Abstract:The presence or absence of warly layer in the vessel elements and fibres of 182 Chinese woody species from 135 genera belonging to 48 dicotyledonous families were examined using electron microscopy. Warly layers were found to be present in 50 species from 32 genera belonging to 17 families but absent in the remainder.Occurance or nonoccurance, distribution densities,sizes and shapes of the warts are of some value for the classification of families,genera and even species of hardwoods.The results of ultrathin wood sections of two species studied by TEM both revealed the differences between the warts and second walls in electron densities,i.e.the chemical composition difference between them. In addition, the relationship between warly layers and types of perforation and their positions in an annual ring was also discussed. But no significant relationship has been found between the occurance of warty layer and the evolution of xylem in this study.