Phenotypic plasticity, the ability to adjust morphological traits in response to the environment, may enhance survival and reproduction in changing environments. Given this adaptive value of phenotypic plasticity, it is important to understand the mechanisms that may maintain genetic variation for plasticity. Despite the important theoretical role the cost of plasticity plays in maintaining this variation, relatively few empirical studies have focused on the role of cost of plasticity under natural conditions. This dissertation research will address this issue by examining the balance between the cost of plasticity and the benefit of plasticity under a variety of environmental conditions. Populations of the herbaceous plant Geranium carolinianum from both environmentally variable and uniform light environments are compared for the patterns of plasticity to natural variation in light availability. The genetic basis of plasticity will be assessed because the potential of phenotypic plasticity to evolve in response to future environmental change depends in part on its genetic basis. Finally, a field study will examine how selection may favor plasticity under environmentally variable conditions but favor non-plastic responses under uniform conditions. An increase in environmental variation is expected with global climate change. Understanding how plasticity is maintained is central to predicting population response to diverse environments.