Recently while working with a group of 6th and 7th graders in a middle school greenhouse, I overheard a conversation between two students. The group was engaged in a transplanting project using common house plants. One student was berating the other because he was not being gentle enough and she thought he was “hurting the plant!” Her fellow student replied that “plants don’t have feelings” so his actions were acceptable. I intervened at this point and asked the other students to weigh in on the disagreement by asking the group “Do you think plants have feelings?” The response was broad, with several disinterested students shrugging their shoulders, while others became quite passionate about the subject. This led me to explore the questions, do plants have feelings and how could this be verified?
As horticultural therapists we often speak of plants using descriptive words like; resilient, adaptable and renewable. Humans have always been in awe of a plants’ ability to survive and even thrive in harsh environmental conditions. To illustrate this point, consider how humans have always had the advantage of movement to benefit their survival. Historically, when environmental conditions become problematic, humans have had the advantage of picking-up and moving to a more optimal space. Plants on the other hand are rooted in place, they must solve rather than avoid, whatever problem their environment presents. Succulents like cacti, aloe and agave are an example of plants that have developed the ability to store water in fleshy leaves and roots, thus allowing them to survive in arid conditions. But how does a plant become resilient without the aid of a central nervous system? Are their actions/reactions to their environment the result of some sort of awareness or thought process?
More than two decades ago, Suzanne Simard, a professor of forest ecology at the University of British Columbia, brought forth a radical idea… trees can actually “talk” to one another. In her research, Simard describes a cooperative underground fungal communication system. Through this system of roots, mycorrhizal fungi and bacteria, trees can send warning signals of an oncoming threat, share nutrients and even search for their offspring within a community. Simard compares this communication system to the neural networks in the human brain. She successfully experimented with radioactive isotope carbon to demonstrate how a paper birch (Betula papyrifera) and a Douglas fir (Pseudotsuga menziesii) were utilizing this underground network. It appears that Simard’s research proves that trees can send out information and provide nutrients, to benefit other trees within the network.
In a later experiment, the mycorrhizal network was mapped or diagrammed, showing that a few of the older trees with larger root systems had stronger connections within the network than some of the younger trees. The older trees with the stronger connections were named “Mother trees”. The mother tree sends carbon through this network, to certain seedlings, which will increase the survival of those seedlings by four times. It was also found that mother trees will create stronger networks with their own kin. This research is evidence that the mother tree is selecting which seedlings to nurture.
Simard uses terms like “Mother tree” and “forest wisdom” in her research for the purpose of making it more relatable to the general public. “If we can relate to it, then we’re going to care about it more. If we care about it more, then we’re going to do a better job of stewarding our landscapes.” It is Simard’s opinion that if too many mother trees are lost to insects, disease or human interference, the mycorrhizal network will collapse, resulting in forest deterioration.
One other piece of research implies, that plants can develop a “stress memory”. In other words, plants can recognize and will react to, certain stimuli perceived as dangerous, while other stimuli will not produce a reaction. This research was conducted by Stefano Mancuso, a neurobiologist at the University of Florence in Italy. In his book The Revolutionary Genius of Plants, Mancuso describes an experiment he conducted using Mimosa pudica or sensitive plant. I use Mimosa pudica frequently when working with special needs students. They are always delighted to see the plant respond to their touch by closing its leaves.
The purpose of Mancuso’s experiment was to demonstrate that Mimosa pudica could identify whether a certain stimulus was dangerous or not and respond accordingly. This was accomplished by simply placing jars containing the plants on an apparatus that produced a sudden drop in height of approximately four inches. In the first trial, all plants reacted immediately by closing their leaves. Yet, after repeated drops, the plants stopped reacting to the stimulus and leaves remained open. To quantify this finding, the same plants were then introduced to a new stimulus, shaking the jars in a horizontal motion. This change resulted in the immediate closing of leaves. Mancuso claims that his experiment proves the ability of Mimosa pudica to learn whether a certain stimulus was dangerous and had developed stress memory. Moving forward with his research, Mancuso asked the question “how long will this memory last?” Using approximately one hundred plants that had been trained using the drop method, he periodically tested the group using the same stimulus. To his surprise, the stress memory lasted up to 40 days. Mancuso states that his research does not determine how Mimosa pudica develops stress memory only that it is possible. He believes further studies are needed in the field of epigenetics to fully explain this phenomenon.
These two research studies are just a small representation of the work being conducted world-wide regarding plant behavior and intelligence. The question of “do plants have feelings?” has been debated by humans for centuries. Of course, there are those who strongly disagree with the idea of plant intellect, stating that plants can evolve and nothing more. What is undeniable is the fact that people and plants have had a symbiotic relationship that has endured throughout time. Without this relationship humans would not exist…but the plant world would not only survive but would thrive.
References
Mancuso, S. (2017). The Revolutionary Genius of Plants; A New Understanding of Plant Intelligence and Behavior. New York, New York: Simon & Schuster.
Toomey, D. (September 1, 2016). Exploring How and Why Trees ‘Talk’ to Each Other. YaleEnvironment 360. Retrieved from: https://www.e360.yale.edu/features/exploring_how_and_why_trees_talk_to_each_other
