homeostasis ‘biological intelligence’. I’d pass on the latter term because intelligence is not the same thing as computation. When a bean plant adds up its sensory environment and decides to go that way, it has performed a computation but you would be prescribing mandrake root to say the bean plant ‘thought it over’.

I will go even further and say that intelligence is something unique to the brains of humans. And now we come to the last point I seek to make in this already long letter (I am an academic, sorry). Writing Histories of Increasing Humiliation is motivated, at least partly, by recognizing human abuses and by wanting to atone. Assertions of human uniqueness have been used to justify one atrocity after another. But every assertion has a premise as well as a conclusion. Returning to the middle ages, it was reasoned that man’s sphere of influence was properly out in the world whereas woman’s was indoors because the man’s genitals protruded outwards while the woman’s withdrew inwards. Today, happily, that argument would be ridiculed but not because the schoolmen were mistaken about the positions of the genitals. The falsity of the argument does not imply the falsity of the premise. We are, or should be, humiliated by what has been done in the name of human uniqueness, but that does mean we are not unique. Rather, it means humans can reason fallaciously as well as intelligently.

Looking at the spread and sway of human culture, the fruit of intelligence, over the last few hundred (or even thousand) years, it is difficult to escape the conclusion that something unique is going on. How many millions of years did it take for cyanobacteria to fill the air with oxygen? Crows can use sticks and dogs wag their tails in happy greeting; but a continuum of mental states does not demonstrate shared intelligence. A glass of water at room temperature has a continuum of energy levels–a few molecules evaporate–but the water is not boiling. Nowadays patients are given i.v. sedation for certain minor surgical procedures because, so sedated, the patient can hear and obey commands (roll over, play dead, etc.). Having been such a patient, I can vouch for the perceived experience being indistinguishable from sleep. I suppose that animal intelligence is like a human under i.v. sedation and there is an insurmountable gap between what we experience under the needle and when we are aware. I argue further that the arrival of human intelligence represents as great a change within the biosphere as the origin of the first living cell—a kind of phase-transition; however, I accept that the latter is an extreme view.

Nevertheless, we should not have any qualms, in principle, with the idea that a given species has evolved a unique capacity. This is how evolution works. Nothing forbids human intelligence from being unique and looking around the joint, nothing seems similar enough to merit being called by the same word. I emphasize again that I make this argument because it seems congruent to biological reality. How we view our place in nature should not depend on whether some capacity or other of ours happens to be unique. Whatever we think of its intelligence, we are wrong to find amusement by pulling the wings off a fly.

In summary then, plant neurobiology is misguided because on the one hand plants lack nerves and on the other the phenomenon that group calls intelligence is a property shared by all living things and carried out in distinct ways. And, just as it is wrong to pump up humanity’s chest with bogus claims, so too is it wrong to deny our actual attributes. The Enlightenment was perhaps the first time when human intelligence was proposed to replace god as providing the framework for society. Flawed though they are, our minds provide our surest way forward. We should celebrate them.

Celebration, an appropriate note on which to conclude. I send wishes for  hearty holiday festivities. Thank you for your attention.


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