116 BACTERIAL DISEASES OF PLANTS 



selected. Of course, in the preliminary selection of material 

 for microtome sections, the greatest care should be exercised 

 to avoid that which is sandy or gritty. Dull knives and too 

 soft paraffin also tend to give sections which are crowded to- 

 gether endwise and this is undesirable. Always, thin, machine- 

 made sections are more or less wrinkled, and the folds must be 

 straightened out with gentle heat before the sections are glued to 

 the slides; i.e., while they are floating on the water. This may 

 be done, slide by slide over a low flame, or more conveniently on 

 a metal table (Fig. 54) warmed slightly by means of an electric 

 current, after which the slides bearing the sections are set on 

 end to drain and dry. There are various ways of sticking sec- 

 tions to slides. We now commonly use prepared egg-albumen, 

 a little of which is rubbed over the surface of the slide with a 

 clean ringer before water is pipetted on to the slide to receive 

 the paraffin sections. 



After two or three days, i.e., when the slides are thoroughly 

 dry they are ready for staining, and it is best that they should 

 be stained and covered soon, so that they will not accumulate 

 dust. In no case, if the material is good, should the stained 

 sections be left uncovered. 



STAINING METHODS 



Bacteria in the Plant. The paraffin of the properly dried 

 sections is melted away by very gentle heat, just enough to render 

 it fluid and no more, or they may be put into the xylol unwarmed, 

 if one is not in a hurry, and this is the better way, whereupon 

 the slides are put immediately into xylol, and often into a second 

 xylol, to complete the removal of the paraffin. They are then 

 graded through pure ethyl alcohols (95 per cent, usually 2 jars) 

 to remove the xylol, graded from this pure alcohol into water, 

 or alcohol and water to receive the stain. After this they are 

 graded back into absolute alcohol to remove the water, and from 

 this into pure xylol to remove every trace of alcohol. From 

 pure (water-free) xylol they are mounted in Canada balsam 

 or dammar balsam. The least water in the xylol will give a 

 cloudy slide. 



