Red pine (Pinus resinosa Ait.) specimens were successively treated in the following manner: boiled in water for 2 hours, extracted with alcohol-benzene for 4 hours, extracted with 5% NaOH aqueous solution for 1 hour, and delignified with acidified sodium chlorite for 1 hour. The ultrastructure of the warty layer was studied after each sequence of treatment. It was found that the lumen surface of red pine tracheids was relatively free of encrustation. The microfibrils in the surface lamella of the S3 layer were deposited in a bundle form. Two types of warts were found in red pine tracheids. The first type of warts was localized thickenings of the S3 microfibrillar bundles, and the second type of warts was those situated at the end of microfibrillar bundles. Results of studying the effect of chemical treatments suggested that the encrustant lining the lumen wall was hemicellulose in nature and that the warts were made up of a combination of lignin and hemicelluloses. A possible mechanism of the formation of the last lamella of the S3 layer and the associated warts is also discussed
