The formation of the lower nodes and internodes in maize (Zea rnays L.) and the progression of their differentiation was investigated by generating clonal sectors from cells of the apical meristem. Marked clones were induced by irradiating dry seeds (kernels)and 2-, 8-and 13-day-old seedlings heterozygous for anthocyanin markers (b, pl) and a chlorophyll factor (wd). The extent and apparent number of cells generating the internodes 2-6, which normally remain condensed, were traced by promoting the elongation of these internodes with gibberellic acid. At the mature seed stage, internodes 2 and 3 are undergoing longitudinal expansion and each is represented by two or three circumferential populations of cells. Internodes 4 and 5 are in the process of radial expansion and each is represented by a single circumferential population of cells. At nodes 2-4, the cells for leaves and internodes have separated but such a separation has not occurred for nodes 5 and 6. The formation and expansion of basal six internodes progressed acropetally, i.e. from the base toward distal nodes. Analysis of sectors induced at the seedling stage shows that the formation of middle and top internodes also progress acropetally. The basal, middle and top internodes were found to develop at different apparent cell numbers in the apical meristem.