Stem cross section of Pereskia guamacho, a member of the eight-species cactus lineage hypothesized to be sister to the remaining 1800 or so species of Cactaceae. Pereskia guamacho is a leafy, dry-forest tree lacking many anatomical traits, such as stem succulence, mucilage cells, delayed periderm, and stem stomata, that are highly conserved in most other cacti. Rather than the evolution of a single