Plant Intelligence: The Evidence that Plants Are Conscious

Plant intelligence is a concept not easily accepted due to its necessarily creating a new outlook or perspective on the definition of intelligence. Tony Trewavas’s article, Plant Intelligence: An Overview, provides examples of plant’s physiological complexity which Trewavas interprets as intelligence. He defines that intelligence, “(a) is a property that an individual has as it interacts with its environment or environments, (b) is related to the agent’s ability to succeed or profit with respect to some goal or objective, and (c) depends on how able the agent is to adapt to different objectives or environments”. One main factor of qualifying for intelligence is flexible behavior, or, by using a facet of Trewavas’s definition, the more flexible behavior an organism possesses the more “able the agent is to adapt to different objectives or environments”.

I would like to focus on flexible behavior specifically because I believe it to be one of the most crucial facets of intelligence and, if properly shown in plants, can undermine any other explanations of plant’s behavior but intelligence. My viewpoint on flexible behavior is that it requires active thought to go against physical mechanisms and allows the organism to act spontaneously but logically to different situations. I would like to reason how some of plants’ physical structures and systems could be interpreted as flexible behavior because I believe that there is a high probability of plants possessing active thought and can thereby exhibit flexible behavior. I will first attempt to prove why it is logically possible to believe in plant intelligence, then I will provide scientific examples of plant’s flexible behavior, and, lastly, I will attempt to defend my given conclusions.

I would like to start by introducing Robert Pargetter’s “Theory of the inference to the best explanation,” which he writes in his article, The Scientific Inference to other Minds. He defines his theory of inference to the best explanation by writing that, “if a hypothesis is the best available explanation of all the available evidence of a person at a particular time, then it is rational for that person to believe that hypothesis at that time”.

Pargetter is basically writing that if a hypothesis is the best explanation for something, then it is rational to believe in the accuracy of the hypothesis. I would like to use this theory of rationalization to test whether plant behavior can best be explained as flexible.

Scientists have already discovered that most plant systems are highly complex and sophisticated. One of the most complicated is a plant’s root cap. Trewvas writes about one root cap when he describes the root cap present in Arabidopsis: the extreme tip of the root is covered by a cap constructed of some 200 cells. The cap is dynamic. It is constructed from a layer of dividing cells that abut the root meristem proper. The cap cells are gradually pushed outward. On reaching the cap surface, they are sloughed off. However, during their lifetime, slowly moving to the front of the cap, they act as both sensing and assessment of a variety of different signals. Like the cell and the nervous system above, present information indicates that it has a similar architecture in degree structure, with both a core and a periphery—a structure that seems to underpin intelligent behavior. This architecture may engender resilience distinguished by the range of signals that the cap senses.

The root cap possesses the ability to respond to its environment in a flexible manner, by shedding off dead cells and moving in the direction of essential survival needs. This, however, can easily be interpreted as just a mechanistic process triggered through several chemical reactions. The root cap is attracted to the survival needs and grows towards them. When it does not sense the survival needs, it stops growing towards them. An active thought causing a flexible behavior that goes against a physical mechanism is simply not the best explanation.

Another objection to flexible behavior in plants is that they do not possess neurons, or a brain, which typically signifies that there is no possibility of cognitive activity. I believe, however, that there is no universal system for cognition, so the absence of a neural network does not necessarily eliminate cognition entirely. Peter Godfrey-Smith, in his article, Cephalopods and the Evolution of the Mind, describes cephalopod’s neural difference from vertebrates, but still purports that they exhibit flexible behavior and possess intelligence (Godfrey-Smith 5). He writes that, “Cephalopods have an entirely different organization, both in body in brain” (Godfrey-Smith 5). Whereas, “the vertebrate plan features a head and spinal cord, with the peripheral nervous system coming off it,” cephalopods feature, “a 'ladderlike' nervous system,” where, “neurons were packed together up front between the eyes, and many of the ganglia were fused. So there was a partial submerging of the invertebrate neural plan — but only a partial one” (Godfrey-Smith 5). He also mentions that, “a common octopus has about 500 million neurons.

Two thirds of those are not in the brain at all, but in the arms themselves,” meaning, “their nervous system remains much more ‘distributed,’ more spread through the body, than ours (meaning human)” (Godfrey-Smith 5). He also provides an example of flexible behavior in cephalopods. He writes,

“A group of researchers in Indonesia were recently surprised to see octopus carrying around pairs of half coconut shells, to use as portable shelters (Finn et al. 2009). One half-shell would be nested inside another, and the octopus would carry the pair beneath its body as it stilt-walked across the sea bottom. The octopus would then assemble the half shells into a sphere and climb inside. Many animals use found objects for shelters (hermit crabs are an example), but to assemble and disassemble a compound tool like this is rare.

The cephalopods certainly do exhibit flexible behavior in a way that implies active thought that is spontaneous and logical to the situation; it also seems to be more than just a physical mechanism. The complexity of this described behavior of coconut octopuses is too great for this to be simply a physical mechanism. It would follow, then, that they possess intelligence since it is the best explanation according to Pargetter’s theory.

