Statistical analyses of the relationships between the patterns of distribution of plants with C4 photosynthesis and Crassulacean acid metabolism (CAM) have revealed a variety of correlations with environmental variables. The worldwide abundance of C4 grasses, relative to C3 grasses, is highly positively correlated with growing season temperature. However, microscale analyses have revealed that C4 grasses are more abundant than C3 grasses in habitats with high levels of solar irradiance and low moisture availability. There are numerous exceptions to these generalizations. C4 dicots generally are more abundant in habitats characterized by high rates of evaporation. Species possessing CAM occur in habitats having low levels of soil moisture store. In the Cactaceae such habitats also have very high potential rates of evaporation and the CAM pathway is the primary mechanism of the uptake of atmospheric CO2. In contrast many species of the Crassulaceae grow in habitats with lower potential rates of evaporation and the CAM pathway is less important or not used at all in the uptake of atmospheric CO2.