• Regeneration of lobes;
  • pseudomeristematic thallus margin;
  • positional control;
  • dispersal of symbiotic phenotype;
  • thallus fragmentation


A series of outdoor culturing experiments on artificial ceramic substrata was set up to test the regenerative capacity of thallus fragments of the foliose lichen Xanthoria parietina (L.) Th. Fr. which had been dissected along an axis from the growing marginal pseudomeristem to the apothecia-covered central areas. This dorsiventrally organized macrolichen (photobiont: Trebouxia arboricola De Puymaly) is capable of regenerating new lobes along wound margins or cut edges in any part of the thallus, including the thalline margin of apothecial disks. In semithin sections of central thalline areas of non-disturbed thalli, where fungal and algal cells have reached their mature dimensions and where a high percentage of oversized Trebouxia cells imply an arrested cell cycle, small islets of actively growing and dividing fungal and algal cells were found within the algal layer. These are likely to be the sites where primordial stages of new lobes initiate after wounding. In nature, apothecia-covered central thalline areas of X. parietina usually lose their contact with the substratum and break off along drought-stress induced cracks. When landing on a suitable substratum, detached thallus fragments were found to regenerate new lobes in the same manner as artificially dissected samples, thus serving as propagules of the symbiotic state of this ecologically very successful lichen species.