THE ROOT OF THE MATTER
Michael Pollan reports on a debate over whether plants have intelligence; they don’t (“The Intelligent Plant,” December 23rd & 30th). The property being ascribed to plants as intelligence is shared by all organisms, even the single-celled amoeba. Life, like obscenity, is easy to recognize but hard to define. Still, we can identify general attributes. Growth is one, replication another. But these are insufficient: a crystal that grows, splits in half, and grows more is not alive. Organisms constantly sense their environment and adjust their conditions to maintain some kind of optimality. This is called acclimation (or, over the long term, adaptation) and represents a fundamental attribute of the living state. Intelligence makes humans superb acclimators. It depends on nerves, cells with highly ramified shapes that are deeply interconnected. There is nothing remotely like this in plants. Furthermore, human intelligence arose not only from making decisions among sensory inputs but also from managing musculature. Even though our intelligence today dwells in the aerie of abstraction, and choreography of movement has been outsourced within the brain, the history of being a locomotive engineer permeates the mind’s workings. Plants have no muscles and no requirement for a central coördinator of motion. Plants do move, and powerfully, but on a scale that allows management by distributed cells. In those responses, any resemblance to our intelligence is superficial, like that of a mandrake root to a person. The purpose of acclimation and intelligence is similar, but the means differ; it is the means that defines phenomena in biology.
Professor, Biology Department
University of Massachussetts
I applaud the attention that Pollan pays to botany in his article, but he failed to mention that not all plants live rooted in one place. Alternatives to this “sessile life style” are clear among tropical species and are much more analogous to animals than the examples he provides. Certain fig trees can “walk” on their stilt roots to extract themselves from an overly shady spot, or escape from under a tree that has fallen on them. (In an instance of comedy reflecting scientific discovery, Monty Python riffed on a strikingly similar idea when they made fun of David Attenborough in a 1974 sketch called “The Walking Tree of Dahomey.”) Even more spectacular are species I have termed “nomadic vines,” which wander a distance far in excess of their relatively unchanging length. One such nomad is the philodendron in your dentist’s waiting room. In a rain forest, some philodendrons remain a few yards in length and yet they meander through a tree’s canopy, acting like a snake searching for a place to bask. The plant grows small leaves and long thin stems to move quickly when in shade, changing over to the thick stems and large leaves when it reaches a patch of sunlight. Both walking trees and nomads change locations without muscles. Taking advantage of the “modular” strategy Pollan describes, they grow in the direction of motion while leaving their trailing parts to die. The results are fantastical: when a canopy-dwelling plant, such as an orchid, falls from a tree, it’s likely to perish in the understory shade. But the philodendron simply uncoils itself, crawls over to the nearest tree trunk, and climbs up again.
Mark W. Moffett
National Museum of Natural History