Differentiating metaxylem cell lines reveal an increase in nucleolar volume with a parallel increase in r-RNA content of cells. Nucleoli of rapidly elongating metaxylem cells contain feulgen positive material with a concurrent upsurge in endogenous r-RNA level of cells. In late stages of metaxylem differentiation, an extrusion of nucleolus is observed which is followed by an extensive autolysis of protoplast.