Plant cognition after Darwin: historical and epistemological remarks
Plant “intelligence” and “sensation” are controversial notions in contemporary botany. While some scientists argue that sensory awareness, learning, memory, communication, and possibly a kind of consciousness should be ascribed to plants, a substantial part of the scientific community dismisses such claims because of insufficient evidence and the lack of neural systems in plants. In this paper, I will examine this issue from a historical and epistemological perspective. First, I will go back to a crucial precedent that is often evoked in these debates, i.e., Darwin’s remarks on plant “sensation” and “behavior” and his analogy between roots and brains in the 1880 book The Power of Movement in Plants. I will point out that the issue was already well known and controversial in the nineteenth century. I will then examine contemporary debates starting from new experiments and evidence on plant cognitive powers. I will argue that current debates are partly based on elements that have been available since Darwin, such as postulates of biological continuity among living beings and the use of analogy, leading to an epistemological contrast between the need to avoid ungrounded anthropomorphic projections and the argument that kinds of cognitive states could be realized in plants. I also finally point out that investigations on animal ethics and the revaluation of non-Western views on animism and ecology have recently been introduced into this debate to break the epistemological balance between opposing views.
Plant cognition is a controversial issue in contemporary botany and in the philosophy of life sciences. A number of scientists have argued that sensory awareness, learning, memory, communication, and possibly a kind of consciousness should be ascribed to plants. A substantial part of the scientific community has rejected their claims because of paucity of evidence and the lack of neural systems in plants, although further empirical investigations are ongoing. In light of new evidence, the debate concerns the attribution of different notions and capacities to plants, ranging from “intelligence” to “sensation”. This conceptual side of the investigation raises an epistemological question: how far can we legitimately stretch our analogies concerning the “mental” life of plants?
In this paper, I will examine, this issue in historical perspective, thereby aiming to contribute to a broader epistemological framework for investigating this controversy. I will start from Charles and Francis Darwin’s remarks on plant “sensation” and “behavior”, and their analogy between roots and brains in the 1880 book The Power of Movement in Plants. I will show that the issue was already well-known and controversial in the nineteenth century, as evolutionary theory incorporated previous physiological notions such as irritability and scientists were already divided on the attribution of mental capacities to plants, with positions ranging from reductive materialism to agnosticism and panpsychism.
Against this background, I will examine contemporary observations and experiments on plant cognitive powers together with the corresponding epistemological debates. I will identify a recurring epistemological contrast between the need to avoid ungrounded anthropomorphic projections and the possibility that kinds of mental states exist in plants. In addition, I will point out that new investigations on animal minds and ethics, and a revaluation of non-Western views on ecology have been recently introduced into this debate to upset the established epistemological balance.
In the final chapter of their 1880 book The Power of Movement in Plants, Charles and Francis Darwin formulated a famous analogy between the tip of radicles and brains:
“We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle. If the tip be lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides […]. In almost every case we can clearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, no doubt in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground must be determined by the tip; hence it has acquired such diverse kinds of sensitivities. It is hardly an exaggeration to say that the tip of the radicle thus endowed, 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” (Darwin & Darwin, 1880, pp. 572–573, my italics).
In this seminal text, the Darwins develop a progressive succession of concepts of capacities: functions, physiological transmission and purposefulness belong to a standard set of properties of living beings in modern life sciences. The attribution of “kinds of sensitivities” and the analogy of roots with the animal brain form a more excentric pair of notions: while similar ideas were widespread in the philosophical-scientific tradition since antiquity,1 the attribution of sensitivity to organisms without nervous system was (and is still) at odds with contemporary standards. Indeed, scientists who are recently reconsidering such ideas present Darwin’s views as a valuable theoretical precedent. As František Baluška, Stefano Mancuso and their co-authors write in a programmatic paper, The Power of Movement in Plants.
“represents a breakthrough in plant biology. In this revolutionary book, the Darwins departed from the classical and still dominant view of plants as organisms which had no need of movements that were based on sensory perceptions or a brain-like organ” (Baluška et al., 2009, p. 1121, my italics).
According to the authors, new evidence suggests that this pioneering view of plants as capable of a kind of sensory perception needs to be reevaluated:
“Present day results […] are increasingly coming to show that, in contrast with the classical view, plants are definitely not passive automatic organisms. On the contrary, they possess a sensory-based cognition which leads to behavior, decisions and even displays of prototypic intelligence” (Ibid., my italics).
