Beyond the View of Plants as Mere Machines: on Plant Sensation, Perception, and Awareness

June 1, 2015 § 3 Comments

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I’ve always found plant behavior intriguing. Over the last six years of casual reading on it, starting with a college research report on the ill-named “Plant Neurobiology”, I learned more than I had imagined possible about the agency of plants (even after debunking misleading works such as The Secret Life of Plants). I learned that while plants do not possess a central nervous system, they nevertheless possess remarkable abilities of sensation, perception, and awareness. While these facets of life of course differ between plants and animals, plants nevertheless possess capacities for vision, olfaction, tactition, thermoception, and for detecting location, direction, and motion. Plants possess some forms of procedural memory, short-term memory, and long-term memory. They signal, communicate, and network with other organisms and species. They even wield a vascular system of awareness which some contrast to a central nervous system. All of these points serve to debunk the notion of plants as pure automata, as mere machines. But what, precisely, does that mean?

Can a Plant “See”? Vision Without Eyes.
Plants can sense the ultraviolet spectrum, distinguish time of day based on sun, distinguish large and small light sources, distinguish parts of the light spectrum via phytochrome receptors, discern incoming light direction and duration as well as shadowing over themselves.  Again, the decentralized rather than centralized nervous system: “If in the middle of the night you shine a beam of light on different parts of the plant, you discover that it’s sufficient to illuminate any single leaf in order to regulate flowering in the entire plant.” [What a Plant Knows, p.20]

Humans have rhodopsin detecting for light and shadow, and three photopsins for red, blue, and green, as well as cryptochrome for our internal clock. Plants have photoreceptors for blue and red lights as well, and, as photosynthesis-based organisms, depend on photoreceptor cues in ways more complex than humans; as an example, arabidopsis thaliana wields 11 different photoreceptors. Plants detect electromagnetic waves both longer and shorter than humans can. While we convert these signals to pictures, they convert it to growth cues. They detect, as well as respond adaptively. Indeed, plants also have their own comparable circadian rhythms.

Can a Plant “Smell”? Olfaction Without Noses.
Plants also have the ability to perceive odor or scent through stimuli. Ethylene signaling for ripening demonstrates that plants have at least this volatile chemical receptor. As another example, the parasitic plant cuscuta pentagona locates its tomato host via multiple volatile chemical odors, avoiding noxious repellents. Many plants both anticipate and warn others toward phenolic and tannic chemical defense responses against predation via airborne pheromones, and some can responsively produce nectar attracting their predators’ predators. They detect methyl salicylate odor, which they store and convert to salicylic acid for healing. Plants have pheromone senses like humans do, with limbic responses when they “smell trouble” or “the smell of fear”, just via different organs than humans.

Can a Plant “Feel”? Tactition and Thermoception Without Brain or Skin.
Not only do they know when they’re being touched, but plants can differentiate between hot and cold, and know when their branches are swaying in the wind. Plants feel direct contact: some plants, like vines, immediately start rapid growth upon contact with an object like a fence they can wrap themselves around, and the Venus flytrap purposely snaps its jaws shut when an insect lands on its leaves. And plants seemingly don’t like to be touched too much, as simply touching or shaking a plant can lead to growth arrest.” [What a Plant Knows, p.50]

Regarding pressure perception, “…plants perceive tactile sensation, and some of them actually ‘feel’ better than we do. Plants like the burr cucumber (Sicyos angulatus) are up to ten times more sensitive than we are when it comes to touch.” [What a Plant Knows, p.50]

In plants, contact similarly initiates electric action potential across the organism, triggering mechanoreceptors. Bio-electrochemical currents via sodium, potassium, and calcium channels (similar to mammals) trigger hydraulic pumping, rather than bloodflow, modifying cell membranes and walls rather than muscles. However, “[w]hereas animals produce the action potential by an exchange of sodium and potassium ions, plant potentials are produced with calcium transport that is enhanced by chloride and reduced by potassium.” [3] The process focuses on the pulvinus motor cells, rather than the brain neurons. Plants have touch-activated TCH genes, including one which encodes in plants the same calmodulin proteins involved in such animal processes as memory, inflammation, muscle function, and nerve growth. Electric communication from wounded to non-wounded leaves in a plant promotes adaptive responses, as does olfactory eavesdropping or perhaps signalling between plants.

Current scientific research hypothesizes cellular level similarities between animals and plants, but organismal level difference, namely: complex mobile organisms (e.g. some animals) can feel pain, with an evolutionary basis, whereas sessile, rooted organisms (e.g. plants) employ metabolic responses but cannot feel pain. Plants lack the nociceptors indicative of pain-perception.

Curiously, however, the same anesthetics that render animals unconscious can induce an unresponsive state in plants, and plants’ ethylene can anesthetize animals. Additionally, some scientists claim plants signal using serotonin and dopamine, which function as neurotransmitters in animals. These both indicate something, though no one knows exactly what.

