Edwards E. J., Nyffeler R., Donoghue M. J., Basal cactus phylogeny: implications of Pereskia (Cactaceae) paraphyly for the transition to the cactus life form, American Journal of Botany, 2005, 92(7):1177-1188, doi: 10.3732/ajb.92.7.1177
Analysis of five DNA regions revealed distinct lineages within broadly circumscribed Pereskia. The paper reconstructs relationships and the sequence of stem structural changes in leafy cacti.