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Book Report: My Recent Talks with Plants
— The first of three reviews...
Three books have come out recently that aim to summarize what we now know of plant senses and communication. All three ask the same question: What senses and attributes of communication do plants share with humans? Consider for example the chapters in Daniel Chamovitz’s What a Plant Knows: A Field Guide to the Senses: What a Plant Sees, What a Plant Smells, What a Plant Feels, What a Plant Hears, How a Plant Knows Where It Is, What a Plant Remembers, and The Aware Plant.
My first thought after reading that table of contents was, “Hoo boy, back to the Druids and communing with trees and shrubs.” There is inevitably some degree of reality-warping when we start attributing human characteristics to plants and animals. But Chamovitz’s book – to my mind the most research-based and science-literate of the three – admits this potential difficulty while setting forth a convincing explanation of his interpretation of plant senses and communication.
This field which some call “plant neurophysiology” was off to a slap-bang start with a 1983 paper published in Science magazine (the gold standard of science research news) which showed conclusive evidence that plants communicate information to each other. To simplify the substance of that study, it was known that plants produce toxic or repellent substances in their leaves in response to insect feeding. What the researchers discovered was that other leaves on the same plant that were not being chewed were also producing the defensive chemicals. And rather by accident, the scientists then found astonishingly that other plants, at some distance from the attacked plants and not at all attacked themselves, showed increased production of toxins. Plants not themselves attacked were receiving “warning messages” from the attacked plants, and protecting themselves in advance of invasion. These studies have since been replicated in the field and under controlled conditions in greenhouse laboratory settings. Attacked plants “tell” their neighbors of impending attack, and the uninfested plants take defensive action to protect themselves. This behavior has also been found in response to bacterial attack, viral attack, and a simple tearing of a leaf to simulate attack. The messaging agents have been found to be various out-gasses produced and emitted by infected plants. Even some of the chemical mechanisms that lead from initial alert to production of defensive chemicals have been identified. In Chamovitz’s view, the undamaged plant “smells” the danger alert from the damaged plant and “changes its behavior” to protect itself.
A more common example of plant smell, well-known to orchardists and vegetable growers, is the fruit-ripening response of many species to the gaseous emission of ethylene from other ripening fruit. As fruits ripen, they release ethylene, which is “smelled” by other fruits to hasten their own ripening. As ethylene also attracts animals, this may have clear seed-dispersal advantages too. (The parasitic plant dodder has been shown to “smell” preferred host species.)
So plants can smell. And we all know that plants can “see,” though we may not have described it in that way. Plant stems certainly perceive and respond to light and dark, typically bending toward light sources and away from darker areas. Roots typically grow away from light and toward the dark. Stems and roots see light and dark, and respond. The plant hormone auxin was identified decades ago as the chemical responsible for this behavior. Plants can also see and distinguish between different wavelengths of light, with specific responses to ultraviolet, blue, red, and infrared bands. And they measure how much light they have taken in, which is the key to a number of plant responses, including (in various species) flowering time, seed germination, the onset of dormancy, and circadian day/night cycles.
Plants also sense when they are being touched, they can “feel.” This is at its most obvious in such plants as the Venus fly-trap and the sensitive plant, both of which close their leaves in response to certain touching. Another common example is the twining behavior of many vine species, both stems and tendrils: “Vines from a burr cucumber can feel a string weighing only 0.009 ounce, which is enough to induce the vine to start winding itself around a nearby object.” This is ten times more sensitive to touch than a human being. Even touching or shaking a plant (induced by such things as people, insects, rain, and wind) inhibits growth, described by Chamovitz as “a general phenomenon in plant biology.” So plants can feel mechanical stimulation, and respond. The mechanism of response is an electrical signal, which can move from cell to cell within a plant.
It’s long been known that plants “know where they are,” by which the author means they sense up and down. Plants sense gravity, resulting in cotyledons and stems that grow up, and roots that grow down. Sensors are in the root tips and in stem endodermis in the form of what I’ll call “gravity balls” that roll around and gather on the gravity side of the plant, stimulating release of auxin which then triggers plant response. (Boy, am I condensing here…)
Can plants hear? In the ’60s and ’70s, pseudo-scientists claimed plant response to exposure to Bach cantatas (growth increase) and Jimi Hendrix’s Purple Haze at high volume (growth inhibition). Credible research has since established that plants have no apparent response to sound, at whatever level of “high” and volume.
Chamovitz sets aside one of his final chapters to discuss “what a plant remembers.” If memory consists of forming a memory, retaining it, and then recalling it, plants remember. A simple example is vernalization (= cold treatment): Many plants need a cold period in order to germinate, or to flower, or to renew vegetative growth in the spring. Many seeds, for example, “remember” not to germinate in the fall, even though environmental conditions are favorable, until they have undergone a cold treatment (winter); they germinate in the spring, the time most favorable for success. Likewise, spring-flowering plants “remember” not to flower in the fall, even though the flower buds are set; they wait until spring, after the cold treatment. (Occasionally an unusually mild and late fall will override this memory.) A different and very specific example of memory is the Venus fly-trap snapping shut: This is triggered by an insect touching one of three sense-hairs, then touching another sense-hair within twenty seconds. If the insect doesn’t touch the second hair, or touches it in thirty seconds, the trap is not sprung. The plant remembers that one hair was touched, and waits twenty seconds for the second touch; if neither of those things happens, the plant re-sets, and forgets the memory. An even more astonishing example of plant memory involves memory across generations, independent of DNA coding. Recent research has shown that stressed plants sometimes pass along resistance to that stress in successive generations. For a more detailed explanation of how that happens, read the book and a mountain of research papers and let me know what you make of it all.
So having established that plants see, smell, feel, know where they are, and remember, Chamovitz asks, “Are plants aware?” He concludes that “…in fact, they are. Plants are acutely aware of the world around them. They are aware of their visual environment; they differentiate between red, blue, far-red, and UV lights and respond accordingly. They are aware of aromas surrounding them and respond to minute quantities of volatile compounds wafting in the air. Plants know when they are being touched and can distinguish different touches. They are aware of gravity: they can change their shapes to ensure that shoots grow up and roots grow down. And plants are aware of their past: they remember past infections and the conditions they’ve weathered and then modify their current physiology based on these memories.”
Fascinating book: What a Plant Knows: A Field Guide to the Senses, by Daniel Chamovitz. 2012. (NY: Scientific American/Farrar, Straus and Giroux). Next up: The Hidden Life of Trees: What They Feel, How They Communicate, by Peter Wohlleben. 2015. (Vancouver/Berkeley: Greystone Books); and Brilliant Green: The Surprising History and Science of Plant Intelligence, by Stefano Mancuso and Alessandra Viola. 2015. (Washington/Covelo/London: Island Press).P
—Dr. Dirt thinks he’s getting smart now, but isn’t so sure about John Hart, dba Environments, Durham NH.