Pinus strobus seedlings were cultivated in growth pouches and inoculated with Pisolithus tinctorius. Semi-thin sections of mycorrhizal and uninoculated short roots and long roots were stained with toluidine blue and periodic acid Schiff's reagent for phenolic and polysaccharidic intracytoplasmic substances respectively.

No obvious difference in intracytoplasmic contents of phenolic substances was observed between ectomycorrhizal and uninoculated short roots. However, the phenolic content of the cortical cells of the long roots was much lower than that of both mycorrhizal and uninoculated short roots.

Starch grains were not observed in cortical cells of either mycorrhizal or uninoculated short roots but were abundant both in the cortex and the stele of long roots and in the stele of short roots. The absence of starch synthesis in the cortical cells of potentially ectomycorrhizal short roots should be taken into account when explaining the accumulation of soluble sugars as a prerequisite for ectomycorrhizal formation.