Lichens have a main body part called the thallus, which is composed of hyphae, and houses the cortex and medulla. The cortex contains the photosynthetic cells while the medulla allows for gas exchange and makes up the bulk of the lichen's thallus. There are three main types of lichens: crustose, foliose, and fruticose. Foliose lichen are characterised by flattened leafy thalli, and an upper and lower cortex. Many have numerous layers, which are stratified, and aid in identifying different types.
Foliose lichens attach to surfaces by hyphae in the lower cortex with smaller root like structures called rhizines (Lichens of North America Pg 3, 13). Lichens play an important role environmentally. They provide a food source for many animals such as deer, goats, and caribou, and are used as building material for bird nests. Some species can even be used in antibiotics. They are also a useful indicator of atmospheric pollution level. (Daniel and Nicholas)
There is a direct correlation between pollution and the abundance and distribution of lichen. Foliose lichens are extremely sensitive to sulphur dioxide, which is a by-product of atmospheric pollution. Sulphur dioxide reacts with the chlorophyll in lichen, which produces phaeophytin and magnesium ions. When this reaction occurs in plants the lichen will then have less chlorophyll causing a decrease in respiration which eventually kills the lichen (Hill 831–836).
Minerals in rocks can be weathered by the growth of lichens on exposed rock surfaces. This can be attributed to both physical and chemical processes. Lichen can chemically weather minerals such as calcite by producing organic acids like oxalic acid. This reacts with minerals in the rock, dissolving them and weakening the rock. As a result of this many rocks that have or once had lichen growth exhibit extensive surface corrosion. By-products of this weathering are poorly ordered iron oxides and amorphous alumino-silica gels, the neoformation of crystalline metal oxalates and secondary clay minerals. Lichen physically weather rocks by penetrating the rock's small crevasses with their rhizoids. The expansion and contraction of the roots causes smaller cracks to expand.
These combined processes – of chemical and physical weathering – also serve to deteriorate asphalt shingles, with foliose lichen byproducts dissolving the limestone (calcium carbonate) used as filler and their rhizoids expanding cracks which develop in the shingles over time.