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The photosynthetic capacity of lecanorine apothecia, and their relative importance for the CO. balance of the thallus have been evaluated in the fruticose desert lichen Teloschistes lacunosus (RUPR.) SAV. under laboratory conditions. Three kinds of thalli were selected: thalli without apothecia, thalli with few apothecia and thalli with many apothecia. In each set chlorophyll content, net photosynthesis and dark respiration rates were studied. CO2 gas exchange of isolated apothecia of Teloschistes lacunosus was also measured, The lecanorine apothecium proved to have high rates of net photosynthesis and chlorophyll content, comparable to the most active regions of thalli without or with few apothecia. However, chlorophyll content and net photosynthesis rates, (expressed on dry weight basis), of thalli with many apothecia were significantly lower than those of thalli without or with few apothecia. It is discussed whether this difference is due to a structural alteration in the thallus. For this purpose, photobiont distribution and structure of thalli with many apothecia were compared with those of the thalli without or with few apothecia. The laciniae that carry apothecia have a smaller chlorophyll content than laciniae from thalli carrying no or few apothecia. Under laboratory conditions, the lecanorine apothecia of T lacunosus are shown to be self-sufficient with respect to its CO2 balance.