Though cephalopod’s brain and neural structure is entirely different from human, and even all mammalian brain and neural structure, it is still concluded that a cephalopod’s flexible behavior signifies intelligence because it is the best explanation. This clearly shows that there is no universal structure for cognition or intelligence. The lack of neural structure or brain in plants is, therefore, not necessarily a problem. Their way of thinking might just simply not be discovered yet. It has only been recently in human history that the idea of our closest evolutionary relatives, such as chimpanzees, were capable of thought or possess intelligence. Now, we consider even further animals species from humans, such as octopuses and cuttlefish according to Peter Godfrey-Smith’s article. I believe that with more understanding and research into plant biology, we will discover a new approach to intelligence indicating structures with plants.

Returning to the task at hand, we have concluded that the best way of explaining cephalopod’s behavior is defining it to be flexible which signifies intelligence. Now if we believe that there is no universal structure for cognition, so there is no necessity for a neural network or brain, then all that is missing is a flexible behavior in plants whereas flexible is the best explanation for that behavior. This is where it gets tricky. Most plant systems are triggered through physical processes that are best explained as not signs of intelligence. The research revolving around flexible behavior in plants is slim, and even the experiments done have not yielded definitive conclusions. Tony Trewavas’s last section of his article, titled “Games plants play,” contains the most unique facets of plant physiology, and this section I believe presents examples of concrete flexible behavior in plants.

Tony Trewavas concludes his article by writing about a legume’s “prisoner’s dilemma” game with rhizobia bacteria. The simple description of this plant’s interactions with bacteria is that some rhizobia bacteria are better than others at converting dinitrogen in the atmosphere to organic nitrogen through the process of nitrogen fixation. There are also several different kinds of rhizobia bacteria, and only certain ones can fixate dinitrogen into organic nitrogen. The plant then will form a “nodule” around the rhizobia bacteria that can fixate and ignore others that do not, thus eliminating ‘free-riders’. Trewavas goes over this behavior very briefly, but I found more information on this topic in an article titled Partner Choice in Nitrogen-Fixation Mutualisms of Legumes and Rhizobia, written by Ellen L. Simms and D. Lee Taylor who provide more information regarding the symbiotic relationship between legumes and rhizobia bacteria (Sims, Taylor 369).

Simms and Taylor start by describing the need for organic nitrogen that begins the process. They write that though “Nitrogen is extremely abundant, comprising about 79% of the atmosphere,” it exists as “dinitrogen,” which plants, “cannot convert… to useful organic forms” and that “mineral nitrogen is labile and of limited supply in soils” (Simms, Taylor 370). This means that nitrogen fixation is an essential process for the legume’s survival. This creates the need for the symbiotic relationship between these two organisms. The legumes provide the rhizobia bacteria carbohydrates by “pay(ing) the energetic price of the reduction reaction, conduct a complicated signal exchange with rhizobia, produce leghaemoglobin, and form a novel organ—the nodule” (Sims, Taylor 372). The legume is essentially trapping the bacteria that nitrogen fixate for nutrients, while it simultaneously ignores the bacteria that do not. It is mutually beneficial between the two organisms; the relationship provides both with essential nutrients. The question now is whether this relationship constitutes as flexible behavior.

I would like to go back to Peter Godfrey-Smith’s example of the coconut octopus’s behavior of finding, dragging, and hiding itself within coconut halves as proof of intelligence and flexible behavior (Godfrey-Smith 5). The legume also uses objects from its environment to benefit its survival. It casts away the ‘free-rider’ rhizobia bacteria and encapsulates the nitrogen-fixating rhizobia bacteria for its own benefit. It is using a tool from its environment to obtain organic nitrogen for survival. This would constitute active thought and then be exhibiting flexible behavior. It also must actively choose quickly and rationally to determine which bacteria it needs to keep or disregard. I believe that the best explanation for this behavior is flexible, meaning that it is a facet of intelligence. This behavior is more than just a physical mechanism because it requires the legume to reject certain rhizobia bacteria and accept others. The legume also sacrifices some of its carbohydrates to feed the rhizobia bacteria, so that it will continue to fixate dinitrogen to organic nitrogen in the nodule. This also requires active thought and can be constituted as flexible behavior. It is not just a physical mechanism because it sacrifices a portion of its own survival benefit for the sake of another organism and what that other organism can do for the legume. Further still, for these three steps of the relationship, the using of the bacteria from the environment to benefit survival, to determine between the nitrogen fixating and non-nitrogen fixation rhizobia bacteria, and to sacrifice a portion of carbohydrates to feed and entice the rhizobia bacteria to stay, occurring simultaneously together at the exact time is very complicated. It is too complex behavior to be explained as simply mechanic.

I believe that plants exhibit flexible behavior because they actively think and logically and spontaneously respond to their environment. There is no universal structure of cognition that would rule out plants due to their lack of a neural structure or brain. With more continued research into plant biology, I believe we will find more evidence of flexible behavior in plants. If plants are found to be capable of flexible behavior and, therefore, possess thought, then it would surely broaden the scope of intelligence in the world. It will force humanity to redefine intelligence and cognition. The world will truly be a much bigger place.