Plant Intelligence Herbalism History: The Epistemology of Rupture
“The plants have virtues hidden from us, and the physician who does not know them labours in the dark.” — attributed to Galen of Pergamon, 2nd century AD
Something is circulating online — a clip, a quote, a breathless caption — claiming that your houseplant senses your arrival from two kilometres away. That it responds to your emotional state. That an EEG strapped to its leaves reveals a pulse of recognition when you walk through the door.
It is appealing. It is also not true. And the reason I want to begin there — with the thing that isn’t true — is because what is true turns out to be far more interesting, and far more relevant to the work we do here, than the fantasy.
Plant electrophysiology is real. Volatile chemical signalling between plants is real. Mycorrhizal communication networks spanning hectares of forest are real. These are peer-reviewed, replicated, and increasingly well-understood phenomena. But they do not prove that plants love you. What they prove is something more useful to an herbalist: that the old physicians were not working in the dark. They were working with a conceptual framework — the Galenic doctrine of the vegetative soul, the Aristotelian understanding of plant faculties, the Carolingian gardener’s intuitive sense of the garden as a living system — that the modern laboratory is now, slowly and without always knowing it, confirming — Plant Intelligence Herbalism History.
That confirmation is the interrupted inheritance. This is the post about the rupture, and the recovery.
What the Old Physicians Already Knew: Plant Intelligence Through Herbalism History
The Three Souls and the Dignity of the Lowest
The Aristotelian-Galenic philosophical tradition organised living things according to soul — not soul in the theological sense that became dominant in later Christian thought, but soul as the organising principle of a living body, the form that made matter alive and purposive rather than merely inert.
This tradition recognised three grades of soul. The rational soul belonged to human beings alone: the capacity for reason, deliberation, language. The sensitive soul belonged to animals: sensation, appetite, voluntary movement. And the vegetative soul — the lowest and most ancient — belonged to plants: the capacities for nutrition, growth, and reproduction.
To call it the “lowest” soul was not to dismiss it. For Galen and for the medieval physicians who built upon him, the vegetative soul was real, active, purposive. It was the source of what they called the plant’s virtues — its powers of action upon the body that consumed it, its temperament of hot and cold, moist and dry, that made it medicinal or dangerous or neutral. A plant did not merely sit there. It processed. It responded. It acted.
What the old physicians could not do — what no one could do before the invention of the microelectrode and the gas chromatograph — was watch those processes happen in real time at the cellular level. They worked from inference, from long observation, from the accumulated wisdom of what we now call clinical tradition. They knew that willow bark cooled fevers without knowing that salicin inhibited prostaglandin synthesis. They knew that wormwood moved the liver without knowing that absinthin stimulated bile production.
The rupture came not when better science disproved the old physicians, but when a particular strand of intellectual plant intelligence herbalism history decided that because the old physicians’ explanations were wrong, their observations must also be discarded.
The old herbalists were not wrong about what plants did. They were wrong — or rather, they were using different language — about why. And those are not the same mistake.
That conflation — of the explanatory framework with the observational tradition — is where the inheritance broke. And it is what the laboratory is now, piecemeal and often unwittingly, beginning to repair.
Electrical Signals: The Vegetative Soul Made Visible
Action Potentials in the Kingdom of Plants: Plant Intelligence Herbalism History
The discovery of electrical signalling in plants began in the Victorian era, when researchers including Jagadish Chandra Bose — an Indian polymath who deserves far more credit than he has received in Western scientific histories — demonstrated that plants responded to stimuli with measurable electrical changes propagating through their tissues. The scientific establishment was dismissive. The idea sat poorly with the reigning assumption that electrical signalling was the exclusive province of the animal nervous system.
It is no longer a fringe claim. Electrical signalling in plants is documented across dozens of species — not only the famously theatrical examples like Dionaea muscipula (the Venus flytrap) or Mimosa pudica (the sensitive plant), whose dramatic responses made excellent demonstrations, but in tomatoes, barley, Arabidopsis, corn, avocado, and many others.
