The search for compounds effective in the immunization of crop plants from insects or fungi has been prolonged and, with few exceptions, unsuccessful. It was not until the early 1940's that Kukenthal, in the routine testing of the biological activities of new compounds prepared by the I. G. Farbenindustrie A. G., observed that certain fluorine and phosphorus compounds synthesized by Schrader (1917), had this property and were described by Schrader as suitable for the “internal therapy” of plants. Because of the accepted application of the term therapeutant by Cunningham (1935) to toxicants in general, Shaw and the writer (1947) borrowed the term systemic (meaning that the plant as a whole is affected) to differentiate this type of action from the localized effect of previously known insecticides.