This claim is based on evidence that root caps orient growth in directions that respond to environmental conditions (such as the distribution of resources and the presence of physical obstacles and biological dangers), whereas the severing of root caps is followed by growth in the previous direction regardless of conditions and obstacles. On the basis of this evidence, these authors associate a range of notions to Darwin’s influential account of plant cognition – from sensitivity to decision, behavior, and communication – arguing for the legitimacy of an analogy with animals that culminates in the attribution of brain-like functions to vegetal tissues.
“In keeping with Charles Darwin’s theory of common descent of all organisms, a unification of animals/humans and plants according to their body polarity is possible and thereby removes from view the Aristotelian dichotomy between plant and animal organisms, an axiom of classical biology which has posed a serious problem in the understanding of the logic of evolution and the nature of biological systems. The common descent of all organisms is the central pillar of Charles Darwin’s theory of evolution and, as pledged by Sir Jagadish Chandra Bose, the unity of life implied thereby is a revelation of both beauty and simplicity. By the same token, the existence of a plant neurobiology harmonises with the neurobiology of animals” (Ibid., p. 1125).
In order to examine this reference to Darwinian biology, we need to consider which theses were actually defended by Darwin in the context of nineteenth-century life sciences and philosophy.
Darwin’s views on plants’ cognitive powers, however based on extensive observations and experiments and unorthodox in the context of his time, were not entirely new. In fact, Darwin reprised the modern dichotomy between two competing views: the merely physiological notion of “irritation” and the attribution of full-fledged “sensation” (or sensitiveness) to plants. Both notions appear in different passages of The Power of Movement in Plants:
“Various trials were made for ascertaining, as far as we could, the nature and degree of irritation to which the apex must be subjected, in order that the terminal growing part should bend away, as if to avoid the cause of irritation” (Darwin & Darwin, 1880, p. 146, my italics).
“We were therefore led to suspect that the apex was sensitive to contact, and that an effect was transmitted from it to the upper part of the radicle, which was thus excited to bend away from the touching object” (Ibid., p. 131, my italics).
The alternative between irritability and sensibility of living tissues has a long tradition in modern philosophy and physiology.2 Irritability was originally defined by Francis Glisson to account for movements of living beings that cannot be explained mechanically and apparently require perception.3 Albrecht von Haller credited Glisson for the discovery of the “active force of our bodies” and the invention of the word irritability, but he rejected Glisson’s view that irritability is grounded on unconscious perception, claiming that the latter notion was contradictory. Haller maintained that irritability was a basic property of living matter displayed by phenomena, analogous to gravity, whose cause “lies concealed beyond the reach of the knife and microscope” (Haller, 1936, p. 8). While Glisson had a metaphysical view of irritability as a property of matter, Haller included irritability among three “reactive” forces defined by induction: elasticity, irritability (reaction without feeling), and sensation or feeling – the latter being a power of consciousness located in the nerves (Haller, 1757–1766, pp. IV, 467–470). Both authors agreed that a specific power of irritability was needed to explain reactions of living tissues to given stimuli (Giglioni, 2008).
An important turn in this debate was taken by Alexander von Humboldt (who was a prominent scientific model for Darwin). Humboldt’s position illustrates an epistemological division concerning the powers of plants that was much debated in Darwin’s times. Humboldt had defended the need for a specific “vital force” in his early work on plants, the 1793 Florae fribergensis specimen. In this study, he tried to apply Lavoisier’s chemistry to plant physiology. Humboldt also raised the question whether plants, although they lack nerves, might be endowed with sensibility, concluding with a cautious remark.
“So far, we see no nerves in plant bodies, and it is claimed that the idea of sentience is related exclusively to nerves. Despite this, however, we are unable to settle the controversy, which has begun among philosophers since ancient times, as to whether plants are truly sentient. This is, in fact, a purely subjective question, of which, beyond the proper sense, no sure criterion of evaluation is given, and which the Skeptics, because they despise analogy, always deny” (Humboldt, 1793, p. 153).