Three primary signal types have been characterised: action potentials, which are self-propagating depolarisations similar in form (if not mechanism) to those in animal neurons; variation potentials, longer and more complex signals triggered by environmental stress that can travel over substantial distances through the plant; and system potentials, which involve the activation of H⁺-ATPase pumps and appear to coordinate whole-plant responses.
The resting membrane potential of plant cells — typically between −100 and −200 mV — is maintained by sophisticated ion transport systems: calcium channels, anion channels, voltage-gated potassium channels. When a plant is wounded, when a herbivore begins feeding, when temperature changes sharply, when water stress develops, these systems shift.
The result is a propagating electrical signal that travels from the site of stimulus to distal tissues, preparing them for what is coming. A 2021 review in Journal of Plant Physiology characterised this process as the plant’s “electrophysiological phenotype” — a recognition that electrical signalling is not an occasional curiosity but a fundamental dimension of how plants exist in and respond to their environments.
The Speed of the Signal
Here is what should stop any herbalist in their tracks: the electrical signal travels faster than the chemical cascade it triggers. When a caterpillar begins feeding on an Arabidopsis leaf, the membrane depolarisation reaches distant leaves within minutes — in some experiments, within seconds. The jasmonic acid signalling that will ultimately coordinate the plant’s defensive response follows the electrical signal. The electricity is not the whole story, but it is the announcement. It is the plant’s equivalent of a trumpet call before the army moves.
Carolingian gardeners had no way of knowing this. But they knew — because centuries of careful observation had taught them — that a plant under insect attack changes. That the timing of harvest matters. That stress affects quality. They were reading the downstream consequences of processes they could not directly observe.
The vegetative soul was their name for the thing that makes a plant responsive. They were right that the thing exists. The laboratory has given it a mechanism.
The Language of Volatiles: How Plants Speak to the Air
A Discovery That Was Not Believed
In 1983, two papers appeared — one by Ian Baldwin and Jack Schultz, one by David Rhoades — reporting something that struck most of their colleagues as absurd: that trees under insect attack appeared to communicate with their neighbours, and that undamaged neighbouring trees responded by upregulating their own chemical defences. The phenomenon was labelled “talking trees” in the popular press, and dismissed as romance in the scientific literature.
The dismissal was premature. Subsequent decades of research confirmed and extended the finding. When a plant is attacked by herbivores, it releases a distinctive blend of volatile organic compounds — herbivore-induced plant volatiles, or HIPVs — that serve multiple functions simultaneously. These volatiles recruit predatory insects that attack the herbivore (an indirect defence). They prime the plant’s own distant tissues to mount faster defences if attack spreads (within-plant signalling). And they are perceived by neighbouring plants, which in turn activate their own defensive machinery in anticipation of attack.
The mechanism has now been traced in considerable biochemical detail. Key signalling molecules include green leaf volatiles (GLVs), fatty acid-derived compounds released almost immediately upon wounding; terpenes of various classes; methyl salicylate; and methyl jasmonate. The receiving plant detects these airborne molecules and responds by upregulating the jasmonic acid (JA) signalling pathway, producing defensive secondary metabolites including tannins, protease inhibitors, and alkaloids. A 2022 review in Journal of Experimental Botany described this inter-plant priming as a fundamental ecological function of inducible volatile emission — no longer a hypothesis but an established phenomenon.
The Herbalist’s Eye on the Volatile
Consider what this means for how we read the classical materia medica. The aromatic compounds that Dioscorides described, that Hildegard praised, that the compilers of the Capitulare de Villis took for granted as the markers of a plant’s virtue — these are, in very large part, exactly the volatile compounds that modern plant science is studying as communication molecules and defence signals.
Wormwood’s artemisinin and absinthin. Sage’s thujone and camphor. Fennel’s trans-anethole and fenchone. These are not incidental features of the plant, chemical curios with no ecological purpose. They are the plant’s active vocabulary. They are what it says to insects, to pathogens, to fungal partners, to neighbouring plants — and, by extension, what it says to the body that consumes it.
