volume of the thallus. The density of pigmentation of the upper cortex also varies and seems to control the amount of light reaching the algae. Reproduction and dispersal A large number of lichens disperse very effectively by means of symbiotic vegetative propagules such as soredia, isidia and blastidia, and thallus fragmentation. However, X. parietina does not produce such vegetative propagules, but must establish the symbiotic state at each reproductive cycle. Two oribatid mite species, Trhypochtonius tectorum and Trichoribates trimaculatus, which are common inhabitants and consumers of X. parietina, are vectors of the photobiont cells. Faecal pellets of both species contain both viable ascospores and photobiont cells, and are suggested to be a common mode of short- and long-distance dispersal of this species. Habitat and distribution Xanthoria parietina occurs in hardwood forests in broad, low-elevation valleys, as well as scattered on Populus and other hardwoods in riparian areas in agricultural and populated areas. It is often associated with high level of nitrogen and favored by eutrophication, and can be often found near farmland and around livestock. The lichen is used as a food source and shelter for the snail Balea perversa. The species is widespread, and has been reported from Australia, Africa, Asia, North America and throughout much of Europe. In eastern North America and Europe, it is found more frequently near coastal locations. The increases in nitrate deposition as a result of industrial and agricultural developments in southern Ontario, Canada in the 20th century are thought to be responsible for the reappearance of this species in the local lichen flora. Pollution tolerance Xanthoria parietina is a very pollution-tolerant species. In laboratory experiments, this species can tolerate exposure to air contaminants and bisulphite ions with little or no damaging effect. It is also tolerant of heavy metal contamination and to nitrogen pollution. For these reasons, this species has found use as a biomonitor for measuring levels of toxic elements. Bioactive compounds Xanthoria parietina produces an orange colored anthraquinone pigment, parietin, that is deposited as tiny crystals in the top layer of the upper cortex. Parietin synthesis is enhanced by UV-B, and stimulated by photosynthates, such as those provided by the green algal Trebouxia symbiont. X. parietina also produces the metabolite 2-methoxy-4,5,7-trihydroxy-anthraquinone. Medicinal properties The water extract of X. parietina has good antiviral activity in vitro, inhibiting the replication of human parainfluenza virus type 2. In the past it was used as