This book has three directions of special emphasis: first, to develop the student's reliance on experiments in forming generalizations about his science; second, to depict science as a complex of imperfect approximations derived by the scientific method; and third, to reorganize the subject to make it more nearly representative of modern plant physiology in the laboratory and in the field. This book I have drawn on experimental evidence insofar as possible to illustrate the concepts being discussed. The book is centered about the workings of the growing plant, without organized coverage of biochemistry and nutrition. This assumes a logical pedagogical division of the subject of plant physiology into a section on growth and development and another section on nutrition and metabolism. This is also done with the hope of maintaining an appropriate level of interest in the functions of the living plant, in the face of a current tendency toward preoccupation with grindates, supernates, and simulated life activities in test tubes without sufficiently clear relationship to the growth of the whole plant.It is hoped that the presentation here will paint an approximate picture of the status of this science today and at the same time will make the student think in terms of experimental units of information. The best experimental units incorporate the weaknesses of the human minds that conceived them and carried them out, and no generalization from these units is more dependable than the underlying experiments.In using a case-history approach, the student will recognize the inconsistencies among the "e;facts"e; accumulated so far, and that this will make him a little more sanguine about the sharp disagreements between equally dependable scientists and will teach him to relish the exchange of criticism that sharpens good science and shrinks poor science.