Studies in the Physiology of Lichens

S ummary The main carbohydrates soluble in 80% alcohol in Peltigera polydactyla are mannitol and a galactose‐mannitol glycoside. The main carbohydrates obtained by mild acid hydrolysis of the insoluble residue are galactose and mannose, together with smaller amounts of glucose and a pentose provisionally identified as xylose. The utilization of the glucose absorbed by Peltigera discs in 24 hours is of the following approximate pattern: 20% is released as carbon dioxide, 20% is converted to an ‘insoluble’ glucose polysaccharide, and the remaining 60% is retained in the ‘soluble’extract—about 45% to 50% as mannitol, about 5% as the glycoside, and the remainder as unidentified substances. No free glucose could be detected in extracts of discs fed with labelled glucose. Dissection experiments did not reveal any qualitative differences between the carbohydrates of the ‘algal zone’ and medulla. The medulla contained more insoluble carbohydrate than the‘algal zone’. The ‘algal zone’ was the only region in which synthesis of the glucose polysaccharide and the galactose‐mannitol glycoside from absorbed glucose could be detected. There is as yet insufficient evidence to indicate the role of mannitol in the metabolism of P. polydactyla , but it is possible that it is a storage carbohydrate.