Nearly every life form found among North American flowering plants is represented in Arizona. This amazing diversity is partly explained by the fact that the altitudinal range extends from a few feet above sea level to approximately 12,000 feet at the summit of the San Francisco Peaks. The life zone range from Arctic-Alpine on these peaks to Lower Sonoran in the southwest and Subtropical in the extreme south. The main objective of this book is to provide means for identifying the approximately 3438 species of flowering plants, ferns, and fern-allies growing without cultivation in Arizona. Keys for identification of the families, genera, and species are provided. Under each species the authors give the geographical distribution within and outside Arizona, and usually the altitudinal range and time of flowering. They describe economic uses, toxic or other properties, and ornamental value of many plants, giving particular attention to the utilization of native plants by the large Indian population of the state. Introductory chapters describe the topography, geology, soils, and climate of Arizona, the several types of vegetation in relation to the physical conditions, and the proportional representation of the larger plant families. There is also a brief account of botanical explorations in Arizona since 1832. This is the only available work on the flora of Arizona that includes the results of intensive, botanical research in the state during the past twenty years. It is based on an earlier publication, Flowering Plants and Ferns of Arizona, issued by the U.S. Department of Agriculture in 1942 and now out of print. For the present revision, a supplementary section of more than fifty pages has been prepared under the direction of John Thomas Howell and Elizabeth McClintock of the California Academy of Sciences.