The Sensitivity of Plants

Photograph by Michael MK Khor.


 Michael Marder

September 14, 2015

It is not just illegal, unsustainable, and irresponsible. Mahogany tree logging in the rapidly disappearing Peruvian rainforest might prove wrong for yet another reason: it is inhumane. Of course, a logging site is different from a slaughterhouse. There are no shockingly gory sights of animal suffering and no screaming, aside from the deafening noise of buzzing chainsaws. But the indifference of plants, their imperviousness to being damaged or even destroyed, is deceptive.

Common sense tells us that, unlike their animal counterparts, plant cells are immobile, caged in a dense cellulose membrane, and, hence, insensitive and incapable of producing real behavioral responses. In brushing off the very possibility of plant ethics, critics often rely on what they consider to be an incontrovertible given — namely, that plants lack anything like a nervous system and the attendant capacity to feel pain.

Neither science nor philosophy should draw upon the fatuous assumptions of common sense, however. Already in the nineteenth century Charles Darwin and his son argued that the analog of the animal brain is situated in the plants’ roots. This argument has come to be known as the “root-brain hypothesis.”

It is worth citing the Darwins’ treatise The Power of Movement of Plants, where they state: “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.[i]

A plant will not run for twenty miles in search of food. But its roots, with their sensitive tips, will explore extensive underground labyrinths until they encounter the most nutrient-saturated location. A plant will also not flee from a source of peril. But its roots are smart enough to avoid dangerous places by actively growing away from hostile environments. So, if plants are like animals, as the Darwins would have it, then they are animals that grow upside-down and inside-out — with their brains (the roots) immersed in the darkness of the soil, their lungs (the leaves) exposed to the outside, and their sexual parts (the flowers) turned toward the airy expanses above. (There is nothing inherently wrong with this comparison, so long as we relativize what is “up” and what is “down,” without treating the animal as a default point of reference. Which means that, just as plants are the upside-down animals, the animals are the downside-up plants.)

Let us agree, then, that plant roots are sensitive to the underworld, which to us, humans, symbolizes the realm of death or, at best, connotes unconscious existence, as encapsulated in Plato’s famous allegory of the cave. Still, in and of itself, this claim is likely to be insufficient when it comes to convincing our skeptical friends that the carrots and radishes they eat, as well as the cacti and the orchids growing on their windowsills, are sensitive beings. Upping the ante, we ask: What if plant cells were actually more like neurons than like miniscule cellulose prisons? And what if plants felt something like pain?

Take, for example, the sensitive plants par excellence. Mimosa pudica is colloquially dubbed “touch-me-not” because it closes its leaves as soon as you pass your hand over them. Dionaea muscipula, or Venus flytrap, detects and feasts on the insects attracted to its suggestive lobes. Both plants lose their distinctive sensitivity, when they are exposed to anesthetics — the very substances that are so effective for relieving pain in animals and humans. The same is true for the roots of other species, not usually classified in the same group as Mimosa and Dionaea.

What is more, plants seem to anaesthetize themselves. Ethylene, a classical plant hormone, which relieves pain, is released in mechanically stressed plant tissues immediately after wounding, suggesting that it might act as an analgesic in our green cousins, as well. In turn, higher plants produce ethylene to prepare their fruit for being “painlessly” devoured by animals and humans, who will then spread their seeds. Only the mature “anaesthetized” fruit are delicious, because devoid of toxic substances, which plants utilize so as to deter potential early feeders.

Ethanol, synthesized especially by plant roots under stress (for example, in a state of molecular oxygen deficiency) creates a comparable anesthetic effect. In response to a painful stimulus, a plant does not shriek at the top of its voice, as an animal would, but it screams biochemically. This should give utilitarian philosophers a pause and much food for thought.

Now, the self-induced “vegetal anesthesia” is not only local. When you puncture a tree leaf, the other parts of the plant that have not been directly affected by the injury receive a biochemical signal about the threat. Having been forewarned, they can prepare themselves for the worst, and start producing their own painkillers in anticipation of the attack. Who would dare assert that the plant and its individual parts placed on “red alert” are indifferent to their survival and don’t care about their own interest or wellbeing?

Perhaps we would not be so shocked to hear that plants experience something like pain if we realized that their cells, too, can fire, generating action potentials (APs). Referring to rapid rises and falls in the electrical charge that occur in the membranes of excitable cells such as our neurons, APs have been recorded regularly in plants since 1873. It is now obvious that not only Mimosa and Dionaea but also all plants produce APs, responsible for anything from wound healing to respiration and photosynthesis.

And then, there are the visible approximations between plant cells and neurons, the only cells in animals and humans that, like those in plants, lack centrioles, or cylinder-shaped cell structures. Similar to the bloodless plant cells, neurons avoid direct contact with the bloodstream and, instead, are protected and nourished via the blood-brain barrier. Even the shapes and principles of cellular polarities in tip-growing plant cells — root hairs and pollen tubes — are reminiscent of neurons extending their axons.

The sensitivity of plants is not a matter of fairytales about talking trees, the likes of the Ents in J.R.R. Tolkien’s The Lord of the Rings. Far from photosynthesizing “green machines,” plants are agents in their milieu, pursuing their optimal development, or, in other words, seeking a good of their own. It is up to ethical thought to catch up with the recent scientific findings about the plants’ responses to damaging stimuli, search for nourishment, and avoidance of danger.

We need not adhere to utilitarianism to justify the inclusion of plants among the objects of our moral considerability. The interests of Venus flytrap as much as of a mahogany tree unmistakably come through in their purpose-driven, intentional, and intelligent behaviors. Their total instrumentalization, that is to say, the unlimited use of plants for human purposes, is unjustifiable both because of the undue stress it causes and because it overwrites the interests of the plants themselves.

To be sure, plants to do not speak our languages and do not think in our zoo- and anthropocentric terms. But they are a case-in-point of intelligent behavior and sensitivity, albeit independent of a cerebral structure and nervous system. From the lower to the higher varieties, plants demonstrate that it is possible to act purposefully in the environment without leaving their places of growth and without forming human-like conscious representations of the world. They are highly social, communicative organisms, even though their communication patterns are not as striking to a naked human eye as, for instance, the bee’s waggle dance. Those who deign to speak on behalf of plants, considered as insensitive and indifferent organisms, unwittingly reveal their own insensitivity to the complexity of non-human living beings.


Note: An earlier version of this article was published here: 

The article was written in consultation with Frantisek Baluska (the University of Bonn).

[i] Charles & Francis Darwin, The Power of Movements in Plants (London: John Murray, 1880).