Functional morphology and evolution of stem succulence in cacti

We explored, using a mathematical model, the changes in areole arrangement that have occurred during the evolution of stem succulence in different cactus life-forms. The study was based on field data. The model used two parameters: (a) areole density in the stem and (b) the divergence angle between successive areoles. The stem morphology in different cactus subfamilies is correlated to changes in one or both parameters. Gradual changes in stem compaction and in the divergence angle can explain the apparently complex evolutionary transition from primitive woody and leafy plants to the more evolved, stem-succulent morphologies of cacti.