Arguably the most salient characteristic of sentience is the capacity for subjective awareness of sensations and emotional states that are pleasant or unpleasant (DeGrazia 1996, p. 99). Specifically, sentient beings have interests, preferences, and cares associated with avoiding pain, fear, and anxiety (Rowlands 2002, p. 11). They have unpleasant sensory experiences associated with actual or potential tissue damage, emotional responses to perceived threats to their physical or psychological wellbeing, and the desire to evade both.
Although it is generally assumed that only animals have these experiences, ample evidence exists to support the proposition that plants, too, are sentient. This is not a new proposition. It is a common view among animists that dates back many millennia, is a central principle of Jainism , and was defended by Aristotle’s student and subsequent director of the Lyceum, Theophrastus (Hall 2011). But it has been out of favor even among plant scientists until quite recently. The bulk of the evidence in favor of plant sentience has been provided by specialists in plant neurobiology, which is a misnomer since plants do not have neurons, synapses, central nervous systems, or brains. For this reason, plant neurobiology is the subject of controversy among plant scientists (Alpi et al. 2007; Firn 2004).
But plants do possess a homologous information-processing system that integrates incoming data – on light, water, gravity, temperature, soil structure, nutrients, microbes, herbivores, and other plants – and coordinates behavioral responses. Plant neurobiologists offer ample reason to accept that these are not simply automatic reactions to specifiable stimuli. This is not to say that the lived experience of plants is somehow the same as that of animals. Among other differences, plants’ lives unfold in a much slower time frame than ours. So while plant neurobiologists use terminology to characterize plant behavior that is usually reserved for animals, they do not insinuate that we should regard the two as experientially interchangeable.
Sensation, Pain, and Homologous Systems
Chamowitz states that while the sensuous experiences of plants and animals differ qualitatively, due in part to the fact that plants have no discernible organ of perception, plants have sensuous capacities that are at least functionally comparable to our five senses . “Sight is the ability not only to detect electromagnetic waves but also the ability to respond to these waves” (2012, p. 24). Plants may not translate light reflecting off objects into images, but they do convert it into cues for physiological, morphological, and phenotypic development. They not only distinguish light from shadow but also track seasons and the time of day. It is no particular advantage for plants to see like we do, but it certainly benefits them to know the direction, intensity, and duration of light sources and potential obstructions to accessing it.
The only difference between taste and smell is that the former is dependent on solubles (by dissolution or liquefaction) while the latter depends on volatiles (on the inhalation of gases). Plants sense and respond to volatiles in the air and to solubles on their bodies. They are highly sensitive to pheromones, just as we are, using them for communication and signaling.
Plants also are able to hear. Appel and Cocroft (2014) have observed plants release defensive chemicals upon playing a recording of a caterpillar biting a leaf. One means by which plants distinguish self from other – yes, they exhibit indicators of an internal sense of self – is through the recognition of unique oscillatory signals. The movements of each plant register on a frequency not shared by other plants (Gruntman and Novoplansky 2004).
Also like animals, the sense of touch is electrochemically mediated in plants. Plants’ sense of touch is highly developed (Trewavas 2005). Roots know when they encounter solid objects. Bushes and trees alter their morphology in response to high wind exposure. This is an evolutionary adaptation that increases their ability to survive perturbations.
Plants do not need a central nervous system to have these sensuous experiences. Do they need one to experience pain and suffering ? They do respond physiologically to leaf punctures from insect bites, being burned, and drought conditions in ways that are consonant with self- and kin preservation. “But plants do not suffer,” Chamowitz remarks. “They don’t have, to our current knowledge, the capacity for an ‘unpleasant’ […] emotional experience” (2012, p. 139). Their responses to wounding are not the equivalent of an ouch, even if these responses do entail the modulation of their development. Indeed, plants do not have nociceptors : neurons that specialize in sensing noxious stimuli. Nociceptors exist to enable their hosts to withdraw from dangerous situations and alert their hosts to internal physical problems (Rachels 2004, p. 77f.). (The activation of nociceptors is not a conscious event per se, although the detection of noxious stimuli often is accompanied by conscious – typically painful – events.) Without nociceptors , it is not clear that these experiences are possible, Chamowitz concludes.
