Had a good chat with your houseplant today?

 kw: book reviews, nonfiction, science, botany, research, plant consciousness, communication, signaling, plant movement, plant intelligence

In the human realm, "talking to the animals" like Dr. Doolittle is fictional. In the plant realm, it may be commonplace. What constitutes "communication"? There is more philosophy than science wrapped up in any attempt to answer that question. Even more so for the words "consciousness" and "intelligence". We may not have definitive answers in the next few decades, and perhaps we never will. In The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth, author Zoë Schlanger doesn't provide the answers, though some of those she interviewed offered nascent attempts at doing so.

The eleven chapters of Light Eaters delve into several strains of research that are on the verge of redefining what a "plant" really is and re-settling our understanding of the rôles plants play in the biosphere. As we find from numerous lines of research, plants have several routes of plant-to-plant signaling: chemical, electrical, acoustic, and possibly bacterio-genetic. Plants discriminate. They are found to send differing signals to siblings versus non-siblings of the same species; plants of one species can also "eavesdrop" on signals of another species. Furthermore, plants send signals intended for animal species! An example of the latter is the plants that emit a pheromone that attracts a certain species of parasitic wasp when a caterpillar that the wasp parasitizes begins chewing on the plant's leaves. It's rather like a youngster who gets attacked and calls on his older brother for help, but in this case the "older brother" is a different species. Plants getting chewed on also emit other volatile chemicals that alert nearby plants, which respond by altering the chemistry of their leaves to be distasteful or even toxic to the caterpillar.

These are examples of chemical signaling. Other stressors such as drought result in plants making tiny clicking noises as low-pressure bubbles collapse; it is similar to the popping knuckles most people engage in. It's hard for me to determine what kind of research showed other plants responding to these barely audible sounds, but Chapter 5, "An Ear to the Ground" presents the evidence. 

What about electrical signals? Within a plant, it has been found that cutting a leaf initiates a wave of electrical activity that sweeps through the plant. These images of a small plant leaf, taken just before a scissor cut, then one second after, and then seven more seconds later. The plant had been grown from seed containing engineered genes that cause the calcium channels (every cell has them) to trigger Green Fluorescent Protein (GFP) when they emit or pass an electrical signal. The electrochemical signal moves as a wave through the whole plant. These images were clipped from this video. If the video doesn't work (they can be ephemeral), search for "gfp plant signaling". This is just one of several.

As the narrator in the video explains, the signal moves through the whole plant in about a minute along the veins, and spreads from the veins throughout all the plant's tissues at a slower rate.

This got me thinking. We know that while an electrical signal in a metallic wire is very fast, roughly the speed of light, the electrochemical signals in animal nerves are much slower. I had the neural conduction speed measured in my arm one time, after an injury. It was 60 m/s, which is normal. The fastest neural conduction speed in mammals is about twice this, and some nerves, where speed is less critical, are as slow as the range 2-5 m/s. Plants don't have nerves; at least none that we can recognize. But the veins seem to have a similar function, albeit slower, in the range of about 1 mm/s. That is between 2,000 and 120,000 times slower than animal neurons.

Put that together with a statement later in the book. The author had a hint of an idea (one I was toying with as I read): What if we think of the entire plant as a brain? She asked one scientist, who said, "I think you're right. I just don't talk about it." Let's speculate a bit. If you get jabbed in the leg with a pin, you'll react within about a quarter of a second. In the little plant shown in the video, which is about 10 cm across, the signal "I've been cut!" reaches the whole plant in less than two minutes. The "reaction" of the plant is to begin to synthesize noxious chemicals in the leaves, which takes a few hours. From this we can extract a couple of ratios:

1. We can infer the signaling time between your leg and brain as about 1/30 second. If signaling through the plant took 100 seconds, the ratio is 3,000:1.
2. Your physical flinch and "Ouch!" begin after about 1/4 second, while chemical synthesis in the plant gets underway in an hour (3,600 sec), for a ratio of 14,400:1.

