Debunking a myth: plant consciousness

Abstract

Claims that plants have conscious experiences have increased in recent years and have received wide coverage, from the popular media to scientific journals. Such claims are misleading and have the potential to misdirect funding and governmental policy decisions. After defining basic, primary consciousness, we provide new arguments against 12 core claims made by the proponents of plant consciousness. Three important new conclusions of our study are (1) plants have not been shown to perform the proactive, anticipatory behaviors associated with consciousness, but only to sense and follow stimulus trails reactively; (2) electrophysiological signaling in plants serves immediate physiological functions rather than integrative-information processing as in nervous systems of animals, giving no indication of plant consciousness; (3) the controversial claim of classical Pavlovian learning in plants, even if correct, is irrelevant because this type of learning does not require consciousness. Finally, we present our own hypothesis, based on two logical assumptions, concerning which organisms possess consciousness. Our first assumption is that affective (emotional) consciousness is marked by an advanced capacity for operant learning about rewards and punishments. Our second assumption is that image-based conscious experience is marked by demonstrably mapped representations of the external environment within the body. Certain animals fit both of these criteria, but plants fit neither. We conclude that claims for plant consciousness are highly speculative and lack sound scientific support.

Keywords: Cell consciousness; Classical (Pavlovian) learning; Plant and animal consciousness; Plant electrophysiology; Proactive behavior; Reciprocal signaling.

Fig. 1

Phloem vascular system. a Sieve element. b Sieve tube consisting of seive elements. Reprinted with permission from Taiz et al. (2015), Sinauer, Oxford University Press

Fig. 2

Extensive reciprocal communication (back-and-forth arrows) between processing centers (ovals) in the human brain is an indicator of consciousness. Such integrative communication also occurs within the centers (not shown). This is a side view of the brain, with anterior to the right. For simplicity, we only label/number a few of the main centers: 1. primary visual cortex of the cerebrum; 2. somatosensory cortex; 3. amygdala (for fear and other emotions); 4. thalamus; 5. superior colliculus of midbrain (optic tectum). Modified from Fig. 3 in Feinberg and Mallatt (2020)

Fig. 3

Longitudinal view of the phloem in Dahlia pinnata. Patterns of phloem anastomoses (arrows) are evident between the longitudinal vascular bundles. The phloem was removed from the xylem at the cambial zone, and is shown from the cambium side in an intact stem, stained with aniline blue and observed under epifluorescence microscope. Scale bar = 100 μm. Micrograph is a gift from Roni Aloni

Fig. 4

Histomicrograph of a root tip, from flax (Linum usitatissimum). The zone of elongation lies just above the top of the micrograph. Reprinted with permission from Taiz et al. (2015), Sinauer, Oxford University Press

Fig. 5

Classical associative learning, step by step. Here, a dog learns from a ringing bell (conditioned stimulus) that is presented prior to the smell of food (unconditioned stimulus) to salivate in response to the bell sound alone. Above, the pretraining steps show that the food alone induces drooling (a) but the bell alone does not (b). Training (c) rings the bell before presenting the food. After training (d), the bell alone induces drooling

Fig. 6

Classical learning by a spinal cord. Drawing is modified from Huie et al. (2015). The spinal cord was transected in the upper thorax 1 day before the learning experiment, so it was isolated from the brain. CS means conditioned stimulus (leg shock) and US means unconditioned stimulus (tail shock). During training, the mild shock to the leg is given just before an antinociceptive shock to the tail, the latter being a shock that naturally diminishes tail flick in response to the focused heat. With learning, the leg shock diminishes this tail flick when given alone (that is, it increases the latency time), having become antinociceptive

Fig. 7

Limited operant learning by an isolated spinal cord. In the picture, the rod electrode also acts as a plate for the rat’s foot. A shock from the shocking electrode causes the leg to lift up, after which the leg naturally drifts down so the rod electrode enters the salt solution, completing a circuit that delivers another shock. As the cycle keeps repeating, the time it takes for the leg to drift down increases as the cord learns to delay the next punishing shock. Picture modified from Grau et al. (1998)

Fig. 8

Neuronal sensory maps in the human nervous system. For each sense, a path of several neurons (far right) is a hierarchy that carries signals up to the brain, keeping a point-by-point mapping (A, B, or C) of the outside environment or a body structure. On reaching the cerebral cortex, this leads to the mapped neural representations that are shown around the brain. Information from the different senses is combined for multisensory integration (Stein et al. 2020), especially in the posterior cortical hot zone (Koch : p. 61). Here, this seems to lead to a unified, all-sense map of the world that characterizes consciousness. This illustration is from Feinberg and Mallatt (2018). Used with permission from © Mount Sinai Health System

Fig. 9

Summary figure showing that the conscious organisms, all of which we deduced to have both affective and image-based consciousness, do not include plants. The vertebrates are Komodo dragon Varanus and bowfin fish Amia. The arthropods are crustacean Nebalia and ladybug beetle Coccinella. The cephalopods are a cuttlefish and an octopus