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METHODS IN PLANT HISTOLOGY 
CLEARING 
After the sections have been dehydrated, they must be cleared, 
or made transparent by some clearing agent. The clearing agent 
must be a solvent of balsam, but it is not at all necessary that the 
balsam shall be dissolved in the particular clearing agent which has 
been used. Xylol balsam is used not only when preparations have 
been cleared in xylol, but also when they have been cleared in clove 
oil, cedar oil, bergamot oil, or other clearing agents. 
Xylol is the most generally useful clearing agent. Place the 
slide in equal parts of xylol and absolute alcohol and then in pure 
xylol, allowing each to act for about 2 minutes. 
Clove oil is also an excellent clearing agent. The clove oil should 
follow the absolute alcohol, without any mixtures. Pour on a few 
drops of clove oil, and drain them off at once in such a way as to carry 
with them whatever alcohol may still remain. Then flood the slide 
repeatedly with clove oil, draining the clove oil back into the bottle. 
If judiciously used, 50 c.c. of clove oil is enough to clear 100 prepara¬ 
tions. Sections are usually cleared in a few seconds. The only 
objection to clove oil is that mounts harden slowly. To overcome 
this difficulty, the slide should be dipped in xylol for a minute before 
mounting in balsam. 
Synthetic oil of wintergreen is much less expensive and some 
claim that it is just as good as clove oil. 
For clearing sections on the slide, other clearing agents are 
hardly worth mentioning. 
MOUNTING IN BALSAM 
After the sections are cleared, wipe the slide on the side which 
does not bear the sections. Put on a drop of Canada balsam and 
add a clean, 1 thin cover. Before the cover is put on, pass it through 
the flame of an alcohol lamp to remove moisture, for it would be a pity 
indeed to injure a preparation at this stage of the process. Add a 
label, and the mount is complete. 
1 Slides and covers should be treated with hydrochloric acid, or equal parts of hydrochloric 
acid and water, for several hours. They should then be thoroughly rinsed in water and' wiped 
with a cloth perfectly free from lint. After rinsing in water, they may be kept in 95 per cent 
alcohol. When a cover is needed for use, it is Dr. Land’s practice to rest the corner of the cover 
on a piece of filter paper to remove the drop of alcohol; then pass the cover through the flame of a 
Bunsen or alcohol lamp. The film of alcohol will burn and the cover may warp, but it will usually 
straighten, and it will be clean and dry. 
The mixture of sulphuric acid and bichromate of potash, used for cleaning laboratory glass¬ 
ware, is equally good for slides and covers. 
