First off, a plant wilts because of a phenomenon called Osmosis.
Osmosis-:: The diffusion of water across a selectively permeable membrane.
When a plant is in a Hypertonic environment, Osmosis occurs, causing the plant to wilt.
Hypertonic environment-:: When there is a greater amount of dissolved substances outside of a plant cell, causing there to be a greater amount of water inside of the plant cell.
-::When a plant cell is in this environment, Osmosis occurs, causing water to rush out of the plant cell, which causes the cytoplasm and plasma membrane of the cell to shrink away from the cell wall, causing the plant to wilt.
-::When water is added to a wilted plant, water rushes back into the cell, causing the cell organelles such as the cytoplasm and plasma membrane to retain their normal shapes. As water rushes into the cell, the plasma membrane is then pushed against the cell wall (instead of away from it as in Osmosis in a Hypertonic environment) causing the cell to become firm, thus making the plant firm.
Hope this helps.