Plants do however certainly detect temperature, and adapt to it. They can produce proteins to protect from the damage of ice formation or falling rates of enzyme catalysis at low temperatures, or enzyme denaturation and increased photorespiration at high temperatures. They can produce antifreeze proteins and dehydrins, or heat shock proteins, or antioxidant systems in case of the build-up of oxygen during metabolic imbalances at temperature extremes. They can change the composition of their membranes; increased unsaturated fatty acids for cold conditions, saturated fatty acids for hot conditions.

Can a Plant “Navigate”? On Location, Direction, and Motion.
“…when a plant has been turned upside down, it will reorient itself in a slow-motion maneuver…so that its roots grow down and its shoots grow up”, “they’re constantly aware of where their branches are; they know if they’re growing perpendicular to the ground or at an angle off to one side, and tendrils always have a  pretty good idea of where the nearest support is to grab onto.” [What a Plant Knows, p.91-92]

Plants sense gravity via statoliths in root caps, rather than otoliths in ears, with their auxin providing the movement hormone. Each individual plant has a movement pattern, in particular a slow-motion, recurring, spiral sway performed throughout the day, inherently and in response to statolith settling and gravity.

On Plant Memory.
“…plants clearly have the ability to retain past events and to recall this information at a later period for integration into their developmental framework: Tobacco plants know the color of the last light they saw. Willow trees know if their neighbors have been attacked by caterpillars. These examples, and many more, illustrate a delayed response to a previous occurrence, which is a key component to memory.” [What a Plant Knows, p.114]

Memory requires the capacity to encode, retain, and retrieve information. Scientists have not found evidence for semantic or episodic memory in plants, but rather, procedural memory, as seen in the example that a “…tendril that had been touched in the dark had stored this information and recalled it once he placed it in the light.” [What a Plant Knows, p.115] Venus fly-trap behavior, as one example, demonstrates short-term memory via electric action potential and temporary ionic calcium concentrations.

Plants also demonstrate long-term memory, for example through immune memory, or through morphogenetic trauma. “Morphogenetic memory is a type of memory that later influences the shape or form of the plant. In other words, a plant can experience a stimulus at some point, like a rip in its leaf or a fracture of a branch and be unaffected by it at first, but when environmental combinations change, the plant may remember the past experience and respond by changing its growth.” [What a Plant Knows, p.120] Regarding cellular memory, plants respond to weather cues across time (as seen with wheat vernalization).

They, like animals, can have epigenetic adaptation (primarily toward environmental and physical stresses) which “…facilitates memory not only from season to season within a single organism but from generation to generation.” [What a Plant Knows, p.129] In fact, “…not only do the stressed plants make new combinations of DNA but their offspring also make the new combinations, even though they themselves had never been directly exposed to any stress. The stress in the parents caused a stable heritable change that was passed on to all their offspinrg: the plants behaved as if they’d been stressed. They remembered that their parents had been through this stress and reacted similarly.” [What a Plant Knows, p.129]

Animals have glutamate receptors responsible for neural communication, memory formation, and learning, but while plants have glutamate receptors that may support cell-to-cell signaling, they lack connection to distinct organs (though the receptors may serve as defense proteins, warding off predator insects).

Animal neurology requires synapses, neural transmitters, and receptors. Plants’ auxin transmission can resemble neurotransmission, especially in vesicle trafficking (roughly, the movement of important biochemical signal molecules). “Still, auxin is not found in animals, and it appears to be a plant-specific protein that regulates growth. To some, Jürgens’s observations suggest that the vesicle struc­tures might be similar enough to make a good argu­ment. When the kinds of ‘synapses’ made in plants are examined, two junction types turn out to have protein domains embedded in the cell membrane. The auxin transport system, accomplished through vesicle traffick­ing, is influenced by light and gravity to control cell-to­-cell communication, and it uses auxin as a transmitter, behaving in much the same way as a neurotransmitter.” [3]

On Plant Signalling, Communication, and Networking.
Pheromones I discussed in the section on scent. Beyond that, plants often perform multi-species cooperation. “The other ‘synapse’ behaves like the interconnection between an animal immune cell and a pathogenic cell. In animals, this system implements the immune response and the destruction of the invading pathogen. In plants, it al­lows the individual not only to deal with pathogens but also to stabilize interactions with symbionts—an important function. Plants establish useful two-way interactions with a lot of microorganisms such as bacteria and fungi, and in some cases these microbes accomplish tasks that the plant is unable to do on its own. Some plants cannot process environmental nitrogen, so they form a sym­biotic relationship with bacteria from the genus Rhizobium to do the trick, and the synapse-like attachment is essential for the relation­ship. In the process, the rhizobia get the benefit of being fed by the plant.” [3]

Trees network themselves together through underground webs of mycorrhizal fungi, “to convey warnings of insect attacks, and also to deliver carbon, nitrogen, and water to trees in need”. [2]

