The Sensitive Not Fern

By Sean

Mimosa pudica leaves and flowers

During the summer of 2014, I went on a school trip to Costa Rica, a tropical paradise with an extremely diverse assortment of plants. During a trip to a local plantation, the guide pointed out an interesting plant that was growing on the ground next to the trail. When the guide encouraged us to run our finger along the stem, I was very confused. But after I did, I found my favorite plant that I have ever seen (or felt!).

When I ran my finger along the stem of the plant, the leaves on either side raised up and closed together. This very simple reaction from the plant amazed me because plants don’t have a brain, or instincts, or nerves to react like this plant did. I was told at the time that the plant was called a sensitive fern, and that it has a unique ability to close its leaves when they are touched. Because of the plant’s appearance, I didn’t question that it was a fern. I always thought ferns were pretty interesting, and this one seemed like most of the other ferns I was familiar with seeing. It had many long, rounded green leaves that spread out perpendicular to the stem. It wasn’t until a couple of years later that I learned that the so-called “sensitive fern” was not a fern at all.

In 2017 I traveled to England with my family. While I was there, I visited an attraction called the Eden Project, a collection of massive “bubbles'' that had artificial Mediterranean and tropical biomes in them. While exploring the tropical forest, I noticed a small familiar plant alongside the path. Unlike my first encounter, this time the plant was accompanied by an informational sign. The plant that I had been so fascinated with was the sleepy plant/shame plant (Mimosa pudica). Rather than being classified as a fern, as I had previously thought, I discovered that it was actually a perennial plant from the same family as peas and legumes.

After some further research, I found out that the amazing ability of the sleepy plant to close its leaves is thought to have evolved as a mechanism to prevent water loss via evaporation, scare away insects that land on it, and as a way to appear less appealing to herbivores. It is able to perform this action through a complex reaction of kinetic energy, ions, and water movement.

The animal-like reaction of a plant is something that amazed me and drew my interest. Whenever I have come across this plant, I always have to spend a few minutes touching and studying it. I am disappointed that this species does not grow naturally in Colorado because the plant is very fun to toy around with and it could be a point of fascination for people of all ages. I would credit the sleepy plant with a significant involvement in getting me interested in plant biology.

Mimosa pudica video by Túllio F, and photo by Ferdous from Wikimedia Commons.

(And check out Mimosa the Musical!)

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Sean is currently a student at the University of Colorado, Boulder studying Ecology and Evolutionary Biology.