If, then, the whole plant is, or contains, a distributed brain, it runs several thousand times more slowly than an animal brain. This is in accord with the rate that twigs grow on many woody plants, compared with the rate of animal motions. Animals move at about "the speed of gravity", by which I mean that rapid animal motions, such as swatting at a fly, happen at speeds similar to that of an object dropped a meter or so. Time-lapse videos of plants either growing or "doing" various things, such as the "reaching" of bean tendrils for something to cling to, show their motions to be hundreds to thousands of times slower than animal motions. It seems plausible that, if plants "think", they do so correspondingly slowly. While we cannot consider plants to have a nervous system, perhaps a term such as "signal conduction system" or "signal transduction system" can be used.

Do they think? Plant "intelligence" has been a fiercely contentious issue for decades, and that doesn't seem to be slowing down. Focusing on just three things: speed of motion, speed of communication, and speed of reaction, I (and, I think, Ms Schlanger) consider plants to be doing most things animals do, but on a time scale around 10,000 times slower. If we learn to talk to plants, and to hearken and understand what they are saying, we'll need enormous patience. Perhaps a translating SI (simulated intelligence) application can craft a signal at a rate the plant can accept, patiently receive its reply, and signal a human (who is doing something else in the meantime, because it could be hours) to come "read" the response. Even if the human then requires several minutes to decide what to say next, to the plant, the signal coming back, through the app, seems to begin almost instantly.

Finally, do plants see? Plants that mimic neighboring species hint that this is so. How can a South American vine Boquila take on the appearance of at least a few dozen other plants, just by growing in the vicinity? Moreso, if part of this vine is near one kind of plant, and another part is near another, it mimics both! To a lesser extent Mistletoe plants do something similar. One researcher believes the "signal" received by a Boquila plant is not visual, but bacterio-genetic, some kind of genetic signal from the neighboring plant's cloud of symbiont bacteria. All animals and all plants are inhabited by and surrounded by their own microbiome. Each breath we exhale contains members of our microbiome. Your own bacterial "envelope" changes every time you make a new friend and begin spending lots of time with him or her. The author finds a visual hypothesis more parsimonious, and I agree. Plants do have photoreceptors; they are chloroplasts. There are also other colored bodies in plants, in colors other than green. They may also receive light as well as reflect it, or they may provide color filters for chloroplasts to detect colored light. The author points out that this is similar to cuttlefish, which have color-blind eyes, yet they can still mimic the patterns and colors of the surface they are sitting on, probably because their whole skin surface is covered with photoreceptors that must provide the color signal.

I suggest a "red hat" experiment. Start with a number (12 at least) of plants that are wired to detect stress. Once they have recovered from being wired the experiment begins. Whenever the person who cares for the plants wears a red hat, that person also takes a snip from the end of one leaf of half the plants, chosen by a prearranged formula, and let some of the plants never be snipped. Let the interval between snipping incidents be a few days. I conjecture that after a few weeks at most, the plants will all react whenever the caretaker enters wearing the hat, before any snipping is done. This should indicate something visual on the part of the plants. It is likely that the never-snipped plants will react differently from the others. However, it is always possible that the caretaker is in a different mental state on "snipping days", and this causes an airborne chemical signal that the plants can detect and react to. I am not sure how to control for that.

Plants are fascinating, even more so now to me, after reading this book. What a great read!

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A couple of quibbles and contentions:

1. On p 39 the author coined the adjective "Descartian", referring to René Descartes. The adjective "Cartesian" already exists and is easier to say.
2. On p 156 we read, "In the United States alone, as many as 11,000 farmworkers are fatally poisoned by pesticides each year, and another 385 million are severely poisoned…". 'Scuse me, but the entire US population is about 360 million, of whom two million are farmworkers. The CDC states 10,000-20,000 "poisonings" without saying how many are fatal. Sundry reports are all over the place. One appears to be the author's source for 11,000 fatalities yearly, while another states that 60,000 nonfatal incidents occurred in five years, or 12,000 per year. The author needs to dig a bit deeper.