By Dr. Robert Norman, Clinical Professor, Dermatology, Nova Southeastern University
While biking recently on the Courtney Campbell Causeway, I stopped for a rest before heading back over the bridge and to my car at Skyway Park. As I looked down, I noticed a group of sensitive plants, (Mimosa strigillosa). The plants are also called Sunshine Mimosa, Powderpuff, or Humble Plant.
I have been working hard on a book about the natural and cultural history of touch and how touch has helped form our world. What about sensitivity in plants?
Mimosa, (genus Mimosa), is a large genus of plants in the pea family (Fabaceae), native to tropical and subtropical areas throughout both hemispheres. They are so named from the movements of the leaves in certain species that “mimic” animal sensibility.
Most Mimosa species are herbs or undershrubs, some are woody climbers, and a few are trees. The leaves of most are bipinnate (i.e., the leaflets of the feather-formed leaves, in turn, bear leaflets). The plants are characterized by small regular flowers and produce legume fruits. In addition to its response to physical stimuli, the leaves also droop in response to darkness and reopen with daylight, a phenomenon known as nyctinastic movement. This movement during the night and when it is exposed to abiotic factors such as excessive heat and rain protects the plant from physical damage or desiccation.
The leaves of Mimosa have the capability to display thigmonasty (touch-induced movement). In the sensitive plant, the leaves respond to being touched, shaken, heated, or rapidly cooled. The speed of the response depends on the magnitude of the stimulus. Hitting the leaf hard with the flick of a finger will cause the leaf to close in the blink of an eye whereas a gentle touch or modest heat source applied to leaflets at the tip of a leaf will result in a slower response and the propagation of the stimulus along the leaf can be observed.
In Mimosa, the mechanical or heat stimulus induces an electrical signal, similar to the electrical potentials in nerve cells, that can move from cell to cell at a high rate. When the electrical potential reaches specialized “motor cells” in pulvini at the base of each leaflet, the folding is caused by a rapid efflux of potassium followed by rapid water transport out of the motor cells. The pulvinus is the “hinge-like” area of the plant where the leaflet connects to the midrib, and the midrib connects to the stem. If the applied stimulus is great enough, the signal produced is strong enough that it can propagate further and faster and cause the rest of the leaf to fold in several places.
The plant’s unusually quick response to touch is due to rapid water release from specialized cells located at the bases of leaflet and leaf stalks. The leaves reopen in several minutes, and it is thought that this adaptation is a defense against browsing herbivores who may be startled by the movement. “The rapid folding of Mimosa leaflets is triggered by a mechanical stimulus and has been hypothesized to deter herbivore attack for several reasons: it exposes the spines located below the leaf raquis; the leaf moves when insects land on it causing them to move away; and it could decrease the visibility of the leaves or make them look smaller.” (Amador-Vargas 2014: 1446). It is hypothesized that this rapid folding deters herbivores and insects from eating the plant by making the plant appear smaller, while simultaneously exposing the sharp spines on the plant stems.
Here is a truly fascinating observation. The folding process takes between 4-5 seconds. After folding is complete, the unfolding of the leaflets can take anywhere from tens of seconds to up to 10 minutes. It is believed that the unfolding time is a result of behavioral adjustments that the plant makes over time in response to different kinds of stimuli. Herbivores prefer younger, tenderer leaves of plants. When younger leaves of the sensitive plant were repeatedly exposed to non-damaging stimuli, the younger leaves consistently folded completely, but over time, they decreased the time it took for them to unfold. Conversely, older leaves folded only partially while maintaining similar reopening times. This shows that the plant is able to modulate its behaviour to optimize protection, energy production (photosynthesis), and energy expenditure (folding and unfolding). In many ways it reminds me of the economy of movement and energy in our elders.
We must always keep in mind how we are part of the natural environment. Looking closely at sensitive plants, we can see the power of touch. Think about this—plants and people are always sensitive to touch. Of all our senses, the sense of touch is the only one that we cannot temporarily “switch off” or deliberately suppress, so we are particularly sensitive to tactile sensations—just like plants.
Some plants react quickly to stimuli, such as the Venus flytrap (Dionea muscipula). An insect landing on the inside of its red hinged leaves will touch sensory hairs that trigger electrical signals causing the leaf to close. With the mimosa (Mimosa pudica), even the slightest brush of the outer edge of its leaflets starts a chain reaction that results in the entire leaf folding inwards. The plant is also called “touch-me-not” and in German, “mimosa” is used to describe someone who is very sensitive.
“Plants don’t feel regret; they don’t get a feel for a new job. They do not have an intuitive awareness of a mental or emotional state. But plants perceive tactile sensation, and some of them actually ‘feel’ better than we do. Plants like the burr cucumber (Sicyos angulatus) are up to ten times more sensitive than we are when it comes to touch,” writes the biologist Daniel Chamovitz in What a Plant Knows: A Field Guide to the Senses (2012), his book which explores the world of plant perception.
If a plant survives gusts and storms, it can be a particularly intensive form of touch for plants and trees. The touch triggers growth stimuli, causing the plant to become stronger and more robust. In similar fashion, sensation of touch can help people to be healthier. Physical therapies such as baths, cold and warm packs, and massages trigger our natural reactions to external stimuli in order to strengthen the body and promote recovery.
Thank you to Weleda, Britannica, https://plantsinmotion.bio.indiana.edu/ and asknature.org
References
Daniel Chamovitz: What a Plant Knows: A Field Guide to the Senses. New York: Scientific American/Farrar, Strauss and Giroux: 2012
Stefano Mancuso, Alessandra Viola: Brilliant Green: The Surprising History and Science of Plant Intelligence, Washington, DC: Island Press, 2015
Kathrin Meyer und Judith Elisabeth Weiss (Hrsg. für das Deutsche Hygiene-Museum Dresden): Von Pflanzen und Menschen, Wallstein, 2019
Martin Grunwald: Homo hapticus. Warum wir ohne Tastsinn nicht leben können”, Droemer, 2017
H.S. Patil, Siddharth Vaijapurkar Study of the Geometry and Folding Pattern of Leaves of Mimosa pudica Journal of Bionic Engineering 15/04/2007
Amador-Vargas S, Dominguez M, León G, Maldonado B, Murillo J, Vides GL. Leaf-folding response of a sensitive plant shows context-dependent behavioral plasticity, Plant Ecol. 215: 1445-1454.
How can you observe these sensitivities of nature?
Get out into The Great Florida Outdoors!
Dr. Robert Norman, Clinical Professor of Dermatology
Director–Center for Geriatric Dermatology, Integrative Dermatology and Neuro-Dermatology
8002 Gunn Hwy. • Tampa, Florida 33626 • 813-880-7546
61 books (series editor of 13 • 300+ articles • 25 videos and films • 16 photo calendars and
exhibits • 4 music CDs
Tampa Bay Medical Hero Award (2008) • Hadassah Humanitarian Award (2012)
Dr. Norman is an advanced master naturalist graduate of the FMNP program from UF and a board-certified dermatologist based in Tampa and Riverview. He can be reached at 813-880-7546.
61 books (series editor of 13) ● 300+ articles ● 25 videos and films
● 16 photo calendars and exhibits ● 4 music CDs
Tampa Bay Medical Hero Award (2008)
● Hadassah Humanitarian Award (2012)
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