Shock to System
Most flowers wilt due to a lack of water, minerals or both. Flowers that are cut wilt and eventually die. Flowers that lack water can recover from wilting with the application of more water. The cutting of a flower's stem is a shock to its system. The flower is suddenly severed from the roots, which is how it absorbs water and nutrients from the soil. Water flows from the soil through the epidermis, or skin, of the roots, to the center of the roots. Minerals, which provide the flower with nutrients, travel along the tiny hairs that line roots. These minerals "ride" the water molecules to all parts of the plant and power the metabolism of the plant. The instant a plant is cut, it is cut off from these minerals. The flower responds by sealing off the tissues in the stem to prevent the loss of any more water or nutrients.
Loss of Turgor Pressure
Most people know that if you put a cut flower into water, it will start to drink the water and survive for a short time. To prevent the stem from sealing off, it is important to put it immediately in the water after cutting, or you will need to give it a fresh cut. While the flower is still drinking water, it is no longer receiving any minerals from the soil. This lack of minerals means that the plant can no longer make energy from sugar, which means that plant cells are not full of sugar. When the vacuoles (which house cell food) are full of sugar, water flows into the vacuoles, filling them up. The plumped-up vacuoles in turn cause the cell walls to stiffen up, which is how plants and flowers "stand." This is called tugor pressure. When plant cells run out of energy, they can no longer maintain this pressure.
Loss of Water
Over time, cut flowers will stop taking in water. They no longer have the energy to do so. Water will cease to flow into cells and it will no longer fill the spaces between cells, which is another reason plants stay firm. Plants that are in the ground but do not have enough water will also suffer in the same manner. If water is not being circulated to all parts of the plant, those parts will begin to wilt. Usually, the flower wilts first, followed by the tips of the leaves, then the leaves, then the stem itself. Eventually, the flower dies.