Movements of leaves of sensitive plant, Mimosa pudica are due to

The correct option is seismonasty.

To understand why the folding movement of the leaves of the sensitive plant (Mimosa pudica) is classified as seismonasty, let us break down the biological concepts step-by-step:
1. Nastic Movements: In plants, movements that are non-directional responses to external stimuli are called nastic movements. Unlike tropic movements (which are directed toward or away from a stimulus), the direction of a nastic movement is determined by the anatomy of the plant organ itself, not by where the stimulus comes from.
2. Defining Seismonasty: Seismonasty (sometimes referred to as thigmonasty) is a rapid nastic response triggered by mechanical stimuli such as touch, shock, wind, or vibration. The prefix "seismo-" originates from the Greek word for shaking or shock.
3. Mechanism of Action: When the leaves of Mimosa pudica are touched or shaken, the mechanical stimulus is converted into an electrical signal (an action potential) that travels through the vascular bundles. This signal reaches a specialized swollen structure at the base of each leaflet and petiole called the pulvinus.
4. Loss of Turgor Pressure: The electrical signal triggers a rapid efflux of potassium ions (K⁺) and chloride ions (Cl^-) from the cells in the half of the pulvinus that is sensitive. Water quickly follows the osmotic gradient, moving out of these cells into the intercellular spaces. This causes a sudden loss of turgor pressure (internal water pressure), making the cells flaccid and causing the leaf to fold and droop rapidly.

Let us briefly examine the other options to understand why they are incorrect:
- Chemonasty: This is a non-directional movement in response to chemical substances (e.g., the closing of glandular hairs in some insectivorous plants when triggered by nitrogenous compounds).
- Thermonasty: This is a non-directional movement in response to changes in temperature (e.g., the opening and closing of tulip flowers).
- Hydrotropism: This is a directional growth movement of plant parts, such as roots, toward a water source, which is slow and growth-dependent rather than a rapid turgor-driven movement.