When Galen assigned plants a temperament — hot in the second degree, dry in the third, and so on — he was, in a pre-biochemical idiom, attempting to describe the intensity and direction of a plant’s action on the body. The categories were wrong in their mechanism. But the instinct that plants were actively doing something, that they had powers that operated on living tissue in predictable and characterisable ways, was correct. The volatiles are one of the places we can see that instinct confirmed.
We know this not only in the abstract. We know it because in the spring of 1983, two researchers at the University of Washington — Gordon Orians and David Rhoades — placed tent caterpillars and fall webworms on the leaves of willow trees in a field plot and then watched what happened to the trees that had not been touched. Those trees changed. Their leaf chemistry shifted — the same defensive compounds, the same direction of response — as if they too were under attack.
Rhoades and Orians published their results and proposed that the damaged trees were releasing airborne chemical signals, something they called, for want of a better word, pheromones, that neighbouring trees were detecting and acting upon.
The scientific establishment was not impressed. “Talking trees” became a term of ridicule. The same year, Ian Baldwin and Jack Schultz published controlled laboratory results confirming the same phenomenon in poplars and sugar maples — and the field still resisted.
It would take the better part of two more decades, and the development of gas chromatography precise enough to identify and quantify individual volatile compounds in real time, before the phenomenon was accepted as settled science. The chemicals were eventually identified: green leaf volatiles, terpenes, methyl jasmonate, methyl salicylate. Exactly what you smell when you crush an aromatic herb between your fingers.
What strikes me about this story — and I learned it, as so many things, from a passing remark by my late husband Homer, who had encountered it through the timber industry in that same era — is not just that the scientists were right and the institution was slow. It is that the people working closest to the trees, the foresters, the industry observers watching test plots in the Pacific Northwest, apparently found the result entirely unsurprising.
Of course the trees were talking. They had been watching forests long enough to know that a stand behaves like a system, not a collection of isolated individuals. The academics needed two decades and better instruments to catch up to what the old foresters, like the old herbalists before them, had simply assumed.
The old physicians tasted, smelled, and touched their way to a pharmacopoeia. The laboratory has spent a century discovering that the nose was not lying.
Below the Garden: The Wood-Wide Web and the Carolingian Intuition
What Suzanne Simard Found
In 1997, Suzanne Simard and her colleagues published a paper in Nature demonstrating that carbon moved between ectomycorrhizal Douglas fir and paper birch trees through shared underground fungal networks — that is, that trees in a mixed forest were not merely competing for the same pool of soil resources but exchanging those resources in patterns that responded to each tree’s seasonal needs. The term that stuck in the popular imagination was the “wood-wide web.”
The science behind that term is robust, though the popular elaborations of it have sometimes outrun the evidence. What has been demonstrated, across multiple decades of increasingly sophisticated research using DNA sequencing, isotopic tracing, and mycorrhizal network mapping, is the following: common mycorrhizal networks (CMNs) connect the roots of trees with other trees, with compatible seedlings, and with understorey plants across a forest.
Through these networks, carbon, water, nutrients, and defence signals move between connected individuals. The direction and volume of transfer shifts in response to ecological conditions — shading, drought, seasonal changes in photosynthetic capacity. Hub trees, sometimes called mother trees, are the most heavily connected nodes in the network, and their removal disrupts the whole.
A 2025 response paper by Simard and colleagues in Frontiers in Forests and Global Change confirmed that both the existence of these networks and the transfer of carbon, water, nutrients, and infochemicals through them have been demonstrated across multiple independent research groups using multiple methodologies. This is not a single lab’s finding. It is a convergent result.
The Garden as System
The Capitulare de Villis, issued in approximately 812 AD under Charlemagne’s authority, listed seventy-three plants to be grown in the imperial gardens. I have argued elsewhere in this series that the list reveals not merely a royal shopping list but an implicit understanding of the garden as a curated ecosystem — one in which the relationships between plants were part of their medicinal value. The preference for polyculture. The grouping of aromatic herbs. The attention to companion planting that we find in monastic garden traditions.
None of the Carolingian physicians knew about mycorrhizal networks. But they were working in gardens, with plants in soil, in communities. The mycelial networks beneath those gardens were real and functioning whether or not anyone knew their names. The experienced gardener who observed that certain plants thrived together, that soil improved under certain rotations, that transplanted specimens sometimes failed while established ones flourished — that gardener was reading the downstream effects of underground networks she could not see.
