Science news at the speed of thought

Peas and quiet

by Leonard Finkleman Story Source May 13, 2012

Dateline: 1968. Cleve Backster, inventor of the polygraph, attaches lie detectors to some house plants and proceeds to yell at them. When his polygraphs register responses from the plants, Backster publishes a paper in the International Journal of Parapsychology (second only to “Weekly World News” in its academic rigor, I’d imagine) declaring that plants have perceptions and feelings. Thus do I have to waste at least fifteen minutes each semester explaining to students why plants don’t factor into utilitarian calculations. Thanks, Cleve.

Forty-four years later, the New York Times publishes an essay by philosopher Michael Marder in its “Stone” opinion column wherein the author insinuates that peas can ‘talk.’ Wonderful: there goes another fifteen minutes out of my virtue ethics lecture (1).

In the decade following its publication, Backster’s research into plant “primary perception” was very thoroughly (if not shockingly) debunked. Nevertheless, Marder helps himself to some suspiciously similar ideas towards the end of arguing against a clear moral distinction between eating plants and eating animals (2). The impetus for his argument is a recent finding (by Falik et al.) indicating that pea plants share stress-induced chemical signals through their root systems, thus triggering defensive responses in unstressed plants. Also, didn’t you see that adorable little peas-in-a-pod doll in Toy Story 3? Put down that can of pea soup, you monster.

I will admit that last part was a rhetorical flourish (Marder’s essay never mentions Toy Story 3, the latter of which has a more plausible narrative). Rhetoric is a dangerous tool when used towards ill effect, either intentionally or otherwise. It’s only fair, then, that we should look at some of the rhetoric at work in this latest round of pea-hugging.

Summarizing the original research, Marder describes plants as capable of “processing, remembering, and sharing information,” able to “draw on their ‘memories’” and engage in “basic learning.” Going back to the journal article to which he refers, we find that peas “eavesdrop” on their neighbors “in ways that have traditionally been attributed to higher organisms” (3). Can anyone be blamed for concluding, as Marder does, that “plants are more complex organisms than previously thought” in ways reminiscent of Backster’s “primary perception” research?

Honestly: no. That plants employ complex signaling systems and information storage mechanisms goes against many of our intuitions regarding the distinction between kingdoms Plantae and Animalia, from which derives Aristotle’s claim that animal telos includes sense perception where plant telos does not. This research is surprising in that respect, so it stands to reason that our intuitions should be adjusted in light of this surprising data.

Of course, the same idea was “unexpected” in 2008, when scientists at the National Center for Atmospheric Research found that walnut trees emit chemical signals that induce stress responses in their neighbors. It was also unexpected in 1990, when E. E. Farmer and C. A. Ryan found that tomato vines engage in “interplant communication.” It was probably also unexpected in 1935, when it was found that the concurrent ripening of all the fruit in a basket may be induced by a single fruit’s secretion of the gaseous aging hormone ethylene. This of course begs the question of why our intuitions need to be adjusted now, but didn’t eight decades ago.

As we say in this ‘biz, one man’s modus ponens is another man’s modus tollens (4). If chemical signaling in plants warrants re-evaluation of our moral attitudes towards plants, then such a re-evaluation would have been appropriate in 1935. But it wasn’t appropriate in 1935, so chemical signaling shouldn’t warrant any change in ethical attitudes now. Adam Kolber summarizes what I take to be the appropriate response to Marder’s moral argument here.

Still, the language used in Marder’s essay — to say nothing of the research paper that inspired it — is awfully suggestive, isn’t it? You wouldn’t want to eat something capable of basic learning and memory, would you? But that’s the real problem: plants aren’t really capable of any of those things. Only a crackpot (or an editor of the International Journal of Parapsychology, it seems) would suggest that peas actually talk or learn or remember in the sense that a human, or even a puppy, talks and learns and remembers (and, to be fair, Marder seems to admit as much). Chemical signaling in plants may resemble those activities in some important ways, and so we can use the terms “talk” and “remember” and “learn” to draw analogies with familiar concepts. The danger in drawing such analogies, and the fallacy in Marder’s moral argument, lies in overextending those analogies.

Every analogy has a breaking point. Life is like a box of chocolates in that it’s far more expensive for some people when Valentine’s Day rolls around; however, life is unlikely to have originated in Kansas City, Missouri. Similarly, animal communication and plant chemical signaling have in common the basic properties of signaling systems; however, those similarities only extend so far, and necessarily end where the animal nervous system comes into play.

Plants don’t have nerve cells, much less centralized clusters of those cells. Consequently, plants “talk” to each other only in the same strained way that we might claim that water communicates its deep-seated hatred to oil by keeping away from it. These are very basic chains of cause and effect determined entirely by fundamental physical properties of the signaler and the receiver. Once we throw a central nervous system in between the signaler and the receiver, things become a hell of a lot more complicated: we then have to consider questions of plasticity and cognition (5). And this is just for animals generally; we haven’t yet said anything about how fundamentally different animal cognition may be from human cognition. Any analogy between plant and animal communication must be very limited indeed.

I don’t mean to suggest that Marder or the authors of the original research paper are being disingenuous. We’re dealing with some high-falutin’ concepts here, and we only have so many words to work with. Referring to chemical signaling between pea root systems as “talking” may be strictly incorrect, but the reference does convey an essential property of the signaling system in a quick, close-enough sense. There’s certainly philosophical precedent for using everyday terms to refer to less-familiar scientific concepts.

But we have to be careful: science isn’t in the business of confirming our intuitions, and so our everyday terms may not be adequate to capture the weird, wild, wondrous things that science finds. It’s fascinating that peas can send warning signals to one another, but that doesn’t mean that those peas are talking, even if “talk” is the term that most easily describes what the peas are doing (6). Once we lose track of our linguistic limits, we fall prey to the dangers of rhetoric.

So don’t worry about peas. They don’t have any good feelings for you, but they’re certainly not talking about you behind your back, either.

(1) In developing his virtue theory, Aristotle employs the method of logical division in order to determine the unique human function. Rationality separates humans from other animals, but even before that cognitive activity must separate animals from other living things, i.e., plants. Of all the shocks that I’ve received in my years of teaching, one of the greatest is my students’ consistent opposition to that latter claim. Hell, the Mythbusters even covered it!

(2) The point of this essay isn’t to argue for or against any particular dietary choice, but I will disclose my own preferences: I’ve recently adopted vegetarianism, much to the dismay of my hamburger-loving tastebuds. Nevertheless, I don’t see anything wrong with eating meat per se; instead, the problems I see are with the production and distribution of most meat and the epistemic complications that arise from finding the meat that is ethically produced and distributed. Since I’m not willing to raise and slaughter my own livestock, vegetarianism seems the safest way to sleep soundly at night.

(3) I live according to a number of principles, including the following: always cock your eyebrow at a modern biologist who uses the phrase “higher organisms.” You’re in trouble as a biologist if both Stephen Jay Gould and Richard Dawkins would write dismissive essays about your ideas.

(4) Some people envision philosophers as socially awkward nerds. Can you believe it?

(5) You might argue that even a central nervous system is entirely determined by fundamental physical properties, but our illustrious host might have some words for you if you do.

(6) Obligatory dinosaur reference: it’s awesome, in a “grade-school-level-hilarious” kind of way, that sauropod flatulence may have contributed to an increase in global temperatures 150 million years ago, but that increase in global temperatures isn’t the same thing as the climate change phenomena we face today.