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Ever Wonder About Mimosas?

When a friend told me she picked up a Mimosa for her apartment, I recalled the first time I saw one as a kid on a field trip to a botanical garden. I was amazed at how the leaves of these plants closed up when you touched them.

Mimosa is a large genus consisting of hundreds of species worldwide. Some are considered weeds or invasive species .The houseplant version is Mimosa pudica, originally from Central and South America. Mimosa means mimicking and pudica refers to shyness. It also has common names like the sensitive plant, the touch-me-not, or the shameful plant. I once had a champagne and orange juice drink called a Mimosa, but that was named after a different Mimosa altogether. That Mimosa, Acacia dealbata, had fluffy, orangey-yellow flowers. The Mimosa I'm talking about has pom-pom-like, purply-pink flowers.

Mimosas are involved with all kinds of nasty-ness, not in the Janet Jackson sense, but as in movement responding to stimuli. They move in response to touch (thigmonasty), vibration (seismonasty) and heat (thermonasty). Mimosas also close up with a lack of light (nyctinasty) which means they usually close up at night.

From the source of the stimulus, a signal travels from cell to cell by cytoplasmic bridges, toward specialized "motor cells" in regions of the plant, called the pulvini, located at the base of the leaflets. Stimuli change the permeability of cell membranes, so certain molecules pass through more easily than others. The effect tends to be all or nothing. Potassium (K+) ions diffuse out, while balancing charges from chloride (Cl-) ions are not able to move inside; so with this difference in charge, an action potential forms. If the signal is strong enough, it can also propagate to other leaflets. The signal travels at 20–30mm per second. That’s about a thousand times slower than a signal along mammalian nerves (100m per second). But, to paraphrase Samuel Johnson, it is like a dog walking on its hind legs—although it’s not done well, you are surprised that it’s done at all. A chemical transmitter also seems to move from the site of stimulation to other parts of the plant.

When the cells in the pulvini receive a signal, they rapidly lose their internal fluid pressure (turgor). These cells have elastic walls, so without turgor they deflate and the attached leaflets droop. They refill with fluid in about fifteen to thirty minutes, which is known as the refractory period.

So what is the point of all this peculiar movement? Related legume species also show nyctinasty, suggesting that this is a primitive adaptation. It may help conserve water. Thigmonasty seems to deter herbivores. Larger plant eaters might avoid the wilted foliage or might be deterred by the spines which become more prominent when the leaves fold. Smaller herbivores, like caterpillars, would fall off when the leaves collapse. An experiment involving drops of water has also shown that Mimosas can learn to stop responding to repeated stimuli; learn more quickly under worse environmental conditions; and remember this response for several weeks—better than me, these days.

If you have any touching tales about Mimosas, share them below! Then read about that time we asked preschoolers to do some gardening in our Ken Spencer Science Park.