Why then do plants excrete endogenous opioids , most notably ethylene, when wounded or subject to stress (Buhner 2002, p. 197)? Why are they rendered unconscious by the same anesthetics that work on animals (Pollan 2013)? (Perhaps plants are not conscious in the sense of having something like subjective awareness of themselves experiencing the world, but they can be awake and aware of the world around them. Marder (2014, p. 182) notes that plants are much like us with respect to consciousness, at least in one sense. They sleep.) How they respond to insect bites, fire, and drought may provide an answer. Plants cannot escape noxious stimuli, but they do send signals indicating that the environment has become dangerous. So perhaps some sort of pain analogue plays a communicative role within and between plants, warning other parts of the plant as well as neighboring plants to make physiological adaptations to increase their chances for survival. Such an analogue need not be qualitatively equivalent to pain in order to be functionally equivalent to it . This is what Chamowitz suggests at any rate (2012, p. 68; see also Dicke and Braun 2001; Weiler 2003; Trewavas 2005; Barlow 2008).
Furthermore, plants’ electrochemical information-processing system is homologous to animals’ neural network . When a plant is wounded or experiences stress, electrical signals that are similar to action potentials running along nerves are sent from one region of a plant to another via voltage changes across cell membranes. These signals induce a rapid change in hormone metabolism. Specifically, they elevate auxin levels (Barlow 2008). Auxins are a class of plant hormone with morphogenic qualities. They induce cell differentiation and regeneration at the site of injury and are secreted from cell to cell along sieve tubes, elongated cells of phloem tissue, which are akin to synapses (Volkov 2000; Thompson and Holbrook 2004; Trewavas 2007). Auxins play a crucial role in coordinating plant growth and behavior, sometimes via long-distance signaling (Struik et al. 2008, p. 369). When the plant is wounded, auxins often induce cell differentiation and regeneration of vascular tissue.
The system thus has neurotransmitter-like characteristics (Dziubinska 2003; Baluška et al. 2006; Brenner et al. 2007; Fromm and Lautner 2007; Baluška and Mancuso 2009). “At the molecular level,” Brenner and his colleagues hereby note, “plants have many, if not all, the components found in the animal neuronal system” (2006, p. 414). According to Baluška (2010), electrical long-distance signaling and the existence of processes akin to action potentials in plant cells and tissue support the proposition that the sensuous abilities of plants are not limited in comparison to animals. Indeed, plants produce several proteins found in animals’ neuron systems, including acetylcholine esterase, glutamate receptors, GABA receptors, and endocannabinoid signaling components. But we should not make a fetish of neurons, Mancuso remarks. Neurons “are really just excitable cells” (quoted in Pollan 2013, p. 92). Plants have excitable cells too – particularly at the root apex of the transition zone.
Sessility, Modularity, and Self-Awareness
thinks the only substantive difference between us animals and our distant green relatives is how mobile we are. We’re used to judging intelligence by actions, he says. It’s what we do and say that reveal what goes on inside our minds. So plants, silent and rooted to the spot, naturally don’t appear too bright. But they do move and they do react to the world around them, he says, and they do so in intelligent ways. Plants can assimilate information, calculate outcomes, and respond using a complex series of molecular signaling pathways that are remarkably like those in our own brains . (Philips 2002, p. 40)
Sessility is typically regarded as a disadvantage , since it purportedly prevents plants from escaping danger. But sessility may account for the greater genetic complexity of plants with respect to animals. Plants must respond with far greater precision to the conditions they face than animals, since animals have the option to relocate (Trewavas 2009). Sessility thus requires an extensive, nuanced understanding of one’s immediate environment not just to address dangers but also to access light and acquire water and nutrients. No wonder plants have biochemical constitutions that permit them to identify specific insects from the taste of their saliva after being bitten, deter and poison predators, and recruit insects to perform services for them (Buhner 2002, p. 162).
