BROWN: WooDs FOR MICROSCOPIC STUDY 133 
rays. Occasionally a species with higher rays or other special 
features may require blocks of larger dimensions. When present, 
both heart-wood (H) and sap-wood (S$) may be included. The 
block should be carefully trimmed with a knife so that the tissues 
are perfectly oriented with respect to each of the planes intended 
for sectioning. The medullary rays in particular should coincide 
as perfectly as possible with the radial plane. In tropical woods, 
the rays are often narrow, visible only under a lens, and curved, 
so that more than ordinary care is necessary in trimming this 
surface. 
Before proceeding with the treatment, the blocks should be 
numbered for sake of record. Some use a system of notches along 
the edges. Perhaps one of the best methods is to carve Roman 
numerals on the radial face not intended for sectioning, underscor- 
ing IX to distinguish it from XI (N, Fic. 2); also, many tropical 
woods require an arrow to indicate the direction of growth. 
B. REMOVING THE AIR 
After the blocks have been cut the desired shape, air should 
be extracted as far as possible from the cell lumina by alternate 
boiling and cooling in water. The use of an Eimer and Amend 
aspirator No. 3250 after boiling greatly hastens the process. This 
treatment should be continued until the lightest blocks sink and 
little or no air comes from the tissues when the aspirator is applied. 
Most blocks sink in a few hours, but species with numerous tyloses 
may take a longer time. Thus, blocks of Rhus semialata Murr. 
var. sandwicensis Gray remain floating five days during the boiling 
and cooling process, or two days when boiling periods are followed 
by the application of the aspirator. 
C. SOFTENING THE TISSUES 
Nearly all xerophytic species of tropical woods require a long 
treatment in strong hydrofluoric acid, often extending over several 
weeks, before they can be sectioned. Rain forest and bottom- 
land woods are frequently soft, but even such woods usually cut 
with clearer outlines when treated for at least a few days in acid. 
For treating material with hydrofluoric acid, the blocks, after 
removal of air, may be placed in a wide-mouthed glass bottle, care- 
