Near Buckeye, Arizona, saltcedar (Tamarix pentandra) was grown on a battery of evapotranspirometers under various conditions of plant density, soil salinity, and access to water. Growth and development of these plants declined sharply at the beginning of each July, although before that time rates of growth and development had been very high. It was initially postulated that carbon dioxide might have become the limiting factor. Measurements of CO2content of the air made at different heights inside and outside the evapotranspirometers, however, showed that this was not the case.