METHODS OF ISOLATION. 13 



more bacteria accidentally introduced from the surface of the plant, from one's 

 clothing or body, or from the air ; or it may have resulted from a few non-pathogenic 

 organisms accidentally present in the inner tissues of the plant, particularly in case 

 of roots which have been dug some time. It is therefore much better for the 

 student to begin with plate cultures. Generally speaking, the parasite will be more 

 easily obtained in a state of purity from plants or organs of plants recently attacked 

 and from deep tissues, or from just within the margin of advancing diseased areas, 

 rather than from near the surface, or from parts which have been diseased for a 

 considerable time. 



Parts long affected almost always contain mixed growths due to the multiplica- 

 tion of saprophytes of various kinds. From such parts it is usually much easier to 

 obtain the saprophyte than the parasite, even if the latter has not been entirely 

 crowded out and destroved. 



Fig. 8.* 



Great care must be exercised to avoid introduction of surface organisms which 

 might complicate results, especially if rapid growers. The easiest and most satis- 

 factory way, when the tissues will admit of such treatment, is to sear the surface 

 with a hot knife or spatula so as to burn all surface organisms and then cut or dig 

 through this sterile surface with hot or cold sterile scissors, scalpels, forceps, or 

 needles to a part which has not been affected by the heat, from which some of the 

 diseased fluids and solids may be removed on a sterile platinum loop. I frequently 

 sear upon sound tissues at one side of the spot from which I desire to make cultures 



*Fic. 8. Muskmelon plant No. 150. inoculated with a pure culture of Bacillus tracheiphilus. 

 The pricked leaf is on the left side. The section shown in plate 3 was taken from the point marked 

 X, three days after the photograph was made and ten full days after the inoculation. 



