Some extracts from Wikipedia link (for pictures & more details, click on the link):http://en.wikipedia.org/wiki/Mimosa_pudica
The stem is erect in young plants, but becomes creeping or trailing with age. The stem is slender, branching, and sparsely to densely prickly, growing to a length of 1.5 m (5 ft). The leaves of the mimosa pudica are compound leaves. The leaves are bipinnately compound, with one or two pinnae pairs, and 10-26 leaflets per pinna. The petioles are also prickly. Pedunculate (stalked) pale pink or purple flower heads arise from the leaf axils. The globose to ovoid heads are 8-10 mm in diameter (excluding the stamens). On close examination, it is seen that the floret petals are red in their upper part and the filaments are pink to lavender. The fruit consists of clusters of 2-8 pods from 1-2 cm long each, these prickly on the margins. The pods break into 2-5 segments and contain pale brown seeds some 2.5 mm long. The flowers are pollinated by the wind and insects.
Mimosa pudica is well known for its rapid plant movement. In the evening the leaflets will fold together and the whole leaf droops downward. It then re-opens at sunrise. This type of motion has been termed nyctinastic movement. The leaves also close up under various other stimuli, such as touching, warming, blowing, or shaking. The stimulus can also be transmitted to neighbouring leaves. These types of movements have been termed seismonastic movements. The movement is caused when the leafs lose turgor pressure. Turgor pressure is the force that is applied onto the cell wall by water and other cell contents. This allows the plant to stay upright, but when it is disturbed by a stimuli, chemicals in the plant force the water to leave the cell. When this pressure is lost the result is a sagging plant. This characteristic is quite common within the Mimosaceae family.
It is not known exactly why the Mimosa pudica has this feature but many scientists think that the plant uses its ability to shrink as a defense from predator. Many animals may be afraid of such a fast moving plant and would rather go and eat a less active one.