Why does the touch-me-not plant shrink when touched?

         

 

 

 

 

 

  The bipinnate compound leaves of Mimosa pudica, touch-me-not plant, have a swollen base called pulvinus which has two distinct halves. The lower half below the vasular strand is made of thin walled parenchyma cells with larger intercellur spaces and the upper half has slightly thick walled parenchyma cells with a few small intercellur spaces.

            Under normal conditions, the cells of both the halves remain turgid. When the touch stimulus reaches the pulvinus the osmotic pressure in the lower half of pulvinus falls. As a result they release water into the intercellur space and become flaccid. But the upper half maintains turgidity the pressure excerted by which causes the leaves to drop down.

            The leaflets also have similar swollen bases but are smaller and are called pulvimules. The touch stimulus is first perceived by these pulvimules. Here also the process occurs which results in the folding of the leaflets. When the stimulus is passed on to the stalk base the entire leaf droops down.

            The touch-me-not plant shrinks within a few minutes of being touched. This is due to the loss of turgidity by cells within the pulvini-specialized motor organs at leaf joints. Upon stimulation the leaf cells lose a potassium ion which causes water to leave the cells by osmosis. It takes about 1 o minutes for the cells to regain turgidity and the leaflets to open out.