Ultrastructural Studies in Lichens

S ummary The mycobiont‐photobiont interface in examples of selected orders of ascolichens with trebouxioid photobionts was investigated by scanning (SEM) and transmission electron microscopy. Between 150 and 530 algal cell halves per species were examined by SEM in order to calculate the predominant type of interaction. Simple wall‐to‐wall apposition predominated in the calicialean Chaenotheca chrysocephala (Turner ex Ach.) Th.Fr. Prominent intracellular haustoria were observed in the majority of photobiont cells of Lecanorales with crustose, non‐stratified thalli [ Lecanora conizaeoides Nyl. ex Crombie, L. chlarotera Nyl. and Lecidella euphorea (Flörke) Hertel]. Three different types of intraparietal haustoria were distinguished in lichens with stratified thalli. Intraparietal haustoria of type 1, consisting of a tiny infection plug formed by a tightly adhering hypha within the largely cellulosic photobiont cell wall were predominant in crustose species with stratified thallus [ Rhizocarpon geographicum (L.)DC, Pertusaria amara (Ach.)Nyl.], but this haustorial type represents also a developmental stage in intraparietal haustoria of types 2 and 3. In the placodioid Lecanora muralis (Schreber)Rabenh. and in the squamules of Cladonia caespiticia (Pers.)Flörke, intraparietal haustoria of type 2 were seen in the majority of photobiont cells, or in ±50% of cell halves, respectively. These can be recognized as an intracellular protrusion formed by a tightly adhering hypha, the tiny infection plug being ensheathed by the algal cell wall. Intraparietal haustoria of type 3, the most highly evolved haustorial type occurring in lichens, was predominant in foliose Parmeliaceae [ P. acetabulum (Necker)Duby, P. tiliacea (Hoffm.)Ach., Cetrelia olivetorum (Nyl.)Culb & C. Culb., Cetraria islandica (L.)Ach.], and in the fruticose Anaptychia ciliaris (L.)Körber, Alectoria ochroleuca (Hoffm.)Massal., Teloschistes flavicans (Swartz)Norman and T. capensis (L. Fil.)Malme. When fully developed the haustorial complex in intraparietal haustoria of type 3 was characteristically stalked owing to growth processes of the haustorium‐forming hypha which forms a tiny infection plug within the cell wall of the photobiont. All types of intraparietal haustoria were established exclusively between juvenile, differentiating partners (growing hyphal tips and algal autospores with not yet fully developed cell walls). Intraparietal haustoria of types 2 and 3 are probably restricted to evolved lichens, but structures resembling intraparietal haustoria of type 1 have also been observed in other symbiotic systems such as ectomycorrhizas.