I am sure many of you have met sensitive plants, Mimosa pudica, on your travels through the local glasshouses and plants nurseries. This plant is native to Central and South America, but now it seems to have escaped captivity and has established itself in several parts of the world.
Neither this plant nor its flowers are particularly attractive, in my opinion, but it is a popular houseplant because of a particular “behaviour” it exhibits. Basically, this plant closes its leaves at night and opens them in the morning. But that’s not all; this plant also closes its leaves in response to being touched, blown on, shaken, or heated. Below the jump is a charming video that captures the plant’s movements in response to being touched, bumped, hit, and heated with fire along with time lapse videography that captures the plant opening its leaves again a few minutes later.
But how does this plant respond so quickly? The plant folds its leaves by changing the turgor pressure inside its disturbed cells. Turgor pressure is created by water that is stored in vacuoles inside the plants’ cells. When filled, the water presses against the cell wall, making it rigid. When turgor pressure is high, the plant stands upright; when it is low, the plant wilts.
When a sensitive plant is disturbed by touching or heat, the cells release positively-charged ions, such as potassium ions, that act as an intracellular signal. These ions bind to receptor proteins on the surface of the vacuoles that store water inside the cell. Binding of potassium ions causes the vacuoles to release the water, which results in a rapid loss of turgor pressure. This makes the plant flaccid, and … the plant closes its leaves. Here’s a diagramme of this process:
This signal can also be transmitted to neighboring leaves: as you’ll see in the video, leaflets next to a disturbed leaflet also respond by folding up even though they were not touched at all. After five minutes or so, the plant recovers its usual perky appearance by pumping water back into its cell vacuoles, which restores turgor pressure and the leaves unfold.
It’s not known why the sensitive plant evolved this response, but I’ve been told by my colleagues that the plant may use this as a defense against plant predators — rapid shrinking of a plant may scare an herbivore so the plant is less likely to be eaten. Some plant keepers tell me this sudden movement dislodges harmful insects, although I’ve never run across anything in the primary literature suggesting this.
The sensitive plant is one of the few plants that responds rapidly to being touched. Another familiar plant that is rapidly touch sensitive — a favourite of mine — is the venus flytrap, Dionaea muscipula, a carnivorous swamp-dwelling plant with a tiny range on the east coast of North America. Its mechanism to rapidly close its traps around an insect is similar to that used by the sensitive plant to close its leaves. Here’s a closer look at the sensitive plant and its rapid movement: