The flowers and the intention behind them bring smiles, but the inevitable wilted blooms signal a kind of ending. Few things are more disappointing than a beautiful flower arrangement that doesn't hold up. The cells in the flower's stem work like a bundle of soda straws. As long as the straws are in a glass of water you can draw water up through them. Take them out of the water while sucking on the straw and you take in air.
Flowers do the same thing.
Their demand for water is continuous, even when cut away from the mother plant. The difference is that cells in the stem have tiny screens that allow water to pass, but not air. When the flower stem is cut, a small bubble of air forms and is trapped at the end of the stem, preventing more water from getting up the stem even if you replace that stem in water. Blame it on the air bubbles in the stem. Make a new stem-end cut in a bowl of water.