Since the XVI century basic anatomical features of Cactaceae have been studied. Mostly this anatomical research has focused on selected features related to different external forms or to stem photosynthesis Crassulacean acid metabolism. However anatomical stem features have rarely been taken into consideration in systematic studies. Recent work has focused in the subfamily Cactoideae because it is the largest and highly diverse subfamily in Cactaceae. Moreover, molecular phylogenies have supported Cactoideae monophyly, but tribal and generic relationships are mostly unresolved. It is thought that Cactoideae originated in the Caribbean and Northern South America and then diverged in two main evolutionary groups, one in South America and the other in Mexico and North America. Anatomical stem characters useful for considering phylogenetic questions in North American Cactoideae members specially tribes Cacteae, Echinocereeae and Pachycereeae, were generated. Analysis revealed that most dermal characters like uni-multiseriate epidermis and dermal cell contents (silica grains, various crystal types) prove to be valuable at the species and genus level, but when analyzing the whole subfamily most of them have originated independently several times. Cortex and pith are more variable, however occurrence of fibers in cortical bundles and distribution of mucilage cells were features shared by many North and South American members that were also originated independently. Wood in North American members is more homogeneous than in members of South America and is not informative at the genus level. The combination of anatomical and morphological features with molecular data will be useful to better understand phylogenetic relationships among Cactoideae members.

Key words: Cactoideae anatomy, Echinocereeae, Pachycereeae, stem anatomy, systematic anatomy