Decomposing pieces of leaves and thorns, dust and other pollutants from the air, contaminate the vase water. All this decomposition, organic matter and molds make the water cloudy and dirty and can be a food source for microorganisms. The vascular bundles in the stem, required for absorbing water, can become plugged by this contamination. This contamination is especially evident on the water surface. When the water level drops, the unsightly scum remains on the side of the vase, dries up and becomes visible.