SUMMARYA purified phytolectin was isolated from the nitrogen-fixing lichen Peltigera canina var canina. This lectin was thermostable, truly non-specific and may have a molecular weight in its native form of the order of 80000 to 90000. It binds to the appropriate phycobiont cell walls and may be involved in some way in the recognition of, or interactions between, compatible lichen symbionts; it may be used in situ as a cytochemical marker of the free receptor sites in the lichen thalli. We advance a hypothesis that within the lichen the morphological and physiological state of the phycobiont is different in the various parts of the thallus; the capacity of the phycobiont to bind to lectins varies with fiuctuations in the symbiotic relationship between partners.

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