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During the past decade, three previously undescribed vascular plant species have been documented growing on Mancos Shale barrens in southwest Colorado. Soil chemistry has been correlated with the presence of some rare endemics elsewhere, such as species growing on serpentine soils. However, research on shale barrens suggests that structural components, rather than unique chemistry, more strongly influence endemism on this substrate. The effect of the structural characteristics of the barrens soil was examined in connection with the presence of cushion bladderpod (Physaria pulvinata), a rare barrens endemic. The results suggest that P. pulvinata may be uniquely suited to the shallow soil and low moisture characteristic of the shale barrens habitat. The need for long-term monitoring and management is discussed.