Give an example and explain how plants may immediately respond to a stimulus. (AS1)

An example to show that plants immediately respond to a stimulus is Mimosa pudica. It is a very sensitive plant. The leaf of Mimosa has 15-20 pairs of small leaflets along each midrib. As the leaf receives a stimulus such as a touch or wind, etc, it folds in & the stem droops. It is due to change in the turgor pressure at the point of touch. Turgor pressure is a hydrostatic pressure which develops due to the osmotic entry of solvent into it. Mimosa pudica shows seismonasty. Turgor pressure changes in the cells of their pulvini. Regular turgor changes in the leaflet bases cause rhythmic autonomic movement. In case of loss of turgidity, the shoots droop and the leaves show wilting. In wilting, the individual cells of leaves become flaccid due to loss of water from their interior.