"The tip of the root acts like the brain of one of the lower animals."
Charles Darwin, with Francis Darwin, 1880
We must come to terms with the matter of plant consciousness. When we say "vegetative", we think of absence: no feeling, no willful acts, only passive reaction. But that does not describe the vegetal kingdom: sensing and moving to satisfy its wishes, a plant can hold abstract thought, plan for the future, and coordinate with kin. Plants have memory, sophisticated senses, and exquisite control over how their genetic blueprint unfolds into physical form. We dismiss them simply because of a difference in perspective, and do so at our peril.
Simple plant behaviors are familiar: responses to gravity and sunlight are part of the children's science fair repertoire, as are experiments showing the effects of different nutrient concentrations. But these controlled tests are hardly a challenge, and don't showcase true decision-making in complex situations. Even more startling behaviors, like the sensory discrimination of a Venus' fly trap that lets it distinguish an insect from a leaf, can be dismissed as multi-step reflexes. There is no "mind" here, no central control structures, no learning.
Or is there? Recent investigations suggest a site for an integration and management network in plants, and at the same time describe the network's components--a "brain", so to speak, and its "neurons". Plant neurobiology has developed dramatically over the last two decades, and uncovered "sophisticated plant competencies" including the capacity for memory and abstraction (Trewavas 2005). One of the first confirmatory experiments looked at heliotropism, or the ability of plants to turn towards the sun during the course of the day. Not all plants exhibit this overt behavior, but when they do, they seem able to foresee--and remember--where the sunlight will come from the next morning. During the night, they "reset" and point back east, not waiting for the sun, but anticipating it. This persists for a few days even after the plants are put into 24-hour darkness. The inevitable conclusion: plants have an internal model of day length and sunlight direction, and they refer to it to guide their behavior. Plants have circadian rhythms (Cashmore 2003).
In searching for a location where these internal rhythms may be "stored", plant neurobiologists have converged on the root system, confirming the accuracy of Darwin's almost 150-year-old intuition. It starts with the root tip--an interactive, mobile structure that seems to serve as the building block of a plant's "brain". There are different functions and cell types in a root tip, including the apex--a sensory unit, the leading exploratory edge; the elongation zone--a growth and movement area; and a crucial transition zone between them (Baluška 1990).
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| 1: meristem and transition zone; 2: root cap; 3: lateral cells; 4: dead root cap cells; 5: elongation zone |
This transition zone takes signals from the apex, the rest of the plant, and the surrounding environment and sends them up the root through the elongation zone. It does this using chemicals, including auxin but also more familiar ones like GABA, that are stored in small vesicles, released like neurotransmitters, and recycled for future use (Baluška 1997). This is similar to the work of our own neurons, and has a critically important feature: the transition zone is a place where simple decisions can happen, based on the totality of inputs around the root tip. It's not a reflex, it's integration. In an incredible twist, recent research found that plants "fall asleep"--they stop engaging in their normal cyclical behaviors and lose the ability to respond to the environment--when exposed to anesthesia drugs (Yokawa 2018). The transition zones are quiet, and neurotransmission stops. The plant is unconscious.
To describe the "consciousness" of a non-anesthetized plant, we need to look at the network of root tips, and their interactions on wide scales across the entire root mass. Local interactions allow for the coordination of root growth: pioneering rootlets, when detecting an obstacle, relay that information to other branches that then modify their behavior and growth even though they lack a direct stimulus to do so. More widespread interactions carry immunologic information--like damage or infection--from above ground to the roots, where the memory of damage is stored and helps support the ongoing production of defensive (and medicinal) compounds. The roots carry a deeply detailed, multifaceted model of a plant's surroundings, and help direct the growth and chemistry of the above-ground parts, coordinating their interactions with the sunlit world (Li and Zhang 2008).
Recognizing that plants have models of circadian and seasonal change, and may have many other internal models besides, is important because it provides a very different perspective from the one that assumes plants are simply reacting to sunlight or predatory insects. Rather, they are comparing current conditions to stored patterns of association--memories--and making decisions based on that comparison (Brenner, Baluška et al. 2006). That's exactly what we do. One of the reasons we still see ourselves as different (superior?) is that we have rapid-cycling behaviors, in large part due to differences in feeding strategies. As animals, we need to find food and eat it: a cycle of movement and feeding that repeats rapidly, sometimes many times during a day. Plants, on the other hand, have a single solar feeding cycle, and don't need to move to eat. During the day, they are constantly feeding and synthesizing new molecules, relying on their above-ground parts to gather energy. We have evolved to picture the 3D space around us, we can see for long distances, and we can move towards the resources we need and away from what threatens us. Plants don't need these skills: rather, like master diplomats, they focus on influencing the world around them. They alter the environment to suit their needs, rather than moving to a more favorable environment.