Consider these three examples. Tomatoes that are subject to damage by insects and herbivores produce methyl jasmonate as an alarm signal. Plants in the vicinity detect it and prepare for attack by producing chemicals that defend against insects or attract their attacker’s predators (Farmer and Ryan 1990). They do not do so when subject to mechanically induced wounding, as from high winds. This indicates the capacity for discernment (Paré and Tumlinson 1999). Acacia trees excrete an unpalatable tannin to defend themselves when being eaten by animals. The scent of the tannin is picked up by other acacia trees, which then also excrete it (van Hoven 1991). And when attacked by caterpillars, some plants release chemical signals that acttract wasps, who attack the caterpillars (Thaler 1999).
Plants’ sessility also accounts for their modularity ; the operational control of growth and behavior is devolved among thousands of meristems at the tips of roots and shoots (Baluška et al. 2005; Brenner et al. 2006). This too has advantages unavailable to animals. Plants may not be able to escape danger, but they can survive extensive damage. Their modular design permits plants to lose up to 90% of their bodies without dying. “Brains come in handy for creatures that move around a lot,” Pollan states, “but they’re a disadvantage for ones that are rooted in place. Impressive as it is to us, self-consciousness is just another tool for living, good for some jobs, unhelpful for others” (2013, p. 92). Plants have no irreplaceable organs, so being food is not necessarily a death sentence. “There’s nothing like that in the animal world. It creates resilience,” Pollan adds (2013, p. 92). Neither being sessile nor having a modular design entails sentience , of course. But they should not be taken as strikes against sentience either.
Among the core signatures of sentience is self-awareness : the capacity to distinguish self from other or, as DeGrazia puts it, “the ability to distinguish one’s own body from the rest of the environment” (1996, p. 166). Available evidence suggests that this too is a capacity that plants have. Plants differentiate themselves from competing and noncompeting organisms and discriminate between kin and nonkin (Karban and Shiojiri 2009). The former capacity is on display when plants must determine whether to compete for or share scarce resources with neighbors (Gersani et al. 2001; Maina et al. 2002; Callaway et al. 2003). The latter capacity includes rejecting pollen contributed by plants with which the recipient shares an allele (Biedrzycki 2010).
Intelligence, Learning, and Intentionality
For sessile beings, the centralization of systems governing signal integration and response is a disadvantage. The decentralized character of these systems in plants is an extension of their modularity . Does this entail that plants lack the capacity for cognition ? Along with phenomenologists and researchers of swarm behavior , a number of cognitive scientists and philosophers of mind reject the proposition that not just cognition but intelligent behavior must result from centralized processes. Marder suggests that intelligence “is not concentrated in a single organ but is a property of the entire living being” (2012, p. 1370). According to Trewavas, “nervous systems are not necessary; complex networks are sufficient to create intelligent behavior” (2012, p. 773; see also Schull 1990; Warwick 2001; Vertosick 2002). And Garzón and Keijzer (2009) contend that the “embodied cognition ” of plants permits them to organize their behavior in complex and adaptive ways.
Stenhouse (1974) suggests that intelligence should be conceptualized as the adaptively variable behavior of an individual organism during its lifetime. Mancuso defines the term “very simply” as the ability to solve problems. Plants do both. Trewavas notes that “plants respond to an enormous variety of physical, chemical, and biological signals in the environment to maximize foraging for resources in two distinct but unpredictable environments: above and below ground” (2012, p. 773). This requires not just considerable physiological, morphological, and phenotypic plasticity but an acute awareness of one’s lived conditions as well. It also requires the capacity to process information from both biotic and abiotic stimuli in order to discriminate between positive and negative experiences, make predictive assessment, weigh trade-offs, and formulate informed decisions (Hutchings and de Kroon 1994; de Kroon and Hutchings 1995; Grime and Mackey 2002; Lyon 2006). Add to this the need to counter threats from predators and disease and the fact that “Mate selection is elaborate and underpinned by discriminating, complex conversations that precede and follow fertilization” (Trewavas 2012, p. 773), and it becomes clear that explaining plant behavior in terms of tropisms – reflex reactions to external stimuli – alone is inadequate.
