ABSTRACT
Flower-breaking in tulips was familiar in the middle of the 16th century, and by 1719 it was well established, at least in some circles, that flower-breaking in jasmine could be graft-transmitted. Early electron micrographs of isolated virus particles were not very informative because of low contrast between particle and background. This is because neither proteins nor nucleic acids are opaque to electron beam. However, even without any enhancement of image contrast, some valuable work was done. All plant viruses whose nucleic acids were analyzed before late 1960s were shown to contain single-stranded RNA and it was thought that all viruses infecting plants had single-stranded RNA as opposed to many viruses infecting bacteria or vertebrates, which have DNA. Investigations on virus-infected plant cells began very soon after the discovery of viruses. Taxonomy has two aims: to break down a mass of units into named groups of like individuals and to relate these groups in some kind of hierarchy reflecting their evolution.
