The leaflets also close when stimulated in other ways, such as touching, warming, blowing, or shaking. These types of movements have been termed seismonastic movements. The stimulus is transmitted as an action potential. The action potential causes potassium ions to flow out from the vacuoles of cells in the various pulvini. This causes water to flow out from those cells by osmosis through aquaporin channels, making them lose turgor, which is the force that is applied onto the cell wall by water within the cell. Differences in turgidity in different regions of the leaf and stem results in the closing of the leaflets and the collapse of the leaf petiole.
It is not known exactly why Mimosa pudica evolved this trait, but many scientists think that the plant uses its ability to shrink as a defense from herbivores. Animals may be afraid of a fast moving plant and would rather eat a less active one. Another possible explanation is that the sudden movement dislodges harmful insects.
“I understood some of those words”
Also interesting: This plant is the best evidence we have for plant memory and learning! Here’s the article: http://phenomena.nationalgeographic.com/2015/12/15/can-a-plant-remember-this-one-seems-to-heres-the-evidence/
From the article: Researchers at the University of Western Australia did an experiment with the Mimosa Pudica where the plant was dropped roughly six inches, not once, but 60 times in a row at five-second intervals. The plants would glide into a soft, cushiony foam that prevented bouncing. The drop was sufficiently speedy to alarm the plant and cause its teeny leaves to fold into a defensive curl.
Six inches, however, is too short a distance to do harm, so what researchers wondered was: If they dropped 56 plants 60 times each, would these plants eventually realize nothing terrible was going to happen? Would any of them stop curling?
Or, to put it another way: Could a plant use memory to change its behavior?
To find out, they kept going with their experiment. And, as they write in the paper, fairly quickly “observed that some individuals did not close their leaves fully when dropped.” In other words, plants seemed to figure out that falling this way wasn’t going to hurt, so more and more of them stopped protecting themselves—until “By the end, they were completely open … They couldn’t care less anymore.”
A week later they repeated the experiment, and still the plants failed to get alarmed. Their leaves stayed open. They did it again, week after week, and after 28 days, these plants still “remembered” what they’d learned.