DNA SYNTHESIS,CELL DIVISION,AND CELL DIFFERENTIATION DURING LEAF DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

Leaf growth consists of two basic processes, cell division and cell enlargement. DNA synthesis is an integral part of cell division and can be studied with autoradiographic techniques and incorporation of some labeled precursor. Studies were made on the synthesis of nuclear DNA through incorporation of ³H-thymidine in various parts of the lamina during the entire course of leaf development of Xanthium pennsylvanicum. The time course analysis of DNA synthesis was correlated with cell division and rates of cell enlargement. Significant differences in ³H-thymidine incorporation were found in various parts of the lamina. Cell division and DNA synthesis were highest in the early stages of development. Since no ³H-thymidine was incorporated after cessation of cell division (LPI 2.8) in the leaf lamina, it appears that DNA synthesis is not needed for enlargement and differentiation of Xanthium cells. Rates of cell enlargement were negligible in the early development and reached their maximum after cessation of mitoses, between plastochron ages (LPI) 3 and 4. Cells matured between LPI's 5 and 6. Enzymatic activity was correlated with cell division and cell differentiation at various stages of leaf development.