Investigation of the lichen use as bioindicators, determining dependencies between atmosphere composition and changes in anatomical and morphological features were carried out since the 70's, when Hawskworth and Rose worked out the 10 gradual lichenic scale for England and Wales which indicates the level of sulphur dioxide contamination of the atmosphere. This method of investigations, now known as the floristic one, is still used also in Poland.
Our investigations showed, that lichen dipped in electrolytes solutions change its composition exchanging cations between the solution and cation-active lichen layer. This mechanism may stabilize the surrounding pH value.
The aim of our research was studies of the dependence between pH of rain and type and quantity of mobile ions bound in the cation-active layer of lichens. The natural cation-active materials, were investigated in accordance with the methods applied for the synthetic ion exchangers. During our investigations the changes in type and concentration of ions between the solution and lichen were not found, however, one should suppose that in accordance with the Donnan's effect this occurrence should take place.
The research was carried out using Hypogymnia physodes lichens, collected in different places from coniferous and leafy trees. Desorption of mobile cations: H⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺, naturally connected with lichen, were carried out in copper sulphate or hydrochloric acid solutions of concentrations c < 2.10^-3 mol/dm³. The changes in conductivity and pH of the solutions were measured and the contents of metals before and after desorption process were determined.

Key words: lichen, ion equilibrium, lichen indication.

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