The distribution of phenols in uninfected and mycorrhizal Eucalyptus fastigata roots has been examined by application of a range of histochemical tests to thin sections.

The vacuoles of most cap cells and many epidermal cells of E. fastigata mycorrhizas contain considerable quantities of phenolic materials. Cap cells of uninfected fine roots similarly contain materials with the staining properties of phenols, but uninfected root epidermal cells rarely do. It is suggested that accumulation of phenols in the epidermal cells of mycorrhizas is a response to the presence of the fungal symbiont. Further evidence is provided that the hyphae of the fungal symbiont penetrate and grow through the phenolic materials of the outer root cap cells. No evidence was found to suggest that these areas contain appreciable quantities of polysaccharide.

Phenols were also detected in the endodermis and outermost layer of cortical cells. Judged on their staining reactions, these seem to be chemically different from those of the cap and epidermis. They are present in both mycorrhizal and uninfected roots, but appear much closer to the tip in the former. The outer layer of cortical cells has other specialized features, including the presence of a suberin layer in the walls, which are characteristic of mature differentiated endodermal cells in E. fastigata. These features are discussed from the viewpoint that the outer cortical layer acts as a barrier to further hyphal penetration in eucalypt mycorrhizas.