If Plants Could Remember. Apparently, They Do.

Part 2 of the series: Are We Giving the Land Something Like Alzheimer's?

Illustration by Scott Keenan

In the first part of the this series, I described how, in 2017, I heard a Lakota Native American mention reports from elders that some sort of forgetting, like Alzheimer’s, was afflicting the Black Hills of South Dakota. I then paired this with the later discovery that in response to a 1998 Mountain Pine Beetle outbreak in the Black Hills, the US Forest Service undertook a massive tree thinning campaign, such that by 2017, when the outbreak was officially over, the forest had lost more trees to thinning than disease.

I also spoke a bit about memory in my own life, both in working to save legacy forests—forests where the original biotic memory of the land is still relatively intact—to seeing how, for my father and brother-in-law, losing memory to Alzheimer’s was akin to losing themselves, as though they were somehow made of memory. I wanted to bring this context to the discussion, as I had begun to sense there’s much more to memory, in both a biological, cultural and personal sense, than we acknowledge.

Then I left us with with the obvious question: did that massive thinning campaign, combined with natural loss from the Mountain Pine Beetle infestation, damage so much “memory” within the Black Hills that Lakota elders were able to detect it? That is the question around which this series revolves. But it begins with a more preliminary set of question. Does western science detect memory in forests? If so, where is it located. In the plants? In the relationships? Where in the plant? Where in the relationships? Such details may or may not be of concern to indigenous people, who have a much different relationship with plants than we do, and thus a much different understanding. But we base our land-use decisions, purportedly, on western science. So it makes to explore what science has to say about the link between plants and memory.

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Not long ago, to even put the words “plant” and “memory” together in a scientific sense would have been scoffed at. But those days are past, with more than enough scientific work now published to effectively demonstrate that plants not only remember, but learn, make decisions, communicate and sense their environments. The subject of plant intelligence even has its own scientific society and journal: Plant Signaling and Behavior. The field was originally proposed under the term Plant Neurobiology, but too many scientists objected to the term “neurobiology,” as plants lack a nervous system. In fact, much of the argumentation around the subject has less to do with scientific design and results than the words used to describe them.

Take, for instance, “intelligence.” Chemical Ecologist Andre Kessler pointed out in a recent paper that there are over 700 published definitions of intelligence, and thus offered this succinct take: “The ability to solve problems, based on the information that you get from the environment, toward a particular goal.” It’s a bit bland, but it’s also easily understood, and allows Kessler to make the further point that “the question is not whether plants express intelligent behavior but how they achieve it without a nervous system and what the ecological consequences of these behaviors entail.”

Note that Kessler says “plant express intelligent behavior” rather than simply saying they are intelligent. Even for those who accept intelligence in plants, it’s apparently still hard to simply say it. It must be couched in the language of “behavior.” And here we need to pause, because we’re already thinking too much. We need to first see what where talking about, which the slowness of plants makes difficult for us. Fortunately, time lapse video helps us out of this trap. Take three minutes to check out this brief video showing bean plants reaching for a pole, narrated by Botany of Desire author, Michal Pollan. Note, as Pollan suggests, not only the tendrils, which ardently seek the pole, but the leaves, how animated they are, as though assisting the effort. And then, when contact is made with the pole, notice how they visibly seem to relax. Intelligence or intelligent behavior—what is the difference?

Perhaps what makes plant intelligence difficult for many scientists to accept is what plants don’t have—a brain, thus lacking the central hardware for which cognition has long been pinned. But if you think about it, any plant with a single loci of intelligence would not last long. For what if a deer came along and ate the leaves with the brain? Or that branch fell off in a storm? Better for the plant to have its “mind” distributed throughout the organism, in leaf, branch and root. That way, if something happens to part of the plant, the rest continues on, maintaining it’s cognitive function. However this intelligence works, the key thing is that it’s distributed throughout the plant.

Another key to “getting” plant cognition lies in the simple fact that plants are stationary beings, what science calls “sessile.” They can’t relocate if they don’t like where they are. They can’t run or hide from prey. They must instead adjust to, and if possible, modify the conditions where they find themselves. As a result, they’ve developed a remarkable repertoire of sense capabilities. In addition to the five senses we recognize in ourselves—hearing, sight, smell, taste and feel—plants can also recognize and analyze gradients in gravity, electromagnetism and chemical concentrations, not to mention the ability to recognize kin. And remember, they do this without the organs we commonly associate with sensing. As renowned plant researcher, Stefano Mancuso, puts it, plants are able to “see without eyes and hear without ears.” One small example: roots can hear the sound of water in the ground and grow toward it.

