Stems of plane tree seedlings were cytologically investigated regarding the involvement of tyloses and gels in defense reactions to Ceratocystis fimbriata f. sp platani, the canker stain disease agent. In the lumen of infected xylem vessels, cytochemical tests revealed the occurrence of compounds labeled for cellulose and which seem to be involved in gel formation. Immunogold methods indicated that pectin-rich material accumulated in the paramural area of vessel-associated parenchyma cells and around tylosis walls. Tylosis formation and deposition of pectin-containing gels were associated with metabolic changes of vessel-associated parenchyma cells. It is suggested that parenchyma cells play a key role in the defense of plane tree seedlings through plugging of xylem vessels leading to compartmentalization of the vascular fungus.