Many people are unaware that plants have diseases,-that they are afflicted by the same pathogenic agents as attack animals and man. Fungi, bacteria, viruses, insects, and nematodes are important agents causing plant maladies. Plants also suffer from serious deficiency diseases. For example, a whole host of plant diseases recently have been shown to arise from an insufficiency of boron. The study of viruses in plants is an intensely interesting one. In 1898, Beijerinck discovered that the infectious agent in juice from tobacco plants affected with mosaic disease could readily pass through filters that would hold back bacteria. As time passed, and more and more juices capable of initiating disease were tested, it became apparent that a considerable group of pathogens possessed this property. These agents also possessed in common the characteristics of being invisible under the microscope and of being unable to grow on any medium which did not contain living cells. This latter attribute made it necessary to develop a technique of handling viruses quite dissimilar in important respects from the bacteriological techniques developed earlier.