36 
METHODS IN PLANT HISTOLOGY 
drawn off, but, of course, this shows merely the form, with little or 
nothing of the internal structure. 
CLEARING AGENTS 
Clearing agents are so named because they render objects trans¬ 
parent. When clearing agents are used to precede infiltration with 
paraffin, the clearing is merely incidental, the real purpose being to 
replace the dehydrating agent with a solvent of paraffin. The 
clearing is useful, even in this case, because it indicates when the 
replacing has become complete. 
When the clearing agent is used to precede infiltration with 
paraffin, the material should always be most thoroughly dehydrated 
with absolute alcohol before beginning with the clearing agent. 
When the clearing agent is used to clear sections or small objects just 
before mounting in balsam, absolutely perfect dehydration is not 
necessary with all clearing agents. Bergamot oil, carbolic acid, and 
Eycleshymer’s clearing fluid (equal parts of bergamot oil, carbolic 
acid, and cedar oil) will clear readily from 95 per cent alcohol. Sec¬ 
tions to be cleared in xylol or clove oil should be dehydrated in 
“absolute” alcohol. If the absolute alcohol is below 99 per cent, 
xylol will not clear perfectly; but clove oil clears readily from 99 per 
cent and even from 98 or 97 per cent. If the absolute alcohol is 
not well above 99 per cent, it is a good practice to go from the alcohol 
to clove oil; and then, from clove oil to xylol. 
Xylol.—In our opinion, xylol is the best clearing agent to pre¬ 
cede infiltration with paraffin. After the material has been dehy¬ 
drated, it should be brought gradually into xylol. Thirty years 
ago it was customary to bring material directly from absolute alcohol 
into xylol; twenty years ago, two or three mixtures of absolute 
alcohol and xylol were used before reaching the pure xylol; at 
present, those who are doing the most critical work are making this 
process still more gradual. As cytologists have been studying more 
and more minute structures, the methods have become more and 
more critical. As in the case of the alcohol series, the xylol series 
has its grades closer together at the beginning than at the end. 
The following series seems to be sufficiently gradual: T V, J, \ f f, 
pure xylol. It is hardly necessary to use a graduate in making up the 
series. For the J, use equal parts of xylol and absolute alcohol; for 
the i, use equal parts of the \ and absolute alcohol; for the J, use 
