This book is the outcome of twenty-seven years' experience, as a teacher of botany, during which fifteen years have been given to a graduate course on the morphology, classification and physiology of the fungi, and five years to a course which combined with this consideration a parallel study of the most important cultural and inoculation methods used by the practical bacteriologist and mycologist at the present day. The English and Germans have led in the production of text-books on mycology and pathology; Berkeley, Smith, Cooke and Massee in England, Frank, Sorauer, von Tubeuf and Küster in Germany. Americans have been behind in this important field, notwithstanding, that American plants harbor some of the most destructive fungi, which, through our careless methods of agriculture and horticulture up to the present, are annually destructive to the extent of millions of dollars. This lack is being rapidly remedied and the appearance of text-books by Duggar, Stevens, Hall and Stevens, Mel T. Cook and general monographs by Erwin T. Smith, and others, augurs well for the future of this line of literary and scientific labor. The bacteriologists have led and mycologists should follow.<br><br>The following pages represent in a much extended form the lectures and laboratory exercises given by the author before his botanic classes at the University of Pennsylvania, and before public audiences elsewhere, especially, Farmers' Institutes with which he has had three years' experience as a lecturer in Pennsylvania. The arrangement of the text has been suggested by the needs of the classroom and from an acquaintance with similar work in other colleges and universities in America. It is hoped that the book and the suggestions, as to teaching which it contains, will appeal to those responsible for similar courses. The keys are given with the anticipation that they will prove useful to the student and teacher who desire exercises in the classification of the fungi: The illustrations have been chosen with care, and credit is given in all cases for those borrowed from other books and monographs.