Lichens as indicators for air quality.

Indication depends on the existence of a known relationship between the possibilities for existence of the lichens and the quality of their surroundings. These possibilities may be read from the parameters such as: distribution patterns of species, species numbers, physiological and morphological aspects. This is illustrated by examples with emphasis on: choice of parameters, interaction between several factors, comparison of lichens with other organisms and evaluation of the use of lichens as indicators.