Among others, Trewavas and Mancuso both contend that plant intelligence may be quite similar to swarm behavior , exhibited in the comportment of bird flocks and insect and bee colonies. Swarm behavior is a manifestation of distributed intelligence, an emergent property of the organization of “mindless” individuals. Swarms are not coordinated by a central system. As Pollan states, “In a flock, each bird has only to follow a few simple rules, such as maintaining a prescribed distance from its neighbor, yet the collective effect of a great many birds executing a simple algorithm is a complex and supremely well-coordinated behavior” (2013, p. 92). As Seeley and Levien note, “It is not too much to say that a bee colony is capable of cognition in much the same way that a human being is. The colony gathers and continually updates diverse information about its surroundings, combines this with internal information about its internal state and makes decisions that reconcile its wellbeing with its environment” (1987, p. 39). A similar phenomenon takes place within plants. Meristems act as do individual birds in a flock or ants in a colony. They gather, assess, and respond to environmental data in local but coordinated ways that benefit the organism as a whole (Noble 2006, p. 113; Ciszak et al. 2012; Marder 2013, p. 169). Indeed, while brains may provide a centralized site of cognition, they operate internally in much the same way (Trewavas 2005, p. 414).
Especially when we humans are young, perhaps the most salient aspect of our intelligence is our capacity to rapidly absorb vast quantities of information. Until we reach puberty, we are learning machines, as it were. We develop memories through our experience, and we establish and modify behavior and make predictions about future events in consonance with these memories. Like all cognition, our memories have a biochemical basis. This also is the case for plants, who remember and learn as well (Bhalla and Iyengar 1999; Brenner et al. 2006).
Memory and learning in animals involve the development of new connections within the neural network, but this is not the only way that information is stored. Pollan remarks that immune cells remember their experience of pathogens, calling on these memories upon future encounters. So-called “epigenetic” effects from traumatic events can be passed to offspring, such as through genetic coding for specifiable levels of cortisol (Molinier et al. 2006). These are the sorts of biochemical processes that contribute to the development of memories by plants (Sung and Amisino 2000, 2004). Memories are thus inscribed in the bodies of plants rather than actualized in subjective consciousness, Marder asserts (2014, p. 33). Some plants learn faster than others, of course, but observed behaviors do not have the traits of innate or programmed responses. Plants even can be taught to learn more quickly and to better retain what they have learned (Gagliano et al. 2014).
With respect to making predictions , plants take great care to ensure that the energetic costs of possible actions do not exceed their anticipated benefits. Future sun and shade patterns are predicted based on the perception of reflected far-red light (Izaguiree et al. 2006). The Mayapple commits to a branching and flowering pattern early in its life, using an analysis of leaf and branch patterns above it to anticipate gaps and where light will land coming through them (Geber et al. 1997). The dodder, a parasitic and nonphotosynthetic plant, discriminates between potential hosts. Once a suitable host is chosen, it decides how many coils to make around it in order to optimize the return on energy investment (Kelly 1990). The stilt palm “walks” out of shade by differential growth of its roots (Trewavas 2005). And when fewer nutrients are found, plants are able to both accelerate root growth (Callaway 2002) and increase the rate of nutrient absorption (Jackson and Caldwell 1996).
After watching time-lapse photography of a bean plant growing, Pollan describes a “seemingly conscious individual with intentions” (2013, p. 92). A bean sprout actively searches for something to climb, but it does not just grow any which way until it encounters something suitable. It seems to know exactly where to go long before making contact, perhaps through the use of echolocation or by sensing the reflection of light off its surroundings. Whatever the case may be, the bean sprout does not waste energy fruitlessly. “And it is striving (there is no other word for it) to get there: reaching, stretching, throwing itself over and over like a fly rod, extending itself a few more inches with every cast. […] As soon as contact is made, the plant appears to relax; its clenched leaves begin to flutter mildly” (ibid).
