ULTRASTRUCTURAL ASPECTS OF ENDOMYCORRHIZA IN THE ERICACEAE

S ummary The ultrastructural features of the dissociated mycorrhizal symbionts Pezizella ericae Read and Calluna vulgaris L. Hull, grown in pure culture, and of the modifications occurring in both fungus and root cells during their reassociation in axenic culture have been studied in detail. Growing (apical/subapical and vacuolated zones) and non‐growing (ageing zone) regions can be distinguished in colony margins of Pezizella ericae in pure culture. In axenic mycorrhiza cultures the general morphology of the fungus remains the same with living hyphae closely resembling those in the active growing region. However, the external hyphae around roots develop an abundant fibrillar sheath which has a polysaccharide‐protein composition and the possibility that this may play a role in recognition and adhesion phenomena in ericoid mycorrhizae is discussed. Metachromatic vacuolar granules, which are very common in pure cultures of Pezizella ericae , are often present in external hyphae around axenic mycorrhizal roots but are very rarely observed in intracellular fungal hyphae. The general ultrastructural organization of these axenically produced ericoid mycorrhizae is compared to that reported for natural infections in several ericaceous species.