Mosses and lichens can serve as indicators of air and water health. Because their thin cell layers absorb water and chemicals from the air, they are sensitive to pollution levels. They do not have a system in which they can filter contaminants such as sulfur and nitrous oxides from air pollution. Many species of mosses and lichens die when the contaminant levels reach a concentration in the plant that interferes with the way the plants manufacture nutrients. In lichens accumulated sulfur breaks down the chlorophyll molecules which are responsible for photosynthesis in the algae. As mosses dry out after a rain, heavy metals and nitrogen concentrate in their cells, which impact photosynthesis on a cellular level. Because of that, scientists identify mosses and lichens as indicators of air and water pollution changes in the environment. Within Colonial National Historical Park, visitors will find these tiny plants along stream banks, in shady woodlands, and along forest pathways. Sphagnum moss species are especially visible in the vicinity of the Grafton Ponds.