Ontogenetic Course and Spatial Constraints in the Appearance and Disappearance of the Terminal Flower in Inflorescences

Inflorescences are usually designed as closed or open, referring to the presence or absence of a terminal flower (TF), respectively. Until now, it was unknown how much developmental constraints in the inflorescence meristem (IM) determined the production of the TF. To face this question, we carried out a quantitative study of inflorescence development including 19 species from four families of the eudicots (Berberidaceae, Papaveraceae-Fumarioideae, Rosaceae, and Campanulaceae). Our study shows that TFs appear on IMs that possess a certain relative surface, phyllotaxis, and convexity. IMs of open inflorescences show a significantly smaller relative surface (<50%), illustrating a morphological unsuitability for producing a TF. This smaller surface either is existent during the whole ontogeny (open I) or results from a drastic meristematic reduction after flower initiation (open II). We conclude that the TF arises as a consequence of a suitable bulge formation of the IM after a suitable dynamic of the ontogeny. The existence of two kinds of ontogenies of open inflorescences suggests two natures of open IMs and, therefore, two ways in which the loss or gain of the TF could have occurred in the course of evolution.