The vegetative shoot apex of Helianthus annuus contains a central zone in which the cell nuclei are relatively large and stain faintly in the Feulgen reaction. Excised apices in the vegetative state were supplied with thymidine-H3 through their sterile, liquid culture medium. Autoradiography after 24 or 48 hours of feeding revealed no significant incorporation of the labeled precursor into central zone nuclei, but extensive incorporation in peripheral regions of the apex. It is concluded that during vegetative growth deoxyribonucleic acid (DNA) synthesis and mitosis are arrested in the central zone or reduced to an extremely slow rate. Microspectrophotometry, however, indicates that the central zone nuclei are not held at the 2C level. With the onset of flowering, cytological zonation disappears in the apex and the incorporation of thymidine-H3 is uniformly heavy throughout the apical region.