Sensitive Plant (Mimosa)

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.

The movie shown here is in real-time and shows the propagation of the response from a momentary exposure to the heat from a flame held below just the tip of a leaf. The response can be seen moving down the midvein where it triggers movement of each individual leaflet. 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. 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 as can be seen in this other real-time movie of a stronger response.