Have you ever noticed that the color of some lichens is a more intense green after they get wet? There’s a very good explanation for this phenomenon. Lichens are made up of an alga or cyanobacterium and a fungus. The alga or cyanobacterium makes the food, and the fungus absorbs the water. A typical lichen has a three-layered structure. A middle layer containing algal cells entwined in threadlike fungus fibers called hyphae is sandwiched between two layers of fungal tissue. Lichens that turn bright green after it rains contain green algae which contains chlorophyll, a green pigment. When it rains, the fungus (which surrounds the algae) soaks up water like a sponge, causing the fungus to become more transparent, which allows the green pigment of the algae to be seen more clearly.
Dog-tooth lichen (Peltigera canina) is often found growing on lawns and rocks. Like all lichens, it consists of an alga or cyanobacterium and a fungus living together in a symbiotic relationship. The fungus provides a structure for taking up moisture and nutrients; the alga or cyanobacterium is capable of photosynthesizing and producing food for both itself and the fungus. The brown structures in the photograph are the fruiting (spore-producing) bodies of this lichen, and their resemblance to dog teeth gives this lichen its common name. In the Middle Ages, dog-tooth lichen was used to treat rabies — it was felt at the time that this lichen’s resemblance to dog teeth indicated that it could cure dog-related ailments.