Who is capable of learning? It seems to be humanity’s specialty, but researchers have long been interested in the ability of primates and other mammals to learn and memorize simple tasks and pieces of information. However, scientists have only recently begun to genuinely delve into the concept of plant learning.
What is learning, anyway? As a basic definition, it is a process for acquiring memories as a result of experience and changing resultant behavior to better adapt to whatever circumstances to which the memories apply. Plants may not have the cognitive abilities of animals, but research has strongly indicated that they may be able to habituate their behavior to a new stimulus by identifying common situational similarities. One study published in 2016 used a simple Y-shaped maze, a light source, and the neutral cue of a fan to examine the influence of external cues on the direction of plant growth. The plants changed their growth based on the fan’s repeated indication of which direction the light source would come from rather than their innate phototropism [1]. Their behavior supported the idea that not only are plants capable of associative learning, but also that learning is necessitated by metabolic demands.
Perhaps the most direct example of associated learning behavior comes from a particularly charismatic organism, Mimosa pudica, the “sensitive plant”. M. pudica exhibits a defensive behavior in which its compound leaves fold rapidly inward upon being disturbed or touched in general. This response requires movement and thus energy from the plant, but is useful if the plant is being threatened. Also in 2016, researchers conducted an experiment on M. pudica’s ability to remember and alter its folding behavior under varying light conditions. Several of the plants were lifted and dropped onto a flat surface, causing them to reflexively fold, and then the drop was repeated several times for each plant. They found that as the process was repeated, the plants stopped folding their leaves each time. These results reveal the presence of plant learning: the plants likely habituated to the drop after realizing that the drop was harmless and that their defensive mechanism was energetically costly [2]. The effect was even more noticeable when light sources for each plant’s habitat was changed and energy was scarcer [3].
Plants clearly do not have brains, but we know that they have an intricate cell network of calcium (Ca2+)-signaling, which mimics animal processes and various biological functions in many organisms. Another hypothesis posits that electrical signaling via ion channels in plant cells could work similarly to neural networks in animals [2]. Both theories will need further research to be substantiated, but the concept of plant memory, learning, and intelligence remains a new and exciting field – one that may even encourage us to reconsider the way we think about learning in general.
References
Gagliano, M.; Vyazovski, V. V.; Borbely, A. A.; Grimonprez, M.; Depczynski, M. Learning by Association in Plants. Nature. 2016, 6, 38427.
Gagliano, M.; Renton, M.; Depczynski, M.; Mancuso, S. Experience teaches plants to learn faster and forget slower in environments where it matters. Oecologia. 2014, 175, 63-72.
The Paris Review. The Intelligence of Plants. https://www.theparisreview.org/blog/2019/09/26/the-intelligence-of-plants/?utm_source=pocket-newtab. (Accessed October 2, 2019).