The response of the plant Mimosa pudica (also known as “Touch Me Not”) to mechanical stimulation has entertained countless generations. As you see in the video below, when the leaves of these plants are touched they fold, and even the branches drop in response to stimulation. This response may be a defense mechanism against insects or weather condition that may harm the plant in the open erect state.
The interesting thing is that the reaction of the plant bears a similarity to the way our nervous system transmits signals. When the plant is touched, several receptors are activated in the leaf which causes a change in the distribution of charged atoms (ions) like potassium between the inside and outside of the cells of the plants. This change in the distribution of ions generates an electric signal called an action potential. In our neurons these actions potentials travel down the length of the neuron and communicate the signal to another neuron or to a muscle. In the plant these action potentials travel to the base of the leaf and into a structure called the pulvinus.
The pulvinus is an area in the base of the leafs and the stems that is engorged with water, and the cells there are under considerable pressure (turgor). In mimosa, the engorged pulvinus is what keeps the leafs open and the stems erect. When the action potential arrives at the pulvinus, some chemical changes take place that causes it to rapidly lose water and therefore turgor. This sequence of events is what causes the leaf or stems of the mimosa to fold or drop in response to touch and other stimuli. In some cases, as you can see in the video, the folding of one leaf causes a chain reaction that causes other leafs to close.
Mimosa is not the only plant that exhibits these rapid responses. The Venus fly trap also relies on a similar mechanism to close its traps.