34 Methods in Plant Histology 



Some very minute objects, like bacteria and the smaller Cyano- 

 phyceae, may be dehydrated by heating them until all water is 

 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. 

 Sections to be cleared in xylol or clove oil should be dehydrated in 

 absolute alcohol. 



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. Twenty years 

 ago it was customary to bring material directly from absolute 

 alcohol into xylol; ten 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 : y 1 ^ , }, 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 



