METHODS IN PLANT HISTOLOGY 
color caused by the acid has been replaced by the rich purple color 
so characteristic of haematoxylin. The acid not only secures differ¬ 
entiation by dissolving out the stain from lignified structures more 
rapidly than from cellulose walls, but it also removes the disfiguring 
precipitates which almost invariably accompany 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 appeared, 
the sections should be left in water for 20 or 30 minutes. They 
might be left for several hours. 
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 to 5 minutes—they are ready for mounting in balsam. If 
the 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 scalpel, 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 important views, transverse, longitudinal 
radial, and longitudinal tangential sections. It is wasteful to use 
three slides and three covers to show these three views, or to make a 
mount containing only a single section of the rhizome of Pteris. 
Put the label at the left. Write first the genus and species; then 
indicate what part of the plant has been mounted. The date on 
which the material was fixed is often valuable. After a year or so, 
