If you've ever purchased flowers that have had their tips or veins dyed, or if you have ever placed a yellow daffodil into a glass of water colored with green or blue food coloring, then you already have some idea of how a plant takes up water to its flower petals. Think of a flower like a straw, with roots or the cut stem at one end, and the flower petals at the other. In this way, the water moves up the straw-like stem to the petals. The dyes clearly highlight how the water moves from the stem as it shows up along the flower's stem and veins, right to the tips of the petals.
Like most living things, plants need to breathe. Plants breathe through cell-like structures in their leaves and petals known as stomata. When the stomata are open and the plant breathes, it loses water in the same way that humans lose water when they breathe. In plants, this process is called transpiration. Inside the stem are vein-like tubes called xylem. The xylem are actually hollow cells stacked end to end. In thin structures such as leaves or petals, the xylem may be bundled together with a substance called phloem, which helps to transport organic compounds through the plant.
The vascular system pulls the water up to the flower to replace the water lost in transpiration. In cut flowers, when the xylem becomes clogged with bacteria, fungi or air, water no longer travels to the petals. This is why some flowers seem to wilt faster than others. Often, cutting away a portion of the stems while holding them underwater will help clear away the clog and help restore the process of transpiration. Additionally, killing the bacteria with bleach will help the plant avoid wilting longer.
Other factors, such as soil quality and amount of available water, can control the rate at which water reaches the petals of a flower. In the desert where the soil is poor and water is scarce, flowering plants will only produce flower petals during rainy season. These plants adapt by having fewer stomata. Transpiration occurs at a much slower rate for these plants. Although soil conditions in rain forests are often similar to that of deserts, the rain forests have much more water available. Because of this, flowering plants are much more abundant.