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| Rhizosphere by Zebragurl4, unchanged. CC lincense: https://creativecommons.org/licenses/by-sa/4.0/deed.en |
To appreciate this power, we come right back to the root system. Almost every growing plant has developed a symbiotic relationship with specific strains of fungi that live among the roots, helping to unlock mineral resources from the soil and receiving sugars--the fruits of photosynthesis--in return. These mycorrhiza provide another layer of complexity and synergy, allowing plants to share resources across entire communities, knitting together the "wood wide web" (Simard 1997). To attract these fungal symbionts, plants secrete signal molecules (including strigolactones, specific bitter lactones derived from carotenoids). But they don't use a targeted strategy: they saturate the environment right around the root tips, and as the distance increases, the molecules break down at a predictable rate. Fungi, sensing the concentration shifts between "old" and "fresh" strigolactones, quickly find their vegetal partners and the mycorrhiza are born (Schliemann 2008). This unification has been playing out underground for the last 400 million years.
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| Mycorrhiza by Mylène Durant, unchanged. CC license: https://creativecommons.org/licenses/by-sa/4.0/deed.en |
There are many social interactions beyond this foundational one, and together, they make up the rhizosphere of plants. Bacteria, other flora and fauna, the roots of neighboring plants all commingle. In some cases, plants repel members of different species--like the bitter wormwood, they exhibit allelopathy. In others, different individuals in the same grove--oaks, sugar maples, hemlocks and apple trees all do it--entwine their roots, grafting one onto the other (an event known as inosculation). If the root systems of plants help to hold their memories and behavior patterns, inosculation must be akin to the deep, non-verbal understanding we can sometimes experience after years of knowing another human being--though in the plants' case, the union is more literal.
The rhizosphere extends its influence above ground by helping to direct growth and the generation of new life. Along the way, a staggering array of chemistry is produced: from volatiles that both warn of danger and attract beneficial partners, to pigments for protection and signaling, to nectars and sugars. And whether above or below the surface of the soil, plants adopt the same strategy we saw them use with their fungal symbionts: saturation. This dispersal of vast quantities of signal molecules creates powerful drives in the creatures that perceive them, from mycorrhiza, to bacteria, to insects and other animals. In many cases, these drives provide mutual benefit (in others, it's more murky for us animals. See Pollan 2002). It is somewhat like the secretion of hormones and other signal molecules by stationary cells inside our bodies--but on a scale billions of times broader. This strategy makes sense for a being who can't move. It began when plants reshaped the planet by saturating it with oxygen. It continues when an oak tree, during mast years, saturates the forest with acorns to safeguard the next generation. Plants can afford it: they soak up sunlight and carbon, becoming master chemists with near-unlimited access to raw materials. And the consequences are profound.
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| Forest by Sémhur, unchanged. CC license: https://creativecommons.org/licenses/by-sa/3.0/deed.en |
It's incredible to think of all this for an individual plant. Each one, on its own, is so capable! But now, remember that plants are everywhere--imagine their power and collective wisdom at such a wide scale. They turn sunlight into energy that feeds us all. But they also, through the conscious, intelligent networks that course across their root systems, make long-term sustainability decisions that affect every ecological niche they inhabit. They influence their environment in complex ways, and using a myriad of tools they saturate the planet with their will. Their chemistry brings the whole world into communion. Plants are a force.
- Baluška, F., Š. Kubica, and M. Hauskrecht. "Postmitotic ‘isodiametric’cell growth in the maize root apex." Planta 181.3 (1990): 269-274.
- Baluška, Frantisek, et al. "Rearrangements of F-actin arrays in growing cells of intact maize root apex tissues: a major developmental switch occurs in the postmitotic transition region." European journal of cell biology 72.2 (1997): 113-121.
- Brenner, Eric D., et al. "Plant neurobiology: an integrated view of plant signaling." Trends in plant science 11.8 (2006): 413-419.
- Cashmore, Anthony R. "Cryptochromes: enabling plants and animals to determine circadian time." Cell 114.5 (2003): 537-543.
- Li, Xia, and W. S. Zhang. "Salt-avoidance tropism in Arabidopsis thaliana." Plant Signaling & Behavior 3.5 (2008): 351-353.
- Schliemann, Willibald, Christian Ammer, and Dieter Strack. "Metabolite profiling of mycorrhizal roots of Medicago truncatula." Phytochemistry 69.1 (2008): 112-146.
- Simard, Suzanne W., et al. "Net transfer of carbon between ectomycorrhizal tree species in the field." Nature 388.6642 (1997): 579.
- Trewavas, Anthony. "Green plants as intelligent organisms." Trends in plant science 10.9 (2005): 413-419.
- Yokawa, Ken, and František Baluška. "Fish and plant sentience: Anesthetized plants and fishes cannot respond to stimuli." Animal Sentience 3.21 (2018): 6.