Perhaps Pollan simply witnessed the results of tropistic biochemical reactions. Biochemical reactions, yes, but our brains make decisions in the same way. Tropistic, no, or at least this is Marder’s conclusion. “To be conscious is to intend something, that is to say to be directed toward the intended object. […] In light of this definition, the intentionality of plants may be understood as the movement of growth, directed toward optimal patches of nutrient-rich soil and sources of light” (2012, p. 1367; see also Darwin 1880, p. 573). It involves the deliberate prioritization of some sources over others. Animals’ intentions, enactments of directness, are carried out via muscle movement. Intentionality in plants “is expressed in modular growth and phenotypic plasticity” (ibid; see also Hutchings and de Kroon 1994). Again, it pays in terms of survivability for sessile beings to be highly attentive to the context of growth and development. Vigilance and the capacity to respond as one’s context changes are key. Sometimes this requires noncognitive action, something akin to our instinctive drive to flee. Sometimes careful and focused cognition is in order. It would seem that plants do both, even if these actions generally occur in a time frame that we have difficulty discerning.
Teaching and Nurturing
Quite often, before it can establish itself in a new location, a keystone species must have a plant that goes first and prepares the way. These initial species, usually selected from among the community of plants that grow with the keystone plants, are the outriders, the plants whose emergence signals the movement of plant species in mass. […] These plants move first and essentially determine what keystone plants will grow where and when. In this way, they act as “filters” through which keystone species are sifted. (2002, p. 181)
Outriders make note of the quality of the soil, water, and light and send a return signal to tell the keystone species where and when to send seeds. Neither wind nor animal dispersal can explain how these seeds move where they move, Buhner contends. “The distances are too far, the dispersal patterns too unusual. But by whatever means, the seeds answer the chemical call sent by the nurse plants ” (ibid).
Once established at the new location, keystone species then call to them soil bacteria , mycelia, and the plants that constituted their previous archipelago. “As the plants arrive, the keystone chemistries literally inform and shape their community structure and behavior. This capacity of keystone species to ‘teach’ their plant communities how to act was widely recognized by indigenous and folk taxonomies” (ibid, p. 183).
Plants also can nurture other plants directly. In the Great Basin of Utah and Nevada, sagebrush nurses piñon pines until the pines are old enough to grow on their own. The sagebrush alters the soil chemistry and provides physical protection from the elements (Callaway 1995).
Plant Sentience: The Speculative Leap
Plants have an information-processing and response system that is homologous to a central nervous system, and they exhibit some key characteristics of beings who suffer. Evidence supports the proposition that they are self-aware and highly attentive to their environments, exhibit intelligence and intentionality, and can remember, learn, and even teach. Appealing to the parsimony principle , “which is the cornerstone of scientific approaches,” Paul Struik and his colleagues assert that the kind of evidence cataloged here does not permit “unfounded philosophical speculation” (2008, p. 369).
Yet whether or not the consideration offered here violates the parsimony principle is itself a philosophical question, as is the merit of adhering to this principle in the first place. Inferring the sentience of other-than-human animals requires empirical evidence on a par with the evidence provided here regarding plants. But neither determination can rely on empirical evidence alone. Hall remarks that “our perception of plants depends heavily on our philosophical orientation” (2011, p. 35). At the end of the day, the need to take a speculative leap is a professional hazard that every philosopher must face. Marder infers plant sentience “from the fact that plants explore and pursue unevenly distributed resource gradients, assess environmental dangers from biotic and abiotic stressors and gather and constantly update various types of information about their surroundings” (2012, p. 1368). But do they actually explore, assess, gather, and update? These are signs of his leap.
For what it is worth, plants make up 99% of the biomass on Earth. They dominate every terrestrial environment. We animals are the outliers. Of course, it is possible that only we animals – indeed, just a subcategory of us – are sentient, but a shift in the burden of proof is overdue.
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