90 



METHODS IN PLANT HISTOLOGY 



pany staining with Delafield's haematoxylin. The acid also washes 

 out the safranin; it is for this reason that the washing after safranin 

 should be stopped while there is still some red color in the cellulose walls. 

 The acid should not only reduce the density of the haematoxylin and 

 remove precipitates but should also remove the little safranin which 

 may remain in the cellulose walls. After the purple color has ap- 

 peared, the sections should be left in water for 20 or 30 minutes, 

 changing frequently. They might be left in the water for several hours. 

 The acid must be washed out thoroughly or the stains will fade. 



Now place the sections in 50 per cent alcohol for 1 minute, then in 

 95 per cent alcohol for 1 minute, 100 per cent alcohol for 5 minutes, 

 and then transfer to xylol. As soon as the sections become clear — in 

 about 1-5 minutes — they are ready for mounting in balsam. If the 



o 



Fig. 18.— The label 



sections do not clear readily, as may be the case if the air is damp, or 

 if the alcohol or xylol is not quite pure, transfer from the absolute 

 alcohol to clove oil, which will clear, even if the absolute alcohol is 

 rather poor. Then transfer from clove oil to xylol; the objection to 

 mounting directly from clove oil is that preparations harden more 

 slowly than when mounted from xylol. With a section-lifter, or scal- 

 pel, or brush, transfer 3 or 4 sections to a clean, dry slide, put on 1 or 2 

 drops of balsam, and add a cover, first heating it gently to remove 

 moisture. If xylol has been used for clearing, it is necessary to work 

 rapidly; for the sections must never be allowed to dry. Use square or 

 oblong covers for such mounts, reserving round covers for glycerin 

 mounts^ If material is abundant, use as many sections as you can 

 cover conveniently. If you have used several stains with the same 

 material, select for each mount sections from the different stains. In 

 ordinary wood sections each mount should show the three most im- 

 portant views, transverse, longitudinal radial, and longitudinal tan- 