This is not romanticism. This is what good empirical tradition does: it tracks consequences without always knowing mechanisms. The Carolingian garden was a system. The science now shows us why systems behave differently from collections of isolated specimens.
The pot-bound houseplant of the viral video is isolated from the networks it evolved within. No wonder it seems diminished. The question was never whether a single plant can sense you. The question is what a garden — a real garden, with soil and time and connection — is doing.
A Necessary Distinction: Response Is Not Sentience
I want to name the distinction that matters here, because the territory we are walking through has a ditch on either side.
On one side: the old dismissal, the one that said plants are just chemistry, that the Galenic tradition was superstition dressed in Latin, that anything resembling plant intelligence herbalism history is anthropomorphism and should be rejected. That ditch has been clearly abandoned by the science. Plants are not passive. They communicate. They respond. They coordinate. The evidence is unambiguous.
On the other side: the new romance, the one that says plants are sentient, that they feel, that they love their caretakers, that the forest has a consciousness like our own, only distributed. That ditch is philosophically confused and scientifically unsupported, and it leads plant intelligence herbalism history into territory that I believe is both intellectually dishonest and — for those of us who hold a Christian worldview — theologically problematic.
The Galenic vegetative soul was never sentient. The capacity for nutrition, growth, and reproduction is real, purposive, and lawful — but it is not awareness. Plants do not choose. They respond. The distinction is not semantic hairsplitting. It is the difference between a thermostat and a mind. A thermostat responds to temperature; we do not attribute suffering to it when it is too cold.
What the modern science vindicates is the middle position — the one the Galenic tradition actually held, before it was caricatured on one side as magical thinking and on the other as mere mechanism. Plants are active. They are not inert. Their activity follows patterns, obeys laws, serves ecological functions, and produces effects on the bodies that consume them. That is enough. That is, in fact, extraordinary.
The herbalist’s task is not to project a human inner life onto the plant kingdom, but to understand — as fully as possible, using every tool available — what plants actually do. The old physicians understood that better than the dismissers who came after them. The laboratory is now recovering what the dismissers threw away.
The Inheritance Recovered: What This Means for How We Work
If the old physicians were right that plants are active — not merely chemical warehouses but responsive, purposive, communicating organisms embedded in ecological webs — then several practical implications follow for the way we think about herbal medicine.
Timing and Stress Matter
Plant electrophysiology and VOC research have established clearly that a plant’s chemical composition changes in response to stress, season, and environmental conditions. A plant harvested in the morning may have different volatile concentrations than one harvested in the afternoon. A plant under insect pressure will be producing defensive compounds that a quiet, unstressed specimen will not. These are not New Age assertions. They are measurable facts.
The old physicians knew this too. They had harvest calendars. They noted the influence of season, of lunar cycle, of the plant’s condition at time of collection. We may not share all their explanatory frameworks — the lunar timing, for instance, has not been well-supported experimentally — but the underlying instinct, that when you harvest matters, is confirmed by the science.
The Soil Is Part of the Medicine
If mycorrhizal networks are real, then soil is not a neutral growth medium. It is a communication infrastructure. What a plant draws from the network, what it gives to the network, how it relates to its neighbours — all of this shapes what the plant is and what it contains. Industrial monoculture severs these networks. Transplanting disrupts them. Older, established plants in undisturbed soil are not the same as young specimens grown in sterile substrate.
The old pharmacopoeias had opinions about geography — about which regions produced the best specimens of a given herb, which soils, which exposures. We tend to read this as pre-scientific regionalism. But beneath the regional preferences was a real observation: plants grown in certain conditions, with certain companions, in certain soils, produce better medicine. The mycorrhizal science gives us a partial mechanism for why.
Aromatic Complexity Is Not Accidental
The volatiles that make an herb smell the way it smells are the plant’s chemical speech. They are not incidental to its medicinal action. They are, in many cases, the medicinal action — or they are part of the broader chemical matrix within which the medicinal compounds operate. The entourage effects that clinical herbalism has always recognised — the observation that whole plants often behave differently than isolated constituents — reflect, in part, the ecological complexity of the volatile fraction.