The slow processes of plants prompted Humboldt to turn to animal physiology to advance his research on the chemical basis of living functions, or “vital chemistry”, in the Experiments on Stimulated Muscle and Nerve Fibers (Versuche über die gereizte Muskel- und Nervenfaser, Humboldt, 1797, pp. 38–40). This vital chemistry posited that phenomena of “organic substances” – being material – are rooted in the same forces and chemical processes observed in inorganic matter (ibid., pp. 40–41). In fact, during the mid-1790s, Humboldt’s views on “vital force” underwent a crucial shift. In Florae fribergensis, he had defined vital force as a common principle of organic beings (both plants and animals). The distinction of inanimate and animate matter depended on the differing statuses of the respective chemical processes: inanimate matter was subject to standard chemical laws,4 whereas animate matter was subject to vital force, defined as “the internal force that loosens the bonds of chemical affinity as well as prevents the elements of the particles from freely compounding” (Humboldt, 1793, p. 135). In the second volume of the Experiments, Humboldt no longer associated vital chemistry with the notion of vital force anymore.5 As he declared in the 1849 third edition of the popular book Views of Nature (Ansichten der Natur), “further studies in the area of physiology and chemistry” had “profoundly shaken” his views thus leading him to revise his stance (Humboldt, 1849, p. 310). In the Experiments (Humboldt, 1797, p. 433), Humboldt claimed that vital force is an “unknown force” that is “by no means proved”.
Even so, Humboldt continued to use the notion of irritability for both animal and plants: “Animal and plant fiber must, as I believe, be regarded not merely as susceptible to irritation but as continuously irritated”. In his later works, he supported an empirical account of living beings and criticized the excessive metaphysical speculations of natural philosophers about the non-chemical properties of life (see Werner, 2000).
A significant example of the alternative view can be found in the works of the psychologist and natural scientist Gustav Fechner. Fechner – inspired by Schelling’s metaphysical identity theory – defended a kind of panpsychist view of nature, and in his book Nanna, or the spiritual life of plants (Nanna oder über das Seelenleben der Pflanzen) he focused on plants and formulated several bold claims. First, he argued that plants have sensations, although these are realized in a different physiological support from that of animals:
“What is so peculiar in the nature of nerves that makes them alone capable of channeling psychic activity? I believe that matter in plant fibers is equally fit to this task” (Fechner, 1848, p. 40).
“If caterpillars and spiders can feel within themselves the impulse to emit strands by means of which they attain the ends nature has given them, how could one not also accord to the toothwort an equal sentiment that presided over its upward growth?” (Ibid, p. 110).
Finally, plants also communicate by means of chemicals: “What words are to us, are to plants smells” (Ibid, p. 70). Fechner acknowledged that these claims were not grounded on sufficient evidence. Rather, they presented a general alternative between two views: the “Day view”, on which all living beings are endowed with a spiritual life, and the “Night view”, on which humans are surrounded by a mindless mechanistic world.6 Fechner overtly sided with the Day view: “It is difficult to think of this gushing living sphere, so full of exchanges with the inside and the outside, as devoid of sensation” (Ibid, p. 61).
Fechner’s affirmative stance concerning the Day View tipped the epistemological balance:
“Of course, we can only imagine this; we cannot directly see these soul flames of nature; but since we can imagine it, why should we not? No one forces us to open our eyes to external lights or to warm ourselves by external flames. Why do we do it? Because we like it so much better than sitting in the dark and cold. Well, we also sit in a dark and cold nature if we do not want to open the inner eye of the spirit to the inner flames of nature” (Ibid, pp. 59–60).
Fechner’s ideas were not isolated. As I will show, a comparable epistemological alternative recurs in contemporary philosophy and plant science. Given the revival of Darwinian ideas in this context, it is instructive to consider how Darwin himself situated his investigations of plants within broader theories of mind and matter.
Darwin’s intense research on plants after the publication of The Origin of the Species has to be contextualized in the epistemological framework of the theory of evolution and its defense. One of the main points of The Power of Movement in Plants was the study of plants’ daily movement, “circumnutation”. The Darwins underscored its physiological origin, based on chemical processes that direct phototropism, pointing out that this movement has no “special purpose” (Darwin & Darwin, 1880, p. 263). They granted that plants display a form of memory by association and spoke of plant “behavior” by analogy with animals: a “radicle may be compared with a burrowing animal such as a mole, which wishes to penetrate perpendicularly down into the ground” (Ibid, p. 199).7 Behind this analogy was the attempt to vindicate the validity of the theory of evolution in the field of botany (Whippo & Hangarter, 2009). The struggle for nourishment was not connected to purposefulness of nature, nor were plants literally endowed with either intentionality or consciousness.
Nevertheless, one may wonder whether the genealogical connection of living forms suggested to Darwin that plants might share some cognitive capacities after all. On the one hand, Darwin maintained that there is a continuous transition from instinct to intelligence in organisms. In the Origin of Species, he argues that “a little dose, as Pierre Huber expresses it, of judgment or reason, often comes into play, even in animals very low in the scale of nature” (Darwin, 1859, p. 208). In his notebooks, he had inclined to broaden the picture, noting: