mJ';tiioi)S of isolation. 



II 



quite constantly, but iiierel_\' as foll(.)\vers of soinethin,t^' else. When possiljle, 

 therefore, diseased plants should be examined for the suspected patlnji^en, in large 

 nuud:)ers, in different years, and from wideh' separated localities. Of course, if fungi 

 are also ])resent they nurst likewise Ije examined as to constant occurrence and 

 pathogenic properties. 



lender (h) all of the standard nutrient media should be tried, and that repeatedly, 

 until the student is entirely familiar with the appearance and beha\'ior of the 

 organism. It is irsuall\' Ijest to isolate the organism for experiment from selected 

 portions ol the tissue \)y means of Ksmarch roll-cultures <>r b\' the use of ])i)ured 

 plates (Petri-dish cultures), generally the latter. 



Isolations nia\" also be made by inserting a sterile platinum needle or loop into 

 the diseased tissue, obtaiuiui;- therefrom a little lluid, and drawiu"- this o\'cr the 



Fig. 6.* 



surface of slant agar, gelatin, or potato a number of times. This is an old method 

 introduced h\ Koch in i8Si. If ten or twelve tubes are used, the final streaks will 

 often consist onh' of scattering colonies, from one or nrore of which the subcultures 

 may be made. The plate method has the great ad\-antage of showing just how 

 man\' kinds of bacteria are present in the tissues (proA'ided they will all grow in the 

 medium used and under the conditions of the experiment), and just how numerous 

 they are. In case of viscid organisms, or those forming compact zoogloete in the 



''Fig. 6. — Cross-scctian f>f root of plant No. 53 (turnip) para^nized by Bacterium cam^cstrc, 

 showing- an early stage an the formation of a haoterial cavity. The original section was made from 

 material fixed in alcnhol, infillrated with paraffin, stained 'with carbol-fnchsin, and washed in a mix- 

 ture of alcohol and water. I?)rawn from a photomicrrigraph. X 500 



