METHODS OF ISOI-ATION. 



13 



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

 clothing or bod)-, or from the air ; or it may ha\-e resulted from a few non-patliogenic 

 organisms accidental!)- present in the inner tissues of the ])lant, particular!)- in case 

 of roots whicli have been dug S(jme time. It is tliercfore mucli Ijetter lor tlie 

 student to l)egin witli plate cultures, (renerall)- speaking, the parasite will be more 

 easil)- obtained in a state of pnrit)- from plants or organs of plants recentl)- attacked 

 and from deep tissues, or from jtist within the margin of ad\-ancing diseased areas, 

 rather than from near the surface, or from parts which ha\-e been diseased for a 

 considerable time. 



Parts long affected almost alwa)'s contain mixed growths due to the multiplica- 

 tion of saproph)-tes of various kinds. From such parts it is usually much easier to 

 obtain the saprophvte than the parasite, e\-en if the latter has not been entirel)- 

 crowded out and destro\-ed. 



Fig. 8.* 



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

 mio"ht complicate resirlts, especiall)- 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 surf:ice organisms and then cut or dig 

 throuo-h this sterile surf.\ce Avitli hot or cold sterile scissors, scalpels, forceps, or 

 needles to a part which has not been affected b)- the heat, from which some of the 

 diseased fluids and solids may be removed on a sterile platinum loop. I frecpientl)' 

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



*FiG. S. — ?\Iiiskmelon plant No. 150, inoculatea with a pure culture of Bacilhis fraclicipliiliis. 

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

 X three days after *he photograph was made and ten iull days after the inoculation. 



