Following the success of the first edition of this work in 1927, a revised and enlarged second edition was published in 1937. It is this latter edition which is reproduced here. Material added to the second edition gave emphasis to the importance of flagellation as a guide to the interrelationships of the Phycomycetes and incorporated the discovery of heterothallism in rusts. In general, the style of the book was aimed at the student rather than the investigator, and it therefore benefits from an ease of readability as well as numerous illustrations. Although considerable advances have been made in the field of study since 1937, this volume will be of remaining value to anyone interested in the history of science in the early twentieth century.