Comparative morphology and fine structure of a group of Umbilicaria lichens

The gross morphology and fine structure of tissue layers in five Umbilicaria species (U. vellea, U. mammulata, U. papulosa, U. muhlenbergii, and U. deusta) were examined using bright-field and transmission electron microscopy. Differences in the surface topography of the upper and lower cortexes of the five species were found. Four of the species contained an osmiophilic banding material on the walls of the outermost file of living upper cortical hyphae. Although the fine structure of phycobiont cells was basically similar for all species, U. vellea was found to have smaller amounts of stored starch and peripheral lipid in cells of the algal zone than the other four species. Algal–fungal contacts were not haustorial, although aplanospore clusters were penetrated by wedge-shaped intrusive hyphae. Senescent algal cells accumulated large numbers of starch grains. Hyphae in the medullary zones were found to be similar for all species with the exception of U. muhlenbergii, which had an extrahyphal, gel-like matrix. Extensive, sheetlike lamellae were also present in the lower cortex of this species. It would appear that many aspects of thallus fine structure and morphology have a direct effect on gas exchange and water relations responses previously reported in the literature.