The architecture and growth pattern of Posidonia oceanica was studied near Cannes, France in the Mediterranean from 80 samples collected at 6 m depth. Mathematical models were constructed in order to quantify the distribution of internodes within growth units from different morphological types of axes and branching orders. The results revealed that the frequency distribution of the number of internodes per growth unit on plagiotropic axes changed according to branching order, passing from a Poisson distribution on order-1 axes, to a combination of a Poisson and a binomial distribution on order-2 axes, to a binomial distribution on order-3 axes. The production of internodes per growth unit on orthotropic axes followed a binomial frequency distribution. The distribution of internodes produced by transitional-plagiotropic growth units was binomial despite the visual plagiotropic aspect of these units. The model shows that each year two statistically independent series of branches, with different probabilities according to branching order, occur in P. oceanica. The results are consistent with growth being regulated by an internal biological clock, which emits three signals during the course of the year.