This volume presents the following four late nineteenth century papers: "Concerning the Mosaic Disease of Tobacco" by Adolf Mayer; "Concerning the Mosaic Disease of the Tobacco Plant" by Dmitrii Ivanowski; "Concerning a Contagium Vivum Fluidum as a Cause of the Spot-Disease of Tobacco Leaves" by Martinus W. Beijerinck; and "On the Etiology of Infectious Variegation" by Erwin Baur.