Plants and Psychology
1. Plant Psychology?
In a recent article in the Journal of Comparative Psychology, Umberto Castiello contends that “many of the sophisticated behaviors plants exhibit are an expression of cognitive competences that are generally attributed to human and nonhuman animals,” and so those behaviors deserve to be addressed by comparative psychologists.
That might surprise you, but it isn’t sheer madness. In fact, there are good reasons to believe Castiello is correct.
Consider psychology in general.
In the broadest terms, psychology is the study of the mind. According to a syllabus for a recent course at The University of North Carolina, “Cognitive psychology is the study of how we sense and interpret information from the world around us, incorporate this new information with our prior experiences, and determine how to respond to an ever-changing environment.”
Depending on how we interpret the word ‘we’ there, this description might under-state the breadth of psychology’s jurisdiction. Psychology investigates not just us humans, but anything that has a mind, including nonhuman animals, such as dogs, pigeons, or jumping spiders.
It aims to understand and explain behaviors that potentially require having a mind, behaviors that seem to result from learning, perception, memory, planning, or decision. And it aims further to understand those causes themselves.
At the foundation of psychology are several conceptual or definitional questions. What is a mind? What is a mental process or mental state?
Since at least the late 1800s, psychology and physiology have been intertwined. At that time, with increasing rigor, people were investigating the behavior of nonhuman animals. For example, Conwy Lloyd Morgan noticed that his fox terrier, Tony, was consistently able to operate the latch on the gate to his garden. How? In general, such inquiries were treated as a branch of physiology, not psychology. Different researchers had different views of this work, but many believed that physiological explanations of nonhuman animal behavior were to be preferred over psychological explanations. Their view was that an explanation that attributes mental processes or states to the creature should be accepted only if physiological explanations could be ruled out.
But when exactly is that? How can we tell—what is good evidence—that some behavior is caused by a mental process or state (such as a memory or a decision), rather than not? That question has shaped much subsequent research in both nonhuman and human psychology.*
In contemporary psychology, researchers generally take for granted that organisms are wholly material entities.* This means that psychological processes either are or are implemented in material processes. For that reason, many or even most introductory courses and books in psychology discuss some anatomy and physiology of the nervous system.
Given that general outlook, the foundational question ‘What is a mental process?’ becomes the question ‘What is the difference between a physiological process that is genuinely mental and one that is not?’ The task is to articulate what distinguishes mental processes from other, non-mental physiological processes.
With those general remarks about psychology in place, consider plants.
In many significant ways, plants differ from most animals. All plants are sessile; they do not move from place to place; they do not locomote. Most animals do. All plants are autotrophic; through photosynthesis, they produce their own sources of food. No animals do; they are heterotrophic; they must consume other organisms. All plants display indeterminate growth; across their lives, they continue to grow. No animals do this. (A much more contentious claim is that while animals have a nervous system, plants appear not to have one. They do not have cells with the same structure as typical neurons: a cell body with many protruding dendrites and an axon. The contentious question is whether they have other structures that function in the ways that typical neurons do.)*
Despite those differences with animals, most plants display behaviors that are goal-directed, flexible, and adaptive.*
Many plant behaviors appear to be goal-directed because they are tropisms; they are not merely responses to a stimulus, but to the direction of a stimulus. Shoots generally grow toward light, and away from the direction of gravity. Leaves twist toward light. Roots grow with the direction of gravity, and toward water. The vines of climbing and twining plants grow toward the direction of mechanical contact.
Plant behaviors are also flexible; in response to one and the same type of stimulus, a plant doesn’t always do one and the same type of thing. When roots are physically impeded in their downward growth, they don’t simply stop growing, but adjust, and re-orient, until downward growth can resume. Their responses incorporate or integrate detection of multiple stimuli.
These behaviors are also typically adaptive, serving the longevity and reproductive capacity of the plant.
For these reasons and related ones, some researchers, such as Castiello, think that plants should be a subject for comparative psychology. Behaviors that are goal-directed, flexible, and adaptive suggest that perception, learning, memory, or decision might be involved.
These behaviors might turn out not to involve mental processes or states*, but this possibility should be experimentally investigated*, rather than merely taken for granted, or stipulated by definitions that exclude plant behaviors from the jurisdiction of psychology. This is because one of psychology’s on-going tasks is to explore and articulate the difference between physiological process that are genuinely mental and those that aren’t. Plant behaviors thus deserve the attention of psychologists.