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Summary The identity of the carbohydrate moving between the symbionts of a lichen depends on the genus of alga present. Twenty‐seven lichen species, involving eight different genera of algae, have been examined. In lichens containing five genera of Chlorophyceae, carbohydrates move to the fungus as polyols (ribitol in Trebouxia, Myrmecia and Coccomyxa; erythritol in Trentepohlia; and sorbitol in Hyalococcus). In lichens containing three genera of Cyanophyceae (Nostoc, Calothrix and Scytonema) glucose was the carbohydrate moving, although there is a possibility that in the one lichen with Calothrix which was investigated, it may have been a glucan.

Summary The toxicity to lichens of dissolved sulphur dioxide (as measured by net photosynthetic 14C fixation) was studied as a function of pH and is linked with the oxidation‐reduction properties of sulphur dioxide. An examination of pigments from lichens and spinach exposed to aqueous sulphur dioxide in vivo and in vitro suggests that the increased toxicity observed at low pH values is associated in part with the destruction of chlorophyll by an irreversible oxidation process.

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