Encelia is a genus of mainly drought-tolerant shrubs found in arid to semi-arid areas of southwestern North America and western South America. Several species have well-characterized ecophysiological and morphological adaptations for dealing with drought. All but one species are currently assigned to one of two clades, based on morphological, phytochemical and preliminary genomic data. The purpose of this study was to determine whether the internal anatomy of leaves, stems, and peduncles reflects the adaptations and external morphology of individual species, or instead the basic phylogenetic patterns of the genus. The basic anatomy of stems and leaves is consistent with other xerophytic Asteraceae. Resin ducts are present in more species than indicated in earlier studies. The unique leaf shape of Encelia ventorum is reflected in its internal anatomy, but the similarities among the species are otherwise much greater than habitat, external morphology, or phylogeny might imply. This provides further evidence of the recent diversification of the group.

Key words: adaptation, anatomy, Asteraceae, Encelia, phylogeny