A quantitative, round-the-clock study was made of vegetative shoot apexes of a clonal stock of Coleus blumei Benth. A special aim of the study was to find meaningful, exact relation that would be reproducible year after year. Such relations we have found for the following: 1. 1. There is a diurnal rhythm in the initiation of leaf primordia, with a plastochron more likely to start near the middle of the dark period than at other times sampled (Fig. 4). 2. 2. The number of mitotic figures in the apical meristem (here defined as the portion of the shoot distal to the youngest leaf primordium) is closely and significantly correlated with the height of the apical meristem, but the distal 10–20 μ of the meristem consistently has fewer mitotic figures than more proximal regions (Fig. 2). 3. 3. The height of the apical meristem is closely and significantly correlated with the length of the youngest leaf primordium (Fig. 1). 4. 4. By combining the above finding with the previously reported (Jacobs and Morrow, 1957) correlation between lengths of unfolded leaves and of younger primordia, one can now collect apical meristems of reasonably specific heights by measuring the older leaves with a millimeter ruler. 5. 5. Contrary to expectation from the older literature, no evidence was found for a marked diurnal rhythm in the percentage of cells showing mitotic figures. The large increase at 11 p.m. in the absolute number of mitotic figures was a reflection, rather, of the much higher apical meristem at that time (Fig. 4). A critical examination reveals that some of the apparently disparate results in the literature could have resulted from an unrecognized, Coleus-like, diurnal rhythm in the growth of the apical meristem.