“The pattern of nutrient traffic showed how ‘mother trees’ were using the network to nourish shaded seedlings, including their offspring—which the trees can apparently recognize as kin—until they’re tall enough to reach the light. And, in a striking example of interspecies cooperation, Simard found that fir trees were using the fungal web to trade nutrients with paper-bark birch trees over the course of the season. The evergreen species will tide over the deciduous one when it has sugars to spare, and then call in the debt later in the season. For the forest community, the value of this cooperative underground economy appears to be better over-all health, more total photosynthesis, and greater resilience in the face of disturbance.” [2]

“…the tips of plant roots, in addition to sensing gravity, moisture, light, pressure, and hardness, can also sense volume, nitrogen, phosphorus, salt, various toxins, microbes, and chemical signals from neighboring plants. Roots about to encounter an impenetrable obstacle or a toxic substance change course before they make contact with it. Roots can tell whether nearby roots are self or other and, if other, kin or stranger. Normally, plants compete for root space with strangers, but, when researchers put four closely related Great Lakes sea-rocket plants (Cakile edentula) in the same pot, the plants restrained their usual competitive behaviors and shared resources.” [2]

Plant Cognition? On Vascular Awareness Systems.
The “Do Plants Have Brains?” article, which critically evaluates the possibility of a plant nervous system, puts forth, “First, plants have genes that are similar to those that specify components of animal nervous systems”, “Second, although those proteins more than likely do not have ‘neural’ functions in plants, some plant proteins do behave in ways very similar to neural molecules. Third, some plants seem to show synapse-like regions between cells, across which neurotransmitter molecules facilitate cell-to-cell communication. Included in the requirement for comparison is that the regions should have the same characteristics as animal synapses, such as the formation of vesicles, small bubbles that store the neurotransmitters that are to be released across the synapse. Fourth, many plants have vascular systems that look like they could act as conduits for the “impulses” that they need to transmit throughout the body of the plant. Last, some plant cells display what could be interpreted as action potentials—events in which the electrical polarity across the cell membrane does a quick, temporary reversal, as occurs in animal neural cells.” [3] (The article does, however, find the similarities un-persuasive, permitting metaphor at best.)

But what if neurons do not hold a monopoly on consciousness or cognition? The possibility remains of decentralized and emergent intelligence diffused across a plant, a network of plant tips, comparable to a swarm or colony, an inherently modular and resilient pattern, performing distributed, often parallel computation. The possibility remains of a dispersed plant nervous system based on vascular strands, cellular end-poles, root apexes, phloem and xylem, and polarly-transported auxin, emphasizing hormones instead of neurons. Even if purely affective (feeling) rather than cognitive (thinking), this would have significant consequences. Visit http://www.plantbehavior.org/, formerly known as the Society for Plant Neurobiology, for more research, including examples of “plant cognition” such as sensitization and habituation.

Philosophical Conclusions. Plants may not as far as we know perceive pain, however, plants do use a variety of sense receptors to detect and adapt to numerous stimuli and environmental variables: light, chemical substances and gradients, water and humidity, gravity and pressure, temperature, orientation, soil structure, nutrients and toxins, microbes, herbivores, chemical signals from other plants, tissue damage and infections, seasons. Plants in fact have capacities to “see”, “smell”, and “feel”, just in their own ways. While rooted, plants even perform rudimentary kinesis, exploring slowly, yet adaptively, both above- and underground. Plants interpret location, direction, and motion in beneficial ways. Notably, plants have some forms of basic memory, and demonstrate epigenetic adaptation, as well as anoetic consciousness. Plants even perform complex inter-signalling and exchange, including for mutual aid, and across species. Plants create impressive scalable networks of self-maintaining, self-operating, self-repairing units cooperating adaptively. And, plants possess vascular systems of awareness that may go even deeper.

While any inquiry into an organism’s potential capacity for sentience and subjective experience encounters the philosophical “Problem of Other Minds“, we can nevertheless definitively state that even individual plants wield remarkable capacities for sensation, perception, and awareness, and employ both cooperative and competitive strategies, actively pursuing aliveness and self-preservation with some semblance of living intention. Human consideration must go — as most traditional indigenous societies have gone — beyond the view of plants as mere machines, instead embracing a view of pervasive and even willful vitality to which we belong and depend on absolutely. Return to the land of the living, and remember plants as kindred Earthlings. Because even the seedling knows to reach for the light.

Sources Cited
1. “What a Plant Knows”, by Daniel Chamovitz. Published in 2012 by Scientific American / Farrar, Straus and Giroux. Book. 2012.
2. “The Intelligent Plant”, by Michael Pollan. Web article. 23 December 2013. Location: http://www.newyorker.com/magazine/2013/12/23/the-intelligent-plant
3. “Do Plants Have Brains?”, by Rob DeSalle and Ian Tattersall. Web article. 2012. Location: http://www.naturalhistorymag.com/features/152208/do-plants-have-brains

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