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The fourth edition of this small book has been rewritten and is virtually a new book. This has been made necessary by the many advances in our knowledge of plant viruses which have taken place during the comparatively few years since the publication of the third edition. In order to give even a brief account of this new knowledge, it has been necessary to omit some of that part of the previous edition which dealt with the purely practical aspects of plant virology. Another reason for this omission is the imminent publication of a multi-volume treatise dealing with every type of technique used in the study of viruses in general. This book was rewritten during the tenture of a Visiting Professorship at the University of Texas in Austin. Grateful acknowledgement is due to Dr W. Gordon Whaley, dean of the Graduate School, and to the cell Research Institute for the facilities afforded to the writer. Acknowledgement is also due to several friends who have supplied prints of illustrations from their published work; credit has been given to authors in the illustration legends.