Cacti exhibit a highly modified bauplan of their vegetative parts. Furthermore, their adaptation to xeric environments has led to extreme levels of specialization. Fourteen morphological and anatomical stem characters from twelve representatives of the primarily West-Andean genera Corryocactus (incl. Austrocactus), Copiapoa, and Eriosyce (incl. Pyrrhocactus, Neoporteria, and Thelocephala) of the tribe Notocacteae, were studied. The different members of these three distinct groups show various degrees of xeromorphic modifications of the epidermis and hypodermis (= dermal system). Variation is found for example in the number of secondary cell divisions and thickness of the epidermis layer, and the number of cell layers and amount of pectinization of the hypodermis. Within Copiapoa and Eriosyce the occurrence of mucilage idioblasts in the water-storage tissue of the cortex seems to be correlated with the presence of reduced dermal systems. A parallel decrease in the xeromorphy of stem anatomy within each of the three genera is discernible. The general pattern of the dermal system is rather distinctive for each of the three genera when the taxa with the more xeromorphic anatomy are compared, while those with 'simplified' dermal systems generally lack generic differences. In the absence of any robust phylogeny based on independent data, it is rather speculative to discuss the evolutionary direction of these trends in stem anatomy. One hypothesis, that is consistent with traditional views on cactus evolution, is that these trends found in the dermal system represent parallel reductions and simplifications correlated with the evolution of small, unbranched, and globular or subglobular cacti.

Key words: Cactaceae, Copiapoa, Corryocactus, Eriosyce, parallel trends, stem anatomy