Stem Is Sealed Off
When a flower's stem is cut, it is abruptly severed from its support system--namely, the roots of the plant. This is a shock to the flower's system. Normally, a flower gets water and minerals from the soil through the roots. Water is absorbed through the epidermis, or skin, of the roots, and minerals travel from the soil to the center of the plant's roots along tiny hairs that grow on roots. These minerals combine with the water in the center of the roots. Then the water travels up the stem of the plant and to all parts of the plant, carrying the nutrient-rich minerals with it. Once the stem is cut, there is no way for the plant to get minerals from the soil, although it can still "drink" water through the stem. The flower's immediate reaction is to seal up the tissue at the cut site to protect the water and minerals that are left in the stem and to prevent harmful bacteria from entering the flower.
Water Intake Slows
If a flower is placed in water, the bottom of the stem should be cut again right before it is immersed to open it back up. This will encourage the flower to "drink" the water in the vase. The process of drawing up water in a flower is called "transpiration-pull," and it is caused by the evaporation of water from the plant's leaves, as well as the attraction between water molecules, which work to "pull" each other up the stem. Still, although the flower will "drink" the water for a while, it will do it much more slowly and less effectively than when it was in the ground.
Death of the Flower
Even if plant food is added to the water, it is not enough to feed the plant. Without the necessary minerals and nutrients (usually found in soil) to drive the plant's metabolism, the plant will eventually stop pulling up the water entirely. Water will cease to be delivered to the outermost parts of the plant, including the blossom and leaves, which will wilt from the lack of water. Eventually, the stem will also wilt with no water to help keep it upright, and the flower will die.