12 BULLETIN 40, U. S. DEPARTMENT OP AGRICULTURE. 



ing tissues. N. viscosum is sometimes killed in exactly the same man- 

 ner, yet no attendant mosaic symptoms have ever appeared in the 

 affected plants. Although frequent inoculations have been made 

 deep into the tissues of all parts of N. glauca and N. tabacum, these 

 species have never shown any indications of rot at the punctured 

 points. Plants of the species N. glauca and N. viscosum have been 

 repeatedly cut back and mosaic virus poured upon all cut surfaces. 

 This severe treatment has invariably failed to produce any indica- 

 tions of the disease. 



Other experiments were carefully made to determine whether the 

 sap of mosaic plants could be rendered innocuous by mixing it with 

 different proportions of the sap of immune species. At first a num- 

 ber of trials were carried out by mixing equal quantities of mosaic 

 virus and the sap of Nicotiana viscosum. As controls, the sap of 

 healthy plants was substituted for the mosaic virus and diluted in 

 the same manner. The mixtures were shaken thoroughly and allowed 

 to stand several hours. Later trials were made in exactly the same 

 manner, using the sap of the immune species of N. glauca. In pass- 

 ing, it may be remarked that the diluted virus in no instance lost 

 its infectious properties, since every plant developed the disease in 

 15 to 18 days following inoculation. All controls remained healthy. 



The virus was still further diluted with the sap of Nicotiana 

 glauca and tested as follows: Eighteen young plants of Connecticut 

 Broadleaf were arranged in two series of nine plants each. One 

 series was inoculated with a mixture of one-tenth mosaic virus and 

 nine-tenths healthy glauca sap. This was shaken and allowed to 

 stand three hours at a greenhouse temperature of 80° to 85° F. before 

 using. Xine control plants were inoculated in the same manner, 

 using healthy tobacco sap diluted with glauca sap in the same pro- 

 portions. Xeedle inoculations in the leaves were made on December 

 22. Examined on January 9, eight of the series inoculated with the 

 diluted virus had developed the mosaic disease. All the controls 

 were healthy. It is evident that the virus was not rendered in- 

 nocuous. 



In further tests with immune species the writer has made many 

 successful graftings of susceptible varieties of Nicotiana tabacum, 

 and N. rustica with the immune species N. glauca and A T . viscosum. 

 The scions in both instances by subsequent inoculations readily be- 

 came diseased, but the immune stocks, which were constantly putting 

 forth vigorous branches, have never shown indications of the dis- 

 ease, although these were growing permanently and intimately in 

 contact with the badly diseased scion. Many inoculation experi- 

 ments have shown conclusively that the virus of the mosaic scions is 

 likewise present in the leaves and shoots of the N. glauca stock, 

 although no visible mosaic symptoms are produced. Furthermore, 



