84 Botanical Microtechnique 



to insure adhesion of the celloidin, it is better to dehydrate in ethyl 

 alcohol or acetone and to use tlie much more expensive Cellosolve as 

 the infiltration solvent. 



Special Treatment of Hard Woods 



The foregoing methods of infiltration yield excellent results ^vith 

 some soft woods such as willow, poplar, basswood, white pine, and 

 many other woods. These can be infiltrated in celloidin without 

 special preliminary treatment, but oak, hickory, walnut, the yellow 

 pines, and other woods are too hard to section by the regular process. 

 Such materials can be softened by treating with hydrofluoric acid 

 (HF) . This highly corrosive reagent is pmchased in wax bottles and 

 should be used in wax or wax-lined or plastic containers. Because of 

 the corrosive action of the licjuid and vapor on glass, metals, and the 

 skin, HF siiould be used in an isolated part of the premises, away 

 from valuable instruments. The staining and microchemical reactions 

 of tissues are materially altered by this treatment. 



Twigs having li\ing bark tissues are first killed as usual and trans- 

 ferred to HF. Dry woods arc prepared for treatment in HF bv 

 alternate boiling in water and exhausting in an aspirator in cold 

 water until the pieces are saturated. The safest concentration of HF 

 for most subjects is commercial acid diluted with approximately 

 twice its volume of water. The duration of treatment in HF varies 

 greatly with the hardness of the material, the size of the pieces, and 

 f)ther factors. As a trial, treat a hard wood such as oak for T) days, 

 wash in running water lor at least 1 hr. to make it safe to handle 

 the pieces, and try to cut thin slices with a sharp razor blade. An 

 alternative mciliod of testing is to wash the pieces in running water 

 for 4 hr., clamj) a piece into the sHding microtome, and list its ciuting 

 properties. After making a test, wash and \\'i])e the danij) and the 

 knile thoroughly. If the material is too hard to tut rtaihh litlicr 

 freehand or with the microtome, return to the HF for another ■\- to 

 5-day inter\al, and test again. When the wood seems to cut satis- 

 factorily, wash for at least 4<S hi., whether it is to \k- embedded in 

 celloidin or cut without cinhcdding. Wood oi i\vi,i;s that ha\e been 

 ticaied with HI'' and arc to l)c |)ut into stoia<;c without embedding 

 should i)e dehydrated thiough 20, 10. and (iO' , ahohol or acetone 

 at 1- to 8-hr. intervals, then stored in a mixture of eipial \()lumes of 

 alcohol, glycerin, and water. 



I he softening of wood can be accelerated I)\ treating in HF 

 undei pressure. The necessary e(|uii)ment is not axaiiable (ommerc ially 



