Roots and Xylem
Flowers normally absorb water through their roots, which are attached to small tubes called xylem. These xylem act like thin straws, pulling water up through the plant to the leaves and flowers. When the flower is cut, it no longer is in contact with the roots but it can still absorb water through the xylem. Having a source of water can keep a cut flower in bloom for days or even weeks before it finally wilts and dies.
Water molecules are attracted to each other, a phenomenon called adhesion. They are also attracted to the surface of the xylem. This causes a phenomenon called capillary action. A water molecule's attraction to the xylem pulls it up the tube into the flower. This molecule in turn pulls other molecules up with it. As a result, a continuous stream of water molecules rises up into the flower, keeping it alive.
Transpiration helps keep the water moving. Plants have small holes in their leaves and flowers. If a plant has excess water, those holes open, allowing some of the water to evaporate. This creates a vacuum, aiding capillary action by pulling more water up the xylem. Plants also use water in photosynthesis, turning water and carbon dioxide into oxygen and sugar. But cut flowers usually do not photosynthesize much, because they generally have few leaves and are usually not put in bright sunlight.
Keeping the Flowers Alive
Once a flower is cut, it slowly dies. How long this process takes depends on the flower and the way it is cared for. Sometimes, the xylem can get an air bubble at the bottom of the stem. This prevents the flower from sucking up any more water, and as a result, it can wilt in just a few hours. To prevent this, florists cut stems diagonally to prevent air from getting stuck on the underside, and they cut the stem under water so no bubbles form. Because cut flowers rarely photosynthesize, they don't receive any food. Plant food with sugar in it is often added to the water to nourish the flowers and keep them alive longer.