"Physiology," which is the study of the function of cells, organs, and organisms, derives from the Latin physiologia, which in turn comes from the Greek physi- or physio-, a prefix meaning natural, and logos, meaning reason or thought. Thus physiology suggests natural science and is now a branch of biology dealing with processes and activities that are characteristic of living things. "Physicochemical" relates to physical and chemical properties, and "Environmental" refers to topics such as solar irradiation and wind. "Plant" indicates the main focus of this book, but the approach, equations developed, and appendices apply equalIy welI to animaIs and other organisms. We wilI specificalIy consider water relations, solute transport, photosynthesis, transpiration, respiration, and environmental interactions. A physiologist endeavors to understand such topics in physical and chemical terms; accurate models can then be constructed and responses to the internal and the external environment can be predicted. Elementary chemistry, physics, and mathematics are used to develop concepts that are key to under-standing biology -the intent is to provide a rigorous development, not a compendium of facts. References provide further details, although in some cases the enunciated principIes carry the reader to the forefront of current research. Calculations are used to indicate the physiological consequences of the various equations, and problems at the end of chapters provide further such exercises. Solutions to alI of the problems are provided, and the appendixes have a large tist of values for constants and conversion factors at various temperatures.