The effects of genotype and environment on the leaf area index of rice are well documented, but the rules governing leaf area distribution among main stem and tillers are less well understood. This study investigated the ontogenetic patterns of leaf blade area and dimensions on the main stem and tillers of three rice cultivars, IAC47, Javaé (Oryza sativa L.) and CG14 (O. glaberrima Steud.) grown in pots without competition among plants. No differences in phyllochron were observed among culms, but leaves on tillers appeared 0.5 to 0.8 phyllochrons earlier than the genealogically corresponding leaves on the main stem (system of Katayama). Cohorts were thus not fully synchronized. All culms produced their largest leaves soon after panicle initiation (PI), despite differences in tiller age. Leaves that appeared subsequently were smaller owing to reduced length but not width, which remained constant. Three different hypothetical rules governing the behaviour of culms were tested: (1) tillers behave as clones of the main stem; (2) cohorts behave uniformly, and (3) behaviour is determined by leaf position (sum of genealogical leaf and tiller indices). The results were best explained by hypotheses (2) and (3), with leaf blade width and length showing markedly different patterns.