This 1974 book was made available as a second edition in 1979. It provides an understanding of the ways in which the various physiological processes are integrated to produce the responses shown by whole plants growing in the variable environment in the field, whilst stressing the quantitative aspects of these relationships. This was the first general text to attempt such a treatment, thereby digesting much material that had been found only in research papers or detailed monographs and complementing the reductionist approach of most standard texts of plant physiology. Most of the subject matter concerns agricultural systems, but many of the concepts and approaches are applicable to more complex natural ecosystems. Emphasis is placed on integrating knowledge from many sources and on trying to assess quantitatively the importance of each component. The result is a comprehensive account making the book a valuable background for all interested in the study of plants in the field.