What Is Plant Behavior?

Photograph by Peter Shanks.


 Michael Marder

November 2, 2015

Behavior is considered to be the hallmark of an active, purposive attitude to the environment. As we observe animals, we perceive the external signs of behavior in the movements of muscles that allow them, for example, to flee from a dangerous situation or to procure nourishment, achieving easily recognizable objectives. Conversely, connected to the stereotypical view that plants do not move is the idea that they do not behave. The limbs of a tree simply sway in the wind; they cannot, unlike a rabbit’s paws, recoil from a saw or a knife wielded against them. Modifying Aristotle’s dictum that “plants seem to live without sharing in locomotion or perception” in De Anima (410b, 23-4), a vast majority of people today believe that plants live without behaving.

The absurdity of denying behavior to plants is, nonetheless, glaring. Like everything that lives, plants need to nourish themselves, and this operation is far from effortless. It is not enough to expose the leaves to sunlight in an arbitrary arrangement; exposure has to be maximized, especially if other (taller) specimens cast shadow, as they do in a forest or in a jungle. It is a wasteful energetic investment to grow roots in any direction whatsoever; root growth needs to be optimized to reach the most promising patches of soil, with plentiful water and mineral resources.

I am bracketing, in this context, the exceptions of Venus flytrap and Mimosa pudica, two plant species that exhibit the rapid movements of shutting the lobes to capture an insect or of closing the leaves in a defensive response to tactile stimulation. More interesting, to my mind, is the claim that all plants behave in certain ways, regardless of the species they belong to. So, if growth is a kind of slow movement—which is the insight of the very thinker who described vegetal life in strikingly impoverished terms, namely Aristotle—then the fine-grained details of their growth constitute criteria for behavior. From the placement of new shoots and leaves, we may infer carefully made decisions on the most advantageous location for these new parts of plants, grounded on the complex spatial maps they construct.[i]  The pattern of root growth, in turn, is a behavior influenced by the “tactile environment of the root.”[ii] Once we account for differences in time-scales and the experiences of time between the human observer and the observed plant, growth begins to exhibit unmistakable behavioral characteristics that are so obvious in the case of animals.

Richard Karban, Professor in the Department of Entomology and Nematology at the University of California, Davis, defines plant behavior as “morphological or physiological responses to events, relative to the lifetime of an individual”.[iii] In the formulation “morphological and physiological responses,” we should hear another echo of Aristotle, who saw in metamorphosis (literally, change of shape) yet another kind of movement. That is to say: plants behave by initiating a chain of alterations, sometimes at the level of genetic transcription, that permit them to respond to events such as the onset of drought more effectively, seeing that they cannot leave the places wherein they grow. Responding to abiotic, environmental stress, plants can alter metabolic rates, roll their leaves, close stoma, and so forth.[iv]

Of the four types of movement, identified by Aristotle, the only one remaining is decay. Could it be the case that plants also behave by decaying? Clearly, the roots of a tree receive nutrients from the rotting leaves and fruit that have fallen on the ground above them. But a collapsed tree as a whole also becomes the site for a community of fungi, microbes, and insects who decompose it, enriching the soil. Most likely, Aristotle imputed a modicum of agency to decomposition, which is why he included decay in the fourfold concept of movement. Rather than make a laughing stock of his insight, we should interpret it obliquely, in light of what happens when the effects of human behavior impede vegetal decay. Surprisingly, the forests surrounding the Chernobyl reactor site are not decaying as they should, with tree trunks and leaves accumulating over the nearly thirty-year period since the disaster.[v] (This summer’s severe wildfires in the so-called exclusion zone have resulted from this failure of plant decay.) Having interfered with the time-scale of finite life through the use of nuclear power, the byproducts of which are virtually indestructible, human behavior has smuggled an absolute non-adaptation to the environment in the guise of the most versatile adaptation to any conditions whatsoever. It has disrupted the communities of decomposition that are indispensable to soil renewal and to the continuation of life.

Given multifaceted plant behaviors, our task is not to reject behaviorism for its simplistic view of the bio-psychological subject, oblivious to the inner world of emotions, intentions, and desires, but to show that this view is not behaviorist enough. The stimulus-response model it subscribes to works with a very limited time-range proper to a very limited group of organismic changes, which are taken to be synonymous with behavior in general. The friction between depth psychology (including, of course, psychoanalysis) and behaviorism is secondary to the immense richness of behavior, uncoupled from the standards of either an individual species or a biological kingdom. Factoring growth, metamorphosis, and perhaps decay into its semantic range, we will approximate the framework for the term behavior, appropriate to life in all its anonymity and singular universality.

[i] Anthony Trewavas, “Aspects of Plant intelligence: an answer to Firn.” Annals of Botany 93 (4), 2004, pp. 353–57. doi:10.1093/aob/mch059.

[ii] J.L. Mullen, et al. “Root-growth behavior of the Arabidopsis mutant rgr1. Roles of gravitropism and circumnutation in the waving/coiling phenomenon.” Plant Physiology 118(4), 1998, pp. 1139-45.

[iii] Richard Karban, “Plant Behavior and Communication.” Ecology Letters 11(7), 2008, pp. 727-739.

[iv] Cf. Loredana Ciarmiello, et al. “Plant Genes for Abiotic Stress,” in Abiotic Stress in Plants—Mechanisms and Adaptations,” edited by Arun Shanker and B. Venkateswarlu (Rijeka: InTech, 2011).

[v] http://www.smithsonianmag.com/science-nature/forests-around-chernobyl-arent-decaying-properly-180950075/?no-ist

2 thoughts on “What Is Plant Behavior?”

  1. I recently read comments from an animal-rights activist that some plants exist to be eaten–that their very nature makes them delectable and, therefore, appropriate for our dinners. Is that true? Are there plants that would cease to exist if we did not eat them?

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