This dissertation describes the physiological advantages and disadvantages conferred by 'Type I' C3-C4 intermediate photosynthesis, how they appear to be related to the ecological distribution of C3-C4 intermediate species, and how these relationships may provide insight to the evolutionary origins of C4 photosynthesis. The primary study system is the genus Flaveria (Asteraceae), a group that is endemic to southwestern North America and contains multiple representatives of C3, C3-C4 intermediate, and NADP-malic enzyme type C4 photosynthesis. A laboratory-based gas exchange study of six of the basal species of Flaveria grown under common conditions finds that, relative to C3 species, 'Type I' C3-C4 intermediate species are characterized by an increase in instantaneous photosynthetic nutrient use-efficiency and a decrease in instantaneous water use-efficiency. The majority of the increase in nutrient use-efficiency appears to derive from refixation of photorespired CO2 in the bundle sheath, and the enhanced resistance of the bundle sheath cell walls that this implies may be the cause of the coincident decrease in water use-efficiency. A field-based study of wild populations of Flaveria chloraefolia ('Type I' C3-C4) finds that their photosynthetic performance is analogous to what was observed in the laboratory and has the potential to confer several ecologically relevant advantages. Since the increase in nutrient use-efficiency conferred by the 'Type I' C3-C4 pathway is probably less than that conferred by the C4 pathway under comparable conditions, this aspect of performance alone does not appear to be sufficient to explain the stable coexistence of C3, C3-C4, and C4 species in an ecological context. However, it might be sufficient to explain the relative advantage of C3-C4 species compared to C3 species in an evolutionary context.