Taking in Nutrients
The roots on plants take in water, but that's not all they do. Nutrients such as magnesium, potassium and phosphorus are gathered from the soil by the roots and enter the xylem. The xylem are tiny tubes that distribute water and nutrients to the rest of the plant. On a cut flower, the xylem are no longer attached to the roots. They can not take in the nutrients the flower needs, so it slowly begins to die.
Plants normally produce sugars in a process called photosynthesis. The chlorophyll cells of a plant take in sunlight, water and carbon dioxide and produce sugar. Some of the sugars are produced in the stem and some in the flowers, but most of them are produced in the leaves. When flowers are cut, usually only the flower and perhaps a few leaves are left. In addition, the flower is often left in a relatively dark environment, which makes it unable to properly conduct photosynthesis.
Putting your bouquet flowers in pure water or in water with special nutrients for preserving cut blooms can lead to longer life. The stem can continue to absorb nutrients for a while, nourishing the flower and keeping it alive for weeks sometimes. Anything that unnecessarily stresses the flower or disturbs the flow of water, however, can shorten lifespan tremendously. If a cut plant stem is not sliced diagonally, for example, it can sit against the bottom of the vase, blocking the absorption of water. If it is cut in the air instead of under water, it can get a bubble in it that prevents water from flowing up the stem. This can cause it to quickly wilt, a phenomena called bent neck.