»Lichens
ξ formed by symbiotic association between cyanobacteria or green algae and fungi
ξ the photosynthetic bacteria or algae make the lichen an autotrophic organism
ξ can take several forms: crustose (encrusting), foliose (leafy) and fruticose (shrubby)
ξ grows on rocks, tree bark or soil often in cold, dry or otherwise inhospitable situations
ξ generally intolerant of air pollution and the presence of lichens is an indicator that pollutants (particularly SO2) are absent