You may be surprised to learn that this elegant plant dappled with water droplets is a variety of moss: Tortula muralis, better known as wall screw-moss. It"s found all over the world, even in urban areas where more sensitive mosses have a hard time thriving because of dry soil and air pollution. Mosses in general are accurate monitors of air pollution—they absorb air and water, so contaminants can also be measured in their cells.