How Smart are Plants?
Plants interact with their environment in remarkably complex ways.* They are sensitive to the direction of light. They exchange chemical messages, defend themselves in sophisticated ways against herbivores, and regulate their root growth by taking a multiplicity of factors into account (e.g. gravity, salinity, and moisture). And they possess ‘circadian clocks’ that allow them to adapt their physiological processes to the daily alternation between light and dark.
When plants exhibit these remarkable forms of behavior, do they ‘merely react’ to environmental states, or is there something more complex going on – something that can legitimately be described as ‘cognition’, ‘representation’, ‘intelligence’ or even ‘thinking’? This question has long fascinated the public at large. However, it is also of great interest to scientists and philosophers – not just for its own sake, but also because it is intimately connected to foundational questions concerning the nature of cognition, representation, and thinking. Evidently, different answers to these foundational questions support different characterizations of the processes that are going on in plants, and vice versa. Hence, it comes as no surprise that the question of how to characterize the sophisticated responses of plants is hotly debated in science* as well as in philosophy.*
To get a glimpse at what these debates are about, let us focus on the notion of representation. Most theorists take beliefs and perceptual experiences to be paradigm examples of representations. When I believe that aardvarks are mammals, or when I visually perceive an object as being red and spherical, the mental states I am in are about things in the world, and represent them as being a certain way. When these states represent things as they actually are, they are true or accurate; if not, they are false or inaccurate. In cognitive science*, this notion of representation is not just applied to conscious states of human subjects, but also to so-called ‘subpersonal’ states in humans, as well as to states of many non-human animals. On the face of it, such representational attributions play a major explanatory role in psychology, ethology and other areas of cognitive science.
Now let us turn back to plants and look, e.g., at states in a plant’s root system that indicate the direction of gravity (accumulations of little organelles called ‘statoliths’ at the lowest parts of specialized cells). Can these states legitimately be described as representing the direction of gravity, and as being true/correct or false/incorrect, depending on what the actual direction of gravity is? Do such representational attributions have a genuine role to play in explanations of plant behavior? Or should we rather understand them as purely metaphorical descriptions, mere manners of speaking that cannot do any real ‘explanatory work’?
Obviously, these questions cannot be answered in a few paragraphs, so I will not attempt to do that here. My point is merely that these are substantive questions, as well as exciting ones. Part of what makes these questions so interesting and difficult is that the ultimate answers to them will depend both on what biologists find out about the mechanisms that underlie plant behavior and on what philosophers and cognitive scientists establish about the nature of representation and representational explanation. Finding these answers is thus a truly interdisciplinary endeavor.