Cytological aspects of the mycobiont-phycobiont contact were investigated in the lichen species Peltigera aphthosa, Cladonia macrophylla, Cladonia caespiticia and Parmelia tiliacea by means of freeze-etch and thin sectioning techniques, and by replication of isolated fragments of myco- and phycobiont cell walls.

In the symbiotic state of the mycobionts investigated a thin outermost wall layer with a distinct pattern was observed mainly in the hyphae contacting phycobiont cells and in the upper medullary layer. No comparable structures were noted on the hyphal surface of the cultured mycobionts of the Cladonia and Parmelia species investigated. A distinct rodlet layer was found on the hyphal surface of the mycobiont of Peltigera aphthosa, while mycobionts of Cladonia macrophylla, C. caespiticia and Parmelia tiliacea had a mosaic of small, irregular ridges, each corresponding in its size to a bundle of rodlets on the outermost wall layer. Comparable surface layers have been described in aerial hyphae of a great number of non-lichenized fungi.

The rodlet layer of the mycobiont wall surface of Peltigera aphthosa adheres tightly to the outermost layer of the sporopollenin-containing cell wall of the Coccomyxa phycobiont. Mature trebouxioid phycobiont cells of the Cladonia and Parmelia species investigated in the symbiotic state had an outermost wall layer which was structurally indistinguishable from the tessellated surface layer of the mycobiont cells. A rodlet pattern was detected in the outermost wall layer of Trebouxia autospores still surrounded by the cellulosic mother cell wall. In mature Trebouxia cells the bundles of rodlets became increasingly covered by a homogeneous material, and thus attained the same tessellated pattern which was observed on the mycobiont wall surface. No comparable structures were found on the wall surface after culturing the Trebouxia phycobionts axenically in liquid media. Confluence of the tessellated surface layers of fungal and algal origin was noted at the contact sites of growing hyphal tips and young Trebouxia cells.

The possible correlations between these cytological features and published immunological data on the cell surface of cultured and symbiotic lichen myco- and phycobionts are discussed.