Of course, plants don’t only sense the world around them, they use that information to make decisions. And they have many decisions to make. Should they put out more leaves in the hopes of late-summer photosynthesis, or should they preserve their energy reserves for the winter to come and following spring? When should they time the blooming of flowers and and the onset of reproduction? Is there enough water for more growth, or should they conserve their use of such a vital resource. A caterpillar is chewing on one of it’s leaves. Should the plant expend energy into mounting a defense, by circulating toxins to its tissues? And which cocktail of chemicals should it produce? Mancuso has identified over a thousand chemical exudates from various plants, which he describes as a “vocabulary.” We would read them, however, with our noses, as the piney smell of pines, the sage smell of sagebrush, the tomato smell of tomato plants.

One particularly striking demonstration of plant learning was conducted by Australian researcher, Monica Gagliano. She took an experimental protocol usually used for animals and applied them to plants. She was testing for habituation, the ability of a subject to recognize when a repeated stress or stimulus no longer represents a threat, and can be ignored as a pointless repetition, thus preserving energy reserves. But rather than using it on animals, she applied it to plants, choosing Mimosa pudica, known as the “sensitive” plant for closing up its leaves in response to touch. It was a clever choice, for it allowed immediate, visual confirmation of effect.

Mimosa are sensitive to touch and shaking. but they also react to dropping. So Gagliano set up the controlled dropping of 56 potted Mimosa, about six inches onto to a soft foam pad, every five seconds. The idea was to begin with a 60 drop training session, yet no training appeared necessary. Some started ignoring the drop after 4 drops, some 6. By the end of the session all the leaves were open; none bought the ruse. Were they just fatigued? No, because when shook, they did close up. The stress was new.

But what was even more remarkable was what followed. About forty minutes later she repeated the experiment and the mimosa remembered. Then a week later she tried again, then again another week later. After four weeks the plants still held memory of the lesson. Bees, on the other hand, under similar tests, remembered for about 48 hours.

And this is only the above-ground parts of the plant, the parts we can see. There’s also a whole world of plant cognition down below. It was Darwin, in fact, with his keen observation, who first noticed “the tip of the radical (root) is endowed with diverse kinds of sensitivities.” He noted how the root trip not only read its surroundings, but seemed to use the information gained to direct the movements of the other parts. From the last paragraph of a four volume treaties, The Power of Movement in Plants, He wrote: “It is hardly an exaggeration to say that the tip of the radicle thus endowed [with sensitivity] and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements.” This has come to be known as the root-brain hypothesis, and though the animal brain isn’t exactly suited to our understanding of plant intelligence, many of Darwin’s observations have since been confirmed by subsequent science.

What has emerged into view is a plant quite unlike the one we grew up picturing: inert, just standing there, or sitting in a pot, or in a row for harvest, and in the case of modern agriculture, sprayed with biocides. Basically just taking what it gets. But now we’ve gained the time-lapse view. Now we can imagine the plant outside our own narrow band of time perception, as it slowly reaches, tests, tastes, listens, delves and smells its way into the world. Rather than disinterested, the plant seems curious. It notices and seems to question those things it notices. “What is this? Is it safe or dangerous? Where is the water? Who is kin and who not? Where is the best light?” And if you were to walk up to it and take hold a leaf, it would likely notice your presence. Certainly it would hear, or feel, your footsteps approaching. At least the roots would, and according to Darwin, tell “the other parts.”

Research is ongoing as to where memory resides in the plant. But it is partly epigenetic. That is, rather than effecting gene sequence, the plant modulates the gene, augmenting and dimming various gene-expressions in a way that maintains itself over time and can even be passed to future generations. There are also systems of chemical cascades and electric signaling that may hold the memory, but remember, there is no brain in which to store it. The memory appears more distributed. It’s as if the plant itself is the organ of memory.

Of course, few plants live as a “plant itself,” but rather as a plant in a community of other plants and living things, large and small, which we call ecosystems, or, in our case, forests. The next question we must ask, therefore, is does a forest have memory? Once again, I think you’ll surprised at the answer, and hope you’ll join me for the next installment in this series, when we explore forest intelligence and memory.

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