A plant with a complex, full aromatic profile is, in the language of modern plant science, a plant with a rich and active volatile ecology. The old physicians were not wrong to pay attention to smell. They were using their noses to read a real signal.
The Rupture and What Remains
The interrupted inheritance is this: for roughly three centuries, from the scientific revolution through the early twentieth century, Western medicine decided that the old physicians were simply wrong — not just wrong about their mechanisms, but wrong about their observations, wrong about their categories, wrong about whether plants were worth taking seriously as anything other than chemical sources to be mined for active compounds.
That decision produced pharmaceutical medicine, which has saved millions of lives and which I do not for a moment dismiss. But it also produced a generation of educated people who genuinely did not know that plants communicated, that forests were cooperative systems, that the aromatic compounds in the herbs their grandmothers used were doing something real. The rupture left a vacuum, and the vacuum was filled — inevitably — with the kind of romantic overcorrection that produces viral videos about houseplants sensing your arrival from two kilometres away.
The laboratory is now, slowly, recovering the inheritance. Not the mysticism that some would project onto it. Not the explanatory frameworks that were specific to a pre-biochemical era. But the underlying observation — that plants are active, responsive, communicating, purposive — that the Galenic tradition had right all along.
For those of us who practice from within that tradition, this is not vindication so much as company. We were already working with plants as if they were doing something. The science is now explaining what.
— CSK
Selected Bibliography
Plant Electrophysiology
Armada-Moreira, A., et al. “Plant Electrophysiology with Conformable Organic Electronics: Deciphering the Propagation of Venus Flytrap Action Potentials.” Science Advances 9 (2023): eadh4443.
Chaparro-Cárdenas, S.L., et al. “Plant Electrophysiology: Bibliometric Analysis, Methods and Applications in the Monitoring of Plant-Environment Interactions.” DYNA 88, no. 218 (2021): 212–223.
Li, Z., et al. “Plant Electrical Signals: A Multidisciplinary Challenge.” Journal of Plant Physiology 261 (2021): 153418.
Fromm, J., and S. Lautner. “Generation, Transmission, and Physiological Effects of Electrical Signals in Plants.” In Plant Electrophysiology: Signaling and Responses, 207–232. Berlin: Springer, 2012.
Volatile Organic Compound Signalling
Ameye, M., et al. “Volatile-Mediated Plant–Plant Interactions: Volatile Organic Compounds as Modulators of Receiver Plant Defence, Growth, and Reproduction.” Journal of Experimental Botany 73, no. 2 (2022): 511–528.
Baldwin, I.T., and J.C. Schultz. “Rapid Changes in Tree Leaf Chemistry Induced by Damage: Evidence for Communication Between Plants.” Science 221 (1983): 277–279.
Heil, M., and R. Karban. “Explaining Evolution of Plant Communication by Airborne Signals.” Trends in Ecology and Evolution 25, no. 3 (2010): 137–144.
Mycorrhizal Networks
Gorzelak, M.A., et al. “Inter-Plant Communication through Mycorrhizal Networks Mediates Complex Adaptive Behaviour in Plant Communities.” AoB Plants 7 (2015): plv050.
Simard, S.W., et al. “Net Transfer of Carbon Between Ectomycorrhizal Tree Species in the Field.” Nature 388 (1997): 579–582.
Simard, S.W., et al. “Opinion: Response to Questions About Common Mycorrhizal Networks.” Frontiers in Forests and Global Change 7 (2025): 1512518.
Historical and Philosophical Context
Baldassarri, F., and A. Blank, eds. Vegetative Powers: The Roots of Life in Ancient, Medieval and Early Modern Natural Philosophy. Cham: Springer, 2021.
Nutton, V. Ancient Medicine. 2nd ed. Abingdon: Routledge, 2013.
Riddle, J.M. Dioscorides on Pharmacy and Medicine. Austin: University of Texas Press, 1985.
© 2026 Carolyn Smith-Kizer. All rights reserved.