688 PHARMACEUTICAL BOTANY 
acetic acid, or formaldehyde) to the water used in mounting and 
carefully sealing the cover glass with asphaltum or zinc white. 
As a rule, a better way is to use a mounting medium that will not 
evaporate, e.g., glycerine, glycerin gelatin or Canada balsam. 
These fluids have a high refractive index and so render the 
objects penetrated by them more transparent. This quality is 
generally an advantage, but for objects already almost trans- 
parent it is quite the reverse. Glycerine has the disadvantage of 
always remaining soft, so that the mount may at any time be 
spoiled by careless handling. Glycerin-gelatin has the advan- 
tage of mixing readily with 50 per cent. glycerin in which the 
object should be placed before being mounted in this medium. 
It should be warmed on a water bath before using and the 
cover glass applied quickly after it is placed on the specimen. It 
cools rapidly and constitutes the quickest and simplest means of 
effecting a durable permanent mount. Its disadvantage is due 
mainly to its jelly like consistency which is frequently responsible 
for damaged mounts when the cover-glass above the preparation 
is too greatly strained. Canada balsam slowly becomes solid, so 
that the mount is exposed to no accident short of actual breakage. 
Balsam has the disadvantage of being non-miscible with water, so 
that before it can be used the object must be carefully dehydrated. 
Even after this is done, and the object lying in absolute alcohol, a 
clearing oil such as cedar oil, clove oil or xylol must be used as an 
intermediate agent between alcohol and balsam. 
2. STAINING.—For two reasons it is generally better to stain 
plant tissues before mounting. Transparent tissues may become 
almost invisible in glycerine, glycerin-gelatin, or balsam, and 
different tissues take a stain differently. This being the case it 
becomes possible to stain one tissue and not another, or one tissue 
with one stain and another in the same section with a different 
stain (double staining), so that the different parts may be brought 
out like areas on a colored map. The most common stains are 
hzematoxylin derived from logwood, and various anilin stains— 
safranin, fuchsin, eosin, crystal violet, fast green, methyl violet, 
light green, iodine green, methyl-green, malachite green, etc. 
A number of useful methods of staining and mounting plant 
materials will now be discussed. For additional methods the 
