134 BROWN: WooDs FOR MICROSCOPIC STUDY 
fully coated inside with hard paraffin, or, still better, in cups pre- 
pared by cutting empty hydrofluoric acid bottles. Strong hydro- 
fluoric acid is added to cover the material, and the container then 
corked or covered with a hard paraffin plate for a period varying 
from a few days to six weeks or longer, the length of time being 
determined by removing blocks at intervals of one to several days 
and testing with a sharp scalpel until they are found to be suf- 
ficiently soft to cut easily in transverse section. 
In the writer’s experience, tissues are 
rarely injured by leaving them a long time 
in hydrofluoric acid. Thus, in case of a 
young stem of Dracaena curea Mann, deli- 
cate tissues, such as the phloem, cambium, 
undifferentiated parenchyma, and other 
thin-walled tissues, together with the bun- 
dles of needle-like raphides of calcium ox- 
alate, were all left uninjured by long treat- 
ment in acid. In one instance, a block of 
Santalum Freycinetianum Gaud., of the size 
and shape described for Fic. 2, was placed 
wiki eo in acid on September 6, 1917, and left until 
eins: March 26, 1918, when it was washed and 
sectioned. Though in moderately strong 
acid for over six months, all structures, including the mineral 
crystals, were in perfect condition. 
Hydrofluoric acid probably softens the tissues mainly, if not 
entirely, by the removal of silica (desilicification). But other 
minerals would probably be acted upon. For example, calcium 
would form the insoluble calcium fluoride which would remain in 
the wall. Curiously, crystals of calcium oxalate in crystal paren- 
chyma or idioblasts usually remain nearly or quite intact long 
after the wood has been sufficiently softened to cut well. Even 
in maceration by Schultze’s solution, crystals may be uninjured. 
Dr. A. J. Hill suggests that, in case of the hydrofluoric acid treat- 
ment, the crystals may have been protected by the formation of 
an insoluble film of calcium fluoride, the possibility of which is 
seen from the following equation: 
CaC,0, + 2 HF = soy + H2C20, 
nsol, 




