Ask Mr. Smarty Plants

When a plant begins to wither, it is to protect itself from water loss. Water is absolutely vital to a plant: it takes up nutrients in water, and it transpires (like a person sweating) water to cool itself down. A plant in a pot or in the ground that has begun to dry out and is out in the sun is in danger of losing all its moisture to transpiration and dying. Once the cells in the stem have dried up and died, there is no recovery. The water moving up the stem, by a process called osmosis, swell the cells in the stems and keep the stem rigid. Meanwhile, the leaves are trying to protect the plant by folding in and shading the leaves from the sun and transpiration. As there is less and less water available to move up the stem, the cells in that stem will become less full and the stem will start to droop. Hopefully, someone comes along and notices that the plant is drooping and applies water. The water goes to the root, where it is gathered into the plant by tiny hair-like growths on the root, and begins to be distributed up the stem again. The cells in the stem begin to fill with the fresh moisture, and the stem again becomes rigid and is able to straighten up. Then, the water in the stem goes to the leaves and they are able to uncurl as water comes to their rescue.

For a more complete explanation and some illustrations, go to this BBC website, "Green Plants as Organisms."