Differentiate between tropic and nastic movements in plants.
Tropic Movements: There is directional growth of a plant or part of a plant in response to an external stimulus i.e., direction of stimulus controls direction of growth. The effect is more or less permanent. It is easily observed in stems and roots of plants. It occurs due to unequal growth on the two sides of a stem or root. For example, bending of root towards gravity and bending of shoot towards light. Nastic Movements: The growth or movement is independent of the direction of stimulus. The effect is temporary and reversible. It occurs in specialised parts and organs of plants like leaves and petals of flowers. It usually involves alterations in cell volumes. For example, folding on leaflets of touch-me